Tony LissSaturday Physics for EveryoneNovember 9, 2013

(With debts to Chris Quigg, Leonard Susskind, Hitoshi Murayama)

Why is there mass?

What is mass?Mass is stuff

Mass is the stuff that gravity acts on

2blue red

GravityM M

F Gr

r

Mass is stuff that resists you when you push it

F Ma

Mass is rest energy

20E Mc

What’s the Matter in the

Universe?

Ordinary Matter

A proton is made of u u d

Add an electron to make a hydrogen atom

The periodic table is made up of “quarks” and “leptons”

A neutron is made of d d u

Now you have all the building blocks of the periodic table.

The Masses

According to theory, the Higgs boson gives these particles their mass.

.511 MeV/c2

~2.3 Mev/c2

~4.8 Mev/c2

~173000 Mev/c2

The periodic table: M=Smi ?

Hydrogen: One proton, one electron

MHc2 = 938.890124 MeV

MH/(MP+Me) = 0.999999986

Helium: Two protons, two neutrons, two electrons

MHec2 =3,728.398128 MeV

MHe/ (2MP+2MN+2Me) = 0.9927

Iron: 26 protons, 30 neutrons, 26 electrons

MFec2 = 52,019 MeV

MFe/(26MP+30MN+26Me )=0.9909

Are the proton and neutron masses like

atomic masses?No!

Proton: Two up quarks and a down quark

(2Mu+Md)c2< 10 MeVMPc2 = 938 MeV !

Neutron: Two down quarks and an up quark

(2Md+Mu)c2 ~12 MeVMNc2 = 940 MeV ! The mass of neutrons and protons is due

(mostly) to the strong force (QCD) that holds them together: 2M E c

.511 MeV/c2

~2.3 Mev/c2

~4.8 Mev/c2

~173000 Mev/c2

Let’s Review• The mass of atoms is the sum of the mass of their parts (with

very small corrections) – protons+neutrons+electrons

• The mass of protons and neutrons is not the sum of the mass of their parts (quarks)• It is mostly from the energy in the strong force (QCD) binding

the quarks together.

• Therefore, the mass of periodic table and the visible universe comes not from the masses of the fundamental particles, but from QCD!

So who needs the Higgs Boson?

Consider the proton & neutron masses

The proton is electrically charged & therefore surrounded by its own electric field. The neutron is electrically neutral.

2( )E field McYou would guess that the proton is heavier than the neutron. But 21.29 MeV cN PM M

Thanks to the Higgs…

Mdown > Mup Mneutron > Mproton

ee

The heavier neutron “beta decays” in about 15 minutes into a proton plus an electron and a neutrino

Otherwise

If the quarks had no mass, then MP > MN

The proton would beta decay into a neutron

No hydrogen atom

If the electron had no mass…

No atoms at all

No me

No you

How does it work?

The Higgs ‘potential’

The Standard Model

Atoms

Force carriers

Interactions

Electroweak force

Weak Interactions

http://www.philica.com/display_article.php?article_id=126

dud

duu

W– e –

e

ee

Beta decay

Electroweak Interactions

In the 1960s, the electromagnetic & weak interactions were unified into a single framework based on electroweak symmetry.The symmetry principle requires the carriers of the electromagnetic and weak forces to have zero mass. Quarks and leptons also have no mass.

Electroweak symmetry must be hidden.

But this isn’t true. Only the photon has no mass.

Weak interactions are very short-ranged, implying that the carriers, W & Z bosons, are very massive.

Hidden Symmetry

I first saw this demo in a talk by Chris Quigg

?

The Higgs Field is Special

Energ

y o

f field

0Strength of field

All other fields

The Higgs field

The minimum energy has a non-zero Higgs field!

http://commons.wikimedia.org/wiki/File:Mexican_hat_potential_polar_with_details.svg

Energy & Fields+

-

HH

O

H H

O

p

Water molecules: Electrically neutral, but with a ‘dipole moment’

HH

O

H H

O

E +

-

(Thanks to Leonard Susskind)

How to Find the Higgs

Is it THE Higgs Boson?

It is certainly A Higgs boson

Therefore we knowIt is responsible for at least some of the W and Z mass(strong evidence).

It is probably responsible for at least some of the fermion masses (evidence getting stronger).

We don’t yet know:If it is the ONLY Higgs boson.

Why it is so light.

The Electron Mass

Electric field lines around an electron

We know the mass of the electron: 2 0.511 MeVem c

This observed mass is an unobservable bare mass plus a correction due to the field energy

2 20.511 MeV e ebarem c m c

This is fine tuning Hitoshi Murayamahttp://arxiv.org/abs/hep-ph/0002232

The energy in the electric field adds to the electron mass

2 1~e

e

m cr

We know , then Dmec2 ~ 10 GeV and1710 cmer

0.511 MeV = (-9999.489 + 10000.000) MeV

The Solution: Antiparticles

-

-+

-+ -+ -+

-+

-+

-+ -+ -+

-+

-+- +- +

-+

-+- +- + -+

-+-

+- +

- +

-+

- +

-+ -

+ -+

-+

-+ -

+ -+

-+

-+-

+-+

- + Electron-positron pairs ‘pop’ out of the vacuum and shield the bare charge of the electron

e

e

The Mass of the Higgs Boson

2 2 2h h hobs bare

M M M

t

t

The problem is, is very very large (~1038 GeV) if there is nothing else going on.

This requires to be equally large in magnitude to cancel it and give

2hM

2h bare

M

2125 GeV ch obsM

0h 0h

Supersymmetry (or something)

SUSY includes a partner to the top quark, the ‘stop quark’ or ‘top squark’ that nearly cancels the top quark loop

2 2 2 2top stoptop stop

~h hM M m m

t

t

0h 0h

But we haven’t found the stop quark yet. The more we don’t find it, the heavier it must be – if it exists at all…

What if there is nothing else?

Nature could be fine tuned. There is no law that forbids125 GeV =

100000000000000000000000000000000000000

- 999999999999999999999999999999999999875

It’s just not very pleasing.

The Anthropic Principle

Since we are here, the fundamental constants must have values in the narrow range compatible with conscious life.

One Universe, or many?

What We Don’t Know yet (partial

list) Is there more than one Higgs boson?• Supersymmetry, and many other ideas, include a family.

Does the Higgs boson give dark matter it’s mass?

Does the Higgs boson have anything to do with neutrino mass?

Is the anthropic principle correct?

What in the Universe is dark energy?