Lepton flavour violating top decays at the LHC Davidson, Sacha; Mangano, Michelangelo L.; Perries, Stéphane ...
European physical journal. C, Particles and fields,
09/2015, Letnik:
75, Številka:
9
Journal Article
Recenzirano
Odprti dostop
We consider lepton-flavour violating decays of the top quark, mediated by 4-fermion operators. We compile constraints on a complete set of SU(3)
×
U(1)-invariant operators, arising from their loop ...contributions to rare decays and from HERA’s single-top search. The bounds on
e
–
μ
flavour change are more restrictive than on
ℓ
–
τ
; nonetheless the top could decay to a jet
+
e
μ
¯
with a branching ratio of order
10
-
3
. We estimate that the currently available LHC data (20 fb
-
1
at 8 TeV) could be sensitive to
B
R
(
t
→
e
μ
¯
+ jet)
∼
6
×
10
-
5
, and we extrapolate that 100 fb
-
1
at 13 TeV could reach a sensitivity of
∼
1
×
10
-
5
.
A
bstract
We study the phenomenology at the Large Hadron Collider of an exotic vector-like quark with charge +5/3. We relax the assumption of a 100% branching into
Wt
and allow for an arbitrary rate ...into
W
plus light quarks, thus covering all possible scenarios. Sizeable decays into light quarks can be achieved, for instance, in a model where the
X
5/3
quark is embedded in a doublet with hypercharge 7/6, which also contains a
t
′ quark. We study the bounds on the parameter space of this model, and perform a detailed simulation of the
X
5/3
pair production and decays. We show that the final state with
WtWq
, where
q
=
u
or
c
, contributes to the same sign dilepton searches, and that the reach can be improved with alternative cuts optimised on such final state.
Kernel methods have recently been introduced to solve Natural Language Processing and Text Mining problems. Kernels define a generalised similarity measure between objects of arbitrary structure, ...with three interesting properties, namely the ability to incorporate prior knowledge about the problem, the implicit mapping of the data into a new feature space, which allows for very richer representation and where problem solving is easier, and finally the independence of learning algorithms from the dimension of this new feature space (—the Kernel trick“). These properties, coupled with robust learning algorithms (for classification, clustering, dimension reduction, filtering, ...) provide some remarkable results in Text Mining tasks, such as document categorization, concept clustering, word sense disambiguation, information extraction, relationship extraction and automatic multilingual lexicon extraction.
Four tops on the real projective plane at LHC Cacciapaglia, Giacomo; Chierici, Roberto; Deandrea, Aldo ...
The journal of high energy physics,
10/2011, Letnik:
2011, Številka:
10
Journal Article
Recenzirano
Odprti dostop
We explore the four top signal
at the 7TeV Large Hadron Collider as a probe of physics beyond the standard model. Enhancement of the corresponding cross-section with respect to the Standard Model ...value can probe the electroweak symmetry breaking sector or test extra dimensional models with heavy Kaluza-Klein gluons and quarks. We perform a detailed analysis including background and detector simulation in the specific case of a universal extra-dimensional model with two extra dimensions compactified using the geometry of the real projective plane. For masses around 600 GeV, a discovery is possible for an effective cross section above 210 fb (36 fb) for 1/fb (10/fb) of integrated luminosity. This implies a branching ratio in tops of the (1
,
1) heavy photon above 13% (5%). Furthermore, the 4-top signal from the (2
,
0) and (0
,
2) tiers can be discovered with an integrated luminosity of 3
.
5/fb. The results of our simulation can be easily adapted to other models since the background processes are identical. Concerning the signal, typical production mechanisms for the
signal are similar even if cross-section values may vary considerably depending on the model and the spectrum of the new particles.
The Generator Coordinate Method with particle number projection using a set of Hartree-Fock plus BCS states is applied to the superdeformed even-even Hg and Pb isotopes. The
q
30 and
q
32 octupole ...vibrations are investigated in even-even Hg and Pb isotopes. These one-dimensional calculations predict that the collective octupole
K
π
— 0
− excitations are the lowest mode in energy. The electric monopole
E0 decay out of superdeformed states is also compared to the electric quadrupole
E2 transition rates.
An inclusive search is performed for supersymmetry in final states containing jets and an apparent imbalance in transverse momentum, ptvecmiss, due to the production of unobserved weakly interacting ...particles in pp collisions at a centre-of-mass energy of 8 TeV. The data, recorded with the CMS detector at the CERN LHC, correspond to an integrated luminosity of 18.5 inverse femtobarns. The dimensionless kinematic variable alphaT is used to discriminate between events with genuine ptvecmiss associated with unobserved particles and spurious values of ptvecmiss arising from jet energy mismeasurements. No excess of event yields above the expected standard model backgrounds is observed. The results are interpreted in terms of constraints on the parameter space of several simplified models of supersymmetry that assume the pair production of top squarks. The search provides sensitivity to a broad range of top squark decay modes, including the two-body decay top squark to c chi0, where c is a charm quark and chi0 is the lightest neutralino, as well as the four-body decay top squark to b f bar-f' chi0, where b is a bottom quark and f and bar-f' are fermions produced in the decay of an intermediate off-shell W boson. These modes dominate in scenarios in which the top squark and lightest neutralino are nearly degenerate in mass. For these modes, top squarks with masses as large as 260 and 230 GeV are excluded, respectively, for the two- and four-body decays.
A dedicated search is presented for new phenomena in inclusive eight- and ten-jet final states with low missing transverse momentum, with and without identification of jets originating from b quarks. ...The analysis is based on data from proton-proton collisions corresponding to an integrated luminosity of 19.6 inverse femtobarns collected with the CMS detector at the LHC at sqrt(s) = 8 TeV. The dominant multijet background expectations are obtained from low jet multiplicity control samples. Data agree well with the standard model background predictions, and limits are set in several benchmark models. Colorons (axigluons) with masses between 0.6 and 0.75 (up to 1.15) TeV are excluded at 95% confidence level. Similar exclusion limits for gluinos in R-parity violating supersymmetric scenarios are from 0.6 up to 1.1TeV. These results comprise the first experimental probe of the coloron and axigluon models in multijet final states.
A search is presented for narrow resonances decaying to dijet final states in proton-proton collisions at sqrt(s)=13 TeV using data corresponding to an integrated luminosity of 12.9 inverse ...femtobarns. The dijet mass spectrum is well described by a smooth parameterization and no significant evidence for the production of new particles is observed. Upper limits at 95% confidence level are reported on the production cross section for narrow resonances with masses above 0.6 TeV. In the context of specific models, the limits exclude string resonances with masses below 7.4 TeV, scalar diquarks below 6.9 TeV, axigluons and colorons below 5.5 TeV, excited quarks below 5.4 TeV, color-octet ascalars below 3.0 TeV, W' bosons below 2.7 TeV, Z' bosons below 2.1 TeV and between 2.3 and 2.6 TeV, and RS gravitons below 1.9 TeV. These extend previous limits in the dijet channel. Vector and axial-vector mediators in a simplified model of interactions between quarks and dark matter are excluded below 2.0 TeV. The first limits in the dijet channel on dark matter mediators are presented as functions of dark matter mass and are compared to the exclusions of dark matter in direct detection experiments.
The Fourier coefficients v2 and v3 characterizing the anisotropy of the azimuthal distribution of charged particles produced in PbPb collisions at sqrt(sNN)= 5.02 TeV are measured with data collected ...by the CMS experiment. The measurements cover a broad transverse momentum range, pt= 1-100 GeV. The analysis focuses on pt > 10 GeV range, where anisotropic azimuthal distributions should reflect the path-length dependence of parton energy loss in the created medium. Results are presented in several bins of PbPb collision centrality, spanning the 60x% most central events. The v2 coefficient is measured with the scalar product and the multiparticle cumulant methods, which have different sensitivities to the initial-state fluctuations. The values of both methods remain positive up to pt ~ 70 GeV, in all examined centrality classes. The v3 coefficient, only measured with the scalar product method, tends to zero for pt >~ 20 GeV. Comparisons between theoretical calculations and data provide new constraints on the path-length dependence of parton energy loss in heavy ion collisions and highlight the importance of the initial-state fluctuations.
The decay B+ to psi(2S) phi(1020) K+ is observed for the first time using data collected from pp collisions at sqrt(s) = 8 TeV by the CMS experiment at the LHC, corresponding to an integrated ...luminosity of 19.6 inverse femtobarns. The branching fraction of this decay is measured, using the mode B+ to psi(2S) K+ as normalization, to be (4.0+/- 0.4 (stat) +/- 0.6 (syst) +/- 0.2 (B)1E-6, where the third uncertainty is from the imprecision in the normalization channel.
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