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Measurement of $V^0$ production ratios in $pp$ collisions at $\sqrt{s}$ = 0.9 and 7 TeV

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Abstract

The $\bar{\Lambda} / \Lambda$ and $\bar{\Lambda} / K^0_\mathrm{S}$ production ratios are measured by the LHCb detector from $0.3\,\mathrm{nb}^{-1}$ of $pp$ collisions delivered by the LHC at $\sqrt{s} = 0.9$\,TeV and $1.8\,\mathrm{nb}^{-1}$ at $\sqrt{s} = 7$\,TeV. Both ratios are presented as a function of transverse momentum, $p_\mathrm{T}$, and rapidity, $y$, in the ranges {$0.15 < p_\mathrm{T} < 2.50\,\mathrm{GeV}/c$} and {$2.0<y<4.5$}. Results at the two energies are in good agreement as a function of rapidity loss, $\Delta y = y_\mathrm{beam} - y$, and are consistent with previous measurements. The ratio $\bar{\Lambda} / \Lambda$, measuring the transport of baryon number from the collision into the detector, is smaller in data than predicted in simulation, particularly at high rapidity. The ratio $\bar{\Lambda} / K^0_\mathrm{S}$, measuring the baryon-to-meson suppression in strange quark hadronisation, is significantly larger than expected.

Figures and captions

The Fisher discriminant $\mathcal{F}_{\mathrm{IP}}$ in 0.5 million Monte Carlo simulated minimum bias events at $\sqrt{s} = 7\mathrm{\,TeV} $ for \subref{fig:fisher-k} $ K ^0_{\rm\scriptscriptstyle S}$ and \subref{fig:fisher-a} $\overline{\Lambda }{}$ .

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Invariant mass peaks for \subref{fig:mass-fit-a} $\overline{\Lambda }{}$ in the range $0.25< p_\mathrm{T} <2.50 {\mathrm{\,GeV\!/}c} $ & $2.5< y <3.0$ and \subref{fig:mass-fit-k} $ K ^0_{\rm\scriptscriptstyle S}$ in the range $0.65< p_\mathrm{T} <1.00 {\mathrm{\,GeV\!/}c} $ & $3.5< y <4.0$ at $\sqrt{s} = 0.9\mathrm{\,TeV} $ with field up . Signal yields, $N$, are found from fits (solid curves) with a double Gaussian peak with common mean, $\mu$, over a linear background (dashed lines). The width, $\sigma$, is computed as the quadratic average of the two Gaussian widths weighted by their signal fractions.

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\subref{fig:weighting-pt} Transverse momentum and \subref{fig:weighting-y} rapidity distributions for $ K ^0_{\rm\scriptscriptstyle S}$ in data and Monte Carlo simulation at $\sqrt{s} = 7\mathrm{\,TeV} $ . The difference between data and Monte Carlo is reduced by weighting the simulated candidates.

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The double ratios \subref{fig:material-int-al} $(\overline{\Lambda }{} /\Lambda )_\mathrm{Data}/(\overline{\Lambda }{} /\Lambda )_\mathrm{MC}$ and \subref{fig:material-int-ak} $(\overline{\Lambda }{} / K ^0_{\rm\scriptscriptstyle S} )_\mathrm{Data}/(\overline{\Lambda }{} / K ^0_{\rm\scriptscriptstyle S} )_\mathrm{MC}$ are shown as a function of the material traversed, in units of radiation length. Flat line fits, shown together with their respective $\chi^2$ probabilities, give no evidence of a bias.

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The ratios $\overline{\Lambda }{}$ / $\Lambda$ and $\overline{\Lambda }{}$ / $ K ^0_{\rm\scriptscriptstyle S}$ from the full analysis procedure at \subref{fig:plotData-900-al-3y} & \subref{fig:plotData-900-ak-3y} $\sqrt{s} = 0.9\mathrm{\,TeV} $ and \subref{fig:plotData-7-al-3y} & \subref{fig:plotData-7-ak-3y} 7 $\mathrm{\,TeV}$ are shown as a function of rapidity, compared across intervals of transverse momentum. Vertical lines show the combined statistical and systematic uncertainties and the short horizontal bars (where visible) show the statistical component.

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The ratios $\overline{\Lambda }{}$ / $\Lambda$ and $\overline{\Lambda }{}$ / $ K ^0_{\rm\scriptscriptstyle S}$ at $\sqrt{s} = 0.9\mathrm{\,TeV} $ are compared with the predictions of the LHCb\,MC, Perugia\,0 and Perugia\,NOCR as a function of \subref{fig:plotDataTheory-900-al-y} & \subref{fig:plotDataTheory-900-ak-y} rapidity and \subref{fig:plotDataTheory-900-al-pt} & \subref{fig:plotDataTheory-900-ak-pt} transverse momentum. Vertical lines show the combined statistical and systematic uncertainties and the short horizontal bars (where visible) show the statistical component.

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The ratios $\overline{\Lambda }{}$ / $\Lambda$ and $\overline{\Lambda }{}$ / $ K ^0_{\rm\scriptscriptstyle S}$ at $\sqrt{s} = 7\mathrm{\,TeV} $ compared with the predictions of the LHCb\,MC, Perugia\,0 and Perugia\,NOCR as a function of \subref{fig:plotDataTheory-7-al-y} & \subref{fig:plotDataTheory-7-ak-y} rapidity and \subref{fig:plotDataTheory-7-al-pt} & \subref{fig:plotDataTheory-7-ak-pt} transverse momentum. Vertical lines show the combined statistical and systematic uncertainties and the short horizontal bars (where visible) show the statistical component.

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The ratios \subref{fig:plotDeltaY-al} $\overline{\Lambda }{}$ / $\Lambda$ and \subref{fig:plotDeltaY-ak} $\overline{\Lambda }{}$ / $ K ^0_{\rm\scriptscriptstyle S}$ from LHCb are compared at both $\sqrt{s} = 0.9\mathrm{\,TeV} $ (triangles) and 7\,TeV (circles) with the published results from STAR [1] (squares) as a function of rapidity loss, $\Delta y = y_\mathrm{beam} - y$. Vertical lines show the combined statistical and systematic uncertainties and the short horizontal bars (where visible) show the statistical component.

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Tables and captions

Integrated signal yields extracted by fits to the invariant mass distributions of selected $ V ^0$ \! candidates from data taken with magnetic field up and down at $\sqrt{s} = 0.9$ and 7 $\mathrm{\,TeV}$ .

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Absolute systematic errors are listed in descending order of importance. Ranges indicate uncertainties that vary across the measurement bins and/or by collision energy. Correlated sources of uncertainty between field up and down are identified.

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The production ratios $\overline{\Lambda }{}$ / $\Lambda$ and $\overline{\Lambda }{}$ / $ K ^0_{\rm\scriptscriptstyle S}$ , measured at $\sqrt{s} = 0.9\mathrm{\,TeV} $ , are quoted in percent with statistical and systematic errors as a function of \subref{tab:val-stat-syst-900-al} & \subref{tab:val-stat-syst-900-ak} rapidity, $ y$ , and \subref{tab:val-stat-syst-900-pt} transverse momentum, $ p_\mathrm{T}$ \!$[\! {\mathrm{\,GeV\!/}c} ]$.

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The production ratios $\overline{\Lambda }{}$ / $\Lambda$ and $\overline{\Lambda }{}$ / $ K ^0_{\rm\scriptscriptstyle S}$ , measured at $\sqrt{s} = 7\mathrm{\,TeV} $ , are quoted in percent with statistical and systematic errors as a function of \subref{tab:val-stat-syst-7-al} & \subref{tab:val-stat-syst-7-ak} rapidity, $ y$ , and \subref{tab:val-stat-syst-7-pt} transverse momentum, $ p_\mathrm{T}$ \!$[\! {\mathrm{\,GeV\!/}c} ]$.

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The production ratios $\overline{\Lambda }{}$ / $\Lambda$ and $\overline{\Lambda }{}$ / $ K ^0_{\rm\scriptscriptstyle S}$ without non-prompt corrections at $\sqrt{s} = 0.9\mathrm{\,TeV} $ are quoted in percent with statistical and systematic errors as a function of \subref{tab:no-correction-900-al} & \subref{tab:no-correction-900-ak} rapidity, $ y$ , and \subref{tab:no-correction-900-pt} transverse momentum, $ p_\mathrm{T}$ \!$[\! {\mathrm{\,GeV\!/}c} ]$.

[Error creating the table]

The production ratios $\overline{\Lambda }{}$ / $\Lambda$ and $\overline{\Lambda }{}$ / $ K ^0_{\rm\scriptscriptstyle S}$ without non-prompt corrections at $\sqrt{s} = 7\mathrm{\,TeV} $ are quoted in percent with statistical and systematic errors as a function of \subref{tab:no-correction-7-al} & \subref{tab:no-correction-7-ak} rapidity, $ y$ , and \subref{tab:no-correction-7-pt} transverse momentum, $ p_\mathrm{T}$ \!$[\! {\mathrm{\,GeV\!/}c} ]$.

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