WIMP

Estimate the present-day temperature of a population of WIMPS that decoupled from radiation at a temperature corresponding to their rest-mass of, say, 100 GeV and subsequently cooled like an adiabatic monatomic gas. (1 GeV = 1.16 x 1013K.)... 

Every student who has just read that this course will involve descriptions of industrial process and the history of the chemical process industry is probably already worried about what will be on the tests. Students usually think that problems with numerical answers (5.2 liters and 95% conversion) are somehow easier than anything where memorization is involved. We assure you that most problems will be of the numerical answer type. However, by the time students become seniors, they usually start to worry (properly) that their jobs will not just involve simple, weU-posed problems but rather examination of messy situations where the boss does not know the answer (and sometimes doesn t understand the problem). You are employed to think about the big picture, and numerical calculations are only occasionally the best way to find solutions. Our major intent in discussing descriptions of processes and history is to help you see the contexts in which we need to consider chemical reactors. Your instructor may ask you to memorize some facts or use facts discussed here to synthesize a process similar to those here. However, even if your instructor is a total wimp, we hope that reading about what makes the world of chemical reaction engineering operate wiU be both instmctive and interesting. 

B) on the resultant microsphere diameter did not significantly affect Jms. However, the number and location of the impellers had a significant impact on 4ns. As a result, to simplify the system, Maa and Hsu always used double impellers (wimp = 2), with the lower one placed close to the bottom of the tank and the other located in the center of the total emulsion volume. Finally, Maa and Hsu determined that the volumes of the organic and aqueous phases, in the range they were concerned with, played only a minor role in affecting 4ms- Thus, by the omission of D/T, B, and and v. Equation (42) was simplified considerably to yield... 

A big wild man, indeed Freud, the neurasthenic, Freud, the hypochondriac, Freud the somatizing wimp has been transformed by a single... 

Adam, David. 2003. Transgenic crop trials gene flow turns weeds into wimps. Nature 421 462. Adam, David. 2006. EU under attack over plan to ease organic labelling. The Guardian. January 6. http //www.guardian.co.Uk/gmdebate/Story/o, 1680438,oo.html (accessed April 18, 2006). 

Assuming that the WIMPS once were in thermal equilibrium, one finds that the relic WIMP froze out at... 

Another important classification of particle dark matter rests upon its production mechanism. Particles that were in thermal equilibrium in the early Universe, like neutrinos, neutralinos, and most other WIMPs (weakly interacting massive particles), are called thermal relics. Particles which were produced by a non-thermal mechanism and that never had the chance of reaching thermal equilibrium in the early Universe are called non-thermal relics. There are several examples of non-thermal relics axions emitted by cosmic strings, solitons produced in phase transitions, WIMPZILLAs produced gravitationally at the end of inflation, etc. 

Heavy neutrino The WIMP par excellence is a heavy neutrino. The example we consider is a thermal Dirac neutrino v of the fourth generation with Standard Model interactions and no lepton asymmetry. Figure 3 summarizes... 

Notice that according to the Merriam-Webster Dictionary of the English Language, a wimp is a weak, cowardly, or ineffectual person. 

Figure 16.2. Evolution of a typical WIMP number density in the early universe. The number of WIMPs in a volume expanding with the universe (comoving density) first decreases exponentially due the Boltzmann factor e-m/T an(j then freezes out to a constant value when the WIMP annihilation reactions cannot maintain chemical equilibrium between WIMPs and standard model particles. In the figure, (av) is the thermally averaged annihilation cross section times relative velocity. WIMPs with larger annihilation cross section end up with smaller densities.

Neutrinos can be cold dark matter if their masses are around few GeV or a TeV. However, fourth-generation heavy neutrinos lighter than 45 GeV are excluded by the measurement of the Z-boson decay width at the Large Electron-Positron collider at CERN. Moreover, direct searches for WIMP dark matter in our galaxy exclude Dirac neutrinos heavier than 0.5 GeV as the dominant component of the galactic dark halo (see Figure 3). Thus although heavy Dirac neutrinos could still be a tiny part of the halo dark matter, they cannot solve the cold dark matter problem. 

The second part of these lectures is an introduction to several methods to detect non-baryonic dark matter. We will use the lightest neutralino as our guinea pig, because of the variety of techniques that can be employed to detect it, but the discussion is more general and can be applied to a generic WIMP. Thus in this second part we assume that non-baryonic dark matter is made of WIMPs (in particular, neutralinos), and we examine several observational ways to test our assumption. 

In cryogenic detectors, a simultaneous measurement of both ionization and thermal energy allows the discrimination of nuclear recoils from electrons produced in radioactive decays or otherwise. This discrimination, however, cannot tell if the nuclear recoil was caused by a WIMP or an ambient neutron. The detector, most often a germanium or silicon crystal, needs to be cooled at liquid helium temperature so that its low heat capacity converts a small deposited energy into a large temperature increase. Only relatively small crystals can be currently used in these cryogenic detectors, with relatively low detection rates. 

Figure 16.9. Annual modulation of the total counting rate (background plus possible dark matter signal) in seven years of data with the DAMA-Nal detector. A constant counting rate has been subtracted to give the residuals. The significance of the modulation is 6cr and its period is 1 year. The interpretation of the yearly modulation as due to a WIMP signal is controversial. (Figure from Bernabei et al.(2003).)...

Figure 16.10. Sketch illustrating the directions of the Sun s and the Earth s motions during a year. As the Sun moves in the Galaxy (here at 232 km/s, 60° out of the plane of the Earth s orbit), the Earth moves around the Sun (here at 30 km/s). The vectorial sum of their velocities gives the velocity of the Earth with respect to the Galaxy. Assuming the WIMPs to be on average at rest in the Galaxy, it follows that the average speed of the WIMPs relative to the Earth is modulated with a period of 1 year.

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