Abstract A measurement of the cross section for top quark-antiquark ( t t ¯ $$ \mathrm{t}\overline{\mathrm{t}} $$ ) pairs produced in association with a photon in proton-proton collisions at s = 8 $$ ...\sqrt{s}=8 $$ TeV is presented. The analysis uses data collected with the CMS detector at the LHC, corresponding to an integrated luminosity of 19.7 fb1. The signal is defined as the production of a t t ¯ $$ \mathrm{t}\overline{\mathrm{t}} $$ pair in association with a photon having a transverse energy larger than 25 GeV and an absolute pseudorapidity smaller than 1.44. The measurement is performed in the fiducial phase space corresponding to the semileptonic decay chain of the t t ¯ $$ \mathrm{t}\overline{\mathrm{t}} $$ pair, and the cross section is measured relative to the inclusive t t ¯ $$ \mathrm{t}\overline{\mathrm{t}} $$ pair production cross section. The fiducial cross section for associated t t ¯ $$ \mathrm{t}\overline{\mathrm{t}} $$ pair and photon production is found to be 127 ±27 (stat+syst) fb per semileptonic final state. The measured value is in agreement with the theoretical prediction.
Abstract A search for dark matter is performed looking for events with large missing transverse momentum and a Higgs boson decaying either to a pair of bottom quarks or to a pair of photons. The data ...from proton-proton collisions at a center-of-mass energy of 13 TeV, collected in 2015 with the CMS detector at the LHC, correspond to an integrated luminosity of 2.3 fb−1. Results are interpreted in the context of a Z′-two-Higgs-doublet model, where the gauge symmetry of the standard model is extended by a U(1)Z ′ group, with a new massive Z′ gauge boson, and the Higgs sector is extended with four additional Higgs bosons. In this model, a high-mass resonance Z′ decays into a pseudoscalar boson A and a light SM-like scalar Higgs boson, and the A decays to a pair of dark matter particles. No significant excesses are observed over the background prediction. Combining results from the two decay channels yields exclusion limits in the signal cross section in the m Z ′ - m A phase space. For example, the observed data exclude the Z′ mass range from 600 to 1860 GeV, for Z′ coupling strength g Z ′ = 0.8, the coupling of A with dark matter particles g χ = 1, the ratio of the vacuum expectation values tan β = 1, and m A = 300 GeV. The results of this analysis are valid for any dark matter particle mass below 100 GeV.
Neutron rich nuclei around super48Ca have been measured with the CLARA-PRISMA setup, making use of super48Ca on super64Ni binary reactions, at 5.9 MeV/A. Angular distributions of gamma rays give ...evidence, in several transfer channels, for a large spin alignment (asymptotic to70%) perpendicular to the reaction plane, making it possible to firmly establish spin and parities of the excited states. In the case of super49Ca, states arising from different types of particle-core couplings are, for the first time, unambiguously identified on basis of angular distribution, polarization and lifetime measurements. Shell model and particle-vibration coupling calculations are used to pin down the nature of the states. Evidence is found for the presence, in the same excitation energy region, of two types of coupled states, i.e. single particle coupled to either super48Ca or super50Ca simple configurations, and particle-vibration coupled states based on the 3super- phonon of super48Ca.
Abstract The transverse momentum balance of pairs of back-to-back b quark jets in PbPb and pp collisions recorded with the CMS detector at the LHC is reported. The center-of-mass energy in both ...collision systems is 5.02 TeV per nucleon pair. Compared to the pp collision baseline, b quark jets have a larger imbalance in the most central PbPb collisions, as expected from the jet quenching effect. The data are also compared to the corresponding measurement with inclusive dijets. In the most central collisions, the imbalance of b quark dijets is comparable to that of inclusive dijets.
Abstract The top quark pair production cross section σ t t ¯ $$ \left({\sigma}_{\mathrm{t}\overline{\mathrm{t}}}\right) $$ is measured for the first time in pp collisions at a center-of-mass energy ...of 5.02 TeV. The data were collected by the CMS experiment at the LHC and correspond to an integrated luminosity of 27.4 pb−1. The measurement is performed by analyzing events with at least one charged lepton. The measured cross section is σ t t ¯ $$ {\sigma}_{\mathrm{t}\overline{\mathrm{t}}} $$ = 69.5 ± 6.1 (stat) ± 5.6 (syst) ± 1.6 (lumi) pb, with a total relative uncertainty of 12%. The result is in agreement with the expectation from the standard model. The impact of the presented measurement on the determination of the gluon distribution function is investigated.
Abstract Search results are presented for physics beyond the standard model in final states with two opposite-charge, same-flavor leptons, jets, and missing transverse momentum. The data sample ...corresponds to an integrated luminosity of 35.9 fb−1 of proton-proton collisions at s = 13 $$ \sqrt{s}=13 $$ TeV collected with the CMS detector at the LHC in 2016. The analysis uses the invariant mass of the lepton pair, searching for a kinematic edge or a resonant-like excess compatible with the Z boson mass. The search for a kinematic edge targets production of particles sensitive to the strong force, while the resonance search targets both strongly and electroweakly produced new physics. The observed yields are consistent with the expectations from the standard model, and the results are interpreted in the context of simplified models of supersymmetry. In a gauge mediated supersymmetry breaking (GMSB) model of gluino pair production with decay chains including Z bosons, gluino masses up to 1500–1770 GeV are excluded at the 95% confidence level depending on the lightest neutralino mass. In a model of electroweak chargino-neutralino production, chargino masses as high as 610 GeV are excluded when the lightest neutralino is massless. In GMSB models of electroweak neutralino-neutralino production, neutralino masses up to 500-650 GeV are excluded depending on the decay mode assumed. Finally, in a model with bottom squark pair production and decay chains resulting in a kinematic edge in the dilepton invariant mass distribution, bottom squark masses up to 980–1200 GeV are excluded depending on the mass of the next-to-lightest neutralino.
Abstract A measurement of the t t ¯ $$ \mathrm{t}\overline{\mathrm{t}} $$ production cross section at s = 13 $$ \sqrt{s}=13 $$ TeV is presented using proton-proton collisions, corresponding to an ...integrated luminosity of 2.2 fb−1, collected with the CMS detector at the LHC. Final states with one isolated charged lepton (electron or muon) and at least one jet are selected and categorized according to the accompanying jet multiplicity. From a likelihood fit to the invariant mass distribution of the isolated lepton and a jet identified as coming from the hadronization of a bottom quark, the cross section is measured to be σ t t ¯ = 888 ± 2 , stat − 28 + 26 syst ± 20 lumi $$ {\sigma}_{\mathrm{t}\overline{\mathrm{t}}}=888\pm 2,{\left(\mathrm{stat}\right)}_{-28}^{+26}\left(\mathrm{syst}\right)\pm 20\left(\mathrm{lumi}\right) $$ pb, in agreement with the standard model prediction. Using the expected dependence of the cross section on the pole mass of the top quark (m t), the value of m t is found to be 170.6 ± 2.7 GeV.
Abstract The spectra of charged particles produced within the pseudorapidity window |η| < 1 at s N N = 5.02 $$ \sqrt{s_{\mathrm{N}\;\mathrm{N}}}=5.02 $$ TeV are measured using 404 μb−1 of PbPb and ...27.4 pb−1 of pp data collected by the CMS detector at the LHC in 2015. The spectra are presented over the transverse momentum ranges spanning 0.5 < p T < 400 GeV in pp and 0.7 < p T < 400 GeV in PbPb collisions. The corresponding nuclear modification factor, R AA, is measured in bins of collision centrality. The R AA in the 5% most central collisions shows a maximal suppression by a factor of 7-8 in the p T region of 6-9 GeV. This dip is followed by an increase, which continues up to the highest p T measured, and approaches unity in the vicinity of p T = 200 GeV. The R AA is compared to theoretical predictions and earlier experimental results at lower collision energies. The newly measured pp spectrum is combined with the pPb spectrum previously published by the CMS collaboration to construct the pPb nuclear modification factor, R pA, up to 120 GeV. For p T > 20 GeV, R pA exhibits weak momentum dependence and shows a moderate enhancement above unity.
Shell structure and magic numbers in atomic nuclei were generally explained by pioneering work(1) that introduced a strong spin-orbit interaction to the nuclear shell model potential. However, ...knowledge of nuclear forces and the mechanisms governing the structure of nuclei, in particular far from stability, is still incomplete. In nuclei with equal neutron and proton numbers (N = Z), enhanced correlations arise between neutrons and protons (two distinct types of fermions) that occupy orbitals with the same quantum numbers. Such correlations have been predicted to favour an unusual type of nuclear superfluidity, termed isoscalar neutron-proton pairing(2-6), in addition to normal isovector pairing. Despite many experimental efforts, these predictions have not been confirmed. Here we report the experimental observation of excited states in the N = Z = 46 nucleus Pd-92. Gamma rays emitted following the Ni-58(Ar-36,2n)Pd-92 fusion-evaporation reaction were identified using a combination of state-of-the-art high-resolution c-ray, charged-particle and neutron detector systems. Our results reveal evidence for a spin-aligned, isoscalar neutron-proton coupling scheme, different from the previous prediction(2-6). We suggest that this coupling scheme replaces normal superfluidity (characterized by seniority coupling(7,8)) in the ground and low-lying excited states of the heaviest N = Z nuclei. Such strong, isoscalar neutron-proton correlations would have a considerable impact on the nuclear level structure and possibly influence the dynamics of rapid proton capture in stellar nucleosynthesis.
PDF
