A-O : Prof. Shafi, thank you very much for sparing your time to have this conversation on elementary and astro-particle physics and cosmology on the frontier. Let me begin by asking you the foremost question on most people's mind. How do you feel about the July 4, 2012 presentations by CMS and ATLAS experiments on the discovery of the Higgs?
QS: I would say that this is the most exciting news in our field since the discovery of atmospheric neutrino oscillations in 1998.
A-O: What are the improvements you would like to see in the Higgs signal before the shutdown?
QS: We would like to have a precise knowledge of the Higgs production crosssection and decay modes to decide whether or not it indeed is the standard model (SM) Higgs boson, or some variation such as in the supersymmetric models. We need to get a handle on this. This is one of the most urgent tasks for the experimentalists to help us discover whether or not we have seen new physics through the Higgs system.
A-O: What do you think of the Tevatron analysis of the Higgs?
QS: The analysis is less conclusive but very welcome in building a complete picture for the Higgs boson.
A-O:What do you think are the prospects for non-minimal Higgs scenarios?QS: Let us first consider the minimal supersymmetric standard model (MSSM). If it really turns out that the Higgs production crosssection and decay in 2 photons are significantly different from the predictions of the SM, then there are good propspects for discovering a light scalar tau and / or scalar top. For instance, in a very recent paper with Ilia Gogoladze and Adeel Ajaib we have explored how the presence of light sparticles such as a scalar tau, top or bottom can produce effects in the Higgs sector that can be tested at the LHC effectively.
A-O:What do you think are the general prospects for the discovery of supersymmetry in the present run and after the upgrade?
QS: It would be really exciting if some of the supersymmetric particles could be found at the present run. In any case, if no signal is seen even at 14 TeV, it would be somewhat discouraging.
A-O: In addition to supersymmetry, what other beyond the SM scenarios are, in your compelling?
QS: In my opinion, warped extra dimension is a fairly compelling extension of the SM. It provides a nice geometric framework for resolving the gauge hierarchy problem as well as accommodating the hierarchy in fermion masses and mixing. In particular, if the SM particles are bulk fields, one expects to see a plethora of new Kaluza-Klein excitations at the LHC.
A-O:How do you compare the excitement over the Higgs discovery with that of the top-quark discovery in 1994?
QS: The SM predicted the existence of both the top-quark as well as that of the Higgs boson long before their experimental discovery. As far as the top-quark is concerned, in 1991, with Ananthanarayan and Lazarides, we wrote a paper in which we exploited the idea of third family `Yukawa unification' in supersymmetric grand unified theories to predict the top-quark mass. The predicted mass, it turns out, is about 170 GeV, which is very close to the current central value.
A-O:Let me now change tracks a little bit and ask a little about you and your journey through particle particle physics and eventually that through astroparticle and cosmology. How did you first get interested in this subject?QS:While in high school in London, I had heard about Abdus Salam and then as an undergraduate at Imperial College, I was specially inspired by lectures from Ian Butterworth. I went on to do a Ph. D. under the supervision of Abdus Salam, and that is how it all started.
A-O:Tell us a little about your interest in grand unified theories especially based on exceptional groups.
QS: I recall a discussion with Salam in which he drew my attention to the SU(5) paper of Georgi and Glashow to my attention as well as to his earlier papers with Jogesh Pati on left-right symmetric models. I read those papers and found them to be extremely exciting and from then on, I was convinced that this is the most elegant extension of the SM. At some point, I got interested in SO(10) and eventually in E6 gauge symmetry which in some ways is a natural extension of SU(5) and SO(10) from the four dimensional viewpoint.
A-O:How do reconcile grand unification with the discovery of neutrino masses of the last dozen or so years?
QS: Grand unified theories such as SO(10) or its Pati-Salam subgroup naturally predict tiny masses for the neutrinos which has now been experimentally verified.
A-O:Grand unified theories also predict the existence of many `topological defects' such as monopoles, cosmic strings and domain walls, of immense consequence to cosmology and the early universe. You have done a lot of work in this field and one may even say that you are therefore one of the founding fathers of what is now called astro-particle physics. Can you comment?
QS: Monopoles, as you know, are theoretically very important, as they go hand in hand with the quantization of electric charge, something that Paul Dirac pointed out more than eighty years ago. All grand unified theories predict their existence, and are topologically stable, which arise as solitonic solutions of the underlying theory. Combining this with big bang cosmology leads to an overproduction of primordial magnetic monopoles and hence some mechanism must be devised to eliminate this problem. Inflation is certainly one of the most appealing solutions, but of course, it would be nice to dilute their number density to a measurable rather than to a totally unobservable level. As far as grand unified scale cosmic strings are concerned, we now know after the WMAP and COBE measurements they can only play a sub-dominant role as far as structure formation is concerned. On the other hand, it would be very exciting to see some signals of such primordial cosmic strings in the sky. Finally, cosmological domain walls are a disaster and as a result, only unstable ones are relevant in cosmology and may have played a role in primordial black hole formation.
A-O:Tell us a little about your work on inflation. In particular, how does our understanding of the inflationary epoch differ today from what we thought we knew in the 1980's and 1990's?
QS: I have worked on supersymmetric (with Gia Dvali, Nefer Senoguz, Mansoor Rehman and Joshua Wickman and others) and non-supersymmetric (with Alexander Vilenkin, Nobuchika Okada and others) and higher dimensional (with Gia Dvali, Christoph Wetterich, Bum Seok Kyae) models of inflation. A major development has been a rather precise measurement of the scalar spectral index by the WMAP experiment. This measurement has eliminated several classes of models and the hope is that the
WMAP Planck collaboration within the next year or so with even more precise measurements can seriously test some of the remaining models of inflation. For instance, let us assume that inflation is driven by a Higgs potential. In this case, one can show that WMAP Planck should see primordial gravity waves that were produced during inflation through the impact on the cosmic microwave background radiation.
A-O:One might say that the findings of the satellite borne experiments COBE and WMAP have revolutionized our understanding of the early universe. What is your take on this?
QS: In particular, the observation of the anisotropy first by COBE and subsequently by WMAP and other experiments has been crucial in providing support for the inflationary scenario. The Planck satellite is expected to provide more precise measurements of the spectral index and would hopefully help us determine a new `standard model' of inflation!
A-O:Some time ago New Scientist magazine featured some of your work. Could you please tell us what that was about?
QS: In a paper written with my student Mansoor Rehman we had shown that quantum corrections can significantly modify the tree-level predictions of inflation. For instance, if one took a quadratic inflationary potential, the quantum corrections arising from the coupling of the inflaton to other fields in the theory can significantly modify the tree-level predictions, which can be tested by the Planck satellite experiment.
A-O:You were one of the early proponents of cold and hot dark matter. How has the picture changed, especially after the discovery of dark energy and the rise of the so-called Lambda-CDM picture?
QS: According to present observations, hot dark matter can play only a minor role in large-scale structure formation. According to current observations, all light neutrinos added together have mass of less than a eV and hence their role in structure formation is largely insignificant. The scale of dark energy is about 0.001 eV and my first guess is that it is some leftover cosmological constant. Some years ago, I co-authored a paper with Stephan Huber in which we showed that this could arise as the consequence of double warping in a generalized Randall-Sundrum scenario. In other words, with a single warp factor M_p -> TeV, and a second warp factor takes you to TeV^2/M_p which is O(0.001 eV)!
A-O:What do you feel are the big unresolved questions in elementary particle and astro-particle physics and cosmology?
QS: In high energy physics, with the discovery of the Higgs boson, hopefully we will have a better understanding of electroweak symmetry breaking, and possibly new physics beyond the SM, such as supersymmetry, extra dimensions. I think we should wait for the ongoing and next generation of experiments to play a decisive role in answering these questions. In cosmology, of course, two of the most challenges are to identify the nature of dark energy and dark matter. In addition, we would like to know what the `standard model' of cosmology really is. Is it really Lambda-CDM, or does it need some tweaking? What is the `standard model' of inflation?
A-O:You have also worked on string phenomenology and string cosmology. What do you think is the future of these fields?
QS: String theory still provides the most compelling answer to incorporating quantum gravity and unifying gravity with the other three fundamental forces of nature. However, it has proved rather difficult to come up with some truly compelling predictions which could verify that this indeed is the correct theory. It is tantalizing yet elusive!
A-O:You have been very patient with all these questions. I would like to request you to help us wrap up with conversation with your thoughts for the future.
QS: I would like to end the conversation on an optimistic note: the discovery of the Higgs boson at the LHC is only the beginning of a new round of exciting discoveries in high energy physics. It would be extremely exciting if supersymmetry is found soon and one or more extra dimensions are discovered, dark matter is identified and the equation of state of dark energy is determined precisely. Furthermore, I very much hope that the Planck satellite would find gravity waves which would have remarkable consequences for the physics underlying the very early universe. It would mean, in particular, that the energy scale during observable inflation was of the order of the grand unified scale of 10^16 GeV. This would be a very beautiful link up between cosmology and high energy physics.
A-O:Thank you very much for this detailed conversation.
Prof. Qaisar Shafi obtained his Ph. D. in Imperial College, London under the supervision of Abdus Salam. He spent time at the RWTH, Aachen, Germany, ICTP, Trieste, Italy, CERN, Geneva, Switzerland, University of Freiburg, Germany, Goddard Space Flight Center, Greenbelt, USA before settling permanently at the University of Delaware in 1983. He has supervised the work of several graduate students and has mentored several post-doctoral fellows. He is also well known for his work for developing countries through ICTP and as an organizer of international schools including the BC(V)SPIN Schools.