Lighting up the dark universe

July 28, 2010 | 1:43 pm

The CHASE detector. The end of the magnet (orange) can be seen on the right.

Exploring our dark universe is often the domain of extreme physics. Traces of dark matter particles are searched for by huge neutrino telescopes located underwater or under Antarctic ice, by scientists at powerful particle colliders, and deep underground.  Clues to mysterious dark energy will be investigated using big telescopes on Earth and experiments that will be launched into space.

But an experiment doesn’t have to be exotic to explore the unexplained. At the International Conference on High Energy Physics, which ended today in Paris, scientists unveiled the first results from the GammeV-CHASE experiment, which used 30 hours’ worth of data from a 10-meter-long experiment to place the world’s best limits on the existence of dark energy particles.

CHASE, which stands for Chameleon Afterglow Search, was constructed at Fermilab to search for hypothetical particles called chameleons. Physicists theorize that these particles may be responsible for the dark energy that is causing the accelerating expansion of our universe.

“One of the reasons I felt strongly about doing this experiment is that it was a good example of a laboratory experiment to test dark energy models,” says CHASE scientist Jason Steffen, who presented the results at ICHEP. “Astronomical surveys are important as well, but they’re not going to tell us everything.” CHASE was a successor to Fermilab’s GammeV experiment, which searched for chameleon particles and another hypothetical particle called the axion.

Preliminary results from the GammeV-CHASE experiment, which rules out the existence of chameleon dark energy particles with a wide range of masses. The blue area on the graph, and the area between the two red lines, are areas of exclusion from GammeV-CHASE. Slide presented at ICHEP on July 23, 2010.

To create chameleon particles, the experiment shone a laser beam into a magnetic field. If chameleons exist, they would be created when photons from the laser beam scatter off photons from the magnetic field. The laser was left on for a certain amount of time to build up enough chameleons, then turned off to allow the chameleon particles to convert back into photons. The “afterglow” photons would then be recorded by the experiment’s photo-detectors.

It took about six weeks for CHASE’s 10 scientists to collect the 30 hours of data they needed to search for a dark energy discovery. According to the preliminary results presented at ICHEP, the subsequent data analysis didn’t reveal any chameleons, which allowed the experiment to rule out a wide range of dark energy models that predict such particles.

“This was the first experiment employing this particular technology that was sensitive to all chameleon dark energy models,” notes Steffen.

The experiment, which took less than 18 months from design to the end of data taking, is already being dismantled. Although the CHASE team doesn’t plan a successor right now, the search continues at the ADMX experiment in Washington, which is analyzing chameleon search data gathered using a different method.

“It would have been great to find something, but I can’t say that I was really sitting on the edge of my chair expecting to see chameleons,” adds Steffen. “It would take new developments in how we approach the problem for us to make serious plans to continue to search for them.”

CHASE’s final results will be presented August 13 in a seminar at Fermilab.

Katie Yurkewicz

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LHC results: Not just the same old thing

July 26, 2010 | 11:30 am

The cross section for production of the W boson as a function of collision energy. The ATLAS experiment measurements (red circle, blue square, brown triangle) are shown along with data from experiment at other particle accelerators.

CERN issued a press release this morning announcing the LHC results presented today at the ICHEP conference in Paris. The release stresses that the measurements currently being made by the four major LHC experiments are allowing them to “rediscover” the Standard Model of particle physics. But the presentations at ICHEP tell a slightly different story–the experiments are already making new measurements that contribute to the understanding of fundamental particles, and are even beginning to place limits on the existence of new particles.

It’s certainly true that many of the results presented today and last week–in the forms of charts, graphs, plots and tables of numbers–are simply re-measurements of the Standard Model, physicists’ best current understanding of the particles of matter and the forces that act between them. These repeat measurements are vital: if physicists can’t show beyond a shadow of a doubt that their new detector can measure something known, no one will believe them if they claim to measure something never before seen.

But many of the measurements are new. While the LHC experiments may be measuring particles whose existence has already been proven, they are measuring properties at an energy 3.5 times higher than ever before. These higher-energy measurements provide new, useful information to the physics community, for example to provide vital input to the theoretical models that describe how particles and forces interact. Such models are continually refined to more accurately reflect the way the universe works, and are also used to predict where new particles may be hiding. The more accurate the model, the better the chance that physicists will look in the right place for new particles, and the sooner the world might hear of a discovery.

One good example is the measurement of the cross section for the W boson. The W is a well-known particle, and its cross section has been very well measured at previous accelerators–but only at collision energies below 2 TeV. ATLAS and CMS presented at ICHEP their first measurements of the W boson cross section at 7 TeV, confirming predictions that the cross section should be approximately four times higher than at 2 TeV. The LHC experiments have also measured the difference between the production of positively and negatively charged W bosons, which may ultimately help physicists better understand the structure of the proton. This difference can only be measured at proton-proton colliders like the LHC.

First results from the CMS experiment's search for stopped gluinos. Lifetimes excluded by the CMS search are shown in the area where the black line dips below the blue line.

The LHC experiments are also already taking their first steps in the search for brand-new physics. The ATLAS experiment presented the most stringent limits so far on the existence of excited quarks, a theorized new type of quark. The CDF experiment had previously ruled out the existence of such particles with a mass of between 260 and 870 GeV; ATLAS has now extended the limit to 1290 GeV.

The CMS experiment presented results from their program to search for another type of exotic particle—stopped gluinos. These particles, if created in LHC collisions, would stop in the CMS detector, live a relatively long time compared to the infinitesimal lifetimes of particle like a top quark, and then decay into other particles. CMS physicists hunt these particles by collecting data between collisions of bunches of protons in the LHC beam. The DZero experiment has previously searched for the particles, and determined that they could not exist with a lifetime longer than 30 microseconds. With only a few months’ worth of data, CMS has now excluded the existence of these particles with a lifetime between 75 nanoseconds and 6 microseconds. (Check out Jester’s post on the ICHEP blog for a more in-depth explanation of this search.)

Katie Yurkewicz

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New limits on Higgs mass announced

July 26, 2010 | 9:19 am

New constraints on the elusive Higgs particle are more stringent than ever before. Scientists of the CDF and DZero collider experiments at the U.S. Department of Energy’s Fermilab revealed their latest Higgs search results today (July 26) at the International Conference on High Energy Physics, held in Paris from July 22-28. Their results rule out a significant fraction of the allowed mass range established by earlier experiments.

The Fermilab experiments now exclude a Higgs particle with a mass between 158 and 175 GeV/c². Searches by previous experiments and constraints due to 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 new Fermilab result rules out about a quarter of the expected Higgs mass range.

Scientists from the CDF and DZero collaborations at DOE's Fermilab have combined Tevatron data from their two experiments to increase the sensitivity for their search for the Higgs boson. While no Higgs boson has been found yet, the results announced today exclude a mass for the Higgs between 158 and 175 GeV/c2.

“Fermilab has pushed the productivity of the Tevatron collider to new heights,” said Dennis Kovar, DOE Associate Director of Science for High Energy Physics. “Thanks to the extraordinary performance of Fermilab’s Tevatron collider, CDF and DZero collaborators from around the world are producing exciting results and are making immense progress on the search for the Higgs particle.”

At the ICHEP conference, CDF and DZero scientists are giving more than 40 talks on searches for exotic particles and dark matter candidates, discoveries of new decay channels of known particles and precision measurements of numerous particle properties. Together, the two collaborations present about 150 results.
The Higgs particle is the last not-yet-observed piece of the theoretical framework known as the Standard Model of Particles and Forces. According to the Standard Model, the Higgs boson explains why some particles have mass and others do not.

“We are close to completely ruling out a Higgs boson with a large mass,” said DZero cospokesperson Dmitri Denisov, one of 500 scientists from 19 countries working on the DZero experiment. “Three years ago, we would not have thought that this would be possible. With more data coming in, our experiments are beginning to be sensitive to a low-mass Higgs boson.”

Robert Roser, cospokesperson for the 550 physicists from 13 countries of the CDF collaboration, also credited the great work of the CDF and DZero analysis groups for the stringent Higgs exclusion results.

“The new Higgs search results benefited from the wealth of Tevatron collision data and the smart search algorithms developed by lots of bright people, including hundreds of graduate students,” Roser said. “The CDF and DZero analysis groups have gained a better understanding of collisions that can mimic a Higgs signal; improved the sensitivity of their detectors to particle signals; and included new Higgs decay channels in the overall analysis.”

To obtain the latest Higgs search result, the CDF and DZero analysis groups separately sifted through more than 500,000 billion proton-antiproton collisions that the Tevatron has delivered to each experiment since 2001. After the two groups obtained their independent Higgs search results, they combined their results to produce the joint exclusion limits.

“Our latest result is based on about twice as much data as a year and a half ago,” said DZero cospokesperson Stefan Söldner-Rembold, of the University of Manchester. “As we continue to collect and analyze data, the Tevatron experiments will either exclude the Standard Model Higgs boson in the entire allowed mass range or see first hints of its existence.”

The observation of the Higgs particle is also one of the goals of the Large Hadron Collider experiments at the European laboratory CERN, which record proton-proton collisions that have 3.5 times the energy of Tevatron collisions. But for rare subatomic processes such as the production of a Higgs particle with a low mass, extra energy is less important than a large number of collisions produced.

“With the Tevatron cranking out more and more collisions, we have a good chance of catching a glimpse of the Higgs boson,” said CDF cospokesperson Giovanni Punzi, of the University of Pisa and the National Institute of Nuclear Physics (INFN) in Italy. “It will be fascinating to see what Mother Nature has in her cards for us. We might find out that the Higgs properties are different from what we expect, revealing new insights into the origin of matter.”

Funding for the CDF and DZero experiments comes from DOE’s Office of Science, the National Science Foundation, and numerous international funding agencies.

Fermi National Accelerator Laboratory is a U.S. Department of Energy Office of Science national laboratory dedicated to research in high-energy physics and related fields. The Fermi Research Alliance LLC operates Fermilab under a contract with DOE.

Source: Fermilab press release.

Press Release

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Schedule for LHC’s next few years revealed

July 26, 2010 | 4:37 am

The LHC's 10 year plan. Image presented by Steve Myers at ICHEP on July 26, 2010.

Steve Myers, CERN’s Director for Accelerators and Technology, presented the LHC schedule for the next 10 years today in the first plenary presentation at the International Conference on High Energy Physics. Myers also presented his predictions for the amount of data that the LHC may collect over the same time period. These predictions over the next few years will be scrutinized closely by scientists at Fermilab’s Tevatron, who have proposed extending the accelerator’s life for a further 3 years.

CERN has previously announced a year-long LHC shutdown in 2012 to repair connections between the accelerator’s thousands of superconducting magnets. The schedule announced today states that the shutdown will last 15 months, after which the LHC would run for approximately three straight years, until November of 2015. Another 15-month shutdown would follow in 2016, to prepare the accelerator to run with even more intense proton beams.

Unlike the 10-year plan for the LHC, which was agreed to by CERN management, Myers stated that his luminosity predictions are just that: his predictions, to be taken with a large amount of salt. It’s been well known that CERN aims to deliver one inverse femtobarn of data to the experiments by the end of the 2010-2011 run, an amount of data that would bring it up to the level of the Tevatron in the searches for some new particles. Myers’ new prediction for the amount of data to be delivered by the end of 2014 – the same year that an extended Tevatron run would end – is 30 inverse femtobarns. In comparison, the Tevatron would have delivered approximately 20 inverse femtobarns of data to the CDF and DZero experiments.

But, as CDF’s Ben Kilminster notes, the numbers don’t tell the whole story. Myers’ predictions state that almost 21 of those 30 inverse femtobarns will have been delivered in the final year of running, which means that all would need to go as planned during the LHC’s 2012 shutdown and the following two years of running. Whereas the Tevatron, as a mature machine, is already up to speed and would be continually cranking out data from now until the end of its life. And when it comes to the hunt for the Higgs, the CDF and DZero experiments have years of running experience and very well-understood detectors on their side, which the ATLAS and CMS experiments are just beginning to feel out their new detectors with the first data. So look for much more to come on the race between the two accelerators to crank out collisions, and the possible extension of the Tevatron’s run.

Katie Yurkewicz

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Higgs is the hot topic at ICHEP

July 24, 2010 | 7:55 am

History of searches for the Higgs boson with one particular mass at the CDF experiment. Image from presentation by Karolos Potamianos at ICHEP on July 23, 2010.

Everyone’s catching Higgs fever, even French President Nicolas Sarkozy. The elusive particle – and the race between the experiments at Fermilab’s Tevatron and those at the Large Hadron Collider to discover it – have made headlines for years, but the frenzy reached new heights in the run-up to the International Conference on High Energy Physics. First, rumors that the particle may have been spotted at the Tevatron spread like wildfire in the media. Fermilab quickly moved to quash the rumors, but others kept popping up. And yesterday it was announced that the President of the French Republic Nicolas Sarkozy will officially open the ICHEP conference on Monday, July 26.

On the first day of ICHEP, it became clear that physicists were not immune to a little publicity; the first Higgs session on Thursday afternoon was standing-room only. Physicists packed the session to hear members of the CDF and DZero collaborations present details of the many individual searches for the Higgs boson that are eventually combined into one final result. The Higgs boson is predicted to decay in a number of different ways, thus leaving a number of different patterns in particle detectors. Scientists search for as many of these patterns as they possibly can, to increase their chances of spotting the Higgs. The ultimate goal is to discover the particle; until that happens, scientists combine all the different searches to further limit the territory where the Higgs boson may be found.

Friday afternoon, the CDF and DZero experiments presented their individual combined analyses to another packed room, including data collected as recently as a few months ago. For these results, the experiments separately combine all of their different types of Higgs searches. Although neither experiment has yet found the Higgs, their ability to pin down the particle’s likely location is greatly improving as the Tevatron generates more and more collisions. With their accelerator currently scheduled to end its run in 2011, presenters from both experiments are taking advantage of the attention to the Higgs to build support among their colleagues for a three-year extension to the collider’s run.

But the main event is still to come: on Monday afternoon, CDF’s Ben Kilminster will present the hotly anticipated combined result from both experiments in the first day of plenary ICHEP sessions.

Both the presentation of the Tevatron Higgs result and President Sarkozy’s speech will be available via webcast.

Author’s note: This article was corrected on July 25 to more accurately represent the status of current predictions regarding the Higgs boson and how it may decay into other particles.

Katie Yurkewicz

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Europe reaches the top, err, the top reaches Europe

July 23, 2010 | 5:44 am

It might be a long way to the top, but the LHC experiments are already half-way there. Today at the International Conference on High Energy Physics in Paris, the CMS and ATLAS experiments presented their first top quark candidates. These candidates are collisions that have all the hallmarks of having produced top quarks, but the experiments don’t yet have enough data to be 100% sure that the events created top quarks that decayed into other particles, rather than another type of event.

“The signal is starting to rise from the background,” notes Tim Christiansen from CMS.

The top quark, the heaviest particle in the Standard Model, was discovered at Fermilab’s Tevatron in 1995. The CDF and DZero experiments on the Tevatron are still busy measuring its properties in detail (one of this morning’s parallel sessions had several talks on its width, mass and likely couplings to particles of and beyond the Standard Model). Now the LHC experiments are joining them on the way to explore the top: both CMS and ATLAS showed selected candidate events of top quark pairs.

Finding top quarks at the LHC is exciting because the top is the last, and heaviest, particle that the LHC needed to add to its list of ‘rediscoveries’. It is also an important partner in the hunt for all sorts of new physics. The better the top and its behavior are understood the easier it will be to distinguish events that involve direct top quark production from events that involve, for example, the Higgs or supersymmetric particles.

by Barbara Warmbein

The top at ATLAS?

The top at CMS?

Guest author

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The ILC in one minute flat

July 22, 2010 | 3:27 pm

The folks working on the proposed International Linear Collider have created a one-minute animation  that flies you through its 30-kilometer-long tunnel.  It has no sound, but the visuals speak for themselves:  Electrons and positrons are generated on opposite arms of the collider, circle in opposite directions in a damping ring, fly out to the ends of the machine and jet back into the middle, where they collide.  For more details about the humongous project – which is scheduled to present a progress report at the International Conference on High Energy Physics on Saturday –  go here.

Glennda Chui

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Particle physicists collide in Paris

July 22, 2010 | 7:07 am

Paris’ 17^th arrondissement has become particle physics central. More than 1,000 physicists have descended on the Palais de Congrès conference center to attend the 35^th International Conference on High Energy Physics, which kicks off today and continues through next Wednesday. ICHEP is the world’s premier particle physics conference, where scientists present and discuss the newest and most intriguing results from experiments in particle physics, particle astrophysics and cosmology, innovative theoretical approaches and predictions, and concepts for future accelerators and particle detectors.

ICHEP is split into two parts. The conference starts today with the parallel sessions, where more than 400 presentations will take place over a mere three days. Each day at any given time, attendees will choose from six parallel sessions, covering topics from every corner of particle physics. The conference pauses on Sunday to allow attendees to visit the city, catch the arrival of the Tour de France cyclists at the Champs-Élysées, or simply recover from three intense days of presentations and coffee-break discussions.

Monday is the official start of the conference, and the beginning of three days of plenary presentations, where the most important results and topics are summarized and discussed. The first day will focus on the much-anticipated first results from the LHC and newest results of the Tevatron experiments’ searches for the Higgs boson. The second day will include results from heavy-ion collisions, measurements of the electroweak and strong forces, and searches for new physics beyond the Standard Model. Wednesday, the conference’s last day, will feature new neutrino measurements, searches for the unknown constituents of dark matter, and a discussion of the future of particle physics.

A plane ticket to Paris isn’t necessary to catch the ICHEP presentations and discussion; selected parallel sessions and all of next week’s plenary sessions will be available via webcast. You can also follow symmetry breaking for updates throughout the conference, or read reports from attending physicists on the ICHEP blog.

Katie Yurkewicz

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The long road to high-intensity LHC collisions

July 16, 2010 | 1:48 pm

Integrated luminosity delivered to the LHC experiments through July 14, 2010. Image copyright CERN.

It’s getting more intense under France and Switzerland. Over the last few months the Large Hadron Collider has been quietly increasing the power of its two counter-rotating beams of protons, and serving up more and more particle collisions to the experiments dotted around the 27-kilometer ring.

An LHC beam is not a continuous stream of particles, but is made up of a number of packets of particles, called “bunches.” Years from now, when the LHC beams are at full power, each beam will be composed of 2,808 bunches of more than 100 billion protons each. But when the LHC collided its very first protons in March, each beam contained only 4 bunches, each with less than 10 billion protons each. Much of the accelerator team’s work over the last two months has focused on preparing the LHC, its protection systems, and its five pre-accelerators to handle beams with more higher-intensity bunches of protons.

The first major step in the march to greater intensity was to increase the number of protons in each bunch. By the end of May, bunches at “nominal” intensity–approximately 100 billion protons each–had been injected into the LHC and accelerated to 3.5 TeV.

Compared to the speed at which the number of protons in each bunch was increased to maximum, the road to 2,808-bunch beams will take much, much longer. To date, the beams have contained at most 13 bunches, with at most 9 bunches colliding in the LHC’s experiments. An increase from 4 bunches to 13 over three and half months might seem extremely slow. But it’s par for the course in particle physics, as scientists learn how to operate and understand the new accelerator, increasing the intensity, and thus the power, of the beam very slowly to ensure the safety of the LHC machinery and particle detectors.

While the LHC accelerator team has been working to increase beam intensity and provide more collisions, the physicists on the LHC experiments have been striving to record as many of the collisions as possible.  Physicists are also closely tracking a quantity called the integrated luminosity, a measure of the number of collisions that have taken place at the center of their detectors. The more collisions the experiments collect, the  better their ability to measure the properties of fundamental particles and forces.

As of this morning, the integrated luminosity recorded by the experiments is just above 250 inverse nanobarns, most of which was collected over the past week. This amount of data should have provided the ATLAS and CMS experiments with a few top quarks, as well as enough W and Z bosons for LHC scientists to start making their first measurements of the two particles. All the latest results from the LHC–and many other experiments from all fields of particle physics–will  be presented at next week’s International Conference on High Energy Physics in Paris.

More details about the LHC’s current status can be found in this week’s CERN Bulletin, or by tuning into the LHC status updates presented at CERN approximately every two weeks.

Katie Yurkewicz

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Cool summer job: Teens build a neutrino detector

July 15, 2010 | 5:35 pm

If  you’ve been reading symmetry for a while you may remember our reports on the Cyclotron Kids – Heidi Baumgartner, Peter Heuer, and German Diagama  — who came home from astronomy camp three years ago with the dream of building a cyclotron.  (They’re still at it; we’ll have an update on the Kids as well as other amateur cyclotron builders in the August issue of symmetry.)

So we were pleased to see this story from Minnesota Public Radio on students from the University of Minnesota with an unusual summer job: building parts for a neutrino detector.  It’s a smaller version of the giant detector, as tall as a six-story building, that will be installed deep in Soudan Mine in Minnesota as part of the Fermilab-based NOvA project.

In a 100,000-square-foot warehouse near campus, 19-year-old Dylan Skerbitz is cleaning the ends of a white PVC panel the size of a garage door. He’s using a corona arc discharge machine, which looks like a high-tech hair dryer spitting out a purple flame.

“It creates an electrical spark, and it burns off any excess dust or particles so glue will connect better with this,” he says.

Each one of these PVC panels — there will be about 500 stacked together in the finished detector — is made up of rows of tubes. Each of the tubes will be fitted with long strands of fiber optic thread. Those threads will pick up barely detectable flashes of light when neutrinos do a sub-atomic dance through the detector. A computer will record the findings.

Not exactly a do-it-yourself project, but still impressive, especially considering that the UM team includes both physics buffs and liberal-arts types.  The mini-detector will be used to test the technology of the maxi-experiment.  The MPR  website has photos.

Glennda Chui

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