J Trigger Parameter Simulation

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J/? event selection algorithm - status
Maciej KrauzeInstitute of Physics University of
Silesia, Katowice
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Motivations

• there are many background events due to very low
  J/? multiplicity
• reduction of the number of events
• in order to make it possible to store the data
• to make the online analysis feasible

Requirements of the method

• fast ? because we measure at the full beam
  luminosity
• using as less detector information as possible
  (currently 3 stations of the Transition
  Radiation Detector)
• efficient ? to reduce the bulk of data passed to
  the next level analysis system

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Software tools

• UrQMD, Pluto, Geant ROOT
• as a software base, CBM framework package was
  used this package incorporates TRD detectors
  layout

The detector layout used in our studies
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Why TRD can be usefull for background event
reduction

• it can provide information about the particles
  trajectory and momentum (estimation!)
• it can distinguish between ee- and hadrons (?,
  p)
• 95-99 of hadron rejection (depends on the
  particles momentum)
• the detector has large material budget so the the
  multiple scattering process has an influence on
  obtained results
• not very high resolution

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Event selection methods ideas

• to supress as many background events as possible
• to preserve the signal

How?

• the main selection critterion is the invariant
  mass value
• we take every 2 particles of unlike charge within
  the same event and calculate the invariant mass
  of the pair
• for the J/? particles, the invariant mass of the
  decay pair is 3.1 GeV/c2
• if the event does not contain any pairs of
  invariant mass greater than 2 GeV/c2, it is
  REJECTED

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Transversal momentum cut

• removes low-energy particles from background
• removes some fraction of signal (depends on the
  threshold value)
• to perform it we need a magnetic field and a
  method of momentum reconstruction

Further reduction of the number of particles
taken to the combinatorics (speed!)

• non-bending plane cut (Y)

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Further reduction of the number of particles
taken to the combinatorics (continued)

• bending plane cut (X)

X
Target
Z
these two geometric cuts combined reject 75 of
secondaries while only 3 of signal is lost
TRD1 TRD2
1 m
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Requirements of the method

• fast track finder (at present we consider ideal
  tracks)
• precise track fitter (Kalman Filter)
• momentum determination method (fast and precise)

Summary and next steps

• the algorithm has roughly 90 efficiency or more
  (depends on the parameters used)
• we have to consider realistic track finder
• one can use some additional cuts (Pt angle,
  opening angle, momentum value etc.)
• to achieve greater efficiency, it may be
  necessary to use information from additional
  detector(s)