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Cold Dark Matter and the Exotic Particle

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Cold Dark Matter and the Exotic Particle ! Dark Matter and the Universe Topic 5 Cold Dark Matter and the Exotic Particle Zoo from the Higgs Boson & Antimatter to WIMPs & Axions Why is the dark matter probably exotic new particles from the Big Bang?! ! Contents of Topic 5 In this Topic we delve into the world of new particle physics to explore possible candidates for particle dark matter. This includes study of what cosmology can tell us about particle physics - real particle astrophysics in action. We cover: " The new particle zoo - examples of exotic new particles " The concept of hot, warm and cold dark matter " Structure formation and the 2-point spatial correlation function " Introduction to Weakly Interacting Massive Particles (WIMPs) " Supersymmetry theory, the LSP, R-Parity and the Neutralino " Universal extra dimensions and the LKP " Thermal relics and “freeze out” of particles in the Big Bang " The Higgs Boson, Gravitino, Sterile Neutrinos and Gravitinos " Searches for SUSY and UED dark matter at CERN " Axions and how to detect them Towards New Particles in the Cosmos " The evidence is overwhelming that the Universe is filled with material that is non-luminous but exerts powerful gravitational influence on all scales - the Dark Matter. " It is also clear now that this missing mass can not be normal protons and neutrons, the Big Bang simply did not generate enough Baryons, and anyway all searches for non-luminous baryons, like MACHOs don’t find any, or at least not enough. " When we venture into the known but exotic particle world, for instance, Neutrinos and Antimatter, there is slightly more success. We now know neutrinos have some mass. This means that the neutrinos are part of the solution to the dark matter problem. Unfortunately their mass is <1 eV. So the contribution from neutrinos is miniscule. " So we are forced to consider more exotic particle solutions and the possibility of some completely New Physics. Towards New Particles in the Cosmos " Fortunately there are a lot of reasons in particle physics to believe that there exist new as yet undetected particles. " The famous Higgs-Boson is a recent example of a particle predicted to exist and now discovered. " In fact there is a whole zoo of particles postulated by theorists that could explain the dark matter. One example is the Gravitino. This particle is postulated to have no interactions with normal matter, only gravitational effects. This would make it impossible to detect in the laboratory. " Another example is Sterile Neutrinos. These are postulated inert neutrinos that would also interact only via gravity and not via the fundamental interactions of the Standard Model. " Fortunately there are several well founded candidates that could show themselves by interactions with matter. The New Particle Zoo " Here is a list potential candidate new particles, starting with the most important ones that we will mainly deal with here: (1)! LSP (Lightest Supersymmetric Particle), m ~ 10-1000 GeV (2)! LKP (Lightest Kaluza-Klein Particle), m ~ 10-1000 GeV (3)! Axion, m ~ 10-5 eV " In addition to Gravitinos, ordinary and Sterile Neutrinos, other more exotic examples include: (1)! Heavy Leptons, m ∼ 2 GeV! (2) Ultraheavy, quasi-stable particles, m ∼ 1013 GeV! (3) Mirror matter! (4) Non-topological solitons and Q-balls! (5) QCD nuggets! (6) WIMPzillas! (7) Primordial Black Holes (PBH), m # 1016 g! Cosmology and The Particle Zoo " The theoretical basis for these candidates vary wildly and their masses and interaction strengths with ordinary matter span many orders of magnitude. So how can we decide which are most likely? " Fortunately this is an amazing example of where cosmology can help with particle physics. We can divide the candidates into two broad types: (1)! Hot Dark Matter (HDM) (2)! Cold Dark Matter (CDM) " The difference reflects how fast the particles were moving at the time they decoupled from the Baryonic Matter (when they stopped interacting with it as the Universe cooled): • HDM refers to particles that are Relativistic at decoupling. • CDM refers to slow moving Non-Relativistic candidates. " There is also Warm Dark Matter (WDM), an intermediate form, but there are few candidates in this category. Cosmology and The Particle Zoo " This distinction between HDM and CDM is important because whether dark matter is in the form of slow moving CDM or fast (relativistic) HDM critically influences how we would expect structure in the Universe to form. " Remember which ever it is (CDM or HDM) it dominates the Universe so its bound to affect the structure of the visible bit we can see (galaxies, clusters etc.). " This is seen clearly in the results of simulations of Structure Formation in the Universe when CDM or HDM is included, as below. Note how the HDM case has less structure. CDM HDM Hot Dark Matter " So Hot Dark Matter (HDM) is a particle species that Decouples, or “Freezes Out” when its interaction rate drops below the Hubble Expansion Rate when the particle’s velocity is still relativistic. " An example of HDM is the Neutrino with mass m ~ eV. " On small scales HDM particles move too quickly to participate in Gravitational Clustering. But on larger scales they can cluster just like heavy matter. The transition occurs around the size of clusters of galaxies ~ 10 Mpc. " So HDM tends to smooth out structure so that Large Scale Structure (on the scale of galaxy superclusters) persist but small scale structure (like galaxies) do not readily form. " To fit observation this suggests a Top-Down Scenario for the formation of the Universe where large scale structure forms first and breaks up into small scale structure later. Cold Dark Matter " Cold Dark Matter (CDM) has the reverse characteristic of HDM. The particles hardly move after decoupling, they are non-relativistic, and their masses are larger. " This favours a Hierarchical Clustering scenario where small structure forms first and then collects into larger structure later. So we have a so-called Bottom-Up scenario. " Here is a so-called N-Body Simulation, showing such a Bottom-Up scenario with CDM. The Universe starts with small structures that cluster time into larger scale structure later. " Such simulations provide a powerful tool for cosmologists to examine HDM vs. CDM scenarios and hence what particle physics works - this is real Particle Astrophysics in action! HDM vs. CDM & Structure Formation " A critical issue with these simulations is how to quantify the amount of clustering of matter, or the amount of density fluctuations, vs. scale and compare this for the two cases where the dark matter is either HDM or CDM. Then to compare the results with real observations. " Basically we need to compare the Spatial Distribution of masses in the Universe predicted by the simulations with that actually observed. The best way to do this to use a so called the Two-point Spatial Galaxy Correlation Function. " Suppose N points are distributed in a volume V. The density is then n = N/V. If the points are randomly distributed in the sky then the probability that a given point has a neighbour in a surrounding volume is δP = nδV € HDM vs. CDM & Structure Formation " Now if there is correlation between the points we can rewrite this in terms of the Two-point Spatial Galaxy Correlation Function as distance from the first point nδV[1+ ξ(r)] two-point correlation function " In this equation if the points (galaxies) are in random locations then = 0. Any clustering would mean > 0. € " Observations of galaxies and clusters can be used to determine and hence the size of Density Fluctuations vs. Scale that can be compared with different simulations of Structure Formation with Hot or Cold Dark Matter. " The following plot shows one result compared with data. HDM vs. CDM & Structure Formation ‣ The plot here shows Density Fluctuations (relative units) vs. Scale (in lightyears). ‣ The various points are data from different observations spanning a huge range of scale including the CMBR. ‣ The green and red lines show the expectation if CDM or HDM is the dark matter. ‣ Note that although both HDM and CDM models fit the data well at very large scale (right part of plot), HDM completely fails at smaller scale. HDM simply does not produce enough small scale structure. So CDM clearly wins! ‣ This is more strong evidence against neutrino dark matter. CDM and The Particle Zoo ‣ So the observational evidence combined with structure formation simulations strongly favours CDM Particles and a Bottom-Up Scenario whereby small-scale structure forms first and later clusters into larger structures. In the HDM scenario, with relativistic particles such as neutrinos, the small scale structure gets washed-out. ‣ Out of the list of particles this leaves us with three theoretically well motivated main candidates, all CDM type: Supersymmetry theory ! Weakly (1) LSP - Neutralino Particles ! Interacting (2) LKP - Kaluza-Klein Particles! Massive ! Extra Dimension theory Particles (3) Axions CP violation theory " We will do these in detail but note first the classification of (1) & (2) - Weakly Interacting Massive Particles (WIMPs). What Does WIMP Mean? " We’ll cover these most important candidates and the particle physics theories behind them in more detail now but first what do we mean by the term WIMP and what is a WIMP? " WIMP is an generic term that refers to any type of dark matter candidate particle with the following characteristics: (1)! Weakly Interacting - meaning interaction strengths (cross- sections) with normal matter are as weak or weaker than the Weak Force (the force that governs beta decay) (2)! Massive - typically > ~ 1 GeV and up to ~10,000 GeV (3)! Non Relativistic - slow moving, typically ~ 10-3 c (4)! Neutral - in fact no charged particle can be the dark matter because it would interact too easily with normal matter " All WIMPs count as CDM.
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