Jet quenching is the process of color-charged partons losing energy via interactions with quark-gluon plasma droplets created in heavy-ion collisions. The collective expansion of such droplets is well described by viscous hydrodynamics. Similar evidence of collectivity is consistently observed in smaller collision systems, including $pp$ and $p$+Pb collisions. In contrast, while jet quenching is observed in Pb+Pb collisions, no evidence has been found in these small systems to date, raising fundamental questions about the nature of the system created in these collisions. The ATLAS experiment at the Large Hadron Collider has measured the yield of charged hadrons correlated with reconstructed jets in 0.36 nb$^{-1}$ of $p$+Pb and 3.6 pb$^{-1}$ of $pp$ collisions at 5.02 TeV. The yields of charged hadrons with $p_\mathrm{T}^\mathrm{ch} >0.5$ GeV near and opposite in azimuth to jets with $p_\mathrm{T}^\mathrm{jet} > 30$ or $60$ GeV, and the ratios of these yields between $p$+Pb and $pp$ collisions, $I_{p\mathrm{Pb}}$, are reported. The collision centrality of $p$+Pb events is categorized by the energy deposited by forward neutrons from the struck nucleus. The $I_{p\mathrm{Pb}}$ values are consistent with unity within a few percent for hadrons with $p_\mathrm{T}^\mathrm{ch} >4$ GeV at all centralities. These data provide new, strong constraints which preclude almost any parton energy loss in central $p$+Pb collisions.
This letter presents a search for narrow, high-mass resonances in the $Z\gamma$ final state with the $Z$ boson decaying into a pair of electrons or muons. The $\sqrt{s}=13$ TeV $pp$ collision data were recorded by the ATLAS detector at the CERN Large Hadron Collider and have an integrated luminosity of 140 fb$^{-1}$. The data are found to be in agreement with the Standard Model background expectation. Upper limits are set on the resonance production cross section times the decay branching ratio into $Z\gamma$. For spin-0 resonances produced via gluon-gluon fusion, the observed limits at 95% confidence level vary between 65.5 fb and 0.6 fb, while for spin-2 resonances produced via gluon-gluon fusion (or quark-antiquark initial states) limits vary between 77.4 (76.1) fb and 0.6 (0.5) fb, for the mass range from 220 GeV to 3400 GeV.
The pseudorapidity distribution of charged hadrons in pp collisions at sqrt(s) =13 TeV is measured using a data sample obtained with the CMS detector, operated at zero magnetic field, at the CERN LHC. The yield of primary charged long-lived hadrons produced in inelastic pp collisions is determined in the central region of the CMS pixel detector (abs(eta)<2) using both hit pairs and reconstructed tracks. For central pseudorapidities (abs(eta)<0.5), the charged-hadron multiplicity density is dN/d(eta)[charged,abs(eta) < 0.5] = 5.49 +/- 0.01 (stat) +/- 0.17 (sys), a value obtained by combining the two methods. The result is compared to predictions from Monte Carlo event generators and to similar measurements made at lower collision energies.
The differential cross section for inclusive particle production as a function of energy in proton-proton collisions at a center-of-mass energy of 13 TeV is measured in the very forward region of the CMS detector. The measurement is based on data collected with the CMS apparatus at the LHC, and corresponds to an integrated luminosity of 0.35 inverse microbarns. The energy is measured in the CASTOR calorimeter, which covers the pseudorapidity region -6.6 < eta < -5.2. The results are given as a function of the total energy deposited in CASTOR, as well as of its electromagnetic and hadronic components. The spectra are sensitive to the modeling of multiparton interactions in pp collisions, and provide new constraints for hadronic interaction models used in collider and in high energy cosmic ray physics.
The pseudorapidity distributions of dijets as a function of their average transverse momentum ($p_\mathrm{T}^\text{ave}$) are measured in proton-lead (pPb) and proton-proton (pp) collisions. The data samples were collected by the CMS experiment at the CERN LHC, at a nucleon-nucleon center-of-mass energy of 5.02 TeV. A significant modification of the pPb spectra with respect to the pp spectra is observed in all $p_\mathrm{T}^\text{ave}$ intervals investigated. The ratios of the pPb and pp distributions are compared to next-to-leading order perturbative quantum chromodynamics calculations with unbound nucleon and nuclear parton distribution functions (PDFs). These results give the first evidence that the gluon PDF at large Bjorken $x$ in lead ions is strongly suppressed with respect to the PDF in unbound nucleons.
Measurements of azimuthal angle and transverse momentum ($p_\mathrm{T}$) correlations of isolated photons and associated jets are reported for pp and PbPb collisions at $\sqrt{s_{_{\mathrm{NN}}}} =$ 5.02 TeV. The data were recorded with the CMS detector at the CERN LHC. For events containing a leading isolated photon with $p_\mathrm{T}^\gamma >$ 40 GeV$/c$ and an associated jet with $p_\mathrm{T}^\text{jet} >$ 30 GeV$/c$, the photon+jet azimuthal correlation and $p_\mathrm{T}$ imbalance in PbPb collisions are studied as functions of collision centrality and $p_\mathrm{T}^\gamma$. The results are compared to pp reference data collected at the same collision energy and to predictions from several theoretical models for parton energy loss. No evidence of broadening of the photon+jet azimuthal correlations is observed, while the ratio $p_\mathrm{T}^\text{jet}/p_\mathrm{T}^\gamma$ decreases significantly for PbPb data relative to the pp reference. All models considered agree within uncertainties with the data. The number of associated jets per photon with $p_\mathrm{T}^\gamma >$ 80 GeV$/c$ is observed to be shifted towards lower $p_\mathrm{T}^\text{jet}$ values in central PbPb collisions compared to pp collisions.
The per-event yield of the highest transverse momentum charged particle and charged-particle jet, integrated above a given pt(min) threshold starting at pt(min) = 0.8 and 1 GeV, respectively, is studied in pp collisions at sqrt(s) = 8 TeV. The particles and the jets are measured in the pseudorapidity ranges abs(eta) < 2.4 and 1.9, respectively. The data are sensitive to the momentum scale at which parton densities saturate in the proton, to multiple partonic interactions, and to other key aspects of the transition between the soft and hard QCD regimes in hadronic collisions.
The average total energy as well as its hadronic and electromagnetic components are measured with the CMS detector at pseudorapidities $-$6.6 $<\eta<-$5.2 in proton-proton collisions at a centre-of-mass energy $\sqrt{s}=$ 13 TeV. The results are presented as a function of the charged particle multiplicity in the region $|\eta|< $2. This measurement is sensitive to correlations induced by the underlying event structure over a very wide pseudorapidity region. The predictions of Monte Carlo event generators commonly used in collider experiments and ultra-high energy cosmic ray physics are compared to the data.
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