Stringy origin of diboson and dijet excesses at the LHC

Stringy origin of diboson and dijet excesses at the LHC


Very recently, the ATLAS and CMS collaborations reported diboson and dijet excesses above standard model expectations in the invariant mass region of . Interpreting the diboson excess of events in a model independent fashion suggests that the vector boson pair production searches are best described by or topologies, because states decaying into pairs are strongly constrained by semileptonic searches. Under the assumption of a low string scale, we show that both the diboson and dijet excesses can be steered by an anomalous field with very small coupling to leptons. The Drell-Yan bounds are then readily avoided because of the leptophobic nature of the massive gauge boson. The non-negligible decay into required to accommodate the data is a characteristic footprint of intersecting D-brane models, wherein the Landau-Yang theorem can be evaded by anomaly-induced operators involving a longitudinal . The model presented herein can be viewed purely field-theoretically, although it is particularly well motivated from string theory. Should the excesses become statistically significant at the LHC13, the associated topology would become a signature consistent only with a stringy origin.


Very recently, searches for narrow resonances at the ATLAS and CMS experiments uncovered various peaks in invariant mass distributions near : (i) The ATLAS search for diboson production contains a excess at in boosted jets of  Aad:2015owa (). The global significance of the discrepancy above standard model (SM) expectation is . The invariant mass range with significance above is to . Because the search is fully hadronic, the capability for distinguishing gauge bosons is narrowed. Therefore, many of the events can also be explained by a or resonance, yielding excesses of and in these channels respectively. (ii) The CMS search for diboson production (without distinguishing between the and tagged jets) has a 1.4 excess at  Khachatryan:2014hpa (), and the search for diboson production with a leptonically tagged yields a excess at invariant mass  Khachatryan:2014gha (). (iii) The CMS search for dijet resonances finds a excess near 1.8 TeV Aad:2014aqa (). (iv) Around the same invariant mass ATLAS also recorded an excess in the dijet distribution with a significance Khachatryan:2015sja (). (v) The CMS search for resonant production yields a excess in the energy bin of 1.8 to ; here the Higgs boson is highly boosted and decays into , whereas the decays into charged leptons and neutrinos CMS-14-010 (). Barring the three ATLAS analyses in diboson production, all these excesses are completely independent.

Although none of the excesses is statistical significant yet, it is interesting to entertain the possibility that they correspond to a real new physics signal. On this basis, with the assumption that all resonant channels are consistent with a single resonance energy, a model free analysis of the various excesses has been recently presented Brehmer:2015cia (). The required cross sections to accommodate the data are quite similar for and final states, which can be considered as roughly the same measurement. A pure signal is disfavored and could only describe the data in combination with another signal. This is because the CMS single lepton analysis sets an upper bound of 6.0 fb at 95% C.L. Khachatryan:2014gha () and a cross section of this magnitude is needed to reproduce the hadronic excesses. Moreover, the CMS dilepton search has a small excess that this channel cannot explain Khachatryan:2014gha ().

Several explanations have been proposed to explain the excesses including a new charged massive spin-1 particle coupled to the electroweak sector (which can restore the left-right symmetry) Hisano:2015gna (), strong dynamics engendering composite models of the bosons Fukano:2015hga (), dark matter annihilation into right-handed fermions Alves:2015mua (), a resonant triboson simulating a diboson through judicious choice of cuts Aguilar-Saavedra:2015rna (), and a heavy scalar Chen:2015xql (). In this Letter we adopt an alternate path. We assume that the source of the excesses originates in the decay of a new abelian gauge boson that suffers a mixed anomaly with the SM, but is made self-consistent by the Green-Schwarz (GS) mechanism Green:1984sg (). Such gauge bosons occur naturally in D-brane TeV-scale string compactifications Antoniadis:1998ig (), in which the gauge fields are localized on D-branes wrapping certain compact cycles on an underlying geometry, whose intersection can give rise to chiral fermions Blumenhagen:2005mu (). The SM arises from strings stretching between D-branes which belong to the “visible” sector. Additional D-branes are generally required to cancel RR-tadpoles, or to ensure that all space-filling charges cancel. These additional D-branes generate gauge groups beyond the SM which forge the “hidden” sector.

There are two unrivaled phenomenological ramifications for intersecting D-brane models: the emergence of Regge excitations at parton collision energies string scale and the presence of one or more additional gauge symmetries, beyond the of the SM. The latter derives from the property that, for , the gauge theory for open strings terminating on a stack of identical D-branes is rather than . (For the gauge group can be rather than .) In a series of recent publications we have exploited both these ramifications to explore and anticipate new-physics signals that could potentially be revealed at the LHC. Regge excitations most distinctly manifest in the jet Anchordoqui:2007da () and dijet Lust:2008qc () spectra resulting from their decay. The extra gauge symmetries beyond hypercharge have (in general) triangle anomalies, but are cancelled by the GS mechanism and the gauge bosons get Stückelberg masses. We have used a minimal D-brane construct to show that the massive field, the , can be tagged at the LHC by its characteristic decay to dijets or dileptons Anchordoqui:2011ag (). In the framework of this model herein we adjust the coupling strengths to be simultaneously consistent with the observed dijet excess and the lack of a significant dilepton excess. Concurrently we show that the model is also consistent with the ATLAS diboson excess as it allows for production of -pairs. At the level of effective Lagrangian, the operator contributing to the amplitude is induced by the GS anomaly cancellation.

In our calculations we will adopt as benchmarks:


To develop our program in the simplest way, we will work within the construct of a minimal model with 4 stacks of D-branes in the visible sector. The basic setting of the gauge theory is given by  Cremades:2003qj (). The LHC collisions take place on the (color) stack of D-branes. In the bosonic sector the open strings terminating on this stack contain, in addition to the octet of gluons , an extra boson , most simply the manifestation of a gauged baryon number. The stack is a terminus for the gauge bosons . The boson that gauges the usual electroweak hypercharge symmetry is a linear combination of and the bosons and terminating on the separate and branes. Any vector boson orthogonal to the hypercharge, must grow a mass so as to avoid long range forces between baryons other than gravity and Coulomb forces. The anomalous mass growth allows the survival of global baryon number conservation, preventing fast proton decay Ghilencea:2002da ().

The content of the hypercharge operator is given by


We also extend the fermion sector by including the right-handed neutrino, with charges and . The chiral fermion charges of the model are summarized in Table LABEL:table. It is straightforward to see that the chiral multiplets yield a mixed anomaly through triangle diagrams with fermions running in the loop. This anomaly is cancelled by the GS mechanism, wherein closed string couplings yield classical gauge-variant terms whose gauge variation cancels the anomalous triangle diagrams. The extra abelian gauge field becomes massive by the GS anomaly cancellation, behaving at low energies as a with a mass in general lower than the string scale by an order of magnitude corresponding to a loop factor. Even though the divergences and anomaly are cancelled, the triangle diagrams contribute an univocal finite piece to an effective vertex operator for an interaction between the and two vector bosons Anastasopoulos:2006cz (). This is a distinguishing aspect of the D-brane effective theory, which features a noticeable decay width of the into , , and .2

   Fields    Sector    Representation                
Table 1: Chiral fermion spectrum of the D-brane model.

The covariant derivative for the fields in the basis is found to be


The fields are related to and by the rotation matrix


with Euler angles , and . Equation (6) can be rewritten in terms of , , and as follows

Now, by demanding that has the hypercharge given in (5) we fix the first column of the rotation matrix


and we determine the value of the two associated Euler angles




The couplings and are related through the orthogonality condition,


with fixed by the relation .3 In our calculation we take as a reference point for running down to 1.8 TeV the coupling, ignoring mass threshold effects of stringy states. This yields . We have checked that the running of the coupling does not change significantly for different values of the string scale. The third Euler angle and the coupling will be determined by requiring sufficient suppression to leptons to accommodate (3) and a (pre-cut) production rate in agreement with (2).

The Lagrangian is of the form


where each is a fermion field with the corresponding matrices of the Dirac algebra, and , with and the vector and axial couplings respectively. From ( Stringy origin of diboson and dijet excesses at the LHC) and (13) we obtain the explicit form of the chiral couplings in terms of and


The decay width of is given by Barger:1996kr ()


where is the Fermi coupling constant, , is the strong coupling constant at the scale , , and or 1 if is a quark or a lepton, respectively. The couplings of the to the electroweak gauge bosons are model dependent, and are strongly dependent on the spectrum of the hidden sector. Following Antoniadis:2009ze () we parametrize the model-dependence of the decay width in terms of two dimensionless coefficients,


The production cross section at the LHC8 is found to be Hisano:2015gna ()


Next, we scan the parameter space to obtain agreement with (1) to (4). In Fig. 1 we show contour plots, in the plane, for constant , , and . To accommodate (1), (2), and (3) the ratio of branching fractions of electrons to quarks must be minimized subject to sufficient dijet and diboson production. It is easily seen in Fig. 2 that and , , and yield , , , , which are consistent with (1), (2), and (3) at the level. In addition, . Thus, the upper limit set by (4) is also satisfied by our fiducial values of , , , and . The chiral couplings of and are given in Table 2. All fields in a given set have a common couplings.

Figure 1: Contours of constant cross section times branching into dijet (left), (middle), and diboson (right), for and . The color encoded scales are in fb.
Figure 2: Best fit of cross section contours times branching into dijet/leptons (left) and times branching into dibosons (right), for and . The blue and red contours correspond to and , respectively. The yellow and green contours on the left correspond to and , respectively. The yellow and green contours on the right correspond to and 4.5 fb, respectively.
Table 2: Chiral couplings of and gauge bosons.

The second constraint on the model derives from the mixing of the and the through their coupling to the two Higgs doublets. The criteria we adopt here to define the Higgs charges is to make the Yukawa couplings (, , ) invariant under all three ’s Antoniadis:2000ena (). Two “supersymmetric” Higgses and (with charges , , and , , ) are sufficient to give masses to all the chiral fermions. Here, , , and .

The last two terms in the covariant derivative


are conveniently written as


for each Higgs , with , where for the two Higgs doublets




The Higgs field kinetic term together with the GS mass terms () yield the following mass square matrix for the mixing,

which does not impose any constraint on the parameter. We have verified that, for our fiducial values of and , if the shift of the mass would lie within 1 standard deviation of the experimental value.

In summary, we have shown that recent results by ATLAS and CMS searching for heavy gauge bosons decaying into and final states could be a first hint of string physics. In D-brane string compactifications the gauge symmetry arises from a product of groups, guaranteeing extra gauge bosons in the spectrum. The weak hypercharge is identified with a linear combination of anomalous ’s which itself is anomaly free. The extra anomalous gauge bosons generically obtain a Stückelberg mass. Under the assumption of a low string scale, we have shown that the diboson and dijet excesses can be steered by an anomalous field with very small coupling to leptons. The Drell-Yan bounds are then readily avoided because of the leptophobic nature of the massive gauge boson. The resulting loop diagrams, along with tree-level higher-dimension couplings arising from the GS anomaly cancellation mechanism, generate an effective vertex that couples the anomalous fields to two electroweak gauge bosons. The effective vertex renders viable the decay of the into -pairs, which is necessary to fit the data. Should the excesses become statistically significant at the LHC13, the associated topology would become a signature consistent only with a stringy origin.

L.A.A. is supported by U.S. National Science Foundation (NSF) CAREER Award PHY1053663 and by the National Aeronautics and Space Administration (NASA) Grant No. NNX13AH52G; he thanks the Center for Cosmology and Particle Physics at New York University for its hospitality. H.G. and T.R.T. are supported by NSF Grant No. PHY-1314774. X.H. is supported by the MOST Grant 103-2811-M-003-024. D.L. is partially supported by the ERC Advanced Grant “Strings and Gravity” (Grant.No. 32004) and by the DFG cluster of excellence “Origin and Structure of the Universe.” Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the National Science Foundation.


  1. preprint: MPP–2015–155 LMU-ASC 45/15
  2. The Landau-Yang theorem Landau:1948kw (), which is based on simple symmetry arguments, forbids decays of a spin-1 particle into two photons.
  3. Throughout and are the strong and weak gauge coupling constants.


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