Direct detection of WIMPs.pdf_P6-P10

and is related to the strange quark scalar density in the nucleon.The largest source of uncertainty in fro stems from the determination of this quantity,for which the current data implies EN =(64+8)MeV [10],which translates into a variation of a factor 4 in frs.Notice that in general the WIMP interaction with strange quarks would be the leading contribution to the SI cross section,due to its larger Yukawa coupling.In this case,oo is roughly proportional to frs2 and the above uncertainty in the strange quark content leads to a variation of more than one order of magnitude in the resulting SI cross section [7;9]). Similarly,for the SD cross section the uncertainties in the strange spin contribution As are the dominant contribution to the error in 0o.However,in the case of the neutralino, this can imply a correction of as much as a factor 2 in the resulting cross section [9],being therefore much smaller than the above uncertainty for the SI cross section.It should be emphasized,however,that uncertainties in the determination of the spin form factors S(g) would also affect the theoretical predictions for the dark matter detection rate. 3 Astrophysics input 3.1 Local DM density The differential event rate is directly proportional to the local WIMP density,po =p(r= Ro)where Ro =(8.0+0.5)kpc [11]is the solar radius.Any observational uncertainty in Po therefore translates directly into in an uncertainty in the event rate and the inferred constraints on,or measurements of,the scattering cross-sections. Exclusion limits are traditionally calculated assuming a canonical local WIMP density, Po =0.3GeVcm-3.The local WIMP density is calculated by applying observational con- straints (including measurements of the rotation curve)to models of the Milky Way and the values obtained can vary by a factor or order 2 depending on the models used [12-15]. A recent study [16],using spherical halo models with a cusp (p o r(r)-as r-0)finds p0=(0.30±0.05)GeV cm-3 3.2 Speed distribution The standard halo model,conventionally used in calculations of exclusion limits and signals, has an isotropic,Gaussian velocity distribution (often referred to as Maxwellian) 1 f()= v/2 -exp √2m石 2a2 (25) The speed dispersion is related to the local circular speed by a =v3/2vc and ve=(220+ 20)kms-1 [17](see Sec.3.3)so that o270kms-1.This velocity distribution corresponds to an isotropic singular isothermal sphere with density profile p(r)r-2.The isothermal sphere is simple,and not unreasonable as a first approximation,however it is unlikely to be an accurate model of the actual density and velocity distribution of the Milky Way.Observations and numerical simulations(see chapter 1)indicate that dark matter halos do not have a 1/r2 density profile and are (to some extent at least)triaxial and anisotropic. If the velocity distribution is isotropic there is a one to one relation between f(v)and the the spherically symmetric density profile given by Eddington's formula [18],see Refs.[19;20]. In general the steady state phase-space distribution of a collection of collisionless particles 6

and is related to the strange quark scalar density in the nucleon. The largest source of uncertainty in f p T q stems from the determination of this quantity, for which the current data implies ΣπN = (64 ± 8) MeV [10], which translates into a variation of a factor 4 in fT s. Notice that in general the WIMP interaction with strange quarks would be the leading contribution to the SI cross section, due to its larger Yukawa coupling. In this case, σ0 is roughly proportional to fT s 2 and the above uncertainty in the strange quark content leads to a variation of more than one order of magnitude in the resulting SI cross section [7; 9]). Similarly, for the SD cross section the uncertainties in the strange spin contribution ∆s are the dominant contribution to the error in σ0. However, in the case of the neutralino, this can imply a correction of as much as a factor 2 in the resulting cross section [9], being therefore much smaller than the above uncertainty for the SI cross section. It should be emphasized, however, that uncertainties in the determination of the spin form factors S(q) would also affect the theoretical predictions for the dark matter detection rate. 3 Astrophysics input 3.1 Local DM density The differential event rate is directly proportional to the local WIMP density, ρ0 ≡ ρ(r = R0) where R0 = (8.0 ± 0.5) kpc [11] is the solar radius. Any observational uncertainty in ρ0 therefore translates directly into in an uncertainty in the event rate and the inferred constraints on, or measurements of, the scattering cross-sections. Exclusion limits are traditionally calculated assuming a canonical local WIMP density, ρ0 = 0.3 GeV cm−3 . The local WIMP density is calculated by applying observational constraints (including measurements of the rotation curve) to models of the Milky Way and the values obtained can vary by a factor or order 2 depending on the models used [12–15]. A recent study [16], using spherical halo models with a cusp (ρ ∝ r(r) −α as r → 0) finds ρ0 = (0.30 ± 0.05) GeV cm−3 . 3.2 Speed distribution The standard halo model, conventionally used in calculations of exclusion limits and signals, has an isotropic, Gaussian velocity distribution (often referred to as Maxwellian) f(v) = 1 √ 2πσ exp − |v| 2 2σ 2 ! . (25) The speed dispersion is related to the local circular speed by σ = p 3/2vc and vc = (220 ± 20) km s−1 [17] (see Sec.3.3) so that σ ≈ 270 km s−1 . This velocity distribution corresponds to an isotropic singular isothermal sphere with density profile ρ(r) ∝ r −2 . The isothermal sphere is simple, and not unreasonable as a first approximation, however it is unlikely to be an accurate model of the actual density and velocity distribution of the Milky Way. Observations and numerical simulations (see chapter 1) indicate that dark matter halos do not have a 1/r2 density profile and are (to some extent at least) triaxial and anisotropic. If the velocity distribution is isotropic there is a one to one relation between f(v) and the the spherically symmetric density profile given by Eddington’s formula [18], see Refs. [19; 20]. In general the steady state phase-space distribution of a collection of collisionless particles 6

is given by the collisionless Boltzmann equation and the velocity dispersions of the system are calculated via the Jean's equations (e.g.Ref.[21]).Solving the Jean's equations requires assumptions to be made,and therefore even for a specific density distribution the solution is not unique.Several specific models have been used in the context of WIMP direct detection signals.The logarithmic ellipsoidal model [22]is the simplest triaxial generalisation of the isothermal sphere and has a velocity distribution which is a multi-variate Gaussian.Osipkov- Merritt models [23;24]are spherically symmetric with radially dependent anisotropic velocity distributions.Fitting functions for the speed distributions in these models are available,for a selection of density profiles,in Ref.[25].Velocity distributions have also been extracted from cosmological simulations,with both multi-variate Gaussian [26]and Tsallis [27]distri- butions [28]being advocated as fitting functions.While it is not known whether any of these models provide a good approximation to the real local velocity distribution function,the models are none the less useful for assessing the uncertainties in the direct detection signals. Particles with speed,in the Galactic rest frame,greater than the local escape speed, vesc=√2Φ(Ro)I whereΦ(r）is the potential,are not gravitationally bound.Many of the models used,in particular the standard halo model,formally extend to infinite radii and therefore their speed distribution has to be truncated at vese 'by hand'(see e.g.Ref.[29]). The standard value for the escape speed is vese=650kms-1.A recent analysis,using high velocity stars from the RAVE survey,finds 498kms-<vese 608kms-1 with a median likelihood of 544 kms-1 [30]. In Sec.4 we discuss the impact of uncertainty in the speed distribution on the direct detection signals. 3.3 Earth's motion The WIMP speed distribution in the detector rest frame is calculated by carrying out,a time dependent,Galilean transformation:v=v+ve(t).The Earth's motion relative to the Galactic rest frame,ve(t),is made up of three components:the motion of the Local Standard of Rest (LSR),the Sun's peculiar motion with respect to the LSR,ve,and the Earth's orbit about the Sun,vorb If the Milky Way is axisymmetric then the motion of the LSR is given by the local circular velocity (0,ve,0),where vc =220kms-1 is the standard value.Kerr and Lynden-Bell found, by combining a large number of independent measurements,ve=(222+20)kms-1 [17].A more recent determination,using the proper motions of Cepheids measured by Hipparcos [31], is broadly consistent:vc=(218+7)kms-1(Ro/8kpc). The Sun's peculiar motion,determined using the parallaxes and proper motions of stars in the solar neighbourhood from the Hipparcos catalogue,is ve =(10.0+0.4,5.2+0.6,7.2+ 0.4)kms-1 [32]in Galactic co-ordinates(where x points towards the Galactic center,y is the direction of Galactic rotation and z towards the North Galactic Pole). A relatively simple,and reasonably accurate,expression for the Earth's motion about the Sun can be found by ignoring the ellipticity of the Earth's orbit and the non-uniform motion of the Sun in right ascension [33]:verb =veleisin A(t)-e2 cos A(t)]where ve 29.8kms-1 is the orbital speed of the Earth,A(t)=2(t-0.218)is the Sun's ecliptic longitude(with t in years)and e1(2)are unit vectors in the direction of the Sun at the Spring equinox (Summer solstice).In Galactic co-ordinates e=(-0.0670,0.4927,-0.8676)and e2=(-0.9931,-0.1170,0.01032).More accurate expressions can be found in Ref.[34]. 7

is given by the collisionless Boltzmann equation and the velocity dispersions of the system are calculated via the Jean’s equations (e.g. Ref. [21]). Solving the Jean’s equations requires assumptions to be made, and therefore even for a specific density distribution the solution is not unique. Several specific models have been used in the context of WIMP direct detection signals. The logarithmic ellipsoidal model [22] is the simplest triaxial generalisation of the isothermal sphere and has a velocity distribution which is a multi-variate Gaussian. OsipkovMerritt models [23; 24] are spherically symmetric with radially dependent anisotropic velocity distributions. Fitting functions for the speed distributions in these models are available, for a selection of density profiles, in Ref. [25]. Velocity distributions have also been extracted from cosmological simulations, with both multi-variate Gaussian [26] and Tsallis [27] distributions [28] being advocated as fitting functions. While it is not known whether any of these models provide a good approximation to the real local velocity distribution function, the models are none the less useful for assessing the uncertainties in the direct detection signals. Particles with speed, in the Galactic rest frame, greater than the local escape speed, vesc = p 2|Φ(R0)| where Φ(r) is the potential, are not gravitationally bound. Many of the models used, in particular the standard halo model, formally extend to infinite radii and therefore their speed distribution has to be truncated at vesc ‘by hand’ (see e.g. Ref. [29]). The standard value for the escape speed is vesc = 650 km s−1 . A recent analysis, using high velocity stars from the RAVE survey, finds 498km s−1 < vesc < 608 km s−1 with a median likelihood of 544 km s−1 [30]. In Sec. 4 we discuss the impact of uncertainty in the speed distribution on the direct detection signals. 3.3 Earth’s motion The WIMP speed distribution in the detector rest frame is calculated by carrying out, a time dependent, Galilean transformation: v → v˜ = v + ve(t). The Earth’s motion relative to the Galactic rest frame, ve(t), is made up of three components: the motion of the Local Standard of Rest (LSR), the Sun’s peculiar motion with respect to the LSR, v p ⊙, and the Earth’s orbit about the Sun, v orb e . If the Milky Way is axisymmetric then the motion of the LSR is given by the local circular velocity (0, vc, 0), where vc = 220 km s−1 is the standard value. Kerr and Lynden-Bell found, by combining a large number of independent measurements, vc = (222 ± 20) km s−1 [17]. A more recent determination, using the proper motions of Cepheids measured by Hipparcos [31], is broadly consistent: vc = (218 ± 7) km s−1 (R0/8 kpc). The Sun’s peculiar motion, determined using the parallaxes and proper motions of stars in the solar neighbourhood from the Hipparcos catalogue, is v p ⊙ = (10.0 ± 0.4, 5.2 ± 0.6, 7.2 ± 0.4) km s−1 [32] in Galactic co-ordinates (where x points towards the Galactic center, y is the direction of Galactic rotation and z towards the North Galactic Pole). A relatively simple, and reasonably accurate, expression for the Earth’s motion about the Sun can be found by ignoring the ellipticity of the Earth’s orbit and the non-uniform motion of the Sun in right ascension [33]: v orb e = ve[e1 sin λ(t) − e2 cos λ(t)] where ve = 29.8 km s−1 is the orbital speed of the Earth, λ(t) = 2π(t − 0.218) is the Sun’s ecliptic longitude (with t in years) and e1(2) are unit vectors in the direction of the Sun at the Spring equinox (Summer solstice). In Galactic co-ordinates e1 = (−0.0670, 0.4927, −0.8676) and e2 = (−0.9931, −0.1170, 0.01032). More accurate expressions can be found in Ref. [34]. 7

The main characteristics of the WIMP signals can be found using only the motion of the LSR,and for the time dependence the component of the Earth's orbital velocity in that direction.However accurate calculations,for instance for comparison with data,require all the components described above to be taken into account. 3.4 Ultra-fine structure Most of the WIMP velocity distributions discussed in Sec.3.2 are derived by solving the collisionless Boltzmann equation,which assumes that the phase space distribution has reached a steady state.However this may not be a good assumption for the Milky Way;structure formation in CDM cosmologies occurs hierarchically and the relevant dynamical timescales for the Milky Way are not many orders of magnitude smaller than the age of the Universe. Both astronomical observations and numerical simulations(due to their finite resolution) typically probe the dark matter distribution on kpc scales.Direct detection experiments probe the DM distribution on sub milli-pc scales (the Earth's speed with respect to the Galactic rest frame is 0.2mpcyr-1).It has been argued (see e.g.Refs.[35-37])that on these scales the DM may not have yet reached a steady state and could have a non-smooth phase-space distribution.On the other hand it has been argued that the rapid decrease in density of streams evolving in a realistic,ellipsoidal,Galactic potential means that there will be a large number of overlapping streams in the solar neighbourhood producing an effectively smooth DM distribution 38]. If the local DM distribution consists of a small number of streams,rather than a smooth distribution,then there would be significant changes in the signals which we will discuss in Sec.4.This is currently an open issue;directly calculating the DM distribution on the scales probed by direct detection experiments is a difficult and unresolved problem. It has been suggested that a tidal stream from the Sagittarius (Sgr)dwarf galaxy,which is in the process of being disrupted,passes through the solar neighbourhood with the asso- ciated DM potentially producing distinctive signals in direct detection experiments [39;40]. Subsequent numerical simulations of the disruption of Sgr along with observational searches for local streams of stars suggest that the Sgr stream does not in fact pass through the solar neighbourhood (see e.g.Ref.[41]).None the less the calculations of the resulting WIMP signals in Refs.[39;40]provide a useful illustration of the qualitative effects of streams. 4 Signals We have already seen in Sec.2 that the recoil rate is energy dependent due to the kinematics of elastic scattering,combined with the WIMP speed distribution.Due to the motion of the Earth with respect to the Galactic rest frame the recoil rate is also both time and direction dependent.In this section we examine the energy,time and direction dependence of the recoil rate and the resulting WIMP signals.In each case we first focus on the signal expected for the standard halo model,with a Maxwellian velocity distribution,before discussing the effect on the signal of changes in the WIMP velocity distribution. 8

The main characteristics of the WIMP signals can be found using only the motion of the LSR, and for the time dependence the component of the Earth’s orbital velocity in that direction. However accurate calculations, for instance for comparison with data, require all the components described above to be taken into account. 3.4 Ultra-fine structure Most of the WIMP velocity distributions discussed in Sec. 3.2 are derived by solving the collisionless Boltzmann equation, which assumes that the phase space distribution has reached a steady state. However this may not be a good assumption for the Milky Way; structure formation in CDM cosmologies occurs hierarchically and the relevant dynamical timescales for the Milky Way are not many orders of magnitude smaller than the age of the Universe. Both astronomical observations and numerical simulations (due to their finite resolution) typically probe the dark matter distribution on ∼ kpc scales. Direct detection experiments probe the DM distribution on sub milli-pc scales (the Earth’s speed with respect to the Galactic rest frame is ≈ 0.2 mpc yr−1 ). It has been argued (see e.g. Refs. [35–37]) that on these scales the DM may not have yet reached a steady state and could have a non-smooth phase-space distribution. On the other hand it has been argued that the rapid decrease in density of streams evolving in a realistic, ellipsoidal, Galactic potential means that there will be a large number of overlapping streams in the solar neighbourhood producing an effectively smooth DM distribution [38]. If the local DM distribution consists of a small number of streams, rather than a smooth distribution, then there would be significant changes in the signals which we will discuss in Sec. 4. This is currently an open issue; directly calculating the DM distribution on the scales probed by direct detection experiments is a difficult and unresolved problem. It has been suggested that a tidal stream from the Sagittarius (Sgr) dwarf galaxy, which is in the process of being disrupted, passes through the solar neighbourhood with the associated DM potentially producing distinctive signals in direct detection experiments [39; 40]. Subsequent numerical simulations of the disruption of Sgr along with observational searches for local streams of stars suggest that the Sgr stream does not in fact pass through the solar neighbourhood (see e.g. Ref. [41]). None the less the calculations of the resulting WIMP signals in Refs. [39; 40] provide a useful illustration of the qualitative effects of streams. 4 Signals We have already seen in Sec. 2 that the recoil rate is energy dependent due to the kinematics of elastic scattering, combined with the WIMP speed distribution. Due to the motion of the Earth with respect to the Galactic rest frame the recoil rate is also both time and direction dependent. In this section we examine the energy, time and direction dependence of the recoil rate and the resulting WIMP signals. In each case we first focus on the signal expected for the standard halo model, with a Maxwellian velocity distribution, before discussing the effect on the signal of changes in the WIMP velocity distribution. 8

4.1 Energy dependence The shape of the differential event rate depends on the WIMP and target masses,the WIMP velocity distribution and the form factor.For the standard halo model the expression for the differential event rate,eq.1,can be rewritten approximately (c.f.Ref.[42])as dR dR dER F2(ER)exp ΓE (26) where (dR/dER)o is the event rate in the E>0 keV limit.The characteristic energy scale is given by E=(c12u2)/mN where c is a parameter of order unity which depends on the target nuclei.If the WIMP is much lighter than the target nuclei,mx<mN,then Ec o m/mN while if the WIMP is much heavier than the target nuclei Ecx my.The total recoil rate is directly proportional to the WIMP number density,which varies as 1/mx. In fig.1 we plot the differential event rate for Ge and Xe targets and a range of WIMP masses.As expected,for a fixed target the differential event rate decreases more rapidly with increasing recoil energy for light WIMPs.For a fixed WIMP mass the decline of the differen- tial event rate is steepest for heavy target nuclei.The dependence of the energy spectrum on the WIMP mass allows the WIMP mass to be estimated from the energies of detected events (e.g.Ref.[43]).Furthermore the consistency of energy spectra measured by experiments using different target nuclei would confirm that the events were due to WIMP scattering (rather than,for instance,neutron backgrounds)[42].In particular,for spin independent interactions,the total event rate scales as A2.The is sometimes referred to as the 'materials signal'. The WIMP and target mass dependence of the differential event rate also have some general consequences for experiments.The dependence of the total event rate on my means that,for fixed cross-section,a larger target mass will be required to detect heavy WIMPs than lighter WIMPs.For very light WIMPs the rapid decrease of the energy spectrum with increasing recoil energy means that the event rate above the detector threshold energy,Er, may be small.If the WIMP is light,<(10 GeV),a detector with a low,<O(kev), threshold energy will be required. The most significant astrophysical uncertainties in the differential event rate come from the uncertainties in the local WIMP density and circular velocity.As discussed in Sec.3.1 the uncertainty in the local DM density translates directly into an uncertainty in constraints on (or in the future measurements of)the scattering cross-section.The time averaged differential event rate is found by integrating the WIMP velocity distribution,therefore it is only weakly sensitive to changes in the shape of the WIMP velocity distribution.For the smooth halo models discussed in Sec.3.2 the time averaged differential event rates are fairly similar to that produced by the standard halo model [44;45].Consequently exclusion limits vary only weakly [45;46]and there would be a small (of order a few per-cent)systematic uncertainty in the WIMP mass deduced from a measured energy spectrum [47].With multiple detectors it would in principle be possible to measure the WIMP mass without any assumptions about the WIMP velocity distribution [48. In the extreme case of the WIMP distribution being composed of a small number of streams the differential event rate would consists of a series of (sloping due to the form factor)steps.The positions of the steps would depend on the stream velocities and the target and WIMP masses,while the relative heights of the steps would depend on the stream densities. 9

4.1 Energy dependence The shape of the differential event rate depends on the WIMP and target masses, the WIMP velocity distribution and the form factor. For the standard halo model the expression for the differential event rate, eq. 1, can be rewritten approximately (c.f. Ref.[42]) as dR dER ≈ dR dER 0 F 2 (ER) exp − ER Ec , (26) where (dR/dER)0 is the event rate in the E → 0 keV limit. The characteristic energy scale is given by Ec = (c12µ 2 N v 2 c )/mN where c1 is a parameter of order unity which depends on the target nuclei. If the WIMP is much lighter than the target nuclei, mχ ≪ mN , then Ec ∝ m2 χ/mN while if the WIMP is much heavier than the target nuclei Ec ∝ mN . The total recoil rate is directly proportional to the WIMP number density, which varies as 1/mχ. In fig. 1 we plot the differential event rate for Ge and Xe targets and a range of WIMP masses. As expected, for a fixed target the differential event rate decreases more rapidly with increasing recoil energy for light WIMPs. For a fixed WIMP mass the decline of the differential event rate is steepest for heavy target nuclei. The dependence of the energy spectrum on the WIMP mass allows the WIMP mass to be estimated from the energies of detected events (e.g. Ref. [43]). Furthermore the consistency of energy spectra measured by experiments using different target nuclei would confirm that the events were due to WIMP scattering (rather than, for instance, neutron backgrounds) [42]. In particular, for spin independent interactions, the total event rate scales as A2 . The is sometimes referred to as the ‘materials signal’. The WIMP and target mass dependence of the differential event rate also have some general consequences for experiments. The dependence of the total event rate on mχ means that, for fixed cross-section, a larger target mass will be required to detect heavy WIMPs than lighter WIMPs. For very light WIMPs the rapid decrease of the energy spectrum with increasing recoil energy means that the event rate above the detector threshold energy, ET , may be small. If the WIMP is light, < O(10 GeV), a detector with a low, < O( keV), threshold energy will be required. The most significant astrophysical uncertainties in the differential event rate come from the uncertainties in the local WIMP density and circular velocity. As discussed in Sec. 3.1 the uncertainty in the local DM density translates directly into an uncertainty in constraints on (or in the future measurements of) the scattering cross-section. The time averaged differential event rate is found by integrating the WIMP velocity distribution, therefore it is only weakly sensitive to changes in the shape of the WIMP velocity distribution. For the smooth halo models discussed in Sec. 3.2 the time averaged differential event rates are fairly similar to that produced by the standard halo model [44; 45]. Consequently exclusion limits vary only weakly [45; 46] and there would be a small (of order a few per-cent) systematic uncertainty in the WIMP mass deduced from a measured energy spectrum [47]. With multiple detectors it would in principle be possible to measure the WIMP mass without any assumptions about the WIMP velocity distribution [48]. In the extreme case of the WIMP distribution being composed of a small number of streams the differential event rate would consists of a series of (sloping due to the form factor) steps. The positions of the steps would depend on the stream velocities and the target and WIMP masses, while the relative heights of the steps would depend on the stream densities. 9