Heavy Quark Physics in Heavy-Ion collisions: experiments, phenomenology and theory
ECT* - European Centre for Theoretical Studies in Nuclear Physics and Related Areas
Open-charm production measurements in p-Pb and Pb-Pb collisions with ALICE at the LHC
Paola Pagano
Salerno University & INFN, Italy on behalf of the ALICE Collaboration
Outline
Physics motivations
Detector layout
Analysis technique
Measurements in p-Pb and Pb-Pb collisions
Conclusions and perspectives
2
Physics Motivation: why heavy flavors?
Because of their large mass
(mc ≈1.5 GeV/c2, mb ≈ 5 GeV/c2)
charm and beauty quarks are produced
at the early stages of the collisions
in hard parton scattering processes
They interact with the QCD high-density
medium but flavor is conserved in this interaction
Destruction or creation in the medium is difficult
Heavy quarks experience the whole evolution of the system
3
Heavy flavors in p-Pb collisions
Test cold nuclear matter (CNM) effects:
Modification of Parton Distribution Functions
Shadowing K.J.Eskola et al., JHEP 0904(2009)65
Gluon saturation, Color Glass Condensate H.Fuji & Watanabe, NPA 915(2013)1
Energy loss in cold nuclear matter
I.Vitev at al., PRC 75(2007)064906
kT broadening
X.N.Wang, PRC 61 (2000)064910
reference for Pb-Pb collisions
4
Heavy flavors in Pb-Pb collisions
Study the interaction with the medium:
Parton energy loss depends on the parton color charge and mass,
in-medium energy density and temperature, and in-medium path length
Investigated through the Nuclear Modificaton Factor
Expect: ΔEg >ΔEu,d,s>ΔEcharm>ΔEbeauty
π
D
B
RAA(pT) < RAA(pT) < RAA(pT)
?
Dokshitzer & Kharzeev,PLB 519 (2001) 199
Adil & Vitev PLB 649 (2007)139
5
Heavy flavors in Pb-Pb collisions
Study the interaction with the medium:
Initial spatial anisotropy → azimuthally anisotropic momentum distribution
non-central collisions → anisotropy dominated by elliptic flow ν2
low pT: ν2 sensitive to the system collective expansion
high pT: ν2 sensitive to the path-length dependence
of in-medium parton energy loss
6
A Large Ion Collider Experiment
ALICE is well suited for HF measurements at the LHC
in a wide momentum and rapidity range
VZERO (event
characterization)
2.8<η<5.1, -3.7<η<-1.7
Magnetic Field
|η|<9, B=0.5 T
-4<η<-2.5, B<0.7 T
7
A Large Ion Collider Experiment
Central Barrel
D0 → K-π+ (cτ = 123 μm, BR = 3.88 ± 0.05 %)
D+ → K-π+π+ (cτ = 312 μm, BR = 9.13 ± 0.19%)
D*+→ D0π+ (strong decay , BR = 67.7 ±0.5 %)
Ds→φ π+ → K-K+π+ (cτ = 150 μm, BR = 2.28 ± 0.12%)
Covered in this talk
D mesons
|y|<0.5, pT>1GeV/c
TOF:π/K/p ID
TPC: tracking, π/K/p ID
ITS: vertexing, tracking
8
A Large Ion Collider Experiment
Central Barrel
HF decay electrons
|y|<0.8,pT>0.5 GeV/c
TOF:e ID
TPC: tracking, e ID
EMCAL: e ID, trigger
TRD: eID
ITS: vertexing, tracking
c, b → e + X
b → e+X
c, b → μ+ X
R.Averbeck’s talk
Muon Spectrometer
HF decay muons
2.5<y<4.0, pT>2 GeV/c
Absorbers
Muon CPCs: tracking
Muon RPCs: µ ID, trigger
9
D-meson reconstruction in ALICE: Analysis strategy
Reconstruction and selection of
secondary vertex topologies, that have
a significant separation from the
primary vertex
D-meson candidates selected applying
kinematical and topological cuts:
pT and impact parameter of single tracks,
cosine of pointing angle, distance
between primary and secondary vertex, …
10
D-meson reconstruction in ALICE: Analysis strategy
Particle identification (using TOF and TPC information) in order to
reduce the combinatorial background, especially at low pT
Int. J. Mod. Phys. A 29 (2014) 1430044
Int. J. Mod. Phys. A 29 (2014) 1430044
11
D-meson reconstruction in ALICE: Analysis strategy
Invariant mass analysis in order to extract raw yields in various pT intervals
D+
D0
p-Pb @5.02 TeV
D*+
Ds+
To calculate the cross section ,raw yields are corrected for acceptance x efficiency, as
12
well as for the feed-down from B decays to calculate the cross section.
Results
in p-Pb collisions @ 5.02 TeV
D mesons: pT-differential RpPb
0
D
•
•
D+
D0
D*+
D+
Ds+
No significant difference
between the RpPb of the four
D-meson species
RpPb consistent with unity in
1 < pT < 24 GeV/c
within the uncertainties
Phys.Rev.Lett. 113 (2014) 232301
Average D0,D+,D*+
Results described within uncertainties by theoretical
calculations that include initial-state effects:
NLO pQCD with EPS09 nuclear PDF
Nucl. Phys. B 373 (1992) 295 , JHEP 04 (2009)065
Color Glass Condensate arXiv:1308.1258
Model including energy loss in cold nuclear
matter, nuclear shadowing
and kT-broadening Phys. Rev. C 80 (2009) 054902
14
Nuclear Modification Factor RpPb
D meson
HF decay e
HF decay μ
(forward rapidity)
Phys.Rev.Lett. 113 (2014) 23, 232301
Similar trend for D mesons, electrons and muons from heavy-flavor decays
Good agreement with theoretical calculations that include initial-state
effects
No indication for large cold nuclear matter effects at high pT
15
Multiplicity dependence of open charm yields
D0
Self-normalized D-meson yields
increase with charged-particle
multiplicity
Similar trend observed
in pp and p-Pb collisions
16
D-meson QpPb
To investigate possible multiplicity dependent modification of the pT spectra
in p-Pb collisions w.r.t. pp collisions
Q
mult
pPb
( pT ) =
But several biases are present in the determination
mult
(dN pPb
/ dpT )i
N coll i dN pp / dpT
of <Ncoll> in p-Pb collisions: Multiplicity bias, Jet veto bias, Geometrical bias.
Bias depends on the estimator used for the multiplicity determination
V0A : < Ncoll> obtained from Glauber model.
ZN:< Ncoll> obtained from Hybrid approach
Multiplicity distribution fitted with Negative
Define event classes based on the energy deposited by
Pb-spectator neutrons in the ZDC
Binomial Distribution (NBD)-Glauber fit.
< Ncoll> obtained by scaling the minimum bias value
using the ratio of the multiplicities measured at midrapidity.
17
D-meson QpPb: results
D0,D*+
V0A
ZN
Average QpPb of D0 and D*+ show a residual bias with the V0A estimator
that is reduced when using the ZN one.
Using the ZN estimator no multiplicity dependent modification of D meson
production relative to pp collisions, is observed within uncertainties
18
Results
in Pb-Pb collisions @ 2.76 TeV
Nuclear Modification Factor RAA(pT)
Average
D0, D+, D*+
at high pT
suppression in most central collisions
less suppression in peripheral collisions
30-50%
0-7.5%
D0
D+
D*+
20
Nuclear Modification Factor RAA(pT)
Average
D0, D+, D*+
p-Pb
RpPb compatible with 1 for pT>2GeV/c:
no indication for cold nuclear matter effects
within uncertainties for pT ≥ 2 GeV/c
Pb-Pb 40-80%
Pb-Pb 0-20%
EPS09 nuclear PDF parametrization predicts
small shadowing for Pb–Pb collisions
JHEP 09 (2012) 112
Phys.Rev.Lett. 113 (2014) 23, 232301
No suppression in p-Pb collisions,
the suppression observed in Pb-Pb collision
is a final–state effect due to hot and dense
medium.
21
Nuclear Modification Factor RAA(pT)
Ds+ suppressed by a factor 3-4
for 8<pT<12 GeV/c
Ds
+
Average D0, D+, D*+
More statistic is needed at low pT
where an enhancement of Ds+/D
due to coalescence is predicted
Kunetsova, Rafelski EPJ C 51 (2007) 113
He et al. PRL 110 (2013) 112301
Andronic et al. PLB 659 (2008) 149
22
Nuclear Modification Factor RAA(pT)
Average
D0, D+, D*+
30-50%
0-7.5%
HF decay e, μ (forward rapidity)
HF decay e, μ 0-10%
Results compatible with
HF decay electrons (|y|<0.6)
HF decay muons (2.5<y<4.0)
•17/03/2015
•
•
23
16
Mass hierarchy: RAA(pT) D vs π
ΔEg>ΔEu,d,s>ΔEcharm>ΔEbeaut
π
D
RAA(pT) < RAA(pT) ?
y
Average D-meson and π RAA are
compatible within uncertainties
Considerations:
D0,mesons
Average• D
D+, D*+
Charged particles
π
Different shape of parton momentum spectra
Different fragmentation functions
Contribution of soft processes at low pT
Djordjevic, PL B734(2014)286
Wicks et al., NP A872(2011)265
Comparison not conclusive, more precise measurements of
the transverse momentum dependence are required
24
Mass hierarchy: RAA(pT) D vs B
ΔEg >ΔEu,d,s>ΔEcharm>ΔEbeauty
D
B
RAA(pT) < RAA(pT) ?
•
D-meson RAA is lower than RAA of J/ψ
from B decays measured by CMS
•
Caveat: measurements pT and y interval
• D mesons
Non-prompt
J/Ψ
Average D mesons
CMS-PAS-HIIN-12-014
Difference between D mesons and
non-prompt J/Ψ is described by pQCD
model including quark-mass
dependent energy loss
Djordjevic, PL B734 (2014) 286-289
It is also consistent with other models
e.g. MC@HQ-EPOS, TAMU model 25
Elliptic flow ν2
D mesons
Charged particles
Average v2 of the three D-meson species
larger than zero in 2 < pT < 6 GeV/c
with a significance of 5.7 σ
v2 comparable in magnitude to that of
inclusive charged particles ( dominated by
light-flavor hadrons) in 2 < pT < 6 GeV/c
Physics Letters B 719 (2013) 18–28
PRL 111, 102301 (2013)
Positive v2 suggests that low pT
charm quarks participate in the
collective motion of the system
A positive v2 is also observed for pT > 6 GeV/c
but a proper study of the path-length dependence of partonic
energy loss via ν2 measurements needs more statistics
26
Comparison with models
Central Pb-Pb collisions
PRL 111, 102301 (2013)
Semi-central Pb-Pb collisions
PR C90(2014)034904
TAMU elastic: PL B735(2014)445; Cao, Qin, Bass: PR C88(2013)044907; WHDG rad+coll: NP A872(2011)265; MC@sHQ+EPOS:
PR C89(2014)014905; Vitev, rad+dissoc: PR C80(2009)054902; POWLANG: JP G38(2011)124144; BAMPS: PL B717(2012)430
Simultaneous description of the large suppression of D mesons in central
collisions and their anisotropy in non-central collisions is challenging for
theoretical models
27
Conclusions
Excellent performance of ALICE for heavy-flavor
measurements, thanks to tracking, vertexing and PID
D-meson cross section in p-Pb collisions:
No indication for large cold nuclear matter effects at high pT
For the ZN estimator (the less biased), no evidence of multiplicity
dependent modification of D-meson production in p-Pb w.r.t pp collisions
has been observed, within uncertainties.
D-meson production in Pb-Pb collisions: Strong suppression at high pT in the most central collisions with respect to binary scaled pp collisions, due to parton-energy loss in the medium, consistent with the expected parton-mass dependence
ν2>0 in 2≤ pT≤ 6 GeV suggests that low-pT charm quarks participate in the collective motion of the system Simultaneous measurements of ν2 and RAA are a tool to constrain models
28
Perspectives
With RUN II (More statistics available and increased beam energy) Improve D0, D+, D*+ measurements precision,
extending the coverage to lower and higher pT Perform measurements for rare charmed hadrons Ds+, Λc+
New hadrons, e.g. Ξc+, become accessible
With ALICE Experiment Upgrade (2018) •17/03/2015
Unique HF measurements will be available
A.Dainese’s talk
29
•17/03/2015
Back up
30
Feed down subtraction
Correction for beauty feed-down based on FONLL pQCD calculation
JHEP 1210 (2012) 137
In case of p-Pb (Pb-Pb), the RpPb (RAA) of B mesons have also been
taken into account assuming that the nuclear modification factors
for feed-down and prompt D mesons are equal
S.Wicks,W.Horowitz, M.Djordjevic & M.Gyulassy,
for systematics
in p-Pb 0.9<RAAfeed-down/RAAprompt<1.3
in Pb-Pb -1/3<RAAfeed-down/RAAprompt<3
Nucl.Phys. A783 (2007) 493 N.Armesto, A.Dainese,
C.A.Salgado,U.A. Wiedemann, Phys. Rev. D71 (2005) 0540
In p-Pb from 4% to 40% depending on pT and D-meson species
In Pb-Pb from 5% at low pT to 15% at high pT
•
31
Systematics in PbPb measurements
•
32
D0, D+, D*+
Average RAA of D mesons in the 0–20%
centrality class compared to the expectation
from NLO pQCD with nuclear shadowing
JHEP 09 (2012) 112
EPS09 nuclear PDF parametrization predicts small
initial state effects for Pb–Pb collisions
Cold nuclear matter effects smaller than
uncertainties for pT ≥ 3 GeV/c
•17/03/2015
D0, D+, D*+
•
•
33
27
Elliptic flow ν2
Event Plane Method
D meson azimuthal angle relative to event plane
in-plane
]-(π/4),(π/4)] and ]-(3π/4),(5π/4)]
out-of-plane ]-(π/4),(3π/4)] and ]-(5π/4),(7π/4)]
Other methods used to measure D-meson elliptic flow:
Scalar Product method
Two-particle cumulant method
• 34
Physics motivations: ΛC in Pb-Pb collisions
Excess of baryons with respect to mesons,
measured with STAR at RHIC and ALICE at
the LHC in the light sector, suggests
hadronization via coalescence of constituent
quarks
•17/03/2015
Expected performance with ITS upgrade
(Monte Carlo studies)
What about
charm sector?
The measurement of the Λc yield relative
to D mesons in Pb-Pb collisions would
address the baryon over meson ratio in the
heavy-quark sector
35
•
29
+
ΛC
→pK-π+ in pp and p-Pb collisions
Analysis based on the reconstruction of displaced decay vertices
Reduce the combinatorial background via topological selection and identification
of the decay
•In
suf
f
sig
particles
na icien
l an
db
t se
ack parati
o
gro
und n of
LOOSE TOPOLOGICAL CUTS
cτ(ΛC+) is only 60 µm, small compared to
resolution on the vertex position provided by the
current ITS
Particle Identification (PID), to identify
p, K, π, essential for this analysis
• Background
reduced by a
factor 100
applying PID!
Bayesian approach to combine response
of TPC and TOF
Maximum probability criterion to
identify tracks uniquely
•17/03/2015
36
•
30
+
ΛC
K0
→pK-π+ in pp and p-Pb collisions
from pairs of opposite-sign tracks forming a vertex
•Secondary V0 vertex
displaced from the interaction vertex
•Positive
track
S reconstructed
Tracks selected according to topological cuts
• A third track identified as a proton is attached to the K0S
•Negative
track
•DCA
•d0(positive track)
• θ Pointing angle
•Primary vertex
•d0(negative track)
to form the ΛC+ candidate
A clear K0S signal in minv(π+,π-) limits the
combinatorial background, despite of low B.R.
Particle Identification (PID), to
identify p, essential for this
analysis
Background is
suppressed by a
factor 20 using
PID!
Detector used: TOF and TPC
Approach: number of sigma cuts
•17/03/2015
37
•
31
Comparing the pT and rapidity differential trends
Compared to a prediction based
on FONLL y-differential
D0 cross section multiplied
by R_pA vs. y of D mesons
from MNR+EPS09
• 38
D-meson reconstruction in ALICE: Analysis strategy
Invariant mass analysis in order to extract raw yields in different pT intervals
D0
D+
D*+
D s+
Raw yields are corrected for acceptance x efficiency, as well as for the feed-down
•
from B decays to calculate the cross section.
•17/03/2015
39