News digest

Through a beam dump darkly • Circular reasoning • Dø nabs Zc±(3900) • Strange neutron stars • Hawking’s maths vindicated • In pursuit of Majorana • The rarest decay • Got an axion to grind? • EDM search goes pear-shaped

Through a beam dump darkly

The NA64 experiment.

The NA64 collaboration at CERN has published world-best limits on dark photons below a mass of 200 MeV, excluding mixing strengths down to 10–3, and below 10–5 for masses of the order of 1 MeV (arXiv:1906.00176). Hypothetical dark-sector physics could have an equally rich structure as the Standard Model, and the dark-photon vector field is one possible extension that could mix with the photon. A fixed-target experiment at CERN’s SPS, NA64 looks for missing energy in the interaction of 100 GeV electrons in an active beam dump.

Circular reasoning

Following the European Strategy Update in Granada (see Granada symposium thinks big), proponents of a 100 km future circular collider in the vicinity of CERN have released a “Frequently Asked Questions”-style document, which will be regularly updated as the community debates the post-LHC future (arXiv:1906.02693). Focused on the first “FCC-ee” phase, the document currently addresses 24 questions, including “Why do we need at least 5 × 1012 Z decays?”, “Why do we want FCC in Europe?” and quite simply “Are there better ways to 100 TeV than FCC-ee?”

Dø nabs Zc±(3900)

Following first observations of the charmonium-like state Zc±(3900) at e+e colliders in 2013, the Dø collaboration has published 5.4σ evidence for its presence in the decay chain of b-quarks in pp collisions (arXiv:1905.13704). Meanwhile, the BESIII e+e experiment in Beijing has published 3.9σ evidence for its decay to ρ±ηc (arXiv:1906.00831). The Zc resonance’s quark configuration is still unknown, with models to describe its inner structure including loosely bound molecules of charm-meson pairs, compact tetraquarks, and hadro-quarkonium.

Strange neutron stars

Researchers working on the STAR detector at BNL’s Relativistic Heavy-Ion Collider have measured the binding energy of hypertriton – a hypernucleus consisting of a proton, neutron and Λ hyperon. They found a higher value than the accepted 1973 measurement, calling weakly-bound Λ-deuteron models of the particle into question (arXiv:1904.10520). The team has also provided new constraints on neutron-star interiors (see also Studying neutron stars in the laboratory) and reported the first test of matter–antimatter symmetry pertaining to the binding of strange quarks in a nucleus.

Hawking’s maths vindicated

Jeff Steinhauer’s group has measured the Hawking temperature.

Following the first observation of Hawking radiation in 2016 (Nature Phys. 12 959), researchers at the Israeli Institute of Technology have now quantitatively confirmed the thermality of its spectrum and measured its temperature using an analogue black hole (BH) consisting of a flowing Bose–Einstein condensate (Nature 569 688). The flow is supersonic within the analogue BH and subsonic without: the event horizon at the boundary excites the quantum vacuum and gives rise to the emission of quanta, known as Hawking radiation. The group now plans to study the time evolution of the analogue BH.

In pursuit of Majorana

Researchers working on EXO-200 – a prototype liquid-xenon TPC in Carlsbad, New Mexico – are stepping up their pursuit of neutrinoless double beta decay (0νββ): the most sensitive probe for the postulated Majorana nature of neutrinos, whereby a neutrino is its own antiparticle. The full EXO-200 data set allowed a 90% confidence limit on the half life of 0νββ in 136Xe of 3.5 × 1025 years (arXiv:1906.02723), approaching the world-best limit of 10.7 × 1025 years set by KamLand–Zen in 2016. Comparable limits in other isotopes where single-beta decay is suppressed have been set for 76Ge (8.0 × 1025 years, GERDA) and 130Te (1.5 × 1025 years, CUORE). Attention is now turning to nEXO, a planned tonne-scale successor that will push the sensitivity up by two orders of magnitude to ~1028 years.

The rarest decay

The XENON1T experiment, located 1.4 km below the Gran Sasso massif in northern Italy, has observed the rarest decay process ever seen, with a half-life a trillion times longer than the age of the universe: the two-neutrino double-electron capture of 124Xe (Nature 568 532). The measurement is a step towards the search for neutrinoless double-electron capture, which, like neutrinoless double-beta decay, would establish the Majorana nature of the neutrino and give access to the absolute neutrino mass. The detector’s active mass is now being trebled to boost its primary dark-matter search.

Got an axion to grind?

Researchers are turning to creative proposals to search for axions – cold dark-matter candidates originally postulated to resolve the strong CP problem in QCD. Berkeley theorists propose employing superconducting RF cavities: axions would be converted to photons in the magnetic field of a gapped toroid inspired by the ABRACADABRA and DM Radio experiments (arXiv:1904. 0724). Meanwhile, cosmologist Lawrence Krauss proposes to use atomic clocks to look for the effect of an axion background on photon propagation in the vacuum (arXiv:1905.10014). Similarly inventive proposals published recently seek to extend the search for generic low-mass dark matter downwards, for example using nuclear recoils in lab-grown diamond crystals (Phys Rev D 99 123005).

EDM search goes pear-shaped

Nuclei with octupole distortions may bear fruit in EDM searches.

Permanent electric dipole moments (EDMs) of particles are excellent testbeds for the Standard Model. Nonzero EDMs imply that both P and T, and by implication CP, are violated. Nuclear physicists have thus been seeking to measure atomic EDMs since the 1980s, with pear-shaped nuclei the most promising candidates. More correctly termed a static octupole distortion, the pear shape was first seen at CERN’s ISOLDE facility in radium-isotope 224Ra in 2013 (Nature 497 199). However, Peter Butler of the University of Liverpool and coworkers, using beams from the upgraded HIE-ISOLDE, have now established that radon-isotopes 224Rn and 226Rn do not possess static pear shapes in their ground states, and are thus not promising candidates to have measurable atomic EDMs (Nat. Commun. 10 2473), inducing the team to switch focus to other isotopes.