Exorcising Ghosts In pursuit of the missing solar neutrinos Andrew Hime After thirty years of hints that electron neutrinos slip in and out of existence, new solar-neutrino experiments may finally If neutrinos have mass, then the three separate particles known as the electron neutrino, the catch them in the act. muon neutrino, and the tau neutrino may not be separate at all, but may mix and transform into one another. In this illustration, a large fraction of the electron neutrinos produced in the core of the sun change their identity before they reach the surface (blue curve). They reappear either as muon and/or tau neutrinos (red and yellow curves, respectively). 36 Los Alamos Science Number 25 1997 Number 25 1997 Los Alamos Science xorcising Ghosts Exorcising Ghosts n October 1920, Sir Arthur Edding- is particularly puzzling because scien- the result would be evidence for physics Figure 1. The Primary Neutrino- Neutrino endpoint ton, one of the foremost astrophysi- tists have failed to find errors in the beyond the Standard Model. The models Reaction Producing Reactions in the Sun 1 ν energy (MeV) cists of the century, delivered his standard theoretical framework of the that emerge from elementary particle p + p D + e + e pp Nearly all the Sun’s energy comes from I 2 ν pp 0.42 residential address to the British Asso- Sun or in the terrestrial experiments physics, astrophysics, and cosmology p + e + p D + e pep the fusion of protons into deuterium iation at Cardiff. In his speech, entitled monitoring the neutrinos. would be subject to a new set of con- pep 1.44 nuclei. The deuterium is converted into The Internal Constitution of the Stars,” Where have the solar neutrinos straints and would have to be modified helium-4 by following one of three reaction 7Be 0.86 e referred to a proposal suggested the gone? One intriguing answer may lie with potentially profound implications. pathways (labeled a, b, and c). Of the 8 ear before by the former president of outside our conventional understanding The status of the solar-neutrino p + D 3He + γ B15 reactions shown, four proceed via charge- he association to bore a hole into the of physics. Whereas a remedy based problem, along with how new experi- changing weak interactions (colored rust of the earth in order to discover the upon modifications in solar models ments propose to solve it, forms the 14 % boxes) and therefore produce electron onditions deep below. Motivated by the appears difficult to construct, scientists central theme of this article. Particular neutrinos. Over 95 percent of the neutri- apid progress in astronomy at the time, are particularly excited about the possi- emphasis is reserved for the Sudbury 3He + 4He 7Be + γ nos are created in the pp fusion reaction. ir Eddington proposed something bility that something profound may hap- Neutrino Observatory, an experiment One proton undergoes inverse beta decay, easier” to penetrate, namely, the Sun. pen to the neutrinos as they make their under construction that promises to 86 % 99 % 0.1 % creating a neutron, positron, and an elec- Eddington could scarcely have antic- way out of the Sun en route to Earth. resolve the question of whether neutrino tron neutrino. The neutron then binds to pated the ramifications of his sugges- We know of three different types, or oscillations, and in particular the MSW 7 7 2 7 ν 7 8 γ the proton to form a deuteron (labeled D). Be Be + e Li + e Be + p B + on. After more than seventy-five years flavors, of neutrinos—the electron, effect, are responsible for the observed Other neutrino-producing reactions are f study, the scientific community’s muon, and tau neutrinos. We also know shortfall of solar neutrinos. pep (electron capture), 7Be (electron cap- nvestigations of our closest star have that the nuclear reactions that power the ture), and 8B (beta decay). Notice that 7Be ielded a remarkably detailed under- Sun are energetic enough to produce 3 3 4 7 4 8 4 1 ν 8Bis needed to produce 8B (dashed box). He + He He + 2p Li + p 2 ( He) B 2( He) + e + e anding of what makes the Sun shine. only electron neutrinos. Moreover, Neutrinos from the Sun Modern experiments, however, observe (a) (b) (c) We now know that the Sun is powered existing experiments that detect solar neutrinos from the pp reaction and 8B y thermonuclear fusion and that its hot neutrinos are only sensitive to the elec- Given the enormous power produced decay, but hardly any from 7Be decay. ore can be considered an immense fur- tron flavor. One can thus speculate that by the Sun and its twenty-billion-year ace producing not only heat and light, some of the electron neutrinos produced lifetime, it is a steadfast conclusion that Figure 2. Solar-Neutrino Spectrum ut also vast numbers of neutrinos. in the Sun have transformed, or the Sun produces energy via thermonu- The total integrated flux of all solar neutri- Because of the Sun’s enormous size, oscillated, into muon and/or tau neutri- clear fusion. During the late 1920s and Range of Sensitivity nos reaching the earth is about 65 billion he light produced deep in its interior nos as they make their way to Earth, early 1930s, theoretical calculations, 0.23 0.8 7.0 per square centimeter per second. In this akes tens of years to reach its surface. thereby escaping our terrestrial detectors. including the seminal work of a young Gallium figure, the neutrino flux and energy are Chlorine During that lengthy journey, the pho- The probability for oscillations to occur Hans Bethe, elucidated our understand- plotted on log scales; so, for example, 1011 Kamiokande ons that rain down upon us as sunlight may even be enhanced in the Sun in an ing of the details of these processes. pp the pp flux is about 50 times greater than nd make our existence on Earth energy-dependent and resonant manner As shown in Figure 1, the fusion of the 7Be flux. Also shown are the spectra ustainable lose all the information as neutrinos emerge from the dense core. protons into helium proceeds via three 7 of neutrinos produced from the CNO /s) Be oncerning the detailed processes of This phenomenon, an example of the branches. Neutrinos are created in four 2 109 cycle (gray curves). The pp, 8B, and CNO 13 he stellar core. Unlike photons, neutri- Mikheyev, Smirnov, and Wolfenstein different reactions, referred to simply as CNO ( N) neutrinos are created in beta decay reac- os interact so feebly with matter that (MSW) effect, is considered by many the pp, pep, beryllium-7 (7Be), and pep tions. A neutrino so produced shares 8 hey escape from the Sun in about scientists to be the most favored solution boron-8 ( B) reactions. The neutrinos 7 energy with another light particle. Hence, 10 15 seconds. They arrive on Earth a mere to the solar-neutrino problem. flee the Sun and begin their voyage to CNO ( O) all those neutrinos have a broad energy minutes later, and thus the solar Neutrino oscillations, or the periodic Earth. (In Figure 1, we have omitted spectrum. The 7Be and pep neutrinos eutrinos are a unique probe of a star’s changes in neutrino flavor, require that neutrinos that emerge from the carbon- 8B result from electron capture: A proton in 105 CNO (17F) nnermost regions and of the nuclear neutrinos possess mass and that neutrino nitrogen-oxygen, or CNO, cycle. The a nucleus captures an electron from an eactions that fuel them. flavor not be conserved in nature. No cycle is another, though less important, Neutrino Flux (number /cm atomic orbital, turns into a neutron, and During the past thirty years, detailed undebated evidence for neutrino mass set of neutrino-producing reactions a monoenergetic neutrino is created. 3 heoretical and experimental studies exists despite years of painstaking in the Sun.) 10 The sensitivity range of the various solar- ave resulted in very precise predic- research around the world. Indeed, the Figure 2 shows the predictions of the neutrino experiments is also shown here. hep ons about the fluxes and energy spec- Standard Model of elementary particles standard solar model for the flux of The gallium experiments have energy a of neutrinos produced deep within requires that neutrinos be strictly mass- electron neutrinos at the earth’s surface. thresholds around 0.23 MeV and are sen- he Sun. But a problem has emerged. less. Nonetheless, quests for a Grand The flux is the number of neutrinos per 0.1 1.0 10.0 sitive to all solar neutrinos. The chlorine our different experiments have mea- Unified Theory of the fundamental square centimeter per second. (The fig- Neutrino Energy (MeV) experiment detects neutrinos from the ured the flux of solar neutrinos, and forces in nature suggest that neutrinos, ure assumes no electron neutrinos have 7Be and CNO reactions, but is primarily very one of them reports a flux that is like other elementary particles, should oscillated into a different flavor.) The pp sensitive to those from the 8B reaction. gnificantly below theoretical predic- have mass. Consequently, should the reaction is the primary mode of neutrino Kamiokande is a water Cerenkov detector ons. The discrepancy is referred to solar-neutrino problem be resolved by production, and the reaction completely that can detect only the high-energy s the solar-neutrino problem, and it invoking neutrino mass and oscillations, dominates energy production in the Sun. portion of 8B neutrinos. 38 Los Alamos Science Number 25 1997 Number 25 1997 Los Alamos Science xorcising Ghosts Exorcising Ghosts onsequently, it is ultimately linked to to the neutrino.