Azimuthal dependence in unpolarized proton-induced Drell-Yan processes Lingyan Zhu, Jen-Chieh Peng University of Illinois at Urbana-Champaign Paul Reimer Argonne National Laboratory (FNAL E866/NuSea Collaboration) Mini-symposium on orbital motion of quarks in hard scattering 2nd joint meeting of the APS DNP and JPS Maui, Hawaii, Sep 18-22, 2005
Angular distribution for the Drell-Yan processes In the simple parton model: ( for massless quarks and measured relative to the annihilation axis) =1 and ==0 Correction due to QCD is small.
Azimuthal cos2 distribution in the W Drell-Yan Conway et al., PRD39,92(1989) E615 at Fermilab: 252 GeV π- + W NA10 at CERN: 140/194/286 GeV π- + W Z. Phys. C37, 545 (1988)
Possible explanations for the cos2 asymmetry • The high twist in terms of pion bound state effect is not enough. Brandenburg, Brodsky, Khoze & Muller, PRL73,939(1994) • The nuclear distortion of hadronic projectile wavefunction, typically a spin-orbit effect occuring on the nuclear surface. =0 for pp Drell-Yan. Bianconi & Radici, JPG31,645(2005). • The spin correlation due to nontrival QCD vacuum.—Flavor blind. Brandenburg, Nachtmann & Mirkes, Z. Phy. C60,697(1993) Boer,Brandenburg,Nachtmann&Utermann, EPC40,55(2005). • The non-zero chiral-odd distribution function h1?. It is equal to Sivers function f1T?in the quark-diquark spectator model. Boer, PRD60,014012(1999); …
QCD vacuum effects • Brandenburg, Nachtmann & Mirkes, Z. Phy. C60,697(1993) • The factorization-breaking spin correlation due to nontrivial QCD vacuum may fit the NA10 data at 194 GeV • The helicity flip in the instanton-induced contribution may lead to nontrivial vacuum. • Boer,Brandenburg,Nachtmann&Utermann, EPC40,55(2005). • This vacuum effect should be flavor blind. 0=0.17 mT=1.5
Boer-Mulders function h1? • Boer, PRD60,014012(1999) • An spin-correlation approach in terms of h1? can fit the NA10 data at 194 GeV. • On the base of quite general arguments, for |qT|<<Q(=m), • Salvo,hep-ph/0407208. 1=0.5 mC=2.3 T=CH=1
Models with Boer-Mulders function h1? • The quark-spectator-antiquark model with effective pion-quark-antiquark coupling as a dipole form factor Lu & Ma, hep-ph/0504184 • Twist 2 (as well as the kinematic twist 4) contribution in a parton-spectator framework Gamberg&Goldstein, hep-ph/0506127.
Fermi Lab E866/NuSea experiment Drell-Yan events Analyzed (4.5<M<9, M>11) including pp+- X; pd+- X High Mass Setting: dset7-39k, dset8-85k (+ polarity) dset11-25k (- polarity) Low Mass Setting: dset5-68k (+ polarity)
Muon momenta Blue: simulation Red: data (dset8) +! -!
Comparison of data and simulation Blue: simulation Red: data (dset8)
Extraction of azimuthal cos2distribution Fit Func: p1[1+p3/2 cos2+ p2cos( + p4)] Data M.C. Data/M.C.
Azimuthal cos2distribution for Drell-Yan processes Very Preliminary
Summary • Large cos2 azimuthal asymmetry has been observed in unpolarized -induced Drell-Yan. • The are a few possible explanations including the non-trivial vacuum effect and the non-zero Boer-Mulders function h1? . The latter is related to the Sivers function f1T?. • The unpolarized p-induced Drell-Yan data seem to suggest small cos2 azimuthal asymmetry. This may disfavor the flavor blind explanation such as vacuum effect.