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 .
Using a sample of $(10087\pm44)\times 10^6$$J/\psi$ events, which is about 45 times larger than that was previously analyzed, a further investigation on the $J/\psi\rightarrow \gamma 3(\pi^+\pi^-)$ decay is performed. A significant distortion at 1.84 GeV/$c^2$ in the line-shape of the $3(\pi^+\pi^-)$ invariant mass spectrum is observed for the first time, which could be resolved by two overlapping resonant structures, $X(1840)$ and $X(1880)$. The new state $X(1880)$ is observed with a statistical significance larger than $10\sigma$. The mass and width of $X(1880)$ are determined to be $1882.1\pm1.7\pm0.7$ MeV/$c^2$ and $30.7\pm5.5 \pm2.4$ MeV, respectively, which indicates the existence of a $p\bar{p}$ bound state.
The binned invariant mass spectrum of 6pi. The data correspond to Figure 3 of the event sample collected by BESIII. The mass range 1.55 < M(6pi) < 2.07 GeV is subdivided into 130 equidistant M(6pi) bins with a width of 4 MeV. The binned mass spectrum has been corrected by efficiency and the background Jpsi -> pi0 3(pi+ pi-) has been removed.
For the search of the chiral magnetic effect (CME), STAR previously presented the results from isobar collisions (${^{96}_{44}\text{Ru}}+{^{96}_{44}\text{Ru}}$, ${^{96}_{40}\text{Zr}}+{^{96}_{40}\text{Zr}}$) obtained through a blind analysis. The ratio of results in Ru+Ru to Zr+Zr collisions for the CME-sensitive charge-dependent azimuthal correlator ($\Delta\gamma$), normalized by elliptic anisotropy ($v_{2}$), was observed to be close to but systematically larger than the inverse multiplicity ratio. The background baseline for the isobar ratio, $Y = \frac{(\Delta\gamma/v_{2})^{\text{Ru}}}{(\Delta\gamma/v_{2})^{\text{Zr}}}$, is naively expected to be $\frac{(1/N)^{\text{Ru}}}{(1/N)^{\text{Zr}}}$; however, genuine two- and three-particle correlations are expected to alter it. We estimate the contributions to $Y$ from those correlations, utilizing both the isobar data and HIJING simulations. After including those contributions, we arrive at a final background baseline for $Y$, which is consistent with the isobar data. We extract an upper limit for the CME fraction in the $\Delta\gamma$ measurement of approximately $10\%$ at a $95\%$ confidence level on in isobar collisions at $\sqrt{s_{\text{NN}}} = 200$ GeV, with an expected $15\%$ difference in their squared magnetic fields.
Figure 1a, upper panel, full-event
Figure 1a, lower panel, full-event
Figure 1b, upper panel, subevent
Results on the transverse spherocity dependence of light-flavor particle production ($\pi$, K, p, $\phi$, ${\rm K^{*0}}$, ${\rm K}^{0}_{\rm{S}}$, $\Lambda$, $\Xi$) at midrapidity in high-multiplicity pp collisions at $\sqrt{s} = 13$ TeV were obtained with the ALICE apparatus. The transverse spherocity estimator ($S_{{\rm O}}^{{\it p}_{\rm T}=1}$) categorizes events by their azimuthal topology. Utilizing narrow selections on $S_{\text{O}}^{{\it p}_{\rm T}=1}$, it is possible to contrast particle production in collisions dominated by many soft initial interactions with that observed in collisions dominated by one or more hard scatterings. Results are reported for two multiplicity estimators covering different pseudorapidity regions. The $S_{{\rm O}}^{{\it p}_{\rm T}=1}$ estimator is found to effectively constrain the hardness of the events when the midrapidity ($\left | \eta \right |< 0.8$) estimator is used. The production rates of strange particles are found to be slightly higher for soft isotropic topologies, and severely suppressed in hard jet-like topologies. These effects are more pronounced for hadrons with larger mass and strangeness content, and observed when the topological selection is done within a narrow multiplicity interval. This demonstrates that an important aspect of the universal scaling of strangeness enhancement with final-state multiplicity is that high-multiplicity collisions are dominated by soft, isotropic processes. On the contrary, strangeness production in events with jet-like processes is significantly reduced. The results presented in this article are compared with several QCD-inspired Monte Carlo event generators. Models that incorporate a two-component phenomenology, either through mechanisms accounting for string density, or thermal production, are able to describe the observed strangeness enhancement as a function of $S_{{\rm O}}^{{\it p}_{\rm T}=1}$.
Spherocity distributions with respect to different multiplicity selections.
<pT> vs <dN_{#pi}/dEta> for different multiplicity and spherocity classes.
pT differential Phi spectra as a function of spherocity within 0-1% nTracklets.
Collective behavior has been observed in high-energy heavy-ion collisions for several decades. Collectivity is driven by the high particle multiplicities that are produced in these collisions. At the CERN Large Hadron Collider (LHC), features of collectivity have also been seen in high-multiplicity proton-proton collisions that can attain particle multiplicities comparable to peripheral Pb-Pb collisions. One of the possible signatures of collective behavior is the decrease of femtoscopic radii extracted from pion and kaon pairs emitted from high-multiplicity collisions with increasing pair transverse momentum. This decrease can be described in terms of an approximate transverse mass scaling. In the present work, femtoscopic analyses are carried out by the ALICE Collaboration on charged pion and kaon pairs produced in pp collisions at $\sqrt{s}=13$ TeV from the LHC to study possible collectivity in pp collisions. The event-shape analysis method based on transverse sphericity is used to select for spherical versus jet-like events, and the effects of this selection on the femtoscopic radii for both charged pion and kaon pairs are studied. This is the first time this selection method has been applied to charged kaon pairs. An approximate transverse-mass scaling of the radii is found in all multiplicity ranges studied when the difference in the Lorentz boost for pions and kaons is taken into account. This observation does not support the hypothesis of collective expansion of hot and dense matter that should only occur in high-multiplicity events. A possible alternate explanation of the present results is based on a scenario of common emission conditions for pions and kaons in pp collisions for the multiplicity ranges studied.
CF vs qinv
CF vs qinv
CF vs qinv
Measurements of charged-particle production in pp, p$-$Pb, and Pb$-$Pb collisions in the toward, away, and transverse regions with the ALICE detector are discussed. These regions are defined event-by-event relative to the azimuthal direction of the charged trigger particle, which is the reconstructed particle with the largest transverse momentum ($p_{\mathrm{T}}^{\rm trig}$) in the range $8<p_{\mathrm{T}}^{\rm trig}<15$ GeV$/c$. The toward and away regions contain the primary and recoil jets, respectively; both regions are accompanied by the underlying event (UE). In contrast, the transverse region perpendicular to the direction of the trigger particle is dominated by the so-called UE dynamics, and includes also contributions from initial- and final-state radiation. The relative transverse activity classifier, $R_{\mathrm{T}}=N_{\mathrm{ch}}^{\mathrm{T}}/\langle N_{\mathrm{ch}}^{\mathrm{T}}\rangle$, is used to group events according to their UE activity, where $N_{\mathrm{ch}}^{\mathrm{T}}$ is the charged-particle multiplicity per event in the transverse region and $\langle N_{\mathrm{ch}}^{\mathrm{T}}\rangle$ is the mean value over the whole analysed sample. The energy dependence of the $R_{\mathrm{T}}$ distributions in pp collisions at $\sqrt{s}=2.76$, 5.02, 7, and 13 TeV is reported, exploring the Koba-Nielsen-Olesen (KNO) scaling properties of the multiplicity distributions. The first measurements of charged-particle $p_{\rm T}$ spectra as a function of $R_{\mathrm{T}}$ in the three azimuthal regions in pp, p$-$Pb, and Pb$-$Pb collisions at $\sqrt{s_{\rm NN}}=5.02$ TeV are also reported. Data are compared with predictions obtained from the event generators PYTHIA 8 and EPOS LHC. This set of measurements is expected to contribute to the understanding of the origin of collective-like effects in small collision systems (pp and p$-$Pb).
$R_\mathrm{T}$ distribution using events with trigger particles $5<p_\mathrm{T}^\mathrm{trig}<40~\mathrm{GeV}/c$ in the pseudorapidity range of $|\eta|<0.8$ and with $p_\mathrm{T}>0.5~\mathrm{GeV}/c$ in pp collisions at $\sqrt{s}=2.76~\mathrm{TeV}$
$R_\mathrm{T}$ distribution using events with trigger particles $5<p_\mathrm{T}^\mathrm{trig}<40~\mathrm{GeV}/c$ in the pseudorapidity range of $|\eta|<0.8$ and with $p_\mathrm{T}>0.5~\mathrm{GeV}/c$ in pp collisions at $\sqrt{s}=5.02~\mathrm{TeV}$
$R_\mathrm{T}$ distribution using events with trigger particles $5<p_\mathrm{T}^\mathrm{trig}<40~\mathrm{GeV}/c$ in the pseudorapidity range of $|\eta|<0.8$ and with $p_\mathrm{T}>0.5~\mathrm{GeV}/c$ in pp collisions at $\sqrt{s}=7~\mathrm{TeV}$
Measurements of the differential production cross-sections of prompt and non-prompt $J/\psi$ and $\psi(2$S$)$ mesons with transverse momenta between 8 and 360 GeV and rapidity in the range $|y|<2$ are reported. Furthermore, measurements of the non-prompt fractions of $J/\psi$ and $\psi(2$S$)$, and the prompt and non-prompt $\psi(2$S$)$-to-$J/\psi$ production ratios, are presented. The analysis is performed using 140 fb$^{-1}$ of $\sqrt{s}=13$ TeV $pp$ collision data recorded by the ATLAS detector at the LHC during the years 2015-2018.
Summary of results for cross-section of prompt $J/\psi$ decaying to a muon pair for 13 TeV data in fb/GeV. Uncertainties are statistical and systematic, respectively.
Summary of results for cross-section of non-prompt $J/\psi$ decaying to a muon pair for 13 TeV data in fb/GeV. Uncertainties are statistical and systematic, respectively.
Summary of results for cross-section of prompt $\psi(2S)$ decaying to a muon pair for 13 TeV data in fb/GeV. Uncertainties are statistical and systematic, respectively.
We report results on an elastic cross section measurement in proton-proton collisions at a center-of-mass energy $\sqrt{s}=510$ GeV, obtained with the Roman Pot setup of the STAR experiment at the Relativistic Heavy Ion Collider (RHIC). The elastic differential cross section is measured in the four-momentum transfer squared range $0.23 \leq -t \leq 0.67$ GeV$^2$. We find that a constant slope $B$ does not fit the data in the aforementioned $t$ range, and we obtain a much better fit using a second-order polynomial for $B(t)$. The $t$ dependence of $B$ is determined using six subintervals of $t$ in the STAR measured $t$ range, and is in good agreement with the phenomenological models. The measured elastic differential cross section $\mathrm{d}\sigma/\mathrm{dt}$ agrees well with the results obtained at $\sqrt{s} = 546$ GeV for proton--antiproton collisions by the UA4 experiment. We also determine that the integrated elastic cross section within the STAR $t$-range is $\sigma^\mathrm{fid}_\mathrm{el} = 462.1 \pm 0.9 (\mathrm{stat.}) \pm 1.1 (\mathrm {syst.}) \pm 11.6 (\mathrm {scale})$~$\mu\mathrm{b}$.
Top panel: The $pp$ elastic differential cross section $d\sigma/dt$ fitted with an exponential $A e^{-B(t)|t|}$. Bottom panel: Residuals (Data - Fit)/Error. Uncertainties on the data points are smaller than the symbol size. The vertical scale uncertainty of 2.5% is not included in in the full error.
Results of the exponential function $A e^{-B(t)|t|}$ fit to the elastic differential cross section data as well as the integrated fiducial cross section are listed. Also listed are the corresponding values of the statistical and systematic uncertainties. The scale (luminosity and trigger efficiency) uncertainty of 2.5% applicable to the fit parameter $A$ and fiducial cross section $\sigma^\mathrm{fid}_\mathrm{el}$ is not included in the full error.
This paper presents for the first time a precise measurement of the production properties of the Z boson in the full phase space of the decay leptons. This is in contrast to the many previous precise unfolded measurements performed in the fiducial phase space of the decay leptons. The measurement is obtained from proton-proton collision data collected by the ATLAS experiment in 2012 at $\sqrt s$ = 8 TeV at the LHC and corresponding to an integrated luminosity of 20.2 fb$^{-1}$. The results, based on a total of 15.3 million Z-boson decays to electron and muon pairs, extend and improve a previous measurement of the full set of angular coefficients describing Z-boson decay. The double-differential cross-section distributions in Z-boson transverse momentum p$_T$ and rapidity y are measured in the pole region, defined as 80 $<$ m $<$ 100 GeV, over the range $|y| <$ 3.6. The total uncertainty of the normalised cross-section measurements in the peak region of the p$_T$ distribution is dominated by statistical uncertainties over the full range and increases as a function of rapidity from 0.5-1.0% for $|y| <$ 2.0 to 2-7% at higher rapidities. The results for the rapidity-dependent transverse momentum distributions are compared to state-of-the-art QCD predictions, which combine in the best cases approximate N$^4$LL resummation with N$^3$LO fixed-order perturbative calculations. The differential rapidity distributions integrated over p$_T$ are even more precise, with accuracies from 0.2-0.3% for $|y| <$ 2.0 to 0.4-0.9% at higher rapidities, and are compared to fixed-order QCD predictions using the most recent parton distribution functions. The agreement between data and predictions is quite good in most cases.
Measured $p_T$ cross sections in full-lepton phase space for |y| < 0.4.
Measured $p_T$ cross sections in full-lepton phase space for 0.4 < |y| < 0.8.
Measured $p_T$ cross sections in full-lepton phase space for 0.8 < |y| < 1.2.
Correlations in azimuthal angle extending over a long range in pseudorapidity between particles, usually called the "ridge" phenomenon, were discovered in heavy-ion collisions, and later found in pp and p$-$Pb collisions. In large systems, they are thought to arise from the expansion (collective flow) of the produced particles. Extending these measurements over a wider range in pseudorapidity and final-state particle multiplicity is important to understand better the origin of these long-range correlations in small-collision systems. In this Letter, measurements of the long-range correlations in p$-$Pb collisions at $\sqrt{s_{\rm NN}} = 5.02$ TeV are extended to a pseudorapidity gap of $\Delta\eta \sim 8$ between particles using the ALICE, forward multiplicity detectors. After suppressing non-flow correlations, e.g., from jet and resonance decays, the ridge structure is observed to persist up to a very large gap of $\Delta\eta \sim 8$ for the first time in p$-$Pb collisions. This shows that the collective flow-like correlations extend over an extensive pseudorapidity range also in small-collision systems such as p$-$Pb collisions. The pseudorapidity dependence of the second-order anisotropic flow coefficient, $v_{2}({\eta})$, is extracted from the long-range correlations. The $v_{2}(\eta)$ results are presented for a wide pseudorapidity range of $-3.1 < \eta < 4.8$ in various centrality classes in p$-$Pb collisions. To gain a comprehensive understanding of the source of anisotropic flow in small-collision systems, the $v_{2}(\eta)$ measurements are compared to hydrodynamic and transport model calculations. The comparison suggests that the final-state interactions play a dominant role in developing the anisotropic flow in small-collision systems.
$v_{2}\{2\}$ at $p_{\rm T} > 0$ GeV/$c$ as a function of pseudorapidity in different centrality classes using the template fit method
$v_{2}$ at $p_{\rm T} > 0$ GeV/$c$ as a function of charged particle density for five different pseudorapidity regions with the peripheral subtraction at $-3.1<\eta<-2.5$
$v_{2}$ at $p_{\rm T} > 0$ GeV/$c$ as a function of charged particle density for five different pseudorapidity regions with the peripheral subtraction at $-0.8<\eta<0$
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