CERN experiment: ALICE opens avenue for high-precision studies of the strong force


It is the entry in a new chapter in hadron physics: In a paper published today in the Journal “Nature”, the ALICE collaboration at the European Research Center CERN describes a technique that opens a door to many new high-precision studies at the Large Hadron Collider (LHC) of the dynamics of the strong force between hadrons. From the beginning, GSI has played a leading role in the construction and scientific program of ALICE, one of the largest experiments at CERN.

With the publication "Unveiling the strong interaction among hadrons at the LHC" the ALICE collaboration presents interesting new findings about hadrons and their interactions. Hadrons are composite particles made of two or three quarks bound together by the strong interaction, which is mediated by gluons. This interaction also acts between hadrons, binding nucleons (protons and neutrons) together inside atomic nuclei. One of the biggest challenges in nuclear physics today is understanding the strong interaction between hadrons with different quark content from first principles, that is, starting from the strong interaction between the hadrons’ constituent quarks and gluons.

Calculations known as lattice quantum chromodynamics (QCD) can be used to determine the interaction from first principles, but these calculations provide reliable predictions only for hadrons containing heavy quarks, such as hyperons, which have one or more strange quarks. In the past, these interactions were studied by colliding hadrons together in scattering experiments, but these experiments are difficult to perform with unstable (i.e. rapidly decaying) hadrons such as hyperons. This difficulty has so far prevented a meaningful comparison between measurements and theory for hadron–hadron interactions involving hyperons.

Enter the new study from the collaboration behind ALICE, one of the main experiments at the LHC. The study shows how a technique based on measuring the momentum difference between hadrons produced in proton–proton collisions at the LHC can be used to reveal the dynamics of the strong interaction between hyperons and nucleons, potentially for any pair of hadrons. The technique is called femtoscopy because it allows the investigation of spatial scales close to 1 femtometre (10−15 metres) – about the size of a hadron and the spatial range of the strong-force action.

This method has previously allowed the ALICE team to study interactions involving the Lambda (Λ) and Sigma (Σ) hyperons, which contain one strange quark plus two light quarks, as well as the Xi (Ξ) hyperon, which is composed of two strange quarks plus one light quark. In the new study, the team used the technique to uncover with high precision the interaction between a proton and the rarest of the hyperons, the Omega (Ω) hyperon, which contains three strange quarks.

“The precise determination of the strong interaction for all types of hyperons was unexpected,” says ALICE physicist Laura Fabbietti, professor at the Technical University of Munich“. This can be explained by three factors: the fact that the LHC can produce hadrons with strange quarks in abundance, the ability of the femtoscopy technique to probe the short-range nature of the strong interaction, and the excellent capabilities of the ALICE detector to identify particles and measure their momenta”.

The nuclear physicist Professor Peter Braun-Munzinger, Scientific Director of the ExtreMe Matter Institute EMMI at GSI and longstanding chair of the collaboration board of ALICE, is significantly involved in the current investigations. He also emphasizes the importance of the now published research: “Out findings open the door to a new chapter in hadron physics, and with the factor 100 increase in statistics for the coming Run3 and Run4 at the LHC many new investigations will be possible”.

The relationship between GSI and ALICE is traditionally very close: GSI's research department ALICE shares responsibility for the operation of ALICE's two largest detector systems. The Time Projection Chamber (TPC) and the Transition Radiation Detector (TRD) were designed and built with significant contribution of GSI’s ALICE department and Detector Laboratory. Currently, GSI gives an essential contribution to the ALICE upgrade program, specifically in the TPC project and in the development of the new Online-Offline (O2) software framework. To do this, GSI’s ALICE department, Detector Laboratory and IT department work closely together. GSI scientists have several leading roles in data analysis and in the physics program of ALICE. GSI scientist and professor at Heidelberg University, Silvia Masciocchi currently chairs the ALICE Collaboration Board.

“Our new measurement allows for a comparison with predictions from lattice QCD calculations and provides a solid testbed for further theoretical work,” says ALICE spokesperson Dr. Luciano Musa. “Data from the next LHC runs should give us access to any hadron pair”. He concludes: “ALICE has opened a new avenue for nuclear physics at the LHC – one that involves all types of quarks”. (CERN/BP)

Further information

Scientific publication „Unveiling the strong interaction among hadrons at the LHC" in the journal Nature

The ALICE experiment at CERN