掲載日：2015年2月16日
http://www.sci-news.com/physics/science-cern-polarizability-pion-02499.html
Scientists from CERN’s COMPASS collaboration have made the most precise measurement ever of the
polarizability of pion – the fundamental low-energy parameter of strong interaction.
An electron (green) hits a proton in a nucleus, creating a pion (green-skinned particle) and
transforming the proton into a neutron. Image credit: Joanna Griffin / Jefferson Lab
http://cdn4.sci-news.com/images/2015/02/image_2499-Pion.jpg
Everything we see in the Universe is made up of fundamental particles called quarks and leptons.
Quarks are bound together in groups of three to make up the building blocks of the nuclei of elements
– protons and neutrons.
Flitting between the protons and neutrons in a nucleus are pions, which mediate the strong force binding
the nucleus together. These particles are made up of a quark and an antiquark, themselves held tightly bound
by the strong force.
This makes their deformability, or polarizability, a direct measure of the strong binding force between the quarks.
The polarizability of pions has baffled particle physicists since the 1980s, when the first measurements appeared to
be at odds with the theory.
The new result, appearing in the journal Physical Review Letters, is in close agreement with theory.
To measure pion’s polarizability, scientists from COMPASS – a high-energy physics experiment at the
Super Proton Synchrotron at CERN in Geneva, Switzerland – shot a beam of pions at a target of nickel.
As the pions approached the nickel on average at distances twice the radius of the particles themselves,
they experienced the very strong electric field of the nickel nucleus, which caused them to deform, and
change trajectory, in the process emitting a photon.
By measuring the photon energy and the deflection of the pion for a large sample of 63,000 events,
the COMPASS team determined the pion electric polarizability to be απ = (2.0±0.6stat±0.7syst)*10－4 fm3.
“This result is admirably complementary to the studies of fundamental interactions performed at
the Large Hadron Collider and a testimony to the diversity and strength of CERN’s research programme,”
said Rolf Heuer, Director General of CERN.
<参照>
COMPASS pinpoints polarisability of pions | CERN
http://home.web.cern.ch/scientists/updates/2015/02/compass-pinpoints-polarisability-pions
Phys. Rev. Lett. 114, 062002 (2015) - Measurement of the Charged-Pion Polarizability
http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.114.062002