Tau Neutrino Physics: An Introduction Barry C. Barish California Institute of Technology Pasadena, CA 91125, U.S.A. The properties of the tau neutrino are discussed and the recent result reporting direct detection for the first time is presented. The status of the experimental results relating to the mass of the nt, both from missing energy in tau decays and from neutrino oscillation experiments are reviewed. Finally, I give a perspective on future directions in studies of the physics of tau neutrinos. 1. INTRODUCTION might play an important role in the interpretation of the observations of high energy cosmic rays The tau neutrino was inferred as the third above the Greisen, Kuzmin, Zatsepin (GKZ) neutrino (ne, nm and nt) at the time the tau lepton bound indicating new physics. was discovered and determined to be a lepton. Finally long baseline neutrino experiments Since that time, direct studies of the nt have will be able to study the role of the tau neutrino proven elusive, however, due to the fact that the in the atmospheric neutrino oscillations with tau is so heavy and difficult to abundantly precision, with the possibility of an even more produce. Finally, the fact that the tau is very powerful experimental probe in the future if a short lived makes the creation of an intense neutrino factory is built, offering perhaps even ‘beam’ of tau neutrinos not so simple. the possibility of observing CP violation in the Much is known about the properties of the neutrino sector. tau neutrino despite the fact that direct detection has only been reported this past summer. In fact, 2. THE NUMBER OF NEUTRINOS that detection is clearly one of the more important results experimental results reported in The evidence that there are three families of high energy physics this year. I review that quarks and leptons is strong. Of course we are evidence in this presentation and a more left with the puzzle why there are the three complete talk will be given later in this families, as well as the problem of determining workshop. the large mass splittings between the families. It is interesting that the nt was the last of The evidence for the number of standard the leptons and quarks in the three families to be neutrinos comes from two sources: big bang observed in the laboratory, even after the top nucleosynthesis, and the accurate determination quark, which took a huge effort at Fermilab to that comes from the partial widths in Z-decay pin down and measure the mass. The fact that measured at LEP. the n is the last to be detected is an indication t 2.1. Big Bang Nucleosynthesis of the experimental challenge that was involved The earlies indications that there are a finite in making the direct detection of n interactions. t and small number of families in nature came The main interest in the nt has focused on from big bang nucleosynthesis. the issue of neutrino mass and oscillations. The primordial abundances determined for Direct mass measurements have yielded limits 3 4 7 on the neutrino mass, while the indications are D, He, He and Li are affected by the number very strong that atmospheric neutrino of neutrinos produced in the big bang. The observations indicated oscillations between the predictions and understanding of these muon neutrino and the tau neutrino with a mass abudndances[1] is a major triumph of the standard big bang model of the early universe. difference of Dm2 ~ 10-3 eV2. This, of course, The abundances of these light elements range implies that the nt has finite mass, but over nine orders of magnitude! The first undoubtedly much smaller than the limits set in 4 the direct mass measurements. constraint on these abundances is on Y, the He Future developments in tau neutrino fraction. From number of neutrons when physics look quite promising. Experiments nucleosynthesis began, we know that Y is observing supernovae could lead to quite bounded, Y < 0.25. accurate mass determinations, tau neutrinos Nn = 2.984 ±0.008 This impressive measurement gives strong credence to the view that there are three families of leptons and quarks 3. TAU NEUTRINO EXISTENCE The existence of the tau neutrino has relied on the indirect (though very strong) evidence until this past summer. The initial evidence was presented by Feldman et al [6] in 1981 from tau decay data. The DONUT experiment [7] at Fermilab has now reported the first direct observations of detected nt interactions. They observe the tau and its decays from nt charged current interactions The general principal of the DONUT experiment is to produce a strong source of tau neutrinos by interactions of the 800 GeV proton beam with a beam dump. The proton interactions produce a large number of Ds, having a very short lifetime and subsequently Figure 1. The abundances of light elements in yielding nt’s in the final state. This scheme to nucleosynthesis, which vary by a factor of 109 produce the intense beam of tau neutrinos is are shown. The parameter h is the baryon to shown in figure 2. The data run reported used an photon ratio x1010, which is not well determined. integrated total number of protons on target of 3.6 1017 , and the data was taken from April to The observed value of Y (figure 1) is September 1997. Yobserved = 0.238±0.002±0.005 The presence of additional neutrinos would at the time of nucleosynthesis increases the energy density of the Universe and hence the expansion rate, leading to larger Y. The relation below gives the magnitude of the effect due to an additional neutrino, DYBBN= 0.012-0.014 DNn 1.7 < Nn < 4.3The most precise measurements of the number of light neutrinos come from Z e+ + e- partial width measurements at LEP. The invisible partial width, Ginv, is determined by subtracting the measured visible partial widths (Z decays to quarks and charged leptons) from the Z width. The invisible width is assumed to be due to NnIn the Standard Model, (Gn /Gl)SM = 1.991 ± 0.001,where using the ratio reduces the model dependence. Using this prediction and comparing with the accurate LEP measurements, Figure 2. The DONUT tau neutrino beam is the value [5] for the number of neutrinos is created using 800 GeV protons incident on a bounded by beam dump as shown. The total number of nt interactions is estimated to be ~1100 ( nm , ne , nt ) for the entire DONUT data run. They found 203 candidates for nt in a 6 total of 6.6 10 triggers. The reported nt events satisfied the event topology shown in figure 3, where they measure the parent tau and identify a kink . They also search for the topology were the tau is too short lived to emerge from the 1 mm steel absorber, but they have not reported on that topology. In that case, they do not measure the parent tau but identify the candidates by the apparent kink when the track is projected back toward the vertex. Figure 4. The pT distribution of candidate events, where the events identified as nt events typically have larger pT. 4. TAU NEUTRINO PROPERTIES Figure 3. The event topology signature for observing tau neutrino interactions in the 4.1 Spin of the nt DONUT experiment. The parent tau is The spin of the nt has been established [5] indentified and the kink from the subsequent tau as J = 1/2. The possibility of J = 3/2 has been decay is observed and measured using nuclear ruled out by establishing that the r- is not in a emultions. pure H ± -1 helicity state in the decay - - The final experimental challenge is t ®r + nt. ‘proving’ that the observed events are due to tau neutrino interactions and not background events 4.2 Magnetic Moment of nt satisfying the topological requirements. Detailed We expect mn = 0 for Majorana or chiral tests and modeling has been done to understand massless Dirac neutrinos. If SU(2)xU(1) is the backgrounds, as well as characterizing extended to include massive neutrinos, measured parameters that can at least statistically 2 -19 distinguish tau neutrino events from background. mn = 3eGFmn /(8p 2)= (3.20´10 )mn mB Background events can come from various sources like overlapping events (randomly where mn is in eV and mB = eh/2me Bohr associated tracks) that fake this topology, magnetons. Using the upper bound mt < 18 scattering of a track to look like a kink or MeV, one obtains m < 0.6 10-11 m . This + n B reconstructed charm events D decays without should be compared to the experimental limit of the lepton identified. Fig. 4 illustrates the pT -7 - - mn < 5.4 10 mB from nt + e ® nt + e distribution of both ‘signal’ events and the scattering as measured in BEBC. candidates identified as background events. As a result of a detailed analsysis of the candidate 4.3 Electric Dipole Moment of the n events, their final result is summarized below. t The electric dipole moment of the n has been t established as < 5.2 10-17 e cm from G (Z ®ee) nt events observed 4 at LEP. expected 4.1 ± 1.4 background 0.41± 0.15 neutrino masses illustrating how the events limit the neutrino mass. 4.4 nt Charge The charge of the nt has been determined to -14 be < 2 10 from the Luminosity of Red Giants [8] 4.5 nt Lifetime The nt lifetime has been determined to be > 2.8 15 10 sec/eV Astrophysics for mn < 50 eV [9] 4.6 Direct Mass Measurement of nt Direct bounds on the nt mass come from reconstruction of t multi-hadronic decays.