1

Azimuthal dependence in unpolarized proton-induced

Drell-Yan processes

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

Lingyan Zhu, Jen-Chieh Peng

University of Illinois at Urbana-Champaign

Paul Reimer

Argonne National Laboratory

(FNAL E866/NuSea Collaboration)

2

*h N X X Angular distribution for the Drell-Yan processes

Correction due to QCD is small.

In the simple parton model:

( for massless quarks and measured relative to the annihilation axis)

1 and =0

3

Conway et al., PRD39,92(1989) E615 at Fermilab: 252 GeV π- + W

Azimuthal cos2 distribution in the W Drell-Yan

NA10 at CERN: 140/194/286 GeV π- + W Z. Phys. C37, 545 (1988)

4

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); …

5

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.

QCD vacuum effects

0=0.17mT=1.5

6

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=0.5mC=2.3T=CH=1

221 12 2

( , ) ( )T TkC HTT H

T C

M Mh x k c e f x

k M

7

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.

8

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)

9

Muon momenta

Blue: simulation

Red: data

(dset8)+ !

- !

10

Comparison of data and simulation

Blue: simulation

Red: data

(dset8)

11

Extraction of azimuthal cos2 distribution

Data M.C. Data/M.C.

Fit Func: p1[1+p3/2 cos2+ p2cos( + p4)]

12

Very Preliminary

Azimuthal cos2 distribution for Drell-Yan processes

13

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.