Jets and High-pt Physics with ALICE at the LHC

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Jets and High-pt Physics with ALICE at the LHC

• Andreas Morsch
• CERN

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Outline

• Introduction
• Jets at RHIC and LHC New perspectives and
  challenges
• High-pT di-hadron correlations
• Reconstructed Jets
• Jet Structure Observables
• g-Jet Correlations

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Jets in nucleus-nucleus collisions

• Jets are the manifestation of high-pT partons
  produced in a hard collisions in the initial
  state of the nucleus-nucleus collision.
• These partons undergo multiple interaction inside
  the collision region prior to fragmentation and
  hadronisation.
• In particular they loose energy through medium
  induced gluon radiation and this so called jet
  quenching has been suggested to behave very
  differently in cold nuclear matter and in QGP.
• The properties of the QGP can be studied through
  modification of the fragmentation behavior
• Hadron suppression
• Jet structure.

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Jet Physics at RHIC
pp _at_ ?s 200 GeV STAR
AuAu _at_ ?sNN 200 GeV
In central Au-Au collisions standard jet
reconstruction algorithms fail due to the large
energy from the underlying event (125 GeV in Rlt
0.7) and the relatively low accessible jet
energies (lt 20 GeV). Use leading particles as a
probe.
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Quantities studied
Hadron Suppression
Similar RCP Ratio central to peripheral
away side
pT(assoc)
Hadron Correlations pT(trig)
pT(assoc) Df(trig, assoc)
same side
pT (trig)
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Evidence for Jet Quenching

• In central AuAu
• Strong suppression of inclusive hadron yield in
  Au-Au collisions
• Disappearance of away-side jet
• No suppression in dAu
• Hence suppression is final state effect.

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Surface emission bias

• RHIC measurements are consistent with pQCD-based
  energy loss simulations. However, they provide
  only a lower bound to the initial color charge
  density.

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Jet Physics at LHC Motivation

• Study of reconstructed jets increases sensitivity
  to medium parameters by reducing
• Trigger bias
• Surface bias
• Using reconstructed jets to study
• Modification of the leading hadron
• Additional hadrons from gluon radiation
• Transverse heating.

x ln(Ejet/phadron)
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Jet Physics at LHC New perspectives

• Jet rates are high at energies at which they can
  be reconstructed over the large background from
  the underlying event.
• Reach to about 200 GeV
• Provides lever arm to measure the energy
  dependence of the medium induced energy loss
• 104 jets needed to study fragmentation function
  in the z gt 0.8 region.

Pb-Pb
ET gt Njets
50 GeV 2.0 ? 107
100 GeV 1.1 ? 106
150 GeV 1.6 ? 105
200 GeV 4.0 ? 104
O(103) un-triggered (ALICE) gt Need Trigger
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Jet Physics at LHC New challenges

• More than one jet ETgt 20 GeV per event
• More than one particle pT gt 7 GeV per event
• 1.5 TeV in cone of R ?Dh2Df2 lt 1 !
• We want to measure modification of leading hadron
  and the hadrons from the radiated energy. Small
  S/B where the effect of the radiated energy
  should be visible
• Low z
• Low jT
• Large distance from the jet axis
• Low S/B in this region is a challenge !

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New Challenges for ALICE

• Existing TPCITSPID
• h lt 0.9
• Excellent momentum resolution up to 100 GeV
• Tracking down to 100 MeV
• Excellent Particle ID
• New EMCAL
• Pb-scintillator
• Energy resolution 15/vE
• Energy from neutral particles
• Trigger capabilities

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ALICE Set-up
Size 16 x 26 meters Weight 10,000 tons
TOF
TRD
HMPID
TPC
PMD
ITS
Muon Arm
PHOS
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Di-hadron Correlationsfrom RHIC to LHC

• Di-hadron correlations will be studied at LHC in
  an energy region where full jet reconstruction is
  not possible (E lt 30 GeV).
• What will be different at LHC ?
• Number of hadrons/event (P) large
• Leads to increased signal and background at LHC
• Background dominates, significance independent of
  multiplicity
• Increased width of the away-side peak (NLO)
• Wider h-correlation (loss of acceptance for fixed
  h-widow)
• Power law behavior ds/dpT 1/pTn with n 8 at
  RHIC and n 4 at LHC
• Changes the trigger bias on parton energy

See also, K. Filimonov, J.Phys.G31S513-S520
(2005)
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Scaling From RHIC to LHC

• S/B and significance for away-side correlations
• Scale rates between RHIC and LHC
• Ratio of inclusive hadron cross-section
• N(pT) pT4

From STAR pTtrig 8 GeV/c
pTtrig gt 8 GeV
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Di-hadron Correlations
STAR
LHC, ALICE acceptance HIJING Simulation
4 105 events
M. Ploskon, ALICE INT-2005-49
O(1)/2p
Peak Inversion
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The biased trigger bias
ltpTpartgt is a function of pTtrig but alsp
pTassoc, ?s, near-side/away-side, DE
See also, K. Filimonov, J.Phys.G31S513-S520,2005
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From di-hadron correlations to jets

• Strong bias on fragmentation function
• which we want to measure
• Low selectivity of the parton energy
• Very low efficiency, example
• 6 for ET gt 100 GeV
• 1.1 106 Jets produced in central Pb-Pb collisions
  (h lt 0.5)
• No trigger 2.6 104 Jets on tape
• 1500 Jets selected using leading particles

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Reduction of the trigger biasby collecting more
energy from jet fragmentation
Unbiased parton energy fraction production
spectrum induced bias
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Reconstructed Jets Objectives

• Reduce the trigger bias as much as possible by
  collecting of maximum of jet energy
• Maximum cone-radius allowed by background level
• Minimum pT allowed by background level
• Study jet structure inclusively
• Down to lowest possible pT (z, jT)
• Collect maximum statistics using trigger.

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Jet Finder in HI EnvironmentPrinciple

Rc
Loop1 Background estimation from cells outside
jet cones Loop2 UA1 cone algorithm to find
centroid using cells after background
subtraction
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Jet Finder based on cone algorithms

• Input List of cells in an h-f grid sorted in
  decreasing cell energy Ei
• Estimate the average background energy Ebg per
  cell from all cells.
• For at least 2 iterations and until the change in
  Ebg between 2 successive iterations is smaller
  than a set threshold
• Clear the jet list
• Flag cells outside a jet.
• Execute the jet-finding loop for each cell,
  starting with the highest cell energy. If Ei
  Ebg gt Eseed and if the cell is not already
  flagged as being inside a jet
• Set the jet-cone centroid to be the center of the
  jet seed cell (hc, fc) (hi, fi)
• Using all cells with ?(hi-h)2(fi-f)2 lt Rc of the
  initial centroid, calculate the new energy
  weighted centroid to be the new initial centroid.
• Repeat until difference between iterations shifts
  less than one cell.
• Store centroid as jet candidate.
• Recalculate background energy using information
  from cells outside jets.

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Optimal Cone Size
Jet Finders for AA do not work with the standard
cone size used for pp (R 0.7-1). R and pT cut
have to be optimized according to the background
conditions.
E R2
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Background Fluctuations

• Background fluctuations limit the energy
  resolution.
• Fluctuations caused by event-by-event variations
  of the impact parameter for a given centrality
  class.
• Strong correlation between different regions in
  h-f plane
• R2
• Can be eliminated using impact parameter
  dependent background subtrcation.
• Poissonian fluctuations of uncorrelated particles
• DE ?N ?ltpTgt2 DpT2
• R
• Correlated particles from common source (low-ET
  jets)
• R
• Out-of-cone Fluctuations

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Background Fluctuations
Evt-by-evt background energy estimation
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Signal fluctuationsResponse function for
mono-chromatic jets
ET 100 GeV
DE/E 50
DE/E 30
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Putting things togetherIntrinsic resolution
limit
Ejet 100 GeV
Background included
pT gt 0 GeV 1 GeV 2 GeV
Resolution limited by out-of-cone fluctuations
common to all experiments !
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Expected resolution including EMCAL
Jet reconstruction using charged particles
measured by TPC ITS And neutral energy from
EMCAL.
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Trigger performance
Trigger on energy in patch Dh x Df Background
rejection set to factor of 10 gtHLT
Centrality dependent thresholds
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Reference systems

• Compare central PbPb to reference measurements
• PbPb peripheral vary system size and shape
• pA cold nuclear matter effects
• pp (14 TeV) no nuclear effects, but different
  energy
• pp (5.5 TeV) ideal reference, but limited
  statistics

All reference systems are required for a complete
systematic study
Includes acceptance, efficiency, dead time,
energy resolution
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Jet yields one LHC year
Large gains due to jet trigger Large variation
in statistical reach for different reference
systems
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Resolution buys statistics
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ALICE performanceWhat has been achieved so far ?

• Full detector simulation and reconstruction of
  HIJING events with embedded Pythia Jets
• Implementation of a core analysis frame work
• Reconstruction and analysis of charged jets.
• Quenching Studies on fragmentation function.

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Energy spectrum from charged jets
Cone-Algorithm R 0.4, pT gt 2 GeV
Selection efficiency 30 as compared to 6 with
leading particle ! No de-convolution, but
Gauss?E-n E-n
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Jet structure observables
Low z (high x) Systematics is a challenge,
needs reliable tracking. Also good statistics
(trigger is needed)
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Hump-back plateau
Bias due to incomplete reconstruction.
Statistical error
104 events
Erec gt 100 GeV
2 GeV
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Systematics of background subtraction
Background energy is systematically
underestimated (O(1 GeV)) Corrections under study
(thesis work of R. Dias Valdez)
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jT-Spectra
Bias due to incomplete reconstruction.
Statistical error
104 events
Erec gt 100 GeV
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Quenching Studies
Compare distributions with and without
quenching The measurement ratio of dashed over
solid PbPb(central)/pp
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Toy Models
Nuclear Geometry (Glauber)
Jet (E) ? Jet (E-DE) n gluons (Mini Jets)

• Two extreme approaches
• Quenching of the final jet system and radiation
  of 1-5 gluons. (AliPythiaQuench using
  Salgado/Wiedemann - Quenching weights)
• Quenching of all final state partons and
  radiation of many (40) gluons (I. Lokhtin
  Pyquen)

)I.P. Lokhtin et al., Eur. Phys. J C16 (2000)
527-536 I.P.Lokhtin et al., e-print
hep-ph/0406038 http//lokhtin.home.cern.ch/lokhtin
/pyquen/
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ALICEEMCal in one LHC year
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Benchmark measurementpPb reference
With EMCal jet trigger improved jet
reconstruction provides much greater ET reach
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Benchmark measurementPeripheral PbPb reference
Without EMCal, significant quenching measurements
beyond 100 GeV are not possible
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Summary of statistical reach
Large ? 10 error requires several hundred
signal events (Pb central) and normalization
events (pp,pA). Large zgt0.5 requires several
thousand events
Ratio ?gt4 With EMCAL W/O EMCAL
RAA 225 165
RpA 225 125
RAA(5.5 TeV) 225 100
RAA(f) 150 110
RCP 150 (70)

• The EMCAL
• extends kinematic range by 40125 GeV
• improves resolution (important at high z)
• Some measurements impossible w/o EMCAL

Ratio zgt0.5 With EMCAL W/O EMCAL
RAA 150 100
RpA 150 (70)
RAA(5.5 TeV) 140 (60)
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More to come

• Dijet correlations
• Sub-jet Suppression ?
• Look for hot spots at large distance to jet
  axis
• 10 GeV parton suppression within 100 GeV jets ?

R0 1fm
Q
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Photon-tagged jets

• g-jet correlation
• Eg Ejet
• Opposite direction
• Direct photons are not perturbed by the medium
• Parton in-medium-modification through the
  fragmentation function

g
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Identifying prompt g in ALICE
Statistics for on months of running 2000 g with
Eg gt 20 GeV Eg reach increases to 40 GeV with
EMCAL
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Fragmentation function
Pb-Pb collisions
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Summary

• Copious production of jets in Pb-Pb collisions at
  the LHC
• lt 20 GeV many overlapping jets/event
• Inclusive leading particle correlation
• Background conditions require jet identification
  and reconstruction in reduced cone R lt 0.3-0.5
• At LHC we will measure jet structure observables
  (jT, fragmentation function, jet-shape) for
  reconstructed jets.
• High-pT capabilities (calorimetry) needed to
  reconstruct parton energy
• Good low-pT capabilities are needed to measure
  particles from medium induced radiation.
• EMCAL will provide trigger capabilities which are
  in particular needed to perform reference
  measurements (pA, pp, ..)
• ALICE can measure photon tagged jets with
• Eg gt 20 GeV (PHOS TPC)
• Eg gt 40 GeV (EMCALTPC)
• Sensitivity to medium modifications 5