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Proton decay?

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Xiangdong Ji. Maryland center for fundamental physics. U of Maryland ... Whatever the new physics might be, one can always probe the low-energy baryon ... – PowerPoint PPT presentation

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Title: Proton decay?


1
Proton decay?
Are diamonds really forever?
OCPA conference on Underground Science University
of Hong Kong, July 23, 2008
  • Xiangdong Ji
  • Maryland center for fundamental physics
  • U of Maryland

2
Grand unification
  • 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)

3
Candidates for GUT
  • Pati-Salam SU(2) L?SU(2) R?SU(4) C
  • Georgi-Glashow SU(5)
  • SO(10)
  • Exceptional groups E6 and E8
  • Adding supersymmerty, extra dimension

4
Almost all GUTs allow proton decay
  • 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.

5
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

6
GUT and B L violation scale
  • GUT is a beautiful idea but the scale is very
    high, at least larger than 101516 GeV
  • Can 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
Two attitudes
  • 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.

8
B L violation
  • 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.

9
Experiments
  • Detector type
    Exposure

  • (kt-year)
  • Frejus Fe
    2.0
  • HPW H2O lt1.0
  • IMB H2O
    11.2
  • Kamiokande H2O 3.8
  • KGF Fe
    lt1.0
  • NUSEX Fe
    lt1.0
  • Soudan 1 Fe lt1.0
  • Soudan 2 Fe
    5.9
  • Super-Kamiokande H2O 79.3

4?1032
10
Current limits
11
Non-SUSY GUT
  • 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!

12
SUSY
  • Adding supersymmetry improves the unification and
    pushes the unification scale to higher energy

13
SUSY GUT
  • 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.

14
R-parity
  • 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.

15
Effective Dimension-5 operator
  • 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
16
Doublet-triplet splitting
  • 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

17
Dressing of Dim-5 operator
  • The dimension-5 operator can be dressed with
    gauginos or higgsino to generator SM dim-6
    operators

Y2/Mc MSUSY
18
Magnitude of the dim-5 contribution
  • 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

19
SUSY SU(5)
  • 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 lt MC lt 3.6 ? 1015 GeV
  • p ?K? limit constraints the mass scale to be
  • MC gt 2 ? 1017 GeV
  • The conflicts rules out the simple SU(5)

20
SO(10) models
  • 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
  • Special flavor structure leading to cancellation?
  • Larger unification scale?
  • Split SUSY
  • Extra dimension

21
Future experimental opportunities
  • Japan Hyper-K
  • US DUSEL (UNO or LAr)
  • Europe 100 kt LAr TPC, 1Mt WC detector
  • at Frejus.

22
How far can one go in this game?
23
Exp. vs. theory
24
Conclusion
  • 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.
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