WZ plus missing-E_T signal from gaugino pair production at LHC7

LHC searches for supersymmetry currently focus on strongly produced sparticles, which are copiously produced if gluinos and squarks have masses of a few hundred GeV. However, in supersymmetric models with heavy scalars, as favored by the decoupling solution to the SUSY flavor and CP problems, and m_{\tg}>500 GeV as indicated by recent LHC results, chargino--neutralino (\tw_1^\pm\tz_2) production is the dominant cross section for m_{\tw_1} \sim m_{\tz_2}<m_{\tg}/3 at LHC with \sqrt{s}=7 TeV (LHC7). Furthermore, if m_{\tz_1}+m_Z \lesssim m_{\tz_2}\lesssim m_{\tz_1}+m_h, then \tz_2 dominantly decays via \tz_2\to\tz_1 Z, while \tw_1 decays via \tw_1\to \tz_1 W. We investigate the LHC7 reach in the WZ + MET channel (for both leptonic and hadronic decays of the W boson) in models with and without the assumption of gaugino mass universality. In the case of the mSUGRA/CMSSM model with heavy squark masses, the LHC7 discovery reach in the WZ+MET channel becomes competetive with the reach in the canonical MET + jets channel for integrated luminosities \sim 30 fb^-1. We also present the LHC7 reach for a simplified model with arbitrary m_{\tz_1} and m_{\tw_1} \sim m_{\tz_2}. Here, we find a reach of up to m_{\tw_1}\sim 200 (250) GeV for 10 (30) fb^-1.

> ∼ 500 GeV as indicated by recent LHC results, chargino-neutralino ( W ± 1 Z 2 ) production is the dominant cross section for m W 1 ∼ m Z 2 < mg/3 at LHC with √ s = 7 TeV (LHC7). Furthermore, if m Z 1 + m Z < ∼ m Z 2 < ∼ m Z 1 + m h , then Z 2 dominantly decays via Z 2 → Z 1 Z, while W 1 decays via W 1 → Z 1 W . We investigate the LHC7 reach in the W Z+ E T channel (for both leptonic and hadronic decays of the W boson) in models with and without the assumption of gaugino mass universality. In the case of the mSUGRA/CMSSM model with heavy squark masses, the LHC7 discovery reach in the W Z+ E T channel becomes competetive with the reach in the canonical E T + jets channel for integrated luminosities ∼ 30 fb −1 . We also present the LHC7 reach for a simplified model with arbitrary m Z 1 and m W 1 ∼ m Z 2 . Here, we find a reach of up to m W 1 ∼ 200 (250) GeV for 10 (30) fb −1 .

Introduction
A major goal of the CERN Large Hadron Collider (LHC) is to test the idea of weak scale supersymmetry (SUSY) [1], wherein superpartners of the Standard Model (SM) particles have masses of the order of 1 TeV. The SUSY searches by the ATLAS and CMS collaborations have reported no signal beyond SM expectations [2,3] in ∼ 1 fb −1 of data. Interpreting their results within the mSUGRA/CMSSM model [4], ATLAS and CMS exclude roughly the mass range mq ∼ mg < ∼ 1 TeV for mq ≃ mg, and mg < ∼ 550 GeV in the case where mq ≫ mg. 1 This reach will soon be extended since each experiment now has ∼ 5 fb −1 of data collected. Analysis of this extended data sample is eagerly anticipated by the HEP community.
Within a large class of SUSY models, it is expected that pair production of strongly interacting sparticles-gg,gq andqq production-constitutes the dominant SUSY production cross sections [6,7]. The gluinos and squarks are then expected to decay through a (possibly lengthy) cascade to lighter sparticles plus SM particles, until the decay chain terminates in the (stable) lightest SUSY particle (LSP) [8]. The LSP is expected from cosmological arguments to be a massive, neutral, weakly interacting particle (such as the lightest neutralino Z 1 ) and so does not deposit energy in the experimental apparatus, giving rise to the classic missing transverse energy ( E T ) signature. Thus, gluino and squark pair production followed by cascade decays is expected to give rise to final states containing multiple isolated leptons, multiple jets and E T [9].
While weak scale supersymmetric models are theoretically very compelling, they do suffer from a variety of problems, including 1. the SUSY flavor problem, 2. the SUSY CP problem, 3. the gravitino problem, and 4. the danger of too rapid proton decay in SUSY grand unified theories (GUTs). All four of these problems are greatly ameliorated if not solved by the decoupling solution, wherein first and second generation sfermion masses are pushed into the multi-TeV regime or even beyond. Naturalness may be maintained in models wherein sparticles that couple directly to the Higgs sector-the third generation scalars and electroweak-inosremain at or below the TeV scale [10,11]. Also, in many SUSY models, it is expected that gaugino mass parameters unify at the GUT scale, in parallel with unification of gauge couplings. Renormalization group running effects result in weak scale gaugino masses occurring in the approximate ratio M 1 : M 2 : M 3 ∼ 1 : 2 : 7. We would thus expect the physical gluinog, the wino-like chargino W 1 and the bino-like neutralino Z 1 to be found with roughly the same mass ratio, provided the superpotential µ-parameter |µ| ≫ M 2 . Consequently, in models with gaugino mass unification, the experimental bounds on the gluino mass impose severe constraints on chargino and neutralino masses. Current analyses do not put independent constraints on the electroweak-ino masses if the gaugino mass unification condition is dropped [12]. Moreover, the relative strengths of signals in various multilepton topologies (as well as the gluino mass reach if the parent-daughter mass difference is sufficiently small) depend sensitively on thẽ g − Z 1 and/org − W 1 mass differences. Finally, an independent discovery of directly produced charginos and neutralinos is essential to elucidate the supersymmetry origin of any excess in the well-studied multilepton plus multijet plus E T channel at the LHC. It is therefore interesting 1 To be precise, in the mSUGRA/CMSSM interpretation, squark masses are varied up to mq < ∼ 2 TeV, giving a gluino mass limit of about mg > ∼ 700 GeV; this limit suffers further weakening for decoupling scalars: see [5]. and relevant to find ways to discover charginos and neutralinos independently of gluinos.
Another point is important to note: as we push the gluino mass to larger values, convolution of thegg subprocess cross sections with parton distribution functions (PDFs) requires sampling higher and higher values of parton fractional momentum x F . For such high values of x F , the parton-parton luminosity is sharply falling. At some point we expect that, despite being strongly-produced, gluino pair production will no longer dominate over electroweak-ino pair production, since these latter reactions will sample the PDFs at much lower values of x F if electroweak-inos are significantly lighter than gluinos.
To illustrate this, we plot in Fig. 1 thegg, W ± 1 Z 2 and W + 1 W − 1 production cross sections in pb at LHC with pp collisions at √ s = 7 TeV. Our results are in NLO QCD from the program Prospino [13]. We take mq ≃ 15 TeV for the first and second generations, in accord with a decoupling solution to the above-mentioned pathologies and, for simplicity, assume universal gaugino masses at the GUT scale. From Fig. 1, we see that gluino-pair production is dominant for mg < ∼ 500 GeV. For higher values of mg, W ± 1 Z 2 production is dominant, followed by W + 1 W − 1 production (the reaction W ± 1 Z 1 has lower cross section, 2 as can be seen e.g. in Fig. 12.23 of Ref. [1]). For LHC with √ s = 14 TeV,gg production remains dominant up to mg ∼ 1 TeV if squarks are very heavy. Since ATLAS and CMS already exclude mg < ∼ 550 GeV when mq is large it may prove fruitful to probe electroweak gaugino pair production in the 2011 data but most of all in the 2012 LHC run. This was recognized early on in [6,7] and also more recently in in [14,15]. Recognizing that the stability of the Higgs sector also requires sub-TeV top squarks, we also show the cross section for top squark pair production for mt 1 = mg by the dotted line 3 in Fig. 1. We see that this cross section also drops off rapidly with the top squark mass. Unless top squarks are exceptionally light (with masses of order m W 1 or smaller, and certainly much smaller than mg), electroweak-ino production remains the dominant mechanism.
Let us next examine the signatures resulting from W 1 Z 2 production. If m Z 2 < M Z +m Z 1 , the well-known trilepton signal provides a golden signature for chargino-neutralino production [7,16] provided only that the branching fraction for neutralino decay is not unduly suppressed [17]. The two-body chargino decay the decay mode Z 2 → Z 1 h turns on and dominates. This is illustrated in Fig. 2, where we show the Z 2 branching fractions versus m Z 2 for a mSUGRA model line with m 0 = 10 TeV, A 0 = −2m 0 , tan β = 25 and µ > 0. We vary m 1/2 to obtain the variation in m Z 2 . In this case, W 1 Z 2 → W Z + Z 1 Z 1 is kinematically allowed for 175 GeV < ∼ m Z 2 < ∼ 250 GeV, which corresponds to gluino masses in the interval 600 GeV < ∼ mg < ∼ 800 GeV. Thus, in this mass range, we expect the single reaction pp → W 1 Z 2 2 For the wino-like W 1 and Z 2 , W 1 Z 2 production occurs via the unsuppressed isotriplet W W 1 Z 2 gauge coupling, whereas the W W 1 Z 1 coupling is strongly suppressed because it arises only due to the subdominant higgsino content of the wino-like chargino and the bino-like neutralino -the W -bino-wino coupling is forbidden by gauge invariance. 3 The LO top squark pair production cross section is determined by QCD and is independent of mg. In other words, for the dotted line, the graph is plotted versus mt 1 . If other third generation squarks are also light, their pair production cross sections are also given by the dotted line with the understanding that the label on the horizontal axis is the corresponding squark mass.   Figure 2: Some prominent branching fractions for Z 2 decay in the mSUGRA model with parameters m 0 = 10 TeV, A 0 = −2m 0 , tan β = 25 and µ > 0. We also show the W 1 → W + Z 1 branching fraction (dotted line).
followed by W 1 → Z 1 W and Z 2 → Z 1 Z to be the dominant SUSY production and decay process at LHC7 for models with full gaugino mass unification. The endpoints of this interval can shift up or down in non-universal mass scenarios.

Trilepton+ E T channel
We begin by examining the viability of the reaction pp → W 1 Z 2 → W Z+ E T for SUSY discovery at LHC7, focusing on the case where both Z and W decay leptonically, resulting in clean trilepton events. It is worth mentioning that the trilepton signal from the decay Z 2 → Z 1 Z where a pair of opposite-sign same-flavor (OS/SF) dileptons reconstruct the Z mass has generally been regarded as unobservable because of large SM background from W Z production. The case where the W decays hadronically will be discussed in Section 3.
For our LHC7 event generation, we use the event generator Isajet 7.79 [18] for signal reactions, while for the simulation of the background events, we use AlpGen [19] and Mad-Graph [20] to compute the hard scattering events and Pythia [21] for the subsequent showering and hadronization. In our simulation, we include the following backgrounds for the W Z+ E T signal: tt, W (ℓν)W (ℓν), W (ℓν)Z(ℓℓ), ZZ, W (ℓν) + tb, Z(ℓℓ) + jets, W (ℓν) + jets, Z(ℓℓ) + bb, Z(ℓℓ) + tt and W + tt. For tt, Z + jets, W + jets, Z + bb and Z + tt we include the full matrix elements for at least two real parton emissions and use the MLM matching algorithm to avoid double counting. For W Z production we include the full matrix elements for the 2 → 4 process K-factors for both signal and background 4 (BG) are included and are computed using Prospino [13] and MCFM [22], respectively.
• Electrons and muons are considered isolated if they have |η| < 2.5, p T (l) > 10 GeV with visible activity within a cone of ∆R < 0.2 about the lepton direction, ΣE cells • We identify hadronic clusters as b-jets if they contain a B hadron with E T (B) > 15 GeV, |η(B)| < 3.0 and ∆R(B, jet) < 0.5. We assume a tagging efficiency of 60% and light quark and gluon jets can be mis-tagged as a b-jet with a probability 1/150 for E T ≤ 100 GeV, 1/50 for E T ≥ 250 GeV, with a linear interpolation for 100 GeV ≤ E T ≤ 250 GeV.
Next, we invoke the following pre-selection cuts on our signal and background event samples to extract those with a ℓ + ℓ − ℓ ′ + E T topology: Pre-Selection Cuts: • n(b − jets) = 0 (to aid in vetoing tt background), • 3 isolated leptons with p T (ℓ) > 20 GeV and 4 For the background processes where the NLO cross section is not known we take the K-factor to be 1.
where two of the leptons in the event must form an OS/SF pair. If more than one OS/SF pairing is possible, the pair which minimizes |m(ℓ + ℓ − ) − M Z | is chosen. The remaining lepton is labeled ℓ ′ . In Fig. 3 we show the E T and transverse mass (m T (ℓ ′ , E T )) distributions for the signal and the SM BG after the pre-selection cuts have been applied. The signal point has m W 1 = 189.3 GeV, m Z 2 = 187.3 GeV and m Z 1 = 89.4 GeV and we only consider W 1 Z 2 production. Due to its relatively light parent mass scale, the signal presents a soft E T spectrum, barely visible above the SM background. This is in strong contrast with events from production of the much heavier gluinos or squarks, where the cascade decays to the LSP result in a usually much harder E T spectrum. Therefore, the usual E T plus jets/leptons searches (optimized to look for strongly produced gluinos and squarks) are insensitive to the W 1 Z 2 signal.
As seen in the upper frame of Fig. 3, after the pre-selection cuts the BG is dominated by ZZ production at low E T and by W Z production for E T > ∼ 20 GeV. The transverse mass m T (ℓ ′ , E T ) from W → ℓ ′ ν ℓ ′ , shown in the lower frame of Fig. 3, falls sharply beyond the expected Jacobian peak at m T = M W . In constrast, the corresponding signal distribution from W 1 Z 2 production extends to considerably larger values due to the presence of the two neutralinos in the final state. Therefore, a m T cut is extremely efficient to suppress the W Z background. This is seen in the lower frame of Fig. 3, where the signal distribution clearly stands out for m T > 100 GeV. However, since a precise prediction for the m T tail from W Z events requires a full detector simulation or data-driven estimates, we define a conservative signal region requiring: The BG cross sections from the dominant SM processes after each of the cuts mentioned above, together with the corresponding cross sections for the representative signal point with m W 1 = 189.3 GeV, m Z 2 = 187.3 GeV and m Z 1 = 89.4 GeV, are shown in Table 1. We stress that the signal shown in Fig. 3 and listed in Table 1 comes exclusively from W 1 Z 2 production. Depending on the sparticle spectrum, the actual signal may be larger if heavier electroweakinos are also accessible, or if gluino and/or squark pair production followed by their cascade decays to the W Z final state is sizeable. Nonetheless, a trilepton signal would be visible with an integrated luminosity of ∼ 10 fb −1 at LHC7 even if light electroweak-inos are the only SUSY particles being produced.

LHC7 Reach
As shown in Table 1 and Fig. 3, for m W 1 = 189.3 GeV, m Z 2 = 187.3 GeV and m Z 1 = 89.4 GeV, only an excess of ∼ 2 events in the trilepton channel (after cuts) would be expected for luminosity of ∼ 5 fb −1 . Thus larger integrated luminosities are required in order to claim a signal. In Fig. 4, we show the signal significance for various integrated luminosities versus m W 1 (solid lines). For now we use a mSUGRA model line with m 0 = 10 TeV, A 0 = −2m 0 , tan β = 25 and µ > 0, and we consider the signal only from W 1 Z 2 production. To allow for the low signal rates, the significance is computed using Poisson statistics. For m W 1 < ∼ 170 GeV, the decay into real Zs is kinematically forbidden-as shown in Fig. 2-and the signal significance (solid lines) sharply drops in this region. In this case, however, the well-studied trilepton signal mentioned earlier from W 1 Z 2 → 3ℓ+ E T where m(ℓ + ℓ − ) < M Z is observable. To illustrate this, we show by dashed lines the signal significance, where the same cuts listed in Table 1 are applied, except for the m T and m(ℓ + ℓ − ) cuts. Since in this region Z 2 and W 1 can decay to off-shell Zs and W s we require instead: As seen from Fig. 4, we confirm that the signal in the low m W 1 region ( < ∼ 170 GeV) is readily observable via this "golden" trilepton channel, due to the large W 1 Z 2 production cross sections and small background. 5 As m W 1 ≃ m Z 2 increases so that the Z 2 → Z 1 Z decay turns on, the significance for our W Z → 3ℓ + E T signal increases, reaching its maximum for m W 1 ∼ 220 GeV. This is due to the fact that, for m W 1 < ∼ 200 GeV, m Z 2 − m Z 1 − M Z < ∼ 15 GeV and the Z 1 's coming from Z 2 decays (and to some extent also those from W 1 decay) are rather soft and so contribute relatively little to both E T and to m T . As a result, the E T > 50 GeV and m T > 125 GeV requirements significantly reduce the signal in this region. As m W 1 increases beyond 220 GeV, the W 1 Z 2 production cross section (after cuts) decreases, and so does the signal significance. Finally, once m Z 2 > m Z 1 + m h (at m W 1 ∼ 255 GeV), the Z 2 → Z 1 h decay turns on and dominates 6 causing the signal to drop sharply.
We remark that for 5 fb −1 of data, we would expect a 2σ effect over essentially the entire region where the decay Z 2 → Z 1 Z dominates. Therefore, the LHC experiments already have accumulated enough luminosity to probe this entire region at ∼ 95% C.L.! However, in the happy circumstance that some excess is seen in the data, ∼ 20 −30 fb −1 of data will be required in order to establish a 5σ discovery. This may indeed be achieved in the 2012 run of LHC7. We note further that the SUSY signal events will contain a distinctive asymmetry of trilepton charges +(+−) vs. − (+−) (where the (+−) pair reconstructs m Z ) that originates from the PDFs since LHC is a pp collider. In contrast, SM backgrounds from tt and Ztt (but not W Z) should have the number of +(+−) events equal to −(+−) events, up to statistical fluctuations. In addition, should a large enough data sample be accrued, the p T (Z) distribution should be well-suited for a Z 2 mass extraction since the production and decay modes are single channel. In Fig. 5, we generalize our results to models with unrelated W 1 and Z 1 masses, i.e. models without gaugino mass universality, taking m Z 2 = m W 1 and µ ≫ M 2 . In this figure, we show the discovery regions for several integrated luminosities. We require the following discovery criteria: • significance > 5σ, • signal/BG> 0.2 and • at least 5 signal events.
The mSUGRA model line with m 0 = 10 TeV, A 0 = −2m 0 , tan β = 25 and µ > 0, assumed in Fig. 4, is shown as the dashed orange line. The purple band shows the kinematically allowed region, where M Z < m Z 2 − m Z 1 < m h . As can be seen, chargino masses up to ∼ 170 GeV can already be probed with 5 fb −1 , if m Z 1 < ∼ 50 GeV. As discussed above, for heavier Z 1 , the m Z 2 − m Z 1 mass gap reduces, resulting in softer m T and E T distributions. Therefore the signal efficiency is reduced, requiring higher luminosities in order to achieve 5σ significance. This effect is seen throughout the m W 1 vs. m Z 1 plane, rendering the narrow region close to m Z 2 − m Z 1 ∼ M Z , where the Z 1 is produced at low p T , inaccessible even for L = 30 fb −1 .
On the other hand, the region where m Z 2 − m Z 1 < ∼ m h results in boosted Z 1 s and can be easily probed until the decay Z 2 → Z 1 + h turns on, c.f. Up to now we have only considered W 1 Z 2 production. Despite having subdominant production cross sections, production of heavier chargino W 2 and neutralinos Z 3,4 usually leads to a harder E T spectrum due to their cascade decay, possibly enhancing the signal. Furthermore, for low m 0 (m 1/2 ) squark (gluino) production and cascade decay can also enhance the trilepton signal. In order to clearly see these effects we choose the A 0 and tan β values from the red curve in Fig. 5 (A 0 = 0 and tan β = 45), where we do not expect the W 1 Z 2 signal to be visible for any value of m 1/2 , even for 30 fb −1 . However, now we perform a scan over the m 0 − m 1/2 plane and include the production from all SUSY particles, including squarks and gluinos. For each point in parameter space, we apply the trilepton cuts shown in Table 1 and take the point to be visible if the discovery criteria listed above are satisfied.
The results are shown in Fig. 6, again for four values of integrated luminosities. All points shown are deemed visible for the corresponding integrated luminosity. The gray regions show the parts of the m 0 − m 1/2 plane excluded by theoretical considerations or by experimental constraints. The purple band across the middle of the plot shows the region in parameter space where M Z < m Z 2 − m Z 1 < m h , while the pink area at low values of m 0 and m 1/2 corresponds to the region where at least 50% of the signal comes from gluino and/or squark production. From Fig. 6 we see that, for heavy squarks (m 0 > 800 GeV), the signal mostly comes from electroweakly produced inos. For an integrated luminosity of 5 fb −1 no points are visible. However, for an integrated luminosity of 10 fb −1 , the enhancement of the signal from gluino and squark production renders a few points at low m 0 and low m 1/2 accessible. For 20 fb −1 the  Figure 6: LHC reach in the mSUGRA plane for various integrated luminosities for the W Z+ E T trilepton signal. The pink region is where gluino and/or squark production contribute to at least half the signal, whereas in the purple band the W Z+ E T channel is accessible via electroweak W 1 Z 2 production. Below the green (orange) solid contours there will be a 5σ signal for SUSY via the optimized jets plus E T LHC7 search discussed in Ref. [24] for 30 fb −1 (20 fb −1 ).
reach extends up to m 0 ∼ 800 GeV and m 1/2 ∼ 300 GeV. Finally, for 30 fb −1 , all of the region where the W Z+ E T channel is open can be probed up to m 1/2 ∼ 350 GeV. In the heavy squark region (m 0 > 800 GeV), the signal is enhanced by W 2 and Z 3 production, allowing the LHC to probe gluino masses up to 900 GeV. We point out that without the enhancement of heavy electroweak-ino production no reach is expected even for 30 fb −1 , as shown by the red curve in Fig. 5. We note that there are also visible points at low m 1/2 , below the M Z < m Z 2 −m Z 1 < m h band, where the Z 2 → Z 1 Z and W 1 → Z 1 W decays are closed, but Zs and W s are still produced from heavier EW-ino decays. It is also worth noting that the focus point (light higgsino) region does not enhance the signal. This is partly due to the more compressed chargino/neutralino spectrum in this region leading to softer p T and E T [25,26]. We stress that in Fig. 6 we have only considered observability via W Z+ E T → ℓ + ℓ − ℓ ′ + E T and for the region below the "W Z band" the golden trilepton signal where the OS/SF dilepton pair has a mass below M Z can be used as a discovery channel, as shown by the dashed lines in Fig. 4.
We also show in Fig. 6 the optimized LHC7 reach in the jets plus E T channel from Ref. [24] (solid lines) for L = 20 fb −1 and 30 fb −1 . These curves correspond to an optimization over several E T plus jets channels with zero leptons and do not include the dedicated cuts for the W Z+ E T signal discussed here. As we can see, for such large integrated luminosities the W Z+ E T trilepton channel is competitive with general purpose searches if squarks are essentially decoupled (mq > ∼ 2 TeV) and the neutralino masses lie in the M Z < m Z 2 − m Z 1 < m h band.
3 The ℓ + ℓ − jj+ E T channel As seen in the last section, the main challenge of the trilepton signal is its small rate, which requires relatively high luminosities for observability. A way to increase the rates from W Z+ E T events is to consider the dilepton channel, where W → jj. However, while the main SM background for the ℓ + ℓ − ℓ ′ + E T channel was weakly produced (W Z), the ℓ + ℓ − jj+ E T channel has an irreducible tt background, which can easily overcome the W Z+ E T signal due to its large cross section. Nonetheless, we will show that once evidence of a W 1 Z 2 signal has been seen in the trilepton channel, a corroborative signal (with lower significance) is expected in the dilepton channel.
In Table 2, we show the cross sections after the pre-selection cuts above for the leading BG processes and the W 1 Z 2 signal for the same chargino and neutralino masses used in Table 1. As seen in the Table, after the pre-selection cuts, the SM BG is dominated by Z + jets, followed by tt. To remove much of the background from Z + jets production, we further require: • E T > 40 GeV, • ∆φ( p jet , E T ) > 0.4 for the three hardest p T jets.
After these cuts have been applied, the SM background becomes dominated by tt, which still surpass the signal by almost two orders of magnitude, as shown in Table 2. However, we have not yet made use of the fact that, for the signal, the dijet invariant mass distribution should reconstruct to m(jj) ∼ M W . Therefore, in addition to the previous cuts, we include: where m(jj) is the invariant mass of the two highest p T jets.
In Fig. 7, we show the m(ℓ + ℓ − ) distribution for signal and background after all the above cuts have been applied. The dominant backgrounds displayed are tt and Z + jets (including Z → ττ ). Due to the E T cut, the remaining Z + jets contribution comes mostly from Z → ττ , with τ s decaying leptonically. Therefore all Z + jets events have m(ℓ + ℓ − ) < M Z . For these dominant backgrounds-tt, Z → ττ etc.-we expect nearly equal contributions of oppositeflavor dileptons (OF): e ± µ ∓ pairs, while signal is all in the SF dilepton channel. Hence the OF    distribution can serve as a background normalization. As seen in Fig. 7, the signal is visible over the tt distribution at m(ℓ + ℓ − ) = M Z . 7 Therefore, after applying the cuts listed above, we also require: • |m(ℓ + ℓ − ) − M Z | < 10 GeV .
As shown in Table 2, after the m(ℓ + ℓ − ) cut has been included, the BG is almost entirely given by tt, which contribution can be estimated using the opposite-flavor dilepton invariant  Table 2 have been applied.
mass, as mentioned above. However, although the dilepton signal rate is considerably superior to the trilepton case, the signal still is significantly below the background. In Fig. 8 we plot the signal significance after the above cuts have been applied for various integrated luminosity values versus m W 1 . As in Fig. 4 we assume a mSUGRA line with m 0 = 10 TeV, A 0 = −2m 0 , tan β = 25 and µ > 0. We see immediately that the significance in the dilepton channel is almost half of the significance in the trilepton channel, shown in Fig. 4. Nevertheless, corroborative evidence at the 2σ level is expected over almost the entire kinematically allowed range for an integrated luminosity of 20-30 fb −1 .

Summary and conclusions
In this paper, we have pointed out that for a class of SUSY models with decoupled matter scalars, m W 1 ∼ m Z 2 < ∼ mg/3 and gluino masses above ∼500 GeV, electroweak production of W 1 Z 2 dominates the SUSY production rate at LHC7. We have examined the case where M Z < m Z 2 − m Z 1 < m h , for which we expect the two-body decay modes Z 2 → Z 1 Z and W 1 → Z 1 W to dominate, leading to rather simple final state topologies including (Z → ℓ + ℓ − ) + (W → ℓ ′ ν ℓ ′ )+ E T (trileptons) and (Z → ℓ + ℓ − ) + (W → qq ′ )+ E T (dilepton plus jets).
Evaluation of the trilepton signal against SM backgrounds shows that the SUSY signal should be observable with a 5σ significance at LHC7 up to m W 1 ∼ 250 GeV (depending on m Z 1 ), for an integrated luminosity of 30 fb −1 . In models with gaugino mass unification, this corresponds to a range in gluino masses of mg ∼ 700 − 900 GeV. Moreover, we find that for most of this region a ∼ 2σ excess is expected in the 5 fb −1 data sample that has already been accumulated. Assuming 30 fb −1 of integrated luminosity at LHC7, the trilepton channel will be competitive in reach with the canonical multijet plus E T search in models with unified gaugino mass parameters. If a signal is seen in the trilepton channel, a 2 − 3.5σ confirmatory signal is also expected in the dilepton plus jets channel for most of the parameter space, thus making a stronger case for the W 1 Z 2 signal. Most importantly, the simultaneous presence of these signals will point to the SUSY origin of any new physics that might be discovered in the 2012 run.