Tobias Haas: Introduction to HERA An Introduction to HERA Physics DESY Summer Student Program 16/17 August, 2005 Tobias Haas DESY, Hamburg

Tobias Haas: Introduction to HERA Overview Part 1: What is HERA? What is HERA? Structure Function Formalism Structure Function Formalism Kinematics Kinematics Structure Function Evolution (RGE/DGLAP) Structure Function Evolution (RGE/DGLAP) Part 2: Selected HERA I Results: Selected HERA I Results: Structure Functions Structure Functions High Q 2 and EW High Q 2 and EW Jets and the strong coupling α S Jets and the strong coupling α S Part 3: Why HERA II: Why HERA II: High lumi/polarization High lumi/polarization EW precision measurements EW precision measurements © G. Larson: “The Far Side”

Tobias Haas: Introduction to HERA Part 1

Tobias Haas: Introduction to HERA What is HERA? © G. Larson: “The Far Side”

Tobias Haas: Introduction to HERA Colliders e+e+e+e+ e-e-e-e- pppp pppp p+p+p+p+ eeee +Simple initial state +Clean final states +Little background -Limited energy e.g. LEP(200 GeV), ILC(1 TeV)e.g. LEP(200 GeV), ILC(1 TeV) +High energy (no synch rad) -Complicated initial state -Large and complicated backgrounds -e.g TEVATRON(2 TeV), LHC(14 TeV) +Unique initial state -two accelerators -HERA (300 GeV)

Tobias Haas: Introduction to HERA

HERA: The only ep Collider on the Planet

Tobias Haas: Introduction to HERA HERA Highlights: Highlights: Started operation in Experiments: H1 and ZEUS (ep) HERMES (e) HERA-B (p) (until 2003)  s = 300 GeV (  1997)  s = 318 GeV (1998  ) e + and e - beams up to 60% lepton polarization > 400 Mio ep collisions recorded per experiment

Tobias Haas: Introduction to HERA Physics Topics of HERA Proton Structure Proton Structure Parton densities Parton densities gluon density (xg(x)) gluon density (xg(x)) Valence quark distributions Valence quark distributions QCD evolution QCD evolution Different evolution schemes (e.g. BFKL) Different evolution schemes (e.g. BFKL) Strangeness and charm Strangeness and charm  S  S Perturbative QCD Perturbative QCD Jets Jets Gluon density Gluon density  S  S Multiparticle Observables: Multiplicity distributions Event chapess Multiparticle Correlations Border between pQCD and non-perturbative QCD Photon Structure Diffraction EW BSM and Exotics BSM and Exotics Leptoquarks Excited Quarks and Fermions FCNC MSSM Searches R-parity violation SUSY Contact Interactions … Spectroscopy

Tobias Haas: Introduction to HERA Structure Function Formalism See e.g. David J Griffiths: “Introduction to elementary particles”, New York, 1987

Tobias Haas: Introduction to HERA

“Bj” (aka James Daniel Bjorken)

Tobias Haas: Introduction to HERA Kinematics … or a guided tour around the HERA phase space …

Tobias Haas: Introduction to HERA e-e GeV P 920 GeV Scattered e - Current Jet Scattered e - Current Jet Scattered e -

Tobias Haas: Introduction to HERA HERA I Kinematic Range Huge extension of kinematic reach: x Bj : 6 orders Q 2 : 6 orders Overlap with previous (fixed target) experiments

Tobias Haas: Introduction to HERA 1

Very High Q 2 Very clean events Very clean events Very high energy electron (> E beam ) Very high energy electron (> E beam ) Very collimated jet Very collimated jet Electron forward Electron forward Activity around the beam pipe forward (proton remnant) Activity around the beam pipe forward (proton remnant) e-e GeV P 920 GeV Scattered e - Current Jet

Tobias Haas: Introduction to HERA 2

Scattered e - Very Low Q 2 Little activity in main detector Little activity in main detector Electron backward – seen in special beampipe calorimeter Electron backward – seen in special beampipe calorimeter Electron energy close to E beam Electron energy close to E beam No jet structure No jet structure Activity around the beam pipe forward (proton remnant) Activity around the beam pipe forward (proton remnant) e-e GeV P 920 GeV

Tobias Haas: Introduction to HERA 3

Medium Q 2 e-e GeV P 920 GeV Scattered e - Jet Jet and electron in main detector Jet and electron in main detector Well isolated electron Well isolated electron Well collimated jet Well collimated jet Activity around the beam pipe forward (proton remnant) Activity around the beam pipe forward (proton remnant) Jet

Tobias Haas: Introduction to HERA Deep inelastic: Deep inelastic: W >> M P  const. xF 2  const. ~ xF 2

Tobias Haas: Introduction to HERA

HERA Results for F 2 Dramatic Scaling Violations! Sample F 2 data

Tobias Haas: Introduction to HERA NC Cross Section: NC Reduced cross section: Dominant contribution Sizeable only at high y (y>~0.6) Contribution only important at high Q 2 Q 2 =  q 2 =  (k  k’) 2 x: momentum fraction of the struck parton y=Q 2 /xs NC Cross Section and Structure Functions

Tobias Haas: Introduction to HERA F 2 vs Q 2 Note: Note: Enormous range of data (5 orders in Q 2 and 8 orders in x) Approximate scaling at high Q 2 Scaling violations at low Q 2

Tobias Haas: Introduction to HERA F 2 vs x Bj Dramatic rise a low x Bj Note previous picture:

Tobias Haas: Introduction to HERA Structure Function Evolution

Tobias Haas: Introduction to HERA Reminder: Nomenclature/Kinematics = 0 in the QPM Z 0 Exchange

Tobias Haas: Introduction to HERA What QCD tells about F 2 (x,Q 2 ) ? DGLAP Equation: Integral-Differential equation for the dependence of q(x,Q 2 ), g(x,Q 2 ) on Q 2 Need an initial condition! Splitting functions: Can be calculated in pQCD

Tobias Haas: Introduction to HERA QCD Fits Make an ansatz at a fixed value of Q 2 = Q 0 2 Make an ansatz at a fixed value of Q 2 = Q 0 2 Write F 2 simpler: Write F 2 simpler: Ignore F 3 and F L (for the moment) Ignore F 3 and F L (for the moment) _ _ F 2 ~ 4/9 (U+U) + 1/9 (D+D) with _ _ _ _ _ _ D = d+s U = u+c U = u+c D = d+s

Tobias Haas: Introduction to HERA Parton Distribution Functions QCD fits to structure functions: _ _ F 2 ~ 4/9 (U+U) + 1/9 (D+D) _ _ Valence quarks: 2 (U-U) + (D -D) _ _ _ _ _ _ D = d+s U = u+c U = u+c D = d+s % precision except for gluon :