The Structure of Matter

because you can’t understand how it works until you have some small idea of what it is…

http://scaleoftheuniverse.com/

In the past I never really taught much of this stuff…

• So why start now?– With the probable discovery of the Higg’s Boson

(a.k.a. The God Particle, or the G.D. Particle depending on the reference you choose) during the summer of 2012, it is apparent to me that young people should not be leaving high school without at least a very small notion of the modern understanding of the elements that make up matter.

• Clearly this is not the realm of biology (life science) and while most good chemists admit that their field is really a specialty of atomic physics, chemistry stops right there: at the atom.

• Physics is so broad and encompassing that the discussion of matter is really just a small, but increasingly important, aspect of the whole science.

Particle Physics: The structure of stuff

• Let’s start with the atom…

– About 130 years ago atoms were considered “fundamental.” That is to say that they were thought to be the smallest, most elementary pieces of matter possible.

Image of a uniform surface from a scanning-tunneling (electron) microscope

• Around 1880 theoretical models of the atom are born to help scientists understand other physical phenomena, particularly electromagnetism and radioactive decay

• So we enter the realm of the “sub-atomic” particles: the electron, the proton, (and much later) the neutron.

Electrons

• Discovered by J.J. Thompson and colleagues in 1897, Thompson produced the first known measurements of the particle: It’s charge to mass ratio (e/m). The actual charge of the electon (and therefore its mass using Thompson’s data) was discovered by Robert Milikan in 1909 in his famous “Oil Drop” experiment.

• Interestingly the electron is still considered a fundamental particle, belonging to a class known as leptons.

Protons & Neutrons

• Lots of back history here, but most important:– Discovery of the nucleus, Rutherford, Gold Foil

Experiment, 1911– Nuclear charge = atomic number, Van den Broek &

Moseley, 1913– Isolation of a proton via low energy nuclear reaction,

Rutherford, 1917– Conceived by Rutherford in 1920, Neutron discovered in

1932 by James Chadwick to explain specific nuclear phenomena• Later recognized as a key component in thermonuclear processes

Important thing: Protons and Neutrons are no longer considered fundamental!

So where do we go from here?

• Lots of history between 1932 and the present, but I will sum up:– Lots of experiments with nuclear reactions both

high energy (blowing stuff up) and lower energy (turning one type of atom into another)

– Lots of experiments where electromagnetic forces are used to accelerate, or speed up, particles or even whole nuclei to ridiculous speeds and smash them into each other or other things.

What did we find out?

• Lots of stuff but most importantly:– There appear to be 4 unique types of forces that

affect matter. We will look at these “fundamental forces” later in this course.

– We appear to have isolated a large but finite number of “fundamental particles” and amassed a great deal of knowledge about how they work together to comprise matter as we know it.

Fundamental Particles: Stuff which can’t be broken apart any further (maybe)

• Quarks• Leptons• Force Carriers

This is about categorization people! Don’t just try to memorize this stuff, work out a way to fit it together.

Even I get these twisted up sometimes.I’ll give you some graphics at the end that will help.

Fermions (basically they follow the Pauli Exclusion Principle)

Quarks: Stuff that makes up “nucleons”

• Quarks are fundamental particles which combine in a variety of ways forming composite particles known as hadrons and mesons.

• You are familiar already with the most stable hadrons: The proton and neutron.

Properties of Quarks

• Fermionic• Spin ½ • Carries electric charge (values depend on the type of quark)• Carries “Color Charge,” an interactive property (like electric

charge, but completely different) that determines how quarks will align with other quarks

• 6 types: Up, Down, Charm, Strange, Top, Bottom– Type is determined by the quantum properties of the quark (above)– Anti-quarks exist for each of these (same absolute values but

opposite signs for some properties).• Participate in all 4 fundamental force interactions

Hadrons

• Baryons:– 3 quarks, generally 2 “regular” quarks and 1 anti-quark, or 2

anti’s and 1 regular.• Proton and Neutron are most common and stable• Full List: http://en.wikipedia.org/wiki/List_of_baryons

• Mesons:– 2 quarks: 1 regular, 1 anti

• Exotic, not common in normal matter, very short half-lives• Low energy mesons believed to participate in the nuclear (or

“strong”) force process that holds the nucleus together• High energy mesons believed to have played a role in the Big Bang,

not normally associated with matter outside of high energy particle collisions in laboratory experimentation

You really need to be careful how you spell that, lest you say awkward things…

Leptons: Think “electron”

• According to wikipedia, which has a nice article on the subject,:– A lepton is an elementary particle and a fundamental

constituent of matter. The best known of all leptons is the electron which governs nearly all of chemistry as it is found in atoms and is directly tied to all chemical properties. Two main classes of leptons exist: charged leptons (also known as the electron-like leptons), and neutral leptons (better known as neutrinos). Charged leptons can combine with other particles to form various composite particles such as atoms, while neutrinos rarely interact with anything, and are consequently rarely observed.

Charged Leptons

• Electron, Muon, Tau particle– Anti-particles exist of each type– Muon and Tau particles are rare in normal

conditions, but have been substituted for electrons to form exotic atoms through various reactions.

Uncharged Leptons

• Neutrinos (electron neutrino, muon neutrino, tau neutrino)

– Neutral charge– Very little mass (even compared to electrons)– Rarely interact with matter– First theorized/observed associated with

unexplained energy loss during beta-decay process.

Lepton Properties (all types)

• Fermionic• Spin 1/2• Electric charge• NO color charge (unlike quarks)• Do NOT participate in the strong (nuclear)

force, but are affected by other forces.

Fundamental Forces1) Gravitational Force: Interaction between masses.

Any object with mass is attracted to other masses by the gravitational force.

2) Weak Interaction: A sub-atomic force that occurs most often during nuclear beta decay.

3) Electromagnetic Force: A force between charged objects or particles. Any object with a charge is attracted or repelled by other charges depending on the sign of the charge.

4) Strong (Nuclear force): An attractive force that holds nucleons (protons/neutrons) together.

Force carriers

• How does stuff react (know to move) as a result of other stuff?– Think about it: How does an electron repel from a

nearby electron?– There is a LOT going on in this deceptively simple

question, but the short answer is that fundamental particles exchange energy and therefore do work on each other through these fundamental force charge carriers

– Also called Gauge Bosons

Types

• Photons– Fancy name for electromagnetic radiation,

everything from radio waves, to gamma rays with light and everything else in between can be called a photon.

– These are the force carrier associated with the electromagnetic force and thus the primary mode of force transmission between leptons, but is also seen between charged hadrons.

So how does that electron (lepton) know that other electron (lepton) is out there and they should repel? Answer: They shoot beams of light at each other.

Gluons

• These particles allow for the communication of force between quarks.– Gluons are considered responsible for the strong

force. They are the “glue” that keeps the nucleus of an atom from ripping apart.

W & Z Bosons

• Unlike many of these particles, bosons are not fermionic, meaning the same type of particle with the same energy and properties can occupy the same space at the same time. Let that sink in for a moment.– Responsible for the “weak force”– Associated with the emission and absorption of

neutrinos from and into matter

Higgs Boson

• Proposed in 1964 and discovered* in 2012, the Higgs Boson is associated with the mechanism by which particles acquire mass and therefore by which gravitation affects matter– Zero charge, zero spin, zero color, approx. 126

times more massive than a proton, lifetime: about 1 zeptosecond

Putting it all togetherHere’s a pretty good article that may help: http://abyss.uoregon.edu/~js/ast123/lectures/lec07.html

The standard model of particle physics

An overview of the various families of elementary and composite particles, and the theories describing their interactions

Important for your test Important so you are not scientifically illiterate

Important for your test