The Standard Model of Particle Physics

Chemistry can be understood in the physics of 3 particles (proton, neutron and electron), and the influence of the electromagnetic force. Nuclear physics can be understood in the physics of 4 particles (proton, neutron, electron and electron neutrino), and the influence of the strong and weak nuclear forces together with the electromagnetic force. The Standard Model Theory (SM) of particle physics provides a framework for explaining chemistry and nuclear physics (low energy processes). It additionally provides an explanation for sub-nuclear physics and some aspects of cosmology in the earliest moments of the universe (high energy processes).

The Standard Model is conceptually simple and contains a description of the elementary particles and forces. The SM particles are 12 spin-1/2 fermions (6 quarks and 6 leptons), 4 spin-1 ‘gauge’ bosons and a spin-0 Higgs boson. These are shown in the figure below and constitute the building blocks of the universe. The 6 quarks include the up and down quarks that make up the neutron and proton. The 6 leptons include the electron and its partner, the electron neutrino. The 4 bosons are particles that transmit forces and include the photon, which transmits the electromagnetic force. With the recent observation of the tau neutrino at Fermilab, all 12 fermions and all 4 gauge bosons have been observed. Seven of these 16 particles (charm, bottom, top, tau neutrino, W, Z, gluon) were predicted by the Standard Model before they were observed experimentally! There is one additional particle predicted by the Standard Model called the Higgs, which has not yet been observed. It is needed in the model to give mass to the W and Z bosons, consistent with experimental observations. While photons and gluons have no mass, the W and Z are quite heavy. The W weighs 80.3 GeV (80 times as much as the proton) and the Z weighs 91.2 GeV. The Higgs is expected to be heavy as well. Direct searches for it at CERN dictate that it must be heavier than 110 GeV.

The matter and force particles of the Standard Model. Up and down quarks were observed for the first time in electron-scattering experiments at SLAC in the late 1960s. The 1990 Nobel Prize in physics for this discovery was awarded to SLAC's Richard Taylor and to Jerome Friedman and Henry Kendall from MIT. The charm quark was discovered simultaneously in experiments at SLAC and at Brookhaven in 1974. SLAC's Burton Richter and MIT's Samuel Ting shared the 1976 Nobel Prize in physics for this discovery. The tau lepton was discovered at SLAC in 1975, for which SLAC's Martin Perl was awarded the 1995 Nobel Prize in physics.

The SM particles are considered to be point-like, but contain an internal ‘spin’ (angular momentum) degree of freedom which is quantized and can have values of 0, ½ or 1. Spin-1/2 particles obey Fermi statistics, which have as a consequence that no 2 electrons can be in the same quantum state. This feature is necessary for forming atoms more complex than hydrogen. Spin-1 and spin-0 particles obey Bose-Einstein statistics, which prefer to have many particles in the lowest energy or ground state. This phenomenon is responsible for superconductivity.
The Standard Model says that forces are the exchange of gauge bosons (the force particles) between interacting quarks and leptons. Feynman diagrams are useful to describe this pictorially. As illustrated in the figures below, two electrons may interact by scattering and exchanging a photon; or an electron and positron may collide and annihilate to form a Z particle, which then decays into a quark and anti-quark. Electromagnetic forces occur via exchange of photons; weak nuclear forces occur via exchange of W and Z particles; and strong nuclear forces occur via exchange of gluons. Electromagnetic forces and interactions are familiar to everyone. They are responsible for visible light and radiowaves, and are the physics behind the electronics and telecommunications industries. All quarks and leptons can interact electromagnetically. Strong nuclear forces are responsible for holding protons and neutrons together inside the nucleus, and for fueling the power of the sun. Only quarks interact via the strong interaction. Weak nuclear forces are responsible for radioactivity and also for exhibiting some peculiar symmetry features not seen with the other forces. In contrast to electromagnetic and strong forces, the laws of physics (ie. the strengths of the forces) for the weak force are different for particles and anti-particles (C Violation), for a scattering process and its mirror image (P Violation), and for a scattering process and the time reversal of that scattering process (T Violation). All quarks and leptons can interact via the weak interaction. The Standard Model provides much more than simply a description of electromagnetic, strong and weak interactions. Its mathematics provides explicit and accurate calculations for the rates at which these processes take place and relative probabilities for decays of unstable particles into other lower mass particles (such as for a Z particle to decay into different types of quarks and leptons).

excerpt borrowed from http://www-sldnt.slac.stanford.edu/alr/standard_model.htm

Conditioned Reflexes

Who was Ivan Pavlov?

The Russian scientist Ivan Petrovich Pavlov was born in 1849 in Ryazan, where his father worked as a village priest. In 1870 Ivan Pavlov abandoned the religious career for which he had been preparing, and instead went into science. There he had a great impact on the field of physiology by studying the mechanisms underlying the digestive system in mammals.

For his original work in this field of research, Pavlov was awarded the Nobel Prize in Physiology or Medicine in 1904. By then he had turned to studying the laws on the formation of conditioned reflexes, a topic on which he worked until his death in 1936. His discoveries in this field paved the way for an objective science of behavior.

Pavlov's drooling dogs

While Ivan Pavlov worked to unveil the secrets of the digestive system, he also studied what signals triggered related phenomena, such as the secretion of saliva. When a dog encounters food, saliva starts to pour from the salivary glands located in the back of its oral cavity. This saliva is needed in order to make the food easier to swallow. The fluid also contains enzymes that break down certain compounds in the food. In humans, for example, saliva contains the enzyme amylase, an effective processor of starch.

Pavlov became interested in studying reflexes when he saw that the dogs drooled without the proper stimulus. Although no food was in sight, their saliva still dribbled. It turned out that the dogs were reacting to lab coats. Every time the dogs were served food, the person who served the food was wearing a lab coat. Therefore, the dogs reacted as if food was on its way whenever they saw a lab coat.

In a series of experiments, Pavlov then tried to figure out how these phenomena were linked. For example, he struck a bell when the dogs were fed. If the bell was sounded in close association with their meal, the dogs learnt to associate the sound of the bell with food. After a while, at the mere sound of the bell, they responded by drooling.

Different kinds of reflexes

Reflexes make us react in a certain way. When a light beam hits our eyes, our pupils shrink in response to the light stimulus. And when the doctor taps you below the knee cap, your leg swings out. These reflexes are called unconditioned, or built-in. The body responds in the same fashion every time the stimuli (the light or the tap) is applied. In the same way, dogs drool when they encounter food.

Pavlov's discovery was that environmental events that previously had no relation to a given reflex (such as a bell sound) could, through experience, trigger a reflex (salivation). This kind of learnt response is called conditioned reflex, and the process whereby dogs or humans learn to connect a stimulus to a reflex is called conditioning.

Animals generally learn to associate stimuli that are relevant to their survival. Food aversion is an example of a natural conditioned reflex. If an animal eats something with a distinctive vanilla taste and then eats a tasteless poison that leads to nausea, the animal will not be particularly eager to eat vanilla-flavoured food the next time. Linking nausea to taste is an evolutionarily successful strategy, since animals that failed to learn their lesson did not last very long.

Why were Pavlov's findings given so much acknowledgment?

Pavlov's description on how animals (and humans) can be trained to respond in a certain way to a particular stimulus drew tremendous interest from the time he first presented his results. His work paved the way for a new, more objective method of studying behavior.

So-called Pavlovian training has been used in many fields, with anti-phobia treatment as but one example. An important principle in conditioned learning is that an established conditioned response (salivating in the case of the dogs) decreases in intensity if the conditioned stimulus (bell) is repeatedly presented without the unconditioned stimulus (food). This process is called extinction.

In order to treat phobias evoked by certain environmental situations, such as heights or crowds, this phenomenon can be used. The patient is first taught a muscle relaxation technique. Then he or she is told , over a period of days, to imagine the fear-producing situation while trying to inhibit the anxiety by relaxation. At the end of the series, the strongest anxiety-provoking situation may be brought to mind without anxiety. This process is called systematic desensitization.

Conditioning forms the basis of much of learned human behavior. Nowadays, this knowledge has also been exploited by commercial advertising. An effective commercial should be able to manipulate the response to a stimulus (like seeing a product's name) which initially does not provoke any feeling. The objective is to train people to make the "false" connection between positive emotions (e.g. happiness or feeling attractive) and the particular brand of consumer goods being advertised.

Pavlov's prize

Although the first image that comes to mind while mentioning Ivan Pavlov's name is his drooling dogs, he became a Nobel Laureate for his research in a different field. In 1904 he received the Nobel Prize in Physiology or Medicine for his pioneering studies of how the digestive system works.

Until Pavlov started to scrutinize this field, our knowledge of how food was digested in the stomach, and what mechanisms were responsible for regulating this, were quite foggy.

In order to understand the process, Pavlov developed a new way of monitoring what was happening. He surgically made fistulas in animals' stomachs, which enabled him to study the organs and take samples of body fluids from them while they continued to function normally.

excerpt taken from http://nobelprize.org/educational_games/medicine/pavlov/readmore.html