A diphoton resonance from bulk RS
Received: May
A diphoton resonance from bulk RS
Csaba Csaki 0 1 3
Lisa Randall 0 1 2
0 Cambridge , MA 02138 , U.S.A
1 Ithaca , NY 14853 , U.S.A
2 Department of Physics, Harvard University
3 Department of Physics, LEPP, Cornell University
Recent LHC data hinted at a 750 GeV mass resonance that decays into two photons. A signi cant feature of this resonance is that its decays to any other Standard Model particles would be too low to be detected so far. Such a state has a compelling explanation in terms of a scalar or a pseudoscalar that is strongly coupled to vector states charged under the Standard Model gauge groups. Such a scenario is readily accommodated in bulk RS with a scalar localized in the bulk away from but close to the Higgs. Turning this around, we argue that a good way to nd the elusive bulk RS model might be the search for a resonance with prominent couplings to gauge bosons.
Phenomenology of Field Theories in Higher Dimensions; Phenomenological
Models
1 Introduction
2
3
4
5
1
Introduction
preliminary version of this work was presented at [1].
Recent data from the ATLAS [2] and CMS [3] experiments hint at a 750 GeV
resonance that decays into two photons. The event rate, the absence of other signatures, or of
a signi cant signal in the lower-energy Run 1 of the LHC [4, 5], argues for a scalar or
pseudoscalar coupled reasonably strongly to vector-like fermions charged under the Standard
Model gauge group [6{11]. While SM charged scalar interactions instead of that of vector
fermions are also possible, explaining their low mass would generally require additional
assumptions.
as well.
The mass of the fermions must be at least of order TeV in order to explain the lack of
direct detection. To achieve the necessary production rates, the coupling of these fermions
to the scalar would have to be relatively large, hinting at a strongly coupled theory. Even
if there is large multiplicity, large running would argue for some sort of strong dynamics
In this paper we explore a class of models in which a scalar resonance is sequestered
from the IR brane in a fth warped spatial dimension. Such models [14] automatically
have several features consistent with the required constraints.
Bulk RS models have the potential to explain avor when fermions are in the bulk.
Bulk fermions guarantee a large number of vector fermion states | the KK fermion modes | charged under the Standard Model and distributed throughout the bulk.
With KK masses of order the experimental limit, the required Yukawas of the scalar to the ve-dimenisonal fermions can be of order unity. { 1 {
The scalar in the bulk couples to vector KK modes located in the same region.
Because the SM is chiral, projection operators guarantee that one chirality of the KK
modes vanishes in the IR so ONLY scalars with substantial support away from the
IR brane have the necessary interactions.
All Yukawas can be of order unity, also consistent with the absence of observed decays to the weak gauge bosons but potentially leading to additional observable signals.
A bulk theory with a third brane permits the possibility of lowering KK masses, thereby enhancing the decays to photons relative to Higgses.
We consider a warped 5 dimensional RS2 theory [14], with all SM elds in the bulk [16{18],
and a Higgs eld sharply peaked on the IR brane. We use the conformally at form of the
R
z
2
ds2 =
(dx2
dz2)
(2.1)
with the UV brane placed at z = R and the IR brane at z = R0. R is also the AdS
curvature R
MP l1 and R0
1=TeV.
The key additional ingredient will be the assumption of an additional brane at z =
z0R0, where z0 < 1, z0 = O(1). The additional scalar S (assumed to be a singlet under
the SM gauge interactions) will be localized on this brane.
We will comment on the interpretation and the e ect of this third brane in section 2.1. This scalar will be identi ed with the 750 GeV resonance decaying to diphotons. This scalar is assumed to have a { 2 {
Yukawa interaction with the bulk fermion elds. Since S is a SM singlet, it will have
Yukawa couplings between the LH and the RH modes of the same 5D fermions:
Z
d4xpgindYiRS~ i i
!
Z
d x y
4 h (e )
i
S i(1) (1) + h:c:i
i
where
is a bulk fermion, whose KK decomposition is given by [19]
=
X
n
gn(z) n(x) + fn(z) n(x)
with
always denoting a LH 4D 2 component fermion and
a RH 2 component fermion
multiplets has a bulk mass given by cLR;R , where the bulk mass parameter cL;R controls
the shape of the zero mode wave functions, and also plays a role in the shape of the KK
modes. LH zero modes are UV localized for cL > 0:5, while RH zero modes for cR <
0:5.
The bulk equations of motion for the fermions imply the following wave functions for the
nth KK mode:
gn(z) =
fn(z) =
(2.2)
(2.3)
(2.4)
(2.5)
(2.6)
The boundary conditions of fermions corresponding to LH (or RH) SM elds (in the absence of the Higgs VEV) is
LjR =
LjR0 = 0;
RjR =
RjR0 = 0 :
The lowest KK mode will be given by x1=R0 with x1
2:45, for LH fer (...truncated)