CBM Collaboration Meeting, 28 February 2008, GSI-Darmstadt

Di-electron pair reconstructionDi-electron pair reconstruction

Tetyana GalatyukGSI-Darmstadt Outline:

– Energy dependence of the: track reconstruction electron identification signal-to-background ratio

– Possible scenarios to improve performance

– Summary

CCBBMMInput to the simulationInput to the simulation

UrQMD - final phase space distribution of hadrons and photonsAu+Au at 15 – 35 AGeV, zero impact parameterPLUTO: leptonic and semi-leptonic (Dalitz) decay of vector meson

Full event reconstruction and particle identificationSoftware: cbmroot version AUG07 (17 august)

25 m gold target (to suppress electrons from gamma conversion)Enlarged STS geometry (2 MAPS + 2 Hybrid Pixel + 4 Strip detectors)

Active Field, 70% of nominal value (acceptance vs. resolution)RICH : standard geometryTRD : quadratic planes, 25o geometrical acceptanceTOF : "monolithic" TOF wall

CCBBMM

Invariant eInvariant e++ee-- spectrum in 25 AGeV Au+Au collisions, spectrum in 25 AGeV Au+Au collisions, zero impact parameter (full phase space)zero impact parameter (full phase space)

0 mass distribution generated including:

Breit – Wigner shape around the pole mass;

1/M3, to account for vector dominance in the decay to e+e-;

Thermal phase space factor

Ansatz: is governed by the phase space

MesonMeson

Production rateProduction rateDecay Decay modemode BRBR

15 AGeV15 AGeV 25 AGeV25 AGeV 35 AGeV35 AGeV

23 36 40 e+ e- 5.×10-3

15 23 26 e+ e- 4.7×10-5

27 38 46 e+ e- 0

e+ e-

7.7×10-4

7.18×10-5

0.5 1.28 1.5 e+ e- 2.97×10-4

CCBBMMBackground sources of eBackground sources of e++ee--

Radial vs. z position (eγ) andBy along the beam axis

~250-400 0 98.8%

e+ e-

1.2%

~3 target e+e-

~550 - 800 +/- can potentially be misidentified as electrons

Au+Au collision at beam energy 15 - 35AGeV, zero impact parameterzero impact parameter

CCBBMMTracking performanceTracking performance

Reconstruction efficiency ~93% (p < 1 GeV/c)Reconstruction efficiency ~93% (p < 1 GeV/c)

Momentum resolution ~1.5%Momentum resolution ~1.5%

Momentum resolutionReconstruction efficiency

CCBBMMParticle identificationParticle identification

CCBBMMElectron identification with RICH, TRD and TOFElectron identification with RICH, TRD and TOF

RICH identification cuts:RICH identification cuts:

distance between ring center and track

radial position of the ring center from the centre of photo detector

number of UV photons / ring

ring radius

TRDTRD

statistical analysis of the energy loss spectra (neural net)

TOFTOF

m2 vs momentum

CCBBMMElectron identification efficiency, Electron identification efficiency, suppression suppression

suppression factorElectron id efficiency

~50% electron efficiency (p~50% electron efficiency (plablab< 2GeV/c)< 2GeV/c)

ππ-suppression -suppression of 10of 1044 well in reach well in reach

RICHRICH+TRD+TOF

ring reconstructionRICHRICH+TRD+TOF

CCBBMM

Correlation of the number of STS traversedby e+e- pairs from conversion and π0-Dalitz

Combinatorial background (CB) topologyCombinatorial background (CB) topology

Track Fragment - x, y position; no charge informationTrack Segment - reconstructed trackGlobal Track - identified in RICH

ee0

ee 0

Track Segment

Global Track

eemedium

Track Fragment

signal

fake

pair

Small (moderate) opening angle and/or asymmetric laboratory momenta.

CCBBMMThe strategy of background rejectionThe strategy of background rejection

The strategy of background rejectionThe strategy of background rejectioncomprises the following steps:comprises the following steps:

identify and reject true pairs originating from conversion

remove single tracks where the true partner was not fully

reconstructed using topological cuts

apply single electron pt (200 MeV/c) cut

identified close pairs θ1,2 < 20

assign pairs with a characteristic pattern to 0-Dalitz pairs

CCBBMMInvariant mass spectra (Au+Au @ 15 AGeV)Invariant mass spectra (Au+Au @ 15 AGeV)

ππ0 0 γγee++ee--

ππ00ee++ee--

ηη γγee++ee--

Identified e+e- After all cuts applied

All eAll e++ee--

Combinatorial bgCombinatorial bg

ρρ ee++ee--

ee++ee--

φφ ee++ee--

Central Au+Au@15AGeV

Simulated statistics: 68 kevents

CCBBMMInvariant mass spectra (Au+Au @ 25 AGeV)Invariant mass spectra (Au+Au @ 25 AGeV)

ππ0 0 γγee++ee--

ππ00ee++ee--

ηη γγee++ee--

Identified e+e- After all cuts applied

All eAll e++ee--

Combinatorial bgCombinatorial bg

ρρ ee++ee--

ee++ee--

φφ ee++ee--

Central Au+Au@25AGeV

Simulated statistics: 200 kevents

CCBBMMInvariant mass spectra (Au+Au @ 35 AGeV)Invariant mass spectra (Au+Au @ 35 AGeV)

ππ0 0 γγee++ee--

ππ00ee++ee--

ηη γγee++ee--

Identified e+e- After all cuts applied

All eAll e++ee--

Combinatorial bgCombinatorial bg

ρρ ee++ee--

ee++ee--

φφ ee++ee--

Central Au+Au@35AGeV

Simulated statistics: 65k events

CCBBMM

Invariant mass spectra of the combinatorial Invariant mass spectra of the combinatorial backgroundbackground

ParticleParticleProduction rateProduction rate

15 AGeV15 AGeV 25AGeV25AGeV 35AGeV35AGeV

0 264 337 382

261 332 386

- 293 368 423

( + 45) =( + 73) =

( + 71) =

( + 75) =

Identified e+e- After all cuts applied

( + 54) =

( + 64) =

CCBBMMSignal-to-background ratiosSignal-to-background ratios

Free cocktail only (without medium contribution)

CCBBMMOverview of existing dilepton experimentsOverview of existing dilepton experiments

E = 5.91.5(stat)1.2(syst)1.8(decay)

CERES coll., Phys. Rev. 91 (2003) 042301 CERES, arXiv:nucl-ex/0506002 v1 1 Jun 2005

E = 2.310.190.550.69

CERES, arXiv:nucl-ex/0611022 v1 13 Nov 2006

E=2.580.320.410.77

E = 3

NA 60 coll., J.Phys. G32:S51-S60, 2006 CERES, Phys.Rev.Let vol.75, N7,14 Aug 1995

E = 5.0.7(stat)0.2(syst)

E = 3.4 0.2(stat) 1.3(syst) 0.7(model)

PHENIX, atXiv:0706.3034v1 [nucl-ex] 20 Jun 2007

CCBBMM

Overview of existing dilepton experiments Overview of existing dilepton experiments (summary)(summary)

ExperimentExperiment SystemSystem √√ss dNdNchch/d/dηη EE S/BS/B**** Sys error (%)Sys error (%)

CERES Pb+Au 8.86 216 5.9 1/6 20

CERES (σ/σtot = 28%) Pb+Au 17.2 245 2.31 1/13 24

CERES (σ/σtot = 7%) Pb+Au 17.2 350 2.58 1/21 16

NA60(central) In+In 17.2 193 3 1/11 25

NA60(semi-central) In+In 17.2 133 2 1/8 25

NA60(semi-peripheral) In+In 17.2 63 2 1/3 12

NA60(peripheral) In+In 17.2 17 1.5 2 3

CERES S+Au 19.5 125 5 1/4.3 25

PHENIX(0-10% centrality) Au+Au 200 650 3.4 1/500 ?= 50

SIMULATIONSIMULATION

CBM (b=0fm) Au+Au 250 ? 1/9* -

CBM (b=0fm) Au+Au 300 ? 1/16* -

CBM (b=0fm) Au+Au 350 ? 1/18* -

* Free cocktail only (without medium contribution)** Signal-to-background ratios for invariant mass larger than 200 MeV/c2

CCBBMM

Comparison of expected performance to Comparison of expected performance to existing dilepton experimentsexisting dilepton experiments

NA60 In+In @ 158 AGeVNA60 In+In @ 158 AGeVCERES Pb+Au @ 40 AGeVCERES Pb+Au @ 40 AGeVCERES Pb+Au @ 158 AGeV (CERES Pb+Au @ 158 AGeV (σσ//σσtottot = 28%) = 28%)

CERES Pb+Au @ 158 AGeV (CERES Pb+Au @ 158 AGeV (σσ//σσtottot = 7%) = 7%)

CERES Pb+Au @ 158 AGeV CERES Pb+Au @ 158 AGeV PHENIX Au+Au @ √s = 200 AGeVPHENIX Au+Au @ √s = 200 AGeV

CCBBMM

Question:Can we still improve our results?

Answer:Yes

Where?On the track reconstruction levelOn the electron identification levelOn the pair analysis level

How?

CCBBMMTrajectories of eTrajectories of e++, e, e--, , from from 00-Dalitz decay-Dalitz decay

field : 70% from nominal valuetarget : 25mSTS : 2 MAPS (200m), r = 1.5r0

2 HYBRID (750m), r = 1.5·r0

2 STRIP (400m), r = 1.5·r0

2 STRIP (400m), r = r0

Optimized detector setupStandard detector setup

CCBBMMChanges to the detector setupChanges to the detector setup

Standard STS 100% field– case 1

Standard STS 70% field – case 2

Large (1.5) STS 100% field – case 3

Large (1.5) STS 70% field – case 4

x vs. y position of the extrapolated tracks

STS1STS2

STS2 STS3

STS3 STS4

Number of primary tracks withmomentum < 500 MeV/c

case 1case 1 34

case 2case 2 41.47

case 3case 3 44.85

case 4case 4 52.62

increase up to~26 %

CCBBMMHow about size of other detectors?How about size of other detectors?

RICHTRD

TOF

Increasing of the STS stations is needed to increaseacceptance of the Track Segments

Size of the RICH, TRD detector are not effected!!!

TRD

TOF

CCBBMMElectron identification Electron identification

ring-track assignementring-track assignement

(closest distance)(closest distance)

Improve the ring-track matching by:not only selecting the track closest to the ring centre, but all within a certain range (2 sigma)

include the TRD and TOF information for RICH-candidates to discriminate misidentified pions

only then do ring-track assignment

STSSTS

RICHRICH

TRD TOF

CCBBMMDetermination of maximum level of misidentification

Enough!!!

Konstantin Antipin

Combinatorial background assuming that every 1/N of the pions aremisidentified as electron/positron. N = 100, 1000, 5000, 10000

With misidentification of 1/5000 the combinatorial background isdominated by physical sources (88.8%)

CCBBMMAnd now my old lovely song…And now my old lovely song…

CCBBMMCB suppression II: hit topologyCB suppression II: hit topology

dsts vs. plab of the e dsts vs. plab of the e

Mai

nly

conv

ersi

on

Global Track

Track Fragment

CCBBMM

How to suppress electrons from theHow to suppress electrons from the conversion in the target conversion in the target

excellent double-hit resolution (<100m) provides substantial close pair rejection capability

a realistic concept has to be worked out to suppress the field between the target and first MVD station

trade: suppression of delta-electrons vs. opening of close pairs

Generic simulation w/o realistic detector response

Field free region between the target and first MDV

No invariant mass () cut (m<25 MeV/c2) applied

Distance between ID e+/- and closest hit in first MDV (z=10 cm)

CCBBMM

How the particle identification in the first MVD How the particle identification in the first MVD could help?could help?

e+ e-Rejection of the conversion can be further improved by exploiting energy loss information in theMVD

Could save more signal

Could increase rejection power for the combinatorial background by applying more open cut

Konstantin Antipin

CCBBMMSummarySummary

We presented simulated dielectron invariant mass spectra We presented simulated dielectron invariant mass spectra after full event reconstruction and particle identification after full event reconstruction and particle identification including realistic detector responses for 3 different including realistic detector responses for 3 different energiesenergies

If we could achieve such results in reality – would be nice! If we could achieve such results in reality – would be nice!

CCBBMM

BONUS SLIDESBONUS SLIDES