Electron pair analysis for high multiplicity events in nucleus-

nucleus collisions

A.Baldin, E.Baldina, V.Pozdnyakov

LHE JINR, Dubna

JINR-GSI meeting November 20-21, 2003, Dubna

Enhanced low-mass e+e- pair production (CERES , SPS)

450 AGeV p-Be

HELIOS/NA34

158 AGeV Pb-Au

CERES/NA45

40 AGeV Pb-Au

CERES/NA45

LHE JINR A.Baldin, E.Baldina, V.Pozdnyakov

Combinatorial background originating from partners of low-mass Dalitz or conversion pairs presents the crucial problem in the high-multiplicity environment of nuclear collisions.

[P.Glässel and H.J.Specht, LBL-24604 p.106]

Pair finding considerations

Due to the fact that the inclusive electron spectrum from 0 Dalitz decays is significantly softer that that of the signal, the signal-to-background ratio can be noticeably improved by the pT cut on single electrons.

The cut pT >200MeV/c reduces the signal by a factor of 3 and the background by a factor of 13, thus improving the S/B ratio by a factor of 4 for the mass range 0.2 < m < 0.6 GeV/c2 (the window above the 0 Dalitz tail and below the mass.

LHE JINR A.Baldin, E.Baldina, V.Pozdnyakov

Single electrons from < pT > MeV/c

conversions 85

0 Dalitz 85

Dalitz 155

Geometrical low-mass pair rejection

Low-mass pairs are efficiently rejected by a cut on the pair opening angle.

The steps are the following:

1. All electrons with the angle < 1 to any other electron are discarded.

2. Pairs are discarded in the order of increasing opening angle up to

an angle 2 .

Track efficiency and vertex finding yield additional rejection of conversion e+e- pairs.

LHE JINR A.Baldin, E.Baldina, V.Pozdnyakov

Average number of e+e- pairs produced via decay of particles with ymin<y<ymax and 0<pT<pTmax

WBR=BR·(Nx/N0) ·(dN0/dy) ·(ymax-ymin)

source BR Nx/N0 WBR(SIS-200)

0e+e- 1.198·10-2 1 7

0e+e- 5.0·10-3 0.17 0.49

’e+e- 3.9·10-4 0.09 0.02

e+e-0 5.9·10-4 0.14 0.048

e+e- 1.3·10-4 0.016 1.2·10-3

’e+e- 2.0·10-3 0.09 0.11

e+e- 4.44·10-5 0.15 3.89·10-3

e+e- 7.07·10-5 0.14 5.79·10-3

e+e- 3.1·10-4 0.016 2.89·10-3

DY e+e- 5.0·10-4

dNx/dy·BR

1.46·10-4

J/ e+e- 1.8·10-3

dNx/dy·BR

5.26·10-4

e+- misint. 2.0 0.25LHE JINR A.Baldin, E.Baldina, V.Pozdnyakov

Pair finding strategy

• Dielectron pairs with low masses and high pT are discarded

• Dielectron pairs with low masses and low opening angles are discarded

• Ordering procedures are useful

• pT cuts both for pairs and single electrons

• Account of acceptance, registration efficiency• Order of cutting criteria is important

LHE JINR A.Baldin, E.Baldina, V.Pozdnyakov

Dielectron pair production : modeling strategy • Particle production meets the criteria:

mass spectrum ~1/M2 ;

MT scaling ;

gaussian dN/dy .Particles are produced in 25 AGeV Au+Au collisions using RQMD.

• e+e- from 0 , , , , , Dalitz decays in accordance with branching ratios.

Monte Carlo decay modeling using standard CERNLIB software with preset BR.

• Other sources of e+e- ( conversion, etc.)Single electrons (3-7 per event) are added with uniform probability over the solid

angle and exponential momentum distribution.

LHE JINR A.Baldin, E.Baldina, V.Pozdnyakov

The basic modeling parameters:Au+Au 25 AGev

• Angular acceptance: 3º27º ;• Rapidity range: 0.5 y2 ;• Detection efficiency:100%;• Identification efficiency: 100%;• No multiple scattering.

Central events with dnc/dy=300 are considered.

LHE JINR A.Baldin, E.Baldina, V.Pozdnyakov

Pair finding criteria

• All e forming an unlike-sign pair with m≤50MeV/c2 with any e are discarded;

• All e forming an unlike-sign pair with m<100MeV/c2 if pT of both electrons exceeds 70MeV/c are discarded;

• Unlike-sign pairs are removed in the order of increasing pair mass up to 100MeV/c2 (ordering);

• All e forming an unlike-sign pair with m<120MeV/c2 and opening angle : cos>1-0.0005mee are discarded;

• All e with pT >200MeV are discarded.

LHE JINR A.Baldin, E.Baldina, V.Pozdnyakov

Rough estimate of S/B ratio for an ideal detector CBM

50 100 150 200 250 300 3500

10

20

30

40

50

0.2<Mee

<1.2S/B

dN0/dy

LHE JINR A.Baldin, E.Baldina, V.Pozdnyakov

Invariant mass distributions

after cuts in ,, region for 107 central events

0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0

5000

10000

15000

20000 Number of fake electronsAu+Au (25 AGeV)

Cou

nts/

20M

eV

me+e-

(GeV/c2)

N 2xN

LHE JINR A.Baldin, E.Baldina, V.Pozdnyakov

Background from conversion dominatesAfter cut on e+e- vertex :SNR 3 in 1 M eventsstudy ongoing, tracking needed

Feasibility study : e+ e-

Conclusion

• Due procedures have been developed and used for dielectron pair analysis for CBM;

• The ideal CBM allows for detection of , , and investigation of the low-mass region;

• Further effort will be put into study of conversion electrons and accurate account of the set up design.

LHE JINR A.Baldin, E.Baldina, V.Pozdnyakov