LEP Physics and the Early Universe

LEP PHYSICS AND THE EARLY UNIVERSE LAWRENCEM. KRAUSS· Center For Theoretical Physics And Dept Of Astronomy Sloane Laboratory, Yale University, New Haven, Cf 06511 ABS1RACf: I review the implications of the Z decay measurements at LEP (and SLC) for the early universe: (a) The Z width measurements, when combined with non-accelerator data rule out GeV range Dirac neutrinos, Majorana neutrinos, and sneutrinos as WIMP dark matter, rule out most explicit "cosmions", and strongly constrain neutralinos. When combined with data from other experiments, this implies that any WIMP is likely to be heavier than 10-20 GeV, and could easily be in the 0(100 GeV-l TeV) region; (b) The limit on N v , when combined with big-bang nucleosynthesis (BBN) estimates, is consistent with both baryonic and non-baryonic dark matter and allows one to probe the consistency of BBN itself; (c) direct searches for WIMPs will probably require new detectors, sensitive to spin dependent interactions on possibly heavy targets, with event rates which could be 3-4 orders of magnitude smaller than those expected at present detectors; (d) limits on the Higgs and the top quark also have an impact on possible new processes in the early universe. 1. Introduction: The Z Width Reviewed: There was a great deal of discussion at this meeting about the beautiful COBE results on the cosmic microwave background. The measured black-body spectrum provides incredibly strong constraints on possible energy release mechanisms in the pre- and post-recombination universe, and also on exotic scenarios for the formation of large scale structure. An equally significant experimental curve has recently been produced in particle physics. Like the CMB spectrum, it is also one whose shape has been predicted for over 20 years, waiting for experimentalists to measure it. Also, like the CMB spectrum, a single determination of its overall shape has provided very powerful new limits on exotica beyond the standard model. I refer of course to the measurement at LEP (and SLC) of the shape of the Z boson resonance. In less than a year of running, these measurements of Z decay parameters have significantly constrained our picture of the world. It is remarkable how even a small amount of unambiguous data about so fundamental a system can so strongly direct theory. In particular, the new limits on Z width constrain not only the number of light neutrino species in nature, but also the existence of new particles of mass up to 45 GeV which might be produced in Z decay. Among these are included many of the leading Weakly Interacting Massive Particle (WIMP) candidates for dark matter 1). Those which remain viable will be far more challenging for experimenters to detect directly. Before discussing the detailed constraints, and because it is important for everything I will have to say here, I will first review in detail how one infers a limit on the number of neutrinos a Research supported in part by an NSF Presidential Young Investigator Award and by the DOE 351 K. Sato et al. (eds.), Primordial Nucleosynthesis and Evolution of Early Universe © Springer Science+Business Media Dordrecht1991 352 from the Z decay measurements made at SLC and LEP. Such an analysis is at the very least pedagogically useful, and it also demonstrates some non-intuitive features of the measurement. When Z bosons are produced at e+e- colliders, the shape of the Z resonance is most easily determined by observing the hadronic decays of the Z, which have by far the largest branching ratio, and so lead to the best statistics. By counting the number of decays as a function of energy, and carefully measuring the luminosity of the machine at each energy, the cross section for Z production can be mapped out, and compared to the standard Breit-Wigner form: CTh =of s r./ (s -M?)2 + s 2[/IM? [l+orai(s)] (1) Here, .Js is the center of mass energy, [l+~s)] accounts for initial state radiation, and the BreitWigner resonance form is then described by 3 parameters: Mz . r z' and a h0 , the mass, the total width, and the height at the peak respectively. These are displayed in the figure below. t Mz Not all these parameters are independent, however. In particular, in order that the integral under the curve give the total Z production cross section, the peak height a O and the width rare related, so that: -f) _ Uh - 121tre n (2) Mz2r.z2 where re and r h are the Z partial decay widths into electrons and hadrons respectively. rz The total Z width is defined to be: =.N vrv +3re +rh , where rv is the partial width into massless neutrinos in the standard model, and lepton universality is assumed [1]. (Otherwise, the sum of partial widths into charged lepton pairs may be used in the definition.) This defines what is meant by the "number" of neutrinos, i.e. (3) N v =. -L.{rz -3re -rh) rv One's naive expectation might be that to extract a limit on the number of neutrinos one would first fit the measurements to the Breit-Wigner form (1), determine a best fit value for r z and then use (3). However, since each new neutrino species contributes about 6% to the width, an uncertainty in the width determination of 6% corresponds to an uncertainty of one extra neutrino type. Instead, since a h0 depends quadratically on r z (because the integrated cross section for Z production is fixed), an extra neutrino species contributes about 12% to the peak cross section 353 value. Thus, assuming the uncertainty in the peak cross section determination is no worse than that in the width, one can get twice as good a limit on the number of neutrinos by using the fit for 0h 0 and then using (2) to limit the number of neutrinos than one can get by using r z alone. Moreover, 0h 0 is defined in (2) in terms of ratios of widths, and is thus less sensitive to uncertainties in the widths from the standard model, such as from the top quark mass, or the uncertainty in sin29w . Thus, using (2), we can write N v == Te Tv (J 1;1t 0 M z (Jh . ~ rr; -3 _lh ) , r: (4) Te which expresses N v in terms of 0h 0 and in terms of the ratio r hffe' which can either be extracted from the standard model, or more important, can be directly measured. In the latter case, the ratio r effv must be taken from theory. (There is no such thing as completely model independent bound on N v .) The first limits on N v essentially used (2), (or (4)) 2). Next, the LEP groups began quoting limits from (3) and (4) 3). Most recently, (4) has been used, with rhffJl being directly measured. It is important, when comparing the results of different experiments, to make sure that the limits which are quoted are derived the same way, and also to attempt to use the firmest bounds. For example, the errors on the limits which were originally obtained using r z are about twice as big as those obtained using 0h. 4 ) Also a direct r z bound is more susceptible to systematic errors in partial rates. For this reason a limit obtained by combining (3) and (4) is essentially dominated by the 0h determination. (Also note that for this reason, the bound on extra neutrinos from (4) has an inherent insensitivity to unstable particles. Subsequent neutrino decays could effectively contribute to the measuredhadronic partial width, and thus contribute to both the numerator and denominator of (2). The sensitivity to such an extra neutrino would then be reduced by a factor of 2. Other information from the experiments can be used however to directly rule out unstable neutrinos.) Caveats aside, what is the current combined limit on N v from all the experiments? Below, I display the compilation I performed based on the initial LEP preprints 2), and the second round of preprints in March of this year, and finally the limits quoted at the International High Energy meeting in July, being as careful as possible to quote the limits derived from the same analysis in each experiment. Experiment Markll ALEPH DELPHI 13 OPAL Average Nv Limit (Oct 89) 2.8±0.6 3.27±0.30 2.4 ± 0.4 ± 0 .5 3.42 ±0.48 3.12 ±0.42 3.13±0.20 Nv Limit (Mar 90) same 3.03±0.15 3.05 ±0.28 3.11 ±O.17 4) 3.09±0.19 3.06±0.09 Nv Limit (July 90) same 2.92 ± 0.12 (stat) ±O.03 (thry) 2.80 ± 0.16 (stat) ±O.03 (thry) 2.95 ± 0.18 (stat) ±O.03 (thry) 2.84 ± 0.14 (stat) ±O.03 (thry) (a)2.89 ± 0.10 (stat +sys) * (b)2.99±O.09 (stat + sys) * Finally, after having gone to all the trouble to explain these results, what is the point, i.e. what new information about dark matter may we glean? First, as I have stressed, N v is defined by (3) or (4) above. As such, it can represent more than just new light or massless neutrino species. In particular, any new undetected particle which couples to the Z and is lighter than half its mass can contribute to N v. This includes most WIMP candidates. * The numbers quoted in this column were taken using experimental measured value of r hffJl. If the value of r hffJl is taken from the standard model. the mean value increases. and is given in (b). 354 2. New Constraints On Wimps, Etc: We can determine the contribution of any new particle to the Z width as follows. Fermions (mass M and coupling L::: [g !2 cos Ow ](Gy 'iiirJlyt +Ga'iiirJlrsyt) to the Z) contribute to the Z width as follows: r= ~l["1-4M2IrG 6,,,,x ] {{Gy 2 +G}) [1-MllMrl + 3{G y 2 -G}) MllMr} (5) Complex scalars with mass M and L = [ig K !2 cos Ow 1('I' *a"'rp -@"'rp *)'1') yield r= GFK 2Ml [1-4M2IMr] 3/2 . (6) 12Y!1t One can determine what fraction of a neutrino these might mimic, by comparing these widths to that for a massless v. r(Z 0-+ \IV )=GFMz3/12Y!1t '" 165MeV. (7) Then, by comparing the predictions to the 2 (J upper bound on N v of 3.53 (Oct.), 3.25 (March), or 3.18 (July limit (b», one can derive limits on new particle masses and couplings of new particles so that this bound is not exceeded. For particles which couple with the same strength as neutrinos , one finds 1~ Particle Type fermion scalar Nv <3.25 (20) Nv <3.53 (20) m ~O(30)GeV Nv <3.18 (20) m~0(4)GeV m~0(42)GeV m ~ 0(30) GeV m~OGeV (8) m~0(35)GeV We shall see that most of the new constraints are insensitive to the precise bound on Nv used. (a) Dirac Neutrinos: The prototypical WIMPs, heavy Dirac neutrinos were the first elementary particles whose non-thermal remnant abundance was calculated in a manner identical to that used previously to calculate successfully the remnant abundance of light elements such as Helium produced in the big bang. The idea is simple. Heavy neutrinos maintain an eqUilibrium density as long as their annihilation rate exceeds the expansion rate (assuming no particle antiparticle asymmetry). After this time, the ratio of neutrinos to thermal photons in the universe is frozen in at the Boltzmann factor appropriate to the temperature at freeze-out: (9) nv == exp -[ mv ] "r Ttteezeou . T freeze out is fixed b!, the annihilation cross section. Since <av>'" GF2 m2 for Dirac neutrinos, it was shown in 1977 ) that if mv ... 2 GeV, (9) would imply a closure density of neutrinos in the universe today. Moreover, since the cross section increases quadratically with mass, heavier neutrinos will annihilate more efficiently, and so will have a smaller remnant mass density today. It is this dynamical coupling between weak interaction strength and GeV mass scale that makes WIMPs natural candidates for dark matter. Of course, if there is a particle-antiparticle asymmetry, this coupling is removed, and heavy neutrinos of any mass greater than 2 GeV could have a closure density today. The LEP limits require the mass of a new stable Dirac neutrino to be greater than about 42 GeV. When this is incorporated in the calculations described above one finds that the remnant abundance of such particles, in the absence of an asymmetry, is far too small to make up all of even the galactic halo dark matter for masses between 40 GeV and about 2 Te V. Thus, the LEP results alone, in the absence of an asymmetry, rule out WIMP scale Dirac neutrinos as dark matter. Fortunately, however, the LEP results can be supplemented by non-accelerator experiments, 355 which can directly probe for a flux of dark matter WIMPS at the earth's surface. Low background Ge detectors are sensitive to ionization caused by energy deposited in elastic scattering off of nuclei for masses in excess of about 12 Ge V, and the nonexistence of a signal above background 6) rules out Dirac neutrinos as galactic halo dark matter for 12 GeV $ m $ 2 TeV. This bound, which would require m $ 12 GeV, is exactly complementary to the LEP bound, which requires m ~ 0(40) GeV. The combination completely rules out Dirac neutrinos as WIMPs. (Note: neutrinos with mass in excess of 2 TeV, which would be super-weakly interacting with normal matter, and thus are unWIMP-like, remain viable, if they have a remnant asymmetry.) (b) Majorana neutrinos: Majorana neutrinos are identical to their antiparticles and therefore cannot have a primordial asymmetry. Thus, their relic abundance is entirely determined by annihilations in the early universe. For Majorana neutrinos however, <crv>..(iF2 p2, so annihilation is suppressed for non-relativistic particles, leading to a slightly higher value for the mass resulting in a closure density, of about 5 GeV 7). The relic density falls above this roughly as m-5{3. Thus, the newest LEP limits imply that the fraction of closure density (Q) in Majorana neutrinos must be less than about .01, or about equal to the visible mass density in the universe today. Thus, the result (8) by itself rules out Majorana neutrinos as WIMPs. (c) SUSY WIMPs:Low energy supersymmetry (SUSY) provides the most compelling WIMP dark matter candidates. If the SUSY breaking scale M is tied to the weak scale, then the lightest SUSY partner of ordinary matter (LSP) can get a mass of order aM=O(GeV) . Moreover, since the other SUSY partners of ordinary matter which can mediate in scattering and annihilation processes have masses of order M= M w ' the LSP can have an interaction strength which is comparable to that of an ordinary massive neutrino. This combination makes it a natural WIMP! 8) Which particle is lightest is model-dependent. Those often discussed include the sneutrino---the partner of the neutrino, and a "neutralino" --- the fermion partner of a linear combination of the photon, the Z, and the two Higgses present in SUSY models. I discuss these separately below: (1) Scalar Neutrino: "Sneutrinos" couple to the Z with the same strength as neutrinos, and hence, from (2) will contribute = 1/2 as much to the Z width, for the same mass. Hence, while the initial LEP results did not rule out such a particle accessible in Z decay, the newest results imply m~ 0(35) GeV. Again, non-accelerator data provides a strong complimentary upper bound on its mass. The WIMP direct detection experiments which limit neutrinos also limit sneutrinos to be lighter than 0(12) GeV. This limit is supplemented by data from indirect detection using proton decay detectors. Sneutrinos are not only efficiently captured by the sun and earth 9), but they annihilate into light neutrinos, yielding a signal in proton decay detectors which is not seen, implying m $ 0(3-5) GeV 10) . Combining limits, sneutrinos are ruled out as WIMPs. (2) Neutralinos: The "neutralino" has sufficient flexibility so that it is not entirely ruled out on the basis of the Z decay limits alone. In low energy SUSY there are four neutral majorana fermion "partners" of ordinary matter--the photino, the zino, and two Higsno-~ which are expected to be among the lightest states 8). The states will mix in general, so that the mass eigenstates will be linear combinations of the weak eigenstates. In the minimal model the masses and couplings of the inos depend on four parameters: Ml and M 2, the gauge fermion mass terms, the higgsino mass parameter j.I., and the ratio of the two Higgs doublet expectation values v 2/v l' If the model is unified at some scale then Ml and M2 are related by M 1= 5/3 tan 29wM2' leaving three free parameters. Because of this larger freedom in model building, the constraints derivable from the Z decay width are less easily stated. For example, the pure neutralino states tend to decouple from Z decay. A pure photino or a pure Zino have no diagonal Z couplings, 356 since their boson partners have none. It also turns out that a light pure Higgsino tends to be the linear combination which has vanishing Z coupling. Nevertheless, some non-trivial constraints on the parameter space are derivable. Even in the cases where the LSP decouples from Z decay, the other neutralinos can contribute to the Z width 11) . Moreover, "charginos", the fermionic partners of the charged gauge bosons and Higgses, can give even larger contributions to the Z width 12) . These particles can have triplet weak isospin quantum numbers, so that for a charged gaugino, the Z decay width can be larger than that for 4 neutrinos 12). It is conventional to present constraints as curves in M-Jl space, for a fixed value ofv 1/v 2' This ratio reflects the origin of electro weak symmetry breaking. If the top quark mass term drives spontaneous symmetry breaking in the higgs sector, then v 1/v 2 must be greater than unity (in the limit v I/v 2=1 the LSP tends to be pure photino or Higgsino and thus decouples from the Z), given that the current top quark mass lower limit is 89 Ge V. As v I/v 2 increases, the Z width constraints become more powerful. To derive constraints on the Z width, one diagonalizes the neutralino and chariino mass matrices, finds the weak couplings of the mass eigenstates, and plugs them into (5) 11,1 ,13). The largest decay branching ratios come from the charginos (see above), so that the lightest chargino is constrained to have a m~O(Mzt2) before its contribution to the Z width is below the present upper limit. The range in M- Jl space ruled out by the requirement that m > 0(40) GeV 12), is shown schematically below for two values of~ . Also shown is the region of this parameter space which is ruled out br; the requirement that the total measured Z decay width into neutralinos be less than .6 r z<v)1 ). Finally, I display estimates l4 ) of the cosmological mass density in the neutralino LSP, with the solid line for 0=1 and the dashed curve for 0=0.1. Squark and slepton and top masses .. 0(100) GeV were assumed. Presumably the mass density of light neutralino WIMPs should lie between these two values. As these figures show, a significant region of the M-Jl plane is constrained by the Z width limit. 30~r-. 300 200+-+---- - - - - - - - , _ , 200 '\. "- · 100 ·200 ....... "-:--,..-...,. "- 100 -- 10~+- ~- ./1, -- "" -100 I I b) v/ v, = 4 I \ -200 Hf-~ I I -300 l:=;L I ·300 20 40 a) v/V, . l \ 60 80 100 120 -.~ 20 _,40 _,60 80 100 120 For v 2/V 1 > I, much of the region M, IJlI < 0(30) Ge V is ruled out, and most of the remaining cosmologically interesting range involves large positive M and Jl values. In fact 15), a large parameter range exists for which a cosmologically relevant LSP can be heavier than the Z and is nearly pure bino or Higgsino. 357 This brief foray into the SUSY phase space implies that the Z data suggests that the LSP is probably heavier than 0(20) GeV, and is not likely to be a light pure photino, but rather a mix of neutralinos, whose couplings with ordinary matter mi~ht have significant scalar contributions. A recent detailed analysis of the SUSY phase space 1 ) confirms these estimates, and finds that unless the Higgs mass is light, M LSP > 15-20 GeV, if it is to provide significant contribution to n. If one then incorporates the new LEP limits on a light Higgs (m>0(30) Gev) in this analysis and uses limits from CDF on other SUSY particles, we then obtain a lower bound on a cosmologically significant neutralino of 15-20 GeV, making it, in general, substantially heavier than initial estimates suggested. These are important results because: (a) a search of the available model space then suggests that if such a WIMP is to have a sufficient remnant abundance to make up the dark matter, it is likely to be as heavy or heavier than the W and Z particles; (b) this mass range is in the opposite direction of previous efforts to extend the range in which direct detectors were sensitive, and moreover, the properties of such a WIMP are such as to make it much more difficult to detect directly in underground detectors (see below). Direct searches, which I believe are of vital importance, are now more challenging in light of LEP. (d) Exoticalcosmions: Other than these WIMP candidates from particle physics, exotic objects have been proposed for purely astrophysical reasons. One of these is a "cosmion", a fixed abundance of which inside the sun might, for a mass of 4-10 GeV, lower the core temperature and reduce the solar neutrino flux. 17) This caused interest when it was suggested 18) that WIMPs might in principle be captured in the Sun over cosmological time to produce the required abundance, thus potentially solving both the dark matter and solar neutrino problems. However none of the standard WIMPs fit the severe requirements l9 ) which may make this solution appear contrived. Scattering cross sections 20-100 times larger than weak are required, as is the absence of annihilation inside the sun. However, in times of need ugliness is no obstacle, and theorists with time on their hands have produced exotic cosmion models. I describe here how the Z decay data can rule out many explicit models which previously appeared viable: (a) Magnino, and Neutrino-Higgs: Rabyand West proposed several cosmion models 20) . Each of these involved a Dirac neutrino with mass 5-10 GeV as a cosmion (with an additional interaction mediated either by a large magnetic moment, or by light Higgs exchange). The Z limit on extra massive Dirac neutrinos (and on a light Higgs) rules out both these possibilities. (b) SUSY cosmions 21 ) : This model requires SUSY, and also a 1-2 Higgs. The only viable model proposed involved v 2tv 1 ==1, M=80-105, and J.l.=130-150 and a lightest chargino mass ==3040 GeV. These are ruled out by the Z decay data as discussed above, as is a light Higgs particle. (c) E6 cosmions 22): This model proposed new v's in a 27 of E6 with no Z couplings, but couplings to a new Z' particle. While such v's would not be produced in Z decay, there also exists one new doublet v per family. One might naively expect their mass to be about equal to that of the 5-10 Ge V singlet cosmion. In this case this model would be ruled out by the Z width limit. Also, Z limits on light Z' particles are appearing which might also rule out such models. (d) Colored Scalars: These models23) are the least explicit, and hence difficult to rule out. But, all involve new colored scalars and heavy fermions. If they have standard weak quantum numbers, the Z data would then rule them out. These results do not imply that all "cosmion" models, present or future are ruled out. They just make such a particle even more implausible. Moreover, a more model independent limit on cosmions from direct detection experiments 24), of m< 4Ge V, now limits the available parameter space to a negligible region. (e) Generic WIMPs: One can leave the realm of specific candidates, and examine general LEP constraints on arbitrary massive particles whose abundance in the universe today is determined 358 by their annihilations (via Z exchange) in the early universe 16,25). If one defmes 16) a factor sin <Pz which gives the suppression in the relative Z coupling compared to neutrinos, one finds that only WIMPs with sin <Pz <0.3, and M>10 GeV for Majorana fermions and sin <Pz <.003 and M> 6 GeV for Dirac fermions remain viable. If one combines the constraints imposed by Ge detection experiments. these limits are raised. and with small improvements. one might. set "generic" fermion mass limits of 0(20) GeV for WIMPs which annihilate through the Z. 3.The Z Width And Baryonic Dark Matter It is by now very well known that the number of light neutrinos strongly impacts upon cosmology via primordial big bang nucleosynthesis. Traditionally. cosmologists have used the quantitative agreement between the predicted and the observationally inferred primordial abundances of light elements to limit the number of light neutrinos 26) • Now that this number has been well established experimentally via the Z width. one can hope to use it to further constrain other aspects of big bang cosmology. In particular. with the number of neutrinos known (assuming no other light states which can affect the expansion rate during helium production. but which do not couple to the Z). the uncertainty in the predicted abundance of 4He is the smallest ·of all the light elements. Below I display the BBN predictions for the fraction by mass of 4He (Y p)' for 3.4. and 5 neutrinos as a function of the baryon to photon ratio 11 Zl). including the 20 uncertainty due to our uncertainty in the BBN reaction rates (dominated by the uncertainty in the neutron half life). Also shownfor 3 neutrinos is the reduced 20 uncertainty with the most recent neutron half life measurement included. 0.28 0.26 0.24 c:>. >- 0.22 Ny =3 0.20 Ny =4 N y =5 't112 [neutron) Mampe et aL 0.18 0.16 0.5 1.0 1.5 TI It is clear from the figure that if we can pin down the actual primordial abundance of 4He we could place strong limits from He alone on the density of baryons in the universe today. The fraction of closure density in baryons (QB) is determined from 11 as fiB = .OO36ho ·2 (T/2.74 K)3 (lOlOtt) (10) where T is the present microwave background temperature. and ho is limited by measurements of 359 the Hubble constant today to be between .4 and 1. For example, if we were to limit the primordial abundance to be between 23.5 and 24% for example, then, assuming at most 3 light neutrino equivalents.11 would be constrained to be between 2.5-4.8 x 10 -10. This in turn would limit 0B to be between 0.01 and 0.1. The lower bound (obtained for the extreme value ho =1. which leads to conflicts with limits on the age of the universe) is already suggestive that some dark matter must be baryonic. The upper limit is only marginally in agreement with .the possibility that all dark matter inferred by virial estimates is baryonic. While such a narrow range for the primordial He abundance cannot be inferred from the present data 28), the Z width data. in combination with more careful analyses of big bang nucleosynthesis are bringing us closer definitively limiting the amount of baryonic dark matter in the universe. Indeed. it has been argued 28) that a reasonable upper limit on primordial 4He is 24%. and a "best fit" value is 23%. In the former case, 4He already gives the best upper limit on 11. and in the latter case. the limit is so strong as to make BBN with 3 neutrinos inconsistent! Further developments are eagerly awaited. 4. Implications For Direct Wimp Detection The Z width data effectively rules out all known WIMP candidates which interact coherently with total nuclear charge in low energy scattering processes. In addition, it suggests that the LSP, if it exists. is probably heavier than 10-20 GeV, and is not likely to be a pure photino. These have important implications for direct detection schemes. because they suggest that a viable WIMP is likely to be orthogonal to present detection schemes. Much of the thrust of ongoing WIMP detection experiments has been to develop sensitivity to probe for coherently interacting particles with mass less than 5-10 Ge V. For Dirac neutrinos and sneutrinos. the LEP results now remove the need. Existing detectors have already ruled out this range for cosmions. so that motivation for continuing to probe the light mass range is also reduced. None of the present detectors is sensitive to the small rates which would result from the spin-dependent interactions. typical of neutralinos. While in the most optimistic case. the detection rates can increase linearly with neutralino mass (for fixed squark masses), this is only true for a target material heavier than the to maximize the recoil energy also suggests going to heavier nuclear targets. neutralino. T~ing Recent work ) suggests that the rates in nuclear targets is suppressed compared to original estimates for spin dependent scattering. The challenge to direct detection has increased! It may be that experimenters will have to wait first for a SUSY signal at LEP, the Tevatron, or the sse, in order to optimize their detectors for maximize sensitivity to the resulting WIMP dark matter. 5. Other Early Universe Implications of LEP: LEP has limited the standard electroweak model in two other ways which can impact upon cosmology. First and foremost, sensitive new limits on the parameters of the standard model, limiting isospin breaking and isospin conserving contributions from new heavy particles. severely constrain various possibilities for exotica beyond the standard model. The top quark is now expected to be in a mass range around 140 GeV. This has implications for any scheme of electroweak symmetry breaking, and in turn limits possible characteristics of the electroweak phase transition in the early universe, as well as possible models which might yield dark exotic dark matter. The limits obtained by combining LEP with other precision electroweak probes also appearto limit the possible existence oflarge numbers of new particles in the TeV range, as one might expect from technicolor models 30). Next, LEP has apparently provided a new lower bound on a Higgs mass of about 30 Ge V. This not only impacts in model building which is relevant for WIMP dark matter, as described above, but also strongly constrains the nature of the electroweak 360 phase transition. For example, it rules out very low Higgs masses, which are associated with the possibility that our present vacuum is not stable, but merely metastable on cosmological timescaIes. Limits on the Higgs, and associated light particles may aIso be important for constraining various mechanisms for electro weak. baryogenesis scenarios. 6. Conclusion: It is remarkable to have data which conclusively limits an area in which there has been so much speculation over the last decade. The first results from physics at the Z have provided powerful new limits, not just on the number of light families, but on the nature of dark matter, and on possible new processes in the early universe. We can hope that the coming years may nOl only yield further constraints, but that a clear signaI to guide us might emerge. 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