A
bstract
Measuring the scattering of longitudinally-polarized vector bosons represents a fundamental test of ElectroWeak Symmetry Breaking.
In addition to the challenges provided by low rates and ...large backgrounds, there are conceptual issues which need to be clarified for the definition of a suitable signal. Since vector bosons are unstable and can only be observed through their decay products, the polarization states interfere among themselves. Moreover, already at tree level, there are diagrams which cannot be interpreted as production times decay of EW bosons but are necessary for gauge invariance.
We discuss a possible way to define a cross section for polarized
W
’s, dropping all non resonant diagrams, and projecting on shell the resonant ones, thus preserving gauge invariance. In most cases, the sum of polarized distributions reproduces the full results. In the absence of cuts, the ratios of the polarized cross sections to the full one agree with the results of a standard projection on Legendre polynomials. While the latter cannot be employed in a realistic environment, a comparison of the data with the shapes of the angular distributions for polarized vector bosons allows the extraction of the polarization fractions in the presence of selection cuts on the charged leptons.
We study the polarization of positively charged W's in the scattering of massive electroweak bosons at hadron colliders. We rely on the separation of weak boson polarizations in the gauge-invariant, ...doubly-resonant part of the amplitude in Monte Carlo simulations. Polarizations depend on the reference frame in which they are defined. We discuss the change in polarization fractions and in kinematic distributions arising from defining polarization vectors in two different reference frames which have been employed in recent experimental analyses.
Vector-boson scattering processes are of great importance for the current run-II and future runs of the Large Hadron Collider. The presence of triple and quartic gauge couplings in the process gives ...access to the gauge sector of the Standard Model (SM) and possible new-physics contributions there. To test any new-physics hypothesis, sound knowledge of the SM contributions is necessary, with a precision which at least matches the experimental uncertainties of existing and forthcoming measurements. In this article we present a detailed study of the vector-boson scattering process with two positively-charged leptons and missing transverse momentum in the final state. In particular, we first carry out a systematic comparison of the various approximations that are usually performed for this kind of process against the complete calculation, at LO and NLO QCD accuracy. Such a study is performed both in the usual fiducial region used by experimental collaborations and in a more inclusive phase space, where the differences among the various approximations lead to more sizeable effects. Afterwards, we turn to predictions matched to parton showers, at LO and NLO: we show that on the one hand, the inclusion of NLO QCD corrections leads to more stable predictions, but on the other hand the details of the matching and of the parton-shower programs cause differences which are considerably larger than those observed at fixed order, even in the experimental fiducial region. We conclude with recommendations for experimental studies of vector-boson scattering processes.
A
bstract
Unitarization models describe phenomenologically the high energy behaviour of a strongly interacting symmetry breaking sector. In this work, predictions of some unitarized models in vector ...boson scattering at LHC are studied and compared with analogous studies in Equivalent Vector Boson Approximation and previous results for the benchmark no-Higgs scenario. To perform such studies, unitarized model amplitudes have been implemented in the PHANTOM Monte Carlo in a complete calculation with six fermions in the final state.
A complete parton level analysis of 2
ℓ
2
ν
2
j
and 4
ℓ
2
j
,
ℓ
=
μ
,
e
production at the LHC is presented, including all processes at order
,
. The infinite Higgs mass scenario, which is ...considered as a benchmark for strong scattering theories and is the limiting case for composite Higgs models, and one example of Strongly Interacting Light Higgs models are confronted with the Standard Model light Higgs predictions. This analysis is combined with the results in the
ℓν
+ four jets, the
ℓ
−
ℓ
+
+ four jets and the 3
ℓν
+ two jets channels presented in previous papers, in order to determine whether these alternative Higgs frameworks can be detected as an excess of events in boson-boson scattering.
A
bstract
In view of the annnouncement that in 2012 the LHC will run at 8 TeV, we study the possibility of detecting signals of alternative mechanisms of ElectroWeak Symmetry Breaking, described ...phenomenologically by unitarized models, at energies lower than 14 TeV. A complete calculation with six fermions in the final state is performed using the PHANTOM event generator. Our results indicate that at 8 TeV some of the scenarios with TeV scale resonances are likely to be identified while models with no resonances or with very heavy ones will be inaccessible, unless the available luminosity will be much higher than expected.
A
bstract
Interference effects play an important role in Electroweak Physics. They are responsible for the restoration of unitarity at large energies. When, as is often the case, higher order ...corrections are only available for some particular subamplitude, interferences need to be carefully computed in order to obtain the best theoretical prediction. It has been recently pointed out in gluon fusion that whenever more than one neutral,
C
P
even, scalars are present in the spectrum large cancellations can occur. We extend these studies to Vector Boson Scattering, examining interference effects in the Higgs sector in the Standard Model and its one Higgs Singlet extension. Already in the SM there is a significant difference between the results obtained considering only
s
-channel Higgs exchange and those obtained from the full set of scalar exchange diagrams. In the 1HSM these effects are modulated by the interference between the two neutral Higgses. The full interference between the heavy Higgs diagrams and the rest of the amplitude, which is the sum of light Higgs exchange diagrams and of those diagrams in which no Higgs appear, is very small for values of the mixing angle compatible with the experimental constraints.
PHANTOM is a tree level Monte Carlo for six parton final states at proton–proton, proton–antiproton and electron–positron colliders at
O
(
α
E
M
6
)
and
O
(
α
E
M
4
α
S
2
)
including possible ...interferences between the two sets of diagrams. This comprehends all purely electroweak contributions as well as all contributions with one virtual or two external gluons. It can generate unweighted events for any set of processes and it is interfaced to parton shower and hadronization packages via the latest Les Houches Accord protocol. It can be used to analyze the physics of boson–boson scattering, Higgs boson production in boson–boson fusion,
t
t
¯
and three boson production.
Program title:
PHANTOM (V. 1.0)
Catalogue identifier: AECE_v1_0
Program summary URL:
http://cpc.cs.qub.ac.uk/summaries/AECE_v1_0.html
Program obtainable from: CPC Program Library, Queen's University, Belfast, N. Ireland
Licensing provisions: Standard CPC licence,
http://cpc.cs.qub.ac.uk/licence/licence.html
No. of lines in distributed program, including test data, etc.: 175 787
No. of bytes in distributed program, including test data, etc.: 965 898
Distribution format: tar.gz
Programming language: Fortran 77
Computer: Any with a UNIX, LINUX compatible Fortran compiler
Operating system: UNIX, LINUX
RAM: 500 MB
Classification: 11.1
External routines: LHAPDF (Les Houches Accord PDF Interface,
http://projects.hepforge.org/lhapdf/), CIRCE (beamstrahlung for
e
+
e
−
ILC collider).
Nature of problem: Six fermion final state processes have become important with the increase of collider energies and are essential for the study of top, Higgs and electroweak symmetry breaking physics at high energy colliders. Since thousands of Feynman diagrams contribute in a single process and events corresponding to hundreds of different final states need to be generated, a fast and stable calculation is needed.
Solution method:
PHANTOM is a tree level Monte Carlo for six parton final states at proton–proton, proton–antiproton and electron–positron colliders. It computes all amplitudes at
O
(
α
E
M
6
)
and
O
(
α
E
M
4
α
s
2
)
including possible interferences between the two sets of diagrams. The matrix elements are computed with the helicity formalism implemented in the program PHACT 1. The integration makes use of an iterative–adaptive multichannel method which, relying on adaptivity, allows the use of only a few channels per process. Unweighted event generation can be performed for any set of processes and it is interfaced to parton shower and hadronization packages via the latest Les Houches Accord protocol.
Restrictions: All Feynman diagrams are computed al LO.
Unusual features: Phantom is written in Fortran 77 but it makes use of structures. The g77 compiler cannot compile it as it does not recognize the structures. The Intel, Portland Group, True64 HP Fortran 77 or Fortran 90 compilers have been tested and can be used.
Running time: A few hours for a cross section integration of one process at per mille accuracy. One hour for one thousand unweighted events.
References:
1
A. Ballestrero, E. Maina, Phys. Lett. B 350 (1995) 225,
hep-ph/9403244; A. Ballestrero, PHACT 1.0,
Program for helicity amplitudes Calculations with Tau matrices,
hep-ph/9911318, in: B.B. Levchenko, V.I. Savrin (Eds.),
Proceedings of the 14th International Workshop on High Energy Physics and Quantum Field Theory (QFTHEP 99), SINP MSU, Moscow, p. 303.
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