Search results for "UNITARITY"
showing 10 items of 245 documents
Non-Unitarity of the lepton mixing matrix at the European spallation source
2022
If neutrinos get mass through the exchange of lepton mediators, as in seesaw schemes, the neutrino appearance probabilities in oscillation experiments are modified due to effective nonunitarity of the lepton mixing matrix. This also leads to new CP phases and an ambiguity in underpinning the ''conventional'' phase of the three-neutrino paradigm. We study the CP sensitivities of various setups based at the European spallation source neutrino super-beam (ESSnuSB) experiment in the presence of nonunitarity. We also examine its potential in constraining the associated new physics parameters. Moreover, we show how the combination of DUNE and ESSnuSB can help further improve the sensitivities on …
Asymptotic properties of Born-improved amplitudes with gauge bosons in the final state
1999
For processes with gauge bosons in the final state we show how to continuously connect with a single Born-improved amplitude the resonant region, where resummation effects are important, with the asymptotic region far away from the resonance, where the amplitude must reduce to its tree-level form. While doing so all known field-theoretical constraints are respected, most notably gauge-invariance, unitarity and the equivalence theorem. The calculations presented are based on the process $f\bar{f}\to ZZ$, mediated by a possibly resonant Higgs boson; this process captures all the essential features, and can serve as a prototype for a variety of similar calculations. By virtue of massive cancel…
Quantum loops in radiative decays of the a1 and b1 axial-vector mesons
2007
A previous model where the low-lying axial-vector mesons are dynamically generated, implementing unitarity in coupled channels in the vector-pseudoscalar (V P) meson interaction, is applied to evaluate the decay widths of the a1(1260) and b1(1235) axial-vector mesons into �. Unlike the case of the a1, the b1 radiative decay is systematically underestimated at tree level. In this work we evaluate for the first time the loop contribution coming from an initial V P vertex. Despite the large superficial divergence of the loops, the convergence of the relevant loops can be established by using arguments of gauge invariance. The partial decay widths obtained agree with the experimental values wit…
Tree-level flavor-changing neutral currents in theBsystem: FromCPasymmetries to rare decays
2001
corresponding to 0.59 ≤ sin(2�) ≤ 0.82, which is cer- tainly outside the 1� Babar range but not outside the world average. This potential discrepancy is at the ori- gin of several papers (6) studying the implications of a small aJ/ in the search of new physics. In this paper, we analyze the implications of this situ- ation for a realistic model, obtained with the only addi- tion of an isosinglet down vector-like quark (7) to the SM spectrum. This model naturally arises, for instance, as the low-energy limit of an E6 grand unified theory. At a more phenomenological level, models with isosinglet quarks provide the simplest self-consistent framework to study deviations of 3 ×3 unitarity of the…
Massive Yang-Mills model and diffractive scattering
1998
We argue that the massive Yang-Mills model of Kunimasa and Goto, Slavnov, and Cornwall, in which massive gauge vector bosons are introduced in a gauge-invariant way without resorting to the Higgs mechanism, may be useful for studying diffractive scattering of strongly interacting particles. We perform in this model explicit calculations of S-matrix elements between quark states, at tree level, one loop, and two loops, and discuss issues of renormalisability and unitarity. In particular, it is shown that the S-matrix element for quark scattering is renormalisable at one-loop order and is only logarithmically non-renormalisable at two loops. The discrepancies in the ultraviolet regime between…
Higgs Pair Production as a Signal of Enhanced Yukawa Couplings
2018
We present a non-trivial correlation between the enhancement of the Higgs-fermion couplings and the Higgs pair production cross section in two Higgs doublet models with a flavour symmetry. This symmetry suppresses flavour-changing neutral couplings of the Higgs boson and allows for a partial explanation of the hierarchy in the Yukawa sector. After taking into account the constraints from electroweak precision measurements, Higgs coupling strength measurements, and unitarity and perturbativity bounds, we identify an interesting region of parameter space leading to enhanced Yukawa couplings as well as enhanced di-Higgs gluon fusion production at the LHC reach. This effect is visible in both t…
Unitarized Chiral Perturbation Theory in a finite volume: scalar meson sector
2011
We develop a scheme for the extraction of the properties of the scalar mesons f0(600), f0(980), and a0(980) from lattice QCD data. This scheme is based on a two-channel chiral unitary approach with fully relativistic propagators in a finite volume. In order to discuss the feasibility of finding the mass and width of the scalar resonances, we analyze synthetic lattice data with a fixed error assigned, and show that the framework can be indeed used for an accurate determination of resonance pole positions in the multi-channel scattering.
Vector form factor of the pion : A model-independent approach
2003
4 páginas, 2 figuras, 3 tablas.-- Comunicación presentada a la 9ª High-Energy Physics International Conference on Quantum ChromoDynamics (QCD 02) celebrada del 2 al 9 de Julio de 2002 en Montpellier (Francia).-- arXiv:hep-ph/0209224v1
Theoretical study of the Ξ(1620) and Ξ(1690) resonances in Ξc→π+MB decays
2017
Nonleptonic weak decays of ${\mathrm{\ensuremath{\Xi}}}_{c}$ into ${\ensuremath{\pi}}^{+}$ and a meson $(M)$-baryon $(B)$ final state, $MB$, are analyzed from the viewpoint of probing $S=\ensuremath{-}2$ baryon resonances, i.e., $\mathrm{\ensuremath{\Xi}}(1620)$ and $\mathrm{\ensuremath{\Xi}}(1690)$, of which spin-parity and other properties are not well known. We argue that the weak decay of ${\mathrm{\ensuremath{\Xi}}}_{c}$ is dominated by a single quark-line diagram, preferred by the Cabibbo-Kobayashi-Maskawa coefficient, color recombination factor, the diquark correlation, and the kinematical condition. The decay process has an advantage of being free from meson resonances in the ${\ens…
Unitarity constraints on resonance mixing and the Okubo-Zweig-Iizuka rule
1978
We discuss unitarity constraints on the (complex) mixing angle (δ) between resonances. In particular, when the mixing is small, as when it is forbidden by the Okubo-Zweig-Iizuka (OZI) rule, or some other mechanism, one can derive useful bounds on δ or determine Arg (δ) from the data of allowed decays of the resonances. In the case of f′−f mixing the unitarity bound |δ|≳0.05±0.015 is barely compatible with the experimental f′←ππ rate (if one assumes that it is dominated by f′←f←ππ transition). Furthermore, experimentally Im δ≈0 which together with unitarity gives Re δ=−0.08±0.03, in clear discrepancy with the observed f′←ππ rate. This suggests that the mixing of f′ with higher f-like recurre…