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Posts Tagged ‘ALICE’

Even before my departure to La Thuile in Italy, results from the Rencontres de Moriond conference were already flooding the news feeds. This year’s Electroweak session from 15 to 22 March, started with the first “world measurement” of the top quark mass, from a combination of the measurements published by the Tevatron and LHC experiments so far. The week went on to include a spectacular CMS result on the Higgs width.

Although nearing its 50th anniversary, Moriond has kept its edge. Despite the growing numbers of must-attend HEP conferences, Moriond retains a prime spot in the community. This is in part due to historic reasons: it’s been around since 1966, making a name for itself as the place where theorists and experimentalists come to see and be seen. Let’s take a look at what the LHC experiments had in store for us this year…

New Results­­­

Stealing the show at this year’s Moriond was, of course, the announcement of the best constraint yet of the Higgs width at < 17 MeV with 95% confidence reported in both Moriond sessions by the CMS experiment. Using a new analysis method based on Higgs decays into two Z particles, the new measurement is some 200 times better than previous results. Discussions surrounding the constraint focussed heavily on the new methodology used in the analysis. What assumptions were needed? Could the same technique be applied to Higgs to WW bosons? How would this new width influence theoretical models for New Physics? We’ll be sure to find out at next year’s Moriond…

The announcement of the first global combination of the top quark mass also generated a lot of buzz. Bringing together Tevatron and LHC data, the result is the world’s best value yet at 173.34 ± 0.76 GeV/c2.  Before the dust had settled, at the Moriond QCD session, CMS announced a new preliminary result based on the full data set collected at 7 and 8 TeV. The precision of this result alone rivals the world average, clearly demonstrating that we have yet to see the ultimate attainable precision on the top mass.

ot0172hThis graphic shows the four individual top quark mass measurements published by the ATLAS, CDF, CMS and DZero collaborations, together with the most precise measurement obtained in a joint analysis.

Other news of the top quark included new LHC precision measurements of its spin and polarisation, as well as new ATLAS results of the single top-quark cross section in the t-channel presented by Kate Shaw on Tuesday 25 March. Run II of the LHC is set to further improve our understanding of this

A fundamental and challenging measurement that probes the nature of electroweak symmetry breaking mediated by the Brout–Englert–Higgs mechanism is the scattering of two massive vector bosons against each other. Although rare, in the absence of the Higgs boson, the rate of this process would strongly rise with the collision energy, eventually breaking physical law. Evidence for electroweak vector boson scattering was detected for the first time by ATLAS in events with two leptons of the same charge and two jets exhibiting large difference in rapidity.

With the rise of statistics and increasing understanding of their data, the LHC experiments are attacking rare and difficult multi-body final states involving the Higgs boson. ATLAS presented a prime example of this, with a new result in the search for Higgs production in association with two top quarks, and decaying into a pair of b-quarks. With an expected limit of 2.6 times the Standard Model expectation in this channel alone, and an observed relative signal strength of 1.7 ± 1.4, the expectations are high for the forthcoming high-energy run of the LHC, where the rate of this process is enhanced.

Meanwhile, over in the heavy flavour world, the LHCb experiment presented further analyses of the unique exotic state X(3872). The experiment provided unambiguous confirmation of its quantum numbers JPC to be 1++, as well as evidence for its decay into ψ(2S)γ.

Explorations of the Quark-Gluon Plasma continue in the ALICE experiment, with results from the LHC’s lead-proton (p-Pb) run dominating discussions. In particular, the newly observed “double-ridge” in p-Pb is being studied in depth, with explorations of its jet peak, mass distribution and charge dependence presented.

New explorations

Taking advantage of our new understanding of the Higgs boson, the era of precision Higgs physics is now in full swing at the LHC. As well as improving our knowledge of Higgs properties – for example, measuring its spin and width – precise measurements of the Higgs’ interactions and decays are well underway. Results for searches for Beyond Standard Model (BSM) physics were also presented, as the LHC experiments continue to strongly invest in searches for Supersymmetry.

In the Higgs sector, many researchers hope to detect the supersymmetric cousins of the Higgs and electroweak bosons, so-called neutralinos and charginos, via electroweak processes. ATLAS presented two new papers summarising extensive searches for these particles. The absence of a significant signal was used to set limits excluding charginos and neutralinos up to a mass of 700 GeV – if they decay through intermediate supersymmetric partners of leptons – and up to a mass of 420 GeV – when decaying through Standard Model bosons only.

Furthermore, for the first time, a sensitive search for the most challenging electroweak mode producing pairs of charginos that decay through W bosons was conducted by ATLAS. Such a mode resembles that of Standard Model pair production of Ws, for which the currently measured rates appear a bit higher than expected.

In this context, CMS has presented new results on the search for the electroweak pair production of higgsinos through their decay into a Higgs (at 125 GeV) and a nearly massless gravitino. The final state sports a distinctive signature of 4 b-quark jets compatible with a double Higgs decay kinematics. A slight excess of candidate events means the experiment cannot exclude a higgsino signal. Upper limits on the signal strength at the level of twice the theoretical prediction are set for higgsino masses between 350 and 450 GeV.

In several Supersymmetry scenarios, charginos can be metastable and could potentially be detected as a long-lived particle. CMS has presented an innovative search for generic long-lived charged particles by mapping their detection efficiency in function of the particle kinematics and energy loss in the tracking system. This study not only allows to set stringent limits for a variety of Supersymmetric models predicting chargino proper lifetime (c*tau) greater than 50cm, but also gives a powerful tool to the theory community to independently test new models foreseeing long lived charged particles.

In the quest to be as general as possible in the search for Supersymmetry, CMS has also presented new results where a large subset of the Supersymmetry parameters, such as the gluino and squark masses, are tested for their statistical compatibility with different experimental measurements. The outcome is a probability map in a 19-dimension space. Notable observations in this map are that models predicting gluino masses below 1.2 TeV and sbottom and stop masses below 700 GeV are strongly disfavoured.

… but no New Physics

Despite careful searches, the most heard phrase at Moriond was unquestionably: “No excess observed – consistent with the Standard Model”. Hope now lies with the next run of the LHC at 13 TeV. If you want to find out more about the possibilities of the LHC’s second run, check out the CERN Bulletin article: “Life is good at 13 TeV“.

In addition to the diverse LHC experiment results presented, Tevatron experiments, BICEP, RHIC and other experiments also reported their breaking news at Moriond. Visit the Moriond EW and Moriond QCD conference websites to find out more.

Katarina Anthony-Kittelsen


The coolest and hottest fluid

Friday, October 19th, 2012

In September, the Large Hadron Collider (LHC) operators at CERN attempted a new trick: putting in collisions protons in one beam and lead ions in the other. Usually, the LHC operates with two beams of identical particles (protons or ions) circulating in opposite directions in the accelerator. Here is what is expected from this new setup.

These ions are atoms stripped of all their electrons, leaving only the nucleus. Lead ions contain 82 protons plus 126 neutrons, all held together by the nuclear force.  Protons are also composite objects made of three quarks and bound together by “gluons”, the particles carrying the nuclear force.

So when two such heavy ions collide at nearly the speed of light, I dare anyone to describe where each quark and each gluon will end up. Already, trying to predict where fifteen billiard balls go after breaking the pack is tough enough. But when each projectile is made of hundreds of particles, it becomes impossible.

At first glance, it would seem all we could get out of this is just a mess. But this turns out to be the coolest and hottest mess one will ever see. From the most energetic collisions comes a new form of matter called the quark-gluon plasma.

There are three very well known state of matter: solid, liquid and gaseous. Lesser known is the fourth state of matter called plasma. This is what one finds inside a neon tube when the electric current applied is strong enough to strip the gas of its electrons. Positively charged ions and negatively charged electrons float around freely, having enough energy not to recombine.

The quark-gluon plasma is just one step above this. Imagine there is enough energy around that not only the atoms but the nucleons (the name given to protons and neutrons, the particles found inside the nucleus) break apart and coexist in some sort of an extremely energetic fluid. This is as hot as it got instants after the Big Bang. What is so cool about it though, is that this plasma exhibits collective behavior, meaning quarks and gluons do not float freely but have collective properties. The most spectacular of them is that this fluid has no viscosity and behaves as a perfect fluid. If you try to confine it in a container, it just flows up the container’s wall and spread all over the place.

The ALICE experiment is dedicated to the study of the quark-gluon plasma. Each year, the LHC operates for a few weeks with lead ions instead of protons. ALICE collects data both during proton-proton collisions and heavy ions collisions. Even when only protons collide, the projectiles are not solid balls like on a billiard table but composite objects. By comparing what can is obtained from heavy ion collisions with proton collisions, the ALICE physicists must first disentangle what comes from having protons in a bound state inside the nucleus as opposed to “free protons”.

So far, it appears that the quark-gluon plasma only formed during heavy-ion collisions since they provide the necessary energy density over a substantial volume (namely, the size of a nucleus). Some of the effects observed, such as the number of particles coming out of the collisions at different angles or momenta, depend in part on the final state created. When the plasma is formed, it reabsorbs many of the particles created, such that fewer particles emerged from the collision.

By colliding protons and heavy ions, scientists hope to discern what comes from the initial state of the projectile (bound or free protons) and what is caused by the final state (like the suppression of particles emitted when a quark-gluon plasma forms).

Already, with only one day of data taken in this new mode, the ALICE collaboration just released two papers. The first one presents the measurements of the charged hadrons density produced in proton-ion collisions and compares the result with the same measurement after proper normalization performed in proton-proton and ion-ion collisions. The second compares the transverse momentum distributions of charged hadrons measured in proton-ions and proton-proton collisions.

The ultimate goal is to study the so-called “structure function”, which describes how quarks and gluons are distributed inside protons, when they are free or embedded inside the nucleus.

More will be studied during the two-month running period with protons colliding on heavy ions planned for the beginning of 2013.

A “snapshot” of the debris coming out of a proton-lead ion collision captured by the ALICE detector showing a large number of various particles created from the energy released by the collision.

Pauline Gagnon

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L’IN2P3 participe depuis quatre ans déjà aux Masterclasses internationales de physique des particules, organisées en partenariat avec le Cern. Nicolas Arnaud, coordinateur national et chercheur au CNRS au LAL à Orsay, témoigne.

Préambule : Savez-vous quelle est la particule élémentaire la plus commune dans le corps humain ? La réponse est bien entendue dans le quiz « Masterclasses 2012 » ! (Un petit indice : c’est l’hydrogène qui fait pencher la balance…)

Masterclass à Lyon en 2011 (laboratoire IPNL). Photo : Pascal Bellanca-Penel

Buffet campagnard ou pizzas à emporter (une achetée une gratuite) ? Désintégration d’un boson W dans Atlas ou événement de bruit de fond ? Bon, une vidéoconférence Vydio avec le Cern, ça ne doit pas être sorcier quand même ? Et un J/Ψ (prononcer jipsi) dans CMS, combien ça pèse ? Bizarre vous avez dit bizarre, ces particules « étranges » révélées par le détecteur Alice qui enregistre leurs désintégrations en « V0 » ? Toutes ces questions et bien d’autres – au fait, comment puis-je voir des muons sur mon écran alors qu’il n’y a pas de coups visibles dans les détecteurs ? – organisateurs et participants des Masterclasses 2012 se les poseront au cours des quatre semaines à venir. Pendant cette période, plus de 9000 élèves de 31 pays passeront une journée dans un laboratoire pour découvrir la physique des particules en général et le LHC en particulier.

Pour la quatrième année consécutive, l’IN2P3 est partie prenante de ce programme international né en 2005 et qui s’adresse à des lycéens et à leurs professeurs. Initiée en 2009, la participation de l’Institut s’est renforcée à chaque édition. En 2012, dix laboratoires français (voir la liste complète et descriptif) organisent 25 sessions (16 Atlas, 6 CMS et 3 Alice) au cours desquelles ils accueilleront une trentaine de classes et donc environ un millier d’élèves !

Si le programme précis d’une Masterclass varie d’un labo à l’autre, les grandes lignes sont fixées : le matin, des présentations orales sur la physique des particules, le Cern et le LHC ; l’après-midi, une séance de travaux pratiques sur ordinateur permettant de manipuler de vraies données du LHC enregistrées en 2011 et de réaliser une mesure scientifique ; enfin, une vidéoconférence (en anglais !) animée depuis le Cern et qui rassemble toutes les classes qui auront participé à une session Masterclass le même jour.

Une Masterclass à Orsay (laboratoire LAL) en 2011. Photo : LAL

Élèves comme professeurs – pour une fois presque sur un pied d’égalité face à une discipline qu’ils ne connaissent que rarement – repartent le plus souvent enchantés de ces journées de découverte des principaux aspects de la recherche fondamentale en physique des particules. À tel point que les enseignants postulent en général dès la rentrée scolaire pour revenir l’année suivante avec leur nouvelle classe ! Si cette « fidélisation » des professeurs est un bon baromètre du succès des Masterclasses, elle a pour conséquence inattendue de saturer l’offre puisqu’un laboratoire donné ne peut pas organiser plus de quelques sessions dans l’année. Jusqu’à maintenant la forte croissance de la participation française a permis de contenter les participants réguliers tout en acceptant les nouvelles demandes. Mais toute période de croissance ayant une fin, il est probable que nous affichions bientôt complet ! En 2013 nous espérons néanmoins être rejoints par quelques autres laboratoires…

Nous devrons donc bientôt réfléchir à la meilleure manière de toucher de nouveaux publics sans pour autant frustrer nos aficionados… Une possibilité parmi d’autres, probablement testée en 2013 par une classe de la vallée du Rhône, éloignée géographiquement des laboratoires de l’IN2P3 : aller visiter le CERN pendant la période des Masterclasses et organiser une session sur place ! Plus globalement, la problématique de l’accès à des élèves issus d’établissements peu favorisés et/ou qui offrent moins d’activités « optionnelles » à leurs élèves se pose. Nous y réfléchirons à l’avenir dans le cadre de « l’École des deux infinis » qui regroupe maintenant toutes les initiatives de vulgarisation dans lesquelles l’IN2P3 est impliqué : conférences, visites de labos, le programme « Cosmos à l’École », la formation d’enseignants, le projet « Passeport pour les deux infinis » et bien sûr les Masterclasses.

Mais assez bavardé maintenant. Il est 9h, les pizzas sont commandées, les logiciels installés en salle informatique et la vidéoconférence testée. Un dernier coup d’œil aux transparents chargés sur l’ordinateur en attendant que les derniers élèves s’installent dans l’auditorium. Une bonne respiration et c’est parti pour une nouvelle Masterclass : adieu la logistique, bonjour la physique !

Nicolas Arnaud, coordinateur des Masterclasses physique des particules pour la France et représentant français pour l’International Particle Physics Outreach Group (IPPOG).

PS : retrouvez les exercices en ligne pour chaque expérience du LHC
– Alice : http://www.physicsmasterclasses.org/exercises/ALICE/MasterClassWebpage.html
– Atlas : https://kjende.web.cern.ch/kjende/fr/index.htm
– CMS : http://www.physicsmasterclasses.org/exercises/CMS/cmsfr.html
– LHCb : Peut-être un exercice en 2013 !? Vous nous manquez ! ☺


A new year, a new outlook

Saturday, December 31st, 2011

2011 has been a year of change and excitement. We’ve had plenty of good news and bad news to deal with. The new year doesn’t mean just another calendar on the wall, it means a new way of looking at physics. There’s no better way to bring in the new year than watching the fireworks in central London, surrounded by friends. There’s usually a fantastic display, because London is not only one of the most important cities in the world, but it’s also home of universal time. With the Greenwich Meridian running through the capital, we’re reminded of the role that timekeeping has played in the development our history and our science. But this year was even more special, since London is literally inviting the world to its streets this year for the Olympics. So I got caught up in the excitement of it all my thoughts turned to what we’ve seen in the world of physics, and where we’re going next.

New year fireworks in London (New York Times)

New year fireworks in London (New York Times)

2011 got off to a start with ATLAS announcing a startling asymmetry in the jet momenta in heavy ion collisions. However, the joy was tainted by a leaked abstract from an internal document. That document never made it through internal review and should never have been made public. We were faced with several issues of confidentiality, ethics and biases, and how having several thousand people, all armed with the internet and with friends on competing experiments makes the work tough for all of us. In the end we followed the right course, subjected all the analyses to the rigors of internal and external review, and presented some wonderful papers.

There was more gossip over the CDF dijet anomaly presented at Blois. CDF saw a bump, and D0 didn’t. Before jumping to any conclusions it’s important to remember why we have two experiments at Tevatron in the first place! These kinds of double checks are exactly what we need and they represent the high standard of scientific research that we expect and demand. The big news for Tevatron was, of course, the end of running. We’re all sad that the shutdown had to happen and grateful for such a long, productive run, but lets look to the future in the intensity frontier.

Meanwhile both ATLAS and CMS closed in on the Higgs boson, excluding the vast majority of the allowed regions. The combinations and results just got better and better, until eventually on December 13th we saw the result of 5fb-1 from each experiment. The world watched as the presentations were made and quite a few people were left feeling a little deflated. But that’s not the message we should take away. If the Higgs boson is there (and it probably is) then we’ll see by the end of the year. There’s no more of saying “Probably within a year, if we’re lucky”, or “Let’s not get ahead of ourselves”. This time we can be confident that this time next year we’ll have uncovered every reasonable stone. The strategies will change and we narrow the search. We may have new energies to explore, and we’ll tweak our analyses to get more discriminating power from the data. Now is the time to get excited! The game has changed and the end is definitely in sight.

Raise a glass as we say farewell to a great year of physics, and welcome another

Raise a glass as we say farewell to a great year of physics, and welcome another

It’s been a good year for heavy flavor physics as well. LHCb has gone from strength to strength, probing deeper and deeper into the data. We’ve seen the first new particle at the LHC, a state of bottomonium. Precision measurements of heavy flavor physics give some of the most sensitive tests of new physics models, and it’s easy to forget the vital role they play in discover.

ALICE has been busy exploring different questions about our origins, and they’ve studied the quark gluon plasma in great detail. The findings have told us that the plasma acts like a fluid, while showing unexpected suppression of excited bottomonium states. With even more data from 2011 being crunched we can expect even more from ALICE in 2012.

The result that came completely out of left field was the faster than light neutrinos from OPERA. After seeing neutrinos break the cosmic speed limit, OPERA repeated the measurements with finer proton bursts and got the same result. Something interesting is definitely happening with that result. Either it’s a subtle mistake that has eluded all the OPERA physicists and their colleagues across the world, or our worldview is about to be overturned. I don’t think we’ll get the answers in the immediate future, so let’s keep an eye out for results from MINOS and OPERA.

Finally it’s been an incredible year for public involvement. It’s been a pleasure to have such a responsive audience and to see how many people all across the world have been watching CERN and the LHC. A couple of years ago I would not have thought that the LHC and Higgs boson would get so much attention, and it’s been a of huge benefit to everyone. The discoveries we share with the world are not only captivating us all, they’re also inspiring the next generation of physicists. We need a constant supply of fresh ideas and new students to keep the cutting edge research going. If we can reach out to teenagers in schools and inspire some of them to choose careers in science then we’ll continue to answer the most fascinating, far reaching and beautiful questions about our origins.

So when you a raise a glass to the new year, don’t forget that we’ve had an incredible 2011 for physics, and that 2012 is going to deliver even more. We don’t even know what’s out there, but it’s going to be amazing. To physics!


Five Inverse Femtobarns

Friday, October 14th, 2011

Hi All! Great news: the CMS Experiment, just a moment ago, announced that the LHC delivered 5fb-1 today!

Figure 1: Proof. It happened. (Image: Mine)

This is terrific news and if you happen to see a member of CERN’s accelerator division, be sure to congratulate her or him.

Figure 2: Total (integrated) luminosity delivered to (red) and recorded by (blue) the CMS detector. (Image: CMS)

To give a little context, 1 fb-1 (pronounced: one inverse femtobarn) worth of data is measure of the number proton collisions (scaled by a bunch of physics and efficiency parameters) and is the equivalent of 70 trillion proton-proton collisions. So 5 fb-1 is 350 trillion proton-proton collisions, which is 3.5 × 1014 = 350,000,000,000,000 proton-proton collisions. Before the start of collisions this year, the LHC had only delivered about 35 pb-1 (0.035 fb-1), which is only about 2.45 trillion = 2,450,000,000,000 proton-proton collisions. In other words, 99.3% of the data generated by the LHC came between this past March and Today. How can you not be impressed by that? 😀

Figure 3: Total (integrated) luminosity recorded by ATLAS (black/behind green), CMS (green), LHCb (blue), and ALICE (red). (Image: CERN)

Figure 4: Log of total (integrated) luminosity recorded by ATLAS (black/behind green), CMS (green), LHCb (blue), and ALICE (red). (Image: CERN)

Due to detector efficiencies and such, not all the data generated is recorded. The above plot, generated & continuously updated by CERN, shows that ATLAS and CMS have a small bit before reaching 5 fb-1. However, it is very reasonable to suggest that both experiments will have recorded 5 fb-1 before the end of the third week of November October. (Thanks to Achintya & Dave for catching this mistake. I have “week 43” in my notes for this post, so I have no idea how I ended up with the November date.)



As always, happy colliding.

– richard (@bravelittlemuon)

PS. I refer you to a previous post about what the experiments can do with 5 fb-1.


Why run at lower energy?

Wednesday, March 23rd, 2011

Right now the LHC is about to start a short run with proton-proton collisions at a center of mass energy 2.76 TeV.  This is lower than what we ran last year and is a special request from the heavy ion physicists.  So you’ve heard a lot about why the particle physicists want to go to higher energy.  But why do we heavy ion physicists want to go to lower energy?

We want a reference for our lead-lead collisions.  If nucleus-nucleus collisions were nothing but a bunch of proton-proton collisions, what we measure in lead-lead collisions should be just some constant times what we measure in proton-proton collisions.  This is a bit simplistic, but it’s a pretty good start.  A lot of our measurements use proton-proton collisions as a reference and look for differences between proton-proton collisions and lead-lead collisions.  For instance, in the paper I discussed here we looked at the distribution of particles as a function of their momenta in lead-lead collisions and compared that to what we observed in proton-proton collisions.  For this paper we used the data from proton-proton collisions at 900 GeV and at 7 TeV to extrapolate to what we’d expect at 2.76 TeV, the same energy per nucleon as our lead-lead collisions.  As discussed here our models for proton-proton collisions are pretty good but they get some of the details wrong – and miss some features like this.   Since we depend on models to extrapolate to 2.76 TeV, we have greater uncertainty in our measurements than we would have if we had data at 2.76 TeV.  The LHC can go down to 2.76 TeV and what we need 2.76 TeV proton-proton data doesn’t require as many statistics (as many total proton-proton collisions) as what the particle physicists need to look for things like the Higgs.  So we’re having a short run with proton-proton collisions at a lower energy because it will significantly help the heavy ion physics program.  (We’ll also get some core physics measurements out of the 2.76 TeV proton-proton data, but like the paper I discussed here, these will refine our understanding but not dramatically change our understanding of proton-proton collisions.)  I hope you’re as excited as I am!


I am overdue for a blog post because I have been way too busy lately.  I got an email from an elementary schooler, Jacob, asking about the QGP so I thought instead of replying privately I’d reply here since it may be of general interest.  The questions are from Jacob.

What is QGP going to be used for in the future when it is better controlled?

Right now we don’t think the QGP has any practical applications.  We’re studying it because we want to understand the universe in general and nuclear matter in particular.  Shortly after the Big Bang, we think that the universe went through a Quark Gluon Plasma phase.  By understanding the QGP better, we may understand how the universe expanded better.  When we do basic research, we don’t usually know what impact it will have.  What we know by looking at history is that basic research eventually leads to benefits to humanity – but we’re very bad at predicting what those benefits will be.  When Mendel studied genetics of plants, he never imagined that genetic studies would lead to all of the improvements in medical care we have now.  Einstein developed his theory of gravity not so that we could send satellites into space or so that we could all have GPS in our cars or get better TV reception – he was motivated by simple curiosity and a desire to understand our universe better.  We are still reaping new benefits from quantum mechanics, developed in the early 20th century – we now have light emitting diodes (LEDs) in traffic lights and flashlights and while LEDs existed when I was your age, they weren’t nearly as widespread, as cheap, or available in so many colors.  So it takes a long time to see the benefits of basic research.

So we don’t know what applications this research will have in the future.  That said, there are a lot of spin off benefits to this research.  In high energy physics, we are always building the fastest and most precise detectors possible.  To do this we often have to develop and test new detector technologies.  Once we’ve developed the technology, these detectors can be used elsewhere too.  Particle detectors are used in hospitals in x-ray and MRI machines.  They are used in chemical and biomedical research to study the images of proteins and the structures of solids.  They are used in national security for detecting radioactive materials.

Basic research moves the boundary of what is possible.  Once we have done that, there are a lot of benefits.  But since we’re working on doing things that have never been done and studying things never studied before, we can’t predict exactly how it will be useful.  Put another way, if we knew what would happen, we wouldn’t call it an experiment.

What attributes does it have that other matter does not have?

This is a difficult question to answer as worded – it depends on what you mean by “attributes”.  When I think of the properties of a particular form of matter, I think about its density, its opacity to different probes (like if you shine light through it does the light come out the other side?)…  All forms of matter have a density.  So I’m going to answer a slightly different question – what makes a QGP unique?  What makes the QGP unique (among the forms of matter we’ve studied in the laboratory) is that the quarks and gluons interact through the strong force.  There are four fundamental forces in nature

1. Gravitation
2. Electromagnetism
3. Weak interaction
4. Strong interaction

The first two are the most familiar.  Gravity is the reason why you stay on the ground instead of floating through the air.  It’s also the reason the Earth orbits the Sun.  The electromagnetic force is ultimately responsible for basically every other force you feel or see.  When you sit in a chair, the reason you don’t fall through the chair is ultimately due to interactions between your atoms and the atoms of the chair.  It’s also behind light and electricity.  It’s how your microwave and your TV work.  The most familiar thing we can attribute to the electroweak decay is beta decay – a particular kind of decay of a nucleus.   The strong force is what holds nuclei together.  If we only had the electromagnetic force, the protons in the nucleus would not be bound.

So a QGP is a liquid of quarks and gluons bound together by the strong force.  Water molecules, for instance, primarily interact through the electromagnetic force.  The properties of water are determined by the way water molecules interact through the electromagnetic force.  To understand the QGP, we have to understand how quarks and gluons interact through the strong force.  This turns out to be a very difficult computational problem.  But by studying the QGP, we can try to calculate what we would expect and then compare what we expect from our theories to what we see in the laboratory.

In addition to that, it is the hottest, densest form of matter ever created in the laboratory.  And it appears to have the lowest viscosity of any form of matter ever created in the laboratory.  Viscosity is a way of measuring how much a fluid resists flowing.  Honey, for instance, is much more viscous than water.

How will QGP affect modern or future physics?

I don’t know exactly.  It depends on what we learn.  Already we’ve learned a lot about relativistic fluids – where the individual particles in the fluid are traveling close to the speed of light.  As I said in the first answer, we don’t know exactly what we’ll learn – because if we did, we wouldn’t call it an experiment. One thing I hope – and maybe you can help me out here – is that we’ll inspire the next generation to go into science, math and engineering.

Also, what state of matter is it?  I know that it is called plasma but I’ve also read that it is very similar to both liquid and gas.

A QGP is a new state of matter.  We believe it is a liquid – indeed, a liquid that probably has the lowest viscosity of anything we’ve ever measured.  We thought it’d be a gas, but it turned out to be a liquid.  Here I have a post describing what we know about the QGP and its phase diagram.

I also could not verify what temperature it occurs at because there is so much different information on the internet.

The reason what you find on the internet is somewhat unclear is that the answer is somewhat unclear.  First, it doesn’t exist at just one temperature.  Think about water.  Water can be cold, warm, hot, etc.  It depends.  There’s a temperature where ice melts and becomes water and below that you can’t have water.  That temperature is called the melting point.  But then once you have water, you can heat it up and you have to heat it up a lot before it boils and becomes a gas.  That also occurs at a special temperature – the boiling point.  The problem is, these temperatures depend on pressure and volume.  Water boils at a lower temperature at high altitude.  Analogously, we have a melting point and a boiling point for the QGP.  We think the melting point at the baryochemical potential at RHIC is about 170 MeV – but there’s a fairly large uncertainty in that number.  We think we’re well above that at RHIC and we’ll be even further above it at the LHC (but we haven’t yet had enough time to analyze the data at the LHC to say how hot it is). This gets to a crucial issue – we don’t have a thermometer to measure a QGP.  If you put a thermometer like the one you have in your house into a vat of QGP (if we could ever create that much of it) it’d melt.  So we have to come up with other ways of measuring the temperature.  We can look at the energies of particles created in the collision, for instance.  But it takes more work than just using a thermometer.

Many thanks to Jacob for the great questions!



Thursday, February 3rd, 2011

The electromagnetic calorimeter is now fully installed but there’s still work to do before we start running.  We now have to make sure we’re able to read all of the data.  I’ve spent most of the last week in, on, and next to ALICE troubleshooting (along with several of my colleagues working on the calorimeter.)  Here I am sitting inside the magnet on top of  the support structure next to the front end electronics (the boards that read out the data) for the calorimeter.  I’m on the phone with someone upstairs who’s trying to take a pedestal run to see if we’ve fixed a problem reading out data from one of the new supermodules.  (A pedestal run is a run you take without proton-proton or lead-lead collisions to see what the background in your detector is.  It’s useful for troubleshooting because the detector has to send data.)

Now that we’re getting close to the start of the run, they’re putting the concrete shielding in.  In total 30 or 40 tons of concrete blocks sit above ALICE.  Here you can see one of the last blocks going in:

And just to go along with the preposition theme, here’s a picture under ALICE (in the magnet but under the TPC, TRD, and TOF):


Inside ALICE

Sunday, January 30th, 2011

I am currently at CERN to work on getting the electronics for the electomagnetic calorimeter working now that the rest of it is installed.  I got to see the ALICE detector in person for the first time on Thursday, which was very exciting.

This is a picture of me in front of the detector:

But that was part of a tour and to work in the detector I needed a lot of training.  I needed to take

  • Radiation safety training – there can always be residual radiation from things that have been activated by the beam and there may be radioactive sources in the area.  I have to recognize the appropriate placards and understand any dangers that may be present.
  • Working at heights training – the electromagnetic calorimeter is not at ground level and working on the electronics requires me to work well above ground.  I have to know how to use a harness properly.
  • Confined space training – the doors of the magnet are closed now so that they can start replacing the shielding around ALICE and I need to work inside the magnet.  This is a confined space.  There is a risk of oxygen deficiency – the amount of oxygen can drop rapidly and I have to to be aware of potential dangers and ready to respond.
  • Biocell training – The biocell is a small container of oxygen which I have to carry with me at all times in case the oxygen levels rapidly drop.  I have to be trained to use this properly because I may need to use it to save my life.

I also have to wear a dosimeter (which measures how much radiation I’ve been exposed to), a hardhat with a headlamp (in case the power goes out), and safety (steel-toed) shoes.  No shorts are allowed.  Inside the magnet there are high voltage sources, risks of falling, risks of falling objects, and detectors using flammable and/or toxic gases which could leak.  We are required to have at least two people working inside the magnet at a time – so that if someone gets hurt, the other person knows and can get help – and to have a 3rd person outside the magnet as a watcher keeping track of who is inside and where they are so that if anyone gets hurt or there is an emergency there is someone who can call the fire brigade and tell them how many people are inside and where they are.

I haven’t had the opportunity to take any pictures inside ALICE yet – and safety always has to come first so I may not be able to – but this is the hole we use to enter the magnet:

It is about 60 cm in diameter.  To get down to ALICE, you first have to go through this door:

(This is Soren Sorensen, my boss, coming down to see ALICE.) To go through this door, I have to scan my dosimeter on a card reader.  This says who I am and whether or not I have access to “the cavern” – the space underground where the detector is.  Then the outer doors open, I walk in, and I’m closed inside.  They scan one of my eyes and weigh me to make sure that I really am the person who owns the dosimeter.  Only then am I allowed in.  Inside there’s an elevator that takes us the 70m down to ALICE.  (It is easier to go down to see the cavern as a visitor than to work on the detector – one does not need training but must be supervised.)

This is why we tested as many components of the electromagnetic calorimeter  as possible before the EMCal was installed.  However, there will always be something which doesn’t work quite right and we want to fix it if we can.  It’s really exciting work, but we have to stay alert and stay safe.


I introduced you to the ALICE electromagnetic calorimeter (EMCal) a while ago, and told you about some additional training I had so that I could work on the detector after the EMCal is physically installed.  Over the winter shut down – right now – the EMCal is being installed inside ALICE.

There are several steps in this process.  First the EMCal was assembled, partially calibrated, and tested – this was done in November.  There were several stages of testing.   We tested that each individual cell works.  We tested that each individual electronic card for reading out the data works.  We assembled everything exactly how it would be installed inside ALICE and tested it again, making sure that all of the parts (including the wires) worked together.  We partially calibrated the detector by taking data on cosmic rays.  We’ve had all of the six supermodules we’re adding waiting at CERN until we could get access to ALICE to install them.

Now they’re physically installing the supermodules and our amateur EMCal documentarian, Federico, has taken some videos of the process.  (It might help to go back to this post, where I introduce each of the detectors and explain what they do, and this post, where I show you some pictures of each of the detectors.  Then maybe you can identify the different parts of ALICE in the video.)  Note the action in the videos is very slow because it’s very important not to damage anything while installing the detector.

The first step is to put the supermodule in the EMCal insertion tool.  This is a specialized device for installing EMCal supermodules.  Here you can see a video of that step:

[youtube 75Olhr4YoUw&NR]

And then once the supermodule is in the insertion tool, it gets installed in ALICE:

[youtube 0es9Qcdj-H8]

And now some gratuitous cool pictures of the process:

ALICE ready for the installation of EMCal supermodules

Looking up from the cavern.  We sit at the top when we take data and the detector is far below us.  The EMCal supermodules have to be lowered down.

Checking everything twice to make sure there are no mistakes.

Getting the EMCal insertion tool ready for a supermodule

Getting ready to strap the support onto the supermodule

Sliding the support onto the supermodule

Now it’s in and they’re strapping it onto the crane

Up goes the supermodule…

…and into the insertion tool.

And it’s rotated to the correct angle…

Waiting on the support structure in ALICE to make sure it goes in properly…

Get it in the right position…

Now loosen it from the EMCal tool and in it goes!

And now we do the next one.

Many thanks to Federico for the great pics!