C. Hanhart

Evidence that the a0(980) and f0(980) are not elementary particles

Abstract We study the interesting problem of whether it is possible to distinguish composite from elementary particles. In particular we generalize a model-independent approach of Weinberg to the case of unstable particles. This allows us to apply our formalism to the case of the a 0 (980) and f 0 (980) resonances and to address the question whether these particles are predominantly genuine, confined quark states (of q q or qq q q structure) or governed by mesonic components.

Decoding the Riddle of Y(4260) and Z(c)(3900)

The observation of Zc(3900) by the BESIII Collaboration in the invariant mass spectrum of J/ψπ± in e+e-→J/ψπ+π- at the center of mass 4.260 GeV suggests the existence of a charged DD*+DD* molecular state with I(JP)=1(1+), which could be an isovector brother of the famous X(3872) and an analogue of Zb(10610) claimed by the Belle Collaboration. We demonstrate that this observation provides strong evidence that the mysterious Y(4260) is a DD1(2420)+DD1(2420) molecular state. Especially, we show that the decay of this molecule naturally populates low momentum DD* pairs and leads unavoidably to a cusp at the DD* threshold. We discuss the signatures that distinguish such a DD* cusp from the presence of a true resonance.

Extra dimensions, SN1987a, and nucleon-nucleon scattering data

Abstract One of the strongest constraints on the existence of large, compact, “gravity-only” dimensions comes from SN1987a. If the rate of energy loss into these putative extra dimensions is too high, then the neutrino pulse from the supernova will differ from that actually seen. The dominant mechanism for the production of Kaluza–Klein gravitons and dilatons in the supernova is via gravistrahlung and dilastrahlung from the nucleon–nucleon system. In this paper we compute the rates for these processes in a model-independent way using low-energy theorems which relate the emissivities to the measured nucleon–nucleon cross section. This is possible because for soft gravitons and dilatons the leading contribution to the energy-loss rate is from graphs in which the gravitational radiation is produced from external nucleon legs. Previous calculations neglected these mechanisms. We re-evaluate the bounds on toroidally-compactified “gravity-only” dimensions, and find that consistency with the observed SN1987a neutrino signal requires that if there are two such dimensions then their radius must be less than 1 micron.

Reconciling the X(3872) with the near-threshold enhancement in the D{sup 0}D*{sup 0} final state

We investigate the enhancement in the DD̄π final state with the mass M = 3875.2 ± 0.7 −1.6 ± 0.8 MeV found recently by the Belle Collaboration in the B → KDD̄π decay and test the possibility that this is yet another manifestation of the well–established resonance X(3872). We perform a combined Flattè analysis of the data for the DD̄π mode, and for the ππJ/ψ mode of the X(3872). Only if the X(3872) is a virtual state in the DD̄ channel, the data on the new enhancement comply with those on the X(3872). In our fits, the mass distribution in the DD̄ mode exhibits a peak at 2 ÷ 3 MeV above the DD̄ threshold, with a distinctive non-Breit–Wigner shape.

Precision calculation of the π−d scattering length and its impact on threshold πN scattering

We present a calculation of the pi^- d scattering length with an accuracy of a few percent using chiral perturbation theory. For the first time isospin-violating corrections are included consistently. Using data on pionic deuterium and pionic hydrogen atoms, we extract the isoscalar and isovector pion-nucleon scattering lengths and obtain a^+=(7.6 +/- 3.1) x 10^{-3} mpi^{-1} and a^-=(86.1 +/- 0.9) x 10^{-3} mpi^{-1}. Via the Goldberger-Miyazawa-Oehme sum rule, this leads to a charged-pion-nucleon coupling constant g_c^2/4 pi = 13.69 +/- 0.20.

Analytic properties of the scattering amplitude and resonances parameters in a meson exchange model

The analytic properties of scattering amplitudes provide i mportant information. Besides the cuts, the poles and zeros on the different Riemann sheets determine the global behavior of the amplitude on the physical axis. Pole positions and residues al low for a parameterization of resonances in a well-defined way, f ree of assumptions for the background and energy dependence of the resonance part. This is a necessary condition to relate r esonance contributions in different reactions. In the present study, we determine the pole structure of pion-nucleon scattering in an analytic model based on meson exchange. For this, the sheet structure of the amplitude is determined. To show the precision of the resonance extraction and discuss phenomena such as resonance interference, we discuss the S 11 amplitude in greater detail.

Chiral three-nucleon forces from p-wave pion production.

Production of p-wave pions in nucleon-nucleon collisions is studied according to an improved power counting that embodies the constraints of chiral symmetry. Contributions from the first two nonvanishing orders are calculated. We find reasonable convergence and agreement with data for a spin-triplet cross section in pp-->pppi(0), with no free parameters. Agreement with existing data for a spin-singlet cross section in pp-->pnpi(+) constrains a short-range operator shown recently to contribute significantly to the three-nucleon potential.

Precision calculation of threshold π−d scattering, πN scattering lengths, and the GMO sum rule

Abstract We use chiral perturbation theory (ChPT) to calculate the π − d scattering length with an accuracy of a few percent, including isospin-violating corrections in both the two- and three-body sectors. In particular, we provide the technical details of a recent letter (Baru et al., 2011) [1] , where we used data on pionic deuterium and pionic hydrogen atoms to extract the isoscalar and isovector pion–nucleon scattering lengths a + and a − . We study isospin-breaking contributions to the three-body part of a π − d due to mass differences, isospin violation in the πN scattering lengths, and virtual photons. This last class of effects is ostensibly infrared enhanced due to the smallness of the deuteron binding energy. However, we show that the leading virtual-photon effects that might undergo such enhancement cancel, and hence the standard ChPT counting provides a reliable estimate of isospin violation in a π − d due to virtual photons. Finally, we discuss the validity of the Goldberger–Miyazawa–Oehme sum rule in the presence of isospin violation, and use it to determine the charged-pion–nucleon coupling constant.

Bound state nature of the exotic Zb states

The assumption that the newly observed charged bottomonia states Zb(10610) and Zb(10650) are of molecular nature is confronted with the measured invariant-mass distributions for the transitions of the ϒ(5S) to the final states hbπ+π− and hb(2P)π+π−. It is shown that the assumption that the Zb(10610) and Zb(10650) are B