Precision measurement of oscillation parameters with reactors

Abstract

Abstract. We review the potential of long and intermediate baseline reactor neutrino experiments in measuring the mass and mixing parameters. The KamLAND experiment can measure the solar mass squared difference very precisely. However it is not at the ideal baseline for measuring the solar neutrino mixing angle. If low-LMA is confirmed by the next results from KamLAND, a reactor experiment with a baseline of 70 km should be ideal to measure precisely the solar neutrino mixing angle. If on the contrary KamLAND re-establishes high-LMA as a viable solution, then a 20-30 km intermediate baseline reactor experiment could yield very rich phenomenology.The first results from the KamLAND experiment in Japan [1] has showed that the electron antineutrinos undergo flavor oscillations on their way from their source to the detector. This result coupled with the assumption of CPT invariance has put to rest all speculations regarding the true solution of the long standing solar neutrino problem, where the electron neutrinos produced inside the Sun apparently disappear as they travel from the Sun to the Earth (see [2] for a recent review of the solar neutrino experiments). This disappearance of the solar neutrinos can now be attributed confidently to neutrino flavor mixing, with the mass squared difference same as that relevant for the KamLAND experiment. Earlier the spectacular evidence that these solar electron neutrinos do not really disappear, but rather appear disguised as a neutrino with a different active flavor, came from the first measurement of the total 8 B solar neutrino flux, through the neutral current (NC) reaction on deuterium, at the Sudbury Neutrino Observatory (SNO) [3]. The SNO NC data when combined with the data from all the other solar neutrino experiments picked the so called Large Mixing Angle (LMA) solution to the solar neutrino problem [3,4]. This remarkable result has very recently been reinforced by the salt phase data from the SNO experiment [5]. Prior to the SNO salt phase results, KamLAND data when combined with the other solar neutrino results, allowed two sub-regions within the LMA allowed region at the 99% C.L.-which we choose to call low-LMA (with best-fit at ∆m 2 21 = 7.2 × 10 −5 eV 2 and sin 2 θ 12 = 0.3) and high-LMA (with best-fit at ∆m 2 21 = 1.5 × 10 −4 eV 2 and sin 2 θ 12 = 0.3) [6]. After the SNO salt phase results, the combined analysis using all available data now allows the high-LMA solution only at the 99.13% C.L. [7]. Thus high-LMA is now further disfavored compared to low-LMA, though still not ruled out comprehensively.There exists also very strong evidence for ν µ → ν τ (ν µ →ν τ ) oscillations of the atmospheric ν µ (ν µ ) from the observed Zenith angle dependence of the µ−like events in the Super-Kamiokande (SK) experiment -with maximal mixing and 1.3 × 10 −3 eV 2 ∼ < |∆m 2 A | ∼ < 3.1 × 10 −3 eV 2 (90% C.L.) [8].

Summary of Working Group 2

“…She concluded that the Large Mixing Angle solution gives the best fit (sin 2 2θ 12 ≃ 0.8) and the LOW solution is allowed only at the 3σ level. Valle [1] also gave results on his solar neutrino analyses and his results agree with those in [17]. While whether the LMA solution is the correct one or not will be soon confirmed by the KamLAND experiment, it should be noticed that the goodness of fit (GOF) for each of the other solar solutions (LOW, Quasi Vacuum Oscillation, Small Mixing Angle solutions) is still acceptable.…”

“…Choubey [17] performed model independent and model dependent analyses of solar neutrino data including the neutral current event rate from SNO [18]. She concluded that the Large Mixing Angle solution gives the best fit (sin 2 2θ 12 ≃ 0.8) and the LOW solution is allowed only at the 3σ level.…”

“…While whether the LMA solution is the correct one or not will be soon confirmed by the KamLAND experiment, it should be noticed that the goodness of fit (GOF) for each of the other solar solutions (LOW, Quasi Vacuum Oscillation, Small Mixing Angle solutions) is still acceptable. Namely, according to [17], we have χ 2 (LMA)=40.57, χ 2 (LOW)=50.62, χ 2 (QVO)=56.11, and χ 2 (SMA)=70.97 for 45 degrees of freedom. This implies that LMA is 65.99% (0.44σCL) likely, LOW is 26.14% (1.12σCL) likely, QVO is 12.39% (1.54σCL) likely, and SMA is 0.81% (2.65σCL) likely.…”

Neutrino 2012: Outlook – theory

“…This will be possible studying the atmospheric neutrino fluxes with magnetized detectors, e.g. ICAL at INO [5], or with huge atmospheric neutrino detectors, e.g., PINGU. Another possibility is the LBL experiments: from NOνA [6] to "ultimate" LBL proposal, Fermilab -PINGU [7] in which neutrinos will cross the core of the Earth and therefore will undergo the parametric enhancement of oscillations.…”

Phenomenological and cosmological implications of neutrino oscillations

“…Recent SNO results [1] when combined with other solar neutrino data especially that of Super-Kamiokande strongly favour the large mixing angle (LMA) MSW solar solution [2,3] with three active light neutrino states, and θ 12 ≈ π/6, ∆m undetermined. The CHOOZ experiment limits θ 13 < ∼ 0.2 over the favoured atmospheric range [5].…”