Diphoton and dilepton production.Results and outlook

Kh.U. Abraamyan ᵃ, M.I. Baznat ᵇ, A.V. Friesen ᵃ, K.K. Gudima ᵇ, M.A. Kozhin ᵃ, S.A. Lebedev ᵃʼᶜ, A.I. Malakhov ᵃ, G.L.Melkumov ᵃ,

M.A. Nazarenko ᵈ, G.A. Ososkov ᵃ, S.G. Reznikov ᵃ, A.N. Sissakian ᵃ, A.S. Sorin ᵃ, and V.D. Toneev ᵃ

ᵃ JINR, Dubna, Russia ᵇ Institute of Applied Physics, Kishinev, Moldova

ᶜ Gesellschaft für Schwerionenforschung, Darmstadt, Germanyᵈ MIREA, Moscow, Russia

RR o o u u nn d d T T a a b b l l e e IV, 9 – 12 S e p t e m b e r, 2009 IV, 9 – 12 S e p t e m b e r, 2009

The plan of the report

The resolved tasks New tasks. NICA advantages.

04.05.23 2Abraamyan Kh.U. et al.

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FOTON setup on beams of the Synchrophasotron

Invariant γγ-mass distributions in the reactions (p,α,C)+(C,Cu)→πº + X at 4.5 A GeV/c

The target dependence of production ◊ - the reactions αα + + ((СС,Cu),Cu) ; ♦ - the reactions С + С + ((СС,Cu),Cu) .Abraamyan et al. Phys.Lett. B323 (1994)1; Phys.Atom.Nucl. 60(1997)2014.

0

The target dependence of production

Calculations of nn in frame of Quark Recombination Model (G.Berlad and A.Dar)

11

22),(12 )/(

qANA

qANAPXnAA

42.0n

nTApdEd ~/ 33

0

Transverse momenta of quarks in proton

,)10exp(012)( 2 qq pp

./ cGeVinpwhere q

MeVdxxdxxxpq 178)()(00

)10exp()()(/ 2033 pxFXpCdpEd F

The exponent m in A-dependence for projectile ◊ - the reactions (α,С) + С ; ♦ - the reactions (α,С) + Сu., - data from LBL, Berkeley

6-quark configuration formation probabilities

)(1~/ 3/33 XnpApdEd qi

,...9,6)],2/()()2[( 6/3/6 iXnXnAp qq

%;105)(%;2)( 46

26 Hepdp

%4020)(126 Cp

Double differential cross section of thereaction dd + С + С → → ππº + X º + X at 4.5 A GeV/c

Abraamyan et al. Phys.Atom.Nucl. 68(2005)982.

Check of the dd + С + С → → ππº + X º + X reaction mechanism in the region x > 0.6 x > 0.6

In the CM of the cluster:

In the Lab. System:

)/*exp(~/ 033 TEpdEd

),/exp(~/ 33 TEpdEd

)cos1/()1( 2/120 TT

mT ~0

)(

β(θ)

―――― Tₒ = 160 MeV; - - - - - - Tₒ = 140 MeV

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PHOTON-2 setup on internal beams PHOTON-2 setup on internal beams of theof the NUCLOTRONNUCLOTRON

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The experiments on theNuclotron

(p,d)+C ,

The yield ratio of γγ-pairs from the interval of invariant masses 530÷570 MeV in the reactions d+C→γ+γ+x at 2.75 GeV/c per nucleon and p+C→γ+γ+x at 5.5 GeV/c.

(p,α)+C ,

The experiment on theNuclotron

(p,d)+C ,

The yield ratio of γγ-pairs from the interval of invariant masses 530÷570 MeV in the reactions d+C→γ+γ+x at 2.75 GeV/c per nucleon and p+C→γ+γ+x at 5.5 GeV/c

The experiment on Cynchrophasotron

(d,,C)+C,

The experiments on the Nuclotron: p,d+C ++X, P=5.5 ГэВ/c .

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The selection criteria The selection criteria

1) the number of photons in an event, Nγ =2 or Nγ ≤ 3;

2) the energies of photons, Eγ ≥ 100 MeV;3) the summed energy in real and random

events ≤ 1.5 GeV.

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Invariant mass distributions of γγ pairs without (upper panel) and with (bottom panel) the background subtraction.

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Invariant mass distributions of γγ pairs without (upper panel) and with (bottom panel) the background subtraction in

d+C (left) and d+Cu (right) reactions.

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EExperimental (circles) and simulated (triangles)xperimental (circles) and simulated (triangles) data u data under nder the same conditions. the same conditions. The solid lines – cThe solid lines – contribution of photons ontribution of photons

fromfrom the R decaythe R decay

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J.Banaigs, J.Berger et al. ABC and DEF effects// Nucl.Phys. B67, (1973),1. 03 )( Hepd

ω(ρ) →ee and φ →ee detection

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Concluding remarksConcluding remarks

To understand the nature of the observed effect were attempted some dynamic mechanisms:

production of the hypothetic R resonance in ππ interactions during the evolution of the nuclear collision;

formation of the R resonance with participation of photons from the Δ decay;

the πºπº interaction effect in the 3πº channel of the η decay; a particular decoupled dibaryon mechanism; the light sector of Higgs ???...

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Concluding remarks, outlookConcluding remarks, outlook

Due to the strangeness (sŝ) in η-mesons, comparison η and πº productions allows to clarify the mesons production mechanisms.

Investigations of πº mesons is interest in view of the NICA possibilities. The scan of the πº spectra in the nuclear-nuclear collisions allows to detect signals of the π-condensate state: the pion vacuum breaking at the strong QCD field in dense nuclear matter.

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Concluding remarks, outlookConcluding remarks, outlook

To clarify of the nature and further investigations of the resonance observed in the invariant mass spectrum of γγ pairs in dC-interactions at 2.75 GeV/c per nucleon and dCu-interactions at 3.83 GeV/c per nucleon:

1. pC→1. pC→γγγγ+X at T+X at Tp p ~2 GeV~2 GeV,2. …2. … To search for some features in the invariant mass spectrum

of -pairs in the interval of 270-750 MeV bounding up with the chiral symmetry restoration (A.N. Sysakian, A.S. Sorin, M.K. Suleymanov, V.D. Toneev and G.M. Zinovjev arXiv:nucl-ex/0601034).

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The NICA advantages

The collider NICA allows to scan energyenergy , massmass and polarizationpolarization dependencies of the resonance production in ion collisions at high luminosities.

To solve above tasks a calorimeter with large acceptance is needed (in consisting of MPD or SPD). The increasing of the calorimeter acceptance allows to investigate the invariant mass spectra of pairs in the wide range at different energies and transverse momenta of pairs (that means different production mechanisms).

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Dibaryon mechanism (I)Dibaryon mechanism (I)

Recently the idea of nontrivial dibaryon state becomes more attractive. The proposed mechanism NN→d*→NNγγ proceeds through a sequential emission of two photons:

one is caused by production of the decoupled baryon resonance d*, second is its subsequent decay.

A.S. Khrykin and S.B. Gerasimov, On a possible origin of a resonance-like structure in the two-photon invariant mass spectrum of the reaction pp→ppγγ,

arXiv:0710.3331. A.S. Khrykin et al., PRC64 (2001) 034002, NPA721(2003)625

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Dibaryon mechanism (II)Dibaryon mechanism (II)

Very attractive candidate for its realization might be a model of the intermediate σ-dressed dibaryon. In this model the short-range NN-interaction is described with the s-channel σ exchange associated with the intermediate dibaryon production treated as a σ-dressed six-quark bag. As the result we have decrease of the assumed σ- mass, it is estimated Mσ ~ 350 ÷ 380 MeV. Therefore it should enhance the near-threshold pion and double-pion production. [ V.I.Kukulin et al. J.Phys.G30(2004)287, 30(2004),309]

This mechanism is now under investigation.

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Concluding remarks, outlookConcluding remarks, outlook

1. The dibaryon mechanism is discussing as a possible explanation of observed enhancement. In this way it can be considered as σ-meson.

2. From the experimental side: new experiments are required to be carried out under conditions appropriate for registration of pairs of two photons within the invariant mass interval of 300-400 MeV. Some scanning in the beam energy will clarify the possible resonance structure of this effect. By varying the opening angle of the PHOTON-2 spectrometer it is possible to get information about momentum spectra of the resonance-like structure which could be a test of the R production mechanism.

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Data simulationData simulation

To simulate pC-, dC- and dCu- reactions we used a two-phases transport code [K.K. Gudima et al. LANL Report LA-UR-01-6804, Los Alamos, 2001]

The following γ-decay channels are taken into account: the direct decays of πº,η,ή hadrons into two γ’s; ω → πºγ; Δ → Nγ; the Dalitz decays of η → ππγ, η → γee, πº → γee; ή → ρºγ, Σ →Λγ, the πN and NN-bremsstrahlung.

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The calculated The calculated γγγγ invariant mass distribution invariant mass distributionss in in ppC C (left) (left) and and ddC C (right) (right) collisions for selected events with collisions for selected events with NNγ = 2.γ = 2.

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Invariant mass distributions of γγ pairs in two different runs of measurement under condition Eγ ≥ 50 MeV: with the empty target (dashed histogram) and with the internal carbon target (solid histogram) in the reaction dC = γ +γ +X at 2.75 GeV/c per nucleon.

With the internal carbon target

Without target

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Charged particles contribution

0 200 400 600 8000,8

0,9

1,0

1,1

1,2 - All counts of pairs - Backgrounds

N w

ithou

t sep

. /

N w

ith S

1xS2

M , MeV

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Charged particles contributionafter background subtraction

0 200 400 600 8000,80

0,85

0,90

0,95

1,00

1,05

1,10

1,15

1,20

M , MeV

Ratio

(All

pairs

) /

Ratio

(Bac

kgr.)

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Invariant mass distributions of γγ pairs satisfying the criteria (1) − (2) after background subtraction in the reaction dC = γ +γ +X at 2.75 GeV/c per nucleon for two different beam intensities: 503 events/cycle (a) and 85 events/cycle.

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The invariant mass distributions of two photons for the opening angles 0.55 < cos (Θγγ) < 0.65 (left) and 0.65 < cos (Θγγ) < 0.75 (right) under the selection criteria (1) − (2).

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The invariant mass spectra of γγ pairs for the energy selection Eγ > 400 MeV under the selection criteria (1) - (2), Knorm = 0.973.

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Nγ=3

0 200 400 600 800-3000

-2000

-1000

0

1000

2000

R

N = 3 , E

> 50 MeV

dC, Exper.

Cou

nts

M

[MeV]

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Nγ=3: 1γ in the L.Arm, 2γ in the R.Arm +

2γ in the L.Arm, 1γ in the R.Arm

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The invariant mass distribution of The invariant mass distribution of γγγγ pairs and the biparametri pairs and the biparametric c distribution of the GW of the 8-th order for distribution of the GW of the 8-th order for dC (left) and pC (right)dC (left) and pC (right) interactions. interactions. TThe distribution is obtainedhe distribution is obtained with an additional condition with an additional condition for photon energies Eγ1/Eγ2 > 0.8 and binning in 2for photon energies Eγ1/Eγ2 > 0.8 and binning in 2MeV.MeV.

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A.Taranenko et.all, Czech.J.Phys. 50S4 (2000) 139, nucl-ex/9910002. Results of the invariant-mass analysis of photon pairs (TAPS). The upper frame shows the invariant-mass spectrum which corresponds to the η trigger in the experiment 58Ni+58Ni at 1.9 AGeV. The combinatorial background (dotted line) was determined by event mixing. The lower frame shows the invariant-mass distribution after background subtraction and demonstrates the quality of the background determination.

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R. Averbeck et al. (TAPS Collaboration), Z. Phys. A 359, 65 (1997).

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Comparison with experiments on the “TAPS” 1. Z.Phys.A359, 65(1997): C+C reaction, 2.0A GeV

TAPS Opening angles 65º-102º η energies (GeV) > 0.70 Mean values 0.85 O.R. energies > 0.457 Mean values 0.552 Total cr. sect. (b) 2.021 Arm’s area (m²) 0.578 Arm’s solid angle (sr) 0.257 En.res. σ/E (m.v.,%) 3.0 Sig./B. in 300-420: ~ 0.0014 ª (<0.004) ª Sig. ~ √6 ∙ 10^5

PHOTON-242º-66º > 1.011.21

> 0.6520.7820.6120.4240.047

6.10.027

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Necessary statistics for same observation in the experiment on TAPS ( Z.Phys.A359, 65(1997): C+C reaction, 2.0A GeV)

~3∙10¹² ∙ (ΔΩPHOTON / ΔΩ TAPS)² ∙ (ΔMPHOTON / ΔMTAPS)² ∙ [(S/B)PHOTON / (S/B)TAPS]²

S/B=0.0014~3∙10¹² ∙ (0.047 / 0.257)² ∙ (3.0 / 6.1)² ∙ (0.027 / 0.0014)²

= 9 ∙ 10¹² interactions, S/B<0.004 :

> 1.1 ∙ 10¹² interactions.Ncycle > (1.1 ∙ 10¹² ) / (5 ∙ 10^6 ∙ ω ) =

= 4 500 000 accelerator cycles,ω = ρx∙(NA / A)∙ σ(CC) = 0.049, ρx = 0.487 g/cm ²

For indication (+3 st.err.) : > 550 000 acceler. cycles

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WA80 Collaboration, Phys.Lett. B361 (1995) 14, hep-ex/9507009.Invariant mass distributions for the 200 AGeV/c (SPS) S+Au data in the 0 (a) and η (b) mass range after background subtraction. The signal to the background ratio is 5.7·10-2 and 7·10-4.

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WASA-CELSIUS :(a) M spectrum after kinematic fit.The dotted histogram in shows background expected from misidentified π0π0 events. (b) The same as (a), but for Tp = 1.2 GeV.

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Photon energy spectra from γγ pairs in the invariant mass interval Mγγ = 0.32 ч 0.4 GeV. Experimental (circles) and Monte-Carlo simulation (triangles) points calculated with inclusion of the R resonance formation are given separately forevery spectrometer arm. Distributions are normalized to the same total number of events.

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Invariant mass distributions of γγ pairs satisfying the criteria (1) − (2) after background subtraction in the reaction dC = γ +γ +X at 2.75 GeV/c per nucleon. Normalization of the background by the total number of pairs in the spectrum.

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Invariant mass distributions of γγ pairs from the dC reaction. The top shaded histograms show the background contribution. The bottom histograms are invariant spectra after the background subtraction.

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The invariant mass distributions of γγ pairs from the pC (left) and dC (right) reactions after background subtraction.

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Eγ ≥ 100 MeV

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