Boosting Searches for Natural SUSY with RPV via Gluino Cascades

0.32 MB·English

by  Zhenyu Han

Boosting Searches for Natural SUSY with RPV
via Gluino Cascades
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0 Zhenyu Han1, Andrey Katz2, Minho Son3,4, and Brock Tweedie5
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a 1 Institute of Theoretical Science, University of Oregon, Eugene, OR 97403
J
2 Center for the Fundamental Laws of Nature, Jefferson Physical Laboratory,
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1 Harvard University, Cambridge, MA 02138

] 3Department of Physics, Yale University, New Haven, CT 06511
h
p 4Dipartimento di Fisica, Universita` di Roma “La Sapienza” and
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p INFN Sezione di Roma, I-00185 Roma, Italy
e
5 Physics Department, Boston University, Boston, MA 02215
h
[

2
v
5 In the presence of even minuscule baryonic R-parity violation, the stop can be the lightest
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0 superpartner and evade LHC searches because it decays into two jets. In order to cover
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this interesting possibility, we here consider new searches for RPV stops produced in gluino
.
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1 cascades. While typical searches for gluinos decaying to stops rely on same-sign dileptons,
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the RPV cascades usually have fewer hard leptons, less excess missing energy, and more
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: jets than R-parity conserving cascades. If the gluino is a Dirac fermion, same-sign dilepton
v
Xi signals are also often highly depleted. We therefore explore search strategies that use single-
r leptonchannels, andcombat backgrounds using H , jet counting, andmoredetailedmultijet
a T
kinematics or jet substructure. We demonstrate that the stop mass peaks can be fully
reconstructed over a broad range of spectra, even given the very high jet multiplicities. This
would not only serve as a “double-discovery” opportunity, but would also be a spectacular
confirmation that the elusive top-partner has been hiding in multijets.
I. INTRODUCTION
We are currently at a very interesting crossroads for particle physics. The Higgs boson,
or something very much like it, has recently been discovered at the LHC [1, 2]. At the same
time, the LHC experiments have been engaging in a large variety of searches for the physics
that stabilizes the Higgs potential, so far without any signs of new phenomena. According
to the usual logic, this is somewhat perplexing. Cancellation of the top loop without fine-
tuning requires the existence of a top-partner at the few-hundred GeV scale, and in most
models this is accompanied by many other new colored particles. The LHC is extremely
efficient at producing colored particles, so if this scenario is correct, then the LHC is already
creating top-partners and other new particles in abundance. Where are they?
The non-observation of new physics has in particular made a major dent in the available
parameter space for one of our best-motivated and well-defined stabilization mechanisms,
supersymmetry (SUSY) (for summaries of experimental searches see [3, 4]). SUSY typically
produces complicated cascade decay chains with copious production of jets, leptons, and—it
isusuallyassumed—missing energyfromaninvisiblelightestsupersymmetric particle(LSP).
One escape route for SUSY is to simply push up the masses of all superparticles beyond the
LHC reach, though at the penalty of re-introducing the fine-tuning that it was invented to
alleviate. A more attractive option from the perspective of naturalness is what has come
to be called “natural SUSY” or “effective SUSY” [5, 6] (see also [7, 8]), where sleptons and
first/second generation squarks are assumed to be very heavy, but the particles necessary
to avoid Higgs sector fine-tuning remain relatively light. Because of its reduced particle
content, natural SUSY can more easily evade constraints from generic SUSY searches [9–
11]. But while, for example, relatively light stops and sbottoms can still be accommodated
by the LHC data, targeted searches for these scenarios are gaining ground [12–16].
As our standard approaches to SUSY have thus far proven unrealized by nature, it is
becoming clear that we must cast a wider net both in terms of our SUSY models and
our strategies for searching for them. This does not entail particularly baroque model-
building, as even modest modifications to minimal SUSY models can radically change their
phenomenology. Oneofthesimplest andwell-knownwaystodothisistoturnonthebaryon-
number-violating and R-parity-violating (RPV) superpotential operator W ucdcdc. If this
∼
operator has even a tiny nonvanishing coefficient, all supersymmetric decay chains are fated
to end in jets instead of the usual missing energy. This turns the LHC’s virtue of copious
colored particle production into a curse, as these signals can easily become lost in the
noise of QCD multijet backgrounds. Nonetheless, the LHC detectors are already proving
themselves quite reliable for reconstructing detailed kinematic features using jets. RPV
signals are uniquely well-suited to this because they can contain multijet resonances, and
more generally jet-lepton resonances. Several papers have pointed out how some of these
resonances might be uncovered [17–20], and searches are already underway [21, 22].
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R-parity violation is a well-motivated possibility from the perspective of natural
SUSY [10]. Completely eliminating the sleptons and first/second generation squarks from
the theory, we can view the remaining particle content as an effective field theory, and allow
for a wide range of ignorance about physics above a scale beyond the LHC’s direct reach,
e.g. (10 TeV). The theory’s UV completion may not even be supersymmetric, as in [23].
O
The main argument for exact R-parity conservation, and the ensuing vanishing of all of
the dimension-four RPV superpotential operators, is that it automatically ensures proton
stability. However, in a natural SUSY effective theory, there are no longer any guarantees
that higher-dimensional R-parity-conserving operators are suppressed by a very high super-
symmetric GUT scale, and we must impose additional symmetries such as exact baryon-
or lepton-number conservation. The requirement of R-parity conservation then becomes
redundant, and we can consider turning on RPV couplings consistent with the more fun-
damental proton-stabilizing symmetries. The size of these couplings is still constrained by
flavor physics measurements (see [24] for review) and other indirect constraints that we
briefly review below, but they are capable of mediating prompt decays at the LHC as long
as they are larger than (10−7).
O
While various aspects of the impact of RPV on SUSY phenomenology at the LHC are
being studied, perhaps the simplest scenario, highly motivated by naturalness yet very dif-
ficult to find at a hadron collider, is a stop LSP that decays directly to two jets. Seemingly
the most straightforward way to search for these particles is to pair produce them and look
for pairs of dijet resonances within four-jet events. Searches of this type have been carried
out by [25, 26] (and can be re-interpreted in terms of stop limits). However, these searches
are far too background-dominated to conclusively find or rule out stops. Depending on the
flavor structure of the ucdcdc superpotential operator, the decay may include a taggable b-jet
that could be exploited in future four-jet searches, but this is far from guaranteed.
A search for t˜ jj in sbottom decays was proposed in [19], capitalizing on the presence
→
of extra leptons from real or virtual W emission in the ˜b t˜ transition. There it was
→
shown that the stop may be visible as a dijet invariant mass peak rising above the back-
grounds. Nonetheless, the coverage is spectrum-dependent, and many additional possible
search options remain open for investigation.
In this paper, we turn our attention to searches for RPV stops produced in gluino decays.
If we take naturalness as a guide, then the gluino cannot be much heavier than a TeV, and
should be within reach of the LHC [10, 11]. Producing stops in this way is potentially very
useful for three reasons. First, gluino cross sections can be large and they tend to produce a
lot of activity in their decays, making the stops easier to spot against background. Second,
gluino decays most directly produce stops in association with top quarks. Since the stops
lose their top quantum number in their RPV decay, becoming a fairly anonymous-looking
dijet resonance, this may be one of the cleanest ways to establish that the resonance is
in fact a top-partner. Third, the gluino itself is an independent target in our searches for
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supersymmetry. Of course, if this turns out to be the first search of this kind that yields a
positive result, the impact of such a “double-discovery” would be quite dramatic.
Recently, there has been a study of gluino cascades into top quarks and RPV stops [27]
that utilizes the same-sign dileptons that can result when a pair of gluinos decays into tops
with equal charge. Searches with same-sign dileptons are a classic way to look for gluinos,
and exploit the fact that they are Majorana particles with no intrinsic sense of charge, as
well as the fact that there are usually multiple chances for lepton production in their cascade
decays. Standard Model backgrounds to same-sign dilepton production are quite modest,
allowing for a very clean search.
Here, we will be pursuing a different strategy. While same-sign dilepton searches can be
expeditious for setting limits, it is not immediately clear that they are actually optimal when
baryonic RPV decays are involved. A standard R-parity conserving gluino decay via stop
produces two topquarks (on-oroff-shell), providing four opportunitiesforleptonproduction
in a g˜g˜ pair event. The overall branching ratio for same-sign dileptons is (10%). The RPV
O
cascade g˜ tt˜∗ t(jj) instead provides one top quark from each side of the event, so the
→ →
overall dilepton branching fraction is the same as in SM tt¯, and only half of the dileptons
are same-sign. The same-sign dilepton branching fraction then shrinks to 2.5%. Some gain
could be made by switching to an inclusive dilepton search strategy, combining same-sign
and opposite-sign, with tt¯+jets becoming the major limiting background for the latter.
However, as pointed out in [28], searches in the l+jets channel with high jet multiplicity also
face tt¯+jets as their leading background, but with much higher branching fractions (30%).
Effectively, for the price of roughly doubling the relative background, we gain a factor of six
in signal rate, and the latter is far more important when working near the edge of discovery
or exclusion reach. To capitalize on this, we therefore do not pursue dilepton-based searches,
but focus on what might be possible with the far more plentiful l+jets.
There is also another reason to consider searches that do not rely on the presence of a
low-background, same-sign dilepton component of the signal: the gluino may be a Dirac
fermion instead of a Majorana fermion. This possibility, pointed out in [29–31], is well-
motivated because it relaxes stringent constraints from low-energy tests such as FCNCs [32]
andneutron-antineutronoscillation[10]. Italsoallowsthegluinotobesomewhatheavier(up
to about 4m ) while still keeping the Higgs potential natural [31]. From the perspective of
t˜
super-QCD, Dirac gluinos carry a well-defined fermion number, and can only be produced in
gluino-antigluino pairs. Their subsequent decays into tops and stops then depend sensitively
on the exact spectrum. In the limit where one species of purely-chiral stop dominates,
gluinos will only decay into one specific sign of top quark, and antigluinos will decay into
the other sign. The subset of events where both tops decay leptonically will then be entirely
opposite-sign. More general spectra can lead to a more mixed composition of same-sign
and opposite-sign, though an imbalance in favor of opposite-sign is generally preferred. Our
l+jets searches will be insensitive to such details, and therefore also uniquely suited to
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covering a broad range of possible spectra with Dirac gluinos.
Both Dirac and Majorana gluinos decaying to RPV stops will contribute an excess of
tt¯+jets, and can be revealed with high purity by placing stringent cuts on the total activity
of these events. The simplest version of such a search would demand a single hard, isolated
lepton produced in association with a large number of jets and a large amount of scalar-
summed jet p (H ), as was pursued in [28]. However, the signal also contains within it
T T
two dijet resonances. These could be revealed if we can manage the combinatoric confusion
presented by the additional jets from the top quark decays and from hard radiation. Being
able to explicitly reconstruct the t˜ jj peak is crucial to establish that we are producing
→
a new particle in association with top quarks, a strong indication that it is indeed an RPV
stop. Havinganadditionalhighly-featuredhandleonthesignalwillalsofurtherimproveS/B
and help reduce uncertainties associated with background modeling. These are important
benefits even from the perspective of just setting limits.
We will pursue two approaches to reconstructing the stop peak, which allow us to reliably
combat combinatorics over a wide range of kinematics. If the mass gap between the gluino
and the stop is small, the top quarks receive little kinetic energy and the jets from the stop
decays are usually amongst the hardest ones in the event. We can then attempt a “best pair
of pairs” approach similar to what is currently being used in searches for direct production
of pairs of dijet resonances [25, 26]. If the mass gap is instead large, then the tops and
stops are both produced with fairly large boost. This reduces combinatorial uncertainties
and renders the events suitable for analysis with jet substructure techniques (see [33, 34] for
reviews). We will see that there is a sizable overlap between the regions of parameter space
where either technique is effective, andthat there areno “weak spots” in the sensitivity. The
combination of the two methods allows discovery-level sensitivity to roughly 1 TeV gluinos
by the end of the 2012 run, for almost any LSP stop mass.
Ourpaperisorganizedasfollows. Inthenextsectionwewillbrieflyreviewourassumption
about RPV as well as existing constraints on baryon-number violation, coming from flavor
physics, other precision tests and cosmological considerations. We then go in detail through
therelevant existing searchesforSUSYandestimatetheLHCconstraintsonourscenario. In
section III, we describe our search strategies and estimate their discovery potential. Finally,
in section IV we conclude. An appendix contains details of our simulations.
II. CONSTRAINTS
Throughout this paper we assume that lepton-number is perfectly conserved, thus pre-
venting proton decay, while baryon-number is violated by the RPV superpotential operator
W = λ ucdcdc . (1)
ijk i j k
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Note that λ is antisymmetric in indices j and k. A stop LSP can then decay into two down-
type jets through, e.g., tcscdc or tcscbc. We will not make any assumptions about the flavor
structure, and therefore do not attempt to capitalize on the possible presence of a b-jet in
the decay.1
Although proton decay is automatically prevented by lepton-number conservation, the
presenceofbaryon-number violationinducesseveral othereffectsthatarehighlyconstrained.
One of the simplest is n n¯ oscillation (see [39] for review). This would bound λ . 10−5,
−
but the constraint is easily avoided if the gluino mass is mostly Dirac. There are also bounds
from K K¯ mixing, which would require λ . 10−2 if we assume mostly universal flavor
−
structure. Other indirect constraints from low-energy experiments tend to be weaker [24].2
At high-energy colliders, direct production of stops can be identified most simply by
looking for pairs of dijet resonances. The decays can be considered prompt as long as
λ & 10−7, a bound which can accommodate the above indirect constraints. The LEP
experiments searched for this process (which is quite analogous to e+e− W+W− 4j),
→ →
placing a limit of m > 90 GeV for pure t˜ [43], with a small allowed region at m
t˜ R t˜
≃
m . Similar searches have been done at the LHC [25, 26], but there multijet production
W
backgrounds swamp the signal.3
Hadron collider searches for gluinos decaying via stops usually make a set of implicit
assumptions that lead to weakened sensitivity when the stops decay via RPV. Nonetheless,
these searches are still capable of setting limits, and in the remainder of this section we
provide some estimates of what parameter space is still available for our own more targeted
searches.
The usual assumption in gluino searches is that all cascades end in the emission of a
neutralino LSP, giving an excess of missing energy beyond what could usually be provided
by, for example, SM tt¯. Some of the most powerful searches also capitalize on the fact
that a gluino pair event can produce four top quarks, providing multiple opportunities for
lepton emission and a high rate of multilepton events, including same-sign (SS) dileptons. A
canonical search of this type demands high missing energy, same-sign dileptons, and usually
a handful of hard (b-)jets. With this event selection, Standard Model backgrounds become
very small, mainly populated by tt¯events where a lepton from b-quark decay is fairly hard
and accidentally categorized as isolated, or an extra W or Z is emitted. Our scenario does
1 However, we note that some theories can favor this, especially those motivated by minimal flavor viola-
tion [35, 36]. Constraints on these scenarios might therefore be augmented by direct searches that have
additional b-tagging demands, such as [15, 37, 38].
2
Cosmologically,baryon-numberviolationcanpotentiallywashoutthebaryonasymmetryproducedinthe
early universe. This is avoided if the coupling is less than (10−6). However, one can also use (1) as a
O
mechanism of electroweak-scalebaryogenesis,requiring more sizable values of λ [40–42].
3 No search of this type has been performed at the Tevatron, though it would likely be capable of making
substantial gains.
5
Majorana gluino Dirac gluino
V)1000 V)1000
e e
G 900 G 900
m (~t m (~t
800 800
700 700
600 600
500 500
400 400
300 300
200 200
100 100
300 400 500 600 700 800 900 1000110012001300 300 400 500 600 700 800 900 1000110012001300
mg~ (GeV) mg~ (GeV)
FIG. 1: Left: Constraints on a Majorana gluino. The blue line is from the ATLAS same-sign
dilepton search (LHC8, 6 fb−1). The purple line is from the ATLAS b′ search (LHC7, 1 fb−1).
The red lines are our estimates for our l+jets (N ,H ) style search (thin: assuming LHC7, 1 fb−1;
j T
thick: assuming LHC8, 5 fb−1), with the boundary defined by S/√S +B = 2. Right: Constraints
on a Dirac gluino. The green line is from the CMS opposite-sign dilepton SUSY search (LHC7,
5 fb−1). The black line is from the ATLAS black hole search (LHC7, 1 fb−1). The purple line is
again ATLAS b′, and the red lines are again our (N ,H ) counting estimates. We do not consider
j T
regions with a g˜ LSP, indicated with dark gray. The light gray line indicates m = m +m .
g˜ t˜ t
not produce four tops, but can still produce same-sign tops decaying to SS dileptons if the
gluino is Majorana.
Generic SUSY searches have also been conducted using the opposite-sign (OS) dilepton
channel, which is especially relevant for our Dirac gluino case. In addition, both Dirac and
Majorana gluinos might be picked up by the large variety of SUSY l+jets searches. These
are especially important for us to understand since our own proposed search strategy uses
the l+jets channel. Finally, as our signal is high-multiplicity and high-energy, we can also
consider possible limits from searches for TeV-scale black holes and pairs fourth generation
down-type fermions with b′¯b′ (tW−)(t¯W+) lνb¯b6j.
→ →
Below, we describe some of the details of these searches. We summarize our estimates of
the most relevant limits in the (m ,m ) plane in Fig. 1, assuming BR(g˜ tt˜) 1. These
g˜ t˜
→ ≡
are supplemented by what could be obtained using our simplest high-multiplicity, high-H
T
search strategy described in section III. The conclusion is that Majorana (Dirac) gluinos at
or above 760 GeV (690 GeV) are completely allowed by existing searches. The strongest
constraints occur when the stop is light, since then the top quark can carry more energy,
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and the lepton p and E/ can be larger. Majorana gluinos are more tightly constrained
T T
than Dirac gluinos when the stop is light because they always contribute to the SS dilepton
channel. Dirac gluinos could be much less constrained since they carry an approximately
conservedtypeoffermionnumber, andinthesimplest scenariosdonotproduceaSSdilepton
signal. However, theyhavetwo-timeslargercrosssection, stillproduceanOSdileptonsignal,
and can also still be visible in black hole and b′ searches. Limits from l+jets SUSY searches
appear to be the weakest for both Dirac and Majorana. In contrast to these non-dedicated
searches, our proposed l+jets search should already be capable of ruling out 1 TeV gluinos
with 5 fb−1 of data at LHC8.
A. Same-sign dileptons
Atpresent, themostpowerfulsearchinthiscategoryusesabout6fb−1 ofdatafromLHC8,
and has been performed by ATLAS [44]. This search demands two SS leptons produced in
association with at least four jets of p > 50 GeV and 150 GeV of E/ . In a simplified
T T
R-parity conserving (RPC) model with a gluino decaying via off-shell stop into two tops and
a light LSP neutralino, gluinos of mass less than 920 GeV are ruled out.
Our gluino pair production signal produces two top quarks in association with four hard
jets from the RPV stop decays. It can contribute to the SS dilepton signal if both tops
have the same charge and both decay semileptonically. This is guaranteed to occur if the
gluino is Majorana, since it then has no sense of fermion number, and can decay with equal
probability into tt˜∗ and t¯t˜. The branching fraction of a complete event into SS dileptons is
nonetheless quite small, as we pay the usual penalty of 5% for dileptonic tops, times an extra
factorof1/2toaccount fortheprobabilitythatthetopshavethesamecharge. Subsequently,
the requirement of four hard jets is almost trivially satisfied. The E/ requirement is most
T
easily met if the stop is somewhat light relative to the gluino, so that the tops can pick up
a large fraction of the available energy in the decay and boost up the leptonically-decaying
W-bosons.
The upper limit on a new physics signal is 6.3 events. For a Majorana gluino with
m = 600GeV andm = (100,200,400)GeV, we estimate that ATLAS would have observed
g˜ t˜
(25,22,3) extra events.4 The smallest allowed m is roughly 350 GeV for this gluino mass.
t˜
Note that the limits become weaker for heavier stops, since then the energy apportioned to
the top quark is squeezed out.
The same channel for the same RPV model was also studied in [27], re-interpreting SS
dilepton searches from LHC7 with up to 2 fb−1 of data. They found gluino mass limits of
550 GeV, roughly independent of the stop mass, in the case where the RPV coupling is
4 We have validated our own Monte Carlo with respect to ATLAS by studying predicted kinematic distri-
butions and event counts for their simplified RPC model.
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unity and mainly in regions of parameter space where the top or stop is off-shell.5 The fully
on-shell region which we have checked, with a more recent analysis from ATLAS, extend
these results.
B. Opposite-sign dileptons
CMS has an OS dilepton search for generic RPC SUSY with 5 fb−1 of LHC7 data [45].
This analysis further requires two hard jets, and places a small variety of H and E/ cuts
T T
defining four signal regions. (H is defined by summing over jets.) For our purposes, the
T
“highH ”regionisthemostconstraining, asoursignaleasilyproduceslargeamountsofH ,
T T
but not necessarily large amounts of E/ . The cuts are H > 600 GeV and E/ > 200 GeV.
T T T
This analysis places some constraints on Majorana gluinos with RPV stops, but they
are much weaker than those from SS dilepton searches. For Majorana gluinos, the rates in
OS and SS are identical, but the backgrounds in the latter are significantly smaller. Dirac
gluinos, on the other hand, might only contribute to OS. Assuming its decay is dominated
by a single species of mostly-LH or mostly-RH stop, an approximate fermion number can
be defined that is conserved throughout both the production and decay. The sensitivity is
also much better for Dirac than Majorana, for two reasons. First, Dirac production cross
sections are twice as big. Second, we assume that dileptonic Dirac gluino pairs are always
OS, whereas for Majorana they are only OS half of the time. The total OS cross section for
Dirac is therefore four times larger than for the equivalent Majorana model.
CMS places an upper limit of 23 new physics events in its high H signal region. For
T
a Dirac gluino with m = 600 GeV and m = (100,200,400) GeV, we estimate that CMS
g˜ t˜
would have observed (24,18,1.2) extra events. The smallest allowed m is therefore close to
t˜
100 GeV for this gluino mass.
5 Wepointoutonesubtletyhere. Thebehaviorintheregionmt˜<mg˜ <mt˜+mt maybehighlydependent
on the RPV coupling strength. For very small RPV coupling (e.g., . 10−5), such as we are implicitly
assumingtoavoidprecisionconstraints,thetopwillalmostalwaysbeoff-shell,andthestopon-shell. The
energy available to the lνb system will therefore completely shut off as mg˜ →mt˜from above, eliminating
the leptonic part of the signal and making limit-setting impossible. However, for (1) RPV coupling,
O
such as it was assumed in [27], the stop can also go off-shell with appreciable rate, allowing the lepton to
stillcarrysomeenergyandnontriviallimits to be set. Whenmg˜ <mt˜,this subtlety is sidestepped asthe
stopis then forced offshell, and the lepton canbe energetic regardlessof the coupling. However,since we
are assuming a stop LSP for naturalness reasons, this case formally falls outside the scope of our paper.
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C. Single lepton
Searches for SUSY in single-lepton channels can be nontrivial to interpret for our models.
Our signal essentially looks like tt¯+jets, which occurs plentifully in the SM and serves as
one of the dominant SUSY backgrounds. SUSY searches therefore tend to craft cuts that
either efficiently eliminate tt¯+jets from the outset or attempt to fit it away using control
regions. Most of the ATLAS searches fall into the former category, by demanding that
m (l,E/ ) > 100 GeV. Since this severely degrades our signal along with the SM tt¯+jets,
T T
we do not study these searches in detail. However, CMS has four recent searches which do
not place such a cut.
Of all of these, probably the most robust for our purposes is [46]. The strategy is to
normalize the backgrounds in a low-H , low-E/ control region, extrapolate to high-H and
T T T
high-E/ using MonteCarlo, andcount. Forthisanalysis, thecontrol regionshouldindeedbe
T
morehighlypurified inbackgrounds relative tothesignal regions, at least form > 400GeV.
g˜
∼
The final analysis cuts demand at least three hard jets, H > 750 or 1000 GeV, E/ > 250,
T T
350, or 450 GeV, and various numbers of b-tags. We find the best sensitivity in the signal
regions with either of the two H cuts, the smallest E/ cut, and no requirements on b-tags.
T T
The strongest constraints are obtained for Dirac gluinos, due to their higher cross section.
However, the search proves to be strictly weaker than the OS dilepton.
The other three searches are likely incapable of placing stronger limits. One search [47]
uses an artificial neural network trained on the RPC model LM0, for which m (l,E/ )
T T
is actually the most powerful individual discriminating variable. Another [48] looks for
deviations in the E/ /√H spectrum, again normalizing backgrounds using control regions.
T T
However, these include a high-H and low-E/ /√H control region, which would actually
T T T
be more signal-pure than their high-H and high-E/ /√H signal region. Even ignoring
T T T
this issue completely, the final sensitivity would still be weaker than [45]. Finally, the most
recent search [49] attempts to exploit differences in the relationship between the lepton and
E/ in SM backgrounds versus RPC SUSY signals. For example, in tt¯+jets, harder E/ tends
T T
to be associated with harder leptons. Exactly the same relationships occur for our signal.
But again completely ignoring this issue, and just checking the counts in the signal regions,
we expect sensitivity at most comparable to the OS dilepton search above.
D. Black hole searches
If TeV-scale black holes exist, they should be produced at the LHC and decay into high-
multiplicity final states of jets, leptons, photons, and neutrinos. Two of the most recent such
searches are [50] and [51]. Neither of these searches places stringent requirements on E/ ,
T
and only the second requires the presence of a lepton. They can therefore be fairly efficient
at capturing the gluino signal, even in the all-hadronic mode in the former case.
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