Underlying Event Studies for LHC Energies

Underlying event was originally defined by the CDF collaboration decades ago. Here we improve the original definition to extend our analysis for events with multiple-jets. We introduce a definition for surrounding rings/belts and based on this definition the jet- and surrounding-belt-excluded areas will provide a good underlying event definition. We inverstigate our definition via the multiplicity in the defined geometry. In parallel, mean transverse momenta of these areas also studied in proton-proton collisions at $\sqrt{s}=7$ TeV LHC energy.Comment: 6 pages and 4 figure

A Very High Momentum Particle Identification Detector

The construction of a new detector is proposed to extend the capabilities of ALICE in the high transverse momentum (pT) region. This Very High Momentum Particle Identification Detector (VHMPID) performs charged hadron identification on a track-by-track basis in the 5 GeV/c < p < 25 GeV/c momentum range and provides ALICE with new opportunities to study parton-medium interactions at LHC energies. The VHMPID covers up to 30% of the ALICE central barrel and presents sufficient acceptance for triggered- and tagged-jet studies, allowing for the first time identified charged hadron measurements in jets. This Letter of Intent summarizes the physics motivations for such a detector as well as its layout and integration into ALICE

D + s production at central rapidity in Pb–Pb collisions at √ sNN = 2.76 TeV with the ALICE detector

We present the measurement of the D+ s production in pp collisions at √ s = 7 TeV and in Pb– Pb collisions at √ sNN =2.76 TeV performed with the ALICE detector at central rapidity through the reconstruction of the hadronic decay channel D+ s → φπ+ → K +K −π + . The preliminary results of the D+ s nuclear modification factor will be presented

Multiplicity Dependence of Pion, Kaon, Proton and Lambda Production in p–Pb Collisions at √sNN = 5.02 TeV

In this Letter, comprehensive results on π±, K±, K0 S, p(p) and Λ( ¯Λ) production at mid-rapidity (0 < yCMS < 0.5) in p–Pb collisions at √sNN = 5.02 TeV, measured by the ALICE detector at the LHC, are reported. The transverse momentum distributions exhibit a hardening as a function of event multiplicity, which is stronger for heavier particles. This behavior is similar to what has been observed in pp and Pb–Pb collisions at the LHC. The measured pT distributions are compared to d–Au, Au–Au and Pb–Pb results at lower energy and with predictions based on QCD-inspired and hydrodynamic models

Measurement of charged jet suppression in Pb-Pb collisions at √s NN = 2.76 TeV

A measurement of the transverse momentum spectra of jets in Pb-Pb collisions at sNN−−−√=2.76 TeV is reported. Jets are reconstructed from charged particles using the anti-kT jet algorithm with jet resolution parameters R of 0.2 and 0.3 in pseudo-rapidity |η|<0.5. The transverse momentum pT of charged particles is measured down to 0.15 GeV/c which gives access to the low pT fragments of the jet. Jets found in heavy-ion collisions are corrected event-by-event for average background density and on an inclusive basis (via unfolding) for residual background fluctuations and detector effects. A strong suppression of jet production in central events with respect to peripheral events is observed. The suppression is found to be similar to the suppression of charged hadrons, which suggests that substantial energy is radiated at angles larger than the jet resolution parameter R=0.3 considered in the analysis. The fragmentation bias introduced by selecting jets with a high pT leading particle, which rejects jets with a soft fragmentation pattern, has a similar effect on the jet yield for central and peripheral events. The ratio of jet spectra with R=0.2 and R=0.3 is found to be similar in Pb-Pb and simulated PYTHIA pp events, indicating no strong broadening of the radial jet structure in the reconstructed jets with R<0.3

Two- and three-pion quantum statistics correlations in Pb-Pb collisions at √sNN = 2.76 TeV at the CERN Large Hadron Collider

Correlations induced by quantum statistics are sensitive to the spatio-temporal extent as well as dy- namics of particle emitting sources in heavy-ion collisions. In addition, such correlations can be used to search for the presence of a coherent component of pion production. Two- and three-pion corre- lations of same and mixed-charge are measured at low relative momentum to estimate the coherent fraction of charged pions in Pb-Pb collisions at √sNN = 2.76 TeV at the CERN Large Hadron Collider with ALICE. The genuine three-pion quantum statistics correlation is found to be suppressed relative to the two-pion correlation based on the assumption of fully chaotic pion emission. The suppression is observed to decrease with triplet momentum. The observed suppression at low triplet momentum may correspond to a coherent fraction in charged pion emission of 23% ± 8%

Mid-rapidity anti-baryon to baryon ratios in pp collisions at √s = 0.9, 2.76 and 7 TeV measured by ALICE

The ratios of yields of anti-baryons to baryons probes the mechanisms of baryon-number transport. Results for p/p, Λ/Λ, Ξ+/Ξ− and Ω+/Ω− in pp collisions at √s = 0.9, 2.76 and 7 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 genera- tors, such as PYTHIA and HIJING/B, that are used to model the particle production in pp collisions. The energy dependence of p/p, Λ/Λ, Ξ+/Ξ− and Ω+/Ω−, reaching values compatible with unity for √s = 7 TeV, complement the earlier p/p measurement of ALICE. These dependencies can be described by exchanges with the Regge-trajectory intercept of αJ ≈ 0.5, which are suppressed with increasing rapidity interval ∆y. Any significant contribution of an exchange not suppressed at large ∆y (reached at LHC energies) is disfavoured

Charged kaon femtoscopic correlations in pp collisions at √s=7 TeV

Correlations of two charged identical kaons (KchKch) are measured in pp collisions at √s = 7 TeV by the ALICE experiment at the Large Hadron Collider (LHC). One-dimensional KchKch correlation functions are constructed in three multiplicity and four transverse momentum ranges. The KchKch femtoscopic source parameters R and λ are extracted. The KchKch correlations show a slight in- crease of femtoscopic radii with increasing multiplicity and a slight decrease of radii with increasing transverse momentum. These trends are similar to the ones observed for ππ and K0 s K0 s correlations in pp and heavy-ion collisions. However at high multiplicities, there is an indication that the one- dimensional correlation radii for charged kaons are larger than those for pions in contrast to what was observed in heavy-ion collisions at RHIC

Long-range angular correlations on the near and away side in p-Pb collisions at √sNN=5.02 TeV

Angular correlations between charged trigger and associated particles are measured by the ALICE detector in p–Pb collisions at a nucleon–nucleon centre-of-mass energy of 5.02 TeV for transverse momentum ranges within 0.5 < pT,assoc < pT,trig < 4 GeV/c. The correlations are measured over two units of pseudorapidity and full azimuthal angle in different intervals of event multiplicity, and expressed as associated yield per trigger particle. Two long-range ridge-like structures, one on the near side and one on the away side, are observed when the per-trigger yield obtained in low-multiplicity events is subtracted from the one in high- multiplicity events. The excess on the near-side is qualitatively similar to that recently reported by the CMS collaboration, while the excess on the away-side is reported for the first time. The two-ridge structure projected onto azimuthal angle is quantified with the sec- ond and third Fourier coefficients as well as by near-side and away-side yields and widths. The yields on the near side and on the away side are equal within the uncertainties for all studied event multiplicity and pT bins, and the widths show no significant evolution with event multiplicity or pT. These findings suggest that the near-side ridge is accompanied by an essentially identical away-side ridge

Charmonium and e(+)e(-) pair photoproduction at mid-rapidity in ultra-peripheral Pb-Pb collisions at root s(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 √sNN=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 µ b − 1 . The cross section for coherent and incoherent J / ψ production in the rapidity interval -0.9 < y < 0.9, are d σ coh J/ψ / dy = 2 .38 + 0 .34 − 0 .24 sta +sys mb. and d σ 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, 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