Lepton Flavour Violation in Muon Decays

Flavor Physics and CP Violation Conference, Victoria BC, 2019 1

Lepton ﬂavour violation in muon decays

L. Galli INFN Sezione di Pisa, Largo B. Pontecorvo 3, 56127, Pisa

The search for lepton ﬂavour violation in charged lepton decays is highly sensitive to physics beyond the Standard Model. Among the possible processes, µ-decays are considered to have the largest discovery potential in most of the standard model extensions. Many searches has been performed in the past, however no evidence has been found so far. Four dedicated experiments are in advanced state of preparation to improve the current associated sensibilities by 1-4 order of magnitudes for the charged lepton ﬂavour violating processes µ → eγ, µ → e conversion and µ → eee. In this paper I present physics motivations, experimental challenges and construction status of the experiments, which are the studying above mentioned processes.

I. INTRODUCTION History of CLFV experiments with muons 1 − µ → eγ 10 1 This proceeding is a short review about charged lep- − µ N → e N 10 2 ton ﬂavour violation in the muon sector. The reader − µ → 3e 10 3

interested in more details is invited to look at [1–4]. 90% C.L. Limit 10−4 In the minimal Standard Model (SM) the charged − 10 5 lepton flavour violating processes (cLFV) are prohib- −6 ited since the doublets are separated and the neutri- 10 −7 nos are massless. In this framework the three lepton 10 − families are separated and the lepton flavour num- 10 8 − ber is separately conserved in any processes. Despite 10 9 − the fact that the neutrino oscillation phenomena show 10 10 that neutrino are definitely massive , and even taking 10−11 into account this effect, the branching fraction associ- 10−12 − ated to a cLFV decay µ → eγ in the SM is unmeasur- 10 13 ably small ≈ 10−54: 10−14 1940 1950 1960 1970 1980 1990 2000 2010 2020 Year

2 FIG. 1: cLFV upper limits history in the last 80 years. 3α X ∗ ∆mi1 2 B(µ → eγ) = | UµiUei 2 | (1) 32π i=2,3 MW A. cLFV and physics beyond the standard model: a model independent approach Thus any evidence of µ → eγ decay would incontro- vertibly demonstrate the existence of physics beyond The SM is widely considered a low energy approx- the Standard model (BSM). Figure 1 shows the evolu- imation of a more general theory, possible extensions tion of the sensitivities on the cLFV processes that has are associated to theories, among many others, such been reached in the last 80 years. The lack of the sig- as super-simmetry, grand uniﬁcation of the forces and arXiv:1906.10483v1 [hep-ex] 25 Jun 2019 nal was one of the cornerstone of the leptonic ﬂavour the Majorana nature of the neutrino. Any of the be- structure in the SM. See [5] as a general reference. fore mentioned frameworks produces a prediction of Muons are very sensitive probes of cLFV, in fact cLFV in the range accessible to experiments as a func- intense muon beams can be obtained by hitting light tion of the theory parameters: couplings and energy targets with low energy protons (≤ 590 MeV/c at Paul scale. Scherrer Institut, PSI) or at proton accelerators as Independently of the nature of the BSM physics by product of high energy collisions (Fermilab and J- cLFV in the µ-decay would happen via a dipole tran- PARC); the relatively long decay time, 2.2 µsec, allows sition or a phenomenological 4-fermion like interaction to transport those beams on a thin target to let the as schematically represented in Figure 2. muons decay at rest. It is possible to accumulate large The diagrams schematically show the dipole oper- amount of statistics and then reach sensitivities in the ator contributes at one-loop order to µ → eee and range 10−14 ÷ 10−17 as it will be described in details µ → e and vice versa how the 4-fermion works for in the following sections together with a comparison µ → eγ; thus a combined evidence of cLFV from more among the various decays and their discovery power. than one experiment can indicate the nature of the SM

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Liquid xenon photon detector COBRA (LXe) superconducting magnet

Pixelated timing counter (pTC) Muon stopping target

Cylindrical drift chamber Radiative decay counter (CDCH) (RDC)

FIG. 2: Schematic representation of the tree level and associated one-loop contribution for µ → eγ decay in the FIG. 4: Schematic view of the MEG II detector. top row and µ → eee and µ → e conversion in the bottom. The BSM interaction is in the circle. In general, in order to reach the experimental sensitivity, the experiments are equipped with detectors having resolution at unprecedented values for parti- cles (e and γ) of energy in the range 10-100 MeV, with fast response to cope with µ-decay intensity and as transparent as possible to minimise any matter effect. Ideally, if one was able to measure the experimental observables and to eliminate any beam induced background, these searches would be background-free searches; this is the cLFV experimental challenge.

A. The µ → eγ search in MEG II

MEG II at Paul Scherrer Institute searches for the µ+ → e+γ decay with a design sensitivity of −14 FIG. 3: Allowed values for the dipole and 4-fermion cou- 6×10 [7], about one order of magnitude lower with plings given by current and future limits on µ → eγ and respect to the previous result obtained by the same µ → eee decays [6]. collaboration with the MEG experiment[8]. Positive muons are used to avoid interferences due to the negative muon capture in nuclei. extension; the experiments presented below are in this The signature is a time coincident, back-to-back sense complementary. pair of a monoenergetic photon and a monoenergetic An example is shown in Figure 3 where the allowed positron, both with an energy equal to half of the values for dipole couplings and for 4-fermion interac- muon mass, Ee = Eγ ≈ 52.8 MeV. There are two ma- tion are shown given present and future limits on the jor backgrounds: one is a prompt background from + + + µ → eγ and µ → eee decays [6]. radiative muon decay, µ → e νeνµγ, when e and the γ are emitted back-to-back with the two neutrinos carrying away little energy; in this decay the two par- II. EXPERIMENTAL SEARCHES: ticles are emitted at the same time. The other back- GENERALITIES ground is an accidental coincidence of a e+ in a normal + + µ-decay, µ → e νeνµ, accompanied by a high-energy Given the energy-momentum conservation only photon from a muon radiative decay or a positron an- three processes are considered for the cLFV, each one nihilation in ﬂight. The accidental background gives deserves a dedicated experiment with peculiar char- the major contribution since the associated rate in- acteristics. All searches diﬀer in terms of beam inten- creases with the square of the muon beam intensity; sity, structure, detection technique and related back- the signal to noise ratio is then maximised by using a grounds. continuous beam.

TueB1501100 Flavor Physics and CP Violation Conference, Victoria BC, 2019 3

A 7×107 µ+/s beam is stopped in a 140 µm slanted The Mu3e experiment: Schematic 3D + polyethylene target. The e momentum is measured Tile detector by a magnetic spectrometer, composed by an almost Superconducting 70 ps resolution solenoid Magnet w/ single hit Fibre hodoscope Homogeneous ﬁeld solenoidal magnet (COBRA) with an axial gradient < 500 ps resolution 1T w/ multi hits

ﬁeld and by an ultra thin unique volume cylindrical thickness: < 0.3% X0 drift chamber. The e+ timing is measured by two ma- trices of 256 plastic scintillators pixels read out with MIDAS DAQ and Slow SiPMs (Timing Counter, TC). The γ energy, direction Control Run, history, alarms, HV and timing are measured in a ≈ 800 l volume liquid etc.

Muon Beam and xenon (LXe) scintillation detector by means of MP- target PCs on the inner face and PMTs on the lateral. A Mupix detector Full available Tracking, integrate sensor and beam intensity schematic overview of the MEG II detector is shown readout in the same device: 50 um O(108) thick in Figure 4. 1 layer: ~ 0.1% X0 The experiment is in an advanced construction phase. The detector integration has been completely tested in 2018, and at the end of 2019 an engineering run is scheduled; the beginning of DAQ collection is FIG. 5: Schematic view of the Mu3e detector. foreseen in 2020. Three full years of data taking are expected to reach the experimental goal within 2023.

C. The µ → e conversion B. The µ → eee deacy in Mu3e

The µ → e conversion is a cLFV process which can The signal is the emission of two positrons and an take place when negative muons are stopped in the electron on a plane, from a common vertex with a to- matter through the process µ− + (A) → e−(A), where tal momentum equal to ~0 and a total energy equal to A is a nucleus like for example Al. A pulsed negative the muon mass E ≈ 105.6 MeV. Being a three-body tot muon beam is formed from the decay of pions pro- decay the energy of the daughters is not a fixed val- duced in proton collision on fixed target and brought ues, being the higher energy larger than 35 MeV while to stop in a layer of thin targets, where muon capture the lowest energy peaks near zero and about one half can take place. The signal is a final state with one have an energy larger than 15 MeV. The detector must electron having the energy equal to the muon mass have an excellent tracker as thin as possible in order reduced by the difference in binding energy of the nu- to have high acceptance for tracks from few MeVs cleus before and after the reaction. Since this is a to half of the muon mass. Similarly to the µ → eγ single particle final state process the sensitivity is not a positive muon beam is chosen and there are two limited by the accidental background which does not sources of background. The prompt is given by the pose in principle any limitation in the muon intensity. µ → e+e−e+ν ν where the two neutrinos carry very e µ A beam induced background can originate from the little energy. The other background comes from the interaction of pions in the beam lines with the target; accidental coincidence of two or three muon-decays, this can be eliminated by implementing a highly asym- and strongly depends on the beam rate, for this rea- metric proton beam structure with very short and in- son a continuous beam will be used. tense proton spills well separated in time. All the pi- The Mu3e experiment [9] in under construction at −16 ons created in the proton collisions decay within few Paul Scherrer Institut and aims at reaching a 10 hundred ns, after that electrons can be emitted only sensitivity in two successive phases and improving the by muon-related decays, the distance between the pro- former result by 3 orders of magnitudes [10]. The ton spill is of order of 1/2 µs. The residual fractional muon beam will be transported to a double cone My- contribution of out of time protons (the so called ex- lar target, 85µm thick. The target is placed in a tinction factor), and then of pions, has to be at least solenoidal field of 1 T, the detector is made of ultra- of 10−9. Other backgrounds come from the interac- thin (50 µm) silicon pixels and scintillators fibers and tion of cosmic rays in the target and the muon decay tiles read out by means of SiPMs. in flight. In order to reach the experimental goal a new beam- line concept is required: a dedicated R&D called The current sensitivity is a few 10−13 depending on HiMB is presently ongoing at PSI in order to have the target used and was obtained by the SINDRUM- muon beam intensities up to few 109 µ/sec [11]. II experiment at Paul Scherrer Institut [12, 13]. Two The detector commissioning for the Phase-I is fore- new experiments are currently under construction seen for 2021, the physics program will last until 2030 aiming at improving the sensitivity by 4 orders of mag- with the Phase-II. nitudes.

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Pion Capture Protons Protons Pion Capture Section Section Production Pions Production Target 2.5 T Pions Target 4.6 T 8 GeV proton beam 1.0 T Muons collimator 2.0 T 3m Pion Decay and Phase-I Detectors Muon Transport Section CyDet StrECAL calorimeter tracker Muons Stopping µ-target Target Electrons production transport detector solenoid solenoid solenoid Conversion Beam Detector measurement characterisation Section

Detector FIG. 6: Schematic view of the Mu2e beam line and detec- Section tor.

COMET Phase-I COMET Phase-II 1. The µ → e in Mu2e FIG. 7: Schematic view of the COMET Phase I and II The Mu2e experiment is currently under construc- beam lines and detectors. tion at the muon campus in Fermilab National Lab- oratory [14]. The backward-going pions produced by the 8 GeV proton beam are captured and decay into

muons, which are transported through a bent solenoid 2017 2018 2019 2020 2021 2022 2023 2024 2025 2026 2027 to a series of aluminum disks where they stop. The MEG II electron coming from the muon decay or capture is Mu3e measured by a straw tube tracker and a pair of crys- Mu2e tal calorimeters, arranged in the shape of a hollow cylinder to let low momentum electrons go through COMET undetected. Figure 6 shows a schematic view of the DeeMe beam line and detector. FIG.τ,B 8: Current schedule of the search of cLFV in the Mu2e is in advanced state of commissioning, it is muon sector. expected to start data taking in 2023 and to reach 90% conﬁdence level sensitivity of 6 × 10−17 in three years of data taking. COMET Phase-I is currently under construction together as the facility. The physics data taking is expected in 2021/2022 and will take 2/3 years to accu- 2. The µ → e in COMET mulate the statistics. Phase-II will follow.

The COMET (COherent Muon-to-Electron Tran- sition) experiment is in construction at the Japanese III. CONCLUSIONS AND PROSPECTS Proton Accelerator Research Center (J-PARC) [15]. COMET will operate in two stages, Phase-I and Phase-II that are schematically reported in Figure 7. It is exciting to see that within the next five to ten Phase-I has a sensitivity goal of 2 × 10−15 and will years our present knowledge of fundamental interac- help to understand some of the novel experimental tions could be disproved or confirmed by means of techniques, the beam and the background rates. In a full set of complimentary searches of BSM physics the first experimental stage the target will be placed in the muon sector. As shown in Figure 8 MEG II at the center of a thin cylindrical drift chamber is expected to start data taking in 2020 after an en- surrounded by scintillating hodoscopes for triggering gineering run in 2019; Mu3e Phase-I commissioning and timing. In the Phase-II the beam line will be is expected for 2021 followed by almost 10 years of extended and the electrons will be tracked by a physics program towards the Phase-II; Mu2e foresees forward straw tube tracker and a calorimeter made three years of data taking starting in 2023; finally of LYSO crystals. Phase-II will allow to reach a COMET Phase-I data collection is expected to start sensitivity of 3 × 10−17. Recent measurements have in 2021/2022. been able to demonstrate an extinction factor rate of Table I reports the current cLFV sensitivities in the about 10−12. muon sector and how it will be improved by the new generation of experiments described.

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