Muon Modes Research Articles

Form factors and branching fraction calculations for $$B_s \rightarrow D_s^{(*)} \ell ^+ \nu _\ell$$ in view of LHCb observation

In light of the LHCb observations about the $$B_s \rightarrow D_s^{(*)}\ell \nu _\ell$$ semileptonic decays, we study these channels within the standard model framework of covariant confined quark model. The necessary transition form factors are computed in the entire dynamical range of momentum transfer squared with built-in infrared confinement. Our computed ratios of the decay widths from tau mode to muon mode for $$D_s$$ and $$D_s^*$$ mesons are found to be $$R(D_s) = 0.271 \pm 0.069$$ and $$R(D_s^*) = 0.240 \pm 0.038$$ . We further determine the ratio of the decay width from $$D_s$$ and $$D_s^*$$ channel for muon mode $$\Gamma (B_s \rightarrow D_s \mu ^+ \nu _\mu )/\Gamma (B_s \rightarrow D_s^* \mu ^+ \nu _\mu ) = 0.451 \pm 0.093$$ . Our results are in excellent agreement with the data from the latest LHCb experiments as well as lattice quantum chromodynamics simulations. We also compare the shape of differential decay distribution for $$B_s \rightarrow D_s^* \mu ^+ \nu _\mu$$ with the LHCb data, and our results are in very good agreement throughout all the individual bins. Some other physical observables such as forward-backward asymmetry and longitudinal polarizations of leptons in the final state are also computed.

Combined search for light dark matter with electron and muon beams at NA64

We discuss prospects of searching for a dark photon (A′) which serves as mediator between Standard model (SM) particles and light dark matter (LDM) by using the combined results from the NA64 experiment at the CERN SPS running in high-energy electron (NA64e) and muon (NA64μ) modes. We discuss the most natural values and upper bounds on the A′ coupling constant to LDM and show they are lying in the range accessible at NA64. While for the projected 5×1012 electrons on target (EOT) NA64e is able to probe the scalar and Majorana LDM scenarios, the combined NA64e and NA64μ results with ≃1013 EOT and a few 1013 MOT, respectively, will allow covering significant region in the parameter space of the most interesting LDM models. This makes NA64e and NA64μ extremely complementary to each other and increases significantly the discovery potential of sub-GeV DM.

Exploring leptophilic dark matter with NA64-\u03bc

We investigate the prospects for detecting light leptophilic dark sectors with a missing-momentum experiment at NA64 running in muon mode. In particular, we consider models in which dark matter connects to the visible sector through a lepton- or muon-specific scalar mediator. These scalars can also account for the ∼ 3.5σ discrepancy between the measured and predicted values of (g −2)μ. We emphasize the complementarity between NA64-μ and other terrestrial and astrophysical probes.

Analyzing lepton flavor universality in the decays Λb→Λc(*)(12±,32−)+ℓν¯ℓ

Lepton flavor universality can be tested in the semileptonic decays $\Lambda_b\to \Lambda_c^{(\ast)}$ where $\Lambda_c^{(\ast)}$ denotes either the ground state $\Lambda_c(2286)$ (with $J^P=1/2^+$) or its orbital excitations $\Lambda_c(2595)$ (with $J^P=1/2^-$) and $\Lambda_c(2625)$ (with $J^P=3/2^-$). We calculate the differential decay rates as well as the branching fractions of these decays for both tauonic and muonic modes with form factors obtained from a covariant confined quark model previously developed by us. We present results for the rate ratios of the tauonic and muonic modes which provide important tests of lepton flavor universality in forthcoming experiments.

Null tests from angular distributions in D→P1P2l+l− , l=e , μ decays on and off peak

We systematically analyze the full angular distribution in $D \to P_1 P_2 l^+ l^-$ decays, where $P_{1,2}=\pi,K$, $l=e,\mu$. We identify several null tests of the standard model (SM). Notably, the angular coefficients $I_{5,6,7}$, driven by the leptons' axial-vector coupling $C_{10}^{(\prime)}$, vanish by means of a superior GIM-cancellation and are protected by parity invariance below the weak scale. CP-odd observables related to the angular coefficients $I_{5,6,8,9}$ allow to measure CP-asymmetries without $D$-tagging. The corresponding observables $A_{5,6,8,9}$ constitute null tests of the SM. Lepton universality in $|\Delta c| =|\Delta u|=1$ transitions can be tested by comparing $D \to P_1 P_2 \mu^+ \mu^-$ to $D \to P_1 P_2 e^+ e^-$ decays. Data for $P_1 P_2=\pi^+ \pi^-$ and $K^+ K^-$ on muon modes are available from LHCb and on electron modes from BESIII. Corresponding ratios of dimuon to dielectron branching fractions are at least about an order of magnitude away from probing the SM. In the future electron and muon measurements should be made available for the same cuts as corresponding ratios $R_{P_1 P_2}^D$ provide null tests of $e$-$\mu$-universality. We work out beyond-SM signals model-independently and in SM extensions with leptoquarks.

Patterns of New Physics in b \u2192 s\u2113+\u2113\u2212 transitions in the light of recent data

In the Standard Model (SM), the rare transitions where a bottom quark decays into a strange quark and a pair of light leptons exhibit a potential sensitivity to physics beyond the SM. In addition, the SM embeds Lepton Flavour Universality (LFU), leading to almost identical probabilities for muon and electron modes. The LHCb collaboration discovered a set of deviations from the SM expectations in decays to muons and also in ratios assessing LFU. Other experiments (Belle, ATLAS, CMS) found consistent measurements, albeit with large error bars. We perform a global fit to all available b → sℓ+ℓ− data (ℓ = e, μ) in a model-independent way allowing for different patterns of New Physics. For the first time, the NP hypothesis is preferred over the SM by 5 σ in a general case when NP can enter SM-like operators and their chirally-flipped partners. LFU violation is favoured with respect to LFU at the 3-4 σ level. We discuss the impact of LFU-violating New Physics on the observable P5′ from B → K∗μ+μ− and we compare our estimate for long-distance charm contributions with an empirical model recently proposed by a group of LHCb experimentalists. Finally, we discuss NP models able to describe this consistent pattern of deviations.

Dynamical tides in coalescing superfluid neutron star binaries with hyperon cores and their detectability with third-generation gravitational-wave detectors

The dynamical tide in a coalescing neutron star binary induces phase shifts in the gravitational waveform as the orbit sweeps through resonances with individual g-modes. Unlike the phase shift due to the equilibrium tide, the phase shifts due to the dynamical tide are sensitive to the stratification, composition, and superfluid state of the core. We extend our previous study of the dynamical tide in superfluid neutron stars by allowing for hyperons in the core. Hyperon gradients give rise to a new type of composition g-mode. Compared to g-modes due to muon-to-electron gradients, those due to hyperon gradients are concentrated much deeper in the core and therefore probe higher density regions. We find that the phase shifts due to resonantly excited hyperonic modes are ~ 10^{-3} rad, an order of magnitude smaller than those due to muonic modes. We show that by stacking events, third generation gravitational-wave detectors should be able to detect the phase shifts due to muonic modes. Those due to hyperonic modes will, however, be difficult to detect due to their smaller magnitude.

Lepton-Flavor-Dependent Angular Analysis of B→K^{*}ℓ^{+}ℓ^{-}.

We present a measurement of angular observables and a test of lepton flavor universality in the B→K^{*}ℓ^{+}ℓ^{-} decay, where ℓ is either e or μ. The analysis is performed on a data sample corresponding to an integrated luminosity of 711 fb^{-1} containing 772×10^{6} BB[over ¯] pairs, collected at the ϒ(4S) resonance with the Belle detector at the asymmetric-energy e^{+}e^{-} collider KEKB. The result is consistent with standard model (SM) expectations, where the largest discrepancy from a SM prediction is observed in the muon modes with a local significance of 2.6σ.

Tension in the inclusive versus exclusive determinations of |Vcb| : A possible role of new physics

We reconsider the possibility that the tension in the $|V_{cb}|$ determinations from inclusive and exclusive $B$ decay modes is due to a new physics effect. We modify the Standard Model effective Hamiltonian for semileptonic $b \to c$ transitions including a tensor operator with a lepton flavour dependent coupling $\epsilon_T^\ell$, and investigate separately the muon and electron modes. The interference term between SM and NP, proportional to the lepton mass, has different impact in the inclusive and exclusive $B$ modes to muon. Moreover, even when the lepton mass is small as for the electron, the NP effect is different in inclusive and exclusive $B$ channels. For both $\mu$ and $e$ we find a region of $\epsilon_T^{\mu,\,e}$ where the constraints from $B^- \to D^{(*)0} \ell^- {\bar \nu}_\ell$ and $B \to X_c \, \ell \, \bar \nu_\ell$ are satisfied for the same $|V_{cb}|$.

Nonstandard semileptonic hyperon decays.

We investigate the discovery potential of semileptonic hyperon decays in terms of searches of new physics at teraelectronvolt scales. These decays are controlled by a small SU(3)-flavor breaking parameter that allows for systematic expansions and accurate predictions in terms of a reduced dependence on hadronic form factors. We find that muonic modes are very sensitive to nonstandard scalar and tensor contributions and demonstrate that these could provide a powerful synergy with direct searches of new physics at the LHC.

Formula omitted] measurements with NA48/NA62

The NA48/2 collaboration has analysed more than one million K±→π+π−e±ν decays, resulting in the improved determination of branching fraction by a factor of 3 and high precision form factor measurements. Concurrently, about 45000 K±→π0π0e±ν decays have been analyzed. Background contamination below the percent level and very good π0 reconstruction allow the first accurate measurement of the branching fraction and decay form factor at the percent level. The achieved precision makes possible the observation of small effects such as π+π−→π0π0 re-scattering. Both Ke4 modes decay properties bring new precise inputs to Low Energy QCD studies and can provide strong tests of ChPT predictions. Future prospects include the observation of several thousand decays in similar muonic modes K±→π0π0μ±ν and K±→π+π−μ±ν, that could be studied also in the forthcoming NA62 experiment.

Recent results and prospects from NA48/2: [formula omitted] and [formula omitted], [formula omitted] decays

In 2003–2004 the NA48/2 Collaboration at CERN collected a large sample of K± decays. Using data taken with minimal trigger conditions, a sample of 2.5×106K±→π0μ±ν and 4.0×106K±→π0e±ν events was recorded, and allows precise measurements of the decay form factors. This report describes the event selection and the fitting procedure, and gives a preliminary result for the form factors in various parameterizations. In addition, with the full data sample collected, more than one million K±→π+π−e±ν and about 45000K±→π0π0e±ν decays has been analysed. An improved determination of the branching ratio and high precision form factor measurements for K±→π+π−e±ν, and a preliminary result for the branching ratio of K±→π0π0e±ν are presented in this paper. Future prospects include the observation of several thousands decays in similar muonic modes that will be studied at the NA62 experiment.

Implications ofRparity violating Yukawa couplings inΔS=1semileptonic decays ofKmesons

We present a class of constraints on products and combinations of Yukawa couplings for $R$ parity violating (${\mathrm{R\ensuremath{\llap{\not\;}}}}_{p}$) and lepton flavor conserving as well as violating semileptonic decays of $K$ mesons into light pseudoscalar mesons along with two charged leptons at $1\ensuremath{\sigma}$ and $2\ensuremath{\sigma}$ levels. We compare the constraints obtained by semileptonic rare decays with pure leptonic rare decays and find that most of these bounds are now improved over the existing ones. We also study the forward-backward asymmetry in the decays of ${K}^{+}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{+}{l}^{+}{l}^{\ensuremath{-}}$ ($l=e$ and $\ensuremath{\mu}$) in the absence of tensor terms. The asymmetry is found to be up to $O({10}^{\ensuremath{-}3})$ ($O({10}^{\ensuremath{-}1})$) for the electron and muon modes, respectively. The asymmetry is found to be as large as $O({10}^{\ensuremath{-}1})$ in the case of ${K}^{+}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{+}{\ensuremath{\mu}}^{+}{\ensuremath{\mu}}^{\ensuremath{-}}$.

Probing non-standard neutrino interactions with neutrino factories

We discuss the sensitivity reach of a neutrino factory measurement to non-standard neutrino interactions (NSI), which may exist as a low-energy manifestation of physics beyond the Standard Model. We use the muon appearance mode \nu_e --> \nu_\mu and consider two detectors, one at 3000 km and the other at 7000 km. Assuming the effects of NSI at the production and the detection are negligible, we discuss the sensitivities to NSI and the simultaneous determination of \theta_{13} and \delta by examining the effects in the neutrino propagation of various systems in which two NSI parameters \epsilon_{\alpha \beta} are switched on. The sensitivities to off-diagonal \epsilon's are found to be excellent up to small values of \theta_{13}. We demonstrate that the two-detector setting is powerful enough to resolve the \theta_{13}-NSI confusion problem. We believe that the results obtained in this paper open the door to the possibility of using neutrino factory as a discovery machine for NSI while keeping its primary function of performing precision measurements of the lepton mixing parameters.

More model-independent analysis ofb→sprocesses

We study model-independently the implications of nonstandard scalar and pseudoscalar interactions for the decays $\stackrel{\ensuremath{\rightarrow}}{b}s\ensuremath{\gamma},$ $\stackrel{\ensuremath{\rightarrow}}{b}sg,$ $\stackrel{\ensuremath{\rightarrow}}{b}s{\mathcal{l}}^{+}{\mathcal{l}}^{\ensuremath{-}}$ $(\mathcal{l}=e,\ensuremath{\mu})$ and ${B}_{s}\ensuremath{\rightarrow}{\ensuremath{\mu}}^{+}{\ensuremath{\mu}}^{\ensuremath{-}}.$ We find sizable renormalization effects from scalar and pseudoscalar four-quark operators in the radiative decays and at $O({\ensuremath{\alpha}}_{s})$ in hadronic b decays. Constraints on the Wilson coefficients of an extended operator basis are worked out. Further, the ratios ${R}_{H}=\mathcal{B}(\stackrel{\ensuremath{\rightarrow}}{B}H{\ensuremath{\mu}}^{+}{\ensuremath{\mu}}^{\ensuremath{-}})/\mathcal{B}(\stackrel{\ensuremath{\rightarrow}}{B}{\mathrm{He}}^{+}{e}^{\ensuremath{-}}),$ for ${H=K}^{(*)}{,X}_{s},$ and their correlations with the ${B}_{s}\ensuremath{\rightarrow}{\ensuremath{\mu}}^{+}{\ensuremath{\mu}}^{\ensuremath{-}}$ decay are investigated. We show that the standard model prediction for these ratios defined with the same cut on the dilepton mass for electron and muon modes, ${R}_{H}{=1+O(m}_{\ensuremath{\mu}}^{2}{/m}_{b}^{2}),$ has a much smaller theoretical uncertainty $(\ensuremath{\lesssim}1%)$ than the one for the individual branching fractions. The present experimental limit ${R}_{K}<~1.2$ puts constraints on scalar and pseudoscalar couplings, which are similar to the ones from current data on $\mathcal{B}{(B}_{s}\ensuremath{\rightarrow}{\ensuremath{\mu}}^{+}{\ensuremath{\mu}}^{\ensuremath{-}}).$ We find that new physics corrections to ${R}_{{K}^{*}}$ and ${R}_{{X}_{s}}$ can reach 13% and 10%, respectively.

Testbeam results of a slat CPC prototype for the ALICE Dimuon Spectrometer

The ALICE Dimuon Spectrometer will measure the production of vector mesons in their muon decay mode in heavy ions collisions at the LHC in 2007. The Spectrometer includes a Tracking System, composed of 10 planes of Cathode Pad Chambers. Six larger planes (Stations 3–5) consists of several independent detectors, called slats. This poster presents the results, obtained in test beams at CERN in 2002, on a final slat of 1.20 m long.

Forward-backward asymmetry inK+→π+l+l−

We study the forward-backward asymmetries in the decays of $K^+\to\pi^+\ell^+ \ell^-$ ($\ell=e$ and $\mu$) in the presence of scalar or tensor terms. We find that with the scalar (tensor) type interaction the asymmetry can be up to ${\cal O}(10^{-3})$ (${\cal O}(10^{-1}))$ and arbitrary large for the electron and muon modes, respectively, without conflict with the experimental data. We also discuss the cases in the minimal supersymmetric standard model where the scalar terms can be induced. In particular we show that the asymmetry in $K^+\to\pi^+\mu^+\mu^-$ can be as large as ${\cal O}(10^{-3})$ in the large $\tan\beta$ limit, which can be tested in future experiments such as CKM at Fermilab.

U(1) flavor symmetry and proton decay in supersymmetric standard model

Issues of proton decay are examined in supersymmetric standard model with U(1) flavor symmetry. Dimension five proton-decay operators which arise generically are controlled by the flavor symmetry. We show that unlike the minimal supersymmetric SU(5) case the proton decay modes containing charged lepton can have large branching ratios if the dimension five operators of left-handed type dominate. Measuring the branching ratio of the electron mode to the muon mode may reveal the mechanism of the neutrino mass generation. The case with vanishing charges for Higgs doublets marginally survives the present experimental bound on proton life time.

Measurement of the branching ratio [formula omitted

We report a measurement of the branching ratio B(D + → ϱ 0ℓ +ν ℓ) B(D + → K ∗0ℓ +ν ℓ) from the Fermilab charm hadroproduction experiment E791. Based on signals of 49 ± 17 events in the D + → ϱ 0 e + ν e mode and 54 ± 18 events in the D + → ϱ 0 μ + ν μ mode, we measure. B(D + → ϱ 0e +ν e) B(D + → ▪ e +ν e) = 0.014 ± 0.014 ± 0.009, B(D + → ϱ 0μ +ν μ) B(D + → ▪ μ +ν μ) = 0.051 ± 0.015 ± 0.009. Combining the results from both the electronic and muonic modes, we obtain B(D + → ϱ 0ℓ +ν ℓ) B(D + → ▪ ℓ +ν ℓ) = 0.047 ± 0.013. This result is compared to theoretical predictions.

Results on charm semileptonic decays from E687

Recent results [11] for the Cabibbo suppressed semileptonic decays D0 → π−e+v and D0 → π−μ+v (charge conjugates are implied) are reported by Fermilab photoproduction experiment E687. We find 45.4 ± 13.3 events in the electron mode and 45.6 ± 11.8 in the muon mode. In addition, a general technique for using our data to measure the pseudoscaler to pseudoscaler decay form factors of any theoretical model is presented.