Using Muons and Protons to Probe the Structure of the Universe

This award supports two separate experimental programs, each aimed at exploring the structure of our universe at the most fundamental level. One program will investigate how the quarks and gluons that are produced in high energy proton-proton collisions spawn new quarks and gluons and eventually combine to form the particles that make up the visible universe.

These studies will shed light on the nature of Quantum Chromodynamics, a theory that describes the strong force, one of the three fundamental forces that comprise the Standard Model of Particle Physics. The other program aims to discover signals of new forces and particles that are not yet included in the Standard Model. The g-2 experiment at Fermi National Laboratory aims to precisely measure the magnetic properties of the muon, a fundamental particle with characteristics very similar to the electron, but with 200 times the mass.

This experiment is a second generation of an earlier measurement made at Brookhaven National Lab nearly twenty years ago that differs somewhat from the Standard Model theoretical calculations. The first result from the Fermilab measurement has been announced and it agrees with the earlier measurement but with better precision. The discrepancy between the experiments and the Standard Model points to possible contributions to the muon's magnetic moment from Beyond the Standard Model forces and/or particles.

The Fermilab g-2 collaboration is in the process of analyzing and collecting additional datasets with the goal of further reducing the uncertainties on the measurement and pushing towards a discovery level discrepancy with the Standard Model predictions. The experiments supported by this grant will provide undergraduate and graduate students with the necessary tools and experience to either continue their work in basic research or to enter the technical workforce and lend their problem-solving expertise to a myriad of fields, including finance, big data analysis, patent law and medical physics.

Undergraduate students funded by this award will continue to have the rare opportunity to experience the scientific culture and basic research performed at U.S. National Laboratories.

This award will support the PI and two graduate students in using high energy pp and pA collisions at the Relativistic Heavy Ion Collider to extract longitudinal and transverse momentum dependent jet fragmentation functions from distributions of identified pions, kaons and protons inside of fully reconstructed jets. Measurements in pp collisions will provide significant constraints on the gluon fragmentation functions, particularly in the case when the pion, kaon or proton assume a large fraction of the parent gluon's momentum.

Comparisons of these same fragmentation yields in pp and pA will provide new insights on how the quark and gluon hadronization process is modified by a nuclear environment. Looking at this fragmentation in yet another dimension, as a function of particle momentum relative to the jet axis, provides access to the transverse momentum distributions (TMD) in the proton.

Information about unpolarized TMD fragmentation functions is sparse, largely because the interpretation of the existing data is complicated by the limited energy of the lepton-proton collisions used to make the measurements. Investigation of TMD fragmentation functions is an essential piece of a worldwide effort to advance our current picture of Quantum Chromodynamics, the formal theory of strong interactions within the Standard Model of particle physics.

This grant will also allow the PI, a postdoctoral associate and a graduate student to maintain their current efforts in the muon g-2 experiment at Fermilab. Contributions will include the development and tuning of the g-2 GEANT based simulation software package, the evaluation of beam dynamics systematic errors that require simulation and the Q-method precession analysis of data taken in Runs 2 & 3.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.