The Standard Model of particle physics has been extremely successful in predicting a vast variety of phenomena – so successful, that it is easy to forget that some of its predictions have not yet been verified. A very important one, related intimately to electroweak symmetry breaking, is that the gauge bosons (γ, W and Z) can interact with each other through quartic interactions.
The fusion of two weak bosons is an important process that can be used to probe the electroweak sector of the Standard Model. Measurements of Higgs production via weak-boson fusion are crucial for precise extraction of the Higgs-boson couplings and have the potential to help pin down the charge conjugation and parity of the Higgs boson. A similar process, weak-boson scattering, is sensitive to alternative electroweak symmetry-breaking models and to anomalous weak-boson gauge couplings. These processes are extremely rare and the experimental observation of the production of heavy bosons via weak-boson fusion has become possible only recently with the large centre-of-mass energy and luminosity provided by the LHC. Extracting the signals from the huge backgrounds in the high pile-up conditions at the LHC is a major challenge.
The discovery of the Higgs boson by the ATLAS and CMS collaborations in 2012 marked a new era in particle physics because it completed the Standard Model and gave us another tool to explore territories beyond. The Standard Model predicts precisely the interactions of the Higgs boson to all other elementary particles once its mass is measured.
ATLAS has measured properties of events likely to contain a Higgs boson, in order to get a better understanding of the frequency and manner in which they are produced. The study specifically examines the fiducial and differential cross sections for Higgs bosons that decay into two photons or into two Z bosons, using proton-proton collisions recorded by ATLAS in 2012.
ATLAS physicists have studied the “shadow” of the Higgs boson far above its mass peak in an analysis of the full sample of 8 TeV proton-proton collisions delivered by the LHC in 2012. The study involves Higgs boson decays into two Z bosons, which themselves decay into four charged leptons or two charged leptons plus two neutrinos. Among other interesting properties, it provides new insight into the lifetime, or natural width, of the Higgs boson.
The production of pairs of heavy bosons, such as two Z bosons, a Z and a W boson, or the more challenging pair of W bosons (WW), are processes that particle physicists are passionate about because they cover a rich spectrum of phenomena. The WW channel, in particular, represents a substantial experimental challenge. In the events considered for this measurement, each W boson decays into an electron or a muon plus a neutrino that remains undetected and is reconstructed through the presence of missing energy in the event.
From decades of discoveries made at particle colliders, we know that protons are composed of quarks bound together by gluons. We also know that there are six kinds of quarks, each one with its associated antiparticle. But are quarks fundamental? ATLAS searched for signs that quarks may have substructure in its most recent data, collected from the LHC’s proton-proton collisions in 2012.
Data from a special run of the LHC using dedicated beam optics at 7 TeV have been analysed to measure the total cross-section of proton-proton collisions in ATLAS. Using the Absolute Luminosity For ATLAS (ALFA), a Roman Pot sub-detector located 240 metres from the collision point, ATLAS has determined the cross-section with unprecedented precision to be σtot (pp → X) = 95.4 ± 1.4 millibarn.
The production of a W boson in association with “jets” of particles initiated by quarks or gluons (“W+jets” events) is an important signature to test quantum chromodynamics, the theory of strong interactions. A new measurement reported by ATLAS focuses on studying the properties of the jets in a large data sample of W+jets events.
ATLAS has observed a particle state of mass and decay properties consistent with expectations for an excited state of the Bc meson. The discovery follows analysis of the full 7 TeV and 8 TeV proton-proton collision data sets from the LHC’s first run.