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Archive for November, 2010

There are definitely fewer Americans around CERN today but some of us were determined to celebrate Thanksgiving.We had a celebration on Thursday with mostly ALICE people but two students from ATLAS, one detector guru who has worked with all of the LHC experiments, and one American theorist friend who came from Frankfurt for the privilege of eating turkey.  It’s a great chance to share American culture and an excuse to have a party.  17 people (all physicists), an 8.5 kg turkey, and 10 kg of potatoes.

We got a turkey – special ordered from a butcher – and we found  butternut squash in the market.  Here are some tips on how to find American foods around CERN – interesting to see what people miss when they’re away from home.  Hope you had a nice Thanksgiving!


Hello again..

After just 10/11 months, I try to write a bit again.. Maybe I can only write when there is snow around… This year, certainly, I underestimated the task. I was sure that, being so involved with development, when things got in a stable mode operation, we would have a bit of relaxed time to think more peacefully in which tasks should be performed.

How mistaken I was…

Lots of work.. Specially to make sure the ATLAS trigger is operating in perfect
shape.. The increase of luminosity demanded that all the algorithms had to
be activated (performing selections) …
Also, we moved to a new apartment on the end of January.. That was a lot of
work, of course…
My beautiful little daughter Julia was born on March 19th and, since then,
taking care of Julia and Lucca is being a wonderful and very tiring activity..

Hope I manage to keep a bit more up-to-date version of this blog…

And that the LHC keep marveling us with all the results, like the so commented
“discovery” of quark-gluon plasma..


The ATLAS experiment has released a major new result in the past few hours, and I’m very excited about it because I helped! A public preprint of our paper, already accepted for publication by Physical Review Letters, is here. The result is that we’ve seen striking signs of a phenomenon called jet quenching in heavy ion collisions, in which hadronic jets get spread out and lose energy by interacting with the ultra hot and dense state of matter known as quark-gluon plasma. I’m exhausted, so I won’t try to explain it better than the CERN press release:

The ATLAS and CMS experiments play to the strength of their detectors, which both have very powerful and hermetic energy measuring capability. This allows them to measure jets of particles that emerge from collisions. Jets are formed as the basic constituents of nuclear matter, quarks and gluons, fly away from the collision point. In proton collisions, jets usually appear in pairs, emerging back to back. However, in heavy ion collisions the jets interact in the tumultuous conditions of the hot dense medium. This leads to a very characteristic signal, known as jet quenching, in which the energy of the jets can be severely degraded, signalling interactions with the medium more intense than ever seen before. Jet quenching is a powerful tool for studying the behaviour of the plasma in detail.

“ATLAS is the first experiment to report direct observation of jet quenching,” said ATLAS Spokesperson Fabiola Gianotti. “The excellent capabilities of ATLAS to determine jet energies enabled us to observe a striking imbalance in energies of pairs of jets, where one jet is almost completely absorbed by the medium. It’s a very exciting result of which the Collaboration is proud, obtained in a very short time thanks in particular to the dedication and enthusiasm of young scientists.”

I worked on this paper for only 4 or 5 days, meeting with a few other enthusiastic “young scientists” on Skype almost continuously in order to do a cross-check of the results using track jets. (I explained a bit about track jets when I explained my thesis topic a while back. This work wasn’t part of my thesis, but it’s an application of the same technique.) Our work ended up becoming three sentences on page four of the paper:

The analysis was independently corroborated by a study of “track jets”, reconstructed with [Inner Detector] tracks of pT > 4 GeV using the same jet algorithms. . . . A similar asymmetry effect is also observed with track jets. The jet energy scale and underlying event subtraction were also validated by correlating calorimeter and track-based jet measurements.

My immediate partners on this part of the work were David Miller and Zach Marshall, but I don’t want you to think just because I’m blogging about it that we were the critical people working on this paper. Obviously there were many other cross-checks of this result, all done through equally hard work in an equally short time, and then of course there are the Heavy Ion experts who did the core measurement itself! We only did a small part of it all, but we worked hard and we’re proud.

ATLAS as a whole is proud. We have won some and lost some this year in the competition with our colleagues at the CMS detector to make new measurements and discoveries first. This time, we won, and it’s a great feeling to end the year with.


Today I am sitting at my desk listening to the persisting chants of Birmingham’s protesting students, who have taken over Aston Webb since early this morning and are now playing music and marching across campus. Over 40 of them have joined a sit-in which is planned to last 36 hours. There is a strong, determined, almost positive atmosphere. It is impossible to ignore.

"No fees, no cuts": Over 40 students joined the sit-in

"No fees, no cuts": Over 40 students joined the sit-in

The last time campus was this frenzied was the Prime Ministerial debate in April. Since then, much has changed. Despite Lib-Dem promises, the government now plans to raise tuition fees to up to £9000 per year. Public funding for many subject teaching budgets may be withdrawn. Protests against tuition fees and university cuts are being held across the UK today, including a rally in London’s Trafalgar Square. Over 20,000 students across the country pledged a “walk-out” from lectures at 11am.

Government cuts have left many departments reeling, and a “freeze” on science funding (which still means effective cuts) has left STFC funded research like particle and nuclear physics among the hardest hit (after the funding body’s own severe funding crisis). Raised tuition fees will discourage many talented individuals from studying physics, and a lack of jobs in the field may force many existing scientists away from the country. Staff and students alike are very concerned for the future.

Particle Physics PhD students working at CERN have today joined together in support of the protests and stood atop a Large Hadron Collider Dipole outside the CERN cafeteria in Switzerland, Geneva, with a simple message: “Education is not for sale”. In my opinion, it is fairly poignant, as CERN strongly supports the idea, “Knowledge should be free” (making its research freely available to the public).

UK PhD students at CERN voice their concern

UK PhD students at CERN voice their concern

The UK PhD students are based at CERN to conduct their research. “We held our own demonstration in solidarity today”, Eleanor Davies of Oxford University (shown middle right) said, “We feel that future students will be discouraged from going to university, particularly those hoping to study science and technology courses – these are some of the most expensive courses to run and it is likely the tuition fees will be the highest for them.”

Sara Mahmoud from Liverpool University (shown crouched bottom left) invited the students to join her on the dipole in protest, worried that her younger brother may not be able to afford a degree. She told me,

“I am appalled by the recommended cut on university teaching funding and I think the looming closure of whole departments is nothing short of barbaric.”

Jody Palmer of University of Birmingham (shown top left), agreed:

“All of us here today were lucky enough to obtain our degrees before top up fees were introduced, though many of us have large debts still to pay off. It’s short-sighted of the coalition to think increasing fees wont affect participation, whatever their clauses are. Science particularly requires top class graduates, and that means students from every walk of life. You have to find the best in ability, and without them, our long term prosperity is in jeopardy.”

Though today’s protests are not supported by NUS, they are planning a Lobby of Parliament, so if you are against raising tuition fees and cuts to education, please send your constituency details to here. Also, PLEASE write to your MP.

One final point: Protests that raged earlier this month became out of control, and the rally in London appears to be doing the same. I urge anyone taking part in protests today to keep it peaceful. Violent action does not speak for the majority and destroys our reputation and message.



Thursday, November 18th, 2010

I recently passed the last last exam of my life (unless something unexpected happens), and it got me thinking about all of the exams I have had to take over my lifetime.

The first exam I have a memory from is a spelling test in the first grade.  In fact, I don’t so much remember a particular test, but I remember that I did well on one and my teacher invited me and the other top scorers to have lunch with her at the big table. This was much more exciting for a first grader that it would be now, and trust me it did not happen to me very often.  Since then the number of exams I have had to take is probably easily over a thousand, an idea which is a bit mind boggling.

Another funny thing about exams is that they seem to keep getting more important.  The first important exam for most people is probably the SAT which has some leverage in getting accepted to a college, and the in college you have Midterms and Finals to worry about.  If you are like me and want to get a Ph.D. you then have to go take two GRE exams, one general, and one in your field.  Once in graduate school the Midterms and Finals get even harder, and then you are typically asked to pass a written exam in your field.  This is usually do or die, except that most schools give the students a couple of tries to pass.  It took me two tries to pass that test in my department.

Finally we come to the last test which for us is an oral exam where you stand in front of a committee of five faculty and present to them your research topic while they grill you in an attempt to flush out any and all misunderstandings.  Thankfully I passed this test, and have now advanced to candidacy, a distinction which has no real affect on my day to day life, but is important for the university.

Now it is time for a little rest and relaxation to recover from all of these exams.


But what are quarks made of?

Thursday, November 18th, 2010

The pressure is on.  I have read and enjoyed the US LHC blogs off and on for the past couple years, and so I was thrilled to be offered a chance to join the ranks of these entertaining and informative writers.  Now that it comes time for my first post, I admit that I am wracked with anxiety.  Whatever academic writing skills I may possess will be of little use to me here, right?

Abstract: A new LHC blogger is introduced.  His research is described …

See? It doesn’t work.  So I suppose that, to get over my anxiety related to this first post, I will stick to a subject that I know very well: my own research!  Here it goes …

The remarkable success of the LHC and the experiments that reside on its ring [including my experiment, the Compact Muon Solenoid (CMS)] have made this an exciting time to be at CERN.  I have had the opportunity to help lead an exceptional group of researchers in a study of the early CMS data; this work has resulted in one of the first CMS publications based on data from 7 TeV proton-proton collisions.

To introduce this research, I’ll start with a little history:

We humans have been searching for the smallest unit of matter for a long time.  About 2500 years ago, Democritus proposed that all matter is made of tiny, indivisible (“atomos”) entities.  Unfortunately, Democritus was way ahead of his time, and even 2300 years after his hypothesis, we still did not know whether atoms really existed.  Finally, around 1800, Dalton and others realized that the elements combine in only certain proportions implying that there is a fundamental unit of each element; i.e., each element is made up of atoms.  Dalton’s atomic theory was a great advance, but it didn’t explain why there are so many (about 50, at the time) different elements.  The human tendency to categorize when presented with variety brought us Mendeleev’s Periodic Table of the Elements:

The Periodic Table of Chemical Elements


The fact that the elements fit nicely into a table based on their weights and chemical properties suggested that the elemental atoms are actually just different combinations of even smaller entities.  Only a few decades after Mendeleev presented his table, humans observed these sub-atomic entities when Thomson discovered the electron (1897), Rutherford the atomic nucleus (1910), and Chadwick the neutron (1932).

Soon after the discovery of the neutron, discoveries of particles that didn’t fit into our simple atomic model (e.g. pion, kaon, Lambda) hinted that a revision of that model was needed.  In the 1960’s, Gell-Mann suggested that these new particles, as well as protons and neutrons, were actually entries in another periodic table which he called the “Eightfold Way.”

The baryon octet of Gell-Mann's Eightfold Way.


Just as we now understand the diverse elements to be combinations of only three particles (protons, neutrons, and electrons), the Eightfold Way explained protons, neutrons, kaons, pions, etc. as combinations of particles that we now call quarks.  Only five years after Gell-Mann proposed his theory, these quarks were observed at the Stanford Linear Accelerator Center.

And this is where it stands today.  As far as we know, quarks are indivisible; i.e., quarks are the smallest unit matter in the nucleus.  But wait!  We do observe there to be six quarks arranged in three generations:

I know what you’re thinking:  But this is another table!  This looks just like the Periodic Table or the Eightfold Way!   Isn’t this therefore a hint that even quarks (and leptons) are made up of something smaller still?

That is certainly a very reasonable guess, but only experiment can tell us for sure, and unfortunately, it gets progressively more difficult to see these small particles: roughly speaking, the atom is one million times smaller than a human hair, and the proton is 100,000 time smaller than the atom.   Our current understanding is that the quark is a point-like particle with no spatial extent!

My recent research focuses on searching for evidence that quarks are made up of even smaller stuff by probing these tiny distance scales.   The unprecedented energy of the LHC allows us to probe smaller distances than ever before: about 1/20,000 the size of the proton.   In my next post, I’ll describe how we actually do this and tell you what we have found.


First heavy ion papers!

Thursday, November 18th, 2010

ALICE’s first two papers on lead-lead collisions were submitted yesterday, about a week and a half after the first lead-lead collisions.

One paper is a measurement of the charged particle multiplicity.  This analogous to the multiplicity measurements in proton-proton collisions, except a more particles are produced in a lead-lead collision.  In p+p the models were off by around 10-15%.  This is a plot from the lead-lead paper:

The red point shows the ALICE measurement.  The x-axis is a measure of the number of particles produced in the collision.  (Specifically it is the number of charged particles produced in the collision per unit pseudorapidity for pseudorapidities from -0.5 to 0.5.)  The black points are different predictions.  Notice is that the predictions vary from 1000-2000 particles.  This is a rather large theoretical uncertainty, especially compared to proton-proton collisions.  So less than two weeks after our first collisions, we have already gained a much deeper understanding of lead-lead collisions.

The other new paper is a bit more abstract than the number of particles created in the collision.  Think of a heavy ion collision as like slamming two ice cubes at each other.  If you slammed two ice cubes together fast enough, they’d melt when they hit each other.  If you did this in space – where it’s about 3K (about -270 Celcius and -454 Fahrenheit) – the water would immediately freeze again.  This is roughly what happens in a lead-lead collision.  Nuclei are basically frozen quarks and gluons, and when we collide them fast enough, they melt.  But by the time we see the remnants of the collision in our detector, the quarks and gluons have frozen again.  However, they went through a phase where they were a liquid.  A liquid can flow.  We can see evidence that the liquid of quarks and gluons was flowing because we can see in our detector that the particles are all moving in a preferred direction.

This plot compares ALICE’s measurement to earlier measurements:

The x-axis is the collision energy per nucleon (proton or neutron) in the center of mass.  The y-axis is a measure of how much the liquid is flowing.  [Technical audience:  The y-axis is the coefficient of the second term of the Fourier decomposition of the distribution of particles in azimuth with respect to the reaction plane.]  Measuring how much the Quark Gluon Plasma is flowing gives us some insight into its viscosity.  There are lots of technical details and subtleties in interpreting these data that I’m skipping over.  But already – less than two weeks after the first lead-lead collisions – we have two measurements that give us deep insight into the properties of the Quark Gluon Plasma.


Good day,

As Jon Butterworth reports in his Guardian blog entry today, the ATLAS cd project, long in the making, is about to be released. The project, which is titled Resonance, will constitute a double cd and DVD. Commemorative mugs, t-shirts and baseball caps will undoubtedly follow. All the proceeds go to charity, in particular, to the Happy Children’s Home in Pokhara, Nepal to help them build their orphanage. So not only do you get the great music and fascinating insight into the scientific/sonic marriage, but also you can snuggle up with that warm fluffy feeling of having helped out those little blighters who aren’t as well off as you are. It’s win win. To the music. The compositions are a mixed bag, spanning many genres and styles which in some sense, I suppose, is meant to reflect the diversity of the collaboration it represents. So therefore, not everything will be to everyones tastes. But the quality is excellent
and will, we hope, impress and perhaps even shock a few people. I have my favourites of course, but I am a bit biased given that I wrote and performed one of the songs. On that front, I’d like to make a little mention that the band I’m with, AWESOME, are the only band on the cd composed entirely of ATLAS physicists! But there is a lot to enjoy, from Heavy metal to Celtic harp.

Heavy metal you say? That’s right, like lead. The ions of which we are now continuously bashing together to produce spectacular new results. This shows the flexibility and breadth of the LHC physics program to probe the moments after the big bang.

For a more complete description of the heavy ion collisions see this very nice piece in Symmetry magazine.

But back to the cd, and to all of you going through the usual annual struggle of finding an interesting Christmas present for their families and friends, what better than a music project guaranteed to satisfy all comers. After all, just like ATLAS, there’s something for everyone. Here’s a picture of three of the handsomest collaborators if you needed more convincing.

Posing. (left to right, Nick Barlow, myself, Christian Ohm).

Posing. (left to right, Nick Barlow, myself, Christian Ohm)

Bye for now.


Fall at CERN

Monday, November 15th, 2010

You know it’s Fall at CERN when…

… you’ve forgotten what a sunny day looks like, because it’s been cloudy and raining since the end of September.

… you’ve forgotten what a sunny day looks like, because the sun rises after you’ve gone to work and sets before you’ve gone home, and your office blinds are mostly shut because any glare on your computer screen impedes your research.

… the snow line starts creeping down the Juras.

… ski season starts creeping into conversations.

… fondue is once again an acceptable — and delicious! — dinner option. (N.B. Only tourists eat fondue in the summer. Seriously.)

… Christmas lights go up in Saint-Genis-Pouilly, but they don’t get turned on for some time, possibly because the French have built a certain amount of flexibility into the schedule to take into account strikes and coffee breaks.

… people are preparing for Winter Conferences, and you already begin to resign yourself to a holiday break filled with both cookies and code.

… protons aren’t colliding in the LHC. (Zing!)

… the days keep getting shorter, so how do your workdays keep getting longer?!

… [Readers, what have I missed?]

— Burton


These days I’ve been slacking a bit with my posts here. The reason is that I am in the middle of applying for jobs for next year. Definitely not my favorite part of the job. Unlike when I did this for the first time five years ago, I don’t need to send a stack of 70 envelopes all around the world anymore. Now, luckily, everything can be done online. Many applications, both in Europe and the US, can be done by uploading the documents once to one website (such as AcademicJobsOnline) and then indicating which positions you are interested in. But unfortunately, there are still many institutions which have their own system. I still spend a lot of time browsing through individual job ads and filling variations of the usual forms in many different places. Ah well. It has to be done and I only hope everything will turn out well in the end (two-body problem and all).