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The 750 Gev Diphoton Excess As a First Light on Supersymmetry Breaking

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The 750 Gev Diphoton Excess As a First Light on Supersymmetry Breaking Physics Letters B 759 (2016) 159–165 Contents lists available at ScienceDirect Physics Letters B www.elsevier.com/locate/physletb The 750 GeV diphoton excess as a first light on supersymmetry breaking ∗ J.A. Casas a, J.R. Espinosa b,c, , J.M. Moreno a a Instituto de Física Teórica, IFT-UAM/CSIC, Nicolás Cabrera 13, UAM Cantoblanco, 28049 Madrid, Spain b Institut de Física d’Altes Energies (IFAE), The Barcelona Institute of Science and Technology (BIST), Campus UAB, E-08193, Bellaterra (Barcelona), Spain c ICREA, Institució Catalana de Recerca i Estudis Avançats, Barcelona, Spain a r t i c l e i n f o a b s t r a c t Article history: One of the most exciting explanations advanced for the recent diphoton excess found by ATLAS and Received 15 February 2016 CMS is in terms of sgoldstino decays: a signal of low-energy supersymmetry-breaking scenarios. The Received in revised form 27 April 2016 sgoldstino, a scalar, couples directly to gluons and photons, with strength related to gaugino masses, that Accepted 21 May 2016 can be of the right magnitude to explain the excess. However, fitting the suggested resonance width, Available online 25 May 2016 45 GeV, is not so easy. In this paper we explore efficient possibilities to enhance the sgoldstino Editor: G.F. Giudice width, via the decay into two Higgses, two Higgsinos and through mixing between the sgoldstino and the Higgs boson. In addition, we present an alternative and more efficient mechanism to generate a mass splitting between the scalar and pseudoscalar components of the sgoldstino, which has been suggested as an interesting alternative explanation to the apparent width of the resonance. © 2016 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). Funded by SCOAP3. 1. Introduction an scalar field φ (the sgoldstino) coupled to gluons and photons in a direct way, so that an effective production via gluon fusion and The ATLAS and CMS Collaborations√ have recently reported an the subsequent decay into photons are possible. Beside reproduc- excess in diphoton searches at s = 13 TeV for a ∼ 750 GeV ing the observed cross section, any good explanation of the data diphoton invariant mass [1–3]. The local significance is 3.9σ (AT- should account for the apparent sizeable width of the resonance, LAS) and 2.6σ (CMS), although it gets smaller once the look- φ/Mφ 0.06, although the data are not yet conclusive and the elsewhere effect is taken into account. However, the fact than both significance of such a large width is not too large. For this rea- experiments see the signal in the same place has created in the son, in the following, scenarios that are able to explain at least a community the expectation that it could be the long expected sig- significant fraction of that apparent large width are considered fa- nal of new physics. vorably. The authors of ref. [6] discussed a simple explanation for Once the accumulated statistics at ATLAS and CMS grow large the apparent width: a mass splitting (as advocated in [4]) between enough, we will see finally whether or not this excess is an statisti- the scalar and pseudoscalar degrees of freedom of the sgoldstino. cal fluctuation. In the meantime, it is tempting to try and interpret In this paper we review the explanation of the diphoton sig- the data as a signal of new physics as the flood of papers studying nal (sect. 3) based on this type of scenarios (sect. 2), exploring different BSM scenarios that could accommodate the excess testify. mechanisms for a broad φ , potentially consistent with the data. Those most relevant to our discussion are [4–10]. In our opinion, We present other mechanisms for the mentioned sgoldstino mass probably the most exciting theoretical possibility to accommodate splitting, which are more efficient than those considered up to this resonance is the one pursued by the authors of [5,6,8], who now (sect. 4). In our analysis we discuss some subtleties not pre- have contemplated scenarios with a scale of SUSY breaking not viously considered that can constrain and affect substantially the far from the TeV scale (low-scale SUSY breaking) [11–14]. Poten- results. We also discuss the possibility that sgoldstinos decay effi- tially, these models contain the main ingredient to fit the signal: ciently into Higgses (sect. 5), as the partial width into that channel is naively parametrically enhanced with respect to other chan- nels; into Higgs decay channels through sgoldstino-Higgs mixing * Corresponding author. (sect. 6); and into Higgsinos (sect. 7), as there is more freedom to E-mail address: [email protected] (J.R. Espinosa). enhance this width without clashing with previous LHC searches. http://dx.doi.org/10.1016/j.physletb.2016.05.070 0370-2693/© 2016 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). Funded by SCOAP3. 160 J.A. Casas et al. / Physics Letters B 759 (2016) 159–165 2. The low-scale SUSY-breaking scenario Similarly, from Eqs. (1) and (2), the scalar potential V = V F + V D for the two supersymmetric Higgs doublets plus the complex singlet field , is1: The low-scale SUSY-breaking (LSSB) scenario√ [11–14] is a framework in which the scale of SUSY breaking, F , and its medi- 1 = 2 + ˜ 2| |2 + ˜ 2 2 + ation, M, are not far from the TeV scale. The main differences with V F αφm (ρφm h.c.) 2 respect to more conventional supersymmetric models, where the 2 2 2 2 2 + m |Hu| + m |H | + m Hu · H + h.c. latter scales are large, are the following: i) the low-energy effec- Hu Hd d 12 d tive theory includes the chiral superfield, , responsible for SUSY ∗ ∗ 2 ∗ ∗ 2 + (mX + m )|Hu| + (mX + m )|Hd| breaking, in particular its fermionic (goldstino) and scalar (sgold- 1 X1 2 X2 + + ∗ · + stino) degrees of freedom; ii) besides the ordinary SUSY-soft break- (mX3 mX4 )Hu Hd h.c. ing terms, the effective theory contains additional hard-breaking 1 1 + | |4 + | |4 + | |2| |2 + | · |2 operators, e.g. quartic Higgs couplings. The latter make the Higgs λ1 Hu λ2 Hd λ3 Hu Hd λ4 Hu Hd 2 2 sector resemble a two-Higgs doublet model with an additional 1 2 2 2 (complex) singlet. LSSB models present a much milder electroweak + λ5 (Hu · Hd) + λ6|Hu| Hu · Hd + λ7|Hd| Hu · Hd + h.c. fine-tuning than usual MSSMs [12,13] and a rich phenomenology 2 [11–14]. As discussed in refs. [5,6,8], the LSSB scenario can nicely + ... (7) explain the diphoton excess at 750 GeV observed at the LHC. where the dots stand for higher order terms in and nonrenor- Let us summarize the main ingredients of LSSB scenarios. Ex- malizable terms suppressed by powers of M. The various mass panding in inverse powers of M, superpotential, W , Kähler poten- parameters and quartic couplings in (7) are explicit combinations tial, K , and the gauge kinetic function, fab, read [11] of the parameters in W and K (see ref. [11] for explicit formulae). As a summary, denoting by μ the typical scale of the supersym- ρ φ 3 metric mass parameters [μ, μ , ··· in Eq. (1)] and m˜ = F /M, the W = W MSSM + F + +··· 6M2 2 ˜ 2 mass terms in the potential have contributions of order μ , m , μ m˜ μ. We assume μ m˜ , so that all these squared mass terms are + + +··· · 2 μ Hu Hd expected to be m˜ . Analogously, trilinear terms, m , have con- M Xi tributions of order μ2/M, m˜ 2/M, m˜ μ/M. Finally, the Higgs quartic 1 2 couplings have supersymmetric D-term and F -term contributions, + + +··· (Hu · Hd) + ··· , (1) 2M M where the latter include supersymmetry breaking contributions as (D) (F ) (D) well: λi = λ + λ . The λ are as in the MSSM: αφ αu i i i K =||2 1 − ||2 +··· +|H |2 1 + ||2 +··· 2 u 2 1 1 4M M (D) = (D) = 2 + 2 (D) = 2 − 2 λ1 λ2 (g g ), λ3 (g g ), 2 αd 2 4 4 +|Hd| 1 + || +··· M2 (D) 1 2 λ =− g , (8) 4 2 + · αud ¯ 2 +··· + + ··· Hu Hd h.c. , (2) (D) (D) (D) (F ) 2M2 and λ = λ = λ = 0. Besides, typically λ ∼ m˜ 2/M2, 5 6 7 i ˜ 2 2 2 δab mμ/M , μ /M , although some of these couplings can receive fab = 1 + ca + ··· . (3) (F ) (F ) 2 contributions at a lower order, λ ∼ m˜ /M, λ ∼ μ/M. Whether ga M 5 i=6,7 the effective theory expansion starts at order m˜ /M or m˜ 2/M2 is Here all the parameters are dimensionless, except the μ, μ ··· pa- a model-dependent question. In what follows we will generically rameters in the superpotential, which have dimensions of mass. (F ) ∼ ˜ 2 2 assume λi m /M but the reader should keep in mind this ex- Replacing by its auxiliary field, F , one gets the soft breaking ception, which might be important in some cases. The effective terms of the theory. In particular, from Eq. (3), one gets masses for quartic self-coupling of the light (SM-like) Higgs, λ|H|4, reads gluinos, M , winos, M , and the bino, M , e.g. M = c F /M. Like- 3 2 1 1 1 = (D) + (F ) + wise, replacing by its scalar component, a complex singlet field, λ λ λ δradλ, (9) that we also denote by , where 1 1 λ = λ c2 + λ s2 + (λ + λ + λ ) sin2 2β = √1 + 1 β 2 β 3 4 5 (φS iφP ) (4) 2 4 2 + 2 + 2 λ6cβ λ7sβ sin 2β, (10) (where φS is the scalar component and φP the pseudoscalar one), with tan β =Hu /Hd ≡ vu/vd.
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