Perl was born in New York City, New York. His parents, Fay (née Resenthal), a secretary and bookkeeper, and Oscar Perl, a stationery salesman who founded a printing and advertising company, wereJewishimmigrants to the US from thePolish area ofRussia.[1][2]
Perl was a 1948chemical engineering graduate of Brooklyn Polytechnic Institute (now known asNYU-Tandon) in Brooklyn. After graduation, Perl worked for theGeneral Electric Company, as a chemical engineer in a factory producing electron vacuum tubes. To learn about how the electron tubes worked, Perl signed up for courses in atomic physics and advanced calculus atUnion College inSchenectady, New York, which led to his growing interest in physics, and eventually to becoming a graduate student in physics in 1950.[1]
He received his Ph.D. fromColumbia University in 1955, where his thesis advisor wasI.I. Rabi. Perl's thesis described measurements of the nuclear quadrupole moment ofsodium, using the atomic beam resonance method that Rabi had won the Nobel Prize in Physics for in 1944.[1]
Seeking a simpler interaction mechanism to study, Perl started to considerelectron andmuon interactions.[3]He had the opportunity to start planning experimental work in this area when he moved in 1963 to theStanford Linear Accelerator Center (SLAC), then being built inCalifornia. He was particularly interested in understanding themuon: why it should interact almost exactly like the electron but be 206.8 times heavier, and why it should decay through the route that it does. Perl chose to look for answers to these questions in experiments on high-energy chargedleptons. In addition, he considered the possibility of finding a thirdgeneration of lepton through electron-positron collisions.[2]
He died after a heart attack[5] at Stanford University Hospital on September 30, 2014, at the age of 87.[6] His son,Jed Perl, is an author and art critic forThe New Republic.[5]
The tau was first detected in a series of experiments between 1974 and 1977 by Perl with his colleagues at theSLAC-LBL group.[8] Their equipment consisted ofSLAC's then-newe+ –e− colliding ring, calledSPEAR, and theLBL magnetic detector. They could detect and distinguish between leptons, hadrons andphotons. SPEAR was able to collide electrons and positrons at higher energies than had previously been possible, initially at up to 4.8 GeV and eventually at 8 GeV, energies high enough to lead to the production of a tau/antitau pair.[3] The tau has a lifetime of only2.9×10−13 s and so these particles decayed within a few millimetres of the collision.[9] Hence Perl and his coworkers did not detect the tau directly, but rather discovered anomalous events where they detected either an electron and a muon, or a positron and an antimuon:[10]
We have discovered 64 events of the form
e+ +e− →e± +μ∓ + at least two undetected particles
for which we have no conventional explanation.
The need for at least two undetected particles was shown by the inability to conserve energy and momentum with only one. However, no other muons, electrons, photons, or hadrons were detected. It was proposed that this event was the production and subsequent decay of a new particle pair:
e+ +e− →τ+ +τ− →e± +μ∓ + 4ν
This was difficult to verify, because the energy to produce theτ+ τ− pair is similar to the threshold forD meson production. Work done atDESY-Hamburg, and with the Direct Electron Counter (DELCO) atSPEAR, subsequently confirmed the discovery[2] and established the mass and spin of the tau.
The symbol τ was derived from the Greekτρίτον (triton, meaning "third" in English), since it was the third charged lepton discovered.[11]
Perl won the Nobel Prize in 1995 jointly withFrederick Reines. The prize was awarded "for pioneering experimental contributions to lepton physics". Perl received half "for the discovery of the tau lepton" while Reines received his share "for the detection of theneutrino".[12] In 1996 he publishedReflections on Experimental Science, which consists of "comments, scientific reprints, reflections, and a memoir ...".[13]
