Bulletin of the American Physical Society
Annual Meeting of the APS Four Corners Section
Volume 60, Number 11
Friday–Saturday, October 16–17, 2015; Tempe, Arizona
Session B6: Particle Physics I |
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Chair: Pearl Sandick, University of Utah Room: MU207 |
Friday, October 16, 2015 10:50AM - 11:14AM |
B6.00001: The XYZ Affair: Tales of the Third (and Fourth) Hadron Invited Speaker: Richard Lebed In the past dozen years, a number of new particles have been discovered that, while clearly being hadrons (interacting via the strong nuclear force), do not seem to fit into either of the known hadron categories of meson (quark-antiquark) or baryon (3 quarks). Three species of these ``exotic'' particles, called X, Y, and Z, are now believed to be tetraquarks, and last July the LHC announced the discovery of pentaquark states. After briefly reviewing how the states were discovered, I turn to the question of how they are assembled. Several competing physical pictures attempt to describe the structure of exotics: as molecules of known hadrons, as the result of kinematical effects, and others. I propose that they arise due to the formation of compact diquarks, a well-known but under-appreciated phenomenon of QCD. The diquarks are created with large relative momentum, and hence wish to fly off as free particles. However, they also carry color charge and therefore cannot be free, due to confinement. These competing facts create an entirely new kind of bound state, not a molecule with well-defined orbits, but an extended object that lasts only as long as it takes for quantum mechanics to allow the separated quarks and antiquarks to ``find'' one another, and allow decays to occur. I will discuss several observed effects that support this picture. [Preview Abstract] |
Friday, October 16, 2015 11:14AM - 11:26AM |
B6.00002: Searching for $X(3872)$ and $Z_c(3900)$ using lattice QCD Song-haeng Lee, Carleton DeTar In the past decade, many excited charmonium states have been discovered that cannot be explained within the conventional quark model. Among those, the narrow charmonium-like state $X(3872)$ and the charged charmonium-like state $Z_c(3900)$ have been examined using various phenomenological models. Since $X(3872)$ mass is within 1~MeV of the $D\bar D^*$ threshold and $Z_c(3900)$ must contain at least four quarks, one strong candidate of these states is a $D\bar D^*$ molecular state. However, such molecular state cannot be directly studied by perturbative QCD in a low energy regime. Numerical simulation with lattice QCD can provide a nonperturbative, {\it ab initio} method for studying these mysterious meson states. In this talk, I present simulation results for $X(3872)$ and $Z_c(3900)$ with quantum numbers $J^{\rm PC} = 1^{++}$ in the isospin 0 and $J^{\rm PC} = 1^{-+}$ in the isospin 1 using lattice QCD, respectively. We use interpolating operators including both the conventional excited P-wave charmonium state and the $D^0\bar D^{0*}$ open charm state for $X(3872)$ and $J/\psi~\pi^{\pm}$ and the $D^{\pm} \bar D^{*0}$ open charm state for $Z_c(3900)$. We find an $X(3872)$ candidate close to and below the $D\bar D^*$ threshold, however, only scattering states for $Z_c(3900)$. [Preview Abstract] |
Friday, October 16, 2015 11:26AM - 11:38AM |
B6.00003: Searches for $t\overline t $ Resonances at $\sqrt s =$ 13 TeV Using Run 2 Data from the ATLAS Experiment Aaron Kilgallon Run 2 of the Large Hadron Collider at $\sqrt s =$ 13 TeV will extend the discovery reach for new particles predicted by Beyond the Standard Model theories. Several of these theories predict resonances that decay primarily into $t\overline t $ pairs. In the ATLAS detector, $t\overline t $ resonances are searched for using the lepton plus jets decay channel of the $t\overline t $ pair. For large $t\overline t $ resonance masses, boosted decays of the hadronically decaying top quark must be considered. This talk describes the prospects for discovering $t\overline t $ resonances in Run 2 and discusses possible improvements over previous searches. Preliminary results on data collected for $t\overline t $ resonance searches in Run 2 are shown. [Preview Abstract] |
Friday, October 16, 2015 11:38AM - 11:50AM |
B6.00004: True Muonium on the Light Front Henry Lamm IV, Richard Lebed The true muonium ($\mu^+\mu^-$) bound state presents an interesting test of light-cone quantization techniques. In addition to the standard problems of solving these non-perturbative calculations, true muonium requires correct treatment of $e^+e^-$ Fock state contributions. Having previously produced a crude model of true muonium using the method of iterated resolvents [1], current work has focused on the inclusion of the box diagrams that should improve the cutoff dependent issues of the model. Further, a parallel computer code allowing for decreased numerical uncertainties is in development. This talk will focus on the current state of these efforts to develop a model of true muonium testable at near-term experiments. \\[4pt] [1] H. Lamm and R.F. Lebed, ``True Muonium ($\mu^+\mu^-$) on the Light Front,'' J. Phys. G 41 125003 (2014). [Preview Abstract] |
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