Upload
august
View
48
Download
3
Tags:
Embed Size (px)
DESCRIPTION
Why is there mass?. Tony Liss Saturday Physics for Everyone November 9, 2013 (With debts to Chris Quigg, Leonard Susskind, Hitoshi Murayama). What is mass?. Mass is stuff Mass is the stuff that gravity acts on. r. Mass is stuff that resists you when you push it. - PowerPoint PPT Presentation
Citation preview
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 energy2
0E 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
OtherwiseIf 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
Ener
gy o
f fiel
d
0Strength of field
All other fields
The Higgs fieldThe 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
OE +
-
(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~ee
m cr
We know , then mec2 ~ 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?