First results of W ± boson production in high-energy polarized p+p collisions at RHIC at BNL

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1 1 First results of W ± boson production in high-energy polarized p+p collisions at RHIC at BNL

2 Outline 2 W production - Recent Results First W + /W - Cross-section and A L Measurement at STAR Experimental aspects: RHIC / STAR Summary and Introduction p p Outlook

3 Introduction 3 How do we probe the structure and dynamics of matter in ep / pp scattering? e p x Q 2 e dσ ep F 2 = xe 2 qf q (x) q X Universality p p x 1 x 2 p T dσ pp f 1 f 2 σ h D h f Factorization W 2 Q 2 /x f(x) = f(x) = Momentum contribution + f + (x) + f (x) f + (x) f (x) contribution Spin

4 Introduction 4 What do we know about the polarized quark and gluon distributions? Spin carried by quarks is very small ( ~.3)! x u x d x(!u+!u ) x!u v.4 x!g.2 u/d sea-quarks Σ 1 2 = S q + S g + L q + L g G Σ = u + ū + d + d + s + s DSSV x(!d+!d ) DNS KRE DNS KKP x s x!u DSSV 2 =1 DSSV 2 =2% x!d v x g x!d GRSV maxg GRSV ming x -2 x Bj x x Bj x!s 1-2 large x Bj KRE (NLO).4 KKP (NLO) unpolarized KRE " 2 min +1.2 KRE " 2 min +2% uncertainties! Substantial improvement for.5<x<.2 Large uncertainties at low x q i (Q 2 ) = 1 q i (x, Q 2 )dx G(Q 2 ) = 1 g(x, Q 2 )dx D. de Florian et al., Phys. Rev. D71, Lett (28) (25). 721

5 Introduction 5 Polarized semi-inclusive DIS results: HERMES / COMPASS x!u x!d COMPASS preliminary HERMES PRD71(25) DNS parametrization Semi-inclusive DIS: Correlation of flavor content of hadron with flavor of quark / antiquark probed x!u Good agreement of COMPASS and HERMES LO analysis Good agreement with global fit analysis / Sea quark distributions compatible with zero Great value of independent probe at large momentum scales (sub-leading twist effects unimportant) without hadronic fragmentation x!d x!s x

6 Introduction 6 STAR W program in e-decay mode at mid-rapidity and forward/backward rapidity u/ u (d/ d) d/ d ( ū/ū) x 1 x 2 W + (W ) A W L ν e ( ν e ) = 1 P e + (e ) N + (W ) N (W ) N + (W )+N (W ) y l = y W ln 1 + cos θ 1 cos θ p T = p T = M W 2 sin θ M W y l x 1 = MW s e y W x 2 = MW s e y W RHICBOS W simulation at 5GeV CME =.16 s Key signature: High p T lepton (e - /e + )(Max. M W /2) - Selection of W + /W - : Charge sign discrimination of high p T lepton Required: Lepton/Hadron discrimination dσ/dp T (pb/gev) W + W - (PDF: CTEQ5M) Total cross-section: 83.3pb 1.9pb Total cross-section: 19.3pb 2.5pb -1 < y e < 1 p T > 25GeV/c GeV/c p T (GeV) Ratio σ(w + ) / σ(w - ) dσ/dp T (pb/gev) W + W - (PDF: CTEQ5M) Total Ratio: cross-section: σ(w + ) / σ(w - ) 14.3pb 5.9pb = Total -1 < cross-section: y 8.pb e < 1 5.pb 1 < y e < 2 p T > GeV/c 25GeV/c p p T T (GeV) V) 1 W + W - (PDF: CTEQ5M) Total ( s=5gev) σ(w + )=135pb and σ(w - )=42pb 2 Ratio: σ(w + ) / σ(w - ) = 3.2 Total cross-section: 95.2pb - ) - )

7 Introduction 7 W boson kinematics relevant for STAR rapidity acceptance Leptonic rapidity inherits relation to mean x <X 1,2 > M W S e [±η/2] Forward rapidity: > <x 1 > larger than <x 2 > Backward rapidity: < <x 1 > less than <x 2 > Mid-rapidity: ~ <x 1 > similar to <x 2 > D. deflorian and W. Vogelsang, hep-ph/

8 Introduction 8 A L behavior for STAR mid-rapidity and forward/backward rapidity region W - p T > 25GeV/c A W L A W + L = ū ū x 1 x 2 A(y e ) A(y e ) = u u x 1 x deflorian/vogelsang (NLO) DSSV y e DSSV8 DNS-K DNS-KKP W + p T > 25GeV/c RHICBOS (Resummation) y e A W L A W + L = d d x 1 x 2 A W L = 1 ū 2 ū d d x 1 = x 2 A W + L = 1 d 2 d u u = d d x 1 x 2 Calculations: 1) RHICBOS: P.M. Nadolsky and C.-P. Yuan, Nucl. Phys. B666 (23) 31. 2) deflorian / Vogelsang: D. deflorian, private communications.

9 Collider: The First polarized p+p collider at BNL 9 RHIC Performance - Overview Siberian Snakes RHIC pc Polarimeter Absolute Polarimeter (H jet) Spin Rotators Pol. Proton Source PHENIX 2 MeV Polarimeter Partial Snake STAR Helical Partial Siberian Snake AGS polarimeters Siberian Snakes Rf Dipole Strong AGS snake Long 2GeV production runs at s=2gev (long. polarization): Run 5 / Run 6 / Run 9 First collisions of polarized proton beams at s=5gev (long. polarization): Run 9

10 Collider: The First polarized p+p collider at BNL 1 RHIC polarized p+p running RHIC RUN s [GeV] L recorded [pb -1 ] (trans.) L recorded [pb -1 ] (long.) Polarization [%] RUN 2 RUN 3 RUN 4 RUN 5 RUN 6 RUN 8 RUN / / 5-25 / / 4 Transverse program: A N measurement of forward π and production (Run 2 / Run 6 / Run 8) Gluon polarization program: Inclusive jet and hadron production (Run 3/4, Run 5, Run 6 and Run 9) W program: First A L measurement W + and W - boson production from Run 9

11 The STAR Experiment at RHIC 11 Overview Calorimetry system with 2π First collisions of polarized proton beams at STAR at s = 5GeV: Run 9 (P~4% / L~14pb -1 ) coverage: BEMC (-1< <1) and EEMC (1< <2) TPC: Tracking and particle ID ZDC: Relative luminosity and local polarimetry BEMC TPC FGT EEMC BBC: Relative luminosity and Minimum bias trigger STAR Mid-rapidity W program (-1< <1): BEMC and TPC STAR Forward/Backward W program (1< <2): EEMC and TPC / FGT (Installation in summer 211)

12 W production results: Algorithm 12 W reconstruction - Algorithm : Idea e p ν e p Transverse plane view

13 W production results: W event 13 Event display (W event candidate) and detector signature BTOW response We found ~6 of those kinds of events! TPC track extended to BTOW

14 W production results: QCD Background event 14 Event display (Di-Jet event candidate) and detector signature We recorded and rejected ~1.5M of those kinds of events!

15 W production results: Z event 15 Event display (Z event candidate) and detector signature We found a handful of those kinds of events!

16 W production results: Lego plots 16 Lego plots - STAR BEMC/TPC W event (GeV) BEMC E T BEMC E T Distribution (GeV) TPC p Distribution (GeV/c) Run 9 STAR Data ( s=5gev) (GeV/c) TPC P T T Azimuth (rad) Pseudorapidity Azimuth (rad) Pseudorapidity 1 Run 9 STAR Data ( s=5gev) Di-Jet event

17 W production results: Algorithm Details 17 W reconstruction - Algorithm : Details (1) General: Transverse plane view phi Entries 9 TPC ET sum/cell, maxz=2 GeV/c run1968 eveid=13591 zver=-17cm Integral Select L2W-E T triggered events Select vertices with Z <1 cm Electron isolation cuts: Electron candidate is any primary TPC track with global P T >1 GeV/c Extrapolate TPC track to BTOW tower Compute 2x2 tower cluster E T, require E T sum > 15 GeV Require the excess E T in 4x4 tower patch over 2x2 patch to be below 5% Require distance of 2x2 cluster vs. TPC track below 7 cm 8 Near-cone veto: -1 6 Compute near-cone E T sum of BEMC+TPC over R=.7 in eta-phi space -2 4 Require near-cone excess E T below 12% 2 Away- cone cuts: p T balance requirement Vector sum > 15GeV/c of: 2X2 tower cluster p T and p T of any number of jets outside near-cone event eta E T of jet > 3.5GeV

18 W production results: Algorithm Details 18 W reconstruction - Algorithm : Details (2) Lepton meas. in TPC (direction) and in BEMC (energy) TPC & BEMC matching Suppress background BEMC cluster isolation Near-side veto Away-side veto Transverse plane view Counts Counts Counts Counts 3 3 a) E 3 b) 2 1 M-C: W s M-C: QCD STAR data M-C arb. norm. e 4 4 T / ET E e T /E 4 4 e E.5 1 / E T R<.7 T Select 2x2 cluster with highest E T sum TPC track extrapolated to BTOW tower grid E e T /E R<.7 T Signed pt balance (GeV) balance (GeV) T signed P 6 c) x4 2x2 W events 2 2 e E (GeV) 4 4 E 6 6 T e (GeV) T Di-Jet events

19 W production results: Algorithm Details 19 Evolution of E T distribution vs. cut ID Starting raw distribution Track-cluster match No near cone E T No away cone E T Using the algorithm a clear Jacobian peak can be seen characteristic for W production in contrast to QCD background!

20 W production results: Charge separation 2 Mid-rapidity high p T e ± charge separation TPC vertex positron p T = 5 GeV/c electron p T = 5 GeV/c Q: Charge-sign of reconstructed track 2 cm of tracking BEMC Q / pt (1/GeV) positrons +/- distance D: ~1/P T p T = 5 GeV/c : D ~15 cm p T = 4 GeV/c : D ~2 cm Assign: Q/p T > positrons Q/p T < to be electrons Final TPC calibration electrons EMC Cluster E T (GeV) Successful separation of different charge states!

21 W production results: Charged-separated Yields 21 Charge separated raw Signal / Jacobian Peak Distributions Charged separated W + /W - candidate distributions of the BEMC cluster transverse energy E T (GeV) Cuts: All previously discussed cuts!

22 W production results: Background 22 Background treatment W + distributions PYTHIA+GEANT MC W τ + ν τ τ e + ν e + ν τ MC Normalized to L=13.7 pb -1 Total Background 1. Run analysis with EEMC in veto cuts 2. Run analysis without EEMC in veto cuts Run 9 Data Missing Endcap Vetoed QCD Background EMC Cluster E T (GeV) 3. Subtract two raw signals Signal region Data Driven QCD Bkgd. Normalized at E T < 19 GeV Data Driven QCD Bkgd. Background systematics: Calculate different data driven QCD background shapes by varying p T balance and away-side p T cuts Vary normalization region (E T < GeV) The largest deviation in each bin used for sys. error estimate

23 W production results: Background 23 Background subtraction S/B~6 S/B~11 Background distribution and background-subtracted signal distribution B/(S+B) (E T > 25GeV) W - : 16% B/(S+B) (E T > 25GeV) W + : 8%

24 W production results: Data/MC Comparison 24 Data/MC Comparison of charge-separated Jacobian peak distributions Comparison of data and PYTHIA+GEANT simulations for W signal events at s=5gev Systematic uncertainties were estimated by varying cuts and normalization regions for QCD background and by varying BEMC energy scale uncertainty (±7.5%)

25 W production results: Cross-Section 25 Total W + /W - Cross-section results STAR Preliminary Run 9 (p+p s=5 GeV) σ W + e + +ν = 61 ± 3 (stat.) σ W e + ν = 17 ± 2 (stat.) +3 4 (syst.) ± 14 (lumi.) pb (syst.) ± 4 (lumi.) pb Reasonable agreement between measured and theory evaluated cross-sections within uncertainties!

26 B - Y - B + Y + Counts W production results: Asymmetry measurement 26 A L determination u/ u (d/ d) d/ d ( ū/ū) x 1 x 2 W + (W ) A W L ν e ( ν e ) = 1 P e + (e ) N + (W ) N (W ) N + (W )+N (W ) First measurement of parity-violation in A W + L = 1 d 2 d u u polarized proton-proton collisions at RHIC negative AL (P1+P2) W + : Observe directly u quark polarization!

27 W production results: Asymmetry result 27 Parity-violating single-spin asymmetry W + /W - A L results STAR Preliminary Run 9 (p+p s=5 GeV) A L (W + )= A L (W )=.33 ±.1(stat.) ±.4(syst.).18 ±.19(stat.) (syst.) A W L = 1 ū 2 ū d d A L (W + ) negative with a significance of 3.3 A L (W - ) central value positive A W + L = 1 d 2 d u u Systematic errors of A L under control TPC charge separation works up to p T ~ 5GeV Measured asymmetries are in agreement with theory evaluations using polarized pdf s (DSSV) constrained by polarized DIS data Universality of helicity distribution functions!

28 Future W program: Forward GEM Tracker 28 FGT layout HFT FGT e ± FGT: 6 light-weight triple-gem disks using industrially produced GEM foils (Tech-Etch Inc.) FGT Quarter Section APV module New mechanical support structure Expected installation: Summer 211 FGT GEM foil APV chip

29 Future W program: Projections 29 A L projections Assumptions: Efficiency: Mid-rapidity:.65 Forward rapidity:.6 Assume availability of 9MHz RF Background: Mid-rapidity: Run 9 Forward rapidity: QCD MC simulations Full charge-sign discrimination at highp T Conclusions: W Program at RHIC is a multi-year program - Initial sample of ~1pb -1 / ~5% is only a step along the way! Critical: Design polarization performance of 7% to collect at least 3pb -1 Polarization uncertainty ~5%

30 Future W program: Projections STAR W Impact on polarized QCD sea x!ū DSSV x!d - D. deflorian and W. Vogelsang, hep-ph/ DSSV8 Fit x x ū W 2 pb -1 x d - RHIC x Include W results at RHIC (PHENIX and STAR) assuming -2 < < with 2pb x x -.6 Strong constrain for x>.5

31 Summary 31 STAR High-energy polarized p-p program pqcd: Critical role to interpret measured asymmetries First global analysis incl. RHIC SPIN data Evidence for small gluon polarization for.5<x<.2 Correlation measurements (Di-Jets / -Jets) will allow to provide needed constrain on the partonic kinematics First Di-Jet cross-section measurement at RHIC at s=2gev Run 9 analysis of 2GeV in full swing - Strong focus on di-jet measurements! First Run 9 STAR W result (Cross-section and A L for W + /W - at mid-rapidity) important milestone! Forward rapidity: Complete FGT construction in ~fall 21 followed by full system test and subsequent full installation in ~summer 211 Ready for anticipated long 5GeV polarized pp run in FY12 (Run 12) Future measurements of A L at STAR at mid-rapidity and forward rapidity (Wide rapidity coverage!) are expected to play an important role in our understanding of the polarized QCD sea!

32 Outlook 32 Outlook - RHIC SPIN Three key Recorded Luminosity Main physics Objective Remarks elements: ~5pb -1 Gluon polarization using di-jets and precision inclusive measurements 2 GeV Gluon polarization ~1pb -1 W production (Important consistency check to DIS results - Phase I) 5 GeV Quark / Anti- Gluon polarization (Di-Jets / Photon-Jets) Quark Polarization Transverse spin dynamics Critical: ~3pb -1 W production (Constrain antiquark polarization - Phase II) Gluon polarization (Di-Jets / Photon-Jets) 5 GeV ~3pb -1 Transverse spin gamma-jet 2 GeV ~25pb -1 Transverse spin Drell-Yan (Long term) 2 GeV Beam polarization: 7% / Narrow vertex region / Spin flipper Critical: Sufficient running time!