Prof. Dr. Martin L. Perl > Research Profile
by Roberto Lalli
Martin Lewis Perl
Nobel Prize in Physics 1995
together with Frederick Reines "for the discovery of the tau lepton".
From Chemical Engineering to Physics
Martin L. Perl was born on June 24, 1927, in New York City to Jewish parents who had emigrated from the former Polish area of Russia around 1900. Growing up in a family that was striving to move into the middle class, Martin Perl was motivated to excel as a student in elementary and high school. Early in his life, Perl showed his talent for the sciences and decided to cultivate his natural inclination pursuing a career in a profession that could bring Perl financial stability. In 1939, driven by his interest in chemistry, Perl enrolled at the Polytechnic Institute of Brooklyn to study chemical engineering.
After having interrupted his course of study to serve as an engineering cadet in the United States Merchant Marine during World War II, Perl earned his bachelor’s degree in Chemical Engineering in 1948. His training at the Polytechnic Institute of Brooklyn would prove to be crucial in Perl’s later work in experimental physics. It provided Perl with a deep knowledge of several aspects of experimental practices (e.g. the strength of materials, manufacturing processes, metallurgy, engineering drawings, etc.), which were not addressed in a traditional physics undergraduate curriculum.
Soon after graduation, Perl joined the General Electric Company. His main area of research concerned the production of electron tubes. Thanks to his involvement in this research, Perl discovered his interests in physics—a discovery that served as a turning point in Perl’s career. While Perl was taking courses in atomic physics and advanced calculus at Union College (Schenectady, NY) to better understand the properties of electron vacuum tubes, the professor of physics Vladimir Rojansky recognized that Perl was much more interested in physics than in chemistry and encouraged Perl to switch his career. In 1950, at the age of 23, Perl began pursuing his PhD studies in physics at Columbia University, under the supervision of the Nobel Laureate I. I. Rabi.
Training inAtomic Physics at Columbia
When Perl began the doctoral program, the Columbia physics department was one the World’s premier institutions for the training of would-be physicists. Rabi had been establishing an inclusive environment in which young experimental physicists could work productively together with senior physicists expert in novel experimental techniques as well as with talented theoretical physicists. The list of the physicists who made research at Columbia between the late 1940s and the early 1950s is impressive: Apart from Rabi, Perl had the opportunity to work with P. Kusch—who would be awarded the Physics Nobel Prize in 1955—and to meet several younger researchers who would become leading physicists in the following years, including T.D. Lee, M. Schwartz, L. Lederman, J. Steinberger, and many others. His PhD research project was aimed at measuring the quadrupole moment of the sodium nucleus by employing Rabi’s atomic beam resonance method—the procedure that had gained Rabi the Nobel Prize in 1944. During his doctoral research, Perl strived to acquire a deep expertise in the experimental techniques and the practices of instrumentation. Perl developed his particular style of approaching experimental science that he later called “mechanical view,” with a strong attention to the mechanical aspects of the experimental design. In addition, Perl benefited from Rabi’s teachings, which gave Perl a broader view of the role of the experimenters in physics. Rabi was a particular kind of experimental physicist, more concerned with the theoretical implications of experiments than with the skills necessary to actually manipulate the instruments. From Rabi, Perl learned the importance of being independent in choosing fundamental problems on which to work. Rabi also convinced Perl to focus on particle physics instead of atomic physics in his future plans, because Rabi thought that particle physics was becoming the most relevant research area in physics.
Early Work in Particle Physics
After having successfully finished his dissertation, earning his PhD in 1955, Perl joined the University of Michigan as a faculty member to conduct research in particle physics. Although he had received offers also from universities with an established tradition in particle physics, Perl chose Michigan, because it assured Perl more freedom in the choice of research topics. Between 1956 and 1957, Perl worked with Donald Glaser, who had invented the bubble chamber a few years earlier. They employed the newly developed bubble chamber to explore the properties of various kinds of particles. In 1957, Glaser and Perl were also part of a large team of physicists who confirmed the violation of parity in weak interaction by analysing the hyperon decay.
In the late 1950s, Perl began a long-lasting collaboration with the experimental physicist Lawrence W. Jones. Between 1957 and 1960, Jones and Perl developed a particle detector called luminescent chamber, which they employed to measure the pion-proton elastic scattering at the proton synchrotron Bevatron of the Lawrence Berkeley National Laboratory (LBL). The community of physicists did not pay much attention to the luminescent chamber because the newly developed spark chamber proved to be by far more reliable in determining the paths of the charged particles. In the early 1960s, Perl and Jones, collaborating with other investigators, employed spark chambers to explore the properties of strong interactions. One of these experiments was the work on the pion-proton elastic scattering at the Bevatron, which constituted the dissertation of their student and future Nobel Laureate S. Ting. In his years at Michigan, Perl acquired new skills in many of the experimental practices that were fundamental to research in particle physics, including the employment of detectors (bubble chambers, luminescent chamber, spark chamber, scintillation counters) and the related measurement techniques.
The Muon-Electron Puzzles
In 1963, Perl moved to Stanford and began working on a new experimental technique for measuring the scattering of protons and neutrons with optical spark chambers. Dissatisfied by the confusion surrounding the theoretical description of strong interactions in the early 1960s, Perl switched his focus to the properties of electrons and muons, which do not participate in the strong interaction. When Perl decided to investigate the interactions between leptons, their properties were fairly well understood. In 1962, a Columbia team composed of Lederman, Schwartz, Steinberger and others, had provided strong evidence that there existed (at least) two different kinds of neutrinos: the electron-neutrino νe and the muon-neutrino μe.
Some questions were still unanswered, though. Perl mainly focused on two puzzles. The first issue concerned the reason why the muon is about 200 times heavier than the electron. The second problem regarded the decay of the muon into one electron and two neutrinos—more precisely an electron-type neutrino (anti-neutrino) and a muon-type anti-neutrino (neutrino). Physicists were wondering why the unstable muon did not simply decay in one electron and one photon (µ => e + γ). Perl and his colleagues planned various set ups to attack the electron-muon problem through experiments to be performed at the Stanford Linear Accelerator Center (SLAC) then under construction.
Perl focused on two different sets of questions. The first one concerned the investigations of possible unknown differences between electrons and muons. The second issue was based on the idea that there could be more than just two types of charged leptons. While the first research stream did not lead to any new discovery and only confirmed that muons and electrons have very similar properties, the second line of attack eventually led to the discovery of the tau lepton, which gained Perl the 1995 Nobel Prize in Physics.
The Discovery of the Tau Lepton
In the 1960s, a few theoretical investigations had been devoted to predict the properties of hypothetical charged heavier leptons, which served as a basis for Perl to conceive a new experiment. Perl was especially influenced by the works of Paul Tsai and his co-workers on pair-production calculation as well as on what Perl later called the sequential lepton model. The sequential lepton model described the leptons as a sequence of pairs composed by one lepton and one leptonic neutrino, in which each pair possessed a specific conserved quantum number (now called leptonic family number). Straightforward reasoning on lepton decay processes led Perl to predict some specific decay events related to a hypothetical heavy charged lepton. Following a seminal work by Cabibbo and Gatto (1961), Perl conceived the idea to look for new charged leptons employing the e+e- collider. According to a very general prediction, the e+e- annihilation should produce an energetic virtual photon that could decay in a pair composed of the heavier lepton and its antiparticle: e+ + e- => γ => L+ + L-, where L was the hypothetical heavier lepton. Perl’s idea was to look for the some decay modes of the Ls that could be explained only by the hypothesis that there existed a charged heavy lepton. Perl especially focused on two decay modes:
1) L+ => e+ + undetected neutrinos, and L- => µ- + undetected neutrinos;
2) L+ => µ+ + undetected neutrinos, and L- => e- + undetected neutrinos.
According to Perl, the detection of pairs of the type e+µ- or µ+e- with some missing energies following e+e- annihilation processes would have provided strong evidence for the existence of the charged heavier lepton, temporarily called L.
In 1969, the construction of the e+e- storage ring SPEAR (Stanford Positron Electron Asymmetric Ring) at SLAC began under the direction of Burton Richter and John Rees. The construction of this new collider gave Perl and his group the opportunity to devise an experiment to detect the decays of the type 1) or 2), called ‘eµ events’ in brief.
In 1971, also influenced by detailed calculations on the decay of sequential leptons put forward by Tsai, Perl included this line of research within a larger proposal for an experiment with the still-under-construction SPEAR e+e- storage ring, employing a detector called SLAC-LBL Solenoidal Magnetic Detector (later known as Mark I). The plan of the SLAC-LBL experiment was drafted by four groups of researchers, including the SLAC group E headed by Perl and G. Feldman and the SLAC group C led by Richter as well as two LBL teams led respectively by W. Chinowsky, and G. Goldhaber & G. Triling. The SLAC-LBL detector was designed as a machine for general purposes, because it allowed to detect and identify the majority of the charged particles produced by a reaction occurring at the centre of the detector, including electrons, muons, protons, pions, and kaons. Because of the properties of the detector, the original proposal of the SLAC-LBL experiment was directed at various targets. Perl’s colleagues considered the success of the search for heavy leptons so improbable that they did not emphasise this topic in the official research proposal. Perl was allowed to add to the proposal a ten-page appendix in which he elaborated the strategies for the test of the heavy lepton hypothesis. In addition, Perl and Feldman had to insist on the inclusion of an external muon detector in the general plan of the experiment.
After the proposal was accepted, the SLAC and LBL groups worked together closely in the design and construction of the detector. While the collider was under construction, other experiments aimed at detecting charged heavy leptons were being performed by two different teams at energies around 1 GeV. Because of the maximum energy reached by the colliders employed, the two experiments were unable to find any evidence of the existence of a third charged lepton.
The SLAC-LBL experiment started in 1973 as soon as the SPEAR began operation reaching the maximum energy of 4.8 GeV. By early 1975, the experimenters had registered more than twenty eµ events compatible with the predictions 1) and 2). Perl was convinced that the SLAC-LBL experiment had provided convincing evidence of a heavy lepton decaying in a pair eµ of opposite charge. Perl rushed to make public the discovery of a new particle by means of a press release. This decision sparked an internal controversy within the groups that had collaborated to the experiment because they had not been informed. Moreover, some of Perl’s collaborators believed that the finding was still dubious and that the press release jeopardized the entire SLAC-LBL collaboration. After Perl had made the announcement, there was widespread scepticism about the reality and/or meaning of the observed decays. Perl attended various conferences with the explicit aim to convince the physics community that the eµ events were real and that there was no conventional explanation for them.
After some improvements of the detection techniques and repetitions of the experiment at higher energies, Perl and his colleagues were confirmed in their belief that the eµ events were not an artefact of their experimental apparatus and that these events demonstrated the existence of a new particle with mass in the range between 1.6 and 2 GeV. Perl and his co-workers published their discovery in December 1975 suggesting that high-energy e+e- annihilations produced a pair of a particle and its anti-particle of the above-mentioned energy—a particle that was often referred to as U (for unknown) in the following two years.
There was much controversy surrounding the data exposed by Perl and his group and, above all, on their interpretation as indicating the existence of a heavy lepton. Apart from doubts concerning the reliability of the observational results, one of the main problems was that the third lepton was not required by the Glashow-Weinberg-Salam theory of electroweak interaction, which was gaining momentum within the high-energy physics community. From the observational perspective, a qualitative model of the double heavy lepton decay required that the hypothesised particle should decay also in hadrons leading to general reactions of the types: e+ + e- => µ± + hadrons + missing energy (called anomalous muon events) and e+ + e- => e± + hadrons + missing energy (called anomalous electron events). The criticisms emphasised that no reactions of this kind had been observed until then.
In 1977, thanks to further improvements in the detection techniques and the increase of the maximum energy reached by the SPEAR, Perl’s group was able to identify the expected anomalous muon events. Moreover, other experiments confirmed the detection of processes consistent with the decay of a charged heavy lepton. In particular, the investigators of the PLUTO experiment at the DORIS e+e- storage ring, near Hamburg, claimed that they had found evidence for heavy leptons (1977). All these events definitely convinced Perl of the reality of the third heavy lepton, which was by then denoted with the Greek letter τ.
However, the anomalous electron events had not been detected yet. Perl and his co-workers continued to work on this problem at SLAC. A LBL group led by A. Barbaro-Galtieri joined the project to improve the SLAC-LBL detector and by the end of 1977 the investigators were able to report that they had found convincing evidence of e-hadron events, besides the eµ and µ-hadron events already detected.
At the end of 1977, many of the objections concerning the relationship between the theoretical hypothesis of the third lepton and its empirical confirmation had been overcome. Only one issue still prevented physicists to accept the existence of the tau lepton. There were other expected decay modes (τ- = ντ +π- and τ- = ντ +ρ-), but they had not been observed until then. To detect such decay modes it was necessary to improve the efficiency of the photon detection, because the inability to detect photons prevented the investigators to clearly distinguish between different kinds of hadrons. The construction of new detectors (especially the Mark II detector at SLAC), which were much more efficient in the detection of photons, allowed the SLAC-LBL teams to observe the two decay modes between 1978 and 1979. In the same period, various experiments performed at other high-energy e+e- colliders reported that the reactions had been observed. By the end of 1979, all the experiments had fully confirmed the existence of the heavy lepton tau of mass around 1780 MeV, about 3,500 times heavier than the electron. In 1995, Martin Perl was awarded one half of the Nobel Prize “for the discovery of the tau lepton,” while the second half was awarded to Frederick Reines “for the detection of the neutrino.”
On the Boundaries of Lepton Physics
Once the tau was accepted as the third lepton, Perl continued to work on experiments aimed at exploring the properties of this particle and the related neutrinos, including the measurement of the tau lifetime, the mass of the tau-neutrino and the branching ratio of the various decay modes. Perl also led groups of experimenters that explored the possibility that there existed charged leptons with mass greater than that of the τ. According to the sequential lepton model Perl had put forward in the 1960s, the sequence is hypothetically infinite. From the 1980s onward, however, no evidence of the existence of other charged leptons has been found. To date, experiments have determined that the lower limit of the mass of a fourth heavy charged lepton is about 100 GeV.
Perl has also been involved in numerous experiments conducted at the SLAC e+e- collider aimed at testing the gauge field theories that now go under the name of Standard Model of particle physics; namely, the Glashow-Weinberg-Salam unified theory of electro-weak interaction and the quantum chromodynamcs of the strong interactions. He was particularly involved in the search for free fractionally charged particles (namely, free quarks)—a research that up to now has been unsuccessful.
As Perl has recognized in his reports about his experimental endeavours, he has been driven by the hope that the properties of the tau leptons might be the key to understand why there are three different generations of leptons in nature. The muon-electron puzzle that sparked Perl’s work in the 1960s has not been resolved yet. It has just become the e-mu-tau puzzle. After physicists had understood that the muon was a sort of heavy electron in 1947, Perl’s supervisor Rabi had emphasised the surprising features of the muon by asking “who ordered that?” The majority of physicists think that theoretical physics has been unable to find a satisfying answer to this question up to the present. The question might simply be re-formulated for the tau. As a professor at Stanford University, Perl continues to reflect on this issue and trains a new generation of experimental physicists to develop novel experimental techniques to clarify the world of elementary particles.
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