ATLAS is one of the two multi-purpose experiments on the LHC accelerator, which produces the highest-energy proton-proton collisions ever made by human beings. The accelerator has been hosted by CERN (European laboratory for particle physics), located across the border of Switzerland and France, in the vicinity of Geneva. The design collision energy is 14 TeV, seven times higher than that in the previous experiment at the Tevatron (2 TeV, Fermilab in US). The objective of the experiment is to explore and discover new interaction and new elementary particles at TeV (tera electron-volt) energies. The LHC has started its experiment at the end of March 2010 with the centre-of-energy of 7 TeV, All the particle detectors in the ATLAS experiment has successfully been operated, including the TGC (Thin Gap Chambers) muon detector for trigger, which Kobe University has played a major role in constructing, testing and installing to the experiment. We have taken the data since then until February 2013, when we stopped the accelerator operation for necessary maintenance to restart at a higher energy of about 13 TeV in 2015. The successful operation of detectors together with tremendous effort in analysing the data has led to the discovery of a new particle, whose property is consistent with the Standard Model Higgs particle. We are expecting more findings at the higher energy 13 TeV run, where the luminosity (the rate for collisions) is also higher so that the experiment is more sensitive to not only the interactions present only at very high energy, but also rare phenomena.
All elementary particles and forces mediating the interaction between, which are known to human beings, are described by the Standard Model of particle physics. The model has been tested by numerous experiments with very high precision – we have not yet found any significant breakdown of the theory so far. Yet there are some mysteries remaining in the Standard Model, e.g.
For these and various other reasons, it is widely regarded that the Standard Model is merely an effective theory at present energies (~ 100 GeV and below) and there IS a theory at higher energies. The LHC experiment aims for finding the evidence of such new phenomena beyond the Standard Model by realising collisions at these high energies, TeV and beyond. The expected physics we may find there are the following:
We may, however, discover something unexpected. In fact, the revolution has already started by the discovery of a new particle, which is likely the Higgs boson in the Standard Model. This discovery alone imposed us more questions than ever, rather than solving the above given questions. It is never better than now to start research on collider physics!
The Kobe ATLAS group the third largest among other 15 institutes in the ATLAS Japan group (next to KEK and Univ. of Tokyo). We have been very much involved in construction and commissioning of detectors and trigger, which is a system for selecting online the events of interest.
Altogether, we have been contributing to the preparation and operation of the muon detectors and trigger system. The present activities are: further improvement on the detector and trigger operation, the data analyses for physics and development of detectors for the upgraded LHC with higher data-taking rates. Detector operation and development (mainly by master-course students, under the supervision of staff members) The level-1 muon trigger has to have greater rejection power on background processes after 2015 runs where higher rates of collisions by factor 3 or more is expected. For that, the trigger logic calculated in the Sector Logic board is to be more restrictive while keeping reasonable trigger efficiencies. A new firmware with such an improvement by incorporating the hit information from the innermost layer of the endcap muon detector is being prepared. Also investigated is the trigger logic for the level-2 muon trigger, which is to be improved for the robustness against increasing number of hits from background, and also for better momentum resolution in general. In parallel, much effort has been on the understanding of trigger operation for the data already taken in past two years. The precise determination of trigger efficiencies using actual data taken is a necessary ingredient for precise physics measurements with muons involved. Needless to say, similar amount of effort has been put by students for the daily operation of the TGC detector.
Data analysis (doctor-course students, postdoctoral fellows and assistant professors) We are focusing on three subjects on data analyses.
ATLAS is planning an upgrade for the innermost layer of the endcap muon detector, called the New Small Wheel, for reducing higher trigger rate expected after the phase-1 upgrade of the detector. The idea is to install high-resolution detectors to have precise determination of the track angle for estimating the momentum of muons already at level-1. Kobe University is developing a new method for producing a resistive sheet, which is to cover the electrode of the gas detectors in order to prevent sparks induced by a high amount of ionisation caused by massive charged particles (like protons recoiled by slow neutrons). This study is taken place mainly by the mu-PIC group and the ATLAS group is in collaboration to them. Also in progress is the trigger logic and trigger board development of level-1 endcap triggers as well as muon HLT development with the New Small Wheel.