CMS-PAS-SMP-20-013

CMS-PAS-SMP-20-013
Search for vector boson scattering at the LHC Run 2 with CMS data in the semi-leptonic $\ell\nu \mathrm{qq}$ final state
CMS Collaboration
July 2021

Abstract: A search for electroweak (EW) vector boson scattering in the semi-leptonic decay $\ell\nu \mathrm{qq}$ channel is reported. The search uses the full Run-II CMS dataset of proton-proton collisions at 13 TeV corresponding to an integrated luminosity of 137 fb $^{-1}$ . Events are selected requiring one lepton (electron or muon), moderate missing transverse momentum, two jets with large pseudorapidity separation and dijet mass, and an additional hadronic signature consistent with the decay of a W/Z boson. Events are separated in two categories: either the hadronically decaying W/Z boson is reconstructed as one large-radius jet, or it is identified as a pair of jets with dijet mass close to the boson mass. The signal strength is measured fitting the shape of multivariate machine learning discriminators, implemented to separate the signal from the backgrounds in each category. The observed EW signal strength is $\mu_{\text{EW}} =$ 0.85 $^{+0.24}_{-0.20}$ $=$ 0.85 $^{+0.21}_{-0.17}$ (syst) $^{+0.12}_{-0.12}$ (stat), corresponding to a signal significance of 4.4 standard deviations (5.1 expected): the first evidence of vector boson scattering in the semileptonic channel at LHC. A simultaneous measurement of the EW and QCD WW and WZ production is also performed in the same phase space and found to be in agreement within uncertainty with the standard model prediction.
Links: CDS record (PDF) ; CADI line (restricted) ; These preliminary results are superseded in this paper, PLB 834 (2022) 137438. The superseded preliminary plots can be found here.

Figures & Tables	Summary	References	CMS Publications

Figures
png pdf	Figure 1: Example of a Feynman diagram contributing to the EW component of the process under study.
png pdf	Figure 2: Analysis workflow: objects and categories selection followed by control regions (CR) and signal regions definition.
png pdf	Figure 3: Postfit distributions of ${m_{jj}^{\text{VBS}}}$ observable in the resolved (left) and boosted (right) signal regions.
png pdf	Figure 3-a: Postfit distributions of ${m_{jj}^{\text{VBS}}}$ observable in the resolved (left) and boosted (right) signal regions.
png pdf	Figure 3-b: Postfit distributions of ${m_{jj}^{\text{VBS}}}$ observable in the resolved (left) and boosted (right) signal regions.
png pdf	Figure 4: The DNN distribution for VBS signal and backgrounds in the resolved (left) and boosted (right) signal regions normalized to 1.
png pdf	Figure 4-a: The DNN distribution for VBS signal and backgrounds in the resolved (left) and boosted (right) signal regions normalized to 1.
png pdf	Figure 4-b: The DNN distribution for VBS signal and backgrounds in the resolved (left) and boosted (right) signal regions normalized to 1.
png pdf	Figure 5: The DNN distribution for the resolved (left) and boosted (right) phase space in the top-quark (upper plots) and W+jets (lower plots) control regions. The post-fit uncertainty band is also shown with all systematic uncertainties included.
png pdf	Figure 5-a: The DNN distribution for the resolved (left) and boosted (right) phase space in the top-quark (upper plots) and W+jets (lower plots) control regions. The post-fit uncertainty band is also shown with all systematic uncertainties included.
png pdf	Figure 5-b: The DNN distribution for the resolved (left) and boosted (right) phase space in the top-quark (upper plots) and W+jets (lower plots) control regions. The post-fit uncertainty band is also shown with all systematic uncertainties included.
png pdf	Figure 5-c: The DNN distribution for the resolved (left) and boosted (right) phase space in the top-quark (upper plots) and W+jets (lower plots) control regions. The post-fit uncertainty band is also shown with all systematic uncertainties included.
png pdf	Figure 5-d: The DNN distribution for the resolved (left) and boosted (right) phase space in the top-quark (upper plots) and W+jets (lower plots) control regions. The post-fit uncertainty band is also shown with all systematic uncertainties included.
png pdf	Figure 6: Results for the EW signal only fit, keeping the QCD WV contribution fixed to the SM prediction. Upper plots: post-fit DNN distributions for the resolved (left) and the boosted (right) signal regions, combining the full Run 2 statistics. The signal contribution is plotted both stacked on top of the background processes and also overlaid to show the signal postfit distribution. The expected yield is the sum of signal and backgrounds. Lower plots: background subtracted DNN distribution for the resolved (left) and the boosted (right) phase space, combining the full Run 2 statistics. Post-fit background yields in each bin are subtracted from data and compared with the signal post-fit distribution, plotted as a red line. The post-fit uncertainty band is also shown in the plots with all systematic uncertainties included.
png pdf	Figure 6-a: Results for the EW signal only fit, keeping the QCD WV contribution fixed to the SM prediction. Upper plots: post-fit DNN distributions for the resolved (left) and the boosted (right) signal regions, combining the full Run 2 statistics. The signal contribution is plotted both stacked on top of the background processes and also overlaid to show the signal postfit distribution. The expected yield is the sum of signal and backgrounds. Lower plots: background subtracted DNN distribution for the resolved (left) and the boosted (right) phase space, combining the full Run 2 statistics. Post-fit background yields in each bin are subtracted from data and compared with the signal post-fit distribution, plotted as a red line. The post-fit uncertainty band is also shown in the plots with all systematic uncertainties included.
png pdf	Figure 6-b: Results for the EW signal only fit, keeping the QCD WV contribution fixed to the SM prediction. Upper plots: post-fit DNN distributions for the resolved (left) and the boosted (right) signal regions, combining the full Run 2 statistics. The signal contribution is plotted both stacked on top of the background processes and also overlaid to show the signal postfit distribution. The expected yield is the sum of signal and backgrounds. Lower plots: background subtracted DNN distribution for the resolved (left) and the boosted (right) phase space, combining the full Run 2 statistics. Post-fit background yields in each bin are subtracted from data and compared with the signal post-fit distribution, plotted as a red line. The post-fit uncertainty band is also shown in the plots with all systematic uncertainties included.
png pdf	Figure 6-c: Results for the EW signal only fit, keeping the QCD WV contribution fixed to the SM prediction. Upper plots: post-fit DNN distributions for the resolved (left) and the boosted (right) signal regions, combining the full Run 2 statistics. The signal contribution is plotted both stacked on top of the background processes and also overlaid to show the signal postfit distribution. The expected yield is the sum of signal and backgrounds. Lower plots: background subtracted DNN distribution for the resolved (left) and the boosted (right) phase space, combining the full Run 2 statistics. Post-fit background yields in each bin are subtracted from data and compared with the signal post-fit distribution, plotted as a red line. The post-fit uncertainty band is also shown in the plots with all systematic uncertainties included.
png pdf	Figure 6-d: Results for the EW signal only fit, keeping the QCD WV contribution fixed to the SM prediction. Upper plots: post-fit DNN distributions for the resolved (left) and the boosted (right) signal regions, combining the full Run 2 statistics. The signal contribution is plotted both stacked on top of the background processes and also overlaid to show the signal postfit distribution. The expected yield is the sum of signal and backgrounds. Lower plots: background subtracted DNN distribution for the resolved (left) and the boosted (right) phase space, combining the full Run 2 statistics. Post-fit background yields in each bin are subtracted from data and compared with the signal post-fit distribution, plotted as a red line. The post-fit uncertainty band is also shown in the plots with all systematic uncertainties included.
png pdf	Figure 7: Simultaneous EW and QCD WV production fit: the expected and observed 68% and 95% confidence level (CL) contours on the signal strengths are shown in the plot. The best-fit result is compatible with the SM prediction within the 68% confidence level area.

Tables
png pdf	Table 1: Variables used as input of the DNN for the resolved and boosted models. The Zeppenfeld variable of a particle X is defined as $Z_{X} = \frac {\eta ^{X} - \bar{\eta}^{\text{VBS}}}{{\Delta \eta ^{\text{VBS}}}}$ , where $\bar{\eta}^{\text{VBS}}$ is the mean $\eta$ of VBS tag-jets, while the centrality [49,6] is $C_{VW} = \min(\Delta \eta _-, \Delta \eta _+)$ , with $\Delta \eta _{+} = \max(\eta ^{\text{VBS}}) - \max(\eta ^{{V_{had}}}, \eta ^{W})$ and $\Delta \eta _{-} = \min(\eta ^{\text{VBS}}) - \min(\eta ^{{V_{had}}}, \eta ^{W})$ . The $\eta ^{W}$ is built assuming the W-mass from the lepton and ${{p_{\mathrm {T}}} ^\text {miss}}$ kinematics.
png pdf	Table 2: Uncertainties breakdown on electroweak SM signal measurement

Summary

The first evidence for the EW vector boson scattering of a WV pair plus two jets in the semi-leptonic channel is reported. Events are separated in two categories: either the hadronically decaying W/Z boson is reconstructed as one large-radius jet, or it is identified as a pair of jets with dijet mass close to the boson mass. Multivariate machine learning discriminators are optimized to separate the signal from background in each category and their output is exploited in the statistical analysis. The large background from single W boson production accompanied by jets is estimated in a data-driven way to reduce the impact of MC mismodeling in this multi-jet phase space.

The observed EW signal strength is 0.85

$^{+0.24}_{-0.20}$

$=$ 0.85

$^{+0.21}_{-0.17}$ (syst)

$^{+0.12}_{-0.12}$ (stat), 1

$^{+0.24}_{-0.22}$ expected, corresponding to an observed significance of the signal of 4.4 standard deviations, 5.1 expected. The electroweak WV fiducial signal cross section defined at parton level requiring all partons to have

${p_{\mathrm{T}}} >$ 10 GeV and at least one pair of outgoing quarks with invariant mass

$m_{\mathrm{qq}} >$ 100 GeV, is measured to be 1.9

$\pm$ 0.5 pb, 2.23

$^{+0.08}_{-0.11}$ (scale)

$^{+0.05}_{-0.05}$ (pdf) pb expected. Considering instead as signal the overall EW and QCD WV plus two jets production, the signal strength is extracted as 0.98

$^{+0.20}_{-0.17}$

$=$ 0.98

$^{+0.19}_{-0.16}$ (syst)

$^{+0.07}_{-0.07}$ (stat), (1

$^{+0.19}_{-0.18}$ expected), corresponding to a cross-section, in the same fiducial phasespace as the EW only fit, of 16.6

$^{+3.4}_{-2.9}$ pb, 16.9

$^{+2.9}_{-2.1}$ (scale)

$^{+0.5}_{-0.5}$ (pdf) pb expected.

Finally a simultaneous two-dimensional fit of the EW and QCD WV production components is also performed. Overall, both the WV EW only measurement and the simultaneous EW and QCD WV ones are consistent between each other and in agreement with the SM predictions within the 68% confidence interval.

References
1	ATLAS, CMS Collaboration	Measurements of the Higgs boson production and decay rates and constraints on its couplings from a combined ATLAS and CMS analysis of the LHC pp collision data at $\sqrt{s}=$ 7 and 8 TeV	JHEP 08 (2016) 045	1606.02266
2	B. W. Lee, C. Quigg, and H. B. Thacker	Weak interactions at very high energies: the role of the Higgs boson mass	PRD 16 (1977) 1519
3	CMS Collaboration	Observation of electroweak production of same-sign W boson pairs in the two jet and two same-sign lepton final state in proton-proton collisions at $\sqrt{s} =$ 13 TeV	PRL 120 (2018) 081801	CMS-SMP-17-004 1709.05822
4	ATLAS Collaboration	Observation of electroweak production of a same-sign $W$ boson pair in association with two jets in pp collisions at $\sqrt{s}=$ 13 TeV with the ATLAS detector	PRL 123 (2019) 161801	1906.03203
5	CMS Collaboration	Measurement of electroweak WZ boson production and search for new physics in WZ + two jets events in pp collisions at $\sqrt{s} =$ 13 TeV	PLB 795 (2019) 281	CMS-SMP-18-001 1901.04060
6	ATLAS Collaboration	Observation of electroweak $W^{\pm}Z$ boson pair production in association with two jets in pp collisions at $\sqrt{s} =$ 13 TeV with the ATLAS detector	PLB 793 (2019) 469	1812.09740
7	ATLAS Collaboration	Observation of electroweak production of two jets and a $Z$ -boson pair with the ATLAS detector at the LHC	2020. Submitted to NPHYS	2004.10612
8	CMS Collaboration	Measurement of vector boson scattering and constraints on anomalous quartic couplings from events with four leptons and two jets in proton-proton collisions at $\sqrt{s}=$ 13 TeV	PLB 774 (2017) 682	CMS-SMP-17-006 1708.02812
9	C. F. Anders et al.	Vector boson scattering: Recent experimental and theory developments	Rev. Phys. 3 (2018) 44	1801.04203
10	ATLAS Collaboration	Search for the electroweak diboson production in association with a high-mass dijet system in semileptonic final states in pp collisions at $\sqrt{s}=$ 13 TeV with the ATLAS detector	PRD 100 (2019) 032007	1905.07714
11	CMS Collaboration	Search for anomalous electroweak production of vector boson pairs in association with two jets in proton-proton collisions at 13 TeV	PLB 798 (2019) 134985	CMS-SMP-18-006 1905.07445
12	CMS Collaboration	The CMS experiment at the CERN LHC	JINST 3 (2008) S08004	CMS-00-001
13	S. Frixione and B. R. Webber	Matching NLO QCD computations and parton shower simulations	JHEP 06 (2002) 029	hep-ph/0204244
14	P. Nason	A New method for combining NLO QCD with shower Monte Carlo algorithms	JHEP 11 (2004) 040	hep-ph/0409146
15	S. Frixione, P. Nason, and C. Oleari	Matching NLO QCD computations with parton shower simulations: the POWHEG method	JHEP 11 (2007) 070	0709.2092
16	S. Alioli, P. Nason, C. Oleari, and E. Re	NLO vector-boson production matched with shower in POWHEG	JHEP 07 (2008) 060	0805.4802
17	S. Alioli, P. Nason, C. Oleari, and E. Re	A general framework for implementing NLO calculations in shower Monte Carlo programs: the POWHEG BOX	JHEP 06 (2010) 043	1002.2581
18	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
19	J. Alwall et al.	Comparative study of various algorithms for the merging of parton showers and matrix elements in hadronic collisions	EPJC 53 (2008) 473	0706.2569
20	J. Campbell and T. Neumann	Precision phenomenology with MCFM	JHEP 12 (2019) 034	1909.09117
21	J. M. Campbell, R. K. Ellis, and W. T. Giele	A multi-threaded version of MCFM	EPJC 75 (2015) 246	1503.06182
22	J. M. Campbell, R. K. Ellis, and C. Williams	Vector boson pair production at the LHC	JHEP 07 (2011) 018	1105.0020
23	J. M. Campell and R. K. Ellis	An update on vector boson pair production at hadron colliders	PRD 60 (1999) 113006	hep-ph/9905386
24	P. Artoisenet, R. Frederix, O. Mattelaer, and R. Rietkerk	Automatic spin-entangled decays of heavy resonances in Monte Carlo simulations	JHEP 03 (2013) 015	1212.3460
25	T. Sjostrand et al.	An Introduction to PYTHIA 8.2	CPC 191 (2015) 159	1410.3012
26	M. Bahr et al.	Herwig++ physics and manual	EPJC 58 (2008) 639	0803.0883
27	R. Covarelli, M. Pellen, and M. Zaro	Vector-boson scattering at the LHC: unraveling the electroweak sector	Int. J. Mod. Phys. A 36 (2021) 2130009	2102.10991
28	A. Ballestrero et al.	Precise predictions for same-sign W-boson scattering at the LHC	EPJC 78 (2018) 671	1803.07943
29	GEANT4 Collaboration	GEANT4: a simulation toolkit	NIMA 506 (2003) 250
30	CMS Collaboration	Particle-flow reconstruction and global event description with the CMS detector	JINST 12 (2017) P10003	CMS-PRF-14-001 1706.04965
31	CMS Collaboration	Performance of the CMS Level-1 trigger in proton-proton collisions at $\sqrt{s} =$ 13 TeV	JINST 15 (2020) P10017	CMS-TRG-17-001 2006.10165
32	CMS Collaboration	The CMS trigger system	JINST 12 (2017) P01020	CMS-TRG-12-001 1609.02366
33	M. Cacciari, G. P. Salam, and G. Soyez	The anti- $k_t$ jet clustering algorithm	JHEP 04 (2008) 063	0802.1189
34	M. Cacciari, G. P. Salam, and G. Soyez	FastJet user manual	EPJC 72 (2012) 1896	1111.6097
35	J. Thaler and K. Van Tilburg	Identifying boosted objects with $N$ -subjettiness	JHEP 03 (2011) 015	1011.2268
36	M. Dasgupta, A. Fregoso, S. Marzani, and G. P. Salam	Towards an understanding of jet substructure	JHEP 09 (2013) 029	1307.0007
37	J. M. Butterworth, A. R. Davison, M. Rubin, and G. P. Salam	Jet substructure as a new Higgs search channel at the LHC	PRL 100 (2008) 242001	0802.2470
38	A. J. Larkoski, S. Marzani, G. Soyez, and J. Thaler	Soft drop	JHEP 05 (2014) 146	1402.2657
39	CMS Collaboration	Identification techniques for highly boosted W bosons that decay into hadrons	JHEP 12 (2014) 017	CMS-JME-13-006 1410.4227
40	CMS Collaboration	Performance of electron reconstruction and selection with the CMS Detector in proton-proton collisions at $\surd$ s = 8 TeV	JINST 10 (2015) P06005	CMS-EGM-13-001 1502.02701
41	CMS Collaboration	Performance of CMS muon reconstruction in pp collision events at $\sqrt{s} =$ 7 TeV	JINST 7 (2012) P10002	CMS-MUO-10-004 1206.4071
42	CMS Collaboration	Performance of missing transverse momentum reconstruction in proton-proton collisions at $\sqrt{s} =$ 13 TeV using the CMS detector	JINST 14 (2019) P07004	CMS-JME-17-001 1903.06078
43	D. Bertolini, P. Harris, M. Low, and N. Tran	Pileup per particle identification	JHEP 10 (2014) 059	1407.6013
44	CMS Collaboration	Identification of heavy-flavour jets with the CMS detector in pp collisions at 13 TeV	JINST 13 (2018) P05011	CMS-BTV-16-002 1712.07158
45	D. P. Kingma and J. Ba	Adam: a method for stochastic optimization	technical report	1412.6980
46	I. Goodfellow, Y. Bengio, and A. Courville		MIT Press
47	S. M. Lundberg and S.-I. Lee	A unified approach to interpreting model predictions	in Proceedings of the 31st International Conference on Neural Information Processing Systems, NIPS'17, p. 4768 Curran Associates Inc., Red Hook, NY, USA	arXiv:1705.07874
48	L. S. Shapley	A value for n-person games.	Contributions to the Theory of Games 2.28 (1953) 303
49	ATLAS Collaboration	Measurement of $W^{\pm}W^{\pm}$ vector-boson scattering and limits on anomalous quartic gauge couplings with the ATLAS detector	PRD 96 (2017) 012007	1611.02428
50	The ATLAS Collaboration, The CMS Collaboration, The LHC Higgs Combination Group	Procedure for the LHC Higgs boson search combination in Summer 2011	CMS-NOTE-2011-005
51	CMS Collaboration	Precision luminosity measurement in proton-proton collisions at $\sqrt{s} =$ 13 TeV in 2015 and 2016 at CMS	2021. "Submitted to EPJC"	CMS-LUM-17-003 2104.01927
52	CMS Collaboration	Jet energy scale and resolution in the CMS experiment in pp collisions at 8 TeV	JINST 12 (2017) P02014	CMS-JME-13-004 1607.03663
53	M. Cacciari et al.	The t anti-t cross-section at 1.8-TeV and 1.96-TeV: a study of the systematics due to parton densities and scale dependence	JHEP 04 (2004) 068	hep-ph/0303085
54	S. Catani, D. de Florian, M. Grazzini, and P. Nason	Soft gluon resummation for Higgs boson production at hadron colliders	JHEP 07 (2003) 028	hep-ph/0306211