# Presentation: 700878

## Forward Physics at RHIC

**Forward Physics at RHIC**

- Ramiro Debbe
- BNL Physics Dept.

- 2007 RHIC AGS Users  18-22 June 2007

## Outline of presentation

-  Kinematics of forward physics and the benefits of work in collider mode.
- BRAHMS p+p and d+Au results at high rapidity.
- Similar measurements performed by PHENIX PHOBOS and STAR.
- Future Forward physics at RHIC.

- 2007 RHIC AGS Users  18-22 June 2007

## Leading order kinematics

**Energy and momentum conservation **
**       ****x****L**** = x****a**** - x****b ****=(2M****T****/√s)sinh y**
**                        ****k****a**** + k****b**** = k**
**                ****x****a****x****b ****= M****T****2****/s **
**A solution to this system is:**
**    ****x****a**** = (M****T****/√s) e****y**
**      ****x****b**** = (M****T****/√s) e****-y**
**where y is the rapidity of the (x****L****,****, ****k) system**

**In a 2->2 interaction where both partons are measured at rapidities y****1**** and y****2****,**

**Y****system****=1/2(y****1****+y****2****) **
**     ****y*********=1/2(y****1****-y****2****)**

**Leading order kinematics**

- Sudakov variables

- 2007 RHIC AGS Users  18-22 June 2007

## Parton Distribution Functions

**Parton Distribution Functions**

**X****2**** range**

**X****1**** range**

**Measurements at high rapidity set the dominant parton type: ****Projectile ****(x****1**** ~1) mostly ****valence ****quarks.**
**Target**** (x****2****<0.01) mainly ****gluons****.**

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## The data at forward rapidities were collected with FS at 4o ( ~3) and 2.3o (~3.4)

**INEL**
**2.47<********<5.29**

**The data at forward rapidities were collected with FS at 4o (******** ~3****) and 2.3o (********~3.4****)**

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## Particle Identification is done with BRAHMS RICH

**Particle Identification is done with**** ****BRAHMS**** ****RICH**

- 2007 RHIC AGS Users  18-22 June 2007

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

- BRAHMS, PRL 93, 242303

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

**Y****dAu**

**N****coll****Y****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

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## BRAHMS results in the context of CGC

**Similar saturation effects have already been seen at HERA, and the multiplicity densities in A+A at RHIC show coherence that hint to the onset of saturation. The rapidity and centrality dependence of the BRAHMS results could then be the result of “quantum evolution” of an already saturated Au wave function.**

**These results came just after the effects of the onset of the Color Glass Condensate at RHIC energies were predicted and a qualitative description of its effects in high rapidity particle production was offered. **

- 2007 RHIC AGS Users  18-22 June 2007

##  = 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. Sta****ś****to,** **K. Golec-Biernat et al. PRL 86 2001**)
- R0  “saturation radius” ~x
- defines a scale: for values of Q2 such that for 1/Q≥R0
- the cross section becomes a constant.

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## 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**

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## 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**

**Shadowing or formation of a CGC**

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

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## Parameterization of nuclear shadowing in (LO) calculation

**Parameterization of nuclear shadowing in (LO) calculation**

**R. Vogt Phys. Rev. C70 064902 (2004)**

******-**

- h-

- p-

- K-

**EKS98 shadowing**

**FGS1 parameterization gives similar results**

**Use the spatial dependence of shadowing. FGS1 parameterization**

**Reasonable agreement for R****dAu**** but cannot describe the centrality dependence**

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## Recombination

**Recombination**

**Hadronization by recombination of ****soft ****and ****shower**** partons**

**The decrease in ****R****CP**** ****as ******** increases is related to the drop of ****dn/d******** through the ****soft**** partons.**

- R. Hwa *et al. *Phys. Rev **C71** 024902 (2005)

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## 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.**

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## p+p identified spectra at high rapidity

**Red**** :**** positive **
**Blue empty****: ****negative **
**Built with data from 4 and 2.3 degrees and up to six magnetic field settings.**
**Geometrical acceptance corrections applied as well as absorption and decay in flight.**
**Trigger bias (~20%) is also corrected. ****Normalization to total inelastic cross-section (40 mb)****  **

- To appear in PRL online 22 June

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## Ratios p/+ at y=3.0 and 3.3 

**The ********+****/********-**** ratio is consistent with dominance of valence quarks at these rapidities**.

**The difference between protons and anti-protons indicates another mechanism besides fragmentation that puts so many protons at high p****T**** at this rapidities.**

**Red****: proton/pi+ ****Blue****: anti-proton/pi-**

- Small pbar/p ratio eliminates gluon fragmentation into p/pbar

- e+e- p+pbar/++ - ALEPH

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## Comparison of measurement and NLO pQCD calculations 

- The frag. functions differ by the amount of **g->****** The data points toward a dominance of gluon-gluon and gluon-quark below 10 GeV/c

**NLO pQCD can reproduce the data at RHIC energies. This is a strong indication that correct description of these **

- neutral pions y=0

**S.S. Adler et al. PRL 91 241803 (2003)**

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## NLO-pQCD can reproduce y~0 hadron production at ISR but fails at higher rapidities.

**NLO-pQCD can reproduce y~0 hadron production at ISR but fails at higher rapidities.**

- Neutral pion production at small angles at ISR
-  Lloyd et al.
- PRL 45 89 (1980)

**Bourrely and Soffer Eur. Phys. J. C36 371-374 (2004)****   **

- xF

- xF

********

********

********

********

********

- Ed3dp3[b/GeV3]

- Ed3dp3[b/GeV3]

**Data-pQCD difference at p****T****=1.5GeV**

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## NLO pQCD comparisons to data

- Calculations done by W. Vogelsang. Only one scale =pT and the same fragmentation functions as used for the PHENIX comparison.

**The KKP does a better job compared to Kretzer, ****can we extend the conclusion about gg and gq dominance at these rapidities?**

- KKP has only 0 frag. Needed some modification to produce charged pions

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## NLO pQCD for proton+anti-proton compared to data

**A recent update of the KKP fragmentation function is used here: AKK where g->p has increased relevance.**

**The AKK function does well at y=0 (****STAR**** p+p-) where the ratio anti-p/p~1 can be seen as consistent with dominance of gg or gq processes, ****but in my opinion is not appropriate for high rapidities.**

-  **J. Adams et al.** **Phys. Lett. B ****637**** (2006) 161**

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## d+Au analysis

**d+Au analysis**

**These spectra are still preliminary but one can already see that the yields change is small for different centrality samples.**

**The analysis of the d+Au data is underway, this time we include particle identification (RICH) in the full spectrometer FS**

- BRAHMS Preliminary

- 2007 RHIC AGS Users  18-22 June 2007

## Nuclear modification factors with pid

- As expected there is a difference between positive and negative pions driven by a “suppression” of negative pions in p+p (isospin)

**Red:****positive**
**Open blue: ****negative**

- 2007 RHIC AGS Users  18-22 June 2007

## Similar effects measured by PHENIX and PHOBOS

**PRL 94, 082302**

**PRC 70, 061901(R**)

- Suppression in the d direction and enhancement in the Au frag. region

- 2007 RHIC AGS Users  18-22 June 2007

## STAR 0  at high rapidity

**STAR ********0 **** at high rapidity**

- FPD: Lead-glass arrays 3.4<<4.0 on both sides of collision.

**Spectra at 3.3 and 3.8 obtained with a smaller FPD**

**J. Adams et al. PRL 97 152302 (2006)**

- KKP frag. func. has higher g-> than Kretzer

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## 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**

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## STAR Nuclear Modification Factor at high rapidity

**STAR Nuclear Modification Factor at high rapidity**

**The new ****STAR ****result is consistent with published ****BRAHMS**** once an isospin suppression of h****-**** in p+p is taken into account.**

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

- 2007 RHIC AGS Users  18-22 June 2007

## 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.

- 2007 RHIC AGS Users  18-22 June 2007

## Back-to-back Correlations in d+Au

**Qualitatively**** **consistent with     CGC picture

**Back-to-back Correlations in d+Au**

********** ****of forward pion is correlated with leading (p****T****>0.5GeV/c) h****+-**** at mid-rapidity.**

- 2007 RHIC AGS Users  18-22 June 2007

## STAR measurements with Forward TPCs

**STAR measurements with Forward TPCs**

**2.5<|********|<4**

- <pt> on d side ~constant at diff. centralities.

**nucl-ex/0703016v1**

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## 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)**

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## 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 = N****pair****/********assoc****/N****trig**
**N****pair ****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**

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## 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.**

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## PHENIX and STAR upgrades related to Forward physics

**Both Big experiments PHENIX and STAR have embarked in large projects to improve their high rapidity coverage.**
**Some of these projects start to be operational next year but their construction extends for several years into RHIC II**

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## Slide 34

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## 1+2 =ln(x1/x2)

- 1+2 =ln(x1/x2)

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## Slide 36

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## PHENIX Silicon Vertex Tracker 

- PHENIX: Si-VTX collaboration
- 72 collaborators from 14 institutions
**BNL, Florida State Univ., Iowa State Univ., **KEK, Kyoto Univ.,** LANL, **Niigata Univ.,** ORNL, **RIKEN, RIKEN BNL Reas. Center,** Stony Brook Univ., Univ. New Mexico, **LLR
- ~$3M funds to date (RIKEN)

- PHENIX: F-VTX
- Proposal in preparation
- LANL LDRD approval to construct 1⁄4 of 2 prototype
- Developing connection with FNAL Si-Det lab

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## 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

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## Summary and outlook

**Very interesting results at high rapidity have been obtained in d+Au collisions by all the RHIC experiments.**
**These results may be related to the onset of saturation in the wave function of the Au target and the formation of a Color Glass Condensate.**
**Other explanations of that data have been advanced with some success.**
**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.  **

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## Backup

- .

## Slide 41

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## PHENIX J/ measurements in d+Au collisions

- J/ measurements with the muon arms and with di-electrons at mid-rapidity open a wide window into the Au wave function:

- Cronin effect

- Gluon (anti-)shadowing

- Nuclear absorption.

- Initial state energy loss.

- gluons in Pb / gluons in p

- X

- South (y < -1.2) :
- large X2 (in gold)   ~ 0.090
- Central (y ~ 0) :
- intermediate X2        ~ 0.020
- North (y > 1.2) :
- small X2 (in gold)    ~ 0.003

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## But this time the data is better described by modest shadowing

**But this time the data is better described by modest shadowing**

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## PHENIX Upgrade Physics 

|  | 2002 | 2003 | 2004 | 2005 | 2006 | 2007 | 2008 | 2009 | 2010 | 2011 |
| --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- |
| Aerogel |  |  |  |  |  |  |  |  |  |  |
| TOF-W |  |  |  |  |  |  |  |  |  |  |
| HBD |  |  |  |  |  |  |  |  |  |  |
| VTX-barrel |  |  |  |  |  |  |  |  |  |  |
| VTX-endcap |  |  |  |  |  |  |  |  |  |  |
| NCC |  |  |  |  |  |  |  |  |  |  |
| MuTrigger |  |  |  |  |  |  |  |  |  |  |
| DAQ |  |  |  |  |  |  |  |  |  |  |

**R&D Phase**

**Construction Phase**

**Ready for Data**

**Flavor Tagged high pT Physics**

**Flavor Tagged high pT Physics**

**Low mass di-electrons**

******-jet,  jet tomography, heavy quark spectroscopy**

******-jet, CGC,  jet tomography, heavy quark spectroscopy**

******-jet, CGC,   jet tomography, heavy quark physics**

**Quark spin structure via W-measurements**

**New subsystems, higher luminosity, higher data rates**

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## Multitrack configuration

***Multitrack configuration***

******0 ****→ ********,    d = 25 mm**

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## PreShower 

***PreShower ***

***ShowerMax***

******mm**
** ********0****  30 GeV/c***** ***

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## Expected 0 reconstruction efficiency

**Expected ********0**** reconstruction efficiency**

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## STAR

**STAR**

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