1. Lecture III: Intermediate pT; coalescence and bulk response
Marco van Leeuwen
Utrecht University
Jyväskylä Summer School 2008

2. Intermezzo: particle detectors and particle identification
Tracking
Momentum measurement
Charged particles in magnetic field
Muon detection
Charged particle tracking in magnetic field
EM Calorimeter
Energy measurement
Showering of g, e (e -> eg, g-> ee)
High-Z material
e.g. Pb-Scintillator, Pb-glass, PbWO crystals, Pb-LAr,
W-Si sandwich
Hadron Calorimeter
Energy measurement
Showering of hadrons
h → p0 → gg
Need large total mass
(e.g. big piece iron with scintillators)
‘Standard’ high-energy physics detector stack Main goal: measure all particles/energy flow (except n)

3. PID in HEP detectors
Identify hadrons/leptons/photons by signature in detectors
EMCal
HCal
tracker
muon system
Charged hadron (p, K, p)
Neutral hadron (n, K0L)
electrons
photons
muons
Note: large expense for muons (EW probe, < 1 % of primary tracks in QCD event) neutral hadrons (~5% in QCD event)
HI experiments normally do without HCal and with limited muon capability

4. From sketch to reality: CMS

5. ‘General purpose’ detectors at LHC
Detector examples
‘General purpose’ detectors at LHC
ALICE
CMS
ATLAS
(not to scale: RATLAS>RCMS >RALICE )

6. PID: weak decays in tracker
‘topological reconstruction’
With a tracker, reconstruct weak decays:
Λ, |y|<1
0.4 <pt< 0.6
K0 → pp (ct = 2.7 cm )
L0 → pp (ct = 7.9 cm)
D0 → Kp (ct = 124 mm)
D+ → Kpp (ct = 315 mm)
And also:
t -> hadrons
t -> Wb ->…

7. Charged hadron identification
Other techniques identify p, K, p by measuring mass (velocity)
Specific energy loss dE/dx
Time-of-flight (TOF)
pions
kaons
protons
deuterons
electrons
STAR TPC
STAR
Depends on bg
Mostly at low pT < 1 GeV
Depends on b
< 100 ps resolution, PID up to few GeV
TPC-dE/dx and TOF are basic features of most Heavy-Ion detectors

8. Ring Imaging Cherenkov (RICH)
Ring reconstruction
Cherenkov angle depends on index of refraction  tunable
Advantage: RICH can be optimised for large momentum
Not so easy with high track densities

9. STAR and PHENIX at RHIC STAR PHENIX Large acceptance at mid-rapidity:
TPC tracking
(coarse) EMCal
Some forward Calorimeters
PID: TPC-dE/dx, TOF
Central tracking/calo arms
(partial coverage, finely segmented calo)
Forward muon arms
PID: TOF, RICH
General purpose detector
Focus on rare probes
(electrons/photons)
(PHOBOS, BRAHMS even more specialised)

10. ALICE ‘STAR+PHENIX in one’ at LHC
Barrel: tracking + secondary vertices + PID
Charged particles |h| < 0.9
Excellent momentum resolution up to 100 GeV/c (Dp/p < 6%)
Tracking down to 100 MeV/c
Excellent Particle ID and heavy flavor tagging
EMCal for jet reconstruction
Pb-scintillator, 13k towers
Df = 107, |h| < 0.7
Energy resolution ~10%/√Eg
Trigger capabilities
PHOS: small acceptance, High granularity EMCal
High resolution PbWO4 crystals
|h| < 0.12, 220 < f < 320
Energy resolution: DEg/Eg = 3%/Eg
Forward muon arm
‘STAR+PHENIX in one’ at LHC

11. Baryon excess Intermediate pT, 2 – 6 GeV
STAR Preliminary
B. Mohanty (STAR), QM08
High pT: Au+Au similar to p+p
 Fragmentation dominates
Baryon/meson =
Intermediate pT, 2 – 6 GeV
Large baryon/meson ration in Au+Au

12. Hadronisation through coalescence
Fries, Muller et al
Hwa, Yang et al
fragmenting parton:
ph = z p, z<1
R. Belmont, QM09
recombining partons:
p1+p2=ph
Recombination of thermal (‘bulk’) partons produces baryons at larger pT
Recombination enhances baryon/meson ratio
Baryon pT=3pT,parton
Meson pT=2pT,parton
Note also: v2 scaling
(Roy Lacey’s lecture)
Hot matter

13. d+Au: ‘jet’-peak, symmetric in f, h
Near-side ‘Ridge’ 
trigger
d+Au, 200 GeV
Au+Au 0-10%
STAR preliminary
3 < pt,trigger < 4 GeV
pt,assoc. > 2 GeV
d+Au: ‘jet’-peak, symmetric in f, h
Au+Au: extra correlation strength at large Dh  ‘Ridge’
Unexpected – what can it be?

14. Mechanisms for ridge formation
Three categories
Jet broadening
Medium response
Trigger effect
Long. flow
Long. flow
Gluons from fragmentation/energy loss couple to longitudinal flow
Extra yield due to medium heating/drag or propagating parton
Trigger selects existing structure in the medium (underlying event, color flux tubes)
Different scenarios suggest different behaviour, e.g. multiplicity, pT-dependence, Dh extent, baryon content
Experimental tests ongoing

15. Near-side Ridge Weak dependence of ridge yield on pT,trig
3 < pt,trig< 4 GeV/c
Jet-like peak
4 < pt,trig < 6 GeV/c
pt,assoc. > 2 GeV/c
Au+Au 0-10%
STAR preliminary
Au+Au 0-10%
STAR preliminary
J. Putschke et al, QM06 
trigger
`Ridge’: associated yield at large , small Df
associated
Weak dependence of ridge yield on pT,trig
 Relative contribution reduces with pT,trig
Ridge softer than jet – medium response?

16. Ridge – Dh shape Projection provides more quantitative info
Clearly 2 shapes: jet-like + ridge
Ridge very broad in Dh, almost independent in acceptance
Also note: ridge yield ~ independent of pt,trig

17. Jet-peak shape
Pt,trig > 4 GeV, jet-like peak symmetric in h,j and width similar to d+Au (no medium)
Jet-like peak unmodified (like in high-pt correlations, lect II)

18. Associated spectra jet, ridge
Jet-like spectra similar in d+Au and Au+Au
Ridge softer than jet – Different production mechanism?

19. Associated yields from coalescence
Recombination of thermal (‘bulk’) partons
‘Shower-thermal’ recombination
Baryon pT=3pT,parton
Meson pT=2pT,parton
Baryon pT=3pT,parton
Meson pT=2pT,parton
Hot matter
Hot matter
Hard parton
(Hwa, Yang)
No jet structure/associated yield
Expect large baryon/meson ratio
associated with high-pT trigger
Expect reduced associated yield
with baryon triggers 3 < pT < 4 GeV

20. Associated baryon/meson ratios
pTtrig > 4.0 GeV/c
2.0 < pTAssoc < pTtrig
C. Suarez et al, QM08
p+p / p++p-
Inclusive spectra
Associated yields
Au+Au: Baryon enhancement
Ridge (large Dh):
Baryon enhancement
p+p, d+Au: B/M  0.3
Jet (small Dh)
B/M  0.3
Baryon/meson ratio in ridge close to Au+Au inclusive, in jet close to p+p
Different production mechanisms for ridge and jet?

21. Ridge summary Most notable features: Ridge much broader than jet in Dh
Jet-like peak similar to d+Au in shape and yield
Ridge yield ~ independent of pttrig
Ridge spectrum softer than jet
p/p ratio in ridge similar to bulk, lower in jet
Strongly suggest different production mechanisms for ridge and jet
However, the ridge is correlated with jets:
causation, or trigger-bias (coincidence?)

22. More medium effects: away-side
3.0 < pTtrig < 4.0 GeV/c
1.3 < pTassoc < 1.8 GeV/c
Au+Au 0-10%
d+Au
STAR preliminary
Away-side:
Strong broadening in central Au+Au
‘Dip’ at  = 

23. Away-side shapes Fragmentation becomes ‘cleaner’ as pTtrig goes up
3.0 < pTtrig < 4.0 GeV/c
4.0 < pTtrig < 6.0 GeV/c
6.0 < pTtrig < 10.0 GeV/c
1.3 < pTassoc < 1.8 GeV/c
Au+Au 0-12%
0-12%
Preliminary
M. Horner, M. van Leeuwen, et al
Low pTtrig: broad shape, two peaks
High pTtrig: broad shape, single peak
Fragmentation becomes ‘cleaner’ as pTtrig goes up
Suggests kinematic effect?

24. The fine-print: background
High pT: background <~ signal
Low pT: background >> signal
8 < pTtrig < 15 GeV
3.0 < pTtrig < 4.0 GeV/c
pTassoc > 3 GeV
1.3 < pTassoc < 1.8 GeV/c
Background normalisation:
Zero Yield At Minimum
v2 modulated background
v2trig * v2assoc ~ few per cent
N.B. no signal-free region at low pT

25. Shockwave/Mach Cone Exciting possibility!
Mach-cone/shockwave in the QGP?
Gyulassy et al
arXiv:
T. Renk, J. Ruppert
Exciting possibility!
Proves that QGP is really ‘bulk matter’
Measure speed of sound?
B. Betz, QM09, PRC79,
Are more mundane possibilities ruled out?
– Not clear yet

26. Di-hadron correlation overview
Low-low: soft jets? Fluid dynamics?
PHENIX, arXiv:
High-low: jets+medium response?
High-high: jets + parton eloss

27. Ridge: soft to hard Dh vs Dj Au+Au vs p+p
Low pt: jet-like peak broadened in Dh
High pt: jet-like peak similar to p+p reference + ridge

28. Naive picture for di-hadron measurements
Fragment distribution
(fragmentation fuction)
Radiation softens fragmentation
Fragments produce low-pT hadrons
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

29. High-pT fragments as in vacuum
Energy loss in action
Preliminary
Near side yield
|Dj|>0.9
Away side yield
|Dj|<0.9
8 < pTtrig < 15 GeV
8 < pT < 15 GeV
zT=pTassoc/pTtrig
Au+Au / d+Au
8 < pT < 15 GeV
Near side yield ratio
zT=pTassoc/pTtrig
Preliminary
Away side yield ratio
zT=pTassoc/pTtrig
Au+Au / d+Au
M. Horner, QM06
Lower pTtrig
Lower pTtrig
1.0
M. Horner, M. van Leeuwen, et al
0.2
Near- and away-side show yield enhancement at low pT
Away-side: gradual transition to suppression at higher pT
Possible interpretation:
di-jet → di-jet (lower Q2) + gluon fragments
‘primordial process’
High-pT fragments as in vacuum
Near side: ridge
Away-side: broadening

30. Three-particle measurements
‘Cone’ case
‘kT’ case (deflected jets)
Two classes of events:
All events same distribution:
Background level
2-particle correlations measure event-average – Not sensitive to event-to-event changes in structure
Next slides: simplistic simulation to illustrate 3-particle methods and background

31. Raw signal I no background2
Cone case
kT case
Df12 ≈Df13
Df12 3
(p,p)
(p,p)
2p-Df13 1
z-scale 1
d2N
Ntrig
dDf12 dDf13
Df, Df correlations can identify conical vs deflected jet emission
In the absence of backgrounds…
i.e. high pT,trigger and pT,assoc in Au+Au

32. Combinations of jet-pairs with random third particle
Toy model I, raw data
Cone case
kT case
Background level
Dominant structure:
Combinations of jet-pairs with random third particle

33. Background terms
First assoc particle not correlated
r1(f2) r2(f1,f3)
Second assoc particle not correlated
r1(f3) r2(f1,f2) + =
Dominant structure in raw plots is the 2-particle structure folded with random third particles, r1r2
Need to subtract 2x1 combinatorics
There is one more term

34. Toy model I, raw data Cone case kT case So there is a difference:
Cone case: on-diagonal and off-diagonal are the same
kT case: difference between on- and off-diagonal
Difference is small:
Need well-controlled background subtraction

35. Background normalisation
Signal r2
Background r1 r1
(or mixed events)
Different normalisation schemes possible:
Absolute normalisation (average at )  Cumulant
Phenomenological normalisation ZYAM: scale background to have zero yield at minimum
r3(1,2,3) – r1(1)r2(2,3) – r1(2)r2(1,3) – r1(3)r2(1,2) + 2r1 r1 r1

36. Background subtracted results
Cone case
kT case
‘ZYAM’ normalisation
4-peak structure for conical emission
2-peak for kT-smearing
Cumulant normalisation
Valleys and peaks due to different normalisation
4-peak/2-peak difference also visible
Valley-peak strength similar: 0.2 in both cases
Signal seen in both schemes. Both schemes distinguish between conical emission and kT effect
Caveat: soft-soft term ignored

37. 3-particle results d+Au Au+Au central
3 < pT,trig < 4 GeV
1 < pT,assoc < 2 GeV
d+Au
Au+Au central
STAR, PRL102:052302,2009
Au+Au: off-diagonal peaks indicate conical emission
d+Au: single recoil peak
qM = 1.37 ± 0.02 (stat.)± 0.06 (syst.)

38. 3-particle Dh Event-by-event ridge structure
Ridge ‘localised’ in individual events
Ridge uniform in all events
e.g. fragmenting gluons + long flow
Ridge is bulk
Ridge+jet in every event
Ridgy event
Jetty event

39. 3-Particle Dh result Au+Au: limited statistics d+Au: jet-like peak
Trigger
dAu
Au+Au 0-12%
1
2 A1 A2
STAR Preliminary
3<pTTrig<10 GeV/c
1<pTAssoc<3 GeV/c
||<0.7
P.K. Netrakanti, STAR
J.Phys.G
Au+Au: limited statistics
Jet-like peak+flat background
d+Au: jet-like peak

40. 3-particle summary 2-hadron correlations measure event-average
3-particle techniques to study event-by-event structure
Away-side Dj-Dj: indicates conical emission
Near Dh-Dh: not conclusive; ridge does not stand out
Combinatorial background can be complicated
Needs large statistics

41. Summary
Larger baryon/meson ratio (inclusive spectra) at intermediate pT Hadronisation through coalescence?
Correlations:
Near-side ridge (large Dh, enhanced total yield)
Ridge pT-spectra, composition, bulk-like
Separate jet-peak looks like vacuum fragmentation
Away-side broadening, Mach-Cone?
Three-particle correlations confirm conical emission (or strong broadening in every event)
Suggests bulk-jet interplay
No established theoretical framework – Modeling is qualitative
But: large effects, potentially interesting physics

42. Extra slides

43. Nuclear Modification: RCP
RCP = Yield(central)/Yield(peripheral)*Ncoll(peripheral)/Ncoll(central)
Protons not suppressed at intermediate pT
At higher pT proton RCP approaches but does not merge with pion RCP
RCP consistent with recombination models
R. Belmont, QM09