Higgs Boson for Dummies - University of Ljubljana So far theobservations are consistentwith the observed particle being the Standard Model Higgs boson. The particle decays into …

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.


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."


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 theHiggs boson?


Questions:

What is mass?






Questions:

What is mass?






Questions:

What is mass?






Questions:

What is mass?






What is mass?

One of the basic physical quantity; related to two different concepts:

Inertial mass

m

F

a

~F = m~a

~a is the acceleration of the body

Gravitational mass

m

F

weight: ~F = m~g

~g is the strength ofthe gravitational field


What is mass?

One of the basic physical quantity; related to two different concepts:

Inertial mass

m

F

a

~F = m~a

~a is the acceleration of the body

Gravitational mass

m

F

weight: ~F = m~g

~g is the strength ofthe gravitational field


Relativity

Equivalence principle: both masses are equivalent


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 =m0√1− v2

c2

.

The Higgs boson provides nonzero rest mass (m0)


Relativity: light bending

The photon (light) has nonzero mass due to its energy; it is deflected inthe gravitational field, e.g. of the Sun:


Massless particles

Some particles, e.g. the photon (light), the neutrino ν. . . travel with thespeed of light c.

m0 = m×√

1− v2

c2 = 0 , if v = c .

Hence

Particles that travel with the speed of light have zero rest mass.

Vice versa: Massless particles cannot rest; they always travel withthe speed of light.

Strange behaviour? Not at all; according to Higgs, there is nothingwrong with massless particles; what is strange is that the "normal"bodies rest or travel with the speed less than the speed of light.


Massless particles


m0 = m×√

1− v2

c2 = 0 , if v = c .

Hence





Massless particles


m0 = m×√

1− v2

c2 = 0 , if v = c .

Hence





Microscopic world

Matter consists of building blocks

mbody ≈ Np mproton + Nn mneutron ,

mproton ≈ mneutron , melectrons � mproton


Elementary particles

But: mproton 6= mu quark + mu quark + md quark .

In fact:mu quark ≈ md quark ≈ melectrons ∼ 1

1000 mproton .

mproton ≈Wkinetic

c2 .


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


















What is the role of bosons?

Classical explanation oflong range forces (e.g.electro-magnetic force):A charged particle createsa field in the surroundingspace.

Quantum explanation(Feynman): a chargedparticle emits a photon(a boson, in general) anda second particle absorbsit.


Elementary interactions

gluon is the exchange boson of the strong interaction between quarks

d© u©g

weak bosons (W±, W0, Z) carry the weak interaction

Two complementary interpretation: Bosons are elementary excitations ofthe field; on the other hand, the field is a condensate of bosons, e.g. theelectron is surrounded by a cloud of the so called virtual photons.


Higgs field

Higgs assumed the existence of a new field, – the Higgs field –, that fillsall of space and has no external source. The Higgs boson is anelementary excitation of the field.The source of the Higgs field is the Higgs field itself. In the alternativepicture, the Higgs bosons in the condensate attract each other. Theresulting potential energy of the system has its minimum at a non-zerovalue of the field.


Mass generation

All elementary particles are massless and therefore move with the speedof light. But most of them bounce off the Higgs bosons in the vacuumand hence effectively move with a finite velocity. Their kinetic energy istransformed into the rest energy (mass).

Some particles – including the Higgs boson itself – interact morefrequently than the others; it means they are more massive. Photons,gluons, neutrinos do not interact at all; they are massless – moreprecisely, their rest energy is zero.


Production of the Higgs boson

In order to observe a free Higgs boson, a huge amount of energy has tobe transferred to the vacuum (i.e to the Higgs field). A particle inquantum mechanics is described as a wave with frequency ν = E/h (h isthe Planck constant). The largest probability to excite an oscillation is atthe resonance – i.e. when the transferred energy is equal to the energy(mass) of the particle.


Discovery of new boson

At the proton collider at CERN two proton traveling in opposite directioncollide and produce a shower of particles, mostly quark-antiquark pairs,which in turn annihilates and produce long lived particle that are finallydetected and analyzed by two independent experimental groups.


Two "cleanest" events


Conclusion

So far the observations are consistent with the observed particle beingthe Standard Model Higgs boson.The particle decays into at least some of the predicted channels.Moreover, the production rates and branching ratios for the observedchannels match the predictions by the Standard Model within theexperimental uncertainties.However, the experimental uncertainties currently still leave room foralternative explanations.It is therefore too early to conclude that the found particle is indeed theStandard Model Higgs.

[PDGLive. Particle Data Group. 12 July 2012.]


Spontaneous breaking of chiral symmetry

The reason for introducing the Higgs field actually lies in the observationthat the equations of motion preserve the chiral symmetry while in naturethis symmetry is violated.The symmetry requires that the helicity, i.e. the projection of particlespin onto the direction of motion, is a good quantum number, and theelementary particles are supposed to be either left- or right-handed.

This solution is however not realized in nature: if one observer sees aright handed electron then for another observer, moving with the velocitygreater than the electron velocity in the same direction, the electron hasopposite helicity. The helicity is preserved only for massless particlesmoving with the speed of light; massive particles violate the symmetry.


Spontaneous breaking of chiral symmetry

The situation in which the underlying laws are invariant under somesymmetry while the solution is not is called "spontaneous symmetrybreaking" and the Higgs mechanism is a model that describes suchbreaking.


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Higgs Boson for Dummies - University of Ljubljana So far theobservations are consistentwith the observed particle being the Standard Model Higgs boson. The particle decays into …