KEK - Engineering Physics Sophomores

The High Energy Accelerator Research Organisation(KEK), invited 10 sophomores from EP, to work on various projects, for a duration of 3 weeks, beginning 11 May. This cam under the Japanese government’s Sakura Science Program. This came under the KEK has two campuses - one in Tsukuba which is an electron-positron accelerator and the other in Tokai on the east coast. The Tokai campus goes by the name JPARC (Japan Proton Accelerator Research Complex). As the name states, JPARC is a proton accelerator.

We were invited to the Tsukuba campus which is about 70km from Tokyo. Our airfare was covered and we received a generous amount of money for day to day use.

We were divided into 3 groups, with each group working under the supervision of a Professor.

1. Detection of CMB through Radio analysis of the sky spectrum

Gurbir, Yashvi, and Viraj worked on this project. The CMB group at KEK had developed KUMoDES, a radio system spinoff of a cosmic microwave background detector, which uses the same techniques to determine the precipitable water vapour(PWV) in the atmosphere. This, in turn, is used to predict thunderstorms. We worked on a smaller version called KUMoDES-II, to measure the PWV content as well as the CMB. This involved studying detector technologies, methods of calibrating detectors (using liquid N2), and loads of data analysis. We finally presented our work to the CMB group.

2. Assembling a cosmic ray muon calorimeter and analysing its data (tl;dr in the end :P)

Vedant, Hrishikesh, Shobhna and Prathamesh worked on this one.

This involved testing a small part of the COMET experiment, based in JPARC and due to start operations in early 2018. The aim of COMET is to verify the existence of physics beyond the standard model, by measuring the rate of coherent conversion of a muon to an electron, μ- + N(A,Z) → e- + N(A,Z), which, though allowed by the standard model, has a very low branching ratio (the fraction of decaying particles that decay by this mechanism). Our assignment was to improve the trigger efficiency and calibrate a calorimeter (energy measurement device), using cosmic ray muons which are showering down on us every second. The cool part of this was that we got to handle the actual components that will be used in the final experiment and understand and assemble every part of the experiment from the detector to the data analysis. The muon calorimeter is built using a scintillation crystal called LYSO whose scintillation count is converted to a voltage signal by avalanche photodiodes (APDs), which is then amplified and shaped by a data acquisition system (DAQ). These signals are then read out to a computer. To ensure that we are recording only relevant data, the DAQ system needs to be triggered to capture just that incident signal. This triggering is achieved by using a plastic scintillator whose scintillations are converted to a voltage signal by a photo-multiplier tube (PMT). This setup is usually placed right over the crystal so that the muon passing through the PMT setup then goes through the LYSO. To reduce background noise and improve trigger efficiency, we used two conjoined PMT based scintillators placed at a height above the crystal (so that only muons striking the PMT setup nearly vertically will go through the LYSO). Only if both the PMTs coincidentally generate a trigger will the DAQ be triggered. This clearly reduces the false positive rate. Once the muon strikes the crystal, we must be able to correlate the voltage signal characteristics with the energy of the muon. This is where the calibration comes in. The APDs mounted on the crystals have LEDs placed on them. So, when we send a known signal to the LEDs, we’ll be able to see the output signal and make corresponding correlations. We also devised a simple circuit to test the DAQ using a shaped pulse (which is similar in characteristics to the actual pulse that the system will receive). This is to ensure that DAQ shows essential characteristics like linearity, good signal-noise ratio.

The data was analysed using CERN’s ROOT software. Before making any inferences from the data, we must remove any noise present in the data. So, inherent offsets present in the internal measurement capacitors were subtracted from the data. Then the signal waveform along with other data health metrics were computed and plotted. A point to be appreciated here is that while the data analysis part seems like the easiest part of the assignment, we had to spend a lot of time working on it. It required us to understand the actual measurement system in depth in order to be able to extract information from the data.

As you can see, we got the chance to work on a real life experimental physics problem. And in a lab in the same complex where landmark experiments have been and are being performed. Needless to say, we had an inspiring and very exciting three weeks at KEK.

Tl;dr:

Cosmic ray muons were used to calibrate an energy measurement device called a calorimeter. The components needed for this setup were given to us and we had to assemble the experiment. This helped us understand the role played by each component. Which also helped us in analysing the data that we collected.

3. Calibration of Photomultiplier Tubes against Magnetic Field

Parth Jatakia, Nitin Srirang and Sumukh Vaidya worked on this project.

We worked with photomultiplier tubes which are being used in the Muon g-2/EDM collaboration’s experiment at TRIUMF Canada for finding the gyromagnetic ratio of muons to find new physics beyond the standard model. The experiment was to measure the Gyromagnetic Ratio of the muon with a greater accuracy than what has been done before. The experiment is going to be conducted during June 2017. We worked with PMTs, and designed precise electromagnets to test their response in presence of magnetic field. This included circuits for accurate control of test pulses. The whole setup was made in such a manner that it is completely isolated from the ambient light. One of the important things we learnt here is that in experimental physics data can lie easily and lot of scrutiny is required to get meaning out of the data. At the end we submitted a report to our Professor and it was uploaded on the internal server of the TRIUMF.

FUN and games

It obviously wasn’t all work though. We had the time of our lives exploring the country! The nearby Mount Tsukuba happens to be a popular trekking destination. So, up we went accompanied by entire Japanese families for whom Sundays are spent trekking up hilltops. Watching 70yr old elders lead the charge and 5yr olds enthusiastically follow without breaking a sweat or complaining was the perfect example of Japanese culture. Heck, people brought their dogs too! KEK rents out bicycles for free to its visitors. We took full advantage of that to cycle to the city center, to the base of Mount Tsukuba and for grocery shopping.

Over the weekends we visited Tokyo, stayed overnight and had great fun there visiting popular sites and walking through some neighbourhoods. Some of us happen to know the Tokyo Metro Rail routes better than Mumbai Rail routes!