In the first millionths of a second after the Big Bang, the universe was a
roiling, trillion-degree plasma of quarks and gluons—elementary particles
that briefly glommed together in countless combinations before cooling and
settling into more stable configurations to make the neutrons and protons of
ordinary matter.
In the chaos before cooling, a fraction of these quarks and gluons collided
randomly to form short-lived "X" particles, so named for their mysterious,
unknown structures. Today, X particles are extremely rare, though physicists
have theorized that they may be created in particle accelerators through
quark coalescence, where high-energy collisions can generate similar flashes
of quark-gluon plasma.
Now physicists at MIT's Laboratory for Nuclear Science and elsewhere have
found evidence of X particles in the quark-gluon plasma produced in the
Large Hadron Collider (LHC) at CERN, the European Organization for Nuclear
Research, based near Geneva, Switzerland.
The team used machine-learning techniques to sift through more than 13
billion heavy ion collisions, each of which produced tens of thousands of
charged particles. Amid this ultradense, high-energy particle soup, the
researchers were able to tease out about 100 X particles, of a type known as
X (3872), named for the particle's estimated mass.
The results, published this week in Physical Review Letters, mark the first
time researchers have detected X particles in quark-gluon plasma—an
environment that they hope will illuminate the particles' as-yet unknown
structure.
"This is just the start of the story," says lead author Yen-Jie Lee, the
Class of 1958 Career Development Associate Professor of Physics at MIT.
"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."
The study's co-authors are members of the CMS Collaboration, an
international team of scientists that operates and collects data from the
Compact Muon Solenoid, one of the LHC's particle detectors.
Particles in the plasma
The basic building blocks of matter are the neutron and the proton, each of
which are made from three tightly bound quarks.
"For years we had thought that for some reason, nature had chosen to produce
particles made only from two or three quarks," Lee says.
Only recently have physicists begun to see signs of exotic
"tetraquarks"—particles made from a rare combination of four quarks.
Scientists suspect that X (3872) is either a compact tetraquark or an
entirely new kind of molecule made from not atoms but two loosely bound
mesons—subatomic particles that themselves are made from two quarks.
X (3872) was first discovered in 2003 by the Belle experiment, a particle
collider in Japan that smashes together high-energy electrons and positrons.
Within this environment, however, the rare particles decayed too quickly for
scientists to examine their structure in detail. It has been hypothesized
that X (3872) and other exotic particles might be better illuminated in
quark-gluon plasma.
"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."
'Really a signal'
In their new study, Lee and his colleagues looked for signs of X particles
within the quark-gluon plasma generated by heavy-ion collisions in CERN's
Large Hadron Collider. They based their analysis on the LHC's 2018 dataset,
which included more than 13 billion lead-ion collisions, each of which
released quarks and gluons that scattered and merged to form more than a
quadrillion short-lived particles before cooling and decaying.
"After the quark-gluon plasma forms and cools down, there are so many
particles produced, the background is overwhelming," Lee says. "So we had to
beat down this background so that we could eventually see the X particles in
our data."
To do this, the team used a machine-learning algorithm which they trained to
pick out decay patterns characteristic of X particles. Immediately after
particles form in quark-gluon plasma, they quickly break down into
"daughter" particles that scatter away. For X particles, this decay pattern,
or angular distribution, is distinct from all other particles.
The researchers, led by MIT postdoc Jing Wang, identified key variables that
describe the shape of the X particle decay pattern. They trained a
machine-learning algorithm to recognize these variables, then fed the
algorithm actual data from the LHC's collision experiments. The algorithm
was able to sift through the extremely dense and noisy dataset to pick out
the key variables that were likely a result of decaying X particles.
"We managed to lower the background by orders of magnitude to see the
signal," says Wang.
The researchers zoomed in on the signals and observed a peak at a specific
mass, indicating the presence of X (3872) particles, about 100 in all.
"It's almost unthinkable that we can tease out these 100 particles from this
huge dataset," says Lee, who along with Wang ran multiple checks to verify
their observation.
"Every night I would ask myself, is this really a signal or not?" Wang
recalls. "And in the end, the data said yes!"
In the next year or two, the researchers plan to gather much more data,
which should help to elucidate the X particle's structure. If the particle
is a tightly bound tetraquark, it should decay more slowly than if it were a
loosely bound molecule. Now that the team has shown X particles can be
detected in quark-gluon plasma, they plan to probe this particle with
quark-gluon plasma in more detail, to pin down the X particle's structure.
"Currently our data is consistent with both because we don't have a enough
statistics yet. In 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."
Reference:
A. M. Sirunyan et al, Evidence for X(3872) in Pb-Pb Collisions and Studies
of its Prompt Production at sNN=5.02 TeV, Physical Review Letters (2022).
DOI: 10.1103/PhysRevLett.128.032001