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Recent Physics Topics from ZEUS

Recent Physics Topics from ZEUS. 45th PRC meeting 8 July 1998 Yuji Yamazaki (KEK/IPNS-Tanashi) On behalf of ZEUS collaboration. Outlook: Low- Q 2 /Low- x phenomenology of F 2 Charm production in DIS Charged particle production Multijets in photoproduction

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Recent Physics Topics from ZEUS

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  1. 45th PRC meeting ZEUS collaboration/Y. Yamazaki Recent Physics Topics from ZEUS 45th PRC meeting 8 July 1998 Yuji Yamazaki (KEK/IPNS-Tanashi) On behalf of ZEUS collaboration • Outlook: • Low-Q2/Low-x phenomenology of F2 • Charm production in DIS • Charged particle production • Multijets in photoproduction • High-Q2 neutral current and limits on contact interactions • Charged current Not covering topics on diffraction due to the short time

  2. 45th PRC meeting ZEUS collaboration/Y. Yamazaki Low-Q2/low-xF2 phenomenology - F2 behavior and QCD description in transition region • F2 : observation at HERA/ZEUS • Q2 >> LQCD2(ZEUS nominal vertex) •  Steep rise of F2 • 0.1 < Q2 < 0.7 GeV2(ZEUS BPC) •  The rise becomes slower, closer to Regge model • Intermediate region: 1 < Q2 < 4 GeV2 (ZEUS shifted vertex) •  The slope start to decrease towards low Q2 e.g. Donnache- Landshoff

  3. 45th PRC meeting ZEUS collaboration/Y. Yamazaki  Systematic study of slope in F2  x-leff dlnF2 / dln(1/x)|Q2= leff: rise of F2, for each Q2 reflecting sea quark xS (x) dlnF2 / dln(Q2)|x : scaling violation for each x driven by splitting ~ , reflecting gluon density • … NLO QCD parameterization can fit the behavior • Question: • How is the parton densities S(x) and g(x) derived from • the QCD fits ? • Can we believe the NLO at low Q2 down to ~ 1GeV2 ? • Fit with ZEUS + E665 data for wider arms on x • compared with • GRV 94 parameterization • both q(x) and g(x) valence like at starting scale Q0

  4. 45th PRC meeting ZEUS collaboration/Y. Yamazaki d lnF2/d ln(1/x) = leff : how steep is the rise in x • Observation: • At Q2 ~ 0 : slow rise • higher leff as Q2 higher • Comparison with models in intermediate Q2 region: • GRV : tend to have large leff —slower rise of F2 in data • NLO can fit wide range in Q2

  5. 45th PRC meeting ZEUS collaboration/Y. Yamazaki • “Turnover” observed in scaling violation around x ~ 10-4 •  sea quark production rate smaller in small x and Q2 F2 vs. Q2 : scaling violation • Normal behavior of scaling • violation down to x ~ 10-4 • – decrease at high-x • and rise at low-x • However, slopes less • steeper again at very low-x

  6. 45th PRC meeting ZEUS collaboration/Y. Yamazaki Parton densities xS (x) and xg(x) from NLO fit • The observed behavior of F2: • – modest rise of F2 at low-Q2 • – small scaling violation slope at low-x/low-Q2 • can be explained by valence-like gluon density • (assuming NLO fit to be valid at low-Q2)

  7. 45th PRC meeting ZEUS collaboration/Y. Yamazaki Is NLO valid in low-Q2 (~ 1GeV) range ? — Of course under debate (large as) Example: NLO is flexible enough to explain very low Q2 region but with large uncertainties  prediction power not strong at low Q2 Interesting indication from HERA data, but not conclusive  Need inputs from other process, F2cc and FL

  8. 45th PRC meeting ZEUS collaboration/Y. Yamazaki • Gluon may be valence like at low-Q2 Any other process for more information ? Measuring charm cross section (i.e. ) pin down the gluon density via D* production Charm production in DIS (Almost) direct measurement: Charm production Comparison with NLOusing charm density from GRV • Good agreement overall with NLO • Theory uncertainty mainly from mc (1.3-1.7 GeV) • High-(D*) ( ~ low-x(D*) ) — small excess

  9. 45th PRC meeting ZEUS collaboration/Y. Yamazaki … and the extracted • Good agreement with NLO (GRV94) • Steep rise to low-x down to Q2 ~ 2.5 GeV2 • but rise slower at lowest Q2 bin – even lower Q2behavior ?

  10. 45th PRC meeting ZEUS collaboration/Y. Yamazaki Particle production in DIS Multiplicity distribution and fragmentation function • In the fragmentation universal ? • compare with e+e- using Breit frame Evolution on ln(1/xp) • 1994-1997 data precise test on universality • MLLA fit describes the data • consistent with e+e-

  11. 45th PRC meeting ZEUS collaboration/Y. Yamazaki Fragmentation functions and Q2 evolution • High-x : negative • scaling violation • (cf. High-xF2) •  reflecting • parton branching • like in parton • density • Shapes agree • with e+e- , • but 30% lower • Using the same • parameterization as • e+e- in fragmentation • + NLO (CYCLOPS) •  Good agreement • with data • Universality • in fragmentation

  12. 45th PRC meeting ZEUS collaboration/Y. Yamazaki calculations (LO for this process) reproducing the shape and the amplitude of the cross section Multijet production in photoproduction Jets in photoproduction: hard parton-parton collision (like pp scattering) 3jet mass Jet angles 3-Jet mass distribution M345 > 50 GeV

  13. 45th PRC meeting ZEUS collaboration/Y. Yamazaki  Angular distributions not described by phase space described by MC or calculations Angular distribution: proving matrix elements • cosq3 : spin of primary • exchanging partons (q, g ) • y3: ~ 0 or p •  radiated jet is close to • the plane of beam and • the highest energy jet  Need colour coherence in QCD for describing the observed distribution

  14. 45th PRC meeting ZEUS collaboration/Y. Yamazaki High-Q2 events (NC and CC) 1994-97 results reported in DESY seminar Events before/after LP97 conference Neutral current: No high-Q2 events ( > 20000 GeV2) after LP97 Charged current: Two high-Q2 events ( > 20000 GeV2) after LP97

  15. 45th PRC meeting ZEUS collaboration/Y. Yamazaki Charged current cross section • Good agreement with standard model prediction • up to Q2 ~ 10000 GeV2 • Cross section tend to be high in Q2 > 10000 GeV2 • but large uncertainty is large in this region • e+p CC constrains d-quark density

  16. 45th PRC meeting ZEUS collaboration/Y. Yamazaki Neutral current cross section • Good agreement with standard model prediction up to • Q2 ~ 10000 GeV2 • except two very high-Q2 events above 35000 GeV2 • Precision sufficient to constrain PDFs, and …

  17. 45th PRC meeting ZEUS collaboration/Y. Yamazaki Contact interaction analysis - using NC data Contact interaction Lagrangean : • Enormous numbers of combinations, but … • Vector term only • Constrains from atomic parity violation • SU(2) symmetry implies: • u-quark only term also tested (sensitive at HERA) •  24 combinations of (L,R)(u,d)(+,-)

  18. 45th PRC meeting ZEUS collaboration/Y. Yamazaki Limits on contact interaction terms • Four examples • on fits • Comparable or higher than LEP and Tevatron limits

  19. 45th PRC meeting ZEUS collaboration/Y. Yamazaki Summary of high-Q2 DIS cross section • This year: e- p operation •  Comprehensive studies on EW, PDFs (u/d densities etc.)

  20. 45th PRC meeting ZEUS collaboration/Y. Yamazaki Summary • Filling the gap between soft and hard scale in Q2 • – intermediate region: • NLO fit suggesting gluon density valence like ? • Extending the kinematical region • utilizing high statistics runs in 1997 • – F2cc : high statistics D*, gluon density ? • – Fragmentation : extended to high-x, similar to e+e- • – NC/CC cross section: • start to constrain PDFs • Sensitivity to contact interaction term • Electroweak sector • Extending phase space in final state • – Multijets in photoproduction: • QCD matrix element test • See also ICHEP contributions.

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