Highlights
• Positivity in electron-positron scattering: testing the axiomatic quantum field theory principles and probing the existence of UV states
2021, 45(2): 023108. doi: 10.1088/1674-1137/abcd8c
We consider the positivity bounds on dimension-8 four-electron operators and study two related phenomenological aspects at future lepton colliders. First, if positivity is violated, probing such violations will revolutionize our understanding of the fundamental pillars of quantum field theory and the S-matrix theory. We observe that positivity violation at scales of 1-10 TeV can potentially be probed at future lepton colliders even if one assumes that dimension-6 operators are also present. Second, the positive nature of the dimension-8 parameter space often allows us to either directly infer the existence of UV-scale particles together with their quantum numbers or exclude them up to certain scales in a model-independent way. In particular, dimension-8 positivity plays an important role in the test of the Standard Model. If no deviations from the Standard Model are observed, it allows for simultaneous exclusion limits on all kinds of potential UV-complete models. Unlike the dimension-6 case, these limits apply regardless of the UV model setup and cannot be removed by possible cancellations among various UV contributions. This thus consists of a novel and universal test to confirm the Standard Model. We demonstrate with realistic examples how all the previously mentioned possibilities, including the test of positivity violation, can be achieved. Hence, we provide an important motivation for studying dimension-8 operators more comprehensively.
• Diffuseness effect and radial basis function network for optimizing α decay calculations
2021, 45(2): 024105. doi: 10.1088/1674-1137/abcc5c
A radial basis function network (RBFN) approach is adopted for the first time to optimize the calculation of $\alpha$ decay half-life in the generalized liquid drop model (GLDM), while concurrently incorporating the surface diffuseness effect. The calculations presented herein agree closely with the experimental half-lives for 68 superheavy nuclei (SHN), achieving a remarkable reduction of 40% in the root-mean-square (rms) deviations of half-lives. Furthermore, using the RBFN method, the half-lives for four SHN isotopes, 252-288Rf, 272-310Fl, 286-316119, and 292-318120, are predicted using the improved GLDM with the diffuseness correction and the decay energies from WS4 and FRDM as inputs. Therefore, we conclude that the diffuseness effect should be embodied in the proximity energy. Moreover, increased application of neural network methods in nuclear reaction studies is encouraged.
• Charmed and ϕ meson decay constants from 2+1-flavor lattice QCD
2021, 45(2): 023109. doi: 10.1088/1674-1137/abcd8f
On a lattice with 2+1-flavor dynamical domain-wall fermions at the physical pion mass, we calculate the decay constants of $D_{s}^{(*)}$, $D^{(*)}$, and $\phi$. The lattice size is $48^3\times96$, which corresponds to a spatial extension of $\sim5.5$ fm, with a lattice spacing of $a\approx 0.114$ fm. For the valence light, strange, and charm quarks, we use overlap fermions at several mass points close to their physical values. Our results at the physical point are $f_D = 213(5)$ MeV, $f_{D_s} = 249(7)$ MeV, $f_{D^*} = 234(6)$ MeV, $f_{D_s^*} = 274(7)$ MeV, and $f_\phi = 241(9)$ MeV. The couplings of $D^*$ and $D_s^*$ to the tensor current ($f_V^T$) can be derived from ratios $f_{D^*}^T/f_{D^*} = 0.91(4)$ and $f_{D_s^*}^T/f_{D_s^*} = 0.92(4)$, respectively, which are the first lattice quantum chromodynamics (QCD) results. We also obtain ratios $f_{D^*}/f_D = 1.10(3)$ and $f_{D_s^*}/f_{D_s} = 1.10(4)$, which reflect the size of heavy quark symmetry breaking in charmed mesons. Ratios $f_{D_s}/f_{D} = 1.16(3)$ and $f_{D_s^*}/f_{D^*} = 1.17(3)$ can be taken as a measure of SU(3) flavor symmetry breaking.
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• Multiscalar B-L extension based on S4 flavor symmetry for neutrino masses and mixing
Published: 2021-02-26, doi: 10.1088/1674-1137/abe1c7
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A multiscalar and nonrenormalizable $B-L$ extension of the standard model (SM) with $S_4$ symmetry which successfully explains the recently observed neutrino oscillation data is proposed. The tiny neutrino masses and their hierarchies are generated via the type-I seesaw mechanism. The model reproduces the recent experiments of neutrino mixing angles and Dirac CP violating phase in which the atmospheric angle $(\theta_{23})$ and the reactor angle $(\theta_{13})$ get the best-fit values while the solar angle $(\theta_{12})$ and Dirac CP violating phase ($\delta$) are in $3\, \sigma$ range of the best-fit value for the normal hierarchy (NH). For the inverted hierarchy (IH), $\theta_{13}$ gets the best-fit value and $\theta_{23}$ together with $\delta$ are in the $1\, \sigma$ range, while $\theta_{12}$ is in $3\, \sigma$ range of the best-fit value. The effective neutrino masses are predicted to be $\langle m_{ee}\rangle=6.81 \,\, {\rm{meV}}$ for the NH and $\langle m_{ee}\rangle=48.48\,\, {\rm{meV}}$ for the IH, in good agreement with the most recent experimental data.
• Systematic study of two-proton radioactivity within a Gamow-like model
Published: 2021-02-26, doi: 10.1088/1674-1137/abe10f
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In this study, based on the Gamow-like model, we systematically analyze two-proton ($2p$) radioactivity half-lives of nuclei near or beyond the proton drip line. It is found that the calculated results can reproduce experimental data well. Furthermore, using this model, we predict the half-lives of possible $2p$ radioactivity candidates whose $2p$ radioactivity is energetically allowed or observed but not yet quantified in the latest table of evaluated nuclear properties, i.e., NUBASE2016. The predicted results are in good agreement with those from other theoretical models and empirical formulas, namely the effective liquid drop model (ELDM), generalized liquid drop model (GLDM), Sreeja formula, and Liu formula.
• Origin of hardening in spectra of cosmic ray nuclei at a few hundred GeV using AMS-02 data
Published: 2021-02-26, doi: 10.1088/1674-1137/abe03d
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Many experiments have confirmed spectral hardening at a few hundred GeV in the spectra of cosmic ray (CR) nuclei. Three different origins have been proposed: primary source acceleration, propagation, and the superposition of different kinds of sources. In this work, a broken power law has been employed to fit each of the spectra of cosmic ray nuclei from AMS-02 directly, for rigidities greater than 45 GeV. The fitting results of the break rigidity and the spectral index differences less than and greater than the break rigidity show complicated relationships among different nuclear species, which cannot be reproduced naturally by a simple primary source scenario or a propagation scenario. However, with a natural and simple assumption, the superposition of different kinds of sources could have the potential to explain the fitting results successfully. Spectra of CR nuclei from a single future experiment, such as DAMPE, will provide us the opportunity to do cross checks and reveal the properties of the different kinds of sources.
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2021 Vol. 45, No. 2
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