RESEARCH SUMMARY Marc Sher---self-evaluation
Since my first publication in 1978, I have published approximately 120 papers in peer-reviewed journals, as well as a number of conference proceedings, maintaining a fairly steady pace of 3-5 publications per year. These papers have received more than 5500 citations. A full list is at this link. During my first semester at William and Mary, I applied for an NSF grant; the grant was awarded and I have received substantial NSF support ever since.
Although the focus of my research interests have shifted over the years, they have concentrated in the area of the phenomenology of electroweak and unified theories. "Phenomenology" refers to the interplay between theory and experiment. Many physicists work on extremely mathematical aspects of field theory, such as string theory, without a great concern for specific experimental signatures. Although this is certainly important (and is most likely to eventually lead to the "theory of everything"), I prefer to work with ideas that have some chance of being experimentally tested in my lifetime. In fact, I am not at all disappointed that about a dozen of my papers are on models that have since been experimentally excluded----if none of these papers had yet been excluded, then my work would be too far away from the real world. "Electroweak theory" unifies two of the four fundamental interactions, the weak interaction (responsible for some radioactive decays) and electromagnetism (the other two are the strong interaction, which holds the nucleus together, and gravity). There is a theory, developed in the early 1970s, called the "Standard Model", which has survived a large number of high-precision tests over the past decade. However, there are many unsatisfactory features of the model, and this has led to a large number of alternative models and extensions which will be tested in the very near future. Much of my work has looked at these alternatives. "Unified theory" refers to proposals which unify the strong and electroweak interactions into a single framework; these theories also make various predictions which can be tested. The experimental evidence that tests these models come from accelerator-based experiments (at Fermilab, CERN, Brookhaven, and SLAC), low-energy experiments (such as the large neutrino detector in Japan), as well as astrophysical and cosmological data.
A central area of investigation has concentrated on the properties of the Higgs boson. In the Standard Model, all particles obtain their masses through their interactions with a background field, called a Higgs field. The Higgs boson is an excitation of that background Higgs field. In the Standard Model, the interactions of the Higgs boson with all particles is completely determined, however the mass of the Higgs boson itself is completely undetermined. The Higgs boson is the "holy grail" of particle physics; its discovery will tell us if we understand the origin of masses, and the Large Hadron Collider at CERN (a 5 billion dollar collider in Geneva) is primarily designed to detect this particle. I am perhaps best known for my work on using theory to determine constraints on the mass of the Higgs boson, in the standard model and in many of its various alternatives and extensions.
My PhD thesis primarily concerned the Coleman-Weinberg mechanism (although there were four additional publications included in the thesis). There is a free parameter in the Standard Model which has units of mass. It is known that this parameter must be much smaller than other masses in the model. The Coleman-Weinberg mechanism supposes that this parameter is exactly zero. In that case, the Higgs boson mass is calculable. At this time, in 1978, the calculation of the mass gave approximately 10 GeV (the proton mass is a little less than a GeV). It turns out that there were some well-studied mesons whose masses were near 10 GeV, and the precise value of the mass prediction was crucial in studying the possibilities of detection of the Higgs. My thesis involved a precise calculation of this mass. The calculation took a year, and involved several hundred Feynman diagrams. After this calculation was done, I was invited to speak on it at CERN, the largest particle physics laboratory in the world. I went there just before starting my first post-doc at UC Santa Cruz. While there, I met a lot of people who were beginning to study the effects of the Higgs system on the early universe----certain ranges of mass for the Higgs lead to serious cosmological difficulties. I then began to learn about cosmology.
As a postdoc at UC Santa Cruz, I was given an office at the Stanford Linear Accelerator Center. Virtually everyone in particle physics visits SLAC every few years, so I was able to meet most of the people in the field. In particular, I got to share an office with Alan Guth, a physicist from MIT. While at SLAC, Dr. Guth created a new concept in cosmology, called inflation. Inflation has completely dominated cosmology for the past fifteen years, and is now the "standard" paradigm. Since inflation takes place in the very early stages of the universe, it is difficult to test experimentally (although lately there have been some tests involving various cosmological measurements); I was more interested in ideas that could be tested more precisely, so I continued to study Higgs bosons (now focusing on Higgs masses in extensions of the standard model), but also kept up with the goings-on in inflationary cosmology.
During my post-doc at Santa Cruz, there were two papers that were very important. The first was written while I was spending three months at the Institute for Theoretical Physics at UC Santa Barbara (my wife was at Caltech for four years after my degree, and UCSB was closer to Pasadena than UCSC). The simplest models which actually gave inflation were based on Coleman-Weinberg models--the same model I had studied in my thesis. Based on what I'd learned from that calculation, and some follow-up work, I realized that everyone who was doing calculations of inflation was making a serious mistake. They were using Higgs fields which had grand unified interactions, and yet neglecting the fact that these interactions depend on temperature. If one includes this effect, then the models which previously gave inflation automatically failed (by many orders of magnitude) In fact, every calculation that was being done at the time on inflationary models (more precisely, on Higgs structures that would allow for sufficient inflation) was wrong. I sent the paper to CERN (back then, it was express mail, not electronic), and was told (later) that a number of physicists had to stop what they were doing. From that point on, all models which have inflation use "singlet" Higgs fields, which do not have grand unified interactions and thus avoid this problem. Although the paper does have quite a few citations (around 40-50), its main role was to stop a number of calculations (not the best way to get citations!!).
The second, and more important, paper concerned supersymmetry. Since 1980, roughly 40% of all particle physics papers have dealt with supersymmetry. This is a mathematical symmetry which doubles the number of particles (just as the prediction of antimatter doubled the number of particles), but has a large number of attractive features. Many of the precise cancellations needed to make the standard model work are automatic, the theory automatically incorporates gravity, etc. In 1982, I had been working with a graduate student at Santa Cruz, Ricardo Flores (I was his de facto, but not de jure, thesis advisor). Ricardo and I had been looking at bounds on Higgs masses caused by vacuum stability---it turns out that a heavy quark will cause the Higgs system to become unstable, leading to an unacceptable vacuum instability. After writing a few papers (Ricardo and I wrote five papers while he was a student) about this issue, as well as Higgs mass bounds from other extensions of the standard model, we decided to write a review article on the subject for Annals of Physics. While writing the article, supersymmetry came into vogue, and we began to think about Higgs masses in supersymmetry. To our surprise, we discovered that the standard supersymmetric model had a UPPER bound on the mass of a Higgs boson of 95 GeV. Upper bounds are very important, since they can be experimentally reached in a limited time---failure to discover the Higgs below that mass would rule out these supersymmetric models. This was published (although some Japanese authors came up with the same bound simultaneously, within a week of our article). It had a great impact on the field, since previous upper bounds on the Higgs mass had been about 1000 GeV, which could only be reached at a supercollider. In planning for the extension of the Large Electron-Positron collider at CERN, this bound played a major role in determining the energy of this five billion dollar accelerator. (The Director-General of CERN, Chris Llewellyn-Smith, told me that the mass bound in supersymmetry was the determining factor in using the lab resources of the extension of LEP, rather than pushing as quickly as possible for the large hadron collider.) That paper now has over 240 citations (alas, citations to this bound now refer to later supersymmetry review articles). Incidentally, Ricardo is now a tenured professor at the University of Missouri.
It turns out that the bound was based on a first-order calculation. During my second postdoc, at UC Irvine, I calculated the bound to higher order. We used a top quark mass (an input to the calculation) of 40 GeV, since physicists had reported its discovery at that energy. Later, the top quark was found to have a mass of 170 GeV, and so our result was altered, although the technique used was identical. The current upper bound is approximately 130 GeV. This was slightly out of reach of LEP. Had I updated my results when the top quark "discovery" went away, it could have had a substantial impact on the future of CERN. Sigh...
When I returned to Santa Cruz for a third post-doc (technically, a visiting assistant professorship---during all of my post-docs I taught a number of undergraduate and graduate courses), I continued to focus on the Higgs system, cosmology, and supersymmetric models. At this time, superstring theory was increasing in popularity, and there were some indications that only certain types of grand unified models could come from superstring theory. With a group at Berkeley, I explored the experimental consequences of these models. In addition, I began collaborating with Howard Haber, a new assistant professor at Santa Cruz. In one of our five papers, we looked at the upper bound to the Higgs mass in supersymmetry in the context of these new superstring-inspired models. In another, we looked at the constraints on the models arising from vacuum stability. Both of these papers are fairly heavily cited (over 110 apiece).
When I moved to Washington University in a five-year senior research associate position, I wrote a paper with Ta-Pei Cheng, a well-known physicist at the University of Missouri, St. Louis. We considered a class of models in which there are two Higgs fields, not one. In previous work, investigators of this class of models assumed that either one Higgs only coupled to quarks and leptons and the other did not (Model I), or one coupled to up-quarks and the other coupled to down-quarks (Model II). The reason for this assumption is that allowing for general couplings lead to processes called flavor-changing neutral currents (FCNC), which are very tightly constrained by experiment. Cheng and I noted that if one makes a reasonable assumption about the size of these general couplings, then the constraints from experiment are not as severe as had been expected. Yet the presence of these general couplings led to a host of new predictions. Interestingly, this paper was pretty much ignored for two to three years. In 1989, it was found that the top quark was very heavy, and our "reasonable assumption" (known as the Cheng-Sher ansatz) led to very interesting effects in top quark physics, and the paper become more relevant. The model with this ansatz is now known as Model III, and the paper is approaching 450 citations.
In the summer of 1988, I decided to put all of my work on Higgs masses, bounds, etc. into a single, large review article (I was planning a big push to get a faculty position the following year). I spent the entire summer doing nothing but writing the review. It ended up being 400 pages long with over 500 references, and was accepted in the fall by the review journal Physics Reports. Fortunately, the discovery early the next year that the top quark was heavy made the Report much more relevant---many of the issues discussed were important when the top quark was heavy---and so it was very well-received. It is now at 760 citations, and is probably the piece of work for which I'm most recognized.
Since coming to William and Mary, I have maintained my pace of roughly four publications per year. During the first four years, I wrote several papers on vacuum stability bounds, using more recent input data and higher precision. The discovery of the top quark determined a previously unknown input parameter in the vacuum stability calculations, as well as the upper bounds in supersymmetric models, and thus much higher precision could be obtained. With a graduate student, Yao Yuan, I explored the implications of Model III (see above) for b-quark decays and tau decays---this involved a very close interaction with experimenters at SLAC and Cornell.
I then shifted directions a bit. Although my work is not generally relevant to CEBAF, I've always been well aware of the capabilities of the machine, and have kept possible applications of CEBAF to fundamental particle physics in mind. One such application came a few years ago. In supersymmetric models, a particle called the gluino exists. It most models, it is expected to be very heavy (hundreds of GeV), but an important class of models has a very light (1-2 GeV) gluino, and this would have escaped detection. I realized that CEBAF's Large Angle Spectrometer might be ideal for detecting gluinos, once the energy at CEBAF is increased to 10-12 GeV (as expected in the future). Fortunately, Carl Carlson is one of the leading experts at calculating processes for CEBAF, and so we collaborated on this project. This led to a paper in Physical Review Letters, another in Physical Review, and a Letter of Intent to do the experimental search has been written. In two other papers, I looked at the process in which a long-lived kaon decays into a neutral pi-zero and two neutrinos. It seems that the beam structure of CEBAF is ideally suited for detecting this decay, which is one of the most important tests of the standard model (it is almost entirely CP-violating and the rate is free of most uncertainties). We (one paper was with Carlson) looked at models with CP-violation in the Higgs sector, and determined the rate.
In addition, I started thinking about something called R-parity violation. In supersymmetry, one can generically have processes which violate lepton and baryon number, leading to unacceptably rapid proton decay. These processes are suppressed if the model has a symmetry called R-parity. In two papers, one with Jose Goity of CEBAF and the other with Carlson and Probir Roy (of the Tata Institute in Bombay), we explored the implications of R-parity violation for various rare processes (such as rare B-decys). These papers have both been heavily cited (over 100 cites each).
In another series of papers, I looked at long-lived leptons. If there are extra leptons, the mixings with the known leptons could be very small, and the additional charged lepton could be very long-lived. The expected lifetime (for nonchiral leptons) was calculated in one paper, the cosmological bound on the lifetime was determined in another, the possible capture (experimentally) of these leptons was discussed in another.
In 1999, I wrote another Physics Reports with Profs. Paul Frampton at UNC-Chapel Hill and P.Q. Hung at UVA. The review article looked at "Quarks and Leptons beyond the Third Generation", and included the work on heavy leptons noted above, as well as vacuum stability bounds, and a large number of other topics. Although fourth generation models weren't popular at the time, they received a resurgence of interest in the early 2000's, and the paper is now over 253 citations. Alas, Paul Frampton is now in an Argentinian prison (google his name for details).
For personal reasons, I was less active in 2000 and 2001, but got back to research in 2002. With Tao Han and Dierdre Black, I wrote a long article on flavor-violating tau decays, looking at the general bounds on a wide variety of possible operators. The same year, I noticed that Babu and Kolda had written a paper on tau --> 3 mu in supersymmetry, showing that it could be quite large, and realized that tau --> mu eta would give stronger bounds. Both of these papers now have about 100 citations.
In 2004, I was fortunate enough to have a visitor, Ismail Turan, and we wrote several papers within six months. The most cited was a study of Lorentz violation in the Higgs sector, giving the various bounds and expectations for several experiments. This was later followed by a paper on isotropic Lorentz violation. Two other well-cited papers were on extensions of the standard model with the electroweak gauge group based on SU(3).
In 2010, I had a sabbatical and spent two weeks in Portugal. There, five physicists and I decided to write an extensive Physics Reports on Two-Higgs-Doublet Models. This was written in 2011 and published in early 2012. It came just in time, since the discovery of the Higgs was imminent, and these models are the simplest extensions of the Higgs sector. The paper was published 28 months ago and now has over 350 cites. Two other follow-up papers with some of the same authors were written in 2012.
It is hard to be certain about future directions. I've noticed, when looking back at older grant proposals, that one's "horizon" in this field is typically six to twelve months. If nothing new and exciting happens in the field, then the projects mentioned in the grant proposals will generally be done. However, it is rare for nothing new and exciting to occur for any significant period.
In July, 2012, the Higgs was discovered. Since I've spent much of my career working on it, and since W&M has a good public relations press person, I got a lot of media attention, with many media interviews over the next few days. The "highlight" was an interview on NPR All Things Considered on March 14, 2013.
Finally, for the period Aug. 2013 to Jan. 2015, I was the NSF Program Director for High Energy Theory and Cosmology (see "service" link for details.