Measurement of Longitudinal Spin Asymmetries From W Boson

Measurement of Longitudinal Spin Asymmetries From W Boson Decay in Polarized pp Collisions at s=500 Ge. V at RHICPHENIX Kensuke Okada (RBRC) for the PHENIX collaboration Spin 2010 10/1/2010 K. Okada (RBRC) 1

W bosons at RHIC Polarized p+p collider • An unique probe to access the flavor dependence of polarized sea quarks. • Parity violating single spin asymmetry (AL). 10/1/2010 K. Okada (RBRC) 2

RHIC-PHENIX 2009 run • The first 500 Ge. V p+p run. (March to April, 2009) • Beam Polarization : <P>=0. 39± 0. 04 • Integrated luminosity (with vertex cut) : Ldt=8. 6/pb Today’s talk High p. T electrons : W electron decay channel 10/1/2010 K. Okada (RBRC) 3

PHENIX Central arm: W e EMCal + DC/PC 1 tracking Acceptance : 0. 35 in rapidity 0. 5 *2 arms in EMCal Trigger Fully efficient at 12 Ge. V 10/1/2010 K. Okada (RBRC) 4

e energy and charge α = bend angle data at 40 Ge. V/c [Ge. V/c] p. T= E(in EMCal)* sin 10/1/2010 K. Okada (RBRC) 5

Jacobian peak is confirmed + p. T distribution of electron candidates Red histogram: with minimum track requirements Blue histogram: with an isolation cut - Main background : Photon conversion Charged hadron interaction 10/1/2010 K. Okada (RBRC) 6

Consistent with the expectation + Signal shape after background subtraction electrons from W, Z bosons. With minimum track requirements - 10/1/2010 The red curve shows the expectation (NLO) K. Okada (RBRC) 7

Consistent with the expectation (numbers) W, Z decay electrons positron electron uncertainties: 10/1/2010 Statistical Systematic (Background assumptions) Normalization (acceptance, efficiency, luminosity) K. Okada (RBRC) 8

Convert to the total W production PHENIX acceptance （p. T>30 Ge. V, |y|<0. 35） Total Z boson contribution + : ~7%, -: ~30% Fraction of the PHENIX + : ~22%, -: ~15% The first measurement (with an ar. Xiv number) in p+p collisions. 10/1/2010 K. Okada (RBRC) 9

LHC Preliminary They are also consistent with expectations. (See ICHEP presentations) LHC(ATLAS, CMS) preliminary points 7 Te. V 10/1/2010 K. Okada (RBRC) 10

Parity violating single spin asymmetry RHIC has 120 bunches polarized (every 106 ns) Reduces detector time dependence Concept: 1 -beam polarized Raw asymmetry: N+ : yields in helicity + N- : helicity – R : Correction for L Physics asymmetry : D: Dilution correction P: Beam Polarization Actual calculation From 4 spin patterns (= 2 beams * 2 helicity patterns) 10/1/2010 K. Okada (RBRC) 11

Parity violating raw asymmetries ( L) BG + Signal 42 counts BG Signal 13 counts - L=0 for Background (as expected) Large L for Signal (especially in e+) 10/1/2010 K. Okada (RBRC) 12

Physics asymmetries (AL) D: 1. 04 0. 03 (+ signal) 1. 14 0. 10 (- signal) P: 0. 39 0. 04 (2 beam average) + Limited to the physical region [-1, 1]. 10/1/2010 K. Okada (RBRC) 13

AL (68%CL) and predictions + - They are consistent with all models for now. A non-zero asymmetry (98. 4%CL) is observed in the positive candidates. 10/1/2010 K. Okada (RBRC) 14

Summary • PHENIX central arm detects electrons from W boson decay. • The production cross section is as expected. The first measurement in p+p. • Measured the parity violating single spin asymmetry (non zero AL). • Submitted to a journal. (ar. Xiv: 1009. 0505) • This is a start of RHIC W boson program. • Forward/backward muon channel. Today at 17: 30 (Location C) by Y. Fukao. 10/1/2010 K. Okada (RBRC) 15