1. Transverse Single Spin Asymmetries in Hadronic Interactions An Experimental Overview and Outlook
Transversity 2014
L.C. Bland
Brookhaven National Lab
Chia, Cagliari
10 June 2014

2. Disclaimer
As you will see, personal bias suggests that new endeavors in the forward direction are the best possible future for spin and low-x physics at RHIC.
This talk provides selected experimental results from RHIC. The outlook is a personal perspective on how to improve forward detection instrumentation, and does not necessarily reflect official STAR or PHENIX plans.
10 June 2014

3. Why Study Transverse SSA? Answer #1 – Practical Benefit+4 +2 -2 -4
PAN (10-3)
Bunch Number at IP6
Yellow
Blue
Colliding-beam polarimeter, s=200 GeV
Analyzing power is a tool to measure polarization (P) and is one example of transverse single spin asymmetries (SSA) with origins yet to be fully understood
10 June 2014

4. Why Study Transverse SSA? Answer #2 – Window to New Phenomena
What would we see from this gedanken experiment?
F0 as mq0 in vector gauge theories, so AN ~ mq/pT
or,AN ~ for pT ~ 2 GeV/c
Kane, Pumplin and Repko PRL 41 (1978) 1689
10 June 2014

5. Two of the Explanations for Large Transverse SSA Spin-correlated kT
Collins mechanism requires transverse quark polarization and spin-dependent fragmentation
Sivers mechanism
requires spin-correlated transverse momentum in the proton (orbital motion) and color-charge interaction. SSA is present for jet or g
initial state
final state
Other mechanisms have been suggested in recent years
10 June 2014

6. Why Study Hadronic Collisions?
Many examples where new particles are first observed in the interactions of hadrons: J/ (hidden charm);  (hidden beauty); W,Z0 (weak bosons); H (Higgs boson), as just a few well-known examples.
Hadro-production has provided many examples of first observation of unexpected phenomena: transverse SSA (both AN for p production and induced P for L production); Lam-Tung violation in DY; …
Hadro-production provides direct sensitivity to gluons: unpolarized gluon PDF, especially to low-x ; gluon spin contribution (G)
Hadro-production is pursued because of emergent phenomena: quark-gluon plasma ; onset of gluon saturation?
Hadro-production can provide an important test of universality – are the quantities measured in hard scattering processes really telling us about the structure of the proton?
10 June 2014

7. Review of Experiment-I FermiLab Fixed Target Experiments
s=20 GeV, pT= GeV/c
0 – E704, PLB261 (1991) 201.
+/- - E704, PLB264 (1991) 462.
QCD theory expects very small (AN~10-3) transverse SSA for particles produced by hard scattering.
The FermiLab E-704 experiment found strikingly large transverse single-spin effects in p+p fixed-target collisions with 200 GeV polarized proton beam (s = 20 GeV).
10 June 2014

8. Review of Experiment-II Hyperon Polarization
p+BeL+X, pp=400 GeV/c
PRD 40 (1989) 3557
Review of Experiment-II Hyperon Polarization
Hyperons produced in unpolarized collisions are observed (via weak decay asymmetry) to be polarized.
Weak decay asymmetry…
* : polar angle of decay in rest frame
aL=0.642±0.013 (weak decay asymmetry)
PL, polarization with respect to axis
P is found to be a function of xF, pT
10 June 2014

9. Review of Experiment – III … and, then RHIC
Absolute Polarimeter (H jet)
RHIC pC Polarimeters
BRAHMS & PP2PP
PHOBOS
Siberian Snakes
Siberian Snakes
PHENIX
STAR
Spin Rotators
(longitudinal polarization)
Spin Rotators
(longitudinal polarization)
Pol. H- Source
LINAC
BOOSTER
Helical Partial Siberian Snake
AGS
Show layout & point of polarimeters and experimental locations
Emphasize bunch polarizations prepared at source going through AGS etc. to RHIC
Time spacing between them ~100 ns
Transverse polarization in ring, longitudinal at experiments (optional) using spin rotators
Emphasize none of these techniques existed just 4-5 years ago, all developed here at RHIC with international collaborations led by US and Japanese teams.
200 MeV Polarimeter
AGS pC Polarimeter
Strong AGS Snake
10 June 2014

10. Review of RHIC-spin Inclusive 0 AN persists to large s
Data: B.I. Abelev et al. (STAR), submitted to PRL [arXiv:hep-ex/ ]
Theory (red): M. Boglione, U. D’Alesio, F. Murgia PRD 77 (2008)
Theory (blue): C. Kouvaris, J. Qiu, W. Vogelsang, F. Yuan, PRD 74 (2006)
J. Adams et al. (STAR), PRL 92 (2004) ; and PRL 97 (2006)
Even though the kinematics of the SIDIS measurement and the forward p0,p± data have little overlap, it was possible to account for most of the features of the RHIC data by calculations based on phenomenological fits to the SIDIS data
10 June 2014

11. Review of RHIC-spin xF and pT dependence of AN for p+pp±+X, s=62 GeV
I. Arsene, et al. PRL101 (2008)
AN(p+) ~ -AN(p-), consistent with results at lower s and u,d valence differences
At fixed xF, evidence that AN grows with pT
10 June 2014

12. Review of RHIC-spin Inclusive 0 AN at s=62.4 GeV
Muon piston calorimeter provides PHENIX access to large rapidity
Observe large AN that is iincreasing with xF, as seen at higher and lower s
arXiv:
10 June 2014

13. Review of RHIC-spin Inclusive pion asymmetries change little with s
Can we understand the origin of these transverse SSA at high energy?
Why do these transverse SSA change so little over a broad range of s?
Why do these transverse SSA occur at large xF?
arXiv:
10 June 2014

14. Forward Pion Asymmetries Versus s
ANL
s=4.9 GeV
BNL
s=6.6 GeV
FNAL
s=19.4 GeV
RHIC
s=62.4 GeV
Forward pion analyzing power in p+p collisions exhibits similar xF dependence over a broad range of s
10 June 2014

15. Review of RHIC-spin Inclusive neutral meson asymmetries are zero at midrapidity
PRL 91 (2003)
arXiv:
pT range of midrapidity p0 production comparable to pT values for large-xF p0 production.
Why is large xF so important for transverse SSA?
10 June 2014

16. Review of RHIC-spin Inclusive neutral meson asymmetries are zero for midcentral production
STAR
Mid-central p0 cross section consistent with NLO pQCD within scale uncertainties
Mid-central p0 transverse SSA are consistent with zero, as expected by twist-3 model.
10 June 2014

17. pT Dependence of Inclusive p0 AN
STAR
B.I. Abelev et al. (STAR) PRL 101 (2008)
STAR, PRL 101 (2008)
Rising pT dependence
Preliminary results show that trends go to high pT
10 June 2014

18. Summary Transverse SSA for Inclusive Pion Production
Large s pion production cross sections are consistent with NLO pQCD
Large AN is found to increase with xF over a very broad range of s
xF and pT dependence are now disentangled, with AN increasing with pT to a plateau
Pion asymmetries are consistent with zero at central and mid-central rapidities
Go beyond inclusive pion production to test present understanding
 jets, Drell-Yan production, direct photons
10 June 2014

19. From Inclusive Pions to Jets
arXiv:
Forward jet cross section consistent with NLO pQCD
xF>0 jet AN is non-zero
xF>0 jet AN is consistent with Sivers effect from SIDIS
10 June 2014

20. STAR Results vs. Di-Jet Pseudorapidity Sum
Run-6 Result
Idea: directly measure kT by observing momentum imbalance of a pair of jets produced in p+p collision and attempt to measure if kT is correlated with incoming proton spin
Boer & Vogelsang, PRD 69 (2004)
jet
AN pbeam  (kT  ST)
pbeam
into page
PRL 99 (2007)
Emphasizes (50%+ )
quark Sivers
AN consistent with zero
~order of magnitude smaller in pp  di-jets than in semi-inclusive DIS quark Sivers asymmetry!
VY 1, VY 2 are calculations by
Vogelsang & Yuan, PRD 72 (2005)
STAR
10 June 2014

21. Outlook-I
RHIC remains the only accelerator with polarized proton beams. The large s of the collider has established via cross section measurements that particle production can be explained by NLO pQCD.
Large acceptance RHIC experiments (STAR and PHENIX) are well instrumented at midrapidity. Spin asymmetries, in general, increase in the forward direction (valence quark phenomena?)
Efforts are underway at both STAR and PHENIX to improve forward detector capabilities, because of the important path to a future electron-ion collider
Economic realities may require clever reuse of existing equipment to start on the path…
10 June 2014

22. Forward Instrumentation for 2016? “STAR-Forward”
Existing E864 calorimeter from IP2
FPS (preshower)
TPC
bEMC
Existing forward GEM tracker (redistributed in z)
0.5 T solenoid
Some caveats:
Suitability of E864 calorimeter is under investigation in ongoing RHIC run
Radiation resistance of silicon photomultipliers under investigation in ongoing RHIC run
Suitability of GEM tracker to be demonstrated by efficiency measurements
All of this is pending review
eEMC
What can be done with such a forward detector?
10 June 2014

23. Attractive vs Repulsive Sivers Effects
Unique Prediction of Gauge Theory !
DIS: attractive
Drell-Yan: repulsive
Same in QCD:
As a result:
Transverse Spin Drell-Yan Physics at RHIC (2007)
For now, RHIC is the only accelerator with polarized beams
RHIC should pursue polarized DY, in kinematics that match as closely as possible those from SIDIS  forward
Although STAR-forward is similar to ANDY, detailed simulation studies are still required
10 June 2014

24. L at large xF
As a hadron calorimeter, the E864 calorimeter can reconstruct p- [and charge conjugate]
GEM tracking in “STAR-forward” can measure a displaced vertex, thereby significantly reducing backgrounds
Large xF cross sections, induced polarization and polarization transfer (DNN and DLL) will impact knowledge of polarized PDF and low-x physics.
arXiv:
10 June 2014

25. Other Forward Items… In addition to a focus on forward DY
Forward dijets… of interest for low-x physics and for low-x gluon spin
Nuclear effects on transverse SSA
Forward photons
10 June 2014

26. Conclusions
Inclusive pion transverse single spin asymmetries are large at RHIC in the forward direction, but are consistent with zero for central and mid-central production
Jet transverse single spin asymmetries are small and positive
Improved forward detectors are an excellent upgrade path for RHIC, and are important for a future electron-ion collider.
Forward Drell Yan production at RHIC can match kinematics of semi-inclusive deep inelastic scattering, thereby testing the predicted sign change of the Sivers function.
10 June 2014