An official website of the United States government
Abstract The ALICE Collaboration at the CERN LHC has measured the inclusive production cross section of isolated photons at midrapidity as a function of the photon transverse momentum ($$p_{\textrm{T}}^{\gamma }$$ ), in Pb–Pb collisions in different centrality intervals, and in pp collisions, at centre-of-momentum energy per nucleon pair of$$\sqrt{s_{\textrm{NN}}}~=~5.02$$ TeV. The photon transverse momentum range is between 10–14 and 40–140 GeV/$$c$$ , depending on the collision system and on the Pb–Pb centrality class. The result extends to lower$$p_{\textrm{T}}^{\gamma }$$ than previously published results by the ATLAS and CMS experiments at the same collision energy. The covered pseudorapidity range is$$|\eta ^{\gamma } | <0.67$$ . The isolation selection is based on a charged particle isolation momentum threshold$$p_{\textrm{T}}^\mathrm{iso,~ch} = 1.5$$ GeV/$$c$$ within a cone of radii$$R=0.2$$ and 0.4. The nuclear modification factor is calculated and found to be consistent with unity in all centrality classes, and also consistent with the HG-PYTHIA model, which describes the event selection and geometry biases that affect the centrality determination in peripheral Pb–Pb collisions. The measurement is compared to next-to-leading order perturbative QCD calculations and to the measurements of isolated photons and Z$$^{0}$$ bosons from the CMS experiment, which are all found to be in agreement.
more »
« less
This content will become publicly available on October 1, 2025
Multiplicity dependence of charged-particle intra-jet properties in pp collisions at $$\sqrt{{{\varvec{s}}}}$$ = 13 TeV
Abstract The first measurement of the multiplicity dependence of intra-jet properties of leading charged-particle jets in proton–proton (pp) collisions is reported. The mean charged-particle multiplicity and jet fragmentation distributions are measured in minimum-bias and high-multiplicity pp collisions at center-of-mass energy$$\sqrt{s}$$ = 13 TeV using the ALICE detector. Jets are reconstructed from charged particles produced in the midrapidity region ($$|\eta | < 0.9$$ ) using the sequential recombination anti-$$k_{\textrm{T}}$$ algorithm with jet resolution parametersR= 0.2, 0.3, and 0.4 for the transverse momentum ($$p_\textrm{T}$$ ) interval 5–110 GeV/c. The high-multiplicity events are selected by the forward V0 scintillator detectors. The mean charged-particle multiplicity inside the leading jet cone rises monotonically with increasing jet$$p_\textrm{T}$$ in qualitative agreement with previous measurements at lower energies. The distributions of jet fragmentation function variables$$z^{\textrm{ch}}$$ and$$\xi ^{\textrm{ch}}$$ are measured for different jet-$$p_\textrm{T}$$ intervals. Jet-$$p_\textrm{T}$$ independent fragmentation of leading jets is observed for wider jets except at high- and low-$$z^{\textrm{ch}}$$ values. The observed “hump-backed plateau” structure in the$$\xi ^{\textrm{ch}}$$ distribution indicates suppression of low-$$p_\textrm{T}$$ particles. In high-multiplicity events, an enhancement of the fragmentation probability of low-$$z^{\textrm{ch}}$$ particles accompanied by a suppression of high-$$z^{\textrm{ch}}$$ particles is observed compared to minimum-bias events. This behavior becomes more prominent for low-$$p_\textrm{T}$$ jets with larger jet radius. The results are compared with predictions of QCD-inspired event generators, PYTHIA 8 with Monash 2013 tune and EPOS LHC. It is found that PYTHIA 8 qualitatively reproduces the jet modification in high-multiplicity events except at high jet$$p_\textrm{T}$$ . These measurements provide important constraints to models of jet fragmentation.
more »
« less
- Award ID(s):
- 2208883
- PAR ID:
- 10599258
- Author(s) / Creator(s):
- ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; more »
- Publisher / Repository:
- Springer
- Date Published:
- Journal Name:
- The European Physical Journal C
- Volume:
- 84
- Issue:
- 10
- ISSN:
- 1434-6052
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
A<sc>bstract</sc> 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$$ \left({p}_{\textrm{T}}^{\textrm{trig}}\right) $$ in the range 8<$$ {p}_{\textrm{T}}^{\textrm{trig}} $$ <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,$$ {R}_{\textrm{T}}={N}_{\textrm{ch}}^{\textrm{T}}/\left\langle {N}_{\textrm{ch}}^{\textrm{T}}\right\rangle $$ , is used to group events according to their UE activity, where$$ {N}_{\textrm{ch}}^{\textrm{T}} $$ is the charged-particle multiplicity per event in the transverse region and$$ \left\langle {N}_{\textrm{ch}}^{\textrm{T}}\right\rangle $$ is the mean value over the whole analysed sample. The energy dependence of theRTdistributions 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-particlepTspectra as a function ofRTin the three azimuthal regions in pp, p–Pb, and Pb–Pb collisions at$$ \sqrt{s_{\textrm{NN}}} $$ = 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).more » « less
-
Abstract The double differential cross sections of the Drell–Yan lepton pair ($$\ell ^+\ell ^-$$ , dielectron or dimuon) production are measured as functions of the invariant mass$$m_{\ell \ell }$$ , transverse momentum$$p_{\textrm{T}} (\ell \ell )$$ , and$$\varphi ^{*}_{\eta }$$ . The$$\varphi ^{*}_{\eta }$$ observable, derived from angular measurements of the leptons and highly correlated with$$p_{\textrm{T}} (\ell \ell )$$ , is used to probe the low-$$p_{\textrm{T}} (\ell \ell )$$ region in a complementary way. Dilepton masses up to 1$$\,\text {Te\hspace{-.08em}V}$$ are investigated. Additionally, a measurement is performed requiring at least one jet in the final state. To benefit from partial cancellation of the systematic uncertainty, the ratios of the differential cross sections for various$$m_{\ell \ell }$$ ranges to those in the Z mass peak interval are presented. The collected data correspond to an integrated luminosity of 36.3$$\,\text {fb}^{-1}$$ of proton–proton collisions recorded with the CMS detector at the LHC at a centre-of-mass energy of 13$$\,\text {Te\hspace{-.08em}V}$$ . Measurements are compared with predictions based on perturbative quantum chromodynamics, including soft-gluon resummation.more » « less
-
A<sc>bstract</sc> Measurements of inclusive charged-particle jet production in pp and p-Pb collisions at center-of-mass energy per nucleon-nucleon collision$$ \sqrt{s_{\textrm{NN}}} $$ = 5.02 TeV and the corresponding nuclear modification factor$$ {R}_{\textrm{pPb}}^{\textrm{ch}\ \textrm{jet}} $$ are presented, using data collected with the ALICE detector at the LHC. Jets are reconstructed in the central rapidity region |ηjet|<0.5 from charged particles using the anti-kTalgorithm with resolution parametersR= 0.2, 0.3, and 0.4. ThepT-differential inclusive production cross section of charged-particle jets, as well as the corresponding cross section ratios, are reported for pp and p-Pb collisions in the transverse momentum range 10<$$ {p}_{\textrm{T},\textrm{jet}}^{\textrm{ch}} $$ <140 GeV/cand 10<$$ {p}_{\textrm{T},\textrm{jet}}^{\textrm{ch}} $$ <160 GeV/c, respectively, together with the nuclear modification factor$$ {R}_{\textrm{pPb}}^{\textrm{ch}\ \textrm{jet}} $$ in the range 10<$$ {p}_{\textrm{T},\textrm{jet}}^{\textrm{ch}} $$ <140 GeV/c. The analysis extends thepTrange of the previously-reported charged-particle jet measurements by the ALICE Collaboration. The nuclear modification factor is found to be consistent with one and independent of the jet resolution parameter with the improved precision of this study, indicating that the possible influence of cold nuclear matter effects on the production cross section of charged-particle jets in p-Pb collisions at$$ \sqrt{s_{\textrm{NN}}} $$ = 5.02 TeV is smaller than the current precision. The obtained results are in agreement with other minimum bias jet measurements available for RHIC and LHC energies, and are well reproduced by the NLO perturbative QCD Powhegcalculations with parton shower provided by Pythia8 as well as by Jetscapesimulations.more » « less
-
Abstract The elliptic flow$$(v_2)$$ of$${\textrm{D}}^{0}$$ mesons from beauty-hadron decays (non-prompt$${\textrm{D}}^{0})$$ was measured in midcentral (30–50%) Pb–Pb collisions at a centre-of-mass energy per nucleon pair$$\sqrt{s_{\textrm{NN}}} = 5.02$$ TeV with the ALICE detector at the LHC. The$${\textrm{D}}^{0}$$ mesons were reconstructed at midrapidity$$(|y|<0.8)$$ from their hadronic decay$$\mathrm {D^0 \rightarrow K^-\uppi ^+}$$ , in the transverse momentum interval$$2< p_{\textrm{T}} < 12$$ GeV/c. The result indicates a positive$$v_2$$ for non-prompt$${{\textrm{D}}^{0}}$$ mesons with a significance of 2.7$$\sigma $$ . The non-prompt$${{\textrm{D}}^{0}}$$ -meson$$v_2$$ is lower than that of prompt non-strange D mesons with 3.2$$\sigma $$ significance in$$2< p_\textrm{T} < 8~\textrm{GeV}/c$$ , and compatible with the$$v_2$$ of beauty-decay electrons. Theoretical calculations of beauty-quark transport in a hydrodynamically expanding medium describe the measurement within uncertainties.more » « less
