Probing ALP lepton flavor violation at <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow> <mml:mi>μ</mml:mi> <mml:mi>TRISTAN</mml:mi> </mml:mrow> </mml:math>

Abstract

Axionlike particles (ALPs) with lepton flavor-violating (LFV) interactions are predicted within a wide range of flavored ALP models. The proposed <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"> <a:mi>μ</a:mi> <a:mi>TRISTAN</a:mi> </a:math> high-energy <c:math xmlns:c="http://www.w3.org/1998/Math/MathML" display="inline"> <c:msup> <c:mi>e</c:mi> <c:mo>−</c:mo> </c:msup> <c:msup> <c:mi>μ</c:mi> <c:mo>+</c:mo> </c:msup> </c:math> and <e:math xmlns:e="http://www.w3.org/1998/Math/MathML" display="inline"> <e:msup> <e:mi>μ</e:mi> <e:mo>+</e:mo> </e:msup> <e:msup> <e:mi>μ</e:mi> <e:mo>+</e:mo> </e:msup> </e:math> collider will provide a good opportunity to explore flavor physics in the charged lepton sector. In this work, based on a model-independent effective Lagrangian describing the ALP leptonic interactions, we investigate the potential of <g:math xmlns:g="http://www.w3.org/1998/Math/MathML" display="inline"> <g:mi>μ</g:mi> <g:mi>TRISTAN</g:mi> </g:math> to probe ALP LFV couplings. We analyze the testability of selected ALP production channels with potential sensitivity at <i:math xmlns:i="http://www.w3.org/1998/Math/MathML" display="inline"> <i:mi>μ</i:mi> <i:mi>TRISTAN</i:mi> </i:math> , considering different beams and collision energies, including <k:math xmlns:k="http://www.w3.org/1998/Math/MathML" display="inline"> <k:msup> <k:mi>e</k:mi> <k:mo>−</k:mo> </k:msup> <k:msup> <k:mi>μ</k:mi> <k:mo>+</k:mo> </k:msup> <k:mo stretchy="false">→</k:mo> <k:mi>a</k:mi> <k:mi>γ</k:mi> </k:math> , <n:math xmlns:n="http://www.w3.org/1998/Math/MathML" display="inline"> <n:msup> <n:mi>e</n:mi> <n:mo>−</n:mo> </n:msup> <n:msup> <n:mi>μ</n:mi> <n:mo>+</n:mo> </n:msup> <n:mo stretchy="false">→</n:mo> <n:msup> <n:mi>e</n:mi> <n:mo>−</n:mo> </n:msup> <n:msup> <n:mi>τ</n:mi> <n:mo>+</n:mo> </n:msup> <n:mi>a</n:mi> </n:math> , <q:math xmlns:q="http://www.w3.org/1998/Math/MathML" display="inline"> <q:msup> <q:mi>μ</q:mi> <q:mo>+</q:mo> </q:msup> <q:msup> <q:mi>μ</q:mi> <q:mo>+</q:mo> </q:msup> <q:mo stretchy="false">→</q:mo> <q:msup> <q:mi>μ</q:mi> <q:mo>+</q:mo> </q:msup> <q:msup> <q:mi>τ</q:mi> <q:mo>+</q:mo> </q:msup> <q:mi>a</q:mi> </q:math> , and <t:math xmlns:t="http://www.w3.org/1998/Math/MathML" display="inline"> <t:msup> <t:mi>e</t:mi> <t:mo>−</t:mo> </t:msup> <t:msup> <t:mi>μ</t:mi> <t:mo>+</t:mo> </t:msup> <t:mo stretchy="false">→</t:mo> <t:msup> <t:mi>τ</t:mi> <t:mo>−</t:mo> </t:msup> <t:msup> <t:mi>μ</t:mi> <t:mo>+</t:mo> </t:msup> <t:mi>a</t:mi> </t:math> . The produced ALP <w:math xmlns:w="http://www.w3.org/1998/Math/MathML" display="inline"> <w:mi>a</w:mi> </w:math> is either long-lived or can promptly decay to flavor-violating or flavor-conserving charged lepton final states. In particular, combining the above LFV ALP production modes with a suitable LFV decay mode, one can identify signatures that are virtually free of Standard Model background. We show the resulting sensitivity of <y:math xmlns:y="http://www.w3.org/1998/Math/MathML" display="inline"> <y:mi>μ</y:mi> <y:mi>TRISTAN</y:mi> </y:math> to LFV ALP couplings and compare it with multiple low-energy leptonic constraints and the future improvements thereof. We find that <ab:math xmlns:ab="http://www.w3.org/1998/Math/MathML" display="inline"> <ab:mi>μ</ab:mi> <ab:mi>TRISTAN</ab:mi> </ab:math> can be generally complementary to searches for low-energy LFV processes and measurements of the leptonic magnetic dipole moments and has the capability to explore unconstrained parameter space for ALP masses in the <cb:math xmlns:cb="http://www.w3.org/1998/Math/MathML" display="inline"> <cb:mrow> <cb:mi mathvariant="script">O</cb:mi> <cb:mo stretchy="false">(</cb:mo> <cb:mn>1</cb:mn> <cb:mo stretchy="false">)</cb:mo> <cb:mi>–</cb:mi> <cb:mi mathvariant="script">O</cb:mi> <cb:mo stretchy="false">(</cb:mo> <cb:mn>100</cb:mn> <cb:mo stretchy="false">)</cb:mo> <cb:mtext> </cb:mtext> <cb:mtext> </cb:mtext> <cb:mi>GeV</cb:mi> </cb:mrow> </cb:math> range. In the light ALP regime, however, the parameter space that <kb:math xmlns:kb="http://www.w3.org/1998/Math/MathML" display="inline"> <kb:mi>μ</kb:mi> <kb:mi>TRISTAN</kb:mi> </kb:math> is sensitive to has been already excluded by low-energy searches for LFV decays. Published by the American Physical Society 2024

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