8 Beyond the Standard Model
457
convergence of the all the couplings to a single value at an appropriate energy
scale, there should be physics beyond the standard model. In Super Symmetric
(SUSY) systems this indeed may occur. In fact the unification is achieved at a
distance scale rather close to the Planck scale, the scale at which gravity becomes
strong and its quantum effects, if there, should become noted. Thus the fourth
force Gravity is naturally added to the unification scheme. An older realization
of this TP was that all known interactions are but the low energy descriptions of a
system containing only the gravitational force and residing in a higher number of
space-time dimensions. This had led to Kaluza Klein theories and their variants.
This idea has been revisited with the advent of SUSY and string theory.
• The observation of what is called dark energy and its very possible confirmation
of a very small, but not vanishing, cosmological constant is viewed by many
as a major problem lacking an explanation. The TP behind this is that any
fundamental quantitative property of nature should be explained and not fine
tuned. The absence of any significant amount of CP violation in the strong
interactions is another such problem. The discovery of a new particle, the Axion,
could be a signal of the solution of the latter one. The discovery of another
particle, the Dilaton, could indicate a resolution of the former. This issue has
led also to the reexamination of the so called enthropic principle.
• Many predictions of Classical General Relativity were confirmed experimentally
over the years. The TP in 2007 is that there should be a quantum theory of gravity.
Such a four dimensional quantum theory of gravity is not well defined within
only field theory. This has been a driving force in the study of the properties of a
theory were the basic constituents of nature are not particles but extended objects,
including strings.
• A very successful framework to explain the basic interactions in particle physics
is the so called Wilsonian one. It is very powerful when the laws of physics
are such that different largely separated scales are essentially decoupled from
each other. The physics beneath any energy scale (cutoff) is well described by
operators whose scaling dimensions, in d spacetime dimensions, is not much
larger than the same d. In many cases there is only a finite number of such
operators, i.e. only a finite amount of terms in the Lagrangians describing the
Physics below the energy scale. This TP has been tested successfully time and
again but may eventually be falsified, perhaps in a theory of gravity. But given
the validity of this method physical quantities should be only slightly dependent
on the cutoff scale, and thus on the unknown physics extending beyond it.
Generically the mass of scalar particles is strongly dependent on the cutoff. In
particular instead of the scale of such masses being set by the weak interactions
scale they will depend strongly on the cutoff. The only known, in 2007, physical
scale beyond that of the weak interactions is of the order of the Planck scale. The
discovery of Higgs particles whose masses is in the TeV range or lighter will thus
require a large amount of fine tuning. The TP does not accept that. This is one
manifestation of the so called hierarchy problem. This had led to ideas such as
technicolor and supersymmetry as properties of beyond the standard model.
457
convergence of the all the couplings to a single value at an appropriate energy
scale, there should be physics beyond the standard model. In Super Symmetric
(SUSY) systems this indeed may occur. In fact the unification is achieved at a
distance scale rather close to the Planck scale, the scale at which gravity becomes
strong and its quantum effects, if there, should become noted. Thus the fourth
force Gravity is naturally added to the unification scheme. An older realization
of this TP was that all known interactions are but the low energy descriptions of a
system containing only the gravitational force and residing in a higher number of
space-time dimensions. This had led to Kaluza Klein theories and their variants.
This idea has been revisited with the advent of SUSY and string theory.
• The observation of what is called dark energy and its very possible confirmation
of a very small, but not vanishing, cosmological constant is viewed by many
as a major problem lacking an explanation. The TP behind this is that any
fundamental quantitative property of nature should be explained and not fine
tuned. The absence of any significant amount of CP violation in the strong
interactions is another such problem. The discovery of a new particle, the Axion,
could be a signal of the solution of the latter one. The discovery of another
particle, the Dilaton, could indicate a resolution of the former. This issue has
led also to the reexamination of the so called enthropic principle.
• Many predictions of Classical General Relativity were confirmed experimentally
over the years. The TP in 2007 is that there should be a quantum theory of gravity.
Such a four dimensional quantum theory of gravity is not well defined within
only field theory. This has been a driving force in the study of the properties of a
theory were the basic constituents of nature are not particles but extended objects,
including strings.
• A very successful framework to explain the basic interactions in particle physics
is the so called Wilsonian one. It is very powerful when the laws of physics
are such that different largely separated scales are essentially decoupled from
each other. The physics beneath any energy scale (cutoff) is well described by
operators whose scaling dimensions, in d spacetime dimensions, is not much
larger than the same d. In many cases there is only a finite number of such
operators, i.e. only a finite amount of terms in the Lagrangians describing the
Physics below the energy scale. This TP has been tested successfully time and
again but may eventually be falsified, perhaps in a theory of gravity. But given
the validity of this method physical quantities should be only slightly dependent
on the cutoff scale, and thus on the unknown physics extending beyond it.
Generically the mass of scalar particles is strongly dependent on the cutoff. In
particular instead of the scale of such masses being set by the weak interactions
scale they will depend strongly on the cutoff. The only known, in 2007, physical
scale beyond that of the weak interactions is of the order of the Planck scale. The
discovery of Higgs particles whose masses is in the TeV range or lighter will thus
require a large amount of fine tuning. The TP does not accept that. This is one
manifestation of the so called hierarchy problem. This had led to ideas such as
technicolor and supersymmetry as properties of beyond the standard model.