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Archive for June, 2009

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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Hello from Portugal

Thursday, June 25th, 2009

Adam in Portugal

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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Allow me to introduce myself

Thursday, June 25th, 2009

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).

Our group of CERN tennis players.
The CERN tennis team with our silver medals. Stefano, Pier-Paolo, me, Bettina, Bernardo, and Christoph

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

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Airport 2009

Wednesday, June 24th, 2009

And now I am sitting at the Hamburg airport on the way to Geneva and wait for a plane to take me to  Switzerland. I have to join some more meetings at CERN, but so far there is no plane and we will be delayed by an minimum of one hour. But this is a modern world and I bought an hour of wifi access. And finally get around to continue my report on the setup of the beam telescope at the CERN test beam.

Ok, it is days later now since I started the blog and I did not report on the progress. But this does not mean that the beam never started but quite to contrary:
While I was driving back from Geneva to Hamburg (1057km) my colleagues actually collected 1.6 million events over the weekend. And since then another few million were added. Our little experiment is rather grown up in many ways. We can take data remotely, many little gizmo jobs were written to automatically copy the data to the GRID* and then only little work is needed to analyze the data. Within hours after the first data arrived, my team was already discussing the results.
Of course, as always, there is room for improvement. The telescope is not exactly sitting were it should be sitting. We have currently only a rather small sensor of roughly one square centimenter. If you put six of them behind each other with a minimum distance of 10cm, you have to be rather parallel to the beam not to decrease the actual active window. So here we have to be very precisely. But we do not have to rotate the whole telescope mechanics in order to do the needed rotation but the mechanics foresees already a tool to adjust for this little angles. We just have to rotate a little knob and then little adjustment can be done. The only problem is, to remember in which direction to turn the knob…..

(*the GRID consists of many computers to solve a single problem at the same time. These days it is widely used in particle physics and a real nice tool to analyse large amounts of data very fast … for more information look here)

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A normal day

Wednesday, June 24th, 2009

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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TASI 2009

Tuesday, June 23rd, 2009

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.

TASI 2009

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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Physics and Celebrity

Tuesday, June 23rd, 2009

Susan Boyle has risen to fame after being mocked for her interest in fame and success. So here I go – I want to be famous. She wanted to be as successful at Elaine Paige. Who would I model my success after, if I were given the chance?

Neil DeGrasse Tyson

Neil DeGrasse Tyson

Just as Susan Boyle did not want to be as successful as say, Madonna or Britney Spears, the physicists with the most air time are not my biggest role models. Stephen Hawking may have been featured on The Simpsons, but his fame is more about him personally, rather than his science. I think Neil deGrasse Tyson is a very successful physics “celebrity” – he’s been on The Colbert Report 5 times – but his appearances are about science as much as they are about him. I don’t necessarily want to be on The Daily Show, or late night television, but being called on as an expert for a Nova show would be pretty awesome. I certainly wouldn’t want to go onto a reality show – as a MIT grad, I’d be stuck in a geek role.

My true dream (and if only there was a reality show for this) would be to be a science consultant for tv or movies. I certainly don’t want to run away to Hollywood, but I’d love to be called up someday to make sure that the background for a “science” shot looks more like an actual lab than the back of a Spencer Gift’s store. I worked at a store called Glow!, where I saw many plasma lamps and plates (pictured below). I’ve sadly even seen them in “labs” in Stargate SG-1 but have never seen one actually being used for science. So Hollywood – feel free to give me a call. My business cards are already made. And if you’d rather have some spiffy looking glowing things in the background – rather than actual scientific devices – I can hook you up with that as well.

Plasma Plate

Plasma Plate - fun decoration, but not in a scientific lab!

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Osaka.

Tuesday, June 23rd, 2009

Today is the last day of my three-day lecture at Osaka university. Since I was grown up in Osaka, this city has a lot of things which remind me of many many things. In particular, I am very happy to give lectures at my home town, in my Osaka dialect of Japanese.

It is obvious that when you speak in English (I am supposing that you are not native English speaker), your “nature” should be different from that when you speak in your native language. Your character depends on the language you use. This is in particular the case for me. When I talk in English, my communication “mode” changes. What I like to tell you is that this also applies to the dialogs. In fact, When I talk in Osaka dialect of Japanese, the way I express myself actually changes, compared to myself talking in so-called “standard Japanese” which I use in Tokyo. When I was young, my mother taught me how to talk in the standard Japanese, since she was not a person born in Osaka. Since Osaka dialect was thought to be a “dirty” dialect (This word “dirty” simply means that it is very different from the standard Japanese accent), my mother liked to tell us that there is an alternative in speaking in Japanese, seriously….. Of course, this standard Japanese can be available on TV. But it is in fact important to practive communication in the standard Japanese somehow. In this way, I tell my friends that I am a “bilingual.” But I am not sure how accurate my standard Japanese is. 

In any case, I could deliver my lectures in Osaka dialect, and I enjoyed it a lot. People in Osaka university, including graduate students, are very active and I enjoyed discussions with them. Hopefully they have got some sense of D-branes and their solitonic expressions, which are the main subject of the lectures.

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On Waiting for Data

Tuesday, June 23rd, 2009

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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The Muellberg, with the wind turbine on top, in the distance.

The Muellberg, with the wind turbine on top, in the distance.

Today the weather is not really good enough for a longer trip. But, despite all the things I still have to do (preparing my lecture for tomorrow, working on a paper, coming up with clever ideas for the mechanics for the Belle-II pixel detector… the list is seemingly endless!) , sitting in front of my laptop all of Sunday is not all that attractive. So my wife and me went for a bike ride around were we live, in the north of Munich. Just last weekend, we have upgraded our bikes with click-in pedals. Emboldened, we attacked our local version of L’Alpe d’Huez, the “Froettmaninger Muellberg”. Comparing this partially wooded former waste dump with a big wind turbine on top to one of the most famous Tour de France stages is admittedly a bit of a stretch, but to someone who is usually biking on flat land, the 75 m elevation gain over about 1 kilometer can be slightly intimidating. And, when you do the math, the average grade of 7.5% is not that far below the 7.9% of this Tour de France climb. Of course, our private alpine stage here is only a 7% version of the real thing in terms of elevation gain.

It is much steeper from up close: going to the top!

It is much steeper from up close: going to the top!

Once things get steep, these new click-ins really come through: Mechanics at its best! … Physics really is everywhere 😉 . The pedaling is a much smoother affair, since you pull up with one leg and push down with the other. That way, going up steep grades gets much easier. Of course, energy conservation still applies, so it is strenuous as hell, especially with all that technology that tempts you to go faster than you should.

From the top of the “Muellberg”, there is a nice view over the Allianz Arena, the soccer stadium of the local club FC Bayern Muenchen, and location of the opening match of the 2006 world cup, and over Munich (no picture, since the batteries ran out). In good weather, you can also see the Alps.

Well, now that I got some exercise, I can go back to my computer to wrap up my lecture for tomorrow.

The view from the top: The Allianz Arena, Munichs soccer stadium.

The view from the top: The Allianz Arena, Munichs soccer stadium.

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