1. In-medium hadronization at the LHC and first results from ALICE
The title looks too long.
Rene Bellwied
Wayne State University

2. 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 !! 2

3. 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) 3

4. 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. 4

5. 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 ?) 5

6. 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. 6

7. 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)
large z (=ph / pq)
= resonance is leading particle in jet
 shortens formation time 7

8. Formation Time of Hadrons in RHIC / LHC QGP (C. Markert, RB, I
Formation Time of Hadrons in RHIC / LHC QGP (C. Markert, RB, I. Vitev, )
RHIC LHC 8 8

9. 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)
similar to
Cronin effect
If quark fragments outside then
there is no dependence on z or n 9

10. HERMES results on <pT> broadening10

11. Two distinctly different hadronization processes (both postulated in 1977)
More likely in vacuum ?
More likely in medium ? 11 11

12. 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
Although UA1 and CDF extracted the spectra, they did not show the ratio explicitly;
Ratios are clearly above unity for both energies;
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.

13. 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
Although UA1 and CDF extracted the spectra, they did not show the ratio explicitly;
Ratios are clearly above unity for both energies;

14. 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 ±0.05
k 0.4±0.1
f ±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.

15. Methodology: measure vacuum and medium hadronization in same event
See Christina’s talk and arXiv: :
in medium-modification inside/outside jets (either jet cone or quadrant analysis)
Can already be done in pp (underlying event analysis)
side 1
side 2
near
away

16. Proposed measurements
In quadrants or inside/outside jets at sufficiently high fractional momentum
(pT = 3-10 GeV/c):
baryon / meson ratios
rare particle species
(s,c,b)
Resonances
pT broadening
Can all already be done in pp (underlying event analysis)

17. The beginning of a new era – LHC takes data
ALICE writes the first scientific LHC paper !
submitted to EPJC 28 Nov 2009
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)
This was based on 284 events taken
in the last week of November with just
the inner Silicon Tracker in ALICE.
last time measured at the ISR for pp
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..’

18. The possibilities are boundless…
TPC
No vertex cut !
ITS
In December I had the privilege to be the
ALICE Period Coordinator for one month.
All 18 (!) sub-systems performed flawless
Total events collected: > 1 M
‘Good pp interactions’: 500 k
- 100 k : B = 0 (alignment)
- 10 k : B reversed (systematics)
- 30 k : √s = 2.36 TeV
velocity v/c
TOF
Protons
Kaons
Pions
all plots:
preliminary calibration & alignment !
TRD
Electrons
Pions
ALICE special: Particle Identification
(very important for heavy ion physics later in 2010)
15/2/2006 LHCC Status Report J. Schukraft

19. Tracking yields a first pT spectrum
Beam spot at 2.36 TeV
TPC pt spectrum
Preliminary
preliminary alignment !
SPD Vertex resolution versus # tracks

20. The key to hadronization studies: identified particles
PDG: MeV
K0s ® pp
PDG: MeV
L ® pp
… National Geographic was sort of correct..
PDG: MeV
L ® pp
F ® K+K-
PDG: MeV

21. Multiplicity dependencies will be interesting and relevant
<pt> versus multiplicity
to tune Monte Carlo programs
(and watch for surprises ?)
<pt> in 0.3 to 4 GeV

22. 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
Ös = 900 GeV
LHC
Ös = 2.36 TeV

23. The relevance of pp @ LHC for hadronization
a reference
at which multiplicity is there a medium ?
hadronization modifications in pp as a function of Nch ?
dNch/dh ~
~ mid-central CuCu at RHIC
energy density e ~ 5-10 GeV/fm3
small but dense system
60,000 MB events
dNch/dh ~ 50 ! 23

24. 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

25. The near future: modified fragmentation functions (simulations from the ALICE-EMCal Physics Performance Report) 25

26. Summary
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 !! 26

27. Backup slides 27

28. Additional measurement in nDIS
Multiplicity ratio:
Nh = hadron multiplicity in (n,Q2) bin
Ne = inclusively scattered lepton in same bin
with
z = hadron fractional energy
n = virtual photon energy
pt2 = hadron transverse momentum
Q2 = lepton four-momentum squared
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) 28