weak interactions

weak decays weak decays beta decay beta decay

of of neutron neutron

epn

problem energy and momentum not conserved

e

n

p

W. Pauli: 1930 Neutrino mass ~ m(e)

weak decays weak decays beta decay beta decay

of of neutron neutron

eepn

e

n p

e

direct interaction of four fermions

Enrico Fermi, ~1936

p

n

electron

antineutrino

251016637.1 GeVGF

weak interactions current-current interaction weak currents ~ lefthanded:

jjGL

xj

F

int

5 )1()(

lefthanded fermion

S= - 1/2

righthanded antifermion

S = + 1/2

maximal breaking of

parity

weak interaction of the quarks

beta – decay

d u

eepn

weak decays of strange particles

epe

( u d s ) ( u u d )

s u

Cabibbo angle:

)sincos( sdd

flavor mixing

)(xj

19

W(+) W(-) Z photon

rotation symmetry

spontaneous symmetry breaking in classical mechanics

spontaneous symmetry breaking baki

direction singled out rotation symmetry broken

Peter Higgs

Kibble Guralnik Hagen Englert Brout

potential

V

example 1: real scalar field

4

=> 2 ground states

example 2: complex scalar field

exact symmetry:

broken symmetry:

scalars:

doublet

degrees of freedom before symmetry breaking:

After symmetry breaking:

scalars:

triplet

„Higgs“ scalar

2)(

1)()(

2

13

R

eL

eY

YeY

YTQ

R

L

e

eRSinglet

eLDoublet

:

:

YB

iTW

bosonsgauge

ii

3,2,1

:4

RDiRLDiLL

GGFFL

f

iibg

4

1

4

1..

0

:

scalar

g

gangleweak

ggcouplingstwo

USU

w

tan:

,:

:)1()2(

photon

first term lefthanded

parity violation

neutral current interaction

parity violation

observed in atomic physics

neutrino massless

(only lefthanded component)

The masses of the weak bosons are predicted,

once the weak angle is measured. The electron mass cannot be

predicted( the coupling constant g is unknown ).

electroweak theory: mass generation by

??? SSB ???QCD: mass generation

by confinement

problem: mass generation

hypothetical hypothetical Higgs Higgs bosonboson

L3

I

ATLAS

CMS

LHCb

Alice

December 2011:

ATLAS and CMS report signal at 125 GeV decay into

Z + Z W + W, photon + photon

Real World 3 lepton-quark families

de

ue

s

c

b

t

mass

Real World 6 leptons – 6 quarks

2 x 3 doublets

ee

bsd

tcu

bsd

bsd

flavor mixing