This paper presents the measurement of fiducial and differential cross sections for both the inclusive and electroweak production of a same-sign $W$-boson pair in association with two jets ($W^\pm W^\pm jj$) using 139 fb$^{-1}$ of proton-proton collision data recorded at a centre-of-mass energy of $\sqrt{s}=13$ TeV by the ATLAS detector at the Large Hadron Collider. The analysis is performed by selecting two same-charge leptons, electron or muon, and at least two jets with large invariant mass and a large rapidity difference. The measured fiducial cross sections for electroweak and inclusive $W^\pm W^\pm jj$ production are $2.92 \pm 0.22\, \text{(stat.)} \pm 0.19\, \text{(syst.)}$ fb and $3.38 \pm 0.22\, \text{(stat.)} \pm 0.19\, \text{(syst.)}$ fb, respectively, in agreement with Standard Model predictions. The measurements are used to constrain anomalous quartic gauge couplings by extracting 95% confidence level intervals on dimension-8 operators. A search for doubly charged Higgs bosons $H^{\pm\pm}$ that are produced in vector-boson fusion processes and decay into a same-sign $W$ boson pair is performed. The largest deviation from the Standard Model occurs for an $H^{\pm\pm}$ mass near 450 GeV, with a global significance of 2.5 standard deviations.
Fiducial differential cross section of the electroweak $W^\pm W^\pm jj$ production as a function of $m_{\ell\ell}$. The correlation of uncertainties of the measured cross section across bins is presented in Table 11.
Fiducial differential cross section of the electroweak $W^\pm W^\pm jj$ production as a function of $m_{\mathrm{T}}$. The correlation of uncertainties of the measured cross section across bins is presented in Table 12.
Fiducial differential cross section of the electroweak $W^\pm W^\pm jj$ production as a function of $m_{\mathrm{jj}}$. The correlation of uncertainties of the measured cross section across bins is presented in Table 13.
A search is reported for long-lived dark photons with masses between 0.1 GeV and 15 GeV, from exotic decays of Higgs bosons produced via vector-boson-fusion. Events that contain displaced collimated Standard Model fermions reconstructed in the calorimeter or muon spectrometer are probed. This search uses the full LHC Run 2 (2015-2018) data sample collected in proton-proton collisions at $\sqrt{s}=13$ TeV, corresponding to an integrated luminosity of 139 $fb^{-1}$. Dominant backgrounds from Standard Model processes and non-collision sources are estimated by using data-driven techniques. The observed event yields in the signal regions are consistent with the expected background. Upper limits on the Higgs boson to dark photon branching fraction are reported as a function of the dark-photon mean proper decay length or of the dark-photon mass and the coupling between the Standard Model and the potential dark sector. This search is combined with previous ATLAS searches obtained in the gluon-gluon fusion and \textit{WH} production modes. A branching fraction above 10% is excluded at 95% CL for a 125 GeV Higgs boson decaying into two dark photons for dark-photon mean proper decay lengths between 173 and 1296 mm and mass of 10 GeV.
Observed 95% CL upper limits on B(H→ 2γ<sub>d</sub>+X) for different γ<sub>d</sub> masses and a 125 GeV Higgs boson, as a function of the dark-photon mean proper decay length cτ. The limits are shown for the SR<sub>μ</sub> search channel, assuming an FRVZ signal model. The hatched band denotes the region in which the branching ratio is larger than unity.
Observed 95% CL upper limits on B(H→ 2γ<sub>d</sub>+X) for different γ<sub>d</sub> masses and a 125 GeV Higgs boson, as a function of the dark-photon mean proper decay length cτ. The limits are shown for the SR<sub>c</sub><sup>L</sup> search channel, assuming an FRVZ signal model. The hatched band denotes the region in which the branching ratio is larger than unity.
Observed 95% CL upper limits on B(H→ 2γ<sub>d</sub>+X) for different γ<sub>d</sub> masses and a 125 GeV Higgs boson, as a function of the dark-photon mean proper decay length cτ. The limits are shown for the SR<sub>c</sub><sup>H</sup> search channel, assuming an FRVZ signal model. The hatched band denotes the region in which the branching ratio is larger than unity.
A combination of searches for a new resonance decaying into a Higgs boson pair is presented, using up to 139 fb$^{-1}$ of $pp$ collision data at $\sqrt{s}=13$ TeV recorded with the ATLAS detector at the LHC. The combination includes searches performed in three decay channels: $b\bar{b}b\bar{b}$, $b\bar{b}\tau^+\tau^-$ and $b\bar{b}\gamma\gamma$. No excess above the expected Standard Model background is observed and upper limits are set at the 95% confidence level on the production cross section of Higgs boson pairs originating from the decay of a narrow scalar resonance with mass in the range 251 GeV-5 TeV. The observed (expected) limits are in the range 0.96-600 fb (1.2-390 fb). The limits are interpreted in the Type-I Two-Higgs-Doublet Model and the Minimimal Supersymmetric Standard Model, and constrain parameter space not previously excluded by other searches.
Local p-value as a function of the resonance mass $m_{X}$. Some table entries are empty because resonance masses from 251 GeV to 5 TeV are considered, whereas some channels only use masses in a certain, more restricted, range.
Observed significance as a function of the resonance $m_{X}$. Some table entries are empty because resonance masses from 251 GeV to 5 TeV are considered, whereas some channels only use masses in a certain, more restricted, range.
Observed and expected upper limits at the 95% CL on the resonant Higgs boson pair production cross section as a function of the resonance mass $m_{X}$. Some table entries are empty because resonance masses from 251 GeV to 5 TeV are considered, whereas some channels only use masses in a certain, more restricted, range.
This Letter presents the measurement of near-side associated per-trigger yields, denoted ridge yields, from the analysis of angular correlations of charged hadrons in proton-proton collisions at $\sqrt{s}$ = 13 TeV. Long-range ridge yields are extracted for pairs of charged particles with a pseudorapidity difference of $1.4 < |\Delta\eta| < 1.8$ and a transverse momentum of $1 < p_{\rm T} < 2$ GeV/$c$, as a function of the charged-particle multiplicity measured at midrapidity. This study extends the measurements of the ridge yield to the low multiplicity region, where in hadronic collisions it is typically conjectured that a strongly-interacting medium is unlikely to be formed. The precision of the new low multiplicity results allows for the first direct quantitative comparison with the results obtained in $\mathrm {e^{+}e^{-}}$ collisions at $\sqrt{s}$ = 91 GeV and $\sqrt{s}$ = 183$-$209 GeV, where initial-state effects such as pre-equilibrium dynamics and collision geometry are not expected to play a role. In the multiplicity range $8\lesssim\langle N_\mathrm{ch}\rangle\lesssim 24$ where the $\mathrm {e^{+}e^{-}}$ results have good precision, the measured ridge yields in pp collisions are substantially larger than the limits set in $\mathrm {e^{+}e^{-}}$ annihilations. Consequently, the findings presented in this Letter suggest that the processes involved in $\mathrm {e^{+}e^{-}}$ annihilations do not contribute significantly to the emergence of long-range correlations in pp collisions.
Ridge yield $Y_\mathrm{ridge}$ extracted at $1.4<|\Delta\eta|<1.8$ with $1.0<p_\mathrm{T,trig}<2.0\,\mathrm{GeV}/c$, $1.0<p_\mathrm{T,assoc}<2.0\,\mathrm{GeV}/c$ as a function of charged particle multiplicity counted at midrapidity $|\eta|<1.0$. The first three points at $N_\mathrm{ch}<8$ represent a 95% upper confidence limit where the statistical and systematic uncertainty have been combined.
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 $\pi$-$\pi$ correlations measured by ALICE. We demonstrate that the assumption of a Gaussian primordial source is compatible with 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 ($m_{\rm T}$) of the pairs, leading to the observation of a common scaling for both $\pi$-$\pi$ and K-p, suggesting a collective effect. Further, the present results are compatible with the $m_{\rm T}$ scaling of the p-p and p$-\Lambda$ primordial source measured by ALICE in high multiplicity pp collisions, providing compelling 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.
K$^+$p (K$^+$p $\oplus$ K$^-\overline{\mathrm p}$) correlation function in HM pp collisions at $\sqrt{s_{\mathrm {NN}}}=13 $ TeV (1.2<$m_T$<1.4 GeV/$c^{2}$).
K$^+$p (K$^+$p $\oplus$ K$^-\overline{\mathrm p}$) correlation function in HM pp collisions at $\sqrt{s_{\mathrm {NN}}}=13 $ TeV (1.4<$m_T$<1.5 GeV/$c^{2}$).
K$^+$p (K$^+$p $\oplus$ K$^-\overline{\mathrm p}$) correlation function in HM pp collisions at $\sqrt{s_{\mathrm {NN}}}=13 $ TeV (1.5<$m_T$<1.8 GeV/$c^{2}$).
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 $\sqrt{s}$ = 13 TeV using the ALICE detector. Jets are reconstructed from charged particles produced in the midrapidity region ($|\eta|<0.9$) using the sequential recombination anti-$k_{\rm T}$ algorithm with jet resolution parameters $R=0.2$, 0.3, and 0.4 for the transverse momentum ($p_{\rm T}$) interval 5-110 GeV/$c$. High-multiplicity events are selected by the forward V0 scintillator detectors. The mean charged-particle multiplicity inside the leading jet cone rises monotonically with increasing jet $p_{\rm T}$ in qualitative agreement with previous measurements at lower energies. The distributions of jet fragmentation function variables $z^{\rm ch}$ and $\xi^{\rm ch}$ are measured for different jet-$p_{\rm T}$ intervals. Jet-$p_{\rm T}$ independent fragmentation of leading jets is observed for wider jets except at high- and low-$z^{\rm ch}$. The observed hump-backed plateau structure in the $\xi^{\rm ch}$ distribution indicates suppression of low-$p_{\rm T}$ particles. In high-multiplicity events, an enhancement of the fragmentation probability of low-$z^{\rm ch}$ particles accompanied by a suppression of high-$z^{\rm ch}$ particles is observed compared to minimum-bias events. This behavior becomes more prominent for low-$p_{\rm T}$ 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 $p_{\rm T}$. These measurements provide important constraints to models of jet fragmentation.
Mean number of charged particles ($\langle N_{\mathrm{ch}} \rangle$) within leading charged-particle jets as a function of $p_{T}^{\mathrm{jet, ch}}$ for minimum-bias (MB) pp collisions.
Mean number of charged particles ($\langle N_{\mathrm{ch}} \rangle$) within leading charged-particle jets as a function of $p_{T}^{\mathrm{jet, ch}}$ for high-multiplicity (HM) pp collisions.
The ratio of $\langle N_{\mathrm{ch}} \rangle$ between high-multiplicity (HM) and minimum-bias (MB) events as a function of $p_{T}^{\mathrm{jet, ch}}$ for pp collisions.
This Letter presents the most precise measurement to date of the matter-antimatter imbalance at midrapidity in Pb-Pb collisions at a center-of-mass energy per nucleon pair $\sqrt{s_{\rm NN}} = 5.02$ TeV. Using the Statistical Hadronization framework, it is possible to obtain the value of the electric charge and baryon chemical potentials, $\mu_Q=-0.18\pm0.90$ MeV and $\mu_B=0.71\pm0.45$ MeV, with unprecedented precision. A centrality-differential study of the antiparticle-to-particle yield ratios of charged pions, protons, $\Omega$-baryons, and light (hyper)nuclei is performed. These results indicate that the system created in Pb-Pb collisions at the LHC is on average baryon-free and electrically neutral at midrapidity.
Antiparticle-to-particle yield ratio, 0-5% centrality
Antiparticle-to-particle yield ratio, 5-10% centrality
Antiparticle-to-particle yield ratio, 10-30% centrality
The dependence of $\mathrm{f}_{0}$(980) production on the final-state charged-particle multiplicity in p$-$Pb collisions at $\sqrt{s_{\mathrm{NN}}} = 5.02$ TeV is reported. The production of $\mathrm{f}_{0}$(980) is measured with the ALICE detector via the $\mathrm{f}_0 (980) \rightarrow \pi^{+}\pi^{-}$ decay channel in a midrapidity region of $-0.5<y<0$. Particle yield ratios of $\mathrm{f}_{0}$(980) to $\pi$ and $\mathrm{K}^{*}$(892)$^{0}$ are found to be decreasing with increasing charged-particle multiplicity. The magnitude of the suppression of the $\mathrm{f}_{0}$(980)/$\pi$ and $\mathrm{f}_{0}$(980)/$\mathrm{K}^{*}$(892)$^{0}$ yield ratios is found to be dependent on the transverse momentum $p_{\mathrm{T}}$, suggesting different mechanisms responsible for the measured effects. Furthermore, the nuclear modification factor $Q_{\mathrm{pPb}}$ of $\mathrm{f}_{0}$(980) is measured in various multiplicity ranges. The $Q_{\mathrm{pPb}}$ shows a strong suppression of the $\mathrm{f}_{0}$(980) production in the $p_{\mathrm{T}}$ region up to about 4 GeV/$c$. The results on the particle yield ratios and $Q_{\mathrm{pPb}}$ for $\mathrm{f}_{0}$(980) may help to understand the late hadronic phase in p$-$Pb collisions and the nature of the internal structure of $\mathrm{f}_{0}$(980) particle.
Transverse momentum spectra in different multiplicity classes. Each spectrum is corrected for the branching ratio of (46 $\pm$ 6)% based on [Phys. Rev. Lett. 111 no. 6, (2013) 062001].
The ratio of transverse momentum spectrum to the NSD spectrum
The double ratio of particle yield of f0((980) to charged pions
In this letter, measurements of (anti)alpha production in central (0$-$10%) Pb$-$Pb collisions at a center-of-mass energy per nucleon$-$nucleon pair of $\sqrt{s_{\rm NN}}$ = 5.02 TeV are presented, including the first measurement of an antialpha transverse-momentum spectrum. Owing to its large mass, the production of (anti)alpha is expected to be sensitive to different particle production models. The production yields and transverse-momentum spectra of nuclei are of particular interest because they provide a stringent test of these models. The averaged antialpha and alpha spectrum is compared to the spectra of lighter particles, by including it into a common blast-wave fit capturing the hydrodynamic-like flow of all particles. This fit is indicating that the (anti)alpha also participates in the collective expansion of the medium created in the collision. A blast-wave fit including only protons, (anti)alpha, and other light nuclei results in a similar flow velocity as the fit that includes all particles. A similar flow velocity, but a significantly larger kinetic freeze-out temperature is obtained when only protons and light nuclei are included in the fit. The coalescence parameter $B_4$ is well described by calculations from a statistical hadronization model but significantly underestimated by calculations assuming nucleus formation via coalescence of nucleons. Similarly, the (anti)alpha-to-proton ratio is well described by the statistical hadronization model. On the other hand, coalescence calculations including approaches with different implementations of the (anti)alpha substructure tend to underestimate the data.
Antialpha spectrum in 0-10% V0M centrality class
Alpha spectrum in 0-10% V0M centrality class
Average alpha and antialpha spectrum in 0-10% V0M centrality class
K$^{+}$K$^{-}$ pairs may be produced in photonuclear collisions, either from the decays of photoproduced $\phi (1020)$ mesons, or directly as non-resonant K$^{+}$K$^{-}$ pairs. Measurements of K$^{+}$K$^{-}$ photoproduction probe the couplings between the $\phi (1020)$ and charged kaons with photons and nuclear targets. The kaon$-$proton scattering occurs at energies far above those available elsewhere. We present the first measurement of coherent photoproduction of K$^{+}$K$^{-}$ pairs on lead ions in ultra-peripheral collisions using the ALICE detector, including the first investigation of direct K$^{+}$K$^{-}$ production. There is significant K$^{+}$K$^{-}$ production at low transverse momentum, consistent with coherent photoproduction on lead targets. In the mass range $1.1 < M_{\rm{KK}} < 1.4$ GeV/$c^2$ above the $\phi (1020)$ resonance, for rapidity $|y_{\rm{KK}}|<0.8$ and $p_{\rm T,KK} < 0.1$ GeV/$c$, the measured coherent photoproduction cross section is $\mathrm{d}\sigma/\mathrm{d}y$ = 3.37 $\pm\ 0.61$ (stat.) $\pm\ 0.15 $ (syst.) mb. The center-of-mass energy per nucleon of the photon-nucleus (Pb) system $W_{\gamma \mathrm{Pb, n}}$ ranges from 33 to 188 GeV, far higher than previous measurements on heavy-nucleus targets. The cross section is larger than expected for $\phi (1020)$ photoproduction alone. The mass spectrum is fit to a cocktail consisting of $\phi (1020)$ decays, direct K$^{+}$K$^{-}$ photoproduction, and interference between the two. The confidence regions for the amplitude and relative phase angle for direct K$^{+}$K$^{-}$ photoproduction are presented.
d$^2\sigma$/d$y$/d$p_T^2$ in bins of $p_{T,KK}^2$ for $K^+K^-$ photoproduction in ultra-peripheral Pb$-$Pb collisions at $\sqrt{s_\mathrm{NN}}$ = 5.02 TeV .
d$^2\sigma$/d$y$/d$p_T^2$ in bins of $M_{KK}$ for $K^+K^-$ photoproduction in ultra-peripheral Pb$-$Pb collisions at $\sqrt{s_\mathrm{NN}}$ = 5.02 TeV .
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