arXiv subjects

Mu-Chun Chen

Publications and source records attributed to Mu-Chun Chen.

At least 37 records · Page 2Linked to original sources

arXiv2019

A note on the predictions of models with modular flavor symmetries

Models with modular flavor symmetries have been thought to be highly predictive. We point out that these predictions are subject to corrections from non-holomorphic terms in the Lagrangean. Specifically, in the models discussed in the literature, the Kähler potential is not fixed by the symmetries, for instance. The most general Kähler potential consistent with the symmetries of the model contains additional terms with additional parameters, which reduce the predictive power of these constructions. We also comment on how one may conceivably retain the predictivity.

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arXiv2019

Baryogenesis, Dark Matter, and Flavor Structure in Non-thermal Moduli Cosmology

The appearance of scalar/moduli fields in the early universe, as motivated by string theory, naturally leads to non-thermal "moduli cosmology". Such cosmology provides a consistent framework where the generation of radiation, baryons, and dark matter can occur while maintaining successful Big Bang Nucleosynthesis and avoiding the cosmological moduli problem. We present a relatively economical construction with moduli cosmology, building on a variety of string-inspired components (e.g. supersymmetry, discrete symmetries, Green-Schwarz anomaly cancellation). We address a range of outstanding problems of particle physics and cosmology simultaneously, including the fermion mass hierarchy and flavor puzzle, the smallness of neutrino masses, baryogenesis and dark matter. Our setup, based on discrete $\mathrm{Z}_{12}^{R}$ symmetry and anomalous $\mathrm{U}(1)_A$, is void of the usual issues plaguing the Minimal Supersymmetric Standard Model, i.e. the $μ$-problem and the overly-rapid proton decay due to dimension-4,-5 operators. The model is compatible with $\mathrm{SU}(5)$ Grand Unification. The smallness of Dirac neutrino masses is automatically established by requiring the cancellation of mixed gravitational-gauge anomalies. The decay of the moduli field provides a common origin for the baryon number and dark matter abundance, explaining the observed cosmic coincidences, $Ω_{B} \sim Ω_{DM}$.

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arXiv2019

Baryogenesis From Flavon Decays

Many popular attempts to explain the observed patterns of fermion masses involve a flavon field. Such weakly coupled scalar fields tend to dominate the energy density of the universe before they decay. If the flavon decay happens close to the electroweak transition, the right-handed electrons stay out of equilibrium until the sphalerons shut off. We show that an asymmetry in the right-handed charged leptons produced in the decay of a flavon can explain the baryon asymmetry of the universe.

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arXiv2019

Group-theoretical origin of CP violation

This is a short review of the proposal that CP violation may be due to the fact that certain finite groups do not admit a physical CP transformation. This origin of CP violation is realized in explicit string compactifications exhibiting the Standard Model spectrum.

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arXiv2018

Revisiting Electroweak Phase Transition with Varying Yukawa Coupling Constants

We revisited the scenario of electroweak baryogenesis in the presence of large Yukawa couplings, in which it was found previously that a strongly first order electroweak phase transition can occur with the Higgs mass at its observed value of 125 GeV. Given the sensitivity of the running of the Higgs quartic coupling on the Yukawa coupling constants, we find that the addition of order one Yukawa couplings beyond the top quark drastically lowers the scale at which the Higgs potential becomes unstable. Specifically, even with only one additional order one Yukawa coupling, the scalar potential becomes unstable already at the TeV scale, assuming the Standard Model values for the Higgs sector parameters at the electroweak scale. Furthermore, by assuming the Standard Model values for the Higgs sector parameters at the TeV scale, the quartic coupling constant is driven to be larger than its Standard Model value at the electroweak scale. This in turn predicts a much lighter Higgs mass than the measured value of 125 GeV. In this scenario, the strength of the electroweak phase transition is also significantly weakened.

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arXiv2016

Project X: Physics Opportunities

Part 2 of "Project X: Accelerator Reference Design, Physics Opportunities, Broader Impacts". In this Part, we outline the particle-physics program that can be achieved with Project X, a staged superconducting linac for intensity-frontier particle physics. Topics include neutrino physics, kaon physics, muon physics, electric dipole moments, neutron-antineutron oscillations, new light particles, hadron structure, hadron spectroscopy, and lattice-QCD calculations. Part 1 is available as arXiv:1306.5022 [physics.acc-ph] and Part 3 is available as arXiv:1306.5024 [physics.acc-ph].

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arXiv2016

Beyond Minimal Lepton Flavored Dark Matter

We consider a class of flavored dark matter (DM) theories where dark matter interacts with the Standard Model lepton fields at the renormalizable level. We allow for a general coupling matrix between the dark matter and leptons whose structure is beyond the one permitted by the minimal flavor violation (MFV) assumption. It is assumed that this is the only new source of flavor violation in addition to the Standard Model (SM) Yukawa interactions. The setup can be described by augmenting the SM flavor symmetry by an additional $\mathrm{SU}(3)_χ$, under which the dark matter $χ$ transforms. This framework is especially phenomenologically rich, due to possible novel flavor-changing interactions which are not present within the more restrictive MFV framework. As a representative case study of this setting, which we call "beyond MFV" (BMFV), we consider Dirac fermion dark matter which transforms as a singlet under the SM gauge group and a triplet under $\mathrm{SU}(3)_χ$. The DM fermion couples to the SM lepton sector through a scalar mediator $ϕ$. Unlike the case of quark-flavored DM, we show that there is no $\mathbb{Z}_3$ symmetry within either the MFV or BMFV settings which automatically stabilizes the lepton-flavored DM. We discuss constraints on this setup from flavor-changing processes, DM relic abundance as well as direct and indirect detections. We find that relatively large flavor-changing couplings are possible, while the dark matter mass is still within the phenomenologically interesting region below the TeV scale. Collider signatures which can be potentially searched for at the lepton and hadron colliders are discussed. Finally, we discuss the implications for decaying dark matter, which can appear if an additional stabilizing symmetry is not imposed.

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arXiv2016

Non-thermal cosmic neutrino background

We point out that, for Dirac neutrinos, in addition to the standard thermal cosmic neutrino background (C$ν$B) there could also exist a non-thermal neutrino background with comparable number density. As the right-handed components are essentially decoupled from the thermal bath of standard model particles, relic neutrinos with a non-thermal distribution may exist until today. The relic density of the non-thermal (nt) background can be constrained by the usual observational bounds on the effective number of massless degrees of freedom $N_\mathrm{eff}$, and can be as large as $n_{ν_{\mathrm{nt}}}\lesssim 0.5\,n_γ$. In particular, $N_\mathrm{eff}$ can be larger than 3.046 in the absence of any exotic states. Non-thermal relic neutrinos constitute an irreducible contribution to the detection of the C$ν$B, and, hence, may be discovered by future experiments such as PTOLEMY. We also present a scenario of chaotic inflation in which a non-thermal background can naturally be generated by inflationary preheating. The non-thermal relic neutrinos, thus, may constitute a novel window into the very early universe.

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arXiv2015

Anomaly-safe discrete groups

We show that there is a class of finite groups, the so-called perfect groups, which cannot exhibit anomalies. This implies that all non-Abelian finite simple groups are anomaly-free. On the other hand, non-perfect groups generically suffer from anomalies. We present two different ways that allow one to understand these statements.

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arXiv2015

$R$ parity violation from discrete $R$ symmetries

We consider supersymmetric extensions of the standard model in which the usual $R$ or matter parity gets replaced by another $R$ or non-$R$ discrete symmetry that explains the observed longevity of the nucleon and solves the $μ$ problem of MSSM. In order to identify suitable symmetries, we develop a novel method of deriving the maximal $\mathbb{Z}_{N}^{(R)}$ symmetry that satisfies a given set of constraints. We identify $R$ parity violating (RPV) and conserving models that are consistent with precision gauge unification and also comment on their compatibility with a unified gauge symmetry such as the Pati-Salam group. Finally, we provide a counter-example to the statement found in the recent literature that the lepton number violating RPV scenarios must have $μ$ term and the bilinear $κ\, L \, H_u$ operator of comparable magnitude.

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arXiv2014

Charged Lepton Spectrum Approximation in a Three Body Nucleon Decay

Only phase space is typically used to obtain final state particle spectra in rare decay searches, which is a crude approximation in the case of three body processes. We will demonstrate how both dynamics and phase space can be approximately accounted for, in processes such as nucleon decays $p \rightarrow e^+ \barν ν$ or $p \rightarrow μ^+ \barν ν$ originating from Grand Unification models, using general effective Fermi theory formalism of electroweak muon decay $μ\rightarrow e^+ \barν ν$. This approach allows for a more precise and only weakly model dependent approximation of final particle spectra for these and similar decays, which may improve rare process searches in current and near-future experiments.

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arXiv2014

CP Violation from Finite Groups

We discuss the origin of CP violation in settings with a discrete (flavor) symmetry $G$. We show that physical CP transformations always have to be class-inverting automorphisms of $G$. This allows us to categorize finite groups into three types: (i) Groups that do not exhibit such an automorphism and, therefore, in generic settings, explicitly violate CP. In settings based on such groups, CP violation can have pure group-theoretic origin and can be related to the complexity of some Clebsch-Gordan coefficients. (ii) Groups for which one can find a CP basis in which all the Clebsch-Gordan coefficients are real. For such groups, imposing CP invariance restricts the phases of coupling coefficients. (iii) Groups that do not admit real Clebsch-Gordan coefficients but possess a class-inverting automorphism that can be used to define a proper (generalized) CP transformation. For such groups, imposing CP invariance can lead to an additional symmetry that forbids certain couplings. We make use of the so-called twisted Frobenius-Schur indicator to distinguish between the three types of discrete groups. With $Δ(27)$, $T^{\prime}$, and $Σ(72)$ we present one explicit example for each type of group, thereby illustrating the CP properties of models based on them. We also show that certain operations that have been dubbed generalized CP transformations in the recent literature do not lead to physical CP conservation.

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arXiv2013

Large θ_13 in a SUSY SU(5)xT' Model

In the model based on SUSY SU(5) combined with the T' family symmetry, it is shown in Phys. Lett. B 652, 34 (2007) and Phys. Lett. B 681, 444 (2009) that fermion mass hierarchy and mixing angles can be naturally generated in both lepton and quark sectors compatible with various experimental measurements. But the predicted value for the non-zero lepton mixing angle of θ_13 \simeq θ_c/3\sqrt{2} contradicts the recent experimental results from Daya Bay and RENO. We propose to introduce one more singlet flavon field in the neutrino sector, and with its additional contribution, we are able to generate the large mixing angle θ_13 ~ 8^{\circ}-10^{\circ}. The analytical expressions of the mixing angles and neutrino masses with the additional flavon field are derived. Our numerical results show that a large region in the model parameter space is allowed for the normal hierarchy case, while a much smaller region is allowed for the inverted hierarchy case. In addition, the predicted value of the solar mixing angle is not affected by the additional singlet contribution. On the other hand, there exists a correlation between θ_13 value and θ_23 - π/4, as a result of the additional singlet contribution. We also make predictions for various other variables that can be potentially tested in the future neutrino experiments such as the Dirac CP phase, which is predicted to be in the range of δ~ (195^{\circ}-200^{\circ}), and the neutrinoless double beta decay matrix element, . One numerical example with the minimal χ^2 fit is presented, and the model predictions are consistent with all experimental results.

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arXiv2013

Non-Abelian discrete R symmetries

We discuss non-Abelian discrete R symmetries which might have some conceivable relevance for model building. The focus is on settings with N=1 supersymmetry, where the superspace coordinate transforms in a one-dimensional representation of the non-Abelian discrete symmetry group. We derive anomaly constraints for such symmetries and find that novel patterns of Green-Schwarz anomaly cancellation emerge. In addition we show that perfect groups, also in the non-R case, are always anomaly-free. An important property of models with non-Abelian discrete R symmetries is that superpartners come in different representations of the group. We present an example model, based on a semidirect product of a Z_3 and a Z_8^R symmetry, to discuss generic features of models which unify discrete R symmetries, entailing solutions to the mu and proton decay problems of the MSSM, with non-Abelian discrete flavor symmetries.

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arXiv2013

Predictivity of models with spontaneously broken non-Abelian discrete flavor symmetries

In a class of supersymmetric flavor models predictions are based on residual symmetries of some subsectors of the theory such as those of the charged leptons and neutrinos. However, the vacuum expectation values of the so-called flavon fields generally modify the Kähler potential of the setting, thus changing the predictions. We derive simple analytic formulae that allow us to understand the impact of these corrections on the predictions for the masses and mixing parameters. Furthermore, we discuss the effects on the vacuum alignment and on flavor changing neutral currents. Our results can also be applied to non--supersymmetric flavor models.

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arXiv2013

On predictions from spontaneously broken flavor symmetries

We discuss the predictive power of supersymmetric models with flavor symmetries, focusing on the lepton sector of the standard model. In particular, we comment on schemes in which, after certain `flavons' acquire their vacuum expectation values (VEVs), the charged lepton Yukawa couplings and the neutrino mass matrix appear to have certain residual symmetries. In most analyses, only corrections to the holomorphic superpotential from higher-dimensional operators are considered (for instance, in order to generate a realistic $θ_{13}$ mixing angle). In general, however, the flavon VEVs also modify the Kähler potential and, therefore, the model predictions. We show that these corrections to the naive results can be sizable. Furthermore, we present simple analytic formulae that allow us to understand the impact of these corrections on the predictions for the masses and mixing parameters.

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arXiv2013

Compatibility of theta13 and the Type I Seesaw Model with A4 Symmetry

We derive formulae for neutrino masses and mixing angles in a type I seesaw framework with an underlying A4 flavor symmetry. In particular, the Majorana neutrino mass matrix includes contributions from an A4 triplet, 1, 1', and 1" flavon fields. Using these formulae, we constrain the general A4 parameter space using the updated global fits on neutrino mixing angles and mass squared differences, including results from the Daya Bay and RENO experiments, and we find predictive relations among the mixing parameters for certain choices of the triplet vacuum expectation value. In the normal hierarchy case, sizable deviation from maximal atmospheric mixing is predicted, and such deviation is strongly correlated with the value of theta13 in the range of ~ (8-10) degrees. On the other hand, such deviation is negligible and insensitive to theta13 in the inverted mass hierarchy case. We also show expectations for the Dirac CP phase resulting from the parameter scan. Future refined measurements of neutrino mixing angles will test these predicted correlations and potentially show evidence for particular triplet vev patterns.

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arXiv2012

Supersymmetric unification and R symmetries

We review the role of R symmetries in models of supersymmetric unification in four and more dimensions, and in string theory. We show that, if one demands anomaly freedom and fermion masses, only R symmetries can forbid the supersymmetric Higgs mass term mu. We then review the proof that R symmetries are not available in conventional grand unified theories (GUTs) and argue that this prevents natural solutions to the doublet-triplet splitting problem in four dimensions. On the other hand, higher-dimensional GUTs do not suffer from this problem. We briefly comment on an explicit string-derived model in which the mu and dimension five proton decay problems are solved simultaneously by an order four discrete R symmetry. We also comment on the higher-dimensional origin of this symmetry.

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