Physics and Physics prospects at HERA

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1 Physics and Physics prospects at HERA 58 th Extended Scientific Council / 130 th Scientific Council 1/ June 004 Yuji Yamazaki (KEK, ZEUS) On behalf of the H1, ZEUS, HERMES and HERA-B collaborations

2 The Super Electron Microscope HERA Explore the structure of the nucleon by electron beam Deep-inelastic scattering (DIS) Scattering angle and energy of electron momentum distribution of the quarks High energy = short wave length Scattering angle ~ Q : e mass of the exchanged particle Centre-of-mass energy 314 GeV up to m resolution Q ~ GeV p 1 Jun 004 Physics and physics prospects at HERA, 58th EWR / 130th WR

3 HERA-I and HERA-II running HERA-I: , Integrated Luminosity ~ 100pb 1 HERA-II: , Int. Lumi > 700 pb -1, with lepton longitudinally polarised Startup problem as previously reported Synchrotron radiation to detector Proton-beam and residual beamgas collisions producing Standing current in chambers Load to trigger / DAQ All of them are solved! Oct 03 May Jun 004 Physics and physics prospects at HERA, 58th EWR / 130th WR 3

4 HERA-II running ( CJC - I 0 - SynRad of Dec 003 ) / ( e-gas + p-gas of Feb 004) leak NL 11m for 5000 ma days from Oct Jun 004 H1 takes 4 pb -1 in 004 cf. HERA-I 000: 51 pb -1 (8 months) ZEUS slightly behind, following up HERA II has begun I e I p < 500 ma I e I p < 1000 ma I e I p < 000 ma I e I p < 4000 ma I e I p > 4000 ma recent runs 1 Jun 004 Physics and physics prospects at HERA, 58th EWR / 130th WR 4

5 HERA physics: Questions to be answered Structure of the nucleons partons How is it composed by quarks and gluons? Where the spin of the nucleon (½) come from? HERMES The nature of the interactions Strong interactions quantum chromo-dynamics (QCD) Force between quarks, mediated by gluons Electroweak interactions The physics beyond standard model Is the quark a fundamental particle? New interactions between leptons, quarks and gluons 1 Jun 004 Physics and physics prospects at HERA, 58th EWR / 130th WR 5

6 Structure of the proton simple view gluon splits into quarks Quark splits into gluon splits into quarks... Increasing resolution (large angle scattering = large Q ) More resolution: we see small partons How does the quarks behave at low-x? 1 Jun 004 Physics and physics prospects at HERA, 58th EWR / 130th WR 6

7 Structure function quark density Strong increase of sea quarks towards low x F (x) = e x(q(x)+q(x)) quark density Density increase with Q More partons by magnified view Dynamic creation of quarks at low-x 1 Jun 004 Physics and physics prospects at HERA, 58th EWR / 130th WR 7

8 Evolution of the quark densities Precise measurements in wide range of kinematics except for high-x Scaling violation in Q described well by DGLAP evolution equation (blue bands) F i Dynamic production of quarks and gluons 10-1 understood by perturbative QCD 10 - Extraction of gluon density from F data x = , i = 1 x = , i = 0 x = , i = 19 H1 PDF 000 extrapolation x = , i = 18 x = , i = 17 x = , i = 16 x = , i = 15 x = , i = 14 small x x = 0.000, i = 13 x = 0.003, i = 1 x = , i = 11 x = , i = 10 x = 0.013, i = 9 x = 0.00, i = 8 x = 0.03, i = 7 H1 e + p x = 0.050, i = 6 ZEUS e + p BCDMS x = 0.080, i = 5 x = 0.13, i = 4 x = 0.18, i = 3 x = 0.5, i = x = 0.40, i = 1 x = 0.65, i = 0 1 Jun 004 Physics and physics prospects at 1HERA, 58th 10 EWR / 130th 10 WR NMC large x Q / GeV H1 Collaboration

9 Extraction of parton densities in the proton Gluon density obtained from the QCD fit Precise determination of low-x parton densities Gluon > quark at low-x Still need precision: High-x partons: constrained by fixed target data Low-x: we can do better with more statistics homework for HERA-II sea (x 0.05) gluon (x 0.05) u valence d 1 Jun 004 Physics and physics prospects at HERA, 58th EWR / 130th WR 9

10 Probing gluons by hadronic final state Photon does not couple to gluons directly... hit a gluon by a quark instead At high energies, quark and gluon momentum can be reconstructed as jets Jet 1 Scattered e Jet 1 Jun 004 Physics and physics prospects at HERA, 58th EWR / 130th WR 10

11 Gluon density from jets Jet cross section well described Gluon density from DGLAP is confirmed Jun 004 Physics and physics prospects at HERA, 58th EWR / 130th WR 11 (pb/gev) B T,jet dσ/de ZEUS 10 7 This means: we can predict cross section using the parton density (universality!) e.g. cross section for LHC gluon density from DGLAP ZEUS (DIS data) Jet Energy Scale Uncertainty NLO QCD δ had δ Z 0: ZEUS-JETS (prel.) total uncertainty Dynamically produced partons are quite well understood 15 < Q 50 < Q 500 < Q B E T,jet < 50 GeV 5 ( 10 ) < 500 GeV 4 ( 10 ) < 1000 GeV 3 ( 10 ) 1000 < Q < 000 GeV ( 10 ) 000 < Q < 5000 GeV ( 10) Q > 5000 GeV ( 1) (GeV)

12 Particle production near the proton What is happening here? dσ π /dx [nb] < Q < 4.5 GeV H1 data DIR + RES CCFM (CASCADE) DIR CDM mod. LO BFKL 4.5 < Q < 15 GeV Large number of particles produced between the struck quark and the proton remnant No unique method to describe multi-body decay Large excess over DGLAP framework Many idea coming out for explaining the ordering of particles 1 Jun 004 Physics and physics prospects at HERA, 58th EWR / 130th WR p * T,π >.5 GeV < Q < 70 GeV x

13 Gluon density and heavy flavour production Heavy flavour: only produced as a pair from a photon-gluon fusion Data tend to be above NLO at low p T (transv. momentum) Important to understand the production mechanism e.g. standard model background to Higgs at LHC Statistics limited Continue towards HERA-II Data/Theory 4 Beauty Production ep ebb X ejjµx H1 (Prel.) ZEUS NLO QCD Had < η µ < < η µ <.3 Q < 1 GeV ; 0. < y < 0.8 p jet1() t > 7(6) GeV; η jet < p µ t [GeV] 1 Jun 004 Physics and physics prospects at HERA, 58th EWR / 130th WR 13

14 b-quark production at HERA-B Experiment: wire-target to the proton beam b-quark production cross section in pp collisions is also uncertain Large difference in the past experiments Reducing uncertainty by new HERA-B data 5x data being analysed σ( bb ) (nb/nucleon) Preliminary Hera-B 004 E789 N. Kidonakis E771 R. Bonciani et al. et al Proton energy (GeV) 1 Jun 004 Physics and physics prospects at HERA, 58th EWR / 130th WR 14

15 Measuring strong coupling constant α s Measurement of probability to radiate a parton But parton cannot be observed directly worst precision among four forces -jet probability etc. sensitive to α s strong coupling const. 1 Jun 004 Physics and physics prospects at HERA, 58th EWR / 130th WR 15

16 α s HERA measurements of α s Running in single experiment Precision competitive, thanks to the high energy of HERA Theoretical uncertainty from hadronisation is small ZEUS (8 pb -1 ) (inclusive jet γp - µ=e T jet ) ZEUS (38 pb -1 ) (dijet DIS - µ=q) ZEUS (39 pb -1 ) (inclusive jet DIS - µ=e T jet ) H1 (33 pb -1 ) (inclusive jet DIS - µ=e T jet ) Bethke 00 th. uncert. exp. uncert. Inclusive jet cross sections in γp ZEUS (Phys Lett B 560 (003) 7) Subjet multiplicity in CC DIS ZEUS (Eur Phys Jour C 31 (003) 149) Subjet multiplicity in NC DIS ZEUS (Phys Lett B 558 (003) 41) Jet shapes in NC DIS ZEUS (DESY hep-ex/ ) NLO QCD fit H1 (Eur Phys J C 1 (001) 33) NLO QCD fit ZEUS (Phys Rev D 67 (003) 01007) Inclusive jet cross sections in NC DIS H1 (Eur Phys J C 19 (001) 89) Inclusive jet cross sections in NC DIS ZEUS (Phys Lett B 547 (00) 164) Dijet cross sections in NC DIS ZEUS (Phys Lett B 507 (001) 70) World average (S. Bethke, hep-ex/01101) µ (GeV) α s (M Z ) 1 Jun 004 Physics and physics prospects at HERA, 58th EWR / 130th WR 16

17 HERA-I study of proton structure and QCD (collider experiments) HERA-I study have unveiled: The partonic structure of nucleons at low-x Rapid increase of quarks and gluons towards low-x The QCD description of the hadronic final state gluon density checked by jets precise α s determination Some subjects not completely understood : Forward production, heavy flavour need more study with HERA-II Most of the subjects at HERA cannot be covered here e.g. diffraction (proton stays intact) 1 Jun 004 Physics and physics prospects at HERA, 58th EWR / 130th WR 17

18 HERA-LHC workshop Synergy between running and future experiments Subjects: Parton densities at LHC Multi-jets, energy flow Heavy quarks Diffraction Monte-Carlo simulation March 004: 300 persons gathered Final report Jan 005 Light Higgs? 1 Jun 004 Physics and physics prospects at HERA, 58th EWR / 130th WR 18

19 Nucleon spin by HERMES motivation Where the ½ spin of the nucleon comes from? Naive expectation: u u d but measurement : only 1 % from valence + sea quarks (EMC, 1988) From gluon? 1 = 1 From angular momentum of the quarks / gluons? ( Σ + G + + ) L q L g 1 Jun 004 Physics and physics prospects at HERA, 58th EWR / 130th WR 19

20 How to measure the proton spin contributions Longitudinally polarised nucleon (gas target) and electron beam Observable: cross section asymmetry in spin parallel and antiparallel Photon can scatter with quarks with spin anti-parallel photon spin absorbed by quark spin flip 1 Jun 004 Physics and physics prospects at HERA, 58th EWR / 130th WR 0

21 Quark spin from polarised structure function g 1 probability to find a quark with spin positive Precise measurement by HERMES Flavour decomposition from p, n and deuteron target F g 1 1 q ( x) + q ( x) q ( x) q ( x) u :, d : polarised From strange quark, sea quark? xg proton HERMES HERMES prel. rev. SMC E155 E143 deuteron HERMES 00 prel. udd E14 E154 JLAB uud uud+udd neutron ( 3 He) x 1 Jun 004 Physics and physics prospects at HERA, 58th EWR / 130th WR 1

22 Flavour decomposition by semi-inclusive DIS e N (E, p) (E, p ) γ * u Tag the quark ud π + flavour by fastest hadrons Sea quark spin is extracted without any assumption on the flavour symmetry Sea quark spin slightly negative Valence is ~30 % where is the spin? q u d h h π 1 Jun 004 Physics and physics prospects at HERA, 58th EWR / 130th WR x u x d x u x d x s Q =.5 GeV GRSV000 LO std BB01 LO x

23 Extracted gluon polarisation from world data SMC EMC E155 E143 HERMES Q, GeV x Using spin-dependent QCD evolution of the parton densities Gluon, seems positive, but large uncertainty (small lever arm) Current decomposition: Valence+sea 1 ± 13 %, gluon 41 ± 7 % Where is the rest? 1 Jun 004 Physics and physics prospects at HERA, 58th EWR / 130th WR 3

24 Now at HERMES: transversity h 1 (x) Unmeasured polarised structure function world premier through azimuthal asymmetry of the produced hadrons: semi-inclusive DIS beam If quark is massless, h 1 ~ g 1 Quark mass effect to spin Valence/sea separation: h1 ~ x( δq δq) p p Expect A ~ A (both mainly from u) First result: largely negative: unexpected Disfavoured fragmentation u-quark π? Run until summer 005 p A π π + π Also addressing the angular momentum effect by measuring Sivers angle then measuring DVCS A C π π + π - π x P π /(zm π ) weighted z 1 Jun 004 Physics and physics prospects at HERA, 58th EWR / 130th WR 4

25 HERA-II physics : objectives Unique facility worldwide: ep collision with e ± polarisation High luminosity for colliders Nature of the valence quarks in the proton density, flavour high-x, high-q cross sections are small QCD study requiring large amount of data b-quark production, jets at high-p T Diffraction with jets, heavy flavour Polarisation also for the collider exp t H1 and ZEUS Electroweak interaction Flavour decomposition of the quarks Searches: physics beyond standard model yearly monthly HERMES: deeply virtual Compton scattering (also H1/ZEUS) Next slides: Reviewing HERA-I status, prospect for HERA-II 1 Jun 004 Physics and physics prospects at HERA, 58th EWR / 130th WR 5

26 Disentangling quark flavours by weak int n Charged current (CC) exchanging W ± Coupling to: up-type quarks for e u c t down-type quarks for e + d s b spin polarisation: (1±Pz) dependence High-Q neutral current (NC) exchanging Z 0 ( Q M 0 ) Cross section difference σ ( e ) σ ( e + ) is sensitive to q q Spin polarisation gives similar effect Z 1 Jun 004 Physics and physics prospects at HERA, 58th EWR / 130th WR 6

27 Charged current and parton densities HERA e + p Charged Current Example : full HERA-I data(100pb -1 ), e + beam Different dependence in y decompose ( d + s ) and ( u + c ) distinguishing flavour Sensitive to d Precision limited by statistics Increasing statistics is main goal for HERA-II σ H1 e + p ZEUS e + p Q = 80 GeV SM e + p (CTEQ6D) (1-y) x (d+s) x (u _ +c _ ) Q = 530 GeV Q = 950 GeV Q = 1700 GeV Q = 3000 GeV Q = 5300 GeV Q = 9500 GeV Q = GeV x 1 Jun 004 Physics and physics prospects at HERA, 58th EWR / 130th WR 7

28 Precise measurement of the valence quark density at HERA-II Very high-x valence density is not well known (d + u) valence quark by NC d valence by e + CC xf 3 σ(e + ) σ(e ) q q NC simulation for HERA-II = SLAC = BCDMS = NMC 500 pb -1 γz xf 3 (x) xf 3 Simulation 0.6 GRV94 CTEQ6.1 MRST03 H1 PDF000 data pb 1 e ± pb 1 e ± Q =1500 GeV x 1 Jun 004 Physics and physics prospects at HERA, 58th EWR / 130th WR 8

29 NC/CC cross section and electroweak unification Cross section of NC and CC becomes similar at Q M W, M Z Manifestation of the electroweak unification dσ/dq (pb/gev ) HERA H1 e + p NC H1 e - p NC ZEUS e + p NC ZEUS e - p NC SM e + p NC (CTEQ6D) SM e - p NC (CTEQ6D) Study at HERA-II electroweak parameters in detail with polarisation e.g. Electroweak coupling constants to u- and d-quarks (v u, a u ) and (v d, a d ) H1 e + p CC H1 e - p CC ZEUS e + p CC ZEUS e - p CC SM e + p CC (CTEQ6D) SM e - p CC (CTEQ6D) y < Q (GeV ) 1 Jun 004 Physics and physics prospects at HERA, 58th EWR / 130th WR 9

30 Physics with polarised positron beam: charged current from HERA-II σ CC (pb) 50 H1 e + p ν X 40 ZEUS MRST Prelim. (L=6.6pb -1 ) 30 Prelim. (L=15.3pb -1 ) First measurement of CC with polarisation If right-handed current exists, the cross section at 100 % RH polarisation is non-zero Consistent with no righthanded current so far Q >400 GeV y< P e Data with opposite pol. being collected 1 Jun 004 Physics and physics prospects at HERA, 58th EWR / 130th WR 30

31 Size of the quark ZEUS point-like quark large quark N/N CTEQ5D ZEUS e ± p R q = ( cm) R q = -( cm) 1 Quark Radius Limits Q (GeV ) Finite size of the quark cross section becomes smaller at high-q Short wave cannot see a blur object quark becomes transparent Expectation with finite quark size Cross section agrees with SM: quark is point-like down to radius R q < m (< 1/1000 of proton H1 and ZEUS) 1 Jun 004 Physics and physics prospects at HERA, 58th EWR / 130th WR 31

32 Search for new particles and interactions Only Tevatron ( TeV pp) and HERA are the high-energy colliders in operation Giving world best limits on: Leptoquark (next slide) Flavour-changing neutral current Lepton-flavour violation Observed in neutrino sector! Quark flavour violation Excited leptons e*, ν* Super-symmetric particles Extra-dimension etc. 1 Jun 004 Physics and physics prospects at HERA, 58th EWR / 130th WR 3 e Excited leptons p Leptoquark e * γ / Z Lepton Flavor Violation e, ν p, x γ, Z, W

33 Leptoquark: quark-lepton resonance jet Models from grand-unified theory, super-symmetry etc. Searching for a resonance in the eq = e+jet mass spectrum In large region of parameters HERA gives best limit Other region: complementary to Tevatron and LEP E. Perez, Lepton-Photon 03 1 Jun 004 Physics and physics prospects at HERA, 58th EWR / 130th WR 33

34 High-p T lepton + missing p T excess? e + p! e + ; X Event MUON-3 T =39GeVPX T =7GeVPmiss T P =4GeV M =8GeV W ;! ; Candidate SM: through W production Not abundant at large p T X H1 1 Jun 004 Physics and physics prospects at HERA, 58th EWR / 130th WR 34

35 High-p T lepton + missing p T excess? SM: through W production Not abundant at large p T X 6 events observed/1.08 expected for e + µ decays in H1 No excess in e + µ decays in ZEUS Less significant: 1 / 0.06 expected for τ channel in ZEUS New particle decaying to a neutral + lepton?? 1 Jun 004 Physics and physics prospects at HERA, 58th EWR / 130th WR 35

36 High-p T leptons in HERA-II H1 analysis on highp T leptons with HERA-II data For p X T > 5 GeV events / 0.63 expected agrees with the HERA-I yield More luminosity eagerly awaited Large p TX events 1 Jun 004 Physics and physics prospects at HERA, 58th EWR / 130th WR 36

37 Deeply virtual Compton scattering (DVCS) Process to take out a quark with (x 1, k T1 ) then put back with (x, k T ) e γ * Q e x1 x γ Tag scattered proton Correlation of two partons in a nucleon p p You can rotate a proton sensitive to the angular momentum of the quark Amplitude through the interference with normal Compton events Phase is also known 3D reconstruction of proton structure Changing interference pattern by e + /e beam, L or R polarisation V. Belitsky, D. Müller, hep-ph/ Jun 004 Physics and physics prospects at HERA, 58th EWR / 130th WR 37

38 DVCS measurement plan 004~ e Q γ e γ x 1 x H1, ZEUS studies: low-x, gluon sector Reconstructing k T1 and k T : p Recoil proton angle+momentum measured by new recoil detector at HERMES (summer 005) H1 very forward proton detector (in operation from spring 004) p A ch (φ) e ± p e ± p γ φ assymetry by e+/e- beam φ, rad 1 Jun 004 Physics and physics prospects at HERA, 58th EWR / 130th WR 38

39 Summary HERA-I study being complete, understanding The partonic structure of nucleons at low-x Nucleon spin carried by quarks The QCD description of the hadronic final state but some needs more data for better understanding HERA-II : unique machine world wide with e ±, pol n Proton structure at high-x, high-q Electroweak and maybe surprise The issue is the integrated luminosity for precision We need planned increase of order of magnitude in luminosity w.r.t. HERA-I 1 Jun 004 Physics and physics prospects at HERA, 58th EWR / 130th WR 39