Update on 750 GeV diphotons from closed string states

Article history: Received 10 April 2016 Accepted 20 May 2016 Available online 25 May 2016 Editor: M. Cvetič Motivated by the recent update on LHC searches for narrow and broad resonances decaying into diphotons we reconsider the possibility that the observed peak in the invariant mass spectrum at Mγ γ = 750 GeV originates from a closed string (possibly axionic) excitation φ (associated with low mass scale string theory) that has a coupling with gauge kinetic terms. We reevaluate the production of φ by photon fusion to accommodate recent developments on additional contributions to relativistic light– light scattering. We also study the production of φ via gluon fusion. We show that for both a narrow and a broad resonance these two initial topologies can accommodate the excess of events, spanning a wide range of string mass scales 7 Ms/TeV 30 that are consistent with the experimental lower bound: Ms > 7 TeV, at 95% CL. We demonstrate that for the two production processes the LHC13 data is compatible with the lack of a diphoton excess in LHC8 data within ∼ 1σ . We also show that if the resonance production is dominated by gluon fusion the null results on dijet searches at LHC8 further constrain the coupling strengths of φ, but without altering the range of possible string mass scales. © 2016 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). Funded by SCOAP3. Recently, the ATLAS [1] and CMS [2] Collaborations reported excesses of events over expectations from standard model (SM) processes in the diphoton mass distribution around 750 GeV, using (respectively) 3.2 fb−1 and 2.6 fb−1 of data recorded at a centerof-mass energy √ s = 13 TeV. This could be interpreted as decays of a new massive particle φ . For a narrow width approximation hypothesis, the ATLAS Collaboration gives a local significance of 3.6σ and a global significance of 2.0σ when accounting for the look-elsewhere-effect in the mass range Mφ/ GeV ∈ [200–2000]. Signal-plus-background fits were also implemented assuming a large decay width for the signal component. The most significant deviation from the background-only hypothesis is reported for Mφ ∼ 750 GeV and a total width total ≈ 45 GeV. The local and global significances evaluated for the large width fit are about 0.3 higher than that for the fit using the narrow width approximation, * Corresponding author. E-mail address: [email protected] (L.A. Anchordoqui). http://dx.doi.org/10.1016/j.physletb.2016.05.063 0370-2693/© 2016 The Authors. Published by Elsevier B.V. This is an open access article SCOAP3. corresponding to 3.9σ and 2.3σ , respectively. The CMS data yields a local significance of 2.6σ and a global significance smaller than 1.2σ . Fitting the LHC13 data with a resonance yields a cross section times branching ratio of σLHC13(pp → φ + anything)× B(φ → γ γ ) ≈ { (10± 3) fb ATLAS (6± 3) fb CMS , (1) at 1σ [3]. On the other hand, no diphoton resonances were seen in the data at √ s = 8 TeV, although both ATLAS [4] and CMS [5] data show a mild upward fluctuation at invariant mass of 750 GeV. The lack of an excess at √ s = 8 TeV allows a quite precise limit to be placed on the corresponding cross section at √ s = 13 TeV. The most stringent limit comes from the CMS search σLHC8(pp → φ + anything) × B(φ → γ γ ) < 2.00 fb at 95% CL [5]. This implies that if the diphoton cross section grows by less than about a factor of 3 or 3.5 the LHC8 data are incompatible with the LHC13 data at 95% CL. under the CC BY license (http://creativecommons.org/licenses/by/4.0/). Funded by 224 L.A. Anchordoqui et al. / Physics Letters B 759 (2016) 223–228 More recently, we proposed a model [6] to explain the data in which the resonance production mechanism is calculable in string based dynamics, with large extra dimensions [7]. In our proposal the observed diphoton excess originates from a closed string excitation φ living on the compact space of generic intersecting Dbrane models that realize the SM chiral matter contents and gauge symmetry [8,9]. There are two properties of the scalar φ that are necessary for explaining the 750 GeV signal. It should be a special closed string state with dilaton-like or axion-like coupling to F 2 (respectively to F F̃ ) of the electromagnetic field, but may be decoupled from G2 of color SU (3). The couplings of closed string states to gauge fields do indeed distinguish between different Dbrane stacks, depending on the localization properties of D-branes with respect to φ in the compact dimensions. More specifically, it is quite natural to assume that φ is a closed string mode that is associated to the wrapped cycles of the (lepton) U (1)L and (right isospin) U (1)I R stack of D-branes, however it is not or only weakly attached to the wrapped cycle of (left) Sp(1)L or the color SU (3) stack of D-branes. In this way, we may avoid unwanted dijet signals. Actually, within a selection of string based explanations of the resonance [10–19] our proposal is uniquely exemplified by the possible suppression of dijet topologies in the final state.1 By choice, as we already advertised in [6], we may also allow a coupling φ to G2. This is possible by modifying the localization properties of D-branes with respect to φ in the internal space. Very recently, the ATLAS and CMS Collaborations updated their diphoton resonance searches [26–28]. The ATLAS Collaboration reported two separate analyses performed with 3.2 fb−1 of data at 13 TeV, targeting spin-0 and spin-2 resonances. For spin-0, the largest deviation from the background-only hypothesis is reported for Mφ ∼ 750 GeV and total ≈ 45 GeV. While the local significance somewhat increases to 3.9σ the global significance remains at the 2σ level. For the spin-2 resonance, both the local and global significances are somewhat smaller: 3.6σ and 1.8σ , respectively. The new CMS analysis is based on 3.3 fb−1 collected at √ s = 13 TeV. The additional data was recorded in 2015 while the CMS magnet was not operated. The largest excess is observed for Mφ = 760 GeV and total ≈ 11 GeV and has a local significance of 2.8σ for spin0 and 2.9σ spin-2 hypothesis. After taking into account the effect of searching for several signal hypotheses, the significance of the excess is reduced to < 1σ . The CMS Collaboration also reported a combined search on data collected at √ s = 13 TeV and √ s = 8 TeV. For the combined analysis, the largest excess is observed at Mφ = 750 GeV and total = 0.1 GeV. The local significance is ≈ 3.4σ and the global significance 1.6σ . In this short note we extend our previous discussion in three directions. The first is a calculation to accommodate recent developments on additional contributions to relativistic light–light scattering [29–31]. The second is the explicit calculation for production via gluon fusion disclosed in [6]. The third is a scan of the parameter space to entertain the possibility of a narrow width favored by the recent CMS analysis that combines data from LHC8 and LHC13. Before proceeding we note that the ATLAS excess is quite broad and probably with a large uncertainty. The CMS excess, however, is smaller and has no clear preference for a large width. This seems to indicate that the ATLAS excess could be a real signal combined with a large fluctuation, making the excess appear larger and wider than the underlying physical signal. Throughout we assume the resonance needs to have a signal [32] σLHC13(pp → φ + anything)× B(φ → γ γ )≈ 3–6 fb . (2) 1 A related stringy explanation in which φ can be produced through photon fusion has been put forward in [20]. Alternative axion and dilaton models have been discussed in [21–25]. Fig. 1. Contours of constant partial width γγ . The color encoded scales are in GeV. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) Since the string mass scale is now known to be larger than Ms ≈ 7 TeV [33], the mass Mφ ≈ 750 GeV must be suppressed with respect to the string scale by some anomalous loop corrections. Because φ is a twisted closed string localized at an orbifold singularity, its coupling to γ γ should be suppressed by M−1 s , provided the bulk is large [34]. With this in mind, we parametrize the coupling of φ to the photon by the following vertex