High-precision measurement of the W boson mass with the CDF II detector

The mass of the W boson, a mediator of the weak force between elementary particles, is tightly constrained by the symmetries of the standard model of particle physics. The Higgs boson was the last missing component of the model. After observation of the Higgs boson, a measurement of the W boson mass provides a stringent test of the model. We measure the W boson mass, M-W, using data corresponding to 8.8 inverse femtobarns of integrated luminosity collected in proton-antiproton collisions at a 1.96 tera-electron volt center-of-mass energy with the CDF II detector at the Fermilab Tevatron collider. A sample of approximately 4 million W boson candidates is used to obtain M-W = 80,433.5 +/- 6.4(stat) +/- 6.9(syst) = 80,433.5 +/- 9.4MeV/c(2), the precision of which exceeds that of all previous measurements combined (stat, statistical uncertainty; syst, systematic uncertainty; MeV, mega-electron volts; c, speed of light in a vacuum). This measurement is in significant tension with the standard model expectation.Peer reviewe

Neutral pion and eta meson production at midrapidity in Pb-Pb collisions at root S-NN=2.76 TeV

Neutral pion and eta meson production in the transverse momentum range 1 < p(T) < 20 GeV/c have been measured at midrapidity by the ALICE experiment at the Large Hadron Collider (LHC) in central and semicentral Pb-Pb collisions at root S-NN = 2.76 TeV. These results were obtained using the photon conversion method as well as the Photon Spectrometer (PHOS) and Electromagnetic Calorimeter detectors. The results extend the upper p(T) reach of the previous ALICE pi(0) measurements from 12 to 20 GeV/c and present the first measurement of eta meson production in heavy-ion collisions at the LHC. The eta/pi(0) ratio is similar for the two centralities and reaches at high p(T) a plateau value of 0.457 +/- 0.013(stat) +/- 0.018(syst) A suppression of similar magnitude for pi(0) and eta meson production is observed in Pb-Pb collisions with respect to their production in pp collisions scaled by the number of binary nucleon-nucleon collisions. We discuss the results in terms of Next to Leading Order (NLO) pQCD predictions and hydrodynamic models. The measurements show a stronger suppression than observed at lower center-of-mass energies in the p T range 6 < p(T) < 10 GeV/c. For p(T) < 3 GeV/c, hadronization models describe the pi(0) results while for the eta some tension is observed.Peer reviewe

Lambda(+)(C) production in pb-pb collisions at root S-NN=5.02 TeV

A measurement of the production of prompt Lambda(+)(C) baryons in Pb-Pb collisions at root S-NN = 5.02 TeV with the ALICE detector at the LHC is reported. The Lambda(+)(C) and (Lambda) over bar (-)(C) were reconstructed at midrapidity (vertical bar y vertical bar pK(S)(0) (and charge conjugate) in the transverse momentum and centrality intervals 6 < p(T) < 12 GeV/c and 0-80%. The Lambda(+)(C)/D-0 ratio, which is sensitive to the charm quark hadronisation mechanisms in the medium, is measured and found to be larger than the ratio measured in minimum-bias pp collisions at root s = 7 TeV and in p-Pb collisions at root S-NN = 5.02 TeV. In particular, the values in p-Pb and Pb-Pb collisions differ by about two standard deviations of the combined statistical and systematic uncertainties in the common P-T interval covered by the measurements in the two collision systems. The Lambda(+)(C)/D-0 ratio is also compared with model calculations including different implementations of charm quark hadronisation. The measured ratio is reproduced by models implementing a pure coalescence scenario, while adding a fragmentation contribution leads to an underestimation. The Lambda(+)(C) nuclear modification factor, R-AA, is also presented. The measured values of the R-AA of Lambda(+)(C), D-S(+) and non-strange D mesons are compatible within the combined statistical and systematic uncertainties. They show, however, a hint of a hierarchy (R-AA(D0) < R-AA(DS+) < R-AA(Lambda C+)), conceivable with a contribution from coalescence mechanisms to charm hadron formation in the medium. (C) 2019 European Organization for Nuclear Research. Published by Elsevier B.V.Peer reviewe

Centrality and pseudorapidity dependence of the charged-particle multiplicity density in Xe-Xe collisions at root s(NN)=5.44 TeV

In this Letter, the ALICE Collaboration presents the first measurements of the charged-particle multiplicity density, dN(ch)/d eta, and total charged-particle multiplicity, N-ch(tot), in Xe-Xe collisions at a centre-of-mass energy per nucleon-nucleon pair of root s(NN) = 5.44 TeV. The measurements are performed as a function of collision centrality over a wide pseudorapidity range of -3.5 < eta < 5. The values of dN(ch)/d eta at mid-rapidity and N-ch(tot) for central collisions, normalised to the number of nucleons participating in the collision (N-part) as a function of root s(NN) follow the trends established in previous heavy-ion measurements. The same quantities are also found to increase as a function of N-part, and up to the 5% most central collisions the trends are the same as the ones observed in Pb-Pb at a similar energy. For more central collisions, the Xe-Xe scaled multiplicities exceed those in Pb-Pb for a similar N-part. The results are compared to phenomenological models and theoretical calculations based on different mechanisms for particle production in nuclear collisions. All considered models describe the data reasonably well within 15%. (C) 2018 Published by Elsevier B.V.Peer reviewe

Transverse momentum spectra and nuclear modification factors of charged particles in Xe-Xe collisions at root s(NN)=5.44 TeV

Transverse momentum (p(T)) spectra of charged particles at mid-pseudorapidity in Xe-Xe collisions at root s(NN) = 5.44 TeV measured with the ALICE apparatus at the Large Hadron Collider are reported. The kinematic range 0.15 show a remarkable similarity at p T > 10 GeV/c. The centrality dependence of the ratio of the average transverse momentum in Xe-Xe collisions over Pb-Pb collision at root s= 5.02 TeV is compared to hydrodynamical model calculations. (C) 2018 The Author. Published by Elsevier B.V.Peer reviewe

Multiplicity dependence of light-flavor hadron production in pp collisions at root s=7 TeV

Comprehensive results on the production of unidentified charged particles, pi(+/-), K-+/-, K-s(0), K*(892)(0), p, (p) over bar, phi(1020), Lambda, (Lambda) over bar, Xi(-) , (Xi) over bar (+), Omega(-), and (Omega) over bar (+) hadrons in proton-proton (pp) collisions at root s = 7 TeV at midrapidity (vertical bar y vertical bar < 0.5) as a function of charged-particle multiplicity density are presented. In order to avoid autocorrelation biases, the actual transverse momentum (p(T)) spectra of the particles under study and the event activity are measured in different rapidity windows. In the highest multiplicity class, the charged-particle density reaches about 3.5 times the value measured in inelastic collisions. While the yield of protons normalized to pions remains approximately constant as a function of multiplicity, the corresponding ratios of strange hadrons to pions show a significant enhancement that increases with increasing strangeness content. Furthermore, all identified particleto-pion ratios are shown to depend solely on charged-particle multiplicity density, regardless of system type and collision energy. The evolution of the spectral shapes with multiplicity and hadron mass shows patterns that are similar to those observed in p-Pb and Pb-Pb collisions at Large Hadron Collider energies. The obtained p(T), distributions and yields are compared to expectations from QCD-based pp event generators as well as to predictions from thermal and hydrodynamic models. These comparisons indicate that traces of a collective, equilibrated system are already present in high-multiplicity pp collisions.Peer reviewe

Study of J/psi azimuthal anisotropy at forward rapidity in Pb-Pb collisions at root s(NN)=5.02 TeV

The second (v(2)) and third (v(3)) flow harmonic coefficients of J/ mesons are measured at forward rapidity (2.5 < y < 4.0) in Pb-Pb collisions at sNN=5.02 TeV with the ALICE detector at the LHC. Results are obtained with the scalar product method and reported as a function of transverse momentum, p(T), for various collision centralities. A positive value of J/ v(3) is observed with 3.7 sigma significance. The measurements, compared to those of prompt D-0 mesons and charged particles at mid-rapidity, indicate an ordering with v(n)(J/) < v(n)(D-0) < v(n)(h(+/-)) (n = 2, 3) at low and intermediate p(T) up to 6 GeV/c and a convergence with v(2)(J/) approximate to v(2)(D-0) approximate to v(2)(h(+/-)) at high p(T) above 6-8 GeV/c. In semi-central collisions (5-40% and 10-50% centrality intervals) at intermediate p(T) between 2 and 6 GeV/c, the ratio v(3)/v(2) of J/ mesons is found to be significantly lower (4.6 sigma) with respect to that of charged particles. In addition, the comparison to the prompt D-0-meson ratio in the same p(T) interval suggests an ordering similar to that of the v(2) and v(3) coefficients. The J/ v(2) coefficient is further studied using the Event Shape Engineering technique. The obtained results are found to be compatible with the expected variations of the eccentricity of the initial-state geometry.Peer reviewe

Direct photon production at low transverse momentum in proton-proton collisions at root s=2.76 and 8 TeV

Measurements of inclusive and direct photon production at midrapidity in pp collisions at root s = 2.76 and 8 TeV are presented by the ALICE experiment at the LHC. The results are reported in transverse momentum ranges of 0.4 7 GeV/c is at least one sigma above unity and consistent with expectations from next-to-leading order pQCD calculations.Peer reviewe

Event-Shape Engineering for the D-meson elliptic flow in mid-central Pb-Pb collisions at root sNN=5.02 TeV

The production yield of prompt D mesons and their elliptic flow coefficient v(2) were measured with the Event-Shape Engineering (ESE) technique applied to mid-central (10-30% and 30-50% centrality classes) Pb-Pb collisions at the centre-of-mass energy per nucleon pair sNN=5.02 TeV, with the ALICE detector at the LHC. The ESE technique allows the classification of events, belonging to the same centrality, according to the azimuthal anisotropy of soft particle production in the collision. The reported measurements give the opportunity to investigate the dynamics of charm quarks in the Quark-Gluon Plasma and provide information on their participation in the collective expansion of the medium. D mesons were reconstructed via their hadronic decays at mid-rapidity, || < 0.8, in the transverse momentum interval 1 < p(T) < 24 GeV/c. The v(2) coefficient is found to be sensitive to the event-shape selection confirming a correlation between the D-meson azimuthal anisotropy and the collective expansion of the bulk matter, while the per-event D-meson yields do not show any significant modification within the current uncertainties.Peer reviewe

p-p, p-Lambda, and Lambda-Lambda correlations studied via femtoscopy in pp reactions at root s=7 TeV

We report on the first femtoscopic measurement of baryon pairs, such as p-p, p-Lambda, and Lambda-Lambda, measured by ALICE at the Large Hadron Collider (LHC) in proton-proton collisions at root s = 7 TeV. This study demonstrates the feasibility of such measurements in pp collisions at ultrarelativistic energies. The femtoscopy method is employed to constrain the hyperon-nucleon and hyperon-hyperon interactions, which are still rather poorly understood. A new method to evaluate the influence of residual correlations induced by the decays of resonances and experimental impurities is hereby presented. The p-p, p-Lambda, and Lambda-Lambda correlation functions were fitted simultaneously with the help of a new tool developed specifically for the femtoscopy analysis in small colliding systems: Correlation Analysis Tool using the Schrodinger equation (CATS). Within the assumption that in pp collisions the three particle pairs originate from a common source, its radius is found to be equal to r(0) = 1.125 +/- 0.018 (stat)(-0.035)(+0.058) (syst) fm. The sensitivity of the measured p-Lambda correlation is tested against different scattering parameters, which are defined by the interaction among the two particles, but the statistics is not sufficient yet to discriminate among different models. The measurement of the Lambda-Lambda correlation function constrains the phase space spanned by the effective range and scattering length of the strong interaction. Discrepancies between the measured scattering parameters and the resulting correlation functions at LHC and RHIC energies are discussed in the context of various models.Peer reviewe