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| CMS-HIN-15-003 ; CERN-EP-2016-039 | ||
| ${\Upsilon\mathrm{(nS)}}$ polarizations versus particle multiplicity in pp collisions at $\sqrt{s} =$ 7 TeV | ||
| CMS Collaboration | ||
| 9 March 2016 | ||
| Phys. Lett. B 761 (2016) 31 | ||
| Abstract: The polarizations of the ${\Upsilon\mathrm{(1S)}} $, ${\Upsilon\mathrm{(2S)}} $, and ${\Upsilon\mathrm{(3S)}} $ mesons are measured as a function of the charged particle multiplicity in proton-proton collisions at $\sqrt{s} = $ 7 TeV. The measurements are performed with a dimuon data sample collected in 2011 by the CMS experiment, corresponding to an integrated luminosity of 4.9 fb$^{-1}$. The results are extracted from the dimuon decay angular distributions, in two ranges of ${\Upsilon\mathrm{(nS)}}$ transverse momentum (10-15 and 15-35 GeV), and in the rapidity interval $ | y | < 1.2 $. The results do not show significant changes from low- to high-multiplicity pp collisions, although large uncertainties preclude definite statements in the ${\Upsilon\mathrm{(2S)}} $ and ${\Upsilon\mathrm{(3S)}} $ cases. | ||
| Links: e-print arXiv:1603.02913 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; HepData record ; CADI line (restricted) ; | ||
| Figures | |
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Figure 1:
Charged particle multiplicity distribution of the events selected for the analysis. |
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Figure 2:
Dimuon mass distributions in the ${\Upsilon \mathrm {(nS)}}$ region for two ${p_{\mathrm {T}}}$ ranges. |
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Figure 3:
Distributions of $ {\cos\vartheta } $ (left) and $\varphi $ (right), for the ${\Upsilon \mathrm {(2S)}}$ in a representative analysis bin. The curves represent two polarization scenarios (dashed and dotted lines, defined in the legends) and the measured case (solid lines: $ {\lambda _{\vartheta }} = 0.237$, $ {\lambda _{\varphi }} = -0.027$, $ {\lambda _{\vartheta \varphi }} = -0.025$). |
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Figure 3-a:
Distribution of $ {\cos\vartheta } $, for the ${\Upsilon \mathrm {(2S)}}$ in a representative analysis bin. The curves represent two polarization scenarios (dashed and dotted lines, defined in the legends) and the measured case (solid lines: $ {\lambda _{\vartheta }} = 0.237$, $ {\lambda _{\varphi }} = -0.027$, $ {\lambda _{\vartheta \varphi }} = -0.025$). |
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Figure 3-b:
Distribution of $\varphi $, for the ${\Upsilon \mathrm {(2S)}}$ in a representative analysis bin. The curves represent two polarization scenarios (dashed and dotted lines, defined in the legends) and the measured case (solid lines: $ {\lambda _{\vartheta }} = 0.237$, $ {\lambda _{\varphi }} = -0.027$, $ {\lambda _{\vartheta \varphi }} = -0.025$). |
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Figure 4:
Two-dimensional marginals of the PPD for the HX frame in the ${\lambda _{\varphi }}$ vs. $ {\lambda _{\vartheta }}$ (left) and ${\lambda _{\vartheta \varphi }}$ vs. $ {\lambda _{\varphi }}$ (right) planes, for ${\Upsilon \mathrm {(2S)}}$ with 15 $ < {p_{\mathrm {T}}} < $ 35 GeV and 10 $ < {N_\text {ch}} < $ 20, displaying the 68.3% and 99.7% CL total uncertainties. The shaded areas represent physically forbidden regions of parameter space for the decay of a $J=$ 1 particle [14]. |
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Figure 4-a:
Two-dimensional marginals of the PPD for the HX frame in the ${\lambda _{\varphi }}$ vs. $ {\lambda _{\vartheta }}$ plane, for ${\Upsilon \mathrm {(2S)}}$ with 15 $ < {p_{\mathrm {T}}} < $ 35 GeV and 10 $ < {N_\text {ch}} < $ 20, displaying the 68.3% and 99.7% CL total uncertainties. The shaded areas represent physically forbidden regions of parameter space for the decay of a $J=$ 1 particle [14]. |
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Figure 4-b:
Two-dimensional marginals of the PPD for the HX frame in the ${\lambda _{\vartheta \varphi }}$ vs. $ {\lambda _{\varphi }}$ (right) plane, for ${\Upsilon \mathrm {(2S)}}$ with 15 $ < {p_{\mathrm {T}}} < $ 35 GeV and 10 $ < {N_\text {ch}} < $ 20, displaying the 68.3% and 99.7% CL total uncertainties. The shaded areas represent physically forbidden regions of parameter space for the decay of a $J=$ 1 particle [14]. |
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Figure 5:
The ${\lambda _{\vartheta }} $, ${\lambda _{\varphi }} $, ${\lambda _{\vartheta \varphi }} $, and ${\tilde{\lambda }}$ parameters (top to bottom) for the ${\Upsilon \mathrm {(1S)}} $, ${\Upsilon \mathrm {(2S)}} $, and ${\Upsilon \mathrm {(3S)}}$ states (left to right), in the HX frame, as a function of ${N_\text {ch}} $, for both ${p_{\mathrm {T}}}$ ranges. The ${\tilde{\lambda }}$ values are also shown for the CS frame; the HX and CS uncertainties are strongly correlated. The vertical bars represent the ${N_\text {ch}} $-dependent total uncertainties (at 68.3% CL), while the boxes at the zero horizontal line represent the global uncertainties. The points are placed at the average ${N_\text {ch}}$ of each bin, with a small offset for easier viewing. |
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Figure 6:
Expected ${N_\text {ch}} $-dependences of the ${\lambda _{\vartheta }}$ parameter for the sum of two processes, of polarizations $\lambda _0$ and $\lambda _1$, of relative fractions changing linearly with ${N_\text {ch}}$ (see text for details). The measured ${\lambda _{\vartheta }}$ values are also shown, for the ${\Upsilon \mathrm {(3S)}}$ (left) and ${\Upsilon \mathrm {(1S)}}$ (right). |
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Figure 6-a:
Expected ${N_\text {ch}} $-dependences of the ${\lambda _{\vartheta }}$ parameter for the sum of two processes, of polarizations $\lambda _0$ and $\lambda _1$, of relative fractions changing linearly with ${N_\text {ch}}$ (see text for details). The measured ${\lambda _{\vartheta }}$ values are also shown for the ${\Upsilon \mathrm {(3S)}}$. |
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Figure 6-b:
Expected ${N_\text {ch}} $-dependences of the ${\lambda _{\vartheta }}$ parameter for the sum of two processes, of polarizations $\lambda _0$ and $\lambda _1$, of relative fractions changing linearly with ${N_\text {ch}}$ (see text for details). The measured ${\lambda _{\vartheta }}$ values are also shown for the ${\Upsilon \mathrm {(1S)}}$. |
| Tables | |
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Table 1:
Signal yields, $N$, and background fractions, ${f_\text {Bg}}$ (in%), within 1$\sigma $ windows around the nominal ${\Upsilon \mathrm {(nS)}}$ masses for the considered ${N_\text {ch}}$ and ${p_{\mathrm {T}}}$ ranges. |
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Table 2:
${\Upsilon \mathrm {(nS)}}$ polarization parameters in the HX frame. The global uncertainties, independent of state and ${N_\text {ch}} $ bin, are also indicated. |
| Summary |
| The polarizations of the ${\Upsilon\mathrm{(1S)}} $, ${\Upsilon\mathrm{(2S)}} $, and ${\Upsilon\mathrm{(3S)}} $ mesons produced in pp collisions at $\sqrt{s} = $ 7 TeV have been determined as functions of the charged particle multiplicity of the event in two ${\Upsilon\mathrm{(nS)}}$ $ p_{\mathrm{T}}$ ranges. The measurements do not show significant variations as a function of ${N_\text{ch}} $, even though the large ${\Upsilon\mathrm{(2S)}} $ and ${\Upsilon\mathrm{(3S)}} $ uncertainties preclude definite statements in these cases. This study opens the way for analogous measurements extending to the charmonium family, particularly interesting for the $\psi \mathrm{(2S)} $, which is unaffected by feed-down decays and, therefore, provides a more direct probe of LDME universality. Equivalent analyses should also be performed in pPb and PbPb event samples, in view of evaluating how quark-antiquark bound-state formation is influenced by the surrounding medium, which is an essential input for the interpretation of quarkonium suppression patterns in nuclear collisions. |
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