In this work, we explore an extension of the Standard Model designed to elucidate the fermion mass hierarchy, account for the dark matter relic abundance, and explain the observed matter-antimatter asymmetry in the universe. Beyond the Standard Model particle content, our model introduces additional scalars and fermions. Notably, the light active neutrinos and the first two generations of charged fermions acquire masses at the one-loop level. The model accommodates successful low-scale leptogenesis, permitting the mass of the decaying heavy right-handed neutrino to be as low as 10 TeV. We conduct a detailed analysis of the dark matter phenomenology and explore various interesting phenomenological implications. These include charged lepton flavor violation, muon and electron anomalous magnetic moments, constraints arising from electroweak precision observables, and implications for collider experiments.

Chile

Carolina Arbeláez

A. Hernández

Claudio Dib

Patricio Contreras

Vishnudath K. N.

Alfonso Zerwekh

A common framework for fermion mass hierarchy, leptogenesis and dark matter

We propose a search for B meson decays to a baryon plus missing energy at the Belle II experiment to probe supersymmetry with a GeV-scale lightest neutralino   <math> <msubsup> <mover> <mi>χ</mi> <mo>~</mo> </mover> <mn>1</mn> <mn>0</mn> </msubsup> </math>  $$ {\overset{\sim }{\chi}}_1^0 $$  and R-parity violation (RPV). We perform analytic computations of the signal branching fractions in the framework of effective field theory, with a single nonzero RPV operator   <math> <msubsup> <mi>λ</mi> <mrow> <mi>ij</mi> <mn>3</mn> </mrow> <mrow> <mo>′</mo> <mo>′</mo> </mrow> </msubsup> <msubsup> <mover> <mi>U</mi> <mo>¯</mo> </mover> <mi>i</mi> <mi>c</mi> </msubsup> <msubsup> <mover> <mi>D</mi> <mo>¯</mo> </mover> <mi>j</mi> <mi>c</mi> </msubsup> <msubsup> <mover> <mi>D</mi> <mo>¯</mo> </mover> <mn>3</mn> <mi>c</mi> </msubsup> </math>  $$ {\lambda}_{ij3}^{\prime \prime }{\overline{U}}_i^c{\overline{D}}_j^c{\overline{D}}_3^c $$ , where i, j = 1, 2. The hadronic form factors are calculated using an SU(3) phenomenological Lagrangian approach for the proton, as well as several hyperons and charmed baryons. Since the decay of the neutralino is kinematically and CKM suppressed in this theoretical scenario, it decays outside the detector and appears experimentally only as missing energy. We detail the analysis techniques at the experimental level and estimate the background in the   <math> <msup> <mi>B</mi> <mo>+</mo> </msup> <mo>→</mo> <mi>p</mi> <msubsup> <mover> <mi>χ</mi> <mo>~</mo> </mover> <mn>1</mn> <mn>0</mn> </msubsup> </math>  $$ {B}^{+}\to p{\overset{\sim }{\chi}}_1^0 $$  search using published results for   <math> <msup> <mi>B</mi> <mo>+</mo> </msup> <mo>→</mo> <msup> <mi>K</mi> <mo>+</mo> </msup> <mi>ν</mi> <mover> <mi>ν</mi> <mo>¯</mo> </mover> </math>  $$ {B}^{+}\to {K}^{+}\nu \overline{\nu} $$ . Our final sensitivity plots are shown for both   <math> <msubsup> <mi>λ</mi> <mn>113</mn> <mrow> <mo>′</mo> <mo>′</mo> </mrow> </msubsup> </math>  $$ {\lambda}_{113}^{\prime \prime } $$  versus the squark mass   <math> <msub> <mi>m</mi> <mover> <mi>q</mi> <mo>~</mo> </mover> </msub> </math>  $$ {m}_{\tilde{q}} $$  and   <math> <msubsup> <mi>λ</mi> <mn>113</mn> <mrow> <mo>′</mo> <mo>′</mo> </mrow> </msubsup> <mo>/</mo> <msubsup> <mi>m</mi> <mover> <mi>q</mi> <mo>~</mo> </mover> <mn>2</mn> </msubsup> </math>  $$ {\lambda}_{113}^{\prime \prime }/{m}_{\tilde{q}}^2 $$  versus the neutralino mass   <math> <msub> <mi>m</mi> <msubsup> <mover> <mi>χ</mi> <mo>~</mo> </mover> <mn>1</mn> <mn>0</mn> </msubsup> </msub> </math>  $$ {m}_{{\overset{\sim }{\chi}}_1^0} $$ . We find that the search at Belle II could probe   <math> <msubsup> <mi>λ</mi> <mn>113</mn> <mrow> <mo>′</mo> <mo>′</mo> </mrow> </msubsup> <mo>/</mo> <msubsup> <mi>m</mi> <mover> <mi>q</mi> <mo>~</mo> </mover> <mn>2</mn> </msubsup> </math>  $$ {\lambda}_{113}^{\prime \prime }/{m}_{\tilde{q}}^2 $$  down to the order of 10 −8 GeV −2 in the kinematically allowed   <math> <msub> <mi>m</mi> <msubsup> <mover> <mi>χ</mi> <mo>~</mo> </mover> <mn>1</mn> <mn>0</mn> </msubsup> </msub> </math>  $$ {m}_{{\overset{\sim }{\chi}}_1^0} $$  range. We also obtain current limits on   <math> <msubsup> <mi>λ</mi> <mn>123</mn> <mrow> <mo>′</mo> <mo>′</mo> </mrow> </msubsup> </math>  $$ {\lambda}_{123}^{\prime \prime } $$  by recasting an existing search interpreted as   <math> <msup> <mi>B</mi> <mn>0</mn> </msup> <mo>→</mo> <msup> <mi>Λ</mi> <mn>0</mn> </msup> <msubsup> <mover> <mi>χ</mi> <mo>~</mo> </mover> <mn>1</mn> <mn>0</mn> </msubsup> </math>  $$ {B}^0\to {\Lambda}^0{\overset{\sim }{\chi}}_1^0 $$ , and comment about searches for   <math> <msup> <mi>B</mi> <mo>+</mo> </msup> <mo>→</mo> <msup> <mi>Σ</mi> <mo>+</mo> </msup> <msubsup> <mover> <mi>χ</mi> <mo>~</mo> </mover> <mn>1</mn> <mn>0</mn> </msubsup> </math>  $$ {B}^{+}\to {\Sigma}^{+}{\overset{\sim }{\chi}}_1^0 $$ ,   <math> <msup> <mi>B</mi> <mn>0</mn> </msup> <mo>→</mo> <msup> <mi>Σ</mi> <mn>0</mn> </msup> <msubsup> <mi>χ</mi> <mn>1</mn> <mn>0</mn> </msubsup> </math>  $$ {B}^0\to {\Sigma}^0{\chi}_1^0 $$ ,   <math> <msup> <mi>B</mi> <mo>+</mo> </msup> <mo>→</mo> <msubsup> <mi>Λ</mi> <mi>c</mi> <mo>+</mo> </msubsup> <msubsup> <mover> <mi>χ</mi> <mo>~</mo> </mover> <mn>1</mn> <mn>0</mn> </msubsup> </math>  $$ {B}^{+}\to {\Lambda}_c^{+}{\overset{\sim }{\chi}}_1^0 $$ , and   <math> <msup> <mi>B</mi> <mo>+</mo> </msup> <mo>→</mo> <msubsup> <mi>Ξ</mi> <mi>c</mi> <mo>+</mo> </msubsup> <msubsup> <mover> <mi>χ</mi> <mo>~</mo> </mover> <mn>1</mn> <mn>0</mn> </msubsup> </math>  $$ {B}^{+}\to {\Xi}_c^{+}{\overset{\sim }{\chi}}_1^0 $$ . In closing, we briefly discuss potential searches at the LHCb and BESIII experiments.

Juan Helo

Germany

Valery Lyubovitskij

Nicolás Neill

Israel

Abner Soffer

Taiwan

Zeren Wang

Probing R-parity violation in B-meson decays to a baryon and a light neutralino

We implement a minimal linear seesaw model (LSM) for addressing the Quasi-Dirac (QD) behaviour of heavy neutrinos, focusing on the mass regime of M  N  ≲ M  W . Here we show that for relatively low neutrino masses, covering the few GeV range, the same-sign to opposite-sign dilepton ratio, R  ℓℓ , can be anywhere between 0 and 1, thus signaling a Quasi-Dirac regime. Particular values of R  ℓℓ  are controlled by the width of the QD neutrino and its mass splitting, the latter being equal to the light-neutrino mass m  ν  in the LSM scenario. The current upper bound on m  ν1 together with the projected sensitivities of current and future |U  N ℓ |2 experimental measurements, set stringent constraints on our low-scale QD mass regime. Some experimental prospects of testing the model by LHC displaced vertex searches are also discussed.

Spain

Kevin Monsálvez-Pozo

Iván Schmidt

Quasi-Dirac neutrinos in the linear seesaw model

We consider light neutralinos of mass about 1 GeV, produced from τ lepton rare decays at Belle II, in the context of R-parity-violating (RPV) supersymmetry. With large and clean samples of τ leptons produced at the Belle II experiment, excellent sensitivity to such light neutralinos with the exotic signatures of displaced vertices is expected. We focus on two benchmark scenarios of single RPV operators,   <math> <msubsup> <mi>λ</mi> <mn>311</mn> <mo>′</mo> </msubsup> <msub> <mi>L</mi> <mn>3</mn> </msub> <msub> <mi>Q</mi> <mn>1</mn> </msub> <msub> <mover> <mi>D</mi> <mo>¯</mo> </mover> <mn>1</mn> </msub> </math>  $$ {\lambda}_{311}^{\prime }{L}_3{Q}_1{\overline{D}}_1 $$  and   <math> <msubsup> <mi>λ</mi> <mn>312</mn> <mo>′</mo> </msubsup> <msub> <mi>L</mi> <mn>3</mn> </msub> <msub> <mi>Q</mi> <mn>1</mn> </msub> <msub> <mover> <mi>D</mi> <mo>¯</mo> </mover> <mn>2</mn> </msub> </math>  $$ {\lambda}_{312}^{\prime }{L}_3{Q}_1{\overline{D}}_2 $$ , which induce both the production and decay of the lightest neutralino. For the reconstruction of a displaced vertex, we require at least two charged pions in the final states. We perform Monte-Carlo simulations for both signal and background events, and find that Belle II can explore regions in the parameter space competitive with other probes. In particular, for the   <math> <msubsup> <mi>λ</mi> <mn>311</mn> <mo>′</mo> </msubsup> </math>  $$ {\lambda}_{311}^{\prime } $$  scenario, it can put limits up to two orders of magnitude stronger than the current bounds.

Sourav Dey

Minakshi Nayak

South Korea

Long-lived light neutralinos at Belle II

Knowledge of the mechanism of neutrino mass generation would help understand a lot more about Lepton Number Violation (LNV), the cosmological evolution of the Universe, or the evolution of astronomical objects. Here we propose a verifiable and viable extension of the Standard model for neutrino mass generation, with a low-scale seesaw mechanism via LNV condensation in the sector of sterile neutrinos. To prove the concept, we analyze a simplified model of just a single family of elementary particles and check it against a set of phenomenological constraints coming from electroweak symmetry breaking, neutrino masses, leptogenesis and dark matter. The model predicts (i) TeV scale quasi-degenerate heavy sterile neutrinos, suitable for leptogenesis with resonant enhancement of the CP asymmetry, (ii) a set of additional heavy Higgs bosons whose existence can be challenged at the LHC, (iii) an additional light and sterile Higgs scalar which is a candidate for decaying warm dark matter, and (iv) a majoron. Since the model is based on simple and robust principles of dynamical mass generation, its parameters are very restricted, but remarkably it is still within current phenomenological limits.

Sergey Kovalenko

Ivan Schmidt

Czechia

Adam Smetana

Low-scale seesaw from neutrino condensation

We propose a strategy for distinguishing the Dirac/Majorana character of heavy neutrinos with masses below the <math><mrow><mi>W</mi></mrow></math>-boson mass, using purely leptonic decays at the LHC. The strategy makes use of a forward-backward asymmetry of the opposite-charge lepton in the <math><msup><mi>W</mi><mo>+</mo></msup><mo>→</mo><msup><mi>l</mi><mo>+</mo></msup><msup><mi>l</mi><mo>+</mo></msup><msup><mi>l</mi><mrow><mo>′</mo><mo>−</mo></mrow></msup><mi>ν</mi></math> decay. In order to check the experimental feasibility of the model, we show—through a numerical analysis and for different ranges of the heavy neutrino masses—that in the decay <math><msup><mi>W</mi><mo>+</mo></msup><mo>→</mo><msup><mi>e</mi><mo>+</mo></msup><msup><mi>e</mi><mo>+</mo></msup><msup><mi>μ</mi><mo>−</mo></msup><mi>ν</mi></math> one can distinguish the positron from the <math><mi>W</mi></math> decay from the positron coming from the heavy neutrino. Finally, we estimate the number of events at the LHC Run II for a Dirac and Majorana <math><mi>N</mi></math> neutrino with an integrated luminosity of <math><mrow><mn>120</mn><mtext> </mtext><mtext> </mtext><msup><mrow><mi>fb</mi></mrow><mrow><mo>−</mo><mn>1</mn></mrow></msup></mrow></math>. Signals can be found if heavy-to-light neutrino mixings are <math><mo>|</mo><msub><mi>U</mi><mrow><mi>N</mi><mi>μ</mi></mrow></msub><msup><mo>|</mo><mn>2</mn></msup><mo>,</mo><mo>|</mo><msub><mi>U</mi><mrow><mi>N</mi><mi>e</mi></mrow></msub><msup><mo>|</mo><mn>2</mn></msup><mo>≳</mo><msup><mn>10</mn><mrow><mo>−</mo><mn>6</mn></mrow></msup></math>.

Carolina Arbelaéz

Juan Carlos Vasquez

Probing the Dirac or Majorana nature of the heavy neutrinos in pure leptonic decays at the LHC

We study the possibility of having CP asymmetries in the decay K ± → π ∓ ℓ ± ℓ ± ( ℓ = e, μ ). This decay violates Lepton Number by two units and occurs only if there are Majorana particles that mediate the transition. Even though the absolute rate is highly suppressed by current bounds, we search for Majorana neutrino scenarios where the CP asymmetry arising from the lepton sector could be sizeable. This is indeed the case if there are two or more Majorana neutrinos with similar masses in the range around 10 2 MeV. In particular, the asymmetry is potentially near unity if two neutrinos are nearly degenerate, in the sense Δ m N ∼ Γ N . The full decay, however, may be difficult to detect not only because of the suppression caused by the heavy-to-light lepton mixing, but also because of the long lifetime of the heavy neutrino, which would induce large space separation between the two vertices where the charge leptons are produced. This particular problem should be less serious in heavier meson decays, as they involve heavier neutrinos with shorter lifetimes.

Miguel Campos

C. Kim

CP violation with Majorana neutrinos in K meson decays