ATLAS releases new results in search for weakly-interacting supersymmetric particles

May 18, 2017
The distribution of missing transverse momentum (ETmiss) in events with three electrons or muons. Solid histograms indicate Standard Model background processes, points with error bars indicate the data, and the dashed lines indicate hypothetical models with weakly-interacting supersymmetric particles. The arrow indicates the region used to search for a deviation with respect to the Standard Model. The bottom plot shows the ratio of the data to the total Standard Model background. Credit: Image: ATLAS Collaboration/CERN

Supersymmetry is an extension to the Standard Model that may explain the origin of dark matter and pave the way to a grand unified theory of nature. For each particle of the Standard Model, supersymmetry introduces an exotic new "super-partner," which may be produced in proton-proton collisions. Searching for these particles is currently one of the top priorities of the LHC physics program. A discovery would transform our understanding of the building blocks of matter and the fundamental forces, leading to a paradigm shift in physics similar to when Einstein's relativity superseded classical Newtonian physics in the early 20th century.

Supersymmetric particles (or "sparticles") are grouped into two categories with different properties that depend on the strength of their interactions with protons. Strongly-interactingsparticles may be produced with large rates and lead to striking, energetic events in the detector. Weakly-interacting sparticles are produced at lower rates and lead to less striking signatures, making them more difficult to distinguish from Standard Model background processes.

Since the LHC collision energy was increased from 8 to 13 trillion (TeV) in Run 2 to enhance the discovery reach, a wide variety of searches for strongly-interacting sparticles have been performed. Null results in these searches indicate that if they exist, strongly-interacting sparticles must be very heavy – at least several hundred times heavier than the proton. Due to the smaller production rates, larger data samples are required to probe weakly-interacting sparticles, and more optimized selection criteria are required to tease apart the small signal from the background.

ATLAS physicists presented one of the first Run 2 searches for weakly-interacting sparticles at the LHCP 2017 conference. The search targets the production of sparticles called charginos, heavy neutralinos, and sleptons. If produced at the LHC, these particles would decay to leptons (electrons or their heavier cousins, the muons) and stable dark particles called light neutralinos. These dark matter neutralinos would carry away unseen energy since they do not interact with the detector, leading to unbalanced collision events that appear to violate momentum conservation. This "missing transverse momentum" is the key signature exploited by the ATLAS detector to infer the production of .

The analysis selected collision events containing two or three electrons and muons and large missing transverse momentum. The figure shows the measured distribution (data points) of missing transverse momentum in events with three leptons, compared to that expected from the Standard Model (coloured histogram). No significant deviation from the expectation was observed. The results were used to set stringent limits on weakly-interacting sparticles with masses as large as 1150 billion electron volts (GeV), the heaviest such particles yet probed at ATLAS.

Weakly-interacting sparticles may have eluded detection in this search if they are produced with very small rates or do not produce much energy in the detector. Both of these features are expected in models with light higgsinos, the super-partners of the Higgs boson. Future searches will exploit larger data samples to achieve sensitivity to even smaller production rates. Improvements to these searches are underway that employ reduced lepton momentum thresholds and novel signal vs. background discriminating variables to enhance the sensitivity to models that produce even less energy in the detector. A discovery in these searches could shed light on the nature of and help resolve the "hierarchy problem," a fundamental theoretical shortcoming of the Standard Model leading to a predicted Higgs boson mass that is some 16 orders of magnitude too large.

Explore further: Hunting for the superpartner of the top quark

More information: Presentation by Antonella De Santo: SUSY electroweak searches with ATLAS - indico.cern.ch/event/517784/contributions/2464474/attachments/1461628/2257949/LHCP2017_EWSUSY_draft1.1_16_9_format.pdf

Presentation by Till Eifert: Electroweak Supersymmetry - indico.cern.ch/event/517784/contributions/2464474/attachments/1461628/2257949/LHCP2017_EWSUSY_draft1.1_16_9_format.pdf

Related Stories

Hunting for the superpartner of the top quark

May 17, 2017

Supersymmetry (SUSY) is one of the most attractive theories extending the Standard Model of particle physics. SUSY would provide a solution to several of the Standard Model's unanswered questions, by more than doubling the ...

ATLAS experiment seeks new insight into the Standard Model

May 10, 2017

Ever since the LHC collided its first protons in 2009, the ATLAS Collaboration has been persistently studying their interactions with increasing precision. To this day, it has always observed them to be as expected by the ...

The search for sparticles

March 7, 2011

One of the key theories underpinning modern physics is being tested by the latest results from the LHC’s ATLAS experiment.

The ATLAS Experiment's quest for the lost arc

March 28, 2017

Nature has surprised physicists many times in history and certainly will do so again. Therefore, physicists have to keep an open mind when searching for phenomena beyond the Standard Model. 

Recommended for you

Neptune: Neutralizer-free plasma propulsion

May 23, 2017

Plasma propulsion is an important and efficient technology used to control spacecraft for Earth observation, communications and fundamental exploration of outer space.

7 comments

Adjust slider to filter visible comments by rank

Display comments: newest first

IronhorseA
not rated yet May 18, 2017
"when Einstein's relativity superseded classical Newtonian physics in the early 20th century."

Don't tell Leonard Susskind, in his book 'the theoretical minimum' he referred to relativity as a 'classical' theory.
El_Nose
not rated yet May 18, 2017
considering that it was published in 2013 special relativity was penned in 1905 and general relativity 1915 ish. !00 years constitutes a classic. Newton was the 17th century.

Its all relative i guess.
Whydening Gyre
5 / 5 (1) May 18, 2017
I don't think it superceded. I think it "added to" or "refined"...
Hyperfuzzy
3 / 5 (2) May 18, 2017
Should not your theory define matter, not the other way around. Anyway, Maxwell has defined the building blocks of matter, charge, undeniable. The uncertain postulates make no sense to me. Everything is made of charge, it doesn't matter how smart you are in mathematics, it always breaks down to charge. Axiom, charge exist, ... follow the dots
Whydening Gyre
not rated yet May 18, 2017
Should not your theory define matter, not the other way around. Anyway, Maxwell has defined the building blocks of matter, charge, undeniable. The uncertain postulates make no sense to me. Everything is made of charge, it doesn't matter how smart you are in mathematics, it always breaks down to charge. Axiom, charge exist, ... follow the dots

Isn't that axio N ?
RobertKarlStonjek
not rated yet May 18, 2017
..or, put another way, 'let's rule out this hand waving theory once and for all'...

...and wasn't the Higgs supposed to have a supersymmetric partner **LIGHTER* than itself??
physman
not rated yet May 19, 2017
@hyperfuzzy how much charge is a neutrino made from?

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.