Research Interests

Heather E. Logan

My research is focused on the theory and phenomenology of extensions of the Standard Model (SM) of particle physics. To date I have worked on a number of projects involving the physics of the SM Higgs sector and its supersymmetric and non-supersymmetric extensions. My recent research focuses primarily on Higgs physics in the Minimal Supersymmetric Standard Model (MSSM). Much of this work has been related to the ongoing studies of physics opportunities at an e+e- Linear Collider, and the 2001 Snowmass Summer Study in particular. The prospects for discovering a Higgs boson and revealing the nature of electroweak symmetry breaking in the near future at the Tevatron and the CERN Large Hadron Collider (LHC) are very promising. However, these hadron colliders are limited in their potential to explore the properties of the Higgs sector in detail, particularly if the Higgs boson comes from an extended theory beyond the SM. A Linear Collider offers the opportunity to make high-precision measurements of the properties of the Higgs sector.

M. Carena, H.E. Haber, S. Mrenna and I recently made a detailed analysis of the prospects for distinguishing the lightest CP-even Higgs boson of the MSSM from the Standard Model Higgs boson based on high-precision measurements of the Higgs decay branching fractions at a future Linear Collider. We identified the Higgs decay modes most sensitive to the SM or MSSM nature of a Higgs boson, taking into account the relative precision with which the various decay modes are likely to be measured at the Linear Collider. We found that, in most regions of MSSM parameter space, the lightest MSSM Higgs boson can be distinguished from the SM Higgs boson far into the decoupling limit, for pseudoscalar MSSM Higgs boson masses up to about 600 GeV. We also found regions of parameter space in which the MSSM Higgs boson can be extremely SM-like, even for relatively light pseudoscalar Higgs masses; in this case distinguishing the MSSM Higgs from the SM Higgs based on measurements of its branching ratios alone will be very difficult. Finally, we found a way to extract a parameter of the MSSM Higgs sector (the supersymmetric radiative correction to the relation between the b quark mass and its Yukawa coupling) based on measurements of the couplings of the lightest MSSM Higgs boson to SM particles that can be made at the Linear Collider. I am currently working with M. Carena, S. Heinemeyer, D. Rainwater, G. Weiglein and D. Zeppenfeld to extend these recent results for the Linear Collider to the LHC. In particular, we are analyzing the potential to distinguish the MSSM Higgs boson from the SM Higgs boson and to extract some of its properties based on measurements at the LHC. Although the precision of Higgs coupling measurements at the LHC will be limited compared to that possible at the Linear Collider, the LHC is already being constructed and will begin running sooner than a Linear Collider could be built.

I am also working with S. Su to compute the cross sections for single production of the heavy MSSM Higgs bosons at a Linear Collider. The cross sections for single heavy Higgs boson production tend to be very suppressed compared to the cross sections for two heavy MSSM Higgs bosons to be produced in association. However, producing two heavy MSSM Higgs bosons in association requires a higher center-of-mass energy for the collider. If the cross sections for single heavy Higgs boson production are large enough to be observed, the reach of a Linear Collider for discovery and/or the measurement of the properties of the heavy MSSM Higgs bosons would be extended to higher Higgs masses for a fixed collider center-of-mass energy. We also plan to study how measurements of the production cross sections and decay branching ratios of the heavy MSSM Higgs bosons can be used to extract or constrain MSSM parameters.

I am also interested in looking at the discovery potential for MSSM Higgs bosons at Run 2 of the Tevatron in as-yet-unexplored channels, in particular when the charged and pseudoscalar MSSM Higgs bosons are relatively light. Such channels might close some existing holes in the discovery reach of the Tevatron for MSSM Higgs bosons. I am also planning to look at rare top quark decays and nonstandard top squark signals that occur in models of low-energy supersymmetry breaking.

Also in the MSSM, I recently performed an on-shell calculation of the supersymmetric (SUSY) QCD corrections to the coupling of the lightest CP-even Higgs boson to $b$ quark pairs, in collaboration with H.E. Haber, S. Penaranda, M.J. Herrero, S. Rigolin and D. Temes. These corrections can be large and thus have significant effects on Higgs physics, especially when tan beta (the ratio of the vacuum expectation values of the two Higgs doublets) is large; they have thus received much attention recently. In particular, they can affect the Higgs discovery potential at the Tevatron and LHC and will enter into the extraction of the underlying MSSM parameters from Higgs sector measurements at the LHC and Linear Collider. We focused on the decoupling limit in which the heavy MSSM Higgs bosons and SUSY particles are very heavy. We showed that while the radiative corrections to the couplings of the lightest CP-even MSSM Higgs boson decouple in this limit, as expected, this decoupling can be quite slow leading to significant effects on the Higgs couplings even for relatively large SUSY particle masses. I am also working to see how our Feynman-diagrammatic computation of the SUSY-QCD corrections, which includes subleading contributions that depend on the external Higgs boson momentum and can be significant if the MSSM particles are not too heavy, can be combined with similar calculations done recently in an effective field theory approach that include a resummation of all the two-loop and higher order tan beta enhanced contributions. Some of us (H.E. Haber, M.J. Herrero, S. Penaranda, D. Temes and I) are extending this work by calculating the SUSY-electroweak corrections to the coupling of the lightest Higgs to b quark pairs, the largest of which are enhanced by the large top quark mass and can thus be comparable to the SUSY-QCD corrections. We plan to examine the decoupling behavior of these corrections, as well as seeing how parts of the corrections can be absorbed into an effective mixing angle between the heavy and light CP-even MSSM Higgs bosons. I also plan to examine the SUSY-QCD corrections to the MSSM Higgs boson couplings to top quark pairs; these corrections may have a significant effect on the couplings of the heavy MSSM Higgs bosons.

In the past year I was involved with the study of Linear Collider physics performed for the Fermilab Directorate, with P.F. Derwent, B.A. Dobrescu, A.S. Kronfeld, K.T. Matchev, A. Para, D.L. Rainwater, S. Tkaczyk, and W.C. Wester III. Our report, which was finished during the Snowmass Summer Study, summarizes the existing literature primarily on Higgs physics at a Linear Collider, plus some new results for intermediate mass Higgs bosons. I led the subgroup that examined the methods that can be used to measure the spin and CP properties of a purported Higgs boson at a Linear Collider, including Higgs bosons with mixed CP. These measurements will be important for determining whether a discovered resonance is a CP-even scalar particle, as the SM Higgs boson is. They will also be important in the context of an extended Higgs sector for exploring possible CP violation -- this is particularly well motivated in the MSSM if the soft supersymmetry breaking parameters have CP-violating phases. Parts of our report were incorporated into the Linear Collider Physics Resource Book for Snowmass 2001.

I have also been involved with an ongoing study of physics opportunities at a photon collider. Recent experimental studies indicate that, for Higgs masses below the W boson pair production threshold, Higgs production at a photon collider with decays to both b quark pairs and W boson pairs can be observed. My preliminary results show that one can use the production rates of these two modes to distinguish the lightest CP-even Higgs boson of the MSSM from the SM Higgs boson in some regions of MSSM parameter space. I am planning to further examine Higgs physics at such a machine, and am particularly interested in the opportunities for Higgs physics at a photon collider if there is CP violation in the MSSM Higgs sector.

Finally, I am interested in the effects of new physics on low-energy processes, and how these processes can be used to constrain new models before their direct discovery or exclusion at high-energy colliders. Last year U. Nierste and I examined the new contributions to leptonic B meson decays in a two Higgs doublet model. The new contributions to these decays from the two Higgs doublet model can compete with the SM contribution, especially for large values of tan beta. This tan beta enhancement is even more dramatic in supersymmetric models. We plan to continue this work by considering additional rare B decays and B-Bbar mixing in the MSSM with large tan beta.