shortly after the discovery of
CP
violation that three ingre-
dients were needed:
~
1
!
CP
~
and
C
!
violation;
~
2
!
baryon
number violation, and
~
3
!
a period in which the Universe is
not in thermal equilibrium. All of these conditions are ex-
pected to be satisfied in a wide range of theories, such as
grand unified theories
~
Sec. XI E
!
in which quarks and lep-
tons, and the electroweak and strong interactions, are unified
with one another.
274
However, details of the mechanism are
not clear.
112,113
In some versions of the theory, for example, it
is lepton number that is violated in the early stages of the
Universe, giving rise to a lepton asymmetry that is then con-
verted to a mixture of lepton and baryon asymmetry when
the Universe has evolved further. For a recent review of this
suggestion, see Ref. 275.
273.
‘‘Violation of CP Invariance, C Asymmetry, and Baryon Asymmetry
of the Universe,’’ A. D. Sakharov, Pis’ma Zh. E´ ksp. Teor. Fiz.
5
,
32–35
~
1967
! @
JETP Lett.
5
, 24–27
~
1967
!#
.
~
I
!
274.
‘‘Grand Unified Theories and the Origin of the Baryon Asymmetry,’’
E. W. Kolb and M. S. Turner, Ann. Rev. Nucl. Part. Sci.
33
, 645–696
~
1983
!
.
~
I
!
275.
‘‘Neutrino Masses and the Baryon Asymmetry,’’ W. Buchmu¨ ller and
M. Plumacher, Int. J. Mod. Phys. A
15
, 5047–5086
~
2000
!
.
~
I
!
D. Dark matter
Only a small fraction of the matter in the Universe can be
accounted for by baryons, leaving the remainder to consist of
as-yet-unidentified matter or energy density.
121
Candidates
for this
dark matter
are discussed in the
Review of Particle
Physics
.
276
One class of candidates consists of the lightest
supersymmetric particle
~
LSP
!
, which may be stable; these
suggestions are reviewed in Ref. 277.
276.
‘‘Dark Matter,’’ M. Srednicki and N. J. C. Spooner, in
Review of
Particle Physics
, K. Hagiwara
et al.
, Ref. 73, pp. 173–176.
~
I
!
277.
‘‘Supersymmetric Dark Matter,’’ G. Jungman, M. Kamionkowski,
and K. Griest, Phys. Rep.
267
, 195–373
~
1996
!
.
~
I
!
E. Dark energy
The Universe appears not only to be expanding, but its
expansion appears to be speeding up. Evidence for this be-
havior comes from the study of distant supernovae, which
furnish ‘‘standard candles’’ for a cosmological distance
scale.
130,272
One interpretation is that a
cosmological con-
stant
L ~
first proposed by Einstein shortly after he formu-
lated the general theory of relativity
!
accounts for about 65%
of the energy density of the Universe. This contribution is
sometimes referred to as ‘‘dark energy,’’ to distinguish it
from the ‘‘dark matter’’ accounting for nearly all of the re-
maining energy density aside from a few-percent contribu-
tion from baryons.
269,270
An alternative suggestion is that the
‘‘dark energy’’ is due to a new field, dubbed
‘‘quintessence.’’
136
For recent accounts of ‘‘dark energy’’ see
Refs. 278 and 279.
278.
‘‘The Extravagant Universe,’’ R. P. Kirshner
~
Princeton University
Press, 2002.
~
E
!
279.
‘‘The Cosmological Constant and Dark Energy,’’ P. J. E. Peebles and
B. Ratra, preprint astro-ph/0207347, to appear in Rev. Mod. Phys.
~
I
!
XII. EXPERIMENTAL APPROACHES
The rise of the standard model would not have been pos-
sible without a variety of experimental facilities, including
accelerators, detectors, and nonaccelerator experiments.
What follows is a brief description of some currently oper-
ating laboratories and experiments. Fuller descriptions may
be found through laboratory web sites, listed in Sec. VII B,
and through web sites of specific collaborations. Some refer-
ences to recent experiments are given in this section.
A. High energy accelerator facilities
1. Beijing Electron
–
Positron Collider (China)
This electron–positron collider with center-of-mass en-
ergy 2–5 GeV recently reported an improved measurement
of
R
~
see Sec. II C
!
in this energy range.
280
It has made
important contributions to the study of
t
leptons, charmed
particles, and
cc¯
bound states.
2. Brookhaven National Laboratory (USA)
The
Alternating-Gradient Synchrotron
~
AGS
!
is a fixed-
target proton accelerator with maximum energy of about 30
GeV. The first neutrino beam constructed at an accelerator
was used at the AGS to show that the muon and electron
neutrino are distinct from one another.
281
One of its most
spectacular discoveries was the
J
/
c
particle, a bound state of
a charmed quark and a charmed antiquark.
185
Recent experi-
ments include the detection of the rare process
K
1
→
p
1
nn
¯
~
Ref. 282
!
and a precise measurement of the muon anoma-
lous magnetic moment.
283
It serves as an injector to the
Rela-
tivistic Heavy-Ion Collider
~
RHIC
!
, whose maximum energy
of about 200 GeV per nucleon permits studies of the quark–
gluon plasma and other aspects of hadron physics at high
densities.
3. CERN (Switzerland and France)
CERN’s 28-GeV Proton Synchrotron
~
PS
!
began operation
in 1959. It served as a source of protons for the Intersecting
Storage Rings
~
ISR
!
, which began operation in the early
1970s and achieved a maximum center-of-mass energy of 62
GeV. Its protons were used to produce neutrinos which pro-
vided the first evidence for neutral currents in 1973.
177,178
The
Super-Proton-Synchrotron
~
SPS
!
, a 400-GeV fixed-
target machine built in the mid-1970s, was converted to a
proton-antiproton collider
~
the ‘‘S
p¯ p
S’’
!
early in the 1980s,
leading to the discovery of the
W
and
Z
bosons in 1983.
284,285
The Large Electron–Positron
~
LEP
!
Collider
126
was commi-
sioned in 1989, making a series of precise measurements at
the center-of-mass energy of the
Z
boson
~
91.2 GeV
! ~
an
early measurement of the
Z
width pointed to three families of
quarks and leptons
128
!
before moving up in energy to nearly
210 GeV and ending its program in 2000.
286
Its magnets
have been removed, making way for the Large Hadron Col-
lider
~
LHC
!
, a proton–proton collider that will have a c.m.
energy of 14 TeV.
133,287
4. CLEO
Õ
CESR at Cornell (USA)
The Wilson Synchrotron at Cornell, a circular electron ac-
celerator built in 1967, was converted in 1979 to an
electron–positron collider, the Cornell Electron Storage Ring
~
CESR
!
, with maximum energy 8 GeV per beam.
288
It ar-
rived on the scene just in time to study the
Y
(1
S
)
bb¯
reso-
nance and its excited states, including the
Y
(4
S
) which de-
cays to a
BB¯
meson pair. Studies of
B
mesons have
dominated the program of the CLEO detector at CESR until
315
315
Am. J. Phys., Vol. 71, No. 4, April 2003
Jonathan L. Rosner
