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the "God Particle" Possibly Discovered
One of the most important discoveries in particle physics of the last 25 years has possibly just been
made by experimentalists at CERN, the giant laboratory just outside of Geneva on the border of
Switzerland and France. Scientists there think that they have discovered the Higgs field -- also
nicknamed the "God particle" by Nobel laureate Leon Lederman who wrote a book with that title. If
the result is verified, the Higgs will have a mass about 125 times the mass of the proton, making it as
heavy as a medium-sized nucleus. It will "fill in" the last missing piece of a puzzle involving the
solution of one of the great outstanding problems in physics of the 20th century: the origin of all mass.
If the properties of the Higgs are confirmed, the picture of fundamental particle forces will have been
completed. That picture is known as the Standard Model of Particle Physics.
The Standard Model of particle physics provides a description of microscopic matter and their
fundamental interactions. All matter is comprised of quarks and leptons. 3 quarks bind to form the
proton and neutron. The neutrons and protons stick together to form nuclei (the tiny, heavy central
"hearts" of atoms). Leptons appear in Nature in 2 types: electrically=charged and neutral. Neutral
leptons are called neutrinos and hardly interact with matter at all. There are 3 known charged leptons,
the lightest of which is the electron. Electrons -- which are negatively-charged -- are attracted to nuclei - which are positively-charged -- to form atoms. A good pictorial representation of an atom is a cloud of
electrons swarming around a tiny nucleus, much the way bees might swarm around a queen who has left
her hive. Since atoms make up everything in the World, quarks and leptons are the fundamental
building blocks of Nature.
There are 3 fundamental forces. The most familiar is Gravity, which holds humans and other
objects to the Earth; makes the Moon go around the Earth thereby leading to tides, lunar phases and
eclipses; and causes the Earth to orbit the Sun thereby leading to seasons. Gravity is generated by
objects with mass. But because gravity is such a weak force, only bodies of huge mass such as the Earth
and Sun create a significant effect. In the subatomic world where protons, neutrons, and electrons are
extremely light, gravity plays no role.
The second fundamental force is a combination of 3 forces previously thought to be independent of
one another: magnetism, the electric force, and the weak subnuclear interaction. The unification of the
electric and magnetic forces was achieved in the 19th Century, leading to electromagnetism. The
electromagnetic force is the source for all macroscopic forces except those created by gravity. Friction,
spring forces, air pressure, the forces in collisions, and so on originate from the electromagnetic force.
The weak subnuclear interaction is responsible for certain decays of nuclei and plays a role in generating
the energy of the Sun and other stars in their cores. As its name implies, it operates at distances smaller
than a nucleus and it is very weak. For this reason, it is difficult to observe. At the end of the 1960s, a
theory was proposed that unified the weak subnuclear force with electromagnetism. Experiments in the
1970's and 1980's confirmed the electroweak theory. In 1979, Steven Weinberg, Sheldon Glashow, and
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Abdus Salam received the Nobel Prize in physics for unifying the weak subnuclear interaction with
electromagnetism.
The third fundamental force is called the strong nuclear force. It binds 3 quarks together to from
the proton and the neutron. It is also responsible for causing protons and neutrons to stick to one another
in a nucleus.
One of the key ideas in physics is that the basic particle forces are generated through the exchange of
vector gauge bosons. These are particles that spin with one fundamental unit and incorporate an
enormous amount of symmetry. The electromagnetic force is generated when charged particles
exchange photons (spin one particles of "light"); the weak subnuclear interactions are generated by
exchanging heavy vector bosons known as Ws and Z; while the strong force is produced by 8 gluons.
The fundamental constituents – quarks and leptons – along with the 2 fundamental particle
interactions – the electroweak interaction and the strong nuclear force -- constitute the Standard Model
of particle physics. Gravity has not been yet incorporated because a good quantum theory of
gravitation is not available.
It would seem that scientists know everything there is to know about microscopic matter and its
interactions. However, one aspect of the Standard Model has remained a mystery: the mechanism that
produces fundamental mass.
The weak subnuclear interactions are feeble and short-ranged because the W and Z have very heavy
masses of 90-100 times the mass of a proton. In contrast, the photon is massless. It is this great mass
difference that makes electromagnetism so different from the weak subnuclear force. How are masses
for the W and Z created? During the past 30 years, theorists have proposed various mechanisms of
which only experiment can decide which is correct.
One way to give masses to the W and Z is to use particles known as Higgs fields. 4 such particles
are needed. Three are absorbed by the W and Z and one is left over. In this mechanism, there should be
one spin zero, electrically neutral particle observed in Nature. This is the "God particle" -- the Higgs
particle. The potential breakthrough discovery at CERN suggests that W and Z masses are created by
the Higgs mechanism. The positively-charged W, the negatively-charged W, and the neutral Z obtain
mass by respectively absorbing positively-charged, negatively-charged, and neutral Higgses.
The Higgs field also has the ability to generate masses for the quarks and leptons. Thus, if the
expected properties of the Higgs field are confirmed, then the origin of all mass will be understood.
The generation of mass proceeds through a process known as spontaneous symmetry breaking.
An object has symmetry if rotating it does not change its appearance. For example, if a rod is rotated as
indicated in Figure A, its appearance is unchanged. A sphere has even more symmetry than a rod
because a sphere can be rotated in many ways without changing its shape.
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Figure A
Spontaneous symmetry breaking occurs when a system or object naturally looses its rotational
invariance. Suppose pressure is exerted on the rod -- say by pushing on it with a finger. Then if
sufficient force is applied, the rod will buckle. See Figure B. When a buckled rod is rotated, one can
tell that it has been rotated. Thus, the symmetry has been broken.
Figure B
By the way, if the force is exerted exactly along the axis of the rod from the top, one might think that
it is impossible for the rod to buckle in a particular direction. However, the buckling WILL occur, but
the direction is unpredictable. This is a feature of spontaneous symmetry breaking. The breaking takes
place, but one does not know in which direction.
The Higgs field thus acts as the finger in Figure B, applying pressure to the system to cause it to
buckle and loose its symmetry. It is a deep result of Quantum Field Theory that when this happens that
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the W and Z -- which are messengers of the symmetries broken by the Higgs -- acquire masses. In other
words, without spontaneous symmetry breaking, the W and Z would be massless.
The Higgs field produces masses for the quarks and the electrically-charged leptons through its
interactions with these fields. These masses are proportional to the strength with which the Higgs
couples to the particles. Because the Higgs interacts most strongly with the top quark, the top quark
weighs the most (about 200 times the mass of a proton). The electron interacts very weakly with the
Higgs, and that is why it is the lightest particle (about 2000 times lighter than a proton). Physicists do
not understand why the Higgs couplings differ so greatly. So even if the CERN experiment is confirmed
and the Higgs mechanism is realized, certain features of the Standard Model will remain a mystery.
So if one of the most important scientific discoveries has been made, why has there been so little
news about it? The answer is that because of its great importance, experimentalists must be sure of the
result before announcing it. There are 4 experimental detectors at CERN. Of these four, only "Aleph" is
seeing convincing evidence of Higgs production. That detector sees 3 Higgs-candidate events. Another
detector -- "Delphi" -- also thinks that it has produced one Higgs in a single positron-electron collision.
Although "Aleph" states that the Higgs has been seen with better than 99% confidence, no strong claims
can be made with so few events. CERN has decided to run its LEP experiment an extra month-or-so to
try to produce more Higgs particles. If successful, an important announcement on the Higgs discovery
will be made near the end of this year.
Read an update.
This report was prepared by the staff of Jupiter Scientific, an organization devoted to the promotion
of science through books, the Internet, and other means of communication.
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