Transverse-momentum distribution and nuclear modification factor for neutral pions in the forward-rapidity region in proton-lead collisions at $\sqrt{s_{\mathit{N}N}}=5.02$ TeV

The transverse momentum ($p_{\mathrm{T}}$) distribution for inclusive neutral pions in the very forward rapidity region has been measured, with the Large Hadron Collider forward detector (LHCf), in proton-lead collisions at nucleon-nucleon center-of-mass energies of $\sqrt{s_{\mathit{N}N}}=5.02\mathrm{TeV}$ at the LHC. The $p_{\mathrm{T}}$ spectra obtained in the rapidity range $-11.0<y_{\mathrm{lab}}<-8.9$ and $0<p_{\mathrm{T}}<0.6\mathrm{GeV}$ (in the detector reference frame) show a strong suppression of the production of neutral pions after taking into account ultraperipheral collisions. This leads to a nuclear modification factor value, relative to the interpolated $p_{\mathrm{T}}$ spectra in proton-proton collisions at $\sqrt{s}=5.02\mathrm{TeV}$, of about 0.1–0.4. This value is compared with the predictions of several hadronic interaction Monte Carlo simulations.

Figure 1

Reconstructed invariant mass distribution within the rapidity range from $-9.4$ to $-9.2$. The solid curve shows the best-fit composite physics model to the invariant mass distribution. The dashed curve indicates the background component. Solid and dashed lines indicate the signal and background windows, respectively.

Figure 2

Experimental $p_{\mathrm{T}}$ spectra of the LHCf detector (filled circles). Error bars and shaded rectangles indicate the statistical and systematic uncertainties respectively. The open squares indicate the estimated contribution from UPCs.

Figure 3

Experimental $p_{\mathrm{T}}$ spectra measured by LHCf after the subtraction of the UPC component (filled circles). Error bars indicate the total uncertainties incorporating both statistical and systematic uncertainties. Hadronic interaction models predictions and derived spectra for $p$-$p$ collisions at 5.02 TeV are also shown (see text for details).

Figure 4

(a) Experimental $p_{\mathrm{T}}$ spectra (filled circles and error bars), and the best-fit exponential (dashed curve) and Gaussian distributions (dotted curve). (b) Ratios of the best-fit exponential and Gaussian distribution to the experimental data (open triangles or open boxes) with statistical and systematic uncertainties (filled circles and error bars.)

Figure 5

Average $p_{\mathrm{T}}$ as a function of rapidity $y_{\mathrm{lab}}$. Filled circles indicate the LHCf data. The predictions of hadronic interaction models are shown (solid curve dpmjet 3.04, dashed curve qgsjet II-03, and dotted curve epos 1.99).

Figure 6

Nuclear modification factor for ${\pi }^0$'s. Filled circles indicate the factors obtained by the LHCf measurements. Error bars indicate the total uncertainties incorporating both statistical and systematic uncertainties. Other lines are the predictions by hadronic interaction models (see text for details.)

Figure 7

Average $p_{\mathrm{T}}$ as a function of rapidity loss $\Delta y$. Filled circles indicate the LHCf results in $p$-$p$ collisions at $\sqrt{s}=7\mathrm{TeV}$ taken from Ref. [29]. Open circles indicate the LHCf results in $p$-$p$ collisions at $\sqrt{s}=2.76\mathrm{TeV}$. The best-fit linear function to the LHCf data is shown by the solid line.