Study of the centrality and collision energy
dependence of resonance production using
the MPD detector at NICA
D. Ivanishchev, D. Kotov, M. Malaev, V. Riabov, Yu. Ryabov fot the MPD collaboration* This work was supported by RFBR according to the research project № 18-02-40038

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Outline Motivations for resonance studies in heavy-ion
collisions
 Expectations for resonance properties in heavy-ion
collisions at NICA energies

 Feasibility studies for resonance reconstruction atNICA-MPD
 Summary2

Resonances in heavy-ion collisions
 Vacuum properties of resosnances are well defined; large branching ratios in hadronic
decay channels -> possible to measure Probe reaction dynamics and particle production mechanisms vs. system size and sNN:
 hadron chemistry and strangeness production,  with hidden strangeness is one of the key probes reaction dynamics and shape of particle pT spectra, p/K*, p/ vs. pT lifetime and properties of the hadronic phase spin alignment of vector mesons in rotating QGP (polarization of quarks from spin-orbital interactions) flow, comparison with e+e- measurements, jet quenching, background for other probes etc.

Reconstructed resonance yields in heavy ion collisions aredefined by: resonance yields at chemical freeze-out
 hadronic processes between chemical and kinetic freezeouts:
rescattering: daughter particles undergo elastic scattering or pseudoelastic scattering through a different resonance  parent particle is
not reconstructed  loss of signal
regeneration: pseudo-elastic scattering of decay products (K K*0, KK  etc.)  increased yields
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Resonances in AuAu@11, UrQMD
 Resonances are decayed by UrQMD, daughters participate in elastic and inelastic scattering Resonance are reconstructed by invariant mass method according to decay channels   K+K- (c ~ 45 fm/c): modest line shape modifications in central AuAu@11 at low pT

Low-pT

High-pT (770)0  +- (c ~ 1.3 fm/c): significant line shape modifications in central AuAu@11 at low p T

Low-pT

High-pT4

Yield and mass modifications in AuAu@11, UrQMD
 K*(892)0  K (c ~ 4.2 fm/c); combine K pairs from true K*(892)0 decays Same fitting function for K*(892)0 and background as in ALICE Central collisions: small line shape modifications at low pT; nothing at higher momentum

Low-pT

High-pT

Low-pT

High-pT Peripheral collisions: nothing5

Yield and mass modifications in AuAu@11, UrQMD
 (770)0  +- (c ~ 1.3 fm/c); combine +- pairs from true (770)0 decays Same fitting function for (770)0 and background as in ALICE Central collisions: significant line shape modifications; excess with respect to the peak model isdescribed with a background function
Low-pT

High-pT Peripheral collisions: much smaller modifications are observed, only at low momentum
Low-pT

High-pT

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Yields of K*(892)0 and (770)0 in AuAu@11, UrQMD
 (770)0  +- (c ~ 1.3 fm/c)

yield is undercounted because of pion rescattering;
this yield is preserved in e+e- measurements !!!

 K*(892)0  K (c ~ 4.3 fm/c)
yield is undercounted because of pion andkaon rescattering Signal losses are larger for shorter-lived (770)0  higher chance for (770)0 to decay and fordaughters to rescatter in the medium
 Predicted signal losses are noticeable for the total (pT-integrated) yields since bulk of the hadrons is
produced at low pT at NICA energies

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Particle ratios in AuAu@4-11, UrQMD, AMPT, PHSD
 Models with hadronic cascades (UrQMD, PHSD, AMPT) Ratios for two shortest-lived resonances (, K*(892)) are shown normalized to most peripheral collisions

 Models predict suppression of / and K*/K ratios in Au+Au@4-11, resonances with small c Suppression depends on the final state multiplicity rather than on collision energy
 Yield losses occur at low momentum as has been demonstrated before
8Masses of K*(892)0 and (770)0 in AuAu@11, UrQMD
 (770)0  +- (c ~ 1.3 fm/c)

CentralPeripheral

 K*(892)0  K (c ~ 4.3 fm/c)

CentralPeripheral In peripheral collisions, the peak models return masses and widths as measured in vacuum
 In central collisions, the masses are measured smaller
 Similar mass “modifications” have been reported @ RHIC and the LHC, large uncertainties:
Phys.Rev.C 99 (2019) 6, 064901

Phys.Rev.C 91 (2015) 024609

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MPD experiment, Phase 1
 Phase 1: TPC, TOF, FFD, FCAL и ECAL Startup in 2021-2022

 Simulate AuAu@4-11 collisions using different event
generators
 Propagate particles through the MPD, ‘mpdroot’: Geant (v.3 or v.4) particle transport
 realistic simulation of subsystem response (raw signals)
 track/signal reconstruction and pattern recognition

 Basic event and track selections: event selection: |Zvrtx| < 50 cm
 track selection:••••••

number of TPC hits > 24
|| < 1.0|DCA to PV| < 2 for primary tracks
V0 topology cuts for weakly decaying secondaries
pT > 50-100 MeV/c
TPC-TOF combined π/K/p PID

TPC: || < 2π, ||  1.6
TOF, EMC: || < 2π, ||  1.4
FFD: || < 2π, 2.9 < || < 3.3

FHCAL: || < 2π, 2 < || < 5

 combinatorial background:•event mixing ( |Zvrtx| < 2 cm, |Mult| < 20, Nev = 10 )10Reconstruction efficiencies
 Typical reconstruction efficiencies (A x ) in AuAu @ 4, 7.7 and 11 GeV, |y| < 1

 Reasonable efficiencies in the wide pT range, |y| < 1
 Efficiencies are noticeably lower for multi-stage decays
with weakly decaying daughters (, , 𝐾𝑠0 ) Measurements are possible from zero momentum
 Modest multiplicity (and/or sNN) dependence
11Mass resolution:
 Detector mass resolution (mreconstructed – mgenerated) in AuAu @ 4, 7.7 and 11 GeV, |y| < 1

 Mass resolution is good enough to preserve the capabilityfor the line shape analysis
 Modest multiplicity (and/or sNN) dependence12(1020), reconstructed peaks
 UrQMD v.3.4: AuAu@11 (10M events), AuAu@7.7 (5M events), AuAu@4 (5M events)
 Full chain simulation and reconstruction, pT = 0.2-0.4 GeV/c, |y| < 1
Foreground (same event)
Mixed event

AuAu@4

AuAu@11

AuAu@11
AuAu@7.7
AuAu@4

 Mixed-event combinatorial background is scaled to foreground at high mass and subtracted
 Distributions are fit to Voigtian function + polynomial
 Signal can be reconstructed at pT > 0.2 GeV/c, high-pT reach is limited by available statistics
 S/B ratios deteriorates with increasing centrality and collision energy
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K*(892)0, reconstructed peaks
 UrQMD v.3.4: AuAu@11 (10M events), AuAu@7.7 (5M events), AuAu@4 (5M events)
 Full chain simulation and reconstruction, pT = 0.2-0.4 GeV/c, |y| < 1Foreground
Mixed event

AuAu@4

AuAu@11

AuAu@11
AuAu@7.7
AuAu@4

 Mixed-event combinatorial background is scaled to foreground at high mass and subtracted Distributions are fit to Voigtian function + polynomial
 Signal can be reconstructed at pT > 0.2 GeV/c, high-pT reach is limited by available statistics
 S/B ratios deteriorates with increasing centrality and collision energy

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MC closure tests
 UrQMD v.3.4: AuAu@11 (10M events), AuAu@7.7 (5M events), AuAu@4 (5M events)
 Full chain simulation and reconstruction, pT ranges are limited by the possibility to extract signals, |y| < 1

 Reconstructed spectra match the generated ones within uncertainties
 First measurements for resonances will be possible withaccumulation of ~ 107 A+A events
 Measurements are possible starting from ~ zero momentum sample most of the yield, sensitive to possible modifications
 Measurements of (1530)0 are very statistics hungry
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Summary Measurement of resonances contribute to the MPD physical program hadronic phase properties, strangeness production, hadronization mechanisms and collectivity,
hadrochemistry, spin alignment etc …

 First measurements for resonances will be possible with ~107 sampled Au+Au
collisions at sNN = 4-11 GeV  year-1 measurements Measurements are possible starting from very low momenta (for most of the cases
from zero momenta) with decent mass resolution  high sensitivity to different
physics phenomena most prominent at low pT
 More detailed and multiplicity-dependent studies would require x10-50 larger
statistics, especially for multi-stage decays of K*(892), (1385)  and (1520)016

Backup slides

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Resonance reconstruction in A-A collisions
 Hadronic decays of resonances are studied with the invariant mass method in the experiments
 After subtraction of uncorrelated combinatorial background estimated with mixed-event pairs, like-sighpairs, rotation pairs etc., the resonance peaks are approximated with a given peak-model (rBW + mass
resolution + mass-dependent width + phase space correction + …) + background function
 Examples of invariant mass distributions and fits from ALICE for , (1520) and (770)0: For most of the cases, the peak models are inspired by theory and measurement in elementarye+e- and/or pp collisions where medium effects are not as important Line shape modifications will result in the change of the measured yield and masses/widths

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Particle ratios in AuAu@4-11, UrQMD, AMPT, PHSD
 Models with hadronic cascades (UrQMD, PHSD, AMPT) Ratios for longer-lived resonances ((1385), (1520), (1530) and ) Event generators predict yield modifications qualitatively similar to those obtained at RHIC/LHC: lifetime and density of the hadronic phase are high enough modification of particle properties in the hadronic phase should be taken into account whenmodel predictions for different observables are compared to data study of short-lived resonances is a unique tool to tune simulations of the hadronic phase19

K*(892)±, (1385)±, (1530)0, reconstructed peaks
 UrQMD v.3.4: AuAu@11 (10M events), full chain simulation and reconstruction, |y| < 1
K*, 0.2-0.4 GeV/c
(1385), 0-0.5 GeV/c
(1530)0, 0.5-1 GeV/c
Foreground (same event)
Mixed event

 Can reconstruct signals for multistage decays of K*(892)  𝐾𝑠0   (𝐾𝑠0 +-), (1385)  (p) and (1530)0 +- (-  -, (p -) )20

NICA complexMPDSPD Modernization of the existing Nuclotron facility
 Fixed target experiment: BM@N
 Construction of collider complex to collide: heavy ions up to Au, √sNN = 4-11 GeV, ℒ ~ 1027 cm-2s-1
 polarized p and d, √sNN = 27 GeV, ℒ ~ 1032 cm-2s-1 (pp)

 Collider experiments: MPD, SPD NICA, MPD – start of operation in 2021-2022
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Resonances22

3

Heavy-ion collisions at NICA

 Properties of the hot and dense QCD matter, phase transition to the QGP, critical point Regime of the maximum baryon density (phase transition at с ~ 50) at NICA
 Extension of modern heavy-ion programs at RHIC and the LHC to lower energies

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4Strangeness enhancement in pp, p-A and A-A
Nature Phys. 13 (2017) 535

 Observed in heavy-ion collisions at AGS, SPS, RHIC and LHC; For the first time observed in pp and p-A collisions by ALICE at the LHC
 Observed as for ground-state hadrons as for resonances (/π, */, */π) Strangeness production in A-A collisions is reproduced by statistical hadronization models. Canonicalsuppression models reproduce results in pp and p-A except for   with hidden strangeness is not subject to canonical suppression   is a key observable !!!
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Hadronization at intermediate momenta
Phys.Lett. B736 (2014) 196-207

 Baryon puzzle - increased baryon-to-meson (p/,/Ks0 , c+/D) ratios in heavy-ion collisions at RHIC
and the LHC
 Driving force of enhancement is not yet fully
understood:
 particle mass (hydrodynamic flow)?
 quark count (baryons vs. mesons)?

  and K*0 are well suited for tests as mesons with

Phys.Rev. C88 (2013) 2, 024906

masses very close to that of a proton: m~ 80 MeV/c2, mK*0~ -45 MeV/c2

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Mass resolution
 Detector mass resolution (mreconstructed – mgenerated) in AuAu @ 4, 7.7 and 11 GeV, |y| < 1

 Acceptable mass resolution
 Modest multiplicity (and/or sNN) dependence

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Reconstruction efficiency
 Typical reconstruction efficiencies (A x ) in AuAu @ 4, 7.7 and 11 GeV, |y| < 1

 Reasonable efficiencies in the wide pT range, |y| < 1
 Modest multiplicity (and/or sNN) dependence27

(770), reconstructed peaks
 UrQMD v.3.4: AuAu@11 (10M events), AuAu@7.7 (5M events), AuAu@4 (5M events)
 Full chain simulation and reconstruction, pT = 0.2-0.4 GeV/c, |y| < 1
Foreground (same event)
Mixed event

AuAu@4

AuAu@11

AuAu@11
AuAu@7.7
AuAu@4

 Mixed-event background subtraction, fits to Voigtian function + polynomial Contributions from Ks, , K*0, f0 and f2 are subtracted (need to be measured in advance)* Signal can be reconstructed at pT > 0 GeV/c, high-pT reach is limited by available statistics
 S/B ratios deteriorates with increasing centrality and collision energy*ALICE, Phys.Rev. C99 (2019) no.6, 064901

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(770), signal extraction – practice tests
Phys.Rev. C99 (2019) no.6, 064901

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