W and Z bosons are the heavy carriers of the weak nuclear force, the interaction responsible for radioactive decay and the fusion reactions that power the Sun. Unlike protons or pions, they feel no strong force at all, which makes them exquisitely clean probes of what happens in the earliest instants of a collision. They are produced through the Drell-Yan process, in which a quark from one proton annihilates with an antiquark from the other. Because the bosons escape the collision zone essentially untouched by the fireball of strongly interacting matter around them, their production rates can be predicted with remarkable precision using perturbative quantum chromodynamics, the theory of the strong interaction.

The ALICE detector, situated in a cavern 100 metres beneath the French countryside, was designed primarily to study the quark-gluon plasma, a state of matter that existed microseconds after the Big Bang. Yet its central barrel, equipped with the Inner Tracking System, the Time Projection Chamber and the Electromagnetic Calorimeter, turns out to be well suited to hunting electroweak bosons as well. The collaboration reconstructed Z bosons by identifying pairs of electrons whose combined invariant mass falls between 60 and 108 gigaelectronvolts per square of the speed of light, the unmistakable fingerprint of a Z decay. W bosons, which decay into a single electron and an invisible neutrino, were teased out of the data through their high-energy electron signatures, requiring electrons with transverse momenta between 30 and 60 gigaelectronvolts per c at midrapidity.

Extracting these rare signals from the overwhelming torrent of ordinary particle production demanded extraordinary care. The analysis relied on 60 million calorimeter-triggered events, corresponding to an integrated luminosity of 7.8 inverse picobarns. Electrons were identified through their characteristic ionisation patterns in the Time Projection Chamber and the ratio of their calorimeter energy to their measured momentum, which hovers near unity for genuine electrons but is far smaller for hadrons. Isolation requirements, demanding that little additional energy surround each candidate within a cone of radius 0.3 in pseudorapidity-azimuth space, further suppressed backgrounds from the decays of heavy-flavour hadrons containing charm and beauty quarks. The team subtracted these contaminants using data-driven templates, corrected for photon conversions in the detector material, and accounted for the tiny feed-down from top quark decays, which contributes less than one percent.

The resulting measurements are strikingly consistent with theory. The fiducial Z boson production cross section came out at 203 picobarns, with a total systematic uncertainty of 11.5 percent, and agrees well with next-to-leading-order calculations performed with the POWHEG framework combined with the PYTHIA 8 parton shower. The integrated cross sections for electrons from W decays, 0.68 nanobarns for W-minus and 0.72 nanobarns for W-plus, likewise match predictions built on three modern sets of parton distribution functions: CT14NNLO, CT18NLO and NNPDF4.0. These PDFs encode our knowledge of how quarks and gluons are distributed inside the proton, and the W charge asymmetry, the ratio of W-plus to W-minus production, is particularly sensitive to the balance of down and up quarks. The measured ratio is dominated by statistical uncertainties, so the data cannot yet discriminate between the different PDF sets, but future ALICE data should sharpen this test considerably.

The truly novel part of the measurement goes beyond cross sections. For the first time at the LHC, ALICE studied how W boson production and the production of hadrons flying alongside them depend on the charged-particle multiplicity, essentially how busy the collision event is. This question matters because high-multiplicity proton-proton collisions show puzzling collective behaviour, with particles emerging in correlated patterns reminiscent of the fluid-like flow seen in heavy-ion collisions. Whether these effects arise from multiple parton interactions, colour reconnection, or mere statistical correlations between the measured particles and the event activity has remained hotly debated for over a decade.

The answer ALICE found is elegantly revealing. The yield of electrons from W decays rises approximately linearly with the charged-particle multiplicity, exactly as expected if the boson production is independent of whatever else happens in the event. But the hadrons produced in association with the W bosons, selected by their azimuthal correlation with the decay electron and required to carry transverse momenta above 10 gigaelectronvolts per c, increase faster than linearly, particularly in the busiest events. This divergence between a colourless electroweak probe and the strongly interacting hadrons around it is precisely the kind of differential measurement needed to disentangle the physics of dense small systems.

PYTHIA 8 simulations incorporating multiple parton interactions and colour reconnection reproduce both trends. In the colour reconnection picture, partons from different simultaneous scatterings within the same event exchange colour charge and merge their hadronisation processes, boosting the production of high-momentum hadrons without touching the colourless W boson itself. The data tend to favour calculations that include colour reconnection, though the statistical precision does not yet allow a definitive verdict. The collaboration also points to an alternative explanation: autocorrelations, in which the associated hadrons themselves contribute to the measured event multiplicity, could artificially steepen the observed trend. The comparison with J/psi measurements is instructive here. J/psi production at midrapidity, where the meson shares the same phase space as the multiplicity estimator, shows a faster-than-linear rise, while forward-rapidity J/psi production, separated by a large pseudorapidity gap, does not. The linear behaviour of the W boson, whose production is inherently isolated, supports the idea that such autocorrelation effects can strongly shape multiplicity-dependent measurements of charged particles.

Beyond their intrinsic interest for small-system physics, these measurements serve as a crucial reference for ALICE’s core mission. In lead-lead collisions, the suppression of high-transverse-momentum particles and the azimuthal anisotropies of hadron flow are interpreted as signatures of energy loss and collective motion inside the quark-gluon plasma. Electroweak bosons, immune to the strong force, provide the calibration yardstick against which such suppression is measured. Establishing that W and Z production in proton-proton collisions follows perturbative QCD predictions, and that their yields scale linearly with event activity, underpins the interpretation of every heavy-ion measurement that uses these bosons as baselines.

The measurements reported here represent only a first taste of what is to come. The ALICE detector has undergone a major upgrade for the LHC’s Run 3 and Run 4, and the vastly larger data samples now being collected will reduce statistical uncertainties dramatically, allowing precision tests of the W charge asymmetry, tighter constraints on parton distribution functions, and a definitive resolution of the faster-than-linear hadron puzzle. For now, the collaboration can celebrate a milestone: the quiet, colourless messengers of the weak force have been weighed in the noisiest environment the LHC can create, and they have carried their message through unscathed, exactly as the Standard Model promised.

Subject of Research: Measurements of W and Z boson production and their multiplicity dependence in proton-proton collisions at 13 TeV with the ALICE detector

Article Title: Measurements of the production of W(^{\pm }) and Z(^0) bosons in pp collisions at (\sqrt{s} = 13) TeV

Article References: ALICE Collaboration, Abualrob, I. J., Acharya, S., Aglieri Rinella, G., Aglietta, L., Agrawal, N., Ahammed, Z., Ahmad, S., Ahuja, I., Akbar, Z., Akindinov, A., Akishina, V., Al-Turany, M., Aleksandrov, D., Alessandro, B., Alfaro Molina, R., Ali, B., Alici, A., Alkin, A., … Zurlo, N. (2026). Measurements of the production of W$$^{\pm }$$ and Z$$^0$$ bosons in pp collisions at $$\sqrt{s} = 13$$ TeV. The European Physical Journal C, 86(9), Article 1109. https://doi.org/10.1140/epjc/s10052-026-15943-2

Image Credits: AI Generated

DOI: 10.1140/epjc/s10052-026-15943-2

Keywords: ALICE, Large Hadron Collider, W boson, Z boson, Drell-Yan process, perturbative QCD, parton distribution functions, proton-proton collisions, charged-particle multiplicity, colour reconnection, multiple parton interactions, electroweak bosons