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A new design of a detector plane of sub-millimetre thickness for an electromagnetic sampling calorimeter is presented. It is intended to be used in the luminometers LumiCal and BeamCal in future linear e\\[^{+}\\]e\\[^{-}\\] collider experiments. The detector planes were produced utilising novel connectivity scheme technologies. They were installed in a compact prototype of the calorimeter and tested at DESY with an electron beam of energy 1–5 GeV. The performance of a prototype of a compact LumiCal comprising eight detector planes was studied. The effective Molière radius at 5 GeV was determined to be (8.1 ± 0.1 (stat) ± 0.3 (syst)) mm, a value well reproduced by the Monte Carlo (MC) simulation (8.4 ± 0.1) mm. The dependence of the effective Molière radius on the electron energy in the range 1–5 GeV was also studied. Good agreement was obtained between data and MC simulation.

A prototype of a luminometer, designed for a future e+e- collider detector, and consisting at present of a four-plane module, was tested in the CERN PS accelerator T9 beam. The objective of this beam test was to demonstrate a multi-plane tungsten/silicon operation, to study the development of the electromagnetic shower and to compare it with MC simulations. The Molière radius has been determined to be 24.0 ± 0.6 (stat.) ± 1.5 (syst.) mm using a parametrization of the shower shape. Very good agreement was found between data and a detailed Geant4 simulation.

The Compact Linear Collider (CLIC) is an option for a future e + e - collider operating at centre-of-mass energies up to 3 TeV , providing sensitivity to a wide range of new physics phenomena and precision physics measurements at the energy frontier. This paper is the first comprehensive presentation of the Higgs physics reach of CLIC operating at three energy stages: s = 350 GeV , 1.4 and 3 TeV . The initial stage of operation allows the study of Higgs boson production in Higgsstrahlung ( e + e - → Z H ) and W W -fusion ( e + e - → H ν e ν ¯ e ), resulting in precise measurements of the production cross sections, the Higgs total decay width Γ H , and model-independent determinations of the Higgs couplings. Operation at s > 1 TeV provides high-statistics samples of Higgs bosons produced through W W -fusion, enabling tight constraints on the Higgs boson couplings. Studies of the rarer processes e + e - → t t ¯ H and e + e - → H H ν e ν ¯ e allow measurements of the top Yukawa coupling and the Higgs boson self-coupling. This paper presents detailed studies of the precision achievable with Higgs measurements at CLIC and describes the interpretation of these measurements in a global fit.

A combination is presented of all inclusive deep inelastic cross sections previously published by the H1 and ZEUS collaborations at HERA for neutral and charged current $e^{\\pm}p$ scattering for zero beam polarisation. The data were taken at proton beam energies of 920, 820, 575 and 460 GeV and an electron beam energy of 27.5 GeV. The data correspond to an integrated luminosity of about 1 fb$^{-1}$ and span six orders of magnitude in negative four-momentum-transfer squared, $Q^2$, and Bjorken $x$. The correlations of the systematic uncertainties were evaluated and taken into account for the combination. The combined cross sections were input to QCD analyses at leading order, next-to-leading order and at next-to-next-to-leading order, providing a new set of parton distribution functions, called HERAPDF2.0. In addition to the experimental uncertainties, model and parameterisation uncertainties were assessed for these parton distribution functions. Variants of HERAPDF2.0 with an alternative gluon parameterisation, HERAPDF2.0AG, and using fixed-flavour-number schemes, HERAPDF2.0FF, are presented. The analysis was extended by including HERA data on charm and jet production, resulting in the variant HERAPDF2.0Jets. The inclusion of jet-production cross sections made a simultaneous determination of these parton distributions and the strong coupling constant possible, resulting in $\\alpha_s(M_Z)=0.1183 \\pm 0.0009 {\\rm(exp)} \\pm 0.0005{\\rm (model/parameterisation)} \\pm 0.0012{\\rm (hadronisation)} ^{+0.0037}_{-0.0030}{\\rm (scale)}$. An extraction of $xF_3^{\\gamma Z}$ and results on electroweak unification and scaling violations are also presented.

A bstract The Compact Linear Collider (CLIC) is a proposed future high-luminosity linear electron-positron collider operating at three energy stages, with nominal centre-of-mass energies s = 380 GeV, 1 . 5 TeV, and 3 TeV. Its aim is to explore the energy frontier, providing sensitivity to physics beyond the Standard Model (BSM) and precision measurements of Standard Model processes with an emphasis on Higgs boson and top-quark physics. The opportunities for top-quark physics at CLIC are discussed in this paper. The initial stage of operation focuses on top-quark pair production measurements, as well as the search for rare flavour-changing neutral current (FCNC) top-quark decays. It also includes a top-quark pair production threshold scan around 350 GeV which provides a precise measurement of the top-quark mass in a well-defined theoretical framework. At the higher-energy stages, studies are made of top-quark pairs produced in association with other particles. A study of t ̄ tH production including the extraction of the top Yukawa coupling is presented as well as a study of vector boson fusion (VBF) production, which gives direct access to high-energy electroweak interactions. Operation above 1 TeV leads to more highly collimated jet environments where dedicated methods are used to analyse the jet constituents. These techniques enable studies of the top-quark pair production, and hence the sensitivity to BSM physics, to be extended to higher energies. This paper also includes phenomenological interpretations that may be performed using the results from the extensive top-quark physics programme at CLIC.

A bstract Two-particle azimuthal correlations have been measured in neutral current deep inelastic ep scattering with virtuality Q 2 > 5 GeV 2 at a centre-of-mass energy s = 318 GeV recorded with the ZEUS detector at HERA. The correlations of charged particles have been measured in the range of laboratory pseudorapidity − 1 . 5 < η < 2 . 0 and transverse momentum 0 . 1 < p T < 5 . 0 GeV and event multiplicities N ch up to six times larger than the average 〈 N ch 〉 ≈ 5. The two-particle correlations have been measured in terms of the angular observables c n {2}  = 〈〈 cosn Δ φ 〉〉, where n is between 1 and 4 and ∆ φ is the relative azimuthal angle between the two particles. Comparisons with available models of deep inelastic scattering, which are tuned to reproduce inclusive particle production, suggest that the measured two-particle correlations are dominated by contributions from multijet production. The correlations observed here do not indicate the kind of collective behaviour recently observed at the highest RHIC and LHC energies in high-multiplicity hadronic collisions.

A bstract The production of beauty and charm quarks in ep interactions has been studied with the ZEUS detector at HERA for exchanged four-momentum squared 5 < Q 2 < 1000 GeV 2 using an integrated luminosity of 354 pb −1 . The beauty and charm content in events with at least one jet have been extracted using the invariant mass of charged tracks associated with secondary vertices and the decay-length significance of these vertices. Differential cross sections as a function of Q 2 , Bjorken x , jet trans- verse energy and pseudorapidity were measured and compared with next-to-leading-order QCD calculations. The beauty and charm contributions to the proton structure functions were extracted from the double-differential cross section as a function of x and Q 2 . The running beauty-quark mass, m b at the scale m b , was determined from a QCD fit at next-to-leading order to HERA data for the first time and found to be m b ( m b ) = 4.07 ± 0.14 (fit) − 0.07 + 0.01 (mod.) − 0.00 + 0.05 (param.) − 0.05 + 0.08 (theo.) GeV.

A bstract Charm production in charged current deep inelastic scattering has been measured for the first time in e ± p collisions, using data collected with the ZEUS detector at HERA, corresponding to an integrated luminosity of 358 pb −1 . Results are presented separately for e + p and e − p scattering at a centre-of-mass energy of s = 318 GeV within a kinematic phase-space region of 200 GeV 2 < Q 2 < 60000 GeV 2 and y < 0.9, where Q 2 is the squared four-momentum transfer and y is the inelasticity. The measured cross sections of electroweak charm production are consistent with expectations from the Standard Model within the large statistical uncertainties.

Production of exclusive dijets in diffractive deep inelastic e ± p scattering has been measured with the ZEUS detector at HERA using an integrated luminosity of 372 pb - 1 . The measurement was performed for γ ∗ – p centre-of-mass energies in the range 90 < W < 250 GeV and for photon virtualities Q 2 > 25 GeV 2 . Energy flows around the jet axis are presented. The cross section is presented as a function of β and ϕ , where β = x / x I P , x is the Bjorken variable and x I P is the proton fractional longitudinal momentum loss. The angle ϕ is defined by the γ ∗ –dijet plane and the γ ∗ – e ± plane in the rest frame of the diffractive final state. The ϕ cross section is measured in bins of β . The results are compared to predictions from models based on different assumptions about the nature of the diffractive exchange.