News digest

NOvA joins supernova watch

The NOvA experiment in the US, designed to study neutrino oscillations in a beam produced at Fermilab, has joined the Supernova Early Warning System (SNEWS). SNEWS is a global network of neutrino experiments that has been running in automated mode since 2005 to catch core-collapse supernovae in our galaxy – which occur a handful of times each century. NOvA joins other detectors including Super-K in Japan, Ice Cube at the South Pole, Borexino in Italy, Daya Bay in China, and HALO in Canada. The next step for SNEWS is to embrace the world of multi-messenger astronomy by linking with the LIGO and VIRGO gravitational-wave detectors. Potential collaborators are invited to join a SNEWS workshop at SNOLAB, Canada, in June.

φ boson where art thou

The CDF collaboration at Fermilab in the US has set the most stringent limits to date on the existence of a spin-zero particle called the φ boson. Such bosons arise, for example, in minimal supersymmetric extensions of the Standard Model and would be expected to decay predominantly into a pair of b quarks. A previous CDF analysis observed a 3σ excess of events with respect to the expected background at a φ mass of 150 GeV/c2. The new CDF search excludes such a signal at 95% confidence, setting upper limits ranging from 20 to 2 pb for the product of production cross-sections times branching fraction for a φ boson with mass between 100 and 300 GeV/c2 (arXiv.org/1902.04683).

ADMX Sidecar explores ALPs

The Axion Dark Matter Experiment (ADMX) at the University of Washington in the US has reported results from a pathfinder experiment, ADMX “Sidecar”, which is designed to pave the way for future, higher mass, searches. Axions are cold dark-matter candidates that were originally postulated to solve the strong CP problem of the Standard Model. ADMX searches for them by looking for the resonant conversion of dark-matter axions to microwave photons in a strong magnetic field (known as the inverse Primakoff effect). The experiment has already excluded axion masses near 3 μeV, and its Sidecar pathfinder has now demonstrated that the same technique can be used to search for axion-like particles with masses up to 30 μeV (arXiv:1901.00920).

DES completes data-taking

The Dark Energy Survey (DES) – which began mapping a 5000-square-degree area of the sky in August 2013 – completed taking data on 9 January. Using the 520-megapixel Dark Energy Camera mounted on the 4-metre Blanco telescope at the Cerro Tololo Inter-American Observatory in Chile, the Fermilab-hosted project took data on 758 nights over six years, recording data from more than 300 million distant galaxies. The DES collaboration has already released a full range of papers based on its first year of data, and will now focus on producing results from its six years of data.

BESIII amasses record J/ψ dataset

On 11 February, the BESIII experiment at the Beijing Electron Positron Collider in China finished accumulating a sample of 10 billion J/ψ events – the world’s largest dataset produced directly from electron–positron annihilations. Decays of the J/ψ particle offer a clean laboratory for studying exotic hadrons composed of light quarks and gluons, including those composed of pure gluons. With 1.3 billion J/ψ events collected in 2009 and 2012, BESIII has reported many such studies. The record J/ψ-event data sample – which adds the 8.7 billion events collected in 2017, 2018 and 2019 – will improve the precision of such studies.

New kaon decay observed

The NA48/2 experiment at CERN’s North Area has reported the first observation of the decay K±→ π±π0e+e, based on data recorded in 2003–2004. This radiative decay proceeds via a virtual photon which converts into an electron–positron pair, where the photon can be produced either by inner bremsstrahlung, where it is emitted by one of the charged mesons, or by direct emission at the weak vertex. The experimenters observed decays in a 60 GeV/c kaon beam passing through a 114 m tank, and used 4919 candidates to measure the branching ratio to be 4.24 ± 0.14 × 10–6. Several CP-violating asymmetries were investigated and found to be consistent with zero (arXiv:1809.02873).

Skyrmion model revised

The skyrmion model in nuclear physics offers a way to determine the properties of nuclei from QCD, by describing the nucleus in terms of pions and linking the number of twists – or skyrmions – in the quantum field that the pions create with the number of nucleons. But the model yields nuclear binding energies that are an order of magnitude larger than nuclear data, and predicts shapes for nuclei that don’t match the clustering structure of light nuclei. Carlos Naya and Paul Sutcliffe of Durham University in the UK have now shown that a modified version of the model that includes rho mesons, the next lightest subatomic mesons, can improve both of these features (Phys. Rev. Lett. 121 232002).

FAIR progress with heavy ions

After a two-year-long shut down, the SIS18 heavy-ion synchrotron is back in action at GSI, Darmstadt, with upgraded cavities, power converters and shielding. SIS18 is the new fast-cycling booster synchrotron for FAIR, the international Facility for Antiproton and Ion Research (CERN Courier July/August 2017 p41). FAIR will use beams from protons up to uranium ions with a wide range of intensities and energies to garner insights into the nuclear reactions that underpin the synthesis of heavy elements and other processes.

Dark photon search begins

A new dark-matter experiment, the Heavy Photon Search at Jefferson Lab in Virginia, is preparing to take first data. If dark matter is part of a hidden sector, it could couple to ordinary matter via so-called heavy photons. During a two-month run this summer, the collaboration hopes to create dark photons in bremsstrahlung radiation from 4.5 GeV electrons impinging on a heavy target. The experiment will search for bumps in the invariant mass spectrum of electron–positron pairs with vertices separated from the target, with sensitivity to masses of the order of a few hundred MeV.