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Neutron decay and interconversion

Particle processes are a lot like equations

•You can turn them around and they still work

•You can move particles to the other side by “subtracting them”

•This means replacing them with anti-particles

•The neutron (in isolation) is an unstable particle

•Decays to proton + electron + anti-neutrino

•Mean lifetime: 886 seconds

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•Put the neutrino on the other side

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•Put the electron on the other side

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•All thee processes convert neutrons toprotons and vice versa

Neutron/Proton Freezeout

•Weak interactions interconvert protons/neutrons

•These are slow processes, so they fall out of equilibrium fairly early

•At kBT = 0.71 MeV, the process stops

•What is ratio of protons to neutrons at this temperature?

•Non-relativistic, E = mc2.

•Ratio is:

•This happens at about:

The Deuterium Bottleneck

•The next step in making more complexelements is to make 2H, deuterium:

•This releases about 2.24 MeV of energy

•Naively: this process will go ahead as soon as kBT drops below 2.24 MeV

•Actually, much lower temperature is required because of very low density ofnucleons

•Actual temperature is about factor of 20 lower: 0.1 MeV

•Age of universe at this time:

•At this point, some neutronsare gone due to decay

•Ratio depends weakly on density of protons/neutrons – more makes it happen sooner

Making Helium

•Once we make deuterium, we continue quickly to continue to helium:

•For every two neutrons, there will be two protons that combine to make 4He

•Mass fraction of 4He is twice that of neutron fraction

•4He is extremely stable – once formed it won’t go back.

•The sooner it happens, the more neutrons are left over

•Define  as the current ratio of baryons (protons + neutrons) to photons

•As  increases, YP increases weakly:

Making Other elements

•When you run out of neutrons, 3He can still be turned into 4He via

•The last few 2H, 3He, and 3H nuclei will have trouble finding partners

•There will be small amount of each of these isotopes left

•The more baryons there are, the easier it is to find a partner

•As  increases, 2H, 3He, and 3H all decrease

•There are other rare processes that produce a couple of other isotopes:

•7Li and 7Be are produced

•I don’t understand how theydepend on 

•Within a few hundred seconds, thebaryons are all in 1H, 2H, 3H, 3He,4He, 7Be and 7Li

Anything we missed?

•Two of these isotopes are unstable:

•Add 3H to 3He and 7Be to 7Li

•The process whereby stars make heavier elements do not work in the early universe

•Density is too low for unstable 8Be to findanother 4He to react with

•In the end, we should be able to predict abundance (compared to hydrogen) of2H, 3He, 4He, 7Li

•These have all been measured, mostly by studying light from quasars

•Back in the good old days (the 90s), this was how we estimated 

•Now we have an independent way of estimating it (later lecture)

•We should be able to compare the results withpredictions

•A very strong test of Big Bang theory

The results

•Predictions for 4He, 2H and 3He allwork very well

•Prediction for 7Li seems to be off

•The Lithium problem

•Overall, success for the model

Summary of Events:

EventkBT or TTime

Neutrinos Decouple1 MeV0.4 s

Neutron/Proton freezeout0.7 MeV1.5 s

Electron/Positron Annihilate170 keV30 s

Primordial Nucleosynthesis80 keV200 s

Matter/Radiation Equality0.76 eV57 kyr

Recombination0.26 eV380 kyr

Structure formation30 K500 Myr

Now2.725 K13.75 Gyr

Lots of unsolved problems:

•What is the nature of dark matter?

•Why is the universe flat (or nearly so)?

•Where did all the structure come from?

•What is the nature of dark energy?

What we know and what we don’t:

•Up to now, everything we have discussed is based on pretty well understood physics

•And the experimental results match it well!

•As we move earlier, we reach higher temperatures/energies, and therefore thingsbecome more uncertain

•For a while, we can assume we understand the physics and apply it, but we don’thave any good tests at these scales

New particles appear as temperature rises:

•Muons, mass 105.7 MeV, at about kBT = 35 MeV (g = 4 fermions)

•Pions, mass 135-139 MeV, at about kBT = 45 MeV (g = 3 bosons)

•At a temperature of about kBT = 100 MeV, we have quark deconfinement

Quark Confinement

•There are a group of particles called baryons that have strong interactions

•Proton and neutron are examples

•There are also anti-baryons and other strong particles called mesons

•In all experiments we have done, the baryon number is conserved

•Baryon number = baryons minus anti-baryons

•All strongly interacting particle contain quarks or anti-quarks or both

•The quarks are held together by particles called “gluons”

•At low temperatures quarks are confined into these packets

•At high temperatures, these quarks become free (deconfined)

•Estimated kBT = 150 MeV

Electroweak Phase Transition

•There are three forces that particle physicist understand:

•Strong, electromagnetic, and weak

•Electromagnetic and weak forces affected by a field called the Higgs field

•The shape of the Higgs potential is interesting:

•Sometimes called a Mexican Hat potential

•At low temperatures (us), one direction is easy tomove (EM forces) and one is very hard (weak forces)

•At high temperatures, (early universe) you naturallymove to the middle of the potential

•All directions are created equal

•Electroweak unification becomes apparent atperhaps kBT = 50 GeV

The Standard Model

Particlesymbolsspingmc2 (GeV)Electrone½40.0005Electron neutrinoe½2~0Up quarkuuu ½12~0.005Down quarkddd ½12~0.010Muon½40.1057Muon neutrino½2~0Charm quarkccc ½121.27Strange quarksss ½12~0.10Tau½41.777Tau neutrino½2~0Top quarkttt ½12173Bottom quarkbbb ½124.7

Photon120Gluon gggggggg 1160W-bosonW1680.4Z-bosonZ1391.2

HiggsH01115–285

•Above the electroweak phasetransition, all known particles of thestandard model should exist withthermal densities

•From here on, we will bespeculating on the physics

•Cosmology sometimes indicates weare guessing right

•Goal: Learn physics fromcosmology

Supersymmetry

•In conventional particle physics, fermions and bosons are fundamentally different

•And never the twain shall meet

•In a hypothesis called supersymmetry, fermions and bosons are interrelated

•There must be a superpartner for every particle:

•Supersymmetry also helps solve aproblem called the hierarchy problem

•But only if it doesn’t happen attoo high an energy

•If supersymmetry is right, then scale ofsupersymmetry breaking probably around kBT = 500 GeV or so.

•If this is right, the LHC should discover it

•In most versions ofsupersymmetry, the lightestsuper partner (LSP) should beabsolutely stable

Could this be dark matter?

Grand Unification Theories (GUT’s)

•In the standard model, there are three fundamental forces, and three correspondingcoupling constants

•These have rather different values

•But their strength changes as you change the energy of the experiment, theortically

•How much they change depends on whether supersymmetry is right or not

•If supersymmetry is right, then at an energyof about 1016 GeV, the three forces areequal in strength

•At kBT = 1016 GeV, there will be anotherphase transition – the Grand Unificationtransition

NoSupersymmtery

WithSupersymmtery

Baryogenesis might occur at this scale

Scale could be right for inflation