Low-energy Effects Research Articles

Tau lepton physics: theory overview

The pure leptonic or semileptonic character of tau decays makes them a good laboratory to test the structure of the weak currents and the universality of their couplings to the gauge bosons. The hadronic tau decay modes constitute an ideal tool for studying low-energy effects of the strong interactions in very clean conditions; a well-known example is the precise determination of the QCD coupling from tau-decay data. New physics phenomena, such as a non-zero tau-neutrino mass or violations of (flavour / CP) conservation laws can also be searched for with tau decays.

A Monte Carlo calculation of cell inactivation by light ions

. This study simulates the exposure of V79 Chinese hamster fibroblasts to low-energy protons, deuterons and alpha particles in the LET range 10-200 keV/ mu m. The starting assumption is that the induction of clustered lesions in DNA is a fundamental step for cell inactivation. A non-homogeneous cell population was simulated by a computer program, using as input measured morphological parameters reported in the literature. Variations in the number of traversals through each cell of the population and in the length of the traversal, depending on actual nuclear thickness and position of the traversal, the energy spread of the incident beam, and the change of LET along the tracks were included in the simulation. Microdosimetric spectra were computed and compared with spectra obtained neglecting particle slowing-down and stochastic aspects of cell morphology. Simulated cell survival was estimated under the assumption that surviving cells are those with no clustered DNA lesions or no passages. The main features of experimental RBE versus LET and particle type were reproduced by the simulations. The influence of stochastic aspects of target-cell morphology and of the energy of the incident particles on survival were investigated under different assumptions about the correlation between morphological parameters. Results support the hypothesis of a relevant role of clustered DNA damage in cell killing and point out the importance of target-cell morphology and its variability in beam dosimetry and computer simulations of low-energy particle radiation effects.

The effect of traps at the free surface of GaAs field effect transistors

In a GaAs field effect transistor there are ungated sections of the channel between the gate and the source/drain. The static characteristics and the transients which occur in various parameters after a change in the bias voltages can be shown to be affected by the surface potential in this region. A model is proposed for these effects which involves a surface region with lower effective energy gap. The gate current leakage characteristics and conductance deep level transient spectroscopy measurements on single and dual gate devices are consistent with this model.

R-parity violation effects through soft supersymmetry-breaking terms and the renormalization group.

We present full renormalisation group equations for the MSSM with R-parity violation, including all soft supersymmetry breaking terms. The inclusion of dimensionless R-parity violating couplings can generate many possible low energy effects through the generation of off-diagonal soft masses violating lepton and quark flavour, and through the generation of lepton-Higgs mixing. We discuss the relation between the weak and unification scale R-parity violation and study the effects on neutrino mass generation and $\mu\to e\gamma$.

Yb 2Ni 2Al: A prototypical Yb-based heavy-fermion system

We have investigated the properties of Yb 2Ni 2Al by means of resistivity, susceptibility and specific heat measurements. Our results suggest that it is the first magnetically non-ordered Yb heavy-fermion system with a low characteristic energy and clear coherence effects in the resistivity. A logarithmic temperature dependence of C/ T indicates that it is close to the transition to a magnetic ordered ground state. We further found a simple systematic for composition dependence of the Yb-valence in binary and ternary YbTAl compounds (T: Ni, Pd, Pt).

Cosmological relic density from minimal supergravity with implications for collider physics.

We review the gauging of an R-symmetry in local and global susy. We then construct the first anomaly-free models. We break the R-symmetry and susy at the Planck scale and discuss the low-energy effects. We include a solution to the mu-problem, and the prediction of observable effects at HERA. The models also nicely allow for GUT-scale baryogenesis and R-parity violation without the sphaleron interactions erasing the baryon-asymmetry.

Molecular dynamics and quasidynamic simulations of low-energy particle bombardment effects during vapour-phase crystal growth: 10–50 eV Si and In atoms incident on (2 × 1)-terminated Si(001)

Molecular dynamics and quasidynamic simulations, utilizing the Tersoff many-body potential, were used to investigate projectile incorporation and defect production as well as lattice relaxation, diffusion, and annihilation of defects resulting from 10 and 50 eV Si and In irradiation of (2 × 1)-terminated Si(001). Due to the extreme deference in the time scales associated with the collision cascade and atom diffusion, the two cases were treated separately. Ion-assisted epitaxy, In incorporation into near-surface lattice sites, interstitial and vacancy formation, and position exchange reactions were observed. The stabilized structures of interstitials and vacancies together with pathways and activation energies for Si and In diffusion were calculated as a function of depth below the surface. Near-surface interstitials were found to be easily annealed at typical Si growth temperatures. However, residual vacancies, present at much lower concentrations, are much more difficult to anneal out. Near-surface In atoms were found to exhibit large driving forces for surface segregation. Preventing In segregation during Si growth at 500 °C required the In ions to penetrate to at least the third layer.

Solvatochromic Effect in a Benzimidazole-Based Betaine: Determination of the Dipole Moments and Second-Order Hyperpolarizability

Results of measurements of the solvent-dependent shift of the low-energy absorption band solvatochromic effect in a benzimidazole-based betaine are reported in the paper. Measurements of absorption spectra in several solvents of different polarities were performed. The solvatochromic shift of the low-lying absorption band was found to exceed 3000 cm -1 ; the results obtained were then employed to calculate the groundand excited-state dipole moment of the molecule. The spectroscopic measurements were supplemented by measurements of the ground-state dipole moment. A remarkable change in the charge distribution was found to occur upon the electronic excitation: both the experimental results and the calculations indicate that the ground-state dipole moment is close to 13 D, whereas in the excited state it amounts to ca. 3 D. The second-order hyperpolarizability of the molecule was calculated from the measurements; its off-resonance value amounts to ca. 20 x 10 -40 m4 /V (4.8 x 10 -30 esu), depending on the solvent used. PACS numbers: 33.20.—t, 42.65.An, 42.70.Jk, 78.40.-q

Low-energy effects of charged Higgs bosons with general Yukawa couplings.

We study a model with two Higgs doublets where FCNC are allowed at tree-level. In this model, the interactions of charged Higgs with fermions ($H^\pm f \bar{f'}$), include a term that is not proportional to the fermion masses, which we constraint using the following low-energy processes: i) tau decays ($\tau \to \nu_{\tau} e\nu_e, \nu_{\tau}\mu\nu_{\mu}, \nu_{\tau}\pi$), ii) leptonic decays of pseudoscalar mesons ($\pi, K \to \ell\nu_{\ell}$) and iii) semileptonic b-decays. With these constraints it is possible to make predictions; we illustrate this by presenting the rates for the (FCNC) decay $c\to u+\gamma$, the (second class-current) decay $\tau\to \nu_{\tau}+ \eta\pi$, and also the theoretical value of the neutron life-time.

Connection between low energy effective Hamiltonians and energy level statistics.

We study the level statistics of a non-integrable one dimensional interacting fermionic system characterized by the GOE distribution. We calculate numerically on a finite size system the level spacing distribution $P(s)$ and the Dyson-Mehta $\Delta_3$ correlation. We observe that its low energy spectrum follows rather the Poisson distribution, characteristic of an integrable system, consistent with the fact that the low energy excitations of this system are described by the Luttinger model. We propose this Random Matrix Theory analysis as a probe for the existence and integrability of low energy effective Hamiltonians for strongly correlated systems.

Impurity effects on chiral one-dimensional electron systems.

We studied low energy effects of impurities in several chiral 1D electron systems that contain only excitations moving in one direction. We first considered single-impurity scattering between two branches of 1D chiral Luttinger liquids. The general form of impurity scattering matrix was found which, some times, allow the chemical potentials on the outgoing branches to be higher (or lower) then the maximum (or minimum) chemical potentials on the incoming branches. We also studied the effects of many impurities (with finite density) on two branches of 1D chiral Fermi liquids (that appear on the edge of $\nu=2$ integral quantum Hall liquid). We found that the impurities drive the system to a new fix point at low energies which has an SU(2) symmetry even when the two branches originally have different velocities. (Note a clean system is SU(2) symmetric only when the two branches have the same velocity.) The probability distribution and correlation of scattering matrices at different energies are described by universal functions which are calculated exactly for this new fixed point.

Low energy effects of new interactions in the electroweak boson sector.

Novel strong interactions in the electroweak bosonic sector are expected to induce effective interactions between the Higgs doublet field and the electroweak gauge bosons which lead to anomalous $\mathrm{WWZ}$ and $\mathrm{WW}\ensuremath{\gamma}$ vertices once the Higgs field acquires a vacuum expectation value. Using a linear realization of the Goldstone bosons, we consider a complete set of dimension-six operators which are SU(2)\ifmmode\times\else\texttimes\fi{}U(1) gauge invariant and conserve $C$ and $P$. This approach allows us to study effects of new physics which originates above 1 TeV and the Higgs boson mass dependence of the results can be investigated. Four of the dimension-six operators affect low energy and present CERN LEP experiments at the tree level. Another five influence neutral and charged current experiments at the one-loop level and three of these lead to anomalous $\mathrm{WWZ}$ and $\mathrm{WW}\ensuremath{\gamma}$ vertices. Their loop contributions are at most logarithmically divergent, and these logarithmic divergences can be understood as renormalizations of the four operators which contribute at the tree level. Constraints on the remaining five operators can be obtained if one assumes the absence of cancellations between the tree level and one-loop contributions. The resulting bounds on anomalous triple gauge boson couplins are modest, which emphasizes the importance of direct measurements of the triple gauge boson vertices, e.g., in ${W}^{+}{W}^{\ensuremath{-}}$ production at LEP II.

Scatter in computed radiography.

The effects of scatter radiation on images obtained with storage phosphor computed radiography (CR) were evaluated. The scatter degradation factor was extended to account separately for forward scatter and backscatter. Values for the ratios of forward scatter to primary radiation (sf/p) and backscatter to primary radiation (sb/p) were measured for various x-ray tube potentials, phantom thicknesses, and field sizes for CR imaging plates (IPs). Similar measurements were made for a common screen-film system. The sb/p values for the IPs were 0.05-3.80. Standard CR cassettes do not have lead foil behind the IP to control backscatter; the results of this study indicate that such control is needed. The sf/p values were 40%-70% higher in IPs (50%-120% higher when backscatter was included) than those in the conventional screen-film system. This difference is attributed to the differences in the K-absorption edges of barium and gadolinium and the fact that scatter has lower effective energy than the primary beam as they exit the patient.

Why a scalar explanation of the L3 events is implausible

We investigate the question of whether an additional light neutral scalar can explain the l+l−γγ events with high invariant mass photon pairs recently observed by the L3 Collaboration. We parameterize the low energy effects of the unknown dynamics in terms of higher dimensional effective operators. We show that operators which allow for the scalar to be produced and decay into photon pairs will allow other observable processes that should have been seen in current experiments.

One-loop effects of non-standard triple gauge boson vertices

Low energy effects of generic extensions of the Standard Model can be comprehensively parametrized in terms of higher dimensional effective operators. After the success of all the recent precision tests on the Standard Model, we argue that any sensible description of these extensions at the Z-scale must be stable under higher order quantum corrections. The imposition of SU(2) L×U(1) Y gauge invariance seems to be the simplest and most natural way to fulfill this requirement. With this assumption all the possible deviations from the standard triple gauge boson vertices can be consistently parametrized in terms of a finite set of gauge invariant operators. We deal here with those operators that do not give any tree level effect on present experimental observables and constrain them by computing their effects at the one-loop level. We conclude that for a light Higgs boson, the direct measurement at LEP200 can improve present bounds on these “blind directions”, while for a heavy Higgs it is most unlikely to provide any new information.

Molecular dynamics and quasidynamics simulations of low-energy particle bombardment effects during vapor-phase crystal growth: Production and annihilation of defects due to 50 eV Si incident on (2×1)-terminated Si(001)

Molecular dynamics and quasidynamics simulations, utilizing the Tersoff many-body potential, were used to investigate projectile incorporation and defect production as well as lattice relaxation, diffusion, and annihilation of defects resulting from 50 eV Si irradiation of (2×1)-terminated Si(001). A unity trapping probability, in sites distributed between the epitaxial overlayer and the fourth lattice layer (l=4), was obtained for Si projectiles irradiating an array of high- and low-symmetry points in the primitive surface unit cell. Exchange epitaxy events were observed in which a lattice atom came to rest at an epitaxial (1×1) bridge site while the projectile stopped in a substitutional lattice site. In addition, several collision sequences resulted in the opening of additional dimers, up to four per irradiation event, thus providing (1×1) sites for migrating adatoms during ion-assisted crystal growth. The primary residual lattice defects produced were split and hexagonal interstitials, although tetragonal, bond-centered, and pentagonal interstitials were also produced in layers l=2 through l=14 with the average interstitial layer depth <l≳=2.3. Calculated interstitial migration activation energies Em decreased toward the surface with minimum energy paths generally involving tetragonal sites. Ion-irradiation-induced interstitials can thus be easily annealed out over time periods corresponding to the deposition of less than one monolayer under typical Si molecular-beam-epitaxy conditions. Fewer vacancies were produced, although they have a higher migration activation energy, and they were distributed over a shallower depth, l≤7. Complete annealing of ion-irradiation-induced vacancies requires interaction with deeper interstitials, moving toward the surface, and incident Si projectiles.

The algebraic relational theory applied to low energy effects in malignant cells

Mathematical properties of a structure named dual Heyting arrow lattice are presented and they are used to analyze how the proportion of vicinal water in normal cells could be changed to be as in malignant cells.

Low-energy effects of heavy chiral fermions

We determine the low-energy effective lagrangian produced by a heavy chiral fermion, in the limit M → ∞. In the case of a heavy t-quark, we analyze in detail the effective interaction of vector fields, in the anomalous and non-anomalous sectors, and the couplings of the Higgs boson. Unitarity limits to the t-quark mass are considered. Considering an additional U(1) component of the electroweak group, we see that an anomalous coupling to the known fermions is not excluded.

CHIRAL PERTURBATION THEORY AT THE QUARK LEVEL

The chiral perturbation theory is developed at the quark level within the extended Nambu-Jona-Lasinio model, which we used for the low-energy approximation of QCD in the leading order of the large N expansion. In terms of constituent-quark loop diagrams we analyze all of the main low-energy effects caused by the first order corrections in the current-quark-mass expansions. For the correct description of the η→3π decays we confirm the important role of the final-state interaction quoted by Gasser and Leutwyler.

Intermediate scales of symmetry breaking in Calabi-Yau models

We discuss the generation of large intermediate scales of symmetry breaking in grand unification models suggested by the heterotic string. We analyze on dimensional grounds a particular scenario where one flat direction in the effective potential defines two different scales of gauge-symmetry breaking at very high energies (\ensuremath{\cong}${10}^{15}$ GeV). This mechanism implies the presence of one light (\ensuremath{\sim}1 TeV) nonchiral neutrino. The size of the observable low-energy effects seems, however, quite model dependent.