High-energy heavy-ion collisions offer a unique way to study the behaviour of nuclear matter under conditions of extreme energy and density. One of the remarkable features of the particle production at high energies is the nearly equal abundance of matter and antimatter in the central rapidity region. It is believed that a similar symmetry existed in the initial stage of the universe and it remains to be understand how this symmetry got lost in the evolution of the universe reaching a stage with no visible amounts of antimatter being present.
In relativistic heavy-ion collisions a huge number of particles carrying strangeness is produced. Strangeness can be found not only in elementary particles (i.e. K; Λ; Ξ; Ω), but also in compound particles, such as hypernuclei. A hypernucleus is a nucleus which contains at least one hyperon, namely a baryon containing one or more strange quarks, in addition to nucleons.
Hypernuclear physics was born in 1952 when two Polish scientists, Danysz and Pniewsky observed the first hypernuclear decay in a photographic emulsion exposed to cosmic ray at about 26 km above the ground level.
Since the first observation, there has been a constant interest in searching for new hypernuclei and exploring the hyperon-baryon (Y N) interaction: the nucleus serves as a laboratory offering the opportunity to study the properties of hyperon interactions. While several Λ-hypernuclei have been found since the first observation no anti-hypernucleus has ever been observed until the recent discovery of the anti-hypertriton in Au+Au collisions at √sNN = 200 GeV by the STAR Collaboration at RHIC.
The lifetime of hypernuclei depends on the strength of the hyperon-nucleon interaction: the study of this interaction is relevant for nuclear physics and nuclear astrophysics. For example, the Y N interaction plays a key role to understand the structure of neutron stars. Depending on the strength of the Y N interaction, the collapsed stellar core could consist of hyperons, strange quark matter, or a kaonic condensate.
The hypertriton 3ΛH is the lightest known hypernucleus and is formed by a proton, a neutron and a lambda baryon. Hypertriton decays mesonically into the following channels:
The two pictures show the study of the production of 3ΛH and anti-3ΛH detected via its decay 3ΛH →π-+3He by ALICE experiment. Black points are the data points while the red histogram and the green curve are two estimations of the background. The first one is the "like sign" (LS) background which consists in the combination of two tracks with the same sign (i.e. 3He, π+), and the second is the combined fit (third degree polynomial function for the background and a Gaussian for the signal) of the invariant mass spectrum. The full circles in the bottom part of the plots are data after the pol3 background subtraction (3ΛH ans anti-3ΛH signal extraction), and the superimposed black line is the Gaussian function (signal fit).