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Why are scientists looking for the Higgs boson's closest friend?
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Scientists at the world's largest physics experiment have reported the
most precise measurement yet of the most massive subatomic particle we
know. The finding sounds esoteric but it wouldn't be an understatement
to say it has implications for the whole universe.
The Greek philosopher Empedocles surmised 2,400 years ago that matter
could be broken up into smaller and smaller pieces until we're left
with air, earth, fire, and water. Since the early 20th century,
physicists have broken up matter into smaller and smaller pieces to
find many different subatomic particles instead — as many as to fill a
zoo.
#### The top quark
Rather than a 'smaller' particle, contemporary particle physicists are
concerned with elusive particles.
More energetic particles often break down into ones with less energy.
The greater the difference in energy between that of a particle and
the products of its decay, the less time the particle exists in its
original form and more quickly it breaks down. By the mass-energy
equivalence, a more massive particle is also a more energetic
particle. And the most massive particle scientists have found to date
is the top quark.
It is 10-times heavier than a water molecule, about three-times as
much as a copper atom, and 95% as much as a full caffeine molecule.
As a result, the top quark is so unstable that it could break up into
lighter, more stable particles in less than 10−25 seconds.
The top quark's mass is very important in physics. A particle's mass
is equal to the sum of masses contributed from multiple sources. An
important source for all elementary particles is the Higgs field,
which pervades the entire universe. A 'field' is like a sea of energy
and excitations in the field are called particles. This way, for
example, an excitation of the Higgs field is called the Higgs boson
just as an electron can be considered to be an excitation of an
'electron field'.
All these fields engage with each other in specific ways. When the
'electron field' interacts with the Higgs field at energies much less
than 100 GeV, for example, the electron particle will acquire some
mass. The same thing goes for other elementary particles. (GeV, or
giga-electron-volt, is a unit of energy used in the context of
subatomic particles: 1 joule = 6.24 billion GeV.) Elucidating this
mechanism won François Englert and Peter Higgs the 2013 physics Nobel
Prize.
If the top quark is the most massive subatomic particle, it is because
Higgs bosons interact most strongly with it. By measuring the top
quark's mass as precisely as possible, then, physicists can learn a
lot about the Higgs boson as well.
"Physicists are intrigued by the top quark mass as there is something
peculiar about it," Nirmal Raj, particle theorist and assistant
professor at the Indian Institute of Science, Bengaluru, told _The
Hindu_. "On the one hand, it is the one closest to the Higgs boson's
mass, which is what one would 'naturally' expect before measuring it.
On the other, all other [particles like it] are much, much lighter,
making one wonder if the top quark is actually an oddball, not a
'natural' species."
#### The universe as we know it
But the rabbit hole goes deeper.
Physicists are keen to study the Higgs boson also because of its own
mass, which it acquires by interacting with other Higgs bosons.
Importantly, the Higgs boson is more massive than expected — which is
to say the Higgs field is more energy-laden than expected. And because
it pervades the universe, the universe can be said to be more
energetic than expected. This 'expectation' comes from calculations
physicists have performed and they don't have reason to believe they
are wrong. Why does the Higgs field have so much energy?
Physicists also have a theory as to how the Higgs field originally
formed (at the birth of the universe). If they are right, there is a
small yet non-zero chance that one day in future, the field could go
through a sort of self-adjustment that reduces its energy and modifies
the universe in drastic ways.
They know the field has some potential energy today and there is a way
it could shed some of it to have less and become more stable. There
are two ways to get to this stable state. One is for the field to gain
some energy first before losing it and more, like climbing one side of
a mountain to get into a deeper valley on the other side. The other is
if an event called quantum tunnelling happens, whereby the field's
potential energy would 'tunnel' through the mountain instead of having
to climb over it and drop into the valley yonder.
This is why Stephen Hawking said in 2016 the Higgs boson could spell
the "end of the universe" as we know it. Even if the Higgs field is
slightly stronger than it is now, the atoms of most chemical elements
will be destroyed, taking stars, galaxies, and earthlife with them.
But while Hawking was technically correct, other physicists quickly
said the frequency of the tunnelling event was 1 in 10100 years.
The Higgs boson's mass — 126 GeV/c2 (a unit used for subatomic
particles) — is also just about enough to keep the universe in its
current state; anything else and the "end" would happen. Such a finely
tuned value is obviously curious and physicists would like to know
which natural processes contribute to it. The top quark is part of
this picture by virtue of being the most massive particle, in a sense
the Higgs boson's closest friend.
"Measuring the top quark mass precisely has implications for whether
our universe will tunnel out of existence," Dr. Raj said.
#### Finding the top quark
Physicists discovered the top quark in 1995 at a particle accelerator
in the US called the Tevatron, measuring its mass to be 151-197
GeV/c2. The Tevatron was shut down in 2011; physicists continued to
analyse data it had collected and updated the value three years later
to 174.98 GeV/c2. Other experiments and research groups yielded more
precise values over time. On June 27, physicists at the Large Hadron
Collider (LHC) in Europe reported the most precise figure yet: 172.52
GeV/c2.
Measuring a top quark's mass is difficult when its lifetime is around
10-25 seconds. Typically, a particle-smasher will produce an ultra-hot
soup of particles. If a top quark is present in this soup, it will
quickly decay into specific groups of lighter particles. Detectors
look out for these events, and when they happen track and record their
properties. Finally, computers collect this data and physicists
analyse them to _reconstruct_ the physical properties of the top
quark.
Scientists learn what to expect at each point of this process based on
sophisticated mathematical models and must contend with many
uncertainties. Many of the devices used in these machines also
incorporate state of the art technologies; when engineers improve them
further, the physicists results also improve that much.
Now researchers will incorporate the top quark's mass measurement into
calculations that inform our understanding of our universe's
particles. Some of them will use it to also quest for an even more
precise value. According to Dr. Raj, precisely measuring the top
quark's mass is also key to knowing whether some other particle with
mass close to that of the top quark could be hiding in the data.
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