Dave Bjergaard's Lab Notebook

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Welcome to my research logbook

This website is my "Lab Notebook." Here you will find my daily/weekly entries regarding whatever research project I'm working on at the moment. The archive page contains links to all of my previous entries. Email me if you feel you should have access to those posts.

My research (in plain English)

I think about this stuff all day every day, so I apologize if I don't explain something completely.

The purpose of the Large Hadron Collider (LHC) is to collide protons together at an energy of 8TeV. Protons are the building blocks of atoms; Hydrogen is an atom with one proton and one electron. When an electron is removed, a positively charged proton remains. In the LHC, the proton is accelerated to an extraordinary speed (almost the speed of light). The LHC is ring-shaped, and has a 17mi. circumference. The protons are arranged in bunches which collectively are called "the beam". Another beam circulates from the other direction and at specific locations around the ring; the beams are steered into each other. These beams can be compared to shotgun blasts aimed at each other. The protons pass by each other most of the time. Sometimes they glance off each other, and, more rarely, they hit each other head on. The beams are circulated throughout the ring many times, which increases the frequency of head on collisions.

What happens when the two protons collide? If the protons collided at normal every day energies, they would bounce off each other because of their electrical repulsion. At very high energies, however, two new forces appear. These forces act completely differently than any every day forces the average person would have experienced. The forces are named the strong and the electroweak. The electroweak force interacts with leptons and neutrinos as well as quark flavor. The strong force interacts with the quark's color. Because these forces are so strange, scientists use the words "flavor" and "color" to help keep track of how they behave.

Physicists use mathematical equations to describe various forces. When the equations that should describe strong and electroweak forces are set forth, something interesting happens. Instead of the equations describing the forces and energies encountered in routine physics, more particles are created! These particles are called bosons, and each force has a characteristic boson that "mediates" it.

The boson that mediates the strong force is called the massless, charge-neutral gluon. Gluons mediate the forces between quarks (the particles that make up a proton). In order to more easily use plain language to discuss this complicated phenomenon, physicists describe the gluons as having different colors. Gluons are said to carry red, green or blue, or anti-color. The quarks themselves are also described as colored red, green, blue, anti-red, anti-green or anti-blue. These aren't colors that we can "see", they represent a simplified method for organizing the rules of how quarks and gluons interact.

The boson that mediates the weak force is called the "massive charged W" and the "charge-neutral Z". The W allows a quark to change flavor. The quark flavors have been named up, down charm, strange top, and bottom; they come in three pairs and the W boson follows specific rules when it changes the flavor of the quark. The last force is called the electric. Everyone is familiar with this boson that mediates the force between charges, it’s the massless photon!

Since there are three pairs of quarks, there must be three particles that act like electrons. This group of particles is referred to as leptons. The group is made up of the electron, the muon, and the tau. These leptons can also interact with the W if specific rules are followed. The information presented thus far is considerable, but is necessary background in order to understand what happens when protons smash into each other head on in the LHC. As it turns out, a quark or gluon will only interact with another quark or gluon following one of the forces above. Depending on the degree of energy many different outcomes are possible. My research has involved exploring what happens when either two quarks or a gluon and a quark come together to form a W boson and a quark (or gluon). The W boson then decays into a muon and a neutrino. The outgoing quark (or gluon) interacts with more gluons to produce more quarks. This all comes together to produce light particles called pions and kaons. These pions and kaons fly through the detector (within the LHC) where they push electrons around in the silicon of the detector. This creates measurable voltages that allow the paths of the particles to be measured. When the particles land in the outer part of the detector, they release all of their energy through electric charge and light that be measured. When this information is combined, a measurement of the particle’s energy and momentum is created.

It would be nice to know the charge of the original quark, but unfortunately it can't be directly measured. However, the charge of the pions and kaons as they curve in the magnetic field surrounding the detector can be measured. It turns out that if the measure of their charge is multiplied by the momentum (the same exact way distance multiplied by mass determine the center of the mass of an object), then a rough idea of the charge of the original quark is achieved! Since the W charge can be measured accurately (remember it decayed to a muon, which is like a heavy electron), it is possible to cross check the charge from the quark. In the future, the quark charge will be useful when researchers try to understand what happened in a collision of particles.

Written by: David Bjergaard. Last updated 2014-05-14T15:33-0400.