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Antonino Zichichi Academy of Sciences - Bologna, Italy CERN - Geneva, Switzerland

HERA physics: results and perspectives. Antonino Zichichi Academy of Sciences - Bologna, Italy CERN - Geneva, Switzerland INFN - Bologna, Italy University of Bologna, Italy. Introduction Selection of Results - Structure functions - Large Q 2 NC/CC cross sections

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Antonino Zichichi Academy of Sciences - Bologna, Italy CERN - Geneva, Switzerland

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  1. HERA physics:results and perspectives Antonino Zichichi Academy of Sciences - Bologna, Italy CERN - Geneva, Switzerland INFN - Bologna, Italy University of Bologna, Italy • Introduction • Selection of Results • - Structure functions • - Large Q2 NC/CC cross sections • - Jets, beauty and as measurements • - Leading baryons • - Searches for exotic physics • The HERA Luminosity Upgrade • Summary • LXXXV Congresso Nazionale • Società Italiana di Fisica • Pavia, September 22, 1999

  2. The HERA Collider Hadron Electron Ring Accelerator • First (and unique) ep collider • 6.3 km long tunnel • Construction: 1984-1990 • Commissioning: 1991 • Data taking: 1992-2005 HERA physics

  3. The HERA Collider Two storage rings: 27.5 GeV e± (bottom): normal conducting magnets 820 GeV protons (top) : 4.7 Tesla, obtained with superconducting magnets operating at 4.4 oK. Proton energy raisedin 1998 from 820 to 920 GeVby running SC magnets at 4.0 °K INFN-HERA project: develop the technology of superconducting magnets in Italy. ANSALDO, LMI and ZANON produced: – 242 superconducting dipole magnets for the proton ring (50%) – the superconducting thin solenoid and compensator magnets for ZEUS HERA physics

  4. HERA Experiments 174 bunches with 96 ns spacing. Running with both electrons and positrons. HERA physics

  5. The ZEUS experiment Sezioni INFN: Bologna, Firenze, LNF, Padova, Roma I, Torino ~ 70 people (16% of the collaboration) HERA physics

  6. Introduction 93-97 s = 300 GeV 98-99 s = 320 GeV (equivalent to 47-50 TeV fixed target energy) ZEUS HERA physics

  7. e(k) e'(k') g *(q) Q2 W2 xP p(P) Kinematics  s = k+P= energy in the ep c.m.s. Q2 = -(k-k')2 = -q2 =virtuality of the exchanged  x = Q2/(2P•Q)=fraction of proton momentum carried by the struck quark y = (P•q)/(P•k) =fraction of beam lepton energy transferred to the photon W2 = ys = energy in the *p c.m.s. Q2 = xys HERA physics

  8. HERA Kinematic Range • HERA has unique “depth of field” on the proton structure: • - Q2 from ~ 0.04 GeV2 to ~ 105 GeV2 • -x down to 10-6 • extension by two orders of magnitude both in x and Q2 HERA physics

  9. Structure Functions HERA physics

  10. F2 Measurement at HERA Explore new kinematic regions: where does the Standard Model break down? High Q2 F2: Higher and higher precision measurements to get handle on QCD evolution and constrain PDF’s Low Q2 Study the transition from Photoproduction (Q2 0) to DIS (Q2 > few GeV2): where does pQCD begin to dominate? HERA physics

  11. for not too large y The ep Neutral Current Cross Section (QED radiative corrections have been neglected) where: quark densities relevant for Q2  MZ02 At low Q2, Ai are the quark electric charges HERA physics

  12. High precision proton structure at low x-Q2 • In 1997 ZEUS installed a silicon tracker (BPT) to improve • the detection of positrons at small scattered angles - • extension of kinematic range and higher precision • F2 values at lowest ever x-Q2: • 0.045 < Q2 < 0.65 GeV2 • 6 ·10-7 < x < 1·10-3 • with tipical error: 2.6% (stat) 3.3% (syst)  Regge models provide a good description of the transition region HERA physics

  13. Convert F2 into:stotg*p = 42a/Q2  F2 By assuming Q2 dependence of stotg*p of GVDM: extrapolate to Q2 = 0 to compare with real photoproduction: 2 m g g s = s * p 2 2 p 2 0 ( W , Q ) ( W ) tot tot + 2 2 m Q 0 Extrapolation to Photoproduction Fit W2 dependence of stotgp à la Regge: stotgp(W2) = ARW2(aR-1) + APW2(aP-1) gives:aP = 1.105  0.001(stat)  0.007(syst) HERA physics

  14. F2 Q2 = 15 GeV2 x F2 at Medium Q2: Precision Data • Recent data made measurement of F2 possible with • improved precision due to higher event statistics and • the new backward silicon tracker in H1: • typical errors  1% (stat) and  3-4% (syst) •  approaching fixed target experiments! • Syst. error dominant up to Q2 1000 GeV2 • Strong rise of F2 at HERA regime • (q(x)  F2/x  quark density is zooming up) • Good agreement between H1 and ZEUS HERA physics

  15. Overview of F2 Measurements Scaling violation by gluons Bjorken scaling • NLO DGLAP QCD fit gives good description • of the HERA, NMC and BCDMS data • Scaling violations well interpreted by QCD HERA physics

  16. ZEUS 1995 QCD Fit to F2 • ZEUS DGLAP NLO fit: • Gluon (xg), Sea quark (xS) and u - d difference (xDud) • parameterised as: A·xd ·(1-x)h · polynomial in x • Input u,d valence distributions from MRS(R2) • Apply momentum sum rule • Inner error bands: • exp. Errors on as, mc and • strange quark content DKs • Outer error bands: • variation of Q02 and • xg(x) parameterised with • Chebycheff polinomials. Gluon density extracted from scaling violations: strong rise of xg(x) for x0 at large Q2 but consistent with zero at Q2 = 1 GeV2 HERA physics

  17. ZEUS 1995 QCD Fit to F2 : Gluon and Singlet • At Q2 = 1 GeV2, Sea is still rising but Gluon at • small x is compatible with zero • Uncertainty for gluon at lowest x,Q2 is large • Although error band goes negative (possible in NLO backward evolution): • FL and F2charm stay positive • the fit can be extended down to Q2 = 0.4 GeV2 without deteriorating its quality •  NLO DGLAP does not break down before the formalism becomes suspect HERA physics

  18. ZEUS bpt (red square) F2(x,Q2) vs.  = log(x0/x)·log(1+Q2/Q02) Phenomenological investigation by D. Haidt: in the HERA domain with x < 0.001 (Sea region) F2 has a simple form in the empirical variable  = log(x0/x) ·log(1+Q2/Q02) where x0 = 0.04 and Q02 = 0.5 GeV2 • F2(x,Q2)  F2() • Linearity: F2() = const · • Representation valid in perturbative and • non-perturbative regions of Q2 • consistent with MRST for Q2 > 1.25 GeV2 HERA physics

  19. Extraction of FL • (for Q2 << MZ2 and neglecting radiative corrections) • FL is relevant at high y (few %) for F2 extraction • and it is accounted for using QCD predictions. • Measurement of FL by lowering beam energies will • likely be done at HERA in the future. • H1 extracted FL using the “Subtraction method”: • access to FL from high y cross sections using • assumption on F2 by extrapolation of DGLAP fit • from low y. HERA physics

  20. Extraction of FL • F2QCD is the H1 QCD NLO preliminary fit for y < 0.35 • extrapolate the fit results to high y • FL =[1+(1-y)2]/y2 ·(F2QCD - sr) HERA physics

  21. Extraction of FL FL = FLQCD Extracted FL consistent with pQCD (yellow band) - at highest y systematically higher Cross check and extension towards low Q2 done by another method (indicated by star in the figure). HERA physics

  22. Proton Structure HERA physics

  23. Models of quark content of proton include significant charm contribution HERA physics

  24. F2charm from D*, D0 in DIS Charm cross section in DIS is expected to be dominated by Boson-Gluon-Fusion • Measurement of visible D*, D0 cross section, • extrapolation outside kinematic region in pT,h • extract F2charm: Very effective test of QCD: F2charm calculable from pQCD knowing xg(x) Direct measurement of F2charm HERA physics

  25. F2charm from D*, D0 in DIS pQCD DGLAP fit • Steep rise of F2charm as we go to lower x • Indication that BGF is the dominant • mechanism for charm production at HERA HERA physics

  26. F2charm/F2 vs. x in Q2 bins • F2charm rises more rapidly than F2 •  dominated by gluon contribution, • while F2 has also quarks • F2charm is 25% of F2 at low x and high Q2 HERA physics

  27. Large Q2 Cross Sections HERA physics

  28. High Q2 Neutral Currents Handle on xF3 is given by the  sign due to the different charge of the lepton beam. Need luminosity with e– beams to access xF3 HERA physics

  29. High Q2 Neutral Currents : dse+p/dQ2 • dse+p/dQ2 falls over 7 orders of magnitude • NLO QCD fit to low Q2 data (Q2 < 120 GeV2) • works well for high Q2 • Larger luminosity needed to constrain PDFs • Slight excess at Q2 > 15000 GeV2 remains HERA physics

  30. ( 5pb-1 taken in 98/99) High Q2 Neutral Currents: e+ vs. e- se-p > se+p For Q2 > 3000 GeV2 all e–p measurements are above e+p, in agreement with SM gZ interference HERA physics

  31. High Q2 Charged Currents • probe valence u,d quarks at large x and Q2 • dsCC/dQ2 shape is sensitive to propagator mass HERA physics

  32. High Q2 Charged Currents: dse+p/dx • Reasonable agreement between SM and data • At high x data systematically above SM (CTEQ4) • Need for Bodek & Yang like treatment of d/u ratio • (d/u = 0.2 for x  1) HERA physics

  33. MW from dsCC/dQ2 • Unconstrained fitto dCC/dQ2: • measurement of the mass of spacelike W •  complementary to e+e- and pp timelike measurements. • Results: see figure  •  no evidence for anomalous space-like EW sector. • Use Standard Model relation: Exploiting correlation between shape and normalization in a model dependent fit. see figure  • A sensitive electroweak consistency check! • Extract MW at MH = 100 GeV and Mt= 175 GeV • (PDG: MW = 80.41 ± 0.10 GeV) ZEUS Note: the above is not a measurement, but indicates the sensitivity of the CC cross section to MW assuming the Standard Model. HERA physics

  34. MW from dsCC/dQ2 ZEUS 1994-97 GF[GeV-2] 1 s contour of 2 distrib. ZEUS model dependent fit MW[GeV] GF depends very strongly on MW  sensitivity to MW within the Standard Model HERA physics

  35. (1998-99 data) (1994-97 data) e-p data an order of magnitude above e+p, since se-p (u+c) + (1-y)2 ·(d+s), while se+p (u+c) + (1-y)2 ·(d+s)  probing different quark flavours High Q2 Charged Currents: e+ vs. e- HERA physics

  36. NC and CC e–p Cross Sections 1998+99 data! Electroweak unification made manifest! HERA physics

  37. Jets and as HERA physics

  38. Photon structure Photons oscillate into hadronic states  two types of processes (at Leading Order) distinguished by xg, the fraction of the photon momentum participating to the hard scattering: Direct photon:   pointlike  x = 1 Resolved photon:   source of partons  x < 1 x + HERA physics

  39. Photon structure Dijets offer a tool to study the photon structure. We use: the fraction of the photon momentum participating to the production of the two jets: Direct ETjet1 > 14 GeV ETjet2 > 11 GeV Resolved Comparison with HERWIG: resolved needed HERA physics

  40. Photon structure Dijets in Photoproduction: Data sensitive to photon PDF’s and ready to be used in global fits HERA physics

  41. Jet Shapes in DIS Jet shapes (r) : R = 1 cone jets Q2 > 100 GeV2 ETjet > 14 GeV -1 <jet < 2 Narrow Jet Wide Jet DIS and e+e–: narrow (quark) jets Tevatron: broad (gluon) jets HERA physics

  42. Jet Shapes in DIS differential jet shapes (r): More detailed comparison: Jet shapes in DIS and e+e– are similar: consistent with universality of pattern of final state QCD radiation around primary quark. HERA physics

  43. Subjets ycut = 5·10-4 ycut = 10-1 • Theoretical advantages: • safe observables: can be defined at any order • allowance for “resummed” calculations • small hadronisation corrections (if ycut not too low) • useful tool to investigate colour dynamics • (e.g. quark/gluon differences) HERA physics

  44. Subjets • PYTHIA & HERWIG: ok • Sensitive to differences • between q and g-initiated • jets • (<nsubjet> larger for g jets) • <nsubjet> increases with hjet • consistent with an increase • of gluon jets as hjet increases • (ggqqbar dominates -ve hjet, • qggp qg dominates +ve hjet) HERA physics

  45. Beauty Photoproduction Cross section measured tagging dijet events containing a lepton (ZEUS & H1: m, ZEUS: e¯ ) and fitting the pTrel spectra Muon channel: -1.75 < h < 1.3 1.4 < h < 2.4 HERA physics

  46. Beauty Photoproduction Electron channel - e-ident. by dE/dx: 24 % beauty 76 % charm + l.f. Larger than NLO predictions HERA physics

  47. Inclusive jets at large Q2 • 42.5 pb-1 used (1995/97 data) • kT-cluster algorithm in the LAB frame • Q2 > 125 GeV2, ETjet >14 GeV and -1 < hjet < 2 No evidence for deviations from the Standard Model predictions HERA physics

  48. as measurement at HERA Q2 > 470 GeV2 ET1 > ET1 > 5 GeV ET1 + ET1 > 17 GeV Extracted from Data corrected for detector, hadronisation, QED radiation and electroweak effects HERA physics

  49. as from dijets at high Q2 ZEUS preliminary: World average (S. Bethke, 1998): as(MZ) = 0.119 ± 0.004 Very competitive measurement! Final aim: combined fit with F2, jets and charm data HERA physics

  50. Leading Baryons HERA physics

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