A full amplitude analysis of $\Lambda_b^0 \to J/\psi p \pi^-$ decays is performed with a data sample acquired with the LHCb detector from 7 and 8 TeV $pp$ collisions, corresponding to an integrated luminosity of 3 fb$^{-1}$. A significantly better description of the data is achieved when, in addition to the previously observed nucleon excitations $N\to p\pi^-$, either the $P_c(4380)^+$ and $P_c(4450)^+\to J/\psi p$ states, previously observed in $\Lambda_b^0 \to J/\psi p K^-$ decays, or the $Z_c(4200)^-\to J/\psi \pi^-$ state, previously reported in $B^0 \to J/\psi K^+ \pi^-$ decays, or all three, are included in the amplitude models. The data support a model containing all three exotic states, with a significance of more than three standard deviations. Within uncertainties, the data are consistent with the $P_c(4380)^+$ and $P_c(4450)^+$ production rates expected from their previous observation taking account of Cabibbo suppression.
Invariant mass spectrum for the selected $\Lambda ^0_ b \rightarrow { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} p \pi^-$ candidates. |
PsiPHMass.pdf [23 KiB] HiDef png [301 KiB] Thumbnail [239 KiB] *.C file |
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Background-subtracted data and fit projections onto $ m_{p\pi} $. Fits are shown with models containing $N^*$ states only (EM) and with $N^*$ states (RM) plus exotic contributions. |
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Background-subtracted data and fit projections onto $ m_{ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} p} $ for (a) all events and (b) the $ m_{p\pi} >1.8$ $\mathrm{ Ge V}$ region. See the legend and caption of Fig. 2 for a description of the components. |
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mjpsip3.pdf [23 KiB] HiDef png [339 KiB] Thumbnail [266 KiB] *.C file |
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Background-subtracted data and fit projections onto $ m_{ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} \pi} $ for (a) all events and (b) the $ m_{p\pi} >1.8$ $\mathrm{ Ge V}$ region. See the legend and caption of Fig. 2 for a description of the components. |
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mjpsik3.pdf [18 KiB] HiDef png [313 KiB] Thumbnail [253 KiB] *.C file |
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Background-subtracted data and fit projections of decay angles describing the $N^*$ decay chain, which are included in the amplitude fit. The helicity angle of particle $P$, $\theta_P$, is the polar angle in the rest frame of $P$ between a decay product of $P$ and the boost direction from the particle decaying to $P$. The azimuthal angle between decay planes of $\Lambda ^0_ b $ and $N^*$ (of $ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} $) is denoted as $\phi_\pi$ ($\phi_\mu$). See Ref. [5] for more details. |
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Invariant mass squared of $p\pi ^- $ versus either (a) $ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} p$ or (b) $ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} \pi$ for candidates within $\pm15\mathrm{ Me V} $ of the $\Lambda ^0_ b $ mass, which contain 17% background. The lines show the kinematic boundaries with the $\Lambda ^0_ b $ mass constrained to the known value. |
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(a) Relative signal efficiency and (b) background distribution on the Dalitz plane in arbitrary units. |
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dlzbkg.pdf [43 KiB] HiDef png [318 KiB] Thumbnail [269 KiB] *.C file |
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Background-subtracted data and fit projections onto $ m_{p\pi} $. Fits are shown with models containing $N^*$ states only (EM) and with $N^*$ states (RM) plus exotic contributions. Individual fit components are shown only for the fit which includes all three exotic resonances. |
mppi-all.pdf [54 KiB] HiDef png [668 KiB] Thumbnail [420 KiB] *.C file |
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Background-subtracted data and fit projections onto $ m_{p\pi} $. Fits are shown with models containing $N^*$ states only (EM) and with $N^*$ states (RM) plus exotic contributions. Individual fit components are shown only for the fit which includes all three exotic resonances. |
mppi-l[..].pdf [53 KiB] HiDef png [501 KiB] Thumbnail [373 KiB] *.C file |
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Background-subtracted data and fit projections onto $ m_{p\pi} $. Fits are shown with models containing $N^*$ states only (EM) and with $N^*$ states (RM) plus the two $P_c^+$ resonances. |
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Background-subtracted data and fit projections onto $ m_{ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} p} $ for (a) all events and (b) the $ m_{p\pi} >1.8$ $\mathrm{ Ge V}$ region. See the legend and caption of Fig. 10 for a description of the components. |
mjpsip[..].pdf [18 KiB] HiDef png [270 KiB] Thumbnail [221 KiB] *.C file |
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mjpsip[..].pdf [20 KiB] HiDef png [322 KiB] Thumbnail [256 KiB] *.C file |
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Background-subtracted data and fit projections onto $ m_{ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} \pi} $ for (a) all events and (b) the $ m_{p\pi} >1.8$ $\mathrm{ Ge V}$ region. See the legend and caption of Fig. 10 for the description. |
mjpsik[..].pdf [17 KiB] HiDef png [271 KiB] Thumbnail [228 KiB] *.C file |
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mjpsik[..].pdf [17 KiB] HiDef png [301 KiB] Thumbnail [244 KiB] *.C file |
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Background-subtracted data and fit projections onto $ m_{p\pi} $. Fits are shown with models containing $N^*$ states only (EM) and with $N^*$ states (RM) plus the $Z_c(4200)^-$ resonance. |
mppi-zc.pdf [32 KiB] HiDef png [306 KiB] Thumbnail [221 KiB] *.C file |
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Background-subtracted data and fit projections onto $ m_{ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} p} $ for (a) all events and (b) the $ m_{p\pi} >1.8$ $\mathrm{ Ge V}$ region. See the legend and caption of Fig. 13 for a description of the components. |
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mjpsip3-zc.pdf [21 KiB] HiDef png [260 KiB] Thumbnail [214 KiB] *.C file |
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Background-subtracted data and fit projections onto $ m_{ { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} \pi} $ for (a) all events and (b) the $ m_{p\pi} >1.8$ $\mathrm{ Ge V}$ region. See the legend and caption of Fig. 13 for a description of the components. |
mjpsik0-zc.pdf [17 KiB] HiDef png [253 KiB] Thumbnail [219 KiB] *.C file |
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mjpsik3-zc.pdf [17 KiB] HiDef png [242 KiB] Thumbnail [208 KiB] *.C file |
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Coordinate axes for the spin quantization of particle $A$ (bottom part), chosen to be the helicity frame of $A$ ($\hat{z}_{0}||\vec{p}_{A}$ in the rest frame of its parent particle or in the laboratory frame), together with the polar (${\theta}_{{B}}^{ \F{ A }}$) and azimuthal (${\phi}_{{B}}^{ \F{ A }}$) angles of the momentum of its child $B$ in the $A$ rest frame (top part). Notice that the directions of these coordinate axes, denoted as ${\hat{x}}_{{0}}^{ \F{ A }}$, ${\hat{y}}_{{0}}^{ \F{ A }}$, and ${\hat{z}}_{{0}}^{ \F{ A }}$, do not change when boosting from the helicity frame of $A$ to its rest frame. After the Euler rotation ${\cal R}(\alpha={\phi}_{{B}}^{ \F{ A }},\beta={\theta}_{{B}}^{ \F{ A }},\gamma=0)$ (see the text), the rotated $z$ axis, ${\hat{z}}_{{2}}^{ \F{ A }}$, is aligned with the $B$ momentum; thus the rotated coordinates become the helicity frame of $B$. If $B$ has a sequential decay, then the same boost-rotation process is repeated to define the helicity frame for its decay products. |
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Definition of the decay angles in the $Z_c^-$ decay chain. |
Matrix[..].pdf [165 KiB] HiDef png [259 KiB] Thumbnail [136 KiB] *.C file |
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Animated gif made out of all figures. |
PAPER-2016-015.gif Thumbnail |
The $N^*$ resonances used in the different fits. Parameters are taken from the PDG [16]. The number of $LS$ couplings is listed in the columns to the right for the two versions (RM and EM) of the $N^*$ model discussed in the text. To fix overall phase and magnitude conventions, the $N(1535)$ complex coupling of lowest $LS$ is set to (1,0). |
Table_1.pdf [45 KiB] HiDef png [130 KiB] Thumbnail [63 KiB] tex code |
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Summary of absolute systematic uncertainties of the fit fractions in units of percent. |
Table_2.pdf [82 KiB] HiDef png [133 KiB] Thumbnail [65 KiB] tex code |
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Fit fractions (%) from the RM and EM model fits with statistical uncertainties only. |
Table_3.pdf [52 KiB] HiDef png [289 KiB] Thumbnail [135 KiB] tex code |
Supplementary material full pdf |
supple[..].pdf [501 KiB] |
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This ZIP file contains supplementary material for the publication LHCb-PAPER-2016-015. The files are: Supplementary.pdf: Details of the matrix element for the decay amplitude *.pdf, *png, *.eps : The figures in various formats |
Fig15.pdf [922 KiB] HiDef png [270 KiB] Thumbnail [159 KiB] *C file |
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Fig16.pdf [165 KiB] HiDef png [259 KiB] Thumbnail [136 KiB] *C file |
Created on 26 April 2024.