Evelyn Thomson

The next few years will be a very exciting time at the high energy frontier of experimental particle physics, with many interesting opportunities for undergraduate and graduate research! I work on precision measurements of top quark properties with the CDF experiment at the Tevatron Collider, which will continue to run through at least 2010 at Fermilab near Chicago. Over the next decade, I plan to search for new physics with the ATLAS experiment at the Large Hadron Collider, which will commence operation with a year-long run beginning in November 2009, at CERN near Geneva, Switzerland. In 2005, my research was recognised with an Outstanding Junior Investigator Award from the U.S. Department of Energy and in 2006 by a research fellowship from the Alfred P. Sloan Foundation.

Throughout my career, I have been fascinated by the mystery of mass - what is the missing piece that gives mass to the fundamental particles of the universe? I started by performing one of the first measurements of the W boson mass with the ALEPH experiment at CERN. The W boson carries the weak nuclear force, esssential for the nuclear fusion reactions that power stars, and was discovered in 1983 at CERN. The W boson acquires mass in the standard model of particle physics via the hypothetical, much-hunted but exceedingly elusive, Higgs boson. Interpretation of improved measurements of the W boson mass in the context of the standard model of particle physics leads to a better constraint on the unknown mass of the as-yet-undiscovered Higgs boson - and a better idea of where and how to search for it.

The top quark is by far the most massive of the sixteen known fundamental particles, and has approximately the same mass as a gold nucleus (which contains about 197 nucleons). Intrigued by the possibility that the unexplained large mass of the top quark could be due to effects from physics beyond the standard model, I decided to measure the production rate of top quarks at CDF since a deviation from theoretical predictions could indicate new physics in top quark production or decay. To do this, I developed a novel measurement of the cross section with an advanced multivariate technique. I also combine several complementary measurements to obtain a single best estimate of the production rate, as described in Fermilab Today's Result of the Week in October 2009. In order to search for the presence of particles beyond the standard model in top quark decay, postdoctoral researcher Dr. Aafke Kraan and I analyzed the angular distribution of the decay products of the top quark. Dr. Kraan won a Marie Curie Fellowship from the European Union and is now at INFN Pisa, Italy. From April 2004 to April 2006, I was co-leader of the CDF Top Quark Physics Group, which consisted of over 100 active researchers, including over fifty graduate students from universities in the U.S. and abroad.

The Higgs boson is by far the most elusive particle in the standard model, having evaded detection for over forty years. The Higgs boson is only missing piece in the standard model and spontaneously breaks the symmetry of the electroweak force. This means that the carriers of the weak nuclear force acquire mass via their interaction with the Higgs boson, while the photon, the carrier of the electromagnetic force, remains massless. Thus the forces appear to have very different strengths since the energy barrier to creating a massive W or Z boson is extremely large. The mass of the Higgs is not predicted by theory. My group's research contributes to the search in the region of possible Higgs mass between 115 and 130 GeV. The experimental signature of production of a Higgs in association with a W boson (WH), followed by Higgs decay to a pair of bottom quarks, is the best signature but it is not yet sensitive to the rare rate of Higgs production. Graduate student Justin Keung and I are currently studying improvements to b-jet identification to increase the selection efficiency for WH and thus the sensitivity of the search. We are extending the WH analysis techniques to try to observe the related WZ process to validate these techniques. Dr. Chris Neu and I have measured the production rate of the dominant background to the WH search, which is from W boson production with associated bottom quarks from the strong interaction. We find a factor of two higher rate for the background compared to theoretical predictions. Dr. Neu has advanced to an assistant professor at University of Virginia. Now, we are measuring the differential cross section for this background to see if the excess is in a particular kinematic region. Since the search for the Higgs boson relies on excellent modelling of the kinematics of this background, these measurements will help improve the robustness of the search.

In parallel in 2007, I joined the ATLAS experiment at CERN. With postdocs James Degenhardt and Sasa Fratina, and graduate students Dominick Olivito and Liz Hines, my group is working on the commissioning of the Transition Radiation Tracker (TRT) to prepare for first data. The TRT reconstructs the trajectory of charged particles and provides electron identification information. Each channel of the TRT is a straw cathode of 2 mm diameter and with a wire at the center. High energy charged particles ionise the gas inside the straw, the primary ionisation electrons drift towards the wire anode where they undergo avalanche multiplication in the strong electric field close to the wire, and produce a detectable signal of a few femto-Coulombs. For high energy electrons passing through the detector, a polymer foam in between the straws presents an obstacle course with different electromagnetic properties, causing the low mass electron to give off X-ray photons (transition radiation) that are then absorbed by the Xenon gas inside the straws. The detected signal on the wire from transition radiation is approximately 100 times larger than that from ionisation. Jim Degenhardt has developed data quality software to check for problems in the 350,000 channels of the Transition Radiation Tracker (TRT), and is the deputy run coordinator for the TRT in 2009-2010. Sasa Fratina leads the TRT software group, where analysis of data from cosmic ray muons passing through the detector and single beam events from September 2008 have been used to improve hardware and software calibrations. Dominick has worked extensively on commissioning the front-end and read-out electronics of the TRT, and is the on-call expert at CERN. Liz is investigating the usage of TRT information in electron identification and in the inner detector trigger.

In the next decade, my group is looking forward to exploring the new energy frontier to be opened up by the CERN LHC. Research on ATLAS will require a great deal of creativity in a range of areas, from novel approaches in searches for signatures of new massive fundamental particles to achieving an excellent understanding of a detector with 150 million channels to be read-out every 25 nano-seconds.

For a nice general introduction to the Large Hadron Collider, read these articles:
Scientific American, February 2008, National Geographic, March 2008