H→ℓℓ′ in the Simplest Little Higgs Model
(Submitted on 31 Mar 2016)
Little Higgs Models are promising constructs to solve the hierarchy problem affecting the Higgs boson mass for generic new physics. However, their preservation of lepton universality forbids them to account for the H→τμ CMS hint and at the same time respect (as they do) the severe limits on H→μe inherited from the non-observation of μ→eγ. We compute the predictions of the Simplest Little Higgs Model for the H→ℓℓ′ decays and conclude that the measurement of any of these decays at LHC (even with a much smaller rate than currently hinted) will rule out this model. This result is consistent with our earlier observation of very suppressed lepton flavor violating semileptonic tau decays within this model.
Relating q̂ , η/s and ΔE in an expanding Quark-Gluon Plasma
(Submitted on 30 Mar 2016)
We use linear viscous hydrodynamics to describe the energy and momentum deposited by a fast moving parton in a quark gluon plasma. This energy-momentum is in turn used to compute the probability density for the production of soft partons by means of the Cooper-Frye formula. We use this probability density to render manifest a relation between the average transverse momentum given to the fast moving parton from the medium q̂ , the entropy density to shear viscosity ratio η/s and the energy lost by the fast moving parton ΔE in an expanding medium under similar conditions to those generated in nucleus-nucleus collisions at the LHC. We find that q̂ increases linearly with ΔE for both trigger and away side partons that have been produced throughout the medium. On the other hand, η/s is more stable with ΔE. We also study how these transport coefficients vary with the geometrical location of the hard scattering that produces the fast moving partons. The behavior of q̂ , with ΔE is understood as arising from the length of medium the parton traverses from the point where it is produced. However, since η/s is proportional to the ratio of the length of medium traversed by the fast parton and the average number of scatterings it experiences, it has a milder dependence on the energy it loses. This study represents a tool to obtain a direct connection between transport coefficients and the description of in-medium energy loss within a linear viscous hydrodynamical evolution of the bulk.
Flavor-changing decays of the top quark in 5D warped models
(Submitted on 29 Mar 2016)
We study flavor changing neutral current decays of the top quark in the context of general warped extra dimensions, where the five dimensional metric is slightly modified from 5D anti-de-Sitter (AdS5). These models address the Planck-electroweak hierarchies of the Standard Model and can obey all the low energy flavor bounds and electroweak precision tests, while allowing the scale of new physics to be at the TeV level, and thus within the reach of the LHC at Run II. We perform the calculation of these exotic top decay rates for the case of a bulk Higgs, and thus include in particular the effect of the additional Kaluza-Klein (KK) Higgs modes running in the loops, along with the usual KK fermions and KK gluons.
Interpreting Numerical Measurements in Fixed Topological Sectors
(Submitted on 17 Mar 2016)
For quantum field theories with topological sectors, Monte Carlo simulations on fine lattices tend to be obstructed by an extremely long auto-correlation time with respect to the topological charge. Then reliable numerical measurements are feasible only within individual sectors. The challenge is to assemble such restricted measurements in a way that leads to a substantiated approximation to the fully fledged result, which would correspond to the correct sampling over the entire set of configurations. We test an approach for such a topological summation, which was suggested by Brower, Chandrasekharan, Negele and Wiese. Under suitable conditions, energy levels and susceptibilities can be obtained to a good accuracy, as we demonstrate for O(N) models, SU(2) Yang-Mills theory, and for the Schwinger model.
The distinctive ultraviolet structure of extra-dimensional Yang-Mills theories by integration of heavy Kaluza-Klein modes
(Submitted on 12 Mar 2016)
One-loop Standard Model observables produced by virtual heavy Kaluza-Klein fields play a prominent role in the minimal model of universal extra dimensions. Motivated by this aspect, we integrate out all the Kaluza-Klein heavy modes coming from the Yang-Mills theory set on a spacetime with an arbitrary number, n, of compact extra dimensions. After fixing the gauge with respect to the Kaluza-Klein heavy gauge modes in a covariant manner, we calculate a gauge independent effective Lagrangian expansion containing multiple Kaluza-Klein sums that entail a bad divergent behavior. We use the Epstein-zeta function to regularize and characterize discrete divergences within such multiple sums, and then we discuss the interplay between the number of extra dimensions and the degree of accuracy of effective Lagrangians to generate or not divergent terms of discrete origin. We find that nonrenormalizable terms with mass dimension k are finite as long as k>4+n. Multiple Kaluza-Klein sums of nondecoupling logarithmic terms, not treatable by Epstein-zeta regularization, are produced by four-dimensional momentum integration. On the grounds of standard renormalization, we argue that such effects are unobservable.
Standard Model with extra dimensions and its zeta function regularization
(Submitted on 10 Mar 2016)
We start from a field theory governed by the extra-dimensional ISO(1,3+n) Poincar\’e group and by the extended SM gauge group, G(4+n). Then we construct an effective field theory whose symmetry groups are ISO(1,3) and G(4). The transition is carried out via two canonical transformations: a map that preserves, but it hides, the SO(1,3+n) symmetry; and a transformation, given by Fourier series, that explicitly breaks ISO(1,3+n) into ISO(1,3), but conserves and hides the gauge symmetry G(4+n), which manifests through nonstandard gauge transformations. From the 4-dimensional perspective, a particle that propagates in compact extra dimensions unfolds into a family of fields that reduces to the SM field if the size of the compact manifold is negligible. We include a full catalogue of Lagrangian terms that can be used to derive Feynman rules. The divergent character of the theory at one-loop is studied. A regularization scheme, based on the Epstein zeta function (EZF), is proposed to handle divergences coming from short distance effects in the compact manifold. Any physical amplitude can be written as an infinite series involving products of EZFs and powers of the compactification scale. The two possible scenarios m(m⎯⎯⎯)>m(0⎯⎯) and m(m⎯⎯⎯)<m(0⎯⎯), with m(0⎯⎯) and m(m⎯⎯⎯) the energy scale of the physical process and the compactification scale, are studied for arbitrary n. In the former scenario, amplitudes are given in terms of EZFs valued on the positive part of the real axis, with divergences arising from singularities of the EZFs. In the latter scenario, amplitudes involve EZFs valued on the negative part of the real axis, they are free of divergences, and they simplify considerably when they depend on the ratio m2(m⎯⎯⎯)/m2(0⎯⎯).
Leptonic CP phases near the μ−τ symmetric limit
(Submitted on 7 Mar 2016)
Neutrino masses and mixings, as indicated by current neutrino oscillation experiments, suggest that neutrino mass matrix posses an approximated μ−τexchange symmetry. We explore neutrino parameter space and find that a slight μ−τ symmetry breaking can only occur for quasidegenerate neutrino mass hierarchy with unequal (non zero) Majorana CP violation phases, which are found to be strongly correlated to the Dirac CP violating phase. Within this framework, robust predictions for the values of Majorana phases are thus obtained.