Neutral Current Cross Sections With Polarised Lepton Beam At ZEUS

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1 Neutral Current Cross Sections With Polarised Lepton Beam At Syed Umer Noor York University, Canada On Behalf of the Collaboration DIS 6, - 4 April 6, Tsukuba, Japan Syed Umer Noor (York University) NC with Polarised Leptons at DIS 6, - 4 April 6 / 7

2 Deep Inelastic Scattering at HERA Neutral Current, NC: γ or Z exchange Charged Current, CC: W ± exchange Q is the probing power Q = q = (k k ) x is the Bjorken scaling variable x = Q p q y is the inelasticity y = p q p k They are all related via, Q = x y s s is the centre-of-mass energy squared s = (p + k) Syed Umer Noor (York University) NC with Polarised Leptons at DIS 6, - 4 April 6 / 7

3 Unpolarised NC DIS Cross Section NC DIS cross section d σ(e ± p) dxdq = πα xq 4 [Y + F Y xf y F L ] Y ± ± ( y) Reduced cross section σ e±p = xq4 d σ(e ± p) πα Y + = F dxdq Y Y + xf y Y + F L Extraction of xf σ e p σ e+p = Y Y + xf Structure functions F, F L and xf F : dominant contribution to σ F = F em + Q F γz Q Q +M Z + [ Q +M Z xf : contribution only important at high Q xf = Q xf γz Q +M Z + [ Q Q +M Z F L : sizeable impact only at high y ] F Z F P q=u...b (q + q) ] xf Z xf P q=u...b (q q) Syed Umer Noor (York University) NC with Polarised Leptons at DIS 6, - 4 April 6 / 7

4 Polarised NC DIS Cross Section NC DIS cross section modified by polarisation, d σ(e ± p) dxdq = πα xq 4 [H± + H ± P ] = N R N L N R + N L H ± /P = Y +F /P Y xf /P Using polarised and unpolarised structure functions F /P = x(q i + q i )A /P i xf /P = x(q i q i )B /P i i i Where A /P i and B /P i contain the electron and quark couplings Syed Umer Noor (York University) NC with Polarised Leptons at DIS 6, - 4 April 6 4 / 7

5 Polarised and Unpolarised Coefficients Unpolarised coefficients A i = ei e i v i v e P Z + (ve + ae)(v i + ai )P Z v: vector coupling Bi = e i a i a e P Z + 4a i v i v e a e PZ a: axial coupling Polarised coefficients A P i = e i a e v i P Z a e v e (vi + ai )P Z Bi P = e i a i v e P Z a i v i (ve + ae)p Z All terms in the polarised coefficients depend on P Z P Z = Q Q +M Z sin θ W So polarised structure functions depend only on terms related to Z exchange Expect effect of polarisation on the NC cross section to be significant only at high Q Syed Umer Noor (York University) NC with Polarised Leptons at DIS 6, - 4 April 6 5 / 7

6 Luminosity Integrated Luminosity (pb - ) Luminosity Days of running Collisions between e ± and protons Energies: e ± 7.6 GeV p 9 GeV Centre-of-mass GeV Will present results based on: e + p, L =.8 pb from 4 Submitted to Physics Letters B Collab, S.Chekanov et al., hep-ex/66 e p, L =.5 pb from 4 and 5 Preliminary results Syed Umer Noor (York University) NC with Polarised Leptons at DIS 6, - 4 April 6 6 / 7

7 Polarisation 6 HERA Average Polarisation 4 Spin Rotator Spin Rotator Longitudinal Polarimeter Spin Rotator Spin Rotator Spin Rotator Transverse Polarimeter e Beam p Direction Spin Rotator Transverse polarisation builds up naturally in lepton beam Spin rotators turn this into longitudinal polarisation e + p data L weighted average of +% and -4% e p data L weighted average of +% and -7% Syed Umer Noor (York University) NC with Polarised Leptons at DIS 6, - 4 April 6 7 / 7

8 NC DIS Event in the Detector Well measured scattered electron with high transverse momentum Energy deposits of electron and hadronic jet balanced in φ Syed Umer Noor (York University) NC with Polarised Leptons at DIS 6, - 4 April 6 8 / 7

9 Neutral Current Sample (e + p Data) (c) (a) 4 Q DA (GeV ) θ e (rad) (e) 4 6 p T,h (GeV) (b) x DA 6 4 (d) Q > GeV NC 4 e + (.8 pb - ) SM MC BG MC γ h (rad) (f) -4-4 Z VTX (cm) e + p data, L =.8 pb = +%, L =. pb = -4%, L =.5 pb Q and x from double angle method Scattered electron angle with respect to the proton direction Hadronic jet angle and transverse momentum Z position of the ep interaction vertex Data understood well Syed Umer Noor (York University) NC with Polarised Leptons at DIS 6, - 4 April 6 9 / 7

10 dσ/dq with +ve and -ve dσ/dq (pb / GeV ) dσ/dq (pb / GeV ) σ ( =+.) / σ ( =.4) (a) (b) (c) NC 4 e + SM (-JETS) =+. / = NC 4 e + (. pb - ) SM (-JETS) = +. Q (GeV ) NC 4 e + (.5 pb - ) SM (-JETS) =.4 Q (GeV ) Q (GeV ) Top, middle, bottom plots: dσ/dq with +ve dσ/dq with -ve Ratio of cross-sections, +ve / -ve Measurements consistent with SM expectations χ test for Q > GeV : +ve / -ve = case χ / ndf =.5 +ve / -ve = SM case χ / ndf =. Syed Umer Noor (York University) NC with Polarised Leptons at DIS 6, - 4 April 6 / 7

11 Neutral Current Sample (e p Data) Q (GeV ) DA y DA θ e (rad) P T,h (GeV) x DA 4 5 Ee (GeV) Q > GeV 5 NC (prel.) - e - p (.5pb ) SM MC BG MC γ (rad) h -4-4 Z vtx (cm) e p data, L =.5 pb = +%, L = 4.7 pb = -7%, L = 78.8 pb Q, x and y calculated using the double angle method Scattered electron energy and angle Hadronic jet angle and transverse momentum Z position of the vertex Data well described Syed Umer Noor (York University) NC with Polarised Leptons at DIS 6, - 4 April 6 / 7

12 dσ/dq with +ve and -ve ) (pb/gev dσ/dq ) (pb/gev dσ/dq =-.7) e =+.) / σ(p e σ(p NC (prel.) - - e p (4.7 pb ) = +. NC (prel.) - - e p (78.8 pb ) = -.7 NC (prel.) = +. / = Q (GeV ) Q (GeV ) Q (GeV ) Top, middle, bottom plots: dσ/dq with +ve dσ/dq with -ve Ratio of cross-sections, +ve / -ve Parity violation now clearly observed in NC data! χ test for all points: +ve / -ve = case χ / ndf = 5. / =.5 +ve / -ve = SM case χ / ndf = 9.44 / =.47 Syed Umer Noor (York University) NC with Polarised Leptons at DIS 6, - 4 April 6 / 7

13 dσ/dx and dσ/dy with +ve and -ve dσ/dx (pb) NC (prel.) - - e p (4.7 pb ) = +. - x dσ/dy (pb) 6 4 NC (prel.) - - e p (4.7 pb ) y = +. dσ/dx (pb) NC (prel.) - - e p (78.8 pb ) = x dσ/dy (pb) 6 4 NC (prel.) - - e p (78.8 pb ) y = -.7 =-.7) e =+.) / σ(p e σ(p. NC (prel.) = +. / = x =-.7) e =+.) / σ(p e σ(p. NC (prel.) = +. / = y Overall shift in cross-section ratios due to polarisation Syed Umer Noor (York University) NC with Polarised Leptons at DIS 6, - 4 April 6 / 7

14 σ (Polarised) Q = GeV Q = 45 GeV Q = GeV Q = GeV Q = GeV σ ~First measurements of Q = 5 GeV Q = 65 GeV Q = 5 GeV Q = 5 GeV Q = GeV NC (prel.) - e - p (4.7pb ) = Q = 5 GeV Q = 8 GeV Q = GeV Q = 8 GeV Q = GeV NC (prel.) - e - p (78.8pb ) = x reduced cross sections with polarised e Closed circles -ve data Open circles +ve data Polarisation gives a small effect on σ Data agrees well with prediction Syed Umer Noor (York University) NC with Polarised Leptons at DIS 6, - 4 April 6 4 / 7

15 σ (Unpolarised) Q = GeV Q = 45 GeV Q = GeV Q = GeV Q = GeV σ ~ - - Q = 5 GeV Q = 65 GeV Q = 5 GeV Q = 5 GeV Q = GeV NC - e + p (6. pb ) = - - Q = 5 GeV Q = 8 GeV Q = GeV Q = 8 GeV Q = GeV NC (prel.) - e - p (.5 pb ) = (corrected) - - x Closed circles Full data set of σ e p corrected for residual polarization ( -6%) Open circles Previously measured unpolarised σ e+ p Difference in σ seen very well between e p and e + p This is our xf! σ e±p = F Y Y + xf y Y + F L Syed Umer Noor (York University) NC with Polarised Leptons at DIS 6, - 4 April 6 5 / 7

16 xf Extraction e ± p data combined to extract xf xf.. σ e p σ e+p = Y Y + xf -... Q = GeV Q = 5 GeV Q = 8 GeV NC (prel.) - e ± p (84.7 pb ) Previous measurement dominated by statistical uncertainties due to limited amount of e p data 6 pb -. Q = GeV - Q = GeV - Q = GeV - x Now can make use of pb of e p data! More precise xf measurement Syed Umer Noor (York University) NC with Polarised Leptons at DIS 6, - 4 April 6 6 / 7

17 Summary First measurements of the polarised cross sections with e + p and e p data with polarised lepton beam Parity violation clearly observed with high luminosity e p data! xf measurement made combining new e p data with previously measured unpolarised e + p data Outlook Look forward to more polarised e p running this year with a switch to positrons this summer Hope to achieve precision measurements with full HERA II data set O(fb ) Syed Umer Noor (York University) NC with Polarised Leptons at DIS 6, - 4 April 6 7 / 7