ATLAS finds evidence for the rare electroweak W±W± production

10th September 2014 – 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.

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First observation of Z-boson production via weak-boson fusion

10th September 2014 – 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.

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Higgs boson production measurements from the channels of discovery

10th September 2014 – 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.

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ATLAS Physicist Wins Young Scientist Prize

21st August 2014 – For her contribution toward the discovery of the Higgs boson, Kerstin Tackmann was awarded the Young Scientist Prize in Particle Physics 2014 by the International Union of Pure and Applied Physics.

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Identity problems

24th July 2014 – An obligatory eye scan is required for all ATLAS underground personnel entering the experimental cavern. The iris recognition is performed by the IrisID iCAM7000. Its only point in life is to keep track of who enters and leaves the Zone. It sounds like a simple task for such an advanced technology, but -- like most things in the world of research -- it's never without some hiccups.

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Probing Higgs boson production properties

17th July 2014 – 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.

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The Higgs boson’s shadow

14th July 2014 – 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.

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The WW cross-section: a high flyer

8th July 2014 – 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.

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Are quarks fundamental particles?

6th July 2014 – 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.

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A new state of beauty and charm

5th July 2014 – 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.

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W + jets plot

The production of W bosons in association with jets

5th July 2014 – 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.

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Counting collisions with ALFA

5th July 2014 – 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.

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Hunting for the top squark

5th July 2014 – Completion of the analysis of 2012 data recorded by the ATLAS detector at the LHC’s collision energy of 8 TeV has significantly improved our capability of finding a supersymmetric partner of the top quark – also known as the top squark or the stop.

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Second anniversary of the Higgs boson discovery!

4th July 2014 – It’s been two years since the ATLAS and CMS experiments at CERN jointly announced the discovery of a new boson consistent with the Higgs particle of the Standard Model. Since then, the Higgs boson has been intensely examined. We’ve measured its spin, its mass, its lifetime, and observed its decay into bosons and fermions. In the next run of the Large Hadron Collider, we hope to learn more about how it interacts with other particles and to make many more precise measurements of its properties. By doing, we hope to extend the limits of our current understanding of the fundamental components of nature, and to seek clues for discovery.

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Precise measurement of the Higgs boson mass

3rd July 2014 – The ATLAS Collaboration has analyzed its full Run 1 data sample of seven and eight TeV (tera electron Volts) proton-proton collisions delivered by the Large Hadron Collider (LHC), to produce an accurate measurement of the Higgs boson mass. The Higgs boson resonance appears as a narrow peak in the mass spectra of its decays to two photons or to four charged leptons, as shown in the two figures below.

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New evidence for top quark pairs produced with W or Z bosons

3rd July 2014 – Evidence for the production of a W or Z boson together with a top quark pair, referred to as tt̄W and tt̄Z processes, has been found in the ATLAS analysis of the 8 TeV data from the LHC’s first run.

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Measuring top production in the LHC

3rd July 2014 – Using the full data sample from the LHC’s first run of proton-proton collisions, ATLAS has measured the production rate of top and anti-top quarks.

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The Symphony of ATLAS

23rd June 2014 – Bringing the nine-storey high, many-layered ATLAS detector back to life and preparing it for the Large Hadron Collider's next run is a complex task. Each sub-detector is setup and thoroughly tested before they are joined and the detector as a whole can begin recording data again.

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Taking stock at the LHCP conference

17th June 2014 – I felt like I was returning home as I walked through the gates of Columbia University at 116th Street and Broadway, the day before the LHCP conference began. The scaffolding from the recently completed graduation ceremonies reminded me of my own PhD graduation thirteen years ago. The ubiquitous Columbia-blue signs of "Welcome back Alumni" seemed to be talking just to me.

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Are You Up for the Higgs Challenge?

16th June 2014 – It's been four weeks since the four-month long Higgs Machine Learning Challenge was announced. Almost 700 teams have signed up and more than 200 have beaten the in-house benchmark already.

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