# Presentation: 388229

## STAR Physics at RHIC II

- James C. Dunlop
- Brookhaven National Laboratory

- RHIC II Science Workshop, Nov. 2004

##     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 jets
to 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)

***    ******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 jets**
**to 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

- RHIC II Science Workshop, Nov. 2004

## Jet Quenching

- 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).

- RHIC II Science Workshop, Nov. 2004

## Towards real tomography

-  **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**

- RHIC II Science Workshop, Nov. 2004

## 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 ***

- RHIC II Science Workshop, Nov. 2004

##  + Leading Particle 

-  **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****2****p****T ****(y=0) (GeV****-2****c****-3****)**** **

**P****T**** (GeV/ c)**

** ****AuAu (b = 0), s****1/2**** = 200 GeV**

-  + Leading Particle

- RHIC II Science Workshop, Nov. 2004

## Heavy Quark Energy Loss

-   fills the dead-cone

- massive

- massless

- dead cone

- Armesto, Salgado, Wiedemann, PRD69 (2004) 114003

-  vacuum radiation suppressed
-    in the dead-cone  < m/E

- Dokshitzer, Kharzeev, PLB 519 (2001) 199

- dI/d2

- 

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

- RHIC II Science Workshop, Nov. 2004

## Open Charm

***nucl-ex/0407006, nucl-ex/0404029***

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

- RHIC II Science Workshop, Nov. 2004

## 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

-   **p****T ****~ 15 GeV/c: ********(Au+Au) ~ **
**   ****20********b/Gev ********10 nb****-1**** yields **
**   ****200K b-bar pairs**

- Non-photonic electrons in d+Au

- Tagging in Au+Au (sim)

- RHIC II Science Workshop, Nov. 2004

## 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

- RHIC II Science Workshop, Nov. 2004

## 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, significant
- increase 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?

- RHIC II Science Workshop, Nov. 2004

## 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 fragments
- disappear

- central

***subtract cos*****(****************) *****and cos*****(*****2*****************)**

- widths

**Hijing default **
**central Au-Au**

- minijet angular
- deformation

- 0.15 < *p*t < 0.5 GeV/c

*p*t > 0.5 GeV/c

- 0.15* < p*t < 2 GeV/c

****

*STAR preliminary*

- Au-Au 130 GeV

- path length

*nucl-ex/0411003*

- RHIC II Science Workshop, Nov. 2004

##  pt Fluctuations → pt Correlations

-  *p*t Fluctuations → *p*t 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

- fluctuation
- inversion

- subtract
- multipoles

- autocorrelation

*STAR preliminary*

****** *****p*****t******** fluctuations**

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

- RHIC II Science Workshop, Nov. 2004

## 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*

- RHIC II Science Workshop, Nov. 2004

## 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 |

- RHIC II Science Workshop, Nov. 2004

## Temperature and density: Onium

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

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

***This physics requires luminosity upgrade***

**10**

**To deeply probe the plasma through studies of (Debye) screening length l ~ 1 /gT and map in-medium QCD potential**
**	****	****	****	****	****Study vs. Pt**
**	****	****	****	****	****Study vs. centrality**
**	****	****	****	****	****Study in lighter systems**
**	****	****	****	****	****Study vs. a *****control *****( the Upsilon)**

- RHIC II Science Workshop, Nov. 2004

## 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)

- RHIC II Science Workshop, Nov. 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 work
Thermal Dileptons
Direct sensitivity to temperature as a complement to photons
Somewhat separate uncertainties may help to disentangle T

**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 work**
**Thermal 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

- RHIC II Science Workshop, Nov. 2004

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

**products approach equilibrium**
**with the bulk medium traversed**
**Suggests a means to study particles **
**(e.g. leptonic decays of vector mesons**
**such 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 symmetry**
**is 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**

- φ

- e+

- e-

- RHIC II Science Workshop, Nov. 2004

## Fundamental QCD: Strong CP Violation

- 

****** ****Simple momentum space asymmetry**
**probably not good enough ******** look at e-by-e**
**helicity balance of fermions (********) and search**
**for fluctuation (too many positive helicity ********)**

- N /(N +N)

**Helicity Correlations**

**No Helicity **
**Correlations**

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

**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

- RHIC II Science Workshop, Nov. 2004

## 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 Calculation
- Frankfurt, Strikman & Zhalov, 2001

- No shadowing

- Shadowed

- HERA param.

- d/dy

- 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)

- RHIC II Science Workshop, Nov. 2004

## Forward dAu: probing low x

***Shown at DIS’04***

- RHIC II Science Workshop, Nov. 2004

## STAR RHIC-SPIN program 

- 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 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 polarization in W production (Long-term)

- Heavy flavor production

- 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

- RHIC II Science Workshop, Nov. 2004

## Flavor decomposition of quark and anti-quark polarization 

- 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

- Spin in the RHIC II era: W production

- Semi-inclusive DIS - sensitivity reduced by fragmentation functions and e2q 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

- RHIC II Science Workshop, Nov. 2004

## Spin: Beyond the Baseline

- 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 and*
*anti-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)*

- RHIC II Science Workshop, Nov. 2004

## Conclusion

***Sensitivity to rare probes, plasma radiation, and characterization of bulk matter***
***STAR proposes a future program of QCD studies of***
***unprecedented 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***

- RHIC II Science Workshop, Nov. 2004