Charmonium feasibility study F. Guber, E. Karpechev, A.Kurepin, A. Maevskaia Institute for Nuclear Research RAS, Moscow CBM collaboration meeting 11 February 2004, GSI

1.Feasibility of J/Ψ measurement 2.Software scheme 3.Dielectrons transverse momenta 4.Combinatorial background problem 5.Avoiding errors, caused by low statistics 6.Conclusions Alla Maevskaia, INR, Moscow CBM meeting, GSI Darmstadt

The main experimental objective is the measurement of extremely rare signals in an environment typical for heavy-ion collisions. The rare signals are embedded in a large background of charged particles. The challenge in the measurement of J/Ψ by their decay into an electron-positron pair is the suppression of the combinatorial background.

0.51 The count rate estimate for J/Ψ production in Au+Au collision at 25GeV/u Alla Maevskaia, INR, Moscow CBM meeting, GSI Darmstadt %

UrQMD central Pluto fireball J/Ψ Geant Standard G4CBM setup: -magnet (zero field) -target 2x2 cm 2 x 300 μ m -carbon pipe, 0.5 mm thick - STS, RICH,TRD,TOF background (15000 events) Hits on the 1st TRD Alla Maevskaia, INR, Moscow CBM meeting, GSI Darmstadt Analysis Pluto fireball ρ 0, open charm Additional background signal Software scheme

Contributions to background Alla Maevskaia, INR, Moscow CBM meeting, GSI Darmstadt Average number of pairs produced via decay of varios particles by PLUTO according to UrQMD, HSD and thermal model and not corrected by the detector acceptance

Electron/positron transverse momentum distribution UrQMD events: γ conversion in the 0.3mm Au target π 0 -> Datitz decay misidentified charge pions. The pion suppression factor was assumed to be (1% for both RICH and TRD) PLUTO events η->Dalitz decay ; ρ->e + e - ; D 0 (all decay modes with e + /e - ) J/Ψ ->e+e- Alla Maevskaia, INR, Moscow CBM meeting, GSI Darmstadt Application of P T >1GeV/c cut suppresses most of the background and rejects only ~14% J/Ψ hitted the 1 st TRD J/Ψ is taken into account only when both e + and e - from it's decay hit the 1 st TRD P T, GeV/c Number of J/Ψ J/Ψ PT distribution ( T=170MeV) full acceptance Ω = 4π with TRD acceptance with acceptance & cut PT>1GeV/c for each e+/e-

Invariant mass spectrum of all e + e - pairs that hitted 1 st TRD and had vertex in the target Cuts applied: Vertex in the target P T >1 GeV/c Open angle>10° Alla Maevskaia, INR, Moscow CBM meeting, GSI Darmstadt Applying the cuts improves signal/background ratio from 1 to ~15 For background estimations: UrQMD used for generating Au+Au 25GeV/u central events and G4CBM for transport. For simplifying :  100% efficiency of electron identification;  perfect track and momentum determination;  no magnetic field,  charge pions suppression factor 10 -4

Since P, φ and θ of dielectrons are considered to be independent in the center mass system of two Au interaction nuclei, their values can be extracted from separate low statistics histograms.. The histogram for the values of P is obtained by ► Fitting the low statistics P histogram. The functions and parameters of the fit change depending on the process ( π 0, η Dalitz decay, γ conversion …) ► Extrapolating the values in the high momentum area by the functions of the fit. Azimutal angle in c.m.s. Polar angle in c.m.s. Avoiding errors, caused by low statistics Alla Maevskaia, INR, Moscow CBM meeting, GSI Darmstadt

The number of values extracted for each contribution is taken from the table (different histograms are used for different contributions). Only the electrons with P T > 1 are included in the combinatorial background Avoiding errors, caused by low statistics II Combinatorial background, obtained from the simulation of 1M Au+Au 25 AGeV central events by the procedure, described above. Combinatorial background obtained from Au+Au 25 AGeV central events, generated by UrQMD Alla Maevskaia, INR, Moscow CBM meeting, GSI Darmstadt J/Psi momentum resolution is assumed to be 1%. J/Ψ is taken into account only when both e+ and e- from it's decay hit the 1st TRD Since after extrapolation more particles with high momentum appeare blue curve is higher than pink

Present status of calculations shows that signal (J/Ψ )/background ratio in Au+Au 25GeV/u collisions is ~10 and J/Ψ can actually be measured by the CBM detector Conclusions Alla Maevskaia, INR, Moscow CBM meeting, GSI Darmstadt

Redo the calculations  for large statistics  with additional background contributions ( as Drell-Yan)  realistic PID and tracking  with more realistic detector geometry  with low beam energy To do list Alla Maevskaia, INR, Moscow CBM meeting, GSI Darmstadt

Acknowledgments We thank for useful discussions and fruitful ideas to Peter Senger Romain Holzmann Volker Friese