arXiv subjects
Publications and source records attributed to Peter Schweitzer.
The chromoelectric polarizability of $J/ψ$ is extracted from lattice QCD data on the nucleon-$J/ψ$ potential in the heavy quark limit. The value of $α(1S) = (1.6 \pm 0.8) \,{\rm GeV}^{-3} $ is obtained. We also comment on the possibility of hadrocharmonia.
The D-term is a fundamental particle property which is defined through the matrix elements of the energy-momentum tensor and as such in principle on equal footing with mass and spin. Yet the experimental information on the D-term of any hadron is very scarce. The D-term of the nucleon can be inferred from studies of hard-exclusive reactions, and its measurement will give valuable insights on the dynamics, structure, and the internal forces inside the nucleon. We review the latest developments and the fascinating applications of the D-term and other energy-momentum tensor (EMT) form factors. We also suggest a definition of the mechanical mean square radius and make a prediction for its size.
This work deals with form factors of the energy-momentum tensor (EMT) of spin-0 particles and the unknown particle property D-term related to the EMT, and is divided into three parts. The first part explores free, weakly and strongly interacting theories to study EMT form factors with the following findings. (i) The free Klein-Gordon theory predicts for the D-term D = -1. (ii) Even infinitesimally small interactions can drastically impact D. (iii) In strongly interacting theories one can encounter large negative D though notable exceptions exist, which includes Goldstone bosons of chiral symmetry breaking. (iv) Contrary to common belief one cannot arbitrarily add "total derivatives" to the EMT. Rather the EMT must be defined in an unambiguous way. The second part deals with the interpretation of EMT form factors in terms of 3D-densities with following results. (i) The 3D-density formalism is internally consistent. (ii) The description is subject to relativistic corrections which are acceptably small in phenomenologically relevant situations including nucleon and nuclei. (iii) The free field result D=-1 persists when a spin-0 boson is not point-like but "heuristically given some internal structure." The third part investigates the question, whether such "giving of an extended structure" can be implemented dynamically, and has the following insights. (i) We construct a consistent microscopic theory which, in a certain parametric limit, interpolates between extended and point-like solutions. (ii) This theory is exactly solvable which is rare in 3+1 dimensions, admits non-topological solitons of Q-ball-type, and has a Gaussian field amplitude. (iii) The interaction of this theory belongs to a class of logarithmic potentials which were discussed in literature, albeit in different contexts including beyond standard model phenomenology, cosmology, and Higgs physics.
The D-term is a particle property defined, similarly to the mass and spin, through matrix elements of the energy-momentum tensor. It is currently not known experimentally for any particle, but the D-term of the nucleon can be inferred from studies of hard-exclusive reactions. In this work we show that the D-term of a spin-1/2 fermion is of dynamical origin: it vanishes for a free fermion. This is in pronounced contrast to the bosonic case where already a free spin-0 boson has a non-zero intrinsic D-term as shown in an accompanying work. We illustrate in two simple models how interactions generate the D-term of a fermion with an internal structure, the nucleon. All known matter is composed of elementary fermions. This indicates the importance to study this interesting particle property in more detail, which will provide novel insights especially on the structure of the nucleon.
The probably most fundamental information about a particle is contained in the matrix elements of its energy momentum tensor (EMT) which are accessible from hard-exclusive reactions via generalized parton distribution functions. The spin decomposition of the nucleon and Ji sum rule are one example. Less prominent but equally important information is encoded in the stress tensor, related to the spatial components of the EMT, which shows in detail how the strong forces inside the nucleon balance to form a bound state. This provides not only unique insights on nucleon structure. It also leads to fascinating new applications to hadron spectroscopy which allow us to formulate new interpretations of the charmonium-nucleon pentaquarks discovered by LHCb. Recent progress is reviewed in this short overview article.
The D-term is, like mass and spin, a fundamental property related to the energy-momentum tensor. Yet it is not known experimentally for any particle. In all theoretical studies so far the D-terms of various particles were found negative. Early works gave rise to the assumption the negative sign could be related to stability. The emerging question is whether it is possible to find a field-theoretical system with a positive D-term. To shed some light on this question we investigate Q-clouds, an extreme parametric limit in the Q-ball system. Q-clouds are classically unstable solutions which delocalize, spread out over all space forming an infinitely dilute gas of free quanta, and are even energetically unstable against tunneling to plane waves. In short, these extremely unstable field configurations provide an ideal candidate system for our purposes. By studying the energy-momentum tensor we show that at any stage of the Q-cloud limit one deals with perfectly well-defined and, when viewed in appropriately scaled coordinates, non-dissipating non-topological solitonic solutions. We investigate in detail their properties, and find new physical interpretations by observing that Q-clouds resemble BPS Skyrmions in certain aspects, and correspond to universal non-perturbative solutions in (complex) |Φ|^4 theory. In particular, we show that also Q-cloud solutions have negative D-terms. Our findings do not prove that D-terms must always be negative. But they indicate that it is unlikely to realize a positive D-term in a consistent physical system.
We investigate the energy-momentum tensor form factors of the nucleon in nuclear medium, based on an in-medium modified $π$-$ρ$-$ω$ soliton model, with medium modifications of the mesons considered. The results allow us to establish general features of medium modifications of the structure of nucleons bound in a nuclear medium.
The nucleon form factors of the energy-momentum tensor are studied in nuclear medium in the framework of the in-medium modified Skyrme model. We obtain a negative D-term, in agreement with results from other approaches, and find that medium effects make the value of d_1 more negative.
We study the structure of the energy-momentum tensor of radial excitations of Q-balls in scalar field theories with U(1) symmetry. The obtained numerical results for the $1\le N \le 23$ excitations allow us to study in detail patterns how the solutions behave with N. We show that although the fields and energy-momentum tensor densities exhibit a remarkable degree of complexity, the properties of the solutions scale with N with great regularity. This is to best of our knowledge the first study of the D-term d1 for excited states, and we demonstrate that it is negative --- in agreement with results from literature on the d1 of ground state particles.
We study the energy-momentum tensor of stable, meta-stable and unstable Q-balls in scalar field theories with U(1) symmetry. We calculate properties such as charge, mass, mean square radii and the constant d1 ("D-term") as functions of the phase space angular velocity omega. We discuss the limits when omega approaches the boundaries of the region in which solutions exist, and derive analytical results for the quantities in these limits. The central result of this work is the rigorous proof that d1 is strictly negative for all finite energy solutions in the Q-ball system. We also show that for Q-balls stability is a sufficient, but not necessary, condition for d1 to be negative.