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Archive for June, 2009

Field trip

Tuesday, June 30th, 2009

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!



Tuesday, June 30th, 2009

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!


Somehow these are very busy times: I am still teaching, I have to prepare for a meeting next week at KEK in Japan, and many other deadlines are coming up. Among them are deadlines for conferences: If you want to present your research at a conference, you usually have to submit an abstract: A short description of your work and of what you are planning to present. Based on that, the conference organizers decide who will be invited for a presentation. So those typically 100 to 200 word long texts are quite important. They have to read well, have to arouse the interest of potential listeners, and have to convince the conference organizers that the research is well founded and mature. It is often not easy to fit all this into a short text.

At the moment, I am working on several abstracts, not just for presentations I want to make, but also for others in my research group and for the collaboration in general. And whenever I have to write one of these, I think with some jealousy about the text on the back cover of bestselling novels. Of course, these serve more or less the same purpose: They have to be exciting, and have to get you to buy the book. Very often, those texts are even shorter than the typical physics abstract. After all, they have to get their message across to people browsing through a bookstore, so they really have to catch the attention with the fewest possible words. And at least in a few cases, this works extremely well: Some of these little texts are really nicely crafted pieces of art. That always makes me whish to be able to write abstracts like that. But I guess those back cover texts are written by real pros, and a lot of time (presumably much more than I can put into an abstract) goes into it. Plus, they’ve got the advantage that they don’t have to convey hard facts in addition to the excitement about the text they are advertizing… so it is not quite like physics after all.


Travelling Europe

Monday, June 29th, 2009

As a particle physicist in Europe, I will be travelling back and forth to CERN a lot. This blog is brought to you by Easyjet, by enriching my schedule with tree hours of delay in the wonderful “shopping lounge” of the M gate at Schiphol. It is getting so boring that my neighbor actually is trying to read over my shoulder right now. I don’t think he reads English, because he doesn’t stop as I write this.

There is a guy sitting in the lounge with a lap top with a CERN sticker on it. That is extremely nerdy; I want that too! His sitting in the lounge all of a sudden seems a lot less like a lost for time then mine. He is probably doing something outragingly intelligent right now. I need a sticker to look that smart as well.

Next time, when you are at Schiphol, look out for the blond girl with a lap top with a CERN sticker on it. I will be in course of changing my Facebook profile..


My research [Part 1]

Sunday, June 28th, 2009

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.”


What We Might Find

Sunday, June 28th, 2009

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!


Last weekend I was in Berlin to go to a concert of Ben Harper. On the way to the little hall where he was performing, we walked through a very interesting area in Friedrichshain (in the East of Berlin) where many artists rented old buildings or little halls to have enough space to work. And by chance we actually bumped into a friend of ours, Thomas Stüssi, who is an artist.

This reminded me of a real cool performing art he and his friends did two years ago. The artist collective FallerMiethStuessiWeck (FMSW) decided to give the zero point of the earth a physical fastening. The zero point is the point where the equator meets the prime meridian. It is the position N0o00’000’’E0o00’000’’ respectively S0o00’000’’W0o00’000’, and can be found in the Golf of Guinea 600km south of the Ghana coast.

The first idea they followed was a special buoy, but as the sea is at the zero point about 5000 m deep, this would have been rather difficult. So FSWM planned to bring a sphere “filled” with a vacuum to the zero point – an objet d’art representing the “nothing”. They produced the sphere out of two half stainless steel spheres (25 cm diameter, wall thickness 3cm) and found a company who welded the two halves together inside a vacuum chamber. With this procedure a low pressure of 0,000001 bar could be reached.

Now they wanted to bring this sphere to the zero point, but just taking the plane to Ghana would have been too simple. They saw the journey also as some kind of pilgrimage to the centre of the cartographic world, a slow approach was important. The car transporter ship Grande Argentina from Hamburg to Ghana was found, a journey of 6 weeks from northern Europe to Tema, the biggest harbor in Ghana, only 25km from Accra. On the journey they passed Tilbury, Antwerpen, Dacca, Benin and Lagos.

May 8th 2007 - FMSW leaving Hamburg with a car transporter ship.

May 8th 2007 - FMSW leaving Hamburg with a car transporter ship.

Now the most difficult part of the journey started. They had to find a small, but not too small ship, to get to the zero point. Finding such a vessel in Ghana was more difficult than anticipated, but in the end, shortly before their money and time ended, they found the perfect ship. With an unplanned accompany of a priest the 600 km journey to the final destination through rough sea was the the most adventurous part. In vicinity of the zero point they took a rubber dinghy to find the perfect point. Using a GPS it took about one hour to get as close as they could and then let the sphere go. The sphere reached the ground probably something like 20 minutes later. They also collected 50 l of “zero water” and of course took a plunge into the zero point.

Now the sphere will lie in the darkness 5000 m on the ground for ever and all of them will always think of the sphere when they look at a map of the world.

[A complete report on the performing art can be found (in German) here. ]


This morning, Secretary of Energy Dr. Steven Chu spoke at SLAC. At a national scale, Secretary Chu is notable since he is a Nobel-prize winning physicist. Locally, he is well known since he was a professor at Stanford (and Physics Department Head) and was most recently the director of the Lawrence Berkeley National Laboratory, which is geographically close to SLAC.

I expected his talk to focus on SLAC and the relationship between the DOE and the National Labs. He began by discussing how the DOE has supported the work of 88 Nobel Laureates and assuring us that basic science research was a priority for him. While the DOE may be often seen as involved in nuclear weapons and power or energy research, to many of us it is the main funder of basic science work. It is refreshing to know that the head of DOE shares that idea.

The majority of his talk focused on climate change and what we can do to reduce emissions. This is an issue he worked on before he was head of the DOE and he is now in an excellent position to make progress. It was very useful to see climate change discussed in terms of data, plots, and 90% confidence intervals, rather than the usual ideological arguments. I can see how certain pundits twist the data – there was one prediction in the early 90’s regarding a certain climate change parameter that did not match the measurements taken since. While one can say, “Wow, that data is outside of the 90% confidence error bars so climate scientists really don’t know what they are doing” – it is important to realize that the prediction underestimated the change. Likewise, he showed that there is a carbon cycle in the earth’s crust that has been mapped out for the past 800,000 years. Yes it is a cycle, but the current value is outside of the amplitude of this cycle and it is predicted to get much, much worse. The earth has seen temperature changes of 6 degrees before, but those changes occurred over thousands of years, so that adaptation was possible. We’re looking at a change of 6 degrees of 100 years – adaptation does not happen that quickly. These are all details that get lost in normal news coverage of the climate change “controversy”. Perhaps we don’t know all of the details yet, but the data still makes it look like the world is going to end.

His talk was not just doom and gloom – it was a challenge for us to apply our “intellectual horsepower” to the problems at hand. He discussed how a predicted fertilizer shortage in the early 1900’s was averted through the creation of artificial fertilizer. Predictions in the 1960’s that the world would run out of food were wrong because of new grain hybrids created to grow more efficiently. If the world doesn’t end, it won’t be because our data is wrong. It will be due to the scientific achievements of the next few decades.

He ended with a brilliant quote from Martin Luther King, Jr:

We are now faced with the fact, my friends, that tomorrow is today. We are confronted with the fierce urgency of now. In this unfolding conundrum of life and history, there is such a thing as being too late.

I thought about summarizing the numbers and data that he presented, but it is too extensive for me to do it justice. Instead, I will point you to the Technical Summary of the Intergovernmental Panel on Climate Change. It is 74 pages of data and plots regarding patterns in the ice, atmosphere, temperature, and precipitation of our planet. As a scientist, I look and see that our climate is changing. Secretary Chu’s response to those who question the ideas of climate change was “People are entitled to their own opinions, but they are not entitled to their own facts.”


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!


Culture differences

Friday, June 26th, 2009

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)….