the physics of

Cogresso SIF 2008, Genova, 24.09.2008 Andrea Dainese 1

The Physics ofThe Physics of

Andrea DaineseAndrea Dainese

INFN – LegnaroINFN – Legnaro

for the ALICE Collaboration

Società Italiana di Fisica – Congresso 2008


The Physics ofThe Physics of

HighHigh-density QCD -density QCD with heavy nuclei at LHCwith heavy nuclei at LHC


Layout

Physics of high-density QCDwith heavy nuclei, from SPS and RHIC to LHC

Observables:• soft physics• hard probes

ALICE detector layout

Physics performance


High-density QCD in the final state of the collision: the Quark Gluon Plasma

Pb Pb

Tem

pera

ture

Baryonic density

U. Heinz

Explore QCD phase diagram:High temp. (>QCD ~200 MeV)

Low baryonic density (<< 0)

Lattice QCD predicts phase transition at Tc~170 MeV

Quark confinement removed

From hadronic to partonic d.o.f.

High energy density QCD medium

(“Quark-Gluon Plasma”)

of “weakly”-interacting gluons

high temperaturehigh energy densitylow baryonic density

d.o.f4 nT

37 3 :d.o.f. n

F.Ceradini


High-density QCD in the initial state of the collision:

Saturation at low-xInitial state: high-p nucleus = set of gluons with pg distribution according to PDF g(xBjorken,Q2), with x=pg/pN and Q2 the scale of the process (~ “area” of the gluon)

HERA DIS (ep) data: strong rise of xg(x,Q2) at low-x & low-Q2

(i) Standard DGLAP (linear) approach to

PDF evolution invalid nonlinear gluon-

gluon fusion balances gluon splitting

(ii) Factorisation assumptions (pert. QCD)

invalid parton scattering not

incoherent

Enhanced in the nucleus:

factor A1/3 (6) more gluons

per unit transverse area

New (unknown) regime of QCD: when gluons are numerous enough

(low-x) & extended enough (low-Q2) to overlap Saturation:


CERN-SPS:a glimpse of the new QCD medium

> 1990: fixed target Pb-Pb collisions at s = 17 GeV

Energy density up to 3 GeV/fm3 (> c ~ 1 GeV/fm3)

Experiments observe the two historical signatures of the QGP formation:

Suppression of J/ yield (NA50, NA60) large colour-charge density melts cc

1 10 100

10

1

FactorFactor 20 for20 for NA60NA50

NA57

Enhancement in multi-strange baryon yields (NA57, NA49) partonic system

E.Scomparin


BNL-RHIC:first properties of the new QCD medium> 2000: Au-Au collisions at s = 200 GeV

Energy density up to 5-15 GeV/fm3

Major findings of the 4 RHIC experiments (BRAHMS, PHENIX,

PHOBOS, STAR) in a nut-shell:Jet quenching: high-momentum partons lose energy (much more than expected?) due to interaction with the medium

ppin yield

AA/Nin yield collbinary AAR

• factor 4-5 suppresion for • little suppression for (medium blind)


BNL-RHIC:first properties of the new QCD medium> 2000: Au-Au collisions at s = 200 GeV

Energy density up to 5-15 GeV/fm3

Major findings of the 4 RHIC experiments (BRAHMS, PHENIX,

PHOBOS, STAR) in a nut-shell:Jet quenching: high-momentum partons lose energy (much more than expected?) due to interaction with the medium

Strong collective flow, well described by hydro-dynamical models

Quark recombination dominates over fragmentation in hadron production at intermediate monenta

wQGP (weakly-coupled) sQGP (strongly-coupled)

gas of gluons liquid of gluons


Heavy-ion Physics at LHCHeavy-ion Physics at LHC

LHC: factor 30 jump in w.r.t. RHICs

hotterhotterbiggerbigger

study of:

longer-livedlonger-lived

““fire-balls” of fire-balls” of QCD mediumQCD medium

4-101.5- 4.0< 1QGP (fm/c)

2x1047x103103Vf(fm3)

15- 605-103 (GeV/fm3)

1-3103700430dNch/dy

550020017s (GeV)

LHCRHICSPSCentral collisions

3

3 - 6

Lessons from RHIC:1) look for collectivity in flow/recombination low-pT, PID2) use hard partons to probe medium c, b, jets (+ ref: ,W,Z0)3) potential surprises ...


increasing s

Novel aspects of HI Physics at LHC (1)

Probe unexplored small-x region

bulk of partons at x < 10-3

reach down to 10-6 at forward rapidity

Gluon dynamics (PDF) poorly known, in the proton: in the nucleus

(huge saturation?):

“ter

ra in

cogn

ita”


First snapshot of small-x gluons: Charged multiplicity

“The hadron multiplicity measurements at the LHC will enable us to understand the nature of multi-particle production and the origin of parton evolution at high energies” – D. Kharzeev 14/05/2007

Estimates prior to RHIC: dNch/dy ~ 2000 – 8000

ALICE designed for 6000 – 8000

[GeV] s

dNch/d ~ 2000-2500

dNch/d ~ 1200Kharzeev et al.,

Color Glass Condensate:

Eskola et al., saturation model

Collision of 2 classical saturated

fields (full parton coherence)Day-1

measurementM.Nicassio


QCD medium: perfect liquid or perfect gas?

Much higher temperature and lifetime at LHC w.r.t. RHIC

sQGP (strongly-coupled) at RHIC; wQGP (weakly-coupled) at LHC?

d.o.f.4 nT

phase transition:

37 3 :d.o.f. npion gas QGP

- HARD PARTON QUENCHING - QUARKONIA AND PHOTONS T


sQGP wQGP?

“terra incognita”



Large hard cross sections

New probes of the medium:b quarks

high-ET jets

W, Z0

RHIC LHC

charm 10 100

beauty 0.05 5

10

100

w.r.t. RHIC

2000

High pT pions

ET 2 GeV 20 GeV 100 GeV 200 GeV

100/event 1/event 106/month 4x104/month

Jets at the LHC:

Heavy quarks:


Hard Probes of the Medium

Medium-blind, Control

Parton Energy Loss by medium-induced gluon radiation collisions with medium gluons

Real photon: colourless

Virtual photon/W: colourless

Q: colour triplet

g: colour octet

q: colour triplet

QCD medium

u,d,s: m~0, CR=4/3(difficult to tag at LHC)

g: m=0, CR=3> E loss, dominant at LHC

c: m~1.5 GeV, CR=4/3small m, tagged by D’sb: m~5 GeV, CR=4/3large mass, < E loss


QCD:

),,;( LmCE RQGPbqcg EEE

BAA

DAAAA RRR


Beauty energy loss:Use W-decay muons as a reference

First observation of b energy loss at LHC?

single RCP(pt)

beauty W beauty W

medium-sensitive and medium-blind probe in same measurement


Hard Probes of the Medium

Medium-blind, Control


Real photon: colourless

Virtual photon/W: colourless

Q: colour triplet

g: colour octet

q: colour triplet

QCD medium

u,d,s: m~0, CR=4/3(difficult to tag at LHC)

g: m=0, CR=3> E loss, dominant at LHC

c: m~1.5 GeV, CR=4/3small m, tagged by D’sb: m~5 GeV, CR=4/3large mass, < E loss


QCD:

),,;( LmCE RQGPbqcg EEE

BAA

DAAAA RRR

Dissociation?QQ (,): colour singlet/octet object


Quarkonia at the LHC

Open question from RHIC:

J/ suppression (colour screening) & regeneration (recombination) ?

SPSRHICLHC

30

enhanced suppression

enhanced regeneration

would melt only at LHC

’ TD ~ J/ TD

Small regeneration ’ can unravel J/ suppression VS regeneration

Comparison of suppression for different quarkonia states T/Tc

LHC?

RHICSPS

T > 3 TC


The ALICE DetectorThe ALICE Detector

||| < 0.9, B = 0.5 T| < 0.9, B = 0.5 TTPC + silicon trackerTPC + silicon tracker,,ee, , , K, p identification, K, p identification

-4 < -4 < < -2.5 < -2.5muonsmuons

ALICE Collaboration: >1000 people, 30 countries, ~80 Institutes

Size: 16 x 26 meters

Weight: 10,000 tons


Ready to move in!


Ready to move

TPC (largest ever) installed, ITS moving in


MUON arm: largest dipole ever


Ready to start!


ALICE commissioning with cosmics

Inner Tracking System

Muon Spectrometer

Time Projection Chamber

Inner Tracking System

alignment and calibrationA.RossiA.Silenzi


Physics Performance(selected examples)


statistical hadronization/ recombination

Identified Particle SpectraChemical composition of particle prod.

test of statistical hadronization scenario

strangeness enhancement

resonance in-medium mod.

Interplay between energy loss and hadronization scenario (recombination in medium vs fragmentation in vacuum)

RCP: central-to-peripheral ratio

baryon/meson ratios

STAR@RHIC

Pb-Pb central

300 Hijingevents

pK-

13 reconstructed/event

11-12 GeV

baryons

mesons

F.Riggi


Open heavy-flavour in ALICE

D0 K D+ K Ds KK D* D0 D0 K c Kp

understudy

B e+X B 5 pr. B J/ee

understudy

D0K in Pb-PbD+K in pp

Be +X in Pb-Pb


Heavy-to-light ratios in ALICE

1 year at nominal luminosity(107 central Pb-Pb events, 109 pp events)

)()()(/ thAAt

DAAthD pRpRpR probes colour-charge dep.

of QCD energy loss

)()()( D from eB from e/ tAAtAAtDB pRpRpR

probes mass dep.of QCD energy loss


ITS (vertexing)TPC (tracking)TRD (e/ id)

MUON (tracking,id)

e e

Quarkonia in ALICEMeasured both in the di-electron (midrapidity, TRD) and di-muon (forward rapidity, MUON) channel

J/ family family

Mμμ (GeV/c)Mμμ (GeV/c)

di-electron ||<0.9

di-muon -4<<-2.5


Medium-modified jet fragmentation via -jet correlations

Decrease of leading particle pT

Increase of number of low-pT particlesStudy jet FF(z), where z=pT/E

Search for depletion at z1 and enhancement at z0

quenched jet

non-quenched jet

Jet: TPC+EMCAL

: PHOS


Conclusions

The Large Hadron Collider with Pb nuclei will be:

a HARD PROBES machine

a SMALL-X machine

(not a black holes machine .... ;)

Explore new territories in high-density QCD

ALICE is ready for the challenge


Beam2-gas event in ALICEInner Tracker (Sept 12)

Beam1 circulated in LHC(Sept 10, 10:25:28)


Back-up Slides


Centrality measurement in ALICE

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

b ~ 1fm

bgen (fm)

bre

c(f

m)

reconstructed

Npart Npart

generated

Npart ~15


electron

D & B: Expected pt coverage

1 year at nominal luminosity(107 central Pb-Pb events, 109 pp events)

D0 K B e + X


Charm reconstruction: D0 K-+

Exclusive reconstruction: ideal to study RAA

Large combinatorial background (dNch/dy=2-3103 in central Pb-Pb!)

Main selection: displaced-vertex selection pair of opposite-charge tracks with impact parameters ~100m

good pointing of reconstructed D0 momentum to the primary vertex

Invariant mass analysis

< 60 m (r)for pt > 1 GeV/c

Resolution mainly provided by the 2 layers of silicon pixels


Beauty via displaced electrons

PrimaryVertex B

e

Xd0

rec. track

ALICE has excellent electron identification capabilities (TRD + TPC)

Displaced electrons from B decays can be tagged by an impact parameter cut


Heavy quark production at LHC:a probe of small-x gluons

Probe small-x region with HQs at low pT and/or forward y

Charm production at low pT: Q2 (4mc2 ~ 5 GeV2) ~ Qs

2

Down to x~10-4 with charm at y=0, x~10-6 at y4

Large pert. yields: ~0.1 cc/ev in pp, ~100 cc/ev in Pb-Pb

Cartoon

char

mbea

utyPYTHIAcharm


Low-x at LHC: forward J/ in ALICE (2.5 <||< 4)

J/ measurement in -spectrometer xg(x) at x2~10-5 :

d/dy J/: NLO CEM, varying PDFs

QQbar: Sensitive to different PDFs and DGLAP versus CGC predictions

(Note: mJ/~Q

s at the LHC)

pp @ 14 TeV

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