Jul 2026· The European Physical Journal Special Topics· 2 citations· 36 references
Abstract
The experimental confirmation of non-vanishing neutrino masses at the sub-eV range poses a challenge for the (otherwise successful) Standard Model (SM). The EW-
$$\nu _R$$
ν
R
model, which gives a solution to this problem via a seesaw mechanism with non-sterile right-handed neutrinos at the electroweak scale, contains mirror fermion doublets and singlets with opposite chirality assignments under the same
$$SU_W(2)$$
S
U
W
(
2
)
gauge symmetry. It also features a viable dark-matter candidate and proposes a solution to the strong CP problem. The (electroweak scale) mirror quarks (leptons) decay to SM quarks (leptons) plus very light neutral scalars, giving final states with jets or leptons, scalar mesons and missing momentum. In this paper, we review the signatures through which these mirror particles can be probed at colliders such as the Large Hadron Collider. Emphasis is given to long-lived mirror particles, depending on the associated Yukawa couplings, that can give rise to observable displaced leptons and jets, and heavy hadrons.z
We study leptogenesis via out-of-equilibrium decays of right-handed neutrinos in a parity symmetric extension of the Standard Model with gauge group $SU(3)_c \times SU(2)_L \times SU(2)_R \times U(1)_X$, spontaneously broken to the Standard Model gauge group at a scale $v_R$. We focus on the minimal Higgs realization, in which the strong CP problem is resolved without invoking additional symmetries. In this framework, lepton number is violated through Yukawa interactions, while neutrino masses arise only at higher loop order, a feature that permits the CP-violating quantum corrections to right-handed neutrino decays to be parametrically large. For thermal production of right-handed neutrinos, we find that successful leptogenesis requires $v_R \gtrsim 6\times 10^{12}$ GeV. Non-thermal production, however, relaxes this bound dramatically, requiring only $v_R \gtrsim 10$ TeV, a scale accessible to collider searches for new particles and searches for rare processes.
The observation of neutrino masses strongly motivates $U(1)_{B-L}$ extensions of the Standard Model, in which heavy neutral leptons acquire Majorana masses through spontaneous $U(1)_{B-L}$ symmetry breaking and generate light neutrino masses via the seesaw mechanism. In this framework, the singlet scalar responsible for symmetry breaking mixes with the SM Higgs boson, allowing it to be produced in rare meson decays. We investigate a scenario in which this light scalar promptly decays into a pair of long-lived heavy neutrinos that subsequently decay into visible charged leptons and hadrons through light-heavy neutrino mixing inside the proposed Forward Physics Facility (FPF) at the FCC-hh and the SHiP beam-dump experiment. Taking into account realistic detector geometries, decay probabilities, and visible branching fractions, we estimate the projected sensitivities to the scalar-Higgs mixing angle as a function of the scalar mass and to the light-heavy neutrino mixing as a function of the heavy neutrino mass. We find that FPF and SHiP can significantly extend the discovery reach for both light scalars and long-lived heavy neutrinos beyond existing experimental limits, providing powerful and complementary probes of neutrino-mass generation and hidden-sector physics.
ShivaSankar K.A., Souvik Das, Arindam Das et al.· 0 citations
Heavy Neutral Lepton (HNL)-mediated t-channel processes provide a unique opportunity to probe mass scales beyond the kinematic reach of direct production at high-energy colliders. We revisit these processes using the vector boson scattering channel $WW\to\ell\ell$ at the LHC as a case study, highlighting the essential role of the light neutrinos in restoring the proper high-energy unitary behavior of the scattering amplitude. Their inclusion, overlooked in some previous studies, leads to destructive interference that strongly suppresses lepton number violating signatures, demonstrating that a consistent treatment of the full seesaw spectrum qualitatively alters the phenomenology of t-channel HNL searches. This motivates the exploration of lepton number conserving but lepton flavor violating final states instead. We present a detailed analysis of the $pp\to e\mu jj$ channel and show that it provides a promising probe of TeV-scale HNLs in low-scale seesaw scenarios with sizable active-sterile mixing, extending the LHC sensitivity beyond existing direct searches.
C. García-García, Manuel González-López, Xabier Marcano et al.· 0 citations
Dynamical dark energy offers an alternative to a cosmological constant with distinct observational signatures. However, the small energy density scale, Hubble-sized mass, and Planckian excursions make simple models fine-tuned and unnatural. In this work, we show that a weak version of the axion, identified with the phase field of the anomalous $U(1)_{B+L}$ of the Standard Model, can generate the scale hierarchies expected for dark energy. The axion potential is controlled by sources of explicit baryon and lepton number violation and is radiatively stable. We show that the leading contribution comes from two inequivalent Weinberg operators, one $B+L$-conserving and one $B+L$-violating, which generate the axion potential. We propose a flavor selection rule based on a spontaneously broken $S_3$ permutation symmetry in the lepton sector that simultaneously removes the quadratic divergence and dominant temperature-dependent contributions. The resulting potential first appears at quartic order in the axion-dependent neutrino masses and, for the observed departure from tribimaximal mixing, its amplitude is parametrically close to the dark-energy density. The dominant uncertainty comes from $\delta_{\rm CP}$ and $\theta_{23}$, so experiments like Hyper-K and DUNE can directly test the model in the future. Cosmologically, the field behaves as thawing quintessence for $f$ close to $M_{pl}$, stays frozen by Hubble friction until late times and never enters the adiabatic regime.
The Standard Model is the most precise and consequential theory of science, yet it is not a complete account of all fundamental physics. Amongst its limitations, it lacks a coherent unification of quantum mechanics and gravity, and thus cannot resolve the large hierarchy gap between the weak and Planck scales. It does not explain neutrino masses or the evidence implying the existence of Dark Matter, nor does it provide a viable mechanism for baryogenesis sufficient to explain the observed matter-antimatter asymmetry in the Universe. Lastly, it is in tension with observations hinting toward charged Lepton Flavour Universality violation. These shortcomings indicate that physics beyond the Standard Model must exist. In this thesis, the ATLAS detector at the Large Hadron Collider is used to perform three distinct searches for New Physics. The first targets hypothetical, TeV-scale quantum black holes, which arise in extra-dimensional theories seeking to solve the hierarchy problem. Exploiting a unique feature of the model that strongly enhances the production rate with the collider centre-of-mass energy, and thereby permits a considerable increase in mass reach, it uses lepton+jet final states and sets the strongest limits on the model to date. The second tackles anomalies in lepton flavour by searching for heavy resonances that violate flavour conservation; targeting dilepton+$b$-jets final states, it complements previous inclusive results by directly addressing exclusive ones. The third targets Clockwork, periodic signatures of heavy graviton states appearing in extra-dimensional models. Performed in dielectron and diphoton final states, it establishes a new method to search for periodic signals using wavelet transformations with machine learning, and sets the best limits on the model.
The observation of neutrino oscillations and masses motivates extensions of the Standard Model containing right-handed neutrinos. If additional new physics exists at scales beyond the direct reach of present experiments, its effects can be systematically described within the neutrino Standard Model Effective Field Theory ($\nu$SMEFT). In this framework, heavy neutral leptons provide a promising target for collider searches. In this work, we reinterpret a CMS search for heavy Majorana neutrinos in the same-sign dimuon plus two jets final state to constrain the parameter space of the dimension-six $\nu$SMEFT. Using a detailed recast of the experimental analysis, we present results for both a pure-agnostic benchmark and a benchmark incorporating neutrinoless double-beta-decay bounds. For heavy-neutrino masses in the range $200~\mathrm{GeV} \le m_N \le 15~\mathrm{TeV}$, the observed upper limits on the effective coupling strength range from $5.8\times10^{-7}\,\mathrm{GeV}^{-2}$ to $2.3\times10^{-6}\,\mathrm{GeV}^{-2}$. These results constitute the first experimental exclusion limits on the agnostic $\nu$SMEFT parameter space obtained from a dedicated recast of an LHC heavy-neutrino search.
L. Duarte, Agustı́n Guillenea· 0 citations
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