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HERA Collider Physics - Introduction

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HERA Collider Physics - Introduction HERA Collider physics - introduction Workshop on EW and BSM physics at the EIC May 2020 Results from H1 and ZEUS Stefan Schmitt Outline ● Introduction Next talk: E. Gallo about searches and electroweak – The HERA collider and collider experiments physics at HERA – HERA kinematics ● Selected physics results in perturbative QCD Disclaimer: – Inclusive cross sections, structure functions, PDFs – Heavy flavor production this talk is on HERA result, but reflects my personal opinions only. There is a slight preference in showing results from H1 rather than ZEUS, simply because I know the H1 results better. Please apologize for that. CFNS workshop, May 2020 S.Schmitt, HERA introduction 2 The HERA collider ● Operated from 1992 to 2007 ● Circumference 6.3 km Integrated luminosity: ● Electrons or positrons colliding with protons about 500 pb−1 per collider experiment ● Proton: 460-920 GeV, Leptons 27.6 GeV ● Peak luminosity ~7×1031 cm-2s-1 Two collider experiments: ● Lepton beam polarisation up to 40-60% (Sokolov- H1 and ZEUS Ternov effect, rise-time ~30 minutes) HERA-b & HERMES Fixed-target, not covered in this talk Straight section Curved section CFNS workshop, May 2020 S.Schmitt, HERA introduction 3 HERA compared to other colliders ● HERA at construction time: energy frontier (E ~Tevatron, E ~ ½ LEP) Centre-of mass energy p e Detectors were designed for discoveries, not so much for precision ● EIC compared to HERA: – Reduced center-of-mass energy ×0.3 – Much higher luminosity ×100 Luminosity – Better lepton polarisation – Target polarisation – Heavy targets – Much improved detectors: tracking, acceptance, particle identification, forward detectors, ... CFNS workshop, May 2020 S.Schmitt, HERA introduction 4 The HERA publication harvest Status: Feb 2020 H1+ZEUS combined 8 publication ● Top-ten cited (excluding detector papers) JHEP 1001 (2010) 109 H1+ZEUS 1000+ Data combination, PDF Eur.Phys.J. C21 (2001) 33 H1 700+ Low-x, PDF, alpha_s H1 223 publications Nucl.Phys. B470 (1996) 3 H1 500+ Low-x, PDF Eur.Phys.J. C21 (2001) 443 ZEUS 500+ Low-x, PDF Phys.Lett. B315 (1993) 481 ZEUS 500+ Observation of diffraction ZEUS 250 publications Nucl.Phys. B407 (1993) 515 H1 400+ Rise of F2 at low-x Eur.Phys.J. C75 (2015) 580 H1+ZEUS 400+ Data combination, Low-x, PDF Phys.Lett. B316 (1993) 412 ZEUS 400+ Rise of F2 at low-x Z.Phys. C76 (1997) 613 H1 400+ Difffractive PDF ● Both collaborations are still active Z.Phys. C74 (1997) 207 ZEUS 400+ High Q² DIS and open for new members ● 500+ citations: proton at low-x and PDFs ● 250+ citations: total cross-section, diffractive PDF, ● Data are available for analysis at diffractive vector mesons, pentaquark DESY, including computing ● 200+ citations: charm, jets, DVCS infrastructure (batch system) HERA data are used mainly to study: PDFs and perturbative QCD, low-x and diffraction, transition from soft to hard QCD CFNS workshop, May 2020 S.Schmitt, HERA introduction 5 The HERA detectors H1 and ZEUS Liquid Argon calorimeter σ =0.5/√E, σ =0.11/√E, -1.5<η<3.4 Muon chambers had EM Lead+fiber in backward (electron) direction 4π em+had. [SpaCal] σ =0.07/√E, -4<η<1.4 calorimeters EM Drift-chambers as main tracking devices + silicon vertex detectors electron beam proton beam Asymmetric detectors Uranium-scintillator calorimeter Centre-of-mass system is σ =0.35/√E, σ =0.18/√E, -3.5<η<4 had EM boosted to proton-direction Ee=27.6 GeV, Ep=920 GeV CFNS workshop, May 2020 S.Schmitt, HERA introduction 6 HERA boost visualized “typical” DIS event in H1 detector Proton beam limits acceptance in target region E =27.6 GeV E =920 GeV E =54000 GeV e p Ee=160 GeV Ep=160 GeV e ep center-of-mass system Proton rest-frame Laboratory system c.f. fixed-target experiment CFNS workshop, May 2020 S.Schmitt, HERA introduction 7 Processes studied at the HERA collider Scattered electron ● Neutral Current DIS (Deep Inelastic Scattering) – electron in main detector ● Charged current DIS Hadrons collimated in a jet – neutrino with high transverse Side view Transverse view momentum (escapes detection) Large electron scattering angle (rare event) ● Photoproduction Small electron scattering angle (frequent event) – Electron scattered at very low Hadrons are spread all over angle (dedicated low-angle detector or not detected) Electron beam Proton beam Scattered electron CFNS workshop, May 2020 S.Schmitt, HERA introduction 8 Processes studied at the HERA collider Scattered electron ● Neutral Current DIS – electron in main detector See next talk E. Gallo ● Charged current DIS Hadrons collimated in a jet – neutrino with high transverse Side view Transverse view momentum (escapes detection) Neutral current (NC) event ● Photoproduction Charged current (CC) event Imbalance in transverse plane → neutrino – Electron scattered at very low angle (dedicated low-angle detector or not detected) CFNS workshop, May 2020 S.Schmitt, HERA introduction 9 Processes studied at the HERA collider ● Neutral Current DIS – electron in main detector Scattered ● Charged current DIS electron in detector – neutrino with high transverse momentum (escapes detection) Neutral current (NC) event ● Photoproduction Photoproduction (most frequent type of event) – Electron scattered at very low angle (not detected or scattered Electron beam Scattered into dedicated low-angle tagger) electron is not detected CFNS workshop, May 2020 S.Schmitt, HERA introduction 10 Photoproduction and DIS ● Main kinematic variable: negative four-momentum squared Q²= −(e-e')² ● Q² provides a natural hard scale for Scattered perturbative calculations electron in 2 detector ● Deep-inelastic scattering (DIS): Q ≫0 – Perturbative QCD applicable Neutral current (NC) event Photoproduction (most frequent type of event) 2 ● Photoproduction: Q ∼0 – Perturbative QCD works only if there is another hard scale (jet, Electron beam Scattered electron is heavy quark, etc) not detected CFNS workshop, May 2020 S.Schmitt, HERA introduction 11 Neutral current DIS kinematics at HERA ● Kinematic variables: Q², x, y, Q²=sxy Kinematic plane Q² vs x: ● Determine from 4-vectors of beam particles e, p, where does HERA measure? scattered electron e' and hadronic final state X Q²~30 GeV² (e' p) ( Xp) x~0.001 ye=1− , yh= (ep) (ep) 2 p Q2 Typical low Q², electron Q2= T , x= low-x event 1− y sy ● “Electron” method: y=y and p =p e T T,e ● At low y, the electron method is limited by energy resolution, initial and final state radiation → use y=y (sigma method) h ● Other methods also in use: double-angle, etc CFNS workshop, May 2020 S.Schmitt, HERA introduction 12 Neutral current DIS kinematics at HERA ● Kinematic variables: Q², x, y, Q²=sxy Kinematic plane Q² vs x: ● Determine from 4-vectors of beam particles e, p, why is there this funny scattered electron e' and hadronic final state X shape? Limited statistics Q²~30 GeV² (e' p) ( Xp) x~0.001 ye=1− , yh= (ep) (ep) 2 Limited by centre- p Q2 of-mass energy Typical low Q², electron Q2= T , x= low-x event 1− y sy ● “Electron” method: y=y and p =p e T T,e ● At low y, use y=y (sigma method) → hadrons h contributing to y have to be within detector Hadron acceptance h limitation in proton acceptance → low y / high x is not accessible direction, electron method is limited by HERA is “low-x” because of acceptance energy resolution & Angular acceptance in radiation limitations in the forward (proton) direction electron direction HERA = low-x CFNS workshop, May 2020 S.Schmitt, HERA introduction 13 CFNS workshop, May workshop, 2020 CFNS Kinematics ofradiativephotons ISR: collinear to beam electron of scattered electron electron scattered of P balances Compton: QED FSR: collinear to FSR: scattered electron scattered Radiative effectsRadiative T S.Schmitt, HERA introduction HERA S.Schmitt, ● Radiation fromelectronlineRadiation – – – “ very low to corresponds QED Compton calorimeter use → momentum electron measured reduces Final state(FSR) energy beam reduces effectively Initial state(ISR) true” Q² at proton vertexproton at Q² true” 14 CFNS workshop, May workshop, 2020 CFNS Kinematics ofradiativephotons ISR: collinear to beam electron of scattered electron electron scattered of P balances Compton: QED FSR: collinear to FSR: scattered electron scattered Radiative effects: methodsRadiative HERA T S.Schmitt, HERA introduction HERA S.Schmitt, ● ● Radiation hastwoeffectsRadiation FSR, QEDCompton fromelectronlineRadiation hasthreepoles:ISR, – – with andwithout radiation,with model-dependent cross section changes wrt.theBornlevel For a givenclass ofevents,thepredicted (Sigmamethodreconstruction or similar) → reconstruction isdistorted aFor givenevent,thekinematic → apply cuts, use “robust” kinematic“robust”use apply cuts, radiative corrections, ratioofprediction radiative 15 HERA physics topics Electron Hard probes: Jets, heavy – Color-neutral probe, flavour, photons source of virtual or Inclusive quasi-real photons reactions Proton structure Proton and PDF HERA Searches & – Source of quarks and data electroweak gluons Soft QCD particle production, exclusive states, diffraction CFNS workshop, May 2020 S.Schmitt, HERA introduction 16 HERA physics topics Electron Hard probes: Jets, heavy – Color-neutral probe, flavour, photons source of virtual or Inclusive quasi-real photons reactions Proton structure Proton and PDF HERA Searches & – Source of quarks and data electroweak gluons Soft QCD particle production, See talk by exclusive states, E. Gallo diffractionRequires a dedicated long talk... CFNS workshop, May 2020 S.Schmitt, HERA introduction 17 Inclusive cross sections, PDFs H1+ZEUS Data combination Inclusive cross sections, PDF fit Longitudinal structure function CFNS workshop, May 2020 S.Schmitt, HERA introduction 18 Deep-inelastic scattering at HERA ● Measure Cross section as a function of Q² Neutral current ● High Q² ~ 104 GeV² probes smallest distances ∼1/Q4 ● Combination of H1 and ZEUS data (41 datasets) ● Neutral Current (NC): Charged current – photon, Z-boson exchange 2 2 2 4 1 Q M – Cross section shape similar to 1/Q ∼ /( + W ) ● Charged Current (CC): – W-boson exchange MW=80.4 GeV – Cross section shape ~1/(Q²+MW²)² High Q²: rare events → Talk by E.
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