Precision measurement of oscillation parameters with reactors

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 8B 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 ∆m21 = 7.2× 10−5 eV2 and sin2 θ12 = 0.3) and high-LMA (with best-fit at ∆m21 = 1.5× 10−4 eV2 and sin2 θ12 = 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 νμ → ντ (ν̄μ → ν̄τ ) oscillations of the atmospheric νμ (ν̄μ ) from the observed Zenith angle dependence of the μ−like events in the Super-Kamiokande (SK) experiment – with maximal mixing and 1.3×10−3eV2 ∼< |∆mA | ∼< 3.1×10−3eV2 (90% C.L.) [8]. Since the solar and the atmospheric neutrino “anomalies” involve two hierarchically different mass scales, simultaneous description of the two requires the existence of threeneutrino mixing. The solar neutrino data constrain the parameters ∆m⊙ ∼ ∆m21 and θ⊙ ∼ θ12 while the atmospheric neutrino data constrain the parameters ∆matm ∼ ∆m31 and θatm ∼ θ23. The two sectors get connected by the mixing angle θ13 which is at present constrained by the reactor data [9]. After these magnificent results the stage is set for the era of precision measurement of the oscillation parameters. We will discuss here what more can be achieved in this respect in reactor experiments sensitive to the ∆m2 driven distortions in the ν̄e spectrum due to oscillations. The full expression for the ν̄e survival probability in the case of 3 flavor neutrino mixing and neutrino mass spectrum with normal hierarchy (NH) is given by [10] PNH = 1−2 sin2 θ13 cos2 θ13 sin2 ( ∆m31 L 4Eν ) − 1 2 cos4 θ13 sin2 2θ12 sin2 ( ∆m21 L 4Eν ) + 2 sin2 θ13 cos2 θ13 sin2 θ12 ( cos ( ∆m31 L 2Eν − ∆m 2 21 L 2Eν ) − cos ∆m 2 31 L 2Eν )