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What’s the Matter With Antimatter? Or, The Leftover Universe. Dr. Natalie A. Roe Lawrence Berkeley National Laboratory . Outline. What is Antimatter? (What is Matter? ) What is the Matter with Antimatter New Result on CP Violation from the Asymmetric B Factory Why Does it Matter?.

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what s the matter with antimatter or the leftover universe

What’s the Matter With Antimatter?Or, The Leftover Universe

Dr. Natalie A. Roe

Lawrence Berkeley National Laboratory

  • What is Antimatter? (What is Matter? )
  • What is the Matter with Antimatter
  • New Result on CP Violation from the Asymmetric B Factory
  • Why Does it Matter?
what is matter
What is Matter?
  • What are the fundamental constituents?
  • Greeks: Earth, Air, Fire, Water
  • 1800’s: Periodic table of the elements
  • 1897: Thomson discovers the electron
  • 1919: Rutherford discovers the proton
  • 1932: Chadwick discovers the neutron
  • 1967: Kendall, Friedman and Taylor discover quarks in electron-nucleon scattering experiments at SLAC.
  • Quarks
      • are fractionally charged
      • always occur in pairs or triplets, never singly
      • come in 6 different kinds, arranged in 3 generations
why three generations
Why Three Generations?
  • Only the first generation of particles is needed to make our world
    • proton = (uud) and neutron = (udd)
    • protons and neutrons form nuclei, electrons to form neutral atoms
    • 3 fundamental particles to make all of the Universe!






  • Nature has 2 additional generations of heavier, unstable particles
    • Why?
    • What determines the quark and lepton masses?
    • Central questions for particle physicists

Increasing Mass -->

the prediction of antimatter

Later, Dirac predicted the existence of the positron and, by symmetry, that of the anti-proton, or p

The Prediction of Antimatter
  • Nature is full of symmetries; mathematics is the language we use to describe symmetry
  • Dirac’s equation for motion of a relativistic electron (1928) had two solutions
    • one was the electron
    • the other was an electron with “negative energy”
  • Dirac originally attributed this solution to the proton (only known positively charged particle in 1928!)
matter and energy








Matter and Energy
  • Einstein first realized the equivalence of matter and energy
  • When matter and antimatter meet, they annihilate into energy
  • Energy can also materialize as particle-antiparticle pairs
the discovery of antimatter
The Discovery of Antimatter
  • The positron was discovered in 1932 in cosmic rays by Carl Anderson at Caltech
    • The photograph shows how positrons were first identified in cosmic rays using a cloud chamber, magnetic field and lead plate
more about antimatter
More About Antimatter
  • All charged leptons and quarks have anti-matter partners
    • n may be its own anti-particle
  • Anti-proton was discovered in 1955 at Berkeley
    • billions are now produced every year at accelerator labs
  • Quark-antiquark combinations called mesons also exist
    • p+ = ud K0= ds
    • B0= bd B0= bd
imaging with antimatter





Imaging With Antimatter
  • PET scans use positrons created in radioactive beta decay, by detecting the photons created in the subsequent e+ e- annihilation

How to create and store anti-particles

1. create p p pairs;

separate out the p

2. store p with magnetic fields;

accelerate with RF cavities

colliding protons and anti protons
Colliding protons and anti-protons
  • At Fermilab, located near Chicago, protons and antiprotons collide with the highest energy available in the world
  • The discovery of the top quark was made in 1995 at the Tevatron Collider
colliding electrons and positrons
Colliding electrons and positrons
  • At Stanford, electron-positron collisions have been used to discover the charm quark, the tau lepton - and to study the matter-antimatter asymmetry.

Stanford Linear Accelerator Center,

Stanford, California

what is the matter with antimatter
What is the Matter with Antimatter?
  • In the “Big Bang” particle-antiparticle pairs were created from pure energy in a spontaneous explosion
  • Today we cannot detect significant amounts of antimatter in the universe - why not?
  • Matter and antimatter then annihilated into photons; but a small amount of matter survived - but how?
  • In 1967, Sakharov stated three conditions necessary to create an excess of matter in the early Universe - one of which is CP Violation in particle interactions.
what is cp violation





What is CP Violation?

C: Charge conjugation: particle antiparticle P: Parity (mirror reflection): x  -x

C and P together change matter to antimatter. If the world were CP symmetric, an antimatter world would be indistinguishable from our world. Any difference between matter and antimatter is evidence for CP Violation.Until 1964, most physicists assumed that CP symmetry was obeyed in all particle interactions.

CP Mirror

cp violation was discovered in 1964
CP Violation Was Discovered In 1964


V. Fitch


Cronin and Fitch discovered the violation of CP in the

decay of the long-lived, CP-odd neutral K meson into a CP-even final state: Br(KL -> p+p- ) ~ 0.2%

There is a difference between matter and antimatter!

“We are hopeful… that at some epoch, perhaps distant, this cryptic message from nature will be deciphered.” J. Cronin


The Motivation for Asymmetric B Factories

  • CP violation in K0 (= sd) meson decays has been exhaustively studied, but the effect is small and theoretical uncertainties are large.
  • In B0 (= bd) meson decays, the Standard Model predictions for CP violation are theoretically clean.
  • Large asymmetries between B and anti-B meson decay rates into special CP eigenstates are expected (10- 50%), but the decay rates are small => need a B meson “factory”
  • To observe the CP asymmetry between B and anti-B mesons, a special type of e+e- collider is required with unequal beam energies - the Asymmetric B Factory, proposed by Pier Oddone of Berkeley Lab in 1987







Measuring CP Violation with B0s

Not equal –

CP Violation!

CP violation occurs in the interference between mixing and decay to a CP eigenstate, eg B0-> p+ p -



Stanford Linear Accelerator Center,

Stanford, California

Approved as a Presidential

Initiative in 1993; completed

in 1999. Reached full design

luminosity in 2000.

Japanese B Factory has also

been built with similar design.

the asymmetric b factory concept

e+ e-->U(4S) ->BB

decay of B “tag”

B ->CP eigenstate

Dt µDz

The Asymmetric B Factory Concept

3 GeV e+

9 GeV e-


The BaBar Detector

The BaBar Collaboration:

~500 physicists from 73

institutions and 9 countries


Silicon Vertex Tracker (SVT)

Uses five layers of silicon microstrip

detectors to measure B decay vertices

to better than 0.1 mm

recipe for measuring cp violation in b meson decays
Recipe for Measuring CP Violation in B Meson Decays
  • Produce many B0 B0 pairs (several 10’s of millions)
  • Reconstruct one B in a special decay called a CP eigenstate
  • “Tag” the other B0 to make the matter/antimatter distinction
  • Determine the time between the two B0 decays, Dt, using SVT
  • Compare Dt distributions for B0 and B0 tagged events; the difference measures CP violation, the difference between matter and antimatter

B tagged

B tagged

Dt (ps)


This result: 56 fb-1 on-resonance.

62 million BB events.

The PEP-II asymmetric e+e storage ring

E(e-) = 9.0 GeV, E(e+) = 3.1 GeV

This result

(56 fb-1)

v 0.56 c

Design Achieved


Luminosity (cm-2 s-1) 3 x 1033 4.5 x 1033

Int. Lum / day (pb-1) 135 303

Int. Lum / month (fb-1) 3.3 6.3

2nd PRL (30 fb-1)


1st PRL (20 fb-1)


first observation of cp violation in b decays announced july 6 2001

Dt Distributions for B0

and B0 - tagged events

Difference between B0

and B0 tagged events vs

time between decays

First Observation of CP Violation in B Decays - Announced July 6, 2001

NYT: “Tiny Discovery May Answer a Question About the Big Bang”

-6 -5 -4-3 -2-1 0 1 2 3 4 5 6 7

Dt in trillionths of a second

CP Asymmetry Measuurement: sin2b=0.59±0.14


Latest result:


what does this result mean
What does this result mean?


  • Maximum asymmetry => sin2b = 1
  • Zero asymmetry => sin2b = 0
  • Much larger CP asymmetry than in K0 decays (75% vs 0.2%); and very small theoretical error
  • Error is ~ 1/7 of the central value => very unlikely to be a statistical fluctuation
  • The result is consistent with the prediction of the Standard Model, our best current particle theory
  • But: Standard Model calculations of early Universe do not produce a large enough matter/antimatter asymmetry!
why does it matter
Why Does it Matter?
  • We believe there must be some new physics beyond the Standard Model - this is only one of several clues
  • As we collect more data, we will measure CP violation more precisely, and compare different types of asymmetries
  • These precision tests of the Standard Model may show us an inconsistency which will point the way to new physics
  • This approach is complementary to the searches for new particles at the energy frontier accelerators, such as the Fermilab Tevatron and the CERN Large Hadron Collider
  • Antimatter exists and can be created at accelerators; but there is very little antimatter naturally occurring in our Universe
  • CP violation is required in any theory starting from the Big Bang to explain the dominance of matter over antimatter
  • CP symmetry between matter and antimatter is violated at the quark level
    • recent measurement of CP violation in B mesons by the BaBar experiment
  • We need more CP violation to produce our Universe - our theory is not complete!
could antiprotons be useful
Could Antiprotons Be Useful?
  • Antimatter engines on Starship Enterprise were powered by p + p annihilation...
  • Creating, storing antiprotons requires a lot of energy, and trapping them is also very inefficient
  • All the antiprotons created in one year at Fermilab would only power a 100 watt bulb for 30 minutes, even with 100% trapping and conversion efficiency!

Penning Trap

creating anti hydrogen

Neutral atom is not bent in magnetic field

Antiproton is deflected

and detected

Positron annihilates with electron;

photons are detected

Creating Anti-hydrogen

Anti-protrons are created and stored, then

passed through cold xenon gas. An e+e-

pair is created as the antiproton passes by

the heavy xenon nucleus. The antiproton and

the positron will occasionally form a stable

bound state - an atom of anti-hydrogen!

Detecting Anti-hydrogen