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Poster Excerpt 3: Searches for Short-Baseline Neutrino Oscillations with the T2K off-axis Near Detector

March 2, 2015

(The article below is from Stefania Bordoni, on behalf of the T2K collaboration)

Neutrinos are light and neutral particles interacting via the weak force. The standard model predicts three types (flavors) of neutrinos, each one corresponding to one of the 3 lepton families: electron neutrino (νe), muon neutrino (νµ) and tau neutrino (ντ). A peculiarity of neutrinos  is that they may change their flavor during propagation: given a neutrino of a certain flavor (e.g. νµ), there is a non-vanishing probability to detect that neutrino as another flavor (e.g νe or ντ ). This is the so-called “neutrino oscillation phenomenon”, a nice name hiding a quantum interference process.

To investigate such phenomenon, the parameters of interest are the mixing angles (sin22ϑ), and the mass squared differences (Δm2) giving information respectively about the amplitude and the frequency of the oscillation.

Figure 1 (below): Neutrino oscillation probability in the approximation of two neutrino flavors

bordoni1Neutrino oscillations seem to be very well described by the Pontecorvo-Maki-Nakagawa-Sakata matrix, however in the last 15 years the completeness of the three-neutrino paradigm has been challenged. Deficits or excesses in the expected number of events have been reported by a number of experiments studying the oscillations at a short distance (short-baseline) from the neutrino production point [1][2]. Those anomalies may be explained by introducing one or more neutrinos with some different properties with respect to the already well known standard (νe, νµ and ντ) neutrinos. These additional particles are expected to not interact via the weak force and for that reason they are usually called “sterile neutrinos”. This label clearly distinguishes them from the other three, usually labeled as “active neutrinos”.  Another characteristic of sterile neutrinos is their expected large mass (on the order of 1eV/c2). Such large mass would suggest an oscillation at short-baseline (SBL), as can be deduced from the expression for the  appearance or disappearance probability.

What makes the neutrino oscillations field even more challenging, is the fact that the anomalies  observed so far only concern electron (anti-)neutrinos. No hints of short-baseline oscillations have been observed for muon neutrinos. Several experiments [3-5] searching for muon neutrino short-baseline oscillations have started to set exclusion limits on these processes.

T2K (Tokai-to-Kamioka) is a long-baseline oscillation experiment located in Japan, studying neutrino oscillations starting from a beam of muon neutrinos [6].

Neutrinos are produced from the decay of pions and kaons generated by primary collisions of high energy protons with a graphite target. For that reason, the amount of data collected by an experiment is usually expressed in units of  “proton on target” or POT.

Apart from the well known Super-Kamiokande “far” detector located at about 300 km away from the point where neutrinos are produced, the experiment has also “near” detectors. ND280 is one of those:  it is located at 280m from the neutrino production point and has an angle of 2.5 degrees with respect to the direction of the neutrino beam (off-axis detector).  Thanks to its position this detector offers good conditions to search for short-baseline neutrino oscillations. The T2K collaboration has then the possibility to add new measurements and/or new exclusions limits.

bordoni2Towards this goal, the collaboration has indeed recently published the measurement of νe disappearance at ND280 [7].  A new analysis,  searching for muon neutrino disappearance is now also on-going. The poster presented at the conference summarizes these two analyses.

 Right: The T2K experiment

The searches for short-baseline neutrino oscillations for both νe and νµ analyses are performed considering the minimal extension of the standard three neutrino paradigm, the so-called 3+1 model.  In such model a sterile neutrino (νs) with a mass of order 1eV/c2 is added to the three active neutrinos (νe, νµ and νt).

Both the electron and muon neutrino event selections are based on the signals from the ND280 tracker:  three Time Projections Chambers (TPCs) allowing the reconstruction of the track of charged particles and two Fine Grain Detectors offering active targets for the neutrino interactions. For the electron neutrino selection also the information coming from the Electromagnetic Calorimeter (ECAL) surrounding the tracker is used.  The TPCs and the ECAL offer very good performance on particles identification. Thanks to these detectors it is then possible to disentangle electrons from muons, which are key elements of the signature of electron and muon neutrino events.

SBL νe disappearance :

The search for νe disappearance has been performed using the data collected between January 2010 and May 2013. This amount of data correspond to 5.9×1020 protons on target (POT). This is the very first results of T2K on these searches for short-baseline neutrino oscillations.

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Figure 2:  ND280 exclusion region at 95% CL for SBL electron neutrino disappearance compared to other experiments results [7].

Figure 2 shows the limits (95%CL) set by this analysis (green-shaded area).  One can note that the T2K results is covering part of the regions delimited by the other experiment: this means that with this analysis some potential values of the interesting parameters sin22ϑ and Δm2 are now excluded. More precisely the T2K results exclude parts of the allowed regions coming from the gallium anomaly [1] and the reactor anomaly [2].

SBL νµ disappearance :

A new analysis focusing on the search for νµ  disappearance is under development in the T2K collaboration.  Such measurement aim to verify and extend the current limits[3][4][5].

For this analysis only Monte Carlo-based studies are developed so far. The Monte Carlo simulation used in the study is normalized to the current T2K collected statistics, 6×1020 POT.

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Figure 3: Expected ND280 Monte Carlo sensitivity at 90%CL to SBL muon neutrino oscillations compared to other experiment limits [3-5].

As shown in Figure 3, the expected sensitivity (90%CL) to the muon neutrino disappearance obtained by this study looks promising if compared to the results from the previous experiments. As previously mentioned, no  hints of short-baseline muon neutrino disappearance has been reported so far. The regions delimited by the black line and gray area show the regions for which the values of the parameters sin22ϑ and Δm2 are already excluded. One can note than that in the high Δm2 region in fact, T2K may be able to extend the existing exclusion limits.

To conclude, the collection of further data will reduce the statistical uncertainty, which is an important limitation especially for the neutrino disappearance analysis.

It has to be noticed that deep correlations exist in the 3+1 model between the oscillation parameters of the different channels (νe disappearance, νµ disappearance and νe appearance). A joint analysis, aiming to fit simultaneously electron and muon neutrino signals, is foreseen for the near future and it promises to be interesting.

References :

[1] Kaether et al., “Reanalysis of the Gallex solar neutrino flux and source experiments”, Phys. Lett. B 685 Issue 1 (2010)

[2] Muller et al., “Improved predictions of Reactor Antineutrino Spectra“, Phy.Rev.C 83 054615 (2011)

[3] Mahn et al.,  “Dual baseline search for muon neutrino disappearance at 0.5 eV2 < Δm2 < 40 eV2”, Phys. Rev. D 85 032007 (2012)

[4] Dydak et al.,  “A search for νµ oscillations in the Δm2 range 0.3-90 eV2 ”, Phys Lett B 134, 281 (1984)

[5] Stockdale et al. , “Limits on Muon -Neutrino Oscillations in the large Mass Range 30< Δm2 < 1000 eV2/c4”,  Phys Rev Lett 52 1384 (1984)

[6] Abe et al. , “The T2K experiment”, Nucl. Instrum Meth A 659, 106 (2011)

[7] Abe et al.,  “Search for short baseline νe disappearance with the T2K near detector”, arXiv:1410.8811 (2014),  accepted by PRD.

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