Measurement of forward neutral pion transverse momentum spectra for $\sqrt{s}=7\mathrm{TeV}$ proton-proton collisions at the LHC

The inclusive production rate of neutral pions in the rapidity range greater than $y=8.9$ has been measured by the Large Hadron Collider forward (LHCf) experiment during $\sqrt{s}=7\mathrm{TeV}$ proton-proton collision operation in early 2010. This paper presents the transverse momentum spectra of the neutral pions. The spectra from two independent LHCf detectors are consistent with each other and serve as a cross-check of the data. The transverse momentum spectra are also compared with the predictions of several hadronic interaction models that are often used for high-energy particle physics and for modeling ultrahigh-energy cosmic ray showers.

Figure 1

Schematic views of the Arm1 (left) and Arm2 (right) detectors. The transverse sizes of the calorimeters are $20\times 20{\mathrm{mm}}^2$ and $40\times 40{\mathrm{mm}}^2$ in Arm1, and $25\times 25{\mathrm{mm}}^2$ and $32\times 32{\mathrm{mm}}^2$ in Arm2.

Figure 2

Observation of ${\pi }^0$ decay by a LHCf detector. Left: two photons enter different calorimeters. Right: two photons enter one calorimeter.

Figure 3

$L_{90\%}$ distribution measured by the Arm1-20 mm calorimeter for the reconstructed energy of 500 GeV–1 TeV.

Figure 4

Reconstructed invariant mass distribution within the rapidity range from 9.0 to 9.2. Solid curve shows the best-fit composite physics model to the invariant mass distribution. Dashed curve indicates the background component. Solid and dashed curves indicate the signal and background windows, respectively.

Figure 5

Acceptance map of ${\pi }^0$ detection by the LHCf detectors in rapidity–$p_{\mathrm{T}}$ phase space: Arm1 (left) and Arm2 (right). Fiducial area cuts and energy threshold ($E_{\mathrm{photon}}>100\mathrm{GeV}$) are taken into account. Dashed curves indicate lines of constant energy ${\pi }^0$s, $E=1$, 2, and 3 TeV.

Figure 6

Experimental $p_{\mathrm{T}}$ spectra of the Arm1 (red dots) and Arm2 (blue triangles) detector. Error bars indicate the statistical uncertainties and shaded rectangles show the systematic uncertainties of the Arm1 and Arm2 detectors.

Figure 7

Combined $p_{\mathrm{T}}$ spectra of the Arm1 and Arm2 detectors (black dots) and the total uncertainties (shaded rectangles) compared with the predicted spectra by hadronic interaction models.

Figure 8

Ratio of the combined $p_{\mathrm{T}}$ spectra of the Arm1 and Arm2 detectors to the predicted $p_{\mathrm{T}}$ spectra by hadronic interaction models. Shaded areas indicate the range of total uncertainties of the combined $p_{\mathrm{T}}$ spectra.

Figure 9

Top: experimental $p_{\mathrm{T}}$ spectra (black dots and green shaded rectangles), the best-fit exponential distributions [Eq. (10), dashed curve], and the best-fit Gaussian distributions [Eq. (12), dotted curve]. Bottom: ratios of the best-fit exponential or Gaussian distribution to the experimental data (blue triangles or red open boxes) and the statistical and systematic uncertainties (green shaded areas). For both the top and bottom panels, the rapidity ranges $9.2<y<9.4$ and $9.4<y<9.6$ are shown on the left and right panels, respectively.

Figure 10

Average $p_{\mathrm{T}}$ as a function of rapidity loss $\Delta y$. Black dots and red diamonds indicate the LHCf data and UA7 results taken from Ref. [5], respectively. The predictions of hadronic interaction models are shown by open boxes (sibyll 2.1), open circles (qgsjet II-03), and open triangles (epos 1.99). For the predictions of the three models, solid and dashed curves indicate the results for the center-of-mass energy at the $\mathrm{Sp}\overline{\mathrm{p}}\mathrm{S}$ and the LHC, respectively.