Nu Nubar Research Articles

Z′-induced FCNC decays of top, beauty, and strange quarks

Anomalous $b\ensuremath{\rightarrow}s$ transitions from LHCb data may suggest a new massive gauge boson ${Z}^{\ensuremath{'}}$ that couples to the left-handed $b\ensuremath{\rightarrow}s$ current, which in turn implies a coupling to the $t\ensuremath{\rightarrow}c$ current. In this paper, we study flavor-changing neutral current (FCNC) decays of the top quark induced by a ${Z}^{\ensuremath{'}}$ boson, namely $t\ensuremath{\rightarrow}c{Z}^{\ensuremath{'}}$, based on a model of the gauged ${L}_{\ensuremath{\mu}}\ensuremath{-}{L}_{\ensuremath{\tau}}$ symmetry (the difference between the muon and tauon numbers) with vectorlike quarks, which was introduced to explain the anomalous LHCb data. We illustrate that searching for $t\ensuremath{\rightarrow}c{Z}^{\ensuremath{'}}$ via ${Z}^{\ensuremath{'}}\ensuremath{\rightarrow}{\ensuremath{\mu}}^{+}{\ensuremath{\mu}}^{\ensuremath{-}}$ with LHC Run 1 data can probe a parameter region that is unexplored by $B$ physics for a ${Z}^{\ensuremath{'}}$ mass of around $\mathcal{O}(10)\text{ }\text{ }\mathrm{GeV}$ or greater. We further extend the model to a very light ${Z}^{\ensuremath{'}}$ with mass below 400 MeV, which is motivated by the muon $g\ensuremath{-}2$ anomaly. Taking rare $B$ and $K$ meson decay data into account, we give upper limits on the $t\ensuremath{\rightarrow}c{Z}^{\ensuremath{'}}$ branching ratio for the light ${Z}^{\ensuremath{'}}$ case, and discuss about its observability at the LHC. We also scrutinize the possibility that the decay ${K}_{L}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{0}{Z}^{\ensuremath{'}}$ with ${Z}^{\ensuremath{'}}\ensuremath{\rightarrow}\ensuremath{\nu}\overline{\ensuremath{\nu}}$ may lead to an apparent violation of the usual Grossman-Nir bound of $\mathcal{B}({K}_{L}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{0}\ensuremath{\nu}\overline{\ensuremath{\nu}})<1.4\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}9}$.

Dimension-7operators in theb→stransition

We extend the low-energy effective field theory relevant for b to s transitions up to operators of mass-dimension 7 and compute the associated anomalous-dimension matrix. We then compare our findings to the known results for dimension 6 operators and derive a solution for the renormalization group equations involving operators of dimension 7. We finally apply our analysis to a particularly simple case where the Standard Model is extended by an electroweak-magnetic operator and consider limits on this scenario from the decays Bs to mu+ mu- and B to K nu nubar.

Search forB→h(*)νν¯with the full BelleΥ(4S)data sample

We report a search for the rare decays B -> h(*) nu nubar, where h(*) stands for K+, Ks0, K*+, K*0, pi+, pi0, rho+, rho0 and phi. The results are obtained from a 711 fb-1 data sample that contains 772 million BB pairs collected at the Y(4S) resonance with the Belle detector at the KEKB e+e- collider. We search for signal candidates by fully reconstructing a hadronic decay of the accompanying B meson and requiring a single h(*) meson left on the signal side. No significant signal is observed and we set upper limits on the branching fractions at 90% confidence level. The measurements of B+ -> K*+ nu nubar, B+ -> pi+ nu nubar, B0 -> pi0 nu nubar and B0 -> rho^0 nu nubar provide the world's currently most restrictive limits.

Measurements ofWγandZγproduction inppcollisions ats=7 TeVwith the ATLAS detector at the LHC

The integrated and differential fiducial cross sections for the production of a W or Z boson in association with a high-energy photon are measured using pp collisions at sqrt{s} = 7 TeV. The analyses use a data sample with an integrated luminosity of 4.6 fb^{-1} collected by the ATLAS detector during the 2011 LHC data-taking period. Events are selected using leptonic decays of the W and Z bosons (W(e nu,mu nu) and Z(e+ e-, mu+ mu-, nu nubar)) with the requirement of an associated isolated photon. The data are used to test the electroweak sector of the Standard Model and search for evidence for new phenomena. The measurements are used to probe the anomalous WWgamma, ZZgamma and Zgammagamma triple-gauge-boson couplings and to search for the production of vector resonances decaying to Zgamma and Wgamma. No deviations from Standard Model predictions are observed and limits are placed on anomalous triple-gauge-boson couplings and on the production of new vector meson resonances.

Estimate of the branching ratio forZ→νν¯γγ

The effective interaction for two neutrino, two photon coupling is used to find an approximate width for the decay of the Z boson into the nu nubar gamma gamma final state.

Search for the rare decayB→Kνν¯

We present a search for the rare decays B+ -> K+ nu nubar and B0 -> K0 nu nubar using 459 million BBbar pairs collected with the BaBar detector at the SLAC National Accelerator Laboratory. Flavor-changing neutral-current decays such as these are forbidden at tree level but can occur through one-loop diagrams in the Standard Model (SM), with possible contributions from new physics at the same order. The presence of two neutrinos in the final state makes identification of signal events challenging, so reconstruction in the semileptonic decay channels B -> D(*) l nu of the B meson recoiling from the signal B is used to suppress backgrounds. We set an upper limit at the 90% confidence level of 1.3 x 10^{-5} on the total branching fraction for B+ -> K+ nu nubar, and 5.6 x 10^{-5} for B0 -> K0 nu nubar. We additionally report partial branching fractions in two ranges of di-neutrino mass squared for B+ -> K+ nu nubar.

New physics at a Super Flavor Factory

The potential of a Super Flavor Factory (SFF) for searches of New Physics is reviewed. While very high luminosity B physics is assumed to be at the core of the program, its scope for extensive charm and tau studies are also emphasized. The possibility to run at the Upsilon(5S) is also very briefly discussed; in principle, this could provide very clean measurements of B_s decays. The strength and reach of a SFF is most notably due to the possibility of examining an impressive array of very clean observables. The angles and the sides of the unitarity triangle can be determined with unprecedented accuracy. These serve as a reference for New Physics (NP) sensitive decays such as B^+ ->tau^+ nu and penguin dominated hadronic decay modes, providing tests of generic NP scenarios with an accuracy of a few percent. Besides, very precise studies of direct and time dependent CP asymmetries in radiative B decays and forward-backward asymmetry studies in B-> X_s l^+ l^- and numerous null tests using B, charm and tau decays are also likely to provide powerful insights into NP. The dramatic increase in luminosity at a SFF will also open up entirely new avenues for probing NP observables, e.g. by allowing sensitive studies using theoretically clean processes such as B -> X_s nu nubar. The SFF is envisioned to be a crucial tool for essential studies of flavor in the LHC era, and will extend the reach of the LHC in many important ways.

Search forB→h(*)νν¯Decays at Belle

We present a search for the rare decays B -> h(*) nu nubar, where h(*) stands for a light meson. A data sample of 535 million BBbar pairs collected with the Belle detector at the KEKB e+e- collider is used. Signal candidates are required to have an accompanying B meson fully reconstructed in a hadronic mode and signal-side particles consistent with a single h(*) meson. No significant signal is observed and we set upper limits on the branching fractions at 90% confidence level. The limits on B0 -> K*0 nu nubar and B+ -> K+ nu nubar decays are more stringent than the previous constraints, while the first searches for B0 -> K0 nu nubar, pi0 nu nubar, rho0 nu nubar, phi nu nubar and B+ -> K*+ nu nubar, rho+ nu nubar are reported.

Search forB+→τ+ν

We present a search for the decay B+ -> tau+ nu using 383x10^6 BBbar pairs collected at the Y(4S) resonance with the BABAR detector at the SLAC PEP-II B-Factory. A sample of events with one reconstructed semileptonic B decay (B- -> D0 l nu X)) is selected, and in the recoil a search for B+ -> tau+ nu is performed. The tau is identified in the following channels: tau+ -> e nu nubar, tau+ -> mu nu nubar, tau+ -> pi+ nu, and tau+ -> pi+ pi0 nu. We measure a branching fraction of B(B+ -> tau+ nu)=(0.9 +- 0.6(stat.) +- 0.1(syst.)) x 10^-4. In the absence of a significant signal, we calculate an upper limit at the 90% confidence level of B(B+ -> tau+ nu) < 1.7 x 10^-4. We calculate the product of the B meson decay constant f_B and |V_ub| to be f_B x |V_ub| = (7.2^{+2.0}_{-2.8}(stat.) +- 0.2 (syst.)) x 10^-4 GeV.

EnhancedKL→π0νν¯from directCPviolation inB→Kπwith four generations

Recent CP violation results in B decays suggest that Z penguins may have large weak phase. This can be realized by the four generation (standard) model. Concurrently, B to Xs l+ l- and Bs mixing allow for sizable Vt's* Vt'b only if it is nearly imaginary. Such large effects in b to s transitions would affect s to d transitions, as kaon constraints would demand Vt'd different from zero. Using Gamma(Z to b bbar) to bound |Vt'b|, we infer sizable |Vt's| < |Vt'b| < |Vus|$. Imposing epsilon_K, K+ to pi+ nu nubar and epsilon'/epsilon constraints, we find Vt'd* Vt's = few x 10^(-4) with large phase, enhancing KL to pi0 nu nubar to 5x10^(-10) or even higher. Interestingly, Delta_mBd and sin(2PhiBd) are not much affected, as |Vt'd* Vt'b| << |Vtd* Vtb| = 0.01.

Plans for Kaon Physics at BNL

I give an overview of current plans for kaon physics at BNL. The program is centered on the rare decay modes K+ --> pi+ nu nubar and KL --> pi0 nu nubar.

K→πνbar nu from standard to new physics

Flavour changing neutral current decays are a very sensitive test of the standard model and its extensions. In particular the decay K -> pi nu nubar constitutes a clean way to provide constraints, independent of long distance effects. Motivated by the recent experimental data of the E787 and E865 collaborations and by the difference between the standard model prediction and data, we consider in detail new physics scenarios such as the minimal supersymmetric standard model and R-parity violating supersymmetry. We begin with analysing the impact of new measurements on the standard model result obtaining B(K^+ -> pi^+ nu nubar)=(8.18 +/- 1.22) x 10^(-11). Predictions for other rare kaon decays are discussed, too. Our results allow to improve the limits on R-parity violating couplings with respect to previous analyses.

Single photon signals for warped quantum gravity at a lineare+e−collider

We study the `single photon' process e+- e- -> gamma nu nubar with contributions due to exchange of massive gravitons in the Randall- Sundrum model of low-scale quantum gravity. It is shown that for significant regions in the parameter space, this process unambiguously highlights the resonance structure of the graviton sector. Even in the non-resonant part of the parameter space, we show that comparison with the benchmark process e+- e- -> mu+- mu- can clearly distinguish signals for warped gravity from similar signals for large extra dimensions.

Leptonic decays of the Ds meson

The purely leptonic decays Ds -> tau nu and Ds -> mu nu are studied in a sample of four million hadronic Z decays collected with the ALEPH detector at the LEP e+e- collider from 1991 to 1995. The branching fractions are extracted from a combination of two analyses, one optimized to select Ds -> tau nu decays with tau -> e nu nubar or mu nu nubar, and the other optimized for Ds-> mu nu decays. The results are used to evaluate the Ds decay constant, within the Standard Model: fDs = [285 +- 19(stat) +- 40 (syst)] MeV

Search for the decayK+→π+π0νν¯

The first search for the decay K^+ -> pi^+ pi^0 nu nubar has been performed with the E787 detector at BNL. Based on zero events observed in the kinematical search region defined by 90 MeV/c < P_{pi^+} < 188 MeV/c and 135 MeV < E_{pi^0} < 180 MeV, an upper limit B(K^+ -> pi^+ pi^0 nu nubar) < 4.3 x 10^{-5} at 90% confidence level is established.

Further search for the decay K+-->pi(+)nunu;

A search for additional evidence for the rare kaon decay K+-->pi(+)nunu; has been made with a new data set comparable in sensitivity to the previous exposure that produced a single event. No new events were found in the pion momentum region examined, 211<P<229 MeV/c. Including a reanalysis of the original data set, the backgrounds were estimated to contribute 0.08+/-0.02 events. Based on one observed event, the new branching ratio is B(K+-->pi(+)nunu;) = 1.5(+3.4)(-1.2)x10(-10).

Examination of T/CP Invariance in the e+ e- -&gt; + - Reaction

We propose a method to examine the T/CP invariance of the heaviest lepton, tau, by means of a triple-momentum correlation for the reaction e^+e^- -->tau^+tau^-, tau --> e/mu nu nubar during the course of the B-Factory experimental program. An unprecedented high sensitivity could be obtained without requiring a high integrated luminosity.