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Production of the $ρ$(770)${^{0}}$ meson in pp and Pb-Pb collisions at $\sqrt{s_{\rm NN}}$ = 2.76 TeV

The production of the $\rho$(770)${^{0}}$ meson has been measured at mid-rapidity $(|y|<~0.5)$ in pp and centrality differential Pb-Pb collisions at $\sqrt{s_{\rm NN}}$ = 2.76 TeV with the ALICE detector at the Large Hadron Collider. The particles have been reconstructed in the $\rho$(770)$\rightarrow\pi^{+}\pi^{-}$ decay channel in the transverse momentum ($p_{T}$) range $0.5-11$ GeV/$c$. A centrality dependent suppression of the ratio of the integrated yields $2\rho$(770)$^{0}/(\pi^{+}+\pi^{-})$ is observed. The ratio decreases by $\sim40\%$ from pp to central Pb-Pb collisions. A study of the $p_{T}$-differential $2\rho$(770)$^{0}/(\pi^{+}+\pi^{-})$ ratio reveals that the suppression occurs at low transverse momenta, $p_{T}<~2$ GeV/$c$. At higher momentum, particle ratios measured in heavy-ion and pp collisions are consistent. The observed suppression is very similar to that previously measured for the $K^{*}$(892)$^{0}/K$ ratio and is consistent with EPOS3 predictions that may imply that rescattering in the hadronic phase is a dominant mechanism for the observed suppression.

 

Submitted to: PRC
e-Print: arXiv:1805.04365 | PDF | inSPIRE

Figures

Figure 1

Invariant mass distributions for $\pip\pim$ pairs after subtraction of the like-sign background. Plots on the left and right are for the low and high transverse momentum intervals, respectively. Examples are shown for minimum bias pp, 0--20\% and 60--80\% central \pb collisions at \rsnn. Solid red curves represent fits to the function described in the text. Colored dashed curves represent different components of the fit function, which includes a smooth remaining background as well as contributions from \ksnm, \rhos, \omm, \ksm, \fnm and \ftm. See text for details.
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Figure 2

Left: measured $\omega/\pi$ ratio as a function of transverse momentum in pp collisions at $\rso = 62$ and $200$~GeV~. The smooth curve shows the estimated $\omega/\pi$ ratio in pp collisions at \rs. Right: estimated $\omega/\pi$ ratio as a function of the transverse momentum for 0--20\% central \pb collisions at \rsnn. The shaded regions in the two panels correspond to a $\pm20\%$ variation of the ratio, see text for details.
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Figure 3

Acceptance times reconstruction efficiency ($A\times\effr$) evaluated for \rhos meson in pp and central \pb collisions at \rsnn.
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Figure 4

Reconstructed masses of \rhos mesons as a function of transverse momentum in minimum bias pp, 0--20\% and 60--80\% \pb collisions at \rsnn. The statistical and systematic uncertainties are shown as bars and boxes, respectively. The width of the boxes is varied for visibility. The dashed lines show the \rhos masses as given in ~.
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Figure 5

Differential yields of \rhos as a function of transverse momentum in inelastic pp collisions at \rs. The statistical and systematic uncertainties are shown as bars and boxes, respectively. The results are compared with model calculations from PYTHIA 6 (Perugia 2011 tune)~, PYTHIA 8.14 (Monash 2013 tune)~ and PHOJET~. The lower panel shows the model-to-data ratios and the the gray shaded region represents the sum in quadrature of the systematic and statistical uncertainties associated with the data.
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Figure 6

Differential yields of \rhos in pp and 0--20\%, 20--40\%, 40--60\%, 60--80\% central \pb collisions at \rsnn. The statistical and systematic uncertainties are shown as bars and boxes, respectively.
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Figure 7

Compilation of $2\rhos/(\pip+\pim)$ and $\rhos/\pim$ measurements at different energies. The ratios are from measurements in $e^{+}e^{-}$ collisions at $\rso=10.45$~GeV~, $29$~GeV~ and $91$~GeV~; pp collisions at $6.8$~GeV~, $19.7$~GeV~, $27.5$~GeV~, $52.5$~GeV~ and $200$~GeV~; $K^{+}$p collisions at $7.82$~GeV~ and $\pim$p collisions at $19.6$~GeV~. The ALICE measurement in pp collisions at \rs is shown with a red marker. The total uncertainties are shown as bars.
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Figure 8

$\rhos/\pi$ ratio (left) and \mpt (right) at mid-rapidity ($|y| < 0.5$) in pp and 0--20\%, 20--40\%, 40--60\%, 60--80\% central \pb collisions at \rsnn as a function of \dncr. Statistical uncertainties are shown as bars. The total and uncorrelated systematic uncertainties are shown with open and shaded boxes, respectively. The widths of the boxes are fixed to arbitrary values for better visibility. The measurements are compared to EPOS3~ calculations. The $\rhos/\pi$ ratio is also compared to grand canonical thermal model~ prediction shown with the red horizontal line.
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Figure 9

$\rhos/\pi$ ratio in pp collisions at \rs as a function of transverse momentum. The statistical and systematic uncertainties are shown as bars and boxes, respectively. The results are compared with model calculations from PYTHIA Perugia 2011~,PYTHIA 8.14 Monash 2013~ and PHOJET~.
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Figure 10

$\rhos/\pi$ ratio in 0--20\% (left panel) and 60--80\% (right panel) central \pb collisions at \rsnn. The statistical and systematic uncertainties are shown as bars and boxes, respectively. The measurements are compared to EPOS3~ calculations performed with and without a hadronic cascade modeled with UrQMD~. In the lower panels, the model to data ratio is reported. Bars indicate statistical uncertainty of calculations whereas the gray shaded boxes represent the square root of sum of squares of statistical and systematic uncertainties associated with data.
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Figure 11

Nuclear modification factors \raa measured for charged pions, charged kaons, (anti)protons~ and \rhos mesons in 0--20\% (left panel) and 60--80\% (right panel) central \pb collisions at \rsnn. The statistical and systematic uncertainties are shown as bars and boxes, respectively. The boxes at unity correspond to scaling uncertainties.
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