The LHCb detector seen in 2018 in its underground cavern. The excellent precision of this detector allowed LHCb physicists to perform detailed measurements on the doubly charmed particle they discovered only last year. (Image: M. Brice, J. Ordan/CERN)
Finding a new particle is always a nice surprise, but measuring its characteristics is another story and just as important. Less than a year after announcing the discovery of the particle going by the snappy name of Ξcc++ (Xicc++), this week the LHCb collaboration announced the first measurement of its lifetime. The announcement was made during the CHARM 2018 international workshop in Novosibirsk in Russia: a charming moment for this doubly charmed particle.
The Ξcc++ particle is composed of two charm quarks and one up quark, hence it is a member of the baryon family (particles composed of three quarks). The existence of the particle was predicted by the Standard Model, the theory which describes elementary particles and the forces that bind them together. LHCb’s observation came last year after several years of research. Its mass was measured to be around 3621 MeV, almost four times that of the proton (the best-known baryon), thanks to its two charm quarks.
The Ξcc++ particle is fleeting: it decays quickly into lighter particles. In fact it was through its decay into a Λc+ baryon and three lighter mesons, K-, π+ and π+, that it was discovered. Since then, LHCb physicists have been carrying on an analysis to determine its lifetime with a high level of precision. The value obtained is 0.256 picoseconds (0.000000000000256 seconds), with a small degree of uncertainty. Though very small in everyday life, such an amount of time is relatively large in the realm of subatomic particles. The measured value is within the range predicted by theoretical physicists on the basis of the Standard Model, namely between 0.20 and 1.05 picoseconds.
To achieve this precise result, LHCb physicists compared the measurement of the lifetime of the Ξcc++ with that of another particle whose lifetime is well-known. They based their measurements on the same sample of events that led to the discovery.
Measuring the lifetime of a particle is an important step in determining its characteristics. Thanks to the abundance of heavy quarks produced by the Large Hadron Collider (LHC) and the excellent precision of the LHCb detector, physicists will now continue their detailed measurements of the properties of this charming particle. With these types of measurements, they are gaining a better understanding of the interactions that govern the behaviour of particles containing heavy quarks.
More information on the new measurements of the Ξcc++ particle can be found on the LHCb website.
10,000 tracks grouping 100,000 points in a future LHC detector as simulated for the TrackML challenge (Image: TrackML Challenge Team/CERN)
Physicists from the ATLAS, CMS and LHCb collaborations have just launched the TrackML challenge – your chance to develop new machine-learning solutions for the next generation of particles detectors.
The Large Hadron Collider (LHC) produces hundreds of millions of collisions every second, generating tens of petabytes of data a year. Handling this flood of data is a major challenge for the physicists, who have developed tools to process and filter the events online within a fraction of a second and select the most promising collision events.
Managing the amount of data will become even more challenging in the near future: a major upgrade foreseen for 2026, the planned start of the High-Luminosity LHC, will increase the collision rate up to a factor of five. Innovative new software solutions will be needed to promptly reconstruct the tracks produced by these collisions with the available computing resources.
To help address this issue, a team of machine-learning experts and LHC physicists has partnered with Kaggle to probe the question: can machine learning assist high-energy physics in discovering and characterising new particles?
Specifically, in this competition, you’re challenged to build an algorithm that quickly and efficiently reconstructs particle tracks from 3D points left in the silicon detectors. The challenge consists of two phases:
The “Accuracy Phase” is now running on Kaggle from May to July 2018. Here the focus is on the highest score, irrespective of the evaluation time. This phase is an official IEEE WCCI competition (Rio de Janeiro, July 2018).
The “Throughput Phase” will run on Codalab from July to October 2018. Participants will submit their software to be evaluated by the platform. Incentive is on the throughput (or speed) of the evaluation while reaching a good score. This phase is an official NIPS competition (Montreal, December 2018).
Sign up for the TrackML challenge today. The top three scorers will receive cash prizes. Selected winners may be awarded a top-notch NVIDIA v100 GPU, or get the chance to visit CERN or attend the 2018 Conference on Neural Information Processing Systems in Montreal (Canada).
A collision event recorded by LHCb on 28 April, following the formal start of this year’s data taking (Image: LHCb/CERN)
On Saturday, 28 April 2018, the operators of the Large Hadron Collider (LHC) successfully injected 1200 bunches of protons into the machine and collided them. This formally marks the beginning of the LHC’s 2018 physics season. The start of the physics run comes a few days ahead of schedule, continuing the LHC’s impressive re-awakening since the end of its annual winter hibernation just over a month ago. In early April, a small number of bunches were injected into the ring to deliver test collisions inside the four large LHC experiments. These experiments – ALICE, ATLAS, CMS and LHCb – have now begun their data collection in earnest, which they will use to continue measuring the properties of the Standard Model of particle physics and search for any chinks in its armour.
The Standard Model provides the best explanation of the properties of all known particles and three forces that govern them: the electromagnetic force, the weak force and the strong force. But we know that this model does not give us the complete picture of our universe. For one, it only addresses 5% of the contents of the universe: the remaining 95% is thought to be made of dark matter and dark energy, and the Standard Model has no answers for these mysteries. It also does not provide any way of integrating gravity with the three other forces.
Particle physicists working on the LHC experiments seek explanations to fill this gap in two principle ways. Firstly, they do so by taking a close look at various phenomena predicted by the Standard Model and looking for subtle differences between prediction and measurement. In addition, they search for previously unobserved phenomena and particles. Both types of searches for physics beyond the Standard Model require huge quantities of data in order to filter a potential signal from the expected background processes. The LHC experiments will therefore hope that the LHC continues its annual tradition of outdoing its previous year’s data volume.
An event recorded by ATLAS earlier in April, from some of the first collisions of the year with three proton bunches circulating in the LHC (Image: ATLAS/CERN)
ATLAS and CMS, the two “general-purpose” detectors, will continue to probe the properties of the Higgs boson that they discovered in July 2012. This particle is the newest tool in the utility belt used by particle physicists to explore the properties of nature. Since its discovery, physicists have studied its behaviour and interactions with other particles, which have so far shown good agreement with the Standard Model. Searches will also continue for supersymmetric partners of the familiar bosons and fermions that are predicted to exist by a family of theories known as supersymmetry, which might provide us with a candidate for a dark-matter particle. ATLAS, CMS and LHCb are also searching for hints of dark matter through other means, and will add the forthcoming trove of data to their stockpiles as they advance their explorations.
An event recorded by CMS earlier in April, from some of the first collisions of the year with three proton bunches circulating in the LHC (Image: CMS/CERN)
Among other searches, LHCb will continue to seek a solution to the problem of matter-antimatter asymmetry, as the Standard Model cannot adequately explain the observed abundance of matter in the universe. When matter was formed in the Big Bang, there should have been an equal amount of antimatter accompanying it; each matter-antimatter pair should then have annihilated upon contact, leaving us with a universe without stars or human beings to observe them.
ALICE, the LHC’s heavy-ion specialist, focuses on collisions of lead nuclei in order to study the strong interaction and the quark-gluon plasma, a state of matter that is believed to have prevailed in the very early universe. However, ALICE will also record proton-proton collisions to continue its investigation of the properties of collision events that contain a large number of particles produced at the same time and to serve as a baseline with which to compare lead-lead collisions.
An event recorded by ALICE earlier in April, from some of the first collisions of the year with three proton bunches circulating in the LHC (Image: ALICE/CERN)
The LHC operators will keep ramping up the number of bunches, aiming to hit 2556 bunches in total. This will help them achieve their target of 60 inverse femtobarns (fb-1) of proton-proton collisions this year delivered to both ATLAS and CMS, 20% more than the 50 fb -1 achieved in 2017. In simple terms, each inverse femtobarn can correspond to up to 100 million million (1014) individual collisions between protons. The proton-proton run will be followed by the first heavy-ion run since 2016; the LHC will inject and collide lead nuclei at the end of the year.
This is the last year with collisions before the LHC enters a period of hibernation until spring 2021, known as Long Shutdown 2, during which the machine and the experiments will be upgraded. All four experiments will therefore hope to maximise their data-collection efficiency to keep themselves occupied with many analyses and new results over the two-year shutdown, using high-quality data collected this year.
Read more about the start of the LHC's 2018 physics run:
A “beam splash” event in the ATLAS detector on 6 April 2018, as part of the LHC restart. The image shows tracking-detector hits (red), and energy deposits in the electromagnetic (green) and hadronic (yellow) calorimeters. (Image: CERN)
Proton slamming has resumed at the Large Hadron Collider (LHC). Almost a fortnight after the collider began circulating proton beams for the first time in 2018, the machine’s operations team has today steered beams into collision. While these are only test collisions, they are an essential step along the way to serious data taking, which is expected to kick off in early May.
Achieving first test collisions is anything but an easy job. It involves round-the-clock checking and rechecking of the thousands of systems that comprise the LHC. It includes ramping up the energy of each beam to the operating value of 6.5 TeV, checking the beams’ instrumentation and optics, testing electronic feedback systems, aligning jaw-like devices called collimators that close around the beams to absorb stray particles and, finally, focusing the beams to make them collide.
Each beam consists of packets of protons called bunches. For these test collisions, each beam contains only two “nominal” bunches, each made up of 120 billion protons. This is far fewer than the 1200 bunches per beam that will mark the start of serious data taking and particle hunting. As the year progresses, the operations team will continue to increase the number of bunches in each beam, up to the maximum of 2556.
With today’s test collisions, the teams of the experiments located at four collision points around the LHC ring (ALICE, LHCb, CMS and ATLAS) will now be able to check and calibrate their detectors. Stay tuned for the next steps.