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Large Hadron Collider Kicks Off the 2017 Season

05/30/2017 08:03PM

Large Hadron Collider Kicks Off the 2017 Season
Physicists at the Large Hadron Collider (LHC) have just kicked off the 2017 season. We left Season 2 with LHC providing collisions at the unprecedented energy of 13 TeV (Tera Electron Volts), almost double the collision energy of its Season 1 run that led to the discovery of the Higgs Boson. Operations for the new season are now re-starting with just a few proton bunches per beam, but the operators who control the most powerful collider in the world will soon increase the number of proton bunches and will also more tightly focus the size of the beams at the interaction points. In a few weeks, over a billion collisions will be produced every second at the heart of the experiments. In this new season, the LHC physicists plan to continue improving their knowledge of known phenomena while also probing the unknown.

Last year, the LHC produced an impressive amount of data -- no fewer than 6.5 million billion collisions -- representing an integrated luminosity over the course of the year of almost 40 inverse femtobarns. (An inverse femtobarn is a measurement of particle collision events. One inverse femtobarn corresponds to approximately 100 trillion proton-proton collisions). Luminosity, which corresponds to the number of potential collisions per surface unit in a given time period, is a crucial indicator of an accelerator's performance. In 2017, the operators are hoping to produce the same number of collisions as in 2016, but over a shorter period, since the LHC has started up a month later due to the extended year-end technical stop.

"Over the first two years of operation at a collision energy of 13 TeV, we built up an excellent understanding of how the LHC works, which will allow us to optimise its operation even further in the third year," says Frederick Bordry, Director for Accelerators and Technology at CERN. "Our goal is to increase the peak luminosity even further and to maintain the LHC's excellent availability, which in itself would be a great achievement."

Particle physics relies on the statistical analysis of various phenomena, so the size of the samples is crucial. In other words, the greater the number of collisions that reveal a certain phenomenon, the more reliable the result is. The experiments intend to take advantage of the large quantity of data supplied by the LHC to continue their exploration of physics at the highest energy ever obtained by an accelerator.

"The LHC experiments are well prepared to double their statistics compared to what they obtained in 2016 at 13 TeV. Thanks to the new data, they will be able to reduce the uncertainties that surround their observations every time we enter unchartered territory," says Eckhard Elsen, Director for Research and Computing.

The LHC physicists are working on two different broad areas: improving their knowledge of known phenomena and probing the unknown. The known phenomena constitute the Standard Model of Particles and Forces, a theory that encompasses all our current knowledge of elementary particles. The Higgs Boson, discovered in 2012, plays a key role in the Standard Model. It is also a scalar particle, fundamentally different to the other elementary particles. In 2017, ATLAS and CMS will continue to work on determining the characteristics of this particle. These two large general-purpose experiments will observe its decay modes and how it interacts with other particles. Their measurements may provide indications of possible new physics beyond the Standard Model. The experiments will also carry out precise measurements of other processes of the Standard Model, in particular those involving the top quark, the elementary particle with the greatest mass.

Physicists hope to be able to identify disparities between their measurements and the Standard Model. This is one of the ways in which the unknown can be probed. Although it describes a lot of the phenomena of the infinitely small very well, the Standard Model leaves many questions unanswered. For example, it describes only 5 percent of the universe -- The rest is formed of dark matter and dark energy, the nature of which are as yet unknown. Every discrepancy with regard to the theory could direct physicists towards a larger theoretical framework of new physics that might resolve the enigmas we face.

LHC - Original Plans for the 2016 Physics Run (Video Credit: CERN)

ATLAS, CMS, and LHCb measure processes precisely to detect anomalies. ATLAS and CMS are also looking for new particles, such as those predicted by the theory of supersymmetry, which could be the components of dark matter.

LHCb is also interested in the imbalance between matter and antimatter. Both of these should have been created in equal quantities at the time of the Big Bang, but antimatter is now practically absent from the Universe. LHCb is tracking the phenomenon known as "Charge-Parity Violation," which is thought to be at least partly responsible for this imbalance.

No lead ion collisions, which are the ALICE experiment's specialty, are planned at the LHC this year. ALICE will continue its analysis of the 2016 data and will record proton-proton collisions, which will also allow it to study the strong force. On the basis of the proton-proton collisions from 2016, ALICE recently announced that it had observed a state of matter resembling a quark-gluon plasma. Quark-gluon plasma is the state of matter that existed a few millionths of a second after the Big Bang.

LHC - Achievements of the 2016 Physics Run (Video Credit: CERN)

The LHCb experiment has recently found intriguing anomalies in the way some particles decay. If confirmed, these would be a sign of new physics phenomena not predicted by the Standard Model of particle physics. The observed signal is still of limited statistical significance, but strengthens similar indications from earlier studies. Forthcoming data and follow-up analyses will establish whether these hints are indeed cracks in the Standard Model or a statistical fluctuation.

The LHCb team recently presented new long-awaited results on a particular decay of B0 Mesons produced in collisions at the Large Hadron Collider. The Standard Model of particle physics predicts the probability of the many possible decay modes of B0 Mesons and possible discrepancies with the data could signal new physics.

In this recent study, the LHCb team looked at the decays of B0 Mesons to an excited Kaon and a pair of electrons or Muons. The Muon is 200 times heavier than the electron, but in the Standard Model its interactions are otherwise identical to those of the electron, a property known as Lepton Universality. Lepton Universality predicts that, up to a small and calculable effect due to the mass difference, electron and Muons should be produced with the same probability in this specific B0 decay. LHCb found instead that the decays involving Muons occured less often.

While potentially exciting, the discrepancy with the Standard Model occurs at the level of 2.2 to 2.5 sigma, which is not yet sufficient to draw a firm conclusion. However, the result is intriguing because a recent measurement by LHCb involving a related decay exhibited similar behavior.

While of great interest, these hints are not enough to come to a conclusive statement. Although of a different nature, there have been many previous measurements supporting the symmetry between electrons and Muons. More data and more observations of similar decays are needed in order to clarify whether these hints are just a statistical fluctuation or the first signs for new particles that would extend and help complete the Standard Model of particle physics.

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