My own research on Steem: enhancing the sensitivity to rare new phenomena at the LHC

It is now about two weeks that I did not post anything in English. My time for posting and reading is unfortunately currently very limited. The development of steemstem.io keeps me super busy (I will post an update on @lemouth-dev very soon), and taking part to the @steemalliance working group makes my agenda even more complex.

At the same time, I have also managed to release my first scientific article of the year, entitled ‘Sleptons without hadrons’.

To make a long story short, collaborators and I proposed a novel method to increase the sensitivity of the Large Hadron Collider at CERN to new particles that are weakly interacting (and thus associated with very rare signals).

Our findings demonstrated that the reach of the LHC can be improved by a factor of a few when our method is used instead of more standard ones. And to put things into perpective, a factor of a few is something very large! This is what I will discuss today.

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Vetoing hadrons in a nutshell

The method that we proposed relies on the selection of events (i.e., recorded collisions) where there is little hadronic activity. In other words, the amount of energetic hadrons (composite states bound by the strong interaction) is limited.

Before moving on, let me recall that a specific phenomenon (associated with a new particle) can be rare for two reasons. The first reason corresponds to the fact that the new particle can be heavy. Anything heavy is automatically linked to a small production rate. The second reason is related to the fact that the new particle could be weakly interacting (and by virtue of the weakness of its interactions, its production rate is small).

Our work focused on the second item.

Weakly-interacting particles are ignorant of the strong interaction, so that any LHC event in which such a particle would be produced would feature a small amount of energetic hadrons (hadrons are linked to the strong interaction).

In contrast, in the context of the Standard Model, most common phenomena are associated with an important hadronic activity.

We thus have a potential good handle on weak signals: vetoing the selection of events featuring a large hadronic activity.

This is however used for ages and there is in principle nothing new here. If the hadronic activity (i.e. the amount of energetic hadrons) is above some threshold, the collision event is ignored. If it is below the threshold, it is selected. The novelty of the method that we proposed was to replace the fixed threshold by a dynamic one.

Instead of comparing the hadronic activity to a fixed energy value, we instead decided to compare it with the leptonic activity of the event, i.e. the activity associated with the non-strongly-interacting electrons and muons present in the event. By doing so, we found out that even more Standard Model background events were rejected during the selection process, so that a better handle on the signal process was possible.

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The slepton example

Let us now move on with the first word of the title of my article: sleptons.

In order to illustrate the strength of our method, we picked up supersymmetry, one of the most studied theories extending the Standard Model of particle physics. I recall that we have very good reasons to think that the Standard Model should be extended.

We then focused on one of the most weakly interacting particles of such theories (and thus one of the most harder supersymmetric particles to find), the smuon. This particle is the supersymmetric partner of the Standard Model muon, the heavy brother of the electron (a muon is like a heavy and instable version of the electron).

The corresponding collider signal is very simple: one produces two smuons that then decay each into a muon and a dark matter particle (that is invisible). Therefore, one must search for two muons (that can be reconstructed in the detector) and some missing energy (that can also be reconstructed). The LHC being a strongly-interaction-based machine, this clean final-state is then polluted by some hadronic activity but the latter is limited, as it cannot originate from the signal itself.

Our findings are summarized in the money plot on the left. We present what an analysis using the dynamic jet veto introduced above could do, relatively to what a standard analysis using ‘normal’ vetos does.

In the figure, we hence show the ratio of the sensitivity of the two analyses as a function of the mass of the two new particles, the smuon mass (x-axis) and the dark matter mass (y-axis).

In the area in dark blue, our novel method does worse or as good as the former one. In all other areas, our method does better (the ratio is larger than 1). I think there is no need to discuss further, our proposal is superior in almost all the parameter space!

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Take-home message

SteemSTEM is pushing for science communication on Steem, so that Steem can be used for actual science communication. This is what I have done today, by discussing my own research work.

I have introduced what my collaborators and myself did in our first scientific article of the year. We presented a novel method targetting hypothetical weakly-interacting particles that could be produced at the LHC. The method relies on the fact that those new particles are associated with collision events featuring very few energetic composite states related to the strong interaction, despite the fact the LHC is a strong-interaction-based machine.

By making use of the amount of hadronic activity (the energy of these composite states) and the activity of the non-strongly interacting particles often produced in the decay of weakly-interacting new particles, we have demonstrated that the sensitivity of the LHC to those new guys could be improved by an important number. We have illustrated our findings with a specific example, and it actually works.

Our next step consists now in convincing our experimental colleagues to use the method. This is what I will do today, right after having posted this article :D

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SteemSTEM

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