Online Analysis at LHC run 2

11th December 2015

Summary: First results from Run-2 have used a novel scheme allowing analysis to be performed directly on the output of the trigger. This relies on automatica calibration and alignment of the detector.

Image: Novel elements of the LHCb trigger strategy for LHC Run-2, including realtime alignment.

(Credit LHCb)

It’s been a busy time since summer at the LHCb experiment. The first proton-proton collisions at a centre-of-mass energy of 13TeV have been recorded and the analysis of the data has begun. The new data-taking period allowed LHCb to commission a new system of online calibration and alignment (i.e., the detector is automatically calibrated as the data is recorded - previously this was only done at a later stage). Silvia Borghi, physicist at the University of Manchester, led the development of this system describes the benefits of the new approach: “The new strategy implemented for Run-2 at the LHCb experiment could be considered as a revolution in particle physics. LHCb is the first experiment where the alignment of the full tracking system is evaluated in real-time. This allows us to have the excellent results straight from the detector trigger system, without further offline processing. It has astonished everyone involved that the first two papers from Run-2 used data taken directly from the LHCb trigger system and were published only after few weeks of data taking.”.

Some of the first results to be made public using Run-2 data were measurements of the cross-sections for the production of J/psi, B and charm hadrons [1,2]. Cross-section is a technical term for the probability for each of these hadrons being produced in the LHC proton-proton collisions. Patrick Spradlin, physicist at the University of Glasgow, who coordinated the early data-taking period says “The updated trigger and alignment system worked spectacularly from the first day of physics operation, despite the extensive changes. LHCb was able to produce important physics results with the new data within weeks of its collection. This remarkable achievement is a testament to the careful preparations and the hard work of LHCb personnel. LHCb's Run-2 promises significant advances in both the precision and breadth of its already world-leading flavour physics programme.”.

Physicists at the University of Manchester have also been leading the analysis of the charm mesons. “These measurements provide a ‘standard candle’ against which to compare other measurements”, says Manchester PhD student Alex Pearce. “The processes we looked at in both of these analyses are very well known, but it is important to check that we still understand most basic quantities before moving on to more interesting things.” The upgrade of the LHC proton beams’ energy, from 8 TeV in 2012 to 13 TeV today, causes some physics processes to happen at different rates. LHCb makes and uses measurements of these rates, such as for charm mesons, when estimating how many rare processes it should see over the coming years. It's not only LHCb who finds these measurements useful, however, “the measurements of charm meson production rates we made at 7 TeV have been used to improve models of how comic rays interact with the Earth’s atmosphere to produce neutrinos [3] and the internal structure of the proton [4]. The new measurements from LHCb will further aid those efforts”.

Proton-proton collisions have now stopped for the year, but the LHC isn’t resting as it has now switched to colliding lead ions instead [5]. For the first time the LHCb experiment is recording data during these collisions (previously the experiment was turned-off), leading to a step into the unknown for the collaboration. It is hoped that with the unique particle identification capabilities of the LHCb detector it will be able to say something new about the inner workings of these lead-lead collisions. The heavy-ion collisions will continue for another week or so, prior to the winter shut-down of the LHC during which time some minor maintenance and repair work will be done. Proton-proton collisions will resume in spring 2016 with the aim of significantly increasing the quantity of data that has already been recorded this year.

[1] Journal of High Energy Physics 1510 (2015) 172

[2] arXiv:1510.01707

[3] arxiv:1511.06346

[4] The European Physical Journal C, August 2015, 75:396, arXiv:1503.04581



Dr Patrick Spradlin Patrick.SpradlinATNOSPAM

Mr Alex Pearce Alex.PearceATNOSPAM


The UK participation in the international LHCb experiment is from eleven institutes.

University of Birmingham, University of Bristol, University of Cambridge, University of Edinburgh, University of Glasgow, Imperial College London, University of Liverpool, University of Manchester, University of Oxford, STFC Rutherford Appleton Laboratory, University of Warwick

UK participation in the experiment is funded by the Science and Technology Facillities Council (STFC), with contributions from the participating institutes, the Royal Society and European Union.