What Have We Learned About Solar Neutrinos? by JOHN N. BAHCALL The apparent deficit of solar neutrinos may be caused by physical processes beyond the Standard Model. HIRTY YEARS AGO Ray Davis—then working at Brookhaven and now at Pennsylvania—suggested it was practical Tto build an experiment to detect solar neutrinos if the event rate I calculated was correct. The proposal was based upon his Texperience at the Savannah River reactor trying to detect antineutrinos using a tank filled with 3,000 gallons of perchloroethylene (C2 Cl4, a common cleaning fluid), and on calculations that I had done of the event rate to be expected in a 100,000 gallon tank. 10 FALL 1994 These calculations were in turn based upon nuclear physics estimates of the neutrino capture rates and so- Chlorine lar model calculations of the neu- Comparisons between the predicted fluxes of the standard trino fluxes. Ray was confident that solar model and the four operating 8.0±1.0 he could build and successfully op- solar neutrino experiments. The unit erate the 100,000 gallon tank, ex- used for the three radiochemical 2.55±0.25 tracting the few radioactive atoms of experiments is a solar neutrino unit, Water argon produced each month due to or SNU, which equals one event per 36 neutrino capture by chlorine atoms second per 10 target atoms. 1.0±0.14 (37Cl → 37Ar) in this huge detector. Following the experimenters, the Kamiokande result is expressed in Thirty years later it is clear that he terms of a ratio to the expected 0.51±0.07 was right and the then-abundant event rate. Predictions shown in this Gallium 73±19 skeptics were wrong. figure and quoted elsewhere in this At the time this chlorine experi- article are from the Bahcall- ment was proposed, the only moti- Pinsonneault 1992 standard solar vation either of us presented for per- model with helium diffusion. The 132±7 forming a solar neutrino experiment observed rates are less than the was to use neutrinos “to see into the expected rates for all four experiments. 79±12 interior of a star and thus directly ver- TheoryExperiment 1σ Errors ify the hypothesis of nuclear ener- gy generation in stars.” The hypoth- esis being tested was that stars like the Sun shine by fusing protons to neutrino experiments were discussed are not accessible in laboratory ex- form alpha particles, positrons, neu- by Ken Lande in the Fall 1992 Beam periments. Such searches for new trinos, and thermal energy. Line; they are summarized in the physics are based upon quantitative The original goal of demonstrat- table on the next page. The results of discrepancies between the predic- ing that proton fusion is the origin of these experiments represent a tri- tions for and the observations of so- sunshine has been achieved. Solar umph for the combined physics, lar neutrinos. As the experiments and neutrinos have now been observed chemistry, and astronomy commu- the theoretical predictions have in four different experiments with (to nities because they bring to a suc- steadily improved over the past three usual astronomical accuracy) fluxes cessful conclusion the development decades, these discrepancies have res- and energies that are in rough agree- (which spanned much of the twen- olutely refused to go away, convinc- ment with the expected values. The tieth century) of a theory of how or- ing many of us who work in this field observed rates in all of the solar neu- dinary stars—those like the Sun— that we have been witnessing the dis- trino experiments are only about a shine. covery of new physics in an unex- factor of 2 or 3 less than expected (see pected context. chart on this page). Moreover, the OST OF THE CURRENT Although thirty years ago I was fact that the neutrinos indeed come interest in solar neutrinos a skeptic about the theory of stellar directly from the Sun was established Mis focused on an applica- evolution and did not believe in any by one of these experiments tion of this research that was not explanation of astronomical phe- (Kamiokande), which showed that even discussed when the Homestake nomena that required changing con- electrons scattered by interacting chlorine detector was proposed. Sci- ventional physics, my preconcep- neutrinos recoil in the forward di- entists have realized that they can tions have since been shaken by the rection—away from the Sun. The use solar neutrinos for studying as- robustness of the theory and by the characteristics of the operating solar pects of the weak interactions that combined results of the four solar BEAM LINE 11 Operating Solar Neutrino Experiments Name Target Mass Threshold Detector Type Location (tons) (MeV) Homestake 37Cl 615 0.86 radiochemical Black Hills, South Dakota Kamiokande H2O 680 7.5 electronic Japanese Alps (Kamiokande) registers both electron neutrinos and (with much reduced GALLEX 71Ga 30 0.2 radiochemical Gran Sasso, Italy sensitivity) muon or tau neutrinos. Do electron neutrinos change SAGE 71Ga 57 0.2 radiochemical Caucasus their flavor, or “oscillate,” into hard- Mtns., Russia to-detect muon or tau neutrinos dur- ing their journey from the interior of the Sun to the Earth? The simplest neutrino experiments. I now think it version of the standard electroweak is most likely that we are witnessing model answers “No.” Neutrinos evidence for new physics in these ex- have zero masses in the Standard periments. Model, and lepton flavor does not Solar neutrino observations are of- change. However, solar neutrinos can ten compared to a combined theo- reveal physical processes not yet dis- retical model, the standard solar covered in the laboratory because, for model plus the Standard Model of certain processes, these experiments electroweak interactions. A solar are 1011 times more sensitive than model is required to predict how terrestrial neutrino experiments. many—and with what energies— Their increased sensitivity is due neutrinos are produced in the Sun’s largely to the fact that the elapsed interior. The observed luminosity of time in the rest frame of a (finite- the sun (due to the same nuclear mass) neutrino is proportional to the processes that produce solar neutri- ratio of the target–detector separa- nos) and the other observational con- tion to the neutrino energy; this ra- straints on the solar model (includ- tio is much larger for neutrinos orig- ing the Sun’s known age, mass, inating in the Sun. Moreover, solar chemical composition, and its many neutrinos traverse a far greater precisely measured seismological fre- amount of matter than their labo- quencies) limit the calculated fluxes ratory counterparts. to fairly narrow regions, at least by astrophysical standards (see box on HE FIRST, and for two next page). decades the only, solar neu- The standard electroweak mod- Ttrino experiment uses a chlo- el—or some modification of the Stan- rine detector to observe electron-type dard Model—is required to determine neutrinos via the reaction v + 37Cl − e what happens to neutrinos as they → e + 37Ar. The 37Ar atoms produced pass through the Sun and interplan- by this neutrino capture process are etary space on their way from the so- extracted chemically from the 615 lar interior to earthbound detectors. tons of perchloroethylene in which The observed discrepancies might oc- they were created; they are then cur if neutrinos decay in transit, or counted using their characteristic if they change from one species to an- radioactivity in small, gaseous pro- other before reaching the detectors. portional counters. The threshold The three radiochemical detectors energy is 0.8 MeV, which means (see register only electron neutrinos, figure on the next page) that this while the only electronic detector experiment is sensitive to the rare 12 FALL 1994 SOLAR NEUTRINO FLUXES he spectrum of solar neutri- nos that is predicted by the high-energy 8B neutrinos, as well as threshold is at least 7.5 MeV. The Tstandard solar model is to the lower energy pep and 7Be neu- probability of detecting muon or tau shown in the graph below. The basic low-energy neutrino fluxes, trinos formed by electron capture on neutrinos by their scattering of atom- 7 from pp and pep neutrinos, are two fusing protons and on Be nuclei. ic electrons is only about 17 percent most closely related to the total Like all solar neutrino experiments, of the equivalent probability of de- solar luminosity and are calculated the chlorine experiment is performed tecting electron neutrinos at the en- to an estimated accuracy of about deep underground (in the Homestake ergies for which Kamiokande is sen- 1 percent. These reactions initiate gold mine, in Lead, South Dakota) in sitive. the nuclear fusion chain in the Sun order to avoid cosmic-ray induced Two gallium experiments are in and produce neutrinos with a max- imum energy of 0.4 MeV (pp neu- events that might be confused with progress, GALLEX (located in the trinos) or an energy of 1.4 MeV true neutrino events. Gran Sasso underground laboratory (pep neutrinos). Electron-capture In the Kamiokande experiment, about an hour’s drive from Rome) by 7Be ions produces the next which is carried out in Kamioka and SAGE (in an underground cham- most abundant source of neutri- mine in the Japanese Alps, neutrino- ber excavated beneath the Andyrchi nos, a 0.86 MeV neutrino line, electron scattering, v + e → v' + e', mountains in the North Caucasus whose flux has an estimated theo- occurs inside the fiducial mass of region of Russia). Performed by two retical error of 6 percent. Neutrinos 8 680 tons of ultrapure water. The scat- international collaborations, these from the beta decay of B can have energies as high as 14 MeV; tered electrons are detected by the experiments provided the first ob- they are rare and their flux is cal- Cerenkov light that they produce servational information about the culated to an estimated accuracy while speeding through the water.