1,294 research outputs found
Inclusive top-pair production phenomenology with TOPIXS
We discuss various aspects of inclusive top-quark pair production based on
TOPIXS, a new, flexible program that computes the production cross section at
the Tevatron and LHC at next-to-next-to-leading logarithmic accuracy in soft
and Coulomb resummation, including bound-state effects and the complete
next-to-next-to-leading order result in the q-qbar channel, which has recently
become available. We present the calculation of the top-pair cross section in
pp collisions at 8 TeV centre-of-mass energy, as well as the cross sections for
hypothetical heavy quarks in extensions of the standard model. The dependence
on the parton distribution input is studied. Further we investigate the impact
of LHC top cross section measurements at sqrt(s)=7 TeV on global fits of the
gluon distribution using the NNPDF re-weighting method.Comment: 27 pages, 5 figures; v2: corrected typos in Eqs. (2.8) and (6.2) and
the text, added footnote on page 4, matches published versio
Naturalness bounds in extensions of the MSSM without a light Higgs boson
Adopting a bottom-up point of view, we make a comparative study of the
simplest extensions of the MSSM with extra tree level contributions to the
lightest Higgs boson mass. We show to what extent a relatively heavy Higgs
boson, up to 200-350 GeV, can be compatible with data and naturalness. The
price to pay is that the theory undergoes some change of regime at a relatively
low scale. Bounds on these models come from electroweak precision tests and
naturalness, which often requires the scale at which the soft terms are
generated to be relatively low.Comment: 18 pages, 5 figures. v2: minor revision, added references. v3,v4:
some numerical correction
W boson production at hadron colliders: the lepton charge asymmetry in NNLO QCD
We consider the production of W bosons in hadron collisions, and the
subsequent leptonic decay W->lnu_l. We study the asymmetry between the rapidity
distributions of the charged leptons, and we present its computation up to the
next-to-next-to-leading order (NNLO) in QCD perturbation theory. Our
calculation includes the dependence on the lepton kinematical cuts that are
necessarily applied to select W-> lnu_l events in actual experimental analyses
at hadron colliders. We illustrate the main differences between the W and
lepton charge asymmetry, and we discuss their physical origin and the effect of
the QCD radiative corrections. We show detailed numerical results on the charge
asymmetry in ppbar collisions at the Tevatron, and we discuss the comparison
with some of the available data. Some illustrative results on the lepton charge
asymmetry in pp collisions at LHC energies are presented.Comment: 37 pages, 21 figure
SUSY, the Third Generation and the LHC
We develop a bottom-up approach to studying SUSY with light stops and
sbottoms, but with other squarks and sleptons heavy and beyond reach of the
LHC. We discuss the range of squark, gaugino and Higgsino masses for which the
electroweak scale is radiatively stable over the "little hierarchy" below ~ 10
TeV. We review and expand on indirect constraints on this scenario, in
particular from flavor and CP tests. We emphasize that in this context,
R-parity violation is very well motivated. The phenomenological differences
between Majorana and Dirac gauginos are also discussed. Finally, we focus on
the light subsystem of stops, sbottom and neutralino with R-parity, in order to
probe the current collider bounds. We find that 1/fb LHC bounds are mild and
large parts of the motivated parameter space remain open, while the 10/fb data
can be much more decisive.Comment: 42 pages, 8 figures, 1 table. V2: minor corrections, references adde
PDF dependence of Higgs cross sections at the Tevatron and LHC: response to recent criticism
We respond to some criticism questioning the validity of the current Standard
Model Higgs exclusion limits at the Tevatron, due to the significant dependence
of the dominant production cross section from gluon-gluon fusion on the choice
of parton distribution functions (PDFs) and the strong coupling (alpha_S). We
demonstrate the ability of the Tevatron jet data to discriminate between
different high-x gluon distributions, performing a detailed quantitative
comparison to show that fits not explicitly including these data fail to give a
good description. In this context we emphasise the importance of the consistent
treatment of luminosity uncertainties. We comment on the values of alpha_S
obtained from fitting deep-inelastic scattering data, particularly the
fixed-target NMC data, and we show that jet data are needed for stability. We
conclude that the Higgs cross-section uncertainties due to PDFs and alpha_S
currently used by the Tevatron and LHC experiments are not significantly
underestimated, contrary to some recent claims.Comment: 44 pages, 19 figures. v2: version published in JHEP (paragraph added
at bottom of p.15
Lopsided gauge mediation
It has been recently pointed out that the unavoidable tuning among supersymmetric parameters required to raise the Higgs boson mass beyond its experimental limit opens up new avenues for dealing with the so called mu-B(mu) problem of gauge mediation. In fact, it allows for accommodating, with no further parameter tuning, large values of B(mu) and of the other Higgs-sector soft masses, as predicted in models where both mu and B(mu) are generated at one-loop order. This class of models, called Lopsided Gauge Mediation, offers an interesting alternative to conventional gauge mediation and is characterized by a strikingly different phenomenology, with light higgsinos, very large Higgs pseudoscalar mass, and moderately light sleptons. We discuss general parametric relations involving the fine-tuning of the model and various observables such as the chargino mass and the value of tan beta. We build an explicit model and we study the constraints coming from LEP and Tevatron. We show that in spite of new interactions between the Higgs and the messenger super fields, the theory can remain perturbative up to very large scales, thus retaining gauge coupling unification
Predictions for Higgs production at the Tevatron and the associated uncertainties
We update the theoretical predictions for the production cross sections of
the Standard Model Higgs boson at the Fermilab Tevatron collider, focusing on
the two main search channels, the gluon-gluon fusion mechanism and
the Higgs-strahlung processes with , including all
relevant higher order QCD and electroweak corrections in perturbation theory.
We then estimate the various uncertainties affecting these predictions: the
scale uncertainties which are viewed as a measure of the unknown higher order
effects, the uncertainties from the parton distribution functions and the
related errors on the strong coupling constant, as well as the uncertainties
due to the use of an effective theory approach in the determination of the
radiative corrections in the process at next-to-next-to-leading
order. We find that while the cross sections are well under control in the
Higgs--strahlung processes, the theoretical uncertainties are rather large in
the case of the gluon-gluon fusion channel, possibly shifting the central
values of the next-to-next-to-leading order cross sections by more than
. These uncertainties are thus significantly larger than the
error assumed by the CDF and D0 experiments in their recent
analysis that has excluded the Higgs mass range 162-166 GeV at the 95%
confidence level. These exclusion limits should be, therefore, reconsidered in
the light of these large theoretical uncertainties.Comment: 40 pages, 12 figures. A few typos are corrected and some updated
numbers are provide
Excluding Electroweak Baryogenesis in the MSSM
In the context of the MSSM the Light Stop Scenario (LSS) is the only region
of parameter space that allows for successful Electroweak Baryogenesis (EWBG).
This possibility is very phenomenologically attractive, since it allows for the
direct production of light stops and could be tested at the LHC. The ATLAS and
CMS experiments have recently supplied tantalizing hints for a Higgs boson with
a mass of ~ 125 GeV. This Higgs mass severely restricts the parameter space of
the LSS, and we discuss the specific predictions made for EWBG in the MSSM.
Combining data from all the available ATLAS and CMS Higgs searches reveals a
tension with the predictions of EWBG even at this early stage. This allows us
to exclude EWBG in the MSSM at greater than (90) 95% confidence level in the
(non-)decoupling limit, by examining correlations between different Higgs decay
channels. We also examine the exclusion without the assumption of a ~ 125 GeV
Higgs. The Higgs searches are still highly constraining, excluding the entire
EWBG parameter space at greater than 90% CL except for a small window of m_h ~
117 - 119 GeV.Comment: 24 Pages, 4 Figures (v3: fixed typos, minor corrections, added
references
Scaling Patterns for QCD Jets
Jet emission at hadron colliders follows simple scaling patterns. Based on
perturbative QCD we derive Poisson and staircase scaling for final state as
well as initial state radiation. Parton density effects enhance staircase
scaling at low multiplicities. We propose experimental tests of our theoretical
findings in Z+jets and QCD gap jets production based on minor additions to
current LHC analyses.Comment: 36 pages, 16 figure
Performance of CMS muon reconstruction in pp collision events at sqrt(s) = 7 TeV
The performance of muon reconstruction, identification, and triggering in CMS
has been studied using 40 inverse picobarns of data collected in pp collisions
at sqrt(s) = 7 TeV at the LHC in 2010. A few benchmark sets of selection
criteria covering a wide range of physics analysis needs have been examined.
For all considered selections, the efficiency to reconstruct and identify a
muon with a transverse momentum pT larger than a few GeV is above 95% over the
whole region of pseudorapidity covered by the CMS muon system, abs(eta) < 2.4,
while the probability to misidentify a hadron as a muon is well below 1%. The
efficiency to trigger on single muons with pT above a few GeV is higher than
90% over the full eta range, and typically substantially better. The overall
momentum scale is measured to a precision of 0.2% with muons from Z decays. The
transverse momentum resolution varies from 1% to 6% depending on pseudorapidity
for muons with pT below 100 GeV and, using cosmic rays, it is shown to be
better than 10% in the central region up to pT = 1 TeV. Observed distributions
of all quantities are well reproduced by the Monte Carlo simulation.Comment: Replaced with published version. Added journal reference and DO
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