Longitudinal Double Spin Asymmetry in Inclusive Jet Production at STAR Page: 2 of 5
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is well-equipped for jet reconstruction at mid-rapidity.
These proceedings report on the first measurements of the longitudinal double-spin
asymmetry ALL for inclusive jet production,
ALL _ + (1)
where ++ (+-) is the inclusive jet cross section when the colliding proton beams
have equal (opposite) helicities.
The jet cross sections are described in a factorized form by a convolution of non-
perturbative parton densities and hard-scattering matrix elements. Fragmentation func-
tions do not enter the theoretical description. At jet transverse momenta PT < 8 GeV/c,
the gluon-gluon scattering contribution to the polarized cross section is the largest. At
higher PT in the range of the present measurements the quark-gluon scattering contribu-
tion is larger than the gluon-gluon contribution. Eventually, the quark-quark contribution
becomes the dominant subprocess [6].
EXPERIMENT AND ANALYSIS
During the initial proton running periods in the years 2003 and 2004, RHIC accelerated
and stored typically 55 beam bunches, each with 5-7x1010 protons, in each ring per
store. Typical luminosities were 2-5 x 1030cm-2 s-1 and V/s = 200 GeV. Beam polar-
izations of up to 45% were reached. The beam polarizations were measured for each
beam and for each beam fill with Coulomb-Nuclear Interference (CNI) proton-carbon
polarimeters [7] which were calibrated in situ using a polarized atomic hydrogen gas-jet
target [8].
The proton beam helicities at the STAR interaction region (IR) alternated for succes-
sive bunches in one ring and for successive pairs of bunches in the other ring. The beam
bunches were separated by ~ 213 ns. Hence, STAR recorded data with all four helicity
combinations concurrently.
The STAR detector subsystems of interest here are the Time Projection Chamber
(TPC), the Barrel Electromagnetic Calorimeter (BEMC), and the Beam-Beam Coun-
ters (BBC). The TPC is used to determine the momentum of charged particles in the
pseudorapidity range -1.3 < 7 < 1.3 for all azimuthal angles 0. The BEMC is a lead-
scintillator sampling calorimeter which measured electromagnetic energy deposits for
0 < 7 < 1 and 0 < 0 < 27c. The BEMC installation was completed after the 2004 run-
ning period, and the detector presently covers -1 < 7 < 1. The BBCs are segmented
scintillator annuli mounted around the beam pipe and span 3.3 < 1r < 5.0. Coincident
energy deposits in one or more segments of the BBC on either side of the STAR IR
signal the collision of polarized protons at the IR and were used to form the so-called
minimum bias (MB) trigger. In addition, the BBCs were used to measure proton beam
luminosities and to measure transverse beam polarization components [9].
Jet data were collected with a high tower trigger (HT) that required an energy deposit
in a BEMC tower (Aij x AO = 0.05 x 0.05) in addition to the MB trigger condition.
The trigger threshold corresponded to a transverse energy ET > 2.2 GeV in 2003 and to
ET > 2.2-3.4 GeV for 7 = 0 - 1 in 2004. The integrated luminosity f Ydt amounts to
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Kowalik, Katarzyna. Longitudinal Double Spin Asymmetry in Inclusive Jet Production at STAR, article, August 15, 2006; (https://digital.library.unt.edu/ark:/67531/metadc898045/m1/2/: accessed March 29, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.