Particles In Quantum Field Theory Research Articles

Particle-continuum duality of skyrmions

Topological solitons are crucial to many branches of physics, such as models of fundamental particles in quantum field theory, information carriers in nonlinear optics, and elementary entities in quantum and classical computations. Chiral magnetic materials are a fertile ground for studying solitons. In the past a few years, a huge number of all kinds of topologically protected localized magnetic solitons have been found. The number is so large, and a proper organization and classification is necessary for their future developments. Here we show that many topological magnetic solitons can be understood from the duality of particle and elastic continuum-medium nature of skyrmions. In contrast to the common belief that a skyrmion is an elementary particle that is indivisible, skyrmions behave like both particle and continuum media that can be tore apart to bury other objects, reminiscing particle-wave duality in quantum mechanics. Skyrmions, like indivisible particles, can be building blocks for cascade skyrmion bags and target skyrmions. They can also act as bags and glues to hold one or more skyrmions together. The principles and rules for stable composite skyrmions are explained and presented, revealing their rich and interesting physics.

Factorization and transverse phase-space parton distributions

We first revisit impact-parameter dependent collisions of ultra-relativistic particles in quantum field theory. Two conditions sufficient for defining an impact-parameter dependent cross section are given, which could be violated in proton-proton collisions. By imposing these conditions, a general formula for the impact-parameter dependent cross section is derived. Then, using soft-collinear effective theory, we derive a factorization formula for the impact-parameter dependent cross section for inclusive hard processes with only colorless final-state products in hadron and nuclear collisions. It entails defining thickness beam functions, which are Fourier transforms of transverse phase-space parton distribution functions. By modelling non-perturbative modes in thickness beam functions of large nuclei in heavy-ion collisions, the factorization formula confirms the cross section in the Glauber model for hard processes. Besides, the factorization formula is verified up to one loop in perturbative QCD for the inclusive Drell-Yan process in quark-antiquark collisions at a finite impact parameter.

Experimental perspective on three-dimensional topological semimetals

A confluence of precise theoretical predictions shows that carefully fabricated three-dimensional (3D) semimetals can host a variety of exotic phases dominated by topological constraints. This experimental review of 3D topological semimetals addresses the role that electronic structure and associated band crossings play in validating Dirac and Weyl fermion descriptions that have analogies with elementary particles in quantum field theory. The importance of Fermi arcs, nodal geometries, symmetry, spin-orbit coupling, and dimensionality is highlighted. A list of confirmed 3D topological semimetals is presented with suggestions for future research and applications.

Symmetry-Protected Ideal Type-II Weyl Phonons in CdTe.

Nontrivial low-energy excitations of crystalline solids have insightfully strengthened understanding of elementary particles in quantum field theory. Usually, topological quasiparticles are mainly focused on fermions in topological semimetals. We alternatively show by first-principles calculations and symmetry analysis that ideal type-II Weyl phonons are present in zinc-blende cadmium telluride, a well-known II-VI semiconductor. Importantly, these type-II Weyl phonons originate from the inversion between the longitudinal acoustic and transverse optical branches. Symmetry guarantees that the type-II Weyl points lie along the high-symmetry lines at the boundaries of the Brillouin zone even with a breaking of inversion symmetry, exhibiting the robustness of protected phonon features. The nontrivial phonon surface states and surface arcs projected on the semifinite (001) and (111) surfaces are investigated. The phonon surface arcs connecting the Weyl points with opposite chirality, guaranteed to be very long, are clearly visible. We not only offer a promising candidate for studying type-II Weyl phonons but also provide a route to realize symmetry-protected nontrivial phonons and related applications in realistic materials.

Observation of multiple types of topological fermions in PdBiSe

Topological semimetals with different types of band crossings provide a rich platform to realize novel fermionic excitations, known as topological fermions. In particular, some fermionic excitations can be direct analogues of elementary particles in quantum field theory when both obey the same laws of physics in the low-energy limit. Examples include Dirac and Weyl fermions, whose solid-state realizations have provided new insights into long-sought phenomena in high-energy physics. Recently, theorists predicted new types of fermionic excitations in condensed-matter systems without any high-energy counterpart, and their existence is protected by crystalline symmetries. By studying the topology of the electronic structure in PdBiSe using density functional theory calculations and bulk-sensitive soft X-ray angle-resolved photoemission spectroscopy, we demonstrate a coexistence of four different types of topological fermions: Weyl, Rarita-Schwinger-Weyl, double class-II three-component, and charge-2 fourfold fermions. Our discovery provides a remarkable platform to realize multiple novel fermions in a single solid, charting the way forward to studies of their potentially exotic properties as well as their interplay.

Effective Particles in Quantum Field Theory

The concept of effective particles is introduced in the Minkowski space-time Hamiltonians in quantum field theory using a new kind of the relativistic renormalization group procedure that does not integrate out high-energy modes but instead integrates out the large changes of invariant mass. The new procedure is explained using examples of known interactions. Some applications in phenomenology, including processes measurable in colliders, are briefly presented.

The three quark-current of delta-isobar

Strong decay D®Np is one of the best modes to search for particles with spin 3/2. D - isobar decay consist of 99% - hadronic decay and 1% - electromagnetic decay, i.e. in this process strong decay is dominating channel which makes it important for nuclear research. The relativistic three-quark current describe all parameters of particle in quantum field theory. The three-quark current of Delta-isobar is similar with all large group of particles with quantum numbers Jp=(3/2)+. Relevant interpolating three-quark current is given in great details. It is shown that, this current has a single form. If we have three-quark current, we can write the interaction Lagrangian of baryon interacting with their constituent quarks. The Lagrangian is the base of the Standard model. The generalization of the current to nonlocal case can be used to such nonlocal theory as covariant quark model. Covariant quark model has been applied to a large number of elementary particle processes.       G M T   Английский Испанский Итальянский Казахский Китайский Трад Китайский Упр Корейский Русский Турецкий Французский   Английский Испанский Итальянский Казахский Китайский Трад Китайский Упр Корейский Русский Турецкий Французский                 Звуковая функция ограничена 200 символами     Настройки : История : Обратная связь : Donate Закрыть

Form factors and related quantities in clothed-particle representation

We show new applications of the notion of clothed particles in quantum field theory. Its realization by means of the clothing procedure put forward by Greenberg and Schweber allows one to express the total Hamiltonian H and other generators of the Poincare group for a given system of interacting fields through the creation (annihilation) operators for the so-called clothed particles with physical (observed) properties. Here such a clothed particle representation is used to calculate the matrix elements (shortly, form factors) of the corresponding Nother current operators sandwiched between the H eigenstates. Our calculations are performed with help of an iterative technique suggested by us earlier when constructing the NN → πNN transition operators. As an illustration, we outline some application of our approach in the spinor quantum electrodynamics.

The complex-mass scheme and unitarity in perturbative quantum field theory

We investigate unitarity within the Complex-Mass Scheme, a convenient universal scheme for perturbative calculations involving unstable particles in Quantum Field Theory which guarantees exact gauge invariance. Since this scheme requires to introduce complex masses and complex couplings, the Cutkosky cutting rules, which express perturbative unitarity in theories of stable particles, are no longer valid. We derive corresponding rules for scalar theories with unstable particles based on Veltman's Largest-Time Equation and prove unitarity in this framework.

Nonlinearity of Quantum Mechanics and Solution of the Problem of Wave Function Collapse

The problem of the wave function collapse (a problem of measurement in quantum mechanics) is considered. It is shown that it can be solved based on quantum mechanics and does not require any additional assumptions or new theories. The particle creation and annihilation processes, which are described based on quantum field theory, play a key role in the measurement processes. Superposition principle is not valid for the system of equations of quantum field theory for particles and fields, because this system is a non-linear. As a result of the creation (annihilation) of a particle, an additional uncertainty arises, which “smears” the interference pattern. The imposition of such a large number of uncertainties in the repetitive measurements leads to the classical behavior of particles. The decoherence theory also implies the creation and annihilation of particles, and this processes are the consequence of non-linearity of quantum mechanics. In this case, the term “collapse of the wave function” becomes a consequence of the other statements of quantum mechanics instead of a separate postulate of quantum mechanics.

About Majorana's Unpublished Manuscripts on Relativistic Quantum Theory for Particles of Any Spin

In this paper we analyze the formal and conceptual steps made by Ettore Majorana in a wide set of unpublished (handwritten) manuscripts (Quaderni, Fascicoli, Volumetti) written in later 20's and earlier 30's where, starting from the Dirac equation for spin-1/2 particles, he developed quantum relativistic wave equations for different (integer and half-integer) spins. In such a way Majorana obtained a Dirac-like equation for the photon and an infinite component quantum field theory for particles of any spin, thus anticipating the modern supersymmetry and string theories.

What is a quark?

We are used to thinking of quarks as fundamental particles in the same way we think of the electron, or gauge bosons, neutrinos, leptons. In strong theory, these objects are unified with gravitation and the physics of spacetime into what is hoped to be an ultimate theory, string/M theory. The string/M theory paradigm completely changes the way we think of the so-called elementary particles in quantum field theory.

Asymptotic behavior of Nambu-Bethe-Salpeter wave functions for multiparticles in quantum field theories

We derive asymptotic behaviors of the Nambu-Bethe-Salpeter (NBS) wave function at large space separations for systems with more than 2 particles in quantum field theories. To deal with $n$-particles in the center of mass flame coherently, we introduce the Jacob coordinates of $n$ particles and then combine their $3(n-1)$ coordinates into the one spherical coordinate in $D=3(n-1)$ dimensions. We parametrize on-shell $T$-matrix for $n$-particle system of scalar fields at low energy, using the unitarity constraint of the $S$-matrix. We then express asymptotic behaviors of the NBS wave function for $n$ particles at low energy, in terms of parameters of $T$-matrix, and show that the NBS wave function carry the information of $T$-matrix such as phase shifts and mixing angles of the $n$-particle system in its own asymptotic behavior, so that the NBS wave function can be considered as the scattering wave of $n$-particles in quantum mechanics. This property is one of the essential ingredients of the HAL QCD scheme to define "potential" from the NBS wave function in quantum field theories such as QCD. Our results, together with an extension to systems with spin 1/2 particles, justify the HAL QCD's definition of potentials for 3 or more nucleons(baryons) in terms the NBS wave functions.

Unparticle self-interactions

We develop techniques for studying the effects of self-interactions in the conformal sector of an unparticle model. Their physics is encoded in the higher n-point functions of the conformal theory. We study inclusive processes and argue that the inclusive production of unparticle stuff in standard model processes due to the unparticle self-interactions can be decomposed using the conformal partial wave expansion and its generalizations into a sum over contributions from the production of various kinds of unparticle stuff, corresponding to different primary conformal operators. Such processes typically involve the production of unparticle stuff associated with operators other than those to which the standard model couples directly. Thus just as interactions between particles allow scattering processes to produce new particles in the final state, so unparticle self-interactions cause the production of various kinds of unparticle stuff. We discuss both inclusive and exclusive methods for computing these processes. The resulting picture, we believe, is a step towards understanding what unparticle stuff "looks like" because it is quite analogous to way we describe the production and scattering of ordinary particles in quantum field theory, with the primary conformal operators playing the role of particles and the coefficients in the conformal partial wave expansion (and its generalization to include more fields) playing the role of amplitudes. We exemplify our methods in the 2D toy model that we discussed previously in which the Banks-Zaks theory is exactly solvable.

What is a particle?

Theoretical developments related to gravitational interaction have questioned the notion of particle in quantum field theory (QFT). For instance, uniquely defined particle states do not exist in general, in QFT on a curved spacetime. More generally, particle states are difficult to define in a background-independent quantum theory of gravity. These difficulties have led some to suggest that in general QFT should not be interpreted in terms of particle states, but rather in terms of eigenstates of local operators. Still, it is not obvious how to reconcile this view with the empirically-observed ubiquitous particle-like behavior of quantum fields, apparent for instance in experimental high-energy physics, or ‘particle’ physics. Here we offer an element of clarification by observing that already in flat space there exist—strictly speaking—two distinct notions of particles: globally defined n-particle Fock-states and local particle states. The last describes the physical objects detected by finite-size particle detectors and are eigenstates of local field operators. In the limit in which the particle detectors are appropriately large, global and local particle states converge in a weak topology (but not in norm). This observation has little relevance for flat-space theories—it amounts to a reminder that there are boundary effects in realistic detectors—but is relevant for gravity. It reconciles the two points of view mentioned above. More importantly, it provides a definition of the local particle state that remains well defined even when the conventional global particle states are not defined. This definition plays an important role in quantum gravity.

Quantum gravitational corrections to the stress-energy tensor around the rotating BTZ black hole

Modes emerging out of a collapsing black hole are red-shifted to such an extent that Hawking radiation at future null infinity consists of modes that have energies beyond the Planck scale at past null infinity. This indicates that physics at the Planck scale may modify the spectrum of Hawking radiation and the associated stress-energy tensor of the quantum field. Recently, it has been shown that, the T-duality symmetry of string fluctuations along compact extra dimensions leads to a modification of the standard propagator of point particles in quantum field theory. At low energies (when compared to the string scale), the modified propagator is found to behave as though the spacetime possesses a minimal length, say, $\lp$, which we shall assume to be of the order of the Planck length. We utilize the duality approach to evaluate the modified propagator around the rotating Banados-Teitelboim-Zanelli black hole and show that the propagator is finite in the coincident limit. We compute the stress-energy tensor associated with the modified Green's function and illustrate graphically that the quantum gravitational corrections turn out to be negligibly small. We conclude by briefly commenting on the results we have obtained.

Borel-Leroy summability of a nonpolynomial potential

We investigate the perturbation series for the spectrum of a class of Schrodinger operators with potential V = ½ x 2 + g m −1 x 2 m / (1 + α gx 2 ) which generalize particular cases investigated in the literature in connection with models in laser theory and quantum field theory of particles and fields. It is proved that the series obey a modified strong asymptotic condition of order ( m − 1) and have an order ( m − 1) strong asymptotic series in g which are shown to be summable in the sense of Borel-Leroy method.

Boost-invariant Hamiltonian approach to heavy quarkonia

Light-front Hamiltonian formulation of QCD with only one flavor of quarks is used in its simplest approximate version to calculate masses and boost-invariant wave functions of $c\overline{c}$ or $b\overline{b}$ mesons. The quark-antiquark Hamiltonian is obtained in the lowest (second) order of a weak-coupling expansion scheme for Hamiltonians of effective particles in quantum field theory. The derivation involves a heuristic ansatz for a gluon mass-gap that is meant to account for non-Abelian color dynamics of virtual effective particles in the Fock components with one or more effective gluons and can be improved order by order in the expansion. Fortunately, the resulting quark-antiquark Hamiltonian does not depend on any details of the ansatz within a large class of possibilities. It is shown that in the Hamiltonian approach in its simplest version the strong coupling constant $\ensuremath{\alpha}$ and quark mass $m$ (for suitable values of the renormalization group parameter $\ensuremath{\lambda}$ that is used in the calculation), can be adjusted so that (a) masses of 12 lightest well-established $b\overline{b}$ mesons are reproduced with accuracy better than 0.5% for all of them, which means 50 MeV in a few worst cases and on the order of 10 MeV in other cases, or (b) masses of 11 lightest $c\overline{c}$ mesons are reproduced with accuracy better than 3% for all of them, which means better than 100 MeV in a few worst cases and on the order of 10 MeV in the other cases, while the parameters $\ensuremath{\alpha}$ and $m$ are near the values expected in the cases (a) and (b) by analogy with other approaches. A fourth-order study in the same Hamiltonian scheme will be required to explicitly include renormalization group running of the parameters $\ensuremath{\alpha}$ and $m$ from the scale set by masses of bosons $W$ and $Z$ down to the values of $\ensuremath{\lambda}$ that are suitable in the bound-state calculations. In principle, one can use the Hamiltonian approach to describe the structure, decay, production, and scattering of heavy quarkonia in all kinds of motion, including velocities arbitrarily close to the speed of light. This work is devoted exclusively to a pilot study of masses of the quarkonia in the simplest version of the approach.

Using semiclassical trajectories for the time-evolution of interacting quantum-mechanical systems

We have developed a method that recasts the time-propagation of dynamic, mutually interacting quantum-mechanical wavefunctions principally as the time-evolution of many classical particles. Our approach utilizes an approximation of Feynman path integrals, known as the semiclassical method, to reduce the path integral to only the “classical” paths connecting the wavefunction at one time step to the next. In exchange for simplifying the path sampling, each classical path’s contribution gains a determinant term dependent on the path and its environment. Like virtual particles in quantum field theory, “virtual classical particles” are said to follow these classical paths. Pushing these virtual classical particles provides the necessary data to evolve quantum wavefunctions in time. Particle-based techniques implemented on parallel computers can then be used to propagate quantum systems using this alternative method.

Mixing and oscillations of neutral particles in quantum field theory

We study the mixing of neutral particles in quantum field theory: the neutral boson field and Majorana field are treated in the case of mixing among two generations. We derive the orthogonality of flavor and mass representations and show how to consistently calculate oscillation formulas, which agree with previous results for charged fields and exhibit corrections with respect to the usual quantum mechanical expressions.