Azimuthal	  correla;ons	  of	  neutral	  pions	  and	  charged	  jets	  in	  pp	  collisions	  at	  √s	  =	  7	  TeV	  Daisuke	  Watanabe	  for	  the	  ALICE	  Collabora;on	
Mo;va;on	Jet	  measurements	  play	  a	  cri;cal	  role	  in	  probing	  the	  hot	  and	  high	  energy	  density	  mader	  created	  in	  heavy	  ion	  collisions	  through	  parton	  energy	  loss	  via	  the	  observa;on	  of	  the	  possible	  jet	  structure	  modiﬁca;on	  or	  jet	  suppression.	  
Fig.1	  shows	  the	  posi;on	  of	  hard	  scadering	  centers	  in	  the	  transverse	  plane	  generated	  in	  qPYTHIA.	  
^	Assuming	  no	  in	  medium	  energy	  loss	  per	  unit	  path	  length	  (q	  =	  0	  GeV2/fm),	  the	  distribu;on	  follows	  the	  unbiased	  distribu;on	  of	  hard	  scaderings,	  whereas	  for	  a	  large	  q^		  values	  and	  high	  energe;c	  π0s	  it	  is	  strongly	  biased	  towards	  short	  jet	  path	  lengths.	  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.	  
NNIn	  this	  poster,	  we	  report	  π0-­‐charged	  jet	  azimuthal	  correla;ons	  with	  high	  pT	  neutral	  pion	  triggers	  measured	  in	  pp	  collisions	  at	  √s	  (for=	  7	  TtheeV	  bALICEy	  the	  ALICE	  experiment	  at	  LHC.	  
XiangrongZhuCollaboration)
stitute Of Particle Physics, CCNU; Key Laboratory of Quark & Lepton Physics, MOE, China
Lawrence Berkeley National Laboratory
e-mail: xiangrong.zhu@cern.chy	al pion-hadron correlations in pp and Pb-Pb collisionsat √s = 2.76 TeV with ALICE

ALICE	  experiment	 Motivationx	Fig.1	  qPYTHIA	  simula4on	  of	  distribu4on	  of	  hard	  scaNering	  centers	  in	  the	  transverse	  plane	  [1].	  	

Jet	  reconstruc;on	

The	  ALICE	  detectors	  were	  built	  tproducedo	  exploit	  inthe	  unique	  heavy-ion collisions.
n measurements
propertiesof the hot,[2]	  dense
matterrelativistic
thisCharged	  Inpar;cles	  measured	  in	  the	  TPC	  and	  ITS	  were	  used	  to	  EMCal	 provide a powerful tool to understand thetrigassocof	  ngoalucleus-­‐nucleus	  interac;on	  the	  ∆φ = φ poten;al	  φl correlation between a p0 trigger and charged hadrons inphysics	  . Theis to study thenear-side (|a∆t	  φ|< p/3) and away-side(|∆φ – tp|he	  < jets.	  We	  u;lized	  the	  an;-­‐k 	  algorithm	  [3]	  from	  the	  reconstruct	  hpTπhπh
uppression factors I AA ( pT , pT ) and I CP ( pT , pT ) in Pb-Pb collisions.
LHC	  and	  are	  capable	  of	  studying	  jet	  quenching	  eﬀects	  via	  FastJet	  package	  [4].	
00ch0chch

TPC	par;cle	  iden;ﬁca;on	  and	  jet	  reconstruc;on.	  This	  analysis	  used	  two	  kinds	  of	  detectors,	  the	  central	  t
racking	  devices,	  ALICE
experiment
setup
ITS	
2x(5+1/3) SMs
ITS	  and	  TPC,	  for	  charged	  par;cle	  tracks	  measurement,	  e-acceptance detector at the LHC, designed for comprehensive0	  and	  the	  electromagne;c	  calorimeter	  EMCal	  for	  πplicity environment of high energy Pb-Pb collisions.
EMCal
The	  EMCal	  was	  also	  used	  as	  a	  trigger	  or sampling calorimeter covering large acceptance (|h| < measurements.	  
0.7
detector	  selec;ng	  events	  with	  energy	  deposi;ons	  larger	  y resolution. It provides efficient triggering on
than	  4	  and	  5.5	  GeV	  (the	  threshold	  changed	  during	  data	  tons.
taking).	   ElectroMagnetic Calorimeter (EMCal)
Fig.2	  ALICE	  detector	  [2]	  Jet	  	   selec4on	  criteria	  	 • An;-­‐kT	  algorithm	  •••••

Resolu;on	  parameter	  R	  =	  0.4	  	  Minimum	  track	  pT	  >	  0.15	  GeV/c	  Jet	  pT	  >	  10	  GeV/c	  Jet	  pseudorapidity	  |ηjet|	  <	  0.5	  	  Reconstructed	  jet	  area	  A	  >	  0.4	  Fig.3	  The	  reconstructed	  jet	  shape	  by	  using	  the	  an4-­‐kT	  algorithm	  [3].	

Figure 1: A sample parton-level event (generated with Herwig [8]), together with many random soft
“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.

detector covering full azimuth and |h| < 0.9 with a tracking	  pT (~85% at 1 0	  < pT < 10 GeV/c). It provides

π iden;ﬁca;on	  method	  	Cluster	  spliBng	  method	
Shower	  shape	  parameter	  	the jets roughly midway between them. Anti-kt instead generates a circular hard jet, which clips a1.Select	  a	  neutral	  cluster	  with	  λ02	  >	  0.3,	  track	  lens-shaped region out of the soft one, leaving behind a crescent.
resolution ~5.5% at low multiplicity and ~6.8% at Pb-Pb central);The	  width	  aThelong	  cluster	  main	  abovepropertiesof the anti-kt algorithm translate into concrete results for various quantiPar;cles	  deposit	  their	  energy	  in	  several	  calorimeter	  cells	  ion in relative momentum below 5 MeV/c).
matching	  etc.	  
Time Projection Chamber (TPC)
2tative properties of jets, as we outline below.
axis,	  λ0 ,	  is	  used	  to	  separate	  forming	  a	  cluster	  with	  ellipsoidal	  shape.	  The	  opening	  angle	  2.Find	  local	  maxima:	  cells	  inside	  the	  clusters	  with	  merged	  photon	  clusters	  from	  2.2Area-related
properties
the	  neutral	  meson	  decay	  photons	  becomes	  smaller	  due	  erizationof	  algorithms
Neutralpion
reconstructionenergy	  than	  adjacent	  cells.	  Used	  clusters	  
The most
concrete acontext
in which to quantitatively discuss the propertiesof jet higher	  boundaries
forsingle	  photon	  clusters	  nd	  as	  to	  the	  Lorentz	  boost	  w
ith	  increasing	  energy	  of	  the	  neutral	  different
algorithms
isin
thecalculation
ofjetareas.
Based on the performanceseen	  of thetwoclusterizer
algorithms,
twoons and charged hadrons in the calorimeterwith	  NLM	  <=2.	  
in	  F
ig.5	  up	  to	  20	  GeV	  the	  	  0 Two definitions were given for jet areas in [4]: the passive area (a) which measures a jet’s
arby cells meson.	  
into a cluster.
Two clusterizer
methods are used to reconstruct thesusceptibility
p in both pp
and Pb-Pb.
topoint-likeradiation, and the active area (A) which measures its 3.
susceptibility
toSplit	  the	  cluster	  in	  two	  new	  sub-­‐clusters	  taking	  separa;on	  is	  quite	  good.	  
0diffuse radiation. The simplest place to observe the impact of soft resilience is in the passive area forCluster
showershape parameterssis. • Study#on#πIn	  the	  EMCal,	  the	  showers	  of	  the	  decay	  photons	  start	  0#iden-ﬁca-on#by#using#shower#shape.##singleptwo	  simulation
2wo	  	  highest	  local	  maxima	  and	  aggregate	  all	  jet consisting of a hard2 particle
p1 and a soft one p2 , separated by a y − φ distancethe	  ∆12 .tIn
usual	   λ 2 =0.5(δ a +δ0)±0.25(δ−δ)+δ√jet algorithms
0 (1 )
ϕIRC
ϕ
ηsafe
η
ϕϕ
η η (JA),ηϕ the passive area a2
to	  overlap	  when	  the	  π 	  energy	  is	  larger	  than	  5	  GeV.	
JA,R (∆12 ) is πR when ∆12 = 0, but changes whentowers	  around	  them	  (form	  a	  3x3	  cluster).	  
eighboring #cells #	  # #○ # # # # #(simula-on,#pp#7TeV,#PbPb#2.76#TeV)#w X Y∆12 isw increased.
XwY
In contrast, since the boundaries of anti-kt jets are unaffected by soft radiation,δ =∑−∑∑ w , (X , Y =ϕ , η)cell with energyww●4.
Selected	  clusters	  w
ith	  sub-­‐clusters	  having	  an	  E cellEseed if Ecell >0	  Fig.5	  λ02	  vs	  E 	  for	  neutral	  clusters.	
w=Max (0, w 0+log()), w 0=4.5
invariant	  mass	  at	  most	  3	  sigma	  from	  the	  π
4 T
E cluster
erent particles
V1 clusterization
mass.	  where i isi	  atheocluster,i<48,0⩽ϕi <24.
where	  is	  cella	  cofell	  f	  the	  c0⩽ηluster	10measurement (0.015 < pT < 100 GeV/c);

To#do#list

iiii

iii

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1 photon cluster
2 photons merged cluster

toti

toti

tot

ALI-PERF-29549

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-1

 Method 1: smaller shower shape cluster with V2 clusterizer
Photon(:(ALICE Preliminary Select V2 clusters identified as photon candidates Cluster	  via shower2)
selec4on	0.1)<)λ0 )<)0.27
pp s = 7 TeV
10Figure 2
EMCal L0 trigger, E> 4-5.5 GeV
shapecut(0.1<l<0.27)
andcharged
particleveto.
•Cell	  with	  highest	  energy	  in	  a	  cluster	  must	  be	  1	  cell	  	  away	  from	  border	  0Figure 1π0(:(Uncorrected Raw / spectra1)par1cle)cluster)
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in the right figure upper. A Gauss+3rd order polynomial
is fitted2)par1cles)cluster)•Clusters	  must	  contain	  at	  least	  4	  cells	  	  n En+1 ≥ En is decay photons “EM showers”
to the distribution between 8.0 < pT < 15.0 GeV/c (Figure 2).
starttomergeat6GeV.
The10
he decay• Removal	  of	  the	  +ccluster
luster	  csplitting
ontaining	  a	  bad	  channel	  and	  exo;c	  clusters	  Method2:larger shower
shape clusterphotons
7(M showers” fraction of mergedwith V1 clusterizer(high	  energy	  deposi;on	  in	  a	  single	  tower	  due	  to	  neutron/an;-­‐0
p geometries	  energy. Itoisf	  direct	  photons	  and	  decay	  photons.	  	  10rge at 20 Fig.4	  Tincreases
he	  shower	  withshape	  
A V1 cluster with l0 > 0.5 is
split
intotwonewclusters.
neutrons)	  larger than 50% above 10 GeV.
 Every new cluster is centeredaround seed cells with local
1015
20
25
30
3540
•Timing	  cut	  :	  -­‐25	  <	  ;me	  <	  20	  ns	  p(GeV/c )es for Eseed and Emin in this analysis:
maximal energy and formed with maximum of 3x3 cells.
T, /• Charged	  veto	  is:	  tdivided
rack	  matching	  Δφ	  >	  0.03,	  Δη	  >	  0.25	
Fig.6	  Uncorrected	  π0	  distribu4on	  with	  NLM	  <=2	  and	  pp:Eseed = 100 MeV, Emin = 50 MeV (V1, V2)
 If cells are common to bothclusters,par;cle	  their energy
Figure 3
applied	  analysis	  cuts.	  	
with the energy of the seed cell as weights.
Pb-Pb: Eseed = 300 MeV, Emin = 150 MeV (V1)
 With the two new split clusters, the invariant mass is calculated (Figure 3).
-20T

patch
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ALI−PREL−71747

0T,/trig< 12 GeV/c0.2

0.15

maway (rad)p assoc> 30 GeV/cT,ch,jet2
Strategy2:
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16 < p< 20 GeV/c4
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ALICE Preliminary
pp s = 7 TeV
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Near side

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0.1
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10V2 cluster, trigger p0
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06

1/N /0 dN ch,jet/d6 (rad-1)

15

1/N /0 dN ch,jet/d6 (rad-1)0

1/N /0 dN ch,jet/d6 (rad-1)• Try#to#include#the#ﬂow#BKG#subtrac-on#method#for#PbPb.#• Comparison#with#pp,#PbPb,#simula-on.#
Results	
0-­‐	  charged	  jet	  azimuthal	  correla4ons	  	π• Concentrate#on#away#side#recoil#jet#condi-onal#yield#and#width,##Neutral pion-hadron correlation methods and results# # based# on# the two# p reconstruction# #and#make#I
ract p -hadron correlations
methods using
EMCal triggered events in pp and Pb-Pb.
AA#etc#to#extract#physics#message.#
0p assoc> 30 GeV/c
T,ch,jet
Systematic uncertainty

10.2
Figure 6 shows the comparison of the neutral pionALICE&Physics&Week&in&Padova
0.05
hadron correlation using the two strategies discussed.
Results
are0seen1 to beconsistent.
Since the merged00
7 < mgg < 0.20 GeV/c2)0 -101234-101234-1
234
6 (rad)6 (rad)6 (rad)00cluster
splittingstrategy
significantly
enhances the hight
ALI−PREL−71751
1015202530
35
101520253035
trigtrigp(GeV/c )p(GeV/c )T,/
pt
triggeryield
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itispreferredin
both
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and
000T,/
ALI−PREL−71755
Fig.7	  π -­‐jet	  azimuthal	  correla4ons	  normalized	  by	  number	  of	  trigger	  π 	  for	  trigger	  π 	  pT	  regions	  	  Pb-Pb analyses.
Fig.8	  Near	  and	  away-­‐side	  Gaussian	  widths	  as	  a	  func4on	  of	  trigger	  pT	  for	  trig
trigtrig08	  <	  pT 	  <Figure	  12	  G4eV/c,	  16	  <	  pT 	  <	  20	  GeV/c,	  24	  <	  pT 	  <	  36	  GeV/c,	  and	  associated	  jet	  thresholds	  	  asso	  >	  10,	  20,	  30	  GeV/c.	  	  
p0p trigger correlationTextraction

0

several	  charged	  jet	  pT	  thresholds	  (	  pTasso	  >	  10,	  20,	  30	  GeV/c	  ).	  	  The signal to background ratio f bkg is obtained
Figure 6
from fitting the invariant mass distribution. Then
Mixed event technique
1Y S =Y S+ B (1+)−Y B / f bkg
 In pp and Pb-Pb0• π -­‐jet	  fcbkg
orrela;ons	  have	  been	  measured	  i10n	  bins
pp	  incollisions	  at	  √s	  =	  7	  TeV	  w
ith	  cluster	  splimng	  method	  |zvertex | < 10 cm;
Y S+ B : trigger yield in signal+background range
Only Minimum-bias
events in• Two	  clear	  jet	  peaks	  are	  observed,	  indica;ng	  that	  high	  triggerpT	  produc;on	  is	  correlated	  with	  jet	  [1]	  ALICE	  Collabora;on,	  J.	  Allen,	  et	  al,	  CERN-­‐LHCC-­‐2010-­‐011,	  ALICE-­‐TDR-­‐014-­‐ADD-­‐1	  Y B : trigger yield in side band range
the mixed event pool.
[2]	  ALICE	  Collabora;on,	  arXiv:1402.4476v2	  produc;on	  0 In pp
Y S : signal p trigger yield[3]	  M.	  Cacciari,	  G.	  P.	  Salam	  and	  G.	  Soyez	  et	  al,	  JHEP	  0804	  (2008)	  063	  0010 bins in the.
• Both	  ear	  and	  away-­‐side	  wmultiplicity
ith	  increasing	  pT	  of	  the	  trigger	  π7.	  The	  decrease	  correlation between
p ntrigger
andcharged widths	  are	  decreasing	  Figure
Figure8 .	  Cacciari,	  G.	  P.	  Salam	  and	  G.	  Soyez	  et	  al,	  CERN-­‐PH-­‐TH/2011-­‐297	  [4]	  M
 InPb-Pbblack) and side-band(blue) ranges.
is	  stronger	  for	  the	  Theaway-­‐side	  correla;on	  width	  Figure 7 and Figure 8 show the real and mixed
event in pp	  3 bins in a centrality bin;
xtracted by subtracting
side-bandfromand 20-40% in Pb-Pb, respectively.
3 binsin the event
plane.
• This	  measurement	  provides	  an	  important	  baseline	  for	  Pb-­‐Pb	  data	  Summary	References	

●●

●

●

●

Summary and referencespion-charged hadron azimuthal correlations with neutral pion trigger reconstructed by two methods based on two different clusterizer algorithms.
nsistent results. The merged-cluster splitting strategy is used to extract neutral pion-hadron azimuthal correlation in real and mixed event in pp
0ch

0ch