Diphotons at the Zpole in models of the 750 GeV resonance decaying to axionlike particles
Received: June
Diphotons at the Zpole in models of the 750 GeV resonance decaying to axionlike particles
Alexandre Alves 0 1 2 5
Alex G. Dias 0 1 2 3
Kuver Sinha 0 1 2 4
0 Salt Lake City , UT, 84112 U.S.A
1 Santo Andre, SP , 09210580 Brasil
2 Diadema, SP , 09972270 Brasil
3 Centro de Ci
4 Department of Physics and Astronomy, University of Utah , USA
5 Departamento de Ci
6 encias Naturais e Humanas, Universidade Federal do ABC
Models in which the 750 GeV resonance (S) decays to two light axionlike particles (ALPs a), which in turn decay to collimated photons mimicking the observed signal, are motivated by Hidden Valley scenarios and could also provide a mechanism by signal persists while S ! Z ; ZZ and W W remain subdued in the near future. We point out that these Hidden Valley like models invoking S ! aa ! 4
Beyond Standard Model; E ective eld theories; Extended Supersymmetry

which a S !
must
also contend with Z ! a(!
) constraints coming from CDF and ATLAS. Within an
e ective eld theory framework, we work out the constraints on the couplings of S to a and
gauge bosons coming from photonic Z decays and ensuring that the ALPs decay inside the
electromagnetic calorimeter, in two regimes  where a decays primarily to photons, and
where a also has hadronic branchings. The analysis is done for both when S has a large as
well as a narrow width, and for di erent relative contributions to the signal coming from
S !
complex
and a !
eld are also presented. A
resonance at the Zpole coming from Z ! a is
expected in this class of models. Taking benchmark ALP masses below around 0.4 GeV
and, assuming reasonable values for the fake jet rate and the identi cation e ciency of the
photonjet, we nd the prospects for the discovery of diphotons at the Zpole.
. Results for the particular case where S and a belong to the same
1 Introduction 2 E ective eld theory parameterization
ATLAS and CMS diphoton excess
Total width
Constraints from ALP lifetime
Constraints from photonic decays of the Z boson
Results: constraining the EFT
Discovery estimate from an improved analysis for detecting photonjets
One of the most important anomalies in particle physics in recent decades is the excess in
pp !
peaked at invariant mass around 750 GeV observed at the LHC [1{6]. We will
denote the resonance by S and call it the Scion in this paper, in the hope that S is the
rst visible scion of a larger dynasty. It is hard to imagine systematic issues, theoretical or
experimental, being behind the Scion: the SM background is primarily treelevel qq !
scatterings, while experimentally diphotons constitute an extremely clean channel.
CMS presented new data taken without the magnetic eld during the Moriond 2016
conference, while ATLAS presented a new analysis with looser photon selection cuts.
Moreover, both collaborations recalibrated photon energies optimized around 750 GeV. The
statistical signi cance of the excess increased for both experiments in the aftermath, leading
to renewed activity from theorists.
For the rates and width of the Scion, we will assume two benchmarks: (i) the narrow
width regime with
13 TeV = 2:5 fb and
S = 5 GeV; and (ii) the large width regime with
13 TeV = 6 fb and S = 40 GeV. These values follow the tting of the data presented in [7].
The literature on the diphoton excess is already vast, covering weakly and strongly
coupled models and their embeddings in the UV, as well as new experimental signatures and
connections to dark matter and baryogenesis. For a concise summary, we refer to [8] and
{ 1 {
those where the signal arises from photonjets [9{18]. In this class of models, the resonance
decays to highly boosted objects which decay to multiple photons. These photons then
hit the electromagnetic calorimeter (ECAL) and depending on their angular spread, can
be reconstructed as single photons to mimic the signal. Similar proposals have been made
previously for the Standard Model Higgs as well, and we refer to [19] and references therein
for details. Theoretically, such topologies may be motivated by Hidden Valley scenarios [20,
21], where the decay S ! aa, with a a light scalar or pseudoscalar state, or an axionlike
particle (ALP), occurs at tree level. The collimation required to mimic the single photon
reconstruction can be obtained if ma is small, ma
O(GeV).
From the perspective of the diphoton anomaly, the main motivation behind this class
of models is the (
1
) preference for a wide resonance ( =MS
0:6) from ATLAS. The
reasons are as follows. The simplest realization of weakly coupled models of the Scion
(the socalled \Everybody's Model") consists of loops of vectorlike colored and charged
matter through which the Scion is produced through pp collisions and subsequently gives
the diphoton signal. To realize a wide resonance that simultaneously ts the rates shown
above, however, one needs either a large multiplicity of such new particles running in the
loop, or large Yukawas or charges, which lead to somewhat baroque models. Moreover,
the large coupling of the Scion to gluons required in these scenarios is already constrained
by dijet constraints. Models with gg ! S ! aa ! 4 ameliorate this problem since the
coupling to photons occurs at treelevel.
Another possible motivation to study axionlike models of the new resonance, that is
it, models containing the new resonance S and an ALP, would be a strong suppression of
the other electroweak decay channels. If no signals in Z , ZZ and/or W W is observed in
the near future, this will force us to consider a di erent mechanism for the
decay. In
models where the Scion decays to a light scalar or pseudoscalar with large branching ratio
to photons, the direct loopinduced decay S !
, as the other weak bosons channels, can
be made small and subdominant. The most recent search for Z
resonances by the CMS
Collaboration, by the way, found no signals in the 200{2000 GeV range [22] after combining
19.7 fb 1 of the 8 TeV run and 2.7 fb 1 of the rst 13 TeV run. Also, a combined search for
narrow spin0 and spin2 resonances in the diboson channels ZZ and W W was performed
by the ATLAS Collaboration [23] with the 3.2 fb 1 collected running at
S = 13 TeV. No
p
excess was found in the 750 GeV.
single photon, Z !
in gure 3.
The purpose of this paper is to point out that Hidden Valley like models invoking
must also contend with Z ! a(!
) constraints. Just as one expects an
and S ! ZZ in the near future simply on the basis of writing down a
gauge invariant theory of S that couples to photons, one also expects couplings of a to Z in
models where a decays to photons. This opens up Z !
and, in the limit that ma is small and the two photons from a !
(constrained by ATLAS [24])
are reconstructed as a
decays (constrained by CDF [25]). These constraints are depicted
Within an e ective eld theory framework [26], the coupling of the ALP to S and gauge
bosons is constrained from several directions: (i) tting the diphoton signal; (ii) ensuring
{ 2 {
done for both the large and narrow width regimes and for di erent relative contributions
to the signal coming from S !
. Finally, the combined constraints on the
space of parameters in the e ective
eld theory are presented. The constraints are also
given for the more restrictive case where S and a are the real and imaginary parts of the
same complex eld.
When the ALP decays exclusively into photons pairs, for example, the CDF data
imposes an upper bound on the ALPphoton coupling of 0.07, and with a fty times
almost all the parameters space of axionlike models of this
type can be excluded. These bounds are, of course, weaker if the ALP looks like a genuine
axion which decays to gluons but, we show that the constraints from photonic decays of Z
bosons need to be taken into account for ALP masses up to
resonance at the Zpole coming from Z ! a in this class of
models. We take benchmark ALP masses below around 0.4 GeV, where the branching is
entirely to photons, and assume what we believe are reasonable values for the fake jet rate
and the identi cation e ciency of the photonjet. We nd that with couplings to the gauge
bosons of order 0.07 and with optimal photon detection e ciencies, the LHC will be able
to detect diphotons from Z decays with 300 fb 1 in a simple cutandcount experiment.
The paper is organized as follows. In section 2, we introduce our notation and the
parametrization of the Scion and ALP a in the e ective eld theory framework. We then
discuss, in turn, constraints on our EFT coming from the diphoton signal in section 2.1,
the width
S of the Scion in section 2.2, the lifetime of a in section 2.3 and the photonic
decays of Z in section 2.4. In section 4, we present the constraints in the case where S and
a belong to the same complex
eld. In section 5, we present our simulations and results
for the search of a resonance at the Zpole. We end with our Conclusions.
2
eld theory parameterization
In this section, we rst introduce our notation and the parametrization of the Scion and
ALP a in an e ective eld theory (EFT) framework. We then discuss, in turn, constraints
on our EFT coming from the diphoton signal in section 2.1, the width
S of the Scion in
section 2.2, the lifetime of a in section 2.3, and the photonic decays of Z in section 2.4. In
section 3 we put together all the constraints and present our results.
The most general e ective Lagrangian involving gauge bosons, the scalar S and the
pseudoscalar axionlike a relevant for our studies is given by
L =
cBB SB
+ kBB
aB
B
~
B
b ) are the dual eld strengths of the U(
1
)Y
hypercharge, SU(2)L weak, and SU(3)C gauge bosons. The e ective Lagrangian above can
be generated through oneloop diagrams in models having mediators that, beside couplings
to S and a, interact with the SM gauge bosons as well. If S and a originate from di erent
elds they do not necessarily have the same couplings with the mediators, so that the
coe cients cV V and kV V , V = B; W; G, in eq. (2.1) do not have to be identical.
The e ective couplings of S and a with the gauge bosons are each suppressed by their
characteristic energy scales, vS and va. These scales can be absorbed in the de nitions of
the coe cients cV V and kV V through ratios
=vS;a in favor of the common energy scale
used in the parametrization of the e ective Lagrangian. In fact, S and a may be associated
with symmetries that are broken at di erent energy scales. One example is when the
pseudoscalar a is a pseudo NambuGoldstone boson  and so naturally light  of a U(
1
)a
symmetry broken at the scale va, as happens for the axion or axionlike particles (ALPs),
with S being a Higgs boson of another broken symmetry. In section 4, we will consider
the more constrained case where S and a belong to the same complex eld and, therefore,
have identical couplings to matter implying that cV V = kV V . For de niteness, we will in
any case call the pseudoscalar a an ALP, considering it as a light pseudoscalar which has
a coupling with photons similar to the axion.
The decay of S !
is a looplevel process mediated by charged particles, for example,
vectorlike quarks and leptons. The charged mediators will generally have couplings to the
Z bosons through their hypercharge assignments, and with W bosons if they belong to
nonsinglet representations of SU(2)L. This is also the case of the ALP a. In most applications,
for example, the detection of axionlike particles, we are interested in the ALPphoton
coupling. The ALPZ coupling thus receives less attention, since light ALPs obviously
cannot decay to heavy gauge bosons. Nevertheless, this interaction opens up a new Z
boson decay channel, namely, Z
! a + . We refer to [27{37] for the status of axion
searches with only photon couplings as well as hypercharge couplings, from a combination
of LightShiningthrough Wall experiments, cosmology, as well as colliders.
From the interactions in eq. (2.1), we compute the partial widths of S decaying to
pairs of ALPs, a to
and the Z boson decaying to a +
(S ! aa) =
(S !
) =
(S ! gg) =
(a !
) =
(a ! gg) =
(Z ! a ) =
2
8 mS
m3S
m3S
m3a
m3a
m3Z
4
4
4
4
6
2
2
2
2
2
1
4
m2S
m2a 1=2
cBBc2w + cW W s2w 2
8c2GG
kBBc2w + kW W s2w 2
8kG2G
1
kW W )2 :
(2.2)
(2.3)
(2.4)
(2.5)
(2.6)
(2.7)
B
B
k
1.0
), in dashed black, as functions of the couplings kBB and kGG.
), in solid blue, and Br(a !
The partial decay of the Z boson to three photons is given by
(Z !
(Z ! a )
kW W )2
Br(a !
)
(2.8)
assuming that a decays predominantly to photons and gluons only.
neutral pion mass. There are, thus, two regimes we should consider:
Hadronic decays of a are only possible if ma > 3m 0 , where m 0
135 MeV is the
<
8>Br(a !
Br(a !
>
The branching ratios of the ALP into photons pairs and the Z boson into three photons are
shown in the gure 1. We should also point out that a Br(a !
axion masses larger than 0.4 GeV if the axion mix with the
)
O(
1
) is possible for
and 0 mesons which decay
to photons around 70% and 2% of the time, respectively [38]. If the mixing is negligible,
however, the
100% decay to photons is still guaranteed if couplings to electrons and
neutrinos are absent or too small when ma < 0:4 GeV.
2.1
ATLAS and CMS diphoton excess
Having introduced our EFT, we rst discuss the constraints on it from the recent diphoton
excess. It is useful to parametrize the claimed rates in terms of the following relation
{ 5 {
HJEP08(216)
involving the partial widths to photons and gluons:
(S ! gg)
m2S
= C
" 13 TeV #
8fb
receives two contributions, the rst from direct decay S !
and the second from S ! aa ! 4 . The former is induced by the couplings cBB and cW W
of the Scion to the gauge bosons. The latter mimics the diphoton signal and contributes
to the claimed rate when the ALP is light. The di erent mass regimes where the diphoton
signal is mimicked are discussed in section 2.4 and gure 3.
In our analysis, the relative contribution of the channel S ! aa ! 4 to the total
branching to
is a useful quantity which we denote by
Using eq. (2.12) in eq. (2.10), we can recast the condition for the claimed rate as
(S ! gg)
(S !
) + (S ! aa)
Br2(a !
)
m2S
= C
" 13 TeV #
8fb
S
mS
: (2.13)
Finally, using the expressions for (S ! gg) and (S ! aa) from the EFT Lagrangian in
eq. (2.1) and R
in the above equation, we arrive at the relation
2
2
cGG 2 =
4 2R
Br2(a !
C
)
" 13 TeV #
8fb
S
mS
:
This provides a relation between parameters of the EFT and the relative contribution of
the \fake photons" that gives the claimed rates for the diphoton excess.
Note that if Br(a !
) = 1, tting the diphoton signal does not depend on the ALP
coupling to the gauge bosons, just cBB; cGG and . In this case, the constraints that we are
going to impose on the model from the photonic decays of the Z boson and the ALP decay
length involve just kBB and the mass of the ALP. Whenever we have to take gluonic decays
of the ALP into account though, we use eq. (2.14) to eliminate one of those parameters and
allowed regions of the parameters space will automatically t the LHC diphoton signal.
2.2
Total width
We now turn to the total width
S. This is an important piece of information in model
building in view of the preliminary results of ATLAS and CMS. In particular, ATLAS data
favors a total width of around 40 GeV. On the other hand, CMS data seems not prefer
any particular value at this moment and a much narrower resonance is not discarded. As
{ 6 {
(2.10)
(2.11)
(2.12)
(2.14)
GS=40GeV
0
.1
0
.9
0.4
0.3
GG0.2
c
0.1 5GeV
0
.0
5
0
.3
0.0
0
(S !
(2.15)
(2.16)
HJEP08(216)
The Scion total width of eq. (2.15) without any dark matter component.
green(yellow) area represents the points with S = 40(5) GeV for cBB from 0.1(0.05) to 0.9(0.3).
mentioned in the Introduction, we will consider two scenarios inspired by the tting of the
data performed in [7]: (
1
) a wide scenario with
narrow scenario with
13 TeV = 2:5 fb and S = 5 GeV.
In our EFT parametrization, the total width is given by
13 TeV = 6 fb and
S = 40 GeV, (2) a
=
(S ! aa)+ (S !
)+ (S ! W W )+ (S ! ZZ)+ (S ! Z )+ (S ! XX)
2
8 mS
+
m3S
4
2 8c2GG+c2BB+c2W W + (S ! XX)
neglecting the ALP mass compared to the S and the Z bosons masses. The partial width
XX) represents other contributions we might not be taking into account, for
example, the decay into dark matter. Thus, we are allowed to impose the upper bound
2
8 mS
+
m3S
4
2 8c2GG + c2BB + c2W W
S :
In the gure 2 we display the total width of eq. (2.15) without the dark matter contribution
(S ! XX) in the plane
vs. cGG. The green (yellow) area represents the points with
S = 40(5) GeV for cBB from 0.1(0.05) to 0.9(0.3). As we pointed out, it is easier to get a
large width with small Sgauge bosons couplings when S decays to ALPs.
2.3
Constraints from ALP lifetime
The decay width of the ALP a is small, hence it is necessary to ensure that it decays inside
the electromagnetic calorimeter. For that propose we have to compute the distance from
the interaction point that a can travel before decaying to photons or gluons. In the case of
a boosted particle of mass ma coming from the decay of a heavy particle S, this distance
is given by
`decay =
a
In the LHC detectors, the maximum `decay is around 1 meter in order that the photons
can be detected. In the CDF of Tevatron this distance is not much smaller, around 70
We will consider in both cases a distance of order of a meter to simplify our
discussions. We shall see that unless the ALP mass is very small, `decay will not represent
a severe constraint.
Substituting
a =
obtain the following bound
(a !
) + (a ! gg) from eqs. (2.5), (2.6) into eq. (2.17), we
where we are assuming `decay = 1 m.
Constraints from photonic decays of the Z boson
In this section, we turn to the nal important constraint: the upper limit from the CDF
Collaboration [25] on the Z boson decay to two photons, and from the ATLAS
Collaboration [24] on the Z boson decay to three photons. The limits at 95% of con dence level
(CL) are as follows:
CDF
ATLAS
Br(Z !
Br(Z ! 3 ) :
) :
1:45
2:2
The realm of validity of each constraint is depicted in gure 3, which we now discuss in
branching ratio of order 10 10 [40].
detail. Let us point out that, in the Standard Model, Z !
is extremely rare, with a
We start with large ALP masses. Ref. [27] studied Z bosons decaying to an axionlike
pseudoscalar and a photon. They found that the Z ! 3 channel can probe ALPs with
masses between 4 and 60 GeV where the two photons from a !
are more easily resolved.
This is therefore the region where the 750 GeV scalar would give rise to a signal with four
photons instead of two, and is hence less interesting for us.
As the ALP mass is reduced, the photons coming from a !
get more and more
collimated. The ALP mass thresholds where the
nal state photons start to mimic the
diphoton signal depend on the mass of the mother particle and the resolution of the
detector. The azimuthal opening angle between the two photon jets coming from an initial
state Y ! a !
is given by
The angular resolution of the LHC detectors is
20 mrad [41, 42]. We then obtain,
using eq. (2.20) and putting mX = mZ , that the ATLAS limits on Br(Z ! 3 ) apply for
ma > 0:46 GeV. In the regime 0:46 GeV < ma < 4 GeV, then, the photons coming from
mimic the diphoton signal, but the photons coming from Z ! a
! 3 are
4ma :
mX
{ 8 {
(2.17)
(2.18)
(2.19)
(2.20)
S®gg OK
Bounded
ATLAS
or no bounds from photonic Z bosons are involved.
subject to ATLAS bounds. For ALPs lighter than 0.46 GeV, the photonjets from a !
are too collimated to be resolved at the LHC and the ATLAS limits on Br(Z ! 3 ) no
longer apply.
We turn now to the regime where CDF constraints on Z !
apply. Taking the
resolution of the CDF detector to be 120 mrad [39], we obtain ma > 2:7 GeV using
eq. (2.20). We show in gure 3 the ALP mass regions relevant for each constraint that we
have just discussed.
We now consider the constraints on the EFT parameters coming from photonic decays
of Z. The branching ratio of the new Z decay channel is
Br(Z ! 3 ) =
(Z ! a + )
1 + 8kG2G=(kBBc2w + kW W s2w)2
:
Neglecting the ALP mass in comparison to the Z boson we now have another constraint
on the parameters in the EFT Lagrangian
(kBB
+a gives rise to three photons signals, but for a light a a very boosted and
collimated photonjet also emerges from the a decay mimicking a diphoton signal. Thus,
(2.21)
(2.22)
{ 9 {
10 5 in the case where the CDF limit applies and 2:2
In this section, we put together all the constraints discussed in sections 2.1, 2.2, 2.4, and 2.3
for study the parameter space de ned by our EFT. We begin with some simpli cations, to
render the constraints amenable to a clear exposition. Moreover, all the points allowed in
the forthcoming results were checked against constraints from null signals in the ZZ, Z ,
W W and gg channels [8]. The Z
channel, in particular, was constrained with the recent
analysis of the CMS Collaboration [22] from which we infer that Br(S ! Z )=Br(S !
) . 2{3 at 95% CL. Of course, the ALP contribution to the diphoton signal makes it
easier to respect those bounds as we can always adjust the ALPScion coupling strength .
The general EFT parametrization of eq. (2.1) involves nine parameters: three couplings
(the k's) of the ALP a to the gauge bosons, three couplings (the c's) of the scalar S to the
gauge bosons, the mass dimensional coupling of the scalars , the ALP mass, and the new
physics scale .
This number can be reduced to seven assuming that both S and a are SU(2)L singlets,
cW W = kW W = 0. From now we consider just the singlet models to perform our analysis.
This is a well motivated simpli cation that will help us to illustrate how the constraints
that we are considering are important in models with axionlike particles.
We also x
= 1 TeV as this parameters always appears in ratios with the various
couplings to the gauge bosons. Now we have six parameters: kGG; kBB; cGG; R ; ; and ma,
where we have eliminated cBB in favor of R
de ned in eq. (2.11).
We can simplify eq. (2.14) for the claimed diphoton rate by assuming kW W = 0,
obtaining
cGG
1 + c84w
C
Let us now impose the constraints that we have discussed so far on the axionlike
models of the 750 GeV resonance in the EFT approach. We need to consider two regimes.
3.1
If ma is less than three neutral pion masses then the ALP decays exclusively to photons
and Br(a !
) = 1 as we have already discussed. We can thus take kGG = 0 in eq. (3.1).
This enables us to plot cGG as a function of the ALPScion coupling .
In gure 4, the solid black lines show the points on the cGG vs.
plane which t the
diphoton excess for
S = 5 GeV (upper two panels) and
S = 40 GeV (lower two panels),
with R
xed at 0.1 (left panels) and 0.9 (right panels). We have assumed ma < 0:4 GeV
and used eq. (3.1). Further, using eq. (2.16), we plot contours of the width
S on the same
plane, represented by the dashed lines. The intersections of the black solid line with the
colored dashed lines shows that there are solution points for a wide range of widths for
0.2
0.5GeV
1GeV
2GeV
0
plane: the solid black lines show the points on the cGG vs.
space which t the diphoton excess for S = 5 GeV,
= 2:5 fb (upper two panels) and
S = 40 GeV,
= 6 fb (lower two panels), with R
xed at 0.1 (left panels) and 0.9 (right panels). We have
assumed ma < 0:4 GeV. The dashed lines are contours of the width S.
each scenario. It is implicit in these plots that a large portion of the parameters space for
kBB and ma can t the diphoton signal.
We now turn to constraints on kBB, which come from the photonic decays of Z. From
eq. (2.18) and eq. (2.22), and setting kW W = kGG = 0, we immediately get upper and
lower limits on kBB independently of the other parameters. For ma < 3m 0
0:4 GeV,
the relevant limit is from the CDF search for photon pairs decays of the Z boson. In
gure 5 we show the allowed region of the kBB versus ma plane for ma < 0:4 GeV. The
yellow allowed region is bounded by the straight line at kBB = 0:07 and the black solid
curve. The region kBB > 0:07 is excluded from the CDF experimental constraint in this
mass region, after using the CDF experimental value in eq. (2.22). For a given ma there is
a lower limit coming from eq. (2.18), shown by the black solid curve, if we demand that the
ALP decays inside the calorimeter of the CDF detector. ALPs lighter than approximately
100 MeV cannot be bounded by the results of these experiments as they decay outside
the region of the electromagnetic calorimeter. The dashed lines represent the constraints
for 15 and 50 times stronger bounds from possible future searches. If the
LHC
nds no diphoton signal at the Zpole and the CDF limit can be made two orders
of magnitude smaller than the current bound, models with ALPs mimicking the diphoton
signal will be strongly disfavored if ma < 3m 0 .
0.07
0.06
large.
3.2
line at kBB = 0:07 and the black solid curve. The region kBB > 0:07 is excluded from the CDF
experimental constraint in this mass region, after using the CDF experimental value in eq. (2.22).
For a given ma there is a lower limit coming from eq. (2.18), shown by the black solid curve,
if we demand that the ALP decays inside the calorimeter of the CDF detector. ALPs lighter
than approximately 100 MeV cannot be bounded by the results of these experiments as they decay
outside the region of the electromagnetic calorimeter. The dashed lines represent the constraints
for 15 and 50 times stronger bounds from possible future searches. In this plot,
Of course, it is possible to invoke new decay channels of the ALP into charged leptons
and neutrinos. Investigating these cases could also be very interesting but hard. The
irreducible SM background for Z ! `+`
, where the photon comes from the bremsstrahlung
emission of the charged lepton, is expected to be much larger than the signal Z ! a
!
, and in both cases, the
nal state reconstructs the Z boson. Decays to neutrinos
would lead to monophoton signals which have been scrutinized by the experiments but, in
this case, the photon transverse momentum is not too hard, around half mZ . Actually,
as there are no bounds for Z !
+ ET , this channel could work if the branching ratio is
ma
3m 0 (kGG 6= 0)
When the ALP is allowed to decay into gluons we have another parameter coming into
play: kGG. However, as in the previous case, the constraints from Z decays to photons
inside the calorimeter region do not depend on cGG; cBB and , just on the couplings of
the ALP with the gauge bosons and the ALP mass. On the other hand, once we have
chosen an allowed point we can compute Br(a !
which ts the diphoton excess from eq. (2.10) for a given R .
) from eq. (2.9) and the product cGG
In gure 6 we display the allowed regions in the kGG vs. kBB plane for the wide and
narrow scenarios and for small (R
= 0:1) and large (R
= 0:9) ALP contributions
G 0.0 0.05 0.1
G
0.2
0.3
k
k
allowed by the CDF and the ATLAS constraints, ALPs decaying inside the calorimeter, and a total
width not larger than the tted experimental values. The dashed lines have cGG
xed in order
to t the diphoton cross section and width. The inset plots show the same allowed regions for ten
times stronger limits in a hypothetical future search for Z !
( ).
to the diphoton rate. The magenta shaded region represents the allowed region for ALP
masses where the CDF bound is relevant, for 0:4 < ma < 0:46 GeV. The green area shows
regions escaping the ATLAS bound for 0:46 < ma < 3:8 GeV. The dashed lines have
xed in order to t the diphoton cross section and width. Larger R
means larger
contribution from the axionic fourphoton channel which requires smaller kBB couplings
to t the diphoton signal.
The rst important point is that there is a maximum cGG
compatible with the
diphoton signal and still allowed by the experimental constraints and the upper limit on . In
the narrow scenario (the upper plots) this maximum product is 0:021. As cGG
decreases,
the region of allowed points shrinks. For example, for cGG
= 0:001, only a small
intersection for kBB < 0:1 and kGG < 0:1 survives if R
= 0:1 (the upper left panel). When
the ALP contribution to the photonic decay of the 750 GeV scalar is large, the bounds are
tighter and the intersections of the dashed lines with the allowed regions are even smaller,
as is evident in the upper right panel. In the large width cases displayed in the lower plots
of gure 6, the allowed regions are larger than those in the narrow width cases and the
maximum cGG
is around 0:17.
The inset plots show the projected allowed regions for bounds ten times stronger than
the present ones. An order of magnitude decrease in the attainable limits at the LHC
should be feasible in the near future. Couplings of the ALP with gauge bosons of order 0.1
and smaller could probed in all scenarios from small to large R
with either Z !
or
Z !
at the LHC.
4
A complex scalar
Having discussed in detail the constraints on the EFT, we now apply these ideas to a
concrete model in which S and a belong to the same complex scalar eld
= p12 (S+vS +ia)
which is a singlet under the SM gauge group. We consider a scenario in which this eld
has a renormalizable potential invariant under a discrete symmetry
, but breaking
explicitly an U(
1
)a symmetry only through quadratic terms m2[ 2 +(
)2]. This potential
was proposed recently in [43] as part of a model of the 750 GeV resonance which also
!
communicates with a dark sector. We also assumed that the interaction between
and
the SM is negligible, implying that there is no signi cant mixing of S with the SM Higgs
boson, so the potential is
V ( ) =
2
j j2 +
j j
4
m2[ 2 + (
)2]:
(4.1)
Taking 2 2 m2 < 0,
value h i = vS=p2 6= 0. As a result, S and a get masses mS = p
2
> 0, the minimum of this potential leads to a vacuum expectation
vS = 750 GeV and
ma = 2 m, respectively. The assumption that ma
mS is a natural one in the sense
that in the limit
m ! 0 increase the number of symmetries of the theory  the U(
1
)a
symmetry turns out to be exact having a as its NambuGoldstone boson.
In order to make the results of our analysis as general as possible, we do not specify
the interactions of
with extra particles but just take into account that, after
get a
vacuum expectation value, the e ective interactions of S and a with gauge bosons read as
In terms of the EFT parameters, the following associations can be made after symmetry
L
kBB SB
B
+ kBB
a B
~
B
+
kW W SW
i
W i +
SGa G
a
+
kW W a W
i
a G
b G~b :
cV V = kV V !
p2
=
r
2
mS
mS
kV V ;
(4.2)
(4.3)
Z®gg
CDF
5GeV
10GeV
20GeV
40GeV
eld: the left panel is the analog of gure 5. The
yellow shaded region is, as before, the region allowed by the constraints, and the dashed and dotted
lines, the projected limits from a future experiment. The right panel is the analog of gure 4. In
these plots we assume R
= 1 in the large width scenario.
with V = G; B; W . In this case, S and a have the same couplings to gauge bosons, that
is, kBB = cBB, kW W = cW W and kGG = cGG. For simplicity, we again consider that the
particles which are the mediators involved in the loop process generating the the e ective
interactions in eq. (4.2) are singlets under SU(2)L so that cW W = kW W = 0. We then have
four parameters involved in the analysis: kGG; kBB;
and ma.
If ma < 3m 0 we are able to constrain the product
2
kBB from eqs. (2.18){(2.22) as
and kBB in the allowed region of the parameter space, kGG can
be computed by requiring that
s
kGG =
4 2R
C
Let us present now the results for the case where S and a belong to the same complex
eld. We take R
= 1 as in ref. [43], i.e., the ALP contribution accounts for the entire
diphoton signal. In gure 7 we show, in the left panel, the 95% CL allowed region in the
kBB vs. ma plane in the wide scenario and, in the right panel, the points on the kGG
plane which explain the diphoton rate for xed widths .
If decays to gluons are allowed, kGG plays a role by decreasing Br(a !
) but, once
we have xed kBB and
in order to satisfy the bounds of eq. (4.5), kGG also gets xed by
imposing the tting of the diphoton signal from eq. (2.10) with cGG = kGG. The resulting
allowed regions in the large width scenario are shown in gure 8. For 0:4 < ma < 0:46 GeV,
the union of the green and magenta areas contain points of the parameters space where
the ALP decays inside the ECAL, t the diphoton signal, and are not excluded by the
limits on the photonic decays of the Z boson. The smaller green area represents the region
allowed by the ATLAS bound on Z decays and ma > 0:46 GeV. The inset plot shows the
0.15
0.1
B
B
k
= 1. The interpretations of the shaded areas and the inset
plot are the same as those of gure 6 for the EFT parametrization. The vertical dahsed lines in the
inset plot give the kGG coupling, as compued from eq. (4.5), which is necessary to t the diphoton
same allowed regions but for ten times stronger bounds in a projected future search for
( ). The dashed vertical lines show the values assumed by the coupling kGG in
those points of the parameters space.
Searching for a resonance at the Zpole
Cutting and counting
From the previous sections, it is clear that if the ALPScion connection is correct, we
should also see a
resonance at the Zpole coming from Z ! a . After rst discussing
the range of ALP masses which are most amenable to a preliminary search, we give details
of our simulations and results.
Referring to gure 3, we can see that ALPs with masses below 4 GeV but above around
0.46 GeV lead to diphoton signals from the S decay and to three photons signal from Z
decays. ALPs lighter than 0.46 GeV can mimic diphoton signals from S as well as Z !
since the photons from their decays get too collimated to be resolved in the LHC detectors.
Generally, for the entire range of masses between 0.4 GeV and 4 GeV, the two softest
photons are very collimated and a dedicated experimental study involving an accurate
estimate of cut and identi cation e ciencies, and photon isolation, should be performed
aimed to determine the conditions of operating near the angular detector resolutions.
To simplify matters somewhat, we restrict ourselves to the simulation of Z decaying to
two photons only, that is, where the ALP decays into photons pairs so collimated that they
cannot be resolved in the LHC detectors. We will therefore take benchmark ALP masses
below around 0.4 GeV, and assume what we believe are reasonable values for the fake jet
rate and the identi cation e ciency of the photonjet.
We simulated parton level events for signal pp ! Z ! a
!
, and the SM
backgrounds of continuum production of
, j , jj,
at Leading Order with MadGraph5 [44,
45]. After that we simulated parton hadronization and showering with Pythia6 [46] and
detector e ects with Delphes3 [47]. The photon isolation criteria was based on the
ATLAS experimental study of ref. [24]  rejecting events where particles fall within a cone
of radius 0.15 around the candidate photon with a deposit of energy larger than 4 GeV.
The acceptance cuts adopted are given by
for both photons.
photons invariant mass m
For ALP masses below 0.4 GeV, the majority of signal events contain indeed just two
photons. In order to select photons at the Zpole we imposed the additional cut on the
85 GeV < m
< 95 GeV :
constant Br(a !
as we see in
gure 6.
The signal cut e ciency is
30% while the background rejection is at least 0.02.
We present in
gure 9, the contour lines in the kGG
kBB plane where a 5
discovery
of Z bosons decaying to gamma rays is possible for 300, 1000 and 3000 fb 1 of integrated
luminosity, 0:4 < ma < 0:46 GeV where Br(a !
scenario. In the yellow and green shaded areas Br(Z !
) < 1, and R
) < 1:5
= 1 in the wide
10 5 respecting
the CDF bound and the total width of the scalar S is less than 40 GeV. The green area
represents a projected upper bound ten times stronger than the current CDF limit. We
immediately see that if the LHC is able to reach an upper limit of order 10 6, than it will
be very hard to discover these Z bosons in the LHC. The dashed blue lines are lines with
). The only e ect of decreasing R
is to enlarge the allowed region
We also found that the signal signi cance depends strongly on the fake jet rate and
the identi cation e ciency of the photonjet. The ability to reject jets faking photons is
crucial to reduce the j
and jj backgrounds which have 10 and 105 larger cross sections,
respectively, compared to the
background. In the plots we varied the probability of
a jet being taken as a photon Pj!
from 10 3 to 10 5. The other important tagging
factor is the photon e ciency of the photonjet. The two collimated photons will hit a
single calorimeter cell for very light ALPs and will be detected with an e ciency " . In
principle, this e ciency could be di erent from the single photon e ciency and a dedicated
experimental study or a very careful simulation of the detectors should be done in order
to estimate it. We thus chose to work with an optimistic factor of "
= " , where "
is the single photon e ciency taken as in the Delphes3 package, varying between 0.85
and 0.95 depending on the photon's transverse momentum and rapidity, and a pessimistic
factor 50% smaller than the optimistic one. In the three upper (lower) panels of gure 9
(5.1)
(5.2)
constraint from the CDF Collaboration and a total width not larger than the
tted value in the
wide scenario. The green area is the allowed region for a ten times stronger limit. The solid lines
are contour lines where a 5
discovery is possible with 300, 1000, and 3000 fb 1
. Dashed lines
have constant branching ratios of ALPs decaying to photons. We assume the same identi cation
e ciency for single photons and photonjets in the upper row of plots for three di erent fake jet
rejection factors from 10 3 to 10 5. In the lower row the photonjet e ciency is xed at the half
of single photon e ciency.
we are optimistic (pessimistic) about the photonjet e ciency. If events with jets faking
jets could be rejected at the 10 5 rate, we see in the left upper plot that it is possible to
discover the photonic decay mode of the Z boson with 300 fb 1 if kBB is not too small for
a given kGG. Of course, as kBB drops more luminosity is needed. The discovery becomes
increases. On the other hand, for "
= 0:5" , discovery will
= 10 5, and no discovery at all will be possible
if pj!
= 10 3
increasingly hard as Pj!
only be possible with around 1 ab 1 if pj!
. The plots for the narrow scenario are identical to these ones, but in a
smaller region extending itself up to 0.6(1.4) in the kBB(kGG) direction.
Now, if the ALP is lighter than 0.4 GeV, Br(a !
) = 100% and the discovery
becomes possible in the more optimistic scenario shown in the left upper panel of in gure 10.
In this case, with 300 fb 1, kBB couplings of 0.07 can be probed and up to 0.03 for 3 ab 1.
Again, we observe that if the LHC collaborations are able to exclude branching fractions
of Z bosons decaying to photons pairs of order 10 6, it will be very di cult to discover
this signal in this cutandcount approach.
5.2
Discovery estimate from an improved analysis for detecting photonjets
Identifying photonjets at hadron colliders has been an interesting line of investigation in
recent years [48{51]. In particular, concerning the diphoton excess of 750 GeV photons
kBB
kBB
P¶gj®gg==¶1g05
¶gg=0.5¶g
D8
cn 6
cn 6
D8
cn 6
cn 6
cn 6
resonance for ALPs with 100% decays to photons. We, again, x three di erent luminosities and
fake jet rejection factors, and two di erent photonjet e ciencies to illustrate our results as in the
previous plot. The meaning of the colors is the same of the previous gure.
pairs, in ref. [48] the authors show that is possible to discern between axionlike models
where the Scion decays to two pairs of pairs of collimated photons through the interaction
of a light pseudoscalar within an e ective model very similar to what we are considering
here. The idea is basically counting the number of photons conversions to e+e
pairs in
the inner detector. In the ATLAS, for example, four out of ten photons are expected to
be converted into electronpositron pairs and the ability to recognize such pairs as coming
from photons produced at the interaction point is very important to reach a high photon
identi cation rate. More photons hitting a given cell means more e+e conversions, so from
this basic fact it is possible to tell if more photons than the expected from single isolated
photons are being converted and, then, evaluate the likelihood of a model compared to
some di erent hypothesis. In ref. [48], an axionlike model of the 750 GeV resonance can
be distinguished from models where S decays promptly to two isolated photons at the
statistical level of 2(5)
with
30(100) events.
Of course, the very same technique could be used to increase the discerning power of
identi cation for photonjets from Zdecays. But not only this. As remarked in ref. [48],
there are other ways to tell if the detector was hit by a single isolated photon or a photonjet,
for example, by choosing appropriated photon isolation criteria or observing that in events
where just one of the two collimated photons is converted, the ratio (pT of the track)=ECAL
is not the expected from a single converted photon track.
A dedicated analysis in identifying photonjets in environments rich in QCD jets and
isolated photons was performed in refs. [50, 51]. In these works, samples of Higgs bosons of
120 GeV were assumed to decay to light scalars which, by their turn, decay to collimated
photons. Using substructure techniques and by training decision trees to an e cient
separation of signal and backgrounds events, a fakeQCD jet rate of order 10 4{10 5 and a
fakesingle photon rate of order 10 1{10 4 can be obtained depending on the mass of the
light scalar. The less e cient separation rates occur for the lighter scalars.
These misidenti cation tagging rates can be used for a rough estimate of how much we
expect that this kind of dedicated analysis facilitates the discovery of signals with photons
at the Zpole as suggested in this work. As Z bosons and 120 GeV Higgs bosons have more
or less similar masses, we expect that the kinematics of the photons are not too di erent
in both case. For example, as Z's and Higgses decay predominantly in the central region
with similar transverse momenta, the e+e
probabilities for each kind of event is similar
too. Then, based on refs. [50, 51], we x 0:1 as the fakesingle photon rate and 2
for the fakeQCD jet rate, and adjusting the Delphes3 photon ID e ciency to 80%. Cut
e ciencies are taken as the cut analysis performed in the later subsection.
We show in gure 11, the estimate of the discovery reach of the 13 TeV LHC to observe
at 5 in the upper plots assuming the EFT model of eq. (2.1), and the singlet
scalar complex of eqs. (4.1), (4.2) in the two lower plots. We immediately see that this
dedicated analysis with photonjets has a greater potential to enlarge the region of the
parameters space for discovery.
Contrary to the cutandcount analysis, improving the tagging e ciency for
photonjets allows us probe the currently permitted parameter space (the yellow shaded areas)
with 100 fb 1 as we see in the upper plots of gure 11 in the narrow and the wide scenarios
for the EFT model. For the concrete model presented in section 4, kBB couplings of around
0.5 can be probed with 300 fb 1 compared to 0.7 of the previous analysis as we see in the
lower plots of gure 11.
It is beyond the scope of this investigation to go much further into this direction, but we
believe that a dedicated study along the lines of refs. [48, 50, 51] could boost the discovery
prospects for photons at the Zpole at the LHC, be it related or not to the 750 GeV S.
Essential ingredients for this study are an accurate estimate of the photons rapidity and
transverse momentum distributions, once the probability of e+e
conversion depends on
these kinematic features of the event, and good discriminants against the Z ! e+e +
background, with a bremsthralung .
6
Conclusions
New physics beyond the Standard Model may be right around the corner with the
emergence of tantalizing signals of a new resonance in the
channel with mass around 750 GeV
in both ATLAS and CMS experiments in the LHC. If a new scalar is con rmed in this run of
pp collisions, a whole new dynasty of fundamental particles might be revealing themselves.
In this work, we have considered scenarios in which the 750 GeV resonance decays to
ALPs, which further decay to photons and mimic the signal. These models are motivated by
several factors. They come naturally in Hidden Valley scenarios, are able to accommodate
a large width of S within the perturbative regime, and open up connections to models of
cold dark matter.
As in the beginnings of the Standard Model, we are still ignorant about the interactions
responsible for the production of the scalar S and its decays to electroweak gauge bosons.
An EFT parametrization of couplings of S and a to capture looplevel interactions allows
one to constrain them. The interaction between S and a is assumed to be renormalizable at
0.6
0.4
B
kB0.3
cn 6
cn 6
the constraint from the CDF Collaboration and a total width not larger than the tted value in
the wide(narrow) scenario at the left(right) plot. The green area is the allowed region for a ten
times stronger limit. The solid lines are contour lines where a 5
discovery is possible with 100,
300, and 1000 fb 1
. Dashed lines have constant branching ratios of ALPs decaying to photons. The
identi cation e ciency and fakeQCD jet and fakesingle photons rates are given in the text. In
the lower plots, the yellow and green areas have the same meaning, but for the case of a complex
scalar interacting according to eqs. (4.1), (4.2) and ma
0:4 GeV.
the tree level and the decay channel S ! aa can be responsible for the large width of S. In
order to t the observed diphoton excess and evade the collider constraints from searches
of resonances in the ZZ, W W , Z
and gg channels more easily, an ALP a lighter than
a few GeV contributes to the diphoton signals through its own decays to very collimated
diphotons. If the mass of the ALP is small enough, those diphotons are emitted so colinearly
that they hit the same cell of the electromagnetic calorimeter of the detectors mimicking
a single hit. In this way, the process pp ! S ! aa !
with diphotons.
will lead e ectively to events
It is hard to think of UV completions, however, where the ALP couples only to photons
and not to the other electroweak gauge bosons. Gauge invariance would seem to dictate an
interaction of ALPs with at least the Z boson, in the case the ALP is an SU(2)L singlet.
A light ALP leading to collimated photons cannot decay to electroweak bosons, but a Z
boson can decay as Z ! a
!
. It turns out that in axionlike models of the 750 GeV
resonance, the branching ratio of the photonic decays of the Z boson is orders of magnitude
larger than what is expected in the Standard Model (
10 10) and can be so large that
collider constraints from the search of Z bosons decaying to photons need to be taken into
account.
In this work, we show that the parameters of an EFT description of the 750 GeV
signal get bounded by experimental limits on the two and three photons decays of the Z
boson from the Tevatron and the LHC. We also take into account that (i) the ALP should
decay into two collimated photons inside the electromagnetic calorimeter and (ii) the total
width of S is bounded by the current experimental best t value.
We consider several scenarios assuming narrow and wide resonances, ALPs decaying exclusively to photons when their masses are smaller than three pion masses and heavier ALPs with additional
gluonic decays, and dominant and nondominant S ! 4 decays compared to the S !
decays. For example, if Br(a !
) = 1, limits from the CDF Collaboration on Z !
impose an upper bound on the e ective ALPphoton coupling of 0.07. If the LHC pushes
this limit to a level 50 times stronger, almost all the parameter space of these models can
be excluded at 95% CL. We also show how a concrete model where S and a are the real
and the imaginary parts of a complex scalar, respectively, has its parameters bounded by
these experimental constraints.
The Z ! a
!
is a striking prediction of these kinds of models. It would be
natural, then, to look for photonic decays of the Z boson in the 13 TeV LHC. We estimate
the prospects to discover this decay mode of the Z assuming several photon detection
e ciencies for both single isolated and bunches of collimated photons. A simple
cutandcount analysis su ces to probe the allowed parameter space of the EFT models with
300 fb 1 with optimal photon detection e ciencies.
We also provide a simple estimate
based on the machine learning analysis of ref. [50] to better recognize events with
photonjets and
nd that with 100 fb 1 of accumulated data, observing photonic Z decays of
models tting the 750 GeV resonance is possible.
Acknowledgments
We would like to thank Andrew Askew, Teruki Kamon, Yann Mambrini, Paul Padley, and
David Toback. A. Alves and A.G. Dias acknowledge nancial support from the Brazilian
agencies CNPq, under the processes 303094/20133 (A.G.D.), 307098/20141 (A.A.), and
FAPESP, under the process 2013/220798 (A.A. and A.G.D.).
Open Access.
This article is distributed under the terms of the Creative Commons
Attribution License (CCBY 4.0), which permits any use, distribution and reproduction in
any medium, provided the original author(s) and source are credited.
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