ALICE collaboration opens avenue for high-precision studies of the strong force
Thursday 10 December 2020
An artist’s impression of the ALICE study of the interaction between the rarest of the hyperons, Omega (Ω) hyperon (left), which
contains three strange quarks, and a proton (right)
The collaboration shows how proton–proton collisions
at the Large Hadron Collider can reveal the strong interaction between
composite particles called hadrons In a paper published today in Nature,
the ALICE collaboration describes a technique that opens a door to
high-precision studies at the Large Hadron Collider (LHC) of the
dynamics of the strong force between hadrons. 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.” “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 Luciano Musa.
“Data from the next LHC runs should give us access to any hadron pair.” “ALICE has opened a new avenue for nuclear physics at the LHC – one that involves all types of quarks,” concludes Musa. Further information: Graphic Photos of ALICE detector Videos of ALICE detector Background content
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