Quantum Diaries

Posts Tagged ‘CERN’

Latest news from outer space on dark matter

Friday, December 16th, 2016

To celebrate the first five years of operation on board the International Space Station, Professor Sam Ting, the spokesperson for the Alpha Magnetic Spectrometer (AMS-02) Collaboration just presented their latest results at a recent seminar held at CERN. With a sample of 90 million events collected in cosmic rays, they now have the most precise data on a wide range of particles found in outer space.

ams-02

source: ©NASA

Many physicists wonder if the AMS Collaboration will resolve the enigma on the origin of the excess of positrons found in cosmic rays. Positrons are the antimatter of electrons. Given that we live in a world made almost uniquely of matter, scientists have been wondering for more than a decade where these positrons come from. It is well known that some positrons are produced when cosmic rays interact with the interstellar material. What is puzzling is that more positrons are observed than what is expected from this source alone.

Various hypotheses have been formulated to explain the origin of these extra positrons. One particularly exciting possibility is that these positrons could emanate from the annihilation of dark matter particles. Dark matter is some form of invisible matter that is observed in the Universe mostly through its gravitational effects. Regular matter, everything we know on Earth but also everything found in stars and galaxies, emits light when heated up, just like a piece of heated metal glows.

Dark matter emits no light, hence its name. It is five times more prevalent than regular matter. Although no one knows, we suspect dark matter, just like regular matter, is made of particles but no one has yet been able to capture a particle of dark matter. However, if dark matter particles exist, they could annihilate with each other and produce an electron and a positron, or a proton and antiproton pair. This would at long last establish that dark matter particles exist and reveal some clues on their characteristics.

An alternative but less exotic explanation would be that the observed excess of positrons comes from pulsars. Pulsars are neutron stars with a strong magnetic field that emit pulsed light. But light is made of photons and photons can also decay into an electron and a positron. So both the pulsar and the dark matter annihilation provide a plausible explanation on the source of these positrons.

To tell the difference, one must measure the energy of all positrons found in cosmic rays and see how many are found at high energy. This is what AMS has done and their data are shown on the left plot below, where we see the flux of positrons (vertical axis) found at different energies (horizontal axis). The flux combines the number of positrons found with their energy cube. The green curve gives how many positrons are expected from cosmic rays hitting the interstellar material (ISM).

If the excess of positrons were to come from dark matter annihilation, no positron would be found with an energy exceeding the mass of the dark matter particle. They would have an energy distribution similar to the brown curve on the plot below as expected for dark matter particles having a mass of 1 TeV, a thousand times heavier than a proton. In that case, the positrons energy distribution curve would drop off sharply. The red dots represent the AMS data with their experimental errors shown by the vertical bars. If, on the other end, the positrons came from pulsars, the drop at high energy would be less pronounced.

ams-2016

source: AMS Collaboration

The name of the game is therefore to figure out precisely what is happening at high energy. But there are much fewer positrons there, making it very difficult to see what is happening as indicated by the large error bars attached to the data points at higher energy. These indicate the size of the experimental errors.

But by looking at the fraction of positrons found in all data collected for electrons and positrons (right plot above), some of the experimental errors cancel out. AMS has collected over a million positrons and 16 million electrons. The red dots on the right plot show the fraction of positrons found in their sample as a function of energy. Given the actual precision of these measurements, it is still not completely clear if this fraction is really falling off at higher energy or not.

The AMS Collaboration hopes however to have enough data to distinguish the two hypotheses by 2024 when the ISS will cease operation. These projections are shown on the next two plots both for the positrons flux (left) and the positron fraction (right). As it stands today, both hypotheses are still possible given the size of the experimental errors.

ams-2024

source: AMS Collaboration

There is another way to test the dark matter hypothesis. By interacting with the interstellar material, cosmic rays produce not only positrons, but also antiprotons. And so would dark matter annihilations but pulsars cannot produce antiprotons. If there were also an excess of antiprotons in outer space that could not be accounted for by cosmic rays, it would reinforce the dark matter hypothesis. But this entails knowing precisely how cosmic rays propagate and interact with the interstellar medium.

Using the AMS large sample of antiprotons, Prof. Sam Ting claimed that such excess already exists. He showed the following plot giving the fraction of antiprotons found in the total sample of protons and antiprotons as a function of their energy. The red dots represent the AMS measurements, the brown band, some theoretical calculation for cosmic rays, and the blue band, what could be coming from dark matter.

$antiproton-fraction$

source: AMS Collaboration

This plot clearly suggests that more antiprotons are found than what is expected from cosmic rays interacting with the interstellar material (ISM). But both Dan Hooper and Ilias Cholis, two theorists and experts on this subject, strongly disagree, saying that the uncertainty on this calculation is much larger. They say that the following plot (from Cuoco et al.) is by far more realistic. The pink dots represent the AMS data for the antiproton fraction. The data seem in good agreement with the theoretical prediction given by the black line and grey bands. So there are no signs of a large excess of antiprotons here. We need to wait for a few more years before the AMS data and the theoretical estimates are precise enough to determine if there is an excess or not.

antiprotons-theorie

source: Cuoco, Krämer and Korsmeier, arXiv:1610.03071v1

The AMS Collaboration could have another huge surprise is stock: discovering the first antiatoms of helium in outer space. Given that anything more complex than an antiproton is much more difficult to produce, they will need to analyze huge amounts of data and further reduce all their experimental errors before such a discovery could be established.

Will AMS discover antihelium atoms in cosmic rays, establish the presence of an excess of antiprotons or even solve the positron enigma? AMS has lots of exciting work on its agenda. Well worth waiting for it!

Pauline Gagnon

To find out more about particle physics and dark matter, check out my book « Who Cares about Particle Physics: making sense of the Higgs boson, the Large Hadron Collider and CERN ».

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Tags: AMS, antiproton, CERN, cosmic rays, dark matter, ISS, positron
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Du beau pain sur la planche pour la matière sombre

Friday, December 16th, 2016

Pauline Gagnon

Pour célébrer les cinq premières années d’opération à bord de la Station spatiale internationale, le Professeur Sam Ting, porte-parole de la Collaboration Alpha Magnetic Spectrometer (AMS-02) vient de présenter leurs derniers résultats lors d’un récent séminaire tenu au CERN. Avec plus de 90 millions d’évènements recueillis dans les rayons cosmiques, ce groupe dispose des données les plus précises sur une vaste gamme de particules trouvées dans l’espace.

ams-02

source: ©NASA

La question qui intrigue de nombreux scientifiques est de savoir s’ils pourront résoudre l’énigme de l’origine de l’excès de positrons trouvés dans les rayons cosmiques. Les positrons sont l’antimatière des électrons. Étant donné que nous vivons dans un monde fait presque uniquement de matière, les scientifiques se demandent depuis plus d’une décennie d’où émanent ces positrons. Il est bien connu que des positrons sont produits lorsque les rayons cosmiques interagissent avec la matière interstellaire mais on en observe bien plus que ce à quoi on s’attendait de cette seule source.

Des hypothèses diverses ont été formulées pour expliquer l’origine de ces positrons excédentaires. Une des plus fascinantes suggère que ces positrons pourraient venir de l’annihilation de particules de matière sombre. La matière sombre est une nouvelle forme de matière invisible qu’on détecte dans l’Univers par ses effets gravitationnels. La matière régulière, tout ce que nous voyons sur la Terre, mais aussi dans les étoiles et les galaxies, émet de la lumière lorsque chauffée, tout comme une pièce métallique irradie à haute température.

La matière sombre n’émet aucune lumière, d’où son nom. Elle est cinq fois plus répandue que la matière régulière. Personne ne le sait encore mais on soupçonne que cette matière, tout comme la matière ordinaire, soit faite de particules, mais on n’a toujours pas capturé de particules de matière sombre. Mais si de telles particules existaient, elles pourraient s’annihiler entre elles, produisant des électrons et des positrons, ou des paires de protons et d’antiprotons. Si un tel processus était établi, cela confirmerait enfin l’existence de particules de matière sombre et révèlerait quelques indices sur leurs caractéristiques.

Une explication alternative mais moins exotique serait que l’excès observé de positrons provienne de pulsars. Les pulsars sont des étoiles à neutrons ayant un fort champ magnétique et qui émettent de la lumière pulsée. Mais la lumière est faite de photons et les photons peuvent eux aussi produire des paires d’électrons et de positrons. Donc, les pulsars tout comme l’annihilation de matière sombre, fournissent une explication plausible quant à la source de ces positrons.

Pour les distinguer, il faut mesurer l’énergie des positrons captés dans les rayons cosmiques et voir combien on en trouve à haute énergie. C’est ce que AMS a fait et leurs résultats sont visibles dans le graphe de gauche ci-dessous où nous voyons le flux de positrons (axe vertical) trouvé à une énergie particulière (axe horizontal). Le flux combine le nombre de positrons trouvés avec leur énergie au cube. La courbe en vert donne le nombre de positrons produits lorsque des rayons cosmiques frappent de la matière interstellaire (ISM).

Si l’excès de positrons devait venir de l’annihilation de matière sombre, on ne trouverait aucun positron au-delà de l’énergie correspondant à la masse des particules de matière sombre. Ils auraient une distribution d’énergie semblable à la courbe en brun sur le graphe ci-dessous tel que prédit pour des particules de matière sombre ayant une masse de 1 TeV, soit mille fois plus lourd qu’un proton. Dans ce cas, la courbe de distribution d’énergie des positrons chuterait rapidement. Les points en rouge représentent les données d’AMS avec leurs erreurs expérimentales indiquées par les barres verticales. Par contre, si les positrons venaient de pulsars, la chûte à haute énergie serait moins prononcée.

ams-2016

source: Collaboration AMS

Toute la difficulté consiste à comprendre précisément leur comportement à haute énergie. Mais comme on y trouve moins de positrons, il est beaucoup plus difficile de voir ce qu’il en est comme l’indiquent les larges marges d’erreur associées aux mesures faites à plus haute énergie.

Mais si on mesure plutôt la fraction de positrons trouvés dans les données en combinant positrons et électrons, certaines des erreurs expérimentales s’annulent. AMS a rassemblé plus d’un million de positrons et 16 millions d’électrons. Les points en rouge sur le graphe de droite ci-dessus montrent la fraction de positrons trouvée dans leur échantillon en fonction de leur énergie. Malgré les pas de géants accomplis, la précision actuelle de ces mesures ne permet toujours pas d’établir clairement si cette fraction tombe abruptement à haute énergie ou pas.

La Collaboration AMS espère toutefois avoir assez de données pour distinguer les deux hypothèses d’ici à 2024, date à laquelle la Station Spatiale Internationale cessera ses opérations. On peut voir ces projections sur les deux graphes suivants tant pour le flux de positrons (à gauche) que pour la fraction de positrons (à droite). À ce jour, les deux hypothèses sont toujours valides étant donné la taille des erreurs expérimentales.

ams-2024

source: Collaboration AMS

L’hypothèse de la matière sombre peut aussi être testée d’une autre façon. En interagissant avec la matière interstellaire, les rayons cosmiques produisent non seulement des positrons mais aussi des antiprotons. Les annihilations de matière sombre pourraient aussi en produire mais pas les pulsars. Il faut donc déterminer s’il y a ou pas plus d’antiprotons dans l’espace que ce que les rayons cosmiques peuvent produire. Si c’était établi, ce serait un argument de plus contre l’hypothèse des pulsars. Mais pour ce faire, il faut savoir précisément comment les rayons cosmiques se propagent et interagissent avec la matière interstellaire.

S’appuyant sur le vaste échantillon d’antiprotons recueillis par AMS, le Prof. Sam Ting a soutenu qu’un tel excès existe, présentant le graphe suivant à l’appui. On y voit la fraction d’antiprotons trouvés dans l’échantillon total de protons et des antiprotons en fonction de leur énergie. Les points en rouge représentent les mesures d’AMS, la bande brune, les calculs théoriques pour les rayons cosmiques et la bande bleue, ce qui pourrait venir de la matière sombre.

$antiproton-fraction$

source: Collaboration AMS

Ce graphe suggère fortement un surplus d’antiprotons par rapport à ce que l’on s’attend des rayons cosmiques interagissant avec la matière interstellaire (ISM). Mais tant Dan Hooper qu’Ilias Cholis, deux théoriciens experts en la matière, s’objectent carrément, disant que l’incertitude sur les prédictions théoriques sont beaucoup plus grandes que ce que ce graphe suggère. Ils soutiennent que le graphe suivant (de Cuoco etal.) est de loin plus réaliste. Les points en rose représentent les données d’AMS pour la fraction d’antiprotons et le trait en noir, les prédictions théoriques avec leur marge d’erreur. Les deux concordent ou presque, suggérant l’absence de tout excès. Nous devrons patienter encore quelques années avant que les données d’AMS et les prédictions théoriques soient assez précises pour savoir s’il y a excès ou pas.

antiprotons-theorie

source : Cuoco, Krämer and Korsmeier, arXiv:1610.03071v1

La Collaboration AMS pourrait nous réserver une autre belle surprise : la découverte d’antiatomes d’hélium dans l’espace. Étant donné l’extrême difficulté à produire une particule d’antimatière plus complexe qu’un antiproton, les scientifiques d’AMS devront trier d’énormes quantités de données et réduire toutes les erreurs expérimentales encore davantage avant qu’une telle découverte ne puisse être établie.

La découverte d’antihélium, ou celle d’un excès d’antiprotons ou encore la résolution de l’énigme des positrons, tout cela vaut bien la peine d’attendre encore quelques années. AMS a du beau pain sur la planche!
Pauline Gagnon

Pour en savoir plus sur la physique des particules et la matière sombre, consultez mon livre “Qu’est-ce que le boson de Higgs mange en hiver et autres détails essentiels“.

Pour être au courant des nouveaux blogs, suivez-moi sur Twitter: @GagnonPauline ou inscrivez-vous sur cette liste de distribution

Tags: AMS, antiproton, CERN, matière noire, matière sombre, positron
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Particles over Politics: The More the Merrier

Monday, August 1st, 2016

Ricky Nathvani

Earlier last month, Romania became the 22nd Member State of the European Organisation for Nuclear Research, or CERN, home to the world’s most powerful atom-smasher. But the hundred Romanian scientists working on experiments there have already operated under a co-operation agreement with CERN for the last 25 years. So why have Romania decided to commit the money and resources needed to become a full member? Is this just bureaucratic reshuffling or the road to a more fruitful collaboration between scientists?

Image: CERN

On 18th July, Romania became a full member state of CERN. In doing so, it joined twenty one other countries, which over the years have created one of the largest scientific collaborations in the world. Last year, the two largest experimental groups at CERN, ATLAS and CMS, broke the world record for the total number of authors on a research article (detailing the mass of the Higgs Boson).

To meet its requirements for becoming a member, Romania has committed $11mil USD towards the CERN budget this year, three times as much as neighbouring member Bulgaria and more than seven times as much as Serbia, which holds Associate Membership, aiming to follow in Romania’s footsteps. In return, Romania now holds a place on CERN’s council, having a say in all the major research decisions of the ground-breaking organization where the forces of nature are probed, antimatter is created and Higgs Bosons discovered.

Romania’s accession to the CERN convention marks another milestone in the organisation’s history of international participation over the last sixty years. In that time it has built bridges between the members of nations where diplomacy and international relations were less than favourable, uniting researchers from across the globe towards the goal of understanding the universe on its most fundamental level.

CERN was founded in 1954 with the acceptance of its convention by twelve European nations in a joint effort for nuclear research, the year where “nuclear research” included the largest ever thermonuclear detonation by the US in its history and the USSR deliberately testing the effects of nuclear radiation from a bomb on 45,000 of its own soldiers. Despite the Cold War climate and the widespread use of nuclear physics as a means of creating apocalyptic weapons, CERN’s founding convention alongside UNESCO, which member states adhere to today, states:

“The Organization shall provide for collaboration among European States in nuclear research of a pure scientific and fundamental character…The Organization shall have no concern with work for military requirements,”

The provisional Conseil Européen pour la Recherche Nucléaire (European Council for Nuclear Research) was dissolved and its legacy was carried by the labs built and operated under the convention it had laid and the name it bore: CERN. Several years later in 1959, the British director of the Proton Synchrotron division at CERN, John Adams, received a gift of vodka from Soviet scientist Vladimir Nikitin of the Dubna accelerator, just north of Moscow, and at the time the most powerful accelerator in the world.

The vodka was to be opened in the event the Proton Synchrotron accelerator at CERN was successfully operated at an energy greater than Dubna’s maximum capacity: 10 GeV. It more than doubled the feat, reaching 24 GeV, and with the vodka dutifully polished off, the bottle was stuffed with a photo of the proton beam readout and sent back to Moscow.

John Adams, holding the empty vodka bottle in celebration of the Proton Synchroton’s successful start (Image: CERN-HI-5901881-1 CERN Document Server)

Soviet scientists contributed more than vodka to the international effort in particle physics. Nikitin would later go on to work alongside other soviet and US scientists in a joint effort at Fermilab in 1972. Over the next few decades, ten more member states would join CERN permanently, including Israel, its first non-European member. On top of this, researchers at CERN now join from four associate member nations, four observer states (India, Japan, USA and Russia) and holds a score of cooperation agreements with other non-member states.

While certainly the largest collaboration of this kind, CERN is certainly no longer unique in being a collaborative effort in particle physics. Quantum Diaries is host to the blogs of many experiments all of whom comprise of a highly diverse and internationally sourced research cohort. The synchrotron lab for the Middle East, SESAME, expected to begin operation next year, will involve both the Palestinian and Israeli authorities with hopes it “will foster dialogue and better understanding between scientists of all ages with diverse cultural, political and religious backgrounds,”. It was co-ordinated in part, by CERN.

I have avoided speaking personally so far, but one needs to address the elephant in the room. As a British scientist, I speak from a nation where the dust is only just settling on the decision to cut ties with the European Union, against the wishes of the vast majority of researchers. Although our membership to CERN will remain secure, other projects and our relationship with european collaborators face uncertainty.

While I certainly won’t deign to give my view on the matter of a democratic vote, it is encouraging to take a look back at a fruitful history of unity between nations and celebrate Romania’s new Member State status as a sign that that particle physics community is still, largely an integrated and international one. In the short year that I have been at University College London, I have not yet attended any international conferences, yet have had the pleasure to meet and learn from visiting researchers from all over the globe. As this year’s International Conference on High Energy Physics kicks off this week, (chock-full of 5-σ BSM discovery announcements, no doubt*), there is something comforting in knowing I will be sharing my excitement, frustration and surprise with like-minded graduate students from the world over.

Kind regards to Ashwin Chopra and Daniel Quill of University College London for their corrections and contributions, all mistakes are unreservedly my own.
*this is, obviously, playful satire, except for the case of an announcement in which case it is prophetic foresight.

Tags: Brexit, CERN, collaboration, ICHEP, International, Romania, vodka
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Enough data to explore the unknown

Friday, June 17th, 2016

Pauline Gagnon

The Large Hadron Collider (LHC) at CERN has already delivered more high energy data than it had in 2015. To put this in numbers, the LHC has produced 4.8 fb^-1, compared to 4.2 fb^-1last year, where fb^-1 represents one inverse femtobarn, the unit used to evaluate the data sample size. This was achieved in just one and a half month compared to five months of operation last year.

With this data at hand, and the projected 20-30 fb^-1 until November, both the ATLAS and CMS experiments can now explore new territories and, among other things, cross-check on the intriguing events they reported having found at the end of 2015. If this particular effect is confirmed, it would reveal the presence of a new particle with a mass of 750 GeV, six times the mass of the Higgs boson. Unfortunately, there was not enough data in 2015 to get a clear answer. The LHC had a slow restart last year following two years of major improvements to raise its energy reach. But if the current performance continues, the discovery potential will increase tremendously. All this to say that everyone is keeping their fingers crossed.

If any new particle were found, it would open the doors to bright new horizons in particle physics. Unlike the discovery of the Higgs boson in 2012, if the LHC experiments discover a anomaly or a new particle, it would bring a new understanding of the basic constituents of matter and how they interact. The Higgs boson was the last missing piece of the current theoretical model, called the Standard Model. This model can no longer accommodate new particles. However, it has been known for decades that this model is flawed, but so far, theorists have been unable to predict which theory should replace it and experimentalists have failed to find the slightest concrete signs from a broader theory. We need new experimental evidence to move forward.

Although the new data is already being reconstructed and calibrated, it will remain “blinded” until a few days prior to August 3, the opening date of the International Conference on High Energy Physics. This means that until then, the region where this new particle could be remains masked to prevent biasing the data reconstruction process. The same selection criteria that were used for last year data will then be applied to the new data. If a similar excess is still observed at 750 GeV in the 2016 data, the presence of a new particle will make no doubt.

Even if this particular excess turns out to be just a statistical fluctuation, the bane of physicists’ existence, there will still be enough data to explore a wealth of possibilities. Meanwhile, you can follow the LHC activities live or watch CMS and ATLAS data samples grow. I will not be available to report on the news from the conference in August due to hiking duties, but if anything new is announced, even I expect to hear its echo reverberating in the Alps.

Pauline Gagnon

To find out more about particle physics, check out my book « Who Cares about Particle Physics: making sense of the Higgs boson, the Large Hadron Collider and CERN », which can already be ordered from Oxford University Press. In bookstores after 21 July. Easy to read: I understood everything!

CMS-lumi-17juin

The total amount of data delivered in 2016 at an energy of 13 TeV to the experiments by the LHC (blue graph) and recorded by CMS (yellow graph) as of 17 June. One fb^-1 of data is equivalent to 1000 pb^-1.

Tags: 750 GeV, ATLAS, CERN, CMS, data, LHC, luminosity, new particle, new physics
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Assez de données pour explorer l’inconnu

Friday, June 17th, 2016

Pauline Gagnon

Le Grand collisionneur de hadrons (LHC) du CERN a déjà produit depuis avril plus de données à haute énergie qu’en 2015. Pour quantifier le tout, le LHC a produit 4.8 fb^-1 en 2016, à comparer aux 4.2 fb^-1 de l’année dernière. Le symbole fb^-1 représente un femtobarn inverse, l’unité utilisée pour évaluer la taille des échantillons de données. Tout cela en à peine un mois et demi au lieu des cinq mois requis en 2015.

Avec ces données en réserve et les 20-30 fb^-1 projetés d’ici à novembre, les expériences ATLAS et CMS peuvent déjà repousser la limite du connu et, entre autres, vérifier si les étranges événements rapportés fin 2015 sont toujours observés. Si cet effet était confirmé, il révèlerait la présence d’une nouvelle particule ayant une masse de 750 GeV, soit six fois plus lourde que le boson de Higgs. Malheureusement en 2015, il n’y avait pas suffisamment de données pour obtenir une réponse claire. Après deux ans de travaux majeurs visant à accroître sa portée en énergie, le LHC a repris ses opérations l’an dernier mais à faible régime. Si sa performance actuelle se maintient, les chances de faire de nouvelles découvertes seront décuplées. Tout le monde garde donc les doigts croisés.

Toute nouvelle particule ouvrirait la porte sur de nouveaux horizons en physique des particules. Contrairement à la découverte du boson de Higgs en 2012, si les expériences du LHC révèlent une anomalie ou l’existence d’une nouvelle particule, cela modifierait notre compréhension des constituants de base de la matière et des forces qui les régissent. Le boson de Higgs constituait la pièce manquante du Modèle standard, le modèle théorique actuel. Ce modèle ne peut plus accommoder de nouvelles particules. On sait pourtant depuis des décennies qu’il est limité, bien qu’à ce jour, les théoriciens et théoriciennes n’aient pu prédire quelle théorie devrait le remplacer et les expérimentalistes ont échoué à trouver le moindre signe révélant cette nouvelle théorie. Une évidence expérimentale est donc absolument nécessaire pour avancer.

Bien que les nouvelles données soient déjà en cours de reconstruction et de calibration, elles resteront “masquées” jusqu’à quelques jours avant le 3 août, date d’ouverture de la principale conférence de physique cet été. D’ici là, la région où la nouvelle particule pourrait se trouver est masquée afin de ne pas biaiser le processus de reconstruction des données. A la dernière minute, on appliquera aux nouvelles données les mêmes critères de sélection que ceux utilisés l’an dernier. Si ces évènements sont toujours observés à 750 GeV dans les données de 2016, la présence d’une nouvelle particule ne fera alors plus aucun doute.

Mais même si cela s’avérait n’être qu’une simple fluctuation statistique, ce qui arrive souvent en physique de par sa nature, la quantité de données accumulée permettra d’explorer une foule d’autres possibilités. En attendant, vous pouvez suivre les activités du LHC en direct ou voir grandir les échantillons de données de CMS et d’ATLAS. Je ne pourrai malheureusement pas vous rapporter ce qui sera présenté à la conférence en août, marche en montagne oblige, mais si une découverte quelconque est annoncée, même moi je m’attends à entendre son écho résonner dans les Alpes.

Pauline Gagnon

Pour en apprendre plus sur la physique des particules, ne manquez pas mon livre « Qu’est-ce que le boson de Higgs mange en hiver et autres détails essentiels » disponible en librairie au Québec et en Europe, de meme qu’aux Editions MultiMondes. Facile à lire : moi, j’ai tout compris!

CMS-lumi-17juin

Graphe cumulatif montrant la quantité de données produites à 13 TeV en 2016 par le LHC (en bleu) et récoltées par l’expérience CMS (en jaune) en date du 17 juin.

Tags: 750 GeV, ATLAS, CERN, CMS, data, LHC, nouvelle particule, particules
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Two steps closer to a possible discovery

Friday, March 18th, 2016

Pauline Gagnon

Has CERN discovered a new particle or not? Nobody knows yet, although we are now two steps closer than in December when the first signs of a possible discovery were first revealed.

First step: both the ATLAS and CMS experiments showed yesterday at the Moriond conference that the signal remains after re-analyzing the 2015 data with improved calibrations and reconstruction techniques. The faint signal still stands, even slightly stronger (see the Table). CMS has added new data not included earlier and collected during a magnet malfunction. Thanks to much effort and ingenuity, the reanalysis now includes 20% more data. Meanwhile, ATLAS showed that all data collected at lower energy up to 2012 were also compatible with the presence of a new particle.

The table below shows the results presented by CMS and ATLAS in December 2015 and February 2016. Two hypotheses were tested, assuming different characteristics for the hypothetical new particle: the “spin 0” case corresponds to a new type of Higgs boson, while “spin 2” denotes a graviton.

The label “local” means how strong the new signal appears locally at a mass of 750 or 760 GeV, while “global” refers to the probability of finding a small excess over a broad range of mass values. In physics, statistical fluctuations come and go. One is bound to find a small anomaly when looking all over the place, which is why it is wise to look at the bigger picture. So globally, the excess of events observed so far is still very mild, far from the 5σ criterion required to claim a discovery. The fact that both experiments found it independently is what is so compelling.

table-750GeV

But mostly, the second step, we are closer to potentially confirming the presence of a new particle simply because the restart of the Large Hadron Collider is now imminent. New data are expected for the first week of May. Within 2-3 months, both experiments will then know.

We need more data to confirm or refute the existence of a new particle beyond any possible doubt. And that’s what experimental physicists are paid to do: state what is known about Nature’s laws when there is not even the shadow of a doubt.

That does not mean than in the meantime, we are not dreaming since if this were confirmed, it would be the biggest breakthrough in particle physics in decades. Already, there is a frenzy among theorists. As of 1 March, 263 theoretical papers have been written on the subject since everybody is trying to find out what this could be.

Why is this so exciting? If this turns out to be true, it would be the first particle to be discovered outside the Standard Model, the current theoretical framework. The discovery of the Higgs boson in 2012 had been predicted and simply completed an existing theory. This was a feat in itself but a new, unpredicted particle would at long last reveal the nature of a more encompassing theory that everybody suspects exists but that nobody has found yet. Yesterday at the Moriond conference, Alessandro Strumia, a theorist from CERN, also predicted that this particle would probably come with a string of companions.

Theorists have spent years trying to imagine what the new theory could be while experimentalists have deployed heroic efforts, sifting through huge amounts of data looking for the smallest anomaly. No need to say then that the excitement is tangible at CERN right now as everybody is holding their breath, waiting for new data.

Pauline Gagnon

To learn more about particle physics and what might be discovered at the LHC, don’t miss my upcoming book : « Who cares about particle physics : Making sense of the Higgs boson, Large Hadron Collider and CERN »

To be alerted of new postings, follow me on Twitter: @GagnonPauline or sign-up on this mailing list to receive an e-mail notification.

Tags: 750 GeV, ATLAS, CERN, CMS, Moriond, new particle, new physics
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Deux pas de plus vers une découverte

Friday, March 18th, 2016

Pauline Gagnon

Le CERN a-t-il découvert une nouvelle particule ou pas? Personne ne le sait encore, bien que nous ayons maintenant fait deux pas de plus depuis le dévoilement des premiers signes d’une possible découverte en décembre.

Premier pas : les expériences ATLAS et CMS ont montré hier à la conférence de Moriond que les signes d’un signal persistent après la réanalyse des données de 2015 à l’aide de calibrations et de techniques de reconstruction améliorées. Le faible signal est même légèrement renforci (voir tableau). CMS a ajouté de nouvelles données recueillies durant une défaillance de leur aimant. Après beaucoup d’efforts et d’ingéniosité, ceci ajoute 20 % de données supplémentaires. De son côté, ATLAS a montré que toutes les données accumulées à moindre énergie jusqu’à 2012 étaient aussi compatibles avec la présence d’une nouvelle particule.

Le tableau ci-dessous montre les résultats présentés par CMS et ATLAS en décembre 2015 et février 2016. Deux hypothèses ont été testées, chacune correspondant à des caractéristiques différentes pour cette hypothétique particule : “spin 0” correspond à un nouveau type de boson de Higgs, tandis que “spin 2” dénote un graviton.

“Local” se réfère à l’intensité du signal lorsque mesuré pour une particule ayant une masse de 750 ou 760 GeV, tandis que “global” indique la probabilité de trouver un petit excès sur une large gamme de valeurs de masse. En physique, les fluctuations statistiques sont monnaies courantes. On trouve toujours une petite anomalie lorsqu’on cherche dans tous les coins. Il est donc sage de prendre en compte un intervalle élargi. Globalement donc, l’excédent d’événements observé est toujours très limité. On est encore bien loin de la barre des 5σ, le critère utilisé pour une découverte. Ce qui est très fort par contre, c’est que les deux expériences l’ont trouvé indépendamment.

tableau-750GeV

Le deuxième et bien plus grand pas franchi, c’est que la confirmation possible de la présence d’une nouvelle particule se rapproche simplement parce que la reprise du Grand Collisionneur de Hadrons est imminente. On attend les nouvelles données début mai. Dans 2 ou 3 mois, les deux expériences connaîtront enfin la réponse

Mais sans plus de données, impossible de confirmer ou réfuter l’existence d’une nouvelle particule avec certitude. Et c’est justement pour cela qu’on paie les physiciens et physiciennes: déterminer les lois de la Nature sans qu’il ne subsiste l’ombre d’un doute.

Cela n’empêche personne de rêver en attendant, car si ceci était confirmé, ce serait la plus grande percée en physique des particules depuis des décennies. Déjà, la frénésie s’est emparée des théoriciens et théoriciennes. On comptait en date du premier mars 263 articles théoriques sur le sujet. Tout le monde essaye de déterminer ce que cela pourrait être.

Pourquoi est-ce si passionnant ? Si elle existe, ce serait la première particule à être découverte à l’extérieur du Modèle Standard, la théorie actuelle. La découverte du boson de Higgs en 2012 avait été prévue et avait simplement complété une théorie existante. Un exploit, bien sûr, mais la découverte d’une particule imprévue révèlerait enfin la nature d’une théorie plus vaste dont tout le monde soupçonne l’existence, mais qui n’a pas encore été trouvée. Hier à la conférence de Moriond, Alessandro Strumia, un théoricien du CERN, a prédit que cette particule s’accompagnerait probablement d’une kyrielle de nouvelles particules.

Les théoriciens et théoriciennes ont passé des années à essayer d’imaginer cette nouvelle théorie tandis que du côté expérimental, on a déployé des efforts héroïques à trier des quantités faramineuses de données à la recherche de la moindre anomalie. Nul besoin de dire que l’atmosphère est fébrile en ce moment au CERN; tout le monde retient son souffle en attendant les nouvelles données.

Pauline Gagnon

Pour en savoir plus sur la physique des particules et les enjeux du LHC, consultez mon livre : « Qu’est-ce que le boson de Higgs mange en hiver et autres détails essentiels», en librairie en France et en Suisse dès le 1^er mai.

Pour recevoir un avis lors de la parution de nouveaux blogs, suivez-moi sur Twitter: @GagnonPauline ou par e-mail en ajoutant votre nom à cette liste de distribution.

Tags: 750 GeV, ATLAS, CERN, CMS, découverte, Moriond, nouvelle particule, nouvelle physique
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In search of the secret passage

Wednesday, June 3rd, 2015

Pauline Gagnon

Today begins the second operation period of the Large Hadron Collider (LHC) at CERN. By declaring “stable beams”, the LHC operators signal to physicists it is now safe to turn all their detectors on. After more than two years of intensive repair and consolidation work, the LHC now operates at higher energy. What do we hope to achieve?

The discovery of the Higgs boson in July 2012 completed the Standard Model of particle physics. This theoretical model describes all matter seen around us, both on Earth and in all stars and galaxies. But this is precisely the problem: this model only applies to what is visible in the Universe, namely 5% of its content in matter and energy. The rest consists of dark matter (27%) and dark energy (68%), two absolutely unknown substances. Hence the need for a more encompassing theory. But what is it and how can it be reached?

By operating the LHC at 13 TeV, we now have much more energy available to produce new particles than during the 2010-2012 period, when the proton collisions occurred at 8 TeV. Given that energy and mass are two forms of the same essence, the energy released during these collisions materialises, producing new particles. Having more energy means one can now produce heavier particles. It is as if one’s budget just went from 8000 euro to 13000 euro. We can “afford” bigger particles if they exist in Nature.

The Standard Model tells us that all matter is built from twelve basic particles, just like a construction set consisting of twelve basic building blocks and some “connectors” linking them together. These connectors are other particles associated with the fundamental forces. Since none of these particles has the properties of dark matter, there must still be undiscovered particles.

Which theory will allow us to go beyond the Standard Model? Will it be Supersymmetry, one of the numerous theoretical hypotheses currently under study. This theory would unify the particles of matter with the particles associated with the fundamental forces. But Supersymmetry implies the existence of numerous new particles, none of which has been found yet.

Will the LHC operating at 13 TeV allow us to produce some of these supersymmetric particles? Or will the entrance of the secret passage towards this “new physics” be revealed by meticulously studying a plethora of quantities, such as the properties of the Higgs boson. Will we discover that it establishes a link between ordinary matter (everything described by the Standard Model) and dark matter?

These are some of the many questions the LHC could clarify in the coming years. An experimental discovery would reveal the new physics. We might very well be on the verge of a huge scientific revolution.

For more information about particle physics and my book, see my website

Tags: 13 TeV, CERN, LHC, new physics
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Drell-Yan, Drell-Yan with Jets, Drell-Yan with all the Jets

Monday, May 18th, 2015

Richard Ruiz

All those super low energy jets that the LHC cannot see? LHC can still see them.

Hi Folks,

Particle colliders like the Large Hadron Collider (LHC) are, in a sense, very powerful microscopes. The higher the collision energy, the smaller distances we can study. Using less than 0.01% of the total LHC energy (13 TeV), we see that the proton is really just a bag of smaller objects called quarks and gluons.

This means that when two protons collide things are sprayed about and get very messy.

One of the most important processes that occurs in proton collisions is the Drell-Yan process. When a quark, e.g., a down quark d, from one proton and an antiquark, e.g., an down antiquark d, from an oncoming proton collide, they can annihilate into a virtual photon (γ) or Z boson if the net electric charge is zero (or a W boson if the net electric charge is one). After briefly propagating, the photon/Z can split into a lepton and its antiparticle partner, for example into a muon and antimuon or electron–positron pair! In pictures, quark-antiquark annihilation into a lepton-antilepton pair (Drell-Yan process) looks like this

feynmanDiagram_DrellYan_Simple

By the conservation of momentum, the sum of the muon and antimuon momenta will add up to the photon/Z boson momentum. In experiments like ATLAS and CMS, this gives a very cool-looking distribution

cms_DY_7TeV

Plotted is the invariant mass distribution for any muon-antimuon pair produced in proton collisions at the 7 TeV LHC. The rightmost peak at about 90 GeV (about 90 times the proton’s mass!) is a peak corresponding to the production Z boson particles. The other peaks represent the production of similarly well-known particles in the particle zoo that have decayed into a muon-antimuon pair. The clarity of each peak and the fact that this plot uses only about 0.2% of the total data collected during the first LHC data collection period (Run I) means that the Drell-Yan process is a very useful for calibrating the experiments. If the experiments are able to see the Z boson, the rho meson, etc., at their correct energies, then we have confidence that the experiments are working well enough to study nature at energies never before explored in a laboratory.

However, in real life, the Drell-Yan process is not as simple as drawn above. Real collisions include the remnants of the scattered protons. Remember: the proton is bag filled with lots of quarks and gluons.

feynmanDiagram_DrellYan_wRad

Gluons are what holds quarks together to make protons; they mediate the strong nuclear force, also known as quantum chromodynamics (QCD). The strong force is accordingly named because it requires a lot of energy and effort to overcome. Before annihilating, the quark and antiquark pair that participate in the Drell-Yan process will have radiated lots of gluons. It is very easy for objects that experience the strong force to radiate gluons. In fact, the antiquark in the Drell-Yan process originates from an energetic gluon that split into a quark-antiquark pair. Though less common, every once in a while two or even three energetic quarks or gluons (collectively called jets) will be produced alongside a Z boson.

feynmanDiagram_DrellYan_3j

Here is a real life Drell-Yan (Z boson) event with three very energetic jets. The blue lines are the muons. The red, orange and green “sprays” of particles are jets.

atlas_158466_4174272_Zmumu3jets

As likely or unlikely it may be for a Drell-Yan process or occur with additional energetic jets, the frequency at which they do occur appear to match very well with our theoretical predictions. The plot below show the likelihood (“Production cross section“) of a W or Z boson with at least 0, 1, 2, 3, or 4(!) very energetic jets. The blue bars are the theoretical predictions and the red circles are data. Producing a W or Z boson with more energetic jets is less likely than having fewer jets. The more jets identified, the smaller the production rate (“cross section”).

cms_StairwayHeaven_2014

How about low energy jets? These are difficult to observe because experiments have high thresholds for any part of a collision to be recorded. The ATLAS and CMS experiments, for example, are insensitive to very low energy objects, so not every piece of an LHC proton collision will be recorded. In short: sometimes a jet or a photon is too “dim” for us to detect it. But unlike high energy jets, it is very, very easy for Drell-Yan processes to be accompanied with low energy jets.

feynmanDiagram_DrellYan_wRadx6

There is a subtlety here. Our standard tools and tricks for calculating the probability of something happening in a proton collision (perturbation theory) assumes that we are studying objects with much higher energies than the proton at rest. Radiation of very low energy gluons is a special situation where our usual calculation methods do not work. The solution is rather cool.

As we said, the Z boson produced in the quark-antiquark annihilation has much more energy than any of the low energy gluons that are radiated, so emitting a low energy gluon should not affect the system much. This is like massive freight train pulling coal and dropping one or two pieces of coal. The train carries so much momentum and the coal is so light that dropping even a dozen pieces of coal will have only a negligible effect on the train’s motion. (Dropping all the coal, on the other hand, would not only drastically change the train’s motion but likely also be a terrible environmental hazard.) We can now make certain approximations in our calculation of a radiating a low energy gluon called “soft gluon factorization“. The result is remarkably simple, so simple we can generalize it to an arbitrary number of gluon emissions. This process is called “soft gluon resummation” and was formulated in 1985 by Collins, Soper, and Sterman.

Low energy gluons, even if they cannot be individually identified, still have an affect. They carry away energy, and by momentum conservation this will slightly push and kick the system in different directions.

feynmanDiagram_DrellYan_wRadx6_Text

If we look at Z bosons with low momentum from the CDF and DZero experiments, we see that the data and theory agree very well! In fact, in the DZero (lower) plot, the “pQCD” (perturbative QCD) prediction curve, which does not include resummation, disagrees with data. Thus, soft gluon resummation, which accounts for the emission of an arbitrary number of low energy radiations, is important and observable.

cdf_pTZ dzero_pTZ

In summary, Drell-Yan processes are a very important at high energy proton colliders like the Large Hadron Collider. They serve as a standard candle for experiments as well as a test of high precision predictions. The LHC Run II program has just begun and you can count on lots of rich physics in need of studying.

Happy Colliding,

Richard (@bravelittlemuon)

Tags: @bravelittlemuon, CERN, Drell-Yan, Jets, LHC, QCD, QCD Corrections
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Italian neutrino experiment to move to the US

Wednesday, April 22nd, 2015

Fermilab

This article appeared in symmetry on April 22, 2015.

The world’s largest liquid-argon neutrino detector will help with the search for sterile neutrinos at Fermilab. Photo: INFN

Mysterious particles called neutrinos seem to come in three varieties. However, peculiar findings in experiments over the past two decades make scientists wonder if a fourth is lurking just out of sight.

To help solve this mystery, a group of scientists spearheaded by Nobel laureate Carlo Rubbia plans to bring ICARUS, the world’s largest liquid-argon neutrino detector, across the Atlantic Ocean to the United States. The detector is currently being refurbished at CERN, where it is the first beneficiary of a new test facility for neutrino detectors.

Neutrinos are some of the most abundant and yet also most mysterious particles in the universe. They have tiny masses, but no one is sure why—or where those masses come from. They interact so rarely that they can pass through the entire Earth as if it weren’t there. They oscillate from one type to another, so that even if you start out with one kind of neutrino, it might change to another kind by the time you detect it.

Many theories in particle physics predict the existence of a sterile neutrino, which would behave differently from the three known types of neutrino.

“Finding a fourth type of neutrinos would change the whole picture we’re trying to address with current and future experiments,” says Peter Wilson, a scientist at Fermi National Accelerator Laboratory.

The Program Advisory Committee at Fermilab recently endorsed a plan, managed by Wilson, to place a suite of three detectors in a neutrino beam at the laboratory to study neutrinos—and determine whether sterile neutrinos exist.

Over the last 20 years, experiments have seen clues pointing to the possible existence of sterile neutrinos. Their influence may have caused two different types of unexpected neutrino behavior seen at the Liquid Scintillator Neutrino Detector experiment at Los Alamos National Laboratory in New Mexico and the MiniBooNE experiment at Fermilab.

Both experiments saw indications that a surprisingly large number of neutrinos may be morphing from one kind to another a short distance from a neutrino source. The existence of a fourth type of neutrino could encourage this fast transition.

The new three-detector formation at Fermilab could provide the answer to this mystery.

In the suite of experiments, a 260-ton detector called Short Baseline Neutrino Detector will sit closest to the source of the beam, so close that it will be able to detect the neutrinos before they’ve had a chance to change from one type into another. This will give scientists a baseline to compare with results from the other two detectors. SBND is under construction by a team of scientists and engineers from universities in the United Kingdom, the United States and Switzerland, working with several national laboratories in Europe and the US.

The SBND detector will be filled with liquid argon, which gives off flashes of light when other particles pass through it.

“Liquid argon is an extremely exciting technology to make precision measurements with neutrinos,” says University of Manchester physicist Stefan Soldner-Rembold, who leads the UK project building a large section of the detector. “It’s the technology we’ll be using for the next 20 to 30 years of neutrino research.”

Farther from the beam will be the existing 170-ton MicroBooNE detector, which is complete and will begin operation at Fermilab this year. The MicroBooNE detector was designed to find out whether the excess of particles seen by MiniBooNE was caused by a new type of neutrino or a new type of background. Identifying either would have major implications for future neutrino experiments.

Finally, farthest from the beam would be a liquid-argon detector more than four times the size of MicroBooNE. The 760-ton detector was used in the ICARUS experiment, which studied neutrino oscillations at Gran Sasso Laboratory in Italy using a beam of neutrinos produced at CERN from 2010 to 2014.

Its original beam at CERN is not optimized for the next stage of the sterile neutrino search. “The Fermilab beamline is the only game in town for this type of experiment,” says physicist Steve Brice, deputy head of Fermilab’s Neutrino Division.

And the ICARUS detector “is the best detector in the world to detect this kind of particle,” says Alberto Scaramelli, the former technical director of Gran Sasso National Laboratory. “We should use it.”

Rubbia, who initiated construction of ICARUS and leads the ICARUS collaboration, proposed bringing the detector to Fermilab in August 2013. Since then, the ICARUS, MicroBooNE and SBND groups have banded together to create the current proposal. The updated plan received approval from the Fermilab Program Advisory Committee in February.

“The end product was really great because it went through the full scrutiny of three different collaborations,” says MicroBooNE co-leader Sam Zeller. “The detectors all have complementary strengths.”

In December, scientists shipped the ICARUS detector from the Gran Sasso laboratory to CERN, where it is currently undergoing upgrades. The three-detector short-baseline neutrino program at Fermilab is scheduled to begin operation in 2018.

—Kathryn Jepsen

Tags: CERN, detector, Fermilab, Gran Sasso, ICARUS, liquid argon detector, neutrinos
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