Neutron emission in ultraperipheral Pb-Pb collisions at $\sqrt{s_{\mathrm{NN}}} = 5.02$ TeV

In ultraperipheral collisions (UPCs) of relativistic nuclei without overlap of nuclear densities, the two nuclei are excited by the Lorentz-contracted Coulomb fields of their collision partners. In these UPCs, the typical nuclear excitation energy is below a few tens of MeV, and a small number of nucleons are emitted in electromagnetic dissociation (EMD) of primary nuclei, in contrast to complete nuclear fragmentation in hadronic interactions. The cross sections of emission of given numbers of neutrons in UPCs of $^{208}$Pb nuclei at $\sqrt{s_{\mathrm{NN}}}=5.02$ TeV were measured with the neutron zero degree calorimeters (ZDCs) of the ALICE detector at the LHC, exploiting a similar technique to that used in previous studies performed at $\sqrt{s_{\mathrm{NN}}}=2.76$ TeV. In addition, the cross sections for the exclusive emission of 1, 2, 3, 4 and 5 forward neutrons in the EMD, not accompanied by the emission of forward protons, and thus mostly corresponding to the production of $^{207,206,205,204,203}$Pb, respectively, were measured for the first time. The predictions from the available models describe the measured cross sections well. These cross sections can be used for evaluating the impact of secondary nuclei on the LHC components, in particular, on superconducting magnets, and also provide useful input for the design of the Future Circular Collider (FCC-hh).

 

Submitted to: PRC
e-Print: arXiv:2209.04250 | PDF | inSPIRE
CERN-EP-2022-186
Figure group

Figure 1

Colour online: distributions of energy in ZNC (left) and ZNA (right) from EMD events (histograms) and resulting fit functions (solid curves) representing the sum of Gaussians. The Gaussians representing 1n peaks are shown by dashed curves .

Figure 2

Colour online: measured (points) and calculated with RELDIS  (solid-line histogram) and $\mathrm{n^O_On}$  (dashed-line histogram) cross sections of emission of given numbers of neutrons $i$ in UPCs of $^{208}$Pb nuclei at $\sqrt{s_{\mathrm{NN}}}=5.02$ TeV. Combined statistical and systematic uncertainties of the measurements are presented..

Figure 3

Colour online: measured cross sections of emission of given numbers of neutrons $i$ in UPCs of $^{208}$Pb nuclei at $\sqrt{s_{\mathrm{NN}}}=5.02$ TeV without proton emission (points) and the cross sections calculated with RELDIS  (solid-line histogram). Calculated cross sections to produce specific secondary nuclei, $^{207,206,205,204,203,202,201}$Pb, are presented by the dash-dotted histogram marked by nuclide symbols. Combined statistical and systematic uncertainties of the measurements are presented..

Figure 4

Colour online: Probability, calculated with RELDIS (in \%), of the difference $\Delta A$ between the sum of the mass number of the most heavy residual nucleus and the numbers of emitted nucleons and $A$ of the initial $^{208}$Pb (left) and probability (in \%) of the difference $\Delta Z$ between the sum of the charge of the most heavy residual nucleus and the number of emitted protons and $Z$ of $^{208}$Pb (right) plotted as functions of $N_{\mathrm n}$ in each event..