arXiv subjects

Pedro Ruiz-Femenia

Publications and source records attributed to Pedro Ruiz-Femenia.

At least 19 recordsLinked to original sources

arXiv2020

DsixTools 2.0: The Effective Field Theory Toolkit

$\tt DsixTools$ is a Mathematica package for the handling of the Standard Model Effective Field Theory (SMEFT) and the Low-energy Effective Field Theory (LEFT) with operators up to dimension six, both at the algebraic and numerical level. $\tt DsixTools$ contains a visually accessible and operationally convenient repository of all operators and parameters of the SMEFT and the LEFT. This repository also provides information concerning symmetry categories and number of degrees of freedom, and routines that allow to implement this information on global expressions (such as decay amplitudes and cross-sections). $\tt DsixTools$ also performs weak basis transformations, and implements the full one-loop Renormalization Group Evolution in both EFTs (with SM beta functions up to five loops in QCD), and the full one-loop SMEFT-LEFT matching at the electroweak scale.

hep-ph↗

arXiv2018

Non-resonant and electroweak NNLO correction to the $e^+ e^-$ top anti-top threshold

We determine the NNLO electroweak correction to the $e^+ e^-\to b\bar{b}W^+W^- X$ production cross section near the top-pair production threshold. The calculation includes non-resonant production of the final state as well as electroweak effects in resonant top anti-top pair production with non-relativistic resummation, and elevates the theoretical prediction to NNNLO QCD plus NNLO electroweak accuracy. We then study the impact of the new contributions on the top-pair threshold scan at a future lepton collider.

hep-ph↗

arXiv2016

The last refuge of mixed wino-Higgsino dark matter

We delineate the allowed parameter and mass range for a wino-like dark matter particle containing some Higgsino admixture in the MSSM by analysing the constraints from diffuse gamma-rays from the dwarf spheroidal galaxies, galactic cosmic rays, direct detection and cosmic microwave background anisotropies. A complete calculation of the Sommerfeld effect for the mixed-neutralino case is performed. We find that the combination of direct and indirect searches poses significant restrictions on the thermally produced wino-Higgsino dark matter with correct relic density. For $μ>0$ nearly the entire parameter space considered is excluded, while for $μ<0$ a substantial region is still allowed, provided conservative assumptions on astrophysical uncertainties are adopted.

hep-ph↗

arXiv2016

Integrating out heavy particles with functional methods: a simplified framework

We present a systematic procedure to obtain the one-loop low-energy effective Lagrangian resulting from integrating out the heavy fields of a given ultraviolet theory. We show that the matching coefficients are determined entirely by the hard region of the functional determinant involving the heavy fields. This represents an important simplification with respect the conventional matching approach, where the full and effective theory contributions have to be computed separately and a cancellation of the infrared divergent parts has to take place. We illustrate the method with a descriptive toy model and with an extension of the Standard Model with a heavy real scalar triplet. A comparison with other schemes that have been put forward recently is also provided.

hep-ph↗

arXiv2016

Threshold singularities, dispersion relations and fixed-order perturbative calculations

We show how to correctly treat threshold singularities in fixed-order perturbative calculations of the electron anomalous magnetic moment and hadronic pair production processes such as top pair production. With respect to the former, we demonstrate the equivalence of the "non-perturbative", resummed treatment of the vacuum polarization contribution, whose spectral function exhibits bound state poles, with the fixed-order calculation by identifying a threshold localized term in the four-loop spectral function. In general, we find that a modification of the dispersion relation by threshold subtractions is required to make fixed-order calculations well-defined and provide the subtraction term. We then solve the apparent problem of a divergent convolution of the partonic cross section with the parton luminosity in the computation of the top pair production cross section starting from the fourth-order correction. We find that when the computation is performed in the usual way as an integral of real and virtual corrections over phase space at a given order in the expansion in the strong coupling, an additional contribution has to be added at N3LO.

hep-ph↗

arXiv2015

One-loop corrections to the Higgs electroweak chiral Lagrangian

In this talk we study beyond Standard Model scenarios where the Higgs is non-linearly realized. The one-loop ultraviolet divergences of the low-energy effective theory at next-to-leading order, O(p^4), are computed by means of the background-field method and the heat-kernel expansion. The power counting in non-linear theories shows that these divergences are determined by the leading-order effective Lagrangian L_2. We focus our attention on the most important O(p^4) divergences, which are provided by the loops of Higgs and electroweak Goldstones, as these particle are the only ones that couple through derivatives in L_2. The one-loop divergences are renormalized by O(p^4) effective operators, and set their running. This implies the presence of chiral logarithms in the amplitudes along with the O(p^4) low-energy couplings, which are of a similar importance and should not be neglected in next-to-leading order effective theory calculations, e.g. in composite scenarios.

hep-ph↗

arXiv2015

On the minimality of the order p^6 chiral Lagrangian

A method to find relations between the operators in the mesonic Lagrangian of Chiral Perturbation Theory at order p^6 is presented. The procedure can be used to establish if the basis of operators in the Lagrangian is minimal. As an example, we apply the method to the two-flavour case in the absence of scalar and pseudo-scalar sources (s=p=0), and conclude that the minimal Lagrangian contains 27 independent operators.

hep-ph↗

arXiv2015

One-loop renormalization of the electroweak chiral Lagrangian with a light Higgs

We consider the general chiral effective action which parametrizes the nonlinear realization of the spontaneous breaking of the electroweak symmetry with a light Higgs, and compute the one-loop ultraviolet divergences coming from Higgs and electroweak Goldstone fluctuations using the background field method. The renormalization of the divergences is carried out through operators of next-to-leading order in the chiral counting, i.e. of O(p^4). Being of the same order in power counting, the logarithmic corrections linked to these divergences can be as important as the tree-level contributions from the O(p^4) operators, and must be accounted for in the phenomenological analysis of experimental data. Deviations in the O(p^2) (leading-order) couplings with respect to the Standard Model values, e.g., in the h->WW coupling, would generate contributions from the 1-loop chiral logarithms computed in this work to a vast variety of observables, which do not have a counterpart in the conventional electroweak effective theory with a linearly transforming Higgs complex doublet.

hep-ph↗

arXiv2014

A first estimate of the NNLO nonresonant corrections to top-antitop threshold production at lepton colliders

We compute the dominant term in the expansion in rho=1-M_W/m_t of the unknown next-to-next-to-leading order (NNLO) nonresonant contributions to the e+ e- -> W+ W- b bbar total cross section at energies close to the top-antitop threshold. Our analytic result disagrees with a previous calculation by other authors [1]. We show that our determination has the correct infrared structure needed to cancel the divergences proportional to the top width arising in the resonant production of the same final state, and we point out to a missing contribution in the computation of [1] to explain the discrepancy.

hep-ph↗

arXiv2012

Renormalization Group Improved Bottom Mass from Upsilon Sum Rules at NNLL Order

We determine the bottom quark mass from non-relativistic large-n Upsilon sum rules with renormalization group improvement at next-to-next-to-leading logarithmic order. We compute the theoretical moments within the vNRQCD formalism and account for the summation of powers of the Coulomb singularities as well as of logarithmic terms proportional to powers of alpha_s ln(n). The renormalization group improvement leads to a substantial stabilization of the theoretical moments compared to previous fixed-order analyses, which did not account for the systematic treatment of the logarithmic alpha_s ln(n) terms, and allows for reliable single moment fits. For the current world average of the strong coupling (alpha_s(M_Z) = 0.1183 +- 0.0010) we obtain M_b^{1S}=4.755 +- 0.057(pert) +- 0.009(alpha_s) +- 0.003(exp) GeV for the bottom 1S mass and m_b(m_b)= 4.235 +- 0.055(pert) +- 0.003(exp) GeV for the bottom MSbar mass, where we have quoted the perturbative error and the uncertainties from the strong coupling and the experimental data.

hep-ph↗

arXiv2012

Zeros of the W_L Z_L -> W_L Z_L amplitude: where vector resonances stand

A Higgsless electroweak theory may be populated by spin-1 resonances around E ~ 1TeV as a consequence of a new strong interacting sector, frequently proposed as a tool to smear the high-energy behaviour of scattering amplitudes, for instance, elastic gauge boson scattering. Information on those resonances, if they exist, must be contained in the low-energy couplings of the electroweak chiral effective theory. Using the facts that: i) the scattering of longitudinal gauge bosons, W_L, Z_L, can be well described in the high-energy region E >> M_W by the scattering of the corresponding Goldstone bosons (equivalence theorem) and ii) the zeros of the scattering amplitude carry the information on the heavier spectrum that has been integrated out; we employ the O(p^4) electroweak chiral Lagrangian to identify the parameter space region of the low-energy couplings where vector resonances may arise. An estimate of their masses is also provided by our method.

hep-ph↗

arXiv2012

Non-resonant effects in the top-antitop resonance region

The recent developments on the computation of non-resonant corrections to the e+ e- -> W+ W- b bbar cross section in the top-antitop resonance region are reviewed in this talk. Non-resonant production of the final state W+ W- b bbar starts to contribute at NLO in the nonrelativistic power-counting v ~ alpha_s ~ sqrt(alpha_EW). The corrections induced by non-resonant effects reduce the cross section in the top-antitop resonance peak region at a level which is comparable to the expected experimental precision at the future linear collider, and are thus relevant for a high-precision top mass and width determination.

hep-ph↗

arXiv2010

Phase Space Matching and Finite Lifetime Effects for Top-Pair Production Close to Threshold

The top-pair $t\bar t$ production cross section close to threshold in $e^+e^-$ collisions is strongly affected by the small lifetime of the top quark. Since the cross section is defined through final states containing the top decay products, a consistent definition of the cross section depends on prescriptions how these final states are accounted for the cross section. Experimentally, these prescriptions are implemented for example through cuts on kinematic quantities such as the reconstructed top quark invariant masses. As long as these cuts do not reject final states that can arise from the decay of a top and an anti-top quark with a small off-shellness compatible with the nonrelativistic power-counting, they can be implemented through imaginary phase space matching conditions in NRQCD. The prescription-dependent cross section can then be determined from the optical theorem using the $e^+e^-$ forward scattering amplitude. We compute the phase space matching conditions associated to cuts on the top and anti-top invariant masses at next-to-next-to-leading logarithmic (NNLL) order and partially at next-to-next-to-next-to-leading logarithmic (N${}^3$LL) order in the nonrelativistic expansion and, together with finite lifetime and electroweak effects known from previous work, analyze their numerical impact on the $t\bar t$ cross section. We show that the phase space matching contributions are essential to make reliable NRQCD predictions, particularly for energies below the peak region, where the cross section is small. We find that irreducible background contributions associated to final states that do not come from top decays are strongly suppressed and can be neglected for the theoretical predictions.

hep-ph↗

arXiv2009

Resonance saturation of the chiral couplings at NLO in 1/Nc

The precision obtainable in phenomenological applications of Chiral Perturbation Theory is currently limited by our lack of knowledge on the low-energy constants (LECs). The assumption that the most important contributions to the LECs come from the dynamics of the low-lying resonances, often referred to as the resonance saturation hypothesis, has stimulated the use of large-Nc resonance lagrangians in order to obtain explicit values for the LECs. We study the validity of the resonance saturation assumption at the next-to-leading order in the 1/Nc expansion within the framework of Resonance Chiral Theory (RChT). We find that, by imposing QCD short-distance constraints, the chiral couplings can be written in terms of the resonance masses and couplings and do not depend explicitly on the coefficients of the chiral operators in the Goldstone boson sector of RChT. As we argue, this is the counterpart formulation of the resonance saturation statement in the context of the resonance lagrangian. Going beyond leading order in the 1/Nc counting allow us to keep full control of the renormalization scale dependence of the LEC estimates.

hep-ph↗

arXiv2009

The Upsilon -> X gamma photon spectrum in the soft-energy region

The dominant contribution of the next-to-leading order perturbative QCD corrections to the Upsilon -> X gamma photon energy spectrum for photon energies w ~ m_b alpha_s can be obtained trivially from the knowledge of the corresponding O(alpha) correction to the positronium decay spectrum in the same region. The latter can be obtained from the NRQED computation of the orthopositronium photon spectrum, or by applying the threshold expansion to the 1-loop QED amplitude, as it is explained in this talk.

hep-ph↗

arXiv2008

Implementing invariant mass cuts and finite lifetime effects in top-antitop production at threshold

The effects of the finite top quark width in the top pair production cross section close to the threshold are discussed in this talk. We introduce a $t\bar{t}$ cross section with a cut on the invariant masses of the top and antitop that can be calculated theoretically with effective field theory (EFT) methods. The matching procedure to implement the physical phase-space boundaries in the NRQCD framework (``phase-space matching'') is briefly outlined.

hep-ph↗

arXiv2007

Heavy Pair Production Currents with General Quantum Numbers in Dimensionally Regularized NRQCD

We discuss the form and construction of general color singlet heavy particle-antiparticle pair production currents for arbitrary quantum numbers, and issues related to evanescent spin operators and scheme-dependences in nonrelativistic QCD (NRQCD) in n=3-2epsilon dimensions. The anomalous dimensions of the leading interpolating currents for heavy quark and colored scalar pairs in arbitrary (2S+1)L_J angular-spin states are determined at next-to-leading order in the nonrelativistic power counting.

hep-ph↗

arXiv2007

Vanishing chiral couplings in the large-N_C resonance theory

The construction of a resonance theory involving hadrons requires implementing the information from higher scales into the couplings of the effective Lagrangian. We consider the large-Nc chiral resonance theory incorporating scalars and pseudoscalars, and we find that, by imposing LO short-distance constraints on form factors of QCD currents constructed within this theory, the chiral low-energy constants satisfy resonance saturation at NLO in the 1/Nc expansion.

hep-ph↗