An official website of the United States government
Abstract The production cross section of inclusive isolated photons has been measured by the ALICE experiment at the CERN LHC in pp collisions at centre-of-momentum energy of$$\sqrt{s} =13$$ TeV collected during the LHC Run 2 data-taking period. The measurement is performed by combining the measurements of the electromagnetic calorimeter EMCal and the central tracking detectors ITS and TPC, covering a pseudorapidity range of$$|\eta ^{\gamma }|<0.67$$ and a transverse momentum range of$$7 GeV/$$c$$ . The result extends to lower$$p_\textrm{T}^{\gamma }$$ and$$x_\textrm{T}^{\gamma } = 2p_\textrm{T}^{\gamma }/\sqrt{s} $$ ranges, the lowest$$x_\textrm{T}^{\gamma }$$ of any isolated photon measurements to date, extending significantly those measured by the ATLAS and CMS experiments towards lower$$p_\textrm{T}^{\gamma }$$ at the same collision energy with a small overlap between the measurements. The measurement is compared with next-to-leading order perturbative QCD calculations and the results from the ATLAS and CMS experiments as well as with measurements at other collision energies. The measurement and theory prediction are in agreement with each other within the experimental and theoretical uncertainties.
more »
« less
This content will become publicly available on May 1, 2026
Measurement of the inclusive isolated-photon production cross section in pp and Pb–Pb collisions at $$\mathbf {\sqrt{\textit{s}_{NN }} = 5.02}$$ TeV
Abstract The ALICE Collaboration at the CERN LHC has measured the inclusive production cross section of isolated photons at midrapidity as a function of the photon transverse momentum ($$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}$$ bosons from the CMS experiment, which are all found to be in agreement.
more »
« less
- Award ID(s):
- 2208883
- PAR ID:
- 10599216
- Author(s) / Creator(s):
- ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; more »
- Publisher / Repository:
- Eur. Phys. J.
- Date Published:
- Journal Name:
- The European Physical Journal C
- Volume:
- 85
- Issue:
- 5
- ISSN:
- 1434-6052
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Abstract The total charm-quark production cross section per unit of rapidity$$\textrm{d}\sigma ({{\textrm{c}}\overline{\textrm{c}}})/\textrm{d}y$$ , and the fragmentation fractions of charm quarks to different charm-hadron species$$f(\textrm{c}\rightarrow {\textrm{h}}_{\textrm{c}})$$ , are measured for the first time in p–Pb collisions at$$\sqrt{s_\textrm{NN}} = 5.02~\text {Te}\hspace{-1.00006pt}\textrm{V} $$ at midrapidity ($$-0.96<0.04$$ in the centre-of-mass frame) using data collected by ALICE at the CERN LHC. The results are obtained based on all the available measurements of prompt production of ground-state charm-hadron species:$$\textrm{D}^{0}$$ ,$$\textrm{D}^{+}$$ ,$$\textrm{D}_\textrm{s}^{+}$$ , and$$\mathrm {J/\psi }$$ mesons, and$$\Lambda _\textrm{c}^{+}$$ and$$\Xi _\textrm{c}^{0}$$ baryons. The resulting cross section is$$ \textrm{d}\sigma ({{\textrm{c}}\overline{\textrm{c}}})/\textrm{d}y =219.6 \pm 6.3\;(\mathrm {stat.}) {\;}_{-11.8}^{+10.5}\;(\mathrm {syst.}) {\;}_{-2.9}^{+8.3}\;(\mathrm {extr.})\pm 5.4\;(\textrm{BR})\pm 4.6\;(\mathrm {lumi.}) \pm 19.5\;(\text {rapidity shape})+15.0\;(\Omega _\textrm{c}^{0})\;\textrm{mb} $$ , which is consistent with a binary scaling of pQCD calculations from pp collisions. The measured fragmentation fractions are compatible with those measured in pp collisions at$$\sqrt{s} = 5.02$$ and 13 TeV, showing an increase in the relative production rates of charm baryons with respect to charm mesons in pp and p–Pb collisions compared with$$\mathrm {e^{+}e^{-}}$$ and$$\mathrm {e^{-}p}$$ collisions. The$$p_\textrm{T}$$ -integrated nuclear modification factor of charm quarks,$$R_\textrm{pPb}({\textrm{c}}\overline{\textrm{c}})= 0.91 \pm 0.04\;\mathrm{(stat.)} ^{+0.08}_{-0.09}\;\mathrm{(syst.)} ^{+0.05}_{-0.03}\;\mathrm{(extr.)} \pm 0.03\;\mathrm{(lumi.)}$$ , is found to be consistent with unity and with theoretical predictions including nuclear modifications of the parton distribution functions.more » « less
-
Abstract The transverse momentum ($$p_{\textrm{T}}$$ ) differential production cross section of the promptly produced charm-strange baryon$$\mathrm {\Xi _{c}^{0}}$$ (and its charge conjugate$$\overline{\mathrm {\Xi _{c}^{0}}}$$ ) is measured at midrapidity via its hadronic decay into$$\mathrm{\pi ^{+}}\Xi ^{-}$$ in p–Pb collisions at a centre-of-mass energy per nucleon–nucleon collision$$\sqrt{s_{\textrm{NN}}}~=~5.02$$ TeV with the ALICE detector at the LHC. The$$\mathrm {\Xi _{c}^{0}}$$ nuclear modification factor ($$R_{\textrm{pPb}}$$ ), calculated from the cross sections in pp and p–Pb collisions, is presented and compared with the$$R_{\textrm{pPb}}$$ of$$\mathrm {\Lambda _{c}^{+}}$$ baryons. The ratios between the$$p_{\textrm{T}}$$ -differential production cross section of$$\mathrm {\Xi _{c}^{0}}$$ baryons and those of$$\mathrm {D^0}$$ mesons and$$\mathrm {\Lambda _{c}^{+}}$$ baryons are also reported and compared with results at forward and backward rapidity from the LHCb Collaboration. The measurements of the production cross section of prompt$$\Xi ^0_\textrm{c}$$ baryons are compared with a model based on perturbative QCD calculations of charm-quark production cross sections, which includes only cold nuclear matter effects in p–Pb collisions, and underestimates the measurement by a factor of about 50. This discrepancy is reduced when the data is compared with a model that includes string formation beyond leading-colour approximation or in which hadronisation is implemented via quark coalescence. The$$p_{\textrm{T}}$$ -integrated cross section of prompt$$\Xi ^0_\textrm{c}$$ -baryon production at midrapidity extrapolated down to$$p_{\textrm{T}}$$ = 0 is also reported. These measurements offer insights and constraints for theoretical calculations of the hadronisation process. Additionally, they provide inputs for the calculation of the charm production cross section in p–Pb collisions at midrapidity.more » « less
-
Abstract The azimuthal ($$\Delta \varphi $$ ) correlation distributions between heavy-flavor decay electrons and associated charged particles are measured in pp and p–Pb collisions at$$\sqrt{s_{\mathrm{{NN}}}} = 5.02$$ TeV. Results are reported for electrons with transverse momentum$$4<16$$ $$\textrm{GeV}/c$$ and pseudorapidity$$|\eta |<0.6$$ . The associated charged particles are selected with transverse momentum$$1<7$$ $$\textrm{GeV}/c$$ , and relative pseudorapidity separation with the leading electron$$|\Delta \eta | < 1$$ . The correlation measurements are performed to study and characterize the fragmentation and hadronization of heavy quarks. The correlation structures are fitted with a constant and two von Mises functions to obtain the baseline and the near- and away-side peaks, respectively. The results from p–Pb collisions are compared with those from pp collisions to study the effects of cold nuclear matter. In the measured trigger electron and associated particle kinematic regions, the two collision systems give consistent results. The$$\Delta \varphi $$ distribution and the peak observables in pp and p–Pb collisions are compared with calculations from various Monte Carlo event generators.more » « less
-
Abstract The elliptic flow$$(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
