Search for m e conversion on gold

1
Search for m? e conversion on gold

• Collaboration
• SINDRUM II
• RWTH Aachen (1)
• PSI (Paul Scherrer Institut) (2)
• University Zurich (3)

• Authors
• W. Bertl (2)
• R. Engfer (3)
• E.A. Hermes (3)
• T. Kozlowski (23)
• G. Kurz (3)
• J. Kuth (1)
• G. Otter (1)
• F. Rosenbaum (23)
• N.M. Ryskulov (2)
• A. van der Schaaf (3)
• P. Wintz (3)
• I. Zychor (3)

2
Why searching for m? e conversion?

• m? e (neutrinoless) conversion belongs to the
  most promising processes to study Lepton Flavor
  Violation
• (together with m? eg, m? eee, KL0 ? me, Z0 ?
  me, n - oscillations)
• LFV - not foreseen in Standard Model, but
  appears naturally in models extending it
• - observed recently by finding evidence for n -
  oscillations
• Predictions for LFV effects
• model dependent (dynamic suppression of LFV,
  therefore even zero results are important)
• most promising for m? e (and m? eg ) if
  supersymmetric grand unification is the truth
  (see Barbieri,Hall)
• Advantages of m? e process
• coherent action of all quarks in a nucleus (?10
  of conversion strength leads to excited states
  which are experimentally not accessible)
• single particle final state (monoenergetic) ? no
  random coinc. ? high beam intensities possible

3
Why gold as target material ?

• Linear rise of conversion probability for heavy
  nuclei predicted ( see ? )
• Counterpart to medium heavy elements
• (different (Z-N)/A) ? isoscalar/isovector
  coupling)
• Next experiment (MECO_at_BNL) wont be able to
  measure high Z targets
• Lightweight construction ( background reduction,
  see below)
• Single isotope (good for ?? e search)

Nuclear dependence of m ? e conversion
Our gold target with only 40 mm wall thickness
Kosmas NuFact 01, Tsukuba Kosmas, Lagaris in
preparation
4
SINDRUM II Spectrometer
Schematic event display
rf
rz
PMC
tracks left by a ?100 MeV e-
5
Background a) muon induced

• m - decay in orbit (MIO)

• Radiative Muon Capture (RMC)

Theoretical spectrum
(R. Watanabe et al. Atom.Data and Nucl. Data
Tab. 54 (1993), 165)
Kinematic endpoint of photon spectrum only 0.7
MeV below Eme , but strongly suppressed at high
energies. Check contribution by investigating
positron spectrum ! ( see ? )
)
dN/dE (MeV-1)
V
e
M
/
1
(

E
d
/
N
d
? Only small e- contribution expected. BUT Most
severe background for m -? e search.
6
Background b) pion induced
Radiative Pion Capture (RPC)
Kinematic endpoint of photon spectrum around 130
MeV ! Branching ratio of order 2. No way to
distinguish an asymmetric e e- -pair (with
little e energy and e- energy at 95 MeV) from
me ! ? Needs strong pion suppression only ? 1
pion every 5 minutes is allowed to reach gold
target!
E ? 75 MeV
BUT Degrader is now pion stop target ? ee-
pairs from RPC are collected by BPMC and
transported towards the gold target where they
may scatter into spectrometer acceptance (typ.
forward scattering)
dz(cm)
ENTRIES
989
25
dz(cm)
in phase
0
? use solid angle and cyclotron phase correlation
to cut.
-25
dz (cm)
-50
70
80
90
100
E (MeV)
7
Background c) cosmic induced
Cosmic radiation produce e- which may fake a me
event. Most of them can be identified by hits not
belonging to main track. ? However, ultimate
limit given by photon conversion in target ?
small target mass ? shielding
8
Determine m stops a) Principle

• Muonic X-ray detection from m -Au capture
• Method
• using a Germanium diode to monitor yield of
  4f5/2?3d3/2 transition (899 keV)
• or (respectively)
• using a large NaI crystal detector to monitor
  yield of 2p?1s transitions (5765 and 5595 keV)
• Well known X-ray -intensities (F.J.Hartmann et
  al. TU München) and -energies (B.Robert-Tissot et
  al. Univ. Fribourg)
• Calibration
• Acceptance for Ge-diode measured with calibrated
  sources
• 137Cs (662 keV), 60Co (1117,1333 keV)
• at 2 different z positions
• Reproduced by simulation within 3
• Simulation used to determine acceptance for gold
  transition (899keV) measured stop distribution
• AGe (1.02 0.05stat 0.07 sys) ? 10-6
• Acceptance for NaI (5765,5595 keV) from
  (simultaneous ) Ge measurement
• ANaI (1.01 0.06stat 0.08 sys) ? 10-5

SINDRUM II spectrometer X-ray detector (Ge)
(NaI detector not shown)
9
Determine m stops b) Results
Example of a raw m -Au X-ray spectrum taken
during a 3 hour run
?
X-ray yield normalized to integrated proton beam
2
1.8
1.6
Stability of X-ray yield during 340 hours of data
taking (18 of total) (Total 52 of me
data-taking time accompanied by X-ray monitoring)
1.4
1.2
X-ray counts / mAs
?
s
A
s
1
m
A
/
m
y
/
a
y
r
a
r
X-ray / mAs
0.8
0.6
0.4
0.2
0
14800
14900
15000
15100
15200
15300
15400
15500
run number
10
me acceptance, cuts efficiencies
Total efficiency to accept me events
0.358 solid angle acceptance 0.95 trigger
efficiency 0.79 z reconstruction
efficiency 0.82 energy independent selection
cuts 0.85 loss due to cosmic cut 0.65 energy
dependent selection cuts 0.849 threshold cut on
me peak
(MC) (MC) (exp) (exp) (exp) (MC) (MC)
Preliminary !
11
Final e - Spectrum
SINDRUM II
m
e conversion on gold
run2000
4
10
Ordinary muon decay spectrum (taken with mirrored
B-fields) to check absolute energy calibration
and spectrometer resolution

m
beam
3
?
10
-


m
nn

e
3
10
0
50
52
54

e
momentum (MeV/c)
Events per 0.25 MeV/c
events per 0.25 MeV/c
events per 0.25 MeV/c
-
p
events per 0.25 MeV/c
beam
Data taken at higher beam momentum to demonstrate
effect of RPC in target (scaled).
2
10
?
-
m
beam
10
-
-
-
m
nn
Au
e
Au
Measured e- spectrum together with MIO and me
Monte-Carlo spectrum
?
1
-
-
m
Au
e
Au
-12
3/2001

at 5
10
3/2001
75
80
85
90
95
100
105
95
100
105

12
Result
The probability for neutrinoless m? e conversion
is usually expressed as branching ratio relative
to ordinary nuclear muon capture
of muons stopped in Au target
nuclear capture probability
spectrometer acceptance for m? e events
single event sensitivity
No candidate events have been observed, therefore
Bme becomes an upper limit with...
90 confidence-level upper limit on Poisson mean
value, incorporating systematic error
(CousinsHighland, NIM A320 (1992) 331). s is
relative error of sensitivity
Preliminary !