Experimental search for the LSND anomaly with the ICARUS detector in the CNGS neutrino beam

TL;DR

This study reports an early ICARUS search for the LSND-like νμ→νe signal using the CNGS beam and a large liquid argon TPC. With 1091 ν events, two ν_e candidates are observed against a 3.7 ± 0.6 expectation from standard sources, leading to stringent limits on new-physics oscillations that favor no LSND anomaly within the probed L/E range. The analysis narrows the viable parameter space for a sterile neutrino interpretation, centering a preferred region near Δm^2_{new} ≈ 0.5 eV^2 and sin^2(2θ_{new}) ≈ 0.005, and highlights the potential of future ICARUS-NESSIE measurements to further constrain or reveal oscillations at shorter baselines.

Abstract

We report an early result from the ICARUS experiment on the search for nu_mu to nu_e signal due to the LSND anomaly. The search was performed with the ICARUS T600 detector located at the Gran Sasso Laboratory, receiving CNGS neutrinos from CERN at an average energy of about 20 GeV, after a flight path of about 730 km. The LSND anomaly would manifest as an excess of nu_e events, characterized by a fast energy oscillation averaging approximately to sin^2(1.27 Dm^2_new L/ E_nu) = 1/2. The present analysis is based on 1091 neutrino events, which are about 50% of the ICARUS data collected in 2010-2011. Two clear nu_e events have been found, compared with the expectation of 3.7 +/- 0.6 events from conventional sources. Within the range of our observations, this result is compatible with the absence of a LSND anomaly. At 90% and 99% confidence levels the limits of 3.4 and 7.3 events corresponding to oscillation probabilities of 5.4 10^-3 and 1.1 10^-2 are set respectively. The result strongly limits the window of open options for the LSND anomaly to a narrow region around (Dm^2, sin^2(2 theta))_new = (0.5 eV^2, 0.005), where there is an overall agreement (90% CL) between the present ICARUS limit, the published limits of KARMEN and the published positive signals of LSND and MiniBooNE Collaborations.

Figures (7)

• Figure 1: (A) Energy deposition density distribution for muons in CNGS CC interactions, compared with Monte Carlo, normalised to the same number of entries. Each entry corresponds to one wire hit. (B) Experimental raw energy distribution $E_{dep}$ for muon neutrinos and antineutrinos CC interaction in the ICARUS T600 detector (blue symbols) compared with the Monte Carlo expectations (red solid histogram), normalised to the same number of entries.
• Figure 2: Average ionisation in the first 8 wire hits for sub-GeV photons in the T600 data (full squares), compared to Monte Carlo expectations (solid line) normalised to the same number of events. In MC case, the Compton contribution is shown also separately (dotted line).
• Figure 3: Typical Monte Carlo generated $\nu_\mu \rightarrow \nu_e$ event from the ICARUS full simulation program FLUKA1FLUKA2FLUKA-nu with $E_e = 11$ GeV and $p_T = 1.0$ GeV/c. The close similarity of the MC simulation with actual ICARUS events (see Figure \ref{['fig:eventi']} (A) and (B)) is apparent.
• Figure 4: Experimental picture of the two observed events (A) and (B) with a clearly identified electron signature out of the total sample of 1091 neutrino interactions. Event in (A) has a total energy of $11.5 \pm 1.8$ GeV, and a transverse electron momentum of $1.8 \pm 0.4$ GeV/c. Event in (B) has a visible energy of $\sim$17 GeV and a transverse momentum of $1.3 \pm 0.18$ GeV/c. In both events the single electron shower in the transverse plane is clearly opposite to the remaining of the event. (C): display of the actual dE/dx along individual wires of the electron shower shown in Figure \ref{['fig:eventi']}A, in the region ($\ge$ 4.5 cm from primary vertex) where the track is well separated from other tracks and heavily ionising nuclear prongs. As a reference, the expected dE/dx distribution for single and double minimum ionising tracks (see Figure \ref{['fig:comp']}A), are also displayed. The dE/dx evolution from single ionising electron to shower is also shown.
• Figure 5: Two-dimensional plot of $\Delta m^2$ vs $\sin^2{(2 \theta_{new} )}$ for the main published experiments sensitive to the $\nu_\mu \rightarrow \nu_e$ anomaly LSNDMiniBooneKARMENNOMADCCFRNuTeV and the present ICARUS result. The ICARUS limits to the oscillation probability are $\left< P_{\nu_\mu \rightarrow \nu_e}\right> \le 5.4 \times 10^{-3}$ and $\left< P_{\nu_\mu \rightarrow \nu_e}\right> \le 1.1 \times 10^{-2}$ , corresponding to $\sin^2{(2 \theta_{new} )} \le 1.1\times 10^{-2}$ and $\sin^2{(2 \theta_{new} )} \le 2.2\times 10^{-2}$ respectively at 90% and 99% CL. Limits correspond to 3.41 and to 7.13 events.