Here is a list of recent results in which researchers from our group have been involved. For more information visit the central documentation public page of ATLAS.
Higgs boson production in association with top quarks
The Standard Model (SM) of particle physics predicts all the charged leptons, quarks, W and Z bosons acquire mass through the interaction of the Higgs field, of which the Higgs boson is a quantum fluctuation. Since the observation of this last piece of the SM in 2012 by the LHC experiments, a lot of analyses studied the properties of this particle, with particular interest of its coupling with the other known particles.
One of the most challenging properties is the Yukawa coupling of the Higgs boson with the top quark, which can be probed at the LHC identifying events in which a Higgs boson is produced in association with a pair of top quarks. This rare process represents only the 1% of the overall Higgs boson production mechanism in proton-proton collisions at the LHC.
Atlas analysed these events exploiting different signatures, based on the possible combinations of the Higgs boson and tops decay modes, probing the existence of this process and measuring therefore the strength of the top-Higgs coupling.
References: HIGG-2017-02, Physics Briefings
Search for New Phenomena in hadronic Dijet events
Despite the observation of the Higgs boson in 2012, the long-awaited missing piece, many phenomena observed in nature are still unexplained by the Standard Model (e.g. Dark-Matter, Matter-Antimatter asymmetry, etc.). The quest for an explanation of such phenomena motivates several searches for new physics evidences at the LHC collisions at the energy frontier. One of the most striking new physics evidence is the observation of a new particle reconstructed through its decay product.
The search for massive resonance decaying into a pair of hadronic jets of particles has been a recurrent topic in high energy exotics searches carried out by the ATLAS experiment. When analysing the invariant mass distribution of di-jets events, the peculiar signature of a new resonance is often recognizable as a peak, or a bump, over a smoothly falling background, which can be modeled with a suitable analytical function.
Although less visible, new physics processes occurring at few tens of TeV, can result in a modification of the dijet event kinematics at lower energy scales hence the search for the striking bump is complemented with a search for anomalies in the angular correlation between hadronic jet pairs.
References: EXOT-2016-21, EXOT-2019-03
Search for Dark Matter in mono-jet events from proton-proton collisions
Cosmological observations predicts that the universe is made of ordinary matter for less than the 5% of it. The rest of the universe seems to be made of what is called Dark Matter (DM) or Dark Energy, of which several phenomenological observations confirm the existence. Among all possible scenarios explaining such evidence, one widespread paradigma consider the DM contribution as coming from a weakly interacting massive particle (WIMP), escaping most of the current experiment due to the weakness of its couplings to the ordinary particles. Besides the searches for direct evidence of such WIMP through their scattering with the nuclei of the ordinary matter, collider experiments are looking for the production of such DM candidate particles in proton-proton collisions.
Due to their extremely weak interaction with ordinary matter, a WIMP would leave no signature in the ATLAS detector, hence making their detection quite challenging.
Events with a single hadronic jet, radiated from the initial state of the collisions, and a large amount of unbalanced energy in the collisions transverse plane caused by the undetected DM particles can be used to probe the existence of them.
References: EXOT-2016-27
Higgs boson mass measurement exploiting ZZ(*) and ƔƔ events
The evidence of the Higgs boson existence brought to the Standard Model of particle physics the missing piece of a puzzle which theorists and experimentalist spent several decades to understand and probe experimentally. Apart from its observation, the properties of the Higgs boson are as crucial as its own discovery to understand the validity of the Higgs mechanism as a way for particles to acquire a mass, and the nature of particle interactions.
The collection of Higgs boson events in proton-proton collisions can happen through the identification of its two cleanest decay modes: its decay into a pair of photons and into a pair of Z bosons, decaying in turn into a pair of same flavor and opposite charge massive leptons.
By looking at the invariant mass of the two photons in the former or of the four leptons in the latter it’s possible to observe the Higgs boson presence as a peak at its mass value. The combination of the information contained in these two channels has been used to extract a measurement of the Higgs boson mass with a few per mille precision.
References: HIGG-2016-33
Measurement of the Higgs boson production differential cross section in 4 charged leptons events
In 2012 a new particle has been discovered looking deeply into the events recorded by the ATLAS and CMS experiments from proton-proton collisions at the LHC. Since then a lot of efforts has been put to understand its nature and to verify whether that was or wasn’t the predicted Higgs boson, confirming the prediction in several ways. To know the nature of the Higgs boson it’s crucial to understand in details its behaviour, including the way it’s produced in particle collisions.
A way to do this is to measure the cross section of the Higgs production at the LHC for the different expected production mechanisms (gluon fusion, associated production, vector boson fusion); an even better way to do that is to measure such cross section as a function of some kinematic observables which characterise the produced Higgs boson, to probe the theoretical predictions to a deeper level.
A differential cross section measurement has been carried out by the ATLAS experiment collecting events in which the Higgs boson decays into a pair of Z bosons, which decay in turn into pairs of charged leptons. No significant deviations has been found with respect to the expectations, confirming that this particle is behaving exactly as the theorists predicted already few decades ago.
References: ATLAS-CONF-2018-018