Prompt dimuons and D meson productionin Heavy Ion collisions at the SPS

Bjorn Lenkeit, CERN-EPon behalf of the NA60 Coll.

Moriond, March 2001

OUTLINE

• Physics motivation

• Experimental apparatus

• Physics performance capability

• Summary

Intermediate mass dimuons in p-A collisions

• The p-A data is properly described by a superposition of Drell-Yan and DD decays

• The required charm cross-section is consistent with previous direct measurements

_

NA50

Excess production of intermediate mass dimuons

• Signal / background ~ 1/20 ; unsubtracted background ? Many tests exclude this hypothesis; excess and background have different shapes

NA38+NA50

In heavy ion collisions, S-U and Pb-Pb, the yield of produced dimuons exceeds the superposition of the expected sources, Drell-Yan and D meson decays

Charm enhancement ?

The measured yields can be reproduced by scaling up the charm contribution

But can the charm yield be enhanced by a factor 3 ? A much better measurement is needed

L. Capelli, NA50, at QM2001

yield based on qq annihilation rate

integration over space-time history

central collisions only

parameters : fireball lifetime : 14 fm/c initial temperature : Ti = 192 MeV

explicit introduction of a QGP phase critical temperature : Tc = 175 MeV

no free parameters

Thermal dimuons production ?

Rapp and Shuryak, PLB473 (2000) 13.

the IMR excess can be well accounted for by thermal radiation when combined with DY and open charm

L. Capelli, NA50, at QM2001

Low mass dilepton production

• The p-Be and p-Au data measured by CERES are properly described by the standard cocktail of hadronic decays but there is an excess in the Pb-Au data !

• The excess increases with the square of the charged particle multiplicity and ismore pronounced at low pT

• Degeneracy of and a1 spectral functions ? Chiral symmetry restoration ?• Better statistics and resolution needed : 2000 pairs out of 40 million events !

Physics Motivation

The present SPS results are very interesting but important issues remain unclear :

Is the meson modified by the medium ? measure the !Is there a threshold in the dilepton enhancement ?

What is the origin of the intermediate mass dimuon excess ? Thermal dimuons ?

Is the open charm yield enhanced in nucleus-nucleus collisions ?How does it compare to the suppression pattern of bound charm states ?

What is the variable that rules the onset of ’, c and J/ suppression ?

What is the physical origin of the ’ suppression ?If it is Debye screening, what is its melting temperature ( value of Tc) ?

What fraction of J/ come from c decays ? Does it change from p-Be to p-Pb ?

From NA50 to NA60 (1996 - 2000)

Improved measurement of prompt dimuon production and

open charm in heavy ion collisions

Let’s add silicon detectors to track the muons before they traverse the hadron absorber

Detector concept

• 3-dimensional track matching with the muon spectrometer• Improved mass resolution• Rejection of muons from and K decays• Improved low mass dimuon acceptance

• Muon track offset measurement • Separate charm from prompt (thermal) dimuons

D

{offset

vertex

The NA60 silicon vertex spectrometer

• 10 planes• 88 pixel readout chips• 720 000 channels• pixel size : 50425 m2

Silicon pixel telescope

2.5 Tdipole field

• 2 x-y stations of -strip Si detectors at T = 130 K

• ~ 20 m resolution on the transverse coordinates of the beam ions

Beamscope

Physics performance capability

Detector setup :• The whole detector geometry is described using GEANT

Event generation :

• Soft signals with Genesis code (thermal distributions)• Hard processes with PYTHIA• Underlying hadronic background with VENUS

Dimuon mass resolution : simulation

with pixels

without pixels

J/

’

• Clear improvement in mass resolution and signal / background ratio

M at M = 1 GeV : 70 MeV in NA50; 20 MeV in NA60

NA50

NA60

Vertexspectrometer

Dimuon mass resolution : data

• few hours at ~ 10 8 protons / burst, 450 GeV, 10 mm Be target ( 5 % int )• half acceptance, bump-bonding, radiation damage low detector efficiency

without pixels

M = 70 MeV

with pixels

M = 20 MeV

Low mass dimuons

• Good sensitivity on the shape of the • Very good statistics and pT coverage for the , and mesons

• A few 100 events expected for very low pT at M ~ 500 MeV

/ total signal

statisticaluncertainty

Measurement of the muon track offset

Determination of the interaction vertex

Impact parameter of the muon tracks

D+ : c = 317 m

D0 : c = 124 m

D

Muon track offset for different sources

• Full tracking and vertex reconstruction

• Signal embedded on top of Venus events

select offset windows for prompt and charm samples

IMR : charm or thermal dimuons ?

• Prompt dimuons selection : events with muon track offset < 90 m

• Charm selection : events with muon track offset

in the range 90 800 m and muons > 180 m away from each other in the transverse plane at zv

IMR dimuons : pT distributions

• Prompt dimuons selection • Charm selection

Summary

• SPS is still alive NA60 : third generation experiment to separately study the production of prompt dimuons and of muons coming from the decay of charmed mesons

• NA60 will clarify the origin of the excess IM dimuon production and measure the yield of charmed mesons produced in heavy ion collisions

• Low mass dimuon data with good statistics, mass resolution and signal to background ratio will allow to study the production of , and mesons

• NA60 will certainly contribute to the understanding of the present results concerning the production of a deconfined state of matter in high energy heavy ion collisions at the SPS

The NA60 Collaboration

Brookhaven

R. Arnaldi, A. Baldit, K. Banicz, K. Borer, L. Casagrande, J. Castor, B. Chaurand, W. Chen, B. Cheynis, P. Chochula, C. Cicalò, M.P. Comets, P. Cortese, V. Danielyan, A. David, A. De Falco, N. De Marco, A. Devaux, B. Dezillie, L. Ducroux, B. Espagnon, P. Force,

E. Gangler, V. Granata, A. Grigorian, S. Grigorian, J.Y. Grossiord, A. Guichard, H. Gulkanian, R. Hakobyan, E. Heijne, M. Hess, P. Jarron, D. Jouan, L. Kluberg, Y. Le Bornec, B. Lenkeit,

Z. Li, C. Lourenço, M.P. Macciotta, M. Mac Cormick, F. Manso, D. Marras, A. Masoni, S. Mehrabyan, H. Muller, A. Musso, A. Neves, B. Pes, S. Popescu, G. Puddu, P. Ramalhete,

P. Rosinsky, P. Saturnini, E. Scomparin, J. Seixas, S. Serci, R. Shahoyan, E. Siddi, P. Sonderegger, G. Usai, G. Vandoni, H. Vardanyan, N. Willis, H. Woehri and M. Zagiba

Lisbon

Orsay

CERN Bern

Bratislava

Torino

Yerevan

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