Test Of Lepton Universality Research Articles

Towards the discovery of new physics with lepton-universality ratios of b→sℓℓ decays

Tests of lepton-universality as rate ratios in $b\to s \ell\ell$ transitions can be predicted very accurately in the Standard Model. The deficits with respect to expectations reported by the LHCb experiment in muon-to-electron ratios of the $B\to K^{(*)}\ell\ell$ decay rates thus point to genuine manifestations of lepton non-universal new physics. In this paper, we analyse these measurements in the context of effective field theory. First, we discuss the interplay of the different operators in $R_K$ and $R_{K^*}$ and provide predictions for $R_{K^*}$ in the Standard Model and in new-physics scenarios that can explain $R_K$. We also provide approximate numerical formulas for these observables in bins of interest as functions of the relevant Wilson coefficients. Secondly, we perform frequentist fits to $R_{K}$ and $R_{K^*}$. The SM disagrees with these measurements at $3.7\sigma$ significance. We find excellent fits in scenarios with combinations of $\mathcal O_{9(10)}^\ell=\bar s\gamma^\mu b_L~\ell\gamma_\mu(\gamma_5) \ell$ operators, with pulls relative to the Standard Model in the region of $4\sigma$. An important conclusion of our analysis is that a lepton-specific contribution to $\mathcal O_{10}$ is essential to understand the data. Under the hypothesis that new-physics couples selectively to the muons, we also present fits to other $b\to s\mu\mu$ data with a conservative error assessment, and comment on more general scenarios. Finally, we discuss new lepton universality ratios that, if new physics is the origin of the observed discrepancy, should contribute to the statistically significant discovery of new physics in the near future.

RK anomalies and simplified limits on Z′ models at the LHC

The LHCb collaboration has recently reported a 2.5 $\sigma$ discrepancy with respect to the predicted value in a test of lepton universality in the ratio $R_{K^*}= \hbox{BR}(B \to K^* \mu^+ \mu^-) / \hbox{BR}(B \to K^* e^+ e^-)$. Coupled with an earlier observation of a similar anomaly in $R_{K}$, this has generated significant excitement. A number of new physics scenarios have been proposed to explain the anomaly. In this work we consider simplified limits on $Z'$ models from ATLAS and CMS searches for new resonances in dilepton and dijet modes, and we use the simplified limits variable $\zeta$ to correlate the results of the resonance and B-decay experiments. By examining minimal $Z'$ models that can accomodate the observed LHCb results, we show that the high-mass resonance search results are begining to be sensitive to these models and that future results will be more informative.

Test of lepton universality with B0 \u2192 K*0\u2113+\u2113\u2212 decays

A test of lepton universality, performed by measuring the ratio of the branching fractions of the B0 → K*0μ+μ− and B0 → K*0e+e− decays, {R}_{K^{*0}} , is presented. The K*0 meson is reconstructed in the final state K+π−, which is required to have an invariant mass within 100 MeV/c2 of the known K*(892)0 mass. The analysis is performed using proton-proton collision data, corresponding to an integrated luminosity of about 3 fb−1, collected by the LHCb experiment at centre-of-mass energies of 7 and 8 TeV. The ratio is measured in two regions of the dilepton invariant mass squared, q2, to beRK*0=0.66−+0.070.11stat±0.03systfor0.045<q2<1.1GeV2/c4,0.69−+0.070.11stat±0.05systfor1.1<q2<6.0GeV2/c4.\\documentclass[12pt]{minimal}\t\t\t\t\\usepackage{amsmath}\t\t\t\t\\usepackage{wasysym}\t\t\t\t\\usepackage{amsfonts}\t\t\t\t\\usepackage{amssymb}\t\t\t\t\\usepackage{amsbsy}\t\t\t\t\\usepackage{mathrsfs}\t\t\t\t\\usepackage{upgreek}\t\t\t\t\\setlength{\\oddsidemargin}{-69pt}\t\t\t\t\\begin{document}$$ {R}_{K^{*0}}=\\left\\{\\begin{array}{l}{0.66_{-}^{+}}_{0.07}^{0.11}\\left(\\mathrm{stat}\\right)\\pm 0.03\\left(\\mathrm{syst}\\right)\\kern1em \\mathrm{f}\\mathrm{o}\\mathrm{r}\\kern1em 0.045<{q}^2<1.1\\kern0.5em {\\mathrm{GeV}}^2/{c}^4,\\hfill \\\\ {}{0.69_{-}^{+}}_{0.07}^{0.11}\\left(\\mathrm{stat}\\right)\\pm 0.05\\left(\\mathrm{syst}\\right)\\kern1em \\mathrm{f}\\mathrm{o}\\mathrm{r}\\kern1em 1.1<{q}^2<6.0\\kern0.5em {\\mathrm{GeV}}^2/{c}^4.\\hfill \\end{array}\\right. $$\\end{document}The corresponding 95.4% confidence level intervals are [0.52, 0.89] and [0.53, 0.94]. The results, which represent the most precise measurements of {R}_{K^{*0}} to date, are compatible with the Standard Model expectations at the level of 2.1–2.3 and 2.4–2.5 standard deviations in the two q2 regions, respectively.

On the universality (or not) of beautiful penguins

Abstract Despite the enduring resilience of the Standard Model of particle physics, there remain reasons to expect that it is not a “final” theory. In particular, the Standard Model can not explain either dark matter or the observed matter–antimatter asymmetry of the universe. LHCb is a forward acceptance spectrometer at the Large Hadron Collider, dedicated to precision measurements of heavy flavour particles. Because new particles can appear virtually in the decays of heavy flavour particles, and thus alter their properties, such measurements are inherently sensitive to much higher mass scales that direct searches. We present in this article how the presence of new particles can be probed by testing Lepton Universality in the decay of hadrons containing a b-quark.

Precise measurement of the Ke2/Kμ2 branching ratio and search for new physics beyond the Standard Model

The E36 experiment recently conducted at J-PARC by the TREK Collaboration will provide a precise mesurement of the decay ratio RK = Γ(K+→e+υe)/Γ(K+→μ+υμ) with the aim of testing lepton universality, a basic assumption of the Standard Model (SM). The SM prediction for the decay ratio RK is highly precise (δRK/RK = 4 × 10−4) and any observed deviation from this prediction would clearly indicate the existence of new physics. The E36 experimental apparatus, designed to offer small systematic errors, was installed at the J-PARC K1.1BR kaon beam line in the fall of 2014, fully commissioned in the spring of 2015, and the production data were collected in the fall of 2015. A scintillating fiber target was used to stop a beam of 1.2 × 106K+ per spill. The K+ decay products were then momentum analyzed by a 12-sector superconducting toroidal spectrometer and charged particles were tracked with high precision by Multi-Wire Proportional Chambers (MWPC) combined with a photon calorimeter with a large solid angle (75% of 4π) and 3 different particle identification systems. Details of the E36 experimental apparatus, as well as the status of the data analysis will be presented.

Measurement of forward W → eν production in pp collisions at s = 8 $$ \sqrt{s}=8 $$ TeV

A measurement of the cross-section for $W \to e\nu$ production in $pp$ collisions is presented using data corresponding to an integrated luminosity of $2\,$fb$^{-1}$ collected by the LHCb experiment at a centre-of-mass energy of $\sqrt{s}=8\,$TeV. The electrons are required to have more than $20\,$GeV of transverse momentum and to lie between 2.00 and 4.25 in pseudorapidity. The inclusive $W$ production cross-sections, where the $W$ decays to $e\nu$, are measured to be \begin{equation*} \sigma_{W^{+} \to e^{+}\nu_{e}}=1124.4\pm 2.1\pm 21.5\pm 11.2\pm 13.0\,\mathrm{pb}, \end{equation*} \begin{equation*} \sigma_{W^{-} \to e^{-}\bar{\nu}_{e}}=\,\,\,809.0\pm 1.9\pm 18.1\pm\,\,\,7.0\pm \phantom{0}9.4\,\mathrm{pb}, \end{equation*} where the first uncertainties are statistical, the second are systematic, the third are due to the knowledge of the LHC beam energy and the fourth are due to the luminosity determination. Differential cross-sections as a function of the electron pseudorapidity are measured. The $W^{+}/W^{-}$ cross-section ratio and production charge asymmetry are also reported. Results are compared with theoretical predictions at next-to-next-to-leading order in perturbative quantum chromodynamics. Finally, in a precise test of lepton universality, the ratio of $W$ boson branching fractions is determined to be \begin{equation*} \mathcal{B}(W \to e\nu)/\mathcal{B}(W \to \mu\nu)=1.020\pm 0.002\pm 0.019, \end{equation*} where the first uncertainty is statistical and the second is systematic.

Leptonic precision test of leptophilic two-Higgs-doublet model

The type X (lepton-specific) two-Higgs-doublet model at large $\tan\beta$ becomes leptophilic and thus allows a light pseudoscalar $A$ accommodating the observed muon $g-2$ deviation without conflicting with various hadronic constraints. On the other hand, it is strongly constrained by leptonic precision observables such as lepton universality test in the neutral and charged currents. Treating all the lepton universality data in a consistent way, we show how the current data constrain the parameter space of $m_A$ and $\tan\beta$ for given degenerate masses of heavy Higgs bosons $H$ and $H^\pm$. While no overlapping region is found at $1\sigma$, a sizable region is still viable at $2\sigma$ for $H/H^\pm$ masses at around 200$\sim$400 GeV.

Rare decays in quark flavour physics

Rare heavy-flavour decays are an ideal place to search for the effects of potential new particles that modify the decay rates or the Lorentz structure of the decay vertices. Recent results on Flavour Changing Neutral Current decays from the LHC are reviewed. An emphasis is put on the very rare decay Bs0→μ+μ−, which was recently observed by the CMS and LHCb experiments, on a recent test of lepton universality in loop processes and on the analysis of the angular distributions of the B0→K⁎0μ+μ− decays, both by the LHCb collaboration.

Erratum: Probing lepton nonuniversality in tau neutrino scattering [Phys. Rev. D92, 073016 (2015)

Recently hints of lepton flavor non-universality emerged in the BaBar and LHCb experiments. In this paper we propose tests of lepton universality in $\nu_{\tau}$ scattering. To parametrize the new physics we adopt an effective Lagrangian approach and consider the neutrino deep inelastic scattering processes $\nu_{\tau}+ N \to \tau + X$ and $\nu_{\mu}+ N \to \mu + X$ where we assume the largest new physics effects are in the $\tau$ sector. We also consider an explicit leptoquark model in our calculations. In order to make comparison with the standard model and also in order to cancel out the uncertainties of the parton distribution functions, we consider the ratio of total and differential cross sections of tau-neutrino to muon-neutrino scattering. We find new physics effects that can possibly be observed at the proposed Search for Hidden Particles (SHiP) experiment at CERN.

Lepton Universality and Lepton Flavour Violation tests at theB-factories

We review the experimental status of the lepton universality tests and lepton flavour violation searches after the completion of the data-taking and most of the data analysis of the B -factories BABAR and Belle. The universality of the Standard Model charged weak couplings has been confirmed and moderately improved in precision by the B -factories results. Lepton Flavour violation in the τ lepton decays has been searched in several decay modes and no evidence has been found. We set 90% confidence level upper limits on the Lepton Flavour violating branching ratios in the 10−7 –10−8 range, greatly extending the previous limits set mainly by CLEO in the 10−6 range.

Neutrino masses from neutral top partners

We present theories of `Natural Neutrinos' in which neutral fermionic top partner fields are simultaneously the right-handed neutrinos (RHN), linking seemingly disparate aspects of the Standard Model structure: a) The RHN top partners are responsible for the observed small neutrino masses, b) They help ameliorate the tuning in the weak scale and address the little hierarchy problem, and c) The factor of $3$ arising from $N_c$ in the top-loop Higgs mass corrections is countered by a factor $3$ from the number of vector-like generations of RHN. The RHN top partners may arise in pseudo-Nambu-Goldstone-Boson (pNGB) Higgs models such as the Twin Higgs, as well as more general Composite, Little, and Orbifold Higgs scenarios, and three simple example models are presented. This framework firmly predicts a TeV-scale seesaw, as the RHN masses are bounded to be below the TeV scale by naturalness. The generation of light neutrino masses relies on a collective breaking of lepton number, allowing for comparatively large neutrino Yukawa couplings and a rich associated phenomenology. The structure of the neutrino mass mechanism realizes in certain limits the Inverse or Linear classes of seesaw. Natural Neutrino models are testable at a variety of current and future experiments, particularly in tests of lepton universality, searches for lepton flavor violation, and precision electroweak and Higgs coupling measurements possible at high energy $e^+ e^-$ and hadron colliders.

Probing lepton nonuniversality in tau neutrino scattering

Recently hints of lepton flavor nonuniversality emerged in the BABAR and LHCb experiments. In this paper we propose tests of lepton universality in ${\ensuremath{\nu}}_{\ensuremath{\tau}}$ scattering. To parametrize the new physics we adopt an effective Lagrangian approach and consider the neutrino deep inelastic scattering processes ${\ensuremath{\nu}}_{\ensuremath{\tau}}+N\ensuremath{\rightarrow}\ensuremath{\tau}+X$ and ${\ensuremath{\nu}}_{\ensuremath{\mu}}+N\ensuremath{\rightarrow}\ensuremath{\mu}+X$ where we assume the largest new physics effects are in the $\ensuremath{\tau}$ sector. We also consider an explicit leptoquark model in our calculations. In order to make comparison with the standard model and also in order to cancel out the uncertainties of the parton distribution functions, we consider the ratio of total and differential cross sections of tau-neutrino to muon-neutrino scattering. We find new physics effects that can possibly be observed at the proposed Search for Hidden Particles (SHiP) experiment at CERN.

HFAG τ lepton averages and [formula omitted] determination with τ decays

We present an updated HFAG report for the τ lepton, using τ lepton experimental results available up to Summer 2014. World averages are computed for quantities where the HFAG procedures may produce improvements over the PDG compilation. Furthermore, we list the available upper limits on the searches for τ lepton flavor violating (LFV) decays and, for the first time, we provide combined τ LVF upper limits. The τ lepton branching fractions, mass and lifetime are used to test lepton universality for charged weak currents and to compute the Cabibbo-Kobayashi-Maskawa (CKM) matrix element |Vus|.

Rare and forbidden kaon decays at NA62

A precision lepton universality test by measurement of the helicity suppressed ratio of leptonic decay rates of the charged kaon with ∼ 150000 K ± → e ± ν decays collected by the NA62 experiment in 2007-08 is presented. The record accuracy of 0.4% constrains the parameter space of new physics models with extended Higgs sector, a fourth generation of quarks and leptons or sterile neutrinos.An improved upper limit on the rate of the lepton number violating decay K ± → π ∓ μ ± μ ± , which probes the resonant enhancement of the rate in the presence of heavy Majorana neutrinos in the ∼ 100 MeV range, is presented.The rare decays K + → π + νν are excellent processes to make tests of new physics at the highest scale complementary to LHC thanks to their theoretically cleaness. The NA62 experiment at CERN SPS aims to collect of the order of 100 events in two years of data taking, keeping the background at the level of 10%.

Overview of Tau Physics

Impact of τ leptons on the frontiers of particle physics are overviewed. These include studies of the Higgs, SUSY and other new particles in different exotic models decaying to τ leptons, searches for lepton flavor violation and charge-parity violation in τ decays, tests of lepton universality, αs,g−2 and |Vus| determination from τ decays.

HFAG tau lepton averages

The HFAG-Tau group has computed the world averages of the tau lepton properties using the experimental results available in early 2012. The tau lepton branching fractions, mass and lifetime are used to test lepton universality for charged weak currents and to compute |Vus|. For the first time, all |Vus| measurements based primarily on tau data have been combined in a single measurement. Searches for lepton flavor violating decays of the tau lepton are also summarized.

Rare Kaon Decay Experiments at CERN

The NA62 experiment at the CERN SPS aims to measure the branching ratio of the rare decay K+→π+νν¯ with a relative precision of ∼10%. To achieve that goal, it is designed to be exposed to 1.2×1013K+ decays in its fiducial volume. The unprecedented K+ flux will lead to record sensitivities to rare and forbidden decays of K+ and π0, including those that violate lepton flavour or lepton number conservation. The expected NA62 performances for lepton flavour conservation and lepton universality tests are discussed. Relevant on-going or recently completed measurements from the K± decay data sets collected by earlier kaon experiments at CERN (NA48/2 and NA62-RK) are also presented.

NA62 experiment at CERN SPS

The NA62 experiment at SPS is a continuation of the long standing CERN kaon physics program. The high statistics and the unprecedent precision allow to probe the Standard Model and test the description of the strong interactions at low energy. The final results on the the lepton universality test by measuring the ratio $R_K = \Gamma(K^{+}\to e^{+}\nu) / \Gamma(K^{+}\to \mu^{+}\nu)$ and the study of the $K^\pm\to\pi^{\pm}\gamma\gamma $ decay are presented. The major goal of the NA62 experiment is to perform a measurement of the $Br(K^{+} \to \pi^{+} \nu \bar{\nu})$ with a precision of 10% in two years of data taking. The detector setup together with the analysis technique is described.

Search for new Physics with theπ → evDecay

In the Standard Model, lepton universality refers to the identical electroweak gauge interactions among charged leptons. The measurement of the ratio is one of the most stringent tests of lepton universality between the first two generations. The TRIUMF PIENU experiment aims at the most precise test of universality measuring with 0.1% precision. The measurement will provide improved constraints to physics beyond the SM or uncover new scenarios if a disagreement is found.

Search for exotic new Physics at BABAR

We present the recent BABAR results for direct new physics searches. In particular we describe searches in the τ sector, with new results concerning lepton flavor violation searches, and in the Υ(nS) sector, where light Higgs and dark matter candidates are sought after and searches for lepton flavor violating decays are performed. We also present precise tests of lepton universality in Υ(nS) decays.