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Introduction to High Energy Particle Physics

Lorayna Hinton

Math 89S

Hubert Bray

Duke University

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Abstract

This paper will serve as a qualitative introduction to high energy particle physics. This will

include the small scale structures of baryonic matter and the process of becoming a high energy

system. The structure of particle accelerators will be covered and collisions in these particle

accelerators will be examined. Current advances and future outlook for the field will also be

investigated. Any prior understanding to particle physics will be extremely helpful in

understanding this text, but most necessary background information will be given.

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Baryonic Matter

Baryonic matter is a form of matter described as being composed of 3 quarks. This is the matter

most of us are familiar with, and presents itself as all observable matter (atoms and ions). It is

often referred to as all of the matter on our periodic table of elements, and includes protons,

neutrons, and any antimatter variants of baryons (please note electrons are leptons, not baryons).

This is the typical graphic one would be presented with to describe a baryon. With two up quarks

(signified by u) and one down quark (d), one would indicate the hadron of the left to be a proton.

The above shows the conventional way that one would have these particles displayed. The proton is pictured on the left.

Before you have let this image sink into your mind for too long, I will explain why this image is

very misleading as to the true structure of baryonic matter.

The mass of a proton is about ~938 MeV. Therefore, the mass of each of the thee quarks should

add up to this mass of the proton, correct? Meaning the mass of a quark should be around

300MeV; however, physicists have discovered otherwise.

The mass of an up quark is accepted to be in a range of 1.7-3.3 MeV and the mass of a down

quark, about 4.1-5.8 MeV (exact measurements unknown, but these were the values obtained

from experimental data). This would mean the mass of a proton should be somewhere around

7.8-12.4 MeV, but this is not the case.

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You may notice in the first graphic, there are spiraled lines joining the quarks together. These are

gluons, the force carrier of the strong force. These gluons, although massless, are the key reason

for this difference between the anticipated and the actual values for the masses of quarks because

of their immense energy.

[Side Note: The reader should keep energy-mass equivalence in their mind in reading the

following paragraphs. Although the technicalities are less important for simply understanding the

structure, keeping it in mind will make for a better understanding. For those unfamiliar, this

simply means that mass and energy can be equivalent to one another. In stating the entirely of the

theory in one sentence, the statement is meant loosely.]

As previously stated, gluons hold and pass large amounts of energy as the strong force is

governed between the three quarks. This is to such a degree, that gluons actually briefly decay

into matter-antimatter mesons (2 quark hadrons). This is happening constantly throughout the

entire proton, fluctuating so quickly, that we cannot count how many there are at once. We do

however, observe the mass of this proton, but only on average since it will fluctuate by a small

degree as these quark-antiquark pairs come in and out of existence.

Below, you will see a more accurate graphic of the proton. With the described gluons and quark-

antiquark pairs.

This is a quick depiction of the strong force. It is often helpful to think of it as a rubber band. The force is not as strong when the rubber band is not stretched, but once the band is stretched, you can feel the rubber band pushing to go back into its original state. This happens with quarks when they stay too far from the rest. It is pulled back into the proton because of the strong force.

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In observing the last few pages of text, we now understand the structure of baryonic matter and

where most of the mass in the proton comes from—quark-antiquark pairs.

Becoming a High Energy System

Becoming a high energy system is not a default task for any particle. This is a task that requires

the input of energy. Using the previous example, protons are most often accelerated to high

velocities with particle accelerators.

With myself asking this question prior to writing this paper, I would push for the importance to

notice the physical difference between a lower and higher energy system.

When a proton is at rest, this is when it is the least energetic. It also has the least amount of the

randomly appearing quark-antiquark pairs; however, when energy is put into accelerating these

particles, the energy is stored physically in the gluons. This excites the gluons, and more of these

Pictured to the left is the true structure of the proton. You may notice there are more particles in the proton that previously depicted. Note that the quark-antiquark pairs do not annihilate as they are not pairs within their own flavor. For example, an up quark might be paired with an anti-down quark.

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quark-antiquark pairs are popping in an out of existence at once. We measure this as an increased

mass of the proton.

The left hand picture is a Proton. The right hand picture is also a proton, at a higher energy. You

may notice the number of quark-antiquark pairs have increased.

[Side Note for Special Relativity: This states the adding/subtracting of energy is involved in

changing the reference frame for baryonic matter. Energy needs to be added into a system to

speed up, and energy is released from a system when it slows down. Acceleration is the

movement from one reference frame to another (excluding the change of direction).]

Now that we see the physical difference between lower and higher energy systems, we can look

at high energy particles’ collisions, and what the findings from these experiments imply for our

universe.

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Particle Accelerators and High Energy Particle Collisions

Using protons and the LHC as an example, particle accelerators use electric fields to accelerate

protons to velocities to, at maximum, 0.99999999c. This equates to the mass of a single proton to

be around 4TeV (4000MeV), and with a rest mass of just over 300MeV, you can see how drastic

this increase in velocity is for the mass of the proton. The LHC bends these positively super

accelerated beams of protons are manipulated with magnetic fields into thin beams, then these

particles are set up to collide. Particle accelerators can be miles long, and are often a circular

shape to allow for continuous acceleration.

The above photo shows how large particle accelerators can be. The LHC is 17 miles in

circumference.

We understand there is a higher energy stored in the photon at high speeds and a decrease in

velocity calls for a decrease in energy. So one can make sense of the rapid release of energy

when protons are collided going in opposite directions at such high energies. Due to this rapid

release of energy, the collision creates a variety of other particles from the energy released.

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Essentially most of the energy put into speeding up the proton is released in the form of mass. I

will include visuals provided from CERN as well as an explanation of common results.

These graphics provided by CERN show the collision of two protons. This happens millions of

times per second. The orange is the “scattering of particles”, that is the resulting particles flying

in all directions away from the collision site. Particles from these reactions decay quickly into

other particles, and by observing the final results, they can deduce what the original resultants

were to percentages of possibilities. This could be analogous to backtracking prime factorization.

For example, a positively charged Rho (p+) can decay into π+ and π0, and π0 can decay into two

photons. By following the end results of a collision, π+ and two photons, physicists can deduce

what the original particle was from this collision.

Future works and Conclusion

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Just within the past few years, two huge concepts in physics were found after years of searching

and postulating. The Higgs Boson was discovered and gravitational waves were detected. CERN

and other major organizations continue to push the limits of the known by using these high

energy particles to observe the universe in higher energies. Many of the experiments in high

energy particle physics currently, seek to recreate the early universe (as it was a superheated

gluon-plasma in the early days) and to create a Theory of Everything that matches experimental

data. These goals may not be reached in the short term, but scientists all over the world are

excited to continue to push this lining back just a bit more.

Citations

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"Particle Collision Data." Eufisica.com. CERN, Jan. 2012. Web. Oct. 2016. Photos of Particle

Collisions

Kruse, Mark. "Particle Physics 2." Duke University, n.d. Web. Sept. 2016. Lecture and Office

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David. "Why Don’t Three Quarks Add Up to One Proton." Http://cosmologyscience.com/. N.p.,

Jan. 2013. Web. Photo.

Armstrong University. "Building Blocks of Our Universe." Http://chemphys.armstrong.edu, n.d.

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Pillai, Maya. "What Makes Up an Atom." Http://www.buzzle.com/articles/what-makes-up-an-

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Encyclopedia, 9 Oct. 2016. Web. 9 Oct. 2016.