Nonperturbative effects in the Standard Model with gauged 1-form symmetry

We study the Standard Model with gauged $$ {\mathrm{\mathbb{Z}}}_{N=2,3,6}^{(1)} $$ ℤ N = 2 , 3 , 6 1 subgroups of its $$ {\mathrm{\mathbb{Z}}}_6^{(1)} $$ ℤ 6 1 1-form global symmetry, making the gauge group $$ \frac{\mathrm{SU}(3)\times \mathrm{SU}(2)\times \mathrm{U}(1)}{{\mathrm{\mathbb{Z}}}_N} $$ SU 3 × SU 2 × U 1 ℤ N . We show that, on a finite $$ {\mathbbm{T}}^3 $$ T 3 , there are self-dual instantons of fractional topological charge. They mediate baryon- and lepton-number violating processes. We compare their amplitudes to the ones due to the usual BPST-instantons. We find that the small hypercharge coupling suppresses the fractional-instanton contribution, unless the torus size is taken sub-Planckian, or extra matter is added above the weak scale. We also discuss these results in light of the cosmological bounds on the torus size.

On the role of leptonic CPV phases in cLFV observables

In extensions of the standard model by Majorana fermions, the presence of additional CP violating phases has been shown to play a crucial role in lepton number violating processes. In this work we show that (Dirac and Majorana) CP violating phases can also lead to important effects in charged lepton flavour violating (cLFV) transitions and decays, in some cases with a significant impact for the predicted rates of cLFV observables. We conduct a thorough exploration of these effects in several cLFV observables, and discuss the implications for future observation. We emphasise how the presence of leptonic CP violating phases might lead to modified cLFV rates, and to a possible loss of correlation between cLFV observables.

Muonium-Antimuonium Conversion

The MACS experiment performed at PSI in the 1990s provided an yet unchallenged upper bound on the probability for a spontaneous conversion of the muonium atom, { M=}({\mu^+e^-})M=(μ+e−), into its antiatom, antimuonium {\overline{{M}} = }({\mu^-e^+})M¯=(μ−e+). It comprises the culmination of a series of measurements at various accelerator laboratories worldwide. The experimental limits on the process have provided input and steering for the further development of a variety of theoretical models beyond the standard theory, in particular for models which address lepton number violating processes and matter-antimatter oscillations. Several models beyond the standard theory could be strongly disfavored. There is interest in a new measurement and improved sensitivity could be reached by exploiting the time evolution of the conversion process, e.g., at intense pulsed muonium sources.

Freezing In with lepton flavored fermions

Dark, chiral fermions carrying lepton flavor quantum numbers are natural candidates for freeze-in. Small couplings with the Standard Model fermions of the order of lepton Yukawas are `automatic' in the limit of Minimal Flavor Violation. In the absence of total lepton number violating interactions, particles with certain representations under the flavor group remain absolutely stable. For masses in the GeV-TeV range, the simplest model with three flavors, leads to signals at future direct detection experiments like DARWIN. Interestingly, freeze-in with a smaller flavor group such as SU(2) is already being probed by XENON1T.

CP violation in rare lepton-number-violating W decays at the LHC

Some models of leptogenesis involve a quasi-degenerate pair of heavy neutrinos N1,2 whose masses can be small, O(GeV). Such neutrinos can contribute to the rare lepton-number-violating (LNV) decay $$ {W}^{\pm}\to {\mathrm{\ell}}_1^{\pm }{\mathrm{\ell}}_2^{\pm }{\left({q}^{\prime}\overline{q}\right)}^{\mp } $$ W ± → ℓ 1 ± ℓ 2 ± q ′ q ¯ ∓ . If both N1 and N2 contribute, there can be a CP-violating rate difference between the LNV decay of a W− and its CP-conjugate decay. In this paper, we examine the prospects for measuring such a CP asymmetry ACP at the LHC. We assume a value for the heavy-light neutrino mixing parameter |BℓN|2 = 10−5, which is allowed by the present experimental constraints, and consider 5 GeV ≤ MN≤ 80 GeV. We consider three versions of the LHC — HL-LHC, HE-LHC, FCC-hh — and show that small values of the CP asymmetry can be measured at 3σ, in the range 1% ≲ ACP ≲ 15%.

Pair production of heavy neutrinos in next-to-leading order QCD at the hadron colliders in the inverse seesaw framework

The explanation of the small neutrino mass can be depicted using some handsome models like type-I and inverse seesaw where the Standard Model gauge singlet heavy right-handed neutrinos are deployed. The common thing in these two models is a lepton number violating parameter, however, its order of magnitude creates a striking difference between them making the nature of the right-handed heavy neutrinos a major play factor. In the type-I seesaw a large lepton number violating parameter involves the heavy right-handed neutrinos in the form of Majorana fermions while a small lepton number violating parameter being involved in the inverse seesaw demands the pseudo-Dirac nature of the heavy right-handed neutrinos. Such heavy neutrinos are accommodated in these models through the sizable mixings with the Standard Model light neutrinos. In this paper we consider the purely inverse seesaw scenario to study the pair production of the pseudo-Dirac heavy neutrinos followed by their various multilepton decay modes through the leading branching fraction at the leading order and next-to-leading order QCD at the LHC with a center-of-mass energy of 13 TeV and a luminosity of 3000 fb[Formula: see text]. We also consider a prospective 100 TeV hadron collider with luminosities of 3000 fb[Formula: see text] and 30,000 fb[Formula: see text], respectively to study the process. Using anomalous multilepton search performed by the CMS at the 8 TeV with 19.5 fb[Formula: see text] luminosity we show prospective search reaches of the mixing angles for the three lepton and four lepton events at the 13 TeV LHC and 100 TeV hadron collider.