431 research outputs found
Gaining analytic control of parton showers
Parton showers are widely used to generate fully exclusive final states
needed to compare theoretical models to experimental observations. While, in
general, parton showers give a good description of the experimental data, the
precise functional form of the probability distribution underlying the event
generation is generally not known. The reason is that realistic parton showers
are required to conserve four-momentum at each vertex. In this paper we
investigate in detail how four-momentum conservation is enforced in a standard
parton shower and why this destroys the analytic control of the probability
distribution. We show how to modify a parton shower algorithm such that it
conserves four-momentum at each vertex, but for which the full analytic form of
the probability distribution is known. We then comment how this analytic
control can be used to match matrix element calculations with parton showers,
and to estimate effects of power corrections and other uncertainties in parton
showers.Comment: 12 pages, 6 figures, v2: final journal versio
Full-Phase-Space Twist Expansion in Semileptonic and Radiative B-Meson Decays
We study the Lambda_QCD/M_B corrections from subleading shape functions in
inclusive B-meson decays. We propose a natural and smooth interpolation from
the endpoint region to the full phase space, and derive expressions for the
triple differential decay rate in B -> X_u l nu and the photon energy spectrum
in B -> X_s gamma. Our results are valid to order Lambda_QCD/M_B for hadronic
invariant masses of order Lambda_QCD M_B and to order Lambda_QCD^2/M_B^2 for
larger hadronic masses. They allow a systematic investigation of the transition
between the separate regimes of the local and nonlocal expansions, and can be
used to study decay distributions in any kinematic variables. We consider
several examples of interest and point out that a combined measurement of
hadronic energy and invariant mass provides an alternative to the extraction of
|V_ub| which is largely independent of shape function effects and in principle
allows a higher accuracy than the combined measurement of leptonic and hadronic
invariant masses. We perform the expansion directly in QCD light-cone
operators, and give a discussion of the general basis of light-cone operators.
Reparametrization invariance under the change of the light-cone direction
reduces the number of independent shape functions. We show that differing
previous results for the lepton energy spectrum obtained from different choices
of light-cone coordinates are in agreement.Comment: 41 pages, 20 figures, v2: few references added, slightly extended
discussion (2 figures added to illustrate m_b dependence), v3: final journal
versio
Precise predictions for B -> Xc tau nu decay distributions
We derive precise standard model predictions for the dilepton invariant mass
and the tau energy distributions in inclusive B -> Xc tau nu decay. We include
Lambda_QCD^2/m_b^2 and alpha_s corrections using the 1S short-distance mass
scheme, and estimate shape function effects near maximal tau energy. These
results can improve the sensitivity of b -> c tau nu related observables to
beyond standard model physics.Comment: 7 pages, 4 figures; v2: journal versio
Theory Uncertainties for Higgs and Other Searches Using Jet Bins
Bounds on the Higgs mass from the Tevatron and LHC are determined using
exclusive jet bins to maximize sensitivity. Scale variation in exclusive
fixed-order predictions underestimates the perturbative uncertainty for these
cross sections, due to cancellations between the perturbative corrections
leading to large K factors and those that induce logarithmic sensitivity to the
jet-bin boundary. To account for this, we propose that scale variation in the
fixed-order calculations should be used to determine theory uncertainties for
inclusive jet cross sections, whose differences yield exclusive jet cross
sections. This yields a theory correlation matrix for the jet bins such that
the additional uncertainty from large logarithms due to the jet boundary
cancels when neighboring bins are added. This procedure is tested for H + 0, 1
jets, WW + 0 jets, and W + 0, 1, 2 jets, and found to be generally applicable.
For a case where the higher-order resummation of the jet boundary corrections
is known, we show that this procedure yields fixed-order uncertainties which
are theoretically consistent with those obtained in the resummed calculation.Comment: 13 pages, 4 figures; v2: journal versio
The Gluon Beam Function at Two Loops
The virtuality-dependent beam function is a universal ingredient in the
resummation for observables probing the virtuality of incoming partons,
including N-jettiness and beam thrust. We compute the gluon beam function at
two-loop order. Together with our previous results for the two-loop quark beam
function, this completes the full set of virtuality-dependent beam functions at
next-to-next-to-leading order (NNLO). Our results are required to account for
all collinear ISR effects to the N-jettiness event shape through N^3LL order.
We present numerical results for both the quark and gluon beam functions up to
NNLO and N^3LL order. Numerically, the NNLO matching corrections are important.
They reduce the residual matching scale dependence in the resummed beam
function by about a factor of two.Comment: 21 pages, 6 figures; v2: journal versio
A case study of quark-gluon discrimination at NNLL' in comparison to parton showers
Predictions for our ability to distinguish quark and gluon jets vary by more
than a factor of two between different parton showers. We study this problem
using analytic resummed predictions for the thrust event shape up to NNLL'
using and as proxies for
quark and gluon jets. We account for hadronization effects through a
nonperturbative shape function, and include an estimate of both perturbative
and hadronization uncertainties. In contrast to previous studies, we find
reasonable agreement between our results and predictions from both Pythia and
Herwig parton showers. We find that this is due to a noticeable improvement in
the description of gluon jets in the newest Herwig 7.1 compared to previous
versions.Comment: 10 pages, 5 figure
Rapidity-Dependent Jet Vetoes
Jet vetoes are a prominent part of the signal selection in various analyses
at the LHC. We discuss jet vetoes for which the transverse momentum of a jet is
weighted by a smooth function of the jet rapidity. With a suitable choice of
the rapidity-weighting function, such jet-veto variables can be factorized and
resummed allowing for precise theory predictions. They thus provide a
complementary way to divide phase space into exclusive jet bins. In particular,
they provide a natural and theoretically clean way to implement a tight veto on
central jets with the veto constraint getting looser for jets at increasingly
forward rapidities. We mainly focus our discussion on the 0-jet case in
color-singlet processes, using Higgs production through gluon fusion as a
concrete example. For one of our jet-veto variables we compare the resummed
theory prediction at NLL'+NLO with the recent differential cross section
measurement by the ATLAS experiment in the channel, finding
good agreement. We also propose that these jet-veto variables can be measured
and tested against theory predictions in other SM processes, such as Drell-Yan,
diphoton, and weak diboson production.Comment: 17 pages, 8 figure
Nonperturbative m_X cut effects in B -> Xs l+ l- observables
Recently, it was shown that in inclusive B -> Xs l+ l- decay, an angular
decomposition provides three independent (q^2 dependent) observables. A
strategy was formulated to extract all measurable Wilson coefficients in B ->
Xs l+ l- from a few simple integrals of these observables in the low q^2
region. The experimental measurements in the low q^2 region require a cut on
the hadronic invariant mass, which introduces a dependence on nonperturbative b
quark distribution functions. The associated hadronic uncertainties could
potentially limit the sensitivity of these decays to new physics. We compute
the nonperturbative corrections to all three observables at leading and
subleading order in the power expansion in \Lambda_QCD/m_b. We find that the
subleading power corrections give sizeable corrections, of order -5% to -10%
depending on the observable and the precise value of the hadronic mass cut.
They cause a shift of order -0.05 GeV^2 to -0.1 GeV^2 in the zero of the
forward-backward asymmetry.Comment: 11 pages, 4 figures, v2: corrected typos and Eq. (25), v3: journal
versio
The Beam Thrust Cross Section for Drell-Yan at NNLL Order
At the LHC and Tevatron strong initial-state radiation (ISR) plays an
important role. It can significantly affect the partonic luminosity available
to the hard interaction or contaminate a signal with additional jets and soft
radiation. An ideal process to study ISR is isolated Drell-Yan production, pp
-> X l+l- without central jets, where the jet veto is provided by the hadronic
event shape beam thrust tau_B. Most hadron collider event shapes are designed
to study central jets. In contrast, requiring tau_B << 1 provides an inclusive
veto of central jets and measures the spectrum of ISR. For tau_B << 1 we carry
out a resummation of alpha_s^n ln^m tau_B corrections at
next-to-next-to-leading-logarithmic order. This is the first resummation at
this order for a hadron-hadron collider event shape. Measurements of tau_B at
the Tevatron and LHC can provide crucial tests of our understanding of ISR and
of tau_B's utility as a central jet veto.Comment: 4 pages, 5 figures, v2: journal versio
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