Advances in High Energy Physics

Volume 2018, Article ID 9150617, 14 pages

https://doi.org/10.1155/2018/9150617

## Exclusion Limits on a Scalar Decaying to Photons and Distinguishing Its Production Mechanisms

Department of Physics and Astronomy, Uppsala University, Box 516, 751 20 Uppsala, Sweden

Correspondence should be addressed to Tanumoy Mandal; es.uu.scisyhp@ladnam.yomunat

Received 7 July 2017; Revised 21 November 2017; Accepted 11 December 2017; Published 9 January 2018

Academic Editor: Luca Stanco

Copyright © 2018 Tanumoy Mandal. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. The publication of this article was funded by SCOAP^{3}.

#### Abstract

LHC run-II has a great potential to search for new resonances in the diphoton channel. Latest 13 TeV data already put stringent limits on the cross sections in the diphoton channel assuming the resonance is produced through the gluon-gluon fusion. Many beyond the Standard Model (SM) theories predict TeV-scale scalars, which copiously decay to diphotons. Apart from the gluon-gluon fusion production, these scalars can also be dominantly produced in other ways too at the LHC, namely, through the quark-quark fusion or the gauge boson fusions like the photon-photon, photon-, , or fusions. In this paper we use an effective field theory approach where a heavy scalar can be produced in various ways and recast the latest ATLAS diphoton resonance search to put model-independent limits on its mass and effective couplings to the SM particles. If a new scalar is discovered at the LHC, it would be very important to identify its production mechanism in order to probe the nature of the underlying theory. We show that combining various kinematic variables in a multivariate analysis can be very powerful to distinguish different production mechanisms from one another.

#### 1. Introduction

There are numerous theoretical motivations to expect that the Standard Model (SM) is not the complete story and the scalar spectrum of a larger theory may be richer than to possess only one neutral scalar, the Higgs boson. From this expectation searches for new scalars are continuously being carried out at the Large Hadron Collider (LHC) in various channels. Along these directions, no confirmed hint has been found so far in any of these searches. Nevertheless, some anomalies in some of these searches have drawn significant attention in the high energy physics community, recently. Among them, the most famous one is the 750 GeV diphoton excess [1, 2] which created a lot of excitements in the community. Before the excess went away with more data, numerous attempts have been made to explain the excess (see [3] for a review and a long list of references). Another important excess was the diboson excess around 2 TeV resonance mass [4–6] which also later turned out to be a statistical fluctuation.

Searches for heavy scalars at the LHC are generally being carried out in the diphoton, diboson, or dijet resonance searches. The diphoton channel, among them, is particularly important as this channel provides a comparatively cleaner background. Higgs boson was first discovered in the diphoton channel at the LHC [7, 8]. TeV-scale scalars decaying into a diphoton system is one of the key predictions of many beyond the Standard Model (BSM) theories. Various possibilities have been extensively explored in the context of the 750 GeV diphoton excess (see the reference list of [3] for various models that predict TeV-scale diphoton resonances). To test these predictions, LHC run-II provides us with a great opportunity to observe a diphoton resonance of mass up to a few TeV. In this paper we particularly focus on the diphoton final state for these reasons.

If a particle decays to diphotons, it must either be spin-0 or spin-2 in nature as the spin-1 particles decay to on-shell diphotons is forbidden by the Landau-Yang theorem [9, 10]. A spin-2 particle or graviton couples universally to all matter fields through energy-momentum tensor. Various extra dimensional models like the ADD model [11] or the RS model [12] predict the existence of graviton. If a resonance in the diphoton system mediated by graviton is observed, one would expect resonances at the same mass in other possible channels also. Therefore, simultaneous studies in various channels might be more illuminating for the spin-2 particle. The current limits on the graviton mass are already quite high, around ~2-3 TeV [13, 14]. On the other hand, scalars of mass ~1 TeV decaying into diphotons, which is a typical signature of many models, are still allowed by the LHC data. These scalars can be produced at the LHC in various ways, namely. through the , , , , , or fusions. In this paper we consider a model-independent effective field theory (EFT) approach where the scalar can be produced and decayed (two-body) in different possible ways as mentioned. But we only concentrate on the diphoton decay mode in this paper as stated earlier.

First, we derive the available parameter space for a scalar (produced in different ways) decays to diphotons using our EFT approach. These limits will be grossly model-independent and can be used to set limits on other models wherever applicable. If a scalar resonance will actually be seen in future, the most obvious question that will arise is how the scalar is produced? A most common way to decipher the production mechanism of a heavy scalar is to look at various kinematic distributions especially various jet observables, which are important in this regard. This has been investigated to some extent in the literature in the context of the 750 GeV diphoton excess [15–19]. In this paper we revisit some of the jet observables and show their effectiveness in distinguishing different production modes. We, then, use a multivariate analysis (MVA) by combining many kinematic variables to distinguish different production modes more efficiently.

This paper is organized as follows: in Section 2, we employ an effective Lagrangian for the scalar ; in Section 3, we discuss about the decays and various production modes of at the LHC and derive exclusion limits on the mass and couplings from the latest diphoton resonance search data. In the same section, we discuss how two different production modes of can be distinguished using a MVA analysis. Finally, we conclude in Section 4.

#### 2. Effective Lagrangian

We consider an EFT approach where a heavy scalar interacts with the SM gauge bosons through the dimension-5 operators and with the SM quarks through the dimension-4 operators. Assuming is a CP-even real scalar, we employ the following effective Lagrangian:where the field-strength tensors corresponding to gluon (), photon (), , and bosons are , , , and , respectively, and their generic form is where . All the dimension-5 operators are suppressed by the new physics scale . In general, could be different for different operators, but we assume they are the same for all the operators. Note that only the is a dimension-4 operator and we introduce the electroweak symmetry breaking scale GeV in association with to bring the scale in the interaction. The motivation behind this is that if operators are effectively originated from new physics then effective couplings are expected to contain the imprint of the scale (possibly in the form or some power of this ratio). This way of parameterizing the couplings also enables us to present exclusion limits on all the couplings in the form. Here, we use the notation to denote a generic dimensionless coupling associated with the vertex. The scalar can, in general, couple differently with the different SM quarks. For simplicity, in this analysis, we assume a single coupling , the same for all the SM quarks. Note that, in all interactions with the gauge bosons, the normalization factor is so chosen such that the corresponding Feynman rule takes the form where and are the 4 momenta of two gauge bosons and , respectively, directed towards the vertex. The Feynman rule for the interaction is .

In general, the new scalar can mix with the 125 GeV scalar () with a mixing angle . This leads to the scaling of all the couplings of by a factor . Although this would not change the branching ratios (BRs) of , it would change the production cross section of by a factor . Since all the measured signal strengths are pretty close to unity, this will make close to one. That is why, in this paper, we have neglected any mixing between and for simplicity.

#### 3. Phenomenology

In addition to the SM Lagrangian, we implement the effective Lagrangian of shown in (1) in FEYNRULES [20] to generate the Universal FeynRules Output [21] model files for the MADGRAPH [22] event generator. We use the MMHT14LO [23] parton distribution functions (PDFs) for event generation. This PDF set includes the photon PDF which has been computed following the approach described in [16, 24]. We use the factorization scale and the renormalization scale at in our analysis. Generated events are further showered and hadronized including multiple parton interactions by using PYTHIA8 [25]. We perform detector simulation using DELPHES [26] which uses FASTJET [27] for jet clustering. Jets are clustered using the anti- algorithm [28] with . We analyze the reconstructed objects by implementing ATLAS selection cuts [29], which we summarize in Section 3.3. For MVA, we use the adaptive Boosted Decision Tree (BDT) algorithm in the TMVA [30] framework.

##### 3.1. Decays of

From the Lagrangian in (1), we have the following two-body decay modes of , namely, , where . The partial widths for these decay modes are given by the following expressions: where denotes the electroweak gauge bosons and . There could be subdominant three-body decays of possibly mediated through an off-shell gauge boson. If the intermediate gauge boson is massless, in case of gluons or photons, the three-body BRs are nonnegligible especially when is large [31]. In this analysis, we consider the two-body and three-body decays of to obtain the total width where the three-body decay widths are computed numerically using MADGRAPH. Partial widths of three-body decay modes where an off-shell gauge boson goes to pair grow very rapidly with increasing scalar mass. This is due to the contribution coming from the longitudinal polarization of bosons. Therefore, in high mass region, BR for reduces substantially.

##### 3.2. Production of at the LHC

When all in (1) are nonzero, the scalar can be produced from the , , , , , and fusions at the LHC. In Figure 1, we show the partonic cross sections of different production modes of at the 13 TeV LHC for (taking one at a time) and TeV. In case of the production of through the or fusions, initial and come from the quark splitting. Therefore, is produced in association with at least two jets for this case. Similarly, for the initiated production, is produced in association with at least one jet. Partonic cross sections are computed by applying the following generation level cuts on the jets () and photons () wherever applicable Here, transverse momentum, pseudorapidity, and separation in the - plane are denoted by , and , respectively. These basic cuts are used to avoid any soft divergence present at the event generation level and stricter selection cuts are applied at the level of reconstructed event analysis after detector simulation. Note that all cross sections scale as , and therefore, we present them by choosing and TeV such that one can translate it easily for other values.