# Presentation: 205923

## Saturation, Low-x, and QCDat RHIC, Part 2

- T. Hallman
- Villaggio Guglielmo, Italy

##             Low-x structure of hadrons:
 forward inclusive spectra and correlations

-  forward inclusive spectra and correlations

## The low-x structure of hadrons?

- Mid Rapidity

- Forward Rapidity

- CTEQ6M

- Gluon density can’t grow forever.
- “Saturation” must set in at forward rapidity when gluons start to overlap.

## Two very different possibilities at low x

- Kharzeev, Levin, McLerran gives physics picture (NPA748, 627)

- d+Au: ** Mono-jet?**

**P****T**** is balanced by many gluons**

- Dilute parton system (deuteron)

- Dense gluon
- field (Au)

- Color glass condensate predicts that the back-to-back correlation from p+p should be suppressed

- p+p:  **Di-jet**

##  = Q2R02(x)

-  = Q2R02(x)

- Transverse size of the color dipole is set equal to **1/Q** where **Q****2** is the virtuality of the exchanged photon.

**“****Geometric Scaling”** at HERA (**A. Staśto****,** **K. Golec-Biernat et al. PRL 86 2001**)
- R0  “saturation radius” ~ x ( 0.3)
- defines a scale: for values of Q2 such that for 1/Q ≥ R0
- the growth in the cross section saturates

## Quantum Evolution

- _Quantum Evolution_

- gluon radiation

- gluon fusion

- Y

**dN/d(ln1/x) = ********s**** (2N - N****2****)**

**R****dA**** = (d********pA****/d****2****kdy)/(Ad********pp****/d****2****kdy)**
**For k >> Q****s ****:**
**R****dA****< 1 **_**increasing with k approaching 1 from below.**_
** ****For Q****s ****< k < k****geom**** :  R****dA ******** e****-1.65******** y**** << 1 **
**For k ******** Q****s**** : **
**R****dA ******** exp (4 ******** y(1-√1+lnA****1/6****/2 ******** y) ) < 1**
**At high energy/ rapidity it becomes constant R****dA ******** A****-1/6**

**Kharzeev, Kovchegov and Tuchin Phys. Rev. D 68, 094013**

**Suppression at all k, suppression even stronger for higher A !**

**dP/dY ********s**

**U. Wiedemann et al. Phys. Rev. D 68, 054009**

## Attempt at a semi-classical, effective field approach: conceptual expectation for a Color Glass Condensate

-  related to rapidity of produced hadrons.

**As y grows**

- D. Kharzeev, hep-ph/0307037

- Iancu and Venugopalan, hep-ph/0303204

- Is there evidence for **gluon saturation at RHIC energies**?

## BRAHMS d+Au results as function of rapidity and centrality

- BRAHMS, PRL 93, 242303

**R****dAu****= **

**Yield****dAu**

**N****coll****Yield****pp**

- Normalized ratio of previously measured dN/d

- Calculated from spectra

- Rcp ratios are constructed in wide  bins and with data from same run period

- Nuclear
- Modification
- Factor

## STAR Nuclear Modification Factor at high rapidity

- BRAHMS published result consistent with new STAR once an isospin suppression of h- in p+p is taken into account (factor of 2/3).

- Calculations that do not include mod. of Au wave function cannot reproduce data.

## Forward hadron production and the Color Glass Condensate 

**A. Dumitru et al Nucl.Phys.A765:464-482,2006**

- Forward hadron production and the Color Glass Condensate

- Projectile: collection of quarks and gluons subject to DGLAP evolution.
- Target: CGC subject to quantum evolution.

**Impressive, but for**
**comparable STAR ********0**
**data (coming up) **
**a k factor of 0.9 must**
**be assumed**

## STAR Forward 0 from d+Au collisions  

- Inclusive 0 cross section per binary collision from d+Au at <> =4

- CGC calculation is the closest to data; use of Kretzer FF will improve agreement.

- CGC:A. Dumitru et al., Nucl. Phys. A765, 464 (2006)

**MS: I. Vitev et al. PRL 93 262301 (2004)**

- NLO: W. Vogelsang

- Same calculation as for BRAHMS
- _with_ K factor of 1.6

## STAR Forward 0 from d+Au collisions  

- Inclusive 0 cross section per binary collision from d+Au at <> =4

- CGC calculation is the closest to data; use of Kretzer FF will improve agreement.

- CGC:A. Dumitru et al., Nucl. Phys. A765, 464 (2006)

**MS: I. Vitev et al. PRL 93 262301 (2004)**

- NLO: W. Vogelsang

- Same calculation as for BRAHMS
- _but_ K factor of 0.9

**Q: why should the k factor change**
**from 1.6 to 0.9 as ******** changes **
**from 3.2 to 4.0 ?**

## Shadowing or formation of a CGC ?

**Leading twist gluon shadowing, e.g.:**
** ****Gerland, Frankfurt, Strikman,**
**  ****Stocker & Greiner (hep-ph/9812322)**
** ****phenomenological fit to DIS & DY data,**
**  ****Eskola, Kolhinen, Vogt hep-ph/0104124**
** ****and many others**

**Iancu and Venugopalan hep-ph/0303204**

- Amount of gluon shadowing differs by up to a factor of three between different models **→**

## Shadowing or formation of a CGC ?

**Leading twist gluon shadowing, e.g.:**
** ****Gerland, Frankfurt, Strikman,**
**  ****Stocker & Greiner (hep-ph/9812322)**
** ****phenomenological fit to DIS & DY data,**
**  ****Eskola, Kolhinen, Vogt hep-ph/0104124**
** ****and many others**

**Iancu and Venugopalan hep-ph/0303204**

- Amount of gluon shadowing differs by up to a factor of three between different models **→**

- Phenomenological fits diverge below x  0.02 (where data ends)

-              Requires solution of BK and JIMWLK equations

-                 The answer remains a question for experiment

## pQCD in p+p at RHIC

- Good agreement with NLO pQCD  pQCD should be broadly applicable at RHIC (e.g. heavy flavor production...)

## pp collision at s=200GeV (2)

- Cross-sections at the forward region, <>=3.3, 3.8 and 4.0  are mostly consistent
- with NLO pQCD calculations.

*pp* collision at s=200GeV (2)

- KKP FF : Nucl. Phys. B597, 337 (2001) Kretzer FF: Phys. Rev. D62, 054001 (2000).

## Summary so far:

- At low-x expect some type of “saturation” of the gluon distribution
- Both geometrical scaling and CGC pictures have some successes but, no conclusion yet from comparison with theory despite a _lot of work_
- Data consistent with both pictures (and others as well)
- Need a more discriminating measure

## A better way to tell what is going on: back-to-back azimuthal correlations 

- (Kharzeev, Levin, and McLerran, NP A748, 627)

- The emission  of gluons (pT~Qs)  between the jets makes the correlations disappear.

## Two very different possibilities at low x

- Kharzeev, Levin, McLerran gives physics picture (NPA748, 627)

- d+Au: ** Mono-jet?**

**P****T**** is balanced by many gluons**

- Dilute parton system (deuteron)

- Dense gluon
- field (Au)

- Color glass condensate predicts that the back-to-back correlation from p+p should be suppressed

- p+p:  **Di-jet**

## PYTHIA:  a guide to the physics

-  PYTHIA ***prediction**** *agrees well with the inclusive 0 cross section at 3-4
-  Dominant sources of large xF  production from:
-  q + g  q + g **(2********2)**   + X
-  q + g  q + g + g **(2********3)**   + X

- g+g and
- q+g  q+g+g

- q+g

- Soft processes

**PYTHIA:  a guide to the physics**

- Forward Inclusive  Cross-Section:

- Subprocesses involved:

- STAR FPD

## PYTHIA:  a guide to the physics

-  PYTHIA ***prediction**** *agrees well with the inclusive 0 cross section at 3-4
-  Dominant sources of large xF  production from:
-  q + g  q + g **(2********2)**   + X
-  q + g  q + g + g **(2********3)**   + X

- g+g and
- q+g  q+g+g

- q+g

- Soft processes

**PYTHIA:  a guide to the physics**

- Forward Inclusive  Cross-Section:

- Subprocesses involved:

- STAR FPD

- Caveat Emptor:
- Always mind the small print!
- This plot is only for 0s at  = 3.8 –so
- xF of 0.3 = pT of 1.3 GeV/c, where
- we saw pQCD was in less good
- agreement with the STAR
- forward 0 spectra
- One take home message is the kinematic
- Regime you are in is very important because
- it can affect the relevant
- prominence of various sub processes

## An initial glimpse: correlations in d+Au

- consistent with     CGC picture
** **are similar in d+Au and p+p at larger <xF> and <pT,π>
- as expected by HIJING** **

**25<E********<35GeV**

- π0:   |<η>| = 4.0
- h±:   |η| < 0.75; pT > 0.5 GeV/c

- <pT,π> ~
- 1.0 GeV/c

- <pT,π> ~
- 1.3 GeV/c

- As expected by
- HIJING

- PRL 97, 152302 (2006)

## PHENIX Azimuthal Angle correlations 

- Azimuthal angle correlation between rapidity separated  charged hadrons.

- Trigger :
-  1.4<< 2    d muon-arm
- -1.4>>-2 Au muon-arm

- Associated particle: ||<0.35 0.5<pT<1 GeV/c

- Two-part. Acceptance from event mixing

**PRL96, 222301 (2006)**

## The strength of the correlation is displayed with the conditional yield CY = Npair/assoc/Ntrig

- The strength of the correlation is displayed with the conditional yield CY = Npair/assoc/Ntrig
- Npair counts the events in the gaussian peak and assoc is obtained from Monte-Carlo simulations of PHENIX

- All points are consistent with no rapidity effect.

- Average rapidity separation: ~1.5

**X****2****~0.1**

**X****2****~0.01**

## This ratio is expected to drop below 1 in the presence of mono-jets.

**This ratio is expected to drop below 1 in the presence of mono-jets.**

**These ratios are consistent with one. With the exception of the central forward trigger.**

**Average x****g**** sampled**
**in PHENIX = .015**
**Average x****g ****sampled in Star correlation = .006 (2.5 times less)**
_**No contradiction-**_**-these are on opposite sides of the HERA saturation “knee”**

## STAR Forward Meson Spectrometer 

-  FMS increases areal
-    coverage of forward EMC
-    from 0.2 m2 to 4 m2
-  Addition of FMS to STAR
-   provides nearly continuous
-   EMC from -1<<+4

**FMS will span this entire acceptance—no gap **
** **
**It will also extend the coverage to smaller η, **
**providing even higher p****T ****reach at a given x****F****.****  **
**Inset shows invariant mass spectrum **
**for 2-photon events with 4 < p****T ****< 5 GeV/c,**

**Notes:**
Status of FMS (If pressed say getting it completed will be very challenging, but the Collaboration has committed to try to do this.)

## STAR Forward Meson Spectrometer 

-  FMS increases areal
-    coverage of forward EMC
-    from 0.2 m2 to 4 m2
-  Addition of FMS to STAR
-   provides nearly continuous
-   EMC from -1<<+4

- South half of FMS array
- on movable trolley ready for
- positioning near beamline

**FMS will span this entire acceptance—no gap **
** **
**It will also extend the coverage to smaller η, **
**providing even higher p****T ****reach at a given x****F****.****  **
**Inset shows invariant mass spectrum **
**for 2-photon events with 4 < p****T ****< 5 GeV/c,**

**Notes:**
Status of FMS (If pressed say getting it completed will be very challenging, but the Collaboration has committed to try to do this.)

## 1+2 =ln(x1/x2)

- 1+2 =ln(x1/x2)

## PHENIX Nose-Cone Calorimeter

- Replace existing PHENIX “nose-cones” (hadronic absorbers for muon arms) with Si-W calorimeter
- Major increase in acceptance forphoton+jet studies, will extend || to 3.

- Prototype silicon wafer with
- 3 different versions of “strip-pixel” detectors for the pre-shower and shower max layers

## PHENIX Nose-Cone Calorimeter

- Replace existing PHENIX “nose-cones” (hadronic absorbers for muon arms) with Si-W calorimeter
- Major increase in acceptance forphoton+jet studies, will extend || to 3.

- Prototype silicon wafer with
- 3 different versions of “strip-pixel” detectors for the pre-shower and shower max layers

## STAR Beam Use Proposal Run 8 – Run 10 (March 2007)

| Run |       Energy |      System |        Goal |
| --- | --- | --- | --- |
|             8 | sNN = 200 GeV
s     = 200 GeV
s     = 200 GeV
s     = 500 GeV |               d + Au
    p p  ,  p p
         p p 
                 pp |   (10 + 2 weeks)
  30/60 nb-1 sampled*
 (~12 + 2 weeks)
  3 days pp2pp
  Commissioning** |
|             9 | sNN = 200 GeV
s     = 200 GeV
s     = 200 GeV
s     = 500 GeV |      Au + Au
   p p ,  p p
          p p
            pp |   (8 + 2 weeks)
  (~14 + 2 weeks)
  3 days pp2pp
  Commissioning** |
|             10 |   Low sNN 

 s =  500 GeV |       Au + Au

        PP |     12 + 3 weeks

      8 + 3 weeks |

- *   First number with slow detectors, second number with fast detectors
- ** Contingent on achieving primary physics goals early

## Physics Driving Proposal for Run 8(from STAR Beam Use Defense, March 2007)

- Qualitative advance in our understanding of  the saturation scale for the gluon
- distribution in relativistic heavy nuclei
- Decisive test of gluon saturation as the origin of particle suppression at forward η
- First significant measurement of the x dependence of gluon polarization in the proton,
- G(x); new insight into transverse spin/motion preferences for quarks/gluon
- Qualitative advance in study of pp elastic scattering

**Measurements at RHIC in Run 8 which address these questions can result in a sea change in our understanding of the initial conditions in RHIC (LHC) HI collisions and the helicity**
**preference of gluons in the proton as a function of momentum fraction**

- The STAR Forward Meson Spectrometer (FMS) will be fully commissioned
- (and producing physics) prior to Run 8

- No extraordinary assumptions about machine capability beyond
-   what C-AD has projected or already achieved are necessary

_**Enabling Developments**_

## Trying to identify the onset of saturation and untangle the nature of its origin will be a major focus of the next run at RHIC
The big experiments PHENIX and STAR have embarked in detector upgrades that will increase the forward coverage and provide probes that go beyond the inclusive particle productions studied so far.  
The answer will come out  summer of 2008
It is a great time to think about this problem theoretically and publish your work

- The big experiments PHENIX and STAR have embarked in detector upgrades that will increase the forward coverage and provide probes that go beyond the inclusive particle productions studied so far.
- The answer will come out  summer of 2008
- It is a great time to think about this problem theoretically and publish your work

- _Outlook_