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Register a new userA Standard Model explanation for the excess of electron-like events in MiniBooNE
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A.Ioannisian
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We study the dependence of neutral current (NC) neutrino-induced $pi^0$/photon production ($ u_mu + A to u_mu +1pi^0 / gamma + X$) on the atomic number of the target nucleus, A, at 4-momentum transfers relevant to the MiniBooNE experiment: $Delta$ resonance mass region. Our conclusion is based on experimental data for photon-nucleus interactions from the A2 collaboration at the Mainz MAMI accelerator. We work in the approximation that decays of $Delta$ resonance unaffected by its production channel, via photon or Z boson. $1pi^0+X$ production scales as A$^{2/3}$, the surface area of the nucleus. Meanwhile the photons created in $Delta$ decays will leave the nucleus, and that cross section will be proportional to the atomic number of the nucleus. Thus the ratio of photon production to $pi^0$ production is proportional to A$^{1/3}$. For carbon $^{12}$C this factor is $approx$2.3. MiniBooNE normalises the rate of photon production to the measured $pi^0$ production rate. The reduced neutral pion production rate would yield at least twice as many photons as previously expected, thus significantly lowering the number of unexplained electron-like events.
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We present the results of a new analysis of the data of the MiniBooNE experiment taking into account the additional background of photons. MiniBooNE normalises the rate of photon production to the measured $pi^0$ production rate. We study neutral current (NC) neutrino-induced $pi^0$/photon production ($ u_mu + A to u_mu +1pi^0 / gamma + X$) on carbon nucleus (A=12). Our conclusion is based on experimental data for photon-nucleus interactions from the A2 collaboration at the Mainz MAMI accelerator. We work in the approximation that decays of the intermediate states (non-resonant N, $Delta$ resonance, higher resonances) unaffected by its production channel, via photon or Z boson. $1pi^0+X$ production scales as A$^{2/3}$, the surface area of the nucleus. Meanwhile the photons incoherently created in intermediate states decays will leave the nucleus, and that cross section will be proportional to the atomic number of the nucleus. We also took into account the coherent emission of photons. We show that the new photon background can explain part of the MiniBooNE low-energy excess, thus significantly lowering the number of unexplained MiniBooNE electron-like events from $5.1sigma$ to $3.6sigma$.
The MiniBooNE experiment at Fermilab reports results from an analysis of $ u_e$ appearance data from $12.84 times 10^{20}$ protons on target in neutrino mode, an increase of approximately a factor of two over previously reported results. A $ u_e$ charged-current quasielastic event excess of $381.2 pm 85.2$ events ($4.5 sigma$) is observed in the energy range $200<E_ u^{QE}<1250$~MeV. Combining these data with the $bar u_e$ appearance data from $11.27 times 10^{20}$ protons on target in antineutrino mode, a total $ u_e$ plus $bar u_e$ charged-current quasielastic event excess of $460.5 pm 99.0$ events ($4.7 sigma$) is observed. If interpreted in a two-neutrino oscillation model, ${ u}_{mu} rightarrow { u}_e$, the best oscillation fit to the excess has a probability of $21.1%$, while the background-only fit has a $chi^2$ probability of $6 times 10^{-7}$ relative to the best fit. The MiniBooNE data are consistent in energy and magnitude with the excess of events reported by the Liquid Scintillator Neutrino Detector (LSND), and the significance of the combined LSND and MiniBooNE excesses is $6.0 sigma$. A two-neutrino oscillation interpretation of the data would require at least four neutrino types and indicate physics beyond the three neutrino paradigm.Although the data are fit with a two-neutrino oscillation model, other models may provide better fits to the data.
The MiniBooNE Collaboration observes unexplained electron-like events in the reconstructed neutrino energy range from 200 to 475 MeV. With $6.46 times 10^{20}$ protons on target, 544 electron-like events are observed in this energy range, compared to an expectation of $415.2 pm 43.4$ events, corresponding to an excess of $128.8 pm 20.4 pm 38.3$ events. The shape of the excess in several kinematic variables is consistent with being due to either $ u_e$ and $bar u_e$ charged-current scattering or to $ u_mu$ neutral-current scattering with a photon in the final state. No significant excess of events is observed in the reconstructed neutrino energy range from 475 to 1250 MeV, where 408 events are observed compared to an expectation of $385.9 pm 35.7$ events.
The MiniBooNE Experiment has contributed substantially to beyond standard model searches in the neutrino sector. The experiment was originally designed to test the $Delta m^2$~1 eV$^2$ region of the sterile neutrino hypothesis by observing $ u_e$ ($bar u_e$) charged current quasi-elastic signals from a $ u_mu$ ($bar u_mu$) beam. MiniBooNE observed excesses of $ u_e$ and $bar u_e$-candidate events in neutrino and anti-neutrino mode, respectively. To date, these excesses have not been explained within the neutrino Standard Model ($ u$SM), the Standard Model extended for three massive neutrinos. Confirmation is required by future experiments such as MicroBooNE. MiniBooNE also provided an opportunity for precision studies of Lorentz violation. The results set strict limits for the first time on several parameters of the Standard Model-Extension, the generic formalism for considering Lorentz violation. Most recently, an extension to MiniBooNE running, with a beam tuned in beam-dump mode, is being performed to search for dark sector particles. This review describes these studies, demonstrating that short baseline neutrino experiments are rich environments in new physics searches.
We present the results of a new analysis of the data of the MiniBooNE experiment taking into account the additional background of photons from $Delta^{+/0}$ decay proposed in arXiv:1909.08571 and additional contributions due to coherent photon emission, incoherent production of higher mass resonances, and incoherent non-resonant nucleon production. We show that the new background can explain part of the MiniBooNE low-energy excess and the statistical significance of the MiniBooNE indication in favor of short-baseline neutrino oscillation decreases from $5.1sigma$ to $3.6sigma$. We also consider the implications for short-baseline neutrino oscillations in the 3+1 active-sterile neutrino mixing framework. We show that the new analysis of the MiniBooNE data indicates smaller active-sterile neutrino mixing and may lead us towards a solution of the appearance-disappearance tension in the global fit of short-baseline neutrino oscillation data.
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