Particle physicists like to plan; constructing some of the largest machines in the world needs a long-term vision. Although physics data-taking at the Large Hadron Collider (LHC) began in 2010, the LHC experimental programme can in fact be traced back to the Evian meeting in 1992. To celebrate this 25th anniversary, on 15 December CERN held a symposium to look back at the history and the bold decisions needed to realise the immense detectors and vast worldwide collaborations. The event concluded with the four large LHC experiments – ALICE, ATLAS, CMS and LHCb – reviewing their recent experimental results.
Delving into the extreme physics of heavy-ion collisions, ALICE studies a state of matter that existed just after the Big Bang called the quark-gluon plasma (QGP). QGP is known to behave as a near perfect fluid, and physicists have measured flow coefficients noting stronger flow at larger energies, consistent with hydrodynamic calculations. Latest results on a parameter called elliptic flow in lead-lead and proton-lead collisions show that even the heavy charm quarks follow the fluid expansion, helping physicists to understand more about the QGP evolution. Another key probe of this primordial state is the study of the strange quark, and this year’s results included novel phenomena in strange-particle production in proton collisions that show similar patterns to what is observed in heavy-nuclei collisions.
Using the LHC’s high-energy proton-proton collisions, physicists are rigorously testing the Standard Model. This model, which explains how the basic building blocks of matter interact, has so far withstood the most rigorous of tests. But physicists know it is uncomplete and are determined to find chinks in this model’s armour to reveal new and as yet undiscovered particles and phenomena. The Standard Model’s cornerstone is the Higgs boson. With its discovery announced at CERN in 2012, this newest addition to the elementary particles remains one of the most active areas of research for LHC physicists, and studying its properties has been a quest for both the ATLAS and CMS collaborations.
This year saw several new results of Higgs boson interactions with the heaviest “third-generation” elementary particles: bottom quarks and tau leptons. ATLAS and CMS used data from 2015 and 2016 to establish evidence for Higgs boson decays to two bottom quarks. CMS also presented a “5-sigma” observation of Higgs boson decays to two tau particles. Both ATLAS and CMS saw evidence of “ttH production”, one of the rarest processes measured at the LHC in which a pair of top quarks emits a Higgs boson. This could provide new insights into the Higgs mechanism and perhaps open the door to unknown physics.
The top quark, heavier than the Higgs boson, and in fact all the other elementary particles, also provided a rich ground for investigations this year. ATLAS and CMS joined forces and combined some of their key top quark measurements from proton-proton collisions, including evidence for the associated production of a top quark and a Z boson, a rare electroweak process in the Standard Model. In addition, for the first time, CMS observed top quarks produced in proton-lead collisions. ATLAS also presented high-precision measurements of the top quark mass, which, in combination with the collaboration’s precision measurements of the mass of the W and Higgs bosons, tests the consistency of the Standard Model. CMS also measured the forward-backward asymmetry in Z boson decays to electrons and muons, providing the most precise LHC measurement of the weak mixing angle obtained so far at the LHC.
The elusive physics “beyond the Standard Model” (BSM) remains tantalising for LHC researchers. BSM searches for new particles including supersymmetric particles were aplenty across the experimental collaborations. Despite no conclusive signs of new physics, the experimental results have helped tighten constraints on different models and possibilities, homing in on the most exciting areas of investigation ahead.
One of the most intriguing results comes from LHCb and shows slight anomalies in the way leptons (electrons, muons and tau particles) behave. This potentially challenges a fundamental Standard Model principle known as lepton-flavour-universality and will be a key area of investigation in 2018. LHCb also hit the headlines this year with the discovery of five new particles at once (all slightly different versions of the so-called omega-c baryon) – possibly a record number of new particles for a single publication. Later in the year, the collaboration announced the first observation of a doubly charmed baryon, the first doubly heavy quark particle ever seen. With one trillion beauty hadrons (particles containing a beauty quark) produced at LHCb this year, the collaboration continues to investigate matter-antimatter asymmetry, with results so far being consistent with the Standard Model.
After this year’s great LHC machine performance, physicists are only now beginning to delve into 2017 data as they look ahead to 2018. These physicists like to plan, and indeed, work towards the LHC upgrade, High-Luminosity LHC expected after 2025, has begun in earnest. With the LHC set to continue churning out data at an astounding rate, not only is it a moment to look back at the last 25 years, it is also a chance to look forward to the wealth of undiscovered knowledge that lies ahead.