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Carlos Schat

Publications and source records attributed to Carlos Schat.

26 records · Page 2Linked to original sources
arXiv2007

Improved dispersion relations for γγ\to π^0π^0

We perform a dispersive theoretical study of the reaction γγ\to pi^0π^0 emphasizing the low energy region. The large source of theoretical uncertainty to calculate the γγ\toπ^0π^0 total cross section for \sqrt{s}\gtrsim 0.5 GeV within the dispersive approach is removed. This is accomplished by taking one more subtraction in the dispersion relations, where the extra subtraction constant is fixed by considering new low energy constraints, one of them further refined by taking into consideration the f_0(980) region. This allows us to make sharper predictions for the cross section for \sqrt{s}\lesssim 0.8 GeV, below the onset of D-wave contributions. In this way, were new more precise data on γγ\toπ^0π^0 available one might then distinguish between different parameterizations of the ππisoscalar S-wave. We also elaborate on the width of the σresonance to γγand provide new values.

hep-ph↗
arXiv2006

Positive parity pentaquark towers in large Nc QCD

We construct the complete set of positive parity pentaquarks, which correspond in the quark model to {\bar s} q^{Nc+1} states with one unit of orbital angular momentum L=1. In the large Nc limit they fall into the K=1/2 and K=3/2 irreps (towers) of the contracted SU(4)c symmetry. We derive predictions for the mass spectrum and the axial couplings of these states at leading order in 1/Nc. The strong decay width of the lowest-lying positive parity exotic state is of order O(1/Nc), such that this state is narrow in the large Nc limit. Replacing the antiquark with a heavy antiquark {\bar Q} q^{Nc+1}, the two towers become degenerate, split only by O(1/mQ) hyperfine interactions. We obtain predictions for the strong decay widths of heavy pentaquarks to ordinary baryons and heavy H(*)_{\bar Q} mesons at leading order in 1/Nc and 1/mQ.

hep-ph↗
arXiv2006

Occupation number formalism for arbitrary Nc baryons

A general method is presented for computing matrix elements of quark operators on baryonic states with low strangeness and arbitrary number of colors Nc. These results are useful in applications of the large Nc expansion to baryons and exotics. As an application we compute the matrix elements of strangeness changing operators contributing to kaon couplings to ground state baryons and pentaquarks, in broken SU(3).

hep-ph↗
arXiv2005

Negative parity pentaquarks in large Nc QCD and quark model

Recently, the 1/Nc expansion has been applied to the study of exotic baryons containing both quarks and antiquarks. We extend this approach to exotic states with mixed symmetric spin-flavor symmetry, which correspond in the quark model to negative parity pentaquarks, and discuss the large Nc predictions for their mass spectrum. The heavy exotics Qbar q^4 transform as 3, 6bar,15 and 15' under flavor SU(3), while the light states include the exotic multiplets 10bar, 27, 35. We give mass relations among these multiplets following from the 1/Nc expansion. In the quark model, the mass splittings between these states are given by color-spin interactions. Using the observation of an anticharmed exotic by the H1 collaboration, we give predictions for the masses of other expected heavy pentaquarks.

hep-ph↗
arXiv2004

Determining Pentaquark Quantum Numbers from Strong Decays

Assuming that the recently observed $Θ^+$ and $Ξ^{--}$ are members of an anti-decuplet of SU(3), decays to ground state baryons and mesons are calculated using an effective Lagrangian which incorporates chiral and SU(3) symmetry. We consider the possible quantum number assignments $J^Π={1/2}^\pm,{3/2}^\pm$ and calculate ratios of partial widths. The branching ratios of exotic cascades can be used to discriminate between even and odd parity pentaquarks.

hep-ph↗
arXiv2003

Excited L=1 baryons in large Nc QCD

The physics of the orbitally excited baryons simplifies drastically in the large Nc limit. The states are arranged into irreducible representations of the contracted SU(4)_c symmetry, with mixing angles determined exactly. The ratios of the strong couplings N* -> [Nπ]_{S,D} are predicted in this limit, with results in agreement with those following from the quark model (with the large Nc mixing angles). We present a phenomenological analysis of the observed nonstrange baryons from the perspective of the 1/Nc expansion, including constraints from their masses and strong decays.

hep-ph↗
arXiv2003

1/Nc corrections to the SU(4)_c symmetry for L=1 baryons: The Three Towers

In the large Nc limit, the mass spectrum of the L=1 orbitally excited baryons N* has a very simple structure, with states degenerate in pairs of spins J=(1/2,3/2), (3/2,5/2), corresponding to irreducible representations (towers) of the contracted SU(4)_c symmetry group. The mixing angles are completely determined in this limit. Using a mass operator approach, we study 1/Nc corrections to this picture, pointing out a four-fold ambiguity in the correspondence of the observed baryons with the large Nc states. For each of the four possible assignments, we fit the coefficients of the quark operators contributing to the mass spectrum to O(Nc^{-1}). We comment on the implications of our results for the constituent quark model description of these states.

hep-ph↗