# Presentation: 114222

## Study of the J/ production and suppression in Indium-Indium collisions at the CERN SPS

- Study of the J/ production and suppression in Indium-Indium collisions at the CERN SPS

- Alberto Colla - INFN Torino, Italy
- for the NA60 Collaboration

- Hot Quarks 2004
- July 18 - 24, Taos Valley, NM, USA

- _Outline of the presentation:_
-  Physics motivation of NA60
-  Overview of the detector concept
-  First results from the 2003 Indium run

## Heavy ion collisions and dilepton probes (a)

-  **Dileptons** produced in **heavy-ion collisions** are very useful probes for the study of the **QCD ****	****phase transition** from hadron to quark-gluon matter
-  Since **1986** a **systematic measurement of dilepton spectra** from high energy collisions 		has been carried out at the **CERN SPS** by various experiments (**NA38**, **NA50**, **CERES ...**).
-  Many interesting results were found:

## NA50

- The **J/****** production is **suppressed** in heavy-ion collisions (**Pb-Pb**) with respect to the yields extrapolated from **proton-nucleus** data

- p-A and S-U data: J/ absorption in “normal” nuclear matter, with                                      (*)

- Heavy ion collisions and dilepton probes (b)

- (*) G. Borges (NA50 Coll.), QM 2004

## NA60

**What is the origin of the low mass dilepton excess?****	******** **need much more statistics, better signal to background ratio and mass resolution
**	******** **resolve the  peak
**	******** **study the signal versus pT and collision centrality
**Is the intermediate mass excess due to thermal dimuons from a quark-gluon plasma?**
**What is the open charm yield in nucleus-nucleus collisions?**
**	******** **measure secondary vertices with ~ 50 μm precision
**	******** **separate prompt dimuons from D meson decays
**What is the physics variable driving the ****J/******** suppression?  L, N****part****, energy density?**
**Are the charmonium states broken by deconfined quarks and gluons?**
**	******** **measure the J/ pattern in Indium-Indium and compare it with Pb-Pb
**Which fraction of ****J/******** comes from ********c**** decays**** ****(********c**** → J/******** + ******** ? **
**What is the impact of the ********c**** feed-down on the observed J/******** suppression pattern?**** ****	******** **study the nuclear dependence of c production in p-A collisions

**New and accurate measurements are needed**

## The NA60 Experiment

- Idea: place a *high granularity* and *radiation-hard** ***silicon tracking telescope** in the **vertex region**to measure the muons before they suffer multiple scattering and energy loss in the absorber

- R. Arnaldi, R. Averbeck, K. Banicz, K. Borer, J. Buytaert, J. Castor, B. Chaurand, W. Chen,B. Cheynis, C. Cicalò, A. Colla, P. Cortese, S. Damjanović, A. David, A. de Falco, N. de Marco,A. Devaux, A. Drees, L. Ducroux, H. En’yo, A. Ferretti, M. Floris, P. Force, A. Grigorian, J.Y. Grossiord,N. Guettet, A. Guichard, H. Gulkanian, J. Heuser, M. Keil, L. Kluberg, Z. Li, C. Lourenço,J. Lozano, F. Manso, P. Martins, A. Masoni, A. Neves, H. Ohnishi, C. Oppedisano, P. Parracho,G. Puddu, E. Radermacher, P. Ramalhete, P. Rosinsky, E. Scomparin, J. Seixas, S. Serci, R. Shahoyan,P. Sonderegger, H.J. Specht, R. Tieulent, G. Usai, H. Vardanyan, R. Veenhof, D. Walker and H. Wöhri

- ~ 60 people
- 13 institutes8 countries

## NA60’s detector concept

## The NA60 target region: reality

**Pixel detectors **

**Beam Tracker **

**2.5 T dipole magnet **

- ~ 100 pixel detectors (radiation tolerant)in 11 tracking points; cells = 50 × 425 μm2

- Two stations of 50 m pitch micro-strip detectors Operated at 130 K  increased radiation hardness

## A first look at the Indium data

- target boxwindows

**7 In targets**

**z-vertex (cm)**

- 6500 A

- 4000 A

-   5-week long run in Oct.–Nov. 2003
-   Indium beam of 158 GeV/nucleon
-   ~ 4 × 1012 ions delivered in total
-   ~ 230 million dimuon triggers on tape

- Transverse vertexingwith 20 μm accuracy(and < 200 μm in Z)

**M******** (GeV)**

**dN/dM********  (Events/50 MeV)**

- (80% of
- collected statistics)

- Beam tracker station

**E****ZDC**

**N.  Clusters in VT**

**N.  Clusters in VT**

- Broad centrality coveragemeasured by the ZDC and by the number of clustersseen in the Vertex Telescope

- After event selection

- Before
- event selection

- (~47% statistics left)

**E****ZDC**

-  Opposite-sign dimuon
- 	mass distributions
-  **Before quality cuts**
-  No muon matching
-  Two spectrometer settings

- (100% of
- collected statistics)

## EZDC distributions and data selection for high mass dimuon analysis

- Minimum bias (ZDC) trigger

- Dimuon trigger

-  **Beam pile-up** is rejected using **Beam Tracker timing information**
-  **Non-interacting beam ions** are rejected using **Interaction Counter**
-  Severe **quality cuts** have been used in this preliminary analysis   (statistics will increase in the future, especially for peripheral collisions)
-  Dimuon data analysis performed for events with **E****ZDC ****< 15 TeV**
-   and in the phase space window: **0 < y****cms**** < 1** ; **|cos ********CS****| < 0.5**** **

## Understanding the opposite-sign dimuon mass distribution

- Dimuon data from the **6500 A** event sample
- No muon track matching used in this analysis
- Mass resolution at the J/ : **~100 MeV**
- Combinatorial background from  & K decays estimated from the measured like-sign pairs
- Signal mass shapes from Monte Carlo:  **PYTHIA** with **MRS A** (Low Q2) parton densities  **GEANT 3.21** for detector simulation  reconstructed as the measured data
- Acceptances from Monte Carlo simulation:
-  for **J/****** : **12.4 %**
-  for **DY**  : **13.4 %** (in window 2.9–4.5 GeV)

- J/

- ’

- DY

- Background

- Charm

- A multi-step (max likelihood) fit is performed:**a) M > 4.2 GeV : normalise the DY**

**b) 2.2<M<2.5 GeV: normalise the charm (with DY fixed)**

**c) 2.9<M<4.2 GeV: get the ****J/******** yield****                               (with DY & charm fixed)**

## J/ / Drell-Yan in Indium-Indium collisions

- J/ / Drell-Yan in Indium-Indium collisions

- B (J// (DY) = 19.5 ± 1.6

- In-In collisions of EZDC < 15 TeV  L = 7.0 fm(from **Glauber fit** to the minimum bias EZDC distribution)

-  0.87 ± 0.07 w.r.t. the absorption curve

- and Npart = 133

- all data rescaled to 158 GeV

- _Stability checks_:
-  Background increase by 10% : less than 3% change
-  Different event selection or fitting procedure : less than 8% change
-  Using GRV parton densities instead of MRS : 0.87 ± 0.07  0.93 ± 0.08

- preliminary

## Comments and on-going analysis - 1

- study of the centrality dependence of the J/ suppression in several binscannot use the measured Drell-Yan events, due to the low statistics

- alternative analysis in progress, with the Drell-Yan yield estimated from a **Glauber analysis** of the **Minimum Bias E****ZDC**** distribution**

- 2 independent Minimum Bias triggers in NA60:
- 	  minimum amount of signal in the ZDC
- 	  Indium ion crossing the Beam Tracker

- After the corrections, we see the same trend inboth EZDC spectra, for central events

- EZDC spectra of Beam Trackerand Dimuon triggers

- The J/and “DY” EZDC distributions will be obtained withtwo different triggers  it is crucial to verify:
-  the **time stability** of the dimuon and MB triggers
-  the influence of any possible **trigger timing bias** on 	the ZDC signal acquisition.
- An analysis of the ZDC signals in the three NA60 triggers was performed:
**Stability **of the trigger timing **confirmed**
**Small (~5%) timing biases** found and **corrected**.

- EZDC (GeV)

## Comments and on-going analysis - 2

-  dimuon matching efficiency: **~ 70%** at the J/
-  mass resolution at the J/improves from ~100 MeV to ~**70 MeV**
-  track matching useful to get rid of combinatorial background and out-of-target events 		 **cleaner spectrum**

## Low mass dimuon production in Indium-Indium collisions

- S/B ~ 1/4

- _no centrality selection_

- opposite-sign
- signal
- combinatorial background

- less than 1 % of total statistics

- from a **preliminary** analysis
- of a very small event sample ...

- mass resolution :20–25 MeV at M ~ 1 GeV

- _With respect to the Pb-Au CERES data:_
-  factor **~ 700** higher **effective statistics**
-  **Mass resolution ~2%**, better by a factor** 2**
-  Full information on associated track multiplicity
-  Completely different systematic uncertainties
-  The **2.5 T dipole field** allows for good **p****T**** ****	****coverage** down to very low dimuon masses

-  Combinatorial background resulting from  and K 	decays estimated through a **mixed-event ****	****technique**, using **like-sign muon pairs**.
-  The normalization is still preliminary.

- 

- 

## Summary and outlook

- _Together with the proton run of 2004, NA60 should be able to :_
-  study the production of** low mass dimuons**, including the ,  and  resonances
-  clarify the cause of the excess of **intermediate mass dimuons** in heavy-ion collisions
-  improve the understanding of the production and suppression of **charmonium states**

- _Harvest from the 5-week long Indium run in Oct.–Nov. 2003__ __:_
** ****~ 1 million **signal** low mass dimuons** (after track matching)
-  **mass resolution** **~ 20****–****25 MeV** at the **** and ****** masses**
-  more than **100 000 reconstructed J/******** events** (before track matching)
-  first results on the analysis of the **J/******** suppression** in In-In and comparison with NA50
-  analysis of the **centrality dependence** of the J/ on the way; results soon available

- _To understand the heavy-ion results we need a solid reference baseline from p-A data_
- NA60 is about to take **~ 70 days** of **400 GeV protons**, for an expected integrated luminosity of** ~ 5****•10****4**** nb****-1**, with **7 different nuclear targets**, to study:
-  the impact of c production on the J/ suppression
-  the nuclear dependence of open charm production
-  the intermediate mass *prompt* dimuons
-  the low mass dimuons with unprecedented accuracy
- (*) latest news: 1 week with a **158 GeV proton beam**, to compare p-A, In-In and Pb-Pb data without introducing model-dependent **rescaling factors**

## Slide 16

## Backup slides

## Standard analysis of J/y/DY : event selection

**+ Beamscope**

**+ Beamscope**

**+ Int. Counter**

**+ Int. Counter**

**+ Glob.Cut 10%**

**+ Glob.Cut 10%**

**+ Y, CosCS**

- -27%

- -13%

- -23%

- -4%

**% diff**

**% diff**

**% diff**

**% diff**

- 47% of initial ev.

- Dimuon trigger event selection:  2.9<M<3.3 GeV

- Pixel Pl.7

- EZDC

## NA60 will track the converted photons and will measure cand the c to J/ ratio with ~ 2% accuracy

-   What is the “normal nuclear absorption” of the c?
-   E866, NA50 : The ’ is more absorbed
-   NRQCD : the c should be less absorbed

- NA60 will track the converted photons and will measure cand the c to J/ ratio with ~ 2% accuracy

- Impact of c production on the study of J/ suppression

## In-In collisions : low mass phase space coverage

- The dipole field in the target region leads to much better pT coverage than previous dimuon measurementsDimuons now competitive with respect to dielectrons

- with 2.5 T field

- without field

- Monte-Carlo

- Acceptance improvesin all M and pT windows

- by a factor 50 forM ~ 500 MeV andpT ~ 500 MeV/c

- after muontrack matching