Loopholes in W R Searches at the LHC
591
We investigate the case where the right-handed neutrino is heavier than W R , (so
the on-shell decay W R → ν R is disallowed) and where the possible decay channels
for W R are
W R → jj (q ¯
q
), W R → W L h, W R → W L Z and W R → W L hh ,
all the other Higgs states being very heavy. Of these, the three-body decay W L hh
is very weak with
Γ (W R →W L h)
Γ (W R →W L hh) ∼ 1/v, while the decay W R → W L h depends on
tan β. Most analyses assume tan β to be very small (∼ 0.01), yielding BR(W R →
W L h) to be negligible. Perturbativity bounds alone require tan β < 0.8; however,
the mass of the SM-like Higgs boson h also depends on tan β, and values of
tan β > 0.6 result in the instability of the h mass. To keep our analysis general,
we investigated the production and decays of the W R mass for two cases: small
tan β = 0.01, and large tan β = 0.5. In addition, we allow for two values of g R , viz.
g R = g L , and g L = g R = 0.37, as well as vary matrix elements of V R
CKM to show
how the results are affected. The resulting plots for σ (pp → W R )×BR(W R → t ¯
b)
are given in the left plane of Fig. 1, where we compare our four different cases
(different color-coded, as indicated in the panel insertion) with the CMS result [10]
using collision data collected at
√
s =13 TeV with L=35.9 fb −1 . We show the
observed and expected limit curves for the combined electron and muon final states.
For g L = g R , V L
CKM = V R
CKM , and tan β = 0.01, we note that the branching ratio
W R into t ¯
b ranges from 32% to 33%, as W R decays into q ¯
q pairs democratically.
We confirm the CMS result [10] and exclude W R boson mass up to 3.6 TeV. For
the case where g R = g L and V L
CKM = V R
CKM , represented by the blue line, we
set g R 0.37 and tan β = 0.01. The W R production cross section decreases due to
relatively small g R , and the exclusion limit for W R masses can be reduced to 2.7 TeV
in that scenario. Increasing tan β to 0.5 and g R 0.37 enhances the branching ratios
of W R → W L h to about ∼ 1.95% and the branching ratio of W R → W L Z L to about
∼2.0%. In this case, the branching ratio of W R → t ¯
b is reduced slightly, to 31.0–
31.8%, as shown by the pink line. As can be read from the plot, this reduces the W R
mass limits only slightly, to 2675 GeV. However, when we allow V L
CKM = V R
CKM ,
tan β= 0.5, and g R 0.37, this maximizes decays of W R into other final states, and
the branching ratio of W R → t ¯
b is reduced substantially: from about 20% for high
M W R (4 TeV) to about 29% for low M W R (1.5 TeV). The orange line in the left plane
of Fig. 1 represents our result for this scenario, and the exclusion limit is reduced to
2360 GeV with respect to observed limit, whereas it can be estimated at 1940 GeV
based on the expected limit.
In the right panel of Fig. 1 we plot the cross section of σ × BR(W R → jj ) vs
W R mass, and compare it to the ATLAS result [11] at
√
s =13 TeV for L=37 fb −1 .
(For comparison, we included their acceptance factor A.) The red curve represents
the exclusion limit for W R mass when the gauge couplings g L = g R , and tan β
= 0.01. The branching fraction of W R → jj varies slightly with mass. We keep
the same color coding for curves as in the previous panel. The mass restrictions
are comparable, but slightly weaker than those for the W R → t ¯
b decay, ranging
591
We investigate the case where the right-handed neutrino is heavier than W R , (so
the on-shell decay W R → ν R is disallowed) and where the possible decay channels
for W R are
W R → jj (q ¯
q
), W R → W L h, W R → W L Z and W R → W L hh ,
all the other Higgs states being very heavy. Of these, the three-body decay W L hh
is very weak with
Γ (W R →W L h)
Γ (W R →W L hh) ∼ 1/v, while the decay W R → W L h depends on
tan β. Most analyses assume tan β to be very small (∼ 0.01), yielding BR(W R →
W L h) to be negligible. Perturbativity bounds alone require tan β < 0.8; however,
the mass of the SM-like Higgs boson h also depends on tan β, and values of
tan β > 0.6 result in the instability of the h mass. To keep our analysis general,
we investigated the production and decays of the W R mass for two cases: small
tan β = 0.01, and large tan β = 0.5. In addition, we allow for two values of g R , viz.
g R = g L , and g L = g R = 0.37, as well as vary matrix elements of V R
CKM to show
how the results are affected. The resulting plots for σ (pp → W R )×BR(W R → t ¯
b)
are given in the left plane of Fig. 1, where we compare our four different cases
(different color-coded, as indicated in the panel insertion) with the CMS result [10]
using collision data collected at
√
s =13 TeV with L=35.9 fb −1 . We show the
observed and expected limit curves for the combined electron and muon final states.
For g L = g R , V L
CKM = V R
CKM , and tan β = 0.01, we note that the branching ratio
W R into t ¯
b ranges from 32% to 33%, as W R decays into q ¯
q pairs democratically.
We confirm the CMS result [10] and exclude W R boson mass up to 3.6 TeV. For
the case where g R = g L and V L
CKM = V R
CKM , represented by the blue line, we
set g R 0.37 and tan β = 0.01. The W R production cross section decreases due to
relatively small g R , and the exclusion limit for W R masses can be reduced to 2.7 TeV
in that scenario. Increasing tan β to 0.5 and g R 0.37 enhances the branching ratios
of W R → W L h to about ∼ 1.95% and the branching ratio of W R → W L Z L to about
∼2.0%. In this case, the branching ratio of W R → t ¯
b is reduced slightly, to 31.0–
31.8%, as shown by the pink line. As can be read from the plot, this reduces the W R
mass limits only slightly, to 2675 GeV. However, when we allow V L
CKM = V R
CKM ,
tan β= 0.5, and g R 0.37, this maximizes decays of W R into other final states, and
the branching ratio of W R → t ¯
b is reduced substantially: from about 20% for high
M W R (4 TeV) to about 29% for low M W R (1.5 TeV). The orange line in the left plane
of Fig. 1 represents our result for this scenario, and the exclusion limit is reduced to
2360 GeV with respect to observed limit, whereas it can be estimated at 1940 GeV
based on the expected limit.
In the right panel of Fig. 1 we plot the cross section of σ × BR(W R → jj ) vs
W R mass, and compare it to the ATLAS result [11] at
√
s =13 TeV for L=37 fb −1 .
(For comparison, we included their acceptance factor A.) The red curve represents
the exclusion limit for W R mass when the gauge couplings g L = g R , and tan β
= 0.01. The branching fraction of W R → jj varies slightly with mass. We keep
the same color coding for curves as in the previous panel. The mass restrictions
are comparable, but slightly weaker than those for the W R → t ¯
b decay, ranging