Flavor Physics Research Articles

Holograms of a dynamical top quark

We present holographic descriptions of dynamical electroweak symmetry breaking models that incorporate the top mass generation mechanism. The models allow computation of the spectrum in the presence of large anomalous dimensions due to walking and strong Nambu--Jona-Lasinio interactions. Technicolor and QCD dynamics are described by the bottom-up Dynamic AdS/QCD model for arbitrary gauge groups and numbers of quark flavors. An assumption about the running of the anomalous dimension of the quark bilinear operator is input, and the model then predicts the spectrum and decay constants for the mesons. We add Nambu--Jona-Lasinio interactions responsible for flavor physics from extended technicolor, top-color, etc., using Witten's multitrace prescription. We show the key behaviors of a top condensation model can be reproduced. We study generation of the top mass in (walking) one doublet and one family technicolor models and with strong extended technicolor interactions. The models clearly reveal the tensions between the large top mass and precision data for $\ensuremath{\delta}\ensuremath{\rho}$. The necessary tunings needed to generate a model compatible with precision constraints are simply demonstrated.

Hidden sector behind the CKM matrix

The small quark mixing, described by the Cabibbo-Kobayashi-Maskawa (CKM) matrix in the Standard Model, may be a clue to reveal new physics around the TeV scale. We consider a simple scenario that extra particles in a hidden sector radiatively mediate the flavor violation to the quark sector around the TeV scale and effectively realize the observed CKM matrix. The lightest particle in the hidden sector, whose contribution to the CKM matrix is expected to be dominant, is a good dark matter (DM) candidate, so we focus on the contribution, and discuss the DM physics. In this scenario, there is an explicit relation between the CKM matrix and flavor violating couplings, such as four-quark couplings, because both are radiatively induced by the particles in the hidden sector. Then, we can explicitly find the DM mass region and the size of Yukawa couplings between the DM and quarks, based on the study of flavor physics and DM physics. In conclusion, we show that DM mass in our scenario is around the TeV scale, and the Yukawa couplings are between ${\cal O}(0.01)$ and ${\cal O}(1)$. The spin-independent DM scattering cross section is estimated as ${\cal O}(10^{-9})$ [pb]. An extra colored particle is also predicted at the ${\cal O}(10)$ TeV scale.

Flavor structure, Higgs boson mass, and dark matter in a supersymmetric model with vector-like generations

We study a supersymmetric model in which the Higgs mass, the muon anomalous magnetic moment and the dark matter are simultaneously explained with extra vector-like generation multiplets. For the explanations, non-trivial flavor structures and a singlet field are required. In this paper, we study the flavor texture by using the Froggatt-Nielsen mechanism, and then find realistic flavor structures which reproduce the Cabbibo-Kobayashi-Maskawa matrix and fermion masses at low energy. Furthermore, we find that the fermion component of the singlet field becomes a good candidate of dark matter. In our model, flavor physics and dark matter are explained with moderate size couplings through renormalization group flows, and the presence of dark matter supports the existence of just three generations in low energy scales. We analyze the parameter region where the current thermal relic abundance of dark matter, the Higgs boson mass and the muon $g-2$ can be explained simultaneously.

High- $$p_T$$ p T dilepton tails and flavor physics

We investigate the impact of flavor-conserving, non-universal quark-lepton contact interactions on the dilepton invariant mass distribution in p~p rightarrow ell ^+ ell ^- processes at the LHC. After recasting the recent ATLAS search performed at 13 TeV with 36.1 fb^{-1} of data, we derive the best up-to-date limits on the full set of 36 chirality-conserving four-fermion operators contributing to the processes and estimate the sensitivity achievable at the HL-LHC. We discuss how these high-p_T measurements can provide complementary information to the low-p_T rare meson decays. In particular, we find that the recent hints on lepton-flavor universality violation in b rightarrow s mu ^+ mu ^- transitions are already in mild tension with the dimuon spectrum at high-p_T if the flavor structure follows minimal flavor violation. Even if the mass scale of new physics is well beyond the kinematical reach for on-shell production, the signal in the high-p_T dilepton tail might still be observed, a fact that has been often overlooked in the present literature. In scenarios where new physics couples predominantly to third generation quarks, instead, the HL-LHC phase is necessary in order to provide valuable information.

Phenomenology of flavorful composite vector bosons in light of B anomalies

We analyze the flavor structure of composite vector bosons arising in a model of vectorlike technicolor — often called hypercolor (HC) — with eight flavors that form a one-family content of HC fermions. Dynamics of the composite vector bosons, referred to as HC ρ in this paper, are formulated together with HC pions by the hidden local symmetry (HLS), in a way analogous to QCD vector mesons. Then coupling properties to the standard model (SM) fermions, which respect the HLS gauge symmetry, are described in a way that couplings of the HC ρs to the left-handed SM quarks and leptons are given by a well-defined setup as taking the flavor mixing structures into account. Under the present scenario, we discuss significant bounds on the model from electroweak precision tests, flavor physics, and collider physics. We also try to address B anomalies in processes such as B → K(∗)μ+μ− and Bto {D}^{left(ast right)}tau;overline{nu} , recently reported by LHCb, Belle, (ATLAS, and CMS in part). Then we find that the present model can account for the anomaly in B → K(∗)μ+μ− consistently with the other constraints while it predicts no significant deviations in Bto {D}^{left(ast right)}tau;overline{nu} ν from the SM, which can be examined in the future Belle II experiment. The former is archived with the form C9 = −C10 of the Wilson coefficients for effective operators of b → sμ+μ−, which has been favored by the recent experimental data. We also investigate current and future experimental limits at the Large Hadron Collider (LHC) and see that possible collider signals come from dijet and ditau, or dimuon resonant searches for the present scenario with TeV mass range. To conclude, the present b → sμ+μ− anomaly is likely to imply discovery of new vector bosons in the ditau or dimuon channel in the context of the HC ρ model. Our model can be considered as a UV completion of conventional U(1)′ models.

Axion and neutrino physics in aU(1)-enhanced supersymmetric model

Motivated by the flavored Peccei-Quinn symmetry for unifying the flavor physics and string theory, we construct an explicit model by introducing a $U(1)$ symmetry such that the $U(1)_X$-$[gravity]^2$ anomaly-free condition together with the standard model flavor structure demands additional sterile neutrinos as well as no axionic domain-wall problem. Such additional sterile neutrinos play the role of a realization of baryogenesis via a new Affleck-Dine leptogenesis. We provide grounds for that the $U(1)_X$ symmetry could be interpreted as a fundamental symmetry of nature. The model will resolve rather recent, but fast-growing issues in astro-particle physics, including leptonic mixings and CP violation in neutrino oscillation, high-energy neutrinos, QCD axion, and axion cooling of stars. The QCD axion decay constant, through its connection to the astrophysical constraints of stellar evolution and the SM fermion masses, is shown to be fixed at $F_A=1.30^{+0.66}_{-0.54}\times10^{9}$ GeV (consequently, its mass is $m_a=4.34^{+3.37}_{-1.49}$ meV and axion-photon coupling is $|g_{a\gamma\gamma}|=1.30^{+1.01}_{-0.45}\times10^{-12}\,{\rm GeV}^{-1}$). Interestingly enough, we show that neutrino oscillations at low energies could be connected to astronomical-scale baseline neutrino oscillations. The model predicts non-observational neutrinoless double beta ($0\nu\beta\beta$) decay rate as well as a remarkable pattern between leptonic Dirac CP phase ($\delta_{CP}$) and atmospheric mixing angle ($\theta_{23}$); {\it e.g.} $\delta_{CP}\simeq220^{\circ}-240^{\circ}$, $120^{\circ}-140^{\circ}$ for $\theta_{23}=42.3^{\circ}$ for normal mass ordering, and $\delta_{CP}\simeq283^{\circ},250^{\circ},100^{\circ},70^{\circ}$ for $\theta_{23}=49.5^{\circ}$ for inverted one.

The LHCb Upgrade

During the LHC Run 1 the LHCb experiment has successfully performed a large number of high precision measurements in heavy flavour physics using 3 fb–1 of data collected at centre-of-mass energies of 7 TeV and 8 TeV. In LHC Run 2 the LHCb is expected to integrate an additional 5 fb−1 data, however many of the measurements will remain limited by statistics. For this reason LHCb will undergo in 2020 a major upgrade during the Long Shutdown 2 of LHC, with the aim to collect 50 fb−1 of data by 2028. To achieve this goal the LHCb detector readout rate will be upgraded from the current 1 MHz to the LHC bunch crossing rate of 40 MHz. The luminosity delivered to the experiment will increase by a factor five, up to 2 ⋅ 1033 cm−2 s−1. The online selection of events will be uniquely performed by a pure software trigger, improving the trigger efficiencies. In order to sustain the increased luminosity and readout rate, all the sub-detectors will be upgraded. The architecture of the upgraded DAQ system and trigger strategy will be presented, as well an overview of the sub-detector upgrades.

Separate T, CP, CPT Asymmetries in Neutral Meson Transitions

Symmetries, and Symmetry Breakings, in the Laws of Physics play a crucial role in Fundamental Science. Parity and Charge Conjugation Violations prompted the consideration of Chiral Fields in the construction of the Standard Model, whereas CP-Violation needed at least three families of Quarks leading to Flavour Physics. In this Lecture I will discuss the Conceptual Basis and the present experimental results for a Direct Evidence of Separate Reversal-in-Time T, CP and CPT Genuine Asymmetries in Decaying Particles like Neutral Meson Transitions, using Quantum Entanglement and the Decay as a Filtering Measurement. The eight transitions associated to the Flavour-CP eigenstate decay products of entangled neutral mesons have demonstrated with impressive significance a separate evidence of TRV and CPV in Bd-physics, whereas a CPTV asymmetry shows a 2-σ effect interpreted as an upper limit. Novel CPTV observables are discussed for K and Bd transitions. Their observation would lead to a change of paradigm beyond Quantum Field Theory, however there is nothing in Quantum Mechanics forbidding CPTV. A clean methodology to disentangle CPTV effects in the Hamiltonian dynamics and the ω-effect weakening Entanglement in a given experiment is discussed.

Lattice design and expected performance of the Muon Ionization Cooling Experiment demonstration of ionization cooling

Muon beams of low emittance provide the basis for the intense, well-characterized neutrino beams necessary to elucidate the physics of flavor at a neutrino factory and to provide lepton-antilepton collisions at energies of up to several TeV at a muon collider. The international Muon Ionization Cooling Experiment (MICE) aims to demonstrate ionization cooling, the technique by which it is proposed to reduce the phase-space volume occupied by the muon beam at such facilities. In an ionization-cooling channel, the muon beam passes through a material in which it loses energy. The energy lost is then replaced using rf cavities. The combined effect of energy loss and reacceleration is to reduce the transverse emittance of the beam (transverse cooling). A major revision of the scope of the project was carried out over the summer of 2014. The revised experiment can deliver a demonstration of ionization cooling. The design of the cooling demonstration experiment will be described together with its predicted cooling performance.3 MoreReceived 30 January 2017DOI:https://doi.org/10.1103/PhysRevAccelBeams.20.063501Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasBeam dynamicsBeam processesBeam techniquesSecondary beamsAccelerators & Beams

Explaining B→K(*)ℓ+ℓ− anomaly by radiatively induced coupling in U(1)μ−τ gauge symmetry

We propose an extension of the standard model (SM) with $U(1)_{\mu-\tau}$ gauge symmetry, extra vectorlike quark doublets $Q'_a$ and singlet scalar $\chi$, both of which are charged under $U(1)_{\mu-\tau}$ and carry {odd} dark $Z_2$ parity. Then assuming that $\chi$ is the dark matter (DM) of the universe and imposing various constraints from dark matter search, flavor physics and collider search for $Q'_a$, one can show that radiative corrections to $b\rightarrow s Z^{'*} \rightarrow s l^+ l^- $ involving $Q'_a$ and $\chi$ can induce $\Delta C_9 \sim -1$ which can resolve the LHCb anomalies related with $B\to K^{(*)} \ell^+ \ell^-$. Therefore both DM and $B$ physics anomalies could be accommodated in the model.

SL(2,\xa07) representations and their relevance to neutrino physics

The investigation of the rôle of finite groups in flavor physics and, particularly, in the interpretation of the neutrino data has been the subject of intensive research. Motivated by this fact, in this work we derive the three-dimensional unitary representations of the projective linear group PSL_2(7). Based on the observation that the generators of the group exhibit a Latin square pattern, we use available computational packages on discrete algebra to determine the generic properties of the group elements. We present analytical expressions and discuss several examples which reproduce the neutrino mixing angles in accordance with the experimental data.

LHC phenomenology and baryogenesis in supersymmetric models with a U(1)R baryon number

We study the phenomenology of a supersymmetric extension of the Standard Model with an R-symmetry under which R-charges correspond to the baryon number. This identification allows for the presence in the superpotential of the R-parity violating term λ′′UcDcDc without breaking baryon number, which loosens several bounds on this operator while changing considerably the phenomenology. However, the R-symmetry cannot remain exact as it is at least broken by anomaly mediation. Under these conditions, we investigate the constraints coming from baryon number violating processes and flavour physics and find that, in general, they are lessened. Additionally, we examine recent ATLAS and CMS experimental searches and use these to place limits on the parameter space of the model. This is done for both stop production, which now features both pair and resonant production, and pair production of the first two generations of squarks. Finally, we study the implications this model has on baryogenesis. We find that successful baryogenesis can potentially be achieved, but only at the cost of breaking the R-symmetry by a significant amount.

Right-handed charged currents in the era of the Large Hadron Collider

We discuss the phenomenology of right-handed charged currents in the frame-work of the Standard Model Effective Field Theory, in which they arise due to a single gauge-invariant dimension-six operator. We study the manifestations of the nine complex couplings of the W to right-handed quarks in collider physics, flavor physics, and low-energy precision measurements. We first obtain constraints on the couplings under the assumption that the right-handed operator is the dominant correction to the Standard Model at observable energies. We subsequently study the impact of degeneracies with other Beyond-the-Standard-Model effective interactions and identify observables, both at colliders and low-energy experiments, that would uniquely point to right-handed charged currents.

Indirect searches of the degenerate MSSM

A degenerate sfermionic particle spectrum can escape constraints from flavor physics, and at the same time evade the limits from the direct searches if the degeneracy extends to the gaugino-higgsino sector. Inspired by this, we consider a scenario where all the soft terms have an approximately common mass scale at $M_{\text{SUSY}}$, with splittings $\lesssim \mathcal{O}(10\%)$. As a result, the third generation sfermions have large to maximal (left-right) mixing, the same being the case with charginos and some sectors of the neutralino mass matrix. We study this scenario in the light of discovery of the Higgs boson with mass $\sim$ 125 GeV. We consider constraints from $B$-physics, the anomalous magnetic moment of the muon and the dark matter relic density. We find that a supersymmetric spectrum as light as 600 GeV could be consistent with all current data and also account for the observed anomalous magnetic moment of the muon within $2\sigma$. The neutralino relic density is generally too small to saturate the measured cold dark matter relic density. Direct detection limits from XENON100 and LUX put severe constraints on this scenario which will be conclusively probed by XENONnT experiment.

Constraints on two-Higgs-doublet model parameters in light of rare B decays

We established the allowed parameters of two-Higgs doublet model(2HDM) from flavor physics observables, precisely from rare $B$ meson decays. In our analysis most formidable constraints on the 2HDM parameters arise from the branching ratio of rare radiative $B$ meson decay i.e., $B\to X_{s}\gamma$. However, the constraints arising from the branching ratio of $B_{s}\to\mu^{+}\mu^{-}$ decay in $m_{H^{\pm}}-\tan\beta$ plane give $m_{H^{\pm}} >$ 80 GeV for the value of $\tan\beta\sim 2$, that is in agreement with large electron-positron collider (LEP) data. Furthermore, we also investigate the bounds on the $CP$-even $m_{H}$ and $CP$-odd $m_{A^{0}}$ Higgs boson not only from above mentioned physical observables, but also from the zero crossing of the forward-backward asymmetry of $B\to K^{\ast}\mu^{+}\mu^{-}$ decay. Therefore, these bounds on parameters of the 2HDM will provides a fertile ground to test the 2HDM at current and future $B$-physics experiments.

Recent results on flavor physics by CMS

Differential production cross sections of the B+ meson and of J/ψ, ψ(2S), and ϒ(nS) (n=1,2,3) vector mesons are measured in pp collisions at 13 TeV, as a function of transverse momentum and rapidity, using data collected by the CMS detector in 2015. The B+ is reconstructed in the decay channel B+→J/ψK+ and the five S-wave quarkonium states are reconstructed in the dimuon decay channel.The simultaneous production of two ϒ(1S) mesons in observed at s=8 TeV in a data sample corresponding to an integrated luminosity of L=20.7 fb−1.The angular distributions and the differential branching fraction of the flavor changing neutral current decay B0→K⁎0μ+μ− are studied using data collected at s=8 TeV, as a function of the dimuon invariant mass squared.Flavor changing neutral currents in top quark couplings are searched through single top quark production in association with a photon, and in tt‾→Zq+Wb and tt‾→Hq+Wb decays, where Z→l+l− and H→γγ, WW, ZZ, τ+τ− and bb‾. The observed number of events agrees with the Standard Model prediction, and no evidence of flavor changing neutral currents is found.

Constraints on the CP-Violating MSSM

We discuss the prospects for observing CP violation in the MSSM with six CP-violating phases, using a geometric approach to maximise CP-violating observables subject to the experimental upper bounds on electric dipole moments. We consider constraints from Higgs physics, flavour physics, the dark matter relic density and spin-independent scattering cross section with matter.

Electroweak symmetry breaking and collider signatures in the next-to-minimal composite Higgs model

We conduct a detailed numerical analysis of the composite pseudo-Nambu-Goldstone Higgs model based on the next-to-minimal coset SO(6)/SO(5) ≅ SU(4)/Sp(4), featuring an additional SM singlet scalar in the spectrum, which we allow to mix with the Higgs boson. We identify regions in parameter space compatible with all current exper-imental constraints, including radiative electroweak symmetry breaking, flavour physics, and direct searches at colliders. We find the additional scalar, with a mass predicted to be below a TeV, to be virtually unconstrained by current LHC data, but potentially in reach of run 2 searches. Promising indirect searches include rare semi-leptonic B decays, CP violation in Bs mixing, and the electric dipole moment of the neutron.

The LHCb upgrade: plans and physics potential

LHCb is performing a large number of world-class precision measurements in heavy flavour physics. However, yet by the end of the LHC Run 2, many of these measurements will remain limited by statistics, even though adding the expected integrated luminosity of 5 – 6 fb−1. The main obstacle preventing LHCb to run the present detector at higher luminosities, with enhanced trigger efficiencies, is the current 1 MHz readout system limitation. The detector will therefore undergo a major upgrade in the Long Shutdown 2 (2018 – 2019) aiming at collecting an order of magnitude more data by 2028. The upgrade consists of a new readout system operating at the LHC bunch crossing rate of 40 MHz. The data acquisition system will exploit the ultimate flexibility of a software trigger. The instantaneous luminosity will increase to 2×1033 cm−2s−1, five times higher than presently. In order to cope with the higher expected occupancies and radiation doses several sub-detector upgrades are underway. The physics potential of LHCb shall improve considerably, as will be discussed.

Multiloop Corrections to Rare B Decays

Multiloop calculations that matter for low-energy flavour physics are of two generic types. The first-type ones allow us to determine the Wilson coefficients in the effective theory of interest. At the technical level, they can be reduced to evaluating tadpole diagrams. The second-type ones are necessary to express physical observables in terms of the Wilson coefficients. Here, after summarizing the Wilson coefficient status, I am going to describe several recent perturbative calculations that have contributed to a reduction of theoretical uncertainties in the branching ratios of Bs,d→ℓ+ℓ− and B‾→Xs,dγ