We present our new measurement of the cross-section for charm dimuon production in neutrino–iron interactions based upon the full statistics collected by the NOMAD experiment. After background ...subtraction we observe 15 344 charm dimuon events, providing the largest sample currently available. The analysis exploits the large inclusive charged current sample – about 9×106 events after all analysis cuts – and the high resolution NOMAD detector to constrain the total systematic uncertainty on the ratio of charm dimuon to inclusive Charged Current (CC) cross-sections to ∼2%. We also perform a fit to the NOMAD data to extract the charm production parameters and the strange quark sea content of the nucleon within the NLO QCD approximation. We obtain a value of mc(mc)=1.159±0.075 GeV/c2 for the running mass of the charm quark in the MS¯ scheme and a strange quark sea suppression factor of κs=0.591±0.019 at Q2=20 GeV2/c2.
We have studied the muon neutrino and antineutrino quasi-elastic (QEL) scattering reactions (
ν
μ
n
→
μ
−
p
and
) using a set of experimental data collected by the NOMAD Collaboration. We have ...performed measurements of the cross-section of these processes on a nuclear target (mainly carbon) normalizing it to the total
ν
μ
(
) charged-current cross section. The results for the flux-averaged QEL cross sections in the (anti)neutrino energy interval 3–100 GeV are
and
for neutrino and antineutrino, respectively. The axial mass parameter
M
A
was extracted from the measured quasi-elastic neutrino cross section. The corresponding result is
M
A
=1.05±0.02(stat)±0.06(syst) GeV. It is consistent with the axial mass values recalculated from the antineutrino cross section and extracted from the pure
Q
2
shape analysis of the high purity sample of
ν
μ
quasi-elastic 2-track events, but has smaller systematic error and should be quoted as the main result of this work. Our measured
M
A
is found to be in good agreement with the world average value obtained in previous deuterium filled bubble chamber experiments. The NOMAD measurement of
M
A
is lower than those recently published by K2K and MiniBooNE Collaborations. However, within the large errors quoted by these experiments on
M
A
, these results are compatible with the more precise NOMAD value.
We present a search for neutrino induced events containing a single, exclusive photon using data from the NOMAD experiment at the CERN SPS where the average energy of the neutrino flux is ≃25 GeV. ...The search is motivated by an excess of electron-like events in the 200–475 MeV energy region as reported by the MiniBooNE experiment. In NOMAD, photons are identified via their conversion to e+e− in an active target embedded in a magnetic field. The background to the single photon signal is dominated by the asymmetric decay of neutral pions produced either in a coherent neutrino–nucleus interaction, or in a neutrino–nucleon neutral current deep inelastic scattering, or in an interaction occurring outside the fiducial volume. All three backgrounds are determined in situ using control data samples prior to opening the ‘signal-box’. In the signal region, we observe 155 events with a predicted background of 129.2±8.5±3.3. We interpret this as null evidence for excess of single photon events, and set a limit. Assuming that the hypothetical single photon has a momentum distribution similar to that of a photon from the coherent π0 decay, the measurement yields an upper limit on single photon events, <4.0×10−4 per νμ charged current event. Narrowing the search to events where the photon is approximately collinear with the incident neutrino, we observe 78 events with a predicted background of 76.6±4.9±1.9 yielding a more stringent upper limit, <1.6×10−4 per νμ charged current event.
We present a study of exclusive neutral pion production in neutrino–nucleus Neutral Current interactions using data from the NOMAD experiment at the CERN SPS. The data correspond to 1.44×106 ...muon-neutrino Charged Current interactions in the energy range 2.5⩽Eν⩽300 GeV. Neutrino events with only one visible π0 in the final state are expected to result from two Neutral Current processes: coherent π0 production, ν+A→ν+A+π0 and single π0 production in neutrino–nucleon scattering. The signature of coherent π0 production is an emergent π0 almost collinear with the incident neutrino while π0's produced in neutrino–nucleon deep inelastic scattering have larger transverse momenta. In this analysis all relevant backgrounds to the coherent π0 production signal are measured using data themselves. Having determined the backgrounds, and using the Rein–Sehgal model for the coherent π0 production to compute the detection efficiency, we obtain 4630±522(stat)±426(syst) corrected coherent-π0 events with Eπ0⩾0.5 GeV. We measure σ(νA→νAπ0)=72.6±8.1(stat)±6.9(syst)×10−40 cm2/nucleus. This is the most precise measurement of the coherent π0 production to date.
Results from the
ν
τ
appearance search in a neutrino beam using the full NOMAD data sample are reported. A new analysis unifies all the hadronic
τ decays, significantly improving the overall ...sensitivity of the experiment to oscillations. The “blind analysis” of all topologies yields no evidence for an oscillation signal. In the two-family oscillation scenario, this sets a 90% CL allowed region in the sin
22
θ
μτ
–
Δm
2 plane which includes sin
22
θ
μτ
<3.3×10
−4 at large
Δm
2 and
Δm
2< 0.7 eV
2/
c
4 at sin
22
θ
μτ
=1. The corresponding contour in the
ν
e
→
ν
τ
oscillation hypothesis results in sin
22
θ
eτ
<1.5×10
−2 at large
Δm
2 and
Δm
2<5.9 eV
2/
c
4 at sin
22
θ
eτ
=1. We also derive limits on effective couplings of the
τ lepton to
ν
μ
or
ν
e
.
We present a measurement of the muon neutrino–nucleon inclusive charged current cross section, off an isoscalar target, in the neutrino energy range 2.5⩽Eν⩽40 GeV. The significance of this ...measurement is its precision, ±4% in 2.5⩽Eν⩽10 GeV, and ±2.6% in 10⩽Eν⩽40 GeV regions, where significant uncertainties in previous experiments still exist, and its importance to the current and proposed long baseline neutrino oscillation experiments.
We present the results of a search for nu_mu → nu_e oscillations in the NOMAD experiment at Cern. The experiment looked for the appearance of nu_e in a predominantly nu_mu wide-band neutrino beam at ...the CERN SPS. No evidence for oscillations was found. The 90% confidence limits obtained are Delta m^2 < 0.4 eV^2 for maximal mixing and sin^2(2theta) < 1.4 X 10^-3 for large Delta m^2. This result excludes the LSND allowed region of oscillation parameters with Delta m^2 >~ 10 eV^2.
We report on a search for heavy neutrinos (
ν
4) produced in the decay
D
s
→
τν
4 at the SPS proton target followed by the decay
ν
4→
ν
τ
e
+
e
− in the NOMAD detector. Both decays are expected to ...occur if
ν
4 is a component of
ν
τ
. From the analysis of the data collected during the 1996–1998 runs with 4.1×10
19 protons on target, a single candidate event consistent with background expectations was found. This allows to derive an upper limit on the mixing strength between the heavy neutrino and the tau neutrino in the
ν
4 mass range from 10 to 190 MeV. Windows between the SN1987a and Big Bang Nucleosynthesis lower limits and our result are still open for future experimental searches. The results obtained are used to constrain an interpretation of the time anomaly observed in the KARMEN1 detector.
The NOMAD Collaboration presents a study of opposite sign dimuon events in the framework of Leading Order QCD. A total of 2714 neutrino- and 115 antineutrino-induced opposite sign dimuon events with
...E
μ1
,
E
μ2
>4.5 GeV, 15<
E
ν
<300 GeV and
Q
2>1
(
GeV/
c)
2
are observed in the Front-Calorimeter of NOMAD during the 1995 and 1996 runs. The analysis yields a value for the charm quark mass of
m
c=1.3
+0.3+0.3
−0.3−0.3
GeV/
c
2
and for the average semileptonic branching ratio of
B
c
=0.095
+0.007+0.014
−0.007−0.013. The ratio of the strange to non-strange sea in the nucleon is measured to be
κ=0.48
+0.09+0.17
−0.07−0.12. The measured rate of charm-induced dimuon relative to single muon, as a function of neutrino energy, is consistent with the slow rescaling hypothesis of heavy quark production.
Results of a detailed study of strange particle production in neutrino neutral current interactions are presented using the data from the NOMAD experiment. Integral yields of neutral strange ...particles (
K
s
0
, Λ,
Λ
¯
) have been measured. Decays of resonances and heavy hyperons with an identified
K
s
0
or Λ in the final state have been analyzed. Clear signals corresponding to
K
⋆
±
and
Σ
(
1385
)
±
have been observed. First results on the measurements of the Λ polarization in neutral current interactions have been obtained.
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