The HiRadMat-56 (HRMT-56) experiment was designed and set up in barely a year, during the period from October 2020 to October 2021, in order to answer a question that was as urgent as it was crucial: how should the future HL-LHC beam dumps and the new spare LHC beam dumps be designed? The autopsy performed on one of the accelerator’s old beam dumps had revealed that one of its components, namely extruded graphite, had cracked under the repeated impact of the beam (see the corresponding article entitled “Autopsy of an LHC beam dump”). But what could be used instead of extruded graphite? How could the resistance of the materials that might one day absorb the beams of the LHC and the future HL-LHC be assessed? “We wanted to understand, quantitatively, how various materials would behave under the impact of a high-energy beam,” explains Pablo Andreu Munoz, an engineer in the SY-STI group. “So we designed a custom test station at HiRadMat.”The HRMT-56 experiment installed on its beamline at HiRadMat. (Image: CERN)
The HRMT-56 experiment consists of an aluminium vessel under a controlled atmosphere, where some targets are under vacuum and others under nitrogen gas; the vessel contains 20 target trains, each of which can hold several different samples. By means of a “lift” system, the target trains pass one after another into the 440-GeV/c proton beam supplied by the SPS. The beam hits each sample around four times. The dimensions of the beam and targets are selected such that the energy density generated on impact is comparable to that generated when a 7-TeV beam collides with a beam dump. Moreover, the experiment is equipped with “beam diluters”: titanium tubes containing cylinders made of denser materials, which are located upstream of the targets and allow the amount of energy that hits them to be increased. It is thus possible to reach energy density values close to those that are anticipated during Run 3, and even at the future HL-LHC. On the menu: various types of low- and high-density graphite, silicon carbide reinforced with carbon fibres, and “carbon–carbon”, a material made of woven carbon fibres in a graphic matrix that is notably used in space shuttles.The targets are inserted into the aluminium vessel. (Image: CERN)
“The targets are fitted with various sensors, notably temperature probes and laser Doppler accelerometers, which provide live information on the effect of the beam on the samples,” explains François-Xavier Nuiry, head of the HRMT-56 experiment. “We also compare the target trains, in a radiation bunker, before and after irradiation. The samples are analysed from all perspectives, before and after impact, using various means, including metrology, microtomography, mass measurements and surface studies.”The SY-STI-TCD team analyses samples after irradiation, in the radiation bunker. (Image: CERN)
The first data, obtained in January 2022, confirmed the results of the autopsy: the low- and high-density graphites are fit for use in the spare LHC beam dumps. The carbon–ؘcarbon also produced very promising results, notably for various HL-LHC beam dumps. It will also replace extruded graphite in the spare dumps.
During the second phase of the HRMT-56 experiment, which will take place in 2024, the samples will be massively irradiated – to the tune of several hundred impacts per target – by the SPS beams.anschaef Wed, 06/08/2022 - 12:02 Byline Anaïs Schaeffer Publication Date Wed, 06/08/2022 - 11:40
The ALICE collaboration at the Large Hadron Collider (LHC) has made the first direct observation of the dead-cone effect – a fundamental feature of the theory of the strong force that binds quarks and gluons together into protons, neutrons and, ultimately, all atomic nuclei. In addition to confirming this effect, the observation, reported in a paper published today in Nature, provides direct experimental access to the mass of a single charm quark before it is confined inside hadrons.
“It has been very challenging to observe the dead cone directly,” says ALICE spokesperson Luciano Musa. “But, by using three years’ worth of data from proton–proton collisions at the LHC and sophisticated data-analysis techniques, we have finally been able to uncover it.”
Quarks and gluons, collectively called partons, are produced in particle collisions such as those that take place at the LHC. After their creation, partons undergo a cascade of events called a parton shower, whereby they lose energy by emitting radiation in the form of gluons, which also emit gluons. The radiation pattern of this shower depends on the mass of the gluon-emitting parton and displays a region around the direction of flight of the parton where gluon emission is suppressed – the dead cone1.
Predicted thirty years ago from the first principles of the theory of the strong force, the dead cone has been indirectly observed at particle colliders. However, it has remained challenging to observe it directly from the parton shower’s radiation pattern. The main reasons for this are that the dead cone can be filled with the particles into which the emitting parton transforms, and that it is difficult to determine the changing direction of the parton throughout the shower process.
The ALICE collaboration overcame these challenges by applying state-of-the-art analysis techniques to a large sample of proton–proton collisions at the LHC. These techniques can roll the parton shower back in time from its end-products – the signals left in the ALICE detector by a spray of particles known as a jet. By looking for jets that included a particle containing a charm quark, the researchers were able to identify a jet created by this type of quark and trace back the quark’s entire history of gluon emissions. A comparison between the gluon-emission pattern of the charm quark with that of gluons and practically massless quarks then revealed a dead cone in the charm quark’s pattern.
The result also directly exposes the mass of the charm quark, as theory predicts that massless particles do not have corresponding dead cones.
“Quark masses are fundamental quantities in particle physics, but they cannot be accessed and measured directly in experiments because, with the exception of the top quark, quarks are confined inside composite particles,” explains ALICE physics coordinator Andrea Dainese. “Our successful technique to directly observe a parton shower’s dead cone may offer a way to measure quark masses.”As the parton shower proceeds, gluons are emitted at smaller angles and the energy of the quark decreases, resulting in larger dead cones of suppressed gluon emission. (Image: CERN)
1Technical note: specifically, for an emitter of mass m and energy E, gluon emission is suppressed at angles smaller than the ratio of m and E, relative to the emitter’s direction of motion.
mailys Mon, 05/16/2022 - 12:09 Publication Date Wed, 05/18/2022 - 17:00
Aerosol particles can form and grow in Earth’s upper troposphere in an unexpected way, reports the CLOUD collaboration in a paper1 published today in Nature. The new mechanism may represent a major source of cloud and ice seed particles in areas of the upper troposphere where ammonia is efficiently transported vertically, such as over the Asian monsoon regions.
Aerosol particles are known to generally cool the climate by reflecting sunlight back into space and by making clouds more reflective. However, how new aerosol particles form in the atmosphere remains relatively poorly known.
“Newly formed aerosol particles are ubiquitous throughout the upper troposphere, but the vapours and mechanisms that drive the formation of these particles are not well understood,” explains CLOUD spokesperson Jasper Kirkby. “With experiments performed under cold upper tropospheric conditions in CERN’s CLOUD chamber, we uncovered a new mechanism for extremely rapid particle formation and growth involving novel mixtures of vapours.”
Using mixtures of sulfuric acid, nitric acid and ammonia vapours in the chamber at atmospheric concentrations, the CLOUD team found that these three compounds form new particles synergistically at rates much faster than those for any combination of two of the compounds. The CLOUD researchers found that the three vapours together form new particles 10–1000 times faster than a sulfuric acid–ammonia mixture, which, from previous CLOUD measurements, was previously considered to be the dominant source of upper tropospheric particles. Once the three-component particles form, they can grow rapidly from the condensation of nitric acid and ammonia alone to sizes where they seed clouds.
Moreover, the CLOUD measurements show that these particles are highly efficient at seeding ice crystals, comparable to desert dust particles, which are thought to be the most widespread and effective ice seeds in the atmosphere. When a supercooled cloud droplet freezes, the resulting ice particle will grow at the expense of any unfrozen droplets nearby, so ice has a major influence on cloud microphysical properties and precipitation.
The CLOUD researchers went on to feed their measurements into global aerosol models that include vertical transport of ammonia by deep convective clouds. The models showed that, although the particles form locally in ammonia-rich regions of the upper troposphere such as over the Asian monsoon regions, they travel from Asia to North America in just three days via the subtropical jet stream, potentially influencing Earth’s climate on an intercontinental scale.
“Our results will improve the reliability of global climate models in accounting for aerosol formation in the upper troposphere and in predicting how the climate will change in the future,” says Kirkby. “Once again, CLOUD is finding that anthropogenic ammonia has a major influence on atmospheric aerosol particles, and our studies are informing policies for future air pollution regulations.”
Atmospheric concentrations of sulfuric acid, nitric acid and ammonia were much lower in the pre-industrial era than they are now, and each is likely to follow different concentration trajectories under future air pollution controls. Ammonia in the upper troposphere originates from livestock and fertiliser emissions – which are unregulated at present – and is carried aloft in convective cloud droplets, which release their ammonia upon freezing.Simulation of aerosol particle formation during the Asian monsoon in a global aerosol model with efficient vertical transport of ammonia into the upper troposphere. Including a mixture of sulfuric acid, nitric acid and ammonia enhances upper-tropospheric particle number concentrations over the Asian monsoon region by a factor of 3–5 compared with the same model with only sulfuric acid and ammonia. (Image: CLOUD collaboration)
1Wang, M. et al. Synergistic HNO3–H2SO4–NH3 upper tropospheric particle formation. Nature, doi:10.1038/s41586-022-04605-4 (2022).
gfabre Fri, 05/13/2022 - 09:48 Publication Date Wed, 05/18/2022 - 17:30
FAP-BC-HR, in collaboration with the Users Office and the experiments, has released PREG, a new EDH document for the registration of external participants in an experiment (PARTs) (please note that this document is accessible only to team leaders and secretaries). PREG allows team leaders and secretaries to request the registration of new PARTs who need a computing account in order to carry out remote activities.
The Users Office requested that this document be created to replace paper registration forms and thus facilitate and streamline the registration process for PARTs. By eliminating the need for manual data entries and paper forms, PREG also serves the dual objectives of saving resources allocated to non-value-added activities and ensuring the compliance of processes with OC11 principles, thus reinforcing the protection of personal data.
For more information, visit the Pre-registration help page.anschaef Wed, 03/09/2022 - 14:55 Publication Date Wed, 03/09/2022 - 13:58