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ALICE physics

Guy PaićInstituto de Ciencias Nucleares

UNAMMexico

05/04/2006 Guy Paic LISHEP- ALICE physics

The aim

• The energy density reached in heavy ion collisions at LHC is large – according to the predictions of QCD theory of

strong interactions, nuclear matter will go through a QGP (Quark Gluon Plasma) phase

• state of deconfined partons (in a large volume).

• the question at LHC is not actually to put in evidence the QGP but rather to study its properties and hadronisation.


The aim cont’d

• observe phenomena that are very difficult to explain from a hadronic perspective but have a simple qualitative explanation based on quarks and gluons.

• make quantitative predictions for the emission of various kinds of “hard” radiation from the quark gluon plasma.


QCD phase transition

in vacuum• linear increase with distance • strong attractive force• quark confinement in hadrons baryons (qqq) and mesons (qq)

QCD potential:

in dense and hot matter• screening of color charges• potential vanishes for large distance scales• deconfinement of quarks !


Dynamics of a collision

•

After S. Bass

before the collision – before the collision – coherent field coherent field configuration the clouds configuration the clouds of gluons and quarks of gluons and quarks represented by the QCD represented by the QCD approximation of A x approximation of A x structure functions of structure functions of nucleonsnucleonsThe QCD fields persist The QCD fields persist after the nuclear after the nuclear valence valence quarkquark pancakes collide – pancakes collide – the interaction lasts for ~ the interaction lasts for ~ 30 fm/c =1030 fm/c =10-22-22 s sHadronization! The fields Hadronization! The fields hadronize in a way that is hadronize in a way that is not well understoodnot well understoodThe dense final state The dense final state debris further interact as debris further interact as it expandsit expands


Change of perspective

• SPS - evidence for collective phenomena in Nucleus- nucleus collisions

• many interesting signals telltale of phase transition

• not much hard processes• Dynamics of a collision • first inkling of new processes (hard) at RHIC

more to be seen at LHC– dominance of minijets– dominance of gluon-gluon interactions– importance of parton shadowing– parton saturation phenomena– high initial temperatures– jet quenching


Two main classes

• Soft physics – low pt (<3 GeV/c)• Hard probes


Time evolution

e

distance

time jet

AuAu

E

xpan

sion

p K

QGP

e


Soft physics


Soft Physics in Pb-Pb and pp

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

Chemical composition Hadronisation mechanisms

Event characterization Centrality selection Global observables

Event by event physics Fluctuations

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

1


Pseudorapidity distributions at ALICE and Atlas


Global event characterization in Pb-PbCentrality 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

3

Npart

generated

Npart ~15


Global event properties in Pb-Pb

Multiplicity distribution (dNch/d) in Pb-Pb

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

Energy density

dN/d % centrality (Npart) Fraction of particles produced in hard processes

Npart

(dN

/d)

||<

0.5

(dN/d)||<0.5

Generated Tracklets

Generated Tracklets

1 central Hijing event

4


Identified particle spectra in Pb-Pb and pp Excitation functions of bulk observables for identified hadrons New regime at LHC: strong influence of hard processes

Chemical composition

Equilibrium vs non equilibrium stat. models ?

Jet propagation vs thermalization ?

Production mechanisms for different hadron species also in pp

Interplay between hard and soft processes at intermediate pT

Parton recombination + fragmentation ?

or soft (hydro -> flow) + quenching ? or … ?

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

RHIC

5


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)

6

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


Topological identification of strange particles

Secondary vertex and cascade finding

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

Pb-Pb central

13 recons./event

pT dependent cuts -> optimizeefficiency over the whole pT range

Statistical limit : pT ~11 - 13 GeV for K+, K-, K0s, , 7 - 10 GeV for

6x104 pp collisions

Reconst. rates: : 0.1/event : 0.01/eventpT: 1 7-10 GeV

About the samepT limit for 109 pp

pp collisions

300 Hijingevents

11-12 GeV

Limit of combined PID

7


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 method) mass resolutions ~ 1.5 - 3 MeV and 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

Generated & reconstructed for 107 central Pb-Pb

Invariant mass (GeV/c2)

8


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

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

Relation between V2 and higher harmonics(V4, V6, …) to test perfect liquid % viscous fluid

2 2

2 2 2cos(2 )x y

x y

p pv

p p

y

x

py

px

2

2 21 2 cos( )

2 nnT T

d N dNv n

dp d dp

V2, V4, ...

At LHC: more sensitivity to the QGP

Flow of identified hadrons-> partonic d’s of freedom ?

RHIC

9

Hard Probes


• The yields of hard probes give rather direct information about the initial state of the collision– PDFs – the environment they have to traverse on there way out(QGP).

• Rescattering• Energy loss• Color screening

• A, pA and pp necessary and compulsory to be able to interpret the results

• Open flavor • Heavy quarks produced copiously at LHC

– 120 ccbar et 5 bbbar per central Pb-Pb, event– produced at (~1/2 mQ ~0.1 fm/c compared to τQGP ~10 fm/c)

• Should test: – pQCD – Test the medium thru energy loss of partons (jet quenching)– test the color screening of quarkonia.

•


gluon radiation

Parton energy loss

• High energy partons, resulting from a initial hard scattering, will create a high energy collimated spray of particles → jets

• Partons traveling through a dense colour medium are expected to lose energy via medium induced gluon radiation, “jet quenching”, and the magnitude of the energy loss depends on the gluon density of the medium

• Total jet energy is conserved, but “quenching” changes the jet structure and fragmentation function

Measurement of the parton fragmentation products reveals information about the QCD medium

2ˆLqCE Rs


Jet rates at LHC

4 108 central PbPb collisions/month

6 105 events

|y| < 0.5

ET threshol

d

Njets

50 GeV 2 107

100 GeV 6 105

150 GeV 1.2 105

200 GeV 2.0 104

Copious production:

Several jets per central PbPb collisions for ET > 20 GeV

However, for measuring the jet fragmentation function close to z = 1, >104 jets are needed. In addition you want to bin, i.e. perform studies relative to reaction plane to map out L dependence.


nucl-ex/0406012

x5

● PHENIX (π0)

High-pT suppression in central AuAu collisions

High-pT hadrons of recoiling jet suppressed in AuAu but not in dAu

gluon radiation

Evidence for partonic energy loss in heavy ion collisions

ddpNd

ddpNd

bNR

Tpp

TAA

collAA /

/

)(

12

2

1/Ntriggerd

N/d

()

PRL91, 072304 (2003)

Results from RHIC


Full jet reconstructionEskola et al., hep-ph/0406319

Leading Particle

Reconstructed Jet

Ideally, the analysis of reconstructed jets will allow us to measure the original parton 4-momentum and the jet structure. → Study the properties of the medium through modifications of the jet structure:

– Decrease of particles with high z, increase of particles with low z– Broadening of the momentum distribution perpendicular to jet axis

Leading particle becomes fragile as a probe• Surface emission:

–Small sensitivity of RAA to medium properties.

• For increasing in medium path length L, the momentum of the leading particle is less and less correlated with the original parton 4-momentum.

jetT

T

E

pz


Jet rates at the LHC

Huge jet statistics from ET ~10 GeV to ET~100 GeV

• Jets with ET > 50 GeV will allow full reconstruction of hadronic jets, even in the underlying heavy-ion environment.•Multijet production per event extends to ~ 20 GeV

100

coneR

pt (GeV)2 20 100 200

100/event 1/event 100K/year


50 GeV jet

50 – 100 GeV jets in Pb–Pb

η–φ lego plot with Δη 0.08 Δφ 0.25

At large enough jet energy – jet clearly visibleBut still large fluctuation in underlying energy

Central Pb–Pb event (HIJING simulation) with 100 GeV di-jet (PYTHIA simulation)

C. Loizides

100 GeV


Q

Heavy Quarks – dead cone

• Heavy quarks with momenta < 20–30 GeV/c v << c

• Gluon radiation is suppressed at angles < mQ/EQ “dead-cone” effect– Due to destructive interference– Contributes to the harder fragmentation of heavy

quarks

• Yu.L.Dokshitzer and D.E.Kharzeev: dead cone implies lower energy loss

Yu.L.Dokshitzer and D.E.Kharzeev, Phys. Lett. B519 (2001) 199 [arXiv:hep-ph/0106202].

D mesons quenching reducedRatio D/hadrons (or D/p0) enhanced and sensitive to medium properties


Detection strategy for D0 K- +

• Weak decay with mean proper length c = 124 m• Impact Parameter (distance of closest approach of a track to the primary vertex) of the decay products d0 ~ 100 m

• STRATEGY: invariant mass analysis of fully-reconstructed topologies originating from (displaced) secondary vertices– Measurement of Impact Parameters– Measurement of Momenta– Particle identification to tag the two decay products


Hadronic charmCombine ALICE tracking + secondary vertex finding capabilities (d0~60m@1GeV/c pT) + large acceptance PID to detect processes as D0K-+

~1 in acceptance / central event ~0.001/central event accepted after rec. and all cuts

S/B+S ~ 37

S/B+S ~ 8for 1<pT<2 GeV/c(~12 if K ID required)

significance vs pTResults for 107 PbPb ev. (~ 1/2 a run)


Sensitivity on RAA for D0 mesons

‘High’ pt (6–15 GeV/c)here energy loss can be studied(it’s the only expected effect)

Low pt (< 6–7 GeV/c)Nuclear shadowing+ kt broadening+ ? thermal charm ?

A.Dainese nucl-ex/0311004


Jet quenching

• Excellent jet reconstruction… but challenging to measure medium modification of its shape…

• Et=100 GeV (reduced average jet energy fraction inside R):– Radiated energy ~20% – R=0.3 E/E=3%– Et

UE ~ 100 GeV

RMedium induced redistribution of jet energy occurs inside cone

C.A. Salgado, U.A. Wiedemann hep-ph/0310079

vacuum

medium

Et = 50 GeV

Et = 100 GeV

0.200

0.4 0.6 0.8 1

0.2

R=√(2+2)

0.4

0.6

0.810

0.20.4

0.6

0.8

1

(R

)


Fragmentation functions

0 0.5 1z

10-4

10-2

1vacuummedium

pjet

z

kt

z=pt/pjet


The quarkonia physics


Acceptance for quarkonia measurements

• ALICE can measure J/ down to pt = 0 (unique @ the LHC)

• ALICE-muon can measure J/ & at large y


mass resolution( 100 MeV @ M ~ 10 GeV is needed to separate the sub-states)

• ALICE (& CMS) can measure the sub-states

• warning: ≠ simulation frameworks & inputs

ALICE dielectrons

background level 1 = 2 HIJING evts with dNch/d = 6000 @ = 0 each

ALICE dimuons

ATLAS

> 120 MeV

CMS

~ 80 MeV

ATLAS CERN/LHCC/2004-009, CMS NOTE 2000-060 (updated)


Extract signals

1. get invariant mass cocktail for all centrality & pt bins

2. subtract non-correlated dimuons (assuming a perfect event-mixing subtraction)

3. fit invariant mass spectra with 3 modified Landau convoluted with Gaussian & exponential for background

1. 2. 3.

0 < b < 3 fm 0 < b < 3 fm0 < b < 3 fm


Centrality dependence of ’/

• statistics : one month PbPb• nuclear absorption not in

• interest to combine pt

dependence of the ratio• systematic errors underway


1 month of dielectrons in the central barrel


• W LO production process is: • NLO processes contribute just ~ 13% to the total cross section

• LO dominant contribution (~ 80%) comes from udbar for W+,dubar for W-

• detection?– Via their leptonic decay:– Where?

ATLAS strategy is to measure at < 2.4 and e at< 2.5

CMS will be able to measure spectra for < 2.4

ALICE can measure e for < 0.9 and for – 4.0 << – 2.5

for – 4.02.5 ALICE is the only LHC Experiment able to measure W boson production

W detection in ALICE (Z.Conesa del Vale)

Frixione & Mangano, hep-ph/0405130

Martin, et al, hep-ph/9907231

u d d u

Wq' q

)( )()W(Wq' q llll


Single Muons at LHC• Some estimations for pp and PbPb nominal

runs at LHC Point 2...– pp @ 14 TeV

627.000 ’s generated from W decay in the ALICE IP 337.000 at Pt (30,50) GeV/c

88.800 ’s generated from W decay in the Muon Spectrometer Acceptance

51.000 at Pt (30,50) GeV/c

– PbPb @ 5.5 TeV, Min Bias 142.000 ’s generated from W decay in the ALICE IP

77.000 at Pt (30,50) GeV/c 15.500 ’s generated from W decay in the Muon

Spectrometer Acceptance 7.800 at Pt (30,50) GeV/c

W at LHC


The first three minutes….


Beam characteristics (LHC-OP-BCP-0001 rev 1.)• The highest possible beam energy will be achieved soon, however, with a

small number of bunches, and low intensity • Beam conditions will be ideal for ALICE pp physics – TPC

drift time ~80s – no or small pile-up – L 1x1029cm-2s-1 corresponds to 1 inel event in 160s

Beam Energy (TeV) 6 to 7 6 to 7 6 to 7

Number of bunches 43 43 156

* [m] 10 10 10Crossing Angle [rad] 0 0 0Transverse emittance [m]

3.75 3.75 3.75

Bunch spacing [ns] 2025 2025 525Bunch Intensity 1x101

0

4x101

0

4x1010

Luminosity [cm–2 s–1] 6x102

8

1x103

0

3.5x103

0

Inelastic Rate [Hz] 3600 60000 210000

936

75

Only 3 minutes to collect sample of 104 events…

1.3x1032

later


Motivation for pp study

• First insight in pp collisions in new energy domain (s 14 TeV), study of evolution of soft hadronic physics

– Cosmic ray interactions show `knee’ in 10151016 eV region and `ankle’ in 10181019 eV region

s 14 TeV corresponds to 1017 eV in lab frame• Contribution to knowledge of underlying

minimum bias (background) pp events for other LHC physics programmes (Higgs search, B-physics, etc.)

• Provide pp data as a reference for study of other collision systems (p-A, A-A)

• Low multiplicity data to commission and calibrate various components of ALICE


• It only takes a handful of events to measure a few

important global event properties (dN/d, d/dpT, etc.) – after LHC start-up, with few tens of thousand events we will do: Claus Jorgensen

Mean pT vs multiplicity

Multiplicity distribution

pT spectrumof chargedparticles

Pseudorapidity density dN/dη

CDF:Phys. Rev. D41, 2330 (1990)30000 events at √s=1.8TeV9400 events at √s=640TeV

UA5:Z. Phys43, 357 (1989)6839 events at √s=900GeV4256 events at √s=200GeV

CDF:Phys. Rev. Lett.61, 1819 (1988)55700 events at √s=1.8TeV

CDF:Phys. Rev. D65,72005(2002)3.3M events at 1.8TeV2.6M events at 630GeV

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