Research

Research Topics

Higgs searches

Observation of the H->bb decays and VH production:

A search for the decay of the Standard Model Higgs boson into a bb¯ pair when produced in association with a W or Z boson is performed with the ATLAS detector. The data, corresponding to an integrated luminosity of 79.8 fb−1 were collected in proton-proton collisions during Run 2 of the Large Hadron Collider at a centre-of-mass energy of 13 TeV. For a Higgs boson mass of 125 GeV, an excess of events over the expected background from other Standard Model processes is found with an observed (expected) significance of 4.9 (4.3) standard deviations. A combination with the results from other searches in Run 1 and in Run 2 for the Higgs boson in the bb¯ decay mode is performed, which yields an observed (expected) significance of 5.4 (5.5) standard deviations, thus providing direct observation of the Higgs boson decay into b-quarks. The ratio of the measured event yield for a Higgs boson decaying into bb¯ to the Standard Model expectation is 1.01±0.12(stat.)+0.16−0.15(syst.). Additionally, a combination of Run 2 results searching for the Higgs boson produced in association with a vector boson yields an observed (expected) significance of 5.3 (4.8) standard deviations.


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Observation of the ttH associated production:

The observation of Higgs boson production in association with a top quark pair (tt¯H), based on the analysis of proton–proton collision data at a centre-of-mass energy of 13 TeV recorded with the ATLAS detector at the Large Hadron Collider, is presented. Using data corresponding to integrated luminosities of up to 79.8 fb−1, and considering Higgs boson decays into bb¯, WW∗, ττ, γγ, and ZZ∗, the observed significance is 5.8 standard deviations, compared to an expectation of 4.9 standard deviations. Combined with the tt¯H searches using a dataset corresponding to integrated luminosities of 4.5 fb−1 at 7 TeV and 20.3 fb−1 at 8 TeV, the observed (expected) significance is 6.3 (5.1) standard deviations. Assuming Standard Model branching fractions, the total tt¯H production cross section at 13 TeV is measured to be 670 ± 90 (stat.) +110−100 (syst.) fb, in agreement with the Standard Model prediction.


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Evidence for the H→bbbar decay with the ATLAS detector:

For a Higgs boson mass of 125 GeV, an excess of events over the expected background from other Standard Model processes is found with an observed significance of 3.5 standard deviations, compared to an expectation of 3.0 standard deviations. This excess provides evidence for the Higgs boson decay into b-quarks and for its production in association with a vector boson. The combination of this result with that of the LHC Run 1 analysis yields a ratio of the measured signal events to the Standard Model expectation equal to 0.90 +- 0.18 (stat.) +-0.21 (syst.). Assuming the Standard Model production cross-section, the results are consistent with the value of the Yukawa coupling to b-quarks in the Standard Model.


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Observation and measurement of the Higgs boson decays to WW* with the ATLAS detector at the LHC:

We report the observation of Higgs boson decays to W W ∗ based on an excess over background of 6.1 standard deviations in the dilepton final state, where the Standard Model expectation is 5.8 standard deviations. The results are obtained from a data sample corresponding to an integrated luminosity of 25 fb −1 from pp collisions at 7 and 8 TeV center of mass energy, recorded by the ATLAS detector at the LHC.


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Search for the bb decay of the Standard Model Higgs boson in associated (W/Z)H production with the ATLAS detector:

A search for the bbdecay of the Standard Model Higgs boson is performed with the ATLAS experiment using the full dataset recorded at the LHC in Run 1. The integrated luminosities used are 4.7 and 20.3 fb−1 from _pp_ collisions atsqrt(s)= 7 and 8 TeV, respectively. The processes considered are associated (W/Z)H production, where W → eν/μν, Z → ee/μμ and Z → νν. The observed (expected) deviation from the background-only hypothesis corresponds to a significance of 1.4 (2.6) standard deviations and the ratio of the measured signal yield to the Standard Model expectation is found to be μ = 0.52 ± 0.32 (stat.) ± 0.24 (syst. for a Higgs boson mass of 125.36 GeV.


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Observation of a new particle in the search for the Standard Model Higgs boson with the ATLAS detector at the LHC:

A search for the Standard Model Higgs boson in proton–proton collisions with the ATLAS detector at the LHC is presented. Individual searches in the channels H→ZZ(⁎)→4ℓ, H→γγ and H→WW(⁎)→eνμν using the 8 TeV pp collision data provided by the LHC are combined with previously published results and results from improved analyses of the 7 TeV data. Clear evidence for the production of a neutral boson with a measured mass of 126.0±0.4(stat)±0.4(sys) GeV is presented. This observation, which has a significance of 5.9 standard deviations, corresponding to a background fluctuation probability of 1.7×10−91.7×10−9, is compatible with the production and decay of the Standard Model Higgs boson.


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A Massive Particle Consistent with the Standard Model Higgs Boson observed with the ATLAS Detector at the Large Hadron Collider:

Nearly 50 years ago, theoretical physicists proposed that a field permeates the universe and gives energy to the vacuum. This field was required to explain why some, but not all, fundamental particles have mass. Numerous precision measurements during recent decades have provided indirect support for the existence of this field, but one crucial prediction of this theory has remained unconfirmed despite 30 years of experimental searches: the existence of a massive particle, the standard model Higgs boson. The ATLAS experiment at the Large Hadron Collider at CERN has now observed the production of a new particle with a mass of 126 giga–electron volts and decay signatures consistent with those expected for the Higgs particle. This result is strong support for the standard model of particle physics, including the presence of this vacuum field. The existence and properties of the newly discovered particle may also have consequences beyond the standard model itself.


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Evidence for the Higgs boson production with a top-quark pair:

The ATLAS collaboration has found evidence for the production of a Higgs boson associated with a pair of top quarks. Once clearly observed, this rare process will provide a way to study experimentally the strongest expected interaction between at the Higgs and a quark. It will provide new insights into the Higgs mechanism and allow for new studies of how unknown physics might (or might not) change the behaviour of this fundamental particle. The low production rate of ttH (only 1% of the total Higgs boson rate) makes it especially difficult to measure. Using the 13 TeV dataset collected in 2015 and 2016, ATLAS performed several searches targeting different Higgs boson decay modes: bb, multilepton, γγ and 4l final states. By combining these results, ATLAS has found statistically significant evidence for ttH production at the 4.2 sigma level.The combination yields a ratio of the measured signal events to the Standard Model expectation equal to 1.17 +- 0.19 (stat.) +-0.27 (syst.). The ATLAS group at LIP has contributed to the analysis of the ttH (H→bb) decay channel, which corresponds to over half of Higgs branching ratio.


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Top physics

Search for flavour changing neutral current top quark decays to qZ in pp collision data collected with the ATLAS detector at √s=8 TeV:

A search for the flavour-changing neutral-current decay t→qZ is presented. Data collected by the ATLAS detector during 2012 from proton–proton collisions at the Large Hadron Collider at a centre-of-mass energy of ✓s=8 TeV, corresponding to an integrated luminosity of 20.3 fb⁻¹, are analysed. Top-quark pair-production events with one top quark decaying through the t→qZ (q=u,c) channel and the other through the dominant Standard Model mode t→bW are considered as signal. Only the decays of the Z boson to charged leptons and leptonic W boson decays are used. No evidence for a signal is found and an observed (expected) upper limit on the t→qZ branching ratio of 7×10⁻⁴ (8×10⁻⁴) is set at the 95% confidence level.


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Measurement of the W boson polarization in top quark decays with the ATLAS detector:

This paper presents measurements of the polarization of W bosons in top quark decays, derived from ttbar events with missing transverse momentum, one charged lepton and at least four jets, or two charged leptons and at least two jets. Data from pp collisions at a centre-of-mass energy of 7 TeV were collected with the ATLAS experiment at the LHC and correspond to an integrated luminosity of 1.04 fb^-1. The measured fractions of longitudinally, left- and right-handed polarization are F_0=0.67+/-0.07, F_L=0.32+/-0.04 and F_R=0.01+/-0.05, in agreement with the Standard Model predictions. As the polarization of the W bosons in top quark decays is sensitive to the Wtb vertex Lorentz structure and couplings, the measurements were used to set limits on anomalous contributions to the Wtb couplings.


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Searches for new vector-like quarks

Search for pair and single production of new heavy quarks that decay to a Z boson and a third-generation quark in pp collisions at √s=8 TeV with the ATLAS detector:

A search is presented for the production of new heavy quarks that decay to a Z boson and a third-generation Standard Model quark. In the case of a new charge +2/3 quark (T), the decay targeted is T→Zt, while the decay targeted for a new charge -1/3 quark (B) is B→Zb. The search is performed with a dataset corresponding to 20.3/fb of pp collisions at √s=8 TeV recorded in 2012 with the ATLAS detector at the CERN Large Hadron Collider. Selected events contain a high transverse momentum Z boson candidate reconstructed from a pair of oppositely charged same-flavor leptons (electrons or muons), and are analyzed in two channels defined by the absence or presence of a third lepton. Hadronic jets, in particular those with properties consistent with the decay of a b-hadron, are also required to be present in selected events. Different requirements are made on the jet activity in the event in order to enhance the sensitivity to either heavy quark pair production mediated by the strong interaction, or single production mediated by the electroweak interaction. No significant excess of events above the Standard Model expectation is observed, and lower limits are derived on the mass of vector-like T and B quarks under various branching ratio hypotheses, as well as upper limits on the magnitude of electroweak coupling parameters.


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Jets in Heavy Ion collisions

Study of Heavy Flavour Jets Produced in Pb+Pb Collisions with the ATLAS Detector at LHC:

Ultrarelativistic nucleus-nucleus collisions at the Large Hadron Collider (LHC) provide an unique opportunity to recreate the Quark Gluon Plasma (QGP) in the laboratory energy frontier. This plasma of quarks and gluons, which is known to behave as a nearly perfect liquid, was the prevailing state of the Universe shortly after the Big Bang. The capabilities of ATLAS, namely large acceptance and high granularity calorimeters, afford excellent handles for QGP studies. The ATLAS experiment is strongly committed with the HI program of LHC and great expectations on the capabilities of the Upgrade to bring deeply understanding on the nature of the QGP are raised. On the other hand, LIP has a long tradition in the QGP research, having played a key role in past dedicated CERN experiments. I expect that this PhD project will further contribute for the strength of the laboratory in the field. A major goal of the Heavy Ion Program of the LHC is the understanding of the effects of the QGP on jets, namely the study of the nature of the energy loss suffered by the quarks and gluons while crossing the QGP. The bottom quark, in particular, constitutes an excellent probe. Due to its large virtuality, Q, it has a formation time, ∆t ≈1/Q ≈ 0.1 fm/c, much smaller than the formation time of the QGP at the LHC (≈10 fm/c). The understanding of the nature of the energy loss (either collisional or gluon radiative), by its turn, is crucial to infer the properties of the QGP. The HI ATLAS/LIP group is contributing with important developments preceding the b-jet physics analysis, namely on b-jet reconstruction, b-tagging and b-jet triggers in HI collisions. The group contributes strongly to the Data Quality during the HI runs (Xe+Xe, p+Pb, and Pb+Pb collisions) and contributes to other analyses by membership and chairing their Editorial Boards.

There are two proposed PhD Thesis:


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Jet Trigger with Graphics Processor Units

An evaluation of GPUs for use in an upgraded ATLAS High Level Trigger :

The ATLAS Trigger system consists of two levels, the first level (L1) implemented in hardware and the High Level Trigger (HLT) implemented in software running on a farm of commodity CPU. The HLT reduces the trigger rate from the 100 kHz L1 accept rate to 1 kHz for recording requiring an average per-event processing time of 250 ms for this task. The HLT selection is based on reconstructing tracks in the Inner Detector and Muon Spectrometer and clusters of energy deposited in the Calorimeter. Performing this reconstruction within the available HLT farm resources presents a significant challenge that will increase significantly after future LHC upgrades resulting in higher detector occupancies. General purpose Graphics Processor Units (GPGPU) are being evaluated for possible future inclusion in an upgraded HLT farm. We report on a demonstrator that has been developed consisting of GPGPU implementations of the Calorimeter clustering and Inner Detector and Muon tracking integrated within the HLT software framework. We give a brief overview of the algorithms implementation and present preliminary measurements comparing the performance of the algorithms implemented on GPU with the CPU versions.

Site da conferência: http://www.nss-mic.org/2015/public/welcome.asp


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