arXiv subjects

Eung Jin Chun

Publications and source records attributed to Eung Jin Chun.

At least 55 records · Page 3Linked to original sources

arXiv2018

Reconstructing heavy Higgs boson masses in Type X Two-Higgs Doublet Model with a light pseudoscalar

We analyze the prospects of reconstructing the mass of a heavy charged Higgs boson %have been analyzed in the context of a Type X two-Higgs doublet model where a light pseudoscalar $A$ in the mass range $40-60$ GeV is phenomenologically allowed, and is in fact favoured if one wants to explain the muon anomalous magnetic moment. The associated production of charged Higgs with the pseudoscalar $A$ and subsequent decay of the charged Higgs into a $W$ and $A$, is found to be our relevant channel. The branching ratio for $H^+ \to W^+ A$ with $M_{H^+} \sim 200$ GeV, is close to 50\%. The hadronic decay of the $W$ boson, coupled with the leptonic decays of $A$ into a tau and muon pair, help in identifying the charged Higgs. The neutral heavy Higgs, being degenerate with the charged Higgs for most of the allowed parameter space of the model, also contributes to similar final states. Thus both of these particles are reconstructed within a band of about 10 GeV.

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arXiv2018

Scalar Dark Matter in Leptophilic Two-Higgs-Doublet Model

Two-Higgs-Doublet Model of Type-X in the large $\tanβ$ limit becomes leptophilic to allow a light pseudo-scalar $A$ and thus provides an explanation of the muon $g-2$ anomaly. Introducing a singlet scalar dark matter $S$ in this context, one finds that two important dark matter properties, nucleonic scattering and self-annihilation, are featured separately by individual couplings of dark matter to the two Higgs doublets. While one of the two couplings is strongly constrained by direct detection experiments, the other remains free to be adjusted for the relic density mainly through the process $SS\to AA$. This leads to the $4τ$ final states which can be probed by galactic gamma ray detections.

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arXiv2018

Implications of right-handed neutrinos in $B-L$ extended standard model with scalar dark matter

We investigate the Standard Model (SM) with a $U(1)_{B-L}$ gauge extension where a $B-L$ charged scalar is a viable dark matter (DM) candidate. The dominant annihilation process, for the DM particle is through the $B-L$ symmetry breaking scalar to right-handed neutrino pair. We exploit the effect of decay and inverse decay of the right-handed neutrino in thermal relic abundance of the DM. Depending on the values of the decay rate, the DM relic density can be significantly different from what is obtained in the standard calculation assuming the right-handed neutrino is in thermal equilibrium and there appear different regions of the parameter space satisfying the observed DM relic density. For a DM mass less than $\mathcal{O}$(TeV), the direct detection experiments impose a competitive bound on the mass of the $U(1)_{B-L}$ gauge boson $Z^\prime$ with the collider experiments. Utilizing the non-observation of the displaced vertices arising from the right-handed neutrino decays, bound on the mass of $Z^\prime$ has been obtained at present and higher luminosities at the LHC with 14 TeV centre of mass energy where an integrated luminosity of 100fb$^{-1}$ is sufficient to probe $m_{Z'} \sim 5.5$ TeV.

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arXiv2017

Accidental Peccei-Quinn Symmetry from Discrete Flavour Symmetry and Pati-Salam

We show how an accidental $U(1)$ Peccei-Quinn (PQ) symmetry can arise from a discrete $A_4$ family symmetry combined with a discrete flavour symmetry $ \mathbb{Z}_3 \times \mathbb{Z}_5^2 $, in a realistic Pati-Salam unified theory of flavour. Imposing only these discrete flavour symmetries, the axion solution to the strong $ CP $ problem is protected from PQ-breaking operators to the required degree. A QCD axion arises from a linear combination of $ A_4 $ triplet flavons, which are also responsible for fermion flavour structures due to their vacuum alignments. We find that the requirement of an accidental PQ symmetry arising from a discrete flavour symmetry constrains the form of the Yukawa matrices, providing a link between flavour and the strong $ CP $ problem. Our model predicts specific flavour-violating couplings of the flavourful axion and thus puts a strong limit on the axion scale from kaon decays.

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arXiv2017

LSP baryogenesis and neutron-antineutron oscillations from R-parity violation

R-parity and baryon number violating operators can be allowed in the Supersymmetric Standard Model and thus lead to interesting baryon number violating processes such as neutron-antineutron oscillations and baryogenesis of the Universe via the decay of the lightest supersymmetric particle (LSP). Adopting the LSP baryogenesis mechanism realized by the late decay of the axino, we identify a single coupling lambda''_313 as a common origin for the matter-antimatter asymmetry of the Universe as well as potentially observable neutron-antineutron oscillation rates. From this, rather strong constraints on the supersymmetry breaking masses and the axion decay constant are obtained. The favoured parameter space of lambda''_313 ~ 0.1 and sub-TeV masses for the relevant sparticles is readily accessible by the current and future LHC searches.

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arXiv2017

Reconstructing a light pseudoscalar in the Type-X Two Higgs Doublet Model

We investigate the detectability as well as reconstructibility of a light pseudoscalar particle $A$, of mass in the 50 - 60 GeV range, which is still allowed in a Type-X (lepton-specific) two-Higgs doublet scenario. Such a pseudoscalar can be pair-produced in the decay $h\to AA$ of the 125 GeV scalar $h$. The light pseudoscalar in the aforementioned range, helpful in explaining the muon anomalous magnetic moment, has not only substantial branching ratio in the $τ^+τ^-$ channel but also one of about $0.35\%$ in the $μ^+μ^-$ final state. We show how to faithfully reconstruct the $A$ mass using the $μ^+μ^-$ mode, and establish the existence of a pseudoscalar around 50 - 60 GeV, using the process $pp\to h \to AA\to μ^+μ^-\,τ^+τ^-$. This is the most reliable way of reconstructing the light $A$ mass, and is possible for an integrated luminosity of about $100~\rm{fb}^{-1}$ or less at 14 TeV, with a statistical significance that amounts to discovery.

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arXiv2016

Leptonic Precision Test of Leptophilic Two-Higgs-Doublet Model

The type X (lepton-specific) two-Higgs-doublet model at large $\tanβ$ becomes leptophilic and thus allows a light pseudoscalar $A$ accommodating the observed muon $g-2$ deviation without conflicting with various hadronic constraints. On the other hand, it is strongly constrained by leptonic precision observables such as lepton universality test in the neutral and charged currents. Treating all the lepton universality data in a consistent way, we show how the current data constrain the parameter space of $m_A$ and $\tanβ$ for given degenerate masses of heavy Higgs bosons $H$ and $H^\pm$. While no overlapping region is found at $1σ$, a sizable region is still viable at $2σ$ for $H/H^\pm$ masses at around 200$\sim$400 GeV.

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arXiv2016

Very Degenerate Higgsino Dark Matter

We present a study of the Very Degenerate Higgsino Dark Matter (DM), whose mass splitting between the lightest neutral and charged components is ${\cal O}$(1) MeV, much smaller than radiative splitting of 355 MeV. The scenario is realized in the minimal supersymmetric standard model by small gaugino mixing. In contrast to the pure Higgsino DM with the radiative splitting only, various observable signatures with distinct features are induced. First of all, the very small mass splitting makes (a) sizable Sommerfeld enhancement and Ramsauer-Townsend (RT) suppression relevant to ~1 TeV Higgsino DM, and (b) Sommerfeld-Ramsauer-Townsend effect saturate at lower velocities $v/c \lesssim 10^{-3}$. As a result, annihilation signals can be large enough to be observed from the galactic center and/or dwarf galaxies, while relative signal sizes can vary depending on the location of Sommerfeld peaks and RT dips. In addition, at collider experiments, stable chargino signature can be searched for to probe the model in the future. DM direct detection signal, however, depends on the Wino mass; even no detectable signal can be induced if the Wino is heavier than about 10 TeV.

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arXiv2016

Di-Higgs signatures from R-parity violating supersymmetry as the origin of neutrino mass

Motivated by the naturalness and neutrino mass generation, we study a bilinear R-parity violating supersymmetric scenario with a light Higgsino-like lightest supersymmetric particle (LSP). We observe that the LSP dominantly decays to $νh$ in a large part of the parameter space, and thus study the pair production of electroweakinos followed by the decays $\tildeχ^\pm_1\to \tildeχ^0_1 W^{\pm*}$ and $\tildeχ^0_1\to νh$. This leads to an interesting signature of Higgs boson pair production associated with significantly large missing transverse energy which is grossly distinct from the di-Higgs production in the Standard Model. We investigate the perspective of probing such signatures by performing a realistic detector level simulation of both the signal and corresponding backgrounds for the high-luminosity high energy phase of the Large Hadron Collider (LHC). We also advocate some observables based on kinematical features to provide an excellent handle to suppress the backgrounds.

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arXiv2015

Minimal Models for Axion and Neutrino

The PQ mechanism resolving the strong CP problem and the seesaw mechanism explaining the smallness of neutrino masses may be related in a way that the PQ symmetry breaking scale and the seesaw scale arise from a common origin. Depending on how the PQ symmetry and the seesaw mechanism are realized, one has different predictions on the color and electromagnetic anomalies which could be tested in the future axion dark matter search experiments. Motivated by this, we construct various PQ seesaw models which are minimally extended from the (non-) supersymmetric Standard Model and thus set up different benchmark points on the axion-photon-photon coupling in comparison with the standard KSVZ and DFSZ models.

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arXiv2015

LHC $τ$-rich Tests of Lepton-specific 2HDM for $(g-2)_μ$

The lepton-sepcific (or type X) 2HDM (L2HDM) is an attractive new physics candidate explaining the muon $g-2$ anomaly requiring a light CP-odd boson $A$ and large $\tanβ$. This scenario leads to $τ$-rich signatures, such as $3τ$, $4τ$ and $4τ+W/Z$, which can be readily accessible at the LHC. We first study the whole L2HDM parameter space to identify allowed regions of extra Higgs boson masses as well as two couplings $λ_{hAA}$ and $ξ_h^l$ which determine the 125 GeV Higgs boson decays $h\to τ^+τ^-$ and $h\to AA/AA^*(τ^+τ^-)$, respectively. This motivates us to set up two regions of interest: (A) $m_A \ll m_{H} \sim m_{H^\pm}$, and (B) $m_A \sim m_{H^\pm} \sim {\cal O}(100) \mbox{GeV} \ll m_H$, for which derive the current constraints by adopting the chargino-neutralino search at the LHC8, and then analyze the LHC14 prospects by implementing $τ$-tagging algorithm. A correlated study of the upcoming precision determination of the 125 GeV Higgs boson decay properties as well as the observation of multi-tau events at the next runs of LHC will be able to shed light on the L2HDM option for the muon $g-2$.

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arXiv2015

Muon g$-$2 in two-Higgs-doublet models

Updating various theoretical and experimental constraints on the four different types of two-Higgs-doublet models (2HDMs), we find that only the ``lepton-specific" (or ``type X") 2HDM can explain the present muon g$-$2 anomaly in the parameter region of large $\tanβ$, a light CP-odd boson, and heavier CP-even and charged bosons which are almost degenerate. The severe constraints on the models come mainly from the consideration of vacuum stability and perturbativity, the electroweak precision data, the $b$-quark observables like $B_S \to μμ$, the precision measurements of the lepton universality as well as the 125 GeV boson property observed at the LHC.

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arXiv2015

Lepton flavour violating signature in supersymmetric $U(1)^\prime$ seesaw models at the LHC

We consider a $U(1)'$ supersymmetric seesaw model in which a right-handed sneutrino is a thermal dark matter candidate whose relic density can be in the right range due to its coupling to relatively light $\tilde{Z'}$, the superpartner of the extra gauge boson $Z'$. Such light $\tilde Z'$ can be produced at the LHC through cascade decays of colored superparticles, in particular, stops and sbottoms, and then decay to a right-handed neutrino and a sneutrino dark matter, which leads to lepton flavor violating signals of same/opposite-sign dileptons (or multileptons) accompanied by large missing energy. Taking some benchmark points, we analyze the opposite- and same-sign dilepton signatures and the corresponding flavour difference i.e., ($2e-2μ$). It is shown that $5σ$ signal significance can be reached for some benchmark points with very early data of $\sim 2$ fb$^{-1}$ integrated luminosity. In addition, $3\ell$ and $4\ell$ signatures also look promising to check the consistency in the model prediction, and it is possible to reconstruct the $\tilde{Z'}$ mass from $jj\ell$ invariant mass distribution.

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arXiv2015

Electro-Weak Dark Matter: non-perturbative effect confronting indirect detections

We update indirect constraints on Electro-Weak Dark Matter (EWDM) considering the Sommerfeld-Ramsauer-Townsend (SRT) effect for its annihilations into a pair of standard model gauge bosons assuming that EWDM accounts for the observed dark matter (DM) relic density for a given DM mass and mass gaps among the multiplet components. For the radiative or smaller mass splitting, the hypercharged triplet and higher multiplet EWDMs are ruled out up to the DM mass ~ 10 - 20 TeV by the combination of the most recent data from AMS-02 (antiproton), Fermi-LAT (gamma-ray), and HESS (gamma-line). The Majorana triplet (wino-like) EWDM can evade all the indirect constraints only around Ramsauer-Townsend dips which can occur for a tiny mass splitting of order 10 MeV or less. In the case of the doublet (Higgsino-like) EWDM, a wide range of its mass > 500 GeV is allowed except Sommerfeld peak regions. Such a stringent limit on the triplet DM can be evaded by employing a larger mass gap of the order of 10 GeV which allows its mass larger than about 1 TeV. However, the future CTA experiment will be able to cover most of the unconstrained parameter space.

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arXiv2015

Diluting the inflationary axion fluctuation by a stronger QCD in the early Universe

We propose a new mechanism to suppress the axion isocurvature perturbation, while producing the right amount of axion dark matter, within the framework of supersymmetric axion models with the axion scale induced by supersymmetry breaking. The mechanism involves an intermediate phase transition to generate the Higgs μ-parameter, before which the weak scale is comparable to the axion scale and the resulting stronger QCD yields an axion mass heavier than the Hubble scale over a certain period. Combined with that the Hubble-induced axion scale during the primordial inflation is well above the intermediate axion scale at present, the stronger QCD in the early Universe suppresses the axion fluctuation to be small enough even when the inflationary Hubble scale saturates the current upper bound, while generating an axion misalignment angle of order unity.

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arXiv2015

Limiting two-Higgs-doublet models

Updating various theoretical and experimental constraints on the four different types of two-Higgs-doublet models (2HDMs), we find that only the "lepton-specific" (or "type X") 2HDM can explain the present muon (g-2) anomaly in the parameter region of large $\tanβ$, a light CP-odd Higgs boson, and heavier CP-even and charged Higgs bosons which are almost degenerate. The severe constraints on the models come mainly from the consideration of vacuum stability and perturbativity, the electroweak precision data, $B$ physics observables like $b\to s γ$ as well as the 125 GeV Higgs boson properties measured at the LHC.

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arXiv2015

Mixed axion/gravitino dark matter from SUSY models with heavy axinos

We examine dark matter production rates in supersymmetric axion models typified by the mass hierarchy m_gravitino << m_neutralino << m_axino. In such models, one expects the dark matter to be composed of an axion/gravitino admixture. After presenting motivation for how such a mass hierarchy might arise, we examine dark matter production in the SUSY Kim-Shifman-Vainshtein-Zakharov (KSVZ) model, the SUSY Dine-Fischler-Srednicki-Zhitnitsky (DFSZ) model and a hybrid model containing contributions from both KSVZ and DFSZ. Gravitinos can be produced thermally and also non-thermally from axino, saxion or neutralino decay. We obtain upper bounds on T_R due to overproduction of gravitinos including both the thermal and non-thermal processes. For T_R near the upper bound, then dark matter tends to be gravitino dominated, but for T_R well below the upper bounds, then axion domination is more typical although in many cases we find a comparable mixture of both axions and gravitinos. In this class of models, we ultimately expect detection of relic axions but no WIMP signal, although SUSY should ultimately be discovered at colliders.

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arXiv2015

Mixed axion/neutralino dark matter in the SUSY DFSZ axion model

We examine mixed axion/neutralino cold dark matter production in the SUSY DFSZ axion model where an axion superfield couples to Higgs superfields. We calculate a wide array of axino and saxion decay modes along with their decay temperatures, and thermal and non-thermal production rates. For a SUSY benchmark model with a standard underabundance (SUA) of Higgsino-like dark matter (DM), we find for the PQ scale f_a< 10^{12} GeV that the DM abundance is mainly comprised of axions as the saxion/axino decay occurs before the standard neutralino freeze-out and thus its abundance remains suppressed. For 10^{12}<~ f_a<~ 10^{14} GeV, the saxion/axino decays occur after neutralino freeze-out so that the neutralino abundance is enhanced by the production via decay and subsequent re-annihilation. For f_a>~ 10^{14} GeV, both neutralino dark matter and dark radiation are typically overproduced. For judicious parameter choices, these can be suppressed and the combined neutralino/axion abundance brought into accord with measured values. A SUSY benchmark model with a standard overabundance (SOA) of bino DM is also examined and typically remains excluded due at least to too great a neutralino DM abundance for f_a<~ 10^{15} GeV. For f_a>~ 10^{15} GeV and lower saxion masses, large entropy production from saxion decay can dilute all relics and the SOA model can be allowed by all constraints.

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