Results on the proton structure from HERA

Transcription

1 Results on the proton structure from HERA Shima Shimizu (CERN) KEK

2 The world only e-p collider: HERA electron proton A unique collider at DESY, Hamburg H ZEUS Circumference: 6.3 km Operated since 99 to 7 DESY, Hamburg collider experiments: H & ZEUS Beam energy proton : (46, 575, 8,) 9 GeV electron/positron: 7.5 GeV center of mass energy s = (5 ~) 38GeV

3 3 History of HERA 99-: HERA-I (started with E p =8GeV, until 997) Make full use of large kinematic region. Integrated Luminosity (pb - ) HERA delivered Luminosity upgrade (-) 8 GeV 9 GeV Low E p.5gev <Q <3GeV -7: HERA-II High luminosity to collect high- Q data. (high-q Weak boson exchange) lepton beams are polarized. Reduced E p runs at the end days of running HERA I HERA II In total, each H and ZEUS gains ~.5 fb -.

4 Deep Inelastic Scattering (DIS) Kinematic variables to describe DIS Q : Virtuality probing power x : Bjorken scaling variable momentum fraction of struck quark y : Inelasticity : center of mass energy s Q p q ' Q = sxy Q = q = ( k k ) x = y = p q p k Inclusive DIS cross sections can be written with structure functions. d σ e p dxdq πα F y ) Y ± ( ) = Y+ ( x, Q F ( x, Q ) xf3 ( x, Q 4 L m Q + Y+ cross section with point-like particle Y ) Structure functions reflect momentum distribution of partons in the proton. Y = ± ( y ± ) 4

5 5 Structure functions and PDFs Measured quantity: reduced cross section σ ( e r ± 4 Q Y+ d σ y Y p) = = F ( x, Q ) F ( x, Q ) L m πα dxdq Y Y Structure functions are used for determination of the parton distribution functions (PDFs), q(x,q ) and g(x,q ) F : [Sea + valence] quarks, (gluon) F = Aq x( q + ) q F L : longitudinal structure function gluon (see later) xf 3 : parity violation term (Electroweak) xf = B x( q ) 3 q q Valence quarks F ln Q The Q evolution can be described by perturbative QCD, using DGLAP equation. + xg sizable only at high y high Q only + xf 3 ( x, Q Y = ± ( y ) ± DGLAP evolution )

6 6 DIS at HERA Pure proton target Free from target correction, nuclear effect. Charged current DIS (CC) W ± exchange Q (GeV ) Neutral current DIS (NC) at low Q γ exchange sea, gluon H ZEUS Fixed Target Experiments: CCFR, NMC, BCDMS, E665, SLAC y= (HERA s=3 GeV) valence Charge selective interaction e - : u quark e + : d quark Neutral current DIS (NC) at high Q γ,z exchange γ F x( q + q ) Sea + valence quark F xg gluon lnq - Fixed target experiments x Sea + valence quark Z introduces parity violation. xf x( q ) 3 q valence quark

7 7 DIS in the detectors Neutral current (NC) process γ/z exchange ep e X e Charged current (CC) process W +- exchange ep νx ν e +/- p e +/- jet jet p Kinematic variables are reconstructed by two of measured variables; energy of scattered electron angle jet (~ struck quark)

8 HERA I inclusive DIS cross sections and HERA PDF. published: JHEP () 9

9 9 Combining H and ZEUS cross sections Combining all the HERA-I inclusive DIS cross sections from H and ZEUS: HERA I: e + p ~pb -, e - p ~6pb - NC:.45 < Q < 3 GeV, CC: 8 < Q < 3 GeV 4 points (4 papers) After combination: 74 points Combination is done by averaging each data point by simultaneous χ fit. Assumption: H and ZEUS measure the same cross sections. Taking account of correlated systematics within/between experiments. Different detectors and analysis techniques bring different sensitivities to similar sorce of correlated systematic uncertainties. Cross calibration of experiments Reduction of systematic uncertainties. JHEP () 9

10 Combining H and ZEUS cross sections σ r,nc (x,q ) x=. H and ZEUS x=. HERA I NC e + p ZEUS H x= x=.3 x=.8 x=.5 Uncertainty gets improved by more than sqrt(). Main reduction in systematic uncertainties. 3 4 Q / GeV JHEP () 9

11 Combined inclusive DIS cross sections H and ZEUS σ r,nc (x,q ) x i x =.5, i= x =.8, i= x =.3, i=9 Low x 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 x =., i=8 x =.3, i=7 x =.5, i=6 HERA I NC e + p Fixed Target HERAPDF. x =.8, i=5 x =.3, i=4 Precision of -% for 3 < Q < 5 GeV. Scaling violation is clearly seen. F ln Q xg x =.8, i=3 x =.5, i= High x - x =.4, i= - x =.65, i= Q / GeV JHEP () 9

12 HERA PDF. Data sets Combined HERA I inclusive DIS cross sections e - pcc, e+pcc, e - pnc (Q >GeV ), e + pnc (Q > Q min ) Settings PDF parameterized at the starting scale Q U = u + c D = d + s + b Central fit has free parameters: chosen by saturation of χ RT-VFN scheme NLO DGLAP evolution using QCDNUM7. Scales: μ R = μ F = Q α S (M Z ) fs=s/d m c (GeV) m b (GeV) Q min (GeV ) Q (GeV )

13 HERA PDF. Uncertainties experimental: Δχ = model: Different set of values of f s, m c, m b, Q min, Q xf parameterization: envelope of parameter variation e.g. Q, negative gluon, relax B u =B d, additional parameter xs (.5) xg (.5) H and ZEUS HERAPDF. exp. uncert. model uncert. parametrization uncert. Q = GeV xu v xd v x 3

14 4 xu v Relative Uncertainties at High Q Q = GeV H and ZEUS xd v Q = GeV HERAPDF. exp. uncert. model uncert. param uncert. xs xu sea xd sea xs sea xc sea xb sea - x Q = GeV xg x Q = GeV At Q = GeV, % uncertainty on gluon for x< x x

15 HERA PDF and Tevatron Data d σ/dy jet dp T jet [nb/(gev/c)] Tevatron Jet Cross Sections D RunII HERAPDF. Cone R=.7 - fastnlo (+ non-perturbative corr.) y jet <.4 (x 6 ) A(W) CDF data HERAPDF. total uncert. MSTW < y jet <. (x -6 ). < y jet <.4 (x -9 ).8 < y jet <.. < y jet <.6 (x -3 ).4 < y jet <.8 (x 3 ) Good description of Tevatron data by HERA PDF. y P T jet [Gev/c] 5

16 HERA PDFs for LHC Several predictions are calculated with reasonable uncertainties. s = 7 TeV HERA PDF. G. Watt, PDF4LHC, Comparison with data has started. Asymmetry Data ( s=7 TeV) MC NLO, CTEQ 6.6 MC NLO, HERAPDF. DYNNLO, MSTW 8 W eν Asymmetry Data ( s=7 TeV) MC NLO, CTEQ 6.6 MC NLO, HERAPDF. DYNNLO, MSTW 8 W μν.. CERN-PH-EP--37 arxiv:hep-ex.3. - L dt = 35 nb.5.5 ATLAS lepton η. - L dt = 3 nb.5.5 ATLAS η 6

17 Further measurements at HERA

18 8 Further input from HERA HERA PDF. uses inclusive DIS cross section in HERA I only. HERA II High Q NC/CC cross sections Increase of electron data NC: More sensitivity to xf 3 i.e. valence quarks (u+d) Increase of positron data CC: More sensitivity to d quark e - p e + p HERA-I pb - pb - HERA-II 8pb - 7pb - Inclusive Jet cross sections Longitudinal structure functions Heavy flavour structure functions

19 σ r,nc (x,q ) ± HERA I+II NC cross sections HERA II NC are also combined. H e + p, e - p ZEUS e - p Improved precision. Clear separation of e + p and e - p at high Q HERA I NC e + p HERA I NC e - p xf H and ZEUS HERAPDF. e + p HERAPDF. e - p x =. (x3.) x =.3 (x7.) x =.5 (x9.) x =.8 (x5.) x =.3 (x.) x =.8 (x8.) x =.5 (x.4) x =.4 (x.7) x = Q / GeV = B x( q ) 3 q q σ r,nc (x,q ) ± HERA I+II NC e + p (prel.) HERA I+II NC e - p (prel.) H and ZEUS x =. (x3.) x =.3 (x7.) HERAPDF. e + p HERAPDF. e - p x =.5 (x9.) x =.8 (x5.) x =.3 (x.) x =.8 (x8.) x =.5 (x.4) x =.4 (x.7) x = valence quark Q / GeV to be included: ZEUS 6-7 e + pnc HERA Structure Functions Working Group June 9

20 HERA I+II combined CC cross sections [ ( ) ( )] u + c + y d s CC σ r ( e p) + [ ( ) ( )] u + c + y d s CC + σ r ( e p) + H and ZEUS H and ZEUS σ r,cc (x,q ) Q = 3 GeV Q = 5 GeV Q = GeV Q = 5 GeV Q = GeV Q = 3 GeV Q = 5 GeV Q = 8 GeV Q = 5 GeV Q = 3 GeV x HERA Inclusive Working Group June HERA I+II CC e - p (prel.) HERAPDF. σ r,cc (x,q ) Q = 3 GeV Q = 5 GeV Q = GeV Q = 5 GeV Q = GeV Q = 3 GeV Q = 5 GeV Q = 8 GeV Q = 5 GeV Q = 3 GeV x HERA Structure Functions Working Group June HERA I+II CC e + p (prel.) HERAPDF x x Sensitive to valence quarks.

21 HERA PDF.5 (prel.) Using HERA I+II combined data. Same setting as HERA PDF.. xf H and ZEUS HERA I Combined PDF Fit xg (.5) xs (.5) -3 HERAPDF. exp. uncert. model uncert. parametrization uncert. - Q = GeV xu v xd v - x xf H and ZEUS HERA I+II Combined PDF Fit xg (.5) xs (.5) -3 HERAPDF.5 (prel.) exp. uncert. model uncert. parametrization uncert. - Q = GeV xu v xd v - x HERA Structure Functions Working Group July Uncertainties at the high x region reduced.

22 Inclusive Jet measurements in HERA II Directly sensitive to gluon density at medium x. g dσ/de jet T,B (pb/gev) ZEUS ZEUS (prel.) 3 pb - NLO hadr Z jet dσ/de T (pb/gev) rel. diff. to NLO ZEUS E jet T > GeV, E jet T > 7 GeV - < η jet <.5 Q < GeV. < y <.85 jet energy scale uncertainty ZEUS-S/AFG4 ZEUS-S/CJK ZEUS (prel.) 89 pb - NLO hadr: p/γ PDFs (Klasen et al.) ZEUS-S/GRV-HO MSTW8/GRV-HO jet E T (GeV) jet - < η B <.5 cos γ h <.65 jet energy scale uncertainty 5 < Q < 5 GeV (x) 5 < Q < 5 GeV (x) 5 < Q < GeV (x) < Q < GeV (x) < Q < 5 GeV (x) Q > 5 GeV (x) E jet T,B (GeV) Also allows simultaneous constraints of PDFs and α S.

23 < 3 Longitudinal structure function: F L F L is proportional to the cross section of longitudinal photon interacting with proton. FL σ L In naïve QPM, proton has co-linear spin ½ quarks only. gluon emission in the proton F L i.e. F L directly reflects gluon dynamics in the proton. In pqcd: γ * < q q Longitudinal photon cannot interact with a quark F L = α = s dz 6 + x F L x F 8 eq zg ( z) x 3 4π z 3 q z Measurement of F L is good test for the current understanding of proton structure and QCD. gluon PDF

24 4 Measurement of F L Measured cross section is a combination of F and F L. y F ( x, Q ) FL ( x, Q Y ~ ± At low Q σ ( e p) = ) Y+ = + ( + < Y+ < Separation of F L from F. Cross sections at the same (x, Q ) but the different y y Q sx = multiple beam energies HERA was successfully operated with low E p for its last months. Data sets with three different energy are available. s = 38, 5, 5GeV (E p = 9, 575, 46 GeV) Direct F L measurements at low x (x: -4 ~ -3 ) = gluon dominance. Extraction of structure functions without QCD assumption Consistency check of pqcd framework for the proton structure. y )

25 Reduced cross sec. at E p = 46,575,9GeV The data covers.85 > y >. Difference at low-x (i.e. high-y) F L σ r σ r σ r σ r Q =.5GeV Q =3.5GeV H and ZEUS Q =5GeV Q =6.5GeV Q =8.5GeV Q =GeV Q =5GeV Q =GeV Q =4GeV Q =3GeV Q =45GeV Q =6GeV Q =8GeV Q =GeV Q =5GeV Q =GeV Q =5GeV Q =3GeV Q =4GeV -4 - x Q =5GeV -4 - x Q =65GeV -4 - x Q =8GeV -4 - x Note: ZEUS covers 4<Q < GeV. Still trying to go lower Q HERA prel. E p =9 GeV E p =575 GeV E p =46 GeV HERAPDF. σ r 9 σ r 575 σ r 46 HERA Inclusive Workging Group March 5

26 6 F L extraction σ r (x,q,y) Q =3 GeV.6 X= X= X=.67 X=.97.5 H and ZEUS X=.74 X=.9 HERA prel. E p =9 GeV E p =575 GeV E p =46 GeV Linear fit.5 y /(+(-y) ) HERA Inclusive Workging Group March ~ ± p) = F (, ) (, x Q FL x Q Y+ σ ( e At given (x, Q ) bin, F L should be seen as a negative slope. y )

27 HERA F L Q / GeV F L, averaged over x, for each Q bin Good agreement between data and prediction for Q > GeV. 7 x FL HERA preliminary HERAPDF. H and ZEUS HERA Inclusive Workging Group March

28 8 Heavy Flavour production Dominant process of heavy quark production: Boson-Gluon-Fusion (BGF) Two schemes to treat heavy quarks in pqcd; massive scheme (FFN) appropriate for Q ~ M q Heavy quarks are produced via BGF. Sensitive to gluon PDFs massless scheme (ZMVFN) appropriate for Q >> M q Heavy quarks are massless and exist in the proton if Q is above the mass threshold. Intrinsic heavy quarks PDFs May affect on description of low Q F L. F L x HERA preliminary RT optimized H and ZEUS ACOT full ACOT-χ FFNS.3 Q / GeV HERA Inclusive Working Group April

29 9 Charm mass impact on LHC W cross sec. W + cross section m c :.4 GeV.65 GeV. m c value in HERA PDF. does affect on LHC W cross section by ~ 3%.

30 3 HERA combined F cc F cc is extracted for large kinematic region. different methods _ cc F D mesons by slow pions Impact parameter tagging Muons from semileptonic decay different data sets HERA I and II (partial) Combination gives very precise data Q =GeV -4-3 Q = GeV -3 Q =6 GeV -3 - Q =4GeV - - Q =4GeV -4-3 Q = GeV -3 - Q =GeV Q = GeV x Q =6.5GeV -4-3 Q =35 GeV -3 - Q =GeV October 9 HERA Heavy Flavour Working Group - - HERA (prel.) H D* HERA II (prel.) H D* HERA I H LTT HERA II (prel.) H LTT HERA I ZEUS μ 5 ZEUS D + 5 ZEUS D 5 ZEUS D* 99- ZEUS D* 96-97

31 HERA F cc Precision is 5-%. Possible to constrain theory calculation. _ cc F Better understanding of charm treatment and m c in QCD fits can be expected.. H and ZEUS Q = GeV Q = 4 GeV.5 Q = GeV Q = GeV.5 Q = 6.5GeV Q = 35GeV.5 Q = 6GeV Q =GeV Q =GeV Q =4GeV Q =GeV x HERA (prel.) October 9 HERA Heavy Flavour Working Group MSTW8 NNLO MSTW8 NLO CTEQ 6.6 GJR8 FFN ABKM BMSN ABKM FFNS NLO FFN ABKM FFNS NNLOFFN 3

32 F bb ZEUS Methods: Impact parameter tagging, μ+jet F bb _ +.3 i x=.3 i=7 x=. i=6 x=.5 i=5 ZEUS (prel.) vtx 354 pb - ZEUS (prel.) e 363 pb - ZEUS μ 4 pb - ZEUS μ+vtx 6 pb - H vtx ZEUS-S+HVQDIS GJR8 NLO ABKM NNLO Measurements are consistent with each other x=.3 i=4 x=. i=3 MSTW8 NLO MSTW8 NNLO CTEQ6.6 NLO Combination of H+ZEUS will provides improved precision.5.5 x=.5 i= x=.3 i=. x=.3 i= 3 Q (GeV ) 3

33 33 Summary HERA results are valuable input for to understand the proton structure. Combination of H and ZEUS measurements provides very precise data. A very good PDF determination in HERA PDF.. Using the full HERA II statistics, the analyses of high-q NC/CC, jets and heavy flavour processes will improve the precision of the results. Further understanding of the proton structure may come.

34 Backup

35 Input for HERA I combined data 35

36 Impact of combined data on CTEQ PDFs from M. Guzzi, DIS, Floarence 36