2,564 research outputs found
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
N-Jettiness: An Inclusive Event Shape to Veto Jets
Jet vetoes are essential in many Higgs and new-physics analyses at the LHC
and Tevatron. The signals are typically characterized by a specific number of
hard jets, leptons, or photons, while the backgrounds often have additional
jets. In such cases vetoing undesired additional jets is an effective way to
discriminate signals and background. Given an inclusive event sample with N or
more jets, the veto to have only N energetic jets defines an "exclusive" N-jet
cross section. This strongly restricts the phase space of the underlying
inclusive N-jet cross section and causes large double logarithms in
perturbation theory that must be summed to obtain theory predictions. Jet
vetoes are typically implemented using jet algorithms. This yields complicated
phase-space restrictions and one often relies on parton-shower Monte Carlos,
which are limited to leading-logarithmic accuracy. We introduce a global event
shape "N-jettiness", tau_N, which is defined for events with N signal jets and
vanishes in the limit of exactly N infinitely narrow jets. Requiring tau_N << 1
constrains radiation between the N signal jets and vetoes additional undesired
jets. This provides an inclusive method to veto jets and to define an exclusive
N-jet cross section that can be well-controlled theoretically. N-jettiness
yields a factorization formula with inclusive jet and beam functions.Comment: 4 pages, 1 figure, v2: typos corrected, 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
Dissecting Soft Radiation with Factorization
An essential part of high-energy hadronic collisions is the soft hadronic
activity that underlies the primary hard interaction. It includes soft
radiation from the primary hard partons, secondary multiple parton interactions
(MPI), and factorization-violating effects. The invariant mass spectrum of the
leading jet in +jet and +jet events is directly sensitive to these
effects, and we use a QCD factorization theorem to predict its dependence on
the jet radius , jet , jet rapidity, and partonic process for both the
perturbative and nonperturbative components of primary soft radiation. We prove
that the nonperturbative contributions involve only odd powers of , and the
linear term is universal for quark and gluon jets. The hadronization model
in PYTHIA8 agrees well with these properties. The perturbative soft initial
state radiation (ISR) has a contribution that depends on the jet area in the
same way as the underlying event, but this degeneracy is broken by dependence
on the jet . The size of this soft ISR contribution is proportional to the
color state of the initial partons, yielding the same positive contribution for
and , but a negative interference contribution for . Hence, measuring these dependencies allows one to separate
hadronization, soft ISR, and MPI contributions in the data.Comment: 11 pages, 11 figures, v2: PRL version, text rearrange
Factorization at the LHC: From PDFs to Initial State Jets
We study proton-(anti)proton collisions at the LHC or Tevatron in the
presence of experimental restrictions on the hadronic final state and for
generic parton momentum fractions. At the scale Q of the hard interaction,
factorization does not yield standard parton distribution functions (PDFs) for
the initial state. The measurement restricting the hadronic final state
introduces a new scale \mu_B << Q and probes the proton prior to the hard
collision. This corresponds to evaluating the PDFs at the scale \mu_B. After
the proton is probed, the incoming hard parton is contained in an initial-state
jet, and the hard collision occurs between partons inside these jets rather
than inside protons. The proper description of such initial-state jets requires
"beam functions". At the scale \mu_B, the beam function factorizes into a
convolution of calculable Wilson coefficients and PDFs. Below \mu_B, the
initial-state evolution is described by the usual PDF evolution which changes
x, while above \mu_B it is governed by a different renormalization group
evolution which sums double logarithms of \mu_B/Q and leaves x fixed. As an
example, we prove a factorization theorem for "isolated Drell-Yan", pp -> Xl+l-
where X is restricted to have no central jets. We comment on the extension to
cases where the hadronic final state contains a certain number of isolated
central jets.Comment: 41 pages (19 for everyone + 22 for experts), 16 figures; v2:
Notational typos fixed. Added sentences to emphasize that measuring isolated
Drell-Yan directly tests the initial state parton shower; v3: typos fixed,
journal versio
Employing Helicity Amplitudes for Resummation
Many state-of-the-art QCD calculations for multileg processes use helicity
amplitudes as their fundamental ingredients. We construct a simple and
easy-to-use helicity operator basis in soft-collinear effective theory (SCET),
for which the hard Wilson coefficients from matching QCD onto SCET are directly
given in terms of color-ordered helicity amplitudes. Using this basis allows
one to seamlessly combine fixed-order helicity amplitudes at any order they are
known with a resummation of higher-order logarithmic corrections. In
particular, the virtual loop amplitudes can be employed in factorization
theorems to make predictions for exclusive jet cross sections without the use
of numerical subtraction schemes to handle real-virtual infrared cancellations.
We also discuss matching onto SCET in renormalization schemes with helicities
in - and -dimensions. To demonstrate that our helicity operator basis is
easy to use, we provide an explicit construction of the operator basis, as well
as results for the hard matching coefficients, for jets,
jets, and jets. These operator bases are
completely crossing symmetric, so the results can easily be applied to
processes with and collisions.Comment: 41 pages + 20 pages in Appendices, 1 figure, v2: journal versio
Jet Regions from Event Shapes and the N-Jet Soft Function at Hadron Colliders
The N-jettiness event shape divides phase space into N+2 regions, each
containing one jet or beam. These jet regions are insensitive to the
distribution of soft radiation and, with a geometric measure for N-jettiness,
have circular boundaries. We give a factorization theorem for the cross section
which is fully differential in the mass of each jet, and compute the
corresponding soft function at next-to-leading order (NLO). For N-jettiness,
all ingredients are now available to extend NLO cross sections to resummed
predictions at next-to-next-to-leading logarithmic order.Comment: 3 pages, 2 figures, part of PANIC 2011 proceeding
Jet p_T Resummation in Higgs Production at NNLL'+NNLO
We present predictions for Higgs production via gluon fusion with a p_T veto
on jets and with the resummation of jet-veto logarithms at NNLL'+$NNLO order.
These results incorporate explicit O(alphas^2) calculations of soft and beam
functions, which include the dominant dependence on the jet radius R. In
particular the NNLL' order accounts for the correct boundary conditions for the
N3LL resummation, for which the only unknown ingredients are higher-order
anomalous dimensions. We use scale variations in a factorization theorem in
both rapidity and virtuality space to estimate the perturbative uncertainties,
accounting for both higher fixed-order corrections as well as higher-order
towers of jet-p_T logarithms. This formalism also predicts the correlations in
the theory uncertainty between the exclusive 0-jet and inclusive 1-jet bins. At
the values of R used experimentally, there are important corrections due to jet
algorithm clustering that include logarithms of R. Although we do not sum
logarithms of R, we do include an explicit contribution in our uncertainty
estimate to account for higher-order jet clustering logarithms. Precision
predictions for this H+0-jet cross section and its theoretical uncertainty are
an integral part of Higgs analyses that employ jet binning.Comment: 24 pages, 11 figure
Universality and m_X cut effects in B -> Xs l+ l-
The most precise comparison between theory and experiment for the B -> Xs l+
l- rate is in the low q^2 region, but the hadronic uncertainties associated
with an experimentally required cut on m_X potentially spoil the search for new
physics in these decays. We show that a 10-30% reduction of d\Gamma(B -> Xs l+
l-) / dq^2 due to the m_X cut can be accurately computed using the B -> X_s
gamma shape function. The effect is universal for all short distance
contributions in the limit m_X^2 << m_B^2, and this universality is spoiled
neither by realistic values of the m_X cut nor by alpha_s corrections. Both the
differential decay rate and forward-backward asymmetry with an m_X cut are
computed.Comment: 5 pages, journal versio
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