Francesco Prino - PowerPoint Presentation

Slide1

Francesco PrinoINFN – Sezione di Torino

DNP Fall Meeting, Newport Beach, October 25th 2011

Heavy flavours in heavy ion collisions at the LHCSlide2

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 propertiesPhase transition predicted by Lattice QCD calculations

T

C

≈ 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(10

12

K)

~ 105 x T at centre of Sun~ T of universe 10 µs after Big Bang

F

.

Karsch

,

Nucl.Phys.A698 (2002) 199Slide3

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 Q2pQCD can be used to calculate initial cross sectionsTraverse 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 quark

b

quarkSlide4

Parton energy loss and nuclear modification factor

Parton energy loss

while traversing the mediumMedium induced gluon radiationCollisions with medium constituentsObservable: nuclear modification factor

If no nuclear effects are present -> RAA=1Effects from the hot and deconfined medium:

->

breakup of binary scaling -> R

AA1But also cold nuclear matter effects give rise to RAA1e.g. Shadowing, Cronin enhancementNeed control experiments: pA collisions

4

pp

reference

PbPb

measurement

Production of hard probes in AA expected to scale with the number of nucleon-nucleon collisions

N

coll

(

binary scaling

)Slide5

Heavy quark energy loss

Energy loss DE depends onProperties of the medium: density, temperature, mean free path

Path length in the medium (L)Properties of the parton:Casimir coupling factor (C

R)Mass of the quark (dead cone effect)

5

gluonstrahlung

probability

Wicks,

Gyulassy

, Last Call for LHC predictions

Dokshitzer

and

Kharzeev, PLB 519 (2001) 199Slide6

Azimuthal anisotropy

Re-scatterings among produced particles convert the initial geometrical anisotropy into an observable momentum anisotropyCollective 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-planeObservable: Fourier coefficients, in particular 2nd harmonic v2, called elliptic flow

6

Initial geometrical anisotropy in non-central

heavy ion

collisions

T

he

impact parameter selects a preferred direction in the transverse planeSlide7

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 mediumv2(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:1205.4945Slide8

PbPb

collisions at the LHC

8

Pb-Pb

collisions at the LHC



√

s

NN

=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)Slide9

Heavy

flavour reconstruction9

L

xy

B

J/

y

m

+

m

-

Full reconstruction of D meson

hadronic

decays

Displaced J/

y

(from B decays)

Semi-

leptonic

decays (

c,b

)

jet b-tagging

D

0



K

-

π

+

D

+



K

-

π+ π+D*+  D

0 π

+D

s+

 K- K+ π+

B,D

Primary vertex

e,

mSlide10

ALICE + ATLAS + CMS

10

Complementary rapidity and

p

T

coverage

DISCLAIMER: acceptance plots refer to published measurements in

pp

Slide11

How to: displaced tracks

Lower mass heavy flavour hadrons decay weakly:Lifetimes: ≈0.5-1 ps for D and ≈1.5 ps for Bct: ≈100-300 mm for D and ≈ 500 mm for B Possibility to detect decay vertices/displaced tracksTracking precision plays a crucial role

11

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

ALICE

, JHEP

09 (2012) 112Slide12

12

How to: particle identification

ALICE TPC

dE

/dx vs. p

ALICE TOF

time (n

s) vs. pALICE EMCALE/p for TPC e

ALICE MUON ARMALICE, JHEP 09 (2012) 112

ALICE

, arXiv:1205.5423Slide13

... before going to the results

13Slide14

Is there evidence for parton energy loss?

14Charged particle spectra suppressed in AA w.r.t. pp (RAA<1)Larger suppression at LHC than at RHICMaximum suppression for charged particles at p

T≈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:1210.4520Slide15

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

D

o we understand their production in pp?

YES! pQCD predictions agree with data within uncertaintiesSlide16

Nuclear modification factor

16

E

E-

ESlide17

Heavy

flavour decay electrons17Inclusive electron spectrum with two different PID analyses: TPC+TOF+TRD and TPC+EMCALSubtract background electrons

Electron pair invariant mass methodCocktail methodInclusive-background =

c+bpp reference: 7 TeV pp data sacled to 2.76 TeV

for

p

T

<8 GeV/cFONLL for pT>8 GeV/c

eSlide18

Heavy

flavour decay electrons18Inclusive electrons – cocktail= c+b

pp reference: 7 TeV pp data sacled

to 2.76 TeV for pT<8 GeV/cFONLL(pQCD) for pT>8

GeV

/c

e

Clear suppression in the

p

T

range 3-18

GeV

/c-> amounts to a factor of 1.5-3 in 3<pT

<10 GeV/cSlide19

Heavy flavour decay muons

at forward rapidity19Single muons at forward rapidity (-4<h<-2.5)

Punch-through hadrons rejected by requiring match with trigger chambersSubtract background m from

p/K decay Extrapolated from mid-rapidity measurement with an hypothesis on the rapidity dependence of RAApp reference measured at 2.76 TeV

m

Suppression by a factor 2-4 in 0-10% centrality

Less suppression in peripheral collisions

ALICE

, PRL 109 (2012) 112301Slide20

H

eavy flavour decay muonsat midrapidity

20

Single muons in |h|<1.05, 4<pT<14 GeV

/c

Match tracks from Inner Detector and

Muon

SpectrometerUse discriminant variables with different distribution for signal and backgroundBackground: p/K decays in flight, muons from hadronic showers, fakesApproximately flat vs. pTTrend difficult to evaluate due to fluctuations in peripheral binSlide21

Electrons vs. muons

21Similar RAA for heavy flavour decay electrons (|h|<0.6) and muons (2.5<y<4) in 0-10% centrality

Direct comparison between R

AA

and R

CP

not possible

Assuming ~no suppression for 60-80% centrality -> same size of suppression also for muons in |h

|<1.05Slide22

Can we separate charm and beauty?

22Slide23

D mesons

23

Analysis strategyInvariant mass analysis of fully reconstructed decay topologies displaced from the primary vertexFeed down from B (10-15 % after cuts) subtracted using pQCD (FONLL) predictions

Plus in PbPb hypothesis on RAA of D from B

K

p

D

0



K

-

π+ D+  K- π+ π+D*+  D0 π+Slide24

D meson RAA

24pp 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

p

T≈10 GeV/cSlide25

Charm + strange: D

s+25Strong Ds+ suppression (similar as

D0, D+ and D*+) for 8< p

T <12 GeV/CRAA seems to increase (=less suppression) at low pTCurrent data do not allow a conclusive

comparison to

other D

mesons within

uncertaintiesFirst measurement of Ds+ in AA collisionsExpectation: enhancement of the strange/non-strange D meson yield at intermediate pT if charm hadronizes via recombination in the mediumKuznetsova

, Rafelski, EPJ C 51 (2007) 113He,

Fries, Rapp

, arXiv:1204.4442 Slide26

D vs. heavy

flavour leptons and light flavours26

To properly compare D and leptons the decay kinematics should be consideredp

Te ≈0.5·pTB at high pT

e

Similar trend vs.

p

T for D, charged particles and p±Maybe a hint of RAAD > RAAπ at low pTSlide27

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

midrapidityLittle shadowing at high pT

suppression

is a hot matter

effect

need pPb data to quantify initial state effectHF muons

D mesons

ALICE,

PRL 109 (2012) 112301Slide28

J/

y from B feed-down

28J/

y from B decays to access beauty in-medium energy lossLong 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

L

xy

B

J/

y

m

+

m

-Slide29

RAA of non-prompt J/

y29Slow decrease of RAA with increasing centrality

Hint for increasing suppression (-> smaller RAA) with increasing p

T



CMS, PAS HIN-12-014Slide30

Beauty vs. charm

30

In central collisions, the expected R

AA hierarchy is observed:RAAcharm <

R

AA

beauty

Caveat: different y and

p

T

rangeSlide31

b-jet tagging

31

Jets from b quark fragmentation identified (tagged) for the first time in heavy ion collisions by CMS

jets are tagged by cutting on discriminating variables based on the flight distance of the secondary vertexEnrich the sample in b-jetsAn 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-12-003Slide32

Beauty vs. light flavours

32

L

ow p

T

: 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 jetsSlide33

Azimuthal anisotropy

33Slide34

D meson v

234

First direct measurement of D

anisotropy

in heavy-ion collisions

Yield extracted from invariant mass spectra of

K

p candidates in 2 bins of azimuthal angle relative to the event plane-> indication of non-zero D meson v2 (3s effect) in 2<p

T<6 GeV/cSlide35

Challenge the models

35

The simultaneous description of D meson RAA and v2 is a challenge for theoretical modelsSlide36

Challenge the models

36The simultaneous description of heavy flavour decay electrons RAA and v2 is a challenge for theoretical modelsSlide37

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

R

AAbeauty>RAAcharm and RAAbeauty>RAAlight

More statistics needed to conclude on RAAcharm vs. RAAlight

Indication (3

s

) for non-zero charm elliptic flow at low pT

Hadrochemistry of D meson speciesFirst intriguing result on Ds+ RAA, not enough statistics to concludeSlide38

38

Heavy flavours: what next?

So far, an appetizer

What 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 RAAMore differential studies on beauty

And 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...Slide39

Backup

39Slide40

D meson dN/dp

T40Slide41

D and charged particle R

AA41ALICE, JHEP

09 (2012) 112Slide42

D meson RAA: LHC vs

RHIC42Slide43

Heavy

Flavour electrons: LHC vs RHIC

43Slide44

Ds/D0

and Ds/D+44Slide45

R

AA of non-prompt J/y

45

Hint of slow decrease of RAA with increasing rapidityNon-prompt J/

y

at

midrapidity

slightly less suppressed than at forward rapiditySlide46

b-jet tagging

46

Jets from b quark fragmentation identified (tagged) for the first time in heavy ion collisions by CMS

jets are tagged

by cutting on discriminating

variables based on the flight distance

of the

secondary vertexEnrich the sample in b-jetsAn alternative tagger based only the impact parameter of the tracks in the jet is used as cross checkb-quark contribution extracted using template fits to secondary vertex invariant mass distributionsSlide47

b-jet fraction vs. centrality

47Fraction of b-jets over inclusive jetDoes not show a strong centrality dependence