abstract

A deviation of \(4.2\sigma \), between the experimental measurement and the theoretical determination of the anomalous magnetic moment of the muon \(a_\mu = (g-2)_\mu /2\), has been observed. With increased experimental precision expected in the near future, the theoretical prediction within the Standard Model also needs to be improved. Its total uncertainty is limited by the two hadronic contributions, namely the hadronic vacuum polarization and the hadronic light-by-light contribution. The BESIII experiment at the BEPCII accelerator offers the largest datasets in the \(\tau \)-charm energy region and is, therefore, perfectly suited to provide cross section and form factor measurements in \(e^+ e^-\) collisions for data-driven improvements of the Standard Model prediction.

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Probabilities of finding an antiparticle in an atom or ion containing a particle of spin 1/2 or spin 0 are determined. The spin 1/2 case was previously solved by Hans Bethe and his work is summarized. The spin 0 case is treated numerically for an arbitrary atomic number and analytically for small atomic numbers. The main tool for the spin 0 case is the Feshbach–Villars representation of the Klein–Gordon equation.

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An overview of a quantum algorithm application for the identification of causal singular configurations of multiloop Feynman diagrams is presented. The quantum algorithm is implemented in two different quantum simulators, the output obtained is directly translated to causal thresholds needed for the causal representation in the loop–tree duality.

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We introduce a new method for calculating the mixing matrix for non-singlet quark operators including total derivatives, based solely on their renormalization structure in the chiral limit. As input, the method requires the well-known forward anomalous dimensions, which determine the evolution of parton distribution functions, and a calculation of the matrix elements of operators without total derivatives. Assuming a large number of quark flavors \(n_{\mathrm {f}}\), we are able to calculate the mixing matrix to fifth order in the strong coupling \(\alpha _{\mathrm {s}}\) in the \(\mathrm {MS}\)-scheme.

abstract

The precise measurement of the top-quark mass, which is a fundamental SM parameter, constitutes one of the main goals of the LHC top-physics program. One approach to measure this quantity uses the \(\rho _{\mathrm {s}}\) distribution, an observable depending on the invariant mass of the \(t\bar {t}j\) system. To fully exploit the experimental accuracy achievable in measuring top-quark production cross sections at the LHC, the theory uncertainties associated to these measurements need to be well under control. To this end, we present a study of the effect of varying the theoretical input parameters in the calculation of differential cross sections of the \(t\bar {t}j\) process. Thereby, we studied the influence of the jet reconstruction procedure, as well as the effect of various renormalization and factorization scale definitions and different PDF sets. The variation of the \(R\) parameter in the jet reconstruction algorithm was found to have negligible influence on the scale variation uncertainty. A strong reduction of scale uncertainties and a better behaviour of the NLO/LO ratios using selected dynamical scales instead of a static one in the high-energy tails of differential distributions was observed. This is particularly interesting in the context of the top-quark mass measurements through the \(\rho _{\mathrm {s}}\) distribution, in which the perturbative stability can be improved by applying the proposed dynamical scale definition.

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We present the state-of-the-art predictions for off-shell \(t\bar {t}b\bar {b}\) production with di-lepton decays at the LHC with \(\sqrt {s}=13\) TeV. Results are accurate at NLO in QCD and include all resonant and non-resonant diagrams, interferences, and finite-width effects for top quarks and \(W\) bosons. We discuss the impact of QCD corrections and assess theoretical uncertainties from scale and PDF dependence at the integrated and differential level. Additionally, we investigate the size of contributions induced by initial-state \(b\) quarks to the NLO cross section.

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We present a method of extracting the strong coupling at N\(^3\)LO precision in QCD using a combination of \(\mathcal {O}(\alpha _{\mathrm {S}}^3)\) perturbative calculations with estimations of the \(\mathcal {O}(\alpha _{\mathrm {S}}^4)\) corrections from data. We apply the procedure to a set of event shape averages measured at the LEP, PETRA, PEP, and TRISTAN colliders. We account for non-perturbative effects using both modern Monte Carlo event generators as well as analytic models. Our results show that the precision of \(\alpha _{\mathrm {S}}\) extraction cannot be significantly improved solely with higher-order perturbative QCD predictions, but requires also a significant refinement of the understanding and modeling of the hadronization process.

abstract

Precise calculations for the Large Hadron Collider cannot be imagined without precise parton density functions. For accurate measurements and comparisons, both the valence- and sea-quark parton density functions are needed with high confidence. The hadroproduction of a \(W\) vector boson with a massive charm quark plays an important role in the determination of sea-quark parton density functions because it provides a direct way to measure strangeness in the proton. In this paper, we obtain a next-to-leading order prediction for \(W\)-boson production in association with a \(c\) quark in QCD and match our results to parton shower and hadronization models using PYTHIA 8 in order to get predictions at the hadron level. Our particle-level predictions are compared against available experimental results done by both the ATLAS and CMS collaborations.

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We consider an effective Minimal Left–Right Symmetric Model. Instead of engaging all the dimension-6 operators, we choose only the relevant, in the limit that right-handed symmetry breaking scale is larger than the electroweak one, operators belonging to \(\phi ^2 X^2\) class. We adjudge the impact of those operators on the spectrum and vertices of the model, and then capture their contributions to the low-energy observables, e.g. , \(\rho ,\, \mathcal {G}_\mathrm {F},\, {\mit \Theta }_W\), and oblique parameters \(S,\,T,\,U\).

abstract

One of the primary goals of the proposed future collider experiments is to search for dark matter (DM) particles using different experimental approaches. High-energy \(e^+e^-\) colliders offer a unique possibility for the most general search based on the mono-photon signature. As any \(e^+e^-\) scattering process can be accompanied by a hard photon emission from the initial state radiation, an analysis of the energy spectrum and angular distributions of those photons can be used to search for hard processes with invisible final-state production. Processes of DM production via mediator exchange are considered for the International Linear Collider (ILC) and Compact Linear Collider (CLIC) experiments. The detector effects are taken into account within the Delphes fast simulation framework. Limits on the light DM production in a simplified model are set as a function of the mediator mass and width based on the expected two-dimensional distributions of the reconstructed mono-photon events. The experimental sensitivity is extracted in terms of the DM production cross section. Limits on the mediator couplings are then presented for a wide range of mediator masses and widths.

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Asymmetries in processes of \(e^+e^-\) annihilation into a pair of fermions are considered. Left–right and forward–backward asymmetries are calculated for the polarized initial and/or final particles. Effects due to 1-loop electroweak radiative corrections are scrutinized. Numerical results are presented for the \(Z\)-boson peak. Higher energies relevant for future colliders are also covered. Electroweak scheme dependence and interplay of QED and weak-interaction corrections are discussed.

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We discuss two different ways to understand the origin of matter in models with observable neutron–anti-neutron oscillation: (i) one, where colored scalars couple to a neutral scalar field whose vacuum expectation value (VEV) gives rise to \(n\)–\(\bar {n}\) oscillation and whose decay is responsible for baryogenesis and (ii) another based on the Affleck–Dine mechanism, where an initial early universe asymmetry between the real and imaginary parts of a \(\Delta B=2\) scalar and its subsequent evolution generates the baryon asymmetry. We discuss some phenomenological implications of both these models. For example, when the first model is embedded as part of its natural gauge setting based on SU\((2)_\mathrm {L}\times {\mathrm {SU}}(2)_\mathrm {R}\times {\mathrm {SU}}(4)_\mathrm {c}\) group, it leads to an upper limit on the \(n\)–\(\bar {n}\) oscillation time that is accessible to a planned experiment at the ESS facility in Lund, Sweden. In the second case, a similar prediction results where a large part of the model parameter space can also be probed in the same experiment.

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To explain the observed neutrino mixing, we exploit an \(A_4\) discrete flavor symmetric model, where neutrino masses are generated via type-I seesaw. Thanks to the flavor structure of the model, only the normal hierarchy of neutrino mass is allowed, atmospheric mixing falls in the lower octant, and leptonic CP phases including the Majorana phases also get constrained. Owing to the symmetry, the right-handed neutrinos are degenerate to start with. Once we include the renormalization group running this degeneracy can be lifted and adequate lepton asymmetry can be generated.

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We explore the possibilities of dark matter production in a U(1) extension of the Standard Model, also called the super-weak model. The freeze-in and freeze-out mechanisms are described in detail, assuming the lightest sterile neutrino in the model as the dark matter candidate. In both scenarios, we present the favoured parameter space on the plane of super-weak coupling versus the new gauge boson mass. We discuss the experimental constraints limiting each case and outline possibilities of detection.

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In the case of fermion mixing, we propose to use only one of the usual on-shell renormalization conditions at 1-loop and to use off-diagonal mass counterterms. These new counterterms allow for a natural separation of gauge-dependent parts as well as UV divergent parts, for example, the off-diagonal mass counterterms can be chosen to be gauge-independent and to contain all the UV divergences that would otherwise be included in field renormalization. Containment of UV divergences in off-diagonal mass counterterms prevents the migration of said divergences from the mass term to other terms in the Lagrangian. This naturally allows to not associate counterterms with mixing matrices and take them to be always renormalized. In addition, we argue that it is more consistent to not have counterterms for mixing matrices and instead have off-diagonal mass counterterms. Finally, the renormalization scheme is truly universal as it is based on mass structures and also includes absorptive parts where possible.

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The super-weak force combines three simple extensions of the Standard Model, one in the gauge sector, one in the fermion sector, and one in the scalar sector. All these extensions are well motivated by their rich phenomenology. Combined to a single framework, they can explain several open questions in particle physics and cosmology: the origin of dark matter, cosmic inflation, matter-antimatter asymmetry, neutrino masses, and vacuum stability. We discuss the effects of the model on neutrino masses and phenomenology in the case where the heaviest sterile neutrinos have a GeV scale mass.

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We present the possibility of the low seesaw scenario in the Grimus–Neufeld model. We argue that it can be natural and phenomenologically interesting, while consistent with neutrino data. We present the approximated expressions for neutrino masses and estimate the magnitude of the Yukawa couplings. We show that they can be sizable and can lead to possible restrictions on the scalar sector.