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Sept. 21, 2022
The collaboration’s latest study of a “strange”, unstable nucleus known as the hypertriton offers new insight into the particle interactions that may take place at the hearts of neutron stars
The ALICE detector (Image: CERN)
The international ALICE collaboration at the Large Hadron Collider (LHC) has just released the most precise measurements to date of two properties of a hypernucleus that may exist in the cores of neutron stars.
Atomic nuclei and their antimatter counterparts, known as antinuclei, are frequently produced at the LHC 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.
Almost 70 years since they were first observed 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’s extremely challenging to measure their properties.
At the LHC, 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.
Large Hadron Collider (LHC). Animation Credit: CERN
In their new study, the ALICE team examined a sample of about one thousand hypertritons produced in lead–lead collisions that occurred in the LHC during its second run. Once formed in these collisions, the hypertritons fly for a few centimetres 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 ALICE team investigated these daughter particles and the tracks they leave in the detectors.
By analysing this sample of hypertritons, one of the largest available for these “strange” nuclei, the ALICE researchers were able to obtain the most precise measurements yet of two of the hypertriton’s properties: its lifetime (how long it takes to decay) 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 study of this force is not only interesting in its own right but can also offer valuable insight into the particle interactions that may take place in the inner cores of neutron stars. These cores, which are very dense, are predicted to favour the creation of hyperons over purely nucleonic matter.
Graphic above: Measurements of the hypertriton’s lifetime performed with different techniques over time, including ALICE’s new measurement (red). The horizontal lines and boxes denote the statistical and systematic uncertainties, respectively. The dashed-dotted lines represent different theoretical predictions. (Image: ALICE collaboration).
The new ALICE 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 LHC, the ALICE collaboration was also able to study antihypertritons and determine their lifetime. The team 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.
With data from the third run of the LHC, which started in earnest this July, ALICE will not only further investigate the properties of the hypetriton but will also extend its studies to include heavier hypernuclei.
Note:
CERN, the European Organization for Nuclear Research, is one of the world’s largest and most respected centres for scientific research. Its business is fundamental physics, finding out what the Universe is made of and how it works. At CERN, the world’s largest and most complex scientific instruments are used to study the basic constituents of matter — the fundamental particles. By studying what happens when these particles collide, physicists learn about the laws of Nature.
The instruments used at CERN are particle accelerators and detectors. Accelerators boost beams of particles to high energies before they are made to collide with each other or with stationary targets. Detectors observe and record the results of these collisions.
Founded in 1954, the CERN Laboratory sits astride the Franco–Swiss border near Geneva. It was one of Europe’s first joint ventures and now has 23 Member States.
Related links:
Large Hadron Collider (LHC): https://home.cern/science/accelerators/large-hadron-collider
ALICE experiment: https://home.cern/science/experiments/alice
Antimatter: https://www.google.com/search?client=safari&rls=en&q=cern+antimatter&ie=UTF-8&oe=UTF-8
Cosmic rays: https://home.cern/science/physics/cosmic-rays-particles-outer-space
For more information about European Organization for Nuclear Research (CERN), Visit: https://home.cern/
Image (mentioned), Graphic (mentioned), Animation (mentioned), Text, Credits: CERN/By ALICE collaboration.
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