arXiv subjects

G. Valencia

Publications and source records attributed to G. Valencia.

At least 37 records · Page 2Linked to original sources

arXiv2004

K+ -> pi+ nu nu(bar) and FCNC from non-universal Z' bosons

Motivated by the E787 and E949 result for K+ -> pi+ nu nu(bar) we examine the effects of a new non-universal right-handed Z' boson on flavor changing processes. We place bounds on the tree-level FCNC from K-K(bar) and B-B(bar) mixing as well as from the observed CP violation in kaon decay. We discuss the implications for K -> pi nu nu(bar), B -> X nu nu(bar) and B -> tau+ tau-. We find that existing bounds allow substantial enhancements in the K+ -> pi+ nu nu(bar) rate, particularly through a new one-loop Z' penguin operator.

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arXiv2004

Hadron Collider Signatures for New Interactions of Top and Bottom Quarks

One of the main goals for hadron colliders is the study of the properties of the third generation quarks. We study the signatures for new TeV resonances that couple to top or bottom quarks both at the Tevatron Run II and at the LHC. We find that in the simplest production processes of Drell-Yan type at the Tevatron, the signals are overwhelmed by QCD backgrounds. We also find that it is possible to study these resonances when they are produced in association with a pair of heavy quarks or in association with a single top at the LHC.In particular, with an integrated luminosity of 300 fb$^{-1}$ at the LHC, it is possible to probe resonance masses up to around 2 TeV.

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arXiv2003

TeV resonances in top physics at the LHC

We consider the possibility of studying novel particles at the TeV scale with enhanced couplings to the top quark via top quark pair production at the LHC and VLHC. In particular we discuss the case of neutral scalar and vector resonances associated with a strongly interacting electroweak symmetry breaking sector. We constrain the couplings of these resonances by imposing appropriate partial wave unitarity conditions and known low energy constraints. We evaluate the new physics signals via WW -> tt~ for various models without making approximation for the initial state W bosons, and optimize the acceptance cuts for the signal observation. We conclude that QCD backgrounds overwhelm the signals in both the LHC and a 200 TeV VLHC, making it impossible to study this type of physics in the tt~ channel at those machines.

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arXiv2003

$A^b_{FB}$ and $R_b$ at LEP and New Right-Handed Gauge Bosons

We explore models with additional right-handed gauge bosons that couple predominantly to the third generation in the context of $b\bar{b}$ production at LEP. In particular we investigate potential new contributions to $δg_{Rb}$ that are needed if the measured $A^b_{FB}$ at the $Z$ peak is interpreted as a signal of new physics. We identify two sources of large $δg_{Rb}$ corrections: $Z-Z^\prime$ mixing at tree-level, and one-loop effects from a new $SU_R(2)$ triplet of gauge bosons. We find that the latter can contribute to $δg_{Rb}$ at the 1%-level. We place bounds on the mass of the additional $Z^\prime$ gauge boson that occurs in these models using the $R_b$ measurements from LEP-II. We find that even in cases where the $Z^\prime$ couples almost exclusively to the $b$ and $t$-quarks, masses lighter than about 500 GeV are already excluded.

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arXiv2003

CP Violation in Hyperon Nonleptonic Decays within the Standard Model

We calculate the CP-violating asymmetries A(Lambda_-^0) and A(Xi_-^-) in nonleptonic hyperon decay within the Standard Model using the framework of heavy-baryon chiral perturbation theory (chiPT). We identify those terms that correspond to previous calculations and discover several errors in the existing literature. We present a new result for the lowest-order (in chiPT) contribution of the penguin operator to these asymmetries, as well as an estimate for the uncertainty of our result that is based on the calculation of the leading nonanalytic corrections.

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arXiv2002

CP Violating Rate Difference Relations for $B\to PP$ and $B \to PV$ in Broken SU(3)

Within the standard model there exist certain relations between CP violating rate differences in B decays in the SU(3) limit. We study SU(3) breaking corrections to these relations in the case of charmless, hadronic, two body $B$ decays using the improved factorization model of Ref.\cite{3}. We consider the cases $B \to PP$ and $B \to PV$ for both $B_d$ and $B_s$ mesons. We present an estimate for $A_{CP}(π^- π^+)$ in terms of $A_{CP}(K^- π^+)$.

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arXiv2002

Vector-meson contributions do not explain the rate and spectrum in K_L -> pi0 gamma gamma

We analyze the recent NA48 data for the reaction K_L -> pi0 gamma gamma with and without the assumption of vector meson dominance (VMD). We find that the data is well described by a three-parameter expression inspired by O(p^6) chiral perturbation theory. We also find that it is impossible to fit the shape of the decay distribution and the overall rate simultaneously if one imposes the VMD constraints on the three parameters. We comment on the different fits and their implications for the CP-conserving component of the decay K_L -> pi0 e+ e-.

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arXiv2002

The $Z \to b \bar{b}$ Decay Asymmetry and Left-Right Models

It has been pointed out recently by Chanowitz that the $Z \to b\bar{b}$ decay asymmetry poses a problem for the standard model whether it is genuine or not [Chanowitz:2001bv]. If this conflict is interpreted as new physics in the b-quark couplings, it suggests a rather large right handed coupling of the b-quark to the Z-boson. We show that it is possible to accommodate this result in left-right models that single out the third generation.

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arXiv2002

Probing Intrinsic Charm with Semileptonic B Decays

We discuss semileptonic B decays of the form B -> J/Psi e nu X as possible probes of intrinsic charm. We calculate the leading order perturbative contribution to the process B- -> J/Psi e- nu_e X and find it to be unobservably small, with a branching ratio ~ 10^-10. We propose a modified spectator model to estimate the intrinsic charm contribution and find that it can be significantly larger, with a branching ratio for B -> (c cbar) e- nu_e X as large as 5 X 10^-7. We show that the process could be observed at these levels by CDF assuming a Run II integrated luminosity of 15 fb^-1, making this a useful reaction to probe the idea of intrinsic charm.

hep-ph↗

arXiv2001

K_L -> pi0 gamma gamma and the bound on the CP-conserving K_L -> pi0 e+e-

It has been known for many years that there is a CP-conserving component for the decay mode K_L -> pi0 e+e- and that its magnitude can be obtained from a measurement of the amplitudes in the K_L -> pi0 gamma gamma decay mode. We point out that the usual description of the latter in terms of a single parameter, a_V, is not sufficient to extract the former in a model independent manner. We further show that there exist known physics contributions to K_L -> pi0 gamma gamma that cannot be described in terms of the single parameter a_V. We conclude that a model independent analysis requires the experimental extraction of three parameters.

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arXiv2001

Watson's theorem and electromagnetism in K -> pi pi decay

We consider what constraints unitarity and CPT invariance yield on the strong and electromagnetic phases entering K -> pi pi decay. In particular, we show that the relative size of the electromagnetically-induced changes in the I=0 and I=2 phase shifts in the two--pion final state do not depend on the explicit coupling to the pi^+ pi^- gamma channel. This demonstrates that Watson's theorem can be extended to include the presence of electromagnetism. We point out the consequences for the general structure of the K -> pi pi decay amplitudes in the presence of isospin violation.

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arXiv2000

Can the Lambda pi scattering phase shifts be large?

Motivated by the presence of nearby thresholds in other baryon-meson channels with I=1 and S=-1, we investigate whether the Lambda pi scattering phase shifts at a center-of-mass energy equal to the Xi mass could be larger than suggested by lowest-order chiral perturbation theory. Within a coupled-channel K-matrix approach, we find that the S-wave phase shift could be as large as -7 degrees.

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arXiv2000

Reanalysis of CP Violation in K_L -> pi^+ pi^- gamma

We present a reanalysis of direct CP violation in the decay K_L -> pi^+ pi^- gamma. We point out an existing discrepancy between the theoretical and experimental definitions of epsilon'_{+-gamma}. Adopting the experimental definition of epsilon'_{+-gamma}, we estimate that epsilon'_{+-gamma}/epsilon could be as large as a few times 10^{-4} both within the standard model and beyond. We discuss these estimates in detail and we also show how a judicious choice of E_gamma cuts can increase the sensitivity of the observable epsilon'_{+-γ} to the underlying CP violation.

hep-ph↗

arXiv2000

The Impact of |Delta I|=5/2 Transitions in K-> pi pi Decays

We consider the impact of isospin violation on the analysis of K-> pi pi decays. We scrutinize, in particular, the phenomenological role played by the additional weak amplitude, of |Delta I|=5/2 in character, incurred by the presence of isospin violation. We show that Watson's theorem is appropriate in O(m_d-m_u), so that the inferred pi-pi phase shift at sqrt{s}=m_K determines the strong phase difference between the I=0 and I=2 amplitudes in K-> pi pi decay. We find the magnitude of the |Delta I|=5/2 amplitude thus implied by the empirical branching ratios to be larger than expected from estimates of isospin-violating strong and electromagnetic effects. We effect a new determination of the octet and 27-plet coupling constants with strong-interaction isospin violation and with electromagnetic effects, as computed by Cirigliano, Donoghue, and Golowich, and find that we are unable to resolve the difficulty. Exploring the role of |Delta I|=5/2 transitions in the CP-violating observable epsilon'/epsilon, we determine that the presence of a |Delta I|=5/2 amplitude impacts the empirical determination of omega, the ratio of the real parts of the |Delta I|=3/2 to |Delta I|=1/2 amplitudes, and that it generates a decrease in the estimation of epsilon'/epsilon.

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arXiv2000

CP Violation in $Λ$ to p $π^-$: SM vs New Physics

I discuss CP violation in $Λ$ to p $π^-$ comparing the standard model expectations with what could happen in new physics scenarios. I point out that Fermilab experiment E871 is sensitive to some of these scenarios.

hep-ph↗

arXiv2000

Top-quark couplings to TeV resonances at future lepton colliders

We study the processes $W_L W_L \to t \bar t$ and $W_L Z_L \to t\bar b (\bar t b)$ at future lepton colliders as probes of the couplings of the top quark to resonances at the TeV scale. We consider the cases in which the dominant low energy feature of a strongly interacting electroweak symmetry breaking sector is either a scalar or a vector resonance with mass near 1 TeV. We find that future lepton colliders with high energy and high luminosity have great potential to sensitively probe these physics scenarios. In particular, at a 1.5 TeV linear collider with an integrated luminosity of 200 fb$^{-1}$, we expect about 120 events for either a scalar or a vector to decay to $t\bar t, t \bar b$. Their leading partial decay widths, which characterize the coupling strengths, can be statistically determined to about 10% level.

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arXiv1999

Chiral Perturbation Theory for |Delta I|=3/2 Hyperon Decays

We study the |Delta I|=3/2 amplitudes of hyperon non-leptonic decays of the form B->B'+pi in the context of chiral perturbation theory. The lowest-order predictions are determined in terms of only one unknown parameter and are consistent within errors with current data. We investigate the theoretical uncertainty of these predictions by calculating the leading non-analytic corrections. We also present an estimate for the size of the S-wave Lambda and Cascade decays which vanish at leading order. We find that the corrections to the lowest-order predictions are within the expectations of naive power counting and, therefore, that this picture can be tested more accurately with improved measurements.

hep-ph↗

arXiv1999

CP Violation in Hyperon Decays from Supersymmetry

It was pointed out recently that supersymmetry can generate flavor-changing gluonic dipole operators with sufficiently large coefficients to dominate the observed value of epsilon'/epsilon. We point out that the same operators contribute to direct CP violation in hyperon decay and can generate a CP violating asymmetry A(Λ^0_-) in the range probed by the current E871 experiment. Interestingly, models that naturally reproduce the relation lambda = sqrt{m_d/m_s} do not generate epsilon'/epsilon but could lead to an A(Λ^0_-) of O(10^{-3}).

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