UNDER THE MICROSCOPE:MATTER IS MADE of fundamental building block particles and the theoretical framework describing their interactions with three of the four fundamental forces is called the Standard Model. The model is a well-tested theory, but one essential ingredient still eludes experimental verification – the Higgs field and its associated force carrier the Higgs boson particle.
It is proposed that the Higgs field interacts with the fundamental particles, thereby giving them mass. Because of the great fundamental importance of the Higgs boson it has been nicknamed the God Particle. The Higgs boson may soon be detected experimentally in the giant particle accelerator at Cern, the European Organisation for Nuclear Research.
The 12 fundamental building blocks of all visible matter in the universe are six quarks and six leptons. The six quarks are named up, down, strange, charm, top and bottom. The six leptons are the electron, muon, tau, electron neutrino, muon neutrino, tau neutrino. Theorists postulate that no more quarks or leptons exist but think there may be other types of fundamental building blocks in the mysterious dark matter that astronomical observations have identified in galaxies.
Four fundamental forces act upon these fundamental particles – gravity, electromagnetism, the strong nuclear force, the weak nuclear force. However, the effects of gravity are only significant between massive objects and gravity plays no significant role at the microscopic level. The Standard Model describes the interactions between the quarks, leptons and three of the fundamental forces. The force of gravity is still not included in this framework. One major goal for physics in the 21st century is somehow to incorporate gravity into the Standard Model.
Every atom has a central nucleus containing positively charged protons and neutral neutrons, around which negatively charged electrons orbit. The electromagnetic force holds the negatively charged electrons in these orbits around the positively charged nucleus, thereby allowing atoms to form. The strong nuclear force binds quarks together to form protons, neutrons and related particles. A proton is made of two up quarks and one down quark. A neutron is made of two down quarks and one up quark. The weak nuclear force facilitates the breakdown of heavier particles into smaller particles (important for example in radioactive decay).
The fundamental particles transmit forces between each other by exchanging force- carrying particles called bosons. To help visualise this, picture two basketball players connected to each other simply by quickly and repeatedly passing the ball between each other. The bosons that mediate the strong nuclear force are called gluons. W and Z bosons mediate the weak nuclear force. Photons mediate the electromagnetic force (photons also transmit light), and gravitons mediate gravity.
The Standard Model has predicted many effects, later verified by experiment. The biggest success of the model so far has been the unification of the weak nuclear force and the electromagnetic force into a single electroweak force. Physicists think it will eventually be possible to describe all four forces in a Grand Unified Theory.
The 12 fundamental particles already described have no mass when considered alone and the Higgs boson is considered necessary to confer mass on them.
How can one particle confer mass on another particle? The following analogy is used to explain: you are attending a Hollywood party and the crowd is evenly dispersed around the room. The star guest arrives. The ordinary guests near the door cluster around her. As she moves through the room she attracts those guests closest to her and those she moves away from return to their other conversations. The fawning cluster of admirers around the star give her momentum (an indication of mass) as she moves through the room. She is harder to slow down than she would be without the clustering admirers and once she stops she is harder to get going again.
This clustering effect is analogous to the mechanism postulated by British physicist Peter Higgs in the 1960s. He hypothesised that a type of lattice, called the Higgs field, fills the entire universe and affects the particles that move through it. Those particles that do not interact with the field, eg photons, have no mass. Particles that interact with the field display mass and the greater the interaction, the greater the particle mass.
The Higgs boson associated with the field was nicknamed the God Particle simply to emphasise its great ubiquitous importance. It is mentioned in the new film Angels and Demons, starring Tom Hanks, as if it has some deep significance for the relevance of religion, but the boson has nothing in particular to do with God.
If the experiments at Cern confirm the existence of the Higgs boson Peter Higgs will almost certainly win a Nobel Prize. If it turns out that the Higgs boson does not exist physics will have to think again about the origin of mass.
William Reville is associate professor of biochemistry and public awareness of science officer at University College Cork –
