In search of elusive quarks - possibly matter's ultimate particles
PHYSICISTS who probe the basic structure of matter sometimes have to deal with the awkward fact that the error in their measurements is larger than the effect they seek. Yet they still gain insight into the fundamentals of the universe.
Current research on a particle known quaintly as the ''b,'' or bottom, quark is a case in point.
Quarks are particles - perhaps the ultimate particles - which combine in various ways to construct what are called hadrons. These composite entities are a family which includes the protons and neutrons that make up the nuclei of atoms. Thus quarks seem to be close to, if not at, the very foundation of matter.
Physicists have identified five quarks and strongly suspect that a sixth exists. According to the present ''standard theory,'' quarks come in matched pairs. There are the ''up'' and ''down'' quarks, the ''strange'' quark and ''charmed'' quarks, and finally the ''bottom'' quark, which theory says must be paired with the yet-to-be-found ''t,'' or top, quark.
However, while the t remains elusive, its mass figures in the calculation of the lifetime of the b. Hence the interest in measuring that lifetime as accurately as possible. This helps physicists learn more about the b quark; it also gives them information about its presumed companion.
Last summer, two teams of physicists reported estimates of b quark lifetimes made at the Stanford Linear Accelerator Center (SLAC) at Stanford University in California. It came out to around 1.5 trillionths of a second, with an uncertainty of roughly 33 percent. That's about the time it takes light to travel a thousandth of a foot. It's an extraordinarily short time by human standards but a relatively long in the particle world, as noted in a recent National Science Foundation announcement reaffirming their estimates.
Physicists involved point out that the findings are based on measurements whose individual errors are four to five times larger than the effect being measured. Basic data include measurements of the length of tracks left in a detector by particles created in the collision of beams of electrons and positrons (positively charged electrons). The tracks typically are 150 micrometers (millionths of a meter) long, while the error in measuring them is as large as 500 micrometers.
William T. Ford, of the University of Colorado, who extracted meaning from these fuzzy data for one of the research teams, explains he has to rely on the fact that, statistically, the more measurements he has to work with the more he can trust the average of those measurements as an estimate of the effect he seeks. Thus, with some 400 measurements in hand, he was able to reduce the uncertainty 20-fold.
Donald E. Groom of the University of Utah, another team member, says that estimating the b quark lifetime that way has been a big step forward, but those measurement errors ''leave you a little queasy.'' That is why he and some other members of the team to which he and Ford belong are now refining the detector at SLAC to make the measurements themselves more precise.
The detector - called MAC for magnetic calorimeter - will be moved closer to the beam that generates the particles under study and otherwise improved. This scheme, developed primarily by David Ritson and his colleagues at SLAC, should reduce the overall track measurement error to about 80 micrometers. That will be comfortably less than the 150 micrometer length itself. The new equipment should be operating this fall. By the summer of 1985, it should have produced a far more precise estimate of the b quark lifetime than now is available.
Meanwhile, physicists do have a figure to work with which is far better than a mere guess, despite the large measurement errors involved. Among other things, it suggests that the undiscovered t quark has a mass equivalent to an energy in the range of 30 to 40 billion electron volts (Bev). (An electron volt is the energy given to an electron by a voltage difference of one volt.) A number of particle accelerators now can probe this energy range. Indeed, it is just above the energy of 29 Bev of SLAC's electron-positron collisions. Thus the top quark itself may be found at any time.
Pinning down the nature of such fundamental particles is a time-consuming and labor-intensive job. The MAC team alone includes some 50 physicists from the US and Italy. Their work on the b quark began in 1977. The hardware was in place by 1980. But it still took several more years to check it out, complete the computer programs, and arrive at even the fuzzy estimate of the b's lifetime. It will take more than another year to refine that estimate using the new equipment.