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The Standard Model
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The Standard Model
Introduction:
Human nature is to question. Just ask any pre-schooler what their favorite word is and you'll probably receive the response, "Why?" followed by "what", "how" and "when". Eventually these children move on from their post-toddler obsession with monosyllabic words and develop into adults, yet they always retain some of that curiosity that is so intrinsic to human nature. Physicists, on the other hand, can't stop asking those questions and are the people who never grew up. One puzzle that philosophers and physicists have pondered for centuries is the riddle, "What is matter?" The Greek philosopher Democritus was the first to propose that matter is comprised of tiny "indivisibles" which he called "atoms".
By convention there is color,
By convention sweetness,
By convention bitterness,
But in reality there are atoms and space"
-Democritus (circa 400 BCE)
Democritus was on the right path, and far ahead of his time. Today we know that atoms are not the smallest building blocks of matter; rather, there exists a whole world of particles more fundamental than atoms. Although less poetic than Democritus, we would say, "there are quarks, leptons, gluons and space". Physicists, through experimentation and theory, have created the Standard Model of particle physics, which outlines what they believe to be the most basic building blocks of matter.
History:
The history of physics is a long and involving tale, which will not be told here. This is simply a brief history of particle physics pertinent to the development of the standard model. For more information on the history of physics, please visit the American Physical Society's A Century of Physics timeline.
-Pre 1800 Up until 1800 not much work is done involving the theory of matter. The majority of the exploration falls under chemistry through the identification of elements
-1802 Dalton revives the study of matter with his Atomic theory, which states that atoms are the fundamental building blocks of nature and can only combine in whole number ratios
-1898 J. J. Thompson discovers that cathode rays are electrons, a fundamental particle
-1905 Einstein publishes his theory of the wave-particle duality of light. This forms a foundation for quantum mechanics
-1911 Rutherford discovers that the atom has a concentrated positive nucleus
-1913 Bohr furthers Rutherford's model of the atom to include electron orbits at discrete radii to account for distinct atomic spectra emission lines
-1919 The bending of starlight due to the curvature of space-time is observed, confirming Einstein's general relativity
-1923 Louis de Broglie proposes the wave-particle duality of matter
-1925 Heisenberg creates his uncertainty principle, which puts limits on the precision of experimentation
-1925-26 Schrodinger rescues the wave-particle duality of nature from confusion with the wave equation
-March 1926 Quantum mechanics is formulated
-1932 James Chadwick announces discovery of neutron
-1956-57 Tsung-Dao Lee and Chen Ning Yang propose parity non-conservation in certain sub-atomic processes, which is confirmed by experimentalist Chien-Shiung Wu
-1962 The first experimental observation of the muon neutrino occurs
-1967 Raymond Davis creates the first solar neutrino detector, finding only half of the predicted solar neutrino flux
-1967 Steven Weinberg, Sheldon Glashow (collaboration) and Abdus Salam (independent) create the electro-weak theory, unifying the electromagnetic and weak nuclear force (they win Nobel prizes in 1979)
-1964 Quarks are proposed by Murray Gell-Mann and George Zweig
-1969 Jerome Friedman, Henry Kendall, and Richard Taylor find the first evidence of quarks
-1970-73 Standard model of particle physics is developed
-1974 The charmed quark is observed
-1975 Evidence of the tau lepton is found
-1977 Experimenters find proof of the bottom quark
-1983 Carlo Rubbia discovers the W and Z bosons, mediators of the weak-force
-1994 Planning for LHC (Large Hadron Collider) at CERN begins
-1995 Evidence for the top quark, the final undiscovered quark, is found at Fermilab
-2000 The tau neutrino, the last piece to the standard model, (with the exceptopm of the higgs particle) is observed at Fermilab
Components of the Standard Model:
The standard model is divided into three sections: quarks,
leptons and force carriers. The quarks and leptons, which in turn are divided
into three generations, are members of a family of particles
called
fermions (particles with half integer spins). Both the quarks and leptons come
in pairs. For example, quarks are grouped up and down, charm and strange, and
top and bottom (And yes, those are their real names). Experimental evidence for
the top quark was recently found here at Fermilab in 1995. Scientists have
proven that quarks combine in triplets to form baryons or quark-antiquark pairs
to form mesons, both types of elementary particles.
Leptons, which belong to a class of particles called
fermions, also come in pairs. The electron, muon and tau particles each have an
associated low mass, charge-less neutrino. The electron, like the proton and the
neutron, is a stable particle and is present in almost all matter. The muon and
tau particles are unstable and are found primarily in decay processes.
The intermediate vector bosons, or force carriers, make up
the third section of the standard model. They transmit three of the four
fundamental forces through which matter interacts. The gluon, like its namesake,
is responsible for the most powerful force, the strong force, which binds
together quarks inside protons and neutrons, and holds together particles inside
an atomic nucleus. The photon is the electromagnetic force carrier that governs
electron orbits and chemical processes. Lastly, the W and Z bosons are
attributed to the weak force, playing a role in radioactive decay. The weak
force is very important in observing neutrino reactions, because the neutrinos
are impervious to the electromagnetic force (due to their lack of charge) and
unaffected by the stro
ng
(which governs nuclear interactions), leaving only the weak force to
characterize the neutrino.
The standard model is not a complete theory; in fact it is far from being so. Detectors at Fermilab and eventually at the LHC at CERN are looking for the elusive Higgs particle, which, if found, will either explain the standard model or force us to readjust our conception of matter. Also the standard model does not have a place for gravity, the fourth force, which does not play a significant part in atomic and subatomic processes because it is so weak on those scales. Physicists are searching for a grand unified theory that would unite all four of the forces, currently only those included in the standard model are united. The next twenty years should prove very exciting for this field of physics.
What are they?
Neutrinos are members of the Standard Model, belonging to a class of particles called leptons. For a long time scientists believed neutrinos were massless and moved at the speed of light. However, physicists have found increasing evidence that these tiny particles in fact have mass, although one much less than that of the electron. Right now we only know the upper limits on what the mass could be and the mass differences between flavors of neutrinos, although there are many current experiments designed to probe this question. The difficulty lies in the fact that neutrinos are extremely non-interacting and therefore troublesome to detect. Scientists know with certainty that they are chargeless and have a spin angular momentum of 1/2. Every measurement made of the elements of the standard model (including neutrinos) has shown that they have no internal structure; indeed, that is why they are called fundamental particles. In addition, neutrinos seem to be stable.
Leptons come in pairs; each neutrino has a charged partner. The electron neutrino is paired with the electron, the muon neutrino with the muon and the tau neutrino with the tau. They are created only in pairs due to lepton number conservation, a fundamental principle that dictates the type and quantity of lepton must be conserved in any event.
A little history:
Neutrinos didn't emerge onto the particle physics scene until 1930, when Wolfgang Pauli invented the neutrino to "save" conservation of energy, which was under threat from observations of beta decay in radioactive materials. Scientists such as Henri Bequerel and Marie and Pierre Curie performed the first studies into radiation starting in 1898. In the years that followed radiation was classified into 3 categories: alpha, beta and gamma. In studying beta radiation, scientists discovered a disturbing phenomenon. It seemed that when a nucleus underwent beta decay, which consisted of the emission by a neutron of an electron to create a proton, conservation of energy was violated. There was a missing amount of energy that could not be accounted for by their measurements or calculations. In 1930 Pauli made his hypothesis, saying:
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| Beta decay |
I have hit upon a desperate remedy to save the "exchange theorem" of statistics and the law of conservation of energy. Namely, the possibility that there could exist in the nuclei electrically neutral particles, that I wish to call neutrons, which have spin 1/2 and obey the exclusion principle and which further differ from light quanta in that they do not travel with the velocity of light. The mass of the neutrons should be of the same order of magnitude as the electron mass and in any event not larger than 0.01 proton masses. The continuous beta spectrum would then become understandable by the assumption that in beta decay a neutron is emitted in addition to the electron such that the sum of the energies of the neutron and the electron is constant...
It was not until 1933 that Pauli admitted the possibility of a zero mass neutrino (the discovery of the neutron in 1932 by James Chadwick forced him to change the hypothesized particle's name to neutrino). Today we know that neutrinos have some unknown mass and that they move close to the speed of light. The first detection of neutrinos occurred in 1956 by Clyde Cowan and Fredrick Reines who found a convenient source of neutrinos--nuclear power plants. Power is created in nuclear plants when atoms undergo nuclear fission, a process of which the neutrino is a byproduct. Cowan and Reines employed a 400-L tank of cadmium chloride as their target. The neutrinos struck a proton inside the target, producing a positron and a neutron. That positron encountered an electron; the two annihilated each other, producing two gamma rays (or photons). The neutron was absorbed by a cadmium chloride atom, producing a photon at a 15-microsecond delay from the emission from the positron. Using this knowledge of the photon emission, Cowan and Reines were able to detect the electron neutrino.
Leon Lederman, Mel Schwartz, and Jack Steinberger followed with the detection of the muon neutrino in 1962. They fired a GeV beam of protons through a target creating pions, which decayed into muons and muon neutrinos. Thick shielding halted the muons but the neutrinos continued until they entered a detector where they produced muons, decaying into electrons and a photon that were observed in the spark chambers.
Enter DONUT. The tau neutrino remained in hiding for many years until researchers overcame two major obstacles. First, the tau lepton has an extremely short lifetime--only 300 femtoseconds. Because neutrinos are detected by tracking their charged lepton partners, the lepton partner must be relatively easy to track. However, since the tau has such a short lifetime (even at relativistic speeds) it is difficult to detect. Secondly, tau neutrino production is very rare. Out of the 1013 neutrinos produced only 103 neutrino interactions, 4 of which were identified as tau neutrinos. Finally, in 2000, the DONUT team announced that they had the first direct evidence of the tau neutrino.
Where do they come from?
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| The picture above is of Super Kamiokande, a solar and atmospheric neutrino detector in Japan. |
Neutrinos come from several sources. The majority of neutrinos were created during the first few fractions of a second after the big bang, approximately 15 billion years ago, when the universe was comprised of elementary particles. These neutrinos are very low energy; they are so low, in fact, that we cannot detect them. These, along with microwave radiation, constitute the cosmic background radiation that permeates the entire universe, creating a picture of the events immediately following the big bang. Other neutrinos are produced in stars such as our own sun. In its core four protons combine with two electrons to form a helium nucleus and two electron neutrinos. Also, as we have mentioned before, beta decay is another instance of neutrino production. Cosmic rays, coming from various stellar phenomena and bombarding Earth perpetually, are partially comprised of neutrinos. Lastly, there are man-made sources such as physics laboratories where we create them by smashing high energy particles into fixed or moving targets.
Why are we interested in them?
Neutrinos are a fundamental part of
nature and we know relatively little about them. There are many important
questions being asked by
scientists around the world. What are the neutrino masses? We now believe
neutrinos oscillate between different flavors, but how do they do so and for how
long? What implications do oscillations have on the standard model? Do neutrinos
constitute dark matter? And there are many others which scientists ask so that
they can better understand the world we live in.
The electron, the electron-neutrino, and one pair of quarks - up and down - are all that is needed to build all the stable matter in the Universe. However, they are not all that was needed to build the Universe. High energy processes occurring naturally in the Universe, or generated artificially in laboratories such as CERN, produce a large variety of short-lived particles which include more quarks, more particles like the electron, and more neutrinos. We can group these particles into two "families" - the quarks and the leptons - and we have found that each family has six members. These are the matter particles of the current Standard Model.
| QUARKS | up | charm | top |
| down | strange | bottom | |
| LEPTONS | electron | muon | tau |
| electron-neutrino | muon-neutrino | tau-neutrino |
If we look at the electric charges and masses of the members of the two families
we find that we can group them into pairs, with three pairs for each family.
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The electron and electron-neutrino, and the up quark and down quark of stable
matter form the pairs with the lightest mass; the other particles fall into
heavier pairs, or heavier "generations" of the family.
Experiments with the LEP collider at CERN have shown that these are all the
generations like this that there are. This observation is confirmed by
measurements of how much helium (and other light elements) in the Universe was
made in the Big Bang; if there had been more generations in the early Universe,
less helium would have been made.
The various matter and force-carrying particles weigh in with a wide range of
masses. The photon, carrier of the electromagnetic force, and the gluons that
carry the strong force, are completely massless, while the conveyors of the weak
force, the W and Z particles, each weigh as much as 80 to 90 protons or as much
as a reasonably sized nucleus. The most massive fundamental particle found so
far, the super heavyweight, is the top quark. It is twice as heavy as the W and
Z particles, and weighs about the same as a nucleus of gold! The electron, on
the other hand, is approximately 350,000 times lighter than the top quark, and
the neutrinos may even have no mass at all.
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Why there is such a range of masses is one of the remaining puzzles of particle
physics. Indeed, how particles get masses at all is not yet properly understood.
In the simplest theories, all particles are massless, which is clearly wrong, so
something has to be introduced to give them their various weights. In the
Standard Model, the particles acquire their masses through a mechanism named
after theorist Peter Higgs. According to the theory, all the matter particles
and force carriers interact with another particle, known as the Higgs boson. It
is the strength of this interaction that gives rise to what we call mass: the
stronger the interaction, the greater the mass.
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Experiments have yet to show whether this theory is correct. The search for the Higgs boson (or bosons!) has already begun at the LEP collider at CERN, and will continue into the 21st century with CERN's next machine, the Large Hadron Collider. |
http://www-donut.fnal.gov http://public.web.cern.ch
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