The ALICE Collaboration reports three measurements in ultra-peripheral proton$-$lead collisions at forward rapidity. The exclusive two-photon process \ggmm and the exclusive photoproduction of J/$\psi$ are studied. J/$\psi$ photoproduction with proton dissociation is measured for the first time at a hadron collider. The cross section for the two-photon process of dimuons in the invariant mass range from 1 to 2.5 GeV/$c^2$ agrees with leading order quantum electrodynamics calculations. The exclusive and dissociative cross sections for J/$\psi$ photoproductions are measured for photon$-$proton centre-of-mass energies from 27 to 57 GeV. They are in good agreement with HERA results.
Differential cross sections DSIGMA/DM for exclusive GAMMA* GAMMA* to MU+ MU- production in p–Pb UPCs for each mass and rapidity interval
Exclusive J/psi photoproduction cross section in p-Pb UPC.
Dissociative J/psi photoproduction cross section in p-Pb UPC.
Measurements of the production of electrons from heavy-flavour hadron decays in pp collisions at $\sqrt{s} = 13$ TeV at midrapidity with the ALICE detector are presented down to a transverse momentum ($p_{\rm T}$) of 0.2 GeV$/c$ and up to $p_{\rm T} = 35$ GeV$/c$, which is the largest momentum range probed for inclusive electron measurements in ALICE. In p$-$Pb collisions, the production cross section and the nuclear modification factor of electrons from heavy-flavour hadron decays are measured in the $p_{\rm T}$ range $0.5 < p_{\rm T} < 26$ GeV$/c$ at $\sqrt{s_{\rm NN}} = 8.16$ TeV. The nuclear modification factor is found to be consistent with unity within the statistical and systematic uncertainties. In both collision systems, first measurements of the yields of electrons from heavy-flavour hadron decays in different multiplicity intervals normalised to the multiplicity-integrated yield (self-normalised yield) at midrapidity are reported as a function of the self-normalised charged-particle multiplicity estimated at midrapidity. The self-normalised yields in pp and p$-$Pb collisions grow faster than linear with the self-normalised multiplicity. A strong $p_{\rm T}$ dependence is observed in pp collisions, where the yield of high-$p_{\rm T}$ electrons increases faster as a function of multiplicity than the one of low-$p_{\rm T}$ electrons. The measurement in p$-$Pb collisions shows no $p_{\rm T}$ dependence within uncertainties. The self-normalised yields in pp and p$-$Pb collisions are compared with measurements of other heavy-flavour, light-flavour, and strange particles, and with Monte Carlo simulations.
pT-differential cross section of electrons from heavy-flavour hadron decays in pp collisions at $\sqrt{s}$ = 13 TeV measured at midrapidity
pT-differential cross section of electrons from heavy-flavour hadron decays in p-Pb collisions at $\sqrt{s_{\rm NN}}$ = 8.16 TeV measured at midrapidity
The Nuclear modification factor RpPb of electrons from heavy-flavour hadron decays in p--Pb collisions at $\sqrt{s_{\rm NN}}$ = 8.16 TeV
Multiplicity ($N_{\rm ch}$) distributions and transverse momentum ($p_{\rm T}$) spectra of inclusive primary charged particles in the kinematic range of $|\eta| < 0.8$ and 0.15 GeV/$c$$< p_{T} <$ 10 GeV/$c$ are reported for pp, p-Pb, Xe-Xe and Pb-Pb collisions at centre-of-mass energies per nucleon pair ranging from $\sqrt{s_{\rm NN}} = 2.76$ TeV up to $13$ TeV. A sequential two-dimensional unfolding procedure is used to extract the correlation between the transverse momentum of primary charged particles and the charged-particle multiplicity of the corresponding collision. This correlation sharply characterises important features of the final state of a collision and, therefore, can be used as a stringent test of theoretical models. The multiplicity distributions as well as the mean and standard deviation derived from the $p_{\rm T}$ spectra are compared to state-of-the-art model predictions. Providing these fundamental observables of bulk particle production consistently across a wide range of collision energies and system sizes can serve as an important input for tuning Monte Carlo event generators.
Charged-particle multiplicity distribution for pp collisions at 2.76 TeV.
Koba-Nielsen-Olesen scaled charged-particle multiplicity distribution for pp collisions at 2.76 TeV.
Charged-particle transverse momentum spectra as a function of charged-particle multiplicity for pp collisions at 2.76 TeV.
The production of inclusive, prompt and non-prompt J/$\psi$ was studied for the first time at midrapidity ($ -1.37 < y_{\rm cms} < 0.43$) in p$-$Pb collisions at $\sqrt{s_{\rm NN}} = 8.16$ TeV with the ALICE detector at the LHC. The inclusive J/$\psi$ mesons were reconstructed in the dielectron decay channel in the transverse momentum ($p_{\rm T}$) interval $0 < p_{\rm T} < 14$ GeV/$c$ and the prompt and non-prompt contributions were separated on a statistical basis for $p_{\rm T} > 2$ GeV/$c$. The study of the J/$\psi$ mesons in the dielectron channel used for the first time in ALICE online single-electron triggers from the Transition Radiation Detector, providing a data sample corresponding to an integrated luminosity of $689 \pm 13 \mu{\rm b}^{-1}$. The proton$-$proton reference cross section for inclusive J/$\psi$ was obtained based on interpolations of measured data at different centre-of-mass energies and a universal function describing the $p_{\rm T}$-differential J/$\psi$ production cross sections. The $p_{\rm T}$-differential nuclear modification factors $R_{\rm pPb}$ of inclusive, prompt, and non-prompt J/$\psi$ are consistent with unity and described by theoretical models implementing only nuclear shadowing.
d$^2\sigma$/d$y$d$p_{\rm T}$ in bins of $p_{\mathrm{T}}^{J/\psi}$ for inclusive J/$\psi$ in p--Pb collisions at $\sqrt{s_{NN}}$ = 8.16 TeV.
Nuclear modification factor ($R_{pPb}$) of inclusive J/$\psi$ in p--Pb collisions at $\sqrt{s_{NN}}$ = 8.16 TeV at midrapidity.
$p_\mathrm{T}$ integrated nuclear modification factor ($R_{pPb}$) of inclusive J/$\psi$ in p--Pb collisions at $\sqrt{s_{NN}}$ = 8.16 TeV at midrapidity.
The study of the azimuthal anisotropy of inclusive muons produced in p-Pb collisions at $\sqrt{s_{\rm NN}} = 8.16$ TeV, using the ALICE detector at the LHC is reported. The measurement of the second-order Fourier coefficient of the particle azimuthal distribution, $v_2$, is performed as a function of transverse momentum $p_{\rm T}$ in the 0-20% high-multiplicity interval at both forward ($2.03 < y_{\rm CMS} < 3.53$) and backward ($-4.46 < y_{\rm CMS} < -2.96$) rapidities over a wide $p_{\rm T}$ range, $0.5 < p_{\rm T} < 10$ GeV/$c$, in which a dominant contribution of muons from heavy-flavour hadron decays is expected at $p_{\rm T} > 2$ GeV/$c$. The $v_2$ coefficient of inclusive muons is extracted using two different techniques, namely two-particle cumulants, used for the first time for heavy-flavour measurements, and forward-central two-particle correlations. Both techniques give compatible results. A positive $v_2$ is measured at both forward and backward rapidities with a significance larger than $4.7\sigma$ and $7.6\sigma$, respectively, in the interval $2 < p_{\rm T} < 6$ GeV/$c$. Comparisons with previous measurements in p-Pb collisions at $\sqrt{s_{\rm NN}} = 5.02$ TeV, and with AMPT and CGC-based theoretical calculations are discussed. The findings impose new constraints on the theoretical interpretations of the origin of the collective behaviour in small collision systems.
Inclusive muon $v_{2}^{\mu}$ as a function of $p_{\mathrm{T}}$ is measured by two-particle correlation method at forward rapidities in high-multiplicity (0$-$20%) p$-$Pb collisions at $\sqrt{s_\mathrm{NN}}$= 8.16 TeV. The event activity is estimated with the V0M estimator.
Inclusive muon $v_{2}^{\mu}$ as a function of $p_{\mathrm{T}}$ is measured by two-particle cumulant method at forward rapidities in high-multiplicity (0$-$20%) p$-$Pb collisions at $\sqrt{s_\mathrm{NN}}$= 8.16 TeV. The event activity is estimated with the V0M estimator.
Inclusive muon $v_{2}^{\mu}$ as a function of $p_{\mathrm{T}}$ is measured by two-particle correlation method at backward rapidities in high-multiplicity (0$-$20%) p$-$Pb collisions at $\sqrt{s_\mathrm{NN}}$= 8.16 TeV. The event activity is estimated with the V0M estimator.
A search for lepton-flavor-violating $Z\to e\tau$ and $Z\to\mu\tau$ decays with $pp$ collision data recorded by the ATLAS detector at the LHC is presented. This analysis uses 139 fb$^{-1}$ of Run 2 $pp$ collisions at $\sqrt{s}=13$ TeV and is combined with the results of a similar ATLAS search in the final state in which the $\tau$-lepton decays hadronically, using the same data set as well as Run 1 data. The addition of leptonically decaying $\tau$-leptons significantly improves the sensitivity reach for $Z\to\ell\tau$ decays. The $Z\to\ell\tau$ branching fractions are constrained in this analysis to $\mathcal{B}(Z\to e\tau)<7.0\times10^{-6}$ and $\mathcal{B}(Z\to \mu\tau)<7.2\times10^{-6}$ at 95% confidence level. The combination with the previously published analyses sets the strongest constraints to date: $\mathcal{B}(Z\to e\tau)<5.0\times10^{-6}$ and $\mathcal{B}(Z\to \mu\tau)<6.5\times10^{-6}$ at 95% confidence level.
The best-fit predicted and observed distributions of the combined NN output in the low-$p_\text{T}$-SR for the $e\tau_\mu$ channel. The first and last bin include underflow and overflow events, respectively.
The best-fit predicted and observed distributions of the combined NN output in the low-$p_\text{T}$-SR for the $\mu\tau_e$ channel. The first and last bin include underflow and overflow events, respectively.
The best-fit predicted and observed distributions of the combined NN output in the high-$p_\text{T}$-SR for the $e\tau_\mu$ channel. The first and last bin include underflow and overflow events, respectively.
The ATLAS experiment at the Large Hadron Collider reports a search for charged-lepton-flavour violation in decays of $Z$ bosons into a τ lepton and an electron or muon of opposite charge.
The best-fit expected and observed distributions of the combined NN output in the CRZ$\tau\tau$ for the $\mu\tau$ channel for events with 1-prong $\tau_\text{had-vis}$ candidates. The last bin in each plot includes overflow events.
The best-fit expected and observed distributions of the combined NN output in the CRZ$\tau\tau$ for the $\mu\tau$ channel for events with 3-prong $\tau_\text{had-vis}$ candidates. The last bin in each plot includes overflow events.
The best-fit expected and observed distributions of the combined NN output in the VRSS for the $e\tau$ channel for events with 1-prong $\tau_\text{had-vis}$ candidates. The last bin in each plot includes overflow events.
A measurement of the $B^0_s \to J/\psi\phi$ decay parameters using 80.5 $\mathrm{fb}^{-1}$ of integrated luminosity collected with the ATLAS detector from 13 TeV proton-proton collisions at the LHC is presented. The measured parameters include the $CP$-violating phase $\phi_s$, the width difference $\Delta\Gamma_{s}$ between the $B^0_s$ meson mass eigenstates and the average decay width $\Gamma_{s}$. The values measured for the physical parameters are combined with those from 19.2 $\mathrm{fb}^{-1}$ of 7 TeV and 8 TeV data, leading to the following: \begin{eqnarray*} \phi_s & = & -0.087\phantom{0} \pm 0.036\phantom{0} ~\mathrm{(stat.)} \pm 0.021\phantom{0} ~\mathrm{(syst.)~rad} \\ \Delta\Gamma_{s} & = & \phantom{-}0.0657 \pm 0.0043 ~\mathrm{(stat.)} \pm 0.0037 ~\mathrm{(syst.)~ps}^{-1} \\ \Gamma_{s} & = & \phantom{-}0.6703 \pm 0.0014 ~\mathrm{(stat.)} \pm 0.0018 ~\mathrm{(syst.)~ps}^{-1} \\ \end{eqnarray*} Results for $\phi_s$ and $\Delta\Gamma_{s}$ are also presented as 68% confidence level contours in the $\phi_s$-$\Delta\Gamma_{s}$ plane. Furthermore, the transversity amplitudes and corresponding strong phases are measured. $\phi_s$ and $\Delta\Gamma_{s}$ measurements are in agreement with the Standard Model predictions.
Fitted values for the physical parameters of interest with their statistical and systematic uncertainties, for the result of solution (a).
Fitted values for the physical parameters of interest with their statistical and systematic uncertainties, for the result of solution (b).
Fit correlations between the physical parameters of interest, obtained from the fit for solution (a).
A dedicated sample of Large Hadron Collider proton-proton collision data at centre-of-mass energy $\sqrt{s}=8$ TeV is used to study inclusive single diffractive dissociation, $pp \rightarrow Xp$. The intact final-state proton is reconstructed in the ATLAS ALFA forward spectrometer, while charged particles from the dissociated system $X$ are measured in the central detector components. The fiducial range of the measurement is $-4.0 < \log_{10} \xi < -1.6$ and $0.016 < |t| < 0.43 \ {\rm GeV^2}$, where $\xi$ is the proton fractional energy loss and $t$ is the squared four-momentum transfer. The total cross section integrated across the fiducial range is $1.59 \pm 0.13 \ {\rm mb}$. Cross sections are also measured differentially as functions of $\xi$, $t$, and $\Delta \eta$, a variable that characterises the rapidity gap separating the proton and the system $X$. The data are consistent with an exponential $t$ dependence, ${\rm d} \sigma / {\rm d} t \propto \text{e}^{Bt}$ with slope parameter $B = 7.65 \pm 0.34 \ {\rm GeV^{-2}}$. Interpreted in the framework of triple Regge phenomenology, the $\xi$ dependence leads to a pomeron intercept of $\alpha(0) = 1.07 \pm 0.09$.
Hadron-level differential SD cross section as a function of Delta Eta.
Hadron-level differential SD cross section as a function of t.
Hadron-level differential SD cross section as a function of log_10 xi.
The azimuthal anisotropy of charged particles produced in $\sqrt{s_{\mathrm{NN}}}=8.16$ TeV $p$+Pb collisions is measured with the ATLAS detector at the LHC. The data correspond to an integrated luminosity of $165$ $\mathrm{nb}^{-1}$ that was collected in 2016. Azimuthal anisotropy coefficients, elliptic $v_2$ and triangular $v_3$, extracted using two-particle correlations with a non-flow template fit procedure, are presented as a function of particle transverse momentum ($p_\mathrm{T}$) between $0.5$ and $50$ GeV. The $v_2$ results are also reported as a function of centrality in three different particle $p_\mathrm{T}$ intervals. The results are reported from minimum-bias events and jet-triggered events, where two jet $p_\mathrm{T}$ thresholds are used. The anisotropies for particles with $p_\mathrm{T}$ less than about $2$ GeV are consistent with hydrodynamic flow expectations, while the significant non-zero anisotropies for $p_\mathrm{T}$ in the range $9$-$50$ GeV are not explained within current theoretical frameworks. In the $p_\mathrm{T}$ range $2$-$9$ GeV, the anisotropies are larger in minimum-bias than in jet-triggered events. Possible origins of these effects, such as the changing admixture of particles from hard scattering and the underlying event, are discussed.
Distribution of $v_{2}$ from MBT events plotted as a function of the A-particle $p_\mathrm{T}$ for 0-5% centrality.
Distribution of $v_{2}$ from $p_{T}^{jet}>75$ GeV events plotted as a function of the A-particle $p_\mathrm{T}$ for 0-5% centrality.
Distribution of $v_{2}$ from $p_{T}^{jet}>100$ GeV events plotted as a function of the A-particle $p_\mathrm{T}$ for 0-5% centrality.
Forum