Universality Violation Research Articles

Quenched bond randomness: Superfluidity in porous media and the strong violation of universality

The effects of quenched bond randomness are most readily studied with superfluidity immersed in a porous medium. A lattice model for3He-4He mixtures and incomplete4He fillings in aerogel yields the signature effect of bond randomness, namely the conversion of symmetry-breaking first-order phase transitions into second-order phase transitions, the λ-line reaching zero temperature, and the elimination of non-symmetry-breaking first-order phase transitions. The model recognizes the importance of the connected nature of aerogel randomness and thereby yields superfluidity at very low4He concentrations, a phase separation entirely within the superfluid phase, and the order-parameter contrast between mixtures and incomplete fillings, all in agreement with experiments. The special properties of the helium mixture/aerogel system are distinctly linked to the aerogel properties of connectivity, randomness, and tenuousness, via the additional study of a regularized “jungle-gym” aerogel. Renormalization-group calculations indicate that a strong violation of the empirical universality principle of critical phenomena occurs under quenched bond randomness. It is argued that helium/aerogel critical properties reflect this violation and further experiments are suggested. Renormalization-group analysis also shows that, adjoiningly to the strong universality violation (which hinges on the occurrence or non-occurrence of asymptotic strong coupling—strong randomness under rescaling), there is a new “hyperuniversality” at phase transitions with asymptotic strong coupling—strong randomness behavior, for example assigning the same critical exponents to random-bond tricriticality and random-field criticality.

Phenomenology of a Supersymmetric Model for Fermion Mass Hierarchy

Some phenomenological aspects of a supersymmetric model for fermion mass hierarchy proposed previously are discussed. It is required that the lepton universality violation is near to its current experimental bound. The lepton number violation decay modes τ→2eμ and 3μ maybe observable in the near future. The Majorana mass of electron-neutrino is predicted to be about 0.1 eV. The fine-tuning problem is discussed.

Variations on minimal gauge-mediated supersymmetry breaking

We study various modifications to the minimal models of gauge mediated supersymmetry breaking. We argue that, under reasonable assumptions, the structure of the messenger sector is rather restricted. We investigate the effects of possible mixing between messenger and ordinary squark and slepton fields and, in particular, violation of universality. We show that acceptable values for the $\mu$ and $B$ parameters can naturally arise from discrete, possibly horizontal, symmetries. We claim that in models where the supersymmetry breaking parameters $A$ and $B$ vanish at tree level, $\tan\beta$ could be large without fine tuning. We explain how the supersymmetric CP problem is solved in such models.

Strong Violation of Critical Phenomena Universality Under Quenched Bond Randomness

The empirically long-established principle of universality states that, along a second-order phase boundary between any two given phases, the critical exponents are invariant. We find that this universality is strongly violated under quenched bond randomness, in the sense that the critical exponents abruptly change their values between two segments of the same second-order phase boundary.

Violations of universality in a vectorlike extension of the standard model

Violations of universality of couplings in a vectorlike extension of the standard model with three heavy mirror fermion families are considered. The recently observed discrepancies between experiments and the standard model in the hadronic branching fractions R b and R c of the Z-boson are explained by the mixing of fermions with their mirror fermion partners.

Question of universality in RPn-1 and O(n) lattice sigma models.

We argue that there is no essential violation of universality in the continuum limit of mixed $\RPn$ and $\On$ lattice sigma models in 2 dimensions, contrary to opposite claims in the literature.

Tau -universality violation with light neutralinos.

In a supersymmetric model with the lightest supersymmetric particle (LSP) \ensuremath{\chi} in the range of a few hundred MeV's, the decay \ensuremath{\tau}\ensuremath{\rightarrow}\ensuremath{\mu}\ensuremath{\chi}\ensuremath{\chi} is allowed. We investigate the departure from \ensuremath{\tau} universality caused by this decay which can be triggered by neutrino masses. It is found that universality violation in this way can be greater than both nonuniversal electroweak radiative corrections and supersymmetric one-loop corrections over a considerable region of the parameter space allowed by experiments so far. Thus it suggests a method of constraining the parameter space with light LSP's using data from \ensuremath{\tau} factories. \textcopyright{} 1996 The American Physical Society.

Electroweak Precision Measurements and the Minimal Supersymmetric Standard Model

We discuss supersymmetric contributions to the electroweak precision measurements in the Minimal Supersymmetric Standard Model for two cases: the quark-lepton universality violation $\delta_{q\ell}$ in charged currents and the ratio $R_b=\Gamma(Z\rightarrow b\bar{b})/\Gamma(Z\rightarrow \rm hadrons)$. The recent experimental data suggest deviations from the Standard Model for these observables, at the 1-$\sigma$ level for the former and at more than 3-$\sigma$ level for the latter. We analyze the non-oblique corrections from the SUSY particles to explain these discrepancies. The observed non-zero $\delta_{q\ell}$ may be explained by relatively light sleptons, charginos and neutralinos. Although the observed excess of $R_b$ can be explained by very light scalar top and chargino for low $\tan\beta$, this interpretation is severely constrained by the opening of exotic decay modes for the top quark.

Supersymmetric contributions to the quark-lepton universality violation in charged currents.

The supersymmetric one-loop contribution to the quark-lepton universality violation in the low-energy charged current weak interactions is studied. It is shown that the recent experimental data with the 1-σ deviation from the universality may be explained by the existence of the light sleptons (< 220GeV) and relatively light chargino and neutralinos (< 600GeV) with significant gaugino components. The sign of the universality violation is briefly discussed.

Violation of universality for Ising spin-glass transitions

Simulation data on Ising spin glasses in dimensions 3, 4, and 5 indicate that the critical exponent \ensuremath{\eta} changes with the form of the interaction distribution, which appears to be a pertinent parameter for the spin-glass transition. This result is incompatible with standard second-order transition universality rules which do not seem to hold for Ising spin glasses.

Constraints from lepton universality at the Z peak on unified theories

We suggest the use of a universality-breaking observable based on lepton asymmetries as derived from the left-right asymmetry and the τ polarization at the Z peak, which can efficiently constrain the parameter space of unified theories. The new observable is complementary to the leptonic partial width differences and it depends critically on the chirality of a possible non-universal Z-boson coupling to like-flavour leptons. The LEP/SLC potential of probing universality violation is discussed in representative low-energy extensions of the Standard Model (SM) that could be derived by supersymmetric grand unified theories, such as the SM with left-handed and/or right-handed neutral isosinglets, the left-right symmetric model, and the minimal supersymmetric SM.

Fish recognize and prefer to shoal with poor competitors

All animals are never equal; even in apparently uniform assemblages such as fish schools, some individuals will be consistently better at acquiring resources or at minimizing their risk of being predated upon, than others. Although many species benefit from foraging in groups, the net pay-off to the individual of joining a group will clearly depend on its composition and foragers should, therefore, be choosy as to which group they join. Here we show for the first time that fish (European minnows) can discriminate between shoals composed of good and of poor competitors. They show a distinct preference for shoaling with fish of low competitive ability, even in the absence of obvious cues such as differences in aggressiveness, size or instantaneous feeding rate. This remarkable ability may explain the paradox of animals rapidly distribution themselves between foraging groups according to the predictions of the Ideal Free Distribution despite universal violation of its key assumption of equal competitive abilities.

Signals of unconventional E6 models at e+e- colliders.

Generation dependent discrete symmetries often appear in models derived from superstring theories. In particular, in the framework of E$_6$ models the presence of such symmetries is required in order to allow for the radiative generation of naturally small neutrino masses. Recently it was shown that by imposing suitable generation dependent discrete symmetries, a class of models can be consistently constructed in which the three sets of known fermions in each generation do not have the same assignments with respect to the {\bf 27} representation of E$_6$. In this scenario, the different embedding in the gauge group of the three generations implies in particular that the known charged leptons couple in a non--universal way to the new neutral gauge bosons $(Z_\beta)$ present in these models. We exploit this fact to study the signature of this class of models at present and future $e^+e^-$ colliders. We show that some signals of deviation from lepton universality as well as some other discrepancies with the standard model predictions which have been observed at the TRISTAN collider in the production rate of $\mu$ and $\tau$, can be accounted for if the $Z_\beta$ mass is not much heavier than 300 GeV. We also study the discovery limits for lepton universality violation of this type at LEP-2 and at the 500 GeV $e^+e^-$ Next Linear Collider (NLC). We show that models predicting unconventional assignments for the leptons will give an unmistakable signature, when the $Z_\beta$ mass is as heavy as $\sim 800$ GeV (LEP-2) and $\sim 2$ TeV (NLC).

Model of universality violation reexamined.

The possibility that the interactions of the third generation of quarks and leptons may violate universality by a small amount remains an open experimental question. The model of Li and Ma, which naturally accommodates such violations, is found to be highly constrained by newly obtained, high precision electroweak and $\tau$-lepton data once full Standard Model radiative corrections are incorporated into the analysis. A comparison of the predictions of this model with existing data and the expectations for future colliders is presented.

Universality-breaking effects in leptonic Z decays.

We analyze the possibility of universality violation in diagonal leptonic decays of the Z boson, in the context of interfamily ``seesaw'' models. In a minimal extension of the standard model with right-handed neutrino fields, we find that universality-breaking effects increase quadratically with the heavy Majorana neutrino mass and may be observed in the current experiments at the CERN ${\mathit{e}}^{+}$${\mathit{e}}^{\mathrm{\ensuremath{-}}}$ collider LEP.

Effects of compositeness in leptonic tau decays

We study the effects of induced contact interactions, which arise when leptons and quarks are considered as composite objects, on the leptonic decays of the tau. We find that 4-fermion contact interactions can modify the tau decay pattern, producing a violation of universality, which could resolve the observed discrepancy on the values of leptonic branching ratios, provided that the scale of compositeness is of the order of 3 TeV.

Arithmetical chaos and violation of universality in energy level statistics.

A class of strongly chaotic systems revealing a strange arithmetical structure is discussed whose quantal energy levels exhibit level attraction rather than repulsion. As an example, the nearest-neighbor level spacings for Artin's billiard have been computed in a large energy range. It is shown that the observed violation of universality has its root in the existence of an infinite number of Hermitian operators (Hecke operators) which commute with the Hamiltonian and generate nongeneric correlations in the eigenfunctions.

Test of g universality with a Galileo type experiment.

The results of an experiment searching for the difference \ensuremath{\Delta}g in the free-fall acceleration of aluminum and copper, in vacuum, obtained on the Earth in a differential Galileo type measurement are reported. Within the sensitivity of the measurement, \ensuremath{\Delta}g/g=7.2\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}10}$; no g universality violation has been observed.

Solar neutrino oscillations and neutrino-gravity couplings

Abstract In this talk I review a proposal by Halprin and Leung to set limits on a possible breakdown of universality in neutrino-gravitational interactions from solar neutrino experiments and report the result of a recent application to the current data. We find (with certain caveats) that an adiabatic solution exists which can accommodate both the Homestake and Kamiokande data. This solution leads to a value of the universality violation parameter, Δf ≈ 2 × 10−14. This is three orders of magnitude less than the universality violation bound for ordinary matter inferred from weak equivalence principle experiments.

Universality violations in the Sp(6)L x U(1)Y family model.

We study universality violations arising from an extra neutral gauge boson Z' present in the Sp(6${)}_{\mathit{L}}$\ifmmode\times\else\texttimes\fi{}U(1${)}_{\mathit{Y}}$ family model for ${\mathit{e}}^{+}$${\mathit{e}}^{\mathrm{\ensuremath{-}}}$\ensuremath{\rightarrow}ff\ifmmode\bar\else\textasciimacron\fi{} (f=\ensuremath{\mu},\ensuremath{\tau}). With the capability to measure the \ensuremath{\tau} polarization and the availability of polarized beams, it is found that such violations are detectable even for a rather small mixing angle \ensuremath{\varphi}(\ensuremath{\sim}0.01) between Z and Z'.