The STAR experiment

2005

Bernd and I are writing a paper with Yichun Xu and Zhangbu Xu from Spectra that extends STAR’s charged pion cross sections to 15 GeV/c and reports the inclusive charged pion A_{LL} measured using data from Run 5. We are targeting PRL. Yichun is also submitting a NIM paper detailing the methods used to correct the dE/dx distributions in the TPC at high pT. We don’t have a finished draft of the PRL available yet, but we do have a paper webpage.

Briefly, Yichun and Zhangbu use pions from K_{S} decays, protons from Λ decays, and electrons (and pions) identified using the BEMC to obtain a very precise relativistic dE/dx distribution for charged particles as a function of βγ. Their results deviate from the Bichsel function by up to ~3% (0.4σ) at large βγ, and are precise to better than 0.1σ. They also fit the e+/e- ratio as a function of pT to determine the momentum distortion due to space charge in the TPC. Finally, they use JP2 data from 2005 corrected using PYTHIA to obtain charge-separated pion cross sections out to 15 GeV/c that are in agreement with AKK, KKP, and DSS predictions. For more details, see Yichun’s talk at the Davis Collaboration meeting.

On the asymmetry side, I presented an update in April summarizing the 2005 inclusive result and my evaluation of some of the necessary systematic uncertainty checks. The major task left at that time was a study of pion fragmentation in trigger jets and away-side jets that could be used to assign an uncertainty to the < z > plot and hopefully reduce the trigger bias systematic uncertainty in the A_{LL} plot. That study is ongoing; I ended up rewriting most of my simulation analysis code in the process. I don’t have any final plots to show today.

2006

My Run 6 data analysis is focused directly on the jet+pion A_{LL} measurement. I believe another inclusive measurement using this dataset is not as compelling, considering the additional bias introduced by the higher BJP1 trigger threshold. I’ve been using the BJP1 trigger so far, but I may also try to include the L2gamma trigger if there’s time. In the jet+pion measurement I have been identifying trigger jet(s) in each BJP1 event using the following cuts:

• p_{T} > 3.5
• -0.7 < detector η < 0.9
• R_{T} < 0.92
• jet axis within 36 degrees of triggered jetpatch center

and then counting the charged pions which are ΔR > π/2 away from a trigger jet axis. I suppose (based on Murad’s analysis note) that I should also check that the detector η of the jet is on the correct side of the barrel, with a 0.1 margin of error. Charged pions are identified using the same criteria as the 2005 inclusive measurement:

• 2 < p_{T} < 10
• |η| < 1.0
• nFitPoints > 25
• |dcaGlobal| < 1.0
• recalibrated nσ(π) in [-1,2]

I’m using the list of 302 runs that passed the jet and luminosity QA, and I apply the standard 6 < BBC bin < 9 event cut. Just for fun, here’s the A_{LL} plots obtained using those cuts:

I’m evaluating the same suite of final-state-dependent systematic uncertainties (single-spin null tests, trigger bias, background from PID contamination, and false asymmetries from residual transverse polarization) that I checked in the inclusive A_{LL} measurement. I’ll need to fold in systematic uncertainties on the jet reconstruction as well.

SSA vs. pT
SSA vs. fill
A_{Σ} vs. pT

I expect to be able to release a preliminary jet+pion A_{LL} in time for the fall conferences. I don’t believe there will be any theoretical predictions for this measurement in that time frame, but I haven’t pressed our theorist friends on that front, either.