# Measurement of charged particle multiplicities in $pp$ collisions at $\sqrt{s}$ = 7 TeV in the forward region

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## Abstract

The charged particle production in proton-proton collisions is studied with the LHCb detector at a centre-of-mass energy of ${\sqrt{s} =7}$TeV in different intervals of pseudorapidity $\eta$. The charged particles are reconstructed close to the interaction region in the vertex detector, which provides high reconstruction efficiency in the $\eta$ ranges $-2.5<\eta<-2.0$ and $2.0<\eta<4.5$. The data were taken with a minimum bias trigger, only requiring one or more reconstructed tracks in the vertex detector. By selecting an event sample with at least one track with a transverse momentum greater than 1 GeV/c a hard QCD subsample is investigated. Several event generators are compared with the data; none are able to describe fully the multiplicity distributions or the charged particle density distribution as a function of $\eta$. In general, the models underestimate the charged particle production.

## Figures and captions

 The multiplicity distribution in $\eta$ bins (shown as points with statistical error bars) with predictions of different event generators. The inner error bar represents the statistical uncertainty and the outer error bar represents the systematic and statistical uncertainty on the measurements. The data in both figures are identical with predictions from Pythia 6, Phojet and Pythia 8 in (a) and predictions of the Pythia 6 Perugia tunes with and without diffraction in (b). newsys[..].eps [42 KiB] HiDef png [504 KiB] Thumbnail [222 KiB] *.C file newsys[..].eps [42 KiB] HiDef png [541 KiB] Thumbnail [220 KiB] *.C file The multiplicity distribution in the forward $\eta$ range (shown as points with error bars) with predictions of different event generators. The shaded bands represent the total uncertainty on the measurements. The data in both figures are identical with predictions from Pythia 6, Phojet and Pythia 8 in (a) and predictions of the Pythia 6 Perugia tunes with and without diffraction in (b). newsys[..].eps [16 KiB] HiDef png [262 KiB] Thumbnail [103 KiB] *.C file newsys[..].eps [17 KiB] HiDef png [285 KiB] Thumbnail [112 KiB] *.C file The KNO distributions in different bins of $\eta$. Only the the statistical uncertainties are shown. kno.eps [12 KiB] HiDef png [136 KiB] Thumbnail [60 KiB] *.C file The charged particle densities as a function of $\eta$ (shown as points with statistical error bars) and comparisons with predictions of event generators, as indicated in the key. The shaded bands represent the total uncertainty. The events are selected by requiring at least one charged particle in the range $2.0<\eta<4.5$. The data in both figures are identical with predictions from Pythia 6, Phojet and Pythia 8 in (a) and predictions of the Pythia 6 Perugia tunes with and without diffraction in (b). MCrap.eps [13 KiB] HiDef png [154 KiB] Thumbnail [65 KiB] *.C file MCrap_[..].eps [13 KiB] HiDef png [171 KiB] Thumbnail [72 KiB] *.C file The multiplicity distribution in $\eta$ bins (shown as points with error bars) with predictions of different event generators. The inner error bar represents the statistical uncertainty and the outer error bar represents the systematic and statistical uncertainty on the measurements. The events have at least one track with a $p_{\rm T} >1.0$ ${\mathrm{\,GeV\!/}c}$ in the pseudorapidity range $2.5 < \eta < 4.5$. The data in both figures are identical with predictions from Pythia 6, Phojet and Pythia 8 in (a) and predictions of the Pythia 6 Perugia tunes in (b). newsys[..].eps [43 KiB] HiDef png [494 KiB] Thumbnail [213 KiB] *.C file newsys[..].eps [39 KiB] HiDef png [401 KiB] Thumbnail [183 KiB] *.C file The multiplicity distribution in the forward $\eta$ range (shown as points with statistical error bars) with predictions of different event generators. The shaded bands represent the total uncertainty. The events have at least one track with a $p_{\rm T} >1.0$ ${\mathrm{\,GeV\!/}c}$ in the pseudorapidity range $2.5 < \eta < 4.5$. The data in both figures are identical with predictions from Pythia 6, Phojet and Pythia 8 in (a) and predictions of the Pythia 6 Perugia tunes in (b). newsys[..].eps [17 KiB] HiDef png [285 KiB] Thumbnail [109 KiB] *.C file newsys[..].eps [15 KiB] HiDef png [233 KiB] Thumbnail [84 KiB] *.C file The data charged particle densities as a function of $\eta$ (shown as points with statistical error bars) and comparisons with predictions of event generators, as indicated in the key. The events have at least one track with a $p_{\rm T} >1.0$ ${\mathrm{\,GeV\!/}c}$ in the pseudorapidity range $2.5 < \eta < 4.5$. The shaded bands represent the total uncertainty. MCrap_[..].eps [13 KiB] HiDef png [156 KiB] Thumbnail [65 KiB] *.C file MCrap_[..].eps [10 KiB] HiDef png [121 KiB] Thumbnail [51 KiB] *.C file Animated gif made out of all figures. PAPER-2011-011.gif Thumbnail

## Tables and captions

 Charged particle multiplicity distribution in the pseudorapidity range $-2.5<\eta<-2.0$ for minimum bias events and for hard QCD events (see text). The first quoted uncertainty is statistical and the second is systematic. Table_1.pdf [38 KiB] HiDef png [338 KiB] Thumbnail [61 KiB] tex code Charged particle multiplicity distribution in the pseudorapidity range $2.0<\eta<2.5$ for minimum bias events and for hard QCD events (see text). The first quoted uncertainty is statistical and the second is systematic. Table_2.pdf [38 KiB] HiDef png [341 KiB] Thumbnail [62 KiB] tex code Charged particle multiplicity distribution in the pseudorapidity range $2.5<\eta<3.0$ for minimum bias events and for hard QCD events (see text). The first quoted uncertainty is statistical and the second is systematic. Table_3.pdf [38 KiB] HiDef png [341 KiB] Thumbnail [62 KiB] tex code Charged particle multiplicity distribution in the pseudorapidity range $3.0<\eta<3.5$ for minimum bias events and for hard QCD events (see text). The first quoted uncertainty is statistical and the second is systematic. Table_4.pdf [38 KiB] HiDef png [341 KiB] Thumbnail [62 KiB] tex code Charged particle multiplicity distribution in the pseudorapidity range $3.5<\eta<4.0$ for minimum bias events and for hard QCD events (see text). The first quoted uncertainty is statistical and the second is systematic. Table_5.pdf [38 KiB] HiDef png [344 KiB] Thumbnail [61 KiB] tex code Charged particle multiplicity distribution in the pseudorapidity range $4.0<\eta<4.5$ for minimum bias events and for hard QCD events (see text). The first quoted uncertainty is statistical and the second is systematic. Table_6.pdf [38 KiB] HiDef png [334 KiB] Thumbnail [61 KiB] tex code Charged particle multiplicity distribution in the pseudorapidity range $2.0<\eta<4.5$ for minimum bias events and for hard QCD events (see text). The first quoted uncertainty is statistical and the second is systematic. Table_7.pdf [38 KiB] HiDef png [691 KiB] Thumbnail [122 KiB] tex code

Created on 16 February 2019.Citation count from INSPIRE on 21 February 2019.