Quantum Diaries

There are many different ways people are testing the validity of the current theoretical model we have for particle physics, called the Standard Model. The LHCb experiment, one of the four large experiments operating at the Large Hadron Collider (LHC) at CERN, just released today at the Moriond conference in La Thuile, Italy, the most precise measurement to-date on an extremely rare phenomenon.

The team sifted through about 10 billion events looking for a particle called a B_s meson – a particle made of a b quark and antiquark s – that would have decayed into two muons.

The B_s meson is a very heavy particle, making it unstable and prone to decay into smaller, more stable particles. It can “break apart” in many different ways called decay channels. These are just like the many ways a machine can give change. Some decay channels occur more often, others only rarely. Of all the possible options, how often a particle will decay into a particular decay channel is called the branching ratio.

The Standard Model predicts that about three B_s mesons should disintegrate into two muons out of a billion decaying B_s mesons, that is, the branching ratio is 3 x 10^-9.

LHCb found a few possible B_s mesons decaying into two muons, including the one shown below.  There are not enough to establish a measurement of the branching ratio, but they managed to set the most precise limit on  its value, namely that it has to be less than  4.5 x 10^-9.

This means LHCb is now putting the Standard Model through the highest scrutiny in an area where many people expected to detect a deviation from the Standard Model prediction. As it is, the Standard Model still stands tall and strong even when pushed to such limits.

Display of one of the candidates for a B_s meson decaying into two muons in the LHCb detector.  The muons are penetrating particles (shown in pink) that pass all the way through the detector.  A zoom on the vertex region shown below indicates that they are offset from the ‘primary vertex’ where the protons collided, as would be expected if they come from a B_s meson that will have time to travel a little before decaying.

Finding a deviation from what is expected from the Standard Model would be a way to see if the current theoretical model is not just the tip of the iceberg. Many theorists suspect a more complex theory lies beyond the Standard Model but nobody has been able to crack the model open so far.Last Summer, LHCb had reported on another attempt at revealing small flaws in the Standard Model. Today, they released even more precise measurements on charge-parity or CP-violation in B_s mesons.

CP violation is a way to quantify why more matter than anti-matter  remained when the Universe slowly cooled down after the Big Bang, leaving us with a world predominantly composed of matter. This is quite puzzling since in laboratory experiments, the measured preference for the creation of matter over antimatter is too small to explain why we mostly see matter around us.

Even with the increase in precision in measuring a parameter called Φ_s which is predicted by the Standard Model to be  very small, the new LHCb measurement sees no deviation from this prediction and falls very close to the predicted value.

The red and green areas show the results from D0 and CDF, two experiments from the Tevatron, an accelerator near Chicago. In blue is the LHCb result of last summer, when an ambiguity on the sign of the parameter ∆Γ_s left two possibilities. This ambiguity is now gone such that only one small yellow area remains, in good agreement with the predicted value indicated by the black dot.

This increase in precision will help limit the range of possibilities for new models, making it increasingly easier to zoom onto the right solution among all the models proposed.

Pauline Gagnon

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Like most of my colleagues, the most frequently asked question I get from friends and family these days is: what is this Higgs boson business? Here is what I hope will help not only family members but also struggling physicists. It is not the simplest but it is complete and accurate.

First of all, let’s clarify one point: it has nothing to do with God. This “God’s particle” business has got to go. It was just a bad joke to start with and like any joke, it gets stale fast.

And we need to talk about three separate aspects: the Higgs mechanism, the Higgs field and the Higgs boson.

Currently, there is a theoretical model describing just about everything observed so far in particle physics, called the Standard Model. It rests on two simple principles: 1) all matter is made of particles, namely quarks and leptons and 2) all interactions between particles are mediated by exchange particles associated to the fundamental forces.

The current equations from the Standard Model only produce massless particles when we know these particles all have a mass, as witnessed countless times in our particle detectors.

The “Higgs” mechanism is the mathematical representation of what happens. It is a way to remix the Standard Model equations for the electromagnetic and weak nuclear forces. It brings into the equations three particles called “Goldstone bosons”, that mathematically “absorb” the three massless bosons associated with these forces. Then, out of the new equations pop three massive particles, the W⁺, the W^- and the Z⁰ bosons, plus the massless photon. These are the four particles we know to be associated with the electromagnetic and weak nuclear forces.

In all fairness, it should be named after all the physicists who contributed to the idea but so many did, it is referred to as “Higgs mechanism” for short.

The mechanism is a mathematical description of a physical entity, the “Higgs field” that permeates all space, just like a gravitational field affects the space around a massive object. Although we cannot see this field, particles feel its effects by acquiring a mass, just like we feel the gravitational attraction du to the Earth’s field.

The Higgs field is what is needed to provide mass to all elementary particles such as the electrons, the quarks, the W and Z bosons. The fact that we have found the W and Z bosons with exactly the mass predicted by the theory is a strong indication that the Higgs mechanism takes place and the Higgs field exists, but there is of course no guarantee until we find the Higgs boson to prove it all.

The Higgs boson is just an excitation of the Higgs field. Ok, I admit, this one is harder to swallow. But think of a hydrogen atom. In its ground state, the hydrogen atom lives eternally. It will never decay into anything more stable. But it becomes “excited” after absorbing energy. Its electron then jumps to a higher level making the atom unstable. In just picoseconds (millionth of millionth of a second), the hydrogen atom will come back to its ground state by emitting a photon, spitting back the excess energy to return to its stable state.

The Higgs field, like a hydrogen atom, can be excited, also only in discrete values of energy corresponding to the Higgs boson mass. The energy released when protons collide in the Large Hadron Collider can excite the Higgs field. The excited state is just the Higgs boson itself. And just like the hydrogen atom in its excited state, it will try to return to its ground state. The Higgs boson is therefore very unstable and will decay into other particles instants after appearing.

What we need to establish now at CERN is exactly if the Higgs field exists and how it operates, how it can be excited, how it all works. This is what all our research around the Higgs boson is about. We want to know the specificities of the Higgs field. The simplest thing is that the first excited state is a single particle, the Higgs boson. That’s what the Standard Model favors. Some other theories bet on many different types of excitations, i.e. many different Higgs bosons or composite objects.

Now, last but not least, how does the Higgs field provide the mass to other particles? Imagine a completely empty pool table, with a perfectly smooth marble surface and perfectly straight marble sides. Toss a billiard ball across the table and it will travel on a straight line. Now glue many rigid posts to form two rows, leaving a narrow path for the ball. This time, the ball will hit the posts, bouncing back and forth along the way. If the table is perfectly smooth and the ball perfectly rigid, it will just bounce back without losing energy. It will eventually make it across the table, taking more time but keeping the same energy. If you measure its speed from how long it took to cross the table, it will appear like it is now travelling slower.

A particle moving in a space filled with the Higgs field would no longer be able to travel in a straight line because of its interaction with the field. It would progress more slowly overall, like a billiard ball interacting with small pegs would take more time to reach the other side of the table.

A physicist would say: there is now dispersion, the tossed ball no longer travels along a straight line but without dissipation, i.e. no energy loss.

The Higgs field acts like the posts glued to the table. It prevents the moving ball from traveling in a straight line but without it losing any energy.

In relativity, the mass is seen as a form of energy. This is expressed by the well-known equation:  E = mc² where c² is just like the exchange rate between mass m and energy E. Look at it as having money in your pocket: if you are at CERN, it would be in two currencies, Swiss francs and euro. The sum is the total money you have. For a moving particle, both its movement and its mass contribute to its energy as illustrated below:

For a moving particle, the energy comes from two sources: having speed and having mass. This is a bit like having money in two different currencies. One can convert them into each other.

Ok, let’s stretch our imagination one bit further and now imagine a massless billiard ball, a ball with absolutely no mass. If we toss that massless billiard ball across the table, all its energy comes from its motion since it has no mass. Its effective speed is lower since it no longer travels on a straight line. A physicist would talk of the group velocity here, the speed at which the ball seems to progress in the right direction.

For a massless particle, i.e a particle without any mass, all the energy comes from its motion. It’s like having only one currency in one’s wallet.

So, here is the million Swiss franc question: If it is slower, the energy share coming from its velocity is smaller, then how comes the ball still has the same energy? Simple: this ball has acquired mass through its interaction with the Higgs field. The contribution to the total energy from its mass is no longer zero and it adds up to the reduced contribution associated with movement to give the same total energy value.

A particle interacting with the Higgs field moves more slowly but its total energy is unchanged. It therefore has less energy in the form of motion and some now coming from the newly acquired mass just as if one had converted some Swiss francs into euro.

To summarize: The Higgs field provides the mass and the Higgs boson is just an excited state of this field. All particles that interact with the Higgs field acquire mass since they travel less rapidly but still have the same energy. The Higgs mechanism expresses all that mathematically.

Sleep tight.

Pauline Gagnon

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Most people associate CERN with the Large Hadron Collider (LHC). But lesser known although extremely diversified research activities are also ongoing at CERN.

About a thousand physicists are working on experiments ranging from antimatter studies to cancer therapy, cloud formation and radioisotope production.

Already in 2011, the ALPHA experiment made the headlines when they managed to trap antihydrogen atoms for more than fifteen minutes. Antiparticles and particles are produced in equal amounts in high energy accelerators. But since we live in a world made of matter, it is no small feat to prevent antiparticles from annihilating with particles of matter and vanishing. Usually, a magnetic “bottle” is used as the trap  This is a space confined by strong magnetic fields and operated in a high vacuum to keep antimatter from encountering any matter. First hurdle: one has to combine an antiproton with an antielectron (called “positron”) at low temperature to form antihydrogen atoms that are sluggish enough to be able to trap them (less than 0.5 K or -272.5 ⁰C).

Nevertheless, having improved their antihydrogen production techniques in 2011, the goal of the ALPHA, ASACUSA, and ATRAP experiments is now to see if these antiatoms have the same properties as their counterpart of matter, the same spectroscopy for example. A new experiment AEgIS will come online this year with the long-term goal of measuring the gravitational constant g with antihydrogen to see if it is the same g as matter experiences.

Meanwhile, the CLOUD experiment is attempting to solve a long-standing enigma: how do aerosol particles form in the atmosphere? All cloud droplets form on aerosols — tiny solid or liquid particles suspended in the air – but how these aerosols form or “nucleate” remains a mystery. To find out, a chamber with a carefully controlled temperature is used to introduce traces of various chemical vapours into an initially “pure” atmosphere. Surprise: ammonia and sulphuric acid, the two airborne chemicals thought to be responsible for all aerosol formation, can account for only one tenth to one thousandth of the rate observed in nature. The goal for 2012 is clear: identify the missing elements and pursue studies on the influence of cosmic rays (simulated using a pion beam) on the aerosol formation rate.

Lots of developments are happening in hadron therapy, a cutting-edge cancer therapy technique where protons and other light ions are used instead of X-rays photons as in conventional radiotherapy treatment. The challenge is to destroy cancer cells without affecting the neighbouring healthy tissue. Contrary to X-rays, protons and other ions deposit nearly all their energy at a specific point near the end of their path instead of all along their path. This means one can bring large amounts of energy exactly where needed without causing damage along the way.

Energy deposited by different particles as they penetrate matter such as human tissue. Protons and carbon ions deposit most of their energy at a specific depth, whereas photons used in conventional X-rays tend to leave energy all along their path, damaging healthy tissue.

CERN acted as a catalyst in the formation of the European Network for Research in Light-Ion Hadron Therapy (ENLIGHT) in 2002 , which was established to coordinate European efforts in radiation therapy using light-ion beams. During the 1990s a group at CERN developed designs for a hadron therapy accelerator in the Proton Ion Medical Machine Study(PIMMS). This basic work has been incorporated into several of the subsequent designs. CERN is currently supporting the MedAustron therapy project in Austria and is also planning to exploit its accelerator technology and expertise in developing a second generation design for hadron therapy.

The ACE experiment has also tested the idea of using beams of antiprotons for hadron therapy, with the added advantage of blasting more malignant cells because of the amount of energy released when the antiquarks of the antiproton annihilate with the quarks of protons or neutrons from one of the cancer cells. This work is nearly completed and will be finished this year.

Much is also ongoing at the ISOLDE facility, which uses protons from a small CERN accelerator (the Proton Synchroton Booster) to produce “exotic” nuclei from most chemical elements by adding protons to stable nuclei. The radioisotopes are then used by more than 50 experiments to study nuclear structure, nuclear astrophysics, fundamental symmetries, atomic and condensed-matter physics, and for applications in life sciences. Some scientists pursue research using neutron beams from the n_TOF facility in the hope of transforming long-lived radioactive waste from nuclear power plants into shorter-lived or stable, non-radioactive elements.

Others at the CAST and OSQAR experiments are hot on the tail of “axions”, “paraphotons” and “chameleons”, some of the many hypothetical and rather exotic particles proposed by theorists to explain the nature of dark matter. For the past decade, these experimentalists have been adding new tricks to their experiments every few years to test new hypotheses and axions of heavier masses. More ideas keep these experiments’ “dance-cards” full all the time.

As millions of individuals have heard, CERN also supplies a neutrino beam to several experiments at the Gran Sasso Laboratory in Italy, including OPERA where puzzling results on muon neutrinos apparently travelling faster than the speed of light were reported last year. Two separate experiments at Gran Sasso are now setting up to cross-check this result in the coming months.

Much more is happening but it is impossible to do every one justice in a short overview. These are just a few of the many activities ongoing at CERN besides the LHC programme. All together, they make CERN a place well worth keeping an eye on in 2012, so follow us on Twitter @CERN.

Pauline Gagnon

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I have just returned from an interesting few days at the World Economic Forum’s annual meeting in Davos, where my main message was that science needs to be far higher up the political and business agenda than it is today. This is only the second year I’ve participated, but I have the impression that this message is being heard: one of the things I raised this year is the importance of linking the scientific content of the meeting more closely with the political thread, and I’ll be taking that forward with the Forum before next year’s Davos meeting.

Science is complex. There’s no getting around that. But it’s essential that everyone engage constructively with it. That’s particularly true of the political and business leaders in Davos, whose decisions on science-based subjects can influence everything from the well being of our children to the future of the planet. It’s vital that those decisions are taken from an informed position and on rational grounds.

The challenge that science faces is that we live in a world where it’s de rigueur to know your Shakespeare, Molière or Goethe, but quite all right to be proudly ignorant of Faraday, Pasteur or Einstein. It hasn’t always been that way, and it doesn’t have to be that way. But right now, there’s a trend in society towards scientific apathy, and even antagonism. This is dangerous for us all and it’s incumbent on the scientific community to address the issue.

There was a time not so long ago when science was a fully integrated part of society, discussed in the same breath as football matches and front-page news. In the early part of the 20^th century, news of Einstein’s advances were accompanied by cartoons in the press, and as recently as the 1960s science grabbed the popular imagination, thanks largely to NASA’s Apollo programme. But the moon shots bucked a trend of increasing distance between science and society, which is leaving society ill equipped to make the science-based decisions it needs to make.

Among the biggest challenges to society today are climate change and energy. Both are highly complex political and scientific issues. The climate is changing. There’s no doubt about that, and it is equally incontrovertible that human activity has something to do with it. Yet in the public sphere, the debate still rages on. Similarly, it’s a simple fact that renewable energy does not currently have the capacity to supply the increasing demands of the world.  That’s not to say that renewables do not have a place. Of course they do, and that place will grow with time. But the current timescale for delivery is longer than that for demand. Is society equipped to make the difficult decisions that need to be made on issues of global importance such as this? In my opinion, we’re far from it.

On the personal level, there’s a range of issues that leave people confused and forced to take ill-informed decisions that can literally have life-or-death consequences. Take mad cow disease, scares over the MMR vaccine, and the safety of mobile phones, for example.

Of course, at CERN, we’ve had our own experience of this phenomenon. When starting up the LHC in 2008, the world was in the grip of black hole fever. According to a small handful of people, our flagship accelerator would create a black hole that would devour the Earth. The idea went viral on social media, and was also widely reported in the mainstream media, many of which conveniently left the normal journalistic code of ethics to one side as they explored the comic possibilities of the story. Unfortunately, science has left society alone for too long, and many people were unable to see the funny side. There were even stories of schools being closed for the day to allow children to be with their parents, just in case. And all this was based largely on the testimony of a man who, when asked about his concerns on television, explained that the LHC would either destroy the Universe or it would not, therefore the probability for disaster was one in two.  If this were not so tragic, it would be laughable.

What can science do about it? In my opinion, a great deal.  At the institutional level, things are changing.  The recently created Blavatnik School of Government at Oxford University includes science as an obligatory part of its course on public policy, to cite just one example. We need to use exciting science projects like the LHC to engage people with science, not just through the science pages, but also in new ways such as the arts residency programme we recently launched at CERN. And scientists in positions of influence need to use that influence to shape political debate in the world’s Capitals and in places like Davos.

Broad engagement has been our approach at CERN for a number of years, seizing the opportunity offered by the visibility of the LHC to engage more fully with everyone from decision makers to our neighbours and the general public. As a result we’re seeing our science being covered responsibly, and once again we’re seeing people talking about it along with football and front-page news. Sometimes the stories are not exactly what we’d like to see, but what’s important is that people are talking about science.

When the LHC started up, and the world continued to exist, at least one newspaper boldly declared that the LHC would be the new Apollo, set to engage a whole generation with science. While I take such headlines with a healthy pinch of salt, they do make good reading. More recently, another newspaper declared that physics has the X-factor, that elusive quality that makes it part of the zeitgeist.

Science as a whole needs to capitalise on this, to ensure that the LHC is not science’s one-hit-wonder, and that engagement with society is sustained. As scientists, we owe the world this, helping people to master the complexity of their own science-based lives. Twelve months from now, I’ll be taking this message back to Davos.

Rolf Heuer

For a few years now, Famelab has grown into an international competition for young scientists aged 18-35 eager to share their passion.

Here is an unusual contest: participants are asked to communicate their work or interest in a 3-minute speech delivered to a general audience. In return, they get training from professionals (science communicators and media people), get invited to a Masterclass and can even make it to the finals at the Cheltenham Science Festival in the United Kingdom. The contestants are judged by professional scientists on their content, clarity and charisma. The goal is to detect the new voices for science and to find communicators able to captivate their audience.

It started in 2005 at the Cheltenham Science Festival. In 2007, the British Council adopted this competition as one of its flagship science engagement projects first in South East Europe for a pilot project, then expanding in 2010 to include 14 countries from Europe, Asia and Africa. Check out if there is a competition near you. You can also get help to host your own event.

On February 4, CERN will be hosting the Swiss semi-finals, with the finals to be held in Zurich on March 30. Anybody working or studying in Switzerland can participate. You can register up to the day of the event itself. Every one is also invited to attend the competition, which will start at 15:00 in CERN Globe of Innovation.

Don’t miss Tom Whyntie’s winning performance at the 2009 finals. Tom is a Ph.D student working on the CMS experiment at CERN. This is the most convincing speech you might ever heard about the importance of nothing.

Pauline Gagnon

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The Globe of Innovation, CERN expositions and visitors center

I do not have a crystal ball but it is nevertheless possible to sketch what can be expected from the Large Hadron Collider (LHC) experiments at CERN this year.

Right now, the accelerator is stopped for the annual maintenance shutdown. This is the opportunity to fix all problems that occurred during the past year both on the accelerator and the experiments. The detectors are opened and all accessible malfunctioning equipment is being repaired or replaced.

In the 27-km long LHC tunnel, surveyors are busy getting everything realigned to a high precision, while various repairs and maintenance operations are on their way. By early March, all magnets will have been cooled down again and prepared for operation.

The experimentalists are not only working on their detectors but also improving all aspects of their software: the detector simulations, event reconstruction algorithms, particle identification schemes and analysis techniques are all being revised.

By late March, the LHC will resume colliding protons with the goal of delivering about 16 inverse femtobarns of data, compared to 5 inverse femtobarns in 2011. This will enable the experiments to improve the precision of all measurements achieved so far, push all searches for new phenomena slightly further and explore areas not yet tackled. The hope is to discover particles associated with new physics revealing the existence of new phenomena. The CMS and ATLAS physicists are looking for dozens of hypothetical particles, the Higgs boson being the most publicized but only one of many.

When protons collide in the LHC accelerator, the energy released materializes in the form of massive but unstable particles. This is a consequence of the well-known equation E=mc², which simply states that energy (represented by E) and mass (m) are equivalent, each one can change into the other. The symbol c² represents the speed of light squared and acts like a conversion factor. This is why in particle physics we measure particle masses in units of energy like GeV (giga electronvolt) or TeV (tera electronvolt). One electronvolt is the energy acquired by an electron through a potential difference of one volt.

It is therefore easier to create lighter particles since less energy is required. Over the past few decades, we have already observed the lighter particles countless times in various experiments. So we know fairly well how many events containing them we should observe. We can tell when new particles are created when we see more events of a certain topology than what we expect from those well-known phenomena, which we refer to as the background.

We can claim that something additional and new is also occurring when we see an excess of events. Of course, the bigger the excess, the easier it is to claim something new is happening. This is the reason why we accumulate so many events, each one being a snap-shots of the debris coming out of a proton-proton collisions. We want to be sure the excess cannot be due to some random fluctuation.

Some of the particles we are looking for are expected to have a mass in the order of a few hundred GeV. This is the case for the Higgs boson and we already saw possible signs of its presence last year. If the observed excess continues to grow as we collect more data in 2012, it will be enough to claim the Higgs boson discovery beyond any doubt in 2012 or rule it out forever.

Other hypothetical particles may have masses as large as a few thousand GeV or equivalently, a few TeV. In 2011, the accelerator provided 7 TeV of energy at the collision point.  The more energy the accelerator has, the higher the reach in masses, just like one cannot buy a 7000 CHF car with 5000 CHF. So to create a pair of particles with a mass of 3.5 TeV (or 3500 GeV), one needs to provide at least 7 TeV to produce them. But since some of the energy is shared among many particles, the effective limit is lower than the accelerator energy.

There are ongoing discussions right now to decide if the LHC will be operating at 8 TeV this year instead of 7 TeV as in 2011. The decision will be made in early February.

If CERN decides to operate at 8 TeV, the chances of finding very heavy particles will slightly increase, thanks to the extra energy available. This will be the case for searches for particles like the W’ or Z’, a heavier version of the well-known W and Z bosons. For these, collecting more data in 2012 will probably not be enough to push the current limits much farther. We will need to wait until the LHC reaches full energy at 13 or 14 TeV in 2015 to push these searches higher than in 2011 where limits have already been placed around 1 TeV.

For LHCb and ALICE, the main goal is not to find new particles. LHCb aims at making extremely precise measurements to see if there are any weak points in the current theoretical model, the Standard Model of particle physics. For this, more data will make a whole difference. Already in 2011, they saw the first signs of CP-violation involving charm quarks and hope to confirm this observation. This measurement could shed light on why matter overtook antimatter as the universe expanded after the Big Bang when matter and antimatter must have been created in equal amounts. They will also investigate new techniques and new channels.

Meanwhile, ALICE has just started analyzing the 2011 data taken in November with lead ion collisions. The hope is to better understand how the quark-gluon plasma formed right after the Big Bang. This year, a special run involving collisions of protons and lead ions should bring a new twist in this investigation.

Exploring new corners, testing new ideas, improving the errors on all measurements and most likely the final answer on the Higgs, that is what we are in with the LHC for in 2012. Let’s hope that in 2012 the oriental dragon, symbol of perseverance and success, will see our efforts bear fruit.

Pauline Gagnon

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Google is launching today the second edition of its Science Fair in partnership with CERN, Lego, National Geographic and Scientific American. The goal is to encourage 13 to18 year-old students from all over the world to not only ask good questions but also devise a way to find an answer to these questions.

Today, it has nearly become a reflex: if you want to know what flowers bloom in the snow or how to get rid of bats or what inverse Compton scattering refers too, you just Google it. Thanks to countless people who have put effort in documenting every aspect of human knowledge, be it on Wikipedia or their own website, search engines will find it. With the internet, knowledge is now available just about everywhere on the planet, provided one has access to a computer and a network.

But much remains to be understood about the way the world and its inhabitants work. Behind each piece of knowledge hide some inquisitive people who initially asked the question and set out to find an answer to it. Curiosity has driven much scientific and technical development over the years, when necessity was not the main impetus. In all cases, some form of the scientific method was used to get to the answer: ask a question, formulate a hypothesis, design a way to test this hypothesis, analyze the data or information gathered and draw conclusions. By iterating this process and with a good dose of determination, scientists progress in their quest for an answer.

Pushing the limits of knowledge further is at the centre of CERN’s mission. So it is not surprising that CERN is associated to this major science fair. And don’t forget, it was at CERN that the world wide web was invented.

By encouraging young people to be creative, science fairs worldwide create a reflex among people to question the world they live in, to be critical of information received and draw their own conclusions based on evidence.

As the participants in the first edition of the Google Science Fair testify, like Harine from India, their experience was unique and memories will last a lifetime. So get the word out, support science fairs at all levels and encourage the young people you know to enter this contest.

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On December 21, a few hours after CERN was officially closed for the end-of-the-year holiday, the ATLAS experiment ended a fruitful 2011 on a happy note by releasing a paper announcing the discovery of a new quarkonium state, identified as the χ_b(3P), which had been predicted by theorists.

Quarkonia particles are composed by a quark and its anti-quark, such as charm plus anti-charm for the charmonium family and bottom plus anti-bottom for the bottomonium family.  Patterns of different mass and spin quantum number appear, corresponding to different configurations of how the state is bound together.  States of higher mass decay frequently to configurations of lower mass, such as the J/ψ and Y for the two quarkonium families.

Several such states have been both predicted and observed in the past. This discovery adds one more piece of information to a very complex problem. Quantum Chromodynamics (QCD) is the theory that describes the strong nuclear force that acts between quarks. It is part of the Standard Model, the theoretical model of particle physics. In principle, QCD can directly predict the properties of all particles made from quarks, including the protons and neutrons, which make up ordinary matter, and these more exotic quarkonium states. The problem is that it is mathematically very difficult to perform these calculations, and in many cases it is not currently possible.  Theorists have therefore developed simplified models that enable them to make useful predictions. Data on the newly found χ_b(3P) state of the bottomonium family  help to refine the models.

The newest state is the third of a series, with χ_b(1P) and χ_b(2P) being two lighter states, found in the 80’s and in the 90’s in electron-positron colliders. The χ_b(3P) had been predicted,  but had never been seen before, because it is difficult to observe it in transitions from states of higher mass.

The Large Hadron Collider (or LHC) at CERN is a 27-km machine that accelerates beams of protons at near the speed of light. Two separate beams circulate in opposite directions and are brought into collisions in the center of large detectors such as the ATLAS detector. Heavy but unstable particles are created from these highly energetic collisions. The detectors are used to take a snapshot of all their debris, each “picture” being called one event. Physicists then attempt to reconstruct what happened in each event and see what particles were created and how they decayed. The overall goal is to gain a clearer understanding of what matter is made of and how it interacts at the smallest level.

The χ_b states are found by looking for events where these states decayed into lighter bound states of b quark and anti-b quark called ϒ(1S) or ϒ(2S), plus a photon. In turn, the ϒ decays into two muons. The muons and photons are seen by the detector, and from these, the original χ_b states can be reconstructed.

The photon is detected in two ways: either by a system called the electromagnetic calorimeter, or, when the photon converts into a positron and an electron as it interacts with material in the detector, the latter can be seen by the tracking detector. For these decays, the tracking detector has a better resolution than the calorimeter, which means it measures the photon momentum more precisely.

The results are on the two plots below. The vertical axes show how many events containing a muon pair and a photon ATLAS detects in the collision fragments that are consistent with coming from a χ_b(1P), χ_b(2P) or χ_b(3P) decay. The scale on the horizontal axis is the mass; as the mass increases, the number of selected events rises, then falls, three times, leading to three peaks in the data. Each peak corresponds to one of the three states, and the position of the peak on the mass scale indicates the most likely mass of the state. The upper plot uses photons seen by the calorimeter and the lower one uses photons spotted by the tracking detector; the peaks are narrower in the latter because of the better resolution of the tracking detector.  The red curve shows candidates where the χ_b decayed into a ϒ(1S) state and the purple one, ϒ(2S).

The mass of the new state is equal to 10.54 GeV, about ten times that of a proton. This is somewhat higher than predicted, indicating that the quarks are less tightly bound than suggested by the models.

Each of the peaks shown in the plot is expected to be formed by separate, closely spaced sub-peaks, due to contributions from χ_b states of different spin. With the current quantity of data, it is not possible to see them since the limited resolution of the detector smears them into a single peak. More data from LHC will allow ATLAS, and also CMS and LHCb, to perform additional and more accurate measurements.

Theorists will then have more landmarks to refine their models, leading to a better description of QCD. Improving the understanding of the strong nuclear force is one of the major challenges facing particle physicists, and this new state provides important new information.

Pauline Gagnon

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The special CERN seminar on recent Higgs boson results held yesterday was one of the most exciting presentations I ever attended. The ambiance was electricifying and the room was packed more than two hours before it even started.

Members of each collaboration working on this, namely CMS and  ATLAS, both knew their half of the story. But the two teams had worked independently and all the crucial details of the final results were not known outside the collaborations. Everybody wanted to see if the small excesses observed in their team coincided with similar findings from the other collaboration.

Physicists are notoriously cautious for good reason. To claim a discovery, we ask that if there is only background (and no Higgs), the odds of seeing an excess of event as large as the one observed be less than 0.00003% or 5-sigma.

In the case of the Higgs boson, if we find some signs of its possible presence, we will want it to do much more than just ”look like” a Higgs but also behave like one, smell like one, dance and sing like only that particle can do. As it is, it may look like a Higgs with a mass somewhere around 124-126 GeV but the level of confidence is way too low to draw conclusions. Each experiment has small signals at the 2-3 sigma level, which is what is expected if there is a Higgs boson given current data size. To reach the unambiguous 5 sigma-deviation level will require adding new data.

The higher the number of sigma, the more incompatible the data are with having only background and no Higgs.

Of course, it is encouraging that both groups find similar results, not only in one decay mode, but in multiple channels. A decay channel represents one of the many ways the Higgs boson can decay. As one of my colleagues put it, if the Higgs boson was a large coin, each decay channel would represent one way to break this coin to make small change. CMS and ATLAS collected all events corresponding to specific decay channels. The fact that they all point somewhere to roughly the same mass value is an indication they could all be coming from the same particle.

ATLAS spokesperson, Fabiola Gianotti, presented the ATLAS findings first.

Two separate decay channels both favour a mass value around 126 GeV: Higgs decaying into two photons and Higgs into two Z bosons, with each Z going into a pair of electrons or muons. A third channel with Higgs decaying into two W bosons, each W decaying into an electron or muon plus a neutrino is also consistent with this hypothesis but at a lesser level.

Guido Tonelli, CMS spokesperson, showed the combination of five different channels, adding the Higgs to two taus and Higgs to pairs of heavy quarks to those investigated by ATLAS. The combined results are compatible with a Higgs signal, the highest probability being found at 124 GeV, but not enough data were available to draw any definitive conclusions. The observed excess of events could be a statistical fluctuation of the known background processes, either with or without the existence of the Standard Model Higgs boson in this mass range.

The probability of obtaining an upward fluctuation as large or larger than that is observed if there is only background, prior to accounting for the look-elsewhere effect. As one can see, the excess falls in the same position for two different search channels and is also compatible with a much smaller excess in the third channel. The statistical significance is still modest but having three channels, especially two robust ones, is an indication this could be real. Nevertheless, this is a stronger signal than what was expected from a Standard Model Higgs boson with a mass of 126 GeV, which is shown by the black dashed curved.

The small excess of events observed by CMS in five different decay channels. The dotted line shows what was expected in the absence of a Higgs boson. The green and yellow bands represent the 1-sigma and 2-sigma error margin on this prediction. The black curve is the observed data. Excursions beyond the yellow band indicate where a Higgs signal is the strongest. The most significant value is found for a Higgs mass around 124 GeV.

When all their channels are combined, ATLAS obtains an excess of 2.3 sigma over background, while CMS gets 1.9 sigma, after taking into account the “look-elsewhere effect”, namely how often when looking at all the possible mass points under study would one point fluctuate that much. The chance of obtaining an upward fluctuation this large or larger if there is only background is 1% for ATLAS and about 2.9% for CMS.

Without the “look-elsewhere” correction, the ATLAS probability of such an excess of events if there is only background 3.6 sigma. This value can be compared to the 2.4 sigma deviation one would expect if the excess was due to a Higgs boson. So ATLAS sees slightly more events than what is expected from a Standard Model Higgs boson. Statistical fluctuations can happen in both directions, which is why caution is required until more data is analyzed.

Having already combined all data for 2010 and 2011 from more channels, CMS showed they now exclude all possible Higgs masses from 127 to 600 GeV with a 95% confidence level, leaving only a narrow window open between 114-127 GeV.  ATLAS excludes masses above 131 GeV up to 453 GeV with the same confidence level, but also between 114-115.5 GeV.

The exclusion limits presented by the CMS collaboration. The dotted curve shows what was expected while the black line with dots indicates what is observed. Whenever this curve falls below the red line is excluded. All masses above 127 GeV are now excluded at 95% confidence level.

Of course, everybody would love to be able to say: that’s it! We found it. But it is still premature despite encouraging signs. More data will be collected in 2012. The answer will then become unambiguous: we will either discover the Higgs or rule it out completely. If the small effects presented today keep growing, we will then see the Higgs do its little song and dance.

Pauline Gagnon

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For more information, visit the CERN website or ATLAS and CMS websites

Discovering the Higgs particle at the LHC would be a triumph, but showing that it doesn’t exist could be at least as exciting, perhaps heralding a revolution in our understanding of nature at a fundamental level. After two good years of operation at the LHC, the moment of truth is drawing near. By the end of the 2012 LHC run at the latest, we’ll know whether the simplest incarnation of the Higgs particle is real, or just a chimera. Whatever the case, many more years of research at the LHC will be needed to fully get to grips with the consequences.

Finding the Higgs particle, or definitively ruling out its existence, is one of the top priorities for research at the LHC. The Higgs particle is associated with the simplest realization of a mechanism, proposed in the mid-1960s by Robert Brout, François Englert, Peter Higgs and others, which was put forward to explain why one of nature’s fundamental forces has a very short range, while another similar force has an infinite range. The forces in question are the electromagnetic force, which carries light to us from the stars, drives electricity around our homes, and gives structure to the atoms and molecules from which we are all made, and the weak force, which drives the energy generating processes of the stars. Today we know that the electromagnetic force is carried by particles called photons, which have no mass, whereas the weak force is carried by particles called W and Z, which do have mass. Rather like people passing a ball, interacting particles exchange these force carriers. The heavier the ball, the shorter the distance it can be thrown; the heavier the force carrier, the shorter its range. The W and Z particles were discovered in a Nobel prize winning enterprise at CERN in the 1980s, but the mechanism that gives rise to their mass has not yet been experimentally identified, and that’s where the Higgs particle comes in.

The Higgs mechanism in its basic form is the simplest theoretical model that could account for the mass difference between photons and the W and Z particles, and by extension could account for the masses of a range of fundamental particles. But the simplest form of the Higgs mechanism is not the only possible explanation. There are many others, linked to theories such as supersymmetry, which could account for the mysterious dark matter of the Universe, or theories predicting extra dimensions of space, which, if verified, would truly revolutionise our understanding of the Universe we live in. These searches in turn are just a part of the very wide programme of research that is ongoing at the LHC, which also includes looking for the subtle imbalance in nature between matter and antimatter that has allowed the matter we are made of to exist, and studying matter as it would have been in the first instants of the Universe’s life.

The basic form of the Higgs mechanism forms part of the Standard Model of particle physics, the theory developed in the 1960s and 70s that describes the behaviour of fundamental particles and has since been thoroughly tested at laboratories such as CERN. The Standard Model works extremely well, but we know that it cannot be a complete theory. It describes beautifully the ordinary matter from which we, and the entire visible Universe, are made. But it does not describe the invisible 96% of the Universe that we know to be there, but which has thus far evaded detection. The Standard Model is nevertheless such a good theory that it will always remain valid over the range it has been tested. Today’s scientists are therefore looking for a theory that builds on the Standard Model, rather like Einstein’s theory of gravity, general relativity, builds on Newtonian gravity. That’s why finding an alternative to the Standard Model Higgs particle would be so exciting.

The Standard Model Higgs particle, if it exists, has well-defined properties that depend only on its mass. That’s why it will be possible to confirm or refute its existence before the end of 2012. Some of the possible non-Standard Model Higgs particles would look very much like the Standard Model variety, but could emerge more rarely from LHC collisions and therefore take longer to find. Others would be heavier than the LHC’s current reach, and require more energy to produce. If such particles are nature’s choice, we’ll have to wait until the LHC moves to its full design energy to find out.

Whatever the case, the LHC will make a discovery about the nature of the masses of the fundamental particles. That’s because another shortcoming of the Standard Model is that without the Higgs particle, or something that does the same job, its calculations of particle processes break down at the energies the LHC will reach in its second phase of running starting around 2014. That means that if the LHC does not discover the Higgs particle by 2012, it is heading towards a discovery later. Whatever mechanism nature uses, the LHC will bring us insights.

The status of the search for the Standard Model Higgs particle at the end of the 2011 LHC proton run in October was based on experimental work involving scientists from around the world. Direct searches from CERN’s previous flagship research facility, the Large Electron Positron collider, LEP, had excluded the mass range up to 114 GeV. Results from the Tevatron collider at Fermilab in the USA, and from the LHC, had excluded the range from 141 GeV to 476 GeV.  Indirect searches, in which scientists try to detect tell tale signs that a Higgs particle has influenced their measurements rather than looking for the particle directly, exclude the range above 200 GeV or so. That left just the region 114-141 GeV, which is precisely where theoretical and experimental considerations say a Standard Model Higgs particle is most likely to be. By December 2011, analyses by the ATLAS and CMS collaborations had further narrowed the range of masses available for the Standard Model Higgs particle to just 116-127 GeV, with both experiments seeing tantalising signs that that a Standard Model Higgs particle might be starting to emerge in the region of 124-126 GeV. Only time will tell.

All of this augurs very well for the long-term future of the LHC programme, since whatever form the Higgs particle takes, studying its properties, or examining its absence, will require considerable amounts of data. A Standard Model-like Higgs particle could yet point the way to new physics through subtleties in its behaviour that would only emerge after studying a large number of Higgs particles. A Standard Model-like Higgs particle might also be one of several types of Higgs particles, pointing the way to new physics, and this would only become apparent after detailed scrutiny. A non-Standard Model Higgs particle linked to a theory like supersymmetry that goes beyond the Standard Model would immediately open the door to new physics. And finally, if a Standard Model Higgs particle were definitively ruled out at the LHC’s current operating energy, that would point either to a non-Standard Higgs particle that could be discovered with more luminosity or to the existence of new physics at the LHC’s full design energy where the Standard Model without the Higgs particle starts to break down.

Whatever form the Higgs particle takes, or whatever mechanism drives the differences in fundamental particle masses, finding it is not a simple case of spotting the telltale signs and shouting Eureka! It is a painstaking process of statistical analyses based on measuring specific configurations of particles emerging from collisions. For example, one of the ways a Higgs particle can decay is into two photons, which would be detected. However, there are many other processes that also produce two photons, so the searches compare the number of so-called two-photon events measured with the number expected from already known processes. They do this for all the possible decay modes, and only when they see a statistically significant excess can scientists claim a discovery. In particle physics, people talk of 95% confidence levels, which means that a given signal, such as that for a Higgs particle decaying to two photons, has only a 5% chance of being due to a statistical fluctuation. However, 95% confidence is not enough to claim a discovery, for that, the probability of a statistical fluctuation being responsible for the measurement has to be much smaller, say, than one in a million. This is what physicists call a five-sigma effect. Discovery or exclusion of a Standard Model Higgs particle to that level of confidence is what’s on the cards for 2012 at the latest.

A non-Standard Model Higgs particle could take longer to be discovered, but would certainly be worth the wait. Either way, there will be much work ahead for the LHC scientists to fully understand the new physics that is just over the horizon.

James Gillies