Weakly Interacting Massive Particles (WIMPs)

Dark Matter has always seemed to be a fascinating topic for all the astronomy enthusiasts. What is dark matter? How much of it is present in the universe? How does it affect us? But most importantly, what is it MADE of? In this article, we’ll be exploring one of the potential candidates of what we think dark matter is made up of: Weakly Interacting Massive Particles

What are weakly interacting massive particles?

Weakly interacting massive particles, or WIMPs, are hypothetical particles that are thought to make up dark matter. They are called “weakly interacting” because they barely interact with regular matter, meaning they don’t often bump into other matter and don’t emit or absorb light. They are “massive” because they are much heavier than regular matter particles like electrons and quarks.

How did we know something like this should exist?

1. Galactic rotation curves, which describe how the speed of stars orbiting the galaxy changes with distance from the center, are a key piece of evidence for dark matter. These curves are typically flat or even rise slightly towards the edges of the galaxy, indicating that the mass of the galaxy increases linearly with distance from the center. However, the visible matter in the galaxy is not enough to explain this behavior, suggesting that there is a large amount of unseen mass.

2. Cold Dark Matter Theory: In the 1980s, the cold dark matter (CDM) theory was developed, which proposed that dark matter was composed of weakly interacting particles that moved slowly compared to the speed of light. This theory was able to explain the large-scale structure of the universe, but it required the existence of particles that interacted only through the weak nuclear force and gravity.

The WIMP Hypothesis

In the 1990s, supersymmetric extensions of the Standard Model of particle physics were developed, which predicted the existence of new particles with the right properties to make up dark matter. These particles, known as WIMPs, would have been created in abundance during the early universe and would have stopped interacting with other particles as the universe expanded and cooled.

WIMPs are a popular candidate for dark matter because they would have the right properties to make up the observed dark matter. The WIMP hypothesis proposes that dark matter is composed of particles that interact with normal matter only through the weak nuclear force and gravity, making them extremely difficult to detect.

The weak nuclear force is one of the four fundamental forces of nature, responsible for certain types of radioactive decay. It is a short-range force, meaning it only acts over very small distances, and is much weaker than the electromagnetic force that holds atoms together.

WIMPs are thought to have been created in abundance during the early universe, when temperatures were high enough to produce them. As the universe expanded and cooled, the WIMPs would have stopped interacting with other particles and would have been left over as dark matter.

The idea of WIMP annihilation, which involves the collision of two WIMPs producing detectable particles, was proposed as a way to detect dark matter. This led to the development of indirect detection experiments, which search for products of WIMP annihilation in nearby galaxies and galaxy clusters. In the 2000s, direct detection experiments were developed, which aim to detect the scattering of WIMPs off atomic nuclei in the laboratory. These experiments have provided some of the most stringent limits on WIMP properties to date.

Theoretical Properties Of WIMPS

The main theoretical characteristics of a WIMP are:

1. Interactions only through the weak nuclear force and gravity, or possibly other interactions with cross-sections no higher than the weak scale.

2. Large mass compared to standard particles (WIMPs with sub-GeV masses may be considered to be light dark matter).

Experimental Efforts to Detect WIMPs

Experimental efforts to detect WIMPs are ongoing, with several approaches being pursued:

1. Indirect Detection: This involves searching for products of WIMP annihilation, such as gamma rays, neutrinos, and cosmic rays, in nearby galaxies and galaxy clusters. The idea is that WIMPs would accumulate in the centers of these galaxies and clusters, leading to a higher rate of annihilation and subsequent emission of detectable particles.

2. Direct Detection: This involves designing experiments to measure the collision of WIMPs with nuclei in the laboratory. The goal is to detect the scattering of WIMPs off atomic nuclei, which would produce a signal that could be distinguished from background noise.

3. Collider Searches: This involves attempting to directly produce WIMPs in high-energy collisions, such as those produced by the Large Hadron Collider (LHC) at CERN.

The WIMP Miracle

The WIMP miracle refers to the apparent coincidence that supersymmetric extensions of the Standard Model of particle physics readily predict a new particle with the properties of a WIMP. This has led to a stable supersymmetric partner being a prime WIMP candidate. However, recent results from direct-detection experiments and the failure to produce evidence of supersymmetry in the LHC experiment have cast doubt on the simplest WIMP hypothesis.