In the wake of proton-proton collisions within the Large Hadron Collider,
the ATLAS experiment has verified that a trio of particles—a top-antitop
quark pair and a W boson—occurs more frequently than predicted (LHC).
Just one out of every 50,000 collisions at the LHC produces the trio, known
as ttW, which indicates how uncommon the mechanism is that produces these
three particles post-impact. Top quarks and W bosons have a limited lifetime
and disintegrate nearly instantly, thus the scientists detected ttW events
by looking at the electrons and muons they decay into.
In order to increase the precision and depth of the analysis of the
measurement, members of the ATLAS group at the Department of Energy's SLAC
National Accelerator Laboratory have spent the last three years finishing a
complex analysis to measure the process. This work included creating novel
methods to estimate and remove background and detector effects. The findings
will aid experimentalists looking at various particle physics processes as
well as researchers testing elementary particle physics ideas more
effectively.
Brendon Bullard, research associate at SLAC National Accelerator Laboratory
and coordinator of this data analysis, noted that the LHC is the only
collider that can create these kinds of events at a rate great enough to be
observed.
a mysterious excess
Data from the LHC's Run 1, which took place between 2010 and 2012, were
used by ATLAS to make the first observation of the ttW process in 2015. A
portion of the data acquired during Run 2 (2015–2018) was used in subsequent
measurements to imply that ttW was occurring more frequently than expected
by the Standard Model of particle physics, which describes the behavior of
subatomic particles.
The most recent measurement, which used the whole dataset gathered by ATLAS
during Run 2, allowed for a more accurate measurement of ttW, which revealed
that the total production rate was nearly 20% greater than predicted by
theory. Recent CMS experiment findings support this excess.
Although the precise cause of the disparity is still unknown, Bullard
stated that the data "truly do appear to imply that there's something going
on that we're not taking into consideration."
It's plausible that new, Standard Model-defying physics is to blame.
Alternately, it's conceivable that the current models don't include the
components needed to anticipate ttW production properly. Theorists use
piecewise approximations of increasing difficulty to construct predictions
from the Standard Model, and the disparity may be due to subtle effects that
have not yet been taken into account by these approximations.
In any case, theorists must now attempt to determine the truth by
approximating ttW while taking into consideration these yet-to-be-calculated
subtle effects.
"Because of its difficulty, this has never been attempted before. Yet, in
light of our finding, there are already theories eager to put out the effort
"said Bullard. This measurement will be highly helpful for furthering our
understanding of the Standard Model and, if we're lucky, for identifying any
impacts that go beyond it.
Even too much can be beneficial.
Studying various aspects of ttW events offers new opportunities for
scientists to investigate the fundamental forces at work between the two
quarks and the W boson, including the strong interaction, which holds quarks
together, and the electroweak interaction, which controls electromagnetism
and radioactive decay.
The study of even more uncommon events happening during proton collisions
will benefit from improved observations. To identify the signal they were
looking for, researchers previously had to estimate ttW production and
delete it from data since ttW is a significant background of two other
processes seen at the LHC. They can now identify these unusual signals with
more accuracy because to this more exact measurement of ttW.
The creation of two top quarks and a Higgs boson, the particle that gives
some particles, such quarks and W bosons, mass, is one of these processes.
By hunting for the electrons and muons that this event, known as ttH, decays
into, it is found to be 10 times more uncommon than ttW. Improved
measurements of ttH will aid scientists in determining the strength of the
Higgs-Top quark coupling, a crucial test of the Standard Model that
potentially reveal the source of mass.
The generation of four top quarks, which is 50 times rarer and was just
seen for the first time by ATLAS and CMS, is the second process that ttW
muddles. The most powerful particle in the Standard Model, top quarks, may
play a role in novel physics that may be explored further.
Zhi Zheng, a research associate at SLAC who led the four top quark study at
ATLAS, stated, "Improved understanding of the ttW process, especially with
this discovery, can further enhance the four top measurements and precision,
allowing us to examine additional aspects of this process." She further
supported Bullard's ttW analysis. Cross-checking these related metrics was
made easier by the pair's collaboration at SLAC.
Being at SLAC together has improved communication and cooperation between
these two metrics, according to Bullard.
