China bags moon dust from far side, top quark tips the scales, and the cosmic deuteron puzzle

Chang’e-6 mission brings home 2kg of lunar secrets; CERN teams up to nail down elusive particle’s mass; and how Deuterons defy expectations.

The Alpha Magnetic Spectrometer perched atop the International Space Station, on the left. | Photo Credit: NASA

World’s first samples from moon’s far side

ON June 25, after a 53-day-long mission to the far side of the moon, the Chinese lunar probe Chang’e-6 made its fiery entry through the atmosphere before landing in the grasslands of Inner Mongolia. It was airlifted to Beijing early the next day to the China Academy of Space Technology, which designed the mission spacecraft. Following a ceremony, researchers opened the return capsule to examine key technical indicators and to access its precious cargo: the world’s first-ever lunar samples from the surface of the moon’s far side. The sample container, holding about 2 kg of lunar material, will be transferred to specially developed facilities for storage, analysis, and distribution for research. So far 10 successful missions have brought back samples from the near side.

Like in the case of Chang’e-5 in 2020, which brought back surface samples from the near side, the material will be sorted and then opened to applications for research from scientists and institutions across China. The samples will be likely made available to applications from international researchers after two years. NASA-funded researchers received special clearance late last year to apply for access to lunar samples. Discoveries from Chang’e-5 include a puzzling combination of minerals, the nature of lunar volcanism, an unexpected amount of water, and a new mineral named Changesite-(Y).

On June 25, scientists from the Institute of Geology and Geophysics at the Chinese Academy of Sciences published in the journal The Innovation their predictions for the unique materials that are likely to be found in the Chang’e-6 samples. On the basis of the geological characteristics of the probe’s landing site, the researchers anticipate that the samples will consist of 2.5-million-year-old volcanic rock combined with small amounts of material generated by nearby meteorite strikes. There is also the possibility that evidence of distant impacts will be found in the samples.

“There are significant differences between the far side and the near side of the moon in terms of lunar crustal thickness, volcanic activity, composition, etc., especially considering that Chang’e-6 landed on the South Polar-Aitkin Basin, the special terrane [fault-bounded region with a distinctive stratigraphy, structure, and geological history] of the moon,” said Zongyu Yue, a geologist at the Chinese Academy of Sciences. “The Chang’e-6 samples… are expected to answer one of the most fundamental scientific questions in lunar science research: what geologic activity is responsible for the differences between the two sides?”

A view of the Large Hadron Collider at CERN, Geneva. | Photo Credit: CERN

Mass of top quark gets more precise value

RESEARCHERS at CERN, the European centre for particle physics in Geneva, have significantly increased the precision in the measured value of the mass of the top quark, one of six quarks, the elementary particles that form the building blocks of all matter and also interact via all the fundamental forces of interaction (electromagnetism, gravitation, the weak nuclear force, and the strong nuclear force).

CERN houses the most powerful particle accelerator to date, the Large Hadron Collider (LHC), that discovered the Higgs boson in 2012, the particle that endows mass to all matter. The mass of the top quark, the heaviest of the six quarks, is a key input for precision calculations of predictions of the Standard Model, particularly of rare processes.

At the time of discovery of the top quark in 1995, its mass was estimated to lie between 151 and 197 giga electronvolt (GeV), the energy equivalent of mass in accordance with E= mc², making it the heaviest-known elementary particle. Since then, more data have helped in narrowing down the range. Now the CMS and ATLAS Collaborations at the LHC have pooled 15 different measurements to obtain the most precise value for its mass to date: 172.52 ± 0.33 GeV. The result was published in a recent issue of Physical Review Letters.

The first determination of the top-quark mass was made using measurements of particles produced in proton-antiproton collisions at the Tevatron, the accelerator that was active until 2011.

In the new study, the CMS and ATLAS Collaborations considered statistical uncertainties that arose from having limited collision data. They also considered 25 classes of systematic uncertainties relating the top quark’s mass to the properties of the particles observed in the LHC detectors. This combined effort resulted in a mass value that is 31 per cent more precise than the most precise of the 15 input measurements.

Technicians remove the samples collected on the moon’s far side from the return capsule of the Chang’e-6 lunar mission. | Photo Credit: China Central Television

A puzzling excess of cosmic deuterons

SINCE the Alpha Magnetic Spectrometer (AMS) was installed aboard the International Space Station in May 2011, it has detected the signals of 2.3 × 10¹¹ (or 230 billion) cosmic ray particles. Now an analysis of these signals by researchers has discovered that 2.1 × 10⁷ (about 0.01 per cent) of those particles are deuterons, or heavy hydrogen ions. Samuel Ting of the Massachusetts Institute of Technology, the AMS team head, said that the observation suggested that a high number of high-energy cosmic-ray deuterons were streaming from some currently unknown source. That conclusion awaits confirmation.

According to models of cosmology, deuterons were produced shortly after the big bang. This was when the temperature was low enough for protons to fuse with neutrons (to form deuterons) but still high enough that the deuterons could not subsequently fuse into helium-4 ions. It is estimated that for every atom of hydrogen there are only 0.00002 deuterons. This low cosmic abundance of deuterons is due to the very short temporal window with the right temperature range, just 10 minutes.

It is unclear how many of those original deuterons have had their energies boosted to cosmic ray levels due to a supernova blast or some other cosmic accelerator. But even if they all have, their small number is insufficient to account for the high number of deuterons in the cosmic rays the AMS found.

According to theory, nearly all deuterons in cosmic rays are produced when helium-4 ions collide with atoms of the interstellar medium. This also produces helium-3 ions. So for the theory to be correct, the measured cosmic-ray fluxes of deuterons and helium-3 ions should have similar correlations with the helium-4 flux. But analysis of the AMS data found that a large fraction of the deuterium flux had a distinctly different correlation. Ting said that the finding potentially implied that there was another, “primary” source of deuteron cosmic rays.