CERN Accelerating science

$Z'$ decay branching ratios to $\nu_i \nu_i; N_i N_i$(red), $\ee$(blue), and $\mm$ (green) for the unbroken (solid line) and broken (dashed line) $U(1)_{B-L}$ models. See text for details.

Diagram contributing to the $Z'$ production in the reaction $e^- Z \rightarrow e^- Z Z'$. The produced $Z'$ decays invisibly into the neutrino pair.

The energy distribution of events in the $E_\ec;E_\hc$ plane selected from the data sample of the 2021 run collected with intensity $\simeq 5\times 10^6~e^-$ per spill after applying all selection criteria. The shaded area is the signal box, which contains no events. The size of the signal box along the $E_\hc$ axis is increased by a factor of 5 for illustration.

Schematic illustration of the setup to search for $\ainv$ decays of the bremsstrahlung $Z'$s produced in the reaction $eZ\rightarrow eZ Z'$ of 100 GeV $e^-$ incident on the active ECAL target.

The NA64 90\% C.L. exclusion region in the ($m_{Z'}, g_{B-L}$) plane for the unbroken (solid blue line) and broken (dashed blue line) $U(1)_{B-L}$. For the latter case, three degenerate heavy neutrino species with the mass ratio $\frac{m_{N_i}}{m_{Z'}} = \frac{1}{3}$ have been assumed. Constraints from the results of neutrino-electron scattering experiments obtained with nuclear-reactor neutrinos at TEXONO \cite{tex-csi,tex-hpge,tex-npcge} and GEMMA \cite{gemma}, solar neutrinos at BOREXINO \cite{borexino}, and accelerator neutrino beams at LSND \cite{lsnd}, and CHARM II \cite{charm} derived in Refs. \cite{aliev, lin1} are also shown. The dashed area above the curves is excluded. As an example, the black curve illustrates the parameter space for the thermal scalar DM model for which the abundance of scalar $\chi$ is in agreement with the observed DM energy density. The curve is calculated assuming the $Z'-\chi$ coupling $\alpha_D=0.1$ and the mass ratio $\mz = 3 m_\chi$, see Ref.\cite{berlin}.

The left panel shows the energy distribution of events in the $E_\ec;E_\hc$ plane selected from the data sample of the 2021 run collected with intensity $\simeq 5\times 10^6~e^-$/spill after applying all selection criteria. The shaded area is the signal box, which contains no events. The size of the signal box along the $E_\hc$ axis is increased by a factor of 5 for illustration purposes. The side bands A and C are used for the background estimate inside the signal region. The right panel shows distribution of events in a control area of the signal box $(E_{HCAL}\lesssim 1~{\rm GeV}; E_{ECAL}\gtrsim 50~{\rm GeV} )$. The curve shows the exponential fit to the data extrapolated to the signal region which predicts $n_b = 0.03\pm0.015$ background events.

The NA64 90\% C.L. exclusion region in the ($m_{Z'}, g_{B-L}$) plane for the unbroken (solid blue line) and broken (dashed blue line) $U(1)_{B-L}$. For the later case, three degenerate heavy neutrino species with the mass ratio $\frac{m_{N_i}}{m_{Z'}} = \frac{1}{3}$ have been assumed. Constraints from the results of neutrino-electron scattering experiments obtained with nuclear-reactor neutrinos at TEXONO \cite{tex-csi,tex-hpge,tex-npcge} and GEMMA \cite{gemma}, solar neutrinos at BOREXINO \cite{borexino}, and accelerator neutrino beams at LSND \cite{lsnd}, and CHARM II \cite{charm} derived in Refs. \cite{aliev, lin1} are also shown. The dashed area above the curves is excluded.