study of the dimuon mass spectra in pb-pb collisions with the alice muon spectrometer

F. Guérin –Hot Quarks 2006 - Villasimius 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


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

Trigger chambers Dipole magnet Tracking chambers + Muon filter

IP

Pb BeamPb Beam

Front absorber


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

QuarkoniaTrigger

efficiency

Physics continuum

→ μ+μ- J/Ψ → μ+μ- → μ+μ- π/K → μ D → μ B → μ

13 % 71 % 97 % Low pt cut 13 % 27 % 70 %

2 % 22 % 88 % High pt cut 4 % 8 % 36 %


Muon trigger (II) in Pb-Pb collisions @ 5.5 TeV

Trigger rates for a centrality class (b1, b2) f (b1, b2) = fcoll (b1, b2) × Ptr (b1, b2)

fcoll : Collision frequency fcoll (b1, b2) = <L> × σgeo(b1,b2) Ptr : Trigger probability (poissonian behaviour)

For <L> = 5× 1026 cm-2s-1 in 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

Single muon

Trigger multiplicity in central Pb-Pb

collisions

Unlike-sign dimuon

Low pt cut High pt cut Low pt cut High pt cut

2.09 0.64 1.25 0.16


Muon trigger (III)in Pb-Pb collisions @ 5.5 TeV

fJ/Ψ 1 Hz for Low pt cut

f 1/100 Hz for High pt cut

Unlike-sign dimuonMinimum bias

fcoll = 4000 Hz

330 Hz (Lpt)

65 Hz (Hpt)

Unlike-sign dimuon trigger rates vs centrality

Hpt cutLpt cut

f (b<bmax) [Hz]

bmax (fm) bmax (fm)

f (b<bmax) [Hz]

These rates fit the bandwidth of ALICE

DaQ for dimuon


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 & p t

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


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)

’ ’’

αacc 0.036 0.044 0.05 0.05 0.05

0.0018 0.027 0.736 0.746 0.751

cc bb

TkHptTr


Upsilon states (CEM model) & heavy quark (pQCD @ NLO) cross section in p-p at 5.5 TeV

Simulation inputs (II)

(hep-ph/0311048) ’ ’’

σpp (μb) 6640 210 0.501 0.246 0.100

cc

TBrbCLTbfbN accshppPbPbcollPbPb

)()()(

• : 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

PbPbT

Unlike-sign dimuon spectra normalization :

bb


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


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

Input yields

N = 1323

N ’ = 351

N ’’ = 194

w/o uncorrelated


Yields vs. centrality

• large statistics for

• separation of the states is possible for each centrality class


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


Thanks to all my ALICE colleagues who participated to this work


BACKUP SLIDES


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

bbcorrelated cccorrelated


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 ’


Quarkonia yields

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

J/Ψ Ψ’ ’ ’’

Nμμ (103) 53.08 ± 4.18 1.344 ± 0.421 1.316 ± 0.049 0.348 ± 0.032 0.193 ± 0.029


),,(

)(1

)(1)( 765 2

23

3 21

0 4

xGauss

x

xxf

Fit function

For correlated b-bbar :

4 2

23

3 21

0)(1

)(1)(

x

xxfFor correlated c-cbar :

2

22

3

22

23

3 21

0)(

1

)(

)(1)(

4

xx

xxfFor upsilon state :

Breit-Wigner function

B-chain

BB-diff


J/Ψ Ψ’ ’ ’’

Csh(b=0) 0.65 0.84 0.60 0.60 0.76 0.76 0.76

cc bb

• Shadowing factor : Csh(b)

4

16)0(1)0()(

bCCbC shshsh

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)

centrality class

0 < b < 3 fm3 < b < 6 fm

6 < b < 9 fm9 < b < 12 fm

12 < b < 16 fm

1932 1348 657 182 14

inelppPbPb

PbPbpp bTN

)(

PbPbppN

Values for slides n°9

study of the dimuon mass spectra in pb-pb collisions with the alice muon spectrometer

Documents