Study of the dimuon mass spectra in Pb-Pb collisions with the ALICE muon spectrometer

1. Study of the dimuon mass spectra in Pb-Pb collisions with the ALICE muon spectrometer • OUTLINE : • Introduction • ALICE muon spectrometer • Muon trigger performance • Beauty physics with the ALICE muon spectrometer • simulation inputs • upsilon yields for one month of Pb-Pb collisions • Conclusion

2. ALICE physics goalPPR vol I (CERN/LHCC 2003-049) • study of hot and dense nuclear medium, the Quark Gluon Plasma (QGP), predicted by QCD • In a QGP, QCD predicts the suppression of quarkonia (J/Ψ, ) by “color screening” • In ALICE, quarkonia can be measured via their dimuon decays with the muon spectrometer at forward rapidity Pb Beam Pb Beam IP Dipole magnet + Trigger chambers Front absorber Tracking chambers Muon filter

3. Muon trigger (I) • Muon trigger performs 2 pt cuts : • ptcut ~ 1 GeV/c, optimized for J/Ψ physics → Low pt cut (Lpt) • ptcut ~ 2 GeV/c, optimized for  physics→ High pt cut (Hpt) • Method for muon trigger efficiency calculation : • Input : (pt, η) parameterization for physics continuum (π/K, D, B) & quarkonia (, J/Ψ, ) • Output : trigger response for single muons and dimuons •  Trigger efficiency for muons firing at least 3/4 trigger planes

4. Muon trigger (II) in Pb-Pb collisions @ 5.5 TeV f (b1, b2) = fcoll (b1, b2) × Ptr(b1, b2) • Trigger rates for a centrality class (b1, b2) • fcoll : Collision frequency fcoll (b1, b2) = <L> × σgeo(b1,b2) • Ptr : Trigger probability (poissonian behaviour) For <L> = 5× 1026 cm-2s-1in Pb-Pb collisions, fcoll = 4000 Hz (minimum bias) • Particle multiplicity : • 1 central Pb-Pb collision in 4π (b < 5 fm) ≈ 86000 π/K, 204 D hadrons, 8 B hadrons • For a centrality class, multiplicity of each source is assumed to be proportionnal to the number of hard processes → Glauber model • Trigger multiplicity = number of triggered muons (or dimuons) for a given trigger pt cut

5. f (b<bmax)[Hz] f (b<bmax)[Hz] Lpt cut Hpt cut bmax (fm) bmax (fm) Muon trigger (III)in Pb-Pb collisions @ 5.5 TeV Unlike-sign dimuon trigger rates vs centrality These rates fit the bandwidth of ALICE DaQ for dimuon

6. Beauty physics with ALICE muon spectrometer • Interests for beauty physics :(PPR vol II –CERN/LHCC 2005-030) • in p-p : open beauty measurement will be a test of pQCD (@NLO) • upsilon measurement will be used to test of production models (CEM, COM,...) • in A-A : study of medium effects • upsilon measurement will be used to study“color screening” vs centrality & pt • “energy loss” will be studied from open beauty measurement vs centrality & pt •  Upsilon yields measurement • Method : The upsilon yields are extracted from a fit of unlike-sign dimuon mass spectra with : • an exponential shape for the correlated continuum (beauty + charm) • a breit-wigner “modified” shape for upsilon states

7. Simulation inputs (I) • Muon sources : • upsilon family pt-distributions are extrapolated from CDF data & quarkonia y-distributions are given by CEM model. • (un-)correlated open charm & open beauty are generated using Pythia • π and K are generated from parameterizations of Hijing distributions • Detector efficiency : • detector response (trigger/tracking efficiencies & acceptance & smearing) obtained from fast simulations in AliRoot. • Cut used for analysis :trigger high pt cut & ptμ > 1 GeV/c (on single muon)

8. : nuclear overlap function for Pb-Pb collisions (given by Glauber model) • Csh: nuclear shadowing factor in Pb-Pb (no other nuclear effect introduced) • Brμμ: dimuon branching ratio for quarkonia or heavy quark pair • T = 106 s : effective acquisition time for one month of Pb-Pb collisions Simulation inputs (II) Upsilon states (CEM model) & heavy quark (pQCD @ NLO) cross section in p-p at 5.5 TeV Unlike-sign dimuon spectra normalization :

9. Dimuon mass spectrum from upsilon states • Fit with a sum of 3 “modified” Breit-Wigner : • f(Mμμ) * Г2/(Г2+(Mμμ -M)2) • f(Mμμ) is a polynomial function with 3 free parameters to take into account detector effects on muon transport • mean mass M and mass resolution Г are extracted from fit • upsilon mass resolution : • Г ~ 100 MeV

10. Global fit of correlated unlike-sign dimuon mass spectra • Case of central Pb-Pb collisions (b < 3 fm) • Assumption : perfect subtraction of uncorrelated dimuon pairs  σbin cor= √Nbin tot w/o uncorrelated

11. Yields vs. centrality • large statistics for  • separation of the  states is possible for each centrality class

12. Conclusion • Trigger performance in Pb-Pb : • low pt cut : εtr = 71 % for J/Ψ & unlike-sign dimuon trigger rates = 330 Hz / 4000 Hz MB • high pt cut : εtr = 88 % for  & unlike-sign dimuon trigger rates = 65 Hz / 4000 Hz MB • Upsilon yields from a fit of dimuon mass spectra : • upsilon states yield expected for one month of minimum bias Pb-Pb collisions : • N≈ 7400 N’≈ 2000 N’’≈ 1000 • separation of upsilon states for each of the 5 centrality classes • large statistics & low statistical errors (<10 %) for upsilon in 5 centrality classes BEAUTY PHYSICS LOOKS VERY PROMISING WITH THE ALICE MUON SPECTROMETER

13. Thanks to all my ALICE colleagues who participated to this work

14. BACKUP SLIDES

15. Shape of dimuon mass spectra from correlated charm & beauty Fit with a polynomial function with 8 free parameters for beauty and 5 free parameters for charm

16. Shape of dimuon mass spectra from psi states • Fit with a double Gaussian : • mean mass M Ψ and mass resolution σ are extracted from fit • J/Ψ mass resolution : • σ ~ 70 MeV Ψ ’

17. Quarkonia yields  fit output : quarkonia yields for one month of Pb-Pb collisions (b < 3 fm)pTμ > 1 GeV/c & Mμμ> 2 GeV/c2

18. Fit function For correlated b-bbar : B-chain BB-diff For correlated c-cbar : For upsilon state : Breit-Wigner function

19. Values for slides n°9 • Shadowing factor : Csh(b) Parameterization of shadowing factor vs centrality extract : Data : V. Emelyanov et al., Phys. Rev. C61, 044904 (2000) Extrapolation : S. Grigoryan • : Number of binary collisions in Pb-Pb (Glauber model)