Jia Shen

1
Dilepton Spectra from Open Charm Decays in
Heavy-Ion Collisions

• Jia Shen
• Saint Marys College of California
• Dr. Ralf Rapp
• Cyclotron Institute at Texas AM University

2
Quark-Gluon Plasma and Heavy-Ion Collisions

• A quark-gluon plasma is believed to exist during
  the first 10 microseconds after the universe is
  created from the Big Bang.
• In the heavy-ion collision, a quark-gluon plasma
  is believed to exist for a very short time.

rhic.physics.wayne.edu/sean/collision_a.gif
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Particles produced in a Heavy-Ion Collision as
seen by the STAR Detector
http//en.wikipedia.org/wiki/Quark-gluon_plasma
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This project

• How to detect the Quark-Gluon Plasma?
• Idea
• - Light quarks and gluons thermalize quickly
  and lose imprinted information
• - charm quarks are much heavier and do not
  easily thermalize gt more sensitive probe of
  interactions in QGP
• - use di-electron decay spectra from charm
• quarks to probe Quark-Gluon Plasma

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Dilepton Spectra from PHENIX Collaboration (2007)

• Dileptons from charm dominant in the M1-3GeV
  mass region!

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Step-1Angular Correlation between charm and
anticharm quark

• Back-to-Back
• (no interaction)
• Random angle
• (thermalization)

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Step-2 Input charm transverse-momentum
spectra dN/dPt c Pt e(-Et/Teff)

• key parameter Teff gt slope of the charm
  spectrum

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Step-3Procedure to calculate ee- spectrum

• Decay c-quark into positron in its rest system
• boost positron into lab system
• repeat for anticharm -gt electron
• Calculate invariant mass of electron-positron
  pair M2(E_E)2-(P_P)2
• study dependence of invariant mass distribution
  on charm-quark input (slope and angular
  correlation)

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Result-1 Sensitivity to charm-anticharm
relative angle

• random angle gives softer spectrum than
  back-to-back (180deg)

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Result-2Sensitivity to slope of charm spectrum
Back-to-back
Random Angle

• softer charm pt-spectrum reflects itself in
  softer dilepton
• invariant-mass spectrum for both angular
  scenarios

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Conclusions

• Sensitivity of dilepton spectra to single-charm
  and charm-anticharm correlations confirmed and
  quantified
• Experimental acceptance cuts implemented

12

• Future Directions
• More realistic charm-anticharm input spectra
• Check against single electron spectra in p-p
  collisions
• Use a model for charm-quark Interactions in the
  QGP (consistent with single-electron spectra in
  Au-Au Collisions) to obtain charm and dilepton
  spectra in Au-Au collisions