Modern Physics Letters A

Recent experiments on positronium annihilation have confirmed QED calculations at high orders of α and tested discrete fundamental symmetries. These measurements search for rare modes of annihilation which are distinguished from backgrounds by their specific decay signatures. New developments in beyond Standard Model theory provide motivation for new measurements of such decays. A brief history of searches for rare annihilation modes of Ps is given. Recent experimental and theoretical developments are reviewed. Experiments currently being planned are discussed.

In the Penrose limit, AdS×S space turns into a Cahen–Wallach (CW) space whose Killing vectors satisfy a Heisenberg algebra. This algebra is mapped onto the holographic screen on the boundary of AdS. We show that the Heisenberg algebra on the boundary of AdS may be obtained directly from the CW space by appropriately constraining the states defined on it. The transformations generated by the constraint are similar to gauge transformations. The "holographic screen" on the CW space is thus obtained as a "gauge-fixing" condition.

The production asymmetry of both π and ρ mesons in the reaction p↑p→hX is calculated in a two-component model in which hadrons are produced by a nonperturbative recombination process and by fragmentation of quarks in the final state. Thomas precession in the recombination mechanism seems to be responsible of the single spin asymmetry. We obtain a good description of experimental data. Predictions of single spin asymmetries for ρ production at RHIC energies are presented.

The density of states method is applied for lattice QCD at a finite isospin density. The advantage of this method is that one can easily obtain results for various values of parameters (quark mass, coupling constant and the number of flavors). We compare results for the chiral condensate and the quark number density with those from the R-algorithm and find that they are in good agreement. By calculating the chiral condensate we obtain information on the phase structure for various quark flavors and isospin chemical potentials. We also show results for the chiral condensate at two different quark masses and at two different isospin densities which are not easily obtainable in the conventional Monte Carlo method.

The ratios of B_d and B_s meson decay constant have been estimated using the variational method in conjunction with the relativistic Hamiltonian of the heavy meson in the framework of the statistical model. The ratio of CKM matrix elements has been extracted from mixing using the estimated decay constants and ratio of B_d and B_s mass differences. The results are found to be in good agreement with the existing theoretical and experimental predictions.

We study the structural stability of tachyonic inflation against changes in the shape of the potential. Following Lidsey,¹⁰ the concepts of rigidity and fragility are defined through a condition on the functional form of the Hubble factor. We find that the models are rigid in the sense that the attractor solutions never change as long as the conditions for inflation are met.

We consider d=10, N=1 supersymmetry algebra with maximal number of tensor charges Z and introduce a class of orbits of Z, invariant w.r.t. the T₈ subgroup of massless particles' little group T₈⋉SO(8). For that class of orbits we classify all possible orbits and little groups, which appear to be semidirect products T₈⋉SO(k₁)×⋯×SO(k_n), with k₁+⋯+k_n=8, where compact factor is embedded into SO(8) by triality map. We define actions of little groups on supercharge Q and construct corresponding supermultiplets. In some particular cases we show the existence of supermultiplets with both Fermi and Bose sectors consisting of the same representations of tensorial Poincaré. In addition, complete classification of supermultiplets (not restricted to T₈-invariant orbits) with rank-2 matrix of supersymmetry charges anticommutator, is given.