Shedding light on dark sectors with high-energy muons at the NA64 experiment at the CERN SPS

A search for dark sectors is performed using the unique M2 beam line at the CERN Super Proton Synchrotron. New particles (<a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:mi>X</a:mi></a:math>) could be produced in the bremsstrahlung-like reaction of high-energy 160 GeV positively charged muons impinging on an active target, <c:math xmlns:c="http://www.w3.org/1998/Math/MathML" display="inline"><c:mi>μ</c:mi><c:mi>N</c:mi><c:mo stretchy="false">→</c:mo><c:mi>μ</c:mi><c:mi>N</c:mi><c:mi>X</c:mi></c:math>, followed by their decays, <f:math xmlns:f="http://www.w3.org/1998/Math/MathML" display="inline"><f:mi>X</f:mi><f:mo stretchy="false">→</f:mo><f:mtext>invisible</f:mtext></f:math>. The experimental signature would be a scattered single muon from the target, with about less than half of its initial energy and no activity in the subdetectors located downstream from the interaction point. The full sample of the 2022 run is analyzed through the missing-energy/momentum channel, with total statistics of <i:math xmlns:i="http://www.w3.org/1998/Math/MathML" display="inline"><i:mo stretchy="false">(</i:mo><i:mn>1.98</i:mn><i:mo>±</i:mo><i:mn>0.02</i:mn><i:mo stretchy="false">)</i:mo><i:mo>×</i:mo><i:msup><i:mn>10</i:mn><i:mn>10</i:mn></i:msup></i:math> muons on target. We demonstrate that various muonphilic scenarios involving different types of mediators, such as scalar or vector particles, can be probed simultaneously with such a technique. For the vector case, besides a <m:math xmlns:m="http://www.w3.org/1998/Math/MathML" display="inline"><m:msub><m:mi>L</m:mi><m:mi>μ</m:mi></m:msub><m:mo>−</m:mo><m:msub><m:mi>L</m:mi><m:mi>τ</m:mi></m:msub></m:math> <o:math xmlns:o="http://www.w3.org/1998/Math/MathML" display="inline"><o:msup><o:mi>Z</o:mi><o:mo>′</o:mo></o:msup></o:math> vector boson, we also consider an invisibly decaying dark photon (<q:math xmlns:q="http://www.w3.org/1998/Math/MathML" display="inline"><q:msup><q:mi>A</q:mi><q:mo>′</q:mo></q:msup><q:mo stretchy="false">→</q:mo><q:mtext>invisible</q:mtext></q:math>). This search is complementary to NA64 running with electrons and positrons, thus opening the possibility to expand the exploration of the thermal light dark matter parameter space by combining the results obtained with the three beams. Published by the American Physical Society 2024