Searcharxiv⌕ Search

arXiv subjects

D. London

Publications and source records attributed to D. London.

36 records · Page 2Linked to original sources
arXiv1999

Profiles of the Unitarity Triangle and CP-Violating Phases in the Standard Model and Supersymmetric Theories

We report on a comparative study of the profile of the CKM unitarity triangle, and the resulting CP asymmetries in B decays, in the standard model and in several variants of the minimal supersymmetric standard model (MSSM), characterized by a single phase in the quark flavour mixing matrix. The supersymmetric contributions to the mass differences ΔM_d, ΔM_s and to the CP-violating quantity |ε| are, to an excellent approximation, equal to each other in these theories, allowing for a particularly simple way of implementing the resulting constraints on the elements of V_{CKM} from the present knowledge of these quantities. Incorporating the next-to-leading-order corrections and applying the current direct and indirect constraints on the supersymmetric parameters, we find that the predicted ranges of \sin 2 βin the standard model and in MSSM models are very similar. However, precise measurements at B-factories and hadron machines may be able to distinguish these theories in terms of the other two CP-violating phases αand γ. This is illustrated for some representative values of the supersymmetric contributions in ΔM_d, ΔM_s and |ε|.

hep-ph↗
arXiv1998

Bilepton Production at Hadron Colliders

We examine, as model-independently as possible, the production of bileptons at hadron colliders. When a particular model is necessary or useful, we choose the 3-3-1 model. We consider a variety of processes: q anti-q -> Y^{++} Y^{--}, u anti-d -> Y^{++} Y^{-}, anti-u d -> Y^+ Y^{--}, q anti-q -> Y^{++} e^{-} e^{-}, q anti-q -> phi^{++} phi^{--}, u anti-d -> -> phi^{++} phi^{-}, and anti-u d -> phi^{+} phi^{--}, where Y and phi are vector and scalar bileptons, respectively. Given the present low-energy constraints, we find that at the Tevatron, vector bileptons are unobservable, while light scalar bileptons (M_phi <= 300 GeV) are just barely observable. At the LHC, the reach is extended considerably: vector bileptons of mass M_Y <= 1 TeV are observable, as are scalar bileptons of mass M_phi <= 850 GeV.

hep-ph↗
arXiv1997

Using B_s^0 Decays to Determine the CP Angles αand γ

Dighe, Gronau and Rosner have shown that, by assuming SU(3) flavor symmetry and first-order SU(3) breaking, it is possible to extract the CP angles αand γfrom measurements of the decay rates of B_d^0(t) --> π^+π^-, B_d^0 --> π^- K^+ and B^+ --> π^+ K^0, along with their charge-conjugate processes. We extend their analysis to include the SU(3)-related decays B_s^0 --> π^+ K^-, B_s^0(t) --> K^+ K^- and B_s^0 --> K^0 {\bar K^0}. There are several advantages to this extension: discrete ambiguities are removed, fewer assumptions are necessary, and the method works even if all strong phases vanish. In addition, we show that γcan be obtained cleanly, with no penguin contamination, by using the two decays B_s^0(t) --> K^+ K^- and B_s^0 --> K^0 {\bar K^0}.

hep-ph↗
arXiv1996

New Physics in CP Asymmetries and Rare B Decays

We review and update the effects of physics beyond the standard model on CP asymmetries in B decays. These asymmetries can be significantly altered if there are important new-physics contributions to \bqbqbar mixing. This same new physics will therefore also contribute to rare, flavor-changing B decays. Through a study of such decays, we show that it is possible to partially distinguish the different models of new physics.

hep-ph↗
arXiv1996

CP Violation and Flavour Mixing in the Standard Model -- 1996 Update

We review and update the constraints on the parameters of the quark flavour mixing matrix $V_{CKM}$ in the standard model and estimate the resulting CP asymmetries in $B$ decays, taking into account recent experimental and theoretical developments. With the updated CKM matrix we present the currently-allowed range of the ratios $|V_{td}/V_{ts}|$ and $|V_{td}/V_{ub}|$, as well as the standard model predictions for the \bsbsbar\ mixing parameter $\xs$ (or, equivalently, $\delms$) and the quantities $\sin 2α$, $\sin 2β$ and $\sin^2γ$, which characterize the CP-asymmetries in $B$-decays.

hep-ph↗
arXiv1996

R_b and New Physics: A Comprehensive Analysis

We survey the implications for new physics of the discrepancy between the LEP measurement of $R_b$ and its Standard Model prediction. Two broad classes of models are considered: ($i$) those in which new $Z\bbar b$ couplings arise at tree level, through $Z$ or $b$-quark mixing with new particles, and ($ii$) those in which new scalars and fermions alter the $Z \bbar b$ vertex at one loop. We keep our analysis as general as possible in order to systematically determine what kinds of features can produce corrections to $R_b$ of the right sign and magnitude. We are able to identify several successful mechanisms, which include most of those which have been recently been proposed in the literature, as well as some earlier proposals (\eg\ supersymmetric models). By seeing how such models appear as special cases of our general treatment we are able to shed light on the reason for, and the robustness of, their ability to explain $R_b$.

hep-ph↗
arXiv1995

Inverse Neutrinoless Double Beta Decay Revisited

We critically reexamine the prospects for the observation of the $ΔL=2$ lepton-number-violating process $\eeWW$ using the $e^-e^-$ option of a high-energy $e^+e^-$ collider (NLC). We find that, except in the most contrived scenarios, constraints from neutrinoless double beta decay render the process unobservable at an NLC of $\sqrt{s}<2$ TeV. Other $ΔL=2$ processes such as $\ggllww$, $\egnllw$, $\eennll$ ($\ell=μ,τ$), and $\egeww$, which use various options of the NLC, require a $\sqrt{s}$ of at least 4 TeV for observability.

hep-ph↗
arXiv1995

CP Violation and Flavour Mixing in the Standard Model

We review and update the constraints on the parameters of the quark flavour mixing matrix $V_{CKM}$ in the standard model and estimate the resulting CP asymmetries in $B$ decays, taking into account recent experimental and theoretical developments. In performing our fits, we use inputs from the measurements of the following quantities: (i) $\abseps$, the CP-violating parameter in $K$ decays, (ii) $\delmd$, the mass difference due to the \bdbdbar\ mixing, (iii) the matrix elements $\absvcb$ and $\absvub$, (iv) $B$-hadron lifetimes, and (v) the top quark mass. The experimental input in points (ii) - (v) has improved compared to our previous fits. With the updated CKM matrix we present the currently-allowed range of the ratios $|V_{td}/V_{ts}|$ and $|V_{td}/V_{ub}|$, as well as the standard model predictions for the \bsbsbar\ mixing parameter $\xs$ (or, equivalently, $\delms$) and the quantities $\sin 2α$, $\sin 2β$ and $\sin^2γ$, which characterize the CP-asymmetries in $B$-decays. Various theoretical issues related to the so-called ``penguin-pollution," are of importance for the determination of the phases $α$ and $γ$ from the CP-asymmetries in $B$ decays, are also discussed. (to be published in the proc. of the Pisa heavy flavour physics conference.)

hep-ph↗
arXiv1995

ANOMALOUS GAUGE BOSON INTERACTIONS

We discuss the direct measurement of the trilinear vector boson couplings in present and future collider experiments. The major goals of such experiments will be the confirmation of the Standard Model (SM) predictions and the search for signals of new physics. We review our current theoretical understanding of anomalous trilinear gauge boson self-interactions. If the energy scale of the new physics is $\sim 1$ TeV, these low energy anomalous couplings are expected to be no larger than ${\cal O}(10^{-2})$. Constraints from high precision measurements at LEP and low energy charged and neutral current processes are critically reviewed.

hep-ph↗
arXiv1994

Implications of the Top Quark Mass Measurement for the CKM Parameters, $x_s$ and CP Asymmetries

Motivated by the recent determination of the top quark mass by the CDF collaboration, $\mt =174 \pm 10 ^{+13}_{-12}$ GeV, we review and update the constraints on the parameters of the quark flavour mixing matrix $V_{CKM}$ in the standard model. In performing our fits, we use inputs from the measurements of the following quantities: (i) $\abseps$, the CP-violating parameter in $K$ decays, (ii) $\delmd$, the mass difference due to the \bdbdbar\ mixing, (iii) the matrix elements $\absvcb$ and $\absvub$, and (iv) $B$-hadron lifetimes. We find that the allowed region of the unitarity triangle is very large, mostly due to theoretical uncertainties. (This emphasizes the importance of measurements of CP-violating rate asymmetries in the $B$ system.) Nevertheless, the present data do somewhat restrict the allowed values of the coupling constant product $f_{B_d}\sqrt{\hat{B}_{B_d}}$ and the renormalization-scale invariant bag constant $\hat{B}_K$. With the updated CKM matrix we present the currently-allowed range of the ratio $\vert V_{td}/V_{ts} \vert$, as well as the standard model predictions for the \bsbsbar\ mixing parameter $\xs$ and the quantities $\sin 2α$, $\sin 2β$ and $\sin^2γ$, which characterize the CP-asymmetries in $B$-decays. The ALEPH collaboration has recently reported a significant improvement on the lower limit on the $\bs$-$\bsb$ mass difference, $ΔM_s/ΔM_d > 11.3$ (95\% C.L.). This has interesting consequences for the CKM parameters which are also worked out. NOTE: this is a revised and updated version of our previous paper.

hep-ph↗
arXiv1994

Implications of the Top Quark Mass Measurements for the CKM Parameters, $\xs$ and CP Asymmetries

Motivated by the recent determination of the top quark mass by the CDF collaboration, $\mt =174 \pm 10 ^{+13}_{-12}$ GeV, we review and update constraints on the parameters of the quark flavour mixing matrix $V_{CKM}$ in the standard model. In performing these fits, we use inputs from the measurements of $\abseps$, the CP-violating parameter in $K$ decays, $\xd = (\delm)/Γ$, the mixing parameter in \bdbdbar\ mixing, the present measurements of the matrix elements $\absvcb$ and $\absvub$, and the $B$-hadron lifetimes. The CDF value for $\mt$ considerably reduces the CKM-parameter space previously allowed. An interesting result of our analysis is that the present data can be used to restrict the coupling constant product ratio $f_{B_d}\sqrt{B_{B_d}}$ to the range 110-270 MeV -- in comfortable agreement with existing theoretical estimates of this quantity. We use the updated CKM matrix to predict the \bsbsbar\ mixing ratio $\xs$, as well as the quantities $\sin 2α$, $\sin 2β$ and $\sin^2γ$, which characterize CP-violating asymmetries in $B$-decays.

hep-ph↗
arXiv1994

Single Dilepton Production at $e^+e^-$, $eγ$ and $γγ$ Colliders

We calculate the cross sections for the single production of doubly-charged dileptons, both scalar and vector, at $e^+e^-$, $eγ$ and $γγ$ colliders at $\sqrt{s}=500$ GeV and 1 TeV. The $eγ$ mode is by far the most promising -- dileptons whose coupling is as weak as $O(10^{-4})α_{em}$ can be observed, for masses virtually up to the kinematic limit. Dileptons of mass up to $\sqrt{s}$ can also be seen in $e^+e^-$ and $γγ$ colliders, for couplings of order $α_{em}$. In all three colliders, most of the cross section comes from events in which the only particles detected are $e^-e^-$, the decay products of the dilepton, yielding an unmistakeable experimental signature.

hep-ph↗
arXiv1993

In Pursuit of Gamma

After reviewing techniques for extracting clean information on CP-violating phase angles from $B$ decays, we explain the rules for finding decay modes that can probe the phase angle $γ$ of the unitarity triangle. We identify the more promising of these ``$γ$ modes,'' estimate their branching ratios, and examine the degree to which they are theoretically clean. We then show that when the quark mixing matrix is not approximated as usual, but is treated exactly, none of the ``$γ$ modes'' actually measures $γ$. Rather, each of them measures $γ$ plus some correction. In all modes, the correction is small enough to be disregarded in first-generation experiments, but in some of them, it may be large enough to be observed in second-generation experiments. Our treatment of the $γ$ modes calls attention to the fact that when the quark mixing matrix is treated exactly, there are six unitarity triangles, rather than just one triangle. However, only four of the angles in these six triangles are independent. Examining the role played by these four angles, we discover that, in principle at least, measurements of nothing but CP-violating asymmetries in $B$ decays are sufficient to determine the entire quark mixing matrix.

hep-ph↗
arXiv1993

Model-Independent Global Constraints on New Physics

Using effective-lagrangian techniques we perform a systematic survey of the lowest-dimension effective interactions through which heavy physics might manifest itself in present experiments. We do not restrict ourselves to special classes of effective interactions (such as `oblique' corrections). We compute the effects of these operators on all currently well-measured electroweak observables, both at low energies and at the $Z$ resonance, and perform a global fit to their coefficients. Despite the fact that a great many operators arise in our survey, we find that most are quite strongly bounded by the current data. We use our survey to systematically identify those effective interactions which are {\it not} well-bounded by the data -- these could very well include large new-physics contributions. Our results may also be used to efficiently confront specific models for new physics with the data, as we illustrate with an example.

hep-ph↗
arXiv1993

Single Leptoquark Production

The single production of leptoquarks in $e^+e^-$, $eγ$ and $γγ$ linear colliders is discussed. We show that these new particles could be seen in such machines even if their mass is close to the kinematic limit. (Invited talk given by G.B. at the ``Workshop on Physics and Experiments at Linear $e^+e^-$ Colliders'', Waikoloa, Hawaii, April 26-30, 1993.)

hep-ph↗
arXiv1993

Single Leptoquark Production at $e^+e^-$ and $γγ$ Colliders

We consider single production of leptoquarks (LQ's) at $e^+e^-$ and $γγ$ colliders, for two values of the centre-of-mass energy, $\sqrt{s}=500$ GeV and 1 TeV. We find that LQ's which couple within the first generation are observable for LQ masses almost up to the kinematic limit, both at $e^+e^-$ and $γγ$ colliders, for the LQ coupling strength equal to $α_{em}$. The cross sections for single production of $2^{nd}$- and $3^{rd}$-generation LQ's at $e^+e^-$ colliders are too small to be observable. In $γγ$ collisions, on the other hand, $2^{nd}$-generation LQ's with masses much larger than $\sqrt{s}/2$ can be detected. However, $3^{rd}$-generation LQ's can be seen at $γγ$ colliders only for masses at most $\sim\sqrt{s}/2$, making their observation more probable via the pair production mechanism.

hep-ph↗
arXiv1993

A Global Fit to Extended Oblique Parameters

The STU formalism of Peskin and Takeuchi is an elegant method for encoding the measurable effects of new physics which couples to light fermions dominantly through its effects on electroweak boson propagation. However, this formalism cannot handle the case where the scale of new physics is not much larger than the weak scale. In this case three new parameters (V, W and X) are required. We perform a global fit to precision electroweak data for these six parameters. Our results differ from what is found for just STU. In particular we find that the preference for S < 0 is not maintained.

hep-ph↗