1 / 39

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

cardea
Download Presentation

QCD Tests in Lepton-Proton Collisions

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  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

More Related