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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• Exotic ΩΩ dibaryon states in a molecular picture
Published: 2021-01-26
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We investigate the exotic $\Omega\Omega$ dibaryon states with $J^P=0^+$ and $2^+$ in a molecular picture. We construct a tensor $\Omega$$\Omega$ molecular interpolating current and calculate the two-point correlation function within the method of QCD sum rules. Our calculations indicate that the masses of the scalar and tensor dibaryon states are $m_{\Omega\Omega, \, 0^+} =$ $(3.33\pm 0.51) \,{\rm{GeV}}$ and $m_{\Omega\Omega,\, 2^+}=(3.15\pm0.33)\, {\rm{GeV}}$, respectively, which are below the $2m_\Omega$ threshold. Within error, these results do not negate the existence of loosely bound molecular $\Omega\Omega$ dibaryon states. These exotic strangeness $S=-6$ and doubly-charged $\Omega\Omega$ dibaryons, if they exist, may be identified in heavy-ion collision processes in the future.
• Probing tqZ anomalous couplings in the trilepton signal at the HL-LHC, HE-LHC, and FCC-hh
Published: 2021-01-26
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We investigate the prospect of discovering the Flavour Changing Neutral Current (FCNC) $tqZ$ couplings via two production processes yielding trilepton signals: top quark pair production $pp\to t\bar{t}$ with one top quark decaying to the Z boson and one light jet and the anomalous single top quark plus Z boson production process $pp\to tZ$. We study these channels at various successors of the Large Hadron Collider (LHC), i.e., the approved High-Luminosity LHC (HL-LHC) as well as the proposed High-Energy LHC (HE-LHC) and Future Circular Collider in hadron-hadron mode (FCC-hh). We perform a full simulation for the signals and the relevant Standard Model (SM) backgrounds and obtain limits on the Branching Ratios (BRs) of $t\to qZ\; (q = u,c)$, eventually yielding a trilepton final state through the decay modes $t\to b W^{+}\to b\ell^{+}\nu_{\ell}$ and $Z\to \ell^{+}\ell^{-}$. The upper limits on these FCNC BRs at 95% Confidence Level (CL) are obtained at the HL-LHC with $\sqrt s = 14$ TeV and 3 ab−1, at the HE-LHC with $\sqrt s = 27$ TeV and 15 ab−1, and at the FCC-hh with $\sqrt s = 100$ TeV and 30 ab−1.
• Diagonal reflection symmetries and universal four-zero texture
Published: 2021-01-26
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In this paper, we consider a set of new symmetries in the SM: diagonal reflection symmetries $R \, m_{u,\nu}^{*} \, R = m_{u,\nu}, m_{d,e}^{*} = m_{d,e}$ with $R =$ diag $(-1,1,1)$. These generalized $CP$ symmetries predict the Majorana phases to be $\alpha_{2,3} /2 = 0$ or $\pi /2$. Realization of diagonal reflection symmetries implies a broken chiral $U(1)_{\rm{PQ}}$ symmetry only for the first generation. The axion scale is suggested to be $\langle {\theta_{u,d}} \rangle \sim \Lambda_{\rm{GUT}} \, \sqrt{m_{u,d} \, m_{c,s}} / v \sim 10^{12}$ [GeV]. By combining the symmetries with the four-zero texture, the mass eigenvalues and mixing matrices of quarks and leptons are reproduced well. This scheme predicts the normal hierarchy, the Dirac phase $\delta _{CP} \simeq 203^{\circ},$ and $|m_{1}| \simeq 2.5$ or $6.2$ [meV]. In this scheme, the type-I seesaw mechanism and a given neutrino Yukawa matrix $Y_{\nu}$ completely determine the structure of the right-handed neutrino mass $M_{R}$. A $u-\nu$ unification predicts the mass eigenvalues to be $(M_{R1} \, , M_{R2} \, , M_{R3}) = (O (10^{5}) \, , O (10^{9}) \, , O (10^{14}))$ [GeV].
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2021 Vol. 45, No. 2
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