Paper of the month: Updated Global 3+1 Analysis of Short-BaseLine Neutrino Oscillations

Neutrinos are truly elusive particles: electrically neutral, very light and weakly interacting. That is why we know less about them than about other particles of the Standard Model of particle physics. The quantum-mechanical phenomena of “neutrino oscillations” have allowed us to understand much better the properties of these particles, but still there are some discrepancies between the experimental results and the commom framework in which the neutrino oscillations are studied. Could these disagreements be understood if we introduce a new, “sterile”, neutrino in our formalism? This is the question studied in this paper of the month.

April 25, 2017 by tiinatimonen

So what was our starting point?

To explain what neutrino oscillations are, we need to know that neutrinos (and antineutrinos, the antiparticles of the neutrinos) come in three known types, corresponding to the three families of charged leptons and quarks in the Standard Model of elementary particles. We refer to these diﬀerent types of neutrino as the electron, the muon and the tau neutrino.

In the past decays many experiments measured neutrinos and antineutrinos coming from sources such as the Sun, which is a huge natural source of electron neutrinos, however the measurements of the incoming neutrinos did not ﬁt perfectly with the theoretical expectations: In some cases less neutrinos than expected reached the detector, in others cases it was shown that ﬂavors diﬀerent to the electron neutrino one, which is the only one produced in the Sun, were also present in the detected ﬂux. Were the neutrinos changing ﬂavour during their journey from the source to the detector?

Indeed, the results could be explained by Bruno Pontecorvo’s theory of “neutrino oscillations”: During their propagation through space the ﬂavour neutrinos mix with each other such that the probability at the detector of observing a diﬀerent ﬂavour to the one produced in the source is non-zero.

The only modiﬁcation of the Standard Model of particle physics that is needed in order for it to accommodate the theory of neutrino oscillations is the introduction of neutrino masses. These allow the neutrinos to be described in a new way: not only in terms of their ﬂavour, but also in terms of their mass. When the neutrinos are described in terms of their mass, they become superpositions of ﬂavour neutrinos. It is this mixing that makes the neutrinos “oscillate”, meaning that they change their ﬂavour with time.

The missing neutrino

While the three neutrino ﬂavour framework with its, now massive, electrons, muon and tau neutrinos has been very successful in explaining almost all data collected by neutrino oscillation experiments, some experiments disagree with this picture, presenting some inconsistencies, referred to as the LSND, reactor and Gallium anomalies.

Although there are some discrepancies between the experiments that found the neutrino anomalies, it is interesting to note that the anomalies could be explained by the presence of a new type of neutrino, the sterile neutrino, with ∼1 eV mass).

The reason why this hypothetical neutrino is called “sterile neutrino” is that there are robust experimental tests which ensure that no more than three ﬂavours of light neutrinos (neutrinos with masses smaller than half of the mass of the Z boson) interact with Standard Model particles other than the neutrinos.

In spite of the elusiveness of the hypothetical neutrinos, it will soon be possible to detect them, if they exist, precisely by looking at neutrino oscillations. Naturally, the mixing of the sterile neutrino with the known neutrinos would alter the pattern of neutrino oscillations, possibly explaining the anomalies mentioned above.

At this point it may be wise to remark that the introduction of the sterile neutrino is not the only solution to the neutrino anomalies, and that the small statistical conﬁdence of the experiments does not yet allow us to infer its existence.

However, since the discovery of a sterile neutrino would have a large impact on our understanding of particle physics and cosmology, a lot of eﬀort has been put into its search, narrowing down the window where this sterile neutrino could be found.

In a recent combined ﬁt of all the neutrino experiments sensitive to the hypothetical eV sterile neutrino, [1] C.Giunti et.al. give an update on the status of the search for the sterile neutrino.

The allowed parameter space is illustrated in Fig.1. In the panels labeled as (a),(b) and (c) the mass squared diﬀerence ∆m241 = m2 4 −m2 1 of the new neutrino (denoted as number four) and the lightest of the known neutrinos (denoted as number one) is taken along the Y-axis and the characteristic mixing parameter is along the X-axis. The best ﬁt value for the mass squared diﬀerence is 1.7 eV2, which yields a mass of ∼1 eV for the new hypothetical neutrino. In panel (d) the same mixing parameter as in (c) is represented along the X-axis, but diﬀerent future experiment sensitivities are plotted.

The constraints on the sterile neutrino parameters are so stringent that the next generation of experiments will be able to cover the totallity of the currently allowed regions for the sterile neutrino parameters, determining whether or not the sterile neutrino exists: Exciting times!

References

[1] S.Gariazzo, C.Giunti, M.Laveder and Y.F.Li, Updated Global 3+1 Analysis of ShortBaeLine Neutrino Oscillations, 1703.00860v2 [hep-ph].

Figure 1: (Figure 13 from Ref.[1]) Current status of the allowed parameters (colour regions) of the sterile neutrino, along with the sensitivity of the future neutrino experiments.

Text by Álvaro Hernández and Fiona Kirk