Physics at HERA. Summer Student Lectures August Katja Krüger Kirchhoff Institut für Physik H1 Collaboration

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1 Physics at HERA Summer Student Lectures August 009 Kirchhoff Institut für Physik H1 Collaboration

2 Overview Introduction to HERA Inclusive DIS & Structure Functions formalism HERA results High Q & Electroweak Physics QCD: Jet Physics, Heavy Flavour Production Beyond the Standard Model (Diffraction)

3 Collider Types e +e p±p± ep + clean initial and final state + high energy + unique initial state + small background complicated final state + electron as probe of proton structure limited energy large background two accelerators LEP (00 GeV) ILC (1 TeV) Tevatron ( TeV) LHC (14 TeV) HERA (300 GeV) 3

4 HERA HERMES H1 ZEUS p 90 GeV e 7.6 GeV 4

5 HERA & its Pre Accelerators p e circumference: 6.3 km bunch crossing rate: 10.4 MHz 5

6 Collected Luminosity HERA operated lumi upgrade in 001 higher luminosity e polarization for H1 & ZEUS detector upgrades in total ~500 pb 1 of high energy data collected per experiment last months devoted to low p energy (460, 575 GeV) 6

7 ZEUS Detector tracking detector p magnet coil e calorimeter muon system 7

8 H1 Detector p tracking detector calorimeter magnet coil e muon system 8

9 Schematic View of the H1 Detector 9

10 HERMES Detector HERMES used electron beam on polarised gas target spectrometer covering region around electron beam direction 10

11 Physics Topics at HERA expected proton structure not (so) expected exotics (beyond the standard modell) structure functions parton densities SUSY photon structure leptoquarks... perturbative QCD jets s heavy quarks diffraction electroweak 11

12 Some Events... 1

13 Some Events... 13

14 Some Events... 14

15 Some Events... 15

16 Some Events... 16

17 Event Rates some khz some Hz some min 1 some hour 1 17

18 ep Scattering & Structure Functions 18

19 The HERA Textbook Plots gluons?? quarks? 19

20 Rutherford Scattering first scattering experiment existence of the nucleus d 1 Z1 Z e = d E kin from HyperPhysics assumes 1 sin 4 Coulomb potential no spins no recoil 0

21 Elastic Electron Scattering variables: q = k k' Q = q = 4 E E ' sin / E E '= 1 E / M sin / only one independent! d dq = 4 z Q 4 Coulomb Potential ~1/r e' (k') e (k) q q=k k ' E' cos E recoil particles stays intact mass M, charge z, spin 0 1

22 Elastic Electron Scattering: Cross Section Mott Scattering: electron on a pointlike charged particle with spin 0 d 4 E ' = cos 4 E d Q Mott Q Dirac Sacttering: electron on a pointlike charged particle with spin ½ d d Q = 1 tan with = d Q Dirac d Q Mott 4 M [ ] electron on proton: form factors needed: G E Q G M Q d d = G M Q tan 1 d Q ep d Q Mott [ ] protons are not pointlike!

23 Electric Form Factor of the Proton describes the charge distribution in the proton (Fourier transform) measured: GE(0) = 1 fromj.j. Murphy et al., Proton form factor from 0.15 to 0.79 fm GM(0) =.79 Q G E Q, G M Q GeV elastic scattering only import at low Q 3

24 Inelastic Electron Scattering variables: q = k k' Q = q e' (k') e (k) q q=k k ' s = (P + k) W = (P + q) = M + q P Q y = q P / k P p (P) W two independent! elastic: W = M inelastic: W > M 4

25 Inelastic Electron Proton Scattering inelastic scattering: W > Mp ratio to Mott cross section nearly flat in Q 5

26 Deep Inelastic Scattering (DIS) deep: Q > (Mp) inelastic: W > Mp for HERA: me, Mp W neglect me, Mp k '= E ' e,0, E 'e sin e, E 'e cos e s = 4 Ep Ee Q = E e E ' e 1 cos e E' e e y=1 sin Ee W = ys Q k=(ee,0,0, Ee) attention e = P=(Ep,0,0,Ep) one more variable: x = Q / ( P q) = Q / ys 6

27 DIS: What is x? k' x can be interpreted as the momentum fraction of the struck parton of the proton: k q P'q xp P P 'q =q xp q xp = Q x q P xp q xp = xp = m q Q Q x= = q P ys inelastic proton scattering is scattering on a parton of the proton! 7

28 Structure Functions F1 & F the DIS cross section can be written as [ ] 4 1 y = 1 y F x,q x F x, Q 1 4 dx dq Q x 4 1 E ' Q = F x, Q cos x F x, Q sin 1 x M p Q4 x E d [ ] comparison with Dirac formula [ 4 z E ' = 4 E d Q Dirac Q d Q cos sin M ] F corresponds to electric field of the parton F1 corresponds to spin of the parton 8

29 Parton Spin parton spin ½: x F1 = F parton spin 0: x F1 = 0 (Callan Gross) partons have spin ½ from P. Schmüser, Feynman Graphen und Eichtheorien für Experimentalphysiker 9

30 Scaling: F independent of Q SLAC 197 independent of Q, we always see the same partons (=quarks) 30

31 (Naive) Quark Parton Model proton consists of 3 partons, identified with the QCD quarks during the interaction proton is frozen electron proton scattering is sum of incoherent electron quark scatterings proton structure is defined by parton distributions F x, Q = x e q x q 31

32 The HERA Textbook Plots quarks 3