My PhD Thesis, Preliminary Exam !

My PhD Thesis, Preliminary Exam,

Virginia Tech, Jan 6^th 2004

Manmohan Dash

In 2001 I had joined VT Physics Department to pursue my PhD research in experimental high energy physics which is also colloquially known as particle physics as it’s a field of research that studies in depth the reason and processes behind the interaction of subatomic elementary particles. I had the notion of what are experiments having carried out experiments in science in general and physics in particular in the preceding decades.

I also had a good deal of background in the theoretical knowledge in the field required to carry out such a research. But I was also aware the kinds of experiment I am going to involve myself in are really large sized efforts, partly by educating myself through few articles in the CERN Courier journals that were circulating afar to my alma-mater in Odisha.

What I wasn’t aware of was exactly what was going to be required of me. A fear I often carry to this day.

Upon joining therefore the PhD program I was required to pass the course works at the Physics Department of VT. It wasn’t strictly necessary since I already had passed my Masters courses at my alma-mater in Odisha, by the name of Physics Department, Utkal University.

It was then, necessary for two reasons.

One was official requirement of the department at VT. One who wishes to relocate out of the country (USA) for research work in another country must at-least pass 5 of the courses by being physically available in the Department. One could only pass-out (that is transfer credit for) two courses completed from elsewhere.

I was supposed to join my PhD field work in Japan’s famed laboratory KEK. Keeping in eye this fact the University would not transfer all my credits from my previous master’s degree. Only two courses could be transferred for credit.

The advantage of transferring credit from another institution is you can qualify the PhD course work requirements as soon as you join a PhD program. But this advantage is available to those who would be stationed throughout at VT Department till he passes the prelim or something.

In my opinion this is really one nonsensical rule. How can one change the quality of one’s past courses by being at the Department in a future time? But I wouldn’t take objections.

This is the second reason, I thought to be necessary. By being in a new system whose quality quotients are far higher in many aspects, I am going to learn more, better and enhance my own perspectives of two different systems of education, one of my home country in India and one here in USA.

So, I happily transferred credits for only two courses (in spite of the other possibility of 7 courses, if I were not to travel out of USA) . This so happened therefore that I had to transfer only courses which are called “practical papers” back home in India, that is those where you simply perform experiments, answer a few questions in an interactive session, and subject yourself to the whims of Professorial Kings and Queens, as to what mark you deserve or not rather than what marks you secure by way of your competence.

I had to thus take two additional semesters of pressure due to this uncanny rule at VT Physics (and perhaps other outdated University Departments in the nation). But I was Happy, as the USA education system was very well liked by me, particularly the way courses were designed and delivered or at-least ideally meant to be imparted and effected. Eg the rot system of learning back home in India vs the problem solving approach of the coursework at VT.

I thus finished my coursework, an additional 5 theory papers and one independent paper (what I remember) and set my feet in the 3^rd ever country of residence, Japan.

What I had learned by now is thus, a repetition of all Master degree courses. That is practically I have two master degrees, if not formally. In addition my supreme prepared me for attending and understanding the particle physics basics in experimental facilities.

But when I reached the science city of Japan, where I have lived during next 5 years, from 2002 summer to 2007 summer, I was apprehensive as to what’s required of me.

I thus first took some good degree of practice for writing C++ programs on my own, by certain help from peers. Once I was confidently programming (within a month’s time) I took up coding at a far more advanced and realistic level.

Some of the codes that I actually developed bring out results that have been widely used in this presentation of PhD prelim exam. Eg the entire MC studies in the beginning and the experimental data analysis etc, which are done in this presentation, are actually codes that I developed over a period of 15 months.

So, my first 15 months in Japan is when I learned great deal of new knowledge in my life. A fact that makes me proud till today which is why I pronounce Japan in each of my uttered nuisances. I learned some new language, new programming languages and computer codes, to a level of proficiency that can see me carrying out these jobs like a professional.

Life was pressured out, but I took things to my advantage. Its in this 15 months that soon enough I decided on what particular channel of particle physics decay I would be working on towards my PhD thesis. From a level when I was shy towards senior particle physicists and other scientists I started seeing reason why I would love particle physics for a really long time. From jostling through codes and decay channels of particle reactions, I had narrowed myself down to my decay of choice.

It was only a few months time, after I arrived in summer 2002 in Japan, in which all this happened. And it was by summer 2003 I had finished working on many aspects of my PhD thesis. By this time I had decided and worked many rounds on this particular decay phenomenon called “Cabibbo Favored and Doubly Cabibbo Suppressed channels of Neutral Charm Meson D⁰”.

Once I had decided on this mode of decay of the Neutral Charm Meson D⁰ I had begun developing several codes and undertaken analysis studies of the same for about a year. So from late 2002 till September 2003 I had done a great deal of analysis which is presented in these slides.

In September 2003 for this reason I moved to USA, to my parent alma-mater VT Physics, where in early January I successfully defended my prelim examination. Since I was really well prepared with my studies, I took a driving license test which made my advisor very insecure, as he thought I am not focusing on my PhD. But you know how smart I was, don’t you, for two things at hand, a driving license and a prelim qualification. But he thought otherwise. He threatened me with consequences, which boiled into spats years later. But then everyone has their view of our incomprehensible universe.

So what is it that I had worked on the year prior, my very first year of Particle Physics research in one of the most happening places for particle physics in the world?

The first thing is this experiment called as the Belle Experiment, in Tsukuba (the science city of Japan, only 60 kms from Tokyo where I lived for 3 years between 2002, 2007) is called a CP Violation experiment.

That is this is an experiment that tries to test the prediction of a bunch of theorist, specifically by the name Kobayashi, Masukawa (and Cabibbo) that nature isn’t symmetric with respect to the amount of particles and anti particles in the universe. As a consequence a Physical Rule by the name of CP symmetry isn’t actually a valid truth.

They constituted what’s called a CKM matrix. This matrix is named after the initial of these three scientists. What they proposed is summarized into the property of a physical system, in other words, the matrix simply contains all the angles that are the properties of such system.

If CP symmetry were to be a valid truth of nature this matrix would transform (change into another matrix) in a different way, than it does. The 3 scientists above, several decades ago proposed and theorized that the matrix transforms in certain ways than not. And the experiment at Tsukuba tried to verify that.

In 2008 these scientists (except Cabibbo) along with another Japanese origin scientist by the name Yoichiro Nambu received Nobel Prize for the understanding behind such Physics. (Nambu had discovered the process of spontaneous symmetry breaking by which name this process of CKM matrix goes, in a superconducting system.) As an honor therefore Belle Experiment was to be cited for this Nobel Prize.

Hence I have worked towards this goal of verifying CKM matrix, for years.

But my own PhD thesis is a kind of different tale altogether. For my work there is no CP Violation.

In CP Violation two leptons (electrons and positrons are leptons) are imparted quite good amount of energy and made to collide against each other. Thus just like in CERN lab, there is a much smaller tunnel at the Tsukuba facility, called an accelerator-detector facility. In this facility an electron and positron pairs are given enough energy and made to circle in opposite direction.

When they collide, they are made to do so, right inside the detector. They produce pairs of B-mesons. The B-meson one and two, they are almost at rest when they are produced, in the lab frame of reference. But the matter of fact is the B-mesons are produced from what are called Upsilon (4S) mesons.

These Upsilons are called resonances, and they are highly unstable (all resonances are short lived elementary particles). So they turn into B-mesons as soon as they are created.

These B mesons are then liable to further decay. In fact governed by nature’s idiosyncrasies, there are literally 100s of channels in which such decays can occur.

So one such possibility is when these B mesons turn into what are called neutral charm mesons or D⁰’s.

But instead of concerning my-self with the direct study of B mesons my PhD thesis was about studying these neutral D mesons.

(Yes they are called as neutral mesons because they do not have electric charges; they are called D because of the prominence of down quark d and so on, Physicists are wary of naming elementary particles in fanciful ways just for the heck of it. Note of caution; not all variants of D mesons contain a d quark, but only the charged D quarks do contain d quarks, apart from a c quark)

So what do the D’s do? This seemed to be one of those elementary questions a young Physicists like me was thought to be motivated to pursue. Good enough for me, I did not try to leave a stone unturned to know such. But I had many other motivations under my sleeve, one of which was also, knowing about, why at all should I be motivated to know such arcane stuff? As long as its associated with some deeper convictions, like one actually brushes with how nature works, I am on.

In any case, for employment one needs to decide fast, this or that? And quickly you go “okay that”.

So for me that was Neutral Charm Mesons. (The D meson in addition to a d quark has a c or charm quark hence, its called as a charm meson, what a geek must have named it ! Also the D when charged has a c and d in it. But for our discussion the neutral charm meson that we are talking about has a c and a s in it, its a charmingly strange meson, a reason why it shall enthrall me for a long time to come.)

So there is this known phenomenon where a D meson absolves into a K or anti-K meson, along with a partner of the decay final product the Pion or pi meson. A K is also known as a strange meson, because as per lackluster particle physics schemes of naming particles, devoid of fanciful profiling, the K meson actually contains a strange quark or a s quark. See again, s for strange, just like b for beauty or bottom, and c for charm and t for top and so on. Also the K is called a Kaon. There are tons of variants of such particles and short-lived resonances. Eg a K* is also relevant to our discussion.

So a D goes into a Pi and a K (or as the rules allow, an anti particle of any of the three, D, Pi, K). We say D as the initial state particle and Pi and K as final state particles. Because of this; A==>BC, if A goes to BC, B and C are final state particles, A==> BCD, if A goes into BCD, B, C and D are final states.

In our final state there is thus a kaon, K or anti-K. They both have few variants, neutral and charged, and so on. The neutral kaons are seen in two forms, a K-short and a K-long. They are so named because the K-short dies very soon. But the K-long lives longer.

Since the Ks, Kl (short and long) are two different forms of the same kaon, nature has it, in its idiosyncratic rules that the kaon (K) and hence the D (the parent of K) decay in different fashion and in different frequency via the Ks and Kl channels.

In other words D to Ks and D to Kl have different number or rates of production.

My goal according to my thesis was then to count via highly tedious and complex methods of data analysis and software programming and laws of particle physics, the difference of frequency of production to these two different short and long channels.

These frequencies when standardized in certain ways (eg compared to a parent reaction) are called as branching fractions and the difference of branching fraction is called as asymmetry.

My job was then to measure the branching fraction of these two modes and their resulting asymmetry.

In particle physics it so happens that when you measure two modes or channels you have to practically measure tons of modes, because they each have feedback channels among each other.

Its like a complex banking where A loans to B, B loans to c, C loans to A and so on and eventually you are to find who owes whom how much. What a head ache. Your bank boss comes and pours on you tons of records and only in raw data form, lets say 50 people with their possible parent names and so on. You take years to calculate via various methods and hypothesis “who owes whom how much”.

So as a result I had to measure literally tens of other modes. The actuall data analysis is far far more complicated, but if I have to tell you I have to write 20 more pages or more. I am only 5 now. Lets save some labor.

So the basic of these two modes D-to-Kshort-pi and D-to-Klong-pi is that Klong is a hard ass, in how it behaves inside the detector. It has minimal but useful information available in the detector, which is described in detail in this presentation. But we can’t give up right?

So we devise whats called a missing mass method or in our case a D-zero constraint method, in this method we know that the Klongs originate from their parents that are D-zero. So we need to tag them in smart ways to decide they are indeed Klong (in the detector) and not some garbled up orphans of some other ass.

It works, and one of the beauty of the codes I developed over the several months was a toy monte-carlo and another a realistic monte-carlo which are described in detail how well they worked to identify the Klongs.

Things are far far more complicated as to why I was doing what I was doing in identifying the whole processes, but we did have plenty of leads and interesting findings.

In order to calibrate (that is compare to another standard way of doing something) the asymmetry that we find in the above two modes: of kshort and klong with pi, from the parent D-zero we need to study another two modes, and in practicality of particle physics another tons of modes. In all total I was doing analysis of 50-some particle modes or more.

These two other modes for calibration processes were also involving Klongs as they involve Kshorts and the methods only got tedious although promising results were being achieved all the time. These two latter modes were the decay of the Dzero via K* resonances to final Kshort and Klong. Since K* is accompanied by pi, we have a 3 final state particle reactions.

So we have Dzero-to-Kshort-pi-pi and Dzero-Klong-pi-pi instead of just Dzero-Kshort-pi and so on.

The studies that I did for a year and presented to my thesis committee has been described in detail in these presentation slides.

When I finished my prelim requirements and returned to Japan in the summer of 2004, I took up a quite different approach to analysis and coding and redid all my analysis and studied Monte Carlo as well experimental data.

That was far more complicated, advanced and robust in par with how the analyses are actually done at the facility in Japan. Such results have been presented in tons of presentation slides that I will describe in the future.