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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 -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

In central Au-Au collisions standard jet reconstruction algorithms fail due to the large energy from the underlying event (125 GeV in R< 0.7) and the relatively low accessible jet energies (< 20 GeV).

Use leading particles as a probe.

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

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Quantities studied

ddpdT

ddpNdpR

TNN

AA

TAA

TAA /

/)(

2

2

pT (trig)

pT(assoc)

Hadron Suppression

Similar RCP: Ratio central to peripheral

Hadron Correlations:

pT(trig) – pT(assoc)(trig, assoc)…

“away side”

“same side”

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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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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.

Eskola et al., hep-ph/0406319

RAA~0.2-0.3 for broad range of q

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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

s = 5500 GeV

A. Dainese, C. Loizides, G. Paic

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Jet Physics at LHC: New perspectives

ET > Njets

50 GeV 2.0 107

100 GeV 1.1 106

150 GeV 1.6 105

200 GeV 4.0 104

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 > 0.8 region.

Pb-Pb

O(103) un-triggered (ALICE) => Need Trigger

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Jet Physics at LHC: New challenges

More than one jet ET> 20 GeV per event More than one particle pT > 7 GeV per event 1.5 TeV in cone of R = 2+2 < 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 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

12ALICE Set-up

HMPID

Muon Arm

TRD

PHOS

PMD

ITS

TOF

TPC

Size: 16 x 26 meters

Weight: 10,000 tons

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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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The biased trigger bias

hep-ph/0606098

pTtrig > 8 GeV

<pTpart> is a function of pT

trig but alsp pTassoc, s, near-side/away-side, E

See also, K. Filimonov, J.Phys.G31:S513-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 > 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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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 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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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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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 subtrcation.

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

2] ~R

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

Out-of-cone Fluctuations

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Background Fluctuations

Evt-by-evtbackground energy

estimation

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Signal fluctuationsResponse function for mono-chromatic jets

ET = 100 GeV

E/E ~ 50%

E/E ~ 30%

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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

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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 xBackground rejection set to factor of 10=>HLT

Centrality dependent thresholds

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Reference systems

Jet trigger

Compare central Pb+Pb to reference measurements• Pb+Pb peripheral: vary system size and shape• p+A: cold nuclear matter effects• p+p (14 TeV): no nuclear effects, but different energy• p+p (5.5 TeV): ideal reference, but limited statistics

Includes acceptance, efficiency, dead time, energy resolution

All reference systems are required for a complete systematic study

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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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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 > 2 GeV

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

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Jet structure observables

Low z (high ): 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.

Erec > 100 GeV

Statistical error

2 GeV

104 events

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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.

Erec > 100 GeV

Statistical error

104 events

jT

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Estimate quenching at LHC:

/fmGeV50~ˆ7~ˆ 2RHICLHC qq

Quenching Studies

Compare distributions with and without quenching

The measurement: ratio of dashed over solid= Pb+Pb(central)/p+p

fm/GeV50ˆ 2q

Solid: unquenched (p+p)

Pythia-based simulation with quenching

Large R, no pT cut

Dashed: quenched jet (central Pb+Pb)

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Toy Models

Pythia hard scattering Initial and Final State Radiation

Afterburner A

Afterburner B

Afterburner C

.

.

.

Pythia Hadronization

Two extreme approaches Quenching of the final jet system and radiation of 1-5 gluons.

(AliPythia::Quench using Salgado/Wiedemann - Quenching weights) Quenching of all final state partons and radiation of many (~40) gluons

(I. Lokhtin: Pyquen)*

Nuclear Geometry(Glauber)

)*I.P. Lokhtin et al., Eur. Phys. J C16 (2000) 527-536 I.P.Lokhtin et al., e-print hep-ph/0406038http://lokhtin.home.cern.ch/lokhtin/pyquen/

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

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ALICE+EMCal in one LHC year

ratio

BBS 002.0

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Benchmark measurement:p+Pb reference

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

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Benchmark measurement:Peripheral Pb+Pb reference

Without EMCal, significant quenching measurements beyond ~100 GeV are not possible

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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

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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

tform = 1/(kT)tsep = 1/

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Photon-tagged jets

Dominant processes:

g + q → γ + q (Compton)

q + q → γ + g (Annihilation)

pT > 10 GeV/c

-jet correlation E = Ejet

Opposite direction Direct photons are not perturbed by the medium Parton in-medium-modification through the fragmentation function

min max

IP

PHOS

EMCal

TPC

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Identifying prompt in ALICE

x5signal

Statistics for on months of running:2000 with E > 20 GeV

E reach increases to 40 GeV with EMCAL

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Fragmentation function

quenched jet

non-quenched

Pb-Pb collisions

Background

Signal

HIC background

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Summary

Copious production of jets in Pb-Pb collisions at the LHC < 20 GeV many overlapping jets/event

Inclusive leading particle correlation Background conditions require jet identification and reconstruction in

reduced cone R < 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

E > 20 GeV (PHOS + TPC) E > 40 GeV (EMCAL+TPC) Sensitivity to medium modifications ~5%