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16–20 Apr 2007
Holiday Inn Munich City Centre & Gasteig Conference Center
Europe/Zurich timezone

Soft gluon resummation and a novel asymptotic formula for double-spin asymmetries in dilepton production at small transverse momentum

19 Apr 2007, 09:00
20m
Forum 7 (Holiday Inn Munich City Centre & Gasteig Conference Center)

Forum 7

Holiday Inn Munich City Centre & Gasteig Conference Center

Hochstrasse 3 81669 Munich Germany
Spin Physics Spin Physics

Speaker

Hiroyuki Kawamura (RIKEN)

Description

We study the double-spin asymmetries in transversly/longitudinally polarized Drell-Yan processes at transverse momentum Q_T of the produced lepton pair. We first consider the double transverse-spin asymmetry A_{TT}(Q_T) in the transversely polarized Drell-Yan process in detail, in particular, in the small Q_T region, where the bulk of dileptons are produced. The large logarithms due to the soft gluon emission at small Q_T are resummed in the Drell-Yan cross sections at the next-to leading logarithmic (NLL) accuracy, employing the impact parameter b-space formulation. The numerical calculation shows that the effects of the soft gluon resummation to the polarized and unpolarized cross sections largely cancel in A_{TT}(Q_T), but the significant corrections still remain. In particular, as a result of the resummaiton, the flat behavior of A_{TT}(Q_T) is generically observed in the small Q_T region, from Q_T =0 to the peak region of the cross-section Q_T-spectrum, indicating that the Q_T =0 limit of A_{TT}(Q_T) is theoretically a useful quantity. We evaluate the impact parameter b-integral in the resummed cross sections at Q_T=0 by the saddle-point method, and derive a novel asymptotic formula of A_{TT}(Q_T =0), which obeys a remarkably simple analytic form and is exact when the dilepton mass Q is large. We demonstrate the accuracy of our asymptotic formula even at the kinematics of RHIC, J-PARC, and GSI, comparing with the corresponding results of the numerical calculation. Because of the simple structure of our formula, it enables us to extract information on the transversity directly from the data of A_{TT}(Q_T) measured in polarized Drell-Yan processes in the small Q_T region. We also extend our framework to the double-spin asymmetry A_{LL}(Q_T) in the longitudinally polarized Drell-Yan process in the small Q_T region, and derive the corresponding asymptotic formula for A_{LL}(Q_T =0), and discuss its usefulness for extracting the information on the helicity distributions of quarks and gluons inside nucleon.

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