Probing the Dense Fireball: Charm Production and Detection at Threshold Beam Energies
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Transcript of 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
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
JINR International Workshop, Dubna, November 2003 3 V. Friese
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!
JINR International Workshop, Dubna, November 2003 4 V. Friese
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?
JINR International Workshop, Dubna, November 2003 5 V. Friese
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!
JINR International Workshop, Dubna, November 2003 6 V. Friese
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
JINR International Workshop, Dubna, November 2003 7 V. Friese
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
JINR International Workshop, Dubna, November 2003 8 V. Friese
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
JINR International Workshop, Dubna, November 2003 9 V. Friese
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
JINR International Workshop, Dubna, November 2003 10 V. Friese
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
JINR International Workshop, Dubna, November 2003 11 V. Friese
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.
JINR International Workshop, Dubna, November 2003 12 V. Friese
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
JINR International Workshop, Dubna, November 2003 13 V. Friese
Tracking in the CBM detector
JINR International Workshop, Dubna, November 2003 14 V. Friese
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
JINR International Workshop, Dubna, November 2003 15 V. Friese
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
JINR International Workshop, Dubna, November 2003 16 V. Friese
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
JINR International Workshop, Dubna, November 2003 17 V. Friese
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
JINR International Workshop, Dubna, November 2003 18 V. Friese
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
JINR International Workshop, Dubna, November 2003 19 V. Friese
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.