CMS-BPH-17-008

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	Summary	References	CMS Publications

Figures
png pdf	Figure 1: The dimuon invariant mass distribution, in two equidistant $\| y \|$ ranges. The midrapidity dimuons have a significantly better mass resolution.
png pdf	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.
png pdf	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.
png pdf	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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