# Presentation: 463222

## Forward Physics at RHIC

**Forward Physics at RHIC**

- Ramiro Debbe
- BNL Physics Dept.

- SEWM2006  10-13 May 2006

## 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.

- SEWM2006  10-13 May 2006

## 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

- SEWM2006  10-13 May 2006

## 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**

- SEWM2006  10-13 May 2006

## 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. **

- SEWM2006  10-13 May 2006

##  = 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.

- SEWM2006  10-13 May 2006

## 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.**

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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)****  **

- BRAHMS Preliminary

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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  ****Blue****: anti-proton**

- BRAHMS Preliminary

- 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 **

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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**

- SEWM2006  10-13 May 2006

## 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?**

- BRAHMS Preliminary

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

- SEWM2006  10-13 May 2006

## 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.**

- SEWM2006  10-13 May 2006

## d+Au re-analysis

**d+Au re-analysis**

**Red**** ****: positive**** **
**Blue empty****: negative **
**Data sample:**
**Negatives: full field + 1/4 **
** ****Positives: full field 1/2 1/4**

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

- BRAHMS Preliminary

- SEWM2006  10-13 May 2006

## 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)

- Protons are showing a hint enhancement, but a “suppression” in the anti-proton RdAu remains after several checks on the analysis.

- BRAHMS Preliminary

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

- SEWM2006  10-13 May 2006

## Production of protons with high pt at high rapidity 

**Protons can be as much as 80% of the pion yield at high p****T****. This explains the difference between h****+**** and h****-****.**
**p/********-**** is similar to the ratios found in e****+****e- collisions; ****DELPHI Euro. Phys. C17 207, (2000)**

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## 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

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## 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**

- arXiv:nucl-ex/0602011

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

- SEWM2006  10-13 May 2006

## 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.

- SEWM2006  10-13 May 2006

## 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.

- SEWM2006  10-13 May 2006

## 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.**

- SEWM2006  10-13 May 2006

## PHENIX Azimuthal Angle correlations 

**Azimuthal angle correlation between 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

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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.**

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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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## 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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## 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 44

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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**

- SEWM2006  10-13 May 2006