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Review of Particle Physics

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Abstract

This biennial Review summarizes much of particle physics. Using data from previous editions, plus 2778 new measurements from 645 papers, we list, evaluate, and average measured properties of gauge bosons, leptons, quarks, mesons, and baryons. We also summarize searches for hypothetical particles such as Higgs bosons, heavy neutrinos, and supersymmetric particles. All the particle properties and search limits are listed in Summary Tables. We also give numerous tables, figures, formulae, and reviews of topics such as the Standard Model, particle detectors, probability, and statistics. Among the 108 reviews are many that are new or heavily revised including those on CKM quark-mixing matrix, Vud & Vus, Vcb & Vub, top quark, muon anomalous magnetic moment, extra dimensions, particle detectors, cosmic background radiation, dark matter, cosmological parameters, and big bang cosmology.
University of Zurich
Zurich Open Repository and Archive
Winterthurerstr. 190
CH-8057 Zurich
http://www.zora.uzh.ch
Year: 2008
Review of Particle Physics
Particle Data Group, ; Amsler, C; Doser, M; Bloch, P; Ceccucci, A; Giudice, G F;
Höcker, A; Mangano, M L; Spanier, S; Törnqvist, N A
Particle Data Group,; Amsler, C; Doser, M; Bloch, P; Ceccucci, A; Giudice, G F; Höcker, A; Mangano, M L;
Spanier, S; Törnqvist, N A (2008). Review of Particle Physics. Physics Letters B, 667(1-5):1-5.
Postprint available at:
http://www.zora.uzh.ch
Posted at the Zurich Open Repository and Archive, University of Zurich.
http://www.zora.uzh.ch
Originally published at:
Physics Letters B 2008, 667(1-5):1-5.
Particle Data Group,; Amsler, C; Doser, M; Bloch, P; Ceccucci, A; Giudice, G F; Höcker, A; Mangano, M L;
Spanier, S; Törnqvist, N A (2008). Review of Particle Physics. Physics Letters B, 667(1-5):1-5.
Postprint available at:
http://www.zora.uzh.ch
Posted at the Zurich Open Repository and Archive, University of Zurich.
http://www.zora.uzh.ch
Originally published at:
Physics Letters B 2008, 667(1-5):1-5.
Review of Particle Physics
Abstract
This biennial Review summarizes much of particle physics. Using data from previous editions, plus
2778 new measurements from 645 papers, we list, evaluate, and average measured properties of gauge
bosons, leptons, quarks, mesons, and baryons. We also summarize searches for hypothetical particles
such as Higgs bosons, heavy neutrinos, and supersymmetric particles. All the particle properties and
search limits are listed in Summary Tables. We also give numerous tables, figures, formulae, and
reviews of topics such as the Standard Model, particle detectors, probability, and statistics. Among the
108 reviews are many that are new or heavily revised including those on CKM quark-mixing matrix,
Vud & Vus, Vcb & Vub, top quark, muon anomalous magnetic moment, extra dimensions, particle
detectors, cosmic background radiation, dark matter, cosmological parameters, and big bang cosmology.
–1
NOTE ON SCALAR MESONS
Revised January 2008 by S. Spanier (University of Tennessee),
N.A. T¨ornqvist (University of Helsinki), and C. Amsler (Uni-
versity of Zurich).
I. Introduction:
The scalar mesons are especially important to understand
because they have the same quantum numbers as the vacuum
(JPC =0
++). Therefore they can condense into the vacuum
and break a symmetry like a global chiral U(Nf)×U(Nf). The
details of how this symmetry breaking is implemented in Nature
is one of the most profound problems in particle physics.
In contrast to the vector and tensor mesons, the identifi-
cation of the scalar mesons is a long-standing puzzle. Scalar
resonances are difficult to resolve because of their large decay
widths which cause a strong overlap between resonances and
background, and also because several decay channels open up
within a short mass interval. In addition, the K¯
Kand ηη
thresholds produce sharp cusps in the energy dependence of the
resonant amplitude. Furthermore, one expects non-q¯qscalar
objects, like glueballs and multiquark states in the mass range
below 1800 MeV. For some recent reviews see AMSLER 04,
BUGG 04C, CLOSE 02B, and KLEMPT 07.
Scalars are produced, for example, in πN scattering on
polarized/unpolarized targets, p¯pannihilation, central hadronic
production, J/Ψ, B-, D-andK-meson decays, γγ formation,
and φradiative decays. Experiments are accompanied by the
development of theoretical models for the reaction amplitudes,
which are based on common fundamental principles of two-
body unitarity, analyticity, Lorentz invariance, and chiral- and
flavor-symmetry using different techniques (K-matrix formal-
ism, N/D-method, Dalitz Tuan ansatz, unitarized quark models
with coupled channels, effective chiral field theories like the lin-
ear sigma model, etc.). Dynamics near the lowest two-body
thresholds in some analyses is described by crossed channel (t,
u) meson exchange or with an effective range parameterization
instead of or in addition to resonant features in the s-channel,
only. Furthermore, elastic S-wave scattering amplitudes involv-
ing soft pions have zeros close to threshold (ADLER 65, 65A),
CITATION: C. Amsler et al. (Particle Data Group), PL B667, 1 (2008) (URL: http://pdg.lbl.gov)
July 16, 2008 14:35
–2
which may be shifted or removed in associated production
processes.
The mass and width of a resonance are found from the
position of the nearest pole in the process amplitude (T-
matrix or S-matrix) at an unphysical sheet of the complex
energy plane: (EiΓ/2). It is important to notice that only
in the case of narrow well-separated resonances, far away from
the opening of decay channels, does the naive Breit-Wigner
parameterization (or K-matrix pole parameterization) agree
with this pole position.
In this note, we discuss all light scalars organized in the
listings under the entries (I=1/2) K
0(800) (or κ), K
0(1430),
(I=1)a0(980), a0(1450), and (I=0)f0(600) (or σ), f0(980),
f0(1370), and f0(1500). This list is minimal and does not
necessarily exhaust the list of actual resonances. The (I=2)
ππ and (I=3/2) phase shifts do not exhibit any resonant
behavior. See also our notes in previous issues for further
comments on e.g., scattering lengths and older papers.
II. The I=1/2States: The K
0(1430) (ASTON 88) is per-
haps the least controversial of the light scalar mesons. The
S-wave scattering has two possible isospin channels, I=1/2
and I=3/2. The I=3/2 wave is elastic and repulsive up
to 1.7 GeV (ESTABROOKS 78) and contains no known res-
onances. The I=1/2phase shift, measured from about
100 MeV above threshold in Kp production, rises smoothly,
passes 90at 1350 MeV, and continues to rise to about 170at
1600 MeV. The first important inelastic threshold is (958).
In the inelastic region the continuation of the amplitude is
uncertain since the partial-wave decomposition has several so-
lutions. The data are extrapolated towards the threshold
using effective range type formulas (ASTON 88, ABELE 98)
or chiral perturbation predictions (BERNARD 91, JAMIN 00,
CHERRY 01). In analyses using unitarized amplitudes there is
agreement on the presence of a resonance pole around 1410 MeV
having a width of about 300 MeV. With reduced model depen-
dence (LINK 07) finds a larger width of 500 MeV.
July 16, 2008 14:35
–3
In recent years there has been controversy about the ex-
istence of a light and very broad “κ” meson in the 700-
900 MeV region. Hadronic D-meson decays provide additional
data points in the vicinity of the threshold - experimental
results from E791 (e.g. AITALA 02, 06), FOCUS (LINK 02,
07), CLEO (CAWLFIELD 06A), and BaBar (AUBERT 07T)
are discussed in the Review of Charm Dalitz Plot Analyses.
Precision information from semileptonic Ddecays avoiding
theoretically ambiguous three-body final state interactions is
not available. BES II finds a κlike structure in J/ψdecays
to ¯
K0(892)K+πwhere κrecoils against the K(892) (AB-
LIKIM 06C, re-analyzed by (GUO 06)). Also clean with respect
to final state interaction is the decay τK0
Sπντstudied
by Belle (EPIFANOV 07), with K(800) parameters fixed to
(ABLIKIM 06C).
Some authors find a κpole in their phenomenological anal-
ysis (see e.g. ANISOVICH 97C, DELBOURGO 98, OLLER 99,
99C, JAMIN 00, SHAKIN 01, SCADRON 03, BLACK 01,03,
BUGG 03, ISHIDA 03, ZHENG 04, PALAEZ 04A, ZHOU 06,
CAWLFIELD 06A, LINK 07B), while others do not (e.g.
AUBERT 07T, LINK 02E, 05I, CHERRY 01, KOPP 01). Since
it appears to be a very wide object (Γ 500 MeV) near the
threshold, its presence and properties have been difficult to
establish.
Recently a pole position for the κwas found in a theo-
retical analysis by DESCOTES-GENON 06 in the
amplitude on the second sheet. Their analysis involves the Man-
delstam representation, which includes unitarity, analyticity and
crossing symmetry. The precise position of the pole should be
confirmed by independent analyses and different experiments.
III. The I=1States: Two isovector states are known,
the established a0(980) and the a0(1450). Independent of any
model, the K¯
Kcomponent in the a0(980) wave function must
be large: it lies just below the opening of the K¯
Kchannel to
which it strongly couples. This generates an important cusp-
like behavior in the resonant amplitude. Hence, its mass and
width parameters are strongly distorted. To reveal its true
July 16, 2008 14:35
–4
coupling constants, a coupled channel model with energy-
dependent widths and mass shift contributions is necessary. In
all measurements in our listings, the mass position agrees on
a value near 984 MeV, but the width takes values between 50
and 100 MeV, mostly due to the different models. For example,
the analysis of the p¯p-annihilation data (ABELE 98) using an
unitary K-matrix description finds a width as determined from
the T-matrix pole of 92 ±8 MeV, while the observed width of
the peak in the πη mass spectrum is about 45 MeV.
The relative coupling K¯
K/πη is determined indirectly from
f1(1285) (BARBERIS 98C, CORDEN 78, DEFOIX 72) or
η(1410) decays (BAI 90C, BOLTON 92B, AMSLER 95C), from
the line shape observed in the πη decay mode (FLATTE 76,
AMSLER 94D, BUGG 94, JANSSEN 95), or from the coupled-
channel analysis of ππη and K¯
final states of p¯pannihilation
at rest (ABELE 98).
The a0(1450) is seen in p¯pannihilation experiments with
stopped and higher momenta ¯p, with a mass of about 1450 MeV
or close to the a2(1320) meson which is typically a dominant
feature. The broad structure at about 1300 MeV observed in
πN K¯
KN reactions (MARTIN 79) needs further confirma-
tion in its existence and isospin assignment.
IV. The I=0States: The I=0JPC =0
++ sector is
the most complex one, both experimentally and theoretically.
The data have been obtained from ππ,K¯
K,ηη,4π,and
ηη(958) systems produced in S-wave. Analyses based on several
different production processes conclude that probably four poles
are needed in the mass range from ππ threshold to about
1600 MeV. The claimed isoscalar resonances are found under
separate entries σor f0(600), f0(980), f0(1370), and f0(1500).
For discussions of the ππ S wave below the K¯
Kthreshold
and on the long history of the σ(600), which was suggested
in linear sigma models about 50 years ago, see our reviews in
previous editions and the conference proceedings KYOTO 00.
Information on the ππ S-wave phase shift δI
J=δ0
0was al-
ready extracted 30 years ago from the πN scattering (GRAYER
74, BECKER 79), and near threshold from the Ke4-decay
(ROSSELET 77). The reported ππ K¯
Kcross sections
July 16, 2008 14:35
–5
(WETZEL 76, POLYCHRONAKOS 79, COHEN 80, and
ETKIN 82B) have large uncertainties. Recently, the πN data
have been analyzed in combination with high-statistics data
(see entries labeled as RVUE for re-analyses of the data). The
2π0invariant mass spectra of the p¯pannihilation at rest (AM-
SLER 95D, ABELE 96) and the central collision (ALDE 97)
do not show a distinct resonance structure below 900 MeV,
but these data are consistently described with the standard
solution for πN data (GRAYER 74, KAMINSKI 97), which
allows for the existence of the broad σ. An enhancement is ob-
served in the π+πinvariant mass near threshold in the decays
D+π+ππ+(AITALA 01B, LINK 04, BONVICINI 07)
and J/ψ ωπ+π(AUGUSTIN 89, ABLIKIM 04A), and in
ψ(2S)J/ψπ+πwith very limited phase space (GALLE-
GOS 04, ABLIKIM 07A).
The precise σpole is difficult to establish because of its
large width, and because it can certainly not be modelled by
a naive Breit-Wigner resonance. It is distorted by background
as required by chiral symmetry, and from crossed channel ex-
changes, the f0(1370), and other dynamical features. However,
most of the analyzes under f0(600) listed in our previous issues
agree on a pole position near (500 i250 MeV).
The existence of the light and very broad σresonance in the
500 MeV region has been proposed by many authors for over 10
years. In particular, data analyses that included unitarity, ππ
threshold behavior and the chiral symmetry constraints from
Adler zeroes and scattering lengths needed the light and broad
σin the ππ data.
A precise pole position with an uncertainty of less than
20 MeV (see our table for T-matrix pole) is derived by
CAPRINI 06 using unitarized chiral perturbation theory. An
important ingredient is the use of Roy-Steiner equations derived
from crossing symmetry, analyticity and unitarity. With these
constraints CAPRINI 06 find that their position of the σpole
depends, almost exclusively, only on the value of the isosinglet
S-wave phase shift at 800 MeV and the S-wave scattering
July 16, 2008 14:35
–6
lengths a0
0and a2
0. Using analyticity and unitarity only to de-
scribe data from K2πand Kl4decays GARCIA-MARTIN 07
find comparable pole position and scattering length a0
0.
PENNINGTON 06, 07 found that the data for σγγ
are consistent with what is expected for a two step process of
γγ π+πvia pion exchange in the t-andu-channel, followed
by a final state interaction π+ππ0π0. Therefore it may
be difficult to learn anything new about the nature of the σ
from its γγ coupling. There are theoretical indications (e.g.
PELAEZ 06, CHEN 07A, GIACOSA 07, MAIANI 07) that the
σpole behaves differently from a q¯q-state.
The f0(980) overlaps strongly with the σand background
represented by a very slow varying phase extending to higher
masses and/or the f0(1370). This can lead to a dip in the ππ
spectrum at the K¯
Kthreshold. It changes from a dip into
a peak structure in the π0π0invariant mass spectrum of the
reaction πpπ0π0n(ACHASOV 98E), with increasing four-
momentum transfer to the π0π0system, which means increasing
the a1-exchange contribution in the amplitude, while the π-
exchange decreases. The σ,andthef0(980), are also observed
in radiative decays (φf0γ) in SND data (ACHASOV 00F,
ACHASOV 00H), CMD2 (AKHMETSHIN 99B), and in KLOE
data (ALOISIO 02C, AMBROSINO 07). Analyses of γγ ππ
data (BOGLIONE 99, MORI 07) underline the importance of
the K¯
Kcoupling of f0(980).
The f0’s above 1 GeV. A meson resonance that is very
well studied experimentally, is the f0(1500) seen by the Crystal
Barrel experiment in five decay modes: ππ,K¯
K,ηη,ηη(958),
and 4π(AMSLER 95D, ABELE 96, and ABELE 98). Due to
its interference with the f0(1370) (and f0(1710)), the peak at-
tributed to f0(1500) can appear shifted in invariant mass spec-
tra. Therefore, the application of simple Breit-Wigner forms
arrive at slightly different resonance masses for f0(1500). Anal-
yses of central-production data of the likewise five decay modes
(BARBERIS 99D, BARBERIS 00E) agree on the description of
the S-wave with the one above. The p¯p,p¯n/n¯p(GASPERO 93,
ADAMO 93, AMSLER 94, ABELE 96) show a single enhance-
ment at 1400 MeV in the invariant 4πmass spectra, which is
July 16, 2008 14:35
–7
resolved into f0(1370) and f0(1500) (ABELE 01, ABELE01B).
The data on 4πfrom central production (BARBERIS 00C) re-
quire both resonances, too, but disagree on the relative content
of ρρ and σσ in 4π. All investigations agree, that the 4πdecay
mode represents about half of the f0(1500) decay width and is
dominant for f0(1370).
The determination of the ππ coupling of f0(1370) is ag-
gravated by the strong overlap with the broad f0(600) and
f0(1500). Since it does not show up prominently in the 2πspec-
tra, its mass and width are difficult to determine. Multichannel
analyses of hadronically produced two- and three-body final
states agree on a mass between 1300 MeV and 1400 MeV and
a narrow f0(1500), but arrive at a somewhat smaller width for
f0(1370).
Both Belle and BaBar have observed strong indications of
scalars in B meson decays. They observe a broad structure
between 1 and 1.6 GeV in K+Kand π+πdecays (GAR-
MASH 02, 06, 07, AUBERT 06O, 07BB). It could be a result of
interference of several resonances in this mass range, but lack
of statistics prevent from an unambiguous identification of this
effect.
V. Interpretation of the scalars below 1 GeV: In the
literature, many suggestions are discussed such as conventional
q¯qmesons, q¯qq¯qor meson-meson bound states mixed with a
scalar glueball. In reality, they can be superpositions of these
components, and one depends on models to determine the
dominant one. Although we have seen progress in recent years,
this question remains open. Here, we mention some of the
present conclusions.
If one uses the naive quark model it is natural to assume the
f0(1370), a0(1450), and the K
0(1430) are in the same SU(3)
flavor nonet being the (u¯u+d¯
d), u¯
dand u¯sstate, respec-
tively. In this picture, the choice of the ninth member of the
nonet is ambiguous. The controversially discussed candidates
are f0(1500) and f0(1700). Compared to the above states, the
f0(1500) is very narrow. Thus, it is unlikely to be their isoscalar
partner. It is also too light to be the first radial excitation.
July 16, 2008 14:35
–8
The f0(980) and a0(980) are often interpreted as multi-
quark states (JAFFE 77, ALFORD 00, MAIANI 04A) or K¯
K
bound states (WEINSTEIN 90). The insight into their internal
structure using two-photon widths (BARNES 85, LI 91, DEL-
BOURGO 99, LUCIO 99, ACHASOV 00H) is not conclusive.
The f0(980) appears as a peak structure in J/ψ φπ+π
and in Dsdecays without f0(600) background. Based on that
observation it is suggested that f0(980) has a large s¯scom-
ponent, which according to (DEANDREA 01) is surrounded
by a virtual K¯
Kcloud. Data on radiative decays (φf0γ
and φa0γ) from SND, CMD2, and KLOE (see above) favor
a 4-quark picture of the f0(980) and a0(980). The underlying
model for this conclusion (BOGLIONE 03, OLLER 03B) how-
ever may be oversimplified. But it remains quite possible that
the states f0(980) and a0(980), together with the f0(600) and
the K
0(800), form a new low-mass state nonet of predominantly
four-quark states, where at larger distances the quarks recom-
bine into a pair of pseudoscalar mesons forming by a meson
cloud.
Attempts have been made to start directly from chiral
Lagrangians (SCADRON 99, OLLER 99, ISHIDA 99, TORN-
QVIST 99, OLLER 03B, NAPSUCIALE 04, 04A) which predict
the existence of the σmeson near 500 MeV. Hence, e.g.,inthe
chiral linear sigma model with 3 flavors, the σ,a0(980), f0(980),
and κ(or K
0(1430)) would form a nonet (not necessarily q¯q),
while the lightest pseudoscalars would be their chiral partners.
In such models inspired by the linear sigma model the
light σ(600) is often referred to as the ”Higgs boson of strong
interactions”, since the σplays a role similar to the Higgs
particle in electro-weak symmetry breaking. It is important for
chiral symmetry breaking which generates most of the proton
and ηmass, and what is referred to as the constituent quark
mass.
In the approach of (OLLER 99) the above resonances are
generated starting from chiral perturbation theory predictions
near the first open channel, and then by extending the predic-
tions to the resonance regions using unitarity.
July 16, 2008 14:35
–9
In the unitarized quark model with coupled q¯qand meson-
meson channels, the light scalars can be understood as addi-
tional manifestations of bare q¯qconfinement states, strongly
mass shifted from the 1.3 - 1.5 GeV region and very distorted
due to the strong 3P0coupling to S-wave two-meson decay
channels (TORNQVIST 95, 96, BEVEREN 86, 99, 01B). Thus,
the light scalar nonet comprising the f0(600), f0(980), K
0(800),
and a0(980), as well as the regular nonet consisting of the
f0(1370), f0(1500) (or f0(1700)), K
0(1430), and a0(1450), re-
spectively, are two manifestations of the same bare input states
(see also BOGLIONE 02).
Other models with different groupings of the observed
resonances exist and may e.g. be found in earlier versions of
this review and papers listed as other related papers below.
VI. Interpretation of the f0’s above 1 GeV: The f0(1370)
and f0(1500) decay mostly into pions (2πand 4π) while the
f0(1710) decays mainly into K¯
Kfinal states. The K¯
Kdecay
branching ratio of the f0(1500) is small (ABELE 96B,98, BAR-
BERIS 99D). Naively, this suggests a n¯n(= u¯u+d¯
d) structure
for the f0(1370) and f0(1500), and s¯sfor the f0(1710). The
latter is not observed in p¯pannihilation (AMSLER 02), as
expected from the OZI suppression for an s¯sstate.
However, in γγ collisions leading to K0
SK0
S(ACCIA-
RRI 01H) and K+K(ABE 04), a spin 0 signal is observed
at the f0(1710) mass (together with a dominant spin 2 com-
ponent), while the f0(1500) is not observed in γγ K¯
Knor
π+π(BARATE 00E). The upper limit from π+πexcludes a
large n¯ncontent, and hence would point to a mainly s¯scontent
for the f0(1500) (AMSLER 02B). This appears to contradict
the small K¯
Kdecay branching ratio of the f0(1500) and makes
aq¯qassignment difficult for this state. Hence the f0(1500)
could be mainly glue due its absence of 2γ-coupling, while the
f0(1710) coupling to 2γwould be compatible with an s¯sstate.
However, the 2γ-couplings are sensitive to glue mixing with q¯q
(CLOSE 05).
The narrow width of f0(1500), and its enhanced produc-
tion at low transverse momentum transfer in central collisions
(CLOSE 97,98B, KIRK 00) also favor f0(1500) to be non-q¯q.
July 16, 2008 14:35
– 10–
In the mixing scheme of CLOSE 05, which uses central produc-
tion data from WA102 and the recent hadronic J/ψ decay data
from BES (ABLIKIM 04E, 05), glue is shared between f0(1370),
f0(1500) and f0(1710). The f0(1370) is mainly n¯n,thef0(1500)
mainly glue and the f0(1710) dominantly s¯s. This agrees with
previous analyses (AMSLER 96, CLOSE 01B), but alternative
schemes have been proposed (e.g. LEE 00, MINKOWSKI 99;
for a review see e.g. AMSLER 04). In particular, for a scalar
glueball, the two-gluon coupling to n¯nappears to be suppressed
by chiral symmetry (CHANOWITZ 05) and therefore the K¯
K
decay could be enhanced.
Whether the f0(1500) is observed in ’gluon rich’ radiative
J/ψ decays is debatable (ABLIKIM 06V) because of the limited
amount of data - more data for this and the γγ mode are needed.
References
References can be found at the end of the f0(600) listing.
July 16, 2008 14:35
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