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OCPA conference on Underground Science University of Hong Kong, July 23, 2008 Xiangdong Ji Maryland center for fundamental physics U of Maryland One of the most profitable themes in physics! Electricity and magnetism Light! Electromagnetism and weak force W, Z and spontaneous symmetry breaking Will this trend continue? Electroweak + strong? (GUTs) + gravity? (string theory) 7/23/08 Proton decay Pati-Salam SU(2) LSU(2) RSU(4) C Georgi-Glashow SU(5) SO(10) Exceptional groups E6 and E8 Adding supersymmerty, extra dimension … 3723/08 Proton In a typical GUT, quarks and leptons are placed in the same representation of some unification group. SU(5) example F = (d1, d2, d3, , e) ALL the particles in a multiplet are the “same stuff” that can be rotated into each other through gauge and Yukawa interactions. 7/23/2008 Proton decay Hence the baryon and lepton numbers are no longer separately conserved and proton Is not absolutely stable! Decay product: light leptons (muon and electron and neutrinos) + light mesons (pions and kaons) Example: P 0 + e+ A diamond will eventually dissolve into light + neutrinos + electrons 7/23/08 Proton decay GUT is a beautiful idea but the scale is very high, at least larger than 1015~16 GeV Can one really trust a theory at that high-energy scale and pretend that nothing will happen in between? Similar question for the seasaw mechanism, where the Rhanded scale is on 1014 GeV 7/23/08 Proton decay Opportunist: Neutrino mass and proton decay probe physics at extremely high-energy scale, otherwise unreachable using the conventional particle accelerator. Pragmatist: Whatever the new physics might be, one can always probe the low-energy baryon/lepton number violating limit, which might or might not be signals for grand unification. 7/23/08 Proton decay Baryon and lepton numbers are known to be conserved to very good precision in lowenergy experiments. SM have baryon and lepton number as accidental symmetry. These symmetries will likely be broken in beyond-SM theories, taken into account by new high-dimensional operators. 7/23/08 Proton decay Detector type Exposure (kt-year) Frejus Fe 2.0 HPW H2O <1.0 IMB H2O 11.2 Kamiokande H2O 3.8 KGF Fe <1.0 NUSEX Fe <1.0 Soudan 1 Fe <1.0 Soudan 2 Fe 5.9 Super-Kamiokande H2O 79.3 7/2308 Proton decay 41032 7/23/08 Proton decay In non-SUSY GUT, proton decay is mediated by dimension-6 operators The lifetime is simply, Given a unified coupling and GUT scale, one can predict the lifetime, which can be tested immediately in experiments. Non-SUSY SU(5) & SO(10) rule out! 37/23/08 Proton decay Adding supersymmetry improves the unification and pushes the unification scale to higher energy 7/23/08 Proton decay Unlike SM, it is easy to write down operators which violate B and L. Dimension-2 operators mixes leptons and quarks with higginos FH Dimension-3 operators ucdcdc, QLdc, LLec They either violate B or L, but not both, generating huge lepton and baryon number violations. 7/23/08 Proton decay If we imposes R-parity on the SUSY GUT, dimension-3 and 4 operators can be entirely eliminated particles have +1 parity and sparticles have parity -1. There is no deep theoretical reason why R-parity shall be conserved (LR symmetry). Small B & L violation might be the strong empirical reason from R-parity conservation. 7/23/08 Proton decay Proton decay can happen with dimension-5 operators of the following formd QQQL, ucucdcec which are suppressed only by color triplet mass Mc Y2/Mc 7/23/08 Proton decay Higgs color-triplet that generates dim-5 operator must have masses on the order of GUT scale. On the other hand, the weak SU(2) doublet which gives rise masses of SM particles must live on the scale of EW symmetry breaking It is not trivial to generate this stable scale separation in theory Huge theoretical literature 7/23/08 Proton decay The dimension-5 operator can be dressed with gauginos or higgsino to generator SM dim-6 operators Y2/Mc MSUSY 7/23/08 Proton decay Y2/MGUT MSUSY Large, because 1/MSUSY Suppression through yukawa coupling Results depend on sensitively on flavor structure of the GUT, which is least known. Models SU(5): simplest version has been rule out SO(10), many different versions for Y-couplings 7/23/08 Proton decay Unification of the gauge coupling constants depends on the color-triplet threshold. At two-loop level, this gives a constraint for the success of unification 3.5 1014 GeV < MC < 3.6 1015 GeV p K+ limit constraints the mass scale to be MC > 2 1017 GeV The conflicts rules out the simple SU(5) 7/23/08 Proton decay There are many SO(10) models on the market which claim to fit all fermion masses, mixings including neutrino mixing matrix. Generally they predict fast proton decay rates SUSY proton decay problem! Way out 7/23/08 Special flavor structure leading to cancellation? Larger unification scale? Split SUSY Extra dimension… Proton decay Japan: Hyper-K US: DUSEL (UNO or LAr) Europe: 100 kt LAr TPC, 1Mt WC detector at Frejus. 7/23/08 Proton decay 7/23/08 Proton decay 7/23/08 Proton decay Proton decay has not yet been seen yet, but its longevity suggests baryon number violation is small and is perhaps related to GUT and small neutrino mass. However, GUT model building is increasingly complicated. Along with SUSY flavor, CP problems, now we likely have a SUSY proton decay problem. It is very exciting to push the current limit by another order of magnitude. 7/23/08 Proton decay