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Register a new userPhoton-induced Background for Dilepton Searches and Measurements in pp Collisions at 13 TeV
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Dimitri Bourilkov
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The production of high invariant mass opposite sign lepton pairs in proton-proton collisions at the LHC is dominated by the Drell-Yan process. In addition to this photon or Z exchange mediated mechanism, gamma-gamma collisions, where photons radiated by the incoming protons collide, can produce lepton pairs. This is an important additional source of background for high mass resonant (like Z) or non-resonant (like contact interactions) searches. In this paper detailed calculations of the Drell-Yan and photon-induced cross sections in the typical acceptance of a multi-purpose LHC detector at center of mass energy 13 TeV are presented. The hint for a diphoton excess at a mass around 750 GeV, reported by the ATLAS and CMS experiments from the analysis of the 2015 data at 13 TeV, raises the possibility that such an excess could be produced through gamma-gamma collisions. A good theoretical understanding and measurements in the dilepton channels at these energies can help to elucidate this production hypothesis.
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Forward top quark pair production is studied in $pp$ collisions in the $mu eb$ final state using a data sample corresponding to an integrated luminosity of 1.93 fb$^{-1}$ collected with the LHCb experiment at a centre-of-mass energy of 13 TeV. The cross-section is measured in a fiducial region where both leptons have a transverse momentum greater than 20 GeV and a pseudorapidity between 2.0 and 4.5. The quadrature sum of the azimuthal separation and the difference in pseudorapidities, denoted $Delta R$, between the two leptons must be larger than 0.1. The $b$-jet axis is required to be separated from both leptons by a $Delta R$ of 0.5, and to have a transverse momentum in excess of 20 GeV and a pseudorapidity between 2.2 and 4.2. The cross-section is measured to be $$sigma_{tbar{t}}= 126pm19,(mathrm{stat})pm16,(mathrm{syst})pm5,(mathrm{lumi}),,mathrm{ fb}$$ where the first uncertainty is statistical, the second is systematic, and the third is due to the luminosity determination. The measurement is compatible with the Standard Model prediction.
The production fractions of $overline{B}_s^0$ and $Lambda_b^0$ hadrons, normalized to the sum of $B^-$ and $overline{B}^0$ fractions, are measured in 13 TeV pp collisions using data collected by the LHCb experiment, corresponding to an integrated luminosity of 1.67/fb. These ratios, averaged over the $b$-hadron transverse momenta from 4 to 25 GeV and pseudorapidity from 2 to 5, are $0.122 pm 0.006$ for $overline{B}_s^0$, and $0.259 pm 0.018$ for $Lambda_b^0$, where the uncertainties arise from both statistical and systematic sources. The $Lambda_b^0$ ratio depends strongly on transverse momentum, while the $overline{B}_s^0$ ratio shows a mild dependence. Neither ratio shows variations with pseudorapidity. The measurements are made using semileptonic decays to minimize theoretical uncertainties. In addition, the ratio of $D^+$ to $D^0$ mesons produced in the sum of $overline{B}^0$ and $B^-$ semileptonic decays is determined as $0.359pm0.006pm 0.009$, where the uncertainties are statistical and systematic.
Searches are performed for both prompt-like and long-lived dark photons, $A^{prime}$, produced in proton-proton collisions at a center-of-mass energy of 13 TeV, using $A^{prime}tomu^+mu^-$ decays and a data sample corresponding to an integrated luminosity of 1.6 fb$^{-1}$ collected with the LHCb detector. The prompt-like $A^{prime}$ search covers the mass range from near the dimuon threshold up to 70 GeV, while the long-lived $A^{prime}$ search is restricted to the low-mass region $214<m(A^{prime})<350$ MeV. No evidence for a signal is found, and 90% confidence level exclusion limits are placed on the $gamma$-$A^{prime}$ kinetic-mixing strength. The constraints placed on prompt-like dark photons are the most stringent to date for the mass range $10.6 < m(A^{prime}) < 70$ GeV, and are comparable to the best existing limits for $m(A^{prime}) < 0.5$ GeV. The search for long-lived dark photons is the first to achieve sensitivity using a displaced-vertex signature.
Production cross-sections of prompt charm mesons are measured with the first data from $pp$ collisions at the LHC at a centre-of-mass energy of $13,mathrm{TeV}$. The data sample corresponds to an integrated luminosity of $4.98 pm 0.19,mathrm{pb}^{-1}$ collected by the LHCb experiment. The production cross-sections of $D^{0}$, $D^{+}$, $D_{s}^{+}$, and $D^{*+}$ mesons are measured in bins of charm meson transverse momentum, $p_{mathrm{T}}$, and rapidity, $y$, and cover the range $0 < p_{mathrm{T}} < 15,mathrm{GeV}/c$ and $2.0 < y < 4.5$. The inclusive cross-sections for the four mesons, including charge conjugation, within the range of $1 < p_{mathrm{T}} < 8,mathrm{GeV}/c$ are found to be begin{equation} sigma(pp to D^{0} X) = 2072 pm 2 pm 124,mumathrm{b} sigma(pp to D^{+} X) = 834 pm 2 pm phantom{1}78,mumathrm{b} sigma(pp to D_{s}^{+} X) = 353 pm 9 pm phantom{1}76,mumathrm{b} sigma(pp to D^{*+} X) = 784 pm 4 pm phantom{1}87,mumathrm{b} end{equation} where the uncertainties are due to statistical and systematic uncertainties, respectively.
Proton-proton (pp) collisions have been used extensively as a reference for the study of interactions of larger colliding systems at the LHC. Recent measurements performed in high-multiplicity pp and proton-lead (p-Pb) collisions have shown features that are reminiscent of those observed in lead-lead (Pb-Pb) collisions. In this context, the study of identified particle spectra and yields as a function of multiplicity is a key tool for the understanding of similarities and differences between small and large systems. We report on the production of pions, kaons, protons, $K^{0}_{rm S}$, $Lambda$, $Xi$, $Omega$ and $K^{*0}$ as a function of multiplicity in pp collisions at $sqrt{s}=$ 7 TeV measured with the ALICE experiment. The work presented here represents the most comprehensive set of results on identified particle production in pp collisions at the LHC. Spectral shapes, studied both for individual particles and via particle ratios as a function of $p_{rm T}$, exhibit an evolution with charged particle multiplicity that is similar to the one observed in larger systems. In addition, results on the production of light flavour hadrons in pp collisions at $sqrt{s}=$ 13 TeV, the highest centre-of-mass energy ever reached in the laboratory, are also presented and compared with previous, lower energy results.
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