2,046 research outputs found

    Measurement of isolated photon production in pp and PbPb collisions at sqrt{sNN} =2.76$ TeV with CMS

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    Isolated photon production is measured in pp and PbPb collisions at nucleon-nucleon centre-of-mass energies of 2.76 TeV in the pseudorapidity range |eta|<1.44 and transverse energies E_T between 20 and 80 GeV with the CMS detector at the LHC. The measured E_T spectra are found to be in good agreement with next-to-leading-order perturbative QCD predictions. The ratio of PbPb to pp isolated photon E_T-differential yields, scaled by the number of incoherent nucleon-nucleon collisions, is consistent with unity for all PbPb reaction centralities.Comment: Presented at HP2012: Hard Probes 201

    Measurement of the charged-hadron multiplicity in proton-proton collisions at LHC with the CMS detector

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    Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 2011.Cataloged from PDF version of thesis.Includes bibliographical references (p. 165-173).Charged-hadron pseudorapidity densities and multiplicity distributions in protonproton collisions at [the square root of sigma] = 0.9, 2.36, 7.0 TeV were measured with the inner tracking system of the CMS detector at the LHC. The charged-hadron yield was obtained by counting the number of hit-pairs (tracklets). The charged-particle multiplicity per unit of pseudorapidity dNch/d[eta] [eta]<0.5 at [the square root of sigma] = 7.0 TeV is 5.78 i 0.01(stat.) i 0.23(syst.) for nonsingle- diffractive events, higher than predicted by commonly used models. The relative increase in charged-particle multiplicity from [the square root of sigma] = 0.9 to 7 TeV is 66.1% ± 1.0%(stat.) ± 4.2%(syst.) and strong KNO violation is observed in the multiplicity distributions. Results are compared with low energy measurements.by Yen-Jie Lee.Ph.D

    Heavy quarks and jets as probes of the QGP

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    Quark-Gluon Plasma (QGP), a QCD state of matter created in ultra-relativistic heavy-ion collisions, has remarkable properties, including, for example, a low shear viscosity over entropy ratio. By detecting the collection of low-momentum particles that arise from the collision, it is possible to gain quantitative insight into the created matter. However, its fast evolution and thermalization properties remain elusive. Only using high momentum objects as probes of QGP can unveil its constituents at different wavelengths. In this review, we attempt to provide a comprehensive picture of what was, so far, possible to infer about QGP given our current theoretical understanding of jets, heavy-flavor, and quarkonia. We will bridge the resulting qualitative picture to the experimental observations done at the LHC and RHIC. We will focus on the phenomenological description of experimental observations, provide a brief analytical summary of the description of hard probes, and an outlook on the main difficulties we will need to surpass in the following years. To benchmark QGP-related effects, we will also address nuclear modifications to the initial state and hadronization effects

    Measurement of the charged-hadron multiplicity in proton-proton collisions at LHC with the CMS detector

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    Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 2011.Cataloged from PDF version of thesis.Includes bibliographical references (p. 165-173).Charged-hadron pseudorapidity densities and multiplicity distributions in protonproton collisions at [the square root of sigma] = 0.9, 2.36, 7.0 TeV were measured with the inner tracking system of the CMS detector at the LHC. The charged-hadron yield was obtained by counting the number of hit-pairs (tracklets). The charged-particle multiplicity per unit of pseudorapidity dNch/d[eta] [eta]<0.5 at [the square root of sigma] = 7.0 TeV is 5.78 i 0.01(stat.) i 0.23(syst.) for nonsingle- diffractive events, higher than predicted by commonly used models. The relative increase in charged-particle multiplicity from [the square root of sigma] = 0.9 to 7 TeV is 66.1% ± 1.0%(stat.) ± 4.2%(syst.) and strong KNO violation is observed in the multiplicity distributions. Results are compared with low energy measurements.by Yen-Jie Lee.Ph.D

    Data-driven extraction of the substructure of quark and gluon jets in proton-proton and heavy-ion collisions

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    The different modification of quark- and gluon-initiated jets in the quark-gluon plasma produced in heavy-ion collisions is a long-standing question that has not yet received a definitive answer from experiments. In particular, the relative sizes of the modification of quark and gluon jets differ between theoretical models. Therefore a fully data-driven technique is crucial for an unbiased extraction of the quark and gluon jet spectra and substructure. We perform a proof-of-concept study based on proton-proton and heavy-ion collision events from the PYQUEN generator with statistics accessible in Run 4 of the Large Hadron Collider. We use a statistical technique called topic modeling to separate quark and gluon contributions to jet observables. We demonstrate that jet substructure observables, such as the jet shape and jet fragmentation function, can be extracted using this data-driven method. These results suggest the potential for an experimental determination of quark and gluon jet spectra and their substructure

    Hard probes in isobar collisions as a probe of the neutron skin

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    We present an estimate of the yield of hard probes expected for collisions of the isobars 4496^{96}_{44}Ru and 4096^{96}_{40}Zr at collision energies reachable at RHIC and the LHC\@. These yields are proportional to the number of binary nucleon-nucleon interactions, which is characteristically different due to the presence of the large neutron skin in 4096^{96}_{40}Zr. This provides an independent opportunity to measure the difference between the neutron skin of 4496^{96}_{44}Ru and 4096^{96}_{40}Zr, which can provide an important constraint on the Equation of State of cold neutron-rich matter.Comment: 5 pages, 6 figures and an appendix. Comments are welcom

    Two-particle azimuthal correlations in e+ee^+e^- collisions at 91--209 GeV with archived ALEPH data at LEP-2

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    We present the first measurement of two-particle angular correlations of charged particles produced in e+ee^+e^- annihilation up to s=\sqrt{s}= 209 GeV. This analysis utilized the archived hadronic e+ee^+e^- data at center-of-mass energy between 91 and 209 GeV collected with the ALEPH detector at LEP between 1992 and 2000. The angular correlation functions are measured over a broad range of pseudorapidity and full azimuth as a function of charged particle multiplicity for the first time with LEP-2 data. At 91 GeV, no significant long-range correlation is observed in either the beam coordinate analysis or the thrust coordinate analysis, where the latter is sensitive to a medium expanding transverse to the color string between the outgoing qqˉq\bar{q} pair from the Z boson decays. Results with e+ee^+e^- data at higher collision energy than 91 GeV, providing higher event multiplicity reach up to around 50, are presented for the first time. The thrust axis analysis shows a long-range near-side excess in the two-particle correlation function. We performed Fourier series decomposition of the two-particle correlation functions. In high multiplicity events with more than 50 particles, the extracted Fourier coefficients v2v_2 and v3v_3 magnitudes in data are larger than the MC reference.Comment: ICHEP2022 Proceeding

    Measurements of two-particle correlations in e+ee^+e^- collisions at 91 GeV with ALEPH archived data

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    Measurements of two-particle angular correlations of charged particles emitted in hadronic ZZ decays are presented. The archived e+ee^+e^- annihilation data at a center-of-mass energy of 91 GeV were collected with the ALEPH detector at LEP between 1992 and 1995. The correlation functions are measured over a broad range of pseudorapidity and full azimuth as a function of charged particle multiplicity. No significant long-range correlation is observed in either the lab coordinate analysis or the thrust coordinate analysis, where the latter is sensitive to a medium expanding transverse to the color string between the outgoing qqˉq\bar{q} pair from ZZ boson decays. The associated yield distributions in both analyses are in better agreement with the prediction from the PYTHIA v6.1 event generator than from HERWIG v7.1.5. They provide new insights to showering and hadronization modeling. These results serve as an important reference to the observed long-range correlation in proton-proton, proton-nucleus, and nucleus-nucleus collisions.Comment: Replaced with the published version. Added the journal reference and the DO

    First measurement of anti-kT_\mathrm{T} jet spectra and jet substructure using the archived ALEPH e+ee^+e^- data at 91.2 GeV

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    We present the first anti-kT_{T} jet spectrum and substructure measurements using the archived ALEPH e+ee^+e^- data taken in 1994 at a center of mass energy of s=91.2\sqrt{s} = 91.2 GeV. Jets are reconstructed with the anti-kT_{T} algorithm with a resolution parameter of 0.4. It is the cleanest test of jets and QCD without the complication of hadronic initial states. The fixed center-of-mass energy also allows the first direct test of pQCD calculation. We present both the inclusive jet energy spectrum and the leading dijet energy spectra, together with a number of substructure observables. They are compared to predictions from PYTHIA6, PYTHIA8, Sherpa, HERWIG, VINCIA, and PYQUEN. None of the models fully reproduce the data. The data are also compared to two perturbative QCD calculations at NLO and with NLL'+R resummation. The results can also serve as reference measurements to compare to results from hadronic colliders. Future directions, including testing jet clustering algorithms designed for future electron-ion collider experiments, will also be discussed
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