Effective Field Theory calculations of NN→NNπNN\to NN\pi

In this review we present the recent advances for calculations of the reactions $NN\to NN\pi$ using chiral effective field theory. Discussed are the next-to-next-to leading order loop contributions with nucleon and Delta-isobar for near threshold s-wave pion production. Results of recent experimental pion-production data for energies close to the threshold are analyzed. Several particular applications are discussed: (i) it is shown how the measured charge symmetry violating pion-production reaction can be used to extract the strong-interaction contribution to the proton-neutron mass difference; (ii) the role of $NN\to NN\pi$ for the extraction of the pion-nucleon scattering lengths from pionic atoms data is illuminated.Comment: 68 pages, 23 figures, to be published in Int. J. Mod. Phys.

A new parametrization for the scalar pion form factors

We derive a new parametrization for the scalar pion form factors that allows us to analyze data over a large energy range via the inclusion of resonances, and at the same time to ensure consistency with the high-accuracy dispersive representations available at low energies. As an application the formalism is used to extract resonance properties of excited scalar mesons from data for $\bar B^0_s\to J/\psi \pi\pi$. In particular we find for the pole positions of $f_0(1500)$ and $f_0(2020)$ $1465\pm 18 - i (50\pm 9)\,\text{MeV}$ and $1910\pm 50 - i(199\pm 40)\,\text{MeV}$, respectively. In addition, from their residues we calculate the respective branching ratios into $\pi\pi$ to be $(58\pm31)\%$ and $(1.3\pm1.8)\%$.Comment: 18 pages, 11 figures; v2: discussion+figure on pi pi --> K Kbar added, version published in EPJ

Systematic study of the singularity mechanism in heavy quarkonium decays

We investigate in detail the role of heavy meson loops in the transition from $J^{PC}=1^{--}$ sources to candidates for QCD "exotics", such as $Z_c(3900)$, $Z_b(10610)$ and $Z_b'(10650)$. We demonstrate that, if a vector state strongly couples to a heavy meson pair in an $S$--wave and this system decays to another heavy meson pair (e.g. via pion emission), again in an $S$-wave, the pertinent diagrams get enhanced significantly, if the intermediate states are (near) on--shell and have small relative momenta. In a limited kinematic range this mechanism generates "singularity regions" that lead to the creation of a large number of low energy heavy meson pairs, providing an ideal environment for the formation of hadron-hadron bound states or resonances. For instance, we predict that the signals for $Z_b$ and $Z_b'$ should be a lot stronger in $\Upsilon(6S)$ decays due to this mechanism, if these states are indeed hadron-hadron resonances. The findings of this work should be valuable for deepening our understanding of the nature of the mentioned states.Comment: 6 pages, 8 eps figures, version to appear in Phys.Lett.

Extracting the light quark mass ratio m_u/m_d from bottomonia transitions

We propose a new method to extract the light quark mass ratio $m_u/m_d$ using the $\Upsilon(4S)\to h_b\pi^0(\eta)$ bottomonia transitions. The decay amplitudes are dominated by the light quark mass differences, and the corrections from other effects are rather small, allowing for a precise extraction. We also discuss how to reduce the theoretical uncertainty with the help of future experiments. As a by-product, we show that the decay $\Upsilon(4S)\to h_b\eta$ is expected to be a nice channel for searching for the $h_b$ state.Comment: 4 pages, 1 figure. Version slightly shortened to match the published version in PR

Are the XYZ states unconventional states or conventional states with unconventional properties?

We discuss three possible scenarios for the interpretation of mesons containing a heavy quark and its antiquark near and above the first threshold for a decay into a pair of heavy mesons in a relative $S$--wave. View I assumes that these thresholds force the quark potential to flatten which implies that while in these energy ranges molecular states may be formed there should not be any quark--anti-quark states above these thresholds. View II assumes that the main part of the interaction between two mesons is due to the poles which originate from the $Q\bar Q$ interaction. The properties of the $Q\bar Q$ mesons are strongly influenced by opening thresholds but the number of states is given by the quark model. In View III, both types of mesons are admitted also near and above the open flavor thresholds: $Q\bar Q$ mesons and dynamically generated mesons. Experimental consequences of these different views are discussed.Comment: 15 pages, 5 figure

Probing the nature of Y(4260)Y(4260) and Zc(3900)Z_c(3900) in the isospin violating process Y(4260)→J/ψηπ0Y(4260) \to J/\psi \eta \pi^0

The isospin violation process $Y(4260) \to J/\psi \eta \pi^{0}$ is studied assuming that $Y(4260)$ is a $D_{1} \bar{D}+c.c.$ hadronic molecule. In association with the production of the $Z_c(3900)$, which is treated as a $D \bar{D}^{*}+c.c.$ hadronic molecule, this process can help us distinguish their molecular natures from other scenarios, since the incomplete cancellation between the charged and neutral--meson loops, which are prominent in the molecular picture only, produces a peak in the $e^+e^-\to Y(4260)\to J/\psi\eta\pi^0$ cross section at the $D_{1} \bar{D}+c.c.$ threshold and a very prominent peak in the $J/\psi \eta$ invariant mass spectrum in between the $D \bar{D}^{*}+c.c.$ thresholds; the latter being much narrower than the corresponding one in the isospin conserving channel, i.e. $J/\psi \pi^+ \pi^{-}$. The partial width of $Y(4260)\to J/\psi\eta\pi^0$ is about $4 \times 10^{-4}$ of that of $Y(4260)\to J/\psi\pi^+\pi^-$. The cross section of $e^+e^-\to Y(4260)\to J/\psi\eta\pi^0$ at the $D_{1} \bar{D}+c.c.$ threshold is about $0.05 \ \mathrm{pb}$ which is much larger than that produced by the nearby resonances. These features are the direct consequences of the assumed nature of these two states which might be accessible at the high-statistics experiments such as BESIII and LHCb.Comment: 11 pages, 4 figure

Extra dimensions, SN1987a, and nucleon-nucleon scattering data

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 (GODs), 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.Comment: 24 pages, 5 figures. Minor changes to first two paragraphs of introductio

Supernova Constraints on MeV Dark Sectors from e+ e- Annihilations

Theories with dark forces and dark sectors are of interest for dark matter models. In this paper we find the region in parameter space that is constrained by supernova cooling constraints when the models include dark sector particles with masses around 100 MeV or less. We include only interactions with electrons and positrons. The constraint is important for small mixing parameters.Comment: 18 pages, 3 figure