Atmospheric Neutrino Observations and Flavor Changing Interactions

Abstract

Flavor changing (FC) neutrino-matter interactions can account for the zenith-angle-dependent deficit of atmospheric neutrinos observed in the SuperKamiokande experiment, without directly invoking either neutrino mass or mixing. We find that FC n m -matter interactions provide a good fit to the observed zenith angle distributions, comparable in quality to the neutrino oscillation hypothesis. The required FC interactions arise naturally in many attractive extensions of the standard model. [S0031-9007(99) PACS numbers: 14.60. Pq, 14.60.St, 25.30.Pt, 96.40.Tv Neutrinos produced as decay products in hadronic showers from cosmic ray collisions with nuclei in the upper atmosphere [1] have been observed by several detectors [2][3][4][5][6][7]. Although the absolute fluxes of atmospheric neutrinos are largely uncertain, the expected ratio ͑m͞e͒ of the muon neutrino flux ͑n m 1n m ͒ over the electron neutrino flux ͑n e 1n e ͒ is robust, since it largely cancels out the uncertainties associated with the absolute flux. In fact, this ratio has been calculated [1] with an uncertainty of less than 5% over energies varying from 0.1 to 100 GeV. In this resides our confidence in the longstanding atmospheric neutrino anomaly.Although the first iron-calorimeter detectors in Fréjus [2] and NUSEX [3] reported a value of the double ratio, R͑m͞e͒ ͑m͞e͒ data ͑͞m͞e͒ MC , consistent with one, all of the water Cherenkov detectors, Kamiokande [4], IMB [5], and SuperKamiokande [6], have measured R͑m͞e͒ significantly smaller than one. Moreover, not long ago, the Soudan-2 Collaboration, also using an iron calorimeter, reported a small value of R͑m͞e͒ [7], showing that the so-called atmospheric neutrino anomaly was not a feature of water Cherenkov detectors.Recent SuperKamiokande high statistics observations [6] indicate that the deficit in the total ratio R͑m͞e͒ is due to the number of neutrinos arriving in the detector at large zenith angles. Although e-like events do not present any compelling evidence of a zenith angle dependence, the m-like event rates are substantially suppressed at large zenith angles.The n m ! n t [6,8], as well as the n m ! n s [8,9], oscillation hypothesis provides an appealing explanation for this smaller-than-expected ratio, as they are simple and well motivated theoretically. This led the SuperKamiokande Collaboration to conclude that their data provide good evidence for neutrino oscillations and neutrino masses.In this Letter we give an alternative explanation of the atmospheric neutrino data in terms of flavor changing (FC) neutrino-matter interactions [10][11][12][13][14]. We show that, even if neutrinos have vanishing masses and/or the vacuum mixing angle is negligible, FC neutrino-matter interactions can still explain the SuperKamiokande data.There are attractive theories beyond the standard model (SM), where neutrinos are naturally massless [15] as a result of a protecting symmetry, such as B-L in the case of supersymmetric SU͑5͒ models [16] and the model proposed in [17], or chiral symmetry in theories w...

Constraining nonstandard neutrino-electron interactions

“…Before closing this section let us mention that constraints coming from solar [18,19], atmospheric [21,22,23], and MINOS [59] data can not be directly compared with the bounds obtained here since those do not probe directly the NSI parameters but only a combination of them which effectively affects neutrino propagation in matter. For example, for the solar case, the relevant quantities, ε and ε ′ are two effective parameters which, for ε P αµ ∼ 0, are related with the vectorial couplings by:…”

“…The existence of effective neutral current interactions contributing to the neutrino scattering off d-quarks in matter, provides new flavor-conserving as well as flavorchanging terms for the matter potentials of neutrinos. Such NSI are directly relevant for solar [18,19,20] and atmospheric neutrino propagation [21,22,23].…”

“…The simplest of such modifications in the low energy regime may be written in the usual V − A form, similar to On the other hand, current neutrino oscillation data, as inferred from the solar [18,19,20] and atmospheric [21,22,23] neutrino experiments, leave significant room for the existence of sub-leading effects induced by NSI. In fact, NSI effects may be comparable to oscillation effects in solar neutrino physics, where indeed a new degenerate solution is still allowed [18].…”

How Sensitive is a Neutrino Factory to the Angleθ13?

“…These can be non-universal (NU) or flavour-changing (FCI), and may arise also in the absence of neutrino mass [6]. Such non-standard interactions of neutrinos affect their propagation in matter, with implications for solar [7,8,9,10], atmospheric [11,12], and astrophysical neutrino sources [13]. The possibility to use a neutrino factory for probing NSI with the earth matter, which neutrinos have to cross necessarily in a longbaseline experiment, has been previously examined in the appearance channels (−) ν e → (−) ν τ [14] and [14,15].…”

Unification of gauge couplings and the tau-neutrino mass in supergravity without R parity

“…First we point out that, as it stands, the present data allow for alternative explanations either in terms of flavour-changing neutrino matter interactions [31] or ν µ deutrino decay to ν τ [32] or ν µ decau to ν e [33] which might be relevant in the present model or in the presence of a majoron. Barring an enhanced statistics up-going muon event sample, it is hard to dismiss such alternative explanations of the atmospheric data on the basis of the present information on rates and zenith angle distributions both of sub as well as multi-GeV events.…”