The research team, led by Dibakar Dhar, Tumpa Biswas and Prabir Kumar Haldar of Cooch Behar Panchanan Barma University together with Zubayer Ahammed of the Variable Energy Cyclotron Centre in Kolkata, used the PYTHIA Monte Carlo event generator, specifically the refined Monash 2013 tune, to simulate billions of proton-proton collisions. They generated 6.7 billion events for each color reconnection configuration, an enormous statistical investment that allowed them to scrutinize the dynamical net-charge fluctuation observable with high precision. Their central question was deceptively simple: does the way color charges are rearranged before hadronization change how positive and negative particles correlate in the final state?

To understand why this matters, one must appreciate the physics of color reconnection. In perturbative quantum chromodynamics, the hard collision and subsequent parton showers produce quarks and gluons that must eventually be connected by color strings, which then fragment into the hadrons detectors actually observe. When many partons are produced simultaneously, as in multiparton interactions, the naive leading-color assignment of strings can become inefficient. Color reconnection models rearrange these connections so that the total string length is minimized, reflecting the expectation that nature prefers configurations of lower string tension. PYTHIA offers several implementations: the default MPI-based reconnection, a QCD-inspired scheme respecting SU(3) color rules, and a gluon-move variant based on advanced QCD topology, alongside a configuration with reconnection switched off entirely.

The observable at the heart of the study is called nu dynamic, a measure of the relative correlation between particle pairs of like and opposite charge. A negative value signals that opposite-charge pairs dominate the correlations, exactly what local charge conservation would predict, while a positive value would point to same-charge pairings. The quantity is related to the celebrated D measure used to compare fluctuations against theoretical limits: a hadron resonance gas with resonance decays is expected to yield D of roughly 3, whereas a quark-gluon plasma, whose charge carriers are fractionally charged quarks, suppresses fluctuations down to D between 1.0 and 1.5. Because the nu measure is insensitive to single-particle detector inefficiencies, it provides a robust bridge between simulation and experiment.

The team first validated their simulated samples against data from the ALICE experiment at CERN. The default MPI-based reconnection tune reproduced the pseudorapidity distribution of primary charged particles with deviations of less than 0.2 sigma across the full range, while the transverse momentum spectra of pions, kaons and protons showed discrepancies of roughly 25 to 48 percent for the reconnected configurations. The no-reconnection setup, by contrast, fared dramatically worse, overestimating pion yields by as much as 97 to 100 percent in some momentum bins. On the basis of single-particle spectra alone, color reconnection is clearly essential for a realistic description of proton-proton collisions.

Yet when the researchers turned to net-charge fluctuations, a counter-intuitive picture emerged. The no-reconnection configuration, despite its poor description of particle yields, produced fluctuation values closest to the ALICE measurements. Every reconnected tune overestimated the strength of the dynamical fluctuations, with the gluon-move scheme deviating from the data by approximately 75 percent at a pseudorapidity window of 1.0. Increasing the reconnection range parameter of the MPI-based scheme from 0.5 to 5.0 drove the fluctuation values progressively more negative, widening the gap with experiment. This systematic trend suggests that the geometric extent over which color strings are allowed to interact is a primary driver of opposite-sign charge correlations, effectively over-suppressing them relative to what nature actually does.

A crucial concern in any such analysis is whether the observed effects are genuine dynamics or merely a trivial consequence of particle density. Disabling color reconnection increases the number of multiparton interactions and hence the charged-particle multiplicity, which could shift fluctuation observables through simple statistical scaling. To disentangle the two, the team constructed a retuned no-reconnection baseline in which the semi-hard regularization scale of the multiparton interaction cross-section was adjusted until the charged-particle density matched both the ALICE data and the default reconnection tune, at approximately 3.96 particles per unit of pseudorapidity. At this fixed multiplicity, the retuned baseline gave a fluctuation value of minus 0.2868 plus or minus 0.0132, while the default MPI reconnection shifted it to minus 0.3286 plus or minus 0.0164. That persistent gap is the intrinsic dynamical signature of color reconnection, proving that the topological rearrangement of color fields actively enhances local charge correlations independently of how many particles are produced.

The study also examined identified-particle channels, comparing net charge, net pions, net kaons and net protons as a function of the pseudorapidity acceptance. For all tunes, the magnitude of the scaled fluctuation grows monotonically as the acceptance widens, driven partly by strengthening opposite-sign correlations and partly by global charge conservation, since the total charge of the proton-proton system must sum to plus two. Strikingly, only the inclusive net-charge and net-pion channels approach the theoretical quark-gluon plasma boundary at very large acceptances, beyond roughly four units of pseudorapidity, while the kaon and proton channels show no such behavior. The authors are careful to stress that this approach toward the plasma boundary does not constitute evidence for quark-gluon plasma formation in small systems; it reflects charge conservation, the correlation dynamics of the reconnection model, and the overwhelming statistical weight of pions.

By presenting both uncorrected and corrected fluctuation values, the analysis bridges two conventions in the field: the CMS collaboration has published results without the global charge conservation correction, while ALICE applies it. The correction, which adds a term of roughly 0.1 given total multiplicities of order 36 to 39, shifts values upward toward hadron resonance gas expectations but leaves the relative ordering of the tunes untouched, confirming that the discrepancies with data are genuine dynamical effects rather than artifacts of finite acceptance.

The broader implications reach into one of the most contested questions in heavy-ion physics. Recent years have seen tantalizing heavy-ion-like signatures in high-multiplicity proton-proton collisions, fueling speculation about droplets of quark-gluon plasma in small systems. This work delivers a sobering message: before attributing such signatures to collective behavior, the internal mechanics of string fragmentation must be understood at the level of charge correlations. The findings expose a phenomenological inconsistency in the current Monash implementation, which excels at describing energy flow through string-length minimization yet appears to over-correlate local charge partners. Net-charge fluctuations, the authors conclude, serve as a high-precision diagnostic for event generators, and future QCD-inspired models will need a more delicate balance between minimizing string length and preserving the observed local charge conservation. For experimentalists hunting the quark-gluon plasma, these quantitative benchmarks provide an essential baseline against which any claim of deconfinement in the smallest collision systems must now be measured.

Subject of Research: Effects of color reconnection on net-charge fluctuations in proton-proton collisions at 2.76 TeV simulated with PYTHIA Monash

Article Title: Exploring the effects of color reconnection on net-charge fluctuations in pp collisions at (\sqrt{s}=2.76 \text { TeV}) using PYTHIA Monash

Article References: Dhar, D., Biswas, T., Ahammed, Z., & Haldar, P. K. (2026). Exploring the effects of color reconnection on net-charge fluctuations in pp collisions at $$\sqrt{s}=2.76 \text { TeV}$$ using PYTHIA Monash. The European Physical Journal C, 86(9), Article 1119. https://doi.org/10.1140/epjc/s10052-026-16397-2

Image Credits: AI Generated

DOI: 10.1140/epjc/s10052-026-16397-2

Keywords: color reconnection, net-charge fluctuations, PYTHIA Monash, proton-proton collisions, quark-gluon plasma, ALICE, hadronization, multiparton interactions, quantum chromodynamics, string fragmentation, LHC, event generators

The analysis focuses on particles produced at midrapidity, within |y| < 0.5, in three of the highest-multiplicity event classes ever selected in proton-proton collisions. Using a dedicated high-multiplicity trigger that selects roughly the top 0.1 percent of events based on the signal in the V0 scintillator arrays, ALICE pushed the average charged-particle multiplicity density to 35.8 particles per unit of pseudorapidity, about a factor of five higher than in average inelastic proton-proton collisions. That density is comparable to what is seen in peripheral lead-lead collisions at 2.76 TeV per nucleon pair, dramatically narrowing the multiplicity gap between small and large collision systems and providing a crucial bridge for comparing physics across system sizes.

The experimental technique combines two independent particle-identification approaches. At low transverse momentum, between roughly 0.1 and 0.75 GeV/c depending on the particle species, the Inner Tracking System alone identifies hadrons through their specific energy loss in silicon. At higher momenta, from about 0.6 up to 4.0 GeV/c, a two-dimensional fit exploits the correlated information from the Time Projection Chamber and the Time-Of-Flight detector, simultaneously modeling the energy-loss and time-of-flight responses for pions, kaons, and protons while accounting for background from mismatched TOF signals. The data sample, collected in 2016 during LHC Run 2 with an integrated luminosity of 13 inverse picobarns for the high-multiplicity trigger, yielded about 80 million events for the inner-tracking analysis and 8 million for the TPC-TOF analysis.

After correcting the raw yields for tracking efficiency, detector acceptance, and contamination from secondary particles produced in weak decays or interactions with detector material, the collaboration obtained transverse-momentum spectra for each particle species in each multiplicity class. The most striking feature is a hardening of the spectra with increasing multiplicity, and that hardening is mass-dependent: the heavier the particle, the more its spectrum shifts toward higher momenta. This mass ordering is precisely the signature associated with radial flow in heavy-ion collisions, where the hot, dense medium expands outward hydrodynamically and gives heavier particles a bigger boost. Quantitatively, the kaon-to-pion ratio rises by a factor of about six, and the proton-to-pion ratio by a factor of about fifteen, as the transverse momentum increases from roughly 0.3 to 3.0 GeV/c.

The proton-to-pion ratio itself tells a compelling story. At low transverse momentum, below about 1 GeV/c, the ratio is suppressed relative to expectations, while at intermediate momenta around 3 GeV/c it shows a clear enhancement. This baryon-to-meson enhancement at intermediate transverse momentum has long been a hallmark of heavy-ion collisions, where it is attributed either to collective flow or to quark recombination, a process in which thermal quarks from the medium combine to form hadrons. Observing a similar pattern in proton-proton collisions, where no large medium was expected to form, reinforces the accumulating evidence that collective-like phenomena are not exclusive to nucleus-nucleus systems.

Perhaps the most consequential result emerges when the transverse-momentum-integrated kaon-to-pion and proton-to-pion ratios are plotted against the charged-particle multiplicity and compared across proton-proton, proton-lead, and lead-lead collisions at various energies. The measurements from all systems and energies fall on smooth, continuous trends, suggesting that particle production is governed primarily by the final-state charged-particle multiplicity rather than by the collision energy or the size of the colliding system. The kaon-to-pion ratio increases by a factor of 1.39 plus or minus 0.19 from low-multiplicity proton-proton collisions to the highest multiplicities in lead-lead collisions, a hint of strangeness enhancement, while the proton-to-pion ratio decreases by a factor of 1.23 plus or minus 0.15, consistent with antibaryon-baryon annihilation in the hadronic phase of heavy-ion collisions. The new high-multiplicity proton-proton points follow both trends seamlessly.

To probe the underlying mechanism, the collaboration compared its results with several theoretical models. PYTHIA 8, the workhorse Monte Carlo generator based on the Lund string model, was tested with four distinct tunes: the standard Monash 2013 configuration, a Ropes tune in which overlapping strings form higher-tension color ropes that boost baryon and strangeness production, a Shoving tune in which strings repel each other and generate a microscopic transverse pressure, and a CLR-BLC Mode 3 tune that introduces string-junction topologies during color reconnection to enhance baryon production. EPOS4, the latest version of the EPOS generator, takes a different approach, dynamically separating each event into a dense core that undergoes collective expansion and a dilute corona that fragments independently.

The comparison reveals that no single model consistently captures all the observed features. The Monash and Ropes tunes reproduce the pion spectra qualitatively within about 20 percent over the full momentum range, while EPOS4 underestimates them. All model calculations underestimate the kaon-to-pion ratio above 0.5 GeV/c, and none of the PYTHIA 8 tunes captures the low-momentum depletion seen in the highest-multiplicity data. EPOS4 provides a better description of the momentum-dependent kaon-to-pion and proton-to-pion ratios than the PYTHIA 8 tunes, and it reproduces the multiplicity trend of the pion mean transverse momentum across the full range, though it falls short for protons in the highest multiplicity classes. The Shoving tune, notably, gives a quantitatively good description of the pion mean transverse momentum, while all PYTHIA 8 tunes underestimate the kaon and proton values.

The mean transverse momentum itself adds another layer of evidence. For all three particle species, it rises continuously with multiplicity, and the rise is steeper for heavier hadrons, mirroring the pattern seen in heavy-ion collisions and consistent with radial flow. Interestingly, at similar multiplicity the mean transverse momentum in proton-proton collisions is higher than in lead-lead collisions, a difference that models will need to explain. The integrated yields were obtained by fitting the measured spectra with Levy-Tsallis functions, with only 6 to 9 percent of the yield extrapolated from unmeasured momentum regions, and the associated systematic uncertainties on the extrapolations range from 0.9 to 1.8 percent.

These measurements do not claim that a quark-gluon plasma forms in proton-proton collisions, and the nuclear modification factor in small systems remains consistent with unity at high momentum, in contrast to the strong jet suppression seen in lead-lead collisions. But by extending light-flavor hadron measurements to charged-particle densities previously unreachable in proton-proton events, ALICE has delivered exactly the kind of high-precision input that theorists need to constrain and improve their models of collective behavior in small systems. Whether the mass-dependent spectral hardening, the intermediate-momentum baryon enhancement, and the multiplicity-driven scaling of particle ratios ultimately trace back to a tiny droplet of deconfined matter, to dense overlapping strings, or to some other mechanism entirely, the new data make clear that the smallest collisions at the LHC are far from trivial, and that the physics of the very large and the very small is more deeply connected than anyone expected.

Subject of Research: Pion, kaon, and proton production in high-multiplicity proton-proton collisions at 13 TeV measured by ALICE

Article Title: (\pi ), K, and p production in high-multiplicity pp collisions at (\mathbf {\sqrt{ s} = 13}) TeV

Article References: ALICE Collaboration, Abdallah, D. A. H., Abualrob, I. J., Acharya, S., Rinella, G. A., Aglietta, L., Agrawal, N., Ahammed, Z., Ahmad, S., Ahuja, I., Akbar, Z., Akishina, V., Al-Turany, M., Alessandro, B., Molina, R. A., Ali, B., Alici, A., Alme, J., Alocco, G., … Zurlo, N. (2026). $$\pi $$, K, and p production in high-multiplicity pp collisions at $$\mathbf {\sqrt{ s} = 13}$$ TeV. The European Physical Journal C, 86(9), Article 1127. https://doi.org/10.1140/epjc/s10052-026-16235-5

Image Credits: AI Generated

DOI: 10.1140/epjc/s10052-026-16235-5

Keywords: ALICE, LHC, proton-proton collisions, pions, kaons, protons, high multiplicity, radial flow, strangeness enhancement, PYTHIA 8, EPOS4, quark-gluon plasma

The quark–gluon plasma, or QGP, is a state of deconfined quarks and gluons produced in ultrarelativistic heavy-ion collisions. In the earliest instants of the collision, pairs of quarks and gluons undergo hard scatterings, producing high-momentum particles that fly outward. When the plasma is present, these energetic partons lose energy through collisional and radiative processes, a phenomenon known as jet quenching. Experiments at the Relativistic Heavy Ion Collider (RHIC) and the LHC have long observed a strong suppression of jets and high-momentum hadrons in central lead–lead and gold–gold collisions, and this suppression is one of the defining signatures of the plasma’s existence.

The challenge for experimentalists is establishing a reliable reference point. Jets are reconstructed from showers of particles, and the reconstruction itself can be biased by the very energy loss being studied. ALICE’s new measurement sidesteps this problem by exploiting electroweak particles, in this case direct-prompt photons, which are produced in the first instants of the collision and do not interact strongly with the QGP. At leading order in perturbative quantum chromodynamics, a prompt photon is produced back-to-back in azimuthal angle with a recoiling parton of similar transverse momentum. The photon therefore serves as an unmodified clock and energy scale for the hard scattering, while its partner parton plows through the plasma and emerges modified.

To isolate genuine prompt photons from a sea of background, ALICE applied an isolation criterion. Photons were required to carry transverse momenta between 18 and 40 GeV/c at midrapidity, and the sum of charged-particle transverse momenta within a small cone of angular radius 0.2 around the photon direction had to be below 1.5 GeV/c, after subtracting the estimated contribution of the underlying event. This tight selection suppresses fragmentation photons, bremsstrahlung photons, and photons from neutral-meson decays, which are typically accompanied by other particle fragments. The associated charged hadrons were measured with transverse momenta above 1.8 GeV/c using the Inner Tracking System and the Time Projection Chamber.

The analysis, based on lead–lead data collected in 2015 and 2018, examined three collision centrality classes: central (0–30%), semicentral (30–50%), and peripheral (50–90%). For each trigger photon, the researchers constructed the conditional yield D(zT) of associated hadrons, where zT is the ratio of the hadron’s transverse momentum to the photon’s. This distribution, extracted from the region of azimuthal angle opposite to the photon, acts as a proxy for the fragmentation function of the recoiling parton. Extracting it in the dense environment of a lead–lead collision required careful subtraction of the underlying event using mixed-event techniques and statistical removal of background triggers, dominated by decays of neutral pions, through a purity-based procedure.

Because the corresponding proton–proton measurement at the same photon momentum interval was statistically too limited, the collaboration used next-to-leading-order pQCD calculations without energy loss as the proton–proton reference, defining a ratio called IpQCD. These calculations have been shown to describe the published proton–proton data well, making them a sound stand-in. The result is striking: in central collisions, the ratio sits at roughly 0.5 across most of the zT range, meaning the yield of hadrons associated with the photon is cut roughly in half by the medium. In semicentral collisions the suppression is milder, and in peripheral collisions the ratio moves closer to unity, exactly the pattern expected if the quark–gluon plasma is responsible for the quenching.

The data were confronted with two state-of-the-art theoretical models that include energy loss. The first is an NLO pQCD calculation in the Higher-Twist formalism, in which the in-medium transport coefficient controls the energy loss and its parameters were extracted through a global Bayesian analysis of single-hadron, di-hadron, and photon–hadron data at several collision energies. The second is the CoLBT-hydro model, which couples the transport of jet shower partons to a (3+1)-dimensional hydrodynamic description of the expanding medium, allowing the jets themselves to excite the plasma. Both models agree with the measured D(zT) distributions and with the suppression ratios across all three centrality classes, although the current uncertainties do not yet allow the two descriptions to be distinguished from each other.

Crucially, the measurement rules out an alternative explanation. An NLO pQCD calculation that includes only cold nuclear matter effects, using nuclear parton distribution functions but no energy loss, predicts a ratio close to unity and fails to reproduce the magnitude of the suppression seen in central collisions. The suppression therefore cannot be attributed to initial-state effects alone; it demands genuine in-medium energy loss. The collaboration also computed a centrality ratio comparing central and semicentral yields to peripheral ones, finding flat distributions averaging around 0.5 and 0.75 respectively, again in agreement with the energy-loss model and confirming the expected centrality dependence of the quenching.

The measurement gains additional power from comparison with other experiments. The IpQCD results for central collisions are qualitatively compatible with CMS measurements of photon-tagged jets and of hadrons correlated with Z bosons in lead–lead collisions at the LHC, with the Z-boson comparison showing particularly good agreement given its similar centrality range and trigger momentum. They also align with direct photon–hadron correlation results from the STAR Collaboration at RHIC, which used a lower photon momentum range in more central gold–gold collisions. The PHENIX result, however, shows a significantly enhanced soft-hadron yield at low zT, likely because its lower trigger momentum means the recoiling partons lose a larger fraction of their energy, and its lower hadron threshold makes it more sensitive to soft enhancement.

By pushing the photon trigger down to 18 GeV/c, ALICE has opened a window onto a kinematic region where the largest nuclear effects are expected, probing lower momentum transfers than other LHC measurements and overlapping the momentum range explored at RHIC. This measurement serves as a benchmark for the collaboration’s Run 3 and Run 4 analyses, where vastly larger data samples will enable more precise correlations, finer centrality differentials, access to lower and higher photon momenta, and studies of how quenching depends on the orientation of the collision geometry. As the LHC continues to recreate and interrogate the hottest matter ever made in a laboratory, photons are proving to be the perfect witnesses: untouched by the inferno they illuminate, yet carrying the full story of what their partners endured on the way out.

Subject of Research: Jet quenching probed via isolated-prompt photon–hadron correlations in Pb–Pb collisions at the LHC

Article Title: Measurement of isolated-prompt photon–hadron correlations in Pb–Pb collisions at (\sqrt{{s}_{{\textrm{NN}}}} = 5.02) TeV

Article References: ALICE Collaboration, Abdallah, D. A. H., Abualrob, I. J., Acharya, S., Agarwal, K., Rinella, G. A., Aglietta, L., Agrawal, N., Ahammed, Z., Ahmad, S., Ahuja, I., Akbar, Z., Akishina, V., Al-Turany, M., Alessandro, B., Alfarasyi, A. R., Molina, R. A., Ali, B., Alici, A., … Zurlo, N. (2026). Measurement of isolated-prompt photon–hadron correlations in Pb–Pb collisions at $$\sqrt{{s}_{{\textrm{NN}}}} = 5.02$$ TeV. The European Physical Journal C, 86(9), Article 1126. https://doi.org/10.1140/epjc/s10052-026-16192-z

Image Credits: AI Generated

DOI: 10.1140/epjc/s10052-026-16192-z

Keywords: ALICE, LHC, quark–gluon plasma, jet quenching, photon–hadron correlations, Pb–Pb collisions, heavy-ion physics, fragmentation function, energy loss, pQCD, RHIC, CMS

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