The p t-differential inclusive production cross sections of the prompt charmed mesons D0, D+, and... more The p t-differential inclusive production cross sections of the prompt charmed mesons D0, D+, and D*+ in the rapidity range |y| < 0.5 were measured in proton-proton collisions at $ \sqrt {s} = 7{ }TeV $ at the LHC using the ALICE detector. Reconstructing the decays D0 → K−π+, D+ → K−π+π+, D*+ → D0π+, and their charge conjugates, about 8,400 D0, 2,900 D+, and 2,600 D*+ mesons with 1 < p t < 24 GeV/c were counted, after selection cuts, in a data sample of 3.14 × 108 events collected with a minimum-bias trigger (integrated luminosity L int = 5 nb−1). The results are described within uncertainties by predictions based on perturbative QCD.
Proceedings of 5th International Workshop on Critical Point and Onset of Deconfinement — PoS(CPOD 2009)
The statistical model assuming chemical equilibrium and local strangeness conservation describes ... more The statistical model assuming chemical equilibrium and local strangeness conservation describes most of the observed features of strange particle production from SIS up to RHIC. Deviations are found as the maximum in the measured K + /π + ratio is much sharper than in the model calculations. At the incident energy of the maximum, the statistical model shows that freeze out changes regime from one being dominated by baryons at the lower energies toward one being dominated by mesons. It will be shown how deviations from the usual freeze-out curve influence the various particle ratios. Furthermore, other observables exhibit also changes just in this energy regime.
Particle production in p+p and central Pb+Pb collisions at LHC is discussed in the context of the... more Particle production in p+p and central Pb+Pb collisions at LHC is discussed in the context of the statistical thermal model. For heavy-ion collisions, predictions of various particle ratios are presented. The sensitivity of several ratios on the temperature and the baryon chemical potential is studied in detail, and some of them, which are particularly appropriate to determine the chemical freeze-out point experimentally, are indicated. Considering elementary interactions on the other hand, we focus on strangeness production and its possible suppression. Extrapolating the thermal parameters to LHC energy, we present predictions of the statistical model for particle yields in p+p collisions. We quantify the strangeness suppression by the correlation volume parameter and discuss its influence on particle production. We propose observables that can provide deeper insight into the mechanism of strangeness production and suppression at LHC.
A kinetic master equation for multiplicity distributions is formulated for charged particles whic... more A kinetic master equation for multiplicity distributions is formulated for charged particles which are created or destroyed only in pairs due to the conservation of their Abelian charge. It allows one to study time evolution of the multiplicity distributions in a relativistic many-body system with arbitrary average particle multiplicities. It is shown to reproduce the equilibrium results for both canonical (rare particles) and grand canonical (abundant particles) systems. For canonical systems, the equilibrium multiplicity is much lower and the relaxation time is much shorter than the naive extrapolation from grand canonical results. Implications for chemical equilibration in heavy-ion collisions are also discussed.
The properties of net quark number fluctuations in the vicinity of the QCD chiral phase transitio... more The properties of net quark number fluctuations in the vicinity of the QCD chiral phase transition are discussed in terms of an effective chiral model in the mean-field approximation. We focus on the ratio of the fourth-to second-order cumulants (kurtosis) and the compressibility of the system and discuss their dependence on the pion mass. It is shown that near the chiral phase transition, both observables are sensitive to the value of mπ. For physical mπ, the kurtosis exhibits a peak whereas the inverse compressibility shows a dip at the pseudocritical temperature. These structures disappear for large mπ. Our results, obtained in an effective model with two flavors, are qualitatively consistent with recent results of 2+1 flavor lattice gauge theory. We also discuss the high-and lowtemperature properties of these observables and the role of the coupling of the quark degrees of freedom to the Polyakov loop.
We present results of a simulation of two flavor QCD on a 16 3 ×4 lattice using p4-improved stagg... more We present results of a simulation of two flavor QCD on a 16 3 ×4 lattice using p4-improved staggered fermions with bare quark mass m/T = 0.4. Derivatives of the thermodynamic grand canonical partition function Z(V, T, µ u , µ d) with respect to chemical potentials µ u,d for different quark flavors are calculated up to sixth order, enabling estimates of the pressure and the quark number density as well as the chiral condensate and various susceptibilities as functions of µ q = (µ u + µ d)/2 via Taylor series expansion. Furthermore, we analyze baryon as well as isospin fluctuations and discuss the relation between the radius of convergence of the Taylor series and the chiral critical point in the QCD phase diagram. We argue that bulk thermodynamic observables do not, at present, provide direct evidence for the existence of a chiral critical point in the QCD phase diagram. Results are compared to high temperature perturbation theory as well as a hadron resonance gas model.
We compare recent lattice studies of QCD thermodynamics at non-zero quark chemical potential with... more We compare recent lattice studies of QCD thermodynamics at non-zero quark chemical potential with the thermodynamics of a hadron resonance gas. We argue that for T ≤ T c the equation of state derived from Monte-Carlo simulations of two flavour QCD at non-zero chemical potential can be well described by a hadron resonance gas when using the same set of approximations as used in current lattice calculations. We estimate the importance of truncation errors arising from the use of a Taylor expansion in terms of the quark chemical potential and examine the influence of unphysically large quark masses on the equation of state and the critical conditions for deconfinement.
The production of Ξ − and Ω − baryons and their anti-particles in Pb-Pb collisions at √ s NN = 2.... more The production of Ξ − and Ω − baryons and their anti-particles in Pb-Pb collisions at √ s NN = 2.76 TeV has been measured using the ALICE detector. The transverse momentum spectra at mid-rapidity (|y| < 0.5) for charged Ξ and Ω hyperons have been studied in the range 0.6 < p T < 8.0 GeV/c and 1.2 < p T < 7.0 GeV/c, respectively, and in several centrality intervals (from the most central 0-10% to the most peripheral 60-80% collisions). These spectra have been compared with the predictions of recent hydrodynamic models. In particular, the Kraków and EPOS models give a satisfactory description of the data, with the latter covering a wider p T range. Mid-rapidity yields, integrated over p T , have been determined. The hyperon-to-pion ratios are similar to those at RHIC: they rise smoothly with centrality up to N part ∼ 150 and saturate thereafter. The enhancements (yields per participant nucleon relative to those in pp collisions) increase both with the strangeness content of the baryon and with centrality, but are less pronounced than at lower energies.
The transverse momentum (p T) distribution of primary charged particles is measured at midrapidit... more The transverse momentum (p T) distribution of primary charged particles is measured at midrapidity in minimum-bias p-Pb collisions at √ s NN = 5.02 TeV with the ALICE detector at the LHC in the range 0.15 < p T < 50 GeV/c. The spectra are compared to the expectation based on binary collision scaling of particle production in pp collisions, leading to a nuclear modification factor consistent with unity for p T larger than 2 GeV/c. The measurement is compared to theoretical calculations and to data in Pb-Pb collisions at √ s NN = 2.76 TeV.
A review of meson emission in heavy ion collisions at incident energies from SIS up to collider e... more A review of meson emission in heavy ion collisions at incident energies from SIS up to collider energies is presented. A statistical model assuming chemical equilibrium and local strangeness conservation (i.e. strangeness conservation per collision) explains most of the observed features. Emphasis is put onto the study of K + and K − emission at low incident energies. In the framework of this statistical model it is shown that the experimentally observed equality of K + and K − rates at "threshold-corrected" energies √ s − √ s th is due to a crossing of two excitation functions. Furthermore, the independence of the K + to K − ratio on the number of participating nucleons observed between SIS and RHIC is consistent with this model. It is demonstrated that the K − production at SIS energies occurs predominantly via strangeness exchange and this channel is approaching chemical equilibrium. The observed maximum in the K + /π + excitation function is also seen in the ratio of strange to non-strange particle production. The appearance of this maximum around 30 A•GeV is due to the energy dependence of the chemical freeze-out parameters T and µ B .
We present the first wide-range measurement of the charged-particle pseudorapidity density distri... more We present the first wide-range measurement of the charged-particle pseudorapidity density distribution, for different centralities (the 0-5%, 5-10%, 10-20%, and 20-30% most central events) in Pb-Pb collisions at √ s NN = 2.76 TeV at the LHC. The measurement is performed using the full coverage of the ALICE detectors, −5.0 < η < 5.5, and employing a special analysis technique based on collisions arising from LHC 'satellite' bunches. We present the pseudorapidity density as a function of the number of participating nucleons as well as an extrapolation to the total number of produced charged particles (N ch = 17165 ± 772 for the 0-5% most central collisions). From the measured dN ch /dη distribution we derive the rapidity density distribution, dN ch /dy, under simple assumptions. The rapidity density distribution is found to be significantly wider than the predictions of the Landau model. We assess the validity of longitudinal scaling by comparing to lower energy results from RHIC. Finally the mechanisms of the underlying particle production are discussed based on a comparison with various theoretical models.
We have investigated hadron screening masses, the chiral condensate, and the pion decay constant ... more We have investigated hadron screening masses, the chiral condensate, and the pion decay constant close to the decon nement phase transition in the con ned phase of QCD. The simulations were done in the quenched approximation, on a lattice of size 32 3 8. We examined temperatures ranging from 0:75T c up to 0:92T c. We see no sign of a temperature dependence in the chiral condensate or the meson properties, but some temperature dependence for the nucleon screening mass is not excluded.
We discuss the properties of hadronic fluctuations, i.e. fluctuations of net quark and isospin nu... more We discuss the properties of hadronic fluctuations, i.e. fluctuations of net quark and isospin numbers as well as electric charge, in the vicinity of the QCD transition in isospin-symmetric matter at vanishing quark chemical potential. We analyse second-and fourth-order cumulants of these fluctuations and argue that the ratio of quartic and quadratic cumulants reflects the relevant degrees of freedom that carry the quark number, isospin or charge, respectively. In the hadronic phase we find that an enhancement of charge fluctuations arises from contributions of doubly charged hadrons to the thermodynamics. The rapid suppression of fluctuations seen in the high-temperature phase suggests that in the QGP, net quark number and electric charge are predominantly carried by quasi-particles, with the quantum numbers of quarks.
The p t-differential inclusive production cross sections of the prompt charmed mesons D0, D+, and... more The p t-differential inclusive production cross sections of the prompt charmed mesons D0, D+, and D*+ in the rapidity range |y| < 0.5 were measured in proton-proton collisions at $ \sqrt {s} = 7{ }TeV $ at the LHC using the ALICE detector. Reconstructing the decays D0 → K−π+, D+ → K−π+π+, D*+ → D0π+, and their charge conjugates, about 8,400 D0, 2,900 D+, and 2,600 D*+ mesons with 1 < p t < 24 GeV/c were counted, after selection cuts, in a data sample of 3.14 × 108 events collected with a minimum-bias trigger (integrated luminosity L int = 5 nb−1). The results are described within uncertainties by predictions based on perturbative QCD.
Proceedings of 5th International Workshop on Critical Point and Onset of Deconfinement — PoS(CPOD 2009)
The statistical model assuming chemical equilibrium and local strangeness conservation describes ... more The statistical model assuming chemical equilibrium and local strangeness conservation describes most of the observed features of strange particle production from SIS up to RHIC. Deviations are found as the maximum in the measured K + /π + ratio is much sharper than in the model calculations. At the incident energy of the maximum, the statistical model shows that freeze out changes regime from one being dominated by baryons at the lower energies toward one being dominated by mesons. It will be shown how deviations from the usual freeze-out curve influence the various particle ratios. Furthermore, other observables exhibit also changes just in this energy regime.
Particle production in p+p and central Pb+Pb collisions at LHC is discussed in the context of the... more Particle production in p+p and central Pb+Pb collisions at LHC is discussed in the context of the statistical thermal model. For heavy-ion collisions, predictions of various particle ratios are presented. The sensitivity of several ratios on the temperature and the baryon chemical potential is studied in detail, and some of them, which are particularly appropriate to determine the chemical freeze-out point experimentally, are indicated. Considering elementary interactions on the other hand, we focus on strangeness production and its possible suppression. Extrapolating the thermal parameters to LHC energy, we present predictions of the statistical model for particle yields in p+p collisions. We quantify the strangeness suppression by the correlation volume parameter and discuss its influence on particle production. We propose observables that can provide deeper insight into the mechanism of strangeness production and suppression at LHC.
A kinetic master equation for multiplicity distributions is formulated for charged particles whic... more A kinetic master equation for multiplicity distributions is formulated for charged particles which are created or destroyed only in pairs due to the conservation of their Abelian charge. It allows one to study time evolution of the multiplicity distributions in a relativistic many-body system with arbitrary average particle multiplicities. It is shown to reproduce the equilibrium results for both canonical (rare particles) and grand canonical (abundant particles) systems. For canonical systems, the equilibrium multiplicity is much lower and the relaxation time is much shorter than the naive extrapolation from grand canonical results. Implications for chemical equilibration in heavy-ion collisions are also discussed.
The properties of net quark number fluctuations in the vicinity of the QCD chiral phase transitio... more The properties of net quark number fluctuations in the vicinity of the QCD chiral phase transition are discussed in terms of an effective chiral model in the mean-field approximation. We focus on the ratio of the fourth-to second-order cumulants (kurtosis) and the compressibility of the system and discuss their dependence on the pion mass. It is shown that near the chiral phase transition, both observables are sensitive to the value of mπ. For physical mπ, the kurtosis exhibits a peak whereas the inverse compressibility shows a dip at the pseudocritical temperature. These structures disappear for large mπ. Our results, obtained in an effective model with two flavors, are qualitatively consistent with recent results of 2+1 flavor lattice gauge theory. We also discuss the high-and lowtemperature properties of these observables and the role of the coupling of the quark degrees of freedom to the Polyakov loop.
We present results of a simulation of two flavor QCD on a 16 3 ×4 lattice using p4-improved stagg... more We present results of a simulation of two flavor QCD on a 16 3 ×4 lattice using p4-improved staggered fermions with bare quark mass m/T = 0.4. Derivatives of the thermodynamic grand canonical partition function Z(V, T, µ u , µ d) with respect to chemical potentials µ u,d for different quark flavors are calculated up to sixth order, enabling estimates of the pressure and the quark number density as well as the chiral condensate and various susceptibilities as functions of µ q = (µ u + µ d)/2 via Taylor series expansion. Furthermore, we analyze baryon as well as isospin fluctuations and discuss the relation between the radius of convergence of the Taylor series and the chiral critical point in the QCD phase diagram. We argue that bulk thermodynamic observables do not, at present, provide direct evidence for the existence of a chiral critical point in the QCD phase diagram. Results are compared to high temperature perturbation theory as well as a hadron resonance gas model.
We compare recent lattice studies of QCD thermodynamics at non-zero quark chemical potential with... more We compare recent lattice studies of QCD thermodynamics at non-zero quark chemical potential with the thermodynamics of a hadron resonance gas. We argue that for T ≤ T c the equation of state derived from Monte-Carlo simulations of two flavour QCD at non-zero chemical potential can be well described by a hadron resonance gas when using the same set of approximations as used in current lattice calculations. We estimate the importance of truncation errors arising from the use of a Taylor expansion in terms of the quark chemical potential and examine the influence of unphysically large quark masses on the equation of state and the critical conditions for deconfinement.
The production of Ξ − and Ω − baryons and their anti-particles in Pb-Pb collisions at √ s NN = 2.... more The production of Ξ − and Ω − baryons and their anti-particles in Pb-Pb collisions at √ s NN = 2.76 TeV has been measured using the ALICE detector. The transverse momentum spectra at mid-rapidity (|y| < 0.5) for charged Ξ and Ω hyperons have been studied in the range 0.6 < p T < 8.0 GeV/c and 1.2 < p T < 7.0 GeV/c, respectively, and in several centrality intervals (from the most central 0-10% to the most peripheral 60-80% collisions). These spectra have been compared with the predictions of recent hydrodynamic models. In particular, the Kraków and EPOS models give a satisfactory description of the data, with the latter covering a wider p T range. Mid-rapidity yields, integrated over p T , have been determined. The hyperon-to-pion ratios are similar to those at RHIC: they rise smoothly with centrality up to N part ∼ 150 and saturate thereafter. The enhancements (yields per participant nucleon relative to those in pp collisions) increase both with the strangeness content of the baryon and with centrality, but are less pronounced than at lower energies.
The transverse momentum (p T) distribution of primary charged particles is measured at midrapidit... more The transverse momentum (p T) distribution of primary charged particles is measured at midrapidity in minimum-bias p-Pb collisions at √ s NN = 5.02 TeV with the ALICE detector at the LHC in the range 0.15 < p T < 50 GeV/c. The spectra are compared to the expectation based on binary collision scaling of particle production in pp collisions, leading to a nuclear modification factor consistent with unity for p T larger than 2 GeV/c. The measurement is compared to theoretical calculations and to data in Pb-Pb collisions at √ s NN = 2.76 TeV.
A review of meson emission in heavy ion collisions at incident energies from SIS up to collider e... more A review of meson emission in heavy ion collisions at incident energies from SIS up to collider energies is presented. A statistical model assuming chemical equilibrium and local strangeness conservation (i.e. strangeness conservation per collision) explains most of the observed features. Emphasis is put onto the study of K + and K − emission at low incident energies. In the framework of this statistical model it is shown that the experimentally observed equality of K + and K − rates at "threshold-corrected" energies √ s − √ s th is due to a crossing of two excitation functions. Furthermore, the independence of the K + to K − ratio on the number of participating nucleons observed between SIS and RHIC is consistent with this model. It is demonstrated that the K − production at SIS energies occurs predominantly via strangeness exchange and this channel is approaching chemical equilibrium. The observed maximum in the K + /π + excitation function is also seen in the ratio of strange to non-strange particle production. The appearance of this maximum around 30 A•GeV is due to the energy dependence of the chemical freeze-out parameters T and µ B .
We present the first wide-range measurement of the charged-particle pseudorapidity density distri... more We present the first wide-range measurement of the charged-particle pseudorapidity density distribution, for different centralities (the 0-5%, 5-10%, 10-20%, and 20-30% most central events) in Pb-Pb collisions at √ s NN = 2.76 TeV at the LHC. The measurement is performed using the full coverage of the ALICE detectors, −5.0 < η < 5.5, and employing a special analysis technique based on collisions arising from LHC 'satellite' bunches. We present the pseudorapidity density as a function of the number of participating nucleons as well as an extrapolation to the total number of produced charged particles (N ch = 17165 ± 772 for the 0-5% most central collisions). From the measured dN ch /dη distribution we derive the rapidity density distribution, dN ch /dy, under simple assumptions. The rapidity density distribution is found to be significantly wider than the predictions of the Landau model. We assess the validity of longitudinal scaling by comparing to lower energy results from RHIC. Finally the mechanisms of the underlying particle production are discussed based on a comparison with various theoretical models.
We have investigated hadron screening masses, the chiral condensate, and the pion decay constant ... more We have investigated hadron screening masses, the chiral condensate, and the pion decay constant close to the decon nement phase transition in the con ned phase of QCD. The simulations were done in the quenched approximation, on a lattice of size 32 3 8. We examined temperatures ranging from 0:75T c up to 0:92T c. We see no sign of a temperature dependence in the chiral condensate or the meson properties, but some temperature dependence for the nucleon screening mass is not excluded.
We discuss the properties of hadronic fluctuations, i.e. fluctuations of net quark and isospin nu... more We discuss the properties of hadronic fluctuations, i.e. fluctuations of net quark and isospin numbers as well as electric charge, in the vicinity of the QCD transition in isospin-symmetric matter at vanishing quark chemical potential. We analyse second-and fourth-order cumulants of these fluctuations and argue that the ratio of quartic and quadratic cumulants reflects the relevant degrees of freedom that carry the quark number, isospin or charge, respectively. In the hadronic phase we find that an enhancement of charge fluctuations arises from contributions of doubly charged hadrons to the thermodynamics. The rapid suppression of fluctuations seen in the high-temperature phase suggests that in the QGP, net quark number and electric charge are predominantly carried by quasi-particles, with the quantum numbers of quarks.
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Papers by K. Redlich