[Solid_sidis] Two recent papers on pp->W/Z transverse spin asymmetry

[Jin Huang]

Dear Collaborators,

 

Here are two recent arXiv papers on pp->W/Z transverse spin asymmetry which
you may find interesting: arXiv:1511.06003 and arXiv:1511.06764.

 

Last week STAR collaboration arXived the final result on the first W-boson
transverse spin asymmetry measurement in the p+p collision @ 500GeV CM
energy [arXiv:1511.06003v1]. The result disfavors the none-evolution +
none-sign-flip scenario on the Sivers effect, although it is still
statistical limited if the evolution effect is strong. STAR proposes
[arXiv:1501.01220] to take higher statistics transverse data in Run-2017 to
achieve the best deliverable from RIHC. 

 

As the reco for the W-boson kinematics was demonstrated by STAR (rather than
just measuring the decayed lepton), Zhongbo, Ivan, Hongxi and I followed up
with a paper describing the W-boson asymmetries at tree-level under the TMD
framework. This manuscript just show up on arXiv last night
[arXiv:1511.06764v1]

 

Like in the SIDIS case, the spin asymmetry in the p+p->W/Z + X production
contains many more azimuthal modulations that is sensitive to various TMDs.
In particular, for the transverse single spin azimuthal asymmetry, in which
the Sivers effect is traditionally measured, the parity violating nature of
week boson coupling bring up a new parity violating asymmetry that is
sensitive to g_1T, transversal helicity distribution. Experimentally, this
parity violating azimuthal asymmetry can be as large as the Sivers effect,
since W-boson provided 100% analyzing power to the quark helicity. Like in
the SIDIS case (w/ fixed beam helicity), the Sivers and g_1T SSA of can be
separated with a sine + cosine azimuthal fit. 

 

Interpretation of both effects can be related to our 6-GeV and 12-GeV
measurements (as cited in our paper): first, we expect a SIDIS/p+p sign flip
for the Sivers effect but NOT for g_1T. Therefore, simultaneous measurement
of both effects in p+p and in SIDIS gives more conclusive test on the
universality of TMDs. Second, the theoretical uncertainty in the TMD
evolution effect is still large. The experimental comparison on both lower
Q2 and high Q2 (W-mass) data constraints the evolution effect of both Sivers
and g_1T. 

 

If there is any comment or suggestion, please let us know. Have fun and have
a happy Thanksgiving,

 

Cheers,

 

Jin

 

 

______________________________

 

Jin HUANG

 

Brookhaven National Laboratory

Physics Department, Bldg 510 C

Upton, NY 11973-5000

 

Office: 631-344-5898

Cell:   757-604-9946

______________________________

 

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