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# Strong Force Finale Weak Force - PowerPoint PPT Presentation

Strong Force Finale & Weak Force. Announcements Exam#3 next Monday. (Bring your calculator) HW10 solutions are posted. Q&A session on Sunday at 5 pm . Tentative course grades will be posted by Tuesday evening.

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• Announcements

• Exam#3 next Monday. (Bring your calculator)

• HW10solutions are posted.

• Q&A session on Sunday at 5 pm.

• Tentative course grades will be posted by Tuesday evening.

• You can do no worse than this grade if you skip the final (but you could do better if you take it)

• Final Exam, Friday, May 210:15 – 12:15 in Stolkin.

• Summary of Exam 3 coverage willis posted under Slides link..

• Gluons are the carrier of the strong force

• They keeps quarks bound up inside hadrons

• Gluons themselves carry color, so they can interact with each other.

• This previous property (#3) is fully responsible for the humungous difference between the nature of the EM force & the Strong force:* EM Force: gets weaker as (electrical) charges move apart * Strong Force: gets stronger as (color) charges move apart

• The Weak force is the weakest of the 4 fundamental forces.

• The theory of the weak force is also based on force carriers.

• It’s mediators were not discovered until ~1982 and 1983.

• There are 3 mediators and they are referred to as:

W+, W- Z0

Two dramatic differences between these mediators and the photon(or gluon) are:

A) Two of them are electrically charged

B) All 3 have huge masses, around 90 [GeV/c2]

Recall, the mass of the proton is 0.938 [GeV/c2] … (~100 times less)

The weak force is largely responsible for decays of heavyquarks to lighter quarks: Recall, I mentioned that:

t b  c  s  u

+2/3 -1/3  +2/3  -1/3  +2/3

Notice that at each stage, the charge changes by +1 or –1 !!

It’s the W+ and W- which are involved here !

So, let’s see how a particle containing a heavy quark decays !

e-

M=0.51 [MeV/c2]

W -

M~0 [MeV/c2]

ne

bR

dR

cR

dR

b

-1/3+1/3

+2/3+1/3

c

d

B0 Meson

D+ Meson

M=5280 [MeV/c2]

M=1870 [MeV/c2]

B0 D+e-ne

B0 D+m -nm

Could have also decayed in either of these ways:

B0 D+t -nt

Decay of mesons containing a heavy quark

Generation/Family

Arrows show usual decaychain.

Notice how charge is alwayschanging by either +1 or-1 unit !

1

2

3

+2/3

u

c

t

-1/3

d

s

b

Weak Force Carriers

• The weak force carriers can ‘talk to’ both quarks & leptons !!!!

• Most of the time, hadrons containing heavy quarks (c, s, b, t) decay by ‘emitting’ a W and transforming into the next lightest quark

• The emission of the W+ (or W-) force carrier necessarily changes the charge of the quark or lepton by 1 unit.

t +

Top quark decays so fast (10-23 [s]), it doesn’t have time to form a meson. Shown:t  b + t+ + nt

nt

W +

t

b

nm

m -

The particle on the left iscalled a B-, and it decayswithin about 1.5x10-12 [s].This decay: B-D0 + m- + nm

W -

b

c

u

u

e+

The D0 meson decays within about 0.5x10-12[s].This decay: D0K- + e - + ne

ne

W +

s

c

u

u

Feynman Diagrams for decays of heavy quarks

t  b + u + d

d

Q

W +

+2/3

-1/3

+2/3

+1/3

t

b

u

d

B- D0 + u + d

Q

W -

-1

0

-2/3

-1/3

B-

D0

b

c

u

u

u

D0K- + u + d

d

W +

Q

0

-1

+2/3

+1/3

D0

K-

s

c

u

u

But W’s can ‘talk to’ quarks also, so we could also have…

As long as energy conservation is notviolated, these can happen

Here, the emerging ud justend up binding togetherand forms a p- meson.

B- D0 + p-

Q

-1

0

-1

What happens to the quarks afterW+ ud or W-  ud

• Quarks can move about when inside a hadron, but cannot escapeand be totally free, like an electron, for example.

• A process occurs whereby the quarks combine to form hadrons. This process is called ‘hadronization’.

Perhaps the simplest example is:

d

p-

W -

B-

D0

b

c

u

u

K-

s

K+

u

u

d

p-

d

d

d

d

p0

d

But, it can be more complicated

Think of the gluons being exchanged as aspring… which if stretched too far, will snap!Stored energy in spring  mass !

In this way, you can see that quarksare always confined inside hadrons(that’s CONFINEMENT) !

e+

q

e-

q

e -

e+

Reconstructed Collision of e+e-

The lines correspond to reconstructed charged particles.

e -

M = 0.51 [MeV/c2]

ne

M~0 [MeV/c2]

W -

d

u

d

u

u

d

n

p

M=940 [MeV/c2]

M=938 [MeV/c2]

Could the neutron decay as:

n  p + m- + nm

?

Mm = 106 [MeV/c2]

Neutron Decay

A neutron which is outside the confines of the nucleus is not stable.It decays with a lifetime of about 14 [minutes].

How does it decay?

n  p + e - + ne

The neutron has barely enough mass to decay into these by products !

e +

ne

ne + p n + e+

Q=+1

Q=0

W-

The ne emits a W- andconverts into a positron !The ‘u’ quark (from theproton) absorbs the W- and becomes a ‘d’ quark!

Q=+1

Q=0

u

u

d

d

u

d

p

n

time

What else does the Weak Force do ?

It can also produce a scattering process:

So, in a Weak interactionthe particle emitting orabsorbing CAN ANDDOES CHANGE. It usuallychanges into the other quark(or lepton) of it’s family.

Recall, this is the process that Fermi predicted thismust exist if n  p + e- + n exists! (see Lecture 33, slide 12)…  Led to discovery of the neutrino at a nuclear reactor by observing the above process.

quarks

Charged leptons(e,m,t)

Neutral leptons(n)

Color Charge ?

Y

N

N

EM Charge ?

N

Y

Y

‘Weak’ Charge ?

Y

Y

Y

• Quarks can participate in Strong, EM & Weak Interactions !

• All quarks & all leptons carry weak charge.

• Neutrinos only carry weak charge !

Quark sizes here are still exaggerated. Recall they are at least 1000 times smaller than the proton!

.

.

n

.

.

Neutrinos can pass through you &the earth without doing a thing !!

Why are neutrinos/antineutrinos so hard to detect?

• Neutrino’s can only participate in the weak interaction. No color, no electric charge, and no mass..

• The weak interaction is very feeble.

• Matter is ~99.999999999% empty space !!!

e +

Z0

e +

u

Z0

ne

ne

ne

ne

e -

e -

u

Z0

u

u

The Z0

I’ve skipped the Z0 weak force carrier up to now. Don’t really have time for too much details…

Just FYI, here’s some interactions involving the Z0 force carrier.

Very similar diagrams to the EM diagrams involving photons, exceptthe Z0 can ‘talk to’ neutrinos. Thisis because the Z0 carries weak charge Recall: all quarks & leptons alsocarry weak charge !

One of the most challenging questions physicists have been askingfor ~25 years is:Why do the quarks & leptons have the masses that they do?

Within the Standard Model of Particle Physics, there is a mechanismby which particles acquire their masses.

A prediction of this “mechanism”, called the Higgs mechanism, isthat there should exist a new particle, called the Higgs particle, H0.This is not a quark, or lepton; it’s very different than any other particle we’ve talked about…

If the Standard Model of Particle Physics is correct, we believe thatthis particle must exist….experiments have searched for it, and havenot found it so far  mass must be very large.

All we know so far is that mass of Higgs particle > 100 [GeV/c2]

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

• Here’s the picture we’ve uncovered

Matter

Leptons

Forces

Baryons

Mesons

Charged

Neutrinos

Gravity

Strong

Weak

EM

Quarks

Anti-Quarks

Space is mostly “empty space”

.

Electron

g

Nucleus

g

u

Proton

u

d

The quarksmake up a negligiblefraction of the protons volume !!

The entire universe is almost allempty space !(YIKES)Forces are a huge part of ourexistence !

So why does matter appear tobe so rigid ?

Forces, forces, forces !!!!

It is primarily the strong and electromagnetic forces which give matter its solid structure. Strong force  defines nuclear ‘size’ Electromagnetic force  defines atomic ‘sizes’

• All matter, including us, takes on its shape and structurebecause of the way that quarks, leptons and force carriersbehave.

• Our bodies, and the whole universe is almost all empty space !

• By studying these particles and forces, we’re trying to getat the question which has plagued humans for millenia …How did the universe start ? And how did we emerge from it all ?Where’s has all the antimatter gone ?