This three-year award provides support for an ongoing program of research at the University of Illinois at Chicago (UIC) in experimental high energy physics (HEP). The program is centered on the DØ experiment currently running at the Fermilab Tevatron and on the Compact Muon Solenoid (CMS) experiment at the Large Hadron Collider (LHC) at CERN in Geneva, Switzerland. The UIC group consists of four faculty, four postdoctoral research associates, and seven graduate students. For the period covered in this award, the UIC program will include continued involvement in the DØ physics program, concentrating on Tevatron "legacy" measurements using the entire Run II dataset; participation in the operation and commissioning of the CMS Silicon Tracker and High Level Trigger; continuing ongoing object identification and calibration activities at CMS, and build on UIC areas of expertise and focus on studies of QCD, associated production of W and Z bosons with jets and b-jets, and top quark physics. The broader impacts of the proposed activities are multiple and diverse. The challenges faced in successfully building and commissioning complicated trigger and silicon systems enhance the disciplinary knowledge in engineering fields. The physics analyses and hardware projects provide valuable experience and knowledge for UIC HEP students and postdocs, enabling them to pursue successful careers in academia or private sectors. The mature QuarkNet and CLASA (Chicago Large Area Shower Array) outreach activities of the UIC HEP group fosters collaboration between high school teachers and students and UIC HEP group members, strengthening teaching and learning in Chicago-area high schools. As Chicago high schools serve predominantly minority students, this project may lead to a larger number of minority students majoring in a science discipline.
The activities of the High-Energy Physics group at the University of Illinois Chicago (UIC) addressed some of the most basic and important questions confronting science. Using data collected by the CMS Detector at the Large Hadron Collider (LHC), the UIC group participated in the discovery of the Higgs boson and the measurements of its properties; searched for the last remaining symmetry of space-time, known as Supersymmetry; hunted for extra dimensions of space using the top quark, the heaviest known fundamental particle in Nature; and pursued the intriguing possibility that quarks might not be fundamental, but instead could be composite. The Standard Model predicts that a Higgs boson with mass near 125 GeV decays mostly to bottom quarks. Observing that decay rate provides a stringent test of the Standard Model, opening a crucial window to possible new physics beyond our current understanding. The UIC Group used large datasets containing a Standard Model W or Z boson and collected by CMS in 2011 and 2012, to search for evidence that the newly discovered Higgs boson decays to bottom quarks. Figure 1 shows how the upper limit on the signal strength relative to the Standard Model prediction varies for different hypothetical masses of the Higgs boson. The observed data (solid black line with points) agree well with the hypothesis of a Higgs boson decaying to bottom quarks (red dashed line) and moderately disagree with the prediction in the absence of a Higgs boson (black dashed line). This result is published in Phys. Rev. D 89, 012003 (2014) and is the first indication at the LHC of a Higgs boson decaying to bottom quarks. It is difficult to theoretically understand how a Higgs boson’s mass could be near 125 GeV when confronted with large quantum effects that naively suggest that its mass should be much heavier. Supersymmetry offers a possible explanation: those large quantum effects could be cancelled via new, undiscovered partners to the known particles of the Standard Model. The UIC group searched for supersymmetric particles using an experimental signature of multiple jets of hadrons together with a large momentum imbalance measured in the CMS detector. No hints of supersymmetry were seen in the early CMS data collected in 2012. This work is published in Phys. Rev. Lett. 109, 171803 (2012), and placed the most stringent lower mass limits (at that time) on supersymmetric partners to the Standard Model gluons. New theoretical models involving strong dynamics might alternatively explain how the measured mass of the Higgs boson could be natural. Such conjectured models, as well as models having extra dimensions of space, predict new massive partners (called Z’) to Standard Model Z bosons. Those new massive particles could decay to pairs of top quarks and the UIC group used the full dataset collected by CMS in 2012 to search for their signatures. Figure 2 shows how the signal of such a new massive particle (black line), more than 20 times the mass of the Standard Model Z boson, would appear if it had a mass of 2TeV, compared with the dominant background contribution from top quarks produced in the Standard Model (solid red area). The data (black dots) are consistent with no new states decaying to pairs of top quarks. This result is published in Phys. Rev. Lett. 111, 211804 (2013) and is currently the most stringent lower mass limit on the production of these massive hypothetical particles. Finally, building on our extensive experience on QCD studies at the Tevatron and LHC, the UIC group continued to search for new phenomena using events containing two, high momentum jets (called dijets). At the LHC, the angular distributions of these dijets probe the shortest distance scales ever studied, providing a fundamental test of QCD and serving as a powerful tool to search for new phenomena like quark compositeness or extra spatial dimensions. The UIC group analyzed the full 2012 dataset collected by CMS and found that the data agreed well with known QCD processes and no new phenomena beyond the Standard Model were observed. The result has been publicly approved and a paper is currently being prepared. This will be our 3rd publication on searches for quark compositeness at CMS, producing the most comprehensive limits on their existence to date. In addition our results will yield the best limits on models with large extra dimensions of space. With the recent discovery of a Higgs boson, a new window of exploration has been opened. Soon, the LHC will operate at the highest energy frontier yet achieved in the laboratory, probing Nature at the smallest distances ever reached. The UIC High-Energy Physics group is excited about the remarkable success of the first run of the LHC and we eagerly look forward with anticipation to the new high-energy frontier.
