CCBBMM

CBM Collaboration Meeting, 27 September 2007, Dresden

Di-electron background studies andDi-electron background studies andfirst results using compact RICHfirst results using compact RICH

Tetyana GalatyukGSI-Darmstadt

CCBBMMOutlineOutline

No motivation

Input to the simulation

Changes to the detector setup

Sources of e+e- pairs and their characteristics

Track reconstruction and electron identification

Background rejection single electron cuts

pair cuts

Comparison to existing dilepton experiments

Some results with the compact RICH geometry

Summary

CCBBMMInput to the simulationInput to the simulation

UrQMD - final phase space distribution of hadrons and photons final phase space distribution of hadrons and photons central Au+Au@25AGeV, zero impact parameterPLUTO: leptonic and semi-leptonic (Dalitz) decay of vector mesonFull event reconstruction and particle identificationSoftware: cbmroot version AUG07 (17 august)

25 m gold target (to suppress electrons from gamma conversion)STS:

Optimized geometryCA track finderKF track fitterTracks only from primary vertex (χ2 at primary vertex < 3)

Active Field, 70% of nominal value (acceptance vs. resolution)RICH: standard geometry (Photodetector: H8500-03 → 22 hits/ electron ring)TRD: 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)

2.97×10-4 e+ e-1.28

7.7×10-4

7.18×10-5

e+ e- 0

e+ e-

38

4.7×10-5 e+ e-23

5.×10-3 e+ e- 36

BRDecay modeN/eventMeson

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

CCBBMMChanges to the detector setupChanges to the detector setup

25 m ≡ 1‰ interaction lengthuse high quality, high intensity beamfrom FAIR and work with 1‰interaction target!or work with segmented target

x vs. y position of the extrapolated tracks

STS1STS2

STS2 STS3

STS3 STS4

N of γ vs. target thickness

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

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

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

~350 0 98.8%

e+ e-

1.2%

~3 target e+e-

700 +/- could be identified as an electron

Au+Au collision at beam energy 25AGeV, zero impact parameterzero impact parameter

CCBBMMTracking performance Tracking performance (plots from official qa_reco.C)(plots from official qa_reco.C)

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

Momentum resolution ~ 1.68%Momentum resolution ~ 1.68%

Momentum resolutionReconstruction efficiency

Remark: 97% reconstruction efficiency in cbmroot jun06 version

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 : upper momentum cutElectron identification : upper momentum cut

Lepton momentum distributionRing radius vs. momentum

ee+/-+/-

CCBBMMElectron identification : upper momentum cutElectron identification : upper momentum cut

Mee of the meson pt vs. rapidity

p<5.5 GeV

all p

CCBBMMElectron identification : quality cutsElectron identification : quality cuts

~ 90 rings / event :from signal from the conversion (on the detector material, in the target)

fake rings

Matching quality Rich ring selection with Neural Net

CCBBMMElectron identificationElectron identification

version I : RICH + TRD (plab>1GeV) + TOF

version II : RICH + TRD + TOF information required

1

2

3

1, 2, 3 were identified as an e

CCBBMMElectron identification (vI) : TRD and TOF cutsElectron identification (vI) : TRD and TOF cuts

Neural Net Method m2 vs momentum of the tracks identified as e in RICH and TRD

ee

CCBBMMElectron identification (v2) : TRD and TOF cutsElectron identification (v2) : TRD and TOF cuts

Neural Net Method m2 vs momentum of the tracks identified as e in RICH and TRD

ee

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

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

CCBBMMInvariant mass distributionInvariant mass distribution

Identification vII

ID = identified / full phase space

bg : 0.92%bg : 0.92% ρρ00 : 9% : 9% ω ω : 11%: 11% φφ : 13% : 13%

Identification vI

acc = accepted / full phase space

bg : 51%bg : 51% ρρ00 : : 57%57% ω ω : 60%: 60% φφ: : 62%62%

ID = identified / full phase space

bg : 5%bg : 5% ρρ00 : 12% : 12% ω ω : 15%: 15% φφ : 16% : 16%

Invariant mass ρ0 invariant mass

Invariant mass ρ0 invariant mass

CCBBMMLepton multiplicityLepton multiplicity

Ne- vs Ne+, identification vI Ne- vs Ne+, identification vII

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.

CCBBMMCB suppression II: hit topologyCB suppression II: hit topology

Global Track

Track Fragmentdsts vs. plab of the e dsts vs. plab of the e

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

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

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

Mai

nly

conv

ersi

on

CCBBMMCB suppression III: track topologyCB suppression III: track topology

e + closest track eπ0 + closest track

Track Segment

Global Track

Mai

nly

Dal

itz

CCBBMMAdditional cuts for CB suppression Additional cuts for CB suppression

Transverse momentum cut

pt distribution of e+e- from bg and signal

Identified close pairs

θ1,2 < 20

are rejected

Pairs with mee < 0.2 GeV/c2are kept in the sample butare not combined with othersanymore

Pair cuts:Pair cuts:

Single electron cut:Single electron cut:

- bg e+/-

- 0 e+/-

CCBBMMInvariant mass spectra (v II) Invariant mass spectra (v II)

ππ0 0 γ γee++ee--

ππ00ee++ee--

η η γγee++ee--

Identified e+e- After all cuts applied

All eAll e++ee--

Combinatorial bgCombinatorial bg

ρρ ee++ee--

ee++ee--

φφ ee++ee--

Free cocktail only (without medium contribution)Free cocktail only (without medium contribution)Simulated statistics is 200k eventsSimulated statistics is 200k events

Central Au+Au@25AGeV

CCBBMMEfficiency of cuts, S/B ratioEfficiency of cuts, S/B ratio

π0-Dalitz region Enhancement region / region

ε

S/B

CCBBMMComposition of the combinatorial backgroundComposition of the combinatorial background

Physical background Fakes and misidentified

γtarget e+

e-

π0

γ

e+

e-

φ e+

e-

bg*

Background “cocktail”all eall e++ee--

wrong match + fake ring : 25%wrong match + fake ring : 25%

physical : 75%physicalphysical

CCBBMMPhase space coverage (Phase space coverage (0 meson)

No phase space limitation!No phase space limitation!

Full phase space After full event reconstruction, ID and pair analysis

CCBBMMPair detection, w/o pPair detection, w/o ptt cut on single e cut on single e+/-+/-

Coverage in pair pt-minv planePair detection efficiency

(reconstructed/full phase spase)

Nice coverage of very low pt and very low mee!

CCBBMMInvariant mass spectrum w/o pInvariant mass spectrum w/o p tt cut on single e cut on single e+/-+/-

Invariant mass spectrum, no pt cut

What is the Signal to Background ratio?What is the Signal to Background ratio?What is the signal?What is the signal?

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

S. Damjanovic, arXiv:nucl-ex/0510044 v113 Oct 2005

CERES, Phys.Rev.Let vol.75, N7,14 Aug 1995

E = 5.0.7(stat)0.2(syst)

E =? 3

A.Toia, ECT, Trento 2007

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

321.51717.2In+InNA60(peripheral)

251/4.3512519.5S+AuCERES

-1/16*??8Au+AuCBM (real) (b=0fm)

-1/11*??8Au+AuCBM (ideal) (b=0fm)

SIMULATIONSIMULATION

?= 501/500?= 3650200Au+AuPHENIX(0-10% centrality)

121/326317.2In+InNA60(semi-peripheral)

251/8213317.2In+InNA60(semi-central)

251/11319317.2In+InNA60(central)

161/212.5835017.2Pb+AuCERES (σ/σtot = 7%)

241/132.3124517.2Pb+AuCERES (σ/σtot = 28%)

201/65.92168.86Pb+AuCERES

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

* - free cocktail only (without medium contribution)ree cocktail only (without medium contribution)

CCBBMMS/B ratio, EnhancementS/B ratio, Enhancement

NA60 In+In @ 158 AGeVNA60 In+In @ 158 AGeVCERES Pb+Au @ 40 AGeVCERES Pb+Au @ 40 AGeVCERES Pb+Au @ 158 AGeV (σ/σCERES Pb+Au @ 158 AGeV (σ/σtottot = 28%) = 28%)

CERES Pb+Au @ 158 AGeV (σ/σCERES Pb+Au @ 158 AGeV (σ/σtottot = 7%) = 7%)

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

CCBBMMEnhancement and S/B ratio for CBMEnhancement and S/B ratio for CBM

safety factorsafety factor

simulation w/o wrong match simulation w/o wrong match and and fakesfakesdetector response, with wrong match detector response, with wrong match and fakes and fakes

CCBBMMCompact RICH – first viewCompact RICH – first view

CCBBMMWhat has been done?What has been done?

RICH standard geometry (dielectron spectra were presented)

RICH small geometry0 meson only (fast, will give a first imagine about acceptance and phase space coverage) Full event reconstruction and particle identificationPair analysis

CCBBMMElectron identificationElectron identification

Matching qualityRich ring selection

with Neural Net Rich radius

Neural Net selectionm2 vs momentum of the tracks

identified as e in RICH and TRD

CCBBMMInvariant mass spectra (Invariant mass spectra (00 meson) meson)

0 embedded into UrQMD event standatd RICH geometry

0 small RICH geometry (14% losses)

+ additional losses after embedding into UrQMD event

CCBBMMPhase space coverage (0 meson)

Full phase space After full event reconstruction, ID and pair analysis

CCBBMMConclusionsConclusions

We presented simulated dielectron invariant mass spectra after We presented simulated dielectron invariant mass spectra after full event reconstruction and particle identification including full event reconstruction and particle identification including realistic detector responses (and I like spectra a lot realistic detector responses (and I like spectra a lot !)!)

~25% difference compare to ideal MC simulation~25% difference compare to ideal MC simulationStatistic of the simulated data (200k events) is equivalent to 1 Statistic of the simulated data (200k events) is equivalent to 1 spill beam on target (archive data rate 10spill beam on target (archive data rate 1044 evt/sec) evt/sec)

TrackingTrackingFirst result with compact RICH geometry look promising:First result with compact RICH geometry look promising:

small RICH geometry will hopefully reduce only pricesmall RICH geometry will hopefully reduce only priceof the detector and will not reduce interesting physicsof the detector and will not reduce interesting physics

CCBBMM

BONUS SLIDESBONUS SLIDES

CCBBMMInvariant mass spectra, cut efficiency (id vI)Invariant mass spectra, cut efficiency (id vI)