Production of K$^{*}$(892)$^{0}$ and $φ$(1020) in p-Pb collisions at $\sqrt{s_{\mathrm{NN}}}$ = 5.02 TeV

The production of K$^{*}$(892)$^{0}$ and $\phi$(1020) mesons has been measured in p-Pb collisions at $\sqrt{s_{\mathrm{NN}}}$ = 5.02 TeV. K$^{*0}$ and $\phi$ are reconstructed via their decay into charged hadrons with the ALICE detector in the rapidity range $-0.5 <~ y <~0$. The transverse momentum spectra, measured as a function of the multiplicity, have p$_{\mathrm{T}}$ range from 0 to 15 GeV/$c$ for K$^{*0}$ and from 0.3 to 21 GeV/$c$ for $\phi$. Integrated yields, mean transverse momenta and particle ratios are reported and compared with results in pp collisions at $\sqrt{s}$ = 7 TeV and Pb-Pb collisions at $\sqrt{s_{\mathrm{NN}}}$ = 2.76 TeV. In Pb-Pb and p-Pb collisions, K$^{*0}$ and $\phi$ probe the hadronic phase of the system and contribute to the study of particle formation mechanisms by comparison with other identified hadrons. For this purpose, the mean transverse momenta and the differential proton-to-$\phi$ ratio are discussed as a function of the multiplicity of the event. The short-lived K$^{*0}$ is measured to investigate re-scattering effects, believed to be related to the size of the system and to the lifetime of the hadronic phase.

Figures

Figure 1

(a) Comparison of the transverse momentum spectrum d$^{2}N$/(d$p_{\rm T}$d$y$) of $\phi$-meson in non-single diffractive (NSD) p-Pb events, reconstructed via the decay channel into K$^+$K$^-$ by exploiting three different strategies for K identification: TPC only, TPC-TOF and no-PID. The reader can refer to Sec.3.1 for details on the PID selection and to Sec.3.2 for a description of the signal extraction procedure. The uncertainties are the sum in quadrature of statistical and systematic. A Levy-Tsallis function (see Eq.4.1) is used to fit the TPC-TOF spectrum in 0.3 < $p_{\rm T}$ < 10 GeV/$c$. (b) Ratio of each spectrum to the fit function, showing good agreement of the three PID strategies within uncertainties.
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Figure 2

Invariant-mass distributions for K$^{\ast0}$ (a, b) and $\phi$(c, d) in the transverse momentum range 1.2 $\leq p_{\rm T}$ < 1.4 GeV/$c$ and multiplicity classes 0-20$\%$ and 0-5$\%$, respectively. Upper panels, (a) and (c), report the unlike-sign invariant-mass distribution and the mixed event background (MEB) normalised as described in the text. In lower panels, (b) and (c), the distributions after background subtraction are shown. The K$^{\ast0}$ is fitted with a Breit-Wigner function whereas the $\phi$ meson peak is described by a Voigtian function. A second order polynomial function is used to describe the residual background.
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Figure 3

Detector acceptance and signal reconstruction efficiency for K$^{\ast0}$ and $\phi$ mesons, which includes reconstruction, track selection and particle identification efficiency. For K$^{\ast0}$ and $\phi$ production below $\pt$ $< $ 3 GeV/$c$, the PID efficiency is relative to the TPC-TOF approach, whereas for $\phi$ production with $\pt$ $>$ 3 GeV/$c$ no PID contribution is included, as no particle identification is applied in the analysis.
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Figure 4

Transverse momentum spectra d$^{2}N$/(d$p_{\rm T}$d$y$) of K$^{\ast0}$ (a) and $\phi$ (b) for different multiplicity classes (V0A estimator), measured in the rapidity range $-0.5 < y < 0$. K$^{\ast0}$ and $\overline{\rm{K}^{\ast0}}$ are averaged. The multiplicity-dependent spectra are normalised to the visible cross section, whereas the minimum bias spectrum is normalised to the fraction of NSD events (see text). Statistical (bars) and systematic (boxes) uncertainties are indicated. Dashed lines represent Levy-Tsallis fits, see text for details.
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Figure 5

Mean transverse momentum of K$^{\ast0}$ and $\phi$ compared to that of identified $\pi^{\pm}$, K$^{\pm}$, K$^{0}_{S}$, p, $\Lambda$, $\Xi^{-}$ and $\Omega^{-}$ previously measured by ALICE in p-Pb collisions at $\snn=5.02$ TeV as a function of the charged particle density measured in the pseudo-rapidity range $\vert\eta_{\mathrm{lab}}\vert < 0.5$ (${\langle \rm{d}N_{ch}/{\rm d}N_{lab} \rangle}_{\vert \eta \vert < 0.5}$). The K$^{0}_{S}$, $\Lambda$ and $\Xi^{-}$ points are slightly displaced along the x-axis to avoid superposition with other points. Statistical uncertainties are represented as bars, whereas boxes indicate systematic uncertainties.
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Figure 6

System size dependence of the mean transverse momentum of K$^{\ast0}$ compared to that of the proton (left panel) and $\langle p_{\rm T}\rangle$ of $\phi$ compared to that of the proton (right panel). The system size is defined as the cubic root of the average charged particle multiplicity density ($\langle$d$N_{\rm ch}$/d$\eta_{\rm lab}$$\rangle^{1/3}$) measured in the ALICE central barrel ($\vert\eta_{\mathrm{lab}}\vert$ < 0.5) in pp at $\sqrt{s}$ = 7 TeV (green), p-Pb at $\snn$ = 5.02 TeV (blue) and Pb-Pb at $\snn$ = 2.76 TeV (red). Statistical uncertainties are represented as bars, boxes indicate total systematic uncertainties.
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Figure 7

Mass dependence of the mean transverse momentum of identified particles measured by ALICE in minimum bias pp collisions at $\sqrt{s}=7$ TeV and 0-20$\%$ p-Pb collisions. Statistical uncertainties are represented as bars, boxes indicate total systematic uncertainties.
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Figure 8

The (p+$\overline{\rm p}$)/$\phi$ ratio measured in p-Pb in 0-5$\%$ and 80-100$\%$ V0A multiplicity classes, compared to the same ratio measured in minimum bias pp collision at $\sqrt{s}=7$ TeV, 0-10$\%$ central and 60-80$\%$ peripheral Pb-Pb collisions at $\snn=2.76$ TeV.
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Figure 9

Ratio of K$^{\ast0}$ and $\phi$ to charged K measured in the three collision systems, as a function of the cube root of the average charged particle density ($\langle$d$N_{\rm ch}$/d$\eta_{\rm lab}$$\rangle^{1/3}$) measured at mid-rapidity, used as a proxy for the system size. Squares represent K$^{\ast0}$/K, circles refer to $\phi$/K. Statistical uncertainties (bars) are shown together with total (hollow boxes) and multiplicity-uncorrelated (shaded boxes) systematic uncertainties. Measurements in pp at $\sqrt{s}=7$ TeV and Pb-Pb collisions at $\snn=2.76$ TeV are taken from [30] and [33], respectively.
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