Matter and Antimatter: Not Quite a Mirror Image · Matter and Antimatter: Not Quite a Mirror Image ... Patricia Burchat, ... (up, down, strange, charm, bottom, top) Examples of antimatter:

Matter and Antimatter:Not Quite a Mirror Image

(and some recent experimental results from the BABAR experimentat the Stanford Linear Accelerator Center)

Patricia Burchat, Physics DepartmentStanford University

Classes without QuizzesOctober 17, 2003

October 22, 2002 Patricia Burchat, Classes w/o Quizzes 2

The “Big Picture”

Why are we here?


Matter and AntimatterHow do we define matter and antimatter?

Are there any differences in the “static” propertiesof matter and antimatter?

Are there any known differences in the way matterand antimatter evolve in time?

Why would we care whether matter and antimatterevolve differently?

What is the BABAR experiment and how are we using itto explore differences in the evolution of matter andantimatter?


How are matter and antimatter defined?

The elementary particles that make up our everyday world (andthe heavier particles made up of them) are defined to bematter, and their antiparticles are defined to be antimatter.

The choice of which particles we call “matter” and which wecall “antimatter” is arbitrary.

Examples of matter: electrons (e-), quarks (up, down, strange,charm, bottom, top)

Examples of antimatter: positrons (e+), antiquarks (u, d, s, c,b, t )


Matter and antimatter… how are theythe same? how are they different?

Antiparticles have the same mass and lifetime astheir corresponding particles, but they haveopposite values of electric charge as well assome other not-so-familiar properties.

Some particles are their own antiparticle:

e.g., the photon (the particle of light).Particles of light are the same whether they are ina “universe” or an “antiuniverse”.


So how much matter is therein the Universe?

Not a lot…


Most particles & antiparticlesannihilated each other while the universewas still very dense, to form photons (→).

Standard Big-Bang cosmology tells usthat the universe initially contained equalamounts of matter and antimatter.

We are left with aUniverse with a lot ofcosmic microwavephotons and a tiny bitof matter: only oneneutron or proton forevery 10 billionmicrowave photons!Somewhere along theway, particles &antiparticles evolvedslightly differently!


Are the laws of physics the same in auniverse made of matter and a universe

made of antimatter?Until the early 1960’s, it was believed that the answer to the abovequestion was YES: there is no way to distinguish between a universemade of matter and a universe made of antimatter.

By comparing the charge of the particle that is produced slightly morecopiously in this decay to the charge of the particles circulating thenucleus of our atoms, we can tell whether we live in a universe made ofmatter (electrons in the atom) or antimatter (positrons in the atom).

In 1964, it was discovered that matter and antimatter evolvedifferently in time. This phenomenon is called “CP violation”. It wasfound that a particular heavy unstable particle, which is its ownantiparticle, decays slightly more often to positrons (e+) than toelectrons (e-).


The New York TiCP Violation Saves Civilization!People around the world are grateful tophysicists today as a doomed visit from thePlanet-X delegation was called off at the lastminute. “I never thought this stuff was useful”,one physicst was overheard saying...

??

X or X?


Can we explain the excess of matter overantimatter QUANTITATIVELY?

In the Standard Model of particle physics, we have away of accommodating a difference between theevolution of matter and antimatter, but it falls shortof explaining the net excess of matter in theuniverse by about 10 orders of magnitude ! !

With the BABAR experiment at SLAC and a similarexperiment in Japan (called Belle), we are testing theStandard Model predictions for differences betweenthe time evolution of matter and antimatter (CPviolation).


Two new “Asymmetric-energy B Factories” startedaccumulating data ~June 1999

Asymmetric-energy e+e- storage rings⇒ B mesons are moving in the laboratory frame of reference.

What is a B anyway? It is a particle made up of a heavyquark called the “bottom” quark and an ordinary light quark(“up” or “down”).

The BABAR experiment at the PEP-II storage ringat the Stanford LinearAccelerator Center

The Belle experimentat the KEKB storage ringat the KEK Laboratory in Japan


The Asymmetric-EnergyB Factories

Δz

ϒ(4S)e -

B0 / B0

B0 / B0

e +


The BABAR Author List

Large InternationalCollaborations:

BABAR has ~500collaborators from~70 institutions;

Belle has ~270collaborators from~45 institutions.


The Asymmetric-Energy B Factory atthe Stanford Linear Accelerator Center

The BABAR Detector


How many B’s does a BFactory produce anyway?

BABAR and Belle each record about 5 to 10BB “events” per second, ~24 hours a day, 7days a week, for many months at a time.

So far, BABAR and Belle have each recordedover 100 million BB pairs.


Blind Analysis Techniques

We use a technique that hides not only the result ofthe fit, but also the visual CP asymmetry in the timedistribution.

The statistical error on the asymmetry is not hidden.

BABAR and Belle both use “blind”analysis strategies for theextraction of the time-dependentasymmetry in order to minimizepossible experimenters’ bias.


Since we have a “Factory”, we musthave a lot of signal events, right?

Wrong…

~200 million B pairs have been recorded and analysed by BABAR and Belle.

~100 million of these are neutral B pairs.

~one B in a thousand decays to the CP final states we need.

Of these, ~10% decay into final states we can reconstruct.

Of these, ~50% pass all the selection criteria.

We are left with about 5000 signal events.


CP violation in decays of B mesons is expected to exhibit itselfas oscillations in the decay rate.

Dec

ay R

ate

time (ps)

B0

B0

Decay Time(picoseconds)

~ few

Ratio of oscillation frequency to decayrate:

verylarge

In B decays, the oscillationfrequency is small

compared to the decayrate!

~ 0.1


With symmetric beam energies, we cannot measure the difference indecay times for the Btag and BCP.

Δz ≈ 20 µm

Btag BCP

5.3 GeV 5.3 GeV

e+

With asymmetric beam energies, we can measure the difference in decaytimes by measuring the difference in decay positions.

Δz ≈ 255 µm

Btag BCP

9.0 GeV 3.1 GeV

Βzγ ≈ 0.55

e+

Δz can be positiveor negative.


⇒ We measure the time between decays by measuring thedistance between the decays.A time interval of ~1 picosecond is translated into adistance of ~150 microns.


From the ideal world to “reality”…

Btag= B0Btag= B0

First add effect of imperfect tagging.

Time-dependent CP asymmetry is diluted.

Btag= B0Btag= B0

Now add effect of imperfectmeasurement of Δt.

Btag= B0 Btag= B0

Finally add background contribution.

Btag= B0 Btag= B0


B→J/ψ K0S

J/ψ →e+e-

K0S → π+ π-

An Example of an “Event”


sin2β = 0.74 ± 0.07 ± 0.03

BABAR Ks modes KL modes


sin2β = 0.82 ± 0.12 ± 0.05

Belle

Red Curve (B0) minus

Blue Curve (B0)

sin2β is the amplitudeof this asymmetry.

Δt (ps)-8 +80

B0

B0


Constraints on upper vertex of Unitarity Triangle fromall measurements EXCEPT sin2β

Regions of >5% CL

With BABAR and Belle, we aremeasuring directly one of theangles of the green triangleshown in the figure.

Real axis

Im

agin

ary

axi

s

β


World Averagesin2β = 0.73 ± 0.06

The Standard Modelwins again … at least atthe current level ofexperimental precision,in this decay mode.

Real axis

Im

agin

ary

axi

s


This will open up opportunities for not only more precisemeasurements of the angle β, but also measurements ofthe other two angles in the “Unitarity” triangle, furtherconstraining the Standard Model and increasing oursensitivity to physics beyond the Standard Model.

Future ProspectsThe B Factories are working on “upgrades” to further increase the rate atwhich B mesons are produced.

In addition, new measurements will come from proton accelerators(Fermilab in the near term and the Large Hadron Collider at CERN in thelonger term).

βα

γ

IF we find an inconsistency between our measurements and the predictionsof the Standard Model, we may have a hint of the “Physics Beyond theStandard Model” that is necessary to explain how we ended up with anexcess of matter over antimatter in the Universe.


The End

Acknowledgements: Graphics on pp. 7, 9, 18, 21were borrowed from David Kirkby (UCI) with

permission.

Documents

Matter and Antimatter: Not Quite a Mirror Image · Matter and Antimatter: Not Quite a Mirror Image ... Patricia Burchat, ... (up, down, strange, charm, bottom, top) Examples of antimatter: