Production of [Formula: see text] and [Formula: see text] in proton-proton collisions at [Formula: see text] 7 TeV

Abstract

The production of the strange and double-strange baryon resonances ([Formula: see text], [Formula: see text]) has been measured at mid-rapidity ([Formula: see text][Formula: see text]) in proton-proton collisions at [Formula: see text] [Formula: see text] 7 TeV with the ALICE detector at the LHC. Transverse momentum spectra for inelastic collisions are compared to QCD-inspired models, which in general underpredict the data. A search for the [Formula: see text] pentaquark, decaying in the [Formula: see text] channel, has been carried out but no evidence is seen.

Fig. 1

Left panel The $\mathrm{\Lambda }$ ${\mathrm{\pi }}^{+}$ invariant mass distribution in $\left|y\right|$ $<$ 0.5 for the transverse momentum bin 1.2 $<$ $p_{\mathrm{T}}$ $<$1.4 GeV$/c$  in pp collisions at $\sqrt{s}$ $=$ 7 TeV. The background shape estimated using pairs from different events (event-mixing) is shown as open red squares. The mixed-event background is normalized in the range 1.56 $<M<$ 2.0 GeV$/c^2$, where $M$ is the $\mathrm{\Lambda }$ ${\mathrm{\pi }}^{+}$ invariant mass. Right panel The invariant mass distribution after mixed-event background subtraction for 1.2 $<$ $p_{\mathrm{T}}$ $<$1.4 GeV$/c$. The solid curve is the result of the combined fit (see text for details) and the dashed lines describes the residual background

Fig. 2

Same as Fig. 1 but for $\mathrm{\Sigma }$(1385)${}^{-}$ $⟶$ $\mathrm{\Lambda }$ $+$ ${\mathrm{\pi }}^{-}$. Note the peak at around the $\mathrm{\Xi }{(1321)}^{-}$ mass, which is absent in Fig. 1

Fig. 3

Left panel The $\mathrm{\Xi }$ ${}^{-}$ ${\mathrm{\pi }}^{+}$ invariant mass distribution in $\left|y\right|$ $<$ 0.5 for the transverse momentum bin $1.2<$ $p_{\mathrm{T}}$ $<1.6$ GeV$/c$  in pp collisions at $\sqrt{s}$ $=$ 7 TeV. The background shape estimated using pairs from different events (event-mixing) is shown as open red squares. The mixed-event background is normalized in the range 1.49 $<M<$ 1.51 GeV$/c^2$. Right panel The invariant mass distribution after mixed-event background subtraction for $1.2<$ $p_{\mathrm{T}}$ $<1.6$ GeV$/c$. The solid curve is the result of the combined fit and the dashed line describes the residual background

Fig. 4

The product of acceptance, efficiency and branching ratio of $\mathrm{\Sigma }$(1385)${}^{+}$ and $\mathrm{\Xi }$(1530)${}^0$, obtained with PYTHIA 6.4 [22] and GEANT3 [25], as function of $p_{\mathrm{T}}$ in $\left|y\right|$ $<$0.5. Only statistical uncertainties are reported. The dashed- and the dash-dotted lines indicate the overall branching ratio for the two reconstruction channels

Fig. 5

Inelastic baryon yields, ${\mathrm{d}}^{2}N/(\mathrm{d}y\mathrm{d}{p}_{\mathrm{T}})$, per $p_{\mathrm{T}}$ interval per unit rapidity for $\mathrm{\Sigma }$(1385) and $\mathrm{\Xi }{(1530)}^0$. Statistical and systematic uncertainties are summed in quadrature, excluding the $p_{\mathrm{T}}$-independent uncertainties, which affect only the overall normalization of the spectra and are not considered in the fit. Spectra are fitted with a Lévy–Tsallis function. The ratio data/fit is shown in the lower panel. For the sake of visibility, only $\mathrm{\Sigma }$(1385)${}^{+}$ is shown in the lower panel, but similar ratios have been obtained for the other three $\mathrm{\Sigma }$(1385) species. For the ratio, the integral of the fitting function in each corresponding $p_{\mathrm{T}}$ interval is considered. Spectra points are represented at the centre of the $p_{\mathrm{T}}$ interval

Fig. 6

The transverse momentum spectrum of $\mathrm{\Sigma }$(1385)${}^{+}$ is compared to standard tunes of PYTHIA 6 [34] and PYTHIA 8 [35], the latest release of HERWIG (6.521) [36], and SHERPA release 1.4.6 [37]. The MC data are binned according to the data. Spectra points are represented at the centre of the $p_{\mathrm{T}}$ interval. The lower panel shows the ratio data/MC. $p_{\mathrm{T}}$-independent uncertainties are not shown

Fig. 7

Same as Fig. 6 but for $\mathrm{\Xi }{(1530)}^0$

Fig. 8

The mean $p_{\mathrm{T}}$ as function of the particle mass including $\mathrm{\Sigma }$(1385)${}^{+}$, $\mathrm{\Xi }{(1530)}^0$ and other particles reconstructed in pp collisions at $\sqrt{s}$ $=$ 7 TeV and $\sqrt{s}$ $=$ 0.9 TeV by the ALICE collaboration [, , –32] and at $\sqrt{s}$ $=$ 0.2 TeV by the STAR collaboration [, –45]. The lower panel shows the ratio data/MC. Statistical and systematic uncertainties are shown separately (vertical solid lines and brackets, respectively)

Fig. 9

$\mathrm{\Xi }$ ${}^{-}$ ${\mathrm{\pi }}^{+}$ and $\mathrm{\Xi }$ ${}^{-}$ ${\mathrm{\pi }}^{-}$ invariant mass distributions. The arrow and the shaded area indicate the region where the $\mathit{\varphi }(1860)$ pentaquark is expected and where the search was performed