Archivi tag: strong nuclear force

Evidence for single top quark production through the weak nuclear force

Dopo una serie di tentativi che durano ormai da circa 20 anni, finalmente gli scienziati che lavorano agli esperimenti CDF e DZero presso il Fermi National Accelerator Laboratory hanno annunciato di aver trovato il modo di produrre un quark top. I due gruppi hanno affermato di aver osservato uno dei tanti metodi decisamente rari di produrre questa particella attraverso la forza nucleare debole, nel cosiddetto “canale-s”. Per arrivare a questo risultato, i ricercatori hanno dovuto analizzare più di 500 trilioni di collisioni protoni-antiprotoni che sono state realizzate con l’acceleratore Tevatron tra il 2001 e il 2011. I risultati indicano che in circa 40 collisioni, dove è stata prodotta la forza nucleare debole, sono stati identificati singolarmente quark top assieme a quark bottom.

Top quarks are the heaviest and among the most puzzling elementary particles. They weigh even more than the Higgs boson, as much as an atom of gold, and only two machines have ever produced them: Fermilab’s Tevatron and the Large Hadron Collider at CERN. There are several ways to produce them, as predicted by the theoretical framework known as the Standard Model, and the most common one was the first one discovered: a collision in which the strong nuclear force creates a pair consisting of a top quark and its antimatter cousin, the anti-top quark. Collisions that produce a single top quark through the weak nuclear force are rarer, and the process scientists on the Tevatron experiments have just announced is the most challenging of these to detect.

This method of producing single top quarks is among the rarest interactions allowed by the laws of physics.

The detection of this process was one of the ultimate goals of the Tevatron, which for 25 years was the most powerful particle collider in the world. “This is an important discovery that provides a valuable addition to the picture of the Standard Model Universe”, said James Siegrist, DOE associate director of science for high energy physics. “It completes a portrait of one of the fundamental particles of our universe by showing us one of the rarest ways to create them”. Searching for single top quarks is like looking for a needle in billions of haystacks. Only one in every 50 billion Tevatron collisions produced a single s-channel top quark, and the CDF and DZero collaborations only selected a small fraction of those to separate them from background, which is why the number of observed occurrences of this particular channel is so small. However, the statistical significance of the CDF and DZero data exceeds that required to claim a discovery. “Kudos to the CDF and DZero collaborations for their work in discovering this process”, said Saul Gonzalez, program director for the National Science Foundation. “Researchers from around the world, including dozens of universities in the United States, contributed to this important find”. The CDF and DZero experiments first observed particle collisions that created single top quarks through a different process of the weak nuclear force in 2009. This observation was later confirmed by scientists using the Large Hadron Collider. Scientists from 27 countries collaborated on the Tevatron CDF and DZero experiments and continue to study the reams of data produced during the collider’s run, using ever more sophisticated techniques and computing methods. “I’m pleased that the CDF and DZero collaborations have brought their study of the top quark full circle”, said Fermilab Director Nigel Lockyer. “The legacy of the Tevatron is indelible, and this discovery makes the breadth of that research even more remarkable”.

Fermilab: Scientists complete the top quark puzzle

Fermilab: Observation of s-channel production of single top quarks at the Tevatron
arXiv: Evidence for s-channel Single-Top-Quark Production in Events with one Charged Lepton and two Jets at CDF

arXiv: Search for s-channel Single Top Quark Production in the Missing Energy Plus Jets Sample using the Full CDF II Data Set

arXiv: Evidence for s-channel single top quark production in pp¯ collisions at s√ = 1.96 TeV

Easy tests in high energy experiments

Tre fisici teorici hanno fatto un passo in avanti per eliminare alcune ambiguità che derivano dalle complesse formule matematiche che vengono utilizzate per studiare le interazioni tra i quark, le particelle più fondamentali della materia che compongono protoni e neutroni, e i gluoni, le particelle enigmatiche responsabili dell’interazione nucleare forte che lega i quark nei nucleoni. Secondo gli scienziati, semplificando questi calcoli è possibile facilitare, per così dire, il lavoro dei fisici per realizzare previsioni più accurate quando vengono eseguiti gli esperimenti di laboratorio.

The theory describing those interactions is known as quantum chromodynamics (QCD), and is an important component of the Standard Model, the reigning theory of the interactions of subatomic particles. “An important goal in high energy physics is to make predictions that are as precise as possible“, said SLAC theoretical physicist Stan Brodsky. “This makes tests of QCD more rigorous. Most important, if QCD doesn’t pass our experimental tests, it could reveal new physics beyond the Standard Model“. In a paper published in Physical Review Letters, Brodsky and his colleagues, Matin Mojaza of CP3-Origins at the University of Southern Denmark and Xing-Gang Wu of Chongqing University in China, have presented a method that will help theorists to automatically eliminate an important theoretical ambiguity of QCD predictions. Particle theorists attempt to put the quantum realm under a mathematical microscope. However, the world of subatomic particles operates according to very different rules than our familiar everyday world. Quantum uncertainties take hold.

On the scale of quarks and gluons, E=mcis not a slogan on a t-shirt, it’s the law of the land, if there’s a possibility for a particular particle to exist, it, and others, will pop into and out of existence, obscuring what lies under the physicists’ calculational lenses.

These “now you see them, now you don’t” particles, called virtual particles, give rise to infinite terms in quantum calculations, a big problem for theorists, who must remove the uncertainty in their calculations caused by these infinities without introducing new ambiguities. This problem has obscured the precision of the theorists’ mathematical microscope. Brodsky and his colleagues have been developing a method called the Principle of Maximum Conformality (PMC) which can focus the mathematical microscope into the quantum world.

Building on this work, in their new paper, Mojaza, Wu and Brodsky show how a novel generalization of a technique that many theorists employ to remove infinities, called modified minimal subtraction, can be used to identify patterns within the calculations.

This, along with PMC, makes the calculations easier to reduce to a form that can be used to make testable predictions, free of ambiguities, the heart of scientific progress. In addition to adding another tool to the theorists’ toolbox and providing testable predictions to experimenters, their technique has another advantage, said Brodsky: “Since the method is systematic, it can be used as the basis of a computer algorithm“, automating the calculations even further.

SLAC: SLAC Theorist Helps Sharpen Tests of Fundamental Theory in High Energy Experiments

arXiv:A Systematic All-Orders Method to Eliminate Renormalization-Scale and Scheme Ambiguities in PQCD