27 Search Results

The first results of K*(892)^± production at midrapidity (|y| < 0.5) in pp collisions at $$\sqrt{s}$$= 13 TeV as a function of the event multiplicity are presented. The K*(892)^± has been reconstructed via its hadronic decay channel K*(892)^± → π^± + K$$^0_{\textrm{S}}$$ using the ALICE detector at the LHC. For each multiplicity class, the differential transverse momentum (p_T) spectrum, the mean transverse momentum ⟨p_T⟩, the p_T-integrated yield (dN/dy), and the ratio of the K*(892)^± to K$$^0_{\textrm{S}}$$ yields are reported. These are consistent with previous K*(892)⁰ resonance results with a higher level of precision. Comparisons with phenomenological models such as PYTHIA6,more » PYTHIA8, EPOS-LHC, and DIPSY are also discussed. For the first time, a significant K*(892)^±/K$$^0_{\textrm{S}}$$ suppression in pp collisions is observed at a 7σ level passing from low to high multiplicity events. The ratios of the p_T-differential yields of K*(892)^± and K$$^0_{\textrm{S}}$$ in high and low multiplicity events are also presented along with their double ratio. For p_T ≲ 2 GeV/c this double ratio persists below unity by more than 3σ suggesting that the suppression affects mainly low p_T resonances. The measured decreasing trend of the K*(892)^±/K$$^0_{\textrm{S}}$$ ratio with increasing multiplicity, which in heavy-ion collisions is typically attributed to the rescattering of decay particles of the short-lived resonances, is reproduced by the EPOS-LHC model without the use of hadronic afterburners.« less

In this letter, the first measurement of the femtoscopic correlation of protons and $Σ^+$ hyperons is presented and used to study the p–$Σ^+$ interaction. The measurement is performed with the ALICE detector in high-multiplicity triggered pp collisions at $$\sqrt{s}$$ = 13 TeV. The $Σ^+$ hyperons are reconstructed using a missing-mass approach in the decay channel to p + π⁰ with π⁰ → $γγ$, while both $Σ^+$ and protons are identified using a machine learning approach. These techniques result in a high reconstruction efficiency and purity, which allows the measurement of the p–$Σ^+$ correlation function for the first time. Thanks tomore » the high significance achieved in the p–$Σ^+$ correlation signal, it is possible to discriminate between the predictions of different models of the N–$$Σ$$ interaction and to accomplish a first determination of the p–$Σ^+$ scattering parameters.« less

Meson-baryon systems with strangeness content provide a unique laboratory for investigating the strong interaction and testing theoretical models of hadron structure and dynamics. In this work, the measured correlation functions for oppositely charged $$Ξ$$-K and $$Ξ$$ − 𝜋 pairs obtained in high-multiplicity pp collisions at $$\sqrt{𝑠}$$ = 13 TeV at the LHC are presented. For the first time, high-precision data on the $$Ξ$$-K interaction are delivered at small relative momenta. The scattering lengths, extracted via the Lednický–Lyuboshits expression of the pair wavefunction, indicate a repulsive and a shallow attractive strong interaction for the $$Ξ$$-K and $$Ξ$$ − 𝜋 systems, respectively.more » The $$Ξ$$(1620) and $$Ξ$$(1690) states are observed in the $$Ξ$$ − 𝜋 correlation function and their properties, mass and width, are determined. These measurements are in agreement with other available results. Such high-precision data can help refine the understanding of these resonant states, provide stronger constraints for chirally motivated potentials, and address the key challenge of describing the coupled-channel dynamics that may give rise to molecular configurations .« less

This Letter reports the observation of W⁺W^–γ triboson production in 140 fb^–1 of data collected by the ATLAS detector from proton–proton collisions at a centre-of-mass energy of $$\sqrt{s}$$ = 13 TeV at the LHC. Events with an opposite-charge eμ pair, a high transverse-momentum photon, and significant missing transverse momentum are considered. The observed (expected) significance of the signal is 5.9 (6.0) standard deviations. The measured fiducial cross-section, defined for the W⁺W^–γ → e^±μ^∓$$ν\bar{ν}$$γ final state is 6.2  ±  0.8 (stat.)  ±  0.6 (sys.) fb, in good agreement with the Standard Model prediction of 6.1$$^{+1.0}_{–0.7}$$ fb. Constraints on the Wilson coefficientsmore » of 13 dimension-8 operators describing physics beyond the Standard Model through anomalous quartic gauge-boson couplings are derived using the effective field theory framework.« 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

Measurements of jet substructure in Pb+Pb collisions provide key insights into the mechanism of jet quenching in the hot and dense QCD medium created in these collisions. This Letter presents a measurement of the suppression of large-radius jets with a radius parameter of $R = 1.0$ and its dependence on the jet substructure. The measurement uses 1.72 nb$$^{-1}$$ of Pb+Pb data and 255 pb$$^{-1}$$ of $pp$ data, both at $$\sqrt{s_{_\mathrm{NN}}} = 5.02$$ TeV, recorded with the ATLAS detector at the Large Hadron Collider. Large-radius jets are reconstructed by reclustering $R = 0.2$ calorimetric jets and are measured for transverse momentummore » above $200$ GeV. Jet substructure is evaluated using charged-particle tracks, and the overall level of jet suppression is quantified using the jet nuclear modification factor ($$R_\mathrm{AA}$$). The jet $$R_\mathrm{AA}$$ is measured as a function of jet $$p_{\mathrm{T}}$$, the charged $$k_t$$ splitting scale ($$\sqrt{d_{12}}$$), and the angular separation ($$dR_{12}$$) of two leading sub-jets. The jet $$R_\mathrm{AA}$$ gradually decreases with increasing $$\sqrt{d_{12}}$$, implying significantly stronger suppression of large-radius jets with larger $$k_t$$ splitting scale. The jet $$R_\mathrm{AA}$$ gradually decreases for $$dR_{12}$$ in the range $$0.01{-}0.2$$ and then remains consistent with a constant for $$ΔR_{12} \gtrsim 0.2$$. The observed significant dependence of jet suppression on the jet substructure will provide new insights into its role in the quenching process.« less

The coherent J∕𝜓 photoproduction cross section is measured for the first time at midrapidity in peripheral to semicentral Pb − Pb collisions at $$\sqrt{s_{NN}}$$ = 5.02 TeV. The centrality differential cross section d𝜎∕d𝑦 is reported for the centrality range 40–90%, together with the doubly-differential cross section d²𝜎∕d𝑦d𝑝_T, extracted in two peripheral centrality classes. The J∕𝜓 mesons are reconstructed in the dielectron channel, in the rapidity interval |𝑦| < 0.9 using the ALICE central barrel detectors. The J∕𝜓 cross section at midrapidity is statistically compatible to the earlier ALICE measurement at forward rapidity and at the same centre-of-mass energy, and showsmore » only a mild centrality dependence over the covered range. Several sets of theoretical calculations taking into account the hadronic overlap in the collisions but ignoring possible final-state effects from a hot expanding medium are found to give a fairly good description of the current measurements within uncertainties.« less

This work reports femtoscopic correlations of p – – p ($$\overline{p}$$ – – $$\overline{p}$$) and p – – d ($$\overline{p}$$ – – $$\overline{d}$$) pairs measured in Pb–Pb collisions at center-of-mass energy per nucleon $$\sqrt{s_{NN}}$$ = 5.02 TeV in the ALICE Collaboration. A fit to the measured proton-proton correlation functions allows one to extract the dependence of the nucleon femtoscopic radius of the particle-emitting source on the pair transverse mass (m_T) and on the average charge particle multiplicity $$\langle$$dN_ch/dη$$\rangle$$^1/3 for three centrality intervals (0–10%, 10 – – 30 %, 30 – – 50 %). In both cases, the expected power-law andmore » linear scalings are observed, respectively. The measured p–d correlations can be described by both two- and three-body calculations, indicating that the femtoscopy observable is not sensitive to the short-distance features of the dynamics of the p-(p-n) system, due to the large inter-particle distances in Pb–Pb collisions at the LHC. Indeed, in this study, the minimum measured femtoscopic source sizes for protons and deuterons have a minimum value at 2.73$$^{+0.05}_{–0.05}$$ and 3.10$$^{+1.04}_{–0.86}$$ fm, respectively, for the 30–50% centrality collisions. Moreover, the m_T-scaling obtained for the p–p and p–d systems is compatible within 1σ of the uncertainties. These findings provide new input for fundamental studies on the production of light (anti)nuclei under extreme conditions.« less

A measurement of off-shell Higgs boson production is performed in the H* → WW channel. The measurement uses a proton–proton collision dataset with an integrated luminosity of 140 fb^-1 collected at a centre-of-mass energy of 13 TeV by the ATLAS detector at the Large Hadron Collider. Final states in which both W bosons decay leptonically are targeted, and events are categorised based on the flavour of the final-state leptons, the jet multiplicity, and the output of neural network-based classifiers. The data are found to be compatible with the Standard Model expectation. An observed (expected) upper bound on the 95 %more » symmetric confidence level interval is set on the rate of off-shell Higgs boson production at a value of 3.4 (4.4) times the Standard Model prediction. These results are combined with the results from the measurement of on-shell Higgs boson production in the same final states to obtain an observed (expected) upper bound at 95 % confidence level on the Higgs boson total width of 13.1 (17.3) MeV.« less

This letter reports the measurement of double parton scattering in same-sign W boson pair production with the ATLAS detector. The data set used corresponds to an integrated luminosity of 140 fb^-1 of proton–proton collisions at a center-of-mass energy of 13 TeV, collected during Run 2 of the Large Hadron Collider. The study is performed in final states including two same-charge leptons, electron or muon, missing transverse momentum, and up to one jet. An excess of events is observed over the expected background contributions with a significance of 8.8 standard deviations. The measured fiducial cross section times leptonic branching fraction ismore » 4.59 ± 0.64 fb. The measurement corresponds to a double parton scattering effective cross section of 10.6 ± 1.8 mb.« less