Search

The mirror twin Higgs model (MTH) is a solution to the Higgs hierarchy problem that provides well-predicted cosmological signatures with only three extra parameters: the temperature of the twin sector, the abundance of twin baryons, and the vacuum expectation value (VEV) of twin electroweak symmetry breaking. These parameters specify the behavior of twin radiation and the acoustic oscillations of twin baryons, which lead to testable effects on the cosmic microwave background (CMB) and large-scale structure (LSS). While collider searches can only probe the twin VEV, through a fit to cosmological data we show that the existing CMB (Planck18 TTTEEE+lowE+lowT+lensing) and LSS (KV450) data already provide useful constraints on the remaining MTH parameters. Additionally, we show that the presence of twin radiation in this model can raise the Hubble constant $H_0$ while the scattering twin baryons can reduce the matter fluctuations $S_8$, which helps to relax the observed $H_0$ and $S_8$ tensions simultaneously. This scenario is different from the typical $\Lambda$CDM + $\Delta N_{\rm eff}$ model, in which extra radiation helps with the Hubble tension but worsens the $S_8$ tension. For instance, when including the SH0ES and Planck SZ data in the fit, we find that a universe with $\gtrsim 20\%$ of the dark matter comprised of twin baryons is preferred over $\Lambda$CDM by $\sim4\sigma$. If the twin sector is indeed responsible for resolving the $H_0$ and $S_8$ tensions, future measurements from the Euclid satellite and CMB Stage 4 experiment will further measure the twin parameters to $O(1-10\%)$-level precision. Our study demonstrates how models with hidden naturalness can potentially be probed using precision cosmological data., Comment: 45 pages, 18 figures. Published version: Fixed a typo in the code, expanded the discussion of CMB phase shift in the MTH model, conclusions are unchanged. Code available at https://github.com/srbPhy/class_twin

We find new limits on the $\lambda'_{3jk}$ R-parity violating (RPV) couplings of the minimal supersymmetric standard model, using Drell-Yan differential cross sections at the LHC and electroweak precision measurements from LEP and SLC. Specifically, limits on six out of the nine $\lambda'_{3jk}$-couplings, with $j=1\text{ or }2$, are obtained using Drell-Yan data, with the remaining three (for $j=3$) bounded by precision electroweak data. We also update the limits on all $\lambda'_{ijk}$-couplings using electroweak data and find new bounds on $\lambda'_{132}$ and $\lambda'_{232}$ that are stronger than obtained elsewhere. A table of all current bounds on $\lambda'_{ijk}$ is given in an appendix., Comment: 9 pages, 3 figures, 3 tables

We find constraints on R-Parity Violating (RPV) couplings of the minimal supersymmetric standard model, using Drell-Yan differential cross sections at the LHC. Specifically, we look at the constraints on $\lambda'LQD^c$ couplings from monolepton and dilepton data published by ATLAS, with either electrons or muons in the final state. Out of the 18 RPV couplings to which the LHC is at least potentially sensitive by this technique, we find new limits on 12 (or 13) of them, for squarks masses above 1 (or 2) TeV. We also show that one can employ our techniques to achieve significantly stronger bounds at a high-luminosity upgrade of the LHC., Comment: 5 pages, 1 figure, 2 tables

There has been persistent disagreement between the Standard Model (SM) prediction and experimental measurements of $R_{D^{(*)}}=\mathcal{B}(\bar B \rightarrow D^{(*)} \tau \bar\nu_\tau)/\mathcal{B}(\bar B \rightarrow D^{(*)} l \bar\nu_l)$ $(l=e,\mu)$. This anomaly may be addressed by introducing interactions beyond the Standard Model involving new states, such as leptoquarks. Since the processes involved are quark flavor changing, any new states would need to couple to at least two different generations of quarks, requiring a non-trivial flavor structure in the quark sector while avoiding stringent constraints from flavor-changing neutral current processes. In this work, we look at scalar leptoquarks as a possible solution for the $R_{D^{(*)}}$ anomaly under the assumption of $\it{minimal~flavor~violation}$ (MFV). We investigate all possible representations for the leptoquarks under the SM quark flavor symmetry group, consistent with asymptotic freedom. We consider constraints on their parameter space from self-consistency of the MFV scenario, perturbativity, the FCNC decay $b\to s\bar\nu\nu$ and precision electroweak observables. We find that none of the scalar leptoquarks can explain the $R_{D^{(*)}}$ anomaly while simultaneously avoiding all constraints within this scenario. Thus scalar leptoquarks with MFV-generated quark couplings do not work as a solution to the $R_{D^{(*)}}$ anomaly., Comment: 14 pages, 4 figures

We show that stringent limits on leptoquarks that couple to first-generation quarks and left-handed electrons or muons can be derived from the spectral shape of the charged-current Drell-Yan process ($p p \to \ell^\pm \nu$) at Run 2 of the LHC. We identify and examine all six leptoquark species that can generate such a monolepton signal, including both scalar and vector leptoquarks, and find cases where the leptoquark exchange interferes constructively, destructively or not at all with the Standard Model signal. When combined with the corresponding leptoquark-mediated neutral-current ($p p \to \ell^+ \ell^-$) process, we find the most stringent limits obtained to date, outperforming bounds from pair production and atomic parity violation. We show that, with 3000 fb$^{-1}$ of data, combined measurements of the transverse mass in $p p \to \ell^\pm \nu$ events and invariant mass in $p p \to \ell^+ \ell^-$ events can probe masses between 8 TeV and 18 TeV, depending on the species of leptoquark, for electroweak-sized couplings. In light of such robust sensitivities, we strongly encourage the LHC experiments to interpret Drell-Yan (dilepton and monolepton) events in terms of leptoquarks, alongside usual scenarios like $Z'$ bosons and contact interactions., Comment: 9 pages, 5 figures

If dark matter is thermally decoupled from the visible sector, the observed relic density can potentially be obtained via freeze-in production of dark matter. Typically in such models it is assumed that the dark matter is connected to the thermal bath through feeble renormalisable interactions. Here, rather, we consider the case in which the hidden and visible sectors are coupled only via non-renormalisable operators. This is arguably a more generic realisation of the dark matter freeze-in scenario, as it does not require the introduction of diminutive renormalisable couplings. We examine general aspects of freeze-in via non-renormalisable operators in a number of toy models and present several motivated implementations in the context of Beyond the Standard Model physics. Specifically, we study models related to the Peccei-Quinn mechanism and Z' portals., Comment: 24 pages. v2: Published version, correction to equilibration constraints. v3. Minor numerical error in eq.2.16 corrected and propagated, conclusions unchanged

If R-parity is only mildly violated then the lightest supersymmetric particle (LSP) can be stable over cosmologically time-scales and still account for the dark matter relic density. We examine the possibility of generating detectable X-ray lines from R-parity violating decays of keV-scale LSP dark matter to neutrino-photon pairs. Specifically, we consider scenarios in which the LSP is a light gravitino, bino, or hidden sector photino. Potential signals are discussed in the context of recent claims of an unidentified 3.5 keV X-ray line in studies of stacked galaxy clusters. We comment on the difficulties in obtaining the observed relic density for keV scale bino or hidden photino dark matter and some possible resolutions., Comment: 8 pages. v2: Matches PRD version

A generalization of the Next-to-Minimal Supersymmetric Model (NMSSM) is studied in which an explicit \mu-term as well as a small supersymmetric mass term for the singlet superfield are incorporated. We study the possibility of raising the Standard Model-like Higgs mass at tree level through its mixing with a light, mostly-singlet, CP-even scalar. We are able to generate Higgs boson masses up to 145 GeV with top squarks below 1.1 TeV and without the need to fine tune parameters in the scalar potential. This model yields light singlet-like scalars and pseudoscalars passing all collider constraints., Comment: 14 pages, 4 figures

We embed the S-MSSM -- a singlet extension of the minimal supersymmetric standard model with an explicit mu-term and supersymmetric mass term -- in a gauge-mediated supersymmetry breaking scheme. We find that by absolving the singlet of its responsibility for solving the mu-problem, we are instead able to solve the little hierarchy problem naturally. Specifically, we find that even with a minimal embedding of the S-MSSM into a gauge-mediated scheme, we can easily raise the lightest Higgs mass above 114 GeV, while keeping top squarks below the TeV scale, maintaining perturbative unification of the gauge couplings, and without tuning the other parameters of the model., Comment: 22 pages, 4 PDF figures

We present a generalization of the Next-to-Minimal Supersymmetric Standard Model (NMSSM), with an explicit mu-term and a supersymmetric mass for the singlet superfield, as a route to alleviating the little hierarchy problem of the Minimal Supersymmetric Standard Model (MSSM). Though this model does not address the mu-problem of the MSSM, we are able to generate masses for the lightest neutral Higgs boson up to 140 GeV with top squarks below the TeV scale, all couplings perturbative to the gauge unification scale, and with no need to fine tune parameters in the scalar potential. This model, which we call the S-MSSM, more closely resembles the MSSM phenomenologically than the NMSSM as usually defined., Comment: 8 pages, 2 figures

We examine the current lower bound on the charged Higgs boson mass in the Minimal Supersymmetric Standard Model. By applying direct search constraints on the neutral Higgs bosons and other supersymmetric states, as well as a number of indirect constraints (b -> s gamma, B_s -> mu mu, B -> tau nu, B -> D tau nu), we find that it is possible to push the charged Higgs boson mass as low as 140 GeV. We work in a completely low-energy approach with no assumptions about scalar mass unification, examining two of the most popular limits for neutral Higgs boson searches: the Max-Mixing and No-Mixing scenarios. While both scenarios allow light charged Higgs bosons, they do so for completely different ranges of tan(beta). In either case, one expects light top squarks to accompany a light charged Higgs., Comment: 19 pages, 5 figures. High-resolution figures available on request

The question of whether neutrino masses are Dirac or Majorana is one of the most important, and most difficult, questions remaining in the neutrino sector. Searches for neutrinoless double beta-decay may help to resolve this question, but are also sensitive to new, higher dimension Delta L=2 operators. In this paper we place two phenomenological constraints on these operators at dimension d<=11. First, we require that the operators obey the quark flavor symmetries of the Standard Model, with any violation of the symmetries being due to Yukawa interactions, a scheme known as Minimal Flavor Violation (MFV). Second, we require that the operators which generate neutrinoless double beta-decay, and any operators related by the flavor symmetries, do not induce neutrino masses above 0.05 eV, the limit implied by the atmospheric neutrino data. We find that these requirements severely constrain the operators which can violate lepton number, such that most can no longer contribute to neutrinoless double beta-decay at observable rates. It is noteworthy that quark flavor symmetries can play such a strong role in constraining new leptonic physics, even when that physics is not quark flavor changing. Those few operators that can mimic a Majorana neutrino mass then appear with cutoffs below a TeV, and represent new physics which could be directly probed at the LHC or a future linear collider., Comment: 18 pages, 8 figures

The search for supersymmetry (SUSY) and other classes of new physics will be tackled on two fronts, with high energy, direct detection machines, and in high precision experiments searching for indirect signatures. While each of these methods has its own strengths, even more can be gained by finding ways to combine their results. In this paper, we examine one way of bridging these two types of experiments by calculating sum rules which link physical squark masses to the flavor-violating squark mixings. These sum rules are calculated for minimally flavor-violating SUSY theories at both high and low tan(beta). We also explore how the sum rules could help to disentangle the relative strengths of different SUSY contributions to b -> s gamma, a favored channel for indirect searches of new physics. Along the way, we show that the gluino contributions to b -> s gamma can be very sizable at large tan(beta)., Comment: 27 pages, 3 figures

Supersymmetric extensions of the Standard Model generally give a theoretical upper limit on the lightest Higgs boson mass which may be uncomfortably close to the current experimental lower bound of $m_h$ > 114 GeV. Here we show ways in which this upper limit on $m_h$ can be relaxed substantially in the perturbative regime, while maintaining the successful unification of gauge couplings as observed in the minimal scenario. A class of models is presented, which includes new vector-like matter with the quantum numbers of the 5, 5-bar and singlet representations of SU(5) and having Yukawa couplings (\kappa) to the usual Higgs doublet $H_u$. This new matter transforms non-trivially under a "lateral" SU(N) gauge symmetry, which enables the new Yukawa couplings \kappa to be sizable, even larger than the top-quark Yukawa coupling, and still be perturbative at all momenta up to the unification scale of 10^{16} GeV. We find that, consistent with low energy constraints, $m_h$ can be raised to values as large as 200 - 300 GeV., Comment: 15 pages LaTeX, 4 EPS figures. References added

I review briefly the notion of minimal flavor violation and its application to supersymmetry, with a special emphasis on a class of operators, so-called Higgs penguins, which can generate new and interesting flavor signals at large tan(beta)., Comment: Talk presented at the 12th International Conference on Supersymmetry and Unification of Fundamental Interactions (SUSY '04), June 2004, Tsukuba, Japan. To be published in the proceedings

In extra-dimensional models which are compactified on an n-torus (n>1) there exist moduli associated with the torus volume (which sets the fundamental Planck scale), the ratios of the torus radii, and the angle(s) of periodicity. We consider a model with gravity in the bulk of n=2 large extra dimensions with a fixed volume, taking all Standard Model fields to be confined to a "thick" and supersymmetric 3-brane. The Casimir energy of fields in the bulk of the 2-torus accounts for the present dark energy density while the shape moduli begin rolling at late times (z ~ 1) and induce a shift in the Kaluza-Klein masses of the Standard Model fields. The low energy value of the fine-structure constant is sensitive at loop level to this shift. For reasonable cosmological initial conditions on the shape moduli we obtain a redshift dependence of the fine-structure constant similar to that reported by Webb et al., which is roughly compatible with Oklo and meteorite bounds. Constraints from coincident variation in the QCD scale are also briefly discussed., Comment: Latex, 13 pages, 3 figures

We re-examine the bounds on supersymmetric particle masses in light of the new E821 data on the muon anomalous magnetic moment and the revised theoretical calculations of its hadronic contributions. The current experimental excess is either 1.5 sigma or 3.2 sigma depending on whether e+e- or tau-decay data are used in the theoretical calculations for the leading order hadronic processes. Neither result is compelling evidence for new physics. However, if one interprets the excess as coming from supersymmetry, one can obtain upper mass bounds on many of the particles of the minimal supersymmetric standard model (MSSM). Within this framework we provide a general analysis of the lightest masses as a function of the deviation so that future changes in either experimental data or theoretical calculations can easily be translated into upper bounds at the desired level of statistical significance. In addition, we give specific bounds on sparticle masses in light of the latest experimental and theoretical calculations for the MSSM with universal slepton masses, with and without universal gaugino masses., Comment: 21 pages, LaTeX, 15 figures. This paper is a major revision of hep-ph/0108122 with theoretical sections left mostly unchanged but with completely new discussions in light of the most recent E821 data and with all new abstract/conclusions. This version is updated to match published version

Recent observations of neutrino oscillations imply non-zero neutrino masses and flavor violation in the lepton sector, most economically explained by the seesaw mechanism. Within the context of supersymmetry, lepton flavor violation (LFV) among the neutrinos can be communicated by renormalization group flow to the sleptons and from there to the charged leptons. We show that LFV can appear in the couplings of the neutral Higgs bosons, an effect that is strongly enhanced at large tan(beta). In particular, we calculate the branching fraction for tau -> 3 mu and mu -> 3 e mediated by Higgs and find that they can be as large as 10^{-7} and 5x10^{-14} respectively. These modes, along with B^0 -> mu mu, can provide important evidence for supersymmetry before direct discovery of supersymmetric partners occurs. Along with tau -> mu gamma and mu -> e gamma, they can also provide key insights into the form of the neutrino Yukawa mass matrix., Comment: 9 pages LaTeX, 2 figures. Added a discussion of mu -> 3e and its ramifications for probing neutrino mass matrix. Also added references, fixed typos, and made one notational change

Supersymmetric models often predict a lightest superpartner (LSP) which is electrically charged and stable on the timescales of collider experiments. If such a particle were to be observed experimentally, is it possible to determine whether or not it is stable on cosmological timescales? Charged, stable particles are usually considered to be excluded by cosmological arguments coupled with terrestrial searches for anomalously heavy water molecules. But when the cosmology is significantly altered, as can happen in models with large extra dimensions, these arguments are in turn significantly weakened. In this paper we suggest an alternate way to use searches for superheavy water to constrain the lifetimes of long-lived, charged particles, independent of most cosmological assumptions. By considering SUSY production by cosmic rays in the upper atmosphere, we are able to use current bounds on superheavy water to constrain the mass scale of squarks and gluinos to be greater than about 230 GeV, assuming a stable, charged LSP. This bound can be increased, but only by significantly increasing the size of the initial water sample tested., Comment: 9 pages, 2 figures

We re-examine the bounds on supersymmetric particle masses in light of the E821 data on the muon anomalous magnetic moment. We confirm, extend and supersede previous bounds. In particular we find (at one sigma) no lower limit on tan(beta) or upper limit on the chargino mass implied by the data at present, but at least 4 sparticles must be lighter than 700 to 820 GeV and at least one sparticle must be lighter than 345 to 440 GeV. However, the E821 central value bounds tan(beta) > 4.7 and the lighter chargino mass by 690 GeV. For tan(beta) < 10, the data indicates a high probability for direct discovery of SUSY at Run II or III of the Tevatron., Comment: 20 pages LaTeX, 14 figures; references added

Recent observations indicate that the universe's expansion has been accelerating of late. But recent theoretical work has highlighted the difficulty of squaring acceleration with the underlying assumptions of string theory, disfavoring most models of quintessence. because they predict eternal acceleration. We show that one of the simplest and most motivated quintessence models described by an exponential potential can produce the acceleration needed to explain the data while also predicting only a finite period of acceleration, consistent with theoretical paradigms. This model is no more tuned than the canonical tracking quintessence models., Comment: 8 pages Latex; 3 figures

Cosmological data suggest that we live in an interesting period in the history of the universe when \rho_\Lambda \sim \rho_M \sim \rho_R. The occurence of any epoch with such a "triple coincidence" is puzzling, while the question of why we happen to live during this special epoch is the "Why now?" problem. We introduce a framework which makes the triple coincidence inevitable; furthermore, the ``Why now?'' problem is transformed and greatly ameliorated. The framework assumes that the only relevant mass scales are the electroweak scale, M_{EW}, and the Planck scale, M_{Pl}, and requires \rho_\Lambda^{1/4} \sim M_{EW}^2/M_{Pl} parametrically. Assuming that the true vacuum energy vanishes, we present a simple model where a false vacuum energy yields a cosmological constant of this form., Comment: 5 pages, 1 figure, uses psfig. Refs added, slightly enhanced

At moderate to large tan(beta), it is no longer possible to simultaneously diagonalize the masses of quarks and their couplings to the neutral Higgs bosons. The resulting flavor violations of the form (\bar b_R d_L H) and (\bar b_R s_L H) do not generate large meson-anti-meson mixing amplitudes but do generate large contributes to rare decays such as B_s -> mu mu. Run II of the Tevatron willl probe a large region of interesting MSSM parameter space through this decay channel. This talk is based on results obtained with K.S. Babu and presented in hep-ph/9909476., Comment: 4 Pages LateX. Talk given at PASCOS '99

We study the theoretical correlation between the Higgs mass of the minimal standard model and the scale at which new physics is expected to occur. In addition to the classic constraints of unitarity, triviality and vacuum stability, we reexamine the constraints imposed by the precision electroweak data. We then pay particular attention to the constraint imposed by the absence of fine-tuning in the Higgs mass parameter (the Veltman condition). We find that the fine-tuning condition places a significant constraint on the new physics scale for the Higgs mass range 100 GeV < m_h < 200 GeV mostly unconstrained by the classic constraints., Comment: 19 pages Latex, 2 figures

In this letter we demonstrate a new source for large flavor-changing neutral currents within the minimal supersymmetric standard model. At moderate to large tan(beta), it is no longer possible to diagonalize the masses of the quarks in the same basis as their Yukawa couplings. This generates large flavor-violating couplings of the form $\bar b_R d_L H$ and $\bar b_R s_L H$ where H is any of the three neutral, physical Higgs bosons. These new couplings lead to rare processes in the B system such as $B^0 -> \mu^+ \mu^-$ decay and B-Bbar mixing. We show that the latter is anomalously suppressed, while the former is in the experimentally interesting range. Current limits on $B^0 -> \mu^+ \mu^-$ already provide nontrivial constraints on models of moderate to large tan(beta), with an observable signal possible at Run II of the Tevatron if m_A < 400-600 GeV, extending to the TeV range if a proposed Run III of 30/fb were to occur., Comment: 9 pages LaTeX, 1 figure. Small numerical improvement and comment on b->X_s l+l- added

We examine the cosmology of the two recently proposed scenarios for a five dimensional universe with localized gravity. We find that the scenario with a non-compact fifth dimension is potentially viable, while the scenario which might solve the hierarchy problem predicts a contracting universe, leading to a variety of cosmological problems., Comment: 10 pages, LaTeX. Reference added

If spacetime contains large compact extra dimensions, the fundamental mass scale of nature, $Lambda$, may be close to the weak scale, allowing gravitational physics to significantly modify electroweak symmetry breaking. Operators of the form $(1/Lambda^2) |phi^* D phi|^2$ and $(1/Lambda^2) phi^* W B phi$, where $W$ and $B$ are the SU(2) and U(1) field strengths and $phi$ is the Higgs field, remove the precision electroweak bound on the Higgs boson mass for values of $Lambda$ in a wide range: $4 TeV < Lambda < 11 TeV$. Within this framework, there is no preference between a light Higgs boson, a heavy Higgs boson, or a non-linearly realized SU(2)xU(1) symmetry beneath $Lambda$. If there is a Higgs doublet, then operators of the form $(1/Lambda^2) phi^* phi (G^2, F^2)$, where $G$ and $F$ are the QCD and electromagnetic field strengths, modify the production of the Higgs boson by gluon-gluon fusion, and the decay of the Higgs boson to 2 photons, respectively. At Run II of the Tevatron collider, a 2-photon signal for extra dimensions will be discovered if $Lambda$ is below 2.5 (1) TeV for a Higgs boson of mass 100 (300) GeV. Furthermore, such a signal would point to gravitational physics, rather than to new conventional gauge theories at $Lambda$. The discovery potential of the LHC depends sensitively on whether the gravitational amplitudes interfere constructively or destructively with the standard model amplitudes, and ranges from $Lambda$ = 3 - 10 (2 - 4) TeV for a light (heavy) Higgs boson., Comment: 14 pages LaTeX, 3 figures

Models of supersymmetric D-term inflation require a new mass scale near 10^{15-16} GeV in order to match the density perturbation spectrum observed by COBE. Attempts to obtain such a scale from the anomalous U(1) of string theories fail in most string models. However there is hope that models based on non-standard embeddings in M-theory can solve the discrepancy. We will show that such models still suffer from the other drawback of D-inflationary models, namely that Planckian field values are required to drive inflation. Thus it is hard to understand why the inflaton potential remains so flat without imposing stringent symmetries on the superpotential. We also examine a fascinating quasi-fixed point behavior for the gauge coupling of the anomalous U(1) in these extra-dimensional models, and show that either the presence of large numbers of fields in the 5-dimensional bulk or strongly suppressed U(1) gauge couplings is required in order to restore naturality to the inflationary potential., Comment: 9 pages LaTeX

An alternative to a cosmological constant is quintessence, defined as a slowly-varying scalar field potential V(\phi). If quintessence is observationally significant, an epoch of inflation is beginning at the present epoch, with \phi the slowly-rolling inflaton field. In contrast with ordinary inflation, quintessence seems to require extreme fine tuning of the potential V(\phi). The degree of fine-tuning is quantified in various cases., Comment: 7 pages LaTeX. Revised to be more pedagogical; results unchanged

We show that the branching ratio R_{b/tau}=BR(h^0->bb)/BR(h^0->tau tau) of the Higgs boson h^0 may usefully differentiate between the Higgs sectors of the Minimal Supersymmetric Standard Model (MSSM) and non-supersymmetric models such as the Standard Model or its two Higgs doublet extensions. Although at tree level R_{b/tau} is the same in all these models, only in the MSSM can it receive a large radiative correction, for moderate to large values of the parameter tan(beta). Such large corrections are motivated in supersymmetric unified schemes wherein the Yukawa couplings of the b-quark and the tau-lepton are equal at the unification scale; otherwise the b-quark mass prediction is too large by 15-30% for most of parameter space. Thus accurate measurements of the Higgs branching ratios can probe physics at the unification scale. The branching ratio of h^0 into charm quarks, as well as of the other Higgs bosons (H^0,A^0) into bb, tau tau, and cc can provide additional information about the supersymmetric nature of the Higgs sector., Comment: 13 pages LaTeX, 4 figures

The supersymmetric flavor problem may be solved if the first and second generation scalars are heavy (with multi-TeV masses) and scalars with large Higgs couplings are light (with sub-TeV masses). We show that such an inverted spectrum may be generated radiatively; that is, from initial conditions where all scalar masses are multi-TeV at some high scale, those with large Higgs couplings may be driven asymptotically to the weak scale in the infra-red. The lightness of third generation scalars is therefore a direct consequence of the heaviness of third generation fermions, and fine-tuning is avoided even though the fundamental scale of the soft supersymmetry breaking parameters is multi-TeV. We investigate this possibility in the framework of the usual Yukawa quasi-fixed point solutions. The required high scale boundary conditions are found to be simple and highly predictive. This scenario also alleviates the supersymmetric CP and Polonyi problems., Comment: 18 pages, 3 figures; typos corrected, refs added, conforms to published version

It is demonstrated how the hierarchy between the gauge coupling unification scale of minimal supersymmetry and the Planck (or string) scale, which resembles in order of magnitude a loop factor, can actually be explained as such in supergravity-coupled supersymmetry. A gauge and global singlet field acquires a linear potential term due to its one-loop supergravity interactions and slides to the desired scale. The singlet field can then provide the seed for the breaking of the unified theory at the appropriate scale via its couplings to fields in the adjoint representation., Comment: 6 pages, LaTeX

Supersymmetric extensions of the standard model generically contain additional sources of CP violation. We discuss how at one loop a potentially large CP violating coupling of the lightest Higgs, h^0, to leptons is induced in the minimal supersymmetric standard model (MSSM). The CP violating couplings of h^0 in extensions of the MSSM, such as the next-to-minimal supersymmetric standard model (NMSSM) are also considered. We indicate how this CP violation might be observed; in particular a polarization-dependent production asymmetry, in the context of a muon collider, provides a means to access this coupling cleanly. In the MSSM, existing limits on the electric dipole moment (EDM) of the electron, coupled with standard universality assumptions, severly constrains any such signal. Nevertheless, extensions of the MSSM, such as the NMSSM, allow CP-violating signals as large as 100%., Comment: 13 pages LaTeX, 4 figures (some typos fixed, discussion clarified)

Within the context of supergravity-coupled supersymmetry, fields which are gauge and global singlets are usually considered anathema. Their vacuum expectation values are shifted by quadratically divergent tadpole diagrams which are cutoff at the Planck scale, destabilizing the classical potential and driving the singlet field to large values. We demonstrate a new and generic mechanism which stabilizes the singlet in the presence of an extended gauge symmetry. Such a symmetry will be broken down to the Standard Model by the supergravity interactions near the scale of spontaneous supersymmetry-breaking in the hidden-sector (about 10^{10-11} GeV). The resulting singlet expectation value is stabilized and naturally of order the gravitino mass, providing therefore a weak-scale mass for the Higgs fields of the supersymmetric Standard Model (a "mu-parameter"). The resulting low-energy theory is the minimal supersymmetric Standard Model, with all new fields decoupling at the intermediate scale., Comment: 9 pages, LaTeX

The phenomenology of supersymmetric models of inflation, where the inflationary vacuum energy is dominated by D-terms of a U(1), is investigated. Particular attention is paid to the questions of how to arrange for sufficient e-folds of inflation to occur, what kind of thermal history is expected after the end of inflation, and how to implement successful baryogenesis. Such models are argued to require a more restrictive symmetry structure than previously thought. In particular, it is non-trivial that the decays of the fields driving D-inflation can reheat the universe in such a way as to avoid the strong gravitino production constraints. We also show how the initial conditions for Affleck-Dine baryogenesis can arise in these models and that the simplest flat directions along which baryon number is generated can often be ruled out by the constraints coming from decoherence of the condensate in a hot environment. At the end, we find that successful reheating and baryogenesis can take place in a large subset of D-inflationary models., Comment: 23 pages LaTeX

We give a brief overview of 20 open theoretical questions in supersymmetric particle physics. The 20 questions we have chosen range from the GeV scale to the Planck scale, and include issues pertaining to the Minimal Supersymmetric Standard Model and its extensions, SUSY-breaking, cosmology, grand unified theories, and string theory. Throughout, our goal is to address those topics in which supersymmetry plays a fundamental role, and which are areas of active research in the field. This survey is written at an introductory level and is aimed at people who are not necessarily experts in the field. (To appear as an Overview Chapter in the review volume "Perspectives on Supersymmetry", edited by G. Kane, to be published by World Scientific.), Comment: 64 pages, LaTeX, 2 figures

We discuss experimental implications of extending the gauge structure of the Standard Model to include an additional U(1) interaction broken at or near the weak scale. We work with the most general, renormalizable Lagrangian for the SU(2)\times U(1)\times U(1) sector, with emphasis on the phenomenon of gauge kinetic mixing between the two U(1) gauge fields, and do not restrict ourselves to any of the "canonical" Z' models often discussed in the literature. Low-energy processes and Z-pole precision measurements are specifically addressed., Comment: 10 pages LaTeX

Recent progress in the gauge-mediated supersymmetry breaking is reviewed, with emphasis on the theoretical problems which gauge-mediated models so successfully solve, as well as those problems which are endemic to the models themselves and still beguile theorists today., Comment: Talk given at the 5th International Conference on Supersymmetries in Physics (SUSY-97), Philadelphia, PA, May 27-31, 1997. 10 pages LaTeX. Requires espcrc2.sty file

We demonstrate that in the presence of mixing between different scalar leptoquark multiplets it is possible to simultaneously account for the HERA high-$Q^2$ neutral current anomaly, and produce a charged current anomaly of comparable magnitude. The reduced branching ratio to electrons and jets of the lightest leptoquark state results in a significant weakening of the CDF/D0 limits on scalar leptoquarks; masses consistent with the HERA neutral current excess are comfortably within the allowed range. We show that the possibilities for such a successful mixed leptoquark scenario are quite limited, and we investigate some aspects of their phenomenology., Comment: 10 pages Latex. Corrected 3-body decay widths

A recent proposal seeks to explain the anomaly in the HERA high-Q^2 neutral current e^+p data by modifying the parton distribution functions at moderate-to-high-x and large Q^2. We investigate the consequences of this proposal for the HERA high-Q^2 data itself, in the neutral current e^-p channel and especially in the charged current e^\pm p channels. We find that there are striking signatures in the charged current which already serve to rule out many possibilities, including the (symmetric) intrinsic charm hypothesis. For those not ruled out, interesting signals are predicted in e^-p scattering., Comment: 10 pages Latex with 1 EPSF figure

Taking the reported high-Q^2 anomaly at HERA as a signal of new physics, we show that several independent considerations point towards s-channel leptoquark production as the most attractive interpretation. We argue that even this option is highly constrained by flavor-changing processes: the couplings must be accurately diagonal in the quark and lepton mass eigenstate basis and should preserve individual quark and lepton family numbers. We propose a dynamical mechanism that might produce this pattern; it has distinctive experimental consequences., Comment: 10 pages Latex with 1 EPSF figure

The most general Lagrangian for a model with two U(1) gauge symmetries contains a renormalizable operator which mixes their gauge kinetic terms. Such kinetic mixing can be generated at arbitrarily high scales but will not be suppressed by large masses. In models whose supersymmetry (SUSY)-breaking hidden sectors contain U(1) gauge factors, we show that such terms will generically arise and communicate SUSY-breaking to the visible sector through mixing with hypercharge. In the context of the usual supergravity- or gauge-mediated communication scenarios with D-terms of order the fundamental scale of SUSY-breaking, this effect can destabilize the gauge hierarchy. Even in models for which kinetic mixing is suppressed or the D-terms are arranged to be small, this effect is a potentially large correction to the soft scalar masses and therefore introduces a new measurable low-energy parameter. We calculate the size of kinetic mixing both in field theory and in string theory, and argue that appreciable kinetic mixing is a generic feature of string models. We conclude that the possibility of kinetic mixing effects cannot be ignored in model-building and in phenomenological studies of the low-energy SUSY spectra., Comment: 16 pages, LaTeX, 1 figure. Revised to match published version

We consider the low-energy signatures of, and high-energy motivations for, scenarios of semi-perturbative gauge coupling unification. Such scenarios can leave striking imprints on the low-energy sparticle spectrum, including novel gaugino mass ratios (including $M_2/M_1\approx1$), substantial compression of the intra-generational squark-to-slepton mass ratios, and an overall lifting of scalar masses relative to the gauginos. We also demonstrate that the unification scale can be raised to $M_X \approx 4\times10^{17}$ GeV while still in the perturbative regime -- close to the 1-loop heterotic string scale. We employ a 3-loop calculation of the running of the gauge couplings as a test of the perturbativity of the high-scale theory., Comment: 21 pages in LaTeX with 4 embedded figures