News from ICHEP


This past week has seen a thousand physicists meet at Paris for the International Conference on High Energy Physics (ICHEP), widely regarded as the most prestigious high energy physics conference in the world. After gathering millions of collisions in data, the biggest particle physics experiments in the world presented their results. For ATLAS this is a very exciting time, as we compete with another experiment called CMS, and the competition is fierce. ICHEP would be the first chance to compare how ATLAS and CMS are performing and to see who has the leading edge in the field. At the same time many other experiments present other fascinating results from different fields. Old colleagues cross paths, rumors circulate, and at every water cooler in labs across the world, people are discussing the latest results. Even though ICHEP is hundreds of miles away from CERN, there is no escaping its presence. For weeks people have been working hard to get their results ready, often working up to the last possible moment (as more than 90% of our data were collected less than month before ICHEP!) So when the results are presented there is both a collective sigh of relief, and an intense curiosity.

ATLAS and CMS presented their findings in two back-to-back talks, giving a perfect opportunity to see how the machines are performing. The first news is good for ATLAS- our dataset is already 10% larger than the CMS dataset! Steve and I work on the triggering system of ATLAS, which is the first step in the acquisition of data, so we are very happy that the data acquisition efficiency is high and reliable!

The ATLAS di-lepton spectrum. [1]

The CMS di-lepton spectrum. [2]

When a new experiment takes its first data it is vital that it is calibrated correctly. It’s good practice to verify our results using phenomena we already understand well. So it’s no surprise that both ATLAS and CMS have studies of di-lepton spectroscopy. This is a simple kind of study where physicists identify two oppositely charged muons, combine their energies and momenta, and find their invariant mass. The picture we get comes from two main processes. We expect to see a large, smooth smeared out distribution of background where we have taken to unrelated muons and generated nothing of interest. On top of this background we expect to see a rich series of peaks of known mass, and if we don’t see this, then something must have gone wrong! When we compare what ATLAS and CMS present, the results agree perfectly, with each other and with our past experience of particle physics. Both experiments measure up when it comes to reproducing the basic spectroscopy of the standard model!

A leptonic W decay at ATLAS. [1]

After demonstrating that the machines are giving precise and reliable results it’s time to see what new and exciting events have been seen. Since we are in a new energy regime there is a great deal of prestige in finding very high energy events. Perhaps one of the most dramatic events we can hope to see is the leptonic decay of a W boson. In this kind of interaction the W boson (a particle about as heavy as a Krypton atom) decays into an lepton, which takes away half the energy, and a neutrino which takes away the other half of the energy. The lepton manages to deposit nearly all of this energy in the detector, giving a huge peak on one side, but the neutrino keeps all of its energy. The result is that we see a very asymmetric event, with lots of energy on one side of the detector, and almost no energy on the other. ATLAS showed the event display for one of these events, and it looks amazing! It is as if someone fired a torpedo in the side of the detector.

Very energetic jets at ATLAS. [1]

In addition to muons and electron we also search for jets. Jets are collections of particles that all came from the same point. They form a messy cone of tracks which can be hard to spot when they have low energy. But when they have very high energy they stand out and they look impressive. Jets are a great place to look for new physics, especially in new energy ranges. Both ATLAS and CMS presented plots of some very high energy jets. While these jets do not show any evidence of new physics, they do show that both experiments are capable of using jets to make new discoveries. CMS’s best pair of jets has a mass of 2.13TeVc-2, whereas ATLAS’s best pair of jets has a mass of 2.55TeVc-2. We shouldn’t read too much into this; our higher mass is probably just the luck of the draw. What is clear is that both experiment have what it takes to search TeV scale physics, and that is what is going to shape the discoveries of the next decade. With many theories predicting particles with masses in this range it is vital that we can recreate jets of this energy.

ATLAS’s first top quark? [1]

Putting all of this work together, it is possible to reconstruct the top quark, the heaviest particle in the standard model, and one of the most difficult to identify. The datasets are small right now, so finding a top quark candidate is hard work. In spite of this ATLAS has seen some tentative glimpses that suggest we have already seen a top quark! Real top quarks have previously only been seen at Fermilab in North America. Seeing the first top quark on a different continent is a breathtaking experience. As the energies get higher and beam intensities increase we’ll see these top quarks more and more often, until eventually they will become an indispensible tool in the search for new particles.

An ‘X-ray’ photograph of the ATLAS hardware. [1]

An ‘X-ray’ photograph of the CMS hardware. [2]

Taking a step back from the dizzying heights of the top quark, it is worth noting some of the other tools at our disposal. Since the detector itself is made of matter we expect to be able to see interactions of the detector material with particles produced in beam collisions. This is like taking a huge X-ray photograph of the machine, giving beautiful images of the machinery. This method has actually been used on other experiments to search for new physics. For example, the BaBar experiment diverted one of the beams for a short time so that it hit parts of the machinery head on. By studying the remnants of these collisions, physicists were able to search for exotic particles, and for a short but fascinating period of time BaBar became a fixed target experiment. When there are no colliding beams the detectors can still take data. Muons are constantly bombarding every square meter of the Earth’s surface, and many of them penetrate as far as the detectors. These cosmic rays pass through the detector and give us a good opportunity to calibrate our machines. Both ATLAS and CMS have been diligently recording these events, with their peculiar signatures. After staring at some many busy event displays there is no denying that cosmic rays looks slightly eerie with their warped, lonely paths.

A cosmis ray passing through the CMS detector. [2]

After taking a look over what has been presented it seems that both ATLAS and CMS are working hard to get as much as they can from the datasets. In some areas ATLAS has the lead, and in some areas CMS are doing better, but both experiments are full of dedicated and intelligent physicists who are doing all they can to remain competitive. There is little respite, as there are always more conferences looming, there are always data taking shifts that require attention and with both experiments at the same site it is impossible to forget each other’s presence.

On a more personal note ICHEP has been something of a milestone for me, as the work that made the bulk of my PhD thesis was presented. In recent decades charm physics has always been less prominent than other areas, so my analysis was presented at half a talk in a less fashionable parallel session. It was probably seen by very few people, but for those who are interested it marks an important measurement that has confused experts in quantum chromodynamics for years. Having this work made public is indicative that my career is moving on and that my focus is shifting to less explored areas of research.

ICHEP will return in 2012 with more results and new discoveries. For now, the difficult part is done. Thousands of physicists from two huge, world class experiments have worked hard to demonstrate that the machines are operating well, and that with little data they can reproduce the results they need to convince the world that they are ready for the challenges of the coming years. If anybody still needs convincing that the LHC is a worthwhile venture then they have all the information they need to make their own minds up. We are ready for anything that comes our way, and the rest is up to what nature decides to throw our way.

You can find out more information about ICHEP at the ICHEP 2010 website. Images were taken from the following two talks:

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