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.
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.
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.
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)