Post on 12-Jan-2016
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