Spin as a Probe into the Structure of the Universe

Spin, a type of angular momentum inherent to all fundamental particles, is a purely quantum mechanical dynamic and therefore serves as a particularly sharp probe into the structure of our universe. This proposal aims to utilize the properties of spin to study the underlying mechanisms of Quantum Chromodynamics (QCD), the formal theory of strong interactions within the Standard Model, as well as Beyond-the-Standard- Model (BSM) signals which may contribute to the muon anomalous magnetic moment. The proton’s anomalous magnetic moment was first measured in 1933 by Otto Stern. The unexpected large value, 2.79 nuclear magnetons, was the first indication that protons are not Dirac particles but are instead composed of smaller, more fundamental particles called quarks and gluons. These point-like particles, or partons, are confined inside nucleons by the strong force which governs their interactions. Consequently, the abundant, stable and easily manipulated proton has served as an experimental “partonic laboratory” for over half a century. The underlying mechanisms of QCD are illuminated via investigations into how the proton mass, charge and spin manifest from partonic degrees of freedom. The “spin puzzle” is an outstanding question in QCD physics and can be stated simply: How does the spin and orbital angular momentum of the quarks and gluons combine to form the total proton spin of ~/2? Experimental efforts to answer this question have uncovered a rich and unexpected spin structure within the proton, requiring a variety of experimental approaches to separate the various contributions. Two important components of this structure are the helicity (Df ) and transversity (DT f ) parton densities, which characterize the number density of longitudinally/transversely polarized partons inside of a longitudinally/transversely polarized nucleon. Longitudinal and transverse are defined to be along and perpendicular to the direction of the nucleon momentum respectively. Just as the measurement of the proton anomalous magnetic moment shed light on the then unknown partonic substructure of the proton, the precision measurement of the muon anomalous magnetic moment has the potential to shed light on new physics at the TeV scale. Dirac predicted that all charged, point-like fermions, such as the muon, would have a g-factor of precisely 2. However, due to interactions with virtual particle fluctuations in the vacuum, the muon magnetic moment deviates from this expectation and acquires a non-zero anomalous magnetic moment (aì). This term, often written as aì = (g..2)/2 , encapsulates the contributions to the muon magnetic dipole moment from interaction with all fundamental particles that exist in our universe. A significant discrepancy between the Standard Model prediction and the measurement of aì would imply the existence of new particles and/or forces of nature. Indeed, recent experimental developments have lead the particle physics community to believe these BSM contributions are real and may be deeply connected to the origin of dark matter and neutrino oscillations. The Principle Investigator, Renee Fatemi, requests support to continue investigations into the origin of the proton spin within the STAR Collaboration at the Relativistic Heavy Ion Collider (RHIC). Measurements of mid-rapidity di-jet asymmetries at s = 500 GeV will map the x dependence of Dg(x;Q2), providing insight into the functional form and reducing extrapolation errors on the calculation of the full integral G = g(x;Q2)dx. Measurements of the Collins asymmetries of charged pions in mid-rapidity jets will help constrain on the transversity distributions and provide important information about the Q2 evolution of transverse momentum dependent functions as we’ll as the validity of non-collinear factorization in hadronic collisions. Finally, this grant will support for the P.I. and her group to join the new muon g-2 experiment (E989) at Fermilab. Initial contributions include development and support of the data acquisition system and simulation software package. This program provides a wide breadth of opportunities for undergraduate, graduate and post-doctoral involvement and education. 1