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QCD Tests in Lepton-Proton Collisions

QCD Tests in Lepton-Proton Collisions. QCD tests in DIS and large p T photoproduction (non-diffractive processes) High- p T jets in DIS “Forward excess” and virtual photon structure Jets in photoproduction Heavy quark production QCD tests in diffractive processes Vector meson production

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QCD Tests in Lepton-Proton Collisions

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  1. QCD Tests in Lepton-Proton Collisions • QCD tests in DIS and large pT photoproduction(non-diffractive processes) • High-pT jets in DIS • “Forward excess” and virtual photon structure • Jets in photoproduction • Heavy quark production • QCD tests in diffractive processes • Vector meson production • Hadronic final state in diffraction 8th International Workshop on Deep Inelastic Scattering and QCD (DIS2000)April 26th, 2000 Yuji Yamazaki KEK-IPNS (DESY F1J) “Collisions”: Fixed target experiments are not covered. QCD Tests in Lepton-Proton Collisions, DIS2000

  2. QCD tests in hard-scatteringep collisions The QCD test “shopping list” in DIS and photoproduction (PHP) processes • Scaling violation in the structure function – the ultimate QCD test at HERA. More comes from Hadronic Final State (HFS) study. • DIS with  2 jets – O(s) or higher • s determination • Gluon density g(x, Q2) [especially in low Q2] • Studies of higher order QCD dynamics (jet shape, 3 jets etc.) • Forward jet as a signal of BFKL dynamics and/or virtual photon () structure function • Particle production, fragmentation (strange, charm) • Heavy Quark (HQ) production • Photoproudction jets and HQ production • Low ET: Real photon pdf at low x Soft Underlying Event (SUE) • High ET:Photon pdf at high x, gluon density at high xpTest of QCD dynamics In Mike’s talk QCD Tests in Lepton-Proton Collisions, DIS2000

  3. Introduction to DIS HFS Trivial Lowest Order (QPM) In Breit frame • Difference from e+e– : Treatment of the “ladder” from p toMulti-scale Q2, ET • Development: Dijet definition infrared safe (not discussed) Inclusive-k algorithm in Breit frame (pp like) giving the length of the current regionphase space • Cross section  F2 • Final state – quark dominated BGF gluondensity QCDC quarkdensity QCD Tests in Lepton-Proton Collisions, DIS2000

  4. Inclusive jet cross section and s • Inclusive-k algorithm in Breit frame – longitudinal invariant cone-type, high ET, O(s) process • Thus defined jets agree with NLO well for high ETAND high Q2 region  proceed to extract s QCD Tests in Lepton-Proton Collisions, DIS2000

  5. S uncertainty from r • Main uncertainty comes from renormalization scale ET , Q2 • Obtained s compatible • The size of the uncertainty depends on the choice of the scalesmaller scale uncertainty for r = ET than Q2.ET is the preferred theory here Is this a good enough reason to choose ET ? Let’s see the cross section behavior. QCD Tests in Lepton-Proton Collisions, DIS2000

  6. Dijet xsection: ET2/Q2 dependence • Dijet cross section (ET1 > 8 GeV, ET2 > 5 GeV in Breit frame) is measured as a function of ET2/Q2 . • ET2/Q2 dependence well reproduced by r = Q2r = ET also OK for high ET2/Q2 • ET2 underestimates the cross section for large ET2/Q2. • The scale uncertainty is larger for Q2 than ET2 (not shown)  H1, ZEUS talk WG2 ZEUS preliminary Here the natural scale is ET2 !NLO misses something in low Q2? The theoretically stable r may not reproduce the data • New measurement “phase space scanning” in HFS variables: See also ET, Q2 and jetfor inclusive cross section H1 talk in WG2 QCD Tests in Lepton-Proton Collisions, DIS2000

  7. s measurement: pdf uncertainty • Using jet cross sections for obtaining s: Need to estimate the uncertainty from pdf.Here the propagationof input s to the outputis studied by H1. • ZEUS has estimated the gluon density uncertainty estimated from a global F2fitting using scaling violation. • H1+ZEUS talk in WG2, new measurement and progress report QCD Tests in Lepton-Proton Collisions, DIS2000

  8. Measurement of g(x, Q2) using dijet events • Dijet event: reconstruction of initial gluon momentum through • Benefit: gluon densityin relatively high x • Again the choice of the scalemay give large difference. • Low- excess = relation to the “forward excess” • BFKL footprint • virtual photon structure function  gluon density at high scalewhere the cross section is stable QCD Tests in Lepton-Proton Collisions, DIS2000

  9. Introduction to  SF and BFKL effect on DIS HFS DGLAP evolution “direct” Q2 >> ET2 LEPTO, HERWIG NLO programs CCFM (LDCMC) BFKL evolution • Q2ET2 • Forward excess • No MC available • No prediction with jet finder “Resolved” in virtual  • Q2 << ET2 : QCD evolutionfrom photon • Forward excess • Low-x events • MC: RAPGAP • NLO: JetViP QCD Tests in Lepton-Proton Collisions, DIS2000

  10. Higher Q2 Higher ET2 The  structure at high Q2 • “DIS” cross section is measured as a function of xOBS • Clear evidence of resolved events over LO calculation(NLO not sufficient – not shown) • contribution decreases as Q2 becomes higher  H1 talk WG2  SF exists even at high Q2 for the high ET events QCD Tests in Lepton-Proton Collisions, DIS2000

  11. “direct” “resolved” BFKL  SF as an explanation ofthe forward excess • Forward region: sensitive to BFKL signal • Plugging the  SF in:increases the forward(= low xBj) cross sectionand explains the data • Cross section as a function ofET2/Q2:Both LO MC (RAPGAP)and NLO (JetViP)explain the shape A solution for the forward excess.Is this entire story ? QCD Tests in Lepton-Proton Collisions, DIS2000

  12. Virtual photon SF, is it enough ? • Forward (1.5 <  < 3.1) 0production at low-x (x ~ 10–4) • For very low Q2 (2.0 < Q2 < 4.5) :  SF estimates too low • The modified BFKL (~ NLO) prediction is higher and closer to the data  New measurement on jets, H1 talk in WG2 • NLO agrees with data for most of the phase space • However: Large excess of data in low ET, low Q2, forward • NNLO calculation (or BFKL, resolved photon?) awaited QCD Tests in Lepton-Proton Collisions, DIS2000

  13. Azimuthal asymmetries in hadron production and FL • The twice oscillation ( |cos 2 |) comes fromthe longitudinal contribution ( boson-gluon fusion)  ZEUS talk WG2 Semi-direct measurement of FL at HERA, consistent with QCD expectation. QCD Tests in Lepton-Proton Collisions, DIS2000

  14. Jets in Photoproduction • Study on jets in photoproduction during first years • Very large excess in low-ET cross section • The excess mostly in the resolved process • Large “pedestal” around jet in ET flow • Mainly attributed to Soft-Underlying-Events (SUE). • Amount of the “jet pedestal” is measured • The effect is expected to be smaller at higher ET …  talk WG2 QCD Tests in Lepton-Proton Collisions, DIS2000

  15. Very high ET PHP – test of QCD • Now with higher integrated luminositycross section measurement with higher ET is possible. NLO gives a good description of PHP dijets at high ET QCD Tests in Lepton-Proton Collisions, DIS2000

  16. Very high ET PHP – photon SF • Closer look to the cross section:A large cross section excess in central-forward rapidity (in Lab frame) over NLO. • xOBS spectrum at high ET (> 25 GeV)The excess persists ! • Natural explanation: photon pdf in high xregion PHP at HERA is sensitive to photon pdf in high x Can’t we measure low x = gluon ?  talk WG1/2/3 QCD Tests in Lepton-Proton Collisions, DIS2000

  17. Low ET PHP: revisited. How we should proceed ? • SUE bothers in measuring the “parton level” cross section • One way:“subtract” the jet pedestal from SUE (estimated by MC)extract the LO parton density • Another way: “less sensitive to SUE” method • single particle production • prompt photon production • Measuring the low-x pdf with large error • Method indirect by subtracting the SUE effect.Any way out or we should forget ? • For theorist: need also “uncorrected” xsectionas a starting point.  talk WG2 QCD Tests in Lepton-Proton Collisions, DIS2000

  18. Open charm production • Now forward excess not related to SUE:Charm shows also “Forward excess”. • Both in DIS and PHP. ZEUS 1996-97 ZEUS 1996-97 • For PHP: small x events • resolved (virtual) photon again ? “Massless” calculation include resolved effect (large ET/mc) • Comparison of xOBS with massless NLO is awaited. • For DIS, associated jet xOBS should be checked.  ZEUS talk WG2 QCD Tests in Lepton-Proton Collisions, DIS2000

  19. Open beauty production • Large excess is observed over LO/NLO • Is this again due to “massless” effect ?(LO simulation: factor 2 increase) • The reason is unknown, we need statistics to see the distribution differentially.  talk WG2 QCD Tests in Lepton-Proton Collisions, DIS2000

  20. Summary on HFS in DIS/PHP • Many issues are special for  (*)p collisions • Tremendous amount of understanding in “how to measure” • Improved jet algorithm (inclusive-k in p or Breit) • Infrared safe definition of the dijet cross section • Stable NLO calculations, improved MCs • For most phase space the data agree with NLO • High Q2 (> 100 GeV2), high ET (also in PHP), backward • For these “good regions” : we can measure S, g(x), photon pdf in high x … • Still data excess in many place: low Q2 and ET, forward jets.The NLO scale uncertainty also large (also choice of scale) • Important terms are missing in NLO • Virtual photon SF, BFKL, NNLO ? • Similar effect in charm ? Massless solves this ?  Necessary step to measure e.g. g(x, Q2) in low Q2 • Q2= 0 suffers from SUE: any way out ? We are near the completion, just need theoretical input ! QCD Tests in Lepton-Proton Collisions, DIS2000

  21. Introduction to QCD tests in diffractive processes • Diffraction without a hard scale can be explained by an universal Pomeron  Diffractive physics is traditionally classified as a soft phenomena. • The diffraction with a hard scale at Tevatron, LEP and HERA has opened an wide range of QCD tests. • Soft  hard: how is the transition of the underlying mechanism ? • How the hard scales play a role ?2 = f (Q2, t [, MV2]) – are they similarly effective ? Hard scattering of“Hard Pomeron” and (virtual) photon 2-gluon exchangeand higher order Soft Pomeron pQCD modelof VM We could do only with these Investigating by HFS and vector meson production QCD Tests in Lepton-Proton Collisions, DIS2000

  22. EM current conservation,SU(4) prediction wave function MV Q2 Forming VMmuch after dipole collision t | g(x,Q2) |2 fast rise Introduction to quasi-elastic Vector Meson (VM) production Soft production mechanism – VDM Good description forno hard scale process(e.g. light VM PHP at HERA) Slow rise of the cross section Hard process QCD Tests in Lepton-Proton Collisions, DIS2000

  23. Elastic VM: W dependence in PHP • Light VM (, , ): slow rise  ~ 0.22  P(0) ~ 1.08consistent with universal Pomeron • J/ : fast rise hard scale is given by mc2 (= 2 GeV2, not so large!). Now final (points not updated) QCD Tests in Lepton-Proton Collisions, DIS2000

  24. Shrinkage in J/ PHP • A lot smaller shrinkage than the soft Pomeron if hard scaleIn high-Q2, photon is small the blowup of the hadron size at high W cannot be detected by a small object • another evidence of being a hard process. • New result from ZEUS, talk in WG4 MV provides a hard scale. QCD Tests in Lepton-Proton Collisions, DIS2000

  25. VM: W dependence in *p • Q2: expected to be another hard scale. • Light VM – no hard scale from mV : see  p • Result: • It rises more than soft Pomeron at Q2 > 1 GeV2 • Approaching to J/ only Q2 ~ 10 GeV2 ??? We don’t know. • W slope slower than W2 (2 – 2) (Regge expectation)consistent with inclusive diffraction from P parametrization determined by inclusive cross section H. Abramowicz H1 PHP J/ soft Pomeron mc2 Need precise measurement QCD Tests in Lepton-Proton Collisions, DIS2000

  26. Q2 dependence: b-slope, L/T • Expectation in b-slope: Soft: ~ 10 GeV–2 Hard: ~ 4 GeV–2 • b as a function of Q2:Slow approach, only at ~ 20 GeV–2 to the asymptotic value ? • L/T  Q2/MV2 in naïve LO calculation • Develops a lot slower than a linear rise • Three models: all compatible to the data. soft hard Need precise measurement Again need precise measurement for distinguishing models QCD Tests in Lepton-Proton Collisions, DIS2000

  27. Cross section ratio and SU(4) prediction – Q2 dependence • The cross section ratio of the four VMs  : : : J/= 9:1:2:8 assuming EM current conservation (flavor independence). • The ratio for  / is badly broken for Q2 ~ 0 • flat for  • mass effect ? QCD Tests in Lepton-Proton Collisions, DIS2000

  28. VM ratio: is Q2 the right scale ? • Restoration of SU(4) at high Q2, what’s going on in low Q2? • Universal behavior by taking Q2+MV2 as a kinematical scale. Compiled by B. Clerbaux • H1 talk: new result on  in WG4 (also covers the topics for next pages) Again: we want to havea bit more precision QCD Tests in Lepton-Proton Collisions, DIS2000

  29. t-dependence and SU(4) • Large t events (t > 1 GeV2) from proton dissociation. • Light VM:no satisfactory explanation by soft+hard • Charmonium is again described by pQCD –mc seems enough to give a hard scale • The ratio of light VM follows SU(4) for t > 1 GeV2 t affects differently from Q2 QCD Tests in Lepton-Proton Collisions, DIS2000

  30. Heavy VM:  cross section • Surprisingly higher cross sections than LO theories • Two theoretical models: Martin et al., Frankfurt et al. • Both incorporate the skewed parton density(SPD) effect • Many other effects can also raise the cross section [relativistic correction, real part of the amplitude, NLO corrections, Fermi momentum effect etc.] • Seems we need SPD. Can’t we measure it directly ? Martin, Ryskin, Teubner Frankfurt, McDermott, Strikman QCD Tests in Lepton-Proton Collisions, DIS2000

  31. DVCS • DVCS – the cleanest VM production • No uncertainty from the wave function of VMs • Large mass from the virtual photon, x1  x2  able to measure SPD • Interference with BH : the real part of the amplitude. • ZEUS has observed the signal … Bethe-Heitler (BH) diagram Analogy to VM production LO diagram • New result from H1: cross section measurementH1+ZEUS talk WG4 QCD Tests in Lepton-Proton Collisions, DIS2000

  32. Summary on Vector Mesons • Testing ground of pQCD by VM with a hard scale • Current hypotheses are: • Hard mechanism given by Q2, MV or t. • A dipole scatters with p, then forms VM long time after • SPD seems necessary for heavy particles • Are all these scenario true ? The status of measurement is • Data not precise enough to test models[W dependence, b-slope, L/T] • The role of Q2, MV or t as a hard scale seems different.Q2 may be MV , but MV in PHP in W dependence ?t seems different • Way out – isolating non-perturbative wave function (WF) • DVCS: no uncertainty on WF effect, full pQCD First glance: The data agrees with the prediction Ideas on DVCS mechanism seems ok. • Measuring excited states (', ', ') • In general theories are advanced • We foresee greater precision in coming year thanks to HERA’s larger int. lumi • we should provide sensible measurements ! QCD Tests in Lepton-Proton Collisions, DIS2000

  33. Detecting partonic mechanism in diffraction by HFS • The transition from soft to hard Pomeron is seen in: • VM production [W, b-slope, t-distribution etc.] • Inclusive diffraction [W dependence steeper: however the signal is not very clear] • Factorized Pomeron or pure pQCD ? • Both models describe the F2Ddata well Direct investigation by HFS and heavy flavor production • Energy flow in P frame (LPS tagging) • Two jet structure in high MX, but not too aligned • Indication of BGF diagram, gluonic Pomeron QCD Tests in Lepton-Proton Collisions, DIS2000

  34. HFS event shape in diffraction • Colourless system: analogy to e+e– final state ? Thrust and sphericity Diffraction final state is close to e+e–, but slightly broader • Transverse momentum in the “target region” • Non-diffraction – soft spectrum in pT • Hard radiation from Pomeron QCD Tests in Lepton-Proton Collisions, DIS2000

  35. HFS in diffraction: more to come • Charm production: diagrams limited, hard scale provided. Ideal measurement, but small statistics. • Dijet production – partonic structure, gluon in P.Just Pomeron+resolved photon seems perfect ? • Three jet structure: proving information ofpartonic dynamics.  H1+ZEUS WG4  H1 talk WG4 3jets  ZEUStalk WG4 QCD Tests in Lepton-Proton Collisions, DIS2000

  36. A Remark on “Universal Pomeron” • Large rapidity gap rate is a lot smaller in Tevatron • 10-20 % at HERA, 1-3 % at Tevatron • At HERA (Collins)Proton and photon vertex factorize • At Tevatron: soft gluon exchangebetween two protons (SUE effect !)Reduction of gap survival probability p • Is the Pomeron flux universal ? • Is this the right explanation ? Do we understand the soft exchange ? • An open question X p Soft gluon exchange QCD Tests in Lepton-Proton Collisions, DIS2000

  37. Remarks on the luminosity and detector issues on QCD tests • After upgrade: we lose the forward detectors. • Tagging diffraction by LRG will be limitedFatal for the most of diffraction study • Very forward hadron measurement will be difficult for studying both diffraction and HFS • However: ~ 100pb–1 data by Sep 2000 !Statistical error can be improved by factor  2 • Most of the studies so far are with 5-30 pb–1e.g.  at Q2 ~ 4 GeV2 are still statistically limited • For diffractions after upgrade: H1 FPS in the cold sectionHigh acceptance at xL ~ 0.97 (talk in WG4) Data until September 2000: last chance for many analyses Analysing current data can answer many questions QCD Tests in Lepton-Proton Collisions, DIS2000

  38. Conclusions • The QCD study in ep collision in last years has been investigated as a collision of  (*) and p • Here the study is more complicated than we thought • The incoming “hadron” = photon varies its size (Q2) However this gives us a rich testing ground of pQCD. Non-diffraction • Tremendous amount of progress in understanding NLO calculations, jet algorithms etcFake problems are gone • The excess of the cross section is being identified (low Q2, low ET, forward jets etc.) • Qusestion: how these excess can be explained ? • Need a bit more investigation, especially in theoryInteresting by itself, but also for extracting important quantities at low Q2 and ET ( g(x, Q2), s ?) QCD Tests in Lepton-Proton Collisions, DIS2000

  39. Conclusions(2) Diffraction and Vector Mesons • The observations in last years have set the direction of to go: we know what to measure for the moment. • Q: Is the proposed pQCD mechanism of VM production valid ? How the hard scales (Q2, t, mV) play role ? • Need more precision measurements • Q: Partonic mechanism of the inclusive diffraction ? • New analyses on HFS and HQ production may give some hints, may not. Let’s see • Large luminosity now  precise data (hopefully)  Promising future, challenge for experimentalist QCD Tests in Lepton-Proton Collisions, DIS2000

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