We present the first measurement of the electron angular distribution parameter alpha_2 in W to e nu events produced in proton-antiproton collisions as a function of the W boson transverse momentum. Our analysis is based on data collected using the D0 detector during the 1994--1995 Fermilab Tevatron run. We compare our results with next-to-leading order perturbative QCD, which predicts an angular distribution of (1 +/- alpha_1 cos theta* + alpha_2 cos^2 theta*), where theta* is the polar angle of the electron in the Collins-Soper frame. In the presence of QCD corrections, the parameters alpha_1 and alpha_2 become functions of p_T^W, the W boson transverse momentum. This measurement provides a test of next-to-leading order QCD corrections which are a non-negligible contribution to the W boson mass measurement.
Angular distributions of the emitted charged lepton is fitted to the formula d(sig)/d(pt**2)/dy/d(cos(theta*)) = const*(1 +- alpha_1*cos(theta*) + alpha_2*(cos(theta*))**2). The angle theta* is measured in the Collins-Soper frame. alpha_1 velues are calculated based on the measured PT(W) of each event. Possible variations of alpha_1 are treated as a source of systematic uncertainty.
The associated production of a Higgs boson with a $W$ or $Z$ boson decaying into leptons and where the Higgs boson decays to a $b\bar{b}$ pair is measured in the high vector-boson transverse momentum regime, above 250 GeV, with the ATLAS detector. The analysed data, corresponding to an integrated luminosity of 139 fb$^{-1}$, were collected in proton-proton collisions at the Large Hadron Collider between 2015 and 2018 at a centre-of-mass energy of $\sqrt{s} = 13$ TeV. The measured signal strength, defined as the ratio of the measured signal yield to that predicted by the Standard Model, is $0.72 ^{+0.39}_{-0.36}$ corresponding to an observed (expected) significance of 2.1 (2.7) standard deviations. Cross-sections of associated production of a Higgs boson decaying into $b$ quark pairs with a $W$ or $Z$ gauge boson, decaying into leptons, are measured in two exclusive vector boson transverse momentum regions, 250-400 GeV and above 400 GeV, and interpreted as constraints on anomalous couplings in the framework of a Standard Model effective field theory.
Observed correlations between the measured reduced stage-1.2 simplified template VH, V->leptons and H->bb cross sections, including both the statistical and systematic uncertainties.
Measured and predicted VH, V->leptons reduced stage-1.2 simplified template cross sections times the H->bb and V->leptons branching fractions with corresponding uncertainties. All possible Z decays into neutral and charged leptons are considered.
Linear combinations of Wilson coefficients corresponding to the principal component decomposition eigenvectors. The corresponding eigenvalues, representing in the gaussian approximation the inverse uncertainty square of the measured eigenvector, is also indicated.
Differential measurements of charged particle azimuthal anisotropy are presented for lead-lead collisions at sqrt(s_NN) = 2.76 TeV with the ATLAS detector at the LHC, based on an integrated luminosity of approximately 8 mb^-1. This anisotropy is characterized via a Fourier expansion of the distribution of charged particles in azimuthal angle (phi), with the coefficients v_n denoting the magnitude of the anisotropy. Significant v_2-v_6 values are obtained as a function of transverse momentum (0.5<pT<20 GeV), pseudorapidity (|eta|<2.5) and centrality using an event plane method. The v_n values for n>=3 are found to vary weakly with both eta and centrality, and their pT dependencies are found to follow an approximate scaling relation, v_n^{1/n}(pT) \propto v_2^{1/2}(pT). A Fourier analysis of the charged particle pair distribution in relative azimuthal angle (Dphi=phi_a-phi_b) is performed to extract the coefficients v_{n,n}=<cos (n Dphi)>. For pairs of charged particles with a large pseudorapidity gap (|Deta=eta_a-eta_b|>2) and one particle with pT<3 GeV, the v_{2,2}-v_{6,6} values are found to factorize as v_{n,n}(pT^a,pT^b) ~ v_n(pT^a)v_n(pT^b) in central and mid-central events. Such factorization suggests that these values of v_{2,2}-v_{6,6} are primarily due to the response of the created matter to the fluctuations in the geometry of the initial state. A detailed study shows that the v_{1,1}(pT^a,pT^b) data are consistent with the combined contributions from a rapidity-even v_1 and global momentum conservation. A two-component fit is used to extract the v_1 contribution. The extracted v_1 is observed to cross zero at pT\sim1.0 GeV, reaches a maximum at 4-5 GeV with a value comparable to that for v_3, and decreases at higher pT.
The EP Resolution Factor vs. Centrality for n values from2 to 6.
The Chi Reolution Factor vs. Centrality for n values from 2 to 6.
The one-dimensional Delta(PHI) correlation function vs Delta(PHI) for |DETARAP| in the range 2 to 5 summed over all n values from 1 to 6.
Measurements of the azimuthal anisotropy in lead-lead collisions at $\sqrt{s_\mathrm{NN}} = 5.02$ TeV are presented using a data sample corresponding to 0.49 $\mathrm{nb}^{-1}$ integrated luminosity collected by the ATLAS experiment at the LHC in 2015. The recorded minimum-bias sample is enhanced by triggers for "ultra-central" collisions, providing an opportunity to perform detailed study of flow harmonics in the regime where the initial state is dominated by fluctuations. The anisotropy of the charged-particle azimuthal angle distributions is characterized by the Fourier coefficients, $v_{2}-v_{7}$, which are measured using the two-particle correlation, scalar-product and event-plane methods. The goal of the paper is to provide measurements of the differential as well as integrated flow harmonics $v_{n}$ over wide ranges of the transverse momentum, 0.5 $
The V2 harmonic measured with the scalar product method as a funtion of transverse momentum in centrality bin 0-0.1%
The V2 harmonic measured with the scalar product method as a funtion of transverse momentum in centrality bin 0-1%
The V2 harmonic measured with the scalar product method as a funtion of transverse momentum in centrality bin 0-5%
This paper presents a measurement of the ordering of charged hadrons in the azimuthal angle relative to the beam axis in high-energy proton-proton collisions at the Large Hadron Collider (LHC). A spectral analysis of correlations between longitudinal and transverse components of the momentum of the charged hadrons, driven by the search for phenomena related to the structure of the QCD field, is performed. Data were recorded with the ATLAS detector at centre-of-mass energies of sqrt(s) = 900 GeV and sqrt(s) = 7 TeV. The correlations measured in a phase space region dominated by low-pT particles are not well described by conventional models of hadron production. The measured spectra show features consistent with the fragmentation of a QCD string represented by a helix-like ordered gluon chain.
The corrected data for the power spectra S_E for the three different data samples at a centre-of-mass energy of 7 TeV.
The corrected data for the power spectra S_ETA for the three different data samples at a centre-of-mass energy of 7 TeV.
The corrected data for the power spectra S_E for the three different data samples at a centre-of-mass energy of 900 GeV.
The b-hadron production cross section is measured with the ATLAS detector in pp collisions at sqrt(s) = 7 TeV, using 3.3 pb^-1 of integrated luminosity, collected during the 2010 LHC run. The b-hadrons are selected by partially reconstructing D*muX final states. Differential cross sections are measured as functions of the transverse momentum and pseudorapidity. The measured production cross section for a b-hadron with pT>9 GeV and |eta|<2.5 is 32.7 pm 0.8 (stat) ^{+4.5}_{-6.8} (syst) ub, higher than the next-to-leading-order QCD predictions but consistent within the experimental and theoretical uncertainties.
$b$ hadron $p_\perp$ at $\sqrt{s}=7$ TeV.
$b$ hadron $\eta$ at $\sqrt{s}=7$ TeV.
The integrated elliptic flow of charged particles produced in Pb+Pb collisions at sqrt(s_NN)=2.76 TeV has been measured with the ATLAS detector using data collected at the Large Hadron Collider. The anisotropy parameter, v_2, was measured in the pseudorapidity range |eta| <= 2.5 with the event-plane method. In order to include tracks with very low transverse momentum pT, thus reducing the uncertainty in v_2 integrated over pT, a 1 mu b-1 data sample without a magnetic field in the tracking detectors is used. The centrality dependence of the integrated v_2 is compared to other measurements obtained with higher pT thresholds. A weak pseudorapidity dependence of the integrated elliptic flow is observed for central collisions, and a small decrease when moving away from mid-rapidity is observed only in peripheral collisions. The integrated v2 transformed to the rest frame of one of the colliding nuclei is compared to the lower-energy RHIC data.
Monte Carlo evaluation of the tracklet reconstruction efficiency as a function of pseudorapidity for the 0-10% centraliry interval.
Monte Carlo evaluation of the tracklet reconstruction efficiency as a function of pseudorapidity for the 40-50% centraliry interval.
Monte Carlo evaluation of the tracklet reconstruction efficiency as a function of pseudorapidity for the 70-80% centraliry interval.
The ATLAS experiment at the LHC has measured the centrality dependence of charged particle pseudorapidity distributions over |eta| < 2 in lead-lead collisions at a nucleon-nucleon centre-of-mass energy of sqrt(s_NN) = 2.76 TeV. In order to include particles with transverse momentum as low as 30 MeV, the data were recorded with the central solenoid magnet off. Charged particles were reconstructed with two algorithms (2-point 'tracklets' and full tracks) using information from the pixel detector only. The lead-lead collision centrality was characterized by the total transverse energy in the forward calorimeter in the range 3.2 < |eta| < 4.9. Measurements are presented of the per-event charged particle density distribution, dN_ch/deta, and the average charged particle multiplicity in the pseudorapidity interval |eta|<0.5 in several intervals of collision centrality. The results are compared to previous mid-rapidity measurements at the LHC and RHIC. The variation of the mid-rapidity charged particle yield per colliding nucleon pair with the number of participants is consistent with the lower sqrt(s_NN) results. The shape of the dN_ch/deta distribution is found to be independent of centrality within the systematic uncertainties of the measurement.
The measured charged particle density distributions as a fuinction of pseudorapidity in the centrality regions 0-10, 10-20, 20-30 and 30-40 %.
The measured charged particle density distributions as a fuinction of pseudorapidity in the centrality regions 40-50, 50-60, 60-70 and 70-80 %.
Mean values of the charged particle multiplicities in the pseudorapidiy range -0.5-0.5 as a function of centrality. N(C=PART), the number of participating nucleons in the collision, is also shown, determined from the muliplicity and ET of the event, with which it has been shown to be strongly correlated.
The centrality dependence of the mean charged-particle multiplicity as a function of pseudorapidity is measured in approximately 1 $\mu$b$^{-1}$ of proton--lead collisions at a nucleon--nucleon centre-of-mass energy of $\sqrt{s_{_{\rm{NN}}}} = 5.02$ TeV using the ATLAS detector at the Large Hadron Collider. Charged particles with absolute pseudorapidity less than 2.7 are reconstructed using the ATLAS pixel detector. The $p$+Pb collision centrality is characterised by the total transverse energy measured in the Pb-going direction of the forward calorimeter. The charged-particle pseudorapidity distributions are found to vary strongly with centrality, with an increasing asymmetry between the proton-going and Pb-going directions as the collisions become more central. Three different estimations of the number of nucleons participating in the $p$+Pb collision have been carried out using the Glauber model as well as two Glauber--Gribov inspired extensions to the Glauber model. Charged-particle multiplicities per participant pair are found to vary differently for these three models, highlighting the importance of including colour fluctuations in nucleon--nucleon collisions in the modelling of the initial state of $p$+Pb collisions.
The $\langle N_{\mathrm{part}} \rangle$ values and their uncertainties for centrality intervals used in this analysis together with asymmetric systematic uncertainties for Glauber model, GGFC with $\omega$=0.11 and GGFC with $\omega$=0.2.
Centrality dependence of the charged particle pseudorapidity distribution measured in several centrality intervals for charged particles with $p_{T} > 0.1$ GeV. The first uncertainty is statistical the second systematic.
Centrality dependence of the charged particle pseudorapidity distribution measured in several centrality intervals for charged particles with $p_{T} > 0$ GeV. The first uncertainty is statistical the second systematic.
In the $pp \rightarrow t\bar{t}$ process the angular distributions of top and anti-top quarks are expected to present a subtle difference, which could be enhanced by processes not included in the Standard Model. This Letter presents a measurement of the charge asymmetry in events where the top-quark pair is produced with a large invariant mass. The analysis is performed on 20.3 fb$^{-1}$ of $pp$ collision data at $\sqrt{s} =$ 8 TeV collected by the ATLAS experiment at the LHC, using reconstruction techniques specifically designed for the decay topology of highly boosted top quarks. The charge asymmetry in a fiducial region with large invariant mass of the top-quark pair ($m_{t\bar{t}} > $ 0.75 TeV) and an absolute rapidity difference of the top and anti-top quark candidates within $-$2 $ < |y_t| - |y_{\bar{t}}| <$ 2 is measured to be 4.2 $\pm$ 3.2%, in agreement with the Standard Model prediction at next-to-leading order. A differential measurement in three $t\bar{t}$ mass bins is also presented.
The measured charge asymmetry after the unfolding to parton level in four intervals of the invariant mass of the $t\bar{t}$ system. The phase space is limited to $|(\Delta |y|)|<$ 2. The uncertainties correspond to the sum in quadrature of statistical and systematic uncertainties (for the data) or to the theory uncertainty (for the SM prediction).
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