5.5 TeV
"
208Pb
"
"
208Pb
LHC
Hard (Heavy) QCD probes
in heavy-ion collisions at the LHC:
ALICE performance
Andrea Dainese,University of Padova
for the ALICE Collaboration
XXXIXth Rencontres de Moriond - QCD, March 28 - April 4, 2004
Andrea Dainese
1
Go for deep deconfinement at LHC
Next step in the “quest for QGP” …
LHC: factor 30 jump in s w.r.t. RHIC
much larger initial temperature
SPS
17 GeV
RHIC
200 GeV
LHC
5.5 TeV
initial T
~ 200 MeV
~ 300 MeV
> 600 MeV
volume
103 fm3
104 fm3
105 fm3
life-time
< 2 fm/c
2-4 fm/c
> 10 fm/c
study of hotter, bigger, longer-living ‘drops’ of QGP
‘Deep de-confinement’  closer to ‘ideal’ QGP
 easier comp. with theory
(lattice)
XXXIXth Rencontres de Moriond - QCD, March 28 - April 4, 2004
Andrea Dainese
2
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 < 1/2mc ~ 0.1 fm/c << tQGP ~ 5–10 fm/c)
Pb
c
Pb
Q
medium formed in
the collision
c
time
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
XXXIXth Rencontres de Moriond - QCD, March 28 - April 4, 2004
Andrea Dainese
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Parton Energy Loss
Due to medium-induced gluon emission
path length L
hard
parton
QGP
QCD process:
emitted gluon itself@ LHC
radiates  DE  L2
cold
matter
Average energy loss (BDMPS model):
2
ˆ
DE   s CR q L
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 [arXiv:hep-ph/0302184].
XXXIXth Rencontres de Moriond - QCD, March 28 - April 4, 2004
Andrea Dainese
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Lower Loss for Heavy Quarks?
Heavy quarks with momenta < 20–30 GeV/c  v << c
In vaccum, gluons radiation suppressed at Q < 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
D mesons quenching reduced
Ratio D/hadrons (or D/p0) enhanced and
sensitive to medium properties
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 [arXiv:hep-ph/0106202].
N.Armesto, C.A.Salgado and U.A.Wiedemann [arXiv:hep-ph/0312106].
XXXIXth Rencontres de Moriond - QCD, March 28 - April 4, 2004
Andrea Dainese
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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)
dN AA / dpt
1

Nuclear modification factor: RAA ( pt ) 
N coll dN pp / dpt
see talk by D. d’Enterria
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)
XXXIXth Rencontres de Moriond - QCD, March 28 - April 4, 2004
Andrea Dainese
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The ALICE Detector
|h| < 0.9
TPC + silicon tracker
g, e, p, K, p identification
XXXIXth Rencontres de Moriond - QCD, March 28 - April 4, 2004
2.5 < h < 4
muons
Andrea Dainese
7
Exclusive charm in ALICE: D0  K-p+
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 tracks with large impact parameters
good pointing of reconstructed D0 momentum to the primary vertex
Invariant mass analysis to “count” D0
XXXIXth Rencontres de Moriond - QCD, March 28 - April 4, 2004
Andrea Dainese
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Results. Example: Pb-Pb pt-integrated
(K,p) Invariant Mass distribution (pt –integrated)
(corresponding to 107 central Pb-Pb events ~ 1 month run)
S / B  10%
Statistical Significance of the Signal:
S / S  B  40
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) [arXiv:nucl-ex/0311004]
XXXIXth Rencontres de Moriond - QCD, March 28 - April 4, 2004
Andrea Dainese
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Sensitivity on RAA for D0 mesons
Low 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)
XXXIXth Rencontres de Moriond - QCD, March 28 - April 4, 2004
Andrea Dainese
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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 ~ 0.2–0.3 (~ RHIC)
ˆq  4 GeV 2 /fm
C.A.Salgado and U.A.Wiedemann, Phys. Rev. D68 (2003) 014008 [arXiv:hep-ph/0302184].
XXXIXth Rencontres de Moriond - QCD, March 28 - April 4, 2004
Andrea Dainese
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Average relative energy loss
QGP
@ LHC
cold
matter
XXXIXth Rencontres de Moriond - QCD, March 28 - April 4, 2004
Andrea Dainese
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RAA with Quenching
RAA ~ 0.4–0.5
increasing at high pt
RAA ~ 0.7–0.8
decreasing at high pt
A.D. Eur. Phys. J. C, in press [arXiv:nucl-ex/0312005].
XXXIXth Rencontres de Moriond - QCD, March 28 - April 4, 2004
Andrea Dainese
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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 D/h ratio:
D
=R
RAA
almost all systematic errors of both Pb-PbRand
cancel
D/h pp
AA /out!
h
RD/h ~ 2–3
in hot QGP
sensitive to
medium density
XXXIXth Rencontres de Moriond - QCD, March 28 - April 4, 2004
Andrea Dainese
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D/hadrons ratio (2)
hadron = z p parton
parton)’ = p parton – DE
p
(p
t
t
t
t
RD/h is enhanced only by the dead-cone
effect
hadron)’ = p hadron – z DE
(p
t
t
Enhancement due to different
quark/gluon loss not seen
It is compensated by the harder fragmentation of charm
Energy loss observed in RAA is not DE but zDE
zcD  0.8; zgluonhadron  0.4 (for pt > 5 GeV/c)
DEc = DEgluon/2.25
(w/o dead cone)
zcD DEc  0.9 zgluonhadron DEgluon
Without dead cone, RAAD  RAAh
XXXIXth Rencontres de Moriond - QCD, March 28 - April 4, 2004
Andrea Dainese
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Open Beauty in electron channel
Inclusive B  e + X:
electron ID + cut on its pt & on its impact parameter d0
S/(S+B)
S per 107 Pb-Pb events
pt > 2 GeV/c, d0 > 180 mm:
50,000 electrons with S/(S+B) = 90 %
XXXIXth Rencontres de Moriond - QCD, March 28 - April 4, 2004
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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 < pt < 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
XXXIXth Rencontres de Moriond - QCD, March 28 - April 4, 2004
Andrea Dainese
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BACK-UP SLIDES
XXXIXth Rencontres de Moriond - QCD, March 28 - April 4, 2004
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Jet Rates at the LHC
# Jets in |h|<0.5
with Et > Etmin per
central Pb-Pb event
1/event
104/month (year)
use High Level Trigger
XXXIXth Rencontres de Moriond - QCD, March 28 - April 4, 2004
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Jet Quenching Studies (1)
High-pt suppression: RAA up to ~ 40–50 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.
XXXIXth Rencontres de Moriond - QCD, March 28 - April 4, 2004
Andrea Dainese
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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 < 1–2 GeV/c is
needed
XXXIXth Rencontres de Moriond - QCD, March 28 - April 4, 2004
Andrea Dainese
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Relevance of jet energy
The Jet shape for a 50
GeV and a 100 GeV quarklead 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.
XXXIXth Rencontres de Moriond - QCD, March 28 - April 4, 2004
40-60
60-80
200-220
Andrea Dainese
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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 <
1 GeV are essential.
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.
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 <
0.7 and dN/dh = 4000.
XXXIXth Rencontres de Moriond - QCD, March 28 - April 4, 2004
Andrea Dainese
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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 < 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.
XXXIXth Rencontres de Moriond - QCD, March 28 - April 4, 2004
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High pt
Estimates of upper limit for 1 year ALICE run:
107 central events Pb-Pb
109 min. bias events pp
~103 tracks with pt > 50 GeV/c
~103 tracks with pt > 40 GeV/c
Identified particles:
g
S/B  1
5.5 L/event
p0
L Yield
Entries
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
Reconstructed
pt spectrum
M(pp)
2) - April 4, 2004
(GeV/c28
XXXIXth Rencontres de Moriond - QCD, March
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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
medium
probe IN
(known from
pp, pA
+ pQCD)
IN vs. OUT
probe
OUT
XXXIXth Rencontres de Moriond - QCD, March 28 - April 4, 2004
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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
2
B  dNch / dy 
dN ch / dy  2500
in central Pb-Pb at LHC
Simulations performed using
dN ch / dy  6000
huge combinatorial background!
need excellent detector response and good selection strategy
XXXIXth Rencontres de Moriond - QCD, March 28 - April 4, 2004
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ALICE Barrel
|h|<0.9:
B = 0.4 T
TOF
TPC
ITS with:
- Si pixels
- Si drifts
- Si strips
Two layers:
r = 4 cm
r = 7 cm
XXXIXth Rencontres de Moriond - QCD, March 28 - April 4, 2004
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Tracking
Tracking efficiency ~70% with dNch/dy=6000
pions
kaons
pt resolution = 1% at 1 GeV/c
D0 invariant mass resolution:
 (M )
M
1  ( p)

 0.7%,  ( M )  13 MeV
2 p
XXXIXth Rencontres de Moriond - QCD, March 28 - April 4, 2004
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Impact parameter resolution
Crucial for heavy-quark ID
Systematic study of resolution was carried out
< 60 mm
for pt > 1 GeV/c
XXXIXth Rencontres de Moriond - QCD, March 28 - April 4, 2004
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TOF PID
TOF
Pb-Pb, dNch/dy=6000
Optimization for hadronic charm
decays was studied:
minimize probability to tag K as p
XXXIXth Rencontres de Moriond - QCD, March 28 - April 4, 2004
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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
d0K  d0p <<0
XXXIXth Rencontres de Moriond - QCD, March 28 - April 4, 2004
cos pointing 1
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D0 K-p+: Results
Pb-Pb, 5.5S/B
TeV
107 events
initial
(M3)
S/evt
final
(M1)
S/B
Significance
pp,
14 TeV
9 events
10
final
S/S+B
(M1)
1–14 GeV/c
Pb-Pb
pp
5  10-6
2  10-3
1.3  10-3
1.9  10-5
(M1)
0–14 GeV/c
37
11 %
(for 107 evts,
~1 month)
44
11 %
(for 109 evts,
~1 year)
Note: with dNch/dy = 3000, S/B larger by  4 and
significance larger by  2
XXXIXth Rencontres de Moriond - QCD, March 28 - April 4, 2004
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D0 K-p+: d2(D0)/dptdy and d(D0)/dy
inner bars: statistical
outer bars: systematic
(D00)/dy
)/dy for
for
dd(D
|y| << 11 and
and pptt >> 01 GeV/c
|y|
(65% of (pt > 0))
statisticalerror
error ==37%
%
statistical
systematicerror
error ==14
19%
%
systematic
frombb
%
from
==89%
MCcorrection
correction ==10%
10%
MC
B.R.
2.4%
%
B.R.
==2.4
inel AA to NN ==513
from
%%
XXXIXth Rencontres de Moriond - QCD, March 28 - April 4, 2004
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Initial-state effects: Shadowing
Bjorken-x: fraction of the momentum of the proton ( s / 2 )
carried by the parton entering the hard scattering
q
x1 s / 2
x2 s / 2
x  Q / s  M qq / s
Q
q
3
4
x
~
10

10
At the LHC
Pb ion @ LHC ~ 105-106 partons
(mainly gluons)
charm
gPb
(x)/gp(x)
“they are so close that they fuse”
xa
Shadowing:
xa+xb • reduces initial hard yield at low pt
xb
• scales trivially from pA to AA
XXXIXth Rencontres de Moriond - QCD, March 28 - April 4, 2004
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BDMPS model
qˆ  q
2
medium
vacuum

medium
 c  1 2 qˆ L2
dI 2 s C R


d
p

c
2
kt
c
2 s C R
dI 2 2 s C R
2
DE   d 

c 
qˆ L
d
p
p
0
XXXIXth Rencontres de Moriond - QCD, March 28 - April 4, 2004
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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)
XXXIXth Rencontres de Moriond - QCD, March 28 - April 4, 2004
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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%!): 
bb
error of ~8% assuming present uncertainty (~80%) on  @ LHC
Monte Carlo corrections: ~10%
B.R. D0 Kp: 2.4%
inel
0

extrapolation from N(D )/event to d(D0)/dy:
 pp: error on  inel (~5%, will be measured by TOTEM)
 Pb-Pb: error on centrality selection (~8%) + error on TAB (~10%)
bb
pp
Pb-Pb
XXXIXth Rencontres de Moriond - QCD, March 28 - April 4, 2004
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Comparison with pQCD for pp
XXXIXth Rencontres de Moriond - QCD, March 28 - April 4, 2004
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Interpolation pp 14  5.5 TeV
Necessary to compare Pb-Pb and pp by RAA
mc ,
mR mF
, , PDF set
mt mt
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 @ 14 TeV
to 5.5 TeV
XXXIXth Rencontres de Moriond - QCD, March 28 - April 4, 2004
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Effect of shadowing
XXXIXth Rencontres de Moriond - QCD, March 28 - April 4, 2004
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Transport coefficient choice
Require for LHC suppression of hadrons as observed at
RHIC: RAA ~ 0.2-0.3 for 4<pt<10 GeV/c
pt 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 – DE
(independent) fragmentation with KKP LO F.F.
RAA = (pt distr. w/ quenching) / (pt distr. w/o quenching)
XXXIXth Rencontres de Moriond - QCD, March 28 - April 4, 2004
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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)
cc
 NN
[mb] shadowing
system
cc
N tot
dN ( D 0  Kp ) / dy
pp 14 TeV
11.2
1
0.16
0.0007
Pb-Pb 5.5 TeV (5% cent)
6.6
0.65
115
0.5
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/dy=6000 ! we expect ~2500 !); pp: PYTHIA;
MNR Program: M.L.Mangano, P.Nason and G.Ridolfi, Nucl. Phys. B373 (1992) 295.
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