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In the late 1980s, as particle colliders probed deeper into the building blocks of nature, there were hints of a strange and paradoxical behaviour in the heart of atoms. Fundamental particles have a curious quantum mechanical property known as “spin”, which the electron carries in magnitude ½. While the description of electron’s spin is fairly simple, protons are made up of many particles whose “spins” can add together in complicated ways and yet remarkably, its total spin turns out to be the same as the electron: ½. This led to one of the great mysteries of modern physics: how do all the particles inside the proton conspire together to give it a ½ spin? And what might this mean for our understanding of hadrons, the particles that make up most of the visible universe?

[This article is largely intended for a lay-audience and contains an introduction to foundational ideas such as spin. If you’ve had a basic introduction to Quantum Mechanics before, you may wish to skip to section marked —— ]

We’ve known about the proton’s existence for nearly a hundred years, so you’d be forgiven for thinking that we knew all there was to know about it. For many of us, our last exposure to the word “proton” was in high school chemistry, where they were described as a little sphere of positive charge that clumps with neutrons to make atomic nuclei, around which negatively charged electrons orbit to create all the atoms, which make up Life, the Universe and Everything1.

2000px-Proton.svg

The simple, three-quark model of a proton (each coloured circle is a type of “quark”). 

Like many ideas in science, this is a simplified model that serves as a good introduction to a topic, but skips over the gory details and the bizarre, underlying reality of nature. In this article, we’ll focus on one particular aspect, the quantum mechanical “spin” of the proton. The quest to measure its origin has sparked discovery, controversy and speculation that has lasted 30 years, the answer to which is currently being sought at a unique particle collider in New York.

The first thing to note is that protons, unlike electrons2, are composite particles, made up from lots of other particles. The usual description is that the proton is made up of three smaller “quarks” which, as far as we know, can’t be broken down any further. This picture works remarkably well at low energies but it turns out at very high energies, like those being reached at the at the LHC, this description turns out to be inadequate. At that point, we have to get into the nitty-gritty and consider things like quark-antiquark pairs that live inside the proton interacting dynamically with other quarks without changing its the overall charge. Furthermore, there are particles called gluons that are exchanged between quarks, making them “stick” together in the proton and playing a crucial role in providing an accurate description for particle physics experiments.

So on closer inspection, our little sphere of positive charge turns out to be a buzzing hive of activity, with quarks and gluons all shuffling about, conspiring to create what we call the proton. It is by inferring the nature of these particles within the proton that a successful model of the strong nuclear force, known as Quantum Chromodynamics (QCD), was developed. The gluons were predicted and verfied to be the carriers of this force between quarks. More on them later.

Proton structure

A more detailed model of the proton. The golden chains between the quarks (the coloured spheres) are representations of gluons, transferred between them. Quark anti-quark pairs are also visible with arrows representing spins.

That’s the proton, but what exactly is spin? It’s often compared to angular momentum, like the objects in our everyday experience might have. Everyone who’s ever messed around on an office chair knows that once you get spun around in one, it often takes you a bit of effort to stop because the angular momentum you’ve built up keeps you going. If you did this a lot, you might have noticed that if you started spinning with your legs/arms outstretched and brought them inwards while you were spinning, you’d begin to spin faster! This is because angular momentum (L) is proportional to the radial (r) distribution of matter (i.e. how far out things are from the axis of rotation) multiplied by the speed of rotation3 (v). To put it mathematically L = m × v × r where m is just your constant mass. Since L is constant, as you decrease r (by bringing your arms/legs inwards), v (the speed at which you’re spinning) increases to compensate. All fairly simple stuff.

So clearly, for something to have angular momentum it needs to be distributed radially. Surely r has to be greater than 0 for L to be greater than 0. This is true, but it turns out that’s not all there is to the story. A full description of angular momentum at the quantum (atomic) level is given by something we denote as “J”. I’ll skip the details, but it turns out J = L + S, where L is orbital angular momentum, in a fashion similar to what we’ve discussed, and S? S is a slightly different beast.

Both L and S can only take on discrete values at the microscopic level, that is, they have quantised values. But whereas a point-like particle cannot have L > 0 in its rest frame (since if it isn’t moving around and v = 0, then L = 0), S will have a non-zero value even when the particle isn’t moving. S is what we call Spin. For the electron and quarks, it takes on the value of ½ in natural units.

Spin has a lot of very strange properties. You can think of it like a little arrow pointing in a direction in space but it’s not something we can truly visualise. One is tempted to think of the electron like the Earth, a sphere spinning about some kind of axis, but the electron is not a sphere, it’s a point-like particle with no “structure” in space. While an electron can have many different values of L depending on its energy (and atomic structure depends on these values), it only has one intrinsic magnitude of spin: ½. However, since spin can be thought of as an arrow, we have some flexibility. Loosely speaking, spin can point in many different directions but we’ll consider it as pointing “up” (+½) or “down” (- ½). If we try to measure it along a particular axis, we’re bound to find it in one of these states relative to our direction of measurement.

Spin250

Focus on one of the red faces. When the cube rotates every 360 degrees, the red ribbon appears to go above and below the cube alternatively! Because the cube is coupled to its environment, it takes 720 degrees to return it to it’s original orientation.


One of the peculiar things about spin-½ is that it causes the wave-function of the electron to exhibit some mind bending properties. For example, you’d think rotating any object by 360 degrees would put it back into exactly the same state as it was, but it turns out that doesn’t hold true for electrons. For electrons, rotating them by 360 degrees introduces a negative sign into their wave-function! You have to spin it another 360 degrees to get it back into the same state! There are ways to visualise systems with similar behaviour (see right) but that’s just a sort of “metaphor” for what really happens to the electron. This links into the famous conclusion of Pauli’s that no two identical particles with spin-½ (or any other half-integer spin) can share the same quantum mechanical state.

——

Spin is an important property of matter that only really manifests on the quantum scale, and while we can’t visualise it, it ends up being important for the structure of atoms and how all solid objects obtain the properties they do. The other important property it has is that the spin of a free particle likes to align with magnetic fields4 (and the bigger the spin, the greater the magnetic coupling to the field). By using this property, it was discovered that the proton also had angular momentum J = ½. Since the proton is a stable particle, it was modelled to be in a low energy state with L = 0 and hence J = S = ½ (that is to say, the orbital angular momentum is assumed to be zero and hence we may simply call J, the “spin”). The fact the proton has spin and that spin aligns with magnetic fields, is a crucial element to what makes MRI machines work.

Once we got a firm handle on quarks in the late 1960s, the spin structure of the proton was thought to be fairly simple. The proton has spin-½. Quarks, from scattering experiments and symmetry considerations, were also inferred to have spin-½. Therefore, if the three quarks that make up the proton were in an “up-down-up” configuration, the spin of the proton naturally comes out as ½ – ½ + ½ = ½. Not only does this add up to the measured spin, but it also gives a pleasant symmetry to the quantum description of the proton, consistent with the Pauli exclusion principle (it doesn’t matter which of the three quarks is the “down” quark). But hang on, didn’t I say that the three-quarks story was incomplete? At high energies, there should be a lot more quark-antiquark pairs (sea quarks) involved, messing everything up! Even so, theorists predicted that these quark-antiquark pairs would tend not to be polarised, that is, have a preferred direction, and hence would not contribute to the total spin of the proton.

If you can get the entirety of the proton spinning in a particular direction (i.e. polarising it), it turns out the scattering of an electron against its constituent quarks should be sensitive to their spin! Thus, by scattering electrons at high energy, one could check the predictions of theorists about how the quarks’ spin contributes to the proton.

In a series of perfectly conducted experiments, the theory was found to be absolutely spot on with no discrepancy whatsoever. Several Nobel prizes were handed out and the entire incident was considered resolved, now just a footnote in history. OK, not really.

In truth, the total opposite happened. Although the experiments had a reasonable amount of uncertainty due to the inherent difficulty of polarising protons, a landmark paper by the European Muon Collaboration found results consistent with the quarks contributing absolutely no overall spin to the proton whatsoever! The measurements could be interpreted with the overall spin from the quarks being zero5. This was a complete shock to most physicists who were expecting verification from what was supposed to be a fairly straightforward measurement. Credit where it is due, there were theorists who had predicted that the assumption about orbital angular momentum (L = 0) had been rather ad-hoc and that L > 0 could account for some of the missing spin. Scarcely anyone would have expected, however, that the quarks would carry so little of the spin. Although the nuclear strong force, which governs how quarks and gluons combine to form the proton, has been tested to remarkable accuracy, the nature of its self-interaction makes it incredibly difficult to draw predictions from.

The feynman diagram for Deep Inelastic Scattering (electron line at the top, proton on the bottom). This type of scattering is sensitive to quark spin.

The Feynman diagram for Deep Inelastic Scattering (electron line at the top, proton on the bottom, with a photon exchanged between them). This type of scattering is sensitive to quark spin.

Future experiments (led by father and son rivals, Vernon and Emlyn Hughes6 of CERN and SLAC respectively) managed to bring this to a marginally less shocking proposal. The greater accuracy of the measurements from these collaborations had found that the total spin contributions from the quarks was actually closer to ~30%. An important discovery was that the sea quarks, thought not to be important, were actually found to have measurable polarisation. Although it cleared up some of the discrepancy, it still left 60-70% of spin unaccounted for. Today, following much more experimental activity in Deep Inelastic Scattering and precision low-energy elastic scattering, the situation has not changed in terms of the raw numbers. The best estimates still peg the quarks’ spin as constituting only about 30% of the total.

Remarkably, there are theoretical proposals to resolve the problem that were hinted at long before experiments were even conducted. As mentioned previously, although currently impossible to test experimentally, the quarks may carry orbital angular momentum (L) that could compensate for some of the missing spin. Furthermore, we have failed to mention the contribution of gluons to the proton spin. Gluons are spin-1 particles, and were thought to arrange themselves such that their total contribution to the proton spin was nearly non-existent.

BNL AERIALS

The Brookhaven National Laboratory where RHIC is based (seen as the circle, top right).


The Relativistic Heavy Ion Collider (RHIC) in New York is currently the only spin-polarised proton collider in the world. This gives it a unique sensitivity to the spin structure of the proton. In 2014, an analysis of the data collected at RHIC indicated that the gluons (whose spin contribution can be inferred from polarised proton-proton collisions) could potentially account for up to 30 of the missing 70% of proton spin! About the same as the quarks. This would bring the “missing” amount down to about 40%, which could be accounted for by the unmeasurable orbital angular momentum of both quarks and gluons.

As 2016 kicks into gear, RHIC will be collecting data at a much faster rate than ever after a recent technical upgrade that should double it’s luminosity (loosely speaking, the rate at which proton collisions occur). With the increased statistics, we should be able to get an even greater handle on the exact origin of proton spin. 


The astute reader, provided they have not already wandered off, dizzy from all this talk of spinning protons, may be tempted to ask “Why on earth does it matter where the total spin comes from? Isn’t this just abstract accountancy?” This is a fair question and I think the answer is a good one. Protons, like all other hadrons (similar, composite particles made of quarks and gluons) are not very well understood at all. A peculiar feature of QCD called confinement binds individual quarks together so that they are never observed in isolation, only bound up in particles such as the proton. Understanding the spin structure of the proton can inform our theoretical models for understanding this phenomenon.

This has important implications, one being that 98% of the mass of all visible matter does not come from the Higgs Boson. It comes from the binding energy of protons! And the exact nature of confinement and precise properties of QCD have implications for the cosmology of the early universe. Finally, scattering experiments with protons have already revealed so much to fundamental physics, such as the comprehension of one of the fundamental forces of nature. As one of our most reliable probes of nature, currently in use at the LHC, understanding them better will almost certainly aid our attempts to unearth future discoveries.

Kind regards to Sebastian Bending (UCL) for several suggestions (all mistakes are unreservedly my own).

 

[1] …excluding dark matter and dark energy which constitute the dark ~95% of the universe.

[2] To the best of our knowledge.

[3] Strictly speaking the component of velocity perpendicular to the radial direction.

[4] Sometimes, spins in a medium like water like to align against magnetic fields, causing an opposite magnetic moment (known as diamagnetism). Since frogs are mostly water, this effect can and has been used to levitate frogs.

[5] A lot of the information here has been summarised from this excellent article by Robert Jaffe, whose collaboration with John Ellis on the Ellis-Jaffe rule led to many of the predictions discussed here.

[6] Emlyn was actually the spokesperson for SLAC, though he is listed as one of the primary authors on the SLAC papers regarding the spin structure of the proton.

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Dark Matter: A New Hope

Monday, December 7th, 2015

[Apologies for the title, couldn’t resist the temptation to work in a bit of Star Wars hype]

To call the direct detection of dark matter “difficult” is a monumental understatement. To date, we have had no definite, direct detection on Earth of this elusive particle that we suspect should be all around us. This seems somewhat of a paradox when our best astronomical observations indicate that there’s about five times more dark matter in the universe than the ordinary, visible matter that appears to make up the world we see. So what’s the catch? Why is it so tricky to find?

An enhanced image of the “Bullet Cluster”: two colliding galaxies are observed with ordinary “baryonic” matter (coloured red) interacting as expected and the dark matter from each galaxy, inferred from gravitational lensing (coloured blue), passing straight through one another. Source: NASA Astronomy Picture of the Day 24/08/2006

The difficulty lies in the fact that dark matter does not interact with light (that is, electromagnetically) or noticeably with atoms as we know them (that is, with the strong force, which holds together atomic nuclei). In fact, the only reason we know it exists is because of how it interacts gravitationally. We see galaxies rotate much faster than they would without the presence of some unseen “dark matter”, amongst other things. Unfortunately, none of the particles we know from the Standard Model of particle physics are suitable candidates for explaining dark matter of this sort. There are, however, several attempts in the works to try and detect it via weak nuclear interactions on Earth and pin down its nature, such as the recently approved LUX-ZEPLIN experiment, which should be built and collecting data by 2020.

Direct detection, however, isn’t the only possible way physicists can get a handle on dark matter. In February 2014, an X-Ray signal at 3.5 keV was detected by the XMM-Newton, an X-ray spectroscopy project by the European Space Agency, in orbit around Earth. Ever since, there’s been buzz amongst particle cosmologists that the signal may be from some kind of dark matter annihilation process. One of the strongest candidates to explain the signal has been sterile neutrino, a hypothetical cousin of the Standard Model neutrino. Neutrinos are ghostly particles that also interact incredibly rarely with ordinary matter* but, thanks to the remarkable work of experimentalists, were detected in the late 1950s. Their exact nature was later probed by two famous experiments, SNO and Super-Kamiokande, that demonstrated that neutrinos do in fact have mass, by observing a phenomenon known as Neutrino Oscillations. As reported on this blog in October, the respective heads of each collaboration were awarded the 2015 Nobel Prize in Physics for their efforts in this field.

“Handedness” refers to how a particle spins about the axis it travels along. Standard Model neutrinos (first observed in 1956) are all observed as left handed. Sterile neutrinos, a hypothetical dark matter candidate, would be right-handed, causing them to spin the opposite way along their axes. Image source: ysfine.com

The hope amongst some physicists is that as well as the neutrinos that have been studied in detectors for the last half a century, there exists a sort of heavier “mirror image” to these particles that could act as a suitable dark matter candidate. Neutrinos are only found to “spin” in a certain way relative to the axis of their propagation, while the hypothesised sterile neutrinos would spin the opposite way round (in more technical terms, they have opposite chirality). This difference might seem trivial, but in the mathematical structure underpinning the Standard Model, it would fundamentally change how often these new particles interact with known particles. Although predicted to react incredibly rarely with ordinary matter, there are potentially processes that would allow these sterile neutrinos to emit an X-Ray signal, with half the mass-energy of the original particle. Due to the sheer number of them found in dense places such as the centres of galaxies, where XMM-Newton was collecting data from, in principle such a signal would be measurable from regions with a high density of sterile neutrinos.

This all seems well and good, but how well does the evidence measure up? Since the announcement of the signal, the literature has gone back and forth on the issue, with the viability of sterile neutrinos as a dark matter candidate being brought into question. It is thought that the gravitational presence of dark matter played a crucial role in the formation of galaxies in the early universe, and the best description we have relies on dark matter being “cold”, i.e. with a velocity dispersion such that the particles don’t whizz around at speeds too close to the speed of light, at which point their kinematic properties are difficult to reconcile with cosmological models. However, neutrinos are notorious for having masses so small they have yet to be directly measured and to explain the signal at 3.5 keV, the relevant sterile neutrino would have to have a relatively small mass of ~7 keV/c2, about 15,000 times lighter than the usual prediction for dark matter at ~100 GeV/c2. This means that under the energy predicted by cosmological models for dark matter production, our sterile neutrinos would have a sort of “luke-warm” characteristic, in which they move around at speeds comparable to but not approaching the speed of light.

A further setback has been that the nature of the signal has been called into question, since the resolution of the initial measurements from XMM-Newton (and accompanying X-ray satellite experiments such as Chandra) was not sharp enough to definitively determine the signal’s origin. XMM-Newton built up a profile of X-ray spectra by averaging across measurements from just 73 galaxy clusters, though it will take further measurements to fully rule out the possibility that the signal isn’t from the atomic spectra of potassium and sulpher ions found in hot cosmic plasmas.

But there remains hope.

A recent pre-print to the Monthly Notices of the Royal Astronomical Society (MNRAS) by several leading cosmologists has outlined the compatibility of a 7 keV/c2 sterile neutrino’s involvement with the development of galactic structure. To slow down the sterile neutrinos enough to bring them in line with cosmological observations, “lepton asymmetry” (a breaking of the symmetry between particles and antiparticles) has to be introduced in the model. While this may initially seem like extra theoretical baggage, since lepton asymmetry has yet to be observed, there are theoretical frameworks than can introduce such an asymmetry with the introduction of two much heavier sterile neutrinos at the GeV scale.

A Black Brant XII sounding rocket, similar to the type that could be used to carry microcalorimeters, capable of recording X-ray signals of the type XMM-Newton and Chandra have been observing in galactic nuclei. These rockets are used to conduct scientific experiments in sub-orbital flight, including attempts at dark matter detection. Source: NASA/Wallops

Under such a model, not only could our dark matter candidate be reconciled, but neutrino oscillations could also be explained. Finally, baryogenesis, the description of why there was slightly more matter than antimatter in the early universe, could also find an explanation in such a theory. This would resolve one of the largest puzzles in Physics; the Standard Model predicts nearly equivalent amounts of particles and antiparticles in the early universe which should have annihilated to leave nothing but radiation, rather than the rich and exciting universe we inhabit today. On the experimental side, there are a few proposed experiments to try and measure the X-ray signal more carefully to determine its shape and compare it with the prediction of such models, such as flying rockets around with calorimeters inside to try and pick up the signal by observing a broader section of the sky than XMM or Chandra did.

With the experts’ opinions divided and further research yet to be done, it would be facetious to end this article with any sort of comment on whether the signal can or will gather the support of the community and become verified as a full blown dark matter signal. At time of writing, a paper has been released claiming signal is better explained as an emission from the plasmas found in galactic nuclei. A further preprint to MNRAS, put on arXiv just days ago, claims the sterile neutrino hypothesis is incompatible with the signal but that axions (a dark matter model that supposes a totally different type of particle outside of the Standard Model) remain as a candidate to explain the signal. Perhaps sterile neutrinos, are not the particles we’re looking for.

This kind of endeavour is just one of the hundreds of ways particle physicists and our colleagues in Astrophysics are looking to find evidence of new, fundamental physics. The appeal for me, as someone whose work will probably only have relevance to huge, Earth-bound experiments like the Large Hadron Collider, is the crossover between modelling the birth of colossal objects like galaxies and theories of subatomic particle production, using comparison between the two for consistency. Regardless of whether future rocket-based experiments can gather enough data to fully validate the signal in terms of theories produced by physicists here on Earth, it is a perfect example of breadth of activity physicists are engaged in, attempting to answer the big questions such as the nature of dark matter, through our research.

Kind regards to Piotr Oleśkiewicz (Durham University) for bringing this topic to my attention and for his insights on cosmology, and to Luke Batten (University College London) for a few corrections.

*The oft-quoted fact about neutrinos is that 65 billion solar neutrinos pass through just one square centimetre of area on earth every single second. The vast majority of these neutrinos will whizz straight through you without ever having noticed you were there, but by chance, in your entire lifetime, it’s likely that at least one or two will have the courtesy to notice you and bump off one of your atoms. The other interesting fact is that due to the decay of potassium in your bones, you actually emit about three hundred neutrinos a second.

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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.

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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.

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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!

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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.

myproton_profmattstrassler

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

atlas2009-collision-vp1-142308-482137-web

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

feynmanDiagram_DrellYan_Simple

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

cms_DY_7TeV

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.

feynmanDiagram_DrellYan_wRad

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.

feynmanDiagram_DrellYan_3j

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.

atlas_158466_4174272_Zmumu3jets

 

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”).

cms_StairwayHeaven_2014

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.

feynmanDiagram_DrellYan_wRadx6

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.

feynmanDiagram_DrellYan_wRadx6_Text

 

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)

 

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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.

Ingredients

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.

Recipe

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!

 

GRAPE 2

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.

Safety

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.

Plasmatastic!

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.

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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.

 

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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…

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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.

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