Resonance suppression from color reconnection

We present studies that show how multi-parton interaction and color reconnection affect the hadro-chemistry in proton-proton (pp) collisions with special focus on the production of resonances using the pythia8 event generator. We find that color reconnection suppresses the relative production of meson resonances such as ${\rho }_0$ and ${\mathrm{K}}^{*}$, providing an alternative explanation for the ${\mathrm{K}}^{*}/\mathrm{K}$ decrease observed in proton-proton collisions as a function of multiplicity by the ALICE collaboration. Detailed studies of the underlying mechanism causing meson resonance suppression indicate that color reconnection leads to shorter, less energetic strings whose fragmentation is less likely to produce more massive hadrons for a given quark content, therefore reducing ratios such as ${\mathrm{K}}^{*}/\mathrm{K}$ and ${\rho }_0/\pi$ in high-multiplicity pp collisions. In addition, we have also studied the effects of allowing string junctions to form and found that these may also contribute to resonance suppression.

Figure 1

${\mathrm{K}}^{*}/\mathrm{K}$ $p_T$-integrated ratio as a function of charged particle multiplicity as measured by the ALICE experiment in p-Pb collisions at $\sqrt{s_{NN}}=5.02\mathrm{TeV}$ [22] compared to pythia8 predictions with (blue squares) and without (black circles) color reconnection.

Figure 2

${\mathrm{K}}^{*}/\mathrm{K}$ $p_T$-integrated ratio as a function of number of partonic interactions $N_{PI}$, for pp collisions at 7 TeV obtained with pythia8 with (blue symbols) and without (black symbols) color reconnection, and with (squares) and without (circles) considering hadronic decays.

Figure 3

String invariant mass distribution for inelastic events with at least one charged particle within $|\eta |<1.0$ generated with the pythia8 event generator with (blue symbols) and without (black symbols) CR. In both cases, the distributions were obtained without any selection on $N_{PI}$.

Figure 4

Average string invariant mass as a function of number of partonic interactions with (blue symbols) and without (black symbols) CR.

Figure 5

Integrated ${\mathrm{K}}^{*}/\mathrm{K}$ ratio obtained when fragmenting strings of different invariant masses with the pythia8 event generator with (blue symbols) and without (black symbols) CR. No restriction on hadron rapidity was required for either ${\mathrm{K}}^{*}$ or K.

Figure 6

(left) Integrated ${\mathrm{K}}^{*}/\mathrm{K}$ ratio and (right) average string invariant mass as a function of number of partonic interactions for $q\overline{q}$ strings (red) and junctions (blue) in the more QCD-based CR scheme.

Figure 7

(left) Integrated ${\rho }_0/\pi$ ratio as a function of number of partonic interactions and (right) as a function of fragmenting string mass with (blue symbols) and without (black symbols) CR. All hadronic-level decays have been switched off explicitly for these predictions and no restriction on hadron rapidity was required when calculating the ${\rho }^0/\pi$ ratio as a function of fragmenting string mass (right plot).

Figure 8

(left) Integrated ${\eta }^{\prime }/\pi$ ratio as a function of number of partonic interactions and (right) as a function of fragmenting string mass with (blue symbols) and without (black symbols) CR. All hadronic-level decays have been switched off explicitly for these predictions and no restriction on hadron rapidity was required when calculating the ${\eta }^{\prime }/\pi$ ratio as a function of fragmenting string mass (right plot).