The DØ Group
at the University of California, Riverside
Experimental High Energy Physics Research
Department of Physics
University of California
Riverside, CA 92521-0413
Experimental physicists from UC Riverside have been performing high energy physics research at the DØ experiment since 1986. The DØ detector is at the proton-antiproton Tevatron Collider at the Fermi National Accelerator Laboratory near Chicago, Illinois. We use data collected at the experiment to study properties of fundamental particles including the top and bottom quarks and W and Z bosons, and to search for the Higgs boson. We played a leading role in the design and construction of the Silicon Microstrip Tracker for the experiment upgrade from 1989 to 2001. With DØ, we have published 132 papers in several premier international journals from the data collected in Run I (19921996), and over 200 papers from the data collected in Run II (20022011).
Top Quark Physics
The top quark is the heaviest known elementary particle. It was first observed at Fermilab in 1995. Since then, many measurements have been made of its properties, including the production cross sections in several decay channels, and its mass. Because its mass is close to the electroweak symmetry breaking scale, it is hoped that information from precision measurements will lead to insights into how that symmetry breaking occurs.
Ann Heinson co-led the search for single top quark production at DØ for many years; Liang Li took over this role in September 2009. Philip Perea graduated in June 2006 with an analysis that searched for t-channel single top quark production using multiple neural networks to separate the expected signals from background. Liang Li designed the triggers used to collect the data and was responsible for event selection. Mark Padilla starting work on this search summer 2008. In December 2006, we announced the first evidence for single top quark production the evidence paper in Physical Review Letters has SPIRES TOPCITE=100+ status with over 170 citations, and a long companion paper in Physical Review D has SPIRES TOPCITE=50+ status. In March 2009, we announced 5-sigma significance observation of single top quark production. The observation paper was published in August in Physical Review Letters and has TOPCITE=100+ status with over 140 citations. Details of the single top observation analysis are available here, together with many plots for talks. In November 2009, we published a measurement of the t-channel single top quark cross section from an analysis done by Liang Li. And in August 2010 we published a search for single top quarks produced via flavor-changing neutral currents (which are not allowed in the Standard Model), also by Liang. We have reently also published analyses of a larger dataset, and a long paper on the top quark mass combination from 12 measurements by DØ and by CDF.
Bottom Quark Physics
Postdoctoral researcher Avdhesh Chandra and John Ellison have been engaged in groundbreaking work on measurement of the CP-violating phase in the Bs-Bs system using the deacay Bs0 → J/ψ φ. We have made the first measurement of this phase and have published three papers in Physical Review Letters and Physical Review D on this analysis, including a recent paper with SPIRES TOPCITE=100+ status with nearly 200 citations and another with TOPCITE=50+ status. There is considerable worldwide interest in our results, which, when combined with CDF's recent results, yield an intriguing deviation from the standard model predicted value at the level of about 2-to-3 standard deviations. Measurements are continuing at DØ and CDF that are expected to reveal whether the level of CP-violation is at the level expected in the SM or not.
Silicon Microstrip Tracker
The SMT was first simulated and studied by Ellison in 1989 for DØ in Run I, and went through many design changes since then before finally being completed and installed in 2001 for Run II. This device, with nearly 800,000 readout channels, measures the 3-d coordinates where charged particles pass through its layers. From this information, we reconstruct the particles' tracks, and hence the position of the collision point on the beamline, and the secondary vertices in jets from heavy flavor decays. We use this jet identification in the reconstruction of top quark and Higgs boson decays. Many members of the UCR group worked on the design and construction of this detector for over a decade, and on the associated software to reconstruct the information from it. It has been used for all data-taking for the past eight years, and is expected to be in almost continuous operation until the end of the Tevatron proton-antiproton operation. Almost every measurement made by DØ relies upon the information produced by this detector.
Liang Li was the Top Group's trigger representative for two years, followed by being co-convener of the Trigger Studies Group for a year and then co-convener of DØ's Trigger Group for a further year, until recently (the Trigger Group was formed from a merge of the Trigger Studies Group and the Trigger Board). He has designed, implemented, and maintains the Super-OR of triggers that allows multiple triggers to be correctly modeled in MC simulations, thus significantly increasing the signal acceptance in many critical analyses throughout the experiment.
Liang Li is an assistant professor at Shanghai Jiao Tong University. Avdhesh Chandra is at Rice University working on DØ and Mu2e. Ia Iashvili co-led the jet energy scale group, the top mass and top properties subgroups, and was co-convener of the electroweak physics group; she is now an associate professor at the State University of New York, Buffalo, working on DØ and CMS. Suyong Choi co-led the vertex reconstruction group, the Higgs dilepton subgroup, was reconstruction executable manager, and was co-convener of the Higgs physics group; he is now a professor at Korea University, Seoul, working on DØ and CMS. Jim Cochran is a full professor at Iowa State University, working on BaBar and Atlas. Brajesh Choudhary is a full professor at Delhi University, India, working on DØ and CMS. Ray Hall is an associate professor at California State University, Fresno, working on DØ and Atlas. Valentin Kuznetsov, Krish Gounder, and Chris Boswell are staff scientists.