The double-differential inclusive jet cross section is measured as a function of jet transverse momentum pT and absolute rapidity y, using proton-proton collision data collected with the CMS experiment at the LHC, at a center-of-mass energy of sqrt(s) = 2.76 TeV and corresponding to an integrated luminosity of 5.43 inverse picoboarns. Jets are reconstructed within the pT range of 74 to 592 GeV and the rapidity range |y| < 3.0. The reconstructed jet spectrum is corrected for detector resolution. The measurements are compared to the theoretical prediction at next-to-leading-order QCD using different sets of parton distribution functions. This inclusive cross section measurement explores a new kinematic region and is consistent with QCD predictions.
Measurements of event shapes and azimuthal correlations are presented for events where a Z boson is produced in association with jets in proton-proton collisions. The data collected with the CMS detector at the CERN LHC at sqrt(s) = 7 TeV correspond to an integrated luminosity of 5.0 inverse femtobarns. The analysis provides a test of predictions from perturbative QCD for a process that represents a substantial background to many physics channels. Results are presented as a function of jet multiplicity, for inclusive Z boson production and for Z bosons with transverse momenta greater than 150 GeV, and compared to predictions from Monte Carlo event generators that include leading-order multiparton matrix-element (with up to four hard partons in the final state) and next-to-leading-order simulations of Z + 1-jet events. The experimental results are corrected for detector effects, and can be compared directly with other QCD models.
Differences in the behaviour of matter and antimatter have been observed in $K$ and $B$ meson decays, but not yet in any baryon decay. Such differences are associated with the non-invariance of fundamental interactions under the combined charge-conjugation and parity transformations, known as $C\!P$ violation. Using data from the LHCb experiment at the Large Hadron Collider, a search is made for $C\!P$-violating asymmetries in the decay angle distributions of $\Lambda^0_b$ baryons decaying to $p\pi^-\pi^+\pi^-$ and $p\pi^-K^+K^-$ final states. These four-body hadronic decays are a promising place to search for sources of $C\!P$ violation both within and beyond the Standard Model of particle physics. We find evidence for $C\!P$ violation in $\Lambda^0_b$ to $p\pi^-\pi^+\pi^-$ decays with a statistical significance corresponding to 3.3 standard deviations including systematic uncertainties. This represents the first evidence for $C\!P$ violation in the baryon sector.
The production of $\Upsilon (nS)$ mesons ($n=1,2,3$) in $p$Pb and Pb$p$ collisions at a centre-of-mass energy per nucleon pair $\sqrt{s_{NN}}=8.16$ TeV is measured by the LHCb experiment, using a data sample corresponding to an integrated luminosity of 31.8 nb$^{-1}$. The $\Upsilon (nS)$ mesons are reconstructed through their decays into two opposite-sign muons. The measurements comprise the differential production cross-sections of the $\Upsilon (1S)$ and $\Upsilon (2S)$ states, their forward-to-backward ratios and nuclear modification factors, performed as a function of the transverse momentum $p_{\mathrm{T}}$ and rapidity in the nucleon-nucleon centre-of-mass frame $y^*$ of the $\Upsilon (nS)$ states, in the kinematic range $p_{\rm{T}}<25$ GeV/$c$ and $1.5<y^*<4.0$ ($-5.0<y^*<-2.5$) for $p$Pb (Pb$p$) collisions. In addition, production cross-sections for $\Upsilon (3S)$ are measured integrated over phase space and the production ratios between all three $\Upsilon (nS)$ states are determined. The measurements are compared to theoretical predictions and suppressions for quarkonium in $p$Pb collisions are observed.
Several extensions of the Standard Model predict associated production of dark-matter particles with a Higgs boson. Such processes are searched for in final states with missing transverse momentum and a Higgs boson decaying to a $b\bar b$ pair with the ATLAS detector using 36.1 fb$^{-1}$ of $pp$ collisions at a center-of-mass energy of 13 TeV at the LHC. The observed data are in agreement with the Standard Model predictions and limits are placed on the associated production of dark-matter particles and a Higgs boson.
The cross section of a top-quark pair produced in association with a photon is measured in proton-proton collisions at a centre-of-mass energy of $\sqrt{s} = 8$ TeV with $20.2$ fb$^{-1}$ of data collected by the ATLAS detector at the Large Hadron Collider in 2012. The measurement is performed by selecting events that contain a photon with transverse momentum $p_\mathrm{T} > 15$ GeV, an isolated lepton with large transverse momentum, large missing transverse momentum, and at least four jets, where at least one is identified as originating from a $b$-quark. The production cross section is measured in a fiducial region close to the selection requirements. It is found to be $139 \pm 7 (\mathrm{stat.}) \pm 17 (\mathrm{syst.})$ fb, in good agreement with the theoretical prediction at next-to-leading order of $151 \pm 24$ fb. In addition, differential cross sections in the fiducial region are measured as a function of the transverse momentum and pseudorapidity of the photon.
The differential cross-section for pair production of top quarks with high transverse momentum is measured in 20.3 fb$^{-1}$ of proton-proton collisions at a center-of-mass energy of 8 TeV. The measurement is performed for $t\bar{t}$ events in the lepton+jets channel. The cross-section is reported as a function of the hadronically decaying top quark transverse momentum for values above 300 GeV. The hadronically decaying top quark is reconstructed as an anti-$k_t$ jet with radius parameter $R=1.0$ and identified with jet substructure techniques. The observed yield is corrected for detector effects to obtain a cross-section at particle level in a fiducial region close to the event selection. A parton-level cross-section extrapolated to the full phase space is also reported for top quarks with transverse momentum above 300 GeV. The predictions of a majority of next-to-leading-order and leading-order matrix-element Monte Carlo generators are found to agree with the measured cross-sections.
A measurement of the cross section for the production of an isolated photon in association with jets in proton-proton collisions at a center-of-mass energy $\sqrt{s}$ = 7 TeV is presented. Photons are reconstructed in the pseudorapidity range $|\eta^{\gamma}| \lt 1.37$ and with a transverse energy $E_T^\gamma$ > 25 GeV. Jets are reconstructed in the rapidity range $|y^{jet}|$ < 4.4 and with a transverse momentum $p_T^{jet}$ > 20 GeV. The differential cross section $d\sigma/dE_T^\gamma$ is measured, as a function of the photon transverse energy, for three different rapidity ranges of the leading-$p_T$ jet: $|y^{jet}| < 1.2, 1.2 \le |y^{jet}|$ < 2.8 and 2.8 $\le |y^{jet}|$ < 4.4. For each rapidity configuration the same-sign $(\eta^{\gamma}y^{jet}\ge 0)$ and opposite-sign $(\eta^{\gamma}y^{jet}<0)$ cases are studied separately. The results are based on an integrated luminosity of 37 pb$^{-1}$, collected with the ATLAS detector at the LHC. Next-to-leading order perturbative QCD calculations are found to be in fair agreement with the data, except for $E_T^{\gamma} \lt 45$ GeV, where the theoretical predictions overestimate the measured cross sections.
The results of a search for charged Higgs bosons decaying to a $\tau$ lepton and a neutrino, $H^{\pm} \rightarrow \tau^{\pm} \nu$, are presented. The analysis is based on 19.5 fb$^{-1}$ of proton-proton collision data at $\sqrt{s}=8$ TeV collected by the ATLAS experiment at the Large Hadron Collider. Charged Higgs bosons are searched for in events consistent with top-quark pair production or in associated production with a top quark. The final state is characterised by the presence of a hadronic $\tau$ decay, missing transverse momentum, $b$-tagged jets, a hadronically decaying $W$ boson, and the absence of any isolated electrons or muons with high transverse momenta. The data are consistent with the expected background from Standard Model processes. A statistical analysis leads to 95\% confidence-level upper limits on the product of branching ratios $ {\cal B}(t\rightarrow bH^\pm) \times {\cal B} (H^\pm\rightarrow \tau^{\pm}\nu) $, between 0.23% and 1.3% for charged Higgs boson masses in the range 80-160 GeV. It also leads to 95% confidence-level upper limits on the production cross section times branching ratio, $\sigma(pp \rightarrow tH^{\pm} + X) \times {\cal B}(H^{\pm} \rightarrow \tau^{\pm} \nu)$, between 0.76 pb and 4.5 fb, for charged Higgs boson masses ranging from 180 GeV to 1000 GeV. In the context of different scenarios of the Minimal Supersymmetric Standard Model, these results exclude nearly all values of $\tan\beta$ above one for charged Higgs boson masses between 80 GeV and 160 GeV, and exclude a region of parameter space with high $\tan\beta$ for $H^{\pm}$ masses between 200 GeV and 250 GeV.
Double-differential three-jet production cross-sections are measured in proton-proton collisions at a centre-of-mass energy of $\sqrt{s} = 7$ TeV using the ATLAS detector at the Large Hadron Collider. The measurements are presented as a function of the three-jet mass $(m_{jjj})$, in bins of the sum of the absolute rapidity separations between the three leading jets $(|Y^\ast|)$. Invariant masses extending up to 5 TeV are reached for $8< |Y^\ast| < 10$. These measurements use a sample of data recorded using the ATLAS detector in 2011, which corresponds to an integrated luminosity of 4.51 fb$^{-1}$. Jets are identified using the anti-$k_t$ algorithm with two different jet radius parameters, R=0.4 and R=0.6. The dominant uncertainty in these measurements comes from the jet energy scale. Next-to-leading-order QCD calculations corrected to account for non-perturbative effects are compared to the measurements. Good agreement is found between the data and the theoretical predictions based on most of the available sets of parton distribution functions, over the full kinematic range, covering almost seven orders of magnitude in the measured cross-section values.
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