Flavor-changing Processes Research Articles

Search for particles and forces beyond the standard model in high energy lepton-hadron and hadron-hadron collisions

A review of searches for physics beyond the Standard Model carried out at high energy leptonhadron and hadron-hadron facilities is presented, with emphasis on topics of interest for future data taking at the upgraded Tevatron p p and HERA ep colliders. The status and discovery prospects are discussed for leptoquarks, Technicolour and supersymmetry, forbidden lepton and quark flavour-changing processes, extra gauge bosons, excited states of composite fermions, generic contact interactions and extra compactified dimensions.

Quark-squark alignment reexamined

We re-examine the possibility that the solution to the supersymmetric flavor problem is related to small mixing angles in gaugino couplings induced by approximate horizontal Abelian symmetries. We prove that, for a large class of models, there is a single viable structure for the down quark mass matrix with four holomorphic zeros. Consequently, we are able to obtain both lower and upper bounds on the supersymmetric mixing angles and predict the contributions to various flavor changing neutral current processes. We find that the most likely signals for alignment are $\Delta m_D$ close to the present bound, significant CP violation in $D^0-\bar{D^0}$ mixing, and shifts of order a few percent in various CP asymmetries in $B^0$ and $B_s$ decays. In contrast, the modifications to radiative B decays, to $\epsilon^\prime/\epsilon$ and to $K\to\pi\nu\bar\nu$ decays are small. We further investigate a new class of alignment models, where supersymmetric contributions to flavor changing processes are suppressed by both alignment and RGE-induced degeneracy.

Neutrino masses and lepton flavor violation in thick brane scenarios

We address the issue of lepton flavour violation and neutrino masses in the ``fat-brane'' paradigm, where flavour changing processes are suppressed by localising different fermion field wave-functions at different positions (in the extra dimensions) in a thick brane. We study the consequences of suppressing lepton number violating charged lepton decays within this scenario for lepton masses and mixing angles. In particular, we find that charged lepton mass matrices are constrained to be quasi-diagonal. We further consider whether the same paradigm can be used to naturally explain small Dirac neutrino masses by considering the existence of three right-handed neutrinos in the brane, and discuss the requirements to obtain phenomenologically viable neutrino masses and mixing angles. Finally, we examine models where neutrinos obtain a small Majorana mass by breaking lepton number in a far away brane and show that, if the fat-brane paradigm is the solution to the absence of lepton number violating charged lepton decays, such models predict, in the absence of flavour symmetries, that charged lepton flavour violation will be observed in the next round of rare muon/tau decay experiments.

Dynamical CP violation and flavour-changing processes

We investigate the phenomenological constraints on a model where, besides the standard model Higgs sector, there is an effective new strong interaction acting on the third generation of quarks and characterized by a $\theta$-like term. This $\theta$ term induces electroweak symmetry breaking and leads to dynamical spontaneous CP violation. We show that the constraints coming from K physics and the electric dipole moment of the neutron impose that the new physics scale should be of the order of 35 TeV. Contrary to naive expectations, the predictions of the model for B physics are very close to the standard model ones. The main differences appear in processes involving the up quarks such as $D^0-\bar{D}^0$ mixing and in the electric dipole moment of the neutron, which should be close to the experimental limit. Possible deviations from the standard model predictions for CP asymmetries in B decays are also considered.

CPviolation and the scale of supersymmetry breaking

Supersymmetric models with a high supersymmetry breaking scale give, in general, large contributions to ${\ensuremath{\varepsilon}}_{K}$ and/or to various electric dipole moments, even when contributions to $\mathrm{CP}$ conserving, flavor changing processes are sufficiently suppressed. Some examples are models of dilaton dominance, alignment, non-Abelian flavor symmetries, heavy first two generation sfermions, anomaly mediation and gaugino mediation. There is then strong motivation for ``approximate $CP,''$ that is, a situation where all $\mathrm{CP}$ violating phases are small. In contrast, in supersymmetric models with a low breaking scale it is quite plausible that the CKM matrix is the only source of flavor and $\mathrm{CP}$ violation. Gauge mediation provides a concrete example. Approximate $\mathrm{CP}$ is then unacceptable. Upcoming measurements of the $\mathrm{CP}$ asymmetry in $\stackrel{\ensuremath{\rightarrow}}{B}\ensuremath{\psi}{K}_{S}$ might exclude or support the idea of approximate $\mathrm{CP}$ and consequently probe the scale of supersymmetry breaking.

The bound on the mass of the new gauge boson Z′ from the process μ→3 e

The new gauge boson Z′ predicted by the strong top dynamical symmetry breaking models has significant contributions to the lepton flavor changing process μ→3 e. We consider the bound on the mass of the new gauge boson Z′ from the experimental value of the branching ratio Br( μ→3 e) in the framework of topcolor assisted technicolor models. We find that the precision experimental value of Br( μ→3 e) gives a severe bound on the Z′ mass M Z′ . For k 1⩽1, M Z′ must be larger than 1.64 TeV.

Lepton flavor violation and bilinear r-parity violation

We examine some flavor changing processes such as rare leptonic decays of the long-lived neutral kaon, muon-electron conversion in nuclei and radiative muon decay which are induced by the combined effects of bilinear and trilinear R-parity violations. These processes are used to put strong constraints on certain products of the bilinear and trilinear couplings. We also discuss the constraints on R-parity violation from neutrino masses and compare them with the constraints from flavor changing decay processes. Large range of parameter space satisfying the constraints from neutrino masses can be excluded by the flavor changing processes considered in this paper, and also vice versa.

Neutrino mixing and flavour changing processes

We study the implications of a large ν μ – ν τ mixing angle on flavour changing transitions of quarks and leptons in supersymmetric extensions of the standard model. Two patterns of supersymmetry breaking are considered, models with modular invariance and the standard scenario of universal soft breaking terms at the GUT scale. The analysis is performed for two symmetry groups G⊗ U(1) F , with G= SU(5) and G= SU(3) 3, where U(1) F is a family symmetry. Models with modular invariance are in agreement with observations only for restricted scalar quark and gaugino masses, M ̄ q 2/m g ̃ 2≃7/9 and m b ̃ >350 GeV. A characteristic feature of models with large tan β and radiatively induced flavour mixing is a large branching ratio for μ→ eγ. For both symmetry groups and for the considered range of supersymmetry breaking mass parameters we find BR( μ→ eγ)>10 −14.

Hexagonal theory of flavor

We construct a supersymmetric theory of flavor based on the discrete gauge group (D_6)^2, where D_6 describes the symmetry of a regular hexagon under proper rotations in three dimensions. The representation structure of the group allows one to distinguish the third from the lighter two generations of matter fields, so that in the symmetry limit only the top quark Yukawa coupling is allowed and scalar superpartners of the first two generations are degenerate. Light fermion Yukawa couplings arise from a sequential breaking of the flavor symmetry, and supersymmetric flavor-changing processes remain adequately suppressed. We contrast our model with others based on non-Abelian discrete gauge symmetries described in the literature, and discuss the challenges in constructing more minimal flavor models based on this approach.

Phenomenological issues in TeV scale gravity with light neutrino masses

The possible existence of bulk singlet neutrinos in the scenario with large compactified dimensions and low string scale M ∗ has important consequences for low-energy observables. We demonstrate that intergenerational mass splitting and mixing lead to the effective violation of the lepton universality and flavor changing processes in charged lepton sector. Current experimental constraints push M ∗ to the scale of 10 TeV over most of the interesting range for neutrino mass splitting.

Flavor changing processes in quarkonium decays

We study flavor changing processes $\Upsilon \to B/{\bar B} X_s$ and $J/\psi \to D/{\bar D} X_u$ in the B factories and the Tau-Charm factories. In the standard model, these processes are predicted to be unobservable, so they serve as a probe of the new physics. We first perform a model independent analysis, then examine the predictions of models; such as TopColor models, MSSM with R-parity violation and the two Higgs doublet model; for the branching ratios of $\Upsilon \to B/{\bar B} X_s$ and $J/\psi \to D/{\bar D} X_u$ . We find that these branching ratios could be as large as $10^{-6}$ and $10^{-5}$ in the presence of new physics.

Symmetry studies in flavor changing processes

In searching for new physics beyond the standard model, the importance of symmetry studies, in particular in the flavor changing processes of both quarks and leptons, is stressed. As examples in this talk, for the quark sector, a search for violation of the time reversal invariance in K + decays is presented, whereas for the lepton sector a search for violation of the lepton flavor conservation in rare muon decays is discussed.

Exotic leptoquarks from superstring-derived models

The H1 and ZEUS collaborations have recently reported a significant excess of e + p → e + jet events at high- Q 2. While there exists insufficient data to conclusively determine the origin of this excess, one possibility is that it is due to a new leptoquark at mass scale around 200 GeV. We examine the type of leptoquark states that exists in superstring-derived standard-like models, and show that, while these models may contain the standard leptoquark states which exist in Grand Unified Theories, they also generically contain new and exotic leptoquark states with fractional lepton number, ± 1 2 . In contrast to the traditional GUT-type leptoquark states, the couplings of the exotic leptoquarks to the Standard Model states are generated after the breaking of U(1) B− L . This important feature of the exotic leptoquark states may result in local discrete symmetries which forbid some of the undesired leptoquark couplings. We examine these couplings in several models and study the phenomenological implications. The flavor symmetries of the superstring models are found to naturally suppress leptoquark flavor-changing processes.

Size of flavor-changing effects induced by the symmetry-breaking sector

It has recently been shown that strong interactions underlying electroweak symmetry breaking will induce four-fermion amplitudes proportional to ${m}_{t}^{2}$, which in turn will influence a variety of flavor-changing processes. We argue that the typical size of these effects is likely to be far below the current experimental bounds.

A low energy dynamical SUSY breaking scenario motivated from superstring derived unification

Recently there has been a resurgence of interest in gauge mediated dynamical supersymmetry breaking scenarios. I investigate how low energy dynamical SUSY breaking may arise from superstring models. In a three generation string derived model I propose that the unbroken hidden non-Abelian gauge group at the string scale is SU(3) H with matter multiplets. Due to the small gauge content of the hidden gauge group the supersymmetry breaking scale may be consistent with the dynamical SUSY breaking scenarios. The messenger states are obtained in the superstring model from sectors which arise due to the “Wilson-line” breaking of the unifying non-Abelian gauge symmetry. An important property of the string motivated messenger states is the absence of superpotential terms with the Standard Model states. The stringy symmetries therefore forbid the flavor changing processes which may arise due to couplings between the messenger sector states and the Standard Model states. Motivated from the problem of string gauge coupling unification I contemplate a scenario in which the messenger sector consists solely of color triplets. This hypothesis predicts a chargino mass below the W-boson mass. Imposing the current limits from the LEP1 and LEP1.5 experiments the lightest supersymmetric particles predicted by this hypothesis are in the mass ranges M χ ± ≈ 55–65 GeV, M χ 0 ≈ 35–50 GeV and M ν ≈ 85–95 GeV which will be tested in the forthcoming LEP2 experiments.

Gauged SO(3) family symmetry and squark mass degeneracy

It is shown that a gauged SO(3) family symmetry can suppress flavor-changing processes from squark-mass non-degeneracy to an acceptable level. The potentially dangerous SO(3) D-tems can be rendered harmless if the CP-violating phases appearing in the SO(3)-breaking sector are small, which can naturally be the case if CP is a spontaneously broken symmetry. This approach has certain advantages over models based on global or non-abelian discrete symmetries, and dovetails with some recent proposals for explaining the pattern of quark and lepton masses. Moreover this approach relates the near-degeneracy of the squark masses to an approximate CP invariance which can also explain the smallness of the electron and neutron electric dipole moments.

Enhancement of the Transition Magnetic Moments of a Neutrino by Degenerate Electrons

The one-loop induced magnetic dipole moments of a neutrino are examined in a background of degenerate electrons in the standard model. For the nonrelativistic neutrino, they are enhanced by a factor \( (8\pF/3\mnu) \), where \pF\ is the electron Fermi momentum and \mnu\ the neutrino mass. For the relativistic neutrino, they enhance the flavor-changing but helicity-conserving process because of the absence of the GIM cancellation.

Flavor-changing neutral current constraints in topcolor-assisted technicolor

It is pointed out that flavor-changing neutral current processes (such as B - B mixing) can be sensitive to the dynamics of the topcolor models recently proposed by Hill [Phys. Lett. B 345 (1995) 483], unless at least some of the mixing angles between down-type quarks are suppressed. The flavor-changing processes are mediated by scalar bound states which are likely to be light as a result of the near critical dynamics of the b-quark sector. The consequences of the latter on the bottom quark mass are also briefly discussed.

Two-loop mixing of dimension-five flavor-changing operators

We calculate the two-loop QCD mixing of the dimension-five flavor-changing operators that arise in the Standard Model after integrating out the W, Z and Higgs bosons and the top-quark, i.e. the mixing among gluonic and photonic magnetic moment operators. Our calculation completes the two-loop anomalous dimension matrix of operators that govern low energy flavor-changing processes. It is an important ingredient of the next-to-leading calculation of the B → X s γ decay rate.

A Vector-Like Extension of the Standard Model

A vector-like extension of the standard model for heavier quarks and leptons with $SU(2)\times U(1)$ gauge symmetry and only one Higgs doublet is examined. This scheme incorporates infinitely many fermions and avoids the appearance of a strongly interacting sector. The model is perturbatively well controllable, and constraints such as $b\rightarrow s\gamma$ and GIM suppression in general are naturally satisfied if the on-set of heavier fermion mass scale is chosen at a few TeV. In this scheme, the physical Higgs particle generally mediates leptonic as well as quark flavor-changing processes at a rate below the present experimental limit. Vector-like models, if suitably defined, provide a viable and interesting scheme for physics beyond 1 TeV.