Polarisation, Charged Current and Parton Densities

1. Lepton Polarisation, Charged Current and Parton Density Function fitting at HERA, using the ZEUS detector and Transverse Polarimeter Polarisation, Charged Current and Parton Densities HERA, the Transverse Polarimeter and the ZEUS detector TPOL performance and testing Charged Current cross section measurements Parton Density Function fitting

2. Polarisation, Charged Current and Parton Densities • Polarisation principles • Leptons rapidly naturally transversely polarise in HERA guide field. • Outline of Sokolov-Ternov effect to explain natural polarisation. • Outline spin dependence of compton scattering cross section of laser light, and how it produces angular asymmetry of scattered photons (so we can detect pol) • CC interactions • Descripton of DIS and definition of Q2,x,y,s • Reconstruction of CC events- Jaquet Blondel (as no e+ remnant) • Definition of the double diff CC cross section (F2/fL/xf3 + pdfs) • Longitudinal lepton polarisation (required by experiments) actually affects crosssections (see figure) Hence detail spin dependent CC cross sections • PDF theory • For CC e+P, we basically measure the d-valence quark density: scc(e+P)A(1-y2).x.(d+s) and scc (e-P)Ax.(u+c) • Theory PDFs parameterised as follows: xf(x) = p1.xp2(1-x)p3(1+p5x) f(x)=d,u valence, sea, glue etc. • Gives flexibility at low (p2),high (p3) andmid (p5)x • Then, theory cross section is calculated, which is compared to data • ZEUS offset method used to obtain central values of fit, errors determined by freeing correlated systematic error parameters.

3. HERA, the TPOL and the ZEUS detector 27.5 GeV Silicon Detector

4. Measuring Polarisation • Polarisation measured using circularly polarised laser light incident on beam. • Compton scattered laser light has angular asymmetry (dep. on beam pol) • 1 Xo Pb preshower to convert Compton photons. • Converted Photons enter two halves of TPOL calorimeter and up-down asymmetry is measured. • The use of =(Eu-Ed)/(Eu+Ed), and the -y conversion • Silicon in front of calorimeter allows fast up-down calibration of calorimeter. • 6x6cm2 silicon detector with horizontal and vertical strips (80,120mm pitch respectively) • Should improve accuracy of Polarisation measurement to under 1%.

5. TPOL performance and testing • 2 Testbeams performed. DESY/CERN-sps. • publications: ZEUS-01-019; ZEUS-02-019 • Plus, small amount of HERA data gathered • (only 1 run, nothing like enough data to get polarised cross-section!) • Profile plot eta-y (right) from 10 GeVe+ testbeam • Allows fast calibration of calorimeter -for a given eta, the y coordinate can be deduced from the calibrated curve. • Improves systematic error on pol. estimate, especially at high |y| where eta flattens off.

6. TPOL Silicon dead strips/extra hits • Unfortunately some dead strips/extra hits • Dead strips visible in beam profile • Magnified region shows how adjacent strips record ‘extra’ hits, as charge is dissipated to them • defective strip numbers have been recorded and can be identified by their rms response. • ~11% of the detector is unusable • Attributed to bonding process • Replacement silicon in production

7. Charged Current cross section • Extracted from 99-00 ZEUS data. • Detail of Cuts applied, showing plots where relevant. (Include signal and background MC). • Mainly detected using missing Pt (ZEUS has 99.7% solid angle coverage) • Bin definitions and resolutions. • Systematic error sources. • Cross sections, efficiencies, acceptances, purities. • Agreement very high (differences accounted for) between my analysis and ZEUS 99-00 CCe+ paper due out soon

8. Reduced cross section in bins of fixed Q2 Large Circles show ZEUS paper points Reduced cross section in bins of fixed x

9. PDF Fitting O-fit 94-00 • Include 99-00 High Q2 data in existing ZEUS-O fit (ZEUS-Only data) • 99-00 CC e+P allows better measurement of d-valence with ZEUS data alone. • Standard fit (ZEUS-S) has large systematic error due to deuterium/fixed target corrections. • Now playing with model dependences, Mw fit

10. NEW O fit 94-00 PUBLISHED ZEUS-O Valence distributions from ZEUS data alone 16pb-1 e- data and 37pb-1 e+ data Valence distributions from ZEUS data alone 16pb-1 e- data and 100pb-1 e+ data

11. NEW O fit 94-00 ZEUS-S Errors on High-x xdv from ZEUS-O as good as ZEUS-S and mostly statistical so that HERA-II will improve more!

12. Results • Independent measurement of CC e+ cross sections in (sgl diff) x,y,Q2, and (dbl diff) x,Q2 using 99-00 ZEUS data. • Use of 99-00 data in PDF fit to improve on the old ZEUS-O fit. • Determination of Mw from fit (still to do)

13. Work Status • Systematic errors should complete within few wks • PDF model dependences being worked on now. should be finished imminently. • Writing up when workload of systematics drops..