Physics

monitor The temperature of the incandescent gas Electromagnetic energy (photons) from nu­ (mainly hydrogen) ball of the Sun ranges clear reactions deep inside the Sun can take from 6000 degrees at the surface to 15 mil­ millions of years to migrate to the surface lion degrees at its centre, where proton and escape. Neutrinos on the other hand nuclei fuse together into deuterium, liberat­ give a unique glimpse deep into the Sun's in­ ing a positron and a neutrino. Subsequently terior, but the interpretation requires a repre­ other reactions produce additional neutrinos, sentative sample of these elusive particles. but most solar neutrinos emanate from the central fusion process.

NEUTRINOS Moriond spotlight

The regular 'Rencontres de Mo­ riond' meetings in the French Alps, which celebrate their 25th anniver­ sary this year, have a strong tradi­ tion of reflecting new trends in physics thinking and January's ses­ sion on Tests of Fundamental Laws in Physics' was no exception. The spotlight this time fell on the neutrino sector, a branch of phy­ sics frequently in evolution, if not controversial. Currently the solar neutrino problem is still a preoccu­ pation, while a wave of new heavy neutrino results also awaits clarifi­ cation. The Moriond neutrino sessions began with a review by D.Vignaud of the status of the 'solar neutrino problem' - the discrepancy be­ tween theoretical predictions (based on solar model calculations) and the experimentally observed chemical method had to be devel­ mately 6 x 1010 per sq cm per s. fluxes of neutrinos from the Sun. oped to extract and detect the pre­ The predicted value of the bor­ So far solar neutrinos have been cious few unstable argon-37 atoms on-8 neutrino flux is extremely sen­ detected in two experiments: the (half-life 35 days) produced by the sitive to the temperature in the pioneer study led by Ray Davis, solar neutrinos during exposures Sun's core where these neutrinos which began data-taking in 1970 ranging typically from 35 to 60 are thought to be produced. and continues to run, and the Kam- days. Approximately 14 per cent of iokande II collaboration, operational The Kamiokande II collaboration the argon-37 production rate seen in Japan since 1987. looks for Cherenkov radiation from by Davis' team is predicted to be The former uses the chlorine- neutrino interactions in a tank con­ due to monochromatic 0.86 MeV 37/argon-37 radiochemical method taining 3000 tons of water, of neutrinos produced with lithium-7 for detecting solar neutrinos, sug­ which only 680 tons are used for by electron capture in solar beryl- gested in 1946 by Bruno Pontecor- solar neutrino detection. The de­ lium-7. vo and somewhat later indepen­ tector is presently capable of regis­ The solar-neutrino-induced event dently by Luis Alvarez. Neutrino tering solar neutrinos above 7.5 rates observed by Davis' team and capture in chlorine-37 has a thre­ MeV. Kamiokande II are typically several shold energy of 0.816 MeV. Davis' Both studies register neutrinos times smaller than predictions team uses a tank filled with 615 supposedly emitted in the decay of based on detailed solar model cal­ tons of perchloroethylene, and the boron-8 with energies up to 14 culations. Estimates of the uncer­ observed average rate of argon-37 MeV. According to current wis­ tainties in these calculations indi­ production over 20 years is ap­ dom, boron-8 neutrinos are only a cate that the discrepancy between proximately one atom every two small fraction (one in ten thousand) the predicted and the observed ev­ days. of the total flux of solar neutrinos ent rates could actually be smaller. A remarkably efficient radio- on the Earth's surface - approxi­ It was suggested in 1982 by

16 CERN Courier, May 1991

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18 CERN Courier, May 1991 The solar neutrino flux (compared with the predictions of the Standard Solar Model - SSM) as seen by the Kamiokande II detector in Japan shows no appreciable time vari­ ation. Some hypotheses call for the solar neutrino signal to be linked with the sunspot cycle.

gy balance, their flux can be calcu­ lated more accurately than that of boron-8 neutrinos. The theoretically predicted gal- lium-71/germanium-71 conversion rate in SAGE and Gallex due to so­ lar neutrinos is approximately one atom per day, requiring the perfec­ tion of very efficient techniques to extract and detect the few german- ium-71 atoms produced in the 30 ton detectors during typical expo­ sures of 20-30 days. Results from the first five phy­ sics runs of the SAGE collabora­ tion, which took place between January and July last year, were presented at the Workshop by V. Gavrin. The observed rate of ger- manium-71 formation was found to be slightly above that expected due to background processes, with an upper limit of 74 SNU (each Solar Neutrino Unit - SNU - represents G.A. Bazilevskaya et al. that the viet-American Gallium Experiment 10~36 solar neutrino captures per solar neutrino flux detected by Dav­ in the Baksan Neutrino Observatory second per atom of target), smaller is' team varies in time, being - June 1990, page 16) and Gallex than the theoretically predicted rate (anti)correlated* with the solar activ­ (built by a collaboration of scien­ of 132 SNU. (The contributions ity. A measure of the Sun's activity tists from France, Germany, Israel, due to proton fusion, beryllium-7 is the number of sunspots on its Italy and the US and installed in the and boron-8 neutrinos in the calcu­ surface, in turn related to the Italian Gran Sasso Laboratory, Jan­ lated rate are 71, 34 and 14 SNU, strength of the toroidal component uary/February, page 10). respectively.) of the solar magnetic field. The lat­ Rather than looking at a remote A crucial step in the interpreta­ ter is known to vary over a 11-year fringe of the solar neutrino spec­ tion of these results will be half-cycle, the last two maxima of trum, the major aim of these two measurements of germanium-71 the sunspot number having been in experiments is to detect the major produced by neutrinos from a cali­ 1979-1980 and in 1989-1990. portion of the solar neutrinos - brated source. D. Vignaud presented the results those accompanying proton fusion A status report of the Gallex ex­ of four independent statistical ana­ into deuterium, with a maximal en­ periment was given by D. Vignaud. lyses of data from Davis' team, all ergy of 0.42 MeV. This fusion is Here data-taking is not possible for reporting evidence for (anti)corre- the first of a series of reactions ev­ the time being because of contami­ lation, although at very different entually producing helium-4 and nation of the detector by an unex­ levels of statistical significance. providing more than 98 per cent of pectedly large amount of radioac­ The results of the Kamiokande II the Sun's energy. tive germanium-68. This isotope collaboration on possible variation Measurements of this part of the was probably produced when the of the solar neutrino flux from solar neutrino flux would test basic gallium-71 source material for the 1987-1990, presented by Y. Suzu­ ideas about the processes taking detector was stored for a certain ki, show no time variations. place in the initial stage of stellar time prior to installation under­ Solar neutrino studies have been evolution. Since these neutrinos ground without being protected boosted recently by the arrival of come from a reaction which plays a from cosmic rays. This problem is two new detectors - SAGE (So­ fundamental role in the solar ener­ being attacked.

CERN Courier, May 1991 19 / /Des/70/77/r?es /'etc/esmachhes '/ Aya/7ttoi/tc/es/?om/T7es. Qc//, c7rec/estec/?/70- Ac/xJ coc/Zec/rsetd/arotat/Ve aAac/teper/br/7?a/7ce j /og/es/70i/re//es, rorttp/c/s/o//t c/a/7s/ec/rsex/- o/7t ete//?tegres tot/s/es se/v/ces co/Hp/erneM- M ge/7ces. @i/<7//te, rc7t/o/7d//sc7t/or? c/c/ trwa//, co/7se/7 ta/res.'fc7fo/7/7age, gest/0/7 c/'ac/resses, roc/tage. M c/e/ei/rs c//e/?ts, //so/?toptepO£/r/e/r?c7x//r7i/rr7. lesPC/so/7ta£//oc/^/;c//i//7ee/7trepr/se/7c7t/o- M Pi//s c/esmc7c/?//7es. lesr°Clo/7t/r7/sesi/r/'c7YC7/7ce na/e. £//es/mpr/'/ne/Jt/oi/rrtcwx, per/oc/Zc/c/es, M tec/wo/og/c/i/e. ZeZegestZos?, pZrotocomposZtZo/?a ZmprZ/nespc/McZtaZresarecZesZtom/nesetZes m gra/?c/e capacZte, cocZe-£c7rress0f7tcZeye/7i/sro£/tZ/?e. /no/e/is cZe cZe/naZs?.

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20 Courrier CERN, janvier/fevrier 1991 The solar neutrino results of the of solar neutrinos from proton fu­ the neutrino mass and mixing hy­ Davis, Kamiokande II and SAGE ex­ sion. pothesis the electron-, muon- and periments could have important im­ Measurements of the overall so­ tau-type neutrinos do not have def­ plications for particle physics, sig­ lar neutrino spectrum and its differ­ inite masses, but rather are super­ nalling unconventional neutrino pro­ ent reaction components, which positions of at least three neutrino perties (like nonzero mass and mix­ probably will not materialize before states of definite mass, at least ing and/or a significant magnetic the end of the century, will reveal some of which are nonzero. moment). The latter could imply nuclear reactions deep inside the The standard electroweak theory partial conversion (due to vacuum Sun, giving new information about has only massless neutrinos, so or matter-enhanced oscillations solar physics. that observation of a 17 keV neu­ and/or spin-flavour precession) of The 17 keV neutrino (April, page trino could be the first evidence for solar electron-type neutrinos into 9) made a reappearance at Mo­ the incompleteness of this theory. neutrinos of a different kind, unde­ riond, the first evidence (in the beta Nonzero neutrino masses and tectable by the current experi­ decay of tritium) having been pre­ mixing imply a remarkably rich ments, the transitions occurring sented by John Simpson in the spectrum of possible neutrino pro­ during the solar neutrinos' journey January 1986 session. Sub­ perties, a whole 'new world' of ele­ from the central region of the Sun sequently, six spectrometer experi­ mentary particle physics. In particu­ to the Earth's surface. ments looking for possible distor­ lar, the electrically neutral massive Hopefully new results from tions of the beta-spectra of sul- neutrinos can be Dirac-type (having SAGE and Gallex over the next four phur-35 and nickel-63 due to the distinct antiparticles) or Majorana- years will help identify the cause of emission of a 17 keV mass neutri­ type particles (coinciding with their the solar neutrino problem, now no were performed between 1985 own antiparticles), the type being more than fifteen years old. and 1989, all reporting negative re­ determined by the symmetries of With the Japanese Govern­ sults. the underlying theory. ment's December approval for the Meanwhile two additional experi­ Dirac neutrinos can have intrinsic improved 'Super-Kamiokande' de­ ments (with tritium and sulphur-35 characteristics like magnetic and el­ tector (see page 9, reviewed at implanted in semiconductor detec­ ectric dipole moments which are Moriond by Y. Suzuki), there are tors) by Simpson and his student zero for the Majorana neutrinos. three more solar neutrino experi­ Andrew Hime confirmed the 17- Phenomenological considerations ments in preparation (the other two keV sighting. Searches for a 17 indicate that the 17 keV neutrino being the Sudbury heavy water ex­ keV neutrino continued by Hime should be a Dirac particle if there periment in Canada - January/Fe­ and Nick Jelley at Oxford (with a are only three massive neutrinos bruary 1990, page 23, and the sulphur-35 source), by Eric Nor­ and the neutrino oscillations are the Baksan chlorine-37 experiment, man's group at Berkeley (with a cause for the solar neutrino prob­ five times bigger than Davis'). carbon-14 source), by a group at lem. It can be a Majorana particle All three are expected to accu­ Zagreb (with germanium-71), all us­ if, for instance, there are more than mulate much higher statistics than ing semiconductor detectors and three neutrinos with definite mass. their predecessors. Although sensi­ by a group at Caltech using a sul­ In this case at least one more 'hea­ tive only to boron-8 and beryllium- phur-35 source with a spectro­ vy' Majorana neutrino should exist. 7 neutrinos, they will be capable of meter. A 17 keV neutrino cannot be measuring the total flux and spec­ Initial data from all these experi­ stable: cosmological arguments trum of boron-8 neutrinos with ments were reported at the Work­ suggest that it should decay suffi­ high accuracy. The first results on shop, the first three of seeing di­ ciently fast into, e.g., three light this spectrum from Kamiokande II stortions in the beta spectra com­ neutrinos, or a light particle and gave an initial glimpse of its shape. patible with emission of a 17 keV neutrino. It should also show up in Talks by R. Lannou, G. Zacek neutrino, while the Caltech-based oscillation experiments with elec­ and R. Gaitskell on new methods of group gives a negative result. tron neutrino (and antineutrino) solar neutrino detection heralded With the electron coupled to a beams, leading to effects (for ex­ the dawn of low energy threshold, 17 keV neutrino as well as a mass- ample the 'appearance' of tau neu­ high statistics, real-time detectors less (or lighter than 10 eV) neutri­ trinos) of the order of one or two capable of measuring the spectrum no, a new scenario is called for. In percent.

CERN Courier, May 1991 21 * The June issue will include an Dimitri Nanopoulos - strengthening links article on the COBE results. between particle physics and cosmology

Also reported at the Workshop ing Supercollider Laboratory. It at­ were new results from cosmic ray tracted many distinguished speak­ studies; searches for dark matter, ers in this rapidly evolving field, re­ deviations from Newtonian gravita­ sulting in a wide-ranging and stimu­ tion, time-reversal violation in beta lating scientific programme. decay, the electric dipole moment CERN's discussed the of the neutron, and neutron-anti- Standard Model of Particle Physics neutron oscillations; strong field and beyond, and the implications tests of gravitational theories and of recent LEP data (April, page 1). other subjects. Rocky Kolb of Fermilab gave an in­ For the first time the session had troduction to the Standard Big an interdisciplinary character, with Bang, while School Director Dimitri an invited lecture by Ed Fredkin on Nanopoulos of A&M and 'Digital Mechanics: the Universe as HARC presented a unified view of a Computer'. The improvised even­ the two fields. ing concert of classical music, David Schramm of Chicago dis­ given by attending physicists Mi­ cussed the important issue of pri­ chael and Myriam Treichel, Jim mordial nucleosynthesis, with the ports on 17 keV neutrinos (see Faller, Elisabeth Ribs and Tibault observational basis covered by page 21). John Bahcall of Princeton Damour added to the pleasant in­ Greg Shields of Texas (Austin). Ro­ was among the neutrino speakers formal atmosphere which is one of bert Wagoner of Stanford exam­ at the Texas meeting. the elements of Moriond success. ined probes of the Universe at all The status of Big Science was il­ scales, from nuclei to supernovae. lustrated in talks on the US Super­ By S.T. Petcov Andre Linde, now at Stanford, conducting Supercollider (SSC) looked back to the very early from Fred Gilman, SSC's Associate stages of the Universe, including Director of Physics Research; on ASTROPARTICLE the inflation mechanism linking the NASA's role from Venon Jones; PHYSICS initial Big Bang to present large- on the 's scale structure. work from Giacomo Cavallo; and New synergy Mark Srednicki of Santa Barbara on Hubble telescope from Peter reviewed the need for Dark Matter, Stockman of the Space Telescope Two major recent experimental re­ leaving his colleague David Cald­ Science Institute. sults have further strengthened the well to look at experimental The meeting showed that while links between particle physics and searches for it. George Smoot of the 'Standard Models' of both par­ cosmology. These are the confir­ Berkeley discussed the beautiful ticle physics and cosmology were mation by experiments at CERN's COBE results confirming the nature doing fine, some refinements are LEP electron-positron collider that of the cosmic background radi­ necessary, especially for the Big there are only three species of light ation, while Alan Dressier of Mt. Bang picture, at a loss to explain neutrino, as predicted by Grand Wilson and Las Campanas Obser­ new large-scale structure. New si­ Unified Theories and needed for vatories reviewed the intriguing mulations show that even cold dark primordial nucleosynthesis, and the large-scale structures, including the matter, until now the best candi­ results from the US Cosmic Back­ great attractor, the great wall and date for the missing material of the ground Explorer (COBE) satellite similar concentrations of matter, Universe, may not fit the bill. New that show beyond any doubt that revealed by recent astronomical observations over the next few the cosmic background radiation is surveys. Nicola Vittorio of D'Aquila years will help to clarify many of primordial.* and Joe Silk of Berkeley tried to the major issues in both particle With this in mind, a new inter­ make sense of it all. physics and cosmology. Hopefully national school was initiated re­ Neutrinos are never far from the a clearer picture will emerge before cently by 's Advanced Re­ physics headlines - currently solar the second school in this series. search Center (HARC) and co-spon­ neutrino observations are in a state sored by the nearby Superconduct­ of flux and there is a spate of re­ From Dimitri Nanopoulos

22 CERN Courier, May 1991