LHCf investigates proton–lead collisions

The final run of the LHC in January 2013 prior to the start of the current long shutdown provided collisions between a beam of protons and a beam of lead ions, allowing the LHCf experiment to make further studies related to the interactions of cosmic rays in the Earth’s atmosphere. In particular, the collaboration was able to measure the distribution in transverse momentum (p_T) for the inclusive production of neutral pions in the very forward region.

Despite several experimental indications at the HERA electron–proton collider at DESY, it is still not well understood how the density of partons (quarks and gluons) in a proton target increases or even saturates when Bjorken-x in the target – essentially the fraction of the proton’s momentum – is extremely small. Such phenomena are known to be visible in events at large rapidities – that is, close to the beam direction. Furthermore, in the case of nuclear targets, the parton density in the target is expected to be larger by about A^1/3, where A is the nuclear mass number. In hadronic interactions, partons in the projectile hadron would lose their energy while travelling in the dense QCD-governed matter of the nuclear target, and particle production mechanisms would change accordingly when compared with those in nucleon–nucleon interactions.

The LHCf detector is designed to measure the hadronic production cross-sections of neutral particles emitted at angles close to the beam direction – the “very forward” region – in proton–proton (pp) and proton–lead (pPb) collisions at the LHC. The detector covers a pseudorapidity range larger than 8.4 and is capable of precise measurements of the forward high-energy inclusive-particle-production cross-sections of neutral particles. Now, the collaboration has analysed the data taken in January 2013 on pPb collisions at nucleon–nucleon centre-of-mass energies of √s_NN = 5.02 TeV and a beam-crossing angle of 145 μrad, for an integrated luminosity of 0.63 nb⁻¹.

To obtain the soft-QCD component of the forward pion production, which is sensitive to the parton density in target, unavoidable contamination from ultra-peripheral collisions was first calculated using Monte Carlo simulations and then subtracted from the measured p_T spectra. Once the ultra-peripheral collisions have been taken into account, the p_T spectum measured by LHCf in the rapidity range −11.0 < y_lab < −8.9 and 0 < p_T < 0.6 GeV (in the detector reference frame) indicates a strong suppression of the production of neutral pions. This leads to a value of the nuclear modification factor value, R_pPb, relative to the interpolated p_T spectra in pp collisions at √s = 5.02 TeV, of about 0.1–0.4 – a value that is in overall agreement with the predictions of several Monte Carlo simulations of hadronic interactions.