In-medium hadronization at the LHC and first results from ALICE The title looks too long. 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 !! 2

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

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

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

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

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

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, 0807.1509) RHIC LHC 8 8

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

HERMES results on <pT> broadening 10

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

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.

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;

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) Can already be done in pp (underlying event analysis) side 1 side 2 near away

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)

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..’

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

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

The key to hadronization studies: identified particles PDG: 497.6 MeV K0s ® pp PDG: 1115.7 MeV L ® pp … National Geographic was sort of correct.. PDG: 1115.7 MeV L ® pp F ® K+K- PDG: 1019.5 MeV

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

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

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 ~ 50-100 ~ mid-central CuCu at RHIC energy density e ~ 5-10 GeV/fm3 small but dense system 60,000 MB events dNch/dh ~ 50 ! 23

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) 25

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

Backup slides 27

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