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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.

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arXiv IDhep-ph/9703259 (version 1)
Pith IDYSEXXI2K, Pith's permanent record of this paper and its review
CitationHitoshi Murayama (UC Berkeley, LBNL), Nima Arkani-Hamed. "Can The Supersymmetric Flavor Problem Decouple?." arXiv:hep-ph/9703259 (1997). https://pith.science/paper/YSEXXI2K
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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

Low-energy supersymmetry has a flavor problem: squarks and sleptons of the first two generations, sitting near the weak scale with arbitrary mixing, generate kaon mixing, $mu$ to $e gamma$ and electric dipole moments far larger than observed. A popular escape is to push those first- and second-generation scalars up to multi-TeV masses, where their flavor-changing effects are suppressed, while keeping the third generation, gauginos and higgsinos light enough to stabilize the weak scale. The paper asks whether that split spectrum survives its own quantum corrections and answers no: the heavy scalars enter the two-loop renormalization group equations with a negative sign, dragging the third-generation scalar mass squared toward negative values, which would break color and charge. Requiring the kaon-mixing contribution to stay below the measured value forces the first two generations above 22 TeV even with order-$lambda$ alignment, and that in turn forces the boundary value of the left-handed stop mass above 4 TeV, far outside any natural range. The authors conclude that raising the first two generations does not solve the flavor problem, and that a real flavor symmetry or a dynamical mechanism for degenerate scalar masses is needed instead.

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

Editorial extensions of the paper, not claims the author makes directly.

  • 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.

Compiled by classifier-1.13-20260917 via OpenRouter, question bank abstract-p1-20260923, 2026-09-23: 1 request, 4,477 input tokens, $0.0002 reported.

What the abstract itself states, as probabilities: claims first 0.03 · states limitation 0.41 · human participants 0.01 · controlled comparison 0.04 · randomized assignment 0.01 · states quantitative result 0.98 · compares with prior results 0.59 · mentions code or data availability 0.02.

The abstract, sentence by sentence

arXiv listing abstract v1. Results and the main claim are underlined; .

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

What the classifier reports for each sentence. The number is the probability it gave the label.
#sentencerolethe authors' own workstates the main conclusionstrength
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 kind of paper the classifier reports this is: theoretical result 1.00 · evidence from proof or derivation 0.81.

What was examined, and what was not

sources read
  • arXiv listing abstract v1
not examined
  • full text
  • figures and tables
  • references and citing sentences
  • relations to other papers
levels run
  • P abstract
not run
  • S sections
  • T body sentences
  • C citations
  • R cross-paper relations

Every quoted sentence on this record is the authors'. Every label is from Pith's closed vocabulary, with the probability shown beside it. Every number in the measurements table is the authors', checked by code. Record as JSON

Figures
Figure 1. The minimum mass of the first- and second-generation sca
Figure 2. Constraint on the mass ratio between the first- and seco
Figure 3. The minimum boundary mass of the left-handed scalar top

Figures reproduced from arXiv:hep-ph/9703259 by the authors.

References
How the literature uses it

37 arXiv papers cite this in 57 sentences

Each citing sentence is taken from the citing paper's LaTeX source and typed by Pith's classifier. A label appears at probability 0.8 or more (0.95 for support), a contest at 0.7 or more; the quoted words are the citing authors'. How each label was checked · JSON

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

Discussion

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