The GlueX experiment at Jefferson Lab has observed $p\bar{p}$ and, for the first time, $Λ\barΛ$ and $p\barΛ$ photoproduction from a proton target at photon energies up to 11.6 GeV. The angular distributions are forward peaked for all produced pairs, consistent with Regge-like $t$-channel exchange. Asymmetric wide-angle anti-baryon distributions show the presence of additional processes. In a phenomenological model, we find consistency with a double $t$-channel exchange process where anti-baryons are created only at the middle vertex. The model matches all observed distributions with a small number of free parameters. In the hyperon channels, we observe a clear distinction between photoproduction of the $Λ\barΛ$ and $p\barΛ$ systems but general similarity to the $p\bar{p}$ system. We report both total cross sections and cross sections differential with respect to momentum transfer and the invariant masses of the created particle pairs. No narrow resonant structures were found in these reaction channels. The suppression of $s\bar{s}$ quark pairs relative to $d\bar{d}$ quark pairs is similar to what has been seen in other reactions.
Measured $\frac{d\sigma}{dm_{\Lambda\bar{\Lambda}}}~[\mathrm{nb/GeV}]$ for reaction $\gamma p\to \{\Lambda \bar{\Lambda}\} p$ including data of $6.5 \leq E_{\gamma} \leq 11.5$ [GeV], splitted in 10 energy bins (each as a column in the table). The observable $m_{\Lambda\bar{\Lambda}}$ is in unit of $[\mathrm{nb/GeV}]$ and is divided into bins of width 0.05 $[\mathrm{GeV}]$ (each as a row in the table). The global systematic uncertainty is 19% (not included in the table), with contributions of 5% from kinematic fitting, 10% from data selection, 5% from flux normalization, 13% from tracking efficiency, 3% from model dependence, and 6% from run-period variations.
Measured $\frac{d\sigma}{dm_{p\bar{\Lambda}}}~[\mathrm{nb/GeV}]$ for reaction $\gamma p\to \{p \bar{\Lambda}\} \Lambda$ including data of $6.5 \leq E_{\gamma} \leq 11.5$ [GeV], splitted in 10 energy bins (each as a column in the table). The observable $m_{p\bar{\Lambda}}$ is in unit of $[\mathrm{nb/GeV}]$ and is divided into bins of width 0.1 $[\mathrm{GeV}]$ (each as a row in the table). The global systematic uncertainty is 22% (not included in the table), with contributions of 2% from kinematic fitting, 10% from data selection, 5% from flux normalization, 15% from tracking efficiency, 3% from model dependence, and 10% from run-period variations.
Measured $\frac{d\sigma}{dm_{p\bar{p}}}~[\mathrm{nb/GeV}]$ for reaction $\gamma p\to \{p \bar{p}\} p$ including data of $3.5 \leq E_{\gamma} \leq 11.5$ [GeV], splitted in 15 energy bins (each as a column in the table). The observable $m_{p\bar{p}}$ is in unit of $[\mathrm{nb/GeV}]$ and is divided into bins of width 0.044 $[\mathrm{GeV}]$ (each as a row in the table). The global systematic uncertainty is 13% (not included in the table), with contributions of 8% from kinematic fitting, 4% from data selection, 5% from flux normalization, 8% from tracking efficiency, 3% from model dependence, and 1% from run-period variations.
The measurement of $\Sigma^{+}$ production in pp collisions at $\sqrt{s} = 13$ TeV is presented. The measurement is performed at midrapidity in both minimum-bias and high-multiplicity pp collisions at $\sqrt{s} = 13$ TeV. The $\Sigma^{+}$ is reconstructed via its weak-decay topology in the decay channel $\Sigma^{+} \rightarrow {p} + \pi^{0}$ with $\pi^{0} \rightarrow \gamma + \gamma$. In a novel approach, the neutral pion is reconstructed by combining photons that convert in the detector material with photons measured in the calorimeters. The transverse-momentum ($p_{T}$) distributions of the $\Sigma^{+}$ and its rapidity densities d$N/$dy in both event classes are reported. The $p_{T}$ spectrum in minimum-bias collisions is compared to QCD-inspired event generators. The ratio of $\Sigma^{+}$ to previously measured $\Lambda$ baryons is in good agreement with calculations from the Statistical Hadronization Model. The high efficiency and purity of the novel reconstruction method for $\Sigma^{+}$ presented here will enable future studies of the interaction of $\Sigma^{+}$ with protons in the context of femtoscopic measurements, which could be crucial for understanding the equation of state of neutron stars.
$p_\mathrm{T}$-differential production yield of $\Sigma^+$ and $\bar{\Sigma}^-$ baryons in the high-multiplicity triggered pp collisions at $\sqrt{{s}}=13~\mathrm{{TeV}}$ in the rapidity interval $|y|<0.8$.
$p_\mathrm{T}$-differential production yield of $\Sigma^+$ and $\bar{\Sigma}^-$ baryons in the minimum-bias triggered pp collisions at $\sqrt{{s}}=13~\mathrm{{TeV}}$ in the rapidity interval $|y|<0.8$.
The first measurement at midrapidity ($|y| < 0.5$) of the production yield of the strange-charm baryons $Ξ_c^+$ and $Ξ_c^0$ as a function of transverse momentum ($p_{\rm T}$) in different charged-particle multiplicity classes in proton-proton collisions at $\sqrt{s} = 13$ TeV with the ALICE experiment at the LHC is reported. The $Ξ_c^+$ baryon is reconstructed via the $Ξ_c^+ \rightarrow Ξ^-π^+π^+$ decay channel in the range $4 < p_{\rm T} < 12$ GeV/$c$, while the $Ξ_c^0$ baryon is reconstructed via both the $Ξ_c^0 \rightarrow Ξ^-π^+$ and $Ξ_c^0 \rightarrow Ξ^-e^+ν_e$ decay channels in the range $2 < p_{\rm T} < 12$ GeV/$c$. The baryon-to-meson ($Ξ_c^{0,+}/{\rm D}^0$) and the baryon-to-baryon ($Ξ_c^{0,+}/Λ_{\rm c}^+$) production yield ratios show no significant dependence on multiplicity. In addition, the observed yield ratios are not described by theoretical predictions that model charm-quark fragmentation based on measurements at $e^+e^-$ and $e^-$p colliders, indicating differences in the charm-baryon production mechanism in pp collisions. A comparison with different event generators and tunings, including different modelling of the hadronisation process, is also discussed. Moreover, the branching-fraction ratio of BR($Ξ_c^0 \rightarrow Ξ^-e^+ν_e$)/BR($Ξ_c^0 \rightarrow Ξ^-π^+$) is measured as 0.825 $\pm$ 0.094 (stat.) $\pm$ 0.081 (syst.). This value supersedes the previous ALICE measurement, improving the statistical precision by a factor of 1.6.
$p_{\rm T}$-differential per-event yield of prompt $\Xi_c^0$ baryons measured in the different multiplicity classes.
$p_{\rm T}$-differential per-event yield of prompt $\Xi_c^+$ baryons measured in the different multiplicity classes.
Ratio between the prompt $\Xi_c^0$ baryons in a multiplicity class to the multiplicity-integrated (INEL $>$ 0) class.
The dependence of f$_0$(980) production on the final-state charged-particle multiplicity is reported for proton-proton (pp) collisions at the centre-of-mass energy, $\sqrt{s}= 13$ TeV. The production of f$_0$(980) is measured with the ALICE detector via the f$_0(980) \rightarrow π^{+}π^{-}$ decay channel in a midrapidity region of $|y| < 0.5$. The evolution of the integrated yields and mean transverse momentum of f$_{0}$(980) as a function of charged-particle multiplicity measured in pp at $\sqrt{s} = 13$ TeV follows the trends observed in pp at $\sqrt{s} = 5.02$ TeV and in proton-lead (p-Pb) collisions at $\sqrt{s_{\rm{NN}}} = 5.02$ TeV. Particle yield ratios of f$_{0}$(980) to $π^{\pm}$ and K$^{*}$(892)$^{0}$ are found to decrease with increasing charged-particle multiplicity. These particle ratios are compared with calculations from the canonical statistical thermal model as a function of charged-particle multiplicity. The thermal model calculations provide a better description of the decreasing trend of particle ratios when no strange or antistrange quark composition for f$_{0}$(980) is assumed, which suggests that the tetraquark interpretation of the f$_{0}$(980) is disfavored.
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 INEL > 0 spectrum
Transverse momentum integrated f0(980) yield in pp collisions
The transverse momentum spectra and integrated yields of $\overlineΣ^{\pm}$ have been measured in pp and p-Pb collisions at $\sqrt{s_{\mathrm{NN}}} = 5.02$ TeV with the ALICE experiment. Measurements are performed via the newly accessed decay channel $\overlineΣ^{\pm} \rightarrow {\rm\overline{n}}π^{\pm}$. A new method of antineutron reconstruction with the PHOS electromagnetic spectrometer is developed and applied to this analysis. The $p_{\rm T}$ spectra of $\overlineΣ^{\pm}$ are measured in the range $0.5 < p_{\rm T} < 3$ GeV/$c$ and compared to predictions of the PYTHIA 8, DPMJET, PHOJET, EPOS LHC and EPOS4 models. The EPOS LHC and EPOS4 models provide the best descriptions of the measured spectra both in pp and p-Pb collisions, while models which do not account for multiparton interactions provide a considerably worse description at high $p_{\rm T}$. The total yields of $\overlineΣ^{\pm}$ in both pp and p-Pb collisions are compared to predictions of the Thermal-FIST model and dynamical models PYTHIA 8, DPMJET, PHOJET, EPOS LHC and EPOS4. All models reproduce the total yields in both colliding systems within uncertainties. The nuclear modification factors $R_{\rm pPb}$ for both $\overlineΣ^{+}$ and $\overlineΣ^{-}$ are evaluated and compared to those of protons, $Λ$ and $Ξ$ hyperons, and predictions of EPOS LHC and EPOS4 models. No deviations of $R_{\rm pPb}$ for $\overlineΣ^{\pm}$ from the model predictions or measurements for other hadrons are found within uncertainties.
$p_\mathrm{{T}}$-differential production yield of $\overline{\Sigma}^{+}$ in INEL pp collisions at $\sqrt{s}=5.02~\mathrm{{TeV}}$ in the rapidity interval $|y|<0.5$.
$p_\mathrm{{T}}$-differential production yield of $\overline{\Sigma}^{-}$ in INEL pp collisions at $\sqrt{s}=5.02~\mathrm{{TeV}}$ in the rapidity interval $|y|<0.5$.
$p_\mathrm{{T}}$-differential production yield of $\overline{\Sigma}^{+}$ in NSD p-Pb collisions at $\sqrt{s_\mathrm{NN}}=5.02~\mathrm{{TeV}}$ in the rapidity interval $|y_\mathrm{CMS}|<0.5$.
Ultrarelativistic heavy-ion collisions produce a state of hot and dense strongly interacting QCD matter called quark--gluon plasma (QGP). On an event-by-event basis, the volume of the QGP in ultracentral collisions is mostly constant, while its total entropy can vary significantly due to quantum fluctuations, leading to variations in the temperature of the system. Exploiting this unique feature of ultracentral collisions allows for the interpretation of the correlation of the mean transverse momentum of produced charged hadrons and the number of charged hadrons as a measure for the speed of sound. It is determined by fitting the relative increase in transverse momentum with respect to the relative change in the average charged-particle density measured at midrapidity. This study reports the event-average transverse momentum of charged particles as well as the self-normalized variance, skewness, and kurtosis of the event-by-event transverse momentum distribution in ultracentral Pb-Pb collisions at a center-of-mass energy of 5.02 TeV per nucleon pair using the ALICE detector. Different centrality estimators based on charged-particle multiplicity or the transverse energy of the event are used to select ultracentral collisions. By ensuring a pseudorapidity gap between the region used to define the centrality and the region used to perform the measurement, the influence of biases on the rise of the mean transverse momentum is tested. The measured values are found to strongly depend on the exploited centrality estimator. The variance shows a steep decrease towards ultracentral collisions, while the skewness variables show a maximum, followed by a fast decrease. These non-Gaussian features are understood in terms of the vanishing of the impact-parameter fluctuations contributing to the event-to-event transverse momentum distribution.
Average number of participating nucleons ($\langle N_{\mathrm{part}} \rangle$) as a function of centrality percentile in $\mathrm{Pb}-\mathrm{Pb}$ collisions at $\sqrt{s_{\mathrm{NN}}} = 5.02~\mathrm{TeV}$. Data points are shown for centrality estimators based on $N_{\mathrm{ch}}$, ${N_{\mathrm{tracklets}}}$, and $E_{\mathrm{T}}$ within $|\eta|\leq 0.8$.
Average number of participating nucleons ($\langle N_{\mathrm{part}} \rangle$) as a function of centrality percentile in $\mathrm{Pb}-\mathrm{Pb}$ collisions at $\sqrt{s_{\mathrm{NN}}} = 5.02~\mathrm{TeV}$. Data points are shown for centrality estimator based on $N_{\mathrm{ch}} \in$ $-3.7<\eta<-1.7$ and $2.8 < \eta <5.1$.
Normalized $p_{\mathrm{T}}$-spectrum ratio as a function as a function of centrality in $\mathrm{Pb}-\mathrm{Pb}$ collisions at $\sqrt{s_{\mathrm{NN}}} = 5.02~\mathrm{TeV}$. Data points are shown for centrality estimator based on $N_{\mathrm{ch}} \in$ $0.5 \leq |\eta|\leq 0.8$.
The total cross section for Compton scattering off atomic electrons, $γ+e\rightarrowγ'+e'$, was measured using photons with energies between 6.5 and 11.1 GeV incident on a $^9$Be target as part of the PrimEx-eta experiment in Hall D at Jefferson Lab. This is the first measurement of this fundamental QED process within this energy range. The total uncertainties of the cross section, combining the statistical and systematic components in quadrature, averaged to 3.4% across all energy bins. This not only demonstrates the capability of this experimental setup to perform precision cross-section measurements at forward angles but also allows us to compare with state-of-the-art QED calculations.
$\gamma +e^- \rightarrow \gamma + e^-$ total cross section in bins of photon beam energy. The first uncertainties are statistical, and the second are systematic.
This work reports femtoscopic correlations of p$-$p ($\bar{\rm p}-\bar{\rm p}$) and p$-$d ($\bar{\rm p}-\bar{\rm d}$) pairs measured in Pb$-$Pb collisions at center-of-mass energy per nucleon $\sqrt{s_{\rm NN}}$ = 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 ($m_\text{T}$) and on the average charge particle multiplicity $\langle\text{dN}_\text{ch}/\text{d}η\rangle^{1/3}$ for three centrality intervals (0$-$10$\%$, 10$-$30$\%$, 30$-$50$\%$). In both cases, the expected power-law and linear scalings are observed, respectively. The measured p$-$d correlations can be described by both two- and three-body calculations, indicating that the femtoscopy observable is not sensitive to the short-distance features of the dynamics of the p$-$(p$-$n) system, due to the large inter-particle distances in Pb$-$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 $m_{\rm{T}}$-scaling obtained for the p$-$p and p$-$d systems is compatible within 1$σ$ of the uncertainties. These findings provide new input for fundamental studies on the production of light (anti)nuclei under extreme conditions.
proton-proton (same charge) correlation function for centrality 0-10% from Pb-Pb collisions at 5020 GeV
proton-proton (same charge) correlation function for centrality 10-30% from Pb-Pb collisions at 5020 GeV
proton-proton (same charge) correlation function for centrality 30-50% from Pb-Pb collisions at 5020 GeV
This Letter presents measurements of long-range transverse-momentum correlations using a new observable, $v_{0}(p_\mathrm{T})$, which serves as a probe of radial flow and medium properties in heavy-ion collisions. Results are reported for inclusive charged particles, pions, kaons, and protons across various centrality intervals in Pb$-$Pb collisions at $\sqrt{s_\mathrm{NN}} = 5.02$ TeV, recorded by the ALICE detector. A pseudorapidity-gap technique, similar to that used in anisotropic-flow studies, is employed to suppress short-range correlations. At low $p_\mathrm{T}$, a characteristic mass ordering consistent with hydrodynamic collective flow is observed. At higher $p_\mathrm{T}$ ($> 3$ GeV/$c$), protons exhibit larger $v_{0}(p_\mathrm{T})$ than pions and kaons, in agreement with expectations from quark-recombination models. These results are sensitive to the bulk viscosity and the equation of state of the QCD medium formed in heavy-ion collisions.
The $p_\mathrm{T}$ dependence of $v_{0}(p_\mathrm{T})$ for inclusive charged particles is measured in Pb$-$Pb collisions at $\sqrt{s_\mathrm{NN}} = 5.02$ TeV within the 10$–$20% centrality interval, using a two-particle correlation method with a pseudorapidity gap of $\Delta\eta = 0.4$.
The $p_\mathrm{T}$ dependence of $v_{0}(p_\mathrm{T})$ for inclusive charged particles is measured in Pb$-$Pb collisions at $\sqrt{s_\mathrm{NN}} = 5.02$ TeV within the 30$–$40% centrality interval, using a two-particle correlation method with a pseudorapidity gap of $\Delta\eta = 0.4$.
The $p_\mathrm{T}$ dependence of $v_{0}(p_\mathrm{T})$ for inclusive charged particles is measured in Pb$-$Pb collisions at $\sqrt{s_\mathrm{NN}} = 5.02$ TeV within the 60$–$70% centrality interval, using a two-particle correlation method with a pseudorapidity gap of $\Delta\eta = 0.4$.
Femtoscopy of non-identical particle pairs has been instrumental for precision measurements of both two-particle sources and the final-state interactions in high-energy elementary and heavy-ion collisions. The majority of measurements assessing the source properties are based on identical particle pairs, providing direct access to the characteristics of the single-particle source. The work in this paper demonstrates, via femtoscopy measurements of charged pion--deuteron pairs in Pb$-$Pb collisions at $\sqrt{s_{\rm NN}} = 5.02$ TeV, the feasibility of accessing the characteristics of the single-particle femtoscopic source by using particle pairs with large mass differences such as pions and deuterons. The first experimental results of the measurement of deuteron source sizes in ultrarelativistic heavy-ion collisions are presented. The results show good agreement with the trend derived from other charged hadrons such as pions, kaons, and protons as a function of transverse mass, indicating similar source properties.
pion-deuteron (same charge) correlation function for centrality 0-10% from Pb-Pb collisions at 5020 GeV
pion-deuteron (same charge) correlation function for centrality 10-30% from Pb-Pb collisions at 5020 GeV
pion-deuteron (same charge) correlation function for centrality 30-50% from Pb-Pb collisions at 5020 GeV
Forum