Search for low-mass leptoquarks using the llqq final states in Pb-Pb ultra-peripheral collisions

After a review over past experiments and theoretical requirements from low-energy flavour physics, we argue that the possibility of low-mass leptoquarks (LQ) cannot be fully excluded due to the assumptions made in the measurements. Therefore we propose to search for the pair production of low-mass leptoquarks in Pb-Pb ultra-peripheral collisions with the least model dependence (assuming a small coupling constant ≲ 0.1). There are a couple of advantages: 1) high photon flux provides high production rate for low mass LQs; 2) the background contamination is much lower than that in p-p collisions. The analysis strategy permits a leptoquark to decay to all possible lepton-plus-quark modes. Taking the scalar LQ, S3, with an electric charge $$ \left|q\right|=\frac{4}{3}e $$ q = 4 3 e , as example, the mass point of 100 GeV can be excluded at the 95% confidence level using a dataset of 4 pb−1 Pb-Pb ultra-peripheral collisions at $$ \sqrt{s} $$ s = 5.02 TeV and the performance of the ATLAS detector in Run 2. The proposed method also applies to searching for high-mass LQs in p-p collisions as long as the LQ pair production mechanism dominates.

Radiative corrections and instability of large Q-balls

We discuss stability of Q-balls interacting with fermions in theory with small coupling constant [Formula: see text]. We argue that for configurations with large global U(1)-charge [Formula: see text], the problem of classical stability becomes more subtle. For example, in model with flat direction there is maximal value of charge for stable solutions with [Formula: see text]. This result may be crucial for the self-consistent consideration of Q-ball evaporation into the fermions. We study the origin of additional instability and discuss possible ways to avoid it.

Mass Renormalization in the Nelson Model

The asymptotic behavior of the effective mass meff(Λ) of the so-called Nelson model in quantum field theory is considered, where Λ is an ultraviolet cutoff parameter of the model. Let m be the bare mass of the model. It is shown that for sufficiently small coupling constant α of the model, meff(Λ)/m can be expanded as meff(Λ)/m=1+∑n=1∞an(Λ)α2n. A physical folklore is that an(Λ)=O(logΛ(n-1)) as Λ→∞. It is rigorously shown that 0<limΛ→∞a1(Λ)<C, C1≤limΛ→∞a2(Λ)/log⁡Λ≤C2 with some constants C, C1, and C2.

Spectral Theory for a Mathematical Model of the Weak Interaction—Part I: The Decay of the Intermediate Vector BosonsW±

We consider a Hamiltonian with cutoffs describing the weak decay of spin 1 massive bosons into the full family of leptons. The Hamiltonian is a self-adjoint operator in an appropriate Fock space with a unique ground state. We prove a Mourre estimate and a limiting absorption principle above the ground state energy and below the first threshold for a sufficiently small coupling constant. As a corollary, we prove the absence of eigenvalues and absolute continuity of the energy spectrum in the same spectral interval.

Optical properties of complex oxides: Vanadate ladders and related inorganic nanotubes

Optical spectroscopy is a sensitive probe of charge, spin, and lattice degrees of freedom. In this talk, I will detail our magneto-optical work on alpha'-NaV2O5, a prototypical ladder material, as well as the new Na2V3O7 nanotubes. Using changes in electronic structure with temperature and applied magnetic field, we have mapped out the H-T phase diagram of alpha'-NaV2O5. We find a temperature independent phase boundary at 27 T which persists above Tc (34 K), a kink in Tc(H) near 27 T, and H$\wedge $2 behavior of Tc(H) in the high field phase with an unusually small coupling constant. The energy scale of Tc and the newly discovered phase boundary at Hc (27 T) are identical. Confinement effects are investigated in the related nanotubes. Results include a larger gap than in analogous bulk materials as well as a low-energy rattling mode, which corresponds to the motion of Na+ ions inside the nanotube.

CHAOS–ORDER TRANSITION IN MATRIX THEORY

Classical dynamics in SU(2) matrix theory is investigated. A classical chaos–order transition is found. For small enough angular momentum (even for small coupling constant), the system exhibits a chaotic behavior, for large enough angular momentum, the system is regular.

VARIATIONAL PERTURBATION THEORY

A nonperturbative method — variational perturbation theory (VPT) — is discussed. A quantity we are interested in is represented by a series, a finite number of terms of which not only describe the region of small coupling constant but reproduce well the strong coupling limit. The method is formulated only in terms of the Gaussian quadratures, and diagrams of the conventional perturbation theory are used. Its efficiency is demonstrated for the quantum-mechanical anharmonic oscillator. The properties of convergence are studied for series in VPT for the [Formula: see text] model. It is shown that it is possible to choose variational additions such that they lead to convergent series for any values of the coupling constant. Upper and lower estimates for the quantities under investigation are considered. The nonperturbative Gaussian effective potential is derived from a more general approach, VPT. Various versions of the variational procedure are explored and the preference for the anharmonic variational procedure in view of convergence of the obtained series is argued. We investigate the renormalization procedure in the φ4 model in VPT. The nonperturbative β function is derived in the framework of the proposed approach. The obtained result is in agreement with four-loop approximation and has the asymptotic behavior as g3/2 for a large coupling constant. We construct the VPT series for Yang-Mills theory and study its convergence properties. We introduce coupling to spinor fields and demonstrate that they do not influence the VPT series convergence properties.

Structure and Magnetic Properties of a Dinuclear, Doubly Methoxo-Bridged Iron(III) Complex with Tetrahydrosalen as Capping Ligand

The reaction of the tetrahydrosalen ligand H2L = N,N′-bis(2-hydroxy-3-t-butyl-5-methylbenzyl)- 1,2-diaminoethane with iron(III) chloride in the presence of sodium methoxide affords a neutral complex of the composition [FeL(OMe)]. Single crystal X-ray analysis of this complex shows dinuclear units, [FeL(OMe)]2 = 1, with two methoxide bridges. Complex 1 crystallizes in the monoclinic space group C 2/c with a = 31.67(1)Å, b = 12.544(4)Å, c = 14.135(5)Å and β = 101.33(1)°. The two edge-sharing FeO4N2-octahedra are distorted and connected by a center of inversion. The iron(III) centers are separated by 3.203 Å and weakly antiferromagnetically coupled (J = —5.7 cm-1), which follows from temperature-dependent magnetic susceptibility measurements in the range 4.2 to 280 K. The effective magnetic moment per iron(III) is µeff = 5.24 μΒ at 280 K. The observed small coupling constant J fits well into the magnetostructural relationship suggested recently by Gorun and Lippard (Inorg. Chem. 30,1625 (1991)).