Minimal conductivity in bilayer graphene

Using the Landauer formula approach, it is proven that minimal conductivity of order e²/h found experimentally in bilayer graphene is an intrinsic property. For the case of ideal crystals, the conductivity turns out to be equal to e²/2h per valley per spin. A zero-temperature shot noise in bilayer graphene is considered and the Fano factor is calculated. Its value 1–2/π is close to the value 1/3 found earlier for single-layer graphene.     

Tests of the naturalness of the coupling constants in ChPT at order p6

We derive constraints on combinations of O(p⁶) chiral coupling constants by matching a recent two-loop calculation of the πK scattering amplitude with a set of sum rules. We examine the validity of the natural expectation that the values of the chiral couplings can be associated with physics properties of the light resonance sector. We focus, in particular, on flavor symmetry breaking of vector resonances. A resonance chiral Lagrangian is constructed which incorporates flavor symmetry breaking more completely than was done before. We use πK unsubtracted sum rules as tests of the modelling of the resonance contributions to the chiral couplings. In some cases the O(p⁶) couplings are found not to be dominated by the resonance contributions. PACS 12.39.Fe; 11.55.Hx; 13.75.Lb     

Quantum criticality and minimal conductivity in graphene with long-range disorder

We consider the conductivity sigma of graphene with negligible intervalley scattering at half filling. We derive the effective field theory, which, for the case of a potential disorder, is a symplectic-class sigma model including a topological term with theta=pi. As a consequence, the system is at a quantum critical point with a universal value of the conductivity of the order of e(2)/h. When the effective time-reversal symmetry is broken, the symmetry class becomes unitary, and sigma acquires the value characteristic for the quantum Hall transition.     

On two-body decays of a scalar glueball

We study two-body decays of a scalar glueball. We show that in QCD a spin-0 pure glueball (a state with only gluons) cannot decay into a pair of light quarks if chiral symmetry holds exactly, i.e., the decay amplitude is chirally suppressed. However, this chiral suppression does not materialize itself at the hadron level such as in decays into π⁺π^- and K⁺K^-. We show this explicitly in the two cases with the glueball much lighter and much heavier than the QCD scale using low-energy theorems and perturbative QCD. For a heavy glueball, using QCD factorization based on an effective Lagrangian, we find that the hadronization into ππ and KK leads to a large difference between Br(π⁺π^-) and Br(K⁺K^-); even the decay amplitude is not chirally suppressed. Our results can provide some understanding of the partonic contents if Br(ππ) or Br(KK̄) is measured reliably.     

Low-mass lepton pair production in Pb-Au collisions at 40 A.GeV

The CERES/NA45 experiment at the CERN SPS has previously measurede ⁺ e ^- pair production in 160 A.GeV Pb-Au collisions. In the mass regionm > 02 GeV/c², an enhancement of 2.7±04(stat.)±0.5(syst.) compared to the expectation from known hadronic decay sources was observed. In the 40 A.GeV data taken in 1999, an enhancement is again found; a preliminary analysis gives an even larger value of 50 ±13(stat.). The results are compared to theoretical model calculations based on π⁺π^- annihilation with a modified ρ-propagator; they may be related to chiral symmetry restoration.     

Aspects of radiative K<Superscript>+</Superscript><Subscript>e3</Subscript> decays

We re-investigate the radiative charged kaon decay K^±→π⁰e^±ν_eγ [K_e3γ ^±] in chiral perturbation theory, merging the chiral expansion with Low’s theorem. We thoroughly analyze the precision of the predicted branching ratio relative to the non-radiative decay channel. Structure dependent terms and their impact on differential decay distributions are investigated in detail, and the possibility to see effects of the chiral anomaly in this decay channel is emphasized. PACS 13.20.Eb; 11.30.Rd; 12.39.Fe     

η, h￠ \eta{^\prime} → πl+l- in a chiral unitary approach

The decays η, → π⁺π^- l ⁺ l ^- (with l = e,μ are investigated within a chiral unitary approach which combines the chiral effective Lagrangian with a coupled-channels Bethe-Salpeter equation. Predictions for the decay widths and spectra are given.     

Analysis of the decays ψ′ → J/ψπ+π? and η′c → ηcπ+π? with the heavy-quark symmetry

In this article, we study the decays ψ′ → J/ψπ ⁺ π ⁻ and η′_c → η _c π ⁺ π ⁻ by taking into account the chiral symmetry breaking effects, the final-state interactions and the heavy-quark symmetry. We can confront the predictions of the η′_c → η _c π ⁺ π ⁻ decay width and differential decay width with the experimental data in the future, and obtain powerful constraints on the chiral breaking effects and the final-state interactions, and test the heavy-quark symmetry.     

Formulation and picture of quantum phase

Based on the concept of classical phase, we formulate a new explanation for the quantum phase from the quantum mechanical point of view. The quantum phase is the canonically conjugate variable of an angular momentum operator, which corresponds to the angular position φ in an actual physical space with a classical reference frame, but it takes a complex exponential form e ^iφ≡cosφ+i sinφ in the abstract Hilbert space of a quantum reference frame. This formulation is simply the famous Euler formula in a complex number field. In particular, when φ = π/2, the correlative quantum phase is a unitary pure imaginary number e ^iπ/2≡cos(π/2)+i sin(π/2) ≡ i. By using a photon state-vector function that is the general solution of photon Schr?dinger equation and can completely describe a photon’s behavior, we discuss the relationship between the angular momentum of a photon and the phase of the photon; we also analyze the intrinsic relationship between the macroscopic light wave phase and the microscopic photon phase.     

Electromagnetic corrections in <Emphasis Type="Italic">η</Emphasis>→3<Emphasis Type="Italic">π</Emphasis> decays

We re-evaluate the electromagnetic corrections to η→3π decays at next-to-leading order in the chiral expansion, arguing that effects of order e ²(m _u −m _d ) disregarded so far are not negligible compared to other contributions of order e ² times a light-quark mass. Despite the appearance of the Coulomb pole in η→π ⁺ π ⁻ π ⁰ and cusps in η→3π ⁰, the overall corrections remain small.     

Renormalization of the contribution of the electron—electron interaction to the conductivity of two-dimensional electron systems

The contribution of the electron—electron interaction to the conductivity of the two-dimensional electron gas in an In_x Ga_1-x As single quantum well with different disorder strengths was experimentally studied. It is shown that the data are described well within the framework of the one-loop approximation of the renormalization group theory so long as the conductivity of the system remains higher than around 15e ²/μh.     

Partially conserved axial-vector current inS-matrix theory

Mandelstam’s argument that PCAC follows from assigning Lorentz quantum numberM=1 to the massless pion is examined in the context of multiparticle dual resonance model. We construct a factorisable dual model for pions which is formulated operatorially on the harmonic oscillator Fock space along the lines of Neveu-Schwarz model. The model has bothm _π andm _ϱ as arbitrary parameters unconstrained by the duality requirement. Adler self-consistency condition is satisfied if and only if the conditionmϱ²−mπ²=1/2 is imposed, in which case the model reduces to the chiral dual pion model of Neveu and Thorn, and Schwarz. The Lorentz quantum number of the pion in the dual model is shown to beM=0.     

Chiral phenomenological relations between rates of rare radiative decays of kaon to pion and leptons and the meson form factors

In the framework of the chiral perturbation theory we obtain the phenomenological relations between decay branches of rare radiative kaon to pion and leptons K ⁺ → π⁺ l ⁺ l ⁻ and K _S ⁰ → π⁰ l ⁺ l ⁻ and meson form factors. The comparison of these results with the present-day experimental data shows us that the ChPT relations for a charge kaon can determine meson form factors from already measured decay rates at high precision level. However, in the case of the neutral kaon decays K ⁰ → π⁰ e ⁺ e ⁻(μ⁺μ⁻) the form factor data are known to a higher precision than data on the differential rates of radiative kaon decay K ⁰ π⁰ e ⁺ e ⁻(μ⁺μ⁻).     

Structure and reactions of pentaquark baryons

We review the current status of the exotic pentaquark baryons. After a brief look at experiments of both positive and negative results, we discuss theoretical methods to study the structure and reactions for the pentaquarks. First we introduce the quark model and the chiral soliton model, where we discuss the relation of mass spectrum and parity with some emphasis on the role of chiral symmetry. It is always useful to picture the structure of the pentaquarks in terms of quarks. As for other methods, we discuss a model-independent method, and briefly mention the results from the lattice and QCD sum rule. Decay properties are then studied in some detail, which is one of the important properties of ⊝⁺. We investigate the relation between the decay width and the quark structure having certain spin-parity quantum numbers. Through these analyses, we consider as plausible quantum numbers of ⊝⁺,J ^P = 3/2⁻. In the last part of this note, we discuss production reactions of ⊝⁺ which provide links between the theoretical models and experimental information. We discuss photoproductions and hadron-induced reactions which are useful to explore the nature of ⊝⁺     

Correlated electron current and temperature dependence of the conductance of a quantum point contact

We investigate finite temperature corrections to the Landauer formula due to electron–electron interaction within the quantum point contact. When the Fermi level is close to the barrier height, the conducting wavefunctions become peaked on the barrier, enhancing the electron–electron interaction. At the same time, away from the contact the interaction is strongly suppressed by screening. To describe electron transport we formulate and solve a kinetic equation for the density matrix of electrons. The correction to the conductance G is negative and strongly enhanced in the region 0.5 × 2e²/h ≤ G ≤ 1.0 × 2e²/h. Our results for conductance agree with the so-called “0.7 structure” observed in experiments.     

Small Exotic Molecules with Bosonic Constituents

We study the stability and the properties of the ground state of neutral systems containing up to four positively charged bosons and their antiparticles. Examples are the di-pionium molecule (π ⁺ π ⁻)², which is almost identical to the positronium molecule (e ⁺ e ⁻)², the tri-pionium (π ⁺ π ⁻)³, and the quadri-pionium (π ⁺ π ⁻)⁴ molecules. We briefly compare our results on the energy to those on the large-N limit of (π ⁺ π ⁻) ^N . We also show that the annihilation probability can be calculated accurately with simple wave functions when one uses the generalized Schwinger rule. Received September 5, 1994; accepted for publication October 15, 1994     

Update of the e + e − → π+π− cross section measured by the spherical neutral detector in the energy region 400 < √s < 1000 MeV

The corrected cross section of the e ⁺ e ⁻ → π⁺π⁻ process measured in the spherical neutral detector experiment at the VEPP-2M e ⁺ e ⁻ collider is presented. The update is necessary due to a flaw in the e ⁺ e ⁻ → π⁺π⁻ and e ⁺ e ⁻ → μ⁺μ⁻ Monte Carlo event generators used previously in data analysis. The text was submitted by the authors in English.