First Results in the Search for Dark Sectors at NA64 with the CERN SPS High Energy Muon Beam

We report the first search for dark sectors performed at the NA64 experiment employing a high energy muon beam and a missing energy-momentum technique. Muons from the M2 beamline at the CERN Super Proton Synchrotron with a momentum of $160\mathrm{GeV}/c$ are directed to an active target. The signal signature consists of a single scattered muon with momentum $<80\mathrm{GeV}/c$ in the final state, accompanied by missing energy, i.e., no detectable activity in the downstream calorimeters. For a total dataset of $(1.98\pm 0.02)\times {10}^{10}$ muons on target, no event is observed in the expected signal region. This allows us to set new limits on the remaining $(m_{Z^{\prime }},g_{Z^{\prime }})$ parameter space of a new $Z^{\prime }$ ($L_{\mu }-L_{\tau }$) vector boson which could explain the muon $(g-2{)}_{\mu }$ anomaly. Additionally, our study excludes part of the parameter space suggested by the thermal dark matter relic abundance. Our results pave the way to explore dark sectors and light dark matter with muon beams in a unique and complementary way to other experiments.

Figure 1

Production of a generic $X$ boson through a bremsstrahlunglike reaction, followed by its prompt invisible decay, $\mu N\to \mu NX$; $X\to \text{invisible}$. The interaction strength of the $X$ boson with SM particles and DM candidates is regulated by the couplings $g_X$ and $g_D$ respectively. The nucleus is assumed to recoil elastically leaving only the outgoing muon and invisible energy in the detector.

Figure 2

Schematic illustration of the $\mathrm{NA}64\mu$ setup and of a signal event topology. Well-defined incoming muons with momentum $p_{\mathrm{in}}\simeq 160\mathrm{GeV}/c$ are reconstructed in the first magnet spectrometer and tagged by a set of scintillator counters before arriving at the active target (ECAL). In the collision of muons with the target nuclei the bremsstrahlunglike reaction and subsequent invisible decay, $\mu N\to \mu N(Z^{\prime }\to \text{invisible})$ is produced. The resulting scattered muon with momentum $p_{\mathrm{out}}\le 80\mathrm{GeV}/c$ is measured in the second spectrometer (MS2).

Figure 3

Event distribution in the $(p_{\mathrm{out}},E_{\mathrm{CAL}})$ plane before the MIP-compatible requirement selection criterion. The signal region is defined as the shaded green rectangular area and the controlled region labeled with $A$ through $D$ (see text).

Figure 4

Left: $\mathrm{NA}64\mu$ 90% C.L. exclusion limits on the coupling $g_{Z^{\prime }}$ as a function of the $Z^{\prime }$ mass, $m_{Z^{\prime }}$, for the vanilla $L_{\mu }-L_{\tau }$ model. The $\pm 2\sigma$ band for the $Z^{\prime }$ contribution to the $(g-2{)}_{\mu }$ discrepancy is also shown. Existing constraints from BABAR [58, 59] and from neutrino experiments such as BOREXINO [21, 60, 61] and CCFR [22, 62] are plotted. Right: The 90% C.L. exclusion limits obtained by the $\mathrm{NA}64\mu$ experiment in the $(m_{\chi },y)$ parameters space for thermal dark matter charged under $U(1{)}_{L_{\mu }-L_{\tau }}$ with $m_{Z^{\prime }}=3m_{\chi }$ and the coupling $g_{\chi }=5\times {10}^{-2}$ for $2\times {10}^{10}$ MOT. The branching ratio to invisible final states is assumed to be $\mathrm{Br}(Z^{\prime }\to \text{invisible})\simeq 1$ (see text for details). Existing bounds obtained through the CCFR experiment [22, 62] are shown for completeness. The thermal targets for the different scenarios are taken from [63].