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Posts Tagged ‘AMS’

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.


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.


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.


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.


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.


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 ».

To be notified of new blogs, follow me on Twitter : @GagnonPauline or sign up on this distribution list



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.


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.


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.


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.


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.


            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


A l’occasion de l’ouverture de l’appel à candidature 2013 de “Sciences à l’Ecole” pour l’accueil d’enseignants français au CERN durant une semaine, nous publions ces jours-ci le journal quotidien plein d’humour de Jocelyn Etienne qui a suivi ce programme l’année dernière, au mois de novembre dernier.


Chambre à brouillard: la chasse aux particules commence !
Mardi 06 novembre 2012

Aujourd’hui, construction d’une chambre à brouillard, alors que le Soleil décide enfin à se montrer ! C’est l’écossais Wilson qui en a inventé le procédé en 1911 (avant de recevoir le Nobel en 1927) pour détecter la trajectoire des particules. Pour nous, de la carboglace, un peu d’isopropanol et de bricolage, et l’on voit des muons issus de particules cosmiques laisser une trace de leur passage.Oulala! (Vue en vidéo d’un muon grâce à la chambre à brouillard)
Mick Storr en pleine explication

On a beau être dans un des plus grands centre de recherche fondamentale du monde, rien de vaut un tableau noir et une craie (cette dernière difficile à trouver par ici parait-il).


Les conférences du jour :

David Rousseau (IN2P3 / LAL-Orsay) nous confirme la découverte presque peut-être sûre du boson de Higgs, en tout cas, si c’est pas lui, c’est quand même quelque chose. Il travaille sur le détecteur ATLAS, il doit savoir de quoi il parle. Il y a des détecteurs sur le LHC, comme ATLAS et CMS  et chacun est un monstre de technologie et de compétences, et tous deux confirment indépendamment la détection du Higgs (c’est comme ça qu’on dit).

Julien Lesgourgues (Ecole Polytechnique Fédérale de Lausanne) nous parle de la courbure de l’espace qui en fait est plat, à moins que ce ne soit l’inverse, mais j’arrive un quart d’heure en retard…

Sylvie Rosier-Lees du CNRS/IN2P3 au laboratoire d’Annecy, s’occupe du détecteur spatial AMS (spectromètre magnétique Alpha ndlr), accroché à l’ISS. AMS s’occupe des particules cosmiques, et il y en a qui viennent de très loin ! (ici: les dernières new d’AMS ndlr).

Crédit: Jocelyn Etienne.

A droite, la personne semblait coder un programme pour un traitement graphique de données, mais il basculait souvent sur son compte facebook… tsss tsss tsss… Pour les connaisseurs, son portable est sous Xubuntu.

Enfin, Corinne Berat du CNRS/IN2P3 au laboratoire de Grenoble a plus les pieds sur Terre. Son joujou se trouve en Argentine et détecte les rayons cosmiques (encore) qui arrivent au sol après avoir éclaboussé l’atmosphère d’une multitude de particules (des gerbes…). L’observatoire Pierre Auger recouvre quelque chose comme 3000 km² et se délecte des particules de haute énergie provenant peut être de collisions de galaxies ou de supernovae.

Après le repas du soir, je me rends à une conférence dans le cadre de « The 4th International Conference on Particle and Fundamental Physics in Space ». Aujourd’hui, William H. Gerstenmaier de la NASA qui nous présente in English, les recherches faites sur l’ISS. La vidéo finale (un film qui compile les plus belles vues de la Terre prises de la station) est absolument sublime.



Earth from Michael König – Même ceux qui ont bossé sur leur ordinateur (occupés à coder ou traiter les informations du LHC) toute la durée de la présentation sans écouter un mot du conférencier, stoppent leur activité pour regarder le film. on Vimeo.

A suivre…

Jocelyn Etienne est enseignant au lycée Feuillade de la ville de Lunel.

Pour soumettre sa candidature pour la prochaine session du stage au CERN, c’est par ici.


Grey matter confronted to dark matter

Thursday, April 4th, 2013

After 18 years spent building the experiment and nearly two years taking data from the International Space Station, the Alpha Magnetic Spectrometer or AMS-02 collaboration showed its first results on Wednesday to a packed audience at CERN. But Prof. Sam Ting, one of the 1976 Nobel laureates and spokesperson of the experiment, only revealed part of the positron energy spectrum measured so far by AMS-02.

Positrons are the antimatter of electrons. Given we live in a world where matter dominates, it is not easy to explain where this excess of positrons comes from. There are currently two popular hypotheses: either the positrons come from pulsars or they originate from the annihilation of dark matter particles into a pair of electron and positron.  To tell these two hypotheses apart, one needs to see exactly what happens at the high-energy end of the spectrum. But this is where fewer positrons are found, making it extremely difficult to achieve the needed precision. Yesterday, we learned that AMS-02 might indeed be able to reach the needed accuracy.

The fraction of positrons (measured with respect to the sum of electrons and positrons) captured by AMS-02 as a function of their energy is shown in red. The vertical bars indicate the size of the uncertainty. The most important part of this spectrum is the high-energy part (above 100 GeV or 102) where the results of two previous experiments are also shown: Fermi in green and PAMELA in blue. Note that the AMS-02 precision exceeds the one obtained by the other experiments. The spectrum also extends to higher energy. The big question now is to see if the red curve will drop sharply at higher energy or not. More data is needed before the AMS-02 can get a definitive answer.

Only the first part of the story was revealed yesterday. The data shown clearly demonstrated the power of AMS-02. That was the excellent news delivered at the seminar: AMS-02 will be able to measure the energy spectrum accurately enough to eventually be able to tell where the positrons come from.

But the second part of the story, the punch line everyone was waiting for, will only be delivered at a later time. The data at very high energy will reveal if the observed excess in positrons comes from dark matter annihilation or from “simple” pulsars.  How long will it take before the world gets this crucial answer from AMS-02? Prof. Ting would not tell. No matter how long, the whole scientific community will be waiting with great anticipation until the collaboration is confident their measurement is precise enough. And then we will know.

If AMS-02 does manage to show that the positron excess has a dark matter origin, the consequences would be equivalent to discovering a whole new continent. As it stands, we observe that 26.8% of the content of the Universe comes in the form of a completely unknown type of matter called dark matter but have never been able to catch any of it. We only detect its presence through its gravitational effects. If AMS-02 can prove dark matter particles can annihilate and produce pairs of electrons and positrons, it would be a complete revolution.


Here are two plots to show how different the positron fraction spectrum (i.e. the curve showing the fraction of positrons as a function of energy) would differ at high energy (the rightmost part of the plot) if the positrons come from the sum of all pulsars around or if it comes from dark matter annihilation. Note they are not on the same scale and difficult to compare, but they still give some idea. It will be easier once theorists update their plots with the new AMS-02 data points on them and of course, once AMS-02 releases further information at high energy.

This is one theoretical prediction of what the positron fraction spectrum should look like if the positrons come from dark matter particles like neutralinos (represented by the symbol χ). Two curves are shown, depending on the hypothetical mass of the neutralino (mχ) at 400 GeV or 800 GeV. In each case, the maximum energy the positrons can get is roughly equal to the the mass of the neutralino, such that the curve ends close to the neutralino mass. Note the logarithmic scale on both axes.

Here is the expected spectrum if the positrons come from the sum of all pulsars. Three hypotheses were shown but only the middle one seemed to fit the PAMELA experimental results. The important feature is that this curve comes down smoothly, and not sharply at neutralino mass as with the dark matter hypothesis. Again, this curve only represents one theoretical prediction as done by Dan Hooper and his colleagues. The data point in red are from the PAMELA experiment and stop around 100 GeV. The hope is that AMS-02 will be able to provide accurate measurements at higher energies, up to several hundred GeV.

Pauline Gagnon

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April 2013 AMS Liveblog

Wednesday, April 3rd, 2013

General information

Today, the Alpha Magnetic Spectrometer (AMS) experiment is going to announce its findings for the first time. The AMS experiment uses a space-based detector, mounted on the International Space Station (ISS), and was delivered on NASA’s shuttle Endeavour, on NASA’s penultimate shuttle mission. To date AMS has observed 25 billion events over the course of the last 18 months. There has been a lot of news coverage and gossip about how this might change our understanding of the universe, and how it might impact on the search for dark matter and dark energy. However until today the results have been a guarded secret for AMS. Sam Ting, who leads the AMS Experiment, will make the presentation in the CERN Main Auditorium at 17:00 CERN time.

AMS-02 on the ISS (Wikipedia)

AMS-02 on the ISS (Wikipedia)

I’ll be live blogging the event, so stay tuned for updates and commentary! This is slightly outside my comfort zone when it comes to the science, so I may not be able to deliver the same level of detail as I did for the Higgs liveblogs. All times are CERN times.

See the indico page of the Seminar for details, and for a live video feed check out the CERN Webcast.

18:25:Congratulations and applause. The seminar is over! Thanks for reading.


Q (Pauline Gagnon): How many events above 350GeV?
A: We should wait for more statistics and better understanding. Note we do not put “Preliminary” on any results.

Q: Is there a step in the spectrum?
A: Good question! Experiments in space are different to those on the ground. This was studied over Christmas, but it’s just fluctuations. “If you don’t have fluctuations something is wrong.”

Q (Bill Murray): What is the efficiency of the final layer of the Silicon tracker?
A: Close to 100%

Q: Some bins not included. Why not?
A: Less sensitive at low energy. We want a simple model for the spectrum.

Q: Are you going to provide absolute flux measurements?
A: Yes, we will provide those. We calibrated the detector very carefully for precise measurements.

Q (John Ellis): Dark matter interpretation constrained by other experiments, eg ground based experiments.
A: Good point, we have a large number of spectra to analyze very carefully.

Q: Why not use a superconducting magnet?
A: NASA could not deliver more Helium, so superconducting is not an option for a long lived experiment.

Q: You have high statistics in the final bin, so why not rebin?
A: That’s an important question! “I’ve been working at CERN for many years and never made a mistake… We will publish this when we are absolutely sure.” (To my mind this sounds like a fine tuning problem- we should not pick which binning gives us the results we want.) “You will have to wait a little bit.”

Q (Pauline Gagnon): How can you tell the difference between the sources of positrons and models?
A: The fraction will fall off very sharply at high energy as a function of the energy.
Q: How much more time do you need to explore that region?
A: It will happen slowly.

The liveblog

18:11: Ting concludes, to applause. Time for questions.
18:10: The excess of positons has been observed for about 20 years and aroused much interest. AMS has probed this spectrum in detail. The source of the excess will be understood soon.
18:09: Conclusion time. More statistics needed for the high energy region. No fine structure is observed. No anisotropy is observed. (anisotropy of less than 0.036 at 95% confidence.)
18:07: Diffuse spectrum fitted and consistent with a single power law source.
18:00: The positron fraction spectrum is shown (Twitpic) Results should be isotropic if it’s a physics effect. The most interesting part is at high energy. No significant anisotropy is observed.
17:57: Time for some very dense tables of numbers and tiny uncertainties. Is this homeopathic physics? Dilute the important numbers with lots of other numbers!
17:53: A detailed discussion of uncertainties. There seems to be no correlation between the number of positrons and the positron fraction. Energy resolution affects resolution and hence bin to bin migration as a function of energy. There are long but small tails in the TRD estimator spectra for electrons and positrons, which must be taken into account. For charge confusion the MC models are used to get the uncertainties, which are varied by 1 sigma.
17:51: Charge confusion must be take into account. The rate is a few percent with a subpercent uncertainty. Sources of uncertainty come from large angle scattering and secondary tracks. Monte Carlo (MC) simulations are used to estimate these contributions and they seem to be well modeled.
17:48: A typical positron event, showing how the various components make the measurements. (Twitpic)
17:46: Ting shows the cover of the upcoming Physical Review Letters, a very prestigious journal, with an AMS event display. Expect a paper on April 5th!
17:45: The positron fraction. Measurements of the number of positrons compared positrons+electrons can be used to constrain physics beyond the Standard Model. In particular it can be sensitive to neutralinos, particles which are present in the Supersymmetric (SUSY) models of particle physics. The models are extensions of the Standard Model. The positron fraction is sensitive to the mass of the neutralino, if it exists.
17:42: Onto the data! There have been 25 billion events, with 6.8 million electron or positron events in the past 18 months. Two independent groups (Group A and Group alpha for fairness) analyze the data. Each group has many subgroups.
17:41: AMS is constantly monitored and reports/meetings take place every day. NASA keep AMS updated with the latest technology. There’s even an AMS flight simulator, which NASA requires AMS to use.
17:40: A less obvious point: AMS have no control over the ISS orientation or position- the position and orientation must be monitored, tolerated and taken into account.
17:38: “Operating a particle physics experiment on the ISS is fundamentally different from operating an experiment in the LHC”. Obvious Ting is obvious! 🙂
17:34: The tracking system must be kept at constant temperature, while the thermal conditions vary by tens of degrees. It has a dedicated cooling system.
17:30: Sophisticated data readout and trigger system with 2 or 4 times redundancy. (You can’t just take a screwdriver out to it if it goes wrong.)
17:27: In addition to all the other constraints, there are also extreme thermal conditions to contend with. The sun is a significant source of thermal radiation. ECAL temperatures vary from -10 to 30 degrees Celcius.
17:25 : Data can be stored for up to two months in case of a communication problem. Working space brings all kinds of constraints, especially for computing.
17:23 : NASA was in close contact to make sure it all went to plan, with tests on the ground. NASA used 2008t of mass to transport 7.5t of AMS mass (plus other deliveries) into space! AMS was installed on May 19th 2011. (I was lucky enough to hear the same story from the point of view of the NASA team, and it was an epic story they told. Apparently AMS was “plug and play”.)
17:21: Calibration is very important, because once AMS is up in space you can’t send a student to go and fix it. (Murmurs of laughter from the audience)
17:19: The detector was tested and calibrated at CERN. (I remember seeing it in the Test Beam Area long before it was launched.)
17:18: Ting shows a slide of the AMS detector, which is smaller than the LHC physicists are used to. “By CERN standards, it’s nothing”. (Twitpic)
17:16: Lots of challenges for electronic when in space. Electronics must be radiation sensitive, and AMS needs electronics that perform better than most commercial space electronics.
17:15: The TRD system measures energy loss (dE/dx) to separate electrons and positrons. A tried and true method in particle physics! The Silicon tracker has nine layers and 200,000 channels, all aligned to within 3 microns. Now that’s precision engineering. The RICH has over 10,000 photosensors to identify nuclei and measuring their energy. This sounds like a state of the art particle detector, but In Space! The ECAL system, with its 50,000 fibers and 600kg of lead can measure up to 1TeV of energy, comparable to the LHC scale.
17:11: Permanent magnet shows <1% deviation in the field since 1997. Impressive. Cosmic rays vetoed with efficiency of 0.99999. 17:10 Studies require rejection of protons versus positrons of 1 million, a huge task! TRD and TOF provides a factor of 10^2, whereas the RICH and ECAL provide the rest of the discrimination. 17:08: AMS consists of a transition radiation detector (TRD), nine layers of silicon tracker, two layers of time of flight (TOF) systems, a magnet (for measuring the charge of the particles), and a ring imaging Cherenkov detector (RICH) and electromagnetic calorimetry system (ECAL). Charges and momenta of particles are measured independently. 17:06: Ting summarizes the contributions from groups in Italy, Germany, Spain, China, Taiwan, Switzerland, France. Nice to see the groups get recognition for their long, hard work. The individual groups are often mentioned only in passing. 17:03: "AMS is the only particle physics experiment on the ISS" which is the size of a football field. The ISS cost "about 10 LHC" units of money! It's a DOE sponsored international collaboration. Ting is doing a good job acknowledging the support of collaborators and the awesomeness of having a space based particle physics experiment. 17:00: "Take your seats please." The crowd goes quiet, as the introduction starts. Sam Ting was awarded the 1976 Nobel Prize for Physics, for the discovery of the J/psi particle. 16:54: Rolf Heuer has arrived. The room is nearly full now! 16:47: Sam Ting is here. He arrived about 10 minutes ago, and spoke to Sau Lan Wu, an old colleague of his. (Twitpic)
16:31: There are a few early bird arrivals. (Twitpic)