While the Standard Model summarizes our present understanding of matter and forces, the real news is the profound discovery upon which it is based, namely that nature obeys "local" symmetries.
The elementary particles (e.g., quarks and electrons) contain "labels", but their properties are unchanged when the labels are switched in certain ways. This independence of the choice is called a "symmetry" of the interaction. Now, if the labels can be changed "locally" - differently at different times and places - without changing the interactions, then massless force-carrying particles (e.g., the photon) are REQUIRED to exist, and their interactions are defined. Thus, the forces and their carriers are a necessary result of the local symmetry. ALL of the forces we know about exhibit this behavior.
There are also "hidden" symmetries that exist in the mathematical structure, but that do not show up directly in observations. One such symmetry relates the carriers of the weak force, the W and Z, to the photon and explains why the W and Z are not massless, while the photon is. Although data show that this hidden symmetry must exist, its details are not known. The different possible mechanisms for hiding the symmetry all require the existence of a new particle, the higgs boson. Its discovery will show us how the symmetry is hidden.
All of the components of particle physics are interrelated and form a whole. As a result, other measurements that involve virtual higgs particles in the primary interaction can say something about the possible range of mass values that the higgs particle could have. These results confine the mass of the higgs to a region that - if we are lucky - might just be in reach of present experiments, such as the CDF, one of those that we are engaged in at Fermilab outside Chicago. Certainly the higgs will have to show up at the LHC, another experiment we are preparing for. If not, then some other "new physics" must appear at the LHC; the Standard Model will not survive at that energy.
The Standard Model raises other questions. For example:
The material of the present universe is dominated by protons, not antiprotons. Yet a symmetry, called CP, should have produced equal numbers of protons and antiprotons in the early universe, and they should have annihilated one another, leaving few or no protons or antiprotons (and we would not exist). What was it that broke this CP symmetry and led to the present universe? The Standard Model could accommodate this violation of CP, but the data are not sufficient to be definitive. Experiments now underway, or soon to be started, might give us the answer.
What are the masses of the neutrinos? What are the couplings that lead to the "mixing" of one type of neutrino with the others? With the recent evidence for neutrino mass, these are now "hot" experimental topics.
Supersymmetry is another hot topic right now. The parameter values (constants) in the Standard Model are many, and why they have the values they do is not understood. A very tempting theory that can explain the interrelations of the values is called Supersymmetry. It would unify the "particles" and the force carriers. Particles have half-integer quanta of spin angular momentum, force carriers have integral values. It is possible that each of these has a corresponding partner with the opposite integer or half-integer character. If, like the others, the choice of these labels can be made locally (this is not clear as yet) then the existence of the gravitational force is required! We would have every force being "generated" by this local symmetry requirement of Nature. The supersymmetry partners are being looked for in experiments today.
So, there are indeed many exciting developments going on right now in High Energy Physics, both in experiment and theory. It is our job to find the data that will define the correct theory.
Experimental Techniques of HEP
The art of designing and carrying out HE experiments and data analysis
involves many different techniques of "hardware" and computing. The basic
goal is to identify the directions, momenta, and identity of all the particles
emanating from a collision. Since there are very many tracks in high energy
collisions, and the collisions occur frequently, this can be a daunting
task, but without success here, new understanding of the fundamental physics
does not come. There is a long history of new detector capability leading
to the discovery of new physics. Designing the best detector for a given
experiment requires a deep understanding of the physics processes involved,
and many new detectors have been developed over the years. Some, such as
the cloud chamber and bubble chamber have merited Nobel prizes.
Scintillating plastic sheets and photomultiplier tubes provide fast (nanosecond) detection of the passage of a charged particle. Plastic scintillator fibers with photodiodes at the ends provide equally fast timing, with better spatial resolution.
Small tubes, each with a high voltage wire on its central axis, and filled with gas is another method of localizing the position of a traversing particle. Electrons liberated in the gas by the passage of the particle drift rapidly toward the wire and multiply in an avalanche of electrons in the high field near the wire. Sensitive electronics detect and amplify this signal. Measurement of the time of drift can localize the position to better than a tenth of a millimeter.
Even better precision, a few thousandths of a millimeter, is obtained with the use of small silicon diodes fabricated in strips, or in arrays of pixels a few hundredths of a millimeter square, with special integrated circuit chips (ASIC's) for the electronics.
All of these devices are arranged to form a set of large surfaces that define precisely the path of the particle. Strong magnetic fields are used in the assembled detector to bend the particle and thereby determine its momentum.
Other devices called "calorimeters" totally absorb the incident particle, causing it to generate a cascade of many particles as it and the secondary particles from these collisions continue to multiply. The resulting ionization is summed and provides a measure of the total energy of the incident particle.
Large electronic arrays have to be designed to collect and transfer the huge amounts of data arriving at high speed, and sophisticated computer programs must be written to collate and store these data. Equally complex programs must be written to view and analyze the data in a sensible way.
The techniques involved in HE experiments thus cover a very wide spectrum of mechanics, electronics of all types, and computing techniques at the forefront of developments. HEP presents one of the major challenges to data storage and retrieval, making use of new systems of "data mining" and object oriented programming. All of these detectors and software techniques have been, and will continue to be, part of experimental HEP here at UCD.
What Does One Do with a Ph.D. in HE Experiment Physics?
Some people are concerned that if there are not enough positions in
research universities or institutes, what is learned in gaining a Ph.D.
in experimental particle physics may not be useful. That happens not to
be true. My latest graduate student took a job in industry directly after
his Ph.D. and is now directing a group of 17 engineers. The company wanted
him precisely because he had demonstrated the capability to develop new
equipment, debug it, install it, and make it work compatibly with other
equipment in a large experimental collaboration. Much of industry needs
the "know-how" that is developed by students in high energy experiments
today. Other HEP students from UCD are in senior positions in Fairchild
Semiconductor, Sandia Laboratories, and Booz, Allan, & Hamilton. In
fact, over the past few decades, about one-third of our H.E. Experiment
Ph.D.'s took positions in industry, the others going into HE research.
For the field as a whole in the U.S., that fraction is about one-half.
Honors and Awards
for
Richard Lander
Fellow, American Physical Society
American Physical Society American Institute of Physics California Alumni Association
E-mail lander@physics.ucdavis.edu
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