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Charged-hadron and identified-hadron yield measurements in photonuclear Pb+Pb and p+Pb collisions at = 5.02 TeV with ATLAS
No full textThis paper presents the measurement of charged-hadron and identified-hadron yields in photonuclear collisions using 1.7 nb of = 5.02TeV Pb+Pb data collected in 2018 with the ATLAS detector at the Large Hadron Collider. Candidate photonuclear events are selected using a combination of tracking and calorimeter information, including the zero-degree calorimeter. The yields as a function of transverse momentum and rapidity are measured in these photonuclear collisions as a function of charged-particle multiplicity. These photonuclear results are compared with 0.1nb of = 5.02 TeV p+Pb data collected in 2016 by ATLAS using similar charged-particle multiplicity selections. These photonuclear measurements shed light on potential quark-gluon plasma formation in photonuclear collisions via observables sensitive to radial flow, enhanced baryon-to-meson ratios, and strangeness enhancement. The results are also compared with the Monte Carlo dpmjet-iii generator and hydrodynamic calculations to test whether such photonuclear collisions may produce small droplets of quark-gluon plasma that flow collectively
Precision meets sustainability at the LHC: extraction of fundamental parameters of the Standard Model and machine learning techniques in the simulation
No full textThis thesis presents precise determinations of two fundamental parameters of the stan-dard model (SM), the strong coupling constant αS (mZ) and the top quark mass mtalong with its width Γt. These contribute importantly to constraints on the stabilityof the SM electroweak vacuum.For the determination of αS (mZ), CMS inclusive jet measurements using LHC proton-proton (pp) collisions at centre-of-mass energies √s = 2.76, 7, 8, and 13 TeV are anal-ysed together, for the first time. This has been made possible by detailed studies ofthe correlations between the different data sets. The CMS jet data are combined withHERA deep inelastic scattering measurements to extract the parton distribution func-tions (PDFs) and αS (mZ), simultaneously. This approach properly accounts for thecorrelation between PDFs and αS (mZ). The resulting value, αS (mZ) = 0.1176+0.0014−0.0016,is the most precise value of αS (mZ) from jet rates to date and has been achievedthrough a comprehensive QCD analysis at next-to-next-to-leading order. Further, therunning of αS up to an energy scale of 1.6 TeV is probed.The measurement of the top quark mass parameter in the Monte Carlo (MC) simula-tion mMCt and Γt from the unfolded differential cross section of top quark-antiquark(tt) and single top quark production in association with a W boson (tW) is per-formed. This analysis uses LHC pp collision data at √s = 13 TeV, collected by theCMS experiment during 2017–2018. Events in the dilepton decay channel are selected.The differential cross section as a function of the invariant mass of the lepton and bquark, mℓb, is unfolded to the particle level. This analysis is the first of its kind us-ing CMS data, employing the state-of-the-art event generator bb4l, which simulatespp → bbℓ+ℓ−ν ¯ν final states and takes into account the interference between tt andtW production. The precision of this measurement is estimated using Asimov pseudo-data, resulting in mMCt = 172.61+0.41−0.44 GeV and Γt = 1.36+0.24−0.28 GeV. The mMCt resultis as precise as the most accurate single-experiment direct mMCt measurement and willpresent the first determination of mMCt from the combined tt and tW cross sectionsfrom the CMS Collaboration once the analysis is unblinded. Further, this analysispromises improved precision as compared to direct measurements of Γt obtained withthe bb4l method.Finally, a machine learning (ML) technique is presented, developed to reweight MCsimulations obtained with a particular set of model parameters to simulations withalternative values of these parameters, or to simulations based on an entirely differentmodel. The reweighting is performed at the generator level by applying the outputof the ML algorithm, stored as weights, to the nominal MC simulation. As a result,detailed detector simulation and event reconstruction are not needed for alternativeMC samples, significantly reducing computational costs by up to 75%. The perfor-mance of the method is studied in simulated tt production and results are presentedfor reweighting to model variations and higher-order calculations. This ML-basedreweighting is already used by the CMS experiment and will facilitate precision mea-surements at the High-Luminosity LHC
Local Dzyaloshinskii-Moriya Interactions Driving Quasi-2D Magnetism in a Centrosymmetric Nanoskyrmion Material
No full textThe unabating discovery of nanoskyrmions in centrosymmetric magnets challenges the conventional Dzyaloshinskii-Moriya (DM) skyrmion stabilization mechanism. We investigate GdPdSi using polarized resonant x-ray scattering and find that the low-field incommensurate modulations are elliptical helices, evolving into spin-density waves at higher fields. Quasi-2D magnetism arises via local DM interactions generated by inversion symmetry breaking around Gd-Gd bonds, which we characterize using atomistic simulations. Our findings suggest a prominent “hidden” role of DM interactions even in centrosymmetric skyrmionic hosts
Evidence for Similar Collectivity of High Transverse-Momentum Particles in p-Pb and Pb-Pb Collisions
No full textCharged hadron elliptic anisotropies (v2) are presented over a wide transverse momentum (pT) range for proton-lead (pPb) and lead-lead (PbPb) collisions at nucleon-nucleon center-of-mass energies of 8.16 and 5.02 TeV, respectively. The data were recorded by the CMS experiment and correspond to integrated luminosities of 186 and 0.607 nb-1 for the pPb and PbPb systems, respectively. A four-particle cumulant analysis is performed using subevents separated in pseudorapidity to effectively suppress noncollective effects. At high pT (pT>8 GeV), significant positive v2 values that are similar between pPb and PbPb collisions at comparable charged particle multiplicities are observed. This observation suggests a common origin for the multiparticle collectivity for high-pT particles in the two systems
Molecular fluctuations in mixed-metal MOF-74: influence of the metal composition
No full textA selected series of metal–organic frameworks M-MOF-74 (M = Mg, Co, Ni) and mixed metal MM-MOF-74 (Mg/Co or Mg/Ni) with different compositions of metal atoms have been prepared and further investigated by broadband dielectric spectroscopy (BDS) in a wide temperature range. The dielectric spectra show at least two relaxation processes. Process-A is observed only for the Ni-containing MOFs and is attributed to localized fluctuations of the metal oxide corners. Relaxation processes-B and -C are observed for all prepared MOFs, except that process-B is not observed for Ni-MOF-74. Large-angle fluctuations such as free rotations of the linkers can be excluded due to the structure of MOF-74, but small-angle fluctuations such as torsions are possible. According to numerical simulations carried out for MOF-74, process-B can be attributed to inward and outward fluctuations of the linkers relative to the pore center. Process-C is related to small-angle rotational fluctuations of the linker together with co-rotations of the metal nodes. The latter interpretation is supported by the dependence of the activation energy of the relaxation rate of process-C on the metal composition of the MOFs, which is discussed in terms of the bond lengths between the metal atoms and the linker which decrease in the sequence Mg, Co and Ni
Pressure‐Driven Reactivity in Dense Methane‐Nitrogen Mixtures
No full textCarbon, nitrogen, and hydrogen are among the most abundant elements in the solar system, and our understanding of their interactions is fundamental to prebiotic chemistry. CH and N are the simplest archetypical molecules formed by these elements and are both markedly stable under extremes of pressure. Through a series of diamond anvil cell experiments supported by density functional theory calculations, we observe diverse compound formation and reactivity in the CH-N binary system at high pressure. Above 7 GPa two concentration-dependent molecular compounds emerge, (CH)N and (CH)7(N2)8, held together by weak van der Waals interactions. Strikingly, further compression at room temperature irreversibly breaks the N2 triple bond, inducing the dissociation of CH above 140 GPa, with the near-quenched samples revealing distinct spectroscopic signatures of strong covalently bonded C−N−H networks. High temperatures vastly reduce the required pressure to promote the reactivity between CH and N, with NH forming together with longer-chain hydrocarbons at 14 GPa and 670 K, further decomposing into powdered diamond when temperatures exceed 1200 K. These results exemplify how pressure-driven chemistry can cause unexpected complexity in the most simple molecular precursors
Deformation mechanisms of L-PBF-processed Ti-6Al-4V investigated using a combined experimental and simulation approach
No full textDespite the significant application potential of laser powder bed fusion (L-PBF) processed Ti-6Al-4V components, a detailed understanding of their deformation mechanisms remains limited. This study investigates the deformation behavior of the {α^\prime} and α phases in the as-built and heat-treated specimens, respectively, using in-situ high-energy X-ray diffraction (HEXRD) combined with crystal plasticity modeling. Both phases exhibited similar elastic anisotropy, with the highest modulus along \{00.2\} and the lowest along \{10.0\}, although the α phase consistently showed higher directional moduli than the {α^\prime} phase. Their plastic deformation responses differed markedly: in the as-built {α^\prime} phase, slip activation followed the sequence prismatic \rightarrow basal \rightarrow pyramidal I \langle c+a \rangle, whereas in the heat-treated α phase, the sequence was basal \rightarrow prismatic \rightarrow pyramidal I \langle c+a \rangle. Analyses of full width at half maximum (FWHM) and diffraction peak intensities further supported these observations. Finally, inverse modeling within a crystal plasticity framework was employed to determine slip family--specific critical resolved shear stresses (CRSS), revealing higher CRSS values in the {α^\prime} phase for all slip systems except the prismatic family
Reconfigurable Soft Actuators Constructed via Layer-by-layer Assembly
No full textStimuli-responsive actuators are candidates for the development of soft robots due to their shape deformation and environmental adaptation capabilities. Adaptation to environmental stimuli not only enables complex shape reprogramming but also benefits the recovery of mechanical injuries of the soft actuators. Nevertheless, reports on soft actuators that integrate shape-reprogramming and injury-healing functions into a monolithic actuating material through facile fabrication strategies remain scarce. Herein, a stimuli-responsive and healable actuator is developed via layer-by-layer casting of two stimuli-responsive materials with complementary properties. Upon specific stimulation, these two materials reorganize their structural network at the nano- and microscales and heal. The resulting actuator exhibits a robust photo-responsive actuating strength due to the asymmetric volumetric responses of the two layers. Importantly, the whole actuator can be healed with the aid of a sequential heating-humidifying–drying treatment, achieving excellent healing efficiency in both mechanical strength (72%) and actuating strength (95%). Moreover, the initial actuation modes can be restored and diversified through humidifying or heating-assisted welding procedures, respectively. This work demonstrates a facile strategy to construct reprogrammable actuators with healing and welding abilities stimulated by two environmental stimuli and provides a platform for developing adaptable materials to a changing environment
A versatile and transportable endstation for controlled molecule experiments
No full textWe report on a new versatile transportable endstation for controlled molecule (eCOMO) experiments providing a combination of molecular beam purification by electrostatic deflection and simultaneous ion and electron detection using velocity-map imaging (VMI). The -type electrostatic deflector provides spatial dispersion of species based on their effective-dipole-moment-to-mass ratio. This enables selective investigation of molecular rotational quantum states, conformers, and molecular clusters. Furthermore, the double-sided VMI spectrometer equipped with two high-temporal-resolution event-driven Timepix3 cameras provides detection of all generated ions independently of their mass-over-charge ratio and electrons. To demonstrate the potential of this novel apparatus, we present experimental results from our investigation of carbonyl sulfide (OCS) after ionization. Specifically, we provide the characterization of the molecular beam, electrostatic deflector, and electron- and ion-VMI spectrometer. The eCOMO endstation delivers a platform for ultrafast dynamics studies using a wide range of light sources from table-top lasers to free-electron-laser and synchrotron-radiation facilities. This makes it suitable for research activities spanning from atomic, molecular, and cluster physics, to energy science and chemistry, to structural biology
TelePix2: Full scale fast region of interest trigger and timing for the EUDET-style telescopes at the DESY II Test Beam Facility
No full textWith increasing demands by future and current upgrades of particle physics experiments on rate capabilities and time resolution, the requirements on test beams are also increasing. The current infrastructure at the DESY II test beam facility includes particle tracking telescopes with long integration times, no additional timing but excellent spatial resolution. This results in readouts with multiple particles per trigger, causing ambiguities in tracking and assigning particles to triggers. Also, it is likely not to trigger on particles that pass through a small device under test, leading to inefficient data taking. These issues can be solved by adding TelePix2 as a timing and flexible region of interest trigger layer. TelePix2 is a full scale HV-CMOS chip based on the successful small scale prototype TelePix. The DAQ system and the sensors performance featuring efficiencies above 99 % and a time resolution of 3.844(2) ns are presented. The integration into EUDAQ2 and the AIDA-TLU to seamlessly work in the test beam environment as well as into the analysis chain is described. First successful use cases are highlighted to conclude that TelePix2 is a well-suited timing and trigger layer for test beam