1,721,055 research outputs found
An Equivalent Gauge and the Equivalence Theorem
No full textI describe a novel covariant formulation of massive gauge theories in which the longitudinal polarization vectors do not grow with the energy. Therefore in the present formalism, differently from the ordinary one, the energy and coupling power-counting is completely transparent at the level of individual Feynman diagrams, with obvious advantages both at the conceptual and practical level.
Since power-counting is transparent, the high-energy limit of the amplitudes involving longitudinal particles is immediately taken, and the Equivalence Theorem is easily demonstrated at all orders in perturbation theory. Since the formalism makes the Equivalence Theorem self-evident, and because it is based on a suitable choice of the gauge, we can call it an “Equivalent Gauge”
The composite Nambu-Goldstone-Higgs
No full textThe Hierarchy Problem is arguably the most important guiding principle concerning the extension to high-energy scales of the Standard Model (SM) of Fundamental Interactions. Every scenario for addressing this issue unavoidably predicts new physics in the TeV energy range, which is currently being probed directly by the LHC experimental program. Among the possible solutions to the Hierarchy Problem, the scenario of a composite Higgs boson is a very simple idea and a rather plausible picture has emerged over the years by combining the following ingredients: First, the Higgs must be a (pseudo-) Nambu-Goldstone boson, rather than a generic hadron of the new strong sector. Second, through the so-called ‘partial compositeness’, SM particles mix with strong sector resonances with suitable quantum numbers, so that they become a linear combination of elementary and composite degrees of freedom. Recently, general descriptions of the Composite Higgs Scenario were developed which successfully capture the relevant features of this theoretical framework in a largely model-independent way. The present book provides a concise and illustrative introduction to the subject for a broad audience of graduate students and non-specialist researchers in the fields of particle, nuclear and gravitational physics
Nucleon Form Factors from 5D Skyrmions
No full textSeveral aspects of hadron physics are well described by a simple 5D effective field theory, Baryons arise in this scenario as "large" (and therefore calculable) 5D skyrmions. We extend and reline the existing analysis of this 5D soliton, which is fairly non-trivial due to the need of numerical methods. We perform the complete quantization of those collective coordinates which are relevant for computing the static observables like the nucleon form factors. We compare the result with simple expectations about large-N-c QCD and with the experimental data. An agreement within 30% is found. (C) 2009 Elsevier B.V. All rights reserved.S
A Strong Sector at the LHC: Top Partners in Same-Sign Dileptons
Full text linkHeavy partners of the top quark are a common prediction of many models in which a new strongly-coupled sector is responsible for the breaking of the electroweak symmetry. In this paper, we investigate their experimental signature at the LHC, focusing on the particularly clean channel of same-sign dileptons. We show that, thanks to a strong interaction with the top quark which allows them to be singly produced at a sizable rate, the top partners will be discovered at the LHC if their mass is below 1.5 TeV, higher masses being possible in particularly favorable (but plausible) situations. Since the partners are expected to be lighter in both the Higgsless and composite-Higgs scenarios, then one of same-sign dileptons is found to be a very promising channel in which these models could be tested. We also discuss several experimental signatures which would allow, after the discovery of the excess, to attribute it uniquely to the top partners production and to measure the relevant physical parameters, i.e. the top partners' masses and couplings. We believe that our results constitute a valid starting point for a more detailed experimental study.LPT
Composite Heavy Vector Triplet in the ATLAS Diboson Excess
No full textVector triplets of the standard model SU(2)_L group are a universal prediction of "natural" new physics models involving a new composite sector and are therefore among the most plausible new particles that the LHC could discover. We consider the possibility that one such triplet could account for the ATLAS excess in the boson-tagged jets analysis and we assess the compatibility of this hypothesis with all other relevant searches. We find that the hypothesis is not excluded and that the predicted signal is close to the expected sensitivity of several channels, some of which show an upper fluctuation of the observed limit while others do not. An accurate study of the signal compatibility with these fluctuations could only be performed by the experimental collaborations
Stable skyrmions from extra dimensions
No full textWe show that skyrmions arising from compact five dimensional models have stable sizes. We numerically obtain the skyrmion configurations and calculate their size and energy. Although their size strongly depends on the magnitude of localized kinetic-terms, their energy is quite model-independent ranging between 50 - 65 times F-pi(2)/m(rho), where F-pi is the Goldstone decay constant and m(rho) the lowest Kaluza-Klein mass. These skyrmion configurations interpolate between small 4D YM instantons and 4D skyrmions made of Goldstones and a massive vector boson. Contrary to the original 4D skyrmion and previous 5D extensions, these configurations have sizes larger than the inverse of the cutoff scale and therefore they are trustable within our effective 5D approach. Such solitonic particles can have interesting phenomenological consequences as they carry a conserved topological charge analogous to baryon number.S
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