We consider a concise dark matter scenario in the minimal gauged B − L extension of the Standard Model (SM), where the global B −L (baryon number minus lepton number) symmetry in the SM is gauged, and three generations of right-handed neutrinos and a B −L Higgs field are introduced. Associated with the B − L gauge symmetry breaking by a VEV of the B − L Higgs field, the seesaw mechanism for generating the neutrino mass is automatically implemented after the electroweak symmetry breaking in the SM. In this model context, we introduce a Z 2 -parity and assign an odd parity for one right-handed neutrino while even parities for the other fields. Therefore, the dark matter candidate is identified as the right-handed Majorana neutrino with odd Z 2 parity, keeping the minimality of the particle content intact. When the dark matter particle communicates with the SM particles mainly through the B − L gauge boson (Z ′ BL boson), its relic abundance is determined by only three free parameters, the B − L gauge coupling (α BL ), the Z ′ BL boson mass (m Z ′ ) and the dark matter mass (m DM ). With the cosmological upper bound on the dark matter relic abundance we find a lower bound on α BL as a function of m Z ′ . On the other hand, we interpret the recent LHC Run-2 results on search for Z ′ boson resonance to an upper bound on α BL as a function of m Z ′ . Combining the two results we identify an allowed parameter region for this "Z ′ BL portal" dark matter scenario, which turns out to be a narrow window with the lower mass bound of m Z ′ > 2.5 TeV.

We consider a concise dark matter (DM) scenario in the context of a non-exotic U(1) extension of the Standard Model (SM), where a new U(1) X gauge symmetry is introduced along with three generation of right-handed neutrinos (RHNs) and an SM gauge singlet Higgs field. The model is a generalization of the minimal gauged U(1) B−L (baryon number minus lepton number) extension of the SM, in which the extra U(1) X gauge symmetry is expressed as a linear combination of the SM U(1) Y and U(1) B−L gauge symmetries. We introduce a Z 2-parity and assign an odd-parity only for one RHN among all particles, so that this Z 2-odd RHN plays a role of DM. The so-called minimal seesaw mechanism is implemented in this model with only two Z 2-even RHNs. In this context, we investigate physics of the RHN DM, focusing on the case that this DM particle communicates with the SM particles through the U(1) X gauge boson (Z ′ boson). This "Z ′-portal RHN DM" scenario is controlled by only three free parameters: the U(1) X gauge coupling (α X), the Z ′ boson mass (m Z ′), and the U(1) X charge of the SM Higgs doublet (x H). We consider various phenomenological constraints to identify a phenomenologically viable parameter space. The most important constraints are the observed DM relic abundance and the latest LHC Run-2

Inflection-point inflation is an interesting possibility to realize a successful slow-roll inflation when inflation is driven by a single scalar field with its value during inflation below the Planck mass (φ I M P l ). In order for a renormalization group (RG) improved effective λφ 4 potential to develop an inflection-point, the running quartic coupling λ(φ) must exhibit a minimum with an almost vanishing value in its RG evolution, namely λ(φ I ) ≃ 0 and β λ (φ I ) ≃ 0, where β λ is the beta-function of the quartic coupling. In this paper, we consider the inflection-point inflation in the context of the minimal U(1) X extended Standard Model (SM), a generalization of the minimal U(1) B−L model, where the U(1) X symmetry is realized as a linear combination of the SM U(1) Y and the U(1) B−L gauge symmetries. We identify the U(1) X Higgs field with the inflaton field. For a successful inflection-point inflation to be consistent with the current cosmological observations, the mass ratios among the U(1) X gauge boson, the right-handed neutrinos and the U(1) X Higgs boson are fixed. Focusing on the case that the U(1) X gauge symmetry is mostly oriented towards the SM U(1) Y direction, we investigate a consistency between the inflationary predictions and the latest LHC Run-2 results on the search for a narrow resonance with the di-lepton final state. In addition, the inflection-point inflation provides a unique prediction for the running of the spectral index α ≃ −2.7 × 10 −3 60 N 2 (N is the e-folding number), which can be tested in the near future.

We propose a grand unified SU(5)×U(1) X model, where the standard SU(5) grand unified theory is supplemented by minimal seesaw and a right-handed neutrino dark matter with an introduction of a global Z 2 -parity. In the presence of three right-handed neutrinos (RHNs), the model is free from all gauge and mixed-gravitational anomalies. The SU(5) symmetry is broken into the Standard Model (SM) gauge group at M GUT ≃ 4 × 10 16 GeV in the standard manner, while the U(1) X symmetry breaking occurs at the TeV scale, which generates the TeV-scale mass of the U(1) X gauge boson (Z ′ boson) and the three Majorana RHNs. A unique Z 2 -odd RHN is stable and serves as the dark matter (DM) in the present Universe, while the remaining two RHNs work to generate the SM neutrino masses through the minimal seesaw. We investigate the Z ′ -portal RHN DM scenario in this model context. We find that the constraints from the DM relic abundance, and the Z ′ boson search at the Large Hadron Collider (LHC), and the perturbativity bound on the U(1) X gauge coupling are complementary to narrow down the allowed parameter region in the range of 3.0 ≤ m Z ′ [TeV] ≤ 9.2 for the Z ′ boson mass. The allowed region for m Z ′ ≤ 5 TeV will be fully covered by the future LHC experiments. We also briefly discuss the successful implementation of Baryogenesis and cosmological inflation scenarios in the present model.

We consider a dark matter scenario in the context of the minimal extension of the Standard Model (SM) with a − (baryon number minus lepton number) gauge symmetry, where three right-handed neutrinos with a − charge −1 and a − Higgs field with a − charge +2 are introduced to make the model anomaly-free and to break the − gauge symmetry, respectively. The − gauge symmetry breaking generates Majorana masses for the right-handed neutrinos. We introduce a Z 2 symmetry to the model and assign an odd parity only for one right-handed neutrino, and hence the Z 2 -odd right-handed neutrino is stable and the unique dark matter candidate in the model. The so-called minimal seesaw works with the other two right-handed neutrinos and reproduces the current neutrino oscillation data. We consider the case that the dark matter particle communicates with the SM particles through the − gauge boson ( 耠 퐵−퐿 boson) and obtain a lower bound on the − gauge coupling ( 퐵−퐿 ) as a function of the 耠 퐵−퐿 boson mass ( 푍 ) from the observed dark matter relic density. On the other hand, we interpret the recent LHC Run-2 results on the search for a 耠 boson resonance to an upper bound on 퐵−퐿 as a function of 푍 . These two constraints are complementary for narrowing down an allowed parameter region for this "耠 portal" dark matter scenario, leading to a lower mass bound of 푍 ≥ 3.9 TeV.

In the light of the Planck 2015 results, we update simple inflationary models based on the quadratic, quartic, Higgs and Coleman-Weinberg potentials in the context of the Randall-Sundrum brane-world cosmology. Brane-world cosmological effect alters the inflationary predictions of the spectral index (ns) and the tensor-to-scalar ratio (r) from those obtained in the standard cosmology. In particular, the tensor-to-scalar ratio is enhanced in the presence of the 5th dimension. In order to maintain the consistency with the Planck 2015 results for the inflationary predictions in the standard cosmology, we find a lower bound on the five-dimensional Planck mass (M 5 ). On the other hand, the inflationary predictions laying outside of the Planck allowed region can be pushed into the allowed region by the brane-world cosmological effect with a suitable choice of M 5 .

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We consider a non-exotic gauged U(1) X extension of the Standard Model (SM), where the U(1) X charge of a SM field is given by a linear combination of its hypercharge and Baryonminus-Lepton (B − L) number. All the gauge and mixed gauge-gravitational anomalies are canceled in this model with the introduction of three right-handed neutrinos (RHNs). Unlike the conventional minimal U(1) X model, where a universal U(1) X charge of −1 is assigned to three RHNs, we consider an alternative charge assignment, namely, two RHNs (N 1,2 R ) have U(1) X charge −4 while one RHN (N R ) has a +5 charge. With a minimal extension of the Higgs sector, the three RHNs acquire their Majorana masses associated with U(1) X symmetry breaking. While N 1,2 R have Yukawa coupling with the SM lepton doublets and play an essential role for the "minimal seesaw" mechanism, N R is isolated from the SM particles due to its U(1) X charge and hence it is a natural candidate for the dark matter (DM) without invoking additional symmetries. In this model context, we investigate the Z ′ -portal RHN DM scenario, where the RHN DM communicates with the SM particles through the U(1) X gauge boson (Z ′ boson). We identify a narrow parameter space by combining the constraints from the observed DM relic abundance, the results of the search for a Z ′ boson resonance at the Large Hadron Collider Run-2, and the gauge coupling perturbativity up to the Planck/Grand Unification scale. A special choice of U(1) X charges for the SM fields allows us to extend the model to SU(5)×U(1) X grand unification. In this scenario, the model parameter space is more severely constrained, which will be explored at future high energy collider experiments.