1. Andrea Dainese,University of Padova for the ALICE Collaboration
LHC
208Pb
5.5 TeV
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, Andrea Dainese

2. 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
> 600 MeV
volume
103 fm3
104 fm3
105 fm3
life-time
< 2 fm/c
2-4 fm/c
> 10 fm/c
‘Deep de-confinement’
 closer to ‘ideal’ QGP
easier comp. with theory
(lattice)
XXXIXth Rencontres de Moriond - QCD, March 28 - April 4, Andrea Dainese

3. 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)
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 Q Pb c
medium formed in
the collision
time
XXXIXth Rencontres de Moriond - QCD, March 28 - April 4, Andrea Dainese

4. Parton Energy Loss Due to medium-induced gluon emission QGP
cold
matter
QGP
@ LHC
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) [arXiv:hep-ph/ ].
XXXIXth Rencontres de Moriond - QCD, March 28 - April 4, Andrea Dainese

5. 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/ ].
N.Armesto, C.A.Salgado and U.A.Wiedemann [arXiv:hep-ph/ ].
XXXIXth Rencontres de Moriond - QCD, March 28 - April 4, Andrea Dainese

6. 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. d’Enterria
XXXIXth Rencontres de Moriond - QCD, March 28 - April 4, Andrea Dainese

7. The ALICE Detector |h| < 0.9 TPC + silicon tracker
g, e, p, K, p identification
2.5 < h < 4
muons
XXXIXth Rencontres de Moriond - QCD, March 28 - April 4, Andrea Dainese

8. 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, Andrea Dainese

9. 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) [arXiv:nucl-ex/ ]
XXXIXth Rencontres de Moriond - QCD, March 28 - April 4, Andrea Dainese

10. 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, Andrea Dainese

11. 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)
C.A.Salgado and U.A.Wiedemann, Phys. Rev. D68 (2003) [arXiv:hep-ph/ ].
XXXIXth Rencontres de Moriond - QCD, March 28 - April 4, Andrea Dainese

12. Average relative energy loss
cold
matter
QGP
@ LHC
XXXIXth Rencontres de Moriond - QCD, March 28 - April 4, Andrea Dainese

13. RAA with Quenching RAA ~ 0.4–0.5 RAA ~ 0.7–0.8 increasing at high pt
decreasing at high pt
A.D. Eur. Phys. J. C, in press [arXiv:nucl-ex/ ].
XXXIXth Rencontres de Moriond - QCD, March 28 - April 4, Andrea Dainese

14. D/hadrons ratio (1) D/h ratio: RD/h = RAAD / RAAh RD/h ~ 2–3
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!
RD/h ~ 2–3
in hot QGP
sensitive to
medium density
D/h ratio:
RD/h = RAAD / RAAh
XXXIXth Rencontres de Moriond - QCD, March 28 - April 4, Andrea Dainese

15. D/hadrons ratio (2) RD/h is enhanced only by the dead-cone effect
pthadron = z ptparton (ptparton)’ = ptparton – DE
(pthadron)’ = pthadron – z DE
Energy loss observed in RAA is not DE but zDE
zcD  0.8; zgluonhadron  (for pt > 5 GeV/c)
DEc = DEgluon/ (w/o dead cone)
zcD DEc  0.9 zgluonhadron DEgluon
Without dead cone, RAAD  RAAh
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
XXXIXth Rencontres de Moriond - QCD, March 28 - April 4, Andrea Dainese

16. 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, Andrea Dainese

17. 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, Andrea Dainese

18. BACK-UP SLIDES
XXXIXth Rencontres de Moriond - QCD, March 28 - April 4, Andrea Dainese

19. 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, Andrea Dainese

20. 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, Andrea Dainese

21. 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, Andrea Dainese

22. 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
XXXIXth Rencontres de Moriond - QCD, March 28 - April 4, Andrea Dainese

23. 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.
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.
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.
XXXIXth Rencontres de Moriond - QCD, March 28 - April 4, Andrea Dainese

24. 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 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, Andrea Dainese

25. g p0 High pt Estimates of upper limit for 1 year ALICE run:
107 central events Pb-Pb ~103 tracks with pt > 50 GeV/c
109 min. bias events pp ~103 tracks with pt > 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 g p0
S/B  1
5.5 L/event
Reconstructed
pt spectrum
M(pp) (GeV/c2)
Entries
L Yield
XXXIXth Rencontres de Moriond - QCD, March 28 - April 4, Andrea Dainese

26. 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)
probe
OUT
IN vs. OUT
XXXIXth Rencontres de Moriond - QCD, March 28 - April 4, Andrea Dainese

27. 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
XXXIXth Rencontres de Moriond - QCD, March 28 - April 4, Andrea Dainese

28. ALICE Barrel rf: 50 mm |h|<0.9: z: 425 mm B = 0.4 T TOF PIXEL CELL
Two layers:
r = 4 cm
r = 7 cm
 9.8 M
|h|<0.9:
B = 0.4 T
TOF
TPC
ITS with:
- Si pixels
- Si drifts
- Si strips
XXXIXth Rencontres de Moriond - QCD, March 28 - April 4, Andrea Dainese

29. Tracking Tracking efficiency ~70% with dNch/dy=6000
pions
kaons
pt resolution = 1% at 1 GeV/c
D0 invariant mass resolution:
XXXIXth Rencontres de Moriond - QCD, March 28 - April 4, Andrea Dainese

30. 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, Andrea Dainese

31. TOF PID TOF Optimization for hadronic charm decays was studied:
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, Andrea Dainese

32. 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  d0 <<0
cos pointing 1
XXXIXth Rencontres de Moriond - QCD, March 28 - April 4, Andrea Dainese

33. D0 K-p+: Results Pb-Pb pp 0–14 GeV/c 1–14 GeV/c S/B initial (M3s)
S/evt
final
(M1s)
Significance
S/S+B
Pb-Pb
5  10-6
1.3  10-3
11 % 37
(for 107 evts,
~1 month) pp
2  10-3
1.9  10-5 44
(for 109 evts,
~1 year)
0–14 GeV/c
1–14 GeV/c
Pb-Pb, 5.5 TeV
107 events
pp, 14 TeV
109 events
Note: with dNch/dy = 3000, S/B larger by  4 and significance larger by  2
XXXIXth Rencontres de Moriond - QCD, March 28 - April 4, Andrea Dainese

34. D0 K-p+: d2s(D0)/dptdy and ds(D0)/dy
ds(D0)/dy for
|y| < 1 and pt > 0
statistical error = 3 %
systematic error = 14 %
from b = 8 %
MC correction = 10%
B.R = 2.4 %
sinel = 5 %
inner bars: statistical
outer bars: systematic
ds(D0)/dy for
|y| < 1 and pt > 1 GeV/c
(65% of s(pt > 0))
statistical error = 7 %
systematic error = 19 %
from b = 9 %
MC correction = 10%
B.R = 2.4 %
from AA to NN = 13 %
XXXIXth Rencontres de Moriond - QCD, March 28 - April 4, Andrea Dainese

35. 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 LHC ~ partons
(mainly gluons) q Q xa xb
xa+xb
gPb(x)/gp(x)
Shadowing:
reduces initial hard yield at low pt
scales trivially from pA to AA
“they are so close that they fuse”
charm
XXXIXth Rencontres de Moriond - QCD, March 28 - April 4, Andrea Dainese

36. BDMPS model w vacuum medium kt
XXXIXth Rencontres de Moriond - QCD, March 28 - April 4, Andrea Dainese

37. 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, Andrea Dainese

38. Estimate of the errors pp Pb-Pb
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 @ 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%)
XXXIXth Rencontres de Moriond - QCD, March 28 - April 4, Andrea Dainese

39. Comparison with pQCD for pp
XXXIXth Rencontres de Moriond - QCD, March 28 - April 4, Andrea Dainese

40. 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 14 TeV
to 5.5 TeV
XXXIXth Rencontres de Moriond - QCD, March 28 - April 4, Andrea Dainese

41. Effect of shadowing
XXXIXth Rencontres de Moriond - QCD, March 28 - April 4, Andrea Dainese

42. Transport coefficient choice
Require for LHC suppression of hadrons as observed at RHIC: RAA ~ 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, Andrea Dainese

43. D0 K-p+: Signal and background
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/dy=6000 ! 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.
XXXIXth Rencontres de Moriond - QCD, March 28 - April 4, Andrea Dainese