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


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.

Project Overview: 

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.

Research Description:

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.


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.


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.

About Me:

 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

American Institute of Physics

Mu2e Experiment Home Page

Contact Info:




 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.