81 Search Results

The first measurement of the 𝑒⁺⁢𝑒⁻ pair production at midrapidity and low invariant mass in central Pb-Pb collisions at $$\sqrt{s_{NN}}$$ = 5.02 TeV at the Large Hadron Collider is presented. The yield of 𝑒⁺⁢𝑒⁻ pairs is compared with a cocktail of expected hadronic decay contributions in the invariant mass (𝑚𝑒⁢𝑒) and pair transverse momentum (𝑝_{T,𝑒⁢𝑒}) ranges 𝑚_𝑒⁢𝑒 < 3.5 GeV/𝑐² and 𝑝_{T,𝑒⁢𝑒} < 8 GeV/𝑐. For 0.18 < 𝑚_𝑒⁢𝑒 < 0.5 GeV/𝑐² the ratio of data to the cocktail of hadronic contributions amounts to 1.40 ± 0.11⁢(stat.) ± 0.23 ⁢(syst.) ± 0.16⁢ (cocktail) and 1.42 ± 0.11⁢ (stat.) ± 0.23more » (syst.)$$^{+0.24}_{−0.29}$$ ⁢(cocktail), including or not including medium effects in the estimation of the heavy-flavor background, respectively. It is consistent with predictions from two different models for an additional contribution of thermal 𝑒+⁢𝑒− pairs from the hadronic and partonic phases. In the intermediate-mass range (1.2 < 𝑚_𝑒⁢𝑒 < 2.6 GeV/𝑐²), the pair transverse impact parameter of the 𝑒⁺⁢𝑒⁻ pairs (DCA𝑒⁢𝑒, where “DCA” denotes “distance of closest approach”) is used for the first time in Pb-Pb collisions to separate displaced dielectrons from heavy-flavor hadron decays from a possible (thermal) contribution produced at the interaction point. The data are consistent with a suppression of 𝑒⁺⁢𝑒⁻ pairs from $$𝑐\bar{𝑐}$$ and an additional prompt component. Finally, the first direct-photon measurement in the 10% most central Pb-Pb collisions at $$\sqrt{s_{NN}}$$ = 5.02 TeV is reported via the study of virtual direct photons in the transverse momentum range 1 < 𝑝_T < 5 GeV/𝑐. A model including prompt photons, as well as photons from the preequilibrium and fluid-dynamic phases, can reproduce the result, while being at the upper edge of the data uncertainties.« less

The intense photon fluxes from relativistic nuclei provide an opportunity to study photonuclear interactions in ultraperipheral collisions. In particular, it allows for the investigations of excited, light-flavour vector mesons. The measurement of coherently photoproduced 𝜋⁺𝜋⁻𝜋⁺𝜋⁻ final states in ultraperipheral Pb–Pb collisions at $$\sqrt{s_{NN}}$$ = 5.02 TeV is presented for the first time. The cross section, d𝜎/d𝑦, times the branching ratio (𝜌 → 𝜋⁺𝜋⁺𝜋⁻𝜋⁻) is found to be 47.8 ± 2.3 (stat.) ± 7.7 (syst.) mb in the rapidity interval |𝑦| < 0.5. The invariant mass distribution is not well described with a single Breit-Wigner resonance without an interference term. Includingmore » in- terference with a non-resonant contribution results in the mass and width values being too far from those reported in PDG, while the production of two interfering resonances, 𝜌(1450) and 𝜌(1700), also provides a good description of the data. The values of the masses (𝑚) and widths (Γ) of the resonances extracted from the fit assuming two interfering resonances are 𝑚₁ = 1385 ± 14 (stat.) ± 3 (syst.) MeV/𝑐², Γ₁ = 431 ± 36 (stat.) ± 82 (syst.) MeV/𝑐², 𝑚₂ = 1663 ± 13 (stat.) ± 22 (syst.) MeV/𝑐² and Γ₂ = 357 ± 31 (stat.) ± 49 (syst.) MeV/𝑐², respectively. The measured cross sections times the branching ratios are compared to recent theoretical predictions.« less

A sample of 𝑝⁢𝑝 collision data, corresponding to an integrated luminosity of 5.4 fb⁻¹ and collected by the LHCb experiment during LHC Run 2, is used to measure the ratio of the lifetime of the $$Ξ^0_b$$ baryon to that of the $$Λ^0_b$$ baryon, 𝑟_𝜏 $$≡$$ $$𝜏_{Ξ^0_b}$$$$/𝜏_{Λ^0_b}$$. The value 𝑟$$^{\textrm{Run 2}}_𝜏$$ =1.004 ± 0.009 ± 0.006 is obtained, where the first uncertainty is statistical and the second systematic. This value is averaged with the corresponding value from Run 1 to obtain 𝑟_𝜏 =1.004 ± 0.008 ± 0.005. Multiplying by the known value of the $$Λ^0_b$$ lifetime yields $$𝜏_{Ξ^0_b}$$ = 1.475 ±more » 0.012 ± 0.008 ± 0.009 ps, where the last uncertainty is due to the limited knowledge of the $$Λ^0_b$$ lifetime. This measurement improves the precision of the current world average of the $$Ξ^0_b$$ lifetime by about a factor of two, and is in good agreement with the most recent theoretical predictions.« less

A study is presented of $B^{+}\to K_{\mathrm{S}}^0{K}^{-}{\pi }^{+}{K}^{+}$ and $B^{+}\to K_{\mathrm{S}}^0{K}^{+}{\pi }^{-}{K}^{+}$ decays based on the analysis of proton-proton collision data collected with the LHCb detector at center-of-mass energies of 7, 8 and 13 TeV, corresponding to an integrated luminosity of $9{\mathrm{fb}}^{-1}$ . The ${more »}_{}^{}$ K S 0 K π invariant-mass distributions of both $B^{+}$ decay modes show, in the $m\left(K_{\mathrm{S}}^{0}K\pi \right)<1.85\mathrm{GeV}$ mass region, large activity which is resolved using an amplitude analysis. A simple model, where $J^{PC}$ amplitudes are described by multiple Breit-Wigner functions with appropriate angular distributions, provides a good description of the experimental data. In this approach a complex mixture of $J^{PC}=0^{-+}$ , $1^{++}$ and $1^{+-}$ amplitudes is observed that is dominated by $\eta \left(1405\right)$ , $\eta \left(1470\right)$ , $\eta \left(1760\right)$ , $f_1\left(1285\right)$ , $f_1\left(1420\right)$ and $h_1\left(1405\right)$ resonances. The $K_{\mathrm{S}}^{0}K\pi$ Dalitz plots are dominated by asymmetric crossing $K^{*}\overline{K}$ bands which are different for the two $B^{+}$ decay modes. This is due to a different interference pattern between the $1^{++}$ and $1^{+-}$ amplitudes in the two channels. Branching fractions are measured for each resonant contribution. © 2025 CERN, for the LHCb Collaboration 2025 CERN« less

The first measurement of the $CP$ asymmetry of the decay rate ( $A_{CP}$ ) and the $CP$ average ( $\mathrm{\Sigma }{A}_{\mathrm{FB}}$ ) and $CP$ asymmetry ( $\mathrm{\Delta }{A}_{\mathrm{FB}}$ ) of the forward-backward asymmetry in the muon system of ${\mathrm{\Lambda }}_c^{+}\to p{\mu }^{+}{\mu }^{-}$ decays is reported. The measurement is performed using a data sample of proton-proton collisions, recordedmore » by the LHCb experiment from 2016 to 2018 at a center-of-mass energy of 13 TeV, which corresponds to an integrated luminosity of $5.4{\mathrm{fb}}^{-1}$ . The asymmetries are measured in two regions of dimuon mass near the  $\varphi$ -meson mass peak. The dimuon-mass integrated results are $A_{CP}=(-1.1\pm 4.0\pm 0.5)\%$ , $\mathrm{\Sigma }{A}_{\mathrm{FB}}=(3.9\pm 4.0\pm 0.6)\%$ , $\mathrm{\Delta }{A}_{\mathrm{FB}}=(3.1\pm 4.0\pm 0.4)\%$ , where the first uncertainty is statistical and the second systematic. The results are consistent with the conservation of $CP$ symmetry and the Standard Model expectations. © 2025 CERN, for the LHCb Collaboration 2025 CERN« less

A search for $CP$ violation in ${\mathrm{\Lambda }}_b^0\to p{K}^{-}$ and ${\mathrm{\Lambda }}_b^0\to p{\pi }^{-}$ decays is presented using the full Run 1 and Run 2 data samples of $pp$ collisions collected with the LHCb detector, corresponding to an integrated luminosity of $9{\mathrm{fb}}^{-1}$ at center-of-mass energies of 7, 8, and 13 TeV. For the Run 2 data sample, the $CP$ -violatingmore » asymmetries are measured to be $A_{CP}^{p{K}^{-}}=(-1.4\pm 0.7\pm 0.4)\%$ and $A_{CP}^{p{\pi }^{-}}=(0.4\pm 0.9\pm 0.4)\%$ , where the first uncertainty is statistical and the second is systematic. Following significant improvements in the evaluation of systematic uncertainties compared to the previous LHCb measurement, the Run 1 dataset is reanalyzed to update the corresponding results. When combining the Run 2 and updated Run 1 measurements, the final results are found to be $A_{CP}^{p{K}^{-}}=(-1.1\pm 0.7\pm 0.4)\%$ and $A_{CP}^{p{\pi }^{-}}=(0.2\pm 0.8\pm 0.4)\%$ , constituting the most precise measurements of these asymmetries to date. © 2025 CERN, for the LHCb Collaboration 2025 CERN« less

The first measurement of the polarisation of coherently photoproduced J/ψ mesons in ultra-peripheral Pb–Pb collisions, using data at $$\sqrt{s_{NN}}$$ = 5.02 TeV, is presented. The J/ψ meson is measured via its dimuon decay channel in the forward rapidity interval –4.0 < y < –2.5 using the ALICE detector at the CERN LHC. An event sample corresponding to an integrated luminosity of 750 μb^–1 ± 5% (syst) is analysed. Hadronic activity is highly suppressed since the interaction is mediated by a photon. The polar and azimuthal angle distributions of the decay muons are measured, and the polarisation parameters λ_θ, λ_φ, λ_θφ,more » are extracted. The analysis is carried out in the helicity frame. The results are found to be consistent with a transversely polarised J/ψ. These values are compared with previous measurements by the H1 and ZEUS experiments. The polarisation parameters of coherent J/ψ photoproduction in Pb–Pb collisions are found to be consistent with the s-channel helicity conservation hypothesis.« less

The femtoscopic study of pairs of identical pions is particularly suited to investigate the effective source function of particle emission, due to the resulting Bose–Einstein correlation signal. In small collision systems at the LHC, pp in particular, the majority of the pions are produced in resonance decays, which significantly affect the profile and size of the source. In this work, we explicitly model this effect in order to extract the primordial source in pp collisions at $$\sqrt{s}$$ = 13 TeV from charged π–π correlations measured by ALICE. We demonstrate that the assumption of a Gaussian primordial source is compatible withmore » the data and that the effective source, resulting from modifications due to resonances, is approximately exponential, as found in previous measurements at the LHC. The universality of hadron emission in pp collisions is further investigated by applying the same methodology to characterize the primordial source of K–p pairs. The size of the primordial source is evaluated as a function of the transverse mass (m_T) of the pairs, leading to the observation of a common scaling for both π–π and K–p, suggesting a collective effect. Further, the present results are compatible with the m_T scaling of the p–p and p–Λ primordial source measured by ALICE in high multiplicity pp collisions, providing additional evidence for the presence of a common emission source for all hadrons in small collision systems at the LHC. This will allow the determination of the source function for any hadron–hadron pairs with high precision, granting access to the properties of the possible final-state interaction among pairs of less abundantly produced hadrons, such as strange or charmed particles.« less

The measurement of ϒ(1S), ϒ(2S), and ϒ(3S) yields as a function of the charged-particle multiplicity density, dN_ch/dη, using the ALICE experiment at the LHC, is reported in pp collisions at √s = 13 TeV. The Υ meson yields are measured at forward rapidity (2.5 < y < 4) in the dimuon decay channel, whereas the charged-particle multiplicity is defined at central rapidity (|η| <1). Both quantities are divided by their average value in minimum bias events to compute the self-normalized quantities. The increase of the self-normalized ϒ(1S), ϒ(2S), and ϒ(3S) yields is found to be compatible with a linearmore » scaling with the self-normalized dN_ch/dη, within the uncertainties. The self-normalized yield ratios of excited-to-ground ϒ states are compatible with unity within uncertainties. Similarly, the measured double ratio of the self-normalized ϒ(1S) to the self-normalized J/ψ yields, both measured at forward rapidity, is compatible with unity for self-normalized charged-particle multiplicities beyond one. The measurements are compared with theoretical predictions incorporating initial or final state effects.« less

The first measurement of the multiplicity dependence of intra-jet properties of leading charged-particle jets in proton–proton (pp) collisions is reported. The mean charged particle multiplicity and jet fragmentation distributions are measured in minimum-bias and high-multiplicity pp collisions at center-of-mass energy $$\sqrt{s}$$ = 13 TeV using the ALICE detector. Jets are reconstructed from charged particles produced in the midrapidity region (|η| < 0.9) using the sequential recombination anti-k_T algorithm with jet resolution parameters R = 0.2, 0.3, and 0.4 for the trans verse momentum (p_T) interval 5–110 GeV/c. The high multiplicity events are selected by the forward V0 scintilla tor detectors.more » The mean charged-particle multiplicity inside the leading jet cone rises monotonically with increasing jet p_T in qualitative agreement with previous measurements at lower energies. The distributions of jet fragmentation function variables z^ch and ξ^ch are measured for different jet-p_T intervals. Jet-p_T independent fragmentation of leading jets is observed for wider jets except at high- and low-z^ch values. The observed “hump-backed plateau” structure in the ξ^ch distribution indicates suppression of low-p_T particles. In high-multiplicity events, an enhancement of the fragmen tation probability of low-z^ch particles accompanied by a suppression of high-zch particles is observed compared to minimum-bias events. This behavior becomes more promi nent for low-p_T jets with larger jet radius. The results are compared with predictions of QCD-inspired event generators, PYTHIA 8 with Monash 2013 tune and EPOS LHC. It is found that PYTHIA 8 qualitatively reproduces the jet modification in high-multiplicity events except at high jet p_T. These measurements provide important constraints to models of jet fragmentation.« less