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Home , Strong interaction, Weak charge

From Before… EM

• Discussed the • Charged particles interact via the • All and have a ‘’ electromagnetic (EM) interaction – They interact through the weak interaction – A charged particle couples to the • Weak interaction often swamped by – It can also excite a photon (excited state of photon electromagnetic or . field) and lose energy. – Another charged particle can absorb the energy • Most clearly manifested in particle decays, from the photon field (photon disappears). where the weak interaction can change one particle into another. Only particles with an couple to the photon field. Essay Due Today

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Strong interaction Weak interaction • Particles with interact via the strong • Particles with weak(flavor) charge interact via the interaction weak interaction – A color charged particle(red, blue, green) couples to the – A flavor charged particle couples to the W+, W- and Z field. fields. – Includes quarks and – Includes all particles. can only interact via the weak – Fact that gluons can interact with other gluons to some interesting effects. – Strangest force. Only force that changes particles involved. Change must conserve charge and mass/energy. – Confinement, particle creation, range of the strong force. – For the W can be thought of as flipping the flavor: • Pulling apart quarks takes a large amount of energy. Like a very strong spring. Actually a string of gluons. • From up to down, from massive to • That energy can be used to make other particles, E=mc2. – Most often noticed when the weak force in the only force • Leads to short range of force that can act on the system. to , neutrinos – Massive nature of W and Z make weak force short lived Phy107 Fall 2006 3 and weak, Et~h Phy107 Fall 2006 4

Neutrino into Ice Cube • Neutrino has no electric or color charge • Interacts only via the - weak force. µ µ • How weak is weak? – Neutrino traveling in W+ solid would interact only once every 22 -years! – And weak force only d u “kicks in” for d <10-18 n u u p m, a distance ~ 1000 times smaller than the d d nucleus • But there are lots of time neutrinos, so it is possible to observe an interaction. • This is our method or studying the Phy107 Fall 2006 5 Phy107 Fall 2006 6

1 Changing flavors Quarks and the weak force Quarks have color charge, electric charge, and weak charge • Flavor change can occur spontaneously. • — other swamp the weak interaction – Experimentally, this occurs within a lepton generation • But similar to leptons, quarks can change their flavor (decay) via the weak force, by emitting a W particle. Generation I Generation II Generation III Charge Generation I Generation II Generation III Charge e— µ— — -1 u c t +2/3

e  µ 0 d s b -1/3 is stable Emit W- Emit W- + + 2x10-6 seconds 3x10-13 seconds Emit W Emit W 2x10-12 seconds 10-23 seconds

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Flavor change between generations Decay of heavy quarks  + • But for quarks, not limited to within a generation Top decays so fast -23  (10 s), it doesn’t have • Similar to leptons, quarks can change their flavor (decay) W + time to form a . via the weak force, by emitting a W particle. + t b t  b +  +  - µ B— particle decays Generation I Generation II Generation III Charge  -12 W - µ within 1.5x10 s. _ +2/3 - 0 - u c t b c B  D + µ + µ u u d e + s b -1/3 The D0 meson decays W + e within 0.5x10-12 s. Emit W- Emit W- This decay: 10-12 seconds s c D0  K- + e - +  u u e Phy107 Fall 2006 9 Phy107 Fall 2006 10

Put all the together discovery 1995 Think of the gluons being exchanged as a u ! spring… which if stretched too far, will snap! u • Proton- Use stored energy in spring to create mass. - u K collision at u u s u u s • Only final decay u u s u K+ products are u observed. W- u u u • Infer existence of u d other particles by d - d  thinking about d Stretch the spring: d dd decays. turn kinetic into More stretch, Spring ‘snaps’. d more stored Use energy to 0 energy. create uu pair  d Phy107 Fall 2006 11 Phy107 Fall 2006 12

2 What the detector sees Particles & their Interactions (Summary) Charged Neutral • These are the only quarks leptons leptons objects observed. (e,µ,) () • Everything else Color must be Y NN extrapolated. Charge ? EM • Build on known N reactions. Charge ? Y Y ‘Weak’ Y YY Charge ?  Quarks can participate in Strong, EM & Weak Interactions.  All quarks & all leptons carry weak charge.

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Comparison of the Force Carriers Something interesting about the weak interaction EM Strong Weak • As far as the weak interaction goes, leptons and Force Photon Gluon quarks are basically identical. W+, W- Z0 Carrier () (g) • All carry a weak charge. Charge of • All six quarks can change flavor via the weak force None Color Electric None carrier interaction. Within a generation or between Particles Particles generations. Particles Particles w/weak charge w/weak charge Couples to: w/elect. w/color charge • Leptons can change flavor within a generation, charge (Quarks,gluons) (Quarks, (Quarks, leptons, W,Z) leptons W,Z) and neutrinos between generations(discovered Infinite <10-14 m recently by looking at neutrinos from the sun). Range < 2x10-18 m < 2x10-18 m (1/d2) (inside hadrons) • So maybe all quarks and leptons are just different ‘states’ of the same ‘master particle’

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And several different interactions

• Remember that interactions are due to • It may be possible that all quarks and exchange of . leptons can be viewed as different • EM interaction - exchange components of the same particle. • Weak interaction - exchange W+, W—, Zo • Also may be possible to unify the forces • Strong interaction - exchange gluons (8) (exchange bosons). • But are they so different? • Electromagnetic and Weak force have already been unified. • People working hard to include the strong force and gravitational force in this.

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3 Exchange Bosons (force carriers) Electro-weak unification

The says that the electromagnetic interaction (photon exchange) & EM Weak the weak interaction (W+, W-, Zo exchange) are different pieces of the same

+ e - W e W

e e Neutral weak Electromagnetic Pos. weak Neg. weak • Zero charge • Zero charge • Pos. charge • Neg. charge • Mass=91 GeV/c2 • Mass=0 GeV/c2 • Mass=80 GeV/c2 • Mass=80 GeV/c2 • Range ~ 10-18 m • Range ~ inf. • Range ~ 10-18 m • Range ~ 10-18 m

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Some similarities here Similar indeed

o  • The Z and photon interactions are so similar that W+  e W- e they are very difficult to distinguish experimentally. e e • One of the ideas behind the Standard Model is that Neutral weak Electromagnetic Pos. weak Neg. weak particles should follow regular and • Zero charge • Zero charge • Pos. charge • Neg. charge explainable patterns or symmetries. • Mass=91 GeV/c2 • Mass=0 GeV/c2 • Mass=80 GeV/c2 • Mass=80 GeV/c2 • Range ~ 10-18 m • Range ~ inf. • Range ~ 10-18 m • Range ~ 10-18 m • A pattern that would account for two changed weak force carriers also called for a neutral particle: These two both These two both The Zo a neutral particle much like the photon. exchange neutral bosons exchange charged bosons. Neither Both bosons • The Zo was predicted by the Standard Model, and changes the lepton flavor change the lepton flavor then found experimentally. (remains electron)

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Symmetries in the SM breaking • The standard model says that at high energies, • The standard model is based on symmetries, this symmetry is apparent but they are a little subtle. – We see a single electroweak interaction. o • Similarities between photon and Z interactions – Zo and  interact exactly the same way with the same point to a common source. strength. • Electroweak force with two charges • At low energies the – This is (‘flavor charge’)x(‘electric like charge’) symmetry is broken – We see distinct • This results in four exchange bosons. electromagnetic and –W+, Zo, W- for flavor charge weak interactions –  for electric like charge • However needs one more element. Something to give the W and Z mass Phy107 Fall 2006 23 Phy107 Fall 2006 24

4 Mass What is mass?

• Here’s the experimental • Think of inertial mass: masses of SM particles. – inertial mass is a particle’s • Original SM gives zero resistance to changes in velocity. mass for all particles. • When you apply the same force to particles, • But can give particles the smaller the mass, the larger the mass by to a new field, the Higgs field. acceleration. • is the • What is the origin of mass? (unobserved) quanta of the Higgs field.

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Mass in the SM Mass and the Higgs field • In the standard model (SM), Imagine a party in a room particles have mass because they interact packed full of people. with something that pervades the .

This something is the Higgs field Particles ‘hit’ the Higgs field when you try to accelerate them Now a popular person enters the room, attracting a Mass = cluster of hangers-on that (chance of hit) x (Higgs density) impede her motion she has become more massive

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The Higgs boson The Higgs Boson

The Higgs boson is a quantum excitation of the Higgs field. How much mass do you thing the Higgs Boson In analogy, suppose an interesting rumor is shouted in thru the door. has

A. No mass B. Light like an up or The people get quite excited. They cluster to pass on the rumor, and the C. Very massive like a cluster propagates thru the room. top quark Looks very similar to the popular/massive person who entered the room Good way to think of other quantum excitations. All the other force carriers Phy107 Fall 2006 29 Phy107 Fall 2006 30

5 How can we ‘see’ the Higgs? Grand Unified Theories • The Higgs boson needs to be created in order to see it. E = mc2 • What do we really need to unify ? • Not found yet • Maxwell unified the electric and magnetic interactions into electromagnetic (EM) •mH > 114GeV

•mH < 186GeV • The standard model unified the EM and weak interactions into the electroweak interaction

e- Zo • Start with the strong force. Zo • What kind of theory is needed to unify this?

H e+

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Not all that easy More Unifications?

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Grand Unified Theories The price of unification

• Flavor changing interactions in quarks • When the SM unified EM and weak interactions, we (e.g. changing a top quark to a by ended with more force-carrying bosons (e.g. the Zo) emitting a W+) suggest that quarks can be viewed as different ‘orientations’ of the same object. • This is because our fundamental ‘particle’ increased in complexity • Have found the same thing for leptons. – e.g. from an electron to an electron-neutrino pair • But maybe there should be a lepto-quark field? • If our ‘particle’ now encompasses both leptons and – Quarks could turn into leptons, leptons into quarks quarks, the interaction also becomes more complex. – All matter particles would be different ‘orientations’ of the same fundamental object. • In one particular GUT, we get 24 exchange bosons • If we unify leptons and quarks then weak and (W+,W-,Z0, photon, 8 gluons, and 12 new ones) strong forces may be shown to be two aspects of one force. Phy107 Fall 2006 35 Phy107 Fall 2006 36

6 Summary • Details of weak interaction suggest that – Different quarks and diff. leptons are diff. ‘orientations’ of the same particle. – Weak and EM interactions are diff. parts the ‘electroweak’ force. •Mass – Particles get mass by interacting with Higgs field – Higgs boson is an excitation of the Higgs field • Grand Unified Theories (GUTs) – Will ‘combine’ letptons and quarks – Unify strong and electroweak interactions • What’s beyond the Standard Model.

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