Department of
Physics and Astronomy

Paul G. Langacker

Theoretical Elementary Particle Physics

Born 1946
B.S. M.I.T. (1968)
Ph.D. University of California at Berkeley (1972)

Emeritus Professor of Physics (Penn)
Office: 2E15, David Rittenhouse Laboratory
Phone: (610) 898-5943
Email: pgl@electroweak.hep.upenn.edu

Member, Institute for Advanced Study
Office: 228, Bloomberg Hall
Phone: (609) 734-8076
Email: pgl@ias.edu

In the last 30 years, there has been a tremendous advance in our understanding of the elementary particles and their interactions. We now have a mathematically consistent theory of the strong, electromagnetic, and weak interactions-the standard model-which is almost certainly the correct description of Nature down to a distance scale 1/1000th the size of the atomic nucleus. However, nobody believes that the standard model is the ultimate theory-it is too complicated and arbitrary.

Therefore, most current activity is directed towards discovering the new physics which must underly the standard model. One approach, exemplified by superstring theories, is to try to develop a "theory of everything". However, promising ideas involve incredibly short distance scales and make little contact with accessible energies at present. Another direction, expected to be very important in the next decades, is to build larger accelerators to directly search for new particles and interactions. An equally important and complementary approach is to subject the standard model to diverse high-precision tests to determine how good it is and where it might break down. My research has been directed towards the theoretical interpretation of the various experimental probes, and the phenomenological implications of fundamental theories.

Much of my work in recent years has involved the interpretation of high-precision tests, and has exploited the fact that the global analysis of many experiments often yields more information than the sum of the individual experiments. Such global analyses involve collecting the data, deriving uniform and accurate theoretical formulas to interpret it, developing expressions for the possible effects of new physics, and fitting the data to search for or set limits on new physics.

A related effort has been a study of the possibilities for discovering and studying the properties of possible additional gauge bosons at future colliders.

Another of my major interests involves theoretical models for neutrino mass and their laboratory, astrophysical, and cosmological implications. My collaborators and I have analyzed the solar neutrino data, and have argued that the data favor a neutrino mass (rather than astrophysical) explanation of the solar neutrino deficit. We have developed analysis programs for the interpretation of the data according to matter-enhanced Mikheyev-Smirnov-Wolfenstein neutrino oscillations. I have also been involved in the analysis of big bang nucleosynthesis.

I have also had a long standing interest in grand unified theories and their consequences. Recently, I have become interested in the phenomenological consequences of semi-realistic superstring compactifications. My collaborators and I have developed techniques to classify the possible vacua of such theories which do not break supersymmetry at the string scale. We have studied the implications of a class of closed string solutions for additional gauge bosons, new particles, the origin of intermediate scales, and the consequences for the masses of quarks, charged leptons, and neutrinos. Our current work emphasizes the classification and study of open-string theories and their vacua. I am also studying types of observable new physics at the TeV scale suggested by these string vacua, including extended Higgs/neutralino sectors, additional Z' gauge bosons, and exotic fermions.

Positions Held

Selected Publications

  1. Light Quark Mass Spectrum in Quantum Chromodynamics (with H. Pagels), Phys. Rev. D 19, 2070-2079 (1979).
  2. Magnetic Monopoles in Grand Unified Theories (with S.-Y. Pi), Phys. Rev. Lett. 45, 1-4 (1980).
  3. Grand Unified Theories and Proton Decay, Physics Reports 72, 185-385 (1981).
  4. A Comprehensive Analysis of Data Pertaining to the Weak Neutral Current and the Intermediate Vector Boson Masses (with U. Amaldi, A. Bohm, L.S. Durkin, A.K. Mann, W. J. Marciano, A. Sirlin, and H.H. Williams), Phys. Rev. D 36, 1385 (1987).
  5. Unification of Two Fundamental Forces (with A. K. Mann), Physics Today 42, #12, p. 22 (1989).
  6. High Precision Electroweak Experiments: A Global Search for New Physics Beyond the Standard Model (with M. Luo and A. K. Mann), Rev. Mod. Phys. 64, 87 (1992).
  7. Implications of Precision Electroweak Experiments for m_t, rho_0, sinthw, and Grand Unification (with M. Luo), Phys. Rev. D 44, 817 (1991).
  8. W and Z Physics, in TeV Physics, ed. T. Huang et al., (Gordon and Breach, Philadelphia, 1991), p. 53.
  9. Editor (with M. Cvetic) of Testing the Standard Model (Proceedings of TASI-90), (World, Singapore, 1991).
  10. Five Phases of Weak Neutral Current Experiments From the Perspective of a Theorist, Discovery of Weak Neutral Currents: The Weak Interaction Before and After, ed. A. K. Mann and D. B. Cline, AIP Conference Proceedings 300 (AIP, New York, 1994), p. 289, hep-ph/9305255.
  11. Astrophysical Solutions are Incompatible with the Solar Neutrino Data (with S. Bludman and N. Hata), Phys. Rev. D49, 3622 (1994), hep-ph/9306212.
  12. Editor of Precision Tests of the Standard Electroweak Model, (World, Singapore, 1995), and articles on pp 1 (Structure of the Standard Model), 15, 883.
  13. The Strong Coupling, Unification, and Recent Data (with N. Polonsky), Phys. Rev. D52, 3081 (1995), hep-ph/9503214.
  14. Big Bang Nucleosynthesis in Crisis (with N. Hata, R. Scherrer, G. Steigman, D. Thomas, T. Walker, and S. Bludman), Phys. Rev. Lett. 75, 3977 (1995), hep-ph/9505319.
  15. Implications of Abelian Extended Gauge Structures From String Models (with M. Cvetic), Phys. Rev. D54, 3570 (1996), hep-ph/9511378.
  16. Phase Transitions and Vacuum Tunneling Into Charge and Color Breaking Minima in the MSSM (with A. Kusenko and G. Segre), Phys. Rev. D54, 5824 (1996), hep-ph/9602414.
  17. Electroweak Breaking and the Mu Problem in Supergravity Models with an Additional U(1) (with M. Cvetic, D. A. Demir, J. R. Espinosa, and L. Everett), Phys. Rev. D56, 2861 (1997), hep-ph/9703317.
  18. Solutions to Solar Neutrino Anomaly (with N. Hata), Phys. Rev. D56, 6107 (1997), hep-ph/9705339.
  19. Z' Physics and Supersymmetry (with M. Cvetic), in Perspectives on Supersymmetry, ed. G. Kane (World, Singapore, 1998), p312, hep-ph/9707451.
  20. Editor (with M. Cvetic), SUSY '97, Proceedings of the Fifth International Conference on Supersymmetries in Physics, Nucl. Phys. B (Proc. Suppl.) 62 (1998) (North Holland, 1998).
  21. Physics Implications of Flat Directions in Free Fermionic Superstring Models I: Mass Spectrum and Couplings (with G. Cleaver, M. Cvetic, J. R. Espinosa, L. Everett, and J. Wang), Phys. Rev. D59, 055005 (1999), hep-ph/9807479.
  22. Physics Implications of Flat Directions in Free Fermionic Superstring Models II: Renormalization Group Analysis (with G. Cleaver, M. Cvetic, J. R. Espinosa, L. Everett, and J. Wang), Phys. Rev. D59, 115002 (1999), hep-ph/9811355.
  23. Flavor Changing Effects in Theories with a Heavy Z' Boson with Family Non-Universal Couplings (with M. Pluemacher), Phys. Rev. D62, 013006 (2000), hep-ph/0001204.
  24. Editor, Neutrinos in Physics and Astrophysics: From 10-33 to 10+28 cm, (Proceedings of TASI-98), (World, Singapore, 2000).
  25. Neutrino Oscillation Workshop 2000: Conference Summary, NOW2000, Otranto, Italy, September 2000, hep-ph/0101244.
  26. Precision electroweak data: Phenomenological analysis, in Proceedings of the APS/DPF/DPB Summer Study on the Future of Particle Physics (Snowmass 2001) ed. R. Davidson and C. Quigg, hep-ph/0110129.
  27. Implications of gauge unification for time variation of the fine structure constant (with G. Segre and M. J. Strassler), Phys. Lett. B 528, 121 (2002), hep-ph/0112233.
  28. The Z-Z' Mass Hierarchy in a Supersymmetric Model with a Secluded U(1)'-Breaking Sector (with J. Erler and T. Li), Phys. Rev. D 66, 015002 (2002), hep-ph/0205001.
  29. Phenomenology of A Three-Family Standard-like String Model (with M. Cvetic and G. Shiu), Phys. Rev. D 66, 066004 (2002), hep-ph/0205252.
  30. No-go for detecting CP violation via neutrinoless double beta decay (with V. Barger, S. L. Glashow, and D. Marfatia), Phys. Lett. B 540, 247 (2002), hep-ph/0205290.
  31. Primordial nucleosynthesis constraints on Z' properties (with V. Barger and H. S. Lee), Phys. Rev. D 67, 075009 (2003), hep-ph/0302066.
  32. Dynamical supersymmetry breaking in standard-like models with intersecting D6-branes (with M.Cvetic and J. Wang), Phys. Rev. D 68, 046002 (2003), hep-th/0303208.
  33. Z' mediated flavor changing neutral currents in B meson decays (with V. Barger, C. W. Chiang, and H. S. Lee), Phys. Lett. B 580, 186 (2004), hep-ph/0310073.
  34. Electroweak Model and Constraints on New Physics (with J. Erler), in 2006 WWW update for 2006 edition of Review of Particle Properties, (URL: http://pdg.lbl.gov/).
  35. Review Of Particle Physics (W. M. Yao et al.), J. Phys. G33, 1 (2006), (on-line).
  36. Electroweak baryogenesis in a supersymmetric U(1)' model, (with J. Kang, T. j. Li, and T. Liu), Phys. Rev. Lett. 94, 061801 (2005), hep-ph/0402086.
  37. The Higgs sector in a U(1)' extension of the MSSM, (with T. Han and B. McElrath), Phys. Rev. D 70, 115006 (2004), hep-ph/0405244.
  38. Elementary Particles in Physics (with S. Gasiorowicz), in Encyclopedia of Physics, Third Edition, ed. R. C. Lerner and G. L. Trigg, (Wiley-VCH, 2005), p671, www.physics.upenn.edu/~pgl/e27/E27.pdf.
  39. Toward realistic intersecting D-brane models, (with R. Blumenhagen, M. Cvetic, and G. Shiu), ARNPS 55, 71 (2005), hep-th/0502005.
  40. Massive neutrinos and (heterotic) string theory, (with J. Giedt, G. L. Kane, and B. D. Nelson), Phys. Rev. D 71, 115013 (2005), hep-th/0502032.
  41. String-inspired triplet see-saw from diagonal embedding of SU(2)_L in SU(2)_A x SU(2)_B, (with B. Nelson), Phys. Rev. D 72, 153013 (2005), hep-ph/0507063.
  42. Higgs sector in extensions of the MSSM, (with V. Barger, H. S. Lee and G. Shaughnessy), Phys. Rev. D73, 115010 (2006), hep-ph/0603247.
  43. New grand unified models with intersecting D6-branes, neutrino masses, and flipped SU(5), (with M. Cvetic), hep-th/0607238 .
  44. Neutralino signatures of the singlet extended MSSM (with V. Barger and G. Shaughnessy), Phys. Lett. B644, 361 (2007), hep-ph/0603247 .
  45. Collider signatures of singlet extended Higgs sectors, (with V. Barger and G. Shaughnessy), Phys. Rev. D75, 055013 (2007), hep-ph/0611239 .
  46. TeV physics and the Planck scale (with V. Barger and G. Shaughnessy), hep-ph/0702001 .
  47. Recoil detection of the lightest neutralino in MSSM singlet extensions, (with V. Barger, I. Lewis, M. McCaskey, G. Shaughnessy and B. Yencho), Phys. Rev. D75, 115002 (2007), hep-ph/0702036 .
  48. A T-odd observable sensitive to CP violating phases in squark decay, (with G. Paz, L. T. Wang and I. Yavin), hep-ph/0702068 .
  49. LHC Phenomenology of an Extended Standard Model with a Real Scalar Singlet, (with V. Barger, M. McCaskey, M. J. Ramsey-Musolf and G. Shaughnessy), 0706.4311 [hep-ph] .

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Paul Langacker
July 30, 2007