Jet fragmentation transverse momentum ($j_{\rm T}$) distributions are measured in proton-proton (pp) and proton-lead (p-Pb) collisions at $\sqrt{s_{\rm NN}}$ = 5.02 TeV with the ALICE experiment at the LHC. Jets are reconstructed with the ALICE tracking detectors and electromagnetic calorimeter using the anti-$k_{\rm T}$ algorithm with resolution parameter $R=0.4$ in the pseudorapidity range $|\eta|<0.25$. The $j_{\rm T}$ values are calculated for charged particles inside a fixed cone with a radius $R = 0.4$ around the reconstructed jet axis. The measured $j_{\rm T}$ distributions are compared with a variety of parton-shower models. Herwig and PYTHIA 8 based models describe the data well for the higher $j_{\rm T}$ region, while they underestimate the lower $j_{\rm T}$ region. The $j_{\rm T}$ distributions are further characterised by fitting them with a function composed of an inverse gamma function for higher $j_{\rm T}$ values (called the "wide component"), related to the perturbative component of the fragmentation process, and with a Gaussian for lower $j_{\rm T}$ values (called the "narrow component"), predominantly connected to the hadronisation process. The width of the Gaussian has only a weak dependence on jet transverse momentum, while that of the inverse gamma function increases with increasing jet transverse momentum. For the narrow component, the measured trends are successfully described by all models except for Herwig. For the wide component, Herwig and PYTHIA 8 based models slightly underestimate the data for the higher jet transverse momentum region. These measurements set constraints on models of jet fragmentation and hadronisation.
$j_\mathrm{T}$ distribution in pp collisions at $\sqrt{s}$ = 5.02 TeV in $40 < p_\mathrm{T, jet}$ < 60 GeV/$c$.
$j_\mathrm{T}$ distributions in p--Pb collisions at $\sqrt{s_\mathrm{NN}}$ = 5.02 TeV in $40 < p_\mathrm{T, jet}$ < 150 GeV/$c$.
$\sqrt{\langle j_\mathrm{T}^2 \rangle }$ values for the narrow and wide components in pp collisions at $\sqrt{s}$ = 5.02 TeV in $40 < p_\mathrm{T, jet}$ < 60 GeV/$c$.
The multiplicity dependence of the pseudorapidity density of charged particles in proton-proton (pp) collisions at centre-of-mass energies $\sqrt{s}$ = 5.02, 7 and 13 TeV measured by ALICE is reported. The analysis relies on track segments measured in the midrapidity range ($|\eta| < 1.5$). Results are presented for inelastic events having at least one charged particle produced in the pseudorapidity interval $|\eta|<1$ ($\mathrm{INEL}_{>0}$). The multiplicity dependence of the pseudorapidy density of charged particles is measured with mid and forward rapidity multiplicity estimators, the latter being less affected by autocorrelations. A detailed comparison with predictions from the PYTHIA 8 and EPOS LHC event generators is also presented. Both generators provide a good description of the data.
Charged-particle pseudorapidity density for forward multiplicity classes as a function of $\eta$ in pp collisions at $\sqrt{s} = 5.02\,\mathrm{TeV}$. Statistical errors are generally insignificant.
Charged-particle pseudorapidity density for forward multiplicity classes as a function of $\eta$ in pp collisions at $\sqrt{s} = 7\,\mathrm{TeV}$. Statistical errors are generally insignificant.
Charged-particle pseudorapidity density for forward multiplicity classes as a function of $\eta$ in pp collisions at $\sqrt{s} = 13\,\mathrm{TeV}$. Statistical errors are generally insignificant.
A measurement of dielectron production in proton-proton (pp) collisions at $\sqrt{s} = 13$ TeV, recorded with the ALICE detector at the CERN LHC, is presented in this Letter. The data set was recorded with a reduced magnetic solenoid field. This enables the investigation of a kinematic domain at low dielectron invariant mass $m_{\rm ee}$ and pair transverse momentum $p_{\rm T,ee}$ that was previously inaccessible at the LHC. The cross section for dielectron production is studied as a function of $m_{\rm ee}$, $p_{\rm T,ee}$, and event multiplicity ${\rm d} N_{\rm ch}/{\rm d} \eta$. The expected dielectron rate from hadron decays, called hadronic cocktail, utilizes a parametrization of the measured $\eta/\pi^0$ ratio in pp and proton-nucleus (p-A) collisions, assuming that this ratio shows no strong dependence on collision energy at low transverse momentum. Comparison of the measured dielectron yield to the hadronic cocktail at $0.15
Differential dielectron cross section in pp collisions at $\sqrt{s}$ = 13 TeV as a function of $m_{\rm ee}$. Electrons are measured within $|\eta_{\rm e}| < 0.8$ and $p_{\rm T,e} > 0.075$ GeV/$c$.
Differential dielectron cross section in pp collisions at $\sqrt{s}$ = 13 TeV in the invariant mass interval 0.15<$m_{\rm ee}$<0.6 as a function of $p_{\rm T,ee}$. Electrons are measured within $|\eta_{\rm e}| < 0.8$ and $p_{\rm T,e} > 0.075$ GeV/$c$.
Dielectron yield per event in the excess region(0.15<$m_{\rm ee}$<0.6, $p_{\rm T,ee}$<0.4) as a function of the event multiplicity. Electrons are measured within $|\eta_{\rm e}| < 0.8$ and $p_{\rm T,e} > 0.075$ GeV/$c$.
The first measurements of dielectron production at midrapidity ($|\eta_{c}|<0.8$) in proton-proton and proton-lead collisions at $\sqrt{s_{\rm{NN}}}$ = 5.02 TeV at the LHC are presented. The dielectron cross section is measured with the ALICE detector as a function of the invariant mass $m_{\rm{ee}}$ and the pair transverse momentum $p_{\rm{T,ee}}$ in the ranges $m_{\rm{ee}}$ < 3.5 GeV/$c^{2}$ and $p_{\rm{T,ee}}$ < 8.0 GeV/$c^{2}$, in both collision systems. In proton-proton collisions, the charm and beauty cross sections are determined at midrapidity from a fit to the data with two different event generators. This complements the existing dielectron measurements performed at $\sqrt{s}$ = 7 and 13 TeV. The slope of the $\sqrt{s}$ dependence of the three measurements is described by FONLL calculations. The dielectron cross section measured in proton-lead collisions is in agreement, within the current precision, with the expected dielectron production without any nuclear matter effects for $\rm{e}^{+}\rm{e}^{-}$ pairs from open heavy-flavor hadron decays. For the first time at LHC energies, the dielectron production in proton-lead and proton-proton collisions are directly compared at the same $\sqrt{s_{\rm{NN}}}$ via the dielectron nuclear modification factor $R_{\rm{pPb}}$. The measurements are compared to model calculations including cold nuclear matter effects, or additional sources of dielectrons from thermal radiation.
Charm production cross section at midrapidity
The first measurements of dielectron production at midrapidity ($|\eta_{c}|<0.8$) in proton-proton and proton-lead collisions at $\sqrt{s_{\rm{NN}}}$ = 5.02 TeV at the LHC are presented. The dielectron cross section is measured with the ALICE detector as a function of the invariant mass $m_{\rm{ee}}$ and the pair transverse momentum $p_{\rm{T,ee}}$ in the ranges $m_{\rm{ee}}$ < 3.5 GeV/$c^{2}$ and $p_{\rm{T,ee}}$ < 8.0 GeV/$c^{2}$, in both collision systems. In proton-proton collisions, the charm and beauty cross sections are determined at midrapidity from a fit to the data with two different event generators. This complements the existing dielectron measurements performed at $\sqrt{s}$ = 7 and 13 TeV. The slope of the $\sqrt{s}$ dependence of the three measurements is described by FONLL calculations. The dielectron cross section measured in proton-lead collisions is in agreement, within the current precision, with the expected dielectron production without any nuclear matter effects for $\rm{e}^{+}\rm{e}^{-}$ pairs from open heavy-flavor hadron decays. For the first time at LHC energies, the dielectron production in proton-lead and proton-proton collisions are directly compared at the same $\sqrt{s_{\rm{NN}}}$ via the dielectron nuclear modification factor $R_{\rm{pPb}}$. The measurements are compared to model calculations including cold nuclear matter effects, or additional sources of dielectrons from thermal radiation.
Inclusive $e^+e^-$ cross section in pp collisions at $\sqrt{s}$ = 5.02 TeV as a function of $m_{\rm ee}$. Electrons are measured within $|\eta_{\rm e}| < 0.8$ and $p_{\rm T,e} > 0.2$ GeV/$c$.
Inclusive $e^+e^-$ cross section in p-Pb collisions at $\sqrt{s}$ = 5.02 TeV as a function of $m_{\rm ee}$. Electrons are measured within $|\eta_{\rm e}| < 0.8$ and $p_{\rm T,e} > 0.2$ GeV/$c$.
Inclusive $e^+e^-$ cross section in pp collisions at $\sqrt{s}$ = 5.02 TeV as a function of $p_{\rm T,ee}$ for $0.5 < m_{\rm ee} < 1.1$ GeV/$c$. Electrons are measured within $|\eta_{\rm e}| < 0.8$ and $p_{\rm T,e} > 0.2$ GeV/$c$.
The production of the $\Lambda$(1520) baryonic resonance has been measured at midrapidity in inelastic pp collisions at $\sqrt{s}$ = 7 TeV and in p-Pb collisions at $\sqrt{s_{\rm{NN}}}$ = 5.02 TeV for non-single diffractive events and in multiplicity classes. The resonance is reconstructed through its hadronic decay channel $\Lambda$(1520) $\rightarrow$ pK$^{-}$ and the charge conjugate with the ALICE detector. The integrated yields and mean transverse momenta are calculated from the measured transverse momentum distributions in pp and p-Pb collisions. The mean transverse momenta follow mass ordering as previously observed for other hyperons in the same collision systems. A Blast-Wave function constrained by other light hadrons ($\pi$, K, K$_{\rm{S}}^0$, p, $\Lambda$) describes the shape of the $\Lambda$(1520) transverse momentum distribution up to 3.5 GeV/$c$ in p-Pb collisions. In the framework of this model, this observation suggests that the $\Lambda(1520)$ resonance participates in the same collective radial flow as other light hadrons. The ratio of the yield of $\Lambda(1520)$ to the yield of the ground state particle $\Lambda$ remains constant as a function of charged-particle multiplicity, suggesting that there is no net effect of the hadronic phase in p-Pb collisions on the $\Lambda$(1520) yield.
$p_{\rm T}$-differential yields of $\Lambda$(1520) (sum of particle and anti-particle states) at midrapidity in inelastic pp collisions at $\sqrt{s}$ $\mathrm{=}$ 7 TeV.
$p_{\rm T}$-differential yields of $\Lambda$(1520) (sum of particle and anti-particle states) in NSD p--Pb collisions at $\sqrt{s_{\mathrm{NN}}}$ $\mathrm{=}$ 5.02 TeV. The uncertainty 'sys,$p_{\rm T}$-correlated' indicates the systematic uncertainty after removing the contributions of $p_{\rm T}$-uncorrelated uncertainty.
$p_{\rm T}$-differential yields of $\Lambda$(1520) (sum of particle and anti-particle states) in p--Pb collisions at $\sqrt{s_{\mathrm{NN}}}$ $\mathrm{=}$ 5.02 TeV in multiplicity interval 0--20\%. The uncertainty 'sys,$p_{\rm T}$-correlated' indicates the systematic uncertainty after removing the contributions of $p_{\rm T}$-uncorrelated uncertainty.
The Transition Radiation Detector (TRD) was designed and built to enhance the capabilities of the ALICE detector at the Large Hadron Collider (LHC). While aimed at providing electron identification and triggering, the TRD also contributes significantly to the track reconstruction and calibration in the central barrel of ALICE. In this paper the design, construction, operation, and performance of this detector are discussed. A pion rejection factor of up to 410 is achieved at a momentum of 1 GeV/$c$ in p-Pb collisions and the resolution at high transverse momentum improves by about 40% when including the TRD information in track reconstruction. The triggering capability is demonstrated both for jet, light nuclei, and electron selection.
Most probable charge deposit signal normalised to that of minimum ionising particles as a function of $\beta\gamma$ for $\pi$, $\it{e}$ test beam (dE/dx). Statistical uncertainties as vertical error bars.
Most probable charge deposit signal normalised to that of minimum ionising particles as a function of $\beta\gamma$ for $\pi$, $\it{e}$ test beam (dE/dx + TR). Statistical uncertainties as vertical error bars.
Most probable charge deposit signal normalised to that of minimum ionising particles as a function of $\beta\gamma$ for $\pi$, $\it{e}$ and proton in pp collisions ($\sqrt{s} = 7$ TeV). Statistical uncertainties as vertical error bars. Uncertainties in momentum and thus $\beta \gamma$ determination are drawn as horizontal error bars.
We present the charged-particle multiplicity distributions over a wide pseudorapidity range ($-3.4<\eta<5.0$) for pp collisions at $\sqrt{s}=$ 0.9, 7, and 8 TeV at the LHC. Results are based on information from the Silicon Pixel Detector and the Forward Multiplicity Detector of ALICE, extending the pseudorapidity coverage of the earlier publications and the high-multiplicity reach. The measurements are compared to results from the CMS experiment and to PYTHIA, PHOJET and EPOS LHC event generators, as well as IP-Glasma calculations.
Multiplicity distribution in the pseudorapidity region -2.0 to 2.0 for NSD collisions at a centre-of-mass energy of 900 GeV.
Multiplicity distribution in the pseudorapidity region -2.4 to 2.4 for NSD collisions at a centre-of-mass energy of 900 GeV.
Multiplicity distribution in the pseudorapidity region -3.0 to 3.0 for NSD collisions at a centre-of-mass energy of 900 GeV.
A measurement of the transverse momentum spectra of jets in Pb-Pb collisions at $\sqrt{s_{\rm NN}}=2.76$ TeV is reported. Jets are reconstructed from charged particles using the anti-$k_{\rm T}$ jet algorithm with jet resolution parameters $R$ of $0.2$ and $0.3$ in pseudo-rapidity $|\eta|<0.5$. The transverse momentum $p_{\rm T}$ of charged particles is measured down to $0.15$ GeV/$c$ which gives access to the low $p_{\rm T}$ fragments of the jet. Jets found in heavy-ion collisions are corrected event-by-event for average background density and on an inclusive basis (via unfolding) for residual background fluctuations and detector effects. A strong suppression of jet production in central events with respect to peripheral events is observed. The suppression is found to be similar to the suppression of charged hadrons, which suggests that substantial energy is radiated at angles larger than the jet resolution parameter $R=0.3$ considered in the analysis. The fragmentation bias introduced by selecting jets with a high $p_{\rm T}$ leading particle, which rejects jets with a soft fragmentation pattern, has a similar effect on the jet yield for central and peripheral events. The ratio of jet spectra with $R=0.2$ and $R=0.3$ is found to be similar in Pb-Pb and simulated PYTHIA pp events, indicating no strong broadening of the radial jet structure in the reconstructed jets with $R<0.3$.
Average values of the number of participating nucleons (Npart), number of binary collisions (Ncoll), and the nuclear overlap function (TAA) for the centrality intervals used in the jet analysis.
Charged jet spectra using two cone radius parameters R = 0.2 and 0.3 and a leading track selection of pT > 0.15 GeV, for centrality 0-10%. The two systematic uncertainties correspond to the shape uncertainty and the correlated uncertainty.
Charged jet spectra using two cone radius parameters R = 0.2 and 0.3 and a leading track selection of pT > 0.15 GeV, for centrality 10-30%. The two systematic uncertainties correspond to the shape uncertainty and the correlated uncertainty.
Azimuthally anisotropic distributions of D$^0$, D$^+$ and D$^{*+}$ mesons were studied in the central rapidity region ($|y|<0.8$) in Pb-Pb collisions at a centre-of-mass energy $\sqrt{s_{\rm NN}} = 2.76$ TeV per nucleon-nucleon collision, with the ALICE detector at the LHC. The second Fourier coefficient $v_2$ (commonly denoted elliptic flow) was measured in the centrality class 30-50% as a function of the D meson transverse momentum $p_{\rm T}$, in the range 2-16 GeV/$c$. The measured $v_2$ of D mesons is comparable in magnitude to that of light-flavour hadrons. It is positive in the range $2 < p_{\rm T} < 6$ GeV/$c$ with $5.7\sigma$ significance, based on the combination of statistical and systematic uncertainties.
v2 vs. pt for D0. The first systematic (sys) error is that from the data analysis and the second is from the B feed-down subtraction, as explained in the paper.
v2 vs. pt for D+. The first systematic (sys) error is that from the data analysis and the second is from the B feed-down subtraction, as explained in the paper.
v2 vs. pt for D*+. The first systematic (sys) error is that from the data analysis and the second is from the B feed-down subtraction, as explained in the paper.
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