The ALICE experiment at the LHC is undergoing a major upgrade during the Long Shutdown 2 (2019-2021). In particular, the Time Projection Chamber (TPC) is being equipped with new GEM-based readout chambers and the readout electronics of several detectors are being replaced with faster and more flexible technology. This will allow ALICE to read out most of the detectors in the continuous mode and record minimum bias Pb-Pb events at rates of about 50 kHz in Run 3 (2021-2024) and Run 4 (2027-2030). The ALICE Collaboration is also considering the possibility to collect a large sample of proton-proton collisions at interaction rates of about 1 MHz using online and offline preselection of rare events. These goals require a completely new online computing system that will be used to perform fast reconstruction and compression of the data stream. The event selection strategy becomes especially challenging for the case of central diffractive events and ultra-peripheral Pb-Pb collisions characterized by rapidity gaps at forward and backward directions with only few tracks at central rapidity. In this contribution, the motivation for studying central diffractive and ultra-peripheral events is presented, and feasibility studies for their selection in Run 3 and 4 will be given.
Ultra-peripheral collisions (UPCs), studied using the ALICE detector, allow us to investigate the low-$x$ behavior of the gluon distribution of the colliding particles. Two new measurements of coherent J/$ψ$ photoproduction cross section from Pb-Pb UPCs at $\sqrt{s_{\rm NN}}=5.02$ TeV are presented: the first measurement of the $|t|$-dependence of the cross section providing a new tool to investigate the transverse gluonic structure at low Bjorken-$x$ and a rapidity-differential measurement at midrapidity allowing us to provide stringent constraints on nuclear gluon shadowing and saturation mo…
The intense photon fluxes of relativistic nuclei provide a possibility to study photonuclear and two-photon interactions in ultra-peripheral collisions (UPC) where the nuclei do not overlap and no strong nuclear interactions occur. The study of such collisions provides information about the initial state of nuclei. First ALICE results from the LHC Run 2 are presented for forward exclusive \jpsi production in UPC, which is sensitive to the gluon distribution in nuclei. The increased statistics and the higher collision energy allows for a more detailed study at lower values of Bjorken-$x$. The a…
The LHC is not only the highest energy collider for protons and heavy ions, but also for photon photon and photon hadron ($γ$p and $γ$Pb) interactions. This is because the protons and ions accelerated in the LHC carry an electromagnetic field, which can be viewed as a source of photons, and such photons can interact with either other photons or with hadrons. Ultra Peripheral Collisions (UPC) occur when the incoming ions pass beyond the range of the strong force, with impact parameters larger than the sum of the radii of the incoming projectiles, and are mediated by the exchange of virtual phot…
The photoproduction of heavy vector mesons in the electromagnetic interactions of ultra-relativistic nuclei is sensitive to the gluon distribution in the nucleus and thus to cold nuclear matter effects like shadowing or parton saturation. Besides the well known observations of vector meson production in ultra-peripheral collisions, first observations of an excess over the expected hadronic J/$ψ$ production at very low transverse momentum ($p_T < $~0.3 GeV/$c$) in peripheral and semi-central nucleus-nucleus collisions both at LHC and RHIC energies were interpreted as the first sign of coherent …
Exploring Collective Effects in Small Collision Systems via Light-Flavour Particle Production in High-Multiplicity pp Collisions at $\sqrt{s}=$ 13 TeV in ALICE
Measurements of cross sections of inelastic and diffractive processes in proton--proton collisions at LHC energies were carried out with the ALICE detector. The fractions of diffractive processes in inelastic collisions were determined from a study of gaps in charged particle pseudorapidity distributions: for single diffraction (diffractive mass $M_X < 200$ GeV/$c^2$) $σ_{\rm SD}/σ_{\rm INEL} = 0.21 \pm 0.03, 0.20^{+0.07}_{-0.08}$, and $0.20^{+0.04}_{-0.07}$, respectively at centre-of-mass energies $\sqrt{s} = 0.9, 2.76$, and 7~TeV; for double diffraction (for a pseudorapidity gap $Δη> 3$) $σ_…
The $ρ^0$ is copiously photoproduced in ultra-peripheral heavy-ion collisions at the LHC. In this talk, I will present recent results on $ρ$ photoproduction with ALICE, including cross-section measurements from PbPb and XeXe collisions, including a discussion of the production of high-mass final states that decay to $π^+π^-$, and of neutron production that accompanies $ρ$ photoproduction. I will conclude by presenting some prospects for ALICE in Runs 3 and 4.
Two-particle femtoscopic correlations between non-identical charged particles for different charge combinations are measured in Pb-Pb collisions at $\sqrt{s_{\rm NN}}$ = 2.76 TeV with ALICE at the LHC. The three-dimensional two-particle correlation functions are studied in different centrality bins. The femtoscopic source size parameter ($R_{Out}$) and emission asymmetry ($μ$) are extracted. It is observed that the average source size of the system and emission asymmetry between particles increase from peripheral to central events.
The coherent photoproduction of $\rm{J/ψ}$ and $\rm{ψ'}$ mesons was measured in ultra-peripheral Pb-Pb collisions at a center-of-mass energy $\sqrt{s_{\mathrm{NN}}}~=~5.02$ TeV with the ALICE detector. Charmonia are detected in the central rapidity region for events where the hadronic interactions are strongly suppressed. The $\rm{J/ψ}$ is reconstructed using the dilepton ($l^{+} l^{-}$) and proton-antiproton decay channels, while for the $\rm{ψ'}$, the dilepton and the $l^{+} l^{-} π^{+} π^{-}$ decay channels are studied. The analysis is based on an event sample corresponding to an integrated…
Femtoscopic correlations between charged pions and kaons for different charge combinations are measured in Pb$-$Pb collisions at $\sqrt{s_{\rm NN}}=2.76$ TeV with ALICE at the LHC. The three-dimensional pion-kaon ($π-{\rm K}$) correlation functions and double ratios in the out-side-long pair rest frame are studied in different centrality bins. The $π-{\rm K}$ femtoscopic source size parameter ($R_{\rm out}$) and emission asymmetry ($μ_{\rm out}$) are extracted. It is observed that the average source size of the system and the emission asymmetry between pions and kaons increase from peripheral …
Light flavor hadron spectra at low $p_{T}$ and search for collective phenomena in high multiplicity pp, p-Pb and Pb-Pb collisions measured with the ALICE experiment
本文报道了在7 TeV pp碰撞中，在低$p_{T}$和中等快度时，确定的轻味强子（$π$，K，p）的横向动量分布及其比值随带电粒子多重性变化的综合结果。在2.76 TeV的Pb-Pb碰撞的光谱形状的粒子质量依赖硬化归因于流体动力学流和定量参数化与Boltzmann-Gibbs爆炸波拟合。在这方面的贡献，我们调查的集体现象的存在，在小系统：pp，p-Pb和外围Pb-Pb的多重依赖研究中观察到类似的模式。
Heavy Flavour decay muon production at forward rapidity in pp collisions at \sqrt{s}= 7 TeV and in Pb-Pb collisions at \sqrt{s_{NN}}=2.76 TeV with the ALICE experiment
Event-by-event fluctuations of the mean transverse momentum of charged particles produced in Pb-Pb and Xe-Xe collisions at $\sqrt{s_{\rm{NN}}}$ = 5.02 TeV and $\sqrt{s_{\rm{NN}}}$ = 5.44 TeV, respectively, are studied as a function of the charged-particle multiplicity using the ALICE detector at the LHC. Dynamical fluctuations are observed in both collision systems which indicate correlated particle emission. The central Pb-Pb and Xe-Xe collisions show a significant reduction of the fluctuation in comparison to peripheral collisions and are in qualitative agreement with previous measurements i…
The ALICE Collaboration has measured the energy dependence of exclusive photoproduction of $J/ψ$ vector mesons off proton targets in ultra-peripheral p-Pb collisions at a centre-of-mass energy per nucleon pair $\sqrt{s_{\rm NN}} = 5.02$ TeV. The e$^+$e$^-$ and $μ^+μ^-$ decay channels are used to measure the cross section as a function of the rapidity of the $J/ψ$ in the range $-2.5 < y < 2.7$, corresponding to an energy in the $γ$p centre-of-mass in the interval $40 < W_{γ\mathrm{p}}<550$ GeV. The measurements, which are consistent with a power law dependence of the exclusive $J/ψ$ photoproduc…
Surprisingly enough, the ratio of elastic to inelastic cross sections of proton interactions increases with energy in the interval correspond- ing to ISR - LHC (i.e. from 10 GeV to 10 TeV). That leads to special features of their spatial interaction region at these and higher ener- gies. Within the framework of some phenomenological models, we show how the particular ranges of the transferred momenta measured in elastic scattering experiments expose the spatial features of the in- elastic interaction region according to the unitarity condition. The difference between their predictions at highe…
The ALICE collaboration performed the first rapidity-differential measurement of coherent J/$ψ$ photoproduction in ultra-peripheral Pb-Pb collisions at a center-of-mass energy $\sqrt{s_{\rm{NN}}}$ = 5.02 TeV. The J/$ψ$ is detected via its dimuon decay in the forward rapidity region ($-4.0 < y < -2.5$) for events where the hadronic activity is required to be minimal. The analysis is based on an event sample corresponding to an integrated luminosity of about 750 $μ$b$^{-1}$. The cross section for coherent J/$ψ$ production is presented in six rapidity bins. The results are compared with theoretic…
By studying the balance functions of several hadronic species, one can gain insight into the chem-ical evolution of the Quark \textendash Gluon Plasma and radial flow. In a picture of early hadronisation,pairs of particles and anti-particles (created at the same space \textendash time point) are separated furtherin rapidity due to the higher initial temperature and diffusive interactions with other particles.Therefore delayed hadronisation will lead to strong correlations in rapidity in the final state. In this work, balance functions for identified charged-pion pairs measured by the ALICE det…
The ALICE collaboration has made the first measurement at the LHC of J/$ψ$ photoproduction in ultra-peripheral Pb-Pb-collisions at $\sqrt{s_{\rm NN}}$ = 2.76 TeV. The J/$ψ$ is identified via its dimuon decay in the forward rapidity region with the muon spectrometer for events where the hadronic activity is required to be minimal. The analysis is based on an event sample corresponding to an integrated luminosity of about 55 $μ$b$^{-1}$. The cross section for coherent J/$ψ$ production in the rapidity interval $-3.6 < y < -2.6$ is measured to be d$σ$/d$y = 1.00 \pm 0.18$ (stat) $+0.24/-0.26$ (sys…
In 2010, the STAR Collaboration collected 38 million low-multiplicity triggers for ultra-peripheral collision studies. We present new results involving photonuclear interactions in ultra-peripheral relativistic heavy ion collisions (UPCs), based on an analysis of 384,000 photoproduced pion pairs, from $ρ$, $ω$ and direct $π^+π^-$ pair production.The $ω$ component is clearly visible through its interference with the $ρ$ peak. The relative amplitudes of the three components have been measured, along with the phase angle between the $ρ$ and $ω$ components. Using a two-year (2010 and 2011) dataset…
Exclusive dimuon production and coherent charmonium photoproduction at forward rapidity in ultra-peripheral Pb$-$Pb collisions at $\mathbf{\sqrt{s_{\rm NN}}=5.36}$ TeV
European Organization for Nuclear Research（欧洲核子研究中心）
