Non-perturbative effects in e+e− annihilation in quantum chromodynamics

We study the change of one two-point function of hadronic electromagnetic currents in the presence of background gauge fields. It is found that it has the asymptotic behavior of 1/q⁴ as compared to ln(-q²) behavior from the bare fermion loop contribution. The effects of the non-perturbative vacuum in QCD on the two-point function is studied in an explicit calculation. We obtain the same qualitative behavior as in our general discussion. The effects on the total hadronic e⁺e^? annihilation is also discussed. We find that the effect ΔR^I/R is of the order of o(1/q⁴) or O(1/q⁶). This may explain the precocious scaling in the e⁺e^? annihilation.     

Production and two-photon decay of the MSSM scalar Higgs bosons at the LHC

We consider the production and two-photon decay of theCP-even Higgs bosons (h ⁰ andH ⁰) of the Minimal Supersymmetric Standard Model (MSSM) at the Large Hadron Collider. We study in detail the dependence of the cross section on various parameters of the MSSM, especially the dependence on the mixing effects in the squark sector due to the Higgs bilinear parameterμ and the soft supersymmetry breaking parameterA. We find that the cross section for the production of these Higgs bosons has a significant dependence on the parameters which determine the chiral mixing in the squark sector. The cross section times the two-photon branching ratio ofh ⁰ is of the order of 15–25 fb in much of the parameter space that remains after imposing the present experimental constraints. For theH ⁰ the two-photon branching ratio is only significant if theH ⁰ is light, but then the cross section times the branching ratio may exceed 200 fb. The QCD corrections due to quark loop contributions are known to increase the cross section by 50%. We find the dependence of the cross section on the gluon distribution function used to be rather insignificant.     

B s →μ + μ − decay in the R-parity violating minimal supergravity

We study B _s →μ ⁺ μ ^? in the context of the R-parity violating minimal supergravity in the high-tan?β regime. We find that the lowest value of the branching ratio can go well below the present LHCb sensitivity and hence B _s →μ ⁺ μ ^? can even be invisible to the LHC. We also find that the present upper bound on Br?(B _s →μ ⁺ μ ^?) puts strong constraint on the minimal supergravity parameter space. The constraints become more severe if the upper bound is close to its standard model prediction.     

Shape selection in non-Euclidean plates

We investigate isometric immersions of disks with constant negative curvature into R³, and the minimizers for the bending energy, i.e. the L² norm of the principal curvatures over the class of W^2,2 isometric immersions. We show the existence of smooth immersions of arbitrarily large geodesic balls in H² into R³. In elucidating the connection between these immersions and the non-existence/singularity results of Hilbert and Amsler, we obtain a lower bound for the L^∞ norm of the principal curvatures for such smooth isometric immersions. We also construct piecewise smooth isometric immersions that have a periodic profile, are globally W^2,2, and numerically have lower bending energy than their smooth counterparts. The number of periods in these configurations is set by the condition that the principal curvatures of the surface remain finite and grow approximately exponentially with the radius of the disk. We discuss the implications of our results on recent experiments on the mechanics of non-Euclidean plates.     

Weak annihilation in charm baryon decays — A re-evaluation

We discuss weak decays of charm baryons taking the mode Λ _c ⁺ →Δ⁺⁺ K ^? as the starting point of our analysis. This channel which can occur only due to weak annihilation has been seen at the ISR. Hence we conclude that weak annihilation is very significant for baryons. Further tests of this picture are listed, in particular concerning charm-strange baryons. We argue that τ(D°)~τ(Λ _c ⁺ )<τ(D ⁺) suggests that baryon and meson decays should be treated on an equal footing even when weak annihilation is included. We describe a framework based on evolution equations where this can be done.     

Nuclear Corrections in ν A DIS and Their Compatibility with Global NPDF Analyses

We perform a global χ ²-analysis of nuclear parton distribution functions using data from charged current neutrino–nucleus (ν A) deep inelastic scattering (DIS), charged-lepton–nucleus (? ^± A) DIS, and the Drell–Yan (DY) process. We show that the nuclear corrections in ν A DIS are not compatible with the predictions derived from ? ^± A DIS and DY data. We quantify this result using a hypothesis-testing criterion based on the χ ² distribution which we apply to the total χ ² as well as to the χ ² of the individual data sets. We find that it is not possible to accommodate the data from ν A and ? ^± A DIS by an acceptable combined fit. This implies that either the twist-2 parton distribution functions in nuclei are not universal, or that higher-twist terms play a more important role in the nuclear environment and have to be taken into account.     

RSOS revisited

We investigate the issues of unitarity and reality of the spectrum for the imaginary coupled affine Toda field theories based on a₁⁽¹⁾ and a₂⁽²⁾ and the perturbed minimal models that arise from their various RSOS restrictions. We show that while all theories based on a₁⁽¹⁾ have real spectra in finite volume, the spectra of a₂⁽²⁾ models is in general complex, with some exceptions. We also correct the S matrices conjectured earlier for the φ₁₅ perturbations of minimal models and give evidence for a conjecture that the RSOS spectra can be obtained as suitable projections of the folded ATFTs in finite volume.     

Diffractive open charm production at HERA

We use perturbative QCD to calculate the cross sections σ^LT for the diffractive production of open charm (cc) from longitudinally and transversely polarised photons (of virtuality Q2) incident at high energy (√S) on a proton target. We study both the Q ² and M ² dependence of the cross sections, where M is the invariant mass of the cc pair. Surprisingly, the result for σ^T, as well as for σ^L, is perturbatively stable. We estimate higher-order corrections and find a sizeable enhancement of the cross sections. The cross sections depend on the square of the gluon density g(x, K²), and we show that the observation of open charm at the HERA electron-proton collider can act as a sensitive probe of the gluon distribution for x = (Q ² + M ²)/s and scale K2 = (M ²c + )(1 + Q ²/M ²) where the average quark transverse momentum squared 〈k ₂ ^t ~ (M ²c . As compared to diffractive J/#x03C8; production, open charm has the advantage that it is independent of the non-perturbative ambiguities arising from the J/#x03C8; wave function. We estimate the fraction of diffractive events that arise from cc¯ production.     

B → Kμ+μ-: Light Higgs and fourth generation

We examine the current limit on the branching ratio for B → Kμ⁺μ^- and the theoretical predictions for B → KH → Kμ⁺μ^- in three- and four-generation models. It is shown that for a heavy top quark m_t≳60 GeV a light Higgs, m_H ⩽3.5 GeV, is disfavoured in a three-generation model. We also find that the existence of a light Higgs constrains the four-generation model quite strongly through the above process.     

Double pomeron exchange in the reaction pp → ppπ+π−

We compute the cross section for pp → ppπ⁺π^? in the kinematical region for which two-pomeron exchange is important, assuming dominance of the pion-exchange pole. We include effects due to absorption, final-state interaction, and interference between the two contributing Feynman diagrams. These effects have been ignored in previous calculations. Consequently, our predicted cross section is smaller than previous predictions by a factor of ～ 3, and is consistent with experimental indications. We calculate the background cross section in the double pomeron exchange region, and find that it is sizable even at s ～ 3000 GeV². We discuss the two-particle inclusive process pp → ppX.     

η − η′ photoproduction and the axial isoscalar neutral current coupling

We show that coherentη andη′ photoproduction by means of the Primakoff Effect on the proton depends on the strange component of the neutral axial current coupling. We construct polarization asymmetries that are sensitive to this coupling through theγ?Z interference. Theη′ is not a Goldstone boson of a spontaneously broken chiral symmetry, but a phenomenological analysis of theη andη′ production through chiral perturbation theory allows to calculate the observables of interest. The polarized proton or polarized photon asymmetries are predicted to be close to 10^?4 for ?q ²～0.1?0.5 GeV².     

Upper Bounds on the Critical Temperature for Kac Potentials

We consider an Ising system in two dimensions with a two body ferromagnetic interaction J _γ(x, y) that depends on the Kac scaling parameter γ. We prove that the inverse critical temperature β_cr(γ) is strictly above the mean-field value (equal to 1), namely that there exists C>0 so that for any b<C, β_cr(γ)> 1 + bγ²log γ^?1 for all γ sufficiently small. The temperature shift Cγ²log γ^?1 is to leading orders equal to the covariance of the magnetization fluctuations.     

Coherent two pion photoproduction on 12C

We develop the formalism for coherent two pion photoproduction in nuclei and perform actual calculations of cross sections for π⁻π⁺ and π⁰π⁰ photoproduction on ¹²C. We find that due to the isospin symmetry the cross section for π⁰π⁰ production is very small and has a maximum when the pions propagate together. However, the kinematical region where the energies and polar angles of the two π⁰ mesons are equal and their relative azimuthal angle Ø = 180° is forbidden. Conversely in the π⁻π⁺ production the pions prefer to have a relative azimuthal angle 180° and the production of the pions propagating together is suppressed. The dominant one-body mechanism in both channels is related to the excitation of the Δ isobar. Hence the reaction can serve as a source of information about Δ's properties in nucleus. We have found that the reaction is sensitive to effects of the pion and Δ renormalization in the nuclear medium, similar to those found in the coherent (γ, π⁰) reaction, but magnified because of the presence of the two pions.     

Heavy Majorana neutrinos in e + e − → W + W−

In this paper we investigate the effects of heavy Majorana neutrinos in the reaction e ⁺ e ^? → W ⁺ W ^?. We consider neutrino masses in the 1–10 TeV region. We show that at LEP II and NLC energies it is possible to use this processes to verify indirect evidence of heavy neutral particles with mixing angles of the order sin² α = 0.01. We discuss the unitarity restrictions that can be obtained for vector singlet and fermion-mirror-fermion models.     

Roy equation analysis of ππ scattering

We analyse the Roy equations for the lowest partial waves of elastic ππ scattering. In the first part of the paper, we review the mathematical properties of these equations as well as their phenomenological applications. In particular, the experimental situation concerning the contributions from intermediate energies and the evaluation of the driving terms are discussed in detail. We then demonstrate that the two S-wave scattering lengths a₀⁰ and a₀² are the essential parameters in the low energy region: Once these are known, the available experimental information determines the behaviour near threshold to within remarkably small uncertainties. An explicit numerical representation for the energy dependence of the S- and P-waves is given and it is shown that the threshold parameters of the D- and F-waves are also fixed very sharply in terms of a₀⁰ and a²₀. In agreement with earlier work, which is reviewed in some detail, we find that the Roy equations admit physically acceptable solutions only within a band of the (a₀⁰,a₀²) plane. We show that the data on the reactions e⁺e⁻→ππ and τ→ππν reduce the width of this band quite significantly. Furthermore, we discuss the relevance of the decay K→ππeν in restricting the allowed range of a₀⁰, preparing the grounds for an analysis of the forthcoming precision data on this decay and on pionic atoms. We expect these to reduce the uncertainties in the two basic low energy parameters very substantially, so that a meaningful test of the chiral perturbation theory predictions will become possible.     

Origin of the Q 2-dependence of the DIS structure functions

We consider in detail Q ²-dependence of the DIS structure functions. Very often this dependence is claimed to be originated by the Q ²-dependence of the QCD coupling. This leads to the small-x asymptotics of the structure functions with Q ²-dependent intercepts. We demonstrate that the DGLAP parametrization α _s = α _s (Q ²) is an approximation valid in the region of large x (where 2pq can be approximated by Q ²) only, providing the factorization scale is also large. Outside this region, the DGLAP parametrization fails, so α _s should be replaced by an effective coupling which is independent of Q ² at small x. As a consequence, intercepts of the structure functions are independent of Q ² . Nevertheless, the small-x asymptotics of the structure functions explicitly depend on Q ² , even when the coupling does not depend on it. We also consider the structure functions at small Q ² and give a comment on power-Q ² corrections to the structure functions at large and small Q ² .     

Effect of strain on multisubband electron transport in GaAs/InxGa1-xAs coupled quantum well structures

We analyze the strain induced changes in the low temperature multisubband electron mobility mediated through the intersubband interactions in a pseudomorphic GaAs/In_xGa_1−xAs coupled double quantum well structure. We consider the non-phonon scattering mechanisms and study the effect of strain on them. We show that strain reduces the mobility due to ionized impurity (imp-) scattering μ^imp but enhances the mobility due to interface roughness (IR-) scattering μ^IR. For alloy disorder (AL-) scattering as long as the lowest subband is occupied, the effect of strain enhances the mobility μ^AL. However, once the second subband is occupied, there is almost no change, rather decrease in μ^AL for larger well widths. It is gratifying to note that for single subband occupancy, the effect of strain enhances the total mobility μ. On the other hand, for double subband occupancy, initially there is almost no change, but with increase in well width the total mobility reduces. We vary the In composition x from 0.15 to 0.2 and 0.25 and the barrier width between the two wells to analyze their effects on the mobility which shows interesting results. Our study of multisubband mobility can be utilized for the low temperature device applications.     

Topological vacuum structure of a latticeCP 3-model onT 2 andS 2

The topological susceptibility 〈Q _t ² 〉/V is computed for theCP ³-model appropriately discretized on simplicial lattices onT ² andS ², respectively. We concentrate on volumes 1?V/ξ²?100, ξ being the correlation length measured onT ². Approximate scaling behaviour is found presumably not related to the continuum limit but rather to the occurence of dislocations. We observe finite-size effects which are fairly weak on the sphere, however.     

Global structure of the superstring partition function and resolution of the supermoduli measure ambiguity

We investigate the global structure of the fermionic string partition function on the supermoduli space M_g^sup. In particular we show how the recently discovered moduli total-derivative ambiguity is due to a non-trivial cocycle on M_g^sup, present if an atlas of coordinates of M_g^sup can be found, whose transition functions contain even, nilpotent components. We find a correction to the usual Berezin measure on M_g^sup, given by the so-called Rothstein volume form, that eliminates the above boundary ambiguities of the supermoduli measure at any genus. We discuss also the so-called theorem of holomorphic factorization in this particular case and its relation to the physical requirement of modular invariance.