Nuclear modification factor of light neutral-meson spectra up to high transverse momentum in p-Pb collisions at $\sqrt{s_{\rm NN}}$ = 8.16 TeV

Neutral pion ($\pi^{0}$) and $\eta$ meson production cross sections were measured up to unprecedentedly high transverse momenta ($p_{\rm T}$) in p-Pb collisions at $\sqrt{s_{\rm NN}}$ = 8.16 TeV. The mesons were reconstructed via their two-photon decay channel in the rapidity interval $-1.3<~ y <~0.3$ in the ranges of $0.4<~p_{\rm T}<~200$ GeV/$c$ and $1.0<~p_{\rm T}<~50$ GeV/$c$, respectively. The respective nuclear modification factor ($R_{\rm pPb}$) is presented for $p_{\rm T}$ up to of 200 and 30 GeV/$c$, where the former was achieved by extending the $\pi^{0}$ measurement in pp collisions at $\sqrt{s}$ = 8 TeV using the merged cluster technique. The values of $R_{\rm pPb}$ are below unity for $p_{\rm T}<~10$ GeV/$c$, while they are consistent with unity for $p_{\rm T}>10$ GeV/$c$, leaving essentially no room for final state energy loss. The new data provide strong constraints for nuclear parton distribution and fragmentation functions over a broad kinematic range and are compared to model predictions as well as previous results at $\sqrt{s_{\rm NN}}$ = 5.02 TeV.


Phys. Lett. B 827 (2022) 136943
HEP Data
e-Print: arXiv:2104.03116 | PDF | inSPIRE
Figure group

Figure 1

Shower shape distribution for the elongation $\sigma_\mathrm{long}^2$ in PYTHIA 8 Monte Carlo simulations a) showing the various contributions to the full cluster sample for a high $\pT$ example interval and b) compared to the $\sigma_\mathrm{long}^2$ distribution in data in the same $\pT$ interval.

Figure 2

Ratio of the neutral pion invariant differential cross sections to the two-component model (TCM) fit of the combined spectrum for the different reconstruction techniques PCM, PCM-EMC, EMC, PHOS and mEMC in \pPb\ collisions at \sU. Statistical uncertainties are given by the vertical error bars while systematic uncertainties are shown as boxes.

Figure 3

Neutral pion~a) and $\eta$ meson~d) \com{invariant differential} cross sections for \pp\ collisions at $\sUp$ and $\pPb$ collisions at $\sU$ together with TCM fits, NLO calculations~ and PYTHIA~8~ predictions using different (n)PDFs~. Statistical uncertainties are shown as vertical bars; the systematic uncertainties as boxes. The ratios of the $\pz$ spectra in \pPb\ and \pp\ collisions to the TCM fits are shown in panel b) and c), respectively, together with the ratios of the calculations to the fits; panel~e) shows the same for $\eta$ mesons in \pPb\ collisions. In panel f) the $\eta/\pi^0$ ratios in \pp\ and \pPb\ collisions are compared to theory predictions The normalization uncertainty in the spectra ratio panels is indicated as a solid gray box around unity.

Figure 4

a)~$\rpp$ for $\pz$ and $\eta$ mesons in \pPb\ collisions at $\sU$ together with NLO~, CGC~ and FCEL~ predictions b)~$\rpp$ for $\pz$ at $\sU$ compared with $\pz$~ and charged hadron measurements~ at $\snn=5.02$ TeV c)~Ratio of the $\pz$ $\rpp$ at $\sU$ to that at $\snn=5.02$ TeV together with corresponding CGC and FCEL model predictions Statistical uncertainties are shown as vertical bars; the systematic uncertainties as boxes. The overall normalization uncertainties are indicated as solid boxes around unity and amount to 3.4\% in a) and b), and to 6.2\% in c).