A few weeks ago I had a trip planned to CERN. An unexpected root canal the day before my flight forced me to scrap that trip (to be accurate, it was half a root canal – the second half had to wait until two weeks after the first procedure). I had originally planned to use SMU’s Spring Break to refresh and recoup. Alas, that’s a non-starter. Instead, I rescheduled my trip to CERN for the break so that I could get some much needed time (and face-time) at CERN. Here are my notes, photos, and other experiences from the trip.

Day One

Me, on day one, trying to stay awake in the SMU CERN office.

(Monday, March 14) Day one is always the hardest. You get off a plane, maybe having grabbed 4 hours of sleep. You feel like you’re in a dream. You need to stay awake but every synapse in your brain is telling you to sleep. You dare not engage in any intensive intellectual tasks (although everybody wants you to). You try to stay awake as long as you can, in the hopes that when you do put pillow to head you can get a single, solid night of sleep. Night two will be worse.

I kept myself busy on Day One by doing a series of small tasks. I’ve been desperately trying to pull from the GRID (the LHC’s globally distributed computing system) some files I generated days ago. All I want to do is answer a simple question, but doing that requires the files, and for some inexplicable reason the transfer freezes at random times and must be retried manually. I edited some papers, including one I am writing with a couple of SMU students and some that are due for publication by other authors in ATLAS and BaBar. I chatted with a graduate student about lobbying Congress on behalf of physics, how to do it, and how to be effective.

Mostly, I tried to stay awake.

A new data sample: 2011

One of the first proton-proton collisions seen by ATLAS in 2011.

(Monday, March 14) From the time I left Dallas, to the time I landed in Washington D.C., ATLAS had recorded its first proton-proton collisions of 2011. While my trip to CERN was not originally planned for the Spring Break, this hads turned out to be quite serendipitous! Not only is the lab abuzz with excitement about the new data (we expect to accumulate the same size data set as in 2010 in a matter of weeks, and then move on to accumulate a HUGE total sample for 2011), but if some of the projects I’m involved in go haywire at the beginning of data-taking, I’ll actually be here to help fix them.

Approval: my first ATLAS paper

(Wedesday, March 16) Less than a year ago, I began my formal work toward becoming an ATLAS author. What does that mean? To obtain authorship on a large collaboration, you must prove your worth not with physics analysis (that’s the reward) but with service to the collaboration. Thus, to initiate the process of becoming an author your must begin a project that enhances the functioning of the experiment. We started that just under a year ago, and since then have also spent some of our time working on a physics analysis. Today (Wednesday) is the approval talk for this work, with the goal of taking it to a major international winter conference. This work is but the but the first step toward a real publication for summer. For me, this work represents my first, real contribution to an ATLAS physics analysis. It’s a small step, but I’m proud of what we all accomplished together. Today, we find out if the collaboration deems it worthy of going public.

Update: the meeting was stressful, but exhilarating. One of the ATLAS Physics Coordinators was present, as was the chair of our paper’s Editorial Board, members of the community who offered comments on the paper, and my fellow analysts. Our analysis was approved, with the caveat that of course we respond to all written comments and those offered during the meeting. I even had some physics fun during the meeting. At one point, ??a question was asked that I could answer by quickly pulling up some numbers and running some analysis code. Times like this make my field a tremendous amount of fun; you feel like a student again, running numbers and answering questions. Everything is new. It’s great fun!

SMU Undergraduates at CERN: Touring CERN on Spring Break

(Friday, March 18) On my last day at CERN, one of my former students from introductory physics (now an SMU graduate living abroad) and her best friend (a new physics major at SMU) arrived at CERN for a tour of the laboratory. Holly, our current major, was visiting her friend Trang over the break. They had asked if they could come and take a tour of CERN, and I was happy to host them. I introduced them to the SMU group members present at CERN, we grabbed a bite to eat, and then we explored the Microcosm and the Globe, the two big visitor attractions at CERN. It was pretty damned exciting to hear them both start lecturing each other about subatomic particles and how they behave, and then they got excited when I showed them the spark chamber which shows muons raining down on us. We talked about detectors, and relativity, and all the things that the LHC is hoping to understand: the origin of mass, the face of new physics at the TeV energy scale, the asymmetry of matter and antimatter in our universe, etc.

SMU undergraduate and physics major Holly Howard (left), and SMU graduate Trang Ho (right), stand in front of the ATLAS Control Room just as the LHC delivers the first proton-proton collisions of the day on March 18, 2011.

One of the most exciting moments of the tour happened right after we walked into the visitor area outside the ATLAS Control Room. I noted that the LHC seemed to have a good fill going – that is, stable beams capable of producing real proton-proton collisions for ATLAS data taking. My fellow Atlanteans were eagerly watching the big screens in the control room beyond the viewing gallery. Suddenly, LHC declared stable beams and ATLAS began taking data. Huge splats of energy containing hundreds of particles appeared on te big event display. I got very excited, and pointed to the screen and said, “Look, those are proton-proton collisions!”Then they got very excited, because I was excited, and they started trying to see if they could spot muons and electrons and photons based on what I had told them earlier about detecting these particles. It was a truly exciting moment, and I was so happy to be able to share it with them.

On display: the heart of CERN computing.

CERN alone cannot house all of the computer power needed to analyze LHC data. They DO house a tremendous amount of computing power, however. Some of that is on display at CERN, in the heart of their computing center. During my January trip to CERN, I visited the facility with Aidan. I shot the panorama of the center and recorded the sound below. This is the sound of thousands of physicists analyzing data, exchanging e-mail, putting information on the Web, and generally getting physics done.

(Can’t play the audio in the browser? Download the audio!)

Author’s Note: I started writing this weeks ago – mid-January – when I was actually on this trip to CERN. Only now have I found the time to polish it up and post it. Welcome to . . . LOST TALES!

Usually when I collaborate with my ATLAS colleagues at SMU, I do it over an EVO or Skype connection. This week, however, I had the pleasure of a trip to CERN to work directly with them and to have meetings with a number of ATLAS collaborators from outside of SMU. It was intentionally a busy week, a week where lack of sleep was a way of life. With the semester starting in just a few days and a sudden change in my own schedule imminent, it was imperative to spend at least a week at CERN

Snow begins to rain down on DFW shortly before my scheduled departure to CERN.

A week at CERN is not really a week. I left on a Sunday – a VERY SNOWY Sunday, I might add – and arrived on a Monday morning. I opted to not sleep on the flight, since over time I have decided that sleeping or not sleeping makes no difference; I feel awful when I land. An old colleague of mine from the BaBar Experiment, who for a few years now has been working on ATLAS, was just two rows up from me on the plane. It was very pleasant to have company as I stepped off the jet and headed to customs.

Sometime during the trip from the airplane to where I met my ride from the airport, I either lost one of my credit cards or it was stolen. A day later, I was forced to cancel it when it was used in Geneva to buy souvenirs and food at a cafe. That said, this was the only unfortunate part of the trip; the time at CERN has been an intense affair. It began the first morning with discussions of projects that might bear fruit for the summer; it moved onto concentrated work to update an internal ATLAS research document with the first draft of some research we’ve been doing since the summer; it then became a series of discussions with my ATLAS colleagues about all kinds of things, from physics efforts to the future of the experiment. Sleep was lost, but physics got done.

Jet lag is not your friend. I repeat: jet lag is NOT your friend. But when you have to change seven timezones to do your work, you learn to live with it. There will be at least a few trips during the semester from Dallas to CERN, because research has to get done and so much can be accomplished face-to-face. In between, I’ll always have Skype.

There are many sides to being a physicist, some of them are good, and some bad. Recently I’ve had the chance to experience the three best reasons to be a research physicist, and they are discovery, collaboration and travel. Recently, I traveled to Beijing to present my work on the decays of the D_s meson at the Charm 2010 conference.

Presenting my work at the conference.

This was an opportunity to show the physics community how I spent my PhD years and what discoveries I had made. The great part was that I got to make four different measurements, and have the thrill of discovery four times! The puzzle I was trying to solve was seemingly simple. The D_s meson should decay at a fixed rate, and theorists can predict this rate using advanced models. When I started this analysis in 2008 the theory and experiment disagreed significantly, which was a serious problem for physicists. Charm mesons decays are supposed to be clean and easy to predict and measure using quantum chromodynamics, and yet the predictions missed by four standard deviations. So I joined a team of physicists to measure the leptonic decays of the D_s meson to answer the question once and for all. The measurements I made were competitive, better than the world average at the item. They were consistent with what we had already seen, and they didn’t seem to solve the problem from the experimental side. Fortunately (or unfortunately, depending how you see these things) the theorists had improved their models and updated their predictions, and once again the theory and experiments matched. Having the chance to go to Charm 2010 not only allowed me to present my findings to the world, but it also allowed me to see what the other scientists had discovered. Everybody agreed that the results were impressive and that the hard work of the theorists was worthwhile, and that this curious journey probing the D_s mesons was at an end, at least for now.

What lifted my spirits most about the conference was the sense of collaboration that everybody felt. They had all come together because they care about charm physics, one of the least fashionable areas of particle physics, so they were all enthusiastic. The talks were fascinating, followed by curious questions and side discussions. The talks covered every aspect, from recent discoveries to long standing theoretical problems to new experiments which are currently being developed. This kind of collaboration, even in the face of competition, is something that isn’t seen in many other areas of work. We have physicists from dozens of countries, speaking dozens of languages, and from hundreds of universities, who come together to share their ideas and work. They disagree, and when they do their debates can be heated, but they only do this because they love their work, and deep down they all agree that no matter what an individual believes about a discovery, the data itself has the final word. Whenever I think about how physicists bond over their frustrations or joys I’m taken back to a meeting at Elba. Some politicians had joined the meeting for a day and gave a short speech about cooperation across borders, and how inspired he felt looking at the physicists, and how he wished this was also possible in the realm of politics. Hearing this, I realized that there is something fundamentally important about international collaboration. It’s not just useful for the progress of science, it opens up new opportunities for people and nations to work together. If it’s in the interests of the USA and Italy to let their scientists travel freely to share their work, then we can be sure that this is going to have far reaching consequences for relationships between these nations.

Visiting the Great Wall.

The real adventure began shortly afterwards, as I took this trip as a unique opportunity to journey across Russia via the trans-Mongolian express, something I’ve wanted to do ever since I first heard of the railway. The journey takes about a week, and passes through some of the most breathtaking scenery in the world, including Lake Baikal, the largest freshwater lake on Earth. The journey is rich in history, geology and culture, across 5,000 miles from Beijing to Moscow. Of course, there were great sights to be seen, including the Forbidden City in Beijing and the Kremlin in Moscow, as well as the gauge transformation at the Sino-Mongolian border. Taking time off from work like this is vital for research, as it gives us time to think things over at a slower pace. On the way back, I couldn’t help thinking about the B_c meson and what kinds of mysteries it might hold. It was also something of a watershed in my life, as it signified the end of my work on BaBar, and also the time when I needed to start settling down at CERN properly, after having spent months planing the trip and trying to balance out my time on the two experiments.

This trip has been a wonderful, once in a lifetime experience. It’s reminded me of why I enjoy pursuing physics in the first place, in spite of the frustrations and the mundane day-to-day work. I love the challenges, the mental acrobatics, the thrill of discovery, sharing that with others, and the opportunities to see the world. It’s true that we have to make sacrifices for physics, we also have great rewards. And so I am, shall we say, Charmed by the experience!

Every now and then physics experiments surprise us with something unexpected. Today, CMS, a competing experiment, announced such a result [1], a “bump” in their distributions. Bumps are very exciting in particle physics, because they are usually easy to identify, and they appear in unexpected places. A bump occurs where we expect to see a smooth, flat spectrum, but instead we see a peak that nobody predicted.

Roughly speaking, the detector is shaped like a cylinder, or a soda can. We describe the detector using two numbers. The first number, Φ. is the angle around the cylinder, and since the cylinder is symmetric around this angle, we expect to see no major bumps in any Φ distribution. The other number is η, called the pseudorapidity. It’s similar to the angle from the axis of the cylinder. Using these two numbers we can describe a region in the detector. If we take two regions in the detector, we can then define the difference in these two numbers, ΔΦ and Δη. These are the variables where the bump was seen!

It helps to have a nice picture in your head when thinking about these kinds of distributions, so if you have a cylinder to hand, such as a soda can, bottle or coffee mug, grab it and take a look at the shape. If not, you can take a look at the photos of some plastic cups that I used to help visualize the detector. You should see that if we have a very large value of ΔΦ between two regions, then these regions must be on the opposite side of the cylinder. If we have a very large value of Δη between two regions, then one must be near the top of the cylinder and one near the bottom. What CMS saw was two regions with a very small ΔΦ and a very large Δη. That means that they saw two regions that were on the same side of the cylinder, but one was near the top of the cylinder, and one was near the bottom. This is a very odd arrangement of regions.

Left: Definition of the Φ and η coordinates.
Center: Two regions separated by large ΔΦ and large Δη, which is what we expect.
Right: Two regions separated by small ΔΦ and large Δη, where the bump was seen.

In order to understand things better, we need to know some more details about how CMS reconstructed the events. The physicists looked for charged particles, known as tracks, and looked that the values of ΔΦ and Δη between pairs of tracks. Most of the tracks at the LHC come from jets, which are huge cones of tracks. The apex of the cone is at the center of the detector and cone points away from the center of the detector. This is easy to understand, as each track in the cone moves with a slightly different value of Φ and η, so they move away from each other as they travel along. This means that most of the track pairs come from these jets, and have very small values of ΔΦ and Δη. This huge peak was removed from the analysis, giving the physicists a chance to see the bump.

The plot showing the location of the bump. [1]

Taking a look at the plots presented by the analysts, it is obvious that there is definitely some effect there. What could be causing this? So far I have not seen any explanation of this bump in terms of new physics, or new particles. This seems a little odd, as bumps like this usually lead to detailed discussions over coffee, and people rush to the library to look for papers. This time, the results are being presented as they appear, and the auditorium is full. With no theoretical explanation, and an odd angular distribution, things seem a little odd to me.

History has taught us to be cautious of bumps like this. Some famous bumps include the J/ψ particle [2], which showed that the charm quark exists. That was a bump discovery that ended well. Unfortunately, another bump, which was thought to be the Υ particle, turned out to be fake [3]. Perhaps the most infamous bump was the pentaquark, which was seen in the mid 2000s, and later “confirmed” on several experiments [4]. As these experiments gathered more data, the bumps disappeared, and it seemed that several experiments managed to see the same bias in their reconstruction.

Competition in particle physics is fierce, so it is not unusual for resuts to be scrutinized. In fact, the results seen today have been known by CMS since ICHEP, but a whole series of additional tests have been performed since then. It is essential that physicists eliminate as many explanations as possible. Our detectors are arranged in a particular shape, and event rejection depends on the geometry of the detector. It’s very easy to unwittingly shape distributions with event selection, or to accidentally change the shape of a spectrum. The physicsts have checked and rechecked a huge number of effects, using everything they can think of. This does not mean that all effects have been taken into account, but it means that if there is a mundane effect which has generated the bump, it is a subtle one. It has escaped simulation and it has escaped reasonable skepticism for now. The next stage is to see what other experiments see. If ATLAS sees this effect, then there could be a physical process that we do not yet understand which is responsible for this effect. It is worth noting that this bump only appears for “high multiplicity” events, that is events with very large numbers of particles in the final state. This hints that this effect may be associated with a state of energy and mass known as the quark-gluon plasma, and several references have been made to heavy ion collider experiments which study this state. The bump could be more prominent on other experiments.

The most concerning and thought provoking feature for me is the value of ΔΦ. Suppose one particle is moving from the center of the detector, and then it decays into two others, which fly away from each other. If the momentum of each particle is drawn as a straight line, then they fit nicely on a plane. There seems to be no reason why this plane would prefer to line up so that they have similar values of Φ, which would give a small value of ΔΦ. Whatever process is responsible for this bump must explain this effect. It could be a very odd new physical process, or could simply be the result of the geometry of the detector.

Finally, the bump seems to be dependent on the transverse momentum of the tracks. In particular, the bump is most prominent when both tracks have momentum larger than 1 GeV^-1 and less than 3 GeVc^-1. A GeV^-1 is a unit of momentum that can be compared to mass, and it is roughly equal to the mass of a proton. This means that a proton traveling with a momentum of 1 GeV^-1 would share its energy equally between its rest energy (using E=mc²) and its kinetic energy. For the LHC this momentum is very small. The momentum of the incoming proton is roughly 1000 times larger than this, and it seems odd that the effect only applies to very low transverse momentum tracks.

These results will be discussed in much more detail over the coming weeks. It may be that the effect goes away, or that it is a hardware effect which means that other experiments will not see it. Either way, the experimentalists and theorists from all over the world will be discussing what has been seen. These kinds of discoveries are very exciting and they make working with physics research worthwhile. Based on past experience with other geometry based bumps (such as the pentaquarks), this will probably just be an artifact of the hardware and will not be seen elsewhere. Even so, there is not much which will fill out the main auditorium in a lab. It’s new, it’s exciting, and if it can’t be explained away it’s exactly the kind of discovery we all search for.

• [1] Long-Range Near-Side Angular Correlations in Proton-Proton Interactions in CMS.
• [2] J/ψ discovery.
• [3] Oops-Leon “discovery”.
• [4] Review of pentaquarks.

Just in case you needed evidence that blogging has more than just a communicative payoff, check out the August edition of “Symmetry Magazine.” It features three of our young physicists – Rozmin Daya, Tingting Cao, and Aidan Randle-Conde – showing off their creativity with innovative writing surfaces [1]. This article resulted from an earlier post in this blog! [2]

[1] http://www.symmetrymagazine.org/cms/?pid=1000827
[2] /smucern/files/2010/06/snapshot_backofthebanana_physi_1.html

Nicola Cabibbo [1]

The CERN flags flying at half mast in honor of Cabibbo.

Earlier this week, Nicola Cabibbo passed away. The flags at CERN were lowered to half mast. Around the world people were discussing this man’s life and the 2008 Nobel Prize. Cabibbo was one of the most influential physicists of the 20th century. His work has influenced, and continues to influence, all the areas of particle physics, unifying interactions between all the fundamental fermions, the building blocks of all the matter we see around us.

Cabibbo’s main contribution to modern particle physics was the discovery of flavor mixing, in 1963. At the time, we knew of three quarks. The lightest quarks were called up and down, and they are the building blocks of protons and neutrons. In 1947 a new particle called the kaon was discovered, which seemed to be very similar to a lighter particle, known as the pion. The decays of the kaon seemed to be more subtle and peculiar than those of the pion, so the quark associated with these particles was named the strange quark. The more physicists studied the kaons, the more complicated the situation became, and the best models still failed to described the observations.

Cabibbo’s stroke of genius was to suggest that quarks could mix. The strange quark and down quark have the same charge, the same coupling to the strong force, but different masses. In a paper less than two pages long, Cabibbo suggested that they down and strange quarks are in fact two other particles, the strong force eigenstates, manifesting in two different ways [3]. He described the quark mixing in terms of a rotation, where the down and strange quarks have a different preference for each strong force eigenstate. This implied that the mixing was the result of some deeper symmetry, so physicists should expect to see yet another quark, which would be the partner of the up quark. Cabibbo showed that by allowing the quark flavors to mix, he could explain the couplings of the down and strange quarks, while also predicting another quark to complete the symmetry. Over a decade later this fourth quark was discovered in three separate experiments.

A graphical representation of the CKM matrix [2].

However, this was just the beginning. Since Cabibbo’s 1963 paper his ideas have been taken and extended. Maskawa and Kobayashi took Cabibbo’s paper and added another pair of quarks, called the bottom quark and top quark. This may seem like an arbitrary extension of the model, but it predicts CP violation, the key to understanding why matter and antimatter behave differently. The theory was developed into a piece of mathematical machinery that describes the mixing of the quark states called the Cabibbo-Kobayashi-Maskawa matrix, or CKM matrix. So far the CKM matrix has passed all the experimental tests that have been performed, and offer a unique probe into new physics. Following the success of the CKM matrix, the same principle was applied to neutrinoes, in the Pontecorvo-Maki-Nakagawa-Sakata, or PMNS matrix. In addition, the mixing matrix has been used to describe several models of new physics, and flavor mixing is now seen as essential part of any new interaction.

Cabibbo’s contributions to particle physics have been far reaching. They took the confusion and frustration of the 1960s and turned them into a fertile theory that has stood the test of time, while explaining how both the strong and weak forces affect quarks. In 2008, Kobayashi and Maskawa were awarded the Nobel Prize for their contribution to the development of the CKM matrix. Some feel that Cabibbo was overlooked, and the discussions of the decision are heated. Cabibbo was later recognized with the award of the Dirac Medal, one of the most prestigious accolades in the field of physics. While opinion is divided on the Nobel Prize decision, nobody doubts that Cabibbo’s work was one of the most important breakthroughs of modern physics. It’s a sad week and although Cabibbo will be missed, his work continues to live on.

• [1] Wikipedia
• [2] UTFit
• [3] “Unitary Symmetry and Leptonic Decays”, N. Cabibbo (1963) Physical Review Letters 10:531-533