ALICE identifies properties of hypermatter

ALICE identifies properties of hypermatter

The collaboration’s latest study of a ‘strange’, unstable nucleus known as the hypertriton offers new insight into the particle interactions that can take place at the heart of neutron stars

The international ALICE collaboration at the Large Hadron Collider (LHC) has just published the most precise measurements to date of two properties of a hypernucleus that may exist in the core 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. Less frequently but still regularly, unstable nuclei called hypernuclei also form. Unlike normal nuclei, which consist of only protons and neutrons (i.e. nucleons), hypernuclei are also made of hyperons – unstable particles containing strange-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 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, d a neutron and a Lambda – a hyperon containing a strange quark.

In their new study, the ALICE team examined a sample of around a thousand hypertritons produced in lead-lead collisions that occurred in the LHC during its second run. 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 the ALICE detectors can pick up and identify. . The ALICE team studied 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, ALICE researchers were able to obtain the most precise measurements to date of two of the hypertriton’s properties: its life (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 understand the internal structure of this hypernucleus and, consequently, the nature of the strong force which binds the nucleons and the hyperons between them. Studying this force is not only interesting in itself, it can also offer valuable insight into the particle interactions that can take place in the inner cores of neutron stars. These nuclei, which are very dense, should favor the creation of hyperons rather than purely nucleonic matter.

Hypertriton lifespan measurements made with different techniques over time, including the new ALICE measurement (red). Horizontal lines and boxes indicate statistical and systematic uncertainties, respectively. The dotted lines represent different theoretical predictions. (Photo: 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 rays X used in medical imaging, and the lifetime of the hypertriton is compatible with that of the free Lambda.

Moreover, since matter and antimatter are produced in almost equal quantities 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, 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, the CPT symmetry.

With data from the LHC’s Run 3, which started in earnest in July, ALICE will not only deepen its research into the properties of hypertriton, but will also expand its studies to include heavier hypernuclei.

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