Study dileptons (e + e - ) and direct photons fn MPD/NICA NICA Roundetable Workshop IV: Physics at NICA9-12 October In-medium properties of hadrons: dilepton spectroscopy 2. Ratios of different decay modes 3. Charmonium suppression 4. Direct photons Yu.M.Zaitsev (ITEP, Moscow) Topics connected with ECAL

PHENIX p+p Excellent agreement with cocktail Au+Au Large enhancement in 150 MeV < Mee < 750 MeV HADES Vacuum  CERES 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

Without background rejection  Signal/Background ~ 1% in Au-Au The main combinatorial background: e + and e  from different uncorrelated source π 0 → γ e + e - γ → 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 π 0 → γγ e + e - Use Monte Carlo simulation and like sign data to estimate and subtract background Challenge: Pair 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 π 0 Dalitz decays – we will lose some efficiency but could decrease combinatorial background

If resonance decays before kinetic freeze-out  Possible rescattering of hadronic daughters  Reconstruction probability decrease for hadronic mode ω(782)  π + π - π 0 B.R. 0.89(c  = 23 fm) ω(782)  π + π - B.R ω(782)  π 0  B.R ω(782)  e + e - B.R φ(1020)  K + K - B.R. 0.49(c  = 44 fm) φ(1020)  η  B.R φ(1020)  e + e - B.R Measuring different decay modes ratios A.V.Stavinsky, XIX Baldin Seminar, v.1, 151 (2008) Acta Phys. Pol. B40,1179 (2009)

6 Φ(1020)  K + K - B.R c  = 44 fm Φ(1020)  e + e - B.R c  = 44 fm STAR Preliminary d+Au Au+Au √s NN = 200 GeV PHENIX Φ K+ K-Φ K+ K- Φ e+e-Φ e+e-

7 ω(782)  π + π - π 0 B.R c  = 23 fm ω(782)  π 0  B.R c  = 23 fm Au+Au p+p √s NN = 200 GeV PHENIX ω π0ω π0 ω π0ω π0 η,ω π+ π- π0η,ω π+ π- π0

√s = 9 GeV√s = 3.8 GeV NICA/MPD (MC, UrQMD, ECAL with σ= 4%/√s) Reconstruction of decay modes with π 0 is not easy – very big combinatorial background (more than 600 photons at √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 π 0 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 ~ 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

10 PRL (2008) Suppression of high p T π 0 at SPS PRL (2000) First direct photons in AA WA98 (CERN) 4. Direct photons 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

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 ?)