arXiv subjects

A. Kagan

Publications and source records attributed to A. Kagan.

4 recordsLinked to original sources

arXiv2013

Implications of LHCb measurements and future prospects

During 2011 the LHCb experiment at CERN collected 1.0 fb-1 of sqrt{s} = 7 TeV pp collisions. Due to the large heavy quark production cross-sections, these data provide unprecedented samples of heavy flavoured hadrons. The first results from LHCb have made a significant impact on the flavour physics landscape and have definitively proved the concept of a dedicated experiment in the forward region at a hadron collider. This document discusses the implications of these first measurements on classes of extensions to the Standard Model, bearing in mind the interplay with the results of searches for on-shell production of new particles at ATLAS and CMS. The physics potential of an upgrade to the LHCb detector, which would allow an order of magnitude more data to be collected, is emphasised.

hep-ex↗

arXiv2002

Probing for New Physics in Polarized $Λ_b$ decays at the Z

Polarized $Λ_b \to Λγ$ decays at the Z pole are shown to be well suited for probing a large variety of New Physics effects. A new observable is proposed, the angular asymmetry between the $Λ_b$ spin and photon momentum, which is sensitive to the relative strengths of the opposite chirality and Standard Model chirality $b \to s γ$ dipole operators. Combination with the $Λ$ decay polarization asymmetry and comparison with the $Λ_b$ polarization extracted from semileptonic decays allows important tests of the $V-A$ structure of the Standard Model. Modifications of the rates and angular asymmetries which arise at next-to-leading order are discussed. Measurements for $Λ_b \to Λγ$ and the CP conjugate mode, with branching ratios of a few times $10^{-5}$, are shown to be sensitive to non-standard sources of CP violation in the $Λ_b \to Λγ$ matrix element. Form factor relations for heavy-to-light baryon decays are derived in the large energy limit, which are of general interest.

hep-ph↗

arXiv1993

Supersymmetry and the Nelson-Barr Mechanism

One possible solution to the strong CP problem is that CP is an exact symmetry, spontaneously broken at some scale. Some years ago, Nelson and Barr suggested a mechanism for obtaining $θ=0$ at tree level in this framework, and showed that radiative corrections were small in some non-supersymmetric models. Further investigations suggested that the same could be true in supersymmetric theories. In this note, we show that these analyses assumed an extraordinarily high degree of degeneracy among squark masses and among other supersymmetry breaking parameters. We argue, using naturalness as well as expectations from string theory, that this is not very plausible.

hep-ph↗

arXiv1993

Flavor Symmetries and The Problem of Squark Degeneracy

If supersymmetry exists at low energies, it is necessary to understand why the squark spectrum exhibits sufficient degeneracy to suppress flavor changing neutral currents. In this note, we point out that gauged horizontal symmetries can yield realistic quark mass matrices, while at the same time giving just barely enough squark degeneracy to account for neutral $K$-meson phenomenology. This approach suggests likely patterns for squark masses, and indicates that there could be significant supersymmetric contributions to $B-\bar{B}$ and $D-\bar{D}$ mixing and CP violation in the $K$ and $B$ systems.

hep-ph↗