Sin2 2θ13 Research Articles

Some recent results from the ICARUS experiment

ICARUS is the largest liquid Argon TPC detector ever built (∼ 600 ton LAr mass). It was smoothly operated underground at the LNGS laboratory in Gran Sasso since summer 2010, up to June 2013, collecting data with the CNGS beam and with cosmics. ICARUS is internationally considered as a milestone towards the realization of next generation of massive detectors (∼ tens of ktons) for neutrino and rare event physics. It permits, as a unique feature, the unambiguous identification of νe events. In particular an update of the experimental search for a νe signal in the LSND anomaly region in the CNGS beam will be here presented with the full statistics. The published result strongly limits the window of opened options for the LSND anomaly, reducing the remaining effect to a narrow region centered around (Δm2,sin2⁡(2θ))=(0.5 eV2,0.005) 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. Moreover, new results will be shown concerning the analysis of a CNGS beam-related stopping muon sample with the purpose of the momentum reconstruction through multiple Coulomb scattering. Finally, the most recent result on the Argon purity analysis will be presented, which allowed to reach impressive results in terms of Argon purity and a free electron lifetime exceeding 12 ms, corresponding to about 25 parts per trillion of O2-equivalent contamination: a milestone for any future project involving LAr-TPCs and the development of higher detector mass scales.

Current status of the Double Chooz experiment

The Double Chooz reactor antineutrino experiment aims for a precision measurement of the neutrino mixing angle θ13. Located at the Chooz nuclear power plant in France, it observes an energy dependent deficit in the electron antineutrino spectrum, currently with one detector filled with gadolinium-loaded liquid scintillator at a baseline of 1.05 km. The Double Chooz analysis utilizes different approaches to extract θ13: A combined rate and spectral shape fit as well as a background-model-independent analysis based on reactor power variations are performed, giving consistent results. Among the recent reactor-based oscillation experiments with comparable baseline it was the only one to observe reactor shutdown phases, during which all reactors are turned off. These enabled to measure the backgrounds solely, allowing to crosscheck the background models used in the oscillation analysis. At present an improved analysis was put forward with twice as much data statistics collected compared to the last publication. Revised selection criteria and background studies enhance the signal to background ratio while a decrease in the corresponding uncertainties is achieved. Along with an improved energy calibration the overall systematic uncertainty on θ13 is reduced, preparing for a two detector analysis. The new analysis obtains from 467.90 live days with 66.5 GW-ton-years of exposure (reactor power × detector mass × live time) a value of sin2⁡2θ13=0.090−0.029+0.032(stat + syst).

Neutrino oscillations with MINOS and MINOS+

The MINOS experiment ran from 2003 until 2012 and collected a data sample including 10.71×1020 protons-on-target (POT) of beam neutrinos, 3.36×1020 POT of beam antineutrinos and an atmospheric neutrino exposure of 37.88 kt yrs. The final measurement of the atmospheric neutrino oscillation parameters, Δm322 and θ23, came from a full three flavour oscillation analysis of the combined CC νμ and CC ν‾μ beam and atmospheric samples and the CC νe and CC ν‾e appearance samples. This analysis yielded the most precise measurement of the atmospheric mass splitting Δm322 performed to date. The results are |Δm322|=[2.28–2.46]×10−3 eV2 (68%) and sin2⁡θ23=0.35–0.65 (90%) in the normal hierarchy, and |Δm322|=[2.32–2.53]×10−3 eV2 (68%) and sin2⁡θ23=0.34–0.67 (90%) in the inverted hierarchy. The successor to MINOS in the NOνA era at FNAL, MINOS+, is now collecting data mostly in the 3–10 GeV region, and an analysis of νμ disappearance using the first 2.99×1020 POT of data produced results very consistent with those from MINOS. Future data will further test the standard neutrino oscillation paradigm and allow for improved searches for exotic phenomena including sterile neutrinos, large extra dimensions and non-standard interactions.

Measurement of neutrino mixing angle θ13 and mass difference [formula omitted] from reactor antineutrino disappearance in the RENO experiment

RENO (Reactor Experiment for Neutrino Oscillation) made a definitive measurement of the smallest neutrino mixing angle θ13 in 2012, based on the disappearance of reactor electron antineutrinos. The experiment has obtained a more precise value of the mixing angle and the first result on neutrino mass difference Δmee2 from an energy and baseline dependent reactor neutrino disappearance using ∼500 days of data. Based on the ratio of inverse-beta-decay (IBD) prompt spectra measured in two identical far and near detectors, we obtain sin2⁡(2θ13)=0.082±0.009(stat.)±0.006(syst.) and |Δmee2|=[2.62−0.23+0.21(stat.)−0.13+0.12(syst.)]×10−3 eV2. An excess of reactor antineutrinos near 5 MeV is observed in the measured prompt spectrum with respect to the most commonly used models. The excess is found to be consistent with coming from reactors. A successful measurement of θ13 is also made in an IBD event sample with a delayed signal of neutron capture on hydrogen. A precise value of θ13 would provide important information on determination of the leptonic CP phase if combined with a result of an accelerator neutrino beam experiment.

Intense antineutrino source based on a lithium converter. Proposal for a promising experiment for studying oscillations

An intense electron-antineutrino source with a hard spectrum (\(E_{{{\tilde v}_e}}^{\max }\) = 13 MeV and \(\left\langle {{E_{{{\tilde v}_e}}}} \right\rangle \) = 6.5MeV) can be created on the basis of the short-lived isotope 8Li (β−-decay, T1/2 = 0.84 s) formed via the (n, γ) activation of 7Li. In contrast to a reactor antineutrino spectrum whose uncertainty is large, particularly in the high-energy region \({E_{{{\tilde v}_e}}}\) > 6 MeV, which is experimentally relevant, the lithium \({\tilde v_e}\) spectrum is accurately determined. The proposed accelerator-driven experimental scheme with a neutron-producing target and a lithium converter as an intense \({\tilde v_e}\) source is an alternative to a nuclear reactor. The required amount of high-purity 7Li will be reduced in many times by using the suggested heavy-water LiOD solutions. A possible experiment involving the lithium source on search for sterile neutrinos in the mass region Δm2 ≥ 0.2 eV2 with a very high sensitivity to mixing-angle values down to sin2(2Θ) ≈ (7–10) × 10–4 at the 95% C.L. has been considered.

Neutrino masses and mixings: Status of known and unknown 3ν parameters

Within the standard 3ν mass–mixing framework, we present an up-to-date global analysis of neutrino oscillation data (as of January 2016), including the latest available results from experiments with atmospheric neutrinos (Super-Kamiokande and IceCube DeepCore), at accelerators (first T2K ν‾ and NOνAν runs in both appearance and disappearance modes), and at short-baseline reactors (Daya Bay and RENO far/near spectral ratios), as well as a reanalysis of older KamLAND data in the light of the “bump” feature recently observed in reactor spectra. We discuss improved constraints on the five known oscillation parameters (δm2, |Δm2|, sin2⁡θ12, sin2⁡θ13, sin2⁡θ23), and the status of the three remaining unknown parameters: the mass hierarchy [sign(±Δm2)], the θ23 octant [sign(sin2⁡θ23−1/2)], and the possible CP-violating phase δ. With respect to previous global fits, we find that the reanalysis of KamLAND data induces a slight decrease of both δm2 and sin2⁡θ12, while the latest accelerator and atmospheric data induce a slight increase of |Δm2|. Concerning the unknown parameters, we confirm the previous intriguing preference for negative values of sin⁡δ (with best-fit values around sin⁡δ≃−0.9), but we find no statistically significant indication about the θ23 octant or the mass hierarchy (normal or inverted). Assuming an alternative (so-called LEM) analysis of NOνA data, some δ ranges can be excluded at >3σ, and the normal mass hierarchy appears to be slightly favored at ∼90% C.L. We also describe in detail the covariances of selected pairs of oscillation parameters. Finally, we briefly discuss the implications of the above results on the three non-oscillation observables sensitive to the (unknown) absolute ν mass scale: the sum of ν masses Σ (in cosmology), the effective νe mass mβ (in beta decay), and the effective Majorana mass mββ (in neutrinoless double beta decay).

Experimental conditions for determination of the neutrino mass hierarchy with reactor antineutrinos

This article reports the optimized experimental requirements to determine neutrino mass hierarchy using electron antineutrinos (ν¯e) generated in a nuclear reactor. The features of the neutrino mass hierarchy can be extracted from the |Δm312| and |Δm322| oscillations by applying the Fourier sine and cosine transforms to the L/E spectrum. To determine the neutrino mass hierarchy above 90% probability, the requirements on the energy resolution as a function of the baseline are studied at sin2⁡2θ13=0.1. If the energy resolution of the neutrino detector is less than 0.04/Eν and the determination probability obtained from Bayes' theorem is above 90%, the detector needs to be located around 48–53 km from the reactor(s) to measure the energy spectrum of ν¯e. These results will be helpful for setting up an experiment to determine the neutrino mass hierarchy, which is an important problem in neutrino physics.

Leptonic Dirac CP violation predictions from residual discrete symmetries

Assuming that the observed pattern of 3-neutrino mixing is related to the existence of a (lepton) flavour symmetry, corresponding to a non-Abelian discrete symmetry group Gf, and that Gf is broken to specific residual symmetries Ge and Gν of the charged lepton and neutrino mass terms, we derive sum rules for the cosine of the Dirac phase δ of the neutrino mixing matrix U. The residual symmetries considered are: i) Ge=Z2 and Gν=Zn, n>2 or Zn×Zm, n,m≥2; ii) Ge=Zn, n>2 or Zn×Zm, n,m≥2 and Gν=Z2; iii) Ge=Z2 and Gν=Z2; iv) Ge is fully broken and Gν=Zn, n>2 or Zn×Zm, n,m≥2; and v) Ge=Zn, n>2 or Zn×Zm, n,m≥2 and Gν is fully broken. For given Ge and Gν, the sum rules for cos⁡δ thus derived are exact, within the approach employed, and are valid, in particular, for any Gf containing Ge and Gν as subgroups. We identify the cases when the value of cos⁡δ cannot be determined, or cannot be uniquely determined, without making additional assumptions on unconstrained parameters. In a large class of cases considered the value of cos⁡δ can be unambiguously predicted once the flavour symmetry Gf is fixed. We present predictions for cos⁡δ in these cases for the flavour symmetry groups Gf=S4, A4, T′ and A5, requiring that the measured values of the 3-neutrino mixing parameters sin2⁡θ12, sin2⁡θ13 and sin2⁡θ23, taking into account their respective 3σ uncertainties, are successfully reproduced.

Subdominant Dark Matter sterile neutrino resonant production in the light of PLANCK

Few independent detections of a weak X-ray line at an energy of ∼ 3.5 keV seen toward a number of astrophysical sites have been reported. If this signal will be confirmed to be the signature of decaying DM sterile neutrino with a mass of ∼ 7.1 keV, then the cosmological observables should be consistent with its properties.In this paper we make a coupled treatment of the weak decoupling, primordial nucleosynthesis and photon decoupling epochs in the sterile neutrino resonant production scenario, including the extra radiation energy density via Neff. We compute the radiation and matter perturbations including the full resonance sweep solution for να/ν̄α → νs flavor conversion in the expanding Universe.We show that the cosmological measurements are in agreement with subdominant Dark Matter sterile neutrino resonant production with following parameters (errors at 95% CL): mass mνs=6.08 ± 3.22 keV, mixing angle sin2 2θ < 5.61 × 10−10, lepton number per flavor L4 = 1.23 ± 0.04 (L4 ≡ 104 Lνa) and sterile neutrino mass fraction fνs< 0.078.Our results are in good agreement with the sterile neutrino resonant production parameters inferred in ref. [1] from the linear large scale structure constraints to produce full Dark Matter density.

Particle gamma correlations in 12C measured with the CsI(Tl) based detector array CHIMERA

The gamma decay of the first excited 4.44MeV 2+level of 12C, populated by inelastic scattering of proton and 16O beams at various energies was studied in order to test γ-ray detection efficiency and the quality of angular distribution information given by the CsI(Tl) detectors of the 4π CHIMERA array. The γ-decay was measured in coincidence with ejectile scattered particles in an approximately 4π geometry allowing to extract the angular distribution in the reference frame of recoiling 12C target. The typical sin2(2θ) behavior of angular distribution was observed in the case of 16O beam. Besides that, for the proton beam, in order to explain the observed distribution, the addition of an incoherent flat contribution was required. This latter is the effect of proton spin flip events allowing the population of M=±1 magnetic substates, that is not possible in reactions induced by 16O beam. A comparison with previously collected data, obtained measuring only in and out of plane proton-γ-ray coincidences, confirms the good quality of the angular distribution information given by the apparatus. Possible applications with radioactive beams are outlined.

Non-Standard Neutrino Interactions in T2K and Daya Bay

We show that the relatively large best fit value of sin2⁡2θ13=0.14(0.17) measured in the T2K experiment for fixed values of i) the Dirac CP violation phase δ=0, and ii) the atmospheric neutrino mixing parameters θ23=π/4, |Δm322|=2.4×10−3 eV2, can be reconciled with the Daya Bay result sin2⁡2θ13=0.090±0.009 if the effects of non-standard neutrino interactions (NSI) in the relevant ν‾e→ν‾e and νμ→νe oscillation probabilities are taken into account.

Double Chooz: Latest results

The latest results from the Double Chooz experiment on the neutrino mixing angle θ13 are presented. A detector located at an average distance of 1050 m from the two reactor cores of the Chooz nuclear power plant has accumulated a live time of 467.90 days, corresponding to an exposure of 66.5 GW-ton-year (reactor power × detector mass × live time). A revised analysis has boosted the signal efficiency and reduced the backgrounds and systematic uncertainties compared to previous publications, paving the way for the two detector phase. The measured sin2⁡2θ13=0.090−0.029+0.032 is extracted from a fit to the energy spectrum. A deviation from the prediction above a visible energy of 4 MeV is found, being consistent with an unaccounted reactor flux effect, which does not affect the θ13 result. A consistent value of θ13 is measured in a rate-only fit to the number of observed candidates as a function of the reactor power, confirming the robustness of the result.

Predictions for the Dirac Phase in the Neutrino Mixing Matrix and Sum Rules

Using the fact that the neutrino mixing matrix U = U†eUν, where Ue and Uv result from the diagonalisation of the charged lepton and neutrino mass matrices, we analyse the sum rules which the Dirac phase δ present in U satisfies when Uv has a form dictated by, or associated with, discrete symmetries and Ue has a “minimal” form (in terms of angles and phases it contains) that can provide the requisite corrections to Uv, so that reactor, atmospheric and solar neutrino mixing angles θ13, θ23 and θ12 have values compatible with the current data. The following symmetry forms are considered: i) tri-bimaximal (TBM), ii) bimaximal (BM) (or corresponding to the conservation of the lepton charge L' = Le — Lμ — Lτ (LC)), iii) golden ratio type A (GRA), iv) golden ratio type B (GRB), and v) hexagonal (HG). We investigate the predictions for 5 in the cases of TBM, BM (LC), GRA, GRB and HG forms using the exact and the leading order sum rules for cos δ proposed in the literature, taking into account also the uncertainties in the measured values of sin2 θ12, sin2 θ23 and sin2 θ13. This allows us, in particular, to assess the accuracy of the predictions for cos δ based on the leading order sum rules and its dependence on the values of the indicated neutrino mixing parameters when the latter are varied in their respective 3σ experimentally allowed ranges.

Neutrino mixing anarchy: Alive and kicking

Neutrino mixing anarchy is the hypothesis that the leptonic mixing matrix can be described as the result of a random draw from an unbiased distribution of unitary three-by-three matrices. In light of the very strong evidence for a nonzero sin2⁡2θ13, we show that the anarchy hypothesis is consistent with the choice made by the Nature – the probability of a more unusual choice is 41%. We revisit anarchy's ability to make predictions, concentrating on correlations – or lack thereof – among the different neutrino mixing parameters, especially sin2⁡θ13 and sin2⁡θ23. We also comment on anarchical expectations regarding the magnitude of CP-violation in the lepton sector, and potential connections to underlying flavor models or the landscape.

Determining the Dirac CP violation phase in the neutrino mixing matrix from sum rules

Using the fact that the neutrino mixing matrix U=Ue†Uν, where Ue and Uν result from the diagonalisation of the charged lepton and neutrino mass matrices, we analyse the sum rules which the Dirac phase δ present in U satisfies when Uν has a form dictated by, or associated with, discrete symmetries and Ue has a “minimal” form (in terms of angles and phases it contains) that can provide the requisite corrections to Uν, so that reactor, atmospheric and solar neutrino mixing angles θ13, θ23 and θ12 have values compatible with the current data. The following symmetry forms are considered: i) tri-bimaximal (TBM), ii) bimaximal (BM) (or corresponding to the conservation of the lepton charge L′=Le−Lμ−Lτ (LC)), iii) golden ratio type A (GRA), iv) golden ratio type B (GRB), and v) hexagonal (HG). We investigate the predictions for δ in the cases of TBM, BM (LC), GRA, GRB and HG forms using the exact and the leading order sum rules for cos⁡δ proposed in the literature, taking into account also the uncertainties in the measured values of sin2⁡θ12, sin2⁡θ23 and sin2⁡θ13. This allows us, in particular, to assess the accuracy of the predictions for cos⁡δ based on the leading order sum rules and its dependence on the values of the indicated neutrino mixing parameters when the latter are varied in their respective 3σ experimentally allowed ranges.

Recent Highlights From The T2K Experiment

Recent results from the T2K experiment on neutrino oscillation and neutrino interaction physics are presented. Details of the project, the current status and future plans are discussed. From a sample of 28νe appearance events T2K reports: sin2⁡2θ13=0.140−0.032+0.038(0.170−0.037+0.045) for normal (inverted) mass hierarchy and from a study of νμ disappearance events the following world-best value is extracted: sin2⁡(θ23)=0.514−0.056+0.055(sin2⁡(θ23)=0.511±0.055) for normal (inverted) mass hierarchy. Preliminary results from joint appearance/disappearance data fits are discussed which are begining to show sensitivity to allowed regions of δCP phase space.

Muon neutrino disappearance at T2K

The T2K experiment has observed the disappearance of muon neutrinos from the J-PARC muon-neutrino beam. Using data corresponding to 3.01 × 1020 protons on target, 58 muon neutrino event candidates were observed; 205 ± 17 (syst.) events are expected under the no-oscillation hypothesis. A fit to the observed rate and energy spectrum in a neutrino oscillation framework with three active neutrino flavours in constant density matter, assuming the normal mass hierarchy, yields a best fit mixing angle sin2 θ23 = 0.514 ± 0.082 and best fit mass squared splitting |Δm232| = 2.44+0.17−0.15 × 10-3 eV2/c4, which corresponds to the maximal disappearance probability.

Erratum to: Improved measurements of the neutrino mixing angle θ 13 with the Double Chooz detector

The Double Chooz experiment presents improved measurements of the neutrino mixing angle θ13 using the data collected in 467.90 live days from a detector positioned at an average distance of 1050 m from two reactor cores at the Chooz nuclear power plant. Several novel techniques have been developed to achieve significant reductions of the backgrounds and systematic uncertainties with respect to previous publications, whereas the efficiency of the ν¯¯¯e signal has increased. The value of θ13 is measured to be sin2 2θ13 = 0.090+ 0.032− 0.029 from a fit to the observed energy spectrum. Deviations from the reactor ν¯¯¯e prediction observed above a prompt signal energy of 4 MeV and possible explanations are also reported. A consistent value of θ13 is obtained from a fit to the observed rate as a function of the reactor power independently of the spectrum shape and background estimation, demonstrating the robustness of the θ13 measurement despite the observed distortion.

Double Chooz: First Background-independent Measurement of θ13

While the oscillation results published by the Double Chooz collaboration in 2011 and 2012 rely on a background model derived from reactor-on data, in this analysis we present a background model independent measurement of the mixing angle θ13. A global fit of the observed neutrino rates for different reactor power conditions is performed, yielding a measurement of both θ13 and the total background rate. The results on the mixing angle are improved significantly by including in the fit 7.53 days of reactor-off data, as they provides a direct measurement of the total background rate. In this reactor rate modulation analysis we consider neutrino candidates with neutron captures on both Gd and H nuclei, thereby boosting the statistical sample. The combination of the n-Gd and n-H candidates yields sin2(2θ13) = 0.102 ± 0.028(stat.) ± 0.033(syst.). The results presented in this study are consistent with the ones already published by Double Chooz, achieving a competitive precision. For the first time, they provide a determination of θ13 which does not dependon background models.

Recent results from Daya Bay experiment

This manuscript is a short summary of my talk given at ICNFP2014 Conference. Here we report on new results of sin2 2θ13 and Δm2ee measurements, search for the sterile neutrino within 10−3 eV2 < Δm241 < 0.1 eV2 domain and precise measurement of the reactor absolute antineutrino flux.