New information on particle interactions that can take place in the heart of neutron stars

The ALICE detector. Credit: CERN

The international ALICE collaboration at the Large Hadron Collider (LHC) has just released the most accurate measurements to date of two properties of a hypernucleus that could exist in the nuclei of neutron stars.

Atomic nuclei and their antimatter counterparts, known as antinuclei, are often produced at the LHC in high-energy collisions between heavy ions or protons. Less frequently, but still regularly, unstable nuclei called hypernuclei are also formed. Unlike normal nuclei, which include only protons and neutrons (i.e. nucleons), hypernuclei are also made up of hyperons, unstable particles containing odd-type quarks.

Nearly 70 years after their first observation in cosmic rays, hypernuclei continue to fascinate physicists because they are rarely produced in the natural world, and although they are traditionally made and studied in low-energy nuclear physics experiments, it is extremely difficult to measure their properties. .

At the LHC, hypernuclei are created in significant quantities in heavy ion collisions, but the only hypernucleus observed so far at the collider is the lightest hypernucleus, the hypertriton, which is composed of a proton, a neutron and a Lambda. a hyperon containing a strange quark.

In their new study, the ALICE team examined a sample of roughly a thousand hypertritons produced in the lead-lead collisions that occurred in the LHC during its second run. Once formed in these collisions, the hypertritons fly a few inches inside the ALICE experiment before decaying into two particles, a helium-3 core and a charged counter, which the ALICE detectors can capture and identify. The ALICE team studied these daughter particles and the traces they leave in the detectors.

ALICE sets the properties of hyper-matter

Hypertriton waist measurements performed with different techniques over time, including the new ALICE measurement (red). The horizontal lines and boxes denote statistical and systematic uncertainties respectively. The dotted lines represent various theoretical predictions. Credit: ALICE collaboration

By analyzing this sample of hypertriton, one of the largest available for these “strange” nuclei, the ALICE researchers were able to obtain the most accurate measurements so far of two of the properties of hypertriton: its life (how long it takes to decay) and the energy needed to separate its hyperon, Lambda, from the remaining constituents.

These two properties are fundamental to understanding the internal structure of this hypernucleus and, consequently, the nature of the strong force that binds nucleons and hyperons together. Studying this force is not only interesting in itself, but can also offer valuable insight into the particle interactions that can take place in the inner nuclei of neutron stars. These nuclei, which are very dense, are expected to favor the creation of hyperons on purely nucleonic matter.

The new ALICE measurements indicate that the interaction between the hypertriton hyperon and its two nucleons is extremely weak: the Lambda separation energy is a few tens of kiloelectron volts, similar to the energy of X-rays used in medical imaging. and the life of the hypertriton is compatible with that of the free Lambda.

Furthermore, since matter and antimatter are produced in nearly equal quantities to the LHC, the ALICE collaboration was also able to study antihypertritons and determine their duration. The team found that, within the experimental measurement uncertainty, antihypertriton and hypertriton have the same duration. Finding even a small difference between the two lives could signal the breaking of a fundamental symmetry of nature, the CPT symmetry.

With data from the third LHC run, which began in earnest this July, ALICE will not only further investigate the properties of hypetriton, but will also extend its studies to include heavier hypernuclei.

The light core was predicted to be stable despite having two strange quarks

More information:
ALICE Collaboration, Measurement of duration and separation energy of 3H.arXiv: 2209.07360v1 [nucl-ex]

Citation: New information on particle interactions that could take place in the heart of neutron stars (2022, 21 September) recovered on 21 September 2022 from – neutron.html

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