118112940
Retardation effects and gauge transformations in the Bethe-Salpeter equations
SummaryThe ladder Bethe-Salpeter equation with Feynman propagators gives rise to spurious corrections of orderv/c to the nonrelativistic limit. The exact cancellation of these retardation effects by the introduction of the crossed-graph interaction kernel is here rigorously proved for the case of scalar bosons. The electrodynamic case is then examined, and the appearance of these spurious terms is related to the lack of exact gauge invariance of the approximate Bethe-Salpeter equation. In particular, the gauge-transformation properties of the wave functions are established, and also the gauge invariance of the crossed-graph equation is shown, up to orderv/c. Finally, for the case of scalar bosons, a more general transformation is defined which leaves theS-matrix invariant, but gives rise to a purely instantaneous interaction in the ladder approximation. Transformations of this kind directly avoid the spurious retardation effects, even for theories which are not gauge invariant.RiassuntoL'equazione di Bethe e Salpeter nell'approssimazione a scala dà luogo, con l'uso dei propagatori di Feynman, a correzioni spurie, dell'ordinev/c, al limite non relativistico. Si mostra qui rigorosamente che, nel caso di bosoni scalari, questi effetti di ritardo si cancellano esattamente con l'introduzione del nucleo di interazione dovuto ai grafici incrociati. Si esamina poi il caso dell'elettrodinamica e mettiamo in relazione la presenza di questi termini spuri con il fatto che l'equazione approssimata di Bethe e Salpeter non è esattamente invariante per trasformazioni di gauge. Si stabiliscono in particolare le proprietà di trasformazione delle funzioni d'onda e si mostra che l'equazione con i grafici incrociati è invariante fino all'ordinev/c. Si definisce infine, per il caso dei bosoni scalari, una trasformazione di tipo più generale che lascia invariata la matriceS. ma dà luogo, nella approssimazione a scala, ad una interazione puramente istantanea. Trasformazioni di questo tipo evitano direttamente gli effetti spuri di ritardo anche per teorie che non sono invarianti per trasformazioni di gauge.РезюмеЛестничное урвнение Бете-Салпетера с пропагаторами Фейнмана приводит к ложным поправкам, порядкаv/c, в нерелятивистском пределе. Здесь для случая скалярных бозонов проводится строгое доказательство точного уничтожения этих эффектов запаздывания, посредством введения перекрестных графиков в ядро взаимодействия. Затем исследуется случай электродинамики, и появление этих ложных членов связано с отсутствием точной калибровочной инвариантности приближенного уравнения Бете-Салпетера. В частности, устанавливаются свойства калибровочных преобразований для волновых функций, и также показывается калибровочная инвариантность уравнения с перекрестными графиками, вплоть до порядкаv/c. В заключение для случая скалярных бозонов определяется более общее преобразование, которое оставляет инвариантнойS-матрицу, но приводит к возникновению чисто мтновенного взаимодействия в лестничном приближении. Преобразование этого вида непосредственно уничтожает ложные эффекты запаздывания, даже для теорий, которые являются калибровочно неинвариантными.
118630400
Minimal Flavour Violation with hierarchical squark masses
In a supersymmetric model with hierarchical squark masses we analyze a pattern of flavour symmetry breaking centered on the special role of the top Yukawa coupling and, by extension, of the full Yukawa couplings for the up-type quarks. For sufficiently heavy squarks of the first and second generation this leads to effective Minimal Flavour Violation of the Flavour Changing Neutral Current amplitudes. For this to happen we determine the bounds on the masses of the heavy squarks with QCD corrections taken into account, properly including previously neglected effects. We believe that the view presented in this paper altogether strengthens the case for hierarchical sfermions.
118381299
Composite vectors at the Large Hadron Collider
An unspecified strong dynamics may give rise to composite vectors sufficiently light that their interactions, among themselves or with the electroweak gauge bosons, be approximately described by an effective Lagrangian invariant under SU(2)L × SU(2)R/SU(2)L+R. We study the production at the LHC of two such states by vector boson fusion or by the Drell-Yan process in this general framework and we compare it with the case of gauge vectors from a SU(2)L × SU(2)R × SU(2)N gauge model spontaneously broken to the diagonal SU(2) subgroup by a generic σ-model. Special attention is payed to the asymptotic behaviour of the different amplitudes in both cases. The expected rates of multi-lepton events from the decay of the composite vectors are also given. A thorough phenomenological analysis and the evaluation of the backgrounds to such signals, aiming at assessing the visibility of composite-vector pairs at the LHC, is instead deferred to future work.
11346163
What is the limit on the Higgs mass
Abstract We obtain the bounds on all the gauge invariant, flavour symmetric, CP-even operators of dimension 6 that can affect the electroweak precision tests. For the preferred Higgs mass of about 100 GeV , their minimal scales range from 2 to 10 TeV. Depending on the individual operator, these limits are often significantly stronger than those quoted in the literature, when they exist at all. The impact, if any, of these bounds on the upper limit con the Higgs mass itself is discussed.
27801366
Grand unification and the supersymmetric threshold.
It is shown that grand unification of the minimal supersymmetric standard model does not lead to a prediction for the scale of the superpartner masses, even if the inputs {alpha}, sin{sup 2}{theta}, and {alpha}{sub {ital s}} are known exactly.
122546935
Supersymmetry, R-parity breaking and the neutrino magnetic moment
Abstract We show that supersymmetric theories with R -parity breaking naturally give rise to significant neutrino magnetic moments. In the minimal supersymmetric standard model the transition magnetic moment μ ν e ν μ can be as large as 10 −13 μ B ( μ B is the Bohr magneton). By requiring an SU (2) H horizontal symmetry of the lagrangian in the Yukawa-less limit, μ ν e ν μ may be one order magnitude larger. In this case the main naturalness constraint on the maximum μ ν e ν μ attainable is the electron mass value rather than the bound on the neutrino mass itself.
218730825
Measurement of pseudorapidity distributions of charged particles in proton–proton collisions at √s = 8 TeV by the CMS and TOTEM experiments
208865876
Measurements of jet multiplicity and differential production cross sections of Z + jets events in proton-proton collisions at √s = 7 TeV
Measurements of differential cross sections are presented for the production of a Z boson and at least one hadronic jet in proton-proton collisions at √s=7  TeV, recorded by the CMS detector, using a data sample corresponding to an integrated luminosity of 4.9  fb^(−1). The jet multiplicity distribution is measured for up to six jets. The differential cross sections are measured as a function of jet transverse momentum and pseudorapidity for the four highest transverse momentum jets. The distribution of the scalar sum of jet transverse momenta is also measured as a function of the jet multiplicity. The measurements are compared with theoretical predictions at leading and next-to-leading order in perturbative QCD.