17 research outputs found
Reducing combinatorial uncertainties: A new technique based on MT2 variables
We propose a new method to resolve combinatorial ambiguities in hadron
collider events involving two invisible particles in the final state. This
method is based on the kinematic variable MT2 and on the MT2-assisted-on-shell
reconstruction of invisible momenta, that are reformulated as `test' variables
Ti of the correct combination against the incorrect ones. We show how the
efficiency of the single Ti in providing the correct answer can be
systematically improved by combining the different Ti and/or by introducing
cuts on suitable, combination-insensitive kinematic variables. We illustrate
our whole approach in the specific example of top anti-top production, followed
by a leptonic decay of the W on both sides. However, by construction, our
method is also directly applicable to many topologies of interest for new
physics, in particular events producing a pair of undetected particles, that
are potential dark-matter candidates. We finally emphasize that our method is
apt to several generalizations, that we outline in the last sections of the
paper.Comment: 1+23 pages, 8 figures. Main changes in v3: (1) discussion at the end
of sec. 2 improved; (2) added sec. 4.2 about the method's dependence on mass
information. Matches journal versio
Initial determination of the spins of the gluino and squarks at LHC
In principle particle spins can be measured from their production cross
sections once their mass is approximately known. The method works in practice
because spins are quantized and cross sections depend strongly on spins. It can
be used to determine, for example, the spin of the top quark. Direct
application of this method to supersymmetric theories will have to overcome the
challenge of measuring mass at the LHC, which could require high statistics. In
this article, we propose a method of measuring the spins of the colored
superpatners by combining rate information for several channels and a set of
kinematical variables, without directly measuring their masses. We argue that
such a method could lead to an early determination of the spin of gluino and
squarks. This method can be applied to the measurement of spin of other new
physics particles and more general scenarios.Comment: 23 pages, 8 figures, minor change
Momentum asymmetries as CP violating observables
Three body decays can exhibit CP violation that arises from interfering
diagrams with different orderings of the final state particles. We construct
several momentum asymmetry observables that are accessible in a hadron collider
environment where some of the final state particles are not reconstructed and
not all the kinematic information can be extracted. We discuss the
complications that arise from the different possible production mechanisms of
the decaying particle. Examples involving heavy neutralino decays in
supersymmetric theories and heavy Majorana neutrino decays in Type-I seesaw
models are examined.Comment: 20 pages, 9 figures. Clarifying comments and one reference added,
matches published versio
Composite Leptoquarks at the LHC
If electroweak symmetry breaking arises via strongly-coupled physics, the
observed suppression of flavour-changing processes suggests that fermion masses
should arise via mixing of elementary fermions with composite fermions of the
strong sector. The strong sector then carries colour charge, and may contain
composite leptoquark states, arising either as TeV scale resonances, or even as
light, pseudo-Nambu-Goldstone bosons. The latter, since they are coupled to
colour, get a mass of the order of several hundred GeV, beyond the reach of
current searches at the Tevatron. The same generic mechanism that suppresses
flavour-changing processes suppresses leptoquark-mediated rare processes,
making it conceivable that the many stringent constraints may be evaded. The
leptoquarks couple predominantly to third-generation quarks and leptons, and
the prospects for discovery at LHC appear to be good. As an illustration, a
model based on the Pati-Salam symmetry is described, and its embedding in
models with a larger symmetry incorporating unification of gauge couplings,
which provide additional motivation for leptoquark states at or below the TeV
scale, is discussed.Comment: 10 pp, version to appear in JHE
RECO level \sqrt{s}_{min} and subsystem \sqrt{s}_{min}: improved global inclusive variables for measuring the new physics mass scale in missing energy events at hadron colliders
The variable \sqrt{s}_{min} was originally proposed in arXiv:0812.1042 as a
model-independent, global and fully inclusive measure of the new physics mass
scale in missing energy events at hadron colliders. In the original incarnation
of \sqrt{s}_{min}, however, the connection to the new physics mass scale was
blurred by the effects of the underlying event, most notably initial state
radiation and multiple parton interactions. In this paper we advertize two
improved variants of the \sqrt{s}_{min} variable, which overcome this problem.
First we show that by evaluating the \sqrt{s}_{min} variable at the RECO level,
in terms of the reconstructed objects in the event, the effects from the
underlying event are significantly diminished and the nice correlation between
the peak in the \sqrt{s}_{min}^{(reco)} distribution and the new physics mass
scale is restored. Secondly, the underlying event problem can be avoided
altogether when the \sqrt{s}_{min} concept is applied to a subsystem of the
event which does not involve any QCD jets. We supply an analytic formula for
the resulting subsystem \sqrt{s}_{min}^{(sub)} variable and show that its peak
exhibits the usual correlation with the mass scale of the particles produced in
the subsystem. Finally, we contrast \sqrt{s}_{min} to other popular inclusive
variables such as H_T, M_{Tgen} and M_{TTgen}. We illustrate our discussion
with several examples from supersymmetry, and with dilepton events from top
quark pair production.Comment: 41 pages, 26 figure
Spin effects in the antler event topology at hadron colliders
We investigate spin correlation effects in the "antler" event topology pp->
A-> B1, B2 -> l^{-}, C1, l^{+}, C2 at the LHC. We study the shapes of several
kinematic variables, including the relative pseudorapidity, relative azimuthal
angle and the energies of the two leptons, as well as several mass variables
M_{ll}, Meff, \sqrt{s}_{min}, MT2, MCT and MCTx. We focus on the two kinematic
extremes of \sqrt{s} - threshold and infinity - and derive analytical
expressions for the differential distributions of several variables, most
notably the cos{\theta_{ll}}^* variable proposed by Barr in hep-ph/0511115. For
all possible spin assignments of particles A, B and C, we derive the
cos{\theta_{ll}}^* differential distribution at threshold, including the
effects of spin correlations. Our analytical results help identify the
problematic cases for spin discrimination.Comment: 43 pages, 14 figures, Preprint typeset in JHEP styl
Precise reconstruction of sparticle masses without ambiguities
We critically reexamine the standard applications of the method of
kinematical endpoints for sparticle mass determination. We consider the typical
decay chain in supersymmetry (SUSY) squark -> neutralino -> slepton -> LSP,
which yields a jet j and two leptons ln and lf. The conventional approaches use
the upper kinematical endpoints of the individual distributions m_{jll},
m_{jl(lo)} and m_{jl(hi)}, all three of which suffer from parameter space
region ambiguities and may lead to multiple solutions for the SUSY mass
spectrum. In contrast, we do not use m_{jll}, m_{jl(lo)} and m_{jl(hi)}, and
instead propose a new set of (infinitely many) variables whose upper kinematic
endpoints exhibit reduced sensitivity to the parameter space region. We then
outline an alternative, much simplified procedure for obtaining the SUSY mass
spectrum. In particular, we show that the four endpoints observed in the three
distributions m^2_{ll}, m^2_{jln} U m^2_{jlf} and m^2_{jln}+m^2_{jlf} are
sufficient to completely pin down the squark mass and the two neutralino
masses, leaving only a discrete 2-fold ambiguity for the slepton mass. This
remaining ambiguity can be easily resolved in a number of different ways: for
example, by a single additional measurement of the kinematic endpoint of any
one out of the many remaining 1-dimensional distributions at our disposal, or
by exploring the correlations in the 2-dimensional distribution of m^2_{jln} U
m^2_{jlf} versus m^2_{ll}. We illustrate our method with two examples: the LM1
and LM6 CMS study points. An additional advantage of our method is the expected
improvement in the accuracy of the SUSY mass determination, due to the
multitude and variety of available measurements.Comment: 37 pages, added a new figure in the Appendix, published versio