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Posts Tagged ‘CDF’

I thought I might touch on the topic of a recent post by one of my fellow bloggers here at Quantum Diaries.  As was mentioned, researchers at Fermilab have recently discovered the Ξ0b (read Xi-0-b) a “heavy relative of the neutron.”  The significance of this discovery was found to 6.8σ [1]!  The CDF Collaboration has prepared a brief article regarding this discovery, which is is being submitted to Physical Review Letters (a peer-review journal).  A pre-print has been made available on arXiv.

But rather than talk about what’s already been written, let’s discuss something new.  Namely, how on earth did physicists know to look for such a particle?

The answer to this question takes us back to the year 1961.  An American physicist (and now Nobel Laureate, 1969)  by the name of Murray Gell-Mann proposed a way to arrange baryons and mesons based on their properties.  In a sense, Gell-Mann did for particle physics what Dmitri Mendeleev did for chemistry.

Gell-Mann decided to use geometric shapes for arranging particles.  He placed a baryon/meson onto these geometric shapes.  The location a particle was given went according to that particle’s properties.  All his diagrams however were incomplete.  Meaning, there were spaces on the shapes that a particle should have went, but the location was empty.  This was because Gell-Mann had a much smaller number of particles to work with, today more have been discovered; but we still have holes in the diagrams.

But to illustrate how Gell-Mann originally made these diagrams, I’ve shown an example using a triangle, which is part of a larger diagram that appeared in the previous post on this  subject.  I’ve also added three sets of colored lines to this diagram.

Let’s talk about the black set of lines first.  If you go along the direction indicated by each of these lines you’ll notice something interesting.  On the far right line (labeled Qe=+1, Up =2), there is only one particle along this direction, the Σ+b.  This baryon is composed of two up quarks, a beauty quark, and has an electric charge of +1.

Let’s go to the second black line (labeled Qe = 0, Up =1).  Here there are four particles (the Σ0b has yet to be discovered).  But all of these four particles have one up quark, and zero electric charge.

See the pattern?

But just to drive the point home, look at the orange lines.  Each line represents the number of strange quarks found in the particles along the line’s direction (0, 1 or 2 strange quarks!).  The blue lines do the same thing, only for the number of down quarks present in each particle. Also, for all the particles shown on this red triangle, each particle has one beauty quark present!

In fact, if you go back to the original post on the Ξ0b discovery, you’ll notice the diagram has three “levels.”  All the particles on the top level have two beauty quarks present.  Then the red triangle appears (that I’ve shown in detail above).  Then finally in the bottom level, all the particles have zero beauty quarks.

Also, if you spend some time, you can see the black, orange and blue lines I’ve drawn at right actually form planes in this 3D diagram.  And all the particles on one of these planes will have the properties of the plane (electric charge, quark content)!

So what’s the big deal about this anyway?

Well, when Gell-Mann first created the Eight-Fold-Way in the early 1960s, none of the shapes were “filled.”  But just like Dmitri Mendeleev, Gell-Mann  took this to mean that there were undiscovered particles that would go into the empty spots!!!!!

So this seemingly abstract ordering of particles onto geometric shapes (called the Eight-Fold-Way) gave Gell-Mann a way to theoretically predict the existence of new particles.  And just like Mendeleev’s periodic table, the Eight-Fold-Way went one step further, by immediately giving us knowledge on the properties these undiscovered particles would have!

If you’re not convinced, let’s come back to the experimental discovery of the Ξ0b, which is conveniently encompassed by the yellow star in the diagram above.  This particle was experimentally discovered just a few weeks ago.  But Murray Gell-Mann himself could have made the prediction that the Ξ0b existed decades earlier.  Gell-Mann would have even been able to tell us that it would have zero electric charge and be made of a u,s and b quark!!!

In fact, Gell-Mann’s Eight-Fold-Way tells high energy physicists that there is still one particle left to be discovered before this red triangle may be completed.  So, to all my colleagues in HEP, happy Σ0b hunting!

 

 

But in summary, it was the Eight-Fold-Way that gave physicists the clue that the Ξ0b was lurking out there in the void, just waiting to be discovered.

Until Next Time,

-Brian

 

References

[1] T. Aaltonen (CDF Collaboration), “Observation of the Xi_b^0 Baryon,” arXiv:1107.4015v1[hep-ex], http://arxiv.org/abs/1107.4015

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This article first appeared in symmetry breaking July 22.

Editor’s note:  The LHC experiments reported at the EPS meeting a tantalizing excess of Higgs-like events, short of claiming a discovery, but very intriguing nevertheless. See the Higgs search at the LHC section further below for more information on these results.

 The LHC experiments reported at the EPS meeting a tantalizing excess of Higgs-like events, short of claiming a discovery, but very intriguing nevertheless. See the Higgs search at the LHC section further below for more information on these results.

Experiments at Fermi National Accelerator Laboratory and the European particle physics center, CERN, are zooming in on the final remaining mass region where the Higgs particle might be lurking. Over the next seven days, Fermilab’s CDF and DZero collaborations and CERN’s ATLAS and CMS collaborations will announce their latest Higgs search results at the High-Energy Physics conference of the European Physical Society.

Scientists at Fermilab and CERN employ very similar methods to create the Higgs: accelerate particles to high energy using the world’s most powerful accelerators, the Tevatron (1 TeV beam energy) and the Large Hadron Collider (3.5 TeV), smash the particles together, and sift through the large number of new particles emerging from these collisions. But to find a Higgs particle among the many particles created, the teams of scientists are focusing on different signals (see below).

If the Higgs particle exists and has the properties predicted by the simplest Higgs model, named after Scottish physicist Peter Higgs, then the colliders at Fermilab and CERN already must have produced Higgs particles. But finding the tell-tale sign of a Higgs boson among all other particle signatures is like searching for a drop of ink in an ocean. Only if the accelerators produce more and more collisions do scientists stand a chance of finding enough evidence for the Higgs particle.

Where to look

The Higgs mechanism, developed in the 1960s by several independent groups of theorists, explains why some fundamental particles have mass and others don’t. Its mathematical framework fits perfectly into one of the most successful theories in science: the Standard Model of elementary particles and forces.

Experimenters sifting through data from one experiment after another have come up empty-handed; instead they have ruled out larger and larger swaths of potential Higgs territory. An analysis by the GFitter group of precision measurements and the direct and indirect constraints on the Higgs mass indicates that, in the simplest Higgs model, the Higgs particle should have a mass between approximately 115 and 137 billion electron volts (GeV)/c2, or about 100 times the mass of a proton.

Higgs search at the Tevatron

At Fermilab’s Tevatron, scientists attempt to produce Higgs particles by smashing together protons and antiprotons, composite particles that comprise elementary building blocks. When a proton and antiproton hit each other at high energy, scientists observe the collisions and interactions of these components, such as quarks, antiquarks and gluons. Those subatomic collisions transform energy into new particles that can be heavier than the protons themselves, as predicted by Einstein’s famous equation E=mc2.

At the Tevatron, which makes protons and antiprotons collide, scientists focus on finding signs for the decay of the Higgs particle into a bottom quark and anti-bottom quark.

At the Tevatron, which makes protons and antiprotons collide, scientists focus on finding signs for the decay of the Higgs particle into a bottom quark and anti-bottom quark.
Tevatron scientists have carried out detailed simulations of such collisions and found that the best chance for producing, say, a 120-GeV Higgs boson at the Tevatron are quark-antiquark collisions that create a high-energy W boson (see graphic). This W boson has a chance to spend its extra energy to generate a short-lived Higgs boson. The W boson and the Higgs boson would then decay into lighter particles that can be caught and identified by the CDF and DZero particle detectors, which surround the two proton-antiproton collision points of the Tevatron.

According to the Standard Model, such a 120-GeV Higgs boson will decay 68 percent of the time into a bottom quark and anti-bottom quark. But other collision processes and particle decays also produce bottom and anti-bottom quarks. Identifying an excess of these particles due to the decay of the Higgs boson is the best chance for Tevatron scientists to discover or rule out a Standard Model Higgs.

At the EPS conference, CDF and DZero will report (see press release) that, for the first time, the two collaborations have successfully applied well-established techniques used to search for the Higgs boson to observe extremely rare collisions that produce pairs of heavy bosons (WW or WZ) that decay into heavy quarks. This well-known process closely mimics the production of a W boson and a Higgs particle, with the Higgs decaying into a bottom quark and antiquark.

Higgs search at the LHC

At the LHC, located on the French-Swiss border, scientists smash protons into protons. Because the LHC operates at higher collision energies than the Tevatron, each collision produces on average many more particles than a collision at the Tevatron. In particular, the LHC floods its particle detectors with bottom and anti-bottom quarks created by many different types of subatomic processes. Hence it becomes more difficult than at the Tevatron to find this particular “ink in the ocean”—an excess of bottom and anti-bottom quarks in the LHC data due to the Higgs particle.

At the EPS conference, the ATLAS scientists showed that they should have been able to exclude a Higgs boson with mass between 130 and 200 GeV/c2, but instead the collaboration saw an excess of events in the 130 to 155 GeV/c2 range, as reported by ATLAS physicist Jon Butterworth in his blog at the Guardian. It could be a fluctuation, but it could also be the first hint of a Higgs signal. Geoff Brumfiel writes for Nature News that the CMS experiment also sees an excess in the 130 to 150 GeV/c2 range. (CMS physicist Tommaso Dorigo has posted the relevant CMS Higgs search plots in his blog.) Combined, the two LHC experiments should have enough data by the end of this summer to say whether this excess is real or not. The Tevatron experiments are getting close to being sensitive to a Higgs particle near 150 GeV as well. Here is the new DZero result: the dotted line, which indicates sensitivity, is approaching 1 near 150 GeV, but the solid line, which is the actual observation, is significantly below 1, yet it differs from the expectation only at the 1 to 1.5 sigma level. Bottom line: DZero scientists cannot exclude a Higgs boson in this range. And here is the new CDF result: Again, for a Higgs mass of about 150 GeV/c2, the sensitivity approaches 1, and the observed Higgs constraints agree well with the expectations. (Note that DZero shows 1-sensitivity and CDF shows sensitivity; that’s why the CDF curve is above 1.) On Wednesday, July 27, CDF and DZero will present their combined results for this mass range at the EPS conference. The sensitivity of the combined CDF and DZero results will be even closer to 1 at 150 GeV/c2.

At the Large Hadron Collider, which smashes protons into protons, scientists focus on finding signs for the decay of the Higgs particle into two photons.

At the Large Hadron Collider, which smashes protons into protons, scientists focus on finding signs for the decay of the Higgs particle into two photons.

For a light Higgs boson, LHC scientists focus on a very different Higgs production and decay process, complementary to the Higgs search at the Tevatron. Detailed simulations of high-energy proton-proton collisions have shown that the best chance to catch, say, a 120-GeV Standard Model Higgs particle at the LHC is to look for a Higgs boson emerging from the collision of two gluons, followed by its decay into two high-energy gamma rays (see second graphic). This is an extremely rare process since the Higgs boson doesn’t interact directly with the massless gluons and gamma rays. Instead, the Higgs production and decay occur through intermediate, massive quark-antiquark loops, which can temporarily appear in subatomic processes, in accordance with the laws of quantum mechanics. The intermediate loop, however, makes this process much rarer to occur. In particular, the decay of a 120-GeV Standard Model Higgs boson into two gamma rays happens only once out of 500 times. Hence LHC scientists will need to gather a sufficiently large number of proton-proton collisions to observe this process.

Why do physicists think that the Higgs particle exists?

The discovery in the 1980s of heavy, force-carrying particles, known as W and Z bosons, confirmed crucial predictions made by the Standard Model and the simplest Higgs model. Since then, further discoveries and precision measurements of particle interactions have confirmed the validity of the Standard Model many times. It now seems almost impossible to explain the wealth of particle data without the Higgs mechanism. But one crucial ingredient of this fabulous particle recipe—the Higgs boson itself—has remained at large. Does it exist? How heavy is it? Does it interact with quarks and other massive particles as expected? These questions will keep scientists busy for years to come.

Want to learn more about what the Higgs particle is and how it gives mass to some particles? Watch this 3-minute video.

Kurt Riesselmann

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Fermilab issued the following press release July 20.

Baryons are particles made of three quarks. The quark model predicts the baryon combinations that exist with either spin J=1/2 (this graphic) or spin J=3/2 (not shown). The graphic shows the various three-quark combinations with J=1/2 that are possible using the three lightest quarks--up, down and strange--and the bottom quark. The CDF collaboration announced the discovery of the neutral Xi-sub-b (), highlighted in this graphic. Experiments at Fermilab’s Tevatron collider have discovered all of the observed baryons with one bottom quark except the Lambda-sub-b, which was discovered at CERN. There exist additional baryons involving the charm quark, which are not shown in this graphic. The top quark, discovered at Fermilab in 1995, is too short-lived to become part of a baryon. Credit: Fermilab

Scientists of the CDF collaboration at the Department of Energy’s Fermi National Accelerator Laboratory announced the observation of a new particle, the neutral Xi-sub-b (Ξb0). This particle contains three quarks: a strange quark, an up quark and a bottom quark (s-u-b). While its existence was predicted by the Standard Model, the observation of the neutral Xi-sub-b is significant because it strengthens our understanding of how quarks form matter. Fermilab physicist Pat Lukens, a member of the CDF collaboration, presented the discovery at Fermilab on Wednesday, July 20.

The neutral Xi-sub-b is the latest entry in the periodic table of baryons. Baryons are particles formed of three quarks, the most common examples being the proton (two up quarks and a down quark) and the neutron (two down quarks and an up quark). The neutral Xi-sub-b belongs to the family of bottom baryons, which are about six times heavier than the proton and neutron because they all contain a heavy bottom quark. The particles are produced only in high-energy collisions, and are rare and very difficult to observe.

Although Fermilab’s Tevatron particle collider is not a dedicated bottom quark factory, sophisticated particle detectors and trillions of proton-antiproton collisions have made it a haven for discovering and studying almost all of the known bottom baryons. Experiments at the Tevatron discovered the Sigma-sub-b baryons (Σb and Σb*) in 2006, observed the Xi-b-minus baryon (Ξb-) in 2007, and found the Omega-sub-b (Ωb-) in 2009. The lightest bottom baryon, the Lambda-sub-b (Λb), was discovered at CERN. Measuring the properties of all these particles allows scientists to test and improve models of how quarks interact at close distances via the strong nuclear force, as explained by the theory of quantum chromodynamics (QCD). Scientists at Fermilab and other DOE national laboratories use powerful computers to simulate quark interactions and understand the properties of particles comprised of quarks.

Once produced, the neutral Xi-sub-b travels a fraction of a millimeter before it decays into lighter particles. These particles then decay again into even lighter particles. Physicists rely on the details of this series of decays to identify the initial particle. The complex decay pattern of the neutral Xi-sub-b has made the observation of this particle significantly more challenging than that of its charged sibling (Ξb-). Combing through almost 500 trillion proton-antiproton collisions produced by Fermilab’s Tevatron particle collider, the CDF collaboration isolated 25 examples in which the particles emerging from a collision revealed the distinctive signature of the neutral Xi-sub-b. The analysis established the discovery at a level of 7 sigma. Scientists consider 5 sigma the threshold for discoveries.

CDF also re-observed the already known charged version of the neutral Xi-sub-b in a never before observed decay, which served as an independent cross-check of the analysis. The newly analyzed data samples offer possibilities for further discoveries.

The CDF collaboration submitted a paper that summarizes the details of its Xi-sub-b discovery to the journal Physical Review Letters. It will be available on the arXiv preprint server on July 20, 2011.

CDF is an international experiment of about 500 physicists from 58 institutions in 15 countries. It is supported by the U.S. Department of Energy, the National Science Foundation and a number of international funding agencies.

Fermilab is a national laboratory funded by the Office of Science of the U.S. Department of Energy, operated under contract by Fermi Research Alliance, LLC.

Related information: Graphics and photos

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An astute viewer of the Big Bang Theory TV show from the school of physics at Georgia Tech noticed that the white board in Leonard and Sheldon’s apartment had on it a plot for a result from the CDF experiment at Fermilab. The image of the plot from the episode was posted on the school’s Facebook site and later the Internet.

And not just any plot. The plot.

The one that has caused debate in the blogosphere and media and put the Tevatron’s two collider experiments at odds.

April 7 CDF announced it had seen an unexpected spike in its data, “a bump” as physicists call it,  that potentially could signal a new particle or force, but more data was needed to know for sure. Science lovers jumped at the news and speculations about what this could mean abounded.

Discussions took a turn when DZero’s results released June 10 found no such bump.

The experiments will continue to analyze larger data sets and compare data until “the bump” is verified or disputed beyond a doubt. So stay tuned. I’m sure the Big Bang Theory writers will.

George Smoot (in Sheldon's seat, no less!) on the set of Big Bang Theory. Photo courtesy of George Smoot.

Scientists help advise the show’s writers, and the show has a history of making references to current experiments or leaders in the field. And sometimes physicists even make cameos.

In 2009 Nobel laureate George Smoot, professor of physics at the University of California, Berkeley, research physicist at Lawrence Berkeley National Laboratory, and a confessed fan of the show, agreed to appear in the episode.

The show averages about 9 million viewers a week but not all physicists are fans. In a symmetry magazine article Jennifer Ouellette Science blogger and author of the Buffyverse came her critique of the show and the high-energy physics communities reaction to it.

Personally I think the community should embrace the program. I think the public is smart enough to recognize the stereotypes and formulaic dating plots for what they are and not a documentary-type representation of physicists. Plus the show exposes people to the science, which is never a bad thing. I have met many people who had never heard of particle physics prior to watching the Big Bang Theory or reading “Angles and Demons”.  While not all of these people were enticed to learn more about particle physics, some were, and no one came away with a negative view of the science.  

—  Tona Kunz

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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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Blois Blois Blois!

Tuesday, June 7th, 2011

(2011/06/10: Updated to include D0 result!)

At the Blois conference last week there were plenty of interesting results and updates from experts across the world. The work spanned a whole range of fields from particle physics to cosmology, and I think the most interesting result was the update on the CDF dijet anomaly. It’s worth taking a look at the program for the conference if you want a nice cross section of the status of experiment and theory for various topics.

Giovanni Punzi presented a [pdf]talk on the search for the Higgs boson. As we would expect, we see the usual exclusion plots, with LEP at the low mass end, Tevatron making inroads on the region around 165GeV and the LHC starting to close in across the whole range (especially in the WW mode). Of course we’re recording data so rapidly at the moment that the results in this talk are not really representative of our current sensitivity, and the conferences will be playing catch-up for a while!

In his talk, Punzi showed the updated dijet mass spectrum from CDF. This was blogged about by Michael and Flip a while back. At the time there was a tantalizing bump in a spectrum, with a 3.2σ excess, suggesting that there could be a new resonance around 150Gev! 3.2σ is exciting, but it’s not enough to justify calling this a discovery. We usually ask for 5σ for that, as well as a whole host of other tests to make sure there hasn’t been some error in the analysis or some non-trivial hardware effect:

CDF anomaly at 4.3fb^-1

CDF anomaly at 4.3fb^-1

The big question on everyone’s mind at the time was “Is this just a fluctuation?” Well the answer seems to be “No!”, and Punzi shows us why:

CDF anomaly at 7.3fb^-1

CDF anomaly at 7.3fb^-1

Adding another 3.0fb-1 of data (about 70% more than before) gives an excess of nearly 5σ, and taken on its own has an excess of 2.85σ. Either there is a serious systematic problem at CDF, or this is a new effect! If this indeed new physics, D0 should see something similar, but so far we haven’t heard anything from them. There are efforts at D0 and at LHC to see if we can recreate this peak, so right now all we can do is wait for more results to come in. No doubt you’ll hear about more updates on this blog, so keep reading. There are already quite a few papers on the arXiv about this effect and if you have any more information please leave it in the comments!

Update: D0 see no evidence of any anomaly in the dijet mass spectrum! Source.

D0 result

D0 result. The black points show the data. The dashed histogram shows the CDF anomaly. The red and blue histograms show expected contributions from the Standard Model.

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The CDF detector at Fermilab. Credit: Fermilab/ Reidar Hahn

Wednesday afternoon, the CDF collaboration announced that it has evidence of a peak in a specific sample of its data. The peak is an excess of particle collision events that produce a W boson accompanied by two hadronic jets. This peak showed up in a mass region where we did not expect one. The peak was observed in the 140 GeV/c2 mass range, as shown in the plot above. It is the kind of peak in a plot that, if confirmed, scientists associate with the existence of a particle. The significance of this excess was determined to be 3.2 sigma, after accounting for the effect of systematic uncertainties. This means that there is less than a 1 in 1375 chance that the effect is mimicked by a statistical fluctuation. Particle physicists consider a result at 5.0 sigma to be a discovery.

The excess might be explained by the production of a new, unknown particle that is not predicted by the Standard Model, the current standard theory of the fundamental laws of physics. The features of this excess exclude the possibility that this peak might be due to a Standard Model Higgs boson or a supersymmetric particle. Instead, we might see a completely new type of force or interaction. A few models proposed and developed in recent years postulate the existence of new fundamental interactions beyond those known today, which would create an excess similar to the one seen in the CDF data. That’s why everybody at CDF is excited about this result.

The di-jet invariant mass distribution for candidate events selected in an analysis of W+2 jet events. The black points represent the data. The red line plots the expected Standard Model background shape based on Monte Carlo modeling. The red shading shows the systematic and statistical uncertainty on this background shape. The blue histogram is the Gaussian fit to the unexpected peak centered at 144 GeV/c2

The alternative explanation for this excess would be that we need to reconsider the theory that is used to predict the background spectrum, which is based on standard particle physics processes. That possibility, albeit less glamorous, would still have important implications. Those calculations use theoretical tools that are generally regarded as reliable and well understood, and form the basis of many other predictions in particle physics. Questioning these tools would require us to challenge our understanding of the fundamental forces of nature, the foundation of particle physics.

The current analysis is based on 4.3 inverse femtobarns of data. The CDF collaboration will repeat the analysis with at least twice as much data to see whether the bump gets more or less pronounced. Other experiments, including DZero and the LHC experiments, will look for a particle of about 140 GeV/c2 in their data as well. Their results will either refute or confirm our result. Our result has been submitted to Physical Review Letters. You can read the paper and watch the lecture online.

It remains to be seen whether this measurement is an important indication of long-awaited new physics beyond the Standard Model.

— Edited by Rob Roser and Giovanni Punzi

Several interesting articles have been written about the result. Media interest was generated after a thesis article was spotted in an academic journal.  Gordon Watts has an intriguing blog post about how the release of scientific information is and could be affected by today’s fast-paced, Internet-driven society. This could bring people into the scientific process before an analysis has been fully vetted or enough data has been complied and analyzed to declare something a discovery by reaching the 5-sigma  threshold. Do you think that is a good or bad thing?

Related stories:

USLHC blog: A hint of something new in “W+dijets” at CDF

New York Times: At Particle Lab, a Tantalizing Glimpse Has Physicists Holding Their Breaths

Nature:  The Tevatron claims possible glimpse of physics beyond the standard model

Jakarta Globe:  US atom smasher may have found new force of nature

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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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