Particle Physics Group,Department of Physics, Graduate School of Science, Kobe University

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.

The mission of the LHC experiment

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.

• why there are three generations of particles
• why the mass of particles are different by about 11 order of magnitudes, while they are all point-like particles i.e. with the same size.

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:

• The Higgs boson, which provides an evidence for the Higgs mechanism responsible for the mass of elementary particles in the Standard Model, and
• The Super-symmetry, a most promising theory providing explanation for the existence of Dark Matter and also the quantum gravity.

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!

Research Opportunity in Kobe University

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.

• All the tracking chambers of the TGC system produced at KEK are tested in Kobe using cosmic ray muons. We have also contributed a major part in installing the TGC chambers. The chambers serve as a detector system at the level-1 trigger and therefore quickly identify a presence of muons originated from high-energy proton-proton collisions. We have also developed electronics for calculating the momentum of muons (the Sector Logic) and have been maintaining and optimising the board for the calculation of momentum with better precision.
• We have developed monitoring program for the muon HLT (high-level triggers).

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.

1. Cross section measurement of Higgs to WW. Since this channel has relatively large background and no clear mass peak of the Higgs boson, understanding the amount of background is mandatory for precise measurement. We are improving estimate of top quark background as well as two bosons in the final state by using real data.
2. Cross section measurement of top quark pairs through their so-called dilepton decay, where the leptons are produced by the decay of W bosons produced through t -> bW decay. Here the main issue is to understand how large fraction of the leptons is through fakes from hadrons. The measurement is crucial for understanding the detailed production mechanism of top quarks as well as for precise determination of top quark background in many search channels such as Higgs boson and supersymmetry.
3. Search for stable massive charged particles (SMPs), which is an immediate indication of presence of new physics beyond the Standard Model. The method is to detect particles which run significantly slower than the speed of light. We have achieved high-precision calibration of timing detectors to increase the sensitivity.

Upgrading detectors

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.