april 21, 2008high energy photon collisions at the lhc 1 photon induced collisions: a...
TRANSCRIPT
April 21, 2008 High energy photon collisions at the LHC
1
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)
April 21, 2008 High energy photon collisions at the LHC
2
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?
April 21, 2008 High energy photon collisions at the LHC
3
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
April 21, 2008 High energy photon collisions at the LHC
4
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
April 21, 2008 High energy photon collisions at the LHC
5
General solution
point-like
hadron-likeSeparation inherently ambiguous
Example of the point-like part
which results from the resummation of the diagrams
April 21, 2008 High energy photon collisions at the LHC
6
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.
April 21, 2008 High energy photon collisions at the LHC
7
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!!
April 21, 2008 High energy photon collisions at the LHC
8
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:
April 21, 2008 High energy photon collisions at the LHC
9
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 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.
( / )sO
April 21, 2008 High energy photon collisions at the LHC
10
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
April 21, 2008 High energy photon collisions at the LHC
11
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
April 21, 2008 High energy photon collisions at the LHC
12
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!
April 21, 2008 High energy photon collisions at the LHC
13
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
April 21, 2008 High energy photon collisions at the LHC
14
April 21, 2008 High energy photon collisions at the LHC
15
April 21, 2008 High energy photon collisions at the LHC
16
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.
April 21, 2008 High energy photon collisions at the LHC
17
TeV8.1at sbb
April 21, 2008 High energy photon collisions at the LHC
18
TeV8.1at stt
10-15% difference
April 21, 2008 High energy photon collisions at the LHC
19
TeV14at stt
30-40% difference phenomenologically significant
April 21, 2008 High energy photon collisions at the LHC
20
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
April 21, 2008 High energy photon collisions at the LHC
21
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+
April 21, 2008 High energy photon collisions at the LHC
22
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%.
April 21, 2008 High energy photon collisions at the LHC
23
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, ….
April 21, 2008 High energy photon collisions at the LHC
24
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.
April 21, 2008 High energy photon collisions at the LHC
25
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
April 21, 2008 High energy photon collisions at the LHC
26
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
April 21, 2008 High energy photon collisions at the LHC
27
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
April 21, 2008 High energy photon collisions at the LHC
28
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
April 21, 2008 High energy photon collisions at the LHC
29
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
April 21, 2008 High energy photon collisions at the LHC
30Conclusions: DGLAP resolved photon (RAPGAP DIR+RES) and colour dipole models (Ariadne) closest to the data.
April 21, 2008 High energy photon collisions at the LHC
31
ZEUS: EPJC 52 (2007), 515
QFR has made just the latter choice, setting
April 21, 2008 High energy photon collisions at the LHC
32
Large theoretical uncertainty exceeding experimental errorsprevents a clear conclusion on NLO. The best description of the inclusive forward-jets by the newly tuned ARIADNE.
April 21, 2008 High energy photon collisions at the LHC
33
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!
April 21, 2008 High energy photon collisions at the LHC
34
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
April 21, 2008 High energy photon collisions at the LHC
35
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
April 21, 2008 High energy photon collisions at the LHC
36
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.
April 21, 2008 High energy photon collisions at the LHC
37
Mp
MF
μ
Mγ
very differentdependences
April 21, 2008 High energy photon collisions at the LHC
38
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
April 21, 2008 High energy photon collisions at the LHC
39
tmM standard choice of scales and their variation
where 2,1,5.0and
April 21, 2008 High energy photon collisions at the LHC
40
April 21, 2008 High energy photon collisions at the LHC
41
Another H1 paper:
Scales in NLO calculations
April 21, 2008 High energy photon collisions at the LHC
42
similarZEUS
In DIS as well as photoproductionstandard scales and their variation
FMNR
April 21, 2008 High energy photon collisions at the LHC
43
Systematic attention to quark mass effects
April 21, 2008 High energy photon collisions at the LHC
44
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
April 21, 2008 High energy photon collisions at the LHC
45
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
April 21, 2008 High energy photon collisions at the LHC
46
What to conclude?
April 21, 2008 High energy photon collisions at the LHC
47
What have we learned from comparison of new data onb-quark with theory?
all photo-production
For b quarksituation stillconfusing!
Clearly not much!
April 21, 2008 High energy photon collisions at the LHC
48
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
Direct contribution: 2 +2
s
Single Resolved: 2s
+2 2
s (box)
Double Resolved: 2 2
s
Sometimes the following restricted set of terms is taken
April 21, 2008 High energy photon collisions at the LHC
49
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.
April 21, 2008 High energy photon collisions at the LHC
50
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%.
April 21, 2008 High energy photon collisions at the LHC
51
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
April 21, 2008 High energy photon collisions at the LHC
52
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
April 21, 2008 High energy photon collisions at the LHC
53
Prompt photon production at the TevatronD0: PLB 639
(2006), 151
remarkable achievement, no intrinsic kT needed!
JETPHOX code
April 21, 2008 High energy photon collisions at the LHC
54
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.
April 21, 2008 High energy photon collisions at the LHC
55
no signal
CDF PRL 28 (2002) 281801 TeVM D 6.0
fmR
fmR
pmR
6.2)8(
55)6(
24)4(
April 21, 2008 High energy photon collisions at the LHC
56
D0 (2008)
April 21, 2008 High energy photon collisions at the LHC
57
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.
April 21, 2008 High energy photon collisions at the LHC
58
Another training ground for Higgs searches at LHC.
Very close to theHiggs scenario.
CDF
April 21, 2008 High energy photon collisions at the LHC
59
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.
April 21, 2008 High energy photon collisions at the LHC
60
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
April 21, 2008 High energy photon collisions at the LHC
61
eeppppin the process
eewhere the leptons come from
16 exclusive electron pairs with ET> 5 GeV, with background estimate of about 2 events.
CDF
April 21, 2008 High energy photon collisions at the LHC
62
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.
April 21, 2008 High energy photon collisions at the LHC
63
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
April 21, 2008 High energy photon collisions at the LHC
64
“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
April 21, 2008 High energy photon collisions at the LHC
65
October 2003
April 21, 2008 High energy photon collisions at the LHC
66
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:
April 21, 2008 High energy photon collisions at the LHC
67
The Year of
April 21, 2008 High energy photon collisions at the LHC
68
April 21, 2008 High energy photon collisions at the LHC
69
2005:
Focus onΘc
April 21, 2008 High energy photon collisions at the LHC
70
Focus on Θ(1540)2006:
April 21, 2008 High energy photon collisions at the LHC
71
Focus on Ξ--
2008:
April 21, 2008 High energy photon collisions at the LHC
72
CLAS at JLAB
6.02.5
statistics (sometimes) matters!
April 21, 2008 High energy photon collisions at the LHC
73
Inclusive particle production in γγ collisionsGordon 94:
Problems with inclusive particle production are not new recall
April 21, 2008 High energy photon collisions at the LHC
74
Binnewies, Kniehl, Kramer 96
The samemessage
April 21, 2008 High energy photon collisions at the LHC
75
Kniehl et al.
L3
April 21, 2008 High energy photon collisions at the LHC
76
Problem with γγ collisions also for jets
L3
Bertora, Frixione
April 21, 2008 High energy photon collisions at the LHC
77
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
April 21, 2008 High energy photon collisions at the LHC
78
Recent OPAL analysis ( )
Using the calculations of Klasen based on his earlier workwith Kramer and Kleinwort brings the data closer to theory.
April 21, 2008 High energy photon collisions at the LHC
79
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
Direct contribution: 2 +2
s
Single resolved: 2s +
Double resolved: 2 2
s
2 2s +
April 21, 2008 High energy photon collisions at the LHC
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.
April 21, 2008 High energy photon collisions at the LHC
81
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.
April 21, 2008 High energy photon collisions at the LHC
82
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.
April 21, 2008 High energy photon collisions at the LHC
83
Backup
April 21, 2008 High energy photon collisions at the LHC
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.
April 21, 2008 High energy photon collisions at the LHC
85
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
April 21, 2008 High energy photon collisions at the LHC
86
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.
April 21, 2008 High energy photon collisions at the LHC
87
Klasen, Kramer, Kleinwort (99): single jets for 20 GeV, =1TE
/ TE
/ TE
/ TM E
/ TM E
direct direct
resolved
resolved
April 21, 2008 High energy photon collisions at the LHC
88
Direct + resolved calculation
All scales identified and set equal to M.What so these plots indicate? (In)stability?
?
?
April 21, 2008 High energy photon collisions at the LHC
89
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
April 21, 2008 High energy photon collisions at the LHC
90
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.
April 21, 2008 High energy photon collisions at the LHC
91