Paul Dirac Predicts AntiMatter! • In 1929, Theoretical Physicist Paul Dirac combined • Special Relativity • Quantum Mechanics to try to describe the behavior of the electron • One problem with his equation: it had two solutions • This is not always bad. For example, the equation x2=25 also has two solutions: +5 and -5 • In Dirac’s case, his equation had two solutions: • An electron with positive energy • An electron with negative energy • Usually, negative energy solutions are NOT GOOD! Energy must always be positive. • But Dirac was pretty smart. He realized that in his case, the negative energy electrons could be INTERPRETED as anti-electrons Neils Bohr: Of all physicists, Dirac has the purest soul.
The Discovery of Cosmic Rays • At the beginning of the 20th century, scientists thought there was too much radioactivity than could be accounted for naturally. Where was it coming from? • Victor Hess decided to test the idea that the additional radiation came from outer space. In 1912, one way to do this was by BALLOON! • He got to about 18,000 feet (without oxygen) He noticed that the radiation steadily increased. • COSMIC RAYS!
Beaten to the Punch! • Actually, a Jesuit Priest named Theodor Wulf beat Hess by 2 years, noting that the radioactivity at the top of the Eiffel Tower was higher than at the base. • But alas, no Nobel Prize. This went to Hess in 1936.
The Discovery of Antimatter! • In 1932 Carl Anderson studied cosmic rays using a “cloud chamber”. • Charged particles produced in cosmic rays would enter the chamber and leave “tracks”. The tracks would bend in circles because the chamber was placed in a strong magnetic field • Positive particles bend one way • Negative particles bend the other way • He found equal numbers of positive and negative particles • Maybe the negative particles were electrons? (YES!) • Maybe the positive particles were protons? (NO!) • By studying how much energy the positive particles lost, he figured out that they had the same mass as the electrons! • Positive electrons! • Antimatter! • Nobel Prize!
More Antimatter: Search for AntiProton • The search for antiprotons heated up in the 1940s and 1950s, as laboratory experiments reached ever higher energies... • In 1930, Ernest Lawrence (Nobel Prizewinner in 1939) had invented the cyclotron, a machine that eventually could accelerate a particle like a proton up to an energy of a few tens of MeV. Initially driven by the effort to discover the antiproton, the accelerator era had begun, and with it the new science of "High Energy Physics" was born. • It was Lawrence that, in 1954, built the Bevatron at Berkeley, California (BeV, at the time, was what we now call GeV). The Bevatron could collide two protons together at an energy of 6.2 GeV, expected to be the optimum for producing antiprotons. Meanwhile a team of physicists, headed by Emilio Segre', designed and built a special detector to see the antiprotons. • In October 1955 the big news hit the front page of the New York Times: "New Atom Particle Found; Termed a Negative Proton". With the discovery of the antiproton, Segre' and his group of collaborators (O. Chamberlain, C. Wiegand and T. Ypsilantis) had succeeded in a further proof of the essential symmetry of nature, between matter and antimatter. • Segre' and Chamberlain were awarded the Nobel Prize in 1959. Only a year later, a second team working at the Bevatron (B. Cork, O. Piccione, W. Wenzel and G. Lambertson) announced the discovery of the antineutron.
But Can We Make and Anti-Nuclei? YES! • By now, all three particles that make up atoms (electrons, protons and neutrons) were know to each have an antiparticle. So if particles, bound together in atoms, are the basic units of matter, it is natural to think that antiparticles, bound together in antiatoms, are the basic units of antimatter. • But are matter and antimatter exactly equal and opposite, or symmetric, as Dirac had implied? The next important step was to test this symmetry . Physicists wanted to know: how do subatomic antiparticles behave when they come together? Would an antiproton and an antineutron stick together to form an antinucleus, just as protons and neutrons stick together to form an atom's nucleus? • The answer to the antinuclei question was found in 1965 with the observation of the antideuteron, a nucleus of antimatter made out of an antiproton plus an antineutron (while a deuteron, the nucleus of the deuterium atom, is made of a proton plus a neutron). The goal was simultaneously achieved by two teams of physicists, one led by Antonino Zichichi, using the Proton Synchrotron at CERN, and the other led by Leon Lederman, using the Alternating Gradient Synchrotron (AGS) accelerator at the Brookhaven National Laboratory, New York.
What about anti-atoms? • At this point, a natural question to ask: Can we form antiatoms? • The necessary ingredients: an antiproton and an antielectron. • But typically when these things are made, an accelerator is used, and the anti-particles are moving too fast. So we need to slow them down. • This was done at the European Laboratory CERN, using the Low Energy Antiproton Ring (LEAR). • In 1995, scientists at LEAR succeed in making the first anti-atoms (about 9 of them). • So anti-atoms exist, and a natural question to ask is: Are there anti-worlds out there? Anti-galaxies? • Before answering this question, lets first try to ask what practical use anti-particles have in our world. • Can anyone think of any? Low Energy Antiproton Ring (LEAR).
PET • "PET“ stands for Positron Emission Tomography. Positron Emission Tomography uses positrons to look at the brain. • Radioactive nuclei in a fluid are injected into the subject. • The radioactive nuclei then emit positrons at low velocities and these then annihilate with nearby electrons. • The positrons and electrons are moving slowly and don't have the energy required to create a new pair of particle and antiparticle. Instead, 2 gamma rays are emitted and these are used to actively scan the brain. • The gamma rays leave the patient’s body and are detected by the PET scanner. • The information is then fed into a computer to be converted into a complex picture of the patient’s working brain.
Anti-Matter SpaceCraft!? • NASA's Marshall Space Flight Center, Pennsylvania State University are studying using annihilation of matter and antimatter to fuel spacecraft. • Matter and antimatter provides the highest energy density of any known propellant. • it would require only a gram of antimatter to put the shuttle into orbit. • about ten billion times more energy than the hydrogen/oxygen mixture that powers the shuttle • 300 times more than the fusion reactions at the Sun's core. But… costs $62.5 trillion per gram. Might be able to bring this down to $5 billion per gram
How do we really know that the universe is not matter-antimatter symmetric? • We have landed on the moon,so we know the moon is made of matter. • Cosmic rays from the sun are matter not antimatter. • The other planets are matter (Mars Rovers are still taking data!) • The Milky Way: Cosmic rays sample material from the entire galaxy. In cosmic rays, protons outnumber antiprotons 104 to 1. • The Universe at large: This is tougher. If there were antimatter galaxies then we should see gamma emissions from annihilation.
Colliding Galaxies • The image shows the collision of two galaxies from the Hubble Space Telescope 63 million light years away. • Such collisions would occur in other places in the universe as well. • If there were anti-matter galaxies, then such collisions would result in a very specific signature of gamma rays (like what we see in the PET scanner). • No such signal is seen. • Also, by looking at cosmic rays, there is some antimatter, but this can be accounted for by radioactive decays or by nuclear reactions involving ordinary matter. • So we believe most of the universe (>99.99%) is made of matter. The Antennae Galaxies
Any Other Evidence for Antimatter in the Universe? • NASA's orbiting Compton Gamma Ray Observatory (CGRO) spacecraft spotted unexpected clouds of antimatter in the Milky Way Galaxy. The clouds suggest a hot fountain of gas filled with antimatter electrons is rising from the region around the center of the our galaxy. Antimatter electrons also are known as positrons.The nature of the furious activity producing the hot antimatter-filled fountain is unclear, but could be related to massive numbers of stars being born near the large black hole at the center of our galaxy. Other possibilities include winds from giant stars or black hole antimatter factories.
Ok. The Universe is Only Matter. So What? • The fact that there is only matter present is a problem • All models of how the universe started in the Big Bang indicate that there should be as much matter as antimatter, initially. • Every reaction we know of which makes a quark, also gives us an anti-quark • This problem is referred to as the “Baryon Asymmetry” problem, where “Baryon” is a general name for things like protons and neutrons. • Biology Asymmetry: aminoacids only righthanded chains • So there must be some mechanism which prefers matter to antimatter • Since there is so much matter (in terms of baryons) you might think that the mechanism must be very obvious – that is that it is a very large effect • But there are about 10 billion photons for every baryon in the universe • Where did these photons come from? • Baryon+anitbaryon -> two photons • So an asymmetry which leaves 1 baryon leftover for every 10 billion baryons would work fine
Sakharov Conditions • In 1967 Andrei Sakharov (father of the Soviet Bomb and later dissident) proposed three conditions that – if satisfied – would account for the propenderence of matter over antimatter • 1: Baryon number violation: There must be a way of making(or destroying) baryons that differs from making (or destroying) antibaryons • Possible in the early universe • 2: Must be a process which favors matter over anti-matter • CP violation. • CP is something called a transformation • To understand CP, we need to understand first the processes called C and P • The first two conditions can generate both baryons and an asymmetry of baryons over antibaryons. But we need another condition to “freeze” this situation in place to have what we observe today. • 3: The universe must fall out of thermal equilibrium, at the precise moment when baryon number switches from being efficiently violated, to being almost exactly conserved.
Charge Conjugation C and Time Reversal T + • Charge Conjugation, C • Charge conjugation turns a particle into its anti-particle • e+e-K-K+g g • Time Reversal, T • Changes, for example, the direction of motion of particles • t -t
Parity Transformation • Parity, P • Vectors change sign • The parity transformation changes a right-handed coordinate system into a left-handed one or vice versa. • Two applications of the parity transformation restores the coordinate system to its original state.
C and P Symmetry Can be Violated • You can apply the Charge conjugation transformation to a particle • Apply it to the electron: get a position: this exists • Apply it to a neutrino: • complication: there are only left-handed neutrinos or right handed anti-neutrinos • So C applied to a left-handed neutrino gives you a left-handed anti-neutrino. • But this particle does not exist • You can apply the Parity transformation to a particle. • Applying P to a “left-handed neutrino” generates a “right-handed neutrino” • But this particle does not exist! • As a result, it is said that the weak force (the only force that a neutrino feels) is not symmetric under the parity transformation • Turns out that the transformation CP does work for neutrinoes • CP(left handed neutrino) = right handed antineutrino
CP Symmetry • The CP symmetry appears to work in the weak force • The CP symmetry does work in both strong and electromagnetic forces • But to help explain matter antimatter asymmetry, we need CP violation • It turns out that CP symmetry is actually violated in some weak force cases… at a very low rate
CP can be violated • There is a particle called the KL (read K-long) • It has a well defined mass (and lifetime) • No other particle has such a mass • Therefore, the KL is it OWN anti-particle! • The KL decays in the following way • It decays both to and to , but slightly more often to the latter mode. Therefore, it violates both C and CP. So CP can be violated. • But the violation is really rare: like waving to yourself in a mirror one thousand times, and once your reflection waves back with the other hand! • Interesting aside: Say there was an alien, and you wanted to meet them • Are they made of matter or antimatter? How could you tell? • Hint: Ask them to look at how the KL decays….
BaBar • Why BaBar? • Bottom AntiBottom Assymetric Ring • The detector is designed to study Bottom-mesons • A meson is a combination of a quark and an antiquark • Bottom mesons contain one bottom quark • Why study bottom quarks? • CP violation is expected just like for KL decays • But it could (should) be much larger • Is the amount needed to explain matter-antimater assymetry in the universe?
Stanford Linear Accelerator Center (SLAC) The 3-km long linear accelerator in Stanford, California uses electromagnetic fields to accelerate electrons and positrons to close to the speed of light:
PEP-II Rings • The electrons and positrons are then guided into the two PEP-II storage rings (PEP stands for Positron Electron Project). The rings are located one on top of the other. Electrons go clockwise round the lower ring (which is an upgrade of the older PEP storage ring, which came into operation in 1980). Positrons go anticlockwise round the newly built upper ring.
A BaBar “Event” The B mesons live for about a billionth of a second, in which time they travel less than a millimetre. The BaBar detector observes the particles to which the Bs decay. From the decay products, the physicists can deduce which was the B and which was the anti-B. They can also measure how far and fast the Bs travel before decaying, and hence they can calculate their lifetimes.
The Unitarity Triangle Bdp+p-,r±p ,... (r,h) a V*tbVtd V*ubVud g b (1,0) (0,0) V*cbVcd BdJ/y Ks,D*±D,.. B D±K (rescale sides by 1/|V*cbVcd| and choose V*cbVcd real ) • Amazingly enough, studying CP violation in Bottom mesons can be reduced to measuring the sides and angles of a special triangle • The Unitarity triangle • BaBar is mostly interested in measuring the angle called • Actually measures sin(2) • Result is 0.74 +/- 0.07 The physicists are interested in the difference in the decay times of the and the . By observing millions of decays, they can build up a distribution of the differences in the decay times. It is predicted that the actual distribution will be different from that which you would get if there were complete symmetry between matter and antimatter.
A possible problem with all of this! • The first is that the CP violation of the Standard Model is far, far too weak to explain the matter-antimatter asymmetry. • There must be extra physics which introduces new CP violation. There are some strong limits on such new CP violation, which generally require it to occur via interactions which will be very hard to measure in future particle physics experiments. • In any case, the CP violation must involve new physics we don't know about.
Evidence that the laws of nature are not completely symmetric with respect to matter and antimatter first emerged in 1964, when a violation of the so-called charge-parity (CP) symmetry was observed in ephemeral particles known as K mesons, or kaons. Researchers discovered a tiny discrepancy between kaons and anti-kaons in the way they decay.