255 research outputs found
Kinematic Discrimination of tW and tt Productions Using Initial State Radiation
Production of a single top quark provides excellent opportunity for
understanding top quark physics and Cabibbo-Kobayashi-Maskawa structure of the
quark sector in the Standard Model. Although an associated production with a
b-quark has already been observed at the Tevatron in 2009, a single top
production in association with a W gauge boson has not been observed till 2014
at the LHC, where pair production of the top quark serves as the dominant
background. Due to the kinematic similarity between tW and the dominant
background, it is challenging to find suitable kinematic variables that offer
good signal-background separation, which naturally leads to the use of
multivariate methods. In this paper, we investigate kinematic structure of tW+j
channel using M_T2 and invariant mass variables, and find that tW +j production
could well be separated from tt production with high purity at a low cost of
statistics when utilizing these kinematic correlations.Comment: 15 pages, 11 figures, 3 tables, journal submitted version, references
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Using Kinematic Variables to Extract Information from Semi-invisible Decays of Heavy Particles at Hadron Colliders
We examine the ways of extracting information from semi-invisible decays of (new) heavy particles at hadron colliders, i.e., such heavy particles are assumed to decay into visible/Standard Model (SM) particles and invisible particles. As a concrete realization, we employ the models with the stable weakly interacting massive particle (WIMP), a well-motivated dark matter (DM) candidate. By definition, dark matter is not seen by the detectors, i.e., invisible. Typically, stability of dark matter is ensured by introducing a new (unbroken) symmetry under which the DM is non-trivially charged while the SM particles are uncharged. Also, many new physics models contain other heavier particles which are charged under the same symmetry so that such heavier particles must decay into (invisible) DM particles along with the relevant visible/SM particles.
In particular, we study how to determine the masses of DM and heavy particles and the nature of the above-mentioned DM stabilization symmetries. For this purpose we take three kinematic variables as the main toolkits. We first discuss the distribution of the invariant mass formed by the visible part in the associated decays. As the second variable, we include the invisible part in forming the invariant mass. Because we are not aware of the longitudinal momentum of invisible particles, such a quantity is constructed in the plane transverse to the beam pipe, which is therefore called “transverse” mass. This is typically suitable for a singly produced heavy particle. Since the DM stabilization symmetries lead to pair-production of heavier particles, we here consider the “stransverse/MT2” type variable, a simple generalization of the transverse mass. Finally, we consider the energy spectrum of visible particle(s), which is not Lorentz-invariant at all even under longitudinal boosts. The relevant strategy is predicated upon the new observations that we shall make about physical implications of the peak position in such an energy spectrum. We emphasize that the relevant methods using the three observables are complementary to one another
An alternative interpretation for cosmic ray peaks
We propose an alternative mechanism based upon dark matter (DM)
interpretation for anomalous peak signatures in cosmic ray measurements,
assuming an extended dark sector with two DM species. This is contrasted with
previous effort to explain various line-like cosmic-ray excesses in the context
of DM models where the relevant DM candidate directly annihilates into Standard
Model (SM) particles. The heavier DM is assumed to annihilate to an on-shell
intermediate state. As the simplest choice, it decays directly into the lighter
DM along with an unstable particle which in turn decays to a pair of SM states
corresponding to the interesting cosmic anomaly. We show that a sharp continuum
energy peak can be readily generated under the proposed DM scenario, depending
on dark sector particle mass spectra. Remarkably, such a peak is robustly
identified as half the mass of the unstable particle. Furthermore, other
underlying mass parameters are analytically related to the shape of energy
spectrum. We apply this idea to the two well-known line excesses in the cosmic
photon spectrum: 130 GeV gamma-ray line and 3.5 keV X-ray line. Each observed
peak spectrum is well-reproduced by theoretical expectation predicated upon our
suggested mechanism, and moreover, our resulting best fits provide rather
improved chi-square values.Comment: 9 pages, 3 figure
How to prove that the LHC did not discover dark matter
If the LHC is able to produce dark matter particles, they would appear at the
end of cascade decay chains, manifesting themselves as missing transverse
energy. However, such "dark matter candidates" may decay invisibly later on. We
propose to test for this possibility by studying the effect of particle widths
on the observable invariant mass distributions of the visible particles seen in
the detector. We consider the simplest non-trivial case of a two-step two-body
cascade decay and derive analytically the shapes of the invariant mass
distributions, for generic values of the widths of the new particles. We
demonstrate that the resulting distortion in the shape of the invariant mass
distribution can be significant enough to measure the width of the dark matter
"candidate", ruling it out as the source of the cosmological dark matter.Comment: 5 pages, 5 figure
Using Energy Peaks to Measure New Particle Masses
We discussed in arXiv:1209.0772 that the laboratory frame distribution of the
energy of a massless particle from a two-body decay at a hadron collider has a
peak whose location is identical to the value of this daughter's (fixed) energy
in the rest frame of the corresponding mother particle. For that result to hold
we assumed that the mother is unpolarized and has a generic boost distribution
in the laboratory frame. In this work we discuss how this observation can be
applied for determination of masses of new particles, without requiring a full
reconstruction of their decay chains or information about the rest of the
event. We focus on a two-step cascade decay of a massive particle that has one
invisible particle in the final state: C -> Bb -> Aab, where C, B and A are new
particles of which A is invisible and a, b are visible particles. Combining the
measurements of the peaks of energy distributions of a and b with that of the
edge in their invariant mass distribution, we demonstrate that it is in
principle possible to determine separately all three masses of the new
particles, in particular, without using any measurement of missing transverse
momentum. Furthermore, we show how the use of the peaks in an inclusive energy
distribution is generically less affected by combinatorial issues as compared
to other mass measurement strategies. For some simplified, yet interesting,
scenarios we find that these combinatorial issues are absent altogether. As an
example of this general strategy, we study SUSY models where gluino decays to
an invisible lightest neutralino via an on-shell bottom squark. Taking into
account the dominant backgrounds, we show how the mass of the bottom squark,
the gluino and (for some class of spectra) that of the neutralino can be
determined using this technique.Comment: 42 pages, 11 figure
Searching for Boosted Dark Matter via Dark-Strahlung
We propose a new search channel for boosted dark matter (BDM) signals coming
from the present universe, which are distinct from simple neutrino signals
including those coming from the decay or pair-annihilation of dark matter. The
signal process is initiated by the scattering of high-energetic BDM off either
an electron or a nucleon. If the dark matter is dark-sector U(1)-charged, the
scattered BDM may radiate a dark gauge boson (called "dark-strahlung") which
subsequently decays to a Standard Model fermion pair. We point out that the
existence of this channel may allow for the interpretation that the associated
signal stems from BDM, not from the dark-matter-origin neutrinos. Although the
dark-strahlung process is generally subleading compared to the lowest-order
simple elastic scattering of BDM, we find that the BDM with a significant boost
factor may induce an O(10-20%) event rate in the parameter regions unreachable
by typical beam-produced dark-matter. We further find that the dark-strahlung
channel may even outperform the leading-order channel in the search for BDM,
especially when the latter is plagued by substantial background contamination.
We argue that cosmogenic BDM searches readily fall in such a case, hence taking
full advantage of dark-strahlung. As a practical application, experimental
sensitivities expected in the leading-order and dark-strahlung channels are
contrasted in dark gauge boson parameter space, under the environment of DUNE
far-detectors, revealing usefulness of dark-strahlung.Comment: 9 pages, 4 figures, Journal-submitted versio
Dark Matter "Collider" from Inelastic Boosted Dark Matter
We propose a novel dark matter (DM) detection strategy for the models with
non-minimal dark sector. The main ingredients in the underlying DM scenario are
a boosted DM particle and a heavier dark sector state. The relativistic DM
impinged on target material scatters off inelastically to the heavier state
which subsequently decays into DM along with lighter states including visible
(Standard Model) particles. The expected signal event, therefore, accompanies a
visible signature by the secondary cascade process associated with a recoiling
of the target particle, differing from the typical neutrino signal not
involving the secondary signature. We then discuss various kinematic features
followed by DM detection prospects at large volume neutrino detectors with a
model framework where a dark gauge boson is the mediator between the Standard
Model particles and DM.Comment: 6 pages, 4 figures, 2 table
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