Mu2e: Muon to Electron Conversion & Token
Bit Management (TBM)
By: Ramiro Torres
Partnered with: Timothy Baker
Supervisor: Kristan L. Corwin, Associate Professor
of Physics
Advisors: Dr. Tim Bolton & Dr.
Glenn Horton-Smith (Mu2e), Dr. Andrew Ivanov & Dr. Nikoloz
Skhirtladze (TBM).
Kansas State University Physics Department REU Program
Introduction:
Hello and Welcome to my web
page. It is dedicated to explain my research project partnered with Timothy
Baker. I am a participant of the 2015 REU program conducted at Kansas State
University. I am attending Texas Tech University. I also want to thank the
National Science Foundation for funding the REU program hosted at Kansas State
University. Tim and I were assigned to participate in two projects that were
part of the physics particle field. In our first project, mentors were two
faculty members at Kansas State University: Dr. Tim Bolton & Dr. Glenn
Horton-Smith. As for the second project, we worked under the supervision of Dr.
Andrew Ivanov, the leader of the project and our supervisor, Dr. Nikoloz Skhirtladze.
Below, I describe the Project Overview, my Research
Description, and an About Me section.
Under the guidance of Dr.
Bolton and Dr. Horton-Smith, Tim
and I were studying two of the possible
background sources of the mu2e experiment that will be conducted at Fermilab. The “mu” stands for muon, a particle much like
the electron but approximately 200 times heavier and “2e” represents conversion
to electron; hence, muon-to-electron conversion. This experiment predicts this
conversion, something that hasn’t been observed experimentally to this day and
according to the standard model of particle physics, it is very unlikely to
happen. The goal of this experiment is
to utilize 1018 muons and if it succeeds then this will be serve as
evidence to revise the standard model of particle physics. Because this
experiment wants to detect something very unlikely to happen, my partner and I
want to examine two possible sources of background that could potentially give
some false data. This experiment is going to be running in another 2+ years.
For
the second project, we took part in the testing of new devices that will
improve the data management on the large hadron collider at CERN. These new
devices are Token Bit Management chips, the TBMs 08c and 09 are the major
candidates to improve the data sorting from the many particle collections at
the large hadron collider. Because these chips will have a major role in very
important experiments that will be conducted in future experiments conducted at
CERN, it is imperative that these TBMs are fully operative.
As already mentioned in project overview, I am
participating in studying two background sources of the mu2e experiment, but in
this section I want to expand more on the description of the experiment itself
and on the background source that I am focusing on. If you haven’t yet, I would
recommend reading my project overview section before moving to this section, it
should give a main idea of what I discuss in the following sections. As, I
intend to raise the amount of detail in the following subsections.
Project 1:
Main Goal
of Mu2e:
The
purpose of the experiment of the mu2e experiment is to find the neutrinoless muon-electron conversion something that has
not been observed experimentally. According to the standard model of particle
physics (SMP), it has a chance of 10-55 probability of occurrence,
but there are theories that probe physics beyond SMP that suggest otherwise.
Normally the standard model of particle physics predicts that a muon converts
to an electron in the following form:
This
process occurs 99 % of the time because of charged lepton family conservation,
the number of leptons is conserved on both sides of the equations. However,
mu2e seeks to observe the following process:
The
example of charged flavor lepton violation; there aren’t equal numbers of muons
and electrons on both the initial and final state of the process. The muon
would be converted to an electron in the presence of the nucleus field; this
electron will then have momentum and energy with the magnitude of the muon’s
mass (~106 MeV). The aluminum nucleus serves the purpose of momentum
conservation. Mu2e experiment will use 1018 and send them towards an
aluminum target and use the setup that is discussed below.
Set of
Mu2e Experiment:
The
main setup of the mu2e experiment is composed of three major components, the
production solenoid, the transport solenoid, and detector solenoid. These three
solenoids are superconducting solenoids; the reason is to reduce the cost
amount for running this experiment. The function of the components of the
experiment will be discussed in the following sections.
*This is an image of the basic setup, and the woman
is placed there to give a sense of scale for the setup.
Production
Solenoid:
The tungsten production target is
placed inside the production solenoid and an 8 GeV pulse-proton beam is
targeted in the opposite direction of the solenoid’s magnetic field. When the beam
is fired, the protons will collide with the production target and cause the
production of many particles, but this experiment requires particles with low
energy, so the experiment only finds the backscattered particles to be useful.
Amongst the particles that are backscattered, pions
both positively and negatively charged are the particles of interest, they will
be sent to the transport solenoid.
Transport
Solenoid:
The
(S) design of the transport solenoid helps to filter particles that have too
much or little momentum, particles with just the right momentum will pass the
elbow section of the solenoid. This is already helping to reduce the number of
particles that are unwanted or would affect the outcome of the experiment. The
produced particles such as pions and muons are
directed into the transport solenoid by the magnetic field of the production
solenoid. During the time of travel, the pions are
expected to decay because of their relative short lifetime, which is 26
nanoseconds (10-9 seconds), and decay into a negatively charged muon
and a muon anti-neutrino. The muons and anti-neutrinos will travel down the
solenoid, and reach the last section of the transport solenoid before the
trajectory of the muons is changed by the influence of magnetic field; here,
the anti-neutrinos, being neutral, will just travel with their trajectory
unchanged.
Radiative
Pion Capture:
My goal is to make an estimate of how significant
radiative pion capture (RPC) is as a background source. Because the decay of
particles is a probabilistic process, there might be a number of pions that survive the travel from the production solenoid
to the detector solenoid and they can interact with the aluminum in the same
manner as the muons. A process defined as radiative implies that there will
photons emitted as a result of this process. The radiative pion capture process
can be described in the following form:
The photon that is emitted
can come from two types of cases. The photon can be emitted from radiative processes
in which the photon γ is real and can propagate into space before reacting
with material and produce an electron-positron pair. The other possible method
is that the photon can be emitted as a result of internal conversion; in that
case the photon is defined to be a virtual photon γ*, which
means it will have a nonzero mass and decays instantaneously into an
electron-positron pair. However, in my project I am considering the photons
that are emitted by radiative processes.
Furthermore, the “X” in the
right indicates that there is more than one nuclear state for the final state
of this process and, therefore, it is expected that the energy of the photons
will be not be monochromatic but follow a spectrum structure. Furthermore, the
pion is supplying all of its rest mass as energy to be gained in this process;
hence, the maximum energy that a photon could carry is the mass of the pion
~140 MeV. This is more than enough energy for a photon to produce and
electron-positron pair in the range of the signal ~105 MeV. When the pion comes into contact with an
aluminum atom, it will be captured due to its great mass and reach the ground
state of the atom which will allow the pion to interact with the nucleus. The
photons that come out of this process are emitted instantaneously; this is due
to the fact that a pion is a hadron, this is the name for particles that
interact via all of the fundamental forces: baryons and mesons.
The other background source
that can produce photon emission is the radiative muon capture (RMC), but Tim
discusses it more at great detail. However, I will mention that the major way
to differ between these two possible radiative background sources is the very
fact that pions are captured immediately because of
the strong interaction with the nucleus; but in the case of the muon, it will
capture with a delay of ~ 864 ns which is attributed to its lifetime.
Approach Method:
Our mentors suggested taking
the following approach. Instead of making calculations to estimate the amount
of background that RPC, an alternative method is to look at the total number of
interactions which can produce an electron-positron pair in the tracker. For
this, there are two things that are necessary the probability of interaction
per photon and the number of photons emitted. Because, Tim and I were looking
at two very similar background sources, we could follow the same approach, the
probability of interaction could be the same, but the number of photons emitted
would differ. After performing several calculations with the geometry of the
tracker, we found the probability of interaction is 1.617×10-4 and
from looking through some papers in the mu2e document database, the number of
emitted photons resulted to be 9.873×1012. The product of these two
numbers gives the result that there are 8.18×108 electron-positron
pair productions to be expected. Tim found a result in the same order of
magnitude.
The next stage was to
determine the number of photons that are actually converted into an
electron-positron pair. However, that is a difficult calculation to perform and
this required some type of simulation. Dr. Bolton suggested that we could use
simulation program Geant4 that was already used for previous estimates for the
mu2e experiment. We edited some of the code to look at the total number of
conversion photons and check that it was possible to measure them. We also
studied a sense of energy asymmetry to verify that there was no preference in
which an electron was more likely than the positron to have more energy, and the
angle between the electron-positron pair when they were converted from the
photon.
Conclusion:
Because of the time
constraint I used a sample of 1 million RPC photons, a sample too small for
experiment conditions, but enough to give a rough estimate of the number of
conversion photons and electron-positron pairs out of the total number of
photons. The simulated data reflected that 0.4-1.50 % of photons emitted by RPC
and RMC are converted into electron-positron pairs. This suggests that with
that there could be 150 billion electron-positron pairs, it seems small
relative to the number of muons that are being used, but to obtain a better
estimate it will be necessary to look at a much larger set of data. Also, if
mu2e experiment chooses to follow this approach, this could produce a more
reliable estimate because this is something that is measured with the
experiment setup itself, and not any
Project 2:
The second project that my
partner and I were assigned to was, like previously mentioned in my overview, the
testing of Token Bit Management chips. These are designed help facilitate the
collection of data when a hadron collision occurs, because otherwise there will
be a collection of tons and tons of data.
The implementation of the new TBM 09 is the advantage that it is faster
because it is digital, unlike the previous model, the TBM 05, that is currently
being used, which is analog. It is designed to be more resistant to the
radiation and is made of less material, which would reduce the interference
with particle detection.
Testing:
The
TBM testing setup is composed by the following: alloy platinum stage, testing
board, and microscope. The TBMs are placed on the stage and moves in a vertical
direction to come into contact with the testing board, the microscope allows to
visually perform the alignment and connect the TBM pads to micro probes
attached to testing board. The stage is managed by a computer that uses the
Cascade hardware with the Nucleus 3.2 software. The Nucleus software manages
the cascade probe station. Then, the actual testing consists of applying a
voltage across the TBM pads and verify that the TBMs operate at frequencies
between 40-52 MHz (106 Hz). The test is conducted with a separate
computer that has a previously written code. This is to ensure that they will
operate at the required 40 MHz for a very long time while radiation damage
diminishes their performance overtime.
Final Presentation: If you would
like to learn more about my project, click here to
download my presentation in powerpoint and here to download my poster
presentation.
I attend college at Texas Tech University, my major is physics with
an astrophysics concentration, and I am currently a senior entering my last
semester as an undergraduate. I first got interested in physics in my
senior of high school; I began liking the idea of studying the laws of nature. My
interest lies in the field of particle astrophysics; I am interested in the
behavior of particles in the universe and their ties to astrophysical objects.
I am part of the astro-group at my home institute,
this is a group formed by the astrophysics faculty members that get together to
discuss any progress on research that is conducted inside the institute and
current topics of interest in the field of astrophysics. I do look forward to
attending graduate school to help further my studies in the field of astrophysics.
The REU program has helped me
grow in the following ways:
Being part of this REU
program has presented me with the opportunity of interacting with other physics
students from different institutions and that made me aware
of how they find physics interesting. I came to this program expecting to find
other undergraduate physics students, but in very little time they became
friends. Each of us was assigned to a specific project that fitted our
interests in physics. During the biweekly group meetings, I had the opportunity
to hear what they had learned working on their projects. Presenting my work
every two weeks gave me the opportunity to improve my presenting skills and my
clarity explaining the contents of my project and my project throughout the course
of the REU. It provided me with the opportunity of being aware of what
performing research is like. It requires a lot of time and dedication to get
things done. Also, there will be many occasions when something will fail and to
be a participant of this program gave me a chance of seeing how physicists
address the problems that they come across. To summarize, this REU was a great
experience and I have enjoyed every moment of it.
Useful Links:
I have found the following
links particularly informative or useful:
American Physical Society Statements on
Ethics
Contact Info:
Email: ramiro.torres@ttu.edu
Funding:
This program is funded
by the National Science Foundation through grant number PHYS-1461251.
Any opinions, findings, and conclusions or recommendations expressed in this
material are those of the author(s) and do not necessarily reflect the views of
the National Science Foundation.