1 / 56

Recent results from ep -scattering at HERA

Recent results from ep -scattering at HERA. Universit ä t Siegen Physikalisches Kolloquium 8 Jun 2006 Yuji Yamazaki (KEK). Contents of the talk. Introduction: The ep -collider HERA and its objective Bread-and-butter physics at HERA: The structure of the proton

micah-parsons
Download Presentation

Recent results from ep -scattering at HERA

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Recent results fromep-scattering at HERA Universität SiegenPhysikalisches Kolloquium 8 Jun 2006 Yuji Yamazaki (KEK) Recent results from ep-scattering at HERA / Y. Yamazaki

  2. Contents of the talk • Introduction: The ep-collider HERA and its objective • Bread-and-butter physics at HERA: The structure of the proton • More on the quark-gluon interactions:The QCD (Quantum Chromo Dynamics) studies • The must for a high-energy collider:Electroweak force studySearch for particles beyond the Standard Model • HERA physics relevant to the LHC Recent results from ep-scattering at HERA / Y. Yamazaki

  3. HERA: The super electron-microscope for inside a proton Optical microscope Electron microscope X-ray sourcesSynchrotron radiation euglena , -rays fromisotopes bird flu virus Electron beamHeavy-ion beam Diffraction pattern of synchrotronradiation light from bio-moleculestaken at HASYLAB in DESY Electron-protonscattering HERA A modern implementation ofthe Rutherford scattering at RAL Exploring the nucleon spin:HERMES experiment at HERAwith electron beam and fixed target Recent results from ep-scattering at HERA / Y. Yamazaki

  4. How do we see the structure in practice Deep-inelastic scattering (DIS) • Longitudinal momentum fraction of the quark, x • from the scattering angle and energy • The larger the scattering angle is … • the larger momentum transfer (Q2)through the virtual photon (g*) • the finer resolutionto 1018 m Q2 scattered hadrons Colourflow:manyhadrons protonbeam e+ / ebeam scattered e Recent results from ep-scattering at HERA / Y. Yamazaki

  5. What we know about proton before HERA gluon splitsinto quarks proton diameter~ 1015 m (1fm) • Constituent of a proton is quarks and gluonsNamed as partons(= quarks + gluons) • The interaction between quarks and gluons:QCD (Quantum ChromoDynamics) early fixed target exp’ts Q ~ 1-3 GeV (101 fm) recent fixed target + muonQ ~ 1-10 GeV (102 fm) Increasing resolution (large Q2) Recent results from ep-scattering at HERA / Y. Yamazaki

  6. Quantum Chromo Dynamics (QCD) from Novelprize.org as • Describing interaction between partons • colour charges exchangedby gluons • The coupling constant becomes smaller as the energy becomes higher • Asymptotic freedomNovel prize in physics 2004 • This means: the force between quarks is strongeras they are apart • Quarks are confined High energy in the potential field meson meson r Recent results from ep-scattering at HERA / Y. Yamazaki

  7. Structure function: status 1991 Quarks emita gluon,which splitsinto quarks ... Courtesy C. Gwenlan, K. Nagano • What is the observation ? Structure function  quark density recent fixed target + muonQ ~ 1-10 GeV (102 fm) HERAQ > 1-300 GeV (< 103 fm) Increasing resolution (large Q2) Recent results from ep-scattering at HERA / Y. Yamazaki

  8. HERA: the only electron-proton collider • Circumference: 6.3 km(similar size to the Tevatron at Fermilab) • Proton beam: 920 GeV • Electron/positron beam: 27.5 GeV  centre-of-mass energyResolve structure down to 1018 m • 220 bunch operation(96ns bunch spacing) • Operated from 1992 Recent results from ep-scattering at HERA / Y. Yamazaki

  9. Hermes (fixed nucleus target exp’t) H1 ZEUS DESY and HERA in west of Hamburg, Germany Recent results from ep-scattering at HERA / Y. Yamazaki

  10. HERA performance, luminosity upgrade • HERA-I delivered 190pb-1 • Luminosity upgrade (HERA-II) • Production run from 2004 • With final focusing magnets3 specific luminosity • Achieving ~ the designInstantaneous luminosity 5.21031 cm2s1 • 370pb-1 delivered, ~250pb-1 collected • Longitudinal polarisation of e+/e–for colliding experiments Recent results from ep-scattering at HERA / Y. Yamazaki

  11. Contents of the talk • Introduction: The ep-collider HERA and its objective • Bread-and-butter physics at HERA: The structure of the proton • More on the quark-gluon interactions:The QCD (Quantum Chromo Dynamics) studies • The must for a high-energy collider:Electroweak force studySearch for particles beyond the Standard Model • HERA physics relevant to the LHC Recent results from ep-scattering at HERA / Y. Yamazaki

  12. “Structure function” F2(x, Q2): observation 2000 Deep-inelastic scattering (DIS) • F2(x) = e2 x(q(x)+q(x)) • F2 is proportional to the quark charge density q(x)at given x and Q2 • Strong increase of sea quarks towards low x x rest of the proton scattered quark “Saturated” behaviouris excluded Momentum fraction Recent results from ep-scattering at HERA / Y. Yamazaki

  13. e' e g* Higher Q2 evolution p p Number of quarks increases as we see closely • Sea quarks are dynamically produced at high-Q2 • Predicted by the DGLAP (Dokschitzer-Gribov-Lipatov-Altarelli-Parisi) evolution equation • Based on the perturbative QCD Higher Q2 You cansee gluonsindirectly Quark density decreasing at high-x with Q2 Recent results from ep-scattering at HERA / Y. Yamazaki

  14. DGLAP can predict the quark density vs Q2 • at high-x : • quarks radiate gluons • momentum of quarks goes down • providing low-x gluons • at low-x: • a gluon radiatesa qq pair • gluon also radiates gluons • driving a rapid increase of the quark+gluon density Fixed target Recent results from ep-scattering at HERA / Y. Yamazaki

  15. Parton density through the DGLAP analysis • Gluons from a DGLAP fit to the data • Indirect extraction • Steep increase of gluons; gluon dominates at low-x • Precise determination at HERA BeforeHERA Recent results from ep-scattering at HERA / Y. Yamazaki

  16. Contents of the talk • Introduction: The ep-collider HERA and its objective • Bread-and-butter physics at HERA: The structure of the proton • More on the quark-gluon interactions:The QCD (Quantum Chromo Dynamics) studies • The must for a high-energy collider:Electroweak force studySearch for particles beyond the Standard Model Recent results from ep-scattering at HERA / Y. Yamazaki

  17. Measuring gluons more “directly” • Photon does not couple to gluons directly ... • hit a gluon by a quark instead • Look at a high-pT qq pair production (jets)and heavy-quark pair (QQ) production Jet 1 Scattered e Jet 2 Recent results from ep-scattering at HERA / Y. Yamazaki

  18. as parton-electroncross section parton density:mainly gluons The extracted gluon is universal • The extracted gluon + perturbative QCD calculation:agrees with the measurement QCD calculationusing the partondensity extractedat HERA Recent results from ep-scattering at HERA / Y. Yamazaki

  19. Heavy quark production: for understanding gluons • Heavy quark production is reproduced by HERA parton densities • Gluon density is double-checked as Recent results from ep-scattering at HERA / Y. Yamazaki

  20. DGLAP evolution as as as as The strong coupling constant as a fundamental parameter in the standard model • Least well-determined qty in the standard model • Better measurement at high energy: theoretical uncertainties are smaller • “See” parton’s behaviour by • Using jets/HQs • From the DGLAP analysis etc. as Recent results from ep-scattering at HERA / Y. Yamazaki

  21. HERA measurement of as Some newest measurements not included in this summary • as is running in single experiment • Some of them give best precision among collider measurements(LEP, Tevatron and HERA) FromDGLAP Recent results from ep-scattering at HERA / Y. Yamazaki

  22. A small detour, back to the proton structure • Introduction: The ep-collider HERA and its objective • Bread-and-butter physics at HERA: The structure of the proton at high-x • More on the quark-gluon interactions:The QCD (Quantum Chromo Dynamics) studies • The must for a high-energy collider:Electroweak force studySearch for particles beyond the Standard Model • HERA physics relevant to the LHC Recent results from ep-scattering at HERA / Y. Yamazaki

  23. Disentangling valence quarks u s s u d • Valence quarks: carrying the quantum number of the proton • Fermion  3 quarks • Charge +1 (Isospin +½)  (uud) • Sea quarks: • Flavour decomposition by charged current (CC) • up-type quarks with e • down-type quarks with e+ • different scattering angle distribution for q and q Recent results from ep-scattering at HERA / Y. Yamazaki

  24. Example of the CC measurement • Example: e+p CC • produced uniformly in angle • d, s : neutrino produced only in small scattering angle • Statistical error is largeHERA-II analysis ongoing Recent results from ep-scattering at HERA / Y. Yamazaki

  25. Neutral current (NC) with e+ and e beam:disentangling valence and sea • Difference of e and e+ scattering gives amount of valence quarks(e)(e+)  • Precision improved with large HERA-II e sample (130pb1) • 16pb1 for HERA-I From Z0 and Z0-g interference xF3gZ(x)  valence (e)(e+)  Recent results from ep-scattering at HERA / Y. Yamazaki

  26. Valence quark from NC (cont’d) fractional uncertainty (e)(e+)  Now ~ 100pb1 for e+e each, will be 200pb1 each Valence quark uncertainty reduced,in particular for u-quark Recent results from ep-scattering at HERA / Y. Yamazaki

  27. Contents of the talk • Introduction: The ep-collider HERA and its objective • Bread-and-butter physics at HERA: The structure of the proton • More on the quark-gluon interactions:The QCD (Quantum Chromo Dynamics) studies • The must for a high-energy collider:Electroweak force studySearch for particles beyond the Standard Model • HERA physics relevant to the LHC Recent results from ep-scattering at HERA / Y. Yamazaki

  28. Unification of the electromagnetic and weak force NC CC • Weak force appears weak • because of the heavy mass of the weak boson • exchanged through uncertainty principle at low energy • At high energy, the- neutral (g, Z0) and- charged (W) currenthave similar cross sections NC CC HERA-I result Recent results from ep-scattering at HERA / Y. Yamazaki

  29. Polarisation dependence of the charged current cross sections • HERA-II: polarisation of electron for colliding exp’ts • Right-handed current is forbidden in SM • V-A: vector and axial vector coupling cancels • Linear dependence on polarisation More left-handed More right-handed MW(Right-handed) > 180-190 GeV in 95% CL Recent results from ep-scattering at HERA / Y. Yamazaki

  30. Determination of the electroweak coupling constant to Z0 for u,d quarks (au,d, vu,d) • Standard model prediction to W:aq = vq = 1/2 • to Z0: • aq = 1/2 (u,c,t)1/2 (d,s,b) • vq = 1/2  2·(2/3)sin2w  0.19 (u,c,t)1/2 2·(1/3)sin2w  0.34 (d,s,b) • LEP measured the (aq, vq) for heavy flavour precisely, but • flavour decomposition is difficult for light quarks • Light quarks (u, d): HERA knows the quark density well • Best measurement in the world • vu improved thanks to the polarisation Recent results from ep-scattering at HERA / Y. Yamazaki

  31. Searching for physics beyond Standard Model • LEP/Tevatron • Exotic particles are often from pair creation througha fusion • Can produce (almost) any particles • Initial state quantum number vanishes • HERA • Initial state has both e and quarksThey should not vanish • Sensitive to flavour violation (flavour-changing neutral current: FCNC) • Best limit in many LFV combination • competition: B-factory, atomic parity v. LEP Tevatron LFV(Lepton-flavourviolation) Quark-flavourviolation Recent results from ep-scattering at HERA / Y. Yamazaki

  32. Example from flavour violation: single-top production • Best or complementary limit from HERA vector coupling magnetic coupling Recent results from ep-scattering at HERA / Y. Yamazaki

  33. High-pT lepton + missing pT excess at H1To be or not to be? • The story started when … • H1 found an event with a high-pT muon in the final state • It looked like an event with FCNC • More events found, revealing: • With missing pT and missing longitudinal momentum not FCNC events • Excess pronounced if we enrich W-production A ZEUS event with muon muon Recent results from ep-scattering at HERA / Y. Yamazaki

  34. p W production at HERA positrons • W produced through gqWq • Leptonic decay Wln • Signal: • Lepton, missing pT (n) and jet (q) • SM prediction: jet pT (pTX) tend to be small • H1: excess at large pTX:only for e+p collisionsnot for ep collisions electrons Recent results from ep-scattering at HERA / Y. Yamazaki

  35. H1 result and ZEUS? • The H1 excess on positron beam (e+m decays) is 3.4s • ZEUS consistent with SM except fort decay (2 events) • H1 t decay with large excess too, but all from the electron beam data • H1 claims the excess is not on electron beam data ZEUS result * Calculated by me My personal conclusion: • SM prediction is slightly wrongTherefore, we see excess here and there > 50pb-1 more e data to be analysed >100pb-1 more e+ data by the end of HERA Recent results from ep-scattering at HERA / Y. Yamazaki

  36. Contents of the talk • Introduction: The ep-collider HERA and its objective • Bread-and-butter physics at HERA: The structure of the proton • More on the quark-gluon interactions:The QCD (Quantum Chromo Dynamics) studies • The must for a high-energy collider:Electroweak force studySearch for particles beyond the Standard Model • HERA physics relevant to the LHC Recent results from ep-scattering at HERA / Y. Yamazaki

  37. From HERA to the LHC • HERA is ep, the LHC is pp: all the physics are relevant • Advantage over Tevatron: the study “under control” by e beam • “HERA and the LHC” workshop • 2004-2005 meeting successful • Continue to have one meeting per year (6-9 June 2006) • In particular: • Parton densities – the luminosity function for the LHC • HQ production – the (2nd) discovery channel for Higgs/SUSY • Multijet final states and energy flow – need 8 jet cross sections for the LHC ? • Diffraction • Today: multi-parton phenomena Recent results from ep-scattering at HERA / Y. Yamazaki

  38. Parton luminosity in hadron colliders • Cross section prediction in hadron colliders:Factorisation formula • Assuming only one parton from each hadron • But: parton density at low-x is very large • At x ~ 104: number of partons is  O(102-3) • Cross section at 10 GeV in parton-parton CMS comparable to the total proton-proton cross section  2 interactions can occur at the same time • Consequence: factorisation formula cannot predict cross section any more Recent results from ep-scattering at HERA / Y. Yamazaki

  39. Multi-parton processes A jet overlapped:no longer an isolated lepton.Events rejected … • 1-parton: “normal” • 2-partons • Multi-parton collisions • Colourless exchange • 2-gluons can formulate colour neutral state (diffraction) • 3-partons • Colourless state destroyed by another coloured collision h  l+l!!! LargeRapidityGap LargeRapidityGap destroyed Recent results from ep-scattering at HERA / Y. Yamazaki

  40. Example of factorisation breaking (1):Suppression of diffractive events in pp destroyed • CDF dijet rate is factor 3-10 lower than the prediction using the HERA event rate • Rapidity gap seems destroyed • Various models developed to explain the “gap survival probability” • Progress in understanding the underlying events Large Rapidity Gap Dijet cross section vsmomentum fraction of the partonin “pomeron”, the colourless object Recent results from ep-scattering at HERA / Y. Yamazaki

  41. Example of factorisation breaking (2):multi-parton interaction (MPI) in photoproduction • Ideal testing ground • Direct (xg ~ 1): point-like • Resolved (xg < 1): hadron-like • 4-jets enhanced in resolved • MPI can explain, but • Large uncertaintyMeasurements at HERA and the Tevatron are important MC predictionsassuming factorisation MPImodel1 MPImodel2 xg = 1 xg < 1 additionaljets resolved direct Recent results from ep-scattering at HERA / Y. Yamazaki

  42. HERA is not yet over. Prime time now! More optimistic • HERA-II running until 2007 summer • Already ~250pb-1/exp’t recorded in 2004-2006 summer • Further > 150pb-1/exp’t. • Focussing on low-stat studies • High-x PDFs • Heavy flavours • Electroweak studies • Search for exotics • Low-energy running • for longitudinal structure function FL Most pessimistic End of HERA30 June 2007 Recent results from ep-scattering at HERA / Y. Yamazaki

  43. Summary • HERA has measured low-x proton structureHigh-x is to be measured • HERA has tested QCD, and will continue • HERA does EW studies and BSM searchesCurrently the main subjects • HERA measurements are relevant to the next generation colliders, the LHC Recent results from ep-scattering at HERA / Y. Yamazaki

  44. Backup slides hereafter Recent results from ep-scattering at HERA / Y. Yamazaki

  45. HERA tunnel • 2-story ringUpper: proton, lower: electron Recent results from ep-scattering at HERA / Y. Yamazaki

  46. So, what have we learned ? • In 1992 we planned to learn • Proton structure • Low-x parton densities • Jet production • Electroweak physics • Search for physics beyond Standard Model Recent results from ep-scattering at HERA / Y. Yamazaki

  47. Yes, we have learned more than expected! • In 2005 we have learned much more • Proton structure  success of the DGLAP equation • Low-x parton densities  very precise determination • Diffractive physics  “forgotten” • Jet production and QCD, s very much in detail • Heavy flavour production  became a major subject • Electroweak physics • Search for physics beyond Standard Model • Promoted the QCD theory very much • Having a high energy collider, we find what we expect.But also something unexpected Recent results from ep-scattering at HERA / Y. Yamazaki

  48. Event with a large-rapidity gap (LRG) Scattered quark (with a colour charge) • DIS: many hadrons between proton and the scattered quark • Events with a large gap found in DIS • The exchanged object is colourless • Called diffractive scattering Colourflow:manyhadrons protonbeam scattered e No hadrons(LRG)No colour flow Photon diffractedinto hadrons Colourlessexchange scattered e Proton stays intact Recent results from ep-scattering at HERA / Y. Yamazaki

  49. Explaining diffraction by quarks+gluons • The colourless exchanged state is… • Historically explained by a hypothetical particle “Pomeron” • Like a meson (, …) but has a quantum number of vacuum • Introduced at 60’s to explain the behaviour of hadron-hadron scattering • Modern trial: using perturbative QCD • 2-quarks  mesons: NG • 2-gluons is colourlessRemember: many gluons in a proton! • Internal structure of the “Pomeron” by the super-microscope HERA See Pomeronby microscope Colourless“Pomeron” Colourless2-gluon Recent results from ep-scattering at HERA / Y. Yamazaki

  50. Experimental result on diffraction • Gluon fraction in the “Pomeron” is determined • Quark/gluon densities are extracted through DGLAP • 60-90 % gluons • 2-gluon is a good approximation Recent results from ep-scattering at HERA / Y. Yamazaki

More Related