8 Search Results

For LHC Run 3, the ALICE Time Projection Chamber was upgraded to operate in continuous readout mode. Interaction rates of up to 50 kHz in Pb-Pb collisions require real-time processing of more than 3 TB s-1 of raw detector data. This requirement is met by a custom FPGA-based processing pipeline that performs the complete front-end data treatment fully in-stream, including common-mode correction, pedestal subtraction, ion-tail filtering, zero suppression, and dense data packing. A central element of the design is a highly parallel common-mode correction algorithm operating directly on the streaming data. It robustly identifies signal-free readout channels on a time-binmore » basis and applies pad-dependent scaling to compensate for local variations in capacitive coupling in the GEM readout. In combination with pedestal subtraction and ion-tail filtering, this enables accurate baseline restoration under extreme high-occupancy conditions, preventing signal loss while efficiently suppressing noise prior to zero suppression. The pipeline operates continuously at the full detector bandwidth and reduces the raw input rate of approximately 3 TB s-1 to about 900 GBps for Pb-Pb collisions at the target interaction rate. Overall, it represents a large-scale FPGA-based real-time signal-processing implementation for high-energy physics detector readout.« less

Here, we present a study of the inclusive charged-particle transverse momentum (p_T) spectra as a function of charged-particle multiplicity density at mid-pseudorapidity, dN_ch/dη, in pp collisions at $$ \sqrt{s}$$ = 5.02 and 13 TeV covering the kinematic range |η| < 0.8 and 0.15 < p_T < 20 GeV/c. The results are presented for events with at least one charged particle in |η| < 1 (INEL>0). The p_T spectra are reported for two multiplicity estimators covering different pseudorapidity regions. The p_T spectra normalized to that for INEL>0 show little energy dependence. Moreover, the high-p_T yields of charged particles increase faster than themore » charged-particle multiplicity density. The average p_T as a function of multiplicity and transverse spherocity is reported for pp collisions at $$ \sqrt{s}$$=13 TeV. For low- (high-) spherocity events, corresponding to jet-like (isotropic) events, the average p_T is higher (smaller) than that measured in INEL>0 pp collisions. Within uncertainties, the functional form of ⟨p_T⟩ (N_ch) is not affected by the spherocity selection. While EPOS LHC gives a good description of many features of data, PYTHIA overestimates the average p_T in jet-like events.« less

The ALICE Collaboration has measured the energy dependence of exclusive photoproduction of J / ψ vector mesons off proton targets in ultra–peripheral p–Pb collisions at a centre-of-mass energy per nucleon pair sNN=5.02 TeV. The e + e - and μ + μ - decay channels are used to measure the cross section as a function of the rapidity of the J / ψ in the range - 2.5 < y< 2.7 , corresponding to an energy in the γp centre-of-mass in the interval 40 < W γp < 550 GeV. The measurements, which are consistent with a power law dependencemore » of the exclusive J / ψ photoproduction cross section, are compared to previous results from HERA and the LHC and to several theoretical models. They are found to be compatible with previous measurements.« less

The measurements of the production of prompt D⁰, D⁺, D^*+, and D$$_s^+$$ mesons in proton–proton (pp) collisions at $$\sqrt{s}$$=5.02 TeV with the ALICE detector at the Large Hadron Collider (LHC) are reported. D mesons were reconstructed at mid-rapidity (|y| < 0.5) via their hadronic decay channels D⁰ → K^- $$π$$⁺, D⁺ → K^- $$π$$⁺ $$π$$⁺, D^*+ → D⁰$$π$$⁺ → K^- $$π$$ ⁺ $$π$$ ⁺, D$$_s^+$$ → Φ$$π$$⁺ → K⁺K^- $$π$$⁺, and their charge conjugates. The production cross sections were measured in the transverse momentum interval 0 < $$p$$T < 36 GeV/$$c$$ for D⁰, 1 < $$p$$T < 36 GeV/$$c$$ formore » D⁺ and D^*+, and in 2 < $$p$$T < 24 GeV/$$c$$ for D$$_s^+$$ mesons. Thanks to the higher integrated luminosity, an analysis in finer $$p$$T bins with respect to the previous measurements at $$\sqrt{s}$$ = 7 TeV was performed, allowing for a more detailed description of the cross-section $$p$$T shape. The measured $$p$$T differential production cross sections are compared to the results at √ s = 7 TeV and to four different perturbative QCD calculations. Its rapidity dependence is also tested combining the ALICE and LHCb measurements in pp collisions at $$\sqrt{s}$$ = 5.02 TeV. This measurement will allow for a more accurate determination of the nuclear modification factor in p–Pb and Pb–Pb collisions performed at the same nucleon– nucleon centre-of-mass energy.« less

The pseudorapidity density of charged particles, d$$N$$_ch/d$$η$$, in p–Pb collisions has been measured at a centre-of-mass energy per nucleon–nucleon pair of $$\sqrt{s_{NN}}$$ = 8.16 TeV at mid-pseudorapidity for non-single-diffractive events. The results cover 3.6 units of pseudorapidity, |$$η$$| < 1.8. The d$$N$$_ch/d$$η$$ value is 19.1 ± 0.7 at |$$η$$| < 0.5. This quantity divided by (N_part)/2 is 4.73 ± 0.20 , where (N_part) is the average number of participating nucleons, is 9.5% higher than the corresponding value for p–Pb collisions at $$\sqrt{s_{NN}}$$ = 5.02 TeV. Measurements are compared with models based on different mechanisms for particle production. All models agreemore » within uncertainties with data in the Pb-going side, while HIJING overestimates, showing a symmetric behaviour, and EPOS underestimates the p-going side of the d$$N$$_ch/d$$η$$ distribution. Saturation-based models reproduce the distributions well for < -1.3. The d$$N$$_ch/d$$η$$ is also measured for different centrality estimators, based both on the charged-particle multiplicity and on the energy deposited in the Zero-Degree Calorimeters. A study of the implications of the large multiplicity fluctuations due to the small number of participants for systems like p–Pb in the centrality calculation for multiplicity-based estimators is discussed, demonstrating the advantages of determining the centrality with energy deposited near beam rapidity.« less

First results on K/$$\pi$$, p/$$\pi$$ and K/p fluctuations are obtained with the ALICE detector at the CERN LHC as a function of centrality in Pb-Pb collisions at $$\sqrt{s_\mathrm{{NN}}} =2.76\hbox { TeV}$$. The observable ν_dyn, which is defined in terms of the moments of particle multiplicity distributions, is used to quantify the magnitude of dynamical fluctuations of relative particle yields and also provides insight into the correlation between particle pairs. This study is based on a novel experimental technique, called the Identity Method, which allows one to measure the moments of multiplicity distributions in case of incomplete particle identification. The resultsmore » for p/$$\pi$$ show a change of sign in ν_dyn from positive to negative towards more peripheral collisions. For central collisions, the results follow the smooth trend of the data at lower energies and ν_dyn exhibits a change in sign for p/$$\pi$$ and K/p.« less

A measurement of beauty hadron production at mid-rapidity in proton-lead collisions at a nucleon–nucleon centre-of-mass energy $$\mathbf{\sqrt{{ s}_{\text {NN}}}= 5.02}$$ TeV is presented. The semi-inclusive decay channel of beauty hadrons into J / ψ is considered, where the J / ψ mesons are reconstructed in the dielectron decay channel at mid-rapidity down to transverse momenta of 1.3 GeV/c. The b b ¯ production cross section at mid-rapidity, d σbb ¯/ d y, and the total cross section extrapolated over full phase space, σbb ¯, are obtained. This measurement is combined with results on inclusive J / ψ production to determine themore » prompt J / ψ cross sections. The results in p–Pb collisions are then scaled to expectations from pp collisions at the same centre-of-mass energy to derive the nuclear modification factor RpPb, and compared to models to study possible nuclear modifications of the production induced by cold nuclear matter effects. RpPbis found to be smaller than unity at low pTfor both J / ψ coming from beauty hadron decays and prompt J / ψ.« less

The invariant differential cross sections for inclusive π 0 and η mesons at midrapidity were measured in pp collisions at √s=2.76 TeV for transverse momenta 0.4 < p _T < 40 GeV/c and 0.6 < p _T < 20 GeV/c, respectively, using the ALICE detector. This large range in p _T was achieved by combining various analysis techniques and different triggers involving the electromagnetic calorimeter (EMCal). Particularly, a new single-cluster, shower-shape based method was developed for the identification of high-p _T neutral pions, which exploits that the showers originating from their decay photons overlap in the EMCal. Above 4 GeV/c, the measured crossmore » sections are found to exhibit a similar power-law behavior with an exponent of about 6.3. Next-to-leading-order perturbative QCD calculations differ from the measured cross sections by about 30% for the π ⁰ , and between 30–50% for the η meson, while generator-level simulations with PYTHIA 8.2 describe the data to better than 10–30%, except at p _T < 1 GeV/c. The new data can therefore be used to further improve the theoretical description of π ⁰ and η meson production.« less