ATLAS Collaboration(Aad, G. et al), Alvarez Piqueras, D., Barranco Navarro, L., Cabrera Urban, S., Castillo Gimenez, V., Cerda Alberich, L., et al. (2017). Performance of algorithms that reconstruct missing transverse momentum in root s=8 TeV proton-proton collisions in the ATLAS detector. Eur. Phys. J. C, 77(4), 241–46pp.
Abstract: The reconstruction and calibration algorithms used to calculate missing transverse momentum (E-T(miss)) with the ATLAS detector exploit energy deposits in the calorimeter and tracks reconstructed in the inner detector as well as the muon spectrometer. Various strategies are used to suppress effects arising from additional proton-proton interactions, called pileup, concurrent with the hard-scatter processes. Tracking information is used to distinguish contributions from the pileup interactions using their vertex separation along the beam axis. The performance of the E-T(miss) reconstruction algorithms, especially with respect to the amount of pileup, is evaluated using data collected in proton-proton collisions at a centre-of-mass energy of 8 TeV during 2012, and results are shown for a data sample corresponding to an integrated luminosity of 20.3 fb(-1). The simulation and modelling of E-T(miss) in events containing a Z boson decaying to two charged leptons (electrons or muons) or a W boson decaying to a charged lepton and a neutrino are compared to data. The acceptance for different event topologies, with and without high transverse momentum neutrinos, is shown for a range of threshold criteria for E-T(miss), and estimates of the systematic uncertainties in the E-T(miss) measurements are presented.
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Gallego Cadavid, A., Enea Romano, A., & Gariazzo, S. (2017). CMB anomalies and the effects of local features of the inflaton potential. Eur. Phys. J. C, 77(4), 242–9pp.
Abstract: Recent analysis of the WMAP and Planck data have shown the presence of a dip and a bump in the spectrum of primordial perturbations at the scales k = 0.002 Mpc(-1) and k = 0.0035 Mpc(-1), respectively. We analyze for the first time the effects of a local feature in the inflaton potential to explain the observed deviations from scale invariance in the primordial spectrum. We perform a best-fit analysis of the cosmic microwave background (CMB) radiation temperature and polarization data. The effects of the features can improve the agreement with observational data respect to the featureless model. The best-fit local feature affects the primordial curvature spectrum mainly in the region of the bump, leaving the spectrum unaffected on other scales.
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LHCb Collaboration(Aaij, R. et al), Garcia Martin, L. M., Martinez-Vidal, F., Oyanguren, A., Remon Alepuz, C., Ruiz Valls, P., et al. (2017). New algorithms for identifying the flavour of B-0 mesons using pions and protons. Eur. Phys. J. C, 77(4), 238–15pp.
Abstract: Two new algorithms for use in the analysis of pp collision are developed to identify the flavour of B mesons at production using pions and protons from the hadronization process. The algorithms are optimized and calibrated on data, using B-0 -> D- pi(+) decays from pp collision data collected by LIICh at centre -of -mass energies of 7 and 8 TeV. The tagging power of the new pion algorithm is 60% greater than the previously available one; the algorithm using protons to identify the flavour of a B-0 meson is the first of its kind.
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Aguilar-Saavedra, J. A., Bernabeu, J., Mitsou, V. A., & Segarra, A. (2017). The Z boson spin observables as messengers of new physics. Eur. Phys. J. C, 77(4), 234–6pp.
Abstract: We demonstrate that the eight multipole parameters describing the spin state of the Z boson are able to disentangle known Z production mechanisms and signals from new physics at the LHC. They can be extracted from appropriate asymmetries in the angular distribution of lepton pairs from the Z boson decay. The power of this analysis is illustrated by (1) the production of Z boson plus jets; (2) Z boson plus missing transverse energy; (3) W and Z bosons originating from the two-body decay of a heavy resonance.
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ATLAS Collaboration(Aaboud, M. et al), Alvarez Piqueras, D., Barranco Navarro, L., Cabrera Urban, S., Castillo Gimenez, V., Cerda Alberich, L., et al. (2017). Measurement of the W boson polarisation in t(t)over-barevents from pp collisions at root s=8 TeV in the lepton plus jets channel with ATLAS. Eur. Phys. J. C, 77(4), 264–28pp.
Abstract: This paper presents a measurement of the polarisation of W bosons from t (t) over bar t decays, reconstructed in events with one high-p(T) lepton and at least four jets. Data from pp collisions at the LHC were collected at root s = 8 TeV and correspond to an integrated luminosity of 20.2 fb(-1). The angle theta* between the b-quark from the top quark decay and a direct W boson decay product in the W boson rest frame is sensitive to the W boson polarisation. Two different W decay products are used as polarisation analysers: the charged lepton and the down-type quark for the leptonically and hadronically decaying W boson, respectively. The most precise measurement of the W boson polarisation via the distribution of cos theta* is obtained using the leptonic analyser and events in which at least two of the jets are tagged as b-quark jets. The fitted fractions of longitudinal, left-and right-handed polarisation states are F-0 = 0.709 +/- 0.019, F-L = 0.299 +/- 0.015 and F-R = – 0.008 +/- 0.014, and are the most precisely measured W boson polarisation fractions to date. Limits on anomalous couplings of the W tb vertex are set.
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ATLAS Collaboration(Aaboud, M. et al), Alvarez Piqueras, D., Barranco Navarro, L., Cabrera Urban, S., Castillo Gimenez, V., Cerda Alberich, L., et al. (2016). Luminosity determination in pp collisions at root s=8 TeV using the ATLAS detector at the LHC. Eur. Phys. J. C, 76(12), 653–45pp.
Abstract: The luminosity determination for the ATLAS detector at the LHC during pp collisions at root s = 8 TeV in 2012 is presented. The evaluation of the luminosity scale is performed using several luminometers, and comparisons between these luminosity detectors are made to assess the accuracy, consistency and long-term stability of the results. A luminosity uncertainty of delta L/L = +/- 1.9% is obtained for the 22.7 fb(-1) of pp collision data delivered to ATLAS at root s = 8 TeV in 2012.
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LHCb Collaboration(Aaij, R. et al), Garcia Martin, L. M., Henry, L., Martinez-Vidal, F., Oyanguren, A., Remon Alepuz, C., et al. (2017). Study of charmonium production in b-hadron decays and first evidence for the decay B-s(0) -> phi phi phi. Eur. Phys. J. C, 77(9), 609–18pp.
Abstract: Using decays to f-meson pairs, the inclusive production of charmonium states in b-hadron decays is studied with pp collision data corresponding to an integrated luminosity of 3.0 fb(-1), collected by the LHCb experiment at centre-of-mass energies of 7 and 8 TeV. Denoting by B-C = B(b -> CX) x B(C -> phi phi) the inclusive branching fraction of a b hadron to a charmonium state C that decays into a pair of phi mesons, ratios R-C2(C1) = B-C1/B-C2 are determined as R-eta c(1S)(chi c0) = 0.147 +/- 0.023 +/- 0.011, R-eta c(1S)(chi c1) = 0.073 +/- 0.016 +/- 0.006, R-eta c(1S)(chi c2) = 0.081 +/- 0.013 +/- 0.005, R-chi c0(chi c1) = 0.50 +/- 0.11 +/- 0.01, R-chi c0(chi c2) = 0.56 +/- 0.10 +/- 0.01 and R-eta c(1S)(eta c(2S)) = 0.040 +/- 0.011 +/- 0.004. Here and below the first uncertainties are statistical and the second systematic. Upper limits at 90% confidence level for the inclusive production of X(3872), X(3915) and.c2(2P) states are obtained as R-chi c1(X(3872)) < 0.34, R-chi c0(X(3915)) < 0.12 and R-chi c2(chi c2(2P)) < 0.16. Differential cross-sections as a function of transverse momentum are measured for the eta(c)(1S) and chi(c) states. The branching fraction of the decay B-s(0). phi phi phi is measured for the first time, B(B-s(0) -> phi phi phi) = (2.15 +/- 0.54 +/- 0.28 +/- 0.21 B) x10(-6). Here the third uncertainty is due to the branching fraction of the decay B-s(0) -> phi phi, which is used for normalization. No evidence for intermediate resonances is seen. A preferentially transverse phi polarization is observed. Themeasurements allow the determination of the ratio of the branching fractions for the eta(c)(1S) decays to ff and p (p) over bar as B(eta(c)(1S) -> phi phi)/B(eta(c)(1S) -> p (p) over bar) = 1.79 +/- 0.14 +/- 0.32.
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LHCb Collaboration(Aaij, R. et al), Garcia Martin, L. M., Henry, L., Martinez-Vidal, F., Oyanguren, A., Remon Alepuz, C., et al. (2017). Improved limit on the branching fraction of the rare decay K-S(0) -> mu(+)mu(-). Eur. Phys. J. C, 77(10), 678–12pp.
Abstract: A search for the decay K-S(0) -> mu+ mu- is performed, based on a data sample of proton- proton collisions corresponding to an integrated luminosity of 3 fb(-1), collected by the LHCb experiment at centre-of- mass energies of 7 and 8 TeV. The observed yield is consistent with the background- only hypothesis, yielding a limit on the branching fraction of B( K-S(0) -> mu(+)mu(-)) < 0.8 (1.0) x 10(-9) at 90% ( 95%) confidence level. This result improves the previous upper limit on the branching fraction by an order of magnitude.
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ATLAS Collaboration(Aaboud, M. et al), Alvarez Piqueras, D., Barranco Navarro, L., Cabrera Urban, S., Castillo Gimenez, V., Cerda Alberich, L., et al. (2017). Reconstruction of primary vertices at the ATLAS experiment in Run 1 proton-proton collisions at the LHC. Eur. Phys. J. C, 77(5), 332–35pp.
Abstract: This paper presents the method and performance of primary vertex reconstruction in proton-proton collision data recorded by the ATLAS experiment during Run 1 of the LHC. The studies presented focus on data taken during 2012 at a centre-of-mass energy of root s = 8 TeV. The performance has been measured as a function of the number of interactions per bunch crossing over a wide range, from one to seventy. The measurement of the position and size of the luminous region and its use as a constraint to improve the primary vertex resolution are discussed. A longitudinal vertex position resolution of about 30 μm is achieved for events with high multiplicity of reconstructed tracks. The transverse position resolution is better than 20 μm and is dominated by the precision on the size of the luminous region. An analytical model is proposed to describe the primary vertex reconstruction efficiency as a function of the number of interactions per bunch crossing and of the longitudinal size of the luminous region. Agreement between the data and the predictions of this model is better than 3% up to seventy interactions per bunch crossing.
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ATLAS Collaboration(Aaboud, M. et al), Alvarez Piqueras, D., Barranco Navarro, L., Cabrera Urban, S., Castillo Gimenez, V., Cerda Alberich, L., et al. (2017). Measurements of top-quark pair differential cross-sections in the e μchannel in pp collisions at root s=13 TeV using the ATLAS detector. Eur. Phys. J. C, 77(5), 292–30pp.
Abstract: This article presents measurements of t (t) over bar differential cross-sections in a fiducial phase-space region, using an integrated luminosity of 3.2 fb(-1) of proton-proton data at a centre-of-mass energy of root s = 13 TeV recorded by the ATLAS experiment at the LHC in 2015. Differential crosssections are measured as a function of the transverse momentum and absolute rapidity of the top quark, and of the transverse momentum, absolute rapidity and invariant mass of the t (t) over bar system. The t (t) over bar events are selected by requiring one electron and onemuonof opposite electric charge, and at least two jets, one of which must be tagged as containing a b-hadron. The measured differential cross-sections are compared to predictions of next-to-leading order generators matched to parton showers and the measurements are found to be consistent with all models within the experimental uncertainties with the exception of the Powheg-Box + Herwig++ predictions, which differ significantly from the data in both the transverse momentum of the top quark and the mass of the t (t) over bar system.
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