My Interactions with Jozsef

1
My Interactions with Jozsef

• In what feels like another life I actually
  worked on the subject of strangeness
• My first direct interaction with Jozsef on the
  subject was at the 1996 Strangeness meeting in
  Budapest.
• A discussion that I had with Jozsef at that
  meeting had a big impact on how we would consider
  results from our p-A experiment, E910.
• There is a clear dynamical component to
  enhanced strangeness production as evidenced
  by both E910 and NA49.
• Unfortunately, the start of the RHIC program cut
  short my work on strangeness
• And led me into high-pT physics

2
My Interactions with Jozsef (2)

• In 2003, I spent 6 months at KFKI on sabbatical.
• Mostly working on high-pT physics
• That was a summer of great excitement with the
  observation of no high-pT suppression in d-Au.
• I had many opportunities to discuss high-pT
  physics, jet quenching and other RHIC physics
  with Jozsef.
• I hope that I can maintain the same level of
  energy, curiosity, and intellectual incisiveness
  that I saw in Jozsef into my golden years.
• I feel very fortunate to have had the opportunity
  to interact often with Jozsef during those 6
  months.

3
Jozsef in Action
4
Hard Scattering in p-p Collisions
From Collins, Soper, Sterman Phys. Lett.
B438184-192, 1998

• Factorization separation of ? into
• Short-distance physics
• Long-distance physics ?s

5
Single High-pt Hadron Production
Phys. Rev. Lett. 91, 241803 (2003)

• NLO calculation agrees well with PHENIX ?0
  spectrum (!?)
• BUT, FF dependence ?
• Lore KKP better for gluons
• Calc. Includes resummation!

6
Parton Showers

• For large jet energies, need to go beyond
  fragmentation
• Jet initiates a parton shower
• Successive branchings and splittings
• Evolves partons from highly off shell ? on shell
• May result in multiple jets in the final state
• Usually simulated in MC programs (Pythia, Herwig)
• In NLO QCD, care needed to avoid double counting

Parton shower strongly affected by quantum
coherence interference. ? Angular ordering of
emission (largest first, smallest last)
7
Penetrating Probes
Space-time history of RHIC collision in parton
cascade model
t
z
Collisions between partons

• Use self-generated hard quarks/gluons/photons as
  probes of initial (early) medium properties

8
Jet Quenching _at_ RHIC

• (QCD) Energy loss of (color) charged particle
• Dominated by medium-induced gluon radiation (??)
• Strong coherence effects for high-pT jets
• Virtual gluons of high-pT parton multiple scatter
  in the medium and are emitted as real radiation

9
Au-Au ?0 Spectra From PHENIX

• Observe only 20 of expected yield _at_ high pT
• Energy density 15 Gev/fm3
• 100 x normal nuclear energy density!!
• Reminder critical energy density 1 GeV/fm3

Expected
Transverse Momentum spectrum
Calculations with no energy loss
Calculations with energy loss
10
PHENIX Au-Au High-pT ?0 Suppression

• Quenching persists to (pT/?QCD)2 104

11
d-Au Results w/ More Precision
PHENIX high-pT ?0 production

• At high pT apparent modest suppression in yield
  in more central collisions (larger thickness)
• From PDFs (EMC suppression)?
• Cold nuclear energy loss? (Vogelsang
  Venugopalan)

12
High-pT Single Particle Summary
To explain data need Unscreened color charge
dn/dy1000 Initial energy density
15 GeV/fm3 gt ?10 critical energy density

• ?5 violation of factorization up to 20 GeV/c
• In hadron production (jets), but not prompt ?
• Hard scatterings occur at expected rates
• Suppression from final-state energy loss

13
Analysis of Single Hadron Data BDMS-Z-SW

• Thick medium energy loss calculation
• Applied to RHIC data by Dainese, Loizides, Paic
  PQM

Central 200 GeV AuAu
Transport coefficient
for radiated gluon

• Baier Nucl. Phys. A715, 209 (2003)
• C 2 expected for ideal QGP
• 14 GeV/fm2 ? c 8-10!!
• Strong coupling Eskola et al, Nucl. Phys. A747,
  511 (2005)

14
Analysis of Single Hadron Data GLV
Gyulassy and collaborators
Opacity expansion Thin-medium limit, expansion
in n-body correlations between scattering
centers. dE/dx proportional to density of color
chargers

• AuAu central single hadron suppression can be
  explained using expected initial parton
  density(?)
• Based on approximate parton-hadron duality
• Beware sensitivity to choice of ?s

15
Analysis of Single Hadron Data AMY
QCD transport calculation by Arnold, Moore, Yaffe
(AMY)
PHENIX preliminary AuAu central p0
Applied to jet quenching by Turbide et al,
hep-ph/0502248

• Numerically solve coupled Langevin equations for
  quark, gluon distribution functions including
  quenching.
• Hard thermal loop re-summed gluon spectral
  functions.
• Initial condition (T) fixed by final-state
  observables
• Fixed ?s, no other free parameters
• ?E ? E built in!!

16
The Fly in the Ointment Surface bias
Wicks et al (GLV collisional)
Dainese, Loizides, Paic BDMS-Z-SW

• Observed high-pT hadrons suffer less ?E than
  average.
• Biased towards surface
• This effect must be present
• But ? no agreement on the magnitude of the effect

17
More Complications

• Need collisional energy loss
• Need to account for geometric path L fluctuations
  
• Recover good description of ?0 suppression?!

18
Single Hadron Better Quantitative Analysis
From M. van Leeuwan Quark Matter 2006 summary talk
19
More complications ?s

• Peshier
• Usual ansatz for scale at which to evaluate ?s in
  the medium incorrect
• And significant correction to LO Debye mass
• ?1.4 change in MDebye consistent with lQCD
• Need running ?s in (e.g.) collisional dE/dx

20
And more complications Pre-hadrons

• Kopeliovich Last Call for Predictions _at_ LHC

21
High-pT Suppression from pre-hadrons ??
22
More Complications Transverse Flow (?)

• Ruppert Renk
• Incorporating transverse flow effects allows
  understanding of anomalously large

23
Where do we stand?

• Simple picture of energy loss from ca. 2004 is
  now ancient history.
• Not all of the complications are created equal
• e.g. if improved understanding of ?s holds up
  under further investigation,
• All calculations w/ fixed/hand-set ?s should be
  subjected to ridicule until they change.
• Similar w/ geometric fluctuations.
• And accounting for p(?) vs ??E?
• But, what about ASW
• gt 100 GeV2/fm _at_ LHC???
• But what about flow effects on energy loss?
• And what about pre-hadrons?

24
AuAu Quenching Azimuthal Variation

• Azimuthal (?) variation of ?0 suppression
• At intermediate pT gt radiative dE/dx
• But, for pT gt 7 GeV, consistent w/ radiative
  energy loss.
• Important calibration of geometry in dE/dx
  calculations.

20-40
PHENIX Preliminary
AMY dE/dx
25
Jet Quenching Photon Bremstrahlung

• For light quarks (and gluons??), in-medium energy
  loss dominated by radiation.
• Interference between vacuum induced radiation.
• For large parton pT (gt 10 GeV/c) coherence
  crucial.
• Unfortunately, we cant measure the gluons.
• But we could measure photon bremstrahlung!
• Direct measurement of medium properties.

26
Jet Tomography

• At RHIC, studied via
  leading hadrons
• Statistics suffer from
  frag. function ? rates
• Quenching ? geometric bias
• No direct measure of frag. function.
• At LHC
• Full jets, high pT, large rates, b jets, di-jet,
  ?-jet
• Precision jet tomography

27
Parton Showers

• For large jet energies, need to go beyond
  fragmentation
• Jet initiates a parton shower
• Successive branchings and splittings
• Evolves partons from highly off shell ? on shell
• May result in multiple jets in the final state
• Usually simulated in MC programs (Pythia, Herwig)
• In NLO QCD, care needed to avoid double counting

Parton shower strongly affected by quantum
coherence interference. ? Angular ordering of
emission (largest first, smallest last)
28
QCD (MLLA) Description of Parton Showers

• QCD can predict (under certain approximations)
  the hadron spectrum (shape) in energetic jet
• MLLA (modified leading logarithmic approx) gives
  hump-back plateau
• x ? hadron pT / Ejet
• Depletion at small x (large ln(1/x) ) due to
  coherence of the parton cascade.
• Old paradigm that fragmentation is purely
  non-perturbative physics no longer true.
• Angular ordering crucial!

29
Modified Parton Shower in Medium
e.g. Pirner, last call for LHC predictions

• Hump-backed plateau modified in the medium
• Suppression at large x (small ln(1/x))
• Enhancement at small x (large ln(1/x))
• Ideally most complete description of quenching

30
Parton Showers, Hard Radiation _at_ LHC

• Copious hard radiation in high Q2 final-state
  parton showers, ?F 1/kT
• Both an opportunity and a challenge
• Understanding jet quenching more difficult
• Potentially time-dependent probe of medium
• Resolving hard radiation in jets a must!

31
LHC Single Hadrons

• Thin and thick medium formulations give very
  different predictions for single hadron
  suppression _at_LHC
• Different sensitivity to the interplay between
  slope of parton spectra and dependence of energy
  loss on jet energy.

32
Jets in PbPb Collisions
70 GeV di-jet from Pythia
Embedded into central PbPb

• HIJING event generator used for PbPb event
• HIJING may over-estimate bkgd by x2
• Probably a worst-case
• Soft background much less a problem for
  not-so-central collisions
• Centrality dependence as/more important than
  central

33
Jet Reconstruction ET Resolution

• Pythia di-jet events with 35 lt ET lt 280
• Merged (post GEANT) into b 2 fm HIJING events.
• Reconstructed w/ R0.4 seeded cone algorithm
• Seed ET gt 5 GeV in ???? 0.1x0.1 tower
• Compared to R0.4 seeded cone algorithm on Pythia
  final-state particles.

34
Jet Reconstruction b dependence

• Pythia HIJING performance vs b
• R 0.4 seeded cone jet algorithm
• Here, Pythia jets in 70 lt ET lt 90 GeV
• Position resolution
• Energy resolution
• Smooth evolution with centrality
• By b 10 (Npart 100) reach ? p-p performance.

Pythia Jets 70 lt ET lt 90 GeV

ATLAS Preliminary
35
The Fast kT Algorithm
combine closetracks/clusters into jets
Reconstructs jet backwards along fragmentation
chain. Better for complicated multi-jet final
states Typically scales as O(N3) ? Cacciari
Salam (2005)sFastKt has optimized problem
down to O(N log N)!
36
kT Jet Reconstruction (2)

• Cacciari
• Use KT algorithm w/o subtraction.
• Use fake jets to estimate background, subtract.
• ATLAS
• Use jet using ????? 0.1?0.1
  towers to distinguish real fake jets.

Central PbPb event qT 140 GeV Pythia, EM
energy only
37
kT Jet Reconstruction (3)
ATLAS Preliminary

• Very preliminary
• KT algorithm with R0.4
• ETmax / ?ET? cut at avg. 1?
• 1st study of performance of fast kT algorithm in
  PbPb
• But a crucial proof-of-principle showing the
  method works

38
Jet Modifications _at_ LHC (SW)

• Modification of radiated gluon kT distribution
• Crucial point of the figure
• spectrum _at_ large kT is unaffected by energy cut
• Can measure with particles well above background
• Can measure in small cone
• Angular distribution is characteristic of
• For gluons, not hadrons!
• If (newer) SW estimate is correct, we will see
  radiation as sub-jets measureable.

Note that in Nucl. Phys. A747 51, SW estimate
gt 100 GeV2 based on RHIC data
39
Looking Towards the Future

• The LHC will open a new era in the study of jet
  interactions in the medium
• Complete jet reconstruction
• Jet energies gt 200 GeV ? restoration of
  factorization?
• Full acceptance (ATLAS CMS)
• Extensive PID of jet fragments (ALICE)
• Jet measurements will take some time to get
  systematics on energy scale under control
• But measurement of modified Jt distribution
  depends only on angular resolution (lt 0.03)
• And Mach Cone
• And jet-jet relative energies
• Parton cascade will complicate interpretation
• But exciting extension of jet-medium interaction.

40
Multi-Jet Final State CDF

• Proper reconstruction of complicated multi-jet
  final states a non-trivial problem.

41
kT Jet Reconstruction

• kT jet algorithm has several advantages
• Unseeded (better QCD predictability)
• Explicitly accounts for angular ordered parton
  showers
• Adapts to distorted (non-conical) jet shapes

Shamelessly borrowed from talk by W.
Holzmann
42
kT Jet Reconstruction

• kT jet algorithm has several advantages
• Unseeded (better QCD predictability)
• Explicitly accounts for angular ordered parton
  showers
• Adapts to distorted (non-conical) jet shapes
• With algorithmic optimization by Cacciari,
  becomes feasible in PbPb (faster than cone)

43
Fast-kT Example Application in ATLAS

• Cacciari
• Run kT jet reconstruction on unsubtracted events
• Discriminate between true/fake jets
• Use fake jets to measure background in real jets

True high-pt jets
True low-pt jets
False jets
44
A-A Hard Scattering Rates

• For partonic scattering or production
  processes, rates are determined by TAB
• t-integrated A-A parton luminosity
• Normalized relative to p-p
• If factorization holds, then
• Define RAA
• Degree to which factorization is
  violated