Elliptic Flow of D Mesons in Pb-Pb Collisionsreygers/lectures/... · 2014. 6. 11. · Journal Club on heavy-ion collisions, 6 June 2014 Christian Moehler — D meson azimuthal anisotropy

Elliptic Flow of D Mesonsin Pb-Pb Collisions

Christian Möhler

Heidelberg University Journal Club on heavy-ion collisions

6 June 2014

Christian Moehler — D meson azimuthal anisotropyJournal Club on heavy-ion collisions, 6 June 2014

ALICE papers discussed

o D meson elliptic flow in non-central Pb-Pb collisions at √sNN = 2.76 TeV PRL 111, 102301 (2013)

o Azimuthal anisotropy of D meson production in Pb-Pb collisions at √sNN = 2.76 TeV submitted to Phys. Rev. C, 2014

2

• elliptic flow v2 of D0, D+, D*+ in 30-50% centrality

• v2 of D0, D+, D*+ in 3 centrality classes in 0-50% • 3 different methods to determine v2 • RAA with respect to event plane


What is Azimuthal Anisotropy?

3

coordinate basis in momentum space: (pT, y, φ)

ultrarelativistic heavy-ion collisions:

collision geometry not symmetric in φ

—> initial spatial azimuthal anisotropy

—> particle production dN/dφ can be anisotropic

asymmetric overlap region

Pb Pb

φpy

px

transverse plane

pT2 = px2 + py2

depending on the effect of the quark gluon plasma (QGP)

transverse plane


What is Flow?

o thermalized QGP can be described by hydrodynamis

o Euler equation: collective dynamics driven by pressure gradients

o expansion of the fireball

o flow is affected by azimuthal anisotropy —> different pressure gradients in different directions

4

y

x

py

px

initial spatial anisotropy momentum-space anisotropy


Characterizing Anisotropy

5

dN/dφ - 2π-periodic function in φ —> expand in Fourier series

Fourier coefficients:

Pb

Pb

event planeΨn

shape of participant zone (overlap of the nuclei)

v2 v3 v4


Centrality Classeso more central collision —> higher multiplicity

o take amplitude in VZERO detectors (scintillators)as a measure for multiplicity

o cluster into percentiles = centrality classes

6

centralperipheral


Outline

o Introduction – heavy quarks in the QGP

– interest in measuring charm flow

o Analysis – reconstruction of D mesons in ALICE

– ALICE detector

– determination of v2

o Results

o Conclusion

7


Heavy Quarks in the QGP

8

• heavy quarks: charm ~1.5 GeV, bottom ~4.5 GeV, [top ~173 GeV]

• mc, mb >> ΛQCD heavy quarks produced in early pQCD stage

• masses generated mainly by Higgs field—> remain heavy when chiral symmetry is restored

• mc, mb >> TQCD —> no thermal production in medium

• no annihilation

• total charm/beauty is conserved in the medium

• heavy quark experiences full evolution of the system

1

10

102

103

104

105

1 10 102

103

104

105

1

10

102

103

104

105

1 10 102

103

104

105

Total quark mass (MeV)

Hig

gs q

ua

rk m

ass (

Me

V)

t

b

c

s

d

u

QCD Vacuum

!c symmetry breaking

Higgs Vacuum

Electroweak symmetry breaking

heavy quark = unique probe of QGP medium properties

Phys. Lett. B 647 (2007) 366-370


Charm v2

o two contributing mechanisms:

– collective expansion (low pT)

– path-length dependence of in-medium energy loss (high pT)

o does charm participate in the collective expansion?

o how does charm interact in the QGP?

o strong interplay between experiment and theory

9

y

x


Reconstruction of D Mesons in ALICE

o D mesons decay before they can be detected

o invariant mass analysis in selected decay channels

o downside: large combinatorial background – need excellent particle identification (PID)

– expoloit topology of secondary decay vertex

10

meson M (GeV/c2) c⌧ (µm) decay channel BR (%)

D0(cu) 1865 123 K�⇡+

3.9

D+(cd) 1870 312 K�⇡+⇡+

9.1

D⇤+(cd) 2010 � = 83.3 keV D0

(K�⇡+)⇡+

67.7

D+s (cs) 1969 150 �(K�K+

)⇡+5.5

+ anti-particles


ALICE Detector

11

Time Projection Chamber (TPC) —> tracking of decay products —> particle identification via dE/dx

Inner Tracking System (ITS) —> tracking of decay products —> detection of secondary vertices

Time of Flight (TOF) —> particle identification via flight time

VZERO —> trigger —> centrality classes


Particle Identificationo specific energy loss dE/dx in TPC gas

o velocity β via time of flight in TOF

o require measured signal to be within 3σ of the expected signal for each species (K, π)

12

TPC

TOF kaon hypothesis


Topological Selection for D0

o decay vertex displaced from primary vertex by a few 100 µm

o most effective topological cuts:

– pointing angle: cos(θpointing) > 0.95

– product of impact parameters: d0K x d0

π < - (200 µm)2

13

• kinematic range: |y|<0.8, 2 < pT < 16 GeV/c

• topological approach limited at ultra-low pT


o reconstruct event plane angle ψ2 from the distribution of charged particles

o select ‘good’ TPC tracks from event

o exclude candidates for D mesons to remove auto-correlations

Reconstruction of the Event Plane

14

Pb

Pb

event planeΨn

2ψ2Qy

Qx

• N - multiplicity of the event • φi - azimuthal angle of particle i • wi - weight to correct for azimuthal

non-uniformity in TPC


Event Plane Method

o divide φ into 4 quartiles

o 2 categories: in plane, out of plane

o integrate dN/dφ to get the yieldsNin-plane and Nout-of-plane

o v2 can be determined as:

15

in-plane

in-plane

out of plane

out of plane

resolution R2


Invariant Mass Distributions

o invariant mass of daughter particles (e.g. Κπ for D0)

o fitted by second order polynomial (for the background) plus Gaussian (for the signal)

o extract Nin-plane and Nout-of-plane as the integral of the Gaussian

o higher yield in-plane than out-of-plane

o —> non-negative v2

16

)2c) (GeV/π(KM

1.7 1.75 1.8 1.85 1.9 1.95 2

2c

En

trie

s/1

5 M

eV

/

0

200

400

600

800

1000

1200

1400

1600

1800+

π-

K→0

Dand charge conj.

c<3 GeV/T

p2<

In-Plane

Out-of-Plane

)2c) (GeV/π(KM

1.7 1.75 1.8 1.85 1.9 1.95 2 2.05

2c

En

trie

s/2

3 M

eV

/

0

100

200

300

400

500 c<6 GeV/T

p4<

)2c) (GeV/π(KM

1.7 1.75 1.8 1.85 1.9 1.95 2 2.05

2c

En

trie

s/2

3 M

eV

/

0

20

40

60

80

100

120

140

c<12 GeV/T

p8<ALICE

Pb-Pb, 30-50%

= 2.76 TeVNN

s

)2

c) (GeV/ππ(KM

1.7 1.75 1.8 1.85 1.9 1.95 2 2.05

2c

En

trie

s/1

5 M

eV

/

0

20

40

60

80

100+

π+

π-

K→+

Dand charge conj.

c<4 GeV/T

p3<

)2

c) (GeV/ππ(KM

1.7 1.75 1.8 1.85 1.9 1.95 2 2.05

2c

En

trie

s/1

5 M

eV

/

0

50

100

150

200

250

300

c<6 GeV/T

p4<

)2

c) (GeV/ππ(KM

1.7 1.75 1.8 1.85 1.9 1.95 2 2.05

2c

En

trie

s/1

5 M

eV

/

0

10

20

30

40

50

60

70

80 c<12 GeV/T

p8<

)2c) (GeV/π(KM) - ππ(KM

0.14 0.145 0.15 0.155 0.16

2c

En

trie

s/5

00

ke

V/

0

50

100

150

200

250

300

350

400

450+

π0

D→*+

Dand charge conj.

c<4 GeV/T

p2<


0.14 0.145 0.15 0.155 0.16

2c

En

trie

s/5

00

ke

V/

0

20

40

60

80

100

120

140

160

180

200

220c<6 GeV/

Tp4<


0.14 0.145 0.15 0.155 0.16

2c

En

trie

s/5

00

ke

V/

0

10

20

30

40

50

60

70

80

90c<12 GeV/

Tp8<

ALI−PUB−69910


Feed-Down from B

o contribution of D mesons from B decays ~10-20%

o feed-down enhanced by topological selection

o want to give result for prompt D only

o mb > mc —> v2feed-down ≤ v2

prompt

o most conservative assumption: v2

feed-down = v2prompt —> v2

prompt = v2all

o use as an upper limit for systematic uncertainties:

o use v2feed-down = 0 —> v2

prompt = v2all/fprompt

17

B —> D + X


Relevant Systematic Uncertainties

o signal extraction – vary fit range, functional form, binning…

– bin counting instead of integral

– fix fit parameters

o feed-down from B – as described on previous slide

18

4 < pT < 6 GeV/c, 30-50%


Results



19




D meson v2 in 30-50% centrality

o first measurement of D meson v2 in ALICE!

o non-zero v2 for all species

o v2 of different species consistent within uncertainties

20


Average D meson v2 in 30-50%

o v2 of D mesons comparable to light flavor

o v2 in 2<pT<6 GeV/c:

o larger than 0 with 5.7σ !

21

o average computed using statistical significance as weights

o full propagation of systematic uncertainties


Results



22




D0 Elliptic Flow vs. centrality

o indication for increase of v2 vs. mid-central collisions

o comparable to charged particles in all centrality bins

23

)c (GeV/T

p2 4 6 8 10 12 14 16 18

2v

-0.2

0

0.2

0.4

0.6

= 2.76 TeVNNsPb-Pb,

Centrality 0-10%

ALICE

)c (GeV/T

p2 4 6 8 10 12 14 16 18

-0.2

0

0.2

0.4

0.6

Centrality 10-30%

{EP}2v|<0.8, y |0

Prompt D

Syst. from data

Syst. from B feed-down

|>2}η∆{EP,|2vCharged particles,

)c (GeV/T

p2 4 6 8 10 12 14 16 18

-0.2

0

0.2

0.4

0.6

Centrality 30-50%

ALI−PUB−70100

0-10% 10-30% 30-50%


Modelling Flow

o e.g. BAMPS model:

o Boltzmann approach to multi-parton scattering

o microscopic partonic transport model

o implemented processes:

– elastic collisions

– gluon radiation

24

O. Fochler, J. Uphoff, Z. Xu and C. Greiner Phys. Rev. C 84 (2011) 024908

2 —> 2

2 —> 3


Comparison with Models

o many different models exist that predict v2

o with better precision (Run 2) models will be constrained further

25

(GeV/c)T

p0 2 4 6 8 10 12 14 16

2v

-0.1

0

0.1

0.2

0.3

0.4 average*+, D+,D0ALICE D

Syst. from data

Syst. from B feed-down

WHDG rad+collPOWLANGCao, Qin, BassMC@sHQ+EPOS, Coll+Rad(LPM) TAMU elasticBAMPS UrQMD

= 2.76 TeVNNsPb-Pb,

Centrality 30-50%

ALI−PUB−70164


Take Home Messageso D mesons reconstructed via invariant mass analysis using

topological selection and particle identification

o D meson elliptic flow v2 measured for the first time in ALICE

o in 2 < pT < 6 GeV/c:

– v2 =

– v2 > 0 with 5.7σ significance

o elliptic flow of D mesons and charged particles consistent within uncertainties

o indication for an increase of v2 from central to mid-central collisions

o no model describes all data yet —> challenge for theory!

26

evidence for collective flow of charm quarks

Christian Moehler — D meson azimuthal anisotropyJournal Club on heavy-ion collisions, 6 June 2014 27

Backup


RAA in plane and out of plane

o stronger suppression in out-of-plane direction where the path length is larger

28

) c (GeV/T

p 0 2 4 6 8 10 12 14 16

AA

R

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

ALICE

= 2.76 TeVNN

sPb-Pb,

Centrality 30-50%0

D , 0

Prompt D

|<0.5y|

In PlaneOut Of Plane Syst. UncertaintiesCorrelatedUncorrelatedAnticorrelatedGlobal Normalization

[ ]

ALI−PUB−70131

Pb

Pb

out-of-plane

in-plane


Elliptic Flow with Different Methods

29

2 4 6 8 10 12 14 16 18

-0.2

0

0.2

0.4

0.6 0D ,

0Prompt D

|<0.8y|

{EP TPC}2v

2 4 6 8 10 12 14 16 18

-0.2

0

0.2

0.4

0.6 {SP TPC}2vALICE

2 4 6 8 10 12 14 16 18

-0.2

0

0.2

0.4

0.6 {2}2v

2 4 6 8 10 12 14 16 18

2v

-0.2

0

0.2

0.4

0.6 ±Prompt D|<0.8y|

{EP TPC}2v

2 4 6 8 10 12 14 16 18

-0.2

0

0.2

0.4

0.6 {SP TPC}2v

2 4 6 8 10 12 14 16 18

-0.2

0

0.2

0.4

0.6 {2}2v

2 4 6 8 10 12 14 16 18

-0.2

0

0.2

0.4

0.6 ±Prompt D*|<0.8y|

{EP TPC}2v

)c (GeV/T

p2 4 6 8 10 12 14 16 18

-0.2

0

0.2

0.4

0.6 {SP TPC}2v

= 2.76 TeVNNsPb-Pb,

Centrality 30-50%

2 4 6 8 10 12 14 16 18

-0.2

0

0.2

0.4

0.6 {2}2v

Open box: syst. from data

Shaded box: syst. from B feed-down

ALI−PUB−70079

Documents

Elliptic Flow of D Mesons in Pb-Pb Collisionsreygers/lectures/... · 2014. 6. 11. · Journal Club on heavy-ion collisions, 6 June 2014 Christian Moehler — D meson azimuthal anisotropy