No abstract

We present improved nucleon-nucleon potentials derived in chiral effective field theory up to nextto-next-to-next-to-leading order. We argue that the nonlocal momentum-space regulator employed in the two-nucleon potentials of Refs. [1,2] is not the most efficient choice, in particular since it affects the long-range part of the interaction. We are able to significantly reduce finite-cutoff artefacts by using an appropriate regularization in coordinate space which maintains the analytic structure of the amplitude. The new potentials do not require the additional spectral function regularization employed in Ref.[1] to cut off the short-range components of the two-pion exchange and make use of the low-energy constants ci and di determined from pion-nucleon scattering without any fine tuning. We discuss in detail the construction of the new potentials and convergence of the chiral expansion for two-nucleon observables. We also introduce a new procedure for estimating the theoretical uncertainty from the truncation of the chiral expansion that replaces previous reliance on cutoff variation.

The BABAR Collaboration BABAR, the detector for the SLAC PEP-II asymmetric e + e − B Factory operating at the Υ (4S) resonance, was designed to allow comprehensive studies of CP -violation in B-meson decays. Charged particle tracks are measured in a multi-layer silicon vertex tracker surrounded by a cylindrical wire drift chamber. Electromagnetic showers from electrons and photons are detected in an array of CsI crystals located just inside the solenoidal coil of a superconducting magnet. Muons and neutral hadrons are identified by arrays of resistive plate chambers inserted into gaps in the steel flux return of the magnet. Charged hadrons are identified by dE/dx measurements in the tracking detectors and in a ring-imaging Cherenkov detector surrounding the drift chamber. The trigger, data acquisition and data-monitoring systems , VME-and network-based, are controlled by custom-designed online software. Details of the layout and performance of the detector components and their associated electronics and software are presented.

We present a nucleon-nucleon potential at fifth order in chiral effective field theory. We find a substantial improvement in the description of nucleon-nucleon phase shifts as compared to the fourth-order results of Ref. [1]. This provides clear evidence of the corresponding two-pion exchange contributions with all low-energy constants being determined from pion-nucleon scattering. The fifth-order corrections to nucleon-nucleon observables appear to be of a natural size which confirms the good convergence of the chiral expansion for nuclear forces. Furthermore, the obtained results provide strong support for the novel way of quantifying the theoretical uncertainty due to the truncation of the chiral expansion proposed in Ref. [1]. Our work opens up new perspectives for precision ab initio calculations in few-and many-nucleon systems and is especially relevant for ongoing efforts towards a quantitative understanding the structure of the three-nucleon force in the framework of chiral effective field theory.PACS numbers: 13.75. Cs, Chiral effective field theory (EFT) provides a solid foundation for analyzing low-energy hadronic observables in harmony with the symmetries of quantum chromodynamics (QCD), the underlying theory of the strong interactions. It allows one to derive nuclear forces and currents in a systematically improvable way order by order in the chiral expansion, based on a perturbative expansion in powers of Q ∈ (p/Λ b , M π /Λ b ), where p refers to the magnitude of three momenta of the external particles, M π is the pion mass and Λ b is the breakdown scale of chiral EFT [2]. Being combined with modern few-and many-body methods, the resulting framework based on solving the nuclear A-body Schrödinger equation with interactions between nucleons tied to QCD via its symmetries represents nowadays a commonly accepted approach to ab initio studies of nuclear structure and reactions, see Refs. [3,4] for review articles.Chiral power counting suggests that nuclear forces are dominated by pairwise interactions between the nucleons [2], a feature that was known for long but could only be explained with the advent of chiral EFT. Manybody forces are suppressed by powers of the expansion parameter Q. Specifically, the chiral expansion of nucleon-nucleon (NN), three-nucleon (3NF) and fournucleon (4NF) forces starts at the orders Q 0 (LO), Q 3 (N 2 LO) and Q 4 (N 3 LO), respectively, while next-toleading (NLO) corrections involve two-body operators only. While accurate NN potentials at N 3 LO have been available for about a decade [5,6], the 3NF still represents one of the major challenges in the physics of nuclei and nuclear matter [7]. In particular, numerically exact calculations in the three-nucleon (3N) continuum, the most natural place to test the 3NF, have revealed that the spin-structure of the 3NF is not properly reproduced by the available models [8]. Specifically, one observes clear discrepancies between theory and experimental data for various spin observables in nucleon-deuteron (Nd) scattering starting...

We introduce new semilocal two-nucleon potentials up to fifth order in the chiral expansion. We employ a simple regularization approach for the pion-exchange contributions which (i) maintains the long-range part of the interaction, (ii) is implemented in momentum space and (iii) can be straightforwardly applied to regularize many-body forces and current operators. We discuss in detail the two-nucleon contact interactions at fourth order and demonstrate that three terms out of fifteen used in previous calculations can be eliminated via suitably chosen unitary transformations. The removal of the redundant contact terms results in a drastic simplification of the fits to scattering data and leads to interactions which are much softer (i.e. more perturbative) than our recent semilocal coordinate-space regularized potentials. Using the pion-nucleon low-energy constants from matching pion-nucleon Roy-Steiner equations to chiral perturbation theory, we perform a comprehensive analysis of nucleon-nucleon scattering and the deuteron properties up to fifth chiral order and study the impact of the leading F-wave two-nucleon contact interactions which appear at sixth order. The resulting chiral potentials lead to an outstanding description of the proton-proton and neutron-proton scattering data from the self-consistent Granada-2013 database below the pion production threshold, which is significantly better than for any other chiral potential. For the first time, the chiral potentials match in precision and even outperform the available high-precision phenomenological potentials, while the number of adjustable parameters is, at the same time, reduced by about ∼ 40%. Last but not least, we perform a detailed error analysis and, in particular, quantify for the first time the statistical uncertainties of the fourth-and the considered sixth-order contact interactions.

The Hoyle state plays a crucial role in the hydrogen burning of stars heavier than our sun and in the production of carbon and other elements necessary for life. This excited state of the carbon-12 nucleus was postulated by Hoyle [1] as a necessary ingredient for the fusion of three alpha particles to produce carbon at stellar temperatures. Although the Hoyle state was seen experimentally more than a half century ago [2, 3] nuclear theorists have not yet uncovered the nature of this state from first principles. In this letter we report the first ab initio calculation of the low-lying states of carbon-12 using supercomputer lattice simulations and a theoretical framework known as effective field theory. In addition to the ground state and excited spin-2 state, we find a resonance at −85(3) MeV with all of the properties of the Hoyle state and in agreement with the experimentally observed energy. These lattice simulations provide insight into the structure of this unique state and new clues as to the amount of fine-tuning needed in nature for the production of carbon in stars.

The excited state of the 12 C nucleus known as the "Hoyle state" constitutes one of the most interesting, difficult and timely challenges in nuclear physics, as it plays a key role in the production of carbon via fusion of three alpha particles in red giant stars. In this letter, we present ab initio lattice calculations which unravel the structure of the Hoyle state, along with evidence for a lowlying spin-2 rotational excitation. For the 12 C ground state and the first excited spin-2 state, we find a compact triangular configuration of alpha clusters. For the Hoyle state and the second excited spin-2 state, we find a "bent-arm" or obtuse triangular configuration of alpha clusters. We also calculate the electromagnetic transition rates between the low-lying states of 12 C.PACS numbers: 21.10. Dr, 21.60.De, 26.20.Fj The carbon nucleus 12 C is produced by fusion of three alpha particles in red giant stars. However, without resonant enhancement the triple alpha reaction is too slow to account for the observed abundance of carbon in the Universe. In the early 1950's,Öpik and Salpeter noted independently that the first step of merging two alpha particles is enhanced by the formation of [6], which would imply low-lying rotational excitations of even parity. Other ideas also exist for the structure of the Hoyle state, such as a diffuse trimer of alpha particles [7]. Recently, the spin-2 excitation of the Hoyle state has attracted considerable interest from several experimental groups [8][9][10][11][12].We have recently presented an ab initio lattice calculation of the Hoyle state [13] where the low-lying spectrum of 12 C was explored using the framework of chiral effective field theory and Monte Carlo lattice calculations. However the central question regarding the alpha cluster structure of the Hoyle state remained unsolved, perhaps the greatest remaining challenge in ab initio nuclear theory. In this letter, we announce a major innovation of the lattice method that constructs and tests a wide class of nuclear wave functions explicitly. We present ab initio lattice results that resolve questions about the structure of the Hoyle state and the existence of rotational excitations. We also find evidence for a low-lying spin-2 rotational excitation of the Hoyle state. For the Hoyle state and its spin-2 excitation, we find strong overlap with a "bent-arm" or obtuse triangular configuration of alpha clusters. This is in contrast with the 12 C ground state and the first spin-2 state, where we note strong overlap with a compact triangular configuration of alpha clusters. We also calculate the electromagnetic transition rates among the low-lying even-parity states of 12 C. Our lattice results can be compared with other recent theoretical calculations for the low-lying spectrum of 12 C using the no-core shell model [14,15] and variational calculations using Fermionic Molecular Dynamics [16,17].Chiral effective field theory treats the interactions of protons and neutrons as a systematic expansion in powers of nucleon momenta...

LUX-ZEPLIN (LZ) is a next-generation dark matter direct detection experiment that will operate 4850 feet underground at the Sanford Underground Research Facility (SURF) in Lead, South Dakota, USA. Using a two-phase xenon detector with an active mass of 7 tonnes, LZ will search primarily for low-energy interactions with weakly interacting massive particles (WIMPs), which are hypothesized to make up the dark matter in our galactic halo. In this paper, the projected WIMP sensitivity of LZ is presented based on the latest background estimates and simulations of the detector. For a 1000 live day run using a 5.6-tonne fiducial mass, LZ is projected to exclude at 90% confidence level spin-independent WIMP-nucleon cross sections above 1.4 × 10 −48 cm 2 for a 40 GeV=c 2 mass WIMP. Additionally, a 5σ discovery potential is projected, reaching cross sections below the exclusion limits of recent experiments. For spin-dependent WIMP-neutron(-proton) scattering, a sensitivity of 2.3 × 10 −43 cm 2 (7.1 × 10 −42 cm 2) for a 40 GeV=c 2 mass WIMP is expected. With underground installation well underway, LZ is on track for commissioning at SURF in 2020.