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Heavy flavours in heavy ion collisions at the LHC. Francesco Prino INFN – Sezione di Torino. DNP Fall Meeting, Newport Beach, October 25 th 2011. 3 flavours; (q-q)=0. Heavy Ion Collisions. Basic idea: compress large amount of energy in a very small volume - PowerPoint PPT Presentation
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Francesco PrinoINFN – Sezione di Torino
DNP Fall Meeting, Newport Beach, October 25th 2011
Heavy flavours in heavy ion collisions at the LHC
2
Heavy Ion Collisions
Study nuclear matter at extreme conditions of temperature and density Collect evidence for a state where
quarks and gluons are deconfined (Quark Gluon Plasma) and study its properties
Phase transition predicted by Lattice QCD calculations TC ≈ 170 MeV C ≈ 0.6 GeV/fm3
3 flavours; (q-q)=0
Basic idea: compress large amount of energy in a very small volume produce a “fireball” of hot matter: temperature O(1012 K)
~ 105 x T at centre of Sun ~ T of universe 10 µs after Big Bang
F. Karsch, Nucl.Phys.A698 (2002) 199
Heavy quarks as probes of the medium
Hard probes in nucleus-nucleus collisions: Produced at the very early stage
of the collisions in partonic processes with large Q2
pQCD can be used to calculate initial cross sections
Traverse the hot and dense medium
Can be used to probe the properties of the medium
3
D
K p
B
e,m D n
D
e,m
c quarkb quark
Parton energy loss and nuclear modification factor
Parton energy loss while traversing the medium Medium induced gluon radiation Collisions with medium constituents
Observable: nuclear modification factor
If no nuclear effects are present -> RAA=1Effects from the hot and deconfined medium:
-> breakup of binary scaling -> RAA1But also cold nuclear matter effects give rise to RAA1 e.g. Shadowing, Cronin enhancement Need control experiments: pA collisions 4
Tpp
TAA
collTAA dpdN
dpdNN
pR//1)(
vacuummedium~
QCDQCD
pp reference
PbPb measurement
Production of hard probes in AA expected to scale with the number of nucleon-nucleon collisions Ncoll (binary scaling)
Heavy quark energy lossEnergy loss DE depends on Properties of the medium: density,
temperature, mean free path Path length in the medium (L) Properties of the parton:
Casimir coupling factor (CR)Mass of the quark (dead cone effect)
5
)()()(
,
pAAAAAA
quarklightquarkmassivegluonquark
RDRBR
EEEE
DDDD
gluonstrahlung probability
Wicks, Gyulassy, Last Call for LHC predictions
Dokshitzer and Kharzeev, PLB 519 (2001) 199
Reactionplane
In-planeOut
-of-p
lane
Y
XFlow
Flow
Reactionplane
In-planeOut
-of-p
lane
Y
XFlow
Flow
Reactionplane
In-planeOut
-of-p
lane
Y
XFlow
Flow
Azimuthal anisotropy
Re-scatterings among produced particles convert the initial geometrical anisotropy into an observable momentum anisotropy Collective motion (flow) of the “bulk” (low pT)
In addition, path-length (L) dependent energy loss in an almond-shaped medium induces an asymmetry in momentum space Longer path length -> larger energy loss for particles exiting out-of-plane
Observable: Fourier coefficients, in particular 2nd harmonic v2, called elliptic flow
....2cos212 2
0 RPvNddN
p RPv 2cos2
6
Initial geometrical anisotropy in non-central heavy ion collisions The impact parameter selects a
preferred direction in the transverse plane
Heavy flavour v2Due to their large mass, c and b quarks should take longer time (= more re-scatterings) to be influenced by the collective expansion of the medium v2(b) < v2(c)
Uniqueness of heavy quarks: cannot be destroyed and/or created in the medium Transported through the full system evolution
7
J. Uphoff et al., arXiv:1205.4945
PbPb collisions at the LHC
8
Pb-Pb collisions at the LHC
√sNN=2.76 TeV (≈ 14x√sNN at RHIC)
Delivered Integrated luminosity:
10 mb-1 in 2010
166 mb-1 in 2011
3 experiments (ALICE, ATLAS, CMS)
Heavy flavour reconstruction
9
Lxy
B
J/ym+
m-
Full reconstruction of D meson hadronic decays
Displaced J/y (from B decays)Semi-leptonic decays (c,b)
jet b-tagging
D0 K- π+ D+ K- π+ π+
D*+ D0 π+
Ds+ K- K+ π+
B,DPrimary vertex
e,m
ALICE + ATLAS + CMS
10
Complementary rapidity and pT coverage
DISCLAIMER: acceptance plots refer to published measurements in pp
How to: displaced tracksLower mass heavy flavour hadrons decay weakly: Lifetimes: ≈0.5-1 ps for D and ≈1.5 ps for B ct: ≈100-300 mm for D and ≈ 500 mm for B
Possibility to detect decay vertices/displaced tracks Tracking precision plays a crucial role
11
Track impact parameter: distance of closest approach of a track to the interaction vertex
ALICE, JHEP 09 (2012) 112
12
How to: particle identification
ALICE TPCdE/dx vs. p
ALICE TOFtime (ns) vs. p
ALICE EMCALE/p for TPC e
ALICE MUON ARM
ALICE, JHEP 09 (2012) 112
ALICE, arXiv:1205.5423
Is there evidence for parton energy loss?
14
Charged particle spectra suppressed in AA w.r.t. pp (RAA<1) Larger suppression at LHC than at RHIC Maximum suppression for charged particles at pT≈6-7 GeV/c
First results from pilot pPb run confirm that it comes from a final state effect
CMS, EPJC 72 (2012) 1945 ALICE, arXiv:1210.4520
Are heavy flavours well calibrated probes?
15
CMS, EPJC 71 (2011) 1575 ALICE, arXiv:1205.5423
ALICE, JHEP 1201 (2012)
CMS, PRL 106 (2011) 112001
Do we understand their production in pp?
YES! pQCD predictions agree with data within uncertainties
Heavy flavour decay electrons
17
Inclusive electron spectrum with two different PID analyses: TPC+TOF+TRD and TPC+EMCALSubtract background electrons Electron pair invariant mass method Cocktail method
Inclusive-background = c+bpp reference: 7 TeV pp data sacled to 2.76 TeV for
pT<8 GeV/c FONLL for pT>8 GeV/c
e
Heavy flavour decay electrons
18
Inclusive electrons – cocktail = c+b
pp reference: 7 TeV pp data sacled to 2.76 TeV for
pT<8 GeV/c FONLL(pQCD) for pT>8 GeV/c
e
Tpp
TAA
collTAA dpdN
dpdNN
pR//1)(
Clear suppression in the pT range 3-18 GeV/c-> amounts to a factor of 1.5-3 in 3<pT<10 GeV/c
Heavy flavour decay muonsat forward rapidity
19
Single muons at forward rapidity (-4<h<-2.5) Punch-through hadrons rejected by
requiring match with trigger chambers Subtract background m from p/K decay
Extrapolated from mid-rapidity measurement with an hypothesis on the rapidity dependence of RAA
pp reference measured at 2.76 TeV
m
Suppression by a factor 2-4 in 0-10% centralityLess suppression in peripheral collisions
ALICE, PRL 109 (2012) 112301
Heavy flavour decay muonsat midrapidity
20
Single muons in |h|<1.05, 4<pT<14 GeV/c Match tracks from Inner Detector and
Muon Spectrometer Use discriminant variables with different
distribution for signal and background Background: p/K decays in flight, muons from
hadronic showers, fakes
Approximately flat vs. pT Trend difficult to evaluate
due to fluctuations in peripheral bin
PerT
CentT
Centcoll
PercollTCP dpdN
dpdNNN
pR//
)(
Electrons vs. muons
21
Similar RAA for heavy flavour decay electrons (|h|<0.6) and muons (2.5<y<4) in 0-10% centrality
Direct comparison between RAA and RCP not possible Assuming ~no suppression for 60-80% centrality ->
same size of suppression also for muons in |h|<1.05
D mesons
23
Analysis strategy Invariant mass analysis of fully
reconstructed decay topologies displaced from the primary vertex
Feed down from B (10-15 % after cuts) subtracted using pQCD (FONLL) predictions Plus in PbPb hypothesis on RAA of D from
B
K p
D0 K- π+ D+ K- π+ π+ D*+ D0 π+
D meson RAA
24
pp reference from measured D0, D+ and D* pT -differential cross sections at 7 TeV scaled to 2.76 TeV with FONLL Extrapolated assuming FONLL pT shape to highest pT bins not measured in pp
D0, D+ and D*+ RAA agree within uncertainties
Strong suppression of prompt D mesons in central collisions -> up to a factor of 5 for pT≈10 GeV/c
Charm + strange: Ds+
25
Strong Ds+ suppression
(similar as D0, D+ and D*+) for 8< pT <12 GeV/CRAA seems to increase (=less suppression) at low pT Current data do not allow a
conclusive comparison to other D mesons within uncertainties
First measurement of Ds+ in AA collisions
Expectation: enhancement of the strange/non-strange D meson yield at intermediate pT if charm hadronizes via recombination in the medium
Kuznetsova, Rafelski, EPJ C 51 (2007) 113 He, Fries, Rapp, arXiv:1204.4442
D vs. heavy flavour leptons and light flavours
26
To properly compare D and leptons the decay kinematics should be considered pT
e ≈0.5·pTB at high pT
e
Similar trend vs. pT for D, charged particles and p±
Maybe a hint of RAAD > RAA
π at low pT
Data vs. models
27
Models of in-medium parton energy loss can describe reasonably well heavy flavour decay muons at forward rapidity and D mesons at midrapidity
Little shadowing at high pT suppression is a hot matter effectneed pPb data to quantify initial state effect
HF muonsD mesons
ALICE, PRL 109 (2012) 112301
J/y from B feed-down
28
J/y from B decays to access beauty in-medium energy loss Long B-meson lifetime -> secondary J/y’s from
B feed-down feature decay vertices displaced from the primary collision vertex
Fraction of non-prompt J/y from simultaneous fit to m+m- invariant mass spectrum and pseudo-proper decay length distributions )/(
)/( // y
y yy Jp
MJL
T
JxyJ
Lxy
B
J/ym+
m-
RAA of non-prompt J/y
29
Slow decrease of RAA with increasing centralityHint for increasing suppression (-> smaller RAA) with increasing pT
CMS, PAS HIN-12-014
Beauty vs. charm
30
In central collisions, the expected RAA hierarchy is observed:RAA
charm < RAAbeauty
Caveat: different y and pT range
b-jet tagging
31
Jets from b quark fragmentation identified (tagged) for the first time in heavy ion collisions by CMSjets are tagged by cutting on discriminating variables based on the flight distance of the secondary vertex Enrich the sample in b-jets An alternative tagger based only
the impact parameter of the tracks in the jet is used as cross check
b-quark contribution extracted using template fits to secondary vertex invariant mass distributions
CMS, PAS HIN-12-003
Beauty vs. light flavours
32
Low pT: different suppression for beauty and light flavours BEWARE: 1) not the same centrality 2) B->J/y decay kinematics
High pT: similar suppression for light flavour and b-tagged jets
Azimuthal anisotropy
33
Reactionplane
In-planeOut
-of-p
lane
Y
XFlow
Flow
Reactionplane
In-planeOut
-of-p
lane
Y
XFlow
Flow
Reactionplane
In-planeOut
-of-p
lane
Y
XFlow
Flow
D meson v2
34
OUTIN
OUTIN
NNNN
Rv
4
1
22
p
First direct measurement of D anisotropy in heavy-ion collisionsYield extracted from invariant mass spectra of Kp candidates in 2 bins of azimuthal angle relative to the event plane
-> indication of non-zero D meson v2 (3s effect) in 2<pT<6 GeV/c
Challenge the models
35
The simultaneous description of D meson RAA and v2 is a challenge for theoretical models
Challenge the models
36
The simultaneous description of heavy flavour decay electrons RAA and v2 is a challenge for theoretical models
37
Heavy flavours: what have we learned so far?
Abundant heavy flavour production at the LHC Allow for precision measurements
Can separate charm and beauty (vertex detectors!) Indication for RAA
beauty>RAAcharm and RAA
beauty>RAAlight
More statistics needed to conclude on RAAcharm vs. RAA
light
Indication (3s) for non-zero charm elliptic flow at low pT
Hadrochemistry of D meson species First intriguing result on Ds
+ RAA, not enough statistics to conclude
38
Heavy flavours: what next?
So far, an appetizerWhat will/can come in next years (2013-2017): pPb run -> establish initial state effects Separate charm and beauty also for semi-leptonic channels Improved precision on the comparison between charm and light
hadron RAA
More differential studies on beautyAnd even more with the upgrades (2018): High precision measurements of D meson v2 and comparison to light
flavours -> charm thermalization in the medium? Charm baryons (Lc) -> study baryon/meson ratio in the charm
sector High precision measurement of Ds
+ RAA and v2
...
RAA of non-prompt J/y
45
Hint of slow decrease of RAA with increasing rapidity Non-prompt J/y at midrapidity slightly less suppressed
than at forward rapidity
b-jet tagging
46
Jets from b quark fragmentation identified (tagged) for the first time in heavy ion collisions by CMSjets are tagged by cutting on discriminating variables based on the flight distance of the secondary vertex Enrich the sample in b-jets An alternative tagger based only
the impact parameter of the tracks in the jet is used as cross check
b-quark contribution extracted using template fits to secondary vertex invariant mass distributions