Review of Susy searches and Alexis Kalogeropoulos will be giving this talk. So I
hand it over to you, Alexis, you have 24 minutes and I will remind you five minutes
before the end.
Okay, so thank you for the introduction. So this is the experimental SUSY
overview for the Atlas CMS and LHCb collaborations. So let me start by
reminding you why we need supersymmetry or SUSY first of all. Well, we need it because
simply put it is one of the most promising theories we have, beyond the Standard Model
theories, because as we know, Standard Model is not the final answer.
SUSY has many nice features, for example, it helps solving the hierarchy problem. And
because it allows for a Higgs boson without fine tuning, now for the experimental
point of view, it is also very appealing theory because it provides us with a
plethora of possible signatures and scenarios let alone the excitement if
SUSY were to be found that we would nearly double the standard model known
particles. Also among other things, SUSY can provide a massive Dark Matter
candidate, the lightest superpartner or LSP, if a new symmetry or parity is
invoked, this would basically result in SUSY particles produced in pairs and also
stable LSP which will escape direct detector detection. So, briefly SUSY
searches can be categorized in the following: strong production covers a
rather broad class of inclusive searches and topologies that benefit from the high
production cross section and as such we have already set very strong limits on
gluinos and squarks. Then we have searches for third generation squark pair
production, which are a bit more focused and most if not all of them have been
motivated from naturalness arguments and recently we have been able to tackle very
different corners of the phase space like the top corridor and compressed
regions. Then we have electroweak searches we mostly talk here about neutralino
chargino pair production. What are the main challenges is that one deals with
softer kinematics and very often targeting small mass splittings. In some cases like
searches for direct staus though, we have just been able to get sensitive to those
models. Finally, we have long-lived particle searches where we're interested into
displaced jets and leptons with disappearing tracks and delayed photons. And also other
models, RPV models, which in many cases overlap with other bsm analysis, but
actually this will be covered on the plenary on Saturday which I invite you all
of you to attend. Of course I don't have the time to cover all of this. So today I
will focus mostly on new full run two results. But there are also four very nice
parallel talks covering in more details electroweak soft SUSY third generation
and RPV searches. Further although LHCb to be has no recent SUSY
results, we should not forget some very nice older ones.
Okay, so as an experimentalist, what we do here is how to put our hands on SUSY. One
of the biggest problems is that SUSY comes with relatively small cross sections compared
to the Standard Model processes. But on the other hand, from a phenomenological
point of view, we actually expect final states with large missing transverse energy
MET, objects with high transverse momentum uncertainty, the presence of many
jets and many b quark jets as well. So, over the last years, a lot of effort have
been put and a lot of kinematic variables have been proposed, which actually exploit
these features. And a very typical case which you can see on the top right plot
is the transverse mass. Where due to the presence of extra MET in RPC scenario signal
in the semi-leptonic decays expected to populate the tails. And but we expect
the t-tbar actually to have a kinematic endpoint around the W mass. Further, searches
typically form many signal regions around the search variables to optimize the
sensitivity and sometimes, in many cases, a further optimization is achieved by using also
other objects of the event like the number of leptons and so on. All in all one
can say that the strategy is to surprise overwhelming background processes as much
as possible and this is done with several techniques some of more simple some of
these more complex. So, basically we can categorize background to irreducible
sources and that means, that we can have similar to the signal experimental
signature like presence of genuine MET, same number of leptons jets etc. and
simulation is used for the shape and the normalization is extracted from orthogonal
control regions. Then we have reducible sources, which are mainly associated to
mis-construction effects like fake leptons, mis-constructed met and so on
Mismeasured MET, charge flips. And they are estimated from data because we don't trust
Monte Carlo that much and validated in validation regions. In the end, the
signal regions which are designed exactly to be sensitive to the presence of signals
are used to extract the signal. And in the absence of significant excess, we set
upper limit to the production cross section of the target models. Now, speaking of
interpretation and models, of course, we cannot possibly design and execute an
analysis which would have many free parameters like it happens. We have more
than 100 free parameters, even the simplest full Susy models. So what we do,
we make some assumptions. And we define simplifications resulting to a simple set
of simplified models, as we call them simplified models of supersymmetry, that
basically come the main characteristic is that we have a very limited number of free
parameters. A typical example is the production of SUSY particles decaying to lighter
particles, and the only free parameters are exactly the masses of the mother and
the daughter particles and in many cases, we also connect those two. That the
daughter particle is half for instance the the mother particles. We also
regularly assume that the other sparticles are decoupled, are too heavy and can be
decoupled. Now, one side we know is that typical we assume the ratios are low along the
decay cascades for these particles. In real models, this is not usually the case and
the practical impact is that maybe we are excluding more than we should have been
excluding if we would have been considering real rather than simplified models. Now, one
very important thing to remember, however, while doing a real analysis is that we
have to deal with detector imperfections and ageing effects, which can complicate
our analysis and our life because you have millions of constructed objects that can
survive the analysis cats. So, for that we have invested a lot of effort
understanding the detectors and this is not trivial at all. In CMS, for example,
we compile a list of spurious events which we can then exclude from the offline
analysis. You can see such a nice example I think at the bottom plot on the left,
you see how a problematic event in the CMS ECAL endcap would result in fake
reconstructive MET. And on the right hand you can see the effect of applying those
noise reduction filters on data. You see how much the tails are affected on MET and
this is very important because not forget that missing transparent energy is a prime
object for SUSY sources.
Now, let's start with the analysis review. So we start with gluino strong production.
Gluino in the first and second generation squark searches, typically we
expect large met, many jets and many b quark jets, whereas typically search
variables try to capture the hadronic activity and the scale of the event like
HT which is the scalar sum of the jets of the event or another another variable
that we will see often today, large mass radius where you see that the mass appears
very nicely in the tails. So, I start with a very new CMS result this is a brand
new CMS result focusing on gluino decays to Z bosons and the LSP. Incorporating
also the next to the lightest SUSY particle, neutralino two. Neutralino one and
Neutrino two are mixed states of neutral Higgs and gauge bosons and also
this analysis specifically considers a small mass splitting between the gluino
and the neutralino two to be 50 GeV. And also the neutralino one is very
light and is set at one GeV. And that matters because basically that results into final
states with large MET along with very energetic Z bosons and the additional
soft quarks coming from the Z decays can be actually contained in a single
reconstructed jet within a large radius. Now control regions and search bins
are defined in bins of this MJ variable for the leading and the trailing jet. And you can
see the definition of the signal regions and the control regions on the bottom
left. The control regions with a dark blue are used to estimate the background
normalization. And the light blue is used to extract the MET say, based on the
assumption that basically MET and MJ should have minimal correlation. Now,
bottom right, what we can see it's a very nice sane fit performed on this dark blue
side band regions. When we actually require the leading jet to lie outside and
the subleading to lie inside the Z mass window and you can see how nicely the
signal would peak inside the Z mass window see the signal region. Slide nine
on the left side you see the MET on the signal region and the upper limits on the
production cross section on the considered model excluding nicely gluinos
up to 1.5-1.9 TV
Now we're going to
an ATLAS result, an all hadronic with multi jets and b-tagged jets, hadronic analysis
from ATLAS that considers also a non-zero RPV couplings model with a gluino mediated
top squark production. And where actually baryon number violating
interactions are allowed and stops that means that can decay to strange and
bottom or to down and bottom quarks. MJ again and MET significance are the
main handles to reject background. And since we are dealing with MET significance
for the first time in this talk, so the MET significance in particular, it's a variable
that tests in particular the hypothesis, how compatible the total transverse momentum
is with non interacting particles, as basically it quantifies by how much the
measurement can be associated to possible mis-measurements. So, large jets
large values of this MET significance is linked to non interacting particles. And in
this analysis actually what is also very nice is that MET significance has been
optimized to capture the response of the ATLAS detector. The main backgrounds are
multijet through QCD processes for instance, when we have semileptonic B and C
hadron decays and it is estimated from a lower jet multiplicity control region by
building MET significance templates, because we assume that this should be
mostly uncorrelated with MJ. And on the right you can see a very nice closure
test on the MET significance with the different processes populate the plot. Okay, so that
brings me to the results were on top left you can see the yields in one of the signal
regions. And on the bottom you see the results on this note one of the nonzero
RPV coupling model where the reach is just above 1.5 GeV for stop masses of 400 GeV
and it extends considerably the previous result. On the right the
potential chargino in a mediated simplified model compared on the bottom
with some recent CMS result and as you can see both of them have about the same
sensitivity excluding about two TeV for gluino masses for for light
neutralinos.
Okay,
now another CMS result in one lepton final state. The experimental signature
here is one lepton, multijets, B tag and large MET again, but also this
analysis exploits the presence of initial state radiation jets and the main source
variables are the transverse mass, and the MJ. Now, a property of MJ that is used in this
analysis is that for t-tbar events typically, which can have a large jet
multiplicities and MJ is mostly uncorrelated with the transverse mass
meaning that the t-tbar at the high MT can be estimated from a MJ distribution
from a low MT control region. There are several SRs and control regions being
in MJ, MT, MET, multiplicity of jets and B tagged jets and the backgrounds is estimated
from control regions again under the assumption that MJ and MT are correlated. In
any case potential correlation is taken into account by a correlation factor K,
which as you can see on the bottom right Plot is very close to unity. Now, on top
half of the page, you can see the MJ distribution in one of the signal regions
because there are more than one signal regions and at the bottom left hand the
yields for it. The results are independent with the use of simplified models and
masses up to 2.2 TV for gluinos are excluded for this T1tttt model, which is
this limit is among the strongest limits we have for gluinos. Okay, so
overviewing the gluino searches one can say that the Run two has put very stringent
limits on gluino masses, even more than two TeV gluinos seem to be well
excluded and typically we have weaker limits for chargino and boson mediated
decays because we have softer decay products, but still these limits sit well
above one TeV. On the other hand, we expect to reach even above three TeV at the high
luminosity LHC. Switching gears now, we go to stop searches where basically when can
divide the phase space as a function of the mass difference between the stop and the
neutralino, and that basically defines what kind of decays are kinematically
allowed and as we can have two three or even four body decays. Of very, of
particular interest is the top corridor where basically the mass difference
between the stop and the neutralino is about the top mass meaning that stops are
produced at rest. And this is very, very difficult to do analysis there. And the
t-tbar is the dominant background of course, now once again we have variables which try
to distinguish potential signal against the background. For example top-ness which
differentiates through two-lepton dileptonic events. Of course we have many more
In use transverse mass and MT 2 and so on. And with all of them try to exploit
the presence of extra MET in signals basically the characteristics peculiar
the particular characteristics that signal have. Okay, so first of all, I'll be
showing hadronic stop analysis from Atlas that covers all of these two three and
four body the case as we can see on the top left plot, what is very nice in this
analysis is that different regions were optimised with different variables like
the MET significance, MT, but also they made use of ISR sensitive variables, which
would help you significantly on the top corridor and basically this is formed as
the ratio, just for reference, of MET divided by the PT of the ISR. While they also use
the soft PT in the soft PT region they use also other angular variables, in order to reject
mis-measurement and B soft tagging techniques as well. I think this is just a very good
example where actually you need it shows me that we need dedicated optimization if
we want to reach this difficult corners of the phase space and
different standard model processes dominate on different signal regions and
they are estimated from data in orthogonal control regions. Now, coming to the
results on the bottom half, you can see the protections as a function of stop
versus neutralino and we exclude stop masses for about 1.2 TeV for massless LSP
extending by about 200 GeV previous results. CMS has also the same sensitivity
from another analysis although this particular CMS result analysis was not
optimized for three- and four-body decays.
Okay, now coming to yet another one lepton stop analysis results from ATLAS. This
analysis employs both cut-and-count and shape analysis to enhance sensitivity. And
again typically we have many signal regions optimised after applying several
cuts on sensitive variables, dealing now with backgrounds among the most
important sources are mis-measurements and sources that contribute to lost
lepton, meaning lost lepton is when you have a one of the leptons misidentified or
mis-reconstructed and for that, the top-ness is used which basically quantifies how
well an event can be reconstructed under the hadronic top hypothesis. And what
is also very nice is that there is also a similar CMS one lepton analysis, which
also use top-ness as discriminating variable for this kind of background.
Upper limits are presented on the bottom half of the page and once more we say that
stops above one TeV are well excluded. Coming now to a very to a CMS result which
is also very new. It was released this week targeting two lepton final
states. And among other models it considers chargino mediated decays. The main
variables are MT two and a variant of it which also incorporates the multiplicity
the presence of b tagged jets of the event. Now, the key idea behind the MT two is that
actually, you expect them to have a kinematic endpoint for background events
even if you have two neutrinos in the event. The MET significance again is used here to
suppress mis-reconstructed particles, which can be associated for instance, with
pileup interactions, which is the main source of mis-reconstructed MET. And
just to give you some more information that this follows a chi square
distribution with two degrees of freedom for events with no genuine MET. Now,
right hand shows the distribution of the MET significance in a Drell-Yan
region, and I think that nicely demonstrates how different backgrounds
contribute while worth mentioning that The main background source comes from mismeasured
jets that could enter your MT2 tail distribution in the signal region. Okay, slide
19. That brings me to the result of this analysis. On the left you see the yields
in the control and the signal regions, which are fit simultaneously to extract
the signal. And the absence of any significant excessive results are
interpreted in simplified models, one of which is shown on the right. And this
extends we have stop production to top and a neutralino decays and this
extends previous CMS result by about 125 GeV. Coming back to Atlas we have also a
new result was released a couple of days ago, which was present.
Yes, apologize for interrupting you. You have four minutes left.
Okay, thank you. So this analysis considers the presence of on-shell
Higgs and Z bosons and once again, we found the transverse mass, MET
significance and a multiplicity of jets and b-tagged jets. The analysis considers also
three and one lepton final states, each one of these final states with different
backgrounds, you can see a very nice closure test on the top of the bottom left
on the PT Z-enriched region and the interpretation or one of the considered
models is also shown. And this actually demonstrates a very nice improvement
compared to the previous results practically extending results between
hundred and 300 GeV depending on ??? modes Now another new ATLAS result which was
released recently considers multi b=jets in RPV scenarios, given that it
considers high number of jets and beta jets. The main purpose of this study is
estimated from data. And what is very nice with this analysis it is actually that's
the first analysis that sets limits are to stop decaying exclusively to chargino and a
B-quark. So that brings me to the overview slide for stop searches where as you can
see from the summary plots from CMS top quarks and Atlas, we have excluded
stops lighter than 1.2 TeV for low neutralino masses, the limits are weaker
along the diagonal because we have softer objects. One thing to mention is that both
collaborations incorporate more and more elaborate techniques and variables in
order to increase the rates. Further bottom right, you can see that the high
luminosity LHC we can expect to maybe reach up to two TeV stops. Now going
to electroweakino searches now, they pose a very specific challenge because
of this small production cross section and the soft kinematics that we have. But on the
other hand, we have very clear signatures like MET, multi leptons and other handles.
Of course, we can talk about many different scenarios on searches here like
the gaugino neutralino, gauge mediated and so on and so forth. But what I will
actually do to focus on this talk is about the compressed regions and we have seen
recently that people are trying to use handles like vector boson fusion
topologies and exploit the ISR, the presence of ISR, in the models to increase
sensitivity. So a very nice example of that both is this CMS analysis for the
gaugino neutralino mediated stau production looking for very soft taus
optimizes around the presence of ISR. Without going to too much into details,
the point here is that the star that you see the point, which is very excluded in
this analysis, actually sits in a region, which is way before beyond any any
sensitivity than any other chargino neutralino analyses. And I think the the
take home message here is that we should be smart and start using maybe more VBF
and ISR techniques in order to access these compressed and very difficult regions. So
if one wants to review electroweakino searches, again, we have very strong limits the
strongest are for slepton mediated decays Followed by boson mediated decays. On
the right hand, you can see the summary plots from Atlas and CMS. And I think it's
clear that reaching different corners of the phase space requires well established
methods done more than just going with a good for everything model or strategy. So what we
expect in the future, well, given a high Lumi gives access to models that were
suppressed up to now we can expect that the high luminosity will LHC will be
very interesting because it will allow us to have a lot of the phase space, as you
can see on the projections. Of course, the problem is that these projections have
been made with certain assumptions. But I think the point here is that it will be
the electroweakino searches will get more and more attention in the future. And that
the so called CMS, well, it's it's certain that both ATLAS and CMS have a very rich SUSY
physics program producing very nice results. Of course, we expect more full
run 2 results. On the making, among other things, for example, the full likelihoods
being released by Atlas. And don't forget, as we ramp up and we prepare for Run three,
and looking forward to the high luminosity LHC, we will benefit from
not only from additional Lumi to tackle with very difficult corners and models,
while also using more targeted triggers, sophisticated tools, upgraded and improved
detectors allow us to believe I think that it will be very exciting in this period,
propelling us to uncharted waters. So stick around more will come. Thank you for
your time.
Okay, thank you, Alexis for the very nice overview of Susy searches. So I will now
open the floor for questions. So you can raise your hand, there's a feature at the
bottom of your screen. If you cannot talk. You can also type your question in the
chat and we can read that question. So I don't see any right now. Maybe I will ask
one because I was trying to figure out on slide 17, and maybe 22. But if you go to
slide 17, there's on the Atlas exclusions, you see a significant dip. Whereas on the
CMS side, I don't know if we're comparing apples to apples here, but on the CMS
side, you appear to have searches that excluded that that dip.
Am I missing something or
object? I think it has to do exactly what specific optimizations and maybe more
signal regions defined with b-tags and so on. So I don't think it's something
that it has anything to do with Atlas or CMS detectors that say other than the
strategy that people follow in different analysis.
Okay, thank you. I saw there was a raised hand but then it it went away. So I guess
you You gave him the answer.
I don't think
there was one that is appeared again. Yeah.
Okay Federico, if you want to comment.
I can you hear me? Yes, go ahead.
Yeah, I think the answer was was okay. I believe that the main difference in this
region is the trigger strategy that has been used in the two searches. For
example, in ATLAS, we rely the least at least for this result on the MET
trigger. So we lose a little bit of acceptance in this region where the MET
spectrum is not voir dire, while CMS recovers sensitivity thanks to the single
lepton triggers.
Okay, thank you
don't see other questions.
If we still have Some time I can ask out of curiousity speaking exactly of
triggers and speaking of the future you showed high lumi but in view of the run
three given that maybe we cannot really just go forward in the just in the mass
reach about focus on this difficult regions are there may be new strategies
that are being thought maybe for triggers or specific things that have couldn't
been done now but maybe change of priority.
Say for example,
I could say for instance that one of the certain challenges and we have to deal
with that is that with higher pileup, you will have a problem with the MET due to
fakes. Okay, so the triggers there are pileup dependent, and this is something
that we definitely need if we actually would need to squeeze out more on the
compressed region. So I think there is already there already efforts exactly
optimizing the MET triggers, but also soft lepton triggers because typically the
leptons, the single lepton triggers they have thresholds above 25 or even 30 GeV.
We have strategy we have triggers, dileptonic triggers, which can go lower,
but sometimes this is not enough. You need to go even even lower. Transcription: F. Blekman