The Weak Force

EM

STRONG

WEAK

?

The Force Carriers Like the Electromagnetic & Strong forces, the Weak force is alsomediated by “force carriers”.

For the weak force, there are actually 3 force carriers:

W+ W- Z0

These “weak force” carrierscarry electric charge also !

This “weak force” carrieris electrically neutral

The “charge” of the weak interaction is called “weak charge”

Weak Charge of Quarks & LeptonsBoth quarks & leptons carry weak charge

Both quarks & leptons “couple to” the W and Z force carriers

Since the W’s have a charge of +1 and –1 they cause a “charge-changing” interaction. That is when they are emitted or absorbed, to conserve charge, the “emitting” or “absorbing” particle changes charge by +1 or –1 unit.

The emitting or absorbing particle changes into a different particle. Alternately, when the W decays, it decays into 2 particles which:

(a) Carry weak charge(b) The sum of their charges equals the charge of the W

I will mainly talk about the W in the context of decays…

Comparison of the Force CarriersProperty EM Strong Weak

Force Carrier

Photon ()

Gluon (g)

W+, W- Z0

Charge of force carrier

None ColorElectrical &

WeakWeak

Couples to:Quarks &

Charged leptons

Quarks onlyAll quarks

& all leptons

All quarks &

all leptons

RangeInfinite (1/d2)

<10-14 [m](inside hadrons)

< 2x10-18 [m] < 2x10-18 [m]

Notice that the weak force only operates at distances ~ < 10-18 [m] !

Particles & Forces

quarksCharged leptons(e,)

Neutral leptons

()

Strong

Electro-Magnetic

Weak

Y

Y

Y

Y

Y Y

N N

N

Quarks carry strong, weak & EM charge !!!!!

Neutrinos

From the previous table, you saw that neutrinos only interact via the weak force.

Also, the weak force only “kicks in” for d <10-18 [m]. Recall that the nucleus’ size is about 10-15 [m], so this is 1000 times smaller than the size of the nucleus.

As a result, neutrinos can pass through a lot of matter, and do absolutely nothing!!!!

After all, matter is mostly empty space, right !

Neutrinos can easily pass right through the earth, almost as if itwasn’t there!

Neutron decayThis is how neutron decay really proceeds through the

Weak Interaction

du

d

Neutron

W- uu

d

Proton

uu

d

Proton

e-

e

uu

d

Proton

du

d

Neutron

ee- ++

Neutron Decay (cont)

uu

d

Proton

du

d

Neutron

+ W-

n p e -e+ +

d u e -e+ +

But in fact, what’s really going on is this:

ee- +

W-

Feynman diagram for weak decayd u + e - + e

ddu

udu

W-

e -

e

“Spectator quarks”

Spectator quark(s): Those quarks which do not directly participate in the interaction or decay.

Feynman diagram for weak decay(continued)

W-

d u

e -

e

Since the spectator quarks do not directly participate in thedecay, we can just omit them… This yields the “quark-level” Feynman diagram!

-1/3 +2/3

-1

0Is charge conserved ?

Is Le conserved ?

Decays of “heavy” quarksThe heavy quarks decay to the lighter onesby “cascading down”t

b

c

s

u

Q=+2/3

Q=+2/3

Q=+2/3

Q=-1/3

Q=-1/3

d Q=-1/3

W-

b c

-

What about the decay of a b-quark?b c + +

-1/3 +2/3

-1

0Is charge conserved ?

Notice: Here, the W- decays to a and

Is L conserved ?

W+

c s

+

What about the decay of a c-quark? c s + +

2/3 -1/3

+1

0Is charge conserved ?

Notice: Here, I have the W+ decaying to a and (could havebeen an e+ and e as well).

Is L conserved ?

W-

s u

e -

e

What about the decay of a b-quark?s u + e + e

-1/3 +2/3

-1

0Is charge conserved ?

Is Le conserved ?

Decays of heavy quarks to u & d A quark can only decay to a lighter quark. The W charge has the same sign as the parent quark.

t b W+(100%)

b c W- (~90%)b u W- (~10%)

c s W+ (~95%)c d W+ (~5%)

s u W- (~100%)

d u W+ (~100%)

Quark Charge Mass[GeV/c2]

top +2/3 ~175

bottom -1/3 ~4.5

charm +2/3 ~1.5

strange -1/3 ~0.2

up +2/3 ~0.005

down -1/3 ~0.010

“Leptonic” Decay of W

It’s call “leptonic decay” because the W is decaying to leptons!

The W can decay to leptons because leptons carry weak charge

But so do quarks …

Once the W is produced, it must decay

W- e- e

W- -

W- -

W+ e+ e

W+ +

W+ +

“Hadronic” Decay of W

W ud W ud It’s call “hadronic decay” because the

W is decaying to quarks, which will form hadrons!

But quarks are bound to one another by the strong force, and are not observed as “free” particle. That is, they are bound up inside hadrons…

What happens next ?

Since quarks also carry weak charge, we can also get:

W-

b c

u

dCheck charge: (-2/3 + -1/3 = -1)

One possibility…

ud -

W-

b c

u

d

Can, in fact,form a -

u uB- D0

B- D0

B-

Meson

D0

Meson

“Hadronization”

W-

The process by which quarks “dress themselves” into hadrons

W- u

d

u

d

u

d d

u

u

u

As the quarks separate, the “potential energy” stored in this “spring-like” force increases. Eventually, the potential energy gets large enough, and nature relives itself by converting this potential energy into mass energy. That is, quark-antiquark pairs are created ! The quarks then pair off and form hadrons (which we can see) !

0

-

Hadronization (cont)

In fact, this process whereby quark-antiquark pairs are createdcan happen more than once !

One might therefore get 2, 3, or more hadrons from a hadronic W decay!

It’s important to note that when the “spring” associated withthe strong force “snaps”, it always produces quarks and antiquarks of the same type. They are usually the lighter quarks, since theyhave lower mass, and thus are created more easily:

, ,uu dd ssAgain Energy being transformed into mass !

Feynman Diagrams involvingW force carriers

W-

b c

u

d

u uB- D0

to hadrons

Decay of a B- Meson

Could end up as:B- D0

B- D0

B- D0

etcAdditional particles are created when the strong force produces morequark-antiquark pairs. They then combine to form hadrons!

Notice that the charge of the particles other than the D0 add up to the charge of the W- (Q = -1), as they must!

W Decays

W+ follows in an analogous way… see previous slides

W- e- e

W- -

W- -

W- hadrons

Leptonic Hadronic

Can be 1 or morehadrons produced

Interactions involving W’s

W-

time

Pos

itio

n e-

e+

e

e

e ee e

Check lepton number, charge conservation…

Here is one… Don’t worry about these types of interactions… I want to emphasize the role of W’s in decays of quarks

The main points1. Neutrons decay to protons through the weak

interaction

2. The electron and neutrino in fact come fromthe Wee decay.

3. The W can decay into either lepton pairs or a quark-antiquark pair (ud). In the latter case, the quarks undergo hadronization into hadrons

4. Heavy quarks decay to lighter quarks viaemission of a W particle. Since the W has charge,we get bcW- for example, but NOT bdW- !