Lepton-flavor-violating ALP signals with TeV-scale muon beams

Abstract

We explore the feasibility of using TeV-energy muons to probe lepton-flavor-violating (LFV) processes mediated by an axionlike particle (ALP) <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"> <a:mi>a</a:mi> </a:math> with mass <c:math xmlns:c="http://www.w3.org/1998/Math/MathML" display="inline"> <c:mi mathvariant="script">O</c:mi> <c:mo stretchy="false">(</c:mo> <c:mn>10</c:mn> <c:mtext> </c:mtext> <c:mtext> </c:mtext> <c:mi>GeV</c:mi> <c:mo stretchy="false">)</c:mo> </c:math> . We focus on <h:math xmlns:h="http://www.w3.org/1998/Math/MathML" display="inline"> <h:mi>μ</h:mi> <h:mi>τ</h:mi> </h:math> LFV interactions and assume that the ALP is coupled to a dark state <j:math xmlns:j="http://www.w3.org/1998/Math/MathML" display="inline"> <j:mi>χ</j:mi> </j:math> , which can be either less or more massive than <l:math xmlns:l="http://www.w3.org/1998/Math/MathML" display="inline"> <l:mi>a</l:mi> </l:math> . Such a setup is demonstrated to be consistent with <n:math xmlns:n="http://www.w3.org/1998/Math/MathML" display="inline"> <n:mi>χ</n:mi> </n:math> being a candidate for dark matter, in the experimentally relevant regime of parameters. We consider the currently operating NA64- <p:math xmlns:p="http://www.w3.org/1998/Math/MathML" display="inline"> <p:mi>μ</p:mi> </p:math> experiment and proposed <r:math xmlns:r="http://www.w3.org/1998/Math/MathML" display="inline"> <r:mrow> <r:mi>FASER</r:mi> <r:mi>ν</r:mi> <r:mn>2</r:mn> </r:mrow> </r:math> detector as both the target and the detector for the process <t:math xmlns:t="http://www.w3.org/1998/Math/MathML" display="inline"> <t:mi>μ</t:mi> <t:mi>A</t:mi> <t:mo stretchy="false">→</t:mo> <t:mi>τ</t:mi> <t:mi>A</t:mi> <t:mi>a</t:mi> </t:math> , where <w:math xmlns:w="http://www.w3.org/1998/Math/MathML" display="inline"> <w:mi>A</w:mi> </w:math> is the target nucleus. We also show that a possible future active muon fixed-target experiment operating at a 3 TeV muon collider or in its preparatory phase can provide an impressive reach for the LFV process considered, with future <y:math xmlns:y="http://www.w3.org/1998/Math/MathML" display="inline"> <y:mrow> <y:mi>FASER</y:mi> <y:mi>ν</y:mi> <y:mn>2</y:mn> </y:mrow> </y:math> data providing a pilot study towards that goal. The implications of the muon anomalous magnetic moment <ab:math xmlns:ab="http://www.w3.org/1998/Math/MathML" display="inline"> <ab:mo stretchy="false">(</ab:mo> <ab:mi>g</ab:mi> <ab:mo>−</ab:mo> <ab:mn>2</ab:mn> <ab:msub> <ab:mo stretchy="false">)</ab:mo> <ab:mi>μ</ab:mi> </ab:msub> </ab:math> measurements for the underlying model, in case of a positive signal, are also examined, and a sample UV completion is outlined. Published by the American Physical Society 2024

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