I. Joseph Kroll

My research is in experimental high energy particle physics. I carry out my research at Fermilab, which is located in Batavia, Illinois, about 35 miles west of Chicago. I am a member of the CDF collaboration, which consists of over 700 physicists. Together we have constructed a large experimental apparatus that records data from proton-antiproton collisions produced by the Tevatron, the highest energy particle collider in the world. Using the data from our detector, we study a wide variety of phenomena in particle physics. The most important result achieved was the discovery of the top or t quark, the last unobserved fundamental fermion in the standard model of particle physics. This discovery was announced in April, 1995.

My research focuses on the physics of particles called hadrons that contain b quarks, the partner of the t quark. By studying the decays of these B hadrons, we deepen our understanding of the electroweak interaction. In particular, the decays of these particles exhibit a large violation of the combined symmetry operation of charge conjugation and parity or "CP.'' CP-violation is of great interest, because it must exist to explain the observed predominance of matter over anti-matter in our Universe. Until recently, CP-violation had only been observed as a few parts per thousand effect in the decays of neutral K mesons. In contrast, B mesons have recently been shown to exhibit very large CP-violation at a level as high as 75%. Neutral B mesons also undergo "flavor oscillations," that is a neutral B meson can oscillate into a neutral anti-B mesons (the antimatter counterpart to the B meson). Measuring the rate of oscillation helps measure fundamental parameters in our standard model of particle physics. Currently I am developing the experimental techniques that are in integral part of the observation of flavor oscillations and CP-violation in the decays of B mesons.

Recently I began investigating the physics of the Higgs boson. The Higgs boson is the one piece of our standard model of particle physics that has not been discovered. In our model it is responsible for giving mass to the fermions and it provides a mechanism for "electroweak symmetry breaking," which must occur to give the weak intermediate vector bosons mass (the W+, W-, and Z, which mediate the weak interaction), while leaving the photon massless. At this time, the Tevatron is the only accelerator where we might hope to discover the Higgs boson (the chance of this discovery is very remote, however). The discovery of the origin of electroweak symmetry breaking is anticipated at a new accelerator called the Large Hadron Collider (LHC). The LHC is being constructed in an existing tunnel 27 kilometers (20 miles) in circumference, over 300 feet underground near Geneva, at the border of Switzerland and France. The LHC is part of the European Laboratory for Particle Physics or CERN, the largest international scientific laboratory in the world. The LHC is expected to begin operation in 2007. At Penn, we are involved in one of the two large all purpose detectors on the LHC known as ATLAS.

• Arrived at Penn (1997)