QCD Resonances Research Articles

Signals of the QCD axion with mass of 17 MeV/c2 : Nuclear transitions and light meson decays

The QCD axion remains experimentally viable in the mass range of O(10 MeV) if (i) it couples predominantly to the first generation of SM fermions; (ii) it decays to $e^+ e^-$ with a short lifetime $\tau_a\lesssim 10^{-13}\,$s; and (iii) it has suppressed isovector couplings, i.e., if it is piophobic. Remarkably, these are precisely the properties required to explain recently observed anomalies in nuclear de-excitations, to wit: the $e^+e^-$ emission spectra of isoscalar magnetic transitions of $^{8\!}$Be and $^{4\!}$He nuclei showed a "bump-like" feature peaked at $m_{e^+e^-}\sim 17$ MeV. In this article, we argue that on-shell emission of the QCD axion (with the aforementioned properties) provides an extremely well-motivated, compatible explanation for the observed excesses in these nuclear de-excitations. The absence of anomalous features in other measured transitions is also naturally explained: piophobic axion emission is strongly suppressed in isovector magnetic transitions, and forbidden in electric transitions. This QCD axion hypothesis is further corroborated by an independent observation: a $2-3\,\sigma$ deviation in the measurement of $\Gamma(\pi^0\to e^+e^-)$ from the Standard Model theoretical expectation. This article also includes detailed estimations of various axionic signatures in rare light meson decays, which take into account contributions from low-lying QCD resonance exchange, and, in the case of rare Kaon decays, the possible effective implementations of $\Delta S=1$ octet enhancement in chiral perturbation theory. These inherent uncertainties of the effective description of the strong interactions at low energies result in large variations in the predictions for hadronic signals of the QCD axion; in spite of this, the estimated ranges for rare meson decay rates obtained here can be probed in the near future in $\eta/\eta^\prime$ and Kaon factories.

Glueballs and deconfinement temperature in AdS/QCD

We put forward an alternative way to estimate the deconfinement temperature in bottom-up holographic models. The deconfinement in AdS/QCD is related to the Hawking-Page phase transition, in which the critical Hawking temperature is identified with the deconfinement one in the gauge theory. The numeric estimation of the latter is only possible when the parameters of a $5D$ dual model are previously determined from consistency with other physical aspects, standardly, providing a description of the QCD resonances. The traditional way to fix parameters in the simplest AdS/QCD models is to reproduce the mass of the $\rho$ meson or the slope of the approximate radial Regge trajectory of the $\rho$ excitations. Motivated by a general idea that the slope value originates in gluodynamics, we propose calculating the deconfinement temperature using the trajectory of scalar glueballs. We consider several holographic models and use the recent idea of isospectral potentials to make an additional check of the relevance of our approach. It is demonstrated that different models from an isospectral family (i.e. the models leading to identical predictions for spectrum of hadrons with fixed quantum numbers) result in different predictions for the deconfinement temperature. This difference is found to be quite small in the scalar glueball channel but very large in the vector meson channel. The observed stability in the former case clearly favours the choice of the glueball channel for thermodynamic predictions in AdS/QCD models. For a balanced approach, we argue that either assuming $f_0(1500)$ to have a dominating component of $0^{++}$ glueball or accepting the idea of the universality in the radial Regge trajectories of light non-strange vector mesons one can reproduce the results for the deconfinement temperature obtained before in the lattice simulations in the background of non-dynamical quarks.

θ-dependence of the lightest meson resonances in QCD

We derive the pion mass and the elastic pion-pion scattering amplitude in the QCD theta-vacuum up to next-to-leading order in chiral perturbation theory. Using the modified inverse amplitude method, we study the theta-dependence of the mass and width of the light scalar meson sigma(500) and the vector meson rho(770).

The h → 4ℓ spectrum at low m34: Standard Model vs. light New Physics

We analyze h→4ℓ decays in the kinematical region where the dilepton pair of low invariant mass (m34) is not far from QCD resonances (Ψ and ϒ states). On the one hand, we present precise predictions of the spectrum within the Standard Model, taking into account non-perturbative QCD effects. On the other hand, we illustrate the sensitivity of this spectrum to New Physics models predicting the existence of new exotic light states. In particular, we show that parameter regions of models relevant to explain the (g−2)μ anomaly could be probed in the future by means of h→4ℓ decays.

The AdS/CFT Correspondence and Holographic QCD

Holographic QCD is an extra-dimensional approach to modeling QCD resonances and their interactions. Holographic models encode information about chiral symmetry breaking, Weinberg sum rules, vector meson dominance, and other phenomenological features of QCD. There are two complementary approaches to holographic model building: a top–down approach which begins with string-theory brane configurations, and a bottom–up approach which is more phenomenological. In this talk I will describe the AdS/CFT correspondence, which motivates Holographic QCD, and the techniques used to build holographic models of QCD and to calculate observables in those models. I will also discuss an intriguing lightcone approach to Holographic QCD discovered by Brodsky and De Teramond.

CHIRAL SYMMETRY BREAKING IN A SOFT-WALL MODEL OF AdS/QCD

We incorporate chiral symmetry breaking in a soft-wall version of the AdS/QCD model by using a modified dilaton profile and a quartic term in the bulk scalar potential. This allows one to separate the dependence on spontaneous and explicit chiral symmetry breaking. The resulting mass spectra in the scalar, vector and axial-vector sectors compares favorably with the respective QCD resonances.

Strong Electroweak Symmetry Breaking and Spin-0 Resonances

We argue that theories of the strong electroweak symmetry breaking sector necessarily contain new spin 0 states at the TeV scale in the tt and tb/bt channels, even if the third generation quarks are not composite at the TeV scale. These states couple sufficiently strongly to third generation quarks to have significant production at LHC via gg --> phi(0) or gb --> tphi(-). The existence of narrow resonances in QCD suggests that the strong electroweak breaking sector contains narrow resonances that decay to tt or tb/bt, with potentially significant branching fractions to 3 or more longitudinal W and Z bosons. These may give new "smoking gun" signals of strong electroweak symmetry breaking.

Chiral symmetry breaking in the soft-wall AdS/QCD model

We show how to incorporate chiral symmetry breaking in the soft-wall version of the AdS/QCD model by using a modified dilaton profile and a quartic term in the bulk scalar potential. This allows one to separate the dependence on spontaneous and explicit chiral symmetry breaking. Moreover our 5D model automatically incorporates linear trajectories and non chiral-symmetry restoration for highly excited radial states. We compare our resulting mass spectra in the scalar, vector and axial-vector sectors with the respective QCD resonances and find reasonable agreement using the known values for the pion mass and decay constant.

σ→γγWidth from Nucleon Electromagnetic Polarizabilities

The lightest QCD resonance, the sigma, has been recently fixed in the pipi scattering amplitude. The nature of this state remains nowadays one of the most intriguing and difficult issues in particle physics. Its coupling to photons is crucial for discriminating its structure. We propose a new method that fixes this coupling using only available precise experimental data on the proton electromagnetic polarizabilities together with analyticity and unitarity. By taking into account the uncertainties in the analysis and in the parameter values, our result is Gamma{pole}(sigma-->gammagamma)=1.2+/-0.4 keV.

Scattering and resonances in QCD 2

Extending previous works on the spectrum of QCD 2, we now investigate the 2D analogue of meson-baryon scattering. We use semi-classical methods, perturbing around classical soliton solutions. In the case of one flavor, we find that the effective potential is reflectionless. In the case of several flavors, the method yields a potential which depends on the momentum of the incoming particle. In this case there is both transmission and reflection. In both cases no resonances appear.

Scattering and resonances in QCD 2

Extending previous works on the spectrum of QCD 2, we now investigate the 2D analogue of meson–baryon scattering. We use semi-classical methods, perturbing around classical soliton solutions. We start with the Abelian case, corresponding to one flavor, and find that in this case the effective potential is reflectionless. We obtain an explicit expression for the forward phase shift. In the non-Abelian case of several flavors, the method yields a potential which depends on the momentum of the incoming particle. In this case there is both transmission and reflection. In both cases no resonances appear. As a byproduct, we derive the general conditions for a 2D scalar quantum field theoretical action to yield a reflectionless effective potential when one expands in small fluctuations about the classical solution.

Mass and width determination of light-quark baryon resonances in QCD.

The spectrum of the proton and its excited states has been investigated in a finite-energy QCD sum rule approach using a Gauss-Weierstrass transform. The imaginary part of the polarization operator of the proton current has been saturated by the proton and two of its lowest-lying excited states. In the evaluation of the polarization operator using quark and gluon fields, perturbative corrections have also been incorporated. By taking various moments of the spectral function, a total of eight sum rules have been obtained. The masses of the two lowest-lying excited states of the proton, their widths, and the coupling constant, of the proton current with these states have been evaluated. In addition, contributions of all the excited states to the two-point function at zero momentum have also been determined. The values of the masses and widths obtained are in reasonable accord with their experimental values.

RESONANCE PRODUCTION IN A STRONGLY INTERACTING HIGGS SECTOR

We compute the cross section for the production of resonances in the strongly interacting Higgs sector analogous to the f2 and a1 resonances in QCD via gauge boson fusion processes in pp colliders at [Formula: see text] and 40 TeV. Their contribution to signatures of this scenario is briefly discussed.

The application of finite-energy sum rules for the description of the meson resonances in QCD

The mass spectrum and the leptonic decay widths of the meson resonances in QCD are calculated on the basis of the use of finite-energy sum rules and the 1/ N-expansion. The obtained results are in good agreement with experiment, in particular we derive the mass formulas for the ϱ-meson A 1-meson excitations m 2 ϱ ( n)= m 2 ϱ (1+2 n), m 2 A 1 ( n)= m 2 A 1 ( n+1)( n = 0, 1, 2, ...).