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the inert doublet model  @xcite ( idm ) is one of the simplest extensions of the standard model ( sm ) , yet it is also one of the most versatile . this scenario , in which the usual sm fields are supplemented by a single scalar @xmath7 doublet which does not contribute to electroweak - symmetry breaking ( ewsb ) and couples to the gauge - boson sector but not the fermion sector , has a wealth of potential phenomenological applications . perhaps the most intriguing of these stems from the recent observation  @xcite that the fields of this additional scalar doublet can provide a positive contribution to the oblique @xmath8 parameter  @xcite sufficient to render a sm higgs mass of @xmath9  gev consistent with precision data  @xcite . a host of other potential applications for inert doublets exist as well . these range from explaining the lightness of neutrino masses via a one - loop radiative see - saw mechanism  @xcite to the loop - level induction of electroweak - symmetry breaking  @xcite to engineering successful grand unification  @xcite . furthermore , the model yields a natural dark matter candidate in the form of the lightest inert particle ( lip ) , whose absolute stability is guaranteed by an unbroken @xmath10 symmetry . studies of the relic abundance of lip dark matter  @xcite , as well as its prospects for indirect detection via neutrino  @xcite , cosmic - ray positron and antiproton  @xcite , and gamma - ray  @xcite signatures , and for direct detection  @xcite have also been performed . since the coupling structure of the fields of the additional scalar doublet in the idm differs from that of typical two - higgs doublet models ( 2hdm ) in the manner discussed above , the collider phenomenology of the idm also differs markedly from that of such 2hdm . it is therefore worthwhile to investigate the prospects for detecting the additional fields of the idm via their decay signatures at the large hadron collider ( lhc ) . in this work , we focus on the dilepton ( plus missing energy ) channel , which turns out to be one of the most auspicious channels in terms of its discovery potential . some preliminary , parton - level studies of this channel have been conducted  @xcite within one particular region of parameter space . here , we present a more comprehensive , detector - level analysis in which we investigate a variety of different benchmark regions motivated by dark - matter studies , etc . , and assess the prospects for observing a @xmath11 signal at the lhc in each regime . we note that the results of this analysis , although conducted in the context of the idm , should also be applicable to other extensions of the sm with similarly - modified scalar sectors , as long as the extra scalars in those extensions have similar decay patterns to those in the idm . we begin in sect . [ sec : model ] with a recapitulation of the inert doublet model . in sect . [ sec : constraints ] , we summarize the theoretical and experimental constraints to which the model is subject . we outline a set of representative benchmark points which correspond to phenomenologically interesting regions of the parameter space in which all of these constraints are satisfied . in sect . [ sec : cuts ] , we discuss dilepton production in the idm , as well as the sm backgrounds for @xmath11 at the lhc , and we outline the event - selection criteria we use to differentiate signal events from those produced by these backgrounds . we present our numerical results in sect . [ sec : results ] , and in sect . [ sec : conclusion ] , we conclude . the inert doublet model is an extension of the sm in which the higgs sector comprises two scalar @xmath7 doublets . the first of these doublets , here denoted @xmath12 , resembles the sm higgs doublet @xmath13 . it bears full responsibility for electroweak - symmetry breaking and its neutral component acquires a vacuum expectation value ( vev ) equal to that of the sm higgs : @xmath14  gev . it also couples to quarks and leptons in precisely the way @xmath13 does in the sm . the second doublet , here denoted @xmath15 , does not contribute to ewsb ( @xmath16 ) , nor does it have yukawa interactions with the sm quarks and leptons . this coupling structure is enforced by a @xmath10 symmetry under which @xmath17 and all the sm fields transform trivially . this symmetry , sometimes called matter parity , remains unbroken in the vacuum , since @xmath18=0 . the most general @xmath19-even scalar potential allowed by this @xmath10 symmetry is @xmath20 . \label{eq : v}\ ] ]    after ewsb is triggered by the vev of @xmath12 , the scalar spectrum of the model comprises the usual sm higgs @xmath21 ( the neutral , @xmath19-even degree of freedom in @xmath12 ) , as well as four additional fields corresponding to the four degrees of freedom in @xmath15 . these include a pair of charged scalars @xmath22 , a neutral , @xmath19-even scalar @xmath23 , and a neutral , @xmath19-odd scalar @xmath24 . the masses of these scalars , given in terms of the six free parameters appearing in eq . ( [ eq : v ] ) is fixed by the constraint @xmath25 from ewsb . ] @xmath26 in eq . ( [ eq : v ] ) , are @xmath27 for our purposes , it will be useful to parametrize the model using the alternative parameter set @xmath28 , where @xmath29 , @xmath30 , and @xmath31 . this parameterization is particularly useful in that it characterizes the model in terms of physically significant quantities such as particle masses , mass splittings , and @xmath32 : the coefficient which controls the trilinear @xmath33 and quartic @xmath34 couplings . the assumption of an unbroken @xmath10 matter parity renders at least one of the degrees of freedom in @xmath15 absolutely stable . if this particle is electrically neutral ( @xmath23 or @xmath24 in the idm ) , it becomes a good weakly - interacting - massive - particle ( wimp ) dark - matter candidate . for the remainder of this work , we will assume that the lip is the @xmath19-even scalar @xmath23 , and hence @xmath35 . the phenomenology of the alternative scenario , in which the @xmath19-odd scalar @xmath24 plays the role of the lip , is very similar . a variety of considerations , stemming both from theoretical consistency conditions and from experimental bounds , constrain the idm . below , we briefly summarize these constraints , which were discussed in detail in ref . @xcite .    * * perturbativity : * @xmath36 * * vacuum stability : * @xmath37 * * limits from direct collider searches : * + first of all , the excellent agreement between the experimentally - measured values for @xmath38 and @xmath39 obtained from lep and tevatron data  @xcite and the predictions of the sm requires that @xmath40 in order that the decays @xmath41 and @xmath42 are kinematically forbidden . + second of all , bounds on the invisible decays of the higgs boson from lep data  @xcite also serve to constrain scenarios in which the higgs is light and @xmath43 . in this work , however , we will be primarily concerned with cases in which @xmath44  gev , for which the bounds from the searches on invisible higgs decay does not apply . + third and finally , one can consider limits arising from direct searches for @xmath22 , @xmath24 , and @xmath23 , both at lep and at the tevatron  @xcite . it should first be noted that the standard limits on additional charged and neutral higgs scalars do not apply , because the standard search channels from which they are derived generally involve the couplings of such scalars to fermions , which are absent in the idm . on the other hand , bounds derived from the non - observation of @xmath45  @xcite and @xmath46 @xcite decays in supersymmetric models can be used to constrain the idm parameter space , since @xmath47 and @xmath48 in the idm lead to similar signals . a detailed analysis of the constraints on @xmath47 in the idm based on lep  ii searches for @xmath45 was conducted in ref . @xcite , which showed that regions of parameter space with @xmath49  gev and @xmath50  gev for @xmath51  gev had been ruled out . for @xmath52 gev , however , only the lep  i constraint @xmath53 applies . a rough bound of @xmath54  gev  @xcite can also be derived from the lep @xmath46 limit by making the necessary modifications to account for the difference in cross - section between fermion - pair and scalar - pair production . taking these considerations into account , we will henceforth restrict our attention to models for which @xmath55  gev . * * electroweak precision constraints : * + electroweak precision measurements set limits on contributions from the additional higgs doublet to the oblique @xmath23 and @xmath8 parameters  @xcite . we consider a given parameter choice to be consistent with electroweak precision constraints as long as the overall values of @xmath23 and @xmath8 it yields , once all contributions are taken into account , lie within the 68%  c.l . ellipse determined by the lep electroweak working group  @xcite . for a light sm higgs , with @xmath56 gev , the constraint is weak as long as @xmath57 and @xmath58 are of roughly the same order . for a heavy sm higgs , a large splitting between @xmath22 and @xmath23 is preferred , and @xmath59 . * * dark matter relic density : * + one of the attractive aspects of the inert doublet model is that it can provide a viable wimp dark matter candidate in the form of a stable , neutral lip . the model is therefore constrained by experimental limits on the relic density of dark matter in the universe . in what follows , we will assume that the lip relic density represents the dominant component of @xmath60 and falls within the @xmath3 range of the dark - matter density of the universe as measured by wmap  @xcite , only the upper bound applies . ] : @xmath61 . a detailed examination of the relic density of a @xmath19-even scalar lip in the idm was conducted in  @xcite . it was found that the correct dark - matter relic density could be realized in several distinct regions of parameter space in which all the aforementioned theoretical and experimental constraints were also satisfied . for a light sm higgs with @xmath62  gev , two scenarios are possible . the first of these involves a light lip with @xmath63  gev and mass splittings @xmath64 and @xmath65 which are sizeable , but of the same order . the second involves a heavier dark matter particle with @xmath66 gev and relatively small mass splittings . for a heavy sm higgs with @xmath67 gev , the regions which the constraints leave open are those in which @xmath68  gev and @xmath59 , with both @xmath57 and @xmath58 relatively large , or @xmath69  gev , @xmath52 gev with a large @xmath57 . .a list of benchmark points used in our analysis , defined in terms of the model parameters @xmath70 . dark matter relic density and collider phenomenology of the idm depend little on @xmath71 , which is set to 0.1 for all benchmark points . the points lh1 @xmath72 lh5 involve a light ( @xmath73 ) higgs boson , while the points hh1 @xmath72 hh3 involve a heavy ( @xmath74 ) higgs . [ cols="^,^,^,^,^,^",options="header " , ]     let us now turn to examine the results for each of the individual benchmark scenarios in our study in more detail . we begin with the lh1 , which involves a light higgs boson , a light lip ( @xmath75  gev ) , and a large mass splitting ( @xmath76 gev @xmath77 ) . the dominant backgrounds in this scenario are those from @xmath78 and @xmath79 , each of which also involves the leptonic decay of an on - shell @xmath80 and is therefore difficult to differentiate from the signal process on the basis of kinematical variables . the remaining backgrounds are efficiently suppressed after the imposition of the @xmath81 cut near the @xmath6 window . with @xmath82 of integrated luminosity , a significance level of @xmath83 could be obtained in this benchmark scenario . the situation for the heavy - higgs benchmark hh1 is similar ; however the smaller @xmath84-production cross - section in this case ( due primarily to an increased lip mass ) translates into a lower statistical significance . benchmark point lh2 also includes a @xmath0  gev dark matter particle , but involves a smaller mass splitting than that of lh1 : @xmath85  gev . this scenario affords the best opportunity for discovery at the lhc out of any of the benchmark points in our analysis , yielding a statistical significance of @xmath86 with @xmath5 of integrated luminosity . two factors contribute to its success : a small production threshold @xmath87 , and the fact that @xmath88 , which implies that the @xmath19-odd scalar @xmath24 decays via an off - shell @xmath80 . the latter consideration makes it possible to eliminate @xmath78 and @xmath79 background contributions quite efficiently by setting @xmath89 comfortably below the @xmath80 pole . further cuts on the angular variables @xmath90 and @xmath91 serve to reduce the remaining backgrounds to a manageable level . after all cuts are imposed , events from the low-@xmath81 tail of the @xmath78 distribution form the dominant background . it should be noted , however , that while the aforementioned angular - separation cuts are quite efficient in reducing background events , this efficiency comes with a price : the cuts also eliminate a substantial fraction of signal events . this explains why the signal cross - section for lh2 is less than that for lh1 , as no angular separation cuts are imposed in the latter scenario . benchmarks lh3 and lh4 are superficially similar , given that they involve a similar lip mass @xmath92  gev and the same mass splitting @xmath93 gev . in this case , however , the marked difference in @xmath57  a parameter which generally has little effect on observability of the dilepton signal in the @xmath84 associated - production channel  between the two points has a substantial impact on their collider phenomenology . the reason for this is twofold . first of all , since @xmath94 for lh4 ( unlike any other benchmark in our analysis ) , the decay channels @xmath95 , where @xmath96 denotes either four jets , two jets and a single charged lepton , or two charged leptons , are open in this scenario , with a branching ratio @xmath97 . as a result , @xmath98 , and thus the dilepton signal cross - section , are reduced by an additional factor of two relative to those points for which such competing decays are kinematically prohibited . second of all , as discussed above , the small value for @xmath99 gev in lh4 allows the additional contribution of @xmath100 process to the signal due to the unobservable soft jets and leptons from @xmath22 decay . these additional contributions augment the overall signal cross - section and more than compensate for the diminished @xmath101 , as discussed above . for @xmath5 of integrated luminosity , a significance level greater than @xmath3 could be reached for lh4 as well as lh3 . the final light - higgs scenario in our analysis , lh5 , turns out to be the most difficult benchmark point for which to observe a dilepton signal , primarily because of the small mass splitting @xmath102  gev between @xmath23 and @xmath24 . the charged leptons in the final state tend to be extremely soft , and consequently the signal remains buried under the sm background even after an optimized set of level  iii cuts is applied . scenarios with a small value of @xmath58 will in general be difficult to discover via this channel at the lhc . it should be noted that the results we obtain for this benchmark differ significantly from the parton - level results quoted in  @xcite for a similar benchmark scenario , also with @xmath102  gev . the discrepancy owes primarily to our imposition of a level i cut of @xmath103 cut designed to replicate the effect of electron and muon isolation requirements at the atlas and cms detectors . since the angular separation between the lepton momenta tends to be extremely small for such a small value of @xmath58 , a vast majority of signal events will have @xmath104 and hence be eliminated by this cut . let us now turn to discuss the benchmark points which feature a heavy ( @xmath105 gev ) higgs boson  in other words , those benchmarks for which the idm successfully addresses the lep paradox . while the electroweak precision constraints discussed in sect .  [ sec : constraints ] are more stringent in this case , these constraints primarily affect @xmath57 , which is typically required to be quite large . since this parameter generally does not affect results in the dilepton channel , which depend primarily on @xmath106 and @xmath58 , the same qualitative results obtained for the light - higgs benchmarks also apply here . for hh1 , with @xmath76 gev , a significance level of only @xmath107 can be achieved with @xmath5 of integrated luminosity , due to both the overwhelming sm backgrounds that exist for dilepton processes involving on - shell @xmath80 decay , and a suppressed signal cross - section relative to benchmark lh1 ( which has a far lighter lip ) . for hh3  a benchmark with a somewhat small value of @xmath58  a @xmath108  gev cut helps to cut down the sm backgrounds from processes involving on - shell @xmath80 decay . it is , however , hard to improve upon the statistical significance by implementing additional cuts . the remaining background events which survive this cut ( most of which come from @xmath109 ) tend to have similar @xmath90 and @xmath91 distributions to those of the signal  a situation which makes the application of further , angular cuts essentially redundant . furthermore , since the missing - energy distribution for the signal events in scenarios with small @xmath58 peaks at relatively low values of @xmath110 , there is little to be gained by increasing @xmath111 much beyond the level  ii threshold of @xmath112  gev . by contrast , in scenarios with larger @xmath58 , an elevated missing - energy cut works quite effectively in tandem with the angular cuts in reducing backgrounds from @xmath109 and @xmath113 . a significance level of @xmath114 is reached for hh3 with @xmath5 of integrated luminosity . it is benchmark hh2 , however , which affords the best opportunity for discovery at the lhc from among the heavy - higgs scenarios , with a statistical significance of @xmath115 at @xmath5 of integrated luminosity . this is because the signal for this benchmark can be distinguished from the @xmath79 and @xmath78 backgrounds on the basis of @xmath81 cuts , and from the remaining @xmath109 , @xmath116 , and @xmath113 backgrounds on the basis of @xmath90 , @xmath91 , and @xmath110 cuts in the same manner as for the low - higgs - mass point lh2 . we therefore conclude that even scenarios in which the idm permits an evasion of the lep upper bound on @xmath117 can yield an observable dilepton signal at the lhc .    from the results in table  [ tab : bmxsecscutiii ] , it is evident that the prospects for detecting a signal in the dilepton channel in the idm model hinge primarily on two criteria . the first of these is the dependence of the cross - section for @xmath118 on @xmath87 and @xmath58 . this cross - section is , of course , larger in cases where the pair - production threshold energy @xmath87 is small . among cases with similar values of @xmath87 , those in which @xmath58 is smaller will have larger production cross - sections . this can be understood by noting that the partonic cross - section for this process depends on @xmath106 and @xmath58 in the following way : @xmath119^{3/2}.    \label{eq : partonicxsec}\end{aligned}\ ] ] for values of @xmath120 , for which the dependence of this expression on @xmath106 and @xmath58 is non - negligible , it is apparent that for fixed @xmath121 , @xmath122 decreases with increasing @xmath58 . this accounts for the difference between the @xmath123 production cross - sections for benchmarks lh1 and hh3 quoted in table  [ tab : bmxsecs ] . the second criterion is the relationship between @xmath58 and @xmath6 : cases in which @xmath88 tend to have a higher statistical significance than those in which @xmath124 , as is manifest from comparing the results for benchmarks lh2 and lh1 in table  [ tab : bmxsecscutiii ] . this is because in the latter case , it is difficult to distinguish the signal process from the dominant @xmath78 background on the basis of event topology . on the other hand , when @xmath58 is exceedingly small ( as it is in our lh5 scenario ) , the charged leptons will be so soft that the detector - acceptance ( i.e.  level  i ) cuts will eliminate the vast majority of would - be signal events , as discussed above . between these extremes , a window of @xmath125 emerges within which the prospects for observing a signal are quite good , so long as the lip mass also falls roughly within the @xmath2  gev range . for cases in which @xmath126 , the prospects for discovery at the lhc are reasonable  meaning a statistical significance around the @xmath3 level with 100 @xmath127 of integrated luminosity  only if the dark - matter particle is light ( @xmath75  gev ) . it is not difficult simultaneously to satisfy the constraints discussed in sect . [ sec : constraints ] and to realize a @xmath58 value within this mass window of @xmath2  gev while keeping the lip mass relatively light ( @xmath49 gev )  or , alternatively , to obtain a large mass splitting @xmath128 and a light lip mass of around 40  gev . this is true not only in models where the higgs boson is light and the parameters of the theory comparatively unconstrained , but also in cases in which the mechanism of ref . @xcite for evading electroweak precision bounds on the higgs mass is realized in nature , and @xmath129  gev . in either case , it would be possible to observe a dilepton signal at the lhc at a significance level of @xmath3 or higher , with an integrated luminosity of @xmath5 . in this work , we have investigated the potential for observing a dilepton signature in the inert doublet model at the lhc . we have explored the prospects for a number of benchmark scenarios , including several in which the idm successfully ameliorates the lep paradox and the higgs - boson mass can be elevated as high as @xmath130  gev , as well as several of the dark - matter motivated scenarios cataloged in ref . we have shown that for cases in which the dark matter candidate is relatively light ( @xmath2  gev ) and @xmath131 gev , a signal with a significance of more than @xmath3 should be apparent at the lhc with less than @xmath5 of integrated luminosity . moreover , in cases when the lip is on the lighter end of this range , a @xmath3 discovery would be possible with only @xmath132 of integrated luminosity . in addition , there are also certain cases in which @xmath124 and the lip is light ( @xmath75  gev ) for which the prospects for detection are also reasonably good . of course the observation of an excess in the @xmath133 channel alone , while exciting , is by no means conclusive evidence for the inert doublet model . indeed , many models of beyond - the - standard - model physics lead to such a signature , including weak - scale supersymmetry , two - higgs - doublet models , etc . fortunately , evidence for the idm can come from a number of other sources . some of these sources involve other channels associated with the sm - like higgs at the lhc . one potentially interesting signal could arise due to deviations of the decay properties of the higgs boson @xmath21 from those of a sm higgs . in situations in which @xmath134 , for example , @xmath135 can contribute substantially to the invisible higgs width . searches for the weak - boson fusion ( wbf ) process @xmath136 , with @xmath21 decaying invisibly , can be used effectively to identify a higgs boson at the lhc  @xcite , and preliminary studies  @xcite indicate that a @xmath137 discovery should be possible with only @xmath132 of integrated luminosity in regions of parameter space where @xmath138 is large . moreover , if @xmath139 , the tetralepton + @xmath140 signatures resulting from decays of the form @xmath141 may also be detectable in certain regions of parameter space . the observation of signals of this sort , along with the non - observation of other signals which appear in standard 2hdm due to @xmath142 couplings ( where @xmath143 and @xmath144 and @xmath145 are sm fermions ) absent in the inert doublet model , could together serve to distinguish the idm from other scenarios for physics beyond the standard model . evidence for the idm could also come from a a variety of other sources , including dark - matter - direct - detection experiments and from the observation of energetic gamma - rays  @xcite or neutrinos  @xcite resulting from lip dark matter annihilation . clearly , the particular set of signals that an inert doublet would manifest differs substantially , depending on which of the allowed regions of parameter space the model happened to inhabit , and as we have shown , the @xmath11 channel can provide an important probe into which region that might be . we would like to thank j. alwall and t. han for useful discussions and comments . we would like to thank the aspen center of physics , where part of this work was completed . this work was supported in part by the department of energy under grant  de - 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the inert doublet model is one of the simplest and most versatile scenarios for physics beyond the standard model . in this work , 
 we examine the prospects for detecting the additional fields of this model at the lhc in the dilepton channel . 
 we investigate a wide variety of theoretically- and phenomenologically - motivated benchmark scenarios , and show that within regions of model parameter space in which the dark - matter candidate is relatively light ( between @xmath0 and @xmath1  gev ) and the mass splitting between the neutral scalars is also roughly @xmath2  gev , a signal at the @xmath3 to @xmath4 significance level can be observed with @xmath5 of integrated luminosity . 
 in addition , even if the mass splitting between the neutral scalars is larger than @xmath6 , a signal of more than @xmath3 can be observed as long as the mass of the dark - matter candidate is around 40  gev .    .7ex    .7ex 
 = cmss10 = cmss10 at 7pt