The appearance of the tau-neutrino

Europhysics News, Jul 2018

A. Bettini

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The appearance of the tau-neutrino

Y. Fukuda et al., Phys. Rev. Lett. 0 DOI: 10.1051/epn/2010604 On August 22nd 2009 a very special event was recorded by the OPERA experiment at the INFN Gran Sasso Laboratory produced by a neutrino coming from CERN. Born as mu-neutrino, it had transformed into a tau-neutrino on its 730 km long route. This is a long-awaited achievement of an enterprise started 30 years ago. - Neutrinos change family times, neutrinos only by their lepton flavours, which are e spin ½ elementary particles come in three different conserved in the interactions. Neutrinos are produced groups, called families, of identical structure. Each in states of definite flavour, as νe , νμ or ντ , each with an family is made of two quarks, of charge +2/3 q and – antiparticle of the same flavour, its antineutrino or e+, µ + 1/3 q (q is the proton charge), and two leptons, one of or τ+, respectively. When a neutrino of a certain flavour charge –q and one neutral. e neutral leptons are col- interacts with a target producing a charged lepton, the lectively called neutrinos, but are three different latter always has its flavour; for example a νμ produces a particles, distinguished by an additive quantum number µ –, never an electron or a tau. called “lepton flavour”. e electron (e–) and the elec- Experiments in underground laboratories have shown that tron-neutrino (νe) have one unit of electronic flavour neutrinos do not behave as assumed in the Standard Model (–1 their antiparticles); the muon (µ –) and the muon- (SM): they do change,“oscillate”, between one flavour and neutrino (νμ) have one unit of muon-flavour, and another. e evidence has gradually grown in the last four similarly for the tau (τ –) and the tau-neutrino (ντ). decades, by studying the νes produced by the fusion reace charged leptons are distinguished by their different tions in the core of the Sun and the νμs indirectly produced masses (increasing with the family number) and life- by the cosmic rays collisions in the atmosphere. OPERA detector. Photo on permission of LNGS-INFN by defining its initial state. In each mode the two pendulums oscillate harmonically and synchronically with (proper) frequencies, say, ω1 and ω2. e first mode is obtained by letting them go from the same initial elongation with zero initial velocities. e squared frequency is ω 12=g/l. In the initial state of the second mode the two elongations are equal and opposite and the velocities are zero. e squared frequency is ω 22 =g/l+2k/m. Considering for the moment a world with only two neutrinos, the analogues of the modes are ν1 and ν2 and the analogues of the frequencies are the masses m1 and m2. e analogue of a neutrino of flavour a is the initial state with pendulum a abandoned with zero velocity at a certain initial elongation and pendulum b in its at rest position. Vice versa for flavour b. One observes that the motion does not have a definite frequency. Rather, the FIG. 1: Geographical sketch of the neutrino beam from CERN to Gran Sasso by A. amplitude of the vibrations of a gradually decreases Zichichi (1979) down to zero, while that of those of b increases up to a maximum. en the evolution inverts. Hence, the energy “oscillates” periodically from a to b with a freNeutrino oscillations quency ω2-ω1. In the quantum system the analogue of Neutrino oscillations happen because neutrinos of energy is the probability to observe the flavour, because definite flavour, νe, νμ and ντ, are not stationary states, both are proportional to the square of the amplitude. To i.e., states of definite mass, but linear combinations of observe the phenomenon one needs to leave a time long those. We call the latter ν1, ν2 and ν3 and m1, m2 and enough to allow for its development. In practice this m3 their masses. means having the detector far enough from the source. Oscillations phenomena are common in physics. Consi- en there are two alternatives: to look for the disapder for example two identical pendulums, a and b, of pearance of the initial flavour (looking at a) or for the mass m and length l weakly coupled by a spring of appearance of the new one (looking at b). constant k. e system has two normal modes, obtained However, there are three neutrinos. e analogue is a system of three coupled pendulums. Moreover, the pendulums may not be equal. Clearly the phenomenology FIG. 2: A. Bettini (left) and L. Maiani (right), together with a representative of local becomes much richer. S1w2/i1s0s/a2u0t0h0o.rPithieostoceClEeRbNrating at the “ground breaking” event for the CNGS project on In 1968 R. Davis [1] published his measurement, started in 1964, of the flux of neutrinos from the Sun with an experiment sensitive to νe. The flux was smaller, about 1/3, than the theoretical calculations by J. Bachall [2]. When the preliminary results became known, B. Pontecorvo [3] claimed that the disappearance might be due to neutrinos changing flavour through oscillation, a concept he had introduced in 1957. Further νe disappearance experiments, in different energy ranges, KAMIOKANDE [4] and Super KAMIOKANDE [5] in Japan, GALLEX/GNO [6] at Gran Sasso and SAGE [7] at Baksan, had established by 1998 that the Pontecorvo idea was right. The final proof came from the SNO [8] in Canada in 2002. All neutrino flavours can interact with nuclei producing a neutrino in the final state, instead of a charged lepton. The final neutrino cannot be detected, but the effect on the nucleus can. This was done by SNO, proving that the total neutrino flux interacting in this way corresponded exactly to the missing νe flux. A confirmation came from the KamLAND experiment in Japan that observed the oscillation in anti-electron and P. Strolin [14]. e study of the proposals led to neutrinos from nuclear power plants. a common decision by the CERN Director General, By 1998 the Super-KAMIOKANDE [9] experiment had L. Maiani, the INFN Premeasured, with high statistics and high accuracy, the sident, E. Iarocci, and the dependence of the atmospheric νμ flux on the flight- LNGS Director, myself, length and on energy. e shape of this function proved for the more risky (but the oscillation phenomenon and made it possible to much more rewarding if determine the corresponding oscillation parameters. successful) appearance Also in this case, confirmation of the oscillation inter- experiments. e project pretation came from disappearance experiments with was approved by the artificial neutrinos, νμ produced at the KEK accelerators INFN and CERN Counin Japan (K2K experiment) and later at the Fermilab in cils in 1999. It was funded with ad hoc contributions the USA (MINOS experiment). mainly from Italy and from several other countries. e civil engineering works at CERN and the construction of the beam took place between Autumn 2000 (see Fig. 2) and Summer 2004. e subsequent delicate and complex phases of testing and commissioning were completed by the Spring 2006. In August of the same year the large detectors at LNGS, LVD, OPERA and BOREXINO detected the first events produced by the neutrino beam. In the same period the OPERA and ICARUS experiments were developed. Neutrinos do not behave as assumed in the Standard Model (SM): they do change, “oscillate”, between one flavour and another From the third family to CNGS As mentioned above, ντ and its charged partner τ are part of the third “family”. As a matter of fact, the existence of the third family, and the concept of family itself, was experimentally established in the lepton sector much earlier than in the quark sector. We briefly recall that the neutrino was introduced as a “desperate hypothesis” by Pauli in 1930, when only the electron was known, to explain the apparent non-conservation of energy in beta decay. It was discovered in 1956, when also the muon had been discovered, by F. Raines [10]. It was in 1962 that an experiment at Brookhaven [11] established that neutrinos were two, νe and νμ. Not much later, the idea of the possible existence of a third lepton family, called “heavy lepton” Hl and its neutrino νHl was introduced by A. Zichichi. e idea was to search for lepton pairs, which, in the case of e–µ + would be a clear signature of the Hl. e search started at CERN in 1963, with the PAPLEP experiment, and continued at the e+e– collider ADONE [12] at Frascati in 1967. e Hl did indeed exist, but was found, and called τ, only in 1975 by M. Perl [13] and collaborators at the SPEAR collider, which, differently from ADONE, had enough energy to produce it. irty years ago, A. Zichichi, then president of INFN (Istituto Nationale di Fisica Nucleare), succeeded in having approved by the Italian Parliament the Gran Sasso project, to build a large, technologically advanced, laboratory under the Gran Sasso massive. e laboratory halls were oriented, in particular, toward CERN, in order to be able in a future to host experiments on a neutrino beam from CERN. e dra presented by Zichichi to the Parliament is shown in Fig. 1. e vision started to become reality around 1997. Recalling that accelerators produce (almost pure) νμ beams, the alternative νμ disappearance vs. ντ appearance was open. Notice that they require different characteristics both for the beam and the experiments. Vivid discussions started in the community leading to proposals for both. In particular, the OPERA experiment was proposed in that year by A. Ereditato, K. Niwa The first appearance As mentioned, the beam produced at CERN is mainly composed of νμ with no ντ . Consequently, the observation of any ντ at LNGS must be due to the appearance in the oscillation phenomenon. Experimentally, the two types of neutrinos can be distinguished, when they produce a charged lepton: a νμ produces a μ and a ντ produces a τ. e principle is shown in Fig. 3. In a tracking detector the μ appears as a long almost ECC are, however, only a component of OPERA, which straight track, while the τ, which has a very short life- is a very complex structure: It includes different time, picoseconds, decays already aer few hundreds tracking elements and magnetic spectrometers, develomicrometres. Consequently, to observe the τ track a ped by the collaborators, both INFN groups and from micrometre scale spatial resolution is necessary, some- other countries. e detection principle is shown in thing that only nuclear emulsions can provide. Fig. 4. Further fundamental elements are the automatic However, there is a further problem. Indeed, even a scanning and measuring microscope systems that are distance of 730 km is small for the oscillation pheno- needed to extract the information from the emulsion. menon, because the corresponding 2.5 ms flight time is A big effort was invested in increasing by an order of only a small fraction of the oscillation period. Conse- magnitude the speed of these devices to cope with the quently, only a very small fraction of neutrinos, 1-2%, above mentioned enormous emulsion area. are expected to “oscillate”. Considering in addition the In 2008 – 2009 OPERA has collected about 1/5 of the very small neutrino cross section the conclusion is rea- total foreseen data; of these about 35% have been anached that the detector target mass needs to be lysed and the first ντ candidate event has been already considerably larger than 1000 tons. found[16]. It is shown in Fig. 5. The τ lepton is the is is why the “Emulsion Cloud Chamber” (ECC) short red track. It decays in a charged hadron, presutechnique, so called because it provides images similar mably a pion and a π˚, which in turn decays into 2 to a cloud chamber, was chosen. It is based on sand- gammas, which are detected. Even if the calculated wiches of thin (50 mµ ) emulsion sheets, providing the probability for any background to simulate a τ is only ~1 µm resolution tracking, interleaved with 1 mm 1.8%, it is too early to claim the discovery of the thick Pb sheets, providing the mass. appearance phenomenon. But a few other similar ECC is a rather old technique, which was continually events will hopefully lead to this long-awaited discodeveloped in Japan, with increasing levels of automa- very in the next years. tion of the read-out, mainly at Nagoya. Historically, ECC detectors were successfully employed for the References study of the high-energy cosmic-rays. Particularly important is the discovery by Kiyoshi Niu of a meson [1] R. (D1a9v6i8s)J1r.2,D05.S.. Harmer and K.C. Hoffman, Phys. Rev. Lett. 20 of a new flavour (in modern terms) in 1971 [15]. It was the charm, three years earlier than the discovery [2] J.N(1.B9a6c8h)a1l2l,0N9..A. Bachall and G. Shaviv, Phys. Rev. Lett. 20 of hidden charm with the J/ψ particle by Burton [3] B. Pontecorvo, Zh. Eksp. Teor. Fiz. 53 (1967)1717, [Sov. Phys. Richter and collaborators and Samuel Ting and colla- JETP 26 (1968) 984]. borators. The discovery of the ντ in 2000 by Kimio [4] Y. Fukuda et al., Phys. Rev. Lett. 77 (1996) 1683. Niwa and collaborators [16] at Fermilab was obtained with an evolution of the same technique. OPERA is [5] M.B. Smy et al., Phys. Rev. D 69 (2004) 011104. the latest and largest chapter of this evolution, com- [6] W.PHhayms. Lpeetlt.eBta6l1.,6Ph(2y0s.0L5e)tt1.7B44. 47 (1999) 127; M. Altmann et al., posed, just to give a few numbers, of 150 000 sandwiches, called “bricks”, including about 110 000 m2 [7] N.[AZbh.dEukrsaps.hTietoorv. Feitza.1l.,2J2.E(x2p0.0T2h)e2o1r.1P].hys. 95 (2002) 181, emulsion films and 105 000 m2 lead plates, for a total [8] Q. R. Ahmad et al., Phys. Rev. Lett. 87 (2001) 071301. of about 1250 tons.

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A. Bettini. The appearance of the tau-neutrino, Europhysics News, 25-28, DOI: 10.1051/epn/2010604