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The charm and the beauty of LHC

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In search of elusive particles

Everything from galaxies to molecules is made of combinations of quarks and leptons, but quarks have never been observed free in nature. On a quest to detect them, scientists uncovered a measurement bias that will guide future experiments.

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For those who think that physics is boring, a refresher course may be needed. The Universe according to particle physics is something like an Alice in Wonderland story. Some particles carry a colour charge rather than an electrical one. Some have flavours. There are conventional matter particles, but also ones that carry force, like gluon that 'glues' other particles together. Quarks have never been observed free in nature but only confined in hadrons by the gluon field. EU-funded scientists set out to measure and disentangle two quarks. The project 'The charm and the beauty of LHC' (HFQGP) exploited the heavy ion detector ALICE in the world's largest and most powerful particle accelerator. The team explored hadrons under conditions postulated to induce a transition to a plasma of free quarks and gluons (quark–gluon plasma). Initial experiments studied for the first time asymmetric proton–lead (p–Pb) collisions, a type of proton–nucleus collision used as a reference for disentangling signatures of a deconfined hot medium. As with the Pb–Pb and p–p data that came before, a task force was established to set up the official experimental protocol and conduct the first analyses of p–Pb data. An HFQGP scientist was one of the two coordinators with the role of managing the event characterisation. Given the asymmetric nature of p–Pb collisions compared to p–p or Pb–Pb collisions, particular emphasis was given to centrality determination related to the region of interaction during the collision. This has opened a window on unique phenomena and a bias in the measurement dependent on selection of the event sample. Its significance is recognised through numerous recent special sessions at major conferences on the topic. Although signs of their free existence have been observed in experiments, to date free quarks remain a Holy Grail of the particle physics community. Tightening up experimental protocols to reduce measurement bias as postulated by HFQGP outcomes could put that Grail within reach.


Elusive particles, particles, quarks, particle physics, gluon, hadrons, ALICE, particle accelerator, free quarks, quark–gluon plasma, proton–lead, p–Pb, proton–nucleus collision

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