La Vie Est Belle

16th May 2017
Saurabh Sandilya

The field of ‘High Energy Physics’ or ‘Particle Physics’ is about exploring the fundamental building blocks of matter and interactions between them at the deepest level. The experimental frontier of this field relies on the cutting edge instrumentations and the most advanced computational tools. So, as an experimental high energy physicist, I am excited by the theoretical aspect as well as its experimental advancement is also overwhelming.

I started my journey in this field seven years back with the Belle (‘french:beautiful’) experiment as a doctoral student at Tata Institute of Fundamental Research, Mumbai. The Belle detector was located at the interaction region (beam collision point) of the KEKB asymmetric energy electron-positron collider in Tsukuba, Japan. And, the KEKB accelerator holds the current world-record for the luminosity achieved by a high-energy accelerator. The Belle experiment had a successful operational period with several important physics results, and the ‘Observation of CP Violation in B meson system’ led to the Nobel Prize in Physics 2008 to Profs. Kobayashi and Maskawa. The Belle detector recorded about 1 ab-1 data between 1999 - 2010, which continues to produce very competitive physics results. But, to explore the unknown territory of New Physics (beyond the Standard Model of Particle Physics) even more data is needed. For this, the KEKB accelerator is now being upgraded to SuperKEKB and it is designed to produce 40 times larger instantaneous luminosity than its predecessor and with the aim of recording an unprecedented data sample of 50 ab–1. And, to cope with this high collision rate environment, the detector is also being upgraded to Belle II. 

Although I got a very brief opportunity to work on the detector development for Belle II during my doctoral period, as my thesis was mainly focused on the physics data analysis for the Belle experiment. However, my desire to work on the instrumentation got fulfilled as a post-doctoral fellow in University of Cincinnati. I participated in the construction of time of propagation (TOP) detector, which plays a crucial role in identification of charged particles produced after the collision. The Belle II detector consists of several sub-detectors and TOP detector is one of them.

Each sub-detector of the Belle II collects specific information about the collision event. Closest to the beam pipe is the Vertex Detector which consists of two-layers silicon pixel detector and then four-layers of double-sided silicon-strip detector and identifies the vertices (or decay points) of short lived particles (with lifetimes of around a trillionth of a second).  Then, the Central Drift Chamber provides the momentum of charged particles by reconstructing their curved trajectories while moving in a magnetic field. Additionally, it contributes to particle identification by measuring the energy loss of charged particles as they pass through the gas that fills the volume. Aerogel Ring Imaging Cherenkov detector in the forward endcap region and TOP in central region of the detector provides charged particle identification based on angle of the cherenkov photon emitted by the charged particle passing through the detector medium. The The electromagnetic calorimeter reuses Belle’s thallium-doped cesium-iodide crystals with upgraded read-out electronics. And, the flux return of the Belle-II solenoid magnet, which surrounds the electromagnetic calorimeter, is instrumented to detect KL mesons and muons.   
In February 2016, electron and positron beams were successfully stored in the upgraded  SuperKEKB accelerator for the first time. Now, most of the detectors are installed in the Belle II detector. And, on April 11th this year, the Belle II detector was rolled-in from its construction area to the interaction region of the SuperKEKB particle accelerator. The roll-in of the assembled Belle II detector, weighing 1,400 tons, has to be carried out very gently and with great care. And, this successful event was broadcasted live worldwide.  Belle II is now getting ready in full swing to record the first collisions at the SuperKEKB scheduled at the end of this year. 

The Belle II collaboration has more than 600 members from 23 countries and this also provides a nice ground for cultural exchanges while interacting with colleagues from around the world. I really admire working in these large collaborations, as people go beyond the boundaries of geography, race and religion and come forward for a common goal ‘to extend the knowledge of mankind’. The sense of joy and thrill, while exploring together the recipe of our Universe as we see today, is unparalleled.