arXiv subjects

M. Alekseev

Publications and source records attributed to M. Alekseev.

At least 55 records · Page 3Linked to original sources

arXiv2019

Study of the process $e^+ e^- \to p \bar p$ via initial state radiation at BESIII

The Born cross section for the process $e^+ e^- \to p \bar p $ is measured using the initial state radiation technique with an undetected photon. This analysis is based on datasets corresponding to an integrated luminosity of 7.5 fb$^{-1}$, collected with the BESIII detector at the BEPCII collider at center of mass energies between 3.773 and 4.600 GeV. The Born cross section for the process $e^+ e^- \to p \bar p $ and the proton effective form factor are determined in the $p\bar p$ invariant mass range between 2.0 and 3.8 GeV/$c^2$ divided into 30 intervals. The proton form factor ratio ($|G_E|/|G_M|$) is measured in 3 intervals of the $p\bar p$ invariant mass between 2.0 and 3.0 GeV/$c^2$.

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arXiv2019

First observations of $h_c \to$ hadrons

Based on $(4.48 \pm 0.03) \times 10^{8}$ $ψ(3686)$ events collected with the BESIII detector, five $h_c$ hadronic decays are searched for via process $ψ(3686) \to π^0 h_c$. Three of them, $h_c \to p \bar{p} π^+ π^-$, $π^+ π^- π^0$, and $2(π^+ π^-) π^0$ are observed for the first time, with statistical significances of 7.4$σ$, $4.9σ$, and 9.1$σ$, and branching fractions of $(2.89\pm0.32\pm0.55)\times10^{-3}$, $(1.60\pm0.40\pm0.32)\times10^{-3}$, and $(7.44\pm0.94\pm1.56)\times10^{-3}$, respectively, where the first uncertainties are statistical and the second systematic. No significant signal is observed for the other two decay modes, and the corresponding upper limits of the branching fractions are determined to be $B(h_c \to 3(π^+ π^-) π^0)<8.7\times10^{-3}$ and $B(h_c \to K^+ K^- π^+ π^-)<5.8\times10^{-4}$ at 90% confidence level.

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arXiv2019

Precision measurements of $σ_{B}(e^+e^- \to K_{S}^{0}K^{\pm}π^{\mp})$ at center-of-mass energies between 3.8 and 4.6 GeV

Using data samples collected by the BESIII detector operating at the BEPCII storage ring, we measure the $e^+e^- \to K_{S}^{0}K^{\pm}π^{\mp}$ Born cross sections at center-of-mass energies between 3.8 and 4.6\,GeV, corresponding to a luminosity of about 5.0~fb$^{-1}$. The results are compatible with the BABAR measurements, but with the precision significantly improved. A simple $1/s^n$ dependence for the continuum process can describe the measured cross sections, but a better fit is obtained by an additional resonance near 4.2\,GeV, which could be an excited charmonium or a charmonium-like state.

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arXiv2019

Measurements of Weak Decay Asymmetries of $Λ_c^+\to pK_S^0$, $Λπ^+$, $Σ^+π^0$, and $Σ^0π^+$

Using $e^+e^-\toΛ_c^+\barΛ_c^-$ production from a 567 pb$^{-1}$ data sample collected by BESIII at 4.6 GeV, a full angular analysis is carried out simultaneously on the four decay modes of $Λ_c^+\to pK_S^0$, $Λπ^+$, $Σ^+π^0$, and $Σ^0π^+$. For the first time, the $Λ_c^+$ transverse polarization is studied in unpolarized $e^+e^-$ collisions, where a non-zero effect is observed with a statistical significance of 2.1$σ$. The decay asymmetry parameters of the $Λ_c^+$ weak hadronic decays into $pK_S^0$, $Λπ^+$, $Σ^+π^0$ and $Σ^0π^+$ are measured to be $0.18\pm0.43(\rm{stat})\pm0.14(\rm{syst})$, $-0.80\pm0.11(\rm{stat})\pm0.02(\rm{syst})$, $-0.57\pm0.10(\rm{stat})\pm0.07(\rm{syst})$, and $-0.73\pm0.17(\rm{stat})\pm0.07(\rm{syst})$, respectively. In comparison with previous results, the measurements for the $Λπ^+$ and $Σ^+π^0$ modes are consistent but with improved precision, while the parameters for the $pK_S^0$ and $Σ^0π^+$ modes are measured for the first time.

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arXiv2019

Cross section measurements of $e^+ e^-\toωχ_{c0}$ from $\sqrt{s}=$4.178 to 4.278 GeV

The cross section of the process $e^+e^- \rightarrow ωχ_{c0}$ is measured at center-of-mass energies from $\sqrt{s} =$ 4.178 to 4.278 GeV using a data sample of 7 fb$^{-1}$ collected with the BESIII detector operating at the BEPCII storage ring. The dependence of the cross section on $\sqrt{s}$ shows a resonant structure with mass of $(4218.5\pm1.6(\text{stat.})\pm4.0(\text{syst.}))$ MeV/$c^2$ and width of $(28.2\pm3.9(\text{stat.})\pm1.6(\text{syst.}))$ MeV, respectively. This observation confirms and improves upon the result of a previous study. The angular distribution of the $e^+e^- \rightarrow ωχ_{c0}$ process is extracted for the first time.

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arXiv2019

Search for the decay $D_s^+\rightarrow γe^+ν_e$

A search for the rare radiative leptonic decay $D_s^+\toγe^+ν_e$ is performed for the first time using electron-positron collision data corresponding to an integrated luminosity of 3.19 fb$^{-1}$, collected with the BESIII detector at a center-of-mass energy of 4.178 GeV. No evidence for the $D_s^+\toγe^+ν_e$ decay is seen and an upper limit of $\mathcal B(D_s^+\toγe^+ν_e)<1.3\times 10^{-4}$ is set on the partial branching fraction at a 90\% confidence level for radiative photon energies $E_γ^*>0.01$~GeV.

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arXiv2019

Measurement of the Dynamics of the Decays ${ D_s^+ \rightarrow η^{(\prime)} e^{+} ν_e}$

Using $e^+e^-$ annihilation data corresponding to an integrated luminosity of 3.19\,fb$^{-1}$ collected at a center-of-mass energy of 4.178~GeV with the BESIII detector, we measure the absolute branching fractions $\mathcal{B}_{D_s^+ \rightarrow ηe^{+} ν_e }$ = $(2.323\pm0.063_{\rm stat}\pm0.063_{\rm syst})\%$ and $\mathcal{B}_{D_s^+ \rightarrow η^{\prime} e^{+} ν_e}$ = $(0.824\pm0.073_{\rm stat}\pm0.027_{\rm syst})\%$ via a tagged analysis technique, where one $D_s$ is fully reconstructed in a hadronic mode. Combining these measurements with previous BESIII measurements of $\mathcal{B}_{D^+\toη^{(\prime)} e^{+} ν_e}$, the $η-η^\prime$ mixing angle in the quark flavour basis is determined to be $ϕ_{\rm P} = (40.1\pm2.1_{\rm stat}\pm0.7_{\rm syst})^\circ$. From the first measurements of the dynamics of $D^+_s\to η^{(\prime)}e^+ν_e$ decays, the products of the hadronic form factors $f_+^{η^{(\prime)}}(0)$ and the Cabibbo-Kobayashi-Maskawa matrix element $|V_{cs}|$ are determined with different form factor parameterizations. For the two-parameter series expansion, the results are $f^η_+(0)|V_{cs}| = 0.4455\pm0.0053_{\rm stat}\pm0.0044_{\rm syst}$ and $f^{η^{\prime}}_+(0)|V_{cs}| = 0.477\pm0.049_{\rm stat}\pm0.011_{\rm syst}$.

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arXiv2019

Precision measurement of the branching fractions of $η^\prime$ decays

Based on a sample of $(1310.6 \pm 7.0) \times 10^6~J/ψ$ events collected with the BESIII detector, we present measurements of $J/ψ$ and $η^\prime$ absolute branching fractions using the process $J/ψ\rightarrowγη^\prime$. By analyzing events where the radiative photon converts into an $e^+e^-$ pair, the branching fraction for $J/ψ\rightarrow γη^\prime$ is measured to be $(5.27\pm0.03\pm0.05)\times 10^{-3}$. The absolute branching fractions of the five dominant decay channels of the $η^\prime$ are then measured independently for the first time and are determined to be $\mathcal{B}(η^\prime \rightarrow γπ^{+} π^{-})$ = (29.90$\pm$0.03$\pm$0.55)%, $\mathcal{B}(η^\prime \rightarrow ηπ^{+} π^{-})$ = (41.24$\pm$0.08$\pm$1.24)%, $\mathcal{B}(η^\prime \rightarrow ηπ^{0} π^{0})$ = (21.36$\pm$0.10$\pm$0.92)%, $\mathcal{B}(η^\prime \rightarrow γω)$ = (2.489$\pm$0.018$\pm$0.074)%, and $\mathcal{B}(η^\prime \rightarrow γγ)$ = (2.331$\pm$0.012$\pm$0.035)%, where the first uncertainties are statistical and the second systematic.

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arXiv2019

Complete measurement of $Λ$ electromagnetic form factors

The exclusive process $e^+e^-\rightarrowΛ\barΛ$, with $Λ\to pπ^-$ and $\barΛ \to \bar{p}π^+$, has been studied at $\sqrt{s} =$ 2.396 GeV for measurement of the $Λ$ electric and magnetic form factors, $G_E$ and $G_M$. A data sample, corresponding to an integrated luminosity of 66.9 pb$^{-1}$, was collected with the BESIII detector for this purpose. A multi-dimensional analysis with a complete decomposition of the spin structure of the reaction enables a determination of the modulus of the ratio $R=|G_E/G_M|$ and, for the first time for any baryon, the relative phase $ΔΦ=Φ_E-Φ_M$. The resulting values are obtained using the recent and most precise measured value of the asymmetry parameter $α_Λ$ = $0.750~\pm~0.010$ to be $R~=~0.96\pm0.14~(\rm stat.)\pm~0.02~(\rm sys.)$ and $ΔΦ=37^{\mathrm{o}}\pm~12^{\mathrm{o}}~(\rm stat.)\pm~6^{\mathrm{o}}~(\rm sys.)$, respectively. In addition, the cross section is measured with unprecedented precision to be $σ= 118.7~\pm~5.3~(\rm stat.)\pm~5.1~(\rm sys.)$ pb, which corresponds to an effective form factor of $|G|=0.123~\pm~0.003~(\rm stat.)\pm~0.003~(\rm sys.)$. The contribution from two-photon exchange is found to be negligible. Our result enables the first complete determination of baryon time-like electromagnetic form factors.

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arXiv2019

Observation of $e^{+}e^{-}\rightarrow π^{+}π^{-}ψ(3770)$ and $D_{1}(2420)^{0}\bar{D}^{0} + c.c.$

Several intermediate states of the reaction channels $e^{+}e^{-} \rightarrow π^{+} π^{-} D^{0} \bar{D}^{0}$ and $e^{+}e^{-} \rightarrow π^{+} π^{-} D^{+}D^{-}$ are studied using the data samples collected with the BESIII detector at center-of-mass energies above 4.08 GeV. For the first time in this final state, a $ψ(3770)$ signal is seen in the $D\bar{D}$ invariant mass spectrum, with a statistical significance of $5.2σ$ at $\sqrt{s} = 4.42$ GeV. There is also evidence for this resonance at $\sqrt{s}$ = 4.26 and 4.36 GeV with statistical significance of 3.2$σ$ and 3.3$σ$, respectively. In addition, the Born cross section of $e^{+}e^{-}\to π^{+}π^{-}ψ(3770)$ is measured. The proposed heavy-quark-spin-symmetry partner of the $X(3872)$, the state $X_{2}(4013)$, is also searched for in the $D\bar{D}$ invariant mass spectra. No obvious signal is found. The upper limit of the Born cross section of the process $e^{+}e^{-}\to ρ^{0}X_{2}(4013)$ combined with the branching fraction is measured. Also, the processes $e^{+}e^{-}\to D_{1}(2420)\bar{D} + c.c.$ are investigated. The neutral mode with $D_{1}(2420)^{0}\to D^{0}π^{+}π^{-}$ is reported with statistical significance of 7.4$σ$ at $\sqrt{s} = 4.42$ GeV for the first time, and evidence with statistical significance of 3.2$σ$ and 3.3$σ$ at $\sqrt{s}$ = 4.36 and 4.60 GeV is seen, respectively. No evident signal for the process $e^{+}e^{-}\to D_{1}(2420)^{0}\bar{D}^{0} + c.c., D_{1}(2420)^{0}\to D^{*+}π^{-}$ is reported. Evidence for $e^{+}e^{-}\to D_{1}(2420)^{+}D^{-} +~c.c., D_{1}(2420)^{+}\to D^{+}π^{+}π^{-}$ is reported with statistical significance of 3.1$σ$ and 3.0$σ$ at $\sqrt{s}$ = 4.36 and 4.42 GeV, respectively.

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arXiv2019

Amplitude Analysis and Branching Fraction Measurement of $D^0\rightarrow K^-π^+π^0π^0$

Utilizing the dataset corresponding to an integrated luminosity of $2.93$ fb$^{-1}$ at $\sqrt{s}=3.773$ GeV collected by the BESIII detector, we report the first amplitude analysis and branching fraction measurement of the $D^0\rightarrow K^-π^+π^0π^0$ decay. We investigate the sub-structures and determine the relative fractions and the phases among the different intermediate processes. Our results are used to provide an accurate detection efficiency and allow measurement of ${\cal B}(D^0\rightarrow K^-π^+π^0π^0) \,=\, (8.86 \pm 0.13(\text{stat}) \pm 0.19(\text{syst}))\%$.

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arXiv2019

Amplitude analysis of $D_{s}^{+} \rightarrow π^{+}π^{0}η$ and first observation of the pure $W$-annihilation decays $D_{s}^{+} \rightarrow a_{0}(980)^{+}π^{0}$ and $D_{s}^{+} \rightarrow a_{0}(980)^{0}π^{+}$

We present the first amplitude analysis of the decay $D^{+}_{s} \rightarrow π^{+}π^{0}η$. We use an $e^{+}e^{-}$ collision data sample corresponding to an integrated luminosity of 3.19~${\mbox{\,fb}^{-1}}$ collected with the BESIII detector at a center-of-mass energy of $4.178$ GeV. We observe for the first time the pure $W$-annihilation decays $D_{s}^{+} \rightarrow a_{0}(980)^{+}π^{0}$ and $D_{s}^{+} \rightarrow a_{0}(980)^{0}π^{+}$. We measure the absolute branching fractions $\mathcal{B}(D_{s}^{+} \rightarrow a_{0}(980)^{+(0)}π^{0^(+)}, a_{0}(980)^{+(0)} \to π^{+(0)}η) = (1.46\pm0.15_{\rm stat.}\pm0.23_{\rm sys.})$\%, which is larger than the branching fractions of other measured pure $W$-annihilation decays by at least one order of magnitude. In addition, we measure the branching fraction of $D_{s}^{+} \rightarrow π^{+}π^{0}η$ with significantly improved precision.

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arXiv2019

Observation of $ψ(3686)\to p\bar{p}ϕ$

Using a data sample of $4.48\times10^{8}$ $\psip$ events collected with the BESIII detector, we present a first observation of $ψ(3686)\to p\bar{p}ϕ$, and we measure its branching fraction to be $[6.06\pm0.38 ($stat.$) \pm 0.48 ($syst.$)]\times10^{-6}$. In contrast to the earlier discovery of a threshold enhancement in the $p\bar{p}$-mass spectrum of the channel $J/ψ\toγp\bar p$, denoted as $X(p\bar{p})$, we do not find a similar enhancement in $ψ(3686)\to p\bar{p}ϕ$. An upper limit of $1.82\times10^{-7}$ at the $90\%$ confidence level on the branching fraction of $ψ(3686)\to X(p\bar{p})ϕ\to p\bar{p}ϕ$ is obtained.

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arXiv2019

Study of electromagnetic Dalitz decays $χ_{cJ} \rightarrow μ^{+}μ^{-}J/ψ$

Using $4.48 \times 10^{8}$ $ψ(3686)$ events collected with the BESIII detector, we search for the decays $χ_{cJ} \rightarrow μ^{+}μ^{-}J/ψ$ through the radiative decays $ψ(3686) \rightarrow γχ_{cJ}$, where $J=0,1,2$. The decays $χ_{c1,2} \rightarrow μ^{+}μ^{-}J/ψ$ are observed, and the corresponding branching fractions are measured to be $\mathcal{B}(χ_{c1} \rightarrow μ^{+}μ^{-}J/ψ) = (2.51 \pm 0.18 \pm 0.20)\times10^{-4}$ and $\mathcal{B}(χ_{c2} \rightarrow μ^{+}μ^{-}J/ψ) = (2.33 \pm 0.18 \pm 0.29)\times10^{-4}$, where the first uncertainty is statistical and the second one systematic. No significant $χ_{c0} \rightarrow μ^{+}μ^{-}J/ψ$ decay is observed, and the upper limit on the branching fraction is determined to be $2.0\times10^{-5}$ at 90% confidence level. Also, we present a study of di-muon invariant mass dependent transition form factor for the decays $χ_{c1,2} \rightarrow μ^{+}μ^{-}J/ψ$.

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arXiv2019

Measurement of the absolute branching fractions of $Λ_{c}^{+}\toΛηπ^{+}$ and $Σ(1385)^{+}η$

We study the decays $Λ_{c}^{+}\toΛηπ^{+}$ and $Σ(1385)^{+}η$ based on $Λ_{c}^{+}\barΛ_{c}^{-}$ pairs produced in $e^+e^-$ collisions at a center-of-mass energy of $\sqrt{s}=4.6~\mbox{GeV}$, corresponding to an integrated luminosity of $567\;\mbox{pb$^{-1}$}$. The data sample was accumulated with the BESIII detector at the BEPCII collider. The branching fractions are measured to be $\mathcal{B}(Λ_{c}^{+} \to Ληπ^{+})=(1.84\pm0.21({\rm{stat.}})\pm0.15({\rm{syst.}}))\%$ and $\mathcal{B}(Λ_{c}^{+} \to Σ(1385)^{+}η)=(0.91\pm0.18({\rm{stat.}})\pm0.09({\rm{syst.}}))\%$, constituting the most precise measurements to date.

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arXiv2019

Determination of the pseudoscalar decay constant $f_{D_s^+}$ via $D_s^+\toμ^+ν_μ$

Using a $3.19~\mathrm{fb}^{-1}$ data sample collected at an $e^+e^-$ center-of-mass energy of $E_{\rm cm}=4.178$ GeV with the BESIII detector, we measure the branching fraction of the leptonic decay $D_s^+\toμ^+ν_μ$ to be $\mathcal{B}_{D_s^+\toμ^+ν_μ}=(5.49\pm0.16_{\rm stat.}\pm0.15_{\rm syst.})\times10^{-3}$. Combining our branching fraction with the masses of the $D_s^+$ and $μ^+$ and the lifetime of the $D_s^+$, we determine $f_{D_s^+}|V_{cs}|=246.2\pm3.6_{\rm stat.}\pm3.5_{\rm syst.}~\mathrm{MeV}$. Using the $c\to s$ quark mixing matrix element $|V_{cs}|$ determined from a global standard model fit, we evaluate the $D_s^+$ decay constant $f_{D_s^+}=252.9\pm3.7_{\rm stat.}\pm3.6_{\rm syst.}$\,MeV. Alternatively, using the value of $f_{D_s^+}$ calculated by lattice quantum chromodynamics, we find $|V_{cs}| = 0.985\pm0.014_{\rm stat.}\pm0.014_{\rm syst.}$. These values of $\mathcal{B}_{D_s^+\toμ^+ν_μ}$, $f_{D_s^+}|V_{cs}|$, $f_{D_s^+}$ and $|V_{cs}|$ are each the most precise results to date.

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arXiv2019

Observation of $D^+ \to f_0(500) e^+ν_e$ and Improved Measurements of $D \toρe^+ν_e$

Using a data sample corresponding to an integrated luminosity of 2.93~fb$^{-1}$ recorded by the BESIII detector at a center-of-mass energy of $3.773$ GeV, we present an analysis of the decays $\bar{D}^0\toπ^+π^0 e^-\barν_e$ and $D^+\toπ^-π^+ e^+ν_e$. By performing a partial wave analysis, the $π^+π^-$ $S$-wave contribution to $D^+\toπ^-π^+ e^+ν_e$ is observed to be $(25.7\pm1.6\pm1.1)$% with a statistical significance greater than 10$σ$, besides the dominant $P$-wave contribution. This is the first observation of the $S$-wave contribution. We measure the branching fractions $\mathcal{B}(D^{0} \to ρ^- e^+ ν_e) = (1.445\pm 0.058 \pm 0.039) \times10^{-3}$, $\mathcal{B}(D^{+} \to ρ^0 e^+ ν_e) = (1.860\pm 0.070 \pm 0.061) \times10^{-3}$, and $\mathcal{B}(D^{+} \to f_0(500) e^+ ν_e, f_0(500)\toπ^+π^-) = (6.30\pm 0.43 \pm 0.32) \times10^{-4}$. An upper limit of $\mathcal{B}(D^{+} \to f_0(980) e^+ ν_e, f_0(980)\toπ^+π^-) < 2.8 \times10^{-5}$ is set at the 90% confidence level. We also obtain the hadronic form factor ratios of $D\to ρe^+ν_e$ at $q^{2}=0$ assuming the single-pole dominance parameterization: $r_{V}=\frac{V(0)}{A_{1}(0)}=1.695\pm0.083\pm0.051$, $r_{2}=\frac{A_{2}(0)}{A_{1}(0)}=0.845\pm0.056\pm0.039$.

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arXiv2019

Observation of OZI suppressed decays $χ_{cJ}\toωϕ$

Decays $χ_{cJ}~(J=0,1,2)\toωϕ$ are studied using $(448.1\pm2.9)\times 10^{6} ~ψ(3686)$ events collected with the BESIII detector in 2009 and 2012. In addition to the previously established $χ_{c0}\toωϕ$, first observation of $χ_{c1} \to ωϕ$ is reported in this paper. The measured product branching fractions are ${\cal{B}}(ψ(3686)\toγχ_{c0})\times{\cal{B}}(χ_{c0}\toωϕ)=(13.83\pm 0.70\pm 1.01)\times10^{-6}$ and ${\cal{B}}(ψ(3686)\toγχ_{c1})\times{\cal{B}}(χ_{c1}\toωϕ)=(2.67\pm 0.31\pm 0.27)\times10^{-6}$, and the absolute branching fractions are ${\cal{B}}(χ_{c0}\toωϕ)=(13.84\pm 0.70\pm 1.08)\times10^{-5}$ and ${\cal{B}}(χ_{c1}\toωϕ)=(2.80\pm 0.32\pm 0.30)\times10^{-5}$. We also find a strong evidence for $χ_{c2}\toωϕ$ with a statistical significance of 4.8$σ$, and the corresponding product and absolute branching fractions are measured to be ${\cal{B}}(ψ(3686)\toγχ_{c2})\times{\cal{B}}(χ_{c2}\toωϕ)=(0.91\pm0.23\pm0.12)\times10^{-6} $ and ${\cal{B}}(χ_{c2}\toωϕ)=(1.00\pm0.25\pm0.14)\times10^{-5}$. Here, the first errors are statistical and the second ones systematic.

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