Soft Physics in ALICE

Expansion dynamics Space-time structure Radial, anisotropic flow Momentum correlations

Chemical composition Hadronisation mechanisms

Global event properties

Event by event physics Fluctuations

Bulk properties: soft hadrons + interplay hard–soft Identified particle spectra (wide pT range)

1

Christian KUHN (IPHC - Strasbourg) for the ALICE collaboration

SQM 2007 @ Levoca

LHC

From SPS & RHIC to LHC

ALICE

ALICE at LHC: one dedicated heavy ion experimentwhich will offer many novel opportunities to study the physics of strongly interacting matter and the QGP

Many open questions !Need excitation functions on the largest possible energy range -> transition between hard and soft processesAt LHC: “old” (different / better conditions) + new observables

Pb-Pb at √sNN= 5.5 TeV

Au-Au √sNN = 200 GeV

Chemical compositionpT spectra, elliptic flow

Success of parton coalescence

High pT suppression

High degree of thermalization and (pre-hadronic) collectivity (“perfect fluid” ?)

Partonic degrees of freedom

Color dense opaque medium

SPS

Dense, color opaque medium

2

New conditions & new probes at LHC

Much higher energy density (x 3-10)

Hard processes Larger QGP volume, Longer QGP life time (x3-5)

Better overall conditions to study the QGP

Initial temperature T (x2) > 3Tc

5.5nns TeV (x 30 wrt. RHIC)

LHC

RHI CSPS

(h++h-)/ 2

0

17 GeV

200 GeV

5500 GeV=vsLO p+p y=0

LHC

RHI CSPS

(h++h-)/ 2

0

17 GeV

200 GeV

5500 GeV=vs(h++h-)/ 2

0

(h++h-)/ 2(h++h-)/ 2

00

17 GeV

200 GeV

5500 GeV=vs

17 GeV

200 GeV

5500 GeV=vsLO p+p y=0

Very hard probes copiously produced

Bulk properties stronglyinfluenced by hard processes

3

dNch/dη = 2600

1200

Multiplicity

Event by event physics

ITS

TPC

FMD

PHOS

ABSORBERMUON TRACKING CHAMBERS

TOFTRDHMPID

L3 MAGNET

PMD

MUON TRIGGER CHAMBERS

EMCAL

ALICE

Designed to cover essentially all observables of interest in the soft and hard regimes (hadron, lepton and photon sectors)-> Tracking & particle identification in a large acceptance and pT domain

Size: 16 x 26 meters

Weight: 10,000 tons

~ 1000 Members ~ 30 Countries ~ 90 Institutes 4

central Pb – Pb TPC

Tracking and particle identificationPhysical efficiency

For dNch/dy = 4000 in Pb-Pb

Reconstructed / generated ( |η|<0.9)~ 90% for pT > 1 GeV (limited by dead zones)Protons : large absorptionKaons : in-flight decays

Momentum resolution:~1% at PT = 1 GeV/c, ~4% at PT = 100 GeV/c

Precise vertexing (better than 100 m)

,K,p: dE/dx (in TPC & ITS) + TOF and RICH 100 MeV < p < a few tens GeV

electrons: TRD p >1 GeV

muons: p > 4 GeV

photons: PHOS 1 < p < 80 GeV

5

dN/d measurement in pp@ 14 TeV with the TPC

1 central Hijing event

Multiplicity distribution (dNch/d) in Pb-Pb

Silicon Pixel Detector (SPD) -1.6 < < +1.6 Forward Multiplicity Detector (FMD) -5, +3.5

Energy density

Global event properties in Pb-Pb and pp

Benchmark for Pb-Pb + genuine pp physics

60000 Pythia pp events

generated

LHC commissioning with optimal luminosity (<1030 cm-2s-1)

Study of underlying event structure Advantage of ALICE: pT cut-off ~100 MeV

Explore minimum bias pp (adequate triggers) 6

J.F. Grosse OetringhausC. Jorgensen

A

y=0 0.9-0.9

Baryon yields in pp collisions

The experimental challenge

Extract PID efficiency map & absorption map

Distinguish between two mechanisms of Baryon Number (BN) transport:1) Quark – diquark string breaking where BN is carried by the valence quarks2) Baryon stopping = stopping of the string junction

Effect of a few percents but the asymmetry is predicted to be dependent on the multiplicity

BN ASSOCIATED TO GLUONIC FIELD

BN ASSOCIATED TO VALENCE QUARKS

Asymmetry of the proton to anti-proton number A = 2 (Np – Nanti-p) / (Np + Nanti-p)

7

Asymmetry (uncorrected)

Statistical hadron resonance gas model at equilibrium : -> SPS -> RHIC -> LHC ( T~170 MeV, B ~1)

?

LHC ?Hadronization from a super-cooled ( T~140 MeV) over-saturated system with higher entropy – > out of equilibrium strangeness abundance

Chemical equilibrium vs non-equilibrium at LHC

Hot (T~170 MeV) & under- saturated (q < 1)

super-cooled & over-saturated

8

Equilibrium versus non equilibrium at LHC

Non equilibriumOversaturation of s (s = 3 - 5)

Equilibriums = 1

And many other questions: strangeness enhancement % energy Correlation volume (Npart GC, Nbin hard processes) ? Evolution in pp ?

9

B. Hippolyte et al.EPJ C49 (2007)

Hard / soft interplay at intermediate pT

Parton recombination / coalescence + pert. QCD: parton fragmentation ?

Rcp: central over peripheral yields/<Nbin> Baryon/meson ratio Elliptic flow

LHC ?

RHIC

R. Fries et al.

LHC

But baryon & meson production must be first understood in pp !

Strange particles give access to a wide pT range

10

Important evolutions:PYTHIA v6.2 -> v6.3: - multiple parton interactionsNew PDF

Strange baryon / meson ratio @ UA1 and CDF: pp @ 630 GeV & 1.8 TeV

Ratio at mid-pT already surprisingly high in pp data at high energies

Extracting mixed ratio from 1996 UA1 strange particle data

Better but still missing a factor > 2 wrt RHIC -> UA1 -> CDF extrapolationsNeed to investigate NLO contribution and baryon creation mechanisms (diquark to popcorn scenario or gluonic baryon junctions).

This feature is not observed in pp PYTHIA simulations and ratio at 14 TeV stays well below unity.

11

B. Hippolyte et al.EPJ C49 (2007)

Recombination of shower and thermal quarksR.Hwa et al, Phys.Rev.C70 (024904) 2004

(STAR)

(STAR)

triggered azimuthal correlations to measure fragmentation (shower quark contribution). If comes from TTT at intermediate pT, no correlations will be seen

12

End of reco ?

Enhanced production from recombination of thermal partons in the ridge ? 2-dim. correlations:

Topological identification of strange hadrons

Secondary vertex and cascade finding

Identification of K+, K- via their kink topology K

Pb-Pb central

Statistical limit for 1 year: ~ 107 central Pb-Pb, 109 min. bias pp pT ~ 13 - 15 GeV for K+, K-, K0

s, pT ~ 9- 12 GeV for

Reconstruction rates:: 0.1/event : 0.01/event

pp collisions

300 Hijingevents

K

13 reconstructed/event

700K pp collisions (14 TeV)

K0s

13

H. Ricaud

Resonances ( K*, …) Time difference between chemical and kinetic freeze-out In medium modifications of mass, width, comparison between hadronic and leptonic channels partial chiral symmetry restoration

Invariant mass reconstruction, background subtracted (like-sign methodor event mixing)

Mass resolutions ~ 1.5 - 3 MeV pT stat. limits from 8 () to 15 GeV (,K*)

central Pb-Pb

Mass resolution ~ 2-3 MeV

K*(892)0 K 15000 central Pb-Pb

K+K-

Mass resolution ~ 1.2 MeV

Invariant mass (GeV/c2)

14

200K Pythia pp min. bias events (0.9TeV)

~4000 K* reconstructed

See talks: A. Badala, P. Ganoti, D.Tapia Takaki

LHC 60

In the low density limit v2 is driven by and dN/dy

In ideal hydrodynamics v2 driven by the (space time averaged) velocity of sound, v2/ = constant

Hydro limit (full local thermalization) at RHIC ? More likely at LHC ?

Anisotropic Flow

?

Continuous increase with Kn-1 (Kn = mean freepath/system size, Kn-1 ~ (1/S) dN/dy -> no saturation seen in data Hydro limit : Kn << 1 v4=0.5 (v2)2 at large pT

Data: v4/v22 ~1.2 suggest Kn~1:

No thermalisation at RHIC!

Qualitative predictions for LHC • Closer to ideal hydro.

• Significant increase of v2

• v4/(v2)2 smaller than at RHIC

15

N. Borghini, J.Y. Ollitrault

Initial conditions CGC + hydro (until T ~ 170 MeV)i.e., contribution of the QGP + hadronic cascade At LHC, contribution from QGP much larger than at RHIC

T. Hirano

subMgv~~

2

hydro LDL

non-flow v2 (Hijing fit)~

The difference oberved at RHIC between v2{2} and v2{4} could be dominated by event by event fluctuationsThe relevant eccentricity (i.e. part) indeed varies event-by-event

Flow and Flow and non-flownon-flow effects at LHC effects at LHC

Measure flow with different methods and detectors covering different to disentangle flow, nonflow and fluctuations

Estimate of non flow effects: Hijing simulations with v2=0 & no jet quenching

BUT

Track multiplicity = 1000 V2 = 0.06

REC - MC

Event plane resolution ~ 10o

ALICE TPC

Signal clearly disentangled in a wide range of multiplicity

16

E. Simili

Particle correlations

qout

qside

qlong

Rsi

deR lon

g

Rout

x1

x2

12 ppq

p1

p2

q

12 pp2

1k

2 2 2 2 2 2out out side side long long( , ) 1 ( ) q R q R q RC q k k e

Rout/Rside does not increase as expected from hydro which predicts a long system lifetime … pT dependence of Rout/Rside also not reproduced by hydro … And many open questions :- surprising scaling of the radii with pp- non-femtoscopic q-anisotropic behaviour: EMCIC ?

The HBT puzzle at RHIC

Could the long awaited QGP signal of extended lifetime-scales appear only at LHC ?

√sNN (GeV)R

ou

t/Rsi

de

17

Two pion momentum correlation analysis Study of event mixing, two track resolutions, track splitting/merging, pair purity, Coulomb interactions, momentum resolution corrections, PID corrections

Particle correlations

Correlation functions

Rsimul. (fm)

Rre

c(f

m)

Rsim = 8fm

Other potential analyses Two kaon & two proton correlationsSingle event HBTDirect photon HBT, …

1 event : 5000

C(q

lon

g)

C(q

ou

t)C

(q s

ide)

q (GeV/c)

C(q

inv)

q inv (GeV/c)

18

Radii can be recontructed up to 15-20 fm

Event by event fluctuations

Fluctuations of temperature, entropy, energy density and of quark number susceptibilities (net charge, isospin, strangeness content) associated with phase transition

4th moment of the net charge

Lattice computationsat small chemical Potential (S. Ejiri,F. Karsch, K. Redlich)

High multiplicities at LHC => ALICE suited for the measurement of event/event fluctuations of <pT>, Tmultiplicity, particle ratio, strangeness, azimuthal anisotropy, intermediate / high pT phenomena, long range correlations, balance function, …

Mini-jets and jets expected to increase strongly the level of fluctuations Fluctuations of flow viscosity Fluct. of particle ratio constraints on statistical models

T/Tc

19

Resolution T/T:0.5 % for

C. Zampolli

With one month of Pb-Pb collisions (107 central + 107 MB events) many questions left open from the RHIC era should be answered

We also hope many many surprises !!!!

Conclusions & Outlook

Busy months ahead, but working detectorwell on track for first collisions

Major effort ongoing to finalize the DAQ and offline projects in order to be able to perform the physics analysis from day one

Installation and commissioning of all ALICE subsystems are essentially on schedule for the first pp collisions

20

The End

Global event characterization in Pb-Pb

Centrality determination

Event by event determination of the centrality Zero degree hadronic calorimeters (ZDC) + electromagnetic calorimeters (ZEM)EZDC , EZEM Nspec Npart impact parameter (b)

Correlations between ZDC and ZEME

ve

nts

EZEM (GeV)

EZ

DC (

Te

V)

b ~ 1fm

bgen (fm)

bre

c(f

m)

reconstructed

Npart Npart

generated

Npart ~15

dNch/d measurement with the TPC

• Track-to-particle (track level)– Geometrical acceptance,

reconstruction efficiency, decay, feed-down

– Function of , z vtx, pT

• Vertex reconstruction efficiency (event level)– Function of z vtx, multiplicity

• Trigger efficiency (event level) – Function of z vtx, multiplicity

vs. z

pT projection

Corrections

J. F. Grosse-OetringhausC. E. Jørgensen

SPD efficiency correction chip by chip inserted in the geometrical acceptance Map (here arbitrary dead channels)Ongoing:- Finalize method to determine detector efficiency maps- Evaluate dN/dη for 900 GeV sample with the detailed SPD efficiency map - Include corrections and systematics from beam-gas background and trigger acceptance

Tracklets

Integration over φ

zver

tex

Generated

Multiplicity distribution and dNch/d with the SPD (tracklets)

T. Virgili, D. Elia

pp 14 TeVpp 900 GeV

Identified particle spectra at intermediate pT

solid: STARopen: PHENIX PRL91(03)

Recombination % Fragmentation (RF)

Soft (hydro-> flow) + quenching, …

Interplay between hard and soft processes

V2: constituent quark scaling -> Coalescence ?

baryons / mesons

Hadron production in pp at LHC (Pythia)

Coalescence of thermal partonsqualitatively explains the baryonexcess at mid-pT in AuAu at RHIC

Baryons and mesons at intermediate pT

MICOR model : quark-coalescence (0 < pT < 4-5 GeV )+ pert. QCD : jet-fragment. (2 < pT < 10-20 GeV)reproduces many RHIC features at mid-pT: baryon/ meson , Rcp, v2, …

P. Levai

Soft-hard overlap at LHC:pT=4±1 for , pT=6±1 for p

LHCRHIC

/K

0 s

x

y

High pHigh pTT

tracktrack

If no SSS contribution to spectrum

-h correlation will be flat after v2 subtraction

1. Find a trigger particle (pT>2 GeV/c)

2. Find an associated particle

(1.5 GeV/c < pTas< pT

trg) in the same event

3. Compute at primary vertex

Statistical jet measurements

same-side away-side0B

v2

Possibility: enhanced production from recombination of thermal partons in the ridge (elongation in under the jet peak)

Another interesting idea …

…is that coalescence models could open up the exciting possibility to use the unconfined state to probe the constituent quark composition of exotic hadronic states (f0, a0), thus discriminating between conflicting models.

C. Salgado

f0 /a0 candidates for a new class of mesons having a 4-quark structure Joffe & Wilczeck; also Maiani, Piccinini, Polosa, Riquer

[s u ] [ s u ] ± [ s d ] [ s d ] / √2 ?

LHC

f0 =

Based on reco. + frag. model (R. Fries et al)

RHIC

Or f0 = ss ?

Particle reconstruction and identification capabilities: unique to ALICE Global tracking (ITS-TPC-TRD) + dE/dx (low pT + relativ. rise), TOF, HMPID, PHOS, …

Invariant mass, topological reconstruction Acceptance / efficiency / reconstruction rate () / contamination pT range (PID or stat. limits) for 107 central Pb-Pb and 109 min. bias pp

Identified particle spectra

Pb-Pb

Mid-rapidityPID in the relativistic rise

p

K

Pb-Pb

pT (GeV/c)

For ~ 20 particle species for -1 < y < +1 and -4 < y < +2.5

, K, p: 0.1- 0.15 50 GeV Weak or strong decaying particles: until 10-15 GeV

K0s and reconstruction in pp H. Ricaud

K0S

PDC06: 700 K pp(14 TeV) with PROOF

K0s

reconstruction rate

pp @ 900 GeVF. Riggi, R. Vernet, D. Tapia Takaki, M. Spyropoulou

Uncertainties on LHC running scenario: 1-2 days ? maximum expected number of events < 106 ?

With larger statistics, possibility to explore: Extended momentum range (1.5-2.5 GeV/c) for K* Mass shifts analysis vs pt and charged multiplicity Pt-distribution also for other resonances

Not very difficult to get above Tevatron statistics -> 900 GeV data could become best data at this √s.

With 2 x 105 events and PID: Extraction of yields at least for K*(892), Φ(1020), Λ*(1520) Pt-distribution for K*(892) up to 1.5-2 GeV/c Particle ratios K*/K- , Φ/K*, Λ*/K*, Φ/Λ* measurable

Resonances

Resonance reconstruction in p-p @ 0.9 TeV

Φ(1020)

K*(892)

200 000 Pythia pp min. bias events (PDC06) with PID

~ 180reconstructed

F. Riggi, R.Vernet

PT reach for

2 x105 events

K*

Evt mix. & like sign methods give similar results

Anisotropic flow (J.Y. Ollitrault et al.)

Ideal hydro : universal prediction v4=0.5 (v2)2 at large pt

Data ~1.2 suggest Kn~1: No thermalisation at RHIC!

v4/(v2)2

pT

Kn = mean free path/system size

The hydro limit is Kn«1. If not satisfied, one expects smaller v2 than in hydro.

Kn-1~σ/S (dN/dy)

PHOBOS part prediction

PHOBOS v2 result band: 90% CL

Non flow and fluctuations

2 arms: event plane azimuth phiZDC from the mean of the centroids of spectator neutrons spot on ZN1 and ZN2 front face.The distance between the 2 centroids can be a tool to select events with a better e.p. resolution

Reaction Plane Estimate in ALICE from spectator neutrons

Event Plane resolution: <cos(phiZDC – phiRP)>

N. De Marco

EbyEBalance function extended into a 2-dim. differential analysis in pT-η .

Decrease of the width with increasing <PT>. The method is sensitive to the transverse expansion of the system.

Net charge fluctuations, particle ratio fluctuations, long range correlations

P. Christakoglou

700K p+p events @ √s = 14TeV

200K p+p events @ √s = 900GeV

<PT> fluctuations

<PT> rises with increasing multiplicity

PDC06