arXiv subjects

Aaron Pierce

Publications and source records attributed to Aaron Pierce.

At least 73 records · Page 4Linked to original sources

arXiv2009

Top quark forward-backward asymmetry from new t-channel physics

Motivated by recent measurements of the top quark forward-backward asymmetry at the Tevatron, we study how t-channel new physics can contribute to a large value. We concentrate on a theory with an abelian gauge boson possessing flavor changing couplings between up and top quarks, but satisfies flavor physics constraints. Collider constraints are strong, but can be consistent with the aid of small flavor diagonal couplings. We find that M_Z' ~ 160 GeV can yield a total lab-frame asymmetry of ~18% without being in conflict with other observables. There are implications for future collider searches, including exotic top quark decays, like-sign top quark production, and detailed measurements of the top production cross section. An alternate model with a gauged non-Abelian flavor symmetry would have similar phenomenology, but lacks the like-sign top signal.

hep-ph↗

arXiv2009

Cosmic Ray Positrons from Annihilations into a New, Heavy Lepton

Recent results from the PAMELA experiment indicate an excess in the positron spectrum above 10 GeV, but anti-proton data are consistent with the expected astrophysical backgrounds. We propose a scenario that reproduces these features. Dark matter annihilates through channels involving a new heavy vectorlike lepton which then decays by mixing with Standard Model leptons. If charged, this heavy lepton might be produced at the LHC, and could lead to multi-lepton final states or to long-lived charged tracks. Large neutrino detectors such as ANTARES or IceCube might be sensitive to a monochromatic neutrino line. This scenario may be simply embedded in various models, including an extension to the NMSSM.

hep-ph↗

arXiv2008

Is the PAMELA Positron Excess Winos?

Recently the PAMELA satellite-based experiment reported an excess of galactic positrons that could be a signal of annihilating dark matter. The PAMELA data may admit an interpretation as a signal from a wino-like LSP of mass about 200 GeV, normalized to the local relic density, and annihilating mainly into W-bosons. This possibility requires the current conventional estimate for the energy loss rate of positrons be too large by roughly a factor of five. Data from anti-protons and gamma rays also provide tension with this interpretation, but there are significant astrophysical uncertainties associated with their propagation. It is not unreasonable to take this well-motivated candidate seriously, at present, in part because it can be tested in several ways soon. The forthcoming PAMELA data on higher energy positrons and the FGST (formerly GLAST) data, should provide important clues as to whether this scenario is correct. If correct, the wino interpretation implies a cosmological history in which the dark matter does not originate in thermal equilibrium.

hep-ph↗

arXiv2008

Changes in Dark Matter Properties After Freeze-Out

The properties of the dark matter that determine its thermal relic abundance can be very different from the dark matter properties today. We investigate this possibility by coupling a dark matter sector to a scalar that undergoes a phase transition after the dark matter freezes out. If the value of Omega_DM h^2 calculated from parameters measured at colliders and by direct and indirect detection experiments does not match the astrophysically observed value, a novel cosmology of this type could provide the explanation. This mechanism also has the potential to account for the "boost factor" required to explain the PAMELA data.

hep-ph↗

arXiv2008

Neutralino Dark Matter from Indirect Detection Revisited

We revisit indirect detection possibilities for neutralino dark matter, emphasizing the complementary roles of different approaches. While thermally produced dark matter often requires large astrophysical "boost factors" to observe antimatter signals, the physically motivated alternative of non-thermal dark matter can naturally provide interesting signals, for example from light wino or Higgsino dark matter. After a brief review of cosmic ray propagation, we discuss signals for positrons, antiprotons, synchrotron radiation and gamma rays from wino annihilation in the galactic halo, and examine their phenomenology. For pure wino dark matter relevant to the LHC, PAMELA and GLAST should report signals.

hep-ph↗

arXiv2008

Using the Energy Spectrum at DAMA/LIBRA to Probe Light Dark Matter

A weakly interacting massive particle (WIMP) weighing only a few GeV has been invoked as an explanation for the signal from the DAMA/LIBRA experiment. We show that the data from DAMA/LIBRA are now powerful enough to strongly constrain the properties of any putative WIMP. Accounting for the detailed recoil spectrum, a light WIMP with a Maxwellian velocity distribution and a spin-independent (SI) interaction cannot account for the data. Even neglecting the spectrum, much of the parameter space is excluded by limits from the DAMA unmodulated signal at low energies. Significant modifications to the astrophysics or particle physics can open light mass windows.

hep-ph↗

arXiv2008

Modified Higgs Boson Phenomenology from Gauge or Gaugino Mediation in the NMSSM

In the Next-to-Minimal Supersymmetric Standard Model (NMSSM), the presence of light pseudoscalars can have a dramatic effect on the decays of the Standard Model-like Higgs boson. These pseudoscalars are naturally light if supersymmetry breaking preserves an approximate U(1)_R symmetry, spontaneously broken when the Higgs bosons take on their expectation values. We investigate two classes of theories that possess such an approximate U(1)_R at the mediation scale: deformations of gauge and gaugino mediation. In the models we consider, we find two disjoint classes of phenomenologically allowed parameter regions. One of these regions corresponds to a limit where the singlet of the NMSSM largely decouples. The other can give rise to a Standard Model-like Higgs boson with a dominant branching into light pseudoscalars.

hep-ph↗

arXiv2008

Testing a U(1) Solution to the Mu Problem

We discuss the collider phenomenology of TeV Z' gauge bosons related to the absence of a bare mu-term in the superpotential. Decays of the type Z' --> Higgsinos can directly test whether a gauge symmetry is responsible for forbidding the Higgsino mass. Decays to multi-lepton final states may allow these signatures to be observed at the Large Hadron Collider. We comment on whether it will be possible to state definitively that the mu-term is forbidden via a gauge symmetry.

hep-ph↗

arXiv2008

Color-octet scalars at the LHC

Color-octet scalars, if present at the TeV scale, will be produced in abundance at the LHC. We discuss in some detail the phenomenology of scalars in the (8,2)_{1/2} representation, recently identified by Manohar and Wise as an addition to the standard-model Higgs sector consistent with the principle of minimal flavor violation. Couplings of this multiplet to the Higgs lift the mass degeneracy among its states, possibly allowing for two-body decays of a heavier colored scalar to a lighter one and a gauge boson. We perform a renormalization group analysis of these couplings and find that limits from Tevatron searches leave little room for these decays. This fact, and the assumption of minimal flavor violation, lead us to study the case where the octets decay to the heaviest kinematically accessible fermion pairs. Focusing on pair-production events leading to (t t-bar t t-bar), (b b-bar b b-bar), and (b b-bar t t-bar) final states, we find that discovery at the LHC should be possible up to masses exceeding 1 TeV.

hep-ph↗

arXiv2007

Disentangling Dimension Six Operators through Di-Higgs Boson Production

New physics near the TeV scale can generate dimension-six operators that modify the production rate and branching ratios of the Higgs boson. Here, we show how Higgs boson pair production can yield complementary information on dimension-six operators involving the gluon field strength. For example, the invariant mass distribution of the Higgs boson pair can show the extent to which the masses of exotic TeV-scale quarks come from electroweak symmetry breaking. We discuss both the current Tevatron bounds on these operators and the most promising LHC measurement channels for two different Higgs masses: 120 GeV and 180 GeV. We argue that the operators considered in this paper are the ones most likely to yield interesting Higgs pair physics at the LHC.

hep-ph↗

arXiv2007

Natural Dark Matter from an Unnatural Higgs Boson and New Colored Particles at the TeV Scale

The thermal relic abundance of Dark Matter motivates the existence of new electroweak scale particles, independent of naturalness considerations. However, most unnatural Dark Matter models do not ensure the presence of new particles charged under SU(3)_C, resulting in challenging LHC phenomenology. Here, we present a class of models with scalar electroweak doublet Dark Matter that require a host of colored particles at the TeV scale. In these models, the Higgs boson is apparently fine-tuned, but the Dark Matter doublet is kept light without any additional fine-tuning.

hep-ph↗

arXiv2007

Sfermion Interference in Neutralino Decays at the LHC

If the two lightest neutralinos of the Minimal Supersymmetric Standard Model have a mass splitting less than the Z boson mass, interference effects in the three-body decay chi_2^0 --> chi_1^0 f f can be important. We formulate an observable that contains information on the nature of the interference: the ratio BR(chi_2^0 --> chi_1^0 b b) / BR(chi_2^0 --> chi_1^0 l+ l-). This will give a constraint on the supersymmetry breaking parameters that is complementary to many techniques already existing in the literature. We present some ideas on how to perform a simple counting experiment to determine this ratio.

hep-ph↗

arXiv2006

Four Taus at the Tevatron

We study extensions of the Standard Model where the Higgs boson dominantly decays via a cascade to four tau leptons, and discuss whether this decay is visible at the Tevatron. We find that with an integrated luminosity of 6 fb^-1, there can be excesses in multi-lepton events in several channels for a Higgs boson of a mass = 110 GeV.

hep-ph↗

arXiv2006

Prospects for Mirage Mediation

Mirage mediation reduces the fine-tuning in the minimal supersymmetric standard model by dynamically arranging a cancellation between anomaly-mediated and modulus-mediated supersymmetry breaking. We explore the conditions under which a mirage "messenger scale" is generated near the weak scale and the little hierarchy problem is solved. We do this by explicitly including the dynamics of the SUSY-breaking sector needed to cancel the cosmological constant. The most plausible scenario for generating a low mirage scale does not readily admit an extra-dimensional interpretation. We also review the possibilities for solving the mu/Bmu problem in such theories, a potential hidden source of fine-tuning.

hep-ph↗

arXiv2004

D-Terms, Unification, and the Higgs Mass

We study gauge extensions of the MSSM that contain non-decoupling D-terms, which contribute to the Higgs boson mass. These models naturally maintain gauge coupling unification and raise the Higgs mass without fine-tuning. Unification constrains the structure of the gauge extensions, limiting the Higgs mass in these models to roughly less than 150 GeV. The D-terms contribute to the Higgs mass only if the extended gauge symmetry is broken at energies of a few TeV, leading to new heavy gauge bosons in this mass range.

hep-ph↗

arXiv2004

Probing a QCD String Axion with Precision Cosmological Measurements

String and M-theory compactifications generically have compact moduli which can potentially act as the QCD axion. However, as demonstrated here, such a compact modulus can not play the role of a QCD axion and solve the strong CP problem if gravitational waves interpreted as arising from inflation with Hubble constant $H_inf \gsim 10^{13}$ GeV are observed by the PLANCK polarimetry experiment. In this case axion fluctuations generated during inflation would leave a measurable isocurvature and/or non-Gaussian imprint in the spectrum of primordial temperature fluctuations. This conclusion is independent of any assumptions about the initial axion misalignment angle, how much of the dark matter is relic axions, or possible entropy release by a late decaying particle such as the saxion; it relies only on the mild assumption that the Peccei-Quinn symmetry remains unbroken in the early universe.

hep-th↗

arXiv2004

Dark Matter in the Finely Tuned Minimal Supersymmetric Standard Model

We explore dark matter in the Finely Tuned Minimal Supersymmetric Standard model recently proposed by Arkani-Hamed and Dimopoulos. Relative to the MSSM, there are fewer particles at freeze-out, so the calculation of the relic abundance simplifies. Similarly, the predictions for direct detection of the dark matter sharpen. There is a large region of mixed bino--higgsino dark matter where the lightest supersymmetric particle will be accessible at both the LHC and future direct detection experiments, allowing for a conclusive identification of the dark matter particle. Typical dark matter-nucleon cross sections are 10^{-45}-10^{-44} cm^{2}. This model also possesses a novel region where the dark matter annihilates via an s-channel Higgs boson resonance.

hep-ph↗

arXiv2004

Composite Vector Mesons from QCD to the Little Higgs

We review how the rho meson can be modeled in an effective theory and discuss the implications of applying this approach to heavier spin-one resonances. Georgi's vector limit is explored, and its relationship to locality in a deconstructed extra-dimension is discussed. We then apply the formalism for rho's to strongly coupled theories of electroweak symmetry breaking, studying the lightest spin-one techni-rho resonances. Understanding these new particles in Little Higgs models can shed light on previously incalculable, ultraviolet sensitive physics, including the mass of the Higgs boson.

hep-ph↗