Chiral Perturbation Theory with tensor sources

We construct the most general chirally-invariant Lagrangian for mesons in the presence of external sources coupled to the tensor current \bar{\psi}\sigma_{\mu\nu}\psi. In order to have only even terms in the chiral expansion, we consider the new source of O(p^2). With this choice, we build the even-parity effective Lagrangian up to the p^6-order (NLO). While there are only 4 new terms at the p^4-order, at p^6-order we find 78 terms for n_f=2 and 113 terms for n_f=3. We provide a detailed discussion on the different mechanisms that ensure that our final set of operators is complete and non-redundant. We also examine the odd-parity sector, to conclude that the first operators appear at the p^8-order (NNLO).Comment: 23 pages, one figure; typos corrected, one paragraph added, new section added, references added, published versio

Possible duality violations in tau decay and their impact on the determination of alpha_s

We discuss the issue of duality violations in hadronic tau decay. After introducing a physically motivated ansatz for duality violations, we estimate their possible size by fitting this ansatz to the tau experimental data provided by the ALEPH collaboration. Our conclusion is that these data do not exclude significant duality violations in tau decay. This may imply an additional systematic error in the value of alpha_s(m_tau), extracted from tau decay, as large as \delta alpha_s(m_tau) \sim 0.003-0.010 .Comment: 20 pages, 4 figures. Minor fixes in the Appendi

On the Extraction of the Strong Coupling from Hadronic Tau Decay

The finite energy sum rule extraction of the strong coupling {\alpha}_s from hadronic {\tau} decay data provides one of its most precise experimental determinations. As precision improves, small non-perturbative effects become increasingly relevant to both the central value and error. Here we present a new framework for this extraction employing a physically motivated model to accommodate violations of quark-hadron duality and enforcing a consistent treatment of higher-dimension operator product expansion contributions. Using 1998 OPAL data for the non-strange vector and axial-vector spectral functions, we find the n_f=3 values for {\alpha}_s(m_{\tau}^2) of 0.307(19) for fixed-order perturbation theory and 0.322(26) for contour-improved perturbation theory, corresponding to n_f=5 values for {\alpha}_s(M_Z^2) of 0.1169(25) and 0.1187(32), respectively.Comment: 7 pages, contribution to Lattice 201

The hadronic light by light contribution to the (g−2)μ(g-2)_{\mu} with holographic models of QCD

We study the anomalous electromagnetic pion form factor $F_{\pi^0\gamma^*\gamma^*}$ with a set of holographic models. By comparing with the measured value of the linear slope, some of these models can be ruled out. From the remaining models we obtain predictions for the low-energy quadratic slope parameters of $F_{\pi^0\gamma^*\gamma^*}$, currently out of experimental reach but testable in the near future. We find it particularly useful to encode this low-energy information in a form factor able to satisfy also QCD short-distance constraints. We choose the form factor introduced by D'Ambrosio, Isidori and Portoles in kaon decays, which has the right short distance for a particular value of the quadratic slope, which is later shown to be compatible with our holographic predictions. We then turn to a determination of the (dominant) pion exchange diagram in the hadronic light by light scattering contribution to the muon anomalous magnetic moment. We quantify the theoretical uncertainty in $(g-2)_{\mu}$ coming from the different input we use: QCD short distances, experimental input and low-energy holographic predictions. We also test the pion-pole approximation. Our final result is $a_{\mu}^{\pi^0}=6.54(25)\cdot 10^{-10}$, where the error is driven by the linear slope of $F_{\pi^0\gamma^*\gamma^*}$, soon to be measured with precision at KLOE-2. Our numerical analysis also indicates that large values of the magnetic susceptibility $\chi_0$ are disfavored, therefore pointing at a mild effect from the pion off-shellness. However, in the absence of stronger bounds on $\chi_0$, an additional $(10-15)$ systematic uncertainty on the previous value for $a_\mu^{\pi^0}$ cannot be excluded.Comment: 20 pages, 5 figures. Substantial improvements throughout the text to match the published version. Enhanced discussion of the analysis in Section IV with the addition of two appendices. Conclusions unchange

Towards understanding Regge trajectories in holographic QCD

We reassess a work done by Migdal on the spectrum of low-energy vector mesons in QCD in the light of the AdS-QCD correspondence. Recently, a tantalizing parallelism was suggested between Migdal's work and a family of holographic duals of QCD. Despite the intriguing similarities, both approaches face a major drawback: the spectrum is in conflict with well-tested Regge scaling. However, it has recently been shown that holographic duals can be modified to accomodate Regge behavior. Therefore, it is interesting to understand whether Regge behavior can also be achieved in Migdal's approach. In this paper we investigate this issue. We find that Migdal's approach, which is based on a modified Pade approximant, is closely related to the issue of quark-hadron duality breakdown in QCD.Comment: 17 pages, 1 figure. Typos fixed, references added, improved discussion. Minor changes to match the journal versio

Novel patterns for vector mesons from the large-Nc limit

We report on a relation between the decay constants of \rho-like J^{PC}=1^{--} vector mesons, which arises solely from the perturbative analysis of the VV, TT and VT correlators at order \alpha_s^0 in the large-N_c limit. We find f_{V}^T/f_{V}=1/\sqrt{2} for highly excited states together with a pattern of alternation in sign. Quite remarkably, recent lattice determinations reported f_{\rho}^T/f_{\rho}=0.72(2), in excellent agreement with our large-N_c result. This seems to suggest a pattern like f_{Vn}^T/f_{Vn}=(-1)^n/\sqrt{2} for the whole (1^{--}) states. In order to test this conjecture in real QCD we construct a set of spectral sum rules, which turn out to comply nicely with this scenario.Comment: 7 page