2 Particles as Building Blocks of Matter
15
particle is moving at the speed of light, as all zero mass particles do. As we will
see later, in the case of a particle with mass, by moving to different reference
frames, we can change from one helicity to another, whereas we cannot do
the same for a massless particle. The massless particles move at the speed of
light, hence, their direction of motion cannot be changed. In other words, a
massive particle needs two helicity states in nature, whereas a massless spin half
particle needs only one helicity state. This concept has profound implication
for the neutrino, which was thought to be massless for a long time, as we
discuss later.
2.5 Stable and Unstable Particles
A stable particle stays like it is forever, whereas an unstable particle disappears
into other less heavy particle in a certain amount of time. The time it takes for
a particle to decay to roughly one third of its original amount is called its
lifetime. Of the many particles discovered to date using different methods
like colliders or cosmic rays, there are very few which are absolutely stable.
For example, we know that protons and electrons are the only two absolutely
stable elementary particles. Neutrons are unstable, and they decay to a proton,
electrons, and anti-neutrinos, in a little under 15 min. As we will see, this decay
unleashed a huge research area known as weak interactions, and eventually
led to the rise of the standard model, as the theory of all forces and particles
except for gravitational forces. It turns out that once the neutron is inside a
nucleus, it becomes part of the nucleus and often becomes a stable particle.
That is because it effectively loses part of its mass (relative to its mass when it
is free) to the binding by nuclear force. As a result, it does not have enough
energy to decay to a proton, electron, and neutrino. This is due to the fact
that “all processes in nature must conserve the total energy,” which is a cardinal
principle of physics and is a law known to be absolutely valid. The loss of mass
of the neutron inside a nucleus is called its binding energy.
If a particle is unstable, it will decay to some other particles as noted and
that provides a gold mine of information about both the particle that decays
and those that come out in the final state and the nature of the force that causes
this. For instance, how it decays tells us about the interaction of the particles.
Similarly, accounting for all the states it decays to gives a fuller detail about
the interaction of the particle in question.
15
particle is moving at the speed of light, as all zero mass particles do. As we will
see later, in the case of a particle with mass, by moving to different reference
frames, we can change from one helicity to another, whereas we cannot do
the same for a massless particle. The massless particles move at the speed of
light, hence, their direction of motion cannot be changed. In other words, a
massive particle needs two helicity states in nature, whereas a massless spin half
particle needs only one helicity state. This concept has profound implication
for the neutrino, which was thought to be massless for a long time, as we
discuss later.
2.5 Stable and Unstable Particles
A stable particle stays like it is forever, whereas an unstable particle disappears
into other less heavy particle in a certain amount of time. The time it takes for
a particle to decay to roughly one third of its original amount is called its
lifetime. Of the many particles discovered to date using different methods
like colliders or cosmic rays, there are very few which are absolutely stable.
For example, we know that protons and electrons are the only two absolutely
stable elementary particles. Neutrons are unstable, and they decay to a proton,
electrons, and anti-neutrinos, in a little under 15 min. As we will see, this decay
unleashed a huge research area known as weak interactions, and eventually
led to the rise of the standard model, as the theory of all forces and particles
except for gravitational forces. It turns out that once the neutron is inside a
nucleus, it becomes part of the nucleus and often becomes a stable particle.
That is because it effectively loses part of its mass (relative to its mass when it
is free) to the binding by nuclear force. As a result, it does not have enough
energy to decay to a proton, electron, and neutrino. This is due to the fact
that “all processes in nature must conserve the total energy,” which is a cardinal
principle of physics and is a law known to be absolutely valid. The loss of mass
of the neutron inside a nucleus is called its binding energy.
If a particle is unstable, it will decay to some other particles as noted and
that provides a gold mine of information about both the particle that decays
and those that come out in the final state and the nature of the force that causes
this. For instance, how it decays tells us about the interaction of the particles.
Similarly, accounting for all the states it decays to gives a fuller detail about
the interaction of the particle in question.