Scientists have discovered evidence of three previously unseen quark combinations in data from Europe's Large Hadron Collider. This discovery coincides with the start of a new phase of high-energy experiments at the largest particle smasher in the world.
The three unusual particle types, which include two
tetraquarks, which are combinations of four quarks, and a pentaquark, which is
a unit of five quarks, are entirely consistent with the Standard Model, the
long-standing theory that explains the structure of atoms.
On the other hand, scientists are hoping that evidence of
physics beyond the Standard Model will be found during the current run of the
LHC to help explain the nature of enigmatic phenomena like dark matter.
Evidence of this kind may indicate the existence of new subatomic particle
arrays or even extra dimensions in our universe.
For three years, the LHC was offline as it underwent system
upgrades to accommodate previously unheard-of energy levels. Scientists and
engineers at the CERN research centre on the French-Swiss border have been
preparing for the today's resumption of scientific operations since the
shutdown ended in April.
As the LHC started its third run of data collection and
analysis, the control centre at CERN was busy.
Fabiola Gianotti, the director general of CERN, stated
during today's webcast, "It's a magic moment now." "A new era of
exploration at CERN has begun with the collisions we recently witnessed at an
unprecedented energy of 13.6 tera-electronvolts.”
According to Gianotti, the scientists working on the third
run of the LHC hope to gather just as much data as they did during the
collider's two earlier runs, which spanned 13 years. Naturally, this will
improve our chances of making discoveries or learning the underlying principles
governing the universe, the speaker stated.
During Run 3, the 27-kilometer-round (17-mile-round) ring of
superconducting magnets and its particle detectors are expected to run
continuously for almost four years.
The run began today, ten years and one day after physicists
at the LHC revealed their most significant finding to date: proof of the
existence of the Higgs boson, a subatomic particle that contributes to the
understanding of mass.
These three new subatomic particle types are not quite
Higgs-level discoveries, as explained in a CERN seminar today. However, they do
imply that the LHC is actively searching for even more hitherto undiscovered
universe building blocks.
To study hadrons, which are collections of quarks, the Large
Hadron Collider smashes protons together at speeds very near to light.
According to a press release, Niels Tuning, the physics
coordinator for the collider's LHCb detector, "the more analyses we
perform, the more kinds of exotic hadrons we find."
We are living through a similar era of discovery as the
1950s, when hadrons were first found in a "Particle Zoo," which
eventually produced the quark model of conventional hadrons in the 1960s. Our
project is called "Particle Zoo 2.0."
Researching novel quark combinations, according to LHCb
spokesperson Chris Parkes, "will help theorists develop a unified model of
exotic hadrons, the exact nature of which is largely unknown."
Most hadrons aren't that unusual. For example, protons and
neutrons are composed of three bonded quarks. (In actuality, the phrase
"Three quarks for Muster Mark!" from James Joyce's Finnegan's Wake is
where the word "quark" first appeared.) Pions are mixtures of two
quarks.
Much rarer are combinations of four and five quarks, which
are believed to exist for a split second before disintegrating into other
particles.
There are six distinct "flavours" of quarks:
charm, strange, top and bottom, and up and down.
By studying the decays of negatively charged B mesons, the
LHCb team discovered evidence for the presence of a pentaquark, which is made
up of an up, down, and strange quark in addition to a charm quark and charm
antiquark. It's the first pentaquark with an unusual quark that has been
identified.
A "doubly electrically charged" mixture of four
quarks—a charm quark, an odd antiquark, an up quark, and a down antiquark—is
one of the two newly discovered tetraquarks.
Together with its neutral counterpart, which possesses a
charm quark, an odd antiquark, an up antiquark, and a down quark, that
tetraquark was detected. This is the first observation of a pair of tetraquarks
together, according to CERN.
In certain theoretical models, exotic hadrons are
represented as individual tightly bound quark units. Some interpret them as
weakly bonded pairs of standard hadrons, akin to the way atoms are bound
together to form molecules.
"More research on exotic hadrons and time will be
needed to determine whether these particles are one, the other, or both,"
according to CERN.