Search

SOUDAN 2 is a 960-ton tracking calorimeter which has been constructed to search for nucleon decay and other phenomena. The full detector consists of 224 calorimeter modules each weighing 4.3 tons. The modules consist of finely segmented iron instrumented with 1 m long drift tubes of 15 mm internal diameter. The tubes enable three spatial coordinates and dE/ dx to be recorded for charged particles traversing the tubes. The spatial resolution is 0.38 cm in the x-y plane and 0.65 cm in the z, or drift, direction. The operation and performance of the modules are discussed.

Muons recorded in the Soudan 2 underground nucleon decay detector from January 1989 to February 1991 have been examined for any correlation with the radio flares of Cygnus X-3 observed during this period. On two nearby days during the radio flare of January 1991 a total of 32 muons within 2.0° of the Cygnus X-3 direction were observed when 11.4 were expected.

SOUDAN 2 is a 960-ton tracking calorimeter which has been constructed to search for nucleon decay and other phenomena. The full detector consists of 224 calorimeter modules each weighing 4.3 tons. The design and construction of the modules are described. The modules consist of finely segmented iron instrumented with 1 m long drift tubes of 15 mm internal diameter. The tubes enable three spatial coordinates and dE/dx to be recorded for charged particles traversing the tubes.

I welcome this opportunity to celebrate the marvellous accomplishments of CERN, which is arguably the most successful of all international organizations and a showpiece of world-wide cooperation. I have been a visitor here on so many occasions: first as an NSF fellow in 1959–60, then as a paid visitor for a semester, as a frequent summer drop-in, conference participant, and member (now “old-boy”) of the Science Policy Committee. These experiences have been central to the evolution of my own career in theoretical physics. Many of the experimental discoveries underlying our Standard Model have taken place here, among them the discoveries of neutral currents, weak intermediaries, and the many precision tests of the electroweak model carried out at LEP. But of equal importance is the fact that so many crucial developments in fundamental theory were either initiated, nurtured or perfected at CERN, by both its resident and visiting theorists. This is certainly so for me, as I am certain it is for many of my distinguished theoretical colleagues. CERN has always been, and must continue to be, the place where the action is, the Grand Central Station of particle physics, the crossroads of thousands of individual physicists’ lives.

Threefold maximal mixing would imply a cyclic permutation symmetry among the generations. The data from solar and atmospheric neutrino experiments are consistent with such mixing, and require a hierarchical spectrum of mass-squared differences for the neutrinos. A fit for the two independent mass-squared differences (excluding only the HOMESTAKE solar neutrino result) yields: Δm2 = (0.72±0.18) × 10−2 eV2 and Δm′2 < 0.9 × 10−11 eV2 at 90% confidence. In the case that the neutrino mass spectrum is qualitatively similar to that of the charged leptons and quarks, these results can be re-expressed in terms of the neutrino masses as follows: m3 ⋍ 85 ± 10 meV, and m1,m2 < 3 μeV at 90% confidence. Implications for future experiments are discussed.

Atmospheric neutrino fluxes have been computed directly from recent measurements of muon momentum spectra in the stratosphere. The results agree to within about 15% with the previous calculations by Barr, Gaisser and Stanev by another method, although they predict a somewhat different variation of flux with geomagnetic latitude. These results do not support the suggestion that the atmospheric neutrino anomaly can be interpreted as due to excess electron events from proton decay p → e + vv .

A re-calculation of expected rates of upward muons in underground detectors brings out major uncertainties in such estimates, which may be as large as 40%. Interpretations in terms of neutrino oscillations are correspondingly dubious.

First results on contained neutrino interactions are presented from the Soudan 2 proton decay experiment. 20 neutrino events have been found in a fiducial exposure of 0.29 kton years.

Following recent results from the SNO solar neutrino experiment and the K2K long-baseline neutrino experiment, the combined existing data on neutrino oscillations now point strongly to a specific form for the lepton mixing matrix, with effective bimaximal mixing of $\nu_{\mu}$ and $\nu_{\tau}$ at the atmospheric scale and effective trimaximal mixing for $\nu_e$ with $\nu_{\mu}$ and $\nu_{\tau}$ at the solar scale (hence `tri-bimaximal' mixing). We give simple mass-matrices leading to tri-bimaximal mixing, and discuss its relation to the Fritzsch-Xing democratic ansatz., Comment: 12 Pages in LaTeX, 2 Figures as postscript files

Classical and quantum pictures of interactions Classically, interaction at a distance, e.g. in electromagnetism, is described in terms of a potential or field due to one charged particle acting on another, the field permeating the whole of space around the source charge. A conceptually more appealing idea, perhaps, is that of an exchange interaction, where the object exchanged carries momentum from one charge to the other, the rate of exchange of momentum providing the force. A frequently quoted everyday example is of two ice-skaters travelling side by side, initially moving parallel, who start to diverge as they repel one another by throwing a ball back and forth. In quantum theory, action at a distance is indeed viewed in terms of an exchange interaction, the exchange being of a specific quantum (a boson) associated with the particular type of interaction. Since the quantum carries momentum and energy, say Δ E , the conservation laws can only be satisfied if the process takes place within a timescale Δ t limited by the Uncertainty Principle, i.e. Δ E Δ t ≃ ħ. Such transient quanta are said to be virtual . The quantum concept of an electromagnetic interaction is of the continual exchange (emission and absorption) of virtual photons by the charges, which is no more or less fictitious than the classical concept of an all-pervading field surrounding a source charge. Neither the field nor the virtual quanta are directly observable; it is the force that is the measured quantity.

We re-fit for the neutrino mass-squared difference $\Delta m^2$ in the threefold maximal (ie. tri-maximal) mixing scenario using recent CHOOZ and SUPER-K data, taking account of matter effects in the Earth. While matter effects have little influence on reactor experiments and proposed long-baseline accelerator experiments with $L \simlt 1000 km$, they are highly significant for atmospheric experiments, suppressing naturally $\nu_e$ mixing and enhancing $\nu_{\mu} - \nu_{\tau}$ mixing, so as to effectively remove the experimental distinction between threefold maximal and twofold maximal $\nu_{\mu}-\nu_{\tau}$ mixing. Threefold maximal mixing is fully consistent with the CHOOZ and SUPER-K data and the best-fit value for the neutrino mass-squared difference is $\Delta m^2 = (0.98 \pm^{0.30}_{0.23}) \times 10^{-3} eV^2$., Comment: 17 Pages LaTex, plus 8 Figures as postscript files

The phenomenological case for threefold maximal lepton mixing is strengthened by more detailed comparison with existing data. The atmospheric neutrino data are self-consistent and in good agreement with the threefold maximal mixing prediction R = 2/3 (chi-squared/dof = 4.3/5, CL = 51%). Including partially con- tained event data, the zenith angle dependence also points to threefold maximal mixing, ruling-out a range of mixing schemes, including that of Fritzsch and Xing. Accounting for `de facto' uncertainties in solar neutrino fluxes, the solar neutrino data (including HOMESTAKE) are shown to be consistent with threefold maximal mixing (chi-squared/dof = 5.1/3, CL = 16%), as is the upper limit from the LSND appearance experiment. Detailed pred- ictions for forthcoming long-base-line reactor and accelerator experiments are presented., Comment: 15 Pages, LaTex, 2 tables + 6 figures as postscript files

We investigate the possible influence of the MSW effect on the expectations for the solar neutrino experiments in the maximal mixing scenario suggested by the atmospheric neutrino data. A direct numerical calculation of matter induced effects in the Sun shows that the naive vacuum predictions are left completely undisturbed in the particular case of maximal mixing, so that the MSW effect turns out to be unobservable. We give a qualitative explanation of this result., Comment: 8 Pages, LaTex, 4 figures as postscript files

Recently, a few groups1 have made attempts to simultaneously measure the electromagnetic and deep underground muon components of cosmic ray showers. This is one such experiment, using a small proportional tube surface air shower detector in coincidence with the Soudan 2 nucleon decay detector. It has run since June, 1991, and some 22 000 events which triggered both detectors have been recorded. This body of data may provide insight into the composition of cosmic rays in the “knee” area of the cosmic ray spectrum.

A search for grand unified theory magnetic monopoles making highly ionizing tracks in argon gas has been conducted using the Soudan 2 nucleon decay detector. This underground detector is a large fine-grained tracking calorimeter comprised of long drift tubes read out by proportional wires. No monopole candidates were observed in data taken over almost three years, yielding an upper flux limit of 8.7{times}10{sup {minus}15} cm{sup {minus}2}s{sup {minus}1}sr{sup {minus}1} for monopole velocities of {beta}{gt}2{times}10{sup {minus}3}.

Moments of the quark-singlet fragmentation function are determined from measured moments of the u-quark fragmentation functions D ± u ( z , q 2 ) into positive and negative hadrons in the reaction ν p→ μ − + hadrons. The q 2 dependence of these moments is investigated in the framework of leading-order QCD and estimates are given for the moments of the gluon fragmentation function. The results are based on a sample of θ 7800 charged current events from a BEBC neutrino-hydrogen experiment.

In a neutrino-proton experiment performed with the bubble chamber BEBC, which was exposed to the wide-band neutrino beam of the CERN SPS, the semi-inclusive reaction ν p→ μ − h ± + X is studied where h ± is a hadron of a chosen type. Non-factorization of the semi-inclusive cross section is investigated in terms of double moments with respect to Bjorken- x and the energy fraction z and comparisons are made with predictions of perturbative QCD in next-to-leading order.

An analysis of the transverse momentum distributions of secondary hadrons produced in neutrino-proton charged current interactions is presented. It is found thatparticles going forward in the hadronic c.m.s. have a different behaviour from those going backward. A rise of the average transverse momentum squared as a function of both the invariant mass of the hadronic system and the fractional energy carried by the hadron in the c.m. frame, is observed. The result is not in good agreement with the predictions of a QCD based model if the non-perturbative parts are forward-backward symmetric. Using events with no missing neutral particles in the final state the average primordial transverse momentum squared of the quark inside the nucleon is estimated to be ∼0.3 GeV 2 / c 2 .

Experimental results from deep inelastic lepton—nucleon scattering, from electron - positron annihilation to hadrons and from massive lepton - pair production in hadron collisions, are discussed. The data obtained so far give qualitative support for Q C D . Serious quantitative comparisons are beset with difficulties, which arise first because of discrepancies between different experiments, and secondly because in addition to the perturbative effects that have been calculated theoretically, there are non-perturbative contributions for which there are no reliable estimates. These appear to be important in the momentum transfer range that can be investigated experimentally at present.

Exposures of the Ne/H 2 filled Big European Bubble Chamber (BEBC) to a dichromatic neutrino (antineutrino) beam produced by 400 GeV protons of the CERN SPS yielded ∼ 3100 events with a negative, and ∼ 1100 with a positive, muon. The neutrino flux is determined from the muon flux in the shielding. Assuming a linear energy dependence of the cross section, the values σ E between 20 and 200 GeV are found to be 0.657 ± 0.012 (stat.) ± 0.027 (syst.) and 0.309 ± 0.009 (stat.) ± 0.013 (syst.) cm 2 (GeV nucleon) −1 , for neutrinos and antineutrinos, respectively. The scaling variable q 2 E decreases significantly with increasing energy both for neutrinos and antineutrinos.

Data from an exposure of BEBC filled with hydrogen to a wideband neutrino beam are analysed to yield the structure function F v p 2 ( x ) for x > 0.2. Using our results in combination with data from electron-proton and muon-proton scattering, the quark density ratio d/u is determined as a function of x . The dominance u at large x is clearly seen. The results are compared with theoretical predictions.

Two examples of charmed baryon production by neutrinos have been observed in BEBC filled with hydrogen. Both events fit uniquely the reaction ν p → μ − pK − π + π + and thus apparently violate the ΔS = ΔQ rule. None of the appropriate mass combinations is consistent with the mass values of the D 0 and D + mesons. However, for the mass combinations of the pK − π + systems, values of (2.285 ± 0.005) GeV and (2.280 ± 0.003) GeV are found for event 1 and 2, respectively. These values agree with the mass of (2.285 ± 0.006) GeV for the Λ c + charmed baryon determined recently in e + e − collisions at SLAC.

Production of charmed mesons has been observed in neutrino-hydrogen interactions in 285 000 photographs of BEBC exposed to the CERN wide-band neutrino beam. In ∼ 6000 charged current events, two events have been found each giving a three-constraint kinematical fit with production of only one strange particle, namely a K−, which is identified by its characteristic reaction K−p→Σ−π+. These events are thus examples of ΔS=−ΔQ reactions corresponding to charm production on a valence quark. For both events the effective mass of a (K− + pions) system is equal to the D0(1863) mass while the effective mass of this system plus a π+ is equal to the D∗+(2009) mass. The mass difference ΔM=M( D ∗+ )−M D 0 is 145.2 ± 0.6 MeV. The first event is interpreted as ν p →μ − pD ∗+ and the second as ν p →μ − pD ∗+ π + π − , both with D ∗+ → D 0 π + . The rate of inclusive D∗+ production has been estimated for all events by assuming that each negative track in turn is a K− and using the fact that the error on ΔM is small. It is found that the cross section σ(ν p →μ − D ∗+ + anything ) relative to σ(νp→μ− + anything), above the D∗ threshold, is (4.1 ± 2.4)%.

An analysis is presented of the z distributions of secondary hadrons produced in 5600 charged current events from a BEBC neutrino-hydrogen experiment. Evidence is presented for scaling deviations in the z distributions and a breakdown of factorization in the single particle inclusive cross sections. The results are consistent with the leading order QCD predictions on the q2 dependence of nonsinglet moments of quark fragmentation functions, yielding a value of Λ ≈ 0.6 GeV.

Multiplicity distributions of charged hadrons produced in νp interactions are studied using a sample of 7850 charged current interactions in BEBC. Multiplicity moments are studied as functions of the invariant mass of the hadronic system, W , and a comparison is made with other lepton- and hadron-induced reactions. The mean charged multiplicity 〈 n ch 〉 is found to increase linearly with ln W 2 . The other multiplicity moments indicate a close similarity between the neutrino scattering and the class of annihilation processes. A study of the multiplicity distributions in the forward and backward directions in the hadronic c.m.s. gives results in qualitative agreement with the quark parton model.

The average transverse momentum squared, 〈p⊥2〉, of hadrons is studied as a function of W2 and of Q2 for ν and ν interactions on an isoscalar target. An increase of 〈p⊥2〉 with W2 is observed for the hadrons emitted forward in the hadronic c.m.s. The p⊥ dependence of the fragmentation function is found to factorise from the structure function at fixed W, but does not factorise at fixed Q2. Unlike the case of forward-going particles, the 〈p⊥2〉 of hadrons going backward in the c.m.s. shows no strong dependence on W2.

Analysis of bubble chamber neutrino events indicates that the Bjorken scaling formula applies in the low energy region if the cross-sections are expressed in terms of the Bloom-Gilman parameter x′ = q2/(2Mv + M2). The value of ∫F2dx′ averaged over neutrons and protons, is compatible with the electron data, CVC and pure V, A coupling for the nucleon constituents, provided isoscalar contributions are small. Limits are placed on the relative magnitudes of F1, F2 and F3.