Hard (Heavy) QCD probes in heavy-ion collisions at the LHC: ALICE performance

1
Hard (Heavy) QCD probes in heavy-ion collisions
at the LHCALICE performance
5.5 TeV

• Andrea Dainese,University of Padova
• for the ALICE Collaboration

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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
• Deviations from such predictions are due to the
  medium

medium formed in the collision
time
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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
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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Lower Loss for Heavy Quarks?

• Heavy quarks with momenta lt 2030 GeV/c ? v ltlt
  c
• In vaccum, gluons radiation suppressed at Q lt
  mQ/EQ
  
• dead cone effect
• Dead cone implies lower energy loss
  (Dokshitzer-Kharzeev, 2001)
• Recent detailed calculation confirms this
  qualitative feature (Armesto-Salgado-Wiedemann,
  2003) see talk by N.Armesto

Yu.L.Dokshitzer, V.A.Khoze and S.I.Troyan, J.
Phys. G17 (1991) 1602. Yu.L.Dokshitzer and
D.E.Kharzeev, Phys. Lett. B519 (2001) 199
arXivhep-ph/0106202. N.Armesto, C.A.Salgado
and U.A.Wiedemann arXivhep-ph/0312106.
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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
• Important step forward at the LHC
• Compare quenching of massless and
  massive probes
• Study jets (or jets)
• jets - (charged) particle correlations (RHIC
  tells us they can tell a lot!)
• jets - with calorimetry (CMS/ATLAS speciality see
  talk by C.Roland)

see talk by D. dEnterria
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The ALICE Detector
h lt 0.9 TPC silicon tracker g, e, p, K, p
identification
2.5 lt h lt 4 muons
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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 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. Example Pb-Pb pt-integrated
(K,?) Invariant Mass distribution (pt
integrated) (corresponding to 107 central
Pb-Pb events 1 month run)
Statistical
Significance of the Signal

after background subtraction
analysis for Pb-Pb and pp done in bins of
pt and main errors estimated
Details on selection strategy in N.Carrer, A.D.
and R.Turrisi, J. Phys. G29 (2003) 575.
A.D. PhD
thesis (2003) arXivnucl-ex/0311004
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Sensitivity on RAA for D0 mesons
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Energy-loss simulation

• Energy loss simulated using BDMPS quenching
  weights by Salgado and Wiedemann
• Plus
• realistic path lengths
• of partons in the medium
• dead-cone correction for heavy quarks
• transport coefficient for central Pb-Pb
  collisions at LHC estimated requiring hadrons RAA
  0.20.3 ( 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
A.D. Eur. Phys. J. C, in press arXivnucl-ex/0312
005.
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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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Open Beauty in electron channel

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

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Summary

• LHC study properties for deconfined QCD matter
  via hard probes and their quenching
• ALICE good potential in the hard probes sector
• Outstanding example ALICE can exclusively
  reconstruct D0 mesons in Pb-Pb collisions with
  dNch/dy 6000!
• measure charm production in 0 lt pt lt 15 GeV/c (at
  least)
• study the mass and flavour dependence of QCD
  energy loss
• And more
• beauty detection via semi-electronic decays
• jet studies with charged tracks

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BACK-UP SLIDES
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Jet Rates at the LHC
1/event
104/month (year)
use High Level Trigger
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Jet Quenching Studies (1)

• High-pt suppression RAA up to 4050 GeV/c in 1
  year
• Azimuthal correlation studies
• What about identified jets?
• Quite unclear
• Energy plays a minor role as jet observable
• Small effect of quenching on the energy inside
• a given cone (Salgado and Wiedemann (2003))

r(R) Energy contained in a cone of size
R normalized to the energy contained in a cone of
size 1.
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Jet Quenching Studies (2)

• Simple MC quenching model (which reproduces
  results of Salgado and Wiedemann)

Most interesting signals from the radiated energy
are at high R and low pt Very challenging due
to the large background from the underlying
event Good tracking in the region pt lt 12 GeV/c
is needed
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Relevance of jet energy
The Jet shape for a 50 GeV and a 100 GeV
quark-lead jet which fragments in the vacuum (red
line) or in a dense medium (blue line).

• Energy plays a minor role as a jet observable
• Small effect of jet quenching on the energy
  inside a given cone (S.A. Salgado and U.A.
  Wiedemann (2003)).
• Jet structure, i.e. normalized longitudinal
  momentum (z) and momentum perpendicular to jet
  axis (jT) of particles belonging to the jet show
  small energy dependence.

40-60
60-80
200-220
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Relevance of low-pT
Different quenching scenarios result in
differences in the low-pT region of jet
particles. Tracking capabilities for pT lt 1 GeV
are essential. The large background from
particles of the underlying event make this
measurement challenging. The figures on the right
show S/B and significance as a function of pT for
DR lt 0.7 and dN/dh 4000.
Simple quenching model The energy loss of a 100
GeV jets is simulated by reducing the energy of
the jet by 20 and replacing the missing energy
by 1 x 20 GeV gluon 2 x 10 GeV gluons 4 x 5
GeV gluons Jets have been simulated with Pythia.
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Jet shapes
Our simple MC quenching model reproduces
qualitatively the results of Salgado and
Wiedemann. The figure on the left shows that for
quenching measurements with cone sizes between
0.3-0.4 good tracking in the region pT lt 2 GeV is
needed. The most interesting signals from the
radiated energy are at high R and low pT which is
very challenging due to the large background from
the underlying event.
r(r) Energy contained in a cone of size
r normalized to the energy contained in a cone of
size 1.
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High pt

• Estimates of upper limit for 1 year ALICE run
• 107 central events Pb-Pb 103 tracks with pt
  gt 50 GeV/c
• 109 min. bias events pp 103 tracks with
  pt gt 40 GeV/c
• Identified particles
• D0 up to 15 GeV/c, B 10 GeV/c (under study)
• L up to 12 GeV/c
• g/ p0 separation up to 100 GeV/c with PHOS

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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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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

• 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 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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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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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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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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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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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.