SciPost Phys. Proc. 8, 072 (2022) ·
published 12 July 2022
LHC collisions can act as a source of photons in the initial state. This mechanism plays an important role in the production of particles with electroweak couplings, and a precise account of photon-initiated (PI) production at the LHC is a key ingredient in the LHC precision physics programme. I will discuss the possibility of modelling PI processes directly via the structure function approach. This can provide percent level precision in the production cross sections, and is therefore well positioned to account for LHC precision requirements. This formalism in addition allows one to make use of another useful feature of photons, namely that they are colour-singlet and can often be emitted elastically (or quasi-elastically) from the proton. I will discuss recent work on applications of the structure function approach to precision calculations of PI production in the inclusive mode, and to `exclusive' processes with rapidity gaps, which can provide a unique probe of the Standard Model and physics beyond it.
R.S. Thorne, S. Bailey, T. Cridge, L. Harland-Lang, A.D. Martin
SciPost Phys. Proc. 8, 018 (2022) ·
published 11 July 2022
We present the new MSHT20 set of parton distribution functions (PDFs) of the
proton, determined from a global analysis of the available hard scattering
data and superseding the MMHT14 sets. The parameterisation is now adapted and
extended and we include a large number of new data sets:
the final HERA and Tevatron data, and a significant number of LHC data
sets on vector boson production, inclusive jets and top quark distributions.
We include up to NNLO QCD corrections for all data sets that play a major
role in the fit. There are some changes to central PDF values and a
significant reduction in the uncertainties, but the PDFs and the
predictions using them are generally within one standard deviation of the
MMHT14 results. We discuss the phenomenological impact of our results.
Lucian A. Harland-Lang, Valery A. Khoze, Mikhail G. Ryskin
SciPost Phys. 11, 064 (2021) ·
published 21 September 2021
We analyse in detail the role of additional hadron-hadron interactions in
elastic photon-initiated (PI) production at the LHC, both in $pp$ and heavy ion
collisions. We first demonstrate that the source of difference between our
predictions and other results in the literature for PI muon pair production is
dominantly due to an unphysical cut that is imposed in these latter results on
the dimuon-hadron impact parameter. We in addition show that this is
experimentally disfavoured by the shape of the muon kinematic distributions
measured by ATLAS in ultraperipheral PbPb collisions. We then consider the
theoretical uncertainty due to the survival probability for no additional
hadron-hadron interactions, and in particular the role this may play in the
tendency for the predicted cross sections to lie somewhat above ATLAS data on
PI muon pair production, in both $pp$ and PbPb collisions. This difference is
relatively mild, at the $\sim 10\%$ level, and hence a very good control over
the theory is clearly required. We show that this uncertainty is very small,
and it is only by taking very extreme and rather unphysical variations in the
modelling of the survival factor that this tension can be removed. This
underlines the basic, rather model independent, point that a significant
fraction of elastic PI scattering occurs for hadron-hadron impact parameters
that are simply outside the range of QCD interactions, and hence this sets a
lower bound on the survival factor in any physically reasonable approach.
Finally, other possible origins for this discrepancy are discussed.
Rabah Abdul Khalek, Shaun Bailey, Jun Gao, Lucian Harland-Lang, Juan Rojo
SciPost Phys. 7, 051 (2019) ·
published 17 October 2019
For the foreseeable future, the exploration of the high-energy frontier will
be the domain of the Large Hadron Collider (LHC). Of particular significance
will be its high-luminosity upgrade (HL-LHC), which will operate until the
mid-2030s. In this endeavour, for the full exploitation of the HL-LHC physics
potential an improved understanding of the parton distribution functions (PDFs)
of the proton is critical. The HL-LHC program would be uniquely complemented by
the proposed Large Hadron electron Collider (LHeC), a high-energy lepton-proton
and lepton-nucleus collider based at CERN. In this work, we build on our recent
PDF projections for the HL-LHC to assess the constraining power of the LHeC
measurements of inclusive and heavy quark structure functions. We find that the
impact of the LHeC would be significant, reducing PDF uncertainties by up to an
order of magnitude in comparison to state-of-the-art global fits. In comparison
to the HL-LHC projections, the PDF constraints from the LHeC are in general
more significant for small and intermediate values of the momentum fraction x.
At higher values of x, the impact of the LHeC and HL-LHC data is expected to be
of a comparable size, with the HL-LHC constraints being more competitive in
some cases, and the LHeC ones in others. Our results illustrate the encouraging
complementarity of the HL-LHC and the LHeC in terms of charting the quark and
gluon structure of the proton.
Dr Harland-Lang: "I thank the referee for their ..."
in Submissions | report on Physics from Photons at the LHC