Charm energy loss at the LHC with ALICE

1
Charm energy loss at the LHC with ALICE

• Andrea Dainese
• Padova University and INFN

2
Outline

• Heavy-quark energy loss
• Estimates of D-meson suppression at LHC
• ALICE sensitivity via D0 ? Kp
• Conclusions

3
Parton Energy Loss

• Due to medium-induced gluon radiation
• Average energy loss (BDMPS model)

Casimir coupling factor 4/3 for quarks 3 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.
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Lower Loss for Heavy Quarks?

• In vacuum, gluon radiation suppressed at q lt
  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.
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DK dead-cone effect

• Dokshitzer-Kharzeev energy distribution w dI/dw
  of radiated gluons suppressed by angle-dependent
  factor
• suppress high-energy tail of gluon radiation
  spectrum
• sizeable reduction of energy loss

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Experimental study of energy loss

• Compare pt distributions of leading particles in
  pp and nucleus-nucleus collisions ( p-nucleus as
  a control)
• Nuclear modification factor

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The dependences of RD/h

• Initial state effects (PDF shadowing)
  (low pt lt10 GeV/c)
• Parton energy loss
• (c) quark vs gluon (Casimir factor) ? RD/h gt
  1 (all pt)
• mass effects ? RD/h gt 1 (moderate
  pt lt20 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
  lt10 GeV/c)

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Energy-loss simulation ingredients

• BDMPS 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.
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First estimate (last year)
Shadowing included via EKS98
chosen which gives
A.D. Eur. Phys. J. C33 (2004) 495
nucl-ex/0312005.
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New estimate (1) Parton Quenching Model

• PQM 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)

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.
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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 absorbed
• Only those from the surface can escape the medium

energy loss saturated
1. K.J.Eskola, K.Kajantie, P.V.Ruuskanen and
K.Tuominen, Nucl. Phys. B570 (2000) 379
hep-ph/9909456.
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New estimate (3) mass effect much smaller
red band massless
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 Detector
h lt 0.9 TPC silicon tracker
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Exclusive charm in ALICE D0 ? K-p

• Exclusive reconstruction direct
  measurement of the pt distribution
  ideal tool to study RAA
• Large combinatorial background (dNch/dy6000 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
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Results
Stat. and syst. errors on D pt distr.
estimated for pp and Pb-Pb
(K,?) Invariant Mass distribution (pt
integrated) in Pb-Pb
( 1 month run)
Statistical significance
1 lt pt lt 14 GeV/c
N.Carrer, A.D. and R.Turrisi, J. Phys. G29 (2003)
575. A.D. PhD thesis (2003), nucl-ex/0311004.
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Measuring D quenching with ALICE
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Conclusions

• Direct D reconstruction in ALICE powerful tool
  to address experimentally the rich m gt 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 LHC

• Next step in the quest for QGP
• LHC factor 30 jump in w.r.t. RHIC
• much larger initial temperature
• study of hotter, bigger, longer-living
  drops of QGP

SPS 17 GeV RHIC 200 GeV LHC 5.5 TeV
initial T 200 MeV 300 MeV gt 600 MeV
volume 103 fm3 104 fm3 105 fm3
life-time lt 2 fm/c 2-4 fm/c gt 10 fm/c

• ? closer to ideal QGP
• easier comp. with theory
• (lattice)

Deep de-confinement
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Hard Processes in AA at the LHC

• Main novelty of the LHC large hard cross section
• Hard processes are extremely useful tools
• large virtuality Q ? happen at t 0
• ? small
  formation time Dt 1/Q
• (for charm Dt lt 1/2mc 0.1 fm/c ltlt tQGP
  510 fm/c)
• Initial yields and pt distributions in AA can be
  predicted using pp measurements pQCD
  collision geometry known nuclear effects
• Interactions with the medium can induce
  deviations from such predictions

medium formed in the collision
time
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Initial-state effects Shadowing

• Bjorken-x fraction of the momentum of the proton
  ( ) carried by the parton entering the
  hard scattering
• At the LHC
• Pb ion _at_ LHC 105-106 partons
• (mainly gluons)

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Hard partons probe the medium

• Partons travel 5 fm in the high colour-density
  medium
• Energy loss by gluon bremsstrahlung
• modifies momentum distributions
• jet shapes
• depends on medium properties
• PROBE

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BDMPS model
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Background multiplicity in Pb-Pb

• What is the background to hadronic D decays?
• combinatorial background given by pairs of
  uncorrelated tracks with large impact parameter

in central Pb-Pb at LHC
Simulations performed using
huge combinatorial background!
need excellent detector response and good
selection strategy
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ALICE Barrel
hlt0.9 B 0.4 T TOF TPC ITS with - Si
pixels - Si drifts - Si strips
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Tracking
Tracking efficiency 70 with dNch/dy6000
pions kaons
pt resolution 1 at 1 GeV/c
D0 invariant mass resolution
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Impact parameter resolution

• Mainly provided by the 2 layers of Si pixels

? 9.8 M
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TOF PID
TOF
Pb-Pb, dNch/dy6000
Optimization for hadronic charm decays was
studied minimize probability to tag K as p
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D0? K-p Signal and background

• Signal
• 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 b?B?D0 (5) also included
• Background
• Pb-Pb HIJING (dNch/dy6000 ! we expect 2500 !)
  pp PYTHIA

system shadowing
pp 14 TeV 11.2 1 0.16 0.0007
Pb-Pb 5.5 TeV (5 cent) 6.6 0.65 115 0.5
MNR Program M.L.Mangano, P.Nason and G.Ridolfi,
Nucl. Phys. B373 (1992) 295.
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D0? K-p Selection of D0 candidates
increase S/B by factor 103!
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D0? K-p Results
S/B initial (M?3s) S/evt final (M?1s) S/B final (M?1s) Significance S/?SB (M?1s)
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)
Note with dNch/dy 3000, S/B larger by ? 4 and
significance larger by ? 2
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D0? K-p Results
S/B initial (M?3s) S/evt final (M?1s) S/B final (M?1s) Significance S/?SB (M?1s)
2 ? 10-3 1.9 ? 10-5 11 44 (for 109 evts, 9 months at 1030 cm-2s-1)
0.5 lt pt lt 1 GeV/c
2 lt pt lt 2.5 GeV/c
12 lt pt lt 14 GeV/c
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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, mF, mR, 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 errors

• Statistical error on the selected signal
  1/Significance
• Main systematic errors considered
• correction for feed-down from beauty (B.R. B ? D0
  is 65!)
• error of 8 assuming present uncertainty
  (80) on _at_ LHC
• Monte Carlo corrections 10
• B.R. D0? Kp 2.4
• extrapolation from N(D0)/event to ds(D0)/dy
• pp error on (5, will be measured by
  TOTEM)
• Pb-Pb error on centrality selection (8)
  error on TAB (10)

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D0? K-p d2s(D0)/dptdy and ds(D0)/dy
ds(D0)/dy for y lt 1 and pt gt 1 GeV/c (65
of s(pt gt 0)) statistical error 7
systematic error 19
from b 9 MC
correction 10 B.R.
2.4 from AA to NN 13
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Sensitivity to NLO pQCD parameters
pp, 14 TeV
MNR Program M.L.Mangano, P.Nason and G.Ridolfi,
Nucl. Phys. B373 (1992) 295.
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Interpolation pp 14 ? 5.5 TeV
Necessary to compare Pb-Pb and pp by RAA
In pQCD calculations the ratio of the
differential cross 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 _at_ 14 TeV to 5.5 TeV
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Sensitivity on RAA for D0 mesons
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Effect of shadowing
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Transport coefficient choice

• Require for LHC suppression of hadrons as
  observed at RHIC RAA 0.2-0.3 for 4ltptlt10 GeV/c
• pt distributions of hadrons at LHC
• partons (ptgt5 GeV/c) generated with PYTHIA pp,
  5.5 TeV
• (average parton composition 78 g 22 q)
• energy loss pt pt DE
• (independent) fragmentation with KKP LO F.F.
• RAA (pt distr. w/ quenching) / (pt distr. w/o
  quenching)

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RAA with Quenching
A.D. Eur. Phys. J. C33 (2004) 495
arXivnucl-ex/0312005.
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D/hadrons ratio (1)

• Ratio expected to be enhanced because
• D comes from (c) quark, while p, 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/RAAh
• almost all systematic errors of both Pb-Pb and pp
  cancel out!

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D/hadrons ratio (2)

• RD/h is enhanced only by the dead-cone effect
• Enhancement due to different quark/gluon loss not
  seen
• It is compensated by the harder fragmentation of
  charm

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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 _at_ 62.4 GeV
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PQM ch. hadrons RAA at LHC
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PQM ltDE / Egt vs E
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PQM surface effect
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B mesons RAA at LHC
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Open Beauty in electron channel

• Inclusive B ? e? X
• electron ID cut on its pt on its impact
  parameter d0