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Fermion Portal Dark Matter

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Abstract

We study a class of simplified dark matter models in which one dark matter particle couples with a mediator and a Standard Model fermion. In such models, collider and direct detection searches probe complimentary regions of parameter space. For Majorana dark matter, direct detection covers the region near mediator-dark matter degeneracy, while colliders probe regions with a large dark matter and mediator mass splitting. For Dirac and complex dark matter, direct detection is effective for the entire region above the mass threshold, but colliders provide a strong bound for dark matter lighter than a few GeV. We also point out that dedicated searches for signatures with two jets or a mono-jet not coming from initial state radiation, along missing transverse energy can cover the remaining parameter space for thermal relic dark matter.
arXiv:1308.0612v2 [hep-ph] 15 Aug 2013
SLAC-PUB-15704
Fermion Portal Dark Matter
Yang Bai
a
and Joshua Berger
b
a
Department of Physics, Unive rsity of Wisconsin, Madison, WI 53706, USA
b
SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, CA 94025, USA
Abstract
We study a class of simplified dark matter models in which one dark matter particle couples w ith
a mediator and a Standard Mode l fermion. In such models, collider and direct detection searches
probe complimentary regions of parameter space. For Majorana dark matter, direct detectio n
covers the region near mediator-dark ma tter degeneracy, while colliders probe regions w ith a la rge
dark matter and media tor mass splitting. For Dirac and complex dark matter, direct detection is
effective for the entire region above the mass threshold, but colliders provide a strong b ound for
dark matter lighter than a few GeV. We also point out that dedicated searches for signatures with
two jets or a mono-jet not coming from initial state radiation, along missing transverse energy can
cover the remaining pa rameter space for thermal relic dark matter.
1 Introduction
There is now a large body of evidence for the existence of particulate dark matter that interacts
gravitationally and makes up nearly a quarter of the energy density of th e universe [1–4]. On the
other h an d, such dark matter does not exist in the Standard Model (SM), has not been conclusively
observed to interact non-gravitationally, and has never been detected, directly or indirectly, in a
laboratory experiment. Determining the physical properties of dark matter therefore constitutes
one of the most pressing q uestions in high-energy physics. Ideally, it would be possible to measure
the non-gravitational interactions in several ways: by directly detecting rare ambient d ark matter
scattering events off a target in underground laboratories, by detecting the products of dark matter
self-annihilation or decay, and by measuring a momentum imbalance in collider events.
Given that so little is known about the properties of d ark matter, the most informative framework
for studying dark matter properties in a s im plified framework that is as model-independent as possib le.
One powerful approach is to consider effective operators of d ark matter coupling to quarks and other
SM particles [5–25]. In the absense a light mediator below around one GeV, the effective operator
approach provides an excellent description for interpreting the direct detection experiment results
because of the small exchange momentum in the scattering processes. A simplified description in terms
of such operators neatly encapsu lates all possible interactions of dark matter with their detectors. On
the other hand, su ch operators may be unsuitable or at least not precise for the purposes of collider
studies [26–28]. T he latest searches for dark matter at ATLAS [21, 29] and CMS [25, 30] have set an
lower bound on the cutoffs of the effective operators to be around one TeV. The current constraints
are stringent when compared to the limits derived fr om direct detection experiments, but are still
much below the center-of-mass ener gy, 8 TeV, of the latest run at the L HC. When the parton-level
collision energy is comparable to the mediator mass, the effective theory does not apply and a more
complete description is required.
To go beyond an effective operator approach, one could directly use a concrete underlying model
such as the Minimal Supersymmetric Standard Model (MSSM) to study the complementarity among
different experimental probes of dark matter [31]. However, to maintain the model-independent
paradigm of the effective operator approach, a “Sim plified Dark Matter Model” could be a better
ch oice. In this approach, one can introduce one or two new particles w ith one or two interactions to
probe the potentially complicated dark matter sector. For dark matter interacting with SM particles,
one of the most important things is the properties of the mediator. The majority of existing stu dies in
the literature h ave the mediator particle couple to two dark matter particles at the same time: Higgs
1
portal [32–34], 2HDM portal [35], axion-portal [36], gravity-mediated [37], dilaton-assisted [38] and
Z
-mediated [39, 40]
1
[43]. I n this paper, we study s im plified dark matter models with SM fermions
as the portal particle, which we call Fermion Portal (FP) dark matter. T his class of models is well
motivated and can easily be a part of some un derlying models such as SUSY [44] or extra-dimension
models [45].
In FP dark matter models, an SM singlet dark matter particle interacts with quarks via a new Q CD
color triplet state. There are several classes of such models depending on the Lorentz properties of the
dark matter: it may be a Dirac fermion, Majorana fermion, complex scalar, real scalar, or vector. The
vector case requires an additional scalar field or additional dynamics to provide the vector boson mass
and we do not consider it here. For the real scalar dark matter case, the non-relativistic interaction
cross-sections including self-annihilation and direct detection are highly suppressed and we do not
consider th is case either. Therefore, in this paper we perform detailed studies of the Dirac fermion,
Majorana fermion, and complex scalar cases.
A further specification can be made based on species of quarks to which the dark matter couples.
At r enormalizable level, the simplest interactions are to have dark matter particles coup le to only
right-handed fermions. In the non-relativistic limit, the interactions can be transformed as vector
or axi-vector couplings between the dark matter particle and th e SM fermion. For either type of
couplings, a dark matter particle that couples exclusively to heavy quarks would have a suppressed
direct detection cross-section. We concentrate on the couplings to up and dow n quarks in this paper
and leave the heavy quark case for future exploration.
The six cases that are considered here constitute a set of models for benchmarking the progress of
dark matter experiments, as well as for studying the complementarity of different types of experiments.
Unlike th e effective operator approach, they can be extrapolated to arbitrarily high ener gies while
giving sensible results. Furthermore, they are well-motivated and simple enough to allow for deep
experimental scr utiny. In th is paper, we demons trate the power of current experiments to probe these
models and determine the allowed parameter space to probe in the future. For each case, we also show
the parameter space to satisfy the dark matter thermal relic abundance. We pay attention to the
potential for current experiments to probe the thermal relic hypothesis. We find that the parameter
space for a thermal relic is highly constrained for most of the scenarios considered, bu t that there is
some potential at the moment in a model with Majorana dark matter.
The remainder of this paper is structured as follows. In Section 2, we introduce the Fermion Portal
1
The milli-charged dark matter with only one massless gauge boson in this model contradicts quantum gravity, as
shown in Ref. [41] based on arguments in Ref. [42].
2
class of simplified models. We determine the allowed parameter space for dark matter to be a thermal
relic in Section 3. Current direct detection and collider constraints are determined in Sections 4 an d
5 respectively, with summary plots presented in Section 5. We discuss potential improvement for the
LHC collider searches and conclude in Section 6.
2 Simplified dark matter model: fermion portal
If the dark matter sector interacts directly with a single fermion in the SM, two particles with different
spins are required in the dark matter sector. In this paper, we will concentrate on the quark portal dark
matter an d leave the lepton portal dark matter for future exploration. Restricting to particles with a
spin less than one, there are two general s itu ations: fermionic dark matter with a color-triplet scalar
partner or scalar dark m atter with a color-triplet fermion partner. In the former case, we consider
both Dirac and Majorana dark matter, wh ile for the latter case we only consider a complex scalar dark
matter and skip the real scalar dark matter case [6], which h as a quark mass su ppressed s-wave or
a d-wave or three-body suppressed annihilation rate and a velocity sup pressed direct detection cross
section if the quark masses are neglected.
We begin by considering fermionic dark matter coupled to right-han ded quarks as the portal to
the dark matter sector. T he dark matter candidate may be a Dirac or Majorana fermion, χ, that is
an S M gauge singlet. The mediator is an SU(3)
c
triplet with an app ropriately chosen hypercharge.
The renormalizable operators are
L
fermion
λ
u
i
φ
u
i
χ
L
u
i
R
+ λ
d
i
φ
d
i
χ
L
d
i
R
+ h.c. , (1)
where u
i
= u, c, t (d
i
= d, s, b) are different SM quarks. Since χ is the dark matter candidate, the
partner masses m
φ
i
must be larger th an the dark matter m ass m
χ
. In our analysis, we assume the
branching ratio of th e decay φ
u
i
χ¯u
i
and φ
d
i
χ
¯
d
i
is 100%. We also require the Yukawa couplings
λ
i
to be less than
4π to preserve perturbativity. Since we will concentrate on the first generation
quarks, we neglect the flavor index from now on to simplify th e n otation. Using the up quark operator,
the width of φ
u
particle is calculated to be
Γ(φ χ +
u) =
λ
2
u
16π
(m
2
φ
m
2
χ
)
2
m
3
φ
, (2)
for both Dirac and Majorana cases.
Similarly, for a complex scalar dark matter, X, and its partner, ψ, a color-triplet Dirac fermion,
we have the interactions
L
scalar
λ
u
i
X
ψ
u
i
L
u
i
R
+ λ
d
i
Xψ
d
i
L
d
i
R
+ h.c. . (3)
3
For the up quark operator, th e decay width of ψ
u
field is
Γ(ψ X
+ u) =
λ
2
u
32π
(m
2
ψ
m
2
X
)
2
m
3
ψ
. (4)
If the operators in Eqs. (1), (3) are defined in the avor basis, the quark r ight-handed currents be-
come physical and additional (weak) avor constraints apply to the model parameter space. However,
if they are defined in the quark mass basis, there are no additional fl avor changing processes beyond
the SM. We simply take the mass basis assumption an d ignore the flavor physics constraints. We next
explore the dark matter phenomenology of this class of models, including its thermal relic ab undance,
direct detection an d collider searches. Some other studies for the spin-dependent direct detection and
indirect detection signatures can be found in Refs. [46, 47].
3 Relic abundance
The complimentarity between dark m atter collider and direct detection searches is independent of the
dark matter thermal history. Since the weakly interacting massive particle (WIMP) is still the best
motivated scenario that generates the observed dark matter relic abu ndance for a weak-scale mass, we
calculate the thermal relic abundance for the simplified fermion-portal dark matter. We then compare
the allowed thermal relic parameter space to direct detection and collider bounds.
In the fermionic dark matter case, the main annihilation channel is
χχ uu for Dirac dark matter.
The dominant contribution to the annihilation cross-section is
1
2
(σv)
χ¯χ
Dirac
=
1
2
"
3 λ
4
u
m
2
χ
32 π (m
2
χ
+ m
2
φ
)
2
+ v
2
λ
4
u
m
2
χ
(5m
4
χ
18m
2
χ
m
2
φ
+ 11m
4
φ
)
256 π (m
2
χ
+ m
2
φ
)
4
#
s + p v
2
, (5)
where v is the relative velocity of two dark matter particles and is typically 0.3 c at the fr eeze-out
temperature and 10
3
c at present. T he factor of 1/2 in Eq. (5) accounts for the fact that Dirac dark
matter is composed of both a particle and an anti-particle. For Majorana dark matter, the annihilation
rate only contains a p-wave contribution at leading order in the limit of zero quark masses
(σv)
χχ
Majorana
= v
2
λ
4
u
m
2
χ
(m
4
χ
+ m
4
φ
)
16π (m
2
χ
+ m
2
φ
)
4
p v
2
. (6)
In the non-degenerate parameter space, we only need to care about the dark matter annihilation
rate. The dark matter relic abundance is ap proximately related to the s and p variables by
χ
h
2
1.07 × 10
9
GeV M
Pl
g
x
F
s + 3 (p s/4)/x
F
, (7)
4
where the Planck scale is M
Pl
= 1.22×10
19
GeV and g
is the number of relativistic degrees of freedom
at the freeze-out temperature and is taken to be 86.25 here. The freeze-out temperature x
F
is given
by
x
F
= ln
"
5
4
r
45
8
g
2π
3
M
Pl
m
χ
(s + 6 p/x
F
)
g
x
F
#
, (8)
where g = 2(4) is th e number of degrees of freedom for the Majorana (Dirac) fermion dark matter.
In the degenerate p arameter space with (m
φ
m
χ
)/m
χ
1 and comparable to or below the
freeze-out temperature 1/x
F
5%, co-annihilation effects become important [48, 49]. Neglecting th e
sub-leading electroweak interaction, the ann ihilation cross-section for χ + φ
u + g is given by
(σv)
χ φ
=
g
2
s
λ
2
u
24π m
φ
(m
χ
+ m
φ
)
+ v
2
g
2
s
λ
2
u
(29m
2
χ
50m
χ
m
φ
+ 9m
2
φ
)
576π m
φ
(m
χ
+ m
φ
)
3
, (9)
for both Dirac and Majorana dark matter. Add itionally, the φ field self-annihilation cross-section is
given by
(σv)
φ φ
[gg] =
7 g
4
s
216π m
2
φ
v
2
59 g
4
s
5184π m
2
φ
, (10)
(σv)
φ φ
[f
¯
f] = v
2
g
4
s
432π m
2
φ
, for f 6= u , (11)
(σv)
φ φ
[u¯u] = v
2
"
g
4
s
432π m
2
φ
g
2
s
λ
2
u
108π (m
2
χ
+ m
2
φ
)
+
λ
4
u
m
2
φ
48π (m
2
χ
+ m
2
φ
)
2
#
, (12)
for both Dirac and Majorana dark matter. Here, f represents the SM quarks and we have neglected
all quark masses in our calculation f or a heavy m
φ
with m
φ
m
f
. For Majorana dark matter, there
is an additional annihilation channel with cross section
(σv)
φ φ
[uu] =
λ
4
u
m
2
χ
6π (m
2
φ
+ m
2
χ
)
2
+ v
2
λ
4
u
m
2
χ
(3m
4
χ
18m
2
χ
m
2
φ
m
4
φ
)
144π (m
2
φ
+ m
2
χ
)
4
. (13)
Following Refs. [48, 49], we have the effective degrees of freedom as a f unction of th e temperature
parameter x
g
eff
= g
χ
+ g
φ
(1 + ∆)
3/2
e
x
, (14)
with g
φ
= 6 (we count φ and φ
together) and g
χ
= 2(4) for Majorana (Dirac) fermion. The effective
annihilation cross section for the Dirac case is
(σv)
eff
=
1
2
(σv)
χ¯χ
g
2
χ
g
2
eff
+ (σv)
χφ
g
χ
g
φ
g
2
eff
(1 + ∆)
3/2
e
x
+
1
2
(σv)
φφ
g
2
φ
g
2
eff
(1 + ∆)
3
e
2 x
, (15)
5
and for the Majoran a case is
(σv)
eff
= (σv)
χχ
g
2
χ
g
2
eff
+ (σv)
χφ
g
χ
g
φ
g
2
eff
(1 + ∆)
3/2
e
x
+
1
2
[(σv)
φφ
+ (σv)
φφ
]
g
2
φ
g
2
eff
(1 + ∆)
3
e
2 x
, (16)
Variables s
eff
and p
eff
can be constructed by form ing a similar combination to (σv)
eff
. They replace s
and p in Eqs. (7) and (8) for the purposes of calculating the thermal relic abundance.
Fitting to the observed value of
χ
h
2
= 0.1199 ± 0.0027 from Planck [4] and WMAP [3], we show
the allowed values of m
χ
and m
φ
in Fig. 1 for different values of couplings. For Dirac dark matter,
0
500
1000
1500
2000
0
1000
1500
2000
m
Φ
HGeVL
m
Χ
HGeVL
Dirac fermion dark matter
Λ
u
= 1
0.5
0.75
1.2
100
1000
500
200
2000
300
3000
150
1500
700
100
1000
500
200
2000
300
3000
150
1500
700
m
Φ
HGeVL
m
Χ
HGeVL
Majorana fermion dark matter
from left to right:
Λ
u
=0.5, 0.75, 1.0, 1.2
Figure 1: Left panel: the masses of Dirac fermion dark matter and its partner for different choices
of coupling, after fitting the observed dark matter energy fraction,
χ
h
2
= 0.1199 ± 0.0027, from
Planck [4] and WMAP [3]. The blue dotted lines neglect co-annihilation effects, while the blue solid
lines include them. The black dotted line is boundary of the region for which m
φ
> m
χ
. Right panel:
the same, but for a Majorana dark matter.
the co-annihilation effects have a significant effect for small values of λ
u
, but only have a small effect
for lager values of λ
u
. Due to p-wave suppression of χχ annihilation, Majorana dark matter mass is
preferred to have either a light mass, below around 600 GeV, or a heavy mass nearly degenerate with
its partner.
For complex scalar dark matter, the annihilation rate of XX
u
u is also p-wave suppressed and
given by
1
2
(σv)
XX
complex scalar
=
1
2
"
v
2
λ
4
m
2
X
16 π (m
2
X
+ m
2
ψ
)
2
#
p v
2
. (17)
6
The allowed parameter space f or a thermal relic in the complex scalar case has sim ilar features to the
Majorana case, includin g the co-annihilation effects.
4 Dark matter direct detection
For calculation of dark matter direct detection cross-sections, one could integrate out the dark matter
partner and calculate the scattering cross sections using the effective oper ators. However, for the
degenerate region, the dark matter partner in the s-channel can dramatically increase the scattering
cross section. To capture the resonance effects, we keep the dark matter partner pr op agator in our
calculation.
χ
q
φ
χ
q
χ
q
φ
χ
q
(a) (b)
Figure 2: Feynman diagrams for scattering of a fermion dark matter off nucleus. Only the left panel in
(a) contributes to the Dirac fermion case, while both (a) and (b) contribute to the Majorana fermion
case.
For the Dirac dark matter case, only the left pan el in Fig. 2 contributes. Both spin-independent
(SI) and spin-dependent (SD) scattering exist. The leading SI interaction cross-section per nucleon is
given by
σ
Nq
SI
(Dirac) =
|λ
u
|
4
f
2
Nq
µ
2
64 π[(m
2
χ
m
2
φ
)
2
+ Γ
2
φ
m
2
φ
]
, (18)
where N = p, n; µ is the reduced mass of the dark matter-nucleon system; f
Nq
is the coefficient related
to the quark operator matrix element inside a nucleon. For the up quark operator at hand, one has
f
p u
= 2 and f
n u
= 1 [44, 50]. The sub-leading SD interaction cross section is given by
σ
Nq
SD
(Dirac, Majorana) =
3 |λ
u
|
4
2
Nq
µ
2
64 π[(m
2
χ
m
2
φ
)
2
+ Γ
2
φ
m
2
φ
]
, (19)
with
p
u
=
n
d
= 0.842 ±0.012 and
p
d
=
n
u
= 0.427 ±0.013 [51]. For Majorana dark matter, there
is only an SD scattering cross s ection with the same formula as the SD scattering of the Dirac ferm ion
case.
7
For the complex scalar case, the SI scattering cross section is given by
σ
Nq
SI
(complex scalar) =
|λ
u
|
4
f
2
Nq
m
2
p
32 π[(m
2
X
m
2
ψ
)
2
+ Γ
2
ψ
m
2
ψ
]
, (20)
while the SD scattering cross section is suppressed by the dark matter velocity and is neglected here.
Searches for SI dark matter interactions with nuclei are particularly constraining when they are
predicted by a given mo del. We include the most stringent SI direct detection constraints fr om
Xenon100 [52] for heavier dark matter masses and Xenon10 [53] for lighter dark matter masses.
Xenon100 [52] is sensitive to cross-sections nearly down to 10
45
cm
2
at a dark matter mass of around
100 GeV. The Xenon10 [53] experiment has some additional sensitivity for low dark m atter masses.
For the S D scattering cross section, we mainly use the limits from SIMPLE [54], COUPP [55], and
PICASSO [56] experiments for coupling to protons and from Xenon100 [57] and CDMS [58, 59] for
coupling to neutrons.
In addition to placing strong constraints, four experiments (DAMA [60], CoGeNT [61], CRESST-
II [62], CDMS [63]) have now seen excesses in regions of parameter space already probed by Xenon100
under some assumptions [64]. For the purposes of this study, we ignore the debatable excesses and only
consider the constraints from experiments. Because we study the up quark and down quark operators
separately, isospin symmetry is generically broken. Therefore, we will consider the constraints on dark
matter–proton and dark matter–neutron scattering separately for both SI and SD.
5 Collider constraints
Since the dark matter couples to quarks, it can be produced at colliders. In addition, the colored
mediator may be produced, yielding strong constraints both from associated and pair produ ction.
Except in the regime of an extremely heavy mediator, these channels provide the dominant constraints.
Associated production of the med iator and the dark matter particle, along with radiative contributions
from dark matter pair production, yield a monojet s ignature, while pair production of mediators can
be seen in searches for jets plus missing transverse energy (MET). Example d iagrams for the three
production mechanisms in the case of coupling to up quarks are illustrated in Fig. 3. For simplicity, we
consider the constraints f rom the CMS experiment in the monojet and jets plus MET channels [25,65].
There are comparable constraints from the ATLAS experiment [21,66], though the reach of the CMS
searches is slightly better at present.
8
¯u
χ
χ
u
φ
u
φ
u
g
g
u
χ
χ
φ
u
g
u
u
χ
χ
φ
u
u
u
g
(a) (b) (c)
Figure 3: The thr ee dark matter particle produ ction mechanisms at hadron colliders. Diagram (a)
has two jets in final state, while (b) and (c) provide mono-jet signatures.
5.1 Estimated limits from monojet on t-change φ exchange
For the fermionic dark matter case and in the heavy m
φ
limit, the Fierz-transformed effective operator
|λ
u
|
2
8 m
2
φ
χγ
µ
(1 + γ
5
) χ
µ
(1 γ
5
) u (21)
is generated. The existing search at the 8 TeV LHC with around 20 fb
1
constrains the combination of
up q uark and down quark operators. For light dark matter masses below analysis cuts on mon ojet p
T
or /E
T
, the collider production cross section is insensitive to the parity structure of the operators [25].
One can approximately translate the constraints on Λ
2 m
φ
/|λ
u
| obtained in Ref. [25] to our model
parameter space. For light dark matter masses, the 90% confidence level (CL) constraints on Λ in
Ref. [25] is around 900 GeV, leading to an estimated constraint of m
φ
/|λ
u
| & 640 GeV.
5.2 Limits from 2j + E
miss
T
on φ pair production
In the limit of a small dark matter-mediator coupling, λ
u
0, th e only signifi cant diagram yielding
this final state is (a) in Fig. 3. T he p roduction cross-section is identical to that of a single squark in
the MSSM. The present bounds on this process from CMS constrain the colored particle m ass to be
above arou nd 500 GeV [67] for a massless neutralino. For λ
u
6= 0, there are additional contributions
from t-channel dark matter exchange and the cross-section for the parton level process u + ¯u φ + φ
is given by:
σ =
1
1728πs
3
n
2
q
s(s 4m
2
φ
)
4g
4
s
(4m
2
φ
s) + 12g
2
s
λ
2
u
(s + 2m
2
χ
2m
2
φ
) + 27λ
4
u
s
+3λ
2
u
16g
2
s
m
2
χ
s + (m
2
φ
m
2
χ
)
2
+ 9λ
2
u
s(s + 2m
2
χ
2m
2
φ
)
log
s
q
s(s 4m
2
φ
) + 2m
2
χ
2m
2
φ
s +
q
s(s 4m
2
φ
) + 2m
2
χ
2m
2
φ
.
(22)
9
This extra contribution is significant for λ
u
= 1 and leads to a much higher sensitivity. We also
note that there is destructive interference for a small value of λ
u
, as shown in Fig. 4 for different values
of m
φ
. We therefore anticipate that the experimental limits from jets plus E
miss
T
could become weaker
at some intermediate values of λ
u
.
0.0
0.2
0.4
0.6
0.8
1.0
0
20
40
60
80
100
Λ
u
ΣHpp ® ΦΦ
+
L HfbL
LHC8 TeV
m
Χ
=10 GeV
m
Φ
= 400 GeV
m
Φ
= 500 GeV
m
Φ
= 600 GeV
Figure 4: The pair-production cross sections of the φ field as a function of λ
u
.
To estimate the current bounds on this model, as well as the case of scalar dark matter, we calculate
LO cross-sections for the full process using MadGraph [68] with a model constructed by FeynRules [69].
NLO K-factors calculated u sing Prospino [70] are applied to the pure QCD contr ibution to the cross-
section for the cases of fermionic dark matter. The limits provided in [65] are then applied to the
calculated cross-section to obtain an estimate of the current 95% CL exclusion limit. The results of
this analysis are presented below, in Section 5.3.
5.3 Limits from monojet on single φ productions
The dominant prod uction channel for monojets is process (b) in Fig. 3 at a s mall value of λ
u
. The
resulting cross-section at LO for u + g φ + χ is given by
σ(u + g φ + χ) =
λ
2
u
g
2
s
768 π s
3
(3s + 2m
2
χ
2m
2
φ
)
q
(s + m
2
χ
m
2
φ
)
2
4m
2
χ
s , (23)
where
s is the center-of-mass energy. In order to estimate the current reach of monojet searches,
we generate events for all tr ee-level diagrams with one quark plus dark matter particles in the final
state using MadGraph [68] with the models defined in FeynRules [69]. The events are showered and
hadronized using Pythia [71], then the hadrons are clustered into jets using FastJet [72]. The cuts
described in R ef. [25] are then applied to the events in order to estimate the acceptance times efficiency
10
of th at search . The resulting LO signal cross section times estimated efficiency and acceptance for each
signal region are compared to the limits s et in Ref. [25]. We present our results for several different
scenarios in two ways: first in the m
φ
m
χ
plane and second in the m
χ
σ
SI(SD)
plane with all limits at
95% CL.
We begin by considering the model with Majorana dark matter and only λ
u
6= 0. For λ
u
= 1, the
exclusion curves are shown in Fig. 5. The dominant constraints come from collider searches in the
monojet and jets + MET channels, as well as dark matter spin-dependent direct detection searches.
In addition, we show the lines at which the observed dark matter relic abundance is attained assuming
that χ is a thermal relic. The exclusion extends up to scalar masses of around 700 GeV prov ided that
100
200
300
400
500
600
700
0
100
200
300
400
500
m
Φ
HGeVL
m
Χ
HGeVL
Λ = 1
SD, p
Jets + MET
Monojet
COUPP
Thermal relic
10
-1
10
0
10
1
10
2
10
3
10
-43
10
-42
10
-41
10
-40
10
-39
10
-38
10
-37
10
-36
10
-35
m
Χ
HGeVL
Σ Hcm
2
L
Λ = 1, SD, p
m
Φ
> 100 GeV
Jets + MET
Monojet
COUPP
SIMPLE
PICASSO
Thermal relic
Figure 5: 95% exclusion limits (except the black solid line from the thermal relic abundance) from
the most sensitive searches for Majorana dark matter with the only coupling to th e up quark with
λ
u
= 1. The left panel is in the m
φ
m
χ
plane, while the right panel is in the σ m
χ
plane.
the dark matter is lighter than about 300 GeV. In Fig. 5, we have included the co-annihilation effects
for the degenerate spectrum. We show the thermal relic required p arameter space in the black and
solid line in both panels of Fig. 5. In the σ m
χ
plane, we stop plotting the thermal relic line when
the dark matter mass is close to the mediator mass. There is some parameter space at the moment
where a thermal relic is allowed, for a mediator mass of around 400 GeV, though we s tress that the
thermal relic abundance may be set in other ways. It is important to note that in this model, the
monojet search has a wider reach than the jets + MET search for heavy mediator m asses. This is due
11
to the fact that some of the d iagrams for φφ production are proportional to the Majorana dark matter
mass. In ad dition, up to dark matter masses of around 300 GeV, the domin ant constraint on these
models comes fr om colliders. I n particular, this means that the possibility of light dark matter below
a few GeV is highly constrained. The SD direct detection, jets+MET and monojet are complimentary
as they cover different parts of parameter space.
For comparison, in Fig. 6 we show the same exclusions in the mass plane for λ
u
= 0.5. In this case,
the current constr aints are far weaker. Even for the mediator masses below a few hundred GeV, there
is a significant allowed fraction of parameter space, which it is important to cover in future search es,
especially at colliders. On the other hand, for su ch a small coupling, it is difficult to obtain the correct
relic abundance via therm al production except in the co-annihilation region; an alternate non-ther mal
mechan ism could be considered such that dark matter is not over-pro duced.
100
200
300
400
500
600
700
0
100
200
300
400
500
m
Φ
HGeVL
m
Χ
HGeVL
Λ = 0.5
SD, p
Jets + MET
Monojet
COUPP
Thermal relic
10
-1
10
0
10
1
10
2
10
3
10
-43
10
-42
10
-41
10
-40
10
-39
10
-38
10
-37
10
-36
10
-35
m
Χ
HGeVL
Σ Hcm
2
L
Λ = 0.5, SD, p
m
Φ
> 100 GeV
Jets + MET
Monojet
COUPP
SIMPLE
PICASSO
Thermal relic
Figure 6: The same as Fig. 5 for the up quark case with λ
u
= 0.5.
We also study the same model, but for the down quark case with only λ
d
6= 0. For λ
d
= 1, the
exclusion curves are sh own in Figs. 7. The dominant constraints are the same as in the up-type case.
The constraints are slightly weaker in this case and the jets + MET search dominates f or at high
mediator masses as it is less sensitive to the down quark parton distribution function suppression. In
this case, there is a similar parameter space allowed for a thermal relic.
Next, we consider models with Dirac dark matter and complex scalar dark matter. For these
models, the SI dir ect detection constraints dominate up to very low dark matter masses, independent
12
100
200
300
400
500
600
700
0
100
200
300
400
500
m
Φ
HGeVL
m
Χ
HGeVL
Λ = 1
SD, n
Jets + MET
Monojet
Xenon100
Thermal relic
10
-1
10
0
10
1
10
2
10
3
10
-43
10
-42
10
-41
10
-40
10
-39
10
-38
10
-37
10
-36
10
-35
m
Χ
HGeVL
Σ Hcm
2
L
Λ = 1, SD, n
m
Φ
> 100 GeV
Jets + MET
Monojet
X100
CDMS
Thermal relic
Figure 7: 95% exclusion limits from the most sensitive searches for Majorana dark matter with coupling
to the down q uark.
of m
φ
. For λ
u
= 1, the exclusion curves are shown in Figs. 8 and 9. These cases are highly constrained
200
400
600
800
1000
0
100
200
300
400
500
m
Φ
HGeVL
m
Χ
HGeVL
Λ = 1
SI
Jets + MET
Monojet
Xenon100
Thermal relic
10
-1
10
0
10
1
10
2
10
3
10
-46
10
-45
10
-44
10
-43
10
-42
10
-41
10
-40
10
-39
10
-38
m
Χ
HGeVL
Σ Hcm
2
L
Λ = 1, SI
Jets + MET
Monojet
Xenon100
Xenon10
Thermal relic
Figure 8: 95% exclusion limits from the most sensitive searches for Dirac dark matter with coupling
to the up quark.
13
200
400
600
800
1000
0
100
200
300
400
500
m
Ψ
HGeVL
m
X
HGeVL
Λ = 1
SI
Jets + MET
Monojet
Xenon100
Thermal relic
10
-1
10
0
10
1
10
2
10
3
10
-46
10
-45
10
-44
10
-43
10
-42
10
-41
10
-40
10
-39
10
-38
m
X
HGeVL
Σ Hcm
2
L
Λ = 1, SI
Jets + MET
Monojet
Xenon100
Xenon10
Thermal relic
Figure 9: 95% exclusion limits from the most sensitive searches for complex scalar dark matter with
coupling to the up quark.
by searches for spin-independent scattering, which is u nsupp ressed. Since dark matter interactions
generally violate isospin in our models, the different couplings to p rotons and neutrons should be
taken into account in calculating the bou nds. The SI cross-section bounds per nucleon are generally
calculated under th e assumption of isospin, su ch that the proton and neutron cross-sections are the
same. In order to take into account isospin violation, we calculate the cross-section for interaction
with a proton and rescale by
σ
DM,nucleon
=
[f
p
Z + f
n
(A Z)]
2
f
2
p
A
2
σ
DM,p
, (24)
where A and Z are th e mass number and atomic number of the target nucleus respectively. The
dominant SI bounds come from Xe targets, so that A = 131, neglecting small effects from other
comparable or subdominant isotopes, and Z = 54. All scattering cross sections presented in Figs. 8
and 9 are the averaged one, σ
DM,nucleon
.
It is interesting to note that collider bounds take over for light dark matter, below the threshold
of direct detection experiments. In the case of a complex scalar, the low mass bound flattens out in
the cross-section plane since it is not sensitive to the r educed mass of the dark matter-nucleon system,
but rather the nucleon mass itself, as can be seen from Eq. (20).
14
6 Discussion and conclusions
The signal spectrum from the associated production of dark matter and its partner could be dra-
matically different from backgrounds. Particularly when the Yukawa coupling is small, associated
production is the dominant part of the signal. Additional kinematic variables can be used to enhance
the dark matter signal in the fermion-portal scenario. We use MadGraph5 [68] to generate the dark
matter signal events and shower them in PYTHIA [73]. We then use PGS [74] to perform the fast detector
simulation. After utilizing the basic cuts in Ref. [25], where E
miss
T
> 200 GeV has been imposed, we
calculate the normalized E
miss
T
distributions for several different spectra. In the left panel of Fig. 10,
we show the E
miss
T
from the χ + φ associate productions. Because the jet from the decay of φ χ + j
is energetic, the E
miss
T
distributions have a peak-structure with the peak at around m
φ
/2 for a small
m
χ
. As a comparison, the right panel of the Fig. 10 shows the E
miss
T
distribution without on-shell
production of φ. The spectrum is monotonically decreasing in this case, which follows the s hape of
the background although w ith a different slope. For a larger m
φ
, the signal spectrum becomes slightly
harder at higher masses. In principle, the peak structure in the left panel can be used to discover dark
200
300
400
500
600
700
0.00
0.05
0.10
0.15
0.20
0.25
0.30
E
T
miss
HGeVL
Fraction of Events
m
Χ
=10 GeV, m
Φ
=400 GeV
m
Χ
=10 GeV, m
Φ
=700 GeV
m
Χ
=10 GeV, m
Φ
=1000 GeV
150
200
250
300
350
400
450
500
0.00
0.05
0.10
0.15
0.20
0.25
E
T
miss
HGeVL
Fraction of Events
m
Χ
=10 GeV, m
Φ
=400 GeV
m
Χ
=10 GeV, m
Φ
=700 GeV
m
Χ
=10 GeV, m
Φ
=1000 GeV
Figure 10: Left panel: the fraction of events after basic cuts as a function of E
miss
T
for the associated
production of χ + φ with φ χ + j. Right panel: the same as the left one but for the productions of
2χ + j with the jet from ISR.
matter, f or instance performing a “bu mp” search in the E
miss
T
distribution. In practice, the peaks are
too wide to make it feasible. Improvin g the jet energy resolution and E
miss
T
measurement can yield
significant boosts in sensitivity.
To explore more fermion portal dark matter parameter space, we emphasize the importance of a
dedicated search of the two jets plus MET signature. As can be seen from the left panel in Fig. 6, for
15
small values of the Yu kawa coupling, the current limit on the colored m ediator mass is weak, around
350 GeV for a dark matter mass at 100 GeV. Additional kinematic variables like m
T
2
can increase th e
search sensitivity [75].
Note added: we note here that during the completion of our paper, another paper [76] appeared
with different emphasis: our paper concentrates m ore on the complimentarity of direct detection and
collider searches f or dark matter, while their paper has more focus on the thermal relic parameter
space.
Acknowledgments
Y. Bai is supported by startup funds from the UW-Madison. SLAC is operated by Stanford University
for the US Department of Energy under contract DE-AC02-76SF00515. YB also would like to thank
the Kvali Institute for Theoretical Physics, U. C. Santa Barbara, where part of this work was done.
This research was also supported in part by the National Science Foundation under Grant No. NSF
PHY11-25915.
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... In such cases, dark quarks could be produced at the LHC through a portal and undergo rapid hadronization within the dark sector before decaying back, at least in part and potentially with sizeable lifetimes, to SM particles. Models with such hidden dark sectors have been discussed e.g. in the context of twin Higgs models [11,12,13,14], composite and/or asymmetric dark matter scenarios [15,16,17,18,19,20,21,22,23,24,5,25,26,27], and string theory [28,29]. Some studies focusing on hidden valley phenomenology can be found in [30,31,32,33,34,35,36,10,37,38,39,40,41,42,6,43]. ...
... In this paper, we will focus on introducing a new mediator particle that serves as the portal. This could be a Z which would mediate s-channel production [47,48,49,50,51,52,53,54,55,56,57,58], as was proposed in the original hidden valley paper [31], or it could be a new scalar bifundamental which would mediate t-channel production [59,19,60,18,61,62]. In both of these cases, in the limit that the mediator mass is large, it may also be appropriate to integrate it out which would induce a contact operator. ...
... Another simple option for UV completing the contact operator is to introduce a so-called tchannel mediator. The t-channel UV completion is determined by the following interaction [59,19,60,18,61,62] L ⊃ − κ αi χ Dα φq R i + h.c., ...
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In this work, we consider the case of a strongly coupled dark/hidden sector, which extends the Standard Model (SM) by adding an additional non-Abelian gauge group. These extensions generally contain matter fields, much like the SM quarks, and gauge fields similar to the SM gluons. We focus on the exploration of such sectors where the dark particles are produced at the LHC through a portal and undergo rapid hadronization within the dark sector before decaying back, at least in part and potentially with sizeable lifetimes, to SM particles, giving a range of possibly spectacular signatures such as emerging or semi-visible jets. Other, non-QCD-like scenarios leading to soft unclustered energy patterns or glueballs are also discussed. After a review of the theory, existing benchmarks and constraints, this work addresses how to build consistent benchmarks from the underlying physical parameters and present new developments for the PYTHIA Hidden Valley module, along with jet substructure studies. Finally, a series of improved search strategies is presented in order to pave the way for a better exploration of the dark showers at the LHC.
... As a last step, the flavor and LHC constraints can be complemented with the ones from DM phenomenology. Concerning the latter, the models considered here belong to the category of the so-called t-channel portals [88][89][90][91][92][93][94][95] (a "flavored" variant of this kind of setup has been also considered here [96][97][98][99][100]). Our assumption that a good fit of the B anomalies is achieved introduces, however, some relevant variation in the phenomenology of these kinds of models, especially for what concerns direct detection. ...
... The integral is computed between the freezeout temperature T f:o: (we recall that for WIMP T f:o: ∼ M DM 20 − M DM 30 ) and the current temperature of the Universe T 0 . Defining M the field that is t-channel exchanged in DM pair annihilations and also contributes to coannihilation processes, the DM effective annihilation cross section can be written as [88] ...
... In our analysis, the DM relic density, including coannihilations, have been computed with great numerical precision through the package micrOMEGAs [37]. 6 To clarify our results we provide nevertheless analytical expressions of the DM annihilation cross section into fermion pairs, the most relevant in the regions of parameter space favored by B anomalies (see below), at the leading order in the conventional velocity expansion (as further simplification we have neglected the masses of the final state fermions) [88][89][90]106]: ...
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We study renormalizable models with minimal field content that can provide a viable dark matter candidate through the standard freeze-out paradigm and, simultaneously, accommodate the observed anomalies in semileptonic B-meson decays at one loop. Following the hypothesis of minimality, this outcome can be achieved by extending the particle spectrum of the Standard Model either with one vectorlike fermion and two scalars or two vectorlike fermions and one scalar. The dark matter annihilations are mediated by t-channel exchange of other new particles contributing to the B anomalies, thus resulting in a correlation between flavor observables and dark matter abundance. Again based on minimality, we assume the new states to couple only with left-handed muons and second and third generation quarks. Besides an ad hoc symmetry needed to stabilize the dark matter, the interactions of the new states are dictated only by gauge invariance. We present here for the first time a systematic classification of the possible models of this kind, according to the quantum numbers of the new fields under the Standard Model gauge group. Within this general setup we identify a group of representative models that we systematically study, applying the most updated constraints from flavor observables, dedicated dark matter experiments, and LHC searches of leptons and/or jets and missing energy, and of disappearing charged tracks.
... From the Lagrangian in Eq. (9), the interaction of dark matter particles with right-handed neutrinos is shown in Fig. 1. The cross-section formula for this kind of process is given as [70] ...
... This type of study has already been done in Refs. [70,76]. In order to get the correct relic abundance, we did an analysis for a DM particle mass around 50 GeV, as suggested by many experiments like XENON1T [77], PandaX-11 [78], LUX [79], SuperCDMS [80] etc. ...
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In this paper, we present a model based on A4 discrete flavor symmetry implementing inverse and type-II seesaw mechanisms to have LHC-accessible TeV-scale right-handed neutrino mass and texture one-zero in the resulting Majorana neutrino mass matrix, respectively. We investigate the neutrino and dark matter sectors of the model. Non-Abelian discrete A4 symmetry spontaneously breaks into the Z2 subgroup and hence provides a stable dark matter candidate. To constrain the Yukawa Lagrangian of our model, we impose Z2', Z3, and Z4 cyclic symmetries in addition to the A4 flavor symmetry. In this work we use the recently upldated data on cosmological parameters from the Planck Collaboration [N. Aghanim et al.[Planck Collaboration], Astron. Astrophys. A6, 641 (2020)]. For the dark matter candidate mass around 45–55 GeV, we obtain a mediator particle mass (right-handed neutrinos) ranging from 138–155 GeV. The Yukawa couplings are found to be in the range 0.995–1 to have observed the relic abundance of dark matter. We further obtain inverse (X ≡ F^2(n)/z^2 ) and type-II (X ' ≡ f_1v_(Delta_1 ) seesaw contributions to the 0νββ decay amplitude |Mee|, with the model being consistent with low-energy experimental constraints. In particular, we emphasize that the type-II seesaw contribution to |Mee| is large compared to the inverse seesaw contribution for normally ordered (NO) neutrino masses.
... It follows that the relic abundance of χ is given by [57] Ωh 2 1.07 × 10 9 ...
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A bstract Restoration of the electroweak symmetry at temperatures around the Higgs mass is linked to tight phenomenological constraints on many baryogenesis scenarios. A potential remedy can be found in mechanisms of electroweak symmetry non-restoration (SNR), in which symmetry breaking is extended to higher temperatures due to new states with couplings to the Standard Model. Here we show that, in the presence of a second Higgs doublet, SNR can be realized with only a handful of new fermions which can be identified as viable dark matter candidates consistent with all current observational constraints. The competing requirements on this class of models allow for SNR at temperatures up to ∼TeV, and imply the presence of sub-TeV new physics with sizable interactions with the Standard Model. As a result this scenario is highly testable with signals in reach of next-generation collider and dark matter direct detection experiments.
... Colorless mediators coupled to a pair of quarks and to a pair of DM particles are considered, as well as colored mediators, which decay into a single quark together with a single DM candidate. The latter scenario is referred to as a "fermion portal" [11,12]. In addition to a search for DM, a scenario with large extra dimensions proposed by Arkani-Hamed, Dimopoulos, and Dvali (ADD) [13,14] is tested. ...
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A bstract A search is presented for new particles produced at the LHC in proton-proton collisions at $$ \sqrt{s} $$ s = 13 TeV, using events with energetic jets and large missing transverse momentum. The analysis is based on a data sample corresponding to an integrated luminosity of 101 fb − 1 , collected in 2017–2018 with the CMS detector. Machine learning techniques are used to define separate categories for events with narrow jets from initial-state radiation and events with large-radius jets consistent with a hadronic decay of a W or Z boson. A statistical combination is made with an earlier search based on a data sample of 36 fb − 1 , collected in 2016. No significant excess of events is observed with respect to the standard model background expectation determined from control samples in data. The results are interpreted in terms of limits on the branching fraction of an invisible decay of the Higgs boson, as well as constraints on simplified models of dark matter, on first-generation scalar leptoquarks decaying to quarks and neutrinos, and on models with large extra dimensions. Several of the new limits, specifically for spin-1 dark matter mediators, pseudoscalar mediators, colored mediators, and leptoquarks, are the most restrictive to date.
... It leaves the dimension-six operator as the leading contribution, which can be matched to the electromagnetic anapole moment of DM. This receives additional suppression from DM velocity square, so that it is difficult to detect from the direct detection experiments [23]. 1 As a result, we conclude that the lepton portal DM with Majorana DM has negligible signal in indirect and direct searches, and is only subject to the constraints from the thermal relic abundance and collider searches. 1 The low energy electron recoil cross section is (y 4 /π)m 2 e /(m 2 φ − m 2 χ ) 2 , which is typically 10 −44 cm 2 for m φ , mχ ∼ O(100) GeV, well below the constraint from LUX-ZEPLIN experiment [72]. ...
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A bstract We study the lepton portal dark matter (DM) model in which the relic abundance is determined by the portal coupling among the Majorana fermion DM candidate χ , the singlet charged scalar mediator S ± and the Standard Model (SM) right-handed lepton. The direct and indirect searches are not sensitive to this model. This article studies the lepton portal coupling as well as the scalar portal coupling (between S ± and SM Higgs boson), as the latter is generally allowed in the Lagrangian. The inclusion of scalar portal coupling not only significantly enhances the LHC reach via the gg → h * → S ⁺ S − process, but also provides a few novel signal channels, such as the exotic decays and coupling devi- ations of the Higgs boson, offering new opportunities to probe the model. In addition, we also study the Drell-Yan production of S ⁺ S − at future lepton colliders, and find out that the scenario where one S ± is off-shell can be used to measure the lepton portal coupling directly. In particular, we are interested in the possibility that the scalar potential triggers a first-order phase transition and hence provides the stochastic gravitational wave (GW) signals. In this case, the terrestrial collider experiments and space-based GW detectors serve as complementary approaches to probe the model.
... The interactions can induce t-channel annihilation diagrams for the DM pair. Such colored [8,[12][13][14][15][16][17][18][19][20][21][22] and uncolored [14,[23][24][25][26] mediators have been studied in literature. ...
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We study the lepton portal dark matter (DM) model in which the relic abundance is determined by the portal coupling among the Majorana fermion DM candidate $\chi$, the singlet charged scalar mediator $S^\pm$ and the Standard Model (SM) right-handed lepton. The direct and indirect searches are not sensitive to this model. This article studies the lepton portal coupling as well as the scalar portal coupling (between $S^\pm$ and SM Higgs boson), as the latter is generally allowed in the Lagrangian. The inclusion of scalar portal coupling not only significantly enhances the LHC reach via the $gg\to h^*\to S^+S^-$ process, but also provides a few novel signal channels, such as the exotic decays and coupling deviations of the Higgs boson, offering new opportunities to probe the model. In addition, we also study the Drell-Yan production of $S^+S^-$ at future lepton colliders, and find out that the scenario where one $S^\pm$ is off-shell can be used to measure the lepton portal coupling directly. In particular, we are interested in the possibility that the scalar potential triggers a first-order phase transition and hence provides the stochastic gravitational wave (GW) signals. In this case, the terrestrial collider experiments and space-based GW detectors serve as complementary approaches to probe the model.
... There are a number of options related to Eq. (2); see, e.g., [44][45][46][47][48][49][50]. In what follows, we usually drop the superscript on c l L;R for the coupling to electrons and electron neutrinos, as we consider them as always present, c L;R ≡ c e L;R . ...
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Full-text available
We revisit the possibility of light-scalar dark matter, in the MeV to GeV mass bracket and coupled to electrons through fermion or vector mediators, in light of significant experimental and observational advances that probe new physics below the GeV scale. We establish new limits from electron colliders and fixed-target beams and derive the strength of loop-induced processes that are probed by precision physics, among other laboratory probes. In addition, we compute the cooling bound from SN1987A, consider self-scattering, structure formation, and cosmological constraints as well as the limits from dark matter–electron scattering in direct detection experiments. We then show that the combination of constraints largely excludes the possibility that the galactic annihilation of these particles may explain the long-standing International Gamma-Ray Astrophysics Laboratory excess of 511 keV photons as observed in the Galactic bulge. As a caveat to these conclusions, we identify the resonant annihilation regime where the vector mediator goes nearly on shell.
... Discussing the detailed UV completion of this effective field theory is beyond the scope of this work. Such completions have been discussed in the literature for the case of spin zero mediators [80][81][82], fermion mediators [83] and vector mediators associated with a gauged B − L current (see e.g. ref. [84] and references therein). ...
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A bstract We analyze interactions between dark matter and standard model particles with spin one mediators in an effective field theory framework. In this paper, we are considering dark particles masses in the range from a few MeV to the mass of the Z boson. We use bounds from different experiments: Z invisible decay width, relic density, direct detection experiments, and indirect detection limits from the search of gamma-ray emissions and positron fluxes. We obtain solutions corresponding to operators with antisymmetric tensor mediators that fulfill all those requirements within our approach.
Preprint
The quest to discover the nature of dark matter continues to drive many of the experimental and observational frontiers in particle physics, astronomy, and cosmology. While there are no definitive signatures to date, there exists a rich ecosystem of experiments searching for signals for a broad class of dark matter models, at different epochs of cosmic history, and through a variety of processes with different characteristic energy scales. Given the multitude of candidates and search strategies, effective field theory has been an important tool for parametrizing the possible interactions between dark matter and Standard Model probes, for quantifying and improving model-independent uncertainties, and for robust estimation of detection rates in the presence of large perturbative corrections. This white paper summarizes a wide range of effective field theory applications for connecting dark matter theories to experiments.
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The CRESST-II experiment is searching for Dark Matter particles in the form of WIMPs via their elastic scattering off nuclei in a target material. The CRESST target consists of scintillating CaWO4 crystals which are operated as cryogenic calorimeters at millikelvin temperatures and read out by transition edge sensors. Each interaction in CaWO4 produces a phonon signal in the target crystal and also a light signal that is measured by a secondary cryogenic calorimeter, allowing a very efficient discrimination between electron recoils from radioactive e/γ background and nuclear recoils. Moreover, to some extent, the different types of recoiling nuclei (O, Ca, W) can be distinguished. In the paper we present the latest results of the experiment, obtained from a net exposure of 730kg days acquired with 8 detectors between July 2009 and March 2011. The data has shown a considerable number of events in our signal region. Since this large number of events is not consistent with the known sources of background in our experiment, we discuss the compatibility of this excess of events with a possible WIMP-signal.
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Experimental observations and theoretical arguments at Galaxy and larger scales suggested that a large fraction of the universe is composed by Dark Matter particles. This motivated the DAMA experimental efforts to investigate the presence of such particles in the galactic halo by exploiting a model-independent signature and very highly radiopure set-ups deep underground. Here, a review of the model-independent positive results, obtained by the DAMA set-ups at the Gran Sasso National Laboratory of the INFN, and some of the implications will be given.
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We report results from a reanalysis of data from the Cryogenic Dark Matter Search (CDMS II) experiment at the Soudan Underground Laboratory. Data taken between October 2006 and September 2008 using eight germanium detectors are reanalyzed with a lowered, 2 keV recoil-energy threshold, to give increased sensitivity to interactions from weakly interacting massive particles (WIMPs) with masses below ~10 GeV/c^2. This analysis provides stronger constraints than previous CDMS II results for WIMP masses below 9 GeV/c^2 and excludes parameter space associated with possible low-mass WIMP signals from the DAMA/LIBRA and CoGeNT experiments.
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We report results of a search for weakly interacting massive particles (WIMPS) with the silicon detectors of the CDMS II experiment. This blind analysis of 140.2 kg day of data taken between July 2007 and September 2008 revealed three WIMP-candidate events with a surface-event background estimate of 0.41^(+0.20)_(−0.08)(stat)^(+0.28)_(−0.24)(syst). Other known backgrounds from neutrons and ^(206)Pb are limited to <0.13 and <0.08 events at the 90% confidence level, respectively. The exposure of this analysis is equivalent to 23.4 kg day for a recoil energy range of 7–100 keV for a WIMP of mass 10 GeV/c^2. The probability that the known backgrounds would produce three or more events in the signal region is 5.4%. A profile likelihood ratio test of the three events that includes the measured recoil energies gives a 0.19% probability for the known-background-only hypothesis when tested against the alternative WIMP+background hypothesis. The highest likelihood occurs for a WIMP mass of 8.6 GeV/c2 and WIMP-nucleon cross section of 1.9×10^(−41) cm^2.
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An inclusive search for supersymmetric processes that produce final states with jets and missing transverse energy is performed in pp collisions at a centre-of-mass energy of 8 TeV. The data sample corresponds to an integrated luminosity of 11.7 fb^(−1) collected by the CMS experiment at the LHC. In this search, a dimensionless kinematic variable, α_T, is used to discriminate between events with genuine and misreconstructed missing transverse energy. The search is based on an examination of the number of reconstructed jets per event, the scalar sum of transverse energies of these jets, and the number of these jets identified as originating from bottom quarks. No significant excess of events over the standard model expectation is found. Exclusion limits are set in the parameter space of simplified models, with a special emphasis on both compressed-spectrum scenarios and direct or gluino-induced production of third-generation squarks. For the case of gluino-mediated squark production, gluino masses up to 950–1125 GeV are excluded depending on the assumed model. For the direct pair-production of squarks, masses up to 450 GeV are excluded for a single light first- or second-generation squark, increasing to 600 GeV for bottom squarks.
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We present the kinematic variable, mT2, which is in some ways similar to the more familiar ‘transverse-mass’, but which can be used in events where two or more particles have escaped detection. We define this variable and describe the event topologies to which it applies, then present some of its mathematical properties. We then briefly discuss two case studies which show how mT2 is vital when reconstructing the masses of supersymmetric particles in mSUGRA-like and AMSB-like scenarios at the Large Hadron Collider.
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We present new experimental constraints on the elastic, spin-dependent WIMP-nucleon cross section using recent data from the XENON100 experiment, operated in the Laboratori Nazionali del Gran Sasso in Italy. An analysis of 224.6 live days×34 kg of exposure acquired during 2011 and 2012 revealed no excess signal due to axial-vector WIMP interactions with Xe129 and Xe131 nuclei. This leads to the most stringent upper limits on WIMP-neutron cross sections for WIMP masses above 6 GeV/c2, with a minimum cross section of 3.5×10−40 cm2 at a WIMP mass of 45 GeV/c2, at 90% confidence level.
Article
An inclusive search for supersymmetric processes that produce final states with jets and missing transverse energy is performed in pp collisions at a centre-of-mass energy of 8 TeV. The data sample corresponds to an integrated luminosity of 11.7 fb−1 collected by the CMS experiment at the LHC. In this search, a dimensionless kinematic variable, α T, is used to discriminate between events with genuine and misreconstructed missing transverse energy. The search is based on an examination of the number of reconstructed jets per event, the scalar sum of transverse energies of these jets, and the number of these jets identified as originating from bottom quarks. No significant excess of events over the standard model expectation is found. Exclusion limits are set in the parameter space of simplified models, with a special emphasis on both compressed-spectrum scenarios and direct or gluino-induced production of third-generation squarks. For the case of gluino-mediated squark production, gluino masses up to 950–1125 GeV are excluded depending on the assumed model. For the direct pair-production of squarks, masses up to 450 GeV are excluded for a single light first- or second-generation squark, increasing to 600 GeV for bottom squarks.