# proton decay?

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Xiangdong Ji Maryland center for fundamental physics U of Maryland. Are diamonds really forever?. Proton decay?. OCPA conference on Underground Science University of Hong Kong, July 23, 2008. Grand unification. One of the most profitable themes in physics! - PowerPoint PPT PresentationTRANSCRIPT

Xiangdong JiMaryland center for fundamental physicsU of MarylandOCPA conference on Underground ScienceUniversity of Hong Kong, July 23, 2008

Parity symmetry at high-energy

One of the most profitable themes in physics!Electricity and magnetism Light!Electromagnetism and weak force W, Z and spontaneous symmetry breakingWill this trend continue?Electroweak + strong? (GUTs)+ gravity? (string theory)

7/23/08Proton decay

Parity symmetry at high-energy

Pati-Salam SU(2) LSU(2) RSU(4) CGeorgi-Glashow SU(5)SO(10)Exceptional groups E6 and E8 Adding supersymmerty, extra dimension3723/08Proton

Parity symmetry at high-energy

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.

7/23/2008Proton decay

Parity symmetry at high-energy

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

7/23/08Proton decay

Parity symmetry at high-energy

GUT is a beautiful idea but the scale is very high, at least larger than 1015~16 GeVCan one really trust a theory at that high-energy scale and pretend that nothing will happen in between?Similar question for the sea-saw mechanism, where the R-handed scale is on 1014 GeV

7/23/08Proton decay.

Parity symmetry at high-energy

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.7/23/08Proton decay

Parity symmetry at high-energy

Baryon and lepton numbers are known to be conserved to very good precision in low-energy 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.

7/23/08Proton decay

Parity symmetry at high-energy

Detector type Exposure (kt-year)

Frejus Fe 2.0HPW H2O

7/23/08Proton decay

Parity symmetry at high-energy

In non-SUSY GUT, proton decay is mediated by dimension-6 operatorsThe 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!37/23/08Proton decay

Parity symmetry at high-energy

Adding supersymmetry improves the unification and pushes the unification scale to higher energy7/23/08Proton decay

Parity symmetry at high-energy

Unlike SM, it is easy to write down operators which violate B and L.Dimension-2 operators mixes leptons and quarks with higginos FHDimension-3 operators ucdcdc, QLdc, LLecThey either violate B or L, but not both, generating huge lepton and baryon number violations. 7/23/08Proton decay

Parity symmetry at high-energy

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. 7/23/08Proton decay

Parity symmetry at high-energy

Proton decay can happen with dimension-5 operators of the following formd QQQL, ucucdcec which are suppressed only by color triplet mass Mc

7/23/08Proton decayY2/Mc

Parity symmetry at high-energy

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 breakingIt is not trivial to generate this stable scale separation in theoryHuge theoretical literature7/23/08Proton decay

Parity symmetry at high-energy

The dimension-5 operator can be dressed with gauginos or higgsino to generator SM dim-6 operators7/23/08Proton decayY2/Mc MSUSY

Parity symmetry at high-energy

Y2/MGUT MSUSYLarge, because 1/MSUSY Suppression through yukawa couplingResults depend on sensitively on flavor structure of the GUT, which is least known.ModelsSU(5): simplest version has been rule outSO(10), many different versions for Y-couplings7/23/08Proton decay

Parity symmetry at high-energy

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)7/23/08Proton decay

Parity symmetry at high-energy

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 outSpecial flavor structure leading to cancellation?Larger unification scale?Split SUSYExtra dimension7/23/08Proton decay

Parity symmetry at high-energy

Japan: Hyper-KUS: DUSEL (UNO or LAr)Europe: 100 kt LAr TPC, 1Mt WC detector at Frejus.7/23/08Proton decay

Parity symmetry at high-energy

7/23/08Proton decay

Parity symmetry at high-energy

7/23/08Proton decay

Parity symmetry at high-energy

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.

7/23/08Proton decay

Parity symmetry at high-energy

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