Results on the proton structure from HERA

Transcription

1 Results on the proton structure from HERA Shima Shimizu (Univ. of Tokyo)

2 Introduction HERA physics Proton structure

3 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 collider eperiments: H & ZEUS proton 9 GeV electron/positron 7.5 GeV center of mass energy s = 38GeV 3

4 HERA looks into the proton HERA λ s e > Rutherford scattering = 38GeV ~ ~ s 8 min ma > λ ~ p ~ 5 GeV m Resolved dimension [fm]... Rutherford Hofstadter SLAC ep nuclei proton partons CERN μ,ν N HERA [m] -6 HERA can have a resolution of / of proton size.. THERA Year 4

5 Proton has a structure Proton has its subcomponents: partons Naive parton model u u d quarks: valence quarks CD quark emits gluon. gluon splits to qq or gg. Many quarks with lower momentum: Sea quarks 5

6 HERA looks into the proton ep collision scattering of electron and a quark. It reflects the proton structure. Cross section of ep scattering = Cross section of eq scattering proton structure Details come later, but; F : structure function total charge-squared weighted number of quarks in the proton : momentum transfer resolution : momentum fraction of a parton to the proton 6

7 Before HERA Fied target eperiments only. It was unclear how to describe the proton structure at low-. perturbative CD: quarks are asymptotically free. hadronic view : partons are confined in the proton Kinematic region accessible by fied target eperiments momentum () distribution of quarks at a certain resolution ( =GeV ) F total (chargesquaredweighted) number of quarks in the proton pcd Hadronic momentum fraction 7

8 HERA opened the new kinematic region HERA has epanded accessible kinematic region largely. orders in both and / GeV 4 3 HERA 994- Fied Target Eperiments: NMC BCDMS E665 SLAC y = With HERA data F total (chargesquaredweighted) number of quarks in the proton pcd - y = HERA measurement momentum fraction steep rise at low pcd descriptions is verified. 8

9 Plenty of Gluon and Sea quarks Steep rise of F at low. higher = finer resolution more low-momentum quarks are visible i.e. Sea quarks Sea quarks are generated by gluon. higher Abundant Sea quarks Abundant gluons Gluon and Sea quarks physics is started by HERA. 9

10 Introduction HERA physics Proton structure

11 Deep Inelastic Scattering (DIS) Kinematic variables to describe DIS : Virtuality probing power : Bjorken scaling variable momentum fraction of struck quark y : Inelasticity ' p = q = ( k k ) = y = p q p = sy s = center of mass energy q k

12 Deep Inelastic Scattering (DIS) Electroweak proton structure CD CD Kinematic variables to describe DIS : Virtuality probing power : Bjorken scaling variable momentum fraction of struck quark y : Inelasticity ' p = q = ( k k ) = y = p q p = sy s = center of mass energy DIS is a convolution of electroweak (EW) physics and the proton structure. Good prove to the proton structure Sensitive to EW physics from space-like view. Hadronic final state is also sensitive to CD. (not covered in this talk) q k

13 DIS in the detectors Neutral current (NC) process γ/z echange ep e X Charged current (CC) process W +- echange ep νx ν e e +/- p e +/- jet jet p Kinematic variables are reconstructed by two of measured variables; energy scattered electron of angle jet (~ struck quark) 3

14 History of HERA 99-: HERA-I (started with E p =8GeV, until 997) measurements go down to low- Make full use of large kinematic region..5gev < <3GeV -7: HERA-II High luminosity to collect high- data. (high- Weak boson echange) lepton beams are polarized. Increased sensitivity to EW. Integrated Luminosity (pb - ) HERA delivered Luminosity upgrade (-) days of running Some of results will be shown in net slides. 4

15 F measurement F is measured over 4 orders of magnitude in (, ). em F -log () 5 4 HERA F =6.3E-5 =. =.6 =.53 =.4 =.5 =.63 =.8 =.3 =. =.3 ZEUS NLO CD fit H PDF fit H 94- H (prel.) 99/ ZEUS 96/97 BCDMS E665 NMC low strong dependence on scaling violation more is visible. Fied target eperiments =.5 =.8 3 =.3 =. =.3 =.5 =.8 =.3 =.8 =.5 =.4 = higher (GeV ) 5

16 EW unification (NC/CC cross sections) ) (pb/gev dσ/d Charged Current + H e p CC 3-4 (prel.) - H e p CC 5 (prel.) + ZEUS e p CC 4 - ZEUS e p CC 4-5 (prel.) + SM e p CC (CTE6M) - SM e p CC (CTE6M) y <.9 P e = 3 HERA II + H e p NC 3-4 (prel.) - H e p NC 5 (prel.) + ZEUS e p NC 4 - ZEUS e p NC 4-5 (prel.) + SM e p NC (CTE6M) - SM e p NC (CTE6M) Neutral Current 4 (GeV ) dσ d boson Weak bosons are heavy. σ weak is small low NC: γ-echange CC: W-echange σ(nc) >>σ(cc) high NC: γ/z CC: W-echange σ(nc) ~σ(cc) Electroweak unification + M 6

17 Polarized CC cross sections (pb) σ CC e - p ± Charged Current e p Scattering SM (H PDF ) - e p νx H Data 5 (prel.) H Data ZEUS Data 4-5(prel.) ZEUS Data e + p νx H Data ZEUS Data > 4 GeV y <.9 e + p Weak process = No right-handed current CC is purely weak process linear dependence on polarization. left handed P e right handed 7

18 Introduction HERA physics Proton structure

19 9 Parton Distribution Function (PDFs) Parton distribution functions are used to describe the proton structure. Valence quarks: Sea quarks: PDFs evolve with. The evolution is described by DGLAP equation, based on pcd. ( ) ( ) ( ) ( ) [ ] + =,, ) ( ln qg qq s q q y g y P y q y P y dy e d df π α ), ( q ), ( q ), ( g ), ( ), ( ), ( ), ( Sea s s u u = = = etc. ), ( ), ( ), ( Sea Val u u u =, same for d quark Larger allows to see more quarks.

20 Structure functions DIS cross sections can be written with structure functions. d σ e p dd πα F y ) Y ± ( ) = Y+ (, F (, ) F3 (, 4 L m + Y+ Y ) Y = ± ( y ± ) cross section with point-like particle Structure functions: they reflect momentum distribution of partons in the proton. Structure functions are sensitive to PDFs. F F L F 3 : total number of quarks F = Aq ( q + q ) : longitudinal structure function gluon only sizable at high-y Details will come up later : parity violation term F = B q ( q ) Valence quarks 3 q

21 Etraction of PDFs q(, g(, ) ) evolution of PDFs can be predicted by perturbative CD, i.e. by DGLAP equation. -dependence of PDFs can be etracted from fits to measured cross sections. PDFs@ PDFs@ Input Evolution in : DGLAP@NLO Fit to measured cross PDFs are = 7GeV f b c ( ) = A ( ) ( + d) for u, d, S, g, Δ( = d u ) A: Normalization, b: Low, c: High, d: smoothing for middle Constraints from momentum and number sum rule, etc. free parameters v

22 PDF etraction at HERA A single eperiment can determine PDFs. Jets cross sections gluon γechange sea scaling violation gluon (GeV ) sea, gluon H ZEUS Fied Target Eperiments: CCFR, NMC, BCDMS, E665, SLAC y= (HERA s=3 GeV) valence Fied target eperiments CD + EW physics Z echange sea + valence valence only W ± echange - charge selective u or d quark Pure proton target Free from target correction, nuclear effect. Single eperiment systematic uncertainties are well understood.

23 NC cross sections for PDF etraction em F -log () 5 HERA F =6.3E-5 =. =.6 =.53 =.4 =.5 =.63 =.8 =.3 ZEUS NLO CD fit H PDF fit H 94- H (prel.) 99/ ZEUS 96/97 Large kinematic coverage 3< <3 GeV 6-5 <<.6 4 =. =.3 =.5 BCDMS E665 NMC γ, Z echange Fied target eperiments =.8 3 =.3 =. =.3 =.5 =.8 =.3 =.8 =.5 =.4 = (GeV ) F ( q + q ) Sea + valence quark scaling violation of F F ln g gluon 3

24 NC cross high- ZEUS.5 = GeV 5 GeV 35 GeV 45 GeV γ, Z echange.5 65 GeV 8 GeV GeV 5 GeV.75.5 At high, weak current (Z ) introduces parity violation. ~ ± Y σ( e p) = F (, ) (, m F3 ) Y ~ ( ) ~ ( + σ e p σ e p ) F + ( q ) 3 q valence quark σ NC.5.8 GeV 3 GeV 5 GeV.6.4. GeV GeV 3 GeV.6.4 F GeV - ZEUS NLO CD fit tot. error ZEUS NC e - p 98/99 ZEUS NC e + p 96/97 4

25 CC cross sections for PDF etraction e e + q q ν q ( ) ( + ) νq ( + ) ( ) HERA e + p Charged Current CC is charge selective interaction. positron-induced negative-charged partons ~ + σ ( e p) [( u + c) + ( y) ( d + s)] d quark electron-induced ~ positive-charged partons σ ( e p) [( u + c) + ( y) ( d + s)] u quark σ Sea H e + p 94- ZEUS e + p 99- = 8 GeV SM e + p (CTE6D) (-y) (d+s) (u _ +c _ ) = 53 GeV = 95 GeV = 7 GeV = 3 GeV = 53 GeV d = 95 GeV = 7 GeV - - 5

26 Jet Cross sections for PDF etractions dσ/de T jet (pb/gev) 5 3 Directly sensitive to gluon density ZEUS obs ZEUS γ >.75 NLO (GRV) HAD NLO (AFG) HAD Jet energy scale uncertainty < η jet, <.4 ( ) g dσ/de T,jet B (pb/gev) ZEUS ZEUS Jet energy scale uncertainty NLO CD: (corrected to hadron level) α s (M Z )=.75 DISENT MRST99 (μ R =ET,jet B ) DISENT MRST99 (μ R =) 5 < < 5 GeV ( 5 ) 5 < < 5 GeV ( 4 ) 5 < < GeV ( 3 ) < < GeV ( ) < < 5 GeV ( ) > 5 GeV ( ) -3 - < η jet <.4 - < η jet < ( ) < η jet <.4 < η jet < ( ) DIS inclusive jet E B T,jet (GeV) -3-5 < η jet < - < η jet < (.5) - < η jet, < (.) < η jet, < (.) jet E T (GeV) Photo-production ( ~) dijets 6

27 PDFs from HERA All 547 data points (ZEUS) are fitted simultaneously. PDF etraction. Strong rise of gluon PDF at low. Sea quarks are also many. (Note: in the plot they are /) H/ZEUS difference data set (H does not have jets, but results from fied target ep.) parameterization systematic uncertainty 7

28 Why is PDF etraction at HERA important? DGLAP evolution TeV physics W production LHC: proton-proton collision Definitely needs PDFs. Main physics of LHC are at the range which HERA covers. 8

29 F L First direct measurement at the end of HERA. We can improve our understanding further Other (and/or more) measurements NC/CC More statistics with polarized beam. Heavy Flavour production Large statistics with the updated detector in HERA-II. Understanding of systematic uncertainties of measured cross section in HERA-I Combining cross sections from the H and ZEUS eperiments. 9

30 NC/CC cross sections with more statistics Integrated Luminosity (pb - ) HERA delivered e - p e + p HERA-I pb - pb - HERA-II 8pb - 7pb days of running F 3 γz.8 HERA H+ZEUS Combined (prel.) =5 GeV H PDF Increase of electron data.6 ZEUS-JETS PDF NC More sensitivity to F 3 i.e. valence quarks (u+d) Increase of positron data CC More sensitivity to d quark

31 Heavy Flavour production Dominant process of heavy quark production: Boson-Gluon-Fusion (BGF) Two schemes to treat heavy quarks in pcd; massive scheme (FFN) appropriate for ~ M q Heavy quarks are produced via BGF. Sensitive to gluon PDFs massless scheme (ZMVFN) appropriate for >> M q Heavy quarks are massless and eist in the proton if is above the mass threshold. Intrinsic heavy quarks PDFs Cross check for current pcd description for the proton. 3

32 F cc F cc is etracted for large kinematic region. different methods D mesons by slow pions Impact parameter tagging different data sets, theory In good agreement Scaling violation is seen. Well described by NLO-CD. F cc =.3 ( 4 ) NLO CD: CTE5F3 MRST4FF3 HERA F cc =.5 ( 4 9 ) =.7 ( 4 8 ) =.3 ( 4 7 ) =.8 ( 4 6 ) H HERA I (D*) H HERA I (VTX) ZEUS HERA I (D*) ZEUS HERA I (D +, D, D s + ) ZEUS (prel.) HERA II: D* D + μ =.3 ( 4 5 ) =.35 ( 4 4 ) =.5 ( 4 3 ) =.6 ( 4 ) =.8 ( 4 ) =. ( 4 ) =. ( 4 9 ) =.5 ( 4 8 ) H (prel.) HERA II: D* VTX =. ( 4 7 ) =.3 ( 4 6 ) =.4 ( 4 5 ) =.6 ( 4 4 ) =.8 ( 4 3 ) =. ( 4 ) =. ( 4 ) =.3 ( 4 ) 3 (GeV ) 3

33 F bb σ bb _.. H+ZEUS BEAUTY CROSS SECTION in DIS = 5 GeV = GeV = 5 GeV First measurement of F bb at HERA = 6 GeV = 3 GeV = GeV = 65 GeV H (Prel.) HERA I+II VTX ZEUS (Prel.) HERA II 39 pb - (3/4) μ ZEUS (Prel.) HERA II 5 pb - (5) μ MSTW8 (Prel.) CTE CTE5F3 Different methods H: Impact parameter tagging ZEUS: μ+jet More data to come 33

34 < Longitudinal structure function: F L Proportional to longitudinal photon interacting with proton. In naive PM, 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 pcd: γ * < q q Longitudinal photon cannot interact with a quark F L = α = s dz 6 + F L F 8 eq zg ( z) 3 4π z 3 q z gluon PDF Measurement of F L is good test for the current understanding of proton structure and CD. 34

35 F HERA F L gluon Probably HERA is the best place to measure F L. Why had F L not been measured before the end of HERA? Ans. Technical difficulties Needs cross section measurements with different beam energies. Needs to tag scattered electrons with lowest energies as possible. The last 4 months of HERA operation were dedicated to F L HERA delivered measurement. 6 Operation with lowered proton beam 5 energy. 4 E p = 46 GeV : 4 pb - 3 E p = 575 GeV : 8 pb - Successfully done! Integrated Luminosity (pb - ) days of running 35

36 F L measurement Cross section is combination of F and F L. 4 Y+ d σ = F (, ) πα dd ~ = F (, L Y+ σ σ ~ F Comparison of at the same (, L ) but different y = y sy ) different beam energy Sizable only at high-y Low energy of scattered electron. Linear fit on cross sections at each (, ) bin., y) (, σ r.6.4. =.49 = 5 GeV =.6 =.76 H slope = F L.6 =. =.6 =.5.4. H Data E p = 9 GeV E p = 575 GeV E p = 46 GeV Linear fit y / Y + 36

37 F L from two eperiments ) (, F L = GeV = 35 GeV = GeV = 3 GeV H Preliminary F = 8 GeV -3 - L - = 5 GeV = 45 GeV = 5 GeV = 4 GeV - - = GeV = 6 GeV = GeV = 5 GeV -3 - H (Prelim.) = 46, 575, 9 GeV E p - H PDF medium & high = 5 GeV = 9 GeV = 5 GeV = 65 GeV F L = 4 GeV = 6 GeV The first F L measurement at low. The measured F L is consistent with pcd description ZEUS = 3 GeV = 8 GeV -3 - = 45 GeV ZEUS (prel.) ZEUS-JETS = GeV - s = 5 GeV (4.pb ) - s = 5 GeV ( 6.pb ) - s = 38 GeV (3.8pb ) -3-37

38 -averaged F L H PDF CTE 6.6 MSTW 38 L H Preliminary F ) H (Prelim.).5 = 46, 575, 9 GeV E p (, FL medium & high / GeV Again, consistent with pcd prediction.

39 Combining H and ZEUS cross sections All HERA-I inclusive DIS cross sections from H and ZEUS are combined by averaging. Averaged each data point by simultaneous χ fit. Assumption: H and ZEUS measure the same cross sections. taking account of correlated systematics within/between eperiments. Cross calibration Reduction of sys. errors. It is also an consistency check of two eperiments. Uncertainty gets improved by more than sqrt(). σ r (, ) HERA I e + p Neutral Current Scattering H and ZEUS =. H PDF ZEUS JETS HERA I (prel.) ZEUS H =. =.5 / GeV HERA Structure Functions Working Group 39

40 PDFs from combined cross sections CD-Fit on combined cross section. Good constraint on PDFs. f H and ZEUS Combined PDF Fit g (.5) S (.5) -3 HERAPDF. (prel.) ep. uncert. model uncert. - = GeV u v d v - HERA Structure Functions Working Group April 8 W LHC A. M. Cooper-Sarker W + W- * Not include model uncert. HERA PDFs have strong impact on W/Z physics at LHC. Combination is done only for HERA-I. HERA-II will come. ZEUS -JETS HERA PDF. 4

41 Summary During its operation over 5 years, HERA provided plenty of physics through electron-proton collision. Not only the proton structure, but Electroweak and CD physics. Proton structure has been vigorously investigated at HERA. We have precise understanding of the proton structure. Steep rise of gluons and Sea quarks at low- Good description by pcd Good input to LHC! Still, many results from HERA will come up. We can improve our understanding of the proton structure. 4

42 Backup

43 W R boson 95% CL on heavy W R boson M WR >8GeV (H, e+p) M WR >86GeV (H, e-p) M WR >8GeV (ZEUS, e-p) assuming g L =g R and ν R is light 43

44 Chi definition for averaging Fit for data points (554 of them) χ ({ μ},{ r}) e m μ e i e i e i e ji s, r i j = e e e m N i μi β jirj Ke j= + ( e r ) e j ~ N (,) σ Ke i= i j= = measured cross section in bin i = true cross section in bin i by ep e σ = statistical uncertainty in bin i by ep e β = correlated syst. unc. in bin i by ep e And j systematic uncertainties 44

45 α S measurement α s..5. HERA ZEUS ZEUS ZEUS H H CD α s (M Z ) =.89 ±. (S Bethke, hep-e/6635) (inclusive-jet NC DIS) (inclusive-jet γp) (norm. dijet NC DIS) (norm. inclusive-jet NC DIS) (event shapes NC DIS) μ = or E T jet (GeV) HERA α s working group th. uncert. ep. uncert. Inclusive jet cross sections in NC DIS ZEUS (Phys Lett B 649 (7) ) Inclusive-jet cross sections in NC DIS H (DESY 7-73) HERA combined 7 inclusive-jet NC DIS (this analysis) HERA average 4 (hep-e/5635) World average 6 (S. Bethke, hep-e/6635)...4 α s (M Z ) 45