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Posts Tagged ‘SPS’

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.

H-bottle

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.

Accelerators

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.

LS1-schedule-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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Tout un Univers à découvrir

Wednesday, January 15th, 2014

Les deux dernières années ont été plutôt exceptionnelles pour le CERN. En 2012, les expériences CMS et ATLAS  ont découvert le boson de Higgs, confirmant le mécanisme élaboré 48 ans auparavant par Robert Brout, François Englert et Peter Higgs. Et en 2013, Englert et Higgs se sont vus décerner le Prix Nobel de physique pour leurs travaux.

2014 sera également une année spéciale, puisque le CERN célébrera ses 60 ans. Mais au-delà de son anniversaire, cette année le CERN prépare le Grand collisionneur de hadrons (LHC) à explorer de nouveaux territoires.

Avec la découverte du boson de Higgs, nous avons complété le Modèle Standard, la théorie actuelle qui explique de quoi toute la matière visible est faite. Mais ce type de matière ne compte que pour 5 % du contenu total de l’Univers. L’existence de matière sombre nous prouve que le modèle actuel est incomplet. Jusqu’ici, l’analyse des données prises à 8 TeV ne révèle pas pour l’instant de traces de cette matière sombre. Pour pousser nos recherches plus loin et plus vite, nous devons augmenter la portée du LHC en allant à plus haute énergie.

C’est pourquoi depuis février 2013 tous les accélérateurs et expériences du CERN sont à l’arrêt afin d’effectuer des travaux de maintenance et de consolidation. Ceci se poursuivra en 2014 pour le LHC, mais plusieurs accélérateurs du complexe du CERN reprendront du service dès cet été.

H-bottle

Le point de départ de la chaîne d’accélérateurs est une simple bouteille d’hydrogène. Les électrons sont arrachés aux atomes d’hydrogène par un champ électrique pour ne laisser que les protons. Ceux-ci sont ensuite accélérés dans un petit accélérateur linéaire (LINAC 2 en bas, au centre du diagramme ci-dessous). L’anneau d’ions de basse énergie (LEIR) joue le même rôle, mais avec des ions lourds.

Accelerators

Les protons obtiennent une poussée supplémentaire dans le Booster avant d’être injectés dans le plus vieil accélérateur du CERN encore en service, le synchrotron à protons (PS). Puis les protons sont dirigés vers le supersynchrotron à protons (SPS) où ils atteignent une énergie de 450 GeV (soit 450 milliards d’électronvolts). C’est l’étape finale avant l’injection dans le LHC où des énergies près de trente fois plus grandes seront atteintes en 2015, soit 13 TeV.

Plans-2014-fr
Les faisceaux issus de la chaîne d’accélérateur alimentent aussi d’autres zones expérimentales comme ISOLDE et n-TOF où un très grand nombre d’expériences nucléaires prennent place. D’autres protons sont dirigés vers une cible pour produire des antiprotons pour le Décélérateur d’Antiprotons (AD), un laboratoire consacré à l’étude de l’antimatière. Ces expériences reprendront toutes leurs activités en 2014.
Tous les travaux de consolidation du LHC et de ses expériences s’effectuent en parallèle. ATLAS et CMS prévoient d’achever leurs travaux sur les détecteurs avant novembre. ALICE sera prêt début décembre et LHCb début janvier 2015.

Dans le même temps, tous les physicien-ne-s qui ne sont pas impliqué-e-s dans ces travaux finalisent les analyses des données prises jusqu’en 2013, préparent de nouvelles simulations à plus haute énergie, améliorent les algorithmes de reconstruction des données ou rendent les critères de sélection du système de prise de données plus performant. Tout le monde doit relever le défi d’être prêt à traiter plus de données récoltées à plus haute énergie. Tout ça dans l’espoir que nous serons peut-être récompensé-e-s encore une fois par de nouvelles découvertes puisqu’il reste encore tout un monde à découvrir.

Pauline Gagnon

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Early last week Margaret Thatcher, former British Prime Minister, passed away, aged 87. She was a charismatic figure who was known internationally for being a strong and decisive leader. She had close political ties with President Ronald Reagan, she opposed the communist policies in Eastern Europe, and she was skeptical of increasing integration of the UK with Western Europe. Her actions and legacy are entwined with the global political stage at the time. However, in the UK she was very divisive and at times controversial, and even to this day there is a mixture of high praise a bitter resentment about her policies. Much has been said about her legacy over the past few days, and I think that, regardless of one’s own views, one of the best things we can say about Thatcher is that she knew what her vision was, and she pursued it with a great deal of energy and enthusiasm.

Thatcher, the politician (Mirror)

Thatcher, the politician (Mirror)

During her undergraduate years, Thatcher was a chemist at the University of Oxford. It was only later that she studied law and became a politician, so from her very early career she had an appreciation for science. She knew about the care and attention needed to make discoveries, the frustration of waiting for data, and the need for peer review and skepticism. Given her status as an international leader, she had the opportunity to visit CERN in the early 1980s, but as a scientist she took so much more away from the visit than we could have expected.

Thatcher, the chemist (popsci)

Thatcher, the chemist (popsci)

She’d asked to be treated like a fellow scientist, and her questions showed that she had taken her background reading about CERN seriously. She asked why the proposed accelerator, LEP, would be circular and not linear. This is not an easy question to answer unless the person asking has knowledge about how accelerators work. After a discussion with Herwig Schopper, then Director General, she came back to the UK as an ambassador for CERN and LEP was approved in the UK shortly afterwards. One of her questions was very astute. When told that the LEP tunnel would be the last at CERN she knew from experience that scientists will usually want to go further with their research and in particle physics at the energy frontier, further usually means larger. It’s true that CERN has reused the LEP tunnels for the LHC, but there are also proposals for even larger projects that will probe even higher center of mass energies.

Thatcher must have made a very good impression on Schopper during her visit. A recent Scientific American article has revealed that she was told about the discovery of the W and Z bosons before the information was made public. This letter shows that Schopper kept his promise and trusted Thatcher to keep the tantalizing and preliminary evidence to herself:

Schopper writes to Thatcher (Scientific American)

Schopper writes to Thatcher (Scientific American)

When the news of the \(W\) boson discovery was public she wrote to Peter Kalmus of Queen Mary College, London, to offer her congratulations. Naturally she made a point to mention that there was a significant British effort behind the discovery:

Thatcher's letter to Kalmus

Thatcher's letter to Kalmus

On the one hand, Thatcher was genuinely excited about CERN and the research, but on the other she was a fiscally conservative politician with monetarist policies and she had to defend the spending to her colleagues, and to herself. She had to make sure that the physicists at CERN were using the funding effectively, and delivering high quality scientific results for the spending. During a visit to the Super Proton Synchrotron she spoke John Ellis, who introduced himself as a theoretical physicist. The conversation continued:

Thatcher: “What do you do?”
Ellis: “Think of things for the experiments to look for, and hope they find something different.”
Thatcher: “Wouldn’t it be better if they found what you predicted?”
Ellis: “Then we would not learn how to go further!”

Once again Thatcher knew what question to ask, and Ellis knew what answer to give. Thatcher seemed convinced and knew that the people at CERN has the right attitude when it comes to discovery and use of public money. You can see some media coverage of her visit to the UA1 (Underground Area 1) site on the CERN Document Server.

In 1993, three years after Thatcher left office, David Miller from UCL came up with an analogy for the Higgs field where Thatcher played the central role. Essentially we can think of the Higgs field like a room full of people milling around at a cocktail party. Someone famous and popular enters the room, and all of a sudden people crowd around, making this person’s journey through the room harder. They take longer to get up to a good walking speed, and when they are walking they become harder to stop. That’s essentially what mass is- a measure of hard it is to change an object’s velocity. The analogy goes further, to include rumors being spread from the vicinity of this famous person. They would spread in small groups of people, and each group would have its own “mass”, which is what the Higgs boson is, it’s just an excitation of the field in the presence of matter. Who was the famous person in this analogy? Margaret Thatcher, of course!

Thatcher and the Higgs field (Quantum Tangents)

Thatcher and the Higgs field (Quantum Tangents)

So her legacy with CERN is one of a scientist and a politician. She was genuinely excited to see the discoveries take place, she met with the scientists personally and interacted with them as another scientist. She took the time to understand the questions and answers, and even challenged the physicists with more questions. At the same time she put the projects in context. She had to defend the experiments, so she had to challenge the physicists to give her the information she needed to get the support from the UK. In a sense she knew the need for public outreach, to open up CERN’s scientific program to scrutiny from the public so that when we want to push back the frontiers even further we can count on their support.

If we’re to keep pursuing scientific discoveries in the future, we need scientifically literate and inspired politicians. It would be tempting to say that they are becoming more and more rare, but in reality I think things are more favorable than they have been before. With the recent discoveries we’re in a golden age of physics that has made front page news. Multimedia outlets and the internet have helped spread the good word, so science is high in the public consciousness, and justifying further research is becoming easier. However before the modern internet era and the journalistic juggernaut that comes to CERN each time there’s a big announcement it fell on the shoulders of a few people, and Thatcher was one of them.

(I would like to thank John Ellis for providing help with his quote, and for giving the best answer when asked the question!)

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