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High Q 2 Structure Functions and Parton Distributions

High Q 2 Structure Functions and Parton Distributions. Ringberg Workshop 2003 : New Trends in HERA physics Benjamin Portheault LAL Orsay On behalf of the Zeus and H1 Collaborations. I. Introduction The legacy of HERAI : What do we learn ? II. Structure Function Results

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High Q 2 Structure Functions and Parton Distributions

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  1. High Q2 Structure Functions and Parton Distributions Ringberg Workshop 2003 : New Trends in HERA physics Benjamin Portheault LAL Orsay On behalf of the Zeus and H1 Collaborations

  2. I. Introduction The legacy of HERAI : What do we learn ? II. Structure Function Results State of the art of Zeus and H1 inclusive measurements III. Extraction of parton distributions H1 and Zeus schemes Motivation and necessity for a common fit IV. Overview of present (and future) possibilities Towards a H1+Zeus Fit A brief look at Mw

  3. Deep Inelastic Scattering At leading order in the EW interaction : high Q2 dominant high y The QCD factorization theorem allows to express the structure functions as convolutions of universal parton density functions (p.d.fs) and perturbatively computable kernels

  4. Through the measurement of high Q2 inclusive Cross Section : We can measure/extract structure functions Test the structure of the EW interaction (and look for new physics) Use fits to test the pQCD evolution and Measure the universal p.d.fs An HERA QCD analysis can also be used to Extract p.d.fs and the EW parameters ( ,…) For as and the gluon measurements and prospects see talk by V. Shekelian

  5. low Q2 (F2) Charged Current parton distributions helicity factor high Q2 (xF3) The HERA EW plot

  6. The HERA QCD plot beautiful Scaling Violations HERA data Far from fixed target precision at high x (stat. lim.) Important for QCD evolution HERA typical dF2/F2 ~2-3% dF2/F2~30% Interesting Region for exotic searches BCDMS dF2/F2~7%

  7. xF3 contribution changes sign between e+/e- (by definition) Extraction done by subtraction

  8. suppressed (<3% contribution to xF3) access to valence p.d.fs ~30% error, dominated by e- statistics evolved to the same Q2 xF3gZ

  9. Typical systematic errors are ~6% Direct constraint on d at high x

  10. Direct constraint on u at high x Both e+ and e- Cross sections Are necessary For flavor separation in QCD fits

  11. extraction with measured from fit extend CCFR measurement uncertainty from e- statistics

  12. H1 & Zeus QCD Fits Zeus Fits : ZEUS-Standard Use Zeus NC 96/97 e+ BCDMS, NMC, E665 proton data NMC,E665 deuterium data, CCFR xF3 iron data For high x constraint and better flavor separation ZEUS-Only Use only Zeus data NC and CC e+ and e- up to 99 The number of d.o.f. is reduced by fixing parameters Both use TR Variable Flavour Scheme H1PDF2000 and H1+BCDMS Use only H1 data (all HERAI NC and CC high Q2 +low Q2 96/97) Massless Flavour Scheme

  13. H1 Zeus Too much freedom to constrain all these parton densities with H1 alone or Zeus alone Need for additional assumptions or data Fixes parameters robust scheme Use internal constraints Between parameters Not constrained by HERA data Assumptions are needed (parameters fixed/constrained)

  14. Error determination Both fits handle (differently) correlated systematic errors in parameter determination and error estimate H1 : Pascaud-Zomer method Zeus : offset method (uses external data sets) Only a proper treatment of the these correlated Systematic error allow the application Of a error estimate See talk by J. Pumplin

  15. The Art of parameterization At the (low) scale Q02 parton distributions are Parameterized with low x high x It is not trivial to have P(x) such that the fit is flexible enough and stable Errors (and distributions) depend on the parametric form chosen One also has to be kind enough with MINUIT as the fit is non-linear (dependency upon the starting parameters …) Any choice is arbitrary (nature do not choose a parametric form)

  16. Results on parton distributions u type quark distribution precision is 1% for x=0.001 and 7% for x=0.65 (H1PDF2000) d type quark distribution precision is 2% for x=0.001 and 30% for x=0.65 (H1PDF2000) Sea distribution precision is 5% between x=10-4 and 10-1 (ZEUS-S) Good achievement but the fits are at the edge of the fitting possibilities Use all HERA data to gain in flexibility

  17. Reasonable agreement between the fits given the different Data and fitting schemes

  18. HERA data and global fits The HERA data are highly valuable for global analysis of Parton distributions : Crucial low x constraint on quarks and antiquarks, gluon High y data also constraining gluons The knowledge and understanding of the correlated systematic errors of HERA data made then central in any of parton distributions analysis Reliable error determination The CTEQ and MRST will benefit from including the 99/00 Zeus and H1 data in their fits

  19. Data set /ndata NC e+ low Q2 104/75 NC e+ highQ2 210/162 CC e+ 19/29 NC e- 53/92 CC e- 23/26 Towards HERA fits Now (nearly) all Zeus and H1 data sets are available The Zeus and H1 data are in very good agreement (in the sense of global parton distribution analysis) Test of the Zeus data with H1PDF2000 : NB : Zeus systematics fitted 96/97 98/99

  20. Potential of an HERA fit Fit procedure using less technical assumptionsCombination could help a lot on technical fit aspects (we can also run into new problems) And a meaningful for errordetermination Extensive studies must be made e.g. for the choice of parametric forms (and settle if the HERA gluon is ‘H1 like’ or ‘Zeus like’, see talk by V. Shekelian) As a proof of principle : Prospects using a case study fit of Zeus+H1 data (except Zeus 99/00 data) The Mw mass measurement

  21. Electroweak reminder To confront the SM with experimental data one needs to specify several parameters: The On Mass Shell (OMS) scheme uses masses as an input: Whereas the Modified OMS uses: The couplings of leptons to the Z0 and its propagator normalisation also depend on the scheme

  22. Fitting strategies for Several strategies are possible : It is possible to fit Mw to the CC cross section As a ‘propagator mass’ with fixed Structure Functions (used by Zeus and H1) Or fit Mw together with the p.d.fs OMS scheme, Mw as propagator mass and normalisation ( error estimate, similar to as) HERA fit (no Zeus 99/00, Dr fixed) H1PDF2000 scheme

  23. Results and expected sensitivity Significant improvement HERA fit Still far from world average (not so far from NuTeV alone or D0 alone ~80 MeV uncertainty) experimental error only (central value consistent with world average)

  24. Results and expected sensitivity Major improvements in sensitivity Could be better using Zeus 99/00 Unfortunately a significant theoretical error may be present even with better data But still far from world average : HERA is not an electroweak facility QCD (EW) commonfits should start now to preparethe strong physics message of HERAI+IIon p.d.fs and SM parameters

  25. Conclusion Many cornerstone results in DIS have been achieved by HERAI. DIS NC and CC differential cross sections are measured with reasonable precision. High statistics (and polarization) would bring more accuracy Has already been turned into many physics results through QCD Fits The potential for the results still unexploited: Zeus-H1 common fit is absent and needed to settle important physics issues (pdfs, as , gluon, …)

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