HIMB Muon Conversion with high-Z target at - Andreas Knecht Paul Scherrer Institute 8. 4. 2021 HIMB Physics Case Workshop Zoom - PSI Indico
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Muon Conversion with high-Z target at
HIMB
Andreas Knecht
Paul Scherrer Institute
8. 4. 2021
HIMB Physics Case Workshop
ZoomThe predictions may, however, deviate byawayfactors
from the2–3 at ons and muons at given
detectors. grader which is used to remove pions from the beam (see
momentum. Figure 1 shows the
Sect. 2.3). Because of the large momentum band behind
higher Z values. probability of 52 MeV/c pionsandand
the degrader muons
the many turns to crossmake
the muons a CH
in the2
SINDRUM II experiment
As a result of the two-body final state3.1the
Muonelectrons
beam
moderator of given thickness. transport solenoid no beam focus exists inside the spec-
trometer. StillAs beamcan be seen
particles fromreturn
periodically thetofig-
the
produced in µ − e conversion are mono-energetic and their
The 590 MeV proton beam ure has a the mean range differs by about a factor two and range
time structure of 0.3 ns solenoid axis, i.e. every ≈ 65 cm for a typical momentum
energy is given by: wide bursts every 19.75 ns. The πE5 secondary beam line after the degrader of 35 MeV/c and for this reason the gold
straggling can be neglected so perfect separation should be
extracts particles emitted in backward direction from the target was made in the form of a 65 cm long tube.
expected with an 8 mm thick CH2 degrader. In practice the
Eµe = mµ c2 − Bµ (Z) − R(A) , (1)
distributions are broadened by the finite momentum band
Measurementwhere
of muon
Bµ (Z) conversion
is the atomic binding on gold energy of the muon and transmitted by the beam line. Settings of the beam mag-
R(A) is the atomic recoil energy for a muonic atom with nets and slits have to be carefully adjusted to minimize the
Removal of pions
atomicfrom
number beam mass number A. In first approxima- tails in the pion range distribution. At the cost of ≈ 30%
Z and through
degrader and tion Bµ (Z) ∝ Z 2 and
subsequent R(A) ∝ A−1 . For muonic gold Bµ = loss in muon intensity the pion stops in the target could
transport
10.08 MeV and R = 0.025 MeV give Eµe = 95.56 MeV. be suppressed to the required level. A simulation using the
solenoid measured range distribution shows that about one in 106
pions cross the moderator. Since these particles are rela-
Some residual2.2background
Muon inducedevents background from tively slow 99.9% of them decay before reaching the target
pions observed through timing (mostly which
Fig. 2. Plan view of the experiment. Theis
1 MWsituated some
590 MeV proton beam 10extracted
m further downstream.
from the PSI ring cyclotron hits the 40 mm car-
Muon344 decay in orbit (MIO) constitutes an intrinsic
bon production back-
target (top left of RPC
the figure). may
The πE5 produce
beam line background
transports secondary even
particles (π,when taking
µ, e) emitted place
in the backward
from degrader) The SINDRUM
ground source which can only be suppresseda)with
II Collaboration:
direction to a A
degrader
sufficient
shows the
search
situated for
at µ
the
in measured
momentum dispersion
− e conversion
entrance of
the moderator.
a in
transport muonic
solenoid
The
at the position of the
gold
connected axially
first slitresulting
to the SINDRUM
electrons
system. The momentum
II spectrometer.
havefrom≈the10%
was calculated
Inset
flight
time through the channel and the distributions show the increase when opening one side of the slit. Inset b) shows a cross section
electron energy resolution. The process predominantly of the beam observedre-at the position of the beam focus
2
sults in electrons with energy EMIO below mµ c /2, the
typically twice larger for positrons than for electrons [25].
kinematic endpoint in free muon decay, with a steeply Simulation shows that decay in flight is dominantly π− →
falling high-energy component reaching up to Eµe . By e− ν e shortly before the pion would have reached the mod-
using a magnetic spectrometer the vast majority of MIO erator. O(10) background events are expected with a flat
electrons can be kept away from the tracking detectors still energy distribution between 80 and 100 MeV. The decay
maintaining a ≈ 50% acceptance in the region of interest chain π− → µ− ν µ , µ− → e− νµ ν e has a similar yield but the
around 95 MeV. In the endpoint region the MIO rate varies energy distribution falls steeply and is for all energies neg-
as (Eµe − EMIO )5 and a resolution of 1–2 MeV (FWHM) ligible compared to MIO.
As is illustrated in Fig. 10 there is indeed a beam corre-
is sufficient to keep MIO background under control (see lated signal which is strongly peaked in the forward direc-
Sect. 6.4 below). Since the MIO endpoint rises at lower Z tion and which does contain more electrons than positrons.
great care has to be taken to avoid low-Z contaminations The observed time spread is ≈ 10 ns explained mainly by
in andFig. 10. Prompt
around beam induced background. Electron and the momentum spread in the beam.
the target.
positron events were selected with total momentum above
Another background source is due to radiative muon To cope with π − induced background two event classes
87 MeV/c but outside the main signal region 92.5–95.5 MeV/c.
capture
Panel(RMC) µ− (A, Z)
a: spectrometer → γ(A,
timing Z −to
relative 1)∗the
νµ 50.6
after MHz which
cy-
have been introduced based on the values of polar angle
the photon −
e+ edifferences and rf phase:
clotron creates
rf signal. an
Time pair either internally
with respect to both (Dalitz
the Fig. 1. Fraction of pions and muons with a momentum of
+ − ora|tfunction
pair) previous
or through γ →next
and the e erf bucket
pair production in the
are incremented. target.
Shown is – Class
52 MeV/c 1 contains
that cross eventsa CH with cos θ < 0.4as
2 moderator rf − 10 ns|of >the mod-
the difference
The RMC endpoint between
can be thekept
distributions
below Ewithµe forcosselected
θ > 0.4 and iso- erator 4.5 nsthickness.
which areGEANT practically [23]free of pion induced back-
simulation
cos θ < −0.4. Panel b: cos θ distribution. Shown is the distri- ground.
bution corresponding to the phase enhancement by the pion – Class 2 contains events with cos θ > 0.4 and |trf −
induced events. The arrows indicate the region populated by 10 ns| < 4.5 ns which are contaminated by pion induced
pion induced events. See the text for a discussion of the nature background.
of these events Eur. Phys. J. C 47, 337–346 (2006)
6.4 Single-event sensitiviy
Andreas Knecht cosmic background can be distinguished. Most common 2SINDRUM II experiment
340 The SINDRUM II Collaboration: A search for µ − e conversion in muonic gold
Fig. 3. The SINDRUM II spec-
trometer. Typical trajectories
of a beam muon and a hypo-
thetical conversion electron are
indicated
3.2 SINDRUM II spectrometer DC2 is made of low-density foam sandwiched between alu-
minized Kapton foils and has a density of only 35 mg/cm2 .
Figure 3 shows a vertical cross section through the SIN- To stabilize its position DC1 is kept at an overpressure of
DRUM II spectrometer in the configuration used for this 1 mbar relative to DC2. The aluminum on the inside of the
experiment. Beam is entering from the solenoid (A in wall is divided into 4.4 mm wide Eur. Phys. J. C 47, 337–346 (2006)
helical strips which al-
Fig. 3) on the left. The gold target (B) with a radius of lows 3-dimensional track reconstruction. There are separate
Andreas Knecht
≈ 20 mm and a wall thickness of 75 mg/cm2 was produced strips for the upstream and downstream halves of the de-3Why high-Z?
COMET TDR
Scaling of branching ratio of
muon conversion as a function of
Z differs depending on underlying
physics
-> allows to understand
underlying physics in the case of
a discovery
High-Z atoms not ideal for pulsed
beams due to short lifetimes
Andreas Knecht 4Muon conversion at HIMB
Muon rates:
SINDRUM II: ~ 2e6 μ-/s at 52 MeV/c (not written explicitly, but estimated from beam
power at that time and given muon stops on target)
346 The SINDRUM II Collaboration: A search for µ − e conversion in muonic gold
HIMB: ~ 1e8 μ-/s at 40 MeV/c (highest momentum with good transport efficiency)
capture probability:
Sensitivity: 11. SINDRUM Collaboration, U. Bellgardt et al
B 299, 1 (1988)
SINDRUM II: Bµe Au
< 7 × 10−13 90% C.L. (4) 12. W. Bertl et al., Prepared for International
Conference on High-Energy Physics (HEP
HIMB: Can
This increase
limit the sensitivity
is more stringent by aof factor
by two orders magnitude 100 if backgrounds
dapest, Hungary,can beJulreduced
12–18 2001.
accordingly
than the best previous limit on a heavy target [26]. It is the 13. A. van der Schaaf, J. Phys. G 29, 1503 (2003
final result of the research program on rare π and µ decays 14. T. Suzuki et al., Phys. Rev. C 35, 2212 (1987
with the SINDRUM I and II spectrometers at PSI. The 15. A. Czarnecki, W.J. Marciano, K. Melnikov
search for LFV in rare muon decays is continued at PSI by Proc. 549, 938 (2002)
the MEG collaboration [27] aiming at a sensitivity of 10−13 16. T.S. Kosmas, I.E. Lagaris, J. Phys. G 28, 290
17. R. Kitano, M. Koike, Y. Okada, Phys. Rev.
for the µ → eγ decay.
(2002) [arXiv:hep-ph/0203110]
18. T.S. Kosmas, J.D. Vergados, O. Civitarese
Nucl. Phys. A 570, 637 (1994)
References 19. E.A. Hermes, H.P. Wirtz, F. Rosenbaum, N
Meth. A 413, 185 (1998)
1. R.D. McKeown, P. Vogel, Phys. Rept. 394, 315 (2004) 20. M. Grossmann-Handschin et al., Nucl. Instr
2. A. van der Schaaf, Prog. Part. Nucl. Phys. 31, 1 (1993) 327, 378 (1993)
Andreas Knecht
3. T.S. Kosmas, G.K. Leontaris, J.D. Vergados, Prog. Part. 5
21. B. Robert-Tissot, ”Analyse de Transition
197Muon conversion at HIMB
Pion contamination: The SINDRUM II Collaboration: A searc
HIMB: Can also use degrader at 40 MeV/c (less
efficient compared to 50 MeV/c, but due to
lower momentum ~10x less pions in the beam)
Add bend in transport section after degrader to
reduce backgrounds coming from moderator
With better detector, active target, and the
above background reduction, can probably
keep pion backgrounds low enough to capitalise
on higher muon rate
Fig. 11. Momentum distributions of electrons and positrons
for the two event classes. Measured distributions are compared
with the results of simulations of muon decay in orbit and µ − e
conversion
Eur. Phys. J. C 47, 337–346 (2006)
decay in orbit (MIO) which is the dominant source of
40 MeV/c background. Figure 11 shows momentum spectra of elec-
trons and positrons for the two event classes introduced
3% Δp/p in Sect. 6.3. In general the electron distribution of sam-
ple 1 is well described by muon decay in orbit. Whereas
no events are observed with energies expected for µ − e
conversion at higher energy an electron and a positron
event have been found. Since cosmic ray background con-
Andreas Knecht tains much more electrons than positrons these events6
are most likely caused by pions. In sample 2 the elec-Mu2e/COMET Mu2e and COMET will perform their measurements with expected muon rates of around 1e10 μ-/s They will improve the sensitivity over SINDRUM II by a factor 104 Changes in expected branching ratio as a function of Z are on the level of
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