Search for high mass dijet resonances with a new background prediction method in proton-proton collisions at √ s = 13 TeV The CMS collaboration

The cross section for coherent J/ψ photoproduction accompanied by at least one neutron on one side of the interaction point and no neutron activity on the other side, X n 0 n , is measured with the CMS experiment in ultra-peripheral PbPb collisions at √ s NN = 2.76 TeV. The analysis is based on a data sample corresponding to an integrated luminosity of 159 µb −1 , collected during the 2011 PbPb run. The J/ψ mesons are reconstructed in the dimuon decay channel, while neutrons are detected using zero degree calorimeters. The measured cross section is dσ coh X n 0 n /dy(J/ψ) = 0.36 ± 0.04 (stat) ± 0.04 (syst) mb in the rapidity interval 1.8 < |y| < 2.3. Using a model for the relative rate of coherent photoproduction processes, this X n 0 n measurement gives a total coherent photoproduction cross section of dσ coh /dy(J/ψ) = 1.82 ± 0.22 (stat) ± 0.20 (syst) ± 0.19 (theo) mb. The data strongly disfavour the impulse approximation model prediction, indicating that nuclear effects are needed to describe coherent J/ψ photoproduction in γ + Pb interactions. The data are found to be consistent with the leading twist approximation, which includes nuclear gluon shadowing. IntroductionPhoton-induced reactions are dominant in Ultra-Peripheral Collisions (UPC) of heavy ions, which involve electromagnetic interactions at large impact parameters of the colliding nuclei. Because of the extremely high photon flux in ultra-peripheral heavy-ion collisions which is proportional to Z 2 , where Z is the charge of the nucleus, photon-nucleus collisions at the LHC are abundant [1][2][3]. Furthermore, in UPCs the LHC can reach unprecedented photon-lead and photon-proton center-of-mass energies.Vector meson photoproduction in UPCs has received recent interest [3]. Exclusive J/ψ photoproduction off protons is defined by the reaction γ + p → J/ψ + p, with the characteristic features that, apart from the vector meson in the final state, no other particles are produced and the vector meson has a mean transverse momentum significantly lower than in inclusive reactions. Another characteristic feature is that in exclusive photoproduction the quantum numbers of the final state can be studied unambiguously. The γ + p → J/ψ + p production process has been studied by H1 and ZEUS collaborations at the electron-proton collider HERA [4][5][6], by the CDF collaboration in proton-antiproton collisions at the Tevatron [7], and by the ALICE and LHCb collaborations at the LHC, in proton-lead [8] and proton-proton collisions [9], respectively. Since the cross section of photoproduced vector mesons such as J/ψ, ψ(2S), and Υ(nS), in leading order perturbative QCD, is proportional to the gluon density squared in the target [10,11], the study of such diffractive processes in high-energy collisions is expected to provide insights into the role played by gluons in hadronic matter. As an example, a J/ψ produced at rapidity y is sensitive to the gluon distribution at x = (M J/ψ / √ s)e ±y at hard scales Q 2 ∼ M 2 J/ψ /4, where M J/ψ is the J/ψ mass, The CMS detectorThe...

The ATLAS CollaborationThis letter describes the observation of the light-by-light scattering process, γγ → γγ, in Pb+Pb collisions at √ s NN = 5.02 TeV. The analysis is conducted using a data sample corresponding to an integrated luminosity of 1.73 nb −1 , collected in November 2018 by the ATLAS experiment at the LHC. Light-by-light scattering candidates are selected in events with two photons produced exclusively, each with transverse energy E γ T > 3 GeV and pseudorapidity |η γ | < 2.4, diphoton invariant mass above 6 GeV, and small diphoton transverse momentum and acoplanarity. After applying all selection criteria, 59 candidate events are observed for a background expectation of 12 ± 3 events. The observed excess of events over the expected background has a significance of 8.2 standard deviations. The measured fiducial cross section is 78 ± 13 (stat.) ± 7 (syst.) ± 3 (lumi.) nb.Light-by-light scattering, γγ → γγ, is a quantum-mechanical process that is forbidden in the classical theory of electrodynamics [1, 2]. In the Standard Model (SM), the γγ → γγ reaction proceeds at one-loop level at order α 4 (where α is the fine-structure constant) via virtual box diagrams involving electrically charged fermions (leptons and quarks) or W ± bosons. However, in various extensions of the SM, extra contributions are possible, making the measurement of γγ → γγ scattering sensitive to new physics. Relevant examples are magnetic monopoles [3], vector-like fermions [4] and axion-like particles [5,6]. The light-by-light cross section is also sensitive to the effect of possible non-SM operators in an effective field theory [7][8][9]. Light-by-light scattering graphs with electron loops also contribute to the anomalous magnetic moment of the electron and muon [10,11].Strong evidence for this process in relativistic heavy-ion (Pb+Pb) collisions at the Large Hadron Collider (LHC) has been reported by the ATLAS [12] and CMS [13] collaborations with observed significances of 4.4 and 4.1 standard deviations, respectively. Exclusive light-by-light scattering can occur in these collisions at impact parameters larger than about twice the radius of the ions, as demonstrated for the first time in Ref. [14]. The strong interaction becomes less significant and the electromagnetic (EM) interaction becomes more important in these ultraperipheral collision (UPC) events. In general, this allows to study processes involving nuclear photoexcitation, photoproduction of hadrons, and two-photon interactions [15,16]. The EM fields produced by the colliding Pb nuclei can be described as a beam of quasi-real photons with a small virtuality of Q 2 < 1/R 2 , where R is the radius of the charge distribution and so Q 2 < 10 −3 GeV 2 [17, 18]. The cross section for the elastic reaction Pb+Pb (γγ) → Pb+Pb γγ can then be calculated by convolving the appropriate photon flux with the elementary cross section for the process γγ → γγ. Since the photon flux associated with each nucleus scales with the square of the number of protons, the cross section is strongl...

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Constraints on the lifetime and width of the Higgs boson are obtained from H → ZZ → 4l events using data recorded by the CMS experiment during the LHC run 1 with an integrated luminosity of 5.1 and 19.7 fb −1 at a center-of-mass energy of 7 and 8 TeV, respectively. The measurement of the Higgs boson lifetime is derived from its flight distance in the CMS detector with an upper bound of τ H < 1.9 × 10 −13 s at the 95% confidence level (C.L.), corresponding to a lower bound on the width of Γ H > 3.5 × 10 −9 MeV. The measurement of the width is obtained from an off-shell production technique, generalized to include anomalous couplings of the Higgs boson to two electroweak bosons. From this measurement, a joint constraint is set on the Higgs boson width and a parameter f ΛQ that expresses an anomalous coupling contribution as an on-shell cross-section fraction. The limit on the Higgs boson width is Γ H < 46 MeV with f ΛQ unconstrained and Γ H < 26 MeV for f ΛQ ¼ 0 at the 95% C.L. The constraint f ΛQ < 3.8 × 10 −3 at the 95% C.L. is obtained for the expected standard model Higgs boson width.