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Photon induced collisions: a phenomenologist’s point of view

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## Photon induced collisions: a phenomenologist’s point of view

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**Photon induced collisions:**a phenomenologist’s point of view Jiří Chýla Institute of Physics, Prague Two parts: - remarks on phenomenology - discussion of (selected) data involving photons in initial (and final) state coming from HERA and Tevatron(and elsewhere) based on my talks at • PHOTON 2005 in Warsaw • Triangle meeting 2006 in Vienna • PHOTON 2007 in Paris High energy photon collisions at the LHC**Data to be discussed:**• jets (mostly forward) • inclusive particles (forward) • heavy quarks • prompt photons • exotic particles (pentaquarks) • novel phenomena (extra dimensions) Questions addressed: Are the available data described by the current theory? Could we learn more from them? High energy photon collisions at the LHC**In discussing data I will concentrate ontheoretical**uncertainties of various QCD calculations because they are often bigger than experimental errors and thus prevent unambiguous interpretation of the comparison of theory with data. I will comment on • specific feature of photon structure: pointlike and hadronlike components • semantics which matters: defining “leading” and “next-to-leading” order • the art of choosing the scales and schemes • defining the theoretical uncertainty. High energy photon collisions at the LHC**Basics of photon structure**Evolution equations for PDF of the photon Pure QED High energy photon collisions at the LHC**General solution**point-like hadron-like Separation inherently ambiguous Example of the point-like part which results from the resummation of the diagrams High energy photon collisions at the LHC**Note that switching QCD off by sending**which isjust the pure QED expression corresponding to the basic γ→qqvertex. Consequently, the pointlike part of PDF of the photon behave as rather than as claimed in most (though not all) papers. Contrary to the hadronic part the pointlike part of photonic PDF’s can thus be assigned well defined order of αs. High energy photon collisions at the LHC**The presence of the inhomogenous splitting functions in the**evolution equations for PDF’s of the photon has important consequence for the consistency of perturbation expansions, as it implies that, contrary to the case of hadrons, at each order of QCD, part of the scale dependence of the quark distribution functions of the photon, that driven by k(0), is cancelled by “direct” photon contributionsat the same order of αs!! High energy photon collisions at the LHC**Semantics which matters**What do we mean under “LO” or “NLO”? QED Recall: QCD correction Leading order (LO) Next-to Leading order (NLO) Subtract pure QED contributions and keep the terms LO and NLO for QCD effects only! High energy photon collisions at the LHC**Unfortunately, in most papers on photon induced collisions**this convention is not observed. This habit is furthermore coupled with the previously discussed claim that photonic PDF’s behave as leading to the situation that what is usually called “NLO” is not genuine “next-to-leading” order of QCD, but the appro- ximation including two lowest order contributions. This leads to significantly different set of diagrams included in NLO analysis compared to the set included when the correct behavior of PDF is taken into account. High energy photon collisions at the LHC**GRV98, CTEQ6**MRST02 PDF of the photon ( ) Basic ingredients of perturbative calculations renormalization scheme colour coupling renormalization scale factorization scale PDF of the proton factorization scheme AGF94, GRV92, SaS96, GRS99, CJKL03, AFG05 FF BKK96, KKP2000, BFGW2001, Kretzer01 High energy photon collisions at the LHC**(usually Ma=Mb)**cancellation of factorization scale dependence cancellation of renormalization scale dependence Observables Factorization procedure:cross sections of physical processes can be written as convolutions of PDF, FF and partonic hard cross sections Generic expression for XS of hard collision of particles A, B Renormalization scale µ enters only when partonic hard scattering cross section is expanded in PQCD High energy photon collisions at the LHC**Scales and schemes appear due to ambiguities in**the treatment of singularities at short distances: renormalization scale and scheme long distances: factorization scale and scheme Dependence on scales has intuitively clear physical interpretation but their choice is insufficient to specify unambiguously perturbative calculations as the same renormalization scale leads different results in different RS! Theschemes are thus in principle as important as scales, but there is no “natural” RS or FS! In other words, the existence of “natural physical scale” of a given hard process does not help in resolving the scale/scheme setting problem. High energy photon collisions at the LHC**arbitrary number!**The common practice ofidentifying µ=Mand setting it equal to some “natural scale” Qhas no justification apart from simplicity (Politzer, 1987). Moreover, the conventional way of estimating theoretical uncertainty due to scale choice, i.e.plotting the band of results corresponding to • makes little sense because it • depends on selected scheme • is actually misleading: at LO i.e. LO predictions would appear to have very small “uncertainty” at short distances, which is nonsense. High energy photon collisions at the LHC**So how should we proceed?**Keep the renormalization and factorization scales µ and Mas independent parameters and investigate the dependence of perturbative results on these parameters in the whole (µ,M) plane, looking for regions of local stability. Such investigation makes sense even if one does not subscribe to PMS! Make a choice of scales and schemes based on some general idea, and look whether it leads to meaningful phenomenology for widest range of processes. High energy photon collisions at the LHC**10-15% difference**High energy photon collisions at the LHC**30-40% difference phenomenologically significant**High energy photon collisions at the LHC**Jet production in γp collisions**Complete framework for quasireal photons laid out in Frixione, Ridolfi (97) Klasen, Kramer, Kleinwort (98) Aurenche, Bourhis, Fontannaz, Guillet (00) Reviewed inKlasen:RMP 74 (2002), 1221 Differences in regularization method: slicing vs subtraction Suitable cuts on dijets developed • Single jets • Dijets • Multijets • Jets in diffraction High energy photon collisions at the LHC**For full consistency to order**also direct photon contribution to that order needed! direct subtracted Structure of calculations Direct contribution: + + Resolved contribution: + • Extensive application to HERA data lead in general • to good agreement with pQCD predictions • but suffers from sizable scale dependence of many QCD calculations. • Concept of resolved photonindispensable. High energy photon collisions at the LHC**As illustrated by two recent H1 and ZEUS papers even for**large ET jets the scale dependence estimated by the conventional procedure assuming quite large! ZEUS, PRD76 (2007), 072011 H1, PLB 639 (2006), 21: The uncertaintyof NLO QCD found to vary between a few percent and almost ±30%. High energy photon collisions at the LHC**Jet production in γ*p collisions**Many people contributed to theory and phenomenology Kramer, Klasen, Pötter, GRV, Sjöstrand, Friberg, J.Ch., Taševský, Aurenche, Fontannaz, Guillet, Heinrich, De Florian, Sassot, …. Of particular interest the region of transition between photo- production(where ETsets the scales)andgenuine DIS, where Q2 is unquestionably the hard scale. In this region the concept in resolved (virtual) photon does not have to be introduced, but it turns out to be very useful phenomenologically. But, just in this region playing with scales does wonders and may fake interesting effects. High energy photon collisions at the LHC**correspond to QED plus first QCD term of quark**d.f. of resolvedγ* gluon d.f. Resolved virtual photon as approximate method for including higher order direct photon terms NLOJETin 3-jet modeallows us to investigate this idea quantitatively. High energy photon collisions at the LHC**NLO parton level Monte Carlos:**(black: no longer attended) MEPJET(Mirkes, Zeppenfeld) DISENT (Catani, Seymour) DISASTER(Graudenz) JETVIP (Pötter), may be resuscited byKlasen NLOJET(Nagy, Troczsanyi) Only JETVIP includes resolvedγ* The only NLO generator for genuine 3-jet region - hasproblem withstability of NLO RES - disagrees with other MC inNLO DIR plus possibly other. In principleJETVIP most general, but resolved virtual photons available in MC event generators: PYTHIA: γTas well asγLCASCADE: uPDF’s HERWIG: γTand in H1 version alsoγL CDM: color dipole RAPGAP: γTonly High energy photon collisions at the LHC**Single jets**• Dijets • Multijets • Forward jets Of great theoretical interest because of potential impli-cations for identifying BFKL dynamics. However, in most analyses of HERA data only the weaker condition typically with κ=2demanded. Processes: The region of interest: or better • where: • the concept ofresolved virtual photonis relevant • jet ET providesthe only natural scaleand • photon virtuality Q2 enters only through PDF of γT,L High energy photon collisions at the LHC**Forward jets at HERA**Mueller 92:best place to look for BFKL effects. BFKL or Resolved virtual γ? But difficult to distinguish from effects of resolved virtual photon! High energy photon collisions at the LHC**Recent new analyses on forward jet production in DIS at HERA**demonstrate theinfluence of the choice of scales on physics conclusions. H1: EPJC 46 (2006), 27 Scale uncertainty estimated by standard simultaneous variation of μr and μf by factors 0.5 and 2. Resolved virtual photon contributions only in RAPGAP High energy photon collisions at the LHC**weak scale dependence observed in H1 paper for xBJ**with NLO a factor of 2 below the data at low xBj. However, “If the renormalisation and factorisation scales are set to Q instead of pT the NLO prediction increases by about35% at low xBjbut the scale uncertainties are significantly larger”. Sensitivity of NLO calculations different for different variables High energy photon collisions at the LHC**Conclusions: DGLAP resolved photon (RAPGAP DIR+RES) and**colour dipole models(Ariadne) closest to the data. High energy photon collisions at the LHC**ZEUS: EPJC 52 (2007), 515**has made just the latter choice, setting High energy photon collisions at the LHC**Large theoretical uncertainty exceeding experimental errors**prevents a clear conclusion on NLO. The best description of the inclusive forward-jets bythe newly tuned ARIADNE. High energy photon collisions at the LHC**DIR alone**below data Inclusion of NLO RES leads to nice agreement! However: scale choice crucial! Forward jet production at small x in NLO QCD Kramer, Pötter 99 High energy photon collisions at the LHC**or**BFKL or merely direct γ resolved γ needed? photon virtuality γrenormalization γγ-factorization pion ET fragmentation p-factorization Inclusive particle production in γ*p collisions an example how the scale dependence can mar the searches for signals of “new physics” Forward pions in ep collisions has been suggested by A. Mueller as a process where the BFKL effects, characteristic for low x region, should be manifest. But due to large scale dependence, we cannot distinguish Conventional choice identifies four different scales!! High energy photon collisions at the LHC**Kniehl, Kramer and Maniatis 05**The optimists: π0 and charged hadrons spectra compared to H1(99) data. A factor of 2 difference Agreement for range “moderate unphysicalscale variations lead to a satisfactory descriptionof the HERA data” High energy photon collisions at the LHC**NLO resolved**H1 π0 The realists: Aurenche, Basu, Fontannaz, Godbole 05 analyzed the same data on forward π0 taking into account the resolved virtual photon as well and setting as default A factor of 2 difference from Kniehl et al. Large instability is observed when varying independentlythe renormalization and fragmentation scales. This prevents a really quantitative prediction for the singlepion inclusive distribution in the forward region. High energy photon collisions at the LHC**Mp**very different dependences MF μ Mγ High energy photon collisions at the LHC**Heavy quark production at HERA**Pre 2004 data Problem? With experiment: different methods and processes used With theory: treatment of heavy quark thresholds Standard NLO QCD, without NLO direct photon contributions High energy photon collisions at the LHC**standard choice of scales and their variation**where and High energy photon collisions at the LHC**Another H1 paper:**Scales in NLO calculations High energy photon collisions at the LHC**FMNR**similar ZEUS In DIS as well as photoproduction standard scales and their variation High energy photon collisions at the LHC**Systematic attention to quark mass effects**High energy photon collisions at the LHC**FMNR (Frixione, Mangano, Nason, Ridolfi): massive scheme**ZMVFNS (Heinrich, Kniehl):zero-mass variable-flavour-number scheme GMVFNS (Kniehl, Kramer, Schienbein, Spiesberger): general-mass variable-flavour-numberschemecombines the massless with the massivescheme High energy photon collisions at the LHC**GMVFNS should be best, but exhibits huge scale depen-dence,**which should be understood. The authors are right in their claim The precision of the inclusive D∗ cross section measure-ments is much higher than the accuracy of the NLO calculations. in proton rest frame High energy photon collisions at the LHC**What to conclude?**High energy photon collisions at the LHC**What have we learned from comparison of new data on**b-quark with theory? Clearly not much! For b quark situation still confusing! all photo- production High energy photon collisions at the LHC**Direct contribution:**+ (box) + Single Resolved: Double Resolved: Prompt photon (+ jet) production Bawa, Stirling, Gordon, Wogelsang, Krawczyk, Zembruski, Fontannaz,... Standard NLO approximation contains terms in green only should also be included + Direct contribution: + + + + Single resolved: Double resolved: + Sometimes the following restricted set of terms is taken High energy photon collisions at the LHC**Prompt photon photoproduction at HERA**H1: EPJC 38 (2005), 437 The paper investigates the extent to whichfixed order NLO calculations (Fontannaz, Guillet, Heinrich and Krawczyk, Zembrzuski) are able to describe thedata, which are compa-red also with the predictions ofPYTHIA and HERWIG andwith ZEUS data on inclusive prompt photon production. Conventional scales: Sensitivity to standard scale variation weaker than in other processes. Photon always isolated. High energy photon collisions at the LHC**The NLO calculations describe the data described in shape,**but are 30%−40% below them. PYTHIA andHERWIG describe the datain shape as well, but are low by 40%−50%. High energy photon collisions at the LHC