1. Sea Quark Polarization Measurement via W → μ at PHENIX
Chong KIM
Korea University/RIKEN
3rd PHENIX collaboration meeting
Nov. 27th, 2014
for the PHENIX collaboration

2. 3rd PHENIX collaboration meeting, Wako
Outline
Introduction
Accessing proton spin structure
RHIC and PHENIX
Recent longitudinal spin runs
W → μ Analysis
W physics at RHIC
W → μ analysis at PHENIX Muon Arms
Physics impact and outlook
Summary

3. Introduction Accessing proton spin structure
3rd PHENIX collaboration meeting, Wako
Introduction Accessing proton spin structure
D. de Florian et al, PRD (2009)
Decomposition of proton spin
< S z > = = ∆Σ+∆G+ L z
ΔΣ = (Δu+Δ u ) + (Δd+Δ d ) + (Δs+Δ s )
(Δq+Δ q ):
well constrained by inclusive/semi inclusive DIS
on proton/deuteron targets
Δ q :
only known with large uncertainties, which
mainly originated from fragmentation process
q ?
Do they contribute significantly to proton spin?
Do u and d carry similar polarization?

4. Introduction RHIC @ Brookhaven Lab.
3rd PHENIX collaboration meeting, Wako
Introduction Brookhaven Lab. ＋ － …
max. √s = 510 GeV (62.4, 200, and 500/510)
avg. P ~ 52 % (in longitudinal direction)

5. 3rd PHENIX collaboration meeting, Wako
Introduction PHENIX

6. 3rd PHENIX collaboration meeting, Wako
Introduction PHENIX
Central Arms (mid rapidity)
|η| < 0.35, Δφ = π/2
Tracking: VTX, DC, PC
PID/Timing: RICH, ToF
EMCal: PbSc, PbGl
Muon Arms (forward rapidity)
1.2 < |η| < 2.2 (S) / 2.4 (N), Δφ = 2π
Tracking: FVTX, MuTr
Timing: MuID, RPC1/RPC3
MPC (forward EMCal)
3.0 < |η| < 3.8

7. Introduction Recent longitudinal spin runs
3rd PHENIX collaboration meeting, Wako
Introduction Recent longitudinal spin runs
Year √s
L (pb-1)
<P>
FoM (LP2)
2011
500
25.1
0.48
5.78
2012
510
53.1
0.52
14.36
2013
277.1
74.93
* Int. L: Muon Arms (pileup corrected)
Relevant longitudinal spin runs for W physics
Run 11 – Run 13
Work on progress for publish

8. W → μ Analysis W physics at RHIC
3rd PHENIX collaboration meeting, Wako
W → μ Analysis W physics at RHIC
ALW+ = Δ𝑢 𝑥 1 𝑑 𝑥 2 − Δ 𝑑 𝑥 1 𝑢( 𝑥 2 ) 𝑢 𝑥 1 𝑑 𝑥 𝑑 𝑥 1 𝑢( 𝑥 2 )
~ Δ𝑢 𝑥 1 𝑢 𝑥 1 ( 𝑥 1 ≫ 𝑥 2 ) or ~ − Δ 𝑑 𝑥 1 𝑑 𝑥 1 ( 𝑥 2 ≫ 𝑥 1 )
* Swap u and d for W－case
ALW = 1 𝑃 × 𝑁 + 𝑊 − 𝑁 − 𝑊 𝑁 + 𝑊 + 𝑁 − 𝑊
ALW : Single longitudinal spin asymmetry of W
P : Beam polarization
𝑁 +(−) (W) : # of events contains leptons decayed from W
with corresponding helicity (positive/negative)
in experiment,
* un-polarized: averaging over the spin states of the beam
Δ q measurement via W bosons
Probe: leptons (e±, μ±) directly decayed from W bosons
Observable: Single longitudinal spin asymmetry (AL)
Advantages:
Fixed flavor combination: full flavor separated measurements
No dependence on the fragmentation functions

9. W → μ Analysis At PHENIX Muon Arms
3rd PHENIX collaboration meeting, Wako
W → μ Analysis At PHENIX Muon Arms
Probe: reconstructed μ tracks - of course NOT all of them came from Ws
Signal: W± → μ±
Backgrounds:
Hadronic
Punch-through/in-flight decayed hadrons
Mainly Ks and πs
Dominant BG source
Muonic
Drell-Yan, Openbottom, Opencharm,
Quarkonia, W/Z → Hadrons/τ, etc…
Irreducible
But we have a good understanding by MC study
To shift grain from chaff,
Uses various kinematic variables: DG0, DDG0, DCA_r, χ2, Rpc1/3DCA, fvtx_dr/dφ/dθ

10. W → μ Analysis Kinematic variables
3rd PHENIX collaboration meeting, Wako
W → μ Analysis Kinematic variables 1 2 3 4
RPC1
MuTr
MuID
RPC3
Track z
DDG0
DG0
Rpc3DCA
Rpc1DCA
FVTX
FVTX match
13.0 (13.4) λI / cosθ for S (N)
7.1 λI / cosθ
Side view x y
MuTR
St 1
DCAr
Beam view
DCAr : radial distance btw vertex and intpol. track
FVTX_dr/dφ/dθ: match btw FVTX/MuTr tracks
Rpc1DCA: distance btw Rpc1 hit and intpol. track
Χ2 : reconstructed track’s Χ2 (MuTr)
DDG0: angle difference btw MuID 1st hit ad extpol. track
DG0: distance btw MuID 1st hit and extpol. track
lastGap: last hit position of MuID
Rpc3DCA: distance btw Rpc3 hit and extpol. track

11. W → μ Analysis Data composition (after preselection)
3rd PHENIX collaboration meeting, Wako
W → μ Analysis Data composition (after preselection)
1st μ candidates selection by using kinematic variables
16 < pT < 60, DG0 < 20, DDG0 < 9, χ2 < 20, and last MuID hit in 5th gap
But backgrounds are still overwhelming (> 99.9 %) even after 1st selection
Signal
W± → μ±, ( < 0.1 % after preselection)
Muonic BGs
Irreducible
Model it with MC study
Hadronic BGs
Punch-through/in-flight decayed hadrons
Dominant backgrounds
* NOT represent actual composition

12. W → μ Analysis W likelihood (Wness)
3rd PHENIX collaboration meeting, Wako
W → μ Analysis W likelihood (Wness)
Likelihood
One can set a likelihood to certain desired distribution,
if distribution of both desired (Signal) and NOT desired (BG) is known
Wness (likelihood to W)
All of our kinematic variables are distinctive to W and BGs, in various degrees
Then ratio between each of these probability (likelihood) can be defined:
𝜆 𝑠𝑖𝑔 𝜆 𝑠𝑖𝑔 + 𝜆 𝐵𝐺 = Wness
We assign this ‘Wness’ to each event. By its definition as Wness → 1, it is more likely to be W
λ sig
λ(DG0)
λ(DDG0)
λ(DCAr)
λ(χ2)
RpcDCAs
FVTX match
Signal distributions from W MC (PYTHIA + PISA) =
BG distributions from data after 1st selection (> 99.9 % BG)
λ BG
DCA_r

13. W → μ Analysis W likelihood (Wness)
3rd PHENIX collaboration meeting, Wako
W → μ Analysis W likelihood (Wness)
2nd μ candidate selection by using Wness
One can select events with certain Wness cut (ex. Wness > 0.1)
As Wness cut increases relative purity of data also increases, but it means we’ll have lesser statistics
wo/ Wness cut
Wness > 0.1
Wness > 0.9
But, unfortunately…
BGs are still dominant even for the final sample (tightest Wness cut (ex. Wness > 0.92) applied)
Therefore alternative method is needed to measure S/BG ratio among final sample

14. W → μ Analysis S/BG estimation via unbinned max. likelihood fit
3rd PHENIX collaboration meeting, Wako
W → μ Analysis S/BG estimation via unbinned max. likelihood fit
S/BG estimation
We cannot directly estimate S/BG
But we know (or we can model) each of process’ distribution among final sample:
likelihood based fit is possible
Fit variables:
η (pseudorapidity)
dw23 = pT × sinθ ×dφ23 x y
MuTr
St 2
St 3 z θ
dφ23
η total
η signal =
dw23 signal
dw23 total
η μ BG
dw23 μ BG
η Had BG
dw23 Had BG
From MC
(PYTHIA + PISA)
From data
(BG dominant region)

15. W → μ Analysis S/BG estimation via unbinned max. likelihood fit
3rd PHENIX collaboration meeting, Wako
W → μ Analysis S/BG estimation via unbinned max. likelihood fit
Run 13 - South μ+
Result of 2D likelihood fit
projection to each variable
Measured S/BG ratio
Run13 preliminary
For each Arm and Charge

16. W → μ Analysis Run 13 preliminary
3rd PHENIX collaboration meeting, Wako
W → μ Analysis Run 13 preliminary

17. W → μ Analysis Physics impact and Outlook
3rd PHENIX collaboration meeting, Wako
W → μ Analysis Physics impact and Outlook
Impact on DSSV global fit
Δ u (left), Δ d (right)
χ2 profiles for truncated integral (top),
x dependent uncertainty (bottom)
Top:
for u , a shift away expected from
current best mean w/ STAR Run12.
Also it suggests Δ u > Δ d
Bottom:
Clear improvement is expected in
determination of light sea quarks
DSSV : pSIDIS data (1 < Q2 < 50 (GeV)) included
DSSV++ (dashed) : STAR Run12 preliminary included
DSSV++ (solid) : RHIC W AL projections (Run9 - 13) included

18. 3rd PHENIX collaboration meeting, Wako
Summary
W physics at RHIC
Aims better constraint in light sea quark polarization
Fragmentation free leptons from W bosons
Expectations on global analysis shows clear improvements
W → μ analysis at PHENIX Muon Arms
W likelihood based S/BG estimation
Preliminary status in all recent 3 years
Further refinements are ongoing
Hadronic dw23 extrapolation study
Wness optimization
Refine of dimuon scaling factor
Work on progress towards finalization/publish
Aims publish in early 2015

19. Backup Forward muon trigger upgrade
3rd PHENIX collaboration meeting, Wako
Backup Forward muon trigger upgrade
MuTRG
ADTX
MRG
Level 1
Trigger
Board
MuTr
FEE
Resistive Plate Chamber
(RPC) (φ segmented) B
2 planes 5%
95%
Interaction Region
Rack Room
Optical
1.2Gbps
Amp/Discri.
Transmit
Data
Merge
RPC
Trigger events with straight track
(e.g. Dstrip <= 1)
RPC / MuTRG data are
also recorded on disk

20. Backup Signal/BG processes and Fit variable
3rd PHENIX collaboration meeting, Wako
Backup Signal/BG processes and Fit variable
S/BG estimation
Signal/BG processes
Fit variables
Signal
W/Z
Get distribution from MC (PYTHIA + PISA)
Muonic BG
Various processes (quarkonia, opencharm, etc…)
Scale it with factors obtained from independent study
Hadronic BG
Muons decayed from hadrons
Get distribution from BG dominant (low Wness) region in data
Getting distribution itself requires thorough study η
Pseudorapidity
Distribution is stable Vs. Wness
dw23
pT × sinθ × dφ23
Especially effective against Hadronic BG
Distribution changes seriously Vs. Wness

21. Backup Dimuon scaling factors
3rd PHENIX collaboration meeting, Wako
Backup Dimuon scaling factors

22. Backup W ± → e ± @ Central Arm
3rd PHENIX collaboration meeting, Wako
Backup W ± → e Central Arm

23. Backup STAR Run 12, RHIC Run 9 – Run 13 projection
3rd PHENIX collaboration meeting, Wako
Backup STAR Run 12, RHIC Run 9 – Run 13 projection
PRL 113, (2014)

24. Backup Central Arms Vs. Muon Arms
3rd PHENIX collaboration meeting, Wako
Backup Central Arms Vs. Muon Arms
e Central Arm
μ Muon Arm
Triggered by
Energy
Momentum
E deposit in EMCal
Tracking in B field
Charge
Vs. pT pT