Searcharxiv⌕ Search

arXiv subjects

Stephen P. Martin

Publications and source records attributed to Stephen P. Martin.

At least 73 records · Page 4Linked to original sources
arXiv2009

Non-universal gaugino masses from non-singlet F-terms in non-minimal unified models

In phenomenological studies of low-energy supersymmetry, running gaugino masses are often taken to be equal near the scale of apparent gauge coupling unification. However, many known mechanisms can avoid this universality, even in models with unified gauge interactions. One example is an F-term vacuum expectation value that is a singlet under the Standard Model gauge group but transforms non-trivially in the symmetric product of two adjoint representations of a group that contains the Standard Model gauge group. Here, I compute the ratios of gaugino masses that follow from F-terms in non-singlet representations of SO(10) and E_6 and their sub-groups, extending well-known results for SU(5). The SO(10) results correct some long-standing errors in the literature.

hep-ph↗
arXiv2008

Exploring compressed supersymmetry with same-sign top quarks at the Large Hadron Collider

In compressed supersymmetry, a light top squark naturally mediates efficient neutralino pair annihilation to govern the thermal relic abundance of dark matter. I study the LHC signal of same-sign leptonic top-quark decays from gluino and squark production, which follows from gluino decays to top plus stop followed by the stop decaying to a charm quark and the LSP in these models. Measurements of the numbers of jets with heavy-flavor tags in the same-sign lepton events can be used to confirm the origin of the signal. Summed transverse momentum observables provide an estimate of an effective superpartner mass, which is correlated with the gluino mass. Measurements of invariant mass endpoints from the visible products of gluino decays do not allow direct determination of superpartner masses, but can place constraints on them, including lower bounds on the gluino mass as a function of the top-squark mass.

hep-ph↗
arXiv2008

Three-body decays of selectrons and smuons in low-energy supersymmetry breaking models

In models with low-energy supersymmetry breaking, it is well-known that charged sleptons can be significantly lighter than the lightest neutralino, with the gravitino and lighter stau being the lightest and next-to-lightest supersymmetric particles respectively. We give analytical formulas for the three-body decays of right-handed selectrons and smuons into final states involving a tau, a stau, and an electron or muon, which are relevant in this scenario. We find that the three-body decays dominate over much of the parameter space, but the two-body decays into a lepton and a gravitino can compete if the three-body phase space is small and the supersymmetry-breaking scale (governing the two-body channel) is fairly low. We study this situation quantitatively for typical gauge-mediated supersymmetry breaking model parameters. The three-body decay lengths are possibly macroscopic, leading to new unusual signals. We also analyze the final-state energy distributions, and briefly assess the prospects for detecting these decays at CERN LEP2 and other colliders.

hep-ph↗
arXiv2008

Regularization Dependence of Running Couplings in Softly Broken Supersymmetry

We discuss the dependence of running couplings on the choice of regularization method in a general softly-broken N=1 supersymmetric theory. Regularization by dimensional reduction respects supersymmetry, but standard dimensional regularization does not. We find expressions for the differences between running couplings in the modified minimal subtraction schemes of these two regularization methods, to one loop order. We also find the two-loop renormalization group equations for gaugino masses in both schemes, and discuss the application of these results to the Minimal Supersymmetric Standard Model.

hep-ph↗
arXiv2008

A Tumbling Top-Quark Condensate Model

We propose a renormalizable model with no fundamental scalars which breaks itself in the manner of a "tumbling" gauge theory down to the standard model with a top-quark condensate. Because of anomaly cancellation requirements, this model contains two color sextet fermions (quixes), which are vector-like with respect to the standard model gauge group. The model also has a large number of pseudo-Nambu-Goldstone bosons, some of which can be light. The top-quark condensate is responsible for breaking the electroweak gauge symmetry and gives the top quark a large mass. We discuss the qualitative features and instructive shortcomings of the model in its present form. We also show that this model can be naturally embedded into an aesthetically pleasing model in which the standard model fermion families appear symmetrically.

hep-ph↗
arXiv2008

Some Simple Criteria for Gauged R-parity

We catalog some simple conditions which are sufficient to guarantee that R-parity survives as an unbroken gauged discrete subgroup of the continuous gauge symmetry in certain supersymmetric extensions of the standard model.

hep-ph↗
arXiv2008

Cosmic Strings from Supersymmetric Flat Directions

Flat directions are a generic feature of the scalar potential in supersymmetric gauge field theories. They can arise, for example, from D-terms associated with an extra abelian gauge symmetry. Even when supersymmetry is broken softly, there often remain directions in the scalar field space along which the potential is almost flat. Upon breaking a gauge symmetry along one of these almost flat directions, cosmic strings may form. Relative to the standard cosmic string picture based on the abelian Higgs model, these flat-direction cosmic strings have the extreme Type-I properties of a thin gauge core surrounded by a much wider scalar field profile. We perform a comprehensive study of the microscopic, macroscopic, and observational characteristics of this class of strings. We find many differences from the standard string scenario, including stable higher winding mode strings, the dynamical formation of higher mode strings from lower ones, and a resultant multi-tension scaling string network in the early universe. These strings are only moderately constrained by current observations, and their gravitational wave signatures may be detectable at future gravity wave detectors. Furthermore, there is the interesting but speculative prospect that the decays of cosmic string loops in the early universe could be a source of ultra-high energy cosmic rays or non-thermal dark matter. We also compare the observational signatures of flat-direction cosmic strings with those of ordinary cosmic strings as well as (p,q) cosmic strings motivated by superstring theory.

hep-ph↗
arXiv2008

Diphoton decays of stoponium at the Large Hadron Collider

If the lighter top squark has no kinematically allowed two-body decays that conserve flavor, then it will form hadronic bound states. This is required in models that are motivated by the supersymmetric little hierarchy problem and obtain the correct thermal relic abundance of dark matter by top-squark-mediated neutralino annihilations, or by top-squark-neutralino co-annihilations. It is also found in models that can accommodate electroweak-scale baryogenesis within minimal supersymmetry. I study the prospects for detecting scalar stoponium from its diphoton decay mode at the Large Hadron Collider, updating and correcting previous work. Under favorable circumstances, this signal will be observable over background, enabling a uniquely precise measurement of the superpartner masses through a narrow peak in the diphoton invariant mass spectrum, limited by statistics and electromagnetic calorimeter resolutions.

hep-ph↗
arXiv2007

The top squark-mediated annihilation scenario and direct detection of dark matter in compressed supersymmetry

Top squark-mediated annihilation of bino-like neutralinos to top-antitop pairs can play the dominant role in obtaining a thermal relic dark matter abundance in agreement with observations. In a previous paper, it was argued that this can occur naturally in models of compressed supersymmetry, which feature a running gluino mass parameter that is substantially smaller than the wino mass parameter at the scale of apparent gauge coupling unification. Here I study in some more detail the parameter space in which this is viable, and compare to other scenarios for obtaining the observed dark matter density. I then study the possibility of detecting the dark matter directly in future experiments. The prospects are consistently very promising for a wide variety of model parameters within this scenario.

hep-ph↗
arXiv2007

Compressed supersymmetry and natural neutralino dark matter from top squark-mediated annihilation to top quarks

The parameters of the Minimal Supersymmetric Standard Model appear to require uncomfortably precise adjustment in order to reconcile the electroweak symmetry breaking scale with the lower mass limits on a neutral Higgs scalar boson. This problem can be significantly ameliorated in models with a running gluino mass parameter that is smaller than the wino mass near the scale of unification of gauge couplings. A "compressed" superpartner mass spectrum results; compared to models with unified gaugino masses, the ratios of the squark and gluino masses to the lightest superpartner mass are reduced. I argue that in this scenario the annihilation of bino-like neutralino pairs to top-antitop quark pairs through top squark exchange can most naturally play the crucial role in ensuring that the thermal relic dark matter density is not too large, with only a small role played by coannihilations. The lightest superpartner mass must then exceed the top quark mass, and the lighter top squark cannot decay to a top quark. These conditions have important implications for collider searches.

hep-ph↗
arXiv2007

Three-loop corrections to the lightest Higgs scalar boson mass in supersymmetry

I evaluate the largest three-loop corrections to the mass of the lightest Higgs scalar boson in the Minimal Supersymmetric Standard Model in a mass-independent renormalization scheme, using effective field theory and renormalization group methods. The contributions found here are those that depend only on strong and Yukawa interactions and on the leading and next-to-leading logarithms of the ratio of a typical superpartner mass scale to the top quark mass. The approximation assumes that all superpartners and the other Higgs bosons can be treated as much heavier than the top quark, but does not assume their degeneracy. I also discuss the consistent addition of the three-loop corrections to a complete two-loop calculation.

hep-ph↗
arXiv2005

Fermion self-energies and pole masses at two-loop order in a general renormalizable theory with massless gauge bosons

I present the two-loop self-energy functions and pole masses for fermions in an arbitrary renormalizable field theory, in the approximation that vector bosons are treated as massless. The calculations are done simultaneously in the mass-independent \bar{MS}, \bar{DR}, and \bar{DR}' renormalization schemes, with a general covariant gauge fixing, and treating Majorana and Dirac fermions in a unified way. As examples, I discuss the two-loop strong interaction corrections to the gluino, neutralino, chargino, and quark pole masses in minimal supersymmetry. All other two-loop contributions to the fermion pole masses in softly broken supersymmetry can also be obtained as special cases of the results given here, neglecting only the electroweak symmetry breaking scale compared to larger mass scales in two-loop diagrams that involve W or Z bosons.

hep-ph↗
arXiv2005

Two-loop scalar self-energies and pole masses in a general renormalizable theory with massless gauge bosons

I present the two-loop self-energy functions for scalar bosons in a general renormalizable theory, within the approximation that vector bosons are treated as massless or equivalently that gauge symmetries are unbroken. This enables the computation of the two-loop physical pole masses of scalar particles in that approximation. The calculations are done simultaneously in the mass-independent \bar{MS}, \bar{DR}, and \bar{DR}' renormalization schemes, and with arbitrary covariant gauge fixing. As an example, I present the two-loop SUSYQCD corrections to squark masses, which can increase the known one-loop results by of order one percent. More generally, it is now straightforward to implement all two-loop sfermion pole mass computations in supersymmetry using the results given here, neglecting only the electroweak vector boson masses compared to the superpartner masses in the two-loop parts.

hep-ph↗
arXiv2005

Virtual effects of light gauginos and higgsinos: a precision electroweak analysis of split supersymmetry

We compute corrections to precision electroweak observables in supersymmetry in the limit that scalar superpartners are very massive and decoupled. This leaves charginos and neutralinos and a Standard Model-like Higgs boson as the only states with unknown mass substantially affecting the analysis. We give complete formulas for the chargino and neutralino contributions, derive simple analytic results for the pure gaugino and higgsino cases, and study the general case. We find that in all circumstances, the precision electroweak fit improves when the charginos and neutralinos are near the current direct limits. Larger higgsino and gaugino masses worsen the fit as the theory predictions asymptotically approach those of the Standard Model. Since the Standard Model is considered by most to be an adequate fit to the precision electroweak data, an important corollary to our analysis is that all regions of parameter space allowed by direct collider constraints are also allowed by precision electroweak constraints in split supersymmetry.

hep-ph↗
arXiv2004

Strong and Yukawa two-loop contributions to Higgs scalar boson self-energies and pole masses in supersymmetry

I present results for the two-loop self-energy functions for neutral and charged Higgs scalar bosons in minimal supersymmetry. The contributions given here include all terms involving the QCD coupling, and those following from Feynman diagrams involving Yukawa couplings and scalar interactions that do not vanish as the electroweak gauge couplings are turned off. The impact of these contributions on the computation of pole masses of the neutral and charged Higgs scalar bosons is studied in a few examples.

hep-ph↗
arXiv2004

Two-loop scalar self-energies in a general renormalizable theory at leading order in gauge couplings

I present results for the two-loop self-energy functions for scalars in a general renormalizable field theory, using mass-independent renormalization schemes based on dimensional regularization and dimensional reduction. The results are given in terms of a minimal set of loop-integral basis functions, which are readily evaluated numerically by computers. This paper contains the contributions corresponding to the Feynman diagrams with zero or one vector propagator lines. These are the ones needed to obtain the pole masses of the neutral and charged Higgs scalar bosons in supersymmetry, neglecting only the purely electroweak parts at two-loop order. A subsequent paper will present the results for the remaining diagrams, which involve two or more vector lines.

hep-ph↗
arXiv2003

Evaluation of two-loop self-energy basis integrals using differential equations

I study the Feynman integrals needed to compute two-loop self-energy functions for general masses and external momenta. A convenient basis for these functions consists of the four integrals obtained at the end of Tarasov's recurrence relation algorithm. The basis functions are modified here to include one-loop and two-loop counterterms to render them finite; this simplifies the presentation of results in practical applications. I find the derivatives of these basis functions with respect to all squared-mass arguments, the renormalization scale, and the external momentum invariant, and express the results algebraically in terms of the basis. This allows all necessary two-loop self-energy integrals to be efficiently computed numerically using the differential equation in the external momentum invariant. I also use the differential equations method to derive analytic forms for various special cases, including a four-propagator integral with three distinct non-zero masses.

hep-ph↗