On 12 September, during a short, highly successful pilot run, the LHC operated with protons in one beam and lead ions in the other, so providing the LHC experiments with their first proton–nucleus collision data and opening new horizons for the heavy-ion community at CERN (CERN Courier November 2012 p6). During these few hours of pilot running, the ALICE experiment collected about 2 million events, sufficient not only to check the readiness of the detector for the long proton-ion run scheduled for the beginning of 2013, but also to perform a first analysis of the data and produce important physics results.
After the start of the heavy-ion physics programme in 2010, the LHC experiments obtained many striking results related to the formation of the hot and dense hadronic state of matter emerging from the collisions of lead nuclei. This state – the quark–gluon plasma (QGP) – is expected to manifest itself through various signatures, such as the suppression of high-energetic jets in the medium (CERN Courier January/February 2011 p6), collective particle motion, enhancement of strange-particle production and suppressed quarkonia production. In addition, surprising scaling effects were observed in the particle multiplicity compared with measurements at lower energies. However, given the complexity of the lead–lead (PbPb) colliding system, an important step in the quest for QGP lies in decoupling the effects of cold nuclear matter that arise at the initial stage of the collisions.
The proton–nucleus system represents the perfect benchmark for studying these effects because the colliding components are elementary and give rise to processes where the effects of the medium produced in the collision are either small or even totally absent. The collisions are also interesting because they allow the exploration of nuclear parton distributions in the region of small parton fractional momenta, which are so far unmeasured. Proton–nucleus collisions can therefore provide the data needed to understand better the properties of PbPb collisions at the energy of the LHC. The study of the dense initial state also provides access to a completely new QCD regime where the parton densities are expected to be saturated.
Using the newly acquired data, the ALICE collaboration has been able to measure the charged-particle multiplicity density in proton–lead (pPb) collisions at a centre-of-mass energy of √sNN = 5.02 TeV (ALICE collaboration 2012a). Figure 1 compares this measurement with two main groups of theoretical models. The first group consists of models that incorporate nuclear modification – for example, shadowing – of the initial parton distributions; the second includes various saturation models. While the current experimental and theoretical precision is not sufficient for a detailed comparison, the figure shows that the data are described best by the model where the gluon shadowing parameter (sg) is tuned to previous experimental data at lower energies. Saturation models predict much steeper dependence on the pseudorapidity, which is not confirmed by the measurement.
Another important result from the analysis of the proton–nucleus data concerns the charged-particle transverse-momentum spectrum and the corresponding nuclear-modification factor (ALICE collaboration 2012b). The latter is calculated using the proton–proton data at collision energies of 2.76 TeV and 7 TeV as reference (figure 2). The result clearly indicates little or no modification of the production of charged particles with transverse momentum greater than 2 GeV/c, thus confirming that the suppression of high-energy jets in PbPb collisions is not a result of cold nuclear-matter effects. The comparison with the available theoretical predictions suggests that the models require further development because they have difficulties in describing the multiplicity and the transverse-momentum spectrum simultaneously.