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ALICE Collaboration

Publications and source records attributed to ALICE Collaboration.

At least 37 records · Page 2Linked to original sources

Modification of jet-energy flow in heavy-ion collisions

The ALICE Collaboration presents the first measurements of the jet-energy flow ($Δp_{\rm T}$) observable in proton-proton and heavy-ion collisions. Jets are excellent probes for the quark$-$gluon plasma, a deconfined state of matter produced in heavy-ion collisions. The jet-energy flow observable characterizes the radial distribution of energy from the jet axis in an infrared and collinear-safe way and is sensitive to medium-induced parton-shower modifications. Inclusive charged jets are measured in Pb$-$Pb collisions at $\sqrt{s_{\rm NN}} = 5.02$ TeV for the transverse-momentum interval 60$-$80 GeV/$c$. For pp collisions at $\sqrt{s}=13$ TeV, measurements include the 40$-$60 and 60$-$80 GeV/$c$ intervals, where the latter serves as the reference for investigating medium-induced modifications. Results show that most parton energy is concentrated in the jet core, with a clear suppression of energy flow in heavy-ion collisions at larger radii (significance 3.5$-$4.5$σ$) indicating a narrowing of the energy flow. While all models -- PYTHIA 8, HERWIG, JEWEL, and JETSCAPE -- reproduce the pp results with only small deviations in the tails, the relative modification in Pb$-$Pb collisions is well described by JEWEL without recoil. Conversely, JEWEL with recoil (medium response) and JETSCAPE show significant deviations, exhibiting increasing or more constant trends with radius that are disfavored by the data.

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Precision mass measurements of multistrange baryons and their antiparticles

The $Ω^-$ baryon, composed of three strange quarks (sss), was predicted by the quark model and discovered in 1964, playing a pivotal role in establishing quarks as fundamental constituents of matter. Despite its importance, experimental knowledge of its mass remains limited, with the current world average relying on measurements performed more than four decades ago and lacking robust estimates of systematic uncertainties. This is notable given the central role of the $Ω^-$ mass, and alternatively that of the $Ξ^-$(dss), in lattice QCD calculations, where it is widely used to set the overall physical scale. Precise scale setting is essential for first-principles studies of quark confinement, chiral symmetry breaking, and stringent tests of the Standard Model. Here we report high-precision measurements of the masses of the $Ω^-$ and $Ξ^-$ baryons and their antiparticles, determined from invariant-mass reconstruction of their decay products in proton$-$proton collisions at the LHC. The analysis exploits the excellent tracking and particle-identification capabilities of the ALICE experiment, enabling accurate reconstruction of the displaced decay vertices characteristic of these short-lived particles. Each mass is measured with a fractional uncertainty of about 60 parts per million, for example $M_{\barΩ^+}=1672.558\,\pm\,0.034\,({\rm stat.})\,\pm\,0.102\,({\rm syst.})$ MeV/$c^2$. The precisely known K$^0_{\rm S}$ and $Λ$ masses are used for calibration. These results establish new precision benchmarks in strange-baryon spectroscopy and enable stringent tests of Charge-Parity-Time invariance in the multistrange-hadron sector. Our measurement reduces the scale uncertainty in lattice QCD calculations, enabling for instance sub per mille precision for the hadronic vacuum-polarization contribution to the muon anomalous magnetic moment.

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Hyperon ($Λ$) polarization along the beam axis in Pb-Pb collisions at $\sqrt{s_{\rm NN}} = 5.36$ TeV

The measurement of hyperon ($Λ$ and $\overlineΛ$) polarization along the beam axis in Pb-Pb collisions at $\sqrt{s_{\rm NN}} = 5.36$ TeV using the ALICE detector is presented. The polarization, arising from vorticity components induced by anisotropic flow, is studied relative to the second- and third-order event planes. The measured polarization exhibits clear azimuthal sine modulations, providing the first observation of polarization along the beam direction measured relative to the third-order event plane at the Large Hadron Collider. The values of the polarization measured with respect to the second-order event plane relative to the second harmonic event plane are consistent with previous measurements at $\sqrt{s_{\rm NN}} = 5.02$ TeV and show improved statistical precision owing to the larger data sample. Comparisons with hydrodynamic calculations indicate that the measured polarization is sensitive to the bulk viscosity and the vortical structure of the quark-gluon plasma, offering new constraints on its transport properties. In particular, the polarization measured relative to the third-order event plane is expected to provide an additional and independent input to constrain both bulk and shear viscosities of the quark-gluon plasma.

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Probing jet evolution with charged energy correlators in small systems

The ALICE Collaboration presents measurements of charged energy-energy correlators (charged EECs) within charged-particle jets at $\sqrt{s_{\rm NN}} = 5.02$ Tev in proton-proton and proton-lead collisions at the LHC. Charged EECs are a class of jet substructure observables that trace the flow of energy and electric charge within a jet, and provide a tool for disentangling the energy scales involved in the jet fragmentation process through the angular separation and charges of particle pairs. The interplay between energy distribution and charge conservation enables charged EECs to provide novel constraints on hadronization mechanisms. Measurements of charged EECs in proton--proton collisions in charged-particle jets with $20 < p_{\rm T,chjet} < 80$ GeV/$c$ are compared to event generators to investigate different hadronization mechanisms and parton shower models. These model comparisons show that the like-sign EECs are sensitive to changes in parton shower modeling, while unlike-sign EECs are sensitive to different hadronization schemes. Measurements in proton-lead collisions indicate that cold nuclear matter effects on charged EECs are consistent with charge-independent behavior.

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Energy-energy correlators in p-Pb collisions at $\sqrt{s_{\rm NN}} = 5.02$ TeV

This paper presents the first measurement of the two-point energy-energy correlator (EEC) inside charged-particle jets in p-Pb collisions at $\sqrt{s_{\rm NN}} = 5.02$ TeV with the ALICE experiment. The two-point EEC, an infrared- and collinear-safe measure of angular energy flow within jets, is used to probe both perturbative and non-perturbative stages of the jet evolution. The EEC is reported in 20-80 GeV/$c$ jets in p-Pb collisions and compared to a measurement of the EEC in pp events at $\sqrt{s} = 5.02$ TeV. A modification is observed in the 20-40 GeV/$c$ interval, with an enhancement at large opening angles and a suppression at small angles relative to pp collisions. The dependence of this modification on jet transverse momentum, rapidity, and forward activity is investigated. These results provide new constraints on cold nuclear matter effects relevant for disentangling initial- and final-state contributions to jet-structure modifications. An understanding of these cold nuclear matter effects is also relevant for interpreting EEC measurements in heavy-ion collisions.

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Exclusive dimuon production and coherent charmonium photoproduction at forward rapidity in ultra-peripheral Pb$-$Pb collisions at $\mathbf{\sqrt{s_{\rm NN}}=5.36}$ TeV

This Paper presents rapidity-differential measurements of coherent J/$ψ$ and $ψ({\rm 2S})$ photoproduction, as well as rapidity- and mass-differential measurements of exclusive dimuon production, in the forward rapidity region $-4 < y < -2.5$ in ultra-peripheral Pb$-$Pb collisions at $\sqrt{s_{\rm NN}}=5.36$ TeV using data recorded by the ALICE detector at the LHC in 2023, corresponding to an integrated luminosity of $\mathcal{L} = 1170 \pm 50~μ{\rm b}^{-1}$. The J/$ψ$ and $ψ({\rm 2S})$ results reveal the significant role of nuclear shadowing effects. The square root of the ratio of the measured quarkonium cross section to the impulse approximation prediction is about 0.76 for J/$ψ$ and 0.71 for $ψ({\rm 2S})$, at $y \approx -3$, corresponding to typical Bjorken-$x$ values of $10^{-2}$. The exclusive dimuon results highlight the sensitivity of such measurements to precise modeling of the photon flux, particularly at impact parameters near the nuclear radius.

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$\mathbf{\textbf{K}^{0}_{\textbf{S}}}-\mathbf{\textbf{K}^{0}_{\textbf{S}}}$ femtoscopy in Pb$-$Pb collisions at $\mathbf{\sqrt{\textit{s}_{\rm NN}} = 5.02}$ TeV at the LHC

Results from a one-dimensional femtoscopic analysis of ${\rm K}^{0}_{\rm S}-{\rm K}^{0}_{\rm S}$ correlations in Pb$-$Pb collisions at the center-of-mass energy $\sqrt{s_{\mathrm{NN}}}~=~5.02$ TeV using data collected by the ALICE experiment at the LHC are presented. The source radius $R$ and correlation strength $λ$ are studied as a function of centrality and pair-transverse momentum ($k_{\rm T}$) to provide insight into the space-time structure and composition of the particle-emitting source. The observed trends of radii as a function of $k_{\rm T}$ and centrality are consistent with the collective expansion of the system. Comparisons with measurements at $\sqrt{s_{\mathrm{NN}}}~=~2.76$ TeV by the ALICE Collaboration show agreement across multiplicities and $k_{\rm T}$. Hydrokinetic model predictions match the most central collision results but deviate in peripheral events, potentially reflecting limitations in the model's description of peripheral collisions. A comparison with recent measurements at the same energy by the CMS Collaboration shows compatibility in both $R$ and $λ$ within 1.3$σ$. These results extend previous ${\rm K}^{0}_{\rm{S}}-{\rm K}^{0}_{\rm{S}}$ femtoscopy to a higher energy, providing a consistent baseline for future comparisons.

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Measurement of isolated-prompt photon$-$hadron correlations in Pb$-$Pb collisions at $\mathbf{\sqrt{\textit{s}_{\rm NN}} = 5.02}$ TeV

The ALICE Collaboration has measured the azimuthal correlation between trigger isolated-prompt photons and associated charged hadrons in Pb$-$Pb collisions at the CERN LHC, at a centre-of-mass energy per nucleon pair of \snnfive. The trigger isolated-prompt photons are measured in the transverse-momentum range $18< p_{\rm T}^γ < 40$ GeV/$c$ and pseudorapidity range $|η^γ| <0.67$. The isolation selection is based on a charged particle isolation momentum threshold $p_{\rm T}^{\rm iso, ch} = 1.5$ GeV/$c$ within a cone of radius $R=0.2$. The associated charged particles are measured in the transverse-momentum ranges $p_{\rm T}^{\rm h} > 1.8$ GeV/$c$ and pseudorapidity $|η^{\rm h}| <0.9$. The yield D$(z_{\rm T})$ of associated hadrons per trigger, with $z_{\rm T} = p_{\rm T}^{\rm h}/p_{\rm T}^γ$, is measured in three Pb$-$Pb collision centrality classes: central (0$-$30%), semicentral (30$-$50%), and peripheral (50$-$90%). An approximation to the standard $I_{\rm AA}$ is computed from the D$(z_{\rm T})$ conditional yields, using NLO pQCD predictions as pp reference. A strong suppression of this ratio is observed in central collisions compared to peripheral collisions. The result extends to a lower $p_{\rm T}^γ$ relative to those reported in previously published Pb$-$Pb collisions measurements at $\sqrt{s_{\rm NN}}=5.02$ TeV. The measurement is compared to NLO pQCD calculations that include energy loss, and to the CoLBT-hydro model. The results from central collisions are also compared with measurements of jets correlated with isolated-prompt photons and of hadrons correlated with Z$^0$ bosons, both reported by the CMS Collaboration at the LHC, as well as with direct photon$-$hadron correlation measurements reported by the PHENIX and STAR Collaborations at RHIC.

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Wave-Function Femtometry: Hypertriton - The Ultimate Halo Nucleus

The interaction between nucleons and hyperons - baryons containing a strange quark - is key to understanding the properties of dense nuclear matter, such as that expected in the interior of neutron stars. Direct scattering experiments are hindered by the short lifetime of hyperons, prompting the study of hypernuclei - bound states of nucleons and hyperons - as an alternative approach. The lightest known hypernucleus, the hypertriton ($^3_Λ$H), is a weakly bound state composed of a proton, a neutron and a $Λ$ hyperon, and is believed to exhibit a halo-like structure with the $Λ$ being loosely bound to a deuteron core. Based on the first measurement of hypertriton production in proton-proton collisions at the CERN Large Hadron Collider (LHC), its halo structure is confirmed. A successful description of the hypertriton production yield within the nuclear coalescence framework enables an estimation of the $Λ$ separation from the deuteron core as $9.54^{+2.67}_{-1.11}$ fm.

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Dijet invariant mass of charged-particle jets in pp and p-Pb collisions at $\sqrt{s_{\rm NN}} = 5.02$ TeV

The ALICE collaboration presents the first measurement of the dijet invariant mass spectra of charged-particle jets in pp and p-Pb collisions at $\sqrt{s_{\rm NN}} = 5.02$ TeV. Charged particles in the mid-pseudorapidity region, $|η| < 0.9$, are clustered into jets using the anti-$k_{\rm T}$ algorithm with a resolution parameter $R = 0.4$. The leading and subleading jets are required to have a transverse momentum $p_{\rm T} > 20$ GeV$/c$ and to be contained within $\left|η_{\rm jet}\right| < 0.5$. The dijet invariant mass spectrum and the nuclear modification factor $R_{\rm pA}$ are presented in the low-mass region of 75 to 150 GeV/$c^2$. The nuclear modification factor for charged-particle dijet invariant mass is consistent with unity. This is in line with previous small-system jet studies. Comparisons with Monte Carlo simulations suggest that the low-mass region is sensitive to anti-shadowing effects on parton densities in the nucleus, however, the expected signal is subtle and below the present experimental sensitivity.

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Measurement of the $\mathbf{B^0}$-meson production cross section in proton--proton collisions at $\mathbf{\sqrt{\textit{s}}=13.6}$ TeV

This article reports the measurement of the transverse-momentum ($p_{\rm T}$) differential production cross section of B$^0$ mesons in proton-proton collisions at a centre-of-mass energy of $\sqrt{s}=13.6$ TeV with the ALICE detector at the CERN LHC. For the first time, the B$^0$ production cross section is measured at midrapidity ($|y|<0.5$) down to $p_{\rm T}=1~\mathrm{GeV}/c$ at LHC energies. The B$^0$ mesons and their charge conjugates were reconstructed via the B$^{0}\to$D$^{-}π^+$ decay channel, followed by the D$^-\to$K$^+π^-π^-$ decay. The measured $p_{\rm T}$-differential production cross section is described within uncertainties by state-of-the-art models based on perturbative quantum-chromodynamics calculations. Its rapidity dependence is also studied by computing the $p_{\rm T}$-differential ratios between the ALICE measurement and the one of B$^+$ mesons performed by the LHCb Collaboration at forward rapidity. The B$^0$ production cross section per unit of rapidity at midrapidity is ${\rm d}σ({\rm B^0})/{\rm d} y|_{|y|<0.5} = 24.2 \pm 1.4~(\text{stat.}) \pm 2.6~(\text{syst.})_{-0.3}^{+0.2}~(\text{extrap.})~μ{\rm b}$.

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Evidence of different $Λ_{\rm c}$-baryon and D-meson elliptic flow in Pb$-$Pb collisions at $\mathbf{\sqrt{\textit{s}_{\rm NN}}}$ = 5.36 TeV with ALICE at the LHC

The ALICE collaboration reports the azimuthal-anisotropy coefficient $v_2$ of prompt D$^0$, D$^+$, D$^+_{\rm s}$ mesons and the first measurement of $v_2$ of prompt $Λ_{\rm c}$ baryons in semicentral Pb$-$Pb collisions at a center-of-mass energy per nucleon pair of $\sqrt{s_{\rm NN}} = 5.36$ TeV. The D mesons and $Λ_{\rm c}$ baryons are reconstructed in their hadronic decays at midrapidity ($ |y|<0.8$) in the transverse-momentum interval $0.5< p_{\rm T} < 24$ GeV/$c$. Similar $v_2$ values are measured for D$^0$ and D$^+$, while a hint of a difference ($2.6σ$) emerges between D$^0$ and D$^+_{\rm s}$ mesons in the $1 < p_{\rm T} < 5 $ GeV/$c$ interval. A larger $v_2$ for $Λ_{\rm c}$ baryons with respect to D$^0$ mesons is observed with $3.7σ$ significance for $4 < p_{\rm T} < 12$ GeV/$c$, providing evidence for the partonic origin of charm-hadron $v_2$ and hadron formation via quark coalescence. This interpretation is further supported by comparisons with theoretical calculations of charm-quark transport in a hydrodynamically expanding medium.

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Production of $Ξ$ and $Ω$ hyperons in high-multiplicity proton-proton collisions at $\sqrt{s}$ = 13 TeV

This paper presents the first measurements of $Ξ$ and $Ω$ hyperon yields at the highest multiplicities reached in pp collisions at $\sqrt{s} = 13$ TeV. This measurement exploits the high-multiplicity pp collisions collected by ALICE with dedicated triggers. The selected collisions are characterised by about 30 charged particles per unit of rapidity, over four times more than in minimum-bias pp collisions at the same centre-of-mass energy, and about twice as many as in minimum-bias p-Pb or very peripheral Pb-Pb collisions at similar energies. The production yields and average transverse momenta of the hyperons agree with trends indicated by previous measurements in pp collisions at lower multiplicities. The difference in average transverse momenta between pp and p-Pb collisions, observed with the new high-multiplicity pp data, provides additional insight into the underlying particle production mechanisms in small systems. The results support a strong correlation between multi-strange hadron production and final-state multiplicity regardless of the collision system at the LHC energies, extending this observation to the highest multiplicity reached in pp collisions. The comparison with several state-of-the-art models (Pythia8.2 with the Monash 2013 tune, Pythia8.2 with Ropes, and EPOS4) suggests that the description of strange-hadron production is improved by recently introduced features such as interactions between overlapping strings in Pythia8.2 with Ropes and the collective expansion of high-density string regions in EPOS4.

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Measurement of the transverse-momentum fraction of strange hadrons from jet-like correlation structures in pp collisions at $\sqrt{s} = 13$ TeV

The first measurements of the average transverse-momentum fraction ($\langle z \rangle$) as a function of transverse momentum ($p_{\rm T}$) for strange baryons ($Λ$ and $\barΛ$) and strange mesons ($K_{\rm S}^0$), produced in mini-jets defined through angular correlations in pp collisions at $\sqrt{s} = 13$ TeV, are reported by the ALICE Collaboration at the LHC. The observable is obtained using a novel method, where the angular correlation between the strange hadrons and inclusive charged hadrons is weighted by the $p_{\rm T}$ of correlated particles at small angular distance. As a function of strange particles' $p_{\rm T}$, the results reveal a flat trend for strange mesons and a decreasing trend for strange baryons in the measured $p_{\rm T}$ region, indicating distinct hadronization mechanisms for $K_{\rm S}^0$ and $Λ$($\barΛ$). The measurements are compared to Monte Carlo models, namely PYTHIA~8 (with both Monash and Color Rope tunes) and the AMPT (A Multi-Phase Transport) model with string melting. None of these models provides a satisfactory description of the $\langle z \rangle$ distributions at low and intermediate $p_{\rm T}$.

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Measurement of the elliptic flow of $^3$He and $^3_Λ$H in Pb-Pb collisions at $\sqrt{s_{\rm NN}} = 5.36$ TeV

The first measurement of the elliptic flow coefficient of (anti)${}^3_Λ$H and the study of the $v_2$ of $^3\overline{\mathrm{He}}$ measured in Pb-Pb collisions at $\sqrt{s_{\rm NN}} = 5.36$ TeV with the ALICE detector are presented. Based on the large data sample of approximately five billion events collected in 2023 during the LHC Run 3 data taking, these measurements provide important insights into the production mechanism of (anti)(hyper)nuclei, as well as into the phase-space distributions of nucleons and hyperons produced in heavy-ion collisions. The results are discussed in the context of hydrodynamic and coalescence models, highlighting how the measurement of the elliptic flow of nuclei, such as helium and hypertriton, provides critical constraints on hadronization models.

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$π$, K, and p production in high-multiplicity pp collisions at $\sqrt{s} = 13$ TeV

This paper presents the measurement of $π^{\pm}$, K$^{\pm}$, and p($\bar{\text{p}}$) production in high-multiplicity proton-proton collisions at $\sqrt{s} = 13$ TeV at midrapidity ($|y| < 0.5$) using the ALICE detector at the LHC. The transverse-momentum ($p_{\rm T}$) spectra of these particles are reported for three high-multiplicity classes. The results show a mass- and multiplicity-dependent hardening of the $p_{\rm T}$ spectra and an enhancement of the p/$π$ ratio at intermediate $p_{\rm T}$. These features are similar to those observed in heavy-ion collisions, where quark-gluon plasma formation is expected. The new measurements have extended the highest average charged-particle multiplicity density per unit of pseudorapidity achieved in pp collisions, roughly a factor five higher than that in average inelastic pp collisions, thereby reducing the multiplicity gap between small and large collision systems. In addition, the results are further compared with previously published measurements and with model calculations obtained using distinct tunes of the PYTHIA 8 Monte Carlo generator, as well as with predictions from the EPOS4. The comparison of the $p_{\rm T}$-integrated K/$π$ and p/$π$ ratios across different collision systems and energies suggests that particle production scales with charged-particle multiplicity, rather than with collision energy or system size. While the PYTHIA 8 tunes and the EPOS4 model are able to reproduce some of these measurements, either quantitatively or qualitatively, none of them consistently describes all observed features of the data.

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Measurement of correlations between elliptic flow and mean transverse momentum in pp, p-Pb, and Pb-Pb collisions at the LHC

Measurements of the event-by-event correlation between elliptic flow ($v_2$) and the mean transverse momentum ($[p_{\rm T}]$) using the modified Pearson correlation coefficient $ρ(v_2^2,[p_{\rm T}])$ are reported in pp collisions at $\sqrt{s} = 13$ TeV, and in p-Pb and Pb-Pb collisions at a center-of-mass energy per nucleon pair $\sqrt{s_{\rm NN}} = 5.02$ TeV. This analysis is based on the full LHC Run 2 dataset recorded by ALICE and is performed for the first time in small collision systems with the ALICE detector. In Pb-Pb collisions, the $ρ(v_2^2,[p_{\rm T}])$ measurement shows a non-monotonic dependence on charged particle multiplicity ($N_{\rm ch}$); it first decreases and then increases with an increase in multiplicity. The decreasing trends of $ρ(v_2^2,[p_{\rm T}])$ with increasing multiplicity are also observed in p-Pb and pp collisions for the presented multiplicity range. All three systems show consistent values of $ρ(v_2^2,[p_{\rm T}])$ for $N_{\rm ch} \lesssim 80$. These measurements are also compared with theoretical model calculations, including PYTHIA, where no collectivity is generated, as well as AMPT and IP-Glasma + MUSIC + UrQMD, which produce collective effects in small systems. These comparisons offer unique insights into the origin of collectivity in small systems. They improve the understanding of the initial geometry, size, and their correlations. The comparison also allows an investigation of the role of initial momentum correlations predicted by the Color Glass Condensate framework. The new measurements could not be explained by current state-of-the-art models, offering insights into the initial stage of collisions in small systems and also imposing strong constraints on the existing theoretical models. This will significantly advance our understanding of the collective phenomena observed in small systems at the LHC.

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First measurement of the strong interaction scattering parameters for the $\mathbf{K^-d}$ and $\mathbf{K^+d}$ systems

Charged kaon--deuteron interactions offer a sensitive probe of the strangeness sector of QCD at low energy, particularly through their scattering parameters. Despite available theoretical predictions, experimental constraints remain absent, particularly for the $\rm K^{-}d$ system. The first measurement of femtoscopic correlation functions for $\rm K^{-}d \oplus K^{+}\overline{d}$ and $\rm K^{+}d \oplus K^{-}\overline{d}$ particle pairs in Pb--Pb collisions at $\sqrt{s_{\rm NN}}=5.02$ TeV recorded by ALICE at the LHC is presented. Correlation functions for both systems are analyzed in three centrality classes and fitted using the Lednický--Lyuboshitz model to extract the source size and strong interaction scattering lengths. The $\rm K^{-}d$ scattering length is found to be $\Re f_0 = -1.44 \pm 0.15\text{ (stat.)}^{+0.10}_{-0.10}$ (syst.) fm and $\Im f_0 = 1.34 \pm 0.33$ (stat.)$^{+0.21}_{-0.15}$ (syst.) fm, while for $\rm K^{+}d$ the value $\Re f_0 = -0.68 \pm 0.16$ (stat.)$^{+0.09}_{-0.09}$ (syst.) fm is obtained. These results provide the first constraints on the $\rm K^{\pm} d$ interaction modeling at low energy, delivering a long-awaited experimental benchmark for testing chiral QCD dynamics.

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