Heaviest Antimatter Yet: Large Hadron Collider Uncovers Antihyperhelium-4 in Groundbreaking Discovery

By ALICE Collaboration, CERN December 14, 2024

Collected at: https://scitechdaily.com/heaviest-antimatter-yet-large-hadron-collider-uncovers-antihyperhelium-4-in-groundbreaking-discovery/

Heavy ion collisions at the Large Hadron Collider generate conditions conducive to the formation of hypernuclei and their antimatter versions, providing insight into the early universe.

Recent detections of antihyperhelium-4 have validated statistical hadronization models, enhancing knowledge in the field of particle physics.

Quark-Gluon Plasma and Hypernuclei Creation

Collisions between heavy ions at the Large Hadron Collider (LHC) produce quark-gluon plasma, an extremely hot and dense form of matter that existed just one-millionth of a second after the Big Bang. These collisions also create conditions ideal for forming atomic nuclei and exotic hypernuclei, along with their antimatter counterparts, antinuclei and antihypernuclei. Studying these particles helps scientists better understand how hadrons form from quarks and gluons and provides insights into the Universe’s present-day matter-antimatter asymmetry.

Hypernuclei are exotic atomic nuclei made of protons, neutrons, and hyperons—unstable particles containing at least one strange quark. Although first detected in cosmic rays over 70 years ago, hypernuclei continue to intrigue physicists due to their rarity in nature and the difficulty of producing and studying them in laboratory settings.

Recent Advances in Antihypernuclei Observations

In heavy-ion collisions, hypernuclei are created in significant quantities, but until recently only the lightest hypernucleus, hypertriton, and its antimatter partner, antihypertriton, have been observed. A hypertriton is composed of a proton, a neutron and a lambda (a hyperon containing one strange quark). An antihypertriton is made up of an antiproton, an antineutron and an antilambda.

Following hot on the heels of an observation of antihyperhydrogen-4 (a bound state of an antiproton, two antineutrons and an antilambda), reported earlier this year by the STAR collaboration at the Relativistic Heavy Ion Collider (RHIC), the ALICE collaboration at the LHC has now seen the first ever evidence of antihyperhelium-4, which is composed of twoantiprotons, an antineutron and an antilambda. The result has a significance of 3.5 standard deviations and also represents the first evidence of the heaviest antimatter hypernucleus yet at the LHC.

Breakthroughs in Antihyperhelium-4 Detection

The ALICE measurement is based on lead–lead collision data taken in 2018 at an energy of 5.02 teraelectronvolts (TeV) for each colliding pair of nucleons (protons and neutrons). Using a machine-learning technique that outperforms conventional hypernuclei search techniques, the ALICE researchers looked at the data for signals of hyperhydrogen-4, hyperhelium-4 and their antimatter partners. Candidates for (anti)hyperhydrogen-4 were identified by looking for the (anti)helium-4 nucleus and the charged pion into which it decays, whereas candidates for (anti)hyperhelium-4 were identified via its decay into an (anti)helium-3 nucleus, an (anti)proton and a charged pion.

In addition to finding evidence of antihyperhelium-4 with a significance of 3.5 standard deviations, as well as evidence of antihyperhydrogen-4 with a significance of 4.5 standard deviations, the ALICE team measured the production yields and masses of both hypernuclei.

Confirming Models of Hypernuclei Production

For both hypernuclei, the measured masses are compatible with the current world-average values. The measured production yields were compared with predictions from the statistical hadronization model, which provides a good description of the formation of hadrons and nuclei in heavy-ion collisions. This comparison shows that the model’s predictions agree closely with the data if both excited hypernuclear states and ground states are included in the predictions. The results confirm that the statistical hadronization model can also provide a good description of the production of hypernuclei, which are compact objects with sizes of around 2 femtometers (1 femtometer is 10^-15 meters).

The researchers also determined the antiparticle-to-particle yield ratios for both hypernuclei and found that they agree with unity within the experimental uncertainties. This agreement is consistent with ALICE’s observation of the equal production of matter and antimatter at LHC energies and adds to the ongoing research into the matter–antimatter imbalance in the Universe.