Th. Delbar

Measurement of charm production in neutrino charged-current interactions

The nuclear emulsion target of the CHORUS detector was exposed to the wide-band neutrino beam of the CERN SPS of 27 GeV average neutrino energy from 1994 to 1997. In total, about 100 000 charged-current (CC) neutrino interactions with at least one identified muon were located in the emulsion target and fully reconstructed, using newly developed automated scanning systems. Charmed particles were searched for by a program recognizing particle decays. The observation of the decay in nuclear emulsion makes it possible to select a sample with very low background and minimal kinematical bias. In all, 2013 CC interactions with a charmed hadron candidate in the final state were selected and confirmed through visual inspection. The charm production rate induced by neutrinos relative to the CC cross-section is measured to be σ(νμN→μ−CX)/σ(CC)=(5.75 ± 0.32(stat)±0.30(syst))%. The charm production cross-section as a function of neutrino energy is also obtained. The results are in good agreement with previous measurements. The charm-quark hadronization produces the following charmed hadrons with relative fractions (in %): fD0=43.7±4.5, fΛc+=19.2±4.2, fD+=25.3±4.2 and fDs+=11.8±4.7.

A new determination of the electroweak mixing angle from muon-neutrino electron scattering

Abstract We are reporting on a new determination of sin2ϑw from the ratio of v μ e to v e scattering cross sections. A new detector designed for this purpose was exposed tothe Wide Band Neutrino Beamof the 450 GeV (CERN SPS. An analysis of data taken in 1987 and 1988 is presented based on 762 vμe and 1017 v e events. From the ratio of σ( v μ e ) to σ( v μ e ) we determined sin2ϑw=0.233±0.012 (stat) ± 0.008 (syst) without radiative correction. With radiative correction for mt=mH=100 GeV we find sin2ϑw=0.232±0.012(stat)±0.008(syst).

An Improved determination of the electroweak mixing angle from muon-neutrino electron scattering

Abstract We are reporting an improved determination of the electroweak mixing angle sin2Θw from the ratio of νμe to ν μ e scattering cross sections. The CHARM II detector was exposed to neutrino and antineutrino wide band beams at the 450 GeV CERN SPS. Including new data collected in 1989 we have obtained 1316 ± 56 νμe and 1453 ± 62 ν μ e events. From the ratio of the visible cross sections we determined sin2Θ0=0.239 ± 0.009(stat) ± 0.007(syst) without radiative corrections and g V e g A e =0.047 ± 0.046 . Combining this last result with recent results on gAe at LEP we obtain gVe= −0.023 ± 0.023.

New limits for the Ne-19(p,gamma)Na-20 astrophysical reaction rate from direct measurements using radioactive beams

Inexplosive stellar hydrogen burning, the hot CNO cycles and the rp-process are mainly linked by the reaction sequence O-15(alpha,gamma) Ne-19(p,gamma)Na-20. Using intense Ne-19 radioactive beams, both the Ne-19(p,gamma) and the Ne-19(d,n) reaction have been studied. Upper and lower limits for the Ne-19(p,gamma) reaction rate have been deduced, allowing to conclude that the O-15(alpha,gamma) reaction is most likely the bottleneck reaction.

Charged-particle multiplicities in charged-current neutrino– and anti-neutrino–nucleus interactions

The CHORUS experiment, designed to search for νμ→ντ oscillations, consists of a nuclear emulsion target and electronic detectors. In this paper, results on the production of charged particles in a small sample of charged-current neutrino– and anti-neutrino–nucleus interactions at high energy are presented. For each event, the emission angle and the ionization features of the charged particles produced in the interaction are recorded, while the standard kinematic variables are reconstructed using the electronic detectors. The average multiplicities for charged tracks, the pseudo-rapidity distributions, the dispersion in the multiplicity of charged particles and the KNO scaling are studied in different kinematical regions. A study of quasi-elastic topologies performed for the first time in nuclear emulsions is also reported. The results are presented in a form suitable for use in the validation of Monte Carlo generators of neutrino–nucleus interactions.

Destruction of 7Li and 7Be in astrophysical environments

Abstract The destruction of 7 Li and 7 Be in astrophysical environments is essential for understanding several stellar and cosmological processes and is not well understood, though earlier 7 Li + 3 He experiments have been performed [1]. The primordial abundance of 7 Li after Big Bang Nucleosynthesis (BBN) plays a major role in our understanding of the early universe [2]. The value of the baryon to photon ratio (η) deduced from BBN combined with measurements of the cosmic microwave background provide some of the strongest and earliest evidence for the existance of non-baryonic dark matter [2]. The destruction of 7 Be during the hot-pp cycle may alter our conclusions on the production of carbon in this process, which is thought to compete with the triple-α process for the production of 12 C, as the reaction 7 Be( 3 He, 2α)2p competes with 7 Be( 4 He, γ) 11 C and may reduce carbon production [3]. These stellar and cosmological environments involve high temperatures, and thus, effective burning energies (Gamow windows) that are quite high. Experiments using 7 Be targets inevitably involve interactions with 7 Li as background due to the 7 Li daughters from the beta decay of 7 Be. The experiments were performed at the Weizmann Institute VDG Laboratory using 3 He beams from 390 keV to 1130 keV on 7 LiF foil targets and 7 Be implanted targets. Results from measurements using 10 μg 7 LiF foil targets will be discussed.

The Ne-19(p,gamma)Na-20 astrophysical reaction rate determined from measurements with a radioactive beam

The Ne-19(p,gamma)Na-20 as well as the Ne-19(d,n)Na-20 reaction have been studied in inverse kinematics using Ne-19 radioactive beams. Upper and lower limits for the Ne-19(p,gamma)Na-20 astrophysical reaction rate have been deduced, for the first time on the basis of direct experimental data. It is concluded that the transition from the hot-CNO cycle to the rp-process in explosive hydrogen burning is most likely governed by the preceeding O-15(alpha,gamma)Ne-19 reaction.

Nuclear reactions of astrophysical interest with radioactive beams

The ARENAS(3) facility, coupling the two cyclotrons of Louvain-la-Neuve, is intensively been used over the past years for cross-section measurements of nuclear reactions of astrophysical interest. We will describe the characteristics of the available beams and the experiments going on using these beams. The Ne-19(p,gamma)Na-20 reaction, of considerable importance for the reaction now between the CNO and the NeNa mass region in high-temperature hydrogen burning conditions, will be discussed in particular. The proposed low-energy resonance at 0.447 MeV has been measured in inverse kinematics using novel activation techniques. An upper limit (90% C.L.) of 18 meV is obtained for the strength of this first particle-unbound state of Na-20 together with preliminary values for the strength of the higher excited states.

Precise measurement of the Jπ = 1− resonance in 14N by 13C+p elastic scattering on a thick target

The 1- resonance at 512 keV in N-14 was measured precisely by detecting recoil protons after scattering C-13 beams on a thick CH2 target. Proton spectra were analyzed with a Breit-Wigner formalism. It is shown that energy loss and straggling of the protons and of the heavy ions in the target can be correctly taken into account.

Giant-resonance effects in the12C(α, p)15N (3/2−, 6.328 MeV) transition

SummaryThe first experimental evidence for GQR effects in the (α, p) reaction is presented. Shapes of angular distributions of protons from the12C(α, p)15N reaction and their energy dependence suggest virtual formation of the GQR in16O as a doorway state leading to the (3/2−, 6.328 MeV) state of nitrogen.RiassuntoSi presenta la prima prova sperimentale degli effetti della GQR nella reazione (α, p). Le forme delle distribuzioni angolari dei protoni dalla reazione12C(α, p)15N e la loro di pendenza dall’energia suggeriscono la formazione virtuale di GQR nell’16O come uno stato di entrata che porta allo stato (3/2− 6.328 MeV) dell’azoto.