Search Results (49 total)

We report the first detailed comparisons of the rates and spectra of neutral-current neutrino interactions at two widely separated locations. A depletion in the rate at the far site would indicate mixing between ν_μ and a sterile particle. No anomalous depletion in the reconstructed energy spectrum is observed. Assuming oscillations occur at a single mass-squared splitting, a fit to the neutral- and charged-current energy spectra limits the fraction of ν_μ oscillating to a sterile neutrino to be below 0.68 at 90% confidence level. A less stringent limit due to a possible contribution to the measured neutral-current event rate at the far site from ν_e appearance at the current experimental limit is also presented.

A search for a sidereal modulation in the MINOS near detector neutrino data was performed. If present, this signature could be a consequence of Lorentz and CPT violation as predicted by the effective field theory called the standard-model extension. No evidence for a sidereal signal in the data set was found, implying that there is no significant change in neutrino propagation that depends on the direction of the neutrino beam in a sun-centered inertial frame. Upper limits on the magnitudes of the Lorentz and CPT violating terms in the standard-model extension lie between 10^-4 and 10^-2 of the maximum expected, assuming a suppression of these signatures by a factor of 10^-17.

This Letter reports new results from the MINOS experiment based on a two-year exposure to muon neutrinos from the Fermilab NuMI beam. Our data are consistent with quantum-mechanical oscillations of neutrino flavor with mass splitting |Δm²|=(2.43±0.13)×10^-3  eV² (68% C.L.) and mixing angle sin⁡²(2θ)>0.90 (90% C.L.). Our data disfavor two alternative explanations for the disappearance of neutrinos in flight: namely, neutrino decays into lighter particles and quantum decoherence of neutrinos, at the 3.7 and 5.7 standard-deviation levels, respectively.

We report the results of a search for ν_μ disappearance by the Main Injector Neutrino Oscillation Search [D. G. Michael (MINOS), Phys. Rev. Lett. 97, 191801 (2006).]. The experiment uses two detectors separated by 734 km to observe a beam of neutrinos created by the Neutrinos at the Main Injector facility at Fermi National Accelerator Laboratory. The data were collected in the first 282 days of beam operations and correspond to an exposure of 1.27×10²⁰ protons on target. Based on measurements in the Near Detector, in the absence of neutrino oscillations we expected 336±14 ν_μ charged-current interactions at the Far Detector but observed 215. This deficit of events corresponds to a significance of 5.2 standard deviations. The deficit is energy dependent and is consistent with two-flavor neutrino oscillations according to |Δm²|=2.74_-0.26^+0.44×10^-3  eV²/c⁴ and sin⁡²2θ>0.87 at 68% confidence level.

The velocity of a ∼3  GeV neutrino beam is measured by comparing detection times at the near and far detectors of the MINOS experiment, separated by 734 km. A total of 473 far detector neutrino events was used to measure (v-c)/c=5.1±2.9×10^-5 (at 68% C.L.). By correlating the measured energies of 258 charged-current neutrino events to their arrival times at the far detector, a limit is imposed on the neutrino mass of m_ν<50  MeV/c² (99% C.L.).

The 5.4 kton MINOS far detector has been taking charge-separated cosmic ray muon data since the beginning of August, 2003 at a depth of 2070 m.w.e. in the Soudan Underground Laboratory, Minnesota, USA. The data with both forward and reversed magnetic field running configurations were combined to minimize systematic errors in the determination of the underground muon charge ratio. When averaged, two independent analyses find the charge ratio underground to be N_μ⁺/N_μ^-=1.374±0.004(stat)_-0.010^+0.012(sys). Using the map of the Soudan rock overburden, the muon momenta as measured underground were projected to the corresponding values at the surface in the energy range 1–7 TeV. Within this range of energies at the surface, the MINOS data are consistent with the charge ratio being energy independent at the 2 standard deviation level. When the MINOS results are compared with measurements at lower energies, a clear rise in the charge ratio in the energy range 0.3–1.0 TeV is apparent. A qualitative model shows that the rise is consistent with an increasing contribution of kaon decays to the muon charge ratio.

We found 140 neutrino-induced muons in 854.24 live days in the MINOS far detector, which has an acceptance for neutrino-induced muons of 6.91×10⁶  cm² sr. We looked for evidence of neutrino disappearance in this data set by computing the ratio of the number of low momentum muons to the sum of the number of high momentum and unknown momentum muons for both data and Monte Carlo expectation in the absence of neutrino oscillations. The ratio of data and Monte Carlo ratios, R, is R=0.65_-0.12^+0.15(stat)±0.09(syst), a result that is consistent with an oscillation signal. A fit to the data for the oscillation parameters sin⁡²2θ₂₃ and Δm₂₃² excludes the null oscillation hypothesis at the 94% confidence level. We separated the muons into μ^- and μ⁺ in both the data and Monte Carlo events and found the ratio of the total number of μ^- to μ⁺ in both samples. The ratio of those ratios, R-^ _CPT, is a test of CPT conservation. The result R-^ _CPT=0.72_-0.18^+0.24(stat)_-0.04^+0.08(syst) is consistent with CPT conservation.

This Letter reports results from the MINOS experiment based on its initial exposure to neutrinos from the Fermilab NuMI beam. The rates and energy spectra of charged current ν_μ interactions are compared in two detectors located along the beam axis at distances of 1 and 735 km. With 1.27×10²⁰ 120 GeV protons incident on the NuMI target, 215 events with energies below 30 GeV are observed at the Far Detector, compared to an expectation of 336±14 events. The data are consistent with ν_μ disappearance via oscillations with |Δm₃₂²|=2.74_-0.26^+0.44×10^-3  eV² and sin⁡²(2θ₂₃)>0.87 (68% C.L.).

The complete 5.4 kton MINOS far detector has been taking data since the beginning of August 2003 at a depth of 2070 meters water-equivalent in the Soudan mine, Minnesota. This paper presents the first MINOS observations of ν_μ and ν̅ _μ charged-current atmospheric neutrino interactions based on an exposure of 418 days. The ratio of upward- to downward-going events in the data is compared to the Monte Carlo expectation in the absence of neutrino oscillations, giving R_up/down^data/R_up/down^MC=0.62_-0.14^+0.19(stat.)±0.02(sys.). An extended maximum likelihood analysis of the observed L/E distributions excludes the null hypothesis of no neutrino oscillations at the 98% confidence level. Using the curvature of the observed muons in the 1.3 T MINOS magnetic field ν_μ and ν̅ _μ interactions are separated. The ratio of ν̅ _μ to ν_μ events in the data is compared to the Monte Carlo expectation assuming neutrinos and antineutrinos oscillate in the same manner, giving R_{ν̅ μ/νμ}^data/R_{ν̅ μ/νμ}^MC=0.96_-0.27^+0.38(stat.)±0.15(sys.), where the errors are the statistical and systematic uncertainties. Although the statistics are limited, this is the first direct observation of atmospheric neutrino interactions separately for ν_μ and ν̅ _μ.

Upward-going stopping muons initiated by atmospheric ν_μ and ν̅ _μ interactions in the rock below the Soudan 2 detector have been isolated, together with a companion sample of neutrino-induced single muons, created within the detector, which travel downwards and exit. The downward-going sample is consistent with the atmospheric-neutrino flux prediction, but the upward-going sample exhibits a sizable depletion. Both are consistent with previously reported Soudan 2 neutrino-oscillation results. Inclusion of the two samples in an all-event likelihood analysis, using recent 3D-atmospheric-neutrino-flux calculations, reduces both the allowed oscillation parameter region and the probability of the no-oscillation hypothesis.

A deficit of atmospheric ν_μ events, consistent with the hypothesis of neutrino oscillations, is observed in the 5.90 kiloton-year fiducial exposure of the Soudan 2 detector. An unbinned maximum likelihood analysis of the neutrino L/E distribution has been carried out using the Feldman-Cousins prescription. The probability of the no oscillation hypothesis is 5.8×10^-4. The 90% confidence allowed region in the sin²2θ,Δm² plane is presented. The region includes the 90% confidence allowed region of the Super-K experiment.

We describe the status of our effort to realize a first neutrino factory and the progress made in understanding the problems associated with the collection and cooling of muons towards that end. We summarize the physics that can be done with neutrino factories as well as with intense cold beams of muons. The physics potential of muon colliders is reviewed, both as Higgs factories and compact high-energy lepton colliders. The status and time scale of our research and development effort is reviewed as well as the latest designs in cooling channels including the promise of ring coolers in achieving longitudinal and transverse cooling simultaneously. We detail the efforts being made to mount an international cooling experiment to demonstrate the ionization cooling of muons.

We have searched for neutron-antineutron oscillations using the 5.56 fiducial kiloton-year exposure of the Soudan 2 iron tracking calorimeter. We require candidate nn̄ occurrences to have >~4 prongs (tracks and showers) and to have kinematics compatible with n̄N annihilation within a nucleus. We observe five candidate events, with an estimated background from atmospheric neutrino and cosmic ray induced events of 4.5±1.2 events. Previous experiments with smaller exposures observed no candidates, with estimated background rates similar to this experiment. We set a lifetime lower limit at 90% C.L. for the nn̄ oscillation time in iron: T_A(Fe)>7.2×10³¹ yr. The corresponding lower limit for oscillation of free neutrons is τ_nn̄>1.3×10⁸ sec.

This biennial Review summarizes much of Particle Physics. Using data from previous editions, plus 2205 new measurements from 667 papers, we list, evaluate, and average measured properties of gauge bosons, leptons, quarks, mesons, and baryons. We also summarize searches for hypothetical particles such as Higgs bosons, heavy neutrinos, and supersymmetric particles. All the particle properties and search limits are listed in Summary Tables. We also give numerous tables, figures, formulae, and reviews of topics such as the Standard Model, particle detectors, probability, and statistics. This edition features expanded coverage of CP violation in B mesons and of neutrino oscillations. For the first time we cover searches for evidence of extra dimensions (both in the particle listings and in a new review). Another new review is on Grand Unified Theories. A booklet is available containing the Summary Tables and abbreviated versions of some of the other sections of this full Review. All tables, listings, and reviews (and errata) are also available on the Particle Data Group website: http://pdg.lbl.gov.

We have searched for nucleon decay into five two-body final states using a 4.4 kiloton-year fiducial exposure of the Soudan 2 iron tracking calorimeter. For proton decay into the fully visible final states μ⁺η⁰ and e⁺η⁰, we observe zero and one event, respectively, that satisfy our search criteria for nucleon decay. The lifetime lower limits (τ/B) thus implied are 89×10³⁰ years and 81×10³⁰ years at 90% confidence level. For neutron decay into ν̅ η⁰, we obtain the lifetime lower limit 71×10³⁰ years. Limits are also reported for neutron decay into ν̅ π⁰, and for proton decay into ν̅ π⁺.

A shadow of the Moon, with a statistical significance of 5σ, has been observed in the underground muon flux at a depth of 2090 mwe using the Soudan 2 detector. The angular resolution of the detector is well described by a Gaussian with a sigma <~0.3°. The position of the shadow confirms that the alignment of the detector is known to better than 0.15° and has remained stable during ten years of data taking.

A search for nucleon decay into two-body final states containing K⁰ mesons has been conducted using the 963 metric ton Soudan 2 iron tracking calorimeter. The topologies, ionizations, and kinematics of contained events recorded in a 5.52 kiloton-year total exposure (4.41 kton-year fiducial volume exposure) are examined for compatibility with nucleon decays in an iron medium. For proton decay into the fully visible final states μ⁺K_S⁰ and e⁺K_S⁰, zero and one event candidates are observed respectively. The lifetime lower limits (τ/B) thus implied are 1.5×10³² yr and 1.2×10³² yr, respectively. Lifetime lower limits are also reported for proton decay into l⁺K_l⁰, and for neutron decay into νK_S⁰.

Deep underground muon events recorded by the Soudan 2 detector, located at a depth of 2100 m of water equivalent, have been used to infer the nuclear composition of cosmic rays in the “knee” region of the cosmic ray energy spectrum. The observed muon multiplicity distribution favors a composition model with a substantial proton content in the energy region 8×10⁵–1.3×10⁷ GeV/nucleus.

The Soudan 2 experiment has performed time-coincident cosmic ray air shower and underground muon measurements. Comparisons to Monte Carlo predictions show that such measurements can make statistically significant tests of the primary composition in the knee region of the cosmic ray spectrum. The results do not support any significant increase in the average primary mass with energy in the range of ∼10⁴ TeV per nucleus. Some systematic uncertainties remain, however, particularly in the Monte Carlo modeling of the cosmic ray shower.

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 × 10^-15 cm^-2s^-1sr^-1 for monopole velocities of β>2×10^-3.

We have operated the Soudan 1 underground muon detector in coincidence with a 36-m² detector situated at the Earth's surface. Such a combination of detectors can yield information on the composition of the primary cosmic rays at energies above Å 3 × 10¹⁵ eV, where there is an abrupt change in the slope of the energy spectrum. The present experiment was meant to test the feasibility of operating such a system, and to obtain a first sample of data before the complete installation of the much larger Soudan 2 detector. These initial data seem to favor a light composition in the energy range 10¹⁵-10¹⁶ eV, but there are significant systematic uncertainties.

Results are presented of a study of opposite-sign dimuon events observed in a fine-grained neutrino detector exposed to the Fermilab Tevatron wide-band neutrino beam. A total of 300 background-corrected μ⁺μ^- events induced by incident neutrinos and antineutrinos with energies up to 500 GeV were accumulated. The data were analyzed in terms of a model based on charmquark production followed by semileptonic decay of the charmed meson. The Cabibbo-Kobayashi-Maskawa matrix terms were found to be |U_cd|²=0.0378±0.0127 (stat)_-0.0082^+0.0099 (syst), and κ|U_cs|²=0.391±0.076 (stat)_-0.066^+0.097 (syst). The ratio of the strange to nonstrange sea in the nucleon, κ=2S / (U̅ +D̅ ), was measured to be 0.407±0.075 (stat)_-0.069^+0.103 (syst).

From July 1987 through March 1988 an array of proportional wire modules was operated as a muon detector at a depth of 2090 meters water equivalent in the Soudan mine in northern Minnesota. A spatial angular resolution of 1.2° was achieved for muon tracking. A clean sample of 1.02×10⁵ muon trajectories recorded underground is used to search for an excess flux of muons from the direction of Cygnus X-3. For muons within the phase interval [0.6, 0.9] of the source's 4.8-h period, 90%-C.L. upper limits for fluxes arriving within 3° and 1.5° half-angle cones centered on the Cygnus X-3 direction are 8.5×10¹¹ cm^-2s^-1 and 3.1×10^-11 cm^-2s^-1, respectively.

The structure of the nucleon is studied by means of deep-inelastic neutrino-nucleon scattering at high energies through the weak neutral current. The neutrino-nucleon scattering events were observed in a 340-metric-ton fine-grained calorimeter exposed to a narrow-band (dichromatic) neutrino beam at Fermilab. The data sample after analysis cuts consists of 9200 charged-current and 3000 neutral-current neutrino and antineutrino events. The neutral-current valence and sea nucleon structure functions are extracted from the x distribution reconstructed from the measured angle and energy of the recoil-hadron shower and the incident narrow-band neutrino-beam energy. They are compared to those extracted from charged-current events analyzed as neutral-current events. It is shown that the nucleon structure is independent of the type of neutrino interaction, which confirms an important aspect of the standard model. The data are also used to determine the value of sin²θ_W=0.238±0.013±0.015±0.010 for a single-parameter fit, where the first error is from statistical sources, the second from experimental systematic errors, and the third from estimated theoretical errors.

We have measured the parity-violating electroweak asymmetry in the elastic scattering of polarized electrons from ¹²C nuclei. Our result is A_expt=0.60±0.14±0.02 ppm, where the first error is statistical and the second is systematic. With a beam polarization of 0.37, we compute the isoscalar vector hadronic coupling constant γ̃ to be 0.136±0.032±0.009. The standard model predicts γ̃=0.155 at the tree level, in agreement with our data.