1. XXXIV Meeting on Fundamental Physics April 2-7, 2006; El Escorial, Madrid - Spain Enrico Tassi Calabria University and INFN tassi@fis.unical.it Electroweak Physics at HERA Electroweak Physics at HERA OUTLINE HERA-I: -Unpolarized NC and CC inclusive cross sections -Electroweak Unification -EW-DGlap fits of DIS cross sections (light quark weak couplings) HERA-II: -Polarized NC and CC inclusive cross sections Testing the EW sector of the SM at high space-like momentum transfers

2. The HERA Collider The first (and only) ep collider in the world e L,R ± p 27.5 GeV 920 GeV √s = 318 GeV Equivalent to fixed target experiment with 50 TeV e ± Located in Hamburg H1 ZEUS

3. NC and CC inclusive processes

4. Kinematics Reaching values of Q 2 ≥ 3x10 4 GeV 2 Kinematic limit defined by Q 2 =sxy s HERA =1.2x10 5 GeV 2 Previous fixed-target experiments Reaching values of x < 10 -6 Extension by several orders of magnitude in x and Q 2

5. H1 Detector Complete 4π detector Tracking: - central jet chamber - z drift chambers - forward track. detector - Silicon μ-Vtx (operate in a B field of 1.2 T) Calorimeters: - Liquid Argon cal. - Lead-Fiber cal. (SPACAL) Muon chambers

6. Zeus Detector Complete 4π detector Tracking: - central tracking detector - Silicon μ-Vtx (operate in a B field of 1.43 T) Calorimeters: - uranium-scintillator (CAL) σ(E)/E=0.18/√E [emc] σ(E)/E=0.35/√E [had] - instrumented-iron (BAC) Muon chambers Both detectors asymmetric

7. NC and CC processes: Experimental signatures NC: CC:

8. Inclusive cross sections and Structure functions Main observables of interest are the double differential cross sections : Neutral current (NC): with whereand

9. Charged Current (CC): Note: σ SM = σ Born (1+δ EW ) …but full NLO in α s Reduced “cross sections”: Inclusive cross sections and Structure functions

10. Structure functions – PDFs – quark couplings In the quark parton model (QPM) the NC SFs are: For the CC SFs: Vector and axial-vector quark couplings:

11. HERA I – Running period mostly e + p collisions… HERA I (1992 →2000) (120 pb -1 per experiment) HERA I (1992 →2000) (120 pb -1 per experiment) - Unpolarized lepton beams - Unpolarized lepton beams - Physics output: - Physics output: pQCD (PDFs,jet production,heavy flavours),Beyond the SM searches, pQCD (PDFs,jet production,heavy flavours),Beyond the SM searches, Diffraction, etc… Diffraction, etc… see Claudia and Andre presentations see Claudia and Andre presentations

12. F 2 @ HERA Rise of F 2 at low-x (established with ~20 nb -1 ) Best F 2 determination ( 1996-97 data samples) pre-HERA F 2

13. F 2 @ HERA Enormous impact on proton parton distributions functions

14. Incl. Analyses: Zeus as an example Very precise reconstruction of event’s characteristics and kinematic variables: Count nr. of events in appropriately defined (x,Q 2 ) bins Extract reduced cross section Uncertainties are systematics dominated for Q 2 < 800 GeV 2

15. e + p and e - p CC reduced Cross Sections

16. Helicity structure Needs e - data

17. Large unc. at high-Q 2 e + p and e - p NC reduced Cross Sections → γZ 0 and pure Z 0 contributions to xF 3 → valence quarks

18. EW Unification NC cross section sharply decreases with decreasing Q 2 (γ exchange): ~ 1/Q 4 CC cross section approaches a constant at low Q 2 ~[M 2 W /(Q 2 +M 2 W )] 2 Illustrate unification of the electromagnetic and weak interactions in Deep Inelastic Scattering Already a textbook figure…

19. xF 3 (NC) with

20. xF 3 γZ (NC)

21. Combined EW-DGLAP fit to inclusive data (H1)

22. H1 2000 Fit

23. Light quark couplings to the Z 0 boson 68% CL contour plots for the vector (v u,v d ) and axial-vector(a u,a d ) weak couplings of the light quarks to the Z 0 boson

24. G F and W mass G F and the mass of the W can be determined from the Q 2 -dependence (shape) and normlisation of the CC cross section If G F is fixed at PDG value:

25. W mass in the OMS The CC cross section can also be rewritten (in the OMS scheme ) as Fixing the top mass m t at 178 GeV and M H =120 GeV the fit result is: ….a consistency check of the SM.

26. HERA II Running period HERA II (2003 →…) HERA II (2003 →…) - slow start-up : beam related backgrounds… - slow start-up : beam related backgrounds… - now solved. Since then…best ever HERA performance - now solved. Since then…best ever HERA performance - Looking forward to the final running period (end by July 2007). - Looking forward to the final running period (end by July 2007).

27. Electron accelerated on a circular path radiates photons Flip of the projection of the electron spin along y can occur Spin flip probabilities per unit time: Since starting from an unpolarized beam, spin-flip synchroton radiation induces a net transverse polarization (sokolov-ternov effect): Electron polarisation in storage rings Time evolution: Asymptotic limit:

28. Depolarising effects Of course all these results assume an ideal world (perfectly planar storage ring and vertical B field /after γ emission the e- stays on the orbit etc…) In the real world various depolarising effects lead to P max < P ST -Stochastic depolarisation through synchrotron radiation -electron oscillations around central orbit -Mis-aligned magnets etc…

29. Spin rotators and polarimeters  Spin rotators around H1,HERMES and ZEUS  Two independent polarimeters - Longitudinal polarimeter (LPOL) near HERMES - Longitudinal polarimeter (LPOL) near HERMES - Transverse polarimeter (TPOL) near HERA-B hall - Transverse polarimeter (TPOL) near HERA-B hall

30. spin rotators  Use series of transverse magnetic fields to change Py into Pz  so called “mini-rotator“ (56 m long!)  move section vertically during access day to change the helicity

31. Polarised CC total cross sections ~First measurement of helicity dependence of e L,R p → νX Expect linear dependence from SM Direct observation of the chiral structure of the weak interactions ZEUS and H1 in agreement with SM

32. Polarised CC differential cross sections

33. Polarised NC cross sections

34. Polarised NC diff cross sections

35. Outlook - HERA will be running till July 2007 - It is expected to deliver, by the end of data taking, L tot ≥ 700 pb -1 (with an equal share of e ± left- and right- polarised). => Much more precise measurements …for an immediate update refer to the forthcoming DIS06 Workshop