The inquisitive nature of humans has led to numerous discoveries and knowledge; however, with each discovery more questions arise. One of the fields physicists have been working on for a long time now is to understand the working of the universe. Many studies have shown that galaxies are surrounded by particles which don’t interact with electromagnetic or strong force. These proposed particles are dark matter, which only interact with gravitational force and weak interactions. This chapter will discuss briefly the evidence, detection methods, and different models of dark matter.
Through the span of time we have observed the indirect effects and presence of dark matter. This section will discuss some of the observations which provide evidences for existence of these undiscovered Dark Matter.
This is one of the earlier and strong evidence for existence of dark matter. From simple Newtonian physics, the expected rotational velocity (v) of the stars of mass(M) rotating around the center of spiral galaxies at some distance(r) is:
So, with the increase in distance from the center of the galaxies, the rotational velocity of the star must decrease. (v_rot inversely proportional to r
Some of the earlier study done to calculate the rotational velocity of the spiral galaxies showed otherwise. The study done by Rubin in the 1970’s showed that the rotational velocity of the higher radius stars didn’t decrease as expected from the Newtonian equation. The rotational velocity of these stars were calculated by measuring the Doppler shift of the carbon monoxide line, optical lines and HI lines at inner portion, disk and outer portion respectively.
The curve at the higher radius seems to flatten out. This implies that the mass is not only visible luminous matter, but it seems to be distributed beyond the edges of the luminous matter with mass distribution of density(row) proportional to r
-2. Those invisible mass is those galaxies provides evidence for existence of dark matter.
The group of tens to hundreds of galaxies which are gravitationally bound is a galaxy cluster. An earlier study was done by Zwicky and Smith in the 1930’s on Coma cluster to calculate the mass of these cluster using the observed velocity of the galaxies within the cluster using virial theorem. The virial theorem gives the relation between the average kinetic energy (T) of system and the potential energy (V) of the system.
There were 800 observed galaxies in the coma cluster. Zwicky estimated total mass of coma cluster using the average mass of each galaxy times number of galaxies observed. He used distance of 10
6 light years to calculate an estimated average potential energy of the coma cluster. Then, he used virial theorem to determine the average dispersion velocity of galaxies within a cluster to be 80 km/s. But, the observed average dispersion velocity was approximately 1000 km/s. He then concluded as follows:
“If this would be confirmed, we would get the surprising result that dark matter is present in much greater amount than luminous matter.” Smith also followed the same procedure to compare disperse velocity of Virgo cluster to reach the similar conclusions.
This is one of the recent and strongest evidence of dark matter. In 2006, a group of astronomers from Hubble space telescope and Chandra x-ray observatory came up with the publication entitled “A direct empirical proof of existence of dark matter”. They studied two merging clusters with gravity lensing and photo observation of x-ray emission.
When light passes through massive object it also feels the effect of the gravity which cause light to bend it path. So, we can use these massive objects as lens as they will focus some of light coming from background source. This phenomenon is gravity lensing. We can locate these massive lens and calculate its mass after studying those focused images.
When these two clusters collided, the hot gases filling the space between galaxies became more hot causing increase in brightness of emitted X-ray. The red and yellow portion in the plot above is the image seen from x-ray observation which helps to locate the position of hot gases during the collision. The gravity lensing of these merging cluster showed that more masses of the clusters are around the green counters. If the cluster only comprised of ordinary matter, then the images of gravitation lensing and X- ray must be overlapping each other which is not the case here. This behavior could be explained with the existence of dark matter as follows:
Three different approach are being used to test the existence of the dark matter. Those are:
Direct detection: In this type of detection method, the dark matter is expected to scatter elastically with the atomic nucleons. The rate of interaction is expected to be very low. The rate of interaction depends on properties of target nuclei and local density of dark matter halo. So there are different experiments using different material as targets or detectors. Some of experiment uses liquid noble – gas detectors (like XENON, LUX, LZ etc.) where as some of the experiments uses solid semiconductor detectors (like SuperCDMS, CoGENT, CDEX etc.). We will focus more on direct detection using solid semiconductor detectors when we will talk more about SuperCDMS experiment for dark matter search in next chapter.
Indirect detection: This type of detection looks to detect product of Dark matter annihilation. The chances of annihilation are more in regions which has higher dark matter density. So, these experiments look in regions like center of galaxy, the sun and the earth where the density of dark matter is higher due to gravitational capture. Annihilation of dark matter may produce energetic neutrinos, Gamma rays, cosmic rays or other particles. Experiments using indirect detection technique studies these products in their selected regions with higher dark matter density. Some experiments like AMS, HESS, MAGIC, IceCube etc. are using this technique for detection of dark matter.
Particle Collider: Particle collider instead of looking for dark matter tries to create dark matter by colliding different particles like electrons or protons with each other at high speed. Some experiments like LHC, CMS, ATLAS etc. uses particle Collider for detection of dark matter.
The dark matter candidates must satisfy some of the basic criteria of dark matter. These criteria are created from the study of evidence of existence of dark matter. Dark matter should be non- baryonic, very less interactive and non -relativistic to explain some of the evidences.
The leading candidate for dark matter now is WIMPs (Weakly Interacting Massive Particles). These are particles with mass in range of 1 GeV to 10 TeV and they interact with weak interaction. Supersymmetry (SUSY) which is extension of Standard Model also predicts this particle to solve problems in particle physics. We would be more focused on WIMP as SuperCDMS is looking to detect WIMPs with direct detection technique.
Other Dark Matter candidates are Axions are Sterile Neutrinos. Axions are spinless , electrically neutral and very light particles. Axions are non thermal relic candidate of dark matter. Sterile Neutrinos only interacts via gravity which means they won’t even have weak interaction.
Before the discovery of quarks, all the particles discovered had charge that was integer multiple of electron charge(e). But today, the fact that charge of quarks is integer multiple of e/3 opens window to fractionally charged particles (FCPs). The FCPs with smaller fractional charge can only lightly ionize the atoms. Thus these smaller fractional charges are proposed as Lightly Ionizing Particles (LIPs).
Although the Standard Model doesn’t anticipate the existence of FCPs but they are yet to be excluded experimentally. The Standard Model does have quarks and anti quarks with charge e/3 and 2e/3 but their strong interaction confines them inside unit charged hadrons. Some theories that provides motivation towards existence of LIPs are discussed below.
The Abelian theory for QED suggests that the particle can have any electromagnetic charge. This theory has no nontrivial commutation relation between the generators which means there could be any algebraic values for charge Eigen values. In other words, as the electromagnetic force carrier doesn’t show self interaction, the particles theoretically could have any electromagnetic charge which includes fractional charge as well.
The theoretical existence of magnetic monopole can also be used as motivation for existence FCPS or LIPs. After reviewing Maxwell’s equation, Pierre Curie found no reason against the existence of magnetic monopole.  In fact the existence of Magnetic monopole will complete the symmetry between electric and magnetic component. Dirac in his study showed that the existence of magnetic monopole will constrain electric and magnetic charge.
eg = nhc/2
where e = electric charge, g is magnetic charge, n in integer, c is speed of light and h is planks constant.
From the equation it is clear that if monopole exist (g =1 ) then e would be fractional. So a free electrically charged magnetic monopole will have fractional charges.
The LIPs might even be produced in our atmosphere when High energy cosmic rays(CR) interacts with particle in the atmosphere. Shower of byproducts are created when billions of CR coming towards earth interacts with atmosphere. We have observed the production of pions as well as proton and neutron after interaction of CR with atmosphere. The pions further decay into muons or gamma rays. LIPs or FCPs if exist could be created during the same process. The LIPs created during this process can be detected using a detector on earth. This is one of the biggest motivation for studying LIPs using SuperCDMS detector.
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