628
K. Peach
these is overwhelming (see reference [11] for a recent review) with a recipe for
the current state of the Universe containing about 70% of Dark Energy, 25% Dark
Matter and 5% “normal” matter, the only part addressed by the Standard Model.
From a particle physics perspective, Dark Matter is more tractable. First, there
is the challenge of directly detecting it using large volume, sensitive detectors
deep underground. The past decade has seen the limits steadily improved (see
reference [12] for a recent review), while the small number of claimed signals
have still not been independently confirmed. There is an ongoing programme of
experiments at the major underground laboratories which should improve these
limits by several order of magnitude over the next few years. Second, there are
straightforward extensions of the Standard Model which provide candidates for
Dark Matter. Unfortunately, there is no reliable estimate for the mass scale at which
these new particles might appear. Such evidence might come from the Dark Matter
searches themselves. The other place where a mass scale might be forthcoming is in
the analysis of the deficiencies of the Standard Model. For example, Supersymmetry
was proposed to address some hierarchy problems within the model, and also
to assist in Grand Unification, both of which pointed to a TeV energy scale for
appearance of new particles. However, so far these have failed to materialize at
the LHC; unfortunately, the parameter space for Supersymmetry is extensive and
so, while this might be disappointing it may not be conclusive. Dark Energy, on the
other hand, is still mysterious and it is unclear where it fits in to the global picture; its
understanding might require the reconciliation of quantum mechanics with general
relativity.
The “open questions within the Standard Model” identified in the original article
have been largely resolved, although there are still details to be addressed, for example the transition from perturbative to non-perturbative QCD. However, the “open
questions beyond the Standard Model” remain, refined somewhat; for example, the
“pattern of fermion masses” might nowadays be cast as “understanding the Yukawa
couplings of the fundamental fermions to the Higgs”. The improvements in the
knowledge of the neutrino oscillation parameters do not explain how the mixing
comes about, nor how the neutrinos derive their (tiny) masses. Again, there has
been much progress in the measurement of the various cosmological parameters,
including those which either determine or are determined by particle physics, but
the “open questions in particle physics and cosmology” are still far from resolved.
For example, the arguments for Grand Unification of the strong and electroweak
forces remain intact (see Fig. 10.1) even though there is, as yet, no direct evidence
for the existence of Supersymmetric particles. A second area which points to new
physics is to note that, of the 18 parameters that define the Standard Model, 11
are directly related to mass (the quarks, the charged leptons, the Z and the Higgs),
which becomes 12 if the Fermi constant is replaced (as it can be) by the mass of
the W. Given this, there are three mass hierarchies to be explained. First, within a
generation, what determines the relative masses of the charged lepton, the downtype quark (charge −1/3) and the up-type quark (charge +2/3) (and why is the
up-quark lighter than the down quark, while the charm and top quarks are heavier
than the strange and bottom quarks)? Second, what determines the large (several
K. Peach
these is overwhelming (see reference [11] for a recent review) with a recipe for
the current state of the Universe containing about 70% of Dark Energy, 25% Dark
Matter and 5% “normal” matter, the only part addressed by the Standard Model.
From a particle physics perspective, Dark Matter is more tractable. First, there
is the challenge of directly detecting it using large volume, sensitive detectors
deep underground. The past decade has seen the limits steadily improved (see
reference [12] for a recent review), while the small number of claimed signals
have still not been independently confirmed. There is an ongoing programme of
experiments at the major underground laboratories which should improve these
limits by several order of magnitude over the next few years. Second, there are
straightforward extensions of the Standard Model which provide candidates for
Dark Matter. Unfortunately, there is no reliable estimate for the mass scale at which
these new particles might appear. Such evidence might come from the Dark Matter
searches themselves. The other place where a mass scale might be forthcoming is in
the analysis of the deficiencies of the Standard Model. For example, Supersymmetry
was proposed to address some hierarchy problems within the model, and also
to assist in Grand Unification, both of which pointed to a TeV energy scale for
appearance of new particles. However, so far these have failed to materialize at
the LHC; unfortunately, the parameter space for Supersymmetry is extensive and
so, while this might be disappointing it may not be conclusive. Dark Energy, on the
other hand, is still mysterious and it is unclear where it fits in to the global picture; its
understanding might require the reconciliation of quantum mechanics with general
relativity.
The “open questions within the Standard Model” identified in the original article
have been largely resolved, although there are still details to be addressed, for example the transition from perturbative to non-perturbative QCD. However, the “open
questions beyond the Standard Model” remain, refined somewhat; for example, the
“pattern of fermion masses” might nowadays be cast as “understanding the Yukawa
couplings of the fundamental fermions to the Higgs”. The improvements in the
knowledge of the neutrino oscillation parameters do not explain how the mixing
comes about, nor how the neutrinos derive their (tiny) masses. Again, there has
been much progress in the measurement of the various cosmological parameters,
including those which either determine or are determined by particle physics, but
the “open questions in particle physics and cosmology” are still far from resolved.
For example, the arguments for Grand Unification of the strong and electroweak
forces remain intact (see Fig. 10.1) even though there is, as yet, no direct evidence
for the existence of Supersymmetric particles. A second area which points to new
physics is to note that, of the 18 parameters that define the Standard Model, 11
are directly related to mass (the quarks, the charged leptons, the Z and the Higgs),
which becomes 12 if the Fermi constant is replaced (as it can be) by the mass of
the W. Given this, there are three mass hierarchies to be explained. First, within a
generation, what determines the relative masses of the charged lepton, the downtype quark (charge −1/3) and the up-type quark (charge +2/3) (and why is the
up-quark lighter than the down quark, while the charm and top quarks are heavier
than the strange and bottom quarks)? Second, what determines the large (several