Heavy flavours in heavy ion collisions at the LHC

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

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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

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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

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J. Uphoff et al., arXiv:1205.4945

PbPb collisions at the LHC

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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

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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

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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

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Track impact parameter: distance of closest approach of a track to the interaction vertex

ALICE, JHEP 09 (2012) 112

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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

... before going to the results

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Is there evidence for parton energy loss?

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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?

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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

Nuclear modification factor

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E

E-DE

Heavy flavour decay electrons

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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

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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

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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

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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

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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

Can we separate charm and beauty?

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D mesons

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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

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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+

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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

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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

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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

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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

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Slow decrease of RAA with increasing centralityHint for increasing suppression (-> smaller RAA) with increasing pT

CMS, PAS HIN-12-014

Beauty vs. charm

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In central collisions, the expected RAA hierarchy is observed:RAA

charm < RAAbeauty

Caveat: different y and pT range

b-jet tagging

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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

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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

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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

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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

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The simultaneous description of D meson RAA and v2 is a challenge for theoretical models

Challenge the models

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The simultaneous description of heavy flavour decay electrons RAA and v2 is a challenge for theoretical models

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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

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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

...

Backup

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D meson dN/dpT

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D and charged particle RAA

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ALICE, JHEP 09 (2012) 112

D meson RAA: LHC vs RHIC

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Heavy Flavour electrons: LHC vs RHIC

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Ds/D0 and Ds/D+

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RAA of non-prompt J/y

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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

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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

b-jet fraction vs. centrality

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Fraction of b-jets over inclusive jet Does not show a strong centrality dependence