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What can we learn from η production in proton-proton collisions?
Joe SeeleMIT and University of
Colorado
J. Seele - SPIN 2008 2
Outline
• Proton spin puzzle• η production in (polarized) proton-proton collisions
• Measurements of η production at PHENIX
• Extraction of η fragmentation functions
• Constraining ΔG with η production in polarized proton-proton collisions at PHENIX
J. Seele - SPIN 2008 3
The Proton Spin Puzzle
€
1
2=
1
2Δq∑ + Lq
z + ΔG + Lgz
Fairly well measuredonly ~30% of spin
A future challenge
The proton is viewed as being a “bag” of bound quarks and gluons interacting via QCD
Spins + orbital angular momentum needto give the observed spin 1/2 of proton
Beginning to be measuredat RHIC (and hopefully EIC)
J. Seele - SPIN 2008 4
Double Helicity Asymmetries
€
ALL =σ ++ −σ +−
σ ++ + σ +−=
Δfa ⊗Δfb
a,b,c
∑ ⊗ d ˆ σ fa fb → fc X ⋅ ˆ a LLfa fb → fc X ⊗D fc
h
fa ⊗ fb
a,b,c
∑ ⊗ d ˆ σ fa fb → fc X ⊗D fc
h
€
ALL ≈ aggΔG2 + aqgΔqΔG + aqqΔqΔq'and translating…
Take the asymmetry of proton helicity configurations
J. Seele - SPIN 2008 5
Cross Section and Fragmentation Functions
High pT particle production provides information about the
parton distribution functions and fragmentation functions
€
dσ = fa ⊗ fb
a,b,c
∑ ⊗ d ˆ σ ab →cX ⊗Dch
J. Seele - SPIN 2008 6
Why the η?
Want a final state that:1) Produced in large quantities2) Theoretical Motivation
PHENIX - PRC 75 024909
J. Seele - SPIN 2008 7
Why the η?
Want a final state that:1) Produced in large quantities2) Theoretical Motivation
€
π 0 = uu − dd
€
η =uu + dd − 2ss
Different flavor and gluon dependence of structure and fragmentation should give different sensitivities to hard
subprocesses
J. Seele - SPIN 2008 8
RHIC
Year s [GeV] Recorded L Pol [%] FOM (P4L)
2003 (Run 3) 200 .35 pb-1 32 3.7 nb-1
2004 (Run 4) 200 .12 pb-1 45 4.9 nb-1
2005 (Run 5) 200 3.4 pb-1 49 200 nb-1
2006 (Run 6) 200 7.5 pb-1 57 690 nb-1
J. Seele - SPIN 2008 9
PHENIX
Central Arms:• γ/π0/η detection
– Electromagnetic Calorimeter (-0.35 < η < 0.35)
– PC3 - Charge VetoGlobal Detectors:• Relative Luminosity
– Beam-Beam Counter (BBC) (+- 3.1 < η < 4.0)
– Zero-Degree Calorimeter (ZDC) (+- 6.9 < η < infinity)
• Local Polarimetry - ZDC
J. Seele - SPIN 2008 10
Measuring the η at PHENIX
Channel BR (%)
η->2γ 39.39
η->π0π0π0 32.52
η->π+π-π0 22.68
η->π+π-γ 4.69
Choose a decay channel that couples wellwith the capabilities of PHENIX
J. Seele - SPIN 2008 11
Measuring the η at PHENIX
The number of η’s in each pT bin are determined by fitting the peak+background in the di-photon
invariant mass spectrum
2-3 3-4 4-5 5-6
6-7 7-9 9-11
J. Seele - SPIN 2008 12
η Cross Section - Results
Run3 and Run5 cross section results
J. Seele - SPIN 2008 13
Measuring Helicity Asymmetries
€
ALL =1
PB PY
N ++ − RN +−
N ++ + RN +−
€
R =L++
L+−
€
ALL =σ ++ −σ +−
σ ++ + σ +−=
Δσ
σ
What to measure :1) N’s - Final state - the η2) P’s - Polarization at IR3) R - Relative Luminosity
PHENIX Central Arms
PHENIX ZDCs + RHIC Polarimeters
PHENIX ZDCs+BBCs
J. Seele - SPIN 2008 14
η Asymmetry - Results
Scaling uncertainty (Run5 = 9.4% and Run6 = 8.3%) from polarization not shown
*pT bins identical up to
pT=6. Above that they use different binning.
J. Seele - SPIN 2008 15
η Fragmentation Functions
None previously existed, so Marco Stratmann, Christine Aidala and myself
extracted the η FFs from data on e++e- and p+p.
J. Seele - SPIN 2008 16
η FFs - Comparison
Describes e+e- data very well over a large range in energies.
Plan to incorporate BABAR, and possibly
some UA1/UA2 data.
J. Seele - SPIN 2008 17
η Cross Section and Theory
NLO+η FF (μ=pT) calculation done by Marco Stratmann
*In a way this is more of a self-consistency check than a prediction.
J. Seele - SPIN 2008 18
η and π0 Subprocess Fractions
J. Seele - SPIN 2008 19
η Asymmetry and Theory
Calculations by Marco Stratmann
J. Seele - SPIN 2008 20
Constraining ΔG with the η
A series of constrained fits were done by
Vogelsang and Stratmann in the GRSV model.
They constrained the integral of ΔG at the
input scale.
€
C = dxΔg(x,Q02)
0
1
∫
J. Seele - SPIN 2008 21
Constraining ΔG with the η
Each set yields a different ALL
Does not give much power for the negative ΔGs
J. Seele - SPIN 2008 22
Constraining ΔG with the η
Allows a small positive or strongly negative ΔG.
It should be noted that this is only for the GRSV fits.
The data needs to be incorporated into a flexible global fit.
J. Seele - SPIN 2008 23
Constraining ΔG with the η and π0
J. Seele - SPIN 2008 24
The Polarization of the Proton’s Glue
• PHENIX has measured the double helicity asymmetry in η production at mid-rapidity
• With the new fragmentation functions it will provide a constraint on ΔG
Thanks!
J. Seele - SPIN 2008 25
Backup
J. Seele - SPIN 2008 26
η FFs - DataExperiment System Energy (GeV) # Points
ALEPH ’92 e+e- 91.2 8
ALEPH ’00 e+e- 91.2 18
ALEPH ‘02 e+e- 91.2 5
L3 ‘92 e+e- 91.2 3
L3 ’94 e+e- 91.2 8
OPAL e+e- 91.2 9
ARGUS e+e- 10 6
CELLO e+e- 35 4
HRS e+e- 29 13
JADE ’85 e+e- 34.4 1
JADE ‘’90 e+e- 34.9 3
MARK II e+e- 29 7
PHENIX 2 p+p 200 12
PHENIX 3π p+p 200 6
PHENIX ’05 prelim.
p+p 200 19
J. Seele - SPIN 2008 27
Relative Luminosity at PHENIX
€
R =NBBC
++
NBBC+−
=L++σ BBC
++
L+−σ BBC+−
=L++
L+−Measured using the Beam-Beam Counter (BBC) Coincidences
Systematic Studies Include :1. Spin Independent BBC cross
section2. Effects from multiple
collisions per bunch crossing
Currently δR ~ O(10-4)
€
δALL ∝δN ++
N ++
⎛
⎝ ⎜
⎞
⎠ ⎟
2
+δN +−
N +−
⎛
⎝ ⎜
⎞
⎠ ⎟
2
+δR
R
⎛
⎝ ⎜
⎞
⎠ ⎟2
Statistical uncertainty in δR is negligible, but systematics could become a problem in the near future
J. Seele - SPIN 2008 28
Local Polarimetry at PHENIX
The neutrons are measured in the PHENIX ZDCs
Spin rotators are not perfect
and leave a small transverse component
Exploit a forward neutron single spin asymmetry in transversely polarized
p+p collisions
J. Seele - SPIN 2008 29
Polarimetry at RHIC
RHIC uses 2 types of transversely polarized elastic scattering
€
p↑ + C → p + C
€
p↑ + H → p + H
•Provides fast, high statistics measurement•Gives polarization profile of beam•Analyzing power empirically determined
•Provides self-calibrating, low statistics measurement •Analyzing power unknown •Provides calibration for the p+C polarimeter
A tour de force measurement as this is the first time bunch-by-bunch measurements were performed at
any high energy collider
€
p + H↑ → p + H
J. Seele - SPIN 2008 30
η Asymmetry
Because the eta peak sits on a background we need to
subtract a possible background asymmetry
€
r =NBG
NBG + NS
€
ALLη +BG = (1− r)ALL
η + rALLBG
700-800 MeV
300-400 MeV€
ALLη =
ALLη +BG − rALL
BG
1− r
The background asymmetry is estimated
using the blue sideband regions and subtracted from the
peak asymmetry