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Register a new userDijet cross sections in ep collisions: who is afraid of symmetric cuts?
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Three widely used scenaria for defining cuts on transverse energies of jets in ep collisions are discussed. All of them are shown to suffer from the same sort of unphysical behaviour when the cut regions are subject to additional constraints. This feature is inherent in the very way dijet cross sections are defined and cannot be avoided. In particular, the symmetric cut scenario is shown to be equally suitable for the comparison with NLO QCD calculations as the asymmetric or sum-like ones.
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A precision measurement of jet cross sections in neutral current deep-inelastic scattering for photon virtualities $5.5<Q^2<80,{rm GeV}^2$ and inelasticities $0.2<y<0.6$ is presented, using data taken with the H1 detector at HERA, corresponding to an integrated luminosity of $290,{rm pb}^{-1}$. Double-differential inclusive jet, dijet and trijet cross sections are measured simultaneously and are presented as a function of jet transverse momentum observables and as a function of $Q^2$. Jet cross sections normalised to the inclusive neutral current DIS cross section in the respective $Q^2$-interval are also determined. Previous results of inclusive jet cross sections in the range $150<Q^2<15,000,{rm GeV}^2$ are extended to low transverse jet momenta $5<P_{T}^{rm jet}<7,{rm GeV}$. The data are compared to predictions from perturbative QCD in next-to-leading order in the strong coupling, in approximate next-to-next-to-leading order and in full next-to-next-to-leading order. Using also the recently published H1 jet data at high values of $Q^2$, the strong coupling constant $alpha_s(M_Z)$ is determined in next-to-leading order.
Gradient-based algorithms are effective for many machine learning tasks, but despite ample recent effort and some progress, it often remains unclear why they work in practice in optimising high-dimensional non-convex functions and why they find good minima instead of being trapped in spurious ones. Here we present a quantitative theory explaining this behaviour in a spiked matrix-tensor model. Our framework is based on the Kac-Rice analysis of stationary points and a closed-form analysis of gradient-flow originating from statistical physics. We show that there is a well defined region of parameters where the gradient-flow algorithm finds a good global minimum despite the presence of exponentially many spurious local minima. We show that this is achieved by surfing on saddles that have strong negative direction towards the global minima, a phenomenon that is connected to a BBP-type threshold in the Hessian describing the critical points of the landscapes.
The dijet cross section as a function of both the fraction of the photon momentum participating in the hard process, x_gamma^{OBS}, and the photons virtuality, Q^2, is compared with the predictions of Herwig 5.9 for various photon parton distribution functions. The ratio of dijet cross sections with x_gamma^{OBS} < 0.75 and x_gamma^{OBS} > 0.75 is measured as a function of Q^2. This ratio is found to decrease as Q^2 increases consistent with the hypothesis that the photon parton distribution functions decrease with increasing photon virtuality.
We review a recently proposed phenomenological framework to establish the notions of QCD factorization and universality of jet cross sections in the heavy-ion environment. First results of a global analysis of the nuclear modification factor of inclusive jets are presented where we extract medium modified jet functions using a Monte Carlo sampling approach. We observe that gluon jets are significantly more suppressed than quark jets. In addition, we study the jet radius dependence of the inclusive jet cross section in heavy-ion collisions and comment on a recent measurement from CMS. By considering for example jet substructure observables it will be possible to test the universality of the extracted medium jet functions. We thus expect that the presented results will eventually allow for extractions of medium properties with a reduced model bias.
The inclusive $b bar{b}$- and $c bar{c}$-dijet production cross-sections in the forward region of $pp$ collisions are measured using a data sample collected with the LHCb detector at a centre-of-mass energy of 13 TeV in 2016. The data sample corresponds to an integrated luminosity of 1.6 fb$^{-1}$. Differential cross-sections are measured as a function of the transverse momentum and of the pseudorapidity of the leading jet, of the rapidity difference between the jets, and of the dijet invariant mass. A fiducial region for the measurement is defined by requiring that the two jets originating from the two $b$ or $c$ quarks are emitted with transverse momentum greater than 20 GeV$/c$, pseudorapidity in the range $2.2 < eta < 4.2$, and with a difference in the azimuthal angle between the two jets greater than 1.5. The integrated $b bar{b}$-dijet cross-section is measured to be $53.0 pm 9.7$ nb, and the total $c bar{c}$-dijet cross-section is measured to be $73 pm 16$ nb. The ratio between $c bar{c}$- and $b bar{b}$-dijet cross-sections is also measured and found to be $1.37 pm 0.27$. The results are in agreement with theoretical predictions at next-to-leading order.
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