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Posts Tagged ‘DZero’

The DZero collaboration found its data for the production of a W boson and two jets to be in agreement with the predictions by the Standard Model. The red peak below 100 GeV/c2 is a well-known Standard Model feature of the decays of W and Z bosons. If the CDF excess is interpreted as a new particle, the dotted line shows what such a particle would look like in the DZero detector. The DZero data shows no excess around 145 GeV/c2.

This article first appeared in Fermilab Today June 10.

Two months ago, CDF scientists reported an unexpected excess of proton-antiproton collisions that produce a W boson accompanied by two jets of particles. One possible explanation for the excess could be the existence of a new, unanticipated particle. Now the DZero collaboration has finished an independent analysis that tests the CDF result. Following the analysis procedure employed by CDF as closely as possible, DZero scientists did not find the same excess in the data. The collaboration will report its result and the details of its analysis at 4 p.m. today at a seminar at Fermilab.

“Our data for collisions that produce a W boson plus two jets are in agreement with the predictions from the Standard Model,” said DZero co-spokesperson Dmitri Denisov. “We have looked among two hundred trillion particle collisions, and we don’t see the excess reported by CDF.”

Over the last 10 years, CDF and DZero have published more than 500 measurements of particle physics processes using two different particle detectors and independent analysis tools. The results agree more than 99 percent of the time, but there are rare occasions that the findings differ.

“This is exactly how science works,” said DZero co-spokesperson Stefan Sӧldner-Rembold. “Independent verification of any new observation is the key principle of scientific research. At the Tevatron, we have two experiments that, by design, can check each other.”

Now that the independent analyses have been completed, the difference between the two experiments’ results must be understood and resolved. Fermilab Director Pier Oddone and the CDF and DZero collaborations have agreed to create a task force that will coordinate a study of the two experiments’ analyses. The task force will consist of members from both experiments and Fermilab theorists Estia Eichten and Keith Ellis.

Today’s seminar will take place at 4 p.m. in Ramsey Auditorium and will also be webcast.

The DZero paper has been submitted to Physical Review Letters, and a copy is available at the collaboration’s website. The CDF paper is available at their website.

— Kurt Riesselmann & Katie Yurkewicz

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A previous Result of the Week studied the difference between the masses of the top quark and antiquark. This week, DZero returns to the question, this time with a more precise scale.

This article appeared in Fermilab Today April 14.

In July of 2009, the DZero collaboration using the Tevatron at Fermilab reported on a measurement of the difference between the mass of top quarks and antitop quarks. This is a very interesting topic. One of the most fundamental tenets of the Standard Model is that the laws of physics apply equally well to matter and antimatter. This is called Charge, Parity and Time (CPT) invariance. Translated into English, CPT invariance means that if we reverse the left-right directions, go backwards in time instead of forward, and swap positive charges to negative ones, that it is impossible to tell the difference between the values. One crucial consequence is that matter and the corresponding antimatter particles must have identically the same mass. In fact, this tenet is even more fundamental than the Standard Model and physicists think that any theory that describes our universe must incorporate this necessary principle.

Of course a fundamental tenet is a challenge to physicists. Finding that this tenet is false would result in a total shakeup of our understanding. While measuring the mass differences between matter and antimatter is straightforward to do for leptons, it is very difficult to do for quarks. Quarks typically interact with their surroundings in the fractions of a second after they are created and before they decay. This makes it difficult to get a good measurement of the quarks’ masses.

Top quarks have the unique property that they decay very rapidly, indeed so rapidly that they have decayed before they undergo any subsequent interactions. Thus we know the mass of the top quarks with more precision than any other type of quark. This provides an interesting opportunity to test CPT invariance, especially since we suspect that any new and unobserved physics is more likely to occur at very high masses. Given that the top quark is the highest mass particle ever discovered, measuring the differences between the masses of top quarks and antiquarks is an ideal way to study this question.

Using nearly four times more data than the previous result, DZero physicists’ new measurement is twice as precise as the earlier one. The bottom line is that there is no indication of a mass difference, which is a marvelous confirmation of CPT invariance.

– Don Lincoln

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One of the first lead-ion collisions in the LHC as recorded by the CMS experiment on November 8, 2010. Image: Courtesy of CERN

In 2006, the popular television show Mythbusters tried to test a legend, which was that two Civil War-era bullets, if fired at one another, would fuse into a single bullet. The team was unable to confirm the myth, simply because it was too hard to get the bullets to collide. They eventually did a simpler test and fired one bullet into a stationary bullet and found that the two bullets did indeed fuse. They listed the legend as plausible. And in 2009, a story in the UK’s Daily Mail showed two bullets fired during the Crimean War that were reported to have hit in midair and fused. The odds of this occurring were incredibly small, but even rare things happen.

At the LHC, bullets made of lead don’t collide in the accelerator, but lead nuclei did during the December 2010 running period. Every second, two clouds of 100 trillion of these subatomic lead bullets passed by one another and something like 200 times per second two of them collided head on. The fireball formed between colliding lead nuclei is much more complex than ordinary collisions between protons, which makes it correspondingly more difficult to study the details of these collisions.

For some physicists, lead nuclei collisions weren’t enough. They wanted to see something never before observed. They wanted to be the first to see Z bosons in collisions between heavy nuclei. Z bosons can decay in many ways, but a decay into pairs of muons is the most striking signature.

A short while ago, CMS reported an observation of lead collisions that produced Z bosons. Because muons can escape the fireball more-or-less unscathed, this observation opens a unique window into the collision’s inner workings. In the paper described today, CMS physicists report the results of a detailed study of 39 Z bosons made in collisions between lead nuclei. We are just beginning to exploit the capabilities of the LHC.

And, with all due respect to the producers of Mythbusters, the accelerator scientists at the LHC have tested the idea of colliding subatomic lead bullets. Verdict: confirmed.

— Don Lincoln

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It’s been a very exciting winter conference season here at DZero, one of the two huge experiments at the Fermilab Tevatron Collider.
 
 
 

The massive size of the DZero detector is evident in comparison to the size of the man standing near the top of the image. Credit: Fermilab

What’s the winter conference season? We pretty much take data all the time, and we analyze the data and produce dozens of results every year. These analyses are ongoing, but there are two big events in the year where experiments here and at the Large Hadron Collider at CERN aim to roll out new results. That’s the winter and summer conferences. It’s kind of like when the fashion designers send their latest creations down the runway, hoping to turn as many heads as possible. These conferences are where we introduce to the world our latest and greatest insights about the nature of the universe.

The biggest winter conference is “Rencontres de Moriond” held in the Italian Alps, and it just completed two weeks ago. The big summer conference changes from year to year, and last year it was the International Conference on High Energy Physics, or ICHEP, in Paris. I was fortunate to give a talk for DZero at that one (yes, a perk of being a physicist is that every now and then you get to go somewhere nice).

 A deluge of data:

 

As a scientist on DZero and head of the team that runs the data handling for many Fermilab experiments, things get busy in many ways when we gear up for the conference seasons. For example, physicists run computer programs to analyze the mountain of data that comes from the detector. The computers are always busy, but before the conferences they get super busy as people try to finish up their analyses. For those weeks we deliver around 200 terabytes of data per day! The amount of data in 200 terabytes is equivalent to watching a high-definition television station non-stop for 2 ½ years (that reminds me that my 5-year-old daughter needs to watch less TV)!! Our data-handling system is very robust and tends to run itself. But, this season the demand for data was so great that some future plans to make it more efficient had to get implemented very quickly. Fortunately, that all worked and now it’s ready for even greater demands that may be coming in the summer.

 The Results:

 
 
 

A smattering of results from DZero for the 2011 winter conferences. Click on image to make it larger. Credit: DZero

I’m also a “subgroup” convener, which means I organize a small group of analyses and the people doing them. Probably the most exciting result being presented this winter is the Tevatron mass exclusion for the Higgs boson, which was written about previously in Quantum Diaries. But there are lots of other new results as well, including two from my subgroup.

One result is an updated result, which means we’ve analyzed more data, from an analysis that looks for collisions that produce two Z bosons (ZZ). These events are very, very, very rare. In fact in the Standard Model, the theory that describes sub-atomic particles, the Higgs is the next rarer process. Showing that we can find ZZ proves that we really understand our detector and our data. And, in fact, the result shows that we find ZZ at the level the Standard Model predicts.

The other analysis looks for events that have a W boson and a photon (the latter is a particle of light). We now see a few hundred of these events and have a clear picture of the “radiation amplitude zero”, which is an interesting effect predicted by the Standard Model that says that the W and photon fly off in some directions more often than others.

I mentioned I was lucky enough to give a talk at last year’s big summer conference. It was exciting to be in Paris, which is my favorite foreign city and where I’m reminded how much of my high school French I’ve forgotten. But it was also exciting because that was the conference where the new LHC experiments showed their first results.

 This winter they had more results and joined us in comparing experiment with theory. While the LHC is just starting, as you may have heard, the Tevatron will wind down soon. But that doesn’t mean we’re done. We will have lots more data and lots more analyses, including possibly some new ones. The results from our final data set will be appearing at many summer and winter conferences to come!

— Adam Lyon

  

  

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The top (CDF) and bottom (DZero) images show the expected and observed 95 percent confidence level upper limits on the production rate of a Higgs boson as a multiple of the Standard Model prediction, assuming standard model decay branching ratios. The solid, horizontal line shows the prediction for the Higgs boson according to the Standard Model. We determine our measurement by how our data relates to this solid line. The figures have two squiggly lines: one dotted and one solid. The dotted line shows what we expected to measure and is surrounded by a bright green band. The band shows how certain we were in our prediction. The best way to interpret this is that the bright green region shows the area where we predicted our measurement should be.

This article ran in Fermilab Today March 17.

The Standard Model of particle physics needs the Higgs mechanism to explain why all the particles in our universe have mass, but no experiment has yet observed the elusive Higgs boson. Answering the question of whether the Higgs mechanism is correct or whether something else is responsible for the masses of particles is central to our understanding of nature. Many physicists around the world have spent decades searching for the Higgs boson. This week, a crucial step forward in this quest has been made by the CDF and DZero experiments.

All recent Higgs boson mass exclusions have come from combinations of results from more than one experiment. Despite the importance of such combined statements, it is an important milestone when a single experiment reaches the level of sensitivity necessary to rule out or see the Higgs boson. Late last week, the CDF and the DZero experiments crossed this threshold individually. The CDF and DZero experiment collaborations recently updated their Higgs boson searches in the high mass range (130 to 185 Gev). In this range, the Higgs boson mass is high enough to allow it to decay to a pair of W bosons.

Together, the Tevatron experiments put to good use an additional 1.5 inverse femtobarns of data collected since their joint result from last summer, and added several new improvements to their analysis techniques. The new data and improvements have allowed both Tevatron experiments to exclude a portion of the Higgs boson mass range: 158 to 168 GeV for CDF and 163 to 168 GeV for DZero. The CDF and DZero experiments have also combined their results; the region thus excluded is 158 to 173 GeV. A Higgs boson of mass 165 GeV is now excluded at the unprecedented level of more than a 99.5 percent confidence level.

Fermilab currently expects the Tevatron to keep recording data until September 2011. CDF and DZero are also ideally suited to look for the Higgs boson in the low mass range, where the Higgs would decay mainly into bottom quarks. CDF and Dzero expect to present new results in this search region later this year. This large data sample, along with expected analysis improvements, will allow the experiments to either exclude the Higgs boson over the entire mass range of interest if it does not exist or to see hints of it – representing a major breakthrough in our understanding of nature.

— Edited by Andy Beretvas

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This article ran in Fermilab Today March 14.

Improved analysis techniques and more data have made the Tevatron experiments more sensitive to the Higgs boson. CDF and DZero exclude a significant portion of the high-Higgs-mass range.

The CDF and DZero experiments at Fermilab have reached new ground in their quest to find the Higgs boson, a key member of the particle zoo known as the Standard Model. For the first time, each experiment by itself excludes regions of the expected Higgs mass range as more sophisticated data analysis techniques and more data from the Tevatron particle collider have increased their sensitivity to the Higgs boson. This week, the two collaborations, each comprising about 500 scientists, will present details of their results at conferences and seminars around the world, including the Rencontres de Moriond in Italy.

“This makes the Tevatron the frontrunner in the hunt for the Standard Model Higgs boson,” said Fermilab physicist Rob Roser, co-spokesperson for the CDF experiment. “We are getting more mileage out of 10 years worth of Tevatron Run II data.”

The Tevatron, four miles in circumference, is the world’s highest-energy proton-antiproton collider.

“It is impressive to see the progress in the analysis of the Tevatron data from CDF and DZero,” said Fermilab Director Pier Oddone. “Step by step they are narrowing the space in which the Higgs could be hiding.”

Searches by previous experiments and constraints due to precision measurements of the Standard Model of Particles and Forces indicate that the Higgs particle should have a mass between 114 and 185 GeV/c2. (For comparison: 100 GeV/c2 is equivalent to 107 times the mass of a proton.) The CDF and DZero experiments are now sensitive to excluding Higgs bosons with masses from 153 to 179 GeV/c2. Statistical fluctuations in the number of observed particle collisions that mimic a Higgs signal, mixed with collisions that may have produced a Higgs boson, affect the actual range that can be excluded with 95 percent certainty. Combining their independent Higgs analyses, the two experiments now exclude a Higgs boson with a mass between 158 and 173 GeV/c2. The recording of additional collisions and further improved analysis of data will reduce the size of the statistical fluctuations and, over time, could reveal a signal from the Higgs boson.

“Fermilab plans to operate the Tevatron collider until September 2011,” said DZero co-spokesperson Stefan Söldner-Rembold, of the University of Manchester. “During this time, we will increase what is already the largest data set from a hadron collider at the Energy Frontier.”

For the present data analysis, CDF and DZero scientists concentrated on the search for a high-mass Higgs boson that has a mass heavier than 130 GeV/c2. But the Tevatron experiments also continue to look for a low-mass Higgs boson. In this case, the Higgs boson decays mainly into bottom quarks, which would create a different pattern in the CDF and DZero detectors than the decay products of a high-mass Higgs.

“The low-mass scenario now seems to be the more likely option,” said CDF co-spokesperson Giovanni Punzi, of the University of Pisa and the National Institute of Nuclear Physics (INFN) in Italy. “In the coming months, our collaborations will focus on both the high-mass and low-mass scenarios and optimize our analysis techniques for the entire Higgs mass range.”

Said DZero co-spokesperson Dmitri Denisov, of Fermilab, “If the Higgs boson exists, hints of its presence will emerge from the Tevatron data. If it does not exist, the CDF and DZero collaborations expect to rule out the remainder of the allowed mass range and scientists would have to wonder again: how do fundamental particles acquire mass?”

— Kurt Riesselmann

Combined the Tevatron experiments now are sensitive to a Higgs mass from 153 to 179 GeV/c2, but statistical fluctuation reduce the actual mass range that can be excluded so far. For the first time, the experiments now also exclude Higgs mass ranges individually (see CDF and DZero graphics).

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CDF kicked off the new year with the paper “CDF Finds Evidence for a Mass Dependant Forward-Backward Production Asymmetry of Top Quarks”, which has drawn some interest in the blogosphere.

The analysis itself is intriguing. Physicists expect to see the number of top quarks and antitop quarks produced along the beam line to meet with Standard Model predictions of nearly equal amounts. But, CDF isn’t seeing that. The collaboration sees more top quarks along the proton direction and fewer top quarks produced along the antiproton direction. CDF co-spokesman Rob Roser explains the analysis nicely in the article below.

But the really intriguing part is what this could all mean. It could signal a hint of new physics not predicted by the Standard Model.  Bloggers at Resonaances and Nature speculate on possible causes for the asymmetry. It could also be nothing more than an anomaly. In the patience-trying nature of particle physics, we will have to wait for more data to know for sure. Fortunately, CDF’s sister detector at the Tevatron, DZero, can do a complimentary analysis, widening the data set. Until that occurs, check out the analysis yourself below.

The article below ran in Fermilab Today January, 7.

CDF finds evidence for top quark production asymmetry

A new analysis that will be presented at a Wine and Cheese lecture at Fermilab today points to an asymmetry in top quark production. This analysis raises the asymmetry of forward and backward top quark production found in a 2008 analysis to a ~3 sigma level.

In nature, symmetry is seen as pleasing and balanced designs, such as the intricate pattern on a tortoise shell or the structure of a snowflake.

In elementary particle physics, symmetry is fundamental to the theories we use to describe the world in which we live. A discrepancy in the symmetry predicted by theories of the Standard Model can point to new types of physics, an anomaly in the data or that the current theories need revision.

CDF researchers have measured the symmetry of how top quarks emerge from collisions, forward or backward, and how they decay. This analysis was performed for the first time in 2006 at CDF. CDF and DZero both published their inclusive analysis of this asymmetry in 2008. These highly cited white papers already pointed to an anomaly that has generated much interest in the theoretical community. This latest result takes the 2008 publications a step further, by adding more data, and looking at the dependence on the mass of the system. It is this dependence that is most discrepant with the Standard Model.

Fermilab’s Tevatron produces collisions that create top quark and anti-top-quark pairs via the strong force. Simple theoretical calculations predict that the Tevatron detectors should observe symmetric distributions of both top and antitop quarks. However, more detailed calculations suggest that these oppositely charged particles should have a slight preference as to how they emerge from the collisions.

The origins of this symmetry are subtle, but CDF has the sensitivity to be able to observe the 6 percent imbalance, which is predicted by the Standard Model. This result shows that nature prefers an imbalance that is even larger than predicted.

It is important to determine whether the top quark we are observing  behaves the way we expect this Standard Model object to act. There are a number of Beyond-the-Standard-Model theories such as Z’ (pronounced Z-prime) and large extra dimensions that predicts much higher asymmetries. By measuring this asymmetry in top quark production, CDF physicists can compare it to theoretical expectations and probe for potentially undiscovered new physics.

The figures show the number of top events as a function of delta rapidity. The blue shape is that of the background, the green is the Standard Model prediction for top, and the points are our data. The plot on the left contains events in which the ttbar mass is less than 450 GeV/c2 and is very symmetric. The plot on the right is for a ttbar mass of greater than 450 GeV/c2 and illustrates the discrepancy between expected and observed.

Utilizing 5.3 inverse femtobarns of data, CDF measured the top forward backward asymmetry and observed significant asymmetries when studying the production and the production as related to the pairs’ center of mass energy, t-tbar.

When considering the pairs’ mass, the asymmetry is dependent on both the mass and the direction of the production. Scientists expect that the same number of top quarks and antitop quarks would be produced along the beam line, but CDF saw that more tops were produced along the proton direction of the beam and fewer tops produced along the antiproton direction of the beam. This effect is magnified when one looks at the mass dependence of the top-antitop system.

For Mttbar> 450 GeV/c2the asymmetry is measured to be 48 ± 11 percent, three standard deviations from Standard Model expectation (9 ± 1 percent). Some theories suggest that such a mass dependence could be evidence of a massive new particle just out of reach at the Tevatron’s collision energy.

The LHC, a machine with significantly higher energy, cannot easily study this phenomenon, since the LHC does not make protons and antiprotons collide. However a new particle would still be observable in energy spectrum at the LHC. If the result at CDF truly is a sign of new physics, it may be that both machines will be required to understand its nature.

This result may provide the first important clue that there is new physics beyond the Standard Model. There may still be other interesting results as scientists from both Fermilab experiments continue to analyze the Tevatron’s now nearly 10 inverse femtobarns of sample data.

— Rob Roser, CDF co-spokesperson

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