Paper proposal - W Asymmetry PRL paper - Run 9

Title: Measurement of the parity violating longitudinal single-spin asymmetry for $W^{\pm}$ boson production in polarized proton-proton collisions at √s=500GeV 


PA: Jan Balewski*, Ross Corliss, Oleksandr Grebenyuk, Will Leight, Rosi Reed, Joe Seele, Ilya Selyuzhenkov,

Hal Spinka, Justin Stevens, Bernd Surrow*, Gene van Buren, Matt Walker and Scott Wissink   

*Contact and writing team! 


Proposed Target Journal: Physical Review Letters



1) Question 1: Distributions for the distances between TPC projections and the 

center-of-gravity of the BEMC clusters for + and - tracks 

Answer: Plot

2) Question 2: For the 15 ET>50 GeV candidates, we have 10 W-like and 5 QCD-BK-like
events. Among the 10 W-like events, 2 are edge events and 6 are with ADC
saturation/bad gains. I understand that the 5 QCD-BK-like events are due to
the fact that they fell below the sPtBalance cut on figure 2 (BK note).
Where are two edge events and 6 ADC saturation/bad gains in the figure :    

Answer: See Tables 1-4 here

3) Question 3: Could you re-generate the correlations if sPt vs. away ET for different ET bins, for example, ET [15,25], [25,35], [35,50]:

Answer: Plot

4) Question 4: We would like to see the W+-->e and W--->e response function from your Monte Carlo plotted in fig 3 (from the paper) with proper normalization.                                                                      

Answer: Plot

The residual transverse polarization was taken into account evaluating AL in the following way: 

sqrt(1-0.15^2)=0.989 --> Multiply CNI polarization for Yellow and Blue by this number. This gives the effective long. polarization at STAR which is written in the paper. We evaluated then AL for this new value compared to using the CNI polarization values and found no difference at the quoted precision of AL.

  • GPC questions/comments to Version 1.6 (All comments taken into account in Version 1.7):
  • Response to Carl on tau background is here!
  • Impact on AL due to tau background changes is here!
  • Paper ver 2.0 sent to Collaboration, further comments



We report the first measurement of the parity violating single-spin asymmetries for midrapidity decay positrons and electrons from $W^{+}$ and $W^{-}$ boson production in longitudinally polarized proton-proton collisions at $\sqrt{s}=500\,$GeV by the STAR experiment at RHIC. The measured asymmetries, $A^{W^+}_{L}$$=-0.27\pm 0.10\;({\rm stat.})\pm 0.02\;({\rm syst.}) \pm 0.03\;({\rm norm.})$ and $A^{W^-}_{L}$$=0.14\pm 0.19\;({\rm stat.})\pm 0.02 \;({\rm syst.})\pm 0.01\;({\rm norm.})$, are consistent with theory predictions, which are large and of opposite sign. These predictions are based on polarized quark and antiquark distribution functions constrained by polarized DIS measurements.





Figure 1:  (a) Ratios of $E_{T}^{e}$ with respect to the $4\times 4$ BEMC $E_{T}$ sum, $E_{T}^{e}/E_{T}^{X=4 \times 4}$, (b) the near-cone BEMC, EEMC and TPC $E_{T}$ sum, $E_{T}^{e}/E_{T}^{X=R<0.7}$, and (c) correlation of the signed $p_{T}$ balance variable and $E_{T}^{e}$. MC shape distributions (arbitrary normalization) are shown in (a) and (b) for $W^\pm\rightarrow e^\pm+X$ as filled histograms in comparison to both data distributions.  



Figure 2: $E^{e}_{T}$ as a function of the ratio of the TPC reconstructed charge sign to the transverse momentum $p_{T}$. The black-solid and red-dashed lines indicate the selected kinematic region used for the asymmetry analysis. 




Figure 3: $E^{e}_{T}$ for $W^{+}$ (bottom) and $W^{-}$ (top) events showing the candidate histograms in black, the full background estimates in blue and the signal distributions in yellow.




Figure 4: Longitudinal single-spin asymmetry, $A_{L}$, for $W^{\pm}$ events as a function of the leptonic pseudorapidity, $\eta_{e}$, for $25<E^{e}_{T}<50\,$GeV in comparison to theory predictions (See text for details). 




Comparison of preliminary APS result and final result! 


In summary, we report the first measurement of the parity violating single-spin asymmetries for midrapidity, $|\eta_{e}|< 1$, decay positrons and electrons from $W^{+}$ and $W^{-}$ boson production in longitudinally polarized $\vec{p}+p$ collisions at $\sqrt{s}=500\,$GeV by the STAR experiment at RHIC. This measurement establishes a new and direct way to explore the spin structure of the proton using parity-violating weak interactions in polarized $\vec{p}+p$ collisions. The measured asymmetries probe the polarized PDFs at much larger scales than in previous and ongoing pDIS experiments and agree well with NLO and resummed calculations using the polarized PDFs of DSSV08. Future high-statistics measurements at midrapidity together with measurements at forward and backward pseudorapidities will focus on constraining the polarization of $\bar{d}$ and $\bar{u}$  quarks.



Rough timeline:

  • DONE: Tuesday, March 23, 2010: Discussion of PN / PRL paper in W analysis group
  • DONE: Sunday, March 28, 2010:
    • Presentation of final W result (AL incl. all tasks discussed previously)
    • Formal paper proposal presentation to SPIN PWG incl. PN / PRL paper and request to read drafts and convener presentation
  • Tuesday, April 06, 2010: Convener presentation and Deadline for PN / PRL paper comments from SPIN PWG
  • Thursday, April 08, 2010: Answer comments to PN / PRL paper and approval of paper by SPIN PWG followed by request of GPC formation
  • Paper in GPC: April 15-May 13, 2010
  • Thursday, May 13, 2010: Release of paper to collaboration
  • Paper reading by collaboration: May 17-31, 2010 
  • Paper to PRL: First week of June 2010