STAR Flow, Chirality and Vorticity PWG

["Huan Zhong Huang" <huang AT physics.ucla.edu>]

Hi Jiayong,

   I do not know if you were at Zhiwan’s pwg presentation for her update. Based on your summary talk at the plenary session, it appears that you may have missed the key essence of the Event-Shape Selection (ESS) method used in her analysis. Some people including Zhiwan used term Event-Shape Engineering (ESE), I wish that we could actively engineer the events. I prefer to state that we can only passively select class of events.

  Our ESS goals are to 1) select spectator protons to determine the event plane (as close as to reaction plane possible), and 2) use Shape variables to select class of events with shapes as spherical as possible. For goal 1) we divide EPD into inner and outer rings with different eta coverage. For each BES II beam energy data we find the optimal division between inner and outer rings and only used inner part to determine the event plane (some other selections were also presented in Zhiwan’s talk for comparisons). For goal 2) we used single particle q2 or parent particle (hypothetical particle pair parent) q2_parent for particles in the central region of TPC to select event shape. Then based on the event shape selection we calculate the delta-gamma correlation when the event is most spherical (delta-gamma when q2 or q2_parent is approach zero).

  There are some major differences from the ESS method previously used. Previously, people used produced particles from a separated eta region to do event shape calculation and found that the calculated shapes have significant fluctuations/deviations from the shape for particles-of-interest. We found that such event shape selection approach is not effective, presumably due to fluctuations, non-flow and flow de-correlations etc. We argue that the shape selection with an eta gap in fact defeats the purpose of shape selection. Often the eta gap was justified based on removing auto-correlations, a concern for v2 measurement. But our goal is to study and remove the correlation between delta-gamma and the shape (apparent v2 due to flow and non-flow), then it is appropriate to use the particles of interest to do the shape calculation. There may be residual delta-gamma and shape correlation (different flow and non-flow contributions to the shape). That is why we try to use single particle q2 and parent q2_parent as possible shape observables to investigate the dependence. We also use models of AMPT and AVFD to investigate the sensitivity on shape determination.

  The CME is not intrinsically correlated with apparent v2, i.e., as long as the spectator protons have generated a magnetic field direction, the particles in the central TPC region can be completely spherical (no flow/non-flow correlations) and could still have a CME. We think that the shape calculation using the particles of interest for CME search is not an issue.

  As pointed out at the PWG meeting, the use of EPD in our ESS approach is not just to introduce an eta gap for event plane determination to reduce the non-flow effect. We really need to select the EPD inner eta region so that we optimally select spectator protons for the event-plane which allowed us to take advantages of the ESS approach maximizing the sensitivity on B field direction and CME. A eta gap without specifically aiming for spectator protons would vary the magnitude of non-flow effect and also introduce flow de-correlations and other fluctuations, which may not provide significant advantage.

  Unfortunately, the EPD sensitivity to spectator protons will not work well for 200 GeV data. The BES II data present us with the unique opportunity. I am hopeful that we can make some progress on the CME search with the BES II soon.

  We welcome comments and suggestions on the analysis approach.

  Thanks. Regards,

  Huan