Search

The International Linear Collider (ILC) is on the table now as a new global energy-frontier accelerator laboratory taking data in the 2030s. The ILC addresses key questions for our current understanding of particle physics. It is based on a proven accelerator technology. Its experiments will challenge the Standard Model of particle physics and will provide a new window to look beyond it. This document brings the story of the ILC up to date, emphasizing its strong physics motivation, its readiness for construction, and the opportunity it presents to the US and the global particle physics community., Comment: 356 pages, Large pdf file (40 MB) submitted to Snowmass 2021; v2 references to Snowmass contributions added, additional authors; v3 references added, some updates, additional authors

A large, world-wide community of physicists is working to realise an exceptional physics program of energy-frontier, electron-positron collisions with the International Linear Collider (ILC). This program will begin with a central focus on high-precision and model-independent measurements of the Higgs boson couplings. This method of searching for new physics beyond the Standard Model is orthogonal to and complements the LHC physics program. The ILC at 250 GeV will also search for direct new physics in exotic Higgs decays and in pair-production of weakly interacting particles. Polarised electron and positron beams add unique opportunities to the physics reach. The ILC can be upgraded to higher energy, enabling precision studies of the top quark and measurement of the top Yukawa coupling and the Higgs self-coupling. The key accelerator technology, superconducting radio-frequency cavities, has matured. Optimised collider and detector designs, and associated physics analyses, were presented in the ILC Technical Design Report, signed by 2400 scientists. There is a strong interest in Japan to host this international effort. A detailed review of the many aspects of the project is nearing a conclusion in Japan. Now the Japanese government is preparing for a decision on the next phase of international negotiations, that could lead to a project start within a few years. The potential timeline of the ILC project includes an initial phase of about 4 years to obtain international agreements, complete engineering design and prepare construction, and form the requisite international collaboration, followed by a construction phase of 9 years.

We review developments in the theory of multiple, parallel membranes in M-theory. After discussing the inherent difficulties pertaining to a maximally supersymmetric lagrangian formulation with the appropriate field content and symmetries, we discuss how introducing the concept of 3-algebras allows for such a description. Different choices of 3-algebras lead to distinct classes of 2+1 dimensional theories with varying degrees of supersymmetry. We then describe how these are equivalent to a type of conventional superconformal Chern-Simons gauge theories at level k, coupled to bifundamental matter. Analysing the physical properties of these theories leads to the identification of a certain subclass of models with configurations of M2-branes in Z_k orbifolds of M-theory. In addition these models give rise to a whole new sector of the gauge/gravity duality in the form of an AdS_4/CFT_3 correspondence. We also discuss mass deformations, higher derivative corrections as well as the possibility of extracting information about M5-brane physics., Comment: 180 pages, 3 figures, Latex; v2: various typos corrected, clarifications, references and acknowledgements added, title modified, submitted to Physics Reports

We construct the supersymmetric nonlinear sigma model in a fixed AdS_5 background. We use component fields and find that the complex bosons must be the coordinates of a hyper-Kahler manifold that admits a Killing vector satisfying an inhomogeneous tri-holomorphic condition. We propose boundary conditions that map the on-shell bulk hypermultiplets into off-shell chiral multiplets on 3-branes that foliate the bulk. The supersymmetric AdS_5 isometries reduce to superconformal transformations on the brane fields., Comment: Latex, 7 pages. Published version

We use N=1 superspace to construct the supersymmetric matter couplings of vector and hyper multiplets in a five-dimensional anti-de Sitter spacetime background. For hypermultiplets, we find that AdS_5 supersymmetry requires the scalar fields to lie on a hyper-Kahler manifold endowed with a certain type of holomorphic Killing vector., Comment: 8 pages. Published version; revised references

In this work we present 3-algebraic constructions and representations for three-dimensional N = 5 supersymmetric Chern-Simons theories, and show how they relate to theories with additional supersymmetries. The N = 5 structure constants give theories with Sp(2N) \times SO(M) gauge symmetry, as well as more exotic symmetries known from gauged supergravity. We find explicit lifts from N = 6 to 8, and N = 5 to 6 and 8, for appropriate gauge groups., Comment: 23 pages. Published version. References corrected

We derive the general form for a three-dimensional scale-invariant field theory with N=6 supersymmetry, SU(4) R-symmetry and a U(1) global symmetry. The results can be written in terms of a 3-algebra in which the triple product is not antisymmetric. For a specific choice of 3-algebra we obtain the N=6 theories that have been recently proposed as models for M2-branes in an R^8/Z_k orbifold background., Comment: 19 pages. Typos corrected

Recently a three-dimensional field theory was derived that is consistent with all the symmetries expected of the worldvolume action for multiple M2-branes. In this note we examine several physical predictions of this model and show that they are in agreement with expected M2-brane dynamics. In particular, we discuss the quantization of the Chern-Simons coefficient, the vacuum moduli space, a massive deformation leading to fuzzy three-sphere vacua, and a possible large n limit. In this large n limit, the fuzzy funnel solution correctly reproduces the mass of an M5-brane., Comment: 18 pages. Published version

In previous work we proposed a field theory model for multiple M2-branes based on an algebra with a totally antisymmetric triple product. In this paper we gauge a symmetry that arises from the algebra's triple product. We then construct a supersymmetric theory that is consistent with all the symmetries expected of a multiple M2-brane theory: 16 supersymmetries, conformal invariance, and an SO(8) R-symmetry that acts on the eight transverse scalars. The gauge field is not dynamical. The result is a new type of maximally supersymmetric gauge theory in three dimensions., Comment: 15 pages

The supersymmetric analog of the Goldberger-Treiman relation plays a critical role in the low energy effective theory of models in which supersymmetry is spontaneously broken in a hidden sector. The interactions that connect the hidden and visible sectors break a global symmetry, which implies that the low energy theory must be constructed consistently in inverse powers of the messenger scale. The Goldberger-Treiman relation determines the couplings of the Goldstino to the visible sector fields. These couplings are fixed by the soft supersymmetry breaking terms within a power counting scheme that is stable under radiative corrections. We describe the power counting of the low energy effective theory, first for a toy model of extended technicolor and then for the supersymmetric standard model. One implication of this work for supersymmetry phenomenology is the observation that Goldstino loops can destabilize the weak scale if the low energy theory is not constructed consistently. Another is that Goldstino loops induce all visible sector operators not forbidden by symmetries. The magnitudes of these operators are determined by the consistent power counting of the low energy effective theory., Comment: Letex, 7 pages

We investigate the worldvolume theory that describes N coincident M2-branes ending on an M5 brane. We argue that the fields that describe the transverse spacetime coordinates take values in a non-associative algebra. We postulate a set of supersymmetry transformations and find that they close into a novel gauge symmetry. We propose a three-dimensional N=2 supersymmetric action to describe the truncation of the full theory to the scalar and spinor fields, and show how a Basu-Harvey fuzzy funnel arises as the BPS solution to this theory., Comment: Typos corrected, version to appear in PRD

We formulate four-dimensional N=2 supersymmetric nonlinear sigma models in N=1 superspace. We show how to add superpotentials consistent with N=2 supersymmetry. We lift our construction to higher-dimensional spacetime and write five-dimensional nonlinear sigma models in N=1 superspace., Comment: 11 pages. References added

We exhibit the superHiggs effect in heterotic string theory by turning on a background NS-NS field and deforming the BRST operator consistent with superconformal invariance. The NS-NS field spontaneously breaks spacetime supersymmetry. We show how the gravitini and the physical dilatini gain mass by eating the would-be Goldstone fermions., Comment: Six pages

We construct the action and transformation laws for bulk five-dimensional AdS supergravity coupled to one or two brane-localized Goldstone fermions. The resulting bulk-plus-brane system gives a model-independent description of brane-localized supersymmetry breaking in the Randall-Sundrum scenario. We explicitly reduce the action and transformation laws to spontaneously broken four-dimensional supergravity., Comment: 22 pages

We compute the two-derivative low-energy effective action for the radion in the (supersymmetric) Randall-Sundrum scenario with detuned brane tensions. At the classical level, a potential automatically stabilizes the distance between the branes. In the supersymmetric case, supersymmetry can be broken spontaneously by a vacuum expectation value for the fifth component of the graviphoton., Comment: 17 pages. JHEP class. New Appendix. Published version

Bulk supergravity on a manifold with boundary must be supplemented by boundary conditions that preserve local supersymmetry. This "downstairs" picture has certain advantages over the equivalent "upstairs" picture, expressed in terms of orbifolds. In particular, Scherk-Schwarz supersymmetry breaking can be described much more simply in the downstairs picture. Nevertheless, physics on the fundamental domain can always be lifted upstairs, so long as fields are allowed to be discontinuous across the boundary. In this talk we apply these considerations to five-dimensional supergravity in a warped Randall-Sundrum background., Comment: Talk given at International Workshop "Supersymmetries and Quantum Symmetries" - SQS'03 (Dedicated to the 75th anniversary of the birth of Victor Isaakovich Ogievetsky); JINR, Dubna; July 24-29, 2003

In the Randall-Sundrum compactification of AdS$_5$ with detuned brane tensions, supersymmetry can be spontaneously broken by a non-trivial Wilson line for the graviphoton. The supersymmetry breaking vanishes in the tuned limit. This effect is equivalent to supersymmetry breaking by Scherk-Schwarz boundary conditions., Comment: 7 pages, minor changes. Version to be published in Phys. Lett. B

We study gauged five-dimensional supergravity on the interval [0,\pi R]. We find a set of boundary conditions with respect to which the theory is locally supersymmetric. For theories with detuned brane tensions (\Lambda_4<0), we show that these boundary conditions can be used to spontaneously break global supersymmetry. For the original, tuned Randall-Sundrum scenario (\Lambda_4=0), we prove that the locally supersymmetric boundary conditions are also globally supersymmetric. We lift the theory from [0,\pi R] to S^1 and R, with arbitrary twists for the fermions, and cast these results in the language of the Scherk-Schwarz mechanism., Comment: 18 pages, one figure. JHEP format. Latex three times

We present a supersymmetric version of the two-brane Randall-Sundrum scenario, with arbitrary brane tensions T_1 and T_2, subject to the bound |T_{1,2}| \leq \sqrt{-6\Lambda_5}, where \Lambda_5 < 0 is the bulk cosmological constant. Dimensional reduction gives N=1, D=4 supergravity, with cosmological constant \Lambda_4 in the range \half\Lambda_5 \leq \Lambda_4 \leq 0. The case with \Lambda_4 = 0 requires T_1 = -T_2 = \sqrt{-6\Lambda_5}. This work unifies and generalizes previous approaches to the supersymmetric Randall-Sundrum scenario. It also shows that the Randall-Sundrum fine-tuning is not a consequence of supersymmetry., Comment: 19pp; Published version

We study spontaneous supersymmetry breaking induced by brane-localized dynamics in five-dimensional supergravity compactified on S^1/Z_2. We consider a model with gravity in the bulk and matter localized on tensionless branes at the orbifold fixed points. We assume that the brane dynamics give rise to effective brane superpotentials that trigger the supersymmetry breaking. We analyze in detail the super-Higgs effect. We compute the full spectrum and show that the symmetry breaking is spontaneous but nonlocal in the fifth dimension. We demonstrate that the model can be interpreted as a new, non-trivial implementation of a coordinate-dependent Scherk-Schwarz compactification., Comment: 15 pages. v2: improved treatment of brane actions, relation with conventional Scherk-Schwarz mechanism clarified, version to be published in JHEP

In this paper we consider superstring propagation in a nontrivial NS-NS background. We deform the world sheet stress tensor and supercurrent with an infinitesimal B_{\mu\nu} field. We construct the gauge-covariant super-Poincare generators in this background and show that the B_{\mu\nu} field spontaneously breaks spacetime supersymmetry. We find that the gauge-covariant spacetime momenta cease to commute with each other and with the spacetime supercharges. We construct a set of "magnetic" super-Poincare generators that are conserved for constant field strength H_{\mu\nu\lambda}, and show that these generators obey a "magnetic" extension of the ordinary supersymmetry algebra., Comment: 13 pages, Latex. Published version

We reconsider the phenomenon of mass generation via coordinate-dependent compatifications of higher-dimensional theories on orbifolds. For definiteness, we study a generic five-dimensional (5D) theory compactified on S^1/Z_2. We show that the presence of fixed points, where the fields or their derivatives may be discontinuous, permits new realizations of the Scherk-Schwarz mechanism where, for example, the mass terms are localized at the orbifold fixed points. Our technique can be used to describe the explicit breaking of global flavor symmetries and supersymmetries by brane-localized mass terms. It can also be applied to the spontaneous breaking of local symmetries, such as gauge symmetries or supergravities., Comment: 4 pages, 1 figure. Published version

We derive the effective action for the radion supermultiplet in the supersymmetric Randall-Sundrum model with two opposite tension branes., Comment: Latex, 15 pages. Published version

We show that N=1, D=4 Einstein-frame supergravity is inconsistent at one loop because of an anomaly in local supersymmetry transformations. A Jacobian must be added to the Einstein-frame Lagrangian to cancel this anomaly. We show how the Jacobian arises from the super-Weyl field redefinition that takes the superspace Lagrangian to the Einstein frame. We present an explicit example which demonstrates that the Jacobian is necessary for one-loop scattering amplitudes to be frame independent., Comment: 16 pages, One figure, Latex. Published version

We present the supersymmetric version of the minimal Randall-Sundrum model with two opposite tension branes., Comment: Latex, 9 pages. Published version

In supersymmetric models, the well-known tension between naturalness and experimental constraints is relieved if the squarks and sleptons of the first two generations are superheavy, with masses of order 10 TeV, and all other superpartners are light, with masses of order 1 TeV. We show that even if all scalar masses and trilinear A parameters are of order 10 TeV at some high scale, a mass-squared hierarchy of order 400 may be generated dynamically through renormalization group evolution. The required high energy relations are consistent with grand unification, or, alternatively, may be realized in moduli-dominated supersymmetry-breaking scenarios., Comment: 12 pages, 3 figures

We consider the effects of anomalies on the supersymmetry-breaking parameters in supergravity theories. We construct a supersymmetric expression for the anomaly-induced terms in the 1PI effective action; we use this result to compute the complete one-loop formula for the anomaly-induced gaugino mass. The mass receives contributions from the super-Weyl, Kahler, and sigma-model anomalies of the supergravity theory. We point out that the anomaly-mediated gaugino mass can be affected by local counterterms that cancel the super-Weyl-Kahler anomaly. This implies that the gaugino mass cannot be predicted unless the full high-energy theory is known., Comment: Latex, 15 pages. Published version

The partial breaking of supersymmetry in anti-de Sitter (AdS) space can be accomplished using two of four dual representations for the massive OSp(1,4) spin-3/2 multiplet. The representations can be ``unHiggsed,'' which gives rise to a set of dual N=2 supergravities and supersymmetry algebras., Comment: 17 pages, 2 figures, typos corrected, to appear in Phys. Rev. D

We compute the bounds from precision observables on alternative theories of electroweak symmetry breaking. We show that a cut-off as large as 3 TeV can be accomodated by the present data, without unnatural fine tuning., Comment: 4.01 pages, revtex. 2 eps figures

We study supersymmetry breaking by hidden-sector gaugino condensation in N=1 D=4 supergravity models with multiple dilaton-like moduli fields. Our work is motivated by Type I string theory, in which the low-energy effective Lagrangian can have different dilaton-like fields coupling to different sectors of the theory. We construct the effective Lagrangian for gaugino condensation and use it to compute the visible-sector gaugino masses. We find that the gaugino masses can be of order the gravitino mass, in stark contrast to heterotic string models with a single dilaton field., Comment: LaTeX, 17 pages, 2 eps figures

The supersymmetric flavor, CP and Polonyi problems are hints that the fundamental scale of the soft supersymmetry breaking parameters may be above a TeV, in apparent conflict with naturalness. We consider the possibility that multi-TeV scalar masses are generated by Planck- or unification-scale physics, and find the conditions under which the masses of scalars with large Yukawa couplings are driven, radiatively and asymptotically, to the weak scale through renormalization group evolution. Light third generation scalars then satisfy naturalness, while first and second generation scalars remain heavy to satisfy experimental constraints. We find that this mechanism is beautifully realized in the context of grand unified theories. In particular, the existence of right-handed neutrinos plays an important role in allowing remarkably simple scenarios. For example, for SO(10) boundary conditions with the squared masses of Higgs scalars double those of sleptons and squarks, we find that the entire scalar mass scale may be increased to 4 TeV at the unification scale without sacrificing naturalness., Comment: 20 pages, 4 figures

In this talk we use nonlinear realizations to study the spontaneous partial breaking of rigid and local supersymmetry., Comment: Presented at the International Seminar on Supersymmetries and Quantum Symmetries (Dedicated to the Memory of Victor I. Ogievetsky), Dubna, Russia, July, 1997

In this talk we will study the partial breaking of supersymmetry in flat and anti de Sitter space. We will see that partial breaking in flat space can be accomplished using either of two representations for the massive N=1 spin-3/2 multiplet. We will "unHiggs" each representation and find a new N=2 supergravity and a new N=2 supersymmetry algebra. We will also see that partial supersymmetry breaking in AdS space can give rise to a new N=2 supersymmetry algebra, one that is necessarily nonlinearly realized., Comment: Talk delivered at PASCOS 98 (Boston, MA) and SUSY 98 (Oxford, UK)

This talk discusses the prospects for supersymmetry studies at the LHC and NLC. The results are based on those of the Supersymmetry Working Group at the 1996 Snowmass Workshop., Comment: Latex, 13 pages, 13 eps figures. Invited talk presented at SUSY97

We show that the tensor gauge multiplet of N=1 supersymmetry can serve as the Goldstone multiplet for partially broken rigid N=2 supersymmetry. We exploit a remarkable analogy with the Goldstone-Maxwell multiplet of hep-th/9608177 to find its nonlinear transformation law and its invariant Goldstone action. We demonstrate that the tensor multiplet has two dualities. The first transforms it into the chiral Goldstone multiplet; the other leaves it invariant., Comment: 7 pages, Latex. Expanded discussion of duality symmetries

We compute the leading QCD corrections to K-Kbar mixing in the supersymmetric standard model with general soft supersymmetry-breaking parameters. We construct the \Delta S=2 effective Lagrangian for three hierarchies of supersymmetric particle masses, namely, when the gluino mass is comparable to, much greater than, or much less than the masses of the first two generation squarks. We find that the QCD corrections tighten the limits on squark mass splittings by more than a factor of two., Comment: 14 pages, Latex. Version to be published in Phys. Lett. B

In first-quantized string theory, spacetime symmetries are described by inner automorphisms of the underlying conformal field theory. In this paper we use this approach to illustrate the Higgs effect in string theory. We consider string propagation on M^{24,1} \times S^1, where the circle has radius R, and study SU(2) symmetry breaking as R moves away from its critical value. We find a gauge-covariant equation of motion for the broken-symmetry gauge bosons and the would-be Goldstone bosons. We show that the Goldstone bosons can be eliminated by an appropriate gauge transformation. In this unitary gauge, the Goldstone bosons become the longitudinal components of massive gauge bosons., Comment: 12 pages, TeX

We summarize the results obtained by the Supersymmetry Working Group at the 1996 Snowmass Workshop., Comment: Latex2e, 13 pages

We examine gauge and Yukawa coupling unification in models with gauge-mediated supersymmetry breaking. We work consistently to two-loop order, and include all weak, messenger and unification-scale threshold corrections. We find that successful unification requires unification-scale threshold corrections that are in conflict with the minimal SU(5) model, but are consistent with the modified missing doublet SU(5) model for small tan beta, and large tan beta with mu>0., Comment: 9 pages, 2 figures, published version

In this talk we use nonlinear realizations to study the spontaneous partial breaking of rigid and local supersymmetry., Comment: Invited talk delivered at SUSY 96

We study in some detail the spectral phenomenology of models in which supersymmetry is dynamically broken and transmitted to the supersymmetric partners of the quarks, leptons and gauge bosons, and the Higgs bosons themselves, via the usual gauge interactions. We elucidate the parameter space of what we consider to be the minimal model, and explore the regions which give rise to consistent radiative electroweak symmetry breaking. We include the weak-scale threshold corrections, and show how they considerably reduce the scale dependence of the results. We examine the sensitivity of our results to unknown higher-order messenger-sector corrections. We compute the superpartner spectrum across the entire parameter space, and compare it to that of the minimal supergravity-inspired model. We delineate the regions where the lightest neutralino or tau slepton is the next-to-lightest supersymmetric particle, and compute the lifetime and branching ratios of the NLSP. In contrast to the minimal supergravity-inspired model, we find that the lightest neutralino can have a large Higgsino component, of order 50%. Nevertheless, the neutralino branching fraction to the gravitino and the light Higgs boson remains small, < 10^{-4}, so the observation of such a decay would point to a non-minimal Higgs sector., Comment: 22 pages, 16 figures, published version

The partial spontaneous breaking of rigid N=2 supersymmetry implies the existence of a massless N=1 Goldstone multiplet. In this paper we show that the spin-(1/2,1) Maxwell multiplet can play this role. We construct its full nonlinear transformation law and find the invariant Goldstone action. The spin-1 piece of the action turns out to be of Born-Infeld type, and the full superfield action is duality invariant. This leads us to conclude that the Goldstone multiplet can be associated with a D-brane solution of superstring theory for p=3. In addition, we find that N=1 chirality is preserved in the presence of the Goldstone-Maxwell multiplet. This allows us to couple it to N=1 chiral and gauge field multiplets. We find that arbitrary Kahler and superpotentials are consistent with partially broken N=2 supersymmetry., Comment: Latex, 13 pages. Version to appear in Phys. Rev. D

In this paper we compute one-loop corrections to masses and couplings in the minimal supersymmetric standard model. We present explicit formulae for the complete corrections and a set of compact approximations which hold over the unified parameter space associated with radiative electroweak symmetry breaking. We illustrate the importance of the corrections and the accuracy of our approximations by scanning over the parameter space. We calculate the supersymmetric one-loop corrections to the W-boson mass, the effective weak mixing angle, and the quark and lepton masses, and discuss implications for gauge and Yukawa coupling unification. We also compute the one-loop corrections to the entire superpartner and Higgs-boson mass spectrum. We find significant corrections over much of the parameter space, and illustrate that our approximations are good to O(1%) for many of the superparticle masses., Comment: 57 pages, 19 figures, published version; postscript with full resolution figures at http://libnext.slac.stanford.edu/slacpubs/7000/slac-pub-7180.ps.Z

These lectures contain an introduction to the theory and practice of weak-scale supersymmetry. They begin with a discussion of the hierarchy problem and the motivation for weak-scale supersymmetry. They continue by developing the coset approach to superfields. They use superfield techniques to construct the minimal supersymmetric version of the standard model and to discuss soft supersymmetry breaking and its implications. The lectures end with a brief survey of expectations for future collider experiments., Comment: 51 pages, Lectures delivered TASI-95

We study the stability of the gauge hierarchy in hidden-sector supergravity theories. We show that a destabilizing tadpole can appear if a theory has a gauge- and global-symmetry singlet with renormalizable couplings to the visible fields. We also find a quadratically divergent two-loop contribution to the supersymmetric effective potential. This term illustrates the difficulty of using the "LHC mechanism" to control quadratic divergences in theories with Planck-scale vevs., Comment: Invited Talk presented at SUSY-95, Ecole Polytechnique, Palaiseau, France, 15-19 May 1995

Supersymmetry searches are about to enter an important new era. With LEP 200 and the Main Injector, they will, for the first time, begin to probe significant regions of the supersymmetric parameter space. There is a real chance that supersymmetry might indeed be found before the advent of the LHC., Comment: Invited talk given at PbarP95, May 9-13, 1995, Fermilab. This summary is intended for experimentalists

We examine the stability of the mass hierarchy in hidden-sector supergravity theories. We show that a quadratically divergent tadpole can appear at two loops, even in minimal supergravity theories, provided the theory has a gauge- and global-symmetry singlet with renormalizable couplings to the visible fields. This tadpole can destabilize the hierarchy. We also find a quadratically divergent two-loop contribution to the field-dependent vacuum energy. This result casts doubt on the efficacy of the "LHC mechanism" for controlling quadratic divergences. We carry out the two-loop calculation in a manifestly supersymmetric formalism, and explain how to apply the formalism in the presence of supersymmetry breaking to derive radiative corrections to the supersymmetric and soft supersymmetry-breaking operators. Our approach greatly simplifies the calculation and guarantees consistency of our results with the underlying supergravity framework., Comment: Minor changes to references and discussion

We present selected results of our program to determine the masses, gauge couplings, and Yukawa couplings of the minimal supersymmetric model in a full one-loop calculation. We focus on the precise prediction of the strong coupling alpha_s(M_Z) in the context of supersymmetric unification. We discuss the importance of including the finite corrections and demonstrate that the leading-logarithmic approximation can significantly underestimate alpha_s(M_Z) when some superpartner masses are light. We show that if GUT thresholds are ignored, and the superpartner masses are less than about 500 GeV, the prediction for alpha_s(M_Z) is quite large. We impose constraints from nucleon decay experiments and find that minimal SU(5) GUT threshold corrections increase alpha_s(M_Z) over most of the parameter space. We also consider the missing-doublet SU(5) model and find that it predicts preferred values for the strong coupling, even for a very light superpartner spectrum. We briefly discuss predictions for the bottom-quark mass in the small tan beta region., Comment: 12 pages, epsf.tex, 9 figures included. Talk given by D. Pierce at "Beyond the Standard Model IV", Lake Tahoe, California, December 13-18, 1994.