First measurements of N-subjettiness in central Pb-Pb collisions at $$ \sqrt{s_{\mathrm{NN}}} $$ = 2.76 TeV

The ALICE Collaboration reports the first fully-corrected measurements of the N-subjettiness observable for track-based jets in heavy-ion collisions. This study is performed using data recorded in pp and Pb-Pb collisions at centre-of-mass energies of $$ \sqrt{s} $$ s = 7 TeV and $$ \sqrt{s_{\mathrm{NN}}} $$ s NN = 2.76 TeV, respectively. In particular the ratio of 2-subjettiness to 1-subjettiness, τ2/τ1, which is sensitive to the rate of two-pronged jet substructure, is presented. Energy loss of jets traversing the strongly interacting medium in heavy-ion collisions is expected to change the rate of two-pronged substructure relative to vacuum. The results are presented for jets with a resolution parameter of R = 0.4 and charged jet transverse momentum of 40 ≤ pT,jet ≤ 60 GeV/c, which constitute a larger jet resolution and lower jet transverse momentum interval than previous measurements in heavy-ion collisions. This has been achieved by utilising a semi-inclusive hadron-jet coincidence technique to suppress the larger jet combinatorial background in this kinematic region. No significant modification of the τ2/τ1 observable for track-based jets in Pb-Pb collisions is observed relative to vacuum PYTHIA6 and PYTHIA8 references at the same collision energy. The measurements of τ2/τ1, together with the splitting aperture angle ∆R, are also performed in pp collisions at $$ \sqrt{s} $$ s = 7 TeV for inclusive jets. These results are compared with PYTHIA calculations at $$ \sqrt{s} $$ s = 7 TeV, in order to validate the model as a vacuum reference for the Pb-Pb centre-of-mass energy. The PYTHIA references for τ2/τ1 are shifted to larger values compared to the measurement in pp collisions. This hints at a reduction in the rate of two-pronged jets in Pb-Pb collisions compared to pp collisions.

Jets and photons spectroscopy of Higgs-ALP interactions

Axion-like particles (ALPs) and Higgs bosons can interact in scalar sectors beyond the Standard Model, leading the Higgs boson to decay into pairs of gluons and photons through the ALP interaction and giving rise to resonances in the decay products of the process h → aa → gg + γγ, resembling a spectral-line analysis. We explore this signature to constrain an ALP effective field theory formulation. Our analyses show that the forthcoming runs of the LHC will be capable of probing the ALP-Higgs interaction in the ALP mass range from 0.5 to 60 GeV using an automatized search strategy that adapts to different ALP masses in inclusive jets plus photons final states. Such interaction can also be tested in mass regions where the two and four-photon search channels are currently ineffective.

Leading jets and energy loss

The formation and evolution of leading jets can be described by jet functions which satisfy non-linear DGLAP-type evolution equations. Different than for inclusive jets, the leading jet functions constitute normalized probability densities for the leading jet to carry a longitudinal momentum fraction relative to the initial fragmenting parton. We present a parton shower algorithm which allows for the calculation of leading-jet cross sections where logarithms of the jet radius and threshold logarithms are resummed to next-to-leading logarithmic (NLL′) accuracy. By calculating the mean of the leading jet distribution, we are able to quantify the average out-of-jet radiation, the so-called jet energy loss. When an additional reference scale is measured, we are able to determine the energy loss of leading jets at the cross section level which is identical to parton energy loss at leading-logarithmic accuracy. We identify several suitable cross sections for an extraction of the jet energy loss and we present numerical results for leading subjets at the LHC. In addition, we consider hemisphere and event-wide leading jets in electron-positron annihilation similar to measurements performed at LEP. Besides the average energy loss, we also consider its variance and other statistical quantities such as the KL divergence which quantifies the difference between quark and gluon jet energy loss. We expect that our results will be particularly relevant for quantifying the energy loss of quark and gluon jets that propagate through hot or cold nuclear matter.

Applications of pT-xR Variables in Describing Inclusive Cross Sections at the LHC

Invariant inclusive single-particle/jet cross sections in p–p collisions can be factorized in terms of two separable dependences, a [pT−s] sector and an [xR−pT−s] sector. Here, we extend our earlier work by analyzing more extensive data to explore various s-dependent attributes and other systematics of inclusive jet, photon and single particle reactions. Approximate power laws in s,pT and xR are found. Physical arguments are given which relate observations to the underlying physics of parton–parton hard scattering and the parton distribution functions in the proton. We show that the A(s,pT) function, introduced in our earlier publication to describe the pT dependence of the inclusive cross section, is directly related to the underlying hard parton–parton scattering for jet production, with little influence from soft physics. In addition to the A-function, we introduce another function, the F(s,xR) function that obeys radial scaling for inclusive jets and offers another test of the underlying parton physics. An application to heavy ion physics is given, where we use our variables to determine the transparency of cold nuclear matter to penetrating heavy mesons through the lead nucleus.

QGP modification to single inclusive jets in a calibrated transport model

We study inclusive jet suppression and modifications in the quark-gluon plasma (QGP) with a transport-based model. The model includes vacuum-like parton shower evolution at high-virtuality, a linearized transport for jet-medium interactions, and a simple ansatz for the jet-induced hydrodynamic response of the medium. Model parameters are calibrated to nuclear modification factors for inclusive hadron $$ {R}_{AA}^h $$ R AA h and single inclusive jets $$ {R}_{AA}^j $$ R AA j with cone size R = 0.4 in 0–10% central Au-Au and Pb-Pb collisions measured at the RHIC and LHC. The calibrated model consistently describes the cone-size dependent $$ {R}_{AA}^j $$ R AA j (R), modifications to inclusive jet fragmentation functions and jet shape. We discuss the origin of these modifications by analyzing the medium-induced jet energy flow in this model and elucidate the interplay of hard parton evolution and jet-induced medium response. In particular, we demonstrate that the excess of soft hadrons at pT∼ 2 GeV/c in jet fragmentation function and jet shape at large $$ r=\sqrt{\Delta {\eta}^2+\Delta {\phi}^2} $$ r = Δ η 2 + Δ ϕ 2 are consequences of both soft medium-induced gluon radiation and jet-induced medium excitation.

Parton distributions from LHC, HERA, Tevatron and fixed target data: MSHT20 PDFs

We present the new MSHT20 set of parton distribution functions (PDFs) of the proton, determined from global analyses of the available hard scattering data. The PDFs are made available at NNLO, NLO, and LO, and supersede the MMHT14 sets. They are obtained using the same basic framework, but the parameterisation is now adapted and extended, and there are 32 pairs of eigenvector PDFs. We also include a large number of new data sets: from the final HERA combined data on total and heavy flavour structure functions, to final Tevatron data, and in particular a significant number of new LHC 7 and 8 TeV 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, and NLO EW corrections where relevant. We find that these updates have an important impact on the PDFs, and for the first time the NNLO fit is strongly favoured over the NLO, reflecting the wider range and in particular increased precision of data included in the fit. There are some changes to central values and a significant reduction in the uncertainties of the PDFs in many, though not all, cases. Nonetheless, the PDFs and the resulting predictions are generally within one standard deviation of the MMHT14 results. The major changes are the $$u-d$$ u - d valence quark difference at small x, due to the improved parameterisation and new precise data, the $${\bar{d}}, {\bar{u}}$$ d ¯ , u ¯ difference at small x, due to a much improved parameterisation, and the strange quark PDF due to the effect of LHC W, Z data and inclusion of new NNLO corrections for dimuon production in neutrino DIS. We discuss the phenomenological impact of our results, and in general find reduced uncertainties in predictions for processes such as Higgs, top quark pair and W, Z production at post LHC Run-II energies.