Study dileptons (e+e-) and direct photons fn MPD/NICA
Topics connected with ECAL
1. In-medium properties of hadrons: dilepton spectroscopy
2. Ratios of different decay modes
3. Charmonium suppression
4. Direct photons
Yu.M.Zaitsev (ITEP, Moscow)
NICA Roundetable Workshop IV: Physics at NICA9-12 October 2009
1. In-medium properties of hadrons
It is expected that in-medium properties of light vector mesons
are sensitive to partial chiral symmetry restoration in dense
baryonic matter
The easiest way to study this – to use decays into leptonic pairs
Experimental situation now is not well understood yet
Such spectrum will be measured in MPD but some additional
efforts to decrease combinatorial background will be needed
Vacuum r
CERES
PHENIX
p+p
Excellent agreement with cocktail
Au+Au Large enhancement in
150 MeV < Mee < 750 MeV
HADES
Challenge: Pair Background
•
Without background rejection  Signal/Background ~ 1% in Au-Au
•
The main combinatorial background: e+ and e- from different uncorrelated source
π0 → γ e+ eγ → e+ e– Need event mixing because of acceptance differences for e+ and e– Use like sign pairs to check event mixing
•
Unphysical correlated background
– Track overlaps in detectors
– Not reproducible by mixed events: removed from event sample (pair cut)
– Misidentification π → “e”
•
Correlated background: e+ and e- from same source but not “signal”
– “Cross” pairs
π0 → γγ
e+ e-
•
e+ eUse Monte Carlo simulation and like sign data to estimate and subtract background
How to decrease such background?
To build a good detector
Vertex detector with low x0 - to measure electrons from the main vertex & to decrease
electrons from conversion
Main tracker system with good spatial resolution – to decrease tracks overlapping
ECAL with good resolution & high granularity and may be with additional preshower detector
– to have a good e/π separation
To write software with good vertexing (main & secondary)
To study a bit more clever analysis methods to
decrease combinations
For example: to remove electrons & positrons
from π0 Dalitz decays –
we will lose some efficiency but could decrease
combinatorial background
2. Measuring different decay modes ratios
A.V.Stavinsky, XIX Baldin Seminar, v.1, 151 (2008)
Acta Phys. Pol. B40,1179 (2009)
If resonance decays before kinetic freeze-out 
Possible rescattering of hadronic daughters 
Reconstruction probability decrease for hadronic mode
ω(782)  π+ π- π0
B.R. 0.89
ω(782)  π+ π-
B.R. 0.017
ω(782)  π0
B.R. 0.089
ω(782)  e+e-
B.R. 0.000072
φ(1020)  K+ K-
B.R. 0.49
φ(1020)  η
B.R. 0.013
φ(1020)  e+e-
B.R. 0.000297
(c = 23 fm)
(c = 44 fm)
Φ(1020)  K+ KΦ(1020)  e+e-
B.R. 0.49
c = 44 fm
B.R. 0.000296 c = 44 fm
d+Au
PHENIX
Φ K+ KSTAR Preliminary
√sNN = 200 GeV
Au+Au
Φ e+e-
PHENIX
√sNN = 200 GeV
6
ω(782)  π+ π- π0 B.R.
0.89
c = 23 fm
ω(782)  π0
B.R.
0.089
c = 23 fm
PHENIX
η,ω π+ π- π0
p+p
Au+Au
p+p
ω π0
PHENIX
ω π0
√sNN = 200 GeV
PHENIX
7
NICA/MPD
(MC, UrQMD, ECAL with σ= 4%/√s)
Reconstruction of decay modes with π0
is not easy –
very big combinatorial background
(more than 600 photons at √s = 9 GeV)
√s = 3.8 GeV
√s = 9 GeV
strong dependence from ECAL
resolution
Reconstruction of π+π- and e+e- decay
modes depends on momentum
resolution and misidentification
probability
Trying to reconstruct π0 using Dalitz
decay
(study in progress)
There is hope that these decay modes
of ω and φ mesons could be measured
3. J/ψ suppression
At an average luminosity of the NICA Collider of L ~ 1027 cm2s-1 with
reasonable assumption on reconstruction efficiency estimated rate for J/ψ
production is about 400 per 10 weeks of running (see P.Senger talk)
It is clear that it will be extremely difficult in six year from now to
measure something reasonable for better understanding of J/ψ
suppression phenomenon with a few hundred events
PRL 85 3595 (2000)
4. Direct photons
PRL 100 242301 (2008)
WA98
(CERN)
First direct photons in AA
Suppression of high pT π0 at SPS
No conclusive results. Excess seen by WA98 in Pb-Pb is seems to be compatible
with thermal emission but errors are very large
Expectations to measure direct photons at NICA are marginal
10
ITEP groups are very experienced in
production and operation of
“Shashlyk- ECAL”:
ECAL for HERA-B Experiment at
DESY
ECAL for LHCb Experiment at CERN
Now – designing of ECAL for CBM
Experiment at FAIR
Very sophisticated MC program is
developed with very good shower
description and light collection in
Shashlyk (M.Prokudin)
That allow us to minimize
considerably R&D
Now we consider a possibility to join
MPD project
We have people and knowledge how
to build Shashlyk type Calorimeter
(money ?)