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Arnaldi, R.

Publications and source records attributed to Arnaldi, R..

At least 343 records · Page 19
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DOE OSTI2014

J/ψ production and nuclear effects in p-Pb collisions at $ \sqrt{{{{\mathrm{s}}_{\mathrm{NN}}}}} $ = 5.02 TeV

We studied inclusive J/ψ production with the ALICE detector in p-Pb collisions at the nucleon-nucleon center of mass energy √s NN = 5.02 TeV at the CERN LHC. The measurement is performed in the center of mass rapidity domains 2.03 < y cms < 3.53 and -4.46 < y cms < -2.96, down to zero transverse momentum, studying the μ + μ - decay mode. In this paper, the J/ψ production cross section and the nuclear modification factor R pPb for the rapidities under study are presented. Moreover, while at forward rapidity, corresponding to the proton direction, a suppression of the J/ψ yield with respect to binary-scaled pp collisions is observed, in the backward region no suppression is present. Finally, the ratio of the forward and backward yields is also measured differentially in rapidity and transverse momentum. Theoretical predictions based on nuclear shadowing, as well as on models including, in addition, a contribution from partonic energy loss, are in fair agreement with the experimental results.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2014

Spin physics at a fixed-target experiment at the LHC (AFTER@LHC)

We outline the opportunities for spin physics which are offered by a next generation and multipurpose fixed-target experiment exploiting the proton LHC beam extracted by a bent crystal. In particular, we focus on the study of single transverse spin asymetries with the polarisation of the target.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2013

Energy dependence of the transverse momentum distributions of charged particles in pp collisions measured by ALICE

Differential cross sections of charged particles in inelastic pp collisions as a function of p T have been measured at $\sqrt{s}$ = 0.9, 2.76 and 7 TeV at the LHC. The p T spectra are compared to NLO-pQCD calculations. Though the differential cross section for an individual √s cannot be described by NLO-pQCD, the relative increase of cross section with √s is in agreement with NLO-pQCD. Based on these measurements and observations, procedures are discussed to construct pp reference spectra at $\sqrt{s}$ = 2.76 and 5.02 TeV up to p T =50 GeV/c as required for the calculation of the nuclear modification factor in nucleus–nucleus and proton–nucleus collisions.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2013

Charmonium and e + e - pair photoproduction at mid-rapidity in ultra-peripheral Pb–Pb collisions at $\sqrt{s_{\mathrm{NN}}} $ = 2.76 TeV

The ALICE Collaboration at the LHC has measured the J/ψ and ψ' photoproduction at mid-rapidity in ultra-peripheral Pb-Pb collisions at $\sqrt{s_{\mathrm{NN}}} $ = 2.76 Tev. The charmonium is identified via its leptonic decay for events where the hadronic activity is required to be minimal. The analysis is based on an event sample corresponding to an integrated luminosity of about 23 μb -1 . The cross section for coherent and incoherent J/ψ production in the rapidity interval -0.9 < y < 0.9, are dσ coh J/ψ /dy = 2.38 +0.34 –0.24 (sta + sys) mb and dσ inc J/ψ /dy = 0.98 +0.19 –0.17 (sta + sys) mb, respectively. The results are compared to theoretical models for J/ψ production and the coherent cross section is found to be in good agreement with those models incorporating moderate nuclear gluon shadowing at Bjorken-x around 10 –3 , such as EPS09 parametrization. In addition the cross section for the process γγ→e + e – has been measured and found to be in agreement with models implementing QED at leading order.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2013

Mid-rapidity anti-baryon to baryon ratios in pp collisions at $\sqrt{s} = 0.9,\ 2.76\mbox{ and }7\mbox{ TeV}$ measured by ALICE

The ratios of yields of anti-baryons to baryons probes the mechanisms of baryon-number transport. Results for \(\overline{\mathrm {p}}/{\mathrm{p}}\) , \(\overline{\Lambda}/{\Lambda}\) , \({{\overline{\Xi}}^{+}}/{\Xi ^{-}}\) and \({{\overline{\Omega}}^{+}}/{\Omega^{-}}\) in pp collisions at \(\sqrt{s} = 0.9,\ 2.76\mbox{ and }7\mbox{ TeV}\) , measured with the ALICE detector at the LHC, are reported. Within the experimental uncertainties and ranges covered by our measurement, these ratios are independent of rapidity, transverse momentum and multiplicity for all measured energies. The results are compared to expectations from event generators, such as PYTHIA and HIJING/B, that are used to model the particle production in pp collisions. The energy dependence of \(\overline{\mathrm{p}}/{\mathrm{p}}\) , \(\overline {\Lambda}/{\Lambda}\) , \({{\overline{\Xi}}^{+}}/{\Xi^{-}}\) and \({{\overline{\Omega}}^{+}}/{\Omega^{-}}\) , reaching values compatible with unity for \(\sqrt{s} = 7\mbox{ TeV}\) , complement the earlier \(\overline{\mathrm{p}}/{\mathrm{p}}\) measurement of ALICE. These dependencies can be described by exchanges with the Regge-trajectory intercept of \(\alpha_{\rm{J}} \approx0.5\) , which are suppressed with increasing rapidity interval Δ y . Any significant contribution of an exchange not suppressed at large Δ y (reached at LHC energies) is disfavoured.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2013

Measurement of inelastic, single- and double-diffraction cross sections in proton–proton collisions at the LHC with ALICE

Measurements of cross sections of inelastic and diffractive processes in proton–proton collisions at LHC energies were carried out with the ALICE detector. The fractions of diffractive processes in inelastic collisions were determined from a study of gaps in charged particle pseudorapidity distributions: for single diffraction (diffractive mass M X <200 GeV/ c 2 ) \(\sigma_{\mathrm{SD}}/\sigma_{\mathrm {INEL}} = 0.21 \pm 0.03, 0.20^{+0.07}_{-0.08}\) , and \(0.20^{+0.04}_{-0.07}\) , respectively at centre-of-mass energies \(\sqrt{s} = 0.9, 2.76\mbox{, and }7~\mbox{TeV}\) ; for double diffraction (for a pseudorapidity gap Δ η >3) σ DD / σ INEL =0.11±0.03,0.12±0.05, and \(0.12^{+0.05}_{-0.04}\) , respectively at \(\sqrt{s} = 0.9, 2.76\mbox{, and }7~\mbox{TeV}\) . To measure the inelastic cross section, beam properties were determined with van der Meer scans, and, using a simulation of diffraction adjusted to data, the following values were obtained: \(\sigma_{\mathrm{INEL}} = 62.8^{+2.4}_{-4.0} (\mathrm{model}) \pm1.2 (\mathrm{lumi})\) mb at \(\sqrt{s} = 2.76\ \mbox{TeV}\) and \(73.2^{+2.0}_{-4.6} (\mathrm{model}) \pm2.6 (\mathrm{lumi})~\mbox{mb}\) at \(\sqrt{s} = 7\ \mbox{TeV}\) . The single- and double-diffractive cross sections were calculated combining relative rates of diffraction with inelastic cross sections. The results are compared to previous measurements at proton–antiproton and proton–proton colliders at lower energies, to measurements by other experiments at the LHC, and to theoretical models.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2013

Spin and diffractive physics with a fixed-target experiment at the LHC (AFTER-LHC)

We report on the spin and diffractive physics at a future multi-purpose f xed-target experiment with proton and lead LHC beams extracted by a bent crystal. The LHC multi-TeV beams allow for the most energetic f xed-target experiments ever performed, opening new domains of particle and nuclear physics and complementing that of collider physics, in particular that of RHIC and the EIC projects. The luminosity achievable with AFTER using typical targets would surpass that of RHIC by more than 3 orders of magnitude. The f xed-target mode has the advantage to allow for measurements of single-spin asymmetries with polarized target as well as of single-diffractive processes in the target region.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2012

Production of K* (892) 0 and Φ (1020) in pp collisions at $\sqrt{s}$ = 7 TeV

>The production of K*(892) 0 and Φ (1020) in pp collisions at \(\sqrt{s}=7~\mathrm{TeV}\) was measured by the ALICE experiment at the LHC. The yields and the transverse momentum spectra d 2 N /d y d p T at midrapidity | y |<0.5 in the range 0< p T <6 GeV/ c for K * (892) 0 and 0.4< p T <6 GeV/ c for Φ (1020) are reported and compared to model predictions. Using the yield of pions, kaons, and Ω baryons measured previously by ALICE at \(\sqrt{s}=7\ \mathrm{TeV}\) , the ratios K*/K – , Φ /K * , Φ /K – , Φ / π – , and \((\Omega + \overline{\Omega })/\phi\) are presented. The values of the K*K – , Φ /K * and Φ /K – ratios are similar to those found at lower centre-of-mass energies. In contrast, the Φ / π – ratio, which has been observed to increase with energy, seems to saturate above 200 GeV. The \((\Omega + \overline {\Omega })/\phi\) ratio in the p T range 1–5 GeV/ c is found to be in good agreement with the prediction of the \(\mathrm{HIJING}/\mathrm{B}\overline{\mathrm{B}}\) v2.0 model with a strong colour field.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2012

Transverse sphericity of primary charged particles in minimum bias proton–proton collisions at $\sqrt{s}$ = 0.9, 2.76 and 7 TeV

Measurements of the sphericity of primary charged particles in minimum bias proton–proton collisions at $\sqrt{s}$ = 0.9, 2.76 and 7 TeV with the ALICE detector at the LHC are presented. The observable is measured in the plane perpendicular to the beam direction using primary charged tracks with $p_T$ > 0.5 GeV/c in |η| < 0.8. The mean sphericity as a function of the charged particle multiplicity at mid-rapidity ($N_ch$) is reported for events with different $p_T$ scales (“soft” and “hard”) defined by the transverse momentum of the leading particle. In addition, the mean charged particle transverse momentum versus multiplicity is presented for the different event classes, and the sphericity distributions in bins of multiplicity are presented. The data are compared with calculations of standard Monte Carlo event generators. The transverse sphericity is found to grow with multiplicity at all collision energies, with a steeper rise at low N ch, whereas the event generators show an opposite tendency. The combined study of the sphericity and the mean p T with multiplicity indicates that most of the tested event generators produce events with higher multiplicity by generating more back-to-back jets resulting in decreased sphericity (and isotropy). The PYTHIA6 generator with tune PERUGIA-2011 exhibits a noticeable improvement in describing the data, compared to the other tested generators.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗