共查询到20条相似文献,搜索用时 687 毫秒
1.
R. H. K. Kadala P. G. Mercadante J. K. Mizukoshi Xerxes Tata 《The European Physical Journal C - Particles and Fields》2008,56(4):511-528
The branching fraction for the decays of gluinos to third generation quarks is expected to be enhanced in classes of supersymmetric
models where either third generation squarks are lighter than other squarks, or in mixed-higgsino dark matter models constructed
so as to be in concordance with the measured density of cold dark matter. In such scenarios, gluino production events at the
CERN Large Hadron Collider should be rich in top and bottom quark jets. Requiring b jets in addition to E
T
miss should, therefore, enhance the supersymmetry signal relative to Standard Model backgrounds from V + jet, VV and QCD backgrounds (V=W,Z). We quantify the increase in the supersymmetry reach of the LHC from b-tagging in a variety of well-motivated models of supersymmetry. We also explore “top tagging” at the LHC. We find that while
the efficiency for this turns out to be too low to give an increase in reach beyond that obtained via b-tagging, top tagging can indeed provide a confirmatory signal if gluinos are not too heavy. We also examine c jet tagging but find that it is not useful at the LHC. Finally, we explore the prospects for detecting the direct production
of third generation squarks in models with an inverted squark mass hierarchy. This is signaled by b jets + E
T
miss events being harder than in the Standard Model, but softer than those from the production of gluinos and heavier squarks.
We find that while these events can be readily separated from the SM background (for third generation squark masses ∼300–500 GeV),
the contamination from the much heavier gluinos and squarks remains formidable if these are also accessible. 相似文献
2.
S. Moretti 《The European Physical Journal C - Particles and Fields》1999,9(2):229-234
At future electron-positron colliders, one of the largest irreducible backgrounds to top searches in the channel ‘4 jets
+ lepton + missing energy’ comes from QCD events of order . We compute here such processes exactly at the parton level by resorting to 2 6 matrix elements exploiting helicity amplitude techniques. We adopt a typical selection procedure based on the tagging of
a high momentum and separated lepton. We finally outline kinematic differences between signal and background events that can
be exploited to further reduce such a QCD noise.
Received: 12 January 1999 / Revised version: 24 March 1999 / Published online: 20 May 1999 相似文献
3.
S. Heinemeyer C. Schappacher 《The European Physical Journal C - Particles and Fields》2012,72(3):1905
We evaluate all two-body decay modes of the gluino, in the Minimal Supersymmetric Standard Model with complex parameters (cMSSM).
This constitutes an important step in the cascade decays of SUSY particles at the LHC. The evaluation is based on a full one-loop
calculation of all two-body decay channels, also including hard QED and QCD radiation. The dependence of the gluino decay
to a scalar quark and a quark on the relevant cMSSM parameters is analyzed numerically. We find sizable contributions to the
decay widths and branching ratios. They are, roughly of O(±5%)\mathcal{O}({\pm 5\%)}, but can go up to ±10% or higher, where the pure SUSY QCD contributions alone can give an insufficient approximation to the
full one-loop result. Therefore the full corrections are important for the correct interpretation of gluino decays at the
LHC. The results will be implemented into the Fortran code FeynHiggs. 相似文献
4.
In several scenarios of Beyond Standard Model physics, the invisible decay mode of the Higgs boson is an interesting possibility.
The search strategy for an invisible Higgs boson at the Large Hadron Collider (LHC), using weak boson fusion process, has
been studied in detail, by taking into account all possible backgrounds. Realistic simulations have been used in the context
of CMS experiment to devise a set of event selection criteria which eventually enhances the signal contribution compared to
the background processes in characteristic distributions. In cut-based analysis, multi-jet background is found to overwhelm
the signal in the finally selected sample. With an integrated luminosity of 10 fb−1, an upper limit of 36% on the branching ratio can be obtained for Higgs boson with a mass of 120 GeV/c2 for LHC energy of 14 TeV. Since the analysis essentially depends on the background estimation, detailed studies have been
done to determine the background rates from real data. 相似文献
5.
Xiao-Peng Li Lei Guo Wen-Gan Ma Liang Han Ren-You Zhang Shao-Ming Wang 《The European Physical Journal C - Particles and Fields》2012,72(3):1-20
Single slepton production in association with a top quark at the CERN Large Hadron Collider (LHC) is one of the important processes in probing the R-parity violation couplings. We calculate the QCD next-to-leading order (NLO) corrections to the $pp \to t\tilde{\ell}^{-}(\bar{t}\tilde{\ell}^{+})+X$ process at the LHC and discuss the impacts of the QCD corrections on kinematic distributions. We investigate the dependence of the leading order (LO) and the NLO QCD corrected integrated cross section on the factorization/renormalization energy scale, slepton, stop-quark and gluino masses. We find that the uncertainty of the LO cross section due to the energy scale is obviously improved by the NLO QCD corrections, and the exclusive jet event selection scheme keeps the convergence of the perturbative series better than the inclusive scheme. The results show that the polarization asymmetry of the top-quark will be reduced by the NLO QCD corrections, and the QCD corrections generally increase with the increment of the $\tilde{t}_{1}$ or $\tilde {g}$ mass value. 相似文献
6.
Stefano Moretti Ramon Muñoz-Tapia Kosuke Odagiri 《The European Physical Journal C - Particles and Fields》1998,4(2):351-362
Heavy flavour tagging provides a broad range of possibilities in testing QCD features at LEP. We present here a study of
4jets events at LEP I where the so-called light gluinos could be directly produced. We show that microvertex techniques offer
a unique chance to exploit simple kinematical distributions in order to optimise the signal coming from gluino production
with respect to the background of ordinary QCD events. Our results indicate that experimental analyses along the lines suggested
here can exclude or reveal the presence of a gluino for masses up to 10 GeV and lifetimes below 10 sec. We also point out that a large fraction of gluino events could decay in configurations carrying large missing energy,
so to escape the usual selection criteria of 4jet samples. In our study, mass effects of quarks and gluinos have been taken
into account exactly. Our results are independent from both the jet algorithm and its resolution parameter.
Received: 25 July 1997 / Revised version: 16 September 1997 / Published online: 20 February 1998 相似文献
7.
The discovery of the Higgs boson(s) is the major goal of the LHC which will start taking data in 2008. In this work a data
driven extraction of the background and statistical signal significance in the H→ZZ→4ℓ decay channel is presented. The background
for Higgs masses as low as 130 GeV can be extracted with an error of 20%, using a sideband measurement from a single 30 fb-1 experiment. The predicted background distribution is best described by a double asymmetric Gaussian. An analytic formula
is introduced which provides an accurate p-value that a Higgs discovery claim is consistent with a background fluctuation.
The formula can be used in a single real measurement at LHC using as input the measured background and the profile likelihood
asymmetric errors of this measurement. The method presented here can be applied to the general case of extrapolating from
a signal-free data region to a candidate signal region. This is the case of supersymmetry searches at the LHC.
PACS 14.80.Bn; 06.20.Dk 相似文献
8.
We investigate squark and gluino pair production at LHC (CMS) with subsequent decays into quarks, leptons, and the lightest supersymmetric particles (LSP) in models with effective supersymmetry, where the third generation of squarks is relatively light, whilst the first two generations of squarks are heavy. We consider the general case of nonuniversal gaugino masses. The visibility of a signal through an excess over Standard Model background in (n≥2) jets+(m≥0) leptons+E T miss events depends rather strongly on the relation between the LSP, second-neutralino, gluino, and squark masses and decreases with increasing LSP mass. We find that, for a relatively heavy gluino, it is very difficult to detect a SUSY signal even for light third-generation squarks $(m_{\tilde q_3 } \leqslant 1TeV)$ if the LSP mass is close to the third-generation squark mass. 相似文献
9.
T. Binoth J.Ph. Guillet E. Pilon M. Werlen 《The European Physical Journal C - Particles and Fields》2000,16(2):311-330
We discuss the production of photon pairs in hadronic collisions, from fixed target to LHC energies. The study which follows
is based on a QCD calculation at full next-to-leading order accuracy, including single and double fragmentation contributions,
and implemented in the form of a general purpose computer program of “partonic event generator” type. To illustrate the possibilities
of this code, we present a comparison with observables measured by the WA70 and D0 collaborations, and some predictions for
the irreducible background to the search of Higgs bosons at LHC in the channel . We also discuss theoretical scale uncertainties for these predictions, and examine several infrared sensitive situations
which deserve further study.
Received: 24 November 1999 / Revised version: 13 March 2000 / Published online: 8 June 2000 相似文献
10.
MICHEL TROTTIER-McDONALD 《Pramana》2012,79(5):1337-1340
Tau leptons play a central role in the LHC physics programme, in particular as an important signature in many Higgs boson and supersymmetry searches. They are further used in Standard Model electroweak measurements, as well as detector-related studies like the determination of the missing transverse energy scale. Copious backgrounds from QCD processes call for both efficient identification of hadronically decaying tau leptons, as well as large suppression of fake candidates. A solid understanding of the combined performance of the calorimeter and tracking detectors is also required. We present the current status of the tau reconstruction, energy calibration and identification with the ATLAS detector at the LHC. Identification efficiencies are measured in W ?? ??? events in data and compared with predictions from Monte Carlo simulations, whereas the misidentification probabilities of QCD jets and electrons are determined from various jet-enriched data samples and from Z ?? ee events, respectively. The tau energy scale calibration is described and systematic uncertainties on both energy scale and identification efficiencies discussed. 相似文献
11.
We study single top production at the LHC in a SUSY-QCD model with a heavy Dirac gluino. The presence of a heavy Dirac gluino allows for notable top-up flavour changing neutral currents. In this scenario, we find that the process ug→tg gives the largest contribution to single top production via FCNCs at the LHC. The key features of this signal are that the top quark is produced very forward and that it is asymmetric to its anti-top counterpart, as the latter lacks a valence quark. 相似文献
12.
A. Brandenburg M. Maniatis M. M. Weber P. M. Zerwas 《The European Physical Journal C - Particles and Fields》2008,58(2):291-300
Supersymmetry predicts the identity of Yukawa and gauge couplings in the QCD sector:
. We examine whether the
Yukawa coupling can be determined, by methods complementary to LHC, by analyzing squark–gluino final states at a TeV e
+
e
− collider. 相似文献
13.
W. J. Stirling E. Vryonidou J. D. Wells 《The European Physical Journal C - Particles and Fields》2011,71(5):1642
We compute transplanckian parton scattering in flat extra-dimensional theories at the LHC and at the recently discussed high-energy
upgrade (HE LHC). We report new leading-order calculations of the QCD background. We apply appropriate cuts to satisfy the
necessary conditions for the eikonal approximation to be valid while at the same time maximising the signal to background
ratio at LHC energies. We study the computability of the eikonal signal and consider the effect of a possible 33 TeV high-energy
upgrade to the LHC, which serves to extend the calculable region and to enhance the signal to background ratio. 相似文献
14.
M. Della Negra 《EPJ direct》2002,4(1):1-22
We discuss the production of γπ0 and π0 π0 pairs with a large invariant mass at collider energies. We present a study based on a perturbative QCD calculation at full next-to-leading order accuracy, implemented in the computer programme DIPHOX. We give estimations for various observables, which concern the reducible background to the Higgs boson search in the channel H → γγ, in the mass range 80-140 GeV at the LHC. We critically discuss the reliability of these estimates due to our imperfect knowledge of fragmentation functions at high z and a subtle interplay between higher order corrections and realistic experimental cuts. Whereas the invariant mass spectrum of photon-pion pairs is theoretically better under control, in the dipion case large uncertainties remain. Finally we comment on the impact of our findings on Higgs boson searches at the LHC. We conclude that the qualitative statement that the pion backgrounds should not be dangerous for the H → γγ search channel remains true at the next-to-leading order level. 相似文献
15.
We investigate squark and gluino pair production at LHC (CMS) with subsequent decays into quarks and an LSP for the case of nonuniversal gaugino masses. Visibility of a signal by an excess over the SM background in (n≥2)jets+E T miss events depends rather strongly on the relation between the LSP, gluino, and squark masses and decreases with increasing LSP mass. For a relatively heavy LSP mass close to the squark or the gluino mass and for $m_{\tilde q} ,m_{\tilde g} \geqslant 1.5$ TeV, the sygnal is overly small to be observable. 相似文献
16.
S. Hesselbach S. Moretti J. Rathsman A. Sopczak 《The European Physical Journal C - Particles and Fields》2008,53(2):311-327
We report on detailed Monte Carlo comparisons of selection variables to separate tbH± signal events from the standard model tt̄ background using an iterative discriminant analysis (IDA) method. While kinematic
differences exist between the two processes whenever , the exploration of the spin difference between the charged Higgs and the W± gauge boson becomes crucial in the particularly challenging case of near degeneracy of the charged Higgs boson mass with
the W± mass. The TAUOLA package is used to decay the tau leptons emerging from the charged Higgs and W± boson decays taking the spin difference properly into account. We demonstrate that, even if the individual selection variables
have limited discriminant power, the IDA method achieves a significant separation between the expected signal and background.
For both Tevatron and LHC energies, the impact of the spin effects and H± mass on the separation of signal and background has been studied quantitatively. The effect of a hard transverse momentum
cut to remove QCD background has been studied and it is found that the spin effects remain important. The separation is expressed
in purity versus efficiency curves. The study is performed for charged Higgs boson masses between the W± mass and near the top mass. 相似文献
17.
Rohini M. Godbole Agnes Grau Rohit Hegde Giulia Pancheri Yogi Srivastava 《Pramana》2006,66(4):657-668
In this note, we summarize and compare various model predictions forpp total cross-section σ
tot
pp
, giving an estimate of the range of predictions for the total cross-section, σ
tot
pp
expected at the LHC. We concentrate on the results for σ
tot
pp
obtained in a particular QCD based model of the energy dependence of the total cross-section, including the effect of soft
gluon radiation. We obtain the range of predictions in this model by exploring the allowed range of model parameters. We further
give a handy parametrisation of these results which incidentally spans the range of various other available predictions at
the LHC as well 相似文献
18.
M. Guchait R. Kinnunen D.P. Roy 《The European Physical Journal C - Particles and Fields》2007,52(3):665-672
We have done a fast simulation analysis of the H± signal at LHC in 1 and 3 prong hadronic τ-jet channels along with a tt̄ background. The τ polarization was effectively used
to suppress the background in both channels. Combining this with appropriate cuts on pT of the τ-jet, the missing ET and the azimuthal angle between them reduces the background below the signal level. Consequently, one gets a viable H± signal up to a mass range of 600–700 GeV at moderate to large tanβ. 相似文献
19.
S. Dittmaier P. Uwer S. Weinzierl 《The European Physical Journal C - Particles and Fields》2009,59(3):625-646
We report on the calculation of the next-to-leading order QCD corrections to the production of top–antitop-quark pairs in
association with a hard jet at the Tevatron and at the LHC. Results for integrated and differential cross sections are presented.
We find a significant reduction of the scale dependence. In most cases the corrections are below 20% indicating that the perturbative
expansion is well under control. Moreover, the forward–backward charge asymmetry of the top quark, which is analyzed at the
Tevatron, is studied at next-to-leading order. We find large corrections, suggesting that the definition of the observable
has to be refined. 相似文献
20.
We present the next-to-leading order QCD corrections to the associated production of the vector gauge boson (Z/W±) and the graviton in the large extra dimension model at the LHC. We estimate the impact of the QCD corrections on the total cross sections as well as the differential distributions of the gauge bosons and find that they are significant. We also study the dependence of the cross sections on the arbitrary factorization scale and show the reduction in the scale uncertainties at NLO level. Further, we discuss the ultraviolet sensitivity of the theoretical predictions. 相似文献