Contributions of flavor violating couplings of a Higgs boson to pp → W W
Received: April
0 727 E. Third St. , Bloomington, IN 47405 , U.S.A
1 Physics Department, Indiana University
We study contributions to pp → W +W − → `ν``0ν`0 in models with a new Higgs boson, H, and a neutral lepton, ν4, with couplings H − ν4 − νμ and W − ν4 − μ through the process pp → H → ν4νμ → W μνμ → `ν`μνμ. Contrary to naive expectations, we find that contributions to pp → W W can be very large while satisfying constraints from standard H → W W and H → γγ searches. Even the excess observed by ATLAS in pp → W W , if taken at face value, can be easily accommodated. The various kinematic distributions fit nicely the experimentally determined ones. This scenario can arise for example in a two Higgs doublet model with vectorlike leptons.
Radovan Derm´ıˇsek; Enrico Lunghi and Seodong Shin

Contributions of flavor violating couplings of a Higgs
boson to pp
1 Introduction 2 3 4
Strategy of the analysis
Allowed parameter space and constraints from SM Higgs searches
Explanation of the ATLAS excess and kinematic distributions
A Detailed description of H
→ W W constraints
Recently, the ATLAS collaboration presented a measurement of pp → W W
tribution) is found to be [1]
This result has been obtained by using a nexttoleading order (NLO) Monte Carlo
generator (POWHEG [2–6]) to remove the effects of experimental cuts. The corresponding
NLO theoretical prediction in the Standard Model (SM) is [1, 7]
[σ(pp → W W ) + σ(gg → h → W W ∗)]th,NLO = 58.7+−32..07 pb
and deviates from the ATLAS result at 2.2 sigma level. Using the same Monte Carlo tools,
the CMS collaboration found [8]
σ(gg → h → W W ∗)th = 4.14+−77..28%% pb
the CMS result deviates from the NLO SM prediction at 2 sigma level. Several new physics
scenarios have been suggested to explain this excess [10–15].
and found [16]
This result has been obtained by using the POWHEG NLO Monte Carlo generator but
reweighting the simulated qq¯ → W W events by comparing with a parton level
nexttonexttoleading logarithm (NNLL) calculation in which logarithmic terms that contribute to the
W W transverse momentum (pTW W ) distribution are resummed [17] (see also ref. [18] for a
discussion of NNLL pjet resummation). This resummation mostly affects the calculation
of the jet veto efficiency (required to suppress backgrounds from tt¯ and W t production).
Comparing to the NNLO SM prediction [19]
σ(pp → W W )th,NNLO = 59.84+−21..29%% pb .
CMS finds agreement with the SM. One should point out, however, that the results in
eqs. (1.5) and (1.6) have to be compared with care. In fact, fully differential NNLO
ref. [17] is based on NLO matrix elements and considers pTW W rather than pjTet.
predictions for pp → W W are still not available; moreover, the resummation performed in
Note that, the authors of ref. [20] find that the NNL effects on the total cross section
and on the calculation of the acceptances tend to cancel each other. This suggests that
comparing the NLO based ATLAS measurement with the NNLO total cross section can
lead to a bias. Depending on which analysis one takes at face value, the deviation of the
pp → W W cross section with respect to the SM expectation is
This situation will be eventually resolved when better theoretical tools are available leading
to an unambiguous interpretation of the experimental results. Given the caveats above, we
take the results in eq. (1.7) as being compatible with new physics contributions at the 10
pb level either as an explanation of an excess or as a two sigma upper limit.
In this paper we consider contributions to pp → W W
In order to fix the production cross section, the Higgs boson is assumed to have SM
couplings to fermions. Thus it is dominantly produced in the gluongluon fusion channel
with the usual SM strength. We further assume that H has no direct coupling to the W
boson. This situation arises for example in two Higgs doublet model, in which the light
CPeven Higgs boson if fully SMlike in its couplings to gauge boson and thus the heavy
CPeven boson has no direct couplings.
The new neutral lepton can originate from extensions of the SM that include new
vectorlike lepton families, both SU(2) doublets and singlets. Mixing of vectorlike leptons
branching ratios, see for example the discussion in refs. [21, 22] (where the focus was on
the charged lepton sector).
obtain contributions to the ee mode as well, if desirable, one can introduce an additional
neutral lepton that couples exclusively to the the first generation of SM leptons. In fact,
The main focus of this paper is to show that the models we consider can provide very
discrepancy in eq. (1.7). The crucial reason for this scenario being able to generate such
cross section of order 10 pb with the fact that we have only one W that is required to decay
leptonically. For instance, an excess in W W production of about 10 pb corresponds to
O(0.1) pb in the dilepton final state. Having only one W boson in our process implies
up to O(1) pb contribution to the dilepton final state. Therefore we are in an excellent
position to explain a significant excess or to place strong constraints in large ranges of
masses and branching ratios.
We postpone the discussion of a concrete implementation of these ideas in a complete
extension of the SM (e.g. a Two Higgs Doublet Model augmented with vectorlike leptons)
to a forthcoming publication [23].1 The simplified model we consider allows us to present
results in a particularly simple way, in terms of the Higgs mass, mH , the neutral heavy
that can be applied to a variety of specific models.
We require that the simplified model satisfies constraints from searches for H
alization of the model is. The Higgs boson is assumed to be produced dominantly through
the top loop and thus the same loop generates corresponding contribution to H
Similarly, although we assume no direct coupling of H to W , the process in eq. (1.8)
contributes to the same final states as H → W W would. We do not impose any constraints on
constraints on pair production of new leptons from searches for anomalous production of
multi lepton events, discussed in ref. [28], highly depend on the SU(2) doublet component
in this paper do not depend on these further assumptions.
The paper is organized as follows. In section 2 we introduce the concept of fiducial cross
section and describe our event generation procedure, in section 3 we discuss contributions
of our scenario to pp → W W measurements and the interplay with constraints from the
Higgs searches, in section 4 we present actual kinematic distributions for a reference point
chosen to fit the nominal excess found by ATLAS and in section 5 we present our concluding
1A similar process appears in TeV seesaw neutrino models [24].
First of all, it is important to realize that the experimental cuts chosen by ATLAS and
CMS have different impacts on the SM and on a generic new physics (NP) model. We
quantify this statement in terms of acceptances defined as fractions of events that pass
experiments is the socalled fiducial cross section defined as the product of the total cross
section and the acceptance:
In our case, the fiducial cross sections are given as:
σSfiMd = σpSpM→W W BR(W → `ν`) BR(W → `0ν`0 ) ASM ,
σNfidP = σpNpP→H BR(H → W `ν`) BR(W → `0ν`0 ) ANP ,
combinatoric factor of two in eq. (2.2). ASM and ANP are the acceptances corresponding
we focus on the ATLAS analysis [1] because it explicitly presents results for the fiducial
cross sections and allows us to avoid detector simulations.
In order to allow for an easier interpretation of new physics contributions we define
the following effective pp → W W NP cross sections:
where the denominators are constants. Although we do not have two W bosons in the final
state, this quantity can be directly compared to the ATLAS and CMS results summarized
acceptances, therefore our results can be directly applied to any future analysis for which
this ratio is the same as in the ATLAS study.
extra combinatorial factor of 2) the need to reduce DrellYan background for same flavor
dileptons requires much stronger cuts. The last column of table 1 gives the corresponding
generated event set for the process in eq. (1.8). We use the MadGraph5 [29] event generator
and handle parton shower with Pythia6 [30] (implemented in the MadGraph5 pythiapgs
−25.6
−8.0
−7.1
−14.3
−2.7
−2.9
−32
−8.5
−7.8
−5.8
−8.6
−7.3
that we consider (labelled by their mˆH value).
package). The resulting StdHEP event files are then converted into CERN root format
using Delphes [31]. The analysis, that we perform at the shower level, is implemented as a
root macro. Jet clustering is handled by the FastJet package [32, 33]; we adopt an antikt
The cuts and the lepton and jet isolation requirements are explicitly given in ref. [1].
The leading (subleading) leptons in the fiducial region are required to have transverse
Dilepton invariant masses, m`` larger than 10
defined as pTm,iRssel = pmiss×sin (min{Δφ, π/2}) where Δφ is the azimuthal angular difference
T
between p~ miss and the closest jet or lepton in the event. Lepton 4momenta are corrected
reconstructed from the electron. In addition to these event selection requirements proper
triggers and lepton isolation are also implemented.
Allowed parameter space and constraints from SM Higgs searches
W W CMS search presented in
refs. [25, 26] where a number of different cuts, each optimized to be sensitive to a
SMlike heavy Higgs of a given mass, are considered. For each cut (that we label H) CMS,
effectively, places a 95% C.L. upper limit on a fiducial cross section:
implied by the CMS H → W W searches summarized in eq. (3.2). Branching ratios corresponding
eq. (2.3) and ANHP is the acceptance for the cut selection H. Since CMS does not present
the results of the analysis in terms of fiducial cross sections, the extraction of these upper
technical details to appendix A. In the table we consider six CMS analyses (labelled by the
value mˆH of the Higgs mass for which each analysis is optimized) and present separately
are topologically different. The cuts adopted in refs. [25, 26], not being optimized to our
summarized in table 1 while simultaneously satisfying the limits from H → W W searches.
The main results of our study are presented in figure 1. In shades of blue we plot
the values of the branching ratios BR(H
for the transverse mass mT = q
consider in figure 1), the effective cross section contours are essentially controlled by the
contours we find is correspondingly more complicated.
The shape of the H → W W constraints (yellow contours) can be understood as follows.
be very large. This implies a much larger missing energy Emiss and, in turn, a larger value
2p`T`Emiss(1 − cos ΔφEmiss,``), where p`T` is the transverse
T T
E~ miss and p~T``. Since H → W W searches focus on large mT values, points in this region
T
have larger ANHP acceptances and are more easily excluded. A similar argument explains
chain, therefore larger heavy neutral lepton masses imply larger p`T` and, in turn, larger
mT , strengthening the impact of the H → W W constraint.
stronger constraints than direct searches for heavy Higgses decaying to W W . For instance,
matively the value of the ATLAS excess or the two sigma upper limit implied by the CMS
contributions to the effective cross section are excluded at 95% C.L. by the H → W W
different effective cross sections can be easily obtained by simple rescaling.
Let us now discuss the bounds that we obtain from searches for heavy Higgs bosons
decaying to two photons [27]. In our simplified model independent analysis we work under
the assumption that the light Higgs is purely SMlike and that the heavy Higgs H has
no direct coupling to the W boson. This implies that H
in our scenario is controlled by the top loop and is, therefore, much smaller than the
our scenario the Higgs production cross section is fixed to its SMlike value [9] while the
a thick red line. We see that present experimental bounds do not constrain our model.
Explanation of the ATLAS excess and kinematic distributions
Let us now consider the excesses observed by ATLAS at face value with the understanding
that NLL effects (as hinted by the CMS study) might reduce it sizably. In order to fully
clusively to the first generation of SM leptons. For simplicity, we assume that the couplings
we consider are:
The results for this scenario can be directly obtained from those presented in figure 1.
W e). As we pointed out above, we see that in a large region of masses
and branching ratios we can easily explain the excess observed by ATLAS while satisfying
constraints from H → W W . In particular these are the regions above the yellow contours
corresponding to the central values given in the last column of table 1 (about 13 and 10
The next important step is to check whether the kinematic distributions that we obtain
for points in the allowed regions match the observed ones. In ref. [1] the following seven
quantities are considered: transverse momentum, pT , of the leading and subleading lepton;
WW
s
ten600
v
pT (`` + Emiss), of the dilepton plus missing transverse energy system.
T
In order to compare with the experimental results presented in ref. [1], we convert our
(differential) fiducial cross sections into number of events:
p (subleading lepton) [GeV]
Kinematic distributions for pp
section is about 75%
of the required contribution.
The acceptance
The effective cross
The = 0.62 (see table 5 of ref. [1]).
WW Zjets Wjets
and 4 for further details.
is that, in this way, we can easily assess statistical uncertainties on our signal in reference
to the observed excess.
obtain are presented in figures 3–5. The product of branching ratios has been chosen to
estimates are taken directly from ref. [1] while the new physics signal is simulated with
MadGraph5 interfaced with Pythia (with detector effects taken into account via the factors
CW W ). As a consistency check we simulated pp → W W events using the same framework
we use for the signal and found that all kinematic distributions agree fairly well with those
presented in ref. [1] (obtained using POWHEG interfaced with Pythia).
Possible additional freedom from considering different masses and branching ratios
separately and to tweak various kinematic distributions. At present, this is however not
necessary since ATLAS finds similar effects in both modes  see, for instance, table 1, and
all the kinematic distributions are fit nicely with the assumptions of universality in masses
Direct inspection of these figures shows that all the kinematic distributions we
consider agree perfectly with the ATLAS observations with the exception of few bins in the
The choice of the reference point has
significant excesses). In the new physics process we consider, contributions to mT are
bounded from above by the mass of the heavy Higgs. ATLAS data show a large excess
of the heavy neutral lepton has a subleading effect on mT but controls entirely the pT (``)
results with different choices of masses.
In most of the preferred region in figure 1 all the kinematic distributions except mT
typically agree well with observations for proper choices of BR(H
→ W `ν`). For Higgs
masses heavier than 160 GeV, mT still agrees fairly well with the observed excess but the
distribution broadens and moves to somewhat higher mT bins.
Finally we note that if at least one of the heavy neutral leptons is lighter than the
case both H and hSM contribute to the excess through the same decay chains. The mT
distribution of the SM Higgs contribution is localized at low energies (mT < 125 GeV)
while the other distributions are not significantly affected. While we checked that the hSM
contribution alone is unable to fully account for the W W excess, the combination of H
and hSM should clearly work better than H alone.
can be very large and thus the cross sections measured at ATLAS [1] and CMS [16] offer
powerful constraints. This scenario is able to generate large contributions because of a
Higgs production cross section of order 10 pb coupled with having only one W in the final
state (and hence avoiding the double W leptonic branching ratio suppression).
Contrary to naive expectations, we find that in a wide range of masses and branching
ratios the present sensitivity of pp → W W measurements offers stronger constraints than
weakened by the strong suppression of the ratio of acceptances ANHP/ANP (see eq. (3.2)).
couplings to the SM gauge bosons. Our main results are summarized in figure 1.
In addition we selected a representative point in the parameter space for which the NP
contribution to the fiducial cross section matches roughly the excess observed by ATLAS.
We studied several kinematic variables and found that all observed distributions can be
easily accommodated in our scenario.
Future experimental updates of pp → W W measurements will be crucial to test the
class of models studied in this paper. Furthermore, in the region of the parameter space in
which we have significant contributions to W W , our scenario necessarily predicts deviations
the near future.
In addition to the results presented in this paper, we also investigated several
altermissing energy, thus lowering the pT of the charged leptons. This makes it hard for these
events to pass the ATLAS and CMS selection cuts and we found that the resulting rates
found that it also yields small cross sections.
The new neutral leptons can originate from extensions of the SM by vectorlike leptons,
both SU(2) doublets and neutral singlets in a two Higgs doublet model framework. In any
specific model there are additional constraints on masses and couplings of the new leptons.
These include constraints from electroweak precision data and from pair production of new
leptons from searches for anomalous production of multi lepton events, discussed in ref. [28].
We will discuss an explicit scenario along these lines in a forthcoming publication [23].
Acknowledgments
E.L. thanks Giulia Zanderighi and Stefano Pozzorini for useful discussions and
clarifications. This work is supported in part by the Department of Energy under grant number
Detailed description of H
W W constraints
In refs. [25, 26] CMS considered a large number of different cuts each optimized to be
sensitive to a SMlike heavy Higgs of a given mass. The signal we consider here (H →
and cuts optimized for SM Higgs hypotheses are not in general optimal for our process.
consider the constraints implied by each CMS analysis and take the strongest bound we
obtain. Each CMS analysis, that we indicate by H, has been optimized for a given SM
Higgs mass hypothesis mˆH .
The number of surviving new physics events for our signal (NNHP) and for a SM like
heavy Higgs (NSHM) that we expect are given by
and (2.3), ANHP and ASHM are the corresponding acceptances, L is the total integrated
luminosity and CH encapsulates detector efficiencies. We consider the mass mˆH in eq. (A.2)
because all the quantities that thus appear are explicitly given in refs. [25, 26] and we are
therefore able to extract the product CHL. Combining eqs. (A.1) and (A.2) we obtain
NSHM( mˆH ) .
The next step is to extract the upper limit at 95% C.L. that the observed number of events
(NeHxp) and the complete (including the 125 GeV Higgs) SM background (NbHkgd) imply. We
follow a standard CLs method (see appendix D in ref. [35] for a detailed description of the
technique) at obtain the upper bounds NNHP < `9H5 (we add a 30% systematic uncertainty
to our signal in addition to the standard gaussian statistical error). Therefore, the upper
limit on the new physics cross section is
≡ min
NSHM( mˆH )
from refs. [25, 26] and are listed in table 2. The implied upper limit on the fiducial cross
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