CMS-EXO-16-044

CMS-EXO-16-044 ; CERN-EP-2018-061
Search for disappearing tracks as a signature of new long-lived particles in proton-proton collisions at $\sqrt{s} = $ 13 TeV
CMS Collaboration
19 April 2018
JHEP 08 (2018) 016
Abstract: A search is presented for long-lived charged particles that decay within the CMS detector and produce the signature of a disappearing track. A disappearing track is an isolated track with missing hits in the outer layers of the silicon tracker, little or no energy in associated calorimeter deposits, and no associated hits in the muon detectors. This search uses data collected with the CMS detector in 2015 and 2016 from proton-proton collisions at a center-of-mass energy of 13 TeV at the LHC, corresponding to an integrated luminosity of 38.4 fb$^{-1}$. The results of the search are interpreted in the context of the anomaly-mediated supersymmetry breaking model. The data are consistent with the background-only hypothesis. Limits are set on the product of the cross section for direct production of charginos and their branching fraction to a neutralino and a pion, as a function of the chargino mass and lifetime. At 95% confidence level, charginos with masses below 715 (695) GeV are excluded for a lifetime of 3 (7) ns, as are charginos with lifetimes from 0.5 to 60 ns for a mass of 505 GeV. These are the most stringent limits using a disappearing track signature on this signal model for chargino lifetimes above $ \approx $ 0.7 ns.
Links: e-print arXiv:1804.07321 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; HepData record ; CADI line (restricted) ;

Figures & Tables	Summary	Additional Tables	References	CMS Publications

Figures
png pdf	Figure 1: Distributions of the maximum difference in $\phi $ between any two jets (left) and the difference in $\phi $ between the ${\vec{p}_{\mathrm {T}}}$ of the leading jet and ${\vec{p}_{\mathrm {T}}^{\text {miss}}}$ (right) for events passing the basic selection, before either of the requirements on these two variables is imposed. The data is from the 2016 data-taking period, and the blue dashed lines show the distributions for simulated signal events with a chargino that has a lifetime of 100 cm/$c$ and mass of 300 GeV, with a corresponding production cross section of 0.58 pb. The gray shaded area indicates the statistical uncertainty in the SM background, and the leftmost bin of the left plot includes events with only one selected jet. The vertical dashed lines indicate the chosen value for the requirement on each variable, and the arrows indicate which events are selected.
png pdf	Figure 1-a: Distribution of the maximum difference in $\phi $ between any two jets for events passing the basic selection, before the requirements on this variable is imposed. The data is from the 2016 data-taking period, and the blue dashed lines show the distribution for simulated signal events with a chargino that has a lifetime of 100 cm/$c$ and mass of 300 GeV, with a corresponding production cross section of 0.58 pb. The gray shaded area indicates the statistical uncertainty in the SM background. The leftmost bin includes events with only one selected jet. The vertical dashed line indicates the chosen value for the requirement on the variable, and the arrow indicates which events are selected.
png pdf	Figure 1-b: Distribution of the difference in $\phi $ between the ${\vec{p}_{\mathrm {T}}}$ of the leading jet and ${\vec{p}_{\mathrm {T}}^{\text {miss}}}$ for events passing the basic selection, before the requirements on this variable is imposed. The data is from the 2016 data-taking period, and the blue dashed lines show the distribution for simulated signal events with a chargino that has a lifetime of 100 cm/$c$ and mass of 300 GeV, with a corresponding production cross section of 0.58 pb. The gray shaded area indicates the statistical uncertainty in the SM background. The vertical dashed line indicates the chosen value for the requirement on the variable, and the arrow indicates which events are selected.
png pdf	Figure 2: Distributions of the number of missing outer hits for tracks in simulation that pass the full selection, except for the requirement on that variable. Each signal distribution and the sum of the SM background distributions are scaled to have unit area. The gray shaded area indicates the statistical uncertainty in the SM background.
png pdf	Figure 3: The expected and observed 95% CL upper limits on the product of the cross section for direct production of charginos and their branching fraction to $ {\tilde{\chi}^{0}_{1}} {\pi ^\mathrm {{\pm}}}$ as a function of chargino mass for chargino lifetimes of 10, 100, and 1000 cm/$c$. The direct chargino production cross section includes both $ {\tilde{\chi}^{0}_{1}} {\tilde{\chi}^\pm _{1}} $ and $ {\tilde{\chi}^\pm _{1}} {\tilde{\chi}^\mp _{1}} $ production in roughly a 2:1 ratio for all chargino masses considered. The dashed red line indicates the theoretical prediction for the AMSB model.
png pdf root	Figure 3-a: The expected and observed 95% CL upper limits on the product of the cross section for direct production of charginos and their branching fraction to $ {\tilde{\chi}^{0}_{1}} {\pi ^\mathrm {{\pm}}}$ as a function of chargino mass for chargino lifetime of 10 cm/$c$. The direct chargino production cross section includes both $ {\tilde{\chi}^{0}_{1}} {\tilde{\chi}^\pm _{1}} $ and $ {\tilde{\chi}^\pm _{1}} {\tilde{\chi}^\mp _{1}} $ production in roughly a 2:1 ratio for all chargino masses considered. The dashed red line indicates the theoretical prediction for the AMSB model.
png pdf root	Figure 3-b: The expected and observed 95% CL upper limits on the product of the cross section for direct production of charginos and their branching fraction to $ {\tilde{\chi}^{0}_{1}} {\pi ^\mathrm {{\pm}}}$ as a function of chargino mass for chargino lifetime of 100 cm/$c$. The direct chargino production cross section includes both $ {\tilde{\chi}^{0}_{1}} {\tilde{\chi}^\pm _{1}} $ and $ {\tilde{\chi}^\pm _{1}} {\tilde{\chi}^\mp _{1}} $ production in roughly a 2:1 ratio for all chargino masses considered. The dashed red line indicates the theoretical prediction for the AMSB model.
png pdf root	Figure 3-c: The expected and observed 95% CL upper limits on the product of the cross section for direct production of charginos and their branching fraction to $ {\tilde{\chi}^{0}_{1}} {\pi ^\mathrm {{\pm}}}$ as a function of chargino mass for chargino lifetime of 1000 cm/$c$. The direct chargino production cross section includes both $ {\tilde{\chi}^{0}_{1}} {\tilde{\chi}^\pm _{1}} $ and $ {\tilde{\chi}^\pm _{1}} {\tilde{\chi}^\mp _{1}} $ production in roughly a 2:1 ratio for all chargino masses considered. The dashed red line indicates the theoretical prediction for the AMSB model.
png pdf root	Figure 4: The expected and observed constraints on chargino lifetime and mass. The region to the left of the curve is excluded at 95% CL.
png pdf root	Figure 5: The observed 95% CL upper limits on the product of the cross section for direct production of charginos and their branching fraction to $ {\tilde{\chi}^{0}_{1}} {\pi ^\mathrm {{\pm}}}$ as a function of chargino mass and lifetime. The direct chargino production cross section includes both $ {\tilde{\chi}^{0}_{1}} {\tilde{\chi}^\pm _{1}} $ and $ {\tilde{\chi}^\pm _{1}} {\tilde{\chi}^\mp _{1}} $ production in roughly a 2:1 ratio for all chargino masses considered.

Tables
png pdf	Table 1: The data-taking periods and the corresponding integrated luminosities.
png pdf	Table 2: Definitions of the lepton vetoes used in the T&P studies to estimate $ {P_{\text {veto}}} $, for each flavor of charged lepton. The criteria listed are the subset of the search criteria that are the most efficient at rejecting each flavor.
png pdf	Table 3: Summary of the systematic uncertainties in the signal yields. The ranges represent either the variation with chargino mass and lifetime or with the data-taking period used to calculate the uncertainty, depending on the source of each uncertainty as described in the text.
png pdf	Table 4: Summary of numbers of events for the estimated backgrounds and the observed data. The uncertainties include those from statistical and systematic sources. In categories where the systematic uncertainty is negligible, it is not shown.

Summary

A search has been presented for long-lived charged particles that decay within the CMS detector and produce the signature of a disappearing track. In a sample of proton-proton data recorded in 2015 and 2016 at a center-of-mass energy of 13 TeV and corresponding to an integrated luminosity of 38.4 fb$^{-1}$, seven events are observed, compared with the estimated background from standard model processes of 6.5 $\pm$ 0.9 (stat) $\pm$ 1.0 (syst) events. The observation is consistent with the background-only hypothesis. The results are interpreted in the context of the anomaly-mediated supersymmetry breaking model, which predicts a small mass difference between the lightest chargino ($\tilde{\chi}^{\pm}_1$) and neutralino ($\tilde{\chi}^0_1$). The chargino decays via $\tilde{\chi}^{\pm}_1 \to \tilde{\chi}^0_1 \pi^{\pm}$, and because of the limited phase space available for the decay, the chargino has a lifetime on the order of 1 ns and the pion generally has too low momentum to yield a reconstructed track. If the chargino decays inside the tracker volume, it can thus produce a disappearing track. We place constraints on the mass of charginos from direct electroweak production, for chargino mean proper lifetimes between 0.1 and 100 ns. Charginos with masses up to 715 (695) GeV for a lifetime of 3 (7) ns are excluded at 95% confidence level, as are charginos with lifetimes from 0.5 to 60 ns for a mass of 505 GeV. These constraints extend the limits set by a previous search for disappearing tracks performed by the CMS Collaboration [12] and are complementary to the limits set by searches for heavy stable charged particles, which exclude charginos with much longer lifetimes [15,16]. For chargino lifetimes above $\approx$0.7 ns, the present search places the most stringent constraints using a disappearing track signature on direct chargino production.

Additional Tables
png pdf	Additional Table 1: Signal acceptance for each of the generated chargino masses and lifetimes for $ {\mathrm {p}} {\mathrm {p}}\to {\tilde{\chi}^\pm _{1}} \tilde{\chi}^\mp _{1}$ events. The acceptances corresponding to the 2015, 2016A, and 2016B data-taking periods are shown separately. The uncertainties shown include only the statistical uncertainty resulting from the limited sizes of the generated samples.
png pdf	Additional Table 2: Signal acceptance for each of the generated chargino masses and lifetimes for $ {\mathrm {p}} {\mathrm {p}}\to {\tilde{\chi}^\pm _{1}} {\tilde{\chi}^{0}_{1}} $ events. The acceptances corresponding to the 2015, 2016A, and 2016B data-taking periods are shown separately. The uncertainties shown include only the statistical uncertainty resulting from the limited sizes of the generated samples.
png pdf	Additional Table 3: Predicted signal yields for the 2015 data-taking period after the application of each of the disappearing track selections for three chargino lifetime hypotheses ($\tau = $ 0.3, 3.3, and 33 ns) with a chargino mass of 700 GeV. The selections listed are cumulative, i.e., only the events and objects passing a given selection are considered in subsequent selections. The uncertainties shown include only the statistical uncertainty resulting from the limited sizes of the generated samples.
png pdf	Additional Table 4: Predicted signal yields for the 2016A data-taking period after the application of each of the disappearing track selections for three chargino lifetime hypotheses ($\tau = $ 0.3, 3.3, and 33 ns) with a chargino mass of 700 GeV. The selections listed are cumulative, i.e., only the events and objects passing a given selection are considered in subsequent selections. The uncertainties shown include only the statistical uncertainty resulting from the limited sizes of the generated samples.
png pdf	Additional Table 5: Predicted signal yields for the 2016B data-taking period after the application of each of the disappearing track selections for three chargino lifetime hypotheses ($\tau = $ 0.3, 3.3, and 33 ns) with a chargino mass of 700 GeV. The selections listed are cumulative, i.e., only the events and objects passing a given selection are considered in subsequent selections. The uncertainties shown include only the statistical uncertainty resulting from the limited sizes of the generated samples.

References
1	G. F. Giudice, M. A. Luty, H. Murayama, and R. Rattazzi	Gaugino mass without singlets	JHEP 12 (1998) 027	hep-ph/9810442
2	L. Randall and R. Sundrum	Out of this world supersymmetry breaking	NPB 557 (1999) 79	hep-th/9810155
3	C. H. Chen, M. Drees, and J. F. Gunion	A nonstandard string / SUSY scenario and its phenomenological implications	PRD 55 (1997) 330	hep-ph/9607421
4	H.-C. Cheng, B. A. Dobrescu, and K. T. Matchev	Generic and chiral extensions of the supersymmetric standard model	NPB 543 (1999) 47	hep-ph/9811316
5	T. Gherghetta, G. F. Giudice, and J. D. Wells	Phenomenological consequences of supersymmetry with anomaly induced masses	NPB 559 (1999) 27	hep-ph/9904378
6	J. L. Feng et al.	Discovering supersymmetry at the Tevatron in wino LSP scenarios	PRL 83 (1999) 1731	hep-ph/9904250
7	M. Ibe, S. Matsumoto, and T. T. Yanagida	Pure gravity mediation with $ m_{3/2} = $ 10 -- 100 TeV	PRD 85 (2012) 095011	1202.2253
8	L. J. Hall, Y. Nomura, and S. Shirai	Spread supersymmetry with wino LSP: gluino and dark matter signals	JHEP 01 (2013) 036	1210.2395
9	A. Arvanitaki, N. Craig, S. Dimopoulos, and G. Villadoro	Mini-split	JHEP 02 (2013) 126	1210.0555
10	N. Arkani-Hamed et al.	Simply unnatural supersymmetry		1212.6971
11	M. Citron et al.	End of the CMSSM coannihilation strip is nigh	PRD 87 (2013) 036012	1212.2886
12	CMS Collaboration	Search for disappearing tracks in proton-proton collisions at $ \sqrt{s}= $ 8 TeV	JHEP 01 (2015) 096	CMS-EXO-12-034 1411.6006
13	ATLAS Collaboration	Search for charginos nearly mass degenerate with the lightest neutralino based on a disappearing-track signature in pp collisions at $ \sqrt{s} = $ 8 TeV with the ATLAS detector	PRD 88 (2013) 112006	1310.3675
14	ATLAS Collaboration	Search for long-lived charginos based on a disappearing-track signature in pp collisions at $ \sqrt{s} = $ 13 TeV with the ATLAS detector	Submitted to \it JHEP	1712.02118
15	CMS Collaboration	Searches for long-lived charged particles in pp collisions at $ \sqrt{s} = $ 7 and 8 TeV	JHEP 07 (2013) 122	CMS-EXO-12-026 1305.0491
16	CMS Collaboration	Constraints on the pMSSM, AMSB model and on other models from the search for long-lived charged particles in proton-proton collisions at $ \sqrt{s} = $ 8 TeV	EPJC 75 (2015) 325	CMS-EXO-13-006 1502.02522
17	CMS Collaboration	Description and performance of track and primary-vertex reconstruction with the CMS tracker	JINST 9 (2014) P10009	CMS-TRK-11-001 1405.6569
18	CMS Collaboration	The CMS trigger system	JINST 12 (2017) P01020	CMS-TRG-12-001 1609.02366
19	CMS Collaboration	The CMS experiment at the CERN LHC	JINST 3 (2008) S08004	CMS-00-001
20	CMS Collaboration	CMS luminosity measurement for the 2015 data taking period	CMS-PAS-LUM-15-001	CMS-PAS-LUM-15-001
21	CMS Collaboration	CMS luminosity measurements for the 2016 data taking period	CMS-PAS-LUM-17-001	CMS-PAS-LUM-17-001
22	T. Sjostrand, S. Mrenna, and P. Z. Skands	PYTHIA 6.4 physics and manual	JHEP 05 (2006) 026	hep-ph/0603175
23	J. Pumplin et al.	New generation of parton distributions with uncertainties from global QCD analysis	JHEP 07 (2002) 012	hep-ph/0201195
24	F. E. Paige, S. D. Protopopescu, H. Baer, and X. Tata	ISAJET 7.69: A Monte Carlo event generator for pp, $ \overline{\mathrm{p}}\mathrm{p} $, and $ \mathrm{e}^+ \mathrm{e}^- $ reactions		hep-ph/0312045
25	M. Ibe, S. Matsumoto, and R. Sato	Mass splitting between charged and neutral winos at two-loop level	PLB 721 (2013) 252	1212.5989
26	B. Fuks, M. Klasen, D. R. Lamprea, and M. Rothering	Gaugino production in proton-proton collisions at a center-of-mass energy of 8 TeV	JHEP 10 (2012) 081	1207.2159
27	B. Fuks, M. Klasen, D. R. Lamprea, and M. Rothering	Precision predictions for electroweak superpartner production at hadron colliders with resummino	EPJC 73 (2013) 2480	1304.0790
28	P. M. Nadolsky et al.	Implications of CTEQ global analysis for collider observables	PRD 78 (2008) 013004	0802.0007
29	A. D. Martin, W. J. Stirling, R. S. Thorne, and G. Watt	Parton distributions for the LHC	EPJC 63 (2009) 189	0901.0002
30	J. Butterworth et al.	PDF4LHC recommendations for LHC Run II	JPG 43 (2016) 023001	1510.03865
31	CMS Collaboration	Search for top-squark pair production in the single-lepton final state in pp collisions at $ \sqrt{s} = $ 8 TeV	EPJC 73 (2013) 2677	CMS-SUS-13-011 1308.1586
32	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
33	T. Sjostrand, S. Mrenna, and P. Z. Skands	A brief introduction to PYTHIA 8.1	CPC 178 (2008) 852	0710.3820
34	P. Nason	A new method for combining NLO QCD with shower Monte Carlo algorithms	JHEP 11 (2004) 040	hep-ph/0409146
35	S. Frixione, P. Nason, and C. Oleari	Matching NLO QCD computations with parton shower simulations: the POWHEG method	JHEP 11 (2007) 070	0709.2092
36	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
37	T. Melia, P. Nason, R. Rontsch, and G. Zanderighi	$ \text{W}^{+}\text{W}^{-} $, WZ and ZZ production in the POWHEG BOX	JHEP 11 (2011) 078	1107.5051
38	P. Nason and G. Zanderighi	W$ ^+ $W$ ^- $, WZ and ZZ production in the POWHEG-BOX-V2	EPJC 74 (2014) 2702	1311.1365
39	S. Frixione, P. Nason, and G. Ridolfi	A positive-weight next-to-leading-order Monte Carlo for heavy flavour hadroproduction	JHEP 09 (2007) 126	0707.3088
40	E. Re	Single-top Wt-channel production matched with parton showers using the POWHEG method	EPJC 71 (2011) 1547	1009.2450
41	NNPDF Collaboration	Parton distributions for the LHC Run II	JHEP 04 (2015) 040	1410.8849
42	P. Skands, S. Carrazza, and J. Rojo	Tuning PYTHIA 8.1: the Monash 2013 tune	EPJC 74 (2014) 3024	1404.5630
43	CMS Collaboration	Event generator tunes obtained from underlying event and multiparton scattering measurements	EPJC 76 (2016) 155	CMS-GEN-14-001 1512.00815
44	S. Agostinelli et al.	GEANT4--a simulation toolkit	NIMA 506 (2003) 250
45	CMS Collaboration	Particle-flow reconstruction and global event description with the CMS detector	JINST 12 (2017) P10003	CMS-PRF-14-001 1706.04965
46	M. Cacciari, G. P. Salam, and G. Soyez	FastJet user manual	EPJC 72 (2012) 1896	1111.6097
47	M. Cacciari, G. P. Salam, and G. Soyez	The anti-$ {k_{\mathrm{T}}} $ jet clustering algorithm	JHEP 04 (2008) 063	0802.1189
48	CMS Collaboration	Jet algorithms performance in 13 TeV data	CMS-PAS-JME-16-003	CMS-PAS-JME-16-003
49	CMS Collaboration	Technical proposal for the Phase-II upgrade of the Compact Muon Solenoid	CMS-PAS-TDR-15-002	CMS-PAS-TDR-15-002
50	CMS Collaboration	Measurements of inclusive W and Z cross sections in pp collisions at $ \sqrt{s}= $ 7 TeV	JHEP 01 (2011) 080	CMS-EWK-10-002 1012.2466
51	Particle Data Group Collaboration	Review of particle physics	CPC 40 (2016) 100001
52	R. D. Cousins and V. L. Highland	Incorporating systematic uncertainties into an upper limit	NIMA 320 (1992) 331
53	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
54	T. Junk	Confidence level computation for combining searches with small statistics	NIMA 434 (1999) 435	hep-ex/9902006
55	A. L. Read	Presentation of search results: The CL$ _{\text{s}} $ technique	JPG 28 (2002) 2693
56	G. Cowan, K. Cranmer, E. Gross, and O. Vitells	Asymptotic formulae for likelihood-based tests of new physics	EPJC 71 (2011) 1554	1007.1727