Quarks and Leptons Announcements Recitation this week in lab. BRING QUESTIONS ! See my by Wed. if you have any grading issues with your exam. Reading Assignments in Particle Adventure (see Schedule link)

Hadrons Hadrons are particles which interact via the strong interaction. (“hadro” is a Greek root for “strong”) Protons and neutrons bind together in the nucleus because of the strong interaction. It can’t be electrical force, because protons repel each other, and the neutron is electrically neutral. Clearly, the strong force must be stronger than the EM force, since the EM force tries to push the protons apart, but yet the nucleus stays intact! n p Strong p EM Strong

Hadrons, Baryons and Mesons In nature, we find that all particles which contain quarks interact via the Strong Interaction. This is why protons and neutrons are hadrons; because they contain quarks ! So, all particles which contain quarks (or antiquarks) interact via the strong interaction. There are two classes of particles which we know about that contain quarks and/or antiquarks. Hadrons Baryons Mesons qqq Could refer to these as baryon and anti-baryon if you want

Are there baryons other than protons and neutrons? The answer is a resounding YES ! Other quarks can combine to form other baryons. For example: u s d This combination is called a Lambda baryon, or L0 for short What is the charge of this object?) Flavor Q/e u +2/3 d -1/3 s u This combination is called a Delta baryon, or D++ for short What’s this one’s charge?

Let’s make baryons! Quark up down strange Charge Q +2/3 -1/3 -1/3 Mass ~5 [MeV/c2] ~10 [MeV/c2] ~200 [MeV/c2] u u u d d d s s s Note: The neutron differs from a proton only by “d”  “u” quark replacement! Proton Neutron u u d d u d In all cases, you will see that I have given the quarks inside baryons the colors red, green, and blue. This is because quarks also have an intrinsic “color charge”, or simply “color” for short. We will get into this in more detail later when we discuss the strong interaction. For now, assume that all baryons must have 1 RED, 1 GREEN and 1 BLUE quark. Taken together, the RED, GREEN, and BLUE produce an object which has no color (ie., it’s colorless). This is the same idea as the visible light be composed of the full spectrum of colors in the rainbow. Q = +1 M=938 MeV/c2 Q = 0 M=940 MeV/c2

Let’s make some more baryons ! u d s Charge, Q Mass +2/3 -1/3 Quark up down strange ~5 [MeV/c2] ~10 [MeV/c2] ~200 [MeV/c2] Lambda (L) Sigma (S+) Sigma (S-) u u d d u s s d s Here, Q means “the value of the electric charge” Note that the Lambda and Sigma_0 have the same quark content, but have different masses. How can this be? The answer is beyond the scope of this course. If you would like a deeper explanation, I encourage you to talk to the instructor. Q = 0 M=1116 MeV/c2 Lifetime~2.6x10-10[s] Q = +1 M=1189 MeV/c2 Lifetime~0.8x10-10[s] Q = -1 M=1197 MeV/c2 Lifetime~1.5x10-10[s] These particles have been observed, they really exist, but decay fairly rapidly. Is S- the antiparticle of S+ ??

Mesons Mesons are the 2nd member of the hadron family. They are formed when a quark and an anti-quark “bind” together. (We’ll talk more later about what we mean by “bind”). u d s d c d It is difficult to show here, but the two quarks inside mesons must have opposite color. The quark can be either RED, GREEN, or BLUE. Choose one.. If we choose GREEN, then the anti-quark’s color is “ANTI-GREEN”. How do we draw anti-green? Sorry, I’m not sure how, but try and keep in mind that the quark and antiquark in a a meson are one of (or some combination) of these colors combinations: Quark + Antiquark ------ ------------ RED + ANTIRED BLUE + ANTIBLUE GREEN + ANTIGREEN What’s the charge of this particle? What’s the charge of this particle? What’s the charge of this particle? Q= 0, this strange meson is called a K0 Q= -1, and this charm meson is called a D- Q=+1, and it’s called a p+ M~140 [MeV/c2] Lifetime~2.6x10-8 [s] M~500 [MeV/c2] Lifetime~0.8x10-10 [s] M~1870 [MeV/c2] Lifetime~1x10-12 [s]

Building hadrons Generations I II III d (down) s (strange) b (bottom) Charge = -1/3 d (down) s (strange) b (bottom) Charge = +2/3 u (up) c (charm) t (top) The top quark decays before it has time to form a baryon or meson. So, one can build many, many possible baryons by combining any of the 3 quarks (5 x 5 x 5 = 125) One can build many mesons by forming qq combinations: 5x5 = 25

Back to the Particle Zoo So, many of the particles discovered at accelerator experiments are simply different types of baryons and mesons ( qqq or qq )

The Cast of Fundamental Particles Generations I II III Charge = -1/3 d (down) s (strange) b (bottom) Charge = +2/3 u (up) c (charm) t (top) + antiquarks e - Charge = -1 +anti-electron (positron) Is nature really like this?

Muons Recall that we discussed a particle called the muon. It was discovered in cosmic ray experiments (1937). It was also used in the experimental test of time dilation. We find that a muon behaves almost identical to an electron, except its mass is about 200 times more than the electron’s mass. m- e- m=0.51 MeV/c2 m=106 MeV/c2

Neutrino 1934: To account for the “unseen” momentum in neutron decay: p e X n  p + e - + X Fermi proposed that the unseen momentum (X) was carried off by a particle dubbed the neutrino (n ). Nobel Laureate: Enrico Fermi If this neutrino in fact existed, one should also observe the reaction: n + p  e+ + n Read as “a neutrino interacts with a proton, producing a positron and a neutron”

Detector: H2O w/ Cadmium Chloride Neutrino Discovery 1956: Existence of the neutrino confirmed by putting a detector near to a prolific source of neutrinos, a nuclear reactor, and observing n+p  e+ + n (Nobel Prize) Photon detectors Fred Reines and Clyde Cowan, 1956 Detector: H2O w/ Cadmium Chloride

Neutrinos In 1962, an experiment was conducted at Brookhaven National Lab (Long Island). The researchers wanted to know if there is more than one type of neutrino, or are there more? They found in fact that the neutrinos associated with electrons are different particles from the ones associated with muons. Jack Steinberger, Melvin Schwartz and Leon Lederman. 1988 Nobel Prize winners for the discovery of the “muon-neutrino” ne nm electron- neutrino muon- neutrino

Leptons The electron, the muon and their neutrinos, like the quarks, appear to be fundamental. That is, so far, we are unable to find that they are made up of anything smaller. However, they behave very differently than the quarks. They have integral charge (0 or ±1). They do not “bind” to form hadrons. They do not participate in the strong interaction. The electron, muon and neutrino belong to a general class of particles called LEPTONS.

Three happy families… Family Leptons Q = -1 Q = 0 1 e- ne 2 m- nm 3 t- In 1975, researchers at the Stanford Linear Accelerator discovered a third charged lepton, with a mass about 3500 times that of the electron. It was named the t-lepton. In 2000, first evidence of the t’s partner, the tau-neutrino (nt) was announced at Fermi National Accelerator Lab. Family Leptons Q = -1 Q = 0 1 e- ne 2 m- nm 3 t- nt Q = +1 e+ m+ t+ Q = 0 ne nm nt Anti-Lepton 3 families, just like the quarks… interesting !!!

This all looks Greek to me ? electron muon-minus tau-minus electron neutrino muon neutrino tau neutrino Lepton (particle) positron muon-plus tau-plus electron anti-neutrino muon anti-neutrino tau anti-neutrino Anti-lepton (anti-particle)

So here’s the big picture Quarks and leptons are the most fundamental particles of nature that we know about. Up & down quarks and electrons are the constituents of ordinary matter. The other quarks and leptons can be produced in cosmic ray showers or in high energy particle accelerators. Each particle has a corresponding antiparticle. Quarks and leptons differ significantly in the way that they interact with each other. That is, the processes which can occur when two quarks collide is quite different from the situation when two leptons collide. This brings us to the concept of “force carriers”. Force carriers are the particles which transmit a force from one particle to another. We will get into this in greater detail…