di-lepton spectroscopy in cbm claudia höhne, gsi darmstadt cbm collaboration

Di-lepton spectroscopy in CBM

Claudia Höhne, GSI Darmstadt

CBM collaboration

2 Claudia Höhne Quark Matter Conference, February 2008, India

Outline

• Introduction & motivation

• physics case of CBM

• dileptons at maximum baryon densities

• Detector concept of CBM

• overall concept

• dilepton measurement: electrons - muons

• Simulations

• detector performance & challenges

• feasibility studies

• Summary

CBM posters on Di-leptons

• T. Galatyuk Di-electron spectroscopy in CBM• K. Antipin Systematic study of the optimization potential for di-lepton

measurements in the CBM experiment• A. Kiseleva, P. Bhaduri Muon measurement with the CBM experiment at FAIR


Physics caseCompressed Baryonic Matter @ FAIR – high B, moderate T:

searching for the landmarks of the QCD phase diagram• first order deconfinement phase transition • chiral phase transition (high baryon densities!)• QCD critical endpoint

in A+A collisions from 2-45 AGeV starting in 2015 (CBM + HADES)

• physics program complementary to RHIC, LHC

• rare probes! (charm, dileptons)(interaction rates up to 10 MHz!) [Andronic et al. Nucl. Phys. A 772, 167 (2006).

with the CBM energy range we will reach • net baryon densities of (6-12) 0

• excitation energy densities * of (0.8-6) GeV/fm3 for time spans of ~6 fm/c (*=-mN)

→ get access to the electromagnetic radiation from the fireball by the study of dileptons!


-meson spectral function

• -meson couples to the medium: "melts" close to Tc and at high B

• vacuum lifetime 0 = 1.3 fm/c

• dileptons = penetrating probe

• connection to chiral symmetry restoration?

• particular sensitive to baryon densityp

n

++

p

e+, μ+

e-, μ-

"SPS""FAIR"

[R. Rapp, priv. com. (CBM physics book)]

• illustrate sensitivity to modifications caused by the baryonic component of the medium:

-meson spectral function weighted by 1/M to resemble the dilepton rate, Bose-factor will further amplify the low-mass part

• m < 0.4 GeV/c2 of special interest!

no measurement between 2-40 AGeV beam energy yet!


Charm production at threshold

[W. Cassing et al., Nucl. Phys. A 691 (2001) 753]

HSD simulations

• CBM will measure charm production at threshold

→ after primordial production, the survival and momentum of the charm quarks depends on the interactions with the dense and hot medium!

→ direct probe of the medium!

• charmonium in hot and dense matter?

• relation to deconfinement?

• relation to open charm?

no measurement of charmonium below 160 AGeV beam energy yet!


Physics topics and Observables

Onset of chiral symmetry restoration at high B

• in-medium modifications of hadrons (,, e+e-(μ+μ-), D)

Deconfinement phase transition at high B • excitation function and flow of strangeness (K, , , , )• excitation function and flow of charm (J/ψ, ψ', D0, D, c)• charmonium suppression, sequential for J/ψ and ψ' ?

The equation-of-state at high B

• collective flow of hadrons• particle production at threshold energies (open charm)

QCD critical endpoint• excitation function of event-by-event fluctuations (K/π,...)

• mostly new measurements• CBM Physics Book (theory) in preparation


The CBM experiment• tracking, momentum determination, vertex reconstruction: radiation hard silicon pixel/strip detectors (STS) in a magnetic dipole field

• hadron ID: TOF (& RICH)• photons, 0, : ECAL

• electron ID: RICH & TRD suppression 104

• PSD for event characterization• high speed DAQ and trigger → rare probes!

• muon ID: absorber + detector layer sandwich move out absorbers for hadron runs

aim: optimize setup to include both, electron and muon ID

STS + MVD

RICHTRD

TOF ECAL

magnet

absorber +

detectors


STS tracking – heart of CBM

Challenge: high track density 600 charged particles in 25o

Task

• track reconstruction:

0.1 GeV/c < p 10-12 GeV/c

p/p ~ 1% (p=1 GeV/c)

• primary and secondary vertex reconstruction (resolution 50 m)

• V0 track pattern recognition

D+ → ++K- (c = 312 m)

D0 → K-+ (c = 123 m)

silicon pixel

and strip detectors

add detectors for particle identification behind the STS

→ challenge for di-leptons!


Challenges of the di-electron measurement

• clean electron identification ( suppression ≥ 104)

• large background from physical sources

-conversions in target and STS, 0 Dalitz decays

→ use excellent tracking and two hit resolution (≤ 100 m) in first pixel detectors in order to reject this background: → optimize detector setup (STS, B-field), use 1‰ interaction target

TRD

high rates!→ reduce gas gap

RICH

high ring densities and interaction rates→ MAPMTs + fast self triggered read out electronics

prototype:double-sided pad plane


Challenges of the di-muon measurement• major background from ,K decays into , punch through of hadrons and track mismatches

→ use TOF information to reject punch through K,p → compact layout to minimize K, decays → use excellent tracking to reject ,K decays in the STS by kink detection → absorber-detector sandwich for continous tracking

• low momentum !

125 cm Fe ≡ 7.5 I → p > 1.5 GeV/c225 cm Fe ≡ 13.5 I → p > 2.8 GeV/c


Muon detector R&D

•

first double GEM under test at VECC, Kolkatta

• up to 1 hit/cm2 in first muon chambers!

• high rate capability required!

• detector technology still under discussion: Si-pad (first plane), Micromega, GEMs, ...

first TGEM production and test at PNPI, St. Petersburg


CbmRoot simulation frameworkinvestigation of both options in detailed simulations:

• detector simulation (GEANT3 implemented through VMC)

• full event reconstruction: - track reconstruction, add RICH, TRD and TOF info- tracking through the muon absorber

• result from feasibility studies in the following: central Au+Au collisions at 25 AGeV beam energy (UrQMD)


Low mass vector mesons

invariant mass spectra

• electrons: pt > 0.2 GeV/c background dominated by physical sources (75%), 1‰ int. target

• muons: intrinsic p>1.5 GeV cut (125 cm Fe absorber), background dominated by misidentified muons, 1% int. target

electrons: 200k events muons: 4 ∙108 events All eAll e++ee--

Comb. bgComb. bgρρ ee++ee--

ee++ee--

φφ ee++ee--

ππ0 0 γγee++ee--

ππ00ee++ee--

ηη γγee++ee--

m = 14 MeV/c2 m = 11 MeV/c2

25 AGeVcentral AuAu


Phase space coverage

-meson

• 25 AGeV beam energy: midrapidity = 2

• electrons: full coverage

• muons: acceptance forward shifted, weak for low-pt

intrinsic p>1.5 GeV cut (125 cm Fe absorber)

muonselectrons

25 AGeVcentral AuAu


Coverage in pt and minv

Dilepton pair coverage in pt and minv (signal pairs):

• electrons: acceptance also for low pt and lowest masses (no pt-cut)

• muons: cutoff at 2 threshold

electrons muons

25 AGeVcentral AuAu


electrons: 4 ∙1010 events

J/ m = 27 MeV/c2

' m = 29 MeV/c2

J/ and '

invariant mass spectra

• electrons: p < 13 GeV/c, pt > 1.2 GeV, 1‰ interaction target (25 m Au)

• muons: 225 cm Fe absorber, pt > 1 GeV/c, 1% int. target

muons: 3.8 ∙1010 events

J/ m = 22 MeV/c2

' m = 23 MeV/c2

25 AGeVcentral AuAu


Phase space coverageJ/ meson

• 25 AGeV beam energy: midrapidity = 2

• full phase space well covered

muonselectrons

25 AGeVcentral AuAu


Yields and S/B

multiplicity total efficiency

S/B interaction rate [MHz]

yield/10 weeks

→ e+e-

+-

23 4.7%

2.8%

0.008

0.002

0.025

0.25

1.5 ∙106

8.9 ∙ 106

→ e+e-

+-

38 6.6%

4%

0.4

0.11

0.025

0.25

5.4 ∙ 106

41.4 ∙ 106

→ e+e-

+-

1.28 9.4%

7%

0.32

0.06

0.025

0.25

1.1 ∙ 106

7.7 ∙ 106

J/→ e+e-

+-

1.92 ∙10-5 14%

13%

13

18

1 to 10

10

2 ∙ 105-6

1.8 ∙ 106

'→ e+e-

+-

2.56 ∙10-7 19%

16%

0.3

0.8

1 to 10

10

4.4 ∙ 102-3

3.6 ∙ 103

• S/B ratio in a 2 region around the peak, for from 0.2-0.9 GeV/c2

• no trigger for low-mass vector mesons, a factor 10 maybe achievable for muons

• trigger for J/, rate in dielectron channel depends on interaction length of target (segmented target?)

central Au+Au, 25 AGeV minbias = 1/5 central

overall similar performance of electron and muon channel!

25 AGeVcentral AuAu


Summary: Dileptons in CBM• dileptons are only one of several very interesting physics topics of CBM CBM: comprehensive measurement of A+A interactions from 10-45 AGeV including rare probes (charm, dileptons), flow, correlations, fluctuations

• measurement of dileptons (low and high masses) very interesting at FAIR: CBM: 10-45 AGeV, HADES 2-10 AGeV

• highest baryon densities reached, phase border to partonic phase• restoration of chiral symmetry? critical point? • charm production at threshold? charm propagation in-medium?

• dileptons from to ' measurable in electron and muon channel• similar performance – although background is of very different origin• good phase-space coverage

• low-mass dielectrons even down to lowest masses and pt• detector development started

• CBM will (hopefully) not be limited by statistics• systematic uncertainties might be limiting in the end→ a measurement of both, muons and electrons will be the best systematic study we can ever do!


Germany: Univ. Heidelberg, Phys. Inst.Univ. HD, Kirchhoff Inst. Univ. FrankfurtUniv. Mannheim

CBM collaborationRussia:IHEP ProtvinoINR TroitzkITEP MoscowKRI, St. Petersburg

China:CCNU WuhanUSTC Hefei

Croatia:

University of SplitRBI, Zagreb

Portugal: LIP Coimbra

Romania: NIPNE Bucharest

Poland:Krakow Univ.Warsaw Univ.Silesia Univ. KatowiceNucl. Phys. Inst. Krakow

LIT, JINR DubnaMEPHI MoscowObninsk State Univ.PNPI GatchinaSINP, Moscow State Univ. St. Petersburg Polytec. U.

Ukraine: Shevchenko Univ. , Kiev

Univ. MünsterFZ RossendorfGSI Darmstadt

Czech Republic:CAS, RezTechn. Univ. Prague

France: IPHC Strasbourg

Hungaria:KFKI BudapestEötvös Univ. BudapestIndia:Aligarh Muslim Univ., AligarhIOP BhubaneswarPanjab Univ., ChandigarhUniv. Rajasthan, JaipurUniv. Jammu, JammuIIT KharagpurSAHA KolkataUniv Calcutta, KolkataVECC KolkataUniv. Kashmir, SrinagarBanaras Hindu Univ., Varanasi

Korea:Korea Univ. SeoulPusan National Univ.

Norway:Univ. Bergen

Kurchatov Inst. MoscowLHE, JINR DubnaLPP, JINR Dubna

51 institutions, > 400 members

Dresden, September 2007

Cyprus: Nikosia Univ.


Backup


Acceptance in pt and minv

0.2 GeV/c2

pair detection probablitity/ efficiency versus invariant mass for different pt-bins:

• electrons: acceptance also for low pt and lowest masses (no pt-cut)

• muons: cutoff at 2 threshold (plot "hard-hard" and "hard-soft" pairs)

electrons muons

25 AGeVcentral AuAu


Detector performance - muons

particle MC tracks/event

(after 125 cm Fe)

reconstructed tracks/event

(after 125 cm Fe)

p 0.013 0.005

0.038 0.017

K 0.102 0.064

0.386 0.152

all 0.539 0.228

background tracks

• punch through and track mismatches (.K decay!)


STS tracking - simulation

• excellent track reconstruction, momentum resolution achieved

• optimization of layout ongoing, material budget ≥ 3.2 mm Si equivalent (x/X0 ≥ 3.4%)

tracking efficiency momentum resolution

25 AGeVcentral AuAu


Detector concept: electrons

TRD:

• 3 x 4 TRD layers appr. at 4, 6, 8 m behind the target, fast gas detectors

• also used as intermediate tracking detectors towards TOF

• detector development builds upon knowledge gained from ALICE

TOF:

• RPCs, 80ps time resolution

RICH:

• gaseous RICH detector, size still to be optimized

• aim: "simple, compact and robust"

→ N2 radiator, glass mirrors, MAPMT as photodetector

• Nhits/ring = 22, N0 ~ 150 cm-1, <R>e = 6.2 cm, R = 2.5%

• ~ 90 rings per central collision, 25 AGeV Au+Au (occupancy ~2-4%)


Target – electrons

number of -conversions versus target thickness

• <0> ~ 350 for 25 AGeV, central Au+Au collisions

• 0 → (98.8%)

• rejection only by opening angle (difficult with magnetic field)

• can be dominant background even for J/

→ use high quality, high intensity beam from FAIR and work with 1‰ interaction target!

• low-mass vector mesons: no trigger possible → higher rate in order to saturate DAQ

• J/: trigger required! max. beam intensity 10 ions/s → 1% target max. interaction rate 10 MHz 1‰ 1MHz or use segmented target

25 m → ~3 e± per event

25 m ≡ 1‰ interaction length


Performance of combined e-ID

• use TRD and TOF detectors for further electron identification

• combined purity of identified electrons ~96%

p [GeV/c]p [GeV/c]


• suppression of low momentum muons! → low-mass vector mesons: 125 cm Fe absorber: cutoff at 1.5 GeV/c "hard "

90 cm 1 GeV/c "soft " → charmonium: 225 cm Fe absorber 2.8 GeV/c

• mismatches → include TOF information (distorted for background)

• depending on detector layout, tracking cuts: ~ 0.4 identified ± /event (125 cm Fe)

• reconstruction efficiency for tracks passing the absorber ~70% (125 cm Fe)

Detector performance - muons

J/p

absorption of muons from different sources in dependence on absorber thickness (Fe)


-meson spectral function (II)

"SPS""FAIR"

[R. Rapp, priv. com. (CBM physics book)]

• illustrate sensitivity to modifications caused by the baryonic component of the medium:

-meson spectral function weighted by a factor 1/M to resemble the dilepton rate, Bose-factor will further amplify the low-mass part

• region with m < 0.4 GeV/c2 of special interest!


-meson spectral function

[Rapp, Wambach, Adv. Nucl. Phys. 25 (2000) 1, hep-ph/9909229]

• -meson couples to the medium: "melts" close to Tc and at high B

• vacuum lifetime 0 = 1.3 fm/c

• dileptons = penetrating probe

• -meson spectral function particular sensitive to baryon density

• connection to chiral symmetry restoration?

pn

++

p

e+, μ+

e-, μ-


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

Simulation of vector mesons

Meson N/event Decay mode BR

36 e+ e- 5.×10-3

23 e+ e- 4.7×10-5

38 e+ e- 0

e+ e-

7.7×10-4

7.18×10-5

1.28 e+ e- 2.97×10-4

• input: vector mesons generated with Pluto, embedded into central Au+Au collisions, 25 AGeV beam energy from UrQMD

• full event reconstruction and particle identification, appr. realistic detectors descriptions: always work in progress!


First test beam data → hit density?

without Pb converter

with Pb converter (1.5 cm ~ 3X0)

crucial issue for Muon detectors

• hit densities after absorbers? (reliability of simulation?)

• first results from test beam (6 GeV/c) at CERN, PS on high granularity gas detectors (ALICE prototypes):

→ increase hit density in GEANT 3 appr. by factor 2

ADC counts


STS

TRD

TOF

RICH

ECAL

MVDZDC

MUCH

ASCII

Urqmd

Pluto

Track findin

g

digitizers

Hit ProducersDipole Map

Active Map

const. field

CBM Code

ROOT

Run Manager

Virtual MC

Geant3

Geant4

FLUKA

Event Generator

Magnetic

FieldDetector base

IO Manager

Tasks

RTDataBase OracleConf, Par,Geo

Root filesConf, Par,Geo

Root files Hits, Digits, Tracks

G4VMC

FlukaVMC

G3VMC

Geometry

ApplicationCuts, processes

STT

MUO

TOF

DCH

EMC

MVDTPCDIRC

ASCII

EVT

DPM

Track finding

digitizers

Hit Producers

Dipole Map

Solenoid Map

const. field

Panda Code

Event Display

Track propagati

on

comm

on

developm

ents

some feature

s

Always in close

contact

Close contact