Two-particle angular correlations were measured in pp collisions at $\sqrt{s} = 7$ TeV for pions, kaons, protons, and lambdas, for all particle/anti-particle combinations in the pair. Data for mesons exhibit an expected peak dominated by effects associated with mini-jets and are well reproduced by general purpose Monte Carlo generators. However, for baryon-baryon and anti-baryon--anti-baryon pairs, where both particles have the same baryon number, a near-side anti-correlation structure is observed instead of a peak. This effect is interpreted in the context of baryon production mechanisms in the fragmentation process. It currently presents a challenge to Monte Carlo models and its origin remains an open question.
Eur.Phys.J. C77 (2017) 569
HEP Data
e-Print: arXiv:1612.08975 | PDF | inSPIRE
CERN-EP-2016-322
Figure 1
Correlation functions for identical-particle pairs: $\pi^{+}\pi^{+}+\pi^{-}\pi^{-}$, $\rm K^{+}K^{+}+K^{-}K^{-}$, $\rm pp+\overline{p}\overline{p}$, $\Lambda\Lambda+\overline{\Lambda}\overline{\Lambda}$ (left panel) and particle--anti-particle pairs: $\pi^{+}\pi^{-}$, $\rm K^{+}K^{-}$, $\rm p\overline{p}$, $\Lambda\overline{\Lambda}$ (right panel). Plots are mirrored around $\Delta\eta=0$. |  |
Figure 3
$\Delta\eta$ integrated projections of correlation functions for combined pairs of (left) $\rm pp+\overline{p}\overline{p}$, $\rm p\Lambda+\overline{p}\overline{\Lambda}$, and $\Lambda\Lambda+\overline{\Lambda}\overline{\Lambda}$ and (right) $\rm p\overline{p}$, $\rm p\overline{\Lambda}+\overline{p}\Lambda$, and $\Lambda\overline{\Lambda}$. Statistical (bars) and systematic (boxes) uncertainties are plotted |   |
Figure 4
Correlation functions for combined pairs of $\rm pp+\overline{p}\overline{p}$ for two transverse momentum intervals \mbox{(a) $0.5 < p_{\rm T} < 1.25$ GeV/$c$} and (b) $1.25 < p_{\rm T} < 2.5$ GeV/$c$. Panel (c) shows $\Delta\eta$ integrated projections. Statistical (bars) and systematic (boxes) uncertainties are plotted. |  |
Figure 5
$\Delta\eta$ integrated projections of correlation functions for (a) $\pi^+\pi^-$, (b) $\rm K^+K^-$, (c) $\mathrm{p\overline{p}}$, and (d) $\Lambda\overline{\Lambda}$ pairs obtained from ALICE data and four Monte Carlo models (PYTHIA6 Perugia-0, PYTHIA6 Perugia-2011, PYTHIA8 Monash, PHOJET) at $\sqrt{s}=7\ \rm{TeV}$. Bottom panels show ratios of MC models to ALICE data. Statistical (bars) and systematic (boxes) uncertainties are plotted. |  |
Figure 6
$\Delta\eta$ integrated projections of correlation functions for combined pairs of (a) $\pi^+\pi^+ + \pi^-\pi^-$, (b) $\rm K^+K^+ + K^-K^-$, (c) $\mathrm{pp} + \rm \overline{p}\overline{p}$ and (d) $\Lambda\Lambda+\overline{\Lambda}\overline{\Lambda}$, obtained from ALICE data and four Monte Carlo models (PYTHIA6 Perugia-0, PYTHIA6 Perugia-2011, PYTHIA8 Monash, PHOJET). Bottom panels show ratios of MC models to ALICE data. Statistical (bars) and systematic (boxes) uncertainties are plotted. |  |
Figure 7
$\Delta\eta$ integrated projection of correlation functions for combined pairs of (left) $\rm p{\Lambda}+\overline{p}\overline{\Lambda}$ and (right) $\rm p\overline{\Lambda}+\overline{p}\Lambda$ obtained from ALICE pp collision data and four Monte Carlo models (PYTHIA6 Perugia-0, PYTHIA6 Perugia-2011, PYTHIA8 Monash, PHOJET) at $\sqrt{s}=7\ \rm{TeV}$. Clear anti-correlation is observed for all baryon pairs. Statistical (bars) and systematic (boxes) uncertainties are plotted. |  |