Dielectron production in C+C collisions with HADES

28/8/2006 NN2006, Rio de Janeiro August 28 - September 1, 2006

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Dielectron production in C+C collisions with HADES

Pavel Tlusty, NPI Rez

for the HADES Collaboration


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Introduction

Physics motivation in-medium modifications of hadrons –

predictions and existing datavector mesons at SIS energy regime

Hades spectrometerexperimental setup

Experimental resultsP+P @ 2.2 GeV (2004)C+C 2.0 AGeV (2002)C+C 1.0 AGeV (2004) PRELIMINARY

Summary and Outlook


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Physics Motivation Chiral symmetry broken in vacuum

quark condensate – order parameter Partial restoration of chiral

symmetry at high temperatures (RHIC, LHC) at large baryon density (SIS, AGS)

Properties of hot and dense matter Model predictions (one example) for VM

„melting” of the meson mass shift and broadening of the small effects on the φ-meson


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Physics Motivation

CERN data (Ceres/NA45) explained by medium effects

DLS Ca+Ca 1AGeVfull spectral function[*]

[*]R

.Rap

p a

t al.

., N

ucl.

Ph

ys..

A617 (

1997)

472

DLS C+C 1AGeVfull spectral function[*]

Models failed to explain DLS data for 0.15 ≤ M ≤ 0.4 GeV – “DLS puzzle”

elementary processes in 1-2 A GeV energy regime not fully known (e.g. -> e+e-, …)

=> HADES studies of pp, pd reactions


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Probes for hot/dense medium

Vector mesons - ideal probes for medium effects:

short life time decay inside hadronic medium

decay channel into lepton pairs no strong final interaction reconstruction of in-medium properties possible

at 1-2 A GeV production of vector mesons at or below threshold – multistep processes, production confined to the high density phase

Meson Mass (MeV/c2) (MeV/c2) c (fm) Main decay e+e- BR

771 152 1.3 4.5 x 10-5

782 8.43 23.4 7.0 x 10-5

1019 4.26 46.3 K+ K- 3.0 x 10-4

2sinppm ee

eeee

e+

e-

c10-15 fm/c

(p, , A) + A collisions at SIS-GSI - beam energy up to 2 A GeV for AA 0 ≤ ≤, 0 ≤ T≤ 80 MeVc 10-15 fm/c


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Spectrometer concept

Concept: Geometry: six sectors form a hexagonal

structure: acceptance: 2π in φ ; 18 0 < θ < 85 0 pair acceptance 35 %

Tracking: Magnet, MDC superconducting toroid magnet (6 coils) low-mass MDC (multiwire drift chamber) inv. mass resolution ≈ 2% for meson mass

PID and Lepton Identification RICH

photon detector: CsI photo cathode hadron suppression ≈ 106

META (TOF, TOFino, Pre-Shower)

Cross section

START

Experimental Challenges: Small production rates:

1 dilepton decay / 107 central collisions Large background:

hadronic (particle misidentification) electromagnetic conversion, mainly from 0

combinatorial (false combination of e+e–)

Detector requirements: high acceptance excellent lepton identification good momentum resolution low mass/low Z- design for reduced

background highly selective trigger high performance data acquisition


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A High-Acceptance Di-Electron Spectrometer

expanded vi

ew


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pp collisions at 2.2 GeV Experimental goal:

Verify dielectron efficiency: dielectron and hadron channels of 0, hadron channel and branching ratio well

known (DISTO, …) Conditions:

beam intensity: 107 p/second target: LH2 (5 cm length)

totalsignalbackground

M 548 MeV/c2

M 549 MeV/c2

/M 2.5 % (3.6)

pppp pp+-° total°bg

M 548 MeV/c2

M 546 MeV/c2

/M 3.3 %

M° 135 MeV/c2

M 140 MeV/c2

/M 18 %

pppp ppe+e-

yield2322 118 yield

24845 348° yield

7580 454

ee

EXP: 15.3 ± 1.8 (stat)

SIM: 15.6 ± 0.9 (stat)

Resulted yield ratio:

=> Dielectron efficiency well under control


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C+C at 2.0 AGeV

Experimental Setup: Full HADES set-up with medium resolution tracking Medium resolution tracking:

Inner MDC, Meta detector (spatial. res. 1.5 cm) mom. res. 10% @ 0.7 GeV/c

Target: 2 x 2.5 % Data and trigger:

220 Mevents 56 % LVL1 trigger, 44 % LVL2 trigger


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e+e– invariant mass spectrum C+C 2 AGeV

measured exp. spectrum no eff. correction no acc. correction

comb. background (CB) like-sign pairs and event mixing

signal: S+-= Ne+e- - CB+- only statistical errors shown Counts:

S+- < 140 MeV/c2: 20 971 countsS+- > 140 MeV/c2: 1 937 counts

measured rates span over 5 orders of magnitude

good Signal/CB rationucl-ex/0608031submitted to Phys. Rev. Lett.

○ all e+ e-

▲comb. back.

● signal


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e+e– mass spectrum C+C 2 AGeV

efficiency corrected spectra detector efficiency reconstruction efficiency

normalized to the pion yieldN0 = ½ (N - + N +)

medium resolution tracking (10% at 700 MeV/c)

systematic errors norm. to the pion mult. (11 %) eff. corrections and CB

subtraction (30 %) simulated cocktail (PLUTO)

based on known 0 , η (TAPS) and mt-scaled meson multiplicities and their vacuum decay properties

=> only 0 , η not sufficient below 400 MeV/c2


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Comparison with transport HSD*C+C 2 AGeV

*HSD – Hadron-String-Dynamics (Gießen, E. Bratkovskaya, W. Cassing)

efficiency-corrected data compared with model. folded with HADES filter:

geometrical acceptance momentum resolution (medium resolution mode, 10% at 700 MeV/c)

HSD HSD vacuumvacuum HSD HSD in-mediumin-medium


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Comparison with transport: HSD, Pluto, UrQMD

Model vacuum calculations: RQMD Tuebingen (C. Fuchs, …) UrQMD Frankfurt (M. Bleicher, D. Schumacher) HSD Giessen (E. Bratkovskaya, W. Cassing) Pluto Hades

π0, η sources in agreement (20 %)

Discrepancy for full cocktail.

12C + 12C 2 A GeV


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C+C at 1.0 AGeV (preliminary!)

Experimental Setup: Full HADES set-up high res. Tracking. Target: 3 x 1.5 % target.

C+C @ 1.0 AGeVC+C @ 1.0 AGeV

Exp. Spectrum no eff. no eff. ccorrectionorrectionno acc. no acc. ccorrectionorrection

Combinatorial Background (CB)like-sign pairslike-sign pairsformula:formula:

Signal: formula: formula: S+–= N= Ne+ee+e– - CB+ – eeee NN2

PRELIMINARY


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C+C at 1.0 AGeV (preliminary!)

PRELIMINARY

preliminary

only underestimate data by large factor in “eta” region

“full” cocktail almost reproduces data, but Dalitz component not known, currently measured by HADES in p+p at 1.25 GeV experiment (April 2006)

C+C @ 1.0 AGeVin HADES acceptanceno eff. correction

preliminary

only “full” cocktail

simulated cocktail (PLUTO): based on known0 , η (TAPS) and mt-scaled meson multiplicities and their vacuum decay

properties preliminary! – no efficiency corrections


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Summary C+C, 2 AGeV

Final spectrum available Comparison with transport models – vacuum meson

spectral function does not describe data C+C, 1 AGeV

Preliminary spectrum

P+P, 2.2 GeV Dielectron efficiency under control

Outlook

Collected data, ongoing data analysis: C+C 1.0 AGeV, Ar+KCl 1.76 AGeV, p+p 1.25 GeV

Upgrade of Tofino subsystem with RPC (2007) Ni+Ni, Au+Au runs feasible

Hades at FAIR: 2-8 AGeV runs


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The collaboration Bratislava (SAS, PI), Slovakia Catania (INFN - LNS), Italy Cracow (Univ.), Poland Darmstadt (GSI), Germany Dresden (FZR), Germany Dubna (JINR), Russia Frankfurt (Univ.), Germany Giessen (Univ.), Germany Milano (INFN, Univ.), Italy Munich (TUM), Germany Moscow (ITEP,MEPhI,RAS),

Russia Nicosia (Univ.), Cyprus Orsay (IPN), France Rez (CAS, NPI), Czech Rep. Sant. de Compostela (Univ.),

Spain Valencia (Univ.), Spain Coimbra (Univ.), Portugal

SISSIS


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Backup slides


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Meson production at SIS energy regime (1-2 AGeV)

VM production (1-2 AGeV): at or below threshold multi step processes production confined to the

high density phase

Meson multiplicity: 0, known from TAPS

F.D.Berg. At al., PRL 72,977 (1994)

, , - mt scalingM.Bourquin, Nucl. Phys. B114, 334 (1976)

NN → NΔ

πN

πN → ρ N


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Experimental challenges Small production rates:

1 dilepton decay / 107 central collisions Large background:

hadronic (particle misidentification) electromagnetic (conversion, mainly from π0 ) combinatorial (false combination of e+e–)

Detector requirements: excellent particle id perfect lepton identification (hadron-blind detector) low mass/low Z- design for reduced background highly selective trigger high performance data acquisition


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Spectrometer concept Geometry: Six sectors form a hexagonal structure:

Acceptance: 2π in φ ; 18 0 < θ < 85 0 Pair acceptance 35 %

Tracking: Magnet, MDC Superconducting toroid magnet (6 coils)

Bmax = 0.7 T, Bending power 0.34 Tm low-mass MDC (multiwire drift chamber)

PID and Lepton Identification RICH

hadron blind photon detector: CsI photo cathode

META (TOF, TOFino, Pre-Shower) Start detector

diamond. (time res. 30 ps)

Cross section

START

Selective multi-level trigger system Pair enhancement factor close to 10 No bias on data LVL2 efficiency (pairs) 82 % Suppression 10 - 100


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Selective multi-level trigger system

Performance: Pair enhancement factor close to 10 No bias on data LVL2 efficiency (pairs) 82 % Suppression 10 - 100

Second Level Trigger: Online hit finders (Rich, Tof, Online hit finders (Rich, Tof,

Shower)Shower) Online hit matching (MU)Online hit matching (MU) Trigger decision (within 20 μs)Trigger decision (within 20 μs) ~~3 Gbyte/s raw data3 Gbyte/s raw data

Fast First Level TriggerFast First Level Trigger::(50 ns, up to 20 kHz)(50 ns, up to 20 kHz) Meta multiplicityMeta multiplicity Start detector (time Start detector (time

signal)signal)

Events transported to the mass storage

Second Level TriggerSecond Level Trigger


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Charged meson production –multiplicitiesC+C 2 AGeV

LVL1 event, HADES acceptance N< -+> = 0.81

LVL1 event, 4 N< -+> = 1.15

min. bias event, 4 N< -+> = 0.82 ± 0.09

multiplicity/participant N< -+>/part = 0.137 ± 0.015

statistic errors negligible, systematic errors ≈ 11%

correction to 4 and min. bias based on UrQMD simulation

average number of participants in LVL1 events ≈ 8.4

particle ID based on measured momentum and velocity

yields corrected on detection/tracking efficiency (UrQMD simulation)

multiplicity/event (+ and - averaged):


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Charged meson production - cm anisotropyC+C 2 AGeV

A2 EXP A2 UrQMD

mom_cm > 200 MeV/c 1.1 ± 0.4 1.11

all momenta 0.7

exp. data in agreement with UrQMD => A2 = 0.7 used for PLUTO simulation of dileptons

full HADES acceptance:

mom_cm > 200 MeV/c

dN/d cos(cm) ~ 1. + A2 ∙ cos2(cm)


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Charged meson production – mT fitsC+C 2 AGeV

Multiplicity "Temperature"

HADES N< -+>/Apart = 0.137 ± 0.015 T+ = 39 ± 11; 86 ± 2

T- = 51 ± 4; 89 ± 3

TAPS N< 0>/Apart = 0.138 ± 0.014

KaoS N< ->/Apart = 0.126 ± 0.010 T- = 40 ± 3; 86 ± 3

good agreement with TAPS/

KaoS results!

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2 parameter fit

Dielectron production in C+C collisions with HADES

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