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Register a new userImplications of $textit{SU}(2)_L$ gauge invariance for constraints on Lorentz violation
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Lorentz invariance is one of the basic ingredients of quantum field theories and violations of it are stringently constrained experimentally. Therefore, the possibility of Lorentz violation (LV) is usually realized at very high energy scales, resulting in a strong suppression of it (by the new scale) in experiments. The Standard-Model Extension (SME) parameterizes LV in a model-independent way, respecting $SU(2)_L$ gauge invariance. This means, e.g., that the neutrino and charged-lepton sectors are linked to each other. Hence, on the one hand, any modification of neutrino properties simultaneously gives rise to effects for charged leptons, which is why the tight limits on flavour-off-diagonal LV for neutrinos imply new bounds on modifications of charged leptons. On the other hand, LV for left-handed charged leptons implies LV for neutrinos. Since LV modifications of the charged-lepton sector are, in general, even more constraining than effects in the flavour-diagonal neutrino sector, we obtain novel tight bounds on LV in the latter. Subsequently, we apply the same approach to an analysis of time-of-flight data for neutrinos (detected by IceCube) and photons from gamma ray bursts where discrepancies have been observed. Our finding is that an explanation of the arrival time difference between neutrino and photon events by dim-5 operators in the neutrino sector would lead to unacceptably large LV effects in the charged-lepton sector.
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Expanding on our prior efforts to search for Lorentz invariance violation (LIV) using the linear optical polarimetry of extragalactic objects, we propose a new method that combines linear and circular polarization measurements. While existing work has focused on the tendency of LIV to reduce the linear polarization degree, this new method additionally takes into account the coupling between photon helicities induced by some models. This coupling can generate circular polarization as light propagates, even if there is no circular polarization at the source. Combining significant detections of linear polarization of light from extragalactic objects with the absence of the detection of circular polarization in most measurements results in significantly tighter constraints regarding LIV. The analysis was carried out in the framework of the Standard-Model Extension (SME), an effective field theory framework to describe the low-energy effects of an underlying fundamental quantum gravity theory. We evaluate the performance of our method by deriving constraints on the mass dimension $d=4$ CPT-even SME coefficients from a small set of archival circular and linear optical polarimetry constraints and compare them to similar constraints derived in previous works with far larger sample sizes and based on linear polarimetry only. The new method yielded constraints that are an order of magnitude tighter even for our modest sample size of 21 objects. Based on the demonstrated gain in constraining power from scarce circular data, we advocate for the need for future extragalactic circular polarization surveys.
A new vector dark matter (DM) scenario in the context of the gauge-Higgs unification (GHU) is proposed. The DM particle is identified with an electric-charge neutral component in an $SU(2)_L$ doublet vector field with the same quantum number as the Standard Model Higgs doublet. Since such an $SU(2)_L$ doublet vector field is incorporated in any models of the GHU scenario, it is always a primary and model-independent candidate for the DM in the scenario. The observed relic density is reproduced through a DM pair annihilations into the weak gauge bosons with a TeV-scale DM mass, which is nothing but the compactification scale of extra-dimensions. Due to the higher-dimensional gauge structure of the GHU scenario, a pair of the DM particles has no direct coupling with a single $Z$-boson/Higgs boson, so that the DM particle evades the severe constraint from the current direct DM search experiments.
The largest gap in our understanding of nature at the fundamental level is perhaps a unified description of gravity and quantum theory. Although there are currently a variety of theoretical approaches to this question, experimental research in this field is inhibited by the expected Planck-scale suppression of quantum-gravity effects. However, the breakdown of spacetime symmetries has recently been identified as a promising signal in this context: a number of models for underlying physics can accommodate minuscule Lorentz and CPT violation, and such effects are amenable to ultrahigh-precision tests. This presentation will give an overview of the subject. Topics such as motivations, the SME test framework, mechanisms for relativity breakdown, and experimental tests will be reviewed. Emphasis is given to observations involving antimatter.
The assumption of Lorentz invariance is one of the founding principles of Modern Physics and violation of it would have profound implications to our understanding of the universe. For instance, certain theories attempting a unified theory of quantum gravity predict there could be an effective refractive index of the vacuum; the introduction of an energy dependent dispersion to photons could in turn lead to an observable Lorentz invariance violation signature. Whilst a very small effect on local scales the effect will be cumulative, and so for very high energy particles that travel very large distances the difference in arrival times could become sufficiently large to be detectable. This proceedings will look at testing for such Lorentz invariance violation (LIV) signatures in the astronomical lightcurves of gamma-ray emitting objects, with particular notice being given to the prospects for LIV testing with, the next generation observatory, the Cherenkov Telescope Array.
From special relativity, photon annihilation process HepProcess{{Pgg}{Pgg}{to}{Pep}{Pem}} prevents cosmic photons with energies above a threshold to propagate a long distance in cosmic space due to their annihilation with low energy cosmic background photons. However, modifications of the photon dispersion relation from Lorentz invariance violation~(LIV) can cause novel phenomena beyond special relativity to happen. In this paper, we point out that these phenomena include optical transparency, threshold reduction and reappearance of ultra-high energy photons in cosmic space. The recent observation of near and above PeV photon events by the LHAASO Collaboration reveals the necessity to consider the threshold anomalies. Future observations of above threshold photons from extragalactic sources can testify LIV properties of photons.
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