Jet Analysis in Heavy-Ion Collisions

Jet Analysis in Heavy-Ion Collisions

Elena Bruna

INFN Torino & Yale University

5th International School on QGP. Torino, March 2011

5th International School on QGP. Torino, March 2011

These (experimental) lectures won’t probably tell you everything you would ever wanted to know about jets…but I hope some of the young minds will be inspired and start/continue working on hard probes to turn over more stones…

“The same thrill, the same awe and mystery, come again and again when we look at any problem deeply enough. With more knowledge comes deeper, more wonderful mystery, luring one on to penetrate deeper still. With pleasure and confidence we turn over each stone to find unimagined strangeness.”

R. Feynman

Past, Present, Future…

Elena Bruna (Yale&INFN Torino) 3

LEPe+e-

RHIC

Pb+Pb @ √sNN=2.76 TeV

LHC

• Full jet reconstruction energy of the hard scattering but challenging in A+A• New jet-finding tools• Do physics with jets !

Elena Bruna (Yale&INFN Torino)

Outline

Jet I: Intro & Motivations

Jet II: Full Jet Reconstruction

Jet III: Results

Jet IV:The Present: from RHIC to LHC

Jet I: Intro & Motivations

Jets in high-energy collisions

6Elena Bruna (Yale&INFN Torino)

PDF Partonic x-section Fragmentation function

pQCD Factorization:

p pa, xa b, xbσab

c, x c

d, x d

Factorization: assumed between the perturbative hard part and the universal non-perturbative fragmentation (FF) and parton distribution functions (PDF)

Universality: fragmentation functions and parton distribution functions are universal (i.e. FF from ee, PDF from ep, use for pp)

Collins, Soper, StermanNucl. Phys. B263 (1986) 37




pQCD Factorization: Collins, Soper, StermanNucl. Phys. B263 (1986) 37

QCD factorization works!

p + p 0

p+p √s=200 GeV

p + p

p + p p

p+p √s=200 GeV




pQCD Factorization:

p pa, xa b, xbσab

c, x c

d, x d

Collins, Soper, StermanNucl. Phys. B263 (1986) 37

PDFs:

Probability for a parton a(b) to carry a fraction xa(xb) of the hadron momentum

Universal can be measured with fit to experimantal data for one or more processes that can be calculated with perturbative QCD, i.e. deep inelastic scattering DIS (like e-p), Drell-Yan processes (qq l+l-) and others

Many PDFs on the market (CTEQ, GRV, MRST,…)





Hard scattering: dσ/dt = parton cross section calculable in powers of αS

LO

NLO





z

Fragmentation Functions:probability to find, at scale Q, a hadron h with a fraction z of the parton c momentum

universal and measured with fits to experimental dataMany D on the market (KKP, AKK, …)

p(hadron)p (parton)z =

parto

n

Jets in Nucleus-Nucleus collisions


Detector

Self-generated“hard” probes

Calibrated LASER/x-ray

Hard processes make perturbative QCD applicable high momentum transfer Q2

Jet Tomography!

Hard processes scale as Nbin



jet energy loss in the medium

Questions:1) How does the parton lose energy?2) What happens to the radiated energy?3) Collisional energy loss?4) Does the energy loss depend on the parton type?

Eloss ~ ρgluon (gluon density)Eloss ~ ΔL2 (medium length) [~ ΔL with expansion]Egluon > Equark, m=0 > Equark, m>0

Important to measure E of gluons light heavy quarks…

Transport coefficient: q = 2 / is the <pT2> transferred from the parton to a

gluon per unit path length

^

Interpretation: Gluon radiation



Eskola, Honkanen, Salgado, WiedemannNucl Phys A747 (2005) 511

q = 5 – 15 GeV2 / fm from RHIC RAA Data ^

Some Predictions: FF


in je

i j t

t

n e

: increases

z : decreases

chn

Gyulassy et al., nucl-th/0302077

Energy loss in the medium softer fragmentation

Borghini and Wiedemann, hep-ph/0506218

ξ stretches the low z part

Renk, Phys. Rev. C79:054906,2009

z=ph/pjet

ph

pjet

Some Predictions: Jet shapes


If energy loss by gluon radiation broadening of the jet energy profile

Energy loss ratio goes down with larger b.

Energy loss ratio becomes smaller withsmaller R and larger ωmin.

Limit of large R and ωmin=0 no out-of-cone energy ΔEin~E

I Vitev, S Wicks, B-W Zhang,JHEP 0811,093 (2008); EPJC 62, 139 (2009).

R = jet radius (on η-ϕ plane) =√(Δϕ2+Δη2)ωmin (pT

min) = minimum pT on particles in the jet



I Vitev, S Wicks, B-W Zhang,JHEP 0811,093 (2008); EPJC 62, 139 (2009).Vitev, Zhang, PRL 104 (2010) 132001, arXiv: 0910.1090

Limits: • small Rmax and large ωmin single particlesuppression.• large Rmax and small ωmin all jet energy recovered RAA

jet=1 ! (jet production is hard process,

scales as Nbin)



I Vitev, S Wicks, B-W Zhang,JHEP 0811,093 (2008); EPJC 62, 139 (2009).Vitev, Zhang, PRL 104 (2010) 132001, arXiv: 0910.1090

Limits: • small Rmax and large ωmin single particlesuppression.• large Rmax and small ωmin all jet energy recovered RAA

jet=1 ! (jet production is hard process,

scales as Nbin)

Jet quenching from single high-pT hadrons


Observations at RHIC:

1.Large suppression of high-pT hadrons: factor ~ 52.Photons are not suppressed• They don’t interact with the medium (good!)• Nbin scaling works

Jet quenching from single high-pT hadrons


Observations at RHIC:

1.Large suppression of high-pT hadrons: factor ~ 52.Photons are not suppressed• They don’t interact with the medium (good!)• Nbin scaling works3. Also Heavy Flavor is suppressed at RHIC•same as light quarks•role of bottom?•collisional energy loss/resonant elastic scattering?


Jet quenching from single high-pT hadronsRHIC suppression < LHCRHIC: high pT hadrons hadronize from quarks LHC: from gluons (larger color charge!)

Prediction: Vitev(hep-ph/050322v1)• GLV – pQCD factorization • medium-induced gluon brems.

ALICE, Phys. Lett. B 696 (2011) 30.

Jet quenching from di-hadrons


Start from a high-pt “trigger” particle and look on the away side (in ).

Azimuthal correlation function shows ~complete absence of “away-side” jetPartner in hard scatter is absorbed in the dense medium

not the case in d+Au final state effect



Start from a high-pt “trigger” particle make azimuthal correlation

~complete absence of “away-side” jetPartner in hard scatter is strongly interacting with the dense medium

not the case in d+Au final state effect !

Path-length dependence of di-jet topologies

y

x

Out-of-plane

in-plane

Back-to-back suppression out-of-plane stronger than in-plane



STAR, Phys.Rev.C82 024912 (2010)

increasing pTtrig

incre

as

ing

pT

ass

oc



STAR, Phys.Rev.C82 024912 (2010)

increasing pTtrig

incre

as

ing

pT

ass

oc

At high trigger pT:

• re-emergence of away-side jet (punch thru)? or• tangential jets?

At low trigger pT & low pTassoc:double bump:• Mach Cone – conical emission?• Cherenkov Radiation?• pure 3D hydro? [won’t discuss this]

Trigger and Surface BiasesExperiments online-trigger dependent:

• Large pT or energy deposition triggers bias towards hard fragmentation!

• EM calorimetry bias towards large EM fraction


Trigger particles biased toward the surface Surface bias, as seen in hydro models

High pT: towards jets


What we have so far:• Suppression of high-pT hadrons in A+A (at RHIC and LHC) w.r.t. p+p • Evidence for parton energy loss in the medium

But:• Geometrical bias: dominated by surface jets• Jet energy not constrained• Limited kinematic reach

What we want:• Precise measurement of the parton energy loss• Measurement of the modified fragmentation function

How?

Renk and Eskola, hep-ph/0610059

ALICE, Phys. Lett. B 696 (2011) 30.

1. -Jet• Jet energy well constrained

• limited kinematic reach (x-sec scales as αSαem)• Difficult to have a clean measurement of photons



z = p(h)/pparton

p ≠pparton

Leading

Hadron

E = pparton

Di-Hadron

How?

Courtesy Thomas Ullrich

Direct -hadron fragmentation functions


STAR, Phys. Rev. C 82 (2010) 34909

Trig particle= or 0

Assoc particle: h±

1. Good agreement w/ theory models2. more assoc h± for than for different parton energies for and

(come from fragmentation of higher energy parton)

1. Au+Au: different path-length for the recoil jet for and and triggers

Direct -hadron fragmentation functions


IAA= ratio of associated yield per trigger in Au+Au to that in p+p

8<Etrig<16 GeV/c

1. IAA < 1 for zT>0.32. data can distinguish between different

theoretical models3. low zT: expected differences between

and IAA due to path-length dependence of the energy loss

Trig particle= or 0

Assoc particle: h±

Measurements do not indicate path-length or color-charge dependence !


• limited kinematic reach (x-sec scales as αSαem)• Difficult to have a clean measurement of photons



z = p(h)/pparton

E = pparton

Ejet = pparton

How?

Courtesy Thomas Ullrich


• limited kinematic reach (x-sec scales as αSαem)• Difficult to have a clean measurement of photons2. Full Jet Reconstruction • Larger kinematic reach• large background complex and challenging !

Jet II: Full Jet Reconstruction


Jets: Theory vs Experiment


Experiment: jet = spray of collimated hadrons

GOAL: measure the parton energy in experiments do jet physics!

Tool: Full jet reconstruction with jet-finding algorithms

• for both Theory and Experiment !

Theory (pQCD): jet = High-pT parton produced in hard scatterings, or the closest object to a parton

Theoretical requirements


y

pT cone iteration

Jet 1

y

pT

Jet 1

Jet 2Collinear safety replaces one parton by two at the same place the algorithm should be insensitive to any collinear

radiation. Infrared safety a soft emissions that add very soft gluon the jet-finding algorithm should not be sensitive to soft radiation

Experimental requirements

• Detector independence: the performance of the jet algorithm should not be dependent on detector segmentation, energy resolution, …

• Stability with luminosity: jet finding should not be strongly affected by multiple hard scatterings at high beam luminosities.

• Fast

• Efficient: the jet algorithm should find as many physically interesting jets as possible, with good energy resolution CDF


Jet Finding algorithms

Sequential Recombination Cone

kT CDF JetClu, MidPoint

Anti-kT D0 Cone

Cambridge - Aachen CMS Iterative Cone

ATLAS Cone

PyCell

SISCone

Particles are combined into jets• the larger experimental coverage, the betterWhich particles? The measured ones: • charged tracks (TPC)• neutral towers (EMC)• charged energy (Hcal)Different ways of combining particles jet-finding algorithms

35

Review of CDF Jet Algorithms, arXiv:hep-ex/0005012v2FastJet JHEP 0804:005, arXiv:0802.1188FastJet JHEP 0804, 063 (2008), arXiv:0802.1189v2


Sequential Recombination

36

If dij<kTi-2 merged

If dij>kTi-2 not merged

call it a jet

kTi,j= particle transverse momentum (pT)kT: p>0 (soft particles merged first)Anti-kT: p<0 (hard particles merged first)R=resolution parameter


€

dij = min(kTi2p,kTj

2p )(Δy ij2 + Δφij

2) /R2

Example: Anti-kT

Blue = highest pT particle

η

ϕ

-1 +10

2π

kT vs anti-kT

• ALL particles are clustered into “jets”• kT not bound to a circular structure• Anti-kT circular shape, “cone” radius ~R parameter

– Expected to be less sensitive to background/“back reaction” (it starts from high-p T particles) ideal choice in heavy-ion collision

• Recombination algorithms are collinear and infrared safe

37

FastJet M. Cacciari, G. Salam, G. Soyez 0802.1188


R matters!


R matters!


R matters!


R matters!


R matters!

The choice of R depends on• The system we are looking at (e+e-, pp, AuAu, PbPb,…)• Tradeoff: don’t want to loose too much out-of-cone

radiation (corrections for hadronization become difficult) but want to have a small background in the jet area

p+p 200 GeVSTAR Preliminary

In pp: ~80% of jet energy within R=0.4 for 20 GeV jets



Jets in Heavy-Ion Collisions at RHIC and LHC

Central Au+Au √sNN=200 GeV

ETjet ~ 21 GeVSTAR EMC + tracking data

STAR preliminary

Central Pb+Pb√sNN=2.76 TeVALICE tracking data

Why measure jets in heavy ion collisions? [inclusive, di-jets, jet-hadron, -jet,..]• Access kinematics of the binary hard-scattering• Characterize the parton energy loss in the hot QCD medium− modified fragmentation, energy flow within jets, quark vs gluon jet difference− flavor and mass dependence• Study medium response to parton energy loss – establish properties of the medium

Date post:	30-Dec-2015
Category:	Documents
Upload:	lance-bruce
View:	34 times
Download:	0 times

Jet Analysis in Heavy-Ion Collisions

Documents