Study of very forward energy and its correlation with particle production at midrapidity in pp and p-Pb collisions at the LHC

The energy deposited at very forward rapidities (very forward energy) is a powerful tool for characterising proton fragmentation in pp and p$-$Pb collisions. The correlation of very forward energy with particle production at midrapidity provides direct insights into the initial stages and the subsequent evolution of the collision. Furthermore, the correlation with 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 interactions. Results about very forward energy, measured by the ALICE zero degree calorimeters (ZDCs), 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 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.

 

JHEP 08 (2022) 086
HEP Data
e-Print: arXiv:2107.10757 | PDF | inSPIRE
CERN-EP-2021-144
Figure group

Figure 2

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 3

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 estimated from ZN  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 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 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 5

ZN (left) and ZP (right) self-normalised signals as a function of the normalised multiplicity measured in 2 units of $\eta$ around the centre-of-mass midrapidity in pp (red circles) collisions and in the p-fragmentation region in p--Pb (blue squares) collisions. The boxes represent systematic uncertainties.

Figure 6

Self-normalised ZN (left) and ZP (right) signals as a function of the normalised charged-particle multiplicity produced in $|\eta|<1$ in pp collisions. Data (red markers) are compared with PYTHIA 6 (blue solid line), PYTHIA 8 (blue dashed line) and EPOS LHC (green dotted line).

Figure 7

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

Figure 8

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 9

Self-normalised ZN (left) and ZP (right) signals as a function of the leading particle \pt measured in $|\eta|<0.8$ in pp collisions at $\sqrt{s}=$13 TeV. Data (red markers) are compared to PYTHIA 6 (blue solid line), PYTHIA 8 (blue dashed line) and EPOS LHC (green dotted line).

Figure 10

Distributions of the (not fully corrected) number of charged particles in $|\eta|<1$ (left) and of the total transverse momentum in $|\eta|<0.8$ (right) for MB interactions (blue circles), single-side (azure empty squares) and double-side (red full squares) veto conditions on leading baryon production in pp collisions at $\sqrt{s}=$13 TeV.

Figure 11

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 $\pt^{\rm leading}$ measured in $|\eta| <$0.8. Tracks have \pt$>$0.15 GeV/c, markers are placed at the centre and not at the average of the \pt leading bin.