Physicists from the ALICE Collaboration at CERN’s Large Hadron Collider have precisely measured two properties of hypernuclei that may exist in the cores of dense astrophysical objects like neutron stars.
Atomic nuclei and their antimatter counterparts, known as antinuclei, are frequently produced at the Large Hadron Collider in high-energy collisions between heavy ions or protons.
On a less frequent but still regular basis, unstable nuclei called hypernuclei are also formed.
In contrast to normal nuclei, which comprise just protons and neutrons (that is, nucleons), hypernuclei are also made up of hyperons — unstable particles containing quarks of the strange type.
“Hypernuclei are bound states of nucleons and hyperons that are particularly interesting because they can be used as experimental probes for the study of the hyperon-nucleon interaction,” the ALICE physicists said.
“Searching for hypernuclei and exploring the hyperon-nucleon interaction have been a source of fascination for nuclear physicists since the discovery of the first hypernuclei in 1953.”
“In recent years, measurements of the hypertriton production and lifetime have stimulated an interesting debate in the high-energy physics community.”
“The knowledge of the hyperon-nucleon interaction has become more relevant recently due to its connection to the modeling of dense astrophysical objects like neutron stars.”
“Indeed, in the inner core of neutron stars the creation of hyperons is energetically favored compared to purely nucleonic matter.”
“The presence of hyperons as additional degrees of freedom leads to a considerable modification of the matter equation of state, prohibiting the formation of high-mass neutron stars.”
“This is incompatible with the observation of neutron stars heavier than two solar masses, constituting what is referred to as the hyperon puzzle.”
At the Large Hadron Collider, hypernuclei are created in significant quantities in heavy-ion collisions, but the only hypernucleus observed at the collider so far is the lightest hypernucleus, the hypertriton, which is composed of a proton, a neutron and a Lambda — a hyperon containing one strange quark.
In the new study, the ALICE team examined a sample of about one thousand hypertritons produced in lead-lead collisions that occurred in the Large Hadron Collider during its second run.
Once formed in these collisions, the hypertritons fly for a few centimeters inside the ALICE experiment before decaying into two particles, a helium-3 nucleus and a charged pion, which the ALICE detectors can catch and identify.
The physicists investigated these daughter particles and the tracks they leave in the detectors.
By analyzing this sample of hypertritons, one of the largest available for these ‘strange’ nuclei, they were able to obtain the most precise measurements yet of two of the hypertriton’s properties: its lifetime and the energy required to separate its hyperon, the Lambda, from the remaining constituents.
These two properties are fundamental to understanding the internal structure of this hypernucleus and, as a consequence, the nature of the strong force that binds nucleons and hyperons together.
The new measurements indicate that the interaction between the hypertriton’s hyperon and its two nucleons is extremely weak: the Lambda separation energy is just a few tens of kiloelectronvolts, similar to the energy of X-rays used in medical imaging, and the hypertriton’s lifetime is compatible with that of the free Lambda.
In addition, since matter and antimatter are produced in nearly equal amounts at the Large Hadron Collider, the ALICE researchers were also able to study antihypertritons and determine their lifetime.
They found that, within the experimental uncertainty of the measurements, antihypertriton and hypertriton have the same lifetime.
Finding even a slight difference between the two lifetimes could signal the breaking of a fundamental symmetry of nature, CPT symmetry.
“The main remaining piece to be set for the complete understanding of the hypertriton structure is the measurement of branching ratios for the various decay channels,” the scientists said.
“The Run 3 of the Large Hadron Collider will make those measurements accessible with unprecedented precision.”
The team’s results were published online on the arXiv.org preprint server.
ALICE Collaboration. 2022. Measurement of the lifetime and Λ separation energy of 3ΛH. CERN-EP-2022-188; arXiv: 2209.07360
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