137 research outputs found
After the Standard Model: New Resonances at the LHC
Experiments will soon start taking data at CERN's Large Hadron Collider (LHC)
with high expectations for discovery of new physics phenomena. Indeed, the
LHC's unprecedented center-of-mass energy will allow the experiments to probe
an energy regime where the standard model is known to break down. In this
article, the experiments' capability to observe new resonances in various
channels is reviewed.Comment: Preprint version of a Brief Review for Modern Physics Letters A.
Changes w.r.t. the fully corrected version are smal
Reconstructing particle masses from pairs of decay chains
A method is proposed for determining the masses of the new particles N,X,Y,Z
in collider events containing a pair of effectively identical decay chains Z to
Y+jet, Y to X+l_1, X to N+l_2, where l_1, l_2 are opposite-sign same-flavour
charged leptons and N is invisible. By first determining the upper edge of the
dilepton invariant mass spectrum, we reduce the problem to a curve for each
event in the 3-dimensional space of mass-squared differences. The region
through which most curves pass then determines the unknown masses. A
statistical approach is applied to take account of mismeasurement of jet and
missing momenta. The method is easily visualized and rather robust against
combinatorial ambiguities and finite detector resolution. It can be successful
even for small event samples, since it makes full use of the kinematical
information from every event.Comment: 12 pages, 5 figure
Heavy Charged Gauge Bosons with General CP Violating Couplings
Heavy gauge bosons such as are expected to exist in many
extensions of the Standard Model. In this paper, it is shown that the most
general Lagrangian for the interaction of with top and bottom
quarks which consists of V-A and V+A structure with in general complex
couplings produces an Electric Dipole Moment (EDM) for the top quark at one
loop level. We predict the allowed ranges for the mass and couplings of
by using the upper limit on the top quark EDM
The LHC Phenomenology of Vectorlike Confinement
We investigate in detail the LHC phenomenology of "vectorlike confinement",
where the Standard Model is augmented by a new confining gauge interaction and
new light fermions that carry vectorlike charges under both the Standard Model
and the new gauge group. If the new interaction confines at the TeV scale, this
framework gives rise to a wide range of exotic collider signatures such as the
production of a vector resonance that decays to a pair of collider-stable
charged massive particles (a "di-CHAMP" resonance), to a pair of
collider-stable massive colored particles (a "di-R-hadron resonance), to
multiple photons, s and s via two intermediate scalars, and/or to
multi-jet final states. To study these signals at the LHC, we set up two
benchmark models: one for the di-CHAMP and multi-photon signals, and the other
for the di-R-hadron and multijet signals. For the di-CHAMP/multi-photon model,
Standard Model backgrounds are negligible, and we show that a full
reconstruction of the spectrum is possible, providing powerful evidence for
vectorlike confinement. For the di-R-hadron/multijet model, we point out that
in addition to the di-R-hadron signal, the rate of the production of four
R-hadrons can also be sizable at the LHC. This, together with the multi-jet
signals studied in earlier work, makes it possible to single out vectorlike
confinement as the underlying dynamics.Comment: 32 pages, 28 figures. Several typos fixed, one paragraph added
elaborating choice of benchmarks. Version accepted by JHEP
Equivalence of volume and temperature fluctuations in power-law ensembles
Relativistic particle production often requires the use of Tsallis statistics
to account for the apparently power-like behavior of transverse momenta
observed in the data even at a few GeV/c. In such an approach this behavior is
attributed to some specific intrinsic fluctuations of the temperature in
the hadronizing system and is fully accounted by the nonextensivity parameter
. On the other hand, it was recently shown that similar power-law spectra
can also be obtained by introducing some specific volume fluctuations,
apparently without invoking the introduction of Tsallis statistics. We
demonstrate that, in fact, when the total energy is kept constant, these volume
fluctuations are equivalent to temperature fluctuations and can be derived from
them. In addition, we show that fluctuations leading to multiparticle power-law
Tsallis distributions introduce specific correlations between the considered
particles. We then propose a possible way to distinguish the fluctuations in
each event from those occurring from event-to-event. This could have
applications in the analysis of high density events at LHC (and especially in
ALICE).Comment: Revised version with new figure, footnotes and references adde
Constraints on Randall-Sundrum model from top-antitop production at the LHC
We study the top pair production cross section at the LHC in the context of
Randall-Sundrum model including the Kaluza-Klein (KK) excited gravitons. It is
shown that the recent measurement of the cross section of this process at the
LHC restricts the parameter space in Randall-Sundrum (RS) model considerably.
We show that the coupling parameter () is excluded by
this measurement from 0.03 to 0.22 depending on the mass of first KK excited
graviton (). We also study the effect of KK excitations on the spin
correlation of the top pairs. It is shown that the spin asymmetry in
events is sensitive to the RS model parameters with a reasonable choice of
model parameters.Comment: 17 pages, 6 figure
Proposal for chiral bosons search at LHC via their unique new signature
The resonance production of new chiral spin-1 bosons and their detection
through the Drell--Yan process at the CERN LHC is considered. Quantitative
evaluations of various differential cross-sections of the chiral bosons
production are made within the CalcHEP package. The new neutral chiral bosons
can be observed as a Breit--Wigner resonance peak in the invariant dilepton
mass distribution, as usual. However, unique new signatures of the chiral
bosons exist. First, there is no Jacobian peak in the lepton transverse
momentum distribution. Second, the lepton angular distribution in the
Collins-Soper frame for the high on-peak invariant masses of the lepton pairs
has a peculiar "swallowtail" shape.Comment: 4 pages, 5 figure
Extrapolation of Multiplicity distribution in p+p(\bar(p)) collisions to LHC energies
The multiplicity (N_ch) and pseudorapidity distribution (dN_ch/d\eta) of
primary charged particles in p+p collisions at Large Hadron Collider (LHC)
energies of \sqrt(s) = 10 and 14 TeV are obtained from extrapolation of
existing measurements at lower \sqrt(s). These distributions are then compared
to calculations from PYTHIA and PHOJET models. The existing \sqrt(s)
measurements are unable to distinguish between a logarithmic and power law
dependence of the average charged particle multiplicity () on \sqrt(s),
and their extrapolation to energies accessible at LHC give very different
values. Assuming a reasonably good description of inclusive charged particle
multiplicity distributions by Negative Binomial Distributions (NBD) at lower
\sqrt(s) to hold for LHC energies, we observe that the logarithmic \sqrt(s)
dependence of are favored by the models at midrapidity. The dN_ch/d\eta
versus \eta for the existing measurements are found to be reasonably well
described by a function with three parameters which accounts for the basic
features of the distribution, height at midrapidity, central rapidity plateau
and the higher rapidity fall-off. Extrapolation of these parameters as a
function of \sqrt(s) is used to predict the pseudorapidity distributions of
charged particles at LHC energies. dN_ch/d\eta calculations from PYTHIA and
PHOJET models are found to be lower compared to those obtained from the
extrapolated dN_ch/d\eta versus \eta distributions for a broad \eta range.Comment: 11 pages and 13 figures. Substantially revised and accepted for
publication in Journal of Physics
Long-lived charged Higgs at LHC as a probe of scalar Dark Matter
We study inert charged Higgs boson production and decays at LHC
experiments in the context of constrained scalar dark matter model (CSDMM). In
the CSDMM the inert doublet and singlet scalar's mass spectrum is predicted
from the GUT scale initial conditions via RGE evolution. We compute the cross
sections of processes at the LHC and show that
for light the first one is dominated by top quark mediated 1-loop
diagram with Higgs boson in s-channel. In a significant fraction of the
parameter space are long-lived because their decays to predominantly
singlet scalar dark matter (DM) and next-to-lightest (NL) scalar, are suppressed by the small singlet-doublet mixing
angle and by the moderate mass difference
The experimentally measurable displaced vertex in decays to leptons
and/or jets and missing energy allows one to discover the signal over
the huge background. We propose benchmark points for studies of this
scenario at the LHC. If, however, are short-lived, the subsequent
decays necessarily produce additional
displaced vertices that allow to reconstruct the full decay chain.Comment: 15 pages, 5 figure
Planck Scale Boundary Conditions and the Higgs Mass
If the LHC does only find a Higgs boson in the low mass region and no other
new physics, then one should reconsider scenarios where the Standard Model with
three right-handed neutrinos is valid up to Planck scale. We assume in this
spirit that the Standard Model couplings are remnants of quantum gravity which
implies certain generic boundary conditions for the Higgs quartic coupling at
Planck scale. This leads to Higgs mass predictions at the electroweak scale via
renormalization group equations. We find that several physically well motivated
conditions yield a range of Higgs masses from 127-142 GeV. We also argue that a
random quartic Higgs coupling at the Planck scale favors M_H > 150 GeV, which
is clearly excluded. We discuss also the prospects for differentiating
different boundary conditions imposed for \lambda(M_{pl}) at the LHC. A
striking example is M_H = 127\pm 5 GeV corresponding to \lambda(M_{pl})=0,
which would imply that the quartic Higgs coupling at the electroweak scale is
entirely radiatively generated.Comment: 12 pages, 5 figures; references added and other minor improvements,
matches version published in JHE
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