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Archive for September, 2010

Tour of the the EMCal test beam

Thursday, September 30th, 2010

I briefly mentioned the EMCal test beam – the reason for my latest post – but here’s some more details and some gratuitous cool pictures.  I am now back in the US and have more time for blog posts.

The goal of our time at the test beam was to calibrate our detector, an electromagnetic calorimeter, by measuring its response to known particles with a known momentum.  Then when we put in the rest of ALICE we can use it to help identify unknown particles and measure their energy.  Here’s a rough diagram of the test beam set up for the EMCal:


We use the beam from the Super Proton Synchrotron (SPS) to get our test beam.  The SPS is also used as a source of protons for the LHC.  The proton beam from the SPS is aimed at a target – a thin block of material.  When it hits this target it creates a lot of secondary particles – mostly pions and protons.  This beam is then directed through a magnet.  A particle with a charge q with a momentum p in a magnetic field B will move in a circle with a radius r=p/qB.  We aim the beam through an aperture, which fixes the radius.  Then we can change the magnetic field, which changes the momentum of the particles which pass through to our detector.  First we passed the beam through two multiwire proportional chambers, which let us measure the position of the incoming particles.  (These work because of the same physical principles as the Time Projection Chamber.  Charged particles moving through a gas ionize the gas and we can collect the electrons knocked out of gas molecules to see where the particles went.  The details of how multiwire proportional chambers work are different, however.)  Then the particles hit our detector.  We only have a few towers of the calorimeter in the test beam – enough to be representative of the whole detector.

The dashed line in the bending magnet shows an optional converter that we could put in or take out.  When a photon hits the converter, it changes into an electron and a positron.  The converter is only part of the way through the bending magnet, so electrons will not be bent as much as hadrons.  This meant we could choose between a beam with hadrons (pions and protons) or electrons.  We’re particularly interested in the difference between the response of our detector to hadrons and electrons.

Here you can see the set up from above:

I’ve labeled our EMCal towers and the multiwire proportional chambers.  The large cement blocks arranged like giant Legos are for shielding.  There are several cables leading from the detectors up to the barracks where we sat when taking data.  Some of these were used to carry data from the detectors up to us.

Here you can see the back of our detector and see our data acquisition system:

The big green object labeled “DESY” is a movable table that we used to move our detector around in the beam.  We could control it remotely.  Despite what it may look like, all of those cables are meticulously arranged.  We had to arrange the cables so that they stayed plugged in even when we moved the table around.  The computer is for data acquisition.  Here’s a different view:

And that’s me next to the set up – smiling, even though it’s about 1:30 AM and my shift just started.  I have my finger on the outside of one of the EMCal towers.  There’s one wire chamber to my left and one to my right, right in front of the EMCal towers.  The chain around my neck has a dosimeter on it, measuring how much radiation I get exposed to to make sure I’m not getting a dangerous dose of radiation.  (The risk is very low.  There was no beam when we took these pictures and a physical barrier preventing the beam from coming in anyways.)

And here you can see the beam pipe delivering beam to our set up:

With Betty, a post doc at Laurence Livermore National Laboratory.  Betty and I took the midnight to 8 AM shifts for the test beam together.

We got lots of valuable data on the EMCal response to electrons and hadrons from a momentum of 6 GeV/c to 225 MeV/c.  Now we have to analyze the data.  We’ll be able to use it to determine how good our detector is at separating electrons from hadrons.  We have an idea of how well it should work from simulations, but nothing beats a measurement.

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Duck tape fixes everything!!!

Wednesday, September 29th, 2010

As reported today in Symmetry Magazine blog Symmetry Breaking the crew at the Tevatron have been hard at confirming one of the oldest sayings in science/engineering/home repairs:

Duck tape will fix anything!!!

As the story is told, upon noticing the pressure rising in one of the cyrostat vacuum systems a team from the Accelerator Division Mechanical Support determined that a faulty rubber O-ring between two of the superconducting magnets had gone bad. However, having just come out of a summer shutdown and not wanting to add an additional 10 days of down time to the calendar that it would take to warm up the Tevatron and replace the ring other options needed to be explored. I think the article on Symmetry Breaking had a really good quote

“This big machine is four miles in circumference, with a thousand-some superconducting magnets, and one piece of rubber is gonna stop us?” Scott McCormick said. “I don’t think so. Not if we can help it.”

Showing where the faulty O-ring was

Showing where the faulty O-ring was

What amazes me is that an alternative option had already been explored 4 years ago!!! They had determined then that the handyman’s secret weapon (Duck tape) would be a perfect solution for exactly this case! So, low and behold, two days later and a few feet of duck tape and viola!!! You have one of the world premier particle accelerators back online and cranking like it never has before.

It is stories like this that I love about science. Millions of dollars to build, decades of building and engineering to make come to reality, countless man hours spent studying, tuning, and perfecting the operations of one of the most complex machines ever conceived by man kind! And, $2.47 roll of duck tape from the hardware store!

Never leave home / build a particle accelerator without it

Never leave home / build a particle accelerator without it

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Are physicists geeks?

Tuesday, September 28th, 2010

That’s a common stereotype, as a scientist, you certainly have some knowledge about science (of course! 🙂 ), computers etc… So in people’s mind, a physicist is often considered as the ultimate evolution of the geek. I guess it’s not totally wrong, if you consider that a geek is a person who loves computers, science-fiction and complicated role-play games, they will certainly choose a carrier in science or computers.
Before I started my PhD I also though that many of the physicists were geeks, but I realized that it concerns only a small fraction of them.

The Big Bang Theory

The Big Bang Theory

Quite recently, a well known TV-show called “the big bang theory”, shows an extreme image of the physicists as geeks with no life and almost antisocial. The TV-show is very funny, and the references to our work makes me laugh often, but is it really the image we want to give? I’m pretty curious to read the comments about it. So, do you think that physicists are geeks? Do you think it could be bad for us to be seen as geeks?

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Good Burgers, Bad Roads

Monday, September 27th, 2010
Fall is in the air in western Massachusetts in the early morning.

Fall is in the air in western Massachusetts in the early morning.

Finally almost two weeks of vacation. Not the ideal time, since life after summer has started everywhere already, so I am missing a couple of meetings and other events, but still, it is worth it. This time we are overseas, at the moment in Washington, D.C., where a friend of ours got married on Saturday. A welcome occasion for a visit to the US capital, and later a visit of friends in New England.

When we arrived in Boston on Wednesday night, temperature wise it felt like summer, with temperatures beyond 30 degrees, which we have not seen in Germany for a while. But still, fall is here, the leaves are changing color, fog on the fields in the morning, and foliage season is just around the corner.

We also took the chance to visit D.C., stroll along the National Mall, and eat a damn good burger at the Presidents favorite burger joint (allegedly).  At the evening drive through the city to see the beautifully illuminated monuments, our car, prompted by one pothole too many, decided however to throw one of its hubcaps in the direction of said Presidents house, right at the corner of Pennsylvania and 17th. I went once around the block, and much to the entertainment of the police and U.S. Secret Service, my wife jumped out to recover our hubcap. Loss damage waver for the rental car is nice, but of course it is preferable to bring everything back in one piece…

Fantastic weather in DC...

Fantastic weather in DC...

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A great fill!

Friday, September 24th, 2010

Yesterday and last night (US time), the LHC had a really amazing fill. CERN DG Rolf Heuer just sent a message about it, and it says it so well I’m just going to copy it here:

A long period of machine development paid dividends last night with a game-changing fill in the LHC. As I write this, the fill, which started colliding at 19:00 yesterday evening, has just wound down. Both ATLAS and CMS have posted integrated luminosities of over 680 inverse nanobarns, and the initial luminosity for the fill doubles the previous record at 2 x 1031cm-2s-1.

But it’s not the records that are important this time – it’s normal that in the start-up phase of a new machine, records will fall like autumn leaves – what’s significant here is that the LHC’s performance this fill significantly exceeded some crucial design parameters, opening up the path to much better still to come.

Last night’s fill was the first with 56 bunches arranged in trains of eight bunches per train. The significance of bunch train running is that we can configure the orbits such that more bunches collide in the experiments, so even though the number of bunches may not be much higher, the collision rate is. For example, last night’s 56-bunch fill had 47 bunches colliding at ATLAS, CMS and LHCb, with 16 colliding in ALICE, whose needs are lower. This compares to a maximum of 36 colliding bunches out of 48 total before we introduced bunch trains.

A big jump in luminosity was clearly expected in moving to bunch trains and colliding more bunches. What came as a pleasant surprise is that it was accompanied by an exceptional beam lifetime of 40 hours, and less disruption to the beams caused by packing more protons into a smaller space (in technical terms, the beam-beam tune shift was much less destructive to the beams than anticipated). This result means that the LHC operators have more leeway in operational parameters in the quest for higher luminosity.

The plan for today and the weekend is to run for one more fill with 56 bunches in trains of eight before moving on to 104 bunches in 13 trains of eight, with 93 bunches colliding in ATLAS and CMS. Ultimately, the LHC will run with 2808 bunches in each beam, so there’s still a long way to go. We’ll get there slowly but surely by adding bunches to each train until the trains meet in a single machine-filling train. That will take time, but for the moment, last night’s fill puts us well on the way to achieving the main objective for 2010: a luminosity of 1032 cm-2s-1.

To put this in perspective: the much-heralded LHC results at the July ICHEP conference were based on about 250 inverse nanobarns of data, drawn from about 350 inverse nanobarns delivered by the LHC from April through mid-June. Yesterday, in less than one day, the LHC delivered almost double that entire amount! The state of play will be changing quite rapidly on all LHC experiments. Stay tuned!

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Recently, I’ve become more aware of the environmental and societal impact of mass consumption. I am examining my own lifestyle choices, but I also wonder where in the spectrum between extreme consumerism and minimalism scientists as a group are falling.
On the whole, I think most of my colleagues are far from the extreme shopping treadmill. Most of us seem to feel that there are things more important than shopping, fancy clothes or cars. I guess most of us consume less than average, but probably not out of environmental awareness, but because our minds revolve around other things. There is no peer pressure towards consumerism in our segment of society, since no one practices it. People run around in age-old saggy sweatshirts and no one cares. Social comparison is pushing us rather towards looking like it did not occur to us to brush our hair because we’ve been thinking so hard about an interesting problem than buying the latest fashion in order to keep up.
There is one notable exception to the rule, namely electronic gadgets. Most of us are a bit computer geeks. If anyone of us is seen sporting the newest fad of anything, it’s usually the newest iPhone, laptop computer or ebook reader. I am myself a little prone to this. My Macbook is very dear to me. If I go on a (non-work related) 2-day trip bringing only my iPod and not my computer, it feels like quite an achievement.
Before becoming more aware of environmental issues, I had already been drawn to a more minimalist lifestyle, since my frequent moves had amply taught me that by accumulating too much stuff I was doing myself a disservice. Plus, I like clean empty spaces and I hate wasting things.
But I realize that I need to examine my lifestyle choices even more closely. For anyone vaguely interested in the topic, I can recommend the book The Story of Stuff by Annie Leonard. It’s quite an eye-opener.
As a group, scientists are a privileged set, in the sense that we have received a lot of education. Since we cannot hide behind ignorance or claim not to grasp the environmental consequences of our lifestyle choices, I think we need to show a bit of responsibility.

I would be curious to learn from the comments how other scientists think about the matter!

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On Behalf of ATLAS

Wednesday, September 22nd, 2010

The 40th International Symposium on Multiparticle Dynamics is going on coffee break now, but a little bit after we get back, I’ll be giving a talk on behalf of the ATLAS Collaboration.  (You can find my slides on the agenda link above, but there’s no video feed at small conferences like this.)  There’s no new material beyond what was shown at ICHEP, but it’s exciting for me because it’s the first time I’ve represented my experiment in a full-fledged international conference talk.  Wish me luck!

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Exciting new results from CMS

Tuesday, September 21st, 2010

I’m giddy today because CMS just came out with some very exciting results.  I don’t think we understand what they mean at all – and as a scientist, there is nothing I love better than shocking data, data that challenge what we think we understand.  (For the technical audience, the slides from the talk at CERN are here and the paper is here.)  I might be biased because this topic is very closely related to my doctoral thesis, but I think it’s safe to say this is the first surprising result from the LHC, something that changes our paradigm.

In heavy ion collisions at the relativistic heavy ion collider we observed something called the ridge (from this paper):

We more or less understand the peak – called the “jet-like correlation” – but we don’t understand the broad structure the peak is sitting on.  This broad structure is called the ridge.  What I mean when I say we don’t understand the ridge is that we haven’t settled in the field how this structure is formed, where it comes from.  We have a lot of models that can produce something similar, but they can’t describe the ridge quantitatively.

Here’s what CMS saw:

It’s a slightly different type of measurement – I’ve put a box around the part with the ridge.  We see the same peak as we saw before – again, we pretty much understand where this comes from.  But there’s a broad structure beneath this peak.  It’s smaller than what we saw in heavy ion collisions above, but it’s there – the fact that it’s there is surprising.

In the models we have from heavy ion collisions the ridge is from:

  • A high energy quark or gluon losing energy in the Quark Gluon Plasma,
  • Collective motion of particles in the Quark Gluon Plasma, or
  • Remnants of the initial state (meaning the incoming particles)

In our current understanding of what goes on in a proton-proton collision, there is no Quark Gluon Plasma – so the conservative interpretation of these data would mean that the ridge is somehow some remnant of the initial state. Even conservatively, this would severely constrain our models.  Some physicists, such as Mike Lisa at Ohio State University, have proposed that there may be collective motion of particles in proton-proton collisions, similar to what we see in heavy ion collisions.  This would imply that we also see a medium in proton-proton collisions.  That would be a huge discovery.  (Just to be clear, CMS is not making this claim, at least at this point.)  It will take a while for the community to debate the meaning of these data and come to a consensus on what they mean.  But these data are definitely very exciting – this is the most exciting day for me since the first collisions!

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There is an interesting article about Gregg Berman, who received a PhD in particle physics from Princeton about 20 years ago, who is heading the investigation into the stock-market free fall on May 6:

In investigating the crash, Mr. Berman says he finds himself in a position similar to his physics work 20 years ago, when he was collecting huge amounts of data and comparing the competing views of many laboratories on a question dividing particle physics — whether the neutrino, one of the least known and most common elementary particles, actually had mass.

Particle physicists sometimes show up in surprising places.

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Collisions Recorded

Sunday, September 19th, 2010

This essentially shows the amount of proton collisions created by the Large Hadron Collider as a function of time.

I have my own personal “analysis” code set up so that every Friday I run on the newest data available.  Then, by the afternoon or Monday morning I have new plots to look at and show colleagues.

I do this on Friday because that’s when a list of “good” data-taking periods is published.  That is, there is a group of people that decide when the detector was fully operational, working as expected, and recording useful data.  They then publish a list of this “good data”, and everyone else uses that list so that they run on useful data.  An example of “not useful” data (for physics) would be if one or more parts of the detector were off, or not working properly.

The plot I show here is the amount of proton-collision data available.  It grows more than linearly because the intensity of the  beams is being increased as well.  Someday, in the distant future, the LHC will be able to deliver an amount of collisions in one work-week equivalent to everything we’ve recorded since turning on in May this year.  Someday.

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