The resonant structure of the radiative decay $\Lambda_b^0\to pK^-\gamma$ in the region of proton-kaon invariant-mass up to 2.5 GeV$/c^2$ is studied using proton-proton collision data recorded at centre-of-mass energies of 7, 8, and 13 TeV collected with the LHCb detector, corresponding to a total integrated luminosity of 9 fb$^{-1}$. Results are given in terms of fit and interference fractions between the different components contributing to this final state. Only $\Lambda$ resonances decaying to $pK^-$ are found to be relevant, where the largest contributions stem from the $\Lambda(1520)$, $\Lambda(1600)$, $\Lambda(1800)$, and $\Lambda(1890)$ states.
Distribution of the three-body invariant mass of the candidates in the (left) Run 1 and (right) Run 2 data sets. The results of the fits are overlaid. |
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Distribution of the $\Lambda ^0_ b \rightarrow p K ^- \gamma $ candidates in the Dalitz plane, defined by $m^2_\Lambda ^0_ b ( p K ^- )$ and $m^2_\Lambda ^0_ b (p\gamma)$, after background-subtraction using the sPlot method for (left) Run 1 and (right) Run 2. |
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Background-subtracted distribution of the proton-kaon invariant-mass (black dots) for the (left) Run 1 and (right) Run 2 data samples. Also shown is a sample generated according to the result of a simultaneous fit of the reduced model to the data (red dots) and its components (lines) as well as the contributions due to interference between states with the same quantum numbers $J^P$ (shaded areas). |
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Background-subtracted distribution of the (top) proton-kaon and (bottom) proton-photon invariant-mass (black dots) for the (left) Run 1 and (right) Run 2 data samples. Also shown is a sample generated according to the result of a simultaneous fit of the default model to the data (red dots) and its components (lines) as well as the contributions due to interference between states with the same quantum numbers $J^P$ (shaded areas). |
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Background-subtracted distribution of (top) the kaon-photon invariant-mass and (bottom) the proton helicity angle (black dots) for the (left) Run 1 and (right) Run 2 data samples. Also shown is a sample generated according to the result of a simultaneous fit of the default model to the data (red dots) and its components (lines) as well as the contributions due to interference between states with the same quantum numbers $J^P$ (shaded areas). See Fig. 4 for the legend. |
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Final results for the fit fractions and interference fit fractions. The vertical line separates the fit from the interference fit fractions. The error bars represent the different sources of uncertainty. |
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Background-subtracted distribution of the proton-kaon invariant-mass (black dots) for the (left) Run 1 and (right) Run 2 data samples on a logarithmic scale. Also shown is a sample generated according to the result of a simultaneous fit of the default model to the data (red dots) and its components (lines) as well as the contributions due to interference between states with the same quantum numbers $J^P$ (shaded areas). |
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Background-subtracted distribution of (top) the proton-photon invariant-mass, (middle) the kaon-photon invariant-mass, and (bottom) the proton helicity angle (black dots) for the (left) Run 1 and (right) Run 2 data samples. Also shown is a sample generated according to the result of a simultaneous fit of the reduced model to the data (red dots) and its components (lines) as well as the contributions due to interference between states with the same quantum numbers $J^P$ (shaded areas). See Fig. 3 for the legend. |
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Background-subtracted distribution of (top three rows) the two-body invariant-masses and (bottom row) the proton helicity angle (black dots) for the (left) Run 1 and (right) Run 2 data samples. Also shown is a sample generated according to the result of a simultaneous fit of the second best model to the data (red dots) and its components (lines) as well as the contributions due to interference between states with the same quantum numbers $J^P$ (shaded areas). See Fig. 3 for the legend. |
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Background-subtracted distribution of the proton-kaon invariant-mass (black dots) for the (left) Run 1 and (right) Run 2 data samples on a logarithmic scale. Also shown is a sample generated according to the result of a simultaneous fit of the reduced model to the data (red dots) and its components (lines) as well as the contributions due to interference between states with the same quantum numbers $J^P$ (shaded areas). |
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Background-subtracted distribution of the proton-kaon invariant-mass (black dots) for the (left) Run 1 and (right) Run 2 data samples on a logarithmic scale. Also shown is a sample generated according to the result of a simultaneous fit of the second best model to the data (red dots) and its components (lines) as well as the contributions due to interference between states with the same quantum numbers $J^P$ (shaded areas). |
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Animated gif made out of all figures. |
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List of well-established $\Lambda $ resonances and their properties as given in Ref. [29]. $J$ and $P$ are spin and parity of the resonance. The mass $m_0$ and width $\Gamma_0$ correspond to the Breit--Wigner parameters and are given in $\text{ Me V /}c^2$ and $\text{ Me V}$ respectively. The possible mass and width ranges, $\Delta m_0$ and $\Delta\Gamma_0$, are also given. If a measurement of mass and width is available, the uncertainties are given instead of a range. The columns $\sigma_{m_0}$ and $\sigma_{\Gamma_0}$ contain the $\sigma$ values used to estimate the systematic uncertainty related to the resonance parameters. The rightmost columns contain the allowed values of $l$ and $L$. |
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Systematic uncertainties on the fit fractions (top part of the table) and interference fit fractions (bottom part of the table). The values are given in %. The subscripts "BW", "radius", "amp.", and "res." refer to the systematic uncertainty due to fixing the resonance mass and width, fixing the radius of the hadrons, the choice of amplitude model, and the neglected resolution in the amplitude fit, respectively. The subscripts "finite", "acc.", and "kin." refer to the systematic uncertainties due to the finite simulation sample used to determine the acceptance model, the choice of acceptance model, and the kinematic reweighting respectively. The subscripts "$pK$", "$p\gamma$", and "comb." refer to the systematic uncertainty due to calculating the sWeights in bins of the proton-kaon invariant mass, the proton-gamma invariant mass, and the choice of model for the combinatorial background in the three-body invariant mass fit respectively. |
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Fit fractions (top) and interference fit fractions (bottom) determined using the amplitude model. The values are given in %. The uncertainties from internal and external sources, determined by the numerical convolution procedure are labelled $\sigma_\text{syst}^\text{internal}$ and $\sigma_\text{syst}^\text{external}$. |
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Magnitude, $|A_{LS}|$, and phase, $\arg(A_{LS})$, of the couplings at the best fit point of the default model for resonances $\Lambda (1405)$, $\Lambda (1520)$, $\Lambda (1600)$, $\Lambda (1670)$, $\Lambda (1690)$, $\Lambda (1800)$, $\Lambda (1810)$, and $\Lambda (1820)$. |
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Magnitude, $|A_{LS}|$, and phase, $\arg(A_{LS})$, of the couplings at the best fit point of the default model for resonances $\Lambda (1830)$, $\Lambda (1890)$, $\Lambda (2100)$, $\Lambda (2110)$, $\Lambda (2350)$, and the non-resonant component. |
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Statistical correlations between the observables in percent obtained from bootstrapping the data. In the interest of space, the $J^P$ specification of the nonresonant component of the best model, NR($\tfrac{3}{2}^-$), is dropped and the resonances are only referred to by their mass. A single mass refers to the fit fraction of a state, two masses refer to the interference fit fraction of the given combination. |
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Created on 01 June 2024.