Title:

Neutral pion cross section and spin asymmetries at forward rapidity in polarized proton collisions at \sqrt{s} = 200 GeV.
 

Principal Authors (alphabetical order):

Jim Drachenberg (Valpo), Adam Gibson (Valpo), Stephen Gliske (ANL), Keith Krueger (ANL/Valpo), Dave Underwood (ANL), Jason Webb (BNL), possibly others (Indiana)

Intended Journal:

Phys. Rev. D, rapid communication.

Why rapid?

• New:
  • Although data was not taken recently, the results are new and will be the first published EEMC results
• Impact/Importance:
  • Data covers a unique kinematic range in (eta, pT) and (eta, xF)
  • No other instrumentation exists in this kinematic range except the STAR EEMC
  • Results expected to impact the global fit for the unpolarized pi0 fragmentation function, and thus the gluon helicity distribution extraction
    • Effects GRSV++, which is expected to be coming out on a short time scale

Abstract:

The differential cross section and spin asymmetries for neutral pion production at pseudo-rapidity 0.8 < eta < 2.0 in polarized proton collisions at sqrt(s) = 200 GeV are presented. Data was taken using the endcap electromagnetic calorimeter in the STAR detector at RHIC. The cross section is measured over a transverse momentum range of 6 < p_T < 16 GeV/c and is found to lie between the pT and 2pT scale of a next-to-leading order perturbative QCD calculation. The longitudinal double-spin asymmetry, A_LL, is sensitive to the gluonic contribution to the proton spin, delta G, and probes a lower x range than mid-rapidity measurements. The measured A_LL is consistent with model predictions.  The transverse spin asymmetry, A_N, includes contributions from Sivers and Collins effects, and this measurement spans a unique kinematic range in x_F and p_T.  The A_N results presented are consistent with zero.  The parity violating asymmetry A_L is also measured and found to be consistent with zero.

Outline:

1. Motiviation
2. Experimental Setup and Data Analysis
3. Results
   
   1. Presentation and discussion of cross section results
   2. Presentation and discussion of longitudinal asymmetry results
   3. Presentation and discussion of transverse asymmetry results
4. Conclusion

Some possible references

[1] Longitudinal double-spin asymmetry and cross section for inclusive neutral pion production at midrapidity in polarized proton collisions at sqrt(s) = 200 GeV. STAR Collaboration (B.I. Abelev (Illinois U., Chicago) et al.). Phys.Rev. D80 (2009) 111108

[2] Inclusive cross-section and double helicity asymmetry for pi0 production in p + p collisions at s**(1/2) = 200-GeV: Implications for the polarized gluon distribution in the proton. PHENIX Collaboration (A. Adare (Colorado U.) et al.). Phys.Rev. D76 (2007) 051106

[3]Longitudinal double-spin asymmetry for inclusive jet production in p+p collisions at s**(1/2) = 200-GeV.
STAR Collaboration (B.I. Abelev (Illinois U., Chicago) et al.). Oct 2007. 7 pp.
Published in Phys.Rev.Lett. 100 (2008) 232003

[4]Longitudinal and transverse spin asymmetries for inclusive jet production at mid-rapidity in polarized p+p collisions at √s=200 GeV.
STAR Collaboration, Phys.Rev. D86 (2012) 032006

Figure 1: Invarient Mass

We choose to show pT in 8-9 GeV.

Figure 2: Cross Section

For more details regarding the context of these results, see this blog.

Conclusions:

• There exists some tension with the pT scale pQCD results.
• Data falls between pT and 2pT energy scales
  • Preference towards the 2 pT energy scale.
• Overall chi^2 / ndf = 2.0 between data and the pT-scale curve
  • Worse disagreement at lower pT
  • Lowest bins differ by 2.6\sigma, 1.6\sigma and 1.5\sigma
• It is possible we are starting to see signs of non-perturbative effects
• It is also possible that (since this is a previouslyunmeasured pT, phys. eta domain) one can retune some parameters in the pQCD prediction, resulting in good agreement with all the STAR pi0 cross section results.

Figure 3: A_LL vs p_T

Cosmetics:

• Get new A_LL theory curve (using lines hand copied from Weihong's thesis for now)

Conclusions:

• A_LL is consistent with all parameter variations of the theory curves

Figure 4: A_N vs x_F and p_T

Further detail plots here.

Cosmetics:

• Decide how to plot systematic uncertainties, and make sure it is consistent throughout
• Consider adding the other published pi0 A_N results (vs xF or possibly also vs pT) from other detectors on the same plot(s).

Conclusions:

• A_N is consitent with zero, as expected.
• Data does not have the precision to observe signal in the highest xF bin.
• Data does not have the precision to make strong conclusions about pT dependence of A_N.