helen caines - yale university march 2004 star the strange physics occurring at rhic

Helen Caines - Yale University

March 2004 STAR

The Strange Physics Occurring at RHIC

Helen Caines – March 2004 2

Why do we do this research?

To explore the phase diagram of nuclear matter

How:♦ By colliding nuclei in lab.♦ By varying energy (√s) and

size (A).♦ By studying spectra and particle correlations.

How:♦ By colliding most massive and highest energy nuclei.♦ By comparing to more elementary systems. ♦ Through high pT studies

To probe properties of dense nuclear matter

Rajagopal and Wilczek,hep-ph/-0011333


Lattice QCD calculations

TC ≈ 170 MeV

• Coincident transitions: deconfinement and chiral symmetry restoration • Recently extended to B> 0, order still unclear (1st, 2nd, crossover ?)

F. Karsch, hep-ph/0103314

Action density in 3 quark system in full QCDH. Ichie et al., hep-lat/0212036

G. Schierholz et al., Confinement 2003


A theoretical view of the collision

Tc – Critical temperature for transition to QGPTch– Chemical freeze-out (Tch Tc) : inelastic scattering stopsTfo – Kinetic freeze-out (Tfo Tch) : elastic scattering stops


RHIC @ Brookhaven National Lab.

h

Long Island

Long Island

Relativistic

Heavy

Ion

Collider

Previous Runs: ♦ Au+Au @ sNN=130 GeV & 200 GeV

♦ p+p @ sNN =200 GeV♦ d+Au @ sNN =200 GeV

Present Run:

♦ Au-Au sNN=200 GeV

2 concentric rings of 1740 superconducting magnets

3.8 km circumference

counter-rotating beams of ions from p to Au


Number binary collisions (Nbin): number of equivalent inelastic

nucleon-nucleon collisions

Geometry of heavy-ion collisions

Preliminary sNN = 200 GeV

Uncorrected

peripheral (grazing shot)

central (head-on) collision

spectatorsParticle production scales with increasing centrality

Nbin ≥ Npart

participants

Number participants (Npart): number of nucleons in overlap region


Particle creation and distributionsd

Nc

h/d

h

19.6 GeV 130 GeV 200 GeV

PHOBOS Preliminary

Central

Peripheral

Total multiplicity per participant pair scales with Npart

Not just a superposition of p-p

To get much further need PID


STAR is a large acceptance detector

STAR Prelimin

ary

STAR Prelimin

ary

K

K*

STAR Preliminary

preliminary

K0s

Preliminary

STAR Preliminary

preliminary


Strangeness enhancement General arguments for enhancement:

1. Lower energy threshold TQGP > TC ~ ms = 150 MeV

Note that strangeness is conserved in the strong interaction

2. Larger production cross-section

3. Pauli blocking (finite chemical potential)

T = 0ms

u d s

q q s s

g g s s

N K

K N

Ethres 2ms 300 MeV

Ethres 530 MeV

Ethres 1420 MeV

QGP ss HG ss

resonances

)(68%,2.5cm K)(

m)(100%,4.9c )(

)(64%,7.9cm )(

n/a)(49%, KK)(

)(69%,2.7cm )(K

(64%,3.7m) ,K0

sss

dss

puds

ss

sdsd

susu

s

Strange particles with chargeddecay modes

Enhancement is expected to be more pronounced for multi-strange baryons and their anti-particles

Arguments still valid but now use Strange particles for MUCH MORE


Strangeness enhancement? Canonical (small system):

Computed taking into account energy to create companion to ensure conservation of strangeness. Quantum Numbers conserved exactly.

Grand Canonical limit (large system):

Just account for creation of particle itself. The rest of the system acts as a reservoir and “picks up the slack”. Quantum Numbers conserved on average via chemical potential

canonical suppression increases with strangeness decreases with volume ~ observed enhancements

[Hamieh et al.: Phys. Lett. B486 (2000) 61]

♦ Phase space suppression of strangeness in small system/low temperature


Correlation volume

Grand Canonical description is only valid in a system in equilibrium that is

large.

BUT being large is not a sufficient condition for being GC! if A+A were just superposition of p+p STILL need to treat

CANONICALLY

System must know it is large... Must know that an Ω+ generated here can be compensated by, say, an

Ω- on the other side of the fireball!

what counts is the correlation volume

How does the system KNOW its big? Not from hadronic transport: no time

One natural explanation: returning from deconfined state


Grand canonical applicable at RHIC?

♦ See drop in “enhancement” at higher energy ♦ Enhancement values as ~predicted by model♦ Correlation volume not well modeled by Npart

[Tounsi & Redlich: hep-ph/0111159]

System is in G.C. state for most central data

130 GeV



1

Chemical freezeout (Tch Tc) : inelastic scattering stops


Models to evaluate Tch and B

Compare particle ratios to experimental data

Qi : 1 for u and d, -1 for u and d

si : 1 for s, -1 for s

gi : spin-isospin freedom

mi : particle mass

Tch : Chemical freeze-out

temperatureq : light-quark chemical potential

s : strangeness chemical potential

s : strangeness saturation factor

Particle density of each particle:Statistical Thermal ModelF. Becattini; P. Braun-Munzinger, J. Stachel, D. MagestroJ.Rafelski PLB(1991)333; J.Sollfrank et al. PRC59(1999)1637

Assume: ♦ Ideal hadron resonance gas ♦ thermally and chemically equilibrated fireball at hadro-chemical freeze-out

Recipe:♦ GRAND CANONICAL ensemble to describe partition function density of particles of species i

♦ fixed by constraints: Volume V, ,

strangeness chemical potential S,

isospin♦ input: measured particle ratios♦ output: temperature T and baryo-chemical potential B


Thermal model fit to data♦Particle ratios well described:

Tch = 160 5 MeV

B = 24 5 MeV

s = 1.4 1.4 MeV

s = 0.99 0.07

Dat

a –

Fit

(s)

R

atio

Created a Large System in Local Chemical Equilibrium


Tch systematicsHagedorn (1964):

if the resonance mass spectrum grows exponentially

(and this seems to be the case) there is a maximum possible temperature for a system of hadrons

[Satz: Nucl.Phys. A715 (2003) 3c]

filled: AAopen: elementary

Blue – Exp. fit Tc= 158 MeV

r(m

) (G

eV-1)

Green - 1411 states of 1967Red – 4627 states of 1996

Seems he was correct – can’t seem to get above Tch ~170MeV

m



Chemical freezeout (Tch ) ~ 170 MeVTime between Tch and Tfo

2


Thermal model reproduced dataD

ata

– F

it (s

)

R

atio

Do resonances destroy

the hypothesis?

Created a Large System in Local Chemical Equilibrium

Used in fit


Resonances and survival probability

Chemical freeze-out

Kinetic freeze-out

measured

lost

K

K lost

K*

K*

K

K*

Kmeasured

♦ Initial yield established at chemical freeze-out

♦ Decays in fireball mean daughter tracks can rescatter destroying part of signal

♦ Rescattering also causes regeneration which partially compensates

♦ Two effects compete – Dominance depends on decay products and lifetime

time

Ratio to “stable” particle reveals information on behaviour and timescale

between chemical and kinetic freeze-out

K*

K


Resonance ratios

Thermal model [1]:Tch = 177 MeVB = 29 MeV

[1] P. Braun-Munzinger et.al., PLB 518(2001) 41 D.Magestro, private communication[2] Marcus Bleicher and Jörg Aichelin Phys. Lett. B530 (2002) 81-87. M. Bleicher, private communication

Need >4fm between Tch and Tfo

UrQMD [2]

Life time [fm/c] : (1020) = 40 (1520) = 13 K(892) = 4 ++ = 1.7 = 1.3

Small centrality dependence: little difference in lifetime!

Nch

/

TT chfoStable

Resonance

Stable

Resonance te



31

Chemical freezeout (Tch ) ~ 170 MeVTime between Tch and Tfo 4fmKinetic freeze-out (Tfo Tch): elastic scattering stops

2


Kinetic freeze-out and radial flow

m1/m

T d

N/d

mT

light

heavyT

purely thermalsource

explosivesource

T,b

m1/m

T d

N/d

mT

light

heavy

If there is radial flow

Look at p or m= (p2 + m2 )

distribution

Slope = 1/T

dN/dm- Shape depends on mass and size of flow

Want to look at how energy distributed in system.

Look in transverse direction so not confused by longitudinal expansion

A thermal distribution gives a linear distribution

dN/dm e-(m/T)

Heavier particles show curvature


Radial flow and hydro dynamical model

Tfo ~ 90 10 MeV, < > = 0.59 ± 0.05c

tanh 1 r

R

s

E.Schnedermann et al, PRC48 (1993) 2462

dn

mT dmT r dr mT K1

mT coshT

0

R

I0pT sinh

T

Shape of the m spectrum depends on particle massTwo Parameters: Tfo and

r =s (r/R)n

p,K,p fit


Flow of multi-strange baryons

♦ , K, p: Common thermal freeze-out at Tfo ~ 90 MeV

<> ~ 0.60 c

♦ : Shows different thermal freeze-out behavior:

Tfo ~ 160 MeV

<> ~ 0.45 c

But: Already some radial flow!

Tfo ~ Tch Instantaneous Freeze-out of multi-strange particles?Early Collective Motion?

Higher temperatureLower transverse flowProbe earlier stage of collision?

Au+Au sNN=200 GeV

STAR Preliminary

68.3% CL 95.5% CL 99.7% CL



4

31

Chemical freezeout (Tch ) ~ 170 MeVTime between Tch and Tfo 4fmKinetic freeze-out (Tfo) ~ 90 MeV (light particles)Very Early Times

2


Almond shape overlap region in coordinate space

y2 x2 y2 x2

Anisotropy in momentum space

AGS

SPS, RHIC

Interactions

2cos2 vx

y

p

patan

1

2

3

3

cos212

1

nrn

tt

nvdydpp

Nd

pd

NdE

v2: 2nd harmonic Fourier coefficient in dN/d with respect to the reaction plane

Early collective motion

Look at “Elliptic” Flow


v2 of strange particles

♦ Multi-strange particles show sizeable elliptic flow!♦ Reach hydro. limit

♦ Seems to saturate at v2~20% for p~3.0 GeV/c ♦ v2(p) follows evolution♦ v2(p) consistent with and v2(p)

Hydro: P. Huovinen et al.

Equal Energy Density lines

P. Kolb, J. Sollfrank, and U. Heinz


Why high p physics at RHIC?

q

q

hadronsleadingparticle

leading particle

schematic view of jet production

hadrons

q

q

hadrons

leadingparticle

jet production in quark matter

New penetrating probe at RHIC

attenuation or absorption of jets “jet quenching” suppression of high p hadrons modification of angular correlation changes of particle composition

Early production in parton-parton scatterings with large Q2.Direct probes of partonic phases of the reaction


The control experiment – d-Au

♦ Collisions of small with large nuclei quantify all cold nuclear effects.♦ Small + Large distinguishes all initial and final state effects.

Nucleus-nucleuscollision

Medium?

Proton/deuteron-nucleus collision

No Medium!


Jet suppressionHard scatter back-to-back jet – Angular correlation at and

♦ Central Au-Au backwards jet suppressed♦ d-Au backwards jet is visible

Jet suppression is a final state effect


Energy loss creates anisotropy?

Jet Propagation

y

x

STAR Preliminary

Energy loss results in anisotropy due to different “length” of matter passed

through by parton depending on location of hard scattering

Hypothesis seems verified


Identified particle correlationsWhy:

To gain insight on possible different fragmentation function of different parton.

To probe further differences in mesons and baryons at high p

Correlation for K0s, and , in both

cases, there is an absence of a ‘back-to-back’ partner correlation.

Need more statistics for further studies

Fig. Fig. 33

∆Φ (radians)

1/N

1/N

trig

ger

trig

ger*d

N/d

(∆*d

N/d

(∆ΦΦ

))

∆Φ (radians)

1/N

1/N

trig

ger

trig

gerd

N/d

(∆d

N/d

(∆ΦΦ

))

Fig. 5Fig. 5

STAR Au+Au 5%

ptrig > 2.5 GeV/c

2.5 GeV/c <passoc< p

trig


Nuclear modification factor

“Hard” Physics - Scales with Nbin: Number of binary collisions number of equivalent inelastic nucleon-nucleon collisions

ddpdT

ddpNdpR

NNAA

AA

AA

/

/)(

2

2

<Nbinary>/inelp+p

N-N cross sectionNuclear Modification Factor:

If no “effects”: R < 1 in regime of soft physics R = 1 at high-p where hard scattering dominates

Can replace p-p with peripheral Rcp


Suppression of identified particlesTwo groups (2<p <6GeV/c):

- K0s, K, K*, mesons

- baryons

Rcp

Clearly not mass dependence

Come together again at p ~ 6 GeV?

“standard” fragmentation?

show different

behaviour to K

Suppression of K sets

in at lower p

K

Mass or meson/baryon effect?

PHENIX: PRL 91, 172301


d-Au control experiment

Au + Au, RAA << 1; d+Au, RdAu > 1

RAA results confirm there are final state effects

Enhancement is the well known “Cronin Effect”


Parton coalescence and medium p

♦ Recombination p(baryons) > p(mesons) > p(quarks) (coalescence from thermal quark distribution ...)♦ Pushes soft physics for baryons out to 4-5 GeV/c♦ Reduces effect of jet quenching

Do soft and hard partons recombine or just soft+soft ? Explore correlations with leading baryons and mesons

recombining partons:

p1+p2=ph

♦ When slope exponential: coalescence wins

♦ When slope power law: fragmentation wins

Fries et al. QM2004

fragmenting parton:

ph = z p, z<1


v2 and coalescence model

Hadronization via quark coalescence: v2 of a hadron at a given p is the partonic v2 at p/n scaled by the # of quarks (n).♦ Works for K0

s, &

♦ v2s ~ v2

u,d ~ 7%

D. Molnar, S.A. Voloshin Phys. Rev. Lett. 91, 092301 (2003)V. Greco, C.M. Ko, P. Levai Phys. Rev. C68, 034904 (2003) R.J. Fries, B. Muller, C. Nonaka, S.A. Bass Phys. Rev. C68, 044902 (2003)Z. Lin, C.M. Ko Phys. Rev. Lett. 89, 202302 (2002)

Au+Au sNN=200 GeV

STAR Preliminary

MinBias 0-80%


Exotica searches (pentaquarks)♦ Constituent quark model of the1960s has been very successful in describing known baryons as 3-quark states♦ QCD and quark model do not forbid composites of more quarks♦ But early searches were unsuccessful and finally given up

1

2PJ

Chiral Soliton Model: Ns and s rotational states of same soliton field

♦ Minimum quark content is 4 quarks and 1 antiquark♦ “Exotic” pentaquarks are those where the antiquark has a different flavor than the other 4 quarks ♦ Quantum numbers cannot be defined by 3 quarks alone.

Particle Data Group 1986 reviewing evidence for exotic baryons states

“…The general prejudice against baryons not made of three quarks and the lack of any experimental activity in this area make it likely that it will be another 15 years before the issue is decided. “

PDG dropped the discussion on pentaquark searches after 1988.

The mass splittings arepredicted to be equally spaced

Rotational excitations include

2710108

Diakonov et al. Z phys A 359 (1997) 305


Early evidence for pentaquark’s

+ results :

Highest? Significance (CLAS) = 7.8

(hep-ex/0311046)

5 results :

NA49:

-- (1860) - -

0 (1860) - +

Width limits are experimental resolution

Mass (Xp) GeV/c2

Cou

nts

significance=5.6

Need strong confirmation of second member of anti-decuplet


RHIC - ideal place for pentaquarks

No Clear Signal Yet.

B/B ratio ~ 1 should see anti-pentaquark If form QGP should coalesce into pentaquarks? Look at + K0

s +

+ /event (stat. model calc.) 0.5 – 1.5 1.5 Million events 0.8 – 2.3 MEfficiency 3% 25 – 70 KBranching Ratio 50% 10 – 25 KBR 50% from K0

s 5 – 18 K BG in mass range/event 2BG in sample 3 M Significance =

Signal/√(2 X BG+Signal) 2-7 Similar calc. for p-p 0.25-3

d-Au 1-16

p-p

d-Au

STAR Preliminary

STAR Preliminary


Other pentaquarks at RHIC

+n+K-

+p+K0

p

p

Au-Au Minbias

Possible peaks need more investigation

PHENIX Preliminary

d-Au


+ at the AGS

K+ d + p ( Ethresh= 400 MeV)K+ p + + (Ethresh = 760 MeV)

Use AGS Kaon beam (D. Ashery, E. Piasetzky, R. Chrien, P.Pile)

Why:♦ Large production cross-section compared to electro magnetic processes(Liu and Ko) 104:1♦ Only measure single particle mtm to determine mass♦ Angular distribution determines spin

K+d

+

p

Really need to determine propertiesspin, parity etc


Determining spin and parity K+ d + p

Intrinsic parity - + +? + K+ p + +

Intrinsic parity - + +? -

Parity Conserved 1= (-1)L n1n2

L = If – Ii L = Odd L = Even

K+ d + p spin 0 1 ½(?) ½

K+ p + + spin 0 1 ½(?) 0

L = 1 L = 0

L determines the decay angular distribution

Determination of spin and parity will help select between theories

Correlated quark & Chiral soliton models predicts Jpc=½+ (p-wave)Quark model naïve expectation is Jpc=½− (s-wave)


Summary

0 1 2 3 4 5 6 7 8 9 10 11 12 GeV/c

pQCDReCo

Hydro

Different physics for different scales

Strange particles are useful probes for each scale

♦ All evidence suggest RHIC creates a hot and dense medium with partonic degrees of freedom

♦ Only just beginning to understand the rich physics of RHIC


Extra Slides

helen caines - yale university march 2004 star the strange physics occurring at rhic

Documents