A meta-analysis of the 8 TeV ATLAS and CMS SUSY searches
Received: November
Published for SISSA by Springer
Benjamin Nachman 0 2
Tom Rudelius 0 1
Cambridge 0
MA 0
U.S.A. 0
Open Access 0
c The Authors. 0
0 Menlo Park , CA 94025 , U.S.A
1 Jefferson Physical Laboratory, Harvard University
2 SLAC National Accelerator Laboratory, Stanford University
Between the ATLAS and CMS collaborations at the LHC, hundreds of individual event selections have been measured in the data to look for evidence of supersymmetry at a center of mass energy of 8 TeV. While there is currently no significant evidence for any particular model of supersymmetry, the large number of searches should have produced some large statistical fluctuations. By analyzing the distribution of p-values from the various searches, we determine that the number of excesses is consistent with the Standard Model only hypothesis. However, we do find a shortage of signal regions with far fewer observed events than expected in both the ATLAS and CMS datasets (at 1.66 and 2.77, respectively, 3.23 combined). While the lack of deficits could be a hint of new physics already in the 8 TeV datasets, it could also be the result of mis-modeling the uncertainty distributions or biases in background estimation methods.
Supersymmetry; Hadron-Hadron Scattering
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meta-analysis of the 8 TeV
ATLAS and
1 Introduction 2 3 4
Constructing the dataset
Results and discussion
The 2012 discovery of the Higgs boson by the ATLAS [1] and CMS [2] experiments at
the Large Hadron Collider (LHC) brought widespread attention to the field of high energy
physics. The discovery also reaffirmed the existence of the hierarchy problem due to the
small value of the Higgs boson mass. One popular solution to the hierarchy problem is
Supersymmetry (SUSY), in which every Standard Model (SM) particle has a
supersymmetric counterpart. Since no SUSY particles have been observed, SUSY must be broken:
the SUSY partner masses are much larger than their SM analogues. However, the LHC
provides a large center of mass energy which could be high enough to produce some of these
SUSY particles. Both ATLAS and CMS have conducted extensive searches for SUSY in a
multitude of final states, with various numbers of jets, leptons, and photons. The kinematic
reach of the detectors have been exploited in order to be sensitive to high mass particles,
which may be produced with a low cross section.
Every search for SUSY consists of three pieces: 1) an event selection to maximize
sensitivity to a particular model of interest, 2) an estimation of the number of SM events
which will pass the selection for a given amount of data, and 3) a comparison of the
measured number of events in the region with the predicted number. As the data are
stochastic, the last step requires a statistical analysis which incorporates the systematic
uncertainties in the predicted event yields. For a particular statistical model of the SM
prediction, if the probability p that the measurement could have been generated from the
prediction is small, then one has evidence for SUSY. One usually converts the value p
into a Gaussian equivalent number of standard deviations and then the generally agreed
CMS have performed many searches. In particular, each analysis usually involves many
selections and so there are hundreds of searches between the two collaborations. Statistical
distribution of p-values, we analyze the compatibility of the 8 TeV ATLAS and CMS SUSY
searches with the SM-only hypothesis. The procedure is similar to the analysis of the
7 TeV SUSY searches [3], with a few additions that are discussed in the analysis and
results sections.
Same sign leptons
Same sign leptons
Regions orthogonal; drop those with
HT > 800 GeV due to overlap with 1402.4770
Regions orthogonal; drop those with
Unknown correlation with 1303.2985 and 1402.4770: remove
Regions orthogonal; use the LS method
for uncertainties when given a choice
Unknown correlation with 1311.4937.
Prefer 1311.4937 as its regions are orthogonal
Arbitrarily pick the low pT region
Unknown overlap with 1404.5801.
Use 1404.5801 as it has more regions.
Regions orthogonal.
Use the two lepton OS regions only.
Use the two lepton OS regions only.
Use signal sensitive regions (as described in the text)
Regions orthogonal.
Constructing the dataset
Even though the 8 TeV dataset was collected in 2012, both ATLAS and CMS are continuing
to analyze the data. Most likely, searches will continue to become public until the 13 TeV
run efforts are fully operational in the beginning of 2015. Therefore, we arbitrarily cutoff
the searches considered for this analysis at the SUSY 2014 conference (July 20, 2014).
This includes 17 ATLAS papers [420] and 12 CMS papers [2132]. The difficulty in
assembling the dataset for the present analysis is to understand the correlations between
measurements. The general strategy is to categorize the various searches by their selections
on jets, leptons, and photons. Two analyses which have non-overlapping requirements (...truncated)