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New ALICE experiment results show novel phenomena in proton collisions

Experiments and Tracks

As the number of particles produced in proton collisions (the blue lines) increase, the more of these so-called strange hadrons are seen (as shown by the red squares in the graph). (Image: CERN)Geneva 24 April 2017. In a paper published today in Nature Physics, the ALICE collaboration reports that proton collisions sometimes present similar patterns to those observed in the collisions of heavy nuclei. This behaviour was spotted through observation of so-called strange hadrons in certain proton collisions in which a large number of particles are created. Strange hadrons are well-known particles with names such as Kaon, Lambda, Xi and Omega, all containing at least one so-called strange quark. The observed ‘enhanced production of strange particles’ is a familiar feature of quark-gluon plasma, a very hot and dense state of matter that existed just a few millionths of a second after the Big Bang, and is commonly created in collisions of heavy nuclei. But it is the first time ever that such a phenomenon is unambiguously observed in the rare proton collisions in which many particles are created. This result is likely to challenge existing theoretical models that do not predict an increase of strange particles in these events.

“We are very excited about this discovery,” said Federico Antinori, Spokesperson of the ALICE collaboration. “We are again learning a lot about this primordial state of matter. Being able to isolate the quark-gluon-plasma-like phenomena in a smaller and simpler system, such as the collision between two protons, opens up an entirely new dimension for the study of the properties of the fundamental state that our universe emerged from.”

The study of the quark-gluon plasma provides a way to investigate the properties of strong interaction, one of the four known fundamental forces, while enhanced strangeness production is a manifestation of this state of matter. The quark-gluon plasma is produced at sufficiently high temperature and energy density, when ordinary matter undergoes a transition to a phase in which quarks and gluons become ‘free’ and are thus no longer confined within hadrons. These conditions can be obtained at the Large Hadron Collider by colliding heavy nuclei at high energy. Strange quarks are heavier than the quarks composing normal matter, and typically harder to produce. But this changes in presence of the high energy density of the quark-gluon plasma, which rebalances the creation of strange quarks relative to non-strange ones. This phenomenon may now have been observed within proton collisions as well.

In particular, the new results show that the production rate of these strange hadrons increases with the ‘multiplicity’ – the number of particles produced in a given collision – faster than that of other particles generated in the same collision. While the structure of the proton does not include strange quarks, data also show that the higher the number of strange quarks contained in the induced hadron, the stronger is the increase of its production rate. No dependence on the collision energy or the mass of the generated particles is observed, demonstrating that the observed phenomenon is related to the strange quark content of the particles produced. Strangeness production is in practice determined by counting the number of strange particles produced in a given collision, and calculating the ratio of strange to non-strange particles.

Enhanced strangeness production had been suggested as a possible consequence of quark-gluon plasma formation since the early eighties, and discovered in collisions of nuclei in the nineties by experiments at CERN1‘s Super Proton Synchrotron. Another possible consequence of the quark gluon plasma formation is a spatial correlation of the final state particles, causing a distinct preferential alignment with the shape of a ridge. Following its detection in heavy-nuclei collisions, the ridge has also been seen in high-multiplicity proton collisions at the Large Hadron Collider, giving the first indication that proton collisions could present heavy-nuclei-like properties. Studying these processes more precisely will be key to better understand the microscopic mechanisms of the quark-gluon plasma and the collective behaviour of particles in small systems.

The ALICE experiment has been designed to study collisions of heavy nuclei. It also studies proton-proton collisions, which primarily provide reference data for the heavy-nuclei collisions. The reported measurements have been performed with 7 TeV proton collision data from LHC run 1.

Footnote(s)

1. CERN, the European Organization for Nuclear Research, is one of the world’s leading laboratories for particle physics. The Organization is located on the French-Swiss border, with its headquarters in Geneva. Its Member States are: Austria, Belgium, Bulgaria, Czech Republic, Denmark, Finland, France, Germany, Greece, Hungary, Israel, Italy, Netherlands, Norway, Poland, Portugal, Romania, Slovakia, Spain, Sweden, Switzerland and United Kingdom. Cyprus and Serbia are Associate Member States in the pre-stage to Membership. India, Pakistan, Turkey and Ukraine are Associate Member States. The European Union, Japan, JINR, the Russian Federation, UNESCO and the United States of America currently have Observer status.

CERN experiment reports sixfold improved measurement of the magnetic moment of the antiproton

BASE

Stefan Ulmer, Spokesperson BASE Collaboration, in Base Experiment (Image: CERN)Geneva, 18 January 2017. In a paper published today in the journal Nature Communications, the BASE collaboration at CERN1 reports the most precise measurement ever made of the magnetic moment of the antiproton, allowing a fundamental comparison between matter and antimatter. The BASE measurement shows that the magnetic moments of the proton and antiproton are identical, apart from their opposite signs, within the experimental uncertainty of 0.8 parts per million. The result improves the precision of the previous best measurement by the ATRAP collaboration in 2013, also at CERN, by a factor of 6.

At the scale of elementary particles, an almost perfect symmetry between matter and antimatter exists. On cosmological scales, however, the amount of matter outweighs that of antimatter. Understanding this profound contradiction demands that physicists compare the fundamental properties of particles and their antiparticles with high precision.

BASE uses antiprotons from CERN’s unique antimatter factory, the Antiproton Decelerator (AD), and is designed specifically to perform precision measurements of the antimatter counterparts of normal matter particles. The magnetic moment, which determines how a particle behaves when immersed in a magnetic field, is one of the most studied intrinsic characteristics of a particle. Although different particles have different magnetic behaviour, the magnetic moments of protons and antiprotons are supposed to differ only in their sign as a consequence of so-called charge-parity-time symmetry. Any difference in their magnitudes would challenge the Standard Model of particle physics and would offer a glimpse of new physics.

To perform the experiments, the BASE collaboration cools down antiprotons to the extremely low temperature of about 1 degree above absolute zero, and traps them using sophisticated electromagnetic containers so that they do not come into contact with matter and annihilate (thanks to such devices, BASE has recently managed to store a bunch of antiprotons for more than one year). From here, antiprotons are fed one-by-one to further traps where their behaviour under magnetic fields allows researchers to determine their intrinsic magnetic moment. Similar techniques have already been successfully applied in the past to electrons and their antimatter partners, positrons, but antiprotons present a much bigger challenge because their magnetic moments are considerably weaker. The new BASE measurement required a specially designed magnetic “bottle” that is more than 1000 times stronger than that used in electron/positron experiments.

“This measurement is so far the culmination point of 10 years of hard work by the BASE team,” said Stefan Ulmer, spokesperson of the BASE collaboration. “Together with other AD experiments, we are really making rapid progress in our understanding of antimatter.”

BASE now plans to measure the antiproton magnetic moment using a new trapping technique that should enable a precision at the level of a few parts per billion – i.e. a factor of 200 to 800 improvement. “The implementation of this method is much more challenging than the method which was used here and will require several additional iteration steps,” says first author Hiroki Nagahama.

Further information:

Link to the paper in Nature Communications: http://dx.doi.org/10.1038/ncomms14084

Pictures of the BASE experiment: https://cds.cern.ch/record/2242307

Footnote(s)

1. CERN, the European Organization for Nuclear Research, is the world’s leading laboratory for particle physics. Its headquarters are in Geneva. Its Member States are: Austria, Belgium, Bulgaria, Czech Republic, Denmark, Finland, France, Germany, Greece, Hungary, Israel, Italy, Netherlands, Norway, Poland, Portugal, Romania, Slovakia, Spain, Sweden, Switzerland and United Kingdom. Cyprus and Serbia are Associate Member States in the pre-stage to Membership. India, Pakistan, Turkey and Ukraine are Associate Member States. The European Union, Japan, JINR, the Russian Federation, UNESCO and the United States of America currently have Observer status.

CMS animation

Animation showing the construction of the main structural components of CMS in the surface hall at Cessy, together with a detailed overview of the installation in the experimental cavern.
20:53.00 min / 26 July 2006 / © 2006-2016 CERN
Produced by: CERN/CMS
3D animations: Michele de Gennaro, Alexei Sergueev
Direction and project management: Silvano de Gennaro.

Language: English
Source medium: DV, PAL
Reference: CERN-VIDEORUSH-2006-010

http://cds.cern.ch/record/1027378

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Follow ATLAS physicists Claire Lee to find out expectations in research on the Higgs Boson as the LHC prepares to restart with collisions at 13 TeV

CCC restart quick timelapse

Timelapse from the 05/04/2015, the day of the Restart of the LHC

VNR Proton beams are back in the LHC B-Roll

footage of the proton beams 2015 B-Roll