Open Issues in DIS The High Energy Perspective

Transcription

1 Open Issues in DIS The High Energy Perspective My private point of view using data from DIS in collider mode: Accelerator and Experiments HERA success story: Precision cross sections, structure functions and parton densities HERA outlook: What s still in the queue? Open Issues 1

2 Imaging of the Proton quarks & gluons quark anti-quark pairs perturbative regime Quantum Chromo Dynamics U v U v d v Quark Parton Model: valence quark no visible substructure formfactors, electric & magnetic moments non-perturbative regime extended charged object point-like structureless Resolving Power: Q 0.1 momentum fraction of constituents: x Low Q : probing transition perturbative to non-perturbative QCD Low x: probing quarks and gluons at high density ΔEΔt~h high Q high resolution but time averaged picture 1

3 Deep Inelastic Scattering Center of mass energy s : d σ ± ± πα y Y 1 1 y F x, Q F x, Q xf x, Q e p e X ( ( ) ) = + 4 L dxdq xq 1443 Y+ Y+ Y =± 1 1 ± ( y) s = ( k+ p) 4-momentum transfer resolving power Bjørken scaling variable momentum fraction of struck parton Inelasticity: relation for fixed s: ( ) Q = q = k k Q x = p q p q y = p k Neutral current DIS cross section expressed by structure functions: valence & sea quarks Kinematic Variables Q ( ) ( ) m 3 ( ) gluons = sxy valence quarks 3

4 Mapping the Kinematic Plane large reach in x large reach in Q LHC Inclusive scattering (integrate over all hadronic final states X) Z & W exchange Access to EW Physics pdf s of quarks from F pdf of gluon from Q variation of F (e.g. DGLAP evolution) N N HERA II: P R L e = NR + NL longitudinal polarization 4

5 HERA the world s largest electron microscope (Deutsches Elektronen-Synchrotron DESY, Hamburg, Germany) Shutdown on June 30, 007, 3:00 HERA start: 199 upgraded in 001: HERA II p : 90 GeV e± e ±: 7.5 GeV polarized ~ 6.3 km circumference 1 spatial resolution: ~ m colliding beams equivalent to 50TeV on fixed target 5

6 Collider Experiments at HERA A final salute to our experiments How we like to remember H1 and ZEUS 6

7 Hera Luminosity Pe ~30 40% HERA I: HERA II upgrade: luminosity longitudinal polarization of the lepton beams (spin rotator pairs around the interaction regions) massive upgardes also for the detectors running efficiently from 003 onwards Luminosity L = 500 pb -1 per exp. 7

8 Structure Function F H1 & ZEUS extended fixed target kinematic regime in x and Q by Orders HERA I Described by DGLAP Scaling violations rise at low x drop at high x 8

9 HERA Averaged NC cross sections σ ± ± e p e X r Y = F m y F xf L 3 Y+ Y+ Precise measurements from two experiments For Q 100 GeV δ stat 1%,δ sys 3% for Q 1000 GeV δ stat > δ sys Combine datasets from both experiments: Key assumption H1 and ZEUS measure the same cross section at the same x,q,y Next step H1 and ZEUS combined fit Improved precision of combined H1 and ZEUS datasets (stat and sys) 9

10 Input to HERA PDF Fits NC CC d σ ± ± πα ( ( ) ) 1 1 % y, % Y y F x Q F x, Q xf% x, Q e p e X = + 4 L dxdq xq 1443 Y+ Y+ Y =± 1 1 ( y) ( ) ( ) m 3 ( ) 4 1 F% = Ai ( Q ) x + x F = x u+ u + c+ c + x d + d + s+ s i 9 9 xf% = B ( Q ) x x x = ± em [ qi qi ] ( ) ( ) [ qi qi ] F B x( u u c c) B x( d d s s ) 3 i 3 U D i Electroweak Coefficient Functions Ai( Q ), Bi( Q ) (QED: Ai=e i ) σ σ + CC ( ) (1 ) CC ( ) ( ) e p x y d + s + u + c ( ) ( + ) (1 + Pe ) ( e p) x + + (1 ) u c y d s ( 1 Pe ) PDF general form: B C xpdf = Ax (1 x) (1 + Dx +K) Prameterize: g, u v, d v, U(=u+c), D(=d+s) 10

11 Charged Current Cross Section CC Cross section provide flavor sensible constrains at high x σ σ e p x y D+ U + CC ( ) (1 ) e p x U + y D CC ( ) (1 ) Improved precision of σ CC By combining H1 and ZEUS 11

12 PDF Fits on HERA I Data Standalone PDF Fits HERAPDF 0.1 Impressive reduction of uncertainties of combined PDFs Model uncertainty: variation of charm and bottom mass, starting scale Q 0, Q min of included data, strange and charm fraction at starting scale 1

13 Comparison to Global Fits HERAPDF 0.1 HERAPDF 0.1 not a completely fair comparison: HERA combined data were not available to global fitters 13

14 Impact of HERA data Pre-HERA uncert. in gluon/sea PDFs HERA PDFs combining H1 and ZEUS 0 Q = 7 GeV uncorr. uncert. tot. uncert. Q = 000 GeV = 0 GeV 0 Q = 7 GeV uncorr. uncert. tot. uncert. Q = 000 GeV GeV 10 xg xg xg 10 xg xg xg 0 xs xs xs 0 xs xs x x Example: W + production at the LHC (Study by A. Cooper-Sakar) Note: Error bands are experimental uncertainties only model uncertainty will become increasingly important 14

15 Future HERA PDF Fits So far only part of the inclusive HERA I were used HERAPDF0.1 Incorporate all NC and CC from HERA I&II Include jet cross sections constrain high x gluon Include charm and beauty flavor decomposition of the sea Charged & Neutral current cross sections with polarized e ± beams constrain valence quark region 15

16 Charm & Beauty Structure Charm and Beauty production in DIS is driven by gluons in the proton Charm tag: reconstruct D mesons Beauty tag: displaced vertex, soft μ 16

17 Polarization dependence of CC Input to PDF fits: Double differential CC e ± p cross sections 17

18 Polarisation Effects in NC 18

19 NC Cross Section Asymmetries 19

20 OPEN ISSUES 0

21 Open Issues (I): high x Inclusive cross sections at high x: NC ± 4 e p : σ xu 9 CC e p : σ xu + e p: σ (1 y ) xd Quark distributions can be extracted with minimal corrections from QCD fits Reach in x limited by detector acceptance, hadronic final state goes down the proton beam line u: x max = 0.65 d: x max = 0.4 1

22 Open Issues(I): High x continued Events at high x at acceptance limit Poor resolution for x cannot measure differential σ at x,q point Measure integrated σ for x > x limit σ larger than expected PDFs at high x f x f C x > 1 (1 ) 0 Constraint by shape underestimated uncertainty? Here HERA I e + p, e - p see backup

23 Open Issues II: Light flavor decomposition of the qq sea PDF fits conventionally assume xd xu x 0 0 d-u (H1) d-u (H1+BCDMS) NMC found d u at medium x Here is what happens when the xd xu constraint at low x is relaxed A deuteron run at HERA Could have disentangled the light flavor sea H1 only (HERA I) H1 +BCDMS Poorly constrained Attempt to fit U and D Only one input: F ep Fit stabilized by fixed target data (sum rules help) 3

24 Issues at low x? Low x = High y! Neutral current DIS cross section expressed by structure functions: d ± ± σ πα ( ( ) ) y dxdq xq Y e p e X = 1+ 1 y 4 F x, Q x, Q Rosenbluth plot fixed x, Q ( ) F ( ) L Y± =± 1 ( 1 y) σ% : Reduced cross section Measure σ r at fixed x, Q but varying y y = Q / sx s = ep center-of-mass energy Varying y varying s dedicated low E p runs at end of HERA 4

25 Coss Sections at low x, high y σ r measured at 3 different s comparison to ZEUS- JETS PDF prediction with F L = F L (QCD) and F L =0 F L causes suppression at low x Small effect but big enough to extract FL and F 5

26 Extracted Structure Functions Simultaneous extraction of F L (x,q ) and F (x,q ) Error bars: experimental errors (stat & syst) Data support non-zero F L Consistent with expectation from QCD 6

27 Longitudinal Structure Function from H1 Text FL from medium (published) and high Q analysis (prelim) Consistent results from H1 and ZEUS Very successful low E p program at the end of the HERA era 7

28 Low x, Large Parton Densities and Saturation at fixed resolution scale (Q ) go to very large hadronic center of mass energy low x x = Q Q + W ultimately expect partons to overlap ( saturation ) Rise of pdf should flatten with decreasing x 1 1 ln x x No saturation observed at HERA λ 8

29 The Birth of Experimental Low x Physics Biggest HERA discovery: strong increase of quark density (F ) and gluon density (d F / d ln Q ) with decreasing x in newly explored regime. Low x, `large Q is high density, low coupling limit of QCD No saturation observed yet probe at even smaller x 9

30 LHeC Inclusive Kinematics s = 1.4 TeV E e = 70 GeV E = 7 TeV p s = 1.4 TeV (5 x HERA) Extension to higher Q in x range covered by HERA Unprecedented lumi = cm - s -1!!! ea mode possible using LHC ion beam Extension of low x (high W) frontier W x TeV 7 10 at Q 1GeV 30

31 The LHeC for Low x Investigations modes considered: 1) Focusing magnet To optimise lumi detector acceptance to 170 o little acceptance below Q =100 GeV INCREDIBLE LOW x MACHINE! ) No focusing acceptance to 179 o access to Q =1 for all x (x > 5 x 10-7!) Lumi ~ 1 fb -1 / yr 31

32 Summary Precision measurements from HERA inclusive NC & CC cross sections structure functions parton densities Issues: valence quarks constrained by NC & CC at high Q d v will remain less precise than u v sea quarks obtained from F at low x possibility of an asymmetric light flavor sea d-u 0 gluons from scaling violations, F L, jets, vector mesons, cc bb ultimately precision to be seen, final uncertainty at high x? high x: extrapolating towards x 1 How to assess uncertainties? low x: When does the strong rise saturate? 3

33 Backup Slides 33

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