The $750\,\text{GeV}$ diphoton resonance in the light of a 2HDM with $S_3$ flavour symmetry

0.36 MB·

by  A. E. Cárcamo Hernández

DO-TH 16/03
The 750GeV diphoton resonance in the light of a 2HDM with S flavour symmetry
3
A. E. C´arcamo Hern´andez,1,∗ I. de Medeiros Varzielas,2,† and E. Schumacher3,‡
1Universidad T´ecnica Federico Santa Mar´ıa and Centro Cient´ıfico-Tecnolo´gico de Valpara´ıso
Casilla 110-V, Valpara´ıso, Chile
2School of Physics and Astronomy, University of Southampton,
Southampton, SO17 1BJ, U.K.
3Fakult¨at fu¨r Physik, Technische Universit¨at Dortmund
D-44221 Dortmund, Germany
(Dated: April 21, 2016)
Very recently we proposed a predictive 2 Higgs Doublet Model with S flavour symmetry that
3
successfully accounts for fermion masses and mixings. In this letter, motivated by the 750 GeV
Higgs diphoton resonance recently reported by the ATLAS and CMS collaborations, we modify
this model by adding exotic top partners with electric charge 5 and a electrically charged scalar
6 3
singlet. These exotic top partners decay into a charged scalar singlet and the SM up type quarks,
1
whereasthechargedscalarsingletwillmainlydecayintoSMupanddowntypequarks. Thissimple
0
modification enables our model to successfully account for the Higgs diphoton excess at 750GeV
2
provided that the exotic quark masses are in the range [1,2] TeV, for O(1) exotic quark Yukawa

r couplings.
p
A
Introduction. Supersymmetric explanations are possible and the ex-
0
2 Recently, the ATLAS and CMS collaborations re- cess has been interpreted in the context of the Minim-
ported an excess of events above the expected back- imal Supersymmetric Standard Model (MSSM) [33], its

] ground in the diphoton final state [1, 2]. This is very R-symmetry violating version [34] and other supersym-
h
p promising as both collaborations have the excess at an metric extensions [35, 36].
- invariant mass of about 750GeV, and with local sta- Extending the SM gauge symmetry can also explain
p tistical significance of 3.9σ (ATLAS) and 2.6σ (CMS). the excess, as described by [37–48], and models based on
e
The confirmation of this excess would constitute proof strongly coupled theories were considered by [3, 4, 49–
h
[ of physics beyond the Standard Model (SM). Although 56]. An extended broken symmetry with an extra Higgs
the excess may turn out to be a statistical fluctuation, it boson and massive vector bosons can account for the di-
3 is very enticing to consider models that include in their boson anomaly and the anomalous tt¯forward-backward
v
field content something that can account for a resonance asymmetry [57]
1
6 at 750GeV and thus accounts for the excess of events. Models with extra fermions [58], in particular vector-
6 This diphoton final state excess has generated much like fermions had been considered before [59–62] and af-
0 activity in the community. Many works have considered ter the announcement [3–5, 10, 63–71], and other works
0 relate the diphoton excess with loop TeV-scale seesaw
the excess of events in a model independent way, see [3–
1. 14]. Another study considered the implications of the mechanisms, either at two [44] or three loops [72].
0 Higgs diphoton resonance for the stability of the Higgs, Relating the excess with dark matter has been exten-
6 naturalness and inflation [15] and [16] considers whether sively considered, see [31, 51, 73–83].
1 the resonance could be a Coleman-Weinberg inflaton. There are also sgoldstino [84–86], radion [87, 88],
:
v graviton [89] and exotic heavy axion [49, 50, 90] inter-
Previous works have studied the resonant production
i pretations of the 750GeV excess.
X at the LHC of (pseudo-) scalars coupled to two photons
String-motivated models were considered in [91–93].
andgluonsinthemassregionfrom30GeVto2TeV[17],
r
a and of the observable effects of new scalar particles [18]. Finally, a rather natural and popular framework to
explain the excess is that of 2 Higgs Doublet Models
A myriad of explanations have been considered since
(2HDMs), which have been studied in [63, 94–102] and
the announcement of the excess. Adding scalar singlets
will also be considered in this letter, where we make a
is a fairly straightforward option, see [19–28], and [29]
simplemodificationofanexisting2HDM[103],byadding
(which explores several scenarios including a scalar sin-
exotic top quark partners.
glet). The scalar singlet model can be further extended
The model. We consider the 2HDM that we recently
with extra dimensions [30], or, alternatively, with addi-
proposed in [103], with the SM gauge symmetry supple-
tional vector leptoquarks, which also allow to simultane-
mented by the S ⊗Z ⊗Z(cid:48) ⊗Z discrete group. The
ously explain the B decay anomalies [31, 32]. 3 3 3 14
scalar sector has two Higgs doublets (assigned as trivial
S singlets) plus four SM singlet scalars assigned as one
3
S trivial singlet (χ), one S non-trivial singlet (ζ) and
3 3
one S doublet (ξ). In order to successfully explain the
∗Electronicaddress:[email protected] 3
†Electronicaddress: [email protected] LHCdiphotonexcessat750GeV, weextendthefermion
‡Electronicaddress: [email protected] sectorofour2HDMbyincludingfourSU(2)L singletex-
2
otic quark fields with electric charge 5, T , T , T ,
3 1L 1R 2L
T , grouped into two S doublets, i.e., T =(T ,T ),
2R 3 L 1L 2L χ3 χ3 χ5
T = (T ,T ). These exotic quark fields are neutral Ll = ε(l)l φ l +ε(l)l φ l +ε(l)l φ l
R 1R 2R Y 33 3L 1 3RΛ3 23 2L 1 3RΛ3 22 2L 1 2RΛ5
under the Z ⊗Z(cid:48) discrete symmetry but charged under
3 3 χ5 χ7ζ
the Z14 symmetry as: +ε(3l2)l3Lφ1l2RΛ5 +ε(1l1)l1Lφ2l1R Λ8
χ3 χ3
+ε(1ν1)l1Lφ(cid:101)2ν1RΛ3 +ε(1ν2)l1Lφ(cid:101)2ν2RΛ3
+ε(2ν1)l2Lφ(cid:101)1ν1R+ε(2ν2)l2Lφ(cid:101)1ν2R+ε(3ν1)l3Lφ(cid:101)1ν1R
TL →TL, TR →eπ7iTR. (1) +ε(3ν2)l3Lφ(cid:101)1ν2R+M1ν1Rνc1R+M2ν2Rνc2R
ρ+
+M ν νc +ε(ρ)l φ ν
12 1R 2R 21 2L 1 1R Λ
χ3ρ+ χ3ρ+
In addition, the instability of the exotic quarks T1L, +ε(3l3)l3Lφ(cid:101)1l3R Λ4 +ε(2l3)l2Lφ(cid:101)1l3R Λ4
Tw1iRth, Ta2sLi,ngTl2eRerleecqturiirceasllythcahtawrgeedexSteMndscthalearscsailnagrlestecρt+or. +ε(2ρ2)l2Lφ1ν2RρΛ++ε(3ρ1)l3Lφ1ν1RρΛ+
We assume that ρ+ is a trivial S3 singlet and is neu- ρ+
tral under the Z ⊗Z(cid:48) ⊗Z discrete symmetry. The +ε(ρ)l φ ν +h.c (3)
3 3 14 32 3L 1 2R Λ
exotic quarks should then decay into ρ+ and SM up
type quarks, through the Yukawa interactions given in As the quark masses are related to the quark mix-
Eq. (2), while ρ+ in turn will mainly decay into SM ing parameters, we set the vacuum expectation values
up and down type quarks. The remaining particles have (VEVs) of the SM singlet scalars with respect to the
the S ⊗ Z ⊗ Z(cid:48) ⊗ Z charge assignments described Wolfenstein parameter λ = 0.225 and the new physics
3 3 3 14
in [103]. In this model, the S symmetry reduces the scale Λ:
3
number of parameters in the Yukawa sector making this
v ∼v ∼v =λΛ. (4)
2HDM more predictive, and the remaining symmetries ξ ζ χ
control the allowed Lagrangian terms by distinguishing Regarding the Yukawa interactions of ρ+ with quarks
the fields. For example, the two scalar SU(2) doublets and leptons, we only consider operators up to dimension
L
have different Z charges (φ being neutral). The Z(cid:48) eightandneglecthigherdimensionalcontributions. From
3 1 3
and Z symmetries shape the hierarchical structure of the quark Yukawa interactions, it follows that the top
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the fermion mass matrices necessary to get a realistic partners will decay dominantly into either up or charm
pattern of fermion masses and mixing. The assignments quarks and the charged scalar singlet ρ+, whereas the
of the scalar and fermion particles particles are given in dominant decay mode of ρ+ will be into top and bot-
[103], giving rise to the following Yukawa terms for the tom quarks. Let us note that the charged scalar singlet
quark and lepton sectors: ρ+ cannot decay into charged leptons and right handed
neutrinos,sincetherighthandedMajorananeutrinosare
muchheavierthanρ+,thusnotallowingthatdecaychan-
nel. In addition, the charged scalar ρ+ can decay into
charged leptons and light active neutrinos but its cor-
LqY = ε(3u3)q3Lφ(cid:101)1u3R+ε(2u3)q2Lφ(cid:101)2u3RΛχ22 +ε(1u3)q1Lφ(cid:101)2u3RΛχ33 rseesepnofnrodmingthdeelceapytornatYeuiksaswuappinrteesrsaedctiboynsλ.6CΛv2o2n,saesqucleenatrllyy,
the top partners can be searched at the LHC through
+ε(2u2)q2Lφ(cid:101)1URξΛχ43 +ε(1u1)q1Lφ(cid:101)1URξχΛ48ζ3 tl3hje(cid:54)Eir de(cmay=ch1a,n2n).elTThmes→e tρo+pupmar→tnetrbsumare→prWodbubcuemd→in
T
+ε(d)q φ d χ3 +ε(d)q φ d χ5 pairs at the LHC via a gluon fusion mechanism, where
33 3L 1 3RΛ3 22 2L 2 2RΛ5 these exotic quarks are in the triangular loop followed
χ6 χ6 by the propagator of the scalar χ, followed again with
+ε(d)q φ d +ε(d)q φ d
12 1L 2 2RΛ6 21 2L 2 1RΛ6 a pair of the top partner and its antiparticle (note that
the scalar singlet χ has a renormalizable coupling with
χ7 χρ−
+ε(d)q φ d +y T T χ+ε q φ T these top partners). Thus observing an excess of events
11 1L 2 1RΛ7 T L R ρ 3L 2 R Λ2
with respect to the SM background in the opposite sign
ρ− ρ+χ3
+ε(3ρ3)q3Lφ1u3R Λ +ε(3ρ3)q3Lφ(cid:101)1d3R Λ4 datiletphteoLnsHfiCn.alstatecanbeasignaltoconfirmthismodel
+ε(2ρ3)q3Lφ(cid:101)2d2Rρ+Λχ43 +ε3(ρ2)q3Lφ2URξρΛ−3χ thaFtrotmhetVhEeVYuokfaξwiastaelrigmnsedgiavsen(1a,b0)ovine tahnedScondsiirdeecrtiinong
3
χ ζ3χ [103], we find that the quark, charged lepton and light
+y1ρTLURρ+Λ +y2ρTLURρ+ Λ4 +h.c (2) active neutrino mass matrices are:
3
16
v c1λ8 0 a1λ3  14
MU = √2 00 b10λ4 aa2λ32 , fbγγ)[] 12
MD = √v2ee120λλ76 ff120λλ65 g00λ3 , (5) ppσ(→χ→ 1680
1
4
x λ8 0 0 
v 1 2
Ml = √  0 y1λ5 z1λ3 , (6) 1000 1200 1400 1600 1800 2000
2 0 y λ5 z λ3 vχ[GeV]
2 2
 
W2 κWX WY
FIG. 1: Total cross section σ(pp→χ→γγ) as a function of
Mν = κWX X2 κXY , κ=cosϕ. v for and different values of the exotic quark Yukawa cou-
WY κXY Y2 χ
plingsy ∼0.5,1and1.5(inthecurvesfromtoptobottom,
Ti √
respectively), assuming s = 13TeV and α (m /2) (cid:39) 0.1.
where v =246 GeV and a (k =1,2,3), b , c , g , f , s χ
k 1 1 1 1 The horizontal lines denote the experimentally allowed lim-
f , e e , x , y , y , z , z and κ are O(1) parameters,
2 1 2 1 1 2 1 2 its of the diphoton signal given by ATLAS and CMS, which
whereas X, Y and W are parameters with dimension
√ amount to 10 ±3fb and 6±3fb, respectively. The limits
m where m has mass dimension. The Cabbibo mixing require v (cid:46)2TeV if natural order one exotic quark Yukawa
χ
arises from the down-type quark sector whereas the up- couplings are assumed.
type quark sector contributes to the remaining mixing
angles [104]. Furthermore, light active neutrino masses
ariseviaatypeIseesawmechanismwithtwoheavyright- of the exotic quarks y ∼ 1, which with v ≈ 1.2TeV
Ti χ
handedMajorananeutrinosν andν . Wehaveshown amounts to σ ≈8fb. This is well within the limits given
1R 2R
in [103] that the fermion mass textures given above are by the ATLAS and CMS experiments [4]
consistent wih the current data on SM fermion masses
and mixings. σATLAS =10±3fb, σCMS =6±3fb.
The 750 GeV scalar resonance. The recently re-
ported excess in the diphoton final state can be at- The total cross section was computed using the
tributed to the Z breaking scalar χ, which, taking into MSTW2008 next-to-leading-order gluon distribution
14
accounttheheavyexoticfermions,ispredominantlypro- functions [105] as a function of vχ for different values
duced via gluon fusion through the triangular loop dia- of the exotic quark Yukawa couplings. As shown in Fig.
grams with T1 and T2. The corresponding total cross 1, the cross section depends crucially on the VEV vχ as
section σ is a function of the gluon production rate wellasontheYukawacouplings,whichifsizablecanalso
Γ(gg → χ) and the consequent decay rate into photons enhance σ significantly in particular for lower vχ values.
Γ(γγ →χ) Ifwefurtherrequirethatσ bewithintheexperimental
limits given by ATLAS and CMS, we predict v to be
χ
Γ(gg →χ)=Kgg3α22sπm3v3χ2(cid:12)(cid:12)(cid:12)(cid:12)F(xT)(cid:12)(cid:12)(cid:12)(cid:12)2, (7) sbmreaalkleinrgthsacnale2TanedV,ownhtihcehootnhetrhehaonndefihxaensdtsheetsexthpeecZte1d4
χ
particlemassesofχandtheexoticquarksT tobeinthe
α2m3 (cid:12) (cid:12)2 i
Γ(γγ →χ)= 64π3vχ2(cid:12)(cid:12)(cid:12)NcQ2TF(xT)(cid:12)(cid:12)(cid:12) , (8) samCeonrecgliuosni.on. The same flavon that is responsible for
χ
the shaping of the fermion mass and mixing matrices
where m (cid:39)750GeV denotes the resonance mass, x =
χ T can explain the recently reported 750GeV excess in the
4m2/m2,m =y v ,andKgg ∼1.5accountsforhigher
T χ T T χ diphotonchannel. Thisisshownusingapredictiveflavor
order QCD corrections. F(x) is a loop function given by
model based on the S ⊗Z ⊗Z(cid:48) ⊗Z symmetry with
3 3 3 14
(cid:0) (cid:1) (cid:0) (cid:112) (cid:1)2 theadditionofheavyexoticfermionswithelectriccharge
F(x)=2x 1+(1−x)f(x) , f(x)= arcsin 1/x
5 and a electrically charged scalar singlet. These heavy
3
with xT >1⇔4mT >mχ. Finally, we obtain exotic quarks decay into a charged scalar singlet and the
SM up type quarks, whereas the charged scalar singlet
π2Γ(γγ →χ)1s(cid:82)m12/s dxxfg(x)fg(cid:16)msx2χ(cid:17)Γ(gg →χ) willmainlydecayintoSMupanddowntypequarks. At-
σ = χ tributingtheZ breakingscalartotheresonanceallows
8 m Γ 14
χ χ
one to fix the energy of the breaking scale and, hence,
√
with s=13TeV being the LHC center of mass energy, enables immediate testing of the model at the current
Γ thetotaldecaywidthofχandf (x)thegluondistri- LHC run.
χ g
bution function. To obtain a rough estimate of σ we as- Acknowledgments. This project has received fund-
sumeforsimplicityunifiedandnaturalYukawacouplings ing from the European Union’s Seventh Framework Pro-
4
gramme for research, technological development and 2012-327195 SIFT.
demonstration under grant agreement no PIEF-GA-
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