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Jet Analysis in Heavy-Ion Collisions. Elena Bruna INFN Torino & Yale University. 5 th International School on QGP. Torino, March 2011.

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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

slide2

“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

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…

5th International School on QGP. Torino, March 2011

past present future
Past, Present, Future…

Pb+Pb @ √sNN=2.76 TeV

LHC

RHIC

LEP

e+e-

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

Elena Bruna (Yale&INFN Torino)

outline

Jet I: Intro & Motivations

Outline

Jet II: Full Jet Reconstruction

Jet III: Results

Jet IV:

The Present: from RHIC to LHC

Elena Bruna (Yale&INFN Torino)

jets in high energy collisions
Jets in high-energy collisions

pQCD Factorization:

Collins, Soper, Sterman

Nucl. Phys. B263 (1986) 37

PDF

Partonic x-section

Fragmentation function

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)

hadrons

c, xc

a, xa

b, xb

p

p

σab

d, xd

hadrons

Elena Bruna (Yale&INFN Torino)

jets in high energy collisions1
Jets in high-energy collisions

pQCD Factorization:

Collins, Soper, Sterman

Nucl. Phys. B263 (1986) 37

p + p  p0

PDF

Partonic x-section

Fragmentation function

QCD factorization works!

p + p p

p + pp

p+p √s=200 GeV

p+p √s=200 GeV

Elena Bruna (Yale&INFN Torino)

jets in high energy collisions2
Jets in high-energy collisions

pQCD Factorization:

Collins, Soper, Sterman

Nucl. Phys. B263 (1986) 37

PDF

Partonic x-section

Fragmentation function

hadrons

c, xc

a, xa

b, xb

p

p

σab

d, xd

hadrons

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,…)

Elena Bruna (Yale&INFN Torino)

jets in high energy collisions3
Jets in high-energy collisions

pQCD Factorization:

Collins, Soper, Sterman

Nucl. Phys. B263 (1986) 37

PDF

Partonic x-section

Fragmentation function

Hard scattering:

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

LO

NLO

Elena Bruna (Yale&INFN Torino)

jets in high energy collisions4
Jets in high-energy collisions

pQCD Factorization:

Collins, Soper, Sterman

Nucl. Phys. B263 (1986) 37

PDF

Partonic x-section

Fragmentation function

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 data

Many D on the market (KKP, AKK, …)

hadrons

parton

p(hadron)

p (parton)

z

Elena Bruna (Yale&INFN Torino)

jets in nucleus nucleus collisions
Jets in Nucleus-Nucleus collisions

Self-generated

“hard” probes

Detector

Jet Tomography!

Hard processes make perturbative QCD applicable

 high momentum transfer Q2

Hard processes scale as Nbin

Calibrated

LASER/x-ray

Elena Bruna (Yale&INFN Torino)

jets in nucleus nucleus collisions1
Jets in Nucleus-Nucleus collisions
  • Questions:
  • 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?

jet energy loss in the medium

Interpretation: Gluon radiation

DEloss ~ ρgluon (gluon density)

DEloss ~ ΔL2 (medium length) [~ ΔLwith expansion]

DEgluon > DEquark, m=0 > DEquark, m>0

 Important to measure DE of gluons  light  heavy quarks…

Transport coefficient: q = m2 / Lis the <pT2> transferred from the parton to a gluon per unit path length

^

Elena Bruna (Yale&INFN Torino)

jets in nucleus nucleus collisions2
Jets in Nucleus-Nucleus collisions

Eskola, Honkanen, Salgado, Wiedemann

Nucl Phys A747 (2005) 511

^

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

Elena Bruna (Yale&INFN Torino)

some predictions ff

Borghini and Wiedemann, hep-ph/0506218

Some Predictions: FF

Gyulassy et al., nucl-th/0302077

Renk, Phys. Rev. C79:054906,2009

ph

z=ph/pjet

pjet

Energy loss in the medium  softer fragmentation

ξ stretches the low z part

Elena Bruna (Yale&INFN Torino)

some predictions jet shapes
Some Predictions: Jet shapes

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

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

ωmin (pTmin) = minimum pT on particles in the jet

Energy loss ratio goes down with larger b.

Energy loss ratio becomes smaller with

smaller 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).

Elena Bruna (Yale&INFN Torino)

some predictions jet shapes1
Some Predictions: Jet shapes

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 particle
  • suppression.
  • large Rmax and small ωmin  all jet energy recovered  RAAjet=1 ! (jet production is hard process, scales as Nbin)

Elena Bruna (Yale&INFN Torino)

some predictions jet shapes2
Some Predictions: Jet shapes

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 particle
  • suppression.
  • large Rmax and small ωmin  all jet energy recovered  RAAjet=1 ! (jet production is hard process, scales as Nbin)

Elena Bruna (Yale&INFN Torino)

jet quenching from single high p t hadrons
Jet quenching from single high-pT hadrons
  • Observations at RHIC:
  • Large suppression of high-pT hadrons: factor ~ 5
  • Photons are not suppressed
  • They don’t interact with the medium (good!)
  • Nbin scaling works

Elena Bruna (Yale&INFN Torino)

jet quenching from single high p t hadrons1
Jet quenching from single high-pT hadrons
  • Observations at RHIC:
  • Large suppression of high-pT hadrons: factor ~ 5
  • Photons are not suppressed
  • They don’t interact with the medium (good!)
  • Nbin scaling works
  • Also Heavy Flavor is suppressed at RHIC
  • same as light quarks
  • role of bottom?
  • collisional energy loss/resonant elastic scattering?

Elena Bruna (Yale&INFN Torino)

slide20

Jet quenching from single high-pT hadrons

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

RHIC suppression < LHC

RHIC: high pT hadrons hadronize from quarks LHC: from gluons (larger color charge!)

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

Elena Bruna (Yale&INFN Torino)

jet quenching from di hadrons
Jet quenching from di-hadrons

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

Azimuthal correlation function shows ~complete absence of “away-side” jet

Partner in hard scatter is absorbed in the dense medium

not the case in d+Au  final state effect

Elena Bruna (Yale&INFN Torino)

jet quenching from di hadrons1
Jet quenching from di-hadrons

y

x

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

~complete absence of “away-side” jet

Partner 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

Out-of-plane

in-plane

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

Elena Bruna (Yale&INFN Torino)

jet quenching from di hadrons2
Jet quenching from di-hadrons

increasing pTtrig

increasing pTassoc

STAR, Phys.Rev.C82 024912 (2010)

Elena Bruna (Yale&INFN Torino)

jet quenching from di hadrons3
Jet quenching from di-hadrons

increasing pTtrig

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

increasing pTassoc

  • At high trigger pT:
  • re-emergence of away-side jet (punch thru)?
      • or
      • tangential jets?

STAR, Phys.Rev.C82 024912 (2010)

Elena Bruna (Yale&INFN Torino)

trigger and surface biases
Trigger and Surface Biases

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

Experiments online-trigger dependent:

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

EM calorimetry  bias towards large EM fraction

Elena Bruna (Yale&INFN Torino)

high p t towards jets
High pT: towards jets

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

  • 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

Elena Bruna (Yale&INFN Torino)

high p t towards jets1
High pT: towards jets

How?

  • g-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

Di-Hadron

Eγ = pparton

p≠ pparton

Leading

Hadron

Courtesy Thomas Ullrich

Elena Bruna (Yale&INFN Torino)

direct g hadron fragmentation functions
Direct g-hadron fragmentation functions

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

Good agreement w/ theory models

more assoc h± for p0than for g

 different parton energies for p0 and g (p0 come from fragmentation of higher energy parton)

Au+Au: different path-length for the recoil jet for p0 and g and triggers

Trig particle= g or p0

Assoc particle: h±

Elena Bruna (Yale&INFN Torino)

direct g hadron fragmentation functions1
Direct g-hadron fragmentation functions

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

Trig particle= g or p0

Assoc particle: h±

8<Etrig<16 GeV/c

IAA < 1 for zT>0.3

data can distinguish between different theoretical models

low zT: expected differences between p0 and g IAA due to path-length dependence of the energy loss

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

Elena Bruna (Yale&INFN Torino)

high p t towards jets2
High pT: towards jets

How?

  • g-Jet
  • Jet energy well constrained
    • limited kinematic reach (x-sec scales as αSαem)
    • Difficult to have a clean measurement of photons
  • g-Jet
  • Jet energy well constrained
    • limited kinematic reach (x-sec scales as αSαem)
    • Difficult to have a clean measurement of photons
    • Full Jet Reconstruction
    • Larger kinematic reach
      • large background  complex and challenging !

z = p(h)/pparton

Ejet = pparton

Eγ = pparton

Courtesy Thomas Ullrich

Elena Bruna (Yale&INFN Torino)

jet ii full jet reconstruction

Jet II: Full Jet Reconstruction

Elena Bruna (Yale&INFN Torino)

jets theory vs experiment
Jets: Theory vs Experiment

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

  • 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 !

Elena Bruna (Yale&INFN Torino)

theoretical requirements

Theoretical requirements

pT

pT

cone iteration

Jet 1

Jet 1

Jet 2

y

y

  • Collinear 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

Elena Bruna (Yale&INFN Torino)

experimental requirements
Experimental requirements

CDF

  • 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

Elena Bruna (Yale&INFN Torino)

jet finding algorithms
Jet Finding algorithms

Review of CDF Jet Algorithms, arXiv:hep-ex/0005012v2

FastJet JHEP 0804:005, arXiv:0802.1188

FastJet JHEP 0804, 063 (2008), arXiv:0802.1189v2

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

Elena Bruna (Yale&INFN Torino)

sequential recombination
Sequential Recombination

kTi,j= particle transverse momentum (pT)

kT: p>0 (soft particles merged first)

Anti-kT: p<0 (hard particles merged first)

R=resolution parameter

Example: Anti-kT

Blue = highest pT particle

If dij<kTi-2  merged

ϕ

If dij>kTi-2  not merged  call it a jet

0

-1

η

+1

Elena Bruna (Yale&INFN Torino)

k t vs anti k t
kT vs anti-kT

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

  • 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-pT particles)  ideal choice in heavy-ion collision
  • Recombination algorithms are collinear and infrared safe

Elena Bruna (Yale&INFN Torino)

r matters
R matters!

Elena Bruna (Yale&INFN Torino)

r matters1
R matters!

Elena Bruna (Yale&INFN Torino)

r matters2
R matters!

Elena Bruna (Yale&INFN Torino)

r matters3
R matters!

Elena Bruna (Yale&INFN Torino)

r matters4
R matters!

p+p 200 GeV

STAR Preliminary

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

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

Elena Bruna (Yale&INFN Torino)

jets in heavy ion collisions at rhic and lhc
Jets in Heavy-Ion Collisions at RHIC and LHC

Central Au+Au √sNN=200 GeV

Central Pb+Pb√sNN=2.76 TeV

ALICE tracking data

STAR preliminary

STAR EMC + tracking data

ETjet ~ 21 GeV

  • Why measure jets in heavy ion collisions? [inclusive, di-jets, jet-hadron, g-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

Elena Bruna (Yale&INFN Torino)