1. High-pT probes of QCD matter
Marco van Leeuwen, Utrecht University

2. Plan of Lecture IV Yesterday’s summary: Today’s lecture:
Best achievable goal: determine P(DE) experimentally
(Or at least some features of it)
Difficult in practice:
RAA (at RHIC) not sensitive
IAA limited sensitivity (fragmentation bias)
Today’s lecture:
New approaches to reduce fragmentation bias
Multi-hadron correlations
g-jet measurements at RHIC and LHC
Jet measurements at LHC
Mix of existing results and ideas for future

3. pQCD illustrated fragmentation jet spectrum ~ parton spectrum
CDF, PRD75,

4. Naive picture for di-hadron measurements
Fragment distribution
(fragmentation fuction)
Out-of-cone radiation:
PT,jet2 < PT,jet1
In-cone radiation:
PT,jet2 = pT,jet1
Softer fragmentation
Ref: no Eloss
PT,jet,1
PT,jet,2
Naive assumption for di-hadrons: pT,trig measures PT,jet
So, zT=pT,assoc/pT,trig measures z

5. Fragmentation bias in di-hadrons
Test case:
Calculate away-side spectra
Use two different frag functions
LEP:
Quarks: D(z) ~ exp(-8.2 z)
Gluons: D(z) ~ exp(-11.4 z)
pTa
pTt
Away-side spectra not sensitive to slope of fragmentation function
More detailed analysis shows: mainly depends on power n
of partons spectrum

6. Di-jet triggered analysis
pTa1
pTt1
pTt2
T1T2 correlation
Raw, uncorrected signal Df -1 -2 1 2 3 4 5
a.u.
0.6
0.4
0.2
O. Barannikova, F. Wang, QM08
Idea: use back-to-back hadron pair to trigger on di-jet and study assoc yield
T1: pT>5 GeV/c
T2: pT>4 GeV/c
p ± 0.2
Tune/control fragmentation bias and possibly geometry/energy loss bias
di-hadron trigger pairs contain combinatorial background

7. 3-hadron analysis backgrounds
T1: pT>5GeV/c, T2: pT>4GeV/c, A: pT>1.5GeV/c
Signal + Background
Background
Signal
a.u.
T1T2 2 4
Df (T1T2)
ZDC central Au+Au 12%
O. Barannikova, F. Wang, QM08
Df (T2A1) -1 -2 1 2 3 4 5
_dN_
Ntrig d(Df )
STAR Preliminary
T2A1_T1
flow subt.
T2A1
T1A1
pTa1
pTt1
pTt2
Subtract two combinatorial terms:
random T1, random T2

8. Di-jet triggered in d+Au
T1: pT>5 GeV/c, T2: pT>4 GeV/c, A: pT>1.5 GeV/c
O. Barannikova, F. Wang, QM08
_dN_
Ntrig d(Df )
T2A1_T1
T2A1
d+Au 200 GeV
Di-jet trigger
pTa1
pTt1
pTt2 2 1
STAR Preliminary -1 -2 1 2 3 4 5
Df (T2A1)
Requiring away-side trigger increases yield
Fragmentation bias changes: higher Q2
Single hadron trigger
pTa1
pTt1

9. Di-jet triggers in Au+Au
T1: pT>5 GeV/c, T2: pT>4 GeV/c, A: pT>1.5 GeV/c
Di-jet trigger
_dN_
Ntrig d(Df ) 4
pTa1
pTt1
pTt2
central 0-12%
T2A1_T1
T2A1 2
Single hadron trigger
pTa1
pTt1
STAR Preliminary -2 Df -1 -2 1 2 3 4 5
Single trigger: broad away-side
Di-jet trigger: jet peaks on both near and away side
Di-jet trigger selects different events, has different bias

10. Au+Au vs d+Au comparison
T1: pT>5 GeV/c, T2: pT>4 GeV/c, A: pT>1.5 GeV/c
T1A1_T2
T2A1_T1 Df -1 -2 1 2 3 4 5
_dN_
Ntrig d(Df )
STAR Preliminary
200 GeV Au+Au, 12% central
Au+Au
d+Au Df -1 -2 1 2 3 4 5
_dN_
Ntrig d(Df )
STAR Preliminary
200 GeV Au+Au & d+Au
Distributions wrt to both triggers similar
Au+Au similar to d+Au
Di-jet trigger selects jet pairs with little or no energy loss

11. Di-jet trigger model calculation
Renk, Phys. Rev. C 75, (2007)
<DE> for back-to-back jets T1
Thorsten Renk, private comm.
2 density models T2
The energy deposition on the near side is smaller than 0.6 GeV in all cases (this has to do with the fact that there's a bias for a quark leading to the most energetic hadron due to the harder fragmentation function, which maps into another bias of having preferentially a gluon on the away side from pQCD, which maps into yet another bis towards smaller energy loss on the near side due to the different color charge of the quark). If one compares the numbers to a situation where only the near side hadron is triggered (say in the 6-8 GeV momentum range), the expected energy deposition on the away side is about 10 GeV, i.e. any signal of a cone structure should be stronger by a factor 5 or so.
Model agrees: ptT1 ~ ptT2 reduces energy loss
Next step: increase ptT1 - ptT2

12. Multi-hadron cluster triggers
Idea:
Reduce fragmentation bias by clustering hadrons
‘proto-jet’
Away-side spectrum
0-12% Au+Au
Add GeV trigger
B. Haag, QM08
Multi-hadron trigger
STAR Preliminary R
Seed
Secondary Seeds
Associated track
Use cluster energy for trigger:
- R = 0.3
- pT,seed > 5 GeV
- pT,sec seed > 3 GeV
Single-hadron and multi-hadron triggers give similar result
Fragmentation bias does not change?
- Needs further study

13. Probing the photon energy with g-jet events
T. Renk, PRC74,
Nuclear modification factor
Away-side spectra in g-jet
Eg = 15 GeV
RAA insensitive to P(DE) 
Away-side spectra for g-jet are sensitive to P(DE)
g-jet: know jet energy  sensitive to P(DE)

14. g-jet in Au+Au
Use shower shape in EMCal to form p0 sample and g-rich sample
Combinatorial subtraction to obtain direct-g sample

15. Away-side suppression with direct-g triggers
A. Hamed et al QM08
First g-jet results from heavy ion collisions are becoming available*
Measured suppression agrees with theory expectations
Next step: measure pTassoc dependence to probe DE distribution
* both PHENIX and STAR

16. Large Hadron Collider at CERN
CMS
2008: p+p 14 TeV
2009: Pb+Pb 5.5 TeV
ALICE
ATLAS
3 Large general purpose detectors
ALICE dedicated to Heavy Ion Physics, PID p,K, out to pT > 10 GeV
ATLAS, CMS: large acceptance, EM+hadronic calorimetry

17. From RHIC to LHC Most abundant probe: jets, light hadrons
RHIC: s=200 GeV Au+Au
LHC: s=5.5 TeV Pb+Pb
Pion spectra
Hard process yields much larger at LHC
10k/year
(orders of magnitude at high-pT)
Most abundant probe: jets, light hadrons
Robust yields to pT>200 GeV for jets
Larger initial density
 Validate understanding of RHIC data

18. RAA at LHC
GLV
BDMPS
T. Renk, QM2006
RHIC
RHIC
S. Wicks, W. Horowitz, QM2006
LHC: typical parton energy > typical E
Expected rise of RAA with pT depends on energy loss formalism
Nuclear modification factor RAA at LHC sensitive to radiation spectrum P(E)

19. Medium modification of fragmentation
MLLA calculation: good approximation for soft fragmentation
extended with ad-hoc implementation medium modifications
Borghini and Wiedemann, hep-ph/
pThadron ~2 GeV for
Ejet=100 GeV
=ln(EJet/phadron) z
0.37
0.14
0.05
0.02
0.007
Trends intuitive: suppression at high z, enhancement at low z
Recent progress: showering with medium-modified Sudakov factors, see Carlos’s talk and arXiv:

20. Jet modifications at LHC
Jet reconstruction
Expectations from QCD+jet quenching
PQM with fragmentation of radiated gluons (A. Morsch)
Fragmentation function
Radial profile
Ejet = 125 GeV
Energy loss depletes high-z and populates low-z
Low-z fragments from gluon radiation at large R z
0.37
0.14
0.05
0.02
0.007
=ln(EJet/phadron)‏
In-medium energy loss redistributes momenta in jets
Model has the main phenomenology included; use as benchmark

21. Need for a calorimeter Jet energy response
ALICE PPR, part II
100 GeV Jets
R=0.4
Note: tail due to jet-splitting
Charged particle tracking only sees ~50 % of jet energy
TPC+EMCal recovers large fraction of jet energy
Moreover: EMCal provides important trigger capability

22. Improves jet energy resolution
ALICE EMCal
US-France-Italy project
ALICE-EMCal project:
Approved in 2007
Full detector by 2011
EMCal module
Testbeam:
Support frame installed
Lead-scintillator sampling calorimeter
||<0.7, =110o
~13k towers (x~0.014x0.014)‏
Improves jet energy resolution
Provides jet triggers

23. Jets in heavy ion events
Simulation: 100 GeV jet in Hijing
Finding jets is relatively easy
Energy (GeV)
Challenges:
Measuring the energy in presence of cuts to reduce background
Reconstructing lower energy jets, jets with softer fragmentation

24. Reducing the background
Radial distribution of jet energy
Charged background energy fluctuations
PYTHIA
HERWIG
pTcharged>5 GeV
pTcharged>30 GeV
80%
no pt -cut
pt > 2 GeV/c
saturation model scaling (Eskola et al, hep-ph/ )
CDF, PRD65, (2002)
CDF: ~80% of jet energy contained in R < 0.2
Use smaller cone size, e.g. R=0.4 to reduce bkg
Apply pT-cut to reduce background
Disadvantage: Not infrared safe
Background from 5.5 TeV Pb+Pb:
dET/dh ~ 3700 GeV, ET(R<0.2) ~ 75 GeV

25. Small cones: split jets
Reconstructed energy
Split jet fraction
# Jets
all particles, R=0.3, pT > 2GeV
R=0.3, pt>2GeV
all particles
charged+pi0
charged
input
- Njets,rec=1
- Njets,rec>=1 highest jet
Njets,rec>=1 mid-cone
- Njets,rec>=1 sum
Fraction of events Njets,rec.>1
Jet Energy (GeV)
Jet Energy (GeV)
Jet-splitting affects energy recnstruction
Jet shape fluctuate; need stable algorithm (infrared safe?)
Possible solutions: SISCone, anti-kT
Still lots of room to improve jet-finding in heavy ions
(if you’re interested, let me know)

26. Full jet reconstruction performance
Simulation input
Simulated result
reference
Medium modified (APQ)‏
Full jet reco in ALICE is sensitive to modification of fragmentation function
E > E, explore dynamics of energy loss process

27. Under Study: Di-jet Correlations
Acoplanarity pp
PbPb: PYQUEN T = 1 GeV charged jets
J. Casalderrey-Solana and XNW,
arXiv: [hep-ph].
BDMPS
Effect seems to be measurable, but large effect from initial state radiation

28. g-jet rates at RHIC and LHC
g, p0 rates
Jacak, Vogelsang, QM06
gdirect/gdecay small at LHC: ~10% at 100 GeV
Challenging measurement: g/p < 0.1 in accessible kinematic region

29. Identifying prompt g in ALICE
Backgrounds are large, need isolation cut pp
R = 0.3, SpT < 2 GeV/c
Efficiency: 69%
Background rejection: 1/170
PbPb
R = 0.2, pTthresh = 2 GeV/c
Efficiency: 50%
Background rejection: 1/14
 5
signal

30. Make sure you don’t miss out!
Summary/conclusions
Goal of high-pT measurements (my opinion)
Measure/constrain P(DE)
Detemine medium properties
Dominant limitations of current measurements:
Fragmentation bias: single, di-hadron not very sensitive to P(DE)
At RHIC: DE ~ Ejet
Promising new measurements:
g-jet at RHIC and LHC
jet reconstruction at LHC (maybe also at RHIC)
Experimental methods and theory understanding are under continuous development
Make sure you don’t miss out!

32. Heavy-ion backgrounds
Background energy fluctuations
Jet cone energy
pT > 2 GeV
ALICE PPR Vol II
100 GeV
50 GeV
Dominant source: Impact parameter fluctuations
already taken out for these plots
50 GeV jets: need to restrict cone size
>100 GeV jets: can use larger size (R=0.5-1)
Note: also here large uncertainty: multiplicity and mean-pT can only be guessed

33. Getting at the jet energy
Simulations:
Pythia 6.319
(CDF tune A)
‘Ideal’ jets reconstructed using all final state particles
Cone algorithm R=1
Ejet = SpT
Jet energy contributions:
65% in charged particles
20% EM (mostly p0)
Small contributions from K0L , neutrons, etc (approx -1 leading particle)
+ Long tails
Leading particle ~15 %

34. Jets on a steep spectrum
Leading particle: mostly lower limit on jet energy
(+finite ‘efficiency’ at larger pT)
Charged jets: ‘sharp’ turn-on. Still mainly lower cut at RHIC
Charged+EM selects narrow range in E-jet at RHIC & LHC
Caveat: energy loss may transport energy outside cuts (cone, pT)