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)
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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