LHC phenomenology of a two-Higgs-doublet neutrino mass model

LHC phenomenology of a two-Higgs-doublet neutrino mass model

Shainen M. Davidson    Heather E. Logan logan@physics.carleton.ca Ottawa-Carleton Institute for Physics, Carleton University, Ottawa K1S 5B6 Canada
October 5, 2010
Abstract

We study the LHC search prospects for a model in which the neutrinos obtain Dirac masses from couplings to a second Higgs doublet with tiny vacuum expectation value. The model contains a charged Higgs boson that decays to with branching fractions controlled by the neutrino masses and mixing angles as measured in neutrino oscillation experiments. The most promising signal is electroweak production of pairs with decays to , where , , and . We find that a cut on the kinematic variable eliminates most of the and -pair background. Depending on the neutrino mass spectrum and mixing angles, a 100 (300) GeV charged Higgs could be discovered at the LHC with as little as 8 (24) fb of integrated luminosity at 14 TeV center-of-mass energy.

preprint:

I Introduction

The Standard Model (SM) accounts for almost all experimental high energy physics data; however, the observation of neutrino oscillations requires that the SM be extended to include nonzero neutrino masses. While there are many ways to expand the SM to account for neutrino oscillations, we attempt to do so with the following goals. First, the neutrino mass scale is significantly lower than the mass scales of the other fermions, so we would like the model to account for this without the addition of many tiny parameters. Second, lepton number violation has not yet been observed, so we would like the model to give rise to Dirac neutrino masses, with Majorana masses forbidden. Third, we would like the model to be testable at the CERN Large Hadron Collider (LHC).

Most neutrino mass models give rise to Majorana masses for the SM neutrinos, with many predicting TeV-scale new physics accessible at the LHC. In contrast, only a few models for Dirac neutrinos have been proposed. These typically involve a second Higgs doublet with very small vacuum expectation value (vev) that couples only to the left-handed lepton doublets and the right-handed neutrinos, resulting in neutrino masses of the same order as the very small vev. The original SM-like Higgs doublet couples to all of the quarks and charged leptons in the usual way. Such a Yukawa coupling structure can be obtained by imposing a global symmetry, as proposed in the models of Refs. Ma:2000cc (); Gabriel:2006ns (); however, this does not by itself forbid neutrino Majorana mass terms, which must instead be eliminated by imposing an additional lepton number symmetry. The required Yukawa coupling structure can also be obtained by imposing a global U(1) symmetry; this idea was first proposed in Ref. Fayet:1974fj () as a way of ensuring the (then-assumed) masslessness of the neutrinos in the presence of right-handed neutrino states, and has the virtue of forbidding Majorana mass terms by itself.

In order to generate neutrino masses, the global symmetry used to ensure the desired Yukawa structure has to be broken. Spontaneous breaking leads to a very light scalar which can cause problems with standard big-bang nucleosynthesis Gabriel:2006ns (), as well as having significant effects on the phenomenology of the new Higgs particles Gabriel:2008es (). By instead breaking a global U(1) symmetry explicitly, the model proposed by us in Ref. Davidson:2009ha () generates Dirac neutrino masses while avoiding very light scalars.111A similar mechanism was used to explain the top-bottom quark mass hierarchy in Ref. Hashimoto:2004xp (). A supersymmetric version of this model was studied in Ref. Marshall:2009bk (), which found spectacular multi-lepton signals from cascade decays of the supersymmetric partners of the new Higgs bosons and right-handed neutrinos at the LHC.

In this paper we study the LHC detection prospects of the non-supersymmetric model of Ref. Davidson:2009ha (). This model expands the SM by adding a second Higgs doublet with the same electroweak quantum numbers as the SM Higgs doublet , as well as adding three gauge-singlet right-handed Weyl spinors which will become the right-handed components of the three Dirac neutrinos. The model imposes a global U(1) symmetry under which the second Higgs doublet and the right-handed neutrinos have charge , while all the SM fields have charge zero. This allows Yukawa couplings of the second Higgs doublet only to the right-handed neutrinos and the SM lepton doublet, and forbids Majorana masses for the right-handed neutrinos. It also tightly constrains the form of the Higgs potential. Breaking the U(1) symmetry explicitly using a term in the Higgs potential yields a vev for the second Higgs doublet and consequently gives the neutrinos Dirac masses proportional to . By requiring that , the Dirac neutrino masses are made suitably small without requiring tiny Yukawa couplings.

The characteristic feature of the model is that the couplings of the charged scalar pair and two neutral scalars and from the second Higgs doublet to leptons and neutrinos are controlled by the neutrino masses and mixing angles. In this paper we take advantage of the distinctive decay of the charged Higgs boson into charged leptons and neutrinos. We focus on electroweak pair production of at the LHC followed by decays to , where can be any combination of opposite-sign , , and leptons and denotes missing transverse momentum (carried away by the neutrinos). Because leptons are more difficult to reconstruct experimentally, we concentrate on the final states with , , and . The major backgrounds are diboson production (, , and ) and top quark pair production with both tops decaying leptonically.

To determine whether the signal will be detectable at the LHC, we generated signal and background events using MadGraph/MadEvent version 4 Alwall:2007st () assuming 14 TeV center-of-mass energy. We present results both at parton level, and after hadronization with PYTHIA PYTHIA () and fast detector simulation with PGS PGS (). With appropriate cuts, we find that a 5 discovery can be achieved with luminosity in the range 8–75 fb for  GeV, depending on the neutrino mixing parameters. For  GeV a 5 discovery can be made with luminosity in the range 24–460 fb. The higher luminosity requirements occur when the neutrino parameters are such that decays mostly to , leading to final states not considered in our analysis. We find that the kinematic variable is very effective at separating the signal from the and backgrounds for charged Higgs masses above the mass, and also provides sensitivity to the charged Higgs mass.222While we have not made a detailed study of charged Higgs detection prospects at 7 TeV centre-of-mass energy, we note that the cross section for the most dangerous background is about 2.5 times smaller at 7 TeV. However, the signal cross section is also about 2.5 (4.5) times smaller at this energy for (300) GeV. Furthermore, the LHC is anticipated to collect only about 1 fb of integrated luminosity at 7 TeV. We thus expect detection or even exclusion of the process considered here to be unfeasible in the current 7 TeV LHC run.

This paper is organized as follows. In the next section we review the model and present the charged Higgs decay branching ratios. In Sec. III we describe the signal and background processes, our event generation procedure and selection cuts, and the resulting signal significance. In Sec. IV we summarize our conclusions.

Ii The model

As outlined in the introduction, we start with the field content of the SM and add to it a new scalar SU(2) doublet (the SM Higgs is denoted ) and three right-handed gauge singlets (these are the right-handed neutrinos). We impose a U(1) symmetry under which and the three have charge +1 and all the other fields are uncharged, which leads to the Yukawa coupling structure Davidson:2009ha ()

(1)

Here is the conjugate Higgs doublet and are the 33 Yukawa matrices for fermion species .

The Higgs doublets can be written explicitly as

(2)

where will be generated by the usual spontaneous symmetry breaking mechanism of the SM and will be generated by the explicit breaking of the global U(1), described below. Inserting these expressions for into Eq. (1), we obtain the fermion masses and couplings to scalars. In particular, the fourth term in Eq. (1) gives rise to the neutrino mass matrix and interactions:

(3)

After diagonalizing the mass matrix in the first term, the neutrino mass eigenvalues are given by , where are the eigenvalues of . In this way, the small masses of the three neutrinos can be traced to the small value of .

We obtain the vevs of the scalar doublets from the Higgs potential as follows. The most general gauge-invariant scalar potential for two Higgs doublets is (see, e.g., Ref. HHG ()),

(4)

Imposing the global U(1) symmetry eliminates , , , and . The global U(1) symmetry is broken explicitly by reintroducing a small value for . This leaves the Higgs potential Davidson:2009ha (),333Note that using a symmetry instead of the global U(1) would allow a nonzero term.

(5)

Stability of the potential at large field values requires , , and . We want to arise through the usual spontaneous symmetry breaking mechanism, which is achieved when . We do not want the global U(1) to also be broken spontaneously, as that will create a very light pseudo-Nambu-Goldstone boson, which is incompatible with standard big-bang nucleosynthesis; thus we require that the curvature of the potential in the direction at zero field value be positive, i.e., .

To find the values of the vevs in terms of the parameters of the Higgs potential, we apply the minimization conditions,

(6)

Since we will require , we can ignore and when finding the value of . This yields

(7)

For , we need to consider , although again we may ignore higher order terms in ; this yields

(8)

We will choose parameters so that  GeV and  eV. This requires . We note that because is the only source of breaking of the global U(1) symmetry, its size is technically natural; i.e., radiative corrections to are proportional to itself and are only logarithmically sensitive to the high-scale cut-off Davidson:2009ha ().

The mass eigenstates of the charged and CP-odd neutral scalars are given by

(9)

where we define . and are the Goldstone bosons, which do not appear as physical particles in the unitarity gauge. and are the physical charged and CP-odd neutral Higgs states and are almost entirely contained in . Neglecting contributions of order and , the masses of and are Davidson:2009ha ()

(10)

The mass matrix for the CP-even neutral states is almost diagonal, yielding only very tiny mixing of order . Ignoring the mixing, the eigenstates are (SM-like) and , with masses Davidson:2009ha ()

(11)

After diagonalizing the neutrino mass matrix, Eq. (3) yields the following couplings to the new physical Higgs states:

(12)

where is the Pontecorvo-Maki-Nakagawa-Sakata (PMNS) matrix, defined according to , where are the neutrino flavor eigenstates.

The PMNS matrix can be parameterized in terms of three mixing angles (with , 23, and 13) and a phase according to (see, e.g., Ref. Fogli:2005cq ()),

(13)

where and . The 2 experimentally-allowed ranges for the three mixing angles and the neutrino mass-squared differences are given in Table 1. The phase and the mass of the lightest neutrino are undetermined, although tritium beta decay experiments set an upper limit on the neutrino masses of about 2 eV Amsler:2008zzb ().

Parameter Value
 eV
 eV
Table 1: Current values of the neutrino mixing parameters and mass-squared differences, from the global fit to neutrino oscillation data performed in Ref. Fogli:2005cq (). Uncertainties quoted are the 2 ranges. Note that the constraint on is only an upper bound, and that the sign of is not yet known.

Since the decays of and to two neutrinos will be invisible to a collider detector, the decay of most interest is . The charged Higgs can decay into all nine combinations of ; summing over neutrino mass eigenstates, the partial width to a particular charged lepton is Davidson:2009ha ()

(14)

where we define the expectation value of the neutrino mass-squared in a flavor eigenstate by Fukuyama:2008sz ()

(15)

In what follows we work under the assumption that , i.e., , so that the decays , will be kinematically forbidden. The branching ratios of the charged Higgs are then completely determined by the neutrino masses and mixing:

(16)

where we used the unitarity of the PMNS matrix to simplify the denominator.

The sign of the larger neutrino mass splitting is unknown (see Table 1). The situation in which is positive, so that is the heaviest neutrino, is called the normal neutrino mass hierarchy, while the situation in which is negative, so that and are heavier, is called the inverted hierarchy. We compute the charged Higgs branching fractions as a function of the lightest neutrino mass for both hierarchies, scanning over the 2 allowed ranges of the neutrino parameters as given in Table 1. Results are shown in Fig. 1.444We disagree with the charged Higgs branching fractions to leptons presented in Ref. Gabriel:2008es () for the model of Ref. Gabriel:2006ns (); these decays should have the same relative branching fractions as in our model. The large spread in the branching ratios to and for lightest neutrino masses below about 0.06 eV is due to the current experimental uncertainty in , which controls the relative amount of and in the isolated mass eigenstate .

Figure 1: Charged Higgs decay branching fractions to , , and as a function of the lightest neutrino mass.

Limits on the model parameters were discussed in Ref. Davidson:2009ha (). The most significant for our purposes is from searches for leptons plus missing energy at the CERN Large Electron-Positron Collider, which put a lower bound on the charged Higgs mass of 65–85 GeV, depending on the mass of the lightest neutrino. Big-bang nucleosynthesis also puts an upper bound on the neutrino Yukawa couplings of

(17)

Iii Signal and background at the LHC

In most other two-Higgs-doublet models, the charged Higgs decay rate to a particular charged lepton is proportional to the square of the charged lepton mass (see, e.g., Ref. HHG ()). Such a charged Higgs therefore decays predominantly to , with decays to , below 1%. In our neutrino-mass model, however, the charged Higgs decay rate to a particular charged lepton is instead proportional to the square of the mass of the corresponding neutrino flavor eigenstate. As a result, the branching fraction to and/or will always be sizable. In particular, in the normal hierarchy BR(, in the inverted hierarchy BR( and BR(, and for a degenerate neutrino spectrum BR( BR(, as shown in Fig. 1. Considering the high detection efficiency and lower fake rates of and compared to , we study pair production at the LHC mediated by a photon or , followed by decays to or with missing transverse momentum. We consider two scenarios, and 300 GeV, and present results as a function of the lightest neutrino mass for both the normal and inverted hierarchy.

The process of interest is , with , , and . The relevant backgrounds are with , , or and the neutrinos of any type, and . In spite of the presence of the extra jets that can be vetoed, the process is important because of its exceptionally high cross section at the LHC.

iii.1 Event generation

We simulated the signal and background processes with the parton-level Monte Carlo MadGraph/MadEvent version 4 Alwall:2007st (). We present both a parton-level analysis and an analysis including showering, hadronization, and a fast detector simulation using a PYTHIA-PGS package designed to be used with MadEvent. PYTHIA (version 6.4.20) PYTHIA () generates initial- and final-state radiation and hadronizes the final-state quarks and gluons, while PGS (Pretty Good Simulation of High Energy Collisions, version 4) PGS () is a basic detector simulator—we used the default settings for ATLAS. For the signal process we generated 10,000 unweighted events in each of the , , and final states. For both the and backgrounds we generated 100,000 unweighted events in each of the three leptonic final states. We incorporated the final state by doubling the cross sections. For the backgrounds we used the default SM branching fractions from MadGraph/MadEvent, given in Table 2.

Process Branching fraction
( or ) 0.1068
( or ) 0.0336
(all 3 neutrinos) 0.2000
1.0000
Table 2: Default SM branching fractions used in MadGraph/MadEvent Alwall:2007st ().

Although MadGraph/MadEvent is a tree-level event generator, we partially incorporated next-to-leading order (NLO) QCD corrections. We did this for two reasons. First, QCD corrections have a significant effect on the signal and background (especially ) cross sections, as well as significantly reducing the QCD scale uncertainty, so that using NLO cross sections lets us obtain more reliable results. Second, for the  GeV simulation we will apply a jet veto to reduce the background, which will also affect the signal and background once initial-state radiation is included. While this could be simulated by running the no-jet events through PYTHIA, a parton-level simulation of the and processes provides a more accurate description of jet kinematics. Because these one-jet processes make up part of the NLO QCD cross section for the corresponding no-jet processes, we must incorporate the NLO cross sections for consistency, as follows.

In the absence of a full NLO Monte Carlo, NLO QCD corrections are usually incorporated by multiplying the leading-order (LO) cross section—and the cross section corresponding to each simulated event both before and after cuts—by a -factor equal to the ratio of the NLO cross section to the tree-level cross section. In our case, however, our jet veto will affect LO events (which have no jet) and NLO events (which can have a final-state jet) differently. We deal with this by simulating and with the same decay final states as considered in the no-jet processes. For simplicity we generate the same number of events with an additional jet at the parton level as were generated for the no-jet processes. Because the background already contains two jets at leading order, we do not separately generate events with additional jets for this background. To avoid the collinear and infrared singularities, we apply a minimum cut of 10 GeV on the jet at the event-generation level.

The square of the NLO matrix element can be expressed up to order as

(18)

We used MadGraph/MadEvent to calculate the cross sections corresponding to and . We computed the NLO cross-section for at the LHC using the public FORTRAN code PROSPINO Beenakker:1999xh (); Alves:2005kr () with CTEQ6 parton densities Pumplin:2002vw (), with the renormalization and factorization scales set equal to . We took the NLO cross sections for the SM and background processes from Ref. Campbell:1999ah (). This paper quotes results using both the MRS98 and CTEQ5 parton densities, with results differing by 6%; since we use CTEQ6 for the tree-level MadGraph/MadEvent calculation, we take the results using the CTEQ5 parton densities for consistency. For events with in the final state, only the cross section for is relevant; for events with or in the final state, both the and processes contribute and we add the cross sections at both LO and NLO. We took the cross section from Ref. Bonciani:1998vc (), which includes both NLO and next-to-leading logarithmic corrections. The remaining scale uncertainty is about 5% when the factorization and renormalization scales are varied between and . The relevant cross sections are given in Table 3.

Process Cross section Source
( GeV) 295 fb PROSPINO Beenakker:1999xh (); Alves:2005kr ()
( GeV) 5.32 fb PROSPINO Beenakker:1999xh (); Alves:2005kr ()
127.8 pb Ref. Campbell:1999ah ()
17.2 pb Ref. Campbell:1999ah ()
833 pb Ref. Bonciani:1998vc ()
Table 3: NLO cross sections for signal and background processes (before decays) at the LHC (14 TeV). The cross section also includes a resummation of next-to-leading logarithmic corrections.

We find that with our generator-level jet cut on , , so the one-loop matrix element must interfere destructively with the LO matrix element. Thus the generated cross section from the LO process must be scaled down in order to incorporate the effects of the one-loop correction. For the parton-level simulation, the relevant scale factor is determined by solving for in the equation,

(19)

before cuts are applied, and then using this equation with the same value of to calculate the surviving after the cuts are applied to the LO and one-jet MadGraph/MadEvent simulated results.

For the PYTHIA-PGS simulation, the simulated events have extra jets produced by PYTHIA and “measured” jet smeared by PGS. To avoid double-counting, we use the following equation with two constants:

(20)

where and are the cross sections identified by PGS as having no jets and at least one jet, respectively, with GeV, out of the combined LO and one-jet generated samples. The constants and are determined by and using from Eq. (19). Equation (20) with the same values of and is then used after cuts to calculate the surviving .

iii.2 Cuts

We apply four cuts to reduce the background, summarized in Table 4. The first cut checks for the presence of two opposite-sign leptons each with  GeV and missing transverse momentum of at least 30 GeV. For the parton-level simulation, we also apply acceptance cuts on the pseudorapidity of both leptons, for electrons and for muons. Second, for the and final states we veto events for which the dilepton invariant mass falls between 80 and 100 GeV, in order to eliminate background from . This will also eliminate the majority of any background from with fake , which we did not simulate. The third cut vetoes events containing a jet with  GeV; for the parton-level simulation, we require that this jet falls in the rapidity range . This eliminates more than 97% of the backgound, but also reduces the signal by about a factor of two. We find that this cut is useful for  GeV. For  GeV the signal cross section is considerably smaller and the signal events will be better separated from background in our final cut variable, so that we obtain better sensitivity without the jet veto.

Cut name Explanation
Basic cuts Present are a lepton and antilepton, each with  GeV, and missing transverse momentum  GeV. For the parton level results, we also apply lepton acceptance cuts of and .
pole veto To eliminate events that include , we veto events in which the invariant mass of or is between 80 and 100 GeV (not applied to the final state).
Jet veto Designed to reduce background, any event with a jet with  GeV was rejected. For the parton level results, this veto is only applied when . (Applied only for  GeV.)
cut To reduce the and backgrounds, we make use of the larger mass of compared to the intermediate bosons in both backgrounds by cutting on (defined in Eq. (21)). For  GeV we require  GeV and for  GeV we require 150 GeV  GeV.
Table 4: Summary of cuts.

The final cut is on the variable , defined as Lester:1999tx ()

(21)

where is the square of the transverse mass (ignoring the charged lepton and neutrino masses),

(22)

In other words, is determined by making a guess for the transverse momenta of the two neutrinos (constrained by the measured total missing transverse momentum) and computing the transverse masses of the two systems; the guess is then varied until the larger of the two reconstructed transverse masses is minimized.

For equal-mass intermediate particles each decaying to , the distribution has an upper endpoint at the mass of the intermediate particle. Thus by cutting out events with , all the background should be eliminated (the endpoint is in fact smeared out by the finite width and momentum resolution of the detector). Since the leptons and missing transverse momentum in the process also come from decays of on-shell , this background should be eliminated as well. There is also a small contribution to the background from nonresonant processes that can have . Since all signal events will have , we also cut out events with in an effort to reduce the background from these nonresonant processes. For  GeV, we find that raising the minimum cut on to 150 GeV reduces the tail of the nonresonant events without reducing the signal too much.

In Fig. 2 we show the distributions for signal and background processes in the channel for and 300 GeV after the other cuts have been applied, for the PYTHIA-PGS simulation. Note that the background distribution has a maximum value a little above the mass, so that it can be eliminated with a high enough cut on , as we do for the case of  GeV. (The higher endpoint for in the right-hand plot in Fig. 2 is due to the absence of the jet veto, resulting in much higher statistics.) The background also falls off dramatically around ; however, due to nonresonant diagrams without on-shell intermediate pairs, this background extends to much higher values of . With our simulation statistics, a single event corresponds to a cross section of about 0.1 fb, while a single event corresponds to a cross section of about 0.01 fb.

Figure 2: distributions after other cuts have been applied for the final state, with  GeV (left, with jet veto) and 300 GeV (right, no jet veto). For the signal we take BR(, which occurs for a degenerate neutrino spectrum. The cut window is shown by the vertical lines.

iii.3 Results

The efficiency of each cut on for the final state is displayed in Tables 5, 6 and 7. Cut efficiencies for are displayed in Tables 8, 9 and 10, and for in Tables 11, 12 and 13. We give efficiencies for both the parton-level simulation and the simulation including showering, hadronization, and fast detector simulation using the PYTHIA-PGS package. All results incorporate NLO corrections as described in Sec. III.1.

 GeV  GeV
Cuts Parton PYTHIA/PGS Parton PYTHIA/PGS
Basic cuts 0.62644 0.47860 0.92961 0.72105
pole veto 0.90754 0.90165 0.97732 0.97724
Jet veto 0.68433 0.60717
cut 0.17075 0.15542 0.47317 0.45945
Cumulative 0.06643 0.04072 0.42988 0.32375
Table 5: Cut efficiencies for the signal process via . The efficiency of each cut is defined as the cross section that passed the cut divided by the cross section that passed the previous cut. The cumulative efficiency is the cross section that passed all the cuts divided by the original cross section. The jet veto is not applied for  GeV. The cut is  GeV for  GeV, and 150 GeV  GeV for  GeV.
Background Background
Cuts Parton PYTHIA/PGS Parton PYTHIA/PGS
Basic cuts 0.42708 0.33912 0.58407 0.40612
pole veto 0.74727 0.73255 0.86236 0.85501
Jet veto 0.63306 0.67299 0.01318 0.02856
GeV 0.01401 0.01147 0.01205 0.02716
Cumulative 0.00283 0.00192 0.00008 0.00027
Table 6: As in Table 5 but for background for , with cuts for  GeV.
Background Background
Cuts Parton PYTHIA/PGS Parton PYTHIA/PGS
Basic cuts 0.42708 0.33912 0.58407 0.40612
pole veto 0.74727 0.73255 0.86236 0.85501
150 GeV GeV 0.00260 0.00196 0.00000 0.00000
Cumulative 0.00083 0.00049 0.00000 0.00000
Table 7: As in Table 5 but for background for , with cuts for  GeV.
GeV GeV
Cuts Parton PYTHIA/PGS Parton PYTHIA/PGS
Basic cuts 0.51713 0.43680 0.84810 0.69845
pole veto 0.90869 0.90075 0.97756 0.97696
Jet veto 0.68310 0.57831
cut 0.16875 0.17320 0.47802 0.46847
Cumulative 0.05417 0.03941 0.39632 0.31966
Table 8: As in Table 5 but for the signal process via .
Background Background
Cuts Parton PYTHIA/PGS Parton PYTHIA/PGS
Basic cuts 0.32959 0.28226 0.52593 0.39048
pole veto 0.73839 0.73098 0.86021 0.85489
Jet veto 0.62703 0.63204 0.01346 0.02282
GeV 0.01324 0.01554 0.00657 0.03675
Cumulative 0.00202 0.00203 0.00004 0.00028
Table 9: As in Table 5 but for background for , with cuts for  GeV.
Background Background
Cuts Parton PYTHIA/PGS Parton PYTHIA/PGS
Basic cuts 0.32959 0.28226 0.52593 0.39048
pole veto 0.73839 0.73098 0.86021 0.85489
150 GeV GeV 0.00288 0.00239 0.00000 0.00000
Cumulative 0.00070 0.00049 0.00000 0.00000
Table 10: As in Table 5 but for background for , with cuts for  GeV.
GeV GeV
Cuts Parton PYTHIA/PGS Parton PYTHIA/PGS
Basic cuts 0.56131 0.45743 0.88249 0.70832
Jet veto 0.68783 0.59528
cut 0.16857 0.16121 0.47427 0.46373
Cumulative 0.06508 0.04390 0.41854 0.32847
Table 11: As in Table 5 but for the signal process via .
Background Background
Cuts Parton PYTHIA/PGS Parton PYTHIA/PGS
Basic cuts 0.35835 0.30423 0.55297 0.39556
Jet veto 0.65590 0.68572 0.01255 0.02592
GeV 0.00860 0.01207 0.01585 0.03018
Cumulative 0.00202 0.00252 0.00011 0.00031
Table 12: As in Table 5 but for background for , with cuts for  GeV.
Background Background
Cuts Parton PYTHIA/PGS Parton PYTHIA/PGS
Basic cuts 0.35835 0.30423 0.55297 0.39556
150 GeV GeV 0.00057 0.00049 0.00000 0.00000
Cumulative 0.00021 0.00015 0.00000 0.00000
Table 13: As in Table 5 but for background for , with cuts for  GeV.

Consider for example the PYTHIA-PGS results in the final state, and assume a degenerate neutrino spectrum so that BR(. In this case, for  GeV, the cuts reduce the charged Higgs signal cross section in this channel from 32.8 fb to 1.34 fb, while reducing the background from 1570 fb to 3.01 fb and the background from 9500 fb to 2.57 fb. The ratio of signal to background cross sections (S/B) is then 0.24. For  GeV, S/B is comparable. These are displayed for all channels for a degenerate neutrino spectrum in Table 14. In all cases S/B is at least 0.22, comfortably larger than the QCD and parton density uncertainties on the and backgrounds; the overall cross sections of these backgrounds can also be normalized experimentally using regions below .

Channel S/B  Luminosity for 5
  0.24 78 fb
100 GeV 0.22 88 fb
0.22 40 fb
0.25 526 fb
300 GeV 0.25 540 fb
0.89 73 fb
Table 14: Signal over background (S/B) and luminosity required for a 5 discovery in a single channel for the three signal processes studied, for and 300 GeV, assuming a degenerate neutrino spectrum so that BR(.

For  GeV, the background after cuts depends sensitively on the shape of the background distribution just above . This is controlled by the width and the detector resolution for lepton momenta and missing ; its shape should not suffer from QCD or parton-density uncertainties. For  GeV, the shape and normalization of the nonresonant tail of the background is especially important. This background is mostly Drell-Yan with an additional on-shell boson radiated from one of the final-state leptons; the QCD corrections to such processes are well understood. Given enough statistics, the shape of this background could also be normalized using the region above . Note also that the nonresonant tail of the background is significantly smaller for the final state than for the and final states, leading to a much higher signal purity in this final state for  GeV as shown in the last line of Table 14 (for the lower charged Higgs mass this effect is swamped by the resonant- contribution).

The integrated luminosity required for a 5 discovery of is displayed in Fig. 3 for  GeV and Fig. 4 for  GeV, for each channel separately and for all three channels combined. We use the PYTHIA-PGS results and compute only the statistical significance. For the normal hierarchy with (300) GeV, we find 5 discovery statistics with a minimum of 9 (56) fb. For the inverted hierarchy, the minimum is 8 (24) fb. For the case of degenerate neutrino masses, 20 (57) fb is needed. For degenerate neutrino masses, the luminosity needed for a 5 discovery in each channel separately is given in Table 14.

Figure 3: Luminosity required at the LHC (14 TeV) for a 5 discovery if  GeV, for the normal hierarchy (NH, left) and inverted hierarchy (IH, right). The lines for each channel bound the range of required luminosities obtained by scanning over the 2 allowed ranges of the parameters of the neutrino mixing matrix and mass-squared differences.
Figure 4: As in Fig. 3 but for  GeV.

Iv Discussion and conclusions

The two-Higgs-doublet model for Dirac neutrino masses studied here provides distinctive leptonic signatures at the LHC due to the characteristic decay pattern of the charged Higgs boson, controlled by the neutrino masses and mixing. We have shown that a simple set of cuts allows discovery of charged Higgs pairs with decays to with relatively modest integrated luminosity. In particular we found that a cut on the kinematic variable provides very effective suppression of pair and backgrounds for charged Higgs masses sufficiently above the mass.

In the inverted neutrino mass hierarchy, the large branching fractions of the charged Higgs to and guarantees a 5 discovery for any allowed neutrino mass and mixing parameter values with only 20 (57) fb for (300) GeV. The discovery potential remains remarkably good at  GeV despite the rapidly falling charged Higgs pair production cross section because of the increasing separation of the signal distribution from the background.

In the normal neutrino mass hierarchy, the large uncertainty on the neutrino mixing angle leads to parameter regions in which the charged Higgs decays predominantly to , with a branching fraction to light leptons below 40%, resulting in poor discovery sensitivity in the light lepton channels studied in this paper. Away from these parameter regions, the discovery prospects are only slightly worse than in the inverted hierarchy.

As more stringent experimental limits are placed on the neutrino parameters from neutrino oscillation experiments and direct searches for the kinematic neutrino mass in beta decay, the predictions for the charged Higgs branching ratios in this model will tighten. For example, one goal of the currently-running T2K long-baseline neutrino oscillation experiment in Japan is to improve the measurement accuracy of by an order of magnitude Zito:2008zza (), which would reduce the 2 spread in the charged Higgs branching ratios to and at low lightest-neutrino mass from the current 30% to about 10%. Sensitivity to the neutrino mass hierarchy relies on detection of a nonzero , a major goal of T2K and the longer-baseline U.S.-based experiment NOA currently under construction NOvATDR (). The ratios of the signal rates in the three channels considered here would allow the normal, inverted, and degenerate neutrino spectra to be differentiated, providing a key test of the connection of the model to the neutrino sector.

Measurement of the charged Higgs branching fractions will also provide some sensitivity to the mass of the lightest neutrino. For a lightest neutrino mass between about 0.01 and 0.1 eV, the charged Higgs branching ratios vary dramatically with the value of the lightest neutrino mass (Fig. 1); once the measurement of from neutrino oscillations has improved, measurement of the ratio of the and modes will provide sensitivity to the lightest neutrino mass in this range. This is nicely complementary to the prospects for direct kinematic neutrino mass determination from the tritium beta decay experiment KATRIN, which is designed to be sensitive down to neutrino masses of about 0.2 eV Valerius:2005aw ()—i.e., at the lower end of the degenerate part of the spectrum—and is scheduled to begin commissioning in 2012 KatrinTalk (). We note that, because the neutrinos in this model are Dirac particles, neutrinoless double beta decay experiments will have no signal and will thus not be sensitive to the neutrino mass scale.

The mass of the charged Higgs is also accessible at the LHC through the signal event kinematics. In particular, the signal distribution is flat up to an endpoint at the charged Higgs mass, as shown in Fig. 2. A fit to this distribution on top of the background should provide a measurement of the charged Higgs mass. This would allow a valuable cross-check of the charged Higgs pair production cross section together with the visible branching fractions as predicted by the neutrino parameters. The pair production cross section is sensitive to the isospin of the charged Higgs through its coupling to the boson, allowing the two-doublet nature of the model to be established Davidson:2009ha ().

We finally comment on the applicability of our results to two other neutrino mass models that contain a charged Higgs boson. First, the model of Ref. Gabriel:2006ns () contains a charged Higgs with partial widths to leptons and LHC production cross section identical to those in our model. The charged Higgs in the model differs from ours in that it can also decay to , where the neutral scalar is extremely light due to the spontaneous breaking of the symmetry. This competing mode dominates unless is not much heavier than and the neutrino Yukawa couplings are  Gabriel:2008es () (this parameter region is forbidden by standard big-bang nucleosynthesis, but the model already requires nonstandard cosmology due to the very light scalar ). For this parameter range, then, our results should carry over directly. For smaller Yukawa couplings or a heavier , the decays to leptons used in our analysis are suppressed, resulting in a smaller signal on top of the same background.

Second, neutrino masses of Majorana type can be generated by the so-called Type II seesaw mechanism type2seesaw (), in which an SU(2)-triplet Higgs field with very small vev is coupled to a pair of SM lepton doublets. LHC phenomenology for this Higgs-triplet model was studied in Ref. Perez:2008ha (), which considered signatures from and (and the conjugate process) at the LHC. While the decay branching fractions of in this model are identical to those of the charged Higgs in our Higgs-doublet model, the LHC production cross section for in the triplet model is about 2.7 times smaller than for in the doublet model Davidson:2009ha (), due to the different isospin of which modifies its coupling to the boson. The signals studied here would thus have a S/B of less than 10% for most channels, potentially leading to problems with background systematics. For sufficiently high charged Higgs mass, though, the channel would still have a decent S/B (33% for  GeV and a degenerate neutrino spectrum); the reduced cross section in the triplet model would however require an integrated luminosity close to 300 fb for discovery. In any case, searches for the doubly-charged scalar would yield an earlier discovery of the triplet model.

Acknowledgements.
This work was supported by the Natural Sciences and Engineering Research Council of Canada.

References

  • (1) E. Ma, Phys. Rev. Lett. 86, 2502 (2001) [arXiv:hep-ph/0011121].
  • (2) F. Wang, W. Wang and J. M. Yang, Europhys. Lett. 76, 388 (2006) [arXiv:hep-ph/0601018]; S. Gabriel and S. Nandi, Phys. Lett. B 655, 141 (2007) [arXiv:hep-ph/0610253].
  • (3) P. Fayet, Nucl. Phys. B 78, 14 (1974); A. Barroso and J. P. Silva, Phys. Rev. D 50, 4581 (1994) [arXiv:hep-ph/9404246].
  • (4) S. Gabriel, B. Mukhopadhyaya, S. Nandi and S. K. Rai, Phys. Lett. B 669, 180 (2008) [arXiv:0804.1112 [hep-ph]].
  • (5) S. M. Davidson and H. E. Logan, Phys. Rev. D 80, 095008 (2009) [arXiv:0906.3335 [hep-ph]].
  • (6) M. Hashimoto and S. Kanemura, Phys. Rev. D 70, 055006 (2004) [Erratum-ibid. D 70, 119901 (2004)] [arXiv:hep-ph/0403005].
  • (7) G. Marshall, M. McCaskey and M. Sher, Phys. Rev. D 81, 053006 (2010) [arXiv:0912.1599 [hep-ph]].
  • (8) J. Alwall et al., JHEP 0709, 028 (2007) [arXiv:0706.2334 [hep-ph]].
  • (9) T. Sjostrand, S. Mrenna and P. Z. Skands, JHEP 0605, 026 (2006) [arXiv:hep-ph/0603175].
  • (10) J. Conway et al., available from http://www.physics.ucdavis.edu/~conway/research/ software/pgs/pgs4-general.htm.
  • (11) J. F. Gunion, H. E. Haber, G. L. Kane and S. Dawson, The Higgs Hunter’s Guide (Westview Press, Boulder, Colorado, 2000).
  • (12) G.L. Fogli, E. Lisi, A. Marrone and A. Palazzo, Prog. Part. Nucl. Phys. 57, 742 (2006) [arXiv:hep-ph/0506083].
  • (13) C. Amsler et al. [Particle Data Group], Phys. Lett. B 667, 1 (2008).
  • (14) T. Fukuyama and K. Tsumura, arXiv:0809.5221 [hep-ph].
  • (15) W. Beenakker, M. Klasen, M. Kramer, T. Plehn, M. Spira and P. M. Zerwas, Phys. Rev. Lett. 83, 3780 (1999) [Erratum-ibid. 100, 029901 (2008)] [arXiv:hep-ph/9906298].
  • (16) A. Alves and T. Plehn, Phys. Rev. D 71, 115014 (2005) [arXiv:hep-ph/0503135].
  • (17) J. Pumplin, D. R. Stump, J. Huston, H. L. Lai, P. M. Nadolsky and W. K. Tung, JHEP 0207, 012 (2002) [arXiv:hep-ph/0201195].
  • (18) J. M. Campbell and R. K. Ellis, Phys. Rev. D 60, 113006 (1999) [arXiv:hep-ph/9905386].
  • (19) R. Bonciani, S. Catani, M. L. Mangano and P. Nason, Nucl. Phys. B 529, 424 (1998) [Erratum-ibid. B 803, 234 (2008)] [arXiv:hep-ph/9801375].
  • (20) C. G. Lester and D. J. Summers, Phys. Lett. B 463, 99 (1999) [arXiv:hep-ph/9906349].
  • (21) M. Zito [T2K Collaboration], J. Phys. Conf. Ser. 110, 082023 (2008).
  • (22) NOvA Collaboration, Technical Design Report,
    http://www-nova.fnal.gov/nova_cd2_review/tdr_oct_23/tdr.htm.
  • (23) K. Valerius [KATRIN Collaboration], PoS HEP2005, 166 (2006).
  • (24) T. Thummler, talk at XXIV International Conference on Neutrino Physics and Astrophysics, Athens, 14-19 June 2010, slides available from http://indico.cern.ch/conferenceDisplay.py?confId=73981.
  • (25) G. Lazarides, Q. Shafi and C. Wetterich, Nucl. Phys. B 181, 287 (1981); J. Schechter and J. W. F. Valle, Phys. Rev. D 22, 2227 (1980); R. N. Mohapatra and G. Senjanovic, Phys. Rev. D 23, 165 (1981).
  • (26) P. Fileviez Perez, T. Han, G. Y. Huang, T. Li and K. Wang, Phys. Rev. D 78, 071301 (2008) [arXiv:0803.3450 [hep-ph]]; Phys. Rev. D 78, 015018 (2008) [arXiv:0805.3536 [hep-ph]]; A. G. Akeroyd, C. W. Chiang and N. Gaur, JHEP 1011, 005 (2010) [arXiv:1009.2780 [hep-ph]].
Comments 0
Request Comment
You are adding the first comment!
How to quickly get a good reply:
  • Give credit where it’s due by listing out the positive aspects of a paper before getting into which changes should be made.
  • Be specific in your critique, and provide supporting evidence with appropriate references to substantiate general statements.
  • Your comment should inspire ideas to flow and help the author improves the paper.

The better we are at sharing our knowledge with each other, the faster we move forward.
""
The feedback must be of minimum 40 characters and the title a minimum of 5 characters
   
Add comment
Cancel
Loading ...
131654
This is a comment super asjknd jkasnjk adsnkj
Upvote
Downvote
""
The feedback must be of minumum 40 characters
The feedback must be of minumum 40 characters
Submit
Cancel