First measurement of the forward rapidity gap distribution in $p\mathrm{Pb}$ collisions at $\sqrt{s_{\mathrm{NN}}}=8.16\mathrm{TeV}$

For the first time at LHC energies, the forward rapidity gap spectra from proton-lead collisions for both proton and lead dissociation processes are presented. The analysis is performed over 10.4 units of pseudorapidity at a center-of-mass energy per nucleon pair of ${\sqrt{s}}_{\mathrm{NN}}=8.16\mathrm{TeV}$, almost 300 times higher than in previous measurements of diffractive production in proton-nucleus collisions. For lead dissociation processes, which correspond to the pomeron-lead event topology, the epos-lhc generator predictions are a factor of 2 below the data, but the model gives a reasonable description of the rapidity gap spectrum shape. For the pomeron-proton topology, the epos-lhc, qgsjet ii, and hijing predictions are all at least a factor of 5 lower than the data. The latter effect might be explained by a significant contribution of ultraperipheral photoproduction events mimicking the signature of diffractive processes. These data may be of significant help in understanding the high energy limit of quantum chromodynamics and for modeling cosmic ray air showers.

Figure 1

Topologies of $p\mathrm{Pb}$ events with large FRG for $\mathbb{P}\mathrm{Pb}$ (left) and $\mathbb{P}p$ or $\gamma p$ (right). The blue and red cones indicate the products of diffractive dissociation for the lead ion and proton, respectively. The regions free of final-state particles are marked with green arrows. It is possible for $\gamma \mathrm{Pb}$ interactions to mimic the topology on the left but these are highly suppressed compared to the $\gamma p$ case on the right.

Figure 2

Differential cross section $d\sigma /d\mathrm{\Delta }{\eta }^{\mathrm{F}}$ for events with $\mathbb{P}\mathrm{Pb}$ (left) and $\mathbb{P}p+\gamma p$ (right) topologies obtained at the reconstruction level for $|\eta |<3.0$ region. Also shown are the simulated predictions of epos-lhc (blue) and hijing (green). The statistical and systematic errors are added in quadrature and shown with the gray band. The simulated spectra are normalized to the total visible cross section of the data. The bottom panels show the ratio of simulated predictions to data.

Figure 3

Reconstruction level $d\sigma /d\mathrm{\Delta }{\eta }^{\mathrm{F}}$ spectra obtained for the central acceptance, $|\eta |<3$, for the $\mathbb{P}\mathrm{Pb}$ (left) and $\mathbb{P}p+\gamma p$ (right) topologies and compared to the corresponding epos-lhc predictions. The epos-lhc predictions are broken down into the nondiffractive (ND, blue), central-diffractive (CD, green), single-diffractive (SD, orange) and double-diffractive (DD, purple) components, shown as stacked contributions.

Figure 4

The number of high purity tracks, $N_{\mathrm{Trk}}$ (left), their transverse momentum, $p_{\mathrm{T}}$, (middle), and the total energy of all PF candidates, $E_{\mathrm{Sum}}^{\text{PFObjects}}$, (right) in the first $\eta$ bin after a gap of $4.5<\mathrm{\Delta }{\eta }^{\mathrm{F}}<5.0$, for events with the $\mathbb{P}p+\gamma p$ topology. Also shown are the corresponding distributions for the epos-lhc and hijing generators.

Figure 5

Unfolded diffraction-enhanced differential cross section $d\sigma /d\mathrm{\Delta }{\eta }^{\mathrm{F}}$ spectra compared to hadron level predictions of the epos-lhc, hijing, and qgsjet ii generators. The data are corrected for the contribution from events with undetectable energy in the HF calorimeter adjacent to the FRG. The corrections are obtained using the epos-lhc MC samples. For the $p\mathrm{Pb}$ data sample, in the $\mathbb{P}\mathrm{Pb}$ case (left), the FRG, $\mathrm{\Delta }{\eta }^{\mathrm{F}}$, is measured from $\eta =3$ and no particles are present within $3.00<\eta <5.19$, while for the $\mathbb{P}p+\gamma p$ case (right), the FRG is measured from $\eta =-3$ and no particles are present within $-5.19<\eta <-3.00$. The statistical and systematic uncertainties are added in quadrature. The gray band shows the resulting uncertainty. The yellow band indicates the values of the only MC-based correction done to account for the HF calorimeter energy deposition below the noise level. The bottom panels show the ratio of the predictions of the three generators to data.

Figure 6

Unfolded diffraction-enhanced differential cross section $d\sigma /d\mathrm{\Delta }{\eta }^{\mathrm{F}}$ spectra for the $\mathbb{P}\mathrm{Pb}$ (left) and $\mathbb{P}p+\gamma p$ (right) topologies compared to the epos-lhc predictions. The epos-lhc predictions are broken down into the nondiffractive (ND, blue), central-diffractive (CD, green), single-diffractive (SD, orange) and double-diffractive (DD, purple) components, shown as stacked contributions.

Figure 7

Unfolded diffraction-enhanced differential cross section $d\sigma /d\mathrm{\Delta }{\eta }^{\mathrm{F}}$ spectra for the $\mathbb{P}\mathrm{Pb}$ (left) and $\mathbb{P}p+\gamma p$ (right) topologies compared to the qgsjet ii predictions. The qgsjet ii predictions are broken down into the nondiffractive (ND, blue), central-diffractive (CD, green), single-diffractive (SD, orange) and double-diffractive (DD, purple) components, shown as stacked contributions.

Figure 8

Top: Reconstruction level diffraction-enhanced differential cross section $d\sigma /d\mathrm{\Delta }{\eta }^{\mathrm{F}}$ spectrum corrected for the contribution from events with undetectable energy in the HF calorimeter adjacent to the rapidity gap. The correction value is indicated with the yellow band. The statistical and systematic uncertainties are added in quadrature. The gray band shows the resulting uncertainty. The distribution is shown together with the spectrum obtained with events satisfying the ZDC veto requirement $E_{\mathrm{ZDC}-}<1\mathrm{TeV}$ which selects only the events without lead nuclear break up. No correction for HF undetectable energy is applied to this distribution and no systematic uncertainties related to the ZDC veto are accounted for. The statistical and systematic uncertainties are added in quadrature. Bottom: A fraction of events selected with the ZDC veto requirement as a function of the rapidity gap size. Only statistical uncertainties are shown in the plot.