On another side of the dark matter and Dark Sector uh world that is searches for
unconventional signatures and long lived particles.
So, hi, can you hear me? Yes
And we can also see your slides. Okay. So
yeah, then I can get started, I guess. Thanks. Good afternoon, everyone um thanks
to the organizers for this opportunity. So, I will give a talk on the searches for
unconventional signature and long lived particles on behalf of CMS and ATLAS
collaboration. So, our knowledge try to understand what is a long lived particle
and what are its properties. So, from the experimental point of view, you can see
the picture here, it is showing a different signature based on uh whether
particle is neutral charge or any charge. So then if a long lived particle is a neutral
particle, let's say, then uh that decays a macroscopically reconstructible distance
from the P P interaction point. Example in the case of a displaced photon, a displaced jet
etc. Or it can be a charged particle which will decay as a neutral particle or is
quasi stable on the scale of the relevant detector. So, these quasi stable long lived
particles will cross the detector. So, long lived particles have unconventional
signature for example, displaced objects, disappearing objects etc and then these
signatures depend on the lifetime of the particles that is C Tau. So um, then these are since
the entries are unconventional, there are different challenges from the experimental
point of view, first and foremost is a non standard reconstruction for the
displacement and the timing and then sudden sudden and secondly, we have
dedicated triggers for the topologies. Then the third challenge is non standard
background which arise from the detector noise cosmic rays etc. And these should be
estimated from the data. So then uh there are many long lived searches of course performed by
both the CMS and ATLAS experiment. So in this particular talk, I'm showing some
of the recent results, and which I present here. And more you can see uh in the
talks, which happened, I believe already in the parallel session, one was by Allison uh which
were talking about the long lived particles in CMS, and then about the Atlas
get started. So obviously,
it is about the displace vertex and muons with the large impact parameter.
So as I said, it's a new result with the full run two. So this particular search
basically targets a pair production of long lived stop squarks, that decay via a
small RPV coupling into a quark and a muon. And so in the models where we have this sufficiently
small lambda k coupling, and this top squark is the LSP this suppression of the
decay basically causes it to appear a discernible distance from the P P interaction
point where this top squark pair is produced. So this would give rise to a
feature that we have a muon and a high mass vertices that are displaced from the
interaction point yielding a distinctive signature in the collider experience. So,
if we have such a signature, we need definitely a dedicated tracking and the
second vertexing algorithm. And that is what we have. So a dedicated track reconstruction is
performed using the large radius tracking to reconstruct the tracks from the decay of
this long BSM particle, which basically have an impact on a meter that will fall
outside the standard constraints like we're showing in this particular picture there.
Then we also have a dedicated uh secondary vertex reconstruction to uh reconstruct
displaced vertices, and in this particular case, none should be non pointing. So in
this particular search, we have different backgrounds coming from cosmic rays, fake
muons from random hits, semileptonic decays of standard model
dijets. So then how do we select these kind of uh uh interesting uh final state. So we
have two mutually exclusive exclusive trigger based selection. The first one is
based on the cluster based missing transverse energy, and the other one is based on the
muon P T. So in the bottom, I'm showing you the plot about the observed event yield in the
control region, in the validation region, and the signal region. So uh in the signal region uh
in the left is for the missing E T trigger selection and on the right is for the
muon selection, you can see in the missing E T trigger selection we observe signal events,
whereas in the uh muon selection we observe none event, but these are consistent with the expectation
which we see from the backgrounds and where the background basically comes from
the transfer factor in the region that will pass the full selection which we
have. Since everything agrees nicely, so we move on and set limits on the top squark
mass as a functional the lifetime. So this left plot shows that for the proper mean
lifetime of uh let's say point one nanosecond masses below roughly of the top squark mass
up to 171 point seven GeV are excluded. And then if you try to see this plot with the
big try to compare this particular analysis with the bigger picture meaning
comparing with the other analysis, this is where this particular analysis fit in and
we can see basically that it is covering a major unexplored phase space, which was not
covered merely by the other searches which was done earlier. So, this is a very nice
result which came up recently. Then uh we move on uh to the other than analysis which is
performed by CMS. So, its a displaced jet analysis now, this is also uh a full run two analysis uh
on the 132 point one inverse of luminosity. So uh this particular analysis has a particular
topology that we have a pair of objects which are originating at a secondary
vertex. So, there are many different signal models that has been studied by CMS. I'll
explain here the two models. One is the jet jet simplified model and the other
one is the exotic higgs model. So in the jet jet model there we have a long lived
scalar neutral particle x, which are produced by an off shell z boson
mediator, then this each x particle will decay to a quark and an anti-quark pair. And then
we choose a signature weill be having a displaced vertices which will be having a pair
of associated jets to it.
Then the other interesting model which has been added by this 17 and 18 search is
that we consider long lived particles arising from the exotic higgs. In this
particular case you can see that the Higgs boson will decay to uh two long lived scalars and then this
each scalar will decay to a quark and an anti-quark pair. And then there are other models like
the beyond standard model models which have been studied also the split susy, the R P V susy,
etc. And the limits are the limits are also shown in the future slides. So then uh talking about
this analysis, the pre selection is that of course, since it's uh a displaced uh uh object,
we need a dedicated trigger for this. So we have two triggers. One is the displaced
trigger and one is inclusive. And then we have our secondary vertex reconstruction
also there. So the major background, uh which is there is the Q C D multijet
process, and then uh to get the event selection basically be train a B D T on the different
variables. So, let's try to understand these, our variables, a little bit. First
one is the vertex track multiplicity, which is the number of tracks associated
with a vertex, then we have a vertex L X Y significance. So what is that? If you
look at this particular picture here, so uh let's try to understand this. So for each
displaced track, which is associated with a dijet and expected to give one
consistent displaced dijet hypothesis is determined by finding the crossing point
between the track helix which is this one and the dijet direction, which is this
one. So this is the crossing point. And this is what this black dot is there in the
picture. So then this L X Y, which is the uh transverse distance, can be estimated based
on the different quantities which is written here, which is the impact
parameter in 2D plane, track direction and the dijet direction. So this is how we
estimate this L X Y. Then the interesting other variable which we have here is a
K. So this K is basically the sum of the significance of the impact parameter
in the 2D of the uh six leading tracks of the secondary vertex. So you can see a picture
here, which shows this variable, you can see that for the background processes,
which are given by uh this blue distribution, this variable is uh peaking at zero, whereas for
the signal, it is, it is basically having a larger value. So this is a great display
of discrimination with a signal and the background to this variable, which we
got. So we basically train B D T with these particular four variables. Then
after doing that, there is the plot which shows the G B D T score for the data, simulated Q C D
events, and the simulated signal events. So you can see here for the different GBDT
score, we have the different things here. And the last thing, which is the G greater
than 192 is the signal region for us. And this is where we observe one event which
is in consistent with the predicted background expectation, as you can see here.
So then what we do, we basically go ahead and look at the uh different signal models
for this particular search. So we have the combined results for the full run two data. So
on the left here, I'm showing the exclusion limit on the cross section on
the neutral uh long lived particle decaying to the two jets. So this is the jet jet model,
basically the simplified jet jet model. Then in the um middle, here, I'm showing the
RPV model uh the RPV model where the top squark basically decays to a bottom quark and a
charged lepton. So this is a result from CMS and you can see here that from the
CMS side, we're excluding the top squark masses up to 1.6 TeV for the range of lifetime. And then
I tried to compare here the results from the Atlas, which I presented a few slides
before. And one can see that for the Atlas they exclude the top squark
masses up to 1.4 TeV for the range of lifetimes. Do you see these screens now? Yes. So
sorry about that now, let me start from this now, when I was with the slide thirteen,
so, I was talking about the about the yeah, about the exotic decay of Santa Model higgs model. So in
this particular case, I was saying that the branching fraction larger than one person
was excluded for the mean proper decay length between one millimeter and one
meter. And this is the first particular model, this is the first time we looked at
this model and the best limits about setting this particular model.
So uh that brings me to the next search from the Atlas which is a displaced hadronic
jet analysis in the calorimeter. So, this analysis was done on the uh 2016 data.
The particular uh feature for this analysis is that we have a neutral long lived
particles which are decaying in the hadronic calorimeter or at the outer edge
of the electromagnetic calorimeter. So the benchmark model which is considered here
is a neutral long lived scalar which uh basically uh produced from the decay of this heavy boson
and this then this scalar will decay further to the Standard Model fermions. So if
these scalar decay occurs in the calorimeter, then these two resulting
tracks, there will be no activity in the tracker, and these jets will have a high
ratio of the E H over E. So, we will have an energy deposit in the H cal. This is called a
cal ratio jet. And another property will be will be a narrow shower. And then uh at the end,
we required these two cal ratio jets in the analysis. So there were two dedicated
triggers that were developed to target this topology for the low mass and the
high mass for the boson. And so if you look at the uh plot here, basically I'm showing you
the upper image on the branching ratio phi, which is decaying to the two long lived
scalar as a function of the proper decay length. So this pink line is the line from this
particular analysis which is called C R which is the calorimeter region and this was
compared with the previous search which was done in the muon spectrometer. And we
also did the combination of this so you can see this is again is the combined
limit. So at the larger proper decay length we can see that the uh the muon
spectrometer analysis was better, but at the uh lower uh basically lower decay length we
can see the combination is giving us a little bit of gain. So, that was this analysis.
Then the same analysis was done, but uh the hadronic jets were decaying in the inner
detector and the muon spectrometer, so, we use the same model uh from the Atlas side it
is based on thirty six femtobarn inverse, but in this particular case we require that
one long lived particle will decay in the inner detector and the other one in the muon
spectrometer. So, that is the difference with respect to the previous search where we
explicitly require that to decay in the C R. So uh, this uh analysis is particularly
sensitive to the lifetime between the uh centimeters to meters and basically here I'm
showing again the comparison. So, to see basically I'm showing two plots, the top plot
is for the scalar mass of 125 GeV, the bottom plot is for the 200 GeV. So, we see that the
sensitivity to the low mass scalars basically for the 125 GeV increases with
shorter proper life as you see in this particular plot to the left, but this
analysis related to the combined analysis which was done so, if you see this and ID
is this particular analysis the C R plus M S analysis and once we combine these the
sensitivity increases at this a lower proper life then for the lower scalars whereas,
for the high scalars, high mass scalars, this is not the case. So, that was the analysis
which was performed again recently with the 2016 data. So uh now I will talk
about the reinterpretation of the displacement on each analysis using this
recast framework, which came up written recently, just few months back. So, before
talking about the analysis, let's try to understand what is the basics of this
recast framework. So, it is really unfeasible if you try to construct the optimized
analysis for the different variety of physics model. That is not possible. But
if the analysis is optimized for the original model it can offer good sensitivity to
the other model also provided the signatures are similar. So, event selection that can
estimate and the observed data distribution do not change the contents of
the interpretation that is what we have from the original analysis. What will be
required here is only the signal distribution, a new signal model which we
are trying to derive. So, this forms the basis of reinterpretation approach
which basically recast the input to this new decay. So, in summary this
particular recast framework is designed to be used the estimate of background
uncertainty is an observation to test the alternative signal hypothesis. So, with
this particular analysis Atlas has tried to search three model, stealth susy, higgs portal
and dark photon. In this particular slides I'm talking here about the dark photon model and the
interpretation from we see. So, it is another hidden sector model it is the FRVZ model
and here you can see two Feynman diagrams. So, in first case either the dark
fermion decays into a dark photon and the lighter darker fermion, which is a
L S P. Or in the other cases, the dark fermion decays into an L S P and a dark scalar
which will in turn decay into a pair of dark fermions. So, in this particular case, we
will get four dark photons, whereas in this particular case we'll get two dark photons.
So uh, then what we did we looked at the uh cross the limits on the cross section into the
branching fraction as a function of the long lived particle decay near the end basically,
the higgs can decay with two dark photons or four dark photons. So, the top plots
are from the uh uh the particular case of uh higgs one twenty five GeV and the bottom plots are for the 800 GeV. So,
for the higgs 125 GeV, these are the first constraints set uh by the Atlas in this
particular model. And then for the 800 GeV, we have some uh uh uh basically results with the
uh which would be published by the collimated leptons or the light
hadron results. So that was completed. This is this particular analysis and this was
the previous analysis and you you can see the complementarity between the two ends. So
this is basically telling you how the importance of the recasting but that we
really don't need to work through the entire analysis we save things and then we
can just put down the new signal model signal model and obtain the first
constraints, which is really nice. So, then uh moving along. So uh, now, I will talk
about the another interesting search from the CMS which is again based on the full run two
data set, it is about the disappearing tracks. So uh, this particular search
uh basically targets a long lived particle, which will decay within the
C M S tracker.
So if the decay products of this particular particle is undetected, maybe because of
their particular momentum to be reconstructed or because they interact really weakly. So what
we have we will have a disappearing track kind of signature. So this particular
signature is identified as an isolated particle track that we extend from the
interaction region, but after the point of disappearance, it will leave no hit in the muon
or tracking detector or the tracking detector and then it will have only an energy
deposit in the calorimeter set. So, that will be the signature that we will have only
energy deposit in the calorimeter there will be no hit in the muon or the tracking detector
the test bar. So, the benchmark signal model which has been considered here is
the A M S B model. So, the particle mass spectrum of a chargino and neutralino uh that
are nearly degenerate in mass. So, then what will happen is that the chargino is
long lived and then it can reach the CMS tracking detector before it will decay to a uh
neutralino and a uh pion and then neutralino will interact weakly and the
pion will be very soft to be deconstructed. So, that will result in a
disappearing like signature. So uh, let me remind you that CMS has performed this
search at eight and thirteen TeV, but at that point using a three layer pixel tracker,
and the sensitivity was limited to the short tracks. This is the uh result of the
uh previous result as published in J HEP. you can see that we could go only up to the point
one nanosecond. But then in 2017, now we have a new four uh layer pixel tracker. So we can
think about, we can basically take advantage of that. And then we do the
new analysis being resulting in a number of layers with measurement. And the
number of layers are the number of uh tracker layer, which you can see here
is the four, five and greater than or equal to six. And this can result
to the new sensitivity to the shorter particle lifetime, which I'll show in the
next slides. So uh what are the backgrounds for this search? Since the Standard Model
particles do not disappear, so they rarely produce a signature. So the background will be
really I mean, entirely composed of failures of the particle reconstruction or
track finding algorithm. So what can we uh do to control them? So there are two ways. First
is that we explore and mitigate all the possible ways to lose the tracker hits, or the
second one is that we estimate the remaining probability to fall into the signal region. You
have about five minutes left.
Okay, sure. So, then uh the selection which we have. So at the trigger level, since
there is no tracking information available at the level one trigger, we trigger on the
missing transverse momentum. And then finally, the criteria which will define the
condition to have a disappearance, we have two conditions here that the track must
have at least three missing outer hits, which reject basically most of the standard model
tracks. And then the sum of the associated uh um calorimeter energy within this delta less than point
five should be less in ten GeV, and what this condition will do that it will reject
all the electrons and the charged hadrons. So, example electron with significant brem energy
causing a track reconstruction failure will be removed through this particular
condition. So, then uh there I'm showing you the results. So, this is a two
dimensional constraint on the chargino mass and the mean prop as a function of
the proper lifetime for the wino like neutrino in the left plot and higgsino like
neutrino in the right hand side. So these basically results exclude uh charginos below
for the wino like case 884 GeV for a lifetime of 3 nanoseconds, and up to 474 GeV for a
lifetime of one to two nanoseconds. So you can see that using this uh upgraded uh pixel
track pixel detector, we could reach to the this lower lifetime, which was not possible with
our previous searches. And then we had a new interpretation for the 2017 and 18
data for the higgsino like case. And there we exclude up to 750 GeV for a
lifetime of three nanosecond and 175 GeV for a lifetime of point oh nine oh five
nanoseconds which you can also see from these plots that we could go beyond this 10 to the
minus one, which was not the case earlier in this published result. So uh then I'll
just talk a little bit about this neural network based long lived search which we have
performed since um I don't have a lot of time here. So this search was um basically done to
uh enhance the sensitivity with degenerative displaced jet tagger jet tagger. So, basically, we
had a deep neural network based multi class classifier to identify the
displaced jet. So, this uh there is already an uh analysis done with this displaced jet. So what we have
we created this analysis using this dnn classifier. So, what we found I think I
can directly take you to the result. So, what we found there is uh this is the result
here, so, there was a so, in this particular case, I'm showing here the lower
limit on the gluino mass as a function of the lifetime. So, this uh dotted line is the
previous result which we have observed and this uh uh blue line here is the result using this jet tagger.
So, what you can see that there is a significant gain in excluding values of
this gluino masses you can see here, gluino masses, we have gained here up to around 500 GeV that
but then to for the uh C tau above uh one meter, one millimeter. So, this topic is also
competitive with a special dedicated reconstruction that was reported earlier.
But what happened in this region, which was less than one millimeter, we have a bit uh
poor performances here, the tagger performance was degrading, because of our limited
ability actually to tag our uh long lived particle jet in the vicinity of the
primary P P interaction vertex. But the left hand search, which is this particular search is
able to explore the distinguishing kinematic feature of this particular model. So, that is
where this default search was better, but we could do better here with uh this jet tagger.
So uh, okay, so that brings me to the overview of the all the CMS long lived
searches which we have performed. So here we have divided the search into three different
models, you see R P V, R P C, other and I'm highlighting here the two analyses which
I've talked about today, the displaced jet and the disappearing track, which was done
using the uh you can see here with the full run two dataset. And similarly, here are the
results from the Atlas side. And again, I presented today the displaced vertex plus muon,
the I D M S vertex search and two mu jets. And you can see all the luminosities which were used here.
So uh that brings me to the summary. So I have highlighted several unconventional signatures and
long lived searches today from the full run two data set. From the CMS side, I have shown the
disappearing track search, which actually made use of this upgrade to extend the
sensitivity to the lower particular lifetime. And in this particular case, we have added a new
higgsino like interpretation. Again, for full run two, I
showed the CMS displaced jet search, which again, I didn't do so it's not just a
simple reload of the previous analysis, we again added an interpretation from the
new model, which was the exotic decay of standard model higgs boson
And then uh the third search from the full run two I showed from the Atlas, which was
the displaced vertex and muon search and that covered the unexplored phase space, which
was not done earlier. And then uh there are like ongoing effort from Atlas to use
this recast framework, which is really nice to reinterpret their new results, because we don't
have to get that we can just use this framework which is really a nice thing.
And then from this CMS side, so I showed you something very, very quite out of the box
thinking like if you have a neural network based search to target the long lived particle signature.
So, this is what we have at present. So, there are more results which are coming
out so, please stay tuned for that. And as far as moving ahead uh for the long lived
particle searches towards run three, I would say we have new ideas for trigger
reconstruction techniques and application of machine learning all this is going on,
so that we can extend the phase space of the long lived particle and there was also a workshop
recently there all this has been discussed. So, yeah, we are looking
forward to all the new results which will come up and thanks for listening to me and
sorry for this uh trouble in between. Thank you.
Thank you very much for the talk.
And uh I will ask the audience for questions, if there's any
Not yet, so I'm intrigued, of course, by the new ideas for trigger reconstruction
techniques. Could you expand a little on that?
Yeah, so, there are like some signatures, I mean, which which we can see benefit,
because there are some that we need to have this muon P T right where we have
not really gone below 10 GeV and there are a few models which which requires that the
muon P T to be very soft like 4 GeV 5 GeV. So, we need to have some triggers which are
basically designed for these kinds of searches for targeting the long lived
particles. Similarly, the reconstruction techniques, I mean, there are a few models
where uh uh where we are not able to reconstruct the particles like uh in case of uh really uh the jet, the
jet searches where we have this uh the particular jet, which is like narrow, and what we
basically got from is the same for jet. So we need to have some techniques where we
are targeting the particular model which we're interested in and be able to
reconstruct this long lived particle. So all these uh new ideas are coming up for the
trigger we added menus for the trigger. And that is where we are trying to see how we can
improve the search. Because if we're able to go to lower PT for the objects, then
that will definitely have the real impact on the final limits or or, or the objects
which we're seeing at the end of the day. And
that is where I think the new ideas for trigger is really helpful.
Thank you very much.
So last chance to ask any questions here for this session. There is a question and
it's a Alison Elliot. And I'm not sure I can. After the glitch, I don't have the
option to talk. Okay, good. Someone did this for me. So,
yes, please unmute yourself. And you can you can ask the question.
Hi, hi. Can you hear me? Yes we can. Yes.
Okay, thank you. Sorry. Um yeah, I just wanted to follow up on the idea for
triggers question. I'm wondering, and it kind of follows along from the the
previous couple of talks, have you considered data parking for looking for long live
particles especially since you'd you would benefit from low P T muons or low P T
jets or muons you could park the data you could reconstruct it later has has
this been considered at all for long live particle searches?
So is Bhawna speaking
yeah Do you hear me? Yes, no, no, it
looks like I was muted. Okay. So I will say yes, this has been considered in CMS.
I'm from CMS, so I can talk from the CMS side. So we have been really looking into
this parking parking feature and they were some analysts also are done using parking
but not in long lived. But as long lived we are we are, we're taking into account this
feature. I know from CMS side. I do not know from ATLAS side.
Thank you so much. That's really interesting. It'll be really interesting
to see the results with that.
Okay, so I think uh this
possibly concludes our session.
And thanks to all the speakers again, virtual clapping hands, don't know if we have
the option and I probably let it to the
Uber chairs to
say what's going on next, I believe. Yes, thank you everyone because uh the next
session will be starting in nine minutes in another webinar. So please connect there
and we will broadcast uh soon uh the next session also on awebcast. See you there for
the closing plenary session. Bye bye bye.