An invariant differential cross section measurement of inclusive $\pi^{0}$ and $\eta$ meson production at mid-rapidity in pp collisions at $\sqrt{s}=8$ TeV was carried out by the ALICE experiment at the LHC. The spectra of $\pi^{0}$ and $\eta$ mesons were measured in transverse momentum ranges of $0.3<~p_{\rm T}<~35$ GeV/$c$ and $0.5<~p_{\rm T}<~35$ GeV/$c$, respectively. Next-to-leading order perturbative QCD calculations using fragmentation functions DSS14 for the $\pi^{0}$ and AESSS for the $\eta$ overestimate the cross sections of both neutral mesons, although such calculations agree with the measured $\eta/\pi^{0}$ ratio within uncertainties. The results were also compared with PYTHIA~8.2 predictions for which the Monash~2013 tune yields the best agreement with the measured neutral meson spectra. The measurements confirm a universal behavior of the $\eta/\pi^{0}$ ratio seen for NA27, PHENIX and ALICE data for pp collisions from $\sqrt{s}=27.5$ GeV to $\sqrt{s}=8$ TeV within experimental uncertainties. A relation between the $\pi^{0}$ and $\eta$ production cross sections for pp collisions at $\sqrt{s}=8$ TeV is given by $m_{\rm T}$ scaling for $p_{\rm T}>3.5$ GeV/$c$. However, a deviation from this empirical scaling rule is observed for transverse momenta below $p_{\rm T}<~3.5$ GeV/$c$ in the $\eta/\pi^0$ ratio with a significance of $6.2\sigma$.
Eur. Phys. J. C 78 (2018) 263
HEP Data
e-Print: arXiv:1708.08745 | PDF | inSPIRE
CERN-EP-2017-216
Figure 1
Determination of $RF$s for the PHOS-L0 and EMC-L0/L1 triggers. In the plateau region, the $RF$s are obtained by fits of constants in the given cluster energy ranges, illustrated by the dotted lines. The uncertainties of the determined $RF$s are indicated by light colored uncertainty bands, which are obtained by varying the fit ranges. |  |
Figure 2
Example invariant mass spectra in selected $p_{\rm T}$ slices for PCM (top left), PHOS (top right), EMC (bottom left) and PCM-EMC (bottom right) in the $\pi^{0}$ mass region The black histograms show raw invariant mass distributions before any background subtraction The grey points show mixed-event and residual correlated background contributions, which have been subtracted from raw real events to obtain the signal displayed with red data points The blue curves represent fits to the background-subtracted invariant mass spectra Additional examples of invariant mass distributions for the different methods are given in Ref. . |     |
Figure 3
Example invariant mass spectra in selected \pT slices in PCM (top left), PCM-EMCal (top right) and EMCal (bottom plots) in the $\eta$ mass region The black histograms show raw invariant mass distributions before any background subtraction The grey points show mixed-event and residual correlated background contributions, which have been subtracted from raw real events to obtain the signal displayed with red data points The blue curves represent fits to the background-subtracted invariant mass spectra Additional examples of invariant mass distributions for the different methods are given in Ref. . |     |
Figure 4
The left plots show reconstructed $\pi^{0}$ peak positions (left-bottom) and widths (left-top) of each reconstruction method compared to MC simulations for the transverse momentum bins used in the analysis. Corresponding plots for the $\eta$ meson are on the right for peak masses (right-bottom) and widths (right-top). |   |
Figure 5
The normalized correction factors, $\epsilon$, for each reconstruction method for $\pi^{0}$ (left) and $\eta$ mesons (right) plotted versus \pT bins used in the analysis The factors contain the detector acceptances and the respective reconstruction efficiencies, where acceptances are further normalized by the rapidity windows accessible with each method, $\Delta y$, and full azimuth coverage of $2\pi$, in order to enable a direct comparison between the different methods. |   |
Figure 7
Invariant cross sections for neutral meson production are shown together with NLO pQCD predictions using PDFs MSTW08 (CTEQ6M5) with FFs DSS14 (AESSS) for $\pi^{0}$ ($\eta$) as well as PYTHIA8.210 calculations, for which two different tunes are available. The data points are fitted using a TCM fit, Eq. \ref{eq:Bylinkin}, and a Tsallis fit, Eq. \ref{eq:Tsallis}. |   |
Figure 8
Left: $\eta/\pi^{0}$ ratio compared to NLO pQCD predictions using PDF CTEQ6M5 and FFs DSS07 for the $\pi^0$ and AESSS for the $\eta$, and PYTHIA8.210 calculations using Tune 4C and Monash 2013 tune. The total uncertainties of the measured $\eta/\pi^{0}$ ratio are of the order of $10$\% for most of the \pT bins covered, increasing for lower and higher momenta due to limited statistics as well as systematic effects. Right: Comparison of the $\eta/\pi^{0}$ ratio to related, previous ALICE measurements as well as other experiments at lower collision energies, for which total uncertainties are drawn. Furthermore, a comparison to the $\eta/\pi^{0}$ ratio obtained with \mT scaling is added. |   |