Search Results (31 total)

We present a new measurement of the difference between the nucleon strange and antistrange quark distributions from dimuon events recorded by the NuTeV experiment at Fermilab. This analysis is the first to use a complete next to leading order QCD description of charm production from neutrino scattering. Dimuon events in neutrino deep inelastic scattering allow direct and independent study of the strange and antistrange content of the nucleon. We find a positive strange asymmetry with a significance of 1.6σ. We also report a new measurement of the charm mass.

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 NuTeV experiment at Fermilab has obtained a unique high-statistics sample of neutrino and antineutrino interactions using its high-energy sign-selected beam. We present a measurement of the differential cross section for charged-current neutrino and antineutrino scattering from iron. We determine the relative ν̅ to ν cross section, r=σ^ν̅ /σ^ν, at high energy with errors a factor of 2 smaller than the previous world average. Structure functions, F₂(x,Q²) and xF₃(x,Q²), are determined by fitting the inelasticity, y, dependence of the cross sections. This measurement has significantly improved systematic precision as a consequence of more precise understanding of hadron and muon energy scales.

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.

We have analyzed 44.3M single muons collected by MACRO from 1991 through 2000 in 2145 live days of operation. We have searched for the solar diurnal, apparent sidereal, and pseudosidereal modulation of the underground muon rate by computing hourly deviations of the muon rate from 6 month averages. We find evidence for statistically significant modulations with the solar diurnal and the sidereal periods. The amplitudes of these modulations are <0.1%, and are at the limit of the detector statistics. The pseudosidereal modulation is not statistically significant. The solar diurnal modulation is due to the daily atmospheric temperature variations at 20 km, the altitude of primary cosmic ray interactions with the atmosphere; MACRO is the deepest experiment to report this result. The sidereal modulation is in addition to the expected Compton-Getting modulation due to solar system motion relative to the local standard of rest; it represents motion of the solar system with respect to the galactic cosmic rays toward the galactic plane.

The NuTeV Collaboration recently reported a value of sin²θ_W measured in neutrino-nucleon scattering that is 3 standard deviations above the standard model prediction. This result is derived assuming that (1) the strange sea is quark-antiquark symmetric, s(x)=s̅ (x), and (2) up and down quark distributions are symmetric under the simultaneous interchange of u↔d and p↔n. We report the impact of violations of these symmetries on sin²θ_W and discuss the theoretical and experimental constraints on such asymmetries.

Limits on ν_μ→ν_e and ν̅ _μ→ν̅ _e oscillations are extracted using the NuTeV detector with sign-selected ν_μ and ν̅ _μ beams. In ν̅ _μ mode, for the case of sin²2α  =  1, Δm²>2.6  eV² is excluded, and for Δm²≫1000 eV², sin²2α>1.1×10^-3. The NuTeV data exclude the high Δm² end of ν̅ _μ→ν̅ _e oscillation parameters favored by the LSND experiment without the need to assume that the oscillation parameters for ν and ν̅ are the same. We present the most stringent experimental limits for ν_μ(ν̅ _μ)→ν_e(ν̅ _e) oscillations in the large Δm² region.

The NuTeV Collaboration has extracted the electroweak parameter sin²θ_W from the measurement of the ratios of neutral current to charged current ν and ν̅ cross sections. Our value, sin²θ_W^(on-shell)  =  0.2277±0.0013(stat)±0.0009(syst), is 3 standard deviations above the standard model prediction. We also present a model independent analysis of the same data in terms of neutral-current quark couplings.

We report on the extraction of R  =  σ_L / σ_T from CCFR ν_μ-Fe and ν̅ _μ-Fe differential cross sections. The CCFR differential cross sections do not show the deviations from the QCD expectations that are seen in the CDHSW data at very low and very high x. R as measured in ν_μ scattering is in agreement with R as measured in muon and electron scattering. All data on R for Q²>1 GeV² are in agreement with a NNLO QCD calculation which uses NNLO parton distribution functions and includes target mass effects. We report on the first measurements of R in the low x and Q²<1 GeV² region (where an anomalous large rise in R for nuclear targets has been observed by the HERMES Collaboration).

We present measurements of the semi-inclusive cross sections for ν_μ- and ν̅ _μ-nucleon deep inelastic scattering interactions with two oppositely charged muons in the final state. These events dominantly arise from the production of a charm quark during the scattering process. The measurement was obtained from the analysis of 5102 ν_μ-induced and 1458 ν̅ _μ-induced events collected with the NuTeV detector exposed to a sign-selected beam at the Fermilab Tevatron. We also extract a cross-section measurement from a reanalysis of 5030 ν_μ-induced and 1060 ν̅ _μ-induced events collected from the exposure of the same detector to a quad-triplet beam by the Chicago Columbia Fermilab Rochester (CCFR) experiment. The results are combined to obtain the most statistically precise measurement of neutrino-induced dimuon production cross sections to date. These measurements should be of broad use to phenomenologists interested in the dynamics of charm production, the strangeness content of the nucleon, and the Cabibbo-Kobayashi-Maskawa matrix element V_cd.

The NuTeV experiment at Fermilab has used a sign-selected neutrino beam to perform a search for the lepton number violating process ν̅ _μe^-→μ^-ν̅ _e, and to measure the cross section of the standard model inverse muon decay process ν_μe^-→μ^-ν_e. NuTeV measures the inverse muon decay asymptotic cross-section slope σ/E to be (13.8±1.2±1.4)×10^-42 cm²/GeV. The experiment also observes no evidence for lepton number violation and places one of the most restrictive limits on the cross-section ratio σ(ν̅ _μe^-→μ^-ν̅ _e)/σ(ν_μe^-→μ^-ν_e)≤1.7% at 90% C.L. for V-A couplings and ≤0.6% for scalar couplings.

A search for long-lived neutral particles ( N⁰’s) with masses above 2.2 GeV/c² that decay into at least one muon has been performed using an instrumented decay channel at the NuTeV experiment at Fermilab. Data were examined for particles decaying into the final states μμ, μe, and μπ. Three μμ events were observed over an expected standard model background of 0.069±0.010 events; no events were observed in the other modes.

A new structure function analysis of CCFR deep inelastic ν-N and ν̅ -N scattering data is presented for previously unexplored kinematic regions down to Bjorken x  =  0.0045 and Q²  =  0.3 GeV². Comparisons to charged lepton scattering data from NMC and E665 experiments are made and the behavior of the structure function F₂^ν is studied in the limit Q²→0.

We report on the first observation of open charm production in neutral current deep inelastic neutrino scattering as seen in the NuTeV detector at Fermilab. The production rate is shown to be consistent with a pure gluon-Z⁰ boson production model, and the observed level of charm production is used to determine the effective charm mass. As part of our analysis, we also obtain a new measurement for the proton-nucleon charm production cross section at sqrt[s]=38.8 GeV.

We report on the extraction of the structure functions F₂ and ΔxF₃  =  xF₃^ν-xF₃^ν̅ from CCFR ν_μ-Fe and ν̅ _μ-Fe differential cross sections. The extraction is performed in a physics model-independent (PMI) way. This first measurement of ΔxF₃, which is useful in testing models of heavy charm production, is higher than current theoretical predictions. The ratio of the F₂ (PMI) values measured in ν_μ and μ scattering is in agreement (within 5%) with the predictions of next-to-leading-order parton distribution functions using massive charm production schemes, thus resolving the long-standing discrepancy between the two sets of data.

We report on a search for flavor-changing neutral-currents (FCNC) in the production of heavy quarks in deep inelastic ν_μN and ν̅ _μN scattering by the NuTeV experiment at the Fermilab Tevatron. This measurement, made possible by the high-purity NuTeV sign-selected beams, probes for FCNC in heavy flavors at the quark level, and is uniquely sensitive to neutrino couplings of potential FCNC mediators. All searches are consistent with zero, and limits on the effective mixing strengths |V_uc|², |V_db|², and |V_sb|² are obtained.

A search for lightly ionizing particles has been performed with the MACRO detector. This search was sensitive to particles with charges between 1 / 5 e and close to the charge of an electron, with β between approximately 0.25 and 1.0. Unlike previous searches both single track events and tracks buried within high multiplicity muon showers were examined. In a period of approximately one year no candidates were observed. Assuming an isotropic flux, for the single track sample this corresponds to a 90% C.L. upper flux limit Φ<~9.2×10^-15 cm^-2 s^-1 sr^-1.

The E815 (NuTeV) neutrino experiment has performed a search for a 33.9 MeV/c² weakly interacting neutral particle produced in pion decay. Such a particle may be responsible for an anomaly in the timing distribution of neutrino interactions in the KARMEN experiment. E815 has searched for this particle's decays in an instrumented decay region; no evidence for this particle was found. The search is sensitive to pion branching ratios as low as 10^-13.

We present evidence for the diffractive processes ν_μFe→μ^-D_S⁺(D_S^*)Fe and ν̅ _μFe→μ⁺D_S^-(D_S^*)Fe using the Fermilab SSQT neutrino beam and the Lab E neutrino detector. The data are consistent with standard model production of the neutrino trident reactions ν_μFe→ν_μμ^-μ⁺Fe and ν̅ _μFe→ν̅ _μμ⁺μ^-Fe. We see no evidence for neutral-current production of J/ψ via either diffractive or deep inelastic scattering mechanisms.

Data from the CCFR E770 neutrino deep inelastic scattering experiment at Fermilab contain events with a large Bjorken x (x>0.7) and high momentum transfer [Q²>50 (GeV/c)²]. A comparison of the data with a model based on no nuclear effects at large x shows a significant excess of events in the data. The addition of Fermi gas motion of the nucleons in the nucleus to the model does not explain the excess. Adding a higher momentum tail due to the formation of “quasi-deuterons” makes some improvement. An exponentially falling F₂∝e^-s(x-x₀) at large x, predicted by “multi-quark clusters” and “few-nucleon correlations,” can describe the data. A value of s=8.3±0.7(stat)±0.7(syst) yields the best agreement with the data.

A search for neutral heavy leptons (NHLs) has been performed using an instrumented decay channel at the NuTeV (E-815) experiment at Fermilab. The data were examined for NHLs decaying into muonic final states ( μμν, μeν, μπ, and μρ); no evidence has been found for NHLs in the 0.25–2.0 GeV mass range. This analysis places limits on the mixing of NHLs with standard light neutrinos at a level up to an order of magnitude more restrictive than previous search limits in this mass range.

We perform an indirect search for weakly interacting massive particles (WIMPs) using the MACRO detector to look for neutrino-induced upward-going muons resulting from the annihilation of WIMPs trapped in the Sun and Earth. The search is conducted in various angular cones centered on the Sun and Earth to accommodate a range of WIMP masses. No significant excess over the background from atmospheric neutrinos is seen. We set experimental flux limits on the upward-going muon fluxes from the Sun and the Earth. These limits are used to constrain neutralino particle parameters from supersymmetric theory, including those suggested by recent results from DAMA-NaI.

We present a measurement of the underground decoherence function using multi-muon events observed in the MACRO detector at Gran Sasso at an average depth of 3800 hg/cm². Muon pair separations up to 70 m have been measured, corresponding to parent mesons with P_⊥<~1–2 GeV/c. Improved selection criteria are used to reduce detector effects mainly in the low distance separation region of muon pairs. Special care is given to a new unfolding procedure designed to minimize systematic errors in the numerical algorithm. The accuracy of the measurement is such that the possible contribution of rare processes, such as μ^±+N→μ^±+N+μ⁺+μ^-, can be experimentally studied. The measured decoherence function is compared with the predictions of the hadronic interaction model of the HEMAS Monte Carlo code. Good agreement is obtained. We interpret this agreement to indicate that no anomalous P_⊥ components in soft hadron-nucleus and nucleus-nucleus collisions are required by the MACRO experimental data. Preliminary comparisons with other Monte Carlo codes point out that the uncertainties associated with the hadronic interaction model may be as large as 20%, depending on the energy. MACRO data can be used as a benchmark for future work on the discrimination of shower models in the primary energy region around and below the knee of the spectrum.