<p>The gauge-singlet right-handed neutrinos would be essential to explain the tiny masses of active neutrinos. We consider the effective field theory of the Standard Model extended with these fields under the assumption that neutrinos are Dirac particles. In this framework, we provide a comprehensive study for the phenomenological consequences of various dimension six interactions employing various high and low energy observables. These include the neutrino mass itself, constraints from electroweak precision test and collider searches for lepton or jet plus missing energy, coherent neutrino-nucleus scattering, beta decays, as well as decays of proton, meson, tau, and top. We also study their astrophysical and cosmological implications for stellar cooling and relativistic degrees of freedom.</p>

India

Anirban Biswas

South Korea

Eung Jin Chun

Sanjoy Mandal

Dibyendu Nanda

Phenomenology of Dirac neutrino EFTs up to dimension six

<p>We investigate TeV scale seesaw scenarios at <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><msup><mi>μ</mi><mo>+</mo></msup><msup><mi>e</mi><mo>−</mo></msup></math> and <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><msup><mi>μ</mi><mo>+</mo></msup><msup><mi>μ</mi><mo>+</mo></msup></math> colliders in the <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>μ</mi><mi>TRISTAN</mi></math> (Transposable Ring Intersecting STorage Accelerator in Nippon) experiment. In a minimal type I seesaw scenario, we consider two generations of Standard Model (SM) singlet heavy Majorana–type right-handed neutrinos, which couple with SM-gauge bosons through light-heavy neutrino mixing. We discuss the prospects of probing heavy neutrinos via the processes such as <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><msup><mi>μ</mi><mo>+</mo></msup><msup><mi>e</mi><mo>−</mo></msup><mo stretchy="false">→</mo><mi>ν</mi><msub><mi>N</mi><mi>i</mi></msub><mo stretchy="false">→</mo><msup><mi>e</mi><mo>+</mo></msup><mi>j</mi><mi>j</mi><mi>ν</mi></math> or <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><msup><mi>μ</mi><mo>−</mo></msup><mi>j</mi><mi>j</mi><mi>ν</mi></math> for <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><msqrt><mi>s</mi></msqrt><mo>=</mo><mn>346</mn><mtext> </mtext><mtext> </mtext><mi>GeV</mi></math> and <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mrow><mn>1</mn><mtext> </mtext><mtext> </mtext><msup><mrow><mi>ab</mi></mrow><mrow><mo>−</mo><mn>1</mn></mrow></msup></mrow></math> luminosity. Studying these process, we estimate limits on the light-heavy neutrino mixing angles as a function of heavy neutrino mass, which could be 2 orders of magnitude stronger than electroweak precision data. Further, we study the effect of a doubly charged scalar boson <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mo stretchy="false">(</mo><msup><mi>H</mi><mrow><mo>+</mo><mo>+</mo></mrow></msup><mo stretchy="false">)</mo></math> from the type II seesaw scenario in <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><msup><mi>μ</mi><mo>+</mo></msup><msup><mi>μ</mi><mo>+</mo></msup></math> collision at <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><msqrt><mi>s</mi></msqrt><mo>=</mo><mn>2</mn><mtext> </mtext><mtext> </mtext><mi>TeV</mi></math>. In this case, we consider <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><msup><mi>μ</mi><mo>+</mo></msup><msup><mi>μ</mi><mo>+</mo></msup><mo stretchy="false">→</mo><msubsup><mo>ℓ</mo><mi>i</mi><mo>+</mo></msubsup><msubsup><mo>ℓ</mo><mi>j</mi><mo>+</mo></msubsup></math> and <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><msup><mi>μ</mi><mo>+</mo></msup><msup><mi>μ</mi><mo>+</mo></msup><mo stretchy="false">→</mo><msup><mi>H</mi><mrow><mo>+</mo><mo>+</mo></mrow></msup><mi>Z</mi><mo>/</mo><mi>γ</mi></math> processes followed by the same-sign dilepton decay of <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><msup><mi>H</mi><mrow><mo>+</mo><mo>+</mo></mrow></msup></math>. We find that events involving <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><msup><mi>e</mi><mo>+</mo></msup><msup><mi>e</mi><mo>+</mo></msup></math> among these final states can probe the neutrino mass ordering in the <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>μ</mi><mi>TRISTAN</mi></math> experiment at <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mrow><mn>5</mn><mi>σ</mi></mrow></math> significance. In addition, we study the production of positively charged triplet fermions in <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>μ</mi><mi>TRISTAN</mi></math> following the <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><msup><mi>μ</mi><mo>+</mo></msup><msup><mi>μ</mi><mo>+</mo></msup><mo stretchy="false">→</mo><msup><mi>μ</mi><mo>+</mo></msup><msup><mi mathvariant="normal">Σ</mi><mo>+</mo></msup></math> process, where <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><msup><mi mathvariant="normal">Σ</mi><mo>+</mo></msup></math> decays into <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><msup><mi>μ</mi><mo>+</mo></msup><mi>j</mi><mi>j</mi></math> mode through <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>Z</mi></math> boson exchange. Considering a triplet at 1 TeV and studying SM backgrounds, we estimate the discovery potential of the <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><msup><mi>μ</mi><mo>+</mo></msup><msup><mi>μ</mi><mo>+</mo></msup><mi>j</mi><mi>j</mi></math> signal at <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>μ</mi><mi>TRISTAN</mi></math> with respect to projected luminosity.</p>

Japan

Arindam Das

China

Jinmian Li

Takaaki Nomura

Rao Zhang

Testing tree level TeV scale seesaw scenarios in <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>μ</mi><mi>TRISTAN</mi></math>

Small neutrino masses can be sourced by a tiny vacuum expectation value of a leptophilic Higgs doublet, and mediated by Quasi-Dirac heavy neutrinos. In such simplest linear seesaw picture the neutrino mass mediators can be accessible to colliders. We describe novel charged Higgs and heavy neutrino production mechanisms that can be sizeable at <math><msup><mrow><mi>e</mi></mrow><mrow><mo>+</mo></mrow></msup><msup><mrow><mi>e</mi></mrow><mrow><mo>−</mo></mrow></msup></math>, <math><msup><mrow><mi>e</mi></mrow><mrow><mo>−</mo></mrow></msup><mi>γ</mi></math>, pp, or muon colliders and discuss some of the associated signatures. The oscillation length of the heavy neutrino mediators is directly related to the light neutrino mass ordering. Moreover, lepton number violation can be large despite the smallness of neutrino masses, and may shed light on the Majorana nature of neutrinos and the significance of basic symmetries in weak interaction.

Portugal

Aditya Batra

Praveen Bharadwaj

Rahul Srivastava

Spain

José W.F. Valle

Large lepton number violation at colliders: Predictions from the minimal linear seesaw mechanism

We investigate dark matter (DM-)electron scattering in a minimal <math><mi>U</mi><mo>(</mo><mn>1</mn><msub><mo>)</mo><mi>X</mi></msub></math> extension of the Standard Model (SM), where the DM can appear as a Majorana fermion, a complex singlet scalar, or a Dirac fermion. To study bounds on the <math><mi>U</mi><mo>(</mo><mn>1</mn><msub><mo>)</mo><mi>X</mi></msub></math> gauge coupling <math><mo>(</mo><msub><mi>g</mi><mi>X</mi></msub><mo>)</mo></math> and new gauge boson mass <math><mo>(</mo><msub><mi>M</mi><msup><mi>Z</mi><mo>′</mo></msup></msub><mo>)</mo></math>, from DM-electron scattering, we consider several direct search experiments like CDMS, DAMIC, SENSEI, PandaX-II, DarkSide-50, and XENON1T-S2 for different <math><mi>U</mi><mo>(</mo><mn>1</mn><msub><mo>)</mo><mi>X</mi></msub></math> charges. In this setup, we consider DM production via freeze-in in both radiation-dominated and modified cosmological background to project sensitivities onto <math><msub><mi>g</mi><mi>X</mi></msub><mo>−</mo><msub><mi>M</mi><msup><mi>Z</mi><mo>′</mo></msup></msub></math> plane satisfying observed relic abundance. DM-electron scattering could provide comparable, or even stronger, bounds compared to those obtained from the electron/muon (<math><mrow><mi>g</mi><mo>−</mo><mn>2</mn></mrow></math>), low-energy scattering, and intensity frontier experiments within <math><mrow><mn>0.01</mn><mtext> </mtext><mtext> </mtext><mi>GeV</mi><mo>≲</mo><msub><mi>M</mi><msup><mi>Z</mi><mo>′</mo></msup></msub><mo>≲</mo><mn>0.1</mn><mtext> </mtext><mtext> </mtext><mi>GeV</mi></mrow></math>. Constrains from freeze-in could provide stronger sensitivities for <math><msub><mi>M</mi><msup><mi>Z</mi><mo>′</mo></msup></msub><mo>≳</mo><mi>O</mi><mo>(</mo><mn>1</mn><mo>)</mo><mtext> </mtext><mtext> </mtext><mi>GeV</mi></math>; however, these limits are comparable to those obtained from LHCb and LEP experiments for <math><mrow><mi>O</mi><mo>(</mo><mn>10</mn><mo>)</mo><mtext> </mtext><mtext> </mtext><mi>GeV</mi><mo>≲</mo><msub><mrow><mi>M</mi></mrow><mrow><msup><mrow><mi>Z</mi></mrow><mrow><mo>′</mo></mrow></msup></mrow></msub><mo>≲</mo><mn>150</mn><mtext> </mtext><mtext> </mtext><mi>GeV</mi></mrow></math>. In the future, electron-muon scattering (MUonE), proton (FASER and DUNE), and electron/positron (ILC) beam-dump experiments could probe these parameters.

Basabendu Barman

Dark matter-electron scattering and freeze-in scenarios in the light of <math><mrow><msup><mrow><mi>Z</mi></mrow><mrow><mo>′</mo></mrow></msup></mrow></math> mediation

The simplest linear seesaw mechanism can accommodate the new CDF-II <italic>W</italic> mass measurement. In addition to Standard Model particles, the model includes quasi-Dirac leptons, and a second, leptophilic, scalar doublet seeding small neutrino masses. Our proposal is consistent with electroweak precision tests, neutrino physics, rare decays and collider restrictions, requiring a new charged scalar below a few TeV, split in mass from the new degenerate scalar and pseudoscalar neutral Higgs bosons.

W-mass anomaly in the simplest linear seesaw mechanism

We consider the most general effective Lagrangian up to dimension five, built with Standard Model fields and right-handed neutrinos (RHNs) <math><mrow><msub><mrow><mi>N</mi></mrow><mrow><mi>i</mi></mrow></msub></mrow></math>. Assuming that the RHNs are present near the electroweak scale, we study the phenomenology of the RHNs and highlight the differences that arise due to the inclusion of dimension five operators. We specifically focus on the production process <math><msup><mi>e</mi><mo>+</mo></msup><msup><mi>e</mi><mo>−</mo></msup><mo>/</mo><mi>p</mi><mi>p</mi><mo>→</mo><msub><mi>N</mi><mi>i</mi></msub><msub><mi>N</mi><mi>j</mi></msub></math>, which comes from the dimension five magnetic moment operator. We find that this production process followed by the decay chains such as <math><msub><mi>N</mi><mi>i</mi></msub><mo>→</mo><msub><mi>N</mi><mi>j</mi></msub><mi>γ</mi></math>, <math><msub><mi>N</mi><mi>i</mi></msub><mo>→</mo><msub><mi>ν</mi><mi>j</mi></msub><mi>γ</mi></math>, and <math><msub><mi>N</mi><mi>i</mi></msub><mo>→</mo><msup><mo>ℓ</mo><mo>±</mo></msup><mi>j</mi><mi>j</mi></math> leads to striking collider signatures, which might help to probe the Majorana nature of neutrinos. We discuss the current collider constraints on this operator, as well as projected limit at future colliders. In addition, we discuss the stellar-cooling bounds applicable to the RHN mass below 0.1 GeV.

Rojalin Padhan

Collider imprints of a right handed neutrino magnetic moment operator

In the Dynamical Scotogenic Model, the global B − L symmetry is supposed to be broken spontaneously resulting in a massless Goldstone boson called majoron, and massive right handed neutrinos which participate in the generation of light neutrino massses at one-loop. One of them being the lightest stable particle can be a thermal dark matter candidate. We discuss how the dark matter phenomenology differs from the original Scotogenic model, taking into account all the constraints coming from the observed neutrino masses and mixing, lepton flavor violations such as μ → eγ, μ → eJ, astrophysical and cosmological observations of stellar cooling and N eff, as well as collider signatures such as Higgs invisible decays. We find that the dark matter annihilation to majorons plays an important role to produce the right relic abundance.

Eung Chun

Abhishek Roy

Manimala Mitra

Fermionic dark matter in Dynamical Scotogenic Model

The linear seesaw mechanism provides a simple way to generate neutrino masses. In addition to Standard Model particles, it includes quasi-Dirac leptons as neutrino mass mediators, and a leptophilic scalar doublet seeding small neutrino masses. Here we review its associated physics, including restrictions from theory and phenomenology. The model yields potentially detectable μ → eγ rates as well as distinctive signatures in the production and decay of heavy neutrinos (N  i ) and the charged Higgs boson (H  ± ) arising from the second scalar doublet. We have found that production processes such as e + e  −  → NN, e  −  γ → NH  −  and e + e  −  → H + H  −  followed by the decay chain   <math> <msup> <mi>H</mi> <mo>±</mo> </msup> <mo>→</mo> <msubsup> <mi>ℓ</mi> <mi>i</mi> <mo>±</mo> </msubsup> <mi>N</mi> </math>  $$ {H}^{\pm}\to {\ell}_i^{\pm }N $$ ,   <math> <mi>N</mi> <mo>→</mo> <msubsup> <mi>ℓ</mi> <mi>j</mi> <mo>±</mo> </msubsup> <msup> <mi>W</mi> <mo>∓</mo> </msup> </math>  $$ N\to {\ell}_j^{\pm }{W}^{\mp } $$  leads to striking lepton number violation signatures at high energies which may probe the Majorana nature of neutrinos.

José Valle

Phenomenology of the simplest linear seesaw mechanism

We investigate the possibilities of probing the electroweak scale seesaw scenarios such as type-I, type-II, and type-III seesaw at <math><msup><mi>e</mi><mo>−</mo></msup><mi>γ</mi></math> and <math><mi>γ</mi><mi>γ</mi></math> colliders. For the case of type-I seesaw, the heavy neutrinos can be produced at <math><msup><mi>e</mi><mo>−</mo></msup><mi>γ</mi></math> colliders in association with a <math><mi>W</mi></math> boson. We study a variety of final states in this case, including single and multilepton modes in association with jets to estimate bounds on the light-heavy neutrino mixing angle. In case of type-II seesaw, doubly charged multiplets of the <math><mi>SU</mi><mo>(</mo><mn>2</mn><msub><mo>)</mo><mi>L</mi></msub></math> triplet scalar can be produced in pair at <math><mi>γ</mi><mi>γ</mi></math> collider. We study the multileptonic decay modes coming from this pair production of doubly charged Higgs and show how one can probe neutrino mass hierarchy. We also study same-sign <math><mi>W</mi></math> boson production from the doubly charged Higgs to study multilepton modes in association with missing energy. From the type-III seesaw, we study same-sign <math><mrow><mi>dilepton</mi><mo>+</mo><mtext>jets</mtext></mrow></math> and <math><mrow><mi>trilepton</mi><mo>+</mo><mtext>jets</mtext></mrow></math> modes at <math><msup><mi>e</mi><mo>−</mo></msup><mi>γ</mi></math> collider which are coming from the neutral and charged component of the triplet fermion in association with a <math><mi>W</mi></math> boson and a <math><mi>Z</mi></math> boson, respectively. Due to the existing limits on the triplet fermions from the LHC we choose heavier mass so that the gauge boson originating from the decay of a neutral multiplet can be sufficiently boosted producing a fat-jet signature in association with same-sign dilepton and trilepton. Finally, we estimate bounds on the light neutrino-heavy triplet fermion mixing angle and compare them with the existing bounds.

Brazil

Sujay Shil

Testing electroweak scale seesaw models at <math><msup><mi>e</mi><mo>−</mo></msup><mi>γ</mi></math> and <math><mi>γ</mi><mi>γ</mi></math> colliders

We consider a <math><mi>U</mi><msub><mrow><mo>(</mo><mn>1</mn><mo>)</mo></mrow><mrow><mi>X</mi></mrow></msub><mo>⊗</mo><msub><mrow><mi>Z</mi></mrow><mrow><mn>2</mn></mrow></msub><mo>⊗</mo><msubsup><mrow><mi>Z</mi></mrow><mrow><mn>2</mn></mrow><mrow><mo>′</mo></mrow></msubsup></math> extension of the Standard Model (SM), where the <math><mi>U</mi><msub><mrow><mo>(</mo><mn>1</mn><mo>)</mo></mrow><mrow><mi>X</mi></mrow></msub></math> charge of an SM field is given by a linear combination of its hypercharge and B−L number. Apart from the SM particle content, the model contains three right-handed neutrinos (RHNs) <math><msubsup><mrow><mi>N</mi></mrow><mrow><mi>R</mi></mrow><mrow><mi>i</mi></mrow></msubsup></math> and two scalars Φ, χ, all singlets under the SM gauge group but charged under <math><mi>U</mi><msub><mrow><mo>(</mo><mn>1</mn><mo>)</mo></mrow><mrow><mi>X</mi></mrow></msub></math> gauge group. Two of these additional fields, fermion <math><msubsup><mrow><mi>N</mi></mrow><mrow><mi>R</mi></mrow><mrow><mn>3</mn></mrow></msubsup></math> is odd under <math><msub><mrow><mi>Z</mi></mrow><mrow><mn>2</mn></mrow></msub></math> and scalar χ is odd under <math><msubsup><mrow><mi>Z</mi></mrow><mrow><mn>2</mn></mrow><mrow><mo>′</mo></mrow></msubsup></math> symmetry. Thus both χ and <math><msubsup><mrow><mi>N</mi></mrow><mrow><mi>R</mi></mrow><mrow><mn>3</mn></mrow></msubsup></math> contribute to the observed dark matter relic density, leading to two-component dark matter candidates. We study in detail its dark matter properties such as relic density and direct detection taking into account the constraints coming from collider studies. We find that in our model, there can be possible annihilation of one Dark Matter (DM) into the other, which may potentially alter the relic density in a significant way.