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Posts Tagged ‘cosmic rays’

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



Cosmic connections

Friday, August 10th, 2012

6 August 2012. It’s a rather grey evening and I’m in the basket of a hot-air balloon, drifting over the small town of Bad Saarow–Pieskow, some 50 km south-east of Berlin. It’s a ‘first’ for me and my companions who include Bill Breisky, an American writer and former editor of the Cap Cod Times. He’s also the grandson of Victor Hess, whose balloon flight 100 years ago opened a new window on matter in the universe. On 7 August 1912, Hess had landed near Pieskow – no one now knows exactly where – but there the similarities with our small adventure end. Hess had flown for six hours, carried by a hydrogen balloon to a height of more than 5000 m. During the flight, he made measurements that showed that the natural level of radiation increases with altitude, leading him to conclude that “a radiation of very high penetrating power enters our atmosphere from above”. This was the moment that 100 years later is being celebrated as marking the discovery of ‘cosmic rays’.

Although it’s not the name that Hess gave his discovery, it’s certainly apt. We now know that cosmic rays are energetic particles from outer space. When they enter the Earth’s atmosphere, they generate showers of further particles that penetrate right down to the ground, and even below ground. As you read this, about one cosmic-ray muon, a heavier sibling of the electron, passes through your head each second, mainly from above.

Studies of cosmic rays opened the door to a world of particles beyond the confines of the atom: first, the positron (the anti-electron), then the muon, followed by the pion, the kaon and several more. Until the advent of high-energy particle accelerators in the early 1950s, this natural radiation provided the only way to investigate the growing particle ‘zoo’. Indeed, when CERN was founded in 1954, its convention included cosmic rays in the list of scientific interests. But even though accelerators came to provide the best hunting ground for new particles, cosmic rays have maintained their mystery. The record energies of the LHC are still puny compared with the highest energy cosmic rays, where a single proton entering the atmosphere can pack the punch of a tennis ball served by a top player.

Since Hess’s discovery, physicists may have answered the ‘what’ of cosmic rays – they are energetic particles – but they still haven’t answered the ‘how’ or ‘where’. Just how does nature accelerate them to such high energies? Where are the natural accelerators? These remain mysteries that continue to drive adventurous research, in places as diverse as the deep ice of the South Pole and the high plateau of central Nambia.

This brings us back to how Bill and I ended up in balloon together. Michael Walter and colleagues at the German laboratory, DESY – which has a big involvement in the IceCube experiment at the South Pole and HESS facility in Namibia – had organized  a conference in Bad Saarow  to celebrate the centenary. The meeting brought together historians as well as key people in the on-going study of cosmic rays. Bill was one of the invited speakers. On 7 August, he and his brother unveiled a plaque on a geological ‘erratic’ in Peiskow – a stone deposited after being carried from afar by a glacier during the last Ice Age. It was a fitting tribute to Victor Hess, who had found himself near the same place after making a long journey to study an intriguing natural phenomenon – and setting us on a road that would, among other things, lead to CERN and the LHC.

Christine Sutton.



Fermilab planning a busy 2012

Tuesday, January 3rd, 2012

This column by Fermilab Director Pier Oddone first appeared in Fermilab Today Jan. 3 .

We have a mountain of exciting work coming our way!

In accelerator operations, we need to give enough neutrinos to MINERvA to complete their low-energy run, enough anti-neutrinos to MiniBooNE to complete their run and enough neutrinos to MINOS to enable their independent neutrino velocity measurement that will follow up on last year’s OPERA results. We need to provide test beams to several technology development projects and overcome setbacks due to an aging infrastructure to deliver beam to the SeaQuest nuclear physics experiment. And we need to do all of this in the first few months of the year before a year-long shutdown starts. During the shutdown, we will modify the accelerator complex for the NOvA era and begin the campaign to double the number of protons from the Booster to deliver simultaneous beams to various experiments.

In parallel with accelerator modifications, we will push forward on many new experiments. The NOvA detector is in full construction mode, and we face challenges in the very large number of detector elements and large mechanical systems. Any project of this scale requires a huge effort to achieve the full promise of its design. We have the resources in our FY2012 budget to make a lot of progress toward MicroBooNE, Mu2e and LBNE. We will continue to work with DOE to advance Muon g-2. All these experiments are at an important stage in their development and need to be firmly established this year.

At the Cosmic Frontier, we will commission and start operation of the Dark Energy Survey at the Blanco Telescope in Chile, where the camera has arrived and is being tested. In the dark matter arena we will commission and operate the 60 kg COUPP detector at Canada’s SNOLAB and continue the run of the CDMS 15 kg detector in the Soudan Mine while carrying out R&D on future projects. We continue to have a major role in the operation of the Pierre Auger cosmic-ray observatory. In addition we should complete the first phase of the Fermilab Holometer, which will study the properties of space-time at the Planck scale.

At the Energy Frontier, we play a major role in the LHC detector operations and analysis. It should be a fabulously exciting year at the LHC as we push on the hints that we already see in the data.

Beyond construction and operation of facilities we continue our R&D efforts on the superconducting RF technology necessary for Project X and other future accelerators. We will be building the Illinois Accelerator Research Center and moving forward to connect our advanced accelerator program with industry and universities. Our rich program on theory, computation and detector technology will continue to support our laboratory and the particle physics community.

If we accomplish all that is ahead of us for 2012, it will be a year to remember and celebrate when we hit New Year’s Day 2013!


A cosmic ray shower.

Editor’s note: Bob’s most excellent particle detector adventure, part 7.

Bob Peterson continues to travel with his QuarkNet particle detector around the edge of Africa recording remnants of cosmic rays. This offers a chance to study how cosmic ray recordings differ on land and sea and at different latitudes. The data will be accessible to high school students and teachers in several countries who use similar detectors to learn about particle physics.

Read his previous posts here: The voyage begins, Turning the detector on, Other science on the sea, Particle detectors don’t like light, Enduring a branding for science A teaching moment on the ocean.

11 May 2011
R/V Polarstern
Latitude: 30-56.1 N
Longitude: 14-27.0 W
off the Moroccan coast
Ship course 017° T
Ship velocity 10.8 knots

10 May:
Dawn; the forecast called for a sunny day? What’s that cloud bank out my cabin window? Oh, wait, that’s land! Ah, the Canary Islands right on schedule just as the navigator predicted, and we approach Las Palmas harbor from the south.

The Polarstern receives the pilot at 0945 (he’s late), and the ship proceeds cautiously into the slip. Forty-five minutes later, dock lines are thrown and we are winched tight to the quay and the gangway lowered.

We are here to receive new scientists from Alfred Wegner Institute, or AWI, coming onboard for special training in echo acoustics and bottom profiling. And it gives some of us a chance to feel land. Shore leave is two hours; be back by 1400 or the captain will not be happy. So, I escape. Sure enough the land is moving in a wave-like motion. I must have sea-legs.

Underway again at 1745. The pilot was late again. By 1830, the Canary Islands are falling behind and slipping into the sunset haze.

How IceCube works. Credit: IceCube collaboration

11 May:
Daily, the Polarstern weather technician, Klaus, launches a weather balloon for upper-atmosphere soundings. Michael Walter, my contact from DESY/IceCube, needs these data. I’m intrigued, so Klaus gladly offers to train me to ready and launch the balloons. This will be fun.

All over the world, weather stations and ships launch these balloons simultaneously, and they need to be at 10 kilometers, or 6 miles, altitude by 1200 Coordinated Universal Time, or UTC. So, Klaus prepares and launches the balloon one hour before because it takes that long to reach 33,000 feet. The balloons are filled with helium to about five feet diameter and carry a small data collection transponder called a radiosonde.

Launchings on land are straight forward; the technician steps out of the filing garage and let’s go. On ship, it’s anything but simple. The deck is pitching and rolling, the forward speed sweeps the deck with maybe 35 knots of wind, and the tall crow’s nest in the center of the ship is definitely in the way. There will be no end of grief from the crew if I hang the balloon there. So, Klaus coaches (and laughs) as I learn to manage the recalcitrant, reluctant object, but I’ve now mastered the preparation and launch. Launchings are analogous to a young boy throwing a rock off a cliff; except, it defies gravity. What fun to watch it sail away. I wonder how long I can still see it before it disappears into the haze.

The data returns to the ship in one-second intervals, showing the profiles of atmosphere parameters. The soundings return data on altitude, pressure, temperature, humidity and wind speed and these data might couple to the QuarkNet cosmic ray muon detector (CRMD) data. Michael Walter will use this to correlate to the cosmic ray flux, or flow rate of cosmic ray remnant particles passing through various areas of the sky. I hope that QuarkNet students can do the same. I, too, will return home with the large data sets to compare to the onboard QuarkNet detector.


*Quay: Pronounced “key”. A concrete, stone or metal platform lying
alongside or projecting into water for loading and unloading ships. Similar to a

*Pilot: a master mariner hired by the captain to guide a big ship into confined harbors. They have special local knowledge and training. Still the ship’s captain is ultimately responsible. A pilot job is nice if you can get it, because all have to wait for the pilot. They are notoriously late.

*Winch: A hauling or lifting device consisting of a rope, cable or chain winding around a horizontal, rotating drum, turned by a crank or by a motor or other power source.

*AWI: Alfred Wegner Institute – Research facility that owns and manages the R/V Polarstern.

*UTC: Coordinated Universal Time

*Radiosonde: An instrument carried by balloon or other means to various levels of the atmosphere and transmitting measurements by radio.

*Crow’s nest: A shelter or platform fixed near the top of the mast of a vessel as a place for instruments or lookout.

–Bob Peterson