Lepton flavour violating top decays at the LHC Davidson, Sacha; Mangano, Michelangelo L.; Perries, Stéphane ...
The 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
.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
A prospective analysis for the discovery of a light Higgs boson in the Compact Muon Solenoid (CMS) experiment at the Large Hadron Collider (LHC) is presented. The associated production channels WH ...and ZH of a Higgs boson decaying into a photon pair are studied using a full detector simulation. The method of analysis here employed should permit the utilization of real data once they become available in order to optimize the analysis performance and to estimate background rates. Minimizing in this way reliance on simulated data should allow a significant reduction in systematic errors. One year of LHC running at high luminosity (integrated luminosity of 100 fb-1) should allow an observation at 3σ of the Standard Model Higgs boson from the LEP lower limit of 114.4 GeV/c2 up to 146 GeV/c2. Three years of running at high luminosity should allow a 5σ discovery from the LEP lower limit up to 148 GeV/c2. In the context of supersymmetric models, the dominant gluon fusion Higgs boson production process could be strongly suppressed. This light Higgs gluophobic scenario could occur when the mixing in the stop sector is maximal. In such a case, the associated production channels WH and ZH may be recovery channels.
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.
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.
A search for massive resonances decaying to a Z boson and a photon is performed in events with a hadronically decaying Z boson candidate, separately in light-quark and b quark decay modes, identified ...using jet substructure and advanced b tagging techniques. Results are based on samples of proton-proton collisions collected with the CMS detector at the LHC at center-of-mass energies of 8 and 13 TeV, corresponding to integrated luminosities of 19.7 and 2.7 inverse femtobarns, respectively. The results of the search are combined with those of a similar search in the leptonic decay modes of the Z boson, based on the same data sets. Spin-0 resonances with various widths and with masses in a range between 0.2 and 3.0 TeV are considered. No significant excess is observed either in the individual analyses or the combination. The results are presented in terms of upper limits on the production cross section of such resonances and constitute the most stringent limits to date for a wide range of masses.
A search is presented for W' bosons in events with an electron or muon and large missing transverse momentum, using proton-proton collision data at sqrt(s) = 13 TeV collected with the CMS detector in ...2015 and corresponding to an integrated luminosity of 2.3 inverse femtobarns. No evidence of an excess of events relative to the standard model expectations is observed. For a W' boson described by the sequential standard model, upper limits at 95% confidence level are set on the product of the production cross section and branching fraction and lower limits are established on the new boson mass. Masses below 4.1 TeV are excluded combining electron and muon decay channels, significantly improving upon the results obtained with the 8 TeV data. Exclusion limits at 95% confidence level on the product of the W' production cross section and branching fraction are also derived in combination with the 8 TeV data. Finally, exclusion limits are set for the production of generic W' bosons decaying into this final state using a model-independent approach.
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.
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