The BRing

The Booster Ring

High-energy stable beams extracted slowly from the Booster Ring are delivered to the high-energy experimental cave. The 9.3 GeV proton beam and A/Z=2 primary beams up to 4.25 GeV/u energy will be available. We have defined the hypernuclear physics and properties of nuclear matter as our high priority research program at the high-energy station.

1. Hypernuclear Physics

It has been proven that hyperons can be produced in peripheral nuclear collisions at incident energies of 1.0~2.0 GeV/u, and the hyperons may coalesce with the projectile fragments and hence hypernuclei are synthesized. It is expected that the hyperons with double strangeness could also be produced at reaction energies above the threshold of hyperon production (3.75 GeV/u). The interacts with a proton in the hot participant zone, resulting in production of two hyperons. If the two hyperons are captured by the fragments, hypernuclei with double strangeness are produced.

 Diagrammatic sketch for production Double-Hypernucleus using high-energy projectile fragmentation.  

We estimated the production yields of single and double hypernuclei using the high-energy beams available at HIAF. For the reaction ²⁰Ne + ¹²C system, if we employ ²⁰Ne beam energy of 4.25 GeV/u and intensity of 10⁸/s, about 6.0´10⁷ single hypernuclei and 6.0´10³ double hypernuclei can be produced weekly, respectively. The light double hypernuclei produced in this reaction and their decay are given below. The production yields are enough not only to identify the precursor hypernuclei but only to measure their binding energies using the invariant mass method. One of the unique features of hypernuclear spectroscopy with projectile fragmentation is that, due to a large Lorentz factor of the produced hypernuclei, their decays are observed in flight behind the production target. The half-life of an observed hypernucleus can be determined from the distribution of the flight length before it decays.

      In collaboration with GSI and RIKEN, we designed the experimental setup for hypernuclear physics research, which is located in the high-energy cave of HIAF. We hope that HIAF would be the world’s best place for such studies, and a dramatic expansion of the hypernuclear chart is expected.

 The experimental setup for study of hypernuclei at relativistic energies at HIAF. The major detectors are indicated.

2. QCD Phase Structure

The properties of nuclear matter, described by the theory of the Quantum Chromodynamics (QCD), can be summarized in a phase diagram just like any other form of matter. It is of utmost importance to find how the nature of nuclear matter changes as we vary the temperature and baryon density. Lattice QCD calculation predicted that the transition from the quark-gluon plasma (QGP) to the hadronic phase is a smooth cross-over at the vanishing baryochemical potential of µB~0, while at the finite chemical potential the phase transition is of the first-order. Thermodynamically, hence, there must exist a critical point, i.e. the end point of the first-order phase transition line, see the right Figure shown below. The critical point would be a milestone in the QCD phase diagram and is the Holy Grail for the field of the heavy-ion collisions at relativistic energies. During 2010-2017, a beam energy scan was carried out in order to search for the critical point. Interesting non-monotonic behaviors as a function of collision energy have been observed in the net-proton fluctuation and net-baryon first order collectivity, implying the expected criticality and softening of the equation of state, respectively. Both of the observations occur at the low energy end of the beam energy scan at RHIC. HIAF extends the coverage of the baryochemical potential to lower energy. The beam energy scan program would be incomplete without the results from the energy region at HIAF. In heavy ion collisions at relativistic energies at HIAF, the peak nucleon density can reach 2-3 times of the saturation density, where is an ideal place for studying symmetry energy in order to constrain the equation of state of asymmetric nuclear matter at high density. We will build a detector system dedicated to study the QCD phase structure and symmetry energy using various physical observables, as shown below.

 Shown in the right is the sketch of the QCD phase diagram in temperature T as a function of the baryochemical potential µ_B. At LHC/RHIC high energy region, a smooth cross-over at µ_B ~ 0 is expected. At the lower energy region, the phase transition occurs by passing the first-order phase boundary. The QCD critical point, if exists, should be within this region. The left figure presents the experimental setup for study of heavy-ion collisions at relativistic energies at HIAF, and the major detectors are indicated.