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“ Cross section measurements in DIS ”

“ Cross section measurements in DIS ”. K. Papageorgiou University of the Aegean. On behalf of ZEUS and H1 Collaborations. OUTLINE. Introduction Cross sections Power of data sets combination F L measurement Motivations & Method Analysis strategy H1 & ZEUS results Summary.

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“ Cross section measurements in DIS ”

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  1. “ Cross section measurements in DIS ” K. Papageorgiou University of the Aegean On behalf of ZEUS and H1 Collaborations

  2. OUTLINE • Introduction • Cross sections • Power of data sets combination • FL measurement • Motivations & Method • Analysis strategy • H1 & ZEUS results • Summary

  3. HERA – The Worlds only electron-proton collider • Q2 corresponds to the spatial resolution (wavelength λ) of the probe • (set by s) • Collides protons with • Ee : 27.5 GeV • Ep : 920 GeV • Centre of mass energy :

  4. HERA Luminosity • In operation since 1992 • HERA Running ended 30 June 2007 • HERA –I : Until year 2000 • Unpolarized e+ and e- beams • HERA – II : 2002 – Mar 2007 • High luminosity to allow more statistical sensitivity to high Q2 • Longitudinally polarized e+/e- beams to allow direct sensitivity to EW • Low energy Run : March 2007 – June 2007 • A special Run with low proton beam energy (460, 575 GeV) to measure directly FL structure function • HERA – I & II combined integrated Luminosity ~ 0.5 fb-1 per experiment Approximately ~ 1 fb-1 of data collected by H1 & ZEUS

  5. DEEP INELASTIC SCATTERING KINEMATICS Q2 = -q2=-(k-k΄)2 W± (xP) p(P) } Proton remnant Current jet Q2 = -q2=-(k-k΄)2 γ, Ζ0 (xP) p(P) } Proton remnant Current jet • A key tool to “look” inside the proton, allow testing of QCD NC CC • DIS cross section can be described in terms of kinematic variables • Q2: Virtuality of the intermediate boson -- measure of the probing power • x : Bjorken scaling factor -- fraction of proton’s momentum carried by struck quark • y : Inelasticity -- energy fraction transferred from lepton in proton rest frame • s : Centre of mass energy of the ep system

  6. NC DIS cross section: F2 The NC DIS cross section expressed in terms of structure functions F2 -- dominant contribution to the cross section F2 -- SF parameterize target structure, i.e. how far for point-like If proton is point-like then,

  7. NC DIS cross section: F2 (H1 & ZEUS combined data) • New HERA-I PDF fit predictions vs H1 & ZEUS combined data NC e+p: (see talk of LI, Gang ) • Total uncertainties on the PDF fit predictions included but can barely be resolved • Proton scan with high precision • NLO fit (DGLAP) – good description of the data • Systematic uncertainties are now smaller than statistical uncertainties

  8. CC e-p cross section(ZEUS) • Chiral structure of weak interactions is directly visible as a function of polarization •  Linear dependence with lepton polarization Pe=1 for e- ,Pe=-1for e+ • Consistent with SM prediction of No evidence of RH currents. Error band from PDF uncertainty

  9. CC e-p cross section(H1 & ZEUS)

  10. NC e+p cross section: (ZEUS) • Cross sections for NC DIS in e+p collisions with a longitudinal polarized positron beam have been measured using the ZEUS detector •  Results based on data corresponding to an L=113.3 pb-1 at sqrt(318) GeV are given for both positive and negative values of the longitudinal polarization of positron beam • Single differential cross sections are shown in the kinematical region Q2>185GeV and y<0.95 • Measured cross section consistent with SM prediction

  11. NC e+p: (ZEUS)

  12. NC e+p: (H1) • Cross sections for NC DIS in e+p collisions with a longitudinal polarized positron beam have been measured using the H1 detector • Single differential cross section are shown for HERA I and II e+p data • Measured cross section is compared to the prediction of the SM

  13. NC DIS cross section: xF3 xF3-- is the parity-violating term arising from Z0 exchange -- experimentally can be extracted by measuring the -- negligibleat small Q2

  14. FL MEASUREMENT : Motivation & Method :reduced cross section FL -- longitudinal SF, sizeable only at low x, observable at high y -- FL=0 in QPM (spin ½ partons – Gallan-Gross), -- propotional to gluon at higher orders, Direct sensitivity to gluon dynamics To separate F2 , FLw/o theory assumption , one needs to measure the cross section at fixed (x,Q2) and different values of ydifferent beam energies (s)

  15. FL MEASUREMENT : Motivation & Method  Separate SF’s at given x ,Q2 using straight line fit and error compensation  Intermediate energy for measurement control.  At low y and fixed (x,Q2) cross sections need to coincide  Renormalization used by H1 and by ZEUS

  16. FL : Analysis strategy • DIS events: Identified by scattered electron (Electron Method) • High –y :  Low energy electron • Low –Q2: H1 analysis covers both SpaCal and LAR Low y / High s (Ep=920 GeV) • High energy well separated electron in the calorimeter; • no background High y / Low s (Ep=460 GeV) • Low energy electron in the calorimeter; lot of hadronic activity • Electron finding is difficult • at low Q2large γp background, mostly γ’s and π’s faking an electron  needs to be subtracted/controlled

  17. Reduced cross section measurement (ZEUS) • Low-x: visible differences between data sets • High-x: cross sections are consistent • Low-x: the x sections turn over

  18. Reduced cross section measurement (H1) H1 preliminary Ep = 460 GeV = 575 GeV = 920 GeV • Combined LAr & SpaCal ymax ~ 0.9 [@450 & 575 GeV] x

  19. FL measurement (ZEUS) • Results are consistent with FL from ZEUS JETS PDF’s

  20. FL measurement (H1 published results) • Results are consistent with the prediction of H1 PDF

  21. Overview on all (medium & high Q2 ) FL data (H1) • Results consistent with the prediction of H1 PDF fit (obtained using only the H1 high energy x-section data

  22. x- averaged FL in the full range (H1) • Results consistent with H1 fit and with expectations from global parton distribution fits at higher order pertubation theory  DGLAP describes low-x region at HERA

  23. Summary • First direct measurement of FL was performed in a new kinematical range Q2 : (24-110) GeV2 [ZEUS] , Q2 : (12-800) GeV2 [H1] and Bjorken x : 0.0002 – 0.05 • Data are in good agreement with higher order pQCD predictions • First data have been published • HERA experiments provide unique information on proton structure over wide • range of x • an important precision input for physics at LHC • The combination of the data sets of ZEUS and H1 inclusive cross section is very successful and convincing • greatly improved precision compared to the measurements of each experiment separately • Analysis is ongoing, thus can be expected to further constrain low-x • A most remarkable success of HERA!

  24. Back-up slides

  25. Comparison of FL data with pQCD

  26. Comparison of FL data with Dipole Model

  27. NC e+p: (ZEUS)

  28. NC e+p: (ZEUS)

  29. NC e+/-p: (H1)

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