Department of Physics

nuclear physics (experimental)

My research is in the area of high-energy nuclear physics, where one uses extremely energetic particles to study fundamental properties of strongly interacting systems. Most of my work is carried out using the STAR detector at RHIC, the relativistic collider at Brookhaven National Lab, which provides beams of polarized (spin-aligned) protons at energies up to 250 GeV. At the resulting collision energies of 500 GeV, the polarized quarks in one proton serve as efficient and calibrated probes of the partons in the other, allowing one to map out the behavior of the quarks and gluons inside a tightly bound hadron. Some of the specific topics we are working on include:

• using the quark + gluon -> quark + photon "QCD Compton" process to determine the extent to which gluons (force carriers of the strong interaction) contribute to the total spin of the proton
• using W and Z boson production to study the virtual anti-quarks in the nucleon sea, in order to better understand their origin
• using transversely polarized protons to measure spin asymmetries at far-forward angles that are sensitive to possible orbital motion of partons (quarks and gluons) inside the proton

The Indiana University group has also constructed and maintains one of the major detector subsystems used in STAR, the Endcap Electromagnetic Calorimeter (EEMC), a 30-ton device that makes possible several of the measurements mentioned above. We have also made major contributions to the fast front-end electronics used by other parts of the STAR detector.

In my group, we are trying to understand how the observed properties of the proton—its mass, momentum, and spin—emerge from the complex, strong interactions of the quarks and gluons that make it up. We do this by colliding two beams of protons together at very high energies, which results in an energetic parton (quark or gluon) from one proton scattering from a single parton in the other. In these hard-scattering processes, the interactions become weak and can be described using perturbative Quantum Chromodynamics, or QCD. By detecting the outgoing scattered particles, and understanding the details of their hard interaction, we can work backward to determine the behavior of the quarks and gluons—what they were ‘doing’ inside the proton—prior to the collision. This work is done using the STAR detector at the Relativistic Heavy Ion Collider, or RHIC, located at Brookhaven National Laboratory.

RHIC is unique in that the colliding proton beams can be polarized—that is, the proton spins can be aligned to point preferentially in a particular direction. This enables us to address questions such as: Do the spin-1 gluons contribute to the spin of the proton? Do the electrically charged quarks have a net orbital angular momentum about the proton spin, and thus contribute to it? Does the non-perturbative ‘quark sea’ play a role? By learning how the quarks, bound together by gluons, combine to give us protons and neutrons, we will gain insight into the nature of the strongest force in the universe.

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