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Jets and High-pT Physics with ALICE at the LHC

Andreas Morsch

CERN

Workshop on High pT Physics at the LHC, Jyväskylä, March 25, 2007

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Outline

Jet reconstruction in heavy ion collisions Modified fragmentation functions with reconstructed jets Di-Hadron Correlations at LHC

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Jet Physics at RHIC

In central Au-Au collisions standard jet reconstruction algorithms fail due to the large energy from the underlying event (300 GeV in R< 1.0) and the relatively low accessible jet energies (< 20 GeV).

Use leading particles to tag the jet.

p+p @ s = 200 GeV STAR Au+Au @ sNN = 200 GeV

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Evidence for Jet Quenching

In central Au+Au Strong suppression of inclusive hadron yield in Au-Au collisions Disappearance of away-side jet

No suppression in d+Au Hence suppression is final state effect.

Phys. Rev. Lett. 91, 072304 (2003).

Pedestal&flow subtracted

STARSTAR

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Sensitivity to medium parameters

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

Eskola et al., hep-ph/0406319

RAA~0.2-0.3 for broad range of q

Use 2-hadron correlation, 3-hadron correlation… multi-hadron correlation

= Reconstructed Jets !

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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.

From toy model

= ln(Ejet/phadron)

Reconstructed Jet

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Jet reconstruction:New Challenges for ALICE

Existing TPC+ITS+PID || < 0.9 Excellent momentum

resolution up to 100 GeV Tracking down to 100 MeV Excellent Particle ID

New: EMCAL Pb-scintillator Energy resolution ~15%/√E Energy from neutral particles Trigger capabilities

central Pb–Pb

pp

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Signal fluctuationsResponse function for mono-chromatic jets

ET = 100 GeV, R = 0.4

E/E ~ 50%

E/E ~ 24%

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Expected resolution including EMCAL

Assumes conservative multiplicity: dN/dy = 6000

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Jet yields: one LHC year

Jet yield in 20 GeV bin

Large gains due to jet trigger

Large variation in statistical reach for different reference systems

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Background energy

In cone of R = 1 RHIC: 300 GeV LHC: 1500 GeV

However jet energies up to ~250 GeV accessible !

ET > Njets

50 GeV 2.0 107

100 GeV 1.1 106

150 GeV 1.6 105

200 GeV 4.0 104

Provides lever arm to measure the energy dependence of the medium induced energy loss.

104 jets needed to study fragmentation function in the z > 0.8 region.

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Background energy

How to reconstructs jets above a large fluctuation background (EBg) ? Restrict identification and reconstruction to domain in which

Emeas >> EBg

Cone size R < 1 pT-cut

Limiting case R=0: leading particle Advantage: background free by construction

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Optimal Cone Size

Jets reconstructed from charged particles:

Need reduced cone sizes and transverse momentum cut !

Ene

rgy

cont

aine

d in

sub

-co

ne R

E ~ R2

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.

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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 plane ~R2

Can be eliminated using impact parameter dependent background subtraction.

Poissonian fluctuations of uncorrelated particles E = N [<pT>2 +pT

2]

~R

Correlated particles from common source (low-ET jets) ~R

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Background Fluctuations

Evt-by-evtbackground energy

estimation

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Jet reconstruction in reduced domain:Why does it work ?

Measure only fraction of jet energy but measure it well In this case

Ejet Emeas

Since d/dEjet ~ 1/ET5.7, Ejet >> Emeas unlikely

Ejet Erec with relative small fluctuations

Jets are biased into the domain in which they are reconstructed = “Trigger Bias”

Works even for leading particle “jet reconstruction” pTrig = 0.6 Ejet

For ideal calorimetry and R=0.4: pTrig = 0.9 Ejet

Further restriction of domain in ALICE Charged particles only, in region without EMCAL coverage

Trigger bias: enhanced charged particle component TPC + EMCAL: charged + only small fraction of energy from neutrons

and K0L

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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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Another good reason for jet reconstruction:Statistics !

Strong bias on fragmentation function … which we want to measure

Low selectivity of the parton energy Very low efficiency, example:

~6% for ET > 100 GeV 1.1 106 Jets produced in central Pb-Pb collisions (|| < 0.5) No trigger: ~2.6 104 Jets on tape ~1500 Jets selected using leading particles

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Jet reconstruction in restricted domain:What can go wrong ?

Correction factors to go from measured to reconstructed jet energy unknown in AA !

Radiation Mainly soft particles Part of the energy goes outside of

the jet cone Needs

Good low-pT capabilities Measurement of the transverse

jet structure. Theoretical understanding of the

transverse jet-structure.

UnquenchedQuenched (AliPythia)Quenched (Pyquen)

pT < 2 GeV

Largest effect seen in low-pT particles.

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ALICE performance studiesand preparation for first analysis

Full detector simulation and reconstruction of HIJING events with embedded Pythia Jets

Implementation of a core jet analysis frame work Reconstruction and analysis of charged jets. Quenching studies with fragmentation function

TPC only and TPC + EMCAL

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Energy spectrum from charged jets

Cone-Algorithm: R = 0.4, pT > 2 GeV

Selection efficiency ~30% as compared to 6% with leading particle !No de-convolution, but GaussE-n ~ E-n

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Modification of the fragmentation function: Toy Model

Pythia hard scattering Initial and Final State Radiation

Afterburner A

Afterburner B

Afterburner C

.

.

.

Pythia Hadronization

Quenching of the final jet system and radiation of 1-5 gluons. (AliPythia::Quench using Salgado/Wiedemann - Quenching weights)

Nuclear Geometry(Glauber)

Jet (E) → Jet (E-E) + n gluons (“Mini Jets”)

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RAA()

ratio

BBS 002.0

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Example: p+Pb reference

With EMCal: jet trigger+ improved jet reconstruction provides much greater ET reach

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Trigger Bias

Production spectrum weighted response matrix:

Out-of-domain fluctuations are damped with 1/En

symmetrizing the distribution.

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Jet energy resolution and dN/dz

Direct comparison with the MC truth for the same selected track.The dotted line shows the point spread function for z = 0.4.

Model1 Model2

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Systematic shift in RAA()

More energy is radiated outside the cone.

On average the input energy has to be higher in order to give a reconstructed energy of 100 GeV.

As a consequence is shifted to lower values.

Systematics has to be controlled using measurements of the transverse jet structure and RAA

Jet(ET)

unquenched

quenched

Erec = 100 GeV

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Systematic Error fromBackground Subtraction

2 GeV

Soft Background

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Background Fluctuation

log(E/GeV)

log(

dN/d

E)

Background fluctuates up

Jet input spectrum

Background fluctuates down

Bias towards higher Bg

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Influence on Jet axis

dR

Under Ideal detector response – Not quenching

R. Diaz Valdes

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Influence for a jet input spectrum

<Etinput> <Et

rec/Etinput>

p-p 120.0 ± 17.23 0.856 ± 0.0815

Pb-Pb 116.2 ± 19.21 0.894 ± 0.1169

R. Diaz Valdes

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Bias on RAA()

Corrections should be applied on p-p distribution to compare it with quenched Pb-Pb jet fragmentation

R. Diaz Valdes

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Di-hadron Correlations:from RHIC to LHC

Di-hadron correlations will be studied at LHC in an energy region where full jet reconstruction is not possible (E < 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 -correlation (loss of acceptance for fixed -widow) Power law behavior d/dpT ~ 1/pT

n with n = 8 at RHIC and n = 4 at LHC Changes the trigger bias on parton energy

PNBS

SPp

N

BS

SPp

PN

BS

S

PB

S

NPB

NPS

T

T

1: high RHICFor

1 :LHC and low RHICFor

P1 and

1

2

PYTHIA 6.2

See also, K. Filimonov, J.Phys.G31:S513-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) ~ pT

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pTtrig > 8 GeV

RHIC/STAR-like central Au-Au (1.8 107 events)

LHC/ALICE central Pb-Pb (107 events), no-quenching

From STAR pTtrig = 8 GeV/c

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Di-hadron Correlations

STAR LHC, ALICE acceptanceHIJING Simulation

“Peak Inversion”

O(1)/2

4 105 events

M. Ploskon, ALICE INT-2005-49

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Summary

Copious production of jets in Pb-Pb collisions at the LHC Jets can be reconstructed over the background from the underlying event Sufficient dynamic range (50 – 250 GeV) to make systematic studies of

energy dependence. Background conditions require jet identification and reconstruction in

reduced domain R = 0.4. We will measure jet structure observables (jT, fragmentation function,

jet-shape) for reconstructed jets. In AA, high-pT (calorimetry) and low-pT capabilities needed for unbiased

measurement of parton energy. Strength of ALICE

Excellent low-pT capabilities to measure particles from medium induced radiation.

PID to measure the particle composition of quenched jets Dedicated pp experiments have larger ET reach

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Jet Finder based on cone algorithms

Input: List of cells in an 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 > 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 (c, c) = (i, i) Using all cells with (i-)2+(i-)2 < 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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Jet Finder in HI Environment:Principle

Loop1: Background estimation from cells outside jet conesLoop2: UA1 cone algorithm to find centroid

using cells after background subtraction

Rc

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Putting things together:Intrinsic resolution limit

pT > 0 GeV1 GeV2 GeV

Resolution limited by out-of-conefluctuations common to all experiments !

Ejet = 100 GeV

Background included

40ALICE Set-up

HMPID

Muon Arm

TRD

PHOS

PMD

ITS

TOF

TPC

Size: 16 x 26 meters

Weight: 10,000 tons

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Trigger performance

Trigger on energy in patch xBackground rejection set to factor of 10=>HLT

Centrality dependent thresholds

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Summary of statistical reach

Ratio >4 With EMCAL W/O EMCAL

RAA 225 165

RpA 225 125

RAA(5.5 TeV) 225 100

RAA() 150 110

RCP 150 (70)

Ratio z>0.5 With EMCAL W/O EMCAL

RAA 150 100

RpA 150 (70)

RAA(5.5 TeV) 140 (60)

Large : ~10% error requires several hundred signal events (Pb central) and normalization events (pp,pA).

Large z>0.5 requires several thousand events

The EMCAL • extends kinematic range by 40–125 GeV• improves resolution (important at high z)

Some measurements impossible w/o EMCAL