### STAR Software for Analysis of Anisotropic Flow

R.J. Snellings and A.M. Poskanzer
Nuclear Science Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA

In the ROOT for STAR environment, c++ programs have been written to analyze STAR data for anisotropic (directed and elliptic) flow. This document is a road map to the available documentation. The software consists of 7.3k lines of code in 33 files in three directories. The three "maker" modules are called from the STAR software chain and described in the overview presentation. The presentation is also available as a postscript file together with an overview diagram using the Unified Modeling Language. The chain of three makers is run off-line by the macro doFlowEvents.C . In this macro, parameters can be set for the event cuts, track cuts, flags, event plane selections, and PID windows. Also in this macro StFlowSelection can be instantiated and passed to the Tag, Analysis, and Cumulant makers. This object can be set to select track PID.

StFlowMaker is the interface with the STAR DST. It creates StFlowEvent from StEvent or StFlowPicoEvent after event and track cuts. StFlowEvent consists of a track collection of tracks. It also contains methods for calculating event-by-event quantities, like the event plane vector, from the data members of the tracks. The event plane is made isotropic in the laboratory with weights which are the inverse of the acceptance as determined from the azimuthal distribution of particles. This is all described in the README document. One can also view the header, the implementation, and the interface. A persistent sub-set of StFlowEvent can be written out to disk as StFlowPicoEvent. Constants which are needed in more than one file are kept in StFlowConstants.

StFlowTagMaker fills the tag database from StFlowEvent. This is described in the README document. One can also view the header, the implementation, and the interface.

StFlowAnalysisMaker reads the event-by-event quantities from the tag database, if it exists, or if it does not, from StFlowEvent. It reads the particle quantities from StFlowEvent and does the flow analysis1 by correlating the particles with the event plane after removing autocorrelations. The main results are double-differential flow values contained in 2D histograms of the flow variables, v_n, as a function of pseudorapidity and transverse momenta. For identified particles rapidity is used instead of pseudorapidity. A second analysis is done in terms of the normalized vector q which is equal to the length of the flow vector Q divided by the square-root of the sum of the squares of the weights. This integral measure of flow is good for studying the fluctuations in flow. This is all described in the README document. One can also view the header, the implementation, and the interface. The macros plot.C and plotCen.C are used to plot the resulting histograms. The macro minBias.C combines the histograms from different centralities to make a minimum bias histogram file.

StFlowCumulantMaker which is in the StFlowAnalysisMaker directory, can perform a multiparticle analysis using the cumulant method.7 One can also view the header, the implementation, and the interface. The macro plotCumulant.C is used to plot the resulting histograms.

Simulated events which have been run through GEANT have been analyzed and results produced. There is also a STAR Note2 which contains predictions for STAR, a paper3 on the expected behavior of the directed flow, and a paper4 on how to interpret the physics results. Now there are also first results for charged particles5 and identified particles.6

Acknowledgments
We thank Glenn Cooper for considerable help with c++ and ROOT. Markus Oldenburg contributed the FTPC code and Aihong Tang the cumulant analysis.

Footnotes and References
1. A.M. Poskanzer and S.A. Voloshin, "Methods for Analyzing Anisotropic Flow in Relativistic Nuclear Collisions", Phys. Rev. C 58, 1671 (1998).

2. R.J.M. Snellings, A.M. Poskanzer, and S.A. Voloshin, "Anisotropic Flow at STAR", STAR Note SN0388(1999).

3. R.J.M. Snellings, H. Sorge, S.A. Voloshin, F.Q. Wang, and N. Xu, "Novel Rapidity Dependence of Directed Flow in High Energy Heavy Ion Collisions", Phys. Rev. Letters 84, 2803 (2000). nucl-ex/9908001.

4. S.A. Voloshin and A.M. Poskanzer, "The Physics of the Centrality Dependence of Elliptic Flow", Phys. Letters B474, 27 (2000). nucl-th/9906075.

5. K.H. Ackermann et al., "Elliptic Flow in Au+Au Collisions at sq_root(s)=130 GeV", Phys. Rev. Letters 86, 402 (2001). nucl-ex/0009011.

6. C. Adler et al., "Identified Particle Elliptic Flow in Au+Au Collisions at sq_root(s)=130 GeV", Phys. Rev. Letters 87, 182301 (2001). nucl-ex/0107003.

7. N. Borghini, P.M. Dinh, and J.Y. Ollitrault, "Flow Analysis from Cumulants: a Practical Guide". nucl-ex/0110016.