We consider the gauged U(1) model and examine the situation where the sterile neutrino is a dark matter candidate produced by the freeze-in mechanism. In our model, the dark matter $\textit{N}$ is mainly produced by the decay of a U(1) breaking scalar boson $\textit{ϕ}$. We point out that the on-shell production of $\textit{ϕ}$ through annihilation of the U(1) gauge boson $\textit{Z}$ plays an important role. We find that the single production of $\textit{Z}$ from the gluon bath in the early Universe can become the main production modes for $\textit{Z}$ in some parameter regions. To prevent $\textit{N}$ from being overproduced, we show that the U(1) gauge coupling constant $\textit{g}$ must be as small as 10 –10 . We also consider the case where the decay of $\textit{ϕ}$ into $\textit{N}$ is kinematically forbidden. In this case, $\textit{N}$ is generated by the scattering of $\textit{Z}$ and the $\textit{g}$ takes values of 10 –10 , which can be explored in collider experiments like FASER and SHiP.

Japan

Osamu Seto

Takashi Shimomura

China

Yoshiki Uchida

Freeze-in sterile neutrino dark matter in a feebly gauged $\textit{B}$ − $\textit{L}$ model

We consider a dark photon model in which the dark U(1) gauge symmetry is spontaneously broken by a vacuum expectation value of a new scalar boson. We focus on the ForwArd Search ExpeRiment (FASER) and calculate its sensitivity to the dark photon produced from the off-shell decay of the new scalar boson. It is found that the off-shell production extends the sensitivity region beyond the kinematical threshold of the on-shell decay of the scalar boson, and that the sensitivity region can be spanned to unexplored region. We also show the parameter space in which perturbative calculation is valid for the unitarity of an S matrix.

Takeshi Araki

Kento Asai

Yohei Nakashima

Dark photon pair production via off-shell dark Higgs at FASER

The T2K experiment is one of the most powerful long-baseline experiments to investigate neutrino oscillations. The off-axis near detector called ND280 is installed 280 m downstream from the neutrino production target to measure the neutrino energy spectrum. In this paper, we study the capability of the ND280 detector to search for the dark photon produced through the meson rare decay and proton bremsstrahlung processes at the proton beam dump. We find that the ten-year operation of T2K with the ND280 detector excludes the unexplored parameter region for the dark photon mass and kinetic mixing. We also show that a broader parameter region can be searched by the ND280 in the future T2K operation for dark photon as well as U(1) B−L  gauge boson.

Tomoya Iizawa

Hidetoshi Otono

Yosuke Takubo

New constraint on dark photon at T2K off-axis near detector

We propose a model with two different extra <math><mi>U</mi><mo>(</mo><mn>1</mn><mo>)</mo></math> gauge symmetries; muon minus tauon symmetry <math><mi>U</mi><mo>(</mo><mn>1</mn><msub><mo>)</mo><mrow><msub><mi>L</mi><mi>μ</mi></msub><mo>−</mo><msub><mi>L</mi><mi>τ</mi></msub></mrow></msub></math> and hidden symmetry <math><mi>U</mi><mo>(</mo><mn>1</mn><msub><mo>)</mo><mi>H</mi></msub></math>. Then, we explain the muon anomalous magnetic moment, semileptonic decays <math><mi>b</mi><mo>→</mo><mi>s</mi><mo>ℓ</mo><mover><mo>ℓ</mo><mo>¯</mo></mover></math>, and dark matter. In particular, we find an intriguing dark matter candidate to be verified by Hyper-Kamiokande and JUNO in the future, that require neutrinophilic DM with rather light-dark matter <math><mrow><mtext>mass</mtext><mo>∼</mo><mi>O</mi><mo>(</mo><mn>10</mn><mo>)</mo><mtext> </mtext><mtext> </mtext><mi>MeV</mi></mrow></math>.

Keiko I. Nagao

Takaaki Nomura

South Korea

Hiroshi Okada

Neutrinophilic dark-matter annihilation in a model with <math><mrow><mi>U</mi><mo>(</mo><mn>1</mn><msub><mrow><mo>)</mo></mrow><mrow><msub><mrow><mi>L</mi></mrow><mrow><mi>μ</mi></mrow></msub><mo>−</mo><msub><mrow><mi>L</mi></mrow><mrow><mi>τ</mi></mrow></msub></mrow></msub><mo>×</mo><mi>U</mi><mo>(</mo><mn>1</mn><msub><mrow><mo>)</mo></mrow><mrow><mi>H</mi></mrow></msub></mrow></math> gauge symmetry

We discuss the discovery potential of a promising signals,  $$3 \times \mu ^+ \mu ^-$$  <math> <mrow> <mn>3</mn> <mo>×</mo> <msup> <mi>μ</mi> <mo>+</mo> </msup> <msup> <mi>μ</mi> <mo>-</mo> </msup> </mrow> </math>   at a  $$\mu ^+ \mu ^-$$  <math> <mrow> <msup> <mi>μ</mi> <mo>+</mo> </msup> <msup> <mi>μ</mi> <mo>-</mo> </msup> </mrow> </math>   collider and  $$\mu ^+ \mu ^+ \mu ^+ \mu ^+ \mu ^- \mu ^- $$  <math> <mrow> <msup> <mi>μ</mi> <mo>+</mo> </msup> <msup> <mi>μ</mi> <mo>+</mo> </msup> <msup> <mi>μ</mi> <mo>+</mo> </msup> <msup> <mi>μ</mi> <mo>+</mo> </msup> <msup> <mi>μ</mi> <mo>-</mo> </msup> <msup> <mi>μ</mi> <mo>-</mo> </msup> </mrow> </math>   at a  $$\mu ^+ \mu ^+$$  <math> <mrow> <msup> <mi>μ</mi> <mo>+</mo> </msup> <msup> <mi>μ</mi> <mo>+</mo> </msup> </mrow> </math>   collider, that are obtained via production of  $$Z'$$  <math> <msup> <mi>Z</mi> <mo>′</mo> </msup> </math>   and a new scalar boson  $$\phi $$  <math> <mi>ϕ</mi> </math>   in a spontaneously broken local  $$U(1)_{L_\mu - L_\tau }$$  <math> <mrow> <mi>U</mi> <msub> <mrow> <mo>(</mo> <mn>1</mn> <mo>)</mo> </mrow> <mrow> <msub> <mi>L</mi> <mi>μ</mi> </msub> <mo>-</mo> <msub> <mi>L</mi> <mi>τ</mi> </msub> </mrow> </msub> </mrow> </math>   model. We consider the  $$Z'$$  <math> <msup> <mi>Z</mi> <mo>′</mo> </msup> </math>   associated production from the process  $$\mu ^+ \mu ^- \rightarrow \phi Z'$$  <math> <mrow> <msup> <mi>μ</mi> <mo>+</mo> </msup> <msup> <mi>μ</mi> <mo>-</mo> </msup> <mo>→</mo> <mi>ϕ</mi> <msup> <mi>Z</mi> <mo>′</mo> </msup> </mrow> </math>   in addition to a muonphilic  $$Z'$$  <math> <msup> <mi>Z</mi> <mo>′</mo> </msup> </math>   fusion process  $$\mu ^+ \mu ^\pm \rightarrow \phi \mu ^+ \mu ^\pm $$  <math> <mrow> <msup> <mi>μ</mi> <mo>+</mo> </msup> <msup> <mi>μ</mi> <mo>±</mo> </msup> <mo>→</mo> <mi>ϕ</mi> <msup> <mi>μ</mi> <mo>+</mo> </msup> <msup> <mi>μ</mi> <mo>±</mo> </msup> </mrow> </math>  . The scalar boson is associated with  $$U(1)_{L_\mu - L_\tau }$$  <math> <mrow> <mi>U</mi> <msub> <mrow> <mo>(</mo> <mn>1</mn> <mo>)</mo> </mrow> <mrow> <msub> <mi>L</mi> <mi>μ</mi> </msub> <mo>-</mo> <msub> <mi>L</mi> <mi>τ</mi> </msub> </mrow> </msub> </mrow> </math>   symmetry breaking and dominantly decays into  $$Z' Z'$$  <math> <mrow> <msup> <mi>Z</mi> <mo>′</mo> </msup> <msup> <mi>Z</mi> <mo>′</mo> </msup> </mrow> </math>   mode. We carry out numerical simulation analysis for signal and background processes to estimate a discovery significance for different benchmark points. It is shown that our signal can be observed with integrated luminosity less than  $$\mathcal {O}(100)$$  <math> <mrow> <mi>O</mi> <mo>(</mo> <mn>100</mn> <mo>)</mo> </mrow> </math>   fb $$^{-1}$$  <math> <msup> <mrow></mrow> <mrow> <mo>-</mo> <mn>1</mn> </mrow> </msup> </math>   for both  $$\mu ^+ \mu ^-$$  <math> <mrow> <msup> <mi>μ</mi> <mo>+</mo> </msup> <msup> <mi>μ</mi> <mo>-</mo> </msup> </mrow> </math>   and  $$\mu ^+ \mu ^+$$  <math> <mrow> <msup> <mi>μ</mi> <mo>+</mo> </msup> <msup> <mi>μ</mi> <mo>+</mo> </msup> </mrow> </math>   colliders with more than 5- $$\sigma $$  <math> <mi>σ</mi> </math>   significance.

Arindam Das

Multi muon/anti-muon signals via productions of gauge and scalar bosons in a  $$U(1)_{L_\mu - L_\tau }$$  <math> <mrow> <mi>U</mi> <msub> <mrow> <mo>(</mo> <mn>1</mn> <mo>)</mo> </mrow> <mrow> <msub> <mi>L</mi> <mi>μ</mi> </msub> <mo>-</mo> <msub> <mi>L</mi> <mi>τ</mi> </msub> </mrow> </msub> </mrow> </math>   model at muonic colliders

FASER is one of the promising experiments which search for long-lived particles beyond the Standard Model. In this paper, we consider charged lepton flavor violation (CLFV) via a light and weakly interacting boson and discuss the detectability by FASER. We focus on four types of CLFV interactions, i.e., the scalar-, pseudoscalar-, vector-, and dipole-type interaction, and calculate the sensitivity of FASER to each CLFV interaction. We show that, with the setup of FASER2, a wide region of the parameter space can be explored. Particularly, it is found that FASER2 has a sensitivity to very small coupling regions in which the rare muon decays, such as μ → eγ, cannot place bounds, and that there is a possibility to detect CLFV decays of the new light bosons.

Search for lepton flavor violating decay at FASER

We propose a pseudo-Nambu-Goldstone boson (pNGB) dark matter (DM) model based on an additional non-Abelian gauge symmetry <math><mi>S</mi><mi>U</mi><mo>(</mo><mn>2</mn><msub><mo>)</mo><mi>D</mi></msub></math>. The gauge symmetry <math><mi>S</mi><mi>U</mi><mo>(</mo><mn>2</mn><msub><mo>)</mo><mi>D</mi></msub></math> is spontaneously broken to a global custodial symmetry <math><mi>U</mi><mo>(</mo><mn>1</mn><msub><mo>)</mo><mi>V</mi></msub></math> via the nonvanishing vacuum expectation values of <math><mi>S</mi><mi>U</mi><mo>(</mo><mn>2</mn><msub><mo>)</mo><mi>D</mi></msub></math> doublet and triplet scalar fields. Due to the exact global symmetry <math><mi>U</mi><mo>(</mo><mn>1</mn><msub><mo>)</mo><mi>V</mi></msub></math>, the lightest <math><mi>U</mi><mo>(</mo><mn>1</mn><msub><mo>)</mo><mi>V</mi></msub></math> charged particle becomes stable. We assume that the lightest charged particle in the model is the charged complex pNGB, which we regard as DM. It avoids the strong constraints from current DM direct detection experiments due to the property of NGB. We find that the measured energy density of DM can be reproduced when the DM mass is larger than the half of the Higgs mass, where the lower limit generally comes from the constraint of DM invisible decay and the upper limit from DM direct detection experiments depends on the model parameters.

Hajime Otsuka

Koji Tsumura

Naoki Yamatsu

Pseudo-Nambu-Goldstone dark matter from non-Abelian gauge symmetry

We reexamine sterile neutrino dark matter in gauged <math><mi>U</mi><mo>(</mo><mn>1</mn><msub><mo>)</mo><mrow><mi>B</mi><mo>−</mo><mi>L</mi></mrow></msub></math> model. Improvements have been made by tracing and careful evaluation of the evolution of the number densities of sterile neutrinos <math><mi>N</mi></math> and extra neutral gauge bosons <math><msup><mi>Z</mi><mo>′</mo></msup></math>. As a result, the cosmologically-interesting gauge coupling of <math><mi>U</mi><mo>(</mo><mn>1</mn><msub><mo>)</mo><mrow><mi>B</mi><mo>−</mo><mi>L</mi></mrow></msub></math> for freeze-in sterile neutrinos turns out to be smaller than the values reported in the literature. This avoids the overproduction of <math><msup><mi>Z</mi><mo>′</mo></msup></math> so that it is consistent with the big bang nucleosynthesis and the cosmic microwave background constraints on the effective number of neutrino species. Similarly, the free-streaming length constraints exclude a large parameter space derived in previous studies. In addition to known freeze-in pair production of <math><mi>N</mi></math> from the standard model fermion pairs, we find the case that <math><mi>N</mi></math> is dominantly produced from a pair of <math><msup><mi>Z</mi><mo>′</mo></msup></math> at the temperature characterized by the <math><mi>B</mi><mo>−</mo><mi>L</mi></math> breaking scalar mass. Thus, the naive truncation of the <math><mi>U</mi><mo>(</mo><mn>1</mn><msub><mo>)</mo><mrow><mi>B</mi><mo>−</mo><mi>L</mi></mrow></msub></math> scalar contribution made in the literature is not valid.

Shintaro Eijima

Revisiting sterile neutrino dark matter in gauged <math><mi>U</mi><mo>(</mo><mn>1</mn><msub><mo>)</mo><mrow><mi>B</mi><mo>−</mo><mi>L</mi></mrow></msub></math> model

We consider inelastic dark matter scenarios with dark photon mediator and a dark Higgs boson. The dark Higgs boson spontaneously breaks the gauge symmetry associated with the dark photon, and gives the mass to the dark photon and the mass difference to dark particles. Such a dark Higgs boson can decay into the dark particles and hence can be another source of the dark particles at collider experiments. We analyze the sensitivity to decays of the excited state into the dark matter and charged particles at the FASER 2 experiment in fermion and scalar inelastic dark matter scenarios. We consider two mass spectra as illustrating examples in which the excited state can be produced only through the decay of dark Higgs boson. We show that unprobed parameter region can be explored in fermion dark matter scenario for the illustrating mass spectra.

Jinmian Li

Inelastic dark matter from dark Higgs boson decays at FASER

We study constraints on light and feebly interacting bosons having charged lepton flavor violating couplings from electron beam dump experiments. Scalar, pseudoscalar, vector, and dipole type interactions of the bosons are analyzed, and excluded regions from the searches for decays into electron-positron pairs are derived. It is found that parameter regions unconstrained by flavor violating decays of muon can be excluded using the results of the E137 experiment. We also discuss the impact of the search for flavor violating decays of the light bosons in electron beam dump experiments.