Post on 08-Jan-2016
description
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R. Shahoyan, CERN
Dileptons and direct photons at SPS
Motivation and milestones
Excess in dilepton production
-puzzle
Photons
QUARK MATTER 2009, March 30 – April 4, Knoxville, TN
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• Modification of Vector Meson spectral functions close to Chiral Symmetry Restoration point:
VM broadening vs mass shift.
Why do we measure them
Convey dual information convoluted over the history of the collisions:
• probe surrounding matter properties: T, density, flow…
• probes themselves are affected by hot and dense matter
• Thermal emission: both from QGP and Hadronic phase
Rich variety of dilepton contributions in hadronic phase: Low Mass Range (LMR, m ≤ m ) : resonant (VM, mostly );
Intermediate Mass Range (IMR, m <m < mJ/ ) : continuum-like;
rates determined by T (parton–hadron duality?)
need to disentangle experimentally QQP and HG contributions.
• Quarkonia suppression by colour screening in QGP (covered by R.Arnaldi talk)
• Strangeness () enhancement from abundantly produced in QGPss
Caveats: experimental: small yields, strong backgrounds …
interpretational: underlying ‘conventional’ sources need to be understood …
ℓ +
ℓ -
γ
*
Helios-3 p-W, S-W @ 200 A GeV : LMR/IMR enhancement
Thermal emission? Strong a1 contribution at IMR
Li, Gale, Phys.Rev.Lett. 81(1998) 1572
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NA38/NA50 pA 450 GeV/c
Helios-3
200 A GeV/c
Milestones I
NA38 p-W, S-U @ 200 A GeV NA50 p-Al,Cu,Ag,W @ 450
GeV Pb-Pb @ 158 A GeV
• IMR in p-A described by Drell-Yan and Open Charm
• Yields in Pb-Pb exceed the extrapolation from the p-A
• IMR excess resembles (m, pT) : nuclear modifications?
Also described by thermal emission from GQP and HG
Rapp and Shuryak, Phys. Lett. B473 (2000) 13
DD
Nucl.Phys.A590 1995) 93c
Eur.Phys.C13(2000) 433
centralcollisions
NA50Pb-Pb 158 A
GeV/c<Npart> = 381
Eur.Phys.C14(2000) 442
>400
citations
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Milestones II / CERES measurements
Eur.Phys.J.C 4 (1998) 231
• Two RICH detectors
separated by solenoidal field
• 2 SiDC in vertex region
• trigger on multiplicity
p-Be,Au 450 GeV : good descripiton
by hadronic ‘cocktail’ decays
Central S-Au 200 A GeV excess:
5.0 ± 0.7(stat) ± 2.0(syst)
coils
TPC1999/2000 only
Phys.Rev.Lett.75(1995)1272
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LMR excess by CERES / S-Au 200 A GeV
mass drop (Brown/Rho scaling): Li, Ko, Brown, Phys.Rev.Lett.75 (1995) 4007
broadening: Chanfray, Rapp, Wambach, Phys.Rev.Lett. 76 (1996) 368
Brown/Rho
Rapp/Wambach
Vacuum
Not described by annihilation in vacuum
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LMR excess by CERES / Pb-Au 158 A GeV
Excess seen in S-Au confirmed in Pb-Au 158 A GeV 2.73 ±0.25(stat) ±0.65(syst) ± 0.82(decays)
Brown-Rho scaling
Rapp-Wambach
Vacuum- from -annihilation
Eur.Phys.J. C 41 (2005) 475
Phys.Rev.Lett. 91 (2003) 042301
RW BR
Phys.Rev.Lett. 91 (2003) 042301
1999 data
The only dilepton data
at 40 A GeV
higher excess:
5.9±1.5(stat)±1.2(syst)±1.8(decays)
Effect of higher
baryonic density?
2000 data
Phys. Lett. B666 (2008) 425
RW BR
Excess: 2.61 ± 0.31(stat) ± 0.43(syst) ± 0.76(decays)
1999 TPC upgrade: M/M 6% 4% at / region
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Measuring dimuons in NA60
2.5 T dipole magnet
hadron absorber
targets
beam tracker Si-pixel tracker
muon trigger and tracking (NA50)
magnetic field
>10m<1m
Radiation-hard silicon pixel vertex tracker
Muons from the NA50 spectrometer matched to
tracks in the vertex region:
Improved mass resolution
offset wrt the vertex ( @ ~20 GeV)
promt vs open charm (c=123,312 m) separation
Dipole improves low-mass and low-pT acceptance
Matching rejects decay kinks improved S/B
Selective trigger, high luminosity
m 40
K,
%2M
M
Phys.Rev.Lett. 96 (2006) 162302
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NA60 LMR data: peripheral (Nch<30) In-In collisions
Eur.Phys.J.C 49 (2007) 235
Well described by meson decay ‘cocktail’: η, η’, ρ, ω, and contributions
(Genesis generator developed within CERES and adapted for dimuons by NA60).
DD
Similar cocktail describes NA60 p-Be,In,Pb 400 GeV data
Eur.Phys.J.C 43 (2005) 407
Acceptance-corrected data (after subtraction of , and peaks)
fitted by three contributions:
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EM transition form-factors for and
peripheral NA60 InIn data (hep-ph/0902.2547, submitted to PLB)
0
2/1
2
2
2
23
2
2
22
41
211
)(
3
2)(
m
m
m
m
m
m
mdm
d 22 )( mF
TM
eMTMmM
M
m
M
m
M
m
m
Md
Rd
2/3
22222
2
22/1
2
22/3
2
2
4
42
2
21
41
41
)2(3
)(
22 )( mF
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22
222
22
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2
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2
0
2
0
00
41
41
21
)(
3
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mm
mm
mm
m
m
m
m
m
mdm
d
Confirmed anomaly ofF wrt the VDM prediction.
Improved errors wrt the Lepton-G results.
Removes FF ambiguity in the ‘cocktail’In-In, peripheral
pole approximation:
22222 /1)(
mmF
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Clear excess of data above decay ‘cocktail’ describing peripheral events
More central In-In data
Phys. Rev. Lett. 96 (2006) 162302
•Excess isolated subtracting the measured decay cocktail
(without ), independently for each centrality bin
•Based solely on local criteria for the major sources:
η, ω and 2-3% accuracy.
No need of reference data (pA, peripheral data, models)
Less uncertainties (e.g. strangeness enhancement: ,)
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Eur.Phys.J.C 49 (2007) 235
Excess above the cocktail ρ (bound by ρ/ω=1.0), centered at nominal ρ pole
Monotonically rises and broadens with centrality
By coincidence, NA60 acceptance roughly removes the phase-space factor
spectral function convoluted over the fireball evolution is directly measured
NA60 LMR Excess dimuons
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Excess rises faster than linear with multiplicity: compatible with emission from annihilation processes
Centrality dependence of LMR excess
NA60, In-In 158A GeV
Eur.Phys.J. C41 (2005) 475
Eur.Phys.J.C 49 (2007) 235
Total excess wrt “cocktail” : roughly indicative of the number of rho generations: – clock?
Question to theory: can the fireball life-time be inferred? How does the modified life-time evolve?
CERES, Pb-Au 158A GeV
95/96 data combined
data
/coc
ktai
l
peak: R=C-1/2(L+U) continuum: 3/2(L+U)cocktail ρ is fixed by ρ/ω=1.0
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Phys. Lett. B666 (2008) 425
CERES,Pb-Au 158A GeV
LMR Excess: dropping mass vs broadening
Calculations by R.Rapp for both scenarios
Only broadening of observed; interactions with baryons is very important
Mass shift (Brown/Rho scaling) is ruled out.
(CERES data also described by flat spectral function: Kämpfer et all, Nucl. Phys. A 688 (2001) 939)
NA60, InIn 158A GeV
Phys. Rev. Lett. 96 (2006) 162302
cocktail subtractedusing stat.model
Fit range
DD
DY
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Mass spectrum is similar to NA50: Good description by Drell-Yan + ~2Open Charm (extrapolated from pA data)
1.120.17
DataPrompt: 2.290.08Charm: 1.160.16Fit 2/NDF: 0.6
DD
PromptOffset fit shows that the enhancement is not due to Open Charm the excess is prompt
NA60 IMR data (1.16 < M < 2.56 GeV/c2)
Eur.Phys.J. C59 (2009) 607
DD
Prompt
~50m ~1mm
Such explanation is rejected by the spectra of dimuon offsets wrt the interaction vertex!
2/)2( 11212
xyyyxx VyxVyVx 2/)( 2
221
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NA60 IMR excess (1.16 < M < 2.56 GeV/c2)
Mass (GeV/c2)
Mass shape and yield close to Open Charm
contribution measured agrees within ~20% with fromNA50 pA data (same kinematical domain |cosCS|<0.5)
no strong modifications.
DD
DD
2tsparticipanN
Scales with centrality faster than Drell-Yan (~Nbin.coll), but less faster than
Eur.Phys.J. C59 (2009) 607
Much softer in pT than Drell-Yan: rules out higher-twist DY? [Qiu, Zhang, Phys. Lett. B 525, (2002) 265]
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NA60 excess: comparison to theory
H. van Hees and R. Rapp, Nucl. Phys. A 806 (2008) 339 :
mostly annihilation in LMR (collisional broadening of strong effect from baryons),
4 in IMR (full chiral mixing enhanced a1), QGP contribution 20 – 60% (fireball scenario A … C)
T. Renk and J. Ruppert, Phys. Rev. C 77, (2008) 024907 :
mostly annihilation in LMR ( spectral function by Eletsky et al, Phys.Rev.C 69 (2001) 035202),
QGP dominates (~80%) in IMR
K. Dusling, D. Teaney and I. Zahed, Phys. Rev. C 75, (2007) 024908 + PhD thesis of K.Dusling :
hydrodynamic calculation with virial expansion of hadronic rates,
QGP contribution dominates in IMR: 60 – 90%
• All known sources subtracted (‘cocktail’, Drell-Yan, Open Charm)
•Corrected for acceptance
• Integrated over pT
• Absolute normalization: data and models.
Eur.Phys.J. C59 (2009) 607
NA60 excess vs pT: comparison to theory
Absolute normalization both for theory and data
Differences at low masses reflect differences in the tail of spectral function
Differences at high masses, pT reflect differences in flow strength
J.Phys. G 35 (2008) 104036
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Fit of excess by
(‘cocktail’ is not subtracted)
Teff rises up to the mass, then drops
Spectra steepen at low mT (not for hadrons)
effTT TmdmdN exp~/ 2
mT spectra of NA60: excess
Fit in 0.5<PT<2 GeV/c
‘Cocktail’ and excess continuum are separated using side-window method: is hotter
Confirmed by independent IMR excess analysis
Phys. Rev. Lett. 100 (2008) 022302
M Teff
0.2 – 0.4 192±4
0.4-0.6 223±5
0.6-0.9 256±4
242±2
1.0-1.4 224±11
(DY not subtracted)
Eur.Phys.J. C (2009), in press, nucl-ex/0812.3053
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Large difference between and
mT spectra from NA60 Hierarchy of hadrons freeze-out
Blast wave analysis of NA60 data:
crossing of hadrons with defines Tf, and T max reached at respective hadron freeze-out
Suggests different freeze-out time for different hadrons: first, last (maximal coupling to pions)
J.Phys. G 35 (2008) 104036
<
>
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IMR
RR, DZ: thermal emission dominates in the QGP
phase, when flow has not yet built up Teff are not affected by blue shift
no strong mass dependence
RH : emission from HG dominates
Teff evolution with mass
LMR
• Thermal emission dominates in HG phase
(RH, RR, DZ models agree)
• Integration over the flow development rise of Teff with mass
(RH: relatively week flow compensated by
primordial contributions, but insufficient for pure
in-medium LMR emission)
• Hadrons affected by different freeze-out times
Eur.Phys.J. C (2009), in press, nucl-ex/0812.3053
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NA60 also measured the polarization (in the Collins-Soper frame) for m≤ m
Lack of any polarization in excess (and in hadrons) supports emission from thermalized source.
Details in the presentation of G.Usai /session 4C/
2cossin2
cos2sincos11 22
d
d
Polarization of dileptonsSubmitted to PRL, nucl-ex/0812.3100
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Evidence of ω in-medium effects?
Flattening of the pT distributions at low pT, developing very fast with centrality.
Low-pT ω’s have more chances to decay inside the fireball ?
Appearance of that yield elsewhere in the spectrum, due to ω mass shift and/or broadening, unmeasurable due to masking by the much stronger contribution.
Disappearance of yield out of narrow ω peak in nominal pole position
Can only measure disappearance
nucl-ex/0812.3053
Eur.Phys.J. C (2009), in press, nucl-ex/0812.3053
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NA60 results on omega yield suppression
Determine suppression vs pT with respect to (extrapolated from pT>1GeV/c)
Account for difference in flow effects using the results of the Blast Wave analysis
effTT TmdmdN exp~/ 2
Reference line: ω/Npart = 0.131 f.ph.s.
Strong centrality-dependent suppression at
pT<0.8 GeV/c , beyond flow effects
Eur.Phys.J. C (2009), in press nucl-ex/0812.3053
Reference line: /Npart = 0.0284 f.ph.s. (central coll.)
Consistent with radial flow effects
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- puzzle
Disagreement between results for in Pb-Pb 158A GeV
NA49 K+K- (pT<1.6 GeV/c) NA50 (pT>1.1GeV/c)
[Phys. Lett. B491, (2000) 59; J. Phys. G 27,(2001) 355] [Nucl. Phys. A661 (1999)534c; Phys. Lett. B 555 (2003) 147]
NA50 sees >2 times higher yield than NA49
Large difference in Teff : T =218±6 TKK = 305±15
In-medium effects on and K + K absorption and rescattering
reduced yield and harder pT spectrum in hadronic channel?
Recent developments:
CERES mesured K+K- and ee channels
in central Pb-Au 158A GeV [Phys.Rev.Lett. 96 (2006) 152301]
Both channels agree with each other (large errors on
e+e- channel) and with NA49
NA50 reanalysed its old + 2000 Pb-Pb data
[J. Phys. G 35 (2008) 104163]
Old results are confirmed within 8%
New results from NA60 in In-In 158 A GeV data :
comparison of and K+K-
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NA60 finds good agreement between the and K+K- channels no – puzzle in In-In
Teff in central In-In collisions is lower compared to cetral Pb-Pb of NA49 and CERES
<>/Npart in central In-In collisions is lower than the NA50 results (f.ph.s),
but slightly higher than measured by NA49 and CERES
Details in the presentation of A.de Falco /session 5D/
- puzzle
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Direct photon (old) measurements at SPS
Extremely difficult to measure: large background from 0 and ’ very few results
(especially at low pT where QCD contribution is small)
15-20% upper limits (90%CL) wrt the hadronic sources for central S-Au 200 A GeV/c from
CERES [Z.Phys.C71 (1996) 571] and WA80 [Phys.Rev.Lett. 76 (1996) 3506]
Most significant results from WA98 in central Pb-Pb 158 A GeV [Phys.Rev.Lett.85 (2000) 3595]:
excess of up to 20±7% for pT>1.5 GeV/c
Even more difficult to interpret than in the
case of dileptons:
• same ambiguity:
close to the Tc hadronic and partonic
descriptions provide similar rates…
• no extra handle like mass, only pT
Calculation by Turbide, Rapp and Gale
within the same approach as for ll
Phys.Rev.C69 (2004) 014903
Teff = 183 10 MeV
Ti = 210–350 MeV ?
NA60 observes an excess at same masses
Smoothly continues to m>0.3 GeV/c2 (in PHENIX also)
Exponential down to low pT’s (‘high’ virtuality)
Teff = 183 MeV fits well to the smooth rise with M (flow) Tth ~ 160 MeV
No evidence of direct ‘real’ photons ?
PHENIX: interpret enhancement at pT>>mee as internally converted direct photons
(‘almost’ real: 0.1<mee<0.3 GeV/c2, pT>1 GeV/c)
Teff = 2212318 MeV interpret as TQGP (weighted over the QGP evolution) Ti = 300 – 600 MeV
Direct photons from internal conversion ?
Prospects from LHC?
ALICE, ATLAS, CMS: excellent capabilities to measure photons (ALICE and CMS from pT > ~1 GeV/c)
See presentation of D. Peressounko on photons in Alice /session 5D/
Much higher Tinitial and life-time of the fireball stronger thermal emission of , l+l-
(Surprisingly, PHENIX sees excess only at M < 750 MeV/c2, see next presentation by A.Drees)
Even higher background to cope with: Sll /B < 10-2 vs 10-1 on SPS
Possibilities of l+l- measurements below J/ are limited by acceptance, tracking, PID
Currently, only ALICE has some preliminary estimates for low mass dileptons:
+- : no acceptance below mT ~ 1 GeV/c due to the 0.5 GeV/c cut on single pT
e+e- : good PID using TPC+TRD for pT > 1GeV/c
Expectations from Alice Muon Arm1 month of Pb-Pb
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Excess in dilepton emission at SPS: good agreement with models of thermal emission
‘Planck’-like mass spectra (slightly modified by non-flat spectral function in LMR)
Teff rising with mass in LMR and, after sharp drop, flat in IMR
Lack of polarization: emission from isotropic source
Faster than linear scaling with Ncharged
LMR: major contribution from annihilation
IMR: naturally explained by mostly partonic radiation
First evidence of low-pT in-medium modifications in central In-In collisions
-puzzle
Pb-Pb: impossible to reconcile results of from NA50 with KK from NA49
ee and KK channels measured by CERES agree with each other and NA49
In-In : and KK channels measured by NA60 agree (both yield and Teff )
<>/Npart exceeds Pb-Pb value, Teff is in-between of conflicting Pb-Pb values
Direct photons: no conclusive results. Excess seen by WA98 in Pb-Pb is
compatible with thermal emission but the errors are very large
Summary