9,231 research outputs found
The stransverse mass, MT2, in special cases
This document describes some special cases in which the stransverse mass,
MT2, may be calculated by non-iterative algorithms. The most notable special
case is that in which the visible particles and the hypothesised invisible
particles are massless -- a situation relevant to its current usage in the
Large Hadron Collider as a discovery variable, and a situation for which no
analytic answer was previously known. We also derive an expression for MT2 in
another set of new (though arguably less interesting) special cases in which
the missing transverse momentum must point parallel or anti parallel to the
visible momentum sum. In addition, we find new derivations for already known
MT2 solutions in a manner that maintains manifest contralinear boost invariance
throughout, providing new insights into old results. Along the way, we stumble
across some unexpected results and make conjectures relating to geometric forms
of M_eff and H_T and their relationship to MT2.Comment: 11 pages, no figures. v2 corrects minor typos. v3 corrects an
incorrect statement in footnote 8 and inserts a missing term in eq (3.9). v4
and v5 correct minor typos spotted by reader
The swan song: the disappearance of the nucleus of NGC 4051 and the echo of its past glory
BeppoSAX observed the low-luminous Seyfert 1 Galaxy NGC4051 in a ultra-dim
X-ray state. The 2-10 keV flux (1.26 x 10^{-12} erg/cm^2/s) was about 20 times
fainter than its historical average value, and remained steady along the whole
observation (~2.3 days). The observed flat spectrum (\Gamma ~ 0.8) and intense
iron line (EW ~600 eV) are best explained assuming that the active nucleus has
switched off, leaving only a residual reflection component visible.Comment: 5 pages, Latex, 3 Postscript figures, accepted for publication in
MNRA
The Fundamental Plane in RX J0142.0+2131: a galaxy cluster merger at z=0.28
We present the Fundamental Plane (FP) in the z = 0.28 cluster of galaxies RX
J0142.0+2131. There is no evidence for a difference in the slope of the FP when
compared with the Coma cluster, although the internal scatter is larger. On
average, stellar populations in RX J0142.0+2131 have rest-frame V-band
mass-to-light ratios (M/L_V) 0.29+-0.03 dex lower than in Coma. This is
significantly lower than expected for a passively-evolving cluster formed at
z_f=2. Lenticular galaxies have lower average M/L_V and a distribution of M/L_V
with larger scatter than ellipticals. Lower mass-to-light ratios are not due to
recent star formation: our previous spectroscopic observations of RX
J0142.0+2131 E/S0 galaxies showed no evidence for significant star-formation
within the past ~4 Gyr. However, cluster members have enhanced alpha-element
abundance ratios, which may act to decrease M/L_V. The increased scatter in the
RX J0142.0+2131 FP reflects a large scatter in M/L_V implying that galaxies
have undergone bursts of star formation over a range of epochs. The seven
easternmost cluster galaxies, including the second brightest member, have M/L_V
consistent with passive evolution and z_f = 2. We speculate that RX
J0142.0+2131 is a cluster-cluster merger where the galaxies to the east are yet
to fall into the main cluster body or have not experienced star formation as a
result of the merger.Comment: 4 pages, 2 figures, accepted for publication in ApJ Letter
Sqrt{shat}_{min} resurrected
We discuss the use of the variable sqrt{shat}_{min}, which has been proposed
in order to measure the hard scale of a multi parton final state event using
inclusive quantities only, on a SUSY data sample for a 14 TeV LHC. In its
original version, where this variable was proposed on calorimeter level, the
direct correlation to the hard scattering scale does not survive when effects
from soft physics are taken into account. We here show that when using
reconstructed objects instead of calorimeter energy and momenta as input, we
manage to actually recover this correlation for the parameter point considered
here. We furthermore discuss the effect of including W + jets and t tbar+jets
background in our analysis and the use of sqrt{shat}_{min} for the suppression
of SM induced background in new physics searches.Comment: 23 pages, 9 figures; v2: 1 figure, several subsections and references
as well as new author affiliation added. Corresponds to published versio
Absolutely stable proton and lowering the gauge unification scale
A unified model is constructed, based on flipped SU(5) in which the proton is absolutely stable. The model requires the existence of new leptons with masses of order the weak scale. The possibility that the unification scale could be extremely low is discussed
Geometric CP Violation with Extra Dimensions
We discuss how CP symmetry can be broken geometrically through orbifold
projections in hidden extra dimensions in the context of D-brane models for
particle unifications. We present a few toy models to illustrate the idea and
suggest ways to incorporate this technique in the context of realistic models.Comment: 6 pages, one figure; references updated and a new model adde
Effects of the and of other processes on the mixing hierarchies in the four-generation model
We analyze in the four-generation model the first measurement of the
branching ratio of rare kaon decay , along with the
other processes of mass difference , CP-violating
parameter mixing, mixing,
, and the upper bound values of mixing
and , and try to search for mixing of the fourth
generation in the hierarchical mixing scheme of the Wolfenstein
parametrization. Using the results for the mixing of the fourth generation, we
discuss predictions of the mixing () and the
branching ratio of directly CP-violating decay process
, and the effects on the CP asymmetry in neutral B
meson decays and the unitarity triangle.Comment: 29 pages written in LaTex. 6 figures(drawn on LaTeX). Revised from
" in the four-generation model" of the same
Authors(TOKUSHIMA 99-1, January 1999). A minor chang
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
Cohomological descent theory for a morphism of stacks and for equivariant derived categories
In the paper we answer the following question: for a morphism of varieties
(or, more generally, stacks), when the derived category of the base can be
recovered from the derived category of the covering variety by means of descent
theory? As a corollary, we show that for an action of a reductive group on a
scheme, the derived category of equivariant sheaves is equivalent to the
category of objects, equipped with an action of the group, in the ordinary
derived category.Comment: 28 page
Natural Suppression of Higgsino-Mediated Proton Decay in Supersymmetric SO(10)
In supersymmetric Grand Unified Theories, proton decay mediated by the
color--triplet higgsino is generally problematic and requires some fine--tuning
of parameters. We present a mechanism which naturally suppresses such dimension
5 operators in the context of SUSY . The mechanism, which implements
natural doublet--triplet splitting using the adjoint higgs, converts these
dimension 5 operators effectively into dimension 6. By explicitly computing the
higgs spectrum and the resulting threshold uncertainties we show that the
successful prediction of is maintained {\it as a prediction}
in this scheme. It is argued that only a weak suppression of the higgsino
mediated proton decay is achievable within SUSY without fine--tuning,
in contrast to a strong suppression in SUSY .Comment: 39 pages (3 Feynman graphs not included), in Plain LaTeX, BA-93-2
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