arXiv subjects

G. Valencia

Publications and source records attributed to G. Valencia.

At least 19 recordsLinked to original sources

arXiv2023

Workshop summary -- Kaons@CERN 2023

Kaon physics is at a turning point -- while the rare-kaon experiments NA62 and KOTO are in full swing, the end of their lifetime is approaching and the future experimental landscape needs to be defined. With HIKE, KOTO-II and LHCb-Phase-II on the table and under scrutiny, it is a very good moment in time to take stock and contemplate about the opportunities these experiments and theoretical developments provide for particle physics in the coming decade and beyond. This paper provides a compact summary of talks and discussions from the Kaons@CERN 2023 workshop.

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arXiv2021

Constructing CP-Odd Observables

In these lectures we review the general features necessary to construct $\cp$ odd observables and study illustrative examples. We present in some detail the case of $\cp$ violation in hyperon decays. We survey different observables sensitive to $\cp$ violating new physics, concentrating on the search for electric-dipole moments in high energy experiments.

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arXiv2010

Flavor changing Z' couplings at the LHC

Models with a non-universal Z' exhibit in general flavor changing neutral currents (FCNC) at tree-level. When the Z' couplings favor the third generation, flavor changing transitions of the form Z'tc and Z'tu could be large enough to be observable at the LHC. In this paper we explore this possibility using the associated production of a single top-quark with the Z' and find that integrated luminosities of a few hundred fb$^{-1}$ are necessary to probe the interesting region of parameter space.

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arXiv2010

CP-odd correlations using jet momenta from tt_bar events at the Tevatron

We discuss T-odd correlations between jet and lepton momenta in tt_bar events at the Tevatron that can be used to search for CP violation. We identify correlations suitable for the lepton plus jets and purely hadronic top-quark pair decay channels. As an example of CP violation we consider the top-quark anomalous couplings, including its chromo-electric dipole moment, and we estimate the limits that can be placed at the Tevatron.

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arXiv2009

Penguin and Box Diagrams in Unitary Gauge

We evaluate one-loop diagrams in the unitary gauge that contribute to flavor-changing neutral current (FCNC) transitions involving two and four fermions. Specifically, we deal with penguin and box diagrams arising within the standard model (SM) and in nonrenormalizable extensions thereof with anomalous couplings of the W boson to quarks. We show explicitly in the SM the subtle cancelation among divergences from individual unitary-gauge contributions to some of the physical FCNC amplitudes and derive expressions consistent with those obtained using R_xi gauges in the literature. Some of our results can be used more generally in certain models involving fermions and gauge bosons which have interactions similar in form to those we consider.

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arXiv2009

Probing New Physics in Charm Couplings with FCNC

Low-energy experiments involving kaon, B-meson, D-meson, and hyperon flavor-changing neutral transitions have confirmed the loop-induced flavor-changing neutral current (FCNC) picture of the standard model (SM). The continuing study of these processes is essential to further refine this picture and ultimately understand the flavor dynamics. In this paper we consider deviations from the SM in the charm sector and their effect on FCNC processes. Specifically, we parameterize new physics in terms of left- and right-handed anomalous couplings of the W boson to the charm quark. We present a comprehensive study of existing constraints and point out those measurements that are most sensitive to new physics of this type.

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arXiv2009

CP violating anomalous top-quark couplings at the LHC

We study the T odd correlations induced by CP violating anomalous top-quark couplings at both production and decay level in the process gg --> t t_bar --> (b mu+ nu_mu) (b_bar mu- nu_mu_bar). We consider several counting asymmetries at the parton level and find the ones with the most sensitivity to each of these anomalous couplings at the LHC.

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arXiv2009

T-odd correlations from CP violating anomalous top-quark couplings revisited

We revisit the effect of CP violating anomalous top-quark couplings in $t\bar{t}$ production and decay. We consider $t\bar{t}$ production through gluon fusion (and light $q{\bar q}$ annihilation) followed by top-quark decay into $bW$ or $b\ellν$. We find explicit analytic expressions for all the triple products generated by the anomalous couplings that fully incorporate all spin correlations. Our results serve as a starting point for numerical simulations for the LHC.

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arXiv2008

Sigma to p mu+ mu-: Standard Model or New Particle?

The HyperCP collaboration observed three events for the decay Sigma to p mu+ mu-. They suggested that new physics may be required to understand the implied decay rate and the observed dimuon invariant mass distribution. Motivated by this result, we re-examine this mode. First within the standard model, and then assuming there is a new particle. Within the SM we find that this mode is long-distance dominated and its rate falls within the range suggested by the HyperCP measurement. We then examine the conditions under which the observation is consistent with a light Higgs boson and find an explicit example that satisfies all the constraints: the light pseudoscalar Higgs boson in the next-to-minimal supersymmetric standard model (NMSSM).

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arXiv2008

Light Higgs Production in Hyperon Decay

A recent HyperCP observation of three events in the decay Sigma^+ -> p mu^+ mu^- is suggestive of a new particle with mass 214.3 MeV. In order to confront models that contain a light Higgs boson with this observation, it is necessary to know the Higgs production rate in hyperon decay. The contribution to this rate from penguin-like two-quark operators has been considered before and found to be too large. We point out that there are additional four-quark contributions to this rate that could be comparable in size with the two-quark contributions, and that could bring the total rate to the observed level in some models. To this effect we implement the low-energy theorems that dictate the couplings of light Higgs bosons to hyperons at leading order in chiral perturbation theory. We consider the cases of scalar and pseudoscalar Higgs bosons in the standard model and in its two-Higgs-doublet extensions to illustrate the challenges posed by existing experimental constraints and suggest possible avenues for models to satisfy them.

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arXiv2008

Rare Decays with a Light CP-Odd Higgs Boson in the NMSSM

We have previously proposed a light pseudoscalar Higgs boson in the next-to-minimal supersymmetric standard model (NMSSM), the A_1^0, as a candidate to explain the HyperCP observations in Sigma^+ -> p mu^+ mu^-. In this paper we calculate the rates for several other rare decay modes that can help confirm or refute this hypothesis. The first modes we evaluate are K_L -> pi pi A_1^0, which are interesting because they are under study by the KTeV Collaboration. We next turn to eta -> pi pi A_1^0, which are interesting because they are independent of the details of the flavor-changing sector of the NMSSM and may be accessible at DAPhNE. For completeness, we also evaluate Omega^- -> Xi^- A_1^0.

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arXiv2007

The Decay Omega^- -> Xi^- pi^+ pi^- in Chiral Perturbation Theory

We study the decay Omega^- -> Xi^- pi^+ pi^- in heavy-baryon chiral perturbation theory. At leading order, the decay is completely dominated by the Xi^{*0}(1530) intermediate state, and the predicted rate and Xi^-pi^+ mass distribution are in conflict with currently available data. It is possible to resolve this conflict by considering additional contributions at next-to-leading order.

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arXiv2007

The role of $D^{\star\star}$ in $B^-\to D_s^+ K^- π^-$

The BaBar collaboration has recently reported the observation of the decay mode $B^-\to D_s^+ K^- π^-$. We investigate the role played by the $D^{\star\star}$ resonances in this decay mode using HQET. Although these resonances cannot appear as physical intermediate states in this reaction, their mass is very close to the $D_s^+ K^-$ production threshold and may, therefore, play a prominent role. We pursue this possibility to extract information on the properties of the strong $D^{\star\star} D M$ couplings. As a byproduct of this analysis we point out that future super-$B$ factories may be able to measure the $D_0^0 D^\star γ$ radiative coupling through the reaction $B^-\to D^\star γπ^-$.

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arXiv2006

Has HyperCP Observed a Light Higgs Boson?

The HyperCP collaboration has observed three events for the decay Sigma^+ -> p mu^+ mu^- which may be interpreted as a new particle of mass 214.3 MeV. However, existing data from kaon and B-meson decays severely constrain this interpretation, and it is nontrivial to construct a model consistent with all the data. In this letter we show that the ``HyperCP particle'' can be identified with the light pseudoscalar Higgs boson in the next-to-minimal supersymmetric standard model, the A_1^0. In this model there are regions of parameter space where the A_1^0 can satisfy all the existing constraints from kaon and B-meson decays and mediate Sigma^+ -> p mu^+ mu^- at a level consistent with the HyperCP observation.

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arXiv2006

Weak Radiative B Decays

We study the (tree-level) weak radiative decays of $B$ mesons. We present a numerical estimate for the inclusive $b \to X_c γ(γ)$ modes based on the free-quark decay. We then review what is known for the $B \to D^\star γ$ modes in the framework of heavy quark effective theory and chiral perturbation theory. Finally, we extend these ideas to the double radiative decay modes $B \to D γγ$. We find that the $b \to X_c γγ$ rate is about an order of magnitude larger than the corresponding $b \to X_s γγ$ rate. We also find the branching ratio for the $B \to D γγ$ mode with most favorable CKM angles at the few $\times10^{-8}$ level, comparable to predictions for $B \to K γγ$.

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arXiv2006

New CP-odd Observable in H -> t \bar{t}

We study a CP-violating triple product correlation that occurs in the decay of a neutral Higgs boson into t\bar{t} pairs when the Higgs boson does not have a definite CP nature. We consider the H -> t \bar{t} decay channel as well as the gluon fusion process gg -> H -> t \bar{t}. The asymmetry in Higgs decay, normalized to the H -> t \bar{t} width, can reach the 6% percent level. In the gluon fusion process the corresponding normalized asymmetry is smaller by an order of magnitude. We present a crude estimate of this observable at the LHC.

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arXiv2005

The Decay Sigma^+ -> p l^+ l^- within the Standard Model

The HyperCP collaboration has recently reported the observation of three events for the decay Sigma^+ -> p mu^+ mu^-. They have suggested that new physics may be required to understand the implied decay rate and the observed M_{mumu} distribution. Motivated by this result, we re-examine this mode within the standard model, considering both the short-distance and long-distance contributions. The long-distance part depends on four complex form-factors. We determine their imaginary parts from unitarity, fix two of the real parts from the Sigma^+ -> p gamma measurements, and estimate the other two with vector-meson-dominance models. Taking into account constraints from Sigma^+ -> p e^+ e^-, we find that Sigma^+ -> p mu^+ mu^- is long-distance dominated and its rate falls within the range suggested by the HyperCP measurement.

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arXiv2004

Lepton flavor violating tau and B decays and heavy neutrinos

We study lepton flavor violating (LFV) tau and B decays in models with heavy neutrinos to constrain the mixing matrix parameters U_{tau N}. We find that the best current constraints when the heavy neutrinos are purely left-handed come from LFV radiative tau decay modes. To obtain competitive constraints in LFV B decay it is necessary to probe b -> X_{s} tau^{\pm} e^{\mp} at the 10^{-7} level. When the heavy neutrinos have both left and right-handed couplings, the mixing parameters can be constrained by studying LFV B decay modes and LFV tau decay into three charged leptons. We find that the branching ratios B(tau^{\pm} -> l_1^{\pm} l_2^{\pm} l_3^{\mp}), B(B_{s} -> tau^{\pm} e^{\mp}) and B(b -> X_{s} l_1^{\pm} l_2^{\mp}) need to be probed at the 10^{-8} level in order to constrain the mixing parameters beyond what is known from unitarity.

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