Photons and Jets from the first year
of ALICE
A. Marin,
for the ALICE Collaboration
DPG Spring Meeting 2011 , Münster (Germany)
a.marin@gsi.de
1
Outline
 The ALICE experiment
 The 2010 data sample
 Physics results in pp
 Photon physics
 Jets
 First results in PbPb
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ALICE:The dedicated HI Experiment
Size: 16 x 26 meters
Weight: 10,000 tons
Detectors: 18 3
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ALICE Detector Installation 2009-2010
ALICE Status
Complete:
ITS, TPC, TOF, HMPID,
FMD, T0, V0, ZDC,
Muon arm, Acorde
PMD, DAQ
Partial installation:
PHOS(3/5)
7/18 TRD
2-4/6 EMCAL
~ 50% HLT
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P. Kuijer
4
4
The 2010 data sample
•p+p@7TeV: >800M MB, >100M m triggers, >25M high multiplicity
•Pb+Pb@2.76ATeV: ~30M nuclear collisions
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LHC: Entering a new regime
C W Fabjan 2008 J. Phys. G: Nucl. Part. Phys. 35, 104038
Cross-sections of interesting
probes expected to increase by
factors
~ 10 ( cc ) to
~ 102 ( bb ) to
~ > 105 (very high pT jets) ;
Hard probes of the medium
accessible at LHC
Direct photons are abundantly
produced at LHC
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Photon physics in ALICE
 Measurement of p0 and h mesons in pp@7TeV and PbPb@2.76ATeV




Highest energy available in lab
Test of pQCD cross section predictions
(HK 22.2, HK 22.3)
Reference data for PbPb
Main source of background in direct photon/heavy flavour electron measurement
 Measurement of Direct photons (not from decays) in pp@7TeV and
PbPb@2.76ATeV




Highest energy available in lab
Test of pQCD cross section predictions
Reference data for PbPb
PbPb: hard processes in QGP medium, thermal properties of early phase
 g-Jet (g-hadron)
 Study quark energy loss in the medium
 Fragmentation function: Ejet ~ Eg
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p0 and h reconstruction
pp  p0 + Xn
gg
e+
e+ e- e+ e-
(mp0 = 0.135 GeV/c2, BR = 0.988)
pp  h + Xn
gg
e-
e+ e- e+ e-
(mh = 0.548 GeV/c2, BR = 0.393)
g
Y
X
3 independent measurements:
Conversions, PHOS, EMCAL
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run 104792, event 2248
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V0 : Secondary vertex reconstruction
Reconstruction of (K0S,L, L ) converted photons in material (Z) gZ ge+e-Z
For analysis:
Package for fitting decay
particles based on the Kalman Filter
Specific energy loss in the TPC
-3sdE/dx<dE/dx-dE/dx(e)<+5s dE/dx
p rejection for p>1GeV, dE/dx > dE/dx(p)
m(g)=0
constrain to vertex
c2(g) < 30
Strict cut for material studies
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Photons from pair conversions:
g-ray image of ALICE
converted photons in material (Z): gZge+e-Z
ALICE material budget (11.4% X0) agrees
within +3.4,-6%
with its implementation in GEANT simulations
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p0 and h mesons @ 7TeV
M g 1g 2  2 Eg 1 Eg 2 (1  cos  g 1g 2 )
p0
h
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Data 7 TeV: p0 invariant mass in pt bins
Combinatorial bck using mixed events Mgg 0.17-0.3 GeV/c2 for normalization
Very low pt reached with the conversion method
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Data 7TeV: h invariant mass in pt bins
Mgg 0.6-0.7 GeV/c2 for normalization
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Raw yields. Reconstruction efficiency
With 9.5 x 107 Minimum Bias collisions:
p0 measured in 0.4 GeV/c < pt < 7GeV/c
h measured in 0.6 GeV/c < pt< 6 GeV/c
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Differential p0 invariant yield
Differential p0 invariant yield measured
in pp collisions@7 TeV
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Comparsion to NLO
NLO predictions
from W. Vogelsang
s pp=70mb used
New ALICE
measurement
s pp= 62.3±0.4(stat)
±4.3(syst.) mb
Method and calculations described in:
[1] F. Aversa et al., Nucl. Phys. B327, 105 (1989).
[2] B. Jäger et al., Phys. Rev. D67, 054005 (2003).
[3] D. de Florian, Phys. Rev. D67, 054004 (2003).
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h/p0 ratio
The h/p0 ratio is measured in pp collisions @7TeV
for 0.6 GeV/c <pt<6 GeV/c.
In agreement with Pythia expectations and with
the world data measured in hadron-hadron collisions,
taken from Phys. Rev.C 75 0224909 (2007).
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Direct photons at RHIC
PHENIX Coll.:Phys. Rev. Lett. 94, 232301 (2005)
PHENIX Coll.:Phys. Rev. Lett. 104 (2010) 132301
•pp consistent with NLO pQCD calculations
•AuAu larger than calculation for pT<3.5GeV/c
Excess exponential in pT
T=221± 23(stat)±18(sys) MeV
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How many direct photons @ LHC?
Measurement was done at RHIC
... but more difficult at LHC
g/p0 = 0,01-0,1 for pT > 20 GeV/c
We need a good distinction
direct/decay g
Cern Yellow Report 2004-009, hep-ph/0311131
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Photon decay cocktail
Work in progress
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Work in progress
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Jets
HK 22.4,
HK 22.5, HK 22.6,
HK 22.7, HK 40.9
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Jet properties from di-hadron
correlations
Near side (intra-jet):
“Single” jet properties, fragmentation
'near'side
r
jT
'away' side
UE
Away side (inter-jet):
Di-jet, hard scattering, properties,
KT effects
Trigger particle
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Correlation functions:√s= 0.9 and 7 TeV
Large statistics allows up to 30 GeV/c of the trigger particle
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Jet properties from di-hadron
correlations
Measure of average transverse
momentum of fragmentation
products relative to jet axis
jT2
jT2
900 GeV
7 TeV
= 678  12 MeV/c
 673  5 MeV/c
All measurements agree
within the errors
Expected independence
on trigger particle
pT and √s confirmed
r
jT
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Away side width and kT
zt
xˆh
kT2 
partonic
1
xh
2
 jTy2 (1 + xh2 )
pout
hadronic/measured
√(p/2)x√<kT2>
ALICE
preliminary
ALICE
preliminary
Confirmed expected increase of
momentum imbalance of parton pair
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Full Jet reconstruction in ALICE in 2010
 Using central tracking detectors
 Charged jet reconstruction only
 Different jet finders
 UA1 cone algorithm
 FASJET Suite (kT, anti-kT, SIScone)
 Cone size 0.4
 Comparison to PbPb
 Maximum efficiency of central barrel |hjet|<0.5
 Jet finding in PbPb
 Low momentum cut off (150 MeV)
pT , jet  pTrec, jet    A jet  s  A jet
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Raw Jet Spectrum: p+p @7TeV
Jets |h|<0.5
Tracks |h|<0.8
Jet spectrum with charged particles
safely reconstructed out to 70 GeV
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First PbPb collisions in ALICE
0-5% cent: dNch/dh ~ 1584 ± 4 (stat) ± 76 (syst)
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"Jet quenching" from charged
particle spectra
HK22.1
Suppression of high pt particles ( ~ leading jet fragments)
Rising with pt !
Accuracy limited by pp reference
=> need pp at 2.76 TeV !
ALICE, Phys. Lett. B 696 (2011) 30
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PbPb@2.76 ATeV: p0 invariant mass
in pt bins
p0 can be reconstructed from 0.4 GeV/c to 12 GeV/c
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p0 raw yield vs pt
First step towards p0 RAA
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Jet quenching from High pT
correlations
Clear away side correlation in p+p @7TeV
Df
Away side correlation in central Pb-Pb washed out up to pT,trig>10GeV
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Raw anti-k T Jet Spectrum in Pb+Pb
Event-by-event background subtracted,not unfolded.
Effect of background fluctuations/smearing apparent.
Detailed correction currently prepared.
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Conclusions and outlook
 ALICE took data succesfully during the 2010 run for pp
and PbPb collisions
 p0 (and h) mesons differential yields measured in pp
collisions at 7 TeV.
 Basic ingredients for a direct photon measurement
 Jet properties measured in pp collisions from di-hadron
correlations
 Full jet reconstruction is done using different algorithms.
Work is still ongoing to apply all corrections
 High pt suppresion of charged particles is observed in
central PbPb collisions. Jet quenching at LHC
 Uncorrected p0 pt spectrum and jet pt spectrum have
been shown. Work is ongoing to obtain RAA(p0) and fully
corrected jet pt spectrum
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ALICE TALKS
PV 5.1
HK
HK
HK
HK
HK
HK
HK
HK
HK
HK
HK
ALICE in Wonderland: first results from the ALICE experiment at the LHC — •Peter
Braun-Munzinger
7.8
Online Drift Velocity Calibration with the Laser System of the ALICE-TPC — •Mesut
Arslandok
12.9 Central diffractive meson production in p+p collisions at √s=7 TeV at the ALICE
experiment — •Xianguo Lu
14.8 The SysMES Inventory Module in the ALICE HLT Cluster — •Jochen Ulrich, Camilo Lara,
Stefan
Böttger, Timo Breitner, Pierre Zelnicek, and Udo Kebschull
14.9 Virtual Machine Scheduling in the ALICE HLT — •Stefan Boettger, Jochen Ulrich, Camilo
Lara,
Timo Breitner,
Pierre Zelnicek, and Udo Kebschull
15.3 Korrelationen von schweren Quarkteilchen in hochenergetischen Kern-Kern-Reaktionen —
•Andre
Mischke
15.4 Performance Studies for the Measurement of ψ′ via the Decay Channel ψ‵ → J/ψ π+π−
→ e+e− π+π− with
the ALICE Detector — •Moritz Pohl, Christoph Blume, and
Frederick Kramer
15.6 Messung von Dielektronen niedriger Masse mit dem ALICE Detektor in Proton-Proton und
Blei-Blei Kollisionen — •Markus Konrad Köhler
20.5 A Common Read-Out Receiver Card for ALICE DAQ and HLT — •Heiko Engel and Udo
Kebschull
22.1 Suppression of Charged Particle Production at Large Transverse Momentum in Central
Pb−Pb
Collisions at √sNN=2.76 TeV — •Jacek Otwinowski
22.2 Rekonstruktion von π0 und η Mesonen mittels Photon Konversionen in ALICE in Proton Proton
Kollisionen am CERN LHC — •Friederike Bock
22.3 Rekonstruktion von π0-Mesonen mittels Photon-Konversionen mit ALICE in PbPb-Kollisionen
am CERN LHC — •Radoslav Rusanov
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ALICE TALKS
HK 22.4
HK
HK
HK
HK
HK
HK
HK
HK
HK
HK
HK
Jet and High-pT Measurments with the ALICE Experiment at the LHC — •Bastian Bathen,
Tom Dietel, and
Christian Klein-Bösing
22.5 Quark and gluon composition of jets in proton-proton collisions at √s = 7 TeV with ALICE
at the LHC — •Hermes Leon Vargas
22.6 Two- and Three-Particle Jet-Like Correlations in the ALICE Experiment at the LHC —
•Jason Glyndwr Ulery
22.7 Charged Particle Momentum Distribution in Jets in ALICE — Bastian Bathen, •Oliver Busch,
and Christian Klein- Bösing
22.8 ALICE TRD GTU Online Tracking and Trigger Performance in pp and PbPb Collisions —
•Felix Rettig, Stefan
Kirsch, and Volker Lindenstruth
22.9 Beauty and beauty-jet measurement via displaced vertices with ALICE in p+p collisions at
√s = 7 TeV — •MinJung
Kweon
28.1 Alice HLT TPC Tracking of PbPb-Events on GPU and CPU — •David Rohr and Sergey
Gorbunov
28.2 Development of a trigger system to test and calibrate ALICE-TRD super-modules —
•Jonas Anielski
39.20 Ein Driftgeschwindigkeitsmonitor für den ALICE TRD — •Friederike Poppenborg
39.57 ALICE Grid Computing at the GridKa Tier-1 center — •Christopher Jung and Kilian
Schwarz
40.1 Measurement of transverse momentum spectra of identified hadrons from pp and PbPb
collisions with the ALICE
experiment — •Alexander Kalweit
40.2 Neutral strange hadron reconstruction in pp and PbPb collisions at LHC energies — •Simone
Schuchmann
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ALICE TALKS
HK 40.3
Measurement of nuclei and antinuclei with the ALICE experiment at the LHC — •Nicole
Martin and Alexander Kalweit
HK 40.5 Event-by-event fluctuations of mean transverse momentum in pp collisions at √s = 900 GeV
and 7 TeV measured by the ALICE experiment — •Stefan Heckel
HK 40.6 Analyse der Pionenquelle durch HBT-Interferometrie mit ALICE — •Johanna Gramling
HK 40.7 Analyse von Pseudorapiditäts-Dichtekorrelationen mittels Multiplizitätsverteilungen in ppund PbPb-Kollisionen mit
ALICE — •Maren Hellwig
HK 40.8 Mini-Jet Activity as Function of Charged Multiplicity — •Eva Sicking
HK 40.9 Underlying Event measurement in pp collisions with the ALICE detector — •Sara Vallero
for the ALICE collaboration
HK 40.10 Results of the Quality Assurance for the High Level Trigger Applications for the ALICE
TRD — •Theodor Rascanu
HK 61.1 vom ALICE Tier2 zum FAIR Tier0 - Computing at GSI — •Kilian Schwarz
HK 63.2 Photons and Jets from the First Year of ALICE — •Ana Marin
HK 64.1 Measurement of the J/ψ Production Cross Section in pp Collisions at √s=7 TeV with ALICE
at the LHC and
Perspectives for PbPb Collisions — •Frederick Kramer, Ionut Arsene,
Christoph Blume, Julian Book, Anton
Andronic, WooJin
J. Park, and Jens Wiechula
HK 64.2 Electrons from heavy flavour decays with the ALICE experiment in proton-proton collisions
at √s= 7TeV — •Raphaelle Bailhache
HK 64.3 Messung des Wirkungsquerschnitts von Hadronen mit schweren Quarks in Proton-Proton
Kollisionen mit
dem ALICE Experiment — •Fasel Markus
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ALICE TALKS
HK 64.4
HK 64.5
HK 64.6
Messung der Produktion schwerer Quarks in Pb+Pb Kollisionen mit dem ALICE
Detektorsystem — •Yvonne
Pachmayer
Reconstruction of open charm in the decay channel D0→K−π+ with ALICE —
•Robert Grajcarek
Open charm production in the D*+ → D0 π+ decay channel with ALICE — •Yifei
Wang
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ALICE COLLABORATION
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39
Backup slides
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Differential invariant cross section
PHOS + Conversions
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Background density
Strong change within one centrality bin.
Better correlation in multiplicity
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Testing Background Fluctuations
Embedding of single tracks: δ-probe for jet finding at random position
Random Cones: Sum in circular area at random position
(excluding the two leading jets)
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Charged
Jets
10-20% peripheral
168 GeV
192 GeV
Jet quenching
jet E ->
jet E’ (=E-DE) + soft gluons (DE)
Df
Dh
0-10% central
102 GeV
47 GeV
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Dh
44
Charged particle multiplicity density
dNch/dh ~ 1584 ± 4 (stat) ± 76 (syst)
•somewhat on high side of expectations
•growth with √s faster in AA than pp
PRL 105, 252301 (2010)
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Centrality dependence of Charged
particle multiplicity density
Centrality dependence of the
multiplicity is found very similar
at √sNN=2.76 and √sNN=0.2TeV.
Scale different by a factor 2.1
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PRL 106, 032301 (2011)
46
Un-triggered CF
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Probing Hot QCD Matter with hard probes
Hard Probes:
“highly penetrating observables (particles, radiation)
used to explore properties of matter that cannot be viewed directly!”
High pT particles: p0,h, direct g
Jets
p+p collisions provide a baseline
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Hard probes
p+p:
• parton scattering  fragmentation  jet
• can be calculated in perturbative QCD
• collinear factorization
hadrons
c
b
a
A+A:
• partons traversing medium lose energy
hadrons
gluon radiation, elastic collisions
• energy loss different for g, light/heavy quarks
(color factor, dead cone effect)
d
leading
particle
Goal: Use in-medium energy loss to
measure medium properties
h
ds pp
Dh0/ c
2
2 ds
 K   dxa dxb f a ( xa , Q ) f b ( xb , Q )
(ab  cd )
2
ˆ
dyd pT
dt
pzc
abcd
X.-N. Wang, M. Gyulassy, Phys. Rev. Lett. 68 (1992) 1480
Parton distribution function
measured in DIS
initial state (saturation?)
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Matrix element Fragmentation function
pQCD
e +e final state (energy loss?)
50
RAA for p0, h and direct g
RAAg~=1
0,h
RAAp
~=0.2
The hadron spectra at RHIC from p+p, Au+Au and d+Au collisions establish
existence of parton energy loss from strongly interacting,
dense QCD matter in central Au-Au collisions
^ = 4 – 13 GeV2 / fm
<q>
dNg/dy~1400+-200
S. Bass et al. PRC79 (2009) 024901
https://wiki.bnl.gov/TECHQM/index.php/Main_page
Theory-Experiment Collaboration on Hot Quark Matter
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jT systematics: formulae
If trigger and associated hadrons fragment independently:


s  t  a   t    + a     arcsin
2
2
N
2
2
j yt
2
pTt
+ arcsin
2
j ya
pTa
First approximation: arcsin(x)~x and because <jT2>=2<jy2>:
 1
1
2s  j  2 +
2
p
p
Ta
 Tt
2
N
2
T




jT2 
2s N
1
1
+
2
2
pTt
pTa
Two approximations follow:
(1) <1/p2>~1/<p>2
(2) <p> is taken from inclusive (rather than jet) spectrum
2
T
j
 2s N
pTt
pTt
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2
pTa
+ pTa
2
52
52
Why photon-tagged jets?

Medium effects redistribute (q
^ L ) the parton
energy, Ejet, inside the hadron jet
(multiplicity, jT).

Redistribution can be best measured with the
Fragmentation Function... If we know Ejet.

HI environment hinders precise
If we measure Eg ≈ Ejet
1/Njet dN/d
reconstruction of Ejet.
Borghini,Wiedemann, hep-ph/0506218
Quenching increases
low-pT particles
E
p
1
z
  ln( )  ln( )
Quenching reduces
high-pT particles
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Photon Detection in ALICE
 PHOS (1000 x 0.24 in h
(PHOS/CentralBarrel=0.037)
 high resolution (energy and spatial)
 small coverage
 EMCAL
 larger coverage: 120O x 1.4 in η (EMCAL/CentralBarrel=0.26)
 coarser spatial resolution than PHOS
 available in 2010
 gZ→ e+e- Z Conversions (Central Barrel)
 large coverage: 360O x 1.8 (2.4) in h (Conversion/CentralBarrel=0.08)
 low conversion probability : 8% (16%)
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Interaction of photons with matter
•Photoelectric effect
•Compton scattering
•Pair production: Eg>1.02MeV
•Electromagnetic Calorimeters:
The complete photon energy is
deposited in the detector (electromagnetic shower)
•Photon measurement via pair conversions:
Determine photon momentum and direction by measuring e+/efrom a single g conversion in tracking detectors
ALICE ( ITS+TPC+TRD)
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Direct photon sources
Why? Do not interact strongly.
Carry information about the early state.
q
γ
g
q
q
γ
q
qg Compton Scattering
Bremsstrahlung,
fragmentation
qq Annihilation
q
g
γ
g
γ
q
g
g
q
q
Thermal photons from QGP and hadron gas
Photons from Jet re-interaction in the medium
...in a large background from p0 and h decays
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Photon Production: different sources
Turbide, Gale, Jeon, and Moore
PRC (2004) 014906
Photons are abundantly produced at LHC
Jet-photon conversion in the plasma dominates 8<pT<14 GeV
Prompt hard NN scattering dominant for p T>20GeV at LHC
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Data 7 TeV: p0 invariant mass in pt bins
Combinatorial bck using mixed events Mgg 0.17-0.3 GeV/c2 for normalization
Very low pt reached with the conversion method
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Data 7TeV: h invariant mass in pt bins
Mgg 0.6-0.7 GeV/c2 for normalization
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Raw yields. Reconstruction efficiency
With 9.5 x 107 Minimum Bias collisions:
p0 measured in 0.4 GeV/c < pt < 7GeV/c
h measured in 0.6 GeV/c < pt< 6 GeV/c
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Differential p0 invariant yield
Differential p0 invariant yield measured
in pp collisions@7 TeV
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h/p0 ratio
The h/p0 ratio is measured in pp collisions @7TeV
for 0.6 GeV/c <pt<6 GeV/c.
In agreement with Pythia expectations and with
the world data measured in hadron-hadron collisions,
taken from Phys. Rev.C 75 0224909 (2007).
DPG Spring meeting 2011, Münster (Germany)
a.marin@gsi.de
62
PbPb@2.76 ATeV: p0 invariant mass
in pt bins
p0 can be reconstructed from 0.4GeV/c to 12GeV/c
DPG Spring meeting 2011, Münster (Germany)
a.marin@gsi.de
63
p0 raw yield vs pt
First step towards p0 RAA
DPG Spring meeting 2011, Münster (Germany)
a.marin@gsi.de
64