Indiana University High Energy Physics

and Astrophysics Seminars

1999-2000 Academic Year

Mondays, 4pm  Refreshments 3:30 pm
Swain West 251 HEP Coffee Room (SW262)


Spring Semester 2000

Jan. 10: Implications of extra dimensions for high energy experiments
MLK, Jr. Birthday


Jan. 24:


Jan. 31: Screening masses of $SU(2)$ gauge theory at (moderately) high temperatures
Saumen Datta
Indiana University

$SU(2)$ gauge theory is known to undergo a phase transition at a finite temperature to a deconfined phase. The screening masses of the theory are suitable probes for studying properties of this phase. I will report results of numerical measurements of such masses at temperatures a few times the transition temperature, and discuss properties of the high temperature phase, as revealed by the mass pattern.



Feb. 7: Radiative Corrections to the $Z b \bar{b}$ Vertex and Constraints on Extended Higgs Sectors
Heather Logan

We explore the radiative corrections to the process $Z \to b \bar{b}$ in models with extended Higgs sectors. The observables $R_b\equiv \Gamma(Z \to b \bar{b})/\Gamma(Z \to \mathrm{hadrons})$ and the $Zb\bar b$ coupling asymmetry, $A_b\equiv (g_L^2 - g_R^2)/(g_L^2 + g_R^2)$, are sensitive to these corrections. We present general formulae for the one-loop corrections to $R_b$ and $A_b$ in an arbitrary extended Higgs sector, and derive explicit results for a number of specific models. We find that in models containing only doublets, singlets, or larger multiplets constrained by a custodial $SU(2)_c$ symmetry so that $M_W = M_Z \cos\theta_W$ at tree level, the one-loop corrections due to virtual charged Higgs bosons always worsen agreement with experiment. The $R_b$ measurement can be used to set lower bounds on the charged Higgs masses. Constraints on models due to the one-loop contributions of neutral Higgs bosons are also examined.



Feb. 14:


Feb. 21: D-Branes, B-Factories, and a Universal Supersymmetric Origin of CP Violation
Lisa Everett
U. of Michigan

Recently it has been realized that the "flavor-independent" phases in the soft supersymmetry (SUSY) breaking parameters of the MSSM Lagrangian can be large, with the constraints from the experimental upper bounds of the electric dipole moments of the electron and neutron satisfied by cancellations. In this talk, two related issues are addressed: (1) the theoretical motivations for such large phases are examined through a study of the patterns of soft breaking parameters in classes of four-dimensional superstring models. While such large phases are generally disfavored in models based on the perturbative heterotic string and classes of heterotic M theory models, large phases can be accommodated in certain Type I/D-brane models. (2) We address the question of whether such large flavor-independent phases can be the dominant (or only) source of all observed CP violation. The results demonstrate that this is a phenomenologically consistent possibility provided that the superpartner mass matrices have an unconventional flavor structure (with enhanced super-CKM mixing).



Feb. 28: Preons - the building blocks of everything?
Sverker Fredriksson
Lulea U.

There are hints that leptons and quarks are composite. A model with three preons explains most features of the standard model, and predicts new results for the upgraded Tevatron.

Dr. Fredriksson's trip is being partially funded by the Office of International Programs at IU.


Mar. 6: Precision Analysis of Electroweak Interactions
Jens Erler
U. of Pennsylvania

Precision measurements in high energy experiments and low energy observables serve as tests of the Standard Model of electroweak interactions and determine its fundamental parameters. I survey the present status of our analysis within and beyond the Standard Model. The fit to all data improves significantly if the existence of an additional neutral gauge boson is assumed with a mass not much larger than 1 TeV.


Spring Break


Mar. 20: (SW238) The Structure of the Proton and the Photon
Bernd Surrow

Lepton-nucleon scattering experiments have played an important role in our current understanding of hadronic matter. The observation of scaling violation contributed to the test of the field theory among quarks and gluons known as Quantum Chromodynamics (QCD). One of the most interesting and challenging investigations at HERA is to explore the transition region between the domain of non-perturbative and perturbative QCD at low momentum transfer $Q^2$. At high $Q^2$, the probing virtual photon appears to be point-like. Towards the photoproduction limit, i.e. towards low values of $Q^2$, the chance of the photon to undergo a quantum fluctuation into a quark/anti-quark pair and to interact as such, e.g. in $\gamma p$ interactions, increases. This gives rise to a hadronic type reaction, thus the term photon structure. This dual nature of the photon, i.e. point-like and hadron-like, leads to a variety of phenomena which have been investigated in $\gamma p$ and $\gamma ^{(*)}\gamma ^{(*)}/e\gamma $ scattering at HERA and LEP.

After a general introduction into the field of lepton-nucleon scattering experiments, results on a measurement of the proton structure function $F_2$ at low $Q^2$ and very low $x$ using the ZEUS Beam Pipe Calorimeter at HERA are reviewed. A brief overview will be given of a variety of measurements of $\gamma ^{(*)}\gamma ^{(*)}$ and $e\gamma$ scattering at LEP using the OPAL detector to gain a deeper insight into the structure of the photon and its interactions.


Mar. 27: Phenomenology of Supersymmetric CP Violation and Electroweak Baryogenesis
Michal Brhlik
U. of Michigan

The origin of CP violation is a long-standing problem in particle physics. The fact that CP violation within the Standard Model does not by itself provide for a sufficient baryon-antibaryon asymmetry production in the early universe motivates theoretical considerations exploring the possibility of CP violation beyond the Standard Model. The first part of this talk addresses the issue of CP violation in the Minimal Supersymmetric Standard Model concentrating on phenomenological implications. The problem of large electron and neutron dipole moments can be resolved if the soft breaking parameters are correlated allowing for large CP violating flavor independent phases. A specific framework is then considered where all (or most) of CP violation originates in the supersymmetric sector and consequences are discussed for K-Kbar and B-Bbar mixing, supersymmetric dark matter and superpartner searches at colliders. The second part of the talk outlines the mechanism of supersymmetric electroweak baryogenesis in this framework and its implications for the Higgs sector of supersymmetric models.


Apr. 3: Supersymmetry breaking through extra dimensions
Graham Kribs
Carnegie Mellon University

Several approaches to supersymmetry breaking involve one or more extra dimensions. In this talk, I propose a new framework for mediating supersymmetry breaking through an extra dimension. Supersymmetry breaks on a "source brane" that is spatially separated from a parallel brane on which the standard model matter fields and their superpartners live. The gauge and gaugino fields propagate in the "bulk", the latter receiving a supersymmetry breaking mass from direct couplings to the source brane. Scalar masses are suppressed at the high scale but are generated via the renormalization group. I also briefly discuss the spectrum and collider signals for a range of compactification scales.



Apr. 10: (SW238) Gluonic excitations, results from the Crystal Barrel experiment
Ulrike Thoma
Bonn University

Since gluons carry color charge, gluonic excitations like glueballs (bound states of gluons) or hybrids (bound states of a quark, antiquark and a gluon) may exist. The search for these states predicted by lattice gauge theories was one of the goals of the Crystal Barrel experiment at LEAR. In the absence of mixing with quarkonia, glueballs are bound states of only gluons.

In lattice gauge theories the lightest glueball is predicted to have scalar quantum numbers and a mass of about 1600 MeV/c^2. The observation of more scalar isoscalar mesons than the quark model can host has led to speculations that a glueball of mass of about 1600 MeV/c^2 has intruded into the spectrum of scalar quarkonia and mixes with them, thus producing the states observed in the mass range below 2 GeV/c^2. Experimental information on the number of existing scalar states and on their decay modes may be crucial for the understanding of the scalar mesons and may clarify the possible existence of the scalar glueball. Another question of interest is the existence of hybrids, of mesons in which the string mediating the interaction between quarks and antiquarks has been excited. In general hybrids can also mix with normal mesons making their identification difficult. But hybrids can also have exotic quantum numbers, which cannot be reached by quark-antiquark systems. A state with exotic quantum numbers was recently discovered by E852 at BNL and by Crystal Barrel.


Apr. 17:


Apr. 24:

Apr. 25: Study of the CP violation in the B-->J/psi Ks channel with the D0 detector (candidacy seminar)
Chunhui Luo
Indiana University


May 1: Top-Quark Spin Properties from Tevatron to LHC
Vladislav Simak
Institute of Physics Academy of Sciences of the Czech, Republic, Praha , CZECH Rep