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In [[particle physics]], the '''doublet–triplet (splitting) problem''' is a problem of ''some'' [[Grand unification theory|Grand Unified Theories]], such as  [[Georgi-Glashow model|SU(5)]], [[SO(10) (physics)|SO(10)]], <math>E_6</math>. Grand unified theories predict [[Higgs boson]]s (doublets of <math>SU(2)</math>) arise from [[Group representation|representations]] of the unified group that contain other states, in particular, states that are triplets of color. The primary problem with these color triplet Higgs, is that they can mediate [[proton decay]] in [[Supersymmetry|supersymmetric]] theories that are only suppressed by two powers of GUT scale (i.e. they are dimension 5 supersymmetric operators). In addition to mediating proton decay, they alter [[gauge coupling unification]].  The doublet–triplet problem is the question 'what keeps the doublets light while the triplets are heavy?'
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==Doublet–triplet splitting and the <math>\mu</math>-problem==
In 'minimal' SU(5), the way one accomplishes doublet–triplet splitting is through a combination of interactions
 
<math> \int d^2\theta \; \lambda  H_{\bar{5}} \Sigma  H_{5} + \mu H_{\bar{5}} H_{5}</math>
 
where <math>\Sigma</math> is an adjoint of SU(5) and is [[traceless]].  When <math>\Sigma</math> acquires a vacuum expectation value
 
<math>\langle \Sigma\rangle = \rm{diag}(2, 2, 2, -3, -3) f</math>
 
that breaks SU(5) to the Standard Model gauge symmetry the Higgs doublets and triplets acquire a mass
 
<math> \int d^2\theta \;  (2 \lambda f + \mu) H_{\bar{3}}H_3 + (-3\lambda f +\mu) H_{\bar{2}}H_2</math>
 
Since  <math> f</math> is at the GUT scale (<math> 10^{16}</math> GeV) and the Higgs doublets need to have a weak scale mass (100 GeV), this requires
 
<math>\mu \sim 3 \lambda f \pm 100 \mbox{GeV}</math>.  
 
So to solve this doublet–triplet splitting problem requires a tuning of the two terms to within one part in <math>10^{14}</math>.
This is also why the [[mu problem]] of the [[Minimal Supersymmetric Standard Model|MSSM]] (i.e. why are the Higgs doublets so light) and doublet–triplet splitting are so closely intertwined.
 
===Dimopoulos–Wilczek mechanism===
In an SO(10) theory, there is a potential solution to the doublet–triplet splitting problem known as the 'Dimopoulos–Wilczek' mechanism. In SO(10), the adjoint field, <math>\Sigma</math> acquires a vacuum expectation value of the form
 
<math>\langle \Sigma \rangle = \mbox{diag}( i \sigma_2 f_3, i\sigma_2 f_3, i\sigma_2 f_3, i\sigma_2 f_2, i \sigma_2 f_2)</math>.
 
<math>f_2</math> and <math>f_3</math> give masses to the Higgs doublet and triplet, respectively, and are independent of each other, because <math>\Sigma</math> is [[traceless]] for any values they may have. If <math>f_2=0</math>, then the Higgs doublet remains massless. This is very similar to the way that doublet–triplet splitting is done in either higher-dimensional grand unified theories or string theory. 
 
To arrange for the VEV to align along this direction (and still not mess up the other details of the model) often requires very contrived models, however.
 
==Higgs representations in Grand Unified Theories==
In SU(5):
 
:<math>5\rightarrow (1,2)_{1\over 2}\oplus (3,1)_{-{1\over 3}}</math>
:<math>\bar{5}\rightarrow (1,2)_{-{1\over 2}}\oplus (\bar{3},1)_{1\over 3}</math>
 
In SO(10):
 
:<math>10\rightarrow (1,2)_{1\over 2}\oplus (1,2)_{-{1\over 2}}\oplus (3,1)_{-{1\over 3}}\oplus (\bar{3},1)_{1\over 3}</math>
 
==Proton decay==
[[Image:proton decay4.svg|left|frame|Dimension 6 proton decay mediated by the triplet Higgs <math>T (3,1)_{-\frac{1}{3}}</math> and the anti-triplet Higgs <math>\bar{T} (\bar{3},1)_{\frac{1}{3}}</math> in <math>SU(5)</math> GUT]]
 
Non-[[supersymmetric]] theories suffer from q&shy;ratric [[radiative correction]]s to the mass squared of the electroweak Higgs boson (see [[hierarchy problem]]). In the presence of [[supersymmetry]], the triplet [[Higgsino]] needs to be more massive than the GUT scale to prevent proton decay because it generates dimension 5 operators in [[Minimal Supersymmetric Standard Model|MSSM]]; there it is not enough simply to require the triplet to have a [[GUT scale]] mass.
 
==References==
* 'Supersymmetry at Ordinary Energies. 1. Masses AND Conservation Laws.' [[Steven Weinberg]]. Published in Phys.Rev.D26:287,1982.
* 'Proton Decay in Supersymmetric Models.' [[Savas Dimopoulos]], Stuart A. Raby, [[Frank Wilczek]]. Published in Phys.Lett.B112:133,1982.
* 'Incomplete Multiplets in Supersymmetric Unified Models.' Savas Dimopoulos, Frank Wilczek.
 
{{DEFAULTSORT:Doublet-triplet splitting problem}}
[[Category:Particle physics]]

Latest revision as of 03:14, 21 July 2014

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