Mu2e Run I Sensitivity Projections for the Neutrinoless - e- Conversion Search in Aluminum.

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Article

Mu2e Run I Sensitivity Projections for the Neutrinoless µ−→e−

Conversion Search in Aluminum.

arXiv:2210.11380v1 [hep-ex] 20 Oct 2022

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F. Abdi 26 , R. Abrams 28 , J. Adentunji 11 , W. Ahmed 15 , R. Alber 11 , D. Alexander 15 , D. Allen 11 , D. Allspach 11 , C.

Alvarez-Garcia 23 , D. Ambrose 26 , G. Ambrosio 11 , A. Amirkhanov 31 , N. Andreev 11 , C. M. Ankenbrandt 28 , R.

Appleby 23 , D. Arnold 11 , A. Artikov 9, N. Atanov 9, K. Badgley 11 , M. Ball 11 , V. Baranov 9, J. Barker 11 , E. Barnes 2,

B. Barton 37 , L. Bartoszek 11 , G. Bellettini 32 , R. H. Bernstein 11 , A. Bersani 13 , I. Bianchi 11 , K. Biery 11 , S. Bini 12 , G.

Blazey 29 , C. Bloise 12 , K. Boedigheimer 26 , S. Boi 11 , T. Bolton 16 , J. Bono 11 , R. Bonventre 17 , S. Borghi 23 , L. Borrel 7

, R. Bossert 11 , T. Bowcock 20 , M. Bowden 11 , J. Brandt 11 , M. Breach 26 , D. Brown 17 , D. N. Brown 22 , G. Brown 11 , H.

Brown 11 , I. Budagov 9,‡ , M. Buelher 11 , G.-M. Bulugean 26 , K. Byrum 1, M. Campbell 11 , H. Cao 33 , R. M. Carey 2, J.

F. Caron 15 , B. Casey 11 , H. Casler 8, F. Cervelli 32 , S. Cheban 11 , J. Chen 33 , M. Chen 11 , C.-H. Cheng 7, R. Chislett 21 ,

N. Chitirasreemadam 32 , D. Chokheli 9, K. Ciampa 26 , R. Ciolini 32 , J. Coghill 11 , F. Colao 12 , R. N. Coleman 11 , S.

Corrodi 1, L. Crescimbeni 32 , C. Crowley 11 , R. Culbertson 11 , M. A. C. Cummings 28 , A. Daniel 15 , Y. Davydov 9, S.

Demers 38 , A. Deshpande 11 , M. Devilbiss 25 , J. Dey 11 , G. De Felice 32 , A. De Gouvea 30 , J. Dhanraj 11 , D. Ding 30 , D.

Ding 4,17 , M. Dinnon 11 , E. Diociaiuti 12 , S. Dixon 11 , S. Di Falco 32 , R. Djilkibaev 27 , S. Donati 32 , G. Drake 11 , B.

Drendel 11 , G. Duerling 11 , E. C. Dukes 37 , A. Dychkant 29 , B. Echenard 7, N. Eddy 11 , A. Edmonds 2,17 , R. Ehrlich 37 ,

U. Ekka 26 , R. Evans 11 , D. Evbota 11 , P. Fabbricatore 13 , J. Fagan 11 , S. Farinon 13 , W. Farrell 37 , P. Farris 37 , S. Feher 11

, B. Fellenz 11 , E. Fernandez 37 , A. Ferrari 14 , C. Ferrari 32 , J. Finley 33 , K. Flood 7, E. Flumerfelt 11 , F. Fontana 24 , K.

Francis 29 , M. Frand 26 , M. Frank 34 , H. Friedsam 11 , G. Gallo 11 , R. P. Gandrajula 37 , A. Gaponenko 11 , M. Gardner 11

, R. Gargiulo 12 , S. Gaugel 11 , K. L. Genser 11 , M. Gersabeck 23 , G. Ginther 11 , A. Gioiosa 32 , S. Giovannella 12 , V.

Giusti 32 , V. Glagolev 9, H. Glass 11 , D. A. Glenzinski 11 , S. Goadhouse 37 , L. Goodenough 11 , F. Grancagnolo 18 , P.

Gray 26 , C. Group 37 , A. Hahn 11 , D. Hampai 12 , S. Hansen 11 , F. Happacher 12 , L. Harkness-Brennan 20 , K. Harrig 5,

B. Hartsell 11 , S. Hays 11 , M. Hedges 33 , D. Hedin 29 , K. Heller 26 , A. Herman 4, S. Hirsh 4,17 , D. G. Hitlin 7, A.

Hocker 11 , R. Hooper 19 , G. Horton-Smith 16 , S. Huang 33 , E. Huedem 11 , D. Huffman 11 , P. Q. Hung 37 , E.

Hungerford 15 , A. Ibrahim 11 , S. Israel 2, M. Jenkins 34 , C. Johnstone 11 , M. Jones 33 , V. Jorjadze 2, D. Judson 20 , C.

Kampa 30 , M. Kargiantoulakis 11 , V. Kashikhin 11 , P. Kasper 11 , A. Keshavarzi 23 , V. Khalatian 2, J.-H. Kim 7, T. Kiper

11 , D. Knapp 11 , O. Knodel 14 , K. Knoepfel 11 , L. Kokosa 11 , Yu. G. Kolomensky 4, D. Koltick 33 , M. Kozlovsky 11 , J.

Kozminski 19 , G. Kracczyk 11 , M. Kramp 11 , S. Krave 11 , K. Krempetz 11 , R. K. Kutschke 11 , R. Kwarciany 11 , T.

Lackowski 11 , M. J. Lamm 11 , M. Lancaster 23 , M. Larwill 11 , F. Leavell 11 , M. J. Lee 17 , D. Leeb 11 , J. Lema-Sinchi 26 , T.

Leveling 11 , R. Lewis 11 , A. Ley 26 , B. Li 26 , Y. Li 7, D. Lin 7, D. Lincoln 11 , I. Logashenko 31 , V. Lombardo 11 , M.L.

Lopes 11,‡ , A. Luca 11 , K. R. Lynch 8, M. MacKenzie 30 , A. Makulski 11 , J. Manolis 26 , Yu. Maravin 16 , W. J. Marciano 3

, A. Marini 32 , E. Martin 26 , A. Martinez 11 , M. Martini 24 , D. McArthur 11 , F. McConologue 11 , N. Mesmer 19 , B.

Messerly 26 , L. Michelotti 11 , S. Middleton 7, C. Miles 37 , J. P. Miller 2, T. M. Nguyen 5,8 , S. Miscetti 12 , D. Mitchell 11 ,

T. Miyashita 7, N. Mokhov 11 , D. Molenaar 26 , W. Molzon 6, J. Moore 26 , L. Morescalchi 32 , J. Morgan 11 , J. Mott 2, E.

Motuk 21 , S. Mueller 14 , A. Mukherjee 11 , P. Murat 11 , R. Musenich 13 , V. Nagaslaev 11 , A. Narayanan 29 , R. Neely 16 ,

D. V. Neuffer 11 , M. T. Nguyen 5, T. Nicol 11 , J. Niehoff 11 , J. Nogiec 11 , A. Norman 11 , K. Northrup 26 , V. O’Dell 11 , S.

Oh 10 , Yu. Oksuzian 1, P. Olderr 11 , M. Olson 11 , D. Orris 11 , B. Oshinowo 11 , R. Ostojic 11 , J. Oyang 7, D. Paesani 12 ,

S. Pagan 17 , T. Page 11 , A. Palladino 2, C. Park 11 , D. Pasciuto 32 , E. Pedreschi 32 , T. Peterson 11 , G. Pezzullo 38 , R.

Pilipenko 11 , A. Pla-Dalmau 11 , P. Plesniak 21 , N. Pohlman 29 , B. Pollack 30 , V. Poloubotko 11 , M. Popovic 11 , J. L.

Popp. 8, F. Porter 7, E. J. Prebys 5, J. Price 20 , P. Prieto 11 , V. Pronskikh 11 , D. Pushka 11 , J. Quirk 2, R. Rabehl 11 , R.

Rachamin 14 , F. Raffaelli 32 , A. Ragheb 26 , G. Rakness 11 , R. E. Ray 11 , R. Rechenmacher 11 , R. Rivera 11 , G. Rizzo 26 ,

B. L. Roberts 2, S. Roberts 37 , T. J. Roberts 28 , W. Robotham 11 , M. Roehrken 7, P. Rubinov 11 , R. Rucinski 11 , V. L.

Rusu 11 , M.F. Samavat 26 , E. Sanzani 12 , A. Saputi 12 , I. Sarra 12 , M. Sarychev 11 , V. Scarpine 11 , W. Schappert 11 , M.

Schmitt 30 , P. Schmitter 26 , D. Schoo 11 , K. Schumacher 37 , X. Shi 33 , V. Singh 4, T. Sobering 16 , R. Soleti 17 , M. Solt 37

, H. Song 2,17 , E. Song 37 , F. Spinella 32 , M. Srivastav 37 , A. Stefanik 11 , S. Stetzler 37 , D. Still 11 , M. Stortini 38 , D.

Stratakis 11 , T. Strauss 11 , Y. Sun 11 , I. Suslov 9, M. J. Syphers 29 , L. Szemraj 30 , J. Ta 26 , A. Taffara 32 , Z. Tang 11 , N.

Tanovic 11 , M. Tartaglia 11 , G. Tassielli 18 , R. Taylor 16 , M. Tecchio 25 , S. Tickle 20 , D. Tinsley 11 , T. Tope 11 , A.

Torkelson 26 , N. Tran 2, J. Trevor 7, R. S. Tschirhart 11 , S. Turnberg 26 , S. Uzunyan 29 , D. Varier 17 , D. Varier 4, M.

Velasco 30 , L. Vinas 17 , B. Vitali 32 , G. Vogel 11 , R. Wagner 1, R. Wagner 11 , R. Wands 11 , Y. Wang 2, C. Wang 10 , M.

Wang 11 , I. Wardlaw 26 , M. Warren 21 , S. Werkema 11 , H. B. White Jr. 11 , J. Whitmore 11 , R. Wielgos 11 , R. Wildberger

26 , L. Wills 26 , P. Winter 1, R. Woods 11 , C. Worel 11 , Y. Wu 25 , L. Xia 1, Z. You 35 , M. Yucel 11 , P. Zadeh 37 , A. M.

Zanetti 36 , D. Zhadan 31 , R.-Y. Zhu 7, R. Zifko 11 , V. Zutshi 29

(Mu2e Collaboration)

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Citation: Mu2e Collaboration Mu2e

Run I Sensitivity Projections for the

Neutrinoless Muon to Electron

Conversion Search in Aluminum.

Preprints 2021,1, 0. https://doi.org/

Received:

Accepted:

Published:

Publisher’s Note: MDPI stays neutral

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published maps and institutional afﬁl-

iations.

1Argonne National Laboratory;

2Boston University;

3Brookhaven National Laboratory;

4University of California, Berkeley;

5University of California, Davis;

6University of California, Irvine;

7California Institute of Technology;

8City University of New York;

9Joint Institute for Nuclear Research, Dubna;

10 Duke University;

11 Fermi National Accelerator Laboratory;

12 Laboratori Nazionali di Frascati;

13 Istituto Nazionale di Fisica Nucleare, Genova;

14 Helmholtz-Zentrum Dresden-Rossendorf;

15 University of Houston;

16 Kansas State University;

17 Lawrence Berkeley National Laboratory;

18 Istituto Nazionale di Fisica Nucleare, Lecce and Universita del Salento;

19 Lewis University;

20 University of Liverpool;

21 University College London;

22 University of Louisville;

23 University of Manchester;

24 Laboratori Nazionali di Frascati and Universita Marconi Roma;

25 University of Michigan;

26 University of Minnesota;

27 Institute for Nuclear Research, Moscow;

28 Muons Inc.;

29 Northern Illinois University;

30 Northwestern University;

31 Novosibirsk State University/Budker Institute of Nuclear Physics;

32 Istituto Nazionale di Fisica Nucleare, Pisa;

33 Purdue University;

34 University of South Alabama;

35 Sun Yat-Sen University;

36 Istituto Nazionale Fisica Nucleare, Trieste;

37 University of Virginia;

38 Yale University;

*Correspondence: murat@fnal.gov

‡ Deceased.

Abstract:

The Mu2e experiment at Fermilab will search for the neutrinoless

µ−→e−

conversion in

the ﬁeld of an aluminum nucleus. The Mu2e data-taking plan assumes two running periods, Run I

and Run II, separated by an approximately two-year-long shutdown. This paper presents an estimate

of the expected Mu2e Run I search sensitivity and includes a detailed discussion of the background

sources, uncertainties of their prediction, analysis procedures, and the optimization of the experimental

sensitivity. The expected Run I 5

discovery sensitivity is

Rµe=

1.2

−15

, with a total expected

background of 0.11

0.03 events. In the absence of a signal, the expected upper limit is

Rµe<

6.2

−16

at 90% CL. This represents a three order of magnitude improvement over the current experimental limit

of Rµe<7×10−13 at 90% CL set by the SINDRUM II experiment.

Keywords: lepton ﬂavor violation; LFV; muon conversion

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1. Introduction

Experimental observation of quark mixing and neutrino oscillations proves that inter-

actions of the Standard Model (SM) fermions are non-diagonal in ﬂavor. Cross-generational

mixing in the quark and neutrino sectors is large,

|Vus|∼

0.2 [

] and

sin2θ23∼

0.6 [

]. In striking

contrast, no indication of ﬂavor mixing has been observed in the charged lepton sector. In the

SM with massive neutrinos, charged lepton ﬂavor is only approximately conserved. Virtual

loops with mixing neutrinos result in charged lepton ﬂavor violating (CLFV) transitions, re-

gardless of whether neutrinos are Dirac or Majorana particles [

]. The branching fractions of

the corresponding processes are suppressed by factors proportional to

(∆m2

ν)2/M4

to a level

below 10

−50

[

], signiﬁcantly lower than the sensitivity of any current or planned experiment.

Experimental observation of any CLFV process would therefore imply the presence of physics

beyond the SM. Many extensions of the SM predict much higher rates of CLFV processes [

falling within the reach of the new generation of CLFV experiments coming online within the

next few years [

–

]. The process of coherent neutrinoless muon to electron conversion in a

nuclear ﬁeld,

µ−A→e−A

, probes a wide spectrum of new physics models (see Ref. [

] for

general calculations). The present experimental limit on the rate of this process

Rµe=Γ(µ−+N(A,Z)→e−+N(A,Z))

Γ(µ−+N(A,Z)→νµ+N(A,Z−1)) <7×10−13 (90% CL)

has been set by the SINDRUM II experiment on a gold target [13].

The Mu2e experiment at Fermilab [

] will search for

µ−A→e−A

on an aluminum target

with an improved sensitivity of about four orders of magnitude below the SINDRUM II limit.

The current Mu2e run plan assumes two data-taking periods, Run I and Run II, separated by

an approximately two-year-long shutdown. Run I is anticipated to start in 2025 and collect

about 10% of the total expected muon ﬂux, improving the search sensitivity by three orders of

magnitude. Run II will further enhance the search sensitivity by another order of magnitude.

This article details estimates of the expected backgrounds and the sensitivity projections

for Mu2e Run I. The material is organized as follows. Section 2describes the Mu2e experiment

and the run plan. Section 3presents an overview of the event simulation framework. Sections 4,

5, and 6contain discussion of the event reconstruction, trigger simulation, and event selection,

respectively. Section 7describes the background processes, details of their simulation, and

gives the estimated contributions from each background source. Section 8presents the

sensitivity optimization procedure and discussion of the results.

2. Mu2e Experiment

2.1. Muon Beamline

The Mu2e experiment is based upon a concept proposed in Ref. [

]. A schematic view of

the experiment is shown in Figure 1. Formation of the Mu2e muon beam proceeds as follows.

A primary proton beam with

Ekin

= 8 GeV is extracted from the Fermilab Delivery Ring using

the slow resonant extraction technique [

]. The beam has a pulsed timing structure, with 250

ns-wide proton pulses separated by 1695 ns. During each 1.4 s main injector cycle, the proton

pulses are delivered continuously for about 0.4 seconds, then the beam is off for the remainder

of the cycle. On a millisecond time scale, slow resonant extraction results in signiﬁcant proton

pulse intensity variations [

]. The spill duty factor

SDF =

+σ2

I/I2

, where

σ2

is the

variance of the pulse intensity distribution and

is the mean pulse intensity, is expected to be

above 60%.

The beam interacts with the

∼

1.6 interaction lengths-long tungsten production target

positioned in the center of the superconducting production solenoid (PS). The PS graded

magnetic ﬁeld reaches its maximal strength of 4.6 T downstream of the production target.

Most of the particles produced in

interactions are pions. Particles produced backwards

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as well as reﬂected in the PS magnetic mirror travel through the S-shaped superconducting

transport solenoid (TS) towards the superconducting detector solenoid (DS). Muons are

mainly produced in π−→µ−νdecays, which occur in both the PS and TS. The TS magnetic

ﬁeld is also graded, from

∼

2.5 T at the entrance to about 2.1 T in the region where particles exit

the TS and enter the DS. Collimators at the entrance, center, and exit of the TS (COL1, COL3,

and COL5) deﬁne the TS momentum acceptance, greatly reducing the transport efﬁciency for

particles with momenta above

∼

100

MeV/c

. The curved magnetic ﬁeld of the TS causes the

charged particles of opposite signs to drift vertically in opposite directions – see, for example,

Ref. [

]. The vertical separation reaches its maximum in the center of the TS. A vertically

offset opening of the rotatable COL3 collimator selects the beam sign, passing through either

negative or positive particles. The DS magnetic ﬁeld has two regions – an upstream region

with a graded magnetic ﬁeld and a downstream region with a uniform ﬁeld of 1 T.

COL1 COL3

COL5

Figure 1.

Schematic view of the Mu2e apparatus. The center of the Mu2e reference frame is located in

the COL3 collimator center, its

-axis points upwards, the

-axis is parallel to the DS axis and points

downstream, and the

-axis completes the right-handed reference frame. The particle detectors, the

tracker and the calorimeter, are located in the downstream part of the DS, in a uniform magnetic ﬁeld of

1 T.

The inner volumes of all three solenoids are kept at near vacuum. Exposed to the intense

proton beam, the radiatively cooled production target will operate at temperatures above

1000

C. Maintaining a low tungsten oxidation rate requires the pressure in the PS region

to be kept at

∼

−5

torr. To optimize the transport efﬁciency, suppress backgrounds from

secondary interactions, and improve the momentum reconstruction accuracy, the pumping

system for the DS region is designed to achieve 10

−4

torr. A thin window in the TS center

separates the two vacuum regions.

The stopping target is positioned in the graded B-ﬁeld region of the DS. The average

momentum of the muons entering the DS is

∼

MeV/c

, and about 1/3 of them stop in the

stopping target made of 37 Al annular foils spaced 2.2 cm apart. Each foil is 105

m thick

and has an inner and an outer radii of 2.2 cm and 7.5 cm respectively. The foils are arranged

co-axially along the DS axis.

Muons reaching the stopping target and stopping there come from decays of pions

with an average momentum

p∼

100

MeV/c

. The average number of stopped muons per

primary proton, that is the stopped muon rate, determined from the muon beam simulations is

Nµ−

POT =

1.6

−3

. This number highly depends on the pion production cross section for the

protons interacting on the tungsten target. Published measurements of the low-momentum

pion production [

] are not consistent with each other, so the simulation-based estimate

Nµ−

POT

has a large uncertainty. The impact of this uncertainty on the expected sensitivity is

discussed in Section 8.4.

In addition to charged pions, interactions of the proton beam with the production target

also produce a large number of

π0

’s. Photons from

π0→γγ

decays converting in the target

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1of38ArticleMu2eRunISensitivityProjectionsfortheNeutrinolessm!eConversionSearchinAluminum.2of38F.Abdi26,R.Abrams28,J.Adentunji11,W.Ahmed15,R.Alber11,D.Alexander15,D.Allen11,D.Allspach11,C.Alvarez-Garcia23,D.Ambrose26,G.Ambrosio11,A.Amirkhanov31,N.Andreev11,C.M.Ankenbrandt28,R.Appleby23,D.Arnold11,A....

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