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.
Physics Briefing | 14 Jul 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.
Physics Briefing | 08 Jul 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.
Physics Briefing | 06 Jul 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.
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.
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.
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.
News | 04 Jul 2014
The ATLAS & CMS experiments celebrate the second anniversary of the discovery of the Higgs boson. Here, are some images of the path from the LHC's startup to the Nobel Prize, featuring a musical composition by Roger Zare, performed by the Donald Sinta Quartet, called 'LHC'. Happy Discovery Day!
News | 04 Jul 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.
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.
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.
News | 23 Jun 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.
Blog | 17 Jun 2014
As someone who comes from a small mountain town, for many years I've linked the word 'summer' to 'seaside' and 'sun'. During my experience as a physicist working in ATLAS, I found myself associating the word 'conferences' to the word 'summer' more often than to the two above.
Blog | 16 Jun 2014
Previously in Notes from Underground, Dave Robinson wrote in some detail about the work going on inside the ATLAS Detector, and Clara Nellist wrote about the inner detector of ATLAS, discussing the different types of detection units or Sensors (Planars & 3D). I will continue to delve into the exciting world of the inner detector with its brand new Insertable B-Layer (IBL) and its related parts.
Blog | 09 Jun 2014
ATLAS presented new results at the Large Hadron Collider Physics (LHCP) Conference in Columbia University, New York, 2 to 7 June. Many new searches and improved measurements were presented, among which were an updated Higgs boson mass measurement, a search for double Higgs boson production and new searches for Supersymmetry and exotic phenomena.
News | 06 Jun 2014
Closest to the beam pipe where particle collisions will occur in the very heart of ATLAS, a new sub-detector – the Insertable B-Layer – was put in place on 7 May. The IBL team had been developing and practicing the insertion procedure and tooling for two years because of the operation’s delicate nature.
News | 02 Jun 2014
In last week’s post for this Notes from Underground series, David talked about the work that goes on in the ATLAS pit. I'm going to take a step back and talk about what happens before a detector is installed. Although the work I want to tell you about didn't technically take place underground, much of it was performed in what is essentially a large airport hangar without natural light, so it certainly feels like you’re 100m down!
Blog | 27 May 2014
We physicists refer to the vast underground cavern that houses the ATLAS experiment as ‘the pit’. That may be a strange term to use for a marvel of civil, mechanical and electrical engineering, but nonetheless there are parallels to what you might imagine a ‘pit’ to be. Working inside the ATLAS detector in the pit can be dark, sometimes hot and not suited to those with claustrophobia. It often involves climbing several sets of makeshift steps and gantries and crawling flat on your stomach through narrow gaps to get to the part of the detector where you need to be. You will be wearing a safety helmet with mounted lamp, steel toe-cap shoes, one or more dosimeters to monitor radiation exposure and even a harness, if working at heights. Not to mention tools, laptop and any equipment you need to do your job. You tend to recognize the experimental physicists, engineers and technicians who have just come up from the pit – they stand blinking in the sunlight with a tired and rather sweaty appearance.
Blog | 14 May 2014