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Moriond 2019 feels the strong force

Pentaquarks, charmed beauty particles and more from the Moriond conference’s second week, which is devoted to studies of the strong nuclear force

Last week, physicists from all over the world gathered in La Thuile, Italy, for the second week of the Rencontres de Moriond conference. This second week of the annual meeting features new and recent findings in all things related to quantum chromodynamics (QCD) – the theory of the strong force that combines quarks into composite particles called hadrons – and to high-energy particle interactions. This year, results from the main experiments at the Large Hadron Collider (ALICE, ATLAS, CMS and LHCb) included new pentaquarks, new charmed beauty particles, a more precise measurement of matter–antimatter asymmetry in strange beauty particles, and new results from heavy-ion collisions.

Discovery of new pentaquarks

The LHCb collaboration announced the discovery of new five-quark hadrons, or “pentaquarks”. Quarks normally aggregate into groups of twos and threes, but in recent years the LHCb team has confirmed the existence of exotic tetraquarks and pentaquarks, which are also predicted by QCD. In a 2015 study, the LHCb researchers analysed data from the decay of the three-quark particle Λb into a J/ψ particle, a proton and a charged kaon and were able to see two new pentaquarks (dubbed Pc(4450)+ and Pc(4380)+) in intermediate decay states. After analysing a sample of nine times more Λb decays than in the 2015 study, the LHCb team has now discovered a new pentaquark, Pc(4312)+ as well as a two-peak pattern in the data that shows that the previously observed  Pc(4450)+ structure is in fact two particles.


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A Bs candidate decaying to a J/psi and a phi, where the J/psi decays to two opposite-charge muons (red lines) and the phi decays to two opposite-charge kaons (blue). The event was recorded by ATLAS on 16 August 2017 from proton–proton collisions at 13 TeV. (Image: CERN)


Charmed beauty particles in focus

Notwithstanding significant progress over the past two decades, researchers’ understanding of the QCD processes that make up hadrons is incomplete. One way to try and understand them is through the study of the little-known charmed beauty (Bc) particle family, which consists of hadrons made up of a beauty quark and a charm antiquark (or vice-versa). In 2014, using data from the LHC’s first proton–proton collision run, the ATLAS collaboration reported the observation of a Bc particle called Bc(2S). A very recent analysis by the CMS collaboration of the full LHC sample from the second run, published today in Physical Review Letters and presented at the meeting, has unambiguously observed a two-peak feature in this dataset that corresponds to Bc(2S) and to another Bc particle called Bc*(2S). Meanwhile, the LHCb team, which in 2017 reported no evidence for Bc(2S) in its 2012 data, has now analysed the full 2011–2018 data sample and has also observed the Bc(2S) and Bc*(2S), lending support to the CMS result.


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An event recorded by CMS showing a candidate for the Bc(2S*). The signature for this new particle is the presence of two pions (green lines) and a Bc meson, that decays into a pion (yellow line) plus a J/psi that itself decays to two muons (red). (Image: CERN)


Matter–antimatter asymmetry in strange beauty particles

The meeting’s second week also saw the announcement of a new result concerning the amount of the matter–antimatter asymmetry known as CP violation in the system of strange beauty (Bs) particles, which are made of a bottom quark and a strange quark. Bs mesons have the special feature that they oscillate rapidly into their antiparticle and back, and these oscillations can lead to CP violation when the Bs decays into combinations of particles such as a J/ψ and a ϕ. The amount of CP violation predicted by the Standard Model and observed so far in experiments is too small to account for the observed imbalance between matter and antimatter in the universe, prompting scientists to search for additional, as-yet-unknown sources of CP violation and to measure the extent of the violation from known sources more precisely. Following hot on the heels of two independent measurements of the asymmetry in the Bs system reported by ATLAS and LHCb during the meeting’s first week, a new result that combined the two measurements was reported during the second week. The combined result is the most precise measurement yet of the asymmetry in the Bs system and is consistent with the small value precisely predicted by the Standard Model.

Heavy-ion progress

The ALICE collaboration specialises in collisions between heavy ions such as lead nuclei, which can recreate the quark–gluon plasma (QGP) that is believed to have occurred shortly after the Big Bang. ALICE highlighted its observation that three-quark particles (baryons) containing charm quarks (Λc) are produced more often in proton–proton collisions than in electron­–positron collisions. It also showed that its first measurements of such charmed baryons in lead–lead collisions suggest an even higher production rate in these collisions, similar to what has been observed for strange-quark baryons. These observations indicate that the presence of quarks in the colliding beams affects the hadron production rate, shedding new light on the QCD processes that form baryons. The collaboration also presented the first measurement of the triangle-shaped flow of J/psi particles, which contain heavy quarks, in lead–lead collisions. This measurement shows that even heavy quarks are affected by the quarks and gluons in the QGP and retain some memory of the collisions’ initial geometry. Finally, ALICE also presented measurements of particle jets in lead–lead collisions that probe the QGP at different length scales.

For other results, check out the conference page.