1,354,154 research outputs found
Nonlocal quantum field theory without acausality and nonunitarity at quantum level: is SUSY the key?
No full textThe realization of a nonlocal quantum field theory without losing unitarity, gauge invariance
and causality is investigated. It is commonly retained that such a formulation is
possible at tree level, but at quantum level acausality is expected to reappear at one loop.
We suggest that the problem of acausality is, in a broad sense, similar to the one about
anomalies in quantum field theory. By virtue of this analogy, we suggest that acausal
diagrams resulting from the fermionic sector and the bosonic one might cancel each
other, with a suitable content of fields and suitable symmetries. As a simple example,
we show how supersymmetry can alleviate this problem in a simple and elegant way,
i.e. by leading to exact cancellations of harmful diagrams, to all orders of perturbation
theory. An infinite number of divergent diagrams cancel each other by virtue of the
nonrenormalization theorem of supersymmetry. However, supersymmetry is not enough
to protect a theory from all acausal divergences. For instance, acausal contributions to
supersymmetric corrections to D-terms are not protected by supersymmetry. On the
other hand, we show in detail how supersymmetry also helps in dealing with D-terms:
divergences are not canceled but they become softer than in the nonsupersymmetric case.
The supergraphs’ formalism turns out to be a powerful tool to reduce the complexity of
perturbative calculations
Neutron Majorana mass from Exotic Instantons in a Pati-Salam model
No full textWe show how a Majorana mass for the neutron could result from nonperturbative quantum gravity effects peculiar to string theory. In particular, "exotic instantons" in un-oriented string compactifications with D-branes extending the (supersymmetric) standard model could indirectly produce an effective operator delta m n(t)n + h.c.. In a specific model with an extra vector-like pair of 'quarks', acquiring a large mass proportional to the string mass scale (exponentially suppressed by a function of the string moduli fields), delta m can turn out to be as low as 10(-24)-10(-25) eV.The induced neutron-antineutron oscillations could take place with a time scale tau(n (n) over tilde) > 10(8)s that could be tested by the next generation of experiments. On the other hand, proton decay and FCNC's are automatically strongly suppressed and are compatible with the current experimental limits.Depending on the number of brane intersections, the model may also lead to the generation of Majorana masses for R-handed neutrini. Our proposal could also suggest neutron-neutralino or neutron-axino oscillations, with implications in UCN, Dark Matter Direct Detection, UHECR and Neutron-Antineutron oscillations.This suggests to improve the limits on neutron-antineutron oscillations, as a possible test of string theory and quantum gravity
Non-commutative quantum gravity phenomenology in underground experiments
No full textWe show how non-commutative spacetime models can induce Pauli Exclusion Principle (PEP) forbidden nuclear and atomic transitions. We focalize our analysis on one of the most popular instantiations of non-commutativeness: -Poincaré model, based on the Groenewold–Moyal plane algebra. We show that PEP violating transitions induced by -Poincaré have an energy scale and angular emission dependence. PEP violating transitions in nuclear and atomic systems can be tested with very high accuracy in underground laboratory experiments such as DAMA/LIBRA and VIP(2). We derive that the Equivalence Principle assumed -Poincaré model can be already ruled-out until the Planck scale, from nuclear transitions tests by DAMA/LIBRA experiment
External Stability for Spherically Symmetric Solutions in Lorentz Breaking Massive Gravity
No full textWe discuss spherically symmetric solutions for point-like sources in Lorentz-breaking massive gravity theories. This analysis is valid for Stückelberg’s effective field theory formulation, for Lorentz Breaking Massive Bigravity and general extensions of gravity leading to an extra term −Srγ added to the Newtonian potential. The approach consists in analyzing the stability of the geodesic equations, at the first order (deviation equation). The main result is a strong constrain in the space of parameters of the theories. This motivates higher order analysis of geodesic perturbations in order to understand if a class of spherically symmetric Lorentz-breaking massive gravity solutions, for self-gravitating systems, exists. Stable and phenomenologically acceptable solutions are discussed in the no-trivial case S ≠ 0
Exotic see-saw mechanism for neutrinos and leptogenesis in a Pati-Salam model
No full textWe discuss non-perturbative corrections to the neutrino sector, in the context of a D-brane Pati-Salam-like model, that can be obtained as a simple alternative to SO(10) GUT's in theories with open and unoriented strings. In such D-brane models, exotic stringy instantons can correct the right-handed neutrino mass matrix in a calculable way, thus aecting mass hierarchies and modifying the see-saw mechanism to what we name exotic see-saw. For a wide range of parameters, a compact spectrum of right-handed neutrino masses can occur that gives rise to a predictive scenario for low energy observables. This model also provides a viable mechanism for
Baryon Asymmetry in the Universe (BAU) through leptogenesis. Finally, a
Majorana mass for the neutron is naturally predicted in the model, leading
to potentially testable neutron-antineutron oscillations. Combined measurements in neutrino and neutron-antineutron sectors could provide precious
informations on physics at the quantum gravity scale
Born–Infeld condensate as a possible origin of neutrino masses and dark energy
No full textAbstractWe discuss the possibility that a Born–Infeld condensate coupled to neutrinos can generate both neutrino masses and an effective cosmological constant. In particular, an effective field theory is provided capable of dynamically realizing the neutrino superfluid phase firstly suggested by Ginzburg and Zharkov. In such a case, neutrinos acquire a mass gap inside the Born–Infeld ether forming a long-range Cooper pair. Phenomenological implications of the approach are also discussed
Black hole shadow and chaos bound violation in f(T) teleparallel gravity
No full textWe show that the so-called chaos bound, proposed by Maldacena, Shenker & Stanford, can be violated in f(T) teleparallel gravity. In particular, it is possible to select a new gravitational Lyapunov parameter, controlling chaotization of circular trajectories, in black hole photo-sphere, that can exceed the Maldacena, Shenker & Stanford thermal bound. This feature alters the standard quasi-normal modes and ringdown phase after black hole merging with intriguing implications for future gravitational wave detections and black hole shadow measurements. It is a general characteristic for several gravitational theories beyond standard General Relativity
Testing Noncommutative Spacetimes and Violations of the Pauli Exclusion Principle with underground experiments
No full textWe propose to deploy limits that arise from different tests of the Pauli Exclusion Principle: i) to provide
theories of quantum gravity with experimental guidance; ii) to distinguish, among the plethora of possible models, the ones that are already ruled out by current data; iii) to direct future attempts to be in accordance with experimental constraints. We first review experimental bounds on nuclear processes forbidden by the Pauli Exclusion Principle, which have been derived by several experimental collaborations making use of various detector materials. Distinct features of the experimental devices entail sensitivities on the constraints hitherto achieved that may differ from one another by several orders of magnitude. We show that with choices of these limits, well-known examples of flat noncommutative space-time instantiations of quantum gravity can be heavily constrained, and eventually ruled out. We devote particular attention to the analysis of the k-Minkowski and theta-Minkowski noncommutative spacetimes.
These are deeply connected to some scenarios in string theory, loop quantum gravity, and noncommutative geometry.
We emphasize that the severe constraints on these quantum spacetimes, although they cannot rule out theories of
top-down quantum gravity to which they are connected in various ways, provide a powerful limitation for those
models. Focus on this will be necessary in the future
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