年末発表 25 Dec 2015
Study of X-ray and proton emission after
muon capture in aluminium in the AlCap
experiment
Mark Wong
久野研
年末発表 25 Dec 2015
Contents
Physics Motivations
Introduction to AlCap
Argonne National Laboratory – Boston University –
Brookhaven National Laboratory –
Fermilab National Accelerator Laboratory – Imperial
College London – INFN Lecce – INFN Pisa –
Institute of High Energy Physics, China – Laboratori
Nazionali di Frascati, INFN – Nanjing University –
Osaka University – University College London –
University of Houston – University of Washington,
Seattle
Muon beam and experimental setup
Muon beam profile characterization and tuning
Sanity checks and future analysis work
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Physics motivations
COMET and the mu2e experiments aim to look
for neutrinoless muon to electron conversion in
the vicinity of a nucleus.
SUSY loop diagram
Several Beyond the Standard Model (BSM)
theories allow for such processes.
Discovery of the μ-to-e process will provide hints
as to which BSM theories are viable.
Littlest Higgs model with T-parity conservation
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Physics Motivations
The Standard Model (SM) μ-to-e branching ratio is on
the order of 10-54 which is possible via neutrino
oscillations.
COMET Phase-I aims to have a per event sensitivity of
to the order 3.1 x 10-15 therefore it is important to
understand and reduce the background rates.
SM process where the μ-to-e can occur
but at a branching ratio so low current
experiments can not detect it. (c.f. http:
//arxiv.org/abs/1412.1406)
The main processes that contribute to this background
is the muon decays in orbit (DIO) and nucleon emission
after muon capture.
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Background events - Muon decay in orbit
This is the most dominant background
source.
These muons are bound in a muonic
atom under the Coulomb potential of the
Al nucleus.
Nuclear recoil can boost the decay
electron to energies close to the
conversion signal.
Monte carlo simulations of momentum
distributions for the μN -> eN signal in red and
DIO events in blue. (c.f. COMET TDR)
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Background events - Nucleon emission
After muon capture the nucleus is
excited and may emit gammas,
neutrons and charged particles like
protons, deuterons and maybe some
tritons and alphas.
These background events can be
removed if we understand their
momentum spectrum near the signal
peak.
c.f. arxiv.org/abs/1501.04880
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Introduction to AlCap
The Aluminium Capture (AlCap) experiment aims to determine the proton and
neutron muon capture rates which are the major contributors to the background of
COMET and Mu2e.
AlCap website: http://muon.npl.washington.edu/exp/AlCap/index.html
The recently concluded AlCap run in Nov 2015 was the third run. The second was
in this summer and the first in 2013.
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Aim
Recent run 2015b was from 04 Nov to 23 Nov.
WP1: Charged Particle Emission after Muon Capture.
Protons emitted after nuclear muon capture in the stopping target dominate the single-hit rates in the tracking chambers for both the
Mu2e and COMET Phase-I experiments. We plan to measure both the total rate and the energy spectrum to a precision of 5% down to
proton energies of 2.5 MeV.
WP2: Gamma and X-ray Emission after Muon Capture.
A Ge detector will be used to measure X-rays from the muonic atomic cascade, in order to provide the muon-capture normalization for
WP1, and is essential for very thin stopping targets. It is also the primary method proposed for calibrating the number of muon stops in
the Mu2e and COMET experiments. Two additional calibration techniques will also be explored; (1) detection of delayed gamma rays
from nuclei activated during nuclear muon capture, and (2) measurement of the rate of photons produced in radiative muon decay.
WP3: Neutron Emission after Muon Capture.
Neutron rates and spectra after capture in Al and Ti are not well known. In particular, the low energy region below 10 MeV is important
for determining backgrounds in the Mu2e/COMET detectors and veto counters as well as evaluating the radiation damage to electronic
components. Carefully calibrated liquid scintillation detectors, employing neutron-gamma discrimination and spectrum unfolding
techniques, will measure these spectra. The measurement will attempt to obtain spectra as low or lower than 1 MeV up to 10 MeV.
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Muon beam
We used the beam are πE1 in PSI
Experimental hall.
The πE1 beam line supplies high intensity
pion and muon beams with momenta
ranging from 10 to 500 MeV/c.
Outdated (1997) layout of the πE1 beamline
indicating quadrupole and dipole magnets used to
steer and focus the muon beam - PSI
This proton accelerator delivers a proton beam of
590 MeV energy at a current up to 2 mA - PSI
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Si detectors
Experimental setup
Targets used:
~2 days Al100μm, ~2 days Al50μm,
~1 day Si, ~1 day Ti.
A germanium detector is placed
outside the chamber. All components
are shielded and grounded.
For each target, we optimized the
muon beam momentum for highest
number of captured muons.
Ge detector
target
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Experimental setup
On each side are a set of three silicon detectors,
a.
b.
c.
d.
e.
thin detector, 65um
2 x 2 segmented Si detector
thick detector, 1.5mm.
target is tilted 45o to the beam direction facing the Ge detector.
a veto scintillator behind the target to count rates.
c.f. P. Litchfield
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Muon beam tuning
We first did an energy calibration for the
Ge detector using Eu-152 in the target
position.
X-rays/gammas emitted from de-excited
captured muons are detected by this
detector. We determined the best beam
momentum for highest number of x-rays
emitted which was for example for the Al
0.1 mm target, p=1.035*25.3946 MeV.
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c.f. J. Quirk, BU
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Beam profile characterization
One of the problems of previously
done analysis was that the position of
the beam spot was unknown.
We built a remote controlled beam
two dimensional scanner connected
to a 3x3mm SiPM. Currently we have
a rough estimate of where the beam
spot is.
c.f. P. Litchfield
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Plots for sanity checks
It is ideal to know if muons are being captured
during beam time, or if we have any muons being
focused on the target.
Top plot shows a sample readout from a silicon
detector. This plot shows the pulse heights of
electron and muons hitting this detector as a target.
Bottom plot shows part of the x-ray spectrum
recorded by the germanium detector. They are used
for the Ge detector energy calibration. Here marked
in red is the 1.137MeV peak, the second one is a
1.333MeV peak.
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Early analysis and future work
Plot is a ΔE-E plot of particles, with about 2100
captured protons. No time cut was applied so this
also includes particles emitted by muon capture
from other nuclei existing in the chamber.
Currently for future analysis work, We need to
compile a run summary and decide on the tasks
important for analysis.
c.f. V. Tishenko
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Questions & Comments
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