Studying top quarks at high and not-so-high energies

Studying top quarks at high and not-so-high energies

CERN’s Giant Hadron Collider (LHC) is legendary for colliding protons at world-record energies—however typically it pays to dial down the power and see what occurs underneath much less excessive circumstances. The LHC began operation in 2010 with a collision power of seven TeV, and ran at 13 TeV from 2015 to 2018. However for one week in 2017, the LHC produced moderate-intensity collisions at solely 5 TeV—permitting scientists to research the manufacturing of assorted elementary particles at a decrease collision power.

One particle they had been particularly eager to review was the top quark. Because the heaviest-known elementary particle, the speed (or cross-section) for producing top-quark pairs relies upon very strongly on the collision power achieved. By measuring the manufacturing charge at completely different energies, scientists can study extra in regards to the distributions of the quarks and gluons that make up the proton.

The ATLAS Collaboration at CERN has launched a new measurement of the top-quark pair-production charge within the 5 TeV information pattern. With only a single week of knowledge, their ultimate measurement has an uncertainty of simply 7.5%. This uncertainty is primarily because of the very small dimension of the 5 TeV information pattern, with systematic uncertainties associated to the calibration of the LHC luminosity and the experimental response being just a few p.c.

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Top quarks decay quickly and go away a definite signature within the detector. To identify top-pair collision occasions, ATLAS physicists seemed for occasions with two electrons, two muons, or an electron–muon pair, one or two ‘b-tagged’ jets of particles (coming from b-quark decays), and a major momentum imbalance indicating the presence of a neutrino. This choice closely suppresses background occasions from the manufacturing of different forms of particles, notably within the case of electron–muon occasions. In occasions with both two electrons or two muons, there’s nonetheless a big background from occasions with Z bosons to take care of. Physicists diminished this background utilizing the measured energies and angles of the electrons and muons, requiring their mixture to be inconsistent with originating from a Z boson decay.

The brand new measurement is proven within the plot featured on this article (the purple triangle). Earlier measurements at increased energies from electron–muon occasions alone are additionally included. The cross-section at 5 TeV is greater than an element ten smaller than that at the very best power of 13 TeV. All of the measurements are in wonderful settlement with theoretical predictions, which mix the idea of quantum chromodynamics with information of the inner construction of the proton.

Such comparisons serve to validate the understanding of proton–proton collisions, and act as a springboard to the following LHC run beginning in 2022, the place CERN hopes to additional improve the LHC collision power in the direction of 14 TeV.

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Supply: … ATLAS-CONF-2021-003/

Studying top quarks at high and not-so-high energies

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