CMS-HIN-15-003

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 & Tables	Summary	References	CMS Publications

Figures
png pdf	Figure 1: Charged particle multiplicity distribution of the events selected for the analysis.
png pdf	Figure 2: Dimuon mass distributions in the ${\Upsilon \mathrm {(nS)}}$ region for two ${p_{\mathrm {T}}}$ ranges.
png pdf	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: ${\lambda _{\vartheta }} = 0.237$ , ${\lambda _{\varphi }} = -0.027$ , ${\lambda _{\vartheta \varphi }} = -0.025$ ).
png pdf	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: ${\lambda _{\vartheta }} = 0.237$ , ${\lambda _{\varphi }} = -0.027$ , ${\lambda _{\vartheta \varphi }} = -0.025$ ).
png pdf	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: ${\lambda _{\vartheta }} = 0.237$ , ${\lambda _{\varphi }} = -0.027$ , ${\lambda _{\vartheta \varphi }} = -0.025$ ).
png pdf	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].
png pdf	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].
png pdf	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].
png pdf	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.
png pdf	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).
png pdf	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 ${\Upsilon \mathrm {(3S)}}$ .
png pdf	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
png pdf	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.
png pdf	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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