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Posts Tagged ‘ALICE’

A couple of my friends came up with this video that shows you what a heavy ion collision “sounds” like:

[youtube jF8QO3Cou-Q The sound of the little bang]

They base the sound on a power spectrum derived from measurements at RHIC.  The discussion is about the Relativistic Heavy Ion Collider, but everything they say is also true for the LHC.  You can find a great discussion of this on their web page.

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There are many subsystems in ALICE, each of them with different purposes.  The main tracking detector in ALICE is a Time Projection Chamber (TPC).  The TPC is cylinder about 5m (16.4 ft) in diameter and 5 m long filled with gas – the largest time projection chamber in the world.  You can see above that a person can actually fit in it.  The basic principles behind a TPC are really simple:

  • A charged particle in a magnetic field moves in a circle with a radius r = p/qB where p is the particle’s momentum, q is the charge of the particle, and B is the magnetic field.  The magnetic field in the TPC is roughly constant, as shown below:

    so that a charged particle will bend either clockwise or counterclockwise, depending on its charge:

  • A charged particle moving through a gas will ionize the gas, knocking electrons (called secondary electrons) off the gas molecules
  • The TPC has an electric field (400 V/cm), as shown below, that causes the secondary electrons to drift to the end of the TPC (the ends of the cylinder):

  • We have pads on the ends of the TPC which collect the charge and each of these pads has to be read out for every event.  The position on the end of the cylinder (usually called x and y) is determined by where the charge hits the end of the TPC.  The position along the beam axis (usually called z) is determined by the time it took the charge to get there and the drift velocity of the TPC.  (This conversion from time to distance is what gives the TPC its name – a time projection chamber.)
  • This gives us a bunch of “hits” – the position of a charged particle in the TPC at different times so what we actually see are the red dots below:
    We then have to figure out which hits belong to which track.  Not so difficult if there’s only a few tracks but for heavy ion collisions we expect to create a few thousand particles in each collision.

  • We have to collect data fast enough that the detector is ready for the next collision.  Collisions occur several hundred times per second.

A lot of details have to be just right to get a TPC to work well.  We have to know the electric field and the magnetic field very precisely.  The amount of charge left by a particle is sensitive to the type of gas and to the temperature.  We have to keep the temperature constant to within 0.1 degree Celcius.  Because the TPC is so large, keeping everything constant and well calibrated is very difficult.  But ALICE has done it.

And not only is it the biggest TPC in the world, but it’s also the best, in my humble opinion.

Here you can see some tracks reconstructed in the TPC from a 7 TeV proton-proton collision:

You can see some more event displays here.  Some animations of event displays collisions at 7 TeV in ALICE are here, here, and here.  (You can see some of the other detectors in these displays – I left them out of the diagrams above for simplicity.)

ps – Thanks to Jim Thomas, one of the many members of the TPC team, for helping me find event displays, technical details, and editing!

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ALICE’s second paper!

Tuesday, April 20th, 2010

ALICE’s second paper has been submitted!  If you’ve been following carefully, you’ll have heard that it’ll take at least a couple of years to get enough statistics to see the Higgs (if it’s there) – but we don’t have to wait that long for other results.  This paper presents a measurement of the number of charged particles produced in proton-proton collisions at center of mass energies of 0.9 TeV and 2.36 TeV.  (CMS actually published their paper on the same subject first.)  Proton-proton collisions are actually pretty complicated and we still don’t understand everything about them.  Protons are made up of quarks and gluons, so when we slam them together we get a combination of quark-quark, quark-gluon, and gluon-gluon interactions.  We can describe the products of these interactions pretty well when both particles hit each other hard, but not if they barely interact.  They can also interact multiple times.  So proton-proton collisions are really difficult to model.  Theorists have come up with models that try to describe proton-proton collisions, but these models still need improvement.  Counting the number of particles produced in a collision is a relatively straight forward measurement.  (Not to say it’s easy – there’s still a lot of work that has to be done for this, but there’s even more work needed for other measurements.)  And this measurement gives us data we can compare to models.   The models seem to be underestimating the number of particles produced – only by a few percent, but they’re still not quite right.

ALICE and CMS’s measurements also agree.  This is very important.  These measurements are very complicated and many things can go wrong.  Since two experiments did the same measurement with very different detectors, different methods, different code, different people, etc. and still agree, this gives us greater confidence in the measurement.  This is one of the reasons for having multiple collaborations doing the same measurement.

You won’t read about these results in the newspapers because there have been no dramatic paradigm shifts in our understanding of proton-proton collisions, but these are very important basic measurements that improve our understanding incrementally and they have to be done before we can hope to discover new physics.

Update April 21 – the third ALICE paper, on the 7 TeV data, was submitted today!

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Time’s up and this time it’s serious ! All big experiments at the LHC are gearing up for collisions within the next month, and for ALICE the numbers are staggering. Assuming we are running about six months of proton-proton collisions and one month of heavy ion collisions per year (i.e. 30 weeks of continuous operation) , the commitment it takes from each and every member of the collaboration is substantial.

The ALICE experiment consists of 18 detectors and 6 so-called general systems (experiment control, detector control, central trigger processor, high level trigger, data acquisition and offline monitoring). In the start-up phase, which is scheduled to last at least the remainder of 2008 and maybe most of the 2009 run, the experiment requires not only a steady 24/7 shift crew but also a substantial number of on-call experts. At this moment the conservative estimates are that at any given time 24 persons need to be on shift and 41 persons need to be on-call experts. In 2009 the on-site shift crew is supposed to reduce to 17 persons with the goal of reaching steady-state operation with a 10 person shift crew by 2010. The counting house is laid out accordingly, but at least for 2008 and most of 2009 it will get very crowded.

Now ALICE is a big collaboration with more than a 1000 Ph.D.’s at this moment, so these resource requirements should be easy to distribute across the whole collaboration, right ? Well, even with so many people the number of eight hour shifts for each individual Ph.D. are still daunting. My institute, Wayne State University, is one of the larger U.S. participants in ALICE, but even with four Ph.D.’s our responsibility comes up to only 0.882% of the total shifts. Still with a total shift allotment of 17,490 shifts in 2008 and 16,185 in 2009, each of our four Ph.D. needs to take around 40 shifts per year, and assuming we take one shift per day we will be at ALICE at least around 1.5 months per year.

Graduate students will carry a big load of these shifts in the coming years, but the early startup phase will likely have to be covered by the existing Ph.D.’s. This is a major commitment which requires substantial travel funds and time allotments for university folks like myself. It is definitely not cheap to do physics abroad. Besides the bad exchange course of the American dollar, the housing situation in and around Geneva is a major headache for many of us. A whole trek of people will steadily have to commute between the U.S. and Geneva from now on. The total commitment of the U.S. institutions to the ALICE shift total is presently around 5%, which is equivalent to about 850 shifts in 2008. But I would assume the shift load for the U.S. in ATLAS and CMS is considerably higher.

For many students this is a great opportunity to see the world and learn about different cultures besides just doing science within an international community. But all of it needs to be well planned. Apartments need to be rented, transportation needs to be provided etc. etc. So it takes a BIG effort to do BIG science, and if you do it from abroad it might even take a little more.

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The LHC Astro-Lab

Monday, May 5th, 2008

A few weeks ago the physics community got shaken by an announcement of the DAMA project (no not the bad guys on ‘Lost’, they’re the DHARMA initiative), an underground experiment in the Gran Sasso tunnel, which claimed to have found experimental evidence for Dark Matter. The claim is based on the fact that the motion of the earth around the sun should produce a modulation in the dark matter count rate, because the earth’s velocity needs to be added (or subtracted) to the dark matter (or halo) escape velocity. DAMA has found indeed an eight sigma signal of a modulation in their candidate count rate. The question remains whether any background source could cause this signal, and it will take scientists some time to exclude all reasons why this measurement might not be significant. Nevertheless the possibility of experimental evidence for dark matter is exciting. But what does this have to do with the LHC and in particular ALICE ?

Well, throughout the past few years relativistic heavy ion and high energy physics have stressed their significance towards understanding QCD and electro-weak symmetry breaking, but the original quest for the heavy ion program at RHIC and the LHC was to find a state of matter which would have taught us a lot about the evolution of the universe shortly after the Big Bang, at a time where matter as we know it (luminous and dark) should have formed. This original link has been disfavored for some time because scientists felt that the ‘Little Bang’ can not be easily applied to the ‘Big Bang’; the system is too small, the evolution is too fast. But several speculative explanations of experimental measurements at RHIC gave new life to the ‘astro-connection’ of relativistic heavy ion physics (see for example Peter Steinberg’s blog entries on Anti-de-Sitter space and Hawking-Unruh radiation). D.J.Schwarz from CERN in his very instructive article: ‘The first second of the universe’ showed the anticipated evolution of matter formation, and pointed out the relevance of the so-called QCD phase transition from quarks and gluons to hadrons for the evolution of the universe.

It is interesting to note that the LHC offers a two-prong approach to accelerator based astrophysics. Not only can the high energy proton proton collisions likely probe the Higgs field, extra dimensions, super symmetry and dark matter candidates, but the relativistic heavy ion collisions can probe physics in the strong force sector that has traditionally been assumed to occur at higher energies, such as CP violationwhich is necessary for baryogenesis in the universe, anti-baryonic dark matter candidates and the infamous 5-d quantum black holes.

So this is an exciting time, and the diversity of the LHC programme, bringing high energy and heavy ion physicists together by offering proton-proton and Pb-Pb collisions, will lead not only to breakthroughs in the understanding of QCD and potentially new physics beyond the standard model. It will also make the LHC the premier astro-lab in the world. I am glad that all three big experiments (ATLAS, CMS and ALICE) now feature a pp and a PbPb program. Although ALICE is the most versatile heavy ion detector, both ATLAS and CMS have strong programs with heavy ions, and only together and with the necessary verification of each other’s results will we be able to crack some of the cosmic mysteries that I am most interested in. I am looking forward to that and to your attempts of taking aim at some of my claims in this and future blogs

Cheers

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