Inclusive Deep-Inelastic Scattering at HERA

Transcription

1 Inclusive Deep-Inelastic Scattering at HERA Vladimir Chekelian (MPI for Physics, Munich) on behalf of the H and ZEUS Collaborations Completion of the HERA inclusive DIS cross section measurements: HERA the ep collider NC at E p =46, 575 GeV and model independent F L measurements. NC measurements at highest x! 3. Combination of all HERA I+II NC&CC inclusive measurements (HERAPDF.) ICHEP 4 Valencia 4.7.4

2 Deep-Inelastic Scattering (DIS) Neutral Current (NC) : e ± p e ± X P H q 9,8,575, 46 GeV 7.5 GeV k k Charged Current (CC) : e ± p ν X ZEUS = -q = -(k-k ) virtuality of γ*, Z, W x = /(Pq) Bjorken x y = (Pq)/(Pk) inelasticity = sxy s=(k+p) ICHEP 4 Valencia H+ZEUS in total fb - - about equally shared between e + and e -, positive and negative P e Inclusive DIS at - HERA special running at low proton energy for F L

3 . NC cross section measurements at high y and low Ep=46, 575 (and 9) GeV H: NC high : Eur. Phys. J. C 74 (4) 84 (previously published NC low : Eur.Phys.J.C7 () 579) ZEUS: NC nominal and satellite : DESY-4-53 measurements at E p = 46, 575, 9 GeV à supersede results of Phys. Lett. B 68 (9) 8 nominal satellite ICHEP 4 Valencia

4 NC at high y for E p = 46, 575 (and 9) GeV Experimental challenge: large γp background at high y (low scattered electron energy) E e =3-6 GeV H: E e down to 3 GeV à soft electron identification : optimal use of information on shower shape in LAr calorimeter, momentum matching with the track, de/dx à accept only electron candidates with the right electric charge and use the wrong charge events for estimation of remaining background. ZEUS: E e down to 6 GeV à backward tracking : use hits in the tracking detectors à remaining bkg is subtracted using MC predictions verified from 6m tagger and γp enriched sample (agreement within %) ICHEP 4 Valencia

5 A model independent measurement of F L using data at E p =46, 575 and 9 (8) GeV à F L and F can be determined in a model independent way at each x and σ NC (x,,y) = F (x, ) f(y) F L (x, ), f(y)=y /(+(-y) ) (8) (9) ICHEP 4 Valencia ZEUS (575) (46) y /(+(-y) ) Measurements at E p =8 GeV (ZEUS97) are included in fits ~ NC.7 = 6 GeV, x= = 6 GeV, x=.58 E p = 9 GeV H, SpaCal.9.5 an example: =6 GeV and 6 values of x = 6 GeV, x=.4 = 6 GeV, x=.8 E p = 575 GeV H, LAr H, SpaCal.5 F H Collaboration = 6 GeV, x=.39 FL = 6 GeV, x=.4 E p = 46 GeV H, LAr H, SpaCal Linear fit.5 y /(+(-y) ) H: high 46/575 together with 46/575 data at low (Spacal) and 9 e + p data from HERA II 5

6 F L and F measurements as a function of and x F, F L.5.5 using a χ minimisation technique accounting for correlations across all measurements = 35 GeV = 45 GeV = 6 GeV H Collaboration = 9 GeV.5 = GeV = 5 GeV = GeV = 5 GeV.5.5 = 3 GeV = 4 GeV = 5 GeV = 65 GeV = 8 GeV x x x H F H F L - ZEUS F ZEUS F L -3 - HPDF F HPDF F L -3 - x F L measurements are extended to = 8 GeV average F L (x, ) measurements at given! ICHEP 4 Valencia

7 Longitudinal structure function F L F L is a pure CD effect sensitive to gluon density H and ZEUS α s dz 6 x FL ( x, ) = x F 3 8 eq ( ) xg 4π + x z 3 q z F L.4. 4 x H ZEUS approximate relation between F L and gluon (order of α s, with a=) Z. Phys. C39 (988) 8, Nucl. Phys. B 383 (99) 55 Eur. Phys. J. C7 (), arxiv: HERAPDF.5 NNLO CT NNLO MSTW8 NNLO ABM NNLO NNPDF.3 NNLO JR9 NNLO [GeV ] Consistency of the H and ZEUS FL data was checked accounting for corr. errors: χ /ndf=/8 (p-value=%). R=σ L /σ T =F L /(F -F L ) =.3±.4 (H,.5 8 GeV ) R= (ZEUS, 9 GeV ) 3 ICHEP 4 Valencia

8 . Integrated e ± p NC cross section at high xà NC events at high have about % acceptance and efficiency for the scattered electron but at highest x the hadronic final state disappears in the beam pipe and there are no means to measure x ZEUS measured the integrated e ± p NC cross sections at xà using events without jets at x above x edge. Phys. Rev. D 89 (4) 77 ICHEP 4 Valencia

9 NC e ± p cross section at highest x e + p NC e - p NC à there is sensitivity to PDFs at high xà. These integrated measurements are not used so far in the CD fits (and in the combination below) ICHEP 4 Valencia

10 3. Combination of all inclusive NC and CC e ± p data from H an ZEUS 4 data sets from H and ZEUS ( fb - ):.45 5 GeV, 6-7 x.65 data sets from HERA I (E p =9 and 8 GeV) and data sets from HERA II (/4/4 sets for E p = 9/575/46 GeV) Combination of the H & ZEUS incl. unpolarized NC and CC data include expert knowledge in the treatment of the correlations between individual data sets. à precise, complete and easy in use à reduction of stat. and syst. uncertainties. HERA I data: JHEP :9, HERAPDF.. HERA I and preliminary HERA II data HERAPDF.5 HERAverager (wiki-zeuthen.desy.de/heraverager) is used for the cross section averaging, 6 corr. syst. sources are treated as multiplicative, the following χ definition is used: ICHEP 4 Valencia Three additional procedural errors : - multiplicative vs. additive - correlation over all data sets of photoproduction bkg and hadronic energy scale uncertainties

11 Averaging of all NC and CC HERA I+II data 97 cross sections are combined to 37 points with 65 correlated systematic errors NC e + p Ep=9 GeV r, NC (x, ) +.5 = GeV H and ZEUS preliminary NC e + p Ep=46 GeV = 5 GeV.5.5 = 5 GeV = 35 GeV H, ZEUS, combined = 6 GeV -3 - x -3 - HERA NC e + p (prel.) 6 pb - s = 5 GeV ZEUS H x à up to 6 measurements are combined into one averaged point à good consistency of the input data sets (χ /ndf = 685/6 ) ICHEP 4 Valencia 4.7.4

12 Pulls for different samples. Entries NC e p 3.5 GeV RMS = H and ZEUS preliminary pull Entries NC e p 3.5 GeV < GeV RMS = pull Entries NC e p GeV < 5 GeV RMS = pull à everywhere consistent with expected one sigma gaussian Entries RMS =.95 - NC e p ICHEP 4 Valencia pull Entries RMS =.4 + CC e p Inclusive DIS pull at HERA Entries RMS =.97 - CC e p pull

13 Comparison of combinations HERA I+II vs. HERA I r, NC (x, ). H and ZEUS preliminary x=.8 HERA NC e - p (prel.).4 fb - s = 38 GeV HERA I NC e - p r, CC (x, ) 3 = 3 GeV H and ZEUS preliminary = 5 GeV = GeV = 5 GeV CC e - p.8 x=.3 = GeV = 3 GeV = 5 GeV = 8 GeV.6 x= x=.5.6 = 5 GeV = 3 GeV x. 3 4 /GeV x HERA CC e - p (prel.).4 fb - s = 38 GeV HERA I à significant improvements in precision at high (especially for e - p NC&CC) and at high y: about % precision in the best measured regions ICHEP 4 Valencia

14 r, NC (x, ) x i Combined NC and CC data set from HERA (HERAPDF.) e ± p NC&CC (E p =9 GeV), e + p NC (E p = 8, 575, 46 GeV), corresponding to fb - à 65 correlated syst. err.;.45 5 GeV, 6-7 x.65 H and ZEUS preliminary NC e ± p x =.5, i= x =.8, i= x =.3, i=9 x =., i=8 x =.3, i=7 x =.5, i=6 x =.8, i=5 x =.3, i=4 x =., i=3 x =.3, i= x =.5, i= x =.8, i= x =.3, i=9 HERA NC e - p (prel.).4 fb - HERA NC e + p (prel.).5 fb - s = 38 GeV Fixed Target HERAPDF. (prel.) e - p HERAPDF. (prel.) e + p NLO, = 3.5 GeV min x =., i=8 x =.3, i=7 x =.5, i=6 x =.8, i=5 x =.3, i=4 x =.8, i=3 x =.5, i= x =.4, i= x =.65, i= r, CC (x, ) = 3 GeV H and ZEUS preliminary = GeV = 3 GeV = 5 GeV = 5 GeV = GeV = 5 GeV = 3 GeV = 5 GeV - - = 8 GeV - - HERA CC e + p (prel.).5 fb - s = 38 GeV CC e + p HERAPDF. (prel.) NLO, = 3.5 GeV min x ICHEP 4 Valencia / GeV à for CD analysis of the combined data (HERAPDF.) see talk of Katarzyna Wichmann x 4

15 Conclusions H and ZEUS completed inclusive DIS cross section measurements at HERA - e + p NC cross sections measurements at low E p =46 and 575 GeV and a model independent determination of F L. - ZEUS e ± p NC measurements at high x! All inclusive e ± p NC and CC cross sections at E p =9, 8, 575 and 46 GeV are combined in one coherent HERA data set which is used as a sole input to the HERAPDF. CD fits ICHEP 4 Valencia