The Joint Physics Analysis Center (JPAC) was formed in 2013 to develop theoretical and phenomenological methods that would lead to a better understanding of production and decays of hadron resonances. JPAC does not rely on a single model or approach, but it is inclusive to various methodologies. JPAC products not only provide a better understanding of hadron phenomena but are of direct use in...
The use of covariant formalisms in amplitude analysis is becoming more and more popular among the experimental collaborations. We discuss some of the tensor formalisms available in the literature, and check whether or not they satisfy the $S$-matrix constraints. We match these to the usual helicity amplitudes, showing that there is no need for "relativistic" corrections.
We have studied the production and decay of the f1(1285) into πa0(980) and KK¯ as a function of the mass of the resonance and find a shoulder around 1400 MeV, tied to a triangle singularity, for the πa0(980) mode, and a peak around 1420 MeV with about 60 MeV width for the KK¯ mode. Both these features agree with the experimental information on which the f1(1420) resonance is based.
In...
We have analyzed the singularities of a triangle loop integral in detail and derived a formula for an easy evaluation of the triangle singularity on the physical boundary. It is applied to the Λb→J/ψK−p process via Λ∗-charmonium-proton intermediate states. Although the evaluation of absolute rates is not possible, we identify the χc1 and the ψ(2S) as the relatively most relevant states among...
The main purpose of scattering experiments is to unveil
the underlying structure of the colliding particles. Typically
one has scattering observables at discrete angles and energies and
some decision has to be made on what are the maximal amount of data which are
mutually compatible among themselves and with a sufficiently flexible model with a minimal number of theoretical assumptions. We...
Partial-wave analysis of meson and photon-induced reactions is needed to enable the comparison of many theoretical approaches to data. In both energy-dependent and independent parametrizations of partial waves, the selection of the model amplitude is crucial. Principles of the S matrix are implemented to a different degree in different approaches; but a many times overlooked aspect concerns...
It is well known that unconstrained single-energy partial wave analysis (USEPWA) gives many equivalent discontinuous solutions, so a constraint to some theoretical model must be used to ensure the uniqueness. It can be shown that it is a direct consequence of not specifying the angle-dependent part of continuum ambiguity phase which mixes multipoles, and by choosing this phase we restore the...
We lay out a framework that can be used to obtain estimates of the possible impact of (com-
binations) of polarization measurements in pseudoscalar-meson photoproduction from the nucleon.
To this end, we quantify the distance between models for pseudoscalar-meson photoproduction in
amplitude space. Experimental observables, with finite accuracy, map to probability distributions
in amplitude...
Important contributions to the study of the excitation spectra of
baryons are provided by measurements of polarization observables
in reactions that involve particles with spin. Pseudoscalar meson
photoproduction poses an example-reaction that has been under intense
investigation recently.
The extraction of resonance-parameters from the polarization-data in
so-called energy-dependent fits,...
I review recent results on dispersive analyses of pion-pion and kaon-pion amplitudes and the extraction of resonance parameters using dispersion relations or analytic methods
In the so-called isobar ansatz the three-particle states can only be populated
via an interacting two-particle system (resonant or non-resonant), and a spectator.
Using this ansatz, we derive the isobar-spectator interaction such that the
three-body unitarity is ensured exactly.
Some of the final equations are similar to the old Amado/Aaron/Young appoach,
however do not rely on arguments...
A precise understanding of low-energy pion-nucleon interactions is central for many areas in nuclear and hadronic physics, ranging from the scalar couplings of the nucleon to the long-range part of two-pion-exchange potentials and three-nucleon forces in Chiral Effective Field Theory. We present a calculation that combines the general principles of analyticity, unitarity, and crossing symmetry...
We investigate the resonance-like structure with quantum numbers $J^{PC} = 1^{++}$ in the $f_{0}\pi$ P-wave recently observed at a mass $1.4\,$GeV by the COMPASS Collaboration. The signal was tentatively assigned to be an axial resonance, $a_{1}(1420)$, although it does not fit the $q\bar{q}$-model, which suggests an exotic nature. We present an elegant explanation of the phenomenon without...
I will present the recent work [Phys Rev D94(2016) 096015] about the a1(1420) peak which is experimentally and theoretically interesting. We study the decay mode of the a1(1260) into a pi+ in p-wave and the f0(980) that decays into pi+ pi- in s-wave. The mechanisms proceeds via a triangular mechanism where the a1(1260) decays into Kbar K, the K decays to an external pi+ and an internal K that...
A large number of exotic candidates, especially in the heavy sector,
have being observed in experiment, such as Zc(3900), Zc(4020), Pc(4380),
Pc(4450), X(5568) and so on. To understand their nature, numerous explanations
are proposed within different frameworks. I will give an overall review of the status of these studies and emphasize key issues to be answered by these solutions.
Crucial...
I will discuss the structure of the tetraquark candidate Zc(3900)
from the meson-meson coupled-channel scattering on the lattice.
The Zc(3900) is experimentally reported as a peak in the pi J/psi invariant mass
in the Y(4260)-->pi pi J/psi decay.
To understand the nature of the Zc(3900), it is most important
to extract the coupled pi J/psi - rho eta_c - anti-D D* interactions faithful to...
We study the $\Lambda_c N - \Sigma_c N$ system on the basis of the coupled-channel HAL QCD method. The potentials are extracted from Nambu-Bethe-Salpeter wave functions on the lattice, which are faithful to S-matrix in QCD. One of the advantages of this approach is that it is possible to extract the potentials without any assumptions. I will present our results of the coupled-channel...
The CEBAF Large Acceptance Spectrometer (CLAS) housed in Hall B of the Thomas Jefferson Accelerator Facility provides us with the experimental tools to study strongly-interacting matter and its dynamics. Polarisation observables, accessible utilising polarised beams, allow us to performed detailed studies on the underlying dynamics of both initial and final state interactions as well as...
The measurement of the excitation spectrum of light-quark hadrons often requires to apply partial-wave analysis methods. The building blocks of the physical models used in such analyses are the partial waves, which are defined by the quantum numbers and the decay paths of the produced resonances. In diffractive dissociation reactions, in principle, infinitely many partial waves can contribute....
