You will be presented with the full text of a science research paper. Please write an abstract for this paper. Your response should include the abstract and no additional text.

Paper text:
the large hadron collider ( lhc ) has clearly exhibited its ability to make discoveries with the observation of a new resonance  @xcite with even spin that decays to photons and z bosons as expected of the standard model ( sm ) higgs particle .
thus precise measurements of the decays of this resonance into various channels ( whether standard or not ) , are of the utmost importance . at the same time , it is essential to verify our understanding of the existing channels , in particular , @xmath0 .
how well are these photons defined ? can physics objects other than single photons leave signatures in the detector similar to that of a photon ?
not surprisingly , the answer is yes  @xcite .
given the granularity of the calorimeters , an object consisting of ( nearly ) collinear photons , typically labeled a photon - jet , will generate a signature similar to that of a single photon .
the possibility that the higgs particle decays to multiple collinear photons is not new  @xcite .
simple models where the higgs decays to almost massless scalars that each in turn decay to a pair of photons , typically do not give rise to events with four separately identifiable photons , but rather to pairs of photon - jets , each with 2 photons .
slightly more complicated models can produce higgs decays to photon - jets with @xmath1 photons .
we will discuss concrete models where the higgs decays to photon - jets with @xmath2 and @xmath3 photons per photon - jet .
thus it is essential to develop tools to separate single photons from photon - jets from qcd - jets .
otherwise we are unlikely to understand either the signal or the background .
atlas recently made attempts to identify photon - jets from higgs decays  @xcite .
these analyses rely on relaxing the isolation / shower shape criteria , which use the differing distributions of energy deposition within the calorimeter cells to quite successfully discriminate single photons from qcd - jets .
unfortunately , the parameters of the underlying model can be easily adjusted so that the resultant photon - jets pass the strictest isolation / shower shape criteria just like photons . more importantly , loosening isolation criteria results in a larger fake rate for qcd - jets .
discriminating photon - jets from qcd - jets is more challenging than separating single photons from qcd - jets .
fortunately jet substructure techniques  @xcite have recently been developed to distinguish qcd - jets from jets containing boosted heavy particle decays , and we can use this work for detection of photon - jets . more broadly ,
` jets ' , as defined by an infrared safe jet clustering algorithm , are being proposed as a universal language to describe _ all _ calorimeter objects including single photons , photon - jets and qcd - jets . by using the tools developed in jet substructure physics , we do not need to rely on isolation cuts .
we supplement the traditional / conventional variables currently used to discriminate photons from qcd - jets with substructure variables that probe in detail the energy distribution within the jet .
note that the photons - jets are composed of energetic photons distributed inside the jet , where the distribution is a result of the kinematic features of the model , e.g. , the masses and spins of the intermediate particles .
the existence of this structure within photon - jet suggests that substructure variables will be efficient at finding and discriminating photon - jets .
we show that our analysis is capable of separating photon - jets from both single photons and qcd - jets _ at least as _ efficiently as the traditional discriminators separate photons from qcd - jets
.    there is another important advantage to applying jet substructure techniques to purely electro - magnetic calorimeter ( ecal ) objects .
the introduction of ` grooming ' algorithms ( including filtering  @xcite , pruning  @xcite , and trimming  @xcite ) promised to suppress the undesirable contributions to purely hadronic jets from the underlying event ( the largely uncorrelated soft interactions surrounding the interesting hard scattering ) and from pile - up ( the truly uncorrelated proton - proton collisions that occur in the same time window ) .
indeed , the recent results from studies at atlas  @xcite and cms  @xcite indicate this grooming is effective .
we expect that this substructure - based grooming will work as well for all ecal based objects .
it should be noted that in the context of higgs physics , the decay to photon - jets is not the only example where the collinearity of the decay products adds complexity to the analysis .
collinearity plays a role for traditional decays of the higgs boson when it is boosted . in ref .
@xcite , the authors exploited the collinearity of the @xmath4-quarks in boosted higgs decays ( both quarks in a single jet ) to greatly enhance the chances of detecting the @xmath5 channel , featuring jet substructure as a mainstream tool ( see also refs .
the application of jet substructure in higgs physics has now become a very active area of research , applied both to the sm higgs  @xcite as well to beyond the sm higgs scenarios  @xcite . for reviews , more detailed descriptions , and references
see refs .
@xcite .
the paper is organized as follows : in sec .
[ sec : simplified_models ] , we start with a simplified model for photon - jets .
we propose a set of benchmark points , where we take different combinations of masses and parameters in the simplified model to produce photon - jets displaying a variety of distinct kinematics . in sec .
[ sec : simulations ] we define the details of our simulation .
we describe , in detail , how we generate samples of photon - jets , one for each of the benchmark points , qcd - jets , and single photons .
we present our analysis in sec .
[ sec : analysis ] .
we describe all the variables that we use in this work to discriminate photon - jets from qcd - jets from single photons .
then we combine these variables in a multivariate analysis .
we train boosted decision trees ( bdts ) using the samples of jets and use these to optimize the discriminating power of our analyses .
we also show how these bdts can be used to simultaneously separate photon - jets , photons , and qcd - jets from each other .
our conclusions are presented in sec .
[ sec : conclusion ] .
by definition , photons - jets refer to calorimeter objects consisting of more than one hard photon .
however , such a broad definition presents a challenge since all photon - jets are not the same .
they differ in terms of the number of hard constituent photons as well as in the distribution of those photons within the photon - jet . to provide a systematic phenomenological study of photon - jets we classify these objects in more detail in terms of the production mechanism and consider a broad range .
we will refer to the various production scenarios as ` benchmark ' scenarios .
we find that a simple model in the spirit of ref .
@xcite with two new particles is sufficient to characterize these benchmarks .
the model includes a small number of interactions and we can vary the strength of these interaction and the new particle masses in order to generate the benchmark scenarios . in particular , we introduce two scalar fields @xmath6 and @xmath7 of mass @xmath8 and @xmath9 respectively . without loss of generality ,
we choose the naming convention such that @xmath10 .
neither @xmath6 nor @xmath7 carry any sm charges .
we use the following interactions to generate photon - jets @xmath11 where @xmath12 are mass parameters , @xmath13 are dimensionless coupling constants , and @xmath14 is the electromagnetic field strength operator .
this simple model bears a resemblance to a higgs portal scenario  @xcite because of the @xmath15 coupling . in the higgs portal
language , @xmath6 and @xmath7 constitute a ` hidden ' sector while the coupling @xmath15 provides a tunnel to the corresponding ` hidden valley ' . the electromagnetic couplings ( proportional to the @xmath16 parameters ) provide ways for the new particles to decay back to sm particles , photons in this case . with respect to higgs physics
, this simple model provides a realistic example where the sm higgs field decays through the new particles to multiple photons . in the limit @xmath17 , the resultant photons ( the decay products of @xmath6 )
are essentially collinear .    in table
[ table : bench ] we list the benchmark scenarios ( labeled photon - jet study points or pjsps ) that we investigate in this work . all are generated by varying the parameters in eq .. the symbol @xmath18 in table  [ table : bench ] denotes that a non - zero value is selected for that parameter , which then determines the decay mode .
we have chosen the benchmarks in such a way that the parameters denoted by @xmath18 only change the _ total _ width of the decaying particles . as long as
the decays are prompt , the exact values of these parameters are irrelevant to the phenomenological properties of the photon - jets .
.[table : bench ] the study points used in our analysis . for @xmath19 , @xmath7 does not participate in the decay chain since @xmath20 and the @xmath9 and @xmath21 columns are empty . by @xmath18
we denote that a non - zero value is chosen for the parameter , which facilitates prompt decays , but the specific value plays no role . [ cols="^,^,^,^,^,^ " , ]     we estimate these numbers by integrating the energy deposited in an electromagnetic shower in a cell of size ( @xmath22 ) and situated @xmath23 from the origin of the shower .
we assume that the cell is made of lead ( @xmath24 ) .
one of the challenges of collider physics is to unambiguously associate detector based objects with the corresponding elementary physics objects . 
 a particular example is the association of calorimeter - based objects such as `` jets '' , identified with a standard ( ir - safe ) jet algorithm , with the underlying physics objects , which may be qcd - jets ( arising from a scattered parton ) , electrons , photons and , as discussed here , photon - jets ( a group of collinear photons ) . 
 this separation is especially interesting in the context of higgs searches , where the signal includes both di - photon ( in the standard model ) and di - photon - jet decays ( in a variety of beyond the standard model scenarios ) , while qcd provides an ever - present background . 
 here we describe the implementation of techniques from the rapidly evolving area of jet - substructure studies to not only enhance the more familiar photon - qcd separation , but also separately distinguish photon - jets , i.e. , separate usual jets into three categories : single photons , photon - jets and qcd . the efficacy of these techniques for separation is illustrated through studies of simulated data .