Introduction

Objectives

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

Jia Shen Saint Mary’s College of CaliforniaJia Shen Saint Mary’s College of California

Dr. Ralf Rapp Physics Department and Cyclotron Institute at Texas A&M University, College Station, TX 77840-3366Dr. Ralf Rapp Physics Department and Cyclotron Institute at Texas A&M University, College Station, TX 77840-3366

1

ResultsQuark-gluon plasma (QGP) is believed to consist of freely moving quarks and gluons. By colliding heavy ions at ultra-relativistic energies, QGP comes to existence for about 5fm/c. During that time, light quarks, such as up and down quarks, thermalize quickly and lose their original imprinted information while heavy quarks, such as charm quarks take longer to thermalize. Thus, by studying charm quark spectra, we will be better understand the interactions in the quark-gluon plasma. In this project, we focused on di-electron invariant-mass spectra from correlated charm decays.

Fig.1 Tracks of particles produced in the heavy-ion collisionhttp://en.wikipedia.org/wiki/Quark-gluon_plasma

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Methods

Fig.3 single charm quark Pt distribution by dN/dPt=cPte^(-Et/Teff)Green data is when Teff=0.5; red data is when Teff=0.35

Fig.4 Invariant-mass distribution of dileptons with Teff= 0.35 in the charm Pt spectrum Green data indicates the case when dileptons decay from charm-anticharm pairs in opposite directions; red data indicates the case when charm-anticharm pairs are in unrelated random directions.

Fig.5 Invariant-mass distribution of dileptonsdecay from charm-anticharm pairs in opposite directionsGreen data indicates Teff= 0.5 in the charm Pt spectrum; red data indicates Teff= 0.35 inthe charm Pt spectrum.

Fig.6 same as Fig.5 but with random imprinted angles between charm pairs

Conclusions

• softer charm pt-spectrum reflects itself in softer dilepton invariant-mass spectrum for both angular scenarios• Sensitivity of dilepton spectra to single-charm and charm-anticharm correlations is confirmed and quantified.• Experimental acceptance cuts were implemented

Future Directions

• Implement more realistic charm- 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

Trent StrongHendrik van HeesSherry YennelloSteve Rose

Acknowledgement

Figure 2 shows the invariant-mass distribution of dilepton spectra measured by PHENIX Collaboration (2007). In order to understand the data in the range between M=1GeV and 3 GeV, we need to quantitatively understand the contribution from correlated charm decays. As indicated in the figure, this contribution is quite sensitive on the angular correlation between charm and anticharm quarks.

Fig.2 Invariant-mass distribution of dilepton spectra by PHENIX Collaboration (2007)

The main tool we used in this project is Fortran computer programming language. First, we generated distributions for the transverse-momentum (Pt) spectra of single anti-/charm quarks (Fig.3), characterized by a slope parameter Teff. For each charm-anticharm pair, an electron-positron pair will result from their decay. In order to generate the invariant-mass distribution of these dileptons, we first chose 2 random angles for the positron with respect to the rest system of the charm quark, and assumed its momentum in the rest system to be 1/3 of the charm-quark mass. Then we used a Lorentz Transformation to boost the electron momentum into the lab system. We repeated the procedure for anticharm quarks decaying into electrons. For each charm-anticharm pair, we then used equ.1to calculate the invariant-mass for a electron-positron pair. Finally, we used gunplot to generate the histogram of invariant-mass distribution of dileptons.

M^2=(E_+E+)^2-(P_+P+)^2 (1)

Results