Detectors and concepts for future CLFV experiments

Detectors and concepts for future CLFV experiments

Bertrand EchenardCaltech

NuFact 2021

Cagliari - September 2021

p. 2Bertrand Echenard - Caltech

CLFV and BSM physics

Charged lepton flavor violating (CLFV) processes are interactions that do not conserve lepton familynumber(s), e.g. µ → e, τ → µµµ , KL → µe, H → τµ, ...

CLFV can be generated at loop level with massive neutrinos, but the rate is extremely suppressed due toGIM mechanism and tiny neutrino masses. For example:

New physics could greatly enhance these rates, e.g.

CLFV are very clean probes - an observation is an unambiguous sign of physics beyond νSM

CLFV searches share the stage with neutrino experiments in studying the origin of neutrino mass, flavors and families

supersymmetry Heavy neutrino Two Higgs doublet Leptoquark Compositness New heavy boson


CLFV – Snowmass and PBC

Several new concept to search for CLFV have been recently proposed in the context of the Snowmass exercise and the Physics Beyond Collider workshop

Muon decaysMEG-II, Mu3e

Heavy state decaysLHC,FCC-ee,CepC,…

Muon transitionsMu2e(-II), DeeMe, COMET

Enigma

Tau decaysBelle-II, LHCb, SCTF, BES III,…

Meson decaysBelle-II, J-PARC, CERN,…

Muon oscillationsMACE

Tau transitions EIC

Theory

CLFV

We will review some of these new ideas, focusing mostly on the muon and tau sectors

Muon decays


Muon decays - current situation

Two main channels

µ+ → e+γ - MEG II at PSI (see talk from M. Meucci)• Expected sensitivity at the level of 10-14 (3 year run)• Expect data taking in 2021(+)

µ+ → e+e-e+ - Mu3e at PSI (see talk from A. Kozlinskiy / A. Schoening)• Expected sensitivity at the level of 10-15 to 10-16 (with HiMB)• Expect data taking in 2022++

Mu3eMEG-II


Muon decays – going beyond

Improve detector performance to reduce backgrounds for µ+ → e+γ searches

• Accidental background scales as Γacc ∝ Γ2

µ εe εγ δEe (δEγ)2 (δΘeγ)2 δTeγ

• For high intensity beams, this background dominates over physics background (e.g. radiative muon decays)

• Explore new ideas to improve photon angular / energy / time resolution and detection efficiency: high-efficiency calorimetry or photon conversion

e

γνν

ν

νν

µe

γ

νν

µ

Radiative muon decayAccidental background




Renga et al., 1811.12324

Large acceptance and good segmentation

LaBr3 crystals• Initial studies indicate an energy resolution around 800 keV and an excellent timing

resolution (~30 ps) • Very expensive

High-efficiency calorimetry


Renga et al., 1811.12324



Photon conversion

Excellent energy and angular resolution, but lower efficiency to limit multiple scattering

High Z material seem to offer the best compromise

Also proposal for multiple converters with active layers to boost efficiency and preserve timing resolution (G. Tassielli et al, Snowmass RF/SNOWMASS21-RF5_RF0_Tassielli-067.pdf)



Bkg<=10 evts bkg>10 evts

Photon conversion: MEG-II target + cylindrical gaseous detector as positron tracker + lead converter (0.1X0 thickness) + gaseous detector as e+e- pair spectrometer. Optional vertex tracker (silicon or gaseous detector)

Calorimeter: similar to photon conversion but e+e- spectrometer replaced by a LaBr3 calorimeter

Conceptual detector based on photon conversion or LaBr3 calorimeter



Several possibilities to reach 10-15, path depends on the muon beam intensity

Taking full advantage of 1010 µ/s and improving sensitivity need a novel experimentalconcept. Situation is similar for µ+ → e+e-e+, breaking the 10-16 wall will likely requirea new experimental approach.

One interesting question: can we build a single next-generation experiment to looksimultaneously for µ+ → e+γ and µ+ → e+e-e+?

Bkg<=10 evts bkg>10 evts

Conceptual detector based on photon conversion or LaBr3 calorimeter

Muon conversion


Muon conversion - current situation

Three experiments at various stage of execution, to be completed by the end of the decade

• DeeMee at J-PARC, expected SES ∼ 10-14 (see talk from N. Teshima)• COMET at J-PARC, expected SES ∼ 10-17 (see talk from C. Carloganu)• Mu2e at FNAL, expected SES ∼ 10-17 (see talk from G. Pezzullo)

Mu2eCOMETDeeMee


Muon conversion – limiting factors

Current approach (COMET / Mu2e)

• Protons hit the production target, pions → muons captured by production solenoid (pulsed beam)

• Muons transported towards stopping target

• Muon conversion or decay products measured by detector (tracker + calorimeter)

A. Knecht et al., EPJ. Plus (2020) 135

Main limiting factors

• Dead time to wait for beam-associated backgrounds to decrease to negligible level → cannot measure conversions in atoms with short muonic lifetimes (high Z)

• Need well-defined pulse beam (extinction)

• Available beam power limits muon rate, need higher intensity

Several proposals for next generation experiments using the PIP-II accelerator complex to overcome some

or all of these limitations


PIP II at FNAL

PIP II

800 MeV H− linacUp 165 MHz bunchesUp to 2 mA CWUp 1.6 MW

Upgraded Booster 20 Hz, 800 MeV injectionNew injection area

Upgraded Recycler & Main InjectorRF in both rings

Groundbreaking for projectMarch 2019

Protons for the High Energy Program~1% of available beam!

PIP-II will deliver 1.2 MW proton beam for LBNF, but that program uses a very small fraction of the available beam → opportunity for new muon experiments


Mu2e-II at FNAL

Mu2e-II is a proposed Mu2e upgrade to take full advantage of PIP-II and improve the SES by an order of magnitude over Mu2e (i.e. SES ∼ 10-18)

Either explore higher mass scale if no observation has been made by Mu2e, or perform precision measurements with several targets in case of discovery → no lose scenario

Re-use as much of Mu2e infrastructure as possible, and upgrade components required to handle higher beam intensity. In particular:

• Beam delivery: higher beam intensity, lower beam energy, extinction• Tracker: higher occupancy and limited resolution• Calorimeter: high rate and radiation damage• Cosmic ray veto: higher occupancy and background from neutrals • DAQ system: higher rate and throughput

Dedicated R&D efforts in all these domains (and others) to meet the requirements

See talk from C. Dukes


Mu2e-II - tracker

Tracking system

The DIO contribution would increase by a factor x10 with higher beam intensity

Need to improve momentum resolution to mitigate this background. Potential solutions:

Reduce tracker mass• Thinner straws (15 µm → 8µm)• Remove gold layer inside straw

Investigate alternative tracker geometries• Use ultra-light pressure vessel to ease requirements

on straw leakage while keeping Mu2e straw layout (i.e panel geometry)

• Construct a high granularity and high transparency drift chamber à la MEG II.

Also investigate potential of silicon sensors (e.g. HVMapsfor Mu3e) or MPGD (e.g. µ-RWell)

signalbackground

G.F Tassielli

D.N. Drown


Mu2e-II - calorimeter

Barium fluoride crystal calorimeter

Rate and radiation dose are too high for pure CsI• Up to ~ 1Mrad and 1E13 n [1MeVeq/cm2]

BaF2 is an excellent candidate for ultra fast, high rate, radiation hard crystal calorimeter• Fast components (<1 ns) at 195 and 200 nm• Can support > 1 Mrad radiation dose

Must use fast component at 200 nm without undue interference from slow 320 nm component. Two complementary approaches:• Yttrium doping can suppress slow component by factor x5

without reducing fast signal• Develop photo-sensor only sensitive to fast component:

solar blind UV-sensitive SiPM /APD, UV-sensitive MCP or LAPPD, nano-particle wavelength shifting filter,…

R.Y Zhu

R.Y Zhu


Mu2e-II – production target

V. Pronskikh

Production target

Investigate design of pion production target inside solenoid to handle increase beam power

Explore rotation target, fixed granular target or conveyor belt with small target elements

Understand solenoid shielding requirements

Current LDRD program at FNAL, conveyor belt seem the favored solution so far


ENIGMA at FNAL

ENIGMA is a new facility for a next generation of muon experiments at FNAL based on the PIP-II accelerator with a

Surface muon beam for muon decay experiments• Similar to what is done at PSI (1.4MW target, well known technology)• Dedicated beam with higher intensity – up to 1012 µ/s with PIP II• Potentially improve sensitivity by a factor x100 w.r.t MEG-II

New beam for muon conversion experiments• Probe Rµe sensitivity down to 10-19, with the ultimate objective to reach 10-20 and

probe O(104 - 105) TeV effective mass scale• Probe high-Z target (e.g. Au) to explore underlying new physics if CLFV is observed• Based on the PRISM concept to provide a low momentum, quasi-mono-energetic

muons beam with extremely low pion contamination


New beam for muon conversion - PRISM

New beam for conversion experiment*, based on the PRISM (Phase Rotated Intense Slow Muon beam) concept proposed by Y. Kuno and Y. Mori

PRISM concept:

• High intensity (MW) proton beam with very short pulse duration hit target in a capture solenoid, producing π→ µ

• Inject muons into a fixed-field alternating gradient (FFA) ring

• Phase rotates to reduce the beam energy spread (slow down leading edge, accelerate trailing edge)

• Pion contamination is drastically reduced during phase rotation (O(µs))

• Extract purified muon beam to detector

Requires a compressed proton bunch and high power beam to achieve high µ rate → PIP II with a compressor ring (bunch size limit is much too small for the FFA)

*https://www.snowmass21.org/docs/files/summaries/RF/SNOWMASS21-RF5_RF0-AF5_AF0_Robert_Bernstein-027.pdf


Compressor ring using PIP-II linac

PIP II beam power is sufficient, but the 2x108 bunch size limit is much too small for the FFA and requires a compressor ring

This is too low! Need R&D to push repetition rate or bunch size!

https://www.snowmass21.org/docs/files/summaries/AF/SNOWMASS21-AF5_AF0-RF5_RF0_Prebys-071.pdf

E. Prebys


PRISM – conceptual design

J. Pasternak et al.

PRISM task force

https://indico.phys.vt.edu/event/34/contributions/685/attachments/529/625/PRISM_nufact18.pdf

Detector part remains largely to be designed – great opportunity to be creative!


FFA – R&D work in Osaka (MUSIC)

PRISM FFA - proof of concept

• 10 cell DFD ring has been designed• FFA magnet-cell has been constructed and verified• RF system has been tested and assembled• 6 cell ring was assembled and its optics was verified with α particles• Phase rotation was demonstrated for α particles

Pasternak et al., https://indico.phys.vt.edu/event/34/contributions/685/attachments/529/625/PRISM_nufact18.pdfA. Alekou et al., arxiv: 1310.0804

https://indico.phys.vt.edu/event/34/contributions/685/attachments/529/625/PRISM_nufact18.pdf


PRISM – challenges and synergies

FFA ring design• in full synergy with the Neutrino Factory and/or a Muon Collider development

Target and capture system• MW class target in a solenoid• in full synergy with the Neutrino Factory and/or a Muon Collider development

Design of the muon beam transport from the solenoidal capture to the PRISM FFA ring• very different beam dynamics conditions• very large beam emittances and the momentum spread

Muon beam injection/extraction into/from the FFA ring• very large beam emittances and the momentum spread

Compressor ring• Fast kicker to transfer beam from compressor ring at 1kHz

Many synergistic activities with muon collider and neutrino factory R&D

PRISM is in a position to be one of the incremental steps of the muon program

Muonium-antimuonium


Muonium - antimuonium

Muonium: bound state of µ+ and e-. Purely QED, no hadronic uncertainties

Muonium – antimuonium transitions (∆L=2) are extremely suppressed in the SM, but rates can be enhanced by NP

Positive muon beam on Si02 target, detect electron from negative muon and atomic positron

Best limits from MACS experiment (1999) based on 5.7x1010 muonium atoms

P(Mµ → Mµ) < 8.3x10-11 /SB(B0)GMMbar < 3x10-3 GF (90% CL)

MACE: proposal for a new muonium-antimuonium experiment at EMuS in China based on the same principle with the goal to improve sensitivity x100

(https://www.snowmass21.org/docs/files/summaries/RF/SNOWMASS21-RF5_RF0_Jian_Tang-126.pdf)


MACE at EMuSJian Tang

(Snowmass 2021 RPP meeting)EMuS – new muon facility in China

conceptual design

On-going physics studies and detector R&D

MACE concept

Tau sector


Tau – current and future prospects

SCTF proposal – Novosibirsk or China

Future prospects (representative set):• τ → lγ at level of 10-10 - 10-9 by Belle II• τ → µµµ at level of 10-10 - 10-9 by LHCb with 50 fb-1.

Possible improvements with polarized beams at e+e- colliders, or with new experimental concepts (e.g. STCF, TauFV,…).

Belle-II at Super KEKB LHCb, ATLAS, CMS at LHC

https://ctd.inp.nsk.su/wiki/images/4/47/CDR2_ScTau_en_vol1.pdf


Tau – TauFV at BDF

TauFV concept

Concept to exploit the enormous τ production rate with the SPS beam at CERN using Ds → τντ decays

Need a series of thin targets distributed along the beam to minimize multiple scattering and keep adequate resolution

Assuming 4x1018 POT with 0.17% interaction length target, one could produce O(1013) Ds → τντ decays with SPS beam, much larger than the sample collected by other experiments

Could study several LFV channels, τ →µµµ, τ →eµµ, τ →eeµ, ...

Conceptual detector proposal based on technology developed for LHCb upgrade.

G. Wilkinson, PBC workshop 2021

Studies currently on hold, but this is an interesting concept to probe CLFV in τ decays

Projected τ →µµµ yield for BR(τ →µµµ) = 1x10-9


Summary

CLFV processes are very clean probes of new physics, and an observation would be transformative

Several experiments are underway to improve current sensitivity in the muon sector by several orders of magnitude

New experimental concepts and ideas have been proposed to further improve the physics reach by another few orders of magnitude

Plan to include a discussion of the physics case and these opportunities in the Snowmass report, and encourage people to further pursue design studies

People interested in exploring synergies or contributing to this program are welcomed to join the effort

Thank you for your attention

Extra material


Some references

Next generation MW muon facility at FNALA New Charged Lepton Flavor Violation Program at Fermilabhttps://www.snowmass21.org/docs/files/summaries/RF/SNOWMASS21-RF5_RF0-AF5_AF0_Robert_Bernstein-027.pdf

A Phase Rotated Intense Source of Muons (PRISM) for a µ → e Conversion Experimenthttps://www.snowmass21.org/docs/files/summaries/RF/SNOWMASS21-RF5_RF0-AF5_AF0_J_Pasternak-096.pdf

Bunch Compressor for the PIP-II Linachttps://www.snowmass21.org/docs/files/summaries/AF/SNOWMASS21-AF5_AF0-RF5_RF0_Prebys2-203.pdf

Muon decaysThe MEG II experiment and its future developmenthttps://www.snowmass21.org/docs/files/summaries/ RF/SNOWMASS21-RF5_RF0_MEGII-062.pdf

A new experiment for the µ → eγ searchhttps://www.snowmass21.org/docs/files/summaries/ RF/SNOWMASS21-RF5_RF0_Tassielli-067.pdf

Mu2e-IIMu2e-IIhttps://www.snowmass21.org/docs/files/summaries/RF/SNOWMASS21-RF5_RF0_Frank_Porter-106.pdf

Low-E facility at FNALUpgraded Low-Energy Muon Facility at Fermilabhttps://www.snowmass21.org/docs/files/summaries/ RF/SNOWMASS21-RF0-AF0-007.pdf

High intensity muon beam (HiMB) at PSITowards an High intensity Muon Beam (HiMB) at PSI https://indico.cern.ch/event/577856/contributions/3420391/attachments/1879682/3097488/Papa_HiMB_EPS2019.pdfHIMB Physics Case Workshop https://indico.psi.ch/event/10547/timetable/?view=standard


Muon transitions and decays

Three main modes• µ+ → e+γ decays• µ+ → e+e-e+ decays• µ−N→ e-N conversion

Complementarity is key – each reaction probes different NP operatorsZ dependence of µ-e conversion provide information about the nature of new physics

Z

Al Ti Pb

Cirigliano, et al, PRD 80, 013002 (2009)

Vector Z-penguin

Vector γ-penguin

Scalar e.g. SUSY SeeSaw

Dipole e.g SUSY GUTS

4

3

2

1

Rµ

e(n

orm

to

Al.)

A. de Gouvêa, P. Vogel, arXiv:1303.4097

Simplified picture


CLFV and neutrino mass

Many mechanisms to generate ν mass: seesaw, Zee models, RPV SUSY,…• distinct new states realized at different scales

SUSY Seesaw

CLFV induced by exchange of SUSY particles

Low scale Seesaw: inverse seesaw

Addition of 3 “heavy” RH neutrinos and 3 extra “sterile” fermions to SM

Induces sizeable CLFV rates and helps differentiate models

Non Standard Interactions might also impact neutrino oscillations

Teixeira et al., JHEP02 (2016) 083 Figueiredo & Teixeira, JHEP 1041(2014) 015


Simplified Lagrangian

L: effective mass scale of new physics k: relative contribution of the contact term

CLFV can probe very high mass scales O(1000-10000 TeV)

Different BSM scenarios predict different value of k → model

diagnosis10-2 10 -1 1 10 1 10 2

MEG upgrade

MEG

CR( Nm eN on Al) <6×10 -17

CR( N eN on Al 6 10) < × -18

CR( Nm eN on Au) < 7×10-13

B( <4.2 10 e³ ) × -13m

B( <6 10 e³ ) × -14m

B( <4.2 10 e³ ) × -13m

CR( Nm eN on Al) < 6×10 -18

SINDRUM II

10 3

10 4

›Te

V)

Λ (T

eV)

κloop contactκ

Λ(T

eV)

104

103

CR(µN→eN on Al) < 6x10-18



SINDRUM IIB(µ→eγ) < 4.2x10-13

B(µ→eγ) < 6x10-14

MEG upgrade

MEG

“Dipole term”

Contributes to µ → eγ

No contribution to µ → eγ

“Contact term”

Derived from A. de Gouvêa, P. Vogel,

arXiv:1303.4097


CLFV and SUSY (subset of models)

GlossaryAC RH currents & U(1) flavor

symmetryRVV2 SU(3) flavored MSSMAKM RH currents & SU(3) family

symmetryδLL CKM like currentsFBMSSM Flavor-blind MSSMLHT Little Higgs with T parityRS Warped extra dimensions

W. Altmannshofer, A.J. Buras, S. Gori, P. Paradisi & D.M. Straub - 0909.1333

LargeSmall but observableUnobservable

Pattern characteristic of model, diagnosis with multiple observables (not only CLFV)


Model discrimination

B(µ→eγ)

Rat

e µN

→eN

Sterile neutrino model

Teixeira et al., JHEP02 (2016) 083

M1/2 (GeV)tanβ = 10

L. Calibbi et al., JHEP 1211 (2012) 040

mSUGRA PMNSNUHM PMNSmSUGRA CKM

SUSY GUT

Blanke, arXiv:0702136v3

LITTLEST HIGGS

PMNS-likeCKM-likeGeneric

Buras, Duling, Feldmann, Heidsieck, Promberger, 1006.5356


Mu2e-II - tracker

Straw R&D (FNAL LDRD program)

Develop 8 µm Mylar straws using 3.5 µm Mylar + 1 µm adhesive + 3.5 µm Mylar double helical wrap

Held 15 PSI for multiple days and 400 g tension without visible distortion

Handling straws with internal outward force without causing obvious damage• Paper is inside• Inflated straws

Almost no compression force can be applied, making the installation of straw terminationchallenging

Integrated readout electronics must be able to handle large radiation dose


Mu2e-II – photo-sensor

Solar-blind UV-sensitive SiPM

Add anti-reflection filter to SiPM surface• Select fast component, further suppress slow component• Improve overall efficiency (light bounces back)

Add superlattice by implementing boron layer just below the Si surface with molecular beam epoxy

• Improve quantum efficiency and timing performance by reducing undepleted region near surface

• Provide stability under intense UV illumination

Currently under R&D from Caltech, JPL and FBK• First tests with 3 layer filter (no superlattice) show good sensitivity

to fast component. • Further R&D to include 5 layer filter and superlattice

Other possibilities

• MCP not yet suitable for reading crystal in high-rate environment, as charge collected by anode is orders of magnitude too large for device capabilities (max few C/cm2). Would need further R&D.

• Nano-particle WLS filter require more R&D to understand QE and timing performance

D. Hitlin

https://indico.fnal.gov/event/46746/contributions/210202/attachments/141121/177609/Hitlin_CPAD__210318-.pdf

https://indico.fnal.gov/event/46746/contributions/210202/attachments/141121/177609/Hitlin_CPAD__210318-.pdf


High-energy probes

High energy colliders offer complementary way to search for CLFV

Harnik, Kopp, Zupan 1209.1397 Phys. Lett. B 800 (2020) 135069

LFV Higgs couplings

t

CLFV Higgs decay

g-2

mN→eN

t→l’ll (m → eee)

t→mg (m → eg)

Competitive bounds for τ leptons

Room for discovery


High-energy probes

pp → lilj + jetsQuark D6 operators Cai & Schmidt, 1510.02486

pp → lilj

Gluonic D8 operators Cai, Schmidt, Valencia 1802.0982 + many other possibilities

High energy colliders offer complementary way to search for CLFV

Better sensitivity to some operators (often heavy leptons)!

Interplay between low- and high-energy probesCoordinate efforts between CLFV group and high energy frontier


Tau – TauFV at BDF

Possibility to study e→τ transitions at EIC• Some models predict better sensitivity with heavier leptons• Benchmark study at EIC with leptoquark model (Gonderinger et al, JHEP (2010) 2010: 45)

• Current limits on leptoquarks set by HERA

Deshpande, Huang, Kumar Zhang, Zhao

Potential to improve sensitivity by two orders of magnitude at EIC

(qiqj) Mass for (qiqj) generation (GeV)

Detectors and concepts for future CLFV experiments

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