Recoupling mechanism for exotic mesons and baryons

The infinite chain of transitions of one pair of mesons (channel I) into another pair of mesons (channel II) can produce bound states and resonances in both channels even if no interactions inside channels exist. These resonances which can occur also in meson-baryon channels are called channel-coupling (CC) resonances. A new mechanism of CC resonances is proposed where transitions occur due to a rearrangement of confining strings inside each channel — the recoupling mechanism. The amplitude of this recoupling mechanism is expressed via overlap integrals of the wave functions of participating mesons (baryons). The explicit calculation with the known wave functions yields the peak at E = 4.12 GeV for the transitions $$ J/\psi +\phi \leftrightarrow {D}_s^{\ast }+{\overline{D}}_s^{\ast } $$ J / ψ + ϕ ↔ D s ∗ + D ¯ s ∗ , which can be associated with χc1 (4140), and a narrow peak at 3.98 GeV with the width 10 MeV for the transitions $$ {D}_s^{-}+{D}_0^{\ast}\leftrightarrow J/\psi +{K}^{\ast -} $$ D s − + D 0 ∗ ↔ J / ψ + K ∗ − , which can be associated with th recently discovered Zcs (3985).

On the challenges of searching for GeV-scale long-lived particles at the LHC

Many models of dark matter predict long-lived particles (LLPs) that can give rise to striking signatures at the LHC. Existing searches for displaced vertices are however tailored towards heavy LLPs. In this work we show that this bias severely affects their sensitivity to LLPs with masses at the GeV scale. To illustrate this point we consider two dark sector models with light LLPs that decay hadronically: a strongly-interacting dark sector with long-lived exotic mesons, and a Higgsed dark sector with a long-lived dark Higgs boson. We study the sensitivity of an existing ATLAS search for displaced vertices and missing energy in these two models and find that current track and vertex cuts result in very low efficiency for light LLPs. To close this gap in the current search programme we suggest two possible modifications of the vertex reconstruction and the analysis cuts. We calculate projected exclusion limits for these modifications and show that they greatly enhance the sensitivity to LLPs with low mass or short decay lengths.

Photoproduction of the Λ(1520) hyperon with a 9 GeV photon beam at GlueX

The GlueX experiment is located at the Thomas Jefferson National Accelerator Facility (JLab) in Newport News, VA, USA. It features a hermetic 4π detector with excellent tracking and calorimetry capabilities. Its 9 GeV linearly polarized photon beam is produced from the 12 GeV electron beam, delivered by JLab’s Continuous Electron Beam Accelerator Facility (CEBAF), via bremsstrahlung on a thin diamond and is incident on a LH2 target. GlueX recently finished its first data taking period and published first results. The main goal of GlueX is to measure gluonic excitations of mesons. These so-called hybrid or exotic mesons are predicted by Quantum Chromodynamics (QCD) but haven’t been experimentally confirmed yet. They can have quantum numbers not accessible by ordinary quark-antiquark pairs which helps in identifying them using partial wave analysis techniques. The search for exotic mesons requires a very good understanding of photoproduction processes in a wide range of final states, one of them being pK + K − which contains many meson and baryon reactions. The Λ(1520) is a prominent hyperon resonance in this final state and is the subject of this presentation. This talk will give an introduction to the GlueX experiment and show preliminary results for the photoproduction of the Λ(1520) hyperon. The measurement of important observables like the photon beam asymmetry and spin-density matrix elements will be discussed and an outlook to possible measurements of further hyperon states in the pK + K − final state will be given.

Four-Quark States from Functional Methods

In this feature article we summarise and highlight aspects of the treatment of four-quark states with functional methods. Model approaches to those exotic mesons almost inevitably have to assume certain internal structures, e.g. by grouping quarks and antiquarks into (anti-)diquark clusters or heavy-light $$q{\bar{q}}$$ q q ¯ pairs. Functional methods using Dyson–Schwinger and Bethe–Salpeter equations can be formulated without such prejudice and therefore have the potential to put these assumptions to test and discriminate between such models. So far, functional methods have been used to study the light scalar-meson sector and the heavy-light sector with a pair of charmed and a pair of light quarks in different quantum number channels. For all these states, the dominant components in terms of internal two-body clustering have been identified. It turns out that chiral symmetry breaking plays an important role for the dominant clusters in the light meson sector (in particular for the scalar mesons) and that this property is carried over to the heavy-light sector. Diquark-antidiquark components, on the other hand, turn out to be almost negligible for most states with the exception of open-charm heavy-light exotics.

Latest results from GlueX

The GlueX experiment is housed in the newest experimental hall at the Thomas Jefferson National Accelerator Facility in Newport News, Virginia. It was successfully commissioned in 2015 and is in its third year of data taking. GlueX uses a 12 GeV electron beam incident on a diamond radiator, producing a linearly polarized, coherent Bremsstrahlung photon beam. The ultimate goal of GlueX is to search for exotic hybrid mesons (e.g. qq̄g), with either exotic or conventional quantum numbers, whose existence, or lack thereof, would allow for the exploration of the gluon-gluon coupling present in QCD through the manifestation of hadrons with gluonic degrees of freedom. Photo-production at these energies is fairly unexplored and the linear beam polarization allows GlueX to discriminate between various production mechanisms which may be an effective way to identify such exotic hybrid mesons. In addition to exotic mesons, GlueX will also be poised to map out the conventional meson spectrum and to study the spectrum of excited vector mesons, which are often poorly understood. In these proceedings, we will present an overview of the GlueX experiment, its goals, current physics results, and future plans.

Study of exotic mesons in pp and AA collisions

The possibility of study of exotic mesons is discussed. The paper includes detailed analysis of strong, weak and electromagnetic decays containing charmed quark-antiquark pair, physics simulation and event reconstruction at NICA.

Recent results on hadronic spectroscopy at LHCb

The LHCb experiment is optimized to make precision measurements in the flavour sector. The large datasets collected between 2010 and 2016 allow to study the properties of heavy hadrons with unprecedented precision. In these proceedings recent results related to three topics are discussed: studies of charmonium production and properties using inclusive b → φφX decays, the observation of charmed pentaquarks and exotic mesons, and the observation of five new excited Ωc baryons.