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Posts Tagged ‘top’

Top Quarks… So Many Top Quarks

Wednesday, April 30th, 2014

Thousands of paper on top quarks exist. Why?

There are literally thousands of papers, collaboration notes, and conference notes with the words “Top” and “Quark” in the title. As of this post, there are 3,477 since 1979 listed on inSpires. There are many, many more that omit the word “quark”. And sure, this is meager compared to the 5,114 papers with the words “Higgs Boson” written since ’74, but that is over 50,000 pages of top quarks (estimating 15 pages/paper). To be fair, there are also many, many more that omit the word “boson”. But for further comparison, there are only 395 papers with a title including the words “Bottom Quark“, 211 with “Bottomonium“, and 125 with “Bottom Hadron“. So why are there so many papers written about the top quark? The answer is that the top quark is weird special.

http://www.symmetrymagazine.org/breaking/2009/09/02/top-quark-chefs

A single top quark candidate event at the Collider Detector experiment at Fermilab. Credit: CDF Collaboration

The top quark is very heavy, about 185 times heavier than the proton and ranks as the heaviest known elementary particle in all the particle kingdom. The second heaviest quark, the bottom quark, is only 4 or 5 times heavier than the proton. If you or I were a proton, then a medium-to-large school bus (without any people) would be a top quark. In fact, the top quark is so heavy it can decay into a real (on-shell) W boson, which is roughly half its mass. The only other particle that can do this is the Higgs. Though it is rare, exceedingly rare, the top quark can decay into real Z  and Higgs bosons as well. Not even the Higgs can top that last feat.

Top quark decaying into real, on-shell W boson and bottom quark. Credit: DZero Collaboration

However, the top quark is still a quark. It has an electric charge that is 2/3 as large as the proton. It has an intrinsic angular momentum (spin) equal to the proton’s or electron’s spin. The top quark is also colored, meaning that is interacts with gluons and is influenced by the strong nuclear force (QCD). When colored objects (quarks and gluons) are produced at collider and fixed target experiments, they undergo a process called hadronization. Hadronization is when two colored objects are far away from one another and the strong nuclear attraction between the two becomes so strong that a pair of colored objects will spontaneously be produced in the space between them. These new colored particles will then form bound states with the old colored states. However, the process hadronization means that we only observe the bound states of colored objects and not the colored objects themselves. Physicists have to infer their properties from the physics of bound states…. or do we?

jets

Colored objects before (L), during (Center L and Center R), and after (R) hadronization.

The onset of hadronization is typically occurs about 10-24 seconds after the creation of a colored object. Yes, that is 0.000000000000000000000001 seconds. That is incredibly fast and well beyond anything that can be done at an experiment. The mean lifetime of the top quark on the other hand is about 10-25 seconds. In other words, the top quark is much more likely to decay in to a W boson, its principle decay mode, than hadronize. By looking at the decays of the W boson, for example to an electron and an electron-neutrino, their angular distributions, and other kinematic properties, we can measure directly the top quark’s quantum numbers. The top quark is special because it is the only quark whose spin and charge quantum numbers we can measure directly.

feynman_t_decay_ljetsqq_pink

Top quark decaying into real, on-shell W boson and bottom quark. The W boson can subsequently decay into a charged lepton and a neutrino or into a quark and anti-quark. Credit: DZero Collaboration

The top quark tells us much about the Standard Model of particle physics, but it also may be a window to new physics. Presently, no one has any idea why the top quark is so much heavier than the bottom quark, or why both are orders of magnitude heavier than the electron and muon. This is called the “Mass Hierarchy Problem” of the Standard Model and stems from the fact that the quark and lepton masses in the theory are not predicted but are taken as input parameters. This does not mean that the Standard Model is “wrong”. On the contrary, the model works very, very well; it is simply incomplete. Of course there are new models and hypotheses that offer explanations, but none have been verified by data.

However, thanks to the 2012 discovery of the Higgs boson, there is a new avenue that may shed light upon the mass hierarchy problem. We now know that quarks and leptons interact with the Higgs boson proportionally to their masses. Since the top quark is ~40 times more massive than the bottom quark, it will interact with Higgs boson 40 times more strongly. There is suspicion that since the Higgs boson is sensitive to the different quark and lepton masses, it may somehow play a role in how masses are assigned.

Happy Colliding

– richard (@BraveLittleMuon)

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This article appeared in symmetry on March 19, 2014.

An international team of scientists from Fermilab’s Tevatron and CERN’s Large Hadron Collider has produced the world’s best value for the mass of the top quark.

An international team of scientists from Fermilab’s Tevatron and CERN’s Large Hadron Collider has produced the world’s best value for the mass of the top quark.

Scientists working on the world’s leading particle collider experiments have joined forces, combined their data and produced the first joint result from Fermilab’s Tevatron and CERN’s Large Hadron Collider. These machines are the past and current holders of the record for most powerful particle collider on Earth.

Scientists from the four experiments involved—ATLAS, CDF, CMS and DZero—announced their joint findings on the mass of the top quark today at the Rencontres de Moriond international physics conference in Italy.

Together the four experiments pooled their data analysis power to arrive at a new world’s best value for the mass of the top quark of 173.34 ± 0.76 GeV/c2.

Experiments at the LHC at the CERN laboratory in Geneva, Switzerland and the Tevatron collider at Fermilab in Illinois, USA are the only ones that have ever seen top quarks—the heaviest elementary particles ever observed. The top quark’s huge mass (more than 100 times that of the proton) makes it one of the most important tools in the physicists’ quest to understand the nature of the universe.

The new precise value of the top-quark mass will allow scientists to test further the mathematical framework that describes the quantum connections between the top quark, the Higgs particle and the carrier of the electroweak force, the W boson. Theorists will explore how the new, more precise value will change predictions regarding the stability of the Higgs field and its effects on the evolution of the universe. It will also allow scientists to look for inconsistencies in the Standard Model of particle physics—searching for hints of new physics that will lead to a better understanding of the nature of the universe.

“The combining together of data from CERN and Fermilab to make a precision top quark mass result is a strong indication of its importance to understanding nature,” says Fermilab director Nigel Lockyer. “It’s a great example of the international collaboration in our field.”

Courtesy of: Fermilab and CERN

Courtesy of: Fermilab and CERN

A total of more than six thousand scientists from more than 50 countries participate in the four experimental collaborations. The CDF and DZero experiments discovered the top quark in 1995, and the Tevatron produced about 300,000 top quark events during its 25-year lifetime, completed in 2011. Since it started collider physics operations in 2009, the LHC has produced close to 18 million events with top quarks, making it the world’s leading top quark factory.

“Collaborative competition is the name of the game,” says CERN’s Director General Rolf Heuer. “Competition between experimental collaborations and labs spurs us on, but collaboration such as this underpins the global particle physics endeavor and is essential in advancing our knowledge of the universe we live in.”

Each of the four collaborations previously released their individual top-quark mass measurements. Combining them together required close collaboration between the four experiments, understanding in detail each other’s techniques and uncertainties. Each experiment measured the top-quark mass using several different methods by analyzing different top quark decay channels, using sophisticated analysis techniques developed and improved over more than 20 years of top quark research beginning at the Tevatron and continuing at the LHC. The joint measurement has been submitted to the arXiv.

A version of this article was originally issued by Fermilab and CERN as a press release.

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This Fermilab press release was published on February 24.

Matteo Cremonesi, left, of the University of Oxford and the CDF collaboration and Reinhard Schwienhorst of Michigan State University and the DZero collaboration present their joint discovery at a forum at Fermilab on Friday, Feb. 21. The two collaborations have observed the production of single top quarks in the s-channel, as seen in data collected from the Tevatron. Photo: Cindy Arnold

Matteo Cremonesi, left, of the University of Oxford and the CDF collaboration and Reinhard Schweinhorst of Michigan State University and the DZero collaboration present their joint discovery at a forum at Fermilab on Friday, Feb. 21. The two collaborations have observed the production of single top quarks in the s-channel, as seen in data collected from the Tevatron. Photo: Cindy Arnold

Scientists on the CDF and DZero experiments at the U.S. Department of Energy’s Fermi National Accelerator Laboratory have announced that they have found the final predicted way of creating a top quark, completing a picture of this particle nearly 20 years in the making.

The two collaborations jointly announced on Friday, Feb. 21, that they had observed one of the rarest methods of producing the elementary particle – creating a single top quark through the weak nuclear force, in what is called the s-channel. For this analysis, scientists from the CDF and DZero collaborations sifted through data from more than 500 trillion proton-antiproton collisions produced by the Tevatron from 2001 to 2011. They identified about 40 particle collisions in which the weak nuclear force produced single top quarks in conjunction with single bottom quarks.

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 is America’s national laboratory for particle physics research. A U.S. Department of Energy Office of Science laboratory, Fermilab is located near Chicago, Illinois, and operated under contract by the Fermi Research Alliance, LLC. Visit Fermilab’s website at www.fnal.gov and follow us on Twitter at @FermilabToday.

The DOE Office of Science is the single largest supporter of basic research in the physical sciences in the United States, and is working to address some of the most pressing challenges of our time. For more information, please visit science.energy.gov.

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Some Higgs are bigger than others

Thursday, March 12th, 2009
New Higgs mass limits from Tevatron

Update 13 March: The Tevatron experiments have just released new, stronger limits on the mass of the Higgs boson.  Read about it here.

There have been a couple new results from the Tevatron this week.

The first result, shown by both CDF and DØ experiments, is the observation of single top production. The top quark was first observed in 1995 by the same two experiments, but that observation was of top pair production (a top plus its antiparticle). This discovery was of single top quarks, with no antiparticle partner.  In the Standard Model of particle physics, top pair production occurs roughly twice as often as single top production, and the top pair signal is also somewhat easier to see in data. The observation of single top production is important, because even though everyone expected the process to exist, demonstrating sensitivity to such a rare and difficult signal is an important milestone on the way to searches for even rarer processes, like production of a Higgs boson.

The other result is the most precise measurement (by a single experiment) to date of the W boson mass, by DØ. Whereas the single top measurement demonstrates increasing sensitivity to the Higgs, the W mass tells us what mass of Higgs boson we might expect. Since the mass of the W is supposed to come from the Higgs mechanism, a more precise measurement narrows the window of likely Higgs masses. The mass of the Higgs has interesting implications for current searches at the Tevatron and the upcoming searches at the LHC, because the Higgs may be more difficult to see depending on its mass.

(more…)

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