Digital Pixel Test Structures implemented in a 65 nm CMOS process

The ALICE ITS3 (Inner Tracking System 3) upgrade project and the CERN EP R&D on monolithic pixel sensors are investigating the feasibility of the Tower Partners Semiconductor Co. 65 nm process for use in the next generation of vertex detectors. The ITS3 aims to employ wafer-scale Monolithic Active Pixel Sensors thinned down to 20 to 40 um and bent to form truly cylindrical half barrels. Among the first critical steps towards the realisation of this detector is to validate the sensor technology through extensive characterisation both in the laboratory and with in-beam measurements. The Digital Pixel Test Structure (DPTS) is one of the prototypes produced in the first sensor submission in this technology and has undergone a systematic measurement campaign whose details are presented in this article. The results confirm the goals of detection efficiency and non-ionising and ionising radiation hardness up to the expected levels for ALICE ITS3 and also demonstrate operation at +20 C and a detection efficiency of 99% for a DPTS irradiated with a dose of $10^{15}$ 1 MeV n$_{\mathrm{eq}}/$cm$^2$. Furthermore, spatial, timing and energy resolutions were measured at various settings and irradiation levels.Comment: Updated threshold calibration method. Implemented colorblind friendly color palette in all figures. Updated reference

Searching for jet quenching effect using high-multiplicity inclusive jet and hadron-jet semi-inclusive jet in pp collisions with ALICE

Several unforeseen collective phenomena have been observed in high-multiplicity small collision systems that resemble the well-established signatures of the quark-gluon plasma (QGP) formation in heavy-ion collisions. However, jet quenching effects have not been observed in small collision systems. Quantification or setting limits on the magnitude of jet quenching in small systems is essential for understanding the limits of the QGP formation. These proceedings present the outcomes of a search for jet quenching effects performed by the ALICE collaboration in pp collisions at $\sqrt{s} = 13$ TeV as a function of charged-particle multiplicity, measured in forward rapidity. Two jet observables are studied: inclusive $p_{\text{T}}$-differential jet cross section, and the semi-inclusive yield of jets recoiling from a high-$p_{\text{T}}$ trigger-hadron. Jets are reconstructed from charged-particle tracks using the anti-$k_{\text{T}}$ algorithm with resolution parameter $R$ in the range $0.2 - 0.6$. To search for jet quenching effects, both analyses compare jet yields measured in different multiplicity intervals. The analysis of inclusive jets reveals that the rise of event activity leads to an increase in jet production with a weak impact on the spectra slope for high-$p_{\text{T}}$ jets. In the semi-inclusive analysis, the acoplanarity distribution of recoil jets measured in high-multiplicity events exhibits a substantial suppression and broadening when compared to the corresponding spectrum obtained from minimum-bias events. These peculiar features are also seen in pp events simulated by the PYTHIA 8 Monte Carlo event generator. Further studies of the PYTHIA 8 data suggest that the observed suppression and broadening arise from a bias posed by the ALICE high-multiplicity trigger. This bias leads to a growth of the probability to measure high-$p_{\text{T}}$ recoil jets in the acceptance of the forward V0 detector. Furthermore, the high-multiplicity trigger biases toward final states with multi-jet topology

Measurements of jet quenching via hadron+jet correlations in Pb-Pb and high-particle multiplicity pp collisions with ALICE

The ALICE Collaboration presents measurements of jet quenching in the 0--10\% most central Pb--Pb collisions at $\sqrt{s_{\text{NN}}} = 5.02~\text{TeV}$ and high multiplicity pp collisions at $\sqrt{s} = 13~\text{TeV}$ by investigating hadron-jet acoplanarity. In Pb--Pb collisions, the obtained acoplanarity distribution exhibits a marked suppression and narrowing when compared to the pp reference spectrum obtained from PYTHIA simulations. Similar measurements for pp collisions show that the acoplanarity distributions obtained in high multiplicity events are significantly suppressed and broadened relative to the analogous distributions from minimum bias events. The observed features are not caused by jet quenching, since they can be reproduced by PYTHIA 8 event generator, which does not account for jet quenching. Analysis of the PYTHIA events reveals that the suppression and broadening of the hadron-jet acoplanarity distributions are the consequence of a bias induced by the ALICE high multiplicity trigger. This trigger increases the probability to measure a high-$p_{\rm{T}}$ recoil jet in the pseudorapidity acceptance of the forward trigger detectors, and biases toward multi-jet final states

Search for jet quenching effects in high-multiplicity proton-proton collisions at s\sqrt{s} = 13 TeV

The ALICE Collaboration presents results of a search for jet quenching effects in high-particle multiplicity pp collisions at $\sqrt{s}$ = 13 TeV. The analysis is based on the semi-inclusive acoplanarity spectrum of charged jets recoiling from a high-$p_{\rm{T}}$ trigger-track. Significant broadening and suppression of the recoil jet acoplanarity distribution is observed in high-multiplicity pp collisions in real data. However, similar features are also seen in events simulated by the PYTHIA 8 Monash Monte Carlo generator. The simulation reveals that the observed effects result from a bias induced by the ALICE high-multiplicity trigger, which enhances the probability to have a high-$p_{\rm{T}}$ recoil jet in the forward trigger detectors and which biases towards multi-jet final states

Digital Pixel Test Structures implemented in a 65 nm CMOS process

The ALICE ITS3 (Inner Tracking System 3) upgrade project and the CERN EP R&D on monolithic pixel sensors are investigating the feasibility of the Tower Partners Semiconductor Co. 65 nm process for use in the next generation of vertex detectors. The ITS3 aims to employ wafer-scale Monolithic Active Pixel Sensors thinned down to 20 to 40 um and bent to form truly cylindrical half barrels. Among the first critical steps towards the realisation of this detector is to validate the sensor technology through extensive characterisation both in the laboratory and with in-beam measurements. The Digital Pixel Test Structure (DPTS) is one of the prototypes produced in the first sensor submission in this technology and has undergone a systematic measurement campaign whose details are presented in this article. The results confirm the goals of detection efficiency and non-ionising and ionising radiation hardness up to the expected levels for ALICE ITS3 and also demonstrate operation at +20 C and a detection efficiency of 99% for a DPTS irradiated with a dose of $10^{15}$ 1 MeV n$_{\mathrm{eq}}/$cm$^2$. Furthermore, spatial, timing and energy resolutions were measured at various settings and irradiation levels

Pseudorapidity densities of charged particles with transverse momentum thresholds in pp collisions at √ s = 5.02 and 13 TeV

The pseudorapidity density of charged particles with minimum transverse momentum (pT) thresholds of 0.15, 0.5, 1, and 2 GeV/c is measured in pp collisions at the center of mass energies of √s=5.02 and 13 TeV with the ALICE detector. The study is carried out for inelastic collisions with at least one primary charged particle having a pseudorapidity (η) within 0.8pT larger than the corresponding threshold. In addition, measurements without pT-thresholds are performed for inelastic and nonsingle-diffractive events as well as for inelastic events with at least one charged particle having |η|2GeV/c), highlighting the importance of such measurements for tuning event generators. The new measurements agree within uncertainties with results from the ATLAS and CMS experiments obtained at √s=13TeV.

Measurement of inclusive J/ψ\psi pair production cross section in pp collisions at s=13\sqrt{s} = 13 TeV

International audienceThe production cross section of inclusive J/$\psi$ pairs in pp collisions at a centre-of-mass energy $\sqrt{s} = 13$ TeV is measured with ALICE. The measurement is performed for J/$\psi$ in the rapidity interval $2.5 0$. The production cross section of inclusive J/$\psi$ pairs is reported to be $10.3 \pm 2.3 {\rm (stat.)} \pm 1.3 {\rm (syst.)}$ nb in this kinematic interval. The contribution from non-prompt J/$\psi$ (i.e. originated from beauty-hadron decays) to the inclusive sample is evaluated. The results are discussed and compared with data

Inclusive and multiplicity dependent production of electrons from heavy-flavour hadron decays in pp and p−-Pb collisions

International audienceMeasurements of the production of electrons from heavy-flavour hadron decays in pp collisions at $\sqrt{s} = 13$ TeV at midrapidity with the ALICE detector are presented down to a transverse momentum ($p_{\rm T}$) of 0.2 GeV$/c$ and up to $p_{\rm T} = 35$ GeV$/c$, which is the largest momentum range probed for inclusive electron measurements in ALICE. In p$-$Pb collisions, the production cross section and the nuclear modification factor of electrons from heavy-flavour hadron decays are measured in the $p_{\rm T}$ range $0.5 < p_{\rm T} < 26$ GeV$/c$ at $\sqrt{s_{\rm NN}} = 8.16$ TeV. The nuclear modification factor is found to be consistent with unity within the statistical and systematic uncertainties. In both collision systems, first measurements of the yields of electrons from heavy-flavour hadron decays in different multiplicity intervals normalised to the multiplicity-integrated yield (self-normalised yield) at midrapidity are reported as a function of the self-normalised charged-particle multiplicity estimated at midrapidity. The self-normalised yields in pp and p$-$Pb collisions grow faster than linear with the self-normalised multiplicity. A strong $p_{\rm T}$ dependence is observed in pp collisions, where the yield of high-$p_{\rm T}$ electrons increases faster as a function of multiplicity than the one of low-$p_{\rm T}$ electrons. The measurement in p$-$Pb collisions shows no $p_{\rm T}$ dependence within uncertainties. The self-normalised yields in pp and p$-$Pb collisions are compared with measurements of other heavy-flavour, light-flavour, and strange particles, and with Monte Carlo simulations

Observation of medium-induced yield enhancement and acoplanarity broadening of low-pTp_\mathrm{T} jets from measurements in pp and central Pb−-Pb collisions at sNN=5.02\sqrt{s_{\rm NN}}=5.02 TeV

International audienceThe ALICE Collaboration reports the measurement of semi-inclusive distributions of charged-particle jets recoiling from a high transverse momentum (high $p_{\rm T}$) hadron trigger in proton$-$proton and central Pb$-$Pb collisions at $\sqrt{s_{\rm NN}} = 5.02$ TeV. A data-driven statistical method is used to mitigate the large uncorrelated background in central Pb$-$Pb collisions. Recoil jet distributions are reported for jet resolution parameter $R=0.2$, 0.4, and 0.5 in the range $7 < p_{\rm T,jet} < 140$ GeV$/c$ and trigger$-$recoil jet azimuthal separation $\pi/2 < \Delta\varphi < \pi$. The measurements exhibit a marked medium-induced jet yield enhancement at low $p_{\rm T}$ and at large azimuthal deviation from $\Delta\varphi\sim\pi$. The enhancement is characterized by its dependence on $\Delta\varphi$, which has a slope that differs from zero by 4.7$\sigma$. Comparisons to model calculations incorporating different formulations of jet quenching are reported. These comparisons indicate that the observed yield enhancement arises from the response of the QGP medium to jet propagation

Probing the Chiral Magnetic Wave with charge-dependent flow measurements in Pb-Pb collisions at the LHC

International audienceThe Chiral Magnetic Wave (CMW) phenomenon is essential to provide insights into the strong interaction in QCD, the properties of the quark-gluon plasma, and the topological characteristics of the early universe, offering a deeper understanding of fundamental physics in high-energy collisions. Measurements of the charge-dependent anisotropic flow coefficients are studied in Pb-Pb collisions at center-of-mass energy per nucleon-nucleon collision $\sqrt{s_{\mathrm{NN}}}=$ 5.02 TeV to probe the CMW. In particular, the slope of the normalized difference in elliptic ($v_{2}$) and triangular ($v_{3}$) flow coefficients of positively and negatively charged particles as a function of their event-wise normalized number difference, is reported for inclusive and identified particles. The slope $r_{3}^{\rm Norm}$ is found to be larger than zero and to have a magnitude similar to $r_{2}^{\rm Norm}$, thus pointing to a large background contribution for these measurements. Furthermore, $r_{2}^{\rm Norm}$ can be described by a blast wave model calculation that incorporates local charge conservation. In addition, using the event shape engineering technique yields a fraction of CMW ($f_{\rm CMW}$) contribution to this measurement which is compatible with zero. This measurement provides the very first upper limit for $f_{\rm CMW}$, and in the 10-60% centrality interval it is found to be 26% (38%) at 95% (99.7%) confidence level