Charm Energy Loss at the LHC with ALICE nuclex0311004 Ph'D' Thesis nuclex0312005 subm' to EPJ

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Charm Energy Lossat the LHC with
ALICEnucl-ex/0311004 (Ph.D. Thesis)nucl-ex/0312
005 (subm. to EPJ)

• Andrea Dainese
• University of Padova

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Outline

• Phenomenology parton energy loss
• Hard probes in heavy-ion collisions
• BDMPS model of radiative energy loss
• Dead-cone effect for heavy quarks
• Experiment how to measure it?
• Leading-charm-particle analysis nuclear
  modification factor
• Expected D0 meson capability of ALICE
• Sensitivity to the nuclear modification
• Combining Phenomenology and Experiment
• Quenching weights (Salgado-Wiedemann)
• Introduction of dead-cone effect
• Nuclear modification of D mesons with quenching
• Comparison of D mesons to light-quark hadrons

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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
• happen at t 0
• large virtuality Q ? 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
  (e.g. PDF nuclear shadowing ? suppression of
  low-x (low-pt) production)
• Deviations from such predictions are due to the
  medium

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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Final-state effects Hard partons probe the
medium

• Partons travel 5 fm in the high colour-density
  medium
• Energy loss by gluon bremsstrahlung
• (more details follow)
• modifies momentum distributions
• determines deviations from
• binary scaling ( ? Ncoll) also at high pt
• depends on medium properties
• PROBE

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Parton Energy Loss

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

path length L
QCD process emitted gluon itself radiates ? ?E
? L2
Kinematic constraint DE ? E significantly
reduces effective average DE
hard parton
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. C.A.Salgado and U.A.Wiedemann, Phys. Rev.
D68 (2003) 014008 arXivhep-ph/0302184.
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Heavy Quarks dead cone

• Heavy quarks with momenta lt 2030 GeV/c ? v ltlt
  c
• Gluons radiation is suppressed at angles lt mQ/EQ
  
  
• dead-cone effect
• Due to destructive interference
• Contributes to the harder fragmentation of heavy
  quarks
• Yu.L.Dokshitzer and D.E.Kharzeev dead cone
  implies lower energy loss

Q
Yu.L.Dokshitzer and D.E.Kharzeev, Phys. Lett.
B519 (2001) 199 arXivhep-ph/0106202.
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Dead-cone effect

• Dokshitzer-Kharzeev energy distribution wdN/dw
  of radiated gluons suppressed by angle-dependent
  factor
• high-energy part of gluon radiation more
  suppressed
• strong reduction of quenching
• effect vanishes for EQ gtgt mQ (high-pt charm
  standard quenching)

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Experimental study of energy loss

• Quenching can be studied by comparing pt
  distributions of leading particles in pp and AA
• Nuclear modification factor
• Energy loss RAA lt 1

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D0? K-p in ALICE

• Exclusive reconstruction direct
  measurement of the pt distribution
  ideal tool to study RAA
• Weak decay with mean proper length ct 124 mm
• STRATEGY invariant-mass analysis of
  fully-reconstructed topologies originating from
  (displaced) secondary vertices
• Measurement of Impact Parameters
• Measurement of Momenta
• Particle identification to tag the two decay
  products

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D0? K-p Detection strategy with ALICE
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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 (for details see
  hep-ph/0311225)
• contribution from b?B?D0 (5) also included
• Background
• Pb-Pb HIJING (dNch/dy6000) 20k evts pp
  PYTHIA 8M evts
• What is the background to hadronic D decays?
• combinatorial background given by pairs of
  uncorrelated tracks with large impact parameter

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

• Main selection displaced-vertex selection
• pair of tracks with large impact parameters
• good pointing of reconstructed D0 momentum to
  the primary vertex

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D0? K-p Results
Note with dNch/dy 3000, S/B larger by ? 4 and
significance larger by ? 2
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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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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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Energy-loss simulation

• C.A.Salgado and U.A.Wiedemann calculated the
  energy-loss probabilities (quenching weights) for
  light quarks and gluons in the framework of the
  BDMPS model including
• effect of finite in-medium path length L
• space-time expansion of the medium
• Numerical routine provided
• input L, transport coefficient q, parton species
  (light quark or gluon)
• output P(DEL), energy-loss probability
  distribution for fixed L and q
• Need to
• determine path length (L) distribution
• include a dead-cone algorithm for charm quarks
• estimate transport coefficient for centr Pb-Pb _at_
  LHC

C.A.Salgado and U.A.Wiedemann, Phys. Rev. D68
(2003) 014008 arXivhep-ph/0302184.
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Transverse path length L (in Pb-Pb, blt3.5 fm
i.e. 5 central)

• Partons produced at mid-rapidity assumed to
  travel in the transverse plane
• Parton production points (x0,y0) sampled
  according to density of collisions rcoll(x,y) and
  their azimuthal propagation directions (ux,uy)
  sampled uniformly
• This definition of L is exact only for cylindric
  profile rcoll(r)r0 q(R-r)
• No exact definition for generic rcoll profile!
• MC sampling varying b in 0,3.5 fm according to
  dN/db ? b

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Energy-loss probability distribution

• For given q and parton species, convolution of
  P(DEL) and L distribution
• Energy loss can be sampled from this P(DE)
  distribution

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Inclusion of dead-cone effect

• First approximation dead-cone effect accounted
  for by folding the energy-loss probability P(DE)
  with the heavy/light suppression factor FH/L
• Folding done as function of the quark energy
• This procedure yields same result as
  recalculating the quenching weights with
• Single-gluon emission dominates

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Average energy loss

• DE sampled according to P(DE) distribution
• pt pt DE kinematic constraint if DE gt pt,
  pt 0

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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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Quenching of charm quarks

• Procedure similar to that for light quarks and
  gluons
• Differences
• kinematic constraint if DE gt pt then c quark is
  thermalized assign pt according to thermal mt
  distribution with T 300 MeV in this way the
  total charm cross section is conserved
• fragmentation to D mesons via PYTHIA string model

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and of D mesons
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RAA with Quenching
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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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Summary

• QCD energy loss is a promising tool to
  investigate
• quark-gluon matter at the LHC
• depends on medium density and on path length L
• is expected to be different for
• (light) quarks
• gluons
• heavy quarks
• One of the most advanced models considered,
  adapted and applied to ALICE case, for D mesons
• Exclusive reconstruction of D0 mesons in ALICE
  allows to carry out comparative quenching studies
  of massless and massive probes

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Back-up Slides
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Create the Little Bang in the lab

• In high-energy heavy-ion collisions large energy
  densities (gt 23 GeV/fm3) are reached over large
  volumes (gt 1000 fm3)
• Evidence for the QGP formation collected by the 7
  experiments of CERN-SPS programme (up to Pb-Pb,
  )
• Now BNL-RHIC is confirming/extending these
  results at
• Next step LHC with Pb-Pb _at_
• large production of hard partons and
  heavy quarks
• excellent tools to study properties
  of QGP

QCD phase transition
confined hadronic matter
deconfined quark-gluon plasma
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Exclusive charm reconstruction
D0 ? K-? decay allows the exclusive
reconstruction of open charm mesons and a direct
measurement of the c quark pt distribution
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Signal generation

• Charm cross section from NLO pQCD (MNR program),
  average of results given by MRST and CTEQ5M PDFs
  (with EKS98 in Pb-Pb)
• Signal generated using PYTHIA, tuned to reproduce
  pt distr. given by NLO pQCD
• Also the contribution from beauty was included

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Interaction vertex reconstruction

• Position of the beam in (x,y) given by the
  machine with very high precision (stable for a
  long time)
• Nominal size of the beam
• s 15 mm in Pb-Pb
• s 15 mm in pp (L 1031 cm-2 s-1)
• s ? 150 mm in pp (if L is reduced at ALICE IP to
  1029 cm-2 s-1)
• In pp the vertex position has to be reconstructed
  in 3D using tracks

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Fast Simulation Techniques

• The selection strategy of D0 mesons has to be
  optimized using 104-105 events for Pb-Pb and
  106-107 for pp
• 1 Fully simulated Pb-Pb event takes 1.2 GB and
  12h!
• A number of fast simulation techniques were used
  in order to achieve these statistics
• fast response of the ITS (fast points, reduce
  ITS-time by factor 25)
• TPC tracking parameterization, specifically
  developed for charm and beauty studies (reduces
  total time by factor 40)
• Checks were done that these approximations dont
  affect track resolutions (efficiencies were
  corrected)

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D0? K-p Selection of D0 candidates

• Main selection displaced-vertex selection
• pair of tracks with large impact parameters
• good pointing of reconstructed D0 momentum to
  the primary vertex

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Displaced vertex selection
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Results for Pb-Pb
(K,?) Invariant Mass distribution (pT
integrated) (corresponding to 107 Pb-Pb
events 1 month of ALICE)
S/event 0.0013 (1.3 ? 104 D0 in 1 month)
B/event 0.0116
Statistical Significance of
the Signal
after background subtraction
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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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Comparison with pQCD for pp
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L distribution vs. constant L
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B? D a la CDF

• Impact parameter of D0 can be used to separate
  primary and secondary D0
• Background shape from
• side-bands in inv. mass
• Background subtracted

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Open Beauty in electron channel
Electron identification in TRD-TPC (pt gt 1 GeV/c)
impact parameter in ITS
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Open Beauty in electron channel

• B electron identification
• cut on pt reject low-pt backgrounds
  (conversions, dalitz of light mesons)
• cut on impact parameter reject background from D
  mesons