Adam K. Leibovich

Top quark associated production of top-color pions at hadron colliders

We investigate the associated production of a neutral physical pion with top quarks in the context of topcolor assisted technicolor. We find that single-top associated production does not yield viable rates at either the Tevatron or LHC. tt-associated production at the Tevatron is suppressed relative to Standard Model ttH, but at the LHC is strongly enhanced and would allow for easy observation of the main decay channels to bottom quarks, and possible observation of the decay to gluons.

Natural theories of ultralow mass pseudo Nambu-Goldstone bosons: Axions and quintessence

We consider the Wilson Line PNGB which arises in a U(1)^N gauge theory, abstracted from a latticized, periodically compactified extra dimension U(1). Planck scale breaking of the PNGBs global symmetry is suppressed, providing natural candidates for the axion and quintessence. We construct an explicit model in which the axion may be viewed as the 5th component of the U(1)_Y gauge field in a 1+4 latticized periodically compactified extra dimension. We also construct a quintessence PNGB model where the ultra-low mass arises from Planck-scale suppressed physics itself.

Opening the crystalline color superconductivity window

Cold dense quark matter is in a crystalline color superconducting phase wherever pairing occurs between species of quarks with chemical potentials whose difference

A comment on the extractions of V ub from radiative decays

\ensuremath{\delta}\ensuremath{\mu}

Resumming the color-octet contribution to radiative Y decay

lies within an appropriate window. If the interaction between quarks is modeled as pointlike, this window is rather narrow. We show that when the interaction between quarks is modeled as single-gluon exchange, the window widens by about a factor of 10 at accessible densities and by much larger factors at higher density. This striking enhancement reflects the increasingly (1+1)-dimensional nature of the physics at weaker and weaker coupling. Our results indicate that crystalline color superconductivity is a generic feature of the phase diagram of cold dense quark matter, occurring wherever one finds quark matter that is not in the color-flavor locked phase. If it occurs within the cores of compact stars, a crystalline color superconducting region may provide a new locus for glitch phenomena.

The photon spectrum inΥdecays

Abstract We present a model independent closed form expression for |V ub | 2 /|V tb V ts ∗ | 2 , which includes the resummation of large endpoint logarithms as well as the interference effects from the operators O 2 and O 8 . We demonstrate that the method to extract | V ub | presented by the authors in hep-ph/9909404, and modified in this Letter to include interference effects, is not just a refinement of the method introduced in hep-ph/9312311. We also discuss the model dependence of the latter proposal. Furthermore, we show that the resummation is not negligible and that the Landau pole does not introduce any significant uncertainties.

Increased yield of ttbb at Hadron Colliders in low-energy supersymmetry.

At the upper endpoint of the photon energy spectrum in Upsilon -> X gamma, the standard NRQCD power counting breaks down and the OPE gives rise to color-octet structure functions. Furthermore, in this kinematic regime large Sudakov logarithms appear in the octet Wilson coefficients. The endpoint spectrum can be treated consistently within the framework of a recently developed effective field theory of collinear and soft particles. Here we show that within this approach the octet structure functions arise naturally and that Sudakov logarithms can be summed using the renormalization group equations. We derive an expression for the resummed energy spectrum and, using a model lightcone structure function, investigate the phenomenological importance of the resummation.

Radiative upsilon decay at the endpoint

We present a theoretical prediction for the photon spectrum in radiative Upsilon decay. Parts of the spectrum have already been understood, but an understanding of the endpoint region has remained elusive. In this paper we provide the missing piece, and resolve a controversy in the literature. We treat the endpoint region of Upsilon -> X gamma decay within the framework of the soft-collinear effective theory (SCET). Within this approach the Upsilon structure function arises naturally, and kinematic logarithms are summed by running operators using renormalization group equations. In a previous paper we studied the color-octet contribution to the decay. Here we treat the color-singlet contribution. We combine our result with previous results to obtain the Upsilon -> X gamma spectrum. We find that resumming the color-singlet contribution in the endpoint gives a result that is in much better agreement with the data than the leading order prediction.

Resummed photon spectrum in radiative Y decays

Light bottom squarks and gluinos have been invoked to explain the b quark pair production excess at the Fermilab Tevatron. We investigate the associated production of ttbb at hadron colliders in this scenario, and find that the rates for this process are enhanced over the standard model prediction. If light gluinos exist, it may be possible to detect them at the Tevatron, and they could easily be observed at the CERN Large Hadron Collider.

Enhanced subleading structure functions in semileptonic B decay

Abstract The standard NRQCD power counting breaks down and the OPE gives rise to color-octet shape functions at the upper endpoint of the photon energy spectrum in radiative ϒ decay. Also in this kinematic regime, large Sukadov logarithms appear in the octet Wilson coefficients, ruining the perturbative expansion. Using SCET, the octet shape functions arise naturally and the Sudakov logarithms can be summed using the renormalization group equations. We derive an expression for the resummed octet energy spectrum.