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FIG. 5. Interplay of gaugino constraints from LEP 1 (dashed line) and LEP 1.5 (thick solid line χ+χ−, thin solid line χχ′). Notice their complementarity in the μ<0 quadrant. LEP searches for χ+χ− and χχ′ production in Z0 decay and at higher energies together provide an interesting lower bound on mχ. As seen in Fig. 5, neither LEP 1 nor LEP 1.5 (runs at energies between 130 and 140 GeV) individually provided a bound on mχ, but each excluded “wedges” of parameter space allowed by the other, leading to an experimental bound mχ≥ 12.8 GeV modulo loopholes when tan β~1 and mv is large, and when tan Specifically, we have assumed universality for the gaugino and scalar masses (m1/2, m0) at the GUT scale, and required 0.1≤Ωχh2≤0.3, where the lower bound is motivated solely by astrophysical relevance, whereas the upper bound is required to avoid overclosing the Universe. Moreover, physical renormalized parameters such as the Higgs vacuum expectation values and masses are determined by dynamics involving the heavy top quark. The resulting lower bound on mχ as a function of tan β is shown in Fig. 6, where we see that mχ≥ 21.4 GeV on the basis of LEP 1 and LEP 1.5 results (28). More recently, results from higher-energy LEP 2 runs and 160 and 170/172 GeV have been announced, which can be used to strengthen significantly the LEP lower bound on mχ. The latest chargino searches indicate that mχ±≥80 GeV for 
FIG. 6. Phenomenological lower limits on mχ based on LEP 1.5 data, for arbitrary m0, including the AMY result (dotted line), inferred from the Dø gluino search assuming universal gaugino masses (dashed line), assuming scalar-mass universality (dot-dashed line), and applying the cosmological constraint (0.1<Ωχh2<0.3) (solid line). 
FIG. 7. Lower limits on mχ based on data from LEP 1, 1.5, and 2. The dotted line makes no appeal to extra theoretical assumptions. Lines labeled UHM assume universal scalar masses also for Higgs bosons. The branches labeled “cosmo” and “DM” assume Ωχh2<0.3 and >0.1, respectively. The lines labeled C and H are explained in ref. 29. mχ±−mχ≥5 GeV, slepton searches indicate that mē±≥70 GeV, and the latest searches for supersymmetric Higgs bosons indicate that mh≥60 GeV. This mass is linked to the other sparticle masses if one assumes scalar-mass universality, and it plays a particularly important role at low tan β. We find that the previous mχ=0 loopholes are excluded without the need for other theoretical inputs. Overall, as seen in Fig. 7, we find (29) mχ≥40 GeV [4] and no consistent model if μ<0 and tan β≤1.7, or if μ> 0 and tan β≤1.4. These lower limits on tan β come about because Ωχh2≤0.3 is possible only if m1/2≤400 GeV for tan β≤2, whereas the LEP Higgs limits impose increasingly strong lower limits on m1/2. Future LEP searches at center-of-mass energies up to 190 or 200 GeV should be able to explore mχ= ≤95 GeV, m-ℓ±≤85 GeV and mh≤100 GeV. These should reveal evidence for a neutralino weighing ≤50 GeV and explore all astrophysically interesting models with universal scalar masses and tan β≤3. In the longer run, the LHC accelerator at the European Center for Nuclear Research (CERN) (operating at an equivalent redshift z~5×1015!) will be able to explore all the mass range (expression 3) expected for supersymmetric particles. Moreover, the range of parameters in which Ωχh2≤1 can be explored by means of several independent experimental signatures. Therefore, we can expect supersymmetric dark matter to be smoked out at the Large Hadron Collider (LHC) if not previously at LEP (see ref. 30 and the information about the LHC Experiments Committee Workshop on Supersymmetry at http://www.cern.ch/Committees/LHCC/SUSY96.html). The above discussion is in the context of models with universal gaugino masses
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and mv~60 GeV (
and scalar masses mq 
at the GUT scale. The lower limit on the neutralino mass would be affected quantitatively if the gaugino mass universality assumption were relaxed, but not altered qualitatively. On the other hand, if Higgs mass universality is relaxed: 
with δi≠0, the preferred neutralino composition might change from being mainly a gaugino to being higgsino-like. In this case, the relic density can be interestingly large only if mχ≤mw, LEP searches already exclude mχ≤70 GeV in this case, and future LEP searches should determine