Top quark, QCD and collider physics:

I have always been most interested in the interface between theory and experiment, and this has led me to investigate a variety of Standard Model (SM) processes with regard to their implications for collider physics. In the late 1980s, I wrote a series of papers exploring discovery strategies for the missing top quark of the Standard Model. In Phys. Rev. D39 (1989) 3310, we showed that collider experiments in the 1990s at Fermilab should be able to extract the top quark signal from background processes if the top quark mass was less than 200 GeV. We proposed the H_T variable cut which was later used by D0 to extract the top quark signal from background. The top was ultimately discovered by the CDF and D0 experimental groups at Fermilab in 1995-1996 using ~100 pb^-1 of data with mass m(top)~ 173 GeV. D0 made crucial use of our H_T variable cut. I have also written several papers on next-to-leading order calculations in perturbative (NLO) QCD processes. These include calculations of direct photon production, jet photoproduction, W and Z boson production, supersymmetric particle production and the problem of interfacing the parton showers of event generators with next-to-leading order QCD calculations.

Supersymmetry at Colliders:

Over the past forty years, I have been one of the main pioneers in developing the observable consequences of supersymmetric theories of particle physics. In the 1980s, Xerxes Tata and I showed a wide array of decay modes were possible for supersymmetric particles, leading to a variety of signature reactions for collider experiments. We were the first to calculate superparticle *cascade decays*, and to demonstrate their usefulness. After developing the computer program ISAJET for supersymmetry with Frank Paige, we were able to show exactly the sorts of high energy scattering events that would lead to the identification of supersymmetric matter. The search for supersmmetry is one of the main goals of the CERN Large Hadron Collider, which has recently begun taking data. We showed that the 7-14 trillion electron volts available for collisions will be sufficient to either discover supersymmetric matter or to rule it out over wide ranges of parameter space. We initiated the work on extracting superparticle mass measurements from cascade decay events at hadron colliders. Now that LHC has turned on and has provided over 5 fb^-1 of data, I am fully engaged in data analysis and interpretation. This month my group will release a paper showing how LHC SUSY searches can gain a large jump in sensitivity by examining gaugino pair production reactions. A few weeks ago I gave a talk at the CERN W2 group entitled ``From simplified models to cascade decays (and back again)''. I am also heavily involved in theoretical work for the International Linear Collider. We have developed ISAJET to generate SM and SUSY processes including bremsstrahlung and beamstrahlung, and also beam polarization. ISAJET has been a fundamental tool for exploring the possibilities of the ILC. We have shown recently that in cosmologically favored SUSY model parameter space regions, the reach of the ILC can exceed that of the CERN LHC. I am currently writing up the chapter of the ILC re-commissioning report on status of supersymmetry since LHC turn-on.

Cosmology:

With my grad student Michael Brhlik, we published in 1996 the world's best calculation of remnant supersymmetric cold dark matter from the Big Bang. This was the standard calculation for several years. Then with postdocs A. Belyaev and C. Balazs, we reworked our code into *IsaReD*, which yields one of several best estimates of relic cold dark matter from supersymmetric theories. We have also sought to match the relic density predictions with data from dark matter measurements, and see what the corresponding LHC collider signatures would look like in ``dark matter allowed'' regions of parameter space. In the past few years, my group has shown that in fact a mixture of axion and axino dark matter is in many ways superior to the more popular neutralino picture. If mixed axion/axino cold dark matter exists, then the corresponding SUSY signatures at LHC will likely be quite different. Recently, with my student Andre Lessa, we developed an eight-coupled Boltzmann equation code which is best in the world for calculating the abundance of mixed axion/neutralino cold dark matter.

Dark Matter Search:

Recently, my students, postdocs and I have mapped out supersymmetric model parameter space regions where direct and indirect detection of neutralino dark matter is possible. These regions are very much complementary to what can be done at colliders. While a dark matter signal might be detected at muon telescopes such as IceCube at the South Pole, or via direct detection in underground experiments, or via observation of photons or antimatter from halo annihilations, if collider experiments can produce the dark matter particle, then their detailed properties can be mapped out. One of our contributions was to discover that dark matter annihilations via Higgs resonances in the galactic halo can give rise to large detection rates, even if dark matter is not directly detectable or detectable via neutrino telescopes. Our recent work on mixed axion/axino dark matter would imply that no WIMP signal should be seen in direct/indirect detection experiments. We find that SUSY models prefer a somewhat higher value of Peccei-Quinn breaking scale $f_a$, meaning axion detection experiments should search for excitations in the sub-micro-eV range.

Event generator development:

My main contribution to particle physics is in the development of the first realistic simulation program for production of supersymmetric matter. My collaborator Xerxes Tata (Hawaii) and I pioneered much of the phenomenology of particle physics models with supersymmetry. We had a need for detailed simulations of production of these new states of matter at colliding beam experiments. In 1992, in collaboration with Frank Paige (Brookhaven), we developed the ISAJET program to simulate production of supersymmetric matter at colliders. Since that time, we have been continually upgrading ISAJET to give ever better predictions. ISAJET is a key tool that experimenters use to aid in both detector design for future experiments, and for data analysis in current experiments. For instance, ISAJET is currently being used in the search for supersymmetric matter at experiments at Fermilab, and was also being used to design detectors for the multi-billion dollar Large Hadron Collider. Now it is being used in various data analyses by the CERN Atlas and CMS groups. Since the time we released ISAJET, several other similar programs have emerged whch make predictions for supersymmetry. But ISAJET is still a mainstream program, and was chosen at the Snowmass Summer Study in 2001 on the Future of High Energy Physics as the standard event generator for supersymmetry. It was also chosen in 2004 by the SLAC linear collider/cosmology study group as the standard tool to investigate the connection between the ILC and cosmological dark matter. We also developed the first ever public code for calculating the superparticle mass pectrum using renormalization group equations: Isasugra. Isasugra is used these days to make many of the $m_0$ vs. $m_{1/2}$ plots for Atlas, CMS, CDF and D0 presentation of SUSY search results. Later, several European groups developed similar codes: SoftSUSY, SUSpect and Spehno. Isasugra has some unique features which make it in my opinion still the best available code for SUSY spectrum calculation: namely sequential decoupling of sparticles as the RGEs run, in the spirit of effective field theories. Furthermore, Isasugra has a seamless interface with Isajet event generation and with Isatools for relic density and dark matter direct detection calculations. No other single code can do as much!

Particle physics model building:

Recently, my students, postdocs and I have been engaged in a variety of exciting supersymmetric model building projects. We have shown that supersymmetric grand unified theories (GUTs) based on the gauge group SO(10) can accomodate t-b-tau Yukawa coupling unification, but only for very specific choices of GUT scale soft terms. In addition, a class of models we have investigated-- inverted hierarchy models-- are especially attractive as Grand Unified Theories, in that they solve the supersymmetric flavor and CP problems, but maintain the so-called ``naturalness'' condition. We also proposed a normal scalar mass hierarchy model which has generational mass splitting, but still respects bounds from FCNC measurements, and also accommodates recent measurements on Omega h^2, (g-2)_mu and BF( b-> s gamma ). Recently, we have tried to tabulate all the various ways that non-universal soft SUSY breaking terms can be used to explain the correct dark matter abundance in the Universe. Each mechanism leads to rather unique predictions for collider phenomena at the LHC and ILC. We have also investigated collider and dark matter phenomena expected from KKLT string inspired SUSY models, and more recently, gaugino AMSB models.

Naturalness:

Since the turn-on of the CERN LHC, with its failure to detect weak scale supersymmetry, the naturalness issue has risen to the fore. Our group proposed a new, more conservative and model-independent naturalness measure called Delta_{EW}. This measure is based on what we call practical naturalness, where each independent contribution to an observable to should comparable to (to within a factor of a few) or less than the observable in question. In weak scale SUSY, the Z boson mass is related to the weak scale SUSY soft terms and (SUSY conserving) mu term. This then implies that the higgsinos must be quite light, ~100-300 GeV, and one likely has a higgsino-like EWino as LSP. The light higgsinos are thermally underproduced as dark matter, but this is fine as it leaves room for (SUSY DFSZ) axions which are required to solve the strong CP problem. From our calculations, the axions, which have a reduced coupling to photons due to higgsinos circulating in the triangle diagram coupling, usually comprise the bulk of dark matter, with higgsino-like WIMPs at typically the 10% level. Under Delta_{EW}, there is plenty of natural SUSY parameter space beyond present LHC search limits. Natural SUSY models include the i extra-parameter non-universal Higgs models (NUHMi) (with gaugino mass universality), the natural AMSB model where winos are the lightest gauginos, but higgsinos are the lightest EWinos, and naturalmirage mediation models, with comparable moduli- and anomaly-mediated soft terms but with light higgsinos.

SUSY from the string landscape:

Weinberg promoted the idea that the tiny value of the cosmological constant is anthropically selected in the multiverse in that in pocket universes with Lambda_CC much bigger, the universe would expand so fast that structure (in the form of galaxies and stars) would fail to form. He was able to predict the magnitude of the CC to within an order of magnitude well before its value was measured, starting in 1998. Bousso and Polchinski noted that under string flux compactifications, then this sort of anthropic selection would arise form string theory. This was called the string flux landscape by Susskind, and Douglas and collaborators computed a value of ~10^500 different 4-d vacua that could arise under such compactifications.

Our group placed string statistical predictions on a firm foundation using this approach. Douglas argued rather generally that soft SUSY breaking terms should be favored to be large values via a power-law distribution on the landscape. Meanwhile, Agrawal-Barr, Donoghue-Seckel argued that the derived value of the weak scale in each pocket universe should lie no too far from its measured value in our universe, lest complex nuclei, and hence atoms, fail to form in the pocket universe (so-called atomic principle). In WSS, the magnitude of the weak scale is derived in terms of the soft terms and mu (as in the definition of Delta_EW above), and so combining the power law draw to large soft terms with the required magnitude ofthe weak scale lying within the ABDS window, we were able to compute probability distributions for the Higgs mass and sparticle masses n our universe. The value of mh rises to a peak near 125 Gev with sparticles generally beyong LHC limits: just what LHC is seeing! Using these results, we were able to predict a host of expected SUSY signatures at forthcoming LHC luminosity upgrades. Foremost among these are soft isolated dileptons with missing ET, produced in association with an initial state hard jet radiation. ATLAS and CMS seem to have some mild excess in this higgsino-pair production channel at the present time.

Courses Taught This Year:

PHYS-5970 Advanced Quantum Field Theory

Interests & Hobbies:

Wilderness canoeing, whitewater kayaking, climbing and mountaineering.

Some wilderness rivers I have run:

Upper Missinaibi (1979); South Seal/North Knife (1980); Cochrane, Pigeon, Kazan, Maguse (1981); Bloodvein (1982); Kazan (1984); Back, Meadowbank, Quioch, Chesterfield Inlet (1986); Coppermine (1989); Dubawnt (1993); Alanwater (2022)

Wisconsin log cabin for rent

Recent papers with high resolution figures

Recent talks

SUSY07 pre-meeting lectures on SUSY at LHC

TASI 2008 lectures on E_T(miss) signatures at LHC

Karlsruhe/Freudenstadt lectures on SUSY, dark matter and the LHC

Publications

Howard Baer running El Horrendo Falls on the Russell Fork river, Breaks Interstate park, Virginia/Kentucky, circa 1983, photo by Denise Handrich

Howard Baer running Wonder Falls on the Big Sandy river, West Virginia, circa 1983, photo by Denise Handrich

Last Updated: Jan. 17, 2014