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Measurement of the $\chi_b(3P)$ mass and of the relative rate of $\chi_{b1}(1P)$ and $\chi_{b2}(1P)$ production

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Abstract

The production of $\chi_b$ mesons in proton-proton collisions is studied using a data sample collected by the LHCb detector, at centre-of-mass energies of $\sqrt{s}=7$ and $8$ TeV and corresponding to an integrated luminosity of 3.0 fb$^{-1}$. The $\chi_b$ mesons are identified through their decays to $\Upsilon(1S)\gamma$ and $\Upsilon(2S)\gamma$ using photons that converted to $e^+e^-$ pairs in the detector. The relative prompt production rate of $\chi_{b1}(1P)$ and $\chi_{b2}(1P)$ mesons is measured as a function of the $\Upsilon(1S)$ transverse momentum in the $\chi_b$ rapidity range $2.0<y<4.5$. A precise measurement of the $\chi_b(3P)$ mass is also performed. Assuming a mass splitting between the $\chi_{b1}(3P)$ and the $\chi_{b2}(3P)$ states of 10.5 MeV/$c^2$, the measured mass of the $\chi_{b1}(3P)$ meson is $ m(\chi_{b1}(3P))= 10515.7^{+2.2}_{-3.9}(stat) ^{+1.5}_{-2.1}(syst) MeV/c^2.$

Figures and captions

Invariant dimuon mass of the $\Upsilon$ candidates after the event selection requirements and before the $\Upsilon$ mass range requirement. The distribution is fitted with the sum (blue line) of a double-sided Crystal Ball function for each $\Upsilon$ state (dashed red line for $\Upsilon{(1S)} $, dotted pink line for $\Upsilon{(2S)} $, dash-dotted green line for $\Upsilon{(3S)} $) and a second-order polynomial for the background (not shown). The hatched red bands show the signal regions and the hatched blue bands show the mass sidebands used for background studies.

FitUps[..].pdf [49 KiB]
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FitUpsilon_datacuts1P.pdf

Distribution of $m^*(\mu ^+ \mu ^- \gamma )\equiv m(\mu ^+ \mu ^- \gamma )-m(\mu ^+ \mu ^- )+m(\Upsilon)$ for $\chi _{ b }$ candidates with fit projections overlaid for (a) $\chi _{ b }(1P)\rightarrow \Upsilon{(1S)} \gamma $, (b) $\chi _{ b }(2P)\rightarrow \Upsilon{(1S)} \gamma $, (c,e) $\chi _{ b }(3P)\rightarrow \Upsilon{(1S)} \gamma $ and (d,f) $\chi _{ b }(3P)\rightarrow \Upsilon{(2S)} \gamma $ channels. The result of the simultaneous fit to the $\chi _{ b }(3P)\rightarrow \Upsilon{(1S)} \gamma $ and $\chi _{ b }(3P)\rightarrow \Upsilon{(2S)} \gamma $ mass distributions is shown in (e) and (f). The cyan dotted line shows the non-$\Upsilon$ background, the grey dashed line shows the combinatorial background, the red dashed line the $\chi_{b1}$ contribution, the green dash-dotted line the $\chi _{ b 2}$ contribution, and the blue full line the sum of all these contributions.

Chib1P[..].pdf [25 KiB]
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Chib1P1S_uFitnew.pdf
Chib2P[..].pdf [24 KiB]
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Chib2P1S_uFitnew.pdf
Chib3P[..].pdf [25 KiB]
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Chib3P1S_uFitnew.pdf
Chib3P[..].pdf [25 KiB]
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Chib3P2S_uFitnew.pdf
Chib3P[..].pdf [26 KiB]
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Chib3P1S_cFitnew.pdf
Chib3P[..].pdf [25 KiB]
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Chib3P2S_cFitnew.pdf

Distribution of $m^*(\mu ^+ \mu ^- \gamma )\equiv m(\mu ^+ \mu ^- \gamma )-m(\mu ^+ \mu ^- )+m(\Upsilon)$ for $\chi _{ b }(1P)$ candidates with fit projections overlaid for each of the three ranges in $p_{\rm T}^{\Upsilon}$ : (a) 5--10 $ {\mathrm{\,GeV\!/}c}$ , (b) 10--15 $ {\mathrm{\,GeV\!/}c}$ and (c) 15--25 $ {\mathrm{\,GeV\!/}c}$ . The cyan dotted line show the non-$\Upsilon$ background, the grey dashed line shows the combinatorial background, the red dashed line the $\chi_{b1}$ contribution, the green dash-dotted line the $\chi_{b2}$ contribution and the blue full line the sum of all these contributions.

Chib1P[..].pdf [23 KiB]
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Chib1P1S_uFit5-10GeV.pdf
Chib1P[..].pdf [23 KiB]
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Chib1P1S_uFit10-15GeV.pdf
Chib1P[..].pdf [22 KiB]
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Chib1P1S_uFit15-25GeV.pdf

Relative production cross-sections of $\chi_{b1}$ to $\chi_{b2}$ mesons as a function of $ p_{\rm T}^{\Upsilon} $. Panel (a) shows the comparison of this measurement (the hatched rectangles show the statistical uncertainties and the red crosses the total experimental uncertainty) to the LO NRQCD prediction [9] (green band), and to the LHCb $\chi _{ c }$ result (blue crosses), where the $p_{\rm T}$ axis has been scaled by $m(\chi _{ b })/m(\chi _{ c })=2.8$. Panel (b) compares this measurement (empty squares) to CMS results [31] (filled squares) and to the scaled LHCb $\chi _{ c }$ results (empty circles). The error bars are the total experimental uncertainties and do not include the uncertainties on the branching fractions.

Chibra[..].pdf [16 KiB]
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Chibrationew.pdf
sigmar[..].pdf [15 KiB]
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sigmarationew_compCMS.pdf

Animated gif made out of all figures.

PAPER-2014-040.gif
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Tables and captions

Selection criteria for each $\chi _{ b }(nP)\rightarrow \Upsilon(mS)\gamma $ transition. SB indicates sideband.

Table_1.pdf [47 KiB]
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Table_1.pdf

Fitted values of the $\chi _{ b }(nP)$ $(n=1,2)$ masses (in $ {\mathrm{\,MeV\!/}c^2}$ ) from the $\chi _{ b }(nP)\rightarrow \Upsilon{(1S)} \gamma $ transitions, compared to the world average values. The uncertainties are statistical only.

Table_2.pdf [36 KiB]
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Table_2.pdf

Fitted values of the $\chi _{ b }(3P)$ mass (in $ {\mathrm{\,MeV\!/}c^2}$ ) for the $\chi _{ b }(3P)\rightarrow \Upsilon(mS)\gamma $ ($m=1,2$) transitions. The last column gives the result of the simultaneous fit to the two transitions. The values are corrected for the mass bias ($-3$ $ {\mathrm{\,MeV\!/}c^2}$ and $-0.5$ $ {\mathrm{\,MeV\!/}c^2}$ for the $\Upsilon{(1S)} $ and $\Upsilon{(2S)} $ transitions, respectively). The last row gives the total $\chi _{ b }$ yields. The uncertainties are statistical only.

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Table_3.pdf

Summary of the systematic uncertainties on the $\chi _{ b }(nP)$ ($n=1,2$) mass splitting and on the $\chi _{ b 1}(3P)$ mass in $ {\mathrm{\,MeV\!/}c^2}$ . The last column refers to the simultaneous fit to the two transitions.

Table_4.pdf [54 KiB]
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Table_4.pdf

Relative rate of $\chi _{ b 1}(1P)$ and $\chi _{ b 2}(1P)$ production and ratio of total efficiency (in the three $p_{\rm T}^{\Upsilon}$ ranges). Uncertainties only refer to the statistical contributions.

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Table_5.pdf

Summary of the systematic uncertainties on the $\chi _{ b }(1P)$ relative rates, expressed as fractions of the relative rate.

Table_6.pdf [54 KiB]
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Table_6.pdf

Relative production cross section of $\chi_{b1}$ to $\chi_{b2}$ mesons for the $1P$ state for each $p_{\rm T}^{\Upsilon}$ bin. The first uncertainty is statistical, the second is the systematic uncertainty and the third is due to the uncertainty on the branching fractions.

Table_7.pdf [52 KiB]
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Table_7.pdf

Created on 09 December 2018.Citation count from INSPIRE on 18 December 2018.