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Recent results from H1/ZEUS on structure functions

Recent results from H1/ZEUS on structure functions. Masahiro Kuze KEK/IPNS (TIT from April). Introduction F 2 measurement and PDF fit Low- Q 2 region High- Q 2 region Current status and prospects. HERA ep Collider at DESY/Hamburg.

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Recent results from H1/ZEUS on structure functions

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  1. Recent results from H1/ZEUSon structure functions Masahiro Kuze KEK/IPNS (TIT from April) Introduction F2 measurement and PDF fit Low-Q2 region High-Q2 region Current status and prospects 29/Mar/2003 JPS Symposium

  2. HERA ep Collider at DESY/Hamburg • Ep=920GeV Ee=27.5GeV (e+ or e)  • 2 colliding experiments and 2 fixed-target experiments • On-tape luminosity: ~110 pb-1 e+p, ~15 pb-1 ep (‘98-’99) for H1 or ZEUS 29/Mar/2003 JPS Symposium

  3. Deep Inelastic Scattering (DIS) • Probe the proton = our most familiar micro-cosmoswith a point-like lepton probe. ‘giant electron-microscope’ • Bjorken x: Fractional momentum of a parton in the nucleon. • 1/Q (momentum transfer) gives the spacial resolution. • Neutral orCharged currentin t-channelpropagatorQ2=sxy 29/Mar/2003 JPS Symposium

  4. Kinematic region probed • > 100x larger kinematic reach compared to fixed-target DIS experiments at CERN, DESY, FNAL, SLAC… (if proton is at rest, HERA CM energy means Ee=54TeV) • At high Q2, probe the validity of QCD at smallest distance Quark structure? New particles?(Q2=40,000 GeV2 1/Q=0.001 fm) • At low Q2, probe the low-x region  very soft constituents of proton; Saturation? Breakdown of standard DGLAP formalism (BFKL) ? 29/Mar/2003 JPS Symposium

  5. The Detectors • ZEUS Detector • Uranium-Scintillator calorimeter • for electrons • for hadrons • Central tracking detector • H1 Detector • Liquid-Ar calorimeter • for electrons • for hadrons • Central tracking detector e 2 out of (Ee, e, Eh, h)  Reconstruction of (x, Q2) 29/Mar/2003 JPS Symposium

  6. Cross section & Structure functions • NC differential cross section [Y+F2(x, Q2) YxF3(x, Q2)  y2FL(x, Q2)]F2 = xqf+(x, Q2)[ef2  2ef vf vePZ + (vf2+af2)(ve2+ae2)PZ2]xF3 = xqf(x, Q2)[ 2ef af aePZ + 4vf af veaePZ2](f=u,d,c,s,b)(Parton Distribution Functions)PZ= sin22WQ2/(Q2+MZ2) (Z-exchange & Z interference)Y = 1  (1y)2,ef: quark charge,vi/ai: EW couplings FL = F2  2xF1(0 in LO QCD, longitudinal Str. Function) 29/Mar/2003 JPS Symposium

  7. Results of F2 Structure Function • Strong rise of F2 as x decreases • Soft ‘sea’ of quarks in the proton • Slope of rise gets steeper as Q2softer parton smaller resol. dynamics of quarks and gluons • Good agreement with fixed-target experiments at middle - high x • Sea + valence quarks 29/Mar/2003 JPS Symposium

  8. F2 for fixed x, as a function of Q2 • At low x, strong scaling violationis seen. Large gluon density + splitting  F2 increases • At x ~ 0.1, approximate scaling. • At higher x, F2 decreases as Q2. • Line = result of QCD fit(next slide) • All data points well described. 29/Mar/2003 JPS Symposium

  9. Perturbative-QCD fit of F2 data • Example: ZEUS NLO DGLAP analysis PRD 67 (2003) 012007 • At Q2 = Q20, input functional form of PDF (Q20 =7GeV2) • xf(x) = p1xp2(1-x)p3(1+p5x) for u-valence, d-valence, sea quarks and gluons • ‘Q2 evolution’ is predicted by DGLAP (‘Altarelli-Parisi’) Equations • ∂fi/∂lnQ2 = (s/2π)∑fj Pij Pqq Pgq Pqg Pgg • Use world’s precision DIS data + ZEUS F2 • BCDMS, NMC, E665, CCFR (-p, -D, -Fe) • 2 fit to determine p1 … p5 29/Mar/2003 JPS Symposium

  10. PDFs obtained from the fits 29/Mar/2003 JPS Symposium

  11. Low-x sea and gluon distributions • At Q2 ~ 1GeV2, gluon becomes valence-like (and even tends to be negative) • Sea quark is still rising 29/Mar/2003 JPS Symposium

  12. Simultaneous extraction of s and PDF • Scaling violation: F2/lnQ2 ~ sxg(x,Q2)Data at low x allow disentangling correlation of s and xg • s-free fit gives:H1:(additionally 0.0005 from renormalization scale)ZEUS:(additionally 0.0004 from renormalization scale)Difference in exp. error mainly from the treatment ofsystematic error and normalization of data pointsin the fitting procedure and error propagation. 29/Mar/2003 JPS Symposium

  13. What if there were no HERA data? • HERA data determine the low-x gluon and sea-quark PDF. • HERA revealed: F2 is very steep. 29/Mar/2003 JPS Symposium

  14. Relevance for future hadron colliders • Low-x PDF only measured at HERA:x > as low as 10-4 important for 100GeV<M<1TeV in ‘central’ region at LHC. • High-x PDF:Q2 evolution to <~104 GeV2 checked at HERA. QCD still works. • Crucial inputs for signal cross sectionand QCD-background estimation. • Higgs, SUSY, top-pair, W/Z (luminosity)… Martin et al., EPJC 14 (2000) 133 Q2 29/Mar/2003 JPS Symposium

  15. Where is the lowest applicability of pQCD? • Recall gluon becomes negative at low Q2. • It is not necessarily a problem, since PDF itself is not a physical observable. • However, FL (observable) becomes also negative forQ2 < ~ 1 GeV2. • Is this the lowest end of p-QCD applicability?Look at the F2 data  29/Mar/2003 JPS Symposium

  16. Transition from perturbative to non-p. region • At very low Q2, s  and non-perturbative effects important. • ZEUS BPT data at very low Q2 available. • Data cannot be described by pQCD fit below Q2 ~ 1 GeV2.(rather, it’s a surprise that pQCD works as low as 1 GeV2 !) • This is evident thanks to HERA data at low x. 29/Mar/2003 JPS Symposium

  17. F2 slope in low-x region • For x<0.01, F2 can be well parameterised by cx. •  fairly independent of x, linearly rises with logQ2 in ‘DIS’ regime. • Qualitative change happens around Q2 < ~1GeV2, then flattens in non-pert. region. 29/Mar/2003 JPS Symposium

  18. DIS in the hadronic picture • At very low Q2, quasi-real photon acts like a hadron (vector-meson dominance) • Plot shows F2 for fixed y, i.e.at fixed *-p CM energy W (W2~sy= Q2/x) • tot(*p)=(42/Q2)F2(x=Q2/W2, Q2)so F2 must vanish as F2Q2 as Q2 0. • Data show this behaviour (25<W<270 GeV) and successfully fitted by a Regge-type fit. (consitent with Pomeron and Reggeon trajectories determined from hadron-hadron scattering) 29/Mar/2003 JPS Symposium

  19. Attempts to describe both regions Dipole models: a class of phenomenological models •  dissociates into a qq pair (dipole) of radius r=1/Q upstream of the proton. • Its cross section with proton (dipole) rises as r2 for small r (pQCD region). • It saturates at large r, for r >~ R0. • The critical radius R0 is characterised by the parton density: R0~1/gluon~x(low x = high-density environment). • DIS cross section is a convolution of dipole and photon wave function . example: Golec-Biernat & Wuesthoff DGLAP-like saturation _ ˆ Cartoon: R.Yoshida ˆ 29/Mar/2003 JPS Symposium

  20. Does it describe the data? • This simple model gives a good description of x<0.01 F2 data for both Q2 regions. • Amazing thing is that the single model describes F2, diffractive cross section and vector-meson production at the same time (not discussed here).Red:original GB&WBlue:modified one (includesDGLAP evolution)Bartels, GB, KowalskiPRD66(2002)014001 29/Mar/2003 JPS Symposium

  21. High-Q2 region  Electroweak effects • Pz = Q2/(Q2+MZ2)At Q2 > ~5000 GeV2, effect of Z-exchange clearly visible. • (ep) > (e+p) due to ±xF3 sensitive to valence quarks • Statistically limited: more luminosity helps (esp. ep) 29/Mar/2003 JPS Symposium

  22. Charged current  flavour decomposition • e+p: only negative-charge quarks take part.At high x, mainlyd(x, Q2) probed. • ep: only positive-charge quarks take part.At high x, mainlyu(x, Q2) probed. • In addition tou(x, Q2) >d(x, Q2), e+p getshelicity suppression (1-y)2. (y=Q2/sx) (ep) >> (e+p) at large Q2. • Data (not used in the fit) well described by QCD prediction. 29/Mar/2003 JPS Symposium

  23. Can valence be determined by HERA only? • ZEUS-only fit (all HERA-I NC/CC data, prel.) still a bit less precise than global fit, but looks promising with future HERA-II data (e.g. d/u at high x). • Using only ep data: no uncertainty from nuclear corrections (D, Fe, …). 29/Mar/2003 JPS Symposium

  24. Evidence of Electro-Weak Unification Interplay of EW and QCD physics • At low Q2:NC ~ 1/Q4 (EM current)CC ~ GF2 (Weak current) • At high Q2 (> Mz2, MW2):Both NC and CC mediated by unified EW current. NC~CC • Dumping of CC at high Q2 comes from Mw. Fit gives: (Z. ep)Mw = 80 2(stat)1(syst)1(PDF) GeV • Space-like: q2(boson) << 0Completely different phase-space from time-like bosonsat LEP and Tevatron (q2 > 0).Complementary evidence/measurement. 29/Mar/2003 JPS Symposium

  25. HERA luminosity upgrade • 2000-2001: shutdown for lumi upgrade. • Final focussing magnets inside detector 5x luminosity than 2000 values • Goal: Accumulate ~1 fb-1 in HERA-II run (till 2006) • Longitudinal e polarization for H1/ZEUS by spin rotators(was available only for HERMES) • Also detector upgrades (ZEUS example) • Micro-vertex detector (b,c-tagging) • Straw-tube tracker (forward tracking) • New luminosity counters (calorimeter + spectrometer) 29/Mar/2003 JPS Symposium

  26. ZEUS detector upgrade 29/Mar/2003 JPS Symposium

  27. Current status • New detector components successfully commissioned. • New machine optics fine, achieved design specific luminosity (luminosity per beam currents). • 2002-03 run: with commissioning (modest) beam currents. • Beam currents limited by backgrounds at detectors (chamber currents). • Unexpectedly high double-scattered synchrotron radiation • Vacuum conditioning of the new ring slower than expected high proton beam-gas background in the detector • Remedies will be taken in the shutdown March to June 2003. • Achieved before shutdown: L=2.7*1031cm-2s-1 (design 7.0) • Max 50% e+ polarization achieved with H1/HERMES/ZEUS rotators. 29/Mar/2003 JPS Symposium

  28. S.F.measurements with 1 fb-1 • Expected precision in F2 and gluon determination 29/Mar/2003 JPS Symposium

  29. Flavour-specific measurements • Complete ‘mapping’ of the proton… • d/u at high x: charged current • Strange: charm in CC and/or leading  particle • Charm and bottom: improved tagging with micro-vertex Charm 500 pb-1 Bottom/charm 500 pb-1 29/Mar/2003 JPS Symposium

  30. Physics with polarized e • Lepton polarization probes EW structure • Direct measurement of helicity dependence of CC DIS (Right-handed currents?) • Measure light-quark EW couplings 500 pb-1 = for e+p, ~5xfor ep, ~30xmore statistics 29/Mar/2003 JPS Symposium

  31. Summary • Probing the proton with very high-energy probe showed us: • Precise measurement of sea-quark and gluon distributionsat low x Crucial inputs for current and future experiments. • Point of transition from perturbative to non-perturbative picture of the proton in QCD: Q2~1GeV2 • At high Q2, EW effects clearly visible and QCD evolution is valid up to ~10,000GeV2. • HERA-2 has started. High-luminosity run expected this fall. • Very precise determination of F2 and gluon distribution. • Flavour decomposition using various tools. • Polarized ep to probe EW structure of DIS. • Exciting future ahead with 10x more data and polarization. 29/Mar/2003 JPS Symposium

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