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Register a new userPhenomenology of soft QCD: the role of minimum-bias measurements
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Jan Fiete Grosse-Oetringhaus
Publication date
2018
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English
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Jan Fiete Grosse-Oetringhaus
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This chapter summarizes minimum-bias measurements at the Large Hadron Collider. In particular, the pseudorapidity density, the transverse-momentum spectra, the multiplicity distribution, the correlation of average transverse momentum and the multiplicity, and a measurement of minijets are presented. In addition to an overview of the results obtained to date at the LHC, the experimental challenges of defining particle and event sample and correcting to this sample are discussed.
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Results from recent soft QCD measurements by LHC experiments ALICE, ATLAS, CMS, LHCb, LHCf and TOTEM are reported. The measurements include total, elastic and inelastic cross sections, inclusive and identified particle spectra, underlying event and hadronic chains. Results from particle correlations in all three collision systems, namely pp, pPb and PbPb, exhibit unexpected similarities.
We report on the first measurements of the LHCb experiment, as obtained from $pp$ collisions at $sqrt{s} = 0.9$ TeV and 7 TeV recorded using a minimum bias trigger. In particular measurements of the absolute $K^0_S$ production cross section at $sqrt{s} = 0.9$ TeV and of the $bar{Lambda}/Lambda$ ratio both at $sqrt{s} = 0.9$ TeV and 7 TeV are discussed and preliminary results are presented.
The measurements of the minimum bias events provide valuable information on the basic properties of the $pp$ interactions. The results at the new highest energy of $pp$ collisions, $sqrt{s}$ = 13 TeV, obtained using the ATLAS detector, are shown. They include distributions of charged particle pseudorapidity density, transverse momentum and multiplicity. The properties of the underlying event, determined with respect to a leading high-$p_{T}$ particle, are also presented. In both cases the new results are compared with those from earlier studies of the $pp$ collisions at $sqrt{s}$ = 7 TeV.
Axion models with generation-dependent Peccei-Quinn charges can lead to flavor-changing neutral currents, thus motivating QCD axion searches at precision flavor experiments. We rigorously derive limits on the most general effective flavor-violating couplings from current measurements and assess their discovery potential. For two-body decays we use available experimental data to derive limits on $qto q a$ decay rates for all flavor transitions. Axion contributions to neutral-meson mixing are calculated in a systematic way using chiral perturbation theory and operator product expansion. We also discuss in detail baryonic decays and three-body meson decays, which can lead to the best search strategies for some of the couplings. For instance, a strong limit on the $Lambdato n a$ transition can be derived from the supernova SN 1987A. In the near future, dedicated searches for $qto q a$ decays at ongoing experiments could potentially test Peccei-Quinn breaking scales up to $10^{12}$ GeV at NA62 or KOTO, and up to $10^{9}$ GeV at Belle II or BES III.
The strong coupling constant $alpha_s(M_Z)$ is determined from inclusive jet and dijet cross sections in neutral-current deep-inelastic $ep$ scattering (DIS) measured at HERA by the H1 collaboration using next-to-next-to-leading order (NNLO) QCD predictions. The dependence of the NNLO predictions and of the resulting value of $alpha_s(M_Z)$ at the $Z$-boson mass $m_Z$ are studied as a function of the choice of the renormalisation and factorisation scales. Using inclusive jet and dijet data together, the strong coupling constant is determined to be $alpha_s(M_Z)=0.1166,(19)_{rm exp},(24)_{rm th}$. Complementary, $alpha_s(M_Z)$ is determined together with parton distribution functions of the proton (PDFs) from jet and inclusive DIS data measured by the H1 experiment. The value $alpha_s(M_Z)=0.1147,(25)_{rm tot}$ obtained is consistent with the determination from jet data alone. The impact of the jet data on the PDFs is studied. The running of the strong coupling is tested at different values of the renormalisation scale and the results are found to be in agreement with expectations.
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