Flavor Physics Research Articles

Dark matter for b \u2192 s\u03bc+\u03bc\u2212 anomaly in a gauged U(1)X model

We propose a new physics model which has a cold dark matter candidate and can explain the b → sμ+μ− anomaly at the same time. Our model includes a scalar quark tilde{q} and a scalar lepton tilde{ell} which are SU(2)L-doublet as well as a Dirac fermion N which is SU(2)L-singlet. The new particles are charged under a gauged U(1)X group which is spontaneously broken to a discrete Z2 symmetry by a dark scalar S. The remnant Z2 symmetry stabilizes the dark matter. Box diagrams with tilde{q} , tilde{ell} , and N running inside the loop can generate the correct Wilson coefficients C9μ = − C10μ to accommodate the b → sμ+μ− anomaly while avoiding constraints such as {B}_s-{overline{B}}_s mixing. The dark matter annihilation into a second generation lepton pair via t-channel tilde{ell} -exchanging process plays an important role in producing the current dark matter relic abundance of the universe, showing a strong interplay between the flavor and dark matter physics. We also discuss dark-gauge-interaction-dominated and Higgs-portal-dominated scenarios for dark matter physics.

Neutrino Mass Ordering from Oscillations and Beyond: 2018 Status and Future Prospects

The ordering of the neutrino masses is a crucial input for a deep understanding of flavor physics, and its determination may provide the key to establish the relationship among the lepton masses and mixings and their analogous properties in the quark sector. The extraction of the neutrino mass ordering is a data-driven field expected to evolve very rapidly in the next decade. In this review, we both analyze the present status and describe the physics of subsequent prospects. Firstly, the different current available tools to measure the neutrino mass ordering are described. Namely, reactor, long-baseline (accelerator and atmospheric) neutrino beams, laboratory searches for beta and neutrinoless double beta decays and observations of the cosmic background radiation and the large scale structure of the universe are carefully reviewed. Secondly, the results from an up-to-date comprehensive global fit are reported: the Bayesian analysis to the 2018 publicly available oscillation and cosmological data sets provides \emph{strong} evidence for the normal neutrino mass ordering versus the inverted scenario, with a significance of 3.5 standard deviations. This preference for the normal neutrino mass ordering is mostly due to neutrino oscillation measurements. Finally, we shall also emphasize the future perspectives for unveiling the neutrino mass ordering. In this regard, apart from describing the expectations from the aforementioned probes, we also focus on those arising from alternative and novel methods, as 21~cm cosmology, core-collapse supernova neutrinos and the direct detection of relic neutrinos.

Supersymmetry, direct and indirect constraints

We present an overview of direct and indirect constraints in the MSSM, in CP-conserving and CP-violating MSSM scenarios, with some emphasis on the importance of combining the constraints from different sectors, namely SUSY and Higgs direct searches at the LHC, flavour physics, dark matter detection and electric dipole moments.

Design Development for the Beam Dump Facility Target Complex at CERN

CERN has launched a study phase to evaluate the feasibility of a new high-intensity beam dump facility at the CERN Super Proton Synchrotron accelerator with the primary goal of exploring Hidden Sector models and searching for Light Dark Matter, but which also offers opportunities for other fixed target flavour physics programs such as rare tau lepton decays and tau neutrino studies. The new facility will require—among other infrastructure—a target complex in which a dense target/dump will be installed, capable of absorbing the entire energy of the beam extracted from the SPS accelerator. In theory, the target/dump could produce very weakly interacting particles, to be investigated by a suite of particle detectors to be located downstream of the target complex. As part of the study, a development design of the target complex has been produced, taking into account the handling and remote handling operations needed through the lifetime of the facility. Two different handling concepts have been studied and both resulting designs are presented.

R(D(\u2217)) from W\u2032 and right-handed neutrinos

We provide an ultraviolet (UV) complete model for the R(D(∗)) anomalies, in which the additional contribution to semi-tauonic b → c transitions arises from decay to a right-handed sterile neutrino via exchange of a TeV-scale SU(2)L singlet W′. The model is based on an extension of the Standard Model (SM) hypercharge group, U(1)Y , to the SU(2)V × U(1)′ gauge group, containing several pairs of heavy vector-like fermions. We present a comprehensive phenomenological survey of the model, ranging from the low-energy flavor physics, direct searches at the LHC, to neutrino physics and cosmology. We show that, while the W′ and Z′-induced constraints are important, it is possible to find parameter space naturally consistent with all the available data. The sterile neutrino sector also offers rich phenomenology, including possibilities for measurable dark radiation, gamma ray signals, and displaced decays at colliders.

Background Suppression with the Belle II Neural Network Trigger

Neural networks are going to be used in the pipelined first level trigger of the upgraded flavor physics experiment Belle II at the high luminosity B factory SuperKEKB in Tsukuba, Japan. An instantaneous luminosity of L = 8 × 1035cm−2s−1 is anticipated, 40 times larger than the world record reached with the predecessor KEKB. Background tracks, with vertices displaced along the beamline (z-axis), are expected to be severely increased due to the high luminosity. Using input from the central drift chamber, the main tracking device of Belle II, the online neural network trigger provides 3D track reconstruction within the fixed latency of the first level trigger. In particular, the robust estimation of the z-vertices allows a significantly improved suppression of the machine background. Based on a Monte Carlo background simulation, the high event rate faced by the first level trigger is analyzed and the benefits of the neural network trigger are evaluated.

Compact model for quarks and leptons via flavored axions

We show how the scales responsible for Peccei-Quinn (PQ), seesaw, and Froggatt and Nielsen (FN) mechanisms can be fixed, by constructing a compact model for resolving rather recent, but fast-growing issues in astro-particle physics, including quark and leptonic mixings and CP violations, high-energy neutrinos, QCD axion, and axion cooling of stars. The model is motivated by the flavored PQ symmetry for unifying the flavor physics and string theory. The QCD axion decay constant congruent to the seesaw scale, through its connection to the astro-particle constraints of both the stellar evolution induced by the flavored-axion bremsstrahlung off electrons $e+Ze\rightarrow Ze+e+A_i$ and the rare flavor-changing decay process induced by the flavored-axion $K^+\rightarrow\pi^++A_i$, is shown to be fixed at $F_A=3.56^{+0.84}_{-0.84}\times10^{10}$ GeV (consequently, the QCD axion mass $m_a=1.54^{+0.48}_{-0.29}\times10^{-4}$ eV, Compton wavelength of its oscillation $\lambda_a=8.04^{+1.90}_{-1.90}\,{\rm mm}$, and axion to neutron coupling $g_{Ann}=2.14^{+0.66}_{-0.41}\times10^{-12}$, etc.). Subsequently, the scale associated to FN mechanism is dynamically fixed through its connection to the standard model fermion masses and mixings, $\Lambda=2.04^{\,+0.48}_{\,-0.48}\times10^{11}\,{\rm GeV}$, and such fundamental scale might give a hint where some string moduli are stabilized in type-IIB string vacua. In the near future, the NA62 experiment expected to reach the sensitivity of ${\rm Br}(K^+\rightarrow\pi^++A_i)<1.0\times10^{-12}$ will probe the flavored-axions or exclude the model, if the astrophysical constraint of star cooling is really responsible for the flavored-axion.

Constraints on models of scalar and vector leptoquarks decaying to a quark and a neutrino at s=13 TeV

The results of a previous search by the CMS Collaboration for squarks and gluinos are reinterpreted to constrain models of leptoquark (LQ) production. The search considers jets in association with a transverse momentum imbalance, using the $M_\mathrm{T2}$ variable. The analysis uses proton-proton collision data at $\sqrt{s}=$ 13 TeV, recorded with the CMS detector at the LHC in 2016 and corresponding to an integrated luminosity of 35.9 fb$^{-1}$. Leptoquark pair production is considered with LQ decays to a neutrino and a top, bottom, or light quark. This reinterpretation considers higher mass values than the original CMS search to constrain both scalar and vector LQs. Limits on the cross section for LQ pair production are derived at the 95% confidence level depending on the LQ decay mode. A vector LQ decaying with a 50% branching fraction to t$\nu$, and 50% to b$\tau$, has been proposed as part of an explanation of anomalous flavor physics results. In such a model, using only the decays to t$\nu$, LQ masses below 1530 GeV are excluded assuming the Yang--Mills case with coupling $\kappa =$ 1, or 1115 GeV in the minimal coupling case $\kappa =$ 0, placing the most stringent constraint to date from pair production of vector LQs.

Muon g−2 in the 2HDM: Maximum results and detailed phenomenology

We present a comprehensive analysis of the muon magnetic moment $a_\mu$ in the flavour-aligned two-Higgs doublet model (2HDM) and parameter constraints relevant for $a_\mu$. We employ a recent full two-loop computation of $a_\mu$ and take into account experimental constraints from Higgs and flavour physics on the parameter space. Large $a_\mu$ is possible for light pseudoscalar Higgs A with large Yukawa couplings to leptons, and it can be further increased by large A coupling to top quarks. We investigate in detail the maximum possible Yukawa couplings to leptons and quarks of a light A, finding values of around 50...100 (leptons) and O(0.5) (quarks). As a result we find that an overall maximum of $a_\mu$ in the 2HDM of more than $45\times10^{-10}$ is possible in a very small parameter region around M_A=20 GeV. The parameter regions in which the currently observed deviation can be explained are characterized.

Connecting Electroweak Symmetry Breaking and Flavor: A Light Dilaton D and a Sequential Heavy Quark Doublet Q

The 125 GeV boson is quite consistent with the Higgs boson of the Standard Model (SM), but there is a challenge from Anderson as to whether this particle is in the Lagrangian. As Large Hadron Collider (LHC) Run 2 enters its final year of running, we ought to reflect and make sure we have gotten everything right. The ATLAS and CMS combined Run 1 analysis claimed a measurement of 5.4σ vector boson fusion (VBF) production which is consistent with SM, which seemingly refutes Anderson. However, to verify the source of electroweak symmetry breaking (EWSB), we caution that VBF measurement is too important for us to be imprudent in any way, and gluon–gluon fusion (ggF) with similar tag jets must be simultaneous measured, which should be achievable in LHC Run 2. The point is to truly test the dilaton possibility—the pseudo-Goldstone boson of scale invariance violation. We illustrate EWSB by dynamical mass generation of a sequential quark doublet (Q) via its ultrastrong Yukawa coupling and argue how this might be consistent with a 125 GeV dilaton, D. The ultraheavy 2mQ≳4–5 TeV scale explains the absence of New Physics so far, while the mass generation mechanism shields us from the UV theory for the strong Yukawa coupling. Collider and flavor physics implications are briefly touched upon. Current Run 2 analyses show correlations between the ggF and VBF measurements, but the newly observed tt¯H production at LHC poses a challenge.

Heavy flavour physics: A portal to new physics

Heavy flavour physics, as one of the most important parts in the standard model (SM), plays important roles in testing the SM and probing the effects of new physics beyond SM. Thanks to the two B factories, Beijing spectrometer (BES) and large hadron collider beauty (LHCb) experiment, in the past decades, there is great development in heavy flavour physics, in both experimental and theoretical sides. In heavy flavour physics, the first work is to test SM by measuring three angles ($\alpha$, $\beta$ and $\gamma$) and sides of Cabbibo-Kobayashi-Maskawa (CKM) triangle. Though the current results are in agreement with SM, the fit becomes less constrained in the presence of new physics, and ${\cal~O}(20%$) contributions of new physics are still allowed. The charge conjugation and parity (CP) asymmetries measured by two B factories helped us to study the matter-antimatter asymmetry in the Universe, and the discovered CP asymmetry in $\rm~B\to~K_s~J/\psi$ had led to the nobel prize for Kobayashi and Maskawa. In addition, the mass differences between the neutral meson and their own anti-meson can present us the information of the scale of the new physics, because possible new particles might contribute to $\rm~D^0$, $\rm~B_d^0$, and $\rm~B_s^0$ meson mixing through box diagrams. The nonleptonic decays of B mesons afford us a new plate for studying the quantum chromodynamics (QCD) and factorization concept, as well as searching for the strong phases, which are important factors in CP asymmetries. Importantly, the rare decays induced by the flavour-changing neutral-current has attracted much interest, because the new physics can affect the observables remarkably through penguin diagrams. In addition, it is of interest to note that the recent anomalies, such as $R_{\rm~K^{(*)}}$, $R({\rm~D}^{(*)})$ and $P_5^\prime$, might be the hints of new physics beyond SM. In order to explain these anomalies, many models have been proposed, however, all of them should be further tested in the future on both experimental and theoretical sides. Moreover, heavy flavor physics also provides us an important plate for studying the new exotic states. Many new discovered exotic states in BES-III, B factories and LHCb, such as $\rm~X(3872)$, $\rm~Z_c(3900)$ and $\rm~P_c(4450,4380)$, motivate us to further study the quark model and QCD. In 2018, Belle-II will take data, where the total luminosity is predicted to be $50\rm~ab^{-1}$ and $10^{10}$ B meson pair could be produced. The proposed International Linear Collider, Circular Electron-Positron Collider and Hyper-Z Factory, can also produce large number of B mesons, as well as $\rm~B_s$, $\rm~B_c$ and $\Lambda_b$, when the collision energy is set to be ${\cal~O}(m_Z)$. These high precision or energy machines, together with the current LHCb experiment and its next generation, will lead heavy flavor physics to a new golden era.

Update of the global electroweak fit and constraints on two-Higgs-doublet models

We present an update of the global fit of the Standard Model electroweak sector to latest experimental results. We include new kinematic top quark and W boson mass measurements from the LHC, a sin !^2theta ^{ell }_{mathrm{eff}} result from the Tevatron, and a new evaluation of the hadronic contribution to alpha (M_Z^2). We present tests of the internal consistency of the electroweak Standard Model and updated numerical predictions of key observables. The electroweak data combined with measurements of the Higgs boson coupling strengths and flavour physics observables are used to constrain parameters of two-Higgs-doublet models.

Effects of heavy neutrinos on vacuum stability in two-Higgs-doublet model with GUT scale supersymmetry

We analyse the implications of right-handed neutrinos on the stability of the electroweak vacuum in two-Higgs-doublet models with supersymmetry at high scale. It is assumed that supersymmetry is broken at scale MS = 2 × 1016 GeV and effective theory below MS is two-Higgs-doublet model of type II with three generations of singlet neutrinos which induce small masses for the standard model neutrinos through type I seesaw mechanism. We study the high and low scale versions of seesaw mechanism. In both these cases, we show that the presence of right-handed neutrinos significantly improves the stability of electroweak vacuum if their Yukawa couplings with the SM leptons are of mathcal{O} (1) or greater. However, this possibility is severely constrained by the measured mass and couplings of Higgs and limits on the mass of the charged Higgs from the flavour physics data. It is shown that the stable or metastable electroweak vacuum and experimentally viable low energy scalar spectrum require tan β ≲ 2.5 and the magnitude of neutrino Yukawa couplings smaller than mathcal{O} (1). The results obtained in this case are qualitatively similar to those without right-handed neutrinos.

Resolution of RD/[formula omitted] puzzle

One of the exciting results in flavor physics in recent times is the RD/RD⁎ puzzle. The measurements of these flavor ratios performed by the B-factory experiments, BaBar and Belle, and the LHCb experiment are about 4σ away from the Standard Model expectation. These measurements indicate that the mechanism of b→cτν¯ decay is not identical to that of b→c(μ/e)ν¯. This charge lepton universality violation is particularly intriguing because these decays occur at tree level in the Standard Model. In particular, we expect a moderately large new physics contribution to b→cτν¯. The different types of new physics amplitudes, which can explain the RD/RD⁎ puzzle, have been identified previously. In this letter, we show that the polarization fractions of τ and D⁎ and the angular asymmetries AFB and ALT in B→D⁎τν¯ decay have the capability to uniquely identify the Lorentz structure of the new physics. A measurement of these four observables will lead to such an identification.

Charged Higgs pair production in association with the Z0 boson at electron-positron colliders

In this paper, the production of the charged Higgs pair associated with the $Z^0$ boson is analyzed in the minimal extension of the standard model the so-called two-Higgs-doublet model (2HDM). The process $e^+e^- \rightarrow H^+H^-Z^0$ is calculated at the tree level including all the possible diagrams in 2HDM. The numerical analysis is performed in consideration of the current experimental constraints and various scenarios for the free parameters of the model. The results are presented as a function of center-of-mass energy, the charged Higgs mass ($m_{H^\pm}$), and the ratio of the vacuum expectation values ($t_\beta$). The unpolarized cross section, taking into account the results in the flavor physics, gets up to $0.278\text{ fb}$ for $m_{H^\pm}=175\text{ GeV}$, and it declines with decreasing $m_{H^\pm}$ in Type-I. However, it gets down to $0.073\text{ fb}$ for $m_{H^\pm}=500\text{ GeV}$ in Type-II. Further, the calculation is also carried out in the non-alignment scenario and low-$m_{h^0}$ scenario. The effect of the polarized incoming $e^+$ and $e^-$ beams shows that the cross section is enhanced by a factor up to 2.5 at P(+0.60,-0.80) polarization configuration. Decay channels of the charged Higgs, possible final states of the process, and some differential distributions belong to the charged Higgs and $Z^0$ boson are examined for each scenario. The analysis shows that some channels have higher branching ratio such as $H^+\rightarrow t \bar{b}$, $H^+\rightarrow W^+h^0$, and $H^+\rightarrow W^+H^0$. These decay channels are essential for the charged Higgs searches in the lepton colliders regarding the scenarios interested. The detection of the charged Higgs is a powerful sign for the extended scalar sectors, and the results show the potential of a future lepton collider.

Flavor physics and flavor anomalies in minimal fundamental partial compositeness

Partial compositeness is a key ingredient of models where the electroweak symmetry is broken by a composite Higgs state. Recently, a UV completion of partial compositeness was proposed, featuring a new strongly coupled gauge interaction as well as new fundamental fermions and scalars. We work out the full flavor structure of the minimal realization of this idea and investigate in detail the consequences for flavor physics. While CP violation in kaon mixing represents a significant constraint on the model, we find many viable parameter points passing all precision tests. We also demonstrate that the recently observed hints for a violation of lepton flavor universality in $B\to K^{(*)}\ell\ell$ decays can be accommodated by the model, while the anomalies in $B\to D^{(*)}\tau\nu$ cannot be explained while satisfying LEP constraints on $Z$ couplings.

Seeking leptoquarks in IceCube

We investigate the sensitivity of IceCube(-Gen2) to a scalar leptoquark scenario with couplings only to heavy quark flavors which may be connected to solving discrepancies in B-meson semileptonic decays. We take into account, for the first time, the non-negligible neutrino-gluon cross section induced by leptoquarks, and we systematically account for indirect and direct constraints which have been overlooked in previous studies. We conclude that IceCube(-Gen2) can only probe the light LQ regime, already disfavored by the combination of flavor physics constraints, electroweak precision data and the direct searches at the LHC.

Monte Carlo study of particle identification at the CEPC using TPC dE\xa0/\xa0dx information

The kaon identification is crucial for the flavor physics, and also benefits the flavor and charge reconstruction of the jets. We explore the particle identification capability for tracks with momenta ranging from 2–20 GeV/c using the dE / dx measurements in the Time Projection Chamber at the future Circular Electron-Positron Collider. Based on Monte Carlo simulation, we anticipate that an average 3.2~sigma (2.6~sigma ) K/pi separation can be achieved based on dE / dx information for an optimistic (conservative) extrapolation of the simulated performance to the final system. Time-of-flight (TOF) information from the Electromagnetic Calorimeter can provide K/pi separation around 1 GeV/c and reduce the K / p mis-identification rate. By combining the dE / dx and TOF information, we estimate that in the optimistic scenario a kaon selection in inclusive hadronic Z decays with both the average efficiency and purity approaching 95% can be achieved.

Rareb-hadron decays as probe of new physics

The unexpected absence of unambiguous signals of New Physics (NP) at the TeV scale at the Large Hadron Collider (LHC) puts today flavor physics at the forefront. In particular, rare decays of b-hadrons represent a unique probe to challenge the Standard Model (SM) paradigm and test models of NP at a scale much higher than that accessible by direct searches. This article reviews the status of the field.

Laura[formula omitted]: A Dalitz plot fitter

The Dalitz plot analysis technique has become an increasingly important method in heavy flavour physics. The Laura++ fitter has been developed as a flexible tool that can be used for Dalitz plot analyses in different experimental environments. Explicitly designed for three-body decays of heavy-flavoured mesons to spinless final state particles, it is optimised in order to describe all possible resonant or nonresonant contributions, and to accommodate possible CP violation effects. Program summaryProgram title: Laura++Program Files doi:http://dx.doi.org/10.17632/jn266r57nk.1Licensing provisions: Apache License, Version 2.0Programming language:C++Nature of problem: Dalitz-plot analysis of particle decays is an important and increasingly utilised technique in particle physics, in particular in heavy flavour physics. While various software tools have been used for Dalitz plot analyses, these are usually not well optimised and are neither scalable for use with larger samples nor flexible enough to be easily adapted for other analyses.Solution method:Laura++ is a dedicated package for performing Dalitz-plot analysis that is flexible enough both to be used in a range of experimental environments and to describe many possible different decays and types of analyses. It allows analysts to create amplitude models to describe the decay of interest and to use those models either to generate pseudoexperiments or to fit them to data.