Charm production and energy loss in nucleus-nucleus collisions with ALICE

1
Charm production and energy loss in
nucleus-nucleus collisions with ALICE

• Andrea Dainese
• Università degli Studi di Padova
• Dottorato di Ricerca in Fisica Ciclo XVI

2
Outline

• Introduction physics of hot and dense QCD matter
  
• Heavy-ion physics at the LHC
• Parton energy loss in the quark-gluon plasma
  (QGP)
• the case of heavy quarks (charm)
• Experimental feasibility for exclusive D meson
  reconstruction in Pb-Pb (and pp) collisions

Can we measure D meson pt distribution in
Pb-Pb? energy loss of c quarks in the QGP?
3
Physics of hot and dense QCD matter
Is it possible to de-confine quarks?

• QCD phase diagram
• hadronic matter exists in different states
• at high energy density (high temperature and/or
  high density) nuclear matter undergoes a phase
  transition to a deconfined Quark-Gluon Plasma
  (QGP)

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Physics of hot and dense QCD matter
Lattice QCD (mB 0) estimates the phase
transition at Tc 170 MeV and ec 1
GeV/fm3 5 enucleus
3 massless flavours
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Physics of hot and dense QCD matter
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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)
• QGP evidence at CERN-SPS (up to Pb-Pb,
  )
• BNL-RHIC extending these results at
• Next step LHC with Pb-Pb _at_
• deep deconfinement ideal gas of
  gluons and quarks
• large production of hard partons and
  heavy quarks
• excellent tools to study properties
  of QGP

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Deep deconfinement at the LHC

• 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
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
  (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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Hard partons probe the medium

• Partons travel 5 fm in the high colour-density
  medium
• Energy loss by gluon bremsstrahlung
• modifies momentum distributions
• 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.Dokschitzer and D.E.Kharzeev dead cone
  implies lower energy loss

Yu.L.Dokshitzer and D.E.Kharzeev, Phys. Lett.
B519 (2001) 199 arXivhep-ph/0106202.
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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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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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D0? K-p Detection strategy with ALICE
Time Projection Chamber Track
Finding Measurement of Momenta
Magnetic field (B 0.4 Tesla in this
study)
Inner Tracking System Measurement of Impact
Parameters Momentum Res. Improvement
Time Of Flight Particle Identification (K/p
separation)
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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
Crucial for heavy-quark ID Systematic study of
resolution was carried out
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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

• 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
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 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 on RAA for D0 mesons
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Energy-loss simulation

• Energy loss
  simulated using one of the most
• 
  advanced models
• My contributions
• path length of partons in the medium
• dead-cone for heavy quarks
• estimate transport coefficient for
• central Pb-Pb collisions at LHC,
• on the basis of the hadron suppression
• observed at RHIC

C.A.Salgado and U.A.Wiedemann, Phys. Rev. D68
(2003) 014008 arXivhep-ph/0302184.
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Average relative energy loss
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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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Summary
D0 mesons can be exclusively
reconstructed with the ALICE
detector despite the high-multiplicity
environment of central Pb-Pb coll.
This will allow to
measure charm production in 015 GeV/c with
statistical uncertainty better than
1015 systematic uncertainty better than 1520
study the mass and flavour dependence of QCD
energy loss
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Back-up Slides
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Heavy ion Physics at the LHC
What are the properties of deconfined matter?

• LHC factor 30 jump in w.r.t. RHIC

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Initial- and final-state effects

• In absence of nuclear effects
• Hard processes, and charm production in
  particular,
• allow to study and disentangle
• Initial-state (cold medium) effects, by comparing
  pA with pp and with pQCD predictions
• Nuclear modification of PDFs
• Final-state (hot medium) effects, by comparing
  AA, pA, pp (and pQCD)
• Partonic Energy Loss
• Thermal Charm Production (?)

at low pt
(also) at high pt
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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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BDMPS model
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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)

Dokshitzer
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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
channel D0 ? K-? c ? D ? e
c low pt acceptance 0 ? 2-3 GeV/c (electron ID)
c high pt acceptance 15 GeV/c (statistics) probably gt 15 GeV/c
S/B separation clean (inv. mass) ? non-trivial c/b disentangling
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ITS layers
Layer Detector radius cm resolution rf mm resolution z mm
1 Si Pixels 4 12 120
2 Si Pixels 7 12 120
3 Si Drifts 14.9 38 28
4 Si Drifts 23.8 38 28
5 Si Strips 39.1 20 830
6 Si Strips 43.6 20 830
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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

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
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Classification of the processes
Pair Creation (PC)
Flavour Excitation (FE)
Gluon Splitting (GS)

• Hard scattering LO graph
• Processes classified w.r.t. HVQs in hard
  scattering final state
• No double counting because hard scattering is the
  process with largest virtuality

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Hard cross section in pQCD
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Extrapolation to AA

• Binary scaling
• Glauber model of collision geometry
• Shadowing a nucleon in the Pb nucleus contains
  less small-x partons than a free nucleon

Pb-Pb, b0 at LHC
b
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Tuning of PYTHIA parameters

• Comparison at the bare quark level
• Heavy Quarks in PYTHIA
• MSEL 4/5 Leading Order processes
• settings corresponding to MNR
• good agreement with MNR LO
• MSEL 1 initial and final state Parton Shower
  processes describe contributions above LO
• agreement with MNR NLO less good
• parton shower processes ¹ NLO processes
• massless Matrix Elements! cross section diverges
  at pthard 0
• Tuning of parameters less physics inspired
• Main parameter tuned min. pthard (2.1 GeV/c for
  c, 2.75 GeV/c for b)

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Charm NLO (Pb-Pb)/Ncoll 5.5 TeV
pair creation flavour excitation gluon
splitting TOTAL
MNR
Pythia
Include shadowing
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Heavy quark fragmentation
M. Cacciari, CTEQ School 03
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Fragmentation in PYTHIA

• Use default parameters
• Lund string fragmentation model
• longitudinal fragmentation
• Lund symmetric fragmentation function
• Modified to account for harder spectra in HVQ
  fragmentation
• transverse momentum pick-up
• s(px) s(py) 230 MeV/c

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Effect of fragm. on spectra
bare quark meson
Charm
Beauty

• Average pt-reduction
• 25 for charm
• 15 for beauty

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3D reconstruction with tracks

• Track reconstruction in TPCITS
• track seeding uses the position of the primary
  vertex
• (x, y) from beam position (resolution 150 mm)
• z from pixels information (resolution 150 mm)
• Vertex reconstruction in 2 steps
• VERTEX FINDING using DCA for track pairs
• VERTEX FITTING
• give optimal estimate of the position of the
  vertex
• give vertex covariance matrix
• give a c2

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Expected resolutions

• Average rec. tracks 7 (average on events
  with gt 1)
• Average pT of rec. tracks 0.6 GeV/c
• Resolutions of track position parameters _at_ 0.6
  GeV/c
• s(d0(rf)) ? 100 mm d0(rf) is ? to the
  track!
• s(d0(z)) ? 240 mm

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Vertex Finding Algorithm

• Aim get a first estimate of the vertex position
  in (x,y) to be used as a starting point for
  vertex fitter
• independent of beam size
• improved w.r.t. beam size (hopefully)
• Method
• propagate tracks to vertex nominal position
• calculate DCA (in space) for each possible pair
  of tracks (using straight line approximation)
• get estimate of xvtx and yvtx from mean of
  results from all pairs

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Vertex Fitting Algorithm

• Tracks are propagated to the point given by the
  vertex finder (at the moment nominal position
  used)
• A c2 is written as the sum of the single track
  c2s w.r.t. a generic vertex position rvtx
• where Wi is track covariance matrix in
  global ref. frame
• The solution that minimizes this c2 is analytic

covariance matrix
vertex
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Tuning of the algorithm

• Criterion used to reject mismeasured and
  secondary tracks from the fit cut on the maximum
  contribution to the c2
• ci2 lt c2max
• if c2max is too low too many tracks are rejected
  and we loose resolution
• if c2max is too high bad or secondary tracks
  enter the fit and we loose resolution
• This cut is tuned, as a function of event
  multiplicity, in order to optimize the resolution
  

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Results resolutions
s(y) 55 mm
s(x) 55 mm
s(z) 90 mm
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Results pulls
The covariance matrix of the vertex describes
correctly the resolutions
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Resolutions VS multiplicity
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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)

57
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
61
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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Effect of shadowing
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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

66
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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L distribution vs. constant L
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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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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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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