Higgs Boson for Dummies
Bojan Golli
Faculty of Education, University of Ljubljana
1st South-Eastern European Meeting on Physics Education 2012
September 11, 2012
Bojan Golli
Higgs Boson for Dummies
For dummies?
I am not supposed to start from the Higgs Lagrangian
– which I would not do anyway.
Bojan Golli
Higgs Boson for Dummies
What is the Higgs boson?
A particle that gives mass to elementary particles of the Standard model
– an idea proposed by Peter Higgs in 1960.
Paper rejected in Phys. Lett. "of no obvious relevance to physics"
Heisenberg: "You do not understand the rules of physics."
Bojan Golli
Higgs Boson for Dummies
Questions:
What is mass?
Bojan Golli
Higgs Boson for Dummies
Questions:
What is mass?
What are the elementary particles?
Bojan Golli
Higgs Boson for Dummies
Questions:
What is mass?
What are the elementary particles?
What is the mechanism that generates mass of particles?
Bojan Golli
Higgs Boson for Dummies
Questions:
What is mass?
What are the elementary particles?
What is the mechanism that generates mass of particles?
How (new) particles are discovered?
Bojan Golli
Higgs Boson for Dummies
Questions:
What is mass?
What are the elementary particles?
What is the mechanism that generates mass of particles?
How (new) particles are discovered?
How do we know that the particle discovered at CERN is indeed the
Higgs boson?
Bojan Golli
Higgs Boson for Dummies
What is mass?
One of the basic physical quantity; related to two different concepts:
Inertial mass
a
F
~F = m~a
m
~a is the acceleration of the body
Bojan Golli
Higgs Boson for Dummies
What is mass?
One of the basic physical quantity; related to two different concepts:
Inertial mass
a
F
~F = m~a
m
~a is the acceleration of the body
Gravitational mass
weight: ~F = m~g
m
F
~g is the strength of
the gravitational field
Bojan Golli
Higgs Boson for Dummies
Relativity
Equivalence principle: both masses are equivalent
Bojan Golli
Higgs Boson for Dummies
Relativity: mass and energy
Mass and energy are equivalent:
E = mc2 ,
c = 299 792 458 m/s .
A body at rest has energy due to its (rest) mass;
also, a moving body acquires larger mass compared to its rest mass m0 :
m= q
m0
1−
v2
c2
.
The Higgs boson provides nonzero rest mass (m0 )
Bojan Golli
Higgs Boson for Dummies
Relativity: light bending
The photon (light) has nonzero mass due to its energy; it is deflected in
the gravitational field, e.g. of the Sun:
Bojan Golli
Higgs Boson for Dummies
Massless particles
Some particles, e.g. the photon (light), the neutrino ν. . . travel with the
speed of light c.
r
v2
m0 = m × 1 − 2 = 0 , if v = c .
c
Hence
Particles that travel with the speed of light have zero rest mass.
Bojan Golli
Higgs Boson for Dummies
Massless particles
Some particles, e.g. the photon (light), the neutrino ν. . . travel with the
speed of light c.
r
v2
m0 = m × 1 − 2 = 0 , if v = c .
c
Hence
Particles that travel with the speed of light have zero rest mass.
Vice versa: Massless particles cannot rest; they always travel with
the speed of light.
Bojan Golli
Higgs Boson for Dummies
Massless particles
Some particles, e.g. the photon (light), the neutrino ν. . . travel with the
speed of light c.
r
v2
m0 = m × 1 − 2 = 0 , if v = c .
c
Hence
Particles that travel with the speed of light have zero rest mass.
Vice versa: Massless particles cannot rest; they always travel with
the speed of light.
Strange behaviour? Not at all; according to Higgs, there is nothing
wrong with massless particles; what is strange is that the "normal"
bodies rest or travel with the speed less than the speed of light.
Bojan Golli
Higgs Boson for Dummies
Microscopic world
Matter consists of building blocks
mbody ≈ Np mproton + Nn mneutron ,
mproton ≈ mneutron ,
Bojan Golli
melectrons mproton
Higgs Boson for Dummies
Elementary particles
But:
mproton 6= mu quark + mu quark + md quark .
In fact:
mu quark ≈ md quark ≈ melectrons ∼
mproton ≈
Bojan Golli
1
1000
mproton .
Wkinetic
.
c2
Higgs Boson for Dummies
Standard model (Periodic table of elementary particles)
mc quark ≈ mproton
mb quark ≈ 4 mproton
mt quark ≈ 180 mproton
mτlepton ≈ 4000 melectrons ≈ 2 mproton
Bojan Golli
Higgs Boson for Dummies
Standard model (Periodic table of elementary particles)
mc quark ≈ mproton
mb quark ≈ 4 mproton
mt quark ≈ 180 mproton
mτlepton ≈ 4000 melectrons ≈ 2 mproton
Are heavy quarks and leptons composite particles?
No, there is no evidence whatsoever for their internal structure.
Bojan Golli
Higgs Boson for Dummies
Standard model (Periodic table of elementary particles)
mc quark ≈ mproton
mb quark ≈ 4 mproton
mt quark ≈ 180 mproton
mτlepton ≈ 4000 melectrons ≈ 2 mproton
Are heavy quarks and leptons composite particles?
No, there is no evidence whatsoever for their internal structure.
How do we then explain their large masses?
Answer: the Higgs mechanism
Bojan Golli
Higgs Boson for Dummies
What is the role of bosons?
Classical explanation of
long range forces (e.g.
electro-magnetic force):
A charged particle creates
a field in the surrounding
space.
Quantum
explanation
(Feynman): a charged
particle emits a photon
(a boson, in general) and
a second particle absorbs
it.
Bojan Golli
Higgs Boson for Dummies
Elementary interactions
gluon is the exchange boson of the strong interaction between quarks
u
d
g
weak bosons (W ± , W 0 , Z) carry the weak interaction
Two complementary interpretation: Bosons are elementary excitations of
the field; on the other hand, the field is a condensate of bosons, e.g. the
electron is surrounded by a cloud of the so called virtual photons.
Bojan Golli
Higgs Boson for Dummies
Higgs field
Higgs assumed the existence of a new field, – the Higgs field –, that fills
all of space and has no external source. The Higgs boson is an
elementary excitation of the field.
The source of the Higgs field is the Higgs field itself. In the alternative
picture, the Higgs bosons in the condensate attract each other. The
resulting potential energy of the system has its minimum at a non-zero
value of the field.
Bojan Golli
Higgs Boson for Dummies
Mass generation
All elementary particles are massless and therefore move with the speed
of light. But most of them bounce off the Higgs bosons in the vacuum
and hence effectively move with a finite velocity. Their kinetic energy is
transformed into the rest energy (mass).
Some particles – including the Higgs boson itself – interact more
frequently than the others; it means they are more massive. Photons,
gluons, neutrinos do not interact at all; they are massless – more
precisely, their rest energy is zero.
Bojan Golli
Higgs Boson for Dummies
Production of the Higgs boson
In order to observe a free Higgs boson, a huge amount of energy has to
be transferred to the vacuum (i.e to the Higgs field). A particle in
quantum mechanics is described as a wave with frequency ν = E/h (h is
the Planck constant). The largest probability to excite an oscillation is at
the resonance – i.e. when the transferred energy is equal to the energy
(mass) of the particle.
Bojan Golli
Higgs Boson for Dummies
Discovery of new boson
At the proton collider at CERN two proton traveling in opposite direction
collide and produce a shower of particles, mostly quark-antiquark pairs,
which in turn annihilates and produce long lived particle that are finally
detected and analyzed by two independent experimental groups.
Bojan Golli
Higgs Boson for Dummies
Two "cleanest" events
Bojan Golli
Higgs Boson for Dummies
Conclusion
So far the observations are consistent with the observed particle being
the Standard Model Higgs boson.
The particle decays into at least some of the predicted channels.
Moreover, the production rates and branching ratios for the observed
channels match the predictions by the Standard Model within the
experimental uncertainties.
However, the experimental uncertainties currently still leave room for
alternative explanations.
It is therefore too early to conclude that the found particle is indeed the
Standard Model Higgs.
[PDGLive. Particle Data Group. 12 July 2012.]
Bojan Golli
Higgs Boson for Dummies
Spontaneous breaking of chiral symmetry
The reason for introducing the Higgs field actually lies in the observation
that the equations of motion preserve the chiral symmetry while in nature
this symmetry is violated.
The symmetry requires that the helicity, i.e. the projection of particle
spin onto the direction of motion, is a good quantum number, and the
elementary particles are supposed to be either left- or right-handed.
This solution is however not realized in nature: if one observer sees a
right handed electron then for another observer, moving with the velocity
greater than the electron velocity in the same direction, the electron has
opposite helicity. The helicity is preserved only for massless particles
moving with the speed of light; massive particles violate the symmetry.
Bojan Golli
Higgs Boson for Dummies
Spontaneous breaking of chiral symmetry
The situation in which the underlying laws are invariant under some
symmetry while the solution is not is called "spontaneous symmetry
breaking" and the Higgs mechanism is a model that describes such
breaking.
Bojan Golli
Higgs Boson for Dummies