Session E7: Non Accelerator Searches for Exotic Particles

Constraining neutron-proton effective mass splitting and density dependence of nuclear symmetry energy using heavy-ion collisions

While significant progress has been made in understaning the density and momentum depenence of nuclear isovector interaction and the correspodning symmetry energy of neutron-rich nucleonic matter around saturation density, many challenging questions remain to be addressed especially at supra-saturation densities [1]. According to the Hugenholtz-Van Hove theorem [2], nuclear symmetry energy and its slope L are determined by the nucleon isovector (symmetry) potential and its momentum dependence [3]. The latter determines uniquely the neutron-proton effective k-mass splitting in neutron-rich nucleonic matter. Using currently available constraints on the symmetry energy from 28 recent analyses of various terrestrial nuclear laboratory experiments and astrophysical observations, we infer the corresponding neutron-proton effective k-mass splitting [4] and discuss potentially useful observables for further improving the constraints using heavy-ion reactions.\\[4pt] [1] B.A. Li, L.W. Chen and C.M. Ko, Phys. Rep. 464, 113 (2008).\\[0pt] [2] N.M. Hugenholtz and L. van Hove, Physica 24, 363 (1958).\\[0pt] [3] C. Xu, B.A. Li and L.W. Chen, Phys. Rev. C82, 054607 (2010).\\[0pt] [4] B.A. Li and X. Han, Phys. Lett. B727, 276 (2013).   

Measuring the Fusion Cross-Section of Light Nuclei with Low-Intensity Beams

Reactions between neutron-rich light nuclei have been proposed as a heat source in the crust of an accreting neutron star that triggers an X-ray superburst. To explore the probability of such fusion events as well as better understand the fusion dynamics between neutron-rich nuclei, an experimental program to measure the dependence of the fusion cross-section on neutron number has been initiated. Key to these measurements is developing an approach to measure the total fusion cross-section for beams of low-intensity light nuclei (\textless 10$^{5}$ ions/s) on light targets. Fusion residues resulting from the fusion of oxygen nuclei with $^{12}$C at energies near and below the Coulomb barrier are directly measured and distinguished from unreacted beam particles on the basis of their energy and time-of-flight (TOF). The TOF is measured between a microchannel plate (MCP) detector and a segmented Si detector. Two initial problems were charge trapping in the Si detector and slit scattering in the MCP detector. These problems have both been minimized by implementing a gridless MCP detector and a new Si design making the measurement feasible. Supported by the US DOE under Grant No. DEFG02-88ER-40404   

Learning about the nuclear symmetry energy through the lens of isospin transport

Examining nucleon transport between nuclei in intermediate energy heavy-ion collisions is an effective means to assess the density dependence of the nuclear symmetry energy. Overlap of the Fermi tails of the two nuclei as they collide provides a density gradient that drives nucleon transport. In addition, nucleon transport is driven by gradients in N/Z. Disentangling these two contributions provides a measure of the symmetry energy and its density dependence and requires a comparison of N/Z symmetric and asymmetric systems. To address this question we have examined semi-peripheral collisions of $^{\mathrm{64}}$Zn ions with $^{\mathrm{64}}$Zn, $^{\mathrm{209}}$Bi, and $^{\mathrm{27}}$Al targets at E$_{\mathrm{lab}}$ $=$ 45 MeV/A. The projectile-like fragment emerging from these collisions frequently undergoes binary decay in a dynamical fission process. By using the rotation of the projectile-like fragment as a clock, it is deduced that N/Z equilibration persists up to 1200 fm/c. As prior measurements were restricted to timescales of less than 100 fm/c, this approach represents a dramatic improvement in the sensitivity to long timescales. This work is supported by the U.S. DOE under Grant No. DEFG02-88ER-40404.   

Glauber Calculations of the Nuclear Excitation Cross Section of Stable Nuclei and Nuclei with Halo

Interaction cross sections for various stable and unstable isotopes were measured in scattering experiments with nuclear targets. To extract parameters of the nuclear density distribution, experimental data are compared to the reaction cross sections calculated in the Glauber theory framework. The reaction cross-sections include the cross-sections of all processes except of the elastic scattering, whereas the interaction cross-sections do not include the elastic scattering as well as the processes with a target nuclei excitation or disintegration. We calculate the difference between reaction and interaction cross sections (equals to the cross section of the nuclear target excitation) for various stable and unstable isotopes with halo using expressions obtained in the Glauber theory and in optical approximation. We show that the difference cannot be neglected. In addition, we present cross sections of nuclear excitation of projectile nuclei, which significantly differs from the cross sections of the target excitation.   

Experimental yields of in-flight fission products from Ni to Pd measured following U-238 fragmentation at NSCL

In a recent experiment conducted at the National Superconducting Cyclotron Laboratory (NSCL) at Michigan State University a cocktail beam of radioactive nuclei was produced in the projectile fragmentation and in-flight fission of 80 MeV/A U-238 ions impinging upon a 33.5 mg/cm$^{\mathrm{2}}$--thick diamond target. The target was positioned at the pivot point of the S800 magnetic spectrograph and within the GRETINA gamma-ray tracking array. Reaction products were identified on an event-by-event basis (Z, A) by the S800 spectrograph and correlated with gamma-rays detected by GRETINA. In the current work over 100 fission fragments with 28 $\le $ Z $\le $ 46 have been identified. Production yields shall serve as input into models of abrasion-fission at intermediate energies and may be used to plan future experiments. In-flight fission remains a valuable tool for nuclear spectroscopy in the medium-mass region.   

Progress of spin-dependent mesoscopic force experiments with rare-earth garnet test masses

We report on an experimental search for mesoscopic forces, with emphasis on interactions that depend on spin. Our technique uses 1 kHz mechanical oscillators as test masses with a stiff conducting shield in between them to suppress backgrounds, which has been used successfully to constrain mass-coupled forces in this range. With suitable modifications, including spin-polarized test masses, this experiment can be used to explore essentially all of the 15 possible forms of the spin-dependent interaction between electrons described in a recent review. We report on the progress of the test mass development, for which we are investigating ferrimagnetic rare earth iron garnet compounds that exhibit orbital compensation of the magnetism associated with the intrinsic electron spins. Another experiment, which uses a SQUID magnetometer to monitor the induced magnetization of a related garnet compound in the presence of a dense mass in close proximity, could provide even greater sensitivity to several of the possible interactions.   

Searches for possible T-odd and P-odd interactions of mesoscopic range using polarized nuclei and nonmagnetic masses

Various theories predict the possible existence of T-odd and P-odd interactions of mesoscopic ranges (millimeters to microns) between two spin 1/2 fermions proportional to \textbf{S}$\cdot $\textbf{r}, where \textbf{S} is the spin of one of the fermions and \textbf{r} is the unit vector between the particles. We use ensembles of polarized nuclei and an unpolarized mass along with NMR technique to search for such an interaction over sub-mm ranges. We established an improved upper bound on $g_{s}g_{p}^{n}$, the product of the scalar coupling to particles in the unpolarized mass and the pseudoscalar coupling of polarized neutrons for force ranges from 10$^{-4}$ to 10$^{-2}$ m, corresponding to a mass range of 2*10$^{-3}$ to 2*10$^{-5}$ eV for the exchange boson [1]. We will describe this experiment and possible improvements. \\[4pt] [1] P. H. Chu, A. Dennis, C. B. Fu, H. Gao, R. Khatiwada, G. Laskaris, K. Li, E. Smith, W. M. Snow, H. Yan, and W. Zheng, Phys. Rev. D \textbf{87}, 011105(R) (2013)   

High-frequency search for mass-coupled mesoscopic forces

The possible existence of unobserved interactions of nature with ranges of mesoscopic scale (microns to millimeters) and very weak couplings to matter has attracted a great deal of scientific attention. We report on an experimental search for exotic mass-coupled in this range. Our technique uses a planar, double-torsional tungsten oscillator as a test mass, a similar oscillator as a source mass, and a stiff conducting shield in between them to suppress backgrounds. This method affords operation at the limit of instrumental thermal noise, which we have we have recently demonstrated with a measurement of the noise kinetic energy of a detector prototype in thermal equilibrium at room temperature. The fluctuations of the oscillator in a high-Q torsional mode with a resonant frequency near 1 kHz are detected with capacitive transducers coupled to a sensitive differential amplifier. The apparatus is calibrated by means of a known electrostatic force and input from a finite-element model of the selected mode. The measured kinetic energy is in agreement with the expected value of 1/2 kT.   

Limits on Possible Mesoscopic Spin-Dependent Forces using Neutron Spin Rotation

We discuss experiments using polarized slow neutrons to investigate possible spin-dependent forces of \lq\lq mesoscopic\rq\rq range (millimeters to microns). We describe the limits on two potentials that could give rise to such forces (Dobrescu, 2006). The first is proportional to $g_Vg_A\vec{\sigma}\cdot\vec{p}$, with $\sigma$ and $p$ the neutron spin and momentum, respectively. This interaction would lead to a phase difference in the amplitude of positive and negative helicity states, causing the spin of transversely polarized neutrons to rotate through an angle $\phi_{PNC}$. A search for neuron spin rotation in $^{4}$He provides the current limit on the product of vector and axial couplings $g_vg_A<10^{-32}$ at 1 mm (Yan and Snow, 2013). The second potential is proportional to ${g_A}^2\vec{\sigma}\cdot{(\vec{p}\times \vec{r})}$, where $r$ is the distance between the neutrons and the bulk material. We discuss an apparatus to search for this interaction using thin sheets of various mass densities made to rotate about a longitudinal symmetry axis as a target. The current limit on the product of axial vector couplings is ${g_A}^2<6\times10^{-13}$ (Piegsa and Pignol, 2012), which we hope to improve by at least two orders of magnitude.