DiHiggs final states augMT2ed –
selecting events at the high luminosity LHC
Abstract
Higgs boson selfinteractions can be investigated via diHiggs () production at the LHC. With a small fb Standard Model production cross section, and a large background, this measurement has been considered challenging, even at a luminosityupgraded LHC. We demonstrate that by using simple kinematic bounding variables, of the sort already employed in existing LHC searches, the dominant background can be largely eliminated. Simulations of the signal and the dominant background demonstrate the prospect for measurement of the diHiggs production cross section at the level using of integrated luminosity at a highluminosity LHC. This corresponds to a Higgs selfcoupling determination with accuracy in the mode, with potential for further improvements from e.g. subjet technologies and from additional diHiggs decay channels.
pacs:
I Introduction
After a particle consistent with the Standard Model (SM) Higgs boson has been discovered at the LHC Aad et al. (2012a); Chatrchyan et al. (2012a), we have the final irrefutable experimental evidence of the realisation of a Higgs mechanism in nature Higgs (1964a, b); Guralnik et al. (1964); Englert and Brout (1964). This discovery alone, however, does not provide us the full details of this symmetry breaking sector. In particular, we do not have any additional information other than the existence of a (local) symmetrybreaking minimum and the Higgs potential’s curvature at this point in field space. These are rather generic properties of symmetry breaking potentials which can easily be reconciled with more complex scenarios of electroweak symmetry breaking. These typically exhibit a significantly different form of the Higgs selfinteraction from the SM^{*}^{*}*For example in scenarios in which the electroweak symmetry is broken radiatively, we typically encounter ColemanWeinberg type potentials Coleman and Weinberg (1973) which exhibit an infinite power series in the Higgs field with modeldependent expansion coefficients., and to obtain a better understanding of how electroweak symmetry breaking comes about, we need to find a way to discriminate between these different realisations.
The only direct way to provide a satisfying discrimination between the SM symmetry breaking sector and more complicated realisations is probing higher order terms of the Higgs potential directly. In practice this means studying multiHiggs final states and inferring the relevant couplings from data. The size of the cross sections at the LHC and future colliders effectively limits such a program to the investigation of the trilinear Higgs coupling Plehn and Rauch (2005). In the SM, is a function of the Higgs mass and the quartic Higgs interaction ,
(1) 
where we have expanded the potential around the nonzero Higgs vacuum expectation value in unitary gauge in the second line which yields .
The effort of phenomenologically reconstructing the trilinear Higgs coupling is based on diHiggs production Glover and van der Bij (1988); Dicus et al. (1988); Plehn et al. (1996); Djouadi et al. (1999); de Florian and Mazzitelli (2013); Grigo et al. (2013a) and dates back more than a decade Baur et al. (2003), but in the light of the recent Higgs discovery it has gained new momentum Dolan et al. (2012); Papaefstathiou et al. (2013); Dolan et al. (2013); Baglio et al. (2013); Goertz et al. (2013); Cao et al. (2013); Gupta et al. (2013); Grigo et al. (2013b); Nhung et al. (2013); Ellwanger (2013). Probably the most promising approach to infer the trilinear coupling which has been proposed so far is via the channel at the LHC, using boosted techniques Butterworth et al. (2008); Plehn et al. (2010) as reported first in the hadronlevel analysis of Ref. Dolan et al. (2012). That analysis was conservative in the sense that it did not employ selection criteria based on missing transverse momentum, which have the potential to reduce the most challenging backgrounds.
In the present letter we complement the analysis of Ref. Dolan et al. (2012) along these lines and also address the question of the extent to which a successful analysis of the diHiggs final state will depend on the overall Higgs boost. We concentrate on the mode,
(2) 
for which the background process
(3) 
dominates. We use kinematical properties of the decay of Eq. (3) to greatly reduce the background.
While we focus on the mode in this letter, we note that variants of the technique would be applicable to a broader range of diHiggs decay modes, particularly others also involving the and decays, which have the largest branching ratios for a 125 GeV Standard Model Higgs boson.
Ii Kinematic bounding variables
The dominant background can be reduced by using the variable, sometimes called the ‘stransverse mass’ Lester and Summers (1999); Barr et al. (2003). This massbound variable was designed for the case where a pair of equalmass particles decay,
and where one daughter from each parent, or , is a visible particle, and the other, or is not observed. Since the s are invisible their individual fourmomenta are not known. However the vector sum of the transverse momentum components of and can be determined from momentum conservation in the plane perpendicular to the beam.
For any given event is defined to be the maximal possible mass of the parent particle consistent with the constraints; that is provides the greatest lower bound on given the experimental observables Cheng and Han (2008).
In the context of the diHiggs decay (2) the dominant background process (3) satisfies the assumptions under which is useful: the dileptonic (ditau) background involves the pairproduction of identicalmass parents; and each of which decays to a final state which contains visible particles (the jets, and visible decay products) and invisible particles (the neutrinos both from the decays and from the leptonic or hadronic decays). We can therefore build a kinematical variable from the observed final state particles which is bounded above by the top quark mass for the background, but remains unbounded above for the diHiggs signal process.
The variable can be explicitly constructed Lester and Summers (1999) as
(4) 
where is the transverse mass constructed from , , and , while is the transverse mass constructed from , , and , and where the minimisation is over all hypothesised transverse momenta and for the invisible particles which sum to the constraint , which is usually the observed missing transverse momentum . The transverse mass is itself defined by
where the ‘transverse energy’ for each particle is defined by
Variants^{†}^{†}†See Ref. Barr and Lester (2010) for a recent review, and Ref. Barr et al. (2011a) for examples and categorisation. of address cases where some or all of the , , or particles are composed of fourvector sums. Such variants are designed for more complicated body decays with or for the case of sequential decays with onshell intermediates. While these massbounding variables were originally proposed to gain sensitivity to the masses of new particles at hadron colliders, they have also proved effective in searches Barr and Gwenlan (2009); Aad et al. (2011a, 2012b, 2013, 2012c).
For the case, an appropriate variable is constructed as follows. The jets resulting from each of the two top quark decays enter (4) as the visible particles and . The components and in (4) which form the transverse momentum constraint should then be the sum of the decay products of the bosons. The appropriate vector sum for the constraint in (4) contains both visible and invisible components,
(5)  
where the first line sums the missing transverse momentum (from all neutrinos from the leptonic decays, including subsequent leptonic or hadronic decays), and the visible transverse momentum from each of the two reconstructed candidates.
signal  backgrounds  

cross section [fb]  ew.  
Before cuts  13.89  10792  2212  82.3  
After trigger  1.09  1966  372  15.0  
After event selection  0.248  383.0  43.7  2.08  
After cut  0.164 [0.128]  107.7 [107.4]  4.62 [16.0]  0.316 [0.789]  [] 
After cut  0.118 [0.093]  28.7 [29.1]  0.973 [4.03]  0.062 [0.351]  [] 
After cut  0.055 [0.041]  0.475 [0.480]  0.037 [0.247]  0.013 [0.079]  [] 
After cut  0.047 [0.034]  0.147 [0.194]  0.029 [0.204]  0.012 [0.074]  [] 
The resulting variable
(6) 
is by construction bounded above by for the background process (in the narrow width approximation, and in the absence of detector resolution effects). By contrast, for the signal the distribution can reach very large values, in principle up to .
Iii Elements of the Analysis
iii.1 Detector simulation
We model the effects of detector resolution and efficiency using a custom detector simulation based closely on the ATLAS ‘Kraków’ parameterisation ATL (2013). The parameters employed provide conservative estimates of the ATLAS detector performance for the phaseII highluminosity LHC machine (HLLHC), which is expected to deliver an integrated luminosity of to each of the two generalpurpose experiments. In particular we model pileup (at ) and dependent resolutions for jets and for .
iii.2 Event generation
To generate the signal and background events we closely follow Ref. Dolan et al. (2012) (details of the comparison of the signal Monte Carlo that underlies this study and comparisons against earlier results can be found therein). Signal events (which dominate the inclusive cross section) are generated with a combination of the Vbfnlo Arnold et al. (2009) and FeynArts/FormCalc/LoopTools Hahn (2001); Hahn and PerezVictoria (1999) frameworks. We generate events in the Les Houches standard Boos et al. (2001) which we pass to Herwig++ Bahr et al. (2008) for showering and hadronisation of the selected final states. We use a flat NLO QCD factor to account for higher order perturbative corrections by effectively normalizing to an inclusive cross section of fb Baglio et al. (2013); Dawson et al. (1998).
The QCD and electroweak backgrounds are generated with Sherpa Gleisberg et al. (2009) and the background of Eq. (3) is generated with MadEvent 5 Alwall et al. (2011). The NLO cross sections have been computed in Ref. Denner et al. (2011) (we use and specify in Herwig++ during showering and hadronisation to increase the efficiency for the cut selection), for the mixed QCD/electroweak and the purely electroweak contributions we use the corrections to () and () production using Mcfm Campbell and Ellis (2000); Campbell (2001); Campbell et al. (2011).
iii.3 Event selection
Events are assumed to pass the trigger if there are at least two s with visible or at least one with visible . Both leptonic and hadronic decays of s are included. Selected events are required to have exactly two reconstructed s (leptonic or hadronic) and exactly two reconstructed and tagged jets.
The reconstruction of the ditau mass is important in discriminating the from the background. The LHC experiments typically employ sophisticated massreconstruction methods which include kinematic constraints but also likelihood functions or multivariate techniques trained to mitigate against detector resolution Chatrchyan et al. (2012b); Aad et al. (2012d). We use a simpler, purely kinematic reconstruction of the ditau mass, which is not expected to perform as well as the techniques used by the experiments in the presence of detector smearing. To estimate the systematic impact of the reconstruction on selection, we perform the same reconstruction with and without simulation of the resolution. The more sophisticated techniques used by the experiments which mitigate against detector resolution can be expected to lie between our two estimates.
In each case we construct a invariant mass bound using the greatest lower bound on given the visible momenta, and constraints Barr et al. (2011b). When detector smearing leads to events where does not exist, the mass constraints are dropped, and the resulting transverse mass is used as the greatest lower bound on .
In each case we require that lie within a 50 GeV window. In the analysis without smearing we choose , while we select when smearing is included. Note that in the latter case is a large contamination of the signal region defined by the invariant mass windows. By calibrating the Higgs mass reconstruction from as already presently performed in the case Aad et al. (2011b); Elagin et al. (2011), this contamination could be reduced.
The invariant mass is calculated from the fourvector sum of the two tagged jets. Events are selected if they satisfy .
Iv Results
The numbers of events passing each of the selection criteria are tabulated in Tab. 1. We find that the transverse momentum and observables are necessary for background suppression, and, hence, for a potentially successful measurement of the diHiggs final state in a hadronically busy environment. The normalized and distributions after the selection shown in Tab. 1 are plotted in Fig. 1. It can be seen that each of the two variables offers good signal versus background discrimination at the large integrated luminosities anticipated at the high luminosity LHC. We also observe that, and encode orthogonal information and they can be combined towards an optimised search strategy.
We find it is straightforward to obtain signaltobackground ratios of while retaining acceptably large signal cross section. These ratios are reexpressed in Fig. 1 which depicts the luminosity contours that are necessary to claim a discovery of diHiggs production on the basis of a simple ‘cut and count’ experiment that makes the rectangular cut requirements that both and . Both axes stop at rather low values of since a tighter selection would be dependent on the tail of the distribution where does not provide an appropriate indicator of sensitivity. We find that the HLLHC has good sensitivity to the production at high luminosity. For an example selection we obtain a cross section measurement in the 30% range (including the statistical background uncertainty).
The sensitivity to the Higgs trilinear coupling follows from destructive interference with other SM diagrams (see Ref. Dolan et al. (2012)), such that
(7) 
Using the full partonlevel calculation Dolan et al. (2012) we find that the quoted 30% cross section uncertainty translates into 60% level sensitivity to the Higgs trilinear coupling in the part of the distribution which is relevant for this analysis, .
As an alternative to a ‘cut and count’ analysis we construct a two dimensional likelihood from to obtain an estimate of the maximal sensitivity that is encoded in these observables, including their correlation Edwards (1972). Figures 1 and 2 show that the best sensitivity will result from energetic events either with large or large or both. Using the likelihood method we find fractional uncertainty in the cross section of
(8) 
using the CL(s) method Read (2002). The sensitivity to the cross section as captured in Eq. (8) can be rephrased into an expected upper 95% CL bound on the Higgs selfinteraction in the channel via Eq. (7). For a backgroundonly hypothesis with no true production we would find a limit on the selfcoupling of
where it should be noted that a 95% CL of is more stringent than the case of due to the destructive interference (7).
A measurement of at this level would be sufficient constrain a wide range of scenarios of electroweak symmetry breaking, such as compositeHiggs models and pseudodilaton models which can lead to large increases in the Higgs selfcoupling. While the limit might be somewhat degraded by additional systematic uncertainties in background determination, it also has the potential to be improved by using a subjet analysis Dolan et al. (2012), and/or by using the more sophisticated ditau mass reconstruction techniques already employed by the LHC experiments.
V Summary and Conclusions
Following the discovery of a Higgs boson, one of the top priorities at the LHC is to address the mechanism of electroweak symmetry breaking at a more fundamental level.
In this work we have shown that the high luminosity LHC will have sensitivity to the Higgs selfcoupling in the favoured channel using two simple kinematic variables and each of which independently suppresses the dominant background.
We have used parameterised detector simulations of the ATLAS detector as expected for a highluminosity environment throughout.
Using a two dimensional loglikelihood approach, the null hypothesis of would constrain the Higgs trilinear coupling to at the 95% confidence level. An exemplary cross section measurement with 30% precision translates into a measurement of at the 60% level.
However further improvements to the presented analysis are possible:

Jet substructure techniques allow one to narrow the invariant mass window Dolan et al. (2012), thus leading to a larger rejection of the and backgrounds.

We have used a definition of which does not include any information about the lepton momenta other than the sum of their in (5). Using calibrated taggers, further kinematic information is available by modifying Eq. (6) through pairing and objects, and exploiting the Jacobian peak of in the top decay Plehn et al. (2011). One can pair the hardest with that particular jet that yields an value that is closer to (the maximum of the Jacobian peak). The leftover jet is paired with the softer , and the following substitutions used in Eq. (6)
where the latter line indicates that the visible decay products are included in the invariant visible mass definition.
A combination of such techniques can be used by the LHC experiments to gain improved sensitivity to the Higgs selfcoupling — and hence to the nature of electroweak symmetry breaking.
Acknowledgements.
Acknowledgments. This work was supported by the Science and Technology Research Council of the United Kingdom, by Merton College, Oxford, and by the Institute for Particle Physics Phenomenology Associate Scheme. AJB thanks NORDITA and the IPPP for their hospitality during the preparation of this paper. CE thanks the IPPP for hospitality during the time when this work was completed. We thank Margarete Mühlleitner and Michael Spira for interesting discussions during the 2013 Les Houches workshop. Also, we thank Mike Johnson, Peter Richardson, and Ewan Steele for computing support and their patience.References
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