 Introduction  T he ZEUS PDF fit: an overview  Impact of future HERA data on the ZEUS fit

1. Impact of HERA-II data on the ZEUS PDF fits 13th International Workshop on Deep Inelastic Scattering (DIS05) Madison, Wisconsin, USA 27th April – 1st May 2005 Claire Gwenlan with the help of M. Klein (DESY), C. Targett-Adams (UCL), R. Thorne (Cambridge), A. Tricoli (Oxford)  Introduction  The ZEUS PDF fit: an overview  Impact of future HERA data on the ZEUS fit - end of current HERA-II running scenario - additional studies (FL, sea quark asymmetry)  Conclusions

2. Introduction_______________________________ Besides being interesting in their own right, it is essential to know the parton density functions (PDFs) of the proton as precisely as possible in order to maximise the physics potential at both current and future colliders e.g. high-x gluon is dominating uncertainty in several LHC processes HERA data are now very precise and cover a wide range in (x,Q2) • determination of proton PDFs now possible within one experiment Most recently, ZEUS have performed a NLO QCD analyses on their full set of HERA-I e+ and e- structure function data and high precision jet data  the ZEUS-JETS PDF With future measurements at HERA, hope to be able to do even better … • Presented here are the results of studies that give a first look at the • potential impact of future HERA measurements on the proton PDFs: • within current HERA-II running scenario • - increased luminosity • - cross sections optimised for sensitivity to PDFs • 2. Other possibilities (FL, sea quark asymmetry)

3. Determination of proton PDFs at HERA_______________________________ Factorisation: observable = short range interaction  PDFs Observables used in QCD fits to determine PDFs:  Inclusive NC/CC DIS ep cross sections   direct sensitivity to quarks  only indirect sensitivity to gluon (scaling violation)  Jet cross sections:  directly sensitive to gluon through boson-gluon-fusion  , Z0, W Q2 = -q2 = -(k-k’)2 xp Jet Gluon in proton Now, after the HERA-I (94-00) phase of data-taking, the full set of inclusive NC/CC e+/e- data, and high precision jet data are available for QCD analysis… Jet

4. ZEUS-JETS NLO QCD fit _______________________________ For more details on the ZEUS-JETS fit (hep-ph/05030274), and the data included, see cont. to this workshop, “Proton PDFs using Structure Functions and Jet Data from ZEUS”, Juan Terron. Data: Parameterisation:  parameter constraints:  momentum and quark number sum rules  low-x behaviour of uv and dv set equal  flavour structure of light quark sea set consistent with Gottfried sum and Drell-Yan  heavy quarks treated in variable flavour number scheme of Thorne and Roberts  11 free parameters in total 4

5. Current HERA-II running scenario_______________________________ • HERA-II is running efficiently... HERA delivered luminosity (pb-1) 2004 2005 2006 2007 Current running scenario We are here… “ 700pb-1 integrated luminosity, equally divided between e+/e-, expected by the end of HERA-II running in mid-2007 ”

6. What impact will future HERA measurements have on the PDFs?_______________________________ Where does the information come from in a HERA-Only fit ? HERA-I: statistics limited HERA-I: statistics limited at high-ET and high-Q2

7. Example: high-Q2 NC and CC data_______________________________ e+ and e- needed for flavour separation, but high-Q2 CC statistically limited at HERA-I, especially e-p data F2 dominates NC cross section, HERA-I: F2/F2~30%  HERA-II will provide greatly increased luminosity

8. What impact will future HERA measurements have on the PDFs?_______________________________ Where does the information come from in a HERA-Only fit ? HERA-I: statistics limited HERA-I: statistics limited at high-ET and high-Q2

9. What impact will future HERA measurements have on the PDFs?_______________________________ Where does the information come from in a HERA-Only fit ? HERA-I: measurements in only certain kinematic regions – potential to optimise cuts for sensitivity to gluon

10. Optimised jet cross sections_______________________________ Christopher Targett-Adams (UCL)  Measure jet cross sections in kinematic regions “optimised” for sensitivity to gluon - ongoing ZEUS study: dijets in photoproduction (Q2 < 1 GeV2)  data simulated using NLO QCD (Frixione-Ridolfi) and CTEQ5M1 proton PDF (500 pb-1) 10

11. Impact of HERA-II in current running scenario:- case study_______________________________ data exists for HERA-I running data does not yet exist for HERA-I running Impact of the projected HERA-II measurements has been studied in the context of the ZEUS-JETS fit

12. HERA-II projected fit_______________________________  The HERA-II projected PDF compared to the ZEUS-JETS fit  Impact from increased statistics on high-Q2 NC/CC e+/e- data Impact from optimised cross sections (mostly) and increased statistics on jet data

13. u-valence uncertainties _______________________________ log-x scale (low-x region) linear-x scale (high-x region)  uncertainties on u-valence distribution significantly reduced over visible x range

14. d-valence uncertainties _______________________________ log-x scale (low-x region) linear-x scale (high-x region)  uncertainties on d-valence distribution significantly reduced over visible x range

15. Sea-quark uncertainties _______________________________ log-x scale (low-x region) linear-x scale (high-x region) • uncertainties on sea-quark distribution significantly reduced at high-x  most significant improvement from increased statistics

16. gluon uncertainties _______________________________ log-x scale (low-x region) linear-x scale (high-x region) • uncertainties on mid-to-high-x gluon significantly reduced  most significant improvement comes from optimised cross sections

17. HERA Kinematic (x,Q2) Range_______________________________  HERA data covers large region in (x,Q2)  also relevant x-region for LHC  HERA-II projected PDFs should have significant impact on e.g. high-pT jets, heavy particle searches etc.  sensitive to improvement to high-x partons  LHC cross sections under study

18. Impact of a HERA measurement of FL _______________________________ FL contributes at NLO (and HO) and is directly sensitive to the gluon density in the proton  measured at fixed target exps. (x > 10-3)  precision FL measurement at HERA requires low-Ep running  vary y at fixed (x,Q2)

19. Impact of a HERA measurement of FL _______________________________ FL contributes at NLO (and HO) and is directly sensitive to the gluon density in the proton  precision measurement of FL at HERA-II:  pin down gluon density at low-x  reduce uncertainties on gluon PDF

20. Impact of a HERA measurement of FL _______________________________ FL contributes at NLO (and HO) and is directly sensitive to the gluon density in the proton  precision measurement of FL at HERA-II:  pin down gluon density at low-x  reduce uncertainties on gluon PDF  provide tests of higher order QCD  test the need for extensions to DGLAP at low-x Robert Thorne 20

21. Impact of a HERA measurement of FL _______________________________ FL contributes at NLO (and HO) and is directly sensitive to the gluon density in the proton  precision measurement of FL at HERA-II:  pin down gluon density at low-x  reduce uncertainties on gluon PDF  provide tests of higher order QCD  test the need for extensions to DGLAP at low-x Simulation of HERA-II FL: Max Klein (DESY)  FL simulated using GRV94 NLO PDF  statistical uncertainties correspond to:  systematic uncertainties from current H1 analysis of 99-00 data (few %) For further details, see “On the future measurement of FL at low-x at HERA”, Max Klein, in proceedings, DIS04. 21

22. Impact on gluon distribution_______________________________ • Fit including simulated FL data compared to the ZEUS-JETS PDF   Gluon uncertainties reduced at low-x and low-Q2 (not relevant for LHC) 22

23. Sensitivity of the NLO QCD fit_______________________________  FL predictions very sensitive to underlying theory  choice of PDF, order of QCD calculation etc.  How sensitive is the NLO QCD fit to inclusion of “extreme” sets of simulated FL data Simulated FL dataextremes provided by Robert Thorne (Cambridge) • ZEUS fit relatively stable to inclusion of extreme FL data-sets  fit cannot describe “just any data”  an FL measurement of this precision should have power to discriminate between theoretical models 23

24. Summary _______________________________ •  HERA-I has provided precision measurements over large region in (x,Q2) •  PDFs of the proton already determined to high precision using HERA data only •  most recently: ZEUS-JETS fit to HERA-I inclusive NC/CC and jet data •  In current running scenario: •  HERA-II will provide significantly increased statistical precision • -high-Q2 NC/CC data  significant improvement to valence distributions • - high-Q2 and high-ET jet data  improvement to high-x gluon •  potential for measuring cross sections optimised for sensitivity to gluon • - significant improvement to high-x gluon • In low-Ep running scenario a precise measurement of FL at low-x possible •  inclusion in fit indicates improvement in gluon uncertainties at low-x and low-Q2 •  stability of fit to extreme FL sets indicates ability of HERA FL to discriminate • between theory

25. Bonus Extra: - sea quark asymmetry, a study by Max Klein, Burkard Reisert_______________________________ What causes rise of F2 at low x? Have measured 4ū+đ but ū and đ are unknown at low x  would be accessible via deuteron (eD) running at HERA-II ū=đ was a natural assumption for long time, until E866, HERMES found a difference at x ~ 0.1  all global fits followed  Indications for strange-anti-strange asymmetry Are sea and anti-quarks equal ? Are up and down quarks equal at low x?

26. The H1 NLO QCD fit _______________________________  No information on sea flavour composition - only one measurement at low-x:  Assume quark and anti-quark distributions are equal at low-x and u=d experimental errors model uncertainties

27. Releasing the dbar-ubar constraint_______________________________ H1-Only fit (default assumptions) experimental errors model uncertainties H1-Only PDF experimental errors model uncertainties H1-Only PDF 27

28. Releasing the dbar-ubar constraint_______________________________ H1+BCDMS fit experimental errors model uncertainties H1-Only PDF experimental errors model uncertainties H1-Only PDF 28

29. Releasing the dbar-ubar constraint_______________________________ The light sea quark asymmetry is expected and has been assumed to vanish at low x. However, F2 rises strongly towards low x which deserves to be studied. H1+BCDMS fit Q2 =5 GeV2 simulated accuracy (20pb-1 e-D, 40pb-1 e-p)

30. Releasing the dbar-ubar constraint_______________________________ The light sea quark asymmetry is expected and has been assumed to vanish at low x. However, F2 rises strongly towards low x which deserves to be studied. H1+BCDMS fit Q2 =5 GeV2 simulated accuracy (20pb-1 e-D, 40pb-1 e-p)

31. Mini-summary (sea-quark asymmtery)_______________________________  So far HERA has not resolved the light sea quarks at low x QCD fits employ (“reasonable”) assumption that ū=đ and u=ū and d=đ at low-x - this reasonable assumption was proven to be wrong at larger x ~ 0.1  without these requirements the fits become unstable  deuteron data from fixed target experiments (e.g. BCDMS) do help but cannot solve the problem since data lie at higher-x  Would need eD running at HERA-II to resolve this issue

32. Extras …_______________________________

33. Comparison with global fit (u-val.)_______________________________ ZEUS-S global PDF HERA-II Projected PDF  Uncertainties with full HERA-II inclusive data-set comparable to global fits 33

34. Comparison with global fit (d-val.)_______________________________ ZEUS-S global PDF HERA-II Projected PDF  Uncertainties comparable to or better than current global fit 34

35. Impact on sea/gluon uncertainties _______________________________ • already at HERA-I, measurement of optimised jet cross sections would have a significant impact on the high-x gluon