A recent research has discovered strong evidence that a proton also
includes a charm quark in addition to the two up quarks and one down quark
that are listed in textbook descriptions of protons.
A fundamental component of every atom, the proton appears to have a more
complex structure than is typically described in textbooks. A sensitive
particle physics experiment like the Large Hadron Collider may be affected
by the discovery (LHC).
It was originally believed that protons were indestructible, but tests with
particle accelerators in the 1960s showed that they really comprised three
smaller particles called quarks. The proton includes two up quarks and one
down quark out of the six different kinds or flavors of quarks.
However, since a particle's structure in quantum mechanics is determined by
probabilities, it is theoretically possible for more quarks to appear within
the proton as matter-antimatter pairs. The proton may include a charm quark
and its antimatter counterpart, an anticharm, according to a 1980 experiment
at the European Muon Collaboration at CERN, but the results were equivocal
and the subject of intense controversy.
More attempts to pinpoint the proton's charm element were made, but various
teams came up with conflicting findings and had trouble separating the
intrinsic components of a proton from the high energy environment of
particle accelerators, where every type of quark is continuously being
produced and destroyed.
Now, Juan Rojo of the Vrije University Amsterdam in the Netherlands and his
colleagues have discovered proof that the charm quark contributes just a
little portion of the proton's momentum, about 0.5%. In spite of decades of
research, Rojo finds it amazing that "we're continually uncovering new
features of the proton and, in particular, new components."
Rojo and his team used a machine learning model to create hypothetical
proton structures made up of all the quark flavors in order to identify the
charm component. They then compared these structures to more than 500,000
actual collisions from decades of particle accelerator experiments,
including those at the LHC.
According to Christine Aidala of the University of Washington, this use of
machine learning was crucial because it may provide models that physicists
might not have thought of on their own, lowering the likelihood of skewed
measurements.
There is only a 0.3% probability of seeing the outcomes they looked at if
the proton doesn't have a charm-anticharm quark pair, the researchers
discovered. This is what physicists refer to as a "3-sigma" finding, which
is typically seen as a possible indicator of something intriguing. To
increase the results to the conventional criterion for a discovery—5-sigma
level, or roughly a 1 in 3.5 million probability of a fluke result—more
study is required.
The researchers modeled the statistical distribution of the proton's
momentum both with and without a charm quark while taking into consideration
recent findings from the LHCb Z-boson experiment. They discovered that if
the proton is considered to contain a charm quark, the model more closely
matches the outcomes. This indicates that they are more confidence than the
sigma level alone would indicate in their proposal of the existence of a
charm quark. The fact that other research have come to the same conclusion,
according to Rojo, "made us very convinced that our results were
reliable."
This is crucial, according to Harry Cliffe of the University of Cambridge.
"Given how common this particle is and how long we've known about it,
there's still a lot we don't genuinely understand about its substructure,"
he adds.
According to Cliffe, additional physics investigations at the LHC that rely
on precise models of proton substructure may be affected by the proton's
charm component. According to Rojo, this new structure may also need to be
taken into consideration by the IceCube Neutrino Observatory in Antarctica,
which searches for rare neutrinos created when cosmic rays strike particles
in Earth's atmosphere. According to him, the charm level of the proton has a
significant influence on the likelihood that a cosmic ray would strike a
nucleus in the atmosphere and produce neutrinos.
Journal reference: Nature,
DOI: 10.1038/s41586-022-04998-2
