Search for a New $B-L$ $Z^{\prime }$ Gauge Boson with the NA64 Experiment at CERN

A search for a new $Z^{\prime }$ gauge boson associated with (un)broken $B-L$ symmetry in the keV–GeV mass range is carried out for the first time using the missing-energy technique in the NA64 experiment at the CERN SPS. From the analysis of the data with $3.22\times {10}^{11}$ electrons on target collected during 2016–2021 runs, no signal events were found. This allows us to derive new constraints on the $Z^{\prime }-e$ coupling strength, which, for the mass range $0.3\lesssim m_{Z^{\prime }}\lesssim 100\mathrm{MeV}$, are more stringent compared to those obtained from the neutrino-electron scattering data.

Figure 1

$Z^{\prime }$ decay branching ratios to ${\nu }_i{\nu }_i;N_i{N}_i$ (red), $e^{+}{e}^{-}$ (blue), and ${\mu }^{+}{\mu }^{-}$ (green) for the unbroken (solid line) and broken (dashed line) $U(1{)}_{B-L}$ models. See text for details.

Figure 2

The energy distribution of events in the $E_{\mathrm{ECAL}};E_{\mathrm{HCAL}}$ 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 $E_{\mathrm{HCAL}}$ axis is increased by a factor of 5 for illustration.

Figure 3

The NA64 90% C.L. exclusion region in the ($m_{Z^{\prime }},g_{B-L}$) 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 $m_{N_i}/m_{Z^{\prime }}=1/3$ have been assumed. Constraints from the results of neutrino-electron scattering experiments obtained with nuclear-reactor neutrinos at TEXONO [31, 32, 33] and GEMMA [34], solar neutrinos at BOREXINO [35], and accelerator neutrino beams at LSND [36] and CHARM II [37] derived in Refs. [19, 29] 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 $Z^{\prime }-\chi$ coupling ${\alpha }_D=0.1$ and the mass ratio $m_{Z^{\prime }}=3m_{\chi }$, see Ref. [57].