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R. N. Mohapatra
happens since hydrogen atoms are neutral. We see this light as the cosmic
background radiation. It tells us about the epoch of the universe when atoms
formed. Details of how different structures in the universe are distributed are a
matter of intense debate and research right now. The nature of the dark matter
plays a very critical role in the formation of structure in different regions of
the universe.
24.2 What is the Dark Matter?
Clearly, dark matter is a new kind of particle which is not present in the
standard model. The particle has no electric charge, otherwise it would not
be dark. It most likely has no nuclear interactions. We do not know whether
it has half integral spin (i.e. a fermion) or integral spin (boson). Whether it
has some kind of very weak interaction with normal visible matter (protons,
neutrons, electrons, etc.) is a subject of research right now. There are many
experiments searching for dark matter and so far there is no positive indication
of its detection from any experiment.
3 Most stringent constraints on possible
dark matter properties are from a recent experiment called Xenon1T experiment [10] that uses liquid xenon as the detector of dark matter. It is also set
up inside the same Gran Sasso laboratory in Italy where other experiments
are carried out. The precision of dark matter particle research has reached
such a stage that it will soon become impossible to distinguish it from the
neutrino related background from solar, atmospheric neutrinos, which will
produce similar kind of signal to dark matter being. This is called the “neutrino
floor” [22].
There have been many dark matter candidates discussed in the literature.
They cover physics ideas, all the way from supersymmetric dark matter where
the dark matter is heavy to the case of axion dark matter which is light.
These ideas were proposed independently of their connection to dark matter.
For example, the supersymmetry was proposed to solve the gauge hierarchy
3 There is an experiment in the underground laboratory in Gran Sasso, Italy, where a collaboration called
DAMA and DAMA-LIBRA [20] claimed to have found a signal of dark matter. They found a signal that
varies with an annual periodicity as the Earth goes around the Sun. Since this is what one would expect
a dark matter signal to do, there has been claim that this is a discovery of dark matter. The reason such
annual periodicity is expected to happen is that the dark matter moves with a speed of roughly 300 km per
second in one direction. As the Earth goes around the Sun, on one side it faces the moving dark matter
as head wind, and 6 months later, as tail wind. The signal in the experiment depends on the speed of the
detector on Earth with respect to dark matter. This result has not been confirmed by any independent
experiment and therefore remains controversial.
R. N. Mohapatra
happens since hydrogen atoms are neutral. We see this light as the cosmic
background radiation. It tells us about the epoch of the universe when atoms
formed. Details of how different structures in the universe are distributed are a
matter of intense debate and research right now. The nature of the dark matter
plays a very critical role in the formation of structure in different regions of
the universe.
24.2 What is the Dark Matter?
Clearly, dark matter is a new kind of particle which is not present in the
standard model. The particle has no electric charge, otherwise it would not
be dark. It most likely has no nuclear interactions. We do not know whether
it has half integral spin (i.e. a fermion) or integral spin (boson). Whether it
has some kind of very weak interaction with normal visible matter (protons,
neutrons, electrons, etc.) is a subject of research right now. There are many
experiments searching for dark matter and so far there is no positive indication
of its detection from any experiment.
3 Most stringent constraints on possible
dark matter properties are from a recent experiment called Xenon1T experiment [10] that uses liquid xenon as the detector of dark matter. It is also set
up inside the same Gran Sasso laboratory in Italy where other experiments
are carried out. The precision of dark matter particle research has reached
such a stage that it will soon become impossible to distinguish it from the
neutrino related background from solar, atmospheric neutrinos, which will
produce similar kind of signal to dark matter being. This is called the “neutrino
floor” [22].
There have been many dark matter candidates discussed in the literature.
They cover physics ideas, all the way from supersymmetric dark matter where
the dark matter is heavy to the case of axion dark matter which is light.
These ideas were proposed independently of their connection to dark matter.
For example, the supersymmetry was proposed to solve the gauge hierarchy
3 There is an experiment in the underground laboratory in Gran Sasso, Italy, where a collaboration called
DAMA and DAMA-LIBRA [20] claimed to have found a signal of dark matter. They found a signal that
varies with an annual periodicity as the Earth goes around the Sun. Since this is what one would expect
a dark matter signal to do, there has been claim that this is a discovery of dark matter. The reason such
annual periodicity is expected to happen is that the dark matter moves with a speed of roughly 300 km per
second in one direction. As the Earth goes around the Sun, on one side it faces the moving dark matter
as head wind, and 6 months later, as tail wind. The signal in the experiment depends on the speed of the
detector on Earth with respect to dark matter. This result has not been confirmed by any independent
experiment and therefore remains controversial.