Direct Detection of Cold Dark Matter
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Direct Detection of Cold Dark Matter Laura Baudis a Physik Institut, University of Zurich, Winterthurerstr. 190, CH-8057 Zurich Abstract. We know from cosmological and astrophysical observations that more than 80% of the matter density in the Universe is non-luminous, or dark. This non-baryonic dark matter could be composed of neutral, heavy particles, which were non-relativistic, or ’cold’, when they decoupled from ordinary matter. I will review the direct detection methods of these hypothetical particles via their interactions with nuclei in ultra-low background, deep underground experiments. The emphasis is on most recent results and on the status of near future projects. PACS. 95.35.+d dark matter – 95.55.Vj, 29.40.Mc detectors 1 Introduction in the galactic halo, v and f(v) are the WIMP velocity and velocity distribution function in the Earth frame In the Standard Model of cosmology the Universe is and dσ/dER is the WIMP-nucleus differential cross flat and made of 4% baryons, 20% non-baryonic dark 2 2 section. The nuclear recoil energy is ER = mr v (1 − matter and 76% dark energy [1]. Generic candidates cos θ)/mN , where θ is the scattering angle in the WIMP- for non-baryonic dark matter, which thus makes more nucleus center-of-mass frame, mN is the nuclear mass than 80% of the total matter density in the Universe, and mr is the WIMP-nucleus reduced mass.1 The ve- are axions [2] and weakly interacting massive particles 2 2 locity vmin is defined as vmin = (mN Eth/2mr ) , where (WIMPs) [3]. WIMPs could have been thermally pro- Eth is the energy threshold of the detector, and vmax duced in the very early universe. If their mass and cross is the escape WIMP velocity in the Earth frame. The section is determined by the weak scale, the freeze-out simplest galactic model assumes a Maxwell-Boltzmann relic density is around the observed value, Ω ∼0.1 [4]. distribution for the WIMP velocity in the galactic rest −1 The best studied WIMP candidates are the neutralino, frame, with a velocity dispersion of vrms ≈ 270kms −1 or the lightest supersymmetric particle, and the light- and an escape velocity of vesc ≈ 650kms . (1) est Kaluza-Klein particle, for instance the B , which The differential WIMP-nucleus cross section has is the first excitation of the hypercharge gauge boson two separate components: an effective scalar coupling in theories with universal extra dimensions. A recent between the WIMP and the nucleus and an effective comprehensive review of particle dark matter candi- coupling between the spin of the WIMP and the total dates has been provided by Bertone, Hooper and Silk arXiv:0711.3788v1 [astro-ph] 25 Nov 2007 spin of the nucleus. In general the coherent part dom- [5]. A recent review on axions as cold dark matter can- inates the interaction (depending however on the con- didates can be found in [6]. In this review I will focus tent of the neutralino) for target masses with A≥30 [3]. on the direct detection of WIMPs, with emphasis on The cross section can be expressed as the sum of both most recent experimental results and on the status of dσ ∝ 0 2 0 2 contributions dER σSI FSI (ER)+σSDFSD(ER), where projects which are now under construction. 0 σSI,SD are the WIMP-nucleus cross sections in the 2 limit of zero momentum transfer and FSI,SD(ER) de- 2 Direct Detection of WIMPs note the nuclear form factors, expressed as a function of the recoil energy. WIMPs can be detected directly, via their elastic colli- The left side of equation 1 is the measured spec- sions with nuclei in terrestrial targets [7]. The differen- trum in a detector, while the right side represents tial rate for WIMP elastic scattering off nuclei is given the theoretical prediction. It includes WIMP proper- by [8] ties which are completely unknown, such as the mass mW and elastic cross section σ0, quantities in princi- vmax ple accessible from astrophysics, such as the density of dR ρ0 dσ = NT dv f(v) v (1) WIMPs in the halo ρ0, the WIMP velocity distribu- dER mW Zvmin dER tion f(v) and the escape velocity (which are however still prone to large uncertainties) and detector specific where NT represents the number of the target nuclei, m is the WIMP mass and ρ the local WIMP density parameters: mass of target nucleus, energy threshold W 0 and nuclear form factors. The nuclear form factors be- a Email: [email protected] come significant at large WIMP and nucleus masses, Laura Baudis Review and lead to a suppression of the differential scattering surrounding the detector. An additional advantage is rate. Figure 1 shows differential spectra for Si, Ar, Ge provided by a highly granular detector (or good timing and Xe, calculated for a WIMP mass of 100GeV, a and position resolution), since multiple scatters within scalar WIMP-nucleon cross section of σ = 10−43 cm2, the detector volume allow both background reduction 3 using standard halo parameters ( ρ0 = 0.3 GeV/cm , and a direct measurement of the neutron background −1 v0 = 220 km s ). for experiments with an event-by-event discrimination against electron recoils. Finally, the position resolu- tion leads to the identification of events clustered at −2 10 the detector surfaces, which are quite unlikely to be m = 100 GeV Si WIMP WIMPs. Ar σ = 1e−43 cm−2 In order to convincingly detect a WIMP signal, a Ge WIMP−n Xe specific signature from a particle populating our galac- tic halo is important. The Earth’s motion through the galaxy induces both a seasonal variation of the total event rate [9,10] and a forward-backward asymmetry in a directional signal [11,12]. The annual modulation −3 10 of the WIMP signal arises because of the Earth’s mo- tion in the galactic frame, which is a superposition of the Earth’s rotation around the Sun and the Sun’s ro- tation around the galactic center: Counts [/kg/keV/day] vE = v⊙ + vorb cos γ cos ω(t − t0), (2) −1 −1 where v⊙ = v0 + 12 km s (v0 ≈ 220 km s ), −4 −1 10 vorb ≈ 30 km s denotes the Earth’s orbital speed around the Sun, the angle γ ≈ 600 is the inclination of the Earth’s orbital plane with respect to the galactic nd plane and ω =2π/1yr, t0 = June 2 . The expected time dependence of the count rate 0 10 20 30 40 50 60 70 80 Recoil Energy [keV] can be approximated by a cosine function with a pe- nd riod of T = 1year and a phase of t0 = June 2 : Fig. 1. Differential WIMP recoil spectrum for a WIMP mass of 100 GeV and a WIMP-nucleon cross section σ = −43 2 S(t)= S0 + Smcosω(t − t0), (3) 10 cm . The spectrum was calculated for illustrative nuclei such as Si (light solid), Ar (light dot-dashed), Ge where S0, Sm are the constant and the modulated (dark solid), Xe (dark dashed). amplitude of the signal, respectively. In reality, an ad- ditional contribution to S(t) from the background must be considered. The background should be constant A WIMP with a typical mass between a few GeV in time or at least not show the same time depen- and 1 TeV will deposit a recoil energy below 50 keV dency as the predicted WIMP signal. The expected in a terrestrial detector. The predicted event rates for seasonal modulation effect is very small (of the order −6 neutralinos range from 10 to 10 events per kilogram of vorb/v0 ≃ 0.07), requiring large masses and long detector material and day [3]. Evidently, in order to counting times as well as an excellent long-term sta- observe a WIMP spectrum, low energy threshold, low bility of the experiment. background and high mass detectors are essential. In A much stronger signature would be given by the such a detector, the recoil energy of the scattered nu- ability to detect the axis and direction of the recoil cleus is transformed into a measurable signal, such nucleus. It has been shown [11], that the WIMP in- as charge, light or phonons, and at least one of the teraction rate as a function of recoil energy and angle above quantities is detected. Observing two signals si- θ between the WIMP velocity and recoil direction (in multaneously yields a powerful discrimination against the galactic frame) is: background events, which are mostly interactions with electrons, as opposed to WIMPs and neutrons scatter- 2 2 d R (v⊙cosθ − v ) ing off nuclei. Even for experiments with good event ∝ − min exp 2 , (4) by event discrimination, an absolute low background dERdcosθ v0 is still important, given the small size and rate of the 2 2 2 2 expected WIMP signal. It can be achieved in both pas- where vmin = (mN + mW ) ER/2mN mW and v0 2 sive and active ways. Passive methods range from high =3v⊙/2. The forward-backward asymmetry yields a material selection of detector components to various large effect of the order of O(v⊙/v0)≈ 1. Thus fewer specific shields against the natural radioactivity of the events are needed to discover a WIMP signal than in environment and against cosmic rays and secondary the case where the seasonal modulation effect is ex- particles produced in their interactions. Active back- ploited [12]. The challenge is to build massive detec- ground reduction implies an active shield, commonly a tors capable of detecting the direction of the incoming plastic or liquid scintillator, or a water Cerenkov shield WIMP.