Today I and two colleagues took a rather exciting trip to visit the Paul Scherrer Institut, outside of Zurich.  We’re engaged in some research on future pixel detectors there, so we have one postdoc stationed at PSI, and two students who are resident there for the summer.  Since I was at going to be at CERN for a couple of weeks, I wanted to visit them and see what was going on there.

PSI is about three and a half hours away from Geneva by train, plus a short bus ride at the end.  Swiss trains are, by my standards, very civilized!  The first leg of the trip, from Geneva to Lausanne, goes along Lake Geneva, and the scenery is very pretty, as you can look across the lake and see the mountains on the other side.  We went up last night, after a day of work at CERN, and had dinner on the train, which was on the expensive side but also quite pleasant.

PSI sits on both the east and west bank of the Aare River, with a bridge connecting the two sides.  It’s essentially the Swiss general-purpose national laboratory.  High-energy physics is only a small part of what they do.  They also have a synchrotron light source with very stable beams, a proton source and a neutron beam.  Our host for the day was Roland Horisberger, who is the leader of the CMS group there.  PSI built the barrel pixel detector for CMS.  With tens of millions of readout channels within a radius of 11 centimeters, it’s really a work of art.  The truly amazing thing is that the entire barrel was built just by the group at PSI, which is only eight physicists.  Suffice it to say that they are all very good at what they do, and what they do covers the gamut of detector design and construction — mechanical engineering, electrical engineering, chip design, and so forth.  Now that they have built and installed the detector that will operate in CMS starting this fall, they are hard at work on improving their designs so they can start to build a replacement detector, which will be necessary because the large particle fluxes through the detector will ultimately damage it.

Our students are learning a tremedous amount from working with such a strong and knowledgeable team.  I’ll have to visit again soon!

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One of the chores that we have to face as summer interns at the CMS experiment is the half-dreaded, half-loved shifts. During these at time endless periods of time, we get to drive about 30 km away from the LHC main site in Meyrin to the so called point 5, or the location of the LHC ring where the CMS experiment is, in the town of Cessy.

A shift involves monitoring the different subdetectors of the experiment, making sure that the temperatures, humidity, voltages and many other parameters stay within the specified ranges. As shifters, we have limited possibilities as to what exactly we can do to fix any problems that might (and do) arise. In case, we are to let the shift experts know about any issues that arise. Sometimes, the problems are trivial, and one must just make a note of them and let it go. However, as Amram and Tico know very well, sometimes serious issues arise, and one has to have the guts to take drastic decisions, such as turning off the detector itself. (Not that we have access to the buttons that do this, but we’re around when this is done).

Last week, for instance, there was a general power failure at point 5. Some of the cooling cycles did not turn on again after power came on, while some of the wires carrying thousands of volts were quickly heating up. Before they cooked, said voltages had to be turned off.

In general these events are quite rare though. It is of course necessary to always have someone controlling each of the subsystems (Data Acquisition, Tracker, Pixels, the different calorimeters, the cathode strip chambers…), because whenever the experiment is running,   someone will always have to on duty to check that the data acquisition process is running as smoothly as possible.However, spending 8 hours in a row sitting in a room with many screens, many of which don’t even change their display at all does get somewhat tedious at times…

Update: there is this cool website : http://cms.web.cern.ch/cms/Media/CMSeye/cam6.html. From there you can get a snapshot of the surface control room. It is updated every 5 minutes and if you’re lucky you might even see some of us there!

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For some time now I’ve been trying to develop a  good explanation of my research to the general public. The current work-in-progress is composed of six ‘acts’ with increasing specificity. I’d like to present these over my next three posts.

Act 1: Science

Science is a branch of human knowledge associated with the rational, objective, and empirical study of the natural world. The primary mode of generating such knowledge is the scientific method, by which ideas are checked against experiments. Science differs from the humanities in its subject and from the arts in its method.

Scientific fact is based on observation. Causal explanations for these observations are theories that must be rigorously checked against experiment. It is worth highlighting that a “theory,” in the scientific sense, both explains observed phenomena and predicts further observable phenomena. In this way scientific theories are falsifiable and differ from the common use of the word “theory” that implies opinion of speculation. A theory may end up being incorrect when subjected to further experiments, but this is a feature rather than a shortcoming of the scientific method.

Act 2: Physics

Physics is the branch of science concerned the fundamental laws of nature. Branches of physics study atoms (and all things subatomic), materials in different phases (condensed matter), dynamics of different systems (e.g. geophysics, general relativity), outer space (astrophysics and cosmology), and applications to other sciences (biophysics, physical chemistry). In some sense physics is the “purest” science in that it is an interface between fundamental models of nature and experiments.

Unlike the other sciences, physicists can roughly be divided into theorists and experimentalists. Theorists are primarily concerned with mathematical models of nature that can be used to explain experimental data. Experimentalists are primarily concerned with testing theories and acquiring new data that may point to science beyond current theories. This divide occurs because of the high degree of specialization required to study nature at the level of physics. Theorists must be fluent in advanced mathematical methods while experimentalists must be clever to build apparati and interpret data.

Act 3: Particle (‘High Energy’) Physics

Particle physics is the branch of physics concerned the smallest building blocks of nature. In the past century, the “particles” that physicists considered “smallest” have gone from atoms, to nuclei, to protons, to quarks (not to mention electrons and their cousins). We have also learned how to think of the fundamental forces of nature in terms of force-mediating particles such as the photon.

Why do we study these particles? One reason is that we hope that by studying the basic building blocks of the universe we can understand composite objects better (reductionism). There is also a philosophical/aesthetic appeal associated in understanding what the ultimate basic building blocks of the universe should look like.

The current canon of particle physics is called “The Standard Model” and was mostly completed in the 1970s. It is a kind of quantum field theory called a non-abelian gauge theory (this means it is based on certain kinds of symmetries) and explains the strong and weak nuclear forces as well as electromagnetism. It has passed every experimental test (up to some recent modifications in the neutrino sector) with flying colors and is regarded as a stunning success.

Next time: stay tuned for Part 2, where I discuss what is meant by an “effective theory.”

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I have been promising for a long time to talk about what the LHC experiments are looking for, if not just the Higgs boson.  There’s a tremendous amount of material available on this, but I am not going to look up or link to any of it; this will give you, at least, a snapshot of what I know and how I think about it.  If any theoretical particle physicists read this and feel the urge to slap their foreheads in anguish, I invite them to consider this an interesting study in how much information experimentalists actually retain from classes and seminars.

Edit (June 29, 9:30 EDT): As you might expect given the above, I made some oversimplifications and at least one outright error, which have been kindly pointed out by theoretical physicists in the comments.  For one mistake — mixing up two different kinds of extra dimensions — I have made some corrections with strikethroughs and italics in the appropriate section of the text.

To discuss what the LHC experiments are looking for, we need to understand what problems there are in our current understanding of particle physics — in other words, what makes us believe there ought to be any new particles at all?  The strongest case is for the Higgs boson or something like it; it plays a critical role in the behavior of the weak force and the masses of the associated W and Z bosons, which we already know behave exactly the way we would expect if there were a Higgs boson.    You might ask what happens if the W and Z boson masses and interactions are just a coincidence, and they just look like a Higgs Boson is involved, but actually there’s no such thing — the answer is that the Standard Model of particle physics becomes mathematically inconsistent, and makes senseless predictions at energies the LHC will investigate!  So there has to be something to make the theory behave.  However, that doesn’t mean there’s exactly one “Standard” Higgs boson, an issue I’ll get back to.

The next best clue to new physics — or the next biggest problem with what we know now, if you’d rather think of it that way — is called the Hierarchy Problem.  This is expressed most easily as the question, “why is gravity so much weaker than other forces?”  However, because the strength of each force changes as the energy of interactions changes — at different rates for different forces — we particle physicists prefer to frame the problem in terms of this question: “why are the W, Z, and (apparent) Higgs boson mass energies so much smaller than the energy at which the gravitational force becomes strong?”    If we take the Standard Model as the complete picture as far as we can, so that we assume there’s nothing for the LHC to find except the Higgs Boson, then the “desert” between the Higgs boson mass energy and the energy where gravity beccomes strong is a factor of about 10,000,000,000,000,000!   That’s aesthetically displeasing, but it’s actually worse than it sounds at first.  The reason is that the Standard Model has to include the effects of quantum fluctuations on the masses of particles — and the fluctuations have to be allowed to have any energy up to the energy where the Standard Model “breaks down.”  If the Standard Model works up until a theory of Quantum Gravity (for example, String Theory) kicks in, then we have to allow energies up to where gravity is strong — that factor of 10,000,000,000,000,000 really hurts, because the quantum fluctuations force the Higgs boson to have a much higher mass than it needs to for the theory to work!  Here are some solutions to this problem:

1. The Higgs Boson has a “bare mass” — i.e. the mass it starts with before quantum fluctuations — that is very large and negative, and cancels out the quantum fluctuations almost perfectly.  This is allowed, but seems rather implausible.
2. The quantum fluctuations get cancelled out because of new particles whose effects balance out the old ones.  This suggests Supersymmetry, in which every existing particle has a supersymmetric partner, and the pair’s effects on the Higgs Boson mass do indeed cancel.
3. There isn’t really a Higgs boson.  Instead, there is a new force with a new set of particles that “pair up” to act like the Higgs boson at low energies.  These are called Technicolor theories, because the new force looks a lot like the “color charge” found in theories of the strong force.
4. Gravity isn’t really as week as it seems.  Instead, it appears weak because it spreads out in several extra spatial dimensions that are curled up on themselves.  These dimensions would be something like a millimeter in size at most, but are called “Large Extra Dimensions” because they’re pretty big compared to the size of most things in particle physics.  So gravity would spread out in these dimensions, making us think that it’s so weak that it only becomes as strong as the other forces at very high energy — but actually it would surprise us by being strong at much lower energies, maybe even LHC energies.  This would mean that the range of energies allowed for quantum fluctuations affecting the Higgs Boson mass would be greatly reduced — if we want them to be “small enough,” that strongly suggests that gravity becomes strong at energies the LHC can investigate, and we can expect all kinds of new particles and phenomena.

I know that reasoning is rather complicated, but hopefully you’ll retain at least this basic idea: starting just by asking why gravity is so weak, and following the reasoning of our current theories of particle physics, we get that something very strange is going on with the Higgs boson — and the only way to fix it is to appeal to an amazing numerical cancelletion, or to make a change to our understanding of particle physics.  And most changes we can make turn out to add a bunch of new particles, at energies right around the mass of the W and Z bosons, or just above them — in other words, exactly the energies the Large Hadron Collider will explore!

Let’s look at these new ideas in more detail.

• In Supersymmetry, we have a new particle for each existing fundamental particle.  We know they’re all as heavy or heavier than the particles we’ve seen before, because otherwise they would have shown up in previous experiments, but they also can’t be too heavy or they won’t cancel those quantum fluctuations properly.  So we’ll see some new particles decaying into particles we know — and maybe into a non-interacting Lightest Supersymmetric Particle, which might turn out to be dark matter (a nice bonus)!
• If there are Large Extra Dimensions, then we would effectively see new particles also.  This is because an ordinary particle that was moving in a circle around such an extra dimension would appear, in our three dimensions, to have its energy of motion “acting like” mass energy.  Motion in the extra dimension would only be allowed at certain speeds — for essentially the same reason that Hydrogen atoms only have certain energy levels, if you remember that from chemistry — so we would see familiar particles, but with a certain amount of apparent extra mass, and then another “copy” with the same amount of extra mass added again, and so on.  This would actually be pretty hard to distinguish from Supersymmetry, except that where in Supersymmetry the new partners always have different spin from the original particle, in this case they’d have the same spin. All of that is right for a different kind of extra dimensions, but rather than get into that, let me just put down what we’d actually see for Large Extra Dimensions.  It’s fun too — basically, because gravity will be strong at the LHC, we’ll be able to directly explore whatever theory unifies gravity with the other forces.  This could result in some very dramatic objects, including microscopic black holes.  (To be reminded why we know that such black holes cannot be dangerous, whatever their properties might turn out to be, click here.)  Black holes would decay into all kinds of things, making spectacular events in our detectors, and could actually be one of the easiest things to find if they’re light enough!
• If there’s technicolor instead of a Standard Model Higgs Boson, the LHC experiments might have a pretty big challenge.  The new particles might be too heavy to produce, and only through careful and detailed study of certain interactions would we get indirect clues about what was going on.

This is hardly a comprehensive look at all the the issues woth thinking about that might require new theories and new particles, but it perhaps gives you a bit of an idea of the possibilities that are out there.  Personally, I wouldn’t bet on any particular theory — but there are common features to new theories that make some kind of new “zoo” of particles at LHC energies a very real possibility.  Of course, finding all those new particles would raise all kinds of new questions; first we’d ask what theory described the new particles, and then there are all sorts of questions to ask about the new theory.  (For example, in Supersymmetry, we’d have to ask why the new particles are so much heavier than the ordinary particles they’re paired with.)   But answering old questions, and finding new ones to ask, is a particle physicist’s idea of heaven — it helps us understand a little more of the universe, and gives us lots more work to do!

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Hello everyone, this is my first post on the US LHC blog. Over the next months I will tell you about my experience moving to CERN and working on the CMS experiment.

Last May I officially became a Ph D candidate after taking my qualifiers. It was a very interesting, productive, intense and stressful experience, as it is supposed to be. The feeling of being done with this part of my Ph D was nice, unfortunately  it lasted very little time.

As is common for graduate students working on one of the LHC experiments, after have taken the qualifiers, the next step is to move to CERN where most of the action is! Personally I enjoy being  at  CERN, the annoying part  is to get there, why?

Well, there are some difficulties when you are Latino and have to move to a different country because most of the people coming from almost any Latin American country need a visa to go basically anywhere outside south  or central America. I am Colombian and unfortunately we need a visa even to breathe! Under these circumstances, I decided to go to Bogotá, the capital city of Colombia to get my French visa. I thought this could be a good idea in order to get to spent some time with my family, because I haven’t been able to be with them in a very long time. I talked to my advisor and committed to get some work done during the first two weeks and then take 10 days of vacation at the end of my stay. The sad part is that there was a problem with my visa and it took longer than expected. Because of that I was waiting every single day for a call from the consulate and I had to send tons of e-mails trying to solve this problem.

In the end I spent the last two weeks of my visit (which was supposed to be my vacation!) dealing with visa stuff. I was stuck in my apartment without being able to go out with my family anywhere before 5 pm, and I had to change my plane tickets to the US and to Geneva which cost a lot of money, leaving me basically broke! Finally everything worked out and I got my visa. Thanks to the help of my advisor I will able to survive in Europe while I wait to get paid. Now I am in the US sitting on a chair at the airport waiting to board the plane to finally go to Geneva!

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Here I am on one of my rare visits to CERN.  This is a CMS week, and I’m going to be staying next week also.  It was really my only chance to visit CERN this summer, so it seemed sensible to try to spend a little more time here than I do on a typical trip.  Being away for two weeks is not so easy on my family, but we accept that it’s part of the job.

And of course, it is sort of fun to be here!  I am enjoying having the chance to have real discussions with people whom I usually only interact with through email and in meetings.  There’s nothing like the chance encounter you have in the cafeteria or the hallway.  In fact, I’m seeing lots of US-based friends here too, whom I wouldn’t usually see at home.

This is not to say that there aren’t any meetings…quite the contrary, as it’s a CMS week.  There are meetings all the time.  But they have generally been informative.

It is very interesting to consider the cultural differences between CERN and, say, Fermilab, or perhaps any US institute.  It seems to me (and others can offer opinions) that in the US, you go to your office and you work and work and work.  And then perhaps work a little more.  It is different here — there is a lot more coffee drinking.  I’m not a coffee drinker myself.  When I first visited CERN, about twelve years ago when I was looking into a job here, I would go to meet this or that person, and they would say “Ah, you’re here!  Let’s get a coffee.”  I spent most of my week watching people drink coffee.  All the coffee drinking is partially for social purposes, but also for work purposes; people just seem to spend more time in conversations over coffee here compared to in the US, which means less time hunched over the computer reading email.  Perhaps this is good, although I don’t know how they’re all getting through their mail.

One feature of CERN that facilitates this is the restaurant and adjoining outdoor patio.  The restaurant is open from long hours — 7 AM to I think midnight, with coffee available at all times, along with dinner in the evening and alcohol at least some of the time.  And there is a nice patio next to the restaurant with lots of tables and chairs for hanging out.  Since weather in Europe is generally more moderate than in the US (at least in many parts of the US), the patio season is actually quite long — it generally doesn’t get too hot to sit out there in the summer, and you can be out earlier in the spring and later into the fall.  So as we get into the late afternoon and early evening (the sun sets late in the evening, as Europe is pretty far north), there are lots of people outside socializing with snacks and drinks.  It’s nice.  What time is it now?  Perhaps I should go soon (and hope that my colleagues in the US aren’t sending too much mail while I’m out there)….

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Hi everyone, This is my first post on these blogs, and I’ll start off by talking about the ATLAS detector. Let me know what you think. Hope you like it!

Vivian wrote in a previous post,

We simulate how the particles would interact in our detector. To do this we have to have a very complete implementation in software of our detector, including the positions of all the components…Even parts like the cables that bring signals from the inside of the detector out to the electronics that register the data have to be in the simulation since there is some probability that a particle will interact in the cables!

It would be bad if there was material in the detector that we didn’t know about, which threw our measurements off, or, for that matter a bug in the simulation software that did strange things with material description, leading to the interpretation of some garden variety physics effect as a new phenomenon! One can see the headlines, “Oops. Scientists retract discovery of the Higgs boson”.

We carefully account for everything that is installed, down to its weight, composition, position, etc. Remember, the ATLAS detector weighs about 7000 tons and has an extremely large number of individual components that need to be accounted for, so this is a non-trivial task. Another problem is that when you have compound materials, e.g., cables that contain plastic, wires, etc., and we have miles and miles of them snaking their way through and around the detector, it is not easy to accurately describe their properties and precisely know all of their positions. It is also possible to make a mistake while entering this information into a database, e.g., forgetting to enter some support structure or using an incorrect or approximate description, etc.

Since physicists are skeptical by nature, we want an independent way to verify the material. So, how to do this? It turns out that we can use (real) data to “X-ray’ the detector.

This “X-ray” uses a unique property of the photon (aka “the light particle”). As a high energy photon nears a nucleus in the material it is traveling through, it can convert to an electron-positron pair. This effect is known as “photon conversion”. It is the main process by which high-energy photons lose energy as they travel through matter, and the likelihood of a photon converting depends on the material, both the amount and its intrinsic properties, that it is passing through.

In order to convert at all, (a) energy conservation requires that the photon have at least as much energy as the combined mass of the electron and the positron, and, (b) a photon, being massless, has to be near a nucleus.

The likelihood of photon conversions is quantified by a property of the material called the “radiation length” (this quantity also determines how electrons lose energy as they travel through matter); among other things, this variable depends on the atomic weight and atomic number of the element. When you have a compound material, it can be hard to estimate a value for the radiation length, and conversions provide us with an independent measure.

So, when photons encounter denser material, they undergo more conversions, and if we detect these electron-positron pairs, we can get an “X-ray” of the material in the detector. And there will be plenty of high-energy photons in our data.

Using our software, we first identify electron and positron candidates and then check if they come from a common point in space; the latter step also needs sophisticated algorithms. If they do meet at a common point in space, we form a vertex (in 3 dimensions). By looking at the position and the number of these vertices, in both simulation and data, we can decide how well the former mimics the latter.

We are also working on a complementary way to map the material using pions, protons, neutrons, kaons, etc. More about that and other details in a later post!

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This week I am in Foz do Arelho, Portugal for the ATLAS Hadronic Calibration Workshop. As you can see, it is beautiful here, but the 100 or so of us that are here aren’t here for a nice vacation (We have meetings all day every day, so it is a bit unfortunate that we see the beach all day but we can’t go sit on it). We are here to work, and prepare for the data we hope to get in a few months.
This workshop is a chance to review where we stand, and then make plans for the early phase of data taking, the next 18 months or so. Specifically, we are talking mainly about “jets” and “missing energy”, two major topics of particle detection.
“Jets” are what we call the huge number of particles that we detect after a quark or gluon is created in a collision in the center of our detector. It flies away from the center and quickly decays into other particles which all crash into our detector and create many other particles in the collisions.
“Missing energy”, what I mainly work on, is a topic that relies on the conservation of energy in a particle collision. The idea is that the energy held by two particles before a collision is equal to the energy held by particles created in that collision, and to the hundreds of particles that result from the subsequent collisions that happen as those particles travel through our detector, hit it, and in turn decay into more and more particles. Since no energy is missing before the collision, no energy ought to be missing after. If we detect all the resultant particles in our detector, and add up all their energies, we should get zero. The one major exception is if particles called neutrinos are produced, which we are not capable of detecting, so they fly away and take their energy with them. Other than that case, checking whether we actually get zero when we add up the energies of all the particles we detect is a really useful check of the performance of our detector.
What unifies the topics of “jets” and “missing energy” is that both rely on the hadronic calibration of the ATLAS detector, which is the subject of this workshop.
Hadronic calibration is the process of turning many of the signals we measure with our detector into the final measurements of the particles we use for physics studies. This process has many steps and takes a huge amount of work by many people, which is why we are here all week.

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Hello everyone,

I’m the newest member of the LHC bloggers. I decided to join some of my friends and tell you about life here as a graduate student working on ATLAS at CERN. It’s full of ups and downs, heartaches and rewards, and coffee… lots of coffee.

However, my hope is to talk about not only about science, but also the stuff we do for fun as part of the CERN community. Bearing this in mind, my first blog post is on the European Research Laboratory Olympics (Atomiades) that took place June 12-15 in Berlin.

53 people from CERN (students, postdocs and staff alike) attended and participated in the following events: Golf, Athletics, Swimming, Half Marathon, Basketball, Volleyball, Roller Blading, Tennis, and Sailing. Shattering records and sterotypes, we took home several medals (including a silver in tennis, in which I took part).

Even though we work hard, we always find some time for fun. Until next time.

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When we were first told to consider writing a blog entry for the US LHC Blog, none of us really thought much of the idea. However, after having discussed it together a little more seriously, we figured that if we could all contribute our fair share, it would not only be rewarding but also further the cause of helping people understand what it is that makes these massive physics experiments (like the Compact Muon Solenoid) tick.

So what exactly is a normal day for us at work? First off, each of us have different duties and responsibilities. In general, James, Robert, Jafet and Diego are more inclined to dealing with software. This means spending most of the day in front of a computer screen, remotely connected to offline servers that allow everyone at CERN access to essential computing tools and databases, but also allow us to enter a digital mainframe that all CERN workers share. Robert, for instance, has been working on a crafty bit of code that will warn us if something is going wrong in the internal circuitry of the Tracker detector (such as unexpected temperature changes) once the detector is running, and Diego has been working on reconstructing how the W bosons generated in certain proton – antiproton collisions decay into other particles. These tasks require a combination of understanding the subsystems of the detector on the one hand and the ability to interact with the virtual interface of these subsystems, which is why people like Robert (with his years of experience in the private sector) can be crucial components of the experiment.

Patrick (a.k.a. Tico) and Amram also interact with the CMS software to a certain extent, but they spend most of their time in “the cavern”, or the enormous underground hole where the monster particle detector lives. They get to install temperature detectors, or help out the technicians before the cavern is sealed (which should be happening very soon if all goes well). Being down there is truly an experience none of us will soon forget. The sheer size of CMS is breathtaking; but even more surprising is the endless intricacy and detail of it all. At first glance, it resembles more a piece of modern art than it does scientific equipment.

Stay tuned for more updates about how things go with us!

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For the past three weeks I’ve been participating in a special event for American theory PhD students: the Theoretical Advanced Studies Institute hosted by the Physics department at the University of Colorado, Boulder.

My friends tease me for being in summer school, claiming that I must have really screwed up my first year as a PhD if they’ve banished me to take classes for a month. In reality these PhD schools aren’t remedial, but are quite the opposite: they’re pedagogical introductions to more advanced topics that active researchers should be familiar with.

TASI is the United States’ premier summer school for theorists. Europe has a few well-established schools of similar nature, including Les Houches, Cargese, and Erice. These schools leave behind proceedings (written summaries of each course prepared by the lecturers) which live on and are read by generations of future graduate students. The most famous examples are Sidney Coleman’s Aspects of Symmetry, a collection of insightful lectures from the Erice summer school in the 70s. These days it’s more commmon to also leave behind video recordings of lectures.

Given the well-documented lectures, why do students bother travelling to attend these schools in person? Besides the importance of being able to ask questions and discuss the lectures, schools such as TASI play a big role in a young theorists’ development since they are sort of a “debutante’s ball” into the field. They are a chance to be introduced to the wider research community outside of their own universities — especially the other students who will become one’s research colleagues for the rest of one’s career. To this day I enjoy hearing older faculty reminisce about meeting each other for the first time 30 years ago at their first summer school.

Beside the 5 hours of lecture per day, meals with other participants, 1.5 hours of student talks in the evenings, and all the associated physics discussions, we’ve been able to make time for our fair share of recreational activities. Two favorites are basketball and soccer… which can be a bit awkward when one is faced with the task of guarding one’s adviser!

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I was having a conversation at lunch recently with some people who are anxiously awaiting LHC data.  Okay, everyone wants data to analyze so we can discover the secrets of the universe, but the course of some people’s lives are determined by when, or whether or not, the LHC starts colliding particles, something completely outside their control.

Graduate students in physics typically take about 6 years to get a PhD.  If you take much longer than that, people may ask “what took you so long?” and overlook you for other positions later.  There is also the slight issue of living on a graduate student salary for longer than six years.  So with the LHC delay, many US graduate students (including one from the Stony Brook group I am a part of) who were doing research at the LHC and expecting to use data from the LHC are heading back to Fermilab in Illinois to do research using the Tevatron collider, which is recording data right now.  Most of these people preferred the physics of the LHC, but because of the LHC delays are choosing what is likely to be a quicker way out of graduate school.

Similar problems confront postdocs like me, who are waiting for data to improve our chances of landing another position in the field.  Some postdocs I’ve known, after waiting long enough, have decided to leave the field rather than wait any longer.  And of course there are plenty of non-tenured faculty waiting for data so they can get tenure at universities.

One question we spent quite a lot of time discussing at lunch was “do American students need data to graduate”?  It might seem obvious that you need data to do research but universities in Europe allow their PhD students to graduate using simulated data if no real data ia available.  They can develop calibration or other analysis strategies on simulated data, for example, that are later applied to real data.

Another question is whether this “real data” requirement of American universities will survive as experiments get bigger and bigger and timescales for them get longer and longer.  We didn’t come up with any answers, just more questions to ponder while waiting for data.

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And now for the exciting conclusion of “Theory Grad Student Q&A!”

Are you at CERN?

I should probably clarify that unlike many of my experimental colleagues here, I am unfortunately not based at CERN and instead spend most of my time with the theory group at the Laboratory for Elementary Particle Physics in Ithaca, NY. And yes, for the record, I’m jealous of all the experimental grad students who get to spend so much time in Geneva! (Especially during the freezing upstate New York winters.)

Even in the age of Skype and teleconferencing, it is important for experimentalists to be able to spend time at CERN where all the action is happening. This is because when you’re part of a group of experts on a particular facet of a large experiment like ATLAS or CMS, you need to be able to communicate efficiently with each other and with the rest of the large experimental team without worrying about different time zones.

Theorists, on the other hand, work in much smaller collaborations that are often with other researchers at their home institutions. Inter-institutional collaborations are also common, but in this case it’s much easier to just visit each others’ universities. There is a very vibrant theoretical physics group at CERN, but it is less common for American theory grad students to visit.

What does a theoretical physicist actually do?

Theoretical physics research is a mixture of pen-and-paper work, running numerical simulations, checking ideas against experimental constraints, and –most importantly– communicating with others. While theoretical physicists are often imagined by popular media to follow the mold of the patent office clerk working in isolation, the real way theoretical research moves forward is through collaborative work between physicists. Such collaboration allows people to bounce ideas off of each other, take complimentary approaches to a problem, check work, and generate new ideas.

This last point is perhaps the key to outstanding theoretical work: creativity plays a big role in research, where questions are open ended and don’t necessarily have straightforward answers. One needs to ‘think outside of the box’ to find new ways of describing old problems and then use them to find novel solutions. (This, by the way, is the key to all good science: theoretical or experimental.) Sometimes this means thinking in terms of higher dimensions, or maybe understanding a mathematical formula in terms of physical processes (or vice versa), or applying a clever trick from a previously solved problem.

In a sentence one could summarize a theorist’s daily life as the iterated process of (1) thinking about problems and (2) discussing their thoughts with others.

What’s it like being a theory grad student?

As a theory graduate student I spend a lot of time learning many of the tools in my field and slowly trying to develop my own set of tools to address parts of the ‘big questions’ of our time. This involves coursework early in graduate school but moves on to more individually motivated work reading papers as one catches up to the forefront of research.

One big activity that theory grad students often take part in are journal clubs. These are informal discussions where students present current research papers to help keep the entire group abreast of what’s going on in the field. This is also doubles as a chance for participants to practice their scientific communication skills with one another.

Because of the nature of our work, theory students aren’t tied down to a particular lab and many of us have favorite places to go when crunching through a tedious calculation. I’m especially partial to coffee shops, though I know of others who prefer to get their work done at bars or even sitting outside in the park.

Another aspect of grad student life that is especially true for theorists is recreation. Since most of our work can be packed away in a folder or on a USB stick, it’s easy to ‘bring your work home’ and end up working all the time. In order to stay balanced, it’s important to have recreational activities (preferably those that get you outside!) to rest your brain.

Theorists vs. Experimentalists?

If softball is any indication, then experimentalists seem to have the upper hand. For years the SLAC National Laboratory held an annual softball game pitting representatives of the experimental group against their theory counterparts. Historically victories for the theory team were few and far between. In fact, as an undergrad one of my proudest moments was learning that my adviser was a member of one of the few victorious theory softball teams. Unfortunately with SLAC’s recent restructuring and focus on a broader scientific program, the “theory versus experiment” matchup also was restructured into an “accelerator versus research” duel. The ‘research’ team is composed of theorists and experimentalists from particle, astroparticle, material, and laser physics while the ‘accelerator’ team is made up of scientists and engineers working on present and future accelerators.

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CERN has issued a press release on the progress of LHC repairs.  Most importantly, it says:

Director General Rolf Heuer confirmed that the Large Hadron Collider (LHC) remains on schedule for a restart this autumn, albeit about 2-3 weeks later than originally foreseen.

So when will we have collisions?  The release goes on to say that further tests are needed:

to determine the start-up date and initial operating energy of the LHC

The center-of-mass energy of collisions will be between 8 and 10 TeV.  The higher the energy that the LHC can achieve, the more new physics we will be able to do next year.  The center-of-mass energy goal of the LHC is 14 TeV.

Update: For those interested in more information, Symmetry Breaking has posted links to reports and presentations by two external committees that reviewed the systems put into place to prevent future incidents.

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People submitted a lot of good ideas in response to my post about the talking Elmo book.  Here is what I think the answer is, on the basis of some tests we did in the department.

One colleague studied the book for a little while, and said, “It has to be either optical, magnetic or ferroelectric.”  Ferroelectric hadn’t occurred to me, but magnetic was my first thought.  Moving that pen along that arrow seemed to me like swiping a credit card.  But the problem with that, it seemed to me, was that magnets have only north and south poles (last I checked), while on any given page, I could get four different responses out of the pen (one of which was null).  Also working against that idea was that you couldn’t use the pen on the back side of a page to get the same effect on the front side.  You can hold pages up to the light and see through them, so there didn’t seem to be anything there that might shield a magnetic field from going through the page.

Optical thus seemed like the right choice, but whatever was happening wasn’t in the visible spectrum, as different colors could give the same effect.  Another colleague who was playing with the book got a little lucky when he (by accident, as far as I could tell), covered a portion of the end of the pen with a piece of paper, and got a different response from when it was uncovered.  Perhaps there was a sensor inside that responded to different amount of light that were reflected into it.

We went to his lab and examined the pen with an infrared viewer.  Sure enough, when the end of the pen is pressed down, it emits light in the infrared.  This is not my line of work, but I’m told that IR LED’s are quite inexpensive, so that makes them a good candidate part of the pen.  We then examined the book under the microscope.  My eyes aren’t really that great for these things, but my colleague insisted that he saw micropatterns that were different enough in different regions of the page such that they might have different reflectivities.  More convincing was that he examined the patterns on two different pages, and predicted which pairs of colors on each page would gave the same response.  He got them all right, which could have been luck, but probably wasn’t.  And that pretty much sealed it for me.

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