Current World Average Research Articles

Precision measurement of the Ξb− baryon lifetime

A sample of pp collision data, corresponding to an integrated luminosity of 5.5 fb−1 and collected by the LHCb experiment during LHC Run 2, is used to measure the ratio of the lifetime of the Ξb− baryon to that of the Λb0 baryon, rτ≡τΞb−/τΛb0. The value rτ=1.076±0.013±0.006 is obtained, where the first uncertainty is statistical and the second systematic. This value is averaged with the corresponding value from Run 1 to obtain rτRun 1,2=1.078±0.012±0.007. Multiplying by the world-average value of the Λb0 lifetime yields τΞb−Run 1,2=1.578±0.018±0.010±0.011 ps, where the uncertainties are statistical, systematic, and due to the limited knowledge of the Λb0 lifetime. This measurement improves the precision of the current world average of the Ξb− lifetime by about a factor of 2, and is in good agreement with the most recent theoretical predictions. © 2024 CERN, for the LHCb Collaboration 2024 CERN

Charmonium decays of beauty baryons in the perturbative QCD approach

Motivated by recent advances in experimental measurements of heavy baryon decays, the charmonium decays of single b baryon are investigated systematically in the framework of perturbative QCD approach. We calculate the decay branching ratios and helicity amplitudes as well as many pertinent decay asymmetry parameters that characterize the angular decay distributions. In particular, we estimate the fragmentation fractions of b quark to b baryons based on the current world averages of the branching ratio multiplied by fragmentation fraction. Some of the featured results are compared with the predictions of other approaches and experimental data. We also evaluate the branching ratios and angular asymmetries of the Σb and Ξb′ decay modes, which have neither been measured experimentally nor calculated theoretically. Our results could be helpful for a future experimental search for them and provide deeper insights in the understanding of the dynamics of heavy-flavor weak decay processes. Published by the American Physical Society 2024

Measurement of the branching fractions for Cabibbo-suppressed decays D+→K+K−π+π0 and D(s)+→K+π−π+π0 at Belle

We present measurements of the branching fractions for the singly Cabibbo-suppressed decays D+→K+K−π+π0 and Ds+→K+π−π+π0, and the doubly Cabibbo-suppressed decay D+→K+π−π+π0, based on 980 fb−1 of data recorded by the Belle experiment at the KEKB e+e− collider. We measure these modes relative to the Cabibbo-favored modes D+→K−π+π+π0 and Ds+→K+K−π+π0. Our results for the ratios of branching fractions are B(D+→K+K−π+π0)/B(D+→K−π+π+π0)=(11.32±0.13±0.26)%, B(D+→K+π−π+π0)/B(D+→K−π+π+π0)=(1.68±0.11±0.03)%, and B(Ds+→K+π−π+π0)/B(Ds+→K+K−π+π0)=(17.13±0.62±0.51)%, where the uncertainties are statistical and systematic, respectively. The second value corresponds to (5.83±0.42)×tan4θC, where θC is the Cabibbo angle; this value is larger than other measured ratios of branching fractions for a doubly Cabibbo-suppressed charm decay to a Cabibbo-favored decay. Multiplying these results by world average values for B(D+→K−π+π+π0) and B(Ds+→K+K−π+π0) yields B(D+→K+K−π+π0)=(7.08±0.08±0.16±0.20)×10−3, B(D+→K+π−π+π0)=(1.05±0.07±0.02±0.03)×10−3, and B(Ds+→K+π−π+π0)=(9.44±0.34±0.28±0.32)×10−3, where the third uncertainty is due to the branching fraction of the normalization mode. The first two results are consistent with, but more precise than, the current world averages. The last result is the first measurement of this branching fraction.Received 3 May 2022Accepted 28 September 2022DOI:https://doi.org/10.1103/PhysRevD.107.033003Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI. Funded by SCOAP3.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasElectroweak interactionPhysical SystemsCharmed mesonsTechniquesParticle decaysParticles & Fields

CP -violating observables in four-body B→ϕ(→KK¯)K*(→Kπ) decays

We analyze the four-body $B\ensuremath{\rightarrow}\ensuremath{\phi}(\ensuremath{\rightarrow}K\overline{K}){K}^{*}(\ensuremath{\rightarrow}K\ensuremath{\pi})$ decays in the perturbative QCD approach, where the invariant mass of the $K\overline{K}$ ($K\ensuremath{\pi}$) system is limited in a window of $\ifmmode\pm\else\textpm\fi{}15\text{ }\text{ }\mathrm{MeV}$ ($\ifmmode\pm\else\textpm\fi{}150\text{ }\text{ }\mathrm{MeV}$) around the nominal $\ensuremath{\phi}({K}^{*}(892))$ mass. In addition to the dominant $P$-wave resonances, two important $S$-wave backgrounds in the selected invariant mass region are also accounted for. Angular momentum conservation allows six helicity amplitudes to contribute, including three $P$ waves, two single $S$ waves, and one double $S$ wave, in the decays under study. We calculated the branching ratio for each component and found sizable $S$-wave contributions, which coincide roughly with the experimental observation. The obtained branching ratios of ${B}^{0(+)}\ensuremath{\rightarrow}\ensuremath{\phi}{K}^{*0(+)}$ are comparable with the previous theoretical predictions and support the experimental measurements, whereas the predicted $\mathcal{B}({B}_{s}^{0}\ensuremath{\rightarrow}\ensuremath{\phi}{\overline{K}}^{*0})$ is an order of magnitude smaller than the current world average in its central value. The longitudinal polarizations are predicted to be around 0.7, consistent with previous PQCD results but larger than the world average values. Aside from the direct $CP$ asymmetries, the true and fake triple product asymmetries, originating from the interference between the perpendicular polarization amplitude and other helicity amplitudes, are also calculated in this work. In the special case of the neutral modes, both the direct $CP$ asymmetries and true triple product asymmetries are expected to be zero due to the vanishing weak phase difference. The direct $CP$ asymmetries for the ${B}^{+}$ mode are predicted to be tiny, of order ${10}^{\ensuremath{-}2}$, since the tree contributions are suppressed strongly with respect to the penguin ones. The true triple product asymmetries have shown no significant deviations from zero. In contrast, large fake asymmetries are observed in these decays, indicating the presence of significant final-state interactions. We give the theoretical predictions of the $S$-wave induced triple product asymmetries for the first time, which is consistent with current LHCb data and would be checked with future measurements from Belle and BABAR experiments if the $S$-wave components can be properly taken into account in the angular analysis.

Mixing and CP violation in D0 → K−π+ decays

We review the experimental methods to measure mixing and time-dependent CP violation in D0 decays into two hadrons. While these phenomena are usually neglected for D0 → K−π+ decays, this approximation is not always justified. In particular, it produces a bias on the measurement of the parameter yCP, when this is performed by relying on D0 → K−π+ decays as a normalisation channel, whose size is around 40% of the precision of the current world average. Finally, we estimate the sensitivity to the weak mixing phases achievable by studying D0 → K−π+ and untagged D → K−π+ decays, where D stands for either of the D0 and overline{D} 0 mesons. Contrary to Cabibbo-suppressed D0 → h+h− decays, these decay channels allow to measure these phases without final-state dependent nuisance contributions from the decay amplitudes, but their sensitivity is lower by a factor of six.

Simultaneous determination of CKM angle \u03b3 and charm mixing parameters

A combination of measurements sensitive to the CP violation angle γ of the Cabibbo-Kobayashi-Maskawa unitarity triangle and to the charm mixing parameters that describe oscillations between D0 and overline{D} 0 mesons is performed. Results from the charm and beauty sectors, based on data collected with the LHCb detector at CERN’s Large Hadron Collider, are combined for the first time. This method provides an improvement on the precision of the charm mixing parameter y by a factor of two with respect to the current world average. The charm mixing parameters are determined to be x=left({0.400}_{-0.053}^{+0.052}right)% and y = left({0.630}_{-0.030}^{+0.033}right)% . The angle γ is found to be γ = left({65.4}_{-4.2}^{+3.8}right){}^{circ} and is the most precise determination from a single experiment.

Measurement of branching fractions of J/\u03c8 and \u03c8(3686) decays to \u03a3+ and overline{Sigma} \u2212

Using 1310.6 × 106J/ψ and 448.1 × 106ψ(3686) events collected with the BESIII detector, the branching fractions of J/ψ decays to Σ+ overline{Sigma} − is measured to be (10.61 ± 0.04 ± 0.36) × 10−4, which is significantly more precise than the current world average. The branching fractions of ψ(3686) decays to Σ+ overline{Sigma} − is measured to be (2.52 ± 0.04 ± 0.09) × 10−4, which is consistent with the previous measurements. In addition, the ratio of mathcal{B} (ψ(3686) → Σ+ overline{Sigma} −)/ mathcal{B} (J/ψ → Σ+ overline{Sigma} −) is determined to be (23.8 ± 1.1)% which violates the “12% rule”.

Nonleptonic K → 2 π decay dynamics

Using Chiral Perturbation Theory to properly account the dynamics of nonleptonic K → 2 π decays, we found the Standard Model prediction for the CP violating ratio Re (ε′/ε) = (14 ± 5) × 10−4, where isospin breaking effects are included, in perfect agreement with the current experimental world average. Similar results has been reported by a recent release of improved lattice data.

Salt reduction to prevent hypertension: the reasons of the controversy.

There is a causal relationship between dietary salt intake and blood pressure. A reduction in salt intake from the current world average of ∼10 g/day to the WHO recommended level of <5 g/day, lowers blood pressure and reduces the risk of cardiovascular disease and all-cause mortality. However, a few cohort studies have claimed that there is a J-shaped relationship between salt intake and cardiovascular risk, i.e. both high and low salt intakes are associated with an increased risk. These cohort studies have several methodological problems, including reverse causality, and inaccurate and biased estimation of salt intake, e.g. from a single spot urine sample with formulas. Recent studies have shown that the formulas used to estimate salt intake from spot urine cause a spurious J-curve. Research with inappropriate methodology should not be used to refute the robust evidence on the enormous benefits of population-wide reduction in salt intake. Several countries, e.g. Finland, the UK, have successfully reduced salt intake, which has resulted in falls in population blood pressure and deaths from stroke and ischaemic heart disease. Every country should develop and implement a coherent, workable strategy to reduce salt intake. Even a modest reduction in salt intake across the whole population will lead to a major improvement in public health, along with huge cost-savings to the healthcare service.

Exploring the tension between nature and the Standard Model: the muon g-2

Anomalous magnetic moment of the muon (muon g-2) is one of the most precisely measured quantities in particle physics. At the same time, it can be evaluated in the Standard Model with an unprecedented accuracy. The Muon g-2 experiment at Fermilab has started the major data collection and the aimed four-fold increase in precision will shed light on the current discrepancy between the theory prediction and the measured value. This renders a comparable improvement of the precision in the SM theory an essential ingredient in order to fully exploit the expected increase of precision in experimental results. For all these reasons, the muon g-2 is considered to be a great testing ground for new physics.Hadronic contributions are the dominant sources of uncertainty in the theoretical prediction of the muon g-2. A reciprocal effort to a precise determination of the leading hadronic contribution to the muon g-2 using lattice gauge theories is a direct measurement of the hadronic contributions to the running of the fine structure constant recently proposed by the MUonE experiment. A hybrid strategy including both experimental and lattice data sets is expected to give an independent check of the dispersive results from e+e-annihilation, which dominate the current world average.

Implication of flavor anomalies on decay observables

The experimental predictions of R D , , R J/ψ , and in B decays mediated via quark level transition deviate significantly from the standard model expectations. The current world average of the ratio of branching ratios R D and in (l ∈ e, τ) show 1.4σ and 2.5σ deviation from the SM expectations. Similarly, the τ polarization fraction and the longitudinal polarization fraction of the D* meson in are found to deviate from the standard model expectations at 1.6σ and 1.5σ level, respectively. In addition, the ratio of branching ratio R J/ψ in deviates from the standard model prediction at 1.7σ level. In this regard, we study the implication of R D , , R J/ψ , and anomalies on decay observables in a model independent effective field theory formalism. We give predictions of several physical observables in the standard model and in the presence of various 1D and 2D new physics scenarios.

Top quark pair production near threshold: single/double distributions and mass determination

We investigate top quark pair production near the threshold where the pair invariant mass {M}_{toverline{t}} approaches 2mt, which provides sensitive observables to extract the top quark mass mt. Using the effective field theory methods, we derive a factorization and resummation formula for kinematic distributions in the threshold limit up to the next-to- leading power, which resums higher order Coulomb corrections to all orders in the strong coupling constant. Our formula is similar to those in the literature but differs in several important aspects. We apply our formula to the {M}_{toverline{t}} distribution, as well as to the double differential cross section with respect to {M}_{toverline{t}} and the rapidity of the toverline{t} pair. We find that the resummation effects significantly increase the cross sections near the threshold, and lead to predictions better compatible with experimental data than the fixed-order ones. We demonstrate that incorporating resummation effects in the top quark mass determination can shift the extracted value of mt by as large as 1.4 GeV. The shift is much larger than the estimated uncertainties in previous experimental studies, and leads to a value of the top quark pole mass more consistent with the current world average.

First measurement of the CKM angle \u03d53 with B\xb1\u2192 D( {K}_{mathrm{S}}^0 \u03c0+\u03c0\u2212\u03c00) K\xb1 decays

We present the first model-independent measurement of the CKM unitarity triangle angle ϕ3 using B±→ D( {K}_{mathrm{S}}^0 π+π−π0) K± decays, where D indicates either a D0 or overline{D} 0 meson. Measurements of the strong-phase difference of the D → {K}_{mathrm{S}}^0 π+π−π0 amplitude obtained from CLEO-c data are used as input. This analysis is based on the full Belle data set of 772 × 106B overline{B} events collected at the Υ(4S) resonance. We obtain ϕ3 = ( {5.7}_{-8.8}^{+10.2} ±3.5±5.7)° and the suppressed amplitude ratio rB = 0.323±0.147±0.023±0.051. Here the first uncertainty is statistical, the second is the experimental systematic, and the third is due to the precision of the strong-phase parameters measured from CLEO-c data. The 95% confidence interval on ϕ3 is (−29.7, 109.5)°, which is consistent with the current world average.

Polarization and entanglement in baryon–antibaryon pair production in electron–positron annihilation

Using a sample of $1.31\times 10^9$ $J/\psi$ events collected with the BESIII detector, we report the first observation of spin polarization of $\Lambda$ and $\bar\Lambda$ hyperons from the coherent production in the $J/\psi\to\Lambda\bar\Lambda$ decay. We measure the phase between the hadronic form factors to be $\Delta\Phi=(42.4\pm0.6\pm0.5)^\circ$. The decay parameters for $\Lambda\to p\pi^-$ ($\alpha_-$), $\bar\Lambda\to\bar p\pi^+$ ($\alpha_+$) and $\bar\Lambda\to\bar n\pi^0$ ($\bar\alpha_0$) are measured to be $\alpha_-=0.750\pm0.009\pm0.004$, $\alpha_+=-0.758\pm0.010\pm0.007$ and $\bar\alpha_0=-0.692\pm0.016\pm0.006$, respectively. The obtained value of $\alpha_-$ is higher by $(17\pm 3)\%$ than the current world average. In addition, the $CP$ asymmetry of $-0.006\pm0.012\pm0.007$ is extracted with substantially improved precision. The ratio $\bar{\alpha}_0/\alpha_{+} = 0.913\pm 0.028 \pm 0.012$ is also measured.

Measurement of CP violation in B0 \u2192 D\u2213\u03c0\xb1 decays

A measurement of the CP asymmetries Sf and {S}_{overline{f}} in B0 → D∓π± decays is reported. The decays are reconstructed in a dataset collected with the LHCb experiment in proton-proton collisions at centre-of-mass energies of 7 and 8 TeV and corresponding to an integrated luminosity of 3.0 fb−1. The CP asymmetries are measured to be Sf = 0.058 ± 0.020(stat) ± 0.011(syst) and {S}_{overline{f}}=0.038pm 0.020left(mathrm{stat}right)pm 0.007left(mathrm{syst}right) . These results are in agreement with, and more precise than, previous determinations. They are used to constrain angles of the unitarity triangle, | sin (2β + γ) | and γ, to intervals that are consistent with the current world-average values.

Measurement of b hadron lifetimes in pp collisions at sqrt{s} = 8,text {Te}text {V}

Measurements are presented of the lifetimes of the {mathrm {B}^0}, mathrm {B}^0_mathrm {s}, mathrm {Lambda }_mathrm {b} ^0, and mathrm {B}_mathrm {c}^+ hadrons using the decay channels {mathrm {B}^0}!rightarrow ! mathrm {J}/psi mathrm {K^{*}(892)}{}^{0}, {mathrm {B}^0}!rightarrow ! mathrm {J}/psi mathrm {K^0_S}, mathrm {B}^0_mathrm {s} !rightarrow ! mathrm {J}/psi mathrm {pi ^+}mathrm {pi ^-}, mathrm {B}^0_mathrm {s} !rightarrow ! mathrm {J}/psi mathrm {phi (1020)}, varLambda _mathrm {b}^0!rightarrow !mathrm {J}/psi mathrm {Lambda }^{0}, and mathrm {B}_mathrm {c}^+ !rightarrow !mathrm {J}/psi mathrm {pi ^+}. The data sample, corresponding to an integrated luminosity of 19.7,text {fb}^text {-1}, was collected by the CMS detector at the LHC in proton–proton collisions at sqrt{s}=8,text {Te}text {V} . The {mathrm {B}^0} lifetime is measured to be 453.0 pm 1.6,text {(stat)} pm 1.8,text {(syst)} ,upmu text {m} in mathrm {J}/psi mathrm {K^{*}(892)}{}^{0}and 457.8 pm 2.7,text {(stat)} pm 2.8,text {(syst)} ,upmu text {m} in mathrm {J}/psi mathrm {K^0_S}, which results in a combined measurement of ctau _{{mathrm {B}^0}} = 454.1 pm 1.4,text {(stat)} pm 1.7,text {(syst)} ,upmu text {m} . The effective lifetime of the mathrm {B}^0_mathrm {s} meson is measured in two decay modes, with contributions from different amounts of the heavy and light eigenstates. This results in two different measured lifetimes: ctau _{mathrm {B}^0_mathrm {s} rightarrow mathrm {J}/psi mathrm {pi ^+}mathrm {pi ^-}} = 502.7 pm 10.2,text {(stat)} pm 3.4,text {(syst)} ,upmu text {m} and ctau _{mathrm {B}^0_mathrm {s} rightarrow mathrm {J}/psi mathrm {phi (1020)}} = 443.9 pm 2.0,text {(stat)} pm 1.5,text {(syst)} ,upmu text {m} . The mathrm {Lambda }_mathrm {b} ^0 lifetime is found to be 442.9 pm 8.2,text {(stat)} pm 2.8,text {(syst)} ,upmu text {m} . The precision from each of these channels is as good as or better than previous measurements. The mathrm {B}_mathrm {c}^+ lifetime, measured with respect to the {mathrm {B}^{+}} to reduce the systematic uncertainty, is 162.3 pm 7.8,text {(stat)} pm 4.2,text {(syst)} pm 0.1,(tau _{{mathrm {B}^{+}}}),upmu text {m} . All results are in agreement with current world-average values.

χ_{c1} and χ_{c2} Resonance Parameters with the Decays χ_{c1,c2}→J/ψμ^{+}μ^{-}.

The decays χ_{c1}→J/ψμ^{+}μ^{-} and χ_{c2}→J/ψμ^{+}μ^{-} are observed and used to study the resonance parameters of the χ_{c1} and χ_{c2} mesons. The masses of these states are measured to be m(χ_{c1})=3510.71±0.04(stat)±0.09(syst) MeV and m(χ_{c2})=3556.10±0.06(stat)±0.11(syst) MeV, where the knowledge of the momentum scale for charged particles dominates the systematic uncertainty. The momentum-scale uncertainties largely cancel in the mass difference m(χ_{c2})-m(χ_{c1})=45.39±0.07(stat)±0.03(syst) MeV. The natural width of the χ_{c2} meson is measured to be Γ(χ_{c2})=2.10±0.20(stat)±0.02(syst) MeV. These results are in good agreement with and have comparable precision to the current world averages.

Reconsidering the one leptoquark solution: flavor anomalies and neutrino mass

We reconsider a model introducing a scalar leptoquark ϕ ∼ (3,1, −1/3) to explain recent deviations from the standard model in semileptonic B decays. The leptoquark can accommodate the persistent tension in the decays overline{B}to {D}^{left(ast right)}tau overline{nu} as long as its mass is lower than approximately 10 TeV, and we show that a sizeable Yukawa coupling to the right-chiral tau lepton is necessary for an acceptable explanation. A characteristic prediction of this scenario is a value of {R}_{D^{*}} slightly smaller than the current world average. Agreement with the measured overline{B}to {D}^{left(ast right)}tau overline{nu} rates is mildly compromised for parameter choices addressing the tensions in b → sμμ, where the model can significantly reduce the discrepancies in angular observables, branching ratios and the lepton-flavor-universality observables RK and {R}_{K^{*}} . The leptoquark can also reconcile the predicted and measured value of the anomalous magnetic moment of the muon and appears naturally in models of radiative neutrino mass derived from lepton-number violating effective operators. As a representative example, we incorporate the particle into an existing two-loop neutrino mass scenario derived from a dimension-nine operator. In this specific model, the structure of the neutrino mass matrix provides enough freedom to explain the small masses of the neutrinos in the region of parameter space dictated by agreement with the anomalies in overline{B}to {D}^{left(ast right)}tau overline{nu} , but not the b → s transition. This is achieved without excessive fine-tuning in the parameters important for neutrino mass.

Measurement of the top quark mass using single top quark events in proton-proton collisions at sqrt{s}= 8\xa0TeV

A measurement of the top quark mass is reported in events containing a single top quark produced via the electroweak t channel. The analysis is performed using data from proton-proton collisions collected with the CMS detector at the LHC at a centre-of-mass energy of 8 TeV, corresponding to an integrated luminosity of 19.7 fb^{-1}. Top quark candidates are reconstructed from their decay to a mathrm {W} boson and a b quark, with the mathrm {W} boson decaying leptonically to a muon and a neutrino. The final state signature and kinematic properties of single top quark events in the t channel are used to enhance the purity of the sample, suppressing the contribution from top quark pair production. A fit to the invariant mass distribution of reconstructed top quark candidates yields a value of the top quark mass of 172.95 pm 0.77,text {(stat)} ^{+0.97}_{-0.93},text {(syst)} ,text {GeV} . This result is in agreement with the current world average, and represents the first measurement of the top quark mass in event topologies not dominated by top quark pair production, therefore contributing to future averages with partially uncorrelated systematic uncertainties and a largely uncorrelated statistical uncertainty.

Determination of the muonic branching ratio of the W boson and its total width via cross-section measurements at the Tevatron and LHC

The total $W$-boson decay width $\Gamma_W$ is an important observable which allows testing of the standard model. The current world average value is based on direct measurements of final state kinematic properties of $W$-boson decays, and has a relative uncertainty of 2%. The indirect determination of $\Gamma_W$ via the cross-section measurements of vector-boson production can lead to a similar accuracy. The same methodology leads also to a determination of the leptonic branching ratio. This approach has been successfully pursued by the CDF and D0 experiments at the Tevatron collider, as well as by the CMS collaboration at the LHC. In this paper we present for the first time a combination of the available measurements at hadron colliders, accounting for the correlations of the associated systematic uncertainties. Our combination leads to values of $\textrm{BR}(W\rightarrow\mu\nu)=(10.72 \pm 0.16)\%$ and $\Gamma_W = 2113 \pm 31$ MeV, respectively, both compatible with the current world averages.