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Fine Tuning Standard Model and Beyond Peter Athron Dr David Miller In collaboration with

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+ physical mass = bare mass + loops =+ divergent Cut off integral at Planck Scale Fine tuning Hierarchy Problem

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Supersymmetry The only possible extension to space-time Unifies gauge couplings Provides Dark Matter candidates Baryogenesis in the early universe Elegant solution to the Hierarchy Problem! Essential ingredient for M-Theory

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Bosonic degrees of freedom = Fermionic degrees of freedom. ) Two scalar superpartners for each fermion In Susy No Fine Tuning? Quadratic divergences cancelled!

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MSSM At Tree Level: Sparticle mass limits ) Parameters But Exact Susy Softly Broken Susy LEP search fruitless!! Lower bounds on sparticles Fine Tuning reintroduced? Little Hierarchy Problem

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R. Barbieri & G.F. Giudice, (1988) Define Tuning is fine tuned % change in from 1% change in Observable Parameter Traditional Measure

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Criticisms of Traditional Measure Considers each parameter separately The fine tuning is about cancellations between parameters. A good fine tuning measure considers all parameters together. Implicitly assumes a uniform distribution of parameters Parameters in L GUT may be different to those in L SUSY Corresponds to choosing parameters from a different probability distribution Takes infinitesimal perturbations about the point MSSM observables are complicated functions of many parameters. Many small isolated regions of parameter space may give the same value of the observable.

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New Measure the volume of parameter space, the subspace of s.t. the observable Tuning occurs when variations in dimensionless parameters ) larger variations in dimensionless observables. Parameter space point, Tuning is defined as:

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Fine tuning as a function of Higgs Mass Fine tuning as a function of the New Physics scale SM Revisited

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Fine Tuning in the MSSM Choose a point P in the parameter space at GUT scale Take random fluctuations about this point. Using a modified version of Softsusy (B.C. Allanach) Run to Electro-Weak Symmetry Breaking scale. Predict M z and sparticle masses Count how often M z is ok Apply fine tuning measure

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If the tuning in the MSSM is fine for flat probability distributions: Nature is fine tuned. EWSB by some other mechanism than the Higgs e.g. Technicolor The Hierarchy problem is solved other new physics e.g. Little Higgs, Large Extra Dimensions Extended Higgs sector SSMs are favoured e.g. NMSSM, nMSSM, ESSM The MSSM parameters are not all equally likely. What probability distribution minimises tuning? Is there a GUT with this distribution? Also contains tunings S.F. King, S. Moretti, R. Nevzarov, hep-ph/0510419, hep-ph/0511256

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Conclusions Fine Tuning in the SM SUSY Broken SUSY appears fine tuned Little Hierarchy Problem Current measures of tuning neglect: Probability distribution of parameters. Many parameter nature of fine tuning Cancellations a finite distance from point New measure addresses these issues Results for MSSM coming soon

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Ruled out by experiment

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Fine tuning as a function of Higgs Mass Fine tuning as a function of the New Physics scale

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Numerical Approximation Where is the number of points in space

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Is the Tuning Fine? Aesthetic question. How much tuning are we prepared to tolerate? Intuitive physical notion. How do we measure tuning?

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