Week of November 13

Today, I continued my work on radioactive decay and calculating branching fractions. Using the Review of Particle Physics, I found the five most common decays for the D meson, and I calculated the branching fractions for each of those decays. I spent the majority of my time today working on my Python skills, however.

Today, I reviewed writing functions, using the range function, and writing if-else statements. Dr. Bellis gave me the task of writing a Python program that calculates if a number is prime or not and returns all of the prime numbers under 100. Although the part of the program that returned all of the prime numbers under 100 was fairly simple, I couldn't write that section until I wrote the first one. I knew that to calculate if a number was prime or not, I first had to define a range for the computer to calculate from. I defined the range as all the numbers between 2 (since 1 isn't a prime) and 1 + the square root of the number, since there aren't any new factors of a number after its square root. This is what that section looked like:

I had to use the integer function int() which converts all numbers into integers since the range function only works with integers. The if statement "if number % x == 0" basically says "if the number entered is divided by any number in the defined range, and the answer is equal to 0, then return 'False.' If the answer isn't equal to 0, then return 'True.'"

I wrote the section of the program that checks if the number entered by the user is prime, but there were too many bugs for the program to run successfully. Next time I have my internship, I will continue writing and debugging the program.

Week of November 6

Today, Dr. Bellis and I continued our discussion of radioactive decay, with a particular focus on the decay modes for the B meson and for the J/psi particle.

There are several different decay possibilities for any single particle. For example, two possible decay modes for J/psi are J/psi --> e+ e- and J/psi --> μ+ μ-. So, how does one know what a certain particle will decay to? All of a particle's decay modes, the probabilities of these decays, and everything else you could possibly need to know about a particle are published in the wonderful 2012 Review of Particle Physics (which has 1526 pages and a size 10 font).

One important piece of information to know about a decay mode for a particle is its branching fraction. Basically, a branching fraction is the number of particles that decay a certain way over the total number of particles that decay. The Review of Particle Physics publishes the branching fraction for each decay.

Here is a crazy looking graph for the branching fractions of the famous Higgs boson particles, which has many decay channels. This graph shows the branching fraction of the Higgs boson as a function of its mass. Source: Higgs Physics Group at Imperial College London. 

If you have two steps in the decay process, how do you calculate the total possibility of that decay? For example, suppose you want to find the proportion of particles that decay from a B neutral meson to a positive and negative pion. A B neutral meson first decays to a K neutral meson, which then decays into the two pions. You can find the total probability of the decay from a B neutral meson to a positive or negative pion using a simple rule of probability: if two events are independent, then the probability of them both occurring is the product of the probabilities of each occurring. Therefore, to find the total branching fraction for the decay of a B neutral meson to a charged pion, you have to multiply the branching fractions for each separate decay together.

Using this information, I calculated the branching fractions for three different situations. Unfortunately, I had to spend over an hour today in the IT office at Siena since my computer wouldn't connect to the network, so I didn't have much time to do anything else.

Week of October 30

This week, I finally went back to Siena after a two week break! Dr. Bellis and I started off by discussing radioactivity, since I'll be looking at the decay modes of certain mesons.

Radioactive decay occurs when an unstable atom emits particles of ionizing radiation, which results in the nucleus of the atom losing energy. The chance that a given particle will decay is constant over time, although it's impossible to predict exactly when this decay will occur (according to quantum theory). The half life is the time taken for a given amount of a radioactive substance to decay to half of its initial value.

To illustrate this process, Dr. Bellis showed me how to plot the radioactive decay for a particle on Python. Since we couldn't obtain real data, I modeled radioactive decay by shaking pennies in a box and removing all of the pennies that were heads up until there was only one penny left. 

Since the chance that any penny will come up heads on any toss is always the same (50%), about half the pennies are left after the first toss. In this model, the half-life is represented by the time it takes for one half of the remaining pennies to be removed (about one toss). Removing a penny is analogous  to the decay of a radioactive nucleus. 

This picture is what the graph looked like on Python. Radioactive decay has an exponential curve.

Unfortunately, I wasn't able to do any more work since Dr. Bellis had to leave early today. 

Week of October 23

My mentor still wasn't at Siena this week, so I answered some more of the study questions and continued learning Python from Emma.

Here are the study questions I answered:

What is an electron? What is a muon?

Electrons and muons are both classified as leptons.

Electrons are subatomic particles with a negative electric charge. They play an essential role in electricity, magnetism, and thermal conductivity, and they also participate in gravitational, electromagnetic, and weak interactions. Electrons have properties of both particles and waves, and can collide with other particles and can be diffracted like light.

Electrons radiate or absorb energy in the form of photons when accelerated. The antiparticle of an electron is a positron (identical to the electron except with an opposite charge).

A muon is an elementary particle similar to the electron, although it has a much larger mass. It has an electric charge of -1.

Where and when did the BaBar particle physics experiment run?

The BaBar particle physics experiment ran from 1999 to 2008 at the SLAC National Accelerator Laboratory at Stanford.

At the BaBar experiment, what kinds of particle beams did they collide?

BaBar studies the particles produced in collisions between electrons and positrons.

Week of October 16

Congratulations to my mentor! He and his wife had a baby boy this past week, so he won't be coming in for the next two weeks. However, I still went to Siena to answer some of the study questions he'd given me and to work on my Python skills.

Here are the study questions I answered:

 

What are quarks?

A quark is an elementary particle that is believed to be a fundamental constituent of matter (it serves as a sort of “building block” for matter).

Quarks, which are never found in isolation,  make up composite particles called hadrons, which include protons and neutrons.

Antiparticles of quarks are called antiquarks, and they have the same general properties of quarks except their charges have the opposite sign.

Flavors of quarks:

Up (u): + 2/3

Down (d): -1/3

Strange (s): -1/3

Charm: + 2/3

Top (t): +2/3
Bottom (b): -⅓

Properties of quarks:

Electric charge

Spin (form of angular momentum; can be visualized as the rotation of an object around its own axis)

Weak interaction: A quark of one flavor can transform into a quark of another flavor only through the weak interaction. By absorbing or emitting a W boson, any up-type quark (up, charm, and top quarks) can change into any bottom-type quark (down, strange, bottom) or vice versa.

Mass

Color charge and strong interaction:

Quarks have electromagnetic charge, but they also have a completely different type of charge called color charge. The force between color-charged particles is called the strong interaction force. The strong force holds quarks together to form hadrons, and its carrier particles are called gluons.

While quarks have color charge, composite particles made out of quarks have no net color charge (they are color neutral). As a result, the strong force only takes place on the really small level of quark interactions.

What are gluons?

Gluons are elementary particles that act as the exchange particles for the strong force between quarks. Gluons themselves carry color charge, and therefore participate in the strong interaction in addition to mediating it.

Quarks carry three types of color charge (blue, red, and green) and antiquarks carry three types of anticolor (antiblue, antired, and antigreen). Gluons can be thought of as carrying both color and anticolor.

Since the force-carrying gluons have a color charge, they participate in strong interactions. As a quark-antiquark pair separates, the gluon field forms a narrow tube (or string) of color field between them. This tube of color field means that there is a strong force between the quark pair that remains constant, regardless of their distance.

What are mesons?

Mesons are hadronic subatomic particles composed of one quark and one antiquark, bound together by the strong interaction. All mesons are unstable, and charged mesons decay (sometimes through intermediate particles) to form electrons or neutrinos. Mesons are not produced by radioactive decay, but instead appear in nature only as short-lived products of very high-energy interactions in matter, between particles composed of quarks.

Each type of meson has a corresponding antimeson in which quarks are replaced by their corresponding antiquarks and vice-versa. For example, a positive pion (π+) is made of one up quark and one down antiquark; and its corresponding antiparticle, the negative pion (π−), is made of one up antiquark and one down quark.

Mesons participate in both weak and strong interactions.

Strange Charm

I started my internship! I'm working on particle physics research with Dr. Bellis at Siena College. In our first meeting, he introduced me to programming and basic particle physics concepts, and we talked briefly about what project I'll be working on.

From 1999 to 2008, the SLAC National Accelerator Laboratory at Stanford conducted the BaBar experiment, which involved hundreds of researchers using the BaBar detector, a multilayer particle detector, to study the difference or disparity between the matter and antimatter content in the universe. The experiment is no longer running, but there are years of data that have yet to be analyzed. My job is to run a code on Python that analyzes a very specific section of the data to determine whether or not it is worth further analysis. In particular, I am searching for evidence of a new type of particle called an exotic meson, which has already been predicted to exist.

Dr. Bellis started off by explaining some basics of particle physics to me. I have always thought that protons, neutrons, and electrons are the simplest parts of matter, but I learned that there is actually an elementary particle called a quark, which is a fundamental constituent of matter (it serves as a "building block" for matter). Quarks, which are never found in isolation, make up composite particles called hadrons, which include protons and neutrons. Antiparticles of quarks are called antiquarks, and they have the same general properties of quarks except their charges have the opposite sign.

Here's a really funny song explaining quarks (Thanks, Helen!): http://www.youtube.com/watch?v=U0kXkWXSXRA

The song title "Strange Charm" comes from the six different flavors (types) of quarks: up, down, strange, charm, top, and bottom. Fun fact: top and bottom were previously called truth and beauty before the scientists started to think those names were too poetic. 

The particular type of particle that I'll be studying is a meson, which is a hadronic subatomic particle composed of one quark and one antiquark, bound together by the strong interaction force. All mesons are unstable, and charged mesons decay (sometimes through intermediate particles) to form electrons or neutrinos. Mesons are not produced by radioactive decay, but instead appear in nature only as short-lived products of very high-energy interactions in matter, between particles composed of quarks.

Dr. Bellis also introduced me to Python, the programming language that I'll be using to run my code. I'll be using a fantastic website called codeacademy.com to learn Python. I already started the online course, and it's very straightforward and easy to navigate. Hopefully, I'll soon be able to start writing the code!

Dark Matter

My internship has finally started! My mentor is Dr. Matt Bellis, a particle physicist and professor at Siena College. Dr. Bellis' research projects, which tend to be computationally intensive, include tests of the quark model, direct dark matter detection, and large-scale cosmology.

Earlier this week, I was able to attend a talk at RPI that Dr. Bellis gave on dark matter. Even though I didn't understand a lot of the equations and experiments, the talk was fascinating and piqued my curiosity about particle physics and dark matter.

I learned that dark matter is a form of matter that doesn't emit or absorb light. Dark matter consists of a large part of the total mass of the universe, but since it's not luminous (it doesn't emit any type of electromagnetic radiation), we cannot see it directly. However, we can infer that it exists from its gravitational effects on visible matter and radiation.

The existence of dark matter was first hypothesized to account for the discrepancies between the calculations of the mass of galaxies from their gravitational effects and the calculations of the mass from the luminous matter they contain. Although dark matter does not interact through electromagnetic or strong nuclear interactions, it is gravitationally interactive. 

Dr. Bellis also discussed the methods used to detect dark matter, which are classified into direct dark matter detection and indirect detection. Dark matter is widely believed to be composed of WIMPs, or Weakly Interacting Massive Particles. Direct detection experiments try to find the scattering of dark matter particles off atomic nuclei within a detector, and indirect detection experiments search for the products of WIMP annihilations.

I didn't know anything about dark matter before this talk, and even though I didn't understand most of it, the talk was still absolutely fascinating. I am looking forward to my first official meeting!