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Flavio Mercati

Publications and source records attributed to Flavio Mercati.

65 records · Page 4Linked to original sources
arXiv2010

Conserved charges and quantum-group transformations in noncommutative field theories

The recently-developed techniques of Noether analysis of the quantum-group spacetime symmetries of some noncommutative field theories rely on the {\it ad hoc} introduction of some peculiar auxiliary transformation parameters, which appear to have no role in the structure of the quantum group. We here show that it is possible to set up the Noether analysis directly in terms of the quantum-group symmetry transformations, and we therefore establish more robustly the attribution of the conserved charges to the symmetries of interest. We also characterize the concept of "time independence" (as needed for conserved charges) in a way that is robust enough to be applicable even to theories with space/time noncommutativity, where it might have appeared that any characterization of time independence should be vulnerable to changes of ordering convention.

hep-th↗
arXiv2010

Gravity in quantum spacetime

The literature on quantum-gravity-inspired scenarios for the quantization of spacetime has so far focused on particle-physics-like studies. This is partly justified by the present limitations of our understanding of quantum-gravity theories, but we here argue that valuable insight can be gained through semi-heuristic analyses of the implications for gravitational phenomena of some results obtained in the quantum-spacetime literature. In particular, we show that the types of description of particle propagation that emerged in certain quantum-spacetime frameworks have striking implications for gravitational collapse and for the behaviour of gravity at large distances.

gr-qc↗
arXiv2010

String-inspired ultraviolet/infrared mixing and preliminary evidence of a violation of the de Broglie relation for nonrelativistic neutrons

We advocate a novel perspective on the phenomenology of a framework with spacetime noncommutativity which is of established relevance for string theory. Our analysis applies to cases in which the noncommutativity parameters are arranged according to the criteria of "light-like noncommutativity" and ultraviolet supersymmetry is assumed, so that the implications of the characteristic mechanism of ultraviolet/infrared mixing are relatively soft. We also observe that an analogous case of soft ultraviolet-infrared mixing is present in a previously-proposed Loop-quantum-gravity-inspired description of quantum spacetime. And we show that soft ultraviolet-infrared mixing produces an anomaly for the nonrelativistic de Broglie relation $λv = h/m$, with correction term governed by a single (but particle-dependent) parameter $χ$. We test this hypothesis by comparing a determination of the fine structure constant that relies on the de Broglie relation for nonrelativistic neutrons to other independent determinations of the fine structure constant, and we obtain an estimate of $χ$ that differs from 0 with four-standard-deviation significance.

hep-th↗
arXiv2010

Probing the quantum-gravity realm with slow atoms

For the study of Planck-scale modifications of the energy-momentum dispersion relation, which had been previously focused on the implications for ultrarelativistic (ultrafast) particles, we consider the possible role of experiments involving nonrelativistic particles, and particularly atoms. We extend a recent result establishing that measurements of "atom-recoil frequency" can provide insight that is valuable for some theoretical models. And from a broader perspective we analyze the complementarity of the nonrelativistic and the ultrarelativistic regimes in this research area.

gr-qc↗
arXiv2009

Threshold anomalies in Horava-Lifshitz-type theories

Recently the study of threshold kinematic requirements for particle-production processes has played a very significant role in the phenomenology of theories with departures from Poincare' symmetry. We here specialize these threshold studies to the case of a class of violations of Poincare' symmetry which has been much discussed in the literature on Horava-Lifshitz scenarios. These involve modifications of the energy-momentum ("dispersion") relation that may be different for different types of particles, but always involve even powers of energy-momentum in the correction terms. We establish the requirements for compatibility with the observed cosmic-ray spectrum, which is sensitive to the photopion-production threshold. We find that the implications for the electron-positron pair-production threshold are rather intriguing, in light of some recent studies of TeV emissions by Blazars. Our findings should also provide motivation for examining the fate of the law of energy-momentum conservation in Horava-Lifshitz-type theories.

hep-th↗
arXiv2009

Constraining the energy-momentum dispersion relation with Planck-scale sensitivity using cold atoms

We use the results of ultra-precise cold-atom-recoil experiments to constrain the form of the energy-momentum dispersion relation, a structure that is expected to be modified in several quantum-gravity approaches. Our strategy of analysis applies to the nonrelativistic (small speeds) limit of the dispersion relation, and is therefore complementary to an analogous ongoing effort of investigation of the dispersion relation in the ultrarelativistic regime using observations in astrophysics. For the leading correction in the nonrelativistic limit the exceptional sensitivity of cold-atom-recoil experiments remarkably allows us to set a limit within a single order of magnitude of the desired Planck-scale level, thereby providing the first example of Planck-scale sensitivity in the study of the dispersion relation in controlled laboratory experiments. For the next-to-leading term we obtain a limit which is a few orders of magnitude away from the Planck scale, but still amounts to the best limit on a class of Lorentz-symmetry test theories that has been extensively used to investigate the hypothesis of "deformation" (rather than breakdown) of spacetime symmetries.

gr-qc↗
arXiv2008

Discreteness of area in noncommutative space

We introduce an area operator for the Moyal noncommutative plane. We find that the spectrum is discrete, but, contrary to the expectation formulated by other authors, not characterized by a "minimum-area principle". We show that an intuitive analysis of the uncertainty relations obtained from Moyal-plane noncommutativity is fully consistent with our results for the spectrum, and we argue that our area operator should be generalizable to several other noncommutative spaces. We also observe that the properties of distances and areas in the Moyal plane expose some weaknesses in the line of reasoning adopted in some of the heuristic analyses of the measurability of geometric spacetime observables in the quantum-gravity realm.

hep-th↗
arXiv2008

On the distance observable in the Moyal plane and in a novel two-dimensional space with string-theory pregeometry

Motivation for the study of spacetime noncommutativity comes primarily from its possible use in investigations of (Planck-scale) spacetime fuzziness, but most work focuses on S-matrix/field-theory observables and still very little has been established for geometric observables. We argue that it might be useful to exploit the "pregeometric" formulation of spacetime noncommutativity, which in particular describes the coordinates of the Moyal plane in terms of the phase-space coordinates of a point particle "living" in a auxiliary/fictitious spacetime ($\{x_1,x_2\}_{Moyal} \equiv \{q,p\}_{particle}$). This leads us straightforwardly to a distance operator for the Moyal plane, and allows us to expose some limitations of a previous attempt to describe the "area of a disc" in the Moyal plane. We also observe that from our pregeometric perspective it is rather natural to contemplate a spacetime whose pregeometric picture is based on the phase-space coordinates (fields) of a string. The fact that such "stringspaces" essentially provide spacetime points with extendedness is relevant for the fuzziness of geometric spacetime observables in ways that we preliminarily characterize through an analysis of the distance observable on a two-dimensional stringspace and through the observation that by implementing the Amati-Ciafaloni-Veneziano/Gross-Mende uncertainty relation in the phase space of the pregeometric string one could have stringspaces with a novel type of fuzzy coordinates.

hep-th↗
arXiv2007

Twisted Hopf symmetries of canonical noncommutative spacetimes and the no-pure-boost principle

We study the twisted-Hopf-algebra symmetries of observer-independent canonical spacetime noncommutativity, for which the commutators of the spacetime coordinates take the form [x^{mu},x^{nu}]=i theta^{mu nu} with observer-independent (and coordinate-independent) theta^{mu nu}. We find that it is necessary to introduce nontrivial commutators between transformation parameters and spacetime coordinates, and that the form of these commutators implies that all symmetry transformations must include a translation component. We show that with our noncommutative transformation parameters the Noether analysis of the symmetries is straightforward, and we compare our canonical-noncommutativity results with the structure of the conserved charges and the "no-pure-boost" requirement derived in a previous study of kappa-Minkowski noncommutativity. We also verify that, while at intermediate stages of the analysis we do find terms that depend on the ordering convention adopted in setting up the Weyl map, the final result for the conserved charges is reassuringly independent of the choice of Weyl map and (the corresponding choice of) star product.

hep-th↗
arXiv2007

A no-pure-boost uncertainty principle from spacetime noncommutativity

We study boost and space-rotation transformations in kappa-Minkowski noncommutative spacetime, using the techniques that some of us had previously developed (hep-th/0607221) for a description of translations in kappa-Minkowski, which in particular led to the introduction of translation transformation parameters that do not commute with the spacetime coordinates. We find a similar description of boosts and space rotations, which allows us to identify some associated conserved charges, but the form of the commutators between transformation parameters and spacetime coordinates is incompatible with the possibility of a pure boost.

hep-th↗