Title: Charm Energy Loss at the LHC with ALICE nuclex0311004 Ph'D' Thesis nuclex0312005 subm' to EPJ
1Charm Energy Lossat the LHC with
ALICEnucl-ex/0311004 (Ph.D. Thesis)nucl-ex/0312
005 (subm. to EPJ)
- Andrea Dainese
- University of Padova
2Outline
- 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
3Hard 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
4Initial-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)
5Final-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
6Parton 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.
7Heavy 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.
8Dead-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)
9Experimental 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
10D0? 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
11D0? K-p Detection strategy with ALICE
12D0? 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.
13D0? 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 -
14D0? K-p Results
Note with dNch/dy 3000, S/B larger by ? 4 and
significance larger by ? 2
15D0? 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
16Interpolation 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
17Sensitivity on RAA for D0 mesons
18Energy-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.
19Transverse 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
20Energy-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
21Inclusion 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
22Average energy loss
- DE sampled according to P(DE) distribution
- pt pt DE kinematic constraint if DE gt pt,
pt 0
23Transport 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)
24Quenching 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
25 and of D mesons
26RAA with Quenching
27D/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!
28D/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
29Summary
- 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 -
30Back-up Slides
31Create 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
32Exclusive charm reconstruction
D0 ? K-? decay allows the exclusive
reconstruction of open charm mesons and a direct
measurement of the c quark pt distribution
33Signal 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
34Interaction 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
35Fast 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)
36D0? 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 -
37Displaced vertex selection
38Results 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
39What 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)
40Estimate 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)
41Comparison with pQCD for pp
42L distribution vs. constant L
43B? 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
44Open Beauty in electron channel
Electron identification in TRD-TPC (pt gt 1 GeV/c)
impact parameter in ITS
45Open 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