arXiv subjects

Daniel F. Litim

Publications and source records attributed to Daniel F. Litim.

At least 37 records · Page 2Linked to original sources

arXiv2022

Functional Renormalisation for $f(R_{μνρσ})$ Quantum Gravity

We derive new functional renormalisation group flows for quantum gravity, in any dimension. The key new achievement is that the equations apply for any theory of gravity whose underlying Lagrangian $\sim f(R_{μνρσ})$ is a function of the Riemann tensor and the inverse metric. The results centrally exploit the benefits of maximally symmetric spaces for the evaluation of operator traces. The framework is highly versatile and offers a wide range of new applications to study quantum gravitational effects in extensions of Einstein gravity, many of which have hitherto been out of reach. The phase diagram and sample flows for Einstein-Hilbert gravity, Gauss-Bonnet, and selected higher-order theories of gravity are given. We also provide an algorithm to find the flow for general polynomial Riemann curvature interactions. The setup vastly enhances the reach of fixed point searches, enabling novel types of search strategies including across the operator space spanned by polynomial curvature invariants, and in extensions of general relativity relevant for cosmology. Further implications, and links with unimodular versions of gravity are indicated.

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arXiv2022

Asymptotic safety guaranteed for strongly coupled gauge theories

We demonstrate that interacting ultraviolet fixed points in four dimensions exist at strong coupling, and away from large-$N$ Veneziano limits. This is established exemplarily for semi-simple supersymmetric gauge theories with chiral matter and superpotential interactions by using the renormalisation group and exact methods from supersymmetry. We determine the entire superconformal window of ultraviolet fixed points as a function of field multiplicities. Results are in accord with the $a$-theorem, bounds on conformal charges, Seiberg duality, and unitary. We also find manifolds of Leigh-Strassler models exhibiting lines of infrared fixed points. At weak coupling, findings are confirmed using perturbation theory up to three loop. Benchmark models with low field multiplicities are provided including examples with Standard~Model-like gauge sectors. Implications for particle physics, model building, and conformal field theory are indicated.

hep-th↗

arXiv2022

Fixed Points in Supersymmetric Extensions of the Standard Model

We search for weakly interacting fixed points in extensions of the minimally supersymmetric standard model (MSSM). Necessary conditions lead to three distinct classes of anomaly-free extensions involving either new quark singlets, new quark doublets, or a fourth generation. While interacting fixed points arise prolifically in asymptotically free theories, their existence is significantly constrained as soon as some of the non-abelian gauge sectors are infrared free. Performing a scan over $\sim 200$k different MSSM extensions using matter field multiplicities and the number of superpotential couplings as free parameters, we find mostly infrared conformal fixed points, and a small subset with ultraviolet ones. All settings predict low-scale supersymmetry-breaking and a violation of $R$-parity. Despite of residual interactions, the running of couplings out of asymptotically safe fixed points is logarithmic as in asymptotic freedom. Some fixed points can be matched to the Standard Model though the matching scale comes out too low. Prospects for higher matching scales and asymptotic safety beyond the MSSM are indicated.

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arXiv2022

B-Anomalies from flavorful U(1)' extensions, safely

$U(1)^\prime$ extensions of the standard model with generation-dependent couplings to quarks and leptons are investigated as an explanation of anomalies in rare $B$-decays, with an emphasis on stability and predictivity up to the Planck scale. To these ends, we introduce three generations of vector-like standard model singlet fermions, an enlarged, flavorful scalar sector, and, possibly, right-handed neutrinos, all suitably charged under the $U(1)^\prime$ gauge interaction. We identify several gauge-anomaly free benchmarks consistent with $B_s$-mixing constraints, with hints for electron-muon universality violation, and the global $b \to s$ fit. We further investigate the complete two-loop running of gauge, Yukawa and quartic couplings up to the Planck scale to constrain low-energy parameters and enhance the predictive power. A characteristic of models is that the $Z^\prime$ with TeV-ish mass predominantly decays to invisibles, i.e. new fermions or neutrinos. $Z^\prime$-production can be studied at a future muon collider. While benchmarks feature predominantly left-handed couplings $C_9^μ$ and $C_{10}^μ$, right-handed ones can be accommodated as well.

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arXiv2021

Conformal Windows beyond Asymptotic Freedom

We study four-dimensional gauge theories coupled to fermions in the fundamental and meson-like scalars. All requisite beta functions are provided for general gauge group and fermion representation. In the regime where asymptotic freedom is absent, we determine all interacting fixed points using perturbation theory up to three loop in the gauge and two loop in the Yukawa and quartic couplings. We find that the conformal window of ultraviolet fixed points is narrowed-down by finite-$N$ corrections beyond the Veneziano limit. We also find a new infrared fixed point whose main features such as scaling exponents, UV-IR connecting trajectories, and phase diagram are provided. Both fixed points collide upon varying the number of fermion flavours $N_{\rm f}$, and conformality is lost through a saddle-node bifurcation. We further revisit the prospect for ultraviolet fixed points in the large $N_{\rm f}$ limit where matter field fluctuations dominate. Unlike at weak coupling, we do not find clear evidence for new scaling solutions even in the presence of scalar and Yukawa couplings.

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arXiv2021

ARGES -- Advanced Renormalisation Group Equation Simplifier

We present the initial release of ARGES, a toolkit for obtaining renormalisation group equations in perturbation theory. As such, ARGES can handle any perturbatively renormalisable four-dimensional quantum field theory. Notable further features include a symbolic rather than numeric computation, input of unconventional scalar and Yukawa sectors, an interactive evaluation and disentanglement as well as capabilities to inject algebraic simplification rules. We provide a conceptual and practical introduction into ARGES, and highlight similarities and differences with complementary packages.

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arXiv2020

Multi-lepton signatures of vector-like leptons with flavor

We investigate collider signatures of standard model extensions featuring vector-like leptons and a flavorful scalar sector. Such a framework arises naturally within asymptotically safe model building, which tames the UV behavior of the standard model towards the Planck scale and beyond. We focus on values of Yukawa couplings and masses which allow to explain the present data on the muon and electron anomalous magnetic moments. Using a CMS search based on $77.4 \, \rm{fb}^{-1}$ at the $\sqrt{s}=13$ TeV LHC we find that flavorful vector-like leptons are excluded for masses below around $300$ GeV if they are singlets under $SU(2)_L$, and around $800$ GeV if they are doublets. Exploiting the flavor-violating-like decays of the scalars, we design novel null test observables based on opposite sign opposite flavor invariant masses. These multi-lepton distributions allow to signal new physics and to extract mass hierarchies in reach of near-future searches at the LHC and the HL-LHC.

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arXiv2020

Anomalous Magnetic Moments from Asymptotic Safety

The measurements of the muon and electron anomalous magnetic moments hint at physics beyond the standard model. We show why and how models inspired by asymptotic safety can explain deviations from standard model predictions naturally. Our setup features an enlarged scalar sector and Yukawa couplings between leptons and new vector-like fermions. Using the complete two-loop running of couplings, we observe a well-behaved high energy limit of models including a stabilization of the Higgs. We find that a manifest breaking of lepton universality beyond standard model Yukawas is not necessary to explain the muon and electron anomalies. We further predict the tau anomalous magnetic moment, and new particles in the TeV energy range whose signatures at colliders are indicated. With small CP phases, the electron EDM can be as large as the present bound.

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arXiv2020

Model Building from Asymptotic Safety with Higgs and Flavor Portals

We perform a comprehensive search for Standard Model extensions inspired by asymptotic safety. Our models feature a singlet matrix scalar field, three generations of vector-like leptons, and direct links to the Higgs and flavor sectors via new Yukawa and portal couplings. A novel feature is that the enlarged scalar sector may spontaneously break lepton flavor universality. We provide a complete two-loop renormalization group analysis of the running gauge, Yukawa, and quartic couplings to find ultraviolet fixed points and the BSM critical surface of parameters, $i.e.$ the set of boundary conditions at the TeV scale for which models remain well-behaved and predictive up to the Planck scale without encountering Landau poles or instabilities. This includes templates for asymptotically safe Standard Model extensions which match the measured values of gauge couplings and the Higgs, top, and bottom masses. We further detail the phenomenology of our models covering production, decay, fermion mixing, anomalous magnetic moments, effects from scalar mixing and chiral enhancement, and constraints on model parameters from data. Signatures at proton-proton and lepton colliders such as lepton flavor violation and displaced vertices, and the prospect for electric dipole moments or charged lepton-flavor-violating type processes, are also indicated.

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arXiv2020

Heat kernel coefficients on the sphere in any dimension

We derive all heat kernel coefficients for Laplacians acting on scalars, vectors, and tensors on fully symmetric spaces, in any dimension. Final expressions are easy to evaluate and implement, and confirmed independently using spectral sums and the Euler-Maclaurin formula. We also obtain the Green's function for Laplacians acting on transverse traceless tensors in any dimension, and new integral representations for heat kernels using known eigenvalue spectra of Laplacians. Applications to quantum gravity and the functional renormalisation group, and other, are indicated.

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arXiv2019

Asymptotic safety with Majorana fermions and new large N equivalences

Using Majorana fermions and elementary mesons we find new massless quantum field theories with weakly interacting ultraviolet fixed points. We also find new classes of large N equivalences amongst SU, SO and Sp gauge theories with different types of matter fields and Yukawa interactions. Results include a triality of asymptotically safe theories and dualities between asymptotically free matter-gauge theories with identical fixed points, phase diagrams, and scaling exponents. Implications for conformal field theory and orbifold reductions are indicated.

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arXiv2019

Aspects of asymptotic safety for quantum gravity

We study fixed points of quantum gravity with renormalisation group methods, and a procedure to remove convergence-limiting poles from the flow. The setup is tested within the $f(R)$ approximation for gravity by solving exact recursive relations up to order $R^{70}$ in the Ricci scalar, combined with resummations and numerical integration. Results include fixed points, scaling exponents, gap in the eigenvalue spectrum, dimensionality of the UV critical surface, fingerprints for weak coupling, and quantum equations of motion. Our findings strengthen the view that ``most of quantum gravity'' is rather weakly coupled. Another novelty are a pair of de Sitter solutions for quantum cosmology, whose occurrence is traced back to the removal of poles. We also address slight disparities of results in the literature, and give bounds on the number of fundamentally free parameters of quantum gravity.

gr-qc↗

arXiv2019

Asymptotically safe extensions of the Standard Model with flavour phenomenology

We investigate asymptotically safe extensions of the Standard Model with new matter fields arising in the TeV energy range. The new sector contains singlet scalars and vector-like fermions in representations which permit Yukawa interactions with the Standard Model leptons. Phenomenological implications are explored including charged lepton flavour violation, Drell-Yan processes and lepton anomalous magnetic moments. For the latter, we find that BSM contributions can be sizeable enough to explain the present experimental discrepancies of the electron and muon anomalous magnetic moments with the Standard Model.

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arXiv2019

Price of Asymptotic Safety

All known examples of four dimensional quantum field theories with asymptotic freedom or asymptotic safety at weak coupling involve non-abelian gauge interactions. We demonstrate that this is not a coincidence: no weakly coupled fixed points, ultraviolet or otherwise, can be reliably generated in theories lacking gauge interactions. Implications for particle physics, critical phenomena, and conformal field theory are indicated.

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arXiv2018

Asymptotic safety of scalar field theories

We study $3d$ $O(N)$ symmetric scalar field theories using Polchinski's renormalisation group. In the infinite $N$ limit the model is solved exactly including at strong coupling. At short distances the theory is described by a line of asymptotically safe ultraviolet fixed points bounded by asymptotic freedom at weak, and the Bardeen-Moshe-Bander phenomenon at strong sextic coupling. The Wilson-Fisher fixed point arises as an isolated low-energy fixed point. Further results include the conformal window for asymptotic safety, convergence-limiting poles in the complex field plane, and the phase diagram with regions of first and second order phase transitions. We substantiate a duality between Polchinski's and Wetterich's versions of the functional renormalisation group, also showing that that eigenperturbations are identical at any fixed point. At a critical sextic coupling, the duality is worked out in detail to explain the spontaneous breaking of scale symmetry responsible for the generation of a light dilaton. Implications for asymptotic safety in other theories are indicated.

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arXiv2018

One force to rule them all: asymptotic safety of gravity with matter

We study the asymptotic safety conjecture for quantum gravity in the presence of matter fields. A general line of reasoning is put forward explaining why gravitons dominate the high-energy behaviour, largely independently of the matter fields as long as these remain sufficiently weakly coupled. Our considerations are put to work for gravity coupled to Yang-Mills theories with the help of the functional renormalisation group. In an expansion about flat backgrounds, explicit results for beta functions, fixed points, universal exponents, and scaling solutions are given in systematic approximations exploiting running propagators, vertices, and background couplings. Invariably, we find that the gauge coupling becomes asymptotically free while the gravitational sector becomes asymptotically safe. The dependence on matter field multiplicities is weak. We also explain how the scheme dependence, which is more pronounced, can be handled without changing the physics. Our findings offer a new interpretation of many earlier results, which is explained in detail. The results generalise to theories with minimally coupled scalar and fermionic matter. Some implications for the ultraviolet closure of the Standard Model or its extensions are given.

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arXiv2018

On de Sitter solutions in asymptotically safe $f(R)$ theories

The availability of scaling solutions in renormalisation group improved versions of cosmology are investigated in the high-energy limit. We adopt $f(R)$-type models of quantum gravity which display an interacting ultraviolet fixed point at shortest distances. Expanding the gravitational fixed point action to very high order in the curvature scalar, we detect a convergence-limiting singularity in the complex field plane. Resummation techniques including Padé approximants as well as infinite order approximations of the effective action are used to maximise the domain of validity. We find that the theory displays near de Sitter solutions as well as an anti-de Sitter solution in the UV whereas real de Sitter solutions, for small curvature, appear to be absent. The significance of our results for inflation, and implications for more general models of quantum gravity are discussed.

gr-qc↗

arXiv2018

More asymptotic safety guaranteed

We study interacting fixed points and phase diagrams of simple and semi-simple quantum field theories in four dimensions involving non-abelian gauge fields, fermions and scalars in the Veneziano limit. Particular emphasis is put on new phenomena which arise due to the semisimple nature of the theory. Using matter field multiplicities as free parameters, we find a large variety of interacting conformal fixed points with stable vacua and crossovers inbetween. Highlights include semi-simple gauge theories with exact asymptotic safety, theories with one or several interacting fixed points in the IR, theories where one of the gauge sectors is both UV free and IR free, and theories with weakly interacting fixed points in the UV and the IR limits. The phase diagrams for various simple and semi-simple settings are also given. Further aspects such as perturbativity beyond the Veneziano limit, conformal windows, and implications for model building are discussed.

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