History of Atomic Physics 3) Quarks and Anti-particles Connector What are cosmic rays? Where do cosmic rays come from? Why don’t cosmic rays reach the ground?

Discoveries in Cosmic Rays 1932 : Discovery of the antiparticle of the electron, the positron. Confirmed the existence and prediction that anti-matter does exist!!! 1937 : Discovery of the muon. It’s very much like a “heavy electron”. 1947 : Discovery of the pion.

The Plethora of Particles Because one has no control over cosmic rays (energy, types of particles, location, etc), scientists focused their efforts on accelerating particles in the lab and smashing them together. Generically people refer to them as “particle accelerators”. (We’ll come back to the particle accelerators later…) Circa 1950, these particle accelerators began to uncover many new particles. Most of these particles are unstable and decay very quickly, and hence had not been seen in cosmic rays. Notice the discovery of the proton’s antiparticle, the antiproton, in 1955 ! Yes, more antimatter !

From Simplicity Complexity Simplicity Around 1930, life seemed pretty good for our understanding of “elementary (fundamental) particles”. There was protons, neutrons & electrons. Together, they made up atoms  molecules  DNA  People ! AAHHHHH, nature is simple, elegant, aaahhhh… But the discoveries of dozens of more particles in accelerator experiments lead many to question whether the proton and neutron were really “fundamental”. Is nature really this cruel ? I. I. Rabi’s famous quote when the muon was discovered. Who ordered that” ? 1994 Nobel Prize Winner in Physics Needless to say, the “zoo of new particles” that were being discovered at accelerators appeared to reveal that nature was not simple, but complicated? Until….

Quarks ? First things first: Where did the name “quarks” come from? Murray Gell-Mann had just been reading Finnegan's Wake by James Joyce which contains the phrase "three quarks for Muster Mark". He decided it would be funny to name his particles after this phrase. Murray Gell-Mann had a strange sense of humor! In 1964, Murray Gell-mann & George Zweig (independently) came up with the idea that one could account for the entire “Zoo of Particles”, if there existed objects called quarks. Murray Gell-Mann George Zweig Flavor Q/e u +2/3 d -1/3 s The quarks come in 3 types (“flavors”): up(u), down(d), and strange(s) and they are fractionally charged with respect to the electron’s charge

How sure was Gell-Mann of quarks ? When the quark model was proposed, it was just considered to be a convenient description of all these particles.. A mathematical convenience to account for all these new particles… After all, fractionally charged particles… come on ! An excerpt from Gell-Mann’s 1964 paper: “A search for stable quarks of charge –1/3 or +2/3 and/or stable di-quarks of charge –2/3 or +1/3 or +4/3 at the highest energy accelerators would help to reassure us of the non-existence of real quarks”. Well….

Probing deeper into matter If we really want to understand if there is anything “inside” a proton or neutron (aka nucleon), we have to examine it with particles whose wavelengths are smaller than the size of a proton.  Since l = h/p, we must produce higher momentum particles. That is, the higher the momentum of the particle, the smaller it’s deBroglie wavelength  can “see”, or “probe” smaller things Since the proton’s size is very small, about 1x10-15 [m], We need very energetic beams of particles (high momentum) to probe it’s structure. By the 1960’s, physicists had learned how to produce high energy, well-focused, beams of particles, such as electrons or protons (particle accelerators !) This has been the driving force behind understanding “What is matter at its most fundamental level ?”

Are protons/neutrons fundamental ? In 1969, a Stanford-MIT Collaboration was performing scattering experiments e- + p e- + X (X = anything) What they found was remarkable; the results were as surprising as what Rutherford had found more than a half-century earlier! The number of high angle scatters was far in excess of what one would expect based on assuming a uniformly distributed charge distribution inside the proton. It’s as if the proton itself contained smaller constituents

Quarks Protons 2 “up” quarks 1 “down” quark Since 1969, many other experiments have been conducted to determine the underlying structure of protons/neutrons. All the experiments come to the same conclusion.  Protons and neutrons are composed of smaller constituents. These quarks are the same ones predicted by Gell-Mann & Zweig in 1964. (1.6 x 10-15 m) 1x 10-18 m (at most) Protons 2 “up” quarks 1 “down” quark Neutrons 1 “up” quark 2 “down” quarks Are there any other quarks other than UP and DOWN ?

Three Families of Quarks Generations I II III Charge = -1/3 d (down) s (strange) b (bottom) +2/3 u (up) c (charm) t (top) Increasing mass Woohhh, fractionally charged particles? Also, each quark has a corresponding antiquark. The antiquarks have opposite charge to the quarks

The 6 Quarks, when & where… Date Where Mass [GeV/c2] Comment up, down - ~0.005, ~0.010 Constituents of hadrons, most prominently, proton and neutrons. strange 1947 ~0.2 discovered in cosmic rays charm 1974 SLAC/ BNL ~1.5 Discovered simultaneously in both pp and e+e- collisions. bottom 1977 Fermi- lab ~4.5 Discovered in collisions of protons on nuclei top 1995 Fermi-lab ~175 Discovered in pp collisions SLAC = Stanford Linear Accelerator BNL = Brookhaven National Lab Notice the units of mass !!!

Major High Energy Physics Labs Fermilab DESY SLAC CERN KEK CESR BNL

Fermilab Accelerator (30 miles from Chicago) Main Injector Tevatron Experimental areas Top Quark discovered here at FNAL in 1995. FNAL = Fermi National Accelerator Laboratory.

“Typical” Particle Detector ~ 6 ft

Back to matter & quarks…

Fundamental particles We consider quarks to be fundamental, because so far we have been unable to “break them apart”. As we increase the momentum of particles in our accelerators, we are able to resolve, or see, deeper into matter. We are currently able to accelerate particles to energies of ~1 [TeV] = 1x1012 [eV]. To what wavelength does this correspond? First convert [eV] to [J] !!!! l =hc/E = (6.6x10-34)(3x108) / 1.6x10-7 = 1.2x10-18[m] So, if quarks were bigger than this, we would be able to discern their substructure. So far, they look to be smaller than this ! That is they are at least 1000 times smaller than the proton ! Same is true for electron  quarks (and electrons) are considered “fundamental”

More on quark decays later… Quark masses 6 different kinds of quarks. Matter is composed mainly of up quarks and down quarks bound in the nuclei of atoms. The masses vary dramatically (from ~0.005 to 175 [GeV/c2]) The heavier quarks are not stable, and decay to lighter quarks quite rapidly Mass [GeV/c2] Gold atom Silver atom Proton Recall that the unit [GeV/c2] is a unit of mass. E= m c2 Example: t  b (~10-23 [s]) b  c (~10-12 [s]) c  s (~10-12 [s]) su (~10-7-10-10 [s]) More on quark decays later…

Anti-particles too ! We also know that every particle has a corresponding antiparticle! That is, there are also 6 anti-quarks, they have opposite charge to the quarks. So, the full slate of quarks are: Q= +2/3 Q= -1/3 Particle Quarks Q= -2/3 Q= +1/3 Anti- Particle Anti-Quarks

Quark Confinement q Hadron Jail Proton Quarks are “confined” inside objects known as “hadrons”. We’ll learn more about hadrons in a bit… This is a result of the “strong force” which we will discuss later…

Protons & Neutrons To make a proton: We bind 2 up quarks of Q = +2/3 and 1 down quark of Q = -1/3. The total charge is 2/3 + 2/3 + (-1/3) = +1 ! To make a neutron: We bind 2 down quarks of Q= -1/3 with 1 up quark of Q = +2/3 to get: (-1/3) + (-1/3) + (2/3) = 0 ! So, it all works out ! But, yes, we have FRACTIONALLY CHARGED PARTICLES!

Why does the nucleus stay together ? So far, the only “fundamental” forces we know about are: (a) Gravity (b) EM force (Electricity + Magnetism) Which one of these is responsible for binding protons to protons and protons to neutrons ??? Since like sign charges repel, it can’t be EM force? Gravity is way, way, way too weak… Then what is it??? Strong Force This is the third fundamental force in nature and is by far the strongest of the four forces. More on forces later…

Wow, I’m somebody… I’m a Baryon! HADRONS/BARYONS The forces which hold the protons and neutrons together in the nucleus are VERY strong. They interact via the STRONG FORCE. Protons and neutrons are among a class of particles called “hadrons” (Greek for strong). Hadrons interact very strongly with other hadrons! Baryons are hadrons which contain 3 quarks (no anti-quarks). Anti-baryons are hadrons which contain 3 anti-quarks (no quarks). Wow, I’m somebody… I’m a Baryon! Me too, me too…

Are there baryons other than protons and neutrons? Good question, my dear Watson… 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 also in 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]