# Presentation: 843415

## 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**

**Notes:**
Study of the dimuon mass spectra in Pb-Pb collisions with the ALICE muon spectrometer
The outline of my presentation is the following.

<< After a brief introduction on ALICE physics goal, I’ll show the ALICE muon spectrometer and I’ll present the muon trigger performance.
Then, after a brief introduction on the interests of beauty physics, I’ll show the simulation inputs used to extract the upsilon state yield from a fit of unlike-sign dimuon mass spectra. Finally, I’ll show the preliminary results. >>

## ALICE physics goalPPR 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 Beam**

**Pb Beam**

**Front absorber**

**Notes:**
ALICE physics goal.

The creation of hot and dense nuclear medium, the quark gluon plasma, predicted by QCD, requires extreme thermodynamical conditions which are reached in laboratory with ultra-relativistic heavy ion collisions. 
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 (p).

ALICE is equipped with a muon forward spectrometer, between 2°and 9°.
The ALICE muon spectrometer can be divided in four parts :
The front absorber (p) which reduces the background in the spectrometer.
The tracking chambers (p) and the dipole magnet (p) which measure the particle momentum.
The muon filter (p) which further reduces the background on the trigger chambers.
The trigger system (p) which selects events with high pT muon tracks.

## Muon trigger (I)

** **_**Muon trigger performs 2 p**__**t**__** cuts :**_
-  ptcut ~ 1 GeV/c, optimized for _J/__Ψ physics_ 	→ Low pt cut (Lpt)
-  ptcut ~ 2 GeV/c, optimized for _****__** physics**_ 	→ **High p****t**** 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_

| Quarkonia |  |  | Trigger 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 % |

**Notes:**
The muon trigger performs two trigger pt cuts.
The first one, at pt about 1 GeV/c, called Low pt cut, is optimized for J/Ψ physics.
The second one, at pt about 2 GeV/c, called High pt cut, is optimized for upsilon physics.
The inputs for muon trigger efficiency calculation are parameterizations for background (pi/ka) charm and beauty. In outputs, we obtain the muon trigger response. The trigger efficiency is calculated for muons firing at least 3⁄4 trigger planes. The results are summarized in this table (p).
We note that the trigger efficiency for pions and kaons is around thirteen % for the low pt cut, and 4% for the high pt cut. 27% for muons from charm for the low pt cut and 36% for muons from beauty for the high pt cut.
The trigger efficiency for the J/Ψ is 71% for the low pt cut (p) and 88% for upsilon for the high pt cut (p).

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

** **_**Trigger rates for a centrality class**_** (b****1****, b****2****)**  

*f *(b1, b2) = *f*coll (b1, b2) × Ptr (b1, b2)

-  *f*coll : Collision  frequency		              *f*coll (b1, b2) = <L> × σgeo(b1,b2)
-  Ptr : Trigger probability (poissonian behaviour)

- For <L> = 5× 1026 cm-2s-1 in Pb-Pb collisions, ***f*****coll**** = 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 p****t**** 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 |

**Notes:**
The trigger rate for a given centrality class is the product of the collision frequency by the trigger probability. From luminosity and geometrical cross section for a given centrality class, we can calculate the collision frequency. For nominal luminosity (p) , the collision frequency is about 4000 Hz for Pb-Pb minimum bias collisions.

The trigger probability has a poissonian behaviour relative to the trigger multiplicity that I’ll define right now.

One central Pb-Pb collision in 4pi corresponds to these particle multiplicities (p) for pions ad kaons, charm and beauty meson.
For a given centrality class, the multiplicity for each source is assumed to be proportionnal to the number of hard processes. We used the Glauber model to calculate this centrality dependance.

The trigger multiplicity depends on the generated particle multiplicity. The trigger multiplicity is defined as the number of triggered muons or dimuons for a given pt cut. 
The trigger multiplicity for single muons and dimuons is given in this table for Pb-Pb central collisions, and for low and high pt cuts.
For example, in central Pb-Pb collisions, about two single muons and one dimuon are above the low pt cut (p).

## 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 dimuon
Minimum bias |
| --- |
| fcoll = 4000 Hz |
| 330 Hz (Lpt) |
| 65 Hz (Hpt) |

_**Unlike-sign dimuon trigger rates vs centrality**_

**These rates fit the bandwidth of ALICE DaQ for dimuon**

**Notes:**
Trigger rates vs impact parameter in PbPb collisions at 5.5 TeV.

The plots show the integrated trigger rates versus impact parameter for unlike-sign dimuons (p), for low pt cut (p) and high pt cut (p). The results for minimum bias events are summarized in this table for low and high pt cuts (p).
For unlike-sign dimuons, the expected trigger rates in minimum bias is around 330 Hz for the low pt cut and 65 Hz for the high pt cut, to be compared to the collision frequency of 4000 Hz. Such event rates fit the bandwidth of ALICE DaQ. 
These trigger rates can be compared to the number of quarkonia triggered per second which are reported in this table. We note that about one J/Ψ per second and one upsilon in one hundred seconds can be triggered.

## 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

**Notes:**
Beauty physics with ALICE muon spectrometer:

Beauty physics at LHC presents a great interest.

In p-p collisions, open beauty measurement will be a test of pQCD at NLO and Upsilon measurement will be used to test production models (color evaporation model, color octet model or others). 
In A-A collisions, Upsilon measurement will be used to study color screening vs centrality and pt, and energy loss will be studied from open beauty measurement.

In what follows, we’ll introduce a method for upsilon states yield measurement.

The upsilon yields are extracted from a fit of unlike-sign dimuon mass spectra with an exponential shape for the correlated continuum (beauty + charm) and with 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 |

**Notes:**
Simulations inputs. 

For muon sources, the quarkonia pt-distributions are extrapolated from CDF. The quarkonia rapidity-distributions are given by the CEM model.
Uncorrelated & correlated open charm & open beauty are generated using Pythia.
Pions and kaons are generated from parameterizations of hijing distributions.

The detector response (trigger & tracking efficiencies & acceptance & smearing) is obtained from fast simulations in AliRoot.
The total efficiency is the product of tracking efficiency by trigger efficiency. The total efficiency and the acceptance are reported in this table (p) for open charm, open beauty and upsilon states. For the analysis, the trigger high pt cut is used and an additional pt cut of 1 GeV/c is applied to each muon.

## Simulation inputs (II)

**Simulation inputs (II)**

| (hep-ph/0311048) |  |  |  | ’ | ’’ |
| --- | --- | --- | --- | --- | --- |
| σpp (μb) | 6640 | 210 | 0.501 | 0.246 | 0.100 |

- _Unlike-sign dimuon spectra normalization :_

**Notes:**
The unlike-sign dimuon mass spectra are normalized using the equation (p).
In this equation, the cross sections in proton-proton collisions at 5.5 TeV are given in this table (p) for upsilon states, open charm and open beauty. 
This term (p) is the number of binary collisions in Pb-Pb obtained from the Glauber model.
This term (p) represents the nuclear shadowing factor in Pb-Pb collisions. No other nuclear effect is introduced.
This term (p) represents the dimuon branching ratio for quarkonia or heavy quark pairs.
This term (p) is the effective acquisition time corresponding to one month of Pb-Pb collisions. It is fixed at ten to the six seconds.

## 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

**Notes:**
This plot (p) shows the shape of dimuon mass spectra for upsilon states for one month of central Pb-Pb collisions. 
The bin width is 50 MeV.
Each Upsilon state is fitted with a modified Breit-Wigner having five parameters (p). The function f (p) is a polynomial function with three  parameters to take into account detector effects on muon transport. The mean mass M and mass resolution are extracted from the fit.
The Upsilon mass resolution is about 100 MeV for each state.

## 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**

**Notes:**

In the case of central Pb-Pb collisions, this plot (p) shows the uncorrelated and correlated components of unlike-sign dimuon mass spectra. The uncorrelated background can be extracted by event mixing or like sign method. In the case of a perfect subtraction of uncorrelated background, the statistical errors in each bin of correlated dimuon mass spectra is given by this expression (p), where N_bin_tot is the number of entries in each bin of the total mass spectra . 

After subtraction of uncorrelated background, we extract the upsilon yields from a fit of correlated dimuon mass spectra. This plot (p) shows the fit of correlated dimuon mass spectra and extracted yields for one month of central Pb-Pb collisions. For comparison, this table (p) shows the simulated yields. One can see that the numbers are very close.

##  Yields vs. centrality 

** ****large statistics for ******
** ****separation of the ******** states is possible for each centrality class**

**Notes:**
These plots (p) show the fit quality and extracted yields from these method, in 4 centrality classes and for one month of Pb-Pb collisions.
We note the large statistics for upsilon. The separation of the different upsilon states is possible for each centrality bin.
The last plot show the yields obtained from fit for one month of minimum bias Pb-Pb collisions.

## 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**

**Notes:**
In conclusion:

About trigger performance :
For the low pt cut, the trigger efficiency for J/Ψ is around 71 % and the unlike-sign dimuon trigger rate is 330 Hz. 
For the high pt cut, the trigger efficiency for  is around 88 % and the unlike-sign dimuon trigger rate is 65 Hz. 

About upsilon state yields :
The upsilon yields have been extracted from a fit of the dimuon mass spectra. We expect about 8 thousands Upsilon, 2 thousands Upsilon’ and one thousand upsilon’’ for one month in minimum bias Pb-Pb collisions.

For each centrality class, we note that a separation of the upsilon states is possible, and a large statistics and quite low statistical errors, less than 10 %, for upsilon.

We can however conclude that 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

**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

**Notes:**
These plots (p) show the shape of the dimuon mass spectra for correlated charm (p) & beauty (p). 
The bin width is 50 MeV. 
The beauty continuum is fitted with a polynomial function with 8 free parameters, shown in green in this plot. The charm continuum is fitted with a polynomial function with 5 free parameters.

(1 mot sur B-chain et BB-diff ?) :
At low mass, around 2GeV, the peak from so-called B-Chain decay appears. The 2 muons originate from the same 
B meson, through a cascade decay.
At high mass, the long tail from so called BB-diff correlated pairs dominates. In this case, the 2 muons 
originate directly from the b-bbar quark-antiquark pair.

## 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 ’

**Notes:**
This plot (p) shows the shape of dimuon mass spectra for upsilon states for one month of central Pb-Pb collisions. 
The bin width is 50 MeV.
Each Upsilon state is fitted with a modified Breit-Wigner having five parameters (p). The function f (p) is a polynomial function with three  parameters to take into account detector effects on muon transport. The mean mass M and mass resolution are extracted from the fit.
The Upsilon mass resolution is about 100 MeV for each state.

## 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 |

## Fit function

_**For correlated b-bbar :**_

_**For correlated c-cbar :**_

_**For upsilon state :**_

**Breit-Wigner function**

**B-chain**

**BB-diff**

## Values for slides n°9

|  |  |  | J/Ψ | Ψ’ |  | ’ | ’’ |
| --- | --- | --- | --- | --- | --- | --- | --- |
| Csh(b=0) | 0.65 | 0.84 | 0.60 | 0.60 | 0.76 | 0.76 | 0.76 |

-  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****) **

|  centrality class | 0 < b < 3 fm | 3 < b < 6 fm | 6 < b < 9 fm | 9 < b < 12 fm | 12 < b < 16 fm |
| --- | --- | --- | --- | --- | --- |
|  | 1932 | 1348 | 657 | 182 | 14 |

**Values for slides n°9**