Spin Correlation Measurements Research Articles

Inclusive and differential cross-section measurements of tt¯Z\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ t\\overline{t}Z $$\\end{document} production in pp collisions at s\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ \\sqrt{s} $$\\end{document} = 13 TeV with the ATLAS detector, including EFT and spin-correlation interpretations

Measurements of both the inclusive and differential production cross sections of a top-quark-top-antiquark pair in association with a Z boson (tt¯Z\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ t\\overline{t}Z $$\\end{document}) are presented. Final states with two, three or four isolated leptons (electrons or muons) are targeted. The measurements use the data recorded by the ATLAS detector in pp collisions at s\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ \\sqrt{s} $$\\end{document} = 13 TeV at the Large Hadron Collider during the years 2015–2018, corresponding to an integrated luminosity of 140 fb−1. The inclusive cross section is measured to be σtt¯Z\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ {\\sigma}_{t\\overline{t}Z} $$\\end{document} = 0.86 ± 0.04 (stat.) ± 0.04 (syst.) pb and found to be in agreement with the most advanced Standard Model predictions. The differential measurements are presented as a function of a number of observables that probe the kinematics of the tt¯Z\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ t\\overline{t}Z $$\\end{document} system. Both the absolute and normalised differential cross-section measurements are performed at particle level and parton level for specific fiducial volumes, and are compared with NLO+NNLL theoretical predictions. The results are interpreted in the framework of Standard Model effective field theory and used to set limits on a large number of dimension-6 operators involving the top quark. The first measurement of spin correlations in tt¯Z\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ t\\overline{t}Z $$\\end{document} events is presented: the results are in agreement with the Standard Model expectations, and the null hypothesis of no spin correlations is disfavoured with a significance of 1.8 standard deviations.

Energy-resolved spin correlation measurements: Decoding transverse spin dynamics in weakly interacting Fermi gases

Energy-resolved spin correlation measurements: Decoding transverse spin dynamics in weakly interacting Fermi gases

Crafting polarizations for top, W , and Z

We put forward a method to tune the polarization state of decaying heavy particles (top quarks and $W/Z$ bosons) in a preexisting Monte Carlo sample. With this technique, dubbed as ``custom angle replacement,'' the decay angular distributions are modified in such a way that the desired polarization state is reproduced, while the production kinematics are unchanged. A nontrivial test of this approach is presented for the top quark semileptonic decay $t\ensuremath{\rightarrow}Wb\ensuremath{\rightarrow}\ensuremath{\ell}\ensuremath{\nu}b$, with $\ensuremath{\ell}=e$, $\ensuremath{\mu}$, in which the decay distribution is four-dimensional and involves three different Lorentz frames. The proposed method can be used to obtain event samples with polarized heavy particles, as required by experimental measurements of polarization and spin correlations.

Measurement of polarization transfer in Møller scattering of relativistic electrons

A dedicated Mott polarimeter was designed to study the final spin state in M\o{}ller scattering of transversely polarized electron beams in the relativistic energy range. The polarization transfer was measured for two incident-beam polarization orientations with respect to the M\o{}ller scattering plane, at 3 MeV incident-beam energy. The results are found to be in agreement with the predictions of relativistic quantum mechanics. Estimates for the measurement of quantum spin correlations in M\o{}ller scattering, based on a simultaneous determination of the spin projections for both electrons in the final state, are also presented.

Lepton-Antineutrino Entanglement and Chiral Oscillations

Dirac bispinors belong to an irreducible representation of the complete Lorentz group, which includes parity as a symmetry yielding two intrinsic discrete degrees of freedom: chirality and spin. For massive particles, chirality is not dynamically conserved, which leads to chiral oscillations. In this contribution, we describe the effects of this intrinsic structure of Dirac bispinors on the quantum entanglement encoded in a lepton-antineutrino pair. We consider that the pair is generated through weak interactions, which are intrinsically chiral, such that in the initial state the lepton and the antineutrino have definite chirality but their spins are entangled. We show that chiral oscillations induce spin entanglement oscillations and redistribute the spin entanglement to chirality-spin correlations. Such a phenomenon is prominent if the momentum of the lepton is comparable with or smaller than its mass. We further show that a Bell-like spin observable exhibits the same behavior of the spin entanglement. Such correlations do not require the knowledge of the full density matrix. Our results show novel effects of the intrinsic bispinor structure and can be used as a basis for designing experiments to probe chiral oscillations via spin correlation measurements.

Measurements of top-quark pair spin correlations in the emu channel at sqrt{s} = 13 TeV using pp collisions in the ATLAS detector

A measurement of observables sensitive to spin correlations in tbar{t} production is presented, using 36.1 hbox {fb}^{-1} of pp collision data at sqrt{s} = 13 TeV recorded with the ATLAS detector at the Large Hadron Collider. Differential cross-sections are measured in events with exactly one electron and one muon with opposite-sign electric charge as a function of the azimuthal opening angle and the absolute difference in pseudorapidity between the electron and muon candidates in the laboratory frame. The azimuthal opening angle is also measured as a function of the invariant mass of the tbar{t} system. The measured differential cross-sections are compared to predictions by several NLO Monte Carlo generators and fixed-order calculations. The observed degree of spin correlation is somewhat higher than predicted by the generators used. The data are consistent with the prediction of one of the fixed-order calculations at NLO, but agree less well with higher-order predictions. Using these leptonic observables, a search is performed for pair production of supersymmetric top squarks decaying into Standard Model top quarks and light neutralinos. Top squark masses between 170 and 230 GeV are largely excluded at the 95% confidence level for kinematically allowed values of the neutralino mass.

Adiabatic spin-dependent momentum transfer in an SU( N ) degenerate Fermi gas

We introduce a spin-orbit coupling scheme, where a retro-reflected laser beam selectively diffracts two spin components in opposite directions. Spin sensitivity is provided by sweeping through a magnetic-field sensitive transition while dark states ensure that spontaneous emission remains low. The scheme is adiabatic and thus inherently robust. This tailored spin-orbit coupling allows simultaneous measurements of the spin and momentum distributions of a strontium degenerate Fermi gas, and thus opens the path to momentum-resolved spin correlation measurements on SU(N) quantum magnets.

A simulational model for witnessing quantum effects of gravity using IBM quantum computer

Witnessing quantum effects in the gravitational field is found to be exceptionally difficult in practice due to lack of empirical evidence. Hence, a debate is going on among physicists whether gravity has a quantum domain or not. There had been no successful experiments at all to show the quantum nature of gravity till two recent independent works by Bose et al. [Phys. Rev. Lett. 119, 240401 (2017)] and by Marletto and Vedral [Phy. Rev. Lett. 119, 240402 (2017)]. The authors have proposed schemes to test the quantumness of gravity in two small test masses by entangling two spatially separated objects using gravitational interactions. They provide a method to witness the entanglement using spin correlation measurements, which could imply evidence for gravity being a quantum coherent mediator. Here we propose a simulational model by providing a new quantum circuit for verifying the above schemes. We simulate the schemes for the first time in IBM's 5-qubit quantum chip 'ibmqx4' by developing a quantum system which shows effects analogous to quantum gravity and calculates the degree of entanglement of the spin correlation. The entanglement witness over a range is obtained for different experimental parameters.

Highlights of top-quark properties measurements at the ATLAS experiment

The top quark is the heaviest known fundamental particle. As it is the only quark that decays before it hadronizes, this gives a unique opportunity to probe the properties of pseudo-bare quarks. This proceeding focuses on a few recent precision measurements of top-quark properties in top-quark pair events by the ATLAS Collaboration using LHC proton-proton collision data. Among these is the measurement of color flow and measurement of spin correlations in tt‾ pair production. Measurements of the top-quark mass and searches for rare top-quark decays are also presented.

Search for a preferred frame of the photon

Polarization of photons is the key physical quantity in measurements of quantum spin correlations. According to a variant of quantum and classical electrodynamics formulated by us on the grounds of a hypothesis of existence of a preferred frame of reference (PF) a modification of the Malus law is expected if photonic states are frame dependent. A search for a quantum PF of the photon has been conducted by way of dedicated measurements of optical rotation using a high-precision commercial polarimeter. The data yield an upper limit of the PF velocity with respect to the Earth of $3.0\phantom{\rule{0.16em}{0ex}}\phantom{\rule{0.0pt}{0ex}}\mathrm{km}/\mathrm{s}$ at the time of the measurements, allowing us to reject the hypothesis that the cosmic microwave background radiation frame might be a PF for the photon.

Spin resonance and spin fluctuations in a quantum wire

This is a review of theoretical works on spin resonance in a quantum wire associated with the spin-orbit interaction. We demonstrate that the spin-orbit induced internal “magnetic field” leads to a narrow spin-flip resonance at low temperatures in the absence of an applied magnetic field. An applied dc magnetic field perpendicular to and small compared with the spin-orbit field enhances the resonance absorption by several orders of magnitude. The component of applied field parallel to the spin-orbit field separates the resonance frequencies of right and left movers and enables a linearly polarized ac electric field to produce a dynamic magnetization as well as electric and spin currents. We start with a simple model of noninteracting electrons and then consider the interaction that is not weak in 1d electron system. We show that electron spin resonance in the spin-orbit field persists in the Luttinger liquid. The interaction produces an additional singularity (cusp) in the spin-flip channel associated with the plasma oscillation. As it was shown earlier by Starykh and his coworkers, the interacting 1d electron system in the external field with sufficiently large parallel component becomes unstable with respect to the appearance of a spin-density wave. This instability suppresses the spin resonance. The observation of the electron spin resonance in a thin wires requires low temperature and high intensity of electromagnetic field in the terahertz diapason. The experiment satisfying these two requirements is possible but rather difficult. An alternative approach that does not require strong ac field is to study two-time correlations of the total spin of the wire with an optical method developed by Crooker and coworkers. We developed theory of such correlations. We prove that the correlation of the total spin component parallel to the internal magnetic field is dominant in systems with the developed spin-density waves but it vanishes in Luttinger liquid. Thus, the measurement of spin correlations is a diagnostic tool to distinguish between the two states of electronic liquid in the quantum wire.

Revealing compressed stops using high-momentum recoils

Searches for supersymmetric top quarks at the LHC have been making great progress in pushing sensitivity out to higher mass, but are famously plagued by gaps in coverage around lower-mass regions where the decay phase space is closing off. Within the common stop-NLSP / neutralino-LSP simplified model, the line in the mass plane where there is just enough phase space to produce an on-shell top quark remains almost completely unconstrained. Here, we show that is possible to define searches capable of probing a large patch of this difficult region, with S/B ~ 1 and significances often well beyond 5 sigma. The basic strategy is to leverage the large energy gain of LHC Run 2, leading to a sizable population of stop pair events recoiling against a hard jet. The recoil not only re-establishes a MET signature, but also leads to a distinctive anti-correlation between the MET and the recoil jet transverse vectors when the stops decay all-hadronically. Accounting for jet combinatorics, backgrounds, and imperfections in MET measurements, we estimate that Run 2 will already start to close the gap in exclusion sensitivity with the first few 10s of inverse-fb. By 300/fb, exclusion sensitivity may extend from stop masses of 550 GeV on the high side down to below 200 GeV on the low side, approaching the "stealth" point at m(stop) = m(top) and potentially overlapping with limits from top pair cross section and spin correlation measurements.

Measurement of spin correlation in top-antitop quark events and search for top squark pair production in pp collisions at √s=8 TeV using the ATLAS detector.

A measurement of spin correlation in tt[over ¯] production is presented using data collected with the ATLAS detector at the Large Hadron Collider in proton-proton collisions at a center-of-mass energy of 8TeV, corresponding to an integrated luminosity of 20.3 fb^{-1}. The correlation between the top and antitop quark spins is extracted from dilepton tt[over ¯] events by using the difference in the azimuthal angle between the two charged leptons in the laboratory frame. In the helicity basis the measured degree of correlation corresponds to A_{helicity}=0.38±0.04, in agreement with the standard model prediction. A search is performed for pair production of top squarks with masses close to the top quark mass decaying to predominantly right-handed top quarks and a light neutralino, the lightest supersymmetric particle. Top squarks with masses between the top quark mass and 191GeV are excluded at the 95% confidence level.

Measurements of spin correlation in top-antitop quark events from proton-proton collisions ats=7 TeVusing the ATLAS detector

Measurements of spin correlation in top quark pair production are presented using data collected with the ATLAS detector at the LHC with proton-proton collisions at a center-of-mass energy of 7 TeV, corresponding to an integrated luminosity of 4.6 fb$^{-1}$. Events are selected in final states with two charged leptons and at least two jets and in final states with one charged lepton and at least four jets. Four different observables sensitive to different properties of the top quark pair production mechanism are used to extract the correlation between the top and antitop quark spins. Some of these observables are measured for the first time. The measurements are in good agreement with the Standard Model prediction at next-to-leading-order accuracy.

Top Quark Pair Properties - Spin Correlation, Charge Asymmetry, and Complex Final States - at ATLAS

We present measurements of top quark pair properties performed with the ATLAS detector at the Large Hadron Collider in proton-proton collisions at a center-of-mass energy of √s = 7 TeV. The latest measurements of spin correlation and charge asymmetry in events, as well as measurements of the cross section for production in association with vector bosons, are presented.

First measurements of spin correlations in the [formula omitted] reaction

The transverse spin correlations Ax,x and Ay,y in the n→p→→dπ0 reaction have been measured for the first time in quasi-free kinematics at the COSY-ANKE facility using a polarised deuteron beam incident on a polarised hydrogen cell target. The results obtained for neutron energies close to 353 MeV and 600 MeV are in good agreement with the partial wave analysis of data on the isospin-related pp→dπ+ reaction, though the present results cover kinematic regions that are largely absent from existing data sets.

Measurement of Spin Correlations in Top Pair Events in the Dilepton Channels in pp Collisions at 7 TeV

We present a measurement of the spin correlation of top-antitop-pair events produced in pp collisions at = 7 TeV. The data used by this analysis corresponds to an integrated luminosity of 5.0 fb−1 collected with the CMS detector at the LHC. The spin correlation in top-antitop-pair events is extracted from a fit to the angular distribution between the two selected leptons. In the helicity basis, the correlation coefficient is found to be = 0.24±0.02 ±0.08 and agrees with the Standard Model prediction.

Top quark physics in the vector color-octet model

We study and constrain the parameter space of the vector color-octet model from the observed data at the Tevatron by studying the top quark pair production and associated observables ${A}_{\mathrm{FB}}^{t\overline{t}}$ and spin correlation. In particular we study the invariant mass and rapidity dependence of ${A}_{\mathrm{FB}}^{t\overline{t}}$ at the Tevatron. In addition to the flavor conserving (FC) couplings we extend our study to include the flavor violating (FV) coupling involving the first and third generation quarks for both these processes. In order to ensure that we remain within the constraints imposed by the LHC data, we analyze the charge asymmetry, ${p}_{T}$ spectrum and invariant mass in the $t\overline{t}$ production data at the LHC. The constraints from the dijet resonance searches performed by the LHC are also considered. We also explore the contribution of this model to the single top quark production mediated by charged and neutral color-octet vector bosons. FV couplings introduced then induce the same-sign top-pair production process which is analyzed for both the hadron colliders. We have incorporated the effect of finite decay width of color octets on these processes. We find that it is possible to explain the observed ${A}_{\mathrm{FB}}^{t\overline{t}}$ anomaly in the color-octet vector model without transgressing the production cross sections of all these processes both through FC and FV couplings at the Tevatron. We predict best point sets in the model parameter space for specific choices of color-octet masses corresponding to ${\ensuremath{\chi}}_{\mathrm{min}}^{2}$ evaluated using the ${m}_{t\overline{t}}$ and $|\ensuremath{\Delta}y|$ spectrum of ${A}_{\mathrm{FB}}^{t\overline{t}}$ from the observed data set of Run II of the Tevatron at the integrated luminosity $8.7\text{ }\text{ }{\mathrm{fb}}^{\ensuremath{-}1}$. We find that the single top quark production is more sensitive to the FC and FV couplings in comparison to the top-pair production. We provide 95% exclusion contours on the plane of FV chiral couplings from the recent data from Tevatron, CMS and ATLAS corresponding to the nonobservability of large same-sign dilepton events. The four observed point sets are consistent with the cross section, charge asymmetry and spin-correlation measurements for $t\overline{t}$ production and dijet searches at the LHC.

A new twist on top quark spin correlations

Top-antitop pairs produced at hadron colliders are largely unpolarized, but their spins are highly correlated. The structure of these correlations varies significantly over top production phase space, allowing very detailed tests of the Standard Model. Here, we explore top quark spin correlation measurement from a general perspective, highlighting the role of azimuthal decay angles. By taking differences and sums of these angles about the top-antitop production axis, the presence of spin correlations can be seen as sinusoidal modulations resulting from the interference of different helicity channels. At the LHC, these modulations exhibit nontrivial evolution from near-threshold production into the boosted regime, where they become sensitive to almost the entire QCD correlation effect for centrally produced tops. We demonstrate that this form of spin correlation measurement is very robust under full kinematic reconstruction, and should already be observable with high significance using the current LHC data set. We also illustrate some novel ways that new physics can alter the azimuthal distributions. In particular, we estimate the power of our proposed measurements in probing for anomalous color-dipole operators, as well as for broad resonances with parity-violating couplings. Using these methods, the 2012 run of the LHC may be capable of setting simultaneous limits on the top quark’s anomalous chromomagnetic and chromoelectric dipole moments at the level of 3 × 10−18 cm (0.03/m t ).

W helicity, top quark spin and charge

An overview of the recent measurements of top quark charge, spin correlations, and top quark and W boson polarisation in tevents is presented. The measurements were performed at the Fermilab Tevatron collider in p ¯ p collision data at p s = 1:96 TeV by the CDF and D0 experiments and in pp collision data at p s = 7 TeV by the ATLAS and CMS experiments