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Physics Letters B

www.elsevier.com/locate/physletb

Interpreting the LHC Higgs search results in the MSSM

S. Heinemeyera, O. Stálb *. G. Weigleinb

a Instituto de Física de Cantabria (CSIC-UC), Santander, Spain

b Deutsches Elektronen-Synchrotron DESY, Notkestraße 85, D-22607 Hamburg, Germany

ARTICLE INFO

ABSTRACT

Article history:

Received 12 January 2012

Received in revised form 24 February 2012

Accepted 27 February 2012

Available online 2 March 2012

Editor: A. Ringwald

Recent results reported by the ATLAS and CMS experiments on the search for a SM-like Higgs boson both show an excess for a Higgs mass near 125 GeV, which is mainly driven by the YY and zz* decay channels, but also receives some support from channels with a lower mass resolution. We discuss the implications of this possible signal within the context of the minimal supersymmetric Standard Model (MSSM), taking into account previous limits from Higgs searches at LEP, the Tevatron and the LHC. The consequences for the remaining MSSM parameter space are investigated. Under the assumption of a Higgs signal we derive new lower bounds on the tree-level parameters of the MSSM Higgs sector. We also discuss briefly an alternative interpretation of the excess in terms of the heavy CP-even Higgs boson, a scenario which is found to be still viable.

© 2012 Elsevier B.V. All rights reserved.

1. Introduction

The Higgs boson [1] has for a long time been considered as the only missing piece in the Standard Model (SM) of particle physics. Therefore, finding this particle has been one of the main tasks of experimental high-energy physics. However, the main results from the published searches so far have been exclusion limits (see e.g. the results from LEP [2], the Tevatron [3], and the LHC [4,5]). Combining the experimental limits, the only allowed region (before the latest results which will be discussed below) a relatively small window for the Higgs mass: 114 GeV < MHM < 141 GeV. This low mass region is also the one favoured by electroweak precision tests, see e.g. [6].

A low Higgs mass is predicted in supersymmetric extensions of the SM, where the quartic Higgs couplings are related to gauge couplings. Exclusion of a heavy SM-like Higgs [3-5] can therefore be considered as being in line with the predictions of su-persymmetry (SUSY). Besides predicting a light Higgs boson, SUSY protects scalar masses from the large hierarchy of scales, it allows for gauge coupling unification, and it can provide a dark matter candidate [7]. The minimal supersymmetric extension of the SM (MSSM) [8] has two complex Higgs doublets. Following electroweak symmetry breaking, the physical spectrum therefore contains five Higgs bosons. Assuming CP conservation, these are denoted h, H (CP-even), A (CP-odd), and H± (charged Higgs). At the tree-level the MSSM Higgs sector can be described by two

* Corresponding author.

E-mail addresses: Sven.Heinemeyer@cern.ch (S. Heinemeyer), oscar.stal@desy.de (O. Stal), Georg.Weiglein@desy.de (G. Weiglein).

0370-2693/$ - see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.physletb.2012.02.084

parameters (besides the SM parameters), commonly chosen as the mass of the CP-odd Higgs boson, MA, and tan /), the ratio of the two vacuum expectations values. In the decoupling limit, MA > 2MZ (where MZ denotes the mass of the Z boson), all MSSM Higgs bosons except the lightest CP-even scalar h become heavy, whereas h has SM-like properties. In this limit it would be difficult to separate hints for a SM Higgs boson from a potential MSSM counterpart. It is also in the decoupling limit where Mh reaches its maximal value, Mh — 135 GeV [9].

The LHC experiments recently extended their exclusion regions for a SM-like Higgs boson down to MHM < 127 GeV, with the lowest limit coming from CMS (M™ < 131 GeV for ATLAS). In addition, ATLAS reported exclusion of the range 114 GeV < MHM < 115.5 GeV, which is a region where sensitivity was not expected. Most interestingly, both experiments also reported about an excess over the background expectation close to MHM = 125 GeV [10]. Since this Higgs mass lies in the range compatible with supersym-metry, we report in this Letter on a first analysis and interpretation of these results in an MSSM context.

2. Experimental Higgs search results

Both the LHC experiments (ATLAS and CMS) have reported [10] on indications for an excess of Higgs-like events corresponding to a Higgs boson mass

MHM = 126 GeV (ATLAS), MHM = 124 GeV (CMS).

Another excess at MHM — 119 GeV was reported by CMS, but not confirmed by ATLAS. Consequently, we will not consider this value

in our analysis. The result is driven by an observed excess of events over SM background expectations in primarily the yy and ZZ* channels, which provide relatively good resolution for the Higgs boson mass. The local significance for the combined result is 3.6a for ATLAS and 2.6a for CMS. However, when interpreted in a global search containing many mass bins, the local significance is washed out by the look-elsewhere effect (LEE). This effect compensates for the higher probability of random fluctuations generating an excess anywhere when searching in more than one place. Taking this into account, the significance of the reported result is reduced to 2.5a (1.9a) for ATLAS (CMS) when interpreted as a SM Higgs search over the mass range from 110 GeV to 146 GeV. On the other hand, one could argue that when interpreting these results in a model where the allowed range for Mh is constrained to a smaller range by the theory (as in the MSSM), the LEE does not apply to the same degree as for the SM interpretation. These new results are therefore even somewhat more interesting in an MSSM context.

For the remainder of this Letter, encouraged by the excess reported by ATLAS and CMS, we investigate a scenario where we assume the observation of a state compatible with a SM-like Higgs boson with mass Mh = (125 ± 1) GeV. We will discuss the implications that such an assumed signal would have for the MSSM. While the current statistical significance does not allow yet to draw firm conclusions on the validity of the above assumption, our analysis is in fact somewhat more general, as possible implications of observing (or excluding) a state compatible with a SM-like Higgs elsewhere in the allowed mass window 115.5 GeV < Mh < 127 GeV [10] can also be inferred.

3. MSSM interpretation

For calculating the Higgs masses in the MSSM we use the code FeynHiggs [9,11,12] (v. 2.8.5). The status of higher-order corrections to the masses (and mixing angles) in the neutral Higgs sector is quite advanced.1 The complete one-loop result within the MSSM is available and has been supplemented by all presumably dominant contributions at the two-loop level, see Ref. [9] for details. Most recently leading three-loop corrections have been presented [14], where the leading term is also included in FeynHiggs. Following Ref. [9], we estimate the (intrinsic) theory uncertainty on the lightest Higgs mass from missing higher-order corrections to be AMh"tr ~ ±2 GeV. The intrinsic Mh uncertainties are also somewhat smaller for a SM-like Higgs than in the general case, which makes this estimate conservative. Concerning the parametric uncertainty from the experimental errors of the (SM-) input parameters, AM^aram, the main effect arises from the experimental error of the top-quark mass. We incorporate this uncertainty explicitly in our results below by allowing mt to vary within the range mt = 173.2 ± 0.9 GeV [17]. Parametric uncertainties in Mh from as are smaller than the mt uncertainties and will be neglected. Adding the intrinsic theory uncertainty (conservatively) linearly to the assumed experimental uncertainty, we arrive at the allowed interval

122 GeV < Mh < 128 GeV,

which will be used for the MSSM interpretation of the assumed Higgs signal. While for most of this Letter we investigate the case where the assumed signal is interpreted as the lighter CP-even Higgs boson, h, of the MSSM, we comment below also on the possibility of associating the assumed signal with the second-lightest

1 We concentrate here on the case with real parameters. For the complex case, see Refs. [12,13] and references therein.

CP-even Higgs boson, H. Since the observed excess includes WW* and ZZ* final states, an interpretation in terms of the CP-odd Higgs boson, A, appears to be highly disfavoured.

For our discussions of the possible interpretations of the assumed signal, we use a phenomenological description of the (CP-conserving) MSSM with all parameters given at the electroweak scale. In order to determine the radiative corrections to the Higgs masses it is necessary to specify, besides the tree-level parameters MA and tan /), also the relevant SUSY-breaking parameters entering at higher orders. In particular, the parameters in the stop and sbottom sector have a large impact in this context. Since for the case where we interpret the assumed signal as the lighter CP-even Higgs h we are interested in particular in determining lower bounds on the most relevant parameters, we fix those with smaller impact on Mh to their values in the mjmax scenario [15],

Mi = 100 GeV, M2 = 200 GeV

m~g = 0.8Msusy, i = 200 GeV, (2)

so that conservative lower bounds are obtained for the other parameters. In Eq. (2) M1,2 and mg are the soft SUSY-breaking gaug-ino masses corresponding to the SM gauge group, and ¡ is the Higgs mixing parameter. This choice ensures that the corresponding contributions to Mh are such that one obtains (approximately) the highest value for Mh. In addition to varying the tree-level parameters, we allow for variation in the overall SUSY mass scale MsUsY and the stop mixing parameter Xt = At — ¡ cot /), where Atb denotes the trilinear coupling of the Higgs to scalar tops or bottoms. We furthermore set Ab = At. The scalar top masses will be denoted as mt and mt below, with mt < mt . It should be

C1 t2 ' C1 — Í2

noted that when we discuss relatively low values of MSUSY this refers only to squarks of the third generation (which give rise to the relevant Higgs mass corrections). The experimental bounds reported from squark searches at the LHC [16], on the other hand, apply only to squarks of the first two generations, which are essentially irrelevant for Higgs phenomenology. We also do not apply a lower bound on the gluino mass, which leads to more conservative lower limits on the parameters from the Higgs sector than e.g. a bound mg > 700 GeV [16] would do. We comment further on this point below. As mentioned above, for the top-quark mass we use the latest Tevatron combination mt = 173.2 ± 0.9 GeV [17], taking the uncertainty into account by varying mt over its ±1a interval.

Besides constraints from the Higgs sector, which we will discuss shortly, one could also consider indirect constraints on the MSSM parameter space coming from other measurements, such as the anomalous magnetic moment of the muon, (g — 2)¡, or from B-physics observables such as BR(b ^ sy). The former requires in general that ¡ > 0, while the latter is often in better agreement with experimental data for ¡Xt & ¡At < 0 (for a recent analysis see [18] and references therein). We will not apply any indirect constraints here, but when presenting the results below we sometimes distinguish between positive and negative Xt, where the bounds obtained for Xt < 0 could be regarded as experimentally preferred. However, one should keep in mind that a small admixture of non-minimal flavour violation could bring the BR(b ^ sy) results into agreement with experimental data without changing (notably) the Higgs sector predictions [19].

3.1. A light CP-even SM-like Higgs boson

We begin the MSSM interpretation by associating the assumed LHC signal with the light CP-even Higgs boson h. By choosing the relevant parameters such that the radiative corrections yield a maximum upward shift to Mh, it is possible to obtain lower

Table 1

Lower limits on the MSSM Higgs sector tree-level parameters MA (MH±) and tan p obtained with and without the assumed Higgs signal of Mh — 125 GeV, see Eq. (1). The mass limits have been rounded to 1 GeV.

MSUSY (GeV) Limits without Mh — 125 GeV Limits with Mh — 125 GeV

tan p MA (GeV) MH± (GeV) tan p MA (GeV) MH± (GeV)

500 2.7 95 123 4.5 140 161

1000 2.2 95 123 3.2 133 155

2000 2.0 95 123 2.9 130 152

100 150 200 250 300 350 400 450 500 MA (GeV)

Fig. 1. Tree-level Higgs sector parameters (MA, tanp) for the case where the parameters governing the higher-order corrections are chosen such that a maximum value for Mh is obtained (mhmax benchmark scenario). The different colours correspond to the regions excluded by LEP (blue) and Tevatron/LHC (red). The gray area is the allowed parameter space prior to the latest LHC results. The green band shows the region where Mh is compatible with the assumed Higgs signal (see text). (For interpretation of the references to colour, the reader is referred to the web version of this Letter.)

bounds on the parameters MA and tan p governing the tree-level contribution. The situation where the radiative corrections to Mh are maximized in this way is realised in the mjmax scenario with a stop mixing of Xt = 2MSUSY. In Fig. 1 we show the result of varying the tree-level parameters in this scenario (with MSUSY = 1 TeV as originally defined). Constraints on the parameter space from direct Higgs searches at colliders are taken into account by using HiggsBounds [20].2 Since we are interpreting an assumed signal, we do not include the updated exclusion bounds from [10]. Fig. 1 shows separately the regions excluded by LEP [22] (blue), and the Tevatron/LHC (red). The gray area is the allowed parameter space before including the bound from Eq. (1), and the green band corresponds to the mass interval compatible with the assumed Higgs signal of 122 GeV < Mh < 128 GeV. The brighter green is for the central value for mt, while including also the dark green band corresponds to a ±1a variation of mt.

The assumed Higgs signal, interpreted as the lighter CP-even MSSM Higgs mass, implies in particular that Mh > 122 GeV (including theoretical uncertainties), which is significantly higher than the limit observed for a SM-like Higgs at LEP of Mh > 114.4 [2]. From Fig. 1 it is therefore possible to extract lower (one parameter) limits on MA and tanp from the edges of the green band. As explained above, by choosing the parameters entering via radiative corrections such that those corrections yield a maximum upward shift to Mh, the lower bounds on MA and tan p that we have obtained are general in the sense that they (approximately) hold for any values of the other parameters. To address the (small) residual MSUSY dependence of the lower bounds

2 We use HiggsBounds v. 3.5.0-beta with a private addition of the latest CMS results on A/H ^ t+ t- [21]. These new results provide the most stringent Tevatron/LHC limits on the (MA, tanp) plane at medium or large tanp.

on MA and tan p, we extract limits for the three different values MsusY = {0.5,1, 2} TeV. The results are given in Table 1, where for comparison we also show the previous limits derived from the LEP Higgs searches [22], i.e. before the incorporation of the new LHC results reported in Ref. [10]. The bounds on MA translate directly into lower limits on MH± , which are also given in the table. A phenomenological consequence of the bound MH± > 155 GeV (for MSUSY = 1 TeV) is that it would leave only a very small kinematic window open for the possibility that MSSM charged Higgs bosons are produced in the decay of top quarks.

For deriving the conservative lower bounds on MA and tan p it was unnecessary to impose constraints on the production and decay rates of the assumed Higgs signal in the relevant search channels at the LHC. One might wonder whether it would be possible to improve the bound on MA by requiring that the rate in the relevant channels should not be significantly suppressed as compared to the SM case. Such an improvement would be scenario-dependent, however, i.e. the result would depend on the specific choice made for the other MSSM parameters. We will therefore not study this issue in further detail.

It might look tempting to extract also an upper limit on tan p from the green band in Fig. 1, but in contrast to the lower bound which is scenario-independent, this limit will only apply to the specific case of the mhmax scenario. In fact, the allowed range for tan p depends sensitively on the other parameters, as can be seen from Fig. 2, where we show the (Xt, tanp) plane for MA = 400 GeV, but the results are qualitatively similar for other values of MA in the decoupling limit. The main difference is the LHC exclusion limit (in red), which goes down to lower values of tan p for lower MA . On the other hand, for MA in the non-decoupling regime, even before the new results tan p was already quite restricted, from above by the LHC limits, and from below by the LEP limits, which can also be seen from Fig. 1. The mjmax value of Xt = +2MSUSY turns out to be quite special, since this parameter region (at least for MSUSY = 1 TeV and MSUSY = 2 TeV) actually shows the highest sensitivity to variations of tan p when Mh — 125 GeV. This would result in only a narrow allowed tan p region. For other regions of Xt, however, tan p values all the way up to the LHC bound are compatible with an assumed signal at Mh — 125 GeV. Further progress could obviously be made if direct information on the stop sector became available from the LHC or a future Linear Collider.

Having established lower limits on the tree-level parameters MA and tan p, we now investigate instead what can be inferred from the assumed Higgs signal about the higher-order corrections in the Higgs sector. Similarly to the previous case, we can obtain an absolute lower limit on the stop mass scale MSUSY by considering the maximal tree-level contribution to Mh. We therefore perform this analysis in the decoupling limit (fixing MA = 1 TeV, tanp = 20). The resulting constraints for MSUSY and Xt are shown in Fig. 3 (left) using the same colour coding as before.

Several favoured branches develop in this plane, centred around Xt - -1.5Msusy, Xt - 1.2Msusy, and Xt - 2.5Msusy. The minimal allowed stop mass scale is MSUSY — 300 GeV with positive Xt and MSUSY — 500 GeV for negative Xt (which is in general pre-

Fig. 2. Allowed ranges of tan p for MA = 400 GeV, shown as a function of the stop mixing parameter Xt. The colour coding is as in Fig. 1. The three plots correspond to MSUSY = 500 GeV (left), MSUsv = 1 TeV (centre), and MSUsv = 2 TeV (right).

Fig.3. Constraints on the MSSM stop sector from the assumed Higgs signal. The allowed ranges are shown in the (Xt, MSUSY) plane (left) and the (Xt, m-t1) plane (right) for MA = 1 TeV, tan p = 20. The colour coding is as in Fig. 1.

ferred by BR(b ^ sy), see above). The results on the stop sector can also be interpreted as a lower limit on the mass m^ of the lightest stop squark. This is shown in Fig. 3 (right). It is interesting to note from the figure that without the assumed Higgs signal, there is essentially no lower bound on the lightest stop mass coming from the Higgs sector. Taking the new results into account, we obtain the lower bounds m^ > 100 GeV (Xt > 0) and m^ > 250 GeV (Xt < 0). These bounds can be compared to those from direct searches, where the LEP limit m^ > 95 GeV is still valid [23]. Results from stop searches at the Tevatron can also be found in this reference. No new stop limits have been established so far from the SUSY searches at the LHC [16]. It should be noted that our stop mass bound is rather conservative, since the low mass scales discussed here correspond to a gluino mass mg = 0.8Msusy < 300 GeV, which is experimentally disfavoured [16,23,24]. Since the low gluino mass contributes towards a higher value of Mh, a lower bound on mg would lead to a stronger bound on mj1. As an example, in a simplified model consisting just of the gluino, the squarks of the first two generations and a mass-less lightest supersymmetric particle, the ATLAS Collaboration has inferred a lower bound of about 700 GeV on mg [16]. Imposing such a bound on mg in our analysis would shift the lower limit on mj1 to mj1 > 200 GeV (m~t1 > 350 GeV) for positive (negative) Xt. It should be noted, however, that in the presence of a light stop decays of the gluino into a top and a scalar top would open up, g ^ t1t, which are expected to weaken the bound on mg as compared to the analysis in the simplified model where this decay mode is assumed to be absent.

3.2. A heavy CP-even SM-like Higgs boson

All results presented up until this point apply only if we interpret the assumed signal as corresponding to the light CP-even MSSM Higgs h. We now discuss briefly the alternative possibil-

ity that the heavier CP-even H has a mass MH ~ 125 GeV (with the same experimental and theoretical uncertainties as before, see Eq. (1)) and SM-like properties.

In order to investigate whether there is a region in the MSSM parameter space that admits this solution we performed a scan over the relevant free parameters (MA, tanp, MSUSY, Xt), keeping l = 1 TeV fixed and the remaining parameters according to Eq. (2). The results are shown in Fig. 4, indicating the region where MH fulfills Eq. (1) by cyan colour to distinguish it from the case discussed above (similarly to above, the darker region corresponds to the variation of mt). As we can see from this figure, it is possible to obtain MH in the right range in a region with low MA and moderate tanp (left plot) where we have set MSUSY = 1 TeV, Xt = 2.3 TeV. In the right plot we set MA = 100 GeV, tanp = 10 and show the regions compatible with a heavier CP-even Higgs having a mass MH ~ 125 GeV in the plane of the stop sector parameters MSUSY and Xt. We find that such an interpretation is possible over extended regions of the (MSUSY, Xt) parameter plane. Requiring in addition that the production and decay rates into yy and vector bosons are at least 90% of the corresponding SM rates, a smaller allowed region is found (yellow) with large values for the stop mixing (Xt > 1.5 TeV). In the yellow region enhancements of the rate of up to a factor of three as compared to the SM rate are possible.

Concerning the mass of the lighter CP-even Higgs boson h in this kind of scenario we find in our scan allowed values for Mh only below the SM LEP limit of 114.4 GeV [2] (with reduced couplings to gauge bosons so that the limits from the LEP searches for non-SM like Higgs bosons are respected [22]). A particularly intriguing option could be MH — 125 GeV, Mh — 98 GeV, in view of the fact that LEP observed a certain excess at Mh — 98 GeV [22] (whose interpretation is of course subject to the look-elsewhere effect). This combination of Higgs masses is realized (with H SM-like), for instance, for MSUSY = 1 TeV, Xt = 2.4 TeV, ¡i = 1 TeV,

MA (GeV) Xt (TeV)

Fig. 4. Parameter space in the alternative MH — 125 GeV scenario. The colour coding is similar to Fig. 1, with new regions (cyan and yellow) where MH is in the range compatible with the assumed H signal. In addition, for the yellow region the heavy Higgs has a rate for production times decay into YY of at least 90% of the corresponding SM values. For the plot in the (MA, tanp) plane (left) we have assumed MSUSY = 1 TeV, Xt = 2.3 TeV and for the stop parameters (right) we fix MA = 100 GeV, tan p = 10. In both cases i = 1 TeV, and the remaining parameters are given by Eq. (2) with the additional requirement m-g > 700 GeV.

MA = 106 GeV, and tanp = 7. For this scenario we find a reduced coupling (ghzz/gSHzz)2 = 0.1 of the lightest Higgs boson to a pair of Z bosons.

Despite the available parameter space, it should be noted that the scenario where the heavier CP-even Higgs is SM-like and has a mass of MH — 125 GeV appears somewhat more contrived than the h interpretation. In particular, we find that simultaneously large values for the i parameter and a large mixing in the stop sector are required in order to obtain a SM-like rate of production and decay of the heavy CP-even Higgs in the relevant channels. We leave a more detailed investigation of this scenario for future work.

4. Conclusions

An excess in the SM-like Higgs searches at ATLAS and CMS has recently been reported [10] around MHM — 125 GeV, which within the experimental uncertainties appears to be remarkably consistent between ATLAS and CMS and is supported by several search channels. While it would be premature to assign more significance to this result than regarding it as a possible (exciting) hint at this stage, it is certainly very interesting to note that this excess has appeared precisely in the region favoured by the global fit within the SM, and within the range predicted in the MSSM. Concerning the MSSM, it is remarkable that the mass region above the upper MSSM bound on a light SM-like Higgs is meanwhile ruled out [10]. Observing a state compatible with a SM-like Higgs boson with MfM > 135 GeV would have unambiguously ruled out the MSSM (but would have been viable in the SM and in nonminimal supersymmetric extensions of it). We therefore regard the reported results as a strong motivation for studying the possible interpretation of an assumed (still hypothetical, of course) signal at 125 GeV ± 1 GeV. In this Letter we have discussed the possible implications of such an assumed signal within the MSSM, where we have investigated both the possibilities that the assumed signal is associated with the light CP-even Higgs boson of the MSSM, h, and the (slightly more exotic) possibility that the assumed signal in fact corresponds to the heavier CP-even Higgs boson H.

Investigating the interpretation Mh = 125 ± 1 GeV first, we have demonstrated that there is a significant parameter space of the MSSM compatible with the interpretation that the assumed signal corresponds to the lighter CP-even MSSM Higgs boson. While it would not be appropriate to assign any physical significance to point densities in MSSM parameter space, our scans nevertheless do not seem to indicate a strong case for going from the MSSM to non-minimal SUSY models even though the reported excess is not very far away from the upper bound on the lightest Higgs mass in

the MSSM. It should be noted that the question to what extent the scenarios discussed in this Letter can be realized in constrained GUT-based models of SUSY breaking is of a very different nature. We do not pursue this any further here, besides mentioning that it has already been shown to be rather difficult to get to such high Mh values in models such as the CMSSM, mGMSB, mAMSB, or NUHM1 [25].

We performed two kinds of complementary investigations of the implications of an assumed Higgs signal at Mh = 125 ± 1 GeV. Setting the parameters that enter via the (in general) numerically large higher-order corrections in the MSSM Higgs sector to their values in the mhmax benchmark scenario, which maximizes the upward shift in Mh as compared to the tree-level value, we have obtained conservative lower limits on the parameters governing the Mh prediction at tree-level, MA and tan p. We have found that an assumed signal of Mh = 125 ± 1 GeV (when including conservatively estimated intrinsic theoretical uncertainties from unknown higher orders, and taking into account the most important parametric uncertainties arising from the experimental error on the top-quark mass) yields the lower bounds MA > 133 GeV and tanp > 3.2 (for MSUSY = 1 TeV). The bound on MA translates directly into a lower limit MH± > 155 GeV, which restricts the kinematic window for MSSM charged Higgs production in the decay of top quarks.

Choosing values for MA and tan p in the decoupling region, in a second step we have investigated the constraints on the scalar top and bottom sector of the MSSM from an assumed signal at Mh = 125 ± 1 GeV. In particular, we have found that a lightest stop mass as light as m~t1 — 100 GeV is still compatible with the assumed Higgs signal. The bound on m-t1 raises to m~t1 > 250 GeV if one restricts to the negative sign of the stop mixing parameter Xt = At — i/ tan p, which in general yields better compatibility with the constraints from BR(b ^ sy).

As an alternative possibility, we have investigated in how far it is possible to associate the assumed Higgs signal with the heavier CP-even Higgs boson H. Performing a scan over MA, tan p, MSUSY and Xt we have found an allowed area at low MA and moderate tan p. A SM-like rate for production and decay of the heavier CP-even Higgs in the relevant search channels at the LHC is possible for large values of i and large mixing in the stop sector. It is interesting to note that in the scenario where the assumed Higgs signal is interpreted in terms of the heavier CP-even Higgs boson H the mass of the lighter Higgs, Mh, always comes out to be below the SM LEP limit of 114.4 GeV (with reduced couplings to gauge bosons so that the limits from the LEP searches for non-SM like Higgs bosons are respected). The fact that scenarios like this are in

principle viable should serve as a strong motivation for extending the LHC Higgs searches, most notably in the yy final states, also to the mass region below 100 GeV.

Needless to say, an MSSM interpretation of the observed excess would of course gain additional momentum if the searches for the scalar quarks of the third generation and the direct searches for the colour-neutral SUSY states, which so far have resulted in only very weak limits, would soon give rise to a tantalising excess (or more than one) as well.

Acknowledgements

We thank Johan Rathsman and Rikard Enberg for useful suggestions at an early stage of this project. We also thank Tim Stefaniak and Oliver Brein for discussions and help with HiggsBounds, in particular on the CMS A ^ t + t - results. We thank Paloma Arenas Guerrero for her contributions to our investigation of a possible heavy CP-even SM-like Higgs boson. This work has been supported by the Collaborative Research Center SFB676 of the DFG, "Particles, Strings, and the Early Universe". The work of S.H. was supported in part by CICYT (grant FPA 2010-22163-C02-01) and by the Spanish MICINN's Consolider-Ingenio 2010 Program under grant MultiDark CSD2009-00064.

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