In-medium hadronization at the LHC
and first results from ALICE
Rene Bellwied
Wayne State University
The fundamental questions


How does the hadron form ?
◦ Fragmentation or recombination
◦ An early color neutral object (pre-hadron) or a long-lived
colored object (constituent quark)
When does the hadron form ?
◦ Inside the deconfined medium or in the vacuum ?
Robert DeNiro (The Godfather):
It’s non-perturbative, baby.
So forget about it !!
A theory: A. Accardi, 0808.0656
(the evolution of hadronization)
parton



pre-hadron
hadron
dressed parton has inelastic cross section comparable to
hadronic one = pre-hadron (h*).
undressed quark lifetime (tP), color neutralization time (tcn)
sometimes to simplify tP = tcn, hadron formation time (th)
Alternate: Cassing et al., 0808.0022
(see Wolfgang’s talk from Monday)
Partons dress up throughout the partonic
phase (PHSD = parton-hadron string
dynamics)
 At hadronization very massive pre-hadronic
resonances form through recombination of
dressed (constituent) quarks = quasi-particles
(DQPM = dynamic quasi-particle model)
 Resonances subsequently decay into ground
state mesons and octet baryons.

Assuming early ‘pre’-hadronization

Is the in-medium state colored or color neutral ?
Kopeliovich, Accardi: color neutral DOF
Cassing et al.: colored DOF

A colored object will continue to interact and not develop a
hadronic wave function early on (constituent quark)
A color-neutral object will have a reduced size and
interaction cross section (color transparency) and develop
wave function properties early
Only a color neutral state can exhibit hadronic features


(e.g. can pre-resonance decay prior to pion hadronization ?)
Modeling of hadronization
Breaking the color string from the struck parton
to the target remnant
Lund String Model:
Energy conservation
Lorentz boost
Eq = struck quark energy
kstr = string tension
Kopeliovich: the Lorentz boost in formation is offset by
energy conservation in fragmentation. A high z
particle has to form early otherwise the initial parton
loses too much energy.
Combining inside-out and outside-in
in light cone variables
Inside-outside cascade (boost)
to ~ 1 fm/c : proper formation time in hadron rest frame
E : energy of hadron
m: mass of hadron E/m = g
 high energy particles are produced later
 heavy mass particles are produced earlier
C. Markert, RB, I. Vitev
(PLB 669, 92 (2008))
Outside-inside Cascade (pre-hadron formation)
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large z (=ph / pq)
= resonance is leading particle in jet
 shortens formation time
Formation Time of Hadrons in RHIC / LHC QGP
(C. Markert, RB, I.Vitev, 0807.1509)
RHIC
LHC
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pT broadening as signature in DIS
Difference in <pt> between nuclear
and elementary collisions
pT broadening occurs when partons interact, pre-hadron
would have little interaction cross section, thus no pT broadening
If pre-hadron is formed
in nucleus:
(exponent = 0.5 from HERMES analysis
but also close to LUND model prediction)
If quark fragments outside then
there is no dependence on z or n
similar to
Cronin effect
HERMES results on <pT> broadening
Two distinctly different hadronization processes
(both postulated in 1977)
More likely in vacuum ?
More likely in medium ?
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B/M ratio in AA can be attributed to recombination or to
color transparency
(Sickles & Brodsky, PLB 668 (2008))
Recombination
in medium
Fragmentation
in vacuum
A directly (or early) produced proton (color-neutral) will
undergo almost no rescattering, thus its high pT yield is
enhanced relative to later formed mesons.
Energy dependence of baryon/meson ratio in pp
collisions already (reco at high dN/dh ?)
Ratio vs pT seems very energy dependent
(RHIC < < FNAL or LHC ?)
Not described by fragmentation !
(PYTHIA ratios at RHIC, FNAL, LHC are equal)
Additional increase with system size in AA
Nuclear suppression patterns in HI
collisions become more complex
SQM 2009: identified particle RAA
RAA (pT > 6 GeV/c)
-------------------------- ------------p
0.25±0.05
r
0.25±0.1
w
0.3±0.2
h
0.25±0.05
k
0.4±0.1
f
0.25±0.1
p
0.6±0.2
D/B (via e)
0.25±0.1
J/y
1.4±0.4
Nuclear suppression due to absorption of colorless pre-hadron
(Kopeliovich et al., Greiner & Gallmeister). Gluon radiation only
during coherence length prior to color neutral state.
Surprising particle dependence in RAA (hadro-chemistry or flavor
change) ? This is not simple partonic energy loss.
Methodology: measure vacuum and medium
hadronization in same event

See Christina’s talk and arXiv: 0807.1509:
in medium-modification inside/outside jets (either jet cone
or quadrant analysis)
side 1
away

near
side 2
Can already be done in pp (underlying event analysis)
Proposed measurements
In quadrants or inside/outside jets at sufficiently high
fractional momentum
(pT = 3-10 GeV/c):
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baryon / meson ratios
rare particle species
(s,c,b)
Resonances
pT broadening
Can all already be done in pp (underlying event
analysis)
The beginning of a new era – LHC takes data
ALICE writes the first scientific LHC paper !
The average number of charged particles
created perpendicular to the beam
in pp collisions at 900 GeV is:
dN/dh = 3.10 ± 0.13 (stat) ± 0.22 (syst)
National Geographic News (4 Dec.)
‘….a machine called ALICE....
found that a proton-proton collision
recorded on November 23
created the precise ratio
of matter and antimatter particles
predicted from theory..’
last time measured at the ISR for pp
This was based on 284 events taken
in the last week of November with just
the inner Silicon Tracker in ALICE.
The possibilities are boundless…
ITS
In December I had the privilege to be the
ALICE Period Coordinator for one month.
No vertex cut !
TRD
All 18 (!) sub-systems performed flawless
Total TPC
events collected: > 1 M
‘Good pp interactions’: 500 k all plots:
preliminary calibration & alignment !
- 100 k : B = 0 (alignment)
- 10 k : B reversed
ALICE (systematics)
special: TOF
Particle Identification
Protons
(very
important
for
heavy
ion physics later in 2010)
- 30 k : √s = 2.36 TeV
Electrons
Kaons
Pions
Pions
velocity v/c
15/2/2006 LHCC Status Report J. Schukraft
Tracking yields a first pT spectrum
Beam spot at 2.36 TeV
SPD Vertex resolution versus # tracks
TPC pt spectrum
Preliminary
preliminary alignment !
The key to hadronization studies:
identified particles
K  pp
… National Geographic was sort of correct..
  pp
0
s
PDG: 497.6 MeV
PDG: 1115.7 MeV
  pp
PDG: 1115.7 MeV
   + -
PDG: 1019.5 MeV
Multiplicity dependencies will be
interesting and relevant
<pt> in 0.3 to 4 GeV
<pt> versus multiplicity
to tune Monte Carlo programs
(and watch for surprises ?)
Finally… a new frontier
Work in progress..
dN/dh at 2.36 TeV
statistical error negligible
final systematic error under evaluation
(still 7% for the time being)
LHC
s = 2.36 TeV
LHC
s = 900 GeV
The relevance of pp @ LHC for hadronization


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a reference
at which multiplicity is there a medium ?
hadronization modifications in pp as a function of Nch ?

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60,000 MB events
dNch/dh ~ 50 !

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dNch/dh ~ 50-100
~ mid-central CuCu at RHIC
energy density e ~ 5-10 GeV/fm3
small but dense system
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The near future: identified particle ratios
(simulations from the ALICE-EMCal Physics Performance Report)


Sapeta / Wiedemann predictions (dashed = vacuum jets,
solid = in-medium jets)
Statistical and systematic error bars based on annual
unquenched 50 GeV di-jet yield in PbPb collisions in
ALICE acceptance
24
The near future: modified fragmentation functions
(simulations from the ALICE-EMCal Physics Performance Report)
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Summary


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The question of hadronization in QCD is highly relevant
for the evolution of the initial deconfined and chirally
symmetric QCD phase.
Studies of pT, width, mass broadening, nuclear
suppression, yields and ratios of identified particles and
resonances in the fragmentation/recombination region of
their spectrum gives us a unique tool to answer this
decade old question.
Studies in proton proton collisions, especially at high
multiplicity density will give important clues.
The LHC is upon us – happy analyzing !!
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Backup slides
Additional measurement in nDIS
Multiplicity ratio:
Nh = hadron multiplicity in (n,Q2) bin
Ne = inclusively scattered lepton in same bin
In dense partonic medium
either

Short production time
and in-medium
hadronization (l > 0)
or

Long lived quarks with
lose energy and fragment
outside the medium (l < 0)
with
z = hadron fractional energy
n = virtual photon energy
pt2 = hadron transverse momentum
Q2 = lepton four-momentum squared