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Andrea Linville Office of Science, SULI Program 2009 Stanford Linear Accelerator August 13, 2009
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Objective Standard Model What is SUSY? What will we calculate? Results The Future
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Use parton-model methods to predict the stop squark production cross- section in proton-proton collisions at LHC energies.
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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?
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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)
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. . Parton Distribution Functions (PDFs) .
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Q μ = muon charge Q e = electron charge g L = x w – ½ g R = x w (x w = weak mixing angle) α = fine structure constant G F = Fermi coupling constant M Z = mass of Z boson Γ Z = decay width of Z boson
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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
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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
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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
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3 possible initial states: ◦. ◦ Each initial state needs a separate cross-section equation…
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but…
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where α s = strong coupling constant m 1, m 2 = masses of produced squarks s = scattering energy
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was calculated to be 61.6 fb Reminder: 1 barn = 10 -24 cm 2 Invariant dimuon mass: 500 GeV This is the order of magnitude expected.
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∘ 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
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Resonance at 91 GeV ±0.5, ≈ M Z (91.2 GeV) Expected: 1fb at M =500 GeV Calculated: 0.96 fb Data taken every 1 GeV Machine energy: 14 TeV (LHC)
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Maximum cross- section: 0.27 nb More difficult to produce stops with increasing stop mass We examined only leading order (LO) Feynman diagrams
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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
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My mentor, JoAnne Hewett Tom Rizzo Michael Peskin Theory graduate students: John Conley, Randy Cotta, and Jamie Gainer
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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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