---
title: 'COMET: Search for μ–e Conversion'
url: https://www.emergentmind.com/topics/comet-af857590-30a6-4701-a9f4-67478e54ac82
type: topic
---

# COMET: Search for μ–e Conversion

Searching arXiv for the COMET charged-lepton-flavour-violation experiment and closely related references.
COMET is an experiment at the J-PARC proton accelerator laboratory in Japan that searches for charged lepton flavour violation through $\mu \to e$ conversion in a muonic atom, specifically in aluminum [1812.07824]. In the Standard Model, charged-lepton-flavour-violating transitions such as $\mu$–$e$ conversion are mediated only by neutrino mixing loops and are suppressed below $10^{-50}$, so any observation at an experimentally accessible level would be an unambiguous sign of new physics [1812.07824]. The process is experimentally attractive because the final state is a monoenergetic electron, with $E_e \simeq m_\mu - E_{\rm binding} \simeq 104.97\ \mathrm{MeV}$ in aluminum, and no neutrinos, allowing powerful kinematic background rejection [1812.07824]. COMET is organized as a staged program: COMET Phase-I is intended to demonstrate the key beam, transport, and detector elements while improving the existing limit by about two orders of magnitude, and COMET Phase-II is designed to extend the sensitivity to a single event sensitivity of $2.6 \times 10^{-17}$ with $2 \times 10^7$ seconds of data-taking [1812.07824].

## 1. Physics target and observable

The central observable is $\mu \to e$ conversion in a muonic atom. In this process, a negative muon is stopped in matter, forms a muonic atom, and converts into an electron without neutrino emission. The resulting electron is monoenergetic, with energy near $104.97\ \mathrm{MeV}$ for aluminum, which distinguishes the signal from broad-spectrum backgrounds [1812.07824].

Among muonic charged-lepton-flavour-violating processes, $\mu \to e$ conversion is described as one of the most important channels and as a particularly “clean” one [1812.07824]. The absence of neutrinos in the final state makes the event kinematics much more constrained than in decay channels with invisible particles. This clean signature is one reason the search is treated as highly complementary to direct searches for Beyond the Standard Model physics at the LHC [1812.07824].

The theoretical motivation is broad. Many Standard Model extensions, including supersymmetry with R-parity violation, heavy neutrino mixing, leptoquarks, and $Z'$ bosons, predict sizable rates for $\mu \to e$ conversion [1812.07824]. The summary associated with COMET further states that these scenarios often correlate to effective mass scales $\Lambda \sim 10^3$–$10^4\ \mathrm{TeV}$ in loop or contact interactions, and that the sensitivity ultimately sought by COMET can probe many new-physics constructions up to $\mathcal{O}(10^{4})\ \mathrm{TeV}$ [1812.07824]. This suggests that the experiment is aimed less at direct production of heavy states than at precision access to virtual effects in CLFV operators.

## 2. Accelerator, beamline, and detector system

COMET uses the J-PARC $8\ \mathrm{GeV}$ pulsed proton beam and a graphite rod as the primary production target, placed in a $90^\circ$ bent solenoid to capture pions [1812.07824]. A superconducting pion–muon capture solenoid of up to $5\ \mathrm{T}$ surrounds the target. Negative muons are then selected and transported through curved solenoids, with $90^\circ$ and $180^\circ$ bends in Phase-I and Phase-II, at momenta $p_\mu \sim 40$–$60\ \mathrm{MeV}/c$ [1812.07824]. These bends provide charge and momentum selection and sweep out neutral and wrong-sign secondaries [1812.07824].

The stopping target is a set of thin aluminum foils in a $1\ \mathrm{T}$ field [1812.07824]. Aluminum is therefore integral both to the signal definition and to the beamline optimization. The combination of the capture solenoid, curved transport section, and thin-foil stopping target defines the experiment’s basic spectrometer and background-suppression concept.

The detector system comprises a Cylindrical Drift Chamber and a straw-tube tracker for trajectory and momentum measurement, with $\Delta p/p \sim 0.1\%$, together with an electromagnetic calorimeter based on CsI crystals for energy–time coincidence and $e/\pi$ separation, with $\Delta E/E \sim 5\%$ [1812.07824]. Additional performance figures listed for the full program include CDC momentum resolution $\Delta p/p \lesssim 0.1\%$ at $105\ \mathrm{MeV}/c$, calorimeter energy resolution $\Delta E/E \sim 5\%$ at $100\ \mathrm{MeV}$, and timing resolution $\lesssim 1\ \mathrm{ns}$ [1812.07824]. A cosmic-ray veto system surrounds the detector solenoid, with cosmic-ray veto inefficiency $\lesssim 10^{-4}$ [1812.07824].

## 3. Background environment and suppression strategy

COMET’s background-rejection strategy is built around beam timing, transport geometry, and detector resolution [1812.07824]. The pulsed beam has micro-bunch spacing of about $600\ \mathrm{ns}$, pulse width of about $100\ \mathrm{ns}$, and a beam-extinction factor $\lesssim 10^{-10}$ between pulses [1812.07824]. This pulsed structure suppresses prompt beam-related backgrounds, including pion capture and radiative muon capture [1812.07824].

A delayed analysis window is then imposed: the timing window is $700$–$1{,}000\ \mathrm{ns}$ after the proton pulse [1812.07824]. This rejects pions and beam electrons while retaining conversion candidates from stopped muons. Curved solenoids additionally remove line-of-sight neutral particles and charge-wrong tracks [1812.07824]. The geometry therefore serves not only transport but also passive rejection of prompt neutral secondaries.

Detector performance addresses the remaining irreducible and environmental backgrounds. The stated resolutions and active veto are intended to minimize the decays-in-orbit tail and cosmic-ray backgrounds [1812.07824]. Phase-I is also explicitly tasked with measuring beam-related backgrounds and the momentum spectrum of low-energy electrons [1812.07824], making it both a physics run and an empirical validation of the background model.

## 4. Staging, operating conditions, and sensitivity

COMET follows a two-phase staging strategy [1812.07824]. Phase-I is designed as the first demonstration of all key elements, including beam extinction, transport, detector performance, and an initial physics sensitivity. Phase-II is the full-scale apparatus with extended transport line, higher beam power, and optimized target and capture solenoid [1812.07824].

| Feature | Phase-I | Phase-II |
|---|---|---|
| Beam power | $3.2\ \mathrm{kW}$ | $\lesssim 56\ \mathrm{kW}$ |
| Running time | $\sim 10^7\ \mathrm{s}$ | $\sim 2 \times 10^7\ \mathrm{s}$ |
| SES | $\sim 3 \times 10^{-15}$ | $2.6 \times 10^{-17}$ |
| Main role | Demonstration + first sensitivity | Full-scale search |

Phase-I aims at a factor $100$ improvement over the current limit, corresponding to an expected single event sensitivity of about $3 \times 10^{-15}$ at $90\%$ C.L., compared with the SINDRUM II limit of $7 \times 10^{-13}$ [1812.07824]. Phase-II seeks a further improvement of nearly two orders of magnitude, reaching a total gain of $10{,}000$ over the previous limit and a target SES of $2.6 \times 10^{-17}$ after about $2 \times 10^7$ seconds of data taking, described as roughly two years at design power [1812.07824].

The single event sensitivity is written as [1812.07824]
$$
\mathrm{SES}=
\frac{1}{N_{-\mu}\times f_{\rm capture}\times A_{\rm det}},
$$
where $N_{-\mu}$ is the total number of stopped $\mu^-$, $f_{\rm capture}$ is the fraction of muonic atoms undergoing conversion, and $A_{\rm det}$ is the overall detection acceptance. For aluminum, the summary gives $f_{\rm capture}\approx 0.61$ [1812.07824]. Additional Phase-II performance metrics include a muon stop rate $\gtrsim 10^{10}\ \mu^-/\mathrm{s}$ [1812.07824].

The staged design also allows further refinements to be considered. The Phase-II description includes improved stray-field shielding, a thicker cosmic veto, and upgraded CDC electronics for higher rate [1812.07824]. The collaboration also notes that an additional order-of-magnitude improvement is being considered while remaining within the originally assumed beam power and beam time [1812.07824].

## 5. Relation to other CLFV probes and to BSM parameter space

The COMET program is positioned as complementary both to other charged-lepton-flavour-violation searches and to collider experiments [1812.07824]. The Phase-II SES of $2.6 \times 10^{-17}$ is presented as similar in ultimate reach to Mu2e, quoted at about $3 \times 10^{-17}$ [1812.07824]. By contrast, the projected reach of $\mu \to e \gamma$ in MEG II, around $6 \times 10^{-14}$ in branching ratio, is stated to correspond to a somewhat lower mass scale, around $10^3\ \mathrm{TeV}$, than COMET [1812.07824].

For contact interactions, the probed mass scale is summarized by [1812.07824]
$$
\Lambda_{\rm CLFV}\simeq \left(\sqrt{2}\,G_F \times \mathrm{SES}\right)^{-1/2}\sim \mathcal{O}(10^4\ \mathrm{TeV}).
$$
The same summary notes that LHC direct searches for heavy mediators such as leptoquarks and $Z'$ bosons typically probe up to a few $\mathrm{TeV}$, whereas COMET can probe effectively up to $\sim 10^4\ \mathrm{TeV}$ in certain CLFV operators [1812.07824]. A plausible implication is that null results at high-energy colliders do not substantially diminish the motivation for this search, because the scale probed in flavour-violating effective interactions is parametrically much higher.

Several benchmark BSM sensitivities are explicitly listed. In supersymmetric seesaw models with slepton mixing, COMET can probe off-diagonal slepton mass insertions $\delta^{12}_{LL}$ down to about $10^{-5}$ for $m_{\rm SUSY}\sim 1\ \mathrm{TeV}$ [1812.07824]. For heavy neutrino exchange, the sensitivity reaches mixing products $U_{\mu N}U^*_{eN}\sim 10^{-7}$–$10^{-8}$ for $M_N \sim 1$–$10\ \mathrm{TeV}$ [1812.07824]. For leptoquarks, the accessible mass-to-coupling combinations are given as $M_{\rm LQ}/\lambda \sim 10^3$–$10^4\ \mathrm{TeV}$, and for non-universal $Z'$ gauge bosons the sensitivity is quoted as $g'/M_{Z'} \sim 10^{-5}$–$10^{-6}\ \mathrm{TeV}^{-1}$ [1812.07824].

## 6. Scientific role and prospective impact

COMET is explicitly framed as a probe of new physics beyond the Standard Model through a channel whose Standard Model background is negligible at experimentally relevant levels [1812.07824]. Its importance follows from a conjunction of properties: a well-defined monoenergetic signal electron, strong prompt-background suppression from beam timing and extinction, transport-line rejection of wrong-sign and neutral secondaries, and a staged architecture that separates validation from full-sensitivity operation [1812.07824].

The experiment’s two-phase structure has strategic significance. Phase-I provides an early physics result and measures the beam-related backgrounds and low-energy electron spectrum, while simultaneously establishing the key technical ingredients required for the full machine [1812.07824]. Phase-II then extends the search to the $10^{-17}$ SES regime, where the inferred sensitivity to CLFV mass scales reaches $\mathcal{O}(10^4\ \mathrm{TeV})$ [1812.07824]. This suggests that COMET functions both as a discovery experiment and as a precision null-test of flavour conservation in the charged-lepton sector.

In that sense, COMET occupies a specific place in the broader experimental program of particle physics. It is not a collider detector and not a neutrino experiment, but a dedicated rare-process search centered on $\mu \to e$ conversion in aluminum. Its design reflects the logic of rare-event physics: maximize stopped-muon yield, isolate a narrow signal phase space near $105\ \mathrm{MeV}$, suppress prompt and cosmogenic backgrounds by timing and geometry, and translate event-counting sensitivity into constraints on BSM operators [1812.07824]. Within that framework, the collaboration’s stated objective is to push the search for $\mu$–$e$ conversion by four orders of magnitude beyond existing limits [1812.07824].

Source: https://www.emergentmind.com/topics/comet-af857590-30a6-4701-a9f4-67478e54ac82