arXiv subjects

David London

Publications and source records attributed to David London.

At least 55 records · Page 3Linked to original sources

arXiv2017

New Physics in $b \rightarrow s μ^+ μ^-$: Distinguishing Models through CP-Violating Effects

At present, there are several measurements of $B$ decays that exhibit discrepancies with the predictions of the SM, and suggest the presence of new physics (NP) in $b \to s μ^+ μ^-$ transitions. Many NP models have been proposed as explanations. These involve the tree-level exchange of a leptoquark (LQ) or a flavor-changing $Z'$ boson. In this paper we examine whether it is possible to distinguish the various models via CP-violating effects in $B \to K^{(*)} μ^+ μ^-$. Using fits to the data, we find the following results. Of all possible LQ models, only three can explain the data, and these are all equivalent as far as $b \to s μ^+ μ^-$ processes are concerned. In this single LQ model, the weak phase of the coupling can be large, leading to some sizeable CP asymmetries in $B \to K^{(*)} μ^+ μ^-$. There is a spectrum of $Z'$ models; the key parameter is $g_L^{μμ}$, which describes the strength of the $Z'$ coupling to $μ^+μ^-$. If $g_L^{μμ}$ is small (large), the constraints from $B^0_s$-${\bar B}^0_s$ mixing are stringent (weak), leading to a small (large) value of the NP weak phase, and corresponding small (large) CP asymmetries. We therefore find that the measurement of CP-violating asymmetries in $B \to K^{(*)} μ^+ μ^-$ can indeed distinguish among NP $b \to s μ^+ μ^-$ models

hep-ph↗

arXiv2017

Hidden sector explanation of $B$-decay and cosmic ray anomalies

There are presently several discrepancies in $b \to s \ell^+ \ell^-$ decays of $B$ mesons suggesting new physics coupling to $b$ quarks and leptons. We show that a $Z'$, with couplings to quarks and muons that can explain the $B$-decay anomalies, can also couple to dark matter in a way that is consistent with its relic abundance, direct detection limits, and hints of indirect detection. The latter include possible excess events in antiproton spectra recently observed by the AMS-02 experiment. We present two models, having a heavy (light) $Z'$ with $m_{Z'}\sim 600\,(12)\,$GeV and fermionic dark matter with mass $m_χ\sim 50\,(2000)\,$GeV, producing excess antiprotons with energies of $\sim 10\, (300)\,$GeV. The first model is also compatible with fits for the galactic center GeV gamma-ray excess.

hep-ph↗

arXiv2016

Simultaneous Explanation of the $R_K$ and $R_{D^{(*)}}$ Puzzles: a Model Analysis

$R_K$ and $R_{D^{(*)}}$ are two $B$-decay measurements that presently exhibit discrepancies with the SM. Recently, using an effective field theory approach, it was demonstrated that a new-physics model can simultaneously explain both the $R_K$ and $R_{D^{(*)}}$ puzzles. There are two UV completions that can give rise to the effective Lagrangian: (i) $VB$: a vector boson that transforms as an $SU(2)_L$ triplet, as in the SM, (ii) $U_1$: an $SU(2)_L$-singlet vector leptoquark. In this paper, we examine these models individually. A key point is that $VB$ contributes to $B^0_s$-${\bar B}^0_s$ mixing and $τ\to 3μ$, while $U_1$ does not. We show that, when constraints from these processes are taken into account, the $VB$ model is just barely viable. It predicts ${\cal B} (τ^-\toμ^-μ^+μ^-) \simeq 2.1 \times 10^{-8}$. This is measurable at Belle II and LHCb, and therefore constitutes a smoking-gun signal of $VB$. For $U_1$, there are several observables that may point to this model. Perhaps the most interesting is the lepton-flavor-violating decay $Υ(3S) \to μτ$, which has previously been overlooked in the literature. $U_1$ predicts ${\cal B}(Υ(3S) \to μτ)|_{\rm max} = 8.0 \times 10^{-7}$. Thus, if a large value of ${\cal B}(Υ(3S) \to μτ)$ is observed -- and this should be measurable at Belle II -- the $U_1$ model would be indicated.

hep-ph↗

arXiv2016

Summary of the 2015 LHCb workshop on multi-body decays of D and B mesons

This document contains a summary of the LHCb workshop on multi-body decays of D and B mesons, held at CBPF, Rio de Janeiro, in July 2015. The workshop was focused on issues related to amplitude analysis of three- and four-body hadronic decays. In addition to selected LHCb results, contributions from guest theorists are included.

hep-ex↗

arXiv2016

Is there really a $W \to τν$ puzzle?

According to the Particle Data Group, the measurements of ${\cal B}(W^+ \to τ^+ ν_τ)$ and ${\cal B}(W^+ \to \ell^+ ν_\ell)$ ($\ell = e,μ$) disagree with one another at the $2.3σ$ level. In this paper, we search for a new-physics (NP) explanation of this $W \to τν$ puzzle. We consider two NP scenarios: (i) the $W$ mixes with a $W'$ boson that couples preferentially to the third generation, (ii) $τ_{L,R}$ and $ν_{τL}$ mix with isospin-triplet leptons. Unfortunately, once other experimental constraints are taken into account, neither scenario can explain the above experimental result. Our conclusion is that the $W \to τν$ puzzle is almost certainly just a statistical fluctuation.

hep-ph↗

arXiv2016

Measuring CP-Violating Observables in Rare Top Decays at the LHC

In this paper we consider CP-violating new-physics contributions to the decay $t \to b \bar b c$. We examine the prospects for detecting such new physics at the LHC, which requires studying the process $gg \to t (\to b \bar b c) \bar t (\to \bar b \ell \bar ν)$. We find two observables that can be used to reveal the presence of CP-violating new physics in $t \to b \bar b c$. They are (i) the partial-rate asymmetry and (ii) the triple-product correlations involving the momenta of various particles associated with the interaction. A Monte Carlo analysis is performed to determine how well these observables can be used to detect the presence of new physics, and to measure its parameters. We find that there is little difficulty in extracting the value of the relevant new-physics parameter from the partial-rate asymmetry. For the triple-product correlations, we test multiple strategies that can be used for the extraction of the corresponding combination of new-physics parameters.

hep-ph↗

arXiv2015

Quark-flavored scalar dark matter

It is an intriguing possibility that dark matter (DM) could have flavor quantum numbers like the quarks. We propose and investigate a class of UV-complete models of this kind, in which the dark matter is in a scalar triplet of an SU(3) flavor symmetry, and interacts with quarks via a colored flavor-singlet fermionic mediator. Such mediators could be discovered at the LHC if their masses are $\sim 1$ TeV. We constrain the DM-mediator couplings using relic abundance, direct detection, and flavor-changing neutral-current considerations. We find that, for reasonable values of its couplings, scalar flavored DM can contribute significantly to the real and imaginary parts of the $B_s$-$\bar B_s$ mixing amplitude. We further assess the potential for such models to explain the galactic center GeV gamma-ray excess.

hep-ph↗

arXiv2015

Search for New Physics in Rare Top Decays: $t \bar t$ Spin Correlations and Other Observables

In this paper we study new-physics contributions to the top-quark decay $t \to b \bar b c$. We search for ways of detecting such new physics via measurements at the LHC. As top quarks are mainly produced at the LHC in $t \bar t$ production via gluon fusion, we analyze the process $gg \to t \bar t \to (b \bar b c) (\bar b \ell \bar ν)$. We find six observables that can be used to reveal the presence of new physics in $t \to b \bar b c$. Three are invariant mass-squared distributions involving two of the final-state particles in the top decay, and three are angular correlations between the final-state quarks coming from the $t$ decay and the $\ell^-$ coming from the $\bar t$ decay. The angular correlations are related to the $t \bar t$ spin correlation.

hep-ph↗

arXiv2015

Detecting New Physics in Rare Top Decays at the LHC

In the companion paper it was shown that there are six observables in $gg\to t \bar t \to (b \bar b c) (\bar b \ell \bar ν)$ that can be used to reveal the presence of new physics (NP) in $t \to b \bar b c$. In the present paper we examine the prospects for detecting and identifying such NP at the LHC, in both the short term and long term. To this end, we develop an algorithm for extracting the NP parameters from measurements of the observables. In the short term, depending on what measurements have been made, there are several different ways of detecting the presence of NP. It may even be possible to approximately determine the values of certain NP parameters. In the long term, it is expected that all six observables will be measured. The values of the NP parameters can then be determined reasonably precisely from a fit to these measurements, which will provide good information about the type of NP present in $t \to b \bar b c$.

hep-ph↗

arXiv2015

Using U Spin to Extract gamma from Charmless B -> PPP Decays

Some years ago, a method was proposed for measuring the CP-violating phase gamma using pairs of two-body decays that are related by U-spin reflection (d <-> s). In this paper we adapt this method to charmless B -> PPP decays. Time-dependent Dalitz-plot analyses of these three-body decays are required for the measurement of the mixing-induced CP asymmetries. However, isobar analyses of the decay amplitudes are not necessary. A potential advantage of using three-body decays is that the effects of U-spin breaking may be reduced by averaging over the Dalitz plot. This can be tested independently using the measurements of direct CP asymmetries and branching ratios in three-body charged B decays.

hep-ph↗

arXiv2015

New-physics signals of a model with a vector-singlet up-type quark

The VuQ model involves the addition of a vector isosinglet up-type quark to the standard model. In this model the full CKM quark mixing matrix is 4 X 3. Using present flavor-physics data, we perform a fit to this full CKM matrix, looking for signals of new physics (NP). We find that the VuQ model is very strongly constrained. There are no hints of NP in the CKM matrix, and any VuQ contributions to loop-level flavor-changing b -> s, b -> d and s -> d transitions are very small. There can be significant enhancements of the branching ratios of the flavor-changing decays t -> u Z and t -> c Z, but these are still below present detection levels.

hep-ph↗

arXiv2015

Simultaneous Explanation of the $R_K$ and $R(D^{(*)})$ Puzzles

At present, there are several hints of lepton flavor non-universality. The LHCb Collaboration has measured $R_K\equiv{\cal B}(B^+ \to K^+ μ^+ μ^-)/{\cal B}(B^+ \to K^+ e^+ e^-)$, and the BaBar Collaboration has measured $R(D^{(*)}) \equiv {\cal B}({\bar B} \to D^{(*)+} τ^- {\barν}_τ) / {\cal B}({\bar B} \to D^{(*)+} \ell^- {\barν}_\ell)$ ($\ell = e,μ$). In all cases, the experimental results differ from the standard model predictions by 2-3$σ$. Recently, an explanation of the $R_K$ puzzle was proposed in which new physics (NP) generates a neutral-current operator involving only third-generation particles. Now, assuming the scale of NP is much larger than the weak scale, this NP operator must be made invariant under the full $SU(3)_C \times SU(2)_L \times U(1)_Y$ gauge group. In this Letter, we note that, when this is done, a new charged-current operator can appear, and this can explain the $R(D^{(*)})$ puzzle. A more precise measurement of the double ratio $R(D)/R(D^*)$ can rule out this model.

hep-ph↗

arXiv2014

Probing New Physics in Higgs Couplings to Fermions using an Angular Analysis

The standard-model Higgs boson couples to quarks through a parity-even scalar $Hq{\bar q}$ coupling. We show that the rare Higgs decay $H\to VZ$, where $V$ is a vector quarkonium state such as $J/ψ$ ($c{\bar c}$) or $Υ(1S)$ ($b{\bar b}$), can be used to search for the presence of a parity-odd pseudoscalar $Hq{\bar q}$ coupling. Since both $V$ and $Z$ can decay to a pair of charged leptons, this presents an experimentally-clean channel that can be observed at the high-luminosity LHC or a future hadron collider. The P-even and P-odd $Hq{\bar q}$ couplings can be measured by analyzing the angular distribution of the final-state leptons.

hep-ph↗

arXiv2014

Beyond S, T and U

The contribution to precision electroweak measurements due to TeV physics which couples primarily to the $W^\pm$ and $Z$ bosons may be parameterized in terms of the three `oblique correction' parameters, S, T and U. We extend this parameterization to physics at much lower energies, $\ge 100$ GeV, and show that in this more general case three more parameters are required (which we call V, W and X). Only two of these appear in neutral-current experiments, while the third new parameter enters into the $W^\pm$ width.

hep-ph↗

arXiv2014

Charmless B -> PPP Decays: the Fully-Symmetric Final State

In charmless B -> PPP decays, where P is a pseudoscalar meson, there are six possibilities for the symmetry of the final state. In this paper, for P=pi,K, we examine the properties of the fully-symmetric final state. We present expressions for all 32 B -> PPP decay amplitudes as a function of both SU(3) reduced matrix elements and diagrams, demonstrating the equivalence of diagrams and SU(3). We also give 25 relations among the amplitudes in the SU(3) limit, as well as those that appear when the diagrams E/A/PA are neglected. In the SU(3) limit, one has the equalities \sqrt{2} A(B+ -> K+ pi+ pi-)_{FS} = A(B+ -> K+ K+ K-)_{FS} and \sqrt{2} A(B+ -> pi+ K+ K-)_{FS} = A(B+ -> pi+ pi+ pi-)_{FS}, where FS denotes the fully-symmetric final state. These provide good tests of the standard model that can be carried out now by the LHCb Collaboration.

hep-ph↗

arXiv2013

Extraction of the CP-violating phase $γ$ using $B \to K ππ$ and $B \to K K {\bar K}$ decays

Using the B{\sc a}B{\sc ar}\ measurements of the Dalitz plots for $B^0 \to K^+π^0π^-$, $B^0 \to K^0π^+π^-$, $B^+ \to K^+π^+π^-$, $B^0 \to K^+ K^0 K^-$, and $B^0 \to K^0 K^0 {\bar K}^0$ decays, we demonstrate that it is possible to cleanly extract the weak phase $γ$. We find four possible solutions. Three of these -- $32^\circ$, $259^\circ$, and $315^\circ$ -- are in disagreement with the SM, while one -- $77^\circ$ -- is consistent with the SM. An advantage of this Dalitz-plot method is that one can obtain many independent measurements of $γ$, thereby reducing its statistical error. An accurate determination of the errors, however, requires detailed knowledge of the data.

hep-ph↗

arXiv2013

Measurement of $γ$ from three-body B decays

Using the BaBar measurements of the Dalitz plots for the decays $B^0 \to K^+π^0π^-$, $B^0 \to K^0π^+π^-$, $B^+ \to K^+π^+π^-$, $B^0 \to K^+ K^0 K^-$, and $B^0 \to K^0 K^0 \bar K^0$, we demonstrate that it is possible to cleanly extract the weak phase $γ$. An advantage of this Dalitz-plot method is that one can obtain many independent measurements of $γ$, thereby reducing its statistical error. An accurate determination of the errors, however, requires detailed knowledge of the data. Hopefully, an experimental group will repeat the analysis, and obtain precise values of the errors.

hep-ph↗

arXiv2013

Measurement of gamma using B -> K pi pi and B -> K K Kbar decays

The BaBar measurements of the Dalitz plots for B0 -> K+ pi0 pi-, B0 -> K0 pi+ pi-, B+ -> K+ pi+ pi-, B0 -> K+ K0 K-, and B0 -> K0 K0 Kbar0 decays are used to cleanly extract the weak phase gamma. We find four possible solutions: $(31^{+2}_{-3})^\circ$, $(77 \pm 3)^\circ$, $(258^{+4}_{-3})^\circ$, and $(315^{+3}_{-2})^\circ$. One solution -- $(77 \pm 3)^\circ$ -- is consistent with the SM. Its error, which includes leading-order flavor-SU(3) breaking, is far smaller than that obtained using two-body B decays.

hep-ph↗