Matter-Antimatter Asymmetry

Matter-Antimatter Asymmetry

Sridhara DasuUniversity of Wisconsin

Some portions adapted from H. Maruyama, UC-Berkeley

Outline

• What is anti-matter?– What led us to it?– But, why is it so rare?

• The Standard Model – Flavor– Mixing– Fundamental asymmetry between matter-antimatter

• Experimental Program– Meson decay asymmetries– Quark mixing parameters

The smallness of the electron

• At the end of 19th century– Physicists pondered about the electron

• Electron is point-like• At least smaller than 10-17 cm• Like charges repel

– Hard to keep electric charge in a small pack

• Need a lot of energy to keep it small!

E=h, E=mc2

• Need LOTS of energy to pack electric charge tightly inside the electron

• But the observed energy of the electron is only 0.5 MeV

• Breakdown of theory of electromagnetism

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Limit on radius of electron : re <10−17cm

Rest energy of electron, Ee = mec2 = 511MeV

E = 14πε0

e2

re

, E = mc 2 ⇒ re = 14πε0

e2

mec2 ≈10−13cm (classical radius)

E = hν = hcλ

, E = mc 2 ⇒ λ = hcmec

2 ≈10−10cm (compton wavelength)

To hold the electron of 10 -17cm together its rest energy should be much larger

Uncertainty Principle

• Energy-Time Uncertainty Principle:You can violate energy conservation but only for a short time

Werner Heisenberg

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ΔEΔt ≥ h4π

Relativistic Quantum world

• Dirac formulated Relativistic Quantum Mechanics• Schrodinger equation

– Not relativistic (space2 but time1)• Predicted antimatter

– Anderson discovered positron

• Vacuum is full of quantum bubbles!

Paul Adrian Maurice Dirac

Anti-Matter Helps

• Electron creates a force to repel itself

• Vacuum bubble of matter anti-matter creation/annihilation

• Electron annihilates the positron in the bubble

Size of the electron is no longer a relevant parameter - the closer you probe, the more you see the structure of vacuum … matter and antimatter pairs

Anti-Matter Helps

• “Anti-matter attraction” cancels “Like-charge repulsion”

• It does not cost too much energy to tightly pack the electric charge inside the electron

• Needed anti-matter: double #particles• Theory of electromagnetism (QED) now

works at very short distances (12 digits accuracy!)

Matter-Antimatter

• All elementary particles come in matter- antimatter pairs– Opposite electric charge– Identical in almost all other respects– Electron-Positron– Proton-Antiproton– Neutron-Antineutron– Up quark - Anti up quark

• Energy conservation can be violated for short periods of time to generate any of these or other particle-antiparticle pairs in vacuum– Relativistic Quantum Mechanics

Elementary particles

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Leptons : e1

ν e0

⎛

⎝ ⎜

⎞

⎠ ⎟

μ1

ν μ0

⎛

⎝ ⎜

⎞

⎠ ⎟ τ 1

ν τ0

⎛

⎝ ⎜

⎞

⎠ ⎟ electroweak interactions (γ,Z,W±)

Quarks : u2 / 3

d−1/ 3

⎛

⎝ ⎜

⎞

⎠ ⎟

c 2 / 3

s−1/ 3

⎛

⎝ ⎜

⎞

⎠ ⎟

t 2 / 3

b−1/ 3

⎛

⎝ ⎜

⎞

⎠ ⎟ electroweak and strong (g) interaction

Each of these have a corresponding anti- particle

e-

e+

,Z bt

W+

cb

W-e-

Heavier elementary particles decay - only the first generation (e,u,d), photons () and neutrinos () are stable.

Flavor Changing Interactions

• Charged W± particles (like photons but massive - 80 GeV) change flavor of quarks– For short period energy conservation can be violated to

create virtual heavy W± particles• Heavier quarks, leptons decay to lighter generations

(u, d, electron, neutrinos)• Cross generational coupling exists

– b quark decays to c quark + X• The down-type quarks mix together

– Quantum mechanical superposition of states

cb

W-e-

Quark Mixing Matrix

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Cabibbo - Kobayashi - Maskawa (CKM)′ d ′ s ′ b

⎛

⎝

⎜ ⎜ ⎜

⎞

⎠

⎟ ⎟ ⎟=

Vud Vus Vub

Vcd Vcs Vcb

Vtd Vts Vtb

⎛

⎝

⎜ ⎜ ⎜

⎞

⎠

⎟ ⎟ ⎟

dsb

⎛

⎝

⎜ ⎜ ⎜

⎞

⎠

⎟ ⎟ ⎟

This complex mixing matrix is unitaryFour unique parameters - three mixing anglesone complex phase (measured experimentally)If complex phase is nonzero matter and antimattercan behave differently (decays will involve different combinations of CKM matrix elements)

Matter reactions are transposed to antimatter reactions using CP transformation - i.e., CP asymmetry is allowed.

Mesons and Baryons

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Mesons : quark - antiquark bound states (Baryon Number = 0)

B meson (bd )

B meson (b d)Baryons : 3 quark states (Baryon Number ≠ 0) proton (uud) ; B =1

anti - proton (u u d ) ; B = -1 neutron (ddu) ; B =1

All mesons decay (Mean lifetime ≤ 10-8 s)Protons are stable (Lifetime greater than life of the Universe)Anti - protons should be stable too (annihilate when p - p meet)Why is the universe proton and electron dominated?(neutrons are unstable when free - survive only in bound nuclei)

Free quarks cannot exist - they always occur in meson or baryon clusters.

Mesons

• Many types• Decays• Detection

– Interactions with matter

– Calculating combined masses using detected particles

Matter-Antimatter AsymmetryEarly Universe

q q They basically have all annihilated away except a tiny difference between them

10,000,000,001 10,000,000,000

Baryon AsymmetryCurrent Universe

q q They basically have all annihilated away except a tiny difference between them

1

us

Sakharov’s Conditions

• Necessary requirements for genesis of our universe:– CP violation– Baryon, Lepton number violation

• #protons ≠ #anti-protons• #electrons ≠ #positrons

• Consequences– CP violation– Proton decay, etc.

CP violation is experimentally observed in meson systems.

However, all particle reactions observed in nature so far conserve total Baryon and Lepton numbers!

CP Violation: Strange Mesons

• Discovered in kaon system (Cronin and Fitch)

• Theoretically difficult (confinement effect for light quarks in mesons is difficult to compute - relativistic quantum mechanics calculations)

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Neutral k mesons mix : k0(ds), k 0(d s )Experimentally observed KS and KL

ΨKS= pΨ

k o + qΨk o

; ΨKL= pΨ

k o − qΨk o

;

Short and long lived variants observedIf p = q matter and antimatter behave identicallyCP is conservedBut, it was observed that CP is violated in nature

Pursuing studies in more theoretically accessible heavy B meson system

Detector

Particle physicists can reconstruct the events that occur when high energy matter-antimatter are annihilated.

Allows one to probe what happened in the early universe when these particles were abundant!

Upsilon Meson: BaBar Events

CP Violation: B meson system

Summary

• Matter Antimatter Asymmetry– Is necessary for the very existence of our universe– Requires CP violation and Baryon/Lepton number violation

• CP Violation– Observed in K meson system 1964, 1998– Observed in B meson system

• Detailed measurements in progress• Baryon/Lepton number violation

– Proton decay not observed yet– Many theoretical models

• Avenues for exploration abound• Both laboratory and astrophysical searches underway/planned