A submission to the 2020 update of the European Strategy for Particle Physics on behalf of the COMET, MEG, Mu2e and Mu3e collaborations
A. Baldini, D. Glenzinski, F Kapusta, Y. Kuno, M. Lancaster, J. Miller, S. Miscetti, T. Mori, A. Papa, A. Schoning, Y. Uchida
CCharged Lepton Flavour Violation usingIntense Muon Beams at Future Facilities
A. Baldini, D. Glenzinski, F. Kapusta, Y. Kuno, M. Lancaster,J. Miller, S. Miscetti, T. Mori, A. Papa, A. Sch¨oning, Y. UchidaA submission to the 2020 update of the European Strategy for ParticlePhysics on behalf of the COMET, MEG, Mu2e and Mu3e collaborations.
Abstract
Charged-lepton flavour-violating (cLFV) processes offer deep probes for new physics with dis-covery sensitivity to a broad array of new physics models — SUSY, Higgs Doublets, ExtraDimensions, and, particularly, models explaining the neutrino mass hierarchy and the matter-antimatter asymmetry of the universe via leptogenesis. The most sensitive probes of cLFVutilize high-intensity muon beams to search for µ → e transitions.We summarize the status of muon-cLFV experiments currently under construction at PSI, Fer-milab, and J-PARC. These experiments offer sensitivity to effective new physics mass scalesapproaching O (10 ) TeV /c . Further improvements are possible and next-generation experi-ments, using upgraded accelerator facilities at PSI, Fermilab, and J-PARC, could begin datataking within the next decade. In the case of discoveries at the LHC, they could distinguishamong alternative models; even in the absence of direct discoveries, they could establish newphysics. These experiments both complement and extend the searches at the LHC.Contact: Andr´e Sch¨oning [[email protected]] a r X i v : . [ h e p - e x ] D ec xecutive Summary • Charged-lepton flavour-violating (cLFV) processes provide an unique discovery potentialfor physics beyond the Standard Model (BSM). These cLFV processes explore new physicsparameter space in a manner complementary to the collider, dark matter, dark energy,and neutrino physics programmes. • The global programme includes searches for µ → e , τ → e , and τ → µ transitions atexperiments hosted in Europe, the US, and Asia. The relative rates among the varioustransitions are model dependent and comparisons among these transitions offer powerfulmodel discrimination. A full exploration of cLFV parameter space requires the pursuitof all available µ → e and τ → e , µ transitions. • The most sensitive exploration of cLFV is provided by experiments that utilize high-intensity muon beams to search for cLFV µ → e transitions: a muon decaying intoan electron and a photon, µ + → e + γ ( MEG experiment at PSI); a muon decaying intothree electrons µ + → e + e − e + ( Mu3e experiment at PSI); and the coherent neutrinolessconversion of a muon into an electron in the field of a nucleus, µ − N → e − N ( Mu2e experiment at Fermilab and
COMET experiment at J-PARC). • These “golden” search channels provide complementary sensitivity to new sources of cLFVsince the relative rates depend on the details of the underlying new physics model. Thus,it is important to pursue a programme with experiments exploring all three µ → e cLFV transitions to maximize discovery potential, and, in the event of discovery, to helpdifferentiate the various BSM models through a comparison of the rates. • Current limits for cLFV µ → e transitions are in the 10 − − − range and probeeffective new physics mass scales above 10 TeV /c . Next-generation experiments at MEG , Mu3e , Mu2e , and
COMET expect to improve these sensitivities by as much as four orders ofmagnitude on the timescale of the mid–2020s. This dramatic improvement in sensitivityoffers genuine discovery possibilities in a wide range of new physics models with SUSY,Extra Dimensions, an extended Higgs sector, lepto-quarks, or those arising from GUTmodels. • European contributions are vital to the success of all four of these experiments. Europehosts two of them (
MEG , Mu3e ) and provides significant detector components for the others(
Mu2e , COMET ). • Beginning in the latter half of the next decade, upgrades to the beamlines at PSI, Fermi-lab, and J-PARC offer the possibility to further explore this parameter space. Improve-ments in sensitivity by an additional factor of 10–100 are possible with: a High intensityMuon Beamline (HiMB) at PSI to enable an upgraded
Mu3e ( Phase-II ); the PIP-II linacat Fermilab to enable an upgraded
Mu2e ( Mu2e-II ); an increased intensity at J-PARCto enable an upgraded
COMET ( Phase-II ). A next-generation
MEG experiment is also be-ing explored. Like their predecessors, significant European participation in the design,construction, data taking, and analysis will be important to the success of these futureendeavors and represents a prudent investment complementary to searches at colliders. • We urge the committee to strongly support the continued participation of European in-stitutions in experiments searching for cLFV µ → e transitions using high-intensity muonbeams at facilities in Europe, the US, and Asia, including possible upgraded experimentsat next-generation facilities available in the latter half of the next decade at PSI, Fermilab,and J-PARC. 1 bjectives Historically, flavour-changing neutral currents have played a significant role in revealing detailsof the underlying symmetries at the foundation of the SM. In the SM there is no known globalsymmetry that conserves lepton flavour. The discoveries of quark mixing and neutrino mixing,each awarded Nobel Prizes, provided profound insights to the underlying physics. Motivatedby these past successes, there exists a global programme to explore cLFV processes providingdeep, broad probes of BSM physics.The objective of our programme is to search for evidence of new physics beyond the SM usingcLFV processes in the muon sector. These processes offer powerful probes of BSM physics andare sensitive to effective new physics mass scales of 10 − TeV /c , well beyond what can bedirectly probed at colliders. Over the next five years, currently planned experiments in Europe,the US, and Asia will begin taking data and will extend the sensitivity to cLFV interactions byorders of magnitude. The current experiments each benefit from significant contributions byEuropean institutions. Further improvements are possible and new or upgraded experimentsare being considered that would utilize upgraded accelerator facilities at PSI, Fermilab, and J-PARC and could begin taking data in the 2025–2030 timeframe. Strong European participationwill be important for the success of these next-generation muon cLFV experiments. Scientific Context
Flavour violation has been observed in quarks and neutrinos, so it is natural to expect flavourviolating effects among the charged leptons as well. In fact, once neutrino mass is introduced,the SM provides a mechanism for cLFV via lepton mixing in loops. However, the rate issuppressed by factors of (cid:16) ∆ m ij /M W (cid:17) , where ∆ m ij is the mass-squared difference betweenthe i th and j th neutrino mass eigenstates, and is estimated to be extremely small, for example BF ( µ → eγ ) ∼ − [1]. Many extensions to the standard model predict large cLFV effectsthat could be observed as new experiments begin data taking over the next five years. Significantimprovements are expected across a wide variety of cLFV processes (e.g. τ → µµµ , µγ , or eγ ; µ → eγ , eee ; µN → eN ; Z or H → eµ , eτ , or µτ ; K L → eµ ). The largest improvements areexpected in experiments that search for cLFV transitions using muons.Experimentally, there are three primary muon-to-electron transitions used to search for cLFV :a muon decaying into an electron plus a photon, µ + → e + γ ; a muon decaying into threeelectrons, µ + → e + e − e + ; and direct muon-to-electron conversion via an interaction with anucleus, µ − N → e − N . These three µ → e transitions provide complementary sensitivity tonew sources of cLFV since the observed rates will depend on the details of the underlying newphysics model. For example, for models in which cLFV rates are dominated by γ -penguindiagrams, the µ → eγ transition rate is expected to be ∼ times larger than the µ → eee and µN → eN rates. On the other hand, if the cLFV rates are dominated by Z - or H -penguindiagrams, or if tree level contributions are allowed (e.g. as in some lepto-quark or Z (cid:48) models),then the µ → eγ rate is suppressed and µ → eee and µN → eN rates can instead be largest.Thus, a programme with experiments exploring all three muon cLFV transitions maximizes thediscovery potential and offers the possibility of differentiating among various BSM models bycomparing the rates of the three transitions. This is discussed extensively in the literature, seefor example references [3] and [4].Searches for µ → e transitions have been pursued since 1947 when Pontecorvo first searchedfor the µ → eγ process. Since then, the sensitivity has improved by eleven orders of magnitudevia a series of increasingly challenging experiments. The current best limits for the three µ → e Muonium oscillations, µ + e − → µ − e + , in which the muon and electron form a bound state, can also be usedto set limits on cLFV interactions [2] but are not discussed here. BF ( µ + → e + γ ) < . × − [5], BF ( µ + → e + e − e + ) < × − [6], R µe (Au) < × − [7] at 90% CL, where R µe is the µ → e conversion rate normalized to the rate ofordinary muon nuclear capture. Currently planned experiments in Europe, the US, and Asia willprovide sensitivities well beyond these existing limits. The MEG experiment at PSI has recentlycompleted an upgrade and expects to extend the µ + → e + γ sensitivity by about an order ofmagnitude with physics data taking beginning in 2019. Further improvements will require anew approach and/or advances in instrumentation. The first phase of the Mu3e experiment isunder construction at PSI and with about 300 days of data taking is expected to improve the µ + → e + e − e + sensitivity by over two orders of magnitude. A second-phase experiment withadditional instrumentation could offer a further one order of magnitude improvement withan upgraded muon beam providing > × stop- µ + /s (e.g. a high-intensity muon beam,HiMB, at PSI; or a dedicated µ + beamline from PIP-II at Fermilab). The COMET experimentunder construction at J-PARC will extend the sensitivity to µ − N → e − N by about two ordersof magnitude by the early-2020s, while the Mu2e experiment under construction at Fermilabwill extend the sensitivity by about four orders of magnitude by the mid-2020s. On a longertimescale, upgrades in proton intensity offer the possibility of additional improvements. Anupgrade to
Mu2e that extends the sensitivity by another factor of ten or more,
Mu2e-II , isproposed and would utilize about 100 kW of 0.8 GeV protons from the Fermilab PIP-II linac.An upgrade to
COMET , COMET Phase-II , is proposed and would utilize about 56 kW of 8 GeVprotons to reach a comparable sensitivity. The status of the currently planned experiments andtheir potential for further improvement is discussed in more detail in the next sections.The outstanding sensitivities that can be achieved by the muon cLFV experiments provideaccess to new physics mass scales in the 10 − TeV /c range, well beyond what can bedirectly probed at colliders. In general, these experiments explore the BSM parameter spacein a manner complementary to the rest of the HEP experimental programme.The search for muon-cLFV explicitly probes for flavour-violation in either CP-conserving orCP-violating BSM interactions; in contrast, for instance, to muon g-2 which is sensitive toflavour conserving (and chirality-flipping) interactions. If the Fermilab Muon g-2 experimentconfirms the BNL measurement [8] and hence an a µ value at odds with the SM beyond 5 σ ,it will establish the presence of a BSM muon interaction which has obvious ramifications formuon-cLFV, since, in many BSM scenarios, the two are closely related [9]. If the a µ anomalydisappears, the muon-cLFV experiments are still compelling since they probe effective massscales well beyond the TeV scale probed by Muon g-2 .As the charged counterpart of neutrino oscillations, cLFV plays a significant role in most ofthe BSM models seeking to explain the neutrino mass hierarchy and the universe’s matteranti-matter asymmetry generated through leptogenesis. The cLFV measurements thus haveconsiderable synergy with the neutrinoless double beta decay and neutrino oscillation researchprogrammes. For example, there is a large class of models (see e.g. [10]) proposed to explainthe smallness of the neutrino mass. These typically involve extensions to the Higgs sector andthe existence of heavier neutrino partners, the properties of which — sterile or non-sterile,Dirac or Majorana, and the mass-scale of the neutrino partners — depend on the model.These heavy neutrino partners typically also play a role in generating a matter anti-matterasymmetry. The majority of these models predict large cLFV effects, and the comparison ofcLFV and neutrino measurements together becomes a strong constraint on the model type andits parameters. Indeed, in the most natural models, where the neutrino partners are extremelymassive, these measurements are one of the few portals into GUT-scale physics. In the InverseSeesaw models [11], right-handed neutrinos with masses in the TeV-scale are produced thatare potentially observable at the LHC. The present LHC limits are below 1 TeV whereas
Mu2e , COMET , and
Mu3e will extend this sensitivity to 2 TeV. More generally
Mu2e , COMET , and
Mu3e still have a sensitivity for RH neutrinos up to masses of a few PeV, well beyond the direct3etection limit of the LHC.The µ → e experiments also provide complementary information regarding the Majorana natureof neutrinos via the µ − → e + transition: µ − N ( Z, A ) → e + N ( Z − , A ). This transitionviolates both lepton number and lepton flavour and can only proceed if neutrinos are Majorana.This search channel comes for “free” in the Mu2e and
COMET Phase-I experiments. The
Mu2e and
COMET sensitivity to Majorana neutrinos will significantly extend beyond the current bestlimit [12] with a (cid:104) m eµ (cid:105) effective Majoarna neutrino mass scale sensitivity down to the MeVregion surpassing the (cid:104) m µµ (cid:105) sensitivity in the kaon sector which is limited to the GeV region [13].The anomalies in B decays reported by the B-factories and LHCb and the
E821 a µ anomalyhave promoted a renewed interest in leptoquarks [14] and Z (cid:48) s [15]. These models can generatelarge cLFV effects via tree-level contributions. Direct searches for leptoquarks at ATLAS and
CMS place limits in the 400–800 GeV /c range which will ultimately increase to approximately1 TeV /c with HL-LHC. While there are model-dependencies, the limits from muon cLFVexperiments [16] are much stronger with sensitivities up to masses of 300 TeV /c beyond thepresent limit (120 TeV /c ) established from lepton-flavour violating B-decays [17].Many experiments will search for the cLFV τ → µ and τ → e transitions including ATLAS , Belle-II , BES-II , CMS , and
LHCb . In general the existing limits will be extended by aboutan order of magnitude to the 10 − − − range. The proposed tauFV experiment [18] mayoffer another order of magnitude improvement. Thus, the ultimate sensitivity offered by thetau-cLFV searches is several orders of magnitude below the sensitivity offered by the muoncLFV experiments. The relationship between tau-cLFV and muon-cLFV processes is modeldependent. The large, close-to maximal, mixing in the neutrino sector favours scenarios inwhich the rates of cLFV are similar in the two sectors, but other scenarios are also possible inwhich tau-cLFV rates are significantly enhanced. A comparison of all the transitions: µ → e , τ → e and τ → µ is a very important probe of flavour models. All measurements should bepursued.In summary, experiments sensitive to violations of lepton flavour, lepton number, and leptonuniversality play a significant role in the search for BSM physics. It will be necessary tomake as broad an array of measurements as possible in order to maximally probe the availableparameter space. The muon-cLFV experiments explore cLFV transition rates that are manyorders of magnitude beyond what is explored by other experiments and offer sensitivity to newphenomena with mass scales in the few PeV /c region. Over the next several years, the MEG , Mu3e , Mu2e , and
COMET experiments have the best reach in their respective channels. Futureupgrades could extend the sensitivity another one to two orders of magnitude by utilizingimproved accelerator beamlines, and could begin data taking in the 2025–2030 timescale. Thesefuture experiments (e.g.
Mu3e Phase-II , Mu2e-II , COMET Phase-II , PRISM ) would offer themost sensitive probes of cLFV for the foreseeable future.
Beam Facilities
The muon-cLFV experiments rely on facilities with high-power proton beams capable of deliv-ering high-intensity muon beams. The experimental infrastructure costs range from e Mu2e experiment has a total projectcost of $ 274M. Several facilities exist with proton beams and transport channels capable ofproviding muon beams at high intensities. The PSI laboratory utilizes 1.4 MW of 590 MeVprotons to provide high-intensity beams of secondary particles, including the most intense low-energy muon beams in the world. The π E5 channel serves the particle physics communityand provides positively charged muons with a momentum of 28 MeV / c, a momentum bite of5 −
7% FWHM, and rates up to 10 stop- µ + /s . At Fermilab 700 kW of 8.9 GeV/c protonsare available for various experiments, of which about 8 kW will be utilized to produce about40 stop- µ − /s for Mu2e . Similarly, at J-PARC about 500 kW of up to 30 GeV protons areavailable for various experiments, of which about 3 kW (at 8 GeV/c) will be utilized to produce10 stop- µ − /s for COMET Phase-I . The Fermilab and J-PARC muon beamlines are expected tobecome operational in the next few years.Future facilities, capable of providing stopped muon rates a factor of 10–100 larger, are beingplanned and could become available as early as 2025. These future facilities would enable next-generation muon-cLFV experiments with improved sensitivities. At PSI, strong requests fromboth the particle physics and material science communities have motivated studies to upgradethe existing muon beamlines (HiMB study). By optimizing the existing M target station andimproving the transport efficiency, a new beamline could deliver over 10 stop- µ + /s for Mu3ePhase-II or a future extension of the
MEG experiment. At Fermilab, the long-baseline neutrinoprogramme motivates the need for a significant upgrade of the proton beam intensity. The PIP-II linac will be CW capable and will use superconducting RF technology to provide 1.6 MW of0.8 GeV protons available for a variety of experiments [19]. Conceptual designs exist to provideabout 100 kW of protons to an upgraded
Mu2e experiment,
Mu2e-II , with over 10 stop- µ − /s .At J-PARC, plans exist to provide 56 kW to produce 2 × stop- µ − /s for COMET Phase-II .Further future, in conjunction with a 1.3 MW upgrade to the Main Ring at J-PARC, the PRISMproject would utilize a fixed-field alternating gradient (FFAG) muon storage ring to produce avery intense, very high purity, monochromatic muon beam with the potential to make a wholeprogramme of muon-based measurements at world-class sensitivities.
Methodology
The same basic experimental methodology is employed in searches for all three cLFV µ → e processes. The experiment beamline begins by colliding protons onto a production target toproduce low momentum pions. The resulting pions are either transported through a decayvolume or directly stopped inside the target, and their decay muons are collected. Theseexperiments require low momentum muons, typically with momenta less than 50 MeV /c , inorder to stop them in thin targets at the center of the experimental apparatus. At theselow momenta, muons stop in a few mm or less of material. To reach the target sensitivitiesrequires high-intensity muon beams, > stop- µ/s . The detector apparatus is designed toprecisely determine the energy, momentum, and timing of particles originating from the muonstopping target. Because these experiments aim for such extreme sensitivities, their apparatusare customized to the final state of interest. The µ + → e + γ process The µ + → e + γ process is sensitive to new physics mass scales around 10 TeV / c and primarilytests cLFV dipole couplings where new physics appears in loops. The most stringent limiton this process was established by the MEG experiment using data collected from 2009–2013, BF ( µ + → e + γ ) < . × − at 90% CL [5]. The MEG II experiment has recently completedconstruction and aims to improve the sensitivity by an order of magnitude.The experimental signature of a µ + → e + γ decay at rest is given by a back-to-back, photon-positron pair coincident in time and each with an energy of half the mass of the muon. Eachevent can be described by four observables: the photon and positron energies (E γ , E e ), theirrelative direction (Θ eγ ), and their relative emission time (t e γ ).The background has two components: an intrinsic physics background coming from the radia-tive muon decay (RMD), µ → eν ¯ νγ , when the neutrinos carry a small fraction of the availableenergy; and an accidental background that arises when an energetic positron from a stan-dard muon decay overlaps with an energetic photon from RMD, e + e − annihilation-in-flight,or bremsstrahlung. The effective branching fraction for the accidental background is a strong5unction of the muon beam intensity, I µ , and the detector resolutions associated with the fourobservables, ∆E γ , ∆E e , ∆Θ eγ , and ∆t e γ : BF eff ∝ I µ × (∆ E γ ) × ∆ E e × (∆Θ eγ ) × ∆ t eγ . (1)In the MEG experiment, which used I µ ∼ × stop- µ + /s , the accidental background ac-counted for over 90% of the events near the signal region (E γ >
48 MeV). To achieve animproved sensitivity,
MEG II will utilize a higher muon beam intensity. Mitigating the acciden-tal background requires
MEG II to upgrade its detector components to achieve the required,improved resolutions.
Status and Plans of the
MEG II experiment
The
MEG II experiment [20] is depicted in Fig. 1 of the Addendum. The main features arean e + spectrometer formed by a new cylindrical drift chamber plus precision pixelated timingcounters, located inside a superconducting solenoid with a graded magnetic field along thebeam axis, and a γ detector, located outside the solenoid, made up of a homogeneous volumeof 900 liters of liquid xenon readout in the central region by silicon photomultipliers and in theforward and background region by photomultiplier tubes. The finer granularity of the siliconphotomultipliers provides improved γ angular and energy resolution. Additional systems areused to further reduce RMD background, and to monitor the beam quality and stopping target in situ . The detector construction is complete and commissioning has begun. Physics datataking is expected to begin in 2019 and to last for a few years. The upgraded detector isexpected to provide resolutions roughly a factor of two better than MEG , thus allowing
MEG II to utilize the full muon beam intensity available at PSI, I µ ∼ stop- µ + /s , to achieve a factorof ten improvement in expected sensitivity. Search for µ + → e + γ at future facilities Improvements in sensitivity to the µ + → e + γ process beyond the MEG II projection may bepossible by utilizing the increased muon intensities that could be available from future facilitiesat PSI, Fermilab, and J-PARC. However, future experiments would have to devise ways toreduce the accidental background below the 10 − level in order to fully exploit the discoverypotential offered by the increased muon intensities.The use of an active or segmented target could allow a determination of the muon decay vertex,which, in principle, should lead to a strong suppression of the accidental background. Initialstudies [21] made for the MEG II project indicated that this additional suppression would berequired but this idea could be more effective if different schemes (see below) are adopted.Improvements to ∆E γ and ∆Θ eγ resolutions should be the most effective given their quadraticeffect on the accidental background. Feasibility studies have been performed for two verydifferent experimental concepts. One is based on a calorimetric detection of the photon, like MEG II , but with improved energy and timing resolutions [22]. The other is based on convertingthe photon and precisely measuring the trajectories of the resulting e + e − pair with a trackingspectrometer [22, 23]. The photon conversion concept is also being studied by Mu3e as apotential extension to its physics programme. These studies promise sensitivities around 10 − at 90% CL after a few years running, but additional studies are required to verify the efficacyof these concepts. The µ + → e + e − e + process The µ + → e + e − e + process is sensitive to new physics at mass scales beyond 10 TeV / c and probes cLFV couplings that arise from dipole interactions where new physics appearsin loop diagrams and from µeee contact interactions. The most stringent limit on thisprocess was established by the SINDRUM experiment using data collected from 1983–1986, BF ( µ + → e + e − e + ) < × − at 90% CL [6]. Any improvement in the sensitivity beyond6his has a significant impact on models predicting cLFV, especially the associated four-fermioncouplings. The Mu3e collaboration aims for a sensitivity of BF ( µ + → e + e − e + ) < × − at 90% CL in a first phase, Mu3e Phase-I , using the existing π E5 beamline at PSI. A furtherimprovement is possible in a second phase,
Mu3e Phase-II , with upgrades to the muon beam(HiMB project) and detector to reach a sensitivity that is four orders of magnitude better thanthe current experimental limit.The experimental signature of a µ + → e + e − e + decay at rest is given by three charged particletracks, corresponding to the e + e − e + decay products, coincident in time, originating from acommon vertex, and with a total energy consistent with the mass of the muon. Since this is athree-body decay, the energy of the decay products ranges from < e + e − pair; and an accidentalbackground from the random combination of electrons and positrons from separate decays.The RMD background can be kept sufficiently small if the resolution on the e + e + e − energysum can be kept below about 1 MeV, while the suppression of the accidental backgroundadditionally relies on excellent timing and vertex resolution. Status and Plans of the
Mu3e experiment
The
Mu3e Phase-I experiment [24] is depicted in Fig. 2 of the Addendum. The main featuresinclude precision particle tracking with ultra-light, monolithic, silicon pixel tracking layersbased on the HV-MAPS technology [25] cooled in an innovative manner using helium gas, plusa system of scintillating fibers and tiles to provide a time resolution below the sub-nanosecondlevel, all located inside a superconducting solenoid providing a constant 1 T field along the beamaxis. A farm of GPUs will use a highly parallel track-fitting algorithm to perform full onlinetracking of all data as it streams continuously from the detector front-ends. The detectoris capable of handling very high rates and the
Mu3e Phase-I sensitivity will be limited bythe muon rate that PSI can deliver. A new Compact Muon Beamline has been installed for
Mu3e and will deliver a continuous high-purity beam of 28 MeV / c muons at a rate of about10 stop- µ/s . The detector design is advanced and prototypes of all the main components havebeen built and successfully tested. Construction of the Mu3e Phase-I experiment will beginin 2019 and a first commissioning run is expected in 2020. After three years of operation theprojected
Phase-I sensitivity is BF ( µ + → e + e − e + ) < × − at 90% CL. However, the Mu3e experimental concept allows for further significant improvements in sensitivity.
Search for µ + → e + e − e + at future facilities The sensitivity of
Mu3e Phase-I is largely limited by the muon rate. The sensitivity to µ + → e + e − e + could be improved by more than an order of magnitude with a modest ex-tension of the detector and access to a higher intensity muon beam. Roughly a factor twoimprovement in experimental acceptance is expected by extending the instrumentation in theforward and backward directions using the same pixel and scintillator technologies plannedfor Phase-I . At PSI, concepts for a new High Intensity Muon Beamline, HiMB, have beeninvestigated. Recent studies indicate that by refurbishing target M, and by installing a newcapture solenoid the muon rate could be increase up to 1 . × stop- µ + /s [26], which is morethan sufficient for Phase-II . Future facilities at Fermilab and J-PARC could in principle pro-vide similar muon intensities. If approved, the earliest HiMB could be installed is 2024. Afterthree years of data taking at the increased muon intensity the projected
Phase-II sensitivityis BF ( µ + → e + e − e + ) < × − at 90% CL.Detector R&D should also continue to further improve the time resolution and to reduce theamount of detector material, both being important requirements for suppressing accidental7ackgrounds at higher rates. Silicon pixel detectors with picosecond timing represent a verypromising technology for future Mu3e upgrades.
The µ − N → e − N process The µ − N → e − N process is sensitive to new physics at mass scales up to 10 TeV / c and probescLFV couplings that arise from interactions where new physics appears in loop diagrams as wellas from µeqq contact interactions. The most stringent limit on this process was established bythe SINDRUM-II experiment using data collected in 2000, R µe (Au) < × − at 90% CL [7],where, by convention, R µe is the rate of the µ − N → e − N conversion process normalized to thenormal muon nuclear capture process, µ − N (A , Z) → νN ∗ (A , Z − COMET Phase-I experiment is currently underconstruction and aims to improve sensitivity by a factor of 100 starting in the next few years.The
Mu2e experiment is under construction and aims to improve the sensitivity by four orders ofmagnitude by the mid 2020s, while
COMET Phase-II is a proposed upgrade to
COMET Phase-I that would achieve a similar sensitivity or better. Further improvements are possible for bothexperiments with upgrades to the beamline and detectors.The direct conversion process, µ − N → e − N , is dominated by coherent interactions with thenucleus to provide a two-body final state yielding a clean experimental signature: an outgoingelectron with an energy near the muon mass (the recoil nucleus is not directly observed). Thetime distribution of signal electrons should be consistent with the characteristic lifetime of themuonic atoms formed as the initial µ − beam comes to rest in a stopping target.Significant sources of background events can arise from muons that decay-in-orbit (DIO) —that is, decay while captured in the atomic orbit around a nucleus in the stopping target, frompions that survive to the stopping target, and from cosmic ray muons that decay-in-flight orinteract in material to produce an electron with an energy near the muon mass. If the energyof the initial proton beam is above the anti-proton production threshold, then annihilations ofthe anti-protons can contribute an additional source of background. The steeply falling DIObackground can be mitigated with excellent momentum resolution; the pion background canbe suppressed by using a pulsed proton beam and employing a delayed live gate; the cosmicray background can be removed using a high-efficiency cosmic veto system; and anti-protonbackgrounds can be kept small by using absorbers to range out the anti-protons far away fromthe stopping target.Both the COMET and the
Mu2e experiments are based on a very clever idea first proposed byLobashov and Djilkabaev in 1989 [27]. A system of three solenoids — a pion capture solenoid, amuon transport solenoid, and a detector solenoid — with graded magnetic fields which providea significantly improved muon beam intensity to enable dramatic improvements in sensitivityrelative to
SINDRUM-II . The feasibility of this method has been demonstrated at low intensityat the MuSIC facility at the Research Center for Nuclear Physics in Osaka [28].
Status and Plans of the
COMET experiment
The
COMET Phase-I experiment [29] is depicted in Fig. 4 of the Addendum. The appara-tus begins with a pion-production target made of graphite located inside the pion capturesolenoid, which provides a graded magnetic field to collect low-momentum pions by reflectingthem backwards with respect to the incoming proton beam. The muon transport solenoid isa curved 90-degree magnet that, together with a set of dipole coils, serves as a tranport andcharge- and momentum-selection channel for π − → µ − ν decays. The muon transport solenoiddelivers a high-intensity µ − beam to the detector solenoid, which houses an aluminum stop-ping target and active detector elements, including a cylindrical drift chamber (CDC) for the µ − N → e − N search and low-mass straw chambers and a fast LYSO crystal calorimeter for8eam measurements. An active cosmic-ray veto system shadows the detector and stoppingtarget regions outside the solenoid volume. Additional instrumentation monitors the protonand muon beams. The construction of the entire apparatus is at an advanced stage, withtwo of the magnets and the CDC complete, including significant European contributions tothe cosmic-veto, muon stopping target and beam monitoring systems, the trigger and DAQ,and computing and software. Beam commissioning is expected to begin in 2020. The COMETPhase-I experiment will utilize about 3 kW of 8 GeV protons from the J-PARC Main Ring,delivered in pulses spaced by 1 . µs , to first make important measurements of the muonyield and determine rates for various background processes before concentrating on a search forcLFV. After 150 days of operation the projected COMET Phase-I sensitivity is R µe < × − at 90% CL [29]. This sensitivity can be significantly improved using a higher intensity protonbeam and extending the COMET Phase-I apparatus.
Status and Plans of the
Mu2e experiment
The
Mu2e experiment [30] is depicted in Fig. 6 of the Addendum. The graded high-field pionproduction solenoid collects and focuses low-momentum pions towards the muon transportsolenoid, which is an “S”-shaped magnet with a total path length of about 13 meters. Themuon transport solenoid includes a set of collimators for momentum and charge-selection toprovide ∼ stop- µ − /s using 8 kW of 8 GeV protons from the Fermilab Booster deliveredin pulses spaced 1 . µs apart. The detector solenoid provides a graded magnetic field in theupstream region, which houses the stopping target, and a near constant magnetic field in thedownstream region, which houses the active detector elements. A low-mass tracking systemconsisting of approximately 21k thin aluminized-mylar straws [31] and a calorimeter consistingof two annular disks of pure CsI crystals [32] precisely measure the timing, energy, and momentaof particles originating from the stopping target. The apparatus is shadowed on the outsideby a large, scintillator-based cosmic-veto system. Ancillary systems are used to monitor thequality and intensity of the proton and muon beams. Construction of the solenoids and all thedetector sub-systems has begun, with significant European contributions to the muon transportsolenoid, the calorimeter, and the muon beam monitoring system. Commissioning is expectedto begin in 2022. After 690 days of operation the projected sensitivity is R µe < × − at90% CL [30]. This sensitivity can be improved by at least a factor of 10 using a higher intensityproton beam and upgrading the Mu2e apparatus.
Search for µ − N → e − N at future facilities The
COMET Phase-I experiment can be extended, as depicted in Fig. 5 of the Addendum, andutilize 56 kW of 8 GeV protons from the J-PARC Main Ring, delivered in pulses spaced by1.17 µs , to reach a sensitivity of R µe < . × − at 90% CL with about 230 days of operation.Further improvements by one order of magnitude from refinements to the experimental designand operation are being considered, within the beam power and the beam time as originallyassumed [33]. These improvements include dipole steering fields in the curved muon transportand electron spectrometer sections to allow a more fine-tuned momentum selection which isimportant to optimize the acceptance and background rejection. The detailed measurementsfrom Phase-I will provide important input to the final
Phase-II design and construction. Datataking could begin in the mid-2020s.The
Mu2e experiment can be upgraded,
Mu2e-II , to take advantage of the increased protonbeam intensity available from the PIP-II project, currently in the design phase at Fermilab.The PIP-II linac is expected to become operational in the latter half of the 2020s and willprovide 1.6 MW of 0.8 GeV protons with a programmable time structure. An Expression ofInterest for
Mu2e-II [34] was recently submitted to the Fermilab Physics Advisory Committee,which concluded that the science case was compelling and recommended that funding for high-priority R&D be identified. The Expression of Interest included signatures from 130 scientists9rom 36 institutions in six countries, including Italy, Germany, and the UK. Using 100 kW ofprotons from PIP-II, the
Mu2e-II projected sensitivity is a factor ten or more better than the
Mu2e sensitivity. Data taking could begin in the late 2020s.The
COMET collaboration is also heavily involved in R&D towards the
PRISM project, whichcombines
COMET Phase-II with an FFAG muon storage ring to potentially provide muon beamintensities of > stop- µ/s with a narrow momentum bite allowing the use of very thinstopping targets, and significantly reduced pion contamination owing to the increased transportpath length. In conjunction with an upgrade to the J-PARC proton source to achieve 1.3 MWand to the detector systems to accomodate the higher rates, PRISM offers the potential toachieve sensitivies to µ − N → e − N of the order of 10 − . The monochromatic, pion-suppressed,high-intensity muon beam provided by PRISM will allow the use of stopping targets comprisedof heavy elements, such as gold or lead, that can be important in understanding the underlyingnew physics operators in the event of a discovery [33].
Summary
The
MEG , Mu3e , Mu2e , and
COMET experiments use intense muon beams to provide the broadest,deepest, most sensitive probes of charged-lepton flavour violating interactions and to explorethe BSM parameter space with sensitivity to new physics mass scales of 10 − TeV /c ,well beyond what can be directly probed at colliders. Over the next five years, currentlyplanned experiments in Europe, the US, and Asia will begin taking data and will extend thesensitivity to µ → e charged-lepton flavour violating transitions by orders of magnitude. Furtherimprovements are possible and new or upgraded experiments are being considered that wouldutilize upgraded accelerator facilities at PSI, Fermilab, and J-PARC. The schedule of plannedand proposed experiments is summarized in the figure below. Strong European participationin the design, construction, data taking, and analysis will be important for the success of thesefuture endeavors and represents a prudent investment complementary to searches at colliders.We urge the committee to strongly support the continued participation of European institu-tions in experiments searching for charged-lepton flavour violating µ → e transitions usinghigh-intensity beams at facilities in Europe, the US, and Asia, including possible upgradedexperiments at next-generation facilities available the latter half of the next decade at PSI,Fermilab, and J-PARC.Figure 1: Planned data taking schedules for current experiments that search for charged-lepton flavorviolating µ → e transitions. Also shown are possible schedules for future proposed upgrades to theseexperiments. The current best limits for each process are shown on the left in parentheses, whileexpected future sensitivities are indicated by order of magnitude along the bottom of each row. eferences [1] S.T. Petcov, Sov. J. Nucl. Phys. 25 (1977) 340.[2] L. Willmann, et al. , Phys.Rev.Lett. 82 (1999) 49.[3] L. Calibbi and G. Signorelli, Riv. Nuovo. Cimento, 41 (2018) 71.[4] V. Cirigliano, et al. , Phys. Rev. D80 (2009) 013002.[5] A. Baldini, et al. ( MEG
Collaboration), Eur. Phys. J. C76 (2016) 434.[6] U. Bellgardt, et al. ( SINDRUM
Collaboration), Nucl. Phys. B299 (1988) 1.[7] W. Bertl, et al. ( SINDRUM-II
Collaboration), Eur. Phys. J. C47 (2006) 337.[8] G.W. Bennett, et al. ( E821
Collaboration), Phys. Rev. Lett. 92 (2004) 161802.[9] G. F. Giudice, P. Paradisi and M. Passera, JHEP11 (2012) 113.[10] T. Hambye, Proc. Nucl. Phys. B248 (2014) 13.[11] A. Abada, et al. , JHEP 11 (2014) 048.[12] J. Kaulard, et al. ( SINDRUM-II
Collaboration), Phys. Lett. B422 (1998) 334.[13] B. Yeo, Y. Kuno, M. Lee and K. Zuber, Phys.Rev. D96, no. 7 (2017) 075027.[14] B. Gripaios, et al. , JHEP (2015) 6; B. Dumont, et al. , arXiv:1603.05248 (2016); M. Bauerand M. Neubert, Phys. Rev.Lett. 116 (2016) 141802; S. Baek and K. Nishiwaki, Phys. Rev.D93 (2016) 015002.[15] A. Crivellin, et al. , Phys. Rev. D92 (2015) 050413.[16] J. Arnold, et al. , Phys. Rev. D88 (2013) 035009.[17] R. Aaij, et al. ( LHCb
Collaboration), Phys.Rev.Lett. 111 (2013) 141801.[18] G. Wilkinson et al. , https://indico.cern.ch/event/706741/contributions/3017537 [19] M.Ball, et al. (PIP-II Accelerator Facility), http://pip2-docdb.fnal.gov/cgi-bin/ShowDocument?docid=113 (2018). [20] A. Baldini, et al. ( MEG II
Collaboration), Eur. Phys. J. C78 (2018) 380.[21] A. Papa, et al. , Nucl. Phys. Proc. Suppl. 248 (2014) 121.[22] G. Cavoto, et al. , Eur.Phys.J. C78 (2018) 37.[23] C. Cheng, B. Echenard, D.G. Hitlin, arXiv:1309.7679 (2013).[24] A. Blondel, et al. ( Mu3e
Collaboration), arXiv:1301.6113 (2013).[25] I. Peric, Nucl. Instrum. Meth. A582, 876 (2007).[26] A. Papa, NuFact 2018, Blacksburg, Virginia USA, https://indico.phys.vt.edu/event/34/contributions/701 [27] R.M. Dzhilkibaev and V.M. Lobashev, Sov.J.Nucl.Phys. 49(2), (1989) 384.[28] S. Cook, et al. , Phys. Rev. Accel. Beams 20(3), (2017) 030101.[29] R. Abramishvili, et al. ( COMET
Collaboration), http://comet.kek.jp/Documents files/PAC-TDR-2016/COMET-TDR-2016 v2.pdf [30] L. Bartoszek, et al. ( Mu2e
Collaboration), arXiv:1501.05241 (2015).[31] M. Lee (on behalf of
Mu2e
Collaboration), Nucl. Part. Phys. Proc., 273 (2016) 2530.[32] N. Atanov, et al. , IEEE Trans. Nucl. Sci., Vol 65, N. 8, (2018) 2073.[33] COMET submission to the European Strategy for Particle Physics 2020, and referencestherein.[34] F. Abusalma, et al. ( Mu2e-II
Experiment), arXiv:1802.02599 (2018).11 ddendum:
Charged Lepton Flavour Violation usingIntense Muon Beams at Future Facilities
A. Baldini, D. Glenzinski, F. Kapusta, Y. Kuno, M. Lancaster,J. Miller, S. Miscetti, T. Mori, A. Papa, A. Sch¨oning, Y. UchidaA submission to the 2020 update of the European Strategy for ParticlePhysics on behalf of the COMET, MEG, Mu2e and Mu3e collaborations.
Abstract
In this Addendum additional information is provided about the MEG, Mu3e, Mu2e, andCOMET experiments and their associated collaborations. The contributions from Europe areemphasized.Contact: Andr´e Sch¨oning [[email protected]]1 ddendum for the MEG Experiment
COBRA Radiative decay counter (RDC) Liquid xenon photon detector (LX e) Pixelated timing counter Cylindrical drift chamber (CDCH) (pTC)
Figure 1:
Schematic of the
MEG II experiment.
Experiment Website and Contact Information
Website: http://meg.web.psi.chCo-spokespersons: A. Baldini (University of Pisa) and T. Mori (University of Tokyo)([email protected], [email protected])
Interested Community
The
MEG II
Collaboration consists of about 75 participants from Japanese, Italian, Swiss,Russian and US institutions. Scientists and students from Europe account for 50% of thecollaboration. The experiment is hosted at the PSI laboratory in Switzerland.
Timeline
The
MEG II experiment has recently completed construction and first commissioning data wascollected in 2018. A three year physics run is expected to begin in 2019.
European Contributions
The European contributions to
MEG II spanned all the major sub-sytems of the experimentincluding: • Data acquisition software • Construction of trigger and read-out electronics • Procurement of silicon photomultipliers for the positron timing counter • Mechanical structure of the positron timing counter • Construction of the new cylindrical drift chamber2
Construction of the liquid xenon detector cryostat • Calibration devices for the liquid xenon detectorThe European groups also play a significant role in the leadership, commissioning, operations,analysis, and publication activities of the experiment.
Computing Requirements
The computing system of
MEG II consists in about 320 CPU cores and 1300/2000 TB ofdisk/tape space. Computing expenses are equally subdivided among Japanese, Italian andSwiss institutions. 3 ddendum for the Mu3e Experiment
TargetInner pixel layers Outer pixel layersRecurl pixel layersScintillator tiles μ Beam
Figure 2:
Schematic of the
Mu3e experiment.
Experiment Website and Contact Information
Interested Community
The
Mu3e
Collaboration consists of about 70 participants, from eleven European institutionsin Germany, Switzerland and United Kingdom. Scientists and Europe account for 100% of thecollaboration. The experiment is hosted at the PSI laboratory in Switzerland.
Timeline
The experiment will be performed in two phases. The R&D programme is nearly complete andconstruction has begun for various components. Commissioning with beam for
Mu3e Phase-I is expected to start in 2020. Three years of physics data taking is required to reach the designsensitivity.The
Mu3e Phase-II experiment requires an upgraded detector with an extended geometricalacceptance and the construction of a new high intensity muon beamline, HiMB, at PSI. Theproposal requires refurbishing target M of the proton beamline and installation of a new capturesolenoid followed by a solenoidal beamline, using a design similar to existing µ E4 beamline, seeFig. 3. Ongoing studies are investigating whether, with modest modifications, the
Phase-II experiment may also allow searches for µ + → e + γ decays or muonium-anitmuonium oscillations.Design studies for HiMB are underway and installation, if approved, could start at the earliestin 2024 after the completion of the Phase-I programme.
European Contributions
The entire
Mu3e Phase-I experiment is designed and built by European institutions. The maincomponents of the experiment are: • Superconducting solenoid with a homogeneous magnetic field of B = 1 Tesla (up toB=2.6 Tesla). • Ultra-light pixel tracker based on high voltage monolithic active pixel sensors (HV-MAPS). 4
Two scintillating detector systems for sub-nanosecond timing, based on scintillating fibersand tiles. • Trigger-less data acquisition system with continuous readout. • Filter farm based on graphical processing units.The European groups also play a significant role in all aspects of the experiment includingleadership, operations, analysis and publication activities.Most groups of the
Mu3e collaboration
Phase-I have expressed interest to contribute to
Phase-II . Also new groups are invited to contribute to the planned
Mu3e Phase-II upgrade,and to investigate further extensions of the
Mu3e physics programme, for example a searchfor µ + → e + γ with a photon conversion layer or muonium-antimuonium oscillations with anupgraded Mu3e detector.
Computing Requirements
The computing system and needs will be similar to those of the MEG experiment. Expensesfor computing will be shared among the contributing institutes. Additional GRID computingresources will be required to fully exploit the physics potential of the experiment.Figure 3: The new proposed solenoidal beamline for HiMB (right) compared to the currenthybrid µ E4 beamline (left). 5 ddendum for the COMET Experiment
Figure 4:
A schematic of the
COMET Phase-I experiment. A cosmic-ray veto system and monitorsfor the proton beam and muon beam are not shown.
Experiment Website and Contact Information
Website: http://comet.kek.jp/Introduction.htmlSpokesperson: Y. Kuno (Osaka University)([email protected])
Interested Community
The
COMET
Collaboration consists of about 200 participants from 35 institutions from Australia,Belarus, China, Czech Republic, France, Georgia, Germany, India, Japan, Kazakhstan, SouthKorea, Malaysia, Russia, United Kingdom, and Vietnam. Scientists and students from Europeaccount for about 30% of the collaboration. The experiment is hosted at the J-PARC laboratoryin Japan.
Timeline
The experiment will be performed in two phases. Construction of
COMET Phase-I is at andadvanced stage. The J-PARC proton beam will arrive at the
COMET experimental area in early2020, when
Phase-I beam studies and integration will commence. The
Phase-I physics data-taking and analysis will follow.The
COMET Phase-II experiment requires the construction of an extended solenoid systemas depicted in Fig. 5. that, if approved, could be ready in the mid–2020s. The completed
COMET Phase-II configuration can be adapted to search for and measure several charged-leptonflavour- and number-violating (cLNV) processes other than the main µ − N → e − N channel, anda broad programme of study is expected to continue well beyond 2025 and into the 2030s, witha specific path that is dependent on the observations that have been made by that time. Someof these additional measurements will require the beamline to run in dedicated positive-muonmode, which will produce an extremely high-quality beam in the Phase-II configuration.In the longer term (2030 and beyond), the
COMET collaboration is also closely engaged with thenext-generation PRISM experiment through the PRISM Task Force, which makes use of anFFAG muon storage ring to pursue detailed measurements of cLFV and LNV processes. Thisis a relatively long-term project which would be expected in the latter stages of the periodrelevant to the present strategy exercise. 6 uropean Contributions
The European contributions to
COMET include: • Cosmic Ray Veto detector (Belarus, France, Georgia, Russia) • Electromagnetic calorimeter (Belarus, Russia) • Muon target monitor (Germany) • Data-acquisition and detector triggering systems (UK, Czech Republic) • Straw-tube tracking detector (Georgia, Russia) • Muon stopping targetry (Germany)The European groups also play a significant role in the leadership, analysis, and publicationactivities and are expected to play a significant role in the commissioning and operation ofthe experiment beginning in 2020. The international PRISM task force also benefits fromsignificant European contributions, including leadership.
Computing Requirements
Controlling and monitoring the beam composition and the various backgrounds for this rare-decay experiment requires very large simulated data samples. Single- and multi-bunch simu-lations have involved significant contributions in terms of CPU (France, UK and Germany),storage and data sharing (France). Software developments related to the analysis, track find-ing and track fitting optimization lead also to intensive software tests and improvements (UK,France, Germany). In particular, much effort has been focused on introducing simulationstrategies that allow for high-statistics background and signal estimates without requiring aproportional increase in computational resources. Combining such strategies with increasinginternational resource contributions will allow the computational challenges of
COMET to be met.Figure 5:
Schematic of the
COMET Phase-II experiment. ddendum for the Mu2e and Mu2e-II Experiment Figure 6:
Schematic of the
Mu2e experiment. A cosmic-ray veto system, and monitors for the protonbeam and muon beam are not shown.
Experiment Website and Contact Information
Website: https://mu2e.fnal.govCo-spokespersons: D. Glenzinski (Fermilab) and J. Miller (Boston University)([email protected])
Interested Community
The
Mu2e
Collaboration consists of 242 members from 40 institutions in China, Germany, Italy,Russia, the United Kingdom, and the United States. Scientists and students from Europeaninstitutions account for 26% of the collaboration. The experiment is hosted by Fermilab in theUS.
Timeline
The
Mu2e experiment is currently under construction. In 2021 commissioning of the protonbeamline, and cosmic-ray commissioning of the detector systems are scheduled to begin. Com-missioning of the detector systems with beam is expected in 2022 and a four-year physics runis planned starting in 2023.In parallel to
Mu2e construction and commissioning, R&D for
Mu2e-II will begin in order todevelop a conceptual design for the detectors and a new proton beamline to accommodate thenew beam energy provided by the PIP-II linac. There are challenging issues associated with theincreased rate and radiation environment for the production solenoid, the production target,and the detector systems and their associated read-out electronics. The timeline for
Mu2e-II will be driven by the completion of
Mu2e as well as the construction of the PIP-II linac, which,if approved, is expected to become available in the mid-2020 timescale. To achieve anotherfactor of ten or more improvement in sensitivity will require about three years of physics datataking with 100 kW of protons from PIP-II. The flexibility of PIP-II provides an opportunity todeliver customized muon beams for the exploration of other
Mu2e-II stopping target materialsas well as for next-generation µ + → e + e − e + or µ + → e + γ experiments. European Contributions
The European contributions to
Mu2e spanned several important sub-systems of the experimentincluding: 8
Calorimeter: the design and construction is lead by Italy with additional contributionsfrom Germany, Russia, and the US. Italy (INFN) is contributing O (3M Euro) for coreconstruction costs and provided support for O (30) people. • Muon Target Monitor: the final design and construction of the muon target monitor isdriven by the UK in collaboration with the US. The UK (STFC) is contributing O (1MEuro) for core construction costs and provided support for O (15) people. • Transport Solenoid: Italy made very significant contributions to the design, prototyping,and fabrication of the superconductor and coils of the transport solenoid. • Irradiation facilities: Germany provides support for O (2) people plus in-kind access tothe EPOS and G-ELBE irradiation facilities at HZDR for tests of the rate capabilitiesand radiation tolerance of various detector sub-systems.European groups also play a significant role in the leaderhip, analysis, and publication activitiesand are expected to play a significant role in the commissioning and operation of the experimentbeginning in 2021.For Mu2e-II , European groups have expressed interest in contributing to the developmentand design of an upgraded calorimeter (e.g. using BaF crystals and optimized photosensors),of upgraded read-out electronics using next-generation FPGAs or custom ASICs, and of anupgraded tracker potentially using micro-RWell or MPGD technologies. Computing Requirements
The computing resources required for
Mu2e data processing, reconstruction, and analysis areestimated to be about 1000 CPU cores and 30/60 PB of disk/tape space. Significant additionalCPU resources ( ∼∼