Feasibility of thermal photon measurements inthe future MPD experiment at NICA
N. Burmasov, D. Ivanishchev, D. Kotov, E. Kryshen, M. Malaev, V. Riabov, Yu. Riabov(NRC «Kurchatov Institute» - PNPI) for the MPD collaboration

This work was supported by RFBR according to the research project № 18-02-40045

Heavy ion collisions at NICA

• A mega-science project NICA, Dubna, JINR
• Modernization of existing Nuclotron facility• Parameters:

 relativistic ions up to Au, √sNN = 4-11 GeV
 polarized p and d, √sNN = 27 GeV (for p) luminosity 1027 cm-2s-1• Study of the phase diagram in the regionof high baryonic density andintermediate temperatures• Extension ofmodern heavy-ion
programs at RHIC and the LHC tolower energies• Working experiment: BM@N (fixed target)• Experiments under construction: MPD, SPD(collider)2

Thermal radiation in heavy ion collisions

direct photons• Photons leave the medium without interaction• Black body radiation: inverse slope proportional to Teff
Thermal photons from QGP

Thermal photons from hadron gas

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Effective temperature vs energy

J. Phys.: Conf. Ser. 1070 (2018) 012012

4

Photon reconstruction: two methods• Electromagnetic calorimeters• Efficient at pT > 2 GeV/c• Hardware trigger capabilitiesCalorimetry• Photon conversion γ→e+e- in the material• P = 1 – exp(-7/9 x/X0)• Efficient at 0.5 <pT <4 GeV/c• Much better resolution at low pTPhoton conversions

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Conversions at MPD••••

CMS Energy: 4-11 GeVDesign luminosity: 1027 cm-1 s-1
Stage 1: TPC, TOF, ECAL, FHCal, FFD
Stage 2: + ITS + EndCapMain conversion structures in Stage 1:• Beam pipe: 0.3% X0• Inner TPC barrel structures: 2.4% X0Future:• Inner tracking system
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Conversion reconstruction efficiency••••

Studied with MPDROOT Stage 1 setup
Using MpdParticle to build secondary vertices
Cuts optimized to maximize signal significanceContribution of (non-gamma) background < 5%

Typical cuts on electrons:• |η|<1• pT > 50 MeV/c• at least 20 hits in TPC• +/-4σ electron PID in TPC/TOF
Typical cuts on ee pair:• Small DCA (χ2 < 10)• Vertex R > 10 cm• Direction to vertex:
θ < exp(-2.777-2.798* pT) + 0.0175• mee< 0.022 + 0.017* pT [GeV]• ee plane orientation wrt B:
ΨPair < 0.1 rad7

pT-differential direct photon yields

PHENIX, PRL 123, 022301
Khachatryan, Praszałowicz EPJC 80 (2020) 670

thermalpower law• Universal scaling of pT-differential direct photon yields at moderate pT is observed at RHIC/LHC• It can be used to predict pT spectra of direct photons at NICA energies for pT > 0.6 GeV/c• Switch to thermal spectrum at pT < 0.6 GeV/c: dN/dpT ~ pT exp(-pT/T)• Using conservative effective temperature T = 150 MeV (see e.g. PRC 93 (2016) 054901)
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Challenge: decay photons
Inclusive photon spectra aredominated by decay photons

Relative contributions of different hadrons to thetotal decay photon spectrum as a function of thedecay photon transverse momentum

ALICE: Phys. Lett. B754 (2016) 235
9Neutral meson reconstruction••

Using 10M minimum bias URQMD events
Pion signal is clearly visible in a wide pT range

The obtained fits used to calculate
photon spectra from π0 decays
Efficiencycorrected
spectra areextrapolated
down to 0 pT

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Inclusive photon spectra and Rγ ratio
Inclusive photon spectrum was simulated asa sum of direct and decay photon spectra

Excess over 1 shows the fraction of
direct photons• Systematic uncertainties on Rγ can be reduced to ~2-5%• Conclusion: direct photon yields can be extracted with good
accuracy down to low pT11

Rγ and Temperature uncertainties• Direct photon spectra with Rγ systematic uncertainty 2%• Thermal spectrum at pT < 0.6 GeV/c: dN/dpT ~ pT exp(-pT/T)• Teff uncertainty ~ 10%• Decrease in Rγ increase Teff uncertainty
12Dedicated photon converter?
conversionlayer

A dedicated conversion layer under investigation:• Cylindrical copper pipe
• Radius – 15 cm• Radiation length: 5% and 10% (0.7 and 1.4 mm)
Advantages:• Photon reconstruction efficiency can be improvedby factor ~3• Minimization of systematic uncertainties due towell known material budget

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Resolution of track parameters
DCArCharged πDCAzCharged π• Marginal decrease inreconstruction quality ofcharged particles
pTCharged π• The decrease noticeable onlyat low pT < 0.5 GeV/c

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Conversions in Stage 2

Inner tracking system in Stage 2• 5 layers of Monolithic Active Pixel Sensors• ~0.4% X0 in current design• Photon reconstruction efficiency slightly
improves compared to stage 1 setup

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Conclusions and outlook• Photons are valuable probes of dense hadronic matter produced inheavy ion collisions• Photon conversion method is a powerful tool to measure photon
and neutral meson spectra• Reconstruction of thermal photon yields looks promising at MPD• Feasibility studies on the dedicated converter and Stage 2 setup
show promising results

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BACKUP

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Photon spectra at RHIC and LHC
PHENIX (AuAu @ 200 GeV)

ALICE (PbPb @ 2760 GeV)

Teff = 239 ± 25 (stat) ± 7 (syst) MeV

Teff = 297 ± 12 (stat) ± 41 (syst) MeV

PHENIX: Phys. Rev. Lett. 104 (2010) 132301

ALICE: Phys. Lett. B754 (2016) 235

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Integrated direct photon yieldNo reliable predictions for photons in UrQMD/PHSD -> using data driven method

Phys. Rev. Lett. 123, 022301

The integrated direct photon yield:• scales as (dNch/dη)1.25 in a wide range of multiplicities/collision energies• the scaling is violated in small collision systems / small multiplicities (dNch/dη < 20)• AA yield is a factor of ~10 larger than the Ncoll-scaled yield in pp
Assuming this scaling still holds at lower energies, we can expect universal multiplicity scaling for• (0-60)% centralities at 11 GeV• (0-40)% centralities at 4 GeV19Neutral meson reconstruction efficiencyπ→γγ•

Embedding technique used to study
reconstruction efficiency vs pT•

700 000 min. bias UrQMD events @ 11 GeV•

500 π0 + 500 η embedded with flat pT distribution•

Neutral meson reconstruction efficiency ~ 10-4•

π0 peak is significantly narrower with conversion
method compared to ECAL

η→γγ20