Specifically,
in a medium called the quark-gluon plasma, generated in the Large Hadron
Collider by colliding lead ions. There, amid the trillions of particles
produced by these collisions, physicists managed to tease out 100 of the exotic
motes known as X particles.
"This
is just the start of the story," says physicist Yen-Jie Lee of MIT, and a member of the international CMS Collaboration headquartered at CERN in Switzerland.
"We've
shown we can find a signal. In the next few years we want
to use the quark-gluon plasma to probe the X particle's internal structure,
which could change our view of what kind of material the Universe should
produce."
Mere moments after the Big Bang, the very early Universe wasn't made of the same stuff we see floating around today. Instead, for a few millionths of a second, it was filled with plasma superheated to trillions of degrees, consisting of elementary particles called quarks and gluons. That's the quark-gluon plasma.
In
less time than it takes to blink, the plasma cooled and the particles came
together to form the protons and neutrons of which normal matter is constructed
today. But in that very brief twitch of time, the particles in the quark-gluon
plasma collided, stuck together, and came apart again in different
configurations.
One
of those configurations is a particle so mysterious, we don't even know how
it's put together. This is the X particle, and it's only been seen very rarely
and briefly in particle colliders – too briefly to be probed.
Theoretically,
however, X particles could appear in the very small flashes of quark-gluon
plasma that physicists have been creating in particle accelerators for some
years now. And this might afford a better opportunity to understand them.
During
the Large Hadron Collider's 2018 run, positively charged atoms of lead (lead ions) were slammed together at high speeds. Each of these roughly 13 billion
collisions produced a shower of tens of thousands of particles. That's a
dauntingly colossal amount of data to sift through.
"Theoretically
speaking, there are so many quarks and gluons in the plasma that the production
of X particles should be enhanced," Lee says. "But people thought it would be too
difficult to search for them because there are so many other particles produced
in this quark soup."
Although
X particles are very short-lived, when they decay, they produce a shower of
lower-mass particles. To streamline the data analysis process, the team
developed an algorithm to recognize the patterns characteristic of X particle
decay. Then they fed the 2018 LHC data into their software.
The
algorithm identified a signal at a specific mass that indicated the presence of around
100 X particles in the data. This is an excellent start.
"It's almost unthinkable that we can tease out these 100 particles from this huge dataset," Lee said.
At
this point, the data are insufficient to learn more about the X-particle's
structure, but the discovery could bring us closer. Now that we know how to
find the X-particle's signature, teasing it out in future data sets should be a
lot easier. In turn, the more data we have available, the easier it will be to
make sense of them.
Protons
and neutrons are each made up of three quarks. Physicists believe that X
particles may be made of four – either an exotic, tightly bound particle known
as a tetraquark, or a new kind of loosely bound particle made from two mesons, each of
which contain two quarks. If it's the former, because it's more tightly bound,
it will decay more slowly than the latter.
"Currently
our data is consistent with both because we don't have enough statistics yet.
In the next few years we'll take much more data so we can separate these two scenarios," Lee says.
"That
will broaden our view of the kinds of
particles that were produced abundantly in the early Universe."
The research has been published in Physical Review Letters
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