top quark

ATLAS probes interactions between heavyweights of the Standard Model

In the contest for the heaviest known elementary particle, the top quark and Z boson rank first and third, respectively. When a proton–proton collision produces a top-quark pair together with a Z boson – a process known as ttZ production – their total mass can reach an impressive 440 GeV! The discovery of this highly energetic process thus required the record collision energy and rate of the LHC; no previous collider could come close.

30th July 2020

New ATLAS result addresses long-standing tension in the Standard Model

This week, at the LHCP 2020 conference, the ATLAS Collaboration presented a precise measurement of lepton flavour universality using a brand-new technique. Physicists examined collision events where pairs of top quarks decay to pairs of W bosons, and subsequently into leptons. They then measured the relative probability that this lepton is a muon or a tau-lepton – a ratio known as R(τ/μ). According to the Standard Model, R(τ/μ) should be unity – but there has been long-standing tension with this prediction, ever since it was measured at the Large Electron-Positron (LEP) collider in the 1990s.

28th May 2020

ATLAS finds evidence of spectacular four-top quark production

In a new result released today, the ATLAS Collaboration announced strong evidence of the production of four top quarks. This rare Standard Model process is expected to occur only once for every 70 thousand pairs of top quarks created at the LHC and has proven extremely difficult to measure.

26th May 2020

25 years on: a single top quark partners with the Z boson

A quarter-century after its discovery, physicists at the ATLAS Experiment are gaining new insight into the heaviest-known particle: the top quark. The huge amount of data collected during Run 2 of the LHC (2015-2018) has allowed physicists to study rare production processes of the top quark in great detail, including its production in association with other heavy elementary particles.

21st February 2020

ATLAS highlights from TOP2019

As the heaviest elementary particle, the top quark is appropriately named. It is ideally suited for precision measurements that test the limits of our understanding and could provide indirect hints at new physics. Physicists from around the world gathered in Beijing, China, last week at the annual TOP2019 conference to exchange the latest news, results and ideas on the top quark. For the ATLAS collaboration, TOP2019 proved a great success, with several excellent talks and posters presented by its members.

4th October 2019

ATLAS delivers new direct measurement of the top-quark decay width with improved precision

As the heaviest known particle, the top quark plays a key role in studies of fundamental interactions. Due to its short lifetime, the top quark decays before it can turn into a hadron. Thus, its properties are preserved and transferred to its decay products, which can in turn be measured in high-energy physics experiments. Such studies provide an excellent testing ground for the Standard Model and may provide clues for new physics.

6th August 2019

Zooming in on top-quark production

As the heaviest known elementary particle, the top quark has a special place in LHC physics. Top quark-antiquark pairs are copiously produced in collisions recorded by the ATLAS detector, providing a rich testing ground for theoretical models of particle collisions at the highest accessible energies. Any deviations between measurements and predictions could point to shortcomings in the theory – or first hints of something completely new.

5th August 2019

ATLAS finds evidence of charge asymmetry in top-quark pairs

Among the most intriguing particles studied by the ATLAS collaboration is the top quark. As the heaviest known fundamental particle, it plays a unique role in the Standard Model of particle physics and – perhaps – in yet unseen physics beyond the Standard Model. A new ATLAS result, presented today at the European Physical Society Conference on High-Energy Physics (EPS-HEP) in Ghent, Belgium, examines the full Run 2 dataset to find evidence of charge asymmetry in top-quark pair events, with a significance of four standard deviations.

11th July 2019

Producing four top quarks at once to explore the unknown

For several decades, particle physicists having been trying to better understand Nature at the smallest distances by colliding particles at the highest energies. While the Standard Model of particle physics has successfully explained most of the results that have arisen from experiments, many phenomena remain baffling. Thus, new particles, forces or more general concepts must exist and – if the history of particle physics is any indication – they could well be revealed at the high-energy frontier.

6th November 2018

Could a new type of quark fix the “unnaturalness” of the Standard Model?

While the discovery of the Higgs boson at the Large Hadron Collider (LHC) in 2012 confirmed many Standard Model predictions, it has raised as many questions as it has answered. For example, interactions at the quantum level between the Higgs boson and the top quark ought to lead to a huge Higgs boson mass, possibly as large as the Planck mass (>1018 GeV). So why is it only 125 GeV? Is there a mechanism at play to cancel these large quantum corrections caused by the top quark (t)? Finding a way to explain the lightness of the Higgs boson is one of the top (no pun intended) questions in particle physics.

8th August 2018

Precision leads to puzzles

The top quark is a unique particle due to its phenomenally high mass. It decays in less than 10-24 seconds, that is, before it had time to interact with any other particles. Therefore many of its quantum numbers, such as its spin, are transferred to its decay particles. When created in matter-antimatter pairs, the spins of the top quark and the antitop quark are expected to be correlated to some degree.

6th July 2018

ATLAS observes direct interaction of Higgs boson with top quark

The ATLAS Collaboration at CERN has announced the observation of Higgs bosons produced together with a top-quark pair. Observing this extremely rare process is a significant milestone for the field of High-Energy Physics. It allows physicists to test critical parameters of the Higgs mechanism in the Standard Model of particle physics.

4th June 2018

New ATLAS result establishes production of Higgs boson in association with top quarks

According to the Standard Model, quarks, charged leptons, and W and Z bosons obtain their mass through interactions with the Higgs field, whose fluctuation gives rise to the Higgs boson. To test this theory, ATLAS takes high-precision measurements of the interactions between the Higgs boson and these particles. While experiments had observed and measured the Higgs boson decaying to pairs of W or Z bosons, photons or tau leptons, the Higgs coupling to quarks had – until now – not been observed.

4th June 2018

ATLAS studies the dynamics of very high-momentum top quarks

The top quark – the heaviest known fundamental particle – plays a unique role in high-energy physics. Studies of its properties have opened new opportunities for furthering our knowledge of the Standard Model. In a new paper submitted to Physical Review D, the ATLAS collaboration presents a comprehensive measurement of high-momentum top-quark pair production at 13 TeV.

31st January 2018

Measurements of weak top quark processes gain strength

The production of top quarks in association with vector bosons is a hot topic at the LHC. ATLAS first reported strong evidence for the production of a top quark in association with a Z boson at the EPS 2017 conference. In a paper submitted to the Journal of High-Energy Physics, the ATLAS experiment describes the measurement of top-quark production in association with a W boson in 13 TeV collisions.

18th January 2018

ATLAS finds evidence of the Higgs boson produced in association with a pair of top quarks

The ATLAS collaboration has presented evidence of “ttH production”, a rare process where a pair of top quarks emits a Higgs boson. Observing this process would provide new insight into the Higgs mechanism and allow for new studies of how unknown physics might (or might not) change the behaviour of this fundamental particle.

26th October 2017

Studying fragments of the top quark

Using Run 1 data, ATLAS reports a new differential production rate measurement of top quark pairs and a precise new determination of the top quark mass.

5th October 2017

Hunting down forbidden decays of the top quark

Ordinary matter is made of just three kinds of elementary particles: up and down quarks, which form the atomic nucleus, and electrons, which surround the nucleus. But the rest of nature is not so straightforward: heavier forms of quarks and leptons are produced regularly at particle accelerators.

3rd October 2017

ATLAS and CMS look forward with the top quark

The top quark, the heaviest known elementary particle, has a unique place in the Standard Model. By precisely measuring its properties, ATLAS physicists can probe physics beyond our current understanding.

27th September 2017

On top of the top: ATLAS highlights from the TOP2017 workshop

The ATLAS collaboration presented exciting new results at the 10th International Workshop on Top Quark Physics (TOP2017), held in Braga (Portugal). The conference, which concluded today, brought together experimental and theoretical physicists specializing in the heaviest known elementary particle: the top quark.

22nd September 2017

Return of the top quark!

For the first time, ATLAS has measured the kinematics of the top quark and of the tt̅ system in 13 TeV events containing two charged leptons, two neutrinos and two jets (called “dilepton” events).

13th January 2017

Hunting the origin of the top quark’s mass

The ATLAS experiment has been searching for the process in which a pair of top quarks is produced, where one is a “virtual” particle that emits a Higgs boson on the way to becoming a “real” particle. This process is referred to as ttH production after the particles that are produced.

4th August 2016

Weighing in on the top quark mass

ATLAS has released a new precise measurement of the mass of the top quark, the heaviest known elementary particle.

15th June 2016

Top 2015: Mass, momentum and the conga

The top quark conference normally follows the same basic structure. The first few days are devoted to reports on the general status of the field and inclusive measurements; non-objectionable stuff that doesn’t cause controversy. The final few days are given over to more focused analyses; the sort of results that professors really enjoy arguing about.

24th September 2015

ATLAS presents new top physics results

This week, physicists from around the world are gathering at the Top 2015 workshop in Ischia, Italy to discuss the latest measurements of the top quark. As the heaviest known fundamental particle, the top quark plays a special role in the search for "new physics".

17th September 2015

TOP 2015: Top quarks come to Italy!

The annual top conference! This year we’re in Ischia, Italy. The hotel is nice, the pool is tropical and heated, but you don’t want to hear about that, you want to hear about the latest news in the Standard Model’s heaviest and coolest particle, the top quark! You won’t be disappointed.

16th September 2015

Top quarks in Run 2 are spot on

With a precision of just under 14% − currently dominated by our ability to understand how many proton-proton collisions have occurred at ATLAS (i.e. luminosity) − this measurement is able to confirm that quantum chromodynamics, the theory of the strong interaction, still seems to be going strong!

27th July 2015

In search of rare Higgs boson production with top quarks

In proton-proton collisions, several processes can lead to the production of a Higgs boson. The most “frequent” process (which is about one collision in four billion!) is the fusion of two gluons, contained in the initial protons, into a Higgs boson through a “top-quark loop”. Least frequent is a mode where the Higgs boson is produced in association with a pair of top-quarks.

24th March 2015

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

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.

3rd July 2014

Measuring top production in the LHC

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.

3rd July 2014

Is new physics running out of corners?

Friday was the last occasion for Moriond participants to see new results on specific physics topics since Saturday is reserved for summary talks. The topic was 'Beyond the Standard Model' -- a very large subject, which covers an incredible number of theoretical models, from Supersymmetry to Two-Higgs-Doublet Models, two of the most discussed topics of the day.

24th March 2014

New results on the properties of the top quark

At the EPS HEP conference today, ATLAS released a new precise measurement of the top quark mass using events where both top quarks decay via W bosons to electrons or muons. ATLAS also presented limits on the possibility of the top quark decaying to channels not foreseen in the Standard Model. A comparison of the behaviour of top quarks and anti-top quarks produced at the LHC is in agreement with the prediction of the Standard Model, disfavouring models of new physics that require a large top/anti-top asymmetry.

19th July 2013

TOP 2012 - Part 2

Welcome back, dedicated top quark enthusiast. I’m sure you’ve all been waiting on the edge of your seats for an update from TOP 2012, and I can now confirm that a combined team of LHC & Tevatron physicists narrowly beat a mixed team of physicists from LHC & Tevatron at croquet.

26th September 2012
26th September 2012

TOP 2012 - Part 1

Greetings from the TOP 2012 conference, Winchester UK! What’s a ‘Winchester’ I hear you asking? A type of gun? Indeed yes, though sadly not of the smoking variety that we’re all so keen to find. However in this particular case Winchester is a historical town in the south of England, complete with the typical rolling green fields, a cathedral, and the not so typical contingent of visiting physicists!

18th September 2012
18th September 2012

The Physics of Top Quarks

The 5th International Workshop on Top Quark Physics (TOP2012) will take place in Winchester, UK, from the 16th to the 21st of September. It will gather experts in the field of top quark physics as well as PhD students and will highlight the newest results and topics related to the physics of top quarks.

11th September 2012
11th September 2012

Top down: Reflections on a long and sleepless analysis journey

For the last months (which feel like years…) I’ve been working, within a small group of people, on the precision measurement of the top quark pair production cross section, and if you think that sounds complicated – the German word is “Top-Quark-Paarproduktionswechselwirkungsquerschnitt”.

21st August 2011

ATLAS starting to get on Top of things

ATLAS is about to check one more particle off of its Standard Model (SM) checklist. Namely the top quark. This famous quark is perhaps one of the most complex of the SM particles.

26th July 2010