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Nobel Prize in Physics 2015

Tuesday, October 6th, 2015

So, the Nobel Prize in Physics 2015 has been announced. To much surprise of many (including the author), it was awarded jointly to Takaaki Kajita and Arthur B. McDonald “for the discovery of neutrino oscillations, which shows that neutrinos have mass.” Well deserved Nobel Prize for a fantastic discovery.

What is this Nobel prize all about? Some years ago (circa 1997) there were a couple of “deficit” problems in physics. First, it appeared that the detected number of (electron) neutrinos coming form the Sun was measured to be less than expected. This could be explained in a number of ways. First, neutrino could oscillate — that is, neutrinos produced as electron neutrinos in nuclear reactions in the Sun could turn into muon or tau neutrinos and thus not be detected by existing experiments, which were sensitive to electron neutrinos. This was the most exciting possibility that ultimately turned out to be correct! But it was by far not the only one! For example, one could say that the Standard Solar Model (SSM) predicted the fluxes wrong — after all, the flux of solar neutrinos is proportional to core temperature to a very high power (~T25 for 8B neutrinos, for example). So it is reasonable to say that neutrino flux is not so well known because the temperature is not well measured (this might be disputed by solar physicists). Or something more exotic could happen — like the fact that neutrinos could have large magnetic moment and thus change its helicity while propagating in the Sun to turn into a right-handed neutrino that is sterile.

The solution to this is rather ingenious — measure neutrino flux in two ways — sensitive to neutrino flavor (using “charged current (CC) interactions”) and insensitive to neutrino flavor (using “neutral current (NC) interactions”)! Choosing heavy water — which contains deuterium — is then ideal for this detection. This is exactly what SNO collaboration, led by A. McDonald did

Screen Shot 2015-10-06 at 2.51.27 PM

As it turned out, the NC flux was exactly what SSM predicted, while the CC flux was smaller. Hence the conclusion that electron neutrinos would oscillate into other types of neutrinos!

Another “deficit problem” was associated with the ratio of “atmospheric” muon and electron neutrinos. Cosmic rays hit Earth’s atmosphere and create pions that subsequently decay into muons and muon neutrinos. Muons would also eventually decay, mainly into an electron, muon (anti)neutrino and an electron neutrino, as

Screen Shot 2015-10-06 at 2.57.37 PM

As can be seen from the above figure, one would expect to have 2 muon-flavored neutrinos per one electron-flavored one.

This is not what Super K experiment (T. Kajita) saw — the ratio really changed with angle — that is, the ratio of neutrino fluxes from above would differ substantially from the ratio from below (this would describe neutrinos that went through the Earth and then got into the detector). The solution was again neutrino oscillations – this time, muon neutrinos oscillated into the tau ones.

The presence of neutrino oscillations imply that they have (tiny) masses — something that is not predicted by minimal Standard Model. So one can say that this is the first indication of physics beyond the Standard Model. And this is very exciting.

I think it is interesting to note that this Nobel prize might help the situation with funding of US particle physics research (if anything can help…). It shows that physics has not ended with the discovery of the Higgs boson — and Fermilab might be on the right track to uncover other secrets of the Universe.


Nobel Week 2015

Monday, October 5th, 2015

So, once again, the Nobel week is upon us. And one of the topics of conversations for the “water cooler chat” in physics departments around the world is speculations on who (besides the infamous Hungarian “physicist” — sorry for the insider joke, I can elaborate on that if asked) would get the Nobel Prize in physics this year. What is your prediction?

With invention of various metrics for “measuring scientific performance” one can make some educated guesses — and even put the predictions on the industrial footage — see Thomson Reuters predictions based on a number of citations (they did get the Englert-Higgs prize right, but are almost always off). Or even try your luck with on-line betting (sorry, no link here — I don’t encourage this). So there is a variety of ways to make you interested.

My predictions for 2015: Vera Rubin for Dark Matter or Deborah Jin for fermionic condensates. But you must remember that my record is no better than that of Thomson Reuters.


It’s the beginning of September, which means two things. 1- I haven’t managed to write a blog in 6 months (turns out second year is busy!), and 2- it’s nearly the start of the academic year. These two facts have inspired me to write a post aimed for all those fresh-faced 18 year olds about to embark on the adventure of university. Or college for the Americans– a weird concept to us Brits, as college is in fact where 16 year olds go to do mainly non-academic courses like “Travel and Tourism” and “Hair and Beauty”. Right now in the UK, thousands and thousands of teenagers are probably getting increasingly nervous as their start dates in September and October loom nearer and nearer. I thought I would write something for anyone who is about to attempt a physics degree at university, with none of that prospectus fluff.

I think the most succinct way to sum up my undergraduate degree is “play hard, work harder”! It wasn’t easy, some of the time it felt downright impossible but at the end of the day I had fun, I made friends, I got a masters degree and finally a PhD place. What more can you ask for from university?

Other Physicists
So, first things first, before you do any physics, you are going to see and meet some of your coursemates. I’m going to be brutally honest here: physicists are weird. They just are. To dedicate yourself to a subject like physics you just need to have a little bit of weirdness in you, that seems to be a fundamental law. This is not always a bad thing. A disclaimer here, some of my best friends are physicists. My colleagues at UCL are a brilliant, funny and sociable bunch. My boyfriend, although now having sold his soul to the actual law, I met studying the laws of physics at uni. There is nothing wrong with physicists as a whole, but a lot of them are a little strange. You may encounter people who are painfully socially awkward, wear fedoras or suits to lectures, LARPers, guys with LOTS of hair (both on their head and faces), posh kids buying Grey Goose, poor kids living on supernoodles and beans, international kids, gamers, heavy drinkers, those that are politically driven, or lazy with questionable hygiene, and on rare occasions, women.
I joke, I think my course was ~10% girls by the masters year, so not too rare. It’s getting better, and I think we are better off than computer sciences, but we are still in the minority. If you are so lucky to be female amongst the physicists, be warned, they may stare, and they will probably know who you are when you can’t possibly be expected to remember all those generic male faces! I can’t count the times I was approached by strangers, usually in clubs but once on a train, with the line ‘you’re that girl who does physics!’.
I had the good luck of having another physicist in my halls of residence – a rather normal one, who played guitar and drunk a lot. We were flatmates for many years, and we stuck together in lectures and labs as much as we could. In my first year, I remember having a very strong aversion to making any other friends on my course. They’re all awkward and weird and nerdy, I said. I don’t want to hang out with them, they wont be fun, I said. The important thing here is I WAS WRONG. After I realised my flatmate was leaving after third year with a bachelors, I made a bit of effort to meet people, and made some extremely good friends in physics, who I still see often. And guess what? They aren’t weird and no fun. They are really great guys, and I wish I had made friends with them earlier.
So don’t write anyone off immediately. Be sociable, chat to people. People will be shy (I was, and still am) and awkward, but give everyone a chance. You wont get on with everyone, but you may be surprised at who you do end up friends with. Physicists are usually a little bit odd – but they are also often a lot of fun.

The Actual Physics
A very important thing to understand when you start a course like physics (or maths, or any science really) is that unless you are some supreme genius, there will be people cleverer than you. Lots of them. If you are going to a top uni with very high entry grades (Warwick at the time was AAB, A*s weren’t available yet) then chances are you are going to feel a little bit inferior. I went from being the top of my physics A-level class to somewhere in the upper quartile, and at first it was a little disconcerting. But don’t worry – there are more qualities to a person than their grades!

Doing stuff like this in the library will make you feel clever.

Doing stuff like this in the library will make you feel clever.

For my first few weeks at Warwick, I was in a bit of a panic. We had a course called “Physics Foundations” which you may think sounds like some nice gentle introductory course. Wrong. We were thrown in the deep end. It bared almost no resemblance to A-level physics (there was, thank God, a mechanics course that did, but it came after Special Relativity, which also had me panicking a fair bit) and involved all sorts of notation and nomenclature I’d never even heard of (like ’tilde’?!). I also had not done further maths, and whilst they brought us up to speed in maths quite quickly I did feel a little disadvantaged by my lack of knowledge on imaginary numbers. I genuinely spent the first few weeks thinking I chose the wrong course. What had felt so right at A-level, so naturally the thing I was best at, was now giving me an identity crisis. I began wondering how I would explain to my friends that I had failed.

And then, a miracle happened. I talked to other people. I talked to the students in my tutor group and my seminar groups. And guess what – they were all just as confused as me. This was a wonderful realisation. I also noticed my problem sheet marks were actually not so bad. I didn’t always understand what I was doing but I seemed to be doing it the right way. This is another important thing to note – do not expect to understand your lectures. I didn’t understand much at all until I did problems, past papers and proper revision – often in the third term! Do not panic early on if things aren’t going in. Do not think problem sheets aren’t important. They help, seriously.

You’re going to need to do some work. You’re going to notice your hallmates doing humanities having only 8 hours a week of contact time, whereas you’re closer to 25. You will be swamped every week with problems and lab reports and will have problems classes on top of your lectures. You WILL hate labs – I am yet to speak to anyone who really enjoyed them. But it is all essential to your development as a competent physicist (honestly..) and you will be glad of it in the long run.

Start of degree vs end of degree. Proof that blondes don’t have more fun? 

Living Conditions
Now this is an interesting one. You are probably going to be in student halls. Brace yourself. Here are some things that WILL happen:
– If you drink, you will vomit (probably multiple times).
– Again, if you drink, you will be forced to down a dirty pint (the worst I ever saw contained whisky, milk, garlic and and beer). And then you will probably vomit.
– You will really hate 9am lectures. Especially if you’re hungover/still drunk
– Everything will be a mess. All the time. No one will wash up
– You will encounter the panicked rush far too early to sort out a house for your second year, and you may end up not even liking the people you are gong to live with by the end of term
– The toilet will be covered in all manners of disgusting bodily fluids on multiple occasions.
– You will get freshers flu and feel ill for weeks and your lecture halls will be filled with the sound of coughing.
– Someone will not understand how to use a washing machine (and there may even be someone who takes their laundry home to their mum)
– You will inevitably fall out with someone who was initially your best friend
– There will be some romance and some drama. Some couples will last, others will not. Inevitably, people will start breaking up with home boyfriends/girlfriends.
– You wont change your sheets for an unholy amount of time.
– There will be people in your halls you didn’t even know existed until you awkwardly encounter them in the corridor at the end of term or cooking in the middle of the night.
– You will feel sad and miss your parents, your pets and your home friends, no matter how much fun you’re having.
Sound fun? Unfortunately, this seems to be what it takes to get yourself a physics degree. Things might improve when you move off campus into a house, but this is heavily dependent on your choice of housemates. Really, you have to work out what works best for you in order to survive student living. Maybe you wont mind the mess and the mould. What I will say though is please, please wash your sheets at least once a term. It’s gross.

There’s going to be blood, sweat and tears. Literally. There’s going to be fights and drama and emotional and intellectual struggles. There’s going to be regret and awful hangovers. There will be late nights writing lab reports or finishing problems. You will want to tear your hair out over the electromagnetic field of an infinite charged plane, or a pulley with mass, or second order differential equations, or whether or not γμ is a four vector (spoiler: it isn’t). You will hate some lecturers – worst are the ones that pick people out to answer questions, some will send you to sleep and others you will love and respect. You are going to hate physics, you’re going to love physics, and you’re going to question yourself why the hell you chose it. But in the end, if you make it out with your degree, you’ve done something incredible, and a lot of doors will be open to you. I always knew I wanted to stay with physics and my four years at Warwick left me still enjoying physics and well prepared for a PhD.

If you’re about to start your degree – it’s going to be a wild ride, but it may just be some of the best years of your life. Good luck!


All those super low energy jets that the LHC cannot see? LHC can still see them.

Hi Folks,

Particle colliders like the Large Hadron Collider (LHC) are, in a sense, very powerful microscopes. The higher the collision energy, the smaller distances we can study. Using less than 0.01% of the total LHC energy (13 TeV), we see that the proton is really just a bag of smaller objects called quarks and gluons.


This means that when two protons collide things are sprayed about and get very messy.


One of the most important processes that occurs in proton collisions is the Drell-Yan process. When a quark, e.g., a down quark d, from one proton and an antiquark, e.g., an down antiquark d, from an oncoming proton collide, they can annihilate into a virtual photon (γ) or Z boson if the net electric charge is zero (or a W boson if the net electric charge is one). After briefly propagating, the photon/Z can split into a lepton and its antiparticle partner, for example into a muon and antimuon or electronpositron pair! In pictures, quark-antiquark annihilation into a lepton-antilepton pair (Drell-Yan process) looks like this


By the conservation of momentum, the sum of the muon and antimuon momenta will add up to the photon/Z boson  momentum. In experiments like ATLAS and CMS, this gives a very cool-looking distribution


Plotted is the invariant mass distribution for any muon-antimuon pair produced in proton collisions at the 7 TeV LHC. The rightmost peak at about 90 GeV (about 90 times the proton’s mass!) is a peak corresponding to the production Z boson particles. The other peaks represent the production of similarly well-known particles in the particle zoo that have decayed into a muon-antimuon pair. The clarity of each peak and the fact that this plot uses only about 0.2% of the total data collected during the first LHC data collection period (Run I) means that the Drell-Yan process is a very useful for calibrating the experiments. If the experiments are able to see the Z boson, the rho meson, etc., at their correct energies, then we have confidence that the experiments are working well enough to study nature at energies never before explored in a laboratory.

However, in real life, the Drell-Yan process is not as simple as drawn above. Real collisions include the remnants of the scattered protons. Remember: the proton is bag filled with lots of quarks and gluons.


Gluons are what holds quarks together to make protons; they mediate the strong nuclear force, also known as quantum chromodynamics (QCD). The strong force is accordingly named because it requires a lot of energy and effort to overcome. Before annihilating, the quark and antiquark pair that participate in the Drell-Yan process will have radiated lots of gluons. It is very easy for objects that experience the strong force to radiate gluons. In fact, the antiquark in the Drell-Yan process originates from an energetic gluon that split into a quark-antiquark pair. Though less common, every once in a while two or even three energetic quarks or gluons (collectively called jets) will be produced alongside a Z boson.


Here is a real life Drell-Yan (Z boson) event with three very energetic jets. The blue lines are the muons. The red, orange and green “sprays” of particles are jets.



As likely or unlikely it may be for a Drell-Yan process or occur with additional energetic jets, the frequency at which they do occur appear to match very well with our theoretical predictions. The plot below show the likelihood (“Production cross section“) of a W or Z boson with at least 0, 1, 2, 3, or 4(!) very energetic jets. The blue bars are the theoretical predictions and the red circles are data. Producing a W or Z boson with more energetic jets is less likely than having fewer jets. The more jets identified, the smaller the production rate (“cross section”).


How about low energy jets? These are difficult to observe because experiments have high thresholds for any part of a collision to be recorded. The ATLAS and CMS experiments, for example, are insensitive to very low energy objects, so not every piece of an LHC proton collision will be recorded. In short: sometimes a jet or a photon is too “dim” for us to detect it. But unlike high energy jets, it is very, very easy for Drell-Yan processes to be accompanied with low energy jets.


There is a subtlety here. Our standard tools and tricks for calculating the probability of something happening in a proton collision (perturbation theory) assumes that we are studying objects with much higher energies than the proton at rest. Radiation of very low energy gluons is a special situation where our usual calculation methods do not work. The solution is rather cool.

As we said, the Z boson produced in the quark-antiquark annihilation has much more energy than any of the low energy gluons that are radiated, so emitting a low energy gluon should not affect the system much. This is like massive freight train pulling coal and dropping one or two pieces of coal. The train carries so much momentum and the coal is so light that dropping even a dozen pieces of coal will have only a negligible effect on the train’s motion. (Dropping all the coal, on the other hand, would not only drastically change the train’s motion but likely also be a terrible environmental hazard.) We can now make certain approximations in our calculation of a radiating a low energy gluon called “soft gluon factorization“. The result is remarkably simple, so simple we can generalize it to an arbitrary number of gluon emissions. This process is called “soft gluon resummation” and was formulated in 1985 by Collins, Soper, and Sterman.

Low energy gluons, even if they cannot be individually identified, still have an affect. They carry away energy, and by momentum conservation this will slightly push and kick the system in different directions.



If we look at Z bosons with low momentum from the CDF and DZero experiments, we see that the data and theory agree very well! In fact, in the DZero (lower) plot, the “pQCD” (perturbative QCD) prediction curve, which does not include resummation, disagrees with data. Thus, soft gluon resummation, which accounts for the emission of an arbitrary number of low energy radiations, is important and observable.

cdf_pTZ dzero_pTZ

In summary, Drell-Yan processes are a very important at high energy proton colliders like the Large Hadron Collider. They serve as a standard candle for experiments as well as a test of high precision predictions. The LHC Run II program has just begun and you can count on lots of rich physics in need of studying.

Happy Colliding,

Richard (@bravelittlemuon)



Physics + wine = plasma + fun

Wednesday, February 4th, 2015

Ever fancied making your own particle accelerator? Fermilab posted a great blog entry last month (here) showing how anyone can make a particle detector for viewing cosmic rays. In this post, I will explain how particle accelerators can also be hacked so that you can make your very own cathode ray tube (CRT).

I came across this experiment when attending an accelerator school at the Australian Synchrotron last year. To read more about my adventures down under please see Accelerating Down Under and If you can’t stand the heat, get into the Synchrotron!.

What is a cathode ray tube?

Good question. It consists of a vacuum chamber containing some electrodes between which a high voltage is applied. Electrons are accelerated from the negatively charged cathode to the positively charged anode. But some electrons fly past the anode to hit a glass wall. CRTs were utilised in old television sets to form images on a fluorescent screen.


You will need:

  • – a clear wine bottle
  • – a vacuum pump
  • – a rubber hose
  • – epoxy resin
  • – mini chrome-plated metal doorknob
  • – a piece of steel brake line
  • – a piece of steel wire several centimetres long
Empty wine bottles at the Australian Synchrotron.

Experimental preparation at the Australian Synchrotron: GRAPE 1, 2 & 3. Image credit: Ralph Steinhagen.


A detailed method for this experiment may be found (here) but I summarise the main steps below:

  1. Drink a bottle of wine. Wash out the wine bottle with warm soapy water and remove all labelling from the exterior.
  2. Drill a hole about 1/2 way down the wine bottle which is big enough to fit the metal wire through. This will act as the mount for the anode. If your bottle cracks, throw it away and return to step 1.
  3. Drill a hole through the metal doorknob. Use epoxy to attach the break line to the doorknob’s screw mount. This will act both as the cathode and vacuum port. Apply epoxy to the rim of the mouth of the wine bottle and attach the cathode to form an airtight seal.
  4. Bend the steel wire into a C-shape and thread it through the hole you drilled in the wall of the wine bottle. This is your anode. Orient it so that all points on it are equidistant from your cathode. Secure it with epoxy and ensure it is airtight.
  5. Attach the rubber hose to your anode and the other end to the vacuum pump. Attach the anode and cathode to a high voltage power supply. Turn on the power supply and vacuum pump and enjoy!



The GRAPE 2 experiment: a vacuum pump is connected to the experiment via the rubber tube to the right of the bottle. The anode and cathode, which are connected to a high voltage supply, are seen to glow. Image credit: Ralph Steinhagen.


A word of warning: using high voltages, creating vacuums and drilling holes in glass bottles are all inherently dangerous activities. If you attempt this experiment please observe all safety advice. In particular, wear protective clothing and safety glasses, don’t use cracked bottles for the experiment – you risk implosion – and apply the voltage for a maximum of 30/40 seconds.

And please leave adequate time between consuming the wine and carrying out the experiment to sober up.


The video below shows what happened when the switch was flicked on the GRAPE 2 experiment at the Australian Synchrotron:


Initially there is a clear purple electric discharge between the anode and cathode. This discharge excites the atoms in the gas in the bottle causing a burst of liberated free electrons. The electrons are travelling much faster than the positive ions they leave behind and so diffuse to the cathode and bottle walls. Thus a plasma (or ionised gas) is created.

The plasma stabilises as more ionisation occurs, then begins to glow as electrons and ions recombine and emit photons. This process of ionisation and recombination is continuous. The instabilities or fluctuations observed indicate that different proportions of the remaining gas are being excited as the experiment proceeds. Can you think of why this happens? If so, please comment below.

When a magnet is placed near the bottle the plasma is visibly distorted. This phenomenon is known as magnetic deflection and is described by the Lorentz force law. The plasma’s charged particles experience a force when they travel through the magnetic field which is perpendicular both to the path they follow and to the applied magnetic field, that is the magnet causes the particles to follow a curved path. This effect is used in circular particle accelerators, such as the Large Hadron Collider, where strong dipole magnets are used to steer the particles around the machine.

A cross section of the LHC showing the dipole magnets which are used to bend the path followed by protons.

A cross section of the LHC showing the dipole magnets which are used to bend the path followed by protons. The magnets may be seen flanking the left-hand beam pipe. Image credit: James Doherty

What are you waiting for?

Particle physics is not a game that only elite scientists at well-funded institutions can play. With a little effort, determination and ingenuity, it is possible to make your own particle accelerator or detector. So what are you waiting for? Give it a go and let us know how you get on in the chat box below. Good luck!

The GRAPE 2 experiment was carried out by Kaitlin Cook, Paul Bennetto and Tom Lucas under the supervision of Ralph Steinhagen at the 2014 Australian Synchrotron Accelerator School. The above photos and video are courtesy of Ralph Steinhagen.


Data recall at the LHC?

Tuesday, April 1st, 2014

In a stunning turn of events, Large Hadron Collider (LHC) management announced a recall and review of thousands of results that came from its four main detectors, ATLAS, CMS, LHCb and ALICE, in the course of the past several years when it learned that the ignition switches used to start the LHC accelerator (see the enclosed image) might have been produced by GM. Image

GM’s CEO, A. Ibarra, who is better known in the scientific world for the famous Davidson-Ibarra bound in leptogenesis, will be testifying on the Capitol Hill today. This new revelation will definitely add new questions to the already long list of queries to be addressed by the embattled CEO. In particular, the infamous LHC disaster that happened almost six years ago on 10 September 2008 and cost taxpayers over 21Million dollars to fix, has long suspected been caused by a magnet quench. However, new data indicate that it might have been caused by too much paper accidentally placed on a switch by a graduate student, who was on duty that day.

“We want to know why it took LHC management more than five years to issue that recall”, an unidentified US Government official said in the interview, “We want to know what is being done to correct the problem. From our side, we do everything humanly possible to accommodate US high energy particle physics researchers and help them to avoid such problems in the future.  For example, we included a 6.6% cut in US HEP funding in the President’s 2015 budget request.” He added, “We suspected that something might be going on at the LHC after it was convincingly proven to us at our weekly seminar that the detected Higgs boson is ‘simply one Xenon atom of the 1 trillion 167 billion 20 million Xenon atoms which there are in the LHC!'”

This is not the first time accelerators cause physicists to rethink their results and designs. For example, last year Japanese scientists had to overcome the problem of unintended acceleration of positrons at their flagship facility KEK.

At this point, it is not clear how GM’s ignition switches problems would affect funding of operations at the National Ignition Facility in Livermore, CA.



Studies of New Physics require several independent approaches. In the language of experimental physics it means several different experiments. Better yet, several accelerators that have detectors that study similar things, but produce results with different systematic and statistical uncertainties. For a number of years that was how things were: physicists searched for New Physics in high-energy experiments where new particles could be produced directly (think Tevatron or LHC experiments), or low-energy, extremely clean measurements that explored quantum effects of heavy new physics particles. In other words, New Physics could also be searched for indirectly.

As a prominent example of the later approach, detectors BaBar at SLAC (USA) and Belle at KEK (Japan) studied decays of copiously produced B-mesons in hopes to find glimpses of New Physics in quantum loops. These experiments measured many Standard Model-related parameters (in particular, confirming the mechanism of CP-violation in the Standard Model) and discovered many unexpected effects (like new mesons containing charmed quarks, as well as oscillations of charm mesons). But they did not see any effects that could not be explained by the Standard Model. A way to go in this case was to significantly increase luminosity of the machine, thereby allowing for very rare processes to be observed. Two super-flavor factories (those machines are really like factories, churning out millions of B-mesons) were proposed, the Belle-II experiment at KEK and a new Super-B factory at the newly-created Cabibbo Lab in Frascatti, Italy. I have already written about the Cabibbo Lab.

It appears, however, that Italian government decided today that it cannot fund the Super-B flavor factory. Tommaso Dorigo reported it in his blog this morning. Here is more hard data: there is a press release (in Italian) from the INFN that basically tells you that “economic conditions… were incompatible with the costs of the project evaluated.” Which is another way of saying that Italian government is not going to fund it. This follows by the news from the PhysicsWorld saying the same thing.

Many physicists have been expressing doubts that the original Super-B plan, which was, in my opinion, very bold, could be executed within the proposed time frame.  Yet, physicists pressed on… that is until this morning’s announcement. Reality of our world sets in — there is not enough money for basic research…
So what’s left? There is still, of course, Belle-II. Moreover, excellent performance of LHCb experiment at CERN (I wrote about that here) leaves us with great hopes. That is, if Nature cooperates…


Portuguese version below…

Nature sometimes demands a lot of effort in order to reveal its secrets. Particle physics, of course, obeys a very similar pattern. Many, many events have to be analyzed in order to find a few that can be really interesting. Let’s take the case of the ever sought Higgs particle. The probability of generating a single Higgs is quite small. Assuming that in the total period from the start of the physics operation in 2010 and ending just before the July 4th, 2012 announcement comprises, in fact, around 450 real LHC operation days, we had, on average, 480 Higgs per day (all numbers in this post are approximative). However, quite unfortunately, most (~60%) of these Higgs decay in a mode (H->bb) which is very easy to confuse with other production modes or (~20%, H->WW) in a mode not so easy to measure precisely its properties. One of the cleanest modes to study the Higgs is its decay into two photons. The photons were detected with the ATLAS calorimeters (see our previous posts). But the quantity of produced events in this mode is much smaller (basically, around 1 per day!). Sometimes, LHC produces more than 30 million collisions per second. Now, imagine that the LHC could only produce a tenth of this number of collisions (3 million), we would have to wait 10 days for a detectable Higgs. Here comes (at last!) our central topic. Given the rarity of Higgs events, the LHC has to produce a ridiculously high amount of events per second to produce a few interesting ones in a practical rate. Given also the fact that Nature loves to produce other events which are very common and very well known, our detectors are filled up with events that are basically junk (background), at least for Higgs search. If we recorded all of the events produced in ATLAS, more than 40 GBs of data storage space would be necessary per second. That would be simply unmanageable!!! So, the only way to have a reasonable data flow and still be able to make physics at a reasonable rate is to select events before recording. That’s what we call the “trigger”.

Immediately two obsessions related to the trigger system appear : reduce as much as possible the huge rates of events faking good signatures (or the data acquisition system of the detector will not handle the stress); highest possible efficiency : never loose a very good candidate of a given signature or you loose the physics event which is exactly why one builds such gigantic machines!! In the case of a lost Higgs, another working day will be necessary! As you will later on the next posts, the algorithms used in the trigger operate always at the limit rate in which they can guarantee a very high efficiency (usually, not too far from 100%).

Finding the Higgs

The complex work of finding the Higgs. Picture downloaded from : http://www.englishblog.com/2012/07/higgs-boson-cartoons.html#.UER8fkRhq6B

ATLAS Higgs to gamma gamma plot

ATLAS Higgs to gamma gamma mass plot. For more information, check the page : https://twiki.cern.ch/twiki/bin/view/AtlasPublic/HiggsPublicResults

The picture above (stolen from many places in the web – see the caption), is not very far from the truth. To find a Higgs, you have to search a lot. See for instance, the official ATLAS plot for the Higgs detection. In this plot, the amount of Higgs candidates shown as an excess is quite small (around 230-250 events in the 4 bins between 122 and 130 GeV). See in the top plot, that this excess shows up in more than 8000 events (around 2000 in each of the four histogram bins). And this after trigger and offline analysis selection, for a very narrow mass range and only for events in which two photons were detected! In principle, we should expect more Higgs (around 400-450), but some are lost because their photons appear too close to the particle beam and ATLAS does not see them (or at least misses one of them and the pair cannot be formed). Others are lost because the requirements to accept a photon as such are very restrictive and the chance of loosing at least one of the two photons is relatively high. So, loosing a fraction of these events is unfortunate, but unavoidable given the experimental conditions. Another remarkable fact is that we cannot necessarily know which 250 events of those 8250 are really related to Higgs. We just know that the Higgs contributed by increasing the rate of possible events in that mass range. The researchers are always trying to find some clever techniques to avoid the 8000 unnecessary events, but this is no trivial task. If a new technique is developed, it would certainly end up included as a trigger algorithm.

As for the “videos” section of this post, I’d like to make a little propaganda about some sources of information. So, I recorded quick working sessions on two tools that are used in the ATLAS Control Room to visualize a small fraction of the acquired events while the experiment is running. The first is directly from the atlas.ch web site, the so called atlas live events (check the link!). It takes around 15 to 20 seconds to change the event. I made a short extract in the video below. Another very nice tool is Camelia that can make 3D images from events coming directly from the ATLAS detector and you can play with them. The important point here is that most of the events displayed (a random sample) are the ones which have very small signal (lots of tracks with low momentum) but almost no important signal (straight lines with some calorimeter activity). If you wait long enough you will eventually see some interesting events. This demonstrates why we need to apply a strong selection to avoid wasting recording time and space with trivial events. You may want to see these in full screen.

I advise also the third video, where I tried to make a quick analysis using a pre-recorded events. First, one finds two jets in a single event (it could be two photons) and later two muons. Summing the muons momenta, we can see that the pair mass (93 GeV/c2) is quite close to a Z boson mass (91 GeV/c2).

In the next post, we will see the three levels of the ATLAS trigger system with increasing complexity and accessing each time more details of the detector. When it gets to the software trigger levels, we will probably have a post about computing in such environment (yes, the trigger software must be fast, even if you have to loose a bit of precision!). If you are interested in understanding how one of these detectors work, I advise a look at my latest 3 posts (first, second and third). You will need that information to understand the trigger.

Portuguese version :

Por vezes, a Natureza exige um grande esforço antes de revelar seus segredos. A física de partículas obedece um padrão similar. Muitos, muitos eventos têm de ser analisados antes de se encontrar alguns que sejam realmente interessantes. Vejamos o caso da famosa partícula de Higgs. A probabilidade de gerar um Higgs é bem pequena. Assumindo que o período total de operação do LHC real em torno de 450 dias em 2010, 2011 e parte de 2012, tivemos, na média, cerca de 480 Higgs por dia (os números discutidos são aproximações). Entretanto, infelizmente, cerca de 60% desses Higgs têm um modo de decaimento (H->bb) que é muito similar à outros processos físicos, e, logo, difícil de detectar. Outro modo de decaimento (H->WW, 20%) é um pouco difícil de ter suas propriedades medidas. Um dos modos mais “limpos” de se detectar o Higgs é o seu decaimento em dois fótons. Estes fótons sensibilizaram os calorímetros do ATLAS (veja posts anteriores). Mas a quantidade de eventos produzidos dessa forma é muito menor (mais ou menos uma vez por dia!). Algumas vezes, o LHC produz mais de 30 milhões de colisões por segundo. Agora, imagine que o LHC produzisse um décimo desde número (3 milhões), teríamos que esperar 10 dias para encontrar um Higgs detectável. Assim, chegamos (finalmente!) no nosso tópico central. Dada a raridade com que eventos possuindo Higgs acontecem, o LHC tem que produzir uma quantidade ridiculamente gigante de eventos por segundo para produzir alguns interessantes numa taxa praticável. Também dado o fato de que a Natureza adora produzir eventos que são já muito conhecidos, nossos detectores são mantidos cheios de eventos inúteis, pelo menos inúteis para a procura do Higgs. Se gravássemos todos os eventos produzidos no ATLAS, seriam necessários mais de 40 GB de espaço de armazenagem de dados por segundo! O manuseio de tantos dados seria impraticável. Ou seja, a única forma de se ter um regime de armazenamento mais razoável e ainda ser capaz de fazer os estudos da física é selecionar os eventos antes de gravá-los. Esse processo é chamado de “trigger” (cuja tradução literal seria disparo).

Imediatamente, vemos dois tópicos que são verdadeiras obsessões no domínio do trigger : redução ao máximo das gigantescas taxas de eventos fingindo ser boas assinaturas (para reduzir o stress sobre o sistema de aquisição de dados); maior eficiência possível : nunca perder um evento interessante que é exatamente o motivo pelo qual construímos essas máquinas gigantescas! No caso do Higgs, isso pode ser traduzir na necessidade de esperar por todo um novo dia de trabalho! Como ficará claro nos próximos posts, os algoritmos usados no trigger operam sempre na taxa em que eles ainda possam garantir altíssima eficiência (usualmente, próximo a 100%).

Finding the Higgs

O trabalho complicado para encontrar o Higgs. Figura capturada da página : http://www.englishblog.com/2012/07/higgs-boson-cartoons.html#.UER8fkRhq6B

ATLAS Higgs to gamma gamma plot

Massa do par Higgs para gamma gamma medida pelo detector. Para maiores informações, veja https://twiki.cern.ch/twiki/bin/view/AtlasPublic/HiggsPublicResults

Veja a figura acima (encontrada na web – veja o link no texto da figura). A brincadeira é muito próxima da realidade. Para encontrar um Higgs, deve haver uma extensiva procura. Veja por exemplo, a figura da detecção do Higgs pelo detector ATLAS. Nessa figura, a quantidade de candidatos à Higgs aparece como um pequeno excesso (cerca de 230-250 eventos nos quatro bins entre 122 e 130 GeV). Veja na parte superior da figura que esse excesso acontece sobre cerca de 8000 eventos (distribuídos em quatro bins do histograma). E isso, depois da seleção do trigger e da análise offline, por uma pequena faixa de massa e para os eventos nos quais dois fótons foram encontrados. Em princípio, deveríamos ter mais Higgs (cerca de 400-450), mas muitos são perdidos porque seus fótons aparecem muito perto do feixe de partículas e ATLAS não os vê (ou não vê um deles, logo o par não pode ser estudado). Outros são perdidos porque os requisitos para aceitar um fóton são muitos restritivos e a chance de se perder um deles é relativamente alta. Assim, perder uma fração destes eventos é muito ruim, mas inevitável dadas as condições experimentais. Outro fato remarcável é que não podemos saber quais 250 dos 8250 eventos são realmente relativos ao Higgs. Apenas sabemos que o Higgs contribui para o aumento da taxa de eventos naquela faixa de massa. O pesquisadores tentam inventar novas técnicas inteligentes que permitam evitar os 8000 eventos desnecessários, mas essa não é uma tarefa simples. Se uma nova técnica for desenvolvida, ela vai acabar sendo aplicada no trigger.

Para a seção de vídeos deste post, eu gostaria de aproveitar para fazer uma pequena propaganda de algumas fontes de informações. Assim, gravei uma rápida seção de trabalho em duas ferramentas que são utilizadas na sala de Controle do ATLAS para visualizar uma pequena fração dos eventos adquiridos enquanto o experimento está tomando dados. O primeiro está disponível no site atlas.ch com o nome de live events (eventos ao vivo – visite o link!). Demora cerca de 15 a 20 segundos para se trocar o evento. Outra ferramenta é a Camelia que pode fazer imagens 3D dos eventos vindos diretamente do detector ATLAS, e você pode brincar com o detector. O ponto importante aqui é observar que uma amostra aleatória é basicamente composta com eventos com pouco sinal (muitos traços com pouco momento) mas quase nenhuma atividade no calorímetro. Se você esperar algum tempo, você verá alguns eventos com um nível de atividade razoável. Isso demonstra o quanto precisamos fazer uma seleção forte para evitar desperdício de tempo e espaço de dados com eventos triviais. Talvez seja mais prático ver estes eventos em tela cheia.

Também tentei fazer uma análise rápida de alguns eventos pré-gravados no terceiro vídeo. No primeiro, vemos dois jatos num mesmo evento (podem ser dois fótons não corretamente identificados) e, depois, vemos dois múons. Somando o momentum desses múons, podemos ver que a massa do par (93 GeV/c2) é muito próxima à do bóson Z (91 GeV/c2).

No próximo post, veremos os três níveis de trigger do ATLAS acessando a cada nível mais detalhes do detector. Quando chegamos no nível de seleção do trigger por software, provavelmente teremos um post sobre computação neste ambiente (o algoritmo tem que tomar decisões rápidas mesmo que seja a perder um pouco de precisão!). Se você está interessado em como funcionam os detectores, revise meu últimos três posts (primeiro, segundo e terceiro). Você vai precisar disso para entender o trigger.


Portuguese text below….

So, in the last two (first and second) posts about how a calorimeter works, I explained how a particle enters in such detectors, loose its energy producing a shower of other particles and finally how this shower provokes the generation of an electrical signal thanks to the “sampling material”. One detail that is important not to forget is that we have a large number of electrodes (hence, calorimeter “cells” – around 187 thousand of them) collecting information on energy deposition in the calorimeter. A good electron shower can be composed of as many as a few hundred cells. For sure it is very important to measure the signal in every cell for every collision event that happens in ATLAS and that is not exactly something very easy to do. Let’s understand how this is done.

First, I propose to watch the left 12 secs video below. It is an extract of the previous videos on how a particle makes the shower inside the ATLAS Liquid Argon Calorimeter, but now really focusing in the two important parts necessary to understand the format of the output electric signal. First, you see a particle crossing the lead absorber and producing 3 particles. We follow one of these while it crosses the 2 mm space between the absorber (dark gray bar) and the copper electrode (copper-colored?!), this one with a very positive Voltage (~2000V). This space we call “the gap”. Well, despite the “slowliness” implied by the movie, this particle is very close to the speed of light. This means, that the time to cross the gap is less than 0.01 nanoseconds (that’s 0. followed by 10 zeros before the “1” appears – compare with the 25 ns of the collisions time). Even if the particle were at 10% the speed of light, that’s still around 0.1 ns, immediate in terms of LHC collisions interval. This phase is called ionization or, Charge Deposition. The electron created all the negative electrons and positive argon ions and disappeared, going to the next cell.

The second part of the signal is the drift of the electrons freed from the argon atoms towards the electrodes. In the last scene of the movie, you will see three long white trails with the electrons drifting from the absorber until the electrode. If you were in the top of a relatively tall building letting some water leak to the floor and, all of a sudden, you cut the flow, people looking at the column of water would still see the top of the water column falling for a few seconds. That’s exactly the same thing, except that instead of water flow we have electrons flow and in the place of gravity we have the electric pull of the electrons by the positive electrode. During sometime (~400 ns), the electrons will be drifting to the electrode and as time goes by you will have less electrons (again, this movie is part of ATLAS episode II – see here the complete part 1 and part 2 of this movie in English!).

Now, let’s see the signal shape. This is in the second movie. First, you got basically no signal (that never exists in electronics – I should say : you got only noise!). Then, the fast electron crosses almost immediately the gap and you get the highest possible signal. The higher the initial electron energy, the higher the number of electrons freed from the argon atoms and the higher is this initial current. So, all we care for measurement purposes would be this initial current peak. The rest of the time the current gets dimmer and dimmer until we got only noise again. When the time scale on the movie changes, you are just seeing the drift moment. Now, in reality you never see this triangle. All you see is the single measurement value and you have to take a decision about when to “catch” the pulse value. Trying to catch too many tens of samples represents an extra load to the electronics usually hitting a power heating or a data amount limitation and you have to be able to sample as least as possible.

The whole thing happens very quickly, so, you have to use some electronic device to find a better way to work this out. Let’s consider the 3 pictures below. The value is the one marked with a star. In the first picture it is obvious that the shot was taken too soon, our artist was not even in the studio. This means we lost the signal (energy measured = noise!). The second picture is the perfect sampling of the signal at the curve peak. If we always could do like that, this would be perfect. However, most of the time, you would be getting the signal after the peak was reached (third picture) and the energy of the cell would be underestimated. This is very bad.

Sampling of the calorimeter signal performed too early

Sampling of the calorimeter signal performed too early

Sampling of a calorimeter pulse taken at the best timing (pulse peak)

Sampling of a calorimeter pulse taken at the best timing (pulse peak)

Sampling of a calorimeter signal taken too late

Sampling of a calorimeter signal taken too late

So, instead of trying to sample the direct signal and certainly making a mistake, we use an electronic circuit that re-shapes the signal. This circuit stretches the fast rising part so that, in the end, the peak value information is distributed over a much longer time spam (something like 125 ns). The shaped pulse is shown in the figure below together with the original pulse. Now, multiple samples (5) at regular time intervals of this structure are acquired by an analogue-to-digital converter circuit which produces digital numbers related to the pulse value at the sampling moment (marked by dots in the shaped pulse). Using these numbers, it is possible to make a best guess (or what one like to call technically a “fit”) of what the shaped pulse really is, including its height, even if the signal is shifted of 1 or 2 ns. And from that, we can calculate the energy in the cell.

LAr Pulse its shaped version and the samples

LAr Pulse its shaped version and the samples

Due to the very long pulse (400 ns) and the very short interval between collisions (25 ns), it is not impossible (rather, highly probable) that a given cell will receive the signal from one collision while the signal from a previous collision is still in the drifting phase. This effect is called pileUp, and we will discuss it in a much later post.

The discussion today involved complex topics in engineering and physics applied to the detector signal. Design of a good stable and cheap shaping filter, sampling the signal at a cost and power effective rate, dealing with pile Up and performing energy calculation are quite general topics and many different detectors use similar techniques. Many of these topics are whole areas of study, specially in engineering. The signals produced by a detector are usually very fast or very slow and the shaping helps to extract their meaningful properties. For instance, for the Tile Calorimeter discussed in the previous post, the whole pulse is very short (a few ns) and you have to completely stretch it, while maintaining the area produced by the original signal (proportional to the light captured).

Now we will stop the section on how a calorimeter works and we will start another one on how the trigger works to select good collisions for Higgs (??) candidates!

Portuguese part :

Nos últimos dois (primeiro e segundo) posts sobre o funcionamento de um calorímetro, expliquei como uma partícula entra em tais detectores, perde sua energia produzindo uma cascata de outras partículas e, finalmente, como essa partícula provoca a geração de um sinal elétrico graças ao material de amostragem. Um detalhe importante é que o enorme número de eletrodos (ou seja, “células” do calorímetro – cerca de 187 mil) coletam a deposição de energia em todo o calorímetro. Uma cascata razoável de elétrons pode ser composta de algumas centenas destas células. Obviamente é muito importante medir o sinal em todas as células para cada evento de colisão que acontece no ATLAS e essa não é uma tarefa tão simples. Vamos entender como isto é feito.

Primeiramente, podemos ver um filme de 12 segundos no quadro abaixo à esquerda. É um pequeno extrato de um dos vídeos que discutimos anteriormente mostrando uma partícula produzindo a cascada no calorímetro de Argônio Líquido do ATLAS, mas agora focando na geração do sinal elétrico. Primeiro, pode-se ver uma partícula cruzando o absorvedor de chumbo e se produzindo 3 partículas. Seguimos uma destas enquanto ela cruza o pequeno espaço de 2 mm entre o absorvedor (barra cinza escura) e o eletrodo de cobre (na cor do cobre, obviamente! 😉 ) que mantém uma Voltagem alta positiva (~2000V). Apesar da lentidão que o vídeo parece implicar, o elétron está praticamente a velocidade da luz, e isso significa que ele cruza o pequeno intervalo em menos de 0.01 nanosegundo (Ou seja, um “0.” seguido de dez zeros antes de aparecer o “1” – compare com o tempo entre colisões no LHC – 25 ns). Mesmo que fosse um elétron lento (10% da velocidade da luz), ainda teríamos 0.1 ns. Essa fase é chamada de ionização ou Deposição de Carga. O elétron criou todas as cargas negativas e íons positivos dos átomos de Argônio e desapareceu indo para a próxima célula.

A segunda parte do sinal é a tração dos elétrons liberados dos átomos de argônio na direção dos eletrodos. Na última cena do filme, podemos ver os três longos traços brancos relativos aos elétrons atraídos desde o absorvedor até o eletrodo. Se você estivesse no alto de um prédio relativamente alto e observando um vazamento de água até o solo e, de repente, você cortasse o fluxo de água, pessoas observando a coluna de líquido veriam o topo desta coluna demorando alguns segundos até chegar no solo. O efeito é o mesmo para os elétrons, exceto que temos elétrons em vez de água e em vez da força da gravidade temos a atração elétrica do eletrodo positivo!! Durante um certo intervalo de tempo (cerca de 400 ns), os elétrons estarão se dirigindo para o eletrodo e cada vez teremos menos elétrons (Uma vez mais, este filme é parte do Episódio II do ATLAS – veja o filme completo parte 1 e parte 2 deste filme em Inglês).

Agora, vejamos o formato do sinal. Este se encontra no segundo filme. Primeiramente não temos nenhum sinal (isso não existe em eletrônica – eu deveria dizer que só temos ruído!). Então, o elétron rápido cruza praticamente de forma imediata o intervalo e o sinal atinge o seu máximo. Quanto maior a energia do elétron inicial, maior o número de elétrons liberados dos átomos de Argônio e maior é este pico de corrente. Dessa forma, o único valor importante para se realizar a medida da energia é o valor do pico inicial. No resto do tempo, a corrente vai diminuindo lentamente até atingirmos o valor de ruído de novo. Quando a escala de tempo do filme muda, já estamos na parte de tração dos elétrons. Na realidade, o triângulo que se forma no filme não pode realmente ser visto. Tudo o que se vê é o valor a ser medido e temos que tomar a melhor decisão sobre quando realizar a medida. Realizar a medida muitas dezenas de vezes seria o melhor, mas, infelizmente, há um aumento de custo, consumo e dados produzidos, tornando a eletrônica impossível de ser construída. Isso nos leva a tentar capturar o mínimo possível de amostras.

Como a coisa toda acontece muito rapidamente, você tem que usar alguma eletrônica para encontrar uma melhor forma de resolver este problema. Considere as três figuras abaixo. O valor obtido é marcado com uma estrela. No primeiro desenho está claro que a “foto” foi tirada muito cedo, tendo o artista ainda nem entrado na sala. Isso significa que perdemos o sinal (energia medida = nível de ruído!!). No segundo desenho vemos a amostragem perfeita, exatamente no pico. Entretanto, na maior parte das vezes, só conseguimos medir o sinal depois que o pico foi atingido (terceira figura) e a energia da célula fica sub-estimada. Obviamente, isso não é muito bom.

Sampling of the calorimeter signal performed too early

Colhendo a amostra do sinal do Calorímetro muito cedo

Sampling of a calorimeter pulse taken at the best timing (pulse peak)

Colhendo a amostra do sinal do Calorímetro no momento certo (pico de sinal)

Sampling of a calorimeter signal taken too late

Colhendo a amostra do sinal do Calorímetro muito tarde

Para se resolver esse problema, em vez de tentar medir amostras do sinal direto e, quase sempre fazer uma medida errada, usamos um circuito eletrônico que modifica a forma do sinal. Este circuito estica a parte relativa à subida rápida do pulso, “espalhando” a informação num período de tempo bem mais longo (cerca de 125ns). O pulso assim reformatado aparece na figura abaixo junto com o pulso original. Agora, diferentes amostras (5) a intervalos regulares desta estrutura podem ser adquiridas por um circuito que faz a conversão analógico pra digital, produzindo números relativos ao valor do pulso a cada amostra (marcados como pontos no sinal reformatado). Usando estes números, é possível se obter uma “melhor estimativa” (tecnicamente chamada de um “fit”) do que o pulso formato realmente é, incluindo o seu pico, mesmo que o sinal esteja ligeiramente deslocado de 1 ou 2 ns. A partir dessa informação, podemos calcular a energia da célula.

O Pulso do Calorímetro, seu sinal reformatado e suas amostras

O Pulso do Calorímetro, seu sinal reformatado e suas amostras

Como o pulso físico é muito longo (400 ns) e o intervalo entre colisões é bastante curto (25 ns), não é impossível (e na verdade é muito provável) que uma certa célula receba o sinal de uma colisão enquanto o sinal da colisão anterior ainda esta na fase de atração dos elétrons. Este efeito é chamado de empilhamento (PileUp) e discutiremos ele num post futuro.

A discussão de hoje envolveu tópicos complexos em engenharia e física aplicadas ao sinal do detector. O design de um filtro de formatação do sinal estável e barato, amostragem do sinal de forma eficiente em termos de custo e potência utilizada, lidar com o efeito de empilhamento e executar rapidamente o cálculo de energia são tópicos muito gerais e técnicas similares são utilizadas em diferentes detectores. Muitos destes tópicos são áreas inteiras de estudo, especialmente em engenharia. Os sinais produzidos pelos detectores são, normalmente, muito rápidos ou muito lentos e a reformatação ajuda muito a extrair as propriedades realmente importantes. Por exemplo, o Calorímetro de Telhas que discutimos no post anterior tem um sinal rápido demais e a reformatação estica o mesmo enquanto mantém a área sob a curva que é proporcional à energia (proporcional à luz capturada!).

Agora nós vamos fazer uma pausa na seção sobre o funcionamento do calorímetro e vamos começar a discutir o funcionamento do sistema de seleção chamado de Trigger. Este sistema foi responsável por escolher os bons eventos candidatos a Higgs!

Aproveito pra re-anunciar o canal ATLAS/Brasil, agora com uma página melhorada e com mais 9 vídeos :


Portuguese version below…

So, I am finally back from vacation, with the email list almost cleaned up and a list of tasks ready to start piling up before I can do anything.. In a nutshell : the usual working life. The last time I posted something, the idea was to explain how a particle, like an electron, a photon or a neutron enters in the detector called calorimeter, hits the material named the absorber and gets a part of its original energy (speed!) converted into a little shower of particles. Part of this energy however gets sampled by a material called (see how physicists can be very creative sometimes) the sampling material. So, what happens in the sampling material of the two ATLAS calorimeters and how we can use this information to “measure” the final energy of the incoming particle?

The sampling material uses some basic physics process to convert the energy it receives into some other physical quantity. The smart thing physicists try to do is to make this “other physical quantity” something easy to measure. Let’s start with a very simple example. In your home, you probably still have either in your wall or in your medical box an old style thermometer. I mean, not a digital one. It is built with a small scale and a little pipe containing either artificially colored alcohol (the wall thermometer) or mercury (the medical box one). When it receives heat from air or a person’s body, the atoms of such “sampling” materials get very agitated. Due to that, the same number of molecules now occupy a much larger space, increasing the necessary volume to contain them. This translates into a longer column which we can easily readout.

In here, we can identify all the elements of the sampling process. First, the heat modifies some inner property of the sampling material considered (their internal heat or movement of the material). Then, the material responds with some global change in a larger scale (its volume increases) and, in consequence, you can now measure with light and some light receiver (your eyes) the shift in the top position of the alcohol or mercury column.

Now, let’s see the case of the liquid argon (electromagnetic calorimeter) of ATLAS. You might want to review the discussion about the absorption process on the previous post. Again, to illustrate, let’s see an extract of the ATLAS Episode II movie, that you can see completely in youtube (part 1 and part 2). There, we followed an electron as it enters the electromagnetic calorimeter and looses its energy by producing a huge shower of particles thanks to the lead absorbers. When the electron is not in the absorber material, it is walking through argon cooled to -185 oC in order to be kept liquid. The electron, or the particles that come out of it, crosses many atoms of Argon, giving so much energy to the argon electrons, that many of of them get free from the argon nuclei in a process called ionization (see the white little dots in the movie!). The argon electron is now a little negative free charge and the rest of the atom is a positive free charge (yellow). Remember that between one accordion plate and the next one, there are copper plates, the electrodes. Between the electrodes, there is a very high voltage (2000V – almost 10 times the voltage in an electric wall plug in an European home). Such high voltage in a very small gap (~2mm) and a bunch of free electrons is a shocking combination, a bit like throwing a small wire on an electric fence. Usually, the high voltage would not cause electrons to travel from one plate to the other, but now that they were freed by the particle’s energy, they will be collected by the positive electrode, generating an electric current. The intensity of this current relates to the energy that the particle lost in the region around the electrode (this region, we call a cell). The current sparks quite quickly (around 400 ns – you will later see that this is actually not as quick as we would love to!) and this is the “other physical quantity” that we can measure. Using this information, we can calculate back the energy of the original particle that entered the calorimeter.

Those interested in seeing a nice, live example of the ionization process are invited to check how to build a cosmic ray detector (sorry for the “idiot”!) and a little video I recorded during a conference (CHEP2010) of a little device called Cloud Chamber (no relation to Harry Potter!).

I would also like to show an example of a particle entering the hadronic calorimeter of ATLAS called Tile Calorimeter. We will use another extract of the ATLAS Episode II Movie to see it. In this video, we start by seeing the iron absorbers (gray) and the plastic scintillating plates (the “Tiles”, drawn in Violet). The absorption part is very similar to the Liquid Argon calorimeter (particle hits, particle looses energy, shower forms) with the difference that particles hit the nuclei and NOT the atoms as in the EM calorimeter case. The sampling material is an special plastic. The particles passing inside this plastic excite the electrons of the atoms that compose it. Not enough to ionize it as in the argon (the electron stays attached to the nucleus but a bit farther away than its usual “orbit”). When the electron return to its natural position, the energy is released back in the form of photons, or, to simplify, light of a very specific frequency (or a specific color, coincidence : Violet). Some of these photons will be collected by an optical fiber and send to a special device that converts light into an electrical signal, a photomultiplier (some of you may have played with that if your school ever had a project to build cosmic ray detectors!). See a photo of Cíbran Santamarina Rios, a colleague from Galicia beside a plastic scintillator detector (the photomultiplier I hope I am saying this right, is in the bottom of the plastic plate – protected by a black cover!).

Photo with Ph.D Cibran Santamarina Rios assembling a cosmic rays detector

Photo with Ph.D Cibran Santamarina Rios assembling a cosmic rays detector

In the next post, I will discuss a bit the outcome of the sampling process : the electrical signal and how it can be used to calculate the energy of a particle, specially when multiple particles hit the calorimeter in different collisions. This concludes our session on “How the detector works”. Then, we will discuss how the trigger works to select particles for a discovery!!!!

Portuguese version :

Finalmente de volta das férias, com uma lista de emails quase limpa e uma lista de tarefas começando a crescer antes que eu possa tentar fazer algo… Em resumo, uma semana normal de trabalho. Na última vez que postei algo, a idéia era explicar como uma partícula tal como um elétron, um fóton ou um nêutron entra num detetor chamado de calorímetro, bate num material denominado absorvedor e perde parte de sua energia, que estava acumulada na forma de velocidade, na forma de uma cascata de partículas. Parte dessa energia, entretanto, sensibiliza um material chamado “material de amostragem” (o termo vem de amostra, parte de um todo que o representa). Assim sendo, como podemos usar o material de amostragem no caso dos dois calorímetros do ATLAS para medir a energia das partículas que entram nestes detetores?

O material de amostragem usa alguns princípios básicos de física para converter a energia que recebe em algum outro valor com significado físico. A esperteza está em tentar fazer esse “outro valor” ser algo fácil de medir. Comecemos com um exemplo bem simples. Em sua casa, Vocês ainda devem ter em sua parede ou na caixinha de remédios um termômetro antigo (não digital). Esse termômetro é constituído de uma pequena escala com as temperaturas e um tubinho contendo álcool colorido (termômetro de parede) ou mercúrio (o da caixinha de remédios). Quando o termômetro recebe calor do ar ou do corpo de uma pessoa, os átomos desse material de “amostragem” ficam muito agitados. Desta forma, um dado número de moléculas passa a ocupar um volume bem maior. Isso se traduz numa coluna mais longa que pode ser facilmente lida.

Neste exemplo, podemos identificar todos os elementos do processo de amostragem. Primeiro, o calor modifica uma propriedade interna do material de amostragem (o seu “calor interno” ou o movimento de seus átomos). Depois, o material responde com uma mudança global numa escala mais larga (seu volume cresce) e, em conseqüência, pode-se medir com luz e um receptor de luz (os olhos do observador) a variação da posição do topo da coluna de mercúrio ou álcool.

Agora vejamos o caso do Argônio líquido usado no calorímetro eletromagnético do ATLAS. Talvez valha a pena rever a discussão sobre o processo de absorção no último post. De novo, para ilustrar, vejamos um exemplo do filme Episódio II do ATLAS, que pode ser visto na íntegra no youtube (parte 1 e parte 2). Naquele exemplo, seguimos um elétron que entra no calorímetro eletromagnético e perde sua energia se transformando numa enorme cascata de partícula graças aos absorvedores de chumbo. Quando o elétron não está no material absorvedor, ele está atravessando o Argônio refrigerado a -185 oC para se manter líquido. O elétron, ou as partículas produzidas por ele, atravessam muitos átomos de Argônio, dando tanta energia aos elétrons destes átomos que muitos deles se liberam de seus núcleos atômicos num processo chamado de ionização (veja os pontos brancos no filme!). O elétron do Argônio é agora uma carga elétrica negativa livre e o resto do átomo uma carga positiva (em amarelo). Lembre-se que entre uma placa do acordeão e a próxima, existem placas de cobre chamadas de eletrodos e que entre estes eletrodos há uma altíssima voltagem (2000 V – quase 10 vezes a voltagem de uma tomada na Europa). Essa voltagem aplicada entre placas tão próximas umas das outras (cerca de 2mm de distância) com um grupo de elétrons livres no meio é uma combinação “chocante”! O que acontece não é muito diferente do que se vê quando um pequeno fio de metal é jogado numa cerca eletrificada. Usualmente, a alta voltagem não causa o “passeio” de elétrons, mas quando eles são libertados pela energia da partícula, eles podem ser coletados pelo eletrodo positivo gerando uma corrente elétrica. A intensidade da corrente se relaciona com a energia que a partícula perdeu na região vizinha ao eletrodo, que chamamos de célula. A corrente cria uma centelha que desaparece rapidamente (cerca de 400ns – mais tarde veremos que esse tempo não é tão curto quanto gostaríamos!) e esse é o “outro valor” com significado físico que podemos medir. Usando essa informação, podemos calcular a energia da partícula inicial que entrou no calorímetro.

Aqueles interessados num exemplo vivo sobre o processo de ionização são convidados a ver como construir um detetor de raios cósmicos e um pequeno vídeo que gravei durante uma conferência (CHEP2010) de uma câmera de núvens.

Eu também queria mostrar um exemplo de uma partícula entrando no calorímetro hadrônico do ATLAS, chamado de calorímetro de Telhas (Tile Calorimeter). Vamos usar outro pequeno filme Episódio II para entender o que acontece. Nesse vídeo, vemos os absorvedores de ferro (cinza) e as placas cintiladoras (as “Telhas”, desenhadas em Violeta). A parte referente à absorção é muito parecida com o que vimos para o calorímetro eletromagnético (partícula bate, partícula perde energia, cascata de partículas se forma) com a diferença que as partículas agora batem nos núcleos dos átomos e não os átomos em si como no calorímetro de Argônio. O material de amostragem, neste caso, é um plástico especial. As partículas passando dentro deste plástico perturbam os elétrons que compõe o mesmo. Não o suficiente para ionizá-lo como acontece com o Argônio : o elétron continua ligado ao núcleo mas fica um pouco mais distante que sua “órbita” usual. Quando o elétron volta a sua posição natural, a energia é devolvida na forma de fótons, ou, para simplificar, luz de uma freqüência específica (de cor, coincidência : Violeta!). Alguns destes fótons são coletados por um sistema de fibras óticas e enviados para um aparelho especial que converte luz em sinal elétrico, uma fotomultiplicadora (alguns de vocês podem ter brincado com tal aparelho se sua escola tentou realizar um projeto para construção de detetores de raios cósmicos!). Veja uma foto de Cíbran Santamarina Rios, um amigo da Galicia ao lado de um detetor plástico cintilador (a fotomultiplicadora aparece embaixo da placa cintiladora protegida por um protetor negro).

Foto do Ph.D Cibran Santamarina Rios montando um detetor de raios cósmicos

Foto do Ph.D Cibran Santamarina Rios montando um detetor de raios cósmicos

No próximo post, pretendo discutir um pouco o resultado do processo de amostragem : o sinal elétrico produzido e que pode ser usado para se calcular a energia da partícula, especialmente quando a partícula atinge o calorímetro em diferentes colisões. Assim, vamos concluir a seção sobre “como funciona um detetor de partículas”. Depois, discutiremos como funciona o sistema de seleção de eventos, tão importante para as descobertas!

Ah! E quase tinha esquecido. O canal ATLAS/Brasil agora tem uma nova pagina e formatação :