# Presentation: 692182

## Saturation, Low-x, and QCDat RHIC, Part 4Future Perspectives (near and long term)

- T. Hallman
- Villaggio Guglielmo, Italy

## Overview of forward-tagged proton program

- The physics program of tagged forward protons with STAR at RHIC in addition
- to the elastic scattering will:
- Study standard both elastic and inelastic hadron diffraction and its spin dependence in unexplored t and s range.
- Study the structure of color singlet exchange in the non-perturbative regime of QCD.
- Search for central production of light and massive systems in double Pomeron exchange process -  glueballs, hybrids.
- Search for an Odderon - an eigenstate of CGC.
**There is a great potential for important discoveries beginning with the next RHIC run (November 2007)**

## PQCD Picture

- In terms of QCD, Pomeron exchange consists of the exchange of a color singlet
- combination of gluons. Triggering on forward protons at high energies predominantly selects such exchanges and hence interactions mediated by gluon matter.

##  A new QCD exploration at RHIC

**Forward Tagged Proton Studies in STAR**
**Polarized proton-proton **_**Elastic Scattering**_** (p********p******** ******** pp)**
- The instrumentation used will be the STAR detector and the Roman Pots of the pp2pp experiment
- The pp2pp Roman Pot detectors will be used to tag very forward protons, thus selecting processes in which the proton stays intact, and the exchange has quantum numbers of the vacuum,so called Pomeron (IP) exchange. (For these events, the probability of measuring reactions where colorless gluonic matter dominates the exchange is enhanced).
- The use of polarized proton beams, unique at RHIC, will allow exploring unknown spin dependence of diffraction including both elastic and inelastic processes.
- As the entire energy range of this study has been inaccessible to proton-proton (elastic) scattering in the past, all results will be new results.

## Spin averaged observables
Measurement of the differential pp cross section d/dt over extended t-range will include the region at lower |t| that is particularly sensitive to the parameter: measurements will include: 1) the -parameter; 2) the nuclear slope parameter b in a combined fit to the differential cross section, and 3) the total cross section tot.
Interest in such studies: an asymptotic difference observed between the differential and total cross sections for pp could indicate an Odderon contribution to the scattering amplitude. (absence of an Odderon contribution would lead to identical cross sections); measurement of the total cross section, tot , at the highest possible energy will also probe the prevalent assumption that the cross sections for pp and pp scattering are asymptotically identical.
With the expected 20106 elastic events the estimated error on the slope parameter is b=0.31 (GeV/c)-2 and that for the ratio of the real to imaginary part, =0.01. This is comparable to the precision for existing measurements from pp and pp data. Thetot = 2-3 mb. 

- Measurement of the differential pp cross section d/dt over extended t-range will include the region at lower |t| that is particularly sensitive to the parameter: measurements will include: 1) the -parameter; 2) the nuclear slope parameter b in a combined fit to the differential cross section, and 3) the total cross section **tot**.
- Interest in such studies: an asymptotic difference observed between the differential and total cross sections for pp could indicate an Odderon contribution to the scattering amplitude. (absence of an Odderon contribution would lead to identical cross sections); measurement of the total cross section, **tot ****, **at the highest possible energy will also probe the prevalent assumption that the cross sections for pp and pp scattering are asymptotically identical.
- With the expected 20106 elastic events the estimated error on the slope parameter is b=0.31 (GeV/c)-2 and that for the ratio of the real to imaginary part, =0.01. This is comparable to the precision for existing measurements from pp and pp data. Thetot = 2-3 mb.

-  _A new QCD exploration at RHIC_

## Spin dependent observables
By measuring spin related asymmetries one will be able to determine elastic scattering at the amplitude level.  The availability of longitudinal polarization will allow measuring ALL in addition to ANN, ASS, and AN resulting in a significant improvement of the scientific reach of these measurements. 
Physics motivation to measure AN : possibility of the rise with energy of the spin-flip to spin-nonflip amplitudes ratio ( it may occur that the small contribution from hadronic spin-flip to the spin single-spin asymmetry observed with a polarized jet target at 100 GeV/c could be increased at s=200 GeV). 
Reaching small |t|-values will allow measurement of the single spin analyzing power AN close to its maximum at |t| =0.0024 (GeV/c)2, where Amax = 0.04, at s = 200 GeV. The AN and its t-dependence in the covered range are sensitive to a possible contribution of the single spin-flip amplitude, 5, from the interference between the hadronic spin-flip amplitude with the electromagnetic non-flip amplitude.
An additional contribution of the hypothetical Odderon to the pp scattering amplitude can be probed by measuring the double spin-flip asymmetry, ANN . (ANN is thought to be sensitive to contributions of the real and imaginary parts to the double spin-flip amplitude,2, in the range 0.003 < |t| < 0.010 (GeV/c)2). 

- By measuring spin related asymmetries one will be able to determine elastic scattering _at the amplitude level_.  The availability of longitudinal polarization will allow measuring ALL in addition to ANN, ASS, and AN resulting in a significant improvement of the scientific reach of these measurements.
- Physics motivation to measure AN : possibility of the rise with energy of the spin-flip to spin-nonflip amplitudes ratio ( it may occur that the small contribution from hadronic spin-flip to the spin single-spin asymmetry observed with a polarized jet target at 100 GeV/c could be increased at s=200 GeV).
- Reaching small |t|-values will allow measurement of the single spin analyzing power A**N** close to its maximum at |t| =0.0024 (GeV/c)2, where A**max** = 0.04, at s = 200 GeV. The A**N** and its t-dependence in the covered range are sensitive to a possible contribution of the single spin-flip amplitude, **5**, from the interference between the hadronic spin-flip amplitude with the electromagnetic non-flip amplitude.
- An additional contribution of the hypothetical Odderon to the pp scattering amplitude can be probed by measuring the double spin-flip asymmetry, A**NN** . (A**NN** is thought to be sensitive to contributions of the real and imaginary parts to the double spin-flip amplitude,**2**, in the range 0.003 < |t| < 0.010 (GeV/c)2).

-  _A new QCD exploration at RHIC_

## Central Production in Double Pomeron Exchange

- In the double Pomeron exchange process each proton “emits” a Pomeron and the two Pomerons interact producing a massive system MX
- where MX = c(b), qq(jets), H(Higgs boson), gg(glueballs)
- The massive system could form resonances. We expect that because of the constraints provided by the double Pomeron interaction, glueballs, hybrids, and other states coupling preferentially to gluons, will be produced with much reduced backgrounds compared to standard hadronic production processes.

- For each proton vertex one has
**t **    four-momentum transfer
******p/p **
**M****X** invariant mass

## Central production of glueballs
The production of glueballs may be enhanced in the central region in the process pp  pMXp.The crucial argument here is that the pattern of resonances produced in the central region, where both forward protons are tagged, depends on the vector difference of the transverse momentum of the final state protons kT1, kT2, with dPT kT1 kT2. The so-called dPT filter argument is that when dPT is large ( QCD) qbar-q states are prominent and when dPT is small, the surviving resonances include glueball candidates.

       The geometrical acceptance of the proposed setup for both SDD and DPE processes, generating protons with t and  uniformly distributed in the regions 0.003< |t|< 0.04 and 0.005 <  < 0.05 respectively. 
In the proposed research, large data samples of diffractive states can be obtained and analyzed as a function of diffractive mass and t (d2σ/dMX2dt) for central production. As noted above, when dPT between two out going protons is small, the fraction of glueball candidates is enhanced; when the dPT is big the qqbar states are enhanced. In quantifying the threshold between these regimes, the WA102 experiment [54] found "small"  to be dPT < 0.2 GeV and  "big"  to be dPT > 0.5 GeV. 
With the expected luminosity one can collect about 450,000 triggered DPE events during the three days of running, with 40 hrs of useful beam time. The search for glueballs remains an experimental question..

- The production of glueballs may be enhanced in the central region in the process pp** ** pMXp.The crucial argument here is that the pattern of resonances produced in the central region, where both forward protons are tagged, depends on the vector difference of the transverse momentum of the final state protons kT1, kT2, with dPT kT1 kT2. The so-called dPT filter argument is that when dPT is large ( QCD) qbar-q states are prominent and when dPT is small, the surviving resonances include glueball candidates.
-        The geometrical acceptance of the proposed setup for both SDD and DPE processes, generating protons with t and  uniformly distributed in the regions 0.003< |t|< 0.04 and 0.005 <  < 0.05 respectively.
- In the proposed research, large data samples of diffractive states can be obtained and analyzed as a function of diffractive mass and t (d2σ/dMX2dt) for central production. As noted above, when dPT between two out going protons is small, the fraction of glueball candidates is enhanced; when the dPT is big the qqbar states are enhanced. In quantifying the threshold between these regimes, the WA102 experiment [54] found "small"  to be dPT < 0.2 GeV and  "big"  to be dPT > 0.5 GeV.
- With the expected luminosity one can collect about 450,000 triggered DPE events during the three days of running, with 40 hrs of useful beam time. The search for glueballs remains an experimental question..

-  _A new QCD exploration at RHIC_

## Central Production in DPE at RHIC

- Glueball production at in DPE process at RHIC is complementary to GLUEX and PANDA

***Sparse spectrum!***
- New I=0 mesons starting with
**0********              1.6 GeV**
**0******** ,  2********   **** ****  2.3 - 2.5 GeV**
- No **J****PC****-exotic** glueballs until
**2********              4 GeV**

## WA102 F(1500)  

## Polarized Proton Collisions in RHIC

- _Polarized Proton Collisions in RHIC_

## PP2PP Setup - tag forward protons

- Phys. Lett. B 579 (2004) 245-250
- Phys. Lett. B 632 (2006) 167-172
- Phys. Lett. B 647 (2007) 98-103
- NIM A535 (2004) 415 - 420

- The pp2pp Roman pots mounted on the outgoing beams, down stream from the STAR detector as shown.
- To achieve full azimuthal coverage for elastic events, at each location one Roman Pot station is horizontal
- and one vertical. RHIC accelerator magnets are used for momentum analysis resulting in forward proton
- taggers installed in the warm straight section between Q3 and Q4 magnets. The pp2pp Roman Pots are a
- moveable detector system allowing approach to the beam as closely as possible, thus extending the t and x
- ranges to the lowest values.

## Roman Pot Stations at RHIC

**to IR**

**Roman Pot **
**below the beam**

**Roman Pot above the beam**

## Reconstruction of the Proton Momentum Loss 

****** **Accelerator transport

- Need to measure vector at the detection point, hence two RPs are needed on each side of STAR.
- For a proton, which scatters with  and  we have:

- Because  and  are small special focusing is needed

## STAR Detector - measure recoil system Mx

## Resonance Signal in p+p and Au+Au collisions from STAR

** ****K(892)**

****

******(1520)**

**p+p**

**p+p**

**Au+Au**

**Au+Au**

** ********(1385)**

**p+p**

**Au+Au**

********

******(1020)**

**p+p**

**Au+Au**

**p+p**

## Leading particle spectra

## Run 2008 - Phase I Projections

- Elastic scattering:
- 100% acceptance for elastic scattering for 0.003 < |t| < 0.024;
- 20×106 elastic events: b=0.31 (GeV/c)-2, r=0.01, tot = 2-3 mb;
- In four t subintervals we shall have 5×106 events in each resulting in corresponding errors An=0.0017, Ann=Ass=0.003.
- DPE process: Luminosity few ×1029 cm-2sec-1:
-  4.5105 DPE events with full proton momentum reconstruction;
-  (5-7)106 DPE events with tagged protons - good size data sample for this physics.

- Need to reach small t and  values to measure small masses of interest  large *=20m, special optics and beam scraping are needed.
- Hence a dedicated three-day run is planned

## 
In summary the physics program with tagged forward protons at STAR will:
Study elastic scattering and its spin dependence in unexplored t and s range;
Study the structure of color singlet exchange in the non-perturbative regime of QCD;
Search for diffractive production of light and massive systems in double Pomeron exchange process;
Search for new physics, including glueballs and Odderon.
      The proposed studies will add to the world body of knowledge concerning QCD in the non-perturbative regime where calculations are not easy and one has to be guided by measurements. In this way the proposed program extends the physics reach of both STAR and RHIC.

- _In summary the physics program with tagged forward protons at STAR will:_
- Study elastic scattering and its spin dependence in unexplored t and s range;
- Study the structure of color singlet exchange in the non-perturbative regime of QCD;
- Search for diffractive production of light and massive systems in double Pomeron exchange process;
- Search for new physics, including glueballs and Odderon.
-       The proposed studies will add to the world body of knowledge concerning QCD in the non-perturbative regime where calculations are not easy and one has to be guided by measurements. In this way the proposed program extends the physics reach of both STAR and RHIC.

-  _A new QCD exploration at RHIC_

## The not-too-distant RHIC future?

Beyond presently planned STAR 
         / PHENIX Upgrades

- Beyond presently planned STAR
-          / PHENIX Upgrades

## A Fundamental Test of Universality:Transverse Spin Drell Yan at RHIC vs Sivers Asymmetry in Deep Inelastic Scattering

- Important test at RHIC of recent _fundamental QCD predictions_ for the Sivers effect, demonstrating... attractive vs repulsive color charge forces
- Possible access to quark _orbital angular momentum_
- Latest development from DIS, first direct evidences of Sivers and Collins effects (recent new results at DIS 2007 in Műnchen, April 2007, HERMES, Diefenthaler et al.)
- Requires very high luminosity (RHIC II)
- Both STAR and PHENIX can make important, exciting, measurements
- Discussion available at _http://spin.riken.bnl.gov/rsc/_

## Slide 22

## Slide 23

## Attractive vs Repulsive Sivers Effects

**Simple QED**
**example:**

**DIS: attractive**

**Drell-Yan: repulsive**

**Same in**** ****Q****C****D****:**

**As a result:**

**Attractive vs Repulsive Sivers Effects**
**Unique Prediction of Gauge Theory !**

## Slide 25

##   Experiment SIDIS vs Drell Yan:  Sivers|DIS= − Sivers|DY

-   ***0.1        0.2        0.3   x***

-   ***Sivers Amplitude***

**0**

**0**

-   **Experiment SIDIS vs Drell Yan:  ****Sivers|****DIS****= − Sivers|****DY**
** ******* Probes Q****C****D**** attraction and Q****C****D**** repulsion *****

- HERMES Sivers Results

- RHIC II  Drell Yan Projections

- Markus Diefenthaler
- DIS Workshop
- Munich, April 2007

## Luminosity Projection to 2016      (Collider Accelerator Department)

## The more distant (but still foreseeable) RHIC future?

## Candidate EIC designs

- eRHIC
- 250 GeV protons, 100 GeV/n ions

- ELIC (JLab)
- 225 GeV protons, 100 GeV/n ions
- 9 GeV electrons, positrons
- Peak lumi (e+p) ~1035 cm-2s-1

- _Candidate EIC designs_

- 10(20) GeV electrons
- Peak lumi (e+p) = 2.6x1033 cm-2s-1

- 10 GeV electrons+positrons
- Peak lumi (e+p) = 0.47x1033 cm-2s-1

## Explore the new QCD frontier: strong color fields in nuclei

**Explore the new QCD frontier: strong color fields in nuclei**
**  **
- - How do the gluons contribute to the structure of the nucleon / nucleus?
- - What are the properties of high density gluon matter?
- - How do fast quarks or gluons interact as they traverse nuclear matter?

**Precisely image the sea-quarks and gluons in the nucleon**
- - How do the gluons and sea-quarks contribute to the spin structure of the nucleon?
- - What is the spatial distribution of the gluons and sea quarks in the nucleon?
- - How do hadronic final-states form in QCD?

***How do we understand the visible matter in our universe in terms of the fundamental quarks and gluons of QCD?***

## The e-A physics EIC physics agenda

**Broad theme: *****precision studies of emergent features of QCD***
- What are the momentum distributions of quarks, anti-quarks and gluons in the nucleon?
- How are the symmetries of QCD manifest or broken in the nucleon?
- How are quarks and gluons distributed spatially in the nucleon?
- What is the nature of glue at high density?
- How do strong fields appear in hadronic or nuclear wave functions at high energies?
- What are the appropriate degrees of freedom?
- How do they respond to external probes or scattering?
- Is this response universal (ep, pp, eA, pA, AA)?

## Kinematic reach of eRHIC and ELIC for e-A studies

- Region of gluon saturation

## eRHIC physics example: F2(x,Q2)(sea quarks generated by glue)

- eRHIC physics example: *F**2**(x,Q**2**)**(sea quarks generated by glue)*

- one of the first EIC measurements
- nDS, EKS, FGS: pQCD models with different amounts of shadowing

- EIC can distinguish pQCD and saturation model expectations

## 50% of momentum carried by gluons, but still a lot we don’t know

**Longitudinal Structure Function F****L**

- includes systematic
- uncertainties

## >

- RHIC-Spin region

- _Precisely image the sea quarks_

- Spin-Flavor Decomposition of the Light Quark Sea

**| p    =         +              +             + ...**

- u

- u

- d

- u

- u

- u

- u

- d

- u

- u

- d

- d

- d

- Many models predict
- u > 0,  d < 0

## GPDs and Transverse Gluon Imaging

- Deep exclusive measurements in ep / eA with an EIC:
- 	diffractive:		transverse gluon imaging	J/y, ro, g (DVCS)
-          non-diffractive:		quark spin/flavor structure	p, K, r+, ...

- Are gluons uniformly distributed in nuclear matter or are there small clumps of glue?

## Long Term Strategic View

## Summary

- Saturation, Low-x, and QCD are a vibrant, integral component of the ongoing and future RHIC physics program
- Lots of new data just around the bend (in the next 1-2 years)
- Lots of potential for major discoveries, now and in the future.