# 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 $\sqrt{s} = 7\mathrm{ Te V}$ for \subref{fig:fisher-k} $K ^0_{\rm\scriptscriptstyle S}$ and \subref{fig:fisher-a} $\overline{\Lambda }{}$ . Fig1a.eps [13 KiB] HiDef png [151 KiB] Thumbnail [142 KiB] *.C file Fig1b.eps [12 KiB] HiDef png [144 KiB] Thumbnail [137 KiB] *.C file Invariant mass peaks for \subref{fig:mass-fit-a} $\overline{\Lambda }{}$ in the range $0.25< p_\mathrm{T} <2.50 {\mathrm{ Ge V /}c}$ & $2.5< y <3.0$ and \subref{fig:mass-fit-k} $K ^0_{\rm\scriptscriptstyle S}$ in the range $0.65< p_\mathrm{T} <1.00 {\mathrm{ Ge V /}c}$ & $3.5< y <4.0$ at $\sqrt{s} = 0.9\mathrm{ Te V}$ 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. Fig2a.eps [18 KiB] HiDef png [201 KiB] Thumbnail [172 KiB] *.C file Fig2b.eps [18 KiB] HiDef png [209 KiB] Thumbnail [183 KiB] *.C file \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 $\sqrt{s} = 7\mathrm{ Te V}$ . The difference between data and Monte Carlo is reduced by weighting the simulated candidates. Fig3a.eps [12 KiB] HiDef png [199 KiB] Thumbnail [175 KiB] *.C file Fig3b.eps [11 KiB] HiDef png [176 KiB] Thumbnail [146 KiB] *.C file 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. Fig4a.eps [8 KiB] HiDef png [92 KiB] Thumbnail [52 KiB] *.C file Fig4b.eps [8 KiB] HiDef png [96 KiB] Thumbnail [55 KiB] *.C file 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} $\sqrt{s} = 0.9\mathrm{ Te V}$ and \subref{fig:plotData-7-al-3y} & \subref{fig:plotData-7-ak-3y} 7 $\mathrm{ Te V}$ 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. Fig5a.eps [11 KiB] HiDef png [125 KiB] Thumbnail [128 KiB] *.C file Fig5b.eps [12 KiB] HiDef png [131 KiB] Thumbnail [129 KiB] *.C file Fig5c.eps [11 KiB] HiDef png [127 KiB] Thumbnail [131 KiB] *.C file Fig5d.eps [12 KiB] HiDef png [128 KiB] Thumbnail [132 KiB] *.C file The ratios $\overline{\Lambda }{}$ / $\Lambda$ and $\overline{\Lambda }{}$ / $K ^0_{\rm\scriptscriptstyle S}$ at $\sqrt{s} = 0.9\mathrm{ Te V}$ 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. Fig6a.eps [9 KiB] HiDef png [109 KiB] Thumbnail [109 KiB] *.C file Fig6b.eps [12 KiB] HiDef png [135 KiB] Thumbnail [138 KiB] *.C file Fig6c.eps [9 KiB] HiDef png [96 KiB] Thumbnail [96 KiB] *.C file Fig6d.eps [11 KiB] HiDef png [121 KiB] Thumbnail [123 KiB] *.C file The ratios $\overline{\Lambda }{}$ / $\Lambda$ and $\overline{\Lambda }{}$ / $K ^0_{\rm\scriptscriptstyle S}$ at $\sqrt{s} = 7\mathrm{ Te V}$ 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. Fig7a.eps [11 KiB] HiDef png [114 KiB] Thumbnail [116 KiB] *.C file Fig7b.eps [11 KiB] HiDef png [131 KiB] Thumbnail [134 KiB] *.C file Fig7c.eps [10 KiB] HiDef png [100 KiB] Thumbnail [101 KiB] *.C file Fig7d.eps [11 KiB] HiDef png [118 KiB] Thumbnail [119 KiB] *.C file 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 $\sqrt{s} = 0.9\mathrm{ Te V}$ (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. Fig8a.eps [10 KiB] HiDef png [132 KiB] Thumbnail [135 KiB] *.C file Fig8b.eps [11 KiB] HiDef png [140 KiB] Thumbnail [140 KiB] *.C file Animated gif made out of all figures. PAPER-2011-005.gif Thumbnail

## 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 $\mathrm{ Te V}$ . Table_1.pdf [44 KiB] HiDef png [40 KiB] Thumbnail [18 KiB] tex code 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. Table_2.pdf [56 KiB] HiDef png [93 KiB] Thumbnail [41 KiB] tex code The production ratios $\overline{\Lambda }{}$ / $\Lambda$ and $\overline{\Lambda }{}$ / $K ^0_{\rm\scriptscriptstyle S}$ , measured at $\sqrt{s} = 0.9\mathrm{ Te V}$ , 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, $p_\mathrm{T}$ $[ {\mathrm{ Ge V /}c} ]$. [Error creating the table] The production ratios $\overline{\Lambda }{}$ / $\Lambda$ and $\overline{\Lambda }{}$ / $K ^0_{\rm\scriptscriptstyle S}$ , measured at $\sqrt{s} = 7\mathrm{ Te V}$ , 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, $p_\mathrm{T}$ $[ {\mathrm{ Ge V /}c} ]$. [Error creating the table] The production ratios $\overline{\Lambda }{}$ / $\Lambda$ and $\overline{\Lambda }{}$ / $K ^0_{\rm\scriptscriptstyle S}$ without non-prompt corrections at $\sqrt{s} = 0.9\mathrm{ Te V}$ 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, $p_\mathrm{T}$ $[ {\mathrm{ Ge V /}c} ]$. [Error creating the table] The production ratios $\overline{\Lambda }{}$ / $\Lambda$ and $\overline{\Lambda }{}$ / $K ^0_{\rm\scriptscriptstyle S}$ without non-prompt corrections at $\sqrt{s} = 7\mathrm{ Te V}$ 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, $p_\mathrm{T}$ $[ {\mathrm{ Ge V /}c} ]$. [Error creating the table]

Created on 11 July 2020.