Hints for a non-standard Higgs boson from the LHC
HHints for a non-standardHiggs boson from the LHC
Martti Raidal a and Alessandro Strumia a,b (a) National Institute of Chemical Physics and Biophysics, Ravala 10, Tallinn, Estonia(b) Dipartimento di Fisica dell’Universit`a di Pisa and INFN, Italia Abstract
We reconsider Higgs boson invisible decays into Dark Matter in the light of recentHiggs searches at the LHC. Present hints in the CMS and ATLAS data favor anon-standard Higgs boson with approximately 50% invisible branching ratio, andmass around 143 GeV. This situation can be realized within the simplest thermalscalar singlet Dark Matter model, predicting a Dark Matter mass around 50 GeVand direct detection cross section just below present bound. The present runs ofthe
Xenon
100 and LHC experiments can test this possibility.
Introduction
Both the CMS [1] and ATLAS [2] experiments have presented their combined searches forthe standard model (SM) Higgs boson, based on luminosities between 1 fb − and 2.3 fb − depending on the search channel. In the light Higgs mass region preferred by precision data,both experiments exclude the SM Higgs boson with mass greater than 146 GeV.At the same time, both experiments find the following hints in their data: • In the most sensitive channel h → W W ∗ → (cid:96) ν they find a broad excess of events overthe computed background. In view of the undetected neutrinos this channel poorly allowsto reconstruct the Higgs boson mass and any m h between 100 and 2 M W can reproducethe apparent excess roughly equally well. • Furthermore, the “golden” (but less sensitive) channel h → ZZ ∗ shows an excess of signalevents for the Higgs boson mass around 143 GeV. These features appear consistently inboth experiments. (Other higgs masses are favored by the single experiments; but onlythis value is favored by both).Needless to say, the significance of such features is statistically inconclusive: slightly morethan 2 σ in each experiment, as shown in fig. 1. The significance will get reduced after takinginto account “look elsewhere effects”, i.e. the fact that because of statistical fluctuations a fakepeak can appear at many different m h .The best fit value of the SM Higgs boson mass is around 120 GeV [1], in the region wherethe experiments do not yet have sensitivity to discover it. The SM Higgs boson with mass m h a r X i v : . [ h e p - ph ] S e p
36 138 140 142 144 146 148 150 (cid:45) (cid:45) (cid:45) (cid:45) (cid:45) (cid:45)
10 Higgs mass in GeV Χ Σ Σ ATLASCMS
Figure 1:
Hints in CMS and ATLAS data for a Higgs boson with mass around 143 GeV. Thefigure is obtained converting into a χ the experimental fits reported in terms of probabilityvalues in Refs. [1, 2]. around 143 GeV is not favored by the data, because it would have given an excess in h → W W ∗ about twice larger than the observed hint.While these hints could well be statistical fluctuations or systematical artifacts related tothe experiments, the purpose of this note is to point out a more exciting possibility: both hintsfor h → W W ∗ and h → ZZ ∗ can be jointly fitted by a non-standard Higgs boson.Various possibilities exists: new particles at the weak scale can i) reduce the gluon fusionHiggs production cross section; ii) increase the Higgs BR into b ¯ b , which is difficult to seeexperimentally, at the expense of branching ratio into W W ∗ and ZZ ∗ ; iii) add an invisiblebranching ratio, correspondingly reducing all SM branching ratios. If the W W ∗ and ZZ ∗ Higgsrates are reduced by 0 . ± .
2, the interpretation of LHC Higgs boson searches changes and m h ≈
143 GeV becomes the best fit value of the Higgs boson mass supported by the data. Thusthe LHC data indicates to a “physicist dream” scenario in which Higgs boson couples equallystrongly to the SM particles and to the unknown new physics.Possibility iii) is easily realized compatibly with all present negative searches for new physics:invisible Higgs boson decays [3] can occur in many theories beyond the SM including supersym-metry [4], extra dimensions [5], Majoron models [6], fourth generation [7], models with non-trivial hidden sectors [8], “hidden valley” models [9], models with scalar Dark Matter [10, 11, 12]etc. Model independently, LHC needs about 30 fb − data to probe at 95% C.L. a ≈
50% Higgsbranching ratio into invisible channels for m h ≈
140 GeV [13]. Thus the interpretation proposedin this work can be tested by the LHC alone.Among the large number of new physics scenarios the most motivated are the ones predictingDark Matter (DM). Indeed, the existence of DM [14] is presently the only firm evidence of newphysics. If invisible Higgs decays are observed at the LHC, it is most natural to assume thatthe Higgs boson decays into DM. In this case the LHC probes both the mass and the couplingsof the DM particles and predicts the DM-nucleon scattering cross section probed by directdetection experiments. The latter, such as
Xenon
Xenon
100 [17], the global fit [18] now excludes M DM < m h / S [10]. The capability of Xenon
100 and the LHC to constrainthis model was recently studied in Refs. [18, 19]. In this letter we show that the scalar singlet2M model [10] is consistent with the LHC and DM direct detection data and predicts DM withmass M DM ≈
50 GeV to be discovered by the
Xenon
100 experiment.
The Dark Matter interpretation
We consider the simplest DM model obtained adding to the SM a real singlet scalar field S coupled to the Higgs doublet H as described by the following Lagrangian [10]: L = L SM + ( ∂ µ S ) − m S − λS | H | − λ S S . (1)Thanks to the discrete symmetry S → − S the singlet S becomes a good DM candidate. Forsimplicity and minimality we assume S to be real. Given that λ S is essentially irrelevant forparticle physics phenomenology, the model has only 2 new free parameters: the DM mass givenby M = m + λV with V = 246 GeV, and the DM/Higgs coupling λ . The latter can befixed assuming that the relic DM abundance equals to its cosmologically measured value.In this work we compute the DM relic density following [20] (that included the 3 body finalstates whose relevance was emphasized in [21]) and require it to be equal to the observed value,Ω DM = 0 . ± . h → SS ) = λ V πm h (cid:115) − M m h . (2)The corresponding invisible branching ratio is shown by the isocurves in the left panel of fig. 2,as functions of the DM and Higgs boson masses. The region shaded in red is excluded by Xenon m h < M DM and consequently no invisibleHiggs width.Performing a na¨ıve combination of the ATLAS [2] and CMS [1] hints (by just adding their χ plotted in fig. 1), the region favored at 68% and 95% C.L. (2 dof) is shown in green. In thiscase the Higgs boson mass is predicted to be in a narrow region m h = (138 − h → W W ∗ channel is real, while the Higgs boson mass favoredby h → ZZ ∗ are statistical fluctuations. In this case the partially invisible Higgs boson caneven have a mass below the LEP bound m h >
115 GeV, and its discovery at the LHC requiresmore luminosity (and work) than expected in case of the SM Higgs boson.The diagonal gray line in fig. 2a corresponds to the most minimal model m = 0 [12, 20],where DM obtains its mass entirely from the electroweak symmetry breaking. This possibilityhas now been excluded or disfavored by Xenon m > Xenon
100 search for direct DM detection.The Higgs/DM coupling λ determines both the Higgs invisible width and the spin-independentDM direct detection cross section, σ SI = λ m N f πM m h , (3)3 .050.1 0.20.30.40.50.60.730 40 50 60 70110120130140150160170 DM mass in GeV H i gg s m a ss i n G e V Higgs invisible BR (cid:37) CL boundfrom Xenon10095 (cid:37)
CL bound from LEP 95 (cid:37)
CL boundfrom LHC m h (cid:60) M D M
10 20 30 40 50 60 7010 (cid:45) (cid:45) (cid:45) DM mass in GeV Σ S I i n c m Prediction for direct detection (cid:37) CL boundfrom Xenon100
Figure 2:
Left : Iso-lines of the invisible Higgs boson BR in the scalar DM singlet model, fixingthe Higgs/DM coupling from the cosmological DM abundance. The green region shows the regionfavored at and
C.L. from our estimate of LHC data, ignoring look-elsewhere-effects.The red region is excluded by
Xenon m = 0 . Right : Predicted DM spin independent direct detection cross section as a function ofthe DM mass (green region). where f parameterizes the nucleon matrix element, (cid:104) N | m q ¯ qq | N (cid:105) ≡ f q m N [ ¯ N N ] , f = (cid:88) q = { u,d,s,c,b,t } f q = 29 + 59 (cid:88) q = { u,d,s } f q . (4)We here assumed f = 0 .
30, based on the lattice result f = 0 . ± .
015 [22]. The mainuncertainty on f comes from the strange quark contribution f s in (4), and other computationsgive higher more uncertain values, such as f = 0 . ± .
11 [23]. The LHC [1, 2] and LEP [24]excluded region are also shown in magneta.The
Xenon
100 exclusion bound have been plotted assuming the local DM density ρ (cid:12) =0 . / cm . This is the canonical value routinely adopted in the literature, with a typicalassociated error bar of ± . . Recent estimations found a higher central value closer to0 . / cm [25] that would imply stronger bounds on the cross section σ SI . The DAMA [26], CoGeNT [27] and CRESST-II [28] direct DM searches claim positive hints, that are difficultto reconcile with bounds from other experiments such as
Xenon even assuming highly non-standard DM models [29] — an avenue that we do not explore here.We notice that expressions similar to eq. (2) apply in all models where Higgs decays toDM, such that a similar value of λ is obtained: consequently a similar detectable DM directdetection is predicted in more generic models. 4 onclusions Motivated by the existence of DM, we have studied the implications of Higgs boson invisibledecays into DM for the recent CMS and ATLAS Higgs searches. We find that the best fit ofthe present data is provided in terms of a non-standard Higgs boson, with a ≈
50% invisiblebranching ratio and possibly a mass around 143 GeV. We demonstrated that the simplest scalarsinglet model for DM can provide such an invisible width. The needed Higgs boson/DM cou-pling, λ ≈ . , is consistent with DM as a thermal relic and predicts the direct detection crosssection just below present bounds. The present runs of the Xenon
100 and LHC experimentscan test this possibility.
Acknowledgements
We thank G. Giudice, A. Korytov and M. Papucci for discussions. Thiswork was performed in the CERN TH-LPCC summer institute on LHC physics. This workwas supported by the ESF grants 8090, 8499, MTT8 and by SF0690030s09 project.
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