1. Forward spin + cold nuclear measurements and forward Calorimetry
Korea-Japan workshop, Hanyan University
2015/10/20
Ralf Seidl
(RIKEN)

2. Remaining spin/CNM questions at RHIC
What causes these large single transverse spin asymmetries?
Final state effects (transversity x higher twist fragmentation functions, partially related to Collins FF)
Initial state effects (higher twist correlation function related to Sivers function)
Other source such as diffraction
Spin asymmetries in pA to get sensitivity to gluon saturation
Diffractive Meson measurements in pp and pA to access GPD E and gluon saturation
2019/1/2
fsPHENIX

3. Transvsere spin questions continued
Final state effects (eliminate initial state effect):
directly access Collins asymmetries via jet + hadron asymmetries, also allows to reach unmeasured transversity x region
Non Collins related final state effects: ?
Initial state effects (eliminate final state effects):
Direct photon and Jet asymmetries
Is part of initial smallness due to u and d cancellation?
Diffractive effects via proton tagging (Roman pots)
Some answers will be there before 2020+
2019/1/2
fsPHENIX

4. Jet asymmetries
Only jet result so far with rather smaller asymmetries:
Very little initial state effect or
Cancellation of up and down “Sivers” contributions?
AnDY
2019/1/2
fsPHENIX

5. 2019/1/2
fsPHENIX

6. 2019/1/2
fsPHENIX

7. Collins asymmetries Measure AN for hadrons(pions) within a Jet
Directly sensitive to Transversity X Collins
Existence known from HERMES and COMPASS
Also shown at higher PT at mid rapidity in STAR
Forward rapidity for higher x extension
2019/1/2
fsPHENIX

8. x1 x2 ranges in rapidity slices
200 GeV p – p
x1 x2 ranges in rapidity slices
Baseline for TTPMC:
200 GeV pp
Hard light processes (ckin > 1GeV),
Hadrons in 1-4 rapidity range with Pt> 1GeV
2019/1/2
fsPHENIX

9. Expected asymmetries
arXiv:
Transvsersity to higher x could be nicely accessed in forward region
Still large uncertainties
Particle identification expensive
2019/1/2
fsPHENIX

10. fsPHENIX Main requirements: Jet reconstruction (HCAL)
Tracking for momentum and charge (GEMs) reconstruction
Maybe muon identification (old MUID +X)
Photon and electron reco/id ( MPC+EX? )
2019/1/2
fsPHENIX

11. Sideview of ePHENIX
2019/1/2
fsPHENIX

12. The fHCAL General constraints are:
z<4.5 m due to Quad magnet for EIC
Return the flux of the BaBar solenoid
Rapidity around 1.2-4, likely staged with forward first
Follow EIC/STAR proposal by UCLA group (O.Tsai):
Scintillator/Absorber sandwich detector
WLS transport the light to the back 
SiPMT readout
2019/1/2
fsPHENIX

13. STAR/UCLA prototype
Taken from STAR pp/pA proposal, FNAL test beam in March 2014
2019/1/2
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14. Diffraction
Generally large rapidity gap in final state yields or (at least) one proton intact
Most common interaction: Pomeron (two gluon)
Because two gluon interaction sensitive to gluon density2
For spin asymmetries helicity flip needed  odderon?
Experimentally: roman pots
t = (p-p’)2
z = Mx2/W:
momentum fraction of the exchanged object w.r.t. the hadron
exact kinematics not known
 factorization is violated
2019/1/2
fsPHENIX

15. STAR roman pots
2019/1/2
fsPHENIX

16. UPC in polarized pp↑ or Ap↑
Get quasi-real photon from one proton
Ensure dominance of g from one identified proton
by selecting very small t1, while t2 of “typical hadronic
size”
small t1  large impact parameter b (UPC)
Two possibilities:
Final state lepton pair  timelike compton scattering
timelike Compton scattering: detailed access to GPDs
including Eq/g if have transv. target pol.
Challenging to suppress all backgrounds p p’ p p’ Z2 Au
Au’ p p’
Final state lepton pair not from g* but from J/ψ
Done already in AuAu
Estimates for J/ψ (hep-ph/ )
transverse target spin asymmetry
 calculable with GPDs
information on helicity-flip distribution E for gluons
golden measurement for eRHIC
polarized p↑A: gain in statistics ~ Z2
2019/1/2
fsPHENIX

17. RHIC as gA collider: UPC
Ultra-peripheral (UPC) collisions: b > 2R
→ hadronic interactions strongly suppressed
High photon flux ~ Z2
→ well described in Weizsäcker-Williams approximation
→ high σ for -induced reactions
e.g. exclusive vector meson production
Coherent vector meson production:
• photon couples coherently to all nucleons
• pT ~ 1/RA ~ 60 MeV/c
• no neutron emission in ~80% of cases
Incoherent vector meson production:
• photon couples to a single nucleon
• pT ~ 1/Rp ~ 450 MeV/c
• target nucleus normally breaks up
2019/1/2
fsPHENIX

18. Why UPC?
Quarkonia photoproduction allows to study the gluon density G(x,Q2) in A
as well as G(x,Q2, bT)
LO pQCD: forward coherent photoproduction cross section is proportional to the squared gluon density
Quarkonium photoproduction in UPC is a direct tool to measure nuclear gluon shadowing
2020+ UPC: “proton-shine”-case:
Requires: RP-II* and 2.5 pb-1 p+Au
Fourier transform of s vs. t
 g(x,Q2,b)
2019/1/2
fsPHENIX

19. UPC at STAR
R. Debbe
2 tracks in STAR and one neutron in each ZDC Au+Au  n+n+e+e-
no attempt for a Fourier transform of s vs. t has been made
 g(x,Q2,b)
2019/1/2
fsPHENIX

20. Other spin/CNM aspects
More DY and direct photon measurements in pp (Sivers sign change) and pA (saturation effects, clean initial state )
Nuclear fragmentation functions (jets+hadrons, preferably identified)
Incremental measurements:
More ALL measurements for (di)jets and hadrons
2019/1/2
fsPHENIX

21. Summary and Outlook
Jet asymmetry enhancement via hadron selection; requires at least hadronic calorimetry and tracking
Forward Collins (hadrons in jets) interesting for intermediate to higher x at reasonable scale; requires hadronic calorimetry, tracking and better also hadron id
Need forward Hadronic calorimeter and tracking
Diffraction and UPC very interesting for gluon PDFs, three-dimensional PDF and GPD E
Roman pots needed for diffractive measurements
Some Ecal (restacked MPC-EX?) for photon/electron measurements?
2019/1/2
fsPHENIX

22. Backup
2019/1/2
fsPHENIX