ATLAS experiment studies fragments of the top quark

October 9, 2017, ATLAS Experiment
Measured values of the top quark mass determined from the differential production rates. Credit: ATLAS Collaboration/CERN

Top quarks in the Large Hadron Collider (LHC) proton-proton collisions are predominantly produced in pairs, with one top quark and one top antiquark. In order to measure the production rates of top quark pairs, the ATLAS Experiment examined events with an electron, muon, and one or two jets that were likely to have originated from bottom quarks. By comparing the number of events with one bottom-quark jet to those with two bottom-quark jets, ATLAS was able to determine both the total production rate, as well as the efficiency of identifying bottom-flavoured jets.

This method was used to explore the kinematics of the electrons and muons originating from top quark decays. The result reaches an unprecedented level of precision, with total uncertainties ranging from 1 percent to 15 percent, providing new insights into the dynamics of top quark production and improving our knowledge of the distribution of gluons inside the proton. The result also allowed ATLAS to make a precise new determination of the top .

The top quark is a fundamental parameter of the Standard Model, which precisely predicts the relationship between the masses of the top quark, W and Higgs bosons. Any deviations from this pattern could hint at new particles or phenomena. Figure 1 shows the values of the top quark mass (mt) determined from five of the measured production rates. One of the strengths of this measurement comes from its combination of all the kinematic distributions, giving a value of mt = 173.2 ± 1.6 GeV.

Measuring the mass of the top quark is extremely challenging. The top quark cannot be directly observed; it must be inferred indirectly through decayed particles. In addition, measurements of the top quark mass are always affected by an irreducible theoretical ambiguity, as top quarks are particles subjected to the strong interaction and cannot decay in isolation without interacting with other quarks and gluons produced in the event.

The use of electron and muon kinematics has helped to reduce these ambiguities, and the new ATLAS measurement of the top quark mass has a smaller theoretical ambiguity than other measurements that examine jets from bottom and lighter quarks. This new measurement is an important step forward in our understanding of the top .

Explore further: On top of the top quark—new ATLAS experiment results

More information: Measurement of lepton differential distributions and the top quark mass in tt¯ production in pp collisions at s√=8 TeV with the ATLAS detector, arXiv:1709.09407 [hep-ex]

Related Stories

On top of the top quark—new ATLAS experiment results

September 27, 2017

Physicists from the ATLAS Experiment at CERN have presented exciting new results at the 10th International Workshop on Top Quark Physics (TOP2017), held in Braga (Portugal). The conference brought together experimental ...

New CERN results show novel phenomena in proton collisions

April 25, 2017

In a paper published today in Nature Physics , the ALICE collaboration reports that proton collisions sometimes present similar patterns to those observed in the collisions of heavy nuclei. This behaviour was spotted through ...

Hunting for the superpartner of the top quark

May 17, 2017

Supersymmetry (SUSY) is one of the most attractive theories extending the Standard Model of particle physics. SUSY would provide a solution to several of the Standard Model's unanswered questions, by more than doubling the ...

Precision measurements using top quarks at CMS

November 5, 2012

Amongst all known elementary particles, the top quark is peculiar: weighing as much as a Tungsten atom, it completes the so-called 3rd generation of quarks and is the only quark whose properties can be directly measured. ...

Recommended for you

Teaching a machine to spot a crystal

June 21, 2018

Protein crystals don't usually display the glitz and glam of gemstones. But no matter their looks, each and every one is precious to scientists.

How physics explains the evolution of social organization

June 20, 2018

A scientist at Duke University says the natural evolution of social organizations into larger and more complex communities that exhibit distinct hierarchies can be predicted from the same law of physics that gives rise to ...


Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.