Scientists finish installation of 80-ton ‘particle thermometer’ at ALICE detector

January 24, 2012 | 8:50 am

Scientists install the electromagnetic calorimeter at the ALICE detector. Image: CERN

Scientists on the ALICE experiment at the Large Hadron Collider just completed the installation of a crucial component for tracking high-energy particle jets. Without it, physicists would be lacking critical tools to select which events out of billions to store and analyze.

Engineers and physicists around the world worked intensively over five years to complete the electromagnetic calorimeter, or EMCal. The United States, supported by the Department of Energy’s Nuclear Physics Office, contributed 70 percent of the project costs. Scientists installed the last two pieces of the 80-ton device on Jan. 18.

The EMCal’s heft comes from its many sheets of lead absorbers, which it needs to stop particles coming from collisions in the detector in order to measure their energy. “The calorimeter measures the energy of individual photons and electrons,” said ALICE physicist Peter Jacobs. “It’s a sort of particle thermometer.”

The ALICE detector’s calorimeter was specifically designed to study the most complex collisions at the LHC, those created using beams of heavy ions. These collisions recreate big-bang-like conditions and produce events with many more particles than the Large Hadron Collider’s usual collisions using beams of protons.

CERN typically smashes lead ions together each November. These collisions produce a goopy mixture, known as the quark-gluon plasma, in the center of ALICE. Occasionally, a very energetic quark or gluon, called a jet, will also be created in the collision. When this happens, the QGP gets in its way, and that interaction is important for researchers seeking to understand material which first existed in the earliest moments of the universe. The EMCal allows ALICE to select and record the rare events containing such jets, and to measure their properties precisely.

A second arm of the EMCal will be added to ALICE during the long LHC shutdown in 2013.

The two pieces of the EMCal scientists installed this year were small; they add only about 10 percent to the calorimeter’s overall coverage, Jacobs said. However, all the small parts do add up — every new measurement gets us a little closer to the heart of the matter.

Symmetry caught up with researcher Peter Jacobs underground during the end of the installation.

Amy Dusto

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The making and tending of heavy ion beams for the LHC

November 15, 2011 | 1:41 pm

Heavy-ion expert Detlef Kuchler holds a container of lead. Image: CERN

This week the Large Hadron Collider began heavy ion physics, the process of colliding lead ions to learn about conditions in the primordial universe.

The accelerator is expected to perform five to 10 times better than it did in its first run of these collisions last November. Although the heavy ion program will last only from now until CERN’s annual winter shutdown just after the first week of December, operators started preparations months in advance. Here symmetry breaking examines what it really takes to put lead beams in the LHC.

The source

Making heavy ions is more complicated than preparing the protons used in regular LHC collisions, which come from hydrogen gas. Since hydrogen atoms have only one proton and one electron each, applying a voltage to them is sufficient to rip off their electrons, leaving a load of beam-ready, positively charged protons. But the source for heavy ions, enriched lead, starts with 82 electrons. Physicists do not have miracle flypaper to grab that many subatomic particles at once, so the process takes a few steps.

Meet Detlef Kuchler, a heavy-ion expert who tends the lead source, the first part of the heavy-ion acceleration process, by hand. He helped develop the method of extracting lead ions decades ago and can explain from memory its hundreds of associated, unlabeled diagrams. Although several people work on the source, a flowchart of what to do when things go wrong at this stage dead-ends everywhere with, “Call the expert.” It may as well say, “Call Detlef.” He spends a lot of nights and weekends at CERN during heavy ion season.

Kuchler prepares the oven. Image: Amy Dusto

The oven

The first thing Kuchler does when it’s time to make heavy ions is prepare the oven, a palm-sized cylinder on a long pole that evaporates metallic lead. The whole thing fits into another machine that begins a chain of hand-offs from the source to the LHC. The lead must heat slowly in the oven or fragments spill out. Kuchler refills the oven once every two weeks.

The lab’s 10-gram stock of enriched lead costs $12,000. But so little is used at a time — about 500 milligrams per oven fill — that the bill equates to roughly $2 per hour. Not bad, in the scheme of accelerator operating costs.

Kuchler carefully unscrews the oven, takes it apart, cleans its seals with ethanol, measures and fills it with lead, puts it back together, enters the correct settings and turns it on. When symmetry visited, he searched with gloved hands for a leak around water tubes used for cooling and requested new O-rings to fix a vacuum connection somewhere. He usually finishes all the physical tweaks in less than half an hour.

“I know every screw of this machine,” he said. “It’s fun.”

Operators test beams of lead ions in this linear accelerator. Image: CERN

The beam

After lead evaporates in the oven, it moves into a chamber of plasma heated by microwaves. The lead gas loses some electrons when it hits the plasma. In 30 milliseconds, the ions inside have each lost varying amounts of the negatively charged particles, but the majority have lost 29, making their charge 29+. As they leave the chamber, the 29+ ions are separated from the rest and collected into a beam.

A newborn beam of lead ions spends its first hours in testing. Here, the human-machine interactions become less TLC and more CPU. Kuchler adjusts computer settings to send a bit of beam a few meters into a small, straight section of accelerator, where it hits a diagnostic device called a Faraday cup. Kuchler sees how he did from what the cup tells him, makes adjustments accordingly, heats a little more lead in the oven and repeats. Eventually, when the beam looks good enough, he retracts the Faraday cup from the path so that the beam may continue its tour of CERN’s accelerator complex on the way to the LHC.

Electronics that control every aspect of source machine settings sit in shelves all around the oven and the accelerator. “During the day I keep an eye on [the machine],” Kuchler said. He can easily pop down from his office any time. During the weeks of heavy-ion physics, he elects not to take any personal holidays so that even when he’s not at work, he’ll always be nearby. He and dozens of other experts work day and night and remain on-call after hours in case anything goes wrong.

The heavy-ion beams make their presence known through synchrotron light, which shows up as subtly shimmering spots on the screen. Image: CERN

The chain

In August, eight weeks after Kuchler began working on the source, he began letting the beam through to the Low Energy Ion Ring, where other people took over its testing and fine-tuning.

On the way to the ion ring, the beam passes briefly through an area where it encounters a number of 300-nanometer-wide stripping foils. These take off more electrons, increasing the ions’ charges to 54+. Next, at the LEIR, a process called beam cooling begins to reshape and intensify the beam, which the machine accelerates.

The newly narrowed beam travels from there to another accelerator, the Proton Synchroton, behind a wall in the same building. This accelerator takes the beam up to a higher energy and sends it through another stripping foil. Ions leave here at the charge with which they will collide: 82+.

The last checkpoint before the LHC is the second biggest accelerator at CERN: the Super Proton Synchroton. This machine further accelerates the ions. The Proton Synchrotron accelerator started sending beam to the SPS in September. Over the course of eight weeks, a team of physicists spent hours refining the machine’s settings in order to optimize the beam and prepare it for its final destination: the LHC.

The LHC

Operators declared "stable beams" today, which means the LHC is ready for heavy ion physics. Image: CERN

During the few days of tightly-scheduled heavy-ion commissioning, many additional experts join the usual operators at the LHC controls. They help troubleshoot to make sure the beams reach the precise conditions needed for physics. Lead beams here accelerate to 1.38 TeV per single proton or neutron inside the ion.

Three minutes after midnight on Sunday, Nov. 6, LHC accelerator physicist John Jowett captured an image of one moment in the process that always thrills him. When the lead beams in the LHC reach about twice their injection energy, they begin emitting enough radiation, called synchrotron light, to be seen by a special telescope and camera system in the accelerator.

“We see a shimmering spot on a screen,” Jowett said. “It’s as close as we get to seeing the beams of lead nuclei with our own eyes.”

At the end of that week, on Saturday, Nov. 12, at 6:41 a.m., operators declared “stable beams,” the machine term that indicates the LHC is ready for physics.

Amy Dusto

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LHC finishes 2011 proton run

October 31, 2011 | 9:46 am

Operators emptied the LHC of protons in the last beam dump of 2011 on Sunday. Image: CERN

The Large Hadron Collider guided beams of protons along a collision course for the last time this year on Sunday, Oct. 30.

During the LHC’s 2011 run smashing protons into other protons, the machine produced 400 trillion collisions, CERN announced in a press release today. The LHC ran for about 180 days and delivered almost six times as much data as originally expected for the year. The CMS and ATLAS general-purpose experiments collected more than 5 inverse femtobarns of data.

This week, LHC operators will test various aspects of the accelerator and detectors to prepare for future runs. Then they will get ready for four weeks of lead-ion collisions. In the meantime, experimentalists will continue analyzing data from proton-proton collisions in the hunt for new physics.

Kathryn Grim

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Accelerator soup: Scientists to mix elements in LHC to study recipe for heavy-ion collisions

October 20, 2011 | 9:29 am

CERN physicist Detlef Kuchler holds a purified sample of lead used to create heavy ions for the LHC. Photo by M. Brice / CERN

Superman may have superpowers, but Batman has the ingenuity to be just as heroic with the help of gadgets. Scientists at CERN are channeling the Dark Knight to try to make their biggest gadget, the Large Hadron Collider, perform a feat that counters its very design principles but may give them a better understanding of the early universe. They’ll have about 40 hours total to make it work.

Instead of colliding two beams of protons or two beams of much heavier lead ions, as the LHC usually does, operators will try to collide one of each in the coming weeks. On October 31, they will test the process for 16 hours, and two weeks later they’ll get another 24. That’s all the time they decided they could take from the precious month of data-collecting they will give the experiments during the upcoming lead-lead run. If it works, a proton-lead ion research program could be in place for November 2012.

The scientists undertaking the task of colliding protons and lead want to collect benchmark information about single beams of lead ions to get a better picture of what’s going on in lead-lead collisions. For that, the tiny proton acts as a probe of the more massive lead ion.

Theorist Urs Wiedemann explained that it’s a bit like making soup. A meticulous chef wants to know exactly what happens at each step of a recipe. This includes both the initial state of the individual ingredients – onions sautéed or raw? – as well as the final outcome inside the pot. Otherwise the chef can’t make informed changes. Similarly a physicist needs to know the individual properties of both of the elements he or she wants to collide, as well as what their smashing produces, in order to get the full picture for analysis.

After years of experiments, the quarks and gluons in the proton are well characterized; those in the lead ion are not. So, in a proton-lead ion collision, all unexpected effects can be attributed to the lead ion. With this knowledge, scientists will be able to make predictions about lead-lead collisions. “I can only test my theory of ion collisions if I know what is collided,” Wiedemann said. Colliding a proton with a heavy ion serves to “switch off the confounding factors,” he said.

Snapshot of two lead nuclei just after impact. Image by Henning Weber / CERN

Colliding protons and lead ions also gives scientists the potential to discover new physics phenomena, Wiedemann said. For this reason, all the experiments around the ring are interested in taking data during the collisions.

Scientists still need to test whether the technique will work in the LHC, said accelerator physicist John Jowett. The machine was built with a 2-in-1 magnet design: Both beams are controlled with the same magnetic field. This helps keep everything inside the ring symmetric when the beams inside match. Collisions and kicks, when one beam’s electromagnetic field pushes the other beam slightly off course, happen in the same place on each turn, a fundamental principle of stability for circular accelerators.

Protons and lead ions have different masses and charges, so they are not equally affected by the same magnetic field. The biggest challenges in this type of run are to inject and then control the beams as they ramp up from low to high energy. At lower energies, a beam of protons makes one extra lap around the LHC every 15 seconds. Since a proton beam makes about 11,000 laps per second, this discrepancy is small. But it is still enough to shift where the proton and lead-ion beams kick each other with every turn. As energy in the accelerator increases to its peak of 3.5 TeV and the beams get closer to light speed, however, relativity makes the 15 seconds stretch out little by little. At some point the difference between the beams becomes small enough that it can be overcome with a minor adjustment in their orbits – essentially the proton ends up going just a bit farther than the lead does, so they both take the same time to make one revolution.

The CERN accelerator complex. Image: CERN

To further complicate things, the more intense the beams get, the worse the effects each beam has on the other. The need to reach a certain level of intensity, roughly a measure of the amount of particles in a beam, puts a limit on how much data the experiments can extract. So, even if researchers can manage to inject two different beams in the accelerator, they will also need to make them with enough intensity for the method to be considered feasible.

The accelerator does have built-in flexibility despite having only one magnetic field: It has independent RF systems for each beam. These adjust electromagnetic frequencies inside a special cavity at one point around the ring, which controls each beams’ speed and orbit length. The LHC also has a powerful automatic feedback system that reacts to effects in the pipe by serving counter-kicks. This helps to keep the beams stable.

The LHC physicists plan to take baby steps – but fast ones, given the strict time constraints. First, they will simply try and inject two different elements into the LHC. If that works, they’ll try accelerating the beams. Only after that would they consider attempting actual proton-lead ion collisions.

“I hope it all works and we’re not prevented by mundane effects” such as a power outage, Jowett said. “In a month from now we should know the answer.”

Amy Dusto

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LHCb experiment sees Standard Model physics

August 29, 2011 | 11:47 am

Over the weekend, the LHCb experiment at the Large Hadron Collider released new results bolstering the Standard Model of particle physics.

The collaboration announced that their precision measurements disagreed with earlier results from experiments at the Tevatron at Fermilab. This summer, the CDF experiment announced seeing small excesses over the expected amount of decays of mesons composed of bottom and strange quarks into a pair of muons.

The Standard Model predicts that this decay should happen infrequently. It should take more than 350 trillion collisions for scientists to see it. But if physicists observe it more often than that, it can mean that something beyond the Standard Model is affecting the process.

Looking for excesses like this is one way to search for supersymmetry. Not seeing this type of enhancement does not, however, rule out the existence of supersymmetry, said Sheldon Stone, group leader of experimental elementary particle physics at Syracuse University.

“There is still a lot of room for new physics to appear,” he said.

Kathryn Grim

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LHC experiments eliminate more Higgs hiding spots

August 22, 2011 | 9:06 am

Fermi National Accelerator Laboratory and Brookhaven National Laboratory issued the following press release on August 22, 2011.

BATAVIA, IL and UPTON, NY – Two experimental collaborations at the Large Hadron Collider, located at CERN laboratory near Geneva, Switzerland, announced today that they have significantly narrowed the mass region in which the Higgs boson could be hiding.

The ATLAS and CMS experiments excluded with 95 percent certainty the existence of a Higgs over most of the mass region from 145 to 466 GeV. They announced the new results at the biennial Lepton-Photon conference, held this year in Mumbai, India.

“Each time we add new data to our analyses, we close in more on where the Higgs might be hiding,” said Darin Acosta, a University of Florida professor and deputy physics coordinator for the CMS experiment.

More than 1,700 scientists, engineers and graduate students from the United States collaborate on the experiments at the LHC, most of them on the CMS and ATLAS experiments, through funding by the Department of Energy Office of Science and the National Science Foundation. Brookhaven National Laboratory serves as the U.S. base for participation in the ATLAS experiment, and Fermi National Accelerator Laboratory serves as the U.S. base for participation in the CMS experiment.

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.

“The more data the experiments collect, the more scientists can say with greater statistical certainty,” said Konstantinos Nikolopoulos, a physicist at Brookhaven National Laboratory on the ATLAS experiment. “The LHC has been providing that data at an impressive rate. The machine has been functioning beyond expectations.”

Scientists on ATLAS and CMS both announced seeing small, possible hints of the Higgs boson at the European Physical Society meeting in July. Those hints have become less pronounced as scientists have increased the amount of data in their analysis.

“These are exciting times for particle physics,” said CERN’s research director, Sergio Bertolucci. “Discoveries are almost assured within the next twelve months. If the Higgs exists, the LHC experiments will soon find it. If it does not, its absence will point the way to new physics.”

The experiments are on track to at least double the amount of data they have collected by the end of the year.

Read the CERN press release here.

Press Release

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LHC experiments reach record data milestone

June 17, 2011 | 9:09 am

The ATLAS and CMS experiments have both accumulated 1 inverse femtobarn of data, an important milestone for both experiments. Image courtesy of CERN.

As of this week, the Large Hadron Collider has delivered 1 inverse femtobarn of integrated luminosity to ATLAS and CMS, two of the four experimental stations housed along the ring. This means the detectors will have gathered data from about 70 trillion proton-proton collisions. For comparison, the experiments collected just 45 inverse picobarns in all of 2010; 1 inverse femtobarn is equal to one thousand inverse picobarns.

Accelerator scientists promised 1 inverse femtobarn for the entire 2011 run, but met their goal just a few months after collisions began in March. The groups could see several more inverse femtobarns by the end of the year.

Scientists wasted no time analyzing the first few hundred inverse picobarns and announced their findings at the Physics at LHC conference last week. With five times more data than they had in 2010, researchers have been able to set more stringent limits on new physics. They haven’t observed anything new just yet but are beginning to explore uncharted territory.

“We spent 2010 rediscovering the Standard Model,” said Richard Hawkings, the deputy physics coordinator for the ATLAS experiment. “In 2011, we will push our detectors to the limit and make more precise measurements.”

Experimentalists have increased the sensitivity of the detectors to Standard Model processes, the events that account for much of the background signal in the hunt for new physics. Making the most precise measurements of these events could lead to an indirect hint of something new.

The LHC has delivered an integrated luminosity of one inverse femtobarn, nearly all of which has been recorded by the experiments. Image courtesy CERN

For example, the number of single top quarks scientists have spotted agrees well with what is predicted by the Standard Model, but with the new data they can begin to study the behavior between the top quark and the other, lighter quarks.

Despite finding no direct evidence of new physics, Hawkings says they have made significant inroads toward a discovery. “We’re closing in on supersymmetry,” he said. The data suggest that some of the theorized superparticles must have masses greater than 1 TeV, which narrows the window for observation. “We’ve looked through a lot of the space where supersymmetry could have been found, but there are still more corners to explore,” Hawkings said.

With only a few hundred inverse picobarns parsed at the time of the conference, LHC researchers weren’t able to match the Tevatron’s limits on the Higgs boson, though they have made some headway studying potential Higgs events that decay into two photons. Now that the experiments have received an integrated luminosity of 1 inverse femtobarn, researchers will have plenty more data to analyze before the European Physical Society and Lepton-Photon conferences this summer.

“The field is wide open for surprises, hints, speculations and false alarms,” Hawkings said. “We really need to see a hint of something new so we know what we can exclude and what we need to pursue further. But I really hope we get to see the first hint of the Higgs boson this year.”

Lauren Rugani

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String theory may hold answers about quark-gluon plasma

June 15, 2011 | 2:36 pm

String theorists describe the physics of black holes in five-dimensional spacetime. They found that these five-dimensional objects provide a good approximation of the quark-gluon plasma in one fewer dimension, a relationship similar to the one between a three-dimensional object and its two-dimensional shadow. Image: SLAC National Accelerator Laboratory

Recreating the conditions present just after the Big Bang has given experimentalists a glimpse into how the universe formed. Now, scientists have begun to see striking similarities between the properties of the early universe and a theory that aims to unite gravity with quantum mechanics, a long-standing goal for physicists.

“Combining calculations from experiments and theories could help us capture some universal characteristic of nature,” said MIT theoretical physicist Krishna Rajagopal, who discussed these possibilities at the recent Quark Matter conference in Annecy, France.

One millionth of a second after the Big Bang, the universe was a hot, dense sea of freely roaming particles called quarks and gluons. As the universe rapidly cooled, the particles joined together to form protons and neutrons, and the unique state of matter known as quark-gluon plasma disappeared.

In recent years, scientists have reproduced the quark-gluon plasma by smashing together heavy ions – first with gold nuclei at the Relativistic Heavy Ion Collider, and then with heavier lead ions at the Large Hadron Collider in 2010. The energy from the collisions at the LHC is nearly 14 times higher than those at RHIC, and produces temperatures more than 100,000 times hotter than the center of the sun, enough to melt the protons and neutrons from the ions down to their quark and gluon components.

The quark-gluon plasma created inside the experiments disappears almost instantaneously – billions of times faster than it did in the early universe – as the quarks and gluons reassemble into composite particles. These particles then fly into the surrounding detectors. Researchers study them to glean information about the quark-gluon plasma.

For example, in 2005 RHIC scientists discovered that the quark-gluon plasma didn’t behave like a gas of particles traveling randomly, but rather as a nearly perfect liquid. All the particles moved with a strong sense of coordination, much like a school of fish seems to move as a single object even though individual fish may be darting around in different directions.

Although researchers have since learned a great deal about the plasma, the collective movement among the quarks and gluons continues to be a stumbling block for explaining the behavior of the exotic fluid. The tools that normally describe particles and interactions on a subatomic scale don’t work, and physicists have set out to find a better measuring stick.

“We have to figure out how to get from understanding single particles to understanding the medium as a whole,” said CERN theoretical physicist Urs Wiedemann. Surprisingly, the answer may come from string theory.

Strung together

String theory is a hypothetical description of nature that accommodates both gravity and the quantum physics that describe the other three fundamental forces: electromagnetism and the strong and weak nuclear forces. Traditionally, gravity and quantum physics don’t play well together, but string theory uses extra dimensions of space to reconcile the two.

Theorists found that the mathematics of certain quantum theories and that of objects described by gravity in one extra dimension, such as black holes, look remarkably similar. Physicists are taking advantage of this duality by translating problems from the quark-gluon plasma into the language of gravity, where the equations become much simpler. Although the translation can’t provide precise calculations, solving a problem in one realm can give valuable insights into the other.

“On the theory side, black hole physics looks the same as quark-gluon plasma physics,” said physicist David Mateos, an ICREA research professor at the University of Barcelona in Spain, who also presented his work at the conference. “The fact that the same theory can be used to describe physics with gravity and physics without gravity is truly fascinating.”

String theory provides good approximations for certain properties of the quark-gluon plasma, such as its extremely low viscosity – the fact that it can flow without experiencing much resistance – or how certain particles are affected as they travel through the dense liquid.

“String theory is like a gift to us,” Rajagopal said. “We’re challenged with understanding the quark-gluon plasma as a liquid, and while string theory doesn’t give us precision, it can help us get a feel for the shape of the subject.”

Ultimately, physicists hope to understand how quarks and gluons bind together inside the fluid to form new particles, and what causes them to stay confined. Gravity-based explanations from string theory can also be used to describe phenomena that exhibit strong interactions similar to the quark-gluon plasma, such as superconductivity or supercold atomic gases.

In turn, experimental validations of these approximations could show how string theory best represents nature and point theorists toward avenues to explore further, Weidemann said. “But for now, string theory is just a useful tool for solving a current theory of reality.”

Lauren Rugani

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Keep it simple, SUSY

May 3, 2011 | 3:49 pm

This CMS event display from October 2010 captured a collision that produced very energetic jets - showers of particles that leave energy deposits in the detectors - and an exceptional amount of missing energy, represented by the blue line at the bottom left. Experimentalists and theorists are continuing to analyze collision events such as this one in search of new physics.(Image courtesy CMS/CERN.)

Experimentalists at the Large Hadron Collider recently proved effective a simple, new method of looking for evidence of supersymmetry. The method addresses a challenge particle physicists often face in looking for new particles and processes: They can manifest themselves in a multitude of ways.

How new physics will reveal itself is anybody’s guess. It’s a bit like the game Plinko on “The Price is Right” – contestants drop a token at the top of a pegboard and watch hopefully as it zigzags toward a slot at the bottom that may or may not contain a prize. If the game is played enough times, the token is likely to retrace some of the same routes, but occasionally it will take an unexpected path to the prize.

And so it goes at the LHC. Energy from proton collisions transforms into a handful of massive particles, which can decay in seemingly endless ways to any number of final particles. Collide enough protons and eventually the rarest particles will appear among the commonly produced ones.

“The set of possibilities for finding new physics is so large that it can be overwhelming,” said theoretical physicist Jay Wacker from SLAC National Accelerator Laboratory in California. “We’re trying to go about systematically exploring all of these possibilities. We want to make sure no stone is left unturned.”

Using the first round of data taken in 2010, LHC experimentalists sifted through several billion collisions looking for rare signatures that could represent supersymmetry. They looked where theory stated that these signatures were most likely to appear – the tokens that took a particular path down the pegboard and ended up in a certain slot on the bottom – and came up empty-handed.

The experimental groups are still looking for supersymmetry under different conditions, but it would take far too long to analyze every possible combination of initial particles, decay paths and final particles. To solve this problem, a group of theorists came up with a way to look for supersymmetry in the widest area possible.

Wacker and his colleagues designed a search that is sensitive to a number of different particle signatures that appear in the aftermath of a high-energy proton collision. The goal, he said, was to come up with the easiest way to cover the most possible areas where new physics might pop up.

The search looks at a class of events called jets plus missing energy – proton collisions that result in a shower of hadronic particles plus a stable, neutral particle that escapes detection – and ignores events that show signs of electrons or muons.

Both the theorists and the experimentalists looked only at the pile of tokens that landed in a particular slot at the bottom of the Plinko board. While the experimentalists had a set of guidelines about how the tokens should have gotten there and excluded any tokens that didn’t follow the rules, the theorists didn’t care as much about that. They were primarily concerned with the mass of the initial particles, the mass of the final particles and the ratio between them.

When the initial massive particles decay into lighter ones, the total energy must be conserved. Sometimes this energy goes missing; if the missing energy adds up to a certain amount, it could mean that a supersymmetric particle carried it away without being detected.

“These models are really nice because they allow us to think in terms of particles, not abstract parameters,” said CMS deputy spokesperson Joseph Incandela, from the University of California, Santa Barbara. “As particle physicists, we like that.”

The theorists’ approach also does not consider each individual possibility, but rather a few combinations of particle masses, decay paths and missing energy ranges that are a well-rounded representation of all the possibilities.

From the set, Wacker’s group proposed two dozen model signatures, each of which describes the masses and behaviors of a set of particles corresponding to a spot within the search region. The ATLAS group applied these models to their SUSY search.

“The good news is that they work,” said ATLAS physicist Zach Marshall, who helped test the search strategy against the 2010 LHC data. “We were basically asking, ‘If the signature really did look like that, would we have seen it last year?’ And the answer is that we can exclude many of those points.”

The models not only verify last year’s searches, they also help to optimize them. The groups now have a better understanding of potentially missed supersymmetry signatures and can state much more clearly the mass limits on particles, Marshall explains. What’s more, the models extend even beyond the constraints set by the experiments and are sensitive to many different supersymmetry theories.

“They essentially broke down a very important class of supersymmetry models and found the common denominators,” Incandela said. After certain limits are set with these simple models, scientists will gradually add more variables back into the equation. This will narrow down the number of potential new physics events and will help scientists to get a better handle on what they see in the detectors.

Both the CMS and ATLAS experiments will use the search again as they collect and analyze data from the 2011 run. Meanwhile, the theorists are working to expand these models and apply them to other slots at the bottom of the Plinko board.

“These models created a search more extensive than what has been used, and with reasonable efficacy,” Wacker said. “But this is just the first step in showing what kinds of new physics the LHC is sensitive to.”

Lauren Rugani

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Boosting luminosity: how to get the most bang for your proton in the LHC

April 28, 2011 | 12:55 pm

Just after midnight on April 22, the LHC set a new world record for instantaneous luminosity at a hadron collider. The peak can be seen at the far right of the luminosity plot. (Image courtesy of CERN.)

The Large Hadron Collider hit two major milestones last week, first passing its own integrated luminosity record of 49 inverse picobarns, and then breaking the Tevatron’s peak luminosity record with 4.67 × 10³² cm^-2 s^-1.

In particle physics, luminosity measures how many protons pass through a given area in a certain amount of time. Squeezing more protons into increasingly smaller spaces boosts the luminosity, which gives protons a better chance of colliding. Adding up the luminosity over a long period of time – say, a year – provides the integrated luminosity, which is related to the total number of collisions seen by the detectors.

Last year’s integrated luminosity of 49 inverse picobarns corresponds to several thousand billion collisions at the LHC. The record peak luminosity means more collisions and, in turn, a much higher integrated luminosity. Scientists aim to collect anywhere between 1000 and 3000 inverse picobarns by the end of 2011.

As data pours in, LHC experimentalists will comb through trillions of collisions in search of exotic particles and new physics. Such a large data set is necessary for researchers to spot these rare events, so LHC accelerator scientists will use several tactics this year to deliver as many collisions as possible.

Packing in the protons

One way to get more collisions is to increase the number of proton bunches in each particle beam. Think of two marbles rolling toward each other from opposite ends of a hallway. There is a small chance that they will collide in the middle. But rolling 10 marbles from each end would increase that chance.

Likewise, circulating more proton bunches at once means the two beams will cross paths more often, upping the odds of a collision. Last year, scientists ran the LHC with 368 proton bunches per beam. This year, they aim to increase the record to more than 900 bunches during a single physics run.

Now imagine taking those marbles from the hallway and sending them on a collision course through a paper towel tube. The chance of two marbles colliding would rise significantly. For this reason, scientists use focusing magnets to squeeze the proton bunches down in size as they cross paths inside the detectors.

Another trick for getting more proton collisions is packing the bunches closer together. In 2010, proton bunches in the LHC were spaced 150 nanoseconds apart. Now, the proton bunches race around the 17-mile tunnel at 50-nanosecond spacing, or just 50 feet apart.

Ramping up

One final effort to increase the number of collisions scientists can get out of the LHC is to speed up the overall process of injecting protons.

Accelerator scientists watched eagerly as the first proton bunch made its way from the SPS to the LHC in 2008. This is just one of many critical steps toward colliding high-energy protons. (Image courtesy CERN.)

“The less time we spend getting the beam ready, the more time we spend doing collisions,” said beam operations leader Mike Lamont.

More collisions mean more chances to look for the unknown. “And that’s the real reason we’re here – for the Higgs, for supersymmetry,” Lamont said.

Protons go through multiple stages of acceleration before they are used in collisions, starting with a low-energy kick-start from a linear accelerator. They gain speed in the booster ring and make their way to the proton synchrotron, where they are separated into multiple bunches and accelerated to 26 giga-electron volts. Finally, they reach the super proton synchrotron, or SPS, where they are accelerated to 450 GeV and stored before being injected into the LHC.

“Injection is one of the most critical phases,” said Giulia Papotti, and engineer-in-charge at the LHC control center. While the process up to this point is well controlled and takes only a matter of seconds, moving the proton bunches from the SPS to the Large Hadron Collider can be tricky. The bunch characteristics have to be exact, from their length and width to the spacing between them; otherwise the beam gets dumped and the process starts over from the beginning.

Improved software controls will play a large part in enhancing the injection process. Checks on bunch length and variations from bunch to bunch will determine if the beam quality is good enough for the LHC. If not, the bunches can be dumped at the SPS without compromising the LHC beam quality or forcing the entire injection process to restart. Also, if there is a problem in one of the rings, the beam operators can continue to fill the other ring while the issue is sorted out.

Slow and steady wins the race

Finally, there are quality control checks at the LHC after each injection that will help to train the machines and keep the beam consistent throughout the run.

“We just have to keep working and listen to the machine,” Papotti said. “Experience will tell us what we have to do.”

Years of experience and building up a thorough understanding of their machine helped Tevatron scientists achieve record luminosities, which were far above the particle collider’s design capabilities. Ronald Moore, the Tevatron department head at Fermilab, says they employed many of the same tactics, including increasing the number of bunches and squeezing the beams into smaller spaces. As technology improved, the Tevatron scientists were better able to monitor and correct variations in the beam, which led to hardware upgrades and more automated processes.

“Consistency has been key,” Moore said. “Using the same beam configurations from one store to the next allowed us to tune the machine to a known state and keep it there. But perhaps the most important part has been the people. Everyone wanted to see the machine perform better and better.”

In the coming years, the LHC accelerator team will work to inject as many as 2808 bunches spaced 25 nanoseconds apart, and to complete this task in about 20 minutes. The ensuing luminosities will likely produce physics results that will hold a few records of their own.

Lauren Rugani

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