CMS-BPH-17-008 ; CERN-EP-2018-134 | ||
Observation of the $ \chi_{\mathrm{b}1}(\mathrm{3P}) $ and $ \chi_{\mathrm{b}2}(\mathrm{3P}) $ and measurement of their masses | ||
CMS Collaboration | ||
29 May 2018 | ||
Phys. Rev. Lett. 121 (2018) 092002 | ||
Abstract: The $ \chi_{\mathrm{b}1}(\mathrm{3P}) $ and $ \chi_{\mathrm{b}2}(\mathrm{3P}) $ states are observed through their $ \Upsilon(\mathrm{3S}) \gamma$ decays, using an event sample of proton-proton collisions collected by the CMS experiment at the CERN LHC. The data were collected at a center-of-mass energy of 13 TeV and correspond to an integrated luminosity of 80.0 fb$^{-1}$. The $ \Upsilon(\mathrm{3S}) $ mesons are identified through their dimuon decay channel, while the low-energy photons are detected after converting to $ \mathrm{ e^+e^- } $ pairs in the silicon tracker, leading to a $ \chi_{\mathrm{b}}(\mathrm{3P}) $ mass resolution of 2.2 MeV. This is the first time that the $J = $ 1 and 2 states are well resolved and their masses individually measured: 10 513.42 $\pm$ 0.41 (stat) $\pm$ 0.18 (syst) MeV and 10 524.02 $\pm$ 0.57 (stat) $\pm$ 0.18 (syst) MeV; they are determined with respect to the world-average value of the $ \Upsilon(\mathrm{3S}) $ mass, which has an uncertainty of 0.5 MeV. The mass splitting is measured to be 10.60 $\pm$ 0.64 (stat) $\pm$ 0.17 (syst) MeV. | ||
Links: e-print arXiv:1805.11192 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; HepData record ; CADI line (restricted) ; |
Figures | |
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Figure 1:
The dimuon invariant mass distribution, in two equidistant $ | y |$ ranges. The midrapidity dimuons have a significantly better mass resolution. |
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Figure 2:
The PES as a function of the measured photon energy, obtained using $ {\chi _{{{\mathrm {c}}}1}}\to {\mathrm {J}/\psi} \, \gamma $ decays from the 2015-2016 (open circles) and 2017 (filled circles) data samples. The points are drawn at the average $E_{\gamma}$ in each bin. The curve represents the parametrization mentioned in the text. |
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Figure 3:
The invariant mass distributions of the $ {\chi _{{{\mathrm {b}}}J}}\to {\Upsilon} \mathrm {(nS)} \, \gamma $ candidates ($ \mathrm {n} =$ 1, 2, 3), after the PES correction. The inset shows the ${\chi _\mathrm {\mathrm{b}1}\mathrm {(1P)}}$ and ${\chi _\mathrm {\mathrm{b}1}\mathrm {(2P)}}$ masses fitted before (open squares) and after (filled circles) the PES correction, with vertical bars representing the statistical uncertainties. The world-average values [33] are shown by the horizontal bands, with dashed lines representing their total uncertainties. |
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Figure 4:
The invariant mass distribution of the $ {\chi _{{{\mathrm {b}}}J} \mathrm {(3P)}}\to {\Upsilon \mathrm {(3S)}}\, \gamma $ candidates. The vertical bars are the statistical uncertainties. The curves represent the fitted contributions of the two signal peaks, the background, and their sum. |
Summary |
In summary, data samples of pp collisions at $\sqrt{s} = $ 13 TeV,collected by CMS in the years 2015-2017, corresponding to an integrated luminosity of 80.0 fb$^{-1}$, were used to measure the invariant mass distribution of the $\chi_{\mathrm{b}}(\mathrm{3P}) \to \Upsilon(\mathrm{3S}) \gamma $ candidates, with the $ \Upsilon(\mathrm{3S}) $ mesons detected in the dimuon decay channel and the photons reconstructed through conversions to $ \mathrm{ e^+e^- } $ pairs. The measured distribution is well reproduced by the superposition of the $ \chi_{\mathrm{b}1}(\mathrm{3P}) $ and $ \chi_{\mathrm{b}2}(\mathrm{3P}) $ quarkonium states, overlaid on a smooth continuum. This is the first time that the two states are individually observed. Their mass difference is $\Delta M = $ 10.60 $\pm$ 0.64 (stat) $\pm$ 0.17 (syst) MeV, and their masses, assuming that the $J = $ 1 state is the lighter one, are $M(\chi_{\mathrm{b}1}(\mathrm{3P})) = $ 10 513.42 $\pm$ 0.41 (stat) $\pm$ 0.18 (syst) and $M(\chi_{\mathrm{b}2}(\mathrm{3P})) = $ 10 524.02 $\pm$ 0.57 (stat) $\pm$ 0.18 (syst) MeV, having an additional 0.5 MeV uncertainty reflecting the present precision of the world-average $ \Upsilon(\mathrm{3S}) $ mass. This measurement fills a gap in the spin-dependent bottomonium spectrum below the open-beauty threshold and should significantly contribute to an improved understanding of the nonperturbative spin-orbit interactions affecting quarkonium spectroscopy. |
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