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Register a new userA Standard Model explanation for the MiniBooNE anomaly
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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 $3.6sigma$.
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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.
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.
While the low-energy excess observed at MiniBooNE remains unchallenged, it has become increasingly difficult to reconcile it with the results from other sterile neutrino searches and cosmology. Recently, it has been shown that non-minimal models with new particles in a hidden sector could provide a better fit to the data. As their main ingredients they require a GeV-scale $Z$, kinetically mixed with the photon, and an unstable heavy neutrino with a mass in the 150 MeV range that mixes with the light neutrinos. In this letter we point out that atmospheric neutrino experiments (and, in particular, IceCube/DeepCore) could probe a significant fraction of the parameter space of such models by looking for an excess of double-bang events at low energies, as proposed in our previous work (arXiv:1707.08573). Such a search would probe exactly the same production and decay mechanisms required to explain the anomaly.
If dark matter (DM) interacts with the Standard Model (SM) via the kinetic mixing (KM) portal, it necessitates the existence of portal matter (PM) particles which carry both dark and SM quantum numbers that will appear in vacuum polarization-like loop graphs. In addition to the familiar $sim eepsilon Q$ strength, QED-like interaction for the dark photon (DP), in some setups different loop graphs of these PM states can also induce other coupling structures for the SM fermions that may come to dominate in at least some regions of parameter space regions and which can take the form of `dark moments, eg, magnetic dipole-type interactions in the IR, associated with a large mass scale, $Lambda$. In this paper, motivated by a simple toy model, we perform a phenomenological investigation of a possible loop-induced dark magnetic dipole moment for SM fermions, in particular, for the electron. We show that at the phenomenological level such a scenario can not only be made compatible with existing experimental constraints for a significant range of correlated values for $Lambda$ and the dark $U(1)_D$ gauge coupling, $g_D$, but can also lead to quantitatively different signatures once the DP is discovered. In this setup, assuming complex scalar DM to satisfy CMB constraints, parameter space regions where the DP decays invisibly are found to be somewhat preferred if PM mass limits from direct searches at the LHC and our toy model setup are all taken seriously. High precision searches for, or measurements of, the $e^+e^- to gamma +{rm DP}$ process at Belle II are shown to provide some of the strongest future constraints on this scenario.
There has been persistent disagreement between the Standard Model (SM) prediction and experimental measurements of $R_{D^{(*)}}=mathcal{B}(bar B rightarrow D^{(*)} tau bar u_tau)/mathcal{B}(bar B rightarrow D^{(*)} l bar u_l)$ $(l=e,mu)$. This anomaly may be addressed by introducing interactions beyond the Standard Model involving new states, such as leptoquarks. Since the processes involved are quark flavor changing, any new states would need to couple to at least two different generations of quarks, requiring a non-trivial flavor structure in the quark sector while avoiding stringent constraints from flavor-changing neutral current processes. In this work, we look at scalar leptoquarks as a possible solution for the $R_{D^{(*)}}$ anomaly under the assumption of $it{minimal~flavor~violation}$ (MFV). We investigate all possible representations for the leptoquarks under the SM quark flavor symmetry group, consistent with asymptotic freedom. We consider constraints on their parameter space from self-consistency of the MFV scenario, perturbativity, the FCNC decay $bto sbar u u$ and precision electroweak observables. We find that none of the scalar leptoquarks can explain the $R_{D^{(*)}}$ anomaly while simultaneously avoiding all constraints within this scenario. Thus scalar leptoquarks with MFV-generated quark couplings do not work as a solution to the $R_{D^{(*)}}$ anomaly.
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