3 research outputs found
Hadronic Lorentz Violation in Chiral Perturbation Theory Including the Coupling to External Fields
If any violation of Lorentz symmetry exists in the hadron sector, its
ultimate origins must lie at the quark level. We continue the analysis of how
the theories at these two levels are connected, using chiral perturbation
theory. Considering a two-flavor quark theory, with dimension-4 operators that
break Lorentz symmetry, we derive a low-energy theory of pions and nucleons
that is invariant under local chiral transformations and includes the coupling
to external fields. The pure meson and baryon sectors, as well as the couplings
between them and the couplings to external electromagnetic and weak gauge
fields, contain forms of Lorentz violation which depend on linear combinations
of quark-level coefficients. In particular, at leading order the
electromagnetic couplings depend on the very same combinations as appear in the
free particle propagators. This means that observations of electromagnetic
processes involving hadrons--such as vacuum Cerenkov radiation, which may be
allowed in Lorentz-violating theories--can only reliably constrain certain
particular combinations of quark coefficients.Comment: 21 page
An Effective Field Theory Calculation of n(p, d)γ Cross-section for Big Bang Nucleo-synthesis
Studying the nuclear reaction n(p,d)γ and calculating its cross-section is not only a matter of interest from theoretical particle physics point of view but also from the viewpoint of cosmology. We now know that the universe is made up of only 5% baryonic matter. So, computing the density of baryons is of particular importance to physicists in general and cosmologists in particular. Deuterium production during Big Bang Nucleo-synthesis (BBN) is very sensitive to the density of baryons, thus baryon density can be inferred from the abundance of deuterium. In order to calculate deuterium abundance one needs to use the cross-section of np→dγ reaction as one of the inputs; hence the importance of this cross-section calculation.
In this document, a leading-order (LO) calculation of n(p,d)γ cross-section is presented using the framework of pion-less effective field theory with dibaryon fields. The computation yielded a numerical value of σLO = 494 mb which is then compared to the experimental value
Hadronic Lorentz Violation in Chiral Perturbation Theory
The leading-order Lorentz-violating hadronic Lagrange densities are constructed using chiral perturbation theory. This is done for both pions and nucleons starting from a two-flavor quark-level Lagrangian that consists of dimension-four Lorentzviolation operators. The effective Lagrangians are first constructed in the absence of external fields. The formalism is then extended to include interactions with external fields. The presence of Lorentz violation modifies the transformation behavior of external fields under the chiral group SU(2)L × SU(2)R. This in turn leads to modified pion and nucleon covariant derivatives. By expanding parts of both mesonic and baryonic Lagragians in terms of physical pion and nucleon fields, new approximate bounds on the effective pion Lorentz-violation coefficients are placed using experimental observations from the proton and neutron sectors. The resulting constraints on four pion parameters are at the 10-23 level