The Mu2e Experiment Searching for Charged Lepton Flavor Violation

2025-05-06 0 0 4.22MB 5 页 10玖币

The Mu2e Experiment — Searching for Charged Lepton Flavor Violation

M. T. Hedgesa,∗, on behalf of the Mu2e Collaboration

aDepartment of Physics and Astronomy, Purdue University, 525 Northwestern Avenue, West Lafayette, IN 47907, USA

Abstract

The Mu2e experiment will search for a Standard Model violating rate of neutrinoless conversion of a muon into an electron in the

presence of an aluminum nucleus. Observation of this charged lepton ﬂavor violating process would be an unambiguous sign of

new physics. Mu2e will improve upon previous searches for this process by four orders of magnitude. This requires the world’s

highest-intensity muon beam, a detector system capable of eﬃciently reconstructing the 105 MeV/c conversion electron signal, and

minimizing sensitivity to background events. A pulsed 8 GeV proton beam strikes a target, producing pions that decay into muons.

Beam outside the pulse must be suppressed to <10−10 to reduce beam-related backgrounds. The muon beam is guided from the

production target along the transport system and onto the aluminum stopping target. Conversion electrons leave the stopping target

and propagate inside a solenoidal magnetic ﬁeld to the tracker and electromagnetic calorimeter. The tracker is a system of straw

tube panels ﬁlled with Ar/CO2at 1 atm that tracks particles inside of a solenoidal B-ﬁeld and measures their momenta with ∼100

keV/cresolution to resolve signal events from decay-in-orbit backgrounds. The CsI calorimeter provides E/pand is used to seed

the track reconstruction algorithm with σE/E∼10% and σt<500 ps. Additionally, a novel cosmic ray veto with greater than

99.99% eﬃciency brings the expected number of background events to fewer than one over three years of running. To normalize

the experiment, the stopping target monitor measures the rate of capture photons from muons incident on the stopping target by

using a system of high-purity germanium and lanthanum bromide scintillators.

1. Introduction

The Standard Model of particle physics classiﬁes fundamen-

tal fermions into six quark ﬂavors and six lepton ﬂavors. Of

these fermions, only the charged leptons have never experi-

mentally shown evidence of ﬂavor violation. However, the dis-

covery of massive neutrinos provides a mechanism for charged

lepton ﬂavor violation (CLFV) at loop level, but is highly sup-

pressed to unobservably small levels. Thus, any observation of

CLFV would unambiguously indicate physics beyond the Stan-

dard Model.

One particularly promising experimental search for CLFV

focuses on detection of direct conversion of a muon to an elec-

tron in the Coulomb ﬁeld of a nucleus, or µ−N−e−Ncon-

version. This process produces a monoenergetic conversion-

electron (CE) with energy given by:

ECE =mµc2−Eb−Erecoil

where Ebis the binding energy of the muon in the 1Sorbit and

Erecoil is the energy of the recoiling nucleus.

2. The Mu2e experiment

The Mu2e experiment [1] will search for µ−N−e−Nby mea-

suring Rµe, deﬁned as the ratio of the rate of µ−N−e−Ncon-

version to the rate of muon capture using an aluminum target

∗Corresponding author: hedges7@purdue.edu

nucleus. Speciﬁcally, Mu2e will measure Rµeon an Al target

given by:

Rµe=µ−+A(Z,N)→e−+A(Z,N)

µ−+A(Z,N)→νµ+A(Z−1,N)

with a 5σdiscovery potential of Rµe>2×10−16 or a corre-

sponding upper limit of Rµe<8×10−17 (90% CL). This sensi-

tivity is four orders of magnitude beyond the current bounds set

by SINDRUM II which measured Rµe<7×10−13 on Au [2].

For µ−N−e−Nin aluminum, the CE signal is a monoener-

getic electron of ECE =104.9 MeV/c. Mu2e must detect this

signal in the presence of both intrinsic backgrounds such as

muon Decay-In-Orbit (DIO) and cosmic-ray events, and beam-

induced backgrounds such as antiproton annihilation and radia-

tive pion capture (RPC) in the muon stopping target. These two

types of backgrounds drive the design of the Mu2e experiment,

which is shown in Fig. 1.

DIO events are the primary intrinsic background of concern.

Muons incident on the aluminum stopping target can decay

while in atomic orbit to an electron and two neutrinos, just as

a free muon. While the DIO electron momentum spectrum is

similar to that of free muon decay, the presence of the aluminum

nucleus can cause a recoil and results in the kinematic endpoint

of DIO electrons to be equivalent to the momentum of the CE

signal. This is demonstrated graphically in Fig. 2. A full cal-

culation of the DIO spectrum in aluminum can be found in Ref.

[3].

Electrons emitted from the stopping target traverse a

solenoidal magnetic ﬁeld into the Mu2e tracker. Resolving

Preprint submitted to Elsevier

arXiv:2210.14317v2 [hep-ex] 27 Oct 2022

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TheMu2eExperimentSearchingforChargedLeptonFlavorViolationM.T.Hedgesa,,onbehalfoftheMu2eCollaborationaDepartmentofPhysicsandAstronomy,PurdueUniversity,525NorthwesternAvenue,WestLafayette,IN47907,USAAbstractTheMu2eexperimentwillsearchforaStandardModelviolatingrateofneutrinolessconversionofamuonintoa...

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The Mu2e Experiment Searching for Charged Lepton Flavor Violation

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