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Posts Tagged ‘LHC’

This article appeared in DOE Pulse on Nov. 10, 2014.

Fermilab's Oliver Gutsche leads worldwide computing operations for the CMS experiment. Photo: Reidar Hahn

Fermilab’s Oliver Gutsche leads worldwide computing operations for the CMS experiment. Photo: Reidar Hahn

Since he was a graduate student in Germany, Oliver Gutsche wanted to combine research in particle physics with computing for the large experiments that probe the building blocks of matter.

“When I started working on the physics data coming from one of the experiments at DESY, I was equally interested in everything that had to do with large-scale computing,” said Gutsche of his time at the German laboratory. Gutsche now works at DOE’s Fermi National Accelerator Laboratory. “So I also began working on the computing side of particle physics. For me that was always the combination I wanted to do.”

Gutsche’s desire to merge the two focuses has paid off. For the past four years Gutsche has been in charge of worldwide computing operations of the Large Hadron Collider’s CMS experiment, one of two experiments credited with the 2012 Higgs boson discovery. In December he was awarded the CMS Collaboration Award for his contributions to the global CMS computing system. And more recently, he has been promoted to assistant head of the Scientific Computing Division at Fermilab.

As head of CMS Computing Operations, Gutsche orchestrates data processing, simulations, data analysis and transfers and manages infrastructure and many more central tasks. Monte Carlo simulations of particle interactions, for example, are a key deliverable of the CMS Computing Operations group. Monte Carlo simulations employ randomness to simulate the collisions of the LHC and their products in a statistical way.

“You have to simulate the randomness of nature,” explained Gutsche. “We need Monte Carlo collisions to make sure we understand the data recorded by the CMS experiment and to compare them to the theory.”

When Gutsche received his Ph.D. from the University of Hamburg in 2005, he was looking for a job to combine LHC work, large-scale computing and a U.S. postdoc experience.

“Fermilab was an ideal place to do LHC physics research and LHC computing at the same time,” he said. His postdoc work led to his appointment as an application physicist at Fermilab and as the CMS Computing Operations lead.

Today Gutsche interacts regularly with people at universities and laboratories across the United States and at CERN, host laboratory of the LHC, often starting the day at 7 a.m. for transatlantic or transcontinental meetings.

“I try to talk physics and computing with everyone involved, even those in different time zones, from CERN to the west coast,” he said. Late afternoon in the United States is a good time for writing code. “That’s when everything quiets down and Europe is asleep.”

Gutsche expects to further enhance the cooperation between U.S. particle physicists and their international colleagues, mostly in Europe, by using the new premier U.S. Department of Energy’s Energy Sciences Network recently announced in anticipation of the LHC’s restart in spring 2015 at higher energy.

Helping connect the research done by particle physicists around the world, Gutsche finds excitement in all the work he does.

“Of course the Higgs boson discovery was very exciting,” Gutsche said. “But in CMS Computing Operations everything is exciting because we prepare the basis for hundreds of physics analyses so far and many more to come, not only for the major discoveries.”

Rich Blaustein

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This Fermilab press release came out on Oct. 20, 2014.

ESnet to build high-speed extension for faster data exchange between United States and Europe. Image: ESnet

ESnet to build high-speed extension for faster data exchange between United States and Europe. Image: ESnet

Scientists across the United States will soon have access to new, ultra-high-speed network links spanning the Atlantic Ocean thanks to a project currently under way to extend ESnet (the U.S. Department of Energy’s Energy Sciences Network) to Amsterdam, Geneva and London. Although the project is designed to benefit data-intensive science throughout the U.S. national laboratory complex, heaviest users of the new links will be particle physicists conducting research at the Large Hadron Collider (LHC), the world’s largest and most powerful particle collider. The high capacity of this new connection will provide U.S. scientists with enhanced access to data at the LHC and other European-based experiments by accelerating the exchange of data sets between institutions in the United States and computing facilities in Europe.

DOE’s Brookhaven National Laboratory and Fermi National Accelerator Laboratory—the primary computing centers for U.S. collaborators on the LHC’s ATLAS and CMS experiments, respectively—will make immediate use of the new network infrastructure once it is rigorously tested and commissioned. Because ESnet, based at DOE’s Lawrence Berkeley National Laboratory, interconnects all national laboratories and a number of university-based projects in the United States, tens of thousands of researchers from all disciplines will benefit as well.

The ESnet extension will be in place before the LHC at CERN in Switzerland—currently shut down for maintenance and upgrades—is up and running again in the spring of 2015. Because the accelerator will be colliding protons at much higher energy, the data output from the detectors will expand considerably—to approximately 40 petabytes of raw data per year compared with 20 petabytes for all of the previous lower-energy collisions produced over the three years of the LHC first run between 2010 and 2012.

The cross-Atlantic connectivity during the first successful run for the LHC experiments, which culminated in the discovery of the Higgs boson, was provided by the US LHCNet network, managed by the California Institute of Technology. In recent years, major research and education networks around the world—including ESnet, Internet2, California’s CENIC, and European networks such as DANTE, SURFnet and NORDUnet—have increased their backbone capacity by a factor of 10, using sophisticated new optical networking and digital signal processing technologies. Until recently, however, higher-speed links were not deployed for production purposes across the Atlantic Ocean—creating a network “impedance mismatch” that can harm large, intercontinental data flows.

An evolving data model
This upgrade coincides with a shift in the data model for LHC science. Previously, data moved in a more predictable and hierarchical pattern strongly influenced by geographical proximity, but network upgrades around the world have now made it possible for data to be fetched and exchanged more flexibly and dynamically. This change enables faster science outcomes and more efficient use of storage and computational power, but it requires networks around the world to perform flawlessly together.

“Having the new infrastructure in place will meet the increased need for dealing with LHC data and provide more agile access to that data in a much more dynamic fashion than LHC collaborators have had in the past,” said physicist Michael Ernst of DOE’s Brookhaven National Laboratory, a key member of the team laying out the new and more flexible framework for exchanging data between the Worldwide LHC Computing Grid centers.

Ernst directs a computing facility at Brookhaven Lab that was originally set up as a central hub for U.S. collaborators on the LHC’s ATLAS experiment. A similar facility at Fermi National Accelerator Laboratory has played this role for the LHC’s U.S. collaborators on the CMS experiment. These computing resources, dubbed Tier 1 centers, have direct links to the LHC at the European laboratory CERN (Tier 0).  The experts who run them will continue to serve scientists under the new structure. But instead of serving as hubs for data storage and distribution only among U.S.-based collaborators at Tier 2 and 3 research centers, the dedicated facilities at Brookhaven and Fermilab will be able to serve data needs of the entire ATLAS and CMS collaborations throughout the world. And likewise, U.S. Tier 2 and Tier 3 research centers will have higher-speed access to Tier 1 and Tier 2 centers in Europe.

“This new infrastructure will offer LHC researchers at laboratories and universities around the world faster access to important data,” said Fermilab’s Lothar Bauerdick, head of software and computing for the U.S. CMS group. “As the LHC experiments continue to produce exciting results, this important upgrade will let collaborators see and analyze those results better than ever before.”

Ernst added, “As centralized hubs for handling LHC data, our reliability, performance and expertise have been in demand by the whole collaboration, and now we will be better able to serve the scientists’ needs.”

An investment in science
ESnet is funded by DOE’s Office of Science to meet networking needs of DOE labs and science projects. The transatlantic extension represents a financial collaboration, with partial support coming from DOE’s Office of High Energy Physics (HEP) for the next three years. Although LHC scientists will get a dedicated portion of the new network once it is in place, all science programs that make use of ESnet will now have access to faster network links for their data transfers.

“We are eagerly awaiting the start of commissioning for the new infrastructure,” said Oliver Gutsche, Fermilab scientist and member of the CMS Offline and Computing Management Board. “After the Higgs discovery, the next big LHC milestones will come in 2015, and this network will be indispensable for the success of the LHC Run 2 physics program.”

This work was supported by the DOE Office of Science.
Fermilab is America’s premier national laboratory for particle physics and accelerator research. A U.S. Department of Energy Office of Science laboratory, Fermilab is located near Chicago, Illinois, and operated under contract by the Fermi Research Alliance, LLC. Visit Fermilab’s website at www.fnal.gov and follow us on Twitter at @FermilabToday.

Brookhaven National Laboratory is supported by the Office of Science of the U.S. Department of Energy.  The Office of Science is the single largest supporter of basic research in the physical sciences in the United States, and is working to address some of the most pressing challenges of our time.  For more information, please visit science.energy.gov.

One of ten national laboratories overseen and primarily funded by the Office of Science of the U.S. Department of Energy (DOE), Brookhaven National Laboratory conducts research in the physical, biomedical, and environmental sciences, as well as in energy technologies and national security. Brookhaven Lab also builds and operates major scientific facilities available to university, industry and government researchers. Brookhaven is operated and managed for DOE’s Office of Science by Brookhaven Science Associates, a limited-liability company founded by the Research Foundation for the State University of New York on behalf of Stony Brook University, the largest academic user of Laboratory facilities, and Battelle, a nonprofit applied science and technology organization.

Visit Brookhaven Lab’s electronic newsroom for links, news archives, graphics, and more at http://www.bnl.gov/newsroom, follow Brookhaven Lab on Twitter, http://twitter.com/BrookhavenLab, or find us on Facebook, http://www.facebook.com/BrookhavenLab/.

The DOE Office of Science is the single largest supporter of basic research in the physical sciences in the United States and is working to address some of the most pressing challenges of our time. For more information, please visit science.energy.gov.

Media contacts:

  • Karen McNulty-Walsh, Brookhaven Media and Communications Office, kmcnulty@bnl.gov, 631-344-8350
  • Kurt Riesselmann, Fermilab Office of Communication, media@fnal.gov, 630-840-3351
  • Jon Bashor, Computing Sciences Communications Manager, Lawrence Berkeley National Laboratory, jbashor@lbnl.gov, 510-486-5849

Computing contacts:

  • Lothar Bauerdick, Fermilab, US CMS software computing, bauerdick@fnal.gov, 630-840-6804
  • Oliver Gutsche, Fermilab, CMS Offline and Computing Management Board, gutsche@fnal.gov, 630-840-8909
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by Karen McNulty Walsh

With the discovery of the long-sought Higgs boson at the Large Hadron Collider (LHC), the world’s largest and most powerful particle collider, folks unfamiliar with the intricacies of particle physics might think the field has reached its end. But physicists gathered at the Large Hadron Collider Physics Conference in New York City June 2-7 say they are eager to move forward. Even amid discussions of tight budgets that make some proposed projects appear impossible, the general tenor, as expressed by leaders in the field, is that the future holds great potential for even more significant discoveries.

Physicist panel

Physicists joined New York Times science correspondent Dennis Overbye for a discussion on the future of the field.

At a session devoted to reflection and the future of the field, held Friday, June 6, Fabiola Gianotti, a particle physicist at Europe’s CERN laboratory (home of the LHC) and spokesperson for the LHC’s ATLAS experiment at the time of the Higgs discovery, said, “There is challenging work for everyone to make the impossible possible.” In fact, said James Siegrist, Associate Director of the Office of High Energy Physics within the U.S. Department of Energy’s (DOE) Office of Science, “I think the promise of the physics has never been greater.”

Co-sponsored by DOE’s Brookhaven National Laboratory and Columbia University, the week-long meeting featured updates on key findings from the LHC’s four experiments (including a possible hint of new physics), advances in theory, plans for future upgrades, and even future colliders—as well as apanel discussion moderated by Dennis Overbye, a science correspondent for the New York Times.

“We had a very successful conference with more than 300 participants discussing an impressive array of results from the recent LHC run,” said Brookhaven physicist Srini Rajagopalan, U.S. ATLAS Operations Program Manager and a co-organizer of the meeting. He also noted the extremely positive response to an open-to-the-public screening of Particle Fever, a documentary film that follows six scientists during the years leading up to the discovery of the Higgs boson. “I was simply amazed at the public interest in what we do. From young school students to senior citizens, people thronged to watch the movie and continued to ask questions late into the night.”

What keeps you up at night?

At Friday’s panel session, the Times’ Overbye had some questions of his own, perhaps more pointed that the public’s. He asked whether particle physicists’ streak of discoveries could be continued, whether the “glory days” for the U.S. were over, and what keeps physicists up at night. The panelists were realistic about challenges and the need for smart choices and greater globalization. But a spirit of optimism prevailed.

Natalie Roe, Director of the Physics Division at DOE’s Lawrence Berkeley National Laboratory—the first to respond—said, “I’m going to flip the question [of what keeps me up and night] and answer what gets me up in the morning.” Following a long period of experimental and theoretical successes, including the discovery of the Higgs, she said, “this is a very exciting time. There are still a few remaining details … dark matter and dark energy. And these are more than details; they are 95 percent of the universe!” With a long list of techniques available to get answers, she said, there is much work to be done.

University of California, Santa Cruz, physicist Steve Ritz, who recently chaired the Particle Physics Project Prioritization Panel (P5) and presented its recommendations for the future of the field, emphasized the importance of “telling our story,” staging and prioritizing future projects, and “aspiring to a greater program” that continues investments in crucial research and development to lay the foundation for future facilities.

Great technology progress, great challenges

In an overview talk that preceded the panel discussion, Gianotti presented a range of such future projects, including two possible linear accelerators, one in Japan the other at CERN, and two possible circular colliders, one in China and one at CERN. The latter, dubbed FCC, would be a proton-proton collider 80-100 kilometers in circumference—on the scale of the Superconducting Supercollider (SSC) once planned for and later cancelled in the U.S. Such a machine would push beyond the research limits of even the most ambitious upgrades proposed for the LHC.

Those upgrades, planned for data taking in Phase I in 2020 and Phase II in 2025, will begin the exploration of the coupling of the Higgs with other particles to explore the mechanism by which the Higgs generates mass, “electroweak symmetry breaking,” and searches for new physics beyond the standard model and into the realm of dark matter.

But, to really get at the heart of those questions and possibly reveal unknown physics, the scientists say the need for even higher precision and higher energy is clear.

Journey to the dark side

“Our elders had it easy compared to our students,” said Siegrist, describing the physics challenges now open to exploration. He likened this moment in time to the end of a video game his son had played where, “at the end of the game, you end up on ‘the dark side’ and have to start again.” In physics, he said, the dark sector—exploring dark matter and dark energy—is going to be equally challenging to everything that has come before.

To those who say building the future machines needed for this journey is impossible, Gianotti says, “didn’t the LHC also look close to impossible in the 1980s?” The path forward, she emphasized, is to innovate.

“Accelerator R&D is very important,” said Ritz, noting that, “anything we can do to design these machines to cost less” in terms of construction and operation should be done. “We need to be impatient about this,” he said. “We need to ask more and jump in more.”

Panelist Nima Arkani-Hamed, a theorist at the Institute of Advanced Study at Princeton University and Director of the Center for Future High Energy Physics in Beijing, China, likely agrees. He acknowledges the difficult task facing U.S. leadership in high-energy physics. “They are trying to make due with a budget that’s two or three times less than what our vision and this country deserves, and they are doing a good job,” he said. “But I worry that our generation will be viewed as the one that dropped the ball.”

“The sequence of steps for the next few decades is possible,” he added later. “It’s just a matter of will, not technology.”

But because of the scale and cost of future projects, he, like others, emphasized that “we will need the whole world and new pockets of resources and talent.”

The value of collaboration, competition, and globalization

Sergio Bertolucci, Director for Research and Computing at CERN, agreed. “We have been international, but we need to be truly global.”

Such cooperation and competition among nations is good for the field, Ritz emphasized. “We are intensely competitive. We want to be the ones to discover [something new.] But we are also cooperative because we can’t do it alone.”

Panelist Jerry Blazey, Assistant Director for Physical Sciences in the
Office of Science and Technology Policy, DOE’s Siegrist, and others agreed that the LHC is a great model for the field to stand behind and emulate for future collaborative projects. Blazey and Siegrist said OSTP and DOE would work together to discuss ways to smooth the process for such future multinational collaborations and to implement the recommendations of the P5 report.

These include future U.S. work at the LHC, an internationalized Long Baseline Neutrino Facility located at Fermi National Accelerator Laboratory, and a role in Japan’s proposed linear collider, as well as continued investments in the technologies needed for future experiments. Said University of California, Irvine, physicist Andrew Lankford, chair of the High Energy Physics Advisory Panel (HEPAP) to whom the report was delivered, the P5 report describes a field optimized for scientific progress. “It’s a ten year strategic plan—way more than a collection of cool experiments,” he said.

And it emphasizes the value of international competition and cooperation—perhaps one of the biggest successes of particle physics, aside from the breathtaking discoveries. Turning again to the example of the LHC collaborations, Ritz said, “50 years ago some of these people were in countries that were trying to kill one another. Now we don’t even think about what country they are from.”

As Brookhaven’s Rajagopalan summed up, “It is an exciting time for our field as we plan to move forward with ambitious global projects to address the fundamental questions of nature.”

Brookhaven Lab’s particle physics research is supported by the DOE Office of Science.

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Karen McNulty Walsh is a science writer in the Media & Communications Office at Brookhaven National Laboratory.

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Top Quarks… So Many Top Quarks

Wednesday, April 30th, 2014

Thousands of paper on top quarks exist. Why?

There are literally thousands of papers, collaboration notes, and conference notes with the words “Top” and “Quark” in the title. As of this post, there are 3,477 since 1979 listed on inSpires. There are many, many more that omit the word “quark”. And sure, this is meager compared to the 5,114 papers with the words “Higgs Boson” written since ’74, but that is over 50,000 pages of top quarks (estimating 15 pages/paper). To be fair, there are also many, many more that omit the word “boson”. But for further comparison, there are only 395 papers with a title including the words “Bottom Quark“, 211 with “Bottomonium“, and 125 with “Bottom Hadron“. So why are there so many papers written about the top quark? The answer is that the top quark is weird special.

http://www.symmetrymagazine.org/breaking/2009/09/02/top-quark-chefs

A single top quark candidate event at the Collider Detector experiment at Fermilab. Credit: CDF Collaboration

The top quark is very heavy, about 185 times heavier than the proton and ranks as the heaviest known elementary particle in all the particle kingdom. The second heaviest quark, the bottom quark, is only 4 or 5 times heavier than the proton. If you or I were a proton, then a medium-to-large school bus (without any people) would be a top quark. In fact, the top quark is so heavy it can decay into a real (on-shell) W boson, which is roughly half its mass. The only other particle that can do this is the Higgs. Though it is rare, exceedingly rare, the top quark can decay into real Z  and Higgs bosons as well. Not even the Higgs can top that last feat.

Top quark decaying into real, on-shell W boson and bottom quark. Credit: DZero Collaboration

However, the top quark is still a quark. It has an electric charge that is 2/3 as large as the proton. It has an intrinsic angular momentum (spin) equal to the proton’s or electron’s spin. The top quark is also colored, meaning that is interacts with gluons and is influenced by the strong nuclear force (QCD). When colored objects (quarks and gluons) are produced at collider and fixed target experiments, they undergo a process called hadronization. Hadronization is when two colored objects are far away from one another and the strong nuclear attraction between the two becomes so strong that a pair of colored objects will spontaneously be produced in the space between them. These new colored particles will then form bound states with the old colored states. However, the process hadronization means that we only observe the bound states of colored objects and not the colored objects themselves. Physicists have to infer their properties from the physics of bound states…. or do we?

jets

Colored objects before (L), during (Center L and Center R), and after (R) hadronization.

The onset of hadronization is typically occurs about 10-24 seconds after the creation of a colored object. Yes, that is 0.000000000000000000000001 seconds. That is incredibly fast and well beyond anything that can be done at an experiment. The mean lifetime of the top quark on the other hand is about 10-25 seconds. In other words, the top quark is much more likely to decay in to a W boson, its principle decay mode, than hadronize. By looking at the decays of the W boson, for example to an electron and an electron-neutrino, their angular distributions, and other kinematic properties, we can measure directly the top quark’s quantum numbers. The top quark is special because it is the only quark whose spin and charge quantum numbers we can measure directly.

feynman_t_decay_ljetsqq_pink

Top quark decaying into real, on-shell W boson and bottom quark. The W boson can subsequently decay into a charged lepton and a neutrino or into a quark and anti-quark. Credit: DZero Collaboration

The top quark tells us much about the Standard Model of particle physics, but it also may be a window to new physics. Presently, no one has any idea why the top quark is so much heavier than the bottom quark, or why both are orders of magnitude heavier than the electron and muon. This is called the “Mass Hierarchy Problem” of the Standard Model and stems from the fact that the quark and lepton masses in the theory are not predicted but are taken as input parameters. This does not mean that the Standard Model is “wrong”. On the contrary, the model works very, very well; it is simply incomplete. Of course there are new models and hypotheses that offer explanations, but none have been verified by data.

However, thanks to the 2012 discovery of the Higgs boson, there is a new avenue that may shed light upon the mass hierarchy problem. We now know that quarks and leptons interact with the Higgs boson proportionally to their masses. Since the top quark is ~40 times more massive than the bottom quark, it will interact with Higgs boson 40 times more strongly. There is suspicion that since the Higgs boson is sensitive to the different quark and lepton masses, it may somehow play a role in how masses are assigned.

Happy Colliding

- richard (@BraveLittleMuon)

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Even before my departure to La Thuile in Italy, results from the Rencontres de Moriond conference were already flooding the news feeds. This year’s Electroweak session from 15 to 22 March, started with the first “world measurement” of the top quark mass, from a combination of the measurements published by the Tevatron and LHC experiments so far. The week went on to include a spectacular CMS result on the Higgs width.

Although nearing its 50th anniversary, Moriond has kept its edge. Despite the growing numbers of must-attend HEP conferences, Moriond retains a prime spot in the community. This is in part due to historic reasons: it’s been around since 1966, making a name for itself as the place where theorists and experimentalists come to see and be seen. Let’s take a look at what the LHC experiments had in store for us this year…

New Results­­­

Stealing the show at this year’s Moriond was, of course, the announcement of the best constraint yet of the Higgs width at < 17 MeV with 95% confidence reported in both Moriond sessions by the CMS experiment. Using a new analysis method based on Higgs decays into two Z particles, the new measurement is some 200 times better than previous results. Discussions surrounding the constraint focussed heavily on the new methodology used in the analysis. What assumptions were needed? Could the same technique be applied to Higgs to WW bosons? How would this new width influence theoretical models for New Physics? We’ll be sure to find out at next year’s Moriond…

The announcement of the first global combination of the top quark mass also generated a lot of buzz. Bringing together Tevatron and LHC data, the result is the world’s best value yet at 173.34 ± 0.76 GeV/c2.  Before the dust had settled, at the Moriond QCD session, CMS announced a new preliminary result based on the full data set collected at 7 and 8 TeV. The precision of this result alone rivals the world average, clearly demonstrating that we have yet to see the ultimate attainable precision on the top mass.

ot0172hThis graphic shows the four individual top quark mass measurements published by the ATLAS, CDF, CMS and DZero collaborations, together with the most precise measurement obtained in a joint analysis.

Other news of the top quark included new LHC precision measurements of its spin and polarisation, as well as new ATLAS results of the single top-quark cross section in the t-channel presented by Kate Shaw on Tuesday 25 March. Run II of the LHC is set to further improve our understanding of this

A fundamental and challenging measurement that probes the nature of electroweak symmetry breaking mediated by the Brout–Englert–Higgs mechanism is the scattering of two massive vector bosons against each other. Although rare, in the absence of the Higgs boson, the rate of this process would strongly rise with the collision energy, eventually breaking physical law. Evidence for electroweak vector boson scattering was detected for the first time by ATLAS in events with two leptons of the same charge and two jets exhibiting large difference in rapidity.

With the rise of statistics and increasing understanding of their data, the LHC experiments are attacking rare and difficult multi-body final states involving the Higgs boson. ATLAS presented a prime example of this, with a new result in the search for Higgs production in association with two top quarks, and decaying into a pair of b-quarks. With an expected limit of 2.6 times the Standard Model expectation in this channel alone, and an observed relative signal strength of 1.7 ± 1.4, the expectations are high for the forthcoming high-energy run of the LHC, where the rate of this process is enhanced.

Meanwhile, over in the heavy flavour world, the LHCb experiment presented further analyses of the unique exotic state X(3872). The experiment provided unambiguous confirmation of its quantum numbers JPC to be 1++, as well as evidence for its decay into ψ(2S)γ.

Explorations of the Quark-Gluon Plasma continue in the ALICE experiment, with results from the LHC’s lead-proton (p-Pb) run dominating discussions. In particular, the newly observed “double-ridge” in p-Pb is being studied in depth, with explorations of its jet peak, mass distribution and charge dependence presented.

New explorations

Taking advantage of our new understanding of the Higgs boson, the era of precision Higgs physics is now in full swing at the LHC. As well as improving our knowledge of Higgs properties – for example, measuring its spin and width – precise measurements of the Higgs’ interactions and decays are well underway. Results for searches for Beyond Standard Model (BSM) physics were also presented, as the LHC experiments continue to strongly invest in searches for Supersymmetry.

In the Higgs sector, many researchers hope to detect the supersymmetric cousins of the Higgs and electroweak bosons, so-called neutralinos and charginos, via electroweak processes. ATLAS presented two new papers summarising extensive searches for these particles. The absence of a significant signal was used to set limits excluding charginos and neutralinos up to a mass of 700 GeV – if they decay through intermediate supersymmetric partners of leptons – and up to a mass of 420 GeV – when decaying through Standard Model bosons only.

Furthermore, for the first time, a sensitive search for the most challenging electroweak mode producing pairs of charginos that decay through W bosons was conducted by ATLAS. Such a mode resembles that of Standard Model pair production of Ws, for which the currently measured rates appear a bit higher than expected.

In this context, CMS has presented new results on the search for the electroweak pair production of higgsinos through their decay into a Higgs (at 125 GeV) and a nearly massless gravitino. The final state sports a distinctive signature of 4 b-quark jets compatible with a double Higgs decay kinematics. A slight excess of candidate events means the experiment cannot exclude a higgsino signal. Upper limits on the signal strength at the level of twice the theoretical prediction are set for higgsino masses between 350 and 450 GeV.

In several Supersymmetry scenarios, charginos can be metastable and could potentially be detected as a long-lived particle. CMS has presented an innovative search for generic long-lived charged particles by mapping their detection efficiency in function of the particle kinematics and energy loss in the tracking system. This study not only allows to set stringent limits for a variety of Supersymmetric models predicting chargino proper lifetime (c*tau) greater than 50cm, but also gives a powerful tool to the theory community to independently test new models foreseeing long lived charged particles.

In the quest to be as general as possible in the search for Supersymmetry, CMS has also presented new results where a large subset of the Supersymmetry parameters, such as the gluino and squark masses, are tested for their statistical compatibility with different experimental measurements. The outcome is a probability map in a 19-dimension space. Notable observations in this map are that models predicting gluino masses below 1.2 TeV and sbottom and stop masses below 700 GeV are strongly disfavoured.

… but no New Physics

Despite careful searches, the most heard phrase at Moriond was unquestionably: “No excess observed – consistent with the Standard Model”. Hope now lies with the next run of the LHC at 13 TeV. If you want to find out more about the possibilities of the LHC’s second run, check out the CERN Bulletin article: “Life is good at 13 TeV“.

In addition to the diverse LHC experiment results presented, Tevatron experiments, BICEP, RHIC and other experiments also reported their breaking news at Moriond. Visit the Moriond EW and Moriond QCD conference websites to find out more.

Katarina Anthony-Kittelsen

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1st April 2014. The LHC is currently in shutdown in preparation for the next physics run in 2015. However the record breaking accelerator is danger is falling far behind schedule as the engineers struggle with technical difficulties 100m below ground level.

The LHC tunnels house the 27km long particle accelerator in carefully controlled conditions. When the beams circulate they must be kept colder than anywhere else in the solar system, and with a vacuum more empty the voids of outer space. Any disruption to the cryogenic cooling systems or the vacuum systems can place serious strain on the operations timetable, and engineers have found signs of severe damage.

Scientists patrol the LHC, inspecting the damaged areas.

Scientists patrol the LHC, inspecting the damaged areas.

The first indications of problems were identified coming from Sector 7 between areas F and H. Cryogenics expert, Francis Urquhart said “My team noticed dents in the service pipes about 50cm from the floor. There was also a deposit of white fibrous foreign matter on some of the cable trays.” The pipes were replaced, but the damage returned the following day, and small black aromatic samples were found piled on the floor. These were sent for analysis and after chemical tests confirmed that they contained no liquid Helium, and that radiometry found they posed no ionisation risk, they were finally identified as Ovis aries depositions.

Ovis aries are found throughout the CERN site, so on-site contamination could not be ruled out. It is currently thought that the specimens entered the Super Proton Synchrotron (SPS) accelerator and proceeded from the SPS to the LHC, leaving deposits as they went. The expert in charge, Gabriella Oak, could not be reached for comment, but is said to be left feeling “rather sheepish”.

Elsewhere on the ring there was another breach of the security protocols as several specimens of Bovinae were found in the ring. The Bovinae are common in Switzerland and it due to their size, must have entered via one of the service elevators. All access points and elevators at the LHC are carefully controlled using biometry and retinal scans, making unauthorised entry virtually impossible. Upon being asked whether the Bovinae had been seen scanning their retinae at the security checkpoints, Francis Urquhart replied “You might very well think that. I could not possibly comment.” While evidence of such actions cannot be found CCTV footage, there have been signs of chewed cud found on the floor, and Bovinae deposits, which are significantly larger than the Ovis deposits, owing to the difference in size.

The retinal scans at the LHC are designed exclusively for human use. A search of the biometric record database show at least one individual (R Wiggum) with unusual retinae, affiliated to “Bovine University”.

It is not known exactly how much fauna is currently in the LHC tunnels, although it is thought to be at least 25 different specimens. They can be identified by the bells they carry around their necks, which can sound like klaxons when they charge. Until the fauna have been cleared, essential repair work is extremely difficult. “I was repairing some damage caused by a passing cow” said Stanford PhD student Cecilia, “when I thought I heard the low oxygen klaxon. By the time I realised it was just two sheep I had already put on my safety mask and pulled the alarm to evacuate the tunnels.” She then commented “It took us three hours to get access to the tunnels again, and the noises and lights had caused the animals to panic, creating even more damage to clean up.”

This is not the first time a complex of tunnels has been overrun by farm animals. In the early 90s the London Underground was found to be infested with horses, which turned into a longterm problem and took many years to resolve.

Current estimates on the delay to the schedule range from a few weeks to almost a decade. Head of ATLAS operations, Dr Remy Beauregard Hadley, comments “I can’t believe all this has happened. They talk about Bovinae deposits delaying the turn on, and I think it’s just a load of bullshit!”

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This is the last part of a series of three on supersymmetry, the theory many believe could go beyond the Standard Model. First I explained what is the Standard Model and show its limitations. Then I introduced supersymmetry and explained how it would fix the main flaws of the Standard Model. I now review how experimental physicists are trying to discover “superparticles” at the Large Hadron Collider (LHC) at CERN.

If Supersymmetry (or SUSY for short) is as good as it looks, why has none of the new SUSY particles been found yet? There could be many reasons, the simplest being that this theory is wrong and supersymmetric particles do not exist. If that were the case, one would still need another way to fix the Standard Model.

SUSY can still be the right solution if supersymmetric particles have eluded us for some reasons: we might have been looking in the wrong place, or in the wrong way or they could still be out of the reach of current accelerators.

So how does one go looking for supersymmetric particles? One good place to start is at CERN with the Large Hadron Collider or LHC. The 27-km long accelerator is the most powerful in the world. It brings protons into collisions at nearly the speed of light, generating huge amounts of energy in the tiniest points in space.  Since energy and matter are two forms of the same essence, like water and ice, the released energy materializes in the form of fundamental particles. The hope is to create some of the SUSY particles.

One major problem is that nobody knows the mass of all these new particles. And without the mass, it is very much like looking for someone in a large city without knowing the person’s address. All one can do then is comb the city trying to spot that person. But imagine the task if you don’t even know what the person looks like, how she behaves or even in which city, let alone which country she lives in.

Supersymmetry is in fact a very loosely defined theory with a huge number of free parameters. These free parameters are quantities like the masses of the supersymmetric particles, or their couplings, i.e. quantities defining how often they will decay into other particles. Supersymmetry does not specify which value all these quantities can take.

Hence, theorists have to make educated guesses to reduce the zone where one should search for SUSY particles. This is how various models of supersymmetry have appeared. Each one is an attempt at circumscribing the search zone based on different assumptions.

One common starting point is to assume that a certain property called R-parity is conserved. This leads to a model called Minimal SUSY but this model still has 105 free parameters. But with this simple assumption, one SUSY particle ends up having the characteristics of dark matter. Here is how it works: R-parity conservation states that all supersymmetric particles must decay into other, lighter supersymmetric particles. Therefore, the lightest supersymmetric particle or LSP cannot decay into anything else. It remains stable and lives forever, just like dark matter particles do. Hence the LSP could be the much sought-after dark matter particle.

SUSY-cascade-Fermilab Credit: Fermilab

How can the Large Hadron Collider help? Around the accelerator, large detectors act like giant cameras, recording how the newly created and highly unstable particles break apart like miniature fireworks. By taking a snapshot of it, one can record the origin, direction and energy of each fragment and reconstruct the initial particle.

Heavy and unstable SUSY particles would decay in cascade, producing various Standard Model particles along the way. The LSP would be the last possible step for any decay chain. Generally, the LSP is one of the mixed SUSY states with no electric charge called neutralino. Hence, each of these events contains a particle that is stable but does not interact with our detectors. In the end, there would be a certain amount of energy imbalance in all these events, indicating that a particle has escaped the detector without leaving any signal.

At the LHC, both the CMS and ATLAS experiments have searched billions of events looking for such events but to no avail. Dozens of different approaches have been tested and new possibilities are constantly being explored. Each one corresponds to a different hypothesis, but nothing has been found so far.

dijet-monjet Two events with jets as seen in the ATLAS detector. (Left) A very common event containing two jets of particles. The event is balanced, all fragments were recorded, no energy is missing. (Right) A simulation of a mono-jet event where a single jet recoils against something unrecorded by the detector. The imbalance in energy could be the signature of a dark matter particle like the lightest supersymmetric particle (LSP), something that carries energy away but does not interact with the detector, i.e. something we would not see.

One reason might be that all supersymmetric particles are too heavy to have been produced by the LHC. A particle can be created only if enough energy is available. You cannot buy something that costs more money than you have in your pocket. To create heavy particles, one needs more energy. It is still possible all SUSY particles exist but were out of the current accelerator reach. This point will be settled in 2015 when the LHC resumes operation at higher energy, going from 8 TeV to at least 13 TeV.

If the SUSY particles are light enough to be created at 13 TeV, the chances of producing them will also be decupled, making them even easier to find. And if we still do not find them, new limits will be reached, which will also greatly help focus on the remaining possible models.

SUSY has not said its last word yet. The chances are good supersymmetric particles will show up when the LHC resumes. And that would be like discovering a whole new continent.

Pauline Gagnon

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Voici le dernier d’une série de trois volets sur la supersymétrie, la théorie qui pourrait aller au-delà du Modèle Standard. J’ai expliqué dans un premier temps ce qu’est le Modèle standard et montré ses limites. Puis dans un deuxième volet, j’ai présenté la supersymétrie et expliqué comment elle pourrait résoudre plusieurs lacunes du Modèle standard. Finalement, je passe ici en revue comment les physicien-ne-s essaient de découvrir des « superparticules » au Grand collisionneur de hadrons (LHC) du CERN.

Si la supersymétrie (ou SUSY pour les intimes) est aussi miraculeuse que prédite, pourquoi aucune nouvelle particule supersymétrique n’a t-elle été trouvée à ce jour ? Il pourrait y avoir beaucoup de raisons, la plus simple étant que cette théorie soit fausse et les particules supersymétriques n’existent tout simplement pas. Si c’était le cas, on devrait alors trouver une alternative pour parer aux lacunes du Modèle standard.

Mais SUSY est toujours une solution plausible et ses particules supersymétriques ont pu nous échapper pour d’autres raisons. Peut-être avons nous regardé au mauvais endroit ou de la mauvaise façon. Ou encore elles pourraient être hors de portée de nos accélérateurs.

Mais au fait où et comment cherche-t-on des particules supersymétriques ? Le Grand collisionneur de hadrons (LHC) CERN est l’endroit idéal. Cet accélérateur de 27 km de longueur est le plus puissant au monde. Il provoque des collisions entre des protons lancés à une vitesse proche de celle de la lumière. Ces collisions produisent des quantités d’énergie énormes concentrés en de minuscules points de l’espace. Puisque l’énergie et la matière sont deux formes d’une même essence, comme l’eau et la glace, l’énergie libérée se matérialise sous forme de particules fondamentales. Il est donc possible de créer certaines de ces particules supersymétriques au LHC.

Malheureusement, personne ne connaît la masse de toutes ces nouvelles particules. Et sans la masse, c’est un peu comme chercher quelqu’un dans une grande ville sans connaître son adresse. Il faudrait alors ratisser la ville pour découvrir cette personne. Mais imaginez la tâche si vous ne savez même pas à quoi la personne ressemble, comment elle se comporte, ni même la ville ou le pays elle habite.

La supersymétrie est en fait une théorie comportant de nombreux paramètres libres. Ces paramètres représentent des quantités comme les masses des particules supersymétriques ou leurs couplages, c’est-à-dire la probabilité qu’elles se désintègrent en d’autres particules. La supersymétrie ne spécifie pas quelles valeurs ces quantités peuvent prendre.

Les théoricien-ne-s doivent donc faire des suppositions pour réduire la zone de recherches. C’est ainsi que divers modèles de supersymétrie sont apparus. Chaque modèle représente une tentative pour circonscrire la zone de recherche basée sur des suppositions différentes.

Une hypothèse populaire consiste à supposer qu’une certaine propriété appelée la parité R est conservée. C’est le cas pour le modèle minimal de SUSY mais il conserve tout de même 105 paramètres libres. Mais de cette simple supposition surgit une particule de SUSY ayant les caractéristiques de la matière sombre.

Voici comment ça marche : la conservation de R-parité stipule que toutes les particules supersymétriques doivent se désintégrer en d’autres particules supersymétriques. Par conséquent, la particule supersymétrique la plus légère, le LSP (de l’acronyme anglais Lightest Supersymmetric Particle) ne peut se désintégrer en rien d’autre et reste stable. Elle existe pour toujours, comme les particules de matière sombre. Le LSP pourrait donc être la particule de matière sombre tant recherchée.

Comment le Grand collisionneur de hadrons peut-il aider? Autour de l’accélérateur, de grands détecteurs agissent comme des appareils photo géants, enregistrant comment les particules nouvellement créées et fortement instables se brisent, créant de mini feux d’artifice. Ces clichés permettent d’enregistrer l’origine, la direction et l’énergie de chaque fragment et ainsi reconstruire la particule initiale.SUSY-decay-fr

Des particules de SUSY lourdes et instables se désintégreraient en cascade, produisant diverses particules du Modèle standard en chemin. Le LSP serait la dernière étape possible pour n’importe quelle chaîne de désintégration. Généralement, le LSP est un des états de SUSY mélangés sans charge électrique appelée neutralino. Au final, chaque événement supersymétrique contiendrait une particule stable, qui n’interagirait pas avec nos détecteurs. On observerait donc un déséquilibre dans la quantité d’énergie de tous ces événements, indiquant qu’une particule s’est échappée du détecteur sans laisser de signaux dans les diverses couches du détecteur.

Au LHC, les physicien-ne-s des expériences CMS et ATLAS ont trié des milliards d’événements à la recherche de tels événements, mais en vain. Des douzaines d’approches différentes ont été testées et de nouvelles possibilités sont constamment explorées. Chacune correspond à une hypothèse différente, mais rien n’a encore été trouvé.

dijet-monjet
Deux événements contenant des gerbes de particules captés par le détecteur ATLAS. (A gauche) un événement très courant contenant deux gerbes de particules. L’événement est équilibré en énergie, tous les fragments ont été enregistrés, aucune énergie ne manque. (A droite) une simulation d’un événement contenant une seule gerbe reculant contre quelque chose qui échappe au détecteur. Le déséquilibre dans l’énergie serait la signature d’une particule de matière sombre comme la particule supersymétrique la plus légère (LSP), une particule qui emporterait une certaine quantité d’énergie, mais n’interagirait pas avec le détecteur et que l’on ne verrait donc pas.

Il se peut aussi que toutes les particules supersymétriques soient trop lourdes pour avoir été produites par le LHC. Une particule peut être créée seulement si suffisamment d’énergie est disponible. On ne peut pas acheter quelque chose qui coûte plus que ce que l’on a dans sa poche. Pour créer des particules lourdes, il faut plus d’énergie. Il est donc toujours possible que toutes les particules de SUSY existent, mais qu’elles soient hors de la portée actuelle de l’accélérateur du LHC. Mais on en saura plus en 2015 quand le LHC reprendra du service à plus haute énergie, passant de 8 TeV à au moins 13 TeV.

Si les particules de SUSY sont assez légères pour être créé à 13 TeV, leurs chances de production seront aussi décuplées, les rendant encore plus facile à trouver. Et si nous ne les trouvons toujours pas, de nouvelles limites seront atteintes, ce qui permettra de se concentrer sur les modèles possibles restants.

SUSY n’a pas encore dit son dernier mot. Il reste de bonnes chances pour que des particules supersymétriques apparaissent quand le LHC redémarrera. Et si c’était le cas, ce serait aussi extraordinaire que la découverte d’un tout nouveau continent.

Pauline Gagnon

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This article appeared in symmetry on March 19, 2014.

An international team of scientists from Fermilab’s Tevatron and CERN’s Large Hadron Collider has produced the world’s best value for the mass of the top quark.

An international team of scientists from Fermilab’s Tevatron and CERN’s Large Hadron Collider has produced the world’s best value for the mass of the top quark.

Scientists working on the world’s leading particle collider experiments have joined forces, combined their data and produced the first joint result from Fermilab’s Tevatron and CERN’s Large Hadron Collider. These machines are the past and current holders of the record for most powerful particle collider on Earth.

Scientists from the four experiments involved—ATLAS, CDF, CMS and DZero—announced their joint findings on the mass of the top quark today at the Rencontres de Moriond international physics conference in Italy.

Together the four experiments pooled their data analysis power to arrive at a new world’s best value for the mass of the top quark of 173.34 ± 0.76 GeV/c2.

Experiments at the LHC at the CERN laboratory in Geneva, Switzerland and the Tevatron collider at Fermilab in Illinois, USA are the only ones that have ever seen top quarks—the heaviest elementary particles ever observed. The top quark’s huge mass (more than 100 times that of the proton) makes it one of the most important tools in the physicists’ quest to understand the nature of the universe.

The new precise value of the top-quark mass will allow scientists to test further the mathematical framework that describes the quantum connections between the top quark, the Higgs particle and the carrier of the electroweak force, the W boson. Theorists will explore how the new, more precise value will change predictions regarding the stability of the Higgs field and its effects on the evolution of the universe. It will also allow scientists to look for inconsistencies in the Standard Model of particle physics—searching for hints of new physics that will lead to a better understanding of the nature of the universe.

“The combining together of data from CERN and Fermilab to make a precision top quark mass result is a strong indication of its importance to understanding nature,” says Fermilab director Nigel Lockyer. “It’s a great example of the international collaboration in our field.”

Courtesy of: Fermilab and CERN

Courtesy of: Fermilab and CERN

A total of more than six thousand scientists from more than 50 countries participate in the four experimental collaborations. The CDF and DZero experiments discovered the top quark in 1995, and the Tevatron produced about 300,000 top quark events during its 25-year lifetime, completed in 2011. Since it started collider physics operations in 2009, the LHC has produced close to 18 million events with top quarks, making it the world’s leading top quark factory.

“Collaborative competition is the name of the game,” says CERN’s Director General Rolf Heuer. “Competition between experimental collaborations and labs spurs us on, but collaboration such as this underpins the global particle physics endeavor and is essential in advancing our knowledge of the universe we live in.”

Each of the four collaborations previously released their individual top-quark mass measurements. Combining them together required close collaboration between the four experiments, understanding in detail each other’s techniques and uncertainties. Each experiment measured the top-quark mass using several different methods by analyzing different top quark decay channels, using sophisticated analysis techniques developed and improved over more than 20 years of top quark research beginning at the Tevatron and continuing at the LHC. The joint measurement has been submitted to the arXiv.

A version of this article was originally issued by Fermilab and CERN as a press release.

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Voici la deuxième partie d’une série de trois sur la supersymétrie, la théorie qui pourrait aller au-delà du Modèle standard. J’ai expliqué dans un premier temps ce qu’est le Modèle standard et montré ses limites. Je présenterai ici la supersymétrie et expliquerai comment elle pourrait résoudre plusieurs lacunes du Modèle standard. Finalement, je passerai en revue comment les physicien-ne-s essaient de découvrir des « superparticules » au Grand collisionneur de hadrons (LHC) du CERN.

Les théoricien-ne-s doivent souvent attendre pendant des décennies pour voir leurs idées confirmées par des découvertes expérimentales. Ce fut le cas pour François Englert, Robert Brout et Peter Higgs  dont la théorie, élaborée en 1964, ne fut confirmée qu’en 2012 avec la découverte du boson de Higgs  par les expériences du Grand collisionneur de hadrons (LHC).

Aujourd’hui, beaucoup de théoricien-ne-s ayant participé à l’élaboration de ce que l’on connaît maintenant comme la supersymétrie, attendent de voir ce que le LHC révélera.

La supersymétrie est une théorie qui est d’abord apparue comme une symétrie mathématique dans la théorie des cordes au début des années 1970. Au fil du temps, plusieurs personnes y ont apporté de nouveaux éléments, pour finalement aboutir aujourd’hui avec la théorie la plus prometteuse pour aller au-delà du Modèle standard. Parmi les pionniers, il faut d’abord citer deux théoriciens russes, D. V. Volkov et V. P Akulov. Puis en 1973, Julius Wess et Bruno Zumino ont écrit le premier modèle supersymétrique à quatre dimensions, pavant la voie aux développements futurs. L’année suivante, Pierre Fayet a généralisé le mécanisme de Brout-Englert-Higgs  à la supersymétrie et a introduit pour la première fois des superpartenaires pour les particules du Modèle standard.

Tout ce travail ne serait resté qu’un pur exercice mathématique si on n’avait remarqué que la supersymétrie pouvait résoudre certains problèmes fondamentaux du Modèle standard.

Comme nous avons vu, le Modèle standard contient deux types de particules fondamentales : les grains de matière, les fermions avec des valeurs de spin de ½, et les porteurs de force, les bosons avec des valeurs entières de spin.

Le simple fait que les bosons et les fermions n’aient pas les mêmes valeurs de spin les fait se comporter différemment. Chaque groupe obéit à des lois statistiques différentes. Par exemple, deux fermions identiques ne peuvent pas exister dans le même état quantique. Un de leurs nombres quantiques doit être différent. Ces nombres quantiques caractérisent diverses propriétés : leur position, leur charge, leur spin ou leur charge “de couleur” pour les quarks. Puisque tout le reste est identique, deux électrons sur une même orbite atomique doivent avoir deux orientations différentes de spin, une pointant vers le haut, l’autre vers le bas. Cela implique qu’au plus deux électrons peuvent cohabiter sur une même orbite atomique puisqu’il n’y a que deux orientations possibles pour leur spin. Les atomes ont donc plusieurs orbites atomiques pour accommoder tous leurs électrons.

Au contraire, il n’y a aucune restriction imposée au nombre de bosons autorisés à exister dans le même état. Cette propriété explique le phénomène de supraconductivité. Une paire d’électrons forme un boson puisque deux spins de une demie donnent un spin de 0 ou 1 suivant s’ils sont alignés ou non. Dans un supraconducteur, toutes les paires d’électrons peuvent être identiques, chaque paire possédant exactement les mêmes nombres quantiques, ceci étant permis pour les bosons. On peut donc échanger deux paires librement, comme pour du sable mouvant. Tous ses grains de sable sont de taille identique et peuvent changer de position librement, d’où son instabilité. De même, dans un supraconducteur, toutes les paires d’électrons peuvent changer de position, sans aucune friction et donc sans aucune résistance électrique.

La supersymétrie se fonde sur le Modèle standard et associe un « superpartenaire » à chaque particule fondamentale. Les fermions obtiennent des bosons comme superpartenaires et les bosons sont associés à des fermions. Ceci unifie les composantes fondamentales de la matière avec les porteurs de force. Tout devient plus harmonieux et plus symétrique.

SUSY-diagram-Particle-Fever

La supersymétrie se fonde sur le Modèle standard et vient avec plusieurs nouvelles particules supersymétriques, représentées ici avec un tilde (~) au-dessus de leur symbole. (Diagramme tiré du film « Particle Fever » et reproduit avec la permission de Mark Levinson).

Mais il y a d’autres conséquences importantes. Le nombre de particules fondamentales double. La supersymétrie associe un superpartenaire à chaque particule du Modèle standard. De plus, plusieurs de ces partenaires peuvent se mélanger, donnant des états combinés comme les charginos et les neutralinos.

Les implications sont nombreuses. Première conséquence majeure : les deux superpartenaires du quark top, appelés stops, peuvent neutraliser la grande correction du quark top à la masse du boson de Higgs. Deuxième implication: la particule supersymétrique la plus légère (en général un des états mélangés sans charge électrique appelée neutralino) a justement les propriétés que la matière sombre devrait avoir.

Non seulement la supersymétrie réparerait plusieurs gros défauts du Modèle standard, mais elle résoudrait aussi le problème de la matière sombre. On ferait d’une pierre deux coups. Seul minuscule petit problème : si ces particules supersymétriques existent, pourquoi ne les a t’on pas encore trouvées? J’aborderai cette question dans la troisième et dernière partie de cette série.

Pauline Gagnon

Pour être averti-e lors de la parution de nouveaux blogs, suivez-moi sur Twitter: @GagnonPauline ou par e-mail en ajoutant votre nom à cette liste de distribution.

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