SLAC-PUB-4819 January 1988 Pm) THE TAU AND BEYOND: FUTURE RESEARCH ON HEAVY LEPTONS* MARTIN L. PERL Stanford Linear Accelerator Center Stanford University, Stanford, California 94309 . ABSTRACT: This paper outlines directions for future experimental research on the tau and tau neutrino. Present limits on the existence of heavier charged leptons are re- viewed, with emphasis on the close-mass lepton pair con- cept. *Work supported by the Department of Energy, contract DE-AC03-76SF00515. Presented at the 8th International Conference on Physics in Collision, Capri, Italy, October 19-21, 1988. Table of Contents I. Introduction II. The Tau A. The Decay Mode Problem B. Better Measurements of Conventional Properties C. Tau Lepton Number Conservation D. Deviations From a Dirac Point Particle E. The Concept of Lepton-Specific Forces and the Tau III. The Tau Neutrino A. Tau Neutrino Mass B. Tau Neutrino Mixing C. Interactions of the Tau Neutrino D. Variations on the Tau Neutrino IV. Limits on the Existence of Heavier Charged Leptons A. Present Limits B. Future Searches for Charged Leptons I. Introduction In 1981, at the first Physics In Collision Conference, I reviewed the status of the tau and of the search for heavier leptons. Now, seven years later, I have the pleasure of presenting another such review. Those seven years demonstrate the perversity of nature, or more precisely, demonstrate the obstinacy of nature with respect to the desires and expectations of the physicist. By 1981 the tau was established and the particle physicist looked forward to the discovery of more and heavier leptons. Two new electron-positron colliders PETRA and PEP were in operation, and the CERN pp collider was under construction. Surely we would find another charged sequential lepton, or an excited lepton, or a scalar lepton, as the mass search range extended to tens of GeV/c2 from 4 GeV/c2. That was not to be. (A small region of hope in this mass range is still offered by the close-mass lepton pair concept, - Sec. IV.) In the next seven years, new colliders are in operation or will begin operation: TRISTAN, the pji collider at Fermilab, the SLC, LEP and HERA. Again our hopes are high. The obstinacy of nature may continue-or there may be something new to find in the world of heavy leptons, but not what we expect. Therefore, one purpose of this review is to present a skeptical view of what we think we know about heavy charged leptons, using the tau as a model. The history of the tau in the last seven years may provide another example of how nature does not always obey the wishes of the particle physicist. In 1981 we thought we understood 2 the tau, although there were still some measurements to make. Seven years and many more measurements later (Fig. l), I am not sure the tau is well-understood; there is the problem in the l-charged particle decay modes, Sec. 1I.A. The need to learn more about the tau is relevant to current discussions and proposals for very large luminosity e+e- colliders: Tau-Charm Factoriest’lB Factories and 2’ Factories. II. The Tau A. The Decay Mode Problem In the last year my colleagues and I have written three papers[2-31covering various aspects of the tau decay mode problems, also a general review!’ I’ll summarize what we know, referring the reader to those papers [2-41for the details-or it may be better not to read those papers, because they describe the failed attempts by many physicists to resolve the problem. A fresh view of the problem based only on a data summary may be best. The average measured values of the inclusive branching fractions into 1, 3, 5 or 7-charged particles are[3’5’61 Bl = (86.6 f 0.3)% B3 = (13.3 f 0.3)% (1) B5 = (0.10 f 0.03)% B7 > 0.019% ) 90% CL . The usual laws of the conservation of energy, charge, total angular momentum and lepton number require that Br be the sum of the decay fractions of the modes listed in Table 1. The modes listed in rows A-F have been measured in at least three different experiments. Some branching fractions are based on many measurements; for example, 10 for B(v,e-fi,) and 16 for B(v~~-Y~). Incidentally, Br is based on 11 measurements. Since the energy available in r decay is less than 1.78 GeV, we expect the branching fractions to be small for modes containing more than 3 r’s or the relatively heavy 7. As shown in rows H-L of Table I, this is the case. Published experiments have not been able to directly measure the branching fractions for these higher multiplicity or q-containing modes. The failure comes from one or more of the following problems: great difficulty in working backwards from detected photons to the original r” + y + y or q + y + y or 77---) r” + r” + 7r”, background and false photon signals, and insufficient statistics. Direct studies of the l-charged particle modes in rows H-L yield only upper limits. The sum of the branching fraction measurement and upper limits in Table 1 is 5 (88.2 f .1.5)%, compatible with Br = (86.6 f 0.3)%. The l-charged particle decay mode problem occurs when conventional theoretical consid- erations and other data are used to evaluate or set upper limits on the branching fractions 3 Table 1. Summary of direct measurements of branching fractions of l-charged particle modes using only l-charged particle decays. Type of Row Decay Mode Branching Fraction Information I I Fraction (%) A 17.6 f 0.4 B 17.7 f 0.4 Measured in c 10.8 f 0.6 l-charged particle D 22.5 f 0.9 decays E 7.6 f 0.8 1.7 f 0.4 F Sums of rows A-F 1 G 1 I 77.9 f 1.5 I Upper limit deduced H vr7r-37ro < 2.5 or estimated I v9-47ro + v$r-579 is 4. in l-charged J vrrl < 0.3 particle decays K v,qnr' < 2.1 L 477 < 1.4 Sum of rows H-L I M I I 2 10.3 I Sum or rows N 5 88.2 f 1.5 A-F and H-L 1 I l-charged particle 0 86.6 f 0.3 topological Br I I 4 Table 2. Values and upper limits of branching fractions for l-charged particle modes deduced from theory and other measurements. The sum does not include modes with v7mpro, n > 2. Mode Met hod v,7r-3a0 u,n-4z-s 0.06 lJ,lr-57r” 0.11 VTT-77 0.00 v,a-qa” 0.24 v+r-7727r0 0.40 vr7r-7pp7ro, n 2 0 0.60 sum 2.7 a: use conservation of strong isospin b: use 77+ 7r+ + 7rlr- + 7r” c: use e+e- cross section and conserved vector current d: u,r-q requires a second class current amplitude Table 3. Branching fractions for l-charged particle decays. Source of Branching Fraction Information @) Sum of well measured modes 77.9 f 1.5 from Table 1 Sum of 95% CL upper limits from 5 2.7 Table 2 Sum of above Topological branching fraction Br 5 of the modes in rows H-L of Table 1. This is the work of Truong, [71 of Gilman and Rhier’ and of Gilman! The upper limit for the sum of rows H-L reduces to 2.7%, Table 2. Then the sum of the individual, l-charged particle, branching fraction measurement is 5 (80.6 f 1.5)% which d oes not agree with Br = (86.6f0.3)%, Table 3. There is a missing 6%! There are three general classes of possible errors in the measurements which might lead - to the 6% discrepancy. (i) The measured Br might be too large. This could happen if the detector simulation programs used to correct the observed Br and I33 are wrong. However, this kind of error extending over most of the Br determinations has not been found. In an unpublished work, Dorfan[lO1 has shown that it is very difficult to reduce Br by more than one or two percent. (ii) One or more of the measured individual branching fractions Bri might be too large. Such a possibility is more likely than (i) because the ratio Bri (observed)/&; (corrected) may be very small due to decay mode selection criteria and detector inef- ficiency. No one has been able to find the kind of error that would affect simultaneously most of the individual measurements making up an average value in Table 1. I’m still looking for such an error; I expect it to be cause of the discrepancy if the experiments are at fault. (iii) Perhaps the error assigned to the 77.9% partial sum in Table 1 should be several times larger, destroying the significance of the 6% discrepancy. In two papers,[2’31Hayes, Efron and I have examined this possibility. In the sets of measurements used for the average values of Br, B(v,e-fi,), B(v,p-c,), B(v~x-) and B(v,p-), th ere is no internal evidence that the errors should be enlarged. The errors associated with an individual measurement by the experimenters are either about right or too large, according to this internal evidence. There is, however, evidence for bias in the B(v,p-) measurements and hints of bias in other measurements in the sense that the individual measurements cluster more about their central value than their individual errors would predict.