A
bstract
In the computation of short-distance cross sections initiated by electrons and photons one can adopt the so-called structure-function approach, in which these particles play formally the ...same roles as hadrons do in QCD factorisation theorems, and must thus be associated with PDFs (equivalently known as structure functions in this context). At variance with their QCD counterparts, such PDFs are entirely calculable in QED. In this paper we present the results, at the next-to-leading order in the QED coupling constant
α
, for the initial conditions of the unpolarised electron and photon PDFs, which are a nec- essary ingredient for their eventual collinear evolution at the next-to-leading logarithmic accuracy. We also compute the analogous final-state quantities, namely the initial condi- tions for fragmentation functions into electrons and photons.
A
bstract
The electron, positron, and photon Parton Distribution Functions (PDFs) of the unpolarised electron have recently been computed at the next-to-leading logarithmic accuracy in QED, by ...adopting the
MS
¯
factorisation scheme. We present here analogous results, obtained by working in a different framework that is inspired by the so-called DIS scheme. We derive analytical solutions relevant to the large-
z
region, where we show that the behaviour of the PDFs depends in a dramatic way on whether running-
α
effects are included to all orders, as opposed to being truncated to some fixed order. By means of suitable initial and evolution conditions, next-to-leading logarithmic accurate PDFs are obtained whose large-
z
functional forms are identical to those of their leading logarithmic counterparts.
A
bstract
We present the key features relevant to the automated computation of all the leading- and next-to-leading order contributions to short-distance cross sections in a mixed-coupling expansion, ...with special emphasis on the first subleading NLO term in the QCD+EW scenario, commonly referred to as NLO EW corrections. We discuss, in particular, the FKS subtraction in the context of a mixed-coupling expansion; the extension of the FKS subtraction to processes that include final-state tagged particles, defined by means of fragmentation functions; and some properties of the complex mass scheme. We combine the present paper with the release of a new version of M
ad
G
raph
5_
a
MC@NLO, capable of dealing with mixed-coupling expansions. We use the code to obtain illustrative inclusive and differential results for the 13-TeV LHC.
The muon parton distribution functions Frixione, S.; Stagnitto, G.
The journal of high energy physics,
12/2023, Letnik:
2023, Številka:
12
Journal Article
Recenzirano
Odprti dostop
A
bstract
We compute the Parton Distribution Functions (PDFs) of the unpolarised muon for the leptons, the photon, the light quarks, and the gluon. We discuss in detail the issues stemming from the ...necessity of evaluating the strong coupling constant at scales of the order of the typical hadron mass, and compare our novel approach with those currently available in the literature. While we restrict our phenomenological results to be leading-logarithmic accurate, we set up our formalism in a way that renders it straightforward to achieve next-to-leading logarithmic accuracy in the QED, QCD, and mixed QED
×
QCD contributions.
A
bstract
Monte Carlo event generators for hard hadronic collisions depend on the evolution of parton showers backwards from a high-scale subprocess to the hadronization scale. The evolution is ...treated as a branching process with a sequence of resolvable parton emissions. The criterion of resolvability involves cutoffs that determine the no-emission probability (NEP) for a given range of the evolution scale. Existing event generators neglect cutoff-dependent terms in the NEP that, although formally power-suppressed, can have significant phenomenological effects. We compute such terms and study their consequences. One important result is that it is not possible for the backward shower to faithfully reproduce the cutoff-independent parton distribution functions (PDFs) used to generate it. We show that the computed NEP corrections mitigate but do not eliminate this problem. An alternative approach is to use cutoff-dependent PDFs that are consistent with the uncorrected NEP. Then one must apply cutoff-dependent corrections to hard subprocess matrix elements. We compute those corrections to the first nontrivial order for the Drell-Yan process and for Higgs production by gluon fusion.
A
bstract
By working in QED, we obtain the electron, positron, and photon Parton Distribution Functions (PDFs) of the unpolarised electron at the next-to-leading logarithmic accuracy. The PDFs ...account for all of the universal effects of initial-state collinear origin, and are key ingredients in the calculations of cross sections in the so-called structure- function approach. We present both numerical and analytical results, and show that they agree extremely well with each other. The analytical predictions are defined by means of an additive formula that matches a large-
z
solution that includes all orders in the QED coupling constant
α
, with a small- and intermediate-
z
solution that includes terms up to
O
(
α
3
).
A
bstract
We compute the contribution of order
α
S
2
α
2
to the cross section of a top-antitop pair in association with at least one heavy Standard Model boson —
Z
,
W
±
, and Higgs — by including ...all effects of QCD, QED, and weak origin and by working in the automated M
ad
G
raph
5_
a
MC@NLO framework. This next-to-leading order contribution is then combined with that of order
α
S
3
α
, and with the two dominant lowest-order ones,
α
S
2
α
and
α
S
α
2
, to obtain phenomenological results relevant to a 8, 13, and 100 TeV
pp
collider.
A
bstract
We show how a careful analysis of the behaviour of a parton shower Monte Carlo in the vicinity of the soft and collinear regions allows one to formulate a modified MC@NLO-matching ...prescription that reduces the number of negative-weight events with respect to that stemming from the standard MC@NLO procedure. As a first practical application of such a prescription, that we dub MC@NLO-∆, we have implemented it in the M
ad
G
raph
5_
a
MC@NLO framework, by employing the P
ythia
8 Monte Carlo. We present selected MC@NLO-∆ results at the 13 TeV LHC, and compare them with MC@NLO ones. We find that the former predictions are consistent with the latter ones within the typical matching systematics, and that the reduction of negative-weight events is significant.
A
bstract
We study the production of jets in hadronic collisions, by computing all contributions proportional to
α
S
n
α
m
, with
n
+
m
= 2 and
n
+
m
= 3. These correspond to leading and ...next-to-leading order results, respectively, for single-inclusive and dijet observables in a perturbative expansion that includes both QCD and electroweak effects. We discuss issues relevant to the definition of hadronic jets in the context of electroweak corrections, and present sample phenomenological predictions for the 13-TeV LHC. We find that both the leading and next-to-leading order contributions largely respect the relative hierarchy established by the respective coupling-constant combinations.
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