Sources Of CP Violation Research Articles

Wash-in leptogenesis after axion inflation

CP violation and the violation of baryon-minus-lepton number B−L do not necessarily have to occur simultaneously in order to accomplish successful leptogenesis. Instead, it suffices if new CP-violating interactions at high energies result in primordial charge asymmetries, which are then reprocessed into a nonvanishing B−L asymmetry by right-handed neutrinos (RHNs) at lower energies. In this paper, we study this novel mechanism known as wash-in leptogenesis, utilizing axion inflation as the source of high-scale CP violation. We specifically consider axion inflation coupled to the Standard Model hypercharge sector, which results in the dual production of hypermagnetic helicity and fermionic charge asymmetries. Although the survival of these charges is endangered by sphaleron processes, magnetic diffusion, and the chiral plasma instability, we find a large range of viable scenarios. We consistently account for RHN flavor effects and coherence among the Standard Model lepton flavors across a wide range of RHN masses. We find a lower bound of 105···9 GeV on the mass of the lightest RHN involved in wash-in leptogenesis, depending on the onset of turbulence in the chiral plasma and the Hubble scale of inflation. Our model is representative of a broader class of new leptogenesis scenarios and suggests interesting observational signatures with regard to intergalactic magnetic fields, primordial black holes, and gravitational waves.

Electric dipole moment of charged particles using storage rings

The Standard Model (SM) of Particle Physics cannot explain the matter-antimatter asymmetry in the Universe. Especially, new sources of CP-violation are necessary, and this is why physics beyond the SM is being explored. The search for permanent Electric Dipole Moments (EDMs) of elementary particles has the potential to provide a powerful tool to discover new sources of CP-violation. Finding an EDM would be a convincing indication for physics beyond the SM. Moreover, a rotating polarized beam used in EDM searches is sensitive to oscillating EDM caused by axions or axion-like particles (ALPs), the candidates for the dark matter. Storage rings are promising devices to measure EDMs of charged particles by observing the effect of the EDM on the spin motion in the ring. The Cooler Synchrotron COSY at the Forschungszentrum Jülich provides polarized protons and deuterons with momenta up to 3.7 GeV/c and it constitutes an suitable testing ground and starting point for an experimental program to search for EDMs of charged particles. This report provides an overview of the current status of the JEDI project for the first direct measurement of the deuteron EDM at COSY. The analysis is currently ongoing. Due to the complexity of storage rings, this study combines high precision measurements and thorough understanding of the systematic uncertainties involved.

Grand Color axion

We present a model that solves the strong CP problem via an axion parametrically heavier than the standard one. Within this picture the Standard Model quarks are embedded into a larger non-abelian Grand Color group that at high scales splits into ordinary QCD and an additional confining dynamics under which exotic chiral fermions are charged. Crucially, the vacuum expectation value of the axion is automatically relaxed to zero because the only renormalizable source of explicit CP violation, beyond those encoded in the topological angles, is contained in the Standard Model Yukawa couplings, and is therefore very suppressed. The Grand Color axion potential is controlled by the scale of the new confining group and is much larger than the QCD contribution, such that its dynamics is less exposed to the so-called “axion quality problem”. Potentially observable corrections to the effective topological angle can also arise, in our model as well as in a large class of heavy axion scenarios, from non-renormalizable Peccei-Quinn-conserving interactions, which introduce a new “heavy axion quality problem”. Our model has a very minimal field content, it relies entirely on gauge invariance and does not require the introduction of additional symmetries beyond the usual one postulated by Peccei and Quinn. The phenomenology is very rich and can be tested at colliders as well as via cosmological observations. A particularly interesting portion of parameter space predicts a visible Grand Color axion of mass above the GeV and decay constant larger than a few TeV.

Using Bs0→Ds∓K± decays as a portal to new physics

The system of $B^0_s\to D_s^\mp K^\pm$ decays offers a theoretically clean determination of the angle $\gamma$ of the Unitarity Triangle. A corresponding LHCb analysis resulted in a surprisingly large result, which is in tension with other determinations and global analyses of the Unitarity Triangle. Paying special attention to discrete ambiguities, we confirm this picture and resolve a final ambiguity. Moreover, we extract the branching ratios of the underlying $\bar{B}^0_s\to D_s^+K^-$ and $\bar{B}^0_s\to D_s^- K^+$ modes. Combining them with information from semileptonic $B_{(s)}$ decays, we arrive at another puzzling situation, which we obtain also for other decays with similar dynamics. These patterns could be footprints of New Physics in the $b\to c \bar{u} s$ and $b \to u \bar{c} s$ processes which govern the dynamics of the $\bar{B}^0_s\to D_s^\mp K^\pm$ channels. Employing a model-independent parametrisation, we present a strategy to reveal such effects. Applying it to the present data, we obtain strongly correlated New-Physics contributions with potentially large CP-violating phases. We find that new contributions sizeably smaller than the Standard Model amplitudes could actually accommodate the current data. This strategy offers an exciting probe for new sources of CP violation at the future high-precision frontier of $B$ physics.

Unitarity bounds for all symmetry-constrained 3HDMs

Models with three Higgs doublets (3HDM) are the source of much recent activity, for they are key components of many solutions to the problems of the Standard Model; from extra sources of CP violation to Dark Matter candidates. We compute explicitly the theoretical bounds for all symmetry-constrained 3HDM arising from the perturbative unitarity of two-to-two scattering amplitudes. In addition, we propose a method based on principal minors that foregoes diagonalization and which is preferable in all models (not only 3HDM) dealing with large scattering matrices.

CP violation and flavor invariants in the seesaw effective field theory

In this paper, we systematically study the algebraic structure of the ring of the flavor invariants and the sources of CP violation in the seesaw effective field theory (SEFT), which is obtained by integrating out heavy Majorana neutrinos in the type-I seesaw model at the tree level and thus includes the dimension-five Weinberg operator and one dimension-six operator. For the first time, we calculate the Hilbert series and explicitly construct all the primary flavor invariants in the SEFT. We show that all the physical parameters can be extracted using the primary invariants and any CP-violating observable can be expressed as the linear combination of CP-odd flavor invariants. The calculation of the Hilbert series shows that there is an equal number of primary flavor invariants in the SEFT and in the full seesaw model, which reveals the intimate connection between the flavor space of the SEFT and that of its ultraviolet theory. A proper matching procedure of the flavor invariants is accomplished between the SEFT and the full seesaw model, through which one can establish a direct link between the CP asymmetries in leptogenesis and those in low-energy neutrino oscillations.

Yukawa alignment revisited in the Higgs basis

We implement a comprehensive and detailed study of the alignment of Yukawa couplings in the so-called Higgs basis taking the framework of general two Higgs doublet models (2HDMs). We clarify the model input parameters and derive the Yukawa couplings considering the two types of CP-violating sources: one from the Higgs potential and the other from the three complex alignment parameters $\zeta_{f=u,d,e}$. We consider the theoretical constraints from the perturbative unitarity and for the Higgs potential to be bounded from below as well as the experimental ones from electroweak precision observables. Also considered are the constraints on the alignment parameters from flavor-changing $\tau$ decays, $Z\to b\bar b$, $\epsilon_K$, and the radiative $b\to s\gamma$ decay. By introducing the basis-independent Yukawa delay factor $\Delta_{H_1\bar f f}\equiv |\zeta_{f}|(1-g_{_{H_1VV}}^2)^{1/2}$, we scrutinize the alignment of the Yukawa couplings of the lightest Higgs boson to the SM fermions.

CP-violating axion interactions in effective field theory

Axions are introduced to explain the observed smallness of the overline{theta} term of QCD. Standard Model extensions typically contain new sources of CP violation, for instance to account for the baryon asymmetry of the universe. In the presence of additional CP-violating sources a Peccei-Quinn mechanism does not remove all CP violation, leading to CP-odd interactions among axions and Standard Model fields. In this work, we use effective field theory to parametrize generic sources of beyond-the-Standard-Model CP violation. We systematically compute the resulting CP-odd couplings of axions to leptons and hadrons by using chiral perturbation theory. We discuss in detail the phenomenology of the CP-odd axion couplings and compare limits from axion searches, such as fifth force and monopole-dipole searches and astrophysics, to direct limits on the CP-violating operators from electric dipole moment experiments. While limits from electric dipole moment searches are tight, the proposed ARIADNE experiment can potentially improve the existing constraints in a window of axion masses.

Contribution of the Weinberg-type operator to atomic and nuclear electric dipole moments

The contribution of the CP violating three-gluon Weinberg operator, frac{1}{3!}w{f}^{abc}{epsilon}^{nu rho alpha beta}{G}_{mu nu}^a{G}_{alpha beta}^b{G}_{rho}^{cmu} , to the atomic and nuclear EDMs is estimated using QCD sum rules. After calculating the transition matrix element between the pion and the vacuum through the Weinberg operator, we obtain the long-range CP-odd nuclear force by determining the isovector CP-odd pion-nucleon vertex, using chiral perturbation theory at NLO. The EDMs of 199Hg, 129Xe, and 225Ra atoms, as well as those of 2H and 3He nuclei are finally given including comprehensive uncertainty analysis. While the leading contribution of the 199Hg EDM is given by the intrinsic nucleon EDM, that of 129Xe atom may be dominated by the one-pion exchange CP-odd nuclear force generated by the Weinberg operator. From current experimental data of the 199Hg atomic EDM, we obtain an upper limit on the Weinberg operator magnitude of |w| < 4 × 10−10GeV−2 if we assume that it is the only source of CP violation at the scale μ = 1 TeV.

Tasting flavoured Majorana dark matter

We study a simplified model of flavoured Majorana dark matter in the Dark Minimal Flavour Violation framework. The model extends the Standard Model by a dark matter flavour triplet and a scalar mediator, through which the new dark fermions couple to right-handed up-type quarks. This interaction is governed by a new coupling matrix λ which is assumed to constitute the only new source of flavour and CP violation. We analyse the parameter space of this model by using constraints from collider searches, D0− overline{D} 0 mixing, cosmology and direct dark matter searches. Throughout our study, we point out crucial differences between the Majorana and Dirac dark matter cases. After performing a combined analysis within the context of all the experimental constraints mentioned above, we analyse which flavour for the dark matter particle is preferred by experimental data. We further investigate if this model is capable of explaining the large measured value of the direct CP asymmetry Delta {A}_{mathrm{CP}}^{mathrm{dir}} in charm decays. We find that significant enhancements with respect to the Standard Model expectation are compatible with all constraints, and even the central value of the measurement can be reached. We also advertise the flavour-violating final state with two same-sign top quarks produced in association with missing transverse energy as a smoking-gun signature for flavoured Majorana dark matter at the LHC.

Simple model of dark matter and CP violation

We propose a simple model of dark matter and CP violation and consider the associated triple and quadruple productions of 125 GeV Higgs bosons at the Large Hadron Collider (LHC). In the model, the dark matter is a vector-like dark fermion $(\bar{\chi}, \chi)$ interacting with the Standard Model only through a complex messenger scalar $S$ which is an electroweak singlet. New sources of CP violation reside in the most general scalar potential involving the doublet $H$ and the singlet $S$, as well as in the dark Yukawa coupling between $S$ and $(\bar{\chi}, \chi)$. We study current experimental constraints from Higgs measurements, searches for new scalars at the LHC, precision electroweak measurements, EDM measurements, dark matter relic density, as well as direct and indirect detections of dark matter. A smoking-gun signature of CP violation could come from the Higgs-to-Higgs decays, $h_3\to h_2h_1$, where $h_3/h_2/h_1$ are the heaviest scalar, second heaviest scalar and the SM-like 125-GeV Higgs, respectively. Taking into account other Higgs-to-Higgs decays, such as $h_3\to 2h_2$ and $h_3/h_2\to 2h_1$, then gives rise to novel $3h_1$ and $4h_1$ final states, which have yet to be searched for experimentally. We present four benchmarks and show the event rates for $3h_1$ and $4h_1$ final states could be as large as ${\cal O}(10)\ {\rm fb}$ and ${\cal O}(1)\ {\rm fb}$, respectively, at the 14-TeV LHC. This work opens up a new frontier of searching for triple and quadruple Higgs bosons at a high energy collider.

Baryon asymmetric Universe from spontaneous CP violation

Spontaneous CP violation, such as the Nelson-Barr (NB) mechanism, is an attractive scenario for addressing the strong CP problem while realizing the observed phase of the Cabibbo-Kobayashi-Maskawa (CKM) quark-mixing matrix. However, not only the CKM phase but also the baryon asymmetric Universe requires sources of CP violation. In this study, we show that a supersymmetric NB mechanism can naturally accommodate the Affleck-Dine (AD) baryogenesis within a CP-invariant Lagrangian. The model provides flat directions associated with new heavy quarks. Focusing on one of the directions, we find that the correct baryon asymmetry is obtained with a sufficiently low reheating temperature which does not cause the gravitino problem. Some parameter space is consistent with the gravitino dark matter. We assess radiative corrections to the strong CP phase induced by gauge-mediated supersymmetry breaking and CP-violating heavy fields and show that the strong CP problem is solved in a viable parameter space where the visible sector supersymmetric particles must be lighter than mathcal{O} (100) TeV. Even in the case that they are heavier than the TeV scale, our model predicts the neutron electric dipole moment within the reach of the near future experiments. Our model addresses the electroweak naturalness problem, strong CP problem, baryon asymmetric Universe, and dark matter. Then, the model may give a new compelling paradigm of physics beyond the Standard Model.

Leptonic CP violation from a vector-like lepton

Leptonic CP violation is yet to be confirmed as an additional source of CP violation in fundamental interactions. We study the case where leptonic CP violation is spontaneous and is induced by the mixing with a heavy charged vector-like lepton (VLL). We show that the non-decoupling of this VLL is linked with the presence of CP violation and its coupling with the SM leptons are partly fixed from the SM Yukawas. Due to the large leptonic mixing angles, these couplings are typically of the same order and there is no flavor preference. Strong but not definitive constraints come from charged lepton flavor violating processes because the VLL can decouple from one or two leptonic flavors in very special points of parameter space. These special points are very sensitive to the neutrino Majorana phases.

Leptogenesis from low-energy CP violation in minimal left-right symmetric model

We perform a thermal unflavored leptogenesis analysis on minimal left-right symmetric models with discrete left-right symmetry identified as generalized parity or charge conjugation. When left-right symmetry is unbroken in the lepton Yukawa sector, the neutrino Dirac coupling matrix is completely determined by neutrino masses and mixing angles, allowing CP violation needed to generate leptogenesis totally resides in the low-energy sector. With two lepton asymmetry generation ways, both type I and mixed type I+II neutrino mass generation mechanisms are considered. After solving the Boltzmann equations numerically, we find that the low-energy CP phases in the lepton mixing matrix can successfully produce the observed baryon asymmetry, and in some cases, the Dirac CP phase can be the only source of CP violation. Finally, we discuss the interplay among low-energy CP phase measurements, leptogenesis, and neutrinoless double beta decay. We show that the viable models for successful leptogenesis can be probed in next-generation neutrinoless double-beta decay experiments.

Neutrino and doubly charged Higgs boson phenomenology in flavored-TNMSSM

We develop a neutrino mass model based on A4×Z3 flavor symmetry in the context of the NMSSM extended by supersymmetric type II seesaw mechanism (flavored-TNMSSM). We show that the NMSSM-singlet contributes significantly to the light neutrino masses, leading to the trimaximal neutrino mixing and providing a source for CP violation. The model favors normal hierarchy for neutrino masses, and offers testable predictions for upcoming experiments aimed at developing the measurements of the parameters mνe, |mee|. We also show that the decay of the doubly charged Higgs boson (DCHB) into dilepton of the same sign is dominant; and that the muon-tau (μτ) channel is the most relevant, thus giving rise to a collider signature. We prove that the model allows LFV processes, such as τ±→e±μ∓e± and τ±→μ±e∓μ±. We find further that when the triplet VEV exceeds 0.5 eV, a discovery of the DCHB would be possible at the current run of the LHC; whereas for a broad part of the parameter space when the triplet VEV is less than about 0.5 eV, the DCHB can acquire mass up to few TeVs, which can be probed at HL-LHC, HE-LHC, and future 100 TeV hadron colliders.

BSM sources of CP violation at future long baseline experiments

Different new physics models can introduce new sources of CP violation in the neutrino oscillation probabilities. We studied the potential of the DUNE experiment to look for them, in the case of sterile neutrinos and Non-Standard Interactions, measuring only the CP asymmetries in different oscillation channels. We show that if a high energy flux will be used, the asymmetry related to the νμ → νe appearance channel could be very useful to search for hints of new physics.

CP violation in mixing and oscillations for leptogenesis. Part II. The highly degenerate case

We extend to the highly degenerate case a recent approach for analyzing the sources of CP violation in baryogenesis models with quasi-degenerate neutrinos. In this approach an expansion of the resummed propagator around the poles is plugged into a quantum field theory model of neutrino oscillations and a source term for the time evolution of the lepton asymmetry is built directly from the probabilities of lepton number violating processes involving only stable particles. This allows for a transparent consideration of unitarity requirements. The source term has contributions that can be identified with CP violation from mixing, oscillations and interference between both. For the highly degenerate case, i.e. when the mass difference between two neutrinos is similar or smaller than their decay widths, we find that in general the mixing and oscillation terms contribute with opposite signs to the generation of lepton asymmetry and the contribution of the interference term is typically very relevant and crucial to ensure unitarity is satisfied. Moreover, the expressions we obtain are finite in the double degenerate limit of equal masses and couplings. The calculations are done in a simple scalar toy model.

Probing baryogenesis with neutron-antineutron oscillations

In the near future, the Deep Underground Neutrino Experiment and the European Spallation Source aim to reach unprecedented sensitivity in the search for neutron-antineutron (n- overline{n} ) oscillations, whose observation would directly imply |∆B| = 2 violation and hence might hint towards a close link to the mechanism behind the observed baryon asymmetry of the Universe. In this work, we explore the consequences of such a discovery for baryogenesis first within a model-independent effective field theory approach. We then refine our analysis by including a source of CP violation and different hierarchies between the scales of new physics using a simplified model. We analyse the implication for baryogenesis in different scenarios and confront our results with complementary experimental constraints from dinucleon decay, LHC, and meson oscillations. We find that for a small mass hierarchy between the new degrees of freedom, an observable rate for n- overline{n} oscillation would imply that the washout processes are too strong to generate any sizeable baryon asymmetry, even if the CP violation is maximal. On the other hand, for a large hierarchy between the new degrees of freedom, our analysis shows that successful baryogenesis can occur over a large part of the parameter space, opening the window to be probed by current and future colliders and upcoming n- overline{n} oscillation searches.

Predictions for the leptonic Dirac CP-violating phase

In this paper, we explore the theoretical constraints on the observable parameters of neutrino mixing on predictions for the leptonic Dirac CP-violating phase within a well-studied class of simple theoretical models that includes a single source of CP violation due to charged lepton corrections. The approach guarantees that a physically meaningful prediction for the most likely values for the leptonic Dirac CP-violating phase is obtained.

Filtered asymmetric dark matter during the Peccei-Quinn phase transition

In this paper, we propose a bubble filtering-out mechanism for an asymmetric dark matter scenario during the Peccei-Quinn (PQ) phase transition. Based on a QCD axion model, extended by extra chiral neutrinos, we show that the PQ phase transition can be first order in the parameter space of the model and regarding the PQ symmetry breaking scale, the mechanism can generate PeV-scale heavy neutrinos as a dark matter candidate. Considering a CP-violating source, during the phase transition, discriminating between the neutrino and antineutrino number density, we find the observed dark matter relic abundance, such that the setup can be applied to the first order phase transition with different strengths. We then calculate effective couplings of the QCD axion addressing the strong CP problem within the model. We also study the energy density spectrum of gravitational waves generated from the first order phase transition and show that the signals can be detected by future ground-based detectors such as Einstein Telescope. In particular, for a visible heavy axion case of the model, it is shown that gravitational waves can be probed by DECIGO and BBO interferometers. Furthermore, we discuss the dark matter-standard model neutrino annihilation process as a source for the creation of PeV-scale neutrinos.