Photon induced collisions: a phenomenologist’s point of view

April 21, 2008 High energy photon collisions at the LHC

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Photon induced collisions: a phenomenologist’s point

of view Jiří ChýlaInstitute of Physics, Prague

based on my talks at PHOTON 2005 in Warsaw

Triangle meeting 2006 in Vienna PHOTON 2007 in Paris

Two parts:

- remarks on phenomenology- discussion of (selected) data involving photons in initial (and final) state coming from HERA and Tevatron (and elsewhere)


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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?


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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.

In discussing data I will concentrate on theoretical 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


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Basics of photon structureEvolution equations for PDF of the photon

2(0) (1) (2)( ) ( )

( , ) ( ) ( ) ( ) ....2 2 2

s sq q q q

M Mk x M k x k x k x

2

(1) (2)( ) ( )( , ) ( ) ( ) ....

2 2 2s s

G G G

M Mk x M k x k x

2

(0) (1)( ) ( )( , ) ( ) ( ) ...

2 2s s

ij ij ij

M MP x M P x P x

Pure QED


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General solution

point-like

hadron-likeSeparation inherently ambiguous

Example of the point-like part

which results from the resummation of the diagrams


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Consequently, the pointlike part of PDF of the photon behave

as ( / )sO ( )PLNSq O rather than

0QCD Note that switching QCD off by sending

which is just the pure QED expression corresponding tothe basic γ→qq vertex.

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.


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The presence of the inhomogenous splitting functions in theevolution 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 distributionfunctions of the photon, that driven by k(0), is cancelled by

“direct” photon contributions at the same order of αs!!


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Semantics which matters

What do we mean under “LO” or “NLO”? QED

QCDcorrection

Leading order (LO)

Next-to Leading order (NLO)

Subtract pure QED contributions and keep the terms LO and NLO for QCD effects only!

Recall:


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Unfortunately, in most papers on photon induced collisions thisconvention is not observed.

This habit is furthermore coupled with the previously discussedclaim 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 approximation 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.

( / )sO


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Basic ingredients of perturbative calculations

PDF of the proton

AGF94, GRV92, SaS96, GRS99, CJKL03, AFG05FFBKK96, KKP2000, BFGW2001, Kretzer01

( , )p p pD M FS

( , )D M FS

( , )p

hh hD M FS

( , )s RS colour coupling

renormalization scale

renormalization scheme

factorization scale

factorization scheme

PDF of the photon ( )

GRV98, CTEQ6 MRST02

T


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Observables

Factorization procedure: cross sections of physical processes can be written as convolutions of PDF, FF and partonic hard cross sections

Renormalization scale µ enters only when partonic hard scattering cross section is expanded in PQCD

Generic expression for XS of hard collision of particles A, B

)(),,()()( /,

/ statefinalMMMDMD fbahard

fbabBbaba

Aa

cancellation of factorization scale dependence

(usually Ma=Mb)

cancellation of renormalization scale dependence

),,(),(),( 1 ba

NLOi

ks

LOi

ks

hardfba MMCcCc


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Scales and schemes appear due to ambiguities in the treatment of singularities atshort 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

In other words, the existence of “natural physical scale” of a given hard process does not help in resolving the scale/scheme setting problem.

renormalization scale leads different results in different RS! The schemes are thus in principle as important as scales, but there is no “natural” RS or FS!


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The common practice of identifying µ=M and setting it equal to some “natural scale” Q has 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

arbitrary number!

/ 2 2Q Q

i.e. LO predictions would appear to have very small “uncertainty” at short distances, which is nonsense.

as04ln)(

)()2()2/(

0 ss

ss


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15


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Such investigation makes sense even if one does not subscribe to PMS!

Keep the renormalization and factorization scales µ and M as independent parameters and investigate the dependence of perturbative results on these parameters in the whole (µ,M) plane, looking for regions of local stability.

So how should we proceed?

Make a choice of scales and schemes based on somegeneral idea, and look whether it leads to meaningfulphenomenology for widest range of processes.


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TeV8.1at sbb


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TeV8.1at stt

10-15% difference


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TeV14at stt

30-40% difference phenomenologically significant


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Jet production in γp collisions

Frixione, Ridolfi (97)Klasen, Kramer, Kleinwort (98)Aurenche, Bourhis, Fontannaz, Guillet (00)

Complete framework for quasireal photons laid out in

Differences in regularization method: slicing vs subtractionSuitable cuts on dijets developed

Reviewed in Klasen: RMP 74 (2002), 1221

Single jets Dijets Multijets Jets in diffraction


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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 photon indispensable.

3sFor full consistency to order

also direct photon contribution to that order needed!

3s

Direct contribution: s +2s

Resolved contribution: 2s +

Structure of calculationsdirect

subtracted

3s+


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As illustrated by two recent H1 and ZEUS papers even for large ET jets the scale dependence estimated by the conventional procedureassuming quite large!

ZEUS, PRD76 (2007), 072011

2,1,5.0,2/)( )2()1( TTp EEMM

H1, PLB 639 (2006), 21:

The uncertainty ofNLO QCD found to vary between a few percent and almost ±30%.


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Jet production in γ*p collisions

Of particular interest the region of transition between photo-production (where ET sets the scales) and genuine 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.

Many people contributed to theory and phenomenologyKramer, Klasen, Pötter, GRV, Sjöstrand, Friberg, J.Ch., Taševský, Aurenche, Fontannaz, Guillet, Heinrich, De Florian, Sassot, ….


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correspond to QED plus first QCD term of quark d.f. of resolved γ*

gluon d.f.

Resolved virtual photon as approximate method for including higher order direct photon termsNLOJET in 3-jet mode allows us to

investigate this idea quantitatively.


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plus possibly other.

MEPJET (Mirkes, Zeppenfeld)DISENT (Catani, Seymour)DISASTER (Graudenz)JETVIP (Pötter), may be resuscited by KlasenNLOJET (Nagy, Troczsanyi)

NLO parton level Monte Carlos:

Only JETVIP includes resolved γ*

The only NLO generator for genuine 3-jet region

(black: no longer attended)

In principle JETVIP most general, but- has problem with stability of NLO RES - disagrees with other MC in NLO DIR

MC event generators:PYTHIA: γT as well as γL CASCADE: uPDF’s HERWIG: γT and in H1 version also γL CDM: color dipoleRAPGAP: γT only

resolved virtual photons available in


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Single jets Dijets Multijets Forward jets

Of great theoretical interest because of potential impli-cations for identifying BFKL dynamics.The region of interest:2 2

TQ E or better

Processes:

where:

the concept of resolved virtual photon is relevant jet ET provides the only natural scale and photon virtuality Q2 enters only through PDF of γT,L However, in most analyses of HERA data only the weaker condition

2 21/ / TQ E typically with κ=2 demanded.

22TEQ


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Forward jets at HERA

Mueller 92: best place to look for BFKL effects.

But difficult to distinguish from effects of resolved virtual photon!

BFKLResolvedvirtual γ?

or


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Recent new analyses on forward jet production in DIS at HERAdemonstrate the influence of the choice of scales on physicsconclusions.

H1: EPJC 46 (2006), 27

Scale uncertainty estimated by standard simultaneous variation of μr and μf by factors 0.5 and 2.

5/5.0 22, QE jetT

Resolved virtual photon contributions only in RAPGAP


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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 about 35% at low xBj but the scale uncertainties are significantly larger”.

Sensitivity of NLO calculations different for different variables


30Conclusions: DGLAP resolved photon (RAPGAP DIR+RES) and colour dipole models (Ariadne) closest to the data.


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ZEUS: EPJC 52 (2007), 515

QFR has made just the latter choice, setting


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Large theoretical uncertainty exceeding experimental errorsprevents a clear conclusion on NLO. The best description of the inclusive forward-jets by the newly tuned ARIADNE.


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2 2 2 2F TM Q E

Forward jet production at small x in NLO QCDKramer, Pötter 99

DIR alonebelow data

Inclusion ofNLO RES leads to niceagreement!

However:scale choicecrucial!


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Inclusive particle production in γ*p collisions

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.

BFKL direct γ resolved γ needed?or merely or

Conventional choice identifies four different scales!!

an example how the scale dependence can mar the searches for signals of “new physics”

But due to large scale dependence, we cannot distinguish

2 2 2 2 2 2 2( )p h TM M M Q E

p-factorizationγ γ-factorization

fragmentationγ renormalization photon virtuality

pion ET


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π0 and charged hadrons spectracompared to H1(99) data.

A factor of 2 difference

Agreement for

2 22 ( )

2TQ E

M

1

1/ 2 2 range

“moderate unphysical scale variations lead to a satisfactory description of the HERA data”

Kniehl, Kramer and Maniatis 05 The optimists:

GeV5.3Tp


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analyzed the same data on forward π0 taking into account the resolved virtual photon as welland setting as default

2 2 2 2TM Q E

Aurenche, Basu, Fontannaz, Godbole 05

H1 π0

NLO resolved

Large instability is observed when varying independently the renormalization and fragmentation scales. This prevents a really quantitative prediction for the single pion inclusive distribution in the forward region.

The realists:

A factor of 2 difference from Kniehl et al.


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Mp

MF

μ

Mγ

very differentdependences


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Heavy quark production at HERA

Pre 2004 dataProblem?

Standard NLO QCD,without NLO directphoton contributions

With experiment:different methods andprocesses used

With theory:treatment of heavy quarkthresholds


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tmM standard choice of scales and their variation

where 2,1,5.0and


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Another H1 paper:

Scales in NLO calculations


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similarZEUS

In DIS as well as photoproductionstandard scales and their variation

FMNR


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Systematic attention to quark mass effects


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FMNR (Frixione, Mangano, Nason, Ridolfi): massive schemeZMVFNS (Heinrich, Kniehl): zero-mass variable-flavour-number schemeGMVFNS (Kniehl, Kramer, Schienbein, Spiesberger):general-mass variable-flavour-number scheme combines the massless with the massive scheme


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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 accuracyof the NLO calculations.

/DEin protonrest frame


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What to conclude?


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What have we learned from comparison of new data onb-quark with theory?

all photo-production

For b quarksituation stillconfusing!

Clearly not much!


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Prompt photon (+ jet) productionBawa, Stirling, Gordon, Wogelsang, Krawczyk, Zembruski, Fontannaz,... Standard NLO approximation contains terms in green only

2s

Direct contribution: 2 +2

s

Single resolved: 2 2

s +

2 2s +

Double resolved: 2 2

s +2 3

s

2 3s

2 3s

+

+

should also be included


s

Single Resolved: 2s

+2 2

s (box)

Double Resolved: 2 2

s

Sometimes the following restricted set of terms is taken


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Prompt photon photoproduction at HERA

H1: EPJC 38 (2005), 437 GeVE

GeVWGeVQ

T

p

105

266142,1 222

The paper investigates the extent to which fixed order NLO calculations (Fontannaz, Guillet, Heinrich and Krawczyk, Zembrzuski) are able to describe the data, which are compa-red also with the predictions of PYTHIA and HERWIG and with ZEUS data on inclusive prompt photon production. 25.0,

Tp EMMConventional scales:

Sensitivity to standard scale variation weaker than in other processes.

Photon always isolated.


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The NLO calculations describe the data described in shape, but are 30%−40% below them.

PYTHIA and HERWIG describe the data in shape as well, but are low by 40%−50%.


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ZEUS: EPJC 49 (2007), 511

Similar kinematical range, the same calcula-tions (FGH and KZ) with the same scales and. In addition the comparison with calculations

of Lipatov&Zotov using the unintegrated PDF’s of both p and .

problem


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Best description for the calculations based on unintegrated PDF of both the proton and photon.

For both NLO QCD and kT-factorisation calculations describe the data well.

GeVET 7


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Prompt photon production at the TevatronD0: PLB 639

(2006), 151

remarkable achievement, no intrinsic kT needed!

JETPHOX code


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Prompt photons as tool in search for new phenomenaADD PLB 429 (1998), 263 introduced a novel way how to unify

gravity with other forces in which there is no hierarchy problem,because there is only one fundamental scale MEW≈ 300 GeV.

Such extra dimensions are large compared to Planck length LPL≈10-33cm and may be observable at Tevatron and LHC.

size of extra dimensions

Planck scale in 3+n

dimensionsBasic idea at the classical level does not involve supersym-metry/strings, though both may be useful at the quantum level.

Dramatic signature: nonconservation of energy, taken by thegraviton into extra dimensions.


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no signal

CDF PRL 28 (2002) 281801 TeVM D 6.0

fmR

fmR

pmR

6.2)8(

55)6(

24)4(


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D0 (2008)


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CDF

via the mechanism which is closely related to exclusivediffractive Higgs production at LHC

3 candidate events set upper limit on the exclusive photon production, which is a factor of 10 higher than the prediction of Khoze et al. This may be used to constrain calculations of exclusive Higgs production at the LHC.


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Another training ground for Higgs searches at LHC.

Very close to theHiggs scenario.

CDF


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Dedicated MC for generating exclusive dijet events:

ExHuME: LO event generator based on the perturbative calculations of Khoze et al.DPEMC: non-perturbative Regge theory inspired model of Bialas and Landshoff.


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Exclusive dijet cross section and the ratio of exclusive to inclusive DPE dijet cross sections measured as a function of minimum ET of the two leading jets. ExHuME favored

over DPEMC.Data consistent with predictions from pertur-bative calculations of Khoze et al.Good news for LHC


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eeppppin the process

eewhere the leptons come from

16 exclusive electron pairs with ET> 5 GeV, with background estimate of about 2 events.

CDF


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Photoproduction at the Tevatron

FOCUS: experiment which ran during 1996-1997 using wide band beam of real photons with mean energy 180 GeV and spread about 50 GeV.

Extensive studies of charmed particle production and properties (correlations, asymmetries, lifetimes) pentaquarks

Pentaquarks were born in low energy photoproduction expts. in Japan and Jefferson Lab, supported by one of expts. at DESY and were dealt the final blow by FOCUS in a series of recent large statistics analyses.


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The rise and fall of pentaquarks

1977: Jaffe considered exotic (but colorless) configuration of

quarks and antiquarks1997: Diakonov, Petrov and Polyakov predicted from chiral

soliton model the existence of hyperon with Q=S=+1,

mass 1530 MeV and width 15 MeV, which in the quark model, would correspond to the configuration uuddsbar

2003: In July Θ(1540) “discovered” at LEPS facility in Japan in and in July “confirmed” at JLAB

2004: The “Year of pentaquarks” brought two more candidates

Ξ—-(1860) corresponding to ddssubar by NA49 at CERN

Θc(3099) corresponding to uuddcbar by H1 at DESY2005: first doubts2006: growing suspicion2007: death of the idea

KnKn


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“Strongly indicates the existence of an S=1 resonance which may be attributed to the molecular meson-baryon resonance or alternatively as an exotic five-quark baryon”

Λ(1520) Θ(1540)

submitted in January 2003


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October 2003


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A clear and narrow resonanceis observed for both D∗−p and D∗+pbar combinations with an invariant mass3099 ± 3 (stat.) ± 5 (syst.) MeV.

H1: data in the region 1<Q2<100 GeV

March 2004

check with photopro-duction data:


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The Year of


68


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2005:

Focus onΘc


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Focus on Θ(1540)2006:


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Focus on Ξ--

2008:


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CLAS at JLAB

6.02.5

statistics (sometimes) matters!


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Inclusive particle production in γγ collisionsGordon 94:

Problems with inclusive particle production are not new recall


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Binnewies, Kniehl, Kramer 96

The samemessage


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Kniehl et al.

L3


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Problem with γγ collisions also for jets

L3

Bertora, Frixione


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Results:l3

Good shape,but not really good

normalization

Wrong shape andnormalization:

Preliminary data seemto confirm L3-hadron

production discrepancywith NLO prediction

Leonardo Bertora, 2003 April 8

L. Bertora at PHOTON03, DIS04


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Recent OPAL analysis ( )

Using the calculations of Klasen based on his earlier workwith Kramer and Kleinwort brings the data closer to theory.


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Remarks on theory:

Needed for full consistency to order

At large pT: direct γγ contribution dominant (as for hadrons) QED part dominates direct γγ contribution QCD contribution does not represent full NLO but cannot be expected to close the gap threshold resummation may be important

2 2s

missing

+2 3

s

2 2s


s

Single resolved: 2s +

Double resolved: 2 2

s

2 2s +


80

The same problem in bb production in γγ collisions!

Final L3 analysisconfirms their older result which is in excellent agreement with those of OPAL and DELPHI.


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But what is called “NLO” suffers from serious shortcomings as the direct photon contribution is manifestly of LO in αs only, the missing direct terms of the order α2(αs)2 are needed to make the single resolved photon contribution of genuine NLO, the available part of single resolved photon contribution exhibits no region of local stability as a function of μr and M.


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ConclusionsWe have learned much from measurements of hard processesat HERA and Tevatron involving photons in initial or final state.

However ambiguities of perturbative calculations related to the choice of scales often prevent us from drawing clear conclu-sions on the verification of QCD or the manifestations of new phenomena like BFKL.

Work on NNLO calculations should be accompanied with systematic investigation of scale and scheme ambiguities of existing LO/NLO calculations.

Often, the LO MC event generators do better job than NLOparton level calculations.

In theoretically seemingly clean case of jet, inclusive hadronsand heavy quark production in photon-photon collisionscurrent theory disagrees with at least some data.


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Backup


84

)(2

)(....

2

)(

22ln)( )0()1()0(2 MqP

Mk

Mk

MdMdq ss

The meaning of the evolution equation

wherever we have such LO resolvedphotonterm

we must in-clude direct photon whichcompensates part of scale dependenceof the LO re-solved photon

At the NLOwe must include thisdirect photoncontributionalong with

the standardNLO term which for hadronscompensates the whole scale dependence of the LO term.


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What is wrong with the conventional assumption μ=M?

It fakes stability where there is none! It leads away from regions of genuine stability

Sometimes

LO prediction for GeV64at ),,,( ssMbbpptot

μ=M


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MC event generators versus NLO parton level calculations

Some of the features of full NLO QCD effects

are mimicked within “LO” MC event generators by means of

parton showers and noncollinear kinematics

of initial state partons.

But LO MC also use different input (LO PDF and αs ), which were extracted in LO global analysis from data, and so have a chance to describe also other data.

To identify genuine NLO effects is thus not simple.


87

Klasen, Kramer, Kleinwort (99): single jets for 20 GeV, =1TE

/ TE

/ TE

/ TM E

/ TM E

direct direct

resolved

resolved


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Direct + resolved calculation

All scales identified and set equal to M.What so these plots indicate? (In)stability?

?

?


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PDF of virtual γT and γL

γT: GRV, GRS, SaS, AFG

γL: J.Ch.

In most of practical circumstances modest QCD evolution

Kramer, Pötter

Virtuality dependence of PDF very different for the point-likeand hadron-like components.

Crucial improvement over DIRECT unresolved: the inclusion of NLO resolved contribution


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QCD analysis of dijetproduction at low Q2 at HERA(J. Ch., J. Cvach, K. Sedlák,M. Taševský 05)

Note the disagreementbetween JETVIP DIR aDISENT/NLOJET

Still higher order directterms, effectively inclu-ded in resolved γ*, will hopefully fill the remain-ing gap to data.


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Photon induced collisions: a phenomenologist’s point of view

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