Frontiers in Physics | High-Energy and Astroparticle Physics section | New and Recent Articles
https://www.frontiersin.org/journals/physics/sections/high-energy-and-astroparticle-physics
RSS Feed for High-Energy and Astroparticle Physics section in the Frontiers in Physics journal | New and Recent Articlesen-usFrontiers Feed Generator,version:12020-10-26T07:24:13.9992498+00:0060https://www.frontiersin.org/articles/10.3389/fphy.2020.525333
https://www.frontiersin.org/articles/10.3389/fphy.2020.525333
Simulating Indefinite Causal Order With Rindler Observers2020-10-26T00:00:00ZAleksandra DimićMarko MilivojevićDragoljub GočaninNatália S. MóllerČaslav BruknerRealization of indefinite causal order (ICO), a theoretical possibility that even causal relations between physical events can be subjected to quantum superposition, apart from its general significance for the fundamental physics research, would also enable quantum information processing that outperforms protocols in which the underlying causal structure is definite. In this paper, we start with a proposition that an observer in a state of quantum superposition of being at two different relative distances from the event horizon of a black hole, effectively resides in ICO space-time generated by the black hole. By invoking the fact that the near-horizon geometry of a Schwarzschild black hole is that of a Rindler space-time, we propose a way to simulate an observer in ICO space-time by a Rindler observer in a state of superposition of having two different proper accelerations. By extension, a pair of Rindler observers with entangled proper accelerations simulates a pair of entangled ICO observers. Moreover, these Rindler-systems might have a plausible experimental realization by means of optomechanical resonators.]]>https://www.frontiersin.org/articles/10.3389/fphy.2020.00341
https://www.frontiersin.org/articles/10.3389/fphy.2020.00341
Effective Asymptotic Safety and Its Predictive Power: Gauge-Yukawa Theories2020-09-30T00:00:00ZAaron HeldEffective field theory provides a new perspective on the predictive power of Renormalization Group fixed points. Critical trajectories between different fixed points confine the regions of UV-complete, IR-complete, as well as conformal theories. The associated boundary surfaces cannot be crossed by the Renormalization Group flow of any effective field theory. We delineate cases in which the boundary surface acts as an infrared attractor for generic effective field theories. Gauge-Yukawa theories serve as an example that is both perturbative and of direct phenomenological interest. We identify additional matter fields such that all the observed coupling values of the Standard Model, apart from the Abelian hypercharge, lie within the conformal region. We define a quantitative measure of the predictivity of effective asymptotic safety and demonstrate phenomenological constraints for the associated beyond Standard-Model Yukawa couplings.]]>https://www.frontiersin.org/articles/10.3389/fphy.2020.00214
https://www.frontiersin.org/articles/10.3389/fphy.2020.00214
Why the Cosmological Constant Seems to Hardly Care About Quantum Vacuum Fluctuations: Surprises From Background Independent Coarse Graining2020-08-13T00:00:00ZCarlo PaganiMartin ReuterBackground Independence is a sine qua non for every satisfactory theory of Quantum Gravity. If one tries to establish a corresponding notion of Wilsonian renormalization, or coarse graining, it presents a major conceptual and technical difficulty usually. In this paper, we adopt the approach of the gravitational Effective Average Action and demonstrate that, generically, coarse graining in Quantum Gravity and in standard field theories on a non-dynamical spacetime are profoundly different. By means of a concrete example, which, in connection with the cosmological constant problem, is also interesting in its own right, we show that the surprising and sometimes counterintuitive implications of Background Independent coarse graining are neither restricted to high energies nor to strongly non-perturbative regimes. In fact, while our approach has been employed in most studies of Asymptotic Safety, this particular ultraviolet behavior plays no essential role in the present context.]]>https://www.frontiersin.org/articles/10.3389/fphy.2020.00295
https://www.frontiersin.org/articles/10.3389/fphy.2020.00295
Coarse Graining Spin Foam Quantum Gravity—A Review2020-08-06T00:00:00ZSebastian SteinhausIn quantum gravity, we envision renormalization as the key tool for bridging the gap between microscopic models and observable scales. For spin foam quantum gravity, which is defined on a discretization akin to lattice gauge theories, the goal is to derive an effective theory on a coarser discretization from the dynamics on the finer one, coarse graining the system in the process and thus relating physics at different scales. In this review I will discuss the motivation for studying renormalization in spin foam quantum gravity, e.g., to restore diffeomorphism symmetry, and explain how to define renormalization in a background independent setting by formulating it in terms of boundary data. I will motivate the importance of the boundary data by studying coarse graining of a concrete example and extending this to the spin foam setting. This will naturally lead me to the methods currently used for renormalizing spin foam quantum gravity, such as tensor network renormalization, and a discussion of recent results. I will conclude with an overview of future prospects and research directions.]]>https://www.frontiersin.org/articles/10.3389/fphy.2020.00269
https://www.frontiersin.org/articles/10.3389/fphy.2020.00269
Critical Reflections on Asymptotically Safe Gravity2020-08-03T00:00:00ZAlfio BonannoAstrid EichhornHolger GiesJan M. PawlowskiRoberto PercacciMartin ReuterFrank SaueressigGian Paolo VaccaAsymptotic safety is a theoretical proposal for the ultraviolet completion of quantum field theories, in particular for quantum gravity. Significant progress on this program has led to a first characterization of the Reuter fixed point. Further advancement in our understanding of the nature of quantum spacetime requires addressing a number of open questions and challenges. Here, we aim at providing a critical reflection on the state of the art in the asymptotic safety program, specifying and elaborating on open questions of both technical and conceptual nature. We also point out systematic pathways, in various stages of practical implementation, toward answering them. Finally, we also take the opportunity to clarify some common misunderstandings regarding the program.]]>https://www.frontiersin.org/articles/10.3389/fphy.2020.00185
https://www.frontiersin.org/articles/10.3389/fphy.2020.00185
Towards a Unitary, Renormalizable, and Ultraviolet-Complete Quantum Theory of Gravity2020-07-21T00:00:00ZChristian F. SteinwachsFor any fundamental quantum field theory, unitarity, renormalizability, and relativistic invariance are considered to be essential properties. Unitarity is inevitably connected to the probabilistic interpretation of the quantum theory, while renormalizability guarantees its completeness. Relativistic invariance, in turn, is a symmetry that derives from the structure of spacetime. So far, the perturbative attempt to formulate a fundamental local quantum field theory of gravity based on the metric field seems to be in conflict with at least one of these properties. In quantum Hořava gravity, a quantum Lifshitz field theory of gravity characterized by an anisotropic scaling between space and time, unitarity and renormalizability can be retained while Lorentz invariance is sacrificed at high energies and must emerge only as approximate symmetry at low energies. This article reviews various approaches to perturbative quantum gravity, with a particular focus on recent progress in the quantization of Hořava gravity, supporting its theoretical status as a unitary, renormalizable, and ultraviolet-complete quantum theory of gravity.]]>https://www.frontiersin.org/articles/10.3389/fphy.2020.00209
https://www.frontiersin.org/articles/10.3389/fphy.2020.00209
Editorial: Phenomena Beyond the Standard Model: What Do We Expect for New Physics to Look Like?2020-07-21T00:00:00ZAntónio P. MoraisStefano MorettiRoman Pasechnikhttps://www.frontiersin.org/articles/10.3389/fphy.2020.00247
https://www.frontiersin.org/articles/10.3389/fphy.2020.00247
Renormalization in Quantum Theories of Geometry2020-07-09T00:00:00ZJan AmbjornJakub Gizbert-StudnickiAndrzej GörlichJerzy JurkiewiczRenate LollA hallmark of non-perturbative theories of quantum gravity is the absence of a fixed background geometry, and therefore the absence in a Planckian regime of any notion of length or scale that is defined a priori. This has potentially far-reaching consequences for the application of renormalization group methods à la Wilson, which rely on these notions in a crucial way. We review the status quo of attempts in the Causal Dynamical Triangulations (CDT) approach to quantum gravity to find an ultraviolet fixed point associated with the second-order phase transitions observed in the lattice theory. Measurements of the only invariant correlator currently accessible, that of the total spatial three-volume, has not produced any evidence of such a fixed point. A possible explanation for this result is our incomplete and perhaps naïve understanding of what constitutes an appropriate notion of (quantum) length near the Planck scale.]]>https://www.frontiersin.org/articles/10.3389/fphy.2020.00207
https://www.frontiersin.org/articles/10.3389/fphy.2020.00207
On the Possibility of Experimental Detection of the Discreteness of Time2020-07-03T00:00:00ZMarios ChristodoulouCarlo RovelliThe Bose-Marletto-Vedral (BMV) experiment tests a quantum gravitational effect predicted by low energy perturbative quantum gravity. It has received attention because it may soon be within observational reach in the lab. We point out that: (i) in relativistic language, the experiment tests an interference effect between proper-time intervals; (ii) the feasibility study by Bose et al. suggests that current technology could allow to probe differences of such proper-time intervals of the order of 10^{−38} seconds, about twenty orders of magnitude beyond the current resolution of the best atomic clocks; (iii) the difference of proper times approaches Planck time (10^{−44} s) if the masses of the particles in the experiment approach the Planck mass (~micrograms). This implies that the experiment might open a window on the structure of time at the Planck scale. We show that if time differences are discrete at the Planck scale—as research in quantum gravity may suggest—the Planckian discreteness of time would appear as quantum levels of an in principle measurable entanglement entropy.]]>https://www.frontiersin.org/articles/10.3389/fphy.2020.00187
https://www.frontiersin.org/articles/10.3389/fphy.2020.00187
On Characterizing the Quantum Geometry Underlying Asymptotic Safety2020-06-09T00:00:00ZAleksandr KurovFrank SaueressigThe asymptotic safety program builds on a high-energy completion of gravity based on the Reuter fixed point, a non-trivial fixed point of the gravitational renormalization group flow. At this fixed point the canonical mass-dimension of coupling constants is balanced by anomalous dimensions induced by quantum fluctuations such that the theory enjoys quantum scale invariance in the ultraviolet. The crucial role played by the quantum fluctuations suggests that the geometry associated with the fixed point exhibits non-manifold like properties. In this work, we continue the characterization of this geometry employing the composite operator formalism based on the effective average action. Explicitly, we give a relation between the anomalous dimensions of geometric operators on a background d-sphere and the stability matrix encoding the linearized renormalization group flow in the vicinity of the fixed point. The eigenvalue spectrum of the stability matrix is analyzed in detail and we identify a “perturbative regime” where the spectral properties are governed by canonical power counting. Our results recover the feature that quantum gravity fluctuations turn the (classically marginal) R^{2}-operator into a relevant one. Moreover, we find strong indications that higher-order curvature terms present in the two-point function play a crucial role in guaranteeing the predictive power of the Reuter fixed point.]]>https://www.frontiersin.org/articles/10.3389/fphy.2020.00176
https://www.frontiersin.org/articles/10.3389/fphy.2020.00176
On the Testability of the Equivalence Principle as a Gauge Principle Detecting the Gravitational t3 Phase2020-05-28T00:00:00ZChiara MarlettoVlatko VedralThere have been various claims that the Equivalence Principle, as originally formulated by Einstein, presents several difficulties when extended to the quantum domain, even in the regime of weak gravity. Here we point out that by following the same approach as used for other classical principles, e.g., the principle of conservation of energy, one can, for weak fields, obtain a straightforward quantum formulation of the principle. We draw attention to a recently performed test that confirms the Equivalence Principle in this form and discuss its implications.]]>https://www.frontiersin.org/articles/10.3389/fphy.2020.00188
https://www.frontiersin.org/articles/10.3389/fphy.2020.00188
From Renormalization Group Flows to Cosmology2020-05-27T00:00:00ZAlessia PlataniaAccording to the asymptotic-safety conjecture, the gravitational renormalization group flow features an ultraviolet-attractive fixed point that makes the theory renormalizable and ultraviolet complete. The existence of this fixed point entails an antiscreening of the gravitational interaction at short distances. In this paper we review the state-of-the-art of phenomenology of Asymptotically Safe Gravity, focusing on the implications of the gravitational antiscreening in cosmology.]]>https://www.frontiersin.org/articles/10.3389/fphy.2020.00149
https://www.frontiersin.org/articles/10.3389/fphy.2020.00149
Search for Lepton-Flavor-Violating Decays of Bosons With the ATLAS Detector2020-05-08T00:00:00ZTomas DavidekLuca FioriniThe quest for lepton-flavor-violating processes at the LHC represents one of the key searches for new physics beyond the Standard Model. This review summarizes the direct searches for lepton-flavor-violating decays of heavy bosons with the ATLAS detector, using proton-proton collisions at the center-of-mass energy of 13 TeV.]]>https://www.frontiersin.org/articles/10.3389/fphy.2020.00111
https://www.frontiersin.org/articles/10.3389/fphy.2020.00111
Holographic Space-Time and Quantum Information2020-04-21T00:00:00ZTom BanksThe formalism of Holographic Space-time (HST) is a translation of the principles of Lorentzian geometry into the language of quantum information. Intervals along time-like trajectories, and their associated causal diamonds, completely characterize a Lorentzian geometry. The Bekenstein-Hawking-Gibbons-'t Hooft-Jacobson-Fischler-Susskind-Bousso Covariant Entropy Principle, equates the logarithm of the dimension of the Hilbert space associated with a diamond to one quarter of the area of the diamond's holographic screen, measured in Planck units. The most convincing argument for this principle is Jacobson's derivation of Einstein's equations as the hydrodynamic expression of this entropy law. In that context, the null energy condition (NEC) is seen to be the analog of the local law of entropy increase. The quantum version of Einstein's relativity principle is a set of constraints on the mutual quantum information shared by causal diamonds along different time-like trajectories. The implementation of this constraint for trajectories in relative motion is the greatest unsolved problem in HST. The other key feature of HST is its claim that, for non-negative cosmological constant or causal diamonds much smaller than the asymptotic radius of curvature for negative c.c., the degrees of freedom localized in the bulk of a diamond are constrained states of variables defined on the holographic screen. This principle gives a simple explanation of otherwise puzzling features of BH entropy formulae, and resolves the firewall problem for black holes in Minkowski space. It motivates a covariant version of the CKN [1] bound on the regime of validity of quantum field theory (QFT) and a detailed picture of the way in which QFT emerges as an approximation to the exact theory.]]>https://www.frontiersin.org/articles/10.3389/fphy.2020.00056
https://www.frontiersin.org/articles/10.3389/fphy.2020.00056
A Critique of the Asymptotic Safety Program2020-03-11T00:00:00ZJohn F. DonoghueThe present practice of Asymptotic Safety in gravity is in conflict with explicit calculations in low energy quantum gravity. This raises the question of whether the present practice meets the Weinberg condition for Asymptotic Safety. I argue, with examples, that the running of Λ and G found in Asymptotic Safety are not realized in the real world, with reasons which are relatively simple to understand. A comparison/contrast with quadratic gravity is also given, which suggests a few obstacles that must be overcome before the Lorentzian version of the theory is well behaved. I make a suggestion on how a Lorentzian version of Asymptotic Safety could potentially solve these problems.]]>https://www.frontiersin.org/articles/10.3389/fphy.2020.00039
https://www.frontiersin.org/articles/10.3389/fphy.2020.00039
A Guidebook to Hunting Charged Higgs Bosons at the LHC2020-03-10T00:00:00ZAbdesslam ArhribRachid BenbrikHicham HarouizStefano MorettiAbdessamad RouchadWe perform a comprehensive global analysis in the Minimal Supersymmetric Standard Model (MSSM) as well as in the 2-Higgs Doublet Model (2HDM) of the production and decay mechanisms of charged Higgs bosons (H^{±}) at the Large Hadron Collider (LHC). We start from accounting for the most recent experimental results (SM-like Higgs boson signal strengths and search limits for new Higgs boson states obtained at Run-1 and -2 of the LHC and previous colliders), from (both direct and indirect) searches for supersymmetric particles as well as from flavor observables (from both e^{+}e^{−} factories and hadron colliders). We then present precise predictions for H^{±} cross sections and decay rates in different reference scenarios of the two aforementioned models in terms of the parameter space currently available, specifically, mapped over the customary (mA,H±,tanβ) planes. These include the mhmod+ and hMSSM configurations of the MSSM and the 2HDM Type-I, -II, -X, and -Y for which we also enforce theoretical constraints, such as vacuum stability, perturbativity, and unitarity. We also define specific Benchmark Points (BPs) which are always close to (or coinciding with) the best fits of the theoretical scenarios to experimental data. We finally briefly discuss the ensuing phenomenology for the purpose of aiding future searches for such charged Higgs boson states.]]>https://www.frontiersin.org/articles/10.3389/fphy.2019.00228
https://www.frontiersin.org/articles/10.3389/fphy.2019.00228
Flavor Techniques for LFV Processes: Higgs Decays in a General Seesaw Model2020-01-23T00:00:00ZXabier MarcanoRoberto A. MoralesLepton flavor violating processes are optimal observables to test new physics, since they are forbidden in the Standard Model while they may be generated in new theories. The usual approach to these processes is to perform the computations in the physical basis; nevertheless this may lose track of the dependence on some of the fundamental parameters, in particular on those at the origin of the flavor violation. Consequently, in order to obtain analytical expressions directly in terms of these parameters, flavor techniques are often preferred. In this work, we focus on the mass insertion approximation technique, which works with the interaction states instead of the physical ones, and provides diagrammatic expansions of the observables. After reviewing the basics of this technique with two simple examples, we apply it to the lepton flavor violating Higgs decays in the framework of a general type-I seesaw model with an arbitrary number of right-handed neutrinos. We derive an effective vertex valid to compute these observables when the right-handed neutrino masses are above the electroweak scale and show that we recover previous results obtained for low scale seesaws. Finally, we apply current constraints on the model to conclude on maximum Higgs decay rates, which unfortunately are far from current experimental sensitivities.]]>https://www.frontiersin.org/articles/10.3389/fphy.2019.00191
https://www.frontiersin.org/articles/10.3389/fphy.2019.00191
Recent Probes of Standard and Non-standard Neutrino Physics With Nuclei2019-11-27T00:00:00ZDimitrios K. PapouliasTheocharis S. KosmasYoshitaka KunoWe review standard and non-standard neutrino physics probes that are based on nuclear measurements. We pay special attention on the discussion of prospects to extract new physics at prominent rare event measurements looking for neutrino-nucleus scattering, such as the coherent elastic neutrino-nucleus scattering (CEνNS) that may involve lepton flavor violation (LFV) in neutral-currents (NC). For the latter processes several appreciably sensitive experiments are currently pursued or have been planed to operate in the near future, like the COHERENT, CONUS, CONNIE, MINER, TEXONO, RED100, vGEN, Ricochet, NUCLEUS, etc. We provide a thorough discussion on phenomenological and theoretical studies, in particular those referring to the nuclear physics aspects in order to provide accurate predictions for the relevant experiments. Motivated by the recent discovery of CEνNS at the COHERENT experiment and the active experimental efforts for a new measurement at reactor-based experiments, we summarize the current status of the constraints as well as the future sensitivities on nuclear and electroweak physics parameters, non-standard interactions, electromagnetic neutrino properties, sterile neutrinos and simplified scenarios with novel vector Z′ or scalar ϕ mediators. Indirect and direct connections of CEνNS with astrophysics, direct Dark Matter detection and charge lepton flavor violating processes are also discussed.]]>https://www.frontiersin.org/articles/10.3389/fphy.2019.00188
https://www.frontiersin.org/articles/10.3389/fphy.2019.00188
Manifest Gravitational Duality Near Anti de Sitter Space-Time2019-11-21T00:00:00ZSergio HörtnerWe derive a manifestly duality-invariant formulation of the Arnowitt-Deser-Misner action principle linearized around anti de Sitter background. The analysis is based on the introduction of two symmetric potentials—on which the duality transformations act—upon resolution of the linearized constraints, along the lines of previous works focusing on Minkowski and de Sitter backgrounds. Gauge freedom is crucially exploited to solve the constraints in this manner so convenient for exhibiting duality invariance, which suggests a delicate interplay between duality and gauge symmetry.]]>https://www.frontiersin.org/articles/10.3389/fphy.2019.00174
https://www.frontiersin.org/articles/10.3389/fphy.2019.00174
Higgs Lepton Flavor Violating Decays in Two Higgs Doublet Models2019-11-05T00:00:00ZAvelino VicenteThe discovery of a non-zero rate for a lepton flavor violating decay mode of the Higgs boson would definitely be an indication of New Physics. We review the prospects for such signal in Two Higgs Doublet Models, in particular for Higgs boson decays into τμ final states. We will show that this scenario contains all the necessary ingredients to provide large flavor violating rates and still be compatible with the stringent limits from direct searches and low-energy flavor experiments.]]>