Status of BRAHMS Data Analysis Preparation

Sep. 1999

 J.H. Lee

 One of the important measurements of BRAHMS is  single
 particle yields and cross-section.  Two major ingredients for
 the calculations are Data from the experiment and the
 Weighting Factors  come from acceptance, physics processes
 (decay, multiple scattering...), and hardware and software
 inefficiencies associated with the observables. In addition,
 information on the beam luminosities, trigger normalizations
 are required to calulate cross-section.  Current status of
 preparations for  BRAHMS data analysis  is described.
 The status of data reconstruction software (tracking, pid...) is
 not discussed here.
 

 Data
 

  •  Data Format
    •    Input data for the calculations  will be RDO (Reconstructed

       Data Objects) files or PhDs (Physics Data sets).  The proposed
       RDO data format (somewhat old) can be found in here .
       Obviously the data formats needs to be discussed and work on more to
       be "finalized".  Generating RDO files has been exercised during the
       MDC's with GEANT-generated data but needs some more work. I've
       also tried with  raw data taken during RHIC commissioning run , even
       though not many detectors were part of it.
  •  Data Selection
    •    Good  track selection criteria are being studied with simulated data.

       Locating an interaction point (vertex) of the collision is one of the
       most important step to select "good (physics) tracks" over background
       tracks not originated from the production vertex.
       The code for reconstructing a production vertex using tracks in the MRS
       has been written and tested with central Fritiof events generated at x,y,z=0.
       The reconstructed vertex distributions at MRS  90 degreecan be found in here .
       See  also  the  figure  for the distribution of  "x vs z" of the reconstructed
        vertecies from the central Fritiof  events generated with z between
       -30  and +30cm.   The vertexing algorithm is  done with iterations to
       minimize contributions from background tracks.
       A description of the method will be available soon.

     Event (Track) Weight
     

  •  Geometrical Acceptance
    •     Since the BRAHMS spectrometers have relatively small apertures,

        the geometrical acceptance will be a dominant factor for the weight.
       Two main tasks to calculate acceptance factor can be
       -  deciding p-theta (y-pt) aceptance boundaries for the given
           experimental conditions (angle, field...): this factor can be 1 (accepted)
           or 0 (not accepted).
       -  calculating phi acceptance:

          1. The p-theta boundary and phi (azimuthal) acceptance both can be either
              analytically approximated (E802 and E866 HH) or  numerically
             estimated using MC (E866 FS).
             Since the geometry of BRAHMS FS is rather non-trivial to be
             approximated analytically, I propose to use Monte-Carlo to calculate
             boundaries.
          2. Since the vertex position is not fixed (expected interaction sigma =
             15 - 25 cm?), the p-theta boundary should be calculated as a function
              of z. (See figures of "acceptance vs z" for  MRS 90 degree for pi+ ,
              pi- ,  MRS 35 degree for pi+ ,  pi- ,  FS 2.3 degree pi+ , and pi- .)
          3. Since the vertex position varies in z,   the boundaries
              for  p-theta-z has to be estimated for the each  experimental settings.
              Assuming +-15 to 20 cm is the interaction region we are interested
              in,  the variation of the acceptance on z can be 10-20% depending
              on the angle.  The acceptance is calculated using root histogram
              operations for each bins.  Since we have all "final" detector
              dimensions including TOFW, I'll soon start production of the acceptance
              maps.  Initially the map will have a information of (output/input) in
              small bin sizes  (y: 0.005 - 0.01,  p: 50 - 100 MeV).
         4.  Reconstructing pt spectra  has been exercised using central
              Fritiof/GEANT data.  Since the pid is not fully functional yet,
              the spectra reconstructed was for h+.   The difference between
              the acceptance corrected reconstructed spectra  and  the initial input  is
              0-30%.
         5.  Communication between acceptance maps and the database has to be
              done.
     

  •  Correction factors
    •      1.  Software correction factors

             Even though some estimates of  tracking inefficiency has been made, more
             detailed study has to be made. Especially multiplicity/centrality dependence.

         2.  Hardware ineffciency
             TPC and DC inefficiencies has to be calculated , but they are expected to be small.
     

  •  Corrections for decaying particles
    •       Contaminations from decaying particles has to be estimated.

          How well we can estimate proton (anti-proton) from lambda (lambda-bar)
          is an especially  important question.   A rough estimate has been made for proton
          contamination at mid-apidity (MRS at 90 degree) using  the distributions
          of vertex x of proton  and  vertex x of proton from lambda . (generated with
          proton:lambda =2:1 based on RQMD): 20-30% contamination at the mid-rapidity.
          Need further and more detailed studies.