Charm Baryon Spectroscopy at J-PARC†
This is a fundamental question in hadron physics. Quarks are thought to be more fundamental ingredient of hadrons. The principle to describe the dynamics of quarks is known as the Quantum Chromodynamics (QCD). However, it is difficult to reproduce hadrons by solving the equation of QCD due to its non-perturbative nature in low energy.
The constituent quark model (CQM) treats "dressed quark" as an effective degree of freedom in hadrons. The CQM rather well describes the properties of hadrons, such as classification based on spin/flavor symmetry, mass relations, magnetic moments of ground state baryons, and so on. But the CQM sometimes fails in excited states. There remain many undiscovered states, which is known as the so-called missing resonance problem in the CQM. The mass order of resonances, i.e. the N*(1440)1/2+, and/or Λ(1405)1/2- state, is not clearly explained. Recently, unexpected, narrow states in excited hadrons, such as Θ+, X, Y, Z, and Z_b, have been reported. These suggest that hadrons can have rich structure beyond naive CQM, indicating new effective degrees of freedom to describe hadrons.
Through studies of hadrons, we can clarify a mystery of the origin to form the matter in the universe.
In order to answer the above-mentioned questions in hadron physics, we need to understand the interaction between quarks in hadrons further. In particular, quark-quark correlation, namely diquark correlation, in a baryon is of interest. In light baryons, where flavor SU(3) symmetry seems to work rather well, 3 diquark pairs are expected to be correlated each other in an equal weight. Extraction of diquark correlation may not be easy.
Magnitude of the color-spin interaction between quarks is proportional to the inverse of the quark mass. When a quark is replaced by a heavy quark in a baryon, one expects that the correlation of two light quarks is stronger than that of the other pairs. As a result, a diquark correlation is expected to be singled out. Charmed baryons provide unique opportunities to study diquark correlation. The nature of the diquark correlation is expected to appear in level structure, production rates, and decay branching ratios of charmed baryons.
We propose charmed baryon spectroscopy via the (π-, D*-) reaction on hydrogen. Charmed baryons with a wide mass range are expected to be observed in the missing mass, independent of their decay final states. Identifying decay particles from observed charmed baryons, we can easily measure decay branching ratios.
A high-momentum beam line is being constructed at J-PARC, which will provide high-intensity pion beam up to 20 GeV/c with a momentum resolution as good as 0.1 %. We will construct a spectrometer to reconstruct D*- and decay particles from charmed baryons in coincidence with p(π-,D*-).