In this work we discuss two different phases of a complex singlet extension of the Standard Model (SM) together with an extension that also includes new fermion fields. All models allow for a strong first-order electroweak phase transition and the de
tection of primordial gravitational waves (GWs) in planned experiments such as LISA is shown to be possible in one of the phases of the singlet extension and also in the model with extra fermions. In the singlet extension with no additional fermions, the detection of GWs strongly depends on the phase of the Higgs potential at zero temperature. We study for the first time the impact of the precision in the determination of the SM parameters on the strength of the GWs spectrum. It turns out that the variation of the SM parameters such as the Higgs mass and top quark Yukawa coupling in their allowed experimental ranges has a notable impact on GWs detectability prospects.
The discovery potential of both singlet and doublet vector-like leptons (VLLs) at the Large Hadron Collider (LHC) as well as at the not-so-far future muon and electron machines is explored. The focus is on a single production channel for LHC direct s
earches while double production signatures are proposed for the leptonic colliders. Implications for the discovery of VLLs in view of the recently announced muon $(g-2)_mu$ anomaly are also discussed. A Deep Learning algorithm to determine the discovery (or exclusion) statistical significance at the LHC is employed. While doublet VLLs can be probed up to masses of 1 TeV, their singlet counterparts have very low cross sections and can hardly be tested beyond a few hundreds of GeV at the LHC. This motivates a physics-case analysis in the context of leptonic colliders where one obtains larger cross sections in VLL double production channels, allowing to probe higher mass regimes otherwise inaccessible even to the LHC high-luminosity upgrade.
We construct families, and concrete examples, of simple extensions of the Standard Model that can yield ultralight {real or} complex vectors or scalars with potential astrophysical relevance. Specifically, the mass range for these putative fundamenta
l bosons ($sim 10^{-10}-10^{-20}$ eV) would lead dynamically to both new non-black hole compact objects (bosonic stars) and new non-Kerr black holes, with masses of $sim M_odot$ to $sim 10^{10} M_odot$, corresponding to the mass range of astrophysical black hole candidates (from stellar mass to supermassive). For each model, we study the properties of the mass spectrum and interactions after spontaneous symmetry breaking, discuss its theoretical viability and caveats, as well as some of its potential and most relevant phenomenological implications {linking them to the} physics of compact objects.
We construct a three-Higgs doublet model with a flavour non-universal ${rm U}(1)times mathbb{Z}_2$ symmetry. That symmetry induces suppressed flavour-changing interactions mediated by neutral scalars. New scalars with masses below the TeV scale can s
till successfully negotiate the constraints arising from flavour data. Such a model can thus encourage direct searches for extra Higgs bosons in the future collider experiments, and includes a non-trivial flavour structure.
We explore the potential of ultimate unification of the Standard Model matter and gauge sectors into a single $E_8$ superfield in ten dimensions via an intermediate Pati-Salam gauge theory. Through a consistent realisation of a $mathbb{T}^6/(mathbb{Z
}_6times mathbb{Z}_2)$ orbifolding procedure accompanied by the Wilson line breaking mechanism and Renormalisation Group evolution of gauge couplings, we have established several benchmark scenarios for New Physics that are worth further phenomenological exploration.
Given the tremendous phenomenological success of the Standard Model (SM) framework, it becomes increasingly important to understand to what extent its specific structure dynamically emerges from unification principles. In this study, we present a nov
el supersymmetric (SUSY) Grand Unification model based upon gauge trinification $[mathrm{SU}(3)]^3$ symmetry and a local $mathrm{SU}(2)_{mathrm{F}} times mathrm{U}(1)_{mathrm{F}}$ family symmetry. This framework is inspired by $mathrm{E}_8 to mathrm{E}_6times mathrm{SU}(2)_{mathrm{F}} times mathrm{U}(1)_{mathrm{F}}$ orbifold reduction pattern, with subsequent $mathrm{E}_6to [mathrm{SU}(3)]^3$ symmetry breaking step. In this framework, higher-dimensional operators of $mathrm{E}_6$ induce the threshold corrections in the gauge and Yukawa interactions leading, in particular, to only two distinct Yukawa couplings in the fundamental sector of the resulting $[mathrm{SU}(3)]^3times mathrm{SU}(2)_{mathrm{F}} times mathrm{U}(1)_{mathrm{F}}$ Lagrangian. Among the appealing features emergent in this framework are the Higgs-matter unification and a unique minimal three Higgs doublet scalar sector at the electroweak scale as well as tree-level hierarchies in the light fermion spectra consistent with those observed in nature. In addition, our framework reveals a variety of prospects for New Physics searches at the LHC and future colliders such as vector-like fermions, as well as rich scalar, gauge and neutrino sectors.
The tremendous phenomenological success of the Standard Model (SM) suggests that its flavor structure and gauge interactions may not be arbitrary but should have a fundamental first-principle explanation. In this work, we explore how the basic distin
ctive properties of the SM dynamically emerge from a unified New Physics framework tying together both flavour physics and Grand Unified Theory (GUT) concepts. This framework is suggested by the gauge Left-Right-Color-Family Grand Unification under the exceptional $mathrm{E}_8$ symmetry that, via an orbifolding mechanism, yields a supersymmetric chiral GUT containing the SM. Among the most appealing emergent properties of this theory is the Higgs-matter unification with a highly-constrained massless chiral sector featuring two universal Yukawa couplings close to the GUT scale. At the electroweak scale, the minimal SM-like effective field theory limit of this GUT represents a specific flavored three-Higgs doublet model consistent with the observed large hierarchies in the quark mass spectra and mixing already at tree level.
The minimal $U(1)_{rm B-L}$ extension of the Standard Model (B-L-SM) offers an explanation for neutrino mass generation via a seesaw mechanism as well as contains two new physics states such as an extra Higgs boson and a new $Z^prime$ gauge boson. Th
e emergence of a second Higgs particle as well as a new $Z^prime$ gauge boson, both linked to the breaking of a local $U(1)_{rm B-L}$ symmetry, makes the B-L-SM rather constrained by direct searches at the Large Hadron Collider (LHC) experiments. We investigate the phenomenological status of the B-L-SM by confronting the new physics predictions with the LHC and electroweak precision data. Taking into account the current bounds from direct LHC searches, we demonstrate that the prediction for the muon $(g-2)_mu$ anomaly in the B-L-SM yields at most a contribution of approximately $8.9 times 10^{-12}$ which represents a tension of $3.28$ standard deviations, with the current $1sigma$ uncertainty, by means of a $Z^prime$ boson if its mass lies in a range of $6.3$ to $6.5$ TeV, within the reach of future LHC runs. This means that the B-L-SM, with heavy yet allowed $Z^prime$ boson mass range, in practice does not resolve the tension between the observed anomaly in the muon $(g-2)_mu$ and the theoretical prediction in the Standard Model. Such a heavy $Z^prime$ boson also implies that the minimal value for a new Higgs mass is of the order of 400 GeV.
Multi-peaked spectra of the primordial gravitational waves are considered as a phenomenologically relevant source of information about the dynamics of sequential phase transitions in the early Universe. In particular, such signatures trace back to sp
ecific patterns of the first-order electroweak phase transition in the early Universe occurring in multiple steps. Such phenomena appear to be rather generic in multi-scalar extensions of the Standard Model. In a particularly simple extension of the Higgs sector, we have identified and studied the emergence of sequential long- and short-lasting transitions as well as their fundamental role in generation of multi-peaked structures in the primordial gravitational-wave spectrum. We discuss the potential detectability of these signatures by the proposed gravitational-wave interferometers.
We investigate the production of primordial Gravitational Waves (GWs) arising from First Order Phase Transitions (FOPTs) associated to neutrino mass generation in the context of type-I and inverse seesaw schemes. We examine both high-scale as well as
low-scale variants, with either explicit or spontaneously broken lepton number symmetry $U(1)_L$ in the neutrino sector. In the latter case, a pseudo-Goldstone majoron-like boson may provide a candidate for cosmological dark matter. We find that schemes with softly-broken $U(1)_L$ and with single Higgs-doublet scalar sector lead to either no FOPTs or too weak FOPTs, precluding the detectability of GWs in present or near future measurements. Nevertheless, we found that, in the majoron-like seesaw scheme with spontaneously broken $U(1)_L$ at finite temperatures, one can have strong FOPTs and non-trivial primordial GW spectra which can fall well within the frequency and amplitude sensitivity of upcoming experiments, including LISA, BBO and u-DECIGO. However, GWs observability clashes with invisible Higgs decay constraints from the LHC. A simple and consistent fix is to assume the majoron-like mass to lie above the Higgs-decay kinematical threshold. We also found that the majoron-like variant of the low-scale seesaw mechanism implies a different GW spectrum than the one expected in the high-scale seesaw. This feature will be testable in future experiments. Our analysis shows that GWs can provide a new and complementary portal to test the neutrino mass generation mechanism.
