Hot Quarks 2004 - July 24, 2004 Andrea Dainese1
Charm energy loss at the LHC with ALICE
Andrea DainesePadova – University and INFN
Hot Quarks 2004 - July 24, 2004 Andrea Dainese2
Outline
Heavy-quark energy lossEstimates of D-meson suppression at LHCALICE sensitivity via D0 K Conclusions
Hot Quarks 2004 - July 24, 2004 Andrea Dainese3
2 ˆ LqCE Rs
Parton Energy LossDue to medium-induced gluon radiation
Average energy loss (BDMPS model):
Casimir coupling factor:4/3 for quarks3 for gluons
Medium transport coefficient gluon density and momenta
R.Baier, Yu.L.Dokshitzer, A.H.Mueller, S.Peigne' and D.Schiff, (BDMPS), Nucl. Phys. B483 (1997) 291.
hardparton
path length L
Hot Quarks 2004 - July 24, 2004 Andrea Dainese4
Lower Loss for Heavy Quarks?In vacuum, gluon radiation suppressed at < mQ/EQ
“dead cone” effect1
Dead cone implies lower energy loss2 (Dokshitzer-Kharzeev, 2001)
Recent detailed calculation confirms this qualitative feature3 (Armesto-Salgado-Wiedemann, 2003)
1. Yu.L.Dokshitzer, V.A.Khoze and S.I.Troyan, J. Phys. G17 (1991) 1602.2. Yu.L.Dokshitzer and D.E.Kharzeev, Phys. Lett. B519 (2001) 199 [hep-ph/0106202].3. N.Armesto, C.A.Salgado and U.A.Wiedemann, Phys. Rev. D69 (2004) 114003 [hep-ph/0312106].
Q
Hot Quarks 2004 - July 24, 2004 Andrea Dainese5
DK dead-cone effectDokshitzer-Kharzeev: energy distribution dI/d of radiated gluons suppressed by angle-dependent factor
suppress high-energy tail of gluon radiation spectrum sizeable reduction of energy loss
11dd
dd
2
2
2
Q
Q
LIGHTHEAVY EmII
Yu.Dokshitzer
Hot Quarks 2004 - July 24, 2004 Andrea Dainese6
Experimental study of energy lossCompare pt distributions of leading particles in pp and nucleus-nucleus collisions (+ p-nucleus as a control)
Nuclear modification factor:
tpp
tAA
colltAA dpdN
dpdNN
pR//1)(
study D meson RAA
and compare it to that of h (or 0)
)()()(/t
hAA
tDAA
thD pRpRpR
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The dependences of RD/h
Initial state effects (PDF shadowing) (low pt; <10 GeV/c)
Parton energy loss: (c) quark vs gluon (Casimir factor) RD/h > 1 (all pt)
mass effects RD/h > 1 (“moderate” pt; <20 GeV/c)
fragmentation:• (much) harder for charm quarks w.r.t. gluons RD/h
slope of pt distribution:
• harder for charm RD/h
recombination / in-medium hadronization (low pt; <10 GeV/c)
)(/)()(/ thAAt
DAAthD pRpRpR
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Energy-loss simulation: ingredientsBDMPS Quenching Weights1: -dependent distrib.
Transport coefficient for central Pb-Pb at LHC
Realistic path length of partons in the dense medium (Glauber-model based)
1. C.A.Salgado and U.A.Wiedemann, Phys. Rev. D68 (2003) 014008 [hep-ph/0302184].2. N.Armesto, A.D., C.A.Salgado and U.A.Wiedemann, in preparation.
),ˆ( Lq E
q̂
L
First estimate use the DK dead-cone factor to correct for charm
use QCD theory estimate
New estimate use QW specifically calculated for massive quarks2
use model extrapolation based on RHIC data
Hot Quarks 2004 - July 24, 2004 Andrea Dainese9
First estimate (last year)
/fmGeV 101~ˆ
GeV/fm 10010~
2
3
q
cold nucl. matter
QGP@ LHC
R.Baier, Nucl. Phys. A715 (2003) 209.
/fmGeV 4ˆ 2qchosenwhich gives 25.0h
AAR
A.D. Eur. Phys. J. C33 (2004) 495 [nucl-ex/0312005].
Shadowing includedvia EKS98
Hot Quarks 2004 - July 24, 2004 Andrea Dainese10
New estimate (1): Parton Quenching ModelPQM: BDMPS quenching weights + realistic coll. geometry Parton-by-parton calculation of distrib. Centrality evolution included through Glauber model Theoretical uncertainty band considered (finite parton-energy limit)
needed1,2 to match RAA at RHIC (200 GeV)/fmGeV 15ˆ 2q
E
1. PQM: A.D., C.Loizides and G.Paic, hep-ph/0406201.2. K.J.Eskola, H.Honkanen, C.A.Salgado and U.A.Wiedemann, hep-ph/0406319.
Hot Quarks 2004 - July 24, 2004 Andrea Dainese11
New estimate (2): PQM from RHIC to LHC …Assumption: (initial volume-density of gluons) Extrapolation to LHC according to saturation model1 gives:
Most partons are absorbedOnly those from the surface can escape the medium
1. K.J.Eskola, K.Kajantie, P.V.Ruuskanen and K.Tuominen, Nucl. Phys. B570 (2000) 379 [hep-ph/9909456].
/fmGeV 100ˆ 7ˆ 2 RHICLHC qq
… energy loss saturated
gnq ˆ
h
GeV/c 5hadrontp
Hot Quarks 2004 - July 24, 2004 Andrea Dainese12
New estimate (3): mass effect much smaller
red “band”: massless
/fmGeV 100ˆ 2q
blue “band”:massive (1.2 GeV)
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Comparison, step by step[For lower bound of the uncertainty band]
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The ALICE DetectorThe ALICE Detector
||| < 0.9| < 0.9TPC + silicon trackerTPC + silicon tracker
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Exclusive charm in ALICE: D0 K-+
Exclusive reconstruction direct measurement of the pt distribution ideal tool to study RAA
Large combinatorial background (dNch/dy=6000 in central Pb-Pb!)
Main selection: displaced-vertex selection pair of opposite-charge tracks with large impact parameters good pointing of reconstructed D0 momentum to the primary vertex
Invariant mass analysis to “count” D0
Hot Quarks 2004 - July 24, 2004 Andrea Dainese16
Results (K,) Invariant Mass distribution (pt –integrated) in Pb-Pb (~ 1 month run)
%10/ BS
40/ BSSN.Carrer, A.D. and R.Turrisi, J. Phys. G29 (2003) 575.A.D. PhD thesis (2003), nucl-ex/0311004.
Stat. and syst. errorson D pt distr. estimatedfor pp and Pb-Pb
Statistical significance:
1 < pt < 14 GeV/c
Hot Quarks 2004 - July 24, 2004 Andrea Dainese17
Measuring D quenching with ALICE
/fmGeV 4ˆ 2q /fmGeV 100ˆ 2q
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Conclusions
Direct D reconstruction in ALICE: powerful tool to address experimentally the rich m > 0 domain gluon radiation suppressed at small angles smaller energy loss / suppression ???
Theoretical picture on charm energy loss is evolving … DK dead-cone approximation over-estimates effect of mass within current th. uncertainties, D-meson suppression may not be
affected by “dead cone” further improvement in treatment of finite parton energies will
indicate kinematic range where mass effects are more significant
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BACK-UP SLIDES
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Go for deep deconfinement at LHCNext step in the “quest for QGP” …
LHC: factor 30 jump in w.r.t. RHIC much larger initial temperature
study of hotter, hotter, bigger,bigger, longer-livinglonger-living ‘drops’ of QGP
s
‘Deep de-confinement’ closer to ‘ideal’ QGP easier comp. with theory (lattice)
SPS17 GeV
RHIC200 GeV
LHC5.5 TeV
initial T ~ 200 MeV ~ 300 MeV > 600 MeVvolume 103 fm3 104 fm3 105 fm3
life-time < 2 fm/c 2-4 fm/c > 10 fm/c
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Hard Processes in AA at the LHCMain novelty of the LHC: large hard cross sectionHard processes are extremely useful tools large virtuality Q happen at t = 0 small “formation time” t ~ 1/Q (for charm: t < 1/2mc ~ 0.1 fm/c << QGP ~ 5–10 fm/c)
Initial yields and pt distributions in AA can be predicted using pp measurements + pQCD + collision geometry + “known” nuclear effectsInteractions with the medium can induce deviations from such predictions
medium formed in the collision
Q
Pb
Pb c
c
time
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Initial-state effects: ShadowingBjorken-x: fraction of the momentum of the proton ( ) carried by the parton entering the hard scattering
At the LHCPb ion @ LHC ~ 105-106 partons
(mainly gluons)
q
2/1 sx
2/2 sx
2/s
sMsQx qq //
charm43 1010~ x
xa
xb
xa+xb
gPb(x)/gp(x)
Shadowing:• reduces initial hard yield at low pt
• scales trivially from pA to AA
“they are so close that they fuse”
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Hard partons probe the mediumPartons travel ~ 5 fm in the high colour-density mediumEnergy loss by gluon bremsstrahlung modifies momentum distributions jet shapes …
depends on medium properties
PROBEmedium
probe IN(known from
pp, pA+ pQCD)
probeOUT
IN vs. OUT
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BDMPS model
2
0
2
2
ˆ 2 22
22
ˆ21
ˆ
LqCCddIdE
CddI
Lq
Rsc
Rs
cRs
c
medium
c
kt
vacuum
medium
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Background multiplicity in Pb-PbWhat is the background to hadronic D decays? combinatorial background given by pairs of uncorrelated tracks
with large impact parameter 2/ dydNB ch
2500/ dydNchin central Pb-Pb at LHC
huge combinatorial background!huge combinatorial background!
Simulations performed using 6000/ dydNch
need excellent detector response and good selection strategyneed excellent detector response and good selection strategy
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ALICE Barrel
||<0.9:B = 0.4 TTOFTPCITS with: - Si pixels- Si drifts- Si strips
PIXEL CELL
z: 425 m
r: 50 m
Two layers:r = 4 cmr = 7 cm
9.8 M
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TrackingTracking efficiency ~70% with dNch/dy=6000
pionskaons
pt resolution = 1% at 1 GeV/c
D0 invariant mass resolution:MeV 13)( ,%7.0)(
21)(
Mpp
MM
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Impact parameter resolution
< 60 mfor pt > 1 GeV/c
Mainly provided by the 2 layers of Si pixels
PIXEL CELL
z: 425 m
r: 50 m
Two layers:r = 4 cmr = 7 cm
9.8 M
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TOF PID
TOF
Pb-Pb, dNch/dy=6000
Optimization for hadronic charmdecays was studied:minimize probability to tag K as
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D0 K-+: Signal and backgroundSignal: charm cross section from NLO pQCD (MNR program), average of
results given by MRS98 and CTEQ5M PDFs (with EKS98 in Pb-Pb)
signal generated using PYTHIA, tuned to reproduce pt distr. given by NLO pQCD
contribution from bBD0 (~5%) also included
Background: Pb-Pb: HIJING (dNch/dy=6000 ! we expect ~2500 !); pp: PYTHIA;
system shadowingpp 14 TeV 11.2 1 0.16 0.0007Pb-Pb 5.5 TeV (5% cent) 6.6 0.65 115 0.5
[mb] ccNN cctotN dyKDdN /)( 0
MNR Program: M.L.Mangano, P.Nason and G.Ridolfi, Nucl. Phys. B373 (1992) 295.
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D0 K-+: Selection of D0 candidates
increase S/Bby factor ~103!
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S/Binitial
(M3)
S/evtfinal
(M1)
S/Bfinal
(M1)
SignificanceS/S+B(M1)
Pb-Pb 5 10-6 1.3 10-3 11 % 37
(for 107 evts,~1 month)
pp 2 10-3 1.9 10-5 11 % 44
(for 109 evts,~1 year)
D0 K-+: Results
Note: with dNch/dy = 3000, S/B larger by 4 and significance larger by 2
0–14 GeV/c1–14 GeV/c
Pb-Pb, 5.5 TeV 107 events
pp, 14 TeV109 events
Hot Quarks 2004 - July 24, 2004 Andrea Dainese33
S/Binitial
(M3)
S/evtfinal
(M1)
S/Bfinal
(M1)
SignificanceS/S+B(M1)
2 10-3 1.9 10-5 11 % 44
(for 109 evts,~9 months at 1030 cm-2s-1)
D0 K-+: Results
0.5 < pt < 1 GeV/c 2 < pt < 2.5 GeV/c 12 < pt < 14 GeV/c
Hot Quarks 2004 - July 24, 2004 Andrea Dainese34
What if multiplicity in Pb-Pb is lower?
We used dNch/dy = 6000, which is a pessimistic estimate
Recent analyses of RHIC results seem to suggest as a more realistic value dNch/dy = 3000 (or less)
Charm production cross section: estimate from NLO pQCD (only primary production, no collective effects) average of theoretical uncertainties (choice of: mc, F, R, PDF)
BKG proportional to (dNch/dy)2
We can scale the results to the case of dNch/dy = 3000:
S/B = 44 %SGNC = 74
(this only from scaling, obviously better with retuning of cuts)
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Estimate of the errorsStatistical error on the selected signal = 1/SignificanceMain systematic errors considered: correction for feed-down from beauty (B.R. B D0 is 65%!): error of ~8% assuming present uncertainty (~80%) on @ LHC Monte Carlo corrections: ~10% B.R. D0 K: 2.4% extrapolation from N(D0)/event to d(D0)/dy:
• pp: error on (~5%, will be measured by TOTEM)• Pb-Pb: error on centrality selection (~8%) + error on TAB (~10%)
bb
inel
bb
Pb-Pbinel
pp
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D0 K-+: d2(D0)/dptdy and d(D0)/dy
d(D0)/dy for|y| < 1 and pt > 1 GeV/c (65% of (pt > 0)) statistical error = 7 % systematic error = 19 % from b = 9 % MC correction = 10% B.R. = 2.4 % from AA to NN = 13 %
inner bars: statisticalouter bars: systematic
d(D0)/dy for|y| < 1 and pt > 0
statistical error = 3 % systematic error = 14 % from b = 8 % MC correction = 10% B.R. = 2.4 % inel = 5 %
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Sensitivity to NLO pQCD parameters
MNR Program: M.L.Mangano, P.Nason and G.Ridolfi, Nucl. Phys. B373 (1992) 295.
PDFsm RFc ,,,
00
PDFsm RFc ,,,
00
pp, 14 TeV
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Interpolation pp 14 5.5 TeV
In pQCD calculations the ratio of the differentialcross sections at 14 and 5.5 TeV is independent of the input parameters within 10% up to 20 GeV/c
pQCD can be safely used to extrapolate pp @ 14 TeVto 5.5 TeV
Necessary to compare Pb-Pb and pp by RAA
set PDF,,,t
F
t
Rc mmm
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Sensitivity on RAA for D0 mesonsLow pt (< 6–7 GeV/c)Nuclear shadowing
‘High’ pt (6–15 GeV/c)here energy loss can be studied(it’s the only expected effect)
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Effect of shadowing
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Transport coefficient choiceRequire for LHC suppression of hadrons as observed at RHIC: RAA ~ 0.2-0.3 for 4<pt<10 GeV/cpt distributions of hadrons at LHC: partons (pt>5 GeV/c) generated with PYTHIA pp, 5.5 TeV
(average parton composition: 78% g + 22% q) energy loss: pt’ = pt – E (independent) fragmentation with KKP LO F.F.
RAA = (pt distr. w/ quenching) / (pt distr. w/o quenching)
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RAA with Quenching
RRAAAA ~ 0.4 ~ 0.4––0.50.5increasing at high increasing at high pptt
RRAAAA ~ 0.7 ~ 0.7––0.80.8decreasing at high decreasing at high pptt
A.D. Eur. Phys. J. C33 (2004) 495 [arXiv:nucl-ex/0312005].
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D/hadrons ratio (1)Ratio expected to be enhanced because: D comes from (c) quark, while , K, p come mainly (~80% in
PYTHIA) from gluons, which lose 2 more energy w.r.t. quarks dead cone for heavy quarks
Experimentally use double ratio: RAAD/RAA
h almost all systematic errors of both Pb-Pb and pp cancel out!
D/h ratio: RD/h = RAA
D / RAAh
RD/h ~ 2–3 in hot QGP
sensitive tomedium density
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D/hadrons ratio (2)
RD/h is enhanced only by the dead-cone effectEnhancement due to different quark/gluon loss not seenIt is compensated by the harder fragmentation of charm
pthadron = z pt
parton (ptparton)’ = pt
parton – E (pt
hadron)’ = pthadron – z E
Energy loss observed in RAA is not E but zEzcD 0.8; zgluonhadron 0.4 (for pt > 5 GeV/c)
Ec = Egluon/2.25 (w/o dead cone)
zcD Ec 0.9 zgluonhadron Egluon
Without dead cone, RAAD RAA
h
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PQM: RAA all centralities
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PQM: IAA
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PQM: v2
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PQM: RAA @ 62.4 GeV
/fmGeV 7ˆ5.0ˆ 22004.62 qq
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PQM: ch. hadrons RAA at LHC
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PQM: <E / E> vs E
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PQM: surface effect
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B mesons RAA at LHC
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Inclusive B e + X: electron ID + cut on its pt & on its impact parameter d0
Open Beauty in electron channel
S/(S+B) S per 107 Pb-Pb events
pt > 2 GeV/c, d0 > 180 m: 50,000 electrons with S/(S+B) = 90 %