probing the dense fireball: charm production and detection at threshold beam energies

Probing the Dense Fireball:

Charm Production and Detectionat Threshold Beam Energies

Volker Friese

Gesellschaft für SchwerionenforschungDarmstadt, Germany

[email protected]

JINR Particpiation in the Experimental Programme on the Future GSI facilityInternational Workshop, JINR Dubna, November 2003

JINR International Workshop, Dubna, November 2003 2 V. Friese

Why charm is interesting

• charm is produced, not put into the collisions :

a heavy strange quark

• charm is heavy!

mc 1 GeV >> T 170 MeV

perturbative treatment possible (?)

calculable from QCD principles


Charm in heavy-ion collisions

Common picture:Production of charm in the initial hard parton-parton interactions(before possible QGP formation)

Charmonium:

Absorption by comoving participants:“normal suppression”

In deconfined matter: Additional dissociation due to free colour charges (Debye screening):“anomalous suppression”

QGP signature!


Charm production : hard or soft?

NA50, QM 2001

In fact:anomalous J/ suppression observedin Pb+Pb at SPS (158 AGeV)

But: If scaled to h-, J/ is independent of Np

Gazdzicki & Gorenstein, Phys. Rev. Lett. 83 (1999) 4009

hard to understand with hard productionand subsequent suppression soft production of charmed hadrons?


Open charm can tell!

Soft A dependence : <D> ~ <h-> ~ Np

pQCD : <D> ~ A2 ~ Np4/3

p+N data allow estimate of open charm yield in elementary collisions

Compilation by Gazdicki and MarkertActa Phys. Pol. B 31 (2000) 965

5 x 10-2

2 x 10-1

for central Pb+Pb @ 158 AGeV

predictions for A+A differ dramatically!


More scenarios…

Hadron gas in chemical equilibriumCanonical suppression analoguous to strangeness

Equilibrated QGP+ statistical coalescence

Gorenstein et alJ. Phys. G 28 (2002) 2151

Predictions of open charm yield differby orders of magnitude for differentproduction scenarios, especially at lowenergies


Open charm in dense matter

Various QCD inspired models predict a change of D mass in hadronic medium

Mishra et al, nucl-th/0308082

Substantial change (several 100 MeV) already at =0

In analogy to kaon mass modification, but drop for both D+ and D-

Effect for charmonium is substantially smaller


Reduced D meson mass : consequences

If the D mass is reduced in the medium: DD threshold drops below charmonium states

Hayashigaki, PLB 487 (2000) 96

Mishra et al, nucl-th/0308082

Decay channels into DD open for ’, c, J/ broadening of charmonium states suppression of J/ enhancement of D mesons


Reduced D meson mass : consequences contd.

Transport models : reduced D mass has dramatic consequences for the production cross section in p+N and secondary +N, especially near N+N threshold

HSD : D yield enhanced by a factor of 7 at 25 AGeV!

Cassing et al, Nucl. Phys. A 691 (2001) 753


First evidence ?

Enhancement of open charm (3 x pQCD) could explain the excess of intermediate mass dimuons observed at SPS

Abreu (NA50), Eur. Phys. J. C 145 (2000) 443

peripheral central


A short summary

Experimental data on open charm production in nucleus nucleus collisions will give insight into exciting physics questions related to both QGP and dense baryonic matter. But: No measurement available so far.

Effects of in-medium modification increase with baryon density and are best studied near threshold.

Open charm measurement will be one of the major tasks of CBM.

Improvement in theory is surely needed to disentangle various (maybe competing) effects and make strong predictions.


Challenges in open charm measurement

Near threshold means: extremely low yield (~10-3)

Heavy ion collision means: high charged multiplicity (~1000)

Au + Au @ 25 AGeV, UrQMD + GEANT

Detection by charged hadronic decay, e.g. D0 +K-

Without cuts: S/B = 1.3 x 10-5

Background suppression of 105 needed


Tracking in the CBM detector


Background reduction for D0

D0

+ +

K-

K-

p + pK

b

bK

Lab CMpair

vz

*

combinatorial +K- background

Cut variables:• decay angle• decay vertex position• pair impact parameter• pointing angle


Secondary vertex resolution

The most effective cut for background reduction is on the two-track vertex position

resolution depends on: thickness of tracking stationssingle hit resolution


Requirements on the tracking detector

MAPS Hybrid

First station must be close (5cm) to the target: radiation hardness

MHz event rate: fast readout

few materials

good hit position resolution


Estimate of D0 detection capability

based on GEANT simulation, no magnetic field, perfect PID, background from UrQMD

Signal efficiency 48 %Background reduction 2 x 105

S/B 2Significance at 107 events: 7.5

Similar study for D+ ++K- : S/B 3

Improvements towards reality:include magnetic field readyinclude track finding availableinclude hadron PID under work


Daughter identification

Decay + PID efficiency K- from D0 decay

Time-of-flight measurement 10m from targetResolution 80 ps assumed

The majority of the daughters accepted by the tracking system can be identified


Summary

The measurement of open charm in heavy ion collisions gives access to a wide range of yet unsolved physical questions.

D mesons are expected to be quite sensitive to the dense baryonic environment created in nucleus collisions at SIS300 energies.

Measurement near production threshold will give insight into the medium modification of the D meson mass and will hopefully serve to constrain the various model calculations.

Compared to experiments at RHIC or LHC, CBM will have to deal with extremely low open charm rates.

We will have to cope for that with excellent secondary vertex resolution and high event rates.

If CBM will be able to handle the interaction rates promised to us by the accelerator, the open charm measurement will most probably be possible.

probing the dense fireball: charm production and detection at threshold beam energies

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