We study two general <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>U</mi><mo stretchy="false">(</mo><mn>1</mn><mo stretchy="false">)</mo></math> extensions of the Standard Model (SM) those generate tiny neutrino masses via the seesaw mechanism after general <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>U</mi><mo stretchy="false">(</mo><mn>1</mn><mo stretchy="false">)</mo></math> breaking. These models predict a new neutral gauge boson (<math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><msup><mi>Z</mi><mo>′</mo></msup></math>) and right-handed neutrinos (RHNs), the latter introduced for anomaly cancellation and neutrino mass generation. In both scenarios, left- and right-handed fermions couple differently to the <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><msup><mi>Z</mi><mo>′</mo></msup></math>, and RHNs mix with light neutrinos, enabling variety of decay modes. Focusing on the high-luminosity LHC (HL-LHC) and the future FASER2 experiment, we explore RHN pair production from <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><msup><mi>Z</mi><mo>′</mo></msup></math> decays in two cases; (i) long-lived <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><msup><mi>Z</mi><mo>′</mo></msup></math> decays to visible modes and long-lived RHNs, and (ii) short-lived <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><msup><mi>Z</mi><mo>′</mo></msup></math> decays to long-lived RHNs, which further decay visibly inside FASER2. We estimate projected limits on the general <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>U</mi><mo stretchy="false">(</mo><mn>1</mn><mo stretchy="false">)</mo></math> gauge coupling, <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><msup><mi>Z</mi><mo>′</mo></msup></math> mass, RHN mass, and light-heavy neutrino mixing for various <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>U</mi><mo stretchy="false">(</mo><mn>1</mn><mo stretchy="false">)</mo></math> charge assignments, and compare them with current experimental bounds.

Japan

 ShivaSankar K. A.

United States

India

Souvik Das

Arindam Das

South Korea

Sanjoy Mandal

Right-handed neutrino production from <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><msup><mi>Z</mi><mo>′</mo></msup></math> interactions in forward search experiments

Within the general <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>U</mi><mo stretchy="false">(</mo><mn>1</mn><mo stretchy="false">)</mo></math> scenario, we demonstrate that the ultrahigh-energy neutrinos recently detected by KM3NeT could originate from a decaying right-handed neutrino dark matter, with a mass of 440 PeV. Considering dark matter production via freeze-in, we delineate the parameter space that satisfies the observed relic abundance and also lies within the reach of multiple gravitational wave detectors. Our study provides a testable new physics scenario, enabled by multimessenger astronomy.

Basabendu Barman

China

Prantik Sarmah

What KM3-230213A event may tell us about the neutrino mass and dark matter

In the minimal gauged <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mrow><mi mathvariant="normal">B</mi><mo stretchy="false">−</mo><mi mathvariant="normal">L</mi></mrow></math> extension of the Standard Model, we demonstrate that PeV-scale dark matter (DM) and the baryon asymmetry of the Universe (BAU) can be simultaneously explained through the three right-handed neutrinos (RHNs) present in the theory. The DM candidate undergoes rare decay into light neutrinos, providing an explanation for the observed IceCube events, while the other two RHNs generate the BAU via leptogenesis. The breaking of gauge symmetry gives rise to detectable gravitational waves (GWs) from decaying cosmic strings (CS), making this framework testable at several future GW detectors—despite being beyond the reach of conventional collider experiments due to the extremely weak gauge coupling. The symmetry-breaking scale establishes a connection between particle masses, couplings, and the GW spectrum, offering a unified and predictive scenario.

Suruj Jyoti Das

Sweden

Marco Merchand

Hunting for heavy <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><msup><mi>Z</mi><mo>′</mo></msup></math> with IceCube neutrinos and gravitational waves

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.

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>

The origin of tiny neutrino mass is a long standing unsolved puzzle of the Standard Model (SM), which allows us to consider scenarios beyond the Standard Model (BSM) in a variety of ways. One of them being a gauge extension of the SM that may be realized as in the form of an anomaly free, general <math altimg='si1.svg'><mi>U</mi><msub><mrow><mo stretchy='false'>(</mo><mn>1</mn><mo stretchy='false'>)</mo></mrow><mrow><mi>X</mi></mrow></msub></math> extension of the SM, where an <math altimg='si2.svg'><mi>S</mi><mi>U</mi><msub><mrow><mo stretchy='false'>(</mo><mn>2</mn><mo stretchy='false'>)</mo></mrow><mrow><mi>L</mi></mrow></msub></math> triplet scalar with a <math altimg='si1.svg'><mi>U</mi><msub><mrow><mo stretchy='false'>(</mo><mn>1</mn><mo stretchy='false'>)</mo></mrow><mrow><mi>X</mi></mrow></msub></math> charge is introduced to have Dirac Yukawa couplings with the SM lepton doublets. Once the triplet scalar develops a Vacuum Expectation Value (VEV), light neutrinos acquire their tiny Majorana masses. Hence, the decay modes of the triplet scalar have direct connection to the neutrino oscillation data for different neutrino mass hierarchies. After the breaking of the <math altimg='si1.svg'><mi>U</mi><msub><mrow><mo stretchy='false'>(</mo><mn>1</mn><mo stretchy='false'>)</mo></mrow><mrow><mi>X</mi></mrow></msub></math> gauge symmetry, a neutral <math altimg='si1.svg'><mi>U</mi><msub><mrow><mo stretchy='false'>(</mo><mn>1</mn><mo stretchy='false'>)</mo></mrow><mrow><mi>X</mi></mrow></msub></math> gauge boson <math altimg='si3.svg'><mo stretchy='false'>(</mo><msup><mrow><mi>Z</mi></mrow><mrow><mo>&#8242;</mo></mrow></msup><mo stretchy='false'>)</mo></math> acquires mass, which interacts differently with the left and right handed SM fermions. Satisfying the recent LHC bounds on the triplet scalar and <math altimg='si10.svg'><msup><mrow><mi>Z</mi></mrow><mrow><mo>&#8242;</mo></mrow></msup></math> boson productions, we study the pair production of the triplet scalar at LHC, 100 TeV proton proton collider FCC, <math altimg='si5.svg'><msup><mrow><mi>e</mi></mrow><mrow><mo linebreak='badbreak' linebreakstyle='after'>&#8722;</mo></mrow></msup><msup><mrow><mi>e</mi></mrow><mrow><mo linebreak='badbreak' linebreakstyle='after'>+</mo></mrow></msup></math> and <math altimg='si6.svg'><msup><mrow><mi>&#956;</mi></mrow><mrow><mo linebreak='badbreak' linebreakstyle='after'>&#8722;</mo></mrow></msup><msup><mrow><mi>&#956;</mi></mrow><mrow><mo linebreak='badbreak' linebreakstyle='after'>+</mo></mrow></msup></math> colliders followed by its decay into dominant dilepton modes whose flavor structure depend on the neutrino mass hierarchy. Generating the SM backgrounds, we study the possible signal significance of four lepton final states from the triplet scalar pair production. We also compare our results with the purely SM gauge mediated triplet scalar pair production followed by four lepton final states, which could be significant only in <math altimg='si6.svg'><msup><mrow><mi>&#956;</mi></mrow><mrow><mo linebreak='badbreak' linebreakstyle='after'>&#8722;</mo></mrow></msup><msup><mrow><mi>&#956;</mi></mrow><mrow><mo linebreak='badbreak' linebreakstyle='after'>+</mo></mrow></msup></math> collider.

Puja Das

Nobuchika Okada

Testing neutrino mass hierarchy under type-II seesaw scenario in <italic>U</italic>(1) <inf><italic>X</italic></inf> from colliders

We investigate a general <math altimg='si1.svg'><mi>U</mi><msub><mrow><mo stretchy='false'>(</mo><mn>1</mn><mo stretchy='false'>)</mo></mrow><mrow><mi>X</mi></mrow></msub></math> scenario where we introduce three generations of Standard Model (SM) singlet Right Handed Neutrinos (RHNs) to generate the light neutrino mass through the seesaw mechanism after the breaking of <math altimg='si1.svg'><mi>U</mi><msub><mrow><mo stretchy='false'>(</mo><mn>1</mn><mo stretchy='false'>)</mo></mrow><mrow><mi>X</mi></mrow></msub></math> and electroweak symmetries. In addition to that, a general <math altimg='si1.svg'><mi>U</mi><msub><mrow><mo stretchy='false'>(</mo><mn>1</mn><mo stretchy='false'>)</mo></mrow><mrow><mi>X</mi></mrow></msub></math> scenario involves an SM-singlet scalar field and due to the <math altimg='si1.svg'><mi>U</mi><msub><mrow><mo stretchy='false'>(</mo><mn>1</mn><mo stretchy='false'>)</mo></mrow><mrow><mi>X</mi></mrow></msub></math> symmetry breaking the mass of a neutral beyond the SM (BSM) gauge boson <math altimg='si14.svg'><msup><mrow><mi>Z</mi></mrow><mrow><mo>&#8242;</mo></mrow></msup></math> is evolved. The RHNs, charged under the <math altimg='si1.svg'><mi>U</mi><msub><mrow><mo stretchy='false'>(</mo><mn>1</mn><mo stretchy='false'>)</mo></mrow><mrow><mi>X</mi></mrow></msub></math> scenario, can explain the origin of observed baryon asymmetry through the resonant leptogenesis process. Applying observed neutrino oscillation data we study <math altimg='si14.svg'><msup><mrow><mi>Z</mi></mrow><mrow><mo>&#8242;</mo></mrow></msup></math> and BSM scalar-induced processes to reproduce the observed baryon asymmetry. Hence we estimate bounds on the <math altimg='si1.svg'><mi>U</mi><msub><mrow><mo stretchy='false'>(</mo><mn>1</mn><mo stretchy='false'>)</mo></mrow><mrow><mi>X</mi></mrow></msub></math> gauge coupling <math altimg='si3.svg'><mo stretchy='false'>(</mo><msub><mrow><mi>g</mi></mrow><mrow><mi>X</mi></mrow></msub><mo stretchy='false'>)</mo></math> and the mass of the <math altimg='si14.svg'><msup><mrow><mi>Z</mi></mrow><mrow><mo>&#8242;</mo></mrow></msup></math> <math altimg='si4.svg'><mo stretchy='false'>(</mo><msub><mrow><mi>M</mi></mrow><mrow><msup><mrow><mi>Z</mi></mrow><mrow><mo>&#8242;</mo></mrow></msup></mrow></msub><mo stretchy='false'>)</mo></math> for different <math altimg='si1.svg'><mi>U</mi><msub><mrow><mo stretchy='false'>(</mo><mn>1</mn><mo stretchy='false'>)</mo></mrow><mrow><mi>X</mi></mrow></msub></math> charges and benchmark masses of the RHN and SM-singlet scalar. Finally, we compare our results with limits obtained from the existing limits from LEP-II and LHC. We find that depending on the <math altimg='si1.svg'><mi>U</mi><msub><mrow><mo stretchy='false'>(</mo><mn>1</mn><mo stretchy='false'>)</mo></mrow><mrow><mi>X</mi></mrow></msub></math> charges, the masses of RHNs and SM-singlet scalar resonant leptogenesis could provide a stronger limit on <math altimg='si5.svg'><msub><mrow><mi>g</mi></mrow><mrow><mi>X</mi></mrow></msub></math> for <math altimg='si210.svg'><msub><mrow><mi>M</mi></mrow><mrow><msup><mrow><mi>Z</mi></mrow><mrow><mo>&#8242;</mo></mrow></msup></mrow></msub><mo linebreak='goodbreak' linebreakstyle='after'>&gt;</mo><mn>5.8</mn></math> TeV which could be probed by high energy scattering experiment in future.

Czechia

Yuta Orikasa

Resonant leptogenesis in minimal <italic>U</italic>(1) <inf><italic>X</italic></inf> extensions of the Standard Model

The origin of tiny neutrino mass is an unsolved puzzle leading to a variety of phenomenological aspects beyond the Standard Model (BSM). We consider U(1) gauge extension of the Standard Model (SM) where so-called seesaw mechanism is incarnated with the help of thee generations of Majorana type right-handed neutrinos followed by the breaking of U(1) and electroweak gauge symmetries providing anomaly free structure. In this framework, a neutral BSM gauge boson $$Z^\prime $$ <math> <msup> <mi>Z</mi> <mo>′</mo> </msup> </math> is evolved. To explore the properties of its interactions we consider chiral (flavored) frameworks where $$Z^\prime $$ <math> <msup> <mi>Z</mi> <mo>′</mo> </msup> </math> interactions depend on the handedness (generations) of the fermions. In this paper we focus on $$Z^\prime $$ <math> <msup> <mi>Z</mi> <mo>′</mo> </msup> </math> -neutrino interactions which could be probed from cosmic explosions. We consider $$\nu \overline{\nu } \rightarrow e^+ e^-$$ <math> <mrow> <mi>ν</mi> <mover> <mi>ν</mi> <mo>¯</mo> </mover> <mo>→</mo> <msup> <mi>e</mi> <mo>+</mo> </msup> <msup> <mi>e</mi> <mo>-</mo> </msup> </mrow> </math> process which can energize gamma-ray burst (GRB221009A, so far the highest energy) through energy deposition. Hence estimating these rates we constrain U(1) gauge coupling $$(g_X)$$ <math> <mrow> <mo>(</mo> <msub> <mi>g</mi> <mi>X</mi> </msub> <mo>)</mo> </mrow> </math> and $$Z^\prime $$ <math> <msup> <mi>Z</mi> <mo>′</mo> </msup> </math> mass $$(M_{Z^\prime })$$ <math> <mrow> <mo>(</mo> <msub> <mi>M</mi> <msup> <mi>Z</mi> <mo>′</mo> </msup> </msub> <mo>)</mo> </mrow> </math> under Schwarzchild (Sc) and Hartle-Thorne (HT) scenarios. We also study $$\nu $$ <math> <mi>ν</mi> </math> -DM scattering through $$Z^\prime $$ <math> <msup> <mi>Z</mi> <mo>′</mo> </msup> </math> to constrain $$g_X-M_{Z^\prime }$$ <math> <mrow> <msub> <mi>g</mi> <mi>X</mi> </msub> <mo>-</mo> <msub> <mi>M</mi> <msup> <mi>Z</mi> <mo>′</mo> </msup> </msub> </mrow> </math> plane using IceCube data considering high energy neutrinos from cosmic blazar (TXS0506+056), active galaxy (NGC1068), the Cosmic Microwave Background (CMB) and the Lyman- $$\alpha $$ <math> <mi>α</mi> </math> data, respectively. Finally highlighting complementarity we compare our results with current and prospective bounds on $$g_X-M_{Z^\prime }$$ <math> <mrow> <msub> <mi>g</mi> <mi>X</mi> </msub> <mo>-</mo> <msub> <mi>M</mi> <msup> <mi>Z</mi> <mo>′</mo> </msup> </msub> </mrow> </math> plane from scattering, beam-dump and $$g-2$$ <math> <mrow> <mi>g</mi> <mo>-</mo> <mn>2</mn> </mrow> </math> experiments. [ PICS code ].

K. ShivaSankar

Italy

Gaetano Lambiase

Probing chiral and flavored $$Z^\prime $$ <math> <msup> <mi>Z</mi> <mo>′</mo> </msup> </math> from cosmic bursts through neutrino interactions

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.

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

We discuss the Higgs phenomenology in models with a new <math><mi>U</mi><msub><mrow><mo>(</mo><mn>1</mn><mo>)</mo></mrow><mrow><mi>X</mi></mrow></msub></math> gauge symmetry including the <math><mi>U</mi><msub><mrow><mo>(</mo><mn>1</mn><mo>)</mo></mrow><mrow><mi>B</mi><mo>−</mo><mi>L</mi></mrow></msub></math> scenario, where three right-handed neutrinos are inevitably introduced due to the gauge anomaly cancellations. We find that the decay branching ratio of the discovered Higgs boson into a pair of new massive gauge bosons (<math><msup><mrow><mi>Z</mi></mrow><mrow><mo>′</mo></mrow></msup></math>) can significantly be enhanced in the Dirac neutrino case as compared with the Majorana case for a fixed value of the new gauge coupling and the mass of <math><msup><mrow><mi>Z</mi></mrow><mrow><mo>′</mo></mrow></msup></math> under constraints from current experimental data. Because of such an enhancement, the Dirac case can indirectly be discriminated from the Majorana case via the Higgs decay.

K.A. ShivaSankar

Kei Yagyu

Enhancement of the Higgs decay into a ′ Pair in Models With (1) Gauge Symmetry

We consider a general, anomaly free U<math><msup><mrow><mo>(</mo><mn>1</mn><mo>)</mo></mrow><mrow><mo>′</mo></mrow></msup></math> extension of the Standard Model (SM) where the neutrino mass is generated at the tree level via the inverse seesaw mechanism. The model contains three right handed neutrinos, three additional singlet fermions, one extra complex scalar and a neutral gauge boson <math><mo>(</mo><msup><mrow><mi>Z</mi></mrow><mrow><mo>′</mo></mrow></msup><mo>)</mo></math>. Instead of resorting to a specific <math><mi>U</mi><mo>(</mo><mn>1</mn><mo>)</mo></math> extension, we consider a class of models by taking the <math><mi>U</mi><msup><mrow><mo>(</mo><mn>1</mn><mo>)</mo></mrow><mrow><mo>′</mo></mrow></msup></math> charges of the scalars to be free parameters. Here, we assign one pair of the pseudo-Dirac degenerate sterile neutrinos as Dark Matter (DM) candidates which are produced by the freeze-in mechanism. Considering different mass regimes of the DM, <math><msup><mrow><mi>Z</mi></mrow><mrow><mo>′</mo></mrow></msup></math> and reheating temperature, we obtain constraints on the <math><mi>U</mi><msup><mrow><mo>(</mo><mn>1</mn><mo>)</mo></mrow><mrow><mo>′</mo></mrow></msup></math> charges giving the correct relic abundance. We have also obtained constraints on <math><msup><mrow><mi>Z</mi></mrow><mrow><mo>′</mo></mrow></msup></math> mass and coupling from consideration of relic density as well as high energy collider experiments like ATLAS in case of heavy <math><msup><mrow><mi>Z</mi></mrow><mrow><mo>′</mo></mrow></msup></math> or in intensity and lifetime frontier experiments like DUNE, FASERs, and ILC beam dump which are looking for light <math><msup><mrow><mi>Z</mi></mrow><mrow><mo>′</mo></mrow></msup></math>. Additionally, in this model, the decay of pseudo-Dirac DM into active neutrinos can explain the 511 keV line observed by the INTEGRAL satellite.

Srubabati Goswami

Chile

K.N. Vishnudath

Tanmay Kumar Poddar

Freeze-in production of sterile neutrino dark matter in a gauged U(1)′ model with inverse seesaw