What the ridge is telling us and why its important

Raju Venugopalan

Brookhaven National Laboratory

STAR Analysis Meeting, MIT, July 9th, 2009

Outline of Talk

Why is the ridge important ? Multi-gluon production in HI collisions: how Glasma flux tubes emerge & their relation to “pQCD” The Ridge and Glasma flux tubes: where theory becomes model Other model explanations Piecing it all together: what have we learnt and what’s next

Why is the Ridge important? Long-range rapidity correlations (LRC) sensitive to multi-parton correlations in nuclear wavefunction

LRC are a chronometer of early time strong color field Glasma dynamics

Same origin as I) Strong local CP violation from topological “sphaleron”

transitions II) Quantum dynamics leading to early thermalization III) Possible “synchrotron-like” radiation that may

contribute to “anomalous” jet energy loss and / s

In the Glasma flux tube picture, the existence of Ridge-like structures is strongly related to these possible effects

What does a heavy ion collision look like ?

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Color Glass

Condensates

Initial

Singularity

Glasma sQGP - perfect fluid

Hadron Gas

t

The nuclear wavefunction at high energies

At RHIC typical x ~ 0.01 . At the LHC, x ~ 5 * 10-4

PartonDensity

x= fraction of momentum of hadron carried by parton

Nuclear wave function at high energies is a Color Glass Condensate

Glue rules!

What does a heavy ion collision look like ?

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Color Glass

Condensates

Initial

Singularity

Glasma sQGP - perfect fluid

Hadron Gas

t

Forming a Glasma in the little Bang Glasma (\Glahs-maa\): Noun: non-equilibrium matter between Color Glass Condensate (CGC)& Quark Gluon Plasma (QGP)

Problem: Compute particle production in QCD with strong time dependent sources

Gelis, Lappi, RV; arXiv : 0804.2630, 0807.1306, 0810.4829

Solution: for early times (t 1/QS) -- n-gluon production computed in A+A to all orders in pert. theory to leading log accuracy

Numerical Yang-Mills soln.:Glasma flux tubes

After: boost invariant flux tubes of size 1/QS

Krasnitz,Nara, RV; Lappi

Before: transverse E & B“Weizsacker-Williams fields

Lappi,McLerran,NPA 772 (2006)

parallel color E & B fields

Kharzeev, Krasnitz, RV, Phys. Lett. B545 (2002)

generate Chern-Simons topological charge

The unstable Glasma • Small rapidity dependent quantum fluctuations of the LO Yang-Mills fields grow rapidly as

• E and B fields as large as EL and BL at time

Romatschke, RV:PRL 96 (2006) 062302

Possible mechanism for rapid isotropization Problem: collisions can’t ‘catch up’

Turbulent “thermalization” may lead to “anomalously” low viscosities

Asakawa, Bass, Muller; Dumitru, Nara, Schenke, Strickland

Significant energy loss in Glasma because of synchroton like radiation?

Shuryak, Zahed; Zakharov; Kharzeev

A No-Go Theorem

If glue fields are boost invariant,

no sphaleron transitions are permitted!

With rapidity dependent quant. fluctuations, explosive sphaleron transitions are allowed!

Kharzeev, Krasnitz, RV, Phys. Lett. B545 (2002)

Sphaleron = Over the barrier transitions connecting QCD vacua labeled by differing topological charge - a mechanism for EW Baryogenesis in standard model

Klinkhamer and Manton (1984)

P and CP violation: Chiral Magnetic EffectKharzeev,McLerran,Warringa L or B

+ External (QED) magnetic field

Chiral magnetic effect

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=STAR Preliminary

Possible experimental signal of charge separation (Voloshin, Quark Matter 2009)

Topological fluctuations-sphaleron transitions in Glasma

NCS = -2 -1 0 1

2

For effect to be significant, these transitions must occur at early times

What does all this have to do with the ridge?

Long range rapidity correlations arise from two particle correlations in the Glasma

Imagining the Glasma

Causality dictates:

Au+Au 200 GeV, 0 - 30%PHOBOS preliminary

ΔφΔ dd

Nd

N

1 ch2

trig

< 1 fm for Δy > 4

These correlations likelyoccur at early times…

The Ridge and Glasma flux tubes-IDumitru, Gelis ,McLerran, RV, arXiv:0804.3858[hep-ph]

“pQCD” graphs (SRC)

Independent gluon emissionfrom Glasma flux tube (LRC)

For Strong Color Sources (high energy/large nuclei/central collisions) Flux Tube Emission dominates “pQCD” by 1 / S

2

The Ridge and Glasma flux tubes-IICorrelations are induced by color fluctuations that vary event to event - these are local transversely and have color screening radius 1/QS

Simple “Geometrical” result:strength of correlation= area of flux tube / transverse area of nucleus

The Ridge and Glasma Flux tubes- where theory meets model

Color Glass

Condensates

Initial

Singularity

Glasma sQGP - perfect fluid

Hadron Gas

t

The evolution of Glasma into the perfect fluid is not understood. Initial condition for hydro evolution requires modelling…

Soft Ridge = Glasma flux tubes + Radial flow

R€

1

QS

€

Vr

Pairs correlated by transverse Hubble flow in final state- experience same boost

Can be computed non-perturbativelyfrom numerical lattice simulations

Srednyak,Lappi,RV

from blast wave fits to spectra

QS from centrality dependence of inclusive spectra

Ridge from flowing flux tubes

Gavin,McLerran,Moschelli, arXiv:0806.4718

Glasma flux tubes get additional qualitative features right:

i) Same flavor composition as bulk matterii) Ridge independent of trigger pT-geometrical effectiii) Signal for like and unlike sign pairs the same at large Δ

See also Lindenbaum and Longacre, arXiv:0809.3601, 0809.2286

Three particle Glasma correlationsDusling, Fernandez-Fraile, RV, arXiv:0902.4435 [nucl-th]

Three particle cumulant can be expressed as

Radial Boost parameter

Distributions are radially boosted…

Three particle correlations uniform in

Prediction: angular collimation of three particle correlations- max. at (0,0) and min. at (2/3, 4/3)

Ratio independent of

STAR, PRL 102:052302 (2009)

“Glittering” GlasmasGelis,Lappi,McLerran, arXiv:0905.3234

n-particle correlation can be expressed as

with

This is a negative binomial distribution which is knownto describe well multiplicity distributions in hadronicand nuclear collisions

For k = 1, Bose-Einstein dist.For k= , Poisson Dist.

Npart

PHENIXarXiv:0805.1521

Beyond boost inv. - Δ dependence of 2 Part. Corr.

Evol. for nucl. 2

Formalism to compute dN2/d3p d3qab initio for arbitrary rapidity separation ΔYpq

Gelis,Lappi,RV arXiv:0810.4829

(Jet) = 0.25 0.09 (Ridge) = 1.53 0.41

PHOBOS plot…..Model comparisoncoming soon…

Dusling,Gelis,Lappi,RV

The Long and the Short of LRC and SRC

dN2/d3p d3q

Gelis,Lappi,RV

AApA

Strength of correlation

(parton density)

For p >> QS (1 GeV) expect SRC to dominate and LRC to diminishHowever, because jet correlations die off so quickly, phase space dominance extends naïve regime of validity of LRC to large p

-- this argument can and should be quantified further.

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Theorist making comparisonto other theory models…

ALL ANIMALS ARE EQUAL, BUT SOME ARE MORE EQUAL THAN OTHERS"- George Orwell, Animal Farm, Ch. 10

Comparison of models by Nagle, QM09

i) “Causation” models: purely final state models

Longitudinal collective flow, Momentum kick, Broadeningin turbulent color fields,…

Difficult to get independence of trigger, width in ΔMomentum kick model gets it but perhaps too wide in Δ

ii) “Auto-correlation” models: LRC from initial state and collimation in azimuth from boosted flow

Glasma flux tube, Parton Bubble, see also related model by Shuryak

Improving the Glasma flux tube model

NEXUS initial condition + SPHERIO hydro evolution + Cooper-Frye freezeout

Jun Takahashi et al.arXiv:0902.4870

Glasma flux tube initial condition

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Piecing it all together

Remarkable features of RHIC data - the presence of long range correlations strikingly seen in the ridge - topological fluctuations and charge separation - early “isotropization” and strong floware sensitive to the early time strong color field (Glasma)dynamics.

Glasma flux tubes may provide a unifying explanation of all these features

These ideas will be further tested and refined in future RHIC runs and at the LHC