{[ promptMessage ]}

Bookmark it

{[ promptMessage ]}


Chen+-+Theoretical+Particle+Physics - The Standard Model of...

Info iconThis preview shows page 1. Sign up to view the full content.

View Full Document Right Arrow Icon
This is the end of the preview. Sign up to access the rest of the document.

Unformatted text preview: The Standard Model of Particle Physics & Beyond Mu-Chun Chen Theoretical Particle Physics Group News · Physical sciences · Technology The Atom-Smasher: Large Hadron arch 2008 (LHC) Wednesday, 26 M Collider • located at CERN (European Organization for Nuclear Research) near Geneva • lear ers: biggest atomsmasher in the world • e to 27 km long ringshape tunnel; 175m below ground es) . ut ll s de Accelerated particles: Housed in a 27 kilometre- circumference tunnel under the Large Hadron Collider (LHC) • 10-hr exp: • • • 46 m (L) x 25 m (H); 7k tons CMS @ full intensity: each beam will have energy equivalent to a car traveling at 1600 km/h • • beam travel 10 billions km • ATLAS: • use up 120 megawatts of power colliding protons: generate 14 Tera-eV Large Hadron Collider (LHC) Return < first] < prev ] [Dr. Boris Kayser, Fermilab 03] [ NEXT> [ last > Fundamental Building Blocks We, and all every day objects, are made out of three kinds of tiny particles they are bundled up to make atoms < first] < prev ] [Dr. Boris Kayser, Fermilab 03] [ NEXT> [ last > Atoms: building blocks of us, building, planets... But is the entire Universe made out of --electrons protons neutrons ??? Eternal Questions • What is the world made of? • What holds it together? • matter of the world made out of a few fundamental (simple and Is the Atom Fundamental structureless) building blocks of nature amental ? • on realized that they gorize atoms into t shared similar roperties (as in the able of the Elements). ted that atoms were f simpler building mpler building blocks etermined which erties. looked" into an atom that atoms had ishy balls. These etermine that atoms nucleus and a cloud snomer. Why? Answer atoms -> nucleus + telectron -> protons + ucleus Fundamental ? What s Fundamental ? Is the Nneutrons -> quarks What is Fundamental ? Fundamental ? Are Protons and Neutrons Is the Nucleus Fundamental ? Physicists have discovered that protons and neutrons are composed of even smaller par called q dense, Because it appeared small, solid, anduarks . scientists originally thought that the nucleus was fundamental. Later, As far as we know, quarks are like points in geometr they discovered that it was made of protons (p +), which They're not made up of anything else. are positively charged, and neutrons (n), which have no charge. After extensively testing this theory, scientists now suspect So, then, are protons and neutrons fundamental? that quarks a the electron (and a few other things w see in a minute) are fundamental. What is Fundamental ? What is Fundamental ? The Modern Atom Model The Scale of the Atom Scale of the atom. This is the modern ato model. While an atom is tiny, the nucleus is ten thousand times smaller than the atom and the quarks and electrons are at least ten thousand times smaller than that. We don't know exactly how small quarks and electrons are; they are definitely smaller than 10- 18 meters, and they might literally be points, but we do not know. Electrons are in constant motion aro nucleus, protons and neutrons jiggle It is also possible that quarks and the nucleus, and quarks jiggle withi electrons are not fundamental after all, and will turn out to be made up of protons and neutrons. other, more fundamental particles. (Oh, will this madness ever end?) This picture is quite distorted. If we atom to scale and made protons an a centimeter in diameter, then the electrons and quarks would b than physicists constantly look for new particles and categorize • the diameter of a hair and the entire atom's diameter would them to the l find patterns that ootball fields! 99.9999999999 greater than try toength of thirty ftell us about how the fundamental building mpty of nature atom's volume is just eblocks space! interact The Standard Model • physicists has developed a theory called the Standard Model that explains what the world is and what holds it together • it explains all these complex interactions with only • • • • • • 6 quarks 6 leptons force carriers all the known matters are composites of quarks and leptons and they interact by exchanging force carrier particles quarks behave very differently from leptons for each matter particle, there is a corresponding antimatter particle Matter and Anti-matter • for every type of matter particle we have found, there also exists a corresponding antimatter particle (antiparticle) • antiparticles look and behave like the corresponding matter particles • antiparticles and particles have opposite charges • proton (+1) antiproton (-1) Made o•? when aatter and an antiparticle meet: they annihilate into pure f M particle and Antimatter energy r particle we've a corresponding antiparticle . ehave just like their articles, except they . For instance, a proton is electrically positive is electrically negative. Gravity affects matter and y because gravity is not a charged property and a Matter and Anti-matter • • Universe appear to be composed entirely of matter If matter and antimatter are exactly equal but opposite, why is there so much more matter than antimatter? • theorists have been trying to answer this question for four decades... • • neutrino may play an important role! LHC may provide answer Matter Particles What is the World Made of? • • Quarks Quarks most of the matters around us, like protons and neutrons, are made from quarks Quarks are one type of matter particle. Most of the matter we see around us is made from protons and neutrons, which are composed of quarks. there are six of them grouped into three pairs (and corresponding antiquarks) There are six quarks , but physicists usually talk about them in terms of three pairs: up/down , charm/strange , and top/bottom. (Also, for each of these quarks, there is a corresponding antiquark.) Be glad that quarks have such silly names - - it makes them easier to remember! Quarks have the unusual characteristic of having a fractional electric charge, unlike the proton and electron, which have integer charges of +1 and - 1 respectively. Quarks also carry another type of charge called color charge, which we will discuss later. • • The most elusive quark, the top quark, was discovered in 1995 after its existence had been theorized for 20 years. to make theory work: fractional charge Want to see a particle physicist's idea of a good pun? the most elusive one, top quark, was discovered at Fermilab in 1995 after its existence has been theorized for 20 years • • What is the World Made of? : Hadrons, Baryons, and noMesons quarks have been found: only composites made of quarks isolated have been found => hadrons Like social elephants, quarks only exist in groups with other quarks and are never found alone. Composite particles made of quarks are called individual quarks have fractional charges; they combine such that hadrons have integer charges the World Made of? : Hadrons, Baryons, auarks have fractional electrical charges, they Although individual q nd • combine s quarks carry a net integer electric charge. Another have individualuch that hadronsthey hcolornet color charge evencombine such that hadrons charge; they though the property of hadrons is that ave no quarks hemselves carry charge l elephants, quarks only existdo roups with other quarks and charge (we will talk more about this later). in g not thave color color found alone. Composite particles made of quarks are called There are two classes of hadrons (try putting your mouse on the elephants): • two types of hadrons: • Baryons (q q q): proton (uud), neutron (udd) ndividual quarks have fractional electrical charges, they uch that hadrons have a net integer electric charge. which is made ...are any hadron Another f hadrons is that they have no net color charge even though the of three quarks (qqq). mselves carry color charge (we will talk more about this later). ...contain one quark (q) and one antiquark ( ). two classes of hadrons (try putting your mouse on the : y hadron which is made quarks (qqq). • Because they are made of two up quarks and one down quark ...contain oneprotons are baryons. So (uud), quark (q) and one antiquark (neutrons (udd). are ). One example of a meson is a pion Mesons (q anti-q): unstable hich is made of an up quark ( +), w and a down anitiquark. The antiparticle of a meson just has its quark and antiquark switched, so + antipion ( -) is made up a an down quark and an up antiquark. ¯ π (ud) they are made of two s and one down quark otons are baryons. So rons (udd). One example of a meson is a pion ( +), which is made of an up quark and a down anitiquark. The antiparticle of a meson just has its Because a meson consists of a particle and an antiparticle, it is very unstable. The kaon (K- ) meson lives much longer than most mesons, which is why it was which do not. They appear to be point- like particles without internal structure. The best known lepton is the electron (e - ). The other two charged leptons are the muon( ) and the tau( ), which are charged like electrons but have a lot more mass. The other leptons are the three types of neutrinos ( ). They have no electrical charge, very little mass, and they are very hard to find. Leptons • Quarks are sociable and only exist in composite particles with other quarks, whereas leptons are solitary particles. Think of the charged six leptons leptons as independent cats with associated neutrino fleas, which are very hard to see. • • • • For each lepton there is a corresponding antimatter antilepton. Note that the anti- electron has a special name, the "positron." electrically charged: electron, muon, tau Trivia: "Lepton" comes from the Greek for "small mass," but this is a misnomer. Why? [ Answer ] electrically neutral: three types of neutrinos unlike quarks, leptons are solitary heavier leptons (muon and tau) are not found in ordinary matters: once produced, they decay very quickly into lighter leptons Leptons are divided into three lepton families: the electron and its neutrino, the muon and its neutrino, and the tau and its neutrino. We use the terms "electron number," "muon number," and "tau number" to refer to the lepton family of a particle. Electrons and their neutrinos have electron number +1, positrons and their antineutrinos have electron number - 1, and all other particles have electron number 0. Muon number Leptonsnumber operate analogously with the other two lepton families. and tau are divided into three lepton families: Leptons • • • eptons, t electron electron lneutrino (same e muon umber, uon and tau) • One important+thing aboutare alwayshen, is thatforlectronanmassivemlepton number, and tau number conserved when decays into smaller ones. lepton family number: electron number, lepton number, tau number whenLet's take an edecay intodlighter one, lepton family numbers are massive leptons xample ecay. conserved decays into a muon neutrino, an electron, and an A muon electron antineutrino: As you can see, electron, muon, and tau numbers are conserved. These and other conservation laws are what we believe define whether or not a given hypothetical lepton decay is possible. Neutrinos • Leptons are divided into three lepton families: • electron + electron neutrino (same for muon and tau) • neutrinos have no electrical or strong charge: almost never interact with any other particles • fusion inside the Sun produces neutrinos: they pass right through the Earth • they were produced in great abundance in early Universe and rarely interact with matter, there are a lot of them in the universe • their tiny mass but large number may contribute to total mass of the universe and affect the expansion of the universe • neutrinos oscillate! Neutrino: an Abundant yet Elusive Particle Return < first] < prev ] [Dr. Boris Kayser, Fermilab 08] [ NEXT> [ last > 10 million neutrinos from the Big Bang within each cubic foot of space Within each person: ~ 30 million Big Bang neutrinos < first] < prev ] [Dr. Boris Kayser, Fermilab 08] [ NEXT> [ last > Neutrinos from the Sun Every second passing through each person on Earth: one hundred trillion neutrinos from the Sun The Sun shines because of fusion reactions in its core: fusion reactions produce -- • energy, including visible light • neutrinos • atoms more complicated than hydrogen Are Neutrinos harmful? Do we have to worry about all these 100,000,000,000,000 neutrinos zipping through us every second? No. They interact with matters very feebly. To neutrinos, we look like empty space. Typically, a solar neutrino would have to zip through 10,000,000,000,000,000,000 people before doing anything. Probability of a solar neutrino interacting with one of us: 1/10,000,000,000,000,000,000 This feebleness makes them very hard to detect!! Interactions What Holds it Together ? How Does Matter Interact? Standard Model: Electromagnetism One tricky question that plagued physicists for many years was... What Holds it Together ? Electromagnetism • • • there are four fundamental interactions between particles How do matter particles interact? all forces can be attributed toroblem is tfour interactions touching! How do The p these hat things interact without • Electromagnetism: two magnets "feel" each other's presence The electromagnetic force causes like - chargedand attract or repel accordingly? ow does - charged things t earth sun a epel at fundamental level, athings to rany ething that tissuch asttractbetween two force is a andHoppositelyhe passed theoand ? attract. M veryday forces, friction, We know theare caused bhese questions are "magnetism" and answers to t y the particles => force carriermagnetism, even "gravity," but r E- M force. F f instance, the electromagnetic, owhat are theseororces? force that keeps you from falling through the floor is the eAt a fundamental lwhich causes the atoms something lectromagnetic force evel, a force isn't just making uphappens to pour feet and its afloor to which is displaced. that the matter in yarticles. It he thing resist being What Holds it Together ? passed between two particles. Residual E- M Force The carrier particle of the electromagnetic force is the photon ( ). P o protons ifferent energies span t Atoms usually have the same numbers hotons of dand electrons. They arehe electromagnetic spectrum of x rays, force tcarrier bparticlef =ositive rprotons cancel outothe electrically neutral, herefore, ecausevisible lphoton w(massless, orth. the p ight, adio aves, and s f travel at speed of light) negative electrons. Since they are neutral, what causes them to stick together to form stable molecules ? Photons have zero mass, as far as we know, and always travel at the Residual EM force: charged of light", ofwoneisatom 3interact with charged or "speed parts c, hich about 00,000,000 meters per second, The answer is a bitanothere've discoveredbindhe er second,artsaovacuum. molecules parts of strange: w atom186,000that t the atoms to form => miles p charged p in f one atom can interact with the charged parts of another atom. This allows different atoms to bind together, an effect called the residual electromagnetic force . • • So the electromagnetic force is what allows atoms to bond and form molecules, allowing the world to stay together and create the matter you interact with all of the time. Amazing, isn't it? All the structures of the world exist What ts Fundamental ? Is the Nucleus Fun Standard Model: strong interactionFundamental? Is the Nucleus • What bind the nucleus? • • neutrons: electrically neutral protons: positively charged Because it appeared small, solid, and de originally thought that the nucleus was fu they discovered that it was made of prot are positively charged, and neutrons (n), charge. • quarks have color charge: force between color So, then,particles is very neutrons f charged are protons and strong => Strong interaction • • force carrier of strong interactions: gluons color charge behaves very differently than electromagnetic charge • • • • photon: electrically neutral gluons: have color charge composite particles made of quarks have no net color charge strong force takes place only at very small scale (quark interactions), but not observable in everyday life are close to one another, they exchange gluons and create a very strong color force field that binds the quarks together. The force field gets stronger as the quarks get further apart. Quarks constantly change their color charges as they exchange gluons with other quarks. Standard Model: strong interaction What Holds it Together ? • • • • • Quark Confinement Color - charged particlesparticles How doesgluons areharge workolorc-annot be qound individually. quarks and color c color-charged? charged f uarks are For this reason, the c confined in groups (hadrons) with other quarks. These composites are color neutral. The development of the Standard Model's theory of the strong interactions reflected evidence that quarks combine only into baryons (three quark objects), and mesons (quark - antiquark objects), but not, for example, four- quark objects. Now we understand that only baryons (three different colors) and mesons (color and anticolor) are color- neutral. Particles such as ud or uddd that cannot be combined into color- neutral states are never observed. when two Color-Force ield and one corresponding anticolor quarks charges There are three colorareFclose to threeanother, they exchange gluons and (complementarystrongccolor force field that bindsothe quarksctogether create a very color) harges. Each quark has one f the three olor charges and The quarksntiquarkaadrontheadly eo-fforce field whichFnticolor charges. Just each a in a given h has m ne xchangetgluons.a or this o color the hree reason, physicists talk bout as a mix of red, green, gand holding ltight yields white light,they baryon a gluons consists of change their unch of charges as quarks constantlythe luons blue he bcolorquarks together. in a exchange combination If onered,"q"green,"iven hadron is pulled awaycfharges is color neutral, of " of the and "blue" color rom its with otherneighbors, theuarks -in a g field "stretches" between that quark and itsis also color quarks "antired," "antigreen," and "antiblue" and in an antibaryon color force neighbors. n neutral. Mesonsas treIqcso doing,eutralpart.oresenergytihey is etnergetically-force ahe uarks are pulled and m At ome point,ddedarry color olor n more a because s a it c o the c ombinations such field this force cheaper for the color- as field "snap" into get quark -antiquark pair. as "red" andgets stronger force thetoquarks a new further apart => confinement "antired." In so doing, energy is conserved because the energy of the color- force field is converted into the mass of the new quarks, and the color- force Because g state. field can "relax" back to an unstretched luon - emission and - absorption color-charged particles cannot be found individually always changes color, and - in addition - color is a conserved quantity - gluons can be thought of as carrying a color and an anticolor charge. Since there are nine possible coloranticolor combinations we might expect nine different gluon charges, but the mathematics works out such that there are only eight Quarks cannot exist individually because the color force increases combinations. Unfortunately, there is no as they are pulled apart. intuitive explanation for this result. There are six kinds of quarks and six kinds of leptons. But all the stable matter of the universe appears to be made of just the two least- massive quarks (up quark and down quark), the leastmassive charged lepton (the electron), and the neutrinos. Standard Model: weak interaction Weak interactions are responsible for the decay of massive quarks and leptons into lighter quarks and leptons. When fundamental particles decay, it is very strange: we observe the particle vanishing and being replaced by two or more different particles. Although the total of mass and energy is conserved, some of the original particle's mass is converted into kinetic energy, and the resulting particles always have less mass than the original particle that decayed. • all stable matters made of just the two least massive quarks (up and down) and least massive charged lepton (electron) and neutrinos • weak interactions: decay of massiveThe only matter around us that is stable islight up of the smallest quarks quarks and leptons into made and leptons, which cannot decay any further. quarks and leptons When a quark or lepton changes type (a muon changing to an electron, for instance) it is said to change flavor . All flavor changes are due to the weak interaction. • The carrier particles of the weak interactions are the W +, W - , and the Z particles. The W's are electrically charged and the Z is neutral. The Standard changes are due a quark or lepton change type: flavor change; all flavor Model has united electromagnetic interactions and weak interactions into one unified interaction called electroweak to weak interaction: Quarks -- Nobel 2008; .Leptons -- Nobel 2002 • force carrier: (electrically charged) W+, W-, (electrically neutral) Z bosons • SM has united EM and weak interactions => electroweak interaction • • at 10-18m, strength of weak interaction and EM compatible at 10-17m, strength of weak interaction is 10-4 EM interaction What Holds it Together ? Interaction Summary Standard Model: force carriers This is a summary of the different interactions, their force carrier particles, and what particles they act on: Which fundamental interaction is responsible for: Friction? Unsolved Mysteries: What could be discovered at the LHC? Unsolved Mysteries of Standard Model • generation of masses? • • Unsolved has a certain mass SM cannot explain why a particleMysteries What about Masses? The Standard Model cannot explain why a particle has a certain mass. For example, both the photon and the W particle are force carrier particles: why is the photon massless and the W particle massive? force carriers: photon -> massless F e r m io n m a s s g e n e r a tio n W boson -> massive Physicists have theorized the existence of • • the so- called Higgs field, which in theory interacts with other particles to give them et mass. The Higgs field requires a particle,L he Higgs boson. The Higgs boson has not been observed, but physicists are looking for it with great enthusiasm. matters: all quarks and leptonswith masses • Universe is filled have Higgs BEC Higgs boson - the “God Particle” (Nobel 2008) • Left-handed and right-handed interact with other particles to give masses particles mix and bump has not been observed BEC to into Higgs acquire a mass likely will be observed at the LHC • But neutrinos can’t bump because there’s no right-handed one ! massless eR <h> • • • 0 .5 1 1 M e V / c 2 105 M eV / c 2 178,000 MeV/c2 5 Unsolved Mysteries of Standard Model • Why are there three generations? tandard Model • (1)Y ggs boson k symmetry or charged ) U(1)EM tions of • • • three sets of quarks and leptons generations increase in mass higher generation particles decay into lower generation particles everyday world: observe only first generation particles (electrons and up/down quarks) u c t g d s b W e µ " Z !ee !µ µ !"" # II III I heavy Force carriers 4 Mass spectrum of elementary particles u d L M A - M S W s o lu tio n "1 "2 "3 m eV e c t s b µ ! "3 n o r m a l h ie r a r c h y "2 "1 in v e r te d h ie r a r c h y "3 "2 "1 eV n e a r ly d e g e n e r a te keV M eV G eV T eV 28 U ltim a te G o a l o f G r a n d U n ifi c a tio n • Maxwell: electric and magnetic forces different aspects of electromagnetism • E in s te in : e a r ly a tte m p t to u n if y e le c tr ic f o r c e a n d g r a v ity W e a r e g e ttin g th e r e .. … Electroweak theory Quantum Mechanics Electric force Magnetic force Gravity Grand Unified Theory Quantum electrodynamics Electromagnetism Weak force Strong force String Theory or something else ????? Special relativity GR 24 Ultimate Goal of Grand Unification • Grand Unified Theory: • • • Unsolved Mysteries Forces and GUT Physicists hope that a Grand Unified Theory will unify the strong, weak, and electromagnetic interactions. There have been several proposed Unified Theories, but we need data to pick which, if any, of these theories describes nature. existence of another force-carrier particle that lead to proton decay If a Grand Unification of ll the interactions the (such decays are very rare; a proton’s life time iswmore athan 1032is possible, sthen allunified interactions e observe are all different aspects of the ame, interaction. However, how can this be the case if strong and weak and years) electromagnetic interactions are so different in strength and effect ? Strangely enough, current data and theory suggests that these varied forces merge into one force when the particles being affected are at a high enough energy. The w akness of gr for decades experimental searches foreproton decaysavity How about the gravity?onsider 2 protons 1 cm apart: C • to unify, forces must have equal strength The weakness of gravity • gravity is very weak FEM = = 2.3 × 10−19 dynes Consider 2 protons 1 cm apart: q2 (4.8 × 10−10 esu)2 FEM = 2 = r (1 cm)2 m2 8 cm w g on s− s Fgrav = GN 2 = 6.7 × 10−Current 3 ork−1 GUT2 uggests the existence of another force- carrier r particle that causes the proton to decay. Such decays are extremely rare; (1.7 × 10−24 g)2 × (1 cm)2 = 2.3 × 10−19 dynes m2 Fgrav = GN 2 = 6.7 × 136 8 cm3 g−1 s−2 0− r = 1.9 × 10−55 dynes gravity is 10 times weaker! (1.7 × 10−24 g)2 × (1 cm)2 q2 (4.8 × 10−10 esu)2 = r2 (1 cm)2 Gravity is 1036 times weaker! a proton's lifetime is more than 1032 years. Unsolved Mysteries of Standard Model • Supersymmetry • • • • • unify gravity and other fundamental forces every fundamental matter particle should Supersymmetry Unsolved Mysteries have a massive superpartner Some physicists attempting to unify gravity with the other fundamental forces have quarks squarks , come to a startling prediction: every etc... fundamental matter particle should have a massive "shadow" force carrier particle, and every force carrier should have a massive no supersymmetric particle has been foundrelationship "shadow" matter particle. This between matter particles and force carriers is called supersymmetry . F could be discovered at LHC or Tevatronor example, for every type of quark there may be a type of particle called a "squark." No supersymmetric particle has yet been found, but experiments are underway at CERN and Fermilab to detect supersymmetric partner particles. goes around the rope and winds up where it started. So the acrobat has one dimension, and the flea has two dimensions, but one of these dimensions is a small closed loop. Unsolved Mysteries of Standard Model So the acrobat cannot detect any more than the one dimension of the rope, just as we can only see the world in three dimensions, even though it Extra DImensions might well have many more. This is impossible to visualize, precisely because we can only visualize things in three dimensions! • Image:DarkMatterPie.jpg From Wikipedia, the free encyclopedia Nature of Dark Matter? Image File history File links No higher resolution available. DarkMatterPie.jpg! (590 " 329 pixels, file size: 25 KB, MIME type: image/jpeg) This is a file from the Wikimedia Commons. The description on its description page there is shown below. Commons is a freely licensed media file repository. You can help . Beschreibung Description Retrieved from NASA online: [1] (http://www.nasa.gov/vision/universe/starsgalaxies/Collision_Feature.html) PeteSF 23:28, 22 ...
View Full Document

{[ snackBarMessage ]}

Ask a homework question - tutors are online