Phenomenological Relevance Research Articles

QCD corrections to [formula omitted] at LEP2 and the next linear collider: CC10 at O( αs)

QCD one-loop corrections to the semileptonic process e +e − → μ − ν μ u d are computed. We compare the exact calculation with a “naive” approach to strong radiative corrections which has been widely used in the literature and discuss the phenomenological relevance of QCD contributions for LEP2 and NLC physics.

Lattice-constrained unitarity bounds for [formula omitted] decays

Lattice results, kinematical constraints and QCD dispersion relations are combined for the first time to derive model-independent bounds for QCD form factors and corresponding rates. To take into account the error bars on the lattice results we develop a general formalism which ascribes well-defined statistical properties to these bounds. We concentrate on B − 0 → π +ℓ − ν ℓ decays because of the relative simplicity of the analytical behavior of the relevant polarization functions and because of their immediate phenomenological relevance. To determine the range of applicability of the dispersive matching required to obtain the bounds, we have evaluated the leading perturbative and non-perturbative QCD corrections to the relevant polarization functions. Despite the lattice results' large error bars and limited kinematical range, the bounds we obtain appear to favor certain parametrizations for the form factors over others. The bounds also enable the determination of | V ub | from the total experimental rate with a theoretical error ranging from 27% to 37%, depending on the assumptions made. The techniques developed here are, in fact, quite general and are not limited to use with lattice results nor to semileptonic B 0 → π +ℓ − ν ℓ decays.

Comment on quarks and gluons at small x and the SDIS factorization scheme

I present some comments on the partonic interpretation of the HERA data on the proton structure function. The effects of the resummation of the leading and next-to-leading ln x-contributions are discussed. A new factorization scheme, in which these resummation effects are absorbed into a steep redefinition of the gluon density, is introduced and its (possible) interpretation and phenomenological relevance are suggested.

K⊥ factorization vs. renormalization group: a small- x consistency argument

I investigate, at leading twist level, the consistency of the BFKL equation with the renormalization group, when next-to-leading log- x termss are included in the quark-sea channel of the anomalous dimension matrix. By use of k ⊥-factorization, I find that, besides next-to-leading small- x resummation formulae, a leading, x-dependent redefinition of initial quarks and gluons is needed. Its interpretation and phenomenological relevance are briefly discussed.

Random-phase-approximation analysis of orbital- and magnetic-fluctuation-mediated superconductivity in a two-band Hubbard model.

We have applied a generalized random-phase-approximation analysis to a two-band Hubbard model for the cuprate superconductors which treats magnetic and orbital (quadrupolar) fluctuations on an equal footing and which allows for significant itinerant character of the copper [ital d] electrons. The two bands represent the lowest copper orbitals. We qualitatively reproduce the known results of antiferromagnetic order at half filling with an adjoining region of [ital d]-wave-symmetry superconductivity in the single-band limit. When the value of the band splitting is comparable to the hopping integral of the [ital x][sup 2][minus][ital y][sup 2] orbitals, we find a qualitatively new region of superconductivity at increased hole doping which is clearly associated with orbital (quadrupolar) fluctuations and a significant [ital x][sup 2][minus][ital y][sup 2][r arrow]3[ital z][sup 2][minus][ital r][sup 2] charge transfer but no concomitant charge or magnetic order. The pairs are very spread out in momentum and energy space. At the optimal temperature for the new instability, the Fermi surface has two sheets, and [ital d]-wave and [ital s]-wave pair states are nearly degenerate. We discuss the possible phenomenological relevance of these results to photoemission and other data for the cuprates.

Decoupling in nonperturbative background fields: The thermal sphaleron.

Standard decoupling of heavy fermions may fail when there are nonperturbative variations in a scalar field which gives masses to the fermions. One situation of phenomenological relevance is the case of sphalerons in the presence of external fermions at finite temperatures. The free energy of a simple model is determined using a nonperturbative technique to study the effect of fermions on the scalar field. The effects of quantum and thermal fermion fluctuations on the free energy of the sphaleron are calculated, including all orders in gradients of the scalar field.

Soft gluon resummation for the single-spin production of W±-bosons

We carefully study the production of W ±-bosons in singly-polarized hadron-hadron collisions. In the region of small transverse momenta of the produced W ± we perform the soft gluon resummation to double logarithmic accuracy. Special emphasis is laid on matching the resummed expression to the O( α s) perturbative result valid at large transverse momenta. We investigate the phenomenological relevance of our results for pp-collisions at RHIC and show that in the future single-spin W ±-production may help to shed more light on the surprising EMC-result.

Nullification of multi-Higgs threshold amplitudes in the standard model

We show that nullification of all tree-order threshold amplitudes involving Higgs particles in the standard model occurs, provided that certain equations relating the masses of all existing elementary particles to the mass of the Higgs scalar are satisfied. The possible role of these relations in restoring the high-multiplicity unitarity and their phenomenological relevance are briefly discussed.

Indirect Searches for Very Heavy Quarksa

Detailed studies of weak decays can reveal the presence of very massive quanta like heavy top quarks or fourth family quarks. The decay K+ --) ?~+vP and Bd - Ba mixing are particularly promising fields for such searches. We infer a rather conservative lower limit of 70 GeV on the top mass from recent ARGUS data on Bd - Bd mixing; near-maximal B, - Bd mixing is another consequence. If on the other hand top were detected in Z” decays, then the presence of New Physics would be established in B” decays. The ratio between r(B”) and r(B*) is of considerable phenomenological relevance here.

Monte Carlo study of the standard SU(2) Higgs model

The SU(2) Higgs model with scalar doublet field is numerically investigated on lattices with size between 8 4 and 12 4. Masses and zero momentum couplings are determined at several points of the three-dimensional coupling parameter space. Particular interest is given to questions related to the order of the confinement-Higgs phase transition. It is shown that for non-perturbative scalar self-couplings numerical Monte Carlo calculations are possible in the region of weak gauge coupling approximately equal to the physical value in the standard SU(2) ⊗ U(1) electroweak theory. Our first exploratory results in such a point on 12 4 lattice give a Higgs mass to W-mass ratio 6.4 ± 0.8 and a Higgs-WW coupling roughly a factor 3 smaller than the tree-level value. The circumstances under which these numbers could have some phenomenological relevance are discussed. A possible strategy is outlined for future large scale Monte Carlo calculations in the strongly self-interacting standard Higgs model with weak gauge coupling.

An arbitrary natural effective hierarchy in a left-right higgs theory

I describe an ordinary Higgs model that produces naturally near its infrared fixed point an arbitrarily large ration of vacuum expectation values, effectively a hierarchy between scales of symmetry breaking. The model has potential phenomenological relevance to left-right theories: it predicts small W L-W R mixing and the existence of ultralight Dirac fermions.

One-loop renormalization of the electroweak parameters in N = 1 supersymmetry

We present the complete calculation of the radiative corrections to the one-loop order of the electroweak parameters in N = 1 supersymmetry. The general formulas for the resulting weak boson mass shifts are explicitly given and some special cases of possible phenomenological relevance are surveyed. The general conclusion at which we arrive is that one always ends up by finding a few hundred MeV maximal effect on the weak boson mass shifts, i.e., much smaller than the corresponding effect obtained in the standard model.

Double logarithmic asymptotics and regge singularities of quark amplitudes with flavour exchange

We derive simple equations in terms of definite signature partial waves for the quark scattering and annihilation amplitudes in the double logarithmic approximation of QCD. We apply the method of isolating the softest virtual particle in the graphs which is based on gauge invariance and unitarity. Besides the particle Regge trajectories and corresponding cuts, there are further singularities in the angular momentum plane generated by perturbative double logarithmic contributions and which may be of phenomenological relevance. We also consider amplitudes with external currents and discuss how to include ultraviolet single logarithms and non-perturbative contributions.

Leadingqq¯Regge singularities in perturbative quantum chromodynamics

The QCD-motivated integral equation for the $q\overline{q}$ pair is derived taking into account Reggeization of quarks and asymptotic freedom. This equation is shown to generate an infinite number of Regge poles accumulating at $j=0$. It is expected that this short-range effect constraining the intercept of leading Regge singularities to be at least near zero independently of the flavors involved should not be affected by confinement. The phenomenological relevance of this constraint for charm production is pointed out.

Partons and branching

We formulate an evolution equation for the generating functional of parton distributions. It permits a detailed study of the branching process. Several numerical examples are given, and the relevance for jet phenomenology discussed.