Azimuthal	  correla=ons	  of	  neutral	  pions	  and	  charged	  jets	  in	  pp	  collisions	  at	  7	  TeV	  and	  Pb-­‐Pb	  collisions	  at	  2.76	  TeV	  Daisuke	  Watanabe	  for	  the	  ALICE	  Collabora=on	

Mo=va=on	y	Jet	  measurements	  play	  a	  cri=cal	  role	  in	  probing	  the	  hot	  and	  high	  energy-­‐density	  ma^er	  created	  in	  heavy-­‐ion	  collisions	  through	  parton	  energy	  loss	  via	  the	  observa=on	  of	  the	  jet	  structure	  modiﬁca=on	  or	  jet	  suppression.	  
Figure	  1	  shows	  the	  posi=on	  of	  hard-­‐sca^ering	  centers	  in	  the	  transverse	  plane	  generated	  in	  qPYTHIA.	  
^	=	  0	  GeV2/fm),	  the	  distribu=on	  follows	  Assuming	  no	  in-­‐medium	  energy	  loss	  per	  unit	  path	  length	  (q	  the	  unbiased	  distribu=on	  of	  hard-­‐sca^erings,	  whereas	  for	  large	  q^		  values	  and	  high	  energe=c	  π0s	  it	  is	  strongly	  biased	  towards	  short	  jet	  path	  lengths	  [1].	  In	  fact,	  the	  same	  eﬀect	  maximizes	  the	  path	  length	  for	  jet	  recoiling	  from	  a	  high	  transverse	  momentum	  trigger	  hadron	  that	  mainly	  comes	  from	  the	  surface	  of	  the	  medium.	  
In	  this	  poster,	  we	  report	  π0-­‐	  charged	  jet	  azimuthal	  correla=ons	  with	  high	  pT	  neutral	  pion	  triggers	  measured	  in	  pp	  collisions	  at	  √s	  =	  7	  TeV	  and	  the	  π0-­‐	  charged	  hadron	  correla=on	  results	  in	  Pb-­‐Pb	  collisions	  at	  √sNN	  =	  2.76	  TeV	  	  as	  the	  forthcoming	  results	  of	  π0-­‐	  jet	  analysis	  in	  Pb-­‐Pb	  collisions.	  

x	Fig.	  1	  qPYTHIA	  simula=on	  of	  distribu=on	  of	  hard-­‐scaBering	  centers	  in	  the	  transverse	  plane	  [2].	  	

Jet	  reconstruc=on	

ALICE	  experiment	TPC	/dp (GeV-1c)

ITS	

NN

trig

T

assoc

assoc

trig
T

trig

-1

TT

Fig.	  2	  ALICE	  detector	  [3]	  NN-1-11/N dN

trig

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trig

NN

1/N dN

1/NtrigdN/dpassoc (GeV-1c)

1/NtrigdN/dpassoc (GeV-1c)

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22

A

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EMCal	

/dp (GeV-1c)

The	  ALICE	  detector	  [3]	  was	  built	  to	  exploit	  the	  unique	  Charged	  par=cles	  measured	  in	  the	  TPC	  and	  ITS	  were	  used	  to	  reconstruct	  physics	  poten=al	  of	  nucleus-­‐nucleus	  interac=on	  at	  the	  jets.	  We	  u=lized	  the	  an=-­‐kT	  algorithm	  [4]	  from	  the	  FastJet	  package	  [5].	
LHC	  and	  is	  capable	  of	  studying	  jet	  quenching	  eﬀects	  via	  52par=cle	  iden=ﬁca=on	  and	  jet	  reconstruc=on.	  This	  analysis	  Jet	  	   selec=on	  criteria	  used	  two	  types	  of	  detectors,	  the	  central	  tracking	  devices,	  	 • An=-­‐kT	  algorithm	  the	  inner	  tracking	  system	  ITS	  and	  =me	  projec=on	  • Pb-Pb
Resolu=on	  parameter	  R	  =	  0.4	  	  s =2.76 TeV
ALICEAnalysis
Note2013
chamber	  TPC,	  for	  charged	  par=cle	  tracks	  measurement,	  • 0-10%
Minimum	  track	  pT	  >	  010.15	  GeV/c	  8.0 < p < 16.0 GeV/cand	  the	  electromagne=c	  calorimeter	  EMCal	  for	  π0	  • NearJet	  sidepT|	  ∆>ϕ	  |1< 0	  0.7GeV/c	  measurements.	  The	  EMCal	  was	  also	  used	  as	  a	  trigger	  • Jet	  pseudorapidity	  |ηjet|	  <	  0.5	  	  Pb-Pb s = 2.76 TeV
Pb-Pb s = 2.76 TeV
detector	  selec=ng	  deposi=ons	  larger	  10 events	  with	  energy	  0-10%
0-10%• Reconstructed	  jet	  area	  A	  >	  0.4	  8.0 < p < 16.0 GeV/c
8.0 < p < 16.0 GeV/c
10than	  4	  and	  5.5	  GeV	  (the	  threshold	  changed	  data	  Near side |∆φ| < 0.7
Away side |∆φ-πd| <uring	  0.7Fig.	  3	  The	  reconstructed	  jet	  shape	  by	  using	  the	  an=-­‐kT	  algorithm	  [4].	
taking).	  
Figure 1: A sample parton-level event (generated with Herwig [8]), together with many random soft	  10-10	  π iden=ﬁca=on	  method	  	“ghosts”, clustered with four diﬀerent jets algorithms, illustrating the “active” catchment areas of
the resulting hard jets. For kt and Cam/Aachen the detailed shapes are in part determined by thespecific set of ghosts used, and change when the ghosts are modified.

10-20π 	  –	  jet	  correla=on	  results	  in	  pp	  7	  TeV	  	the jets roughly midway between them. Anti-kt instead generates a circular hard jet, which clips alens-shaped region out of the soft one, leaving behind a crescent.
0345The above properties of the anti-kt algorithm translate into concrete results for various quantitative properties of jets, as we outline below.
6assocT,ch,jettrig

T,π0trig

T,π0trig

0.08

ALICE Preliminarys = 7 TeV
EMCal-triggered
Near side

Pb-Pb sNN = 2.76 TeVpp

Uncorrected Raw π0 spectra

0-10%

λ 20+Mass cuts

0.15

NLM ≤ 20.1p assoc> 20 GeV/cT,ch,jetp assoc> 30 GeV/cT,ch,jet

0.0401524250.02

ALI-PERF-29549

0.02
Fig.	  4	  λ02	  vs	  ET	  for	  neutral	  clusters.	

20

ALI−PREL−71747

30p35T, π040

68101214
GeV/c

123(GeV/c )45152025

ALI−PREL−71755

63035trig
p 0 (GeV/c )101520T,π07TT910	  8 p (GeV/c)J(Dj) = C(Dj)b0 (1 + 2hu2trig u2assoc i cos(2Dj))/dp (GeV-1c)

Tassoc669

Fig. 61 shows the same results of the per-trigger yield of charged hadrons onp
Pb-Pb collisions 0-10% centrality at sNN = 2.76 TeV as Fig. 59 and Fig. 60

670

8.3T668

2.2
21.8IAA• Flat	  background	  subtrac=on:	  ZYAM	  method	  [6]	  the jet-like correlations•is obtained
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1

trig

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left, the blue line is the fit value at 8.0 < pT < 16.0 GeV/c with0.0347 ± 0.0043. The fit error is used to estimate
the systematic
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Pb-Pb s =2.76 TeV
NS: Flat Background
2.4
AS: Flat Background
NS: Flow Background
AS: Flow Backgroundtrig

T,πp0Fig. 19: u2 flow of charged pions (left) and charged hadrons (right) in Pb-Pb 0-10% at sNN = 2.76 TeV. In theπ 	  –	  charged	  hadron	  correla=on	  in	  Pb-­‐Pb	  2.76	  TeV	NNtrig
T(left), away side |Dj p| < 0.7 (right) in trigger pT at 8.0 < pTFig.	  7	  Near	  and	  away-­‐side	  Gaussian	  widths	  as	  a	  func=on	  of	  trigger	  pT	  asso	  	  >	  10,	  TeV.
boxes
for	  several	  charged	  jet	  p 	  thresholds	  (p20,	  3The0	  Gblue
eV/c	  ).	   are the total systematic uncertainty.

Fig.	  5	  Uncorrected	  π0	  distribu=on	  with	  NLM	  <=2	  and	  applied	  analysis	  cuts.	  
0 	

100-2

Systematic uncertainty

10-5

100-10% trig
8.0 < p < 16.0433 GeV/ c
T
Near side (NS) |∆ϕ| < 0.7434|∆ϕ-π | < 0.7
Away side (AS)assoc

p assoc> 10 GeV/c
T,ch,jet-1

1. Azimuthal	  correla=on:	  two	  clear	  jet-­‐like	  	  	  	  	  	  	  peaks	  at	  near	  and	  away	  side	  of	  the	  trigger	  10	  	  	  	  	  	  π0	  are	  observed	  in	  pp	  collisions.	  
2.	  	  	  Yields	  of	  jet-­‐like	  peaks	  increase	  with	  	  	  	  	  	  	  	  	  increasing	  π0	  pT.	  
3.4 The	  near	  and	  away-­‐side	  Gaussian	  widths	  356789103456p (GeV/c)0	  p .	  	  	  	  	  	  	  decrease	  moderately	  with	  increasing	  πT
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60:
Per-trigger	  	  	  	  	  	  	   yield of charged hadrons distributions subtracted flow backgro
10-1

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Near side |∆ϕ| < 0.7

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0.06

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σaway (rad)Pb-Pb sNN = 2.76 TeV

pp s = 7 TeV

σnear (rad)T-10-2π0	Near	  and	  away-­‐side	  width	

ALICE Preliminary(h++h ) υ2(π ++π -) υ2⇠ 12.4%. To the charged hadron flow, it has ⇠ 2% systematic uncertainty at pT > 6.0 GeV/c. Finally,

0.1

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1/N dN

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10-1

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assoc
T,ch,jettrig/dp (GeV-1c)

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63456π -­‐	  charged	  jet	  azimuthal	  correla=ons	  	 7 8 p9 (GeV/c)10Cluster	  spli+ng	  method	
15
1.	  Select	  a	  neutral	  cluster	  with	  λ02	  >	  0.3,	  track	  matching	  etc.	  (λ0:	  ellip=cal	  	  	  	  	  	  ALICE Preliminary> 20 GeV/cp> 30 GeV/cFig. p59: Per-trigger
yield of charged hadrons distributions
subtracted
flat backgroundpp s = 7 TeV
2.2 Area-related properties
8 < pGeV/<c 0.7 (right) in trigger p at 8.0 < ptrig < 16.0 GeV/c in Pb-Pb
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345
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isp inthe calculation of jet areas.
(GeV/c)(GeV/c)were given for jet areas in [4]: the passive area (a) which measures
jet’s
Systematicauncertainty
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1665
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the passive
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ALI−PREL−71751
4.	  Selected	  clusters	  with	  sub-­‐clusters	  having	  an	  invariant	  mass	  at	  most	  3	  Pb-Pb0 s =2.76 TeV0-­‐	  jet	  azimuthal	  correla=ons	  normalized	  by	  number	  of	  trigger	  π0	  for	  trigger	  10 trig	  <	  12	  GeV/c,	  431to fit the distribution in the pT range. The fit error is considered to estimate the systematic
uncertainty
Fig.	  6	  ππ	  p	  regions	  	  8	  <	  p
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432
at 8.0 < pT < 16.0 GeV/c. The fit result is 0.0347 ± 0.0043, which gives the systematic 16	  uncertainty<	  pT,π0 	  of<	  20	  GeV/c,	  24	  4 <	  pT,π0 	  <	  36	  GeV/c,	  and	  associated	  jet	  thresholds	  	  pT,	  8.0ch,jet< p < 16.0GeV/c10-2

π 0-h± Flow Backgroundh±-h± Flow Background

Yield modification factor

The	  
Ra=o	  of	  per	  trigger	  yields	  I	  	  AAPb-PbTeV section, the yield modification factors, I671s =2.76
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1.60±PbPbph
Y(pT , pT )p 0 h±IAA (pT , pT ) =;0±phpp
Y (pT , pT )Black points: PRL 108, 092301 (2012)	  	  	  	  	  1.41.410ALICE Preliminary
435
where J(Dj) and C(Dj)arethejet-likecorrelations
andall correlations
obtained by
Eq. 10,1.2
respectively;
1.2	  	  	  	  	  -­‐	  C(Δϕ):	  per	  t
rigger	  normalized	  measured	  	  1• Small	  enhancement	  of	  near-­‐side	  1436
u2trig and u2assoc are the elliptic flow coefficients of trigger particles and associated particles, respectively;	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  azimuthal	  correla=on	  func=on.	  
0.8 employing
0.8437b0 is the background scaled factor, which is generally determined by a pedestal subtraction
ALICE Preliminary	  	  	  	  	  and	  suppression	  of	  away-­‐side	  0.60.6	  	  	  	  	  -­‐(ZYAM)	  J(Δϕ)	  :	  methodper	  trigger	  438the zero-yield-at-minimum
shown ninormalized	  Sec. 5.4.2. jet-­‐like	  0-­‐hadron	  	  	  	  	  	  yields	  observed	  in	  π
100.4	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  correla=on	  aver	  ellip=c	  ﬂow	  background	  0.40.20.2	  	  	  	  	  analysis.	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  subtrac=on.	  
34567891034567
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p (GeV/c)p(GeV/c)
345678910p (GeV/c) 	  	  	  	  	  -­‐	  b0	  	  	  	  	  	  :	  background	  scale	  factor	  obtained	  from	  Fig.	  9	  Near	  and	  away-­‐side	  ra=o	  of	  per	  trigger	  yields	  of	  charged	  Fig.	  8	  Near	  and	  away-­‐side	  per	  trigger	  yields	  o	  f	  	  	  c	  harged	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  the	  Z
YAM	  method	  [6].	  
0trig	  hadrons	  (I )	  for	  π0	  triggers	  with	  8	  <	  p trig	  <	  16	  GeV/c.	  

ALICE Preliminary

Black points: PRL 108, 092301 (2012)

-1

-2

T

ALI−PREL−79136

ALI−PREL−79322

hadrons	  for	  π 	  triggers	  with	  8	  <	  pT,π0	  Summary	

T

ALI−PREL−79167

T<	  16	  GeV/c.	  	  

AA• π0-­‐	  jet	  correla=ons	  have	  been	  measured	  in	  pp	  collisions	  at	  √s	  =	  7	  TeV	  with	  cluster	  splinng	  method	  • Two	  clear	  jet	  peaks	  are	  observed,	  indica=ng	  that	  high	  pT	  produc=on	  is	  correlated	  with	  jet	  produc=on	  • Both	  near	  and	  away-­‐side	  widths	  are	  decreasing	  with	  increasing	  pT	  of	  the	  trigger	  π0.	  The	  decrease	  is	  stronger	  for	  the	  away-­‐side	  correla=on	  width	  • Enhancement	  of	  near-­‐side	  and	  suppression	  of	  away-­‐side	  yields	  in	  Pb-­‐Pb	  collisions	  in	  π0-­‐	  hadron	  correla=on	  observed	  • The	  next	  step	  is	  to	  extend	  the	  π0-­‐hadron	  analysis	  to	  π0-­‐	  jet	  analysis	  and	  measure	  the	  trigger	  π0	  pT	  dependence	  of	  the	  ra=o	  of	  per	  trigger	  jet	  yields	  IAA	  in	  Pb-­‐Pb	  at	  √sNN	  =	  2.76	  TeV	  T,π0

References	
[1]	  Thorsten	  Renk,	  arXiv:1408.6684v1[hep-­‐ph],	  28	  Aug	  2014	  [2]	  ALICE	  Collabora=on,	  J.	  Allen,	  et	  al,	  CERN-­‐LHCC-­‐2010-­‐011,	  ALICE-­‐TDR-­‐014-­‐ADD-­‐1	  [3]	  ALICE	  Collabora=on,	  B.	  Abelev,	  et	  al,	  Int.	  J.	  Mod.	  Phys.	  A	  29	  (2014)	  1430044	  [4]	  M.	  Cacciari,	  G.	  P.	  Salam	  and	  G.	  Soyez	  et	  al,	  JHEP	  0804	  (2008)	  063	  [5]	  M.	  Cacciari,	  G.	  P.	  Salam	  and	  G.	  Soyez	  et	  al,	  CERN-­‐PH-­‐TH/2011-­‐297	  [6]	  T.	  A.	  Trainor,	  Phys.Rev.	  C81	  (2010)	  014905	  	  Quark Matter 2015 Official Logo

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