CMS-SUS-15-011

CMS-SUS-15-011 ; CERN-EP-2016-132
Search for new physics in final states with two opposite-sign, same-flavor leptons, jets, and missing transverse momentum in pp collisions at $\sqrt{s}= $ 13 TeV
CMS Collaboration
4 July 2016
J. High Energy Phys. 12 (2016) 013
Abstract: A search is presented for physics beyond the standard model in final states with two opposite-sign, same-flavor leptons, jets, and missing transverse momentum. The data sample corresponds to an integrated luminosity of 2.3 fb$^{-1}$ of proton-proton collisions at $\sqrt{s}= $ 13 TeV collected with the CMS detector at the LHC in 2015. The analysis uses the invariant mass of the lepton pair, searching for a kinematic edge or a resonant-like excess compatible with the Z boson mass. Both search modes use several event categories in order to increase the sensitivity to new physics. These categories are based on the rapidity of the leptons, the multiplicity of jets and b jets, the scalar sum of jet transverse momenta, and missing transverse momentum. In addition, phase space regions are included for which excesses were reported using the $\sqrt{s}= $ 8 TeV data. A local significance of 2.6$\sigma$ for a kinematic edge search was observed by the CMS Collaboration, and of 3.0$\sigma$ for events compatible with the Z boson mass by the ATLAS Collaboration. The observations in all signal regions are consistent with the expectations from the standard model, and the results are interpreted in the context of simplified models of supersymmetry.
Links: e-print arXiv:1607.00915 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; CADI line (restricted) ;

Figures & Tables	Summary	References	CMS Publications

Figures
png pdf	Figure 1: Diagrams for gluino and $\tilde{ mathrm{ b } }$ pair production and decays realized in the simplified models. The GMSB model targeted by the on-Z search is shown on (a). On (b), the slepton-edge model features characteristic edges in the ${m_{\ell \ell }}$ spectrum given by the mass difference of the $\tilde{\chi}_2^0$ and $\tilde{\chi}^0_1$.
png pdf	Figure 1-a: Diagrams for gluino and $\tilde{ mathrm{ b } }$ pair production and decays realized in the simplified models. The GMSB model targeted by the on-Z search is shown on (a). On (b), the slepton-edge model features characteristic edges in the ${m_{\ell \ell }}$ spectrum given by the mass difference of the $\tilde{\chi}_2^0$ and $\tilde{\chi}^0_1$.
png pdf	Figure 1-b: Diagrams for gluino and $\tilde{ mathrm{ b } }$ pair production and decays realized in the simplified models. The GMSB model targeted by the on-Z search is shown on (a). On (b), the slepton-edge model features characteristic edges in the ${m_{\ell \ell }}$ spectrum given by the mass difference of the $\tilde{\chi}_2^0$ and $\tilde{\chi}^0_1$.
png pdf	Figure 2: Overview of the results in all signal regions of the on-Z search (a) and edge search (b). The labels (c) and (f) refer to central and forward leptons. The data points in black are compared to the background expectation, which is shown as a solid blue line, together with its uncertainty, shown as a light blue band. The background components are shown as a stacked histogram with solid white color for the FS background, solid dark green for DY and dark purple for others.
png pdf	Figure 2-a: Overview of the results in all signal regions of the on-Z search (a) and edge search (b). The labels (c) and (f) refer to central and forward leptons. The data points in black are compared to the background expectation, which is shown as a solid blue line, together with its uncertainty, shown as a light blue band. The background components are shown as a stacked histogram with solid white color for the FS background, solid dark green for DY and dark purple for others.
png pdf	Figure 2-b: Overview of the results in all signal regions of the on-Z search (a) and edge search (b). The labels (c) and (f) refer to central and forward leptons. The data points in black are compared to the background expectation, which is shown as a solid blue line, together with its uncertainty, shown as a light blue band. The background components are shown as a stacked histogram with solid white color for the FS background, solid dark green for DY and dark purple for others.
png pdf	Figure 3: The ${E_{\mathrm {T}}^{\text {miss}}} $ and ${m_{\ell \ell }}$ distributions are shown for data and background predictions in the on-Z ATLAS signal region (a) and for the region where CMS reported an excess in Run 1 (b). The ``Other SM'' category includes WZ, ZZ, and other rare SM backgrounds taken from MC.
png pdf	Figure 3-a: The ${E_{\mathrm {T}}^{\text {miss}}} $ and ${m_{\ell \ell }}$ distributions are shown for data and background predictions in the on-Z ATLAS signal region (a) and for the region where CMS reported an excess in Run 1 (b). The ``Other SM'' category includes WZ, ZZ, and other rare SM backgrounds taken from MC.
png pdf	Figure 3-b: The ${E_{\mathrm {T}}^{\text {miss}}} $ and ${m_{\ell \ell }}$ distributions are shown for data and background predictions in the on-Z ATLAS signal region (a) and for the region where CMS reported an excess in Run 1 (b). The ``Other SM'' category includes WZ, ZZ, and other rare SM backgrounds taken from MC.
png pdf	Figure 4: Cross section upper limits and exclusions contours at 95% CL with the results of the on-Z search interpreted in the GMSB model. The region to the left of the red dotted (black solid) line shows the masses which are excluded by the expected (observed) limit.
png pdf	Figure 5: Cross section upper limits and exclusion contours at 95% CL with the results of the edge search interpreted in the slepton-edge model. The region to the left of the red dotted (black solid) line shows the masses which are excluded by the expected (observed) limit.

Tables
png pdf	Table 1: Summary of $R_\mathrm {SF/OF}$ values obtained in data and simulation using the direct and factorized methods, and the final combination.
png pdf	Table 2: Statistical uncertainties in the normalization of the ${E_{\mathrm {T}}^{\text {miss}}}$ template prediction in the $ {E_{\mathrm {T}}^{\text {miss}}} < $ 50 GeV range, for each signal region. These are taken as a systematic uncertainty in the background prediction. The definitions of SRA, SRB, and ATLAS SR are found in Section 5.1 and Table 4.
png pdf	Table 3: Systematic uncertainties in percentage for the ${E_{\mathrm {T}}^{\text {miss}}} $ template method from the MC closure test, shown for all the on-Z signal regions. The definitions of SRA, SRB, and ATLAS SR are found in Section 5.1 and Table 4.
png pdf	Table 4: Results for the on-Z search, binned as a function of ${H_{\mathrm {T}}} $, the jet multiplicity, the b jet multiplicity, and the missing transverse momentum. In the ATLAS SR an additional requirement is imposed on the angle between the ${E_{\mathrm {T}}^{\text {miss}}}$ and the two leading jets $\Delta \phi _{ {E_{\mathrm {T}}^{\text {miss}}},j_1,j_2} >$ 0.4.
png pdf	Table 5: Results for the edge-like search in all 30 signal regions. The non-FS component of the total background is given separately in the brackets. All signal regions require ${E_{\mathrm {T}}^{\text {miss}}} >$ 150 (100) GeV if ${N_{\text {jets}}} \geq $ 2 (3).
png pdf	Table 6: List of systematic uncertainties taken into account for the signal yields and typical values.

Summary

A search for physics beyond the standard model has been presented in the opposite-sign, same-flavor lepton final state using a data sample of pp collisions collected at a center-of-mass energy of 13 TeV, corresponding to an integrated luminosity of 2.3 fb$^{-1}$, recorded with the CMS detector in 2015. Searches are performed for signals that either produce a kinematic edge, or a peak at the Z boson mass, in the dilepton invariant mass distribution. Comparing the observation to estimates for SM backgrounds obtained from data control samples, no statistically significant evidence for a signal has been observed. Notably, this is true for the two event selections where excesses of 2.6 and 3.0$\sigma$ significance had been observed by the CMS and ATLAS collaborations in their respective 8 TeV results.

The search for events containing an on-shell Z boson is interpreted in a model of gauge-mediated supersymmetry breaking, where the Z bosons are produced in decay chains initiated through gluino pair production, and where the branching ratios have been fixed to 100% to produce the desired topology. Gluino masses below 1.28 TeV for high neutralino masses and 1.03 TeV for low neutralino masses have been excluded, extending the previous exclusion limits derived from a similar analysis at 8 TeV by almost 200 GeV.

The search for an edge is interpreted in a simplified model based on bottom squark pair production, where dilepton mass edges are produced in decay chains containing the two lightest neutralinos and a slepton, where again the branching ratios have been fixed to produce the desired topology. Bottom squark masses below 550 and 620 GeV have been excluded, depending on the $\tilde{\chi}_2^0$ mass. These limits are similar to previous exclusion limits except for low $\tilde{\chi}_2^0$ masses where the excluded limits have been extended by about 100 GeV.

References
1	P. Ramond	Dual theory for free fermions	PRD 3 (1971) 2415
2	Y. A. Gol'fand and E. P. Likhtman	Extension of the algebra of Poincar$ \'e $ group generators and violation of P invariance	JEPTL 13 (1971)323
3	A. Neveu and J. H. Schwarz	Factorizable dual model of pions	NPB 31 (1971) 86
4	D. V. Volkov and V. P. Akulov	Possible universal neutrino interaction	JEPTL 16 (1972)438
5	J. Wess and B. Zumino	A Lagrangian model invariant under supergauge transformations	PLB 49 (1974) 52
6	J. Wess and B. Zumino	Supergauge transformations in four dimensions	NPB 70 (1974) 39
7	P. Fayet	Supergauge invariant extension of the Higgs mechanism and a model for the electron and its neutrino	NPB 90 (1975) 104
8	H. P. Nilles	Supersymmetry, supergravity and particle physics	Phys. Rep. 110 (1984) 1
9	G. R. Farrar and P. Fayet	Phenomenology of the Production, Decay, and Detection of New Hadronic States Associated with Supersymmetry	PLB 76 (1978) 575
10	I. Hinchliffe et al.	Precision SUSY measurements at CERN LHC	PRD 55 (1997) 5520	hep-ph/9610544
11	CMS Collaboration	Search for physics beyond the standard model in events with two leptons, jets, and missing transverse momentum in pp collisions at $ \sqrt{s} = $ 8 TeV	JHEP 04 (2015) 124	CMS-SUS-14-014 1502.06031
12	ATLAS Collaboration	Search for supersymmetry in events containing a same-flavour opposite-sign dilepton pair, jets, and large missing transverse momentum in $ \sqrt{s}= $ 8 TeV pp collisions with the ATLAS detector	EPJC 75 (2015) 318	1503.03290
13	CMS Collaboration	The CMS experiment at the CERN LHC	JINST 3 (2008) S08004	CMS-00-001
14	CMS Collaboration	Performance of electron reconstruction and selection with the CMS detector in proton-proton collisions at $ \sqrt{s} = $ 8 TeV	JINST 10 (2015) P06005	CMS-EGM-13-001 1502.02701
15	CMS Collaboration	Particle--flow event reconstruction in CMS and performance for jets, taus, and $ E_{\mathrm{T}}^{\text{miss}} $	CDS
16	CMS Collaboration	Commissioning of the particle-flow event reconstruction with the first LHC collisions recorded in the CMS detector	CDS
17	M. Cacciari, G. P. Salam, and G. Soyez	The anti-$ k_t $ jet clustering algorithm	JHEP 04 (2008) 063	0802.1189
18	M. Cacciari, G. P. Salam, and G. Soyez	FastJet user manual	EPJC 72 (2012) 1896	1111.6097
19	M. Cacciari and G. P. Salam	Dispelling the N$ ^3 $ myth for the $ k_t $ jet-finder	PLB 641 (2006) 57	hep-ph/0512210
20	CMS Collaboration	Determination of Jet Energy Calibration and Transverse Momentum Resolution in CMS	JINST 6 (2011) P11002	CMS-JME-10-011 1107.4277
21	M. Cacciari and G. P. Salam	Pileup subtraction using jet areas	PLB 659 (2008) 119	0707.1378
22	CMS Collaboration	Identification of b quark jets at the CMS Experiment in the LHC Run 2	CMS-PAS-BTV-15-001	CMS-PAS-BTV-15-001
23	J. Alwall et al.	The automated computation of tree-level and next-to-leading order differential cross sections, and their matching to parton shower simulations	JHEP 07 (2014) 079	1405.0301
24	S. Alioli, P. Nason, C. Oleari, and E. Re	NLO single-top production matched with shower in POWHEG: $ s $- and $ t $-channel contributions	JHEP 09 (2009) 111	0907.4076
25	E. Re	Single-top Wt-channel production matched with parton showers using the POWHEG method	EPJC 71 (2011) 1547	1009.2450
26	R. Gavin, Y. Li, F. Petriello, and S. Quackenbush	FEWZ 2.0: A code for hadronic Z production at next-to-next-to-leading order	CPC 182 (2011) 2388	1011.3540
27	R. Gavin, Y. Li, F. Petriello, and S. Quackenbush	W Physics at the LHC with FEWZ 2.1	CPC 184 (2013) 208	1201.5896
28	M. Czakon and A. Mitov	Top++: A Program for the Calculation of the Top-Pair Cross-Section at Hadron Colliders	CPC 185 (2014) 2930	1112.5675
29	C. Borschensky et al.	Squark and gluino production cross sections in pp collisions at $ \sqrt{s} = $ 13, 14, 33 and 100 TeV	EPJC 74 (2014), no. 12	1407.5066
30	S. Frixione, P. Nason, and C. Oleari	Matching NLO QCD computations with Parton Shower simulations: the POWHEG method	JHEP 11 (2007) 070	0709.2092
31	T. Sjostrand, S. Mrenna, and P. Skands	A brief introduction to PYTHIA 8.1	Comp. Phys. Comm. 178 (2008) 852	0710.3820
32	NNPDF Collaboration	Parton distributions for the LHC Run II	JHEP 04 (2015) 040	1410.8849
33	GEANT4 Collaboration	GEANT4 --- a simulation toolkit	NIMA 506 (2003) 250
34	S. Abdullin et al.	The fast simulation of the CMS detector at LHC	J. Phys. Conf. Ser. 331 (2011) 032049
35	S. P. Martin	A supersymmetry primer	in Perspectives on Supersymmetry II, G. L. Kane, ed Adv. Ser. Direct. High Energy Phys., vol. 21 (2010) 1	hep-ph/9709356
36	T. Junk	Confidence level computation for combining searches with small statistics	NIMA 434 (1999) 435	hep-ex/9902006
37	A. L. Read	Presentation of search results: the $ CL_s $ technique	JPG 28 (2002) 2693
38	ATLAS and CMS Collaborations	Procedure for the LHC Higgs boson search combination in summer 2011	CMS-NOTE-2011-005
39	G. Cowan, K. Cranmer, E. Gross, and O. Vitells	Asymptotic formulae for likelihood-based tests of new physics	EPJC 71 (2011) 1554	1007.1727
40	CMS Collaboration	CMS Luminosity Measurement for the 2015 Data Taking Period	CMS-PAS-LUM-15-001	CMS-PAS-LUM-15-001