Studies of the fragmentation of jets into charged particles in heavy-ion collisions can provide information about the mechanism of jet-quenching by the hot and dense QCD matter created in such collisions, the quark-gluon plasma. This paper presents a measurement of the angular distribution of charged particles around the jet axis in $\sqrt{s_{\mathrm{NN}}}=$ 5.02 TeV Pb+Pb and $pp$ collisions, using the ATLAS detector at the LHC. The Pb+Pb and $pp$ data sets have integrated luminosities of 0.49 nb$^{-1}$ and 25 pb$^{-1}$, respectively. The measurement is performed for jets reconstructed with the anti-$k_{t}$ algorithm with radius parameter $R = 0.4$ and is extended to an angular distance of $r= 0.8$ from the jet axis. Results are presented as a function of Pb+Pb collision centrality and distance from the jet axis for charged particles with transverse momenta in the 1$-$63 GeV range, matched to jets with transverse momenta in the 126$-$316 GeV range and an absolute value of jet rapidity of less than 1.7. Modifications to the measured distributions are quantified by taking a ratio to the measurements in $pp$ collisions. Yields of charged particles with transverse momenta below 4 GeV are observed to be increasingly enhanced as a function of angular distance from the jet axis, reaching a maximum at $r=0.6$. Charged particles with transverse momenta above 4 GeV have an enhanced yield in Pb+Pb collisions in the jet core for angular distances up to $r = 0.05$ from the jet axis, with a suppression at larger distances.
D(pT,r)_PbPb The charged particle distributions around jets as a function of distance from the jet axis in PbPb collisions at 5.02 TeV for different centrality, track pT and jet pT ranges.
D(pT,r)_PbPb The charged particle distributions around jets as a function of distance from the jet axis in PbPb collisions at 5.02 TeV for different centrality, track pT and jet pT ranges.
D(pT,r)_PbPb The charged particle distributions around jets as a function of distance from the jet axis in PbPb collisions at 5.02 TeV for different centrality, track pT and jet pT ranges.
We report systematic measurements of bulk properties of the system created in Au+Au collisions at $\sqrt{s_{\mathrm{NN}}}$ = 14.5 GeV recorded by the STAR detector at the Relativistic Heavy Ion Collider (RHIC).The transverse momentum spectra of $\pi^{\pm}$, $K^{\pm}$ and $p(\bar{p})$ are studied at mid-rapidity ($|y| < 0.1$) for nine centrality intervals. The centrality, transverse momentum ($p_T$),and pseudorapidity ($\eta$) dependence of inclusive charged particle elliptic flow ($v_2$), and rapidity-odd charged particles directed flow ($v_{1}$) results near mid-rapidity are also presented. These measurements are compared with the published results from Au+Au collisions at other energies, and from Pb+Pb collisions at $\sqrt{s_{\mathrm{NN}}}$ = 2.76 TeV. The results at $\sqrt{s_{\mathrm{NN}}}$ = 14.5 GeV show similar behavior as established at other energies and fit well in the energy dependence trend. These results are important as the 14.5 GeV energy fills the gap in $\mu_B$, which is of the order of 100 MeV,between $\sqrt{s_{\mathrm{NN}}}$ =11.5 and 19.6 GeV. Comparisons of the data with UrQMD and AMPT models show poor agreement in general.
The $p_{T}$ spectra of proton measured at midrapidity (|y|<0.1) in Au+Au collisions at $\sqrt{s_{NN}}$ = 14.5 GeV. Spectra are plotted for nine centrality classes, with some spectra multiplied by a scale factor to improve clarity, as indicated in the legend
The $p_{T}$ spectra of antiproton measured at midrapidity (|y|<0.1) in Au+Au collisions at $\sqrt{s_{NN}}$ = 14.5 GeV. Spectra are plotted for nine centrality classes, with some spectra multiplied by a scale factor to improve clarity, as indicatedin the legend
The $p_{T}$ spectra of $\pi^{+}$ measured at midrapidity (|y|<0.1) in Au+Au collisions at $\sqrt{s_{NN}}$ = 14.5 GeV. Spectra are plotted for nine centrality classes, with some spectra multiplied by a scale factor to improve clarity, as indicatedin the legend
The production rates and the transverse momentum distribution of strange hadrons at mid-rapidity ($\ |y\ | < 0.5$) are measured in proton-proton collisions at $\sqrt{s}$ = 13 TeV as a function of the charged particle multiplicity, using the ALICE detector at the LHC. The production rates of $\rm{K}^{0}_{S}$, $\Lambda$, $\Xi$, and $\Omega$ increase with the multiplicity faster than what is reported for inclusive charged particles. The increase is found to be more pronounced for hadrons with a larger strangeness content. Possible auto-correlations between the charged particles and the strange hadrons are evaluated by measuring the event-activity with charged particle multiplicity estimators covering different pseudorapidity regions. When comparing to lower energy results, the yields of strange hadrons are found to depend only on the mid-rapidity charged particle multiplicity. Several features of the data are reproduced qualitatively by general purpose QCD Monte Carlo models that take into account the effect of densely-packed QCD strings in high multiplicity collisions. However, none of the tested models reproduce the data quantitatively. This work corroborates and extends the ALICE findings on strangeness production in proton-proton collisions at 7 TeV.
$K^{0}_{S}$ transverse momentum spectrum - V0M multiplicity classes. Total systematic uncertainties include both correlated and uncorrelated uncertainties across multiplicity. Uncorrelated systematic originating from the multiplicity dependence of the efficiency (2%) is not included.
$K^{0}_{S}$ transverse momentum spectrum - V0M multiplicity classes. Total systematic uncertainties include both correlated and uncorrelated uncertainties across multiplicity. Uncorrelated systematic originating from the multiplicity dependence of the efficiency (2%) is not included.
$\Lambda+\bar{\Lambda}$ transverse momentum spectrum - V0M multiplicity classes. Total systematic uncertainties include both correlated and uncorrelated uncertainties across multiplicity. Uncorrelated systematic originating from the multiplicity dependence of the efficiency (2%) is not included.
A measurement of $W^\pm$ boson production in lead-lead collisions at $\sqrt{s_\mathrm{NN}} = 5.02$ TeV is reported using data recorded by the ATLAS experiment at the LHC in 2015, corresponding to a total integrated luminosity of $0.49\;\mathrm{nb^{-1}}$. The $W^\pm$ bosons are reconstructed in the electron or muon leptonic decay channels. Production yields of leptonically decaying $W^\pm$ bosons, normalised by the total number of minimum-bias events and the nuclear thickness function, are measured within a fiducial region defined by the detector acceptance and the main kinematic requirements. These normalised yields are measured separately for $W^+$ and $W^-$ bosons, and are presented as a function of the absolute value of pseudorapidity of the charged lepton and of the collision centrality. The lepton charge asymmetry is also measured as a function of the absolute value of lepton pseudorapidity. In addition, nuclear modification factors are calculated using the $W^\pm$ boson production cross-sections measured in $pp$ collisions. The results are compared with predictions based on next-to-leading-order calculations with CT14 parton distribution functions as well as with predictions obtained with the EPPS16 and nCTEQ15 nuclear parton distribution functions. No dependence of normalised production yields on centrality and a good agreement with predictions are observed for mid-central and central collisions. For peripheral collisions, the data agree with predictions within 1.7 (0.9) standard deviations for $W^-$ ($W^+$) bosons.
Differential normalised production yields for $W^+$ bosons as a function of absolute pseudorapidity of the charged lepton for the combined electron and muon channels. Systematic uncertainties related to $T_{\mathrm{AA}}$ are not included.
Differential normalised production yields for $W^-$ bosons as a function of absolute pseudorapidity of the charged lepton for the combined electron and muon channels. Systematic uncertainties related to $T_{\mathrm{AA}}$ are not included.
Combined result for lepton charge asymmetry.
A search for long-lived particles decaying into an oppositely charged lepton pair, $\mu\mu$, $ee$, or $e\mu$, is presented using 32.8 fb$^{-1}$ of $pp$ collision data collected at $\sqrt{s}=13$ TeV by the ATLAS detector at the LHC. Candidate leptons are required to form a vertex, within the inner tracking volume of ATLAS, displaced from the primary $pp$ interaction region. No lepton pairs with an invariant mass greater than 12 GeV are observed, consistent with the background expectations derived from data. The detection efficiencies for generic resonances with lifetimes ($c\tau$) of 100-1000 mm decaying into a dilepton pair with masses between 0.1-1.0 TeV are presented as a function of $p_T$ and decay radius of the resonances to allow the extraction of upper limits on the cross sections for theoretical models. The result is also interpreted in a supersymmetric model in which the lightest neutralino, produced via squark-antisquark production, decays into $\ell^{+}\ell^{'-}\nu$ ($\ell, \ell^{'} = e$, $\mu$) with a finite lifetime due to the presence of R-parity violating couplings. Cross-section limits are presented for specific squark and neutralino masses. For a 700 GeV squark, neutralinos with masses of 50-500 GeV and mean proper lifetimes corresponding to $c\tau$ values between 1 mm to 6 m are excluded. For a 1.6 TeV squark, $c\tau$ values between 3 mm to 1 m are excluded for 1.3 TeV neutralinos.
<h1>Overview of reinterpretation material</h1><p><b>Important note:</b> A detailed explanation of the reinterpretation material can be found <a href="https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/SUSY-2017-04/hepdata_info.pdf">here</a>.<br/>Please read this stand-alone document before reinterpreting the search.</p><h2>Parameterized detection efficiencies</h2><p>RPV SUSY model: Tables <a href="90606?version=1&table=Table27">27</a> to <a href="90606?version=1&table=Table44">44</a><br/>Z' toy model: Tables <a href="90606?version=1&table=Table45">45</a> to <a href="90606?version=1&table=Table59">59</a></p><h2>Further material for the RPV SUSY model</h2><p>Acceptances: Tables <a href="90606?version=1&table=Table18">18</a> (ee), <a href="90606?version=1&table=Table19">19</a> (emu) and <a href="90606?version=1&table=Table20">20</a> (mumu)<br/>Detection efficiencies: Tables <a href="90606?version=1&table=Table21">21</a> (ee), <a href="90606?version=1&table=Table22">22</a> (emu) and <a href="90606?version=1&table=Table23">23</a> (mumu)<br/>Overall signal efficiencies: Tables <a href="90606?version=1&table=Table24">24</a> (ee), <a href="90606?version=1&table=Table25">25</a> (emu) and <a href="90606?version=1&table=Table26">26</a> (mumu)</p><h2>Further material for the Z' toy model</h2><p>Acceptances, detection efficiencies and overall signal efficiencies: Tables <a href="90606?version=1&table=Table60">60</a> (mZ' = 100 GeV) to <a href="90606?version=1&table=Table64">64</a> (mZ' = 1000 GeV)</p>
dRcos distribution of dimuon pairs (scaled) and dimuon vertices in the cosmic rays control region. The distribution of all dimuon pairs is scaled to the DV distribution.
Dependence of the overall signal efficiency on the transverse decay radius Rxy of the long-lived Z' for Z' -> ee. The error bars indicate the total uncertainties.
An improved value for the lifetime of the (anti-)hypertriton has been obtained using the data sample of Pb-Pb collisions at $\sqrt{s_{\mathrm{NN}}} = $ 5.02 TeV collected by the ALICE experiment at the LHC. The (anti-)hypertriton has been reconstructed via its charged two-body mesonic decay channel and the lifetime has been determined from an exponential fit to the d$N$/d($ct$) spectrum. The measured value, $\tau$ = 242$^{+34}_{-38}$ (stat.) $\pm$ 17 (syst.) ps, is compatible with all the available theoretical predictions, thus contributing to the solution of the longstanding hypertriton lifetime puzzle.
(Hypertriton + Anti-Hypertriton)dN/d(ct) distribution.
This Letter presents direct searches for lepton flavour violation in Higgs boson decays, $H\rightarrow e\tau$ and $H\rightarrow\mu\tau$, performed with the ATLAS detector at the LHC. The searches are based on a data sample of proton-proton collisions at a centre-of-mass energy $\sqrt{s} = 13$ TeV, corresponding to an integrated luminosity of $36.1\,\mathrm{fb}^{-1}$. No significant excess is observed above the expected background from Standard Model processes. The observed (median expected) 95 % confidence-level upper limits on the lepton-flavour-violating branching ratios are $0.47\%$ ($0.34^{+0.13}_{-0.10}\,\%$) and $0.28\%$ ($0.37^{+0.14}_{-0.10}\,\%$) for $H\to e\tau$ and $H\to\mu\tau$, respectively.
95% CL upper limits on the branching ratio H --> e tau.
95% CL upper limits on the branching ratio H --> mu tau.
To assess the properties of the quark-gluon plasma formed in heavy-ion collisions, the ATLAS experiment at the LHC measures a correlation between the mean transverse momentum and the magnitudes of the flow harmonics. The analysis uses data samples of lead-lead and proton-lead collisions obtained at the centre-of-mass energy per nucleon pair of 5.02 TeV, corresponding to total integrated luminosities of $22 ~\mu b^{-1}$ and $28~nb^{-1}$, respectively. The measurement is performed using a modified Pearson correlation coefficient with the charged-particle tracks on an event-by-event basis. The modified Pearson correlation coefficients for the $2^{nd}$-, 3$^{rd}$-, and 4$^{th}$-order harmonics are measured as a function of event centrality quantified as the number of charged particles or the number of nucleons participating in the collision. The measurements are performed for several intervals of the charged-particle transverse momentum. The correlation coefficients for all studied harmonics exhibit a strong centrality evolution in the lead-lead collisions, which only weakly depends on the charged-particle momentum range. In the proton-lead collisions, the modified Pearson correlation coefficient measured for the second harmonics shows only weak centrality dependence. The data is qualitatively described by the predictions based on the hydrodynamical model.
The $c_{k}$ for the 0.5-2 GeV $p_{T}$ range as a function of event multiplicity $N_{ch}$ in Pb+Pb collisions.
The $c_{k}$ for the 0.5-5 GeV $p_{T}$ range as a function of event multiplicity $N_{ch}$ in Pb+Pb collisions.
The $c_{k}$ for the 1-2 GeV $p_{T}$ range as a function of event multiplicity $N_{ch}$ in Pb+Pb collisions.
The first measurement of the $\Upsilon(1{\rm S})$ elliptic flow coefficient ($v_2$) is performed at forward rapidity (2.5 $<$ $y$ $<$ 4) in Pb-Pb collisions at $\sqrt{s_{\rm NN}} = 5.02$ TeV with the ALICE detector at the LHC. The results are obtained with the scalar product method and are reported as a function of transverse momentum ($p_{\rm{T}}$) up to 15 GeV/$c$ in the 5-60% centrality interval. The measured $\Upsilon(1{\rm S})$ $v_2$ is consistent with zero and with the small positive values predicted by transport models within uncertainties. The $v_2$ coefficient in 2 $<$ $p_{\rm T}$ $<$ 15 GeV/$c$ is lower than that of inclusive J/$\psi$ mesons in the same $p_{\rm{T}}$ interval by 2.6 standard deviations. These results, combined with earlier suppression measurements, are in agreement with a scenario in which the $\Upsilon$(1S) production in Pb-Pb collisions at LHC energies is dominated by dissociation limited to the early stage of the collision whereas in the J/$\psi$ case there is substantial experimental evidence of an additional regeneration component.
The J/$\psi$ $v_2$ coefficient as a function of $p_{\rm T}$ in 5-60% centrality interval in Pb-Pb collisions at $\sqrt{s_{\rm NN}}$ = 5.02 TeV.
The $\Upsilon$(1S) $v_2$ coefficient as a function of $p_{\rm T}$ in 5-60% centrality interval in Pb-Pb collisions at $\sqrt{s_{\rm NN}}$ = 5.02 TeV.
The J/$\psi$ $v_2$ coefficient in three centrality intervals integrated over the transverse momentum range 2~$<$~$p_{\rm T}$~$<$~15 GeV/$c$ in Pb-Pb collisions at $\sqrt{s_{\rm NN}}$ = 5.02 TeV.
High transverse momentum ($p_T$) particle production is suppressed due to parton (jet) energy loss in the hot dense medium created in relativistic heavy-ion collisions. Redistribution of energy at low-to-modest $p_T$ has been elusive to measure because of large anisotropic backgrounds. We report a data-driven method for background evaluation and subtraction, exploiting the away-side pseudorapidity gaps, to measure the jetlike correlation shape in Au+Au collisions at $\sqrt{s_{_\mathrm{NN}}}=200$ GeV with the STAR experiment. The correlation shapes, for trigger particle $p_T>3$ GeV/$c$ and various associated particle $p_T$ ranges within $0.5<p_T<10$ GeV/$c$, are consistent with Gaussians and their widths are found to increase with centrality. The results indicate jet broadening in the medium created in central heavy-ion collisions.
Distributions of the recoil momentum within 0.5<\eta<1 (Px|_{0.5}^{1}) from high-pT trigger particles of 3<pT_Trig<10 GeV/c in 50-80% peripheral collisions
Distributions of the recoil momentum within 0.5<\eta<1 (Px|_{0.5}^{1}) from high-pT trigger particles of 3<pT_Trig<10 GeV/c in 0-10% central collisions
Dihadron azimuthal correlations in close-region and far-region for 3<pT_Trig<10 GeV/c and 1<pT_Assoc<2 GeV/c in 10-30% Au+Au collisions at \sNN=200 GeV
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