Electro Magnetic probes with MPD Calorimeter Dubna JINR

Electro. Magnetic probes with MPD Calorimeter Dubna JINR VBLHEP, DLNP September 2009

1. Physical motivations • Electromagnetic probes • Particle rate 2. Requirements to the Ecal 3. Performance 4. Prototype status 5. Electron identification 6. Conclusions

Electromagnetic Probes virtual (appearing as e+e- or μ+μ- pairs) real photons are tools to diagnose the hot and dense matter produced in relativistic heavy-ion collisions. They are not distorted by final state interactions and once produced can escape unaffected the interaction region, carrying to the detectors information about the conditions and properties of the medium at the time of their creation.

a) thermal radiation emitted by the strongly interacting Quark-Gluon Plasma in the early phase of the collision. The elementary processes involved are: annihilation QCD Compton identification of this signal → 1. proof of deconfined phase 2. direct measurement the temperature, readily given by the inverse slope. Theory has singled out thermal radiation from the Quark-Gluon Plasma phase : .

b) Thermal radiation emitted by the high-density hadron gas in the later phase of the collision. The main elementary process here is the pion annihilation , mediated through vector meson dominance. This component, controlled by the pole at the rho mass of the pion electromagnetic form factor, contributes primarily to the low-mass region, around and below the ρ mass.

There are two main difficulties in the experimental measurements: 1. Huge combinatorial background of uncorrelated lepton pairs. This background therefore depends quadratically on the particle multiplicity and strongly increases as the coverage moves to low-p. T leptons. In the measurement of e+e- pairs, π° Dalitz decays and conversions. 2. 1. 2. 3. 4. Physics background. Photons and dileptons can be emitted by a variety of sources and therefore before claiming observation of any new effect, it is mandatory to have a thorough understanding of the expected contribution from all known sources. 5. 6. 7. Gammas created due to electromagnetic decays of hadrons after the freeze-out carry no information about excited system and should be subtracted.

PHENIX The fraction of the direct photon component as a function of Pt in (a) p+p and (b) Au+Au (min. bias) at √S = 200 Ge. V. The error bars and the error band represent the statistical and systematic uncertainties, respectively. The curves are from a NLO p. QCD calculation.

Low mass enhancement seen in Au+Au invariant mass spectrum measured by CERES (left) and PHENIX (right)

Anomaly observed in energy dependence for the K/π± ratio Systematical uncertainty can be reduced by involving K/π° or K/γ ratio in the analisys

Phase transition should be indicated by increase in photon activity – π° rate vs beam energy? Au+Au collisions γ rate vs beam energy? Prompt γ rate vs beam energy? at √SNN=9 Ge. V Ur. QMD Photons associated to the π° 100/Pt 50/Pt 20/Pt 10/Pt 5/Pt Nγ /Nπ± Nπ°/Nπ± Pt(Ge. V) Nγ pr /Nπ±

Requirements to the Ecal Energy spectra and multiplicity distribution are determine lateral and longitudinal size of calorimeter cell Au+Au collisions at √SNN=9 Ge. V Ur. QMD

Calorimeter energy resolution strongly determine π°signal to combinatorial background ratio and resolution in the γγ invariant mass spectra Crystal Shashlyk

Main option – “Shashlyk” type of calorimeter KOPIO

Au+Au collisions at √SNN=9 Ge. V Granularity -3 x 3 cm

Single photon registration efficiency versus Pt

Gammas not associated to the π° Nγ Nπ± σ=33% Nγ/Nπ±

BARREL P (Ge. V) π K p M(Ge. V)

Energy resolution ~ (2. 9± 0. 1)%/√E(Ge. V). Time resolution ~(80± 10)psec / √E(Ge. V).

Detector response vs beam energy

MIP

Conclusions: • Wide range of physics tasks can be studied including ECal into the MPD detector • Proposed "SHASHLYK" type of calorimeter is adequate to the physics tasks • Prototype of calorimeter module has been built and tested with CERN and DESY beams. Expected parameters have been reached.