ARRA: Fundamental Studies of the Nucleon Spin in QCD

Grants and Contracts Details


Project summary A fundamental and long standing puzzle in QCD concerns how the spin and orbital angular momentum of the gluon, valence and sea quarks sum to give the nucleon spin of h/2. A complete characterization of the partonic spin disthbution in the nucleon requires measurements of both the helicity, Af(x. Q2), and transver- sity, ~f(x, Q2), parton densities. The deep inelastic scattering programs of the last century indicate that the flavor summed quark contribution to the nucleon is relatively small (~ 25%). These same experiments only weakly constrain Ag (x, Q2) and the flavor separated Aq(x, Q2), limiting knowledge about large pieces of the nucleon spin budget. The polarized proton collisions provided by the Relativistic Heavy Ion Collider at Brookhaven National Laboratory offer leading order sensitivity to Ag (x, Q2), Aq(x, Q2), and A4(x, Q2) and leading twist access to the öq(x, Q2) distributions, allowing the unique opportunity to simultaneously investigate the most pressing questions of the spin puzzle. This proposal requests support for the Principle Investigator, Renee Fatemi, to pursue investigations into the origin of the proton spin within the STAR Collaboration at RHIC. The intellectual merit of this proposal is defined by investigations into three aspects of the spin puzzle. The first is the measurement of asymmetries sensitive to the sign and magnitude of the gluon helicity distribution as a function of partonic momentum fraction (4. Analysis of inclusive jet asymmetries at two collision energies will provide sensitivity to over an order of magnitude integrated x range, while a finer resolution of the partonic kinematics (x1 and x2) will be obtained though correlation measurements such as di-jets and the "golden" photon-jet channel. The second is the measurement of the convolution of transversity and Collins distributions, Sq(x, Q2) 0 AD(z), via single-spin asymmetries of the 0 dependence of it mesons within a jet. This signal will be extracted from existing mid-rapidity data, allowing full jet as well as charged and neutral pion reconstruction. Measurement of the ir''- signal is crucial, as it is expected to be significantly larger than the 1t~ signal due to cancelations of the favored and unfavored AD(z) from the u and d quark. This result has the potential to be the first transverse spin measurement from RHIC capable of isolating the Sq(x, Q2) 0 AD (z) contribution to the single spin asymmetry of charged pions. Finally, the P.1. is preparing for future flavor separated Aq(x, Q2) and A4(x, Q2) measurements by contribut- ing to the construction and installation of the STAR Forward Gem Tracker (FGT) Detector. This upgrade will provide the charge sign identification necessary to discriminate between the e~ /e+ decay products from W+/W_ production in the forward region. The P.1. is committed to providing a portion of existing start-up funds and the manpower necessary to build, test and install the FGT data aquisition electronics and interface to the existing STAR DAQ and trigger system.
Effective start/end date9/1/094/30/13


  • National Science Foundation: $479,875.00


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