STAR Physics at RHIC II

RHIC II Science Workshop, Nov. 2004 1

STAR Physics at RHIC II

James C. Dunlop

Brookhaven National Laboratory


To test and extend QCD theory and its predictions STAR will:

– use hard (short wavelength) probes such as• Inclusive jets and direct photons• back to back jets (correlation of leading particles)• direct gamma + leading hadron from jet• flavor tagged jetsto measure the differential energy loss for gluon, light quark and heavy quark probes which couple differently to the

medium• spectra and yields of the full range of quarkonia, through the Upsilon family, to measure the

thermodynamics of deconfinement

– measure very large samples of “soft physics” events to study • mechanism of equilibration through heavy quark yields and flow• the velocity structure of the medium in complete detail• the medium’s temperature using leptons, photons• spectrum of extended hadronic matter (resonances)• broken / restored symmetries (e.g., CP violation, chiral restoration)

Sensitivity to rare probes, plasma radiation, and characterization of bulk matter

STAR Physics in the RHIC II Era


Experimentally well-established phenomenon at RHIC

The promise: Use as a quantitative tool to characterize

the medium’s properties

J. Adams et al., Phys. Rev. Lett. 91, 072304 (2003).

Pedestal&flow subtracted

Jet Quenching


Au+Au: Away-side suppression is larger in the out-of-

plane direction compared to in-plane

Geometry of dense medium imprints itself on correlations

STAR Preliminary, nucl-ex/0407007

?

Towards real tomography


Geometry, Correlations, and “Fragile” RAA

• Quenching so strong that RAA loses sensitivity to the density of the medium: dominated by unquenched “halo”

• Increased sensitivity only through detailed angular correlations and/or decreasing coupling strength

K.J. Eskola, H. Honkanken, C.A. Salgado, U.A. Wiedemann, hep-ph/0406319

A. Dainese, C. Loizides, G. Paic, hep-ph/0406201


• Leading hadrons are very rare: only ~0.1% of jets fragment hard enough that hadrons are above incoherent background

• cross section for + jet coincidences (central Au+Au):

• E=10 GeV: 6 nb/GeV• E=15 GeV: 0.6 nb/GeV

• 50 weeks of Au+Au @ RHIC I design: 10 nb-1 !! luminosity upgrade needed to access this physics!

Remaining question: fragmentation

Quantitative measurement of partonic energy loss

Measurement of the gluon density via direct + jet and flavor-tagged jets to study the quark mass dependence of energy loss

dN

/dyd

2p

T (y

=0)

(G

eV-2c-3

)

PT (GeV/ c)

AuAu (b = 0), s1/2 = 200 GeV

+ Leading Particle


● medium-induced radiation fills the dead-cone

massive

massless

dead cone

Armesto, Salgado, Wiedemann, PRD69 (2004) 114003

● vacuum radiation suppressed in the dead-cone < m/EDokshitzer, Kharzeev, PLB 519 (2001) 199

● total energy loss comparable but smaller than in the massless case

Armesto, Salgado, Wiedemann, PRD69 (2004) 114003B.W. Zhang, E. Wang, X.N. Wang, PRL93 (2004) 072301Djordjevic, Gyulassy, NPA733 (2004) 265

Heavy Quark Energy Loss

1.0E

mdI/d2

Flavor dependence of coupling: Less radiation, and so less suppression, for massive objects


Open Charm

nucl-ex/0407006, nucl-ex/0404029

Abundant production, but high backgrounds: upgrades and large data samples needed for precision measurementsLimited window for testing E-loss flavor dependence: m/E large, but E

perturbative


The next generation: b quark

• Not just c quark: b quark also accessible at RHIC II• Heavier mass -> m/E appreciable deep into perturbative regime

pT ~ 15 GeV/c: (Au+Au) ~

20b/Gev 10 nb-1 yields 200K b-bar pairsNon-photonic electrons in d+Au

Tagging in Au+Au (sim)


What are the relevant degrees of freedom?

PRL 92 (2004) 052302, nucl-ex/0403032

Growing evidence for recombination/coalescence picture:Window into relevant degrees of freedom at hadronization?

Need to precisely characterize behavior of identified hadrons over fullest possible range of species and measures


Asymmetry in near-side correlations

Armesto, Salgado, Wiedemann, hep-ph/0405301, Phys. Rev. Lett. in press

D. Magestro, Hard Probes 2004

STAR Preliminary

At intermediate pT, significantincrease in width in of near-side cone

Is this due to energy loss in a flowing medium? Or recombination effects (PID)? Or?

What beyond density can be learned from strong coupling to the medium?


Minijet Deformation on ()

• Minijet angular correlation: elongated on , narrowed on

• Soft string fragments disappear

• Observed width increase is 10 Armesto, et al. pQCD prediction – hep-ph/0405301

peripheral

p-p 200 GeV

string fragmentsdisappear

central

subtract cos() and cos(2)

widths

Hijing default central Au-Au

minijet angulardeformation

0.15 < pt < 0.5 GeV/c

pt > 0.5 GeV/c

0.15 < pt < 2 GeV/c

STAR preliminaryAu-Au 130 GeV

path length

nucl-ex/0411003


pt Fluctuations → pt Correlations

• Minijets as velocity/temperature correlation structures on ()

• Need PID to distinguish from T• Strong elongation on and new

negative same-side structure

Parton interaction with longitudinally expanding color fluid drags pre-hadronic matter along pseudorapidity

full STAR acceptance

variance excess

fluctuation scaling

fluctuationinversion

subtractmultipoles

autocorrelation

STAR preliminary

70-80%

20-30%

0-5%centrality

pt fluctuations

nucl-ex/0308033, hep-ph/0410180,hep-ph/0410182


Kinetic Thermalization: Open Charm Flow

Test mechanism for thermalization: charm heavy, so needs many collisions to reach kinetic equilibrium.

Current measurements: indirect from electrons, and so suffer from large statistical and systematic errors

Need direct open charm reconstruction (at low pT)!

F. Laue et al, nucl-ex/0411007


Chemical Equilibration: Open Charm Yields

• No thermal creation of c or b

quarks; m(c) = 1.1GeV >> T

• c and b quarks interact with

lighter quarks thermal

recombination ?

– Ds+/D0 very sensitive

– J/suppression vs

recombination ?

– Precision necessary for

J/dilepton baseline

Pythia

p-p 200 GeV

Au-Au

Thermal*

D+/D0 0.33 0.455

Ds+/D0 0.20 0.393

c+/D0 0.14 0.173

J//D0 0.0003 0.0004

No suppression


Yields and Spectra of the onium states (J/, Upsilon, and excited states) to measure the thermodynamics of deconfinement through varying dissociation temperatures

RHIC

Upsilon rate ~ 10-3 J/ Yield in 10 weeks of AuAu running at design luminosity

This physics requires luminosity upgrade 10

Temperature and density: Onium

To deeply probe the plasma through studies of (Debye) screening length l ~ 1 /gT and map in-medium QCD potential

Study vs. PtStudy vs. centralityStudy in lighter systemsStudy vs. a control ( the Upsilon)


Direct photons: temperature

• As yet, no measurement at RHIC directly sensitive to temperature at early times

• Intriguing possibility: direct photons, especially with low pT

interferometric methods– Potentially sensitive to

thermal black-body radiation from the plasma, and so T

– However, significant theoretical uncertainties remain: what is needed to bring these under control?

WA98: Phys. Rev. Lett. 93, 022301(2004)


Vector Meson Properties– Cleanest (only?) experimental signature of chiral symmetry restoration– Experimentally, need excellent e/h rejection at low p, precision vertexing to control

Dalitz, conversion backgrounds– Large contribution from hadronic stage: needs precision, systematics, theoretical

workThermal Dileptons

– Direct sensitivity to temperature as a complement to photons• Somewhat separate uncertainties may help to disentangle T

R. Rapp, PRC 63, 054907 (2001)

Dileptons: chiral symmetry and temperature


The data suggest the away side products approach equilibriumwith the bulk medium traversed

Suggests a means to study particles (e.g. leptonic decays of vector mesonssuch as the Φ) that have an increased probability of having been produced in a bulk medium which may be deconfined, and/or in which chiral symmetryis restored.

Differences between yields and spectra, branching ratios, flavor composition…for products 180° versus 90° from the tagged high pt particle may provide access e.g. to the study of chiral symmetry

away side:

whole: ||<2.0collimated: ||<0.35

away

near

Col

limat

edre

gion

awayφ

e+

e-

Enhancing the signal: “Hard” tagging of “soft” physics


)(~_,_

ccQCDeffectsvacuumanamolyaxial

QCD BEGGLL

EC• HC 0

EC• HC 0

Simple momentum space asymmetryprobably not good enough look at e-by-ehelicity balance of fermions () and searchfor fluctuation (too many positive helicity )

N /(N +N)

Helicity Correlations

No Helicity Correlations

Estimated need: several hundred million events! (efficiency dependent)

Fundamental QCD: Strong CP Violation

QCD “should ” include CP violation, but experimentally, = 0

Under certain conditions around a de-confining phase transition, regions of space may be formed which behave as if 0 – spontaneous CP violation. (Kharzeev et al)

-0.004 0.004


Gluon distributions in Cold Nuclei

• Ultra-Peripheral Collisions: large flux of *– Use hard probes (such as J/,) to

probe gluon distributions

STAR: nucl-ex/0408016, PRC in press BRAHMS, nucl-ex/0403005

Y = 1/2 ln(2k/MV)Leading Twist CalculationFrankfurt, Strikman & Zhalov, 2001

No shadowing

Shadowed

HERA param.d/d

y

d+Au: dependence of spectra Disagrees with expectations based on incoherent multiple scattering in the initial state (i.e. standard factorized pQCD explanation for Cronin enhancement)


PT is balanced by many gluons

“Mono-jet”

Dilute parton system

(deuteron)

Dense gluon field (Au)

• E > 25 GeV• 4

Beam View Top View

Statistical errors only

25<E<35GeV

35<E<45GeVSTAR

Preliminary

Forward dAu: probing low x

Fixed as

E & pT grows

Large 0+h± correlations

• Suppressed at small <xF> , <pT,>

Consistent with CGC picture

•Consistent in d+Au and p+p at larger <xF> and <pT,>

as expected by HIJING

Shown at DIS’04

Fixed as

E & pT grows

Can the boundaries on this diagram be mapped out

experimentally?


• STAR RHIC-SPIN program Comprehensive study of the spin structure and dynamics of the proton, in particular

the nature of the proton sea, using polarized protons: “RHIC SPIN Baseline program” (DOE review, June 2004)

Gluon contributionGluon contribution to the proton spin using various probes involving: Final-state jets such as inclusive jet production and di-jet production (Short-term) Inclusive 0 production (Short-term) Prompt photon production (Long-term) Heavy-Flavor production (Long-term)

Flavor decomposition of quark and anti-quark polarizationFlavor decomposition of quark and anti-quark polarization in W productionW production (Long-term)(Long-term)

• Heavy flavor production

)()(

)()(

LLA

Unique test of partonic aLL

Sensitive to gluon helicity with low background from quark helicities

NLO formalism available (Bojak and Stratmann)

Low

energy

Massless-Q limit

Typical STAR range

M. Karliner and R.W. Robinett,

Phys. Lett. B324 (1994) 209.

Spin in the RHIC II Era


W± production in pp collisions forms the best means to probe the flavor structure of the proton sea

• Flavor decomposition of quark and anti-quark polarization

)(W

d u W

u d W

d u W

u d W

Spin in the RHIC II era: W production

Semi-inclusive DIS - sensitivity reduced by fragmentation functions and e2

q weighting B. Dressler, Eur. Phys. J. C14 (2000) 147.

Q2=2.5GeV2 Q2=2.5GeV2

Parity violating single-spin asymmetries at RHIC provide access to the quark flavor structure of the proton spin:

J. Kiryluk

ALPV (W )(

p p) d /d

ALPV (W )(p

p ) u /u

ALPV (W )(

p p) u /u

ALPV (W )(p

p ) d /d

ALPV

u d

u d

u

u

d

d

d

d

u u


With adequate integrated luminosity and longitudinal spin orientations,RHIC will answer two of the important open questions in nucleon spin structure:

gluon helicity helicities of up and down quarks and anti-quarks

However, the RHIC baseline program will not provide much insight into strange quark andanti-quark helicities and , which are particularly sensitive to flavor symmetry scenario’s.

charm associated W production

RHIC-II’s luminosity upgrade may! It should provide access to rare processes, e.g.

e.g. ALL in prompt photon-jet production e.g. parity violating single spin asymmetries

as well as,

transversity, Drell-Yan, ... (c.f. Werner Vogelsang - this workshop)

Spin: Beyond the Baseline


Conclusion

Sensitivity to rare probes, plasma radiation, and characterization of bulk matter

STAR proposes a future program of QCD studies ofunprecedented breadth and depth to study

• the quark mass dependence of partonic energy loss• collective behavior in partonic systems• the nature of chiral symmetry breaking and its relation to the masses of the

hadrons• the nature of a possible saturated gluon state in cold nuclei at low Bjorken x• the helicity preference of gluons inside a proton; the origin of the proton sea;

the transversity distribution of quarks in a proton• And, of course, surprises: the program has just begun

There are still significant questions about many of these observables: hopefully, this set of workshops can serve to at least clarify the questions

Documents

STAR Physics at RHIC II