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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LHC sets new world record

April 22, 2011 | 10:48 am

The following press release was issued today by CERN.

Around midnight this night CERN’s Large Hadron Collider set a new world record for beam intensity at a hadron collider when it collided beams with a luminosity of 4.67 × 10³²cm^-2s^-1. This exceeds the previous world record of 4.024 × 10³²cm^-2s^-1, which was set by the US Fermi National Accelerator Laboratory’s Tevatron collider in 2010, and marks an important milestone in LHC commissioning.

“Beam intensity is key to the success of the LHC, so this is a very important step,” said CERN Director General Rolf Heuer. “Higher intensity means more data, and more data means greater discovery potential.”

Luminosity gives a measure of how many collisions are happening in a particle accelerator: the higher the luminosity, the more particles are likely to collide. When looking for rare processes, this is important. Higgs particles, for example, will be produced very rarely if they exist at all, so for a conclusive discovery or refutation of their existence, a large amount of data is required.

The current LHC run is scheduled to continue to the end of 2012. That will give the experiments time to collect enough data to fully explore the energy range accessible with 3.5 TeV per beam collisions for new physics before preparing the LHC for higher energy running. By the end of the current running period, for example, we should know whether the Higgs boson exists or not.

“There’s a great deal of excitement at CERN today,” said CERN’s Director for Research and Scientific Computing, Sergio Bertolucci, “and a tangible feeling that we’re on the threshold of new discovery.”

After two weeks of preparing the LHC for this new level of beam intensity, the machine is now moving in to a phase of continuous physics running scheduled to last until the end of the year. There will then be a short technical stop, before physics running resumes for 2012.

Press Release

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LHC sees first stable-beam 3.5 TeV collisions of 2011

March 13, 2011 | 1:07 pm

LHC Page 1 declares stable beams.

The Large Hadron Collider saw its first stable-beam 3.5 TeV collisions of 2011 today after 6 p.m. Central European Time (1 p.m. EST).

Watch the machine’s progress on the LHC Page 1 display. For tips on how to read the display, see the CERN Bulletin.

This marks the beginning of a year of unprecedented data-collection for experiments at the LHC. In 2011, scientists plan to significantly increase their stockpile of data, which will give them access to previously unexplored areas of physics.

In 2010, detectors at the LHC recorded data about a selection of about 3000 billion proton-proton collisions. Using last year’s data, scientists confirmed previous discoveries from the Standard Model of physics and improved previous measurements, set new limits on the Higgs boson and supersymmetry, and conducted preliminary studies of the quark-gluon plasma, a state of matter scientists theorize existed in the early universe.

The LHC will run until the end of 2012 with a short technical stop at the end of 2011. After 2012, the machine will go into a long shutdown to prepare for higher energy running starting 2014.

Kathryn Grim

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SUSY search still going strong

March 10, 2011 | 10:35 am

Source: DESY

This week the CMS and ATLAS collaborations at the Large Hadron Collider released more publications in a series revealing that neither experiment has detected evidence of supersymmetry.

Studies with the 2010 data from the LHC extended the search for supersymmetry, or SUSY, far beyond previous experiments. But experimentalists haven’t given up hope of finding it yet.

The idea behind SUSY is that each of the particles in the Standard Model has a massive partner particle. Scientists think these so-called sparticles are produced in pairs, which then decay into jets of normal particles plus a light SUSY particle that is stable, neutral and weakly interacting. If produced during a proton collision at the LHC, such a particle would escape undetected. Scientists are racing to find evidence of these particles, which would appear as missing energy when jets of particles spew from the collision point.

The earliest analysis from CMS and ATLAS showed that the mass of a SUSY particle is most likely heavier than about 775 giga-electron volts. Now, CMS and ATLAS are looking at more specific sets of collision events to search for signs of SUSY and other new physics.

“Before the first LHC results, the Tevatron at Fermilab ruled out masses of squarks and gluinos [the supersymmetric partners to quarks and gluons] up to 350 GeV,” said theoretical physicist Ben Allanach of the University of Cambridge in the UK, who independently analyzed the results from both CMS and ATLAS. “The first CMS results essentially doubled the exclusion limit” to 600 GeV, he said. A few weeks later, ATLAS published results improving that limit to 775 GeV. Theory suggests it should weigh in at around 1,000 GeV, or 1 TeV.

The two different limits can be boiled down to the slightly different approaches that each experiment took when analyzing the data. The first results, published by CMS, looked at events comprising jets and missing energy.

“There are only very few sources that could have this type of missing energy,” said Oliver Buchmueller, a CMS physicist from Imperial College London. In order to rule out false signals, the CMS group used a variable called α_T, which Buchmueller explained is “very good at controlling the unavoidable background and picking out the very few events that might be related to new physics.” Only 13 events were selected out of the 4.2 billion proton-proton collisions recorded by CMS last year. “As exciting as they are, they turned out to be compatible with signals from sources we already know exist,” Buchmueller said.

The ATLAS collaboration also analyzed events with jets and missing energy. But they took their early analysis one step further to include a signal from a lepton, either an electron or a muon. The number of events they saw was too small compared to the background to count as evidence of new particles, so they could rule out a larger mass range.

“Our approach was very sensitive to SUSY models,” said ATLAS physicist Pascal Pralavorio of the Centre de Physique des Particules de Marseille. Because the high-energy jets deposit all of their energy in the calorimeter layers of the detector, “the missing energy is a signal for the particles that we don’t see,” he said. Another way to look for SUSY is to examine particles with strange behavior, such as those that have a velocity much smaller than the speed of light, he said.

Both collaborations continue to release results with more refined searches, for example those looking for events with signatures from photons, or one or multiple leptons. Physicists will discuss many of these results during the next two weeks at Moriond, an annual physics conference in Europe.

Although the LHC will operate at the same center-of-mass energy in 2011 — 7 TeV — the researchers remain optimistic that they might find evidence for SUSY.

“We can compensate for the smaller energy by simply taking more data,” Buchmueller said. The LHC aims to reach at least one inverse femtobarn this year, up from 35 inverse picobarns in 2010. Such a vast amount of data could result in 100 to 1000 times the number of possible SUSY events. If they see a significant number of events beyond what is expected from the background signal, it could be grounds for claiming a discovery.

“We will build on the lessons we learned last year and produce similar analysis this year, putting out early results then doing more refined searches,” Pralavorio said.

If there is no sign of SUSY during the 7 TeV collisions in 2011 and 2012, the researchers will have to wait until 2014, when the LHC will ramp up to 14 TeV collisions.

“I’m not nervous yet,” Allanach said. “If they go up to 14 TeV and still don’t see anything, then I’ll start to worry.”

Lauren Rugani

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LHC revs up for a year of new physics

February 25, 2011 | 3:05 pm

An operator monitors the LHC's performance in the CERN Control Center in March 2010. Image courtesy of CERN.

CERN scientists learned to drive the Large Hadron Collider in 2010. This year, they’re ready to take it to the highway.

Scientists expect to collect about 100 times as much data in 2011 as they did in 2010, said physicist Gustaaf Broijmans of the ATLAS experiment during a webcast for the National Science Foundation. This will give them their first real chance at making new discoveries, he said.

CMS physicist Aaron Dominguez, fellow webcast interviewee, agreed.

“The research program over this past year was essentially to commission the accelerator and the experiments, to make sure that they work and are giving us sensible results,” he said. “We already had some idea in mind of what to expect in terms of the physics being produced.”

On Saturday, Feb. 19, the LHC began circulating particle beams for the first time since the accelerator’s 10-week technical stop. Scientists plan to start colliding protons in the four LHC experiments by mid-March.

Although scientists will not bring the beams to an energy above last year’s record of 3.5 TeV, they plan to increase the number of particles they accelerate, multiplying the number of collisions that take place in the detectors.

Last year, scientists gradually ramped up the number of particles they accelerated over several months while carefully checking the LHC’s procedures and protection systems. They reached a breakthrough in the fall. In October alone, they increased by a factor of 20 the total number of particle collisions the LHC had produced that year.

The machine’s performance so far has looked strikingly similar to its performance just before shutdown. This is quite a feat, considering the sheer complexity of the machine and how long it took scientists to bring it to that point last year, said Mike Lamont, LHC operations group leader.

“The machine looks to have settled down,” Lamont said. “Now we know what we have to do.”

This week, technicians will celebrate another improvement over the last run. Before they could start sending beams through the accelerator, they needed to test the machine’s many magnet circuits. Last year, the tests caused 30-40 magnet quenches, a loss of superconductivity, in the LHC. This year, technicians completed commissioning in about three weeks without quenching a single magnet.

If the LHC continues to run well over the next two years, scientists will be able to test many of their predictions, including the existence of the Higgs boson within its expected mass range. However, they will need to collect a large sample of events to really understand new physics.

“By end of 2012, if the accelerator is performing according to plan, we should have a very good first picture of the standard Higgs boson,” Dominguez said. “But we wouldn’t have a really crisp statistical picture we would need to study its properties.”

At the end of 2012, scientists plan to shut down the accelerator for about a year of upgrades to prepare it to run at full design energy, 7 TeV per beam. That will allow scientists to study further energy ranges and to collect enough information to give physicists a clear view of possible new discoveries.

If this is the year the LHC takes to the particle highway, the next set of runs will be the LHC’s first trip to the Autobahn.

Kathryn Grim

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LHCb event display — decoded!

February 7, 2011 | 12:37 pm

One of the four major experiments built around the collision points of CERN’s Large Hadron Collider, LHCb studies B particles – particles composed of beauty quarks. By detecting and analyzing the decays of millions of Bs, it explores the imperfect symmetry between matter and antimatter, the slight imbalance that allowed matter to survive over antimatter and build the universe we inhabit today.

This walk through the event display of the first B particle detected by LHCb shows how the collision data is analyzed.

LHCb event display. Image courtesy of LHCb collaboration.

All four views of the event display show data from the same collision.

1 -> Collision point: Marks the spot where protons from the 2 beams smash into each other.
2 -> Beamline: The arrows show the paths of the proton beams.
3 -> Vertex Locator
4 -> Tracking detectors
5 -> Magnet
6 -> Ring Imaging Cherenkov detectors
7 -> Calorimeters
8 -> Muon detectors

A -> Top view : Looking down onto LHCb (scale 24m x 12m)

This view gives a complete picture of the collision, showing data from all layers of the LHCb detector. The proton beams collide on the left, and B particles fly out close to the path of the proton beams. This property of B particles is reflected in the design of the experiment: The sub-detectors sit side by side along the beam path, like books on a giant bookshelf.

Dominating the image on the left are the large coils of LHCb’s magnet, shown in white [5]. The magnetic field is perpendicular to the page and curves the paths of charged particles in the plane of the page.

On either side of the magnet are tracking detectors [4] that measure the positions of particles as they pass through. The signals the particles leave behind are shown as crosses. There are no sub-detectors inside the magnet, so the paths of the different particles spraying out from the collision are reconstructed in a sophisticated version of “connect the dots” using data from all the LHCb detector’s layers.

Particles from the collision also pass through the Ring Imaging Cherenkov detectors [6], which are used for particle identification. They work by measuring emissions of Cherenkov radiation. This phenomenon, often compared to the sonic boom produced by an aircraft breaking the sound barrier, occurs when a charged particle passes through a certain medium (in this case, a dense gas) faster than light does. As it travels, the particle emits a cone of light, which the Cherenkov detectors reflect onto an array of sensors using mirrors.

Then, particles are stopped by the calorimeters [7], and the energy they deposit is represented in the event display by histograms. The red bars represent the energy of lighter particles (such as photons and electrons), while the blue bars show the energy of the group of mostly heavier particles (such as protons).

The position of the muon detectors [8], furthest from the collision point on the right, is shown by the vertical green lines. The tracks left by the two muons created in this collision are colored in magenta.

B -> Along the beam: Looking straight through the collision point (scale 1m x 0.5m)

This view shows data only from LHCb’s two tracking detectors [4] and the Vertex Locator (VELO) [3]. The curved tracks show how the strong magnetic field affects the direction of charged particles as they spray out from the collision. Some particles are only seen in the very central layer, the VELO, whose main job is to identify the primary interaction point, or vertex, where the collision occurred and also any secondary vertices where short-lived particles decayed. The VELO is very close to the collision point – a mere 8 millimeters from the point of impact.

C -> Where it all happens: The collision region (scale 0.7mm x 10mm)

Here, there are no data points showing actual signals from the sub-detectors. Rather, this is a reconstruction of the innermost region surrounding the collision point, using information gleaned from all of the detector layers. This is where the exciting physics happens.

The image has been stretched in the vertical direction to make it easier for our eyes to decipher – particles spray out from collisions so close to the beamline that without changing the scale, different particle tracks could not be separated by our eyes.

The exact collision point is clearly identified as the point of origin of almost all the lines, labelled “primary vertex.” However the two muons (the magenta tracks labelled μ+ and μ-) originate from a point a few millimeters away, the “secondary vertex.” This indicates that other particles were created in the collision and that they decayed before reaching the first layer of detectors, at the point where the two muons emerge.

The combined mass of the two muons corresponds to the mass of a very short-lived particle called a Jpsi (whose path is represented as a short green dash). The Jpsi was created at the same time as a kaon (red track) at a point close to — but not exactly — the muons’ vertex. The Kaon survived longer than the Jpsi and was detected in several of the layers of LHCb detectors.

Working backwards, each time comparing the direction of the momentum of the different particles, their masses and energies, the full story emerges. A B particle (orange track) was created in the collision and, after a few millimeters decayed into a kaon (red) and a Jpsi (green), which in turn decayed into two muons (magenta).

— Emma Sanders & Thomas Ruf

Other posts in this series include: CMS event display — decoded!, ATLAS event display — decoded!, ALICE event display — decoded! and LHC Page 1 — decoded!

For visitors to CERN, a new photo book telling the story of the LHCb experiment is available from the shop at CERN’s reception and from the CERN library.

Guest author

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CERN announces LHC to run in 2012

January 31, 2011 | 11:09 am

CERN issued the following press release on January 31.

The Large Hadron Collider will restart in February. After a short technical stop at the end of the year, it will continue running through 2012.

CERN today announced that the LHC will run through to the end of 2012 with a short technical stop at the end of 2011. The beam energy for 2011 will be 3.5 TeV. This decision, taken by CERN management following the annual planning workshop held in Chamonix last week and a report delivered today by the laboratory’s machine advisory committee, gives the LHC’s experiments a good chance of finding new physics in the next two years, before the LHC goes into a long shutdown to prepare for higher energy running starting 2014.

“If LHC continues to improve in 2011 as it did in 2010, we’ve got a very exciting year ahead of us,” said CERN’s Director for Accelerators and Technology, Steve Myers. “The signs are that we should be able to increase the data collection rate by at least a factor of three over the course of this year.”

The LHC was previously scheduled to run to the end 2011 before going into a long technical stop necessary to prepare it for running at its full design energy of 7 TeV per beam. However, the machine’s excellent performance in its first full year of operation forced a rethink. Expected performance improvements in 2011 should increase the rate that the experiments can collect data by at least a factor of three compared to 2010. That would lead to enough data being collected this year to bring tantalising hints of new physics, if there is new physics currently within reach of the LHC operating at its current energy. However, to turn those hints into a discovery would require more data than can be delivered in one year, hence the decision to postpone the long shutdown. If there is no new physics in the energy range currently being explored by the LHC, running through 2012 will give the LHC experiments the data needed to fully explore this energy range before moving up to higher energy.

“With the LHC running so well in 2010, and further improvements in performance expected, there’s a real chance that exciting new physics may be within our sights by the end of the year,” Said CERN’s Research Director, Sergio Bertolucci. “For example, if nature is kind to us and the lightest supersymmetric particle, or the Higgs boson, is within reach of the LHC’s current energy, the data we expect to collect by the end of 2012 will put them within our grasp.”

The schedule announced today foresees beams back in the LHC next month, and running through to mid December. There will then be a short technical stop over the year before resuming in early 2012.

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ALICE event display – decoded!

December 21, 2010 | 10:00 am

Diagram of the ALICE detector

Most of the time, the Large Hadron Collider accelerates protons, particles so tiny they fit inside of atoms. But for about a month each year the LHC runs, the scientists load the machine with something a bit heartier: lead ions.

Heavy-ion season is the ALICE detector’s time to shine. Of the four detectors at the LHC, only ALICE was designed specifically to study these types of collisions. This post breaks down different components of the ALICE detector and explains how scientists use them to study matter as it formed after the big bang.

Heavy-ion collisions in the LHC can create quark-gluon plasma, a phase of matter scientists think only occurred in nature during the first millionths of a second of the universe’s birth. In this state, protons and neutrons melt into a hot soup of their constituent pieces, quarks and gluons. Earlier this year, physicists announced definitive evidence for having created QGP at Brookhaven National Laboratory’s Relativistic Heavy
Ion Collider. In November, scientists at the LHC observed characteristics attributed to the quark-gluon plasma as well.

Members of the ALICE collaboration want to study the quark-gluon plasma to learn more about the first few moments that shaped our universe. The many components of the ALICE detector give them the information they need.

1: Silicon tracker
The silicon tracker is a cylinder made up of six layers of silicon detectors surrounding the beam pipe, where particles speed into one another and collide. The silicon tracker takes high-precision measurements of the properties and flight path of particles leaving collisions in the beam pipe.

Scientists need the highest precision measurements closest to the collision point to ensure that the particles they are tracking, there and elsewhere in the detector, actually come from a collision. Particles within the beam can scatter or decay on their own, providing distracting background particles the scientists do not want to confuse with the particles they’re trying to study.

ALICE event display

2: Time projection chamber
Surrounding the silicon tracker is the time projection chamber, or TPC. It has a radius of 2.5 meters. The TPC is a container full of gas that allows scientists to measure the momenta and masses of charged particles. When a charged particle flies through the gas in the TPC, it pulls electrons from the gas molecules. A high-voltage electric field running through the center of the TPC causes those electrons to drift to the ends of the chamber, the endcaps.

The detector records where and when the electrons hit sensors on the endcaps. It’s like taking a series of photos of a finish line of a race from directly above. If you flip through the photos you took, you can see the order in which the runners arrived, how far apart they were and where they crossed the finish line. Scientists take information from the detector to find out how the electrons ran their race to the endcap. Using this information, they can figure out the path the particle that knocked them loose followed and how fast it was moving.

Scientists can also use the detector to determine the momentum of a particle. The entire chamber is in a magnetic field, so charged particles curve as they move through it. Positively charged particles curve in one direction and negatively charged particles curve in the opposite direction. Scientists can determine the momentum of a particle based on how much it curves. The field bends the path of slower particles more than it does faster ones.

Not pictured: Time-of-flight barrel
Surrounding the TPC is the time-of-flight barrel, not pictured here. The time-of-flight barrel is important, as it allows scientists to identify passing particles.

Scientists recognize a particle by its mass. The momentum of a particle is equal to its mass multiplied by its velocity. Since scientists already know the momentum of a particle based on the curvature of its path through the magnetic field in the TPC, all they need to know to calculate the mass is its velocity. That’s where the time-of-flight barrel comes in.

The time-of-flight barrel measures the time that each signal arrives, which lets the scientists know how long the particle took to travel through the TPC. The distance the particle traveled through the TPC divided by that time equals the velocity of the particle. Scientists calculate the mass of a particle by dividing its momentum by its velocity.

ALICE event display at an angle

3: Electromagnetic calorimeter
The electromagnetic calorimeter, or EMCal, has the task of measuring neutral particles, such as photons or electrons. It earned its name because photons and electrons interact through the electromagnetic force.

The image here represents the data the ALICE detector collected from a collision of two lead ions. You can see the tracks left in the TPC by about 3,000 charged particles. Another about 1,500 neutral particles passed through the gas chamber unnoticed, as they did not ionize the gas inside it. The calorimeter measures the energy of those neutral particles by absorbing them.

The red bars pictured here represent particles that deposited a particularly large amount of energy. The height of the bar represents its energy. The smaller purple bars represent lower energy particles, most likely charged particles, which will most likely be considered background and no longer be measured once the tolerance of the detector is reset.

4: Transition radiation detector
This detector measures transition radiation, emitted when a particle traveling nearly the speed of light traverses a material. The transition radiation detector allows scientists to identify high-velocity particles. As you read above, the momentum of a particle is equal to its mass multiplied by its velocity. If two particles of similar mass both are moving at very high velocities, they will have similar momenta and be difficult to tell apart using only the TPC.

5: Photon spectrometer
The photon spectrometer, or PhoS, detects photons, which are neutral particles. Some particles that emerge from collisions break down into electron – positron pairs.

In this image, the sensitivity of the photon spectrometer was set lower than that of the calorimeter, so you don’t see as many background signals. The blue bars represent neutral particles that deposited energy.

ALICE event display with muon detector

6: Muon detector
Muons are the heavy cousins of electrons. Scientists want to identify particles that decay into muons, such as beauty and charm quarks. The last piece of the particle detector is the muon detector, pictured here in green.

The muon detector is made up of steel plates and tracking chambers. The steel absorbs most particles, but muons rarely interact with matter. So any charged particles hardy enough to make it through the entire detector and steel plates are likely to be muons, shown here as blue and green lines.

Other posts in this series include: CMS event display — decoded!, ATLAS event display — decoded! and LHC Page 1 — decoded!

Kathryn Grim

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