Quantum Diaries

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

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

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

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

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

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

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

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

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

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

From the CERN Bulletin

Au CERN, nous nous apprêtons non seulement à éclaircir la question fondamentale de l’apparition de la masse après le Big bang, mais nous nous acheminons aussi vers la résolution d’un autre problème essentiel lié à la masse. Le kilogramme est la seule unité fondamentale du système international d’unités (SI) définie officiellement à l’aide d’un prototype matériel de fabrication humaine (un artéfact), plutôt qu’au moyen de grandeurs invariantes de la nature. Si vous estimez que la question vous concerne moins que le prestigieux boson de Higgs, détrompez-vous ! Que diriez-vous si votre balance vous indiquait une valeur différente lorsque vous vous pèserez demain ?

Le prototype international du kilogramme est un cylindre en alliage platine-iridium de 39 mm de hauteur et de 39 mm de diamètre. Il a été usiné en 1878 et se trouve au Bureau international des poids et mesures (BIPM), à Sèvres, près de Paris. À ce jour, alors que toutes les autres unités du système SI ont été redéfinies pour être rattachées à des constantes fondamentales ou à des propriétés atomiques, le kilogramme reste défini par ce bloc de matière.

Un bloc de matière qui doit être nettoyé par des personnes – ou du moins une personne – au risque que des atomes, et donc des fractions de masse, puissent se perdre dans le processus. « Au fil des ans, plusieurs copies ont été produites et déposées dans divers bureaux de métrologie nationaux, explique Ali Eichenberger, physicien à l’Office fédéral suisse de métrologie (METAS). Bien qu’il ne soit pas encore possible de définir la masse du kilogramme de manière absolue, la technologie moderne permet de comparer des masses avec une très haute précision, pouvant atteindre le microgramme. Un examen des divers prototypes nationaux a fait apparaître que leurs masses varient de manière sensible. » De plus, le manque de précision de la définition actuelle du kilogramme a une incidence sur d’autres unités, notamment l’ampère.

En un siècle, des variations de masse significatives ont été enregistrées entre les copies officielles du kilogramme. (Image : METAS).

Un projet de métrologie lancé par le METAS et auquel le CERN participe devrait permettre de régler le problème. L’idée est de construire une balance de watt ultra-précise – un instrument qui compare les puissances mécanique et électrique (voir l’encadré). Grâce à la balance de watt et à ses équations, il est possible de relier l’unité de masse au mètre, à la seconde et à la constante de Planck, bref, à toutes les unités et constantes fondamentales.

« Un élément essentiel de la balance de watt est le circuit magnétique, qui doit être extrêmement stable au cours de la mesure, explique Davide Tommasini, un spécialiste des aimants travaillant dans le groupe Aimants et supraconducteurs au sein du département Technologie du CERN, qui participe directement au projet de balance de watt du METAS. En utilisant un ‘shunt magnétique’ bien dimensionné avec une faible température de Curie, il est possible de réduire radicalement les effets de variation de température. Le circuit doit également fournir un champ magnétique très homogène dans l’ensemble du volume où intervient la mesure. » Le circuit magnétique sera assemblé au CERN. « L’aimant permanent et le cylindre de shuntage devraient bientôt arriver. Nous nous emploierons ensuite à tester les performances du circuit », ajoute Davide Tommasini.

La balance de watt construite par METAS pour effectuer les précédentes mesures de la constante de Planck. Une nouvelle balance est actuellement en développement. (Image : METAS).

« Les exigences associées aux aimants sont extrêmement strictes et nous sommes très heureux que le CERN ait accepté de participer au projet dans le cadre de ses activités de transfert de connaissances, souligne Henri Baumann, un physicien du METAS qui a lancé le projet avec Ali Eichenberger. Cette mesure permettra aussi d’améliorer sensiblement la détermination de la constante de Planck. Les théoriciens du CERN s’en féliciteront certainement ! »

« Ce projet témoigne de la portée qu’ont les compétences et les connaissances spécialisées nécessaires en physique des particules sur des programmes de recherche d’autres disciplines et sur la société dans son ensemble », explique Hartmut Hillemanns, du groupe KT, (Knowledge Transfer ou transfer du savoir) qui mène à bien le volet cernois du projet avec l’équipe scientifique du CERN et interagit avec les autres partenaires.

La nouvelle définition de l’unité de masse devrait être donnée d’ici quelques années. Peut-être aurons-nous alors compris comment la masse se crée au niveau le plus fondamental… Oui, cette fois, nous parlons bien du Higgs !

tiré du Bulletin du CERN

In honour of the season.

As I drive to and from work in Vancouver, I notice that even in winter, the grass is green. In the spring, people are out fertilizing their lawns and in summer watering them (even when they are not allowed to)—mollycoddled grass! They are now even putting grass on the top of buildings. You would almost think that Vancouver exists for the benefit of grass. But it is not just Vancouver; we have wide areas of the world devoted to grass, from bamboo to grain. You would think the world was created for the benefit of grass. After all, the earth is just the right distance from the sun to allow grass to flourish. Farther from the sun, it would be cold and arid like Mars. Closer to the sun it would be hot and sterile like Venus.  Thus, we have what is known in the trade as the acrostic[1] principle: the philosophical argument that observations of the physical universe must be compatible with the preferred status of grass.

As just mentioned, the earth is just at the right distance from the sun for grass to flourish. But it goes beyond that. Carbon is a major component of grass. However, the creation of carbon in stars depends critically on the existence of an excited state in carbon, known as the Hoyle state, with exactly the right energy. If that state were not there, there would be no carbon and hence no grass. The horror of it! Just think, no grass. And it all depends on having the nuclear state at just the right energy.

The Hoyle state is not the only coincidence necessary for the existence of grass. If the fundamental constants of nature, things like the fine structure constant or the gravitational constant (big G) were slightly different, the universe would not support the existence of grass. There are two solutions to this problem. One is to assume that there is an intelligent designer with an inordinate fondness for grass who fine-tuned the universe so grass could exist. Now, there is a minority opinion that it is not grass that he is fond of, but rather beetles (coleoptera) and that he only created grass as a source of feed for beetles. After all, there are the order of a million species of beetles.  But as I just said, the coleopteric principle is distinctly a minority position, but we should be open minded.

The other explanation of the fine tuning of the universe is based on the idea of the multiverse. This is the idea that many different universes exist with all possible values of the physical constants and that we are in the one in which grass is possible.  Again, note the preferred role of grass. The evidence for this scenario, at the present time, is no stronger than that for the existence of the coleopterophillic intelligent designer.

Now one might ask what role consciousness and intelligence have in all this. The answer to that is fairly self-evident. The main role of consciousness and intelligence is the development of civilization, and the main role of civilization is the development of agriculture. It should be obvious to even the most obtuse reader that the main purpose of agriculture is to permit grass to more effectively compete with trees. Just think of the extent to which farmers have replaced forests with grassland. The bringing of European “civilization” to North America had as its main effect, the replacement of forest with grassland.  It had some unfortunate side effects, like the creation of the United States of America, but what is more important—people or grass?

As further evidence of the agrostic principle, I note that it provides the only possible explanation for the existence of golf courses and cricket pitches. The very idea of grown men or women hitting a ball with a club to prove their virility is silly.  Now artificial turf may be considered as evidence against the agrostic principle, but artificial turf seems to be a passing fad. In just 13 years, between 1992 and 2005, the National Baseball League went from having half of its teams (6 of 12) using artificial turf to all of them – now up to 16 – playing on natural grass. As for football (soccer), artificial turf is widely banned. Enough said.

The agrostic principle also highlights flaws in ancient Greek philosophy.  Plato believed that the “good” was contemplating his ideals or ideas. That is incorrect; the greatest good is cultivating and contemplating grass. Like Euclid’s postulates, that should be self-evident. That the smoking of grass is the greatest good is a corruption of Epicurus’s teaching. Rather, he was the first of the new atheists. The Sophists, on the other hand, where the first post-modernists and believed that it was impossible to decide if contemplating or smoking grass was the greatest good. After smoking a few joints, the latter is probably true. Socrates believed that nothing could be learned from nature. Perhaps if he had spent more time cultivating and contemplating grass, he would not have been compelled to drink hemlock. However, Aristotle may have been onto something with his final cause or teleology. Evolution shows its bareness by failing to recognize that consciousness and intelligence arose due to the teleological purpose (final cause) of helping grass compete with trees. This is probably the best example of the need for Aristotle’s final cause that can be found in nature. Unfortunately, Aristotle starting worrying about essences rather than cultivating and contemplating grass. Thus, the Greek civilization decayed. And my wife wants me to replace the lawn with a garden. The end of western civilization is in sight.

The agrostic principle has some naysayers. Douglas Adams gives the example in his Hitch-hikers Guide to the Galaxy of the puddle which observed how well it fitted the hole it was in and concluded that the hole and the universe where created expressly for its benefit. It was consequently quite surprised and distressed when it evaporated. Imagine; the gall of Adams using satire to attack the agrostic principle. Now, of course, the properties of the hole can be deduced from the properties of the puddle, but this should not be used to infer the universe was not created for the sole benefit of the puddle. Some people have followed the example of Adams’s puddle and claimed that since humans nicely fit a hole in the universe, the universe was created for their benefit (this is sometimes call the anthropic principle). These people will probably be surprised when humans go extinct. The superiority of the agrostic principle to the anthropic principle is shown by the observation that while homo spaiens have existed for about 200,000 years, grass tickled the feet of dinosaurs over sixty million years ago. And grass will probably still exist after humans have, through sheer stupidly, destroyed themselves and have been replaced by a group with less intelligence and more wisdom, perhaps the coleoptera.

Additional posts in this series will appear most Friday afternoons at 3:30 pm Vancouver time. To receive a reminder follow me on Twitter: @musquod.

Ladies & Gentlemen, Protons & Neutrons,

The Large Hadron Collider’s Accelerator Division has successfully collided, for the first time, two 4 TeV proton beams! Congratulations to all who made this possible. I can promise that everyone is looking forward to what may be discovered!

Now enjoy some images courteous of @lhcstatus and @ATLASExperiment.

@ATLASExperiment: First 8 TeV collisions from the #LHC in the #ATLAS detector! Thank you LHC operators. Next week: #Physics! @CERN http://pic.twitter.com/ItAbbl64 (Note: Yes, this is actually an 8 TeV collision.)

@lhcstatus: #LHC 17:13:43: preparing collisions (shortly) pic.twitter.com/bDrbjVt9

@lhcstatus: #LHC 17:33:43: preparing collisions http://pic.twitter.com/V3GWWsDW

@ATLASExperiment: Monitoring the position of the beams in #ATLAS as #LHC prepares for 8 TeV collisions. #CERN #Physics http://pic.twitter.com/m1WAitei

Happy Colliding!

– richard (@bravelittlemuon)

PS: Data-taking for physics starts next week. Happy Friday.

The LHC will start colliding beams again in a few weeks after the traditional winter shutdown. 2012 could be THE year. This is not just idle speculation. Hints of the elusive Higgs boson may have been seen in both multipurpose experiments at the LHC (ATLAS and CMS) as well as the Tevatron full luminosity analysis at D0 and CDF. To be clear from the start these hints are – as of this writing – not significant enough to make a claim to have seen anything yet. The level of statistical significance has been described as ‘interesting and tantalizing’ though not conclusive. What makes people so excited about this result is that unlike many other episodes of temporary excitement there is a reasonable level of agreement BETWEEN experiments.

Let me explain. For a so called “low mass” Higgs boson (110-130 GeV) the most prominent decay channel is to the heaviest available fermions which the boson can decay into and conserve energy and momentum- a b and anti-bquark pair. Unfortunately, at both the Tevatron and the LHC the backgrounds to this channel are tremendously large. So its not just the decay branching ratios that are important but also how large the backgrounds are, the difficulty of reconstructing the final state, and the experimental resolution on the observed objects which matter (among other things).

As it turns out the channels with the largest expected statistical power at the LHC in this range are decays into two photons, two W bosons, or two Z bosons. In fact in December of 2011 both the ATLAS and CMS collaborations saw small bumps on an otherwise fastly falling decay spectrum in the diphoton resonant search. In that channel one looks for pairs of photons and reconstructs the invariant mass. If the pair of photons came from the production of a heavy particle you would expect to see a clustering of events around a particular mass. An excess of events was also seen in both experiments in the gauge boson decays (pairs of W and Z particles) around the same mass region.

All of this was known since last December when both collaborations made their results public with the full 2011 datasets for the first time. Then at the annual Moriond conference in 2012 the Tevatron experiments released their results with their full datasets. Because of the different center of mass energy the cross-sections for both signal and background processes are different. It turns out that the they too see an excess of events in the region consistent with the LHC experiments but in the so called “associated Higgs production” where the Higgs is produced along with a W or Z boson. This allows them to make use of the large branching ratio into b and anti-bquark pairs while reducing the background due to the presence of the gauge boson.

So what does this all mean? Well – for now I am optimistically cautious. The exciting thing for me as a part of the ATLAS collaboration is that we will start taking data in a few weeks at an even higher center of mass energy – 8 TeV. This year we could get even more data than we did in 2011 which will give us the opportunity to possibly confirm our preliminary results and make a discovery or exclude the existence of the particle which may have even more significant impact to our field.

One of the roles I currently play in the ATLAS experiment is the coordination of the muon high level trigger. Since we collide protons at the LHC at a rate that is much much higher than we can record to tape for analysis we have to choose which events we can keep. We have to have enormous rejection power on the order of accepting 1 event in many million in order to accomplish this. But on the other hand we have to be very careful which events we choose to save! We must maintain very high efficiency for triggering on the events we care about the most. This is becoming an ever more exciting and difficult game as the event rate gets larger and larger and the stakes get higher and higher.

Looking forward to an exciting year!