Measurement of F L at HERA. S. Glazov, DESY, Ringberg 2008

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1 Measurement of F L at HERA S. Glazov, DESY, Ringberg 8

2 DIS kinematics Kinematics of inclusive scattering is determined by and Bjorken x. In x scale parameter / - equal sharing among quarks. Proton structure for x.5 valence quarks x.5 coupled quark-gluon CD evolution. Large gluon density. At small x complex dynamics which must obey simple asymptotic solutions (unitarity). For low, inclusive cross section is described by two structure functions: d ] σ dxd = πα Y + 4 x σ r = πα Y + [F 4 (x, ) y F L (x, ) x Y + where factors Y + = + ( y) and y define polarization of the exchanged photon and y = /(sx).

3 The Proton Structure Functions at low For low : which implies F L F. F σ L + σ T F L σ L In uark-parton Model F L = for spin / quarks. In CD, F L > due to gluon radiation. At low x, sea quark and gluon density are measured using F and its scaling violation, df /dlog. F L measures gluon via cross section polarization decomposition.

4 F L at low x and low R. Thorne, DIS8 Significant spread of predictions for low and low x = /(Sy) Large higher order perturbative corrections. Small x resummation. Higher twist effects. 4

5 H Detector LAr Use the scattered electron to reconstruct event kinematics e Central Tracker SpaCal p e = 4E e E e cos θ e y = E e sin θ e E e x = Sy H Medium, 9 GeV, SpaCal+CT (DESY-8-5) H High, 5 8 GeV, LAr + CT (H preliminary). ZEUS, 4 GeV, CTD+CAL (ZEUS preliminary) 5

6 Measurement Strategy σ r (x, ; y) = F (x, ) + ( y) F L(x, ) Measure at the same x,, different y use different E p Increase sensitivity by using largest spread in f(y) = y /( + ( y) ): Ep max /Ep min max, y. y H Integrated Luminosity / pb - 4 Status: -July-7 electrons positrons low E HERA- Integrated Luminosity / pb - HERA low E p run 5 HERA deliv: 46 GeV 575 GeV H physics: 46 GeV 575 GeV 5 Status: -July-7 HERA Days of running Days since -Jan-7 6

7 High y Experimental Challenge Measurement at both low y >. and high y <.9 are required. High y is much more difficult. y E e E e Measurement extends down to E e = GeV. Trigger efficiency/rate Electron identification Background Radiative corrections 7

8 σ red.4... Previous H High y σ r Measurements SpaCal H Preliminary LAr Y=.85 H HERA-II prelim. (W=89 GeV) H 997 (W=7 GeV) 5 5 /GeV σ NC H y=.75 e p HERA-II (Prelim.) 4.% Norm. error not shown e p HERA-I.5% Norm. error not shown H has already analyzed high y data in both SpaCal and LAr calorimeters. HERA-II data allows to reduce errors, due to large e + and e samples. For LAr sample, low energy cut was at E e > 5 GeV x /GeV 8

9 Consistency check: H F L determination at high F L y= x H PDF H Low y fit e + p H 99- e p H F L =F (H PDF ) / GeV H Collaboration Determination ( of F L ) as F L = Y + y F fit σ r Use CD fit to obtaine F fit. Consistency check of gluon determined from F scaling violation vs X-section decrease at high y. 9

10 Electron Identification at High y High efficiency with significant reduction of background. Cluster transverse/longitudinal shape requirements. Cluster-track geometric matching to reduce/estimate background directly from data events E p = 46 GeV D CJC-SpaCal / cm Additionally for H-LAr sample and E e < 6 GeV require p track t /Et cluster kinematic match. E p = 46 GeV H Preliminary events.5 Data NC MC + BG BG (data) 5 / cm D CJC_LAr events Ecra / cm events.5 track P t events /E t cluster H Preliminary Data MC+BG BG (data) Medium region E h /E e High region

11 ZEUS background estimation ZEUS MC simulation for photoproduction background. Use events with the true scattered e detected in 6m tagger (tagged) to normalize/check. Events ZEUS (prel.) - s=5 GeV (pb ) PYTHIA γp MC E e (GeV)

12 H Background Estimation e + p scattering: + Scattered lepton has the beam charge (positive). Background from hadronic particles, γ conversions is almost charge symmetric: N + bg N bg require positive charge for the signal sample. Estimate remaining background using negative sample. events 5 5 negative e-candidate charge positive e-candidate charge E/p Background charge asymmetry is measured by comparing e + p, e p samples and using tagged photoproduction events.

13 Radiative Corrections Radiative corrections are large at high y, δ = σ total σ born > 5%. Simulated in DJANGO MC, checked with HECTOR program. Mostly from initial state radiation (ISR) - radiative return to low y and low (σ /y and σ / 4 ). δ % Total Correction ISR Total-ISR y Remove/check ISR radiation using the measured lepton beam energy: E e E p Z in = E p Z out = ( E h p h Z) + (E e p e Z) E p Z h e

14 Control plots: High y medium (H SpaCal) E p = 46 GeV H Preliminary events events 4 Data MC+BG BG (data) E e / GeV events events Θ e / deg 4 Medium region Before background subtraction E p = 46 GeV E e >.4 GeV. Lines indicate cut values E p z is effective against background -4-4 Z vtx / cm E-p z / GeV 4

15 Control plots: High y medium (H SpaCal) events events E p = 46 GeV Data MC E e / GeV events events.5.5 H Preliminary Θ e / deg Medium region After background subtraction E p = 46 GeV E e >.4 GeV. Lines indicate cut values E p z is effective against ISR radiation -4-4 Z vtx / cm E-p z / GeV 5

16 Control plots: High y high (H LAr) E p = 46 GeV H Preliminary events events Data NC MC + BG BG (data) events Electron Energy / GeV events / deg Θ elec High region Before background subtraction E p = 46 GeV E e > GeV. Additional cuts at E > 6 GeV introduce step E p z is also well described - / cm Z vtx E-P z / GeV 6

17 Control plots: High y high (H LAr) events.8.6 E p = 46 GeV events.8 Data NC MC.6 H Preliminary events Electron Energy / GeV - / cm Z vtx events / deg Θ elec E-P z / GeV Good description of data by MC. High region After background subtraction E p = 46 GeV E e > GeV. Additional cuts at E > 6 GeV introduce step 7

18 Control plots: ZEUS E p = 575 Events ZEUS Events 5 4 Events ZEUS Events (GeV) E e E-P z (GeV) (GeV) E e E-P z (GeV) Events Events ( ) θ e (cm) z vtx Events γ ( ) ZEUS (prel.) s=5 GeV (6pb - ) MC DIS (F =) + γp L MC γp MC uses F L = had Events Events ( ) θ e (cm) z vtx Events MC γp 5 5 γ ( ) ZEUS (prel.) s=5 GeV (6pb - ) MC DIS + γp MC reweighted using F L = F CD had 8

19 H σ r for E P = 46, 575 and 9 GeV For (almost) each, x measurements at three E p. Mix of SpaCal and LAr data Turn over of the cross section from F is due to F L y) (x,, σ r H Preliminary medium & high = GeV = 5 GeV = GeV = GeV = 8 GeV x = 5 GeV = 45 GeV.5 = 5 GeV = 4 GeV = GeV = 6 GeV = GeV = 5 GeV = 5 GeV = 9 GeV - - = 5 GeV - - = 65 GeV 9 E = 9 GeV H PDF p 575 E = 575 GeV H PDF p 46 E = 46 GeV H PDF p F σ r σ r σ r H PDF

20 ZEUS σ r for E P = 46, 575 and 9 GeV ZEUS σ ~ = 4 GeV = GeV = 45 GeV = 6 GeV = 8 GeV ZEUS (prel.) s = 5 GeV (4.pb - ) s = 5 GeV ( 6.pb - ) s = 8 GeV (.8pb - ) = GeV - - ZEUS-JETS - x

21 F L extraction, y) (x, σ r x =.49 x =. = 5 GeV x =.6 x =.6 x =.76 x =.5 H Data E p E p H = 9 GeV = 575 GeV E p = 46 GeV Linear fit y / Y + y σ r (y) = F + ( y) F L Linear fit to get F and F L Relative normalization from low y data Data at E p = 575 provides cross check and extends measurement to low x.

22 The First Measurement of F L at HERA ) (x, F L.5.5 = GeV = 5 GeV = GeV H.5 = 5 GeV = 5 GeV = 45 GeV.5.5 = 6 GeV = 9 GeV H Data.5 - H PDF - Agree well with CD prediction. Released for Moriond CD, March 8. DESY-8-5. x -

23 Measurement of F L by ZEUS ZEUS F L = 4 GeV = GeV = 45 GeV ZEUS (prel.) ZEUS-JETS = 6 GeV = 8 GeV = GeV - s = 5 GeV (4.pb ) - s = 5 GeV ( 6.pb ) - s = 8 GeV (.8pb ) Updated for ICHEP to include E p = 575 GeV data. Consistent with NLO prediction and H. x

24 H F L measured at medium ) (x, F L H Data H PDF CTE 6.6 MSTW 7 x / GeV Average data for each takinig into account correlated systematic uncertainties. Typical error: total., uncorrelated.5. Agreement with expectations. 4

25 F L extraction: H overlap region H Preliminary σ r.5 =5GeV x=.8 =5GeV x=.65 =45GeV x=.84 =45GeV x=.9 σ r σ r σ r =45GeV x=. =6GeV x=.4 =9GeV x=..5 =45GeV x=. =6GeV x=.6 =9GeV x=.4.5 =6GeV x=. =9GeV x=.7 =6GeV x=. =9GeV x=.9 SpaCal E p =9 GeV SpaCal E p =575 GeV LAr E p =575 GeV LAr E p =46 GeV Linear fit Medium & High Repeat linear fits to determine F and F L for the SpaCal/LAr overlap region Blue points SpaCal Magenta points LAr y /[ + ( - y ) ] Complementarity of the two fully independent analyzes. 5

26 The Combined Measurement of F L by H ) (x, F L = GeV = 5 GeV = GeV.5 = GeV.5 = 5 GeV = 45 GeV = 5 GeV.5 = 4 GeV H Preliminary F = 8 GeV x L = GeV = 6 GeV.5 = GeV.5 = 5 GeV = 5 GeV = 9 GeV.5 = 5 GeV.5 = 65 GeV.5 H (Prelim.) = 46, 575, 9 GeV E p H PDF - medium & high Bins 5 9 GeV improved, new bins at GeV

27 Average FL by H L H Preliminary F ) H (Prelim.) H PDF.5 = 46, 575, 9 GeV E p (x, FL x medium & high / GeV Average using total errors, compare to prediction based on H CD fit to published by H DIS cross section data. 7

28 Average FL by H vs theory L H Preliminary F ) H PDF CTE 6.6 MSTW H (Prelim.).5 = 46, 575, 9 GeV E p (x, FL x medium & high / GeV H FL measurement agrees with CD calculations. 8

29 World measurements of R R.75.5 = GeV = 5 GeV = GeV.5 R R = 5 GeV = 5 GeV = 6 GeV = 9 GeV = 45 GeV - - x H 8 F L direct [arxiv:85.89] H 997 F L indirect Using F from H CD fit F L can be recalculated to R = F L F F L = σ L σ T - - x - - x NMC SLAC HPDF BCDMS R from indirect F L determinations and direct measurement can be compared to the fixed target experiment. Precision on R from HERA is similar to fixed target experiments. 9

30 Towards measurement at low H Backward Silicon Tracker covers < < GeV range for y >.6. E/p - - Allows scattered angle reconstruction/charge determination for the electron candidate E e GeV

31 Summary First measurement of the longitudinal proton structure function F L at HERA. Good agreement between H and ZEUS results. The measurements agree with CD expectations which are based on HERA measurements of the structure function F and its dependence. Still to come: measurement at < GeV using H backward silicon tracker.