Z', new fermions and flavor changing processes, constraints on E6_6 models from μ\mu --> eee

We study a new class of flavor changing interactions, which can arise in models based on extended gauge groups (rank $>$4) when new charged fermions are present together with a new neutral gauge boson. We discuss the cases in which the flavor changing couplings in the new neutral current coupled to the $Z^\prime$ are theoretically expected to be large, implying that the observed suppression of neutral flavor changing transitions must be provided by heavy $Z^\prime$ masses together with small $Z$-$Z^\prime$ mixing angles. Concentrating on E$_6$ models, we show how the tight experimental limit on $\mu \rightarrow eee$ implies serious constraints on the $Z^\prime$ mass and mixing angle. We conclude that if the value of the flavor changing parameters is assumed to lie in a theoretically natural range, in most cases the presence of a $Z^\prime$ much lighter than 1 TeV is unlikely.Comment: plain tex, 22 pages + 2 pages figures in PostScript (appended after `\bye'), UM-TH 92-1

The impact of lepton-flavor violating Z' bosons on muon g-2 and other muon observables

A lepton-flavor violating (LFV) Z' boson may mimic some of the phenomena usually attributed to supersymmetric theories. Using a conservative model of LFV Z' bosons, the recent BNL E821 muon g-2 deviation allows for a LFV Z' interpretation with a boson mass up to 4.8 TeV while staying within limits set by muon conversion, mu -> e gamma, and mu -> eee. This model is immediately testable as one to twenty e^+e^- -> mu tau events are predicted for an analysis of the LEP II data. Future muon conversion experiments, MECO and PRIME, are demonstrated to have potential to probe very high boson masses with very small charges, such as a 10 TeV boson with an e-mu charge of 10^-5. Furthermore, the next linear collider is shown to be highly complementary with muon conversion experiments, which are shown to provide the strictest and most relevant bounds on LFV phenomena.Comment: 17 pages, 6 figures, uses feynMF, edited references (v2), corrected MEGA experimental limit (v3), accepted to Phys. Rev.

Minimal lepton flavor violating realizations of minimal seesaw models

We study the implications of the global U(1)R symmetry present in minimal lepton flavor violating implementations of the seesaw mechanism for neutrino masses. In the context of minimal type I seesaw scenarios with a slightly broken U(1)R, we show that, depending on the R-charge assignments, two classes of generic models can be identified. Models where the right-handed neutrino masses and the lepton number breaking scale are decoupled, and models where the parameters that slightly break the U(1)R induce a suppression in the light neutrino mass matrix. We show that within the first class of models, contributions of right-handed neutrinos to charged lepton flavor violating processes are severely suppressed. Within the second class of models we study the charged lepton flavor violating phenomenology in detail, focusing on mu to e gamma, mu to 3e and mu to e conversion in nuclei. We show that sizable contributions to these processes are naturally obtained for right-handed neutrino masses at the TeV scale. We then discuss the interplay with the effects of the right-handed neutrino interactions on primordial B - L asymmetries, finding that sizable right-handed neutrino contributions to charged lepton flavor violating processes are incompatible with the requirement of generating (or even preserving preexisting) B - L asymmetries consistent with the observed baryon asymmetry of the Universe.Comment: 21 pages, 4 figures; version 2: Discussion on possible generic models extended, typos corrected, references added. Version matches publication in JHE

Constraints on a Massive Dirac Neutrino Model

We examine constraints on a simple neutrino model in which there are three massless and three massive Dirac neutrinos and in which the left handed neutrinos are linear combinations of doublet and singlet neutrinos. We examine constraints from direct decays into heavy neutrinos, indirect effects on electroweak parameters, and flavor changing processes. We combine these constraints to examine the allowed mass range for the heavy neutrinos of each of the three generations.Comment: latex, 29 pages, 7 figures (not included), MIT-CTP-221

Lepton flavor violating signals of a little Higgs model at the high energy linear e+e−e^{+}e^{-} colliders

Littlest Higgs $(LH)$ model predicts the existence of the doubly charged scalars $\Phi^{\pm\pm}$, which generally have large flavor changing couplings to leptons. We calculate the contributions of $\Phi^{\pm\pm}$ to the lepton flavor violating $(LFV)$ processes $l_{i}\to l_{j}\gamma$ and $l_{i}\to l_{j}l_{k}l_{k}$, and compare our numerical results with the current experimental upper limits on these processes. We find that some of these processes can give severe constraints on the coupling constant $Y_{ij}$ and the mass parameter $M_{\Phi}$. Taking into account the constraints on these free parameters, we further discuss the possible lepton flavor violating signals of $\Phi^{\pm\pm}$ at the high energy linear $e^{+}e^{-}$ collider $(ILC)$ experiments. Our numerical results show that the possible signals of $\Phi^{\pm\pm}$ might be detected via the subprocesses $e^{\pm}e^{\pm}\to l^{\pm}l^{\pm}$ in the future $ILC$ experiments.Comment: 16 pages, 7 figures. Discussions and references added, typos correcte

LHC sensitivity to lepton flavour violating Z boson decays

We estimate that the LHC could set bounds BR(Z -> mu^\pm e^\mp) < 4.1 * 10^{-7} and BR(Z -> tau^\pm mu^\mp)< 3.5 * 10^{-6} (at 95% C.L.) with 20 inverse fb of data at 8 TeV. A similar sensitivity can be anticipated for Z -> tau^\pm e^\mp, because we consider leptonic tau decays such that Z -> tau^+ mu^- gives e^+ \mu^- +$ invisibles. These limits can be compared to the LEP1 bounds of order 10^{-5} to 10^{-6}. Such collider searches are sensitive to a flavour-changing effective Z coupling which is energy dependent, so are complementary to bounds obtained from tau to 3mu and mu to 3e.Comment: 11 pages, 2 figures, version for publicatio

Spontaneous CP Violating Phase as the Phase in PMNS Matrix

We study the possibility of identifying the CP violating phases in the PMNS mixing matrix in the lepton sector and also that in the CKM mixing matrix in the quark sector with the phase responsible for the spontaneous CP violation in the Higgs potential, and some implications. Since the phase in the CKM mixing matrix is determined by experimental data, the phase in the lepton sector is therefore also fixed. The mass matrix for neutrinos is constrained leading to constraints on the Jarlskog CP violating parameter $J$, and the effective mass $$ for neutrinoless double beta decay. The Yukawa couplings are also constrained. Different ways of identifying the phases have different predictions for $\mu \to e e\bar e$ and $\tau \to l_1 l_2 \bar l_3$. Future experimental data can be used to distinguish different models.Comment: 16 pages, 3 figure

Gauged Flavor Group with Left-Right Symmetry

We construct an anomaly-free extension of the left-right symmetric model, where the maximal flavor group is gauged and anomaly cancellation is guaranteed by adding new vectorlike fermion states. We address the question of the lowest allowed flavor symmetry scale consistent with data. Because of the mechanism recently pointed out by Grinstein et al. tree-level flavor changing neutral currents turn out to play a very weak constraining role. The same occurs, in our model, for electroweak precision observables. The main constraint turns out to come from WR-mediated flavor changing neutral current box diagrams, primarily K - Kbar mixing. In the case where discrete parity symmetry is present at the TeV scale, this constraint implies lower bounds on the mass of vectorlike fermions and flavor bosons of 5 and 10 TeV respectively. However, these limits are weakened under the condition that only SU(2)_R x U(1)_{B-L} is restored at the TeV scale, but not parity. For example, assuming the SU(2) gauge couplings in the ratio gR/gL approx 0.7 allows the above limits to go down by half for both vectorlike fermions and flavor bosons. Our model provides a framework for accommodating neutrino masses and, in the parity symmetric case, provides a solution to the strong CP problem. The bound on the lepton flavor gauging scale is somewhat stronger, because of Big Bang Nucleosynthesis constraints. We argue, however, that the applicability of these constraints depends on the mechanism at work for the generation of neutrino masses.Comment: 1+23 pages, 1 table, 5 figures. v3: some more textual fixes (main change: discussion of Lepton Flavor Violating observables rephrased). Matches journal versio

Phenomenological Consequences of Singlet Neutrinos

In this paper, we study the phenomenology of right-handed neutrino isosinglets. We consider the general situation where the neutrino masses are not necessarily given by $m_D^2/M$, where $m_D$ and $M$ are the Dirac and Majorana mass terms respectively. The consequent mixing between the light and heavy neutrinos is then not suppressed, and we treat it as an independent parameter in the analysis. It turns out that $\mu-e$ conversion is an important experiment in placing limits on the heavy mass scale ($M$) and the mixing. Mixings among light neutrinos are constrained by neutrinoless double beta decay, as well as by solar and atmospheric neutrino experiments. Detailed one-loop calculations for lepton number violating vertices are provided.Comment: Revtex file,TRI-PP-94-1,VPI-IHEP-94-1, 23 pages, a compressed for 8 figures is appende

Flavour physics from an approximate U(2)^3 symmetry

The quark sector of the Standard Model exhibits an approximate U(2)^3 flavour symmetry. This symmetry, broken in specific directions dictated by minimality, can explain the success of the Cabibbo-Kobayashi-Maskawa picture of flavour mixing and CP violation, confirmed by the data so far, while allowing for observable deviations from it, as expected in most models of ElectroWeak Symmetry Breaking. Building on previous work in the specific context of supersymmetry, we analyze the expected effects and we quantify the current bounds in a general Effective Field Theory framework. As a further relevant example we then show how the U(2)^3 symmetry and its breaking can be implemented in a generic composite Higgs model and we make a first analysis of its peculiar consequences. We also discuss how some partial extension of U(2)^3 to the lepton sector can arise, both in general and in composite Higgs models. An optimistic though conceivable interpretation of the considerations developed in this paper gives reasons to think that new physics searches in the flavour sector may be about to explore an interesting realm of phenomena.Comment: 29 pages, 5 figure