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Angelozzi, I.

Publications and source records attributed to Angelozzi, I..

At least 451 records · Page 25
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DOE OSTI2015

Study of (W/Z)H production and Higgs boson couplings using H→ W W * decays with the ATLAS detector

A search for Higgs boson production in association with a W or Z boson, in the H→ W W * decay channel, is performed with a data sample collected with the ATLAS detector at the LHC in proton-proton collisions at centre-of-mass energies \( \sqrt{s}=7 \) TeV and 8 TeV, corresponding to integrated luminosities of 4.5 fb -1 and 20.3 fb -1 , respectively. The WH production mode is studied in two-lepton and three-lepton final states, while two- lepton and four-lepton final states are used to search for the ZH production mode. The observed significance, for the combined W H and ZH production, is 2.5 standard deviations while a significance of 0.9 standard deviations is expected in the Standard Model Higgs boson hypothesis. The ratio of the combined W H and ZH signal yield to the Standard Model expectation, μ V H , is found to be μ V H = 3.0 -1.1 +1.3 (stat.) -0.7 +1.0 (sys.) for the Higgs boson mass of 125.36 GeV. The W H and ZH production modes are also combined with the gluon fusion and vector boson fusion production modes studied in the H → W W * → ℓνℓν decay channel, resulting in an overall observed significance of 6.5 standard deviations and μ ggF + VBF + VH = 1.16 -0.15 +0.16 (stat.) -0.15 +0.18 (sys.). The results are interpreted in terms of scaling factors of the Higgs boson couplings to vector bosons (κ V ) and fermions (κ F ); the combined results are: |κ V | = 1.06 -0.10 +0.10 , |κ F | = 0.85 -0.20 +0.26 .

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2015

Search for high-mass diphoton resonances in p p collisions at s = 8 TeV with the ATLAS detector

This article describes a search for high-mass resonances decaying to a pair of photons using a sample of 20.3 fb -1 of pp collisions at s√=8 TeV recorded with the ATLAS detector at the Large Hadron Collider. The data are found to be in agreement with the Standard Model prediction, and limits are reported in the framework of the Randall-Sundrum model. This theory leads to the prediction of graviton states, the lightest of which could be observed at the Large Hadron Collider. A lower limit of 2.66 (1.41) TeV at 95% confidence level is set on the mass of the lightest graviton for couplings of k/M¯¯ Pl =0.1 (0.01).

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2015

Erratum to: Search for production of WW / WZ resonances decaying to a lepton, neutrino and jets in pp collisions at $\sqrt{s}=8$ TeV with the ATLAS detector

It has been found that the bulk RS graviton (G*)(G*) exclusion limits were erroneously expressed as a function of σ(pp→G*)×BR(G*→WW)σ(pp→G*)×BR(G*→WW) . The corrected version of the top plot in Fig. 2 of the paper is presented below. With this correction, resonance masses below 760 GeV are excluded at 95 % confidence level for this model.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2015

Search for heavy long-lived multi-charged particles in pp collisions at √s = 8 TeV using the ATLAS detector

A search for heavy long-lived multi-charged particles is performed using the ATLAS detector at the LHC. Data collected in 2012 at √s = 8 TeV from pp collisions corresponding to an integrated luminosity of 20.3 fb -1 are examined. Particles producing anomalously high ionisation, consistent with long-lived massive particles with electric charges from |q| = 2e to |q| = 6e are searched for. No signal candidate events are observed, and 95 % confidence level cross-section upper limits are interpreted as lower mass limits for a Drell–Yan production model. The mass limits range between 660 and 785 GeV.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2015

Search for type-III seesaw heavy leptons in p p collisions at s = 8 TeV with the ATLAS detector

A search for the pair production of heavy leptons (N⁰,L ± ) predicted by the type-III seesaw theory formulated to explain the origin of small neutrino masses is presented. The decay channels N⁰→W ± l ∓ (ℓ=e,μ,τ) and L ± →W ± ν (ν=ν e ,ν μ ,ν τ ) are considered. The analysis is performed using the final state that contains two leptons (electrons or muons), two jets from a hadronically decaying W boson and large missing transverse momentum. The data used in the measurement correspond to an integrated luminosity of 20.3 fb⁻¹ of pp collisions at s√=8 TeV collected by the ATLAS detector at the LHC. No evidence of heavy lepton pair production is observed. Heavy leptons with masses below 325–540 GeV are excluded at the 95% confidence level, depending on the theoretical scenario considered.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2015

Search for new phenomena in events with three or more charged leptons in pp collisions at $ \sqrt{s}=8 $ TeV with the ATLAS detector

A generic search for anomalous production of events with at least three charged leptons is presented. The data sample consists of pp collisions at √s=8 TeV collected in 2012 by the ATLAS experiment at the CERN Large Hadron Collider, and corresponds to an integrated luminosity of 20.3 fb -1 . Events are required to have at least three selected lepton candidates, at least two of which must be electrons or muons, while the third may be a hadronically decaying tau. Selected events are categorized based on their lepton flavour content and signal regions are constructed using several kinematic variables of interest. No significant deviations from Standard Model predictions are observed. Model-independent upper limits on contributions from beyond the Standard Model phenomena are provided for each signal region, along with prescription to re-interpret the limits for any model. Constraints are also placed on models predicting doubly charged Higgs bosons and excited leptons. For doubly charged Higgs bosons decaying to eτ or μτ, lower limits on the mass are set at 400 GeV at 95% confidence level. For excited leptons, constraints are provided as functions of both the mass of the excited state and the compositeness scale Λ, with the strongest mass constraints arising in regions where the mass equals Λ. In such scenarios, lower mass limits are set at 3.0 TeV for excited electrons and muons, 2.5 TeV for excited taus, and 1.6 TeV for every excited-neutrino flavour.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

A search for high-mass resonances decaying to τ + τ - in pp collisions at $ \sqrt{s}=8 $ TeV with the ATLAS detector

A search for high-mass resonances decaying into τ⁺τ⁻ final states using proton-proton collisions at √s=8 TeV produced by the Large Hadron Collider is presented. The data were recorded with the ATLAS detector and correspond to an integrated luminosity of 19.5–20.3 fb –1 . No statistically significant excess above the Standard Model expectation is observed; 95% credibility upper limits are set on the cross section times branching fraction of Z' resonances decaying into τ⁺τ⁻ pairs as a function of the resonance mass. As a result, Z' bosons of the Sequential Standard Model with masses less than 2.02 TeV are excluded at 95% credibility. As a result, the impact of the fermionic couplings on the Z' acceptance is investigated and limits are also placed on a Z' model that exhibits enhanced couplings to third-generation fermions.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2015

Search for the Standard Model Higgs boson produced in association with top quarks and decaying into $$\varvec{b\bar{b}}$$ b b ¯ in $$\varvec{pp}$$ p p collisions at $$\sqrt{\mathbf{s}}= \varvec{8{{\,\mathrm TeV}}}$$ s = 8 T e V with the ATLAS detector

In this study, a search for the Standard Model Higgs boson produced in association with a top-quark pair, tt¯H, is presented. The analysis uses 20.3 fb –1 of pp collision data at √s=8 TeV, collected with the ATLAS detector at the Large Hadron Collider during 2012. The search is designed for the H→bb¯ decay mode and uses events containing one or two electrons or muons. In order to improve the sensitivity of the search, events are categorised according to their jet and b-tagged jet multiplicities. A neural network is used to discriminate between signal and background events, the latter being dominated by tt¯+jets production. In the single-lepton channel, variables calculated using a matrix element method are included as inputs to the neural network to improve discrimination of the irreducible tt¯+bb¯ background. No significant excess of events above the background expectation is found and an observed (expected) limit of 3.4 (2.2) times the Standard Model cross section is obtained at 95 % confidence level. The ratio of the measured tt¯H signal cross section to the Standard Model expectation is found to be μ=1.5±1.1 assuming a Higgs boson mass of 125GeV.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2015

Search for invisible decays of the Higgs boson produced in association with a hadronically decaying vector boson in pp collisions at √s = 8 with the ATLAS detector

A search for Higgs boson decays to invisible particles is performed using 20.3 fb⁻¹ of pp collision data at a centre-of-mass energy of 8 TeV recorded by the ATLAS detector at the Large Hadron Collider. The process considered is Higgs boson production in association with a vector boson (V = W or Z) that decays hadronically, resulting in events with two or more jets and large missing transverse momentum. No excess of candidates is observed in the data over the background expectation. The results are used to constrain V H production followed by H decaying to invisible particles for the Higgs boson mass range 115 < m H < 300 GeV. The 95 % confidence-level observed upper limit on σ VH × BR(H → inv.) varies from 1.6 pb at 115 GeV to 0.13 pb at 300 GeV. Assuming Standard Model production and including the gg → H contribution as signal, the results also lead to an observed upper limit of 78% at 95% confidence level on the branching ratio of Higgs bosons decays to invisible particles at a mass of 125 GeV.

43 PARTICLE ACCELERATORS↗
DOE OSTI2015

Search for long-lived, weakly interacting particles that decay to displaced hadronic jets in proton-proton collisions at s = 8 TeV with the ATLAS detector

A search for the decay of neutral, weakly interacting, long-lived particles using data collected by the ATLAS detector at the LHC is presented. This analysis uses the full data set recorded in 2012: 20.3 fb -1 of proton-proton collision data at √s = 8 TeV. The search employs techniques for reconstructing decay vertices of long-lived particles decaying to jets in the inner tracking detector and muon spectrometer. Signal events require at least two reconstructed vertices. No significant excess of events over the expected background is found, and limits as a function of proper lifetime are reported for the decay of the Higgs boson and other scalar bosons to long-lived particles and for Hidden Valley Z' and Stealth SUSY benchmark models. The first search results for displaced decays in Z' and Stealth SUSY models are presented. The upper bounds of the excluded proper lifetimes are the most stringent to date.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2015

Constraints on the off-shell Higgs boson signal strength in the high-mass ZZ and WW final states with the ATLAS detector

The measurements of the ZZ and WW final states in the mass range above the \(2m_Z\) and \(2m_W\) thresholds provide a unique opportunity to measure the off-shell coupling strength of the Higgs boson. This paper presents constraints on the off-shell Higgs boson event yields normalised to the Standard Model prediction (signal strength) in the \(ZZ \rightarrow 4\ell \), \(ZZ\rightarrow 2\ell 2\nu \) and \(WW\rightarrow e\nu \mu \nu \) final states. The result is based on pp collision data collected by the ATLAS experiment at the LHC, corresponding to an integrated luminosity of 20.3 fb\(^{-1}\) at a collision energy of \(\sqrt{s}=8\) TeV. Using the \(CL_s\) method, the observed 95 \(\%\) confidence level (CL) upper limit on the off-shell signal strength is in the range 5.1–8.6, with an expected range of 6.7–11.0. In each case the range is determined by varying the unknown \(gg\rightarrow ZZ\) and \(gg\rightarrow WW\) background K-factor from higher-order quantum chromodynamics corrections between half and twice the value of the known signal K-factor. Assuming the relevant Higgs boson couplings are independent of the energy scale of the Higgs boson production, a combination with the on-shell measurements yields an observed (expected) 95 \(\%\) CL upper limit on \(\Gamma _H/\Gamma _H^{\mathrm {SM}}\) in the range 4.5–7.5 (6.5–11.2) using the same variations of the background K-factor. Assuming that the unknown \(gg\rightarrow VV\) background K-factor is equal to the signal K-factor, this translates into an observed (expected) 95 \(\%\) CL upper limit on the Higgs boson total width of 22.7 (33.0) MeV.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2015

Measurement of the top quark mass in the $t\bar{t}\rightarrow\text{ lepton+jets }$→ and $t\bar{t}\rightarrow\text{dilepton}$ → channels using $\sqrt{s}=7$TeV ATLAS data

The top quark mass was measured in the channels $t\bar{t}$→ lepton+jets and $t\bar{t}$→ dilepton (lepton = e,μ) based on ATLAS data recorded in 2011. The data were taken at the LHC with a proton–proton centre-of-mass energy of √s = 7 TeV and correspond to an integrated luminosity of 4.6 fb –1 . The $t\bar{t}$→ lepton+jets analysis uses a three-dimensional template technique which determines the top quark mass together with a global jet energy scale factor (JSF), and a relative b-to-light-jet energy scale factor (bJSF), where the terms b-jets and light-jets refer to jets originating from b-quarks and u, d, c, s-quarks or gluons, respectively. The analysis of the $t\bar{t}$→ dilepton channel exploits a one-dimensional template method using the m ℓb observable, defined as the average invariant mass of the two lepton+b-jet pairs in each event. The top quark mass is measured to be 172.33 ± 0.75 (stat + JSF + bJSF) ± 1.02(syst) GeV, and 173.79 ± 0.54(stat) ± 1.30(syst) GeV in the $t\bar{t}$→ lepton+jets and $t\bar{t}$→ dilepton channels, respectively. Thus, the combination of the two results yields m top = 172.99 ± 0.48(stat) ± 0.78(syst) GeV, with a total uncertainty of 0.91 GeV.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2015

Search for low-scale gravity signatures in multi-jet final states with the ATLAS detector at $\sqrt{s}=8 $ TeV

A search for evidence of physics beyond the Standard Model in final states with multiple high-transverse-momentum jets is performed using 20.3 fb -1 of proton-proton collision data at √s = 8 TeV recorded by the ATLAS detector at the LHC. No significant excess of events beyond Standard Model expectations is observed, and upper limits on the visible cross sections for non-Standard Model production of multi-jet final states are set. A wide variety of models for black hole and string ball production and decay are considered, and the upper limit on the cross section times acceptance is as low as 0.16 fb at the 95% confidence level. For these models, excluded regions are also given as function of the main model parameters.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2015

Identification and energy calibration of hadronically decaying tau leptons with the ATLAS experiment in pp collisions at $\sqrt{s}=8$ $\,\hbox {TeV}$ TeV

This study describes the trigger and offline reconstruction, identification and energy calibration algorithms for hadronic decays of tau leptons employed for the data collected from pp collisions in 2012 with the ATLAS detector at the LHC center-of-mass energy √s=8 TeV. The performance of these algorithms is measured in most cases with Z decays to tau leptons using the full 2012 dataset, corresponding to an integrated luminosity of 20.3 fb –1 . An uncertainty on the offline reconstructed tau energy scale of 2–4%, depending on transverse energy and pseudorapidity, is achieved using two independent methods. The offline tau identification efficiency is measured with a precision of 2.5% for hadronically decaying tau leptons with one associated track, and of 4% for the case of three associated tracks, inclusive in pseudorapidity and for a visible transverse energy greater than 20 GeV. For hadronic tau lepton decays selected by offline algorithms, the tau trigger identification efficiency is measured with a precision of 2–8%, depending on the transverse energy. The performance of the tau algorithms, both offline and at the trigger level, is found to be stable with respect to the number of concurrent proton–proton interactions and has supported a variety of physics results using hadronically decaying tau leptons at ATLAS.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗