Study dileptons ee and direct photons fn MPDNICA

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 NICA 9 -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 Me. V < Mee < 750 Me. V 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+ e. Use 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 x 0 - 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. B 40, 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 √s. NN = 200 Ge. V Au+Au Φ e+e- PHENIX √s. NN = 200 Ge. V 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 √s. NN = 200 Ge. V PHENIX 7

NICA/MPD (MC, Ur. QMD, 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 Ge. V) √s = 3. 8 Ge. V √s = 9 Ge. V 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 cm 2 s-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) WA 98 (CERN) First direct photons in AA Suppression of high p. T π0 at SPS No conclusive results. Excess seen by WA 98 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 ? )