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Posts Tagged ‘Research’

How to build a universe

Thursday, January 8th, 2015

How do you make a world? This is the purview of theologists, science fiction authors and cosmologists. Broadly speaking, explaining how the universe evolves is no different from any other problem in science: we need to come up with an underlying theory, and calculate the predictions of this theory to see if they match with the real world. The tricky part is that we have no observations of the universe earlier than about 300,000 years after the big bang. Particle colliders give us a glimpse of conditions far earlier than that, but to a cosmologist even the tiniest fraction of a second after the big bang is vitally important. Any theorist who tries his or her hand at this is left with a trail of refuse models before reaching a plausible vision of the universe. Of course, how and why one does this is deeply personal, but I would like to share my own small experience with trying to make a universe.

2014_SL_DM_Abell_teaser

For me, the end was very clear; I wanted to design a universe that explained the existence of dark and visible matter in a particular way. Asymmetric dark matter is a class of theories that try to link the origins of dark and visible matter, and my goal was to explore a new way of creating matter in the universe. So what do you start with? As a particle physicist, the most obvious (but not the only) building blocks at our disposal are particles themselves. Starting with the Standard Model, the easiest way to build a new theory is to just start adding particles. While adding a new particle every time you want to explain a new phenomenon seems indulgent (and some people take this to excess), historically this is a very successful tactic. The neutrino, W and Z bosons, the charm, bottom and top quarks, and Higgs boson were all introduced before they were discovered to explain various theoretical or experimental problems. While back in 1930 Pauli apologised for the bad grace of introducing a particle no one had ever seen, theoretical physicists have well and truly overcome this reticence.

So what ingredients does a dark matter model need? Clearly there must be a viable candidate for dark matter, so at least one new particle must be introduced. While the simplest case is for dark matter to be made of one particle, there is no reason for a substance that makes up 85% of matter in the universe to be any simpler than the matter that we are made of. But, for the sake of simplicity, let us say just one particle for now. For my work to explain the creation of visible matter as well as dark matter, there must also be some interaction between the two. To do this there must be a “mediator”, something that communicates between the visible and the dark. So at least two particles are necessary. Now, two particles doesn’t sound so bad, not when we already know of 17.

The model I was originally going to study (one that already existed) was like this, with dark matter interacting with neutrons. Unfortunately, this is also when the realities of model building sank in; it is rare for any model to be this simple and still work as advertised. Under closer scrutiny it turned out that there was no satisfactory way to make the dark matter stick around for the lifetime of the universe – it quickly decayed away unless you made some theoretical sacrifices I wasn’t comfortable making. Thus began my first foray into model building.

The first hurdle to overcome, for a first-time model builder, is simply the vast size of the literature itself. I was constantly worried that I had missed some paper that had beaten me to it, or had already considered some aspect of my work. Even though this was not the case, even the simplest of possible universes has a lot of complicated physics going on in a variety of areas – and any single aspect of the model failing could mean scrapping the whole thing. Most of these failing points are already known to those experienced in these matters, but a first timer has to find them out the hard way.

In the weeks I spent trying to come up with a model worth studying in detail, I had almost a dozen “Eureka” moments, which were almost always followed by me finding a flaw in the next few days. When you have no strict time limits, this is simply disheartening, but occasionally you can find flaws, or potential flaws, when you are already significantly invested and close to a deadline (such as thesis submission). Unfortunately the only real way to avoid this is to develop a level of care bordering on paranoia, to try and think of all the possible ways a theory might implode before getting bogged down in calculations. Of course, some things are inherently unforeseeable (otherwise why is it research) but many can be divined beforehand with enough experience and thought. This was driven home to me after spending a month working out the predictions of a theory, only to discover that I had drastically underestimated the consequences of a minor change to the model. Fortunately in research little is wasted; even though no part of that work appeared in the final version of my thesis, the methods I learnt certainly did.

leptontree

My pride and joy, a model of ADM via a lepton portal. Leptons (like electrons and neutrinos) interact with scalar dark matter (the phi) to create the matter we see today.

Trying to come up with a theory yourself also forces you to confront your theoretical biases – naturalness, simplicity, renormalisability, testability and fine tuning are all considered by theorists to be important considerations, but it is almost impossible to satisfy all of these at once. Even worse, there are often many different competing interpretations of all of these. So, almost inevitably, sacrifices must be made. Perhaps your theory has to give up on technical naturalness, or has a hell of a hierarchy problem (which mine definitely did). That being said, this is not always an issue;  many models are made to explore a particular avenue, or to provide a working example. The fact that some of these traits cannot be satisfied is important information. You have to pick and choose what you care about, because if the history of physics has shown us anything, it is that theoretical biases, even very well grounded ones, can simply be wrong. The discovery of CP (and consequently time reversal) violation and the non-deterministic (or the apparently non-deterministic, depending on whether you prefer a many worlds interpretation) nature of quantum mechanics are just a couple of examples where “essential” elements of a proper theory turned out to simply not apply.

While this seems like a frustrating experience, I actually greatly enjoyed model building. Too much of university coursework is rushed – you have to learn all of a subject in 12 weeks, and are tested in an exam that only lasts four hours, sometimes in quite shallow ways. This kind of research emphasises patience and care, and allows (or requires) you to deeply understand the physics involved. Calculations are irrelevant for a large part of the process. You simply don’t have time to try and brute force your way through dozens of theories, so you must devise more elegant ways to discriminate and choose those worth the time. I very much doubt that the model I worked on is the underlying truth of our world, but it was very fun to try.

 

 

 

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Grad School in the sciences is a life-changing endeavour, so do not be afraid to ask questions.

Hi Folks,

Quantum Diaries is not just a place to learn the latest news in particle physics; it is also a resource. It is a forum for sharing ideas and experiences.

In science, it is almost always necessary to have a PhD, but what is a PhD? It is a certification that the holder has demonstrated unambiguously her or his ability to thoroughly carry out an independent investigation addressing a well-defined question. Unsurprisingly, the journey to earning a PhD is never light work, but nor should it be. Scientists undertake painstaking work to learn about nature, its underpinnings, and all the wonderful phenomena that occur in everyday life. This journey, however, is also filled with unexpected consequences, disappointment, and sometimes even heartbreak.

It is also that time of year again when people start compiling their CVs, resumes, research statements, and personal statements, that time of year when people begin applying for graduate programs. For this post, I have asked a number of good friends and colleagues, from current graduate students to current post docs, what questions they wished they had asked when apply for graduate school, selecting a school, and selecting a research group.

However, if you are interested in applying to for PhD programs, you should always first yourself,  “Why do I want a research degree like a PhD?”

If you have an experience, question, or thought that you would like to share, comment below! A longer list only provides more information for applicants.

As Always, Happy Colliding

– Richard (@bravelittlemuon)

PS I would like to thank Adam, Amy, John, Josh, Lauren, Mike, Riti, and Sam for their contributions.

Applying to Graduate School:

“When scouting for grad schools, I investigated the top 40 schools in my program of interest.  For chemistry, research primarily occurs in one or two research labs, so for each school, I investigated the faculty list and group research pages.  I eliminated any school where there werre fewer than two faculty members whose fields I could see myself pursuing.  This narrowed down my list to about a dozen schools.  I then filtered based on location: I enjoy being near a big city, so I removed any school in a non-ideal location.  This let me with half a dozen schools, to which I applied.” – Adam Weingarten, Chemistry, Northwestern

“If there is faculty member you are interested in working for, ask both the professor and especially the students separately about the average length of time it takes students to graduate, and how long financial support might be available.” – Lauren Jarocha, Chemistry, UNC

“My university has a pretty small physics program that, presently, only specializes in a few areas. A great deal of the research from my lab happens in conjunction with other local institutes (such as NIST and NIH) or with members of the chemistry or biology departments. If you are interested in a smaller department, ask professors about Institutes and interdisciplinary studies that they might have some connection to, be it within academia or industry.” – Marguerite Brown, Physics, Georgetown

“If you can afford the application fees and the time, apply as broadly as you can.  It’s good to have options when it comes time to make final decisions about where to go. That said, don’t aim too high (you want to make sure you have realistic schools on your list, whatever “realistic” means given your grades and experience), and don’t aim too low (don’t waste time and money applying to a school that you wouldn’t go to even if it was the only school that accepted you, whether because of academics, location, or anything else).  Be as honest as possible with yourself on that front and get input from trusted older students and professors.  On the flip side, if you don’t get rejected from at least one or two schools, you didn’t aim high enough.  You want a blend of reach schools and realistic schools.” – Amy Lowitz, Physics, Wisconsin

Choosing a School

“One of the most common mistakes I see prospective graduate students make is choosing their institution based on wanting to work with a specific professor without getting a clear enough idea of the funding situation in that lab.  Don’t just ask the professor about funding.  Also ask their graduate students when the professor isn’t present.  Even then, you may have to read between the lines; funding can be a delicate subject, especially when it is lacking.” – Amy Lowitz, Physics, Wisconsin

“If you have a particular subfield/group you *know* you are interested in, check how many profs/postdocs/grads are in these groups, check if there are likely to be open slots, and if there are only 1 or 2 open slots make sure you know how to secure one. If they tell you there are currently no open slots, take this to mean that this group is probably closed for everything but the most exceptional circumstances, and do not take into account that group when making your decision.” – Samuel Ducatman, Physics, Wisconsin

“When choosing a school, I based my decision on how happy the grad students seemed, how energetic/curious the faculty appeared, and if the location would allow me to have extracurricular pursuits (such as writing, improv, playing games with people, going to the movies…basically a location where I could live in for 4-6 years).” – Adam Weingarten, Chemistry, Northwestern

“At the visitor weekend, pay attention to how happy the [current] grads seem. Remember they are likely to be primarily 1st years, who generally are the most happy, but still check. Pay attention to the other students visiting, some of them will be in your incoming class. Make sure there is a good social vibe.” – Samuel Ducatman, Physics, Wisconsin

“When I was visiting a prospective grad student, there was a professor at a university I was visiting whose research I was really interested in, but the university would only allow tuition support for 5 years. When I asked his students about graduation rates and times, however, the answer I got was, ‘Anyone who graduates in 5 years hasn’t actually learned anything, it takes at least 7 or 8 years before people should really graduate anyway. Seven years is average for our group.’ In some fields, there is a stigma associated with longer graduation times and a financial burden that you may have to plan for in advance.” – Lauren Jarocha, Chemistry, UNC

Choosing a Group

“When considering a sub-field, look for what interests you of course, but bear in mind that many people change their focus, many don’t know exactly what they want to do immediately upon entering grad school, and your picture of the different areas of research may change over time. Ask around among your contemporaries and older students, especially when it comes to particular advisers.” – Joshua Sayre, PhD, Physics, Pittsburgh
“If you know that you’re interested in an academic career that is more teaching oriented or research oriented, ask about teaching or grant writing opportunities, respectively. I know plenty of fellow students who didn’t start asking about teaching opportunities their 4th or 5th year of their program, and often by then it was too late. If you know that finding funding will be a big part of your future, joining a group where the students take an active part in writing grants and grant renewals is invaluable experience.” –  Lauren Jarocha, Chemistry, UNC
“For choosing groups, I attended group and subgroup meetings, met with faculty to discuss research and ideas, and read several recent publications from each group of interest.  What I did not do (and wish I had) was talk with the graduate students, see how they and the group operated.  For example, I am very motivated and curious to try new ideas, so in my current research group my PI plays a minimal role in my life.  The most important aspect is how well one’s working style fits with the group mentality, followed by research interest.  There’s a ton of cool, exciting research going on, but finding a group with fun, happy, motivated people will make or break the PhD experience.” – Adam Weingarten, Chemistry, Northwestern
“I went into [Condensed Matter Theory] and not [X] because (1) In the summer of my first year I had no research, and I came close to having no income because of this. I realized I needed someone who could promise me research/funding and real advising. The [X] group was pretty filled up (and there were some politics), so it was impossible to get more than this. (2) I thought the professors in CMT treated me with more respect then the [X] profs I talked to.” – John Doe, Physics
“I believe that choosing which grad schools to apply to should primarily be about the research, so this question is more for after you’ve (hopefully) been accepted to a couple schools.  If you are going into theoretical physics, and if you don’t have some sort of fellowship from them or an outside agency, ask them how much their theory students [teach].  Do they have to TA every semester for their funding?  Do they at least get summers off?  Or do they only have to TA for the first one or two years?  This shouldn’t be the primary factor in deciding where to go – research always is – but it’s not something that should be ignored completely.  Teaching is usually somewhat rewarding in my experience, but it adds absolutely no benefit to your career if you are focused on a professorship at a research university.  Every hour you spend steaching is an hour someone else is researching and you aren’t.  And 10-20 hours a week of teaching adds up.” – Michael Saelim, Physics, Cornell
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Me and my research

Thursday, September 10th, 2009

Greetings Quantum Diaries World!

Well, after a little bit of delay I’ve finally gotten the go ahead to begin to do some blogging about my life and times at Fermilab and the greater Chicago area. I do hope to fill this blog with all of the great experiences and people that I am privileged to while I spend my days trying to finish my PhD in particle physics. While I doubt my experience is in any way unique to the many graduate students that have seen their ways through Fermilab, I don’t think many of them have had the chance to share them in this format with the world outside their own community.

Research

So I feel I should start the blogging process with a quick explanation of the research I do here at Fermilab. I am a graduate student with Texas A&M University working on the Collider Detector at Fermilab (CDF), the best experiment in all the land (my humble opinion) !

Texas A&M's Delayed Photon CDF team

Texas A&M's Delayed Photon CDF team

Texas A&M’s group is involved in many different activities on CDF, but the portion I am involved with does searches for new physics being created based on looking for odd signatures not predicted by our current model of particle physics.

Specifically, the research I’m involved in looks for events using photons (the particle of light). Photons are the second most frequently created object at Fermilab (right behind jets) so in principle this gives us a lot of events to look at. However, we aren’t looking for just any old kind of photon. In many models of new physics you would notice their signature by virtue of the photons arriving ‘delayed’ in time (relative to other objects in the event).

This idea is admittedly a little weird and in order to get a handle of things we’ve had to do a lot of non-standard things. Most notably, Texas A&M’s group was involved with the installation and are currently the lead maintainers of a system unique to CDF known as EMTiming. The EMTiming system allows us to get a time signature on the different objects in our events with about 0.5 nanosecond resolution. What this means is we know to a real high precision when in time the objects arrived at our detectors. When we compare this to how fast we would expect the objects to arrive from the collision point we can start to ask the question, “Is there anything new / different here?”

An example of "New Physics" that would have a delayed time signature in our EMTiming system

An example of "New Physics" that would have a delayed time signature in our EMTiming system

The example I show in the image here is an example of Gauge Mediated Supersymmetry Breaking (GMSB) model that if our collisions in the accelerator were creating this interaction than you would start to see a ‘delayed’ photon arriving in our detector. Our EMTiming system allows us to be sensitive to this type of new physics. What is truly amazing about this type of signature based search is that you aren’t limited to looking at one model of new physics. Instead you are sensitive to all sorts of new physics since all you care about is looking for something weird in your data.

Now this is a very simplified view of the type of research that we do and obviously there is much more care taken to defining everything and modeling the physics then what I care to describe right now. However, if you have questions or want to learn more I would encourage you to leave a comment or shoot me an email and I love talking about this wacky physics we do at Texas A&M. As just a side note our group (and collaboration) currently holds the world’s best limit on GMSB models. As shown in the image below, the most recent search completed by Eunsin Lee of Texas A&M has extended our limits on the mass and lifetime of the Next to Lightest Super Particle (NLSP) in many GMSB models known as the Neutralino. Basically, WE ROCK, and if there is new physics to find using delayed photons I am sure that our group and CDF will be the ones to hammer it out!

The most recent results blessed showing the region of parameter space excluded by our searches for new physics at CDF

The most recent results blessed showing the region of parameter space excluded by our searches for new physics at CDF

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