What Is Particle Physics: Part 2
Image credit: CERN
The Standard Model is the theory that describes the interaction of
forces affecting subparticles, like electrons and quarks. Although this
physics deals with the major forces that govern our universe, it often
seems disconnected from us–like it belongs to another reality. In truth,
understanding the Standard Model is a necessary part of understanding
our universe; it is the best thing that we have at our disposal when
trying to make experimental predictions.
So let’s take a moment to get to know the Standard Model, and the universe, a little better.
BOSONS:
In the Standard Model, bosons transmit the forces. In other words,
they carry the energy that governs all the interactions that we see in
the world today (except gravity). Bosons differ from fermions because
they
do not abide by the Pauli Exclusion
Principle (PEP). They can overlap each other, or in technical terms,
share the same quantum state. When we make a laser, we get a
concentration of bosons like this. Lasers are strong concentrations of
overlapping photons, but theoretically, a laser can be made of
any
boson. This quality of bosons, the quality of not obeying PEP, is
observed by us everyday (beams of light do not crash into each other
everywhere we look, thus they can overlap).
The known bosons are as followed: the
W+ and
W- boson, the
Z boson, the
gluon, and the
photon. These particles are known to have
integer spin (1), or spin 0 for the special case of the
Higgs Boson. The theoretical graviton
would have an integer spin of (2). It has yet to be found, possibly
because its strength of interaction is many orders of magnitude weaker
than the other forces. Finding the graviton has been the subject of
intense work for many years.
THE STRONG FORCE:
Image credit: Wikipedia
Quarks have a different form of electric charge, which is called “color property.” Quarks come in
red,
green, and
blue
colors; anti-quarks carry anti-colors. Physicists use the red, green,
and blue color analog to help us understand how nucleons form (the
colors combine to make white light, or in this case, a stable
nucleon). This color analog doesn’t work terribly well with the quarks,
but the important thing is that you remember that it is describing a
property of energy.
Gluons are massless. They mediate the strong force by this color
exchange, and have one color and one anti-color (just enough to swap
and, thus, conserve color charge with quarks). Mesons also form hadrons,
but with only a quark and an antiquark bonded, with color charges that
are opposite of each other (like green and magenta).
The strong force is the strongest known force. It creates “flux
tubes” between quarks in any hadron. Flux tubes are areas of calmness in
the hadron produced by the immense amount of energy of the strong force
binding it together. In the craziness that constitutes something like a
proton, the flux tubes are the only areas not consumed with excitations
in the gluon field. In a nucleon they form a “Y” shape, not a triangle
as sometimes depicted. If you try to separate the flux tube by adding
energy to separate the quarks, you just end up creating another quark
pair to attach on either end of the flux tube you were trying to
separate. Outside of the nucleons themselves, but binding separate
protons and neutrons together, you get
another strong force interaction. This is mediated by the
pi meson or
pion.
WATCH: Your Mass Is Not From the Higgs Boson
https://www.youtube.com/watch?feature=player_embedded&v=Ztc6QPNUqls
THE ELECTROMAGNETIC FORCE:
Probably the most familiar force is the electromagnetic force, this
force is mediated by photons and describes the mindbogglingly accurate
quantum field theory of QED, or quantum electrodynamics. Its mediator
is called a photon, and it is massless, meaning that it is travelling at
the speed of light. The mathematical relationships QED describes are
the most precise theory humans have ever discovered, and responsible for
almost
every technological advance in the 20th century. That’s no small feat.
It is the second strongest force, but it is only 1/137 of that of the
strong force; however, it works over much farther range. If the
electromagnetic forces were not in an intricate balance with the strong
force, atomic nuclei would fall apart. This means no atoms, no
molecules, thus no chemistry, and no biology!
THE WEAK NUCLEAR FORCE:
The weak nuclear force is partly
responsible for the slow burning of stars. Without it, our planet would
not have the energy it needed for liquid water to form and for life to
arise. The weak force is mediated by the Z and W bosons. W boson
carries away charge and energy in radioactive decay, the Z boson
transfers energy in the form of momentum to neutral particles, like
neutrinos. The Z boson is its
own antiparticle (which means it
does not have one).
The fact that the photon was massless but the Z and W bosons were not
caused physicists quite a headache, and no one originally knew why this
was the case. This was finally resolved by what is called the
“Electroweak unification.”
The strengths of force interactions can depend on the ‘temperature’,
or speed/energy of the interaction. At room temperature, the massive Z
and W bosons (91 GeV and 80 Gev respectively) do not play an important
role. But at the extremely high energies of 1000 (GeV),
the W, Z, and photon all become unified. This
is all explained accurately in the Electroweak theory of the Standard
Model. However, as the temperature drops, symmetry is subsequently
broken and bosons are divided up into the W and Z bosons and the photon.
The high energy physics of the Electroweak theory is important
because it provides a model of cosmology for us. We know all the forces
were unified at the beginning moments of the big bang (see a previous
article on cosmology
here).
Using this as a guide, we can make predictions about the beginning
moments after the universe’s inception, including problems like the
matter/antimatter asymmetry in the quark gluon plasma. The quark gluon
plasma is what expanded, which subsequently cooled it, giving us free
roaming photons (visible light) and particles to form all the matter in
the universe, shown to us by the
cosmic microwave background radiation.
The Higgs boson itself is a particle byproduct for the Higgs field.
The discovery of this particle was the culmination of half a century of
work and progress. The higgs was finally confirmed in 2013 (after its
initial discovery in 2o12). It imparts all subatomic particles with
their mass as they travel through this Higgs field. This resistance to
inertia, this drag, is how they acquire their intrinsic masses. Whether
or not
another Higgs field (and its corresponding boson) exists
is
a valid question. But it is extremely unlikely for it to be another
similar Standard Model Higgs (like a higgs with similar mass). It would
be field and corresponding boson with completely
different properties.
*This is the second installment to this series about the Standard Model. You can find the first one
here. (You can find a comprehensive source to these articles by the free PDF download link from Cornell university found
here.)
