Researchers from several of the world’s major institutions made announcements during the 38th International Conference on High Energy Physics, including physicists working at CERN, the T2K Collaboration and Fermi National Accelerator Laboratory (CSA CSM).
CERN showcased its LHC operations and shared new results from experiments. The LHC is now operating beyond expectations, reaching around one billion collisions per second. In just a few months of operations this year, experiments at CERN have already recorded five times more data than in 2015 and the objective of delivering 25 inverse femtobarn (or 2,500 trillion) proton-proton collisions by the end of this physics run is within sight.
“The LHC really entered a new regime by reaching its nominal luminosity, now exceeded by 20 percent,” says CERN Director for Accelerators and Technology, Frédérick Bordry. “It’s a major achievement and we can be confident that we will go beyond our goals for the full second run of the LHC.”
With this larger data set, more precise measurements of Standard Model processes and more sensitive searches for the direct production of new particles at the highest energy are possible, according to researchers. For example, the 125 GeV Higgs boson, discovered in 2012, was observed again this year at the new energy of 13 TeV with higher statistical significance. In addition, both ATLAS and CMS experiments have made new precise measurements of Standard Model processes, especially looking for anomalous particle interactions at high mass, a very sensitive but indirect test for physics beyond the Standard Model.
“This is one of the most exciting times in recent years for physicists, as we dig into the unknown in earnest: the particle physics at an energy never explored before,” says Eckhard Elsen CERN director for research and computing.
The ATLAS and CMS experiments have also looked for any signs of the direct production of new particles predicted by supersymmetry and other exotic theories of physics beyond the Standard Model, but no compelling evidence of new physics has appeared yet. In particular, the intriguing hint of a possible resonance at 750 GeV decaying into photon pairs, which caused considerable interest from the 2015 data, has not reappeared in the much larger 2016 data set and thus appears to be a statistical fluctuation and not indicative of a new particle.
LHCb experimenters presented interesting results in the domain of flavor physics. A particular highlight, according to the researchers, is the discovery of the decay mode B0->K+K-, the rarest B-meson decay into a hadronic final state ever observed, as well as studies of unprecedented sensitivity of CP violation, a very subtle phenomenon explaining nature’s “preference” for matter over antimatter. The LHCb team has also conducted measurements that could help to reveal some new phenomena such as the first measurement of the photon polarization in radiative decays of Bs mesons and determinations of the production cross-sections of several key processes at a collision energy of 13 TeV – some of which, at first sight, are at variance with current predictions.
All four major LHC experiments (ATLAS, CMS, ALICE and LHCb) also revealed results from heavy ion collisions in the machine. Amongst these, the ALICE Collaboration presented new measurements of the properties of quark-gluon plasma, a state of matter that existed a few millionths of a second after the Big Bang. ALICE physicists are studying how the nuclear forces are modified in this primordial state of matter. Researchers also measured the viscosity of the plasma at the new energy, showing that it flows almost like an ideal liquid, the same behavior this is observed at lower collision energies.
“We’re just at the beginning of the journey,” said Fabiola Gianotti, CERN director-general. “The superb performance of the LHC accelerator, experiments and computing bodes extremely well for a detailed and comprehensive exploration of the several TeV energy scale, and significant progress in our understanding of fundamental physics.”
The international T2K Collaboration announced findings on its efforts to determine if CP violation occurs in neutrinos. The question of why the universe is dominated by matter today, instead of being comprised of equal parts matter and antimatter, is one of the most intriguing in all of science. One of the conditions required for the observed dominance of matter over antimatter to develop is the violation of Charge-Parity (CP) symmetry, which is the principle that the laws of physics should be the same if viewed upside-down in a mirror, with all matter exchanged with antimatter. If CP violation occurs in neutrinos, it will manifest itself as a difference in the oscillation probabilities of neutrinos and antineutrinos.
In the T2K experiment in Japan, a neutrino/antineutrino beam is monitored by researchers at a detector complex in Tokai. According to the scientists, the observed electron antineutrino appearance event rate was lower than expected based on the electron neutrino appearance event rate, assuming that CP symmetry is conserved. T2K observed 32 electron neutrinos and four electron antineutrinos, when researchers expected around 23 neutrinos and seven antineutrinos with no CP violation. When analyzed in a full framework of three neutrino and antineutrino flavors and combined with measurements of electron antineutrino disappearance from reactor experiments, the T2K data favors maximal CP violation, according to the research team.
This 2016 result was based on a total data set of 1.51×1021 protons on target (POT), approximately 19 percent of the POT exposure that T2K is set to receive. According to researchers, the probability that this observation is a result of random statistical fluctuations that would mimic a neutrino-antineutrino asymmetry when none exists is about 1 in 20, motivating the need for more neutrino and antineutrino data to explore and solidify these intriguing results. The full T2K exposure of 7.8×1021 POT is expected to come around 2021 after planned upgrades to the J-PARC MR accelerator and the neutrino beamline are completed.
The NOvA experiment at Fermilab also shared a result that could improve the understanding of neutrinos. The particles have previously been detected in three types, called flavors—muon, tau and electron. Neutrinos also exist in three mass states but those states don’t necessarily correspond directly to the three flavors. These states relate to each other through a complex (and only partially understood) process called mixing.
At ICHEP, researchers said NOva had produced evidence that one of the three neutrino mass states might not include equal parts of muon and tau flavor, as previously thought. Scientists refer to this as “non-maximal mixing,” and NOvA’s preliminary result is the first hint that this may be the case for the third mass state.
“Neutrinos are always surprising us. This result is a fresh look into one of the major unknowns in neutrino physics,” says Mark Messier of Indiana University, co-spokesperson of the NOvA experiment.
The research also showed that the third mass state might have more muon flavor than tau flavor, or vice versa. The NOvA experiment hasn’t yet collected enough data to claim a discovery of non-maximal mixing, according to scientists, but if this effect persists they expect to have enough data to definitively explore this mystery in the coming years.
“NOvA is just getting started,” says Gregory Pawloski of the University of Minnesota, one of the NOvA scientists who worked on this result. “The data sample reported today is just one-sixth of the total planned, and it will be exciting to see if this intriguing hint develops into a discovery.”