IFAE 2007 NapoliEugenio Nappi Incontri di Fisica delle Alte Energie 2007

IFAE 2007 Napoli Eugenio Nappi

Incontri di Fisica delle Alte Energie 2007 Incontri di Fisica delle Alte Energie 2007


N. Cabibbo and G. Parisi, Phys. Lett. B59 (1975) 67

“In high-energy physics we have concentrated on experiments in which we distribute a higher and higher amount of energy into a region with smaller and smaller dimensions.

In order to study the question of ‘vacuum’‘vacuum’, we must turn to a different direction; we should investigate some ‘bulk’ phenomena by distributing high energy over a distributing high energy over a relatively large volumerelatively large volume.”

To understand the strong force and the phenomenon of confinement:we must create and study a system of deconfined quarks

by colliding heavy nuclei at ultrarelativistic energies

The pioneering papersThe pioneering papers

T.D. Lee Rev. Mod. Phys. 47 (1975) 267


Heavy Ion Physics: – starting in the 80’s at Bevalac by a few physicists mostly from US, Germany and Japan– more than 2000 nuclear and high energy physicists active worldwide today – energy increase by factor 104 in ~ 25 years with LHC in 2008

mostly by (re-)using particle physics machines

Heavy Ion AcceleratorsHeavy Ion Accelerators

1.E+00

1.E+01

1.E+02

1.E+03

1.E+04

1.E+05

1.E+06

1.E+07

1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 2005 2010

Year

sq

rt(s

) -

2m

(G

eV

)

SppS

ISR

FNAL

AGS

TeV LHC

AGS Si/Au

SPS S/Pb

Bevalac

LHC Pb

RHIC

Total center-of-mass energy versus time

x5.5x5.5

x 13x 13

x 28x 28

AGS

SPS

RHIC

LHC


μB

“Warning” in the Cabibbo and Parisi’s paper:The true phase diagram may actually be substantially more complex, due to other kinds of transition

The actual phase diagramThe actual phase diagram

Temperature

Quark-Gluon Plasma

ColorSuperconductor

Hadronic Fluid

Vacuum

CriticalEnd point

Triplepoint ?Ordinary

matter

Big Bang

~ 173 MeV


/T4 ~ number of degrees of

freedom

hadronicmatter:

few d.o.f.

deconfinedQCD matter:many d.o.f.

• Critical temperature: Tc 173 MeV• Critical energy density: 6 normal nuclear matter• The phase transition coincides with the restoration of the chiral symmetry• Pressure changes slowly at the phase boundary• The deconfined system does not behave like a weakly interacting parton gas

Expectations from Lattice QCD calculationsExpectations from Lattice QCD calculations

Tc 173 MeV ~ 2 1012 K

(T at the center of the Sun ~ 15 106 K)

Not yet a Stefan Boltzmann gas


The initial goal was to probe experimentally the phase diagram of nuclear matter (nuclear physics). The scope is now wider: Cosmologynature of QCD under the kind of extreme conditions which occurred in the earliest stages of the evolution of the Universe (~ 1 s)

Astrophysics stability of neutron stars

High energy physics symmetry breaking mechanism origin of (constituent) masses (chiral symmetry restoration)

The scope of heavy ion physicsThe scope of heavy ion physics


Main results from SPS and RHIC

Future prospects at LHC

ALICE physics potential

Outline

Physics of heavy ion collisions at ultrarelativistic energiesPhysics of heavy ion collisions at ultrarelativistic energies


reactions traverse two orders of magnitude of energy density and three phases in few fm/c

Aim: to study the dynamical evolution (intermediate phase) through the knowledge of the initial

conditions and the measurement of the particle yield

Heavy Ion CollisionsHeavy Ion Collisions

Colliding ionsColliding ions Bang! Bang! ? ? Freeze-out Freeze-out


• Chemical freeze-out (at Tch Tc): end of inelastic scatterings; no new particles (except from decays)

• Kinetic freeze-out (at Tfo Tch): end of elastic scatterings; kinematical distributions stop changing

hard (high-pT) probesearly signals (direct,but rare), keepmemory of QGP formation time

soft physics regimelate signals (indirect, but very abundant) created throughout collision history and decoupled late

The time evolution of the matter produced in HI collisionsThe time evolution of the matter produced in HI collisions


Pb-Pb collision seen by the NA49 TPCs

at the CERN SPS

STARSTAR

dNch/dη = 1200

dNch/dη = 2600

Challenge: selecting and calibrating the probesChallenge: selecting and calibrating the probesHigh multiplicity →Large combinatorialbackgroundEvent by event physics:- chemical composition (exploration of T,B)- dynamical fluctuations, …

Question: how do we recognize that the QGP has been formed?

▪ evidence extracted from a careful and systematic comparison of nucleon-nucleon, nucleon-nucleus and nucleus-nucleus reactions;

▪ simultaneous observation of several

(QGP) signatures

dNch/dy ~ 500

dNch/dy ~ 800


AGS : 1986 - 2000• Si and Au beams ; up to 14.6 A GeV• only hadronic observables

RHIC : 2000 - • Au, Cu beams ; up to s = 200 GeV• four experiments• hadrons, photons and dileptons

SPS : 1986 - • O, S and Pb beams ; up to 200 A GeV• sixteen experiments (two still running)• hadrons, photons and dileptons

LHC : 2008 - • Pb beams ; up to s = 5.5 TeV• three experiments• hadrons, photons and dileptons

Two labs to recreate the Big-BangTwo labs to recreate the Big-Bang


New state of matter at the SPSNew state of matter at the SPS

• J/ suppression. Indication of deconfinement?– J/ loosely bound system melts in QGP due to Debye screening

• Strangeness enhancement– Mass of strange quark decreases in QGP therefore easier to produce

• Melting of the – Ifdecays in QGP medium its mass is modified

NA50

April 2000, CERN press office announcement:

compelling evidence that a new state of matter has indeed been created


RHIC revenge RHIC revenge


the energy density is at least 7 times larger than needed for color deconfinement and temperature is about two times the critical temperature predicted by lattice QCD; matter thermalizes in an unexpectedly short time (< 1 fm/c) and exhibites collective motion with ideal hydrodynamic properties: a perfect liquid that appears to flow with a near-zero viscosity to entropy ratio, lower than any previously observed fluid (close to a universal lower bound); anomalous enhancement of baryon and anti-baryon production rates relative to mesons suggesting that hadrons form by quark coalescence after the flow occurs; dramatic energy loss of partons traversing matter of very high color charge density.

Heavy ion physics at RHICHeavy ion physics at RHIC





Particle distribution in the transverse plane Particle distribution in the transverse plane

b

x

y z

Coordinate space: initial asymmetry

Momentum space: final asymmetry

Collective motion → asymmetric pressure gradients are more effective at pushing particles out along the “reaction plane” direction rather than perpendicular to it, as measured by the elliptic flow v2

spectators

participantsnucleons in

nuclear overlap

nn

TT

ndpdN

ddpdN

)cos(v21 v2 is the 2nd harmonic Fourier coefficientof the particle distribution in the x-y plane

22

22

2 2cosyx

yx

pp

ppv


Elliptical flow @ RHICElliptical flow @ RHIC

v2 as large as predicted by perfect fluid dynamics strong (collective) pressure large and fast rescattering (early thermalization)

v2 dependent on mass (as predicted by P. Huovinen et al, PLB 503 (2001) 58)

viscosity/entropy = /s ~ 0 (perfect fluid)

0/ /s s


Scaling with valence quark number n supports a Scaling with valence quark number n supports a quark-coalescence picture of hadronizationquark-coalescence picture of hadronization

2 2v vhad partn

had partT Tp np

Coalescence vs fragmentationCoalescence vs fragmentation

Elliptic flow saturates at pt ~ 1 GeV/c, just at the constituent quark scale





• Recombination models assume particles are formed by the coalescence of “constituent” quarks

• Explain baryon excess by simple counting of valence quark content

Particle production ratioParticle production ratio

Au+Au: p/ ~ 1 Λ/K0S ~ 1.8

p+p: p/ ~ 0.3 Λ/K0S ~ 0.6

e++e-: p/ ~ 0.1-0.2

large enhancement in Au+Au relative to p+p collisions (max at pT~3 GeV/c)





back-to-back jets in pp and d-Au collisionsthe jet opposite to the high-pT trigger particle

disappears in central Au-Au collisions being absorbed by the dense QCD medium

Jet quenching @ RHICJet quenching @ RHIC

- pp d-Aucentral Au-Au

Two-particle azimuthal correlations

the high-pT hadrons are strongly suppressed with

respect to the expected scaling from pp collisions, photons are not affected by the dense medium

jet quenching: fast particles lose energy by traveling through the dense medium

nu

clea

r m

od

ific

atio

n f

acto

r

ppin yield 197 x 197

AuAuin yieldAAR

matter created in heavy-ion collisions is of extreme density and thus very opaque to hard partons

If A+A = p+p, then RAA

=1


Experimental evidence for « liquid » rather

than « gaseous » behavior

Strong correspondence with cosmology RHIC results, if confirmed at LHC,

would imply:

early Universe was a low viscosity liquid (400 times less than water) at T = 2000 109 K

Art due to Hatsuda and S. Bass

QGP @ RHIC QGP @ RHIC

The matter created in heavy ion collisions forms a highly color opaque plasma of quarks and gluons, which is possibly permeated by turbulent color fields, especially at early times, when the expansion is most rapid→→ sQGPsQGP

Berndt MBerndt Mueueller – ller – Duke UniversityDuke University

……it is clear that the matter created it is clear that the matter created

at RHIC differs from anything that at RHIC differs from anything that

has been seen before. Its precisehas been seen before. Its precise

description must await our deeperdescription must await our deeper

understanding… BRAHMS collab. understanding… BRAHMS collab.


LHC heavy ion programmeLHC heavy ion programme– Machine:– energy:

• Ebeam = 7 x Z/A [TeV] => s = 5.5 TeV/A or 1.14 PeV (Pb-Pb)

– beams:• possible combinations: pp, pA, AA (constant beam rigidity)

– Running time:• ~ 4 weeks/year (106 s effective); typically after pp running (like at SPS)• first HI run expected end 2008 (1/20th design luminosity)

– Luminosity: • 1027 (Pb) to >1030 (light ions) cm-2s-1 => rate from 10 kHz to several 100 kHz• integrated luminosity 0.5 nb-1/year (Pb-Pb)

– Detector(s)

– one single dedicated ‘general purpose’ HI expt at LHC: ALICE

– ATLAS/CMS will participate, but priority is pp physics


Why ultra-relativistic nuclear collisions at LHC? Why ultra-relativistic nuclear collisions at LHC?

<0.2~0.3~10 (fm/c)

4–101.5–4.0<1QGP (fm/c)

2x1047x103103Vf(fm3)

15–504–52.5 (GeV/fm3)

1200-2600800500dNch/dy

550020017s1/2(GeV)

LHCRHICSPSCentral collisionsHigher energy densities over larger volumes and increased lifetime of QGP phase (~10 fm/c) QGP more dominant over final-state hadron interactions

LHC is the first machine that will produce a long-lived quark-gluon plasma

transition from strongly coupled QGP -> ideal QGP ?

Much larger “dynamic range” compared to RHIC


• LHC allows to probe initial partonic state in a novel Bjorken-x range (10-2-10-5) where strong nuclear gluon shadowing is expected (saturation of the available phase-space) • At LHC, the central region is expected to be essentially baryon free more direct comparison with lattice QCD calculations and with the conditions of the primordial Universe

Novel aspects of nucleus-nucleus collisions at LHCNovel aspects of nucleus-nucleus collisions at LHC

10-6 10-4 10-2 100

x

108

106

104

102

100M

2 (

GeV

2)

RHIC

High density (saturated) gluon distribution

M = 3 GeV

M = 10 GeV


Hard Processes at LHCHard Processes at LHCMain novelty of the LHC: large hard cross section (scale with s !)

~2% at SPS

~50% at RHIC

~98% at LHC

tothard /

Hard processes ( i.e. jets, heavy flavours) are extremely useful tools:– have large Q and small “formation

time” t 1/Q• probe matter at very early

times – extend the range of matter probes

into regimes that allow for more reliable calculations (pQCD)

X 2000X 2000

Pion Production

Very hard probes

copiously produced

More than 1 jet > 20 GeV per central collision (more than 100 > 2 GeV)More than 4.0 104 jets with ET > 200 GeV in 1 month Pb-Pb run


Heavy quarksHeavy quarks

bbRHIC

bbLHC

ccRHIC

ccLHC

100

10

NLO calculations with MNR code: Mangano, Nason, Ridolfi, Nucl. Phys. B 373 (1992) 295.

NLO predictions (ALICE baseline for charm & beauty)

theoretical uncertainties of a factor 23

[mb] QQNNQQ

totN

system, s pp, 14 TeV Pb-Pb (0-5%), 5.5 TeV

11.2 / 0.5 4.3 / 0.2

0.16 / 0.007 115 / 4.6

SPS RHIC LHC

J/ melting

J/ regeneration

H. Satz, CERN Heavy Ion Forum, 09/06/05

?

Cancellation between J/suppression and regeneration(recombination) in a QGP ?

If more than one c-quark pair is formed within y 1, and if the c-quarks are thermalized, J/ may be formed by recombination.


Guideline: to measure flavor content and phase-space distribution event-by-event– Track and identify most (2 * 1.8 units) of the hadrons from very

low (< 100 MeV/c; soft processes) up to fairly high pT (~100 GeV/c; hard processes)

– Vertex recognition of hyperons and D/B mesons in an environment of very high charged-particles density (up to dN/d = 8000)

– Dedicated & complementary systems for di-electrons and di-muons– Excellent photon detection ( in Δφ =45° and 0.1 η units)– High throughput DAQ system + powerful online intelligence ('PC

farm')

ALICE ( A Large Ion Collider Experiment) is the only LHC detector specifically designed to investigate nucleus-nucleus collisions

Compromise: the fragmentation region is not addressed (difficult at LHC, ybeam=9)

ALICE Design ParametersALICE Design Parameters


The Alice CollaborationThe Alice Collaboration

~ ½ ATLAS,CMS; ~ 2x LHCb

30 Countries

90 Institutions

~1000 collaborators total (63% from CERN MS)

A large community which has been

constantly growing over the years, and still

grows

Italian participation: 190 researchers CORE share ~ 24 M€ ≈ 27%


ALICE Experimental LayoutALICE Experimental Layout

L3 magnet

B ≤ 0.5 T

Total weight : 9,800 tonsOverall diameter : 16 mOverall length : 26 m 130 MCHF CORE

Muon Forward Spectrometer

2.4 < < 4


Lepton AcceptanceLepton Acceptance

ATLAS & CMS present a large lepton acceptance ||<2.4

ALICE combines muonic and electronic channels

- covers the low pT region (quarkonia)

- covers the forward region 2.5<<4.0


J/Ψ family family

Mμμ (GeV/c)Mμμ (GeV/c)

– μ+μ- channel (muon arm)– 1 month PbPb data taking

State S[103] B[103] S/B S/(S+B)1/2

J/ 130 680 0.20 150

’ 3.7 300 0.01 6.7

(1S) 1.3 0.8 1.7 29

(2S) 0.35 0.54 0.65 12

(3S) 0.20 0.42 0.48 8.1

QuarkoniaQuarkonia

e+e- channel (TRD)

2 x 108 PbPb


• Essential jet measurements: modification of fragmentation in dense matter + response of the medium to the jet– cross sections are huge: rate is not a

primary issue– calorimetry alone insufficient:

physics lies in detailed changes of fragmentation patterns and correlations, including low pT

• Requirements for jet measurements:– precise tracking over very broad

kinematic range (TPC+ITS)– PID over broad kinematic range – detailed correlations of soft and hard

physics– jet trigger (EMCAL)

EMCAL EMCAL

Joint project between US and Europe (Italy and France)

It will enhance the ALICE capabilitiesfor jet measurement. It enables triggering on high energy jets (enhancement factor 10-15), reduces the bias for jet studies and improves the jet energy resolution.


August 2006August 2006 Detector InstallationDetector Installation

SEPTEMBER 2006SEPTEMBER 2006SEPTEMBER 2006SEPTEMBER 2006January 2007January 2007March 2006March 2006


Tracking PerformanceTracking Performance

p (GeV/c)

For track densities dN/dy = 2000 – 4000, combined tracking efficiency well above 90% with <5% fake track

probability

dN/dy = 4000, B=0.4 T

resolution ~ 5% at 100 GeV/c excellent performance in hard region!


0 1 2 3 4 5 p (GeV/c)

TPC + ITS (dE/dx)

/K

/K

K/p

K/p

e /

HMPID (RICH)

TOF

1 10 100 p (GeV/c)

TRD e /Rejection factor 100

/K

K/p

HADRON-ID 3 separation power

ITS

kaons

pions

protons

dE/d

x (M

IP u

nits

)

TPC

P(GeV/c)

All known PID techniques used in ALICE

dE/dx = 6.8% at dN/dy=8000 (5.5% for isolated tracks)

Charged Particle IdentificationCharged Particle Identification


Extension of PID by dE/dx to higher momentaExtension of PID by dE/dx to higher momenta

Combine TPC and TRD dE/dx capabilities (similar number of samples/track) to get statistical ID in the relativistic rise region

8<p<10GeV/c


Secondary vertex and cascade finding

Pb-Pb central

13 recons./event

pT dependent cuts -> optimizeefficiency over the whole pT range

Statistical limit : pT ~8 - 10 GeV/c for K+, K-, K0s, ; 3 - 6 GeV/c for

Reconst. rates: : 0.1/event : 0.01/eventpT: 1 to 3-6 GeV

300 Hijingevents

8-10 GeV

106

Topological identification of strange particlesTopological identification of strange particles


Invariant mass reconstruction, background subtracted (like-sign method) mass resolutions ~ 1.5 - 3 MeV and pT stat. limits from 5 () to 12 GeV/c (,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)

ResonancesResonances


Open Charm Detection in Hadronic DecaysOpen Charm Detection in Hadronic Decays

Mass 1.864 GeV/c2 c=124 m

~0.55 D0K-+ accepted/event important also for J/ normalization

Overall significance for 106 events ~10

1<pT<2 GeV/c

High precision open charm measurement


Proton-proton physics with ALICE

The ALICE detector works even better for pp collisions, because of the low occupancy (10–4 to 10–3), even if there is a significant number of events overlapping.

The first physics with ALICE will be proton-proton collisions, which correspond to a major part of the ALICE programme for several reasons:– to provide “reference” data to understand heavy ion collisions. In a new

energy domain, each signal in HI has to be compared to pp;– For genuine proton-proton physics whenever ALICE is unique or

competitive; note that ALICE can reach rather “high” pT, up to ~ 100 GeV/c, ensuring overlap with other LHC experiments.

– The possibility of taking proton data at several center of mass energies (0.9 TeV, 2.4 TeV, perhaps 5.5 TeV, and 14 TeV), will provide ALICE with the possibility to understand the evolution of many of the properties of pp collisions as a function of the center of mass energy, and also to add to the measurements from previous experiments using proton-antiprotons.


Minimum Bias trigger is provided by coincidence between V0 counters covering a pseudo-rapidity range from -1.7 to -3.7 and from 2.8 to 5.1.

excellent measurement in the central region

(charged particles)

Global event properties: charged particle multiplicity


Strange and baryonic particle studies• based on Pythia simulation, ALICE will be able to acquire a significant sample of strange based on Pythia simulation, ALICE will be able to acquire a significant sample of strange

particles produced in pp minimum bias events:particles produced in pp minimum bias events:

• heavy flavour baryons (heavy flavour baryons (bb, , bb, , bb, ...) which are poorly known. , ...) which are poorly known.

Br.(Br.(bb J/ J/ = (4.7 ± 2.8) = (4.7 ± 2.8)1010–4–4, 10, 1099 events, triggered on J/ events, triggered on J/ using the TRD detector, should using the TRD detector, should

produce a few thousand produce a few thousand bb


LHC will provide a quantitative LHC will provide a quantitative understanding of Relativistic understanding of Relativistic Heavy Ion collision dynamics Heavy Ion collision dynamics

and QGP properties likely and QGP properties likely enabling enabling to assess the current to assess the current

theoretical uncertaintiestheoretical uncertainties

ConclusionConclusion

Documents

IFAE 2007 NapoliEugenio Nappi Incontri di Fisica delle Alte Energie 2007