[新聞] Physicist spots unusual charged meson
Physicists spot unusual charged meson
Physicists at the Belle experiment at the KEK laboratory in Japan
have discovered a new particle that provides the best evidence yet that some
mesons contain four quarks rather than the usual two. Dubbed Z(4430), the new
particle is the latest of several mesons discovered at Belle that seem to
defy the "quark model", which has been very successful at classifying mesons
in terms of two constituent quarks.
Introduced in the 1960s by the future Nobel laureate Murray Gell-Mann, the
quark has allowed physicists to make sense of the myriad particles produced
in successive generations of accelerator experiments. As well as describing
the familiar protons and neutrons, which contain three quarks, the quark
model had been successful at classifying the many mesons that contain two
quarks – or more accurately one quark and one antiquark – held together by
the strong force.
However, things started to go wrong about four years ago when two experiments
-- BaBar at the Stanford Linear Accelerator Center in the US, and Belle at
the KEK laboratory in Japan – began to discover mesons that did not appear
to be simple pairs of quarks and antiquarks. These peculiar mesons were made
at both facilities by smashing electrons and positrons together.
Four quarks
Some physicists have suggested that these rouge mesons -- of which at least
four have been seen -- could be made of four, rather than two quarks. Others
believe, however, that the particles are simply excited states of the “
charmonium” meson, which contain one charm quark and one anti-charm quark.
However, charmonium is electrically neutral, whereas Z(4430) is
the first rogue meson to be discovered with an electrical charge. As a
result, Z(4430) is unlikely to be charmonium. Instead, the Belle physicists
believe that it could be a four-quark state comprising up, anti-down, charm
and anti-charm quarks. This is consistent with the observation that Z(4430)
decayed instantly into a charmonium meson and a pi-meson (which contains up
and anti-down quarks). If true, Z(4430) would be the first meson that cannot
be described in terms of two quarks (arXiv 0708.1790v2) .
The discovery of Z(4430) could mean that physicists have to re-examine the
theory of quantum chromodynamics (QCD), which explains why quarks and
anti-quarks are bound together in mesons and other particles.
However not everyone is convinced. Eric Swanson of the University of
Pittsburgh in the US told physicsworld.com that while the Z(4430) data look
convincing, other such particles will need to be discovered before he accepts
that four-quark mesons are a reality. "I can’t find a plausible way to
explain [Z(4430)],", he said.
About the author
Hamish Johnston is editor of physicsworld.com
--
※ 發信站: 批踢踢實業坊(ptt.cc)
◆ From: 67.164.68.140
※ 編輯: yduj 來自: 67.164.68.140 (11/16 13:18)