8
G. Altarelli and S. Forte
most recent and accurate values, the reader should consult the most recent edition
of the PDG [25], preferably using the web-based version [26], which is constantly
updated.
Chapter 3 presents the Electroweak sector of the Standard Model, which was
established as a successful theory by extensive experimentation at the LEP electronpositron collider of CERN in the last decade of the past century, including some
aspects of the theory, such as the CKM mechanism for mass mixing (see Sect. 3.6)
which were originally often considered to be only approximate. The discovery, at
the turn of the century, of neutrino mixing, and thus non-vanishing neutrino masses
(see Sect. 3.7) has been the only significant addition to the minimal version of the
electroweak theory as formulated in the sixties and seventies of the past century.
The general understanding of electroweak interactions was thus essentially settled
at the time of the writing of this chapter.
From the experimental point of view, the main development since then is the
successful completion of the first two runs of the LHC, which have provided further
confirmation of the standard Electroweak theory (see Ref. [27] for a recent review).
From a theoretical point of view, the main surprise (from the LHC, but also a number
of other experiments) is that there have been no surprises.
First and foremost, the Higgs sector of the Standard Model: after discovery of
the Higgs boson in 2012 [28, 29] the Higgs sector has turned out so far to be
in agreement with the minimal one-doublet structure presented in Sect. 3.5. The
discussion presented there, as well as the phenomenology of the Standard Model
Higgs of Sect. 3.13, remain thus essentially unchanged by the Higgs discovery. A
theoretical introduction with more specific reference to the LHC can be found in
Ref. [30], while the current experimental status of Higgs properties can be found in
the continually updated pages of the CERN Higgs cross-section working group [31].
Perhaps, the only real surprise in the Higgs sector of the Standard Model is the
extreme closeness of the measured Higgs mass to the critical value required for
vacuum stability (see Sect. 3.13.1 below)—a fact with interesting cosmological
implications [32]. The discovery of the Higgs has changed somewhat the nature of
global fits of Standard Model parameters discussed in Sect. 3.12: with the value of
the Higgs mass known, the fit is over-constrained—though the conclusion of global
consistency remains unchanged. An updated discussion is given in Ref. [27], as well
as in the review on the Electroweak Model by Erler and Freitas in the PDG [26].
Besides Higgs discovery, the general trend of the last several years has been that
of the gradual disappearance of all anomalies—instances of discrepancy between
Standard Model predictions and the data—either due to more accurate theory
calculations (or even the correction of errors: see Sect. 3.9), or to more precise
measurements. A case in point is that of the measurements of the electroweak
mixing angle, discussed in Sect. 3.12: the tensions or signals of disagreement
which are discussed there have all but disappeared, mostly thanks to more accurate
theoretical calculations. Another case in which the agreement between Standard
Model and experiment is improving (albeit perhaps more slowly) is that of lepton
anomalous magnetic moments, discussed in Sect. 3.9. In both cases, updates on the
current situation can again been found in Ref. [27], and in the aforementioned PDG
review by Erler and Freitas.
G. Altarelli and S. Forte
most recent and accurate values, the reader should consult the most recent edition
of the PDG [25], preferably using the web-based version [26], which is constantly
updated.
Chapter 3 presents the Electroweak sector of the Standard Model, which was
established as a successful theory by extensive experimentation at the LEP electronpositron collider of CERN in the last decade of the past century, including some
aspects of the theory, such as the CKM mechanism for mass mixing (see Sect. 3.6)
which were originally often considered to be only approximate. The discovery, at
the turn of the century, of neutrino mixing, and thus non-vanishing neutrino masses
(see Sect. 3.7) has been the only significant addition to the minimal version of the
electroweak theory as formulated in the sixties and seventies of the past century.
The general understanding of electroweak interactions was thus essentially settled
at the time of the writing of this chapter.
From the experimental point of view, the main development since then is the
successful completion of the first two runs of the LHC, which have provided further
confirmation of the standard Electroweak theory (see Ref. [27] for a recent review).
From a theoretical point of view, the main surprise (from the LHC, but also a number
of other experiments) is that there have been no surprises.
First and foremost, the Higgs sector of the Standard Model: after discovery of
the Higgs boson in 2012 [28, 29] the Higgs sector has turned out so far to be
in agreement with the minimal one-doublet structure presented in Sect. 3.5. The
discussion presented there, as well as the phenomenology of the Standard Model
Higgs of Sect. 3.13, remain thus essentially unchanged by the Higgs discovery. A
theoretical introduction with more specific reference to the LHC can be found in
Ref. [30], while the current experimental status of Higgs properties can be found in
the continually updated pages of the CERN Higgs cross-section working group [31].
Perhaps, the only real surprise in the Higgs sector of the Standard Model is the
extreme closeness of the measured Higgs mass to the critical value required for
vacuum stability (see Sect. 3.13.1 below)—a fact with interesting cosmological
implications [32]. The discovery of the Higgs has changed somewhat the nature of
global fits of Standard Model parameters discussed in Sect. 3.12: with the value of
the Higgs mass known, the fit is over-constrained—though the conclusion of global
consistency remains unchanged. An updated discussion is given in Ref. [27], as well
as in the review on the Electroweak Model by Erler and Freitas in the PDG [26].
Besides Higgs discovery, the general trend of the last several years has been that
of the gradual disappearance of all anomalies—instances of discrepancy between
Standard Model predictions and the data—either due to more accurate theory
calculations (or even the correction of errors: see Sect. 3.9), or to more precise
measurements. A case in point is that of the measurements of the electroweak
mixing angle, discussed in Sect. 3.12: the tensions or signals of disagreement
which are discussed there have all but disappeared, mostly thanks to more accurate
theoretical calculations. Another case in which the agreement between Standard
Model and experiment is improving (albeit perhaps more slowly) is that of lepton
anomalous magnetic moments, discussed in Sect. 3.9. In both cases, updates on the
current situation can again been found in Ref. [27], and in the aforementioned PDG
review by Erler and Freitas.