Bounds on TeV Seesaw Models from LHC Higgs Data

Bounds on TeV Seesaw Models from LHC Higgs Data

P. S. Bhupal Dev, Roberto Franceschini and R. N. Mohapatra Maryland Center for Fundamental Physics and Department of Physics, University of Maryland, College Park, MD 20742, USA.
Consortium for Fundamental Physics, School of Physics and Astronomy, University of Manchester, Manchester M13 9PL, United Kingdom.
Abstract

We derive bounds on the Dirac Yukawa couplings of the neutrinos in seesaw models using the recent Large Hadron Collider (LHC) data on Higgs decays for the case where the Standard Model (SM) singlet heavy leptons needed for the seesaw mechanism have masses in the 100 GeV range. Such scenarios with large Yukawa couplings are natural in Inverse Seesaw (ISS) models since the small neutrino mass owes its origin to a small Majorana mass of a new set of singlet fermions. Large Yukawas with sub-TeV mass right-handed neutrinos are also possible for certain textures in Type-I seesaw models, so that the above bounds also apply to them. We find that the current Higgs data from the LHC can put bounds on both electron- and muon-type Yukawa couplings of order .

preprint: UMD-PP-012-012

I Introduction

Understanding the new physics behind small neutrino masses is a major current focus of particle theory research. The simplest scenarios for this are the various types of seesaw mechanisms, which have also formed the basis for understanding the observed mixing pattern, which is so different from the quark sector. Two of them, type-I seesaw and the inverse seesaw inverse postulate the existence of heavy SM singlet fermions, generally denoted by the symbol ( is a generation index) in addition to the SM fermions. The stability of these patterns can be guaranteed by extending the SM gauge group as we discuss below. These singlet fermions couple to familiar lepton doublets via Yukawa couplings of the form , which after electroweak symmetry breaking leads to the Dirac mass of the neutrino, which is expected to be in the GeV range. In the type-I seesaw, the tiny neutrino mass arises once we introduce an additional Majorana mass term for the ’s. Being SM singlets, these Majorana particles can take arbitrarily large mass values, which then guarantees that the neutrino mass given by the formula below is small:

(1)

Note that since the scale of neutrino masses is known to be in the sub-eV range, the scale is correlated with the magnitude of the Yukawa couplings . For instance, if the masses are in the 100 GeV range, it implies or so. However, for specific flavor pattern of both and , tiny neutrino masses can be realized without making necessarily tiny. Examples of such theories are given in KS ; others .

On the other hand, in the case of inverse seesaw models, in addition to the set of singlets , one adds another set of singlet fermions which form a Dirac mass with the fields, i.e., . The model so far has conserved lepton number and leads to zero neutrino mass WW . One then allows the fields to have a Majorana mass matrix whose overall scale is in the keV range inverse . The neutrino mass formula in this case is given by:

(2)

This also leads to tiny Majorana mass for neutrinos. The small scale of the matrix can be explained in various ways fong and is a window to further new physics. In this model since the small neutrino masses arise from the small values of matrix elements, the singlet neutrinos form a Dirac pair with masses in the 100 GeV range and yet have Yukawa couplings of order without any fine-tuning.

A key question for neutrino mass physics is how to test the seesaw mechanism han . The obvious first step would be to determine the couplings and . The vast literature on neutrino mass models is devoted to precisely this question where additional theoretical assumptions such as symmetries are used for this purpose. The symmetries restrict the theory to a particular sub-space of the full parameter-space. If the seesaw scale is high, as is the case in most type-I models, this appears to be the only realistic possibility. We take a different approach in this paper. Since denotes the coupling of the -fermions to the Higgs boson, if is in the 100 GeV range, experimental information on can be obtained from the LHC data on the Higgs boson decays carp . Our goal in this paper is to focus on this question.

In this paper, we consider primarily the inverse seesaw case and we will comment on the specific type-I scenarios where our results will apply with slight changes. The main result of the paper i.e. bounds on the the Yukawa couplings , is derived from an analysis of the 7-TeV LHC data. The 8-TeV data came out after this work was finished. However, we have redone our analysis with this new data and found that the change in the bounds is very minimal. We comment on it briefly at the end of the paper.

This paper is organized as follows: in Section II, we discuss the minimal gauge models with inverse seesaw; in Section III we give an overview of the Higgs phenomenology of the seesaw models; in Section IV we shall illustrate how to constrain the parameter-space in seesaw models using the results of the Higgs searches in the final state ; in Section V we derive these constraints on the seesaw models by analyzing the rates of all the measured decay modes of the Higgs boson; the concluding remarks are in Section VI. In Appendix A, we list the relevant decay widths of the heavy neutrino.

Ii Gauge models with large Yukawa couplings and sub-TeV singlet fermions

In this section, we briefly discuss two classes of model which can have Dirac Yukawa couplings and GeV mass for the singlet fermions responsible for the seesaw mechanisms111There are other classes of models, e.g., linear seesaw linear and double seesaw inverse ; ma09 models which can also have this feature.. One class, as noted above, is the inverse seesaw model, where this situation can be naturally realized, and a second class, which belongs to type-I seesaw-based neutrino mass models where with specific texture for the Dirac Yukawa matrices as well as for the singlet fermion mass matrix, one can have LHC-accessible parameters.

ii.1 Inverse seesaw gauge model

If we simply add two heavy SM singlet fermions to the SM, there are many gauge invariant terms in the potential and we will not get the simple inverse seesaw formula. The simplest extension of SM where the inverse seesaw formula arises naturally is with gauge symmetry under which the singlet fermions transform as and fields are singlets. The gauge symmetry is broken by a Higgs field acquiring vacuum expectation value (vev) and the SM-doublet vev then breaks the gauge group down to electromagnetic . The Yukawa Lagrangian for the lepton sector is given by:

(3)

It is clear that once we substitute and , we get the inverse seesaw matrix inverse

(4)

with the mass matrix for the neutrinos given by Eq. (2) with (Note that are in general matrices). At the renormalizable level, there are no other terms allowed in the Yukawa Lagrangian by the gauge symmetry and therefore the above inverse seesaw formula for neutrinos is stable under radiative corrections.

Our goal is to find constraints on as a function of . There exists an extensive analysis on the constraints for light singlet fermions atre . Although this analysis addresses only the constraints on Majorana neutrinos, some of their results apply to our case as well. However, the limits derived in atre for GeV or so are very weak. Furthermore constraints from neutrino-less double beta decay beta derived on heavy sterile neutrinos do not apply to this case since in our model, the and form a pseudo-Dirac pair and lepton number is almost exactly conserved.

In order to use the LHC data to explore constraints on and in the 100 GeV range, we will assume that (i) and (ii) the mass of Re() is heavy compared to the SM Higgs boson so that neither the heavy gauge boson associated with -symmetry nor the interactions of Re() affect the Higgs boson decay modes we consider.

It follows from the above Lagrangian that if one of the singlet fermions has mass in the 100 GeV range, it will affect the Higgs branching ratios: for instance if , then this opens up a new mode for SM Higgs decay, i.e., , and the collider signal will arise from mixing diagram in Fig. 1 where . Folding decays, one will get final states with where in the final state both charged leptons and anti-leptons will appear and the existing LHC data on these final states will provide constraints on . Clearly, which charged lepton appears will depend on the flavor structure of and . For we will go to a basis so that it is diagonal, i.e. a linear combination of and are mass eigenstates with field providing the chiral Dirac partner.

Figure 1: The Higgs decay modes into mediated by the ISS couplings.

ii.2 Type-I seesaw case

Turning to the type-I case, as noted earlier, in generic models, the Dirac Yukawa couplings are very small for the seesaw scale in the TeV regime. However, for specific textures for , it is possible to attain singlet fermion mass in the 100 GeV range with Dirac Yukawa ’s of order while still satisfying the neutrino oscillation data. In this case the singlet fermions could show up at the LHC. Two examples of this type of texture are KS :

(5)
(6)

with the constraint that where are the singlet neutrino masses and the singlet fermion mass matrix is diagonal. Seesaw of this kind leads in the leading order to zero neutrino masses; small can then generate the neutrino masses and mixings, and this will not affect the Yukawa couplings or the masses .

A second example has of the following type KS :

(7)

with right handed neutrino mass matrix of the form

(8)

with . In the limit of the neutrino mass matrix is a null matrix regardless of the values of and and small neutrino masses arise from choosing ’s and ’s small. To the best of our knowledge, we are not aware of any symmetries that will guarantee these textures for the and matrices and it will be interesting to seek symmetry origin for them . Therefore again in this case, LHC data should put constraints on vs . Since in this case, ’s are arbitrary, they could be of democratic type which will mean that there are multiple flavor final states.

Two important points regarding these models are worth emphasizing. In both models, as the small parameters ( in the case of inverse seesaw, and and in the type-I model discussed above) go to zero, the neutrinos become massless. As a result, the neutral Higgs decays to two modes: . Unlike the case where both and are Majorana fermions (as in generic type-I seesaw models), the Higgs decay rate to final states becomes twice as large. Thus, the discussions of Higgs decay in both classes of models are similar except for the flavor richness in case (B) compared to case (A) . In case (B) model (II), for , all flavors couple with equal strength to a single fermion flavor implying that final state signal becomes a combination of all three flavors. Similar situation also occurs for case (B), model I if .

Iii Seesaw Higgs phenomenology

As discussed above, a new Yukawa interaction in the leptonic sector can potentially alter the Higgs phenomenology in a dramatic way. In fact, in the models considered here, the Yukawa coupling of the neutrino can be sizable – in principle, much larger than the largest Yukawa coupling involved in the decay of a light SM Higgs boson, i.e., the one with the bottom quark which is of order .

Here we shall describe the observable consequences of the new Yukawa coupling. Focusing on the inverse seesaw case, it is apparent from the neutrino mass formula in Eq. (2) that neutrino mass fits will largely be dictated by the combination of the flavor structure of the matrix and the -matrix.

In quark-lepton unified theories, the natural expectation is that flavor mixings in are “weak”. So we can assume to be a diagonal matrix. The next question is the relative magnitude of different flavor Yukawa couplings. Below we will consider the cases when (a) , (b) , and (c) a democratic form where all the Yukawa couplings are equal. These different coupling structures imply distinctive flavor structure of the final states. From the interaction Lagrangian, Eq. (3), it follows that mediates the decays illustrated in Fig. 1:

(9)

where charged and neutral leptons arise in all the flavor combinations allowed by the form of the matrix. The gauge bosons and arise in the decay of through the mixing, that is . The gauge bosons in turn can decay into leptons or hadrons, such that the new Yukawa coupling contributes to the rates of both fully leptonic and semi-leptonic final states.

The heavy particles , and in the decay chain of Eq. (9) can be either on-shell or off-shell depending on the mass of . In particular, when the Higgs decay width in Eq. (9) scales as while for , it scales as .

The fully leptonic decay mode of the Higgs is among the most sensitive search channels of the Higgs and therefore is a very sensitive probe of the -coupling, especially for not far above or below . This allows us to derive a bound on the parameter space as described in detail in section IV.

For definiteness in what follows, we shall focus on the case of the ISS model with only one light flavor of heavy neutrino which we choose to be the electron flavor, , and the corresponding Yukawa coupling to the Higgs doublet. We generically denote the mass of the heavy neutrino as , suppressing the explicit flavor index .

In this scenario the decay chain of Eq. (9) becomes

(10)

The decay of the boson to leptons produces pairs of charged leptons of all flavors, and hence, it contributes to the same flavor final states, namely

The decay of the boson produces a single charged lepton. In this case, the is produced by a mixing effect of the heavy , and therefore, the is always produced in association with an electron or a positron. Hence, the decay process mediated by the only contributes to electron final states, namely

Altogether, due to the assumed coupling structure , the final state leptons of the Higgs decays mediated by the ISS are both of opposite flavor and of same flavor, with same flavor getting its only contribution from processes with -boson intermediate state.

For the other coupling patterns the resulting flavor structures are straightforward modifications of the one considered here. For the pattern (b) where , the main difference is that now it is the same flavor final state that gets its only contribution from the -intermediate state. The opposite flavor final states again get contribution from both - and -mediated processes, though with the kinematics of and exchanged.

As in our analysis described in the next sections there is going to be little difference between and , the bounds derived for the ISS with light also apply for the case of light .

So far we considered a basis in which the matrix is diagonal. There are however possible situations where one can derive bounds on Yukawa couplings. For instance, it is possible to have structure of the and matrices where democratic Yukawa couplings can emerge i.e. by assuming an appropriate texture for the Dirac mass matrix we can have the lightest heavy neutrino as a linear combination of all the three flavor states with equal probability, i.e., . In this case, if is a unit matrix, then we expect all the possible flavor combinations to be equally populated as both the interactions of and the gauge bosons are flavor-universal. The analyses of the LHC experiments are mostly sensitive to and flavors only, therefore for such a democratic case, only 4 out the 9 possible final state flavor combinations are easily detectable 222In principle, the leptonic decays of the may be used to recover some sensitivity. However, they are only a fraction of the decay modes, and there are detection inefficiencies. As such, the gain might not be huge. At any rate, the use of the ’s requires careful experimental considerations that go beyond our competence; hence we prefer not to consider them.. Furthermore the effect of the mixing of flavors in the lightest mass eigenstate in general reduces the rate of and leptons. Because of these effects the bounds should be recalculated carefully to account for the different flavor structure of the signal.

However, neglecting the sub-dominant processes mediated by the , one can obtain an estimate for the bound in the democratic case. Indeed, the rates of the processes mediated by the are just rescaled by mixing and multiplicity factors compared to those of the ISS model with only a light . As such, the bounds of the ISS case should be relaxed by a constant factor. In particular, from the fact that the rate of Eq. (10) scales as when and as when , we find that the bound gets relaxed by a factor or in the two cases.

In the following, we shall illustrate in detail how to derive a bound on the Yukawa coupling in the ISS model. Using the same procedure, we also computed the bound for the type-I democratic case and we checked that this simple rescaling argument describes the actual bound to a very good accuracy. Hence, we give only the bounds for the ISS case, and those for the democratic type-I case can be obtained by the simple rescaling described above.

Of course a more targeted analysis from the LHC experiment with flavor structure explicitly taken into account could distinguish between these coupling structures. For instance, an imbalance between the and same flavor final states can distinguish case (a) and case (b), while case (c) would be favored if all the flavor combination appear to be equally populated.

Irrespective of the flavor structure of the Yukawa interactions, the new Yukawa coupling mediates new decay modes of the Higgs, which contribute to its total width, and hence, in these seesaw models, the Higgs width is larger than in the SM:

The seesaw contribution to the the total width is necessarily model-dependent. For the case of the ISS with dominant coupling , the width 333This formula sums and . is given by

(11)

where we have normalized the Yukawa couplings such that the Dirac mass terms of all fermions are for . In the case of the democratic type-I seesaw, as in Eq. (7), we can take ; hence the Lagrangian essentially contains three interactions of equal strength for the Higgs decay, and the Higgs decay width is the same as in the ISS model once is taken.

In Section V we shall discuss in detail how to use the information on the measured rates of the several Higgs decay final states to put a bound on the seesaw coupling.

Iv Bounds from the search of the Higgs boson in the final state

The ATLAS and CMS collaboration have both found evidence for a Higgs-like particle at around 125 GeV. The main evidence for the new particle comes from final states with resonant two photons or four leptons HiggsJuly2012 ; CMS-PAS-HIG-12-020 ; ATLAS-CONF-2012-093 . In addition to the and searches the LHC and TeVatron experiments searched for a SM Higgs boson in several other final states, including in the final state CMS-Collaboration:2012vn ; CMS-PAS-HIG-12-017 ; ATLAS-Collaboration:2012kx ; The-CMS-Collaboration:2012pd ; ATLAS-Collaboration:2012mz .

The experiments presented cut-based as well as multivariate analyses to put a bound on the mass of the SM Higgs boson. The bounds from the multivariate analysis are generically (slightly) more stringent than the one obtained from the cut-based analysis. However the multivariate analysis cannot be easily reproduced with our means, and hence, we shall only use the cut-based analysis to derive our bound.

Here we shall reinterpret the results of CMS-Collaboration:2012vn to extract a bound on extra sources of events. To do this, we shall repeat the cut-based analysis of CMS-Collaboration:2012vn on event samples generated by Monte Carlo tools – matrix elements computed with Madgraph5 Alwall:2011fk , showered and hadronized with PYTHIA6.4 Sjostrand:2006zr and detector response parametrized by Delphes1.9 Ovyn:2009ys . Hadrons have been clustered into jet with the anti-kT algorithm as implemented in FastJet2 Cacciari:2011rt ; Cacciari:2006vn .

In CMS-Collaboration:2012vn , the CMS collaboration performed several analysis on the sample collected in the year 2011 with the LHC running at TeV center-of-mass energy. In particular they made a basic selection on the leptons, jets and missing energy of the events depending on the flavor of the final state leptons. The cuts for the opposite flavor (OF) and same flavor (SF) cases are reported in Table 1. Then the analysis is specialized for specific values of the SM Higgs boson mass and further cuts are devised. These cuts are collected in the Tables 2 and 3 for the Higgs mass hypothesis of GeV and GeV, respectively. These analyses, which we call and , are the most sensitive to new physics connected to the Higgs-like particle recently discovered, and hence, these are the only analyses we are going to repeat in order to extract our bound.

Baseline selection for all
,
,
mET 20 GeV mET40 GeV
GeV GeV
GeV
Table 1: Baseline selection of the CMS cut-based analysis, as in CMS-Collaboration:2012vn .
Analysis (tailored for SM Higgs GeV)
GeV GeV
GeV
GeV
GeV

Table 2: Cuts added to those in Table 1 in the analysis devised in CMS-Collaboration:2012vn for the hypothesis GeV.
Analysis (tailored for SM Higgs GeV)
GeV GeV
GeV
GeV
GeV
Table 3: Cuts added to those in Table 1 in the analysis devised in CMS-Collaboration:2012vn for the hypothesis GeV.

To obtain a bound on the ISS model we impose that the events yield of the ISS after the cuts of both the analyses and is not larger than the total number of events allowed by each of the analysis. This is done as follows: for each hypothesis for in the ISS model we compute the yield of events after the cuts of CMS. This depends on the mass and coupling and it is given by

where is the integrated luminosity used in the analysis, is the total Higgs production cross-section taken from  LHC-Higgs-Cross-Section-Working-Group:2011fj , and are flavor indexes , and and are the efficiencies of the CMS selections for the decays mediated by the SM decay channel and by decays of the ISS, respectively.

As we just want to illustrate here how to obtain an upper bound on the Yukawa, we shall use only a few representative values of , namely 60, 100, 140 and 200 GeV. For lighter the LHC searches tends to be rather ineffective. In fact, in the ISS signal both the charged leptons originate from and therefore the invariant mass of the two leptons cannot exceed . The cuts GeV for OF (SF) leptons, needed to reject leptons from QCD decays, would remove completely the ISS contribution for GeV. Also, for light the bounds from other experiments are more stringent atre . The power of the LHC is the sensitivity to around and above the mass of the Higgs, which improves significantly as compared to the reach of previous direct bounds.

We remark that our computation of the total events yield neglects the possible interference between the ISS and the SM contributions, which in general is a small effect due to different flavor and Lorentz structure of the decays.

The selection efficiencies and have been computed with showered events passed through Delphes1.9 and hadrons clustered into jets using the anti-kT algorithm with jet-cone radius parameter , as done in the CMS analysis. The obtained efficiencies have been rescaled such as to reproduce the SM Higgs boson event yield in Table 3 of CMS-Collaboration:2012vn for and GeV for the analysis and respectively. The results of the used chain of simulation codes are rather realistic, indeed the rescaling factor is almost flat w.r.t the cuts and the differences between the CMS and Delphes1.9 efficiencies are within 20%.

For the total width of the SM Higgs boson and the partial width we take the reference values of LHC-Higgs-Cross-Section-Working-Group:2011fj .

The width due to the decays mediated by and the partial widths have been computed with Madgraph5. These widths are the source of the dependence of on . In particular, they scale as when , and as when , i.e., the heavy neutrino that mediates the decays is off-shell. Therefore, as becomes heavier than the Higgs boson we expect the bound to quickly become less stringent. For completeness, we report our computation of the width and of the partial widths in Appendix A.

To derive the bound we compute the maximal such that

where is the 95% CL limit on the number of events after the selection from the analysis under consideration. From CMS-Collaboration:2012vn we extracted and . We take as the final bound, which we denote as , the most stringent one between the two bounds obtained for the analysis and . It turns out that the bounds derived using the analysis are stronger for all the cases we have considered.

The obtained bound for a fixed GeV as a function of is shown in Figure 2. For the cases where , we exclude while for couplings are excluded.

Figure 2: Bound on for GeV as a function of the mass of the heavy neutrino .

V Bounds from the observation of a Higgs-like particle at the LHC

Evidence of a new particle has been observed in the 2011 and 2012 LHC data HiggsJuly2012 ; CMS-PAS-HIG-12-020 ; ATLAS-CONF-2012-093 . The region of phase space where the excesses are concentrated suggests that they are originated by a Higgs-like scalar particle with mass 125 GeV. Assuming that the new observed state is indeed the Higgs boson which is also involved in the seesaw, further bounds can be obtained from a global study of the properties of the new particle instead of just using the bound on the rate of new phenomena in the channel.

In what follows, we use the measured properties of the new particle and we shall illustrate how to use the ISS prediction to put a bound on the size of .

The presence of additional decay modes changes the properties of in several respects. In fact the total width of is increased w.r.t to the SM value. Deviations from the SM value of the total width are potentially observable in a line-shape analysis (when is large enough) or in a global analysis of Higgs decay data Barger:2012qe .

The change of the total width in turn affects all the branching fractions of . In particular the rates of modes that do not get contributions from the new decay mode (such as , and ) are suppressed by a factor

where is given in Eq. (11).

The Higgs decay mode instead gets a contribution from the decays in Eq. (10) and its rate is enhanced by a factor

(13)

where is given in Eq. (LABEL:eq:nISS).

Altogether the ISS model, compared to the SM, predicts a suppression by a factor in the observed rates of all channels but 444Here we are disregarding the ISS enhancement of final states and which, for a light Higgs boson, are less sensitive due to large backgrounds. which instead is enhanced by the new decays.

The constraint that comes from the increase of the total width is far-reaching. In fact, it also applies to seesaw models where, for any reason, the mode does not get enhanced. This might happen, for instance, when the is very light, say , such that the leptons from the decay do not pass the selection cuts in Table 1. Furthermore, the bound coming from the extra contribution to the width applies to other models as the case of ISS models where the coupling dominates the decay. In this case the searches into are much less effective, still the effect on the total width provides a bound on the seesaw coupling.

The constraints from the channel are more specific to each model. Also they depend on the details of each of the analysis. In fact to make a correct use of the measured best-fit signal strengths of the channels one should consider each analysis separately and compute in detail the efficiencies as we have done for the analysis of CMS-Collaboration:2012vn discussed in Section IV .

For these reasons we shall consider the bound from two sets of measurements. In both cases we take the ISS model as reference. In the first case we put a bound on coming from all the measured signal strengths excluding the channels. This is the safest possibility as we are making the least number of assumptions on the structure of the seesaw couplings. Additionally we compute the bound using all the available data on the Higgs-like particle at 125 GeV. To deal with the several analyses we make the simplifying assumption that all the efficiencies for the processes mediated by the seesaw coupling are the same as those computed for the analysis of Section IV 555While this is not completely rigorous we expect it to be a good approximation. In fact, the new CMS analysis CMS-PAS-HIG-12-017 is very similar to CMS-Collaboration:2012vn . Furthermore, one should note that the bound on the seesaw coupling is mostly sensitive to the ratio in Eq. (LABEL:eq:nISS). As we find in section IV that the ISS efficiencies are quite similar to those of the SM, one can expect our simplifying assumption to be reliable for the ATLAS analysis as well..

To put a bound we proceed as follows. For each searched decay mode of the Higgs the collider experiments give the best fit value of where runs on the Higgs production modes and ’s are the branching fraction for the various Higgs boson decay modes . These measured best fits are expressed in units of the SM prediction and are referred to as best-fit signal strengths

where by we mean the central value and by we mean the symmetrized 1 error on the best-fit of the channel given by the experiments.

To place a bound we confront the ISS prediction with the latest best-fit signal strengths given by the experiments at the LHC HiggsJuly2012 ; CMS-PAS-HIG-12-020 ; ATLAS-CONF-2012-093 ; ATLAS-CONF-2012-091 ; ATLAS-CONF-2012-092 ; ATLAS-Collaboration:2012fv ; CMS-PAS-HIG-12-015 ; CMS-PAS-HIG-12-008 and the TeVatron The-TEVNPH-Working-Group:2012uq . For convenience of the reader the best-fit signal strengths used in our analysis are reported in Table 4.

CMS 2011+2012
CMS ZZ 2011+2012
CMS WW 2011+2012
ATLAS 2011+2012
ATLAS ZZ 2011+2012
ATLAS WW 2011
CMS bb AP 2011+2012
CMS 2011+2012
ATLAS bb AP 2011
ATLAS 2011
TeVatron bb AP
TeVatron WW
CMS WW AP
CMS Dijet 2011
CMS Dijet Tight 2012
CMS Dijet Loose 2012
Table 4: Signal strength best-fits extracted from HiggsJuly2012 ; CMS-PAS-HIG-12-020 ; ATLAS-CONF-2012-093 ; ATLAS-CONF-2012-091 ; ATLAS-CONF-2012-092 ; ATLAS-Collaboration:2012fv ; The-TEVNPH-Working-Group:2012uq ; CMS-PAS-HIG-12-015 ; CMS-PAS-HIG-12-008 . AP stands for associated production. The best-fit for the AP and Dijet analyses are extracted from CMS-PAS-HIG-12-015 ; CMS-PAS-HIG-12-008 . We consistently take the best-fit signal strength for GeV throughout.

As in the ISS model the couplings of the Higgs to the fermions, gluons and to the gauge bosons are not modified the production cross-sections are the same as in the SM. This allows us to compute the changes in the rates from the changes in the branching fractions only.

From the ISS predictions for the signal strengths and the measured signal strengths we compute the for several choices of as a function of the ISS coupling :

where runs on all the measured rates under consideration, and for the ISS is given by Eq. (13) and for all the other channels.

For each the is minimized w.r.t. at a value . Considering all channels but those with we get , adding the channels we get . In all cases we find that the is minimal for , therefore we derive a 67% CL upper-bound on . In Figure 3 we show the obtained with and without the channels. The inclusion of the channels improves the bound by a factor of about 2 in most cases.

Figure 3: The as function of in the ISS model for GeV and and GeV. The horizontal black dashed line corresponds to . The colored solid lines are for the of the entire dataset, and the dashed lines are for the without the channels.

Vi Discussions and conclusion

In this paper we have derived bounds on the Dirac Yukawa couplings of the lepton doublet in inverse seesaw models for neutrino masses using LHC Higgs data. In generic versions of these models, such bounds are useful since one could understand small neutrino masses while keeping the Yukawa couplings to be of order one and the singlet fermion masses in the 100 GeV range. We have focused on the cases where the electron or muon Yukawas are the dominant ones and also discussed the case with flavor-democratic Yukawa. Our discussion applies to the supersymmetric version of the model as well. It is perhaps worth pointing out that in SUSY ISS model, there are additional -term contributions an of order of a few GeV as well as new F-term contributions el to the Higgs mass thus relieving some MSSM parameter space. We find that for singlet fermion masses between GeV, useful bounds can be derived on the Yukawa couplings from the recent LHC data on Higgs searches.

It is also worth noting that in the low-scale type-I and inverse seesaw models, there are limits on the mixing parameter from leptonic unitarity LU and lepton flavor violation lfv . The current bounds for the electron-flavor is and for the muon sector it is (see LU ; lfv for details). These bounds are weaker than what we obtain in this paper for GeV from LHC data.

It is also worth pointing out that if we assumed a pattern for Dirac Yukawa couplings similar to the charged fermion case i.e. , then the dominant mode for decay will involve the decay and our constraints will not apply in a straightforward manner. However, the constraints from the global fit due to the increase of the total width shown as dashed lines in Figure 3 will still apply to .

Acknowledgment

RF thanks Alessandro Strumia for discussions on the LHC Higgs data. The work of BD and RNM is supported in part by the National Science Foundation Grant Number PHY-0968854. The work of RF is supported by the NSF under grants PHY-0910467 and PHY-0652363, and by the Maryland Center for Fundamental Physics. This work of BD was partly supported by the Lancaster-Manchester-Sheffield Consortium for Fundamental Physics under STFC grant ST/J000418/1. BD acknowledges the local hospitality and computing facilities provided at IACS, Kolkata, during the final stages of this work.

Note added in proof

After our paper was submitted, the 8-TeV LHC data on the final state appeared CMS-PAS-HIG-12-017 . Compared to the 7-TeV analysis, the major differences in the 8-TeV analysis are:

  1. GeV for both same- and opposite-flavor leptons.

  2. GeV for both same- and opposite-flavor leptons.

  3. GeV for both same- and opposite-flavor leptons.

Using these new cuts, we repeated the derivation of the bounds along the lines of the method described in the text. For a SM Higgs of 125 GeV we find slightly more stringent bounds, that improve on the 7-TeV results, by roughly 10-20%.

After this paper was posted on the arXiv, another paper okada studying the collider signatures of GeV pseudo-Dirac neutrinos in the inverse seesaw scenario was posted. We thank the referee for bringing this paper to our attention.

Appendix A Decay Widths of the Heavy Neutrino

In this Appendix, we collect the partial and total widths of the heavy neutrino (Table 5) as well as the partial widths of the Higgs and the increment in its total width (Tables 6 , 7 and 8) that have been used to compute the event yield and the bounds in Sections IV and V.

(GeV) (GeV)] (GeV)] (GeV)] (GeV)]
60 0.002716
100 0.03204 0.001495 0.03182 0.3263
Table 5: The relevant partial widths and the total width of for in the ISS model with dominant .
(GeV) (GeV) [(GeV)] [(GeV)] [(GeV)] [(GeV)]
125 60 2.9458 0.1588 0.0279 0.2516
100 0.6446 0.0633 0.0030 0.0629

Table 6: The relevant widths and the increment of the total decay width of the Higgs in the ISS model with dominant and . The partial widths are calculated by multiplying the total width by the branching fractions of computed from Table 5.
(GeV) (GeV)
125 140
200
Table 7: Total increment of the decay width of the Higgs in the ISS model with dominant for .
(GeV) (GeV)
125 140
200
Table 8: The relevant decay widths for in the ISS model with dominant .

References

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