Heavy flavor and direct photon measurements at RHIC

David Silvermyr, ORNL

DNP/JPS ’05 Kapalua, September 2005

2

Outline

• Introduction /Motivation • J/production results for p+p, d+Au, Au+Au and

Cu+Cu• Direct photon results for p+p, d+Au and

Au+Au• Summary and Outlook

3

A Pallet of Prompt Probes

q: fast color triplet

g: fast color octet

Q: slow color triplet

QQbar: slow color singlet/octet

Virtual photon: colorless

Real photon: colorless

Unknown Medium

Inducedgluon radiation?

EnergyLoss?

Dissociation?

Controls

A general way to classify QCD probes is by speed and color multiplet; different combinations give rise to different classes of high-Q2 observables:

(from P. Stankus)

Next talkPrevious talkThis talk

Will start with slow and move on to colorless..

4

Heavy Quarkonia

Color Screening

cc

Lattice QCD results show that the confining potential between heavy quarks is screened at high temperature.

This screening should suppress bound states such as J/. However, recent lattice results indicate that the J/ spectral functions only show modest modification near the critical temperature, and thus may not be suppressed until higher T.

See e.g. C-Y Wong DD6, P. Petreczky KH8.

r

V(r

)/

Lattice QCD calculation

5

Pb+Pb collisions show suppression in excess of "normal" nuclear suppression

Recent news: NA60 observed very similar trend in In+In collisions.

J/ normalized to Drell-Yan vs “Centrality”

NOTE: D-Y is not the optimal normalization, closed/open charm is better.

Suppression

Expectation

Observation at CERN SPS (NA50/60)

6

CDF pp (s = 1.8 TeV) results• Color singlet model

underpredicts high-pT yield.• Color octet model

overpredicts transverse polarization at high pT.

F. Abe et al.,Phys. Rev. Lett. 79, 572.

T. Affolder et al.,Phys. Rev. Lett. 85, 2886.

7

J/ @ RHIC: Physics Plan • pp collisions

– Reference, Initial production mechanism

• pA (or dA) collisions– Shadowing– Initial state energy loss– Cold medium absorption

• AA + Light ion collisions– Modify path length through medium

– Most efficient way to dial in Nbinary.

• Energy scans– Modify energy density– More difficult (both luminosity & cross-sections fall

quickly w/ energy)

Many competing effects:

- Reference data essential!

8

p-p J/Psi – PHENIX 200GeV

RapidityTotal cross section in p+p

(nucl-ex/0507032):

2.61+/-0.20(fit)+/-0.26(abs) µb

R. Vogt: EKS98 shadowing. 3mb absorption

J/ rapidity distribution in p+p and d+Au Collisions

X1X2

J/ inSouthy < 0

9

Rapidity and Ncoll Dependence of RdAu: Gluon Shadowing and Nuclear Absorption

• Data favor weak shadowing and weak nuclear absorption effect:Calc. with 1-3 mb most successful at describing the data. [Shape reminiscent to what’s seen for dNch/d(PHOBOS)]

• More suppression for more central events(?)

RdA

0

0.2

0.4

0.6

0.8

1.0

1.2

Rapidity

)1972/( ppdAdAR ppinvcoll

dAinv

dA YieldN

YieldR

10

Recent RUN5 News

1st Upsilons at RHIC !

Phenix muon arm

Beauty measurements will be quite interesting.

Different Quarkonia states test the degree of color screening and measure the temperature.

Significant yields (>hundreds) at RHIC-II ?

PHENIX accumulated ~3pb-1 p+p collision during 2005 run. Will give order of magnitude stat. improvement for reference for d+Au and Au+Au.

11

Heavy Ions: J/ signal in Au+Au

0-20% 20-40% 40-93%

Example Mass-plots:

● Background subtracted using event mixing

● Cu+Cu signal is similar to Au+Au peripheral,

with much larger statistics

J/e+e-

J/-PHENIX

12

Signal also seen in STAR for J/ψ in Au+Au! [di-electron measurement at mid-rapidity.]

=> Should have plenty of stat’s for Run-5 Cu+Cu and p+p too!

0-80% Au+Au

STAR Preliminary

J/ production : STAR

12M eventsSTAR Preliminary

The J/ yield is lower than statistical coalescence model prediction (red-dashed curve, A. Andronic) Extreme enhancement scenario ruled out

13

dAu

200 GeV/c

CuCu

200 GeV/c

AuAu

200 GeV/c

J/ muon arm

1.2 < |y| < 2.2

AuAuee

200 GeV/c

CuCuee

200 GeV/c

J/ eeCentral arm

-0.35 < y < 0.35

CuCu

62 GeV/c

RAA vs Ncoll

About a factor 3 suppression for most central Au+Au points

Band around 1.0 refers to the uncertainty of the p+p reference.

[and sometimes has a global sys. error added for the dataset in question]

14

RAA vs Npart : Comparison with NA50 data

NA50 data is normalized to NA50 p+p point.

Suppression level is rather similar between the two experiments, although the collision energy is 10+ times higher at RHIC (200 GeV vs 17

GeV).

Note: size of error bars not negligible!

15

RAA vs Npart: Comparison with cold nuclear effects

Prediction from pQCD calculations, including 3mb nuclear absorption and shadowing.

Seems to underestimate the suppression somewhat.Note: abs somewhat too high wrt d+Au data; Should have

1 mb curve also.

Forward rapidity Mid rapidity

16

RAA vs Npart: Comparison with predictions without regeneration

Models which approx. reproduce NA50 data, with J/ suppression only. (no regeneration mechanism)

Over-estimates J/ suppression at RHIC!

17

RAA vs Npart : Comparison with predictions w. regeneration

Models using suppression + various regeneration mechanisms;

Better matching with data points, but note that all model calculations should be checked to use up-to-date charm and J/ p+p cross-sections!

(reduced exp. errors on those quantities would also help)

18

Update : Comparison with a prediction w. regeneration

After timely update from Rapp:

agreement with data points rather similar to that of absorption calculation (with 3 mb sigma).

19

Test of Npart scaling

Can the results be explained by some other scenario? Geometry and

surface effects or scaling a la soft processes?

[also argued for NA50 data by e.g. Gazdzicki, Braun-Munzinger et al.]

Alternative looks at data may help to break gridlock..

20

More variables : Rapidity• Rapidity distribution of recombined J/ is supposed to be

peaked at y=0 (e.g. R.L. Thews & al., nucl-th/0505055)– True IF charm distribution ~ J/ in p+p !

– But Au+Au charm rapidity distributions might be very flat!

(previous talk)

pQCD, adjust <kT2>

p+p data

diagonal

mainlyoff-diagonal

(with recomb.)

21

We fit the pt spectrum using to extract <pt2>

Invariant yield vs pt

62 ])/(1[ BpA t

Cu+Cu (|y|[1.2,2.2]) Au+Au (|y|[1.2,2.2])

22

Mean transverse momentum vs Ncoll

• Added Thews predictions for Au+Au and Cu+Cu collisions @200GeV.

Solid (dashed) is with (without) regeneration.

• all PHENIX J/ data

Errors are from the fits only. Should probably try alternative parameterizations too.

23

J/ Conclusions

Data exhibits a factor 3 suppression for most central events in Au+Au collisions. Suppression vs Npart rather similar to what was seen at SPS.

Comparison with models suggests that

1) Models with only cold nuclear matter effects tend to under-predict the suppression

2) Models with color screening or comovers and without recombination have

too much suppression

3) Models with recombination are in rather reasonable agreement with the dataNot clear if recombination is the explanation though.

Pro(?): <pT

2> is also consistent with flat behaviour, but large error bars.

Mixed evidence for recombination from other variables:

Con(?): The rapidity dependence of the J/ yield shows no dramatic change in shape with increasing N

part.

24

J/ Action Items

● Need more work on theory; e.g. updated recombination curves, absorption curves based on favourite RdAu parameterization.. New ideas?

The jury is still out.. Some re-thinking may be required

● Need more work on data; reduce size of errors and go to final results. Using the statistically superior Run5 p+p dataset for reference should be helpful.

• Flow? - J/ v2 studies started; no results yet. Statistically very challenging analysis.

• Question: Do we see (suppression + recombination) or just not so much suppression to start with..?

[‘soft’ scaling and similarity with NA50 suppression pattern - somewhat surprising and hard to overlook. Just coincidences?]

Direct Photons in 200 GeV p+p, d+Au, Au+Au

The fast and colorless control probe..

26

Why Direct Photons?

• p+p:– Test of QCD

• Reduce uncertainty on pQCD photons in A+A

• d+Au– Study nuclear effects

• A+A – Photons don’t strongly

interact with produced medium

– Hard photons• Allow test of Ncoll scaling

for hard processes• Important for interpretation

of high-pT hadron suppression at RHIC

– Thermal photons• Carry information about

early stage of collision• QGP potentially detectable

via thermal photon radiation

Pragmatic Definition :

photons not from hadron decays

Difficult : large backgrounds from

0 and decays to subtract off.

STAR has inclusive (not direct)

photon measurements so far

[PRL95 (2005) 062301, PRC70(2004)044902.]

-All direct photon results here are from PHENIX.

Also see Lin/KG13: STAR direct photon HBT (a la WA98 analysis)

27

Schematic Photon Spectrum in Au+Au

Decay photons

nT

1

phard:

/ E Tethermal:

Hard Photons

p+p, d+Au, Au+Au

29

Direct Photons in p+p• good agreement with

NLO pQCD• Important baseline

for Au+Au

PbSc

at 200 GeVp p s+ =

30

Direct in d+Au

• p+p and d+Au spectra compared to NLO pQCD

• ratio to NLO pQCD• consistent with 1• No indication for

nuclear effects

2

31

Direct Photons in Au+Au

PRL 94, 232301

Expectation for Ncoll scaling of

direct photons

Recently published

holds for all centrality classes

0 suppression caused by medium created in Au+Au collisions

32

Direct Photons in Au+Au : RAA

RAA consistent with 1 for direct photons

Thermal Photons

Au+Au

34

Low pT

• Stay tuned for more improvements

• No significant excess at low pT

35

A New Approach..

Any source of real emits virtual with very low mass

Compton

q

g q

e+

e-

.incl

direct

.incl

direct

**

Use lepton pairs to measure

virtual

0

e+

e-

Background from Dalitz decay

PHENIX features•Low conversion rate•Excellent mass resolution•High statistics in Run4 (2004)

36

phase space factorform factorinvariant mass of virtual photon

invariant mass of Dalitz pair

phase space factorform factorinvariant mass of Dalitz pair

invariant mass of virtual photon

32

222

2

2

2

2

)1()(1

)2

1(4

13

21

M

mmF

mm

m

m

m

dm

dN

Nee

eeeeee

e

ee

e

ee

ee

ee

ee

dm

dN

N

1

The Idea

32

2

)1(M

meeeeee

e

ee

e

mm

m

m

m 1)

21(

41

3

22

2

2

2

22 )( eemF

• Start from Dalitz decay• Calculate invariant mass distribution of Dalitz pairs

• Now direct photons

• Any source of real produces virtual with very low mass

• Rate and mass distribution given by same formula– No phase space factor for

mee<< pT photon

0

0

e+

e-

[Kroll-Wada, ’55]

37

• Calculate ratios of various (for cross-checks) Minv bins to lowest one: Rdata

• If no direct photons: ratios correspond to Dalitz decays• If excess:: direct photons

Method

÷

÷÷

0-3

0

90-1

40

140-2

00 M

eV

200-3

00

Rdata

• Material conversion pairs removed by analysis cut

• Combinatorics removed by mixed events

38

S/B=~1

R

R

Rdirect

calculated from Dalitz formula

measured

Rdata ÷

300,

30-direct,0

direct

data

0

0

allN

N

RR

RR

30-0 all,

300-90 all,

data N

NR

directall 0 NNN

30-direct,0300,

30-direct,0300,

0

00

NN

NRNR direct

300,

30-direct,0)( 00

all

direct N

NRRR

39

S/B=~1

measured

Rdata ÷

0

0

direct

data

incl.

direct

*

*

RR

RR

incl.

direct

measured with EMCal

What we are after..

~25 % systematic error :

~20 % from measured 0 ratio

~10 % from inclusive

~5 % acceptance

R

R

Rdirect

calculated from Dalitz formula

40

*direct/*inclusive

Significant 10% excess of very-low-mass virtual direct photons

0

0

direct

data

incl.

direct

*

*

RR

RR

incl.

direct

0-20 %

41

Centrality Dependence

Indication for centrality dependence

more peripheral

42

Comparison to Conventional result

0

0

direct

data

incl.

direct

*

*

RR

RR

incl.

direct

( + 1 )

New result consistent with conventional method, but with significantly smaller errors!

43

direct

0

0

direct

data

incl.

direct

*

*

RR

RR

incl.

direct

44

The Spectrum

Compare to published Run2 result: PRL94 232301

New result consistent with conventional method, but with significantly smaller errors!

0

0

direct

data

incl.

direct

*

*

RR

RR

incl.

direct

45

The SpectrumCompare to NLO pQCD

• excess above pQCD

• L.E.Gordon and W. Vogelsang• Phys. Rev. D48, 3136 (1993)

46

The Spectrum

Compare to thermal model

Compare to NLO pQCD

• excess above pQCD

• L.E.Gordon and W. Vogelsang• Phys. Rev. D48, 3136 (1993)

• data above thermal at high pT

• D. d’Enterria, D. Perresounko• nucl-th/0503054

2+1 hydroT0

ave=360 MeV(T0max=570 MeV)

0=0.15 fm/c

47

The Spectrum

Compare to thermal + pQCD

• data consistent with thermal + pQCD

Compare to thermal model

• data above thermal at high pT

• D. d’Enterria, D. Perresounko• nucl-th/0503054

Compare to NLO pQCD

• excess above pQCD

• L.E.Gordon and W. Vogelsang• Phys. Rev. D48, 3136 (1993)

2+1 hydroT0

ave=360 MeV(T0max=570 MeV)

0=0.15 fm/c

48

Are these thermal photons? The rate is above pQCD calculation and could provide the first direct measurement of the initial temperature of the matter.

T0max ~ 500-600 MeV !?

T0ave ~ 300-400 MeV !?

A word of caution..

49

Direct Photon Conclusions

• Hard direct photons pT>4GeV/c– p+p:

• Spectrum consistent with pQCD calculations

– d+Au:• No apparent nuclear

effects– Au+Au:

• Confirms Ncoll scaling for hard processes

• Thermal (?) direct photons 1<pT<4GeV/c– New EMCal measurement

with reduced systematics• Stay tuned for further

improvements– New measurement through

very-low-mass virtual photons

• Significant 10% direct photon excess above decay photons

• Spectrum consistent with thermal model

• Reference measurement needed– pp and dAu (and CuCu)– Same analysis method

50

Outlook

• Many recent interesting results on J/ and direct photons!

• Interpretations and cross-checks, and getting the results in final form, are being worked on.

Key issues:

J/ recombination or not? J/ v2?

Thermal photon signal real? Which limits can we place on the temperature?!

We’ve come a long way in the last few years; a lot of new interesting results are sure to follow in the coming years.

51

Year Ions sNN Luminosity Detectors J/

2000[Run-1]

Au+Au 130 GeV 1 b-1 Central (electrons)

0

2001 Au+Au 200 GeV 24 b-1 Central 13 + 0

2002[Run-2]

p+p 200 GeV 0.15 pb-1+ 1 muon arm 46 + 66

2002 d+Au 200 GeV 2.74 nb-1 Central 300+800+600

2003[Run-3]

p+p 200 GeV 0.35 pb-1+ 2 muon arms

100+300+120

2004[Run-4]

Au+Au200 GeV62 GeV

~240 ub-1

~9 ub-1

Central+ 2 muon arms

~500+2000+2000

2005[Run-5]

Cu+Cup+p

200 GeV200 GeV

~3 nb-1

~3 pb-1

Central+ 2 muon arms

~1000+5000+5000

~1000+5000+5000

RHIC Scaling Law : J/

Order of magnitude improvements for approx. every two RHIC runs – quite remarkable! Hope to see continued progress and success like this!

52

Backup slides

53

p+p & d+Au: Disentangle Cold Nuclear Effects

• Cronin effect

• Gluon (anti-)shadowing

• Nuclear absorption.

• Initial state energy loss.

gluons in Pb / gluons in p

X

Shadowing

Eskola, et al., Nucl. Phys. A696 (2001) 729-746.

AntiShadowing

X1 X2

J/ inNorthy > 0

X1X2

J/ inSouthy < 0

rapidity y

South (y < -1.2) : • large X2 (in gold) ~ 0.090

Central (y ~ 0) :• intermediate X2 ~ 0.020

North (y > 1.2) : • small X2 (in gold) ~ 0.003

54

Rapidity and Npart Dependence of RdAu: Gluon Shadowing and Nuclear Absorption

Or if we divide with Npart instead : fairly flat centrality dependence.

RdAu similar to what’s seen for ‘soft’ particles..

55

Normal Nuclear Absorption Expectation

Sigma(j-N) = 3.0 +/- 1.5 mbAuAu (red band)CuCu (blue band)

RAA vs Npart: Comparison with ‘just’ normal nuclear absorption

56

BdN/dy vs rapidity

• For Cu+Cu, no significant change in

shape, except perhaps in most central

bin• Adding Au+Au points on top of

CuCu. Same conclusion.

● Recombination model

(Thews et. al. nucl-th/0505055) expect

rapidity shape to become narrower for increasing N

part

but

● No available combination of

suppression + recombination.

● Possible narrowing rely on charm

rapidity shape

57

CuCu: More Bins...

Copper-Copper 200 GeVJ/ |y| = 1.2-2.2

• Rather smooth onset/scaling with centrality.. (no distinct onset or plateau for c suppression)

58

J / PSI PRODUCTION IN AU+AU COLLISIONS AT RHIC AND THE NUCLEAR ABSORPTION.By A.K. Chaudhuri (Calcutta, VECC),. Jul 2003. 4pp. e-Print Archive: nucl-th/0307029

BASELINE COLD MATTER EFFECTS ON J/PSI PRODUCTION IN AA COLLISIONS.By R. Vogt (LBL, Berkeley & UC, Davis),. LBNL-58155, Jul 2005. 7pp. e-Print Archive: nucl-th/0507027

CHARM COALESCENCE AT RHIC.By A.P. Kostyuk, M.I. Gorenstein (Frankfurt U. & BITP, Kiev), Horst Stoecker, W. Greiner (Frankfurt U.),. May 2003. 4pp. Published in Phys.Rev.C68:041902,2003 e-Print Archive: hep-ph/0305277

CHARMONIUM CHEMISTRY IN A+A COLLISIONS AT RELATIVISTIC ENERGIES.By E.L. Bratkovskaya (Frankfurt U.), A.P. Kostyuk (Frankfurt U. & BITP, Kiev), W. Cassing (Giessen U.), Horst Stoecker (Frankfurt U.),. Feb 2004. 13pp. Published in Phys.Rev.C69:054903,2004 e-Print Archive: nucl-th/0402042

MEDIUM MODIFICATIONS OF CHARM AND CHARMONIUM IN HIGH-ENERGY HEAVY ION COLLISIONS.By L. Grandchamp (LBL, Berkeley), R. Rapp (Texas A-M), G.E. Brown (SUNY, Stony Brook),. Mar 2004. 4pp. Talk given at 17th International Conference on Ultra Relativistic Nucleus-Nucleus Collisions (Quark Matter 2004), Oakland, California, 11-17 Jan 2004. Published in J.Phys.G30:S1355-S1358,2004 e-Print Archive: hep-ph/0403204 IN MEDIUM EFFECTS ON CHARMONIUM PRODUCTION IN HEAVY ION COLLISIONS. By Loic Grandchamp (SUNY, Stony Brook & Lyon, IPN), Ralf Rapp (Nordita), Gerald E. Brown (SUNY, Stony Brook),. Jun 2003. 4pp. Published in Phys.Rev.Lett.92:212301,2004 e-Print Archive: hep-ph/0306077

J/PSI TRANSPORT IN QGP AND P(T) DISTRIBUTION AT SPS AND RHIC.By Xiang-lei Zhu, Peng-fei Zhuang (Tsinghua U., Beijing), Nu Xu (LBL, Berkeley),. Nov 2004. 6pp. Published in Phys.Lett.B607:107-114,2005 e-Print Archive: nucl-th/0411093

J/PSI TRANSPORT IN QGP AND P(T) DISTRIBUTION AT SPS AND RHIC.By Xiang-lei Zhu, Peng-fei Zhuang (Tsinghua U., Beijing), Nu Xu (LBL, Berkeley),. Nov 2004. 6pp. Published in Phys.Lett.B607:107-114,2005 e-Print Archive: nucl-th/0411093

ULTRARELATIVISTIC NUCLEUS-NUCLEUS COLLISIONS AND THE QUARK GLUON PLASMA.By A. Andronic, P. Braun-Munzinger (Darmstadt, GSI),. Feb 2004. 32pp. Lectures given at 8th Hispalensis International Summer School on Exotic Nuclear Physics, Seville, Spain, 9-21 Jun 2003. e-Print Archive: hep-ph/0402291

MOMENTUM SPECTRA OF CHARMONIUM PRODUCED IN A QUARK-GLUON PLASMA.By R.L. Thews (Arizona U.), M.L. Mangano (CERN),. CERN-PH-TH-2005-073, May 2005. 26pp. e-Print Archive: nucl-th/0505055

MOMENTUM SPECTRA OF CHARMONIUM PRODUCED IN A QUARK-GLUON PLASMA.By R.L. Thews (Arizona U.), M.L. Mangano (CERN),. CERN-PH-TH-2005-073, May 2005. 26pp. e-Print Archive: nucl-th/0505055

PREDICTIONS FOR J / PSI SUPPRESSION BY PARTON PERCOLATION.By S. Digal, S. Fortunato (Bielefeld U.), H. Satz (CFIF, Lisbon),. BI-TP-2003-30, Oct 2003. 12pp. Published in Eur.Phys.J.C32:547-553,2004 e-Print Archive: hep-ph/0310354

THE ONSET OF DECONFINEMENT IN NUCLEAR COLLISIONS.By H. Satz (Bielefeld U.),. May 1999. 15pp. Plenary talk given at 14th International Conference on Ultrarelativistic Nucleus-Nucleus Collisions (QM 99), Torino, Italy, 10-15 May 1999. Published in Nucl.Phys.A661:104-118,1999 e-Print Archive: hep-ph/9908339

What do the Theorists Have to Say?

59

The PHENIX detector

Centrality measurement: We use beam beam counters together with zero degree calorimetersCentrality is mapped to N

part (N

col) using Glauber model

Central arms:hadrons, photons, electrons

p > 0.2 GeV/c|y| < 0.35

J/e+e-

Muon arms:muons at forward rapidity

p > 2GeV/c1.2 < |y| < 2.4

J/

60

WA98 Interpretation: T or kT ?

• QGP + HG rates convoluted with simple fireball model plus pQCD hard photons

• Data described with initial temperature Ti=205 MeV + some nuclear kT broadening (Cronin-effect)

• Data also described without kT broadening but with high initial temperature (Ti=270 MeV)

Turbide, Rapp, Gale, Phys. Rev. C 69 (014902), 2004

61

WA98 Data: Conclusions

• Data consistent with QGP picture, but also with pure HG picture

• Large variations in extracted initial temperature Ti (however, most models give Ti > Tc)

Data can be described under a variety of different assumptions, e.g.:

Ti = 214 - 255 MeV

QGP + HG + pQCD(Non-boost inv. hydro)

Huovinen, Ruuskanen, Räsänen (Nucl. Phys. A 650 (227) 1999)

Ti = 213 - 234 MeV

Pure HG + pQCD(Non-boost inv. hydro)

Ti = 335 MeV, = 0,2 fm/c

QGP + HG + pQCC(Bjorken hydro)

Svrivastava (nucl-th/0411041)

250 < Ti < 370 MeV,0,5 < < 3 fm/cQGP + HG + pQCDRenk

(Phys.Rev.C67:064901,2003)

Ti = 250 - 270 MeV, = 0,5 fm/c

QGP + HG + pQCD without kT

Ti = 205 MeV, = 1 fm/c

QGP + HG + pQCD with kT

Turbide, Rapp, Gale (Phys.Rev.C69:014903,2004 )

62

Direct Photon Measurement: Methods

• Subtraction Method– Measure inclusive photon spectrum and

subtract photons from hadron decays

– Inclusive photon spectrum via• Direct method (electromagnetic calorimeter)• Conversion method

• Hanbury Brown-Twiss (HBT) Method– Bose-Einstein correlation expected for

direct photons– Direct photon yield from correlation

strength

63

Conversion Method: A Result from PHENIX

pT (GeV/c)

Inv.

ph

oto

n y

ield Min. bias Au+Au (Run-2)

inclusive photons

conversion method

direct method

T. Hachiya, Y. Akiba, AN158

• Inclusive photons down to pT = 0.5 GeV/c from conversion method

• Direct calorimeter measurement problematic at low pT

– Energy resolution worsens at low pT

– Large background from charged hadrons

Further example: CERES at CERN SPS

target = converter

electron ID with RICH

64

Limitations of the Different Methods

Subtraction method at low pT largelylimited by uncertainty of measurement:

Energy Scale

Reconstruction Efficiency

Peak Extraktion

Low pT limitation of HBT method:Huge charged particle background (pT for MIP’s ~ 100 MeV)

High pT limitation of HBT method:Hit distance cut of D > 20 cm(cluster splitting!) limits usable Qinv range