The DAMA annual modulation effect

The DAMA experiment [311; 314; 319; 320; 328] consists of low-background, highly radiopure NaI(Tl) scintillators with very stable response over long time periods. These characteristics allow the DAMA experiment to look for the annual modulation [719; 881] of the counting rate: this peculiar effect is expected if the Galactic halo is composed of DM particles interacting with the target detector and represents a specific signature for the direct search of DM.

The experiment operated in the first phase as DAMA/NaI for 7 years with a mass of about 100 kg and collected a total exposure of 0.29 ton × yr [310; 312; 315; 319; 320]. The second phase is now under operation as DAMA/LIBRA, which recently released data on an additional four annual cycles with a mass of about 250 kg, adding 0.53 ton×yr to the total exposure. The cumulative analysis of the single-hit residual rate of the DAMA/NaI and DAMA/LIBRA detectors favours at 8.2σ C.L. the presence of annual modulation with the features which are expected from a DM candidate [Chapter 18; 319; 324].

In the case of WIMP DM which scatters off the nuclei of the detector, the DAMA annual modulation effect provides direct information on two basic properties of the DM particle: the particle mass mχ and its elastic scattering cross-section off nuclei. The latter is then usually and more conveniently translated into the cross-section on a single nucleon, in the case of both coherent and spin-dependent interactions.

The long-awaited Large Hadron Collider (LHC) is expected to start taking data in 2009. The LHC research programme has traditionally been centred around the discovery of the Higgs boson. However, the Standard Model description of this particle calls for New Physics. Until a few years ago, the epitome of this New Physics has been supersymmetry, which when endowed with a discrete symmetry called R-parity furnishes a good dark matter candidate. Recently a few alternatives have been put forward. Originally, they were confined to solving the Higgs problem, but it has been discovered that, generically, their most viable implementation (in accord with electroweak precision data, proton decay, etc.) fares far better if a discrete symmetry is embedded in the model. The discrete symmetry is behind the existence of a possible dark matter candidate.

From another viewpoint, stressed in many parts of this book, the past few years have witnessed spectacular advances in cosmology and astrophysics confirming that ordinary matter is a minute part of what constitutes the Universe at large. At the same time in which the LHC will be gathering data, a host of non-collider astrophysical and cosmological observations with ever-increasing accuracy will be carried out in search of dark matter. For example, the upcoming PLANCK experiment will make cosmology enter the era of precision measurements, akin to what we witnessed with the LEP experiments.

The emergence of this new paradigm means it is of utmost importance to analyse and combine data from these upcoming observations with those at the LHC.

In models with extra dimensions, the usual (3 + 1)-dimensional space-time xµ ≡ (x0, x1, x2, x3) is extended to include additional spatial dimensions parameterized by coordinates x4, x5, …, x3+N. Here N is the number of extra dimensions. String theory arguments would suggest that in principle N can be as large as 6 or 7. In this chapter, we are interested in extra-dimensional (ED) models where all particles of the Standard Model (SM) are allowed to propagate in the bulk, i.e. along any of the x3+i (i = 1, …, N) directions [100]. In order to avoid a blatant contradiction with the observed reality, the extra dimensions in such models must be extremely small: smaller than the smallest scale which has been currently resolved by experiment. Therefore, the extra dimensions are assumed to be suitably compactified on some manifold of sufficiently small size (see Fig. 15.1).

Depending on the type of metric in the bulk, the ED models fall into one of the following two categories: flat, also known as ‘universal’ extra dimensions (UED) models, discussed in Section 15.1, or warped ED models, discussed in Section 15.2. As it turns out, the collider signals of the ED models are strikingly similar to the signatures of supersymmetry (SUSY) discussed in Chapter 13. Section 15.3 outlines some general methods for distinguishing an ED model from SUSY at high-energy colliders.

Numerical studies of the formation of cold dark matter haloes have produced several robust results that allow unique tests of the hierarchical clustering paradigm. Universal properties of haloes, including their mass profiles and substructure properties, are being tested against observational data from the scales of dwarf galaxies to galaxy clusters. Resolving the fine-grained structure of haloes has enabled us to make predictions for ongoing and planned direct and indirect dark matter detection experiments taking us beyond the smooth spherical isotropic model for the Galactic halo.

From cold collapse to hierarchical clustering – a brief history

N-body simulations of the gravitational collapse of a collisionless system of particles pre-date the CDM model. Early simulations in the 1960s studied the formation of elliptical galaxies from the collapse of a cold top-hat perturbation of stars [1089; 1556; 1889]. The resulting virialization process gave rise to equilibrium structures with de Vaucouleurs [633] or Einasto [741; 743] type density profiles. Profiles of the same form but with higher concentrations are widely used to describe the light distribution of elliptical galaxies. It is remarkable that the end state of almost any gravitational collapse, independent of the small-scale structure and hierarchical merging pattern, leads to a similar global structure of the final equilibrium system [1143; 1426; 1474].

Computer simulations in the 1970s attempted to follow the expansion and collapse of a spherical overdensity to relate to the observed properties of virialized structures such as galaxy clusters [1930].

Introduction

Dark matter is surely at the heart of modern cosmology. It undoubtedly pervades the Universe, unless we are being completely misled by diverse data sets, yet it has not been detected. The possible connection with proposed extensions of the Standard Model of particle physics, currently being searched for at accelerators, makes the identification of dark matter one of the highest priority goals in cosmology and particle physics. In this chapter we provide an introduction to the dark matter situation and an overview of the material presented in this book.

Dark matter has a venerable history (see e.g. ref. [742] for an historical account). One could even cite Solar System arguments for dark matter, including anomalies in the orbit of Uranus and the advance of Mercury's perihelion. One led to the discovery of a previously dark planet, Neptune, the other to a new theory of gravitation. Similar parallels may be drawn today. There are advocates of new theories of gravitation (see Chapter 6), who seek to dispense with dark matter, and there are observations of largescale structure, such as gravitational lensing, the cosmic web and the cosmic microwave background acoustic fluctuations, that are notoriously difficult to reproduce in the absence of a dominant component of cold dark matter (CDM) particles (we refer the interested reader to Chapter 4, which includes an introduction to gravitational lensing in the context of the CDM paradigm, and its potential to discriminate CDM from modified-gravity theories).

Noble liquids for dark matter detection

Noble liquids, specifically liquid xenon (LXe), liquid argon (LAr) and liquid neon (LNe), are excellent scintillators and, with the exception of LNe, also very good ionizers in response to the passage of radiation, thus providing an excellent alternative to cryogenic detectors (see Chapter 20). The possibility of simultaneously detecting ionization and scintillation signals in LXe and LAr is a unique feature of these liquids compared with other detection media. This capability, together with the promise of scale-up to large mass at a modest cost compared with semiconductors, has contributed to make LXe and LAr popular targets and detectors for rare physics events such as those associated with dark matter, solar neutrinos and neutrinoless double beta decay interactions. In this section, we first describe the ionization and scintillation mechanism in noble liquids, including recent measurements of the relative scintillation efficiency and ionization yield for nuclear recoils, relevant for dark matter searches. We then describe the background rejection capability of noble liquids, based on pulse shape discrimination of the scintillation light and the ratio between ionization and scintillation signals (S2/S1). At the end of the section, we briefly discuss the key requirement for ultra-high purity of noble liquids for dark matter detection.

Physical properties of noble liquids

Table 21.1 summarizes the physical properties of the three noble liquids being used or planned to be used for dark matter direct detection.

Introduction

The class of weakly interacting massive particles (WIMPs) is the leading category of particle dark matter candidates: on the one hand, the mechanism of thermal freeze-out of a stable WIMP χ leads to a non-relativistic relic population whose relative matter density can be approximated as Ωχh2 ≈ 3 × 10−27 cm3 s−1/ 〈σAnnv〉, where h is the Hubble constant in units of 100 km s−1 Mpc−1, Ωχ is the ratio of the χ density over the critical density and 〈σAnnv〉 is the thermally averaged χ pair-annihilation cross-section. If the new physics connected to the WIMP is at the electroweak scale, mEW, one can estimate 〈σAnnv〉 ≈ ≈ 10−25 cm3 s−1, giving a thermal leftover χ population that lies in the same ballpark as the dark matter abundance inferred from cosmic microwave background anisotropies, large-scale structure and other astronomical observations [1266]. On the other hand, numerous motivated particle physics extensions to the Standard Model encompass a stable WIMP, including supersymmetry [1184] and universal extra dimensions (UED) [1126], in virtue of unbroken discrete symmetries (R-parity in the case of supersymmetry, and Kaluza–Klein parity in the case of UED).

Since WIMPs were once kept in thermal equilibrium by pair annihilations into Standard Model particles and inverse WIMP pair production processes, even in today's cold Universe, occasionally, WIMPs can pair annihilate, giving rise to energetic, stable ‘ordinary’ Standard Model particles.

Missing mass in galaxies and clusters of galaxies

A look at the other papers in this volume will show the present one to be singular. Dark matter is a prevalent paradigm. So why do we need to discuss alternatives? While observations seem to suggest that disk galaxies are embedded in giant haloes of dark matter, this is just an inference from accepted Newtonian gravitational theory. Thus if we are missing understanding about gravity on galactic scales, this inference may be deeply flawed. And then we must remember that, aside from some reports which always seem to contradict established bounds, DM is not seen directly. Finally, were we to put all our hope on the DM paradigm, we would be ignoring a great lesson from the history of science: accepted understanding of a phenomenon has usually come through confrontation of rather contrasting paradigms.

To construct a competing paradigm to DM, it is best to bear in mind concrete empirical facts. Newtonian gravity with the visible matter as source of the Poisson equation properly describes all observed systems from asteroid scale up to the scale of globular clusters of stars (∼105 stars bound together in a ball the size of a few tens of light years). But as we move up to galaxies, ours or external ones, troubles appear. In essence, the way that disk-like galaxies rotate is incompatible with the Newtonian gravitational force generated by only the visible stars, gas and dust.