Research Proposal for an Experiment to Search for the Decay μ -> eee

10.6 MB·English

by  A. Blondel

Research Proposal for an Experiment to
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1 Search for the Decay µ → eee
0
2

n
a
J

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A. Blondel, A. Bravar, M. Pohl

] Département de physique nucléaire et corpusculaire,
et Université de Genève, Genève
d
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s S. Bachmann, N. Berger, M. Kiehn, A. Schöning, D. Wiedner, B. Windelband
n
i Physikalisches Institut, Universität Heidelberg, Heidelberg
.
s
c
i P. Eckert, H.-C. Schultz-Coulon, W. Shen
s
y Kirchoff Institut für Physik, Universität Heidelberg, Heidelberg
h
p
[ P. Fischer, I. Perić

1 Zentralinstitut für Informatik, Universität Heidelberg, Mannheim
v
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1 M. Hildebrandt, P.-R. Kettle, A. Papa, S. Ritt, A. Stoykov
1 Paul Scherrer Institut, Villigen
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.
1
G. Dissertori, C. Grab, R. Wallny
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3 Eidgenössiche Technische Hochschule Zürich, Zürich
1
:
v
R. Gredig, P. Robmann, U. Straumann
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Universität Zürich, Zürich
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a
December 10th, 2012
Contents
Executive Summary 4 7 Muon Beam 30
7.1 General Beam Requirements. . . . 30
7.2 Beam for phase I running . . . . . 30
I Introduction 5
7.3 High intensity muon beamline for
phase II running . . . . . . . . . . 32
1 Motivation 6
2 Theory 8 8 Magnet 37
2.1 Comparison µ→eee versus µ→eγ 9
2.2 Discussion of Specific Models . . . 10
9 Stopping Target 39
2.3 Theory Summary . . . . . . . . . . 12
9.1 Baseline Aluminium Design . . . . 39
3 Experimental Situation 14 9.2 Vertex distribution . . . . . . . . . 40
3.1 SINDRUM Experiment . . . . . . 14 9.3 Alternative Designs . . . . . . . . . 40
3.2 MEG Experiment. . . . . . . . . . 14
3.3 Muon Conversion Experiments . . 15
10 The Mu3e Pixel Detector 43
3.4 LFV in τ Decays . . . . . . . . . . 15
10.1 HV-Maps Sensor . . . . . . . . . . 43
3.5 LFV at the Large Hadron Collider 16
10.2 Sensor specification . . . . . . . . . 44
4 The Decay µ→eee 18 10.3 Path towards the Full Sensor . . . 44
4.1 Kinematics . . . . . . . . . . . . . 18 10.4 Characterization of the Prototypes 46
4.2 Detector Acceptance . . . . . . . . 18
10.5 Mechanics . . . . . . . . . . . . . . 49
4.3 Backgrounds . . . . . . . . . . . . 18
10.6 Cooling . . . . . . . . . . . . . . . 51
10.7 Alternative Technologies . . . . . . 51
II The Mu3e Experiment 21
11 The Mu3e Fibre Detector 55
5 Requirements for Mu3e 22
11.1 The time of flight detector . . . . . 55
5.1 Goals of the Experiment . . . . . . 22
11.2 Readout of photon detectors. . . . 57
5.2 Challenges for the Experiment . . 22
11.3 GEANT simulations . . . . . . . . 58
6 Experimental Concept 24
6.1 Momentum Measurement with Re-
12 The Mu3e Tile Detector 59
curlers . . . . . . . . . . . . . . . . 25
12.1 Detector Design . . . . . . . . . . . 59
6.2 Baseline Design . . . . . . . . . . . 25
12.2 Simulation . . . . . . . . . . . . . . 60
6.3 Building up the Experiment . . . . 26
6.4 The Phase I Experiment . . . . . . 27 12.3 Time Resolution Measurements . . 61
6.5 The Phase II Experiment . . . . . 29 12.4 Detector Prototype . . . . . . . . . 62
2
13 Data Acquisition 63 17 Sensitivity Study 84
13.1 Overview . . . . . . . . . . . . . . 63 17.1 Simulation and Reconstruction
13.2 Occupancy . . . . . . . . . . . . . 63 Software . . . . . . . . . . . . . . . 84
13.3 Front-end . . . . . . . . . . . . . . 63 17.2 Signal Acceptance . . . . . . . . . 85
13.4 Read-out links . . . . . . . . . . . 68 17.3 Selection . . . . . . . . . . . . . . . 85
13.5 Read-out cards . . . . . . . . . . . 69 17.4 Results. . . . . . . . . . . . . . . . 89
13.6 Event filter interface . . . . . . . . 69
13.7 Data collection . . . . . . . . . . . 69
13.8 Slow control . . . . . . . . . . . . . 69 III The Mu3e Collaboration 91
14 Online Event Selection 71 18 The Institutes in Mu3e 92
14.1 Selection Algorithms . . . . . . . . 71 18.1 Responsibilities . . . . . . . . . . . 92
14.2 Hardware Implementation . . . . . 71 18.2 Collaborators . . . . . . . . . . . . 92
15 Simulation 74 19 Schedule 94
15.1 Detector geometry . . . . . . . . . 74 19.1 Phase I Schedule . . . . . . . . . . 94
15.2 Magnetic field . . . . . . . . . . . . 76 19.2 Phase II Schedule . . . . . . . . . . 94
15.3 Physics Processes . . . . . . . . . . 76
15.4 Time structure . . . . . . . . . . . 78 20 Cost Estimates 95
16 Reconstruction 79 A Appendix 96
16.1 Track Reconstruction in the Pixel A.1 Mu3e theses . . . . . . . . . . . . . 96
Tracker . . . . . . . . . . . . . . . 79 A.2 Acknowledgements . . . . . . . . . 96
16.2 Track Fitting and Linking . . . . . 79
16.3 Vertex Fitting . . . . . . . . . . . . 82 Bibliography 96
Executive Summary
Weproposeanexperiment(Mu3e)tosearchfor ive Pixel Sensors (HV-MAPS). This technology
the lepton flavour violating (LFV) decay µ+ → provides high granularity, important for precision
e+e−e+. Weaimforanultimatesensitivityofone tracking and vertexing, and allows one to signi-
in 1016 µ-decays, four orders of magnitude better ficantly reduce the material budget by thinning
than previous searches. This sensitivity is made down the sensors and by integrating the hit digit-
possiblebyexploitingmodernsiliconpixeldetect- isation and readout circuitry in the sensor itself.
ors providing high spatial resolution and hodo- The detector geometry is optimized to reach the
scopesusingscintillatingfibresandtilesproviding highest possible momentum resolution in a mul-
precise timing information at high particle rates. tiple Coulomb scattering environment, which is
Existing beamlines available at PSI providing needed to suppress the dominating background
rates of order 108 muons per second allow to test from the radiative muon decay with internal con-
for the decay µ+ → e+e−e+ in one of 1015 muon version, µ → eeeνν¯. The time information of the
decays. In a first phase of the experiment, we decayelectrons1,obtainedfromthepixeldetector
plan to make use of this and establish the experi- isfurtherimprovedbyatime-of-flightsystemcon-
mentaltechniquewhilstatthesametimepushing sisting of a scintillating fiber hodoscope and tiles
the sensitivity by three orders of magnitude. with Silicon Photo-Multipliers (SiPM) for light
Theinstallationofanewmuonbeamlineatthe detection. By combining both detector systems
spallation neutron source is currently under dis- accidental background can be reduced below the
cussionatPSI.SuchaHigh Intensity Muon Beam aimed sensitivity of B(µ+ →e+e−e+)∼10−16.
(HiMB) will provide intensities in excess of 109 We will complete the sensor development and
muons per second, which in turn are required to start constructing the detector in 2013, in order
reachtheaimedsensitivityofB(µ+ →e+e−e+)∼ tobereadyforfirstexploratorydatatakingatan
10−16. existing beam line with a first minimal detector
Theproposedexperimentishighlycomplement- setup in 2015. A detector capable of taking data
ary to other LFV searches for physics beyond the rates of order 108 muons per second and capable
standard model, i.e. direct searches performed at of reaching a sensitivity of B(µ+ → e+e−e+) ∼
the Large Hadron Collider (LHC) and indirected 10−15 will be available in 2016. This Phase I de-
searches in the decay of taus and muons, such tector is the main focus of this proposal.
as the decay µ+ → e+γ, which is the subject In Phase II, beyond 2017, the experiment will
of the MEG experiment currently in operation reach the ultimate sensitivity by exploiting a pos-
at PSI. The proposed experiment for the search sible new high intensity muon beamline with an
µ+ → e+e−e+ will test lepton flavour violating intensity of > 2·109 muons per second. In the
models of physics beyond the Standard Model absence of a signal, LFV muon decays can then
with unprecedented sensitivity. beexcludedforB(µ+ →e+e−e+)<10−16 at90%
This sensitivity is experimentally achieved by a confidence level.
novel experimental design exploiting silicon pixel
1Hereandinthefollowing,theterm“electron”denotes
detectors based on High Voltage Monolithic Act-
genericallybothdecayelectronsandpositrons.
4
Part I
Introduction
5
Chapter 1
Motivation
In the Standard Model (SM) of elementary In many extensions of the SM, such as grand
particle physics, the number of leptons of each unified models [4–6], supersymmetric models [7]
family (lepton flavour) is conserved at tree level. (see Figure 2.2), left-right symmetric models [8–
In the neutrino sector, lepton flavour violation 10], models with an extended Higgs sector [11]
(LFV) has however been observed in the form andmodelswhereelectroweaksymmetryisbroken
of neutrino mixing by the Super-Kamiokande [1], dynamically [12], an experimentally accessible
SNO [2], KamLAND [3] and subsequent experi- amount of LFV is predicted in a large region of
ments. Consequently, lepton flavour is a broken the parameter space.
symmetry, the standard model has to be adap-
Seesaw and Left-Right symmetric (supersym-
ted to incorporate massive neutrinos and lepton
metric) models are good candidates for realising
flavour violation is also expected in the charged
grand unification, which also unify quark and
lepton sector. The exact mechanism and size of
lepton mass matrices. Moreover, it has been
LFVbeingunknown, itsstudyisoflargeinterest,
shown that LFV effects in the low energy limit
as it is linked to neutrino mass generation, CP
can be related to mixing parameters at the GUT
violationandnewphysicsbeyondtheSM(BSM).
scale or to heavy Majorana masses in these mod-
The non-observation of LFV of charged leptons els [17,18]. Seesaw models are therefore very at-
in past and present experiments might at a first tractiveinthecontextofLFVastheyarealsoable
glance be surprising, as the mixing angles in the tonaturallyexplainthesmallnessofthemassesof
neutrino matrix have been measured to be large the left handed neutrinos. In this context the re-
(maximal). This huge suppression of LFV effects cent results from neutrino oscillation experiments
is however accidental and due to the fact that (a) areveryinteresting,astheymeasuredalargemix-
neutrinosaresomuchlighterthanchargedleptons ing angle θ , which enhances the LFV-muon de-
13
and (b) the mass differences between neutrinos caysinmostmodelswhichtrytoexplainthesmall
(morepreciselythesquareofthemassdifferences) neutrino masses and the large mixing.
are very small compared to the W-boson mass.
Currently the most accurate measurement is
The situation completely changes if new provided by the Daya Bay reactor neutrino ex-
particles beyond the SM are introduced. If e.g. periment [19] yielding sin2(2θ ) = 0.089 ±
13
SUSY is realized at the electroweak scale, the 0.010(stat) ± 0.005(syst), excluding the no-
scalar partners of the charged leptons (sleptons) oscillationhypothesisat7.7standarddeviantions.
will have large masses, and if not fully degener- TheDayaBaymeasurementisingoodagreement
ate, induce LFV interactions via loop corrections. with measurements by the RENO [20], Double
These LFV effects from new particles at the TeV Chooz [21] and T2K [22] experiments. These
scale are naturally generated in many models and results are very encouraging, as large values of
are therefore considered to be a prominent signa- sin2(2θ )leadtolargeLFVeffectsinmanyBSM
13
ture for new physics. models.
6
An Experiment to Search for the Decay µ→eee
Decay channel Experiment Branching ratio limit Reference
µ→eγ MEGA <1.2·10−11 [13]
MEG <2.4·10−12 [14]
µ →eee SINDRUM <1.0·10−12 [15]
µAu→eAu SINDRUM II <7·10−13 [16]
Table 1.1: Experimental limits on LFV muon decays
The observation of LFV in the charged lepton gradetheexperimenttofurtherimprovethesens-
sector would be a sign for new physics, possibly itivity are currently under discussion.
at scales far beyond the reach of direct observa- In the near future the DeeMe experiment
tion at the large hadron collider (LHC). Several [44] at J-PARC plans to improve the current
experimentshavebeenperformedorareinopera- muon-to-electron conversion limit of B(µ Au →
tion to search for LFV in the decays of muons or eAu)<7·10−13[16]byalmosttwoordersofmag-
taus. Most prominent are the search for the radi- nitude. By the end of the decade this limit could
ativemuondecayµ→eγ [13,14,23,24],thedecay be improved by even four orders of magnitude
µ→eee[15],theconversionofcapturedmuonsto by COMET at J-PARC [45] and Mu2e at Fer-
electrons [16] and LFV tau decays [25–43]. milab [46,47].
The recent search performed by the MEG- Selectedlimitsforleptonflavourviolatingmuon
Collaboration yields B(µ → eγ)<2.4·10−12 [14] decays and muon-to-electron conversion experi-
and sets currently the most stringent limit on ments, which are of high relevance for the pro-
manyLFVmodels. TheMEG collaborationplans posed experiment, are shown in Table 1.1. A
to continue operation into 2013 in order to in- search for the LFV decay µ→eee with an unpre-
crease the number of stopped muons and to reach cedented sensitivity of <10−16 as proposed here
a sensitivity of a few times 10−13. Plans to up- would provide a unique opportunity for discover-
iesof physicsbeyond theSM inthe coming years.
7
Chapter 2
Theory
IntheSM,chargedleptonflavourviolatingreac-
+
e
tionsareforbiddenattreelevelandcanonlybein-
ducedbyleptonmixingthroughhigherorderloop
γ */Z
diagrams. However, the dominant neutrino mix-
ing loop diagram, see Figure 2.1, is strongly sup- ~ e-
pressed in the SM with B (cid:28)10−50 and thus giv- µ ~
e
ing potentially high sensitivity to LFV reactions
in models beyond the Standard Model (BSM).
+ ~0 +
Such an example is shown in Figure 2.2, where µ χ e
a γ/Z-penguin diagram is shown with new su-
Figure 2.2: Diagram for lepton flavour violation
persymmetric(SUSY)particlesrunninginaloop.
involving supersymmetric particles.
These loop contributions are important basically
for all models, where new particle couplings to
electronsandmuonsareintroduced. Leptonflavor
violation can also be mediated by tree couplings alsopredictsemihadronicdecaysoftauleptonsor
as shown in Figure 2.3. These couplings could the muon conversion process µq → eq0, which is
be mediated by new particles, like Higgs particles experimentallybesttestedinmuoncaptureexper-
or doubly charged Higgs particles, R-parity viol- iments.
ating scalar neutrinos or new heavy vector bo-
sons, the latter being motivated by models with
extra dimensions [48,49]. These models usually
e e
+
e
X
γ*
+
W -
e
µ
e
µ+ νµ νe e+
Figure2.1: Feynmandiagramfortheµ→eeepro- Figure 2.3: Diagram for lepton flavour violation
cess via neutrino mixing (indicated by the cross). at tree level.
8
An Experiment to Search for the Decay µ→eee
The lepton flavor violating three electron decay diagram. The constants η and η0 are T-violating
of the muon can be mediated, depending on the mixing parameters. In case of a signal, the dif-
model, via virtual loop (Figure 2.2) and box dia- ferent terms can be measured from the angular
gramsorviatreediagrams(Figure2.3). Themost distribution of µ → eee decay particles using a
generalLagrangianforthisdecaycanbeparamet- polarized muon beam.
erized as [50] 1:
4G 2.1 Comparison µ → eee versus
L = F [m A µ σµνe F
µ→eee 2 µ R R L µν
µ → eγ
+m A µ σµνe F
µ L L R µν
+ g1 (µReL)(eReL) In the decay µ → eγ physics beyond the SM is
+g (µ e )(e e ) only tested by photon penguin diagrams, in con-
2 L R L R
+ g (µ γµe )(e γ e ) trast to µ → eee where also tree, Z-penguin and
3 R R R µ R box diagrams contribute. To compare the new
+g (µ γµe )(e γ e )
4 L L L µ L physics mass scale reach between the processes
+ g5 (µRγµeR)(eLγµeL) µ→eee and µ→eγ asimplifiedmodelis chosen;
+g (µ γµe )(e γ e ) + H.c.] it is assumed that the photon penguin diagram
6 L L R µ R
(2.1) Figure 2.2 and the tree diagram Figure 2.3 are
the only relevant contributions. The Lagrangian
The form factors A describe tensor type (di- then simplifies to2:
R,L
pole) couplings, mostly acquiring contributions
(cid:20) (cid:21)
from the photon penguin diagram, whereas the m
L = µ µ σµνe F
scalar-type (g ) and vector-type (g −g ) form LFV (κ+1)Λ2 R L µν
1,2 3 6 γ−penguin
factors can be regarded as four fermion contact (cid:20) (cid:21)
κ
interactions, to which the tree diagram contrib- + (µ γµe )(e γ e )
(κ+1)Λ2 L L L µ L
utes in leading order. In addition also off shell tree
(2.3)
form factors from the penguin diagrams, which
are not testable in the µ → eγ decay contribute whereforthecontactinteraction(“tree”)termex-
tog1−g6 [51]. Incaseofnon-zerodipoleandfour- emplarily a left-left vector coupling is chosen. In
fermion couplings also interference effects have to
this definition a common mass scale Λ is intro-
be considered, which can be exploited to investig-
duced and the parameter κ describes the ratio of
ate violation of time reversal (T-invariance). theamplitudesofthevector-type(tree)termover
By neglecting higher order terms in me, the the tensor (γ−penguin) term.
totalbranchingratioofthedecaycanbeexpressed
Limits on the common mass scale Λ as ob-
by:
tained from the experimental bounds on B(µ →
eγ)<2.4·10−12 (90%CLMEG 2011)andB(µ→
g2+g2
B(µ→eee)= 1 2 +2(g2+g2) + g2+g2 eee)<1.0·10−12(90%CLSINDRUM)areshown
8 3 4 5 6
in Figure 2.4 as function of the parameter κ. Ex-
m2
+32eA2 (ln µ −11/4) perimentally, for small values of κ (dipole coup-
m2 ling) the mass scale Λ is best constrained by the
e
q MEG experiment whereas the four fermion con-
+16η eA g2+g2
3 4 tact interaction region with κ (cid:38) 10 is best con-
+8η0 eAqg2+g2 , strained by the SINDRUM experiment.
5 5 For comparison also a hypothetical ten times
(2.2)
improved limit is shown for the MEG experiment
(post-upgrade) and compared to the sensitivities
where the definition A2 = A2 +A2 is used. The
L R of the proposed µ → eee experiment of 10−15
term proportional to A2 is logarithmically en-
(phase I) and 10−16 (phase II). It can be seen
hancedandcanbeassignedtothephotonpenguin
that in this simple model comparison high mass
1A representation of Lagrangian containing explicitly scales Λ will be best constrained by the proposed
the contributions from the loop and box diagrams can be
foundin[51]. 2Asimilarstudywaspresentedin[52]
9
An Experiment to Search for the Decay µ→eee
Figure2.4: Experimentallimitsandprojectedlim- Figure2.5: Experimentallimitsandprojectedlim-
its on the LFV mass scale Λ as a function of the its on the LFV mass scale Λ as a function of the
parameterκ(seeequation2.3)assumingnegligible parameter κ (see equation 2.3) assuming contri-
contributions from Z0 penguins; based on [52]. butions from Z0 penguins ten times larger than
the photon contribution.
µ → eee experiment for all values of κ already in
phase I. going to high mass scales was discussed for Little
In case of dominating tensor couplings (A 6= Higgs models [53,54] as well as for several SUSY
0, κ → 0) a quasi model independent relation models [55–59]. SUSY models with R-parity viol-
between the µ → eee decay rate and the µ → eγ ationandrighthandedneutrinosreceivedrecently
decay rate can be derived: quitesomeattentioninthiscontext,asapproxim-
B(µ→eee) ate cancellations of different Z-diagram contribu-
≈0.006 (2.4) tionsarenotpresentinextendedMinimumSuper-
B(µ→eγ)
Symmetric Standard Models (MSSM).
This ratio applies for many supersymmetric mod- TheeffectofsuchanenhancedZ-penguincoup-
els, where LFV effects are predominantly medi- ling, where the LFV contribution to the µ → eee
ated by gauge bosons and where the masses of process is exemplarily enhanced by a factor of
the scalar leptons or gauginos are of electroweak ten relative to the photon-penguin contribution,
scale. In these models, which are already heavily is shown in Figure 2.5. It can be seen that the
constrained or even excluded by the recent LHC sensitivity of the µ → eee process to new physics
results, the sensitivity of the proposed Mu3e ex- is significantly enhanced at small values of κ and
perimentintermsofbranchingratiohastobetwo thatinsuchacaseasensitivityof10−14 isalready
ordersofmagnitudehigherthanthatoftheMEG sufficient to be competitive with the µ→eγ pro-
experiment in order to be competitive. cess with a sensitivity of a few times 10−13.
2.1.1 Z-penguin Contribution
2.2 Discussion of Specific Models
However, besides the tree and γ-penguin dia-
grams also the Z-penguin diagram can signific- In the following, selected models are discussed in
antly contribute to the process µ → eee. The more detail in the context of the proposed exper-
Z-penguin diagram is of particular importance if iment.
the new physics scale is higher than the electro-
magnetic scale, as can be easily derived from a
2.2.1 Inverse Seesaw SUSY Model
dimensional analysis. The enhancement of the
Z-penguin contribution over the γ-penguin con- Despite the fact that the most simple supersym-
tribution and its non-decoupling behaviour when metricmodelswithlightsquarksandgluinoswere
10