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Accessing Quark Transversity Through Azimuthal Single-Spin Asymmetries of Charged Pions in Jets at STAR

Speaker(s) / Presenter(s):
J. Kevin Adkins (UK)

A complete, fundamental understanding of the proton must include knowledge of the underlying
spin structure. The transversity distribution (h_1 (x)), which describes the transverse spin structure
of quarks inside of a transversely polarized proton, is only accessible through channels that couple
h_1 (x) to another chiral odd distribution, such as the Collins fragmentation function (∆D (z, j_T )).
Significant Collins asymmetries of charged pions have been observed in semi-inclusive deep inelastic
scattering (SIDIS) data. These SIDIS asymmetries combined with e^+ e^- process asymmetries from
Belle have allowed for the extraction of h_1 (x) and ∆D (z, j_T ). The current uncertainties on h_1 (x)
are large compared to the corresponding quark momentum and helicity distributions and reflect
the limited statistics and kinematic reach of the available data. In transversely polarized hadronic
collisions, Collins asymmetries may be isolated and extracted by measuring the spin dependent
azimuthal distributions of charged pions in jets. An exploratory STAR analysis with the 2006
s = 200 GeV dataset hinted at a charge dependent Collins asymmetry and motivated a dedicated
transversely polarized proton run in 2012 where an order of magnitude more data was collected (20 pb^{−1})
at an average polarization of 63%. This measurement, coupled with the same measurement
at √s=510 GeV and interference fragmentation function (IFF) measurements at √s = 200 and
500 GeV at midrapidity (|η| < 1) access higher momentum scales than the existing SIDIS data,
and will allow for a comprehensive study of evolution and factorization of the Collins channel.
Preliminary results from the √s = 200 and 500 GeV Collins and IFF analyses will be presented.

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