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ULB-TH/17-19, PSI-PR-17-15, ZH-TH 27/17

Largeh!bsin generic two-Higgs-doublet models

Andreas Crivellin

Paul Scherrer Institut, CH{5232 Villigen PSI, Switzerland

Julian Heeck

y Service de Physique Theorique, Universite Libre de Bruxelles, Boulevard du Triomphe, CP225, 1050 Brussels, Belgium

Dario Muller

z Paul Scherrer Institut, CH{5232 Villigen PSI, Switzerland and Physik-Institut, Universitat Zurich, Winterthurerstrasse 190, CH-8057 Zurich, Switzerland We investigate the possible size ofh!bsin two-Higgs-doublet models with generic Yukawa couplings. Even though the corresponding rates are in general expected to be small due to the indirect constraints fromBs!+andBs{B smixing, we nd regions in parameter space where h!bscan have a sizable branching ratio well above 10%. This requires a tuning of the neutral scalar masses and their couplings to muons, but then all additional constraints such asB!Xs (g2), andh!+are satised. In this case,h!bscan be a relevant background inh!bb searches and vice versa due to the imperfectb-tagging purity. Furthermore, ifh!bsis sizeable, one expects two more scalar resonances in the proximity ofmh. We brie y comment on other avour violating Higgs decays and on the 95 GeV resonance within generic two-Higgs-doublet models.

I. INTRODUCTION

The possibility of

avour-changing decays of the Brout{Englert{Higgs bosonh(Higgs for short in the fol- lowing) has been discussed for a long time as a possible signal for physics beyond the Standard Model (SM) [1{

7]. Indirect constraints on these couplings come from

avour-changing neutral-current observables. In many analyses one follows an eective-eld-theory approach in which one assumes that only the couplings of the SM-like Higgs to fermions are modied and derives constraints on these couplings from low-energy processes [6, 7]. This leads one to conclude that no avour-changing Higgs de- cays can be observable at the LHC, with the possible ex- ception ofh!eandh![6, 7]. This is a dangerous conclusion because the very existence of avour-changing Higgs couplings in a renormalizable SM extension im- plies additional states which posses avour-changing cou- plings as well. The indirect constraints from avour- changing neutral currents and rare decays are thus in- herently model-dependent and can be decoupled from Higgs decays. This generically involves netuning of the mass spectrum and couplings of the additional states, but opens the way for some new channels to look for physics beyond the SM. In this article we will study the arguably simplest SM extension that can lead to avour-changing couplings of the SM-like Higgs: the two-Higgs-Doublet Model

Electronic address: andreas.crivellin@cern.ch

yElectronic address: julian.heeck@ulb.ac.be zElectronic address: dario.mueller@psi.ch(2HDM) with generic Yukawa couplings, i.e. type III. 1

After computing the eects inBs{B

smixing,Bs! +andb!s , we identify regions of parameter space that can lead to sizable decay rates ofh!bs(upwards of 10%) which are potentially observable at the LHC, hopefully motivating dedicated searches. This is partic- ularly relevant now that the largest Higgs decay mode, h!bb, has nally been observed [18, 19], rendering it background forh!bs. While not the focus of our work, we stress that the additional neutral states (HorA) can easily have even larger avour-violating branching ratios, so general resonance searches forbsnal states are en- couraged as well. The rest of this article is structured as follows: in Sec. II we set up our 2HDM notation. In Sec. III we discuss the main observables that could invalidate large h!bsrates and identify ways to circumvent their con- straints. Sec. IV deals with direct searches for the new scalars at colliders, pointing out their main production and decay channels. We comment on dierent choices of bases for the 2HDM in Sec. V. Finally, we conclude in Sec. VI and provide an outlook for other rare Higgs decays. Appendix A provides one-loop formulae relevant forb!s .1 Similar analyses were performed in the MSSM [8{10], also with additional vector-like fermions [11] and in 2HDMs with of type I and II [12], in aligned 2HDMs [13] as well as in Branco{Grimus{ Lavoura [14] 2HDMs [15] and Zee models [16]. The correlations betweenh!bsandBs!+were considered in Ref. [17].arXiv:1710.04663v2 [hep-ph] 20 Mar 2018 2

II. TYPE-III 2HDM

Our starting point is the 2HDM with generic couplings to fermions (type III) and a CP conserving scalar poten- tial [20]. In the Higgs basis [21{23] in which only one doublet acquires a vacuum expectation value (using no- tation close to Ref. [24]) we have 1= G+ v+H0

1+iG0p2

;2= H+ H 0

2+iAp2

;(1) withv'246GeV, the Goldstone bosonsG0;+, and the physical CP-odd scalarA. Assuming that CP is con- served in the scalar potential, the CP-even mass eigen- states are h=H01sin() +H02cos();(2)

H=H01cos()H02sin();(3)

where we dened the mixing angle asfor easier comparison with the well-known type-I/II/X/Y 2HDM.

We will abbreviatessin(),ccos(),

andttan() below. In the physical basis with diagonal fermion mass ma- trices the Yukawa couplings are given by L Y=X f=u;d;`hf yfs+ ("fPR+"fyPL)c f h +f yfc("fPR+"fyPL)s f H +iff "fPR"fyPL f Ai p2 hu

V "dPR"uyV PL

dH++ h.c.i p2 "`PR`H++ h.c.;(4) whered;`= 1 =uandVis the Cabibbo{Kobayashi{

Maskawa (CKM) matrix. (yf)ij=ijmf

j=vare the stan- dard (diagonal) SM Yukawa couplings, while"u;d;`are arbitrary complex 33 matrices in avour space. O- diagonal elements in"flead to avour-changing Higgs couplings. For our channel of interest,h!bs+bs, we have (h!bs)'3c2mh8(j"d23j2+j"d32j2) 1m2bm 2h 2 :(5) Note that this expression is valid at tree level; next-to- leading order QCD corrections might increase the decay rate by 10{20% [9]. However, since we are interested in an order of magnitude estimate, such corrections are not of particular importance here. The resulting branching ratio is then

BR(h!bs) =(h!bs)(h!bs) +s2SM;(6)

with

SM'4:1MeV and assuming all"u;d;`to be zero,

except of course those forh!bs. Note that a branchingratio ofh!bsof 1% (10%) requires"d23;32couplings of

order 0:02 (0:06), assumingc= 0:1.

So far no searches forh!bshave been performed,

making it dicult to assess the sensitivity. The chan- nelh!bb, which has a large SM branching ratio of

58%, has only recently been observed [18, 19] despite its

betterb-tagging possibilities compared toh!bs. Nev- ertheless, we can obtain a model-independent limit on (h!bs) of 1:1GeV [25], corresponding roughly to the CMS energy resolution. This is still almost three orders of magnitude above the SM value

SM, and thus still al-

lows for BR(h!bs)1. A more intricate upper limit on the Higgs width can be obtained by comparing on- and o-shell cross sections, as proposed in Ref. [26]. A recent CMS analysis of run-1 data along these lines ob- tains h<13MeV [27]. While it cannot be claimed to be a model-independent limit [28], it should hold true in our scenario withc1, seeing ashbecomes arbitrar- ily SM-like. Naively applying h<13MeV on our model and usingc0:55 as a very conservative bound (see below), this implies BR(h!bs).78%; forc1 the limit is BR(h!bs).68%. This is obviously still very large and can most likely be improved by a direct search forh!bs. We will use this as a conservative limit in the following. Stronger limits can be obtained from global ts to ob- served Higgs production and decay channels, seeing as a large (h!bs) would reduce all measured Higgs branch- ing ratios and hence require a larger production cross section to obtain the same rates. An analysis of this type with LHC run-1 data was performed in Ref. [29] and lead to the 95% C.L. limit BR(h!new)<34% on any new decay channels, includingbs. This is a factor of two stronger than the limit from the Higgs width, in part because it is based on a combination of ATLAS and CMS data and makes use of more search channels. We will also show this limit in the following, but stress that it should be taken with a grain of salt; global-t limits are very indirect and depend strongly on the assump- tions one puts in. With the many parameters available in a type-III 2HDM, it is conceivable that the limit could be weakened by increasing some parameters relevant to

Higgs production. A dedicated search forh!bswill

yield far more direct constraints and should always be preferred to global-t limits. The goal of our article is to show that a sizable branch- ing ratio forh!bsis possible, even up to the conser- vative limit of 68%. To simplify the analysis we will set as many entries of"fto zero as possible, i.e."u;d;` ij= 0 is the starting point of our investigation. In this limit, we can obtain bounds on the masses and on the mixing angleby comparison with the type-I 2HDM (in the limit tan()! 1, i.e.!=2, identifying ourc with the type-I sin() = cos(=2)). This gives the rather weak boundjcj.0:55 from LHC run-1 Higgs 3 measurements [30, 31]. In the limitc!0, the new scalars become completely fermiophobic and the model resembles the Inert Higgs Doublet (IDM), with aZ2sym- metry that only allows the new scalars to be produced in pairs. ThisZ2is of course broken in the scalar potential and byc6= 0, but it allows us to use well-known limitsquotesdbs_dbs44.pdfusesText_44

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