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Archive for July, 2012

from PhD to postdoc

Tuesday, July 31st, 2012

Hello!  I’ll probably write more technical posts later, but since I’m a new US LHC blogger, I thought I would spend this first post talking a little about my background and how I decided what kind of postdoc position to look for.  Lately, quite a few of my friends in their last year of graduate school have been asking about the latter. 😉

I’ve been with Columbia University for almost a year now, having defended my PhD thesis at the University of Massachusetts, Amherst on June 10, 2011.  I knew I wanted to stay in the field after graduating, if only for the fact that it would have been a shame to look at only 42 pb-1 of data, see no new physics, and miss out on being around for (what eventually became!) the biggest particle physics discovery in decades. Just to give you an idea of how much data I had for my thesis compared to what we have now, see this histogram showing the integrated luminosity recorded by ATLAS in 2010, 2011 and so far in 2012:

I was that green line.

Before applying for postdoc positions, I needed to decide what kind of research to do in the next stage of my career, and where I would want to do it. Almost all the work I had done as a graduate student was related to the muon spectrometer on ATLAS; from helping in the installation and commissioning of the muon precision chambers, to muon reconstruction performance studies, to measuring the first Z→μμ cross section at sqrt(s)=7 TeV and finally performing a search for new physics in the high-mass tail of the mu-mu invariant mass spectrum. Muons were my thing.

The advice I got from most of my colleagues at the time, including my adviser, was to switch experiments. The reasoning made sense. If you stay with the same experiment for your postdoc, you miss out on a free pass to do research on something completely new. It’s a rare opportunity to start from scratch while still having some allowance for time to catch up.

But that was the thing….most of the people I was seeking advice from had come from other experiments to the LHC, not the other way around. In fact, I was one of the first US students able to write a thesis on LHC data (the delay partly due to the incident in 2008….let’s not talk about that). So where could I have gone from here? If I wanted to stay in collider physics, I needed to stay at the LHC.

Knowing I wanted to come back to CERN, it also took some time to figure out exactly what kind of analysis I wanted to work on after the PhD. I talked to a lot of people that semester, asking who would be working on what and getting lots of advice. I certainly had many interesting options for research, but it wasn’t until I was sitting in a talk about the evidence for forward-backward asymmetry of the top quark when I thought now hey, top physics…

In the end, I decided to make as big of a switch as possible while still staying on ATLAS, moving from the muon spectrometer and dimuon analyses to work on top quark physics and jets at an institute responsible for the liquid argon calorimeter electronics. The move seemed to cover the best of all possible scenarios…I didn’t need to worry about the year-long wait to qualify for authorship or to figure out ATLAS software, but I did get the opportunity to learn something ultimately different when it came to hardware work and physics analysis. However, because of the size of the collaboration, where each subdetector community has roughly the same number of people as one Tevatron experiment, it took some time to get enough exposure to be recognized for the new work I was doing. That will be the case whenever you start a new job, no matter what.

Even more difficult was going from feeling like an expert in my thesis topic to suddenly being thrown in the deep end of a new topic amongst other experts. I found I wasn’t the only one who experienced that.  Before I began, a few senior postdoc friends of mine who wrote their PhDs at the Tevatron said that their first year at the LHC felt just like being a brand new graduate student all over again and that it was hard to feel like anything really substantial had gotten done during that time, just because there was the additional learning curve thrown in. When I looked a little sad, one of them said “well for you, since you’re staying on ATLAS…maybe only 6 months.”

My advice to anyone wrapping up their graduate studies and thinking about getting a postdoc would be to talk to as many people as possible and get as many opinions as possible. My experience is just one of many! I can say though that the more I knew going in, the easier the transition was, and now one year later everything is going really smoothly.

Anyway, have a look at my upcoming posts, where I’ll talk about jet substructure, new physics searches involving the top quark, and whatever other cool beyond-the-Higgs stuff is happening at CERN.

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If I could turn back time…

Thursday, July 26th, 2012

I’m going to do something different today and discuss a result from another experiment… I saw the result this morning and thought the topic would be an interesting one for a blog post.

So what will I be talking about? Time reversal violation!

You might be wondering why I would consider this an interesting topic, we all experience time reversal violation in our lives, everyday events definitely are not symmetric in time – we can always tell when a video is being played backwards. This isn’t the case in the world of particle physics however, where most interactions are symmetric under time reversal.

So why do we expect time reversal violation in particle interactions? It’s related to the underlying structure of the Standard Model, which relies on interactions being CPT symmetric.

What is CPT you ask? It’s the combination of three other more fundamental symmetries, Charge conjugation, Parity and Time reversal. I’ve described C and P previously and also presented results of CP violation in the B and D meson systems.

Now if we expect the Standard Model to be CPT symmetric and we’ve observed CP violation, it follows that we should also observe T violation.

And this is exactly the result that the BaBar collaboration released this morning, where they report “the first direct observation of T violation in the B meson system.”

I’m not going to go through the details of the analysis, it’s quite clever and complicated, instead here is a set of plots from the paper:


The points are the data, the blue line is the model without T violation and the red line is the model with T violation. As you can see, the model with T violation matches the data much better than the model without.

And voila, here you have it! Observation of T violation in the B meson system…

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Où retrouve-t-on des étudiant-e-s de 71 pays différents dans une même salle? Au programme des étudiants d’été du CERN. Cette année, il regroupe 269 jeunes scientifiques talentueux qui suivent des cours le matin et participent à de réels projets de recherches dans l’après-midi. Les cours couvrent tous les sujets d’intérêt au CERN allant des principes des accélérateurs et détecteurs à la théorie sans oublier les techniques d’analyse et de traitement des données. Voici le point de vue de quelques uns d’entre eux.

Michael Borinsky ne se sent pas dépaysé. « Tout ici est à peu près comme en Allemagne. Même que le CERN me rappelle mon université », explique-t-il. Mais la diversité du programme fait toute la différence. « J’étais au supermarché avec un étudiant cubain. Il était tout excité d’y voir des produits introuvables à Cuba. Mais pour moi, c’étaient des articles ordinaires. Ça m’a permis de voir les choses familières différemment. »

Il apprécie vraiment les cours. Certains sujets lui sont déjà connus puisqu’il est étudiant en maitrise en physique des particules. « C’est un peu comme relire un livre qu’on a beaucoup aimé. On y voit toujours des choses nouvelles » dit-il en souriant.

Yu-Dai Tsai de Taiwan avait entendu parlé du programme par un ami chinois. Les deux ont été admis. “Ici on a la chance de rencontrer et d’apprendre avec les meilleurs expérimentateurs-trices et théoricien-ne-s venant de partout au monde »,  explique-t-il avec enthousiasme. « On peut entamer une conversation avec une personne à la cafétéria pour découvrir qu’il est prix Nobel de physique ou un théoricien connu ou encore qu’elle est en charge d’un énorme projet expérimental. » Yu-Dai va appliquer à différents programmes de doctorat et espère revenir un jour travailler au CERN.

Cenk Türkoğlu de Turquie est l’un des récipiendaires de la bourse Engin Arik. Cette bourse au mérite fut établie après la mort tragique de la professeure Arik dans un accident d’avion, afin de poursuivre son soutien intarissable envers les étudiant-e-s turcs.

Cenk veut écrire sa thèse sur les données du Alpha Magnetic Spectrometer (AMS-02), une expérience dédiée à l’étude des rayons cosmiques et de la matière noire et installée à bord de la Station Spatiale Internationale. Cenk a même pu rencontrer les six astronautes qui ont mis AMS dans l’espace.

« CERN joui d’un très bel environnement propice à l’étude qui vous fait désirer étudier et en apprendre plus. Les cours sont vraiment bons et nous permettent de développer nos idées sur plusieurs sujets différents », explique Cenk.

Cristina Turcu est roumaine et étudie le graphisme, les techniques multimédia et la réalité virtuelle à Bucarest. Elle programme en ce moment un simulateur graphique. « C’est vraiment le projet idéal pour moi car ça combine ma spécialité avec la physique et les maths, deux matières que j’aimais beaucoup », explique Cristina.

Elle aime tout particulièrement la façon donc le cours est structuré. « Une étudiante peut travailler sur un projet super intéressant mais aussi apprendre sur les autres expériences dans les cours en plus des visites et des ateliers, sans compter rencontrer des gens célèbres », ajoute-t-elle.

Tous et toutes ont mentionné combien ils apprécient rencontrer des gens de partout. « Ce programme m’a donné la chance d’apprendre plein de trucs nouveaux, faire quelque chose d’intéressant chaque jour et bien sûr, me faire de nouveaux ami-e-s » résume-t-elle.

Si vous ou une personne de votre connaissance aimerait y participer, il suffit d’appliquer tôt en Janvier.

Pauline Gagnon

Pour être averti-e lors de la parution de nouveaux blogs, suivez-moi sur Twitter: @GagnonPauline ou par e-mail en ajoutant votre nom à cette liste de distribution

 

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Where can you find students from 71 different countries in one room? CERN’s Summer Student Programme is the place. This year, it attracted 269 talented young scientists. They attend lectures in the morning and contribute to real research projects in the afternoon. The lectures cover topics of interest at CERN from detector and accelerator principles to theoretical models, as well as computing and analysis techniques. I talked to some of the students to see how they were getting on.

For Michael Borinsky from Germany, the experience is could be very ordinary. “Everything here is pretty much like at home”, he says. CERN even reminds him of his university campus. But the wide diversity of people in the programme makes a big difference. “I was at the supermarket with a Cuban student. He was all excited to find various items that are impossible to find in Cuba. But for me, they were just ordinary products. It allowed me to experience things very differently.”

Michael likes the lectures, though many topics are familiar to this masters student in particle physics. “It’s like reading a really good book for the second time, you always get something new,” he smiles.

Yu-Dai Tsai from Taiwan heard about the programme through a Chinese friend. Both were accepted. “Here you have a chance to meet and learn from the bests from all over the world, both experimentalists and theorists”, he says enthusiastically. “You can start talking to someone in the restaurant to find out he is a Nobel laureate, or had proposed an important theoretical idea, or is in charge of a huge experimental project”, he adds. Yu-Dai is applying to PhD programmes and hopes to come back to work at CERN.

Cenk Türkoğlu from Turkey received one of the Engin Arik fellowships. This fellowship based on merit was set up after professor Arik’s tragic death in a plane crash to continue her life-long work of supporting gifted Turkish students. Cenk plans to write his thesis on data from the Alpha Magnetic Spectrometer (AMS-02) – a cosmic ray and dark matter experiment onboard the International Space Station. He got to meet the six astronauts who brought the experimental setup into space.

“CERN has a very beautiful study environment so it makes you want to study and learn more. Lectures are also really good for us to develop insights on many different subjects,” he says.

Cristina Turcu from Romania is studying advanced techniques for computer graphics, multimedia and virtual reality in Bucharest. She is currently programming a graphic simulator. “This project suits me perfectly because it combines my specialty with physics and maths, two fields I liked in school,” she explains.

Cristina particularly likes the way the programme is structured. “A summer student can work at a very interesting project, learn about other experiments at the lectures and have a chance to go on visits, workshops and meet famous people,” she says.

All the students mentioned how much they enjoy meeting people from all over the world. Cristina’s comment sums it nicely: “This programme gave me the opportunity to learn a lot of new things, do something interesting every day and, of course, make new friends,” she says.

If you or someone you know is interested in participating, apply in early January.

Pauline Gagnon

To be alerted of new postings, follow me on Twitter: @GagnonPauline or sign-up on this mailing list to receive and e-mail notification.

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What fills space?

Wednesday, July 25th, 2012

This article first appeared in Fermilab Today on July 25.

If you follow the news about physics, you might think that physicists don’t know what they are talking about when it comes to space.

I am not talking about the mysteries of outer space, or cataclysms like black holes. I mean ordinary space itself, the inner space between particles everywhere—what we used to call empty space or vacuum. What’s in it? Sometimes we hear that atoms are “mostly empty space.” Now we read in the papers that the newly discovered Higgs field “fills all of space” and “gives particles mass,” that it acts like a kind of space-filling “molasses,” or that it’s like a space-filling crowd of groupies hanging on as a celebrity’s posse.

On the other hand, astronomers tell us that space is expanding. Last year, the Nobel Prize in physics was awarded for the discovery that the cosmic expansion is speeding up. Scientists think that this acceleration is propelled by what they call “dark energy,” which fills and refills that ever-expanding void of intergalactic space. Cosmological space is said to be expanding in some places (between galaxies) and not expanding in others (such as Brooklyn, to choose Woody Allen’s example).

It gets even worse if you dig deeper. For example, the Higgs field is much weirder than the comparisons with molasses or crowds suggest, since it does not actually drag or impede particles, but still somehow shares its mass with them.

Stranger still, consider another space-filling field that also adds mass to everyday substances, in a way different from the Higgs field. The gluons of the strong nuclear force field create most of the mass of atoms through the energy of their incessant motion inside tiny bubbles of space that we call protons and neutrons. Since the mass-giving gluons are immune to the Higgs field, they have no mass themselves, but only add energy because of their motion. Moreover, they are held inside those bubbles by a gluon field that fills empty space everywhere between the bubbles…in just those places in space where the added mass isn’t.

Space is the first concept of physics we all learn as little kids, yet it is entangled with some of the deepest mysteries confronting physics. Confusing, koan-like paradoxes about space are not just pablum: They reflect a real and profound disparity of descriptions, at a deep level of mathematics, about what defines a vacuum, a position, a particle or a time.

It may be that all the space of the universe began, and may end, dominated by the energy of the vacuum, an expanding space devoid of particles. It may be that when examined over very short time intervals, space as we know it does not even exist, but dissolves into a cloud of quantum indeterminacy: It may never sit still, but constantly seethe in microscopic motion. It may be that space has many more than three dimensions on very small scales, while there may be only two truly independent dimensions on large scales. It may even be that all of these exotic possibilities actually apply in the real world.

At Fermilab, we are working on experiments including the Dark Energy Survey, the Holometer and the CMS experiment at the Large Hadron Collider that will probe these ideas in very different ways. If you want to find out more—watch this space!

—Craig Hogan, Director of Fermilab’s Center for Particle Astrophysics

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How is that for the ultimate claim in the ultimate[1] essay in this series? Science: mankind’s greatest achievement. Can there be any doubt? In the four hundred years since science went mainstream, we have learned how the universe works, changed our conception of man’s place in it, and provided the knowledge to develop fantastic technology. We have big history: the inspiring story of the universe beginning with the primordial big bang and creating order out of chaos through self-interaction, and finally life arising and evolving in our corner of the universe. We have developed models that describe the universe on the largest visible scales down to sub-atomic sizes: astronomy, biology, chemistry, cosmology, medicine, physics, psychology, animate, inanimate, eater, and eatee. The models form a mosaic that overlap and interlock to form a seamless whole.  An amazingly complete picture. There is still much to know, but let us take credit as scientists, that much is known. And yes, we should be glad to be living in a time when so much is known.

However, science has two short-comings[2]: it does not offer the illusions of certainty or purpose.  I once came across a last will and testament that began: I commit my body into the ground in the sure and certain knowledge it will be restored to me on the judgement day. Ah, for sure and certain knowledge. Well, the judgement day has not come yet so we do not know if his sure and certain knowledge was valid, but the resurrection of the body is much less prominent in Christian apologetics than it used to be.  When it comes to knowledge, science promises less but delivers more than its competitors in philosophy or theology. I would take Isaac Newton (1642 – 1727) over Rene Descartes (1596 – 1650), Immanuel Kant (1724 – 1804), Thomas Aquinas (1225 – 1274), or William Paley (1743 – 1805) any day of the week and all together.  Their certain knowledge has largely vanished, but Newton’s uncertain and approximate knowledge is still being used in many practical applications. Ask any mechanical engineer.

In the Hitch Hiker’s Guide to the Galaxy, Douglas Adams (1952 – 2001) introduces the total perspective vortex. It was created by a husband whose wife keeps telling him to put things in perspective. However, when anyone looked in the vortex, they realized how utterly insignificant they were in the vast stretches of the universe and invariably went insane and died. This proved that if life is going to exist in a Universe of this size, then the one thing it cannot afford to have is a sense of proportion. Ah yes, the human need for importance and purpose. I guess the best science can come up with for a purpose is entropy[3] generation. I am not sure that is any worse than what I had heard from a Christian apologist who claimed we were created by God to worship him. Personally, I would never worship that narcissistic a God.

Despite its shortcomings, perceived or real, science has a tremendous track record. But the best is still to come. Let us not make the mistake of the late nineteenth century physicists who thought all the important questions had been answered.  There are things that enquiring minds still want to know: What, if anything, was there before the big bang? How do you combine gravity and quantum mechanics? Is there a solution for global warming that is politically acceptable? Are there room temperature superconductors? How did life begin? How intelligent were the Neanderthals? How does the mind work? The last strikes me as the most interesting question: the final frontier[4].  It has the potential to open up a whole new front in the conflict between science and religion, or science and philosophy.  But it is interesting nonetheless. Answering these questions and others will take clever theoretical approaches, clever experiments, and clever approaches to funding. However, the techniques of science are up to the task.

But what is science? In the final analysis, it is a human activity, an exercise of the human mind. We construct models and paradigms because that is how our minds and brains have evolved to deal with the complexities of our experiences. Thus, the nature of science is tied closely to the last question asked above: How does the mind work? Ultimately, how science works and indeed, the very definition of knowledge, are questions for neuroscience and the empirical study of the mind.

I am taking a break from blogging for the rest of the summer but may have some more blogs in the fall. I have run out of interesting things to say (no snide comments that that happened a long time ago). I would like to thank people for their many comments. They have been quite informative. To receive notices of future posts, if and when they occur, follow me on Twitter: @musquod.

 


[1] That is the LP in the language of effective field theorists (LP=last post, not long playing as you old timers thought).

[2] Humility is not one of them.

[3] Entropy generation is the driving force behind evolution.

[4] Sorry Star Trek fans, it is the mind, not space.

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I recently traveled to Lindau, Germany for the 62nd meeting of Nobel Laureates (http://www.lindau-nobel.org/), an annual meeting of Nobel Laureates and young researchers from around the world. This year’s meeting, by sheer coincidence, was dedicated to Physics (ironic right?).

One of the afternoon sessions for Wednesday July 4th was a panel discussion titled simply as “CERN.”  Which was, by sheer coincidence, so well timed.  After all, CERN had just finished giving their scientific and public press-releases regarding the discovery of a new boson, with mass of 125.3+/-0.6 GeV, earlier that morning.  I had the opportunity to sit in the front row of a room filled with approximately 250 other young researchers, listening to top names in astro- & particle-physics discuss the recent CERN discovery.  What follows below is a brief review of the Laureates’ discussion.

 

CERN Panel Discussion on Wednesday, July 4th, 2012 (Higgs Dependence Day) at the 62nd Lindau Nobel Laureate Meeting in Lindau, Germany. The Nobel Laureates shown here are, from left to right, David Gross, Martinus Veltman, Carlo Rubbia, and George Smoot.

The session featured Nobel Laureates David Gross, Martinus Veltman, Carlo Rubbia, and George Smoot.  It was chaired by Prof. Dr. Felicitas Pauss, of CERN.  Additionally, we were also joined by several CERN Scientists (John Ellis counted among their number!).  The air in the room was tense with excitement; and rightly so considering roughly 100 of the young researchers in the room, myself included, participated in high energy physics research in one way or other.  And all of us glowed with sheer joy.

However, it was the Nobel Laureates who out shined us all, for they had been waiting for a discovery like this for the majority of their lives!  David Gross remarked “[This was a] great day for me, for physics, for all humanity!”  David Gross went on to proclaim that this discovery was a “Triumph for CERN…a triumph of theory!”  Martinus Veltman followed by saying that this “closes the last gap amongst the Standard Model.”

I don’t think there was any doubt in either Gross’ or Veltman’s mind that a particle like the Higgs Boson existed.  However, George Smoot originally had his doubts, “I was critical of the theorists not looking for other solutions,” to which David Gross jokingly forgave Smoot on center stage.  Smoot followed up by “commending CERN for being cautious.”  He was referring to the fact that both ATLAS and CMS Collaborations have simply stated that we have found a Boson, and that this particle has similar properties to the predicted Standard Model Higgs Boson, but we have not claimed to have found the Higgs Boson.  Smoot cautioned us all to be careful, not to rush to judgement, and to continue our studies and cross-checks.  Very sound advice in my opinion!

Carlo Rubbia chimed in at this point to say that the value of the experimental cross-section (or the rate of how often this Boson was produced) is almost a factor of 2 larger then the theoretical predictions (measurement from the CMS Collaboration shown in the plot below).

Ratio of the measured production rate of our new boson to the theoretical predictions. Notice that for the case where this new boson decays to a pair of photons, the measured rate is almost two times the predicted rate (with errors).

Rubbia commented that this was a “very important new element that warrants consideration,” and asked the CERN scientists who had joined us “what about this factor 2?”

The ATLAS Representative responded by saying that for the H-> gamma gamma channel the ratio measured by ATLAS was 1.9 +/- 0.5, within two sigma of the theoretical prediction, however the overall ratio was 1.3 +/- 1.2, consistent with the Standard Model.  The CMS Representative responded by saying that this slight excess we observed is compatible with the Standard Model, and that the CMS Collaboration measures this to be 1.5 +/- 0.4 for the H->Gamma Gamma channel, one sigma above the theoretical prediction.

Gross asked John Ellis what he concludes about the possibility for beyond the standard model physics in light of this Boson’s discovery.  Ellis replied by stating that this depends very much on the mass of the Higgs Boson, at 127 GeV the vacuum becomes unstable; and that additional physics is needed to prevent the universe from collapsing.

I found this idea very interesting since the current mass measurements of this new Boson by CMS and ATLAS places its mass between 125 and 126 GeV.  However, these results are preliminary, with more data we will be able to narrow down the mass measurement (it might even shift!!).  If this Boson we discovered  truly is the Higgs Boson, and if a precision mass measurement reveals it’s mass to be above 127 GeV, then we definitely need some new physics to keep the universe in its present state, just as Ellis said!

Martinus Veltman was very curious how CMS and ATLAS were able to make this discovery so quickly.  After all, data collection started in 2010 and this month we announced to the world that we had discovered a new Boson.  CMS & ATLAS responded by saying increasing the center of mass energy of the LHC beams from 7 to 8 TeV was predicted to give a 30% increase in the rate of Higgs Boson production.  Additionally, CMS & ATLAS researchers were able to reduce experimental backgrounds by 15% from 2011 to 2012.  On top of these two facts the number of collisions per second taking place in the CMS and ATLAS Detectors was increased dramatically at the end of 2011 and at the start of 2012.  All of these were factors contributing to the rapid discovery of this new Boson.

At this point Carlo Rubbia brought up the topic of what’s after the LHC.  Rubbia’s idea was to build a muon-muon collider, with a center of mass energy slightly higher then this new Boson’s mass.  Rubbia referred to this as a “Higgs Factory,”  since he believes that such a machine would be able to produce these new Bosons with a much lower background then what occurs at the LHC, allowing for precision measurements of this Boson’s properties.  Gross immediately chimed in with “[a] muon Higgs factory would be fantastic,  ideal project for the US to get back into the game if anyone from FermiLab is listening!”  which caused several moments of laughter in the lecture hall.

However, Rubbia’s comment was a very serious one and good one in my opinion.  Physics needs something after the LHC, many questions are and will still be unanswered.  George Smoot was of a similar opinion, stating that “completing the Standard Model is a great triumph, but everyone wants to see us go beyond.”  On this note John Ellis stated that he would also like to see a Higgs factory on the agenda in the future.  However, Ellis was uncomfortable with the idea of making a machine that would be limited to just producing this new Boson.

I think the final comments from the panel discussion summed up the feeling of everyone in that room, and those of all high energy physicists.  David Gross closed by saying “Congratulations to all of you at CERN, are you having a big party tonight!?”

 

Until Next Time,

-Brian

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Now that the good people at CERN have finished their Higgs-discovery champagne, many of us have found ourselves drawn to harder drinks. While the Higgs is the finishing touch on the elegant edifice of the Standard Model, it’s the culmination of theoretical physics from the 1960s. Where’s all the exciting new physics that we’d been expecting “just around the corner” at the terascale?

My generation of particle physicists entered graduate school expecting a cornucopia of supersymmetry and extra dimensions at the TeV scale just waiting for us to join the party—unfortunately those hopes and dreams have yet come up short. While the book has yet to be written on whether or not the Higgs branching ratios are Standard Model-like, two recent experimental updates in collider and dark matter physics have also turned up empty.

No Z’ at 1 TeV

The first is the search for Z’ (“Z prime”) resonances, these are “smoking gun” signatures of a new particle which behaves like a heavy copy of the Z boson. Such particles are predicted by several models of new physics. There was some very cautious excitement after the 2011 data showed a 2σ bump in the dilepton channel around 1 TeV (both at CMS and ATLAS):

The horizontal axis is the mass of the hypothetical particle (measured by the momenta of the two leptons it supposedly decays to) in GeV, while the vertical axis is the rate at which these two lepton events are seen. (The other lines are examples for what one would expect for a Z’ from different models, for our purposes we can ignore them.) A bump would be indicative of a new particle causing a resonance: an increased rate in the observation of two leptons with a given energy. You can see something that is beginning to “kinda-sorta” look like a bump around 1 TeV. Of course, 2σ signals come and go with statistics—and this is indeed what happened with this year’s data [CMS EXO-12-015]:

Bummer. (Again, one really doesn’t have much right to be disappointed—that’s just the way the statistics works.)

Still no WIMP dark matter

Another area where we have good reason to expect new physics is dark matter. Astrophysical observations have given very strong evidence that the dark matter that gravitationally seeds our galaxies is composed of some new particle that is not described by the Standard Model. One nice feature is that astrophysical and cosmological data tell us the dark matter density in our galaxy, from which we can deduce a relation between the dark matter mass and its interaction strength.

Physicists observed that one particularly interesting scenario is when the dark matter particle interacts via the weak force—the sector of our the Standard Model that gets tied up with electroweak symmetry breaking and the Higgs. In this case, the dark matter mass should be right around a few hundred GeV, right in the ballpark of the LHC. To some, this is very suggestive evidence that dark matter may be related to electroweak physics. This class of models got a cute name: WIMPs, for weakly interacting massive particles. There are other types of dark matter, but until fairly recently WIMPs were king because they fit so nicely with models of new physics that were already modifying the electroweak scale.

Unfortunately, the flagship dark matter detector, XENON, recently released a sobering summary of its latest data at the Dark Attack conference in Switzerland. Yes, that’s really the conference name. XENON is a wonderful piece of detector technology that any particle physicist would be proud of. Their latest data-taking run found only two events (what’s expected from background). The result is the following plot:

How to read this plot: the horizontal axis is the mass of the WIMP particle. You get to pick this (or your model of new physics predicts this).  The vertical axis is the cross section, which measures the number of dark matter–detector interactions that such a WIMP is expected to undergo. The large boomerang-shaped lines are the limits set by the experiment—as the red text says, for a mass of around 55 GeV, it rules out cross sections that are above a certain number. For “garden variety” interaction channels, this number is already much smaller than the ball park estimate for the weak force.

The blob at the bottom right is some fairly arbitrary slice of the supersymmetry parameter space, but this is really just there for illustrative purposes and shouldn’t be interpreted as any kind of exclusion of supersymmetry. The other lines are other past experiments. The circles at the top left are slightly controversial ‘signals’ that have been ruled out within the WIMP paradigm by the last few direct detection experiments (XENON and CDMS).

The story is not necessarily as dour as the plot seems to indicate. There are many clever ways to get dark matter, not all of them WIMP-like. In fact, even the above plot is limited to the “spin-independent” coupling—an assumption about the particular way that dark matter interacts with nuclei. But these WIMP searches will eventually hit a brick wall around 2017: that’s when the XENON 1T (“one ton”) experiment will be sensitive to cross sections that are three orders of magnitude smaller than the current bounds. At that level of sensitivity, you end up with a lot of background noise from cosmic neutrinos which, as far as the detector is concerned, behave very much like dark matter. (They’re not.) Looking for a dark matter signal against this background is like looking for a needle in a stack of needles.

Where do we stand?

Between the infamous magnet quench of 2008 to the sobering exclusion plots of the last couple of years, an entire generation of graduate students and young postdocs is internalizing the idea that finding new physics will not be as simple as turning on the LHC as some of us had believed as undergrads. Despite our youthful naivete, the LHC is also still in its infancy with a 14 TeV run coming after its year-long shutdown. The above results are sobering, but they just mean that there wasn’t any low-hanging fruit for us to gobble up right away.

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We’ve all heard the big news from CERN by now (if not then you might want to catch up on the latest gossip!) Right now most of the focus at ATLAS and CMS is on measuring the properties of the new boson we’ve found. The numbers of events are small, so studies are very difficult. One of the most important properties that we need to study is the particle’s spin, and luckily we can say something about that right now!

A typical Higgs boson candidate in the "golden mode" (ATLAS Collaboration)

The big news: One of many Higgs boson candidates in the "golden mode" (ATLAS Collaboration)

There are two ways to study the spin of this boson, the hard way and the easy way. The hard way involved looking at angles between the final state particles and that’s tricky, but it can be done with the existing data. This method is hard because we have to model both signal and background to get it right. The easy way is to look at the decays of the boson and see which ones happen and which ones don’t. We need a little more data to do this, but we can perform this study by the end the data taking for the year. Richard has already discussed the “hard” method, so I’m going to show the “easy” method. It comes with nice pictures, but there are a few subtleties.

I want to consider four decays: a decay to two photons, a decay to two \(Z\) bosons (the same applies to two \(W\) bosons), a decay to two \(\tau\) leptons, and a decay to two \(b\) quarks. All of these decay modes should be seen by both experiments if what we have seen is the Standard Model Higgs boson.

We need to label our particles properly and describe them a little before we begin. We can never measure the spin of a particle exactly, and the best we can do is measure its total spin, and its projection along a certain axis. The spin along the other two axes remains a mystery, because as soon as we measure its spin along one axis, the other two components of spin become indeterminate. That’s quantum mechanics for you! A component of spin can be increased or decreased with “raising” and “lowering” operators, and the change is always in natural units of 1. (This is just a result of the universe having three spatial dimensions, so if the answer was any different then the universe would look very different!)

Let’s take the electron and work out what spin states it can have. The electron’s total spin has been measured to be 1/2, so we need to project this spin onto an axis and find out the allowed values. A little thought shows that there are only two states that can exist: spin +1/2 and spin -1/2 (which we call “spin up” and “spin down”.) The \(J/\psi\) meson has spin 1, so it’s allowed states are +1, 0, -1. When the \(J/\psi\) is in state spin 0 what really mean is that it has “hidden” its spin at 90 degrees to the axis, so it’s total spin is still 1 and its projection along our chosen axis is 0.

So let’s get on with the job of considering the spins of all these other particles. The photon is a massless boson with spin 1, and it can only arrange its spin transversely (for obscure reasons that Flip explains very well), so it can’t hide its spin when it projects along an axis. That means that it can only have spin of +1 and -1. (There’s one more particle we’re going to use in these arguments, and that’s the gluon. The gluon is the same as the photon, except it interacts with a different field, so like the photon it can only have spin states +1 and -1):

The spin projections of the photon

The spin projections of the photon

The \(Z\) and \(W\) bosons are similar, except they have mass, so they have the luxury of hiding their spin. This means that they can have spin -1, 0, and 1, just like the \(J/\psi\) did:

Spin projections of the massive boson

Spin projections of the massive boson

Both the \(b\) quark and \(\tau\) lepton are fermions, which means that they have spin 1/2. We already know what spin states are allowed for fermions, spin up and spin down:

Spin projections of fermions

Spin projections of fermions

Now that we know the spin states of all these particles we can just add them up and confirm or refute which spin our new boson has. Let’s see how we can get spin 0:

Possible decays of a spin 0 particle

Possible decays of a spin 0 particle

It looks like we can a spin 0 particle by combining any of our particles.

Let’s try spin 1:

Possible decays of a spin 1 particle

Possible decays of a spin 1 particle

Uh-oh, it looks like we can’t make a spin 1 particle from photons! To align the spins correctly the photons must be in an antisymmetric state, which is absolutely forbidden by Bose-Einstein statistics. (Incidentally the term “boson” comes from the name Bose.) That means that this new boson is definitely not spin 1, because we see it decay to two photons.

So that means we have to do things the hard way to measure the spin of this new particle. For those who are interested, one of the main challenges presented here comes from the “acceptance” of the detectors- the kinematics of the final states we observe are significantly biased by the geometry of the detector. Even for a spin-0 boson, which decays isotropically, the distributions of the final decay products in the detector will not be isotropic, because the detectors do not have completely hermetic coverage. Fortunately since this post was first written we’ve gathered more data, and detailed studies have been performed eliminating all but the spin 0 hypothesis with a positive parity, indicating that what we have seen is most likely the long sought Standard Model Higgs boson after all.

Errata: In the original post I incorrectly made an argument stating that the decay of a spin 2 boson to a pair of quarks would be significantly more probable than the decay to a pair of leprons. Following discussions with Frank Close and Bob Cousins it was pointed out that well established graviton models would give a tensor interaction that would decay to leptons roughly 2% of the time per lepton flavour, making these final states accessible to the LHC experiments, and likely before the dijet final states would be accessible. My thanks go out to Close and Cousins for their correction.

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Portuguese version below…

In my last posting I tried to give a very general view of what happens when protons knock each other inside the LHC beam pipe. Many of the heavy particles produced, like the Z boson, don’t survive long enough to reach any detector. Some of the very light ones, like an electron, a muon or a photon actually travel through lots of material and leave some “detectable” traces. So, as detectives, we collect this information with our gigantic detectors and play around to find everything we can about as many collision events as possible. And yes, you may have just caught what I wrote there : a photon (a particle of light) can pass, in such extreme conditions, a great amount of material. That is very true and happens in every collision of the LHC!

So.. What do we do with these traces? We use different detectors which are specialized in the different kinds of particles that can show up. Today I will talk about the detectors with which I worked all my professional life : Calorimeters.. If you look for calorimeter in the Wikipedia, you will find that “Calor” is the latin for “heat”. These are detectors that can measure heat or, in the case of high energy physics, particle’s energy! Well, there is also an entry about Calorimeter in particle physics in the Wikipedia, which, by the way, may need some improvement!

In a nutshell, one can think of the particle physics calorimeter inner working as not being very different from the “regular” calorimeter. Particles with very high energies enter, loose their energy and somehow “warm up” some material and a form of thermometer. In the particle physics calorimeter, the incoming particle hits atoms or atoms nucleus of a material which is named the absorber. It does not warm (too much!) that material, but it rather suffers reactions similar to the ones of the protons in the beam. The energy of the particle hitting the absorber will be converted in multiple particles inside the absorber which will carry a fraction of the initial particle energy. This forms a shower of particles in the detector structure. Usually interleaved with the absorbers, another material, the sampling material (the thermometer!) converts a very small fraction of the shower energy in some measurable quantity (light, electric current, etc). If you can estimate precisely which fraction of the energy is lost (so, not measured!) in the absorbers and which fraction stimulates the sampling material (so, IS measured!), we can figure out the total energy of the original particle! Maybe here, a very curious person would ask why we don’t make a calorimeter only with sampling material. A very simple quick answer is that the amount of sampling material to completely contain the shower and measure a very high energy particle would make a very very big calorimeter (many tens of meters). So, we have to use the absorbers!

See a shower forming as an electron invades the structure of the ATLAS Electromagnetic Calorimeter in this video. The sound is in English and the subtitles in portuguese. This video is an extract of the “ATLAS – Episode II”, the complete version being fully available at youtube (part 1 and part 2). In this case, the absorbers are lead plates organized in a funny accordion shape which convert the energy of the original particle into multiple ones with smaller energy (see the real lead plates here). The sampling material (that initially amazed me a lot) is liquid argon (at -185oC!!). Because of that, the whole calorimeter is installed in a gigantic vessel (see a photo!). We will talk about the sampling process later. The calorimeter is divided into cells formed by the electrodes in the video which collect the energy in a their vicinity. Very detailed algorithms pick up which cells were activated by the shower and calculate the shower energy and geometry.

There is another very interesting fact that happens thanks to the absorbers. Imagine that you are a very high speed photon or electron heading to a wall of lead. Since you are an electron or photon you “see” to other electrons thanks to the electromagnetic force, including the electrons from the lead. I find easier to understand if you imagine a wall filled with plastic bubbles (with 100 m diameter) which are the lead atoms. You (the electron) will certainly very quickly hit one ball and loose your speed and energy. Now, suppose that you are a hadronic particle (like a proton or a neutron), you see very little the bubbles. Actually, a hadronic particle interacts mostly over the strong force, so, mostly with the atoms nucleus. If the electron-sphere were 100 m, the nuclei is only a millimeter, so, very, very tiny and with lots of space between then (at least 100 m!). This way, the plastic bubbles disappear and you can now cross a much larger amount of material without finding anything to stop you. So, the shower will start much deeper in the detector, depending on whether you are an electron or a neutron or proton. This way, the calorimeter can cope with two tasks at the same time : They give you a measurement of the particle’s energy AND allow you to perform particle identification. Both tasks make these devices very attractive and quite often found in High Energy Physics. Recently, a calorimeter of lead/scintillator fibers was sent to space in the AMS detector!

To illustrate, I picked up two events in the ATLAS Calorimeter. In one, a di-photon, you see that the photons leave two small yellow bands in the green section (EM calorimeter) but never touch the red section (the hadronic calorimeter). In the second picture, of an event with two jets, we see that these, being hadronic like protons or neutrons, cross the full EM calorimeter up to the hadronic one. So, hadronic particles go deeper in the detector as discussed above.

Double Photon

Di-photon event in ATLAS

DiJet Event

Di-Jet event in ATLAS

Hope you had enjoyed this view of a calorimeter. Next week, I will take a little pause as it is vacation time! But on the next one, I will discuss how to go from the sampling material to an electric pulse in both main ATLAS calorimeters (Lar and Tile, as they are called!!).

Portuguese version :

No meu último post, eu tentei dar uma visão bem geral do que acontece quantos os prótons se encontram dentro do feixe do LHC. Muitas das partículas produzidas, por exemplo, um bóson Z, não sobrevivem um tempo suficiente para atingir os detetores. Algumas das partículas mais leves tais como um elétron, um múon ou um fóton, conseguem atravessar uma grande quantidade de material e deixar
alguns traços “detectáveis”. Assim, como detetives chegando na cena do crime, podemos coletar estas informações com nossos imensos detetores e tentar descobrir todos os detalhes possíveis num enorme número de colisões! E, sim, você talvez tenha percebido algo interessante que eu escrevi acima : um fóton (partícula de luz) pode atravessar, nessas condições extremas, uma grande quantidade de material. Isso acontece em todas as colisões do LHC!

Assim sendo, o que fazemos com tais traços? Usamos diferentes detetores especializados nos diferentes tipos de partículas que possam vir a aparecer. Hoje, eu vou comentar sobre o detetor com o qual eu trabalhei por toda minha vida profissional : O Calorímetro. Se você procurar a palavra “calorímetro” na Wikipedia, você verá que estes são equipamentos que medem calor, ou, no caso da física de altas energias, a energia das partículas! Em inglês, existe também um verbete pra calorímetros da física de partícula na Wikipedia, embora, ele pareça carecer de detalhes.

Resumindo, podemos pensar no funcionamento do calorímetro da física de partículas de forma similar ao “calorímetro” regular. As partículas com muita energia entram, perdem sua energia e de alguma forma “esquentam” o material e um termômetro. No caso da física de partículas, as partículas entram num material chamado de absorvedor*, se chocando contra seus átomos ou os núcleos de seus átomos. Na realidade o material não esquenta (muito! Um pouco pode esquentar!), mas a partícula original sofre o mesmo tipo de reação que os prótons do feixe, ou seja, a partícula vai bater nos átomos do absorvedor e perder sua energia na forma de outras partículas com uma menor energia. Dessa forma, uma cascata de partículas se forma na estrutura do detetor. Normalmente, misturado com o material absorvedor, outro material, chamado de material de amostragem (o termômetro!) converte uma pequena fração da energia da cascata em algo mensurável, tal como luz ou corrente elétrica. Se for possível calcular a proporção entre a energia perdida (ou seja, não medida porque se perdeu no absovedor) e a energia que estimula o material de amostragem (logo, que é medida de verdade!), podemos calcular a energia total da partícula! Uma pessoa mais curiosa pode perguntar porque não fazemos o calorímetro inteiro com o material de amostragem. A resposta mais rápida e simples é que o calorímetro iria precisar de muito material pra conter a cascata e medir a energia de uma partícula. Assim sendo, o calorímetro ficaria gigantesco (dezenas de metros). Por isso, usamos os absorvedores.

Veja como uma cascata se forma quando um elétron invade a estrutura do Calorímetro Eletromagnético do ATLAS nesse vídeo. O som é em inglês com subtítulos em português. Este vídeo foi extraído do filme “ATLAS – Episódio II”, cuja versão completa está disponível em inglês no youtube (parte 1 and parte 2) ou com subtítulos em português no nosso canal ATLAS / Brasil . Neste caso, os absovedores são placas de chumbo organizadas no formato de acordeão que converte a energia da partícula original em várias outras de menor energia (veja as placas de chumbo aqui). O material de amostragem nesse caso (e isso sempre me impressionou muito) é o Argônio Líquido (a -185oC). Por isso o calorímetro está instalado numa vasilha gigantesca (veja essa foto). Falaremos mais sobre a amostragem no próximo post. O calorímetro é dividido em células formadas pelos eletrodos que aparecem no vídeo que coletam a energia numa certa vizinhança das células. Algoritmos especiais conseguem descobrir quais células foram ativadas pela cascata e qual sua energia. Dessa forma, podemos saber a energia e o formato da cascata.

Há um outro fato muito interessante sobre os absorvedores. Imagine que você é um elétron ou fóton atravessando o espaço a uma velocidade altíssima. Como você é um elétron ou um fóton, você vê outros elétrons graças a força eletromagnética, incluindo os elétrons das placas de chumbo. Eu creio ser mais fácil de entender se você imaginar uma parede de bolhas de plástico com 100 m de diâmetro que representam os átomos de chumbo na parede. Você (o elétron) certamente vai bater numa dessas bolhas e perder sua velocidade e energia. Agora suponha que você é uma partícula hadrônica (como um próton ou nêutron). Você vê muito pouco as bolhas. Uma partícula hadrônica, na verdade, interage na maior parte das vezes através da força nuclear forte, ou seja, só com os núcleos dos átomos.  Se as elétron-esferas, fossem de 100 m, os núcleos teriam apenas um milímetro, ou seja, muito, muito pequenos e com uma grande espaço (pelo menos 100 m) livre entre eles. Assim sendo, as bolhas de plástico desaparecem e você pode agora viajar por um comprimento muito mais longo sem que nada o interrompa. Assim, a cascata vai começar muito mais profundamente no detetor. Dessa forma, o calorímetro pode executar duas tarefas ao mesmo tempo : Ele pode fornecer uma medida da energia da partícula E ajudar na identificação da partícula. Ambas as tarefas fazem com que estes aparatos sejam muito atraentes e muito usados na física de altas energias. Recentemente, um calorímetro de chumbo com cintiladores plásticos foi enviado para o espaço no detetor AMS!

Para ilustrar, selecionei dois eventos no detector ATLAS. No primeiro, um evento com dois fótons, vemos a deposição de energia acontecendo como dois sinais marcados em amarelo na primeira seção (eletromagnética) do calorímetro em verde. Vemos que estes sinais nunca tocam a segunda seção (hadrônica). Na segunda figura, vemos um evento com dois jatos. Cada um dos jatos atravessa o calorímetro electromagnético e chega a depositar uma grande quantidade de energia na parte mais profunda do detetor, como tinhamos discutido.

Double Photon

Evento Di-photon no ATLAS

DiJet Event

Evento Di-Jet no ATLAS

Espero que essa tenha sido uma boa jornada no interior de um calorímetro. Na próxima semana teremos uma curta pausa de férias! Mas na outra semana, vamos discutir como funciona o material de amostragem para produzir um sinal elétrico em ambos os calorímetros do ATLAS (Lar e Tile, como são chamados!).

(* : Eu traduzi livremente – e possivelmente de forma errada – “absorber” em “absorvedor”. Aceito sugestões)

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