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Calculation of Stop Production in Proton-Proton Collisions

Calculation of Stop Production in Proton-Proton Collisions. Andrea Linville Office of Science, SULI Program 2009 Stanford Linear Accelerator August 13, 2009. Outline. Objective Standard Model What is SUSY? What will we calculate? Results The Future. Objective.

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Calculation of Stop Production in Proton-Proton Collisions

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  1. Calculation of Stop Production in Proton-Proton Collisions Andrea Linville Office of Science, SULI Program 2009 Stanford Linear Accelerator August 13, 2009

  2. Outline • Objective • Standard Model • What is SUSY? • What will we calculate? • Results • The Future

  3. Objective • Use parton-model methods to predict the stop squark production cross-section in proton-proton collisions at LHC energies.

  4. Standard Model • Fermions (spin ½) • Bosons (integer spin) = force mediators • Elusive Higgs boson • Generates masses of leptons and W, Z • SM is incomplete • Dark matter? • Hierarchy problem? • Matter/antimatter imbalance?

  5. What is SUSY? • Each SM particle has a superpartner • difference: ½ unit of spin • SM fermions → SUSY bosons • SM bosons → SUSY fermions • SUSY is a broken symmetry • Sparticle masses depend on SUSY breaking model (unknown)

  6. What will we calculate? • . • . • .Parton Distribution Functions (PDFs) • . • .

  7. Qμ = muon charge Qe = electron charge gL = xw – ½ gR = xw (xw = weak mixing angle) α = fine structure constant GF = Fermi coupling constant MZ = mass of Z boson ΓZ = decay width of Z boson

  8. Modifications made to equations: • Replace electron charge with quark charge • Separate components for: • u, c, t quarks (+2/3 charge) • d, s, b quarks (-1/3 charge) • Sum over quark flavors

  9. What are PDFs? • Partons: quasi-free pointlike structures that make up hadrons • PDFs describe the probability density for finding a parton with a given fraction of the total momentum

  10. Integrating the PDFs • Convenient definitions: • = Q: scattering energy/invariant mass of the products • : proton center-of-mass energy (“machine” energy) • The equation we integrate: • Monte Carlo integration algorithm

  11. Squark pair production • 3 possible initial states: • . • . • . • Each initial state needs a separate cross-section equation…

  12. The Equations • but…

  13. Feynman rules: conservation of charge and quark flavor

  14. Scenarios that produce

  15. How to calculate where αs = strong coupling constant m1, m2 = masses of produced squarks s = scattering energy

  16. Results: • was calculated to be 61.6 fb • Reminder: 1 barn = 10-24 cm2 • Invariant dimuon mass: 500 GeV • This is the order of magnitude expected.

  17. Results: Parton densities • Greater contribution from sea quarks as scattering energy increases • At low energies, the sea quarks are confined within hadrons • The probability densities of b and t were exactly zero at these energies

  18. Results: • Resonance at 91 GeV ±0.5, ≈ MZ (91.2 GeV) • Expected: 1fb at M =500 GeV • Calculated: 0.96 fb • Data taken every 1 GeV • Machine energy: 14 TeV (LHC)

  19. Maximum cross-section: 0.27 nb More difficult to produce stops with increasing stop mass We examined only leading order (LO) Feynman diagrams Results:

  20. The Future • can be used to establish a lower bound on stop mass, once we have an experimental bound on σ • Similarly, if stops are observed, σ can determine stop particle masses • Our results are useful for: • Interpreting data • Preparing new experimental searches for SUSY

  21. Acknowledgements • My mentor, JoAnne Hewett • Tom Rizzo • Michael Peskin • Theory graduate students: John Conley, Randy Cotta, and Jamie Gainer

  22. References • S. Dawson, E. Eichten, and C. Quigg. “Search for supersymmetric particles in hadron-hadron collisions,” Physical Review D, vol. 31, no. 7, pp. 1581-1637, 1 April 1985. • W. Beenakker, M. Krämer, T. Plehn, M. Spira, and P.M. Zerwas. “Stop Production at Hadron Colliders,” Nuclear Physics B, vol. 515, pp. 3-14, 1998. • M. Schott. “Z Boson Production at LHC with First Data,” The 2007 Europhysics Conference on High Energy Physics Journal of Physics, Conference Series 110, 2008. • Elsevier publishing, Physics Letters B, vol. 667, Issues 1-5, 18 September 2008.

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