The ALICE experiment measures the interaction between protons and multi-strange baryons

The measurement of the interaction between protons and “hyperons” improves our knowledge of the strong interaction and it will contribute to the understanding of the structure of neutron stars.

A precise measurement of the strong nuclear force, responsible of the interaction between hadrons, allows us to confirm the expectations of the Quantum Chromodynamics (QCD), which is the theory describing the strong interaction, and to study a frontier topic of nuclear physics: the mechanism that determines the formation of “exotic” states of matter, like those that could exist in the core of neutron stars.
As described in an article just published on Nature (https://www.nature.com/articles/s41586-020-3001-6), the ALICE collaboration developed and applied a new method to measure this interaction by means of the hadrons produced in proton-proton collisions at the CERN Large Hadron Collider (LHC). The measurement was indeed feasible thanks to the specific characteristics of the experiment, in which INFN plays a pivotal role.

The strong nuclear force, carried by gluons, is not only responsible of binding together protons and neutrons within the atomic nuclei, but it determines the interaction between hadrons that contain less common quark flavours. Hyperons belong to this family of hadrons: they are baryons containing one or more strange quarks, which are not present in the atomic nuclei, hence in the ordinary matter, but that could exist in neutron stars.

Because of the instability of hyperons it has been difficult, if not impossible, to achieve an experimental validation of the QCD calculations for what concerns this aspect of the strong interaction. ALICE employed a technique named femtoscopy, because it addresses phenomena at the femtometre (10^-15 m) scale, which is about the size of a hadron and the range of the strong interaction. Femtoscopy is based on the quantum mechanical principle that links the momentum difference between two particles to their distance when interacting at short-range. The application of this technique to LHC collisions allowed the ALICE collaboration to measure for the first time the attractive force due to the strong interaction existing between a proton and the heaviest hyperon, consisting of three strange quarks: the Ω particle.
According to Andrea Dainese, Physics Coordinator of ALICE, and researcher of the INFN unit in Padova “The results reported in this article are an important milestone and also a starting point to explore with ALICE this domain of the strong interaction. They confirm the versatility of an experiment designed to study the Quark Gluon Plasma in heavy-ion collisions at the LHC, but now evolved in a tool able to unveil several other aspects of the quantum chromodynamics”.
An additional fascinating application of the method described in the article published on Nature concerns the understanding of the state of matter constituting the core of neutron stars. Because of the high pressure inside these stars, hyperons may be formed thanks to the fact that strange quark production is energetically favoured in these extreme conditions. The future measurements of the interaction between ordinary (protons) and strange (hyperons) matter by means of femtoscopy represent essential elements to determine the equations of state of the matter in the neutron stars and to predict their time evolution.

The effectiveness of this new analysis technique and the precision of the measurements rely on the excellent performance of the ALICE detectors, which are able to identify the particles produced in the proton-proton collisions at the LHC and to measure both their momenta and their decay products. According to Massimo Masera, University and INFN of Torino, coordinator of the Italian groups in ALICE “INFN played an essential role in designing and building the Inner Tracking System, which is used to detect the hyperon decays, and one of the largest detector in ALICE, the Time-Of-Flight detector, used to identify charge particles”.

Momentum correlation functions measured by ALICE for p-Ξ (a) e p-Ω (b) pairs. A correlation value above unity for momentum differences (k*) close to zero indicates an attractive interaction. The data are in excess with respect to the green curve representing the Coulombian interaction: this is consistent with a relevant attractive force, which is compared to the lattice QCD calculations. 

ChETEC-INFRA: research infrastructures for Nuclear Astrophysics

A new European H2020 project where INFN takes on a central role

ChETEC-INFRA (Chemical Elements as Tracers of the Evolution of the Cosmos - Infrastructure) is a project recently funded by the EU with a four-year budget of 5 million euros, within Horizon 2020. INFN and other Italian universities hold a central role in the initiative that is located at the frontier of nuclear astrophysics that studies the origin of chemical elements: from the Big Bang, to stellar combustion up to neutron and proton capture processes for the formation of heavier elements.

 ChETEC-INFRA constitutes a network between three types of infrastructures which, together, will provide the necessary tools for this research: the astro-nuclear laboratories that provide cross section data of nuclear reactions, the supercomputers that perform calculations of stellar structure and nucleosynthesis, and telescopes and mass spectrometers that collect data on the abundance of elements and isotopes.

ChETEC-INFRA will overcome existing barriers to the advancement of these studies: in particular, access to nuclear astrophysics research infrastructures will be unified using a new integrated web portal; targets and detectors for nuclear reactions, software tools for open-source nucleosynthesis and new three-dimensional models for the analysis of stellar spectra will be developed.

ChETEC-INFRA will provide the community with the tools needed to address key issues on solar fusion, nucleosynthesis by neutron capture and explosive stellar processes. In a combined approach designed to facilitate and increase accessibility, synergies and training, the large amount of transnational access provided will make it possible projects that exploit at least two different types of infrastructure.

Within ChETEC-INFRA, the data will be archived and cataloged for long-term sustainability beyond the end of the project and for their usability in open access.

ChETEC-INFRA involves 32 institutions from 18 European nations and is networked with nuclear astrophysics communities in the United States, China and Japan.

INFN contribution to the project, through its own structures and employees and associates, is broad and widespread.

In particular, INFN will be responsible for the development, construction and testing of both solid and gaseous innovative targets, which will allow the study of nuclear reactions of astrophysical interest at very low energies. Furthermore, it will deal with the development of innovative neutron detectors, such as composite scintillators and new plastic materials, for gamma/neutron discrimination, keeping in touch with industrial partners.

INFN will coordinate the validation activities of the cross sections of the most important reactions for nuclear astrophysics such as those involving ¹²C and ²²Ne, possibly measuring the same reactions with complementary methods, and more generally it will establish and maintain an open-access database of cross sections for a large number of reactions of astrophysical interest.

Finally, INFN will play an important role both in the dissemination of research results with particular reference to funding institutions and industries, and in the training of the next generations of researchers in the field of nuclear astrophysics, through schools and masterclasses.

FROM THE GRAN SASSO LABORATORIES TO THE BIG BANG, AND BACK

The LUNA experiment sheds light on the density of matter that makes up everything we know of in the universe

There is a key process – in the sequence of reactions known as Big Bang Nucleosynthesis – that led to the production of the lightest chemical elements in the first moments of life of our universe: it is the reaction by which a proton and a nucleus of deuterium fuse together to form the stable isotope of helium,  Helium-3.

This reaction has now been studied with unprecedented precision at LUNA (Laboratory for Underground Nuclear Astrophysics), at the Gran Sasso National Laboratories of the INFN. Thanks to this study, it has been possible to refine the calculations of the primordial nucleosynthesis and to obtain an accurate determination of the density of ordinary (or “baryonic”) matter, which makes up everything we know of, including living species.

The results of the measurement conducted by the LUNA Collaboration, and their cosmological impact were published today, 12/11/2020 in the journal Nature, https://www.nature.com/articles/s41586-020-2878-4

In this particular study we have benefited from the precious contribution of the theoretical group of astroparticle physics and theoretical cosmology of the Federico II University of Naples, to arrive at an accurate determination of the baryon density. An important contribution to the description of nuclear interaction was also provided by the theoretical nuclear physics group of the University of Pisa.

The LUNA experiment will continue its scientific activity over the next decade with the LUNA-MV project, focused on the study of key reactions important to understand the chemical composition of the universe and the nucleosynthesis of the heavy elements

LUNA is an international collaboration of about 50 scientists from Italy, Germany, Hungary and the United Kingdom. The list of collaborating institutions includes: the National Laboratories of Gran Sasso, the INFN sections and the Universities of Bari, Genoa, Milano Statale, Naples Federico II, Padua, Rome La Sapienza, Turin, and the INAF Observatory of Teramo (Italy); the Helmholtz-Zentrum Dresden-Rossendorf (Germany), the ATOMKI in Debrecen and the Konkoly Observatory of Budapest (Hungary); and the School of Physics and Astronomy of the University of Edinburgh (United Kingdom).

For further information: Gianluca Imbriani This email address is being protected from spambots. You need JavaScript enabled to view it.

DOE, Brookhaven Lab, and Jefferson Lab Launch Electron Ion Collider Project 

On 18 September the event: "Electron-Ion Collider Project Launch" was held in the presence, among others, of the U.S. Department of Energy (DOE) Under Secretary for Science Paul Dabbar, leaders of Brookhaven National Laboratory (Brookhaven Lab) and Thomas Jefferson National Accelerator Facility (Jefferson Lab). Fabiola Gianotti, CERN Director-General, also intervened, testifying to the interest and support of the European scientific community for the project, which aims to create the 3.9-km circumference collider for the study of the properties and dynamics of quarks and gluons.

More details on the event can be found at the two links below:

- link 1

- link 2