KARMEN Experiment Research Articles

Probing the Dark Sector with Nuclear Transition Photons.

Here we present world-leading sensitivity to light (<170 MeV) dark matter (DM) using beam-dump experiments. Dark sector particles produced during pion decay at accelerator beam dumps can be detected via scattering in neutrino detectors. The decay of nuclei excited by the inelastic scattering of DM is an unexploited channel which has significantly better sensitivity than similar searches using the elastic scattering channel. We show that this channel is a powerful probe of DM by demonstrating sensitivity to the thermal relic abundance benchmark in a scalar DM dark-photon portal model. This is achieved through the use of existing data, obtained by the KARMEN experiment over two decades ago, which allow us to set world-leading constraints on this model over a wide mass range. With experimental improvements planned for the future, this technique will be able to probe the thermal relic benchmark for fermionic DM across a wide mass range.

EV See-Saw Sterile Neutrinos and the LSND Anomaly (and Extra Gauged U(1)s)

We suggest that a singlet fermion S exists beyond the standard see-saw structure. It mixes with light neutrinos via interactions with the right-handed neutrino components, so that νe → S conversion solves the solar neutrino problem. Supersymmetry endowed with R-symmetry is shown to give a natural framework for existence, mass scale (∼ 3 · 10−3 eV) and mixing (sin22θes ∼ (0.1−1.5) · 10−2) of such a fermion. Models with an approximate horizontal symmetry are constructed, which embed the fermion S and explain simultaneously solar, atmospheric, hot dark matter problems as well as may predict the oscillation νμ → νe in the region of sensitivity of KARMEN and LSND experiments.

Resolution of puzzles from the LSND, KARMEN, and MiniBooNE experiments

This work has attempted to reconcile puzzling neutrino oscillation results from the LSND, KARMEN and MiniBooNE experiments. We show that the LSND evidence for $\bar{\nu}_\mu \to \bar{\nu}_e$ oscillations, its long-standing disagreement with the results from KARMEN, and the anomalous event excess observed by MiniBooNE in $\nu_\mu$ and $\bar{\nu}_\mu$ data could all be explained by the existence of a heavy sterile neutrino ($\nu_h$). All these results are found to be consistent with each other assuming that the $\nu_h$ is created in $\nu_\mu$ neutral-current interactions and decays radiatively into a photon and a light neutrino. Assuming the $\nu_h$ is produced through mixing with $\nu_\mu$, the combined analysis of the LSND and MiniBooNe excess events suggests that the $\nu_h$ mass is in the range from 40 to 80 MeV, the mixing strength is $|U_{\mu h}|^2 \simeq 10^{-3}-10^{-2}$, and the lifetime is $\tau_{\nu_h} \lesssim 10^{-9}$ s. Surprisingly, this LSND-MiniBooNE parameters window is found to be unconstrained by the results from the most sensitive experiments searching for heavy neutrino. We set new limits on $|U_{\mu h}|^2$ for the LSND-MiniBooNE favorable mass region from the precision measurements of the Michel spectrum by the TWIST experiment. The results obtained provide a strong motivation for a sensitive search for the $\nu_h$ in a near future $ K$ decay or neutrino experiments, which fit well in the existing/planned experimental programs at CERN or FNAL. The question of whether the heavy neutrino is Dirac or Majorana particle is briefly discussed.

Muon capture constraints on sterile neutrino properties

We show that ordinary and radiative muon capture impose stringent constraints on sterile neutrino properties. In particular, we consider a sterile neutrino with a mass between 40 to $80 {\rm MeV}$ that has a large mixing with the muon neutrino and decays predominantly into a photon and light neutrinos due to a large transition magnetic moment. Such a model was suggested as a possible resolution to the puzzle presented by the results of the LSND, KARMEN, and MiniBooNE experiments. We find that the scenario with the radiative decay to massless neutrinos is ruled out by measurements of the radiative muon capture rates at TRIUMF in the relevant mass range by a factor of a few in the squared mixing angle. These constraints are complementary to those imposed by the process of electromagnetic upscattering and de-excitation of beam neutrinos inside the neutrino detectors induced by a large transition magnetic moment. The latter provide stringent constraints on the size of the transitional magnetic moment between muon, electron neutrinos and $N$. We also show that further extension of the model with another massive neutrino in the final state of the radiative decay may be used to bypass the constraints derived in this work.

Final neutrino oscillation results from LSND and KARMEN

The final neutrino oscillation results from the short-baseline accelerator experiments LSND and KARMEN are reviewed. The LSND experiment at LANSCE has observed an excess of ν e events interpreted as evidence for ν μ → ν μ appearance. This result suggests oscillations parameters in the range Δm 2 2 = 0.2–10 eV 2, implying that at least one neutrino could have a mass greater than 0.4 eV. The similar KARMEN experiment at ISIS observes no beam excess. The corresponding oscillation limit sin 2(2Θ) < 1.7 × 10 −3 (90% CL.) is the most stringent ν μ → ν e — limit so far and excludes the high Δm 2 LSND parameter space, while leaving some statistical space for oscillations at the two distinct regions Δm 2 < 1 eV 2 and Δm 2 ≅ 7eV 2.

Statistical analysis of differentν¯μ→ν¯esearches

A combined statistical analysis of the experimental results of the LSND and KARMEN ${\overline{\ensuremath{\nu}}}_{\ensuremath{\mu}}\ensuremath{\rightarrow}{\overline{\ensuremath{\nu}}}_{e}$ oscillation search is presented. LSND has evidence for neutrino oscillations that is not confirmed by the KARMEN experiment. This joint analysis is based on the final likelihood results for both data sets. A frequentist approach is applied to deduce confidence regions. At a combined confidence level of 36%, there is no area of oscillation parameters compatible with both experiments. For the complementary confidence of $1\ensuremath{-}0.36=64%,$ there are two well defined regions of oscillation parameters $({\mathrm{sin}}^{2}(2\ensuremath{\Theta}),\ensuremath{\Delta}{m}^{2})$ compatible with both experiments.

New results on a search for a 33.9 MeV/c2 neutral particle from π+ decay in the NOMAD experiment

Abstract We report on a direct search in NOMAD for a new 33.9 MeV/c2 neutral particle (X) produced in pion decay in flight, π→μX followed by the decay X→νe+e−. Both decays are postulated to occur to explain the time anomaly observed by the KARMEN experiment. From the analysis of the data collected during the 1996–1998 runs with 4.1×1019 protons on target, a single candidate event consistent with background expectations was found. The search is sensitive to a pion branching ratio BR(π→μX)>3.7×10−15, significantly smaller than previous experimental limits.

Short baseline projects

In a search for ν μ → ν e oscillations, the LSND experiment has observed an excess of events which are consistent with an oscillation signal. The KARMEN experiment, which is the only ongoing experiment which can test the LSND hypothesis, does not observe an excess, but also does not cover the full LSND allowed region. In this report, the results of these two experiments are discussed, together with the expectations of the MiniBooNE experiment which has been approved to run at FNAL in 2001. MiniBooNE's goal is to definitely verify or disprove the LSND oscillation hypothesis. Finally, the implications of the experimental techniques developed for current short baseline experiments for a future short baseline program, assuming the verification of the LSND signal, is also discussed.

4-Neutrino mass schemes and the likelihood of (3+1)-mass spectra

We examine the (3+1)-class of 4-neutrino mass spectra within a rigorous statistical analysis based on the Bayesian approach to probability. The data of the Bugey, CDHS and KARMEN experiments are combined by using a likelihood function. Our statistical approach allows us to incorporate solar and atmospheric neutrino data and also the result of the CHOOZ experiment via inequalities which involve elements of the neutrino mixing matrix and are derived from these data. For any short-baseline \(\Delta m^2\) we calculate a bound on the LSND transition amplitude \(A_{\mu;e}\) and find that, in the \(\Delta m^2\)–\(A_{\mu;e}\) plane, there is no overlap between the 99% CL region allowed by the latest LSND analysis and the region allowed by our bound on \(A_{\mu;e}\) at 95% CL; there are some small overlap regions if we take the bound at 99% CL. Therefore, we conclude that, with the existing data, the (3+1)-neutrino mass spectra are not very likely. However, treating the (2+2)-spectra with our method, we find that they are well compatible with all data.

Tau-sterile neutrino mixing in NOMAD

For several years, the KARMEN experiment was reporting an anomalous signal which was interpreted as a possible existence of heavy sterile neutrinos. The NOMAD experiment has the capability to search for such an isosinglet neutrino. It is assumed that the (ν s ) couples to the ν τ produced in the SPS neutrino beam at CERN. Analysis of NOMAD data leads to a single candidate event, compatible with the background expectations. This allows a limit to be set for the first time on the mixing strength between sterile and tau neutrinos, in the mass range 10 to 190 MeV.

Role ofν−inducedreactions on lead and iron in neutrino detectors

We have calculated cross sections and branching ratios for neutrino induced reactions on ^{208}Pb and ^{56}Fe for various supernova and accelerator-relevant neutrino spectra. This was motivated by the facts that lead and iron will be used on one hand as target materials in future neutrino detectors, on the other hand have been and are still used as shielding materials in accelerator-based experiments. In particular we study the inclusive ^{56}$Fe(\nu_e,e^-)$^{56}Co and ^{208}$Pb(\nu_e,e^-)$^{208}Bi cross sections and calculate the neutron energy spectra following the decay of the daughter nuclei. These reactions give a potential background signal in the KARMEN and LSND experiment and are discussed as a detection scheme for supernova neutrinos in the proposed OMNIS and LAND detectors. We also study the neutron-emission following the neutrino-induced neutral-current excitation of ^{56}Fe and ^{208}Pb.

Search for a 33.9 MeV/c(2) neutral particle in pion decay

The E815 (NuTeV) neutrino experiment has performed a search for a 33. 9 MeV/c(2) 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).

The KARMEN limit on [formula omitted] oscillations and its implication for the LSND result

The KARMEN experiment is located at the pulsed spallation neutron source ISIS of the Rutherford Appleton Laboratory. In the ISIS beam stop v ν μ , ν e and ν μ are produced from the π +− μ + decay chain at rest with energies up to 52.8 MeV. Besides a very low ν e contamination, ISIS stands out for its unique time structure. This allows for a highly sensitive search for ν μ→ ν e- oscillations with the KARMEN detector, a 56 t segmented liquid scintillation calorimeter with very good time, energy and position resolution. Here we present the data taken from Feb. 1997 until Feb. 1999. Since there is no indication for any beam excess, an upper limit for the ν μ→ ν e oscillation is deduced using the Unified Approach based on a maximum likelihood analysis. The result is in line with the data presented previously and is in contradiction to an interpretation of the LSND beam excess as indication for ν μ→ ν e oscillation. A combined statistical analysis of the KARMEN and LSND likelihood functions underlines this.

The KARMEN anomaly, light neutralinos and supernova SN1987A

The KARMEN experiment observes a time anomaly in events induced by pion decay at rest. This anomaly can be ascribed to the production of a new weakly interacting particle X with mass m_X\sim 34 MeV. We show that a recently proposed identification of the X particle with the lightest neutralino \chi in the frame work of the MSSM with broken R parity is in contradiction to optical observations of type II supernovae.

Compatibility analysis of the LSND evidence and the KARMEN exclusion for oscillations

A combined statistical analysis of the experimental results of the LSND and KARMEN µe oscillation search is presented. LSND has evidence for neutrino oscillations that is not confirmed by the KARMEN experiment. However, there is a region in the (sin2(2), ) parameter space where the results of both experiments are statistically compatible. This joint analysis is based on likelihood functions for both data sets. A frequentist approach to creating Monte Carlo samples analogous to the experimental outcome is applied to deduce correct confidence limits. Different schemes of combination can be chosen to provide correct coverage which leads to slightly different confidence regions in (sin2(2), ).

Theoretical overview: emphasis on neutrinos

The missing mass of the universe is likely to consist of at least three components: baryons, neutrinos, and a particle which was weakly interacting and slow moving (“cold”) in the early universe. It is doubtful that the baryonic candidates observed by microlensing could be an appreciable part of our local dark matter halo. The dominant cold component might be the supersymmetric neutralino, and if so, recent accelerator constraints making this more gaugino-like should make direct detection of the dark matter particle easier. Despite recent observations favoring a low-density universe, such a model (even with the addition of a cosmological constant) does not fit universe structure. The only model which does has a critical density, 20% neutrinos, 10% baryons, and 70% cold dark matter. The data are fit better if the neutrino dark matter is shared between two neutrino species ( ν μ and ν τ ) rather than one, a mass pattern which also explains the solar and atmospheric neutrino deficits and the LSND experiment. It is shown here that KARMEN and other experiments do not conflict with the LSND results in the appropriate mass region. Further support for this mass pattern is provided by the need for a sterile neutrino to rescue heavy-element nucleosynthesis in supernovae.

KARMEN: Neutrino oscillation limits and new results with the upgrade

The neutrino experiment KARMEN is situated at the beam stop neutrino source ISIS which provides ν μ 's, ν e 's and ν μ' s from the π +− μ +-decay at rest. The oscillation channels ν μ → ν e and ν μ → ν e are investigated with a 56 t liquid scintillation calorimeter. No evidence for oscillations could be found with KARMEN, resulting in 90% CL exclusion limits of sin 2(2Θ) < 8.5 · 10 −3 ( ν e ) and sin 2(2Θ) < 4.0 · 10 −2 ( ν μ → ν e ) for Δm 2 > 100 eV 2. In 1996, the Karmen experiment has been upgraded by an additional veto counting system with a total coverage of 300 m 2. The new system allows the identification of cosmic muons in the vicinity of the detector. Vetoing these muons suppresses energetic neutrons from deep inelastic scattering of muons as well as from μ-capture by a factor of 40. Up to 1996, these neutrons represented the main background for ν μ → ν e oscillation search. The experimental sensitivity for ν μ → ν e will be significantly enhanced towards sin 2(2Θ)⋍ 1.0 · 10 −3 after a further measuring period of 2–3 years.

Measurement of the Energy Spectrum ofνefrom Muon Decay and Implications for the Lorentz Structure of the Weak Interaction

The KARMEN experiment uses the reaction 12C(\nu_e,e-)12N to measure the energy distribution of \nu_e emitted in muon decay at rest. The \nu_e analog \omega_l of the famous Michel parameter \rho has been derived from a maximum-likelihood analysis of events near the kinematic end point, E_max. The result, \omega_l = (2.7^{+3.8}_{-3.3}\pm 3.1) * 10^{-2}, is in good agreement with the standard model prediction \omega_l= 0. We deduce a 90% confidence upper limit of \omega_l <= 0.113, which corresponds to a limit of |g^S_RL + 2 g^T_RL| <= 0.78 on the interference term between scalar and tensor coupling constants.

Measurement of the weak neutral current excitation at Eνμ=29.8 MeV

Abstract The weak neutral current reaction 12 C( ν μ , ν μ ′ ) 12 C ∗ (1 + ,1; 15.1MeV) has been observed for the first time in the KARMEN experiment. Neutrino events were separated from background using two different analysis methods. The measured cross section σ NC =(3.2 ± 0.5 stat. ± 0.4 syst. ) × 10 −42 cm 2 for monoenergetic ν μ from π + -decay at rest is in good agreement with the standard model, the isovector-axialvector coupling constant of weak hadronic current deduced from this experiment is | β |=1.11±0.13.

KARMEN upgrade and prospects at ESS

The KARMEN experiment at the ISIS spallation source has enhanced its sensitivity for ν μ → ν e oscillations in 1996 by the installation of an additional large area veto counter system. Consisting of 136 highly transparent plastic scintillator modules, the new veto is completely embedded inside the experiment's 7000 tonne steel blockhouse. It allows to eliminate the main background component in the search for ν μ → ν e appearance originating from high energy neutrons induced by deep inelastic scattering of cosmic ray muons in the massive shielding. First measurements after the detector upgrade show a substantial reduction of this background meeting the expected factor of 40. If no oscillation signal is seen after 3 years of measuring (1997–1999) KARMEN will be able to exclude mixing amplitudes of sin 22 Θ≃1 x10 -3. The ongoing KARMEN2 measurements will thus allow a decisive and reliable test of the entire parameter space favoured by the positive result of LSND. An even higher oscillation sensitivity can be achieved by experiments at future high intensity short spill proton accelerators such as the planned European Spallation Source ESS.