fri. dec 10phy107 lecture 37 from last time…. fri. dec 10phy107 lecture 37 today unification of...

Fri. Dec 10 Phy107 Lecture 37

From Last Time…


Today• Unification of the electromagnetic and

the weak force – the Standard Model

• Where do particles get mass?– The Higgs mechanism and the Higgs boson(s)

• Prospects for unifying all of particle physics– Grand Unified Theories


Fundamental Matter Particles

Three generations of leptons and quarks

All feel theweak force.

Charged leptons feel weak & EM forces

Quarks feel weak, EM, & strong


Similar particles…• All six quarks can change flavor

via the weak interaction.• Leptons can change flavor within a

generation, and neutrinos between generations

• View 12 fundamental matter particles as only two, with different flavors being different ‘orientations’.


And several different interactions

• Remember that interactions are due to exchange of bosons.

• EM interaction - exchange photons• Weak interaction - exchange W+, W—, Zo

• Strong interaction - exchange gluons (8)


Exchange Bosons (force carriers)

EM Weak


Electro-weak unificationThe standard model says that

the electromagnetic interaction (photon exchange) &the weak interaction (W+, W-, Zo exchange)

are different pieces of the same electroweak interaction

Neutral weak Electromagnetic• Zero charge

• Mass=91 GeV/c2

• Range ~ 10-18 m

•Zero charge•Mass=0

GeV/c2

•Range ~ inf.

Pos. weak Neg. weak• Pos. charge• Mass=80

GeV/c2

• Range ~ 10-18 m

•Neg. charge•Mass=80

GeV/c2

•Range ~ 10-18 m

W+

W-

e

e


Some similarities here

These two both exchange neutral bosons

Neither boson changes the lepton flavor (remains electron)

Neutral weak Electromagnetic• Zero charge

• Mass=91 GeV/c2

• Range ~ 10-18 m

•Zero charge•Mass=0

GeV/c2

•Range ~ inf.

Pos. weak Neg. weak• Pos. charge• Mass=80

GeV/c2

• Range ~ 10-18 m

•Neg. charge•Mass=80

GeV/c2

•Range ~ 10-18 m

W+

W-

e

e

These two both exchange charged bosons.

Both bosons change the lepton flavor


Similar indeed• In fact, the Zo interaction and the

photon interaction are so similar that they are very difficult to distinguish experimentally.

• The Zo was predicted by the Standard Model, and then found experimentally.

• It’s existence was force by symmetries on which the standard model is based.


Symmetries in the SM• The standard model is based on

symmetries,but they are a little subtle.

• The symmetry of the electroweak interaction is SU(2)xU(1)– This is (flavor)x(‘charge’)

• This results in four exchange bosons.– W+, Wo, W- for flavor– Bo for ‘charge’


Symmetry breaking• The standard model says that at high

energies, this symmetry is apparent– We see a single electroweak interaction.

• At low energies it is broken– We see distinct electromagnetic and weak

interactions– Wo and Bo mix together,

giving electromagnetic photon interaction & Zo gives part of electroweak.


• The W0 has the same quantum numbers as the photon, so mixing occurs leading to a unified electroweak theory.

• Pure SU(2)L Bosons (Weak Isospin=1)

• W+, W0, W-

• Pure U(1) Boson (Weak Isospin=0)

• B0

• The actual observed bosons are the g and the Z0.

• g = cosqW B0 - sinqW W0

• Z0 = sinqW B0 + cosqW W0

• Both the Z0 and the g are part SU(2) and part U(1) gauge bosons, so the coupling of the Z0 depends on both the electric charge and the T3 of the fermions. i.e. The coupling of the Z0 to "up" type quarks is different from the coupling to "down" type quarks, which is different from charged leptons, which is different from neutrinos.


Particle mass in the SM• The standard model has great successes,

but in its first incarnation all particles were required to have zero mass.

• Otherwise the mathematics gives non-physical results.

• Clearly a major problem, since many particles do have mass.


Mass• Here’s the experimental

masses of the SM particles.

• Original SM gives zero mass for all particles.

• But can give particles mass by coupling to a new field, the Higgs field.

• Higgs boson is an (unobserved) excitation of the Higgs field.


What is mass?• What is Mass?• Think of inertial mass: inertial mass is a

particle’s resistance to changes in velocity.

• When you apply the same force to particles, the smaller the mass, the larger the acceleration.

• What is the origin of mass?


Mass and the Higgs fieldImagine a party in a room

packed full of people.Nobody is moving around much,

just standing and talking.

Now a popular person enters the room, attracting a cluster of

hangers-on that impede her motionAs she moves she attracts the people she comes close to- the ones she has

left return to their even spacing.Her motion is impeded - she has

become more massive.


• In this example, the popular person plays the role of an electron, or a W boson, or any particle.

• The people in the room represent the Higgs field.• The interaction with the Higgs field gives the

particle its mass• A particularly unpopular person could move

through the room quite easily, with almost no mass. This is the analog of a low-mass particle.

• In present theories, the ‘popularity’ of each particle is an input parameter.


The Higgs bosonThe Higgs boson is a quantum

excitation of the Higgs field.In analogy, suppose an

interesting rumor is shouted in thru the door.

The people get quite excited.They cluster to pass on the rumor, and the

cluster propagates thru the room.Since the information is carried by clusters

of people, and since it was clustering which gave extra mass to the popular

person, the clusters also have mass.


• The Higgs boson is predicted to be just such a clustering in the Higgs field.

• We will find it much easier to believe that the field exists, and that the mechanism for giving other particles mass is true, if we actually see the Higgs particle itself.


Symmetry breaking again• The theory says that the Higgs field has

a‘vacuum expectation value’.

• That is, the Higgs field is always nonzero.• This is different than other fields we have

talked about.• At high energies, this is not true, and all

particles become massless again.


Higgs boson: use at your own risk

• The Higgs Boson wheel isn't really a wheel. It isn't even designed to roll on; it's designed to grind with.

• *WARNING- THE HIGGS BOSON IS NOT A TRADITIONAL INLINE-SKATING WHEEL. THE SHAPE AND URETHANE ARE NOT DESIGNED FOR ROLLING. USE AT YOUR OWN RISK.


Beyond the standard model?

• Standard model has been enormously successful.

• Consistent picture of particles and their interactions.

• Predictive power with unusual accuracy.• But…

– SM w/ Higgs mechanism has 19 input parameters.

– No suggestion of why there are 3 generations of leptons/quarks.

– No explanation of left-right asymmetries.– Quarks and leptons


GUTs• What do we really need to unify particle

physics?• Maxwell unified the electric and magnetic

interactions in to electromagnetic (EM)• The standard model unified the EM and weak

interactions into the electroweak interaction• What’s left is the strong force.• What kind of theory is needed to unify this?


Not all that easy


What GUTs might do• Flavor changing interactions in quarks

(e.g. changing a top quark to a bottom quark by emitting a W+) suggest that quarks can be viewed as different ‘orientations’ of the same object.

• Have found the same thing for leptons.• But maybe there should be a lepto-quark field?

– Quarks could turn into leptons, leptons into quarks– All matter particles would be different ‘orientations’

fo the same fundamental object.


Grand Unified Theories• Although the Standard Model has been very successful in accounting for all experimental phenomena, it is not

expected to be the ultimate theory because of its great complexity and the many questions it leaves unanswered. These objections seem to suggest that there may be deeper symmetries underlying the standard model, leading perhaps to the unification of the strong and electroweak interactions into a single "Grand Unified theory", or GUT. Such scheme is indeed possible if the internal rotation group is further generalized to SU(5) with 24 parameters. All the elementary particles are assigned into

• two 5-multiplets and two 10-multiplets. Figure 15-10 shows one of the 5-multiplets with the 24 gauge bosons assignment arranged into a matrix. Of these, 12 are familiar (the photon, W+, W-, Z0 and 8 gluons). The remaining 12 are new bosons denoted by X; these carry new forces which can transform quarks into leptons and vice versa. The mass of the X bosons have been calculated under the Higgs mechanism, and turns out to be about 1015 Gev. These super-heavy particles lie many orders of magnitude beyond the energy ranges of any conceivable accelerator. However, they would be present in great abundance in the first 10-35 sec after the Big Bang.

• Figure 15-10 SU(5) Symmetry• [view large image]•• Figure 15-11 Grand Unified Theory [view large image]•• At energies well above 1015 Gev, all gauge bosons (including the Xs) can be produced freely and all interactions

have the same strength and quarks can transform into leptons as easily as they change colours; and the grand SU(5) symmetry is manifest. At an energy of about 1015 Gev, the SU(5) symmetry breaks down to separate SU(3) and SU(2)XU(1) symmetries and the grand unified interaction separates into the strong and electroweak interactions. At about 102 Gev, the SU(2)XU(1) symmetry becomes broken, reflecting the separation of electroweak interaction into the distinct weak and electromagnetic interactions. This picture of the unification of interactions also incorporates the variation in the strengths of charges, depending on the distance from which they are acted upon as shown in Figure 15-11. The most dramatic consequence of grand unification is that the proton is no longer stable, it has a small probability for decay into a neutral pion and a positron (with a half life of about 1031 years). No such decay has been detected so far.


What is left?• Left-handed - right-handed asymmetries


Unifications: now and the future


• How does the Higgs boson generate the masses for all other particles? Is it the carrier of a force?

• Dear Umut,

• You need to distinguish between the Higgs boson and the Higgs field. The Higgs field is the stuff that gives all other particles a mass. Every particle in our universe "swims" through this Higgs field. Through this interaction every particle gets its mass. Different particles interact with the Higgs field with different strengths, hence some particles are heavier (have a larger mass) than others. (Some particles have no mass. They don't interact with the Higgs field; they don't feel the field.) It is the opposite of people swimming in water. As people float in water they "become" lighter. Depending on size, shape, etc, some people float better than others.

• The Higgs field is not considered a force. It cannot accelerate particles, it doesn't transfer energy. However, it interacts universally with all particles (except the massless ones), providing their masses.

• The Higgs boson is a particle. It gets its mass like all other particles: by interacting with ("swimming in") the Higgs field. But as you can imagine, the Higgs particle differs from all the other particles we know. It can be thought of a dense spot in the Higgs field, which can travel like any other particle. Like a drop of water in water vapor.

• The Higgs boson has many more ways of interacting with all other kinds of particles than the Higgs field (which just causes a "drag" = mass). In this sense one my call the Higgs particle the mediating particle of the proposed Higgs field, like you wrote. The Higgs field is the silent field that gives the mass. We cannot directly probe for it. But discovering the Higgs boson, the "mediator", would prove the existence of the Higgs field.

• The Higgs particle, like many other elementary particles, is not a stable particle. Since it interacts with all kinds of other massive particles it can be created in collisions. (The Higgs particle does not interact with massless particles, such as a photon or a gluon. Since these particles don't interact with the Higgs field, the Higgs boson also doesn't interact with them.)

• Once the Higgs particle has been created, it will eventually decay. Though the Higgs particle interacts with all massive particles it prefers to interact with the heaviest elementary particles we know, especially the top quark, which was discovered at Fermilab in 1995. Because of this property of the Higgs boson physicists at Fermilab might have a chance to find evidence for the Higgs boson itself within the next five to six years. If they are not successful then an accelerator currently build at the CERN laboratory in Geneva, Switzerland, will have enough energy to produce the Higgs boson. Fermilab's accelerator currently is the world's most powerful accelerator, but physicists don't know whether it has enough power to create Higgs bosons. The new accelerator at CERN will have more power, but construction won't be finished until 2005.

• The Higgs particle is considered to be a carrier of a force. It is a boson, like the other force-transferring particles: photons, gluons, electroweak bosons. One may call the force mediated by the Higgs boson to be universal as the Higgs boson interacts with all kinds of massive particles, no matter whether they are quarks, leptons, or even massive bosons (the electroweak bosons). Only photons and gluons do not interact with the Higgs boson. Neutrinos, the lightest particles with almost zero mass, barely interact with a Higgs boson. Top quarks, which have about the mass of a Gold atom, have the strongest interaction with a Higgs boson.


Review questionAn object has a constant acceleration.

If the object has speed v at time t, then what is the speed at time 2t ?

A. v/4 B. v/2 C. v D. 2v E. 4v


So… what IS Matter ?

• Matter is all the “stuff” around you, including us !

• Here’s the picture we’ve uncovered

Hadrons

Matter

Leptons

Baryons Mesons Charged Neutrinos

Forces

Weak EM

StrongGravity

QuarksAnti-Quarks

fri. dec 10phy107 lecture 37 from last time…. fri. dec 10phy107 lecture 37 today unification of...

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