Deutsches Elektronen-Synchrotron DESY

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    Measurements of ttˉ\mathrm{t\bar{t}}H production and the CP structure of the Yukawa interaction between the Higgs boson and top quark in the diphoton decay channel

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    The first observation of the ttˉ\mathrm{t\bar{t}}H process in a single Higgs boson decay channel with the full reconstruction of the final state (H→γγ\to\gamma\gamma) is presented, with a significance of 6.6 standard deviations (σ\sigma). The CP structureof Higgs boson couplings to fermions is measured, resulting in an exclusion of the pure CP-odd structure of the top Yukawa coupling at 3.2σ\sigma. The measurements are based on a sample of proton-proton collisions at a center-of-mass energy s=\sqrt{s}= 13 TeV collected by the CMS detector at the LHC, corresponding to an integrated luminosity of 137 fb−1^{-1}. The cross section times branching fraction of the ttˉ\mathrm{t\bar{t}}H process is measured to be σttˉHBγγ=\sigma_{\mathrm{t\bar{t}H}}\mathcal{B}_{\gamma\gamma}= 1.56−0.32+0.34^{+0.34}_{-0.32} fb, which is compatible with the standard model prediction of 1.13−0.11+0.08^{+0.08}_{-0.11} fb. The fractional contribution of the CP-odd component is measured to be fCPHtt=f^{\mathrm{Htt}}_\mathrm{CP}= 0.00 ±\pm 0.33

    Structural dynamics in proteins induced by and probed with X-ray free-electron laser pulses

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    Nature Publishing Group UK
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    X-ray free-electron lasers (XFELs) enable crystallographic structure determination beyond the limitations imposed upon synchrotron measurements by radiation damage. The need for very short XFEL pulses is relieved through gating of Bragg diffraction by loss of crystalline order as damage progresses, but not if ionization events are spatially non-uniform due to underlying elemental distributions, as in biological samples. Indeed, correlated movements of iron and sulfur ions were observed in XFEL-irradiated ferredoxin microcrystals using unusually long pulses of 80 fs. Here, we report a femtosecond time-resolved X-ray pump/X-ray probe experiment on protein nanocrystals. We observe changes in the protein backbone and aromatic residues as well as disulfide bridges. Simulations show that the latter’s correlated structural dynamics are much slower than expected for the predicted high atomic charge states due to significant impact of ion caging and plasma electron screening. This indicates that dense-environment effects can strongly affect local radiation damage-induced structural dynamics

    Search for dijet resonances using events with three jets in proton-proton collisions at s=\sqrt{s} = 13 TeV

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    North-Holland Publ.
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    A search for a narrow resonance with a mass between 350 and 700 GeV, and decaying into a pair of jets, is performed using proton-proton collision events containing at least three jets. The data sample corresponds to an integrated luminosity of 18.3 fb −1 recorded at s=13TeV with the CMS detector. Data are collected with a technique known as “data scouting”, in which the events are reconstructed, selected, and recorded at a high rate in a compact form by the high-level trigger. The three-jet final state provides sensitivity to lower resonance masses than in previous searches using the data scouting technique. The spectrum of the dijet invariant mass, calculated from the two jets with the largest transverse momenta in the event, is used to search for a resonance. No significant excess over a smoothly falling background is found. Limits at 95% confidence level are set on the production cross section of a narrow dijet resonance and compared with the cross section of a vector dark matter mediator coupling to dark matter particles and quarks. Translating to a model where the narrow resonance interacts only with quarks, upper limits on this coupling range between 0.10 and 0.15, depending on the resonance mass. These results represent the most stringent upper limits in the mass range between 350 and 450 GeV obtained with a flavor-inclusive dijet resonance search

    A search for the standard model Higgs boson decaying to charm quarks

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    SISSA
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    A direct search for the standard model Higgs boson, H, produced in association with a vector boson, V (W or Z), and decaying to a charm quark pair is presented. The search uses a data set of proton-proton collisions corresponding to an integrated luminosity of 35.9 fb−1^{−1}, collected by the CMS experiment at the LHC in 2016, at a centre-of-mass energy of 13 TeV. The search is carried out in mutually exclusive channels targeting specific decays of the vector bosons: W → ℓν, Z → ℓℓ, and Z → νν, where ℓ is an electron or a muon. To fully exploit the topology of the H boson decay, two strategies are followed. In the first one, targeting lower vector boson transverse momentum, the H boson candidate is reconstructed via two resolved jets arising from the two charm quarks from the H boson decay. A second strategy identifies the case where the two charm quark jets from the H boson decay merge to form a single jet, which generally only occurs when the vector boson has higher transverse momentum. Both strategies make use of novel methods for charm jet identification, while jet substructure techniques are also exploited to suppress the background in the merged-jet topology. The two analyses are combined to yield a 95% confidence level observed (expected) upper limit on the cross section σ(VH)B(H→cc‾) \sigma \left(\mathrm{VH}\right)\mathrm{\mathcal{B}}\left(\mathrm{H}\to \mathrm{c}\overline{\mathrm{c}}\right) of 4.5 (2.4−0.7+1.0) \left({2.4}_{-0.7}^{+1.0}\right) pb, corresponding to 70 (37) times the standard model prediction.[graphic not available: see fulltext

    Study of excited Λb0\Lambda_\mathrm{b}^0 states decaying to Λb0π+π−\Lambda_\mathrm{b}^0\pi^+\pi^- in proton-proton collisions at s=\sqrt{s}= 13 TeV

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    A study of excited Λb0 baryons is reported, based on a data sample collected in 2016–2018 with the CMS detector at the LHC in proton-proton collisions at a center-of-mass energy of 13 TeV, corresponding to an integrated luminosity of up to 140 fb−1 . The existence of four excited Λb0 states: Λb0(5912)0 , Λb0(5920)0 , Λb0(6146)0 , and Λb0(6152)0 in the Λb0π+π− mass spectrum is confirmed, and their masses are measured. The Λb0π+π− mass distribution exhibits a broad excess of events in the region of 6040–6100 MeV, whose origin cannot be discerned with the present data

    Luminosity calibration with the Van der Meer method in proton-proton collisions at the CMS experiment

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    The luminosity measurement of the CMS experiment is calibrated under special beam conditions with beam separation scans called Van der Meer (VdM) scans. In a VdM scan, the two proton beams are separated transversely and moved in steps across each other. From the rate measurement of a luminosity detector as function of the transverse beam separation, the absolute luminosity scale is inferred and used to determine a calibration constant. The beam separation scale is calibrated with special length scale scans, and time-dependent variations are corrected for from measurements of beam position monitors. The rate measurement is normalized with the measured bunch currents. Electromagnetic interactions between the proton beams influence both the separation scale and the rate measurement. A bias occurs in the calibration procedure due to the assumption of factorizable bunch densities, and is corrected for using a reconstruction of the transverse bunch shapes from special beam-imaging scans

    Image of the Transverse Bunch Profile via Coherent Optical Transition Radiation

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    American Physical Society
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    The use of optical transition radiation (OTR) monitors is a standard technique to measure transverse beam profiles at many electron accelerators. With modern accelerator technology it is possible to produce and accelerate even ultrashort electron bunches with sub–femtosecond duration. Such bunches interacting with the OTR target generate coherent optical transition radiation (COTR). For the COTR case, a reconstruction of the bunch profile from a recorded image using a conventional optical scheme is a task with inconclusive solution. In this paper an approach is proposed which is based on the strict propagation of COTR fields through a focusing lens. As result a linear dependence of the measured rms image size on the bunch size is obtained

    Beam Loss Monitors

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    This lecture covers the fundamental aspects of the measurement of beam losses including their use for beam diagnostic and safety issues. The detailed functionality and detection principle of various common beam loss monitors are also presented, with a focus on their intrinsic sensitivity

    The O(α2)O(\alpha^2) Initial State QED Corrections to e+e−→γ∗/Z0∗e^+e^- \rightarrow \gamma^*/Z_0^*

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    North-Holland Publ. Co.
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    We calculate the complete O(α2)O(\alpha^2) initial state radiation corrections to e+e−e^+ e^- annihilation into a neutral vector boson in a direct analytic computation without any approximation. The corrections are represented in terms of iterated incomplete (elliptic) integrals over alphabets of square--root valued letters. Performing the limit s≫me2s \gg m_e^2, we find discrepancies with the earlier results of Ref.~\cite{Berends:1987ab} and confirm results obtained in Ref.~\cite{Blumlein:2011mi} where the effective method of massive operator matrix elements has been used, which works for all but the power corrections in me2/sm^2_e/s. In this way, we also confirm the validity of the factorization of massive partons in the Drell--Yan process. We add non--logarithmic terms at O(α2)O(\alpha^2) which have not been considered in previous calculations. The final results in the limit s≫me2s \gg m_e^2 can be given in terms of Nielsen integrals

    Search for high mass dijet resonances with a new background prediction method in proton-proton collisions at s=\sqrt{s} = 13 TeV

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    SISSA
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    A search for narrow and broad resonances with masses greater than 1.8 TeV decaying to a pair of jets is presented. The search uses proton-proton collision data at s=\sqrt{s} = 13 TeV collected at the LHC, corresponding to an integrated luminosity of 137 fb−1^{-1}. The background arising from standard model processes is predicted with the fit method used in previous publications and with a new method. The dijet invariant mass spectrum is well described by both data-driven methods, and no significant evidence for the production of new particles is observed. Model independent upper limits are reported on the production cross sections of narrow resonances, and broad resonances with widths up to 55% of the resonance mass. Limits are presented on the masses of narrow resonances from various models: string resonances, scalar diquarks, axigluons, colorons, excited quarks, color-octet scalars, W' and Z' bosons, Randall-Sundrum gravitons, and dark matter mediators. The limits on narrow resonances are improved by 200 to 800 GeV relative to those reported in previous CMS dijet resonance searches. The limits on dark matter mediators are presented as a function of the resonance mass and width, and on the associated coupling strength as a function of the mediator mass. These limits exclude at 95% confidence level a dark matter mediator with a mass of 1.8 TeV and width 1% of its mass or higher, up to one with a mass of 4.8 TeV and a width 45% of its mass or higher

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