1,721,010 research outputs found
Lorentz Violation: Motivation and new constraints
No full textWe review the main theoretical motivations and observational constraints on Planck scale–suppressed violations of Lorentz invariance. After introducing the problems related to the phenomenological study of quantum gravitational effects, we discuss the main theoretical frameworks within which possible departures from Lorentz invariance can be described. In particular, we focus on the framework of effective field theory, describing several possible ways of including Lorentz violation therein and discussing their theoretical viability. We review the main low-energy effects that are expected in this framework. We discuss the current observational constraints on such a framework, focusing on those achievable through high-energy astrophysics observations. In this context, we present a summary of the most recent and strongest constraints on quantum electrodynamics with Lorentz-violating nonrenormalizable operators. Finally, we discuss the status of the field and its future perspectives
GZK photon constraints on Planck scale Lorentz violation in QED
No full textWe show how the argument exploited by Galaverni and Sigl (2008
Phys. Rev. Lett. 100 021102 (see also 0708.1737)) to constrain Lorentz invariance
violation (LV) using ultra-high-energy photon non-observation by the AUGER
experiment can be extended to QED with Planck-suppressed LV (at O(E/M)
and O(E2/M2)). While the original constraints given by Galaverni and Sigl
(2008 Phys. Rev. Lett. 100 021102) happen to be weakened, we show that, when
used together with other EFT reactions and the expected detection of photons at
E > 1019 eV by AUGER, this method has the potentiality not only to basically
rule out O(E/M) corrections but also to strongly constrain, for the first time,
the CPT even O(E2/M2) LV QED
Quantum Gravity phenomenology: achievements and challenges
No full textMotivated by scenarios of quantum gravity, Planck-suppressed deviations from Lorentz invariance are expected at observable energies. Ultra-High-Energy Cosmic Rays, the most energetic particles ever observed in nature, yielded in the last two years strong constraints on deviations suppressed by O(E-2/M-P1(2)) and also, for the first time, on space-time foam, stringy inspired models of quantum gravity. We review the most important achievements and discuss future outlooks
Scale hierarchy in Horava-Lifshitz gravity: a strong constraint from synchrotron radiation in the Crab nebula.
No full textHořava-Lifshitz gravity models contain higher-order operators suppressed by a characteristic scale, which is required to be parametrically smaller than the Planck scale. We show that recomputed synchrotron radiation constraints from the Crab Nebula suffice to exclude the possibility that this scale is of the same order of magnitude as the Lorentz breaking scale in the matter sector. This highlights the need for a mechanism that suppresses the percolation of Lorentz violation in the matter sector and is effective for higher-order operators as well
Diffuse Galactic Gamma Rays at intermediate and high latitudes. I. Constraints on the ISM properties
No full textWe study the high latitude (vertical bar b vertical bar > 10 degrees) diffuse gamma-ray emission in the Galaxy in light of the recently published data from the Fermi collaboration at energies between 100MeV and 100GeV. The unprecedented accuracy in these measurements allows to probe and constrain the properties of sources and propagation of cosmic rays (CRs) in the Galaxy, as well as confirming conventional assumptions made on the interstellar medium (ISM). Using the publicly available DRAGON code, that has been shown to reproduce local measurements of CRs, we study assumptions made in the literature on atomic (HI) and molecular hydrogen (H2) gas distributions in the ISM, and non spatially uniform models of diffusion in the Galaxy. By performing a combined analysis of CR and gamma-ray spectra, we derive constraints on the properties of the ISM gas distribution and the vertical scale height of galactic CR diffusion, which may have implications also on indirect Dark Matter detection. We also discuss some of the possible interpretations of the break at high rigidity in CR protons and helium spectra, recently observed by P A M E L A and their impact on gamma-rays
Ultrahigh-Energy Photons as Probes of Lorentz Symmetry Violations in Stringy Space-Time Foam Models
No full textThe time delays between gamma-rays of different energies from extragalactic sources have often been used to probe quantum gravity models in which Lorentz symmetry is violated. It has been claimed that these time delays can be explained by or at least put the strongest available constraints on quantum gravity scenarios that cannot be cast within an effective field theory framework, such as the space-time foam, D-brane model. Here we show that this model would predict too many photons in the ultra-high energy cosmic ray flux to be consistent with observations. The resulting constraints on the space-time foam model are much stronger than limits from time delays and allow for Lorentz violations effects way too small for explaining the observed time delays
New constraints on Planck-scale Lorentz Violation in QED from the Crab Nebula
No full textWe set constraints on O(E/M) Lorentz Violation in QED in an effective field theory framework. A major consequence of such assumptions is the modification of the dispersion relations for electrons/positrons and photons, which in turn can affect the electromagnetic output of astrophysical objects. We compare the information provided by multiwavelength observations with a full and self-consistent computation of the broad-band spectrum of the Crab Nebula. We cast constraints of order 10^{-5} at 95% confidence level on the Lorentz Violation parameters
Violations of Lorentz invariance in the neutrino sector: an improved analysis of anomalous threshold constraints
No full textRecently there has been a renewed activity in the physics of violations of Lorentz invariance in the neutrino sector. Flavor dependent Lorentz violation, which generically changes the pattern of neutrino oscillations, is extremely tightly constrained by oscillation experiments. Flavor independent Lorentz violation, which does not introduce new oscillation phenomena, is much more weakly constrained with constraints coming from time of flight and anomalous threshold analyses. We use a simplified rotationally invariant model to investigate the effects of finite baselines and energy dependent dispersion on anomalous reaction rates in long baseline experiments and show numerically that anomalous reactions do not necessarily cut off the spectrum quite as sharply as currently assumed. We also present a revised analysis of how anomalous reactions can be used to cast constraints from the observed atmospheric high energy neutrinos and the expected cosmogenic ones
Antiprotons from dark matter annihilation in the Galaxy: Astrophysical uncertainties
No full textThe latest years have seen steady progresses in weakly interacting massive particle dark matter (DM) searches, with hints of possible signals suggested both in direct and indirect detection. Antiprotons play a key role in this context, since weakly interacting massive particle annihilations can be a copious source of antiprotons, and the antiproton flux from conventional astrophysical sources is predicted with fair accuracy and matches the measured cosmic ray (CR) spectrum very well. Using the publicly available DRAGON code, we reconsider antiprotons as a tool to set constraints on DM models; we compare against the most up-to-date (p) over bar measurements, taking also into account the latest spectral information on the p and He CR fluxes. In particular, we probe carefully the uncertainties associated to both standard astrophysical and DM originated antiprotons, by using a variety of distinctively different assumptions for the propagation of CRs and for the DM distribution in the Galaxy. We find that the impact of the astrophysical uncertainties on constraining the DM properties of a wide class of annihilating DM models can be much stronger, up to a factor of similar to 50, than the one due to uncertainties on the DM distribution (similar to 2-6). Remarkably, even reducing the uncertainties on the propagation parameters derived by local observables, nonlocal effects can change our predictions for the constraints even by 50%. Nevertheless, current (p) over bar data can place tight constraints on DM models, excluding some of those suggested in connection with indirect and direct searches. Finally we discuss the impact of upcoming CR spectral data from the AMS-02 instrument on DM model constraints
Possible cosmogenic neutrino constraints on Planck-scale Lorentz violation
No full textWe study, within an effective field theory framework, O (E-2/M-Pl(2)) Planck-scale suppressed Lorentz invariance violation (LV) effects in the neutrino sector, whose size we parameterize by a dimensionless parameter eta(nu). We find deviations from predictions of Lorentz invariant physics in the cosmogenic neutrino spectrum. For positive O (1) coefficients no neutrino will survive above 10(19) eV. The existence of this cutoff generates a bump in the neutrino spectrum at energies of 10(17) eV. Although at present no constraint can be cast, as current experiments do not have enough sensitivity to detect ultra-high-energy neutrinos, we show that experiments in construction or being planned have the potential to cast limits as strong as eta(nu) less than or similar to 10(-4) on the neutrino LV parameter, depending on how LV is distributed among neutrino mass states. Constraints on eta(nu) < 0 can in principle be obtained with this strategy, but they require a more detailed modeling of how LV affects the neutrino sector
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