One-loop Research Articles

Gravitational p → ∆+ transition form factors in chiral perturbation theory

The gravitational form factors of the transition from the proton to the ∆+ resonance are calculated to leading one-loop order using a manifestly Lorentz-invariant formulation of chiral perturbation theory. We take into account the leading electromagnetic and strong isospin-violating effects. The loop contributions to the transition form factors are found to be free of power-counting violating pieces, which is consistent with the absence of tree-level diagrams at the considered order. In this sense, our results can be regarded as predictions of chiral perturbation theory.

Spinor fields, CPTM symmetry and smallness of cosmological constant in framework of extended manifold

A model of an extended manifold for the Dirac spinor field is considered. Two Lagrangians related by charge-parity-time-mass symmetry are constructed for a pair of the Dirac spinor fields with each spinor field defined in a separate manifold. An interaction between the matter fields in the manifolds is introduced through gravity. A fermionic effective action of the general system is constructed and a tadpole one-loop spinor diagram and part of the one-loop vacuum diagrams with two external gravitational off-shell fields which contribute to the effective action are calculated. It is demonstrated that among different versions of the second spinor Lagrangian there is a special one for which a cancellation of the mentioned diagrams in the total effective action takes place. As a result, the diagrams do not contribute to the cosmological constant, as well there is a zero contribution of the zero point energies of the spinor fields to the action. The non-zero leading order value of the cosmological constant for each manifold in the framework is proportional to the trace of an momentum–energy tensor of each separated manifold or difference of the tensors of the related manifolds. The result is depending on the chosen model of interaction of gravitational fields with fermions, the different possibilities are discussed. An appearance of the dark matter in the model is shortly discussed as well as further applications of the approach and it is interconnections with other cosmology models.

Phase structure of the on-shell parametrized 2+1 flavor Polyakov quark-meson model

Augmenting the improved chiral effective potential of the on-shell renormalized 2+1 flavor quark-meson (RQM) model with the Polyakov-loop potential that accounts for the deconfinement transition, we get the quantum chromodynamics (QCD)-like framework of the renormalized Polyakov quark-meson (RPQM) model. When the divergent quark one-loop vacuum term is included in the effective potential of the quark-meson (QM) model, its tree-level parameters, or the parameters fixed by the use of meson curvature masses, become inconsistent as the curvature masses involve the self-energy evaluations at zero momentum. Using the modified minimal subtraction method, the consistent chiral effective potential for the RQM model has been calculated after relating the counterterms in the on-shell scheme to those in the MS¯ scheme and finding the relations between the renormalized parameters of both the schemes where the physical (pole) masses of the π, K, η, and η′ pseudoscalar mesons and the scalar σ meson, the pion and kaon decay constants, have been put into the relation of the running couplings and mass parameter. Using the RPQM model and the PQM model with different forms for the Polyakov-loop potentials in the presence or the absence of the quark backreaction, we have computed and compared the effect of the consistent quark one-loop correction and the quark backreaction on the scaled chiral order parameter, the QCD phase diagrams, and the different thermodynamic quantities. The results have been compared with the 2+1 flavor lattice QCD data from the Wuppertal-Budapest Collaboration [ 73, ] and the HotQCD Collaboration []. Published by the American Physical Society 2024

Stochastic parameters for scalar fields in de Sitter spacetime

The stochastic effective theory approach, often called stochastic inflation, is widely used in cosmology to describe scalar field dynamics during inflation. The existing formulations are, however, more qualitative than quantitative because the connection to the underlying quantum field theory (QFT) has not been properly established. A concrete sign of this is that the QFT parameters depend on the renormalization scale, and therefore the relation between the QFT and stochastic theory must have explicit scale dependence that cancels it. In this paper we achieve that by determining the parameters of the second-order stochastic effective theory of light scalar fields in de Sitter to linear order in the self-coupling constant λ. This is done by computing equal-time 2-point correlators to one-loop order both in QFT using dimensional regularization and the MS¯ renormalization scheme and the equal-time 4-point correlator to leading order in both theories, and demanding that the results obtained in the two theories agree. With these parameters, the effective theory is valid when m≲H and λ2≪m4/H4, and therefore it is applicable in cases where neither perturbation theory nor any previously proposed stochastic effective theories are. Published by the American Physical Society 2024

Resummation of Next-to-Leading Nonglobal Logarithms at the LHC.

In cross sections with angular cuts, an intricate pattern of enhanced higher-order corrections known as nonglobal logarithms arises. The leading logarithmic terms were computed numerically two decades ago, but the resummation of subleading nonglobal logarithms remained a challenge that we solve in this Letter using renormalization group methods in effective field theory. To achieve next-to-leading logarithmic accuracy, we implement the two-loop anomalous dimension governing the resummation of nonglobal logarithms into a large-N_{c} parton shower framework, together with one-loop matching corrections. As a first application, we study the interjet energy flow in e^{+}e^{-} annihilation into two jets. We then present, for the first time, resummed predictions at next-to-leading logarithmic accuracy for a gap-between-jets observable at hadron colliders.

Global analysis and LHC study of a vectorlike extension of the standard model with extra scalars

We perform a global analysis of a vectorlike extension of the Standard Model, which also features additional doublet and singlet scalars. The usual Yukawa interactions are forbidden in this setup by an extra U(1) global symmetry and the masses of the second and third family quarks and leptons are generated via the mixing with the vectorlike sector. We identify three best-fit benchmark scenarios which satisfy the constraints imposed by the stability of the scalar potential, the perturbativity of the coupling constants, the measurement of the muon anomalous magnetic moment and the nonobservation of the flavor violating tau decays. We show that dominant contributions to the muon (g−2) originate in this model from the charged Higgs/neutral lepton one-loop diagrams, thus correcting an inaccurate statement than can be found in the literature. We also perform a detailed LHC analysis of the benchmark scenarios. We investigate the experimental constraints stemming from direct searches for vectorlike quarks, vectorlike leptons, and exotic scalars. While we show that the model is not currently tested by any collider experiment, we point out that decays of a heavy Higgs boson into two tau leptons may offer a smoking gun signature for the model verification in upcoming runs at the LHC. Published by the American Physical Society 2024

Primordial Black Holes and loops in single-field inflation

Using the δN formalism we calculate the one-loop correction to the large-scale power spectrum of the curvature perturbation in the standard scenario where primordial black holes are formed in the early universe thanks to a phase of ultra-slow-roll in single-field inflation. We explicitly show that one-loop corrections are negligible when the transition from the ultra-slow-roll to the slow-roll phase is smooth. We conclude that the PBH formation scenario through a ultra-slow-roll phase is viable.

Light baryon spatial correlators at short distances

To study the light baryon light cone distribution amplitudes (LCDAs), the spatial correlator of the light baryon has been calculated up to one-loop order in coordinate space. They reveal certain identities that do not appear in the study of mesons DAs and parton distribution functions. Subsequently, it was renormalized using the ratio renormalization scheme involving division by a 0-momentum matrix element. Then through the matching in the coordinate space, the light baryon light cone correlator can be extracted out from the spatial correlator. These results provide insights into both the ultraviolet and infrared structures of the light baryon spatial correlator, which is valuable for further exploration in this field. Furthermore, the employed ratio scheme is an efficient and lattice-friendly renormalization approach suitable for short-distance applications. These can be used for studying the light baryon LCDAs using lattice techniques. Published by the American Physical Society 2024

Probing dark sector fermions in Higgs precision studies and direct searches

In this paper, we investigate the discovery prospect of simplified fermionic dark sectors models through Higgs precision measurements at e+e− colliders and direct searches at hadron colliders. These models extend the Standard Model with two Majorana or Dirac fermions that are singlets, doublets or triplets under the weak SU(2) group. For all models, we consider two scenarios where the lightest new fermion is either stable, or where it decays into other visible final states. For the Higgs precision observables we primarily focus on σ(e+e− → ZH), which can deviate from the Standard Model through one-loop corrections involving the new fermions. Deviations of 0.5% or more, which could be observable at future e+e− colliders, are found for TeV-scale dark sector masses. By combining the constraints from the oblique parameters, Br(H → γγ), and direct production of the new fermions at the LHC, a comprehensive understanding of the discovery potential of these models can be achieved. In both scenarios, there exist some parameter regions where the Higgs precision measurements can provide complementary information to direct LHC searches.

Flavor fragmentation function factorization

A definition of partonic jet flavor that is both theoretically well-defined and experimentally robust would have profound implications for measurements and predictions especially for heavy flavor applications. Recently, a definition of jet flavor was introduced as the net flavor flowing along the direction of the Winner-Take-All axis of a jet which is soft safe to all orders, but not collinear safe. Here, we exploit the lack of collinear safety and propose a factorization theorem of perturbative flavor fragmentation functions that resum collinear divergences and describe the evolution of flavor from the short distance of jet production to the long distance at which hadronization occurs. Collinear flavor evolution is governed by a small modification of the DGLAP equations. We present a detailed all-orders analysis and identify exact relations that must hold amongst the various anomalous dimensions by probability conservation and the existence of fixed points of the renormalization group flow. We explicitly validate the factorization theorem at one-loop order, and demonstrate its consistency at two loops in particular flavor channels. Starting at two-loops, constraints on phase space imposed by flavor measurements potentially allow for non-trivial soft contributions, but we demonstrate that they are scaleless and so explicitly vanish, ensuring that soft particles are summed inclusively and all divergences are exclusively collinear in nature. This factorization theorem opens the door to precision calculations with identified flavor in the infrared.

One-loop effective action of the IKKT model for cosmological backgrounds

We study cosmological solutions of the IKKT model with k = –1 FLWR geometry, taking into account one-loop corrections. A previously discussed covariant quantum spacetime is found to be stabilized through one-loop effects at early times, without adding a mass term to the model. At late times, this background is modified and goes through a period of acceleration, before settling down to a coasting FLRW geometry with scale parameter a(t) ~ t. This is qualitatively close to observation without any fine-tuning, irrespective of the detailed matter content of the universe.

Complex eigenvalue instantons and the Fredholm determinant expansion in the Gross-Witten-Wadia model

We study the leading nonperturbative corrections to the strong-coupling (ungapped) phase of the Gross-Witten-Wadia (GWW) integral over unitary matrices, to one-loop order. We compute these corrections directly in terms of eigenvalue tunneling in a holomorphic presentation of the integral over eigenvalues. The leading nonperturbative contribution to the partition function comes from a pair of complex eigenvalue instantons. We show that these are in fact “ghost instantons”, which are extrema of the one-eigenvalue effective potential on the “unphysical sheet” of the spectral curve and have been discussed in detail recently by Mariño, Schiappa, and Schwick. Further, we discuss the relationship of these instantons to the Fredholm determinant expansion of the unitary matrix integral, which has recently become an object of interest in the computations of BPS indices of supersymmetric gauge theories and black holes. We find that, after taking the ’t Hooft limit, the first correction given by the Fredholm determinant expansion of the GWW integral agrees precisely with the leading nonperturbative correction, to one-loop order.

Cancellation of quantum corrections on the soft curvature perturbations

We study the cancellation of quantum corrections on the superhorizon curvature perturbations from subhorizon physics beyond the single-clock inflation from the viewpoint of the cosmological soft theorem. As an example, we focus on the transient ultra-slow-roll inflation scenario and compute the one-loop quantum corrections to the power spectrum of curvature perturbations taking into account nontrivial surface terms in the action. We find that Maldacena’s consistency relation is satisfied and guarantees the cancellation of contributions from the short-scale modes. As a corollary, primordial black hole production in single-field inflation scenarios is not excluded by perturbativity breakdown even for the sharp transition case in contrast to some recent claims in the literature. We also comment on the relation between the tadpole diagram in the in-in formalism and the shift of the elapsed time in the stochastic-δN formalism. We find our argument is not directly generalisable to the tensor perturbations.

Full one-loop radiative corrections to e+e−→H+H− in the inert doublet model

We compute the full one-loop radiative corrections for charged scalar pair production e+e−→H+H− in the inert doublet model. The on-shell renormalization scheme has been used. We take into account both the weak contributions as well as the soft and hard QED corrections. We compute both the real emission and the one-loop virtual corrections using the Feynman diagrammatic method. The resummed cross section is introduced to cure the Coulomb singularity which occurs in the QED corrections. We have analyzed the parameter space of the inert doublet model in three scenarios after taking into account theoretical constraints, the collider experimental bounds, and dark matter search bounds as well. It is found that the weak interaction dominates the radiative corrections, and its size is determined by the triple Higgs coupling λh0H+H−, which is further connected to the mass of the charged scalar. In the scenario where all the constraints are taken into account, we find that for s=250 GeV and s=500 GeV, the weak corrections are around −6%∼−5% and −10%∼−3%, respectively. While for s=1000 GeV, the weak corrections can reach −15%∼+25%. The new feature is that the weak corrections can be positive near the threshold when the charged scalar is heavier than 470 GeV. Six benchmark points for future collider searches have been proposed. Published by the American Physical Society 2024

Quantum corrections and the minimal Yukawa sector of SU(5)

It is well known that the SU(5) grand unified theory, with the standard model quarks and leptons unified in 5¯ and 10 and the electroweak Higgs doublet residing in five-dimensional representations, leads to the relation Yd=YeT between the Yukawa couplings of the down-type quarks and the charged leptons. We show that this degeneracy can be lifted in a phenomenologically viable way when quantum corrections to the tree-level matching conditions are taken into account in the presence of one or more copies of gauge singlet fermions. The one-loop threshold corrections arising from heavy leptoquark scalar and vector bosons, already present in the minimal model, and heavy singlet fermions can lead to realistic Yukawa couplings, provided their masses differ by at least 2 orders of magnitude. The latter can also lead to a realistic light neutrino mass spectrum through the type I seesaw mechanism if the color partner of the Higgs stays close to the Planck scale. Most importantly, our findings demonstrate the viability of the simplest Yukawa sector when quantum corrections are considered and sizable threshold effects are present. Published by the American Physical Society 2024

Electroweak loop contributions to the direct detection of wino dark matter

Electroweak loop corrections to the matrix elements for the spin-independent scattering of cold dark matter particles on nuclei are generally small, typically below the uncertainty in the local density of cold dark matter. However, as shown in this paper, there are instances in which the electroweak loop corrections are relatively large, and change significantly the spin-independent dark matter scattering rate. An important example occurs when the dark matter particle is a wino, e.g., in anomaly-mediated supersymmetry breaking (AMSB) and pure gravity mediation (PGM) models. We find that the one-loop electroweak corrections to the spin-independent wino LSP scattering cross section generally interfere constructively with the tree-level contribution for AMSB models with negative Higgsino mixing, mu < 0, and in PGM-like models for both signs of mu , lifting the cross section out of the neutrino fog and into a range that is potentially detectable in the next generation of direct searches for cold dark matter scattering.

Factorization of non-global LHC observables and resummation of super-leading logarithms

We present a systematic formalism based on a factorization theorem in soft-collinear effective theory to describe non-global observables at hadron colliders, such as gap-between-jets cross sections. The cross sections are factorized into convolutions of hard functions, capturing the dependence on the partonic center-of-mass energy ŝ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ \\sqrt{\\hat{s}} $$\\end{document}, and low-energy matrix elements, which are sensitive to the low scale Q0 ≪ ŝ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ \\sqrt{\\hat{s}} $$\\end{document} characteristic of the veto imposed on energetic emissions into the gap between the jets. The scale evolution of both objects is governed by a renormalization-group equation, which we derive at one-loop order. By solving the evolution equation for the hard functions for arbitrary 2 → M jet processes in the leading logarithmic approximation, we accomplish for the first time the all-order resummation of the so-called “super-leading logarithms” discovered in 2006, thereby solving an old problem of quantum field theory. We study the numerical size of the corresponding effects for different partonic scattering processes and explain why they are sizable for pp → 2 jets processes, but suppressed in H/Z and H/Z + jet production. The super-leading logarithms are given by an alternating series, whose individual terms can be much larger than the resummed result, even in very high orders of the loop expansion. Resummation is therefore essential to control these effects. We find that the asymptotic fall-off of the resummed series is much weaker than for standard Sudakov form factors.

On bosonic Thirring model in Minkowski signature

We present the way to continue the bosonic Thirring model or βγ-system with quartic interaction to Minkowski signature, based on the symmetries of this model. It is shown that the considered Minkowski version of the model is one-loop renormalizable. Based on this, we find the amplitudes of scattering of the excitations corresponding to the γ and γ¯ fields up to the one-loop order. In particular, it was computed that the 2→2 amplitudes of these excitations possess property of reflectionless scattering and thus the corresponding S-matrix of such excitations satisfies the Yang-Baxter equation. The obtained S-matrix elements for γ and γ¯ are shown to coincide with the corresponding S-matrix elements of the solitons in the complex sine-Gordon model proposed by Dorey and Hollowood.

Genus-one open string amplitudes on AdS5×S3 from CFT

We bootstrap one-loop string corrections to the four-point function of half-BPS operators in a 4d N\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ \\mathcal{N} $$\\end{document} = 2 SCFT with flavour group SO(8), dual to gluon scattering at genus one on AdS5×S3. We identify an 8-dimensional organising principle which governs the spectrum of double-trace anomalous dimensions, valid to all orders in the string length. This has precise implications for the structure of one-loop Mellin amplitudes, which we explicitly compute for the first three orders beyond the field-theory limit. We also consider the corresponding position space representation, which is entirely determined by the square of a certain differential operator acting on a simpler “pre-correlator”. Finally, we show that the flat-space limit of the Mellin amplitudes exactly matches the logarithmic terms of the genus-one amplitude in 8-dimensional flat space, which we compute via a partial-wave analysis.

The smearing function from the functional renormalization group

The smearing function has been proposed as a tool for writing down the one-loop correction to the Wilsonian effective action in the form of the Schwinger proper-time representation of the functional determinant. Due to the fact that the existence of the smearing function is only a postulate, it is interesting to explore if it is possible to obtain such a function from the functional renormalization group, which is a closely related formulation. In this work, we show that it is possible to obtain the smearing function from the functional renormalization group only in a special case, but not in general cases.