Measurement of very forward energy and particle production at midrapidity in pp and p-Pb collisions at the LHC

The very forward energy is a powerful tool for characterising the proton fragmentation in pp and p-Pb collisions and, studied in correlation with particle production at midrapidity, provides direct insightsinto the initial stages and the subsequent evolution of the collision. Furthermore, the correlation between the forward energy and the production of particles with large transverse momenta at midrapidity provides information complementary to the measurements of the underlying event, which are usually interpreted in the framework of models implementing centrality-dependent multiple parton interaction. Results about the very forward energy, measured by the ALICE zero degree calorimeters (ZDC), and its dependence on the activity measured at midrapidity in pp collisions at $\sqrt{s} = 13$ TeV and in p-Pb collisions at $\sqrt{s_{\rm NN}} = 8.16$ TeV are presented and discussed. The measurements performed in pp collisions are compared with the expectations of three hadronic interaction event generators: PYTHIA 6 (Perugia 2011 tune), PYTHIA 8 (Monash tune), and EPOS LHC. These results provide new constraints on the validity of models in describing the beam remnants at very forward rapidities, where perturbative QCD cannot be used.

 

Submitted to: JHEP
e-Print: arXiv:2107.10757 | PDF | inSPIRE
CERN-EP-2021-144
Figure group

Figure 2

ZN (left) and ZP (right) asymmetry distributions for same (blue full circles) and mixed event distributions (blue open circles). In the bottom panels the difference between the uncorrelated and the correlated distributions is shown (red full squares) and compared to the simulation results using the three event generators (lines).

Figure 3

Average A-side ZN (left) and ZP (right) signals as a function of the C-side signals in pp collisions at $\sqrt{s} = $~13~TeV. Data (red full circles) are compared with model predictions from PYTHIA~6 (azure line), PYTHIA~8 (dashed blue line) and EPOS (dotted green line).

Figure 4

ZN energy normalised to the average MB value in the Pb-fragmentation (left) and in the p-fragmentation (right) regions as a function of centrality in p--Pb collisions at $\sqrt{s_{\rm{NN}}}=5.02$~TeV (pink circles) and $8.16$~TeV (blue squares). The boxes represent the systematic uncertainty.

Figure 5

ZN energy normalised to the average MB value in the Pb-fragmentation (left) and in the p-fragmentation (right) regions as a function of the average $N_{\rm coll}$ in p--Pb collisions at $\sqrt{s_{\rm{NN}}}=5.02$~TeV (pink circles) and $8.16$~TeV (blue squares). The boxes represent the systematic uncertainty.

Figure 6

ZN (left) and ZP (right) self-normalised signals as a function of midrapidity multiplicity in pp (red circles) collisions and in the p-fragmentation region in p--Pb (blue squares) collisions. The boxes represent systematic uncertainties.

Figure 8

Self-normalised ZN (left) and ZP (right) signal as a function of the number of self-normalised MPI extracted from PYTHIA~6 Perugia 2011 (full squares) and PYTHIA~8 Monash (empty squares) tunes.

Figure 9

Left: ZN spectrum in pp collisions at $\sqrt{s}=$13~TeV for the MB sample (blue circles) and in three multiplicity intervals: high (magenta squares), intermediate (orange squares) and low (azure squares) multiplicity. Right: ratio of the spectra, normalised to the number of events in each bin, in the three multiplicity intervals to the MB spectrum.

Figure 12

Self-normalised ZN signal (red circles) and number density $N_{ch}$ (azure squares) distribution in the transverse region (published in Ref.~) as a function of $p_{\rm T}^{\rm leading}$ measured in $|\eta| $0.15~GeV/c, markers are placed at the centre and not at the average of the \pt leading bin.