News digest

Data taking begins at Belle II • P2O targets Mediterranean Sea • FASER approved for LHC • First light for DESI • Squeezing rare kaon decays • No dark photons in NA62 • Ground breaking at PIP-II • Unnatural cross-fertilisation


Data taking begins at Belle II

First collisions

The fully instrumented Belle II detector recorded its first collisions on 25 March. Belle II will observe events at SuperKEKB, a new asymmetric electron–positron collider in Tsukuba, Japan, which plans to achieve a 40-fold increase on the luminosity of its predecessor, KEKB, which ran from 1999 to 2010. To cope with the increased intensity, Belle has been upgraded with a silicon vertex detector that consists of a two-layer pixel detector and a four-layer double-sided silicon-strip detector. It will observe large quantities of B-meson pairs, allowing a broad range of high-precision measurements in flavour physics.

P2O targets Mediterranean Sea

Almost 100 authors from institutes in Europe, Australia and Morocco have submitted a letter of intent to send a neutrino beam 2595 km from the Protvino accelerator facility near Moscow to the KM3NeT/ORCA detector, which is under construction in the Mediterranean Sea, 40 km from the coast of Toulon, France (arXiv 1902.06083). The unusually long baseline of the “P2O” proposal would allow an unparalleled sensitivity to matter effects in the Earth: such matter effects distinguish electron-type neutrinos and antineutrinos via charged current interactions with atomic electrons, allowing for the determination of the neutrino mass hierarchy after a few years of operations. However, say the authors, a significant densification of the ORCA detector would be required to make a competitive measurement of leptonic CP violation.

FASER approved for LHC

On 5 March, CERN approved FASER, the Forward Search Experiment, which will look for light and weakly interacting long-lived particles produced parallel to the LHC beam at solid angles that cannot be instrumented by conventional collider detectors. Located 480 m along a tangent from the ATLAS detector, in an unused service tunnel, the core detector will be a 5.5 m-long cylinder with a diameter of only 20 cm, as the sought-after new particles (such as dark photons) would remain tightly collimated after being produced at the interaction point. FASER will use spare parts donated by the ATLAS and LHCb experiments and is supported by the Heising–Simons and Simons foundations. Working to an ambitious schedule, the experiment will begin operating in 2021 after the end of the second long shutdown of the LHC.

First light for DESI

Whirlpool Galaxy

On 1 April, the Dark Energy Spectroscopic Instrument (DESI) at the Kitt Peak National Laboratory near Tucson, Arizona, demonstrated the precise focusing and alignment of its lens assembly – a notable achievement as the entire moving mass of the Mayall telescope which houses DESI is 375 tons, and the telescope must target objects with 5 μm accuracy. Having first trained its gaze on the Whirlpool Galaxy (pictured), DESI will eventually collect 360 to 980 nm spectrographs rather than large images, thereby probing redshifts up to 1.7 for emission-line galaxies and up to 3.5 for Lyman-α spectra of quasars. DESI’s focal plane will be installed later this year, paving the way for five years of measurements that, in addition to precise measurements of the expansion history of the universe, may allow the first direct detection of the sum of the neutrino masses at 3σ significance.

Squeezing rare kaon decays

The KOTO experiment at Japan’s J-PARC laboratory in Tokai has published a 90% confidence limit of 3.0×10−9 on the branching ratio for the rare neutral kaon decay KL→π0νν̅, beating the previous world best by an order of magnitude (PRL 122 021802). The experiment, whose name derives from “K0 at Tokai”, uses collisions of 30 GeV protons with a gold target; secondary kaons then travel 20 m before their decays are observed in a 6 m-long vacuum vessel. The main background – hadron clusters induced by the contamination of neutrons in the beam –  was eliminated by cuts on the vertex position and the transverse momentum of the neutral pion, and the collaboration is now analysing the larger 2016–2018 dataset. Meanwhile, NA62 at CERN is working on a comparable measurement of the rare charged kaon decay K+→π+νν̅, to extract a 10% measurement of the CKM parameter |Vtd|.

No dark photons in NA62

The NA62 experiment at CERN has set the tightest limit to date on the existence of dark photons in the mass range 60 to 110 MeV, using just 1% of its expected dataset (arXiv 1903.08767). 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 in an analogous way to neutrino oscillations. NA62, an experiment primarily geared to searching for rare kaon decays, has employed a different technique to most searches by making use of its supply of neutral pions from kaon decays. The pions decay to two photons, one of which could almost immediately “oscillate” into a dark photon and decay to invisible dark-sector particles. Meanwhile, neighbouring experiment NA64 is expected to also weigh in on the subject later this month, when results of its search for missing energy in bremsstrahlung radiation will be released.

Ground breaking at PIP-II


On 15 March, international partners from  France, India, Italy and the UK joined US dignitaries to break ground for a new 215 m-long linear accelerator at Fermilab. Part of the laboratory’s Proton Improvement Plan II (PIP-II), this will be the first US accelerator assembled from internationally built components and the new first stage of the accelerator chain that will send a neutrino beam 1300 km to the liquid-argon based DUNE detector in Lead, South Dakota. DOE Under Secretary for Science Paul Dabbar says, “I’m excited for the further cooperation between America’s premier particle physics and accelerator laboratory and its international partners, and the resulting better understanding of the universe”.

Unnatural cross-fertilisation

Complex-system-modelling expert Sauro Succi of the Italian Institute of Technology, Rome, and Harvard, has answered CERN theory chief Gian Giudice’s call for interdisciplinarity in understanding the current challenges to the principle of naturalness in high-energy physics (HEP). Succi’s “modest tentative” (Eur. Phys. J. Plus 2019 134 97) explores two areas of physics which exhibit the hallmarks of un-naturalness: fluid turbulence and the ground state of quantum many-body fermions. According to Succi, these are likely to offer “stimulating opportunities for cross-fertilisation between HEP and the physics of complex systems”.