high energy physics (3)

Collider Neutrinos...


New territory Two candidate collider-neutrino events from the FASERν pilot detector in the plane longitudinal to (top) and transverse to (bottom) the beam direction. The different lines in each event show charged-particle tracks originating from the neutrino interaction point. Credit: FASER Collaboration.

Topics: CERN, High Energy Physics, Particle Physics, Research

Think “neutrino detector” and images of giant installations come to mind, necessary to compensate for the vanishingly small interaction probability of neutrinos with matter. The extreme luminosity of proton-proton collisions at the LHC, however, produces a large neutrino flux in the forward direction, with energies leading to cross-sections high enough for neutrinos to be detected using a much more compact apparatus.

In March, the CERN research board approved the Scattering and Neutrino Detector (SND@LHC) for installation in an unused tunnel that links the LHC to the SPS, 480 m downstream from the ATLAS experiment. Designed to detect neutrinos produced in a hitherto unexplored pseudo-rapidity range (7.2 < 𝜂 < 8.6), the experiment will complement and extend the physics reach of the other LHC experiments — in particular FASERν, which was approved last year. Construction of FASERν, which is located in an unused service tunnel on the opposite side of ATLAS along the LHC beamline (covering |𝜂|>9.1), was completed in March, while installation of SND@LHC is about to begin.

Both experiments will be able to detect neutrinos of all types, with SND@LHC positioned off the beamline to detect neutrinos produced at slightly larger angles. Expected to commence data-taking during LHC Run 3 in spring 2022, these latest additions to the LHC experiment family are poised to make the first observations of collider neutrinos while opening new searches for feebly interacting particles and other new physics.

Collider neutrinos on the horizon, Matthew Chalmers, CERN Courier

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Illustration of the FASER experiment. Image Credit: FASER/CERN.

Topics: CERN, Dark Matter, High Energy Physics, Neutrinos, Particle Physics

Neutrinos are ubiquitous and notorious. Billions are passing through you at this moment. Occasionally described as a “ghost of a particle,” neutrinos are nearly massless, thereby making them extremely difficult to detect experimentally (“Neutrino,” meaning “little neutral one” in Italian, was first used by Enrico Fermi in the early 1930s). Neutrinos were first confirmed in 1956 (thanks to a nearby nuclear reactor), and they’ve since been detected from different sources, including the Sun and cosmic rays, but not yet in a particle collider. Their elusiveness has been the source of much intrigue (and, of course, research funding) within the particle physics community since.

What else makes them so curious? Neutrinos come in three flavors — electron neutrino, muon neutrino, and tau neutrino — and may switch between them through the process of oscillation. Neutrino oscillations have been experimentally confirmed only in the past decade at the Super-K Detector in Japan (physicists Takaaki Kajita and Arthur B. McDonald shared the 2015 Nobel Prize in Physics for it). This discovery signified an important direction in the search for physics beyond the Standard Model because the longstanding theory does not explain neutrino oscillations and describes them as completely massless particles. Something isn’t quite adding up.

Enter: FASER. Initially proposed in 2018, the ForwArd Search ExpeRiment (FASER) is CERN’s newest experiment poised to detect neutrinos, potentially up to 1300 electron neutrinos, 20,000 muon neutrinos, and 20 tau neutrinos. Constructed in an unused service tunnel located about 500 meters from an Atlas experiment interaction point, FASER and its corresponding sub-detector, FASERν, have been designed to probe interactions of high-energy neutrinos (predicted to be between 600 GeV and 1 TeV).

FASER Poised to Further Our Understanding of Neutrinos, Dark Matter, Hannah Pell, Physics Central Buzz Blog

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Astronomers searched for candidate antimatter stars among nearly 6000 gamma-ray sources. After eliminating known objects and sources that lacked the spectral signature of an antistar, 14 possibles remained. (Courtesy: Simon Dupourqué/IRAP)

Topics: Astronomy, Astrophysics, Cosmology, High Energy Physics

Fourteen possible antimatter stars (“antistars”) have been flagged up by astronomers searching for the origin of puzzling amounts of antihelium nuclei detected coming from deep space by the Alpha Magnetic Spectrometer (AMS-02) on the International Space Station.

Three astronomers at the University of Toulouse – Simon Dupourqué, Luigi Tibaldo, and Peter von Ballmoos – found the possible antistars in archive gamma-ray data from NASA’s Fermi Gamma-ray Space Telescope. While antistars are highly speculative, if they are real, then they may be revealed by their production of weak gamma-ray emission peaking at 70 MeV, when particles of normal matter from the interstellar medium fall onto them and are annihilated.

Antihelium-4 was created for the first time in 2011, in particle collisions at the Relativistic Heavy Ion Collider at the Brookhaven National Laboratory. At the time, scientists stated that if antihelium-4 were detected coming from space, then it would definitely have to come from the fusion process inside an antistar.

However, when it was announced in 2018 that AMS-02 had tentatively detected eight antihelium nuclei in cosmic rays – six of antihelium-3 and two of antihelium-4 – those unconfirmed detections were initially attributed to cosmic rays colliding with molecules in the interstellar medium and producing the antimatter in the process.

Subsequent analysis by scientists including Vivian Poulin, now at the University of Montpellier, cast doubt on the cosmic-ray origin since the greater the number of nucleons (protons and neutrons) that an antimatter nucleus has, the more difficult it is to form from cosmic ray collisions. Poulin’s group calculated that antihelium-3 is created by cosmic rays at a rate 50 times less than that detected by the AMS, while antihelium-4 is formed at a rate 105 times less.

The mystery of matter and antimatter

The focus has therefore turned back to what at first may seem an improbable explanation – stars made purely from antimatter. According to theory, matter and antimatter should have been created in equal amounts in the Big Bang, and subsequently, all annihilated leaving a universe full of radiation and no matter. Yet since we live in a matter-dominated universe, more matter than antimatter must have been created in the Big Bang – a mystery that physicists have grappled with for decades.

“Most scientists have been persuaded for decades now that the universe is essentially free of antimatter apart from small traces produced in collisions of normal matter,” says Tibaldo.

The possible existence of antistars threatens to turn this on its head. “The definitive discovery of antihelium would be absolutely fundamental,” says Dupourqué.

Are antimatter stars firing bullets of antihelium at Earth? Physics World, published in Physical Review D

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