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1. Effective field theory bootstrap, large-N χPT and holographic QCD

   https://doi.org/10.1007/JHEP01(2024)072  

   Li, Yue-Zhou (January 2024, Journal of High Energy Physics)

   A<sc>bstract</sc> We review the effective field theory (EFT) bootstrap by formulating it as an infinite-dimensional semidefinite program (SDP), built from the crossing symmetric sum rules and the S-matrix primal ansatz. We apply the program to study the large-Nchiral perturbation theory (χPT) and observe excellent convergence of EFT bounds between the dual (rule-out) and primal (rule-in) methods. This convergence aligns with the predictions of duality theory in SDP, enabling us to analyze the bound states and resonances in the ultra-violet (UV) spectrum. Furthermore, we incorporate the upper bound of unitarity to uniformly constrain the EFT space from the UV scaleMusing the primal method, thereby confirming the consistency of the large-Nexpansion. In the end, we translate the large-N χPT bounds to constrain the higher derivative corrections of holographic QCD models. 

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2. QCD Constraints on Isospin-Dense Matter and the Nuclear Equation of State

   https://doi.org/10.1103/PhysRevLett.134.011903  

   Abbott, Ryan; Detmold, William; Illa, Marc; Parreño, Assumpta; Perry, Robert J; Romero-López, Fernando; Shanahan, Phiala E; Wagman, Michael L (January 2025, Physical Review Letters)

   Understanding the behavior of dense hadronic matter is a central goal in nuclear physics as it governs the nature and dynamics of astrophysical objects such as supernovae and neutron stars. Because of the nonperturbative nature of quantum chromodynamics (QCD), little is known rigorously about hadronic matter in these extreme conditions. Here, lattice QCD calculations are used to compute thermodynamic quantities and the equation of state of QCD over a wide range of isospin chemical potentials with controlled systematic uncertainties. Agreement is seen with chiral perturbation theory when the chemical potential is small. Comparison to perturbative QCD at large chemical potential allows for an estimate of the gap in the superconducting phase, and this quantity is seen to agree with perturbative determinations. Since the partition function for an isospin chemical potential ${\mu }_I$bounds the partition function for a baryon chemical potential ${\mu }_B=3{\mu }_I/2$, these calculations also provide rigorous nonperturbative QCD bounds on the symmetric nuclear matter equation of state over a wide range of baryon densities for the first time. Published by the American Physical Society2025 

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3. Towards a nonperturbative construction of the S-matrix

   https://doi.org/10.1007/JHEP05(2023)197  

   Henning, Brian; Murayama, Hitoshi; Riva, Francesco; Thompson, Jedidiah O.; Walters, Matthew T. (May 2023, Journal of High Energy Physics)

   A bstract We present a nonperturbative recipe for directly computing the S -matrix in strongly-coupled QFTs. The method makes use of spectral data obtained in a Hamiltonian framework and can be applied to a wide range of theories, including potentially QCD. We demonstrate the utility of this prescription in the specific example of the 2+1d O ( N ) model at large N , using energy eigenstates computed with Hamiltonian truncation to reproduce the full 2 → 2 scattering amplitude for arbitrary (complex) center-of-mass energy. 

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4. PQ axiverse

   https://doi.org/10.1007/JHEP06(2023)092  

   Demirtas, Mehmet; Gendler, Naomi; Long, Cody; McAllister, Liam; Moritz, Jakob (June 2023, Journal of High Energy Physics)

   A bstract We show that the strong CP problem is solved in a large class of compactifications of string theory. The Peccei-Quinn mechanism solves the strong CP problem if the CP-breaking effects of the ultraviolet completion of gravity and of QCD are small compared to the CP-preserving axion potential generated by low-energy QCD instantons. We characterize both classes of effects. To understand quantum gravitational effects, we consider an ensemble of flux compactifications of type IIB string theory on orientifolds of Calabi-Yau hypersurfaces in the geometric regime, taking a simple model of QCD on D7-branes. We show that the D-brane instanton contribution to the neutron electric dipole moment falls exponentially in N 4 , with N the number of axions. In particular, this contribution is negligible in all models in our ensemble with N > 17. We interpret this result as a consequence of large N effects in the geometry that create hierarchies in instanton actions and also suppress the ultraviolet cutoff. We also compute the CP breaking due to high-energy instantons in QCD. In the absence of vectorlike pairs, we find contributions to the neutron electric dipole moment that are not excluded, but that could be accessible to future experiments if the scale of supersymmetry breaking is sufficiently low. The existence of vectorlike pairs can lead to a larger dipole moment. Finally, we show that a significant fraction of models are allowed by standard cosmological and astrophysical constraints. 

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5. On Bunching and Identification of the Taxable Income Elasticity

   https://doi.org/10.1086/714446  

   Blomquist, Sören; Newey, Whitney K.; Kumar, Anil; Liang, Che-Yuan (August 2021, Journal of Political Economy)

   The elasticity of taxable income with respect to the net of tax rate is a key parameter for predicting the effect of tax reform or designing an income tax. Bunching at kinks and notches in a single budget set has been used to estimate the taxable income elasticity. We show that when the distribution of preferences is unrestricted the amount of bunching at a kink or a notch is not informative about the size of the taxable income elasticity, and neither is the entire distribution of taxable income for a convex budget set. Kinks do provide information about the size of the elasticity when a priori restrictions are placed on the preference distribution. They can identify the elasticity when the preference distribution is completely and correctly specified across the kink and provide bounds under restrictions on the preference distribution. We find wide estimated bounds in an empirical example using data like Saez (2010) based on upper and lower bounds for the preference density. 

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