Week of April 2nd

Unfortunately, I couldn't travel to Siena today because Dr. Bellis is busy preparing for the American Physical Society conference in Georgia. He's presenting his research on dark matter at the conference, and I'm excited to hear how it goes!

Week of February 26th

I dove into my dark matter research today, and started looking up all of the various dark matter experiments. For each experiment, I read all the major papers it had published, as well as general news articles. It was really challenging to get through the publications, because there was so much technical jargon as well as concepts that I haven't encountered yet. By focusing on the titles of the papers and the abstracts, I was able to get a general understanding of what the paper was about. I also learned a lot just by looking at the graphs and tables each paper included, since I tend to understand concepts better when I see visual representations of them.

There are six major dark matter experiments going on right now: The LUX (Large Underground Xenon) experiment in South Dakota, the Xenon10 and Xenon100 experiments at Columbia, the Dama/Libra experiment in Italy, the CRESST (Cryogenic Rare Event Search with Superconducting Thermometers) experiment, and SuperCDMS (cryogenic dark matter search) in California. 

Today, I researched the LUX experiment and the Xenon10 and Xenon100 experiments, since they both use liquid xenon in their detectors. I put together a page-long summary for each experiment for Dr. Bellis, but I will just briefly recap what I learned here.

LUX experiment:

Detector: LUX is trying to directly detect dark matter (specifically WIMP particles, which most scientists believe constitute dark matter) with a liquid xenon time-projection chamber. A time-projection chamber is a type of particle detector that places electric and magnetic fields parallel to each other in the detector, which means that electrons travel in a straight line (instead of a curved path, which would occur if the electric and magnetic fields were perpendicular to each other). This allows scientists to create a three-dimensional picture of collisions in the detector, and determine the energy and momentum of particles. Xenon is used because it's very pure and because it fluoresces when struck by a charged particle. From the official LUX experiment page:

"Interactions inside the xenon will create an amount of light proportional to the amount of energy deposited. That light can be collected on arrays of light detectors sensitive to a single photon, lending the LUX detector a low enough energy threshold to stand a good chance of detecting the tiny bump of a dark matter particle with an atom of xenon."

Results: In February 2014, scientists at LUX concluded that from the first 90 days of data, there was no statistically significant evidence of WIMPS. This is surprising since it's considered to be the most sensitive detector to date, and other less sensitive detectors have found hints of the particles.

Xenon100:

The Xenon100 detector is very similar to the LUX detector. It's a time-projection chamber and it uses liquid xenon. The setup of all the dark matters are similar, but the difference is in the constraints set on the WIMP interactions (basically, where the scientists are looking for the dark matter interactions to happen). 

Results: The Xenon100 experiment found no evidence for dark matter interactions.

Week of February 19th

Today, I finally returned to Siena! First, Dr. Bellis and I went over all the work I did independently for the past two weeks. He explained the concept of cross sections more in detail to me, and talked about some of its applications in dark matter theory. We also reviewed WIMP (Weakly Interacting Massive Particles) Theory.

Dr. Bellis also took me on a tour of Siena's revamped particle physics lab. Siena just got some new hardware for their computers, including a processor called (I think) the Tesla K40. This enables researchers to perform complicated calculations in a fraction of the time that it used to take them. It's the same kind of hardware that animators at Disney and Pixar use to create movies, except in this case researchers are using it for modeling and programming. Dr. Bellis demonstrated some of the different functions the new computers could perform. It was really cool to see all of the new equipment and learn more about how scientists do their research.

Since I haven't worked on my Python skills in a while, I also practiced some more programming. Dr. Bellis introduced me to Rosalind and Project Euler, which are two sites that have Python coding problems. Project Euler has mathematics-based problems, and Rosalind deals with bioinformatics, which is a field that develops methods for analyzing biological data. I'm interested in biology, so I tackled the first problem on the site:

The website also provides helpful reviews of molecular biology, which was great because I've forgotten a lot since AP Bio! My Python skills were a little bit rusty, so it took a while to figure out how to solve this problem. Eventually, I figured out that I had to create counters for each base and then use an "if/then" statement to define how the computer should return the four integers.

Week of February 12th

Unfortunately, Dr. Bellis' son was still under the weather, so I did some more independent research from school. My first task was to read and understand the Wikipedia page for a cross section in particle physics. There was a lot of scary calculus and formulas, but eventually I understood the basic concept.

In particle physics, a cross section is simply the probability that two particles will interact. Say that you're randomly throwing darts at a target on the wall, with the hope that some of them will hit the target. A cross section is equivalent to the probability that the dart will interact with the target and not the wall (which is basically the ratio of the area of the target and the area of the wall).

The cross section of a particle determines how long that particle will annihilate (which means to convert into radiant energy). Cross sections are used the WIMP (Weakly Interacting Massive Particles) Theory of dark matter, so I'm sure that taking the time to understand this concept will serve me well when I go back to my internship.

In addition to learning about cross sections, I also started researching the current dark matter experiments going on right now. There are about 5 different major experiments, each of which use different elements to detect WIMPs. In my next meetings with Dr. Bellis, I'll start reading the papers that each of these experiments have published, but right now I just wanted to get an overview of the current research. Here's a great video I found about one of the dark matter labs:

I'm looking forward to meeting with Dr. Bellis again!

Week of February 5th

Now that the first part of our hybrid meson project is complete, Dr. Bellis is having me help him with the dark matter research he's working on. Unfortunately, his son was sick today, so I wasn't able to come to Siena to work. However, I still worked from school, and answered a few questions about dark matter that he prepared me. (Source is Wikipedia for all of the answers)

Why do we believe dark matter exists?

Scientists first hypothesized the existence of dark matter to account for the discrepancy between the mass of large astronomical objects determined from their gravitational effects and the mass calculated from the "luminous matter" (stars, gas, etc.) they contain. We believe that the reason for this difference is that there is another type of matter, dark matter, which isn't reactant to light. There is no direct evidence that it exists, so we can't be entirely sure that it is the reason for the discrepancy, but we can infer its existence from its gravitational effects on visible matter and radiation.

Who was Vera Rubin?

Vera Rubin is an American astronomer who, in the 1970s, discovered convincing evidence for the existence of dark matter. She observed that stars in spinning galaxies were all rotating at roughly the same velocity (with the distance from the galactic center having no visible effect on the velocity). The stars were not rotating around the visible center of the galaxy but around many unknown centers, all providing additional gravitational attraction. This contradicted Kepler's law of planetary motion.

Rubin's observation of the velocity of stars in spinning galaxy provided convincing evidence for dark matter, because the velocity curves that she observed could only occur if huge amounts of invisible matter were causing additional gravitational attraction.

What does the "weak" in Weakly Interacting Massive Particle (WIMP) refer to?

The weak in WIMP refers to the weak nuclear force, one of the four fundamental interactions of nature. It's responsible for the radioactive decay and nuclear fusion of subatomic particles. It is caused by the emission and absorption of W and Z bosons. The weak interaction is also capable of changing the flavor of quarks.

How "massive" are these WIMPs believed to be? How big is that relative to the mass of a proton?

10-100 GeV/c^2

proton mass ~ 1 GeV/c^2

WIMPS are predicted to be between 1-100x mass of a proton.

Week of January 29

Yayyy!!! I finished our calculations!!! I'm one step closer to becoming an actual scientist :) Basically, our calculations predict the number of D mesons produced from one billion B mesons. They are so simple that anyone who knows the laws of probability can do them. The key is just knowing the decays for each particle and what numbers to actually multiply together.

Here are the decays each B meson goes through to get to a D meson:

Number of B Mesons: 1 billion

B => B0 Bbar0: 500 million

B => B+ B- =500 million

B+- => K+- psi/g =1%

B0 => K0 psi/g= 1%

psi/g => D2(2460) D

psi/g => D2(2460) D*

psi/g => D1(2420) D

psi/g => D1(2420) D*

So, as you can see, we start off with one billion B mesons. 500 million mesons decay to B neutral Bbar neutral mesons, and 500 million decay to B+ and B- mesons. The charged B mesons then decay to charged kaons and psi/g particles, and the neutral B mesons decay to neutral kaons and psi/g particles. The psi/g particles then decay to our four different types of particles: the D2(2460), D1(2420), D, and D* mesons. (The number in parentheses just tells the mass of the meson.)

The BaBar detector isn't 100% efficient, and its efficiency varies depending on the particle. So, while the efficiency for the charged kaon was a solid 95%, the efficiency for a neutral kaon was only 40%. We had to take all of this into account when making our predictions. Here are all of our efficiencies:

B+- => K+- psi/g =1%	efficiency for K+-: 95%
B0 => K0 psi/g= 1%	efficiency for K0: 40%

However, just because the detector will be able to detect a certain decay doesn't mean that that decay will actually happen in the first place. We also have to take the branching ratios of the decays into account. As I explained in an earlier blog post, the branching ratio is the probability that a certain decay will actually take place. Here are the branching ratios we came up with:

psi/g => D2(2460) D	BR: 25%
psi/g => D2(2460) D*	BR: 20%
psi/g => D1(2420) D	BR: 25%
psi/g => D1(2420) D*	BR: 20%

Once I had all the assumptions, I could finally get started on the actual calculations! This was probably the easiest part of the whole process; all I had to do was multiply the right percentages together to get the final number of D mesons produced. Here are the calculations:

Calculations:
(5.0 x 10^8 charged B mesons)(1%)= 5.0 x 10^6 K+- psi/g		(5 x 10^8)(1% BR)(40% efficiency)= 2.0 x 10^6 K0 psi/g
(.95)(5.0 x 10^6)= 4.75 x 10^6 K+- psi/g
psi/g => D2(2460) D
(.25)(.01)(.05)(4.75 X 10^6)= 593.75		(.25)(.01)(.05)(2.0 x 10^6)= 250
psi/g => D2(2460) D*
(.2)(.01)(.03)(4.75 x 10^6)= 285		(.2)(.01)(.03)(2.0 x 10^6)= 120
psi/g => D1(2420)D
(.25)(.05)(.01)(4.75 x 10^6) = 593.75		(.25)((.05)(.01)(2.0 x 10^6)= 250
psi/g => D1(2420)D*
(.2)(.03)(.01)(4.75 x 10^6)= 285		(.03)(.01)(4.0 x 10^5)= 120

Our numbers were actually a lot better than we were expecting. From one billion B mesons, about 2500 D mesons will be produced (and actually detected). This is a good enough number to continue on with our work.

Of course, this all depends on if the assumptions we made were correct. To get a second opinion, Dr. Bellis is going to send the assumptions and calculations we made to the BaBar researchers at Stanford. If they agree with what we have come up with, then they will send us the rest of the BaBar data, which is when the real fun will begin. That's when all my Python and data analysis skills will come in handy. Fingers crossed!

Week of January 22nd

Q: What did Donald Duck say in his graduate physics class?
A: Quark, quark, quark!

Happy Lame Physics Jokes Day! Today, I was only at my internship for about an hour because of MLK Service Day and because my mentor had to leave a little bit early. I was also recovering from a cold, so I wasn't completely mentally functional. I still got some work done though!

To do our calculations for the hybrid meson, we have to come up with a list of assumed efficiencies for the particles involved in the decay. I started working on our list of assumptions today. The Particle Data Review was, as always, a really helpful source, but it was sometimes difficult to decipher all of the different numbers. I also used the paper Dr. Bellis gave me last week to come up with efficiencies for the D(2420) and D(2460) mesons. Next week, I'll be using these assumptions to actually finish the calculations, which I'm excited about!

Something cool that happened this week was that my particle physics booklet from the Particle Data Group came in the mail. This tiny booklet contains almost everything you need to know about the universe. It has the decays, branching ratios, masses, and pretty much any information you could ever want about a particle. Dr. Bellis has the full-sized version of this booklet in his office, but I ordered this free booklet so I can look up information when I'm working from school. Here's a picture of what the booklet looks like: