Quantum Diaries

Christer Fuglesang, a former physicist who worked at CERN and now an European Space Agency (ESA) astronaut brought back to CERN a neutralino he had taken along on his mission to the Internal Space Station in 2009.

Yesterday, Christer Fuglesang (right) former physicist from CERN and now astronaut with the European Space Agency, brought back to Sergio Bertolucci (left), CERN research director, the neutralino bearing ESA and CERN colors (bottom right insert) he took with him onboard the space shuttle in 2009.

The said neutralino is in fact a stuffed toy created by particle zookeeper Julie Peasley, creator of the Particle Zoo. It represents a hypothetical fundamental particle proposed within a new theory called supersymmetry. This theory builds on the Standard Model, the actual theory in particle physics and would unify together particles of matter and particles associated with fundamental forces.

Most importantly, many hope this neutralino could be a new form of matter that would explain what dark matter is made of.

Dark matter is a completely unknown type of matter that makes up 23% of the whole content of the universe, while only 4% of the universe corresponds to the type of matter that makes humans as well as all stars and galaxies. Physicists still don’t know what makes dark matter and dark energy (the remaining 73% of the universe’s content) but we know it’s there through its gravitational effects.

The universe contains 23% dark matter and 73% dark energy, two forms of matter and energy completely different from the regular matter found on Earth, all stars and galaxies, which accounts for only  4% of the content of the universe.

Dark matter does not radiate any light (hence its name) but still generates a gravitational field, making its presence detectable. On the other hand, it seems to interact very minimally with ordinary matter, making it very difficult to detect it and study its nature.

One hope is that the Large Hadron Collider (LHC) might be able to produce dark matter particles and physicists would at last get a chance to study them. The neutralino is only one of many proposed candidates to explain dark matter but a very plausible one.

So when Christer Fuglesang was told he could take a few mementos with him on his trip to the International Space Station, he chose to bring something special from CERN. “The neutralino offers a nice connection between space and particle physics”, Christer said, making it the perfect choice.

The little softy is now reunited with all its friends, the other particles from the Particle Zoo. Let’s see which one of them will pop-out of the box being the one explaining such a huge amount of matter still unaccounted for. Let’s hope the LHC will manage to shed light on this dark side of the universe.

(Interview with Christer Fuglesang)

Pauline Gagnon

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If you can read this right now, it is thanks to the World Wide Web, a product of basic research done at CERN. The web was invented at CERN to provide a communication tool for high-energy physicists working on different continents. Its impact on society has been tremendous, changing forever the way we communicate and even the way we live.

But the web would have remained an internal product without “knowledge transfer”, a process that aims at finding applications for developments coming from basic research to other fields. CERN’s Knowledge Transfer group tries to multiply such examples and its work is an integral part of CERN’s mission.

The lab’s primary goal is to conduct scientific research to develop knowledge on the nature of matter and better understand the Universe we live in. But in the process of achieving this, we are constantly pushing technology beyond its current limits, developing ever more highly performing tools. In this day and age, this also means trying to do it in a cost and resource effective way, respectful of the environment.

Every time a new detector or a new accelerator is built, we must design the components that will allow us to do better than last time. We either do it ourselves in the hundred of universities and institutes associated with CERN, or we work with industrial partners to develop pieces of equipment that will meet these challenging requirements: faster electronics, lighter materials, better cooling or smarter algorithms.

Procurement of novel equipment is one of the ways CERN drives technological development, promoting innovation within companies from CERN Member States. Another way is through scientists who have developed new ideas thinking of practical applications outside high-energy physics. CERN inventors can then benefit from the support of the Knowledge Transfer group. The team advises on every aspect related to technology transfer and intellectual property management, and provides expertise in multidisciplinary activities relevant to life sciences applications.

The Knowledge Transfer group first needs to establish if the new concept is unique then seeks potential external partners who could further develop and market the idea. Of course, when dealing with the business world, CERN must use business world rules. Guaranteeing exclusivity and an economic return is usually what interests business partners so agreements are drafted where CERN gives licenses to regulate the commercial exploitation of the technology.

Unlike with the World Wide Web, where no patent was taken to ensure wide access to everybody free of charge, patents are sometimes requested for new technologies as a means to attract commercial partners. For some technologies, this is the only way to attract industry and bring technologies to the market. A third of the generated income is reinvested in a Knowledge Transfer Fund to develop new projects, while the remaining two thirds go to CERN’s technical and scientific departments.

Sometimes the partner is another research institute. This is the case right now with CIEMAT, the Spanish Science and Technology funding agency, who entered a partnership with CERN to develop particle accelerators called “cyclotrons” to produce micro doses of radioisotopes needed for medical imaging.

Radioisotopes are short-lived and need to be produced at or near the medical centre. This cyclotron must therefore be small enough to fit within any hospital making the production of single-patient doses possible.

An important part of CERN Knowledge Transfer is the active promotion of multidisciplinary activities in the field of life sciences. CERN is involved in various projects connected to medical imaging, particle therapy, radiobiology, e-health and training of young researchers in these multidisciplinary fields.

Applications to medical imaging are one of the most obvious spin-offs since CERN’s detectors are essentially high-tech cameras capable of catching what is invisible to the eye. Being good at taking pictures of extremely furtive events, physicists can export their skills to improve medical imaging devices.

CERN also supported the development of highly efficient solar panels to produce hot water for heating and cooling purposes. These devices consist essentially of a water circuit placed in front of cylindrical mirrors to catch even diffuse light. The pipes are contained within a vacuum-sealed panel, eliminating heat losses since the vacuum acts like an insulator, rather like in a thermos. CERN’s special touch here is the introduction in the collector of a “getter pump”, a device based on materials and thin-film coating technologies developed to improve the vacuum quality (over long periods of time) in accelerator beam pipes by catching residual gas molecules. The Geneva International Airport is in the process of equipping its roof with roughly 300 such panels that will ensure heating of the airport’s main building.

One of the best and long term means of knowledge transfer is through teachers. Every year, CERN welcomes over a thousand high school teachers who get the opportunity to meet CERN’s scientists, visit different experiments and laboratories and hopefully later on, share how exciting basic research can be with hundreds of their students in the years to follow.

Knowledge Transfer is thriving at CERN and will continue to promote initiatives to maximize the benefits of basic research to different sectors of society and drive innovation.

Pauline Gagnon

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The Ξ (Xi) baryons, like all baryons, are particles made of three quarks. The first X baryon was discovered in cosmic rays in the 50s. More recently, Fermilab experiments discovered the Ξ_b particles, which contain a beauty or b quark. This week, CMS has reported the observation of a new excited state of the neutral baryon, the Ξ_b^*0, the first of the family discovered so far.

The Ξ_b^* baryons contain one beauty (b) quark, one strange (s) quark, and either an up (u) quark, which results in a neutral Ξ_b^*0 baryon, or a down (d) quark, which results in a charged Ξ_b^*. The ground states, that is the lowest-mass Ξ_b baryons — both charged and neutral — have been previously observed. However, none of the excited states predicted by the Standard Model had ever been seen. The Ξ_b^*0 excited state just discovered by CMS is the first one.

The excited states of particles, including the Ξ_b^*0, are expected to break up rapidly in a cascade of decays to lower mass particles, making the particle reconstruction particularly difficult. The CMS observation was made in a data sample of proton-proton collisions delivered in 2011 by the LHC operating at a centre-of-mass energy of 7 TeV. The sample corresponds to an integrated luminosity of 5.3 fb^-1. The mass of the new excited state is measured to be 5945.0 ± 2.8 MeV, which makes it also the heaviest particle state of the family discovered so far.

The CMS result comes with a statistical significance of more than 5 standard deviations (5σ) above the expected background. This is one more piece of information contributing to building up a coherent picture of the various states that matter can form.

In December, the ATLAS collaboration had also reported the first observation of a new particle called χ_b(3P) made of a quark b and an antiquark b.

Detailed information about the CMS result is available here.

A clear signal revealing the presence of Ξ_b^*0 particles (blue) above the background level (red)

EXCITED STATES OF MATTER

Antonella del Rosso

Two Pakistani scientists arrived at CERN in the midst of the cold snap in early February. They will spend the coming year working in collaboration with CERN’s magnet experts both learning the technology and contributing to ongoing projects

Sumera Yamin, a physicist, and Khalid Mansoor Hassan, an electrical engineer, both from the National Centre for Physics in Islamabad, came to CERN thanks to an agreement with Pakistan.

“They started contributing right away, helping us build new magnets for the ALPHA experiment,” says Davide Tommasini, Head of the resistive magnet section. “They fitted right in, just like I had expected. It is amazing to see that all scientists share the same approach.”

The two scientists will also contribute to some aspects of the magnet design and technical specifications for the SESAME project, the Synchrotron-light for Experimental Science and Applications in the Middle East, the first major international research centre in the making for the region.

Sumera Yamin (left) and Khalid Mansoor Hassan (right) next to a quadripole magnet in one of CERN test areas.

SESAME was set up on CERN model and likewise, it is being developed under the auspices of UNESCO. Its current members are: Bahrain, Cyprus, Egypt, Iran, Israel, Jordan, Pakistan, Palestinian Authority and Turkey. The goal is to build scientific and cultural bridges between diverse societies, and contribute to a culture of peace through international cooperation in science. It also aims at preventing or even reversing brain drain by enabling world-class scientific research in basic properties of materials science, physics, chemistry, and life sciences.

CERN is supporting this initiative by sharing its expertise in particular for the magnet system. In 2010, CERN and SESAME Directors signed a collaboration protocol. CERN’s experts will also deliver training to SESAME personnel on request.

Pakistan is both a member of SESAME and the CMS collaboration. Its goal is to support CERN in its effort in favor of SESAME, and, by the same token, build expertise in accelerator science, technology and design for domestic use.

SESAME main building was completed in Allan, Jordan in 2008. By 2015, this research center will start welcoming scientists from all member states. As a “user facility”, scientists will come for short visits, perform a specific experiment and return home for the data analysis. The goal is to create a motivating scientific environment that will encourage the region’s best scientists and technologists to stay in the area or to return if they have left.

The two scientists are now hard at work learning how to build magnets from scratch for the SESAME main ring. They are working on the design and the specifications should be finalized by late spring. The goal is to have most components produced and/or assembled in the Middle East. In parallel, Pakistan counts on scientists like Sumera and Khalid to build expertise in accelerator technology and develop its own skills for medical applications.

“We are getting a lot of help and attention from the whole group”, says Khalid. “Every time we need to discuss something, someone proposes we go for coffee!” But discussing everything and nothing over coffee is not the only memory they will have of CERN. “This is a very different learning experience,” adds Sumera, “more cooperative, more open”. Both Sumera and Khalid enjoy the multicultural environment and are happily soaking up all the new knowledge.

Pauline Gagnon

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Some of you might have heard of the Travelling Gnome prank whereby garden gnomes were ‘borrowed’ from private gardens and taken on a trip to be photographed in front of a known landmark. The owner would then receive the photo in the mail.

These actions, undertaken in the 80’s by members of various branches of the Garden Gnome Liberation Front, inspired not only filmmakers to produce “Amélie Poulain“ but also James Nester from the Kern and Sohn company in Germany, a manufacturer of high precision scales. He and co-workers Chuck Jenks and Nick Hearn launched the Gnome Experiment and are now known as the Gnome Team within their company.

This company has its headquarters near to the home of the first garden gnome factory, so they had the idea of sending a garden gnome around the world along with one of the company’s scales to show that the gravitational pull from the Earth varies with location, a fact that precision scale makers must take into account in their calibration given the non-perfect sphericity of our planet.

At first, this was just a way to bring publicity for their company but the project took on a life of its own after a scientist invited the Gnome to come to the South Pole, an event that caught the media’s attention.

“After Kern the Gnome’s visit to the South Pole, we got up to three invitations a minute on our web site for visits from all over the world,” explains Nester.  Given the physics involved in this experiment, it gave the Gnome Team the idea to send their little ambassador on a tour of famous physics landmarks.

First, Kern the Gnome (that’s his name) visited SNOLAB in Sudbury, Canada, a neutrino physics laboratory located 2 km underground, where he weighed less than on the surface because of the 2 km of rocks above him while there was less of the Earth attracting him below. By how much? A mere 0.1 g out of his 307.73 g measured at the surface in Sudbury. Of course, we are talking about kilogram-force here since nobody measures a weight in Newtons, the official weight unit while the kilogram is the unit of mass.

SNOLAB is the successor to the Sudbury Neutrino Observatory (SNO) where neutrino oscillations were measured for the first time. This refers to the fact that neutrinos can morph into a different type. There are three types of neutrinos: the electron neutrino, the muon neutrino and the tau neutrino. SNO proved that electron neutrinos emitted by the sun were able to change into muon neutrinos on their way to Earth. This brought the resolution of a long-standing problem, explaining why only a third of the neutrinos produced at the surface of the Sun was detected on Earth. This also implies that neutrino have a mass.

And last week, guess where Kern the Gnome was? Indeed, right here at CERN, right after the first stable beams of 2012. A local television crew even came to CERN especially for the occasion.

Mick Storr (on the right), head of CERN Teachers programme, with Kern the Gnome in the Large Hadron Collider (LHC) control room.

If you visit the Gnome Experiment website, you will find by how much Kern’s weight varies around the world. And don’t try to blame absorption of air humidity in tropical regions as a contributing factor. The gnome is made of non-porous, anti-chip, high tech resin and comes with gloves and a duster for handling to avoid dirt.

The idea is now evolving into a science education project. Schools can sign up to receive the gnome’s visit and material is being developed for teachers. If you want to invite him for a visit, just drop him an e-mail.

Kern the Gnome is now on his way to the UK where he will visit the famous orchard where Sir Isaac Newton got inspiration for his theory, or so the legend goes. Let’s hope he won’t reveal himself as some rotten apple!

Pauline Gagnon

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This morning at 00:37, the Large Hadron Collider (LHC) at CERN brought two stable beams of 4 TeV protons into collision for the first time both for this year after the winter shutdown and for that energy. This comes after weeks of preparation by the LHC’s team of operators, technicians and accelerator physicists. The experimenters have also been working hard to complete all necessary work on their detectors in time and test new software. Everybody had been waiting for this important event with great anticipation.

Preparing to take my first shift of the year in the ATLAS control room this week reminded me of first day of school when I was a kid. My clothes were laid down on my chair, my breakfast ready and I had enough food packed to sustain a small siege. I certainly did not want to be late for the start of my shift yesterday at 7:00 am when the first collisions in stable beam conditions were expected.

The ambiance around the ATLAS control room was particular this week with everything and everybody just waiting for this event. So every time a new glitch appeared, experts rushed in, trying to get all wrinkles ironed out before the LHC finally announced they had completed all their checks and measurements, and the next fill would be for real physics.

The whole morning shift crew, about ten people including myself, had been hoping we would be there for this important milestone. As we walked in at 7:00 am, the LHC was delayed by a small glitch that got fixed rapidly. Then they needed to complete one last measurement but lost the beams before completing it. For hours, we kept hoping for some interesting action.

There was not much to do but wait and see if and when the LHC team would be ready. The shift was punctuated by the usual succession of dead quiet periods followed by frantic bouts when one system went bad, a new alarm went off or the daily run meeting ended.

These meetings are held seven days a week just above the ATLAS control room and bring together all system coordinators and experts on call, the run manager (the person on call to assist the shift leader for a week at a time, 24 hours a day) and run coordinators. More than forty people review all recent problems, establish the plans for the day according to the latest news from the LHC, repairs necessitating an access to the detector cavern or special tests being conducted. So every day after the meeting, all the experts drop by to leave instructions with the shift crew, make a few tests or fixes, bringing the control room into a buzzing state. There is a whole sub-culture within the collaboration of people who constantly orbit around the control room, making sure all is always in the best possible running condition. The others only come occasionally to take their share of shifts to staff the control room at all times.

Part of the shift crew in ATLAS control room in the early afternoon while waiting for the first stable beams.

(Photo: Claudia Marcelloni de Oliveira)

As shift leader, my role became really secondary, with all the key players in place. Most of them are students or young postdocs, more than half being women, reflecting well the crucial role taken by young people in the collaboration.

Some already looked tired, having been called a few too many times in the middle of the night already to fix problems while we still had time.

Bringing large detectors like ATLAS out of hibernation is a delicate task and new problems are bound to keep showing up. The whole detector is made of 7000 tons of delicate and complex equipment, 4000 km of cables of all sorts and as many kilometers of tubing, all bringing voltages or special fluids to the detector or taking information out. This in part explains why nearly 4000 people are now involved in each of the ATLAS and CMS collaborations, the two largest LHC experiments. ALICE has about a thousand researchers while LHCb around 1500.

The other reason is the attraction of the possibilities to make great discoveries.. What could it be? No one knows yet but it sure looks promising not only for getting the final answer on the Higgs boson but also on testing many new theories.

Pauline Gagnon

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CERN scientists announced today a new particle was just discovered. It was found on Tristan da Cunha, a volcanic island located roughly half way between Brazil and South Africa in the South Atlantic.

A sign on Tristan da Cunha claims it is the remotest island on Earth, being located half way between Brazil and South Africa.

“We have been looking for new particles everywhere for years. Who would have thought one was hiding on the remotest place on Earth!” said a befuddled James Gillies, spokesperson for CERN, the European Organization for Nuclear Research.

The new particle, called a foolion, had been proposed by theorists to explain how elementary particles could attract so much attention.

CERN received a tip about this sighting from the crew of a small Canadian sailboat that passed by the island last month. Scientists from the ATLAS and CMS collaborations are now studying the possibility of moving their detectors to the island of Tristan da Cunha to confirm the findings.

Pauline Gagnon

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At CERN, while we are about to shed light on the fundamental question of the creation of mass after the Big Bang, we are also close to solving another basic mass-related problem. The kilogram is the only base unit of the International System of Units (SI) whose official definition is still based on a material artefact rather than on invariant quantities. If you are now thinking that this concerns you less than the glamorous Higgs boson, think again: your scales could give you a different value when you use them tomorrow.

The international prototype of the kilogram is a cylinder of platinum-iridium alloy whose height (39 mm) is equal to its diameter. It was machined in 1878 and is kept at the Bureau International des Poids et Mesures (BIPM) in Sèvres, near Paris. To date, while all the other units in the SI system have been redefined to be based on fundamental constants or atomic properties, the kilogram continues to be defined according to this piece of matter.

A piece of matter that people, or at least one person, must clean, and there is a risk that atoms – that is, fractions of mass – might be lost in the process. “Over the years, several official copies have been produced and distributed to various national metrology offices,” says Ali Eichenberger, a physicist at the Swiss Metrology Office (METAS). “Although it is not yet possible to define the kilogram mass in an absolute way, modern technology makes it possible to compare different masses with very high precision, up to 1 microgram. Looking at the different official copies there seems to be a significant variation in masses.” Moreover, not knowing the kilogram with the appropriate precision has an impact on other units, such as the ampère.

Over the past century, significant variation is seen between the masses of the official kilogram copies. (Courtesy of METAS).

A metrology project launched by METAS in which CERN is participating should be able to fix the problem. The idea is to build an ultra-precise watt balance – an instrument that compares the mechanical and the electrical power (see box). Using the watt balance and its equations, it is possible to relate the unit of mass to the metre, the second and the Planck constant, i.e. all fundamental units and constants.

“One of the crucial elements of the watt balance is the magnetic circuit, which needs to be extremely stable during the measurement,” explains Davide Tommasini, a magnet expert from the Magnets and Superconductors group in CERN’s Technology Department, who is directly involved in the METAS watt balance project. “By using a correctly dimensioned ‘magnet shunt’ with a low Curie temperature, it is possible to drastically reduce the effects of temperature variation. The circuit must also provide a very homogenous magnetic field in the whole volume involved in the measurement.” The magnet circuit will be assembled at CERN. “We are expecting the permanent magnet and the ‘shunting’ cylinder to arrive soon. We will then work on testing the performance of the circuit,” says Davide Tommasini.

The watt balance built by METAS to perform previous measurements of the Planck constant. A new balance is currently under development. (Courtesy of METAS).

“The requirements associated with the magnets are very strict and we are very happy that CERN agreed to take part in the project in the framework of its knowledge transfer activities,” says Henri Baumann, a physicist at METAS who launched the project together with Ali Eichenberger. “This measurement will also lead to a significant improvement in the determination of the Planck constant. The CERN theorists will be happy to know that!”

“This project is a clear indication of the impact that the skills and the expertise needed in particle physics have on other research domains and on society,” says Hartmut Hillemanns from the Knowledge Transfer (KT) group, who has fostered the project with the scientific team at CERN and led the negotiation with the other project partners.

The new definition of the mass unit should be available in a couple of years from now. Chances are that by then we will have also understood how mass is created at the most fundamental level… yes, we are talking about the Higgs this time!

From the CERN Bulletin

The first week of the biggest winter conference, the Rencontres de Moriond held in La Thuile in Italy closed on March 10, leaving all attendants both impressed and puzzled by all the new results presented.

The situation is the following: Theorists know that the current theoretical model, the Standard Model of particle physics, has its limits and that it is probably the most accessible part of a more complex but unknown theory. Think of it as for mathematics: arithmetic is all most of us need for every day tasks even though we know geometry, algebra and calculus are needed for more complex applications.

Physicists expect to see new phenomena that are referred to as “new physics”, which would tell us which one of the many new theories currently proposed is the right one. And everybody hopes the Large Hadron Collider (LHC) experiments will soon discover something to set us in the right direction.

Hence, the focus of this conference was to assess the impact of the all the latest experimental results on new models, particularly supersymmetry (SUSY) and extra dimensions. And there were plenty of new results on searches for the Higgs boson, SUSY particles and dark matter, as well as new precision measurements and neutrino physics.

The first excitement came from the LHCb, CMS and ATLAS experiments operating at the LHC with new measurements of how often a B_s meson decays into two muons. This decay occurs so rarely in the context of the Standard Model that even small contributions from new physics could be detected. LHCb is setting the best limit to date, with less than 4.5 x 10^-9, barely above the Standard Model prediction of around 3.5 x 10^-9. This leaves very little room for new physics. However, David Straub, a theorist affiliated with Scuola Normale Superiore and INFN in Pisa, showed that finding less than what is predicted by the Standard Model would also open the door to new physics, something that has previously received little attention but is now becoming possible with the increase in precision from the LHC experiments.

With stringent limits on rare decays such as B_s or B_d to two muons, many supersymmetric models have very little parameter space still allowed, as shown by the small rectangle in the bottom left corner. The rest is what was still allowed a year ago.

On the search for the Higgs boson, now, four separate experiments see faint signs of what could be Higgs bosons in four different channels. It is a bit like hearing a rumor from four trust-worthy people who all got very similar information from different reputable sources. Although it does not prove anything, we can all start thinking seriously about it. All experiments see an excess compatible with a Higgs boson mass of 125 GeV, even though the strength of the signal is still too weak to be convincing. ATLAS and CMS will resume data-taking next week and should have a clear and final verdict this year.

While all four collaborations – ATLAS, CMS, CDF and D0 – insisted that it was too early to jump to conclusions about the Higgs boson, theorists have already been checking the effects of the mass of the Higgs. Nazila Mahmoudi, a theorist from CERN, showed that the currently allowed range for the Higgs boson mass is already putting constraints on SUSY models.

The values of tan β and m_A, two important parameters of supersymmetric models, still allowed if a Higgs boson is found with a mass of 125 GeV. The red points are disqualified by b-physics results. Everything above the yellow curve is excluded by direct searches for SUSY particles obtained by the CMS collaboration. And if the Higgs boson is found around 125 GeV, only the green band would still be allowed under certain constraints.

The Universe content: 96% of it comes from some absolutely unknown types called dark matter and dark energy.

Josef Pradler from the Perimeter Institute in Canada addressed a long-standing and controversial result reported several years ago by the DAMA/LIBRA experiment. The group has been claiming for years the observation of a very strong signal for “dark matter”, a mysterious and unknown type of matter that accounts for about 23% of all the content of the Universe while regular matter (all stars and galaxies) amounts to only 4%. The remaining 73% comes from some unknown type of energy called “dark energy”.

From various gravitational measurements, astronomers have shown that dark matter is more concentrated galactic halo, i.e. outskirt of the galaxy. As the Earth orbits around the Sun on its annual cycle, it encounters more WIMPs (Weakly Interacting Massive Particles, a nickname for a dark matter candidate) than in December when moving away from the dark matter source. It is very much like getting a head-wind in the summer and a tail wind in the winter.

The DAMA/LIBRA detector counts more interactions with WIMPS in the summer than in the winter, hence the annual modulation in the number of particles detected (vertical axis) as a function of time (horizontal axis).

The problem is that other experiments cannot quite confirm this result, so some people have suggested that this could simply be due to cosmic muons. Josef Pradler and his colleagues just showed that the DAMA/LIBRA data are inconsistent with the cosmic muon hypothesis at 99% CL. The mystery remains.

Possible signs of a Higgs boson being produced and decaying just like the Standard Model predicts and no signs of new physics despite extremely precise tests, left all participants rather puzzled.

Theorists know that the Standard Model does not describe everything we observe. So what is the real theory that would explain everything? Lisa Randall from Harvard University reminded the audience that whatever the new theory is, it will have to address both the symmetry breaking (why some particles associated with the electroweak force are massive, others massless) and the hierarchy problem (why is the top quark so much heavier than the electron?). Much food for thought there.

Pauline Gagnon

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Today in La Thuile, in Italy, both the ATLAS and CMS experiments from the Large Hadron Collider (LHC) at CERN, and CDF and D0 from the Tevatron presented their updated results in their search for the Higgs boson, a hypothetical particle that physicists have been trying to find for nearly five decades.

The biggest surprise came from the competing experiments from the Tevatron, another particle accelerator located near Chicago. CDF and D0 announced that they were both seeing a small excess of events for a Higgs boson decaying into a pair of b quarks for a Higgs mass between 115-135 GeV, the same area where CMS and ATLAS are seeing an excess. The observed combined effect corresponds to a 2.2 sigma, i.e. the probability this comes from something else than the Higgs is 1.4%. Since the Tevatron stopped operation last September, these results are nearly final, although both experiments still expect to improve their analyses.

The morning session of the Moriond conference had opened with a lively lecture from Prof. François Englert, one of the key contributors to what is now known as the « Higgs mechanism », a theoretical description of how all elementary particles could acquire their mass. The current theoretical model, the Standard Model, fails to give mass to elementary particles unless this mechanism comes to the rescue. If this theory is right, we must find a new particle called the Higgs boson.

Both CMS and ATLAS are large experiments operating at the Large Hadron Collider (LHC), at CERN. Already, preliminary results were presented last December but today, both experiments showed the results obtained using all data collected in 2011 and involving nearly all possible “decay channels”.

The Higgs boson is expected to be unstable and will “break apart” in many different ways called decay channels. These are just like the different ways a machine can give change.  The sum given should always be the same, no matter which small coins come out. Studying different decay channels is like checking they all correspond to the same particle and all give the same mass value for this particle. i.e. the value of the initial coin.

Now, there is only a very narrow range of about 7.5 GeV where the Higgs might be hiding around 124 GeV. At this time last year, it spanned a range of about 470 GeV.

In December, ATLAS only showed the results of the main two decay channels. Having now included 12 channels, ATLAS consolidates its previous findings and excludes a Higgs boson with a mass below 122.7 GeV (except marginally at 118 GeV), and from 128.6 GeV up to 539 GeV. Similarly, CMS rules out all masses between 127.5 all the way to 600 GeV.

The low mass region is therefore where we can expect some action in the coming months. Given the amount of data collected in 2011, both experiments expected to exclude the whole region between roughly 115 and 540 GeV. They exclude less at low values because they see more events in this area than expected if there were no Higgs boson.

This is observed in many decay channels and for both experiments, which have worked independently. This is now also the case for CDF and D0. Therefore, this is one more sign going toward reinforcing the possibility of a Higgs boson around 125 GeV.

This means these could be the first faint signs of a Higgs boson, most likely between 125-126 GeV. This is a bit like seeing the first pimple when a child develops chicken pox. Until she is covered with them, despite some early tell-tale signs, it is too early for a definitive diagnostic.

The most probable value observed by CMS is for a Higgs mass around 125 GeV where the excess has increased slightly since December after adding one new decay channel involving two photons and two jets. The excess is now a 2.8 sigma deviation, meaning the probability there is nothing there but “background” – other types of events – is 0.26 %.

Here is a quantity equivalent to the amount of events expected as a function of possible Higgs masses. The dashed line shows what ATLAS expected to see given the amount of data analyzed and in the absence of a Higgs boson. The solid black line gives the observed values. The green and yellow bands represent the one and two sigma statistical fluctuations. The fact that the black line exceeds the yellow region means that more events are observed than if there was only background. The most pronounced excess is 2.5 sigma for a Higgs boson mass of 125 GeV.

For ATLAS, the effect is now slightly reduced after adding the Higgs to two W bosons channel. The most pronounced excess is still around 126 GeV but now corresponds to a 2.5 sigma deviation or 0.6 % probability it comes from background only. It was 3.6 sigma in December. But if one adds the “look-else-where” effect, namely not only looking at one local deviation but taking into account the probability that statistical fluctuations can occur anywhere, then the probability that this could be due to a fluctuation from the background reaches to 10%-30%, depending how far elsewhere one looks. Similar effects were shown by CMS.

An excess of events is also shown by the CMS experiment in the low mass range. The largest excess of events is observed at 125 GeV at the 2.8 sigma level.

On the other hand, if there is a Higgs boson at a mass of 126 GeV, ATLAS should see locally an excess of 2.8 sigma when 2.6 sigma are observed. This means the data are also compatible with the hypothesis that this excess really comes from Higgs boson decays.

The LHC accelerator is due to restart on March 14 and will operate at a higher energy this year – 8 TeV instead of 7 TeV like in 2011, slightly increasing the chances of producing a Higgs boson. If all goes well, both experiments expect to have the final answer some time this year.  Knock on wood!

The bottom line is: although the picture has not changed much for CMS and ATLAS since last December, more channels and more data are now analysed, yielding a more robust picture. With the Tevatron now bringing the news of a similar excess in their data, starts to build a coherent and convincing picture. Many physicists will have a hard time deciding in advance when to book their summer vacation…

Pauline Gagnon

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