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A beam of your own

Thursday, January 16th, 2014

As part of its 60th anniversary celebration and to help keep us young at heart, CERN has launched a special competition for students called: Beam line for schools.

CERN is inviting students aged 16 and upward from anywhere in the world to submit a proposal to do an experiment with a beam of particles from the Proton Synchrotron beam line. Each team can be composed of up to 30 students with at least one adult supervisor. This summer, up to nine students of the selected team will be invited to CERN to run the team’s experiment. Travelling and living expenses for the selected group will be covered by CERN.

PSA view of the Proton Synchrotron beam line.

The proposals will be pre-selected by a group of CERN scientists, and will then be reviewed by the same committee that validates all proposals for experiments at the laboratory’s Super Proton Synchrotron and Proton Synchrotron accelerators.

So what could you be doing? Essentially, you can investigate how beams of particles interact with matter. For example, you could study what happens when beams containing different types of particles hit targets made of various materials. The proposals will be judged on creativity, motivation, feasibility and adherence to the scientific method.

To help you understand what can be done, we have put together a short presentation that explains the basics about particles and beams. These short talks are available in English, French, Italian, Spanish and German and are part of a YouTube playlist that includes recordings of Google hangouts in English, French, Italian, Spanish and German, in which CERN scientists answer questions.

Here is your chance to come to run your own experiment at CERN. This will last about a week and take place in July, August or September. CERN physicists will be helping you to refine your idea before and during your stay at CERN.

Interested? Then you can stay up-to-date via the CERN website, #bl4s on Twitter, Facebook, Google+ or YouTube.

Don’t hesitate and fill out the registration form before 31 January 2014. All you need to do at this point is send us the name of the school and of the participants as well as a tweet-of-intent stating why you think you should win this competition. You will still have until 31 March to prepare your full project, including a 1-minute video giving the highlights. Here is your chance!

Pauline Gagnon

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A whole Universe to be discovered

Wednesday, January 15th, 2014

The past two years have been rather exceptional for CERN: first in 2012, the CMS and ATLAS experiments discovered the Higgs boson, confirming the mechanism elaborated 48 years earlier by Robert Brout, François Englert and Peter Higgs. Then in 2013, Englert and Higgs received the Nobel Prize for Physics for their theory.

2014 is also going to be special year since CERN is going to turn 60. But beyond this anniversary, CERN is preparing the Large Hadron Collider (LHC) to explore new territories.

With the Higgs boson discovery, we have completed the Standard Model, the current theory that explains what makes all visible matter around us. But that is just a mere 5% of the total content of the Universe. The existence of dark matter tells us our current model is incomplete. So far, the various analyses of the data taken at 8 TeV has not yet revealed traces of dark matter or any new particles. To push all our searches further and faster, we need to increase the reach of the LHC by going to higher energies.

This is why since February last year all accelerators and experiments at CERN began a long shutdown for maintenance and consolidation. This will continue in 2014 for the LHC but many accelerators of CERN complex will be coming back to life starting this summer.


The starting point of the chain of accelerators is a simple hydrogen bottle. The electrons are stripped from the hydrogen atoms using an electric field to leave single protons. These are then accelerated in a small linear accelerator (LINAC 2 at the bottom centre of the diagram below). The Low Energy Ion Ring (LEIR) plays a similar role but with heavy ions.


The protons get an extra kick in the Booster before being injected into what is CERN’s oldest circular accelerator still in operation, the Proton Synchrotron (PS). Then the protons head for the Super Proton Synchrotron (SPS), where they reach 450 GeV in energy (that is 450 billion electronvolts). This is the final stage before injection into the LHC where the energy will get nearly thirty times larger, namely 13 TeV.

The beams from the accelerator chain are also delivered to various other experimental areas, such as ISOLDE and n-TOF where a huge number of experiments involving nuclei are conducted. Other protons hit a target to produce antiprotons for the Antiproton Decelerator (AD), a facility dedicated to antimatter studies. These experiments will all resume their activities in 2014.


All consolidation work for the LHC and its experiments will take place in parallel. ATLAS and CMS plan to complete all repairs and upgrades to their detector by November, ALICE at the beginning of December and LHCb in early January 2015.

Meanwhile, all physicists not involved with hardware are either completing the many ongoing analyses of all data taken up to 2013, preparing new simulations at higher energies, improving the data reconstruction algorithms or designing the new trigger selection criteria. Everybody is preparing to meet the challenge of dealing with more data at higher energy. All in the hope that we might be rewarded once more with new discoveries since there is still a whole new world to explore out there.

Pauline Gagnon

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One giant leap for the Higgs boson

Friday, December 6th, 2013

Both the ATLAS and CMS collaborations at CERN have now shown solid evidence that the new particle discovered in July 2012 behaves even more like the Higgs boson, by establishing that it also decays into particles known as tau leptons, a very heavy version of electrons.

Why is this so important? CMS and ATLAS had already established that the new boson was indeed one type of a Higgs boson. In that case, theory predicted it should decay into several types of particles. So far, decays into W and Z bosons as well as photons were well established. Now, for the first time, both experiments have evidence that it also decays into tau leptons.

The decay of a particle is very much like making change for a coin. If the Higgs boson were a one euro coin, there would be several ways to break it up into smaller coins, but, so far, the change machine seemed to only make change in some particular ways. Now, additional evidence for one more way has been shown.

There are two classes of fundamental particles, called fermions and bosons depending on their spin, their value of angular momentum. Particles of matter (like taus, electrons and quarks) belong to the fermion family. On the other hand, the particles associated with the various forces acting upon these fermions are bosons (like the photons and the W and Z bosons.).

The CMS experiment had already shown evidence for Higgs boson decays into fermions last summer with a signal of 3.4 sigma when combining the tau and b quark channels. A sigma corresponds to one standard deviation, the size of potential statistical fluctuations.  Three sigma is needed to claim evidence while five sigma is usually required for a discovery.

For the first time, there is now solid evidence from a single channel – and two experiments have independently produced it. ATLAS collaboration showed evidence for the tau channel alone with a signal of 4.1 sigma, while CMS obtained 3.4 sigma, both bringing strong evidence that this particular type of decays occurs.

Combining their most recent results for taus and b quarks, CMS now has evidence for decays into fermions at the 4.0 sigma level.


The data collected by the ATLAS experiment (black dots) are consistent with coming from the sum of all backgrounds (colour histograms) plus contributions from a Higgs boson going into a pair of tau leptons (red curve). Below, the background is subtracted from the data to reveal the most likely mass of the Higgs boson, namely 125 GeV (red curve).

CMS is also starting to see decays into pairs of b quarks at the 2.0 sigma-level. While this is still not very significant, it is the first indication for this decay so far at the LHC. The Tevatron experiments have reported seeing it at the 2.8 sigma-level. Although the Higgs boson decays into b quarks about 60% of the time, it comes with so much background that it makes it extremely difficult to measure this particular decay at the LHC.

Not only did the experiments report evidence that the Higgs boson decays into tau leptons, but they also measured how often this occurs. The Standard Model, the theory that describes just about everything observed so far in particle physics, states that a Higgs boson should decay into a pair of tau leptons about 8% of the time. CMS measured a value corresponding to 0.87 ± 0.29 times this rate, i.e. a value compatible with 1.0 as expected for the Standard Model Higgs boson. ATLAS obtained 1.4 +0.5 -0.4, which is also consistent within errors with the predicted value of 1.0.


One of the events collected by the CMS collaboration having the characteristics expected from the decay of the Standard Model Higgs boson to a pair of tau leptons. One of the taus decays to a muon (red line) and neutrinos (not visible in the detector), while the other tau decays into a charged hadron (blue towers) and a neutrino. There are also two forward-going particle jets (green towers).

With these new results, the experiments established one more property that was expected for the Standard Model Higgs boson. What remains to be clarified is the exact type of Higgs boson we are dealing with. Is this indeed the simplest one associated with the Standard Model? Or have we uncovered another type of Higgs boson, the lightest one of the five types of Higgs bosons predicted by another theory called supersymmetry.

It is still too early to dismiss the second hypothesis. While the Higgs boson is behaving so far exactly like what is expected for the Standard Model Higgs boson, the measurements lack the precision needed to rule out that it cannot be a supersymmetric type of Higgs boson. Getting a definite answer on this will require more data. This will happen once the Large Hadron Collider (LHC) resumes operation at nearly twice the current energy in 2015 after the current shutdown needed for maintenance and upgrade.

Meanwhile, these new results will be refined and finalised. But already they represent one small step for the experiments, a giant leap for the Higgs boson.

For all the details, see:

Presentation given by the ATLAS Collaboration on 28 November 2013

Presentation given by the CMS Collaboration on 3 December 2013

Pauline Gagnon

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Visiting CERN in London

Monday, November 25th, 2013

If you have not had a chance to visit CERN in Geneva, you can now do it in London. The London Science Museum just opened a new exhibition called: Collider. I had the opportunity to visit it and can confirm that this exhibition conveys the impression of being at CERN.

The exhibition, open until May 2014, explores the people, science and engineering behind the largest scientific experiment ever constructed, the Large Hadron Collider at CERN.

The exhibition starts in a small amphitheatre where visitors get the feeling of sitting in CERN main auditorium on 4 July 2012. That was the day the discovery of a new particle, which was later confirmed to be a Higgs boson, was announced. There, a few physicists share their thoughts about particle physics and their participation in that search.

As the curator Alison Boyle explained to my colleagues and I, they tried to portray the essence of various people they had met at CERN over the two years it took them to prepare this exhibition. Although some characters seemed slightly odd, others were strangely familiar.

Prof-Peter-Higgs-and-Prof-SProfessors Peter Higgs and Stephen Hawking during their visit of the Collider exhibition (© Science Museum)

The exhibit is stunning in its clever use of visual effects. Visitors wander at their own leisure through rooms where the walls are covered with life-size pictures of various places at CERN, giving them a sense of being there. Notes scribbled on boards or pieces of papers taped to the wall as one often finds all over the place at CERN add to the likeliness and provide the necessary explanations. Real objects enhance the pictures to create a very special ambiance. A great video animation also gives a feel for what particles go through as they zip through the detectors.

But for CERN people, the most surprising piece is the reproduction of one corridor in its 1950s architecture glory. The walls are pasted with posters announcing a plethora of past and future conferences as well as local events, from the CERN choir down to the LGBT group. It felt like being at work thousands of kilometres away from work.

So if you cannot come see the real thing, this is an excellent substitute to get immersed in CERN ambiance. The exhibition is due to go on tour across the world, giving even more people a chance to experience what it feels like at the world’s largest physics laboratory.

You can follow the exhibition blog here.

Pauline Gagnon

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A Nobel Prize most appreciated at CERN

Tuesday, October 8th, 2013

The whole of CERN was elated today to learn that the Nobel Prize for Physics had been awarded this year to Professors François Englert and Peter Higgs for their theoretical work on what is now known as the Brout-Englert-Higgs mechanism. This mechanism explains how all elementary particles get their masses.


CERN had good reason to celebrate, since last year on 4 July, scientists working on LHC experiments proudly announced the discovery of a new particle, which was later confirmed to be a Higgs boson. This particle proves that the theory Robert Brout, François Englert and Peter Higgs developed, along with others, in 1964 was indeed correct.

The Higgs boson discovery was essential to establish their theory so we are all happy to see their work (and to some extent, our work) acknowledged with this prestigious award.

It took another decade before Steve Weinberg, co-recipient of the Nobel Prize in 1979, saw the full implication of their work while unifying two fundamental forces, the electromagnetic and weak forces, as Peter Higgs explained in July at the European Physical Society meeting of the Particle Physics division, where he gave a highly appreciated presentation. There he detailed the work of all those who preceded him, including Englert and Brout, in bringing key elements that enabled him to conceive his own work.

Peter Higgs recalled how it all began with pioneering work on “spontaneous symmetry breaking” done by Yoichiro Nambu in 1960 (for which he shared the Nobel Prize in 2008). Nambu himself was inspired by Robert Schrieffer, a condensed matter physicist who had developed similar concepts for the theory of superconductivity with John Bardeen and Leon Cooper (1972 Nobel Prize).

Spontaneous symmetry breaking is central in the Brout-Englert-Higgs mechanism rewarded today by the Swedish Academy of Science.

Jeffrey Goldstone then introduced a scalar field model often referred to as the “Mexican hat” potential while another condensed matter theorist, Philip Anderson (Nobel Prize in 1977) showed how to circumvent some problems pointed out by Goldstone.

Then, Englert and Brout published their paper, where the mechanism was finally laid out. Peter Higgs, who was working entirely independently from Brout and Englert, had his own paper out a month later with a specific mention of an associated boson. Tom Kibble, Gerald Guralnik and Carl Hagen soon after contributed additional key elements to complete this theory.

“I had to mention this boson specifically because my paper was first rejected for lack of concrete predictions”, Peter Higgs explained good-heartedly in his address last summer. This explicit mention of a boson is partly why his name got associated with the now famous boson.

The history of the Brout-Englert-Higgs mechanism just goes to show how in theory just like in experimental physics, it takes lots of people contributing good ideas, a bit of luck but mostly great collaboration to make ground-breaking discoveries.

The thousands of physicists, engineers and technicians who made the discovery of the Higgs boson possible at the LHC are also all celebrating today.

Pauline Gagnon

To find out more about the Higgs boson, here is a 25-minute recorded lecture I gave at CERN on Open Days

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New light from CLOUD on climate change

Monday, October 7th, 2013

In a paper published in the journal Nature, the CLOUD experiment at CERN reports on a major advance towards solving a long-standing enigma in climate science: how do aerosol particles form in the atmosphere? It is known that all cloud droplets form on aerosols: tiny solid or liquid particles suspended in the air. However, how these aerosol particles form or “nucleate” from atmospheric trace gases – and which gases are responsible – has remained a mystery.

According to the Intergovernmental Panel on Climate Change (IPCC), aerosol particles and their influence on clouds constitute the biggest uncertainty in assessing human-induced climate change. Understanding how aerosol particles form in the atmosphere is important since in increased concentrations, they cool the planet by reflecting more sunlight and by forming smaller but more numerous cloud droplets. That, in turn, makes clouds more reflective and extends their lifetimes.  These poorly-understood processes currently limit the precision of climate projections for the 21st century.

Thanks to CERN expertise in materials, gas systems and ultra-high vacuum technologies, the CLOUD team was able to build a chamber with unprecedented cleanliness. This enabled them to introduce minute amounts of various atmospheric vapours into an initially “pure” atmosphere under carefully controlled conditions, and start unravelling the mystery.

The researchers made two key discoveries. Firstly, they found that minute concentrations of amines can combine with sulphuric acid to form aerosol particles at rates similar to atmospheric observations. Secondly, using a pion beam from the CERN Proton Synchrotron, they found that cosmic radiation has a negligible influence on the formation rates of these particular aerosol particles.


This detailed plot shows the nucleation rate (i.e. the rate at which aerosol particles form) against sulphuric acid concentration. The small coloured squares in the background show atmospheric observations. The CLOUD measurements (large symbols) were obtained with various vapours in the chamber (curve 1: only sulphuric acid and water; curve 2: with ammonia added; curve 3 to 5: with amines added). The dashed lines and coloured bands show the theoretical expectations for ammonia+sulphuric acid (blue) and amine+sulphuric acid (red/orange) nucleation, based on quantum chemical calculations. Only amines reproduced the nucleation rates observed in the atmosphere, while ammonia was a thousand times too small.

Amines are atmospheric vapours closely related to ammonia, arising from human activities such as animal farming and also from natural sources. Amines are responsible for the familiar odours emanating from the decomposition of organic matter that contains proteins. For example, the smell of rotten fish is due to trimethylamine.

Thanks to their unique ultra-clean chamber, the CLOUD scientists have shown for the first time that the extremely low concentrations of amines typically found in the atmosphere (namely a few parts per trillion by volume or pptv) are sufficient to combine with sulphuric acid to form highly stable aerosol particles at rates similar to those observed in the lower atmosphere, as shown on the figure above.

JasperJasper Kirkby, spokesperson of the CLOUD experiment, crouched inside the ultra-clean chamber used for these measurements.

The precise laboratory measurements have allowed the team to develop a fundamental understanding of the nucleation process at a molecular level. The scientists can even reproduce their experimental results using quantum chemical calculations of molecular clustering.

This is the first time an experiment has reproduced the formation rates of atmospheric particles with complete measurements of the participating molecules. So the CLOUD results represent a major advance in our understanding of atmospheric nucleation.

Nobody expected that the formation rate of aerosol particles in the lower atmosphere would be so sensitive to amines. A large fraction of amines arise from human activities, but they have not been considered so far by the IPCC in their climate assessments. The CLOUD experiment has therefore revealed an important new mechanism that could contribute to a presently unaccounted cooling effect.

Moreover, a technique called “amine scrubbing” is likely to become the dominant technology to capture the carbon dioxide emitted by fossil-fuel power plants. Hence, amine emissions are expected to increase in the future and will now need to be considered when assessing the impact of human activities on past and future climate.

Pauline Gagnon

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Open Days and sore throats

Wednesday, October 2nd, 2013

Many people have a sore throat this week at CERN. Not too surprising given the 70 000 inquisitive visitors we welcomed over the weekend! It was amazing to see so much interest from the public and the enthusiasm of the 2300 volunteers.


Everybody pitched in. From the fire brigade to the experiments, every one was showing their part of the lab. Accelerator specialists and physicists, administrative and support staff, everybody was proudly wearing their bright orange T-shirt.


Thanks to the ticketing system for the underground visits, the queuing time was reduced with respect to the previous event in 2008. It was not easy to accommodate every one who wanted to see the large detectors sitting 100 meters underground, the elevators capacity being the limiting factor. Nevertheless, 20 000 people were taken down in small groups to one of the underground visit points

 Visitors to the ALICE experiment.

But there was loads of action taking place at the surface too. I was at the ATLAS stand on Saturday morning to answer questions about the lab and the various research activities. I met people who had come from the Czech Republic, Sweden, Lithuania, Poland, Algeria, USA, Scotland, Spain and even Australia just to have the opportunity to explore the world’s largest particle physics laboratory.

 Visitor playing with robotic machine in metrology lab.

The volunteers were also happily grabbing the opportunity to discover areas they had never visited before. This was certainly my case and I zipped up and down the road trying to peek at as many sites as possible between my assignments.


I was particularly impressed by the enthusiasm displayed by the machinists in the huge workshop adjacent to my office. I pass by this every morning but had never had a chance to see the mind-boggling pieces and machinery the team had on display. All sorts of round objects blown out of metal or milled, some with puzzling shape, geometry or size.


There was much entertainment for the young and not-so-young crowd too. The crane operators had them go up in tall cherry pickers or lift huge weights from a joy-stick box. Fire fighters had spectacular burning displays or staging rescue operations underground.

The cryogenics department had its popular liquid nitrogen stand, with the superconducting levitating scooter as one of the main attractions.


As I was making my way back to my car shortly before 8 pm on Sunday, I noticed one of my young colleagues was still enthusiastically explaining the working of a dipole magnet. He had started at 8 am the previous day and he was still displaying the same passion although with slightly less voice!

Many lectures given during the weekend will be available shortly.

Pauline Gagnon

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Zooming in on new particles

Friday, September 20th, 2013

The Large Hadron Collider (LHC) at CERN has stopped in the spring to undergo a major consolidation program but this has not stopped the search for new physics. On the contrary, physicists are taking advantage of the interruption to finalise all analyses with the whole data collected so far.

Dozens of new results have been presented by the four LHC experiments at several conferences since the end of operation. While only a handful of these results have made the headlines, a wealth of new information is now available, allowing theorists to refine their models.

Even with the discovery of a Higgs boson, physicists know that the Standard Model of particle physics cannot be the final answer since it has known shortcomings. For example, it fails to provide an explanation for dark matter or why the masses of fundamental particles such as electrons and muons are so different. Another theory called supersymmetry (or SUSY for short) is one of the most popular and most promising ways to extend the Standard Model, but it has yet to manifest itself.

One major difficulty when testing this new theory is the large number of parameters it introduces. To find the new particles predicted by SUSY, we must explore a vast territory spanned by 105 dimensions, corresponding to its 105 free parameters. Finding these new particles is like trying to spot a stranger in a crowd of millions.

Fortunately, theorists have attempted to give us experimentalists some guidance to constrain these parameters using theoretical or experimental considerations. One model that has gained popularity lately is called the phenomenological Minimal Supersymmetric Model or pMSSM and uses only 19 parameters. It takes into account information from all aspects of particle physics, incorporating constraints from the measured characteristics of the Z and Higgs bosons, b-quark physics, astrophysics as well as direct searches for dark matter at underground facilities and supersymmetric particles at the LHC.

Several groups of theorists and experimentalists have combined all these recent results to see which areas of the reduced but still huge parameter space of the pMSSM model are still allowed.

Their approach consists in generating millions of possible values corresponding to the masses and couplings of the hypothesised SUSY particles. The couplings are quantities related to the probability to produce these particles at the LHC.

Then they impose various constraints obtained from the many quantities measured by past and current experiments to see which points among all possibilities are still allowed.

Two theorists, Alex Arbey and Nazila Mahmoudi, and experimentalist Marco Battaglia, contrary to their earlier work, performed their latest scan assuming the four positive results reported by direct dark matter experiments were true dark matter signals to see if these results could be explained within SUSY.

While attempts by other groups were not able to find SUSY scenarios in agreement with the parameters of the possible dark matter signal, their results were rather surprising: they found surviving scenarios pointing to a light neutralino, with a mass of only 10 GeV, twelve times lighter than the Higgs boson. The second lightest particle is the super partner of the bottom quark, called sbottom, at around 20 GeV.


The mass ranges predicted for different SUSY particles coming out of this study. The Higgs boson discovered last summer, h0, is assumed to be the lightest of the five Higgs bosons predicted by SUSY and the lightest SUSY particle is the neutralino, χ0.

If this scenario were correct, why would such a light particle have escaped detection? The reason is that most searches led by the CMS and ATLAS experiments have focused so far on events where a large amount of energy is missing.

This would be the case when some heavy but invisible SUSY particle escapes from our detectors. Such criteria are needed to reduce the overwhelming background and isolate the few events containing traces of SUSY particles. But a light neutralino would only carry a small quantity of energy and would have gone undetected.

While theorists are assessing which corners of the parameter space are still allowed, experimentalists are evaluating the impact of their selection criteria on detecting particles having the characteristics of the remaining allowed regions. New strategies are now being sought to explore this possibility.

Operating the LHC at higher energy and collecting larger datasets starting in 2015 should give definite answers to these questions. These combined efforts may soon pave the way to new discoveries.

Pauline Gagnon

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Our Universe is Yours

Friday, August 9th, 2013

September 28 and 29, from 9:00 to 20:00, CERN is going to open its doors to the public once again. This is a not-to-be-missed opportunity to visit dozens of experimental setups and get to see what is happening in the world’s largest particle physics laboratory. All of that free of charge.

PO-3A group of visitors discovering the CMS detector during the Open Days in 2008

There will be unique opportunities to go underground and see the detectors operating at the Large Hadron Collider (LHC) where the Higgs boson was discovered (ATLAS and CMS), where we explore what happened an instant after the Big Bang (ALICE) and where the Standard Model is being refined with unprecedented precision (LHCb). Visitors will also have the opportunity to discover several other experiments where we probe the world of antimatter (AD), the puzzling mystery of dark matter (AMS) or look inside protons (COMPASS) to name but a few of them.

Just about every aspect of the laboratory will be open to the visitors, from the computing centre to the workshops, the cryogenics and metrology laboratories and the accelerators.

PO-2The cryogenic demonstrations fascinate everyone

There will be hands-on setups for kids, introductory lectures for the most inquisitive ones and fun for everyone.  Take your pick. This will be a day to remember.

The programme is already online. The underground visits are reserved to people 12 years and older and subject to strict quotas. So you will need to reserve in advance. Free tickets will be available around August 15 so keep an eye for it (I will be on vacation and unable to remind you). This will guarantee that every one spends their time playing and not queuing.

I hope many of you will be among the 100 000 people we will have the pleasure to welcome.

PO-4Taking a maximum of people down underground. The quotas are determined by the elevators capacity

Pauline Gagnon

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The Standard Model checked to the ninth decimal

Tuesday, July 30th, 2013

At the European Physics Society conference in Stockholm, two experiments operating at the Large Hadron Collider (LHC) at CERN, LHCb and CMS reported on July 19 solid evidence that the Standard Model of particle physics still shows no sign of wear and tear by checking a prediction of the model to the ninth decimal place.

The Standard Model makes very accurate predictions but theorists know this theory has its limits. At higher energy, its equations start breaking down. Theorists are convinced that despite all the success of this model, it is not giving us the big picture. Hence, scientists have been trying to find a “secret passage” to the next level, a more encompassing and more robust theory.

One way to achieve this is to look for a small deviation in a measured quantity from the value predicted by the Standard Model and a good place to find such a deviation is in an extremely rare process. It is much easier to hear a faint noise in a quiet place than in the middle of traffic during rush hour.

Specifically, the scientists measured how often composite particles denoted Bs and Bd (pronounced “b sub s and b sub d)” mesons decay into a pair of muons (particles similar to electrons but about 200 times heavier). A Bs meson is a composite particle containing b and s quarks while Bd mesons are made of b and d quarks. These heavy particles are unstable and quickly break apart into lighter particles.

The Standard Model predicts that Bs mesons decay into a pair of muons about three times in a billion while for Bd mesons, it occurs thirty times less often. This gives two excellent places to look for small deviations that could reveal the existence of new phenomena not foreseen within the Standard Model.

All theories going beyond the Standard Model come with new particles that would affect how other particles decay, i.e. how they break apart. Decays are very much like making change for a big coin. Imagine a coin of one euro. It can be broken into pieces of 1, 5, 10, 20 or 50 cents Now, say a new 25-cent coin is introduced. An automatic teller would not give change for one euro in a particular way (say with coins of 50, 20, 20 and 10 cents) as often as before just because new possibilities now exist.

By measuring how often a Bs meson decays into muons, scientists were hoping to see the first deviations from the predictions of the Standard Model. On the contrary, the two experiments confirmed this prediction within experimental errors.

CMS, whose name stands for Compact Muon Spectrometer, and LHCb, an experiment designed specifically to study particles containing b quarks, are particularly suited for these types of measurements. CMS got (3.0 +1.0-0.9) x 10-9 and LHCb obtained (2.9 +1.1-1.0) x 10-9, while the Standard Model prediction stands at (3.5 ± 0.3) x 10-9. The significances of the CMS and LHCb signals correspond to 4.3σ and 4.0σ, respectively, which means, the excesses of events that are seen most likely come from signal and not from background. Two other experiments presented new results based on smaller data samples. ATLAS (using a partial data sample) and D0 (final result with their full data sample) and they obtained the same upper limit at 15 x 10-9.Bs-mumu-combo

The results obtained by LHCb and CMS, as well as their combined value, is compared to the prediction from the Standard Model shown by the vertical black line and its theoretical uncertainty (green band).

For Bd decays, 95% confidence level upper limits were set at 7.4 x 10-10 for LHCb while CMS obtained 11 x 10-10. The Standard Model predicts this to be less than 1 x 10-10.

All these values are consistent with the Standard Model predictions but they do not yet rule out new physics. After the LHC resumes operation at higher energy in 2015, the LHC experiments will continue improving their Bs measurements. In particular, they will aim to get a first measurement for Bd mesons instead of an upper limit, and then evaluate the ratio for the Bs and Bd mesons, such that some of the experimental and theoretical uncertainties will cancel out, to obtain an even more precise measurement. Since no deviations were found in the ninth decimal position, it means the experiments need to check the tenth decimal position.

More details can be found on the CMS and LHCb websites.

Pauline Gagnon

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