The Adventures of Daniel & Peter Neutron

My name is Peter Yergeau ’18 and I am spending my summer working in Dr. Crawford’s lab. Dr. Crawford specializes in neutron experiments and is currently part of two collaborations investigating neutron properties. I work with the NSR (Neutron Spin Rotation) collaboration and my counterpart Daniel works with the BL2 (Beam Line 2) collaboration. My work involves working on a code that, a) is older than I am and b) runs a simulation of my experiment. ExperimentalSetupAbove you see the schematic that Chris Haddock, one of the members of the collaboration, used when building the apparatus. What we are looking for with this experiment is considered a physics beyond the standard model result. The standard model of physics is related to the basic building blocks that we understand so far, these are the bosons, leptons, and quarks that make up particles like the neutron and are what cause the fundamental forces of gravity, electromagnetism and the two nuclear forces. Logic would tell you that if these forces and particles are fundamental then they would account for almost all the matter and energy of the universe, but it turns out to be much the opposite. Physics explainable within the standard model only accounts for 5% of the matter and energy of the universe, the rest is left to these models beyond the standard model. We are looking for one of these interactions on a very low energy scale.

Our experiment looks for the change in spin of a neutron. This spin is not the same as the spin of a basketball but rather we think of it like a charge that a particle can have. We are looking for a change to this charge that is on the order of 107 rad/m. This is so small, that in order to make sure that we are actually detecting this change in spin we need to eliminate as much systematic and statistical error as possible. That’s where the work I do comes into play.

My work is on the simulation of our experiment, to see what we should be able to get in a perfect environment and then to put in more realistic effects and see the differences. Specifically, I have been working on adding a code that takes into account the stray magnetic fields of the Earth and calculates how much that spins our neutron. Below I’ll show you the difference between what the theoretical environment looks like and what a more realistic environment looks like for our results.

Staright6angRealFakePicoil Since our experiment is in three dimensions we have two angles to take into consideration, from the y-axis with respect to the z-axis which is our theta value and from the y-axis with respect to the x-axis which is our phi value. In the top two histograms we are interested in the theta angle and how far from 0 that is and for the bottom two we care about the angle in phi since the apparatus has what we call a pi/2 coil that is supposed to change the theta spin totally into phi. We can see from this histogram that the theta isnt fully converted to phi so we need to make our pi/2 coil stronger. 

10KLANSCETrueFakePiCoilThis is what a more realistic environment looks like for our data, since not every neutron will have the same speed they will all rotate different amounts. In both plots I ran 10 thousand neutrons through our apparatus.

What’s next? Well now that the simulation is working, the goal is to improve how it works so that we can run our data sets faster and they will give us better data. Currently to run 1 million neutrons through the simulation of the apparatus it takes over 24 hours. Hopefully we’ll be able to improve the code so that it can run more efficiently and to improve the way it steps through the apparatus. As I am a rising senior this project will also be my senior capstone, so if you’re interested in the progress of my project or any of the other physics projects come to the colloqia in the fall.

Ehh, what`s up, Doc? My name is Daniel and I work with physics Professor Bret Crawford, my mentor. May I make a confession? I am astonished by the statements “physics is hard” and the more general “I am not a science person”. They lead to an unhealthy approach to science, which is so important for our civilization. Physics is an attempt to understand the essences of the physical world. Nothing more, nothing less. Don`t run away from physics. Embrace it. Share it! Consider this: we don`t live forever. Being present and taking some time to appreciate the world is one of the deepest things we can do. Is it hard to understand the behavior of the universe? Oh, man… yes. Thinking about new ideas and developing new abilities is always challenging. But the beauty is worth it. And you don`t have to try to be a physics person, because we are all physics people already! Science is a human achievement. It`s part of our nature to ask questions about the world. It may sound like idealism, but I believe these contemplations make a difference in our busy, stressful lives. We could do it right now, if you would like to!

The mysteries of the atomic and sub-atomic world fascinate me. Scientists believe that ordinary matter, which contrasts with possible unknown forms of matter, is composed of 17 fundamental particles that can be organized in 3 types: quarks, leptons and bosons – the Higgs boson is one of them! -. No other fundamental particles have been observed yet. The classification of elementary particles is one of the goals of the Standard Model, an important theory in particle physics. It`s really hard to imagine these particles and that`s part of the mystery. For simplicity, you can imagine little balls. Quarks may be held together by a fundamental force called “strong force”, which only acts on the atomic world, and may produce hadrons – composite particles made of quarks -. Composed of 3 quarks, protons and neutrons are hadrons and they are present in the nuclei of most atoms. Some nuclei are stable and some are unstable. A stable nucleus doesn`t fragment spontaneously. A neutron may exist forever in a stable nucleus, but a “free” neutron – one not bound to a nucleus – fragments spontaneously, because one of the known fundamental forces, called “weak force”, allows a quark of type “down” inside a neutron to become an “up” quark. This process is known as a “beta decay” and its products are a proton, electron and an electron antineutrino. On average, free neutrons beta decay in about 15 minutes. This average time to decay is known as the lifetime of an entity. In this case, it is the neutron lifetime. Neutron decay doesn`t occur in stable nuclei because it is not energetically favorable.danielpic1

A diagram for neutron decay. Ignore the W-.

There are two predominant experimental methods of measurement of the neutron lifetime. The “bottle” method traps neutrons in a bottle or a magnetic field and count the remaining neutrons after a time span and the “beam” method traps and counts decay protons inside a magnetic and electric “trap” that intercepts a beam of “cold” (slow) neutrons. Our problem is that the value of the neutron lifetime measured in recent “beam” and “bottle” experiments do not agree with each other within their uncertainties! Someone must be wrong, and both methods must be reviewed. Reducing uncertainties in the neutron lifetime may allow physicists to check the theories of the Standard Model and of Big Bang Nucleosynthesis. Computer simulations are useful because they can allow one to gain insights into these experiments and be reasonably accurate and low-cost.


A beam method experiment. The magnetic field B and the electric potential V at the mirror and door “traps” the positive charge protons.

Our research goal is to use the scientific simulation toolkit “Geant4” to simulate the beam method, so as to analyze it and reduce uncertainties due to possible systematic errors, and assist a new beam experiment that is being developed in the National Institute for Standards and Technology (NIST). Geant4 allows the storage of data, which will be analyzed using the ROOT data analysis framework. Geant4 and ROOT are popular tools in physics research and were developed largely to assist CERN experiments. My greatest challenge is to understand and use these tools, partly because of my little knowledge of computer science. We are still preparing our Geant4 simulation and ROOT analysis programs, which are instructions for the computer to do something. Understanding programming has been enlightening. How do you tell a machine made of a bunch of wires and electronic devices to perform such hard tasks? It`s fantastic. I am more aware of the nature of this beautiful and powerful tool, which is increasingly important for physics research. Our visit to NIST (Gaithersburg, Maryland) is scheduled. I am lucky to have the opportunity to work with Professor Crawford and Peter.

That`s all, folks! Thank you.


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