Limits on heavy neutral lepton mixing parameters

Limits on $\vert {{\mathit U}}_{{{\mathit \mu}} {{\mathit x}}}\vert ^2$

INSPIRE   JSON  (beta) PDGID:
S077A01
Quoted limits are either the best limit near the kinematic threshold of the experiment, or a characteristic value in the mass range of the experimental sensitivity
VALUE CL% DOCUMENT ID TECN  COMMENT
$<1 \times 10^{-4}$ 95 1
AAD
2024BU
 ATLS ${\mathit m}_{{{\mathit \nu}_{{{x}}}}}$ $\sim{}$ $20 - 50$ GeV
$<3 \times 10^{-8}$ 90 2
ABRATENKO
2024
MBNE Near ${\mathit m}_{{{\mathit K}}}−{\mathit m}_{{{\mathit \pi}}}−{\mathit m}_{{{\mathit \mu}}}$ kin. thres.
$<1 \times 10^{-6}$ 90 2
ABRATENKO
2024
MBNE ${\mathit m}_{{{\mathit \nu}_{{{x}}}}}$ $\sim{}$ 150 MeV
$<3 \times 10^{-6}$ 95 3
HAYRAPETYAN
2024AB
 CMS ${\mathit m}_{{{\mathit \nu}_{{{x}}}}}$ $\sim{}$ $20 - 60$ GeV
$<1 \times 10^{-3}$ 95 3
HAYRAPETYAN
2024AB
 CMS ${\mathit m}_{{{\mathit \nu}_{{{x}}}}}{ {}\gtrsim{} }$ ${\mathit m}_{{{\mathit W}}}$
$<3 \times 10^{-5}$ 95 4
HAYRAPETYAN
2024AC
 CMS ${\mathit m}_{{{\mathit \nu}_{{{x}}}}}$ $\sim{}$ $1 - 3$ GeV
$<5 \times 10^{-6}$ 95 5
HAYRAPETYAN
2024S
 CMS ${\mathit m}_{{{\mathit \nu}_{{{x}}}}}$ $\sim{}$ 3 GeV
$<5 \times 10^{-7}$ 95 6
HAYRAPETYAN
2024V
 CMS ${\mathit m}_{{{\mathit \nu}_{{{x}}}}}$ $\sim{}$ 10 GeV
$<5 \times 10^{-7}$ 95 7
AAD
2023AO
 ATLS ${\mathit m}_{{{\mathit \nu}_{{{x}}}}}\sim{}3 - 15$ GeV, ${{\mathit p}}{{\mathit p}}$ at 13 TeV
$<0.1$ 95 8
AAD
2023CE
 ATLS Near ${\mathit m}_{{{\mathit \nu}_{{{x}}}}}$ $\sim{}$ $0.1 - 2$ TeV
$<0.1$ 95 9
TUMASYAN
2023AC
 CMS Near ${\mathit m}_{{{\mathit \nu}_{{{x}}}}}$ $\sim{}$ $0.1 - 3$ TeV
$<5 \times 10^{-9}$ 90 10
ABRATENKO
2022A
 MBNE Near ${\mathit m}_{{{\mathit K}}}–{\mathit m}_{{{\mathit \mu}}}$ kin. thres.
$<3 \times 10^{-7}$ 95 11
TUMASYAN
2022AD
 CMS ${\mathit m}_{{{\mathit \nu}_{{{x}}}}}\sim{}8 - 14$ GeV, ${{\mathit p}}{{\mathit p}}$ at 13 TeV
$<1 \times 10^{-3}$ 95 12
AAIJ
2021AA
 LHCB ${\mathit m}_{{{\mathit \nu}_{{{x}}}}}\sim{}5 - 50$ GeV, ${{\mathit p}}{{\mathit p}}$ at 7, 8 TeV
$<2 \times 10^{-4}$ 95 13
AAIJ
2021AA
 LHCB ${\mathit m}_{{{\mathit \nu}_{{{x}}}}}\sim{}5 - 50$ GeV, ${{\mathit p}}{{\mathit p}}$ at 7, 8 TeV
$<5 \times 10^{-9}$ 90 14, 15
CORTINA-GIL
2021
NA62 Near ${\mathit m}_{{{\mathit K}}}–{\mathit m}_{{{\mathit \mu}}}$ kin. thres.
$<0.02$ 90 16
PRIM
2020
BELL ${\mathit m}_{{{\mathit \nu}_{{{x}}}}}$ $\sim{}$ 1 GeV
$<2 \times 10^{-5}$ 95 17
AAD
2019F
 ATLS ${\mathit m}_{{{\mathit \nu}_{{{x}}}}}$ $\sim{}$ $10 - 50$ GeV
$<2 \times 10^{-6}$ 95 18
AAD
2019F
 ATLS ${\mathit m}_{{{\mathit \nu}_{{{x}}}}}$ $\sim{}$ 10 GeV
$<1 \times 10^{-9}$ 90 19
ABE
2019B
 T2K Near ${\mathit m}_{{{\mathit K}}}−{\mathit m}_{{{\mathit \mu}}}$ kin. thres.
$<5 \times 10^{-6}$ 90 20, 21
AGUILAR-AREVA..
2019B
 PIEN ${\mathit m}_{{{\mathit \nu}_{{{x}}}}}$ $\sim{}$ $16 - 30$ MeV
$<1 \times 10^{-5}$ 90 21
AGUILAR-AREVA..
2019B
 PIEN Near ${\mathit m}_{{{\mathit \pi}}}−{\mathit m}_{{{\mathit \mu}}}$ kin. thres.
$<3 \times 10^{-7}$ 90 14
CORTINA-GIL
2018
NA62 ${\mathit m}_{{{\mathit \nu}_{{{x}}}}}$ $\sim{}$ $250 - 350$ MeV
$<3 \times 10^{-6}$ 90 14
LAZZERONI
2017A
 NA62 Near ${\mathit m}_{{{\mathit K}}}−{\mathit m}_{{{\mathit \mu}}}$ kin. thres.
$<0.05$ 90 22
PARK
2016
BELL ${\mathit m}_{{{\mathit \nu}_{{{x}}}}}$ $\sim{}$ 1.4 GeV
$<1 \times 10^{-8}$ 90 14
ARTAMONOV
2015A
 B949 ${\mathit m}_{{{\mathit \nu}_{{{x}}}}}$ $\sim{}$ $200 - 300$ MeV
$<3 \times 10^{-5}$ 90 23
LIVENTSEV
2013
BELL Near ${\mathit m}_{{{\mathit \nu}_{{{x}}}}}$ $\sim{}$ $2 - 2.5$ GeV
$<2.0 \times 10^{-8}$ 95 24
DAUM
2000
KARM ${\mathit m}_{{{\mathit \nu}_{{{x}}}}}$ = 33.905 MeV
$<8 \times 10^{-8}$ 90 25
VAITAITIS
1999
CCFR Near ${\mathit m}_{{{\mathit K}}}−{\mathit m}_{{{\mathit \mu}}}$ kin. thres.
$<6 \times 10^{-8}$ 90 26
VAITAITIS
1999
CCFR Near ${\mathit m}_{{{\mathit D}_{{{s}}}}}−{\mathit m}_{{{\mathit \mu}}}$ kin. thres.
$<3 \times 10^{-5}$ 95 27
ABREU
1997I
 DLPH ${\mathit m}_{{{\mathit \nu}_{{{x}}}}}$ $\sim{}$ $6 - 50$ GeV
$<2 \times 10^{-5}$ 95 28
ABREU
1997I
 DLPH Near ${\mathit m}_{{{\mathit \nu}_{{{x}}}}}$ $\sim{}$ 3.5 GeV
$<3 \times 10^{-5}$ 90 29
VILAIN
1995C
 CHM2 Near ${\mathit m}_{{{\mathit K}}}−{\mathit m}_{{{\mathit \mu}}}$ kin. thres.
$<3 \times 10^{-8}$ 30, 31
BERNARDI
1988
CNTR Near ${\mathit m}_{{{\mathit \mu}}}+{\mathit m}_{{{\mathit \pi}}}$ kin. thres.
$<2 \times 10^{-9}$ 31, 32
BERNARDI
1988
CNTR Near ${\mathit m}_{{{\mathit K}}}−{\mathit m}_{{{\mathit \mu}}}$ kin. thres.
$<1 \times 10^{-7}$ 90 33
DORENBOSCH
1986
CHRM Near ${\mathit m}_{{{\mathit D}}}−{\mathit m}_{{{\mathit \mu}}}$ kin. thres.
$<1 \times 10^{-7}$ 90 34
COOPER-SARKAR
1985
BEBC Near ${\mathit m}_{{{\mathit D}}}−{\mathit m}_{{{\mathit \mu}}}$ kin. thres.
$<1 \times 10^{-6}$ 90 35
HAYANO
1982
CNTR ${\mathit m}_{{{\mathit \nu}_{{{x}}}}}$ $\sim{}$ $200 - 300$ MeV
• • We do not use the following data for averages, fits, limits, etc. • •
$<1 \times 10^{-7}$ 90 36
ABRATENKO
2020
MBNE Superseded by ABRATENKO 2022A
$<2 \times 10^{-5}$ 95 37
SIRUNYAN
2018K
 CMS ${\mathit m}_{{{\mathit \nu}_{{{x}}}}}$ $\sim{}$ $20 - 60$ GeV
$<5 \times 10^{-3}$ 95 37
SIRUNYAN
2018K
 CMS ${\mathit m}_{{{\mathit \nu}_{{{x}}}}}{ {}\gtrsim{} }$ ${\mathit m}_{{{\mathit W}}}$
1  AAD 2024BU search for same-sign muon pairs in semileptonic decays of top quarks via a Majorana HNL, in the HNL mass range $15 - 75$ GeV.
2  ABRATENKO 2024 search for ${{\mathit K}^{+}}$ $\rightarrow$ ${{\mathit \mu}^{+}}{{\mathit \nu}_{{{x}}}}$, with ${{\mathit \nu}_{{{x}}}}$ $\rightarrow$ ${{\mathit \nu}_{{{\mu}}}}{{\mathit e}^{+}}{{\mathit e}^{-}}$ and ${{\mathit \nu}_{{{x}}}}$ $\rightarrow$ ${{\mathit \nu}_{{{\mu}}}}{{\mathit \pi}^{0}}$ $\rightarrow$ ${{\mathit \nu}_{{{\mu}}}}{{\mathit \gamma}}{{\mathit \gamma}}$ in the mass ranges ${\mathit m}_{{{\mathit \nu}_{{{x}}}}}$ $\sim{}$ $10 - 150$ MeV and ${\mathit m}_{{{\mathit \nu}_{{{x}}}}}$ $\sim{}$ $150 - 245$ MeV, respectively, for the case of a Majorana HNL. The ${{\mathit \pi}^{0}}$ mode dominates above ${\mathit m}_{{{\mathit \nu}_{{{x}}}}}$ $\sim{}$ 150 MeV.
3  HAYRAPETYAN 2024AB search for ${{\mathit W}}$ $\rightarrow$ ${{\mathit \mu}}{{\mathit \nu}_{{{x}}}}$ $\rightarrow$ ${{\mathit \mu}}{{\mathit \mu}}{{\mathit \ell}}{{\mathit \nu}_{{{{{\mathit \ell}}}}}}$ prompt decays assuming coupling to a single SM generation, between $10 - 1500$ GeV. Above ${\mathit m}_{{{\mathit \nu}_{{{x}}}}}$ > ${\mathit m}_{{{\mathit W}}}$, sensitivity is greatly reduced by the required virtuality of the HNL. Results are quoted for a mass range below ${\mathit m}_{{{\mathit W}}}$ and just above ${\mathit m}_{{{\mathit W}}}$, for Majorana HNLs. Similar (weaker) limits also for Dirac HNLs are presented.
4  HAYRAPETYAN 2024AC search for ${{\mathit B}}$ $\rightarrow$ ${{\mathit X}}{{\mathit \mu}}{{\mathit \nu}_{{{x}}}}$ and ${{\mathit B}}$ $\rightarrow$ ${{\mathit \mu}}{{\mathit \nu}_{{{x}}}}$ followed by displaced ${{\mathit \nu}_{{{x}}}}$ $\rightarrow$ ${{\mathit \mu}}{{\mathit \pi}^{\pm}}$ decay, where ${{\mathit X}}$ is an unreconstructed hadronic system, for Majorana HNLs. Also considers mixed flavor scenarios with ${{\mathit e}}{{\mathit \mu}}$ final state lepton pairs, and presents similar (weaker) limits for Dirac HNLs.
5  HAYRAPETYAN 2024S search for ${{\mathit W}}$ $\rightarrow$ ${{\mathit \mu}}{{\mathit \nu}_{{{x}}}}$ followed by ${{\mathit \nu}_{{{x}}}}$ displaced decay in flight to electromagnetic and hadronic showers in the CMS muon chamber, in the mass range ${\mathit m}_{{{\mathit \nu}_{{{x}}}}}$ $\sim{}$ $1 - 3$ GeV. Limits are set for both Majorana and Dirac HNLs. This long-lived particle style search results in an exclusion region in the coupling-mass plane whose upper contour is determined by an insufficiently long HNL lifetime to reach the muon chambers, and lower contour by insufficient HNL production. The intersection of these, and thus the extent of the experimental sensitivity, occurs at ${\mathit m}_{{{\mathit \nu}_{{{x}}}}}$ $\sim{}$ 3.5 GeV.
6  HAYRAPETYAN 2024V searches for ${{\mathit W}}$ $\rightarrow$ ${{\mathit \mu}}{{\mathit \nu}_{{{x}}}}$ with displaced decay ${{\mathit \nu}_{{{x}}}}$ $\rightarrow$ ${{\mathit \mu}}{{\mathit j}}$, i.e. with one jet, over the range ${\mathit m}_{{{\mathit \nu}_{{{x}}}}}$ $\sim{}$ $2 - 20$ GeV. Limits are set for both Dirac and Majorana HNLs.
7  AAD 2023AO search for ${{\mathit W}}$ $\rightarrow$ ${{\mathit \nu}_{{{x}}}}{{\mathit \mu}}$, for both Majorana and Dirac HNL scenarios. Also consider scenarios involving multiflavor mixing, with correspondingly weaker limits.
8  AAD 2023CE search for Majorana HNLs via vector boson fusion ${{\mathit W}^{\pm}}$ ${{\mathit W}^{\pm}}$ $\rightarrow$ ${{\mathit \mu}^{\pm}}{{\mathit \mu}^{\pm}}$. Limits set in a ${\mathit m}_{{{\mathit \nu}_{{{x}}}}}$ mass range from 50 GeV up to 20 TeV, using the Phenomenological Type-I Seesaw model as a benchmark scenario.
9  TUMASYAN 2023AC search for Majorana HNLs via vector boson fusion ${{\mathit W}^{\pm}}$ ${{\mathit W}^{\pm}}$ $\rightarrow$ ${{\mathit \mu}^{\pm}}{{\mathit \mu}^{\pm}}$. Limits set in a ${\mathit m}_{{{\mathit \nu}_{{{x}}}}}$ mass range from 50 GeV up to 25 TeV.
10  ABRATENKO 2022A search for ${{\mathit K}^{+}}$ $\rightarrow$ ${{\mathit \mu}^{+}}{{\mathit \nu}_{{{x}}}}$, with ${{\mathit \nu}_{{{x}}}}$ $\rightarrow$ ${{\mathit \mu}^{\mp}}{{\mathit \pi}^{\pm}}$, in the mass range ${\mathit m}_{{{\mathit \nu}_{{{x}}}}}$ $\sim{}$ $246 - 385$ MeV. Also considers limits from ${{\mathit \nu}_{{{x}}}}$ $\rightarrow$ ${{\mathit \mu}^{-}}{{\mathit \pi}^{+}}$ only, for the case of a Dirac HNL.
11  TUMASYAN 2022AD search for ${{\mathit W}}$ $\rightarrow$ ${{\mathit \mu}}{{\mathit \nu}_{{{x}}}}$, ${{\mathit \nu}_{{{x}}}}$ $\rightarrow$ ${{\mathit \mu}}{{\mathit e}}{{\mathit \nu}_{{{e}}}}$ and set limits for Dirac and Majorana Heavy Neutral Leptons. The data correspond to an integrated luminosity of 138 fb${}^{-1}$.
12  Limit from prompt lepton number conserving ${{\mathit W}}$ $\rightarrow$ ${{\mathit \mu}}{{\mathit \mu}}{{\mathit j}}$ search.
13  Limit from prompt lepton number violating ${{\mathit W}}$ $\rightarrow$ ${{\mathit \mu}}{{\mathit \mu}}{{\mathit j}}$ search.
14  Search for ${{\mathit K}^{+}}$ $\rightarrow$ ${{\mathit \mu}^{+}}{{\mathit \nu}_{{{x}}}}$.
15  Assumes a lifetime exceeding 50 ns, and searches over ${\mathit m}_{{{\mathit \nu}_{{{x}}}}}$ range $200 - 384$ MeV.
16  Search for ${{\mathit B}^{+}}$ $\rightarrow$ ${{\mathit \mu}^{+}}{{\mathit \nu}_{{{x}}}}$ in the mass range ${\mathit m}_{{{\mathit \nu}_{{{x}}}}}$ $\sim{}$ $0 - 1.5$ GeV.
17  Limit from prompt lepton number violating trilepton search.
18  Limit from displaced lepton violating or conserving trilepton searches.
19  ${{\mathit K}^{+}}$ $\rightarrow$ ${{\mathit \mu}^{+}}{{\mathit \nu}_{{{x}}}}$, with ${{\mathit \nu}_{{{x}}}}$ decay through ${{\mathit U}}_{{{\mathit \mu}} {{\mathit x}}}$. ABE 2019B also considers bounds on $\vert {{\mathit U}}_{{{\mathit \ell}} {{\mathit x}}}{{\mathit U}}_{{{\mathit \ell}^{\,'}} {{\mathit x}}}\vert $ for combinations of lepton flavors in the ${{\mathit \nu}_{{{x}}}}$ decay final state.
20  Limit requires muon kinetic energy $>$ 1.2 MeV.
21  Search for ${{\mathit \pi}^{+}}$ $\rightarrow$ ${{\mathit \mu}^{+}}{{\mathit \nu}_{{{x}}}}$.
22  PARK 2016 quotes an approximate limit B(${{\mathit B}^{+}}$ $\rightarrow$ ${{\mathit \mu}^{+}}{{\mathit \nu}_{{{x}}}}$) $<$ $3 \times 10^{-6}$ in the mass range ${\mathit m}_{{{\mathit \nu}_{{{x}}}}}$ $\sim{}$ $0.2 - 1.4$ GeV.
23  Search for ${{\mathit B}^{+}}$ $\rightarrow$ ${{\mathit \mu}^{+}}{{\mathit \nu}_{{{x}}}}$.
24  DAUM 2000 quotes a branching ratio bound B(${{\mathit \pi}^{+}}$ $\rightarrow$ ${{\mathit \mu}^{+}}{{\mathit \nu}_{{{x}}}}$) $<$ $6.0 \times 10^{-10}$ at 95$\%$ CL.
25  ${{\mathit K}^{+}}$ $\rightarrow$ ${{\mathit \mu}^{+}}{{\mathit \nu}_{{{x}}}}$, with ${{\mathit \nu}_{{{x}}}}$ $\rightarrow$ ${{\mathit \mu}}{{\mathit X}}$.
26  ${{\mathit D}_{{{s}}}}$ $\rightarrow$ ${{\mathit \mu}^{+}}{{\mathit \nu}_{{{x}}}}$, with ${{\mathit \nu}_{{{x}}}}$ $\rightarrow$ ${{\mathit \mu}}{{\mathit X}}$.
27  Search for prompt ${{\mathit \nu}_{{{x}}}}$ decay signatures.
28  Search for displaced ${{\mathit \nu}_{{{x}}}}$ decay signatures.
29  Search for Heavy Neutral Leptons produced by neutral current muon neutrino interactions, with ${{\mathit \nu}_{{{x}}}}$ $\rightarrow$ ${{\mathit \mu}^{+}}{{\mathit \mu}^{-}}{{\mathit \nu}_{{{\mu}}}}$.
30  ${{\mathit K}^{+}}$ $\rightarrow$ ${{\mathit \mu}^{+}}{{\mathit \nu}_{{{x}}}}$, with ${{\mathit \nu}_{{{x}}}}$ decay through ${{\mathit U}}_{{{\mathit \mu}} {{\mathit x}}}$ and ${\mathit m}_{{{\mathit \nu}_{{{x}}}}}$ $<$ ${\mathit m}_{{{\mathit \mu}}}$ + ${\mathit m}_{{{\mathit \pi}}}$.
31  BERNARDI 1988 also considers bounds on $\vert {{\mathit U}}_{{{\mathit e}} {{\mathit x}}}{{\mathit U}}_{{{\mathit \mu}} {{\mathit x}}}\vert $.
32  ${{\mathit K}^{+}}$ $\rightarrow$ ${{\mathit \mu}^{+}}{{\mathit \nu}_{{{x}}}}$, with ${{\mathit \nu}_{{{x}}}}$ $\rightarrow$ ${{\mathit \mu}^{-}}{{\mathit \pi}^{+}}$.
33  ${{\mathit D}^{+}}$ $\rightarrow$ ${{\mathit \mu}^{+}}{{\mathit \nu}_{{{x}}}}$, with ${{\mathit \nu}_{{{x}}}}$ $\rightarrow$ ${{\mathit \mu}^{-}}{{\mathit \ell}^{+}}{{\mathit \nu}_{{{{{\mathit \ell}}}}}}$.
34  ${{\mathit D}^{+}}$ $\rightarrow$ ${{\mathit \mu}^{+}}{{\mathit \nu}_{{{x}}}}$, with ${{\mathit \nu}_{{{x}}}}$ $\rightarrow$ ${{\mathit \mu}^{-}}{{\mathit \ell}^{+}}{{\mathit \nu}_{{{{{\mathit \ell}}}}}}$ or ${{\mathit \nu}_{{{x}}}}$ $\rightarrow$ ${{\mathit \mu}^{-}}{{\mathit \pi}^{+}}$.
35  Search for ${{\mathit K}^{+}}$ $\rightarrow$ ${{\mathit \mu}^{+}}{{\mathit \nu}_{{{x}}}}$( ${{\mathit \gamma}}$).
36  ${{\mathit K}^{+}}$ $\rightarrow$ ${{\mathit \mu}^{+}}{{\mathit \nu}_{{{x}}}}$, with ${{\mathit \nu}_{{{x}}}}$ $\rightarrow$ ${{\mathit \mu}^{-}}{{\mathit \pi}^{+}}$, in the mass range ${\mathit m}_{{{\mathit \nu}_{{{x}}}}}$ $\sim{}$ $260 - 385$ MeV. ABRATENKO 2020 also considers ${{\mathit \nu}_{{{x}}}}$ $\rightarrow$ ${{\mathit \mu}^{+}}{{\mathit \pi}^{-}}$ for the case of a Majorana HNL.
37  Superseded by HAYRAPETYAN 2024AB.
References