Elliptic flow of electrons from heavy-flavor hadron decays in Au+Au collisions at sNN=\sqrt{s_{\rm NN}} = 200, 62.4, and 39 GeV

We present measurements of elliptic flow ($v_2$) of electrons from the decays of heavy-flavor hadrons ($e_{HF}$) by the STAR experiment. For Au+Au collisions at $\sqrt{s_{\rm NN}} =$ 200 GeV we report $v_2$, for transverse momentum ($p_T$) between 0.2 and 7 GeV/c using three methods: the event plane method ($v_{2}${EP}), two-particle correlations ($v_2${2}), and four-particle correlations ($v_2${4}). For Au+Au collisions at $\sqrt{s_{\rm NN}}$ = 62.4 and 39 GeV we report $v_2${2} for $p_T< 2$ GeV/c. $v_2${2} and $v_2${4} are non-zero at low and intermediate $p_T$ at 200 GeV, and $v_2${2} is consistent with zero at low $p_T$ at other energies. The $v_2${2} at the two lower beam energies is systematically lower than at $\sqrt{s_{\rm NN}} =$ 200 GeV for $p_T < 1$ GeV/c. This difference may suggest that charm quarks interact less strongly with the surrounding nuclear matter at those two lower energies compared to $\sqrt{s_{\rm NN}} = 200$ GeV.Comment: Version accepted by PR

J/ψ production cross section and its dependence on charged-particle multiplicity in p + p collisions at s=200 GeV

We present a measurement of inclusive J/ψ production at mid-rapidity (|y|&lt;1) in p+p collisions at a center-of-mass energy of s=200 GeV with the STAR experiment at the Relativistic Heavy Ion Collider (RHIC). The differential production cross section for J/ψ as a function of transverse momentum (p ) for

Radiation effects of CERN-PS 24 GeV/c protons in silicon strip sensors, evaluated with ATLAS18 ITk strip sensor test structures

Test structures from the ATLAS18 ITk strip detector wafers were irradiated with 24 GeV/c protons. These test structures were positioned at various angles with respect to the proton beam. Blocks of G10 material were placed in front of these test structures to study the effect of scattering of primary protons on the received particle fluence. The results confirm that both the incidence angle of the beam and scattering significantly influence the actual fluence to which the samples are exposed. Miniature strip detectors, first irradiated with protons, were also irradiated with reactor neutrons, to a combined fluence of φneq = 1.6 x 1015  cm−2. The combination of proton and neutron fluences matched the combination expected in the most exposed part of the strip detector in the ALTAS Inner Tracker (ITk). Good charge collection was measured confirming that the strip detectors are sufficiently radiation hard for successful operation to highest fluences expected at the HL-LHC

Setups for eliminating static charge of the ATLAS18 strip sensors

Construction of the new all-silicon Inner Tracker (ITk), developed by the ATLAS collaboration for the High Luminosity LHC, started in 2020 and is expected to continue till 2028. The ITk detector will include 18,000 highly segmented and radiation hard n+-in-p silicon strip sensors (ATLAS18), which are being manufactured by Hamamatsu Photonics. Mechanical and electrical characteristics of produced sensors are measured upon their delivery at several institutes participating in a complex Quality Control (QC) program. The QC tests performed on each individual sensor check the overall integrity and quality of the sensor. During the QC testing of production ATLAS18 strip sensors, an increased number of sensors that failed the electrical tests was observed. In particular, IV measurements indicated an early breakdown, while large areas containing several tens or hundreds of neighbouring strips with low interstrip isolation were identified by the Full strip tests, and leakage current instabilities were measured in a long-term leakage current stability setup. Moreover, a high surface electrostatic charge reaching a level of several hundreds of volts per inch was measured on a large number of sensors and on the plastic sheets, which mechanically protect these sensors in their paper envelopes. Accumulated data indicates a clear correlation between observed electrical failures and the sensor charge-up. To mitigate the above-described issues, the QC testing sites significantly modified the sensor handling procedures and introduced sensor recovery techniques based on irradiation of the sensor surface with UV light or application of intensive flows of ionized gas. In this presentation, we will describe the setups implemented by the QC testing sites to treat silicon strip sensors affected by static charge and evaluate the effectiveness of these setups in terms of improvement of the sensor performance

Establishing the Quality Assurance programme for the strip sensor production of the ATLAS tracker upgrade including irradiation with neutrons, photons and protons to HL-LHC fluences

The successful pre-production delivery of strip sensors for the new Inner Tracker (ITk) for the upgraded ATLAS detector at the High Luminosity LHC (HL-LHC) at CERN was completed and based on their performance full production has commenced. The overall delivery period is anticipated to last 4 years to complete the approximately 22000 sensors required for the ITk. For Quality Assurance (QA), a number of test structures designed by the collaboration, along with a large area diode and miniature version of the main sensor, are produced in every wafer by the foundry Hamamatsu Photonics K.K (HPK). As well as Quality Control (QC) checks on every main sensor, samples of the QA pieces from each delivery batch are tested both before and after irradiation with results after exposure to neutrons, gammas or protons to fluences and doses corresponding to those anticipated after operation at the HL-LHC to roughly 1.5 times the ultimate integrated luminosity of 4000 fb-1. In this paper the procedures are presented and the studies carried out to establish that the seven ITk QA Strip Sensor irradiation and test sites meet all the requirements to support this very extensive programme throughout the strip sensor production phase for the ITk project.The authors acknowledge, the Particle Physics Consolidated Grants received from the UK Research and Innovation — STFC Grants. ST/N000463/1, ST/N001125/1, ST/P005888/1, the US Department of Energy — Grant DE-SC0010107, the Canada Foundation for Innovation, the Natural Science and Engineering Research Council, the Ministry of Education of the Czech Republic projects LTT17018 Inter-Excellence and LM2018104 CERN-CZ and the Charles University grant GAUK 942119, the support from the Slovenian Research Agency (research core funding No. P1 0135) and the Spanish R&D grant PID2021-126327OB-C22, funded by MCIN/AEI/10.13039/501100011033/FEDER, UE.Peer reviewe

Identification and recovery of ATLAS18 strip sensors with high surface static charge

The new all-silicon Inner Tracker (ITk) is being constructed by the ATLAS collaboration to track charged particles produced at the High-Luminosity LHC. The outer portion of the ITk detector will include nearly 18,000 highly segmented and radiation hard silicon strip sensors (ATLAS18 design). Throughout the production of 22,000 sensors, the strip sensors are subjected to a comprehensive suite of mechanical and electrical tests as part of the Quality Control (QC) program. In a large fraction of the batches delivered to date, high surface electrostatic charge has been measured on both the sensors and the plastic sheets between which the sensors are packaged for shipping and handling rigidity. Aggregate data from across QC sites indicate a correlation between observed electrical failures and the sensor/plastic sheet charge build up. To mitigate these issues, the QC testing sites introduced recovery techniques involving UV light or flows of ionizing gas. Significant modifications to sensor handling procedures were made to prevent subsequent build up of static charge. This publication details a precise description of the issue, a variety of sensor recovery techniques, and trend analyses of sensors initially failing electrical tests (IV, strip scan, etc.)

Investigation of K+K−K^+K^- pairs in the effective mass region near 2mK2m_K

The DIRAC experiment at CERN investigated in the reaction $\rm{p}(24~\rm{GeV}/c) + Ni$ the particle pairs $K^+K^-, \pi^+ \pi^-$ and $p \bar{p}$ with relative momentum $Q$ in the pair system less than 100 MeV/c. Because of background influence studies, DIRAC explored three subsamples of $K^+K^-$ pairs, obtained by subtracting -- using time-of-flight (TOF) technique -- background from initial $Q$ distributions with $K^+K^-$ sample fractions more than 70\%, 50\% and 30\%. The corresponding pair distributions in $Q$ and in its longitudinal projection $Q_L$ were analyzed first in a Coulomb model, which takes into account only Coulomb final state interaction (FSI) and assuming point-like pair production. This Coulomb model analysis leads to a $K^+K^-$ yield increase of about four at $Q_L=0.5$ MeV/c compared to 100 MeV/c. In order to study contributions from strong interaction, a second more sophisticated model was applied, considering besides Coulomb FSI also strong FSI via the resonances $f_0(980)$ and $a_0(980)$ and a variable distance $r^*$ between the produced $K$ mesons. This analysis was based on three different parameter sets for the pair production. For the 70\% subsample and with best parameters, $3680\pm 370$ $K^+K^-$ pairs was found to be compared to $3900\pm 410$ $K^+K^-$ extracted by means of the Coulomb model. Knowing the efficiency of the TOF cut for background suppression, the total number of detected $K^+K^-$ pairs was evaluated to be around $40000\pm 10\%$, which agrees with the result from the 30\% subsample. The $K^+K^-$ pair number in the 50\% subsample differs from the two other values by about three standard deviations, confirming -- as discussed in the paper -- that experimental data in this subsample is less reliable

Measurement of exclusive pion pair production in proton–proton collisions at √s=7 TeV with the ATLAS detector

The exclusive production of pion pairs in the process pp→ ppπ+π- has been measured at s=7TeV with the ATLAS detector at the LHC, using 80μb-1 of low-luminosity data. The pion pairs were detected in the ATLAS central detector while outgoing protons were measured in the forward ATLAS ALFA detector system. This represents the first use of proton tagging to measure an exclusive hadronic final state at the LHC. A cross-section measurement is performed in two kinematic regions defined by the proton momenta, the pion rapidities and transverse momenta, and the pion–pion invariant mass. Cross-section values of 4.8±1.0(stat)-0.2+0.3(syst)μb and 9±6(stat)-2+2(syst)μb are obtained in the two regions; they are compared with theoretical models and provide a demonstration of the feasibility of measurements of this type