ALICE establishes the properties of hypermatter

ALICE establishes the properties of hypermatter

The collaboration’s latest study of a “strange,” unstable core known as a hypertriton provides new insight into the interactions between particles that can take place in the hearts of neutron stars.

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 also form. Unlike normal nuclei, which contain only protons and neutrons (i.e., nucleons), hypernuclei are also made up of hyperons—unstable particles containing odd-type quarks.

Nearly 70 years after 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 high-energy nuclear physics experiments low, it is extremely difficult to measure them. properties.

At the LHC, hypernuclei are created in significant quantities in collisions with heavy ions, 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 about a thousand hypertritons produced in lead-lead collisions that occurred in the LHC during its second cycle. Once formed in these collisions, the hypertritons fly a few centimeters inside the ALICE experiment before decaying into two particles, a helium-3 nucleus and a charged pion, which ALICE’s detectors can catch and identify. The ALICE team investigated these daughter particles and the tracks they leave behind in the detectors.

By analyzing this hypertriton sample, 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: the lifetime (how long it takes to decay) and the energy required to separate its hyperon, Lambda, from the rest of the 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 provide valuable insight into the particle interactions that can take place in the inner cores of neutron stars. These nuclei, which are very dense, will favor the creation of hyperons at the expense of purely nucleonic matter.

Hypertriton lifetime measurements have been made with various techniques over time, including ALICE’s new measurement (red). Horizontal lines and boxes indicate statistical and systematic uncertainties, respectively. 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 only a few tens of kiloelectronvolts, similar to the energy of X-rays used in medical imaging, and the lifetime of the hypertriton it is compatible with that of free Lambda.

Additionally, 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 lifetimes. The team found that, within the experimental uncertainty of the measurements, the antihypertriton and hypertriton have the same lifetime. Finding even a small difference between the two lifetimes could signal the breaking of a fundamental symmetry of nature, the CPT symmetry.

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

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