REVIEW
Can The Supersymmetric Flavor Problem Decouple?
High Energy Physics - Phenomenology · Submitted 7 Mar 1997 · 20 references
Pith's verdict: A clean, checkable no-go for split sfermion spectra: the two-loop gauge running really does drive m^2_{Q3} negative, and the numbers are order-of-magnitude but honest.
T0 · AI review
Why Pith says this
Highlight any text to comment on it.
Referee report
The paper
The paper claims
Pushing the first two generations of squarks and sleptons to multi-TeV masses to suppress flavor-changing processes drives the third-generation scalar mass squared negative through two-loop running, so the supersymmetric flavor problem does
The reading
What carries the argument
The engine is the two-loop renormalization group equation for a third-generation scalar mass, Eq. (1) of the paper, which retains only the one-loop gaugino term and the two-loop term proportional to the heavy scalar mass squared $m^2_{1,2}$. The coefficient of the heavy-scalar term is fixed by the Standard Model quantum numbers of the heavy fields and by how many heavy $5^*$ and $10$ multiplets of $SU(5)$ are present, counted as $N_5$ and $N_{10}$; the hypercharge piece also depends on the $D$-term cancellation requirement that scalars within a single $SU(5)$ multiplet be degenerate. Solving the equation analytically gives Eq. (3), an explicit expression for $m^2_{tilde f}(t)$ at the weak scale in terms of the boundary stop mass, the universal gaugino mass $M_0$, and $m^2_{1,2}$, with the heavy-scalar contribution switched off at a decoupling scale taken as 10 TeV. Setting the result to zero yields the exclusion curves of Fig. 2, which are then intersected with the kaon-mixing bounds of Fig. 1 to produce the stop-mass bounds of Fig. 3.
Extended reading notes
Core claim
The claim is that a split scalar spectrum, with the first two generations heavy and the third light, is radiatively unstable in any theory where the soft supersymmetry-breaking parameters are generated at a high scale. The heavy scalars contribute at two loops to the running of the third-generation scalar masses, and the contribution is negative and proportional to the heavy mass squared $m^2_{1,2}$. Combining the two steps, the authors first bound $m_{1,2}$ from the requirement that the squark-gluino contribution to $K^0$-$bar K^0$ mixing not exceed the observed value, obtaining bounds from the multi-TeV range up to about 100 TeV depending on the assumed squark mixing pattern, and stronger by a factor of 13 if order-one CP-violating phases are allowed. They then solve the two-loop renormalization group equations analytically and find that the third-generation mass squared vanishes along curves in the plane of $m_{tilde f}(0)/m_{1,2}$ versus $M_0/m_{1,2}$, with everything below those curves excluded. For the two most favorable mixing patterns the two constraints together require $m_{tilde Q_3}(0) > 4$ TeV, which corresponds to electroweak fine-tuning worse than the $10^{-3}$ level, and only a scenario with an extra factor of about five in degeneracy marginally allows a stop near 1 TeV, still at the percent fine-tuning level.
Load-bearing premise
The size of the negative two-loop contribution depends on the scale at which the heavy first- and second-generation scalars are removed from the running, which the authors take to be 10 TeV and which they explicitly state is not known without a dedicated study of threshold corrections; the sign of the effect survives any choice, but the quoted 22 TeV and 4 TeV numbers scale with that logarithm.
Editorial extensions
If this is right
- In high-scale mediation schemes, suppressing flavor-changing neutral currents by making the first two generations heavy requires a boundary stop mass above 4 TeV for order-$lambda$ alignment, which corresponds to electroweak fine-tuning worse than one part in a thousand.
- The anomalous $U(1)$ string models that predict $m_{tilde f}(0)/m_{1,2} = 0.1$ and $M_0/m_{1,2} = 0.01$ are excluded, because their third-generation mass squared is driven negative.
- Models with $m_{1,2}$ in the few-TeV range and order-$10^{-2}$ squark mixing, previously claimed to solve the flavor problem including CP violation, actually require $m_{1,2} > 13$ TeV and $m_{tilde Q_3}(0) > 2.5$ TeV.
- Any concrete realization of a split spectrum with a low mediation scale must be checked explicitly, since the negative two-loop contribution is smaller there but not absent.
Reading between the lines
- The same negative two-loop effect should appear for any heavy matter charged under the Standard Model gauge group, so GUT-scale threshold multiplets introduced for unification reasons could destabilize the stop even when no flavor problem is being addressed.
- Because the sign of the effect is robust while its magnitude scales with the logarithm between the decoupling scale and the weak scale, a low-scale mediation model is the sharpest surviving version of the split-spectrum idea, and the paper's own numbers suggest it is the place to look next.
- A discovery of a first-generation squark in the multi-TeV range would be a direct test: the spectrum would have to come with either a heavier stop or a mediation scale low enough that the running length is short.
- The argument reframes the flavor problem as a radiative-stability problem, which suggests that any solution should be judged by whether it keeps the third-generation scalar mass squared positive, not only by whether it suppresses the flavor-changing operators.
Original abstract
It has been argued that the squarks and sleptons of the first and second generations can be relatively heavy without destabilizing the weak scale, thereby improving the situation with too-large flavor-changing neutral current (FCNC) and CP violating processes. In theories where the soft supersymmetry breaking parameters are generated at a high scale (such as the Planck scale), we show that such a mass spectrum tends to drive the scalar top mass squared $m_{\tilde{Q}_3}^2$ negative from two-loop renormalization group evolution. Even ignoring CP violation and allowing $O(\lambda) \sim .22$ alignment, the first two generation scalars must be heavier than 22 TeV to suppress FCNC. This in turn requires the boundary condition on $m_{\tilde{Q}_3} > 4 TeV$ to avoid negative $m_{\tilde{Q}_3}^2$ at the weak scale. Some of the models in the literature employing the anomalous U(1) in string theory are excluded by our analysis.
The compiled record
The paper's own sentences, arranged, kept beside the review and separate from it. Each label is drawn from a closed vocabulary; the number beside it is the probability the classifier gave that label. Numbers in the tables are the authors', checked by code. No sentence in this record was written by a model.
The abstract, sentence by sentence
It has been argued that the squarks and sleptons of the first and second generations can be relatively heavy without destabilizing the weak scale, thereby improving the situation with too-large flavor-changing neutral current (FCNC) and CP violating processes. In theories where the soft supersymmetry breaking parameters are generated at a high scale (such as the Planck scale), we show that such a mass spectrum tends to drive the scalar top mass squared $m_{\tilde{Q}_3}^2$ negative from two-loop renormalization group evolution. Even ignoring CP violation and allowing $O(\lambda) \sim .22$ alignment, the first two generation scalars must be heavier than 22 TeV to suppress FCNC. This in turn requires the boundary condition on $m_{\tilde{Q}_3} > 4 TeV$ to avoid negative $m_{\tilde{Q}_3}^2$ at the weak scale. Some of the models in the literature employing the anomalous U(1) in string theory are excluded by our analysis.
background contribution method result interpretation main claim
| # | sentence | role | the authors' own work | states the main conclusion | strength |
|---|---|---|---|---|---|
| s0 | It has been argued that the squarks and sleptons of the first and second generations can be relatively heavy without destabilizing the weak scale, thereby improving the situation with too-large flavor-changing neutral current (FCNC) and CP violating processes. | background 1.00 | 0.04 | 0.06 | hedged possibility 0.87 |
| s1 | In theories where the soft supersymmetry breaking parameters are generated at a high scale (such as the Planck scale), we show that such a mass spectrum tends to drive the scalar top mass squared $m_{\tilde{Q}_3}^2$ negative from two-loop renormalization group evolution. | result 0.89 | 0.91 | 0.27 | asserted 0.94 |
| s2 | Even ignoring CP violation and allowing $O(\lambda) \sim .22$ alignment, the first two generation scalars must be heavier than 22 TeV to suppress FCNC. | result 1.00 | 0.73 | 0.22 | asserted 0.99 |
| s3 | This in turn requires the boundary condition on $m_{\tilde{Q}_3} > 4 TeV$ to avoid negative $m_{\tilde{Q}_3}^2$ at the weak scale. | result 0.98 | 0.81 | 0.26 | asserted 1.00 |
| s4 | Some of the models in the literature employing the anomalous U(1) in string theory are excluded by our analysis. | interpretation 0.60 | 0.77 | 0.13 | asserted 0.95 |
What was examined, and what was not
- arXiv listing abstract v1
- full text
- figures and tables
- references and citing sentences
- relations to other papers
- P abstract
- S sections
- T body sentences
- C citations
- R cross-paper relations
Figures
Figures reproduced from arXiv:hep-ph/9703259 by the authors.
References
How the literature uses it
37 arXiv papers cite this in 57 sentences
Papers citing it · 43 sentences without a confident label
Uses its method or tool 1 paper
-
In this simplified case, we can solve the RGE for the stop mass analytically to find [ref] [equation] where $t̃$ can refer to $t̃_L$ or $t̃_R$ with appropriate choice of Casimirs (the stronger bound is on $t̃_L$), $i = 1,2,3,$ $b_i$ and $C_i$ are the usual GUT-normalized $β$ function parameters and Casimirs respectively, $m̃_1,2^2$ are the mean squark masses, $μ$ is the low scale (taken to be 1 TeV), $μ'$ is the scale where the heavy squarks decouple (taken to be 10 TeV), and $Λ$ is the scale where supersymmetry is broken and RG flow commences (taken to be 100 TeV).
Dynamical Supersymmetry Breaking, with Flavor · Constraints from tachyonic stop mass · 0.81
Compares with it 4 papers
-
In Fig. [ref] (left) we report the analogous of Fig. 2 of [ref] for different values of $M$[The solid line is in agreement with [ref] , taking into account the fact that we keep only the gluino (while they keep all the gauginos with equal mass at $M_GUT$) and that our $M_g$ is the gluino mass at low energies.].
Can the Supersymmetric Flavour Problem decouple in case of a Non Standard Supersymmetric Spectrum? · Constraint from colour conservation · 0.84
-
Notice that, in the case of the MSSM, for $M=M_GUT$ we obtain basically the same bound as in Figure 2 of [ref] , with the proper translation of the parameters[Our colour conservation constraint is actually slightly stronger because we keep only the gluino mass, while [ref] keeps all the gauginos with equal mass at $M_GUT$.].
A Non Standard Supersymmetric Spectrum · Constraint from colour conservation · 1.00
-
Notice that, in the case of the MSSM, for $M=M_GUT$ we obtain basically the same bound as in Figure 2 of [ref] (to be compared with our Figure [ref], see also [ref] ), with the proper translation of the parameters[Our colour conservation constraint is actually slightly stronger because we keep only the gluino mass, while [ref] keeps all the gauginos with equal mass at $M_GUT$.].
A Motivated Non-Standard Supersymmetric Spectrum · Constraint from colour conservation · 0.99
Cites it as support 3 papers
-
For very heavy 1st/2nd generation squarks or too-high messenger scales, the stop squarks can even become tachyonic due to this effect [ref] .
Precision Corrections to Fine Tuning in SUSY · Transfer matrix RGEs: stop and gluino masses · 0.96
-
In RG scaling from high scales, 2-loop correction to the third generation soft masses from the first two generations give large negative soft masses that would lead to color breaking [ref] .
Simply Unnatural Supersymmetry · New Flavor Physics and Radiative Fermion Masses · 0.99
-
But they cannot get too heavy, as they induce a logarithmically enhanced negative mass for the (light) third-generation squarks [ref] , and so cannot be pushed higher than at most $∼30$ TeV.
Physics Opportunities of a 100 TeV Proton-Proton Collider · Natural Theories and the Tests at the 100 TeV $pp$ Collider · 0.96
Cites it as background 4 papers
-
Analyses with Radiatively Generated Inverted Mass Hierarchy Models (RIMH) were made in Ref. [ref] .
Higgs funnel region of SUSY dark matter for small $\tan\beta$ and renormalization group effects on pseudoscalar Higgs boson with scalar mass non-universality · Introduction · 0.91
-
It is interesting to consider fully solving the supersymmetric flavor problem by raising the masses of the first two generation sparticles a bit more - for discussions of whether this is possible or not, see also [ref] .
Single-Sector Supersymmetry Breaking in Supersymmetric QCD · Brief comments on phenomenology · 0.92
-
[ref] using the RG MSSM equations in the $DR$ scheme; they were revisited in ref.
Large, negative threshold contributions to light soft masses in models with Effective Supersymmetry · Tachyon bounds for squarks and gaugino masses · 0.85
Discussion
Disagree with the review, or know something it missed? Leave a comment, highlight any passage above to comment on it, or challenge this review formally.