Collisions between heavy ions at the Large Hadron Collider (LHC) created quark-gluon plasma, a high-temperature, dense state of matter that is believed to have filled the universe within about a millionth of a second after the Big Bang. Heavy ion collisions also create suitable conditions for the creation of atomic nuclei and exotic supernuclei, as well as their antimatter counterparts, antinuclear and antisupernuclei. Measuring these forms of matter is important for a variety of purposes, including helping to understand the process by which hadrons formed from the constituent quarks and gluons of plasma and the matter-antimatter asymmetry seen in the universe today.
Hypernuclei are exotic nuclei formed from a mixture of protons, neutrons and hyperons, which are unstable particles containing one or more exotic quarks. More than 70 years since their discovery in cosmic rays, hypernuclei remain a source of fascination for physicists because they are rare in nature and challenging to create and study in the laboratory.
In heavy ion collisions, supernuclei are produced in large numbers, but until recently, only the lightest supernuclear supertritons and their antimatter partners, the antisupertritons, have been observed. Hypertritons are made up of protons, neutrons and lambda (a hyperon containing a strange quark). Anti-supertritons are composed of antiprotons, antineutrons and antiλ.
Following the discovery of anti-superhydrogen-4 (a combination of one antiproton, two antineutrons and an anti-lambda) earlier this year by the STAR collaboration at the Relativistic Heavy Ion Collider (RHIC), the ALICE collaboration at the LHC has now discovered for the first time evidence of anti-superhydrogen-4, which consists of two antiprotons, one antineutron and an anti-lambda. The result is significant at 3.5 standard deviations and is the first evidence of the heaviest antimatter supernucleus ever discovered at the LHC.
The ALICE measurements are based on lead-lead collision data obtained in 2018 for each pair of nucleons (protons and neutrons) colliding at an energy of 5.02 teraelectronvolts (TeV). Using a machine learning technique that outperforms traditional hypernucleus search techniques, ALICE researchers looked at signal data for superhydrogen-4, superhelium-4, and their antimatter partners. Candidates for (anti)hyperhydrogen-4 were identified by looking for (anti)helium-4 nuclei and the charged pions they decay into, while candidates for (anti)hyperhelium-4 were identified through their decay into (anti)helium-3 nuclei, (anti)protons and charged pions.
In addition to finding evidence against superhelium-4 with a significance of 3.5 standard deviations and evidence against superhydrogen-4 with a significance of 4.5 standard deviations, the ALICE team also measured the yields and masses of the two supernuclei.
For both supercores, the measured masses agree with current world averages. The measured yields were compared with predictions from a statistical hadronization model that well describes the formation of hadrons and nuclei in heavy ion collisions. This comparison shows that the model's predictions agree well with the data if both excited supernuclear states and ground states are included in the predictions. The results confirmed that the statistical hadronization model can also well describe the production of supercores, which are dense objects with a size of about 2 femtometers (1 femtometer is 10-15 meters).
The researchers also determined the antiparticle-to-particle yield ratios of the two supernuclei and found that they were consistent with 1 within the experimental uncertainty. This agreement is consistent with ALICE's observation of equal production of matter and antimatter at LHC energies, and adds to ongoing research into the matter-antimatter imbalance in the universe.