High-multiplicity Pp Collisions Research Articles

Multiplicity dependence of light-flavor hadron production in pp collisions at s=7TeV

Comprehensive results on the production of unidentified charged particles, $\pi^{\pm}$, $\rm{K}^{\pm}$, $\rm{K}^{0}_{S}$, $\rm{K}$*(892)$^{0}$, $\rm{p}$, $\overline{\rm{p}}$, $\phi$(1020), $\Lambda$, $\overline{\Lambda}$, $\Xi^{-}$, $\overline{\Xi}^{+}$, $\Omega^{-}$ and $\overline{\Omega}^{+}$ hadrons in proton-proton (pp) collisions at $\sqrt{s}$ = 7 TeV at midrapidity ($|y| < 0.5$) as a function of charged-particle multiplicity density are presented. In order to avoid auto-correlation biases, the actual transverse momentum ($p_{\rm{T}}$) spectra of the particles under study and the event activity are measured in different rapidity windows. In the highest multiplicity class, the charged-particle density reaches about 3.5 times the value measured in inelastic collisions. While the yield of protons normalized to pions remains approximately constant as a function of multiplicity, the corresponding ratios of strange hadrons to pions show a significant enhancement that increases with increasing strangeness content. Furthermore, all identified particle to pion ratios are shown to depend solely on charged-particle multiplicity density, regardless of system type and collision energy. The evolution of the spectral shapes with multiplicity and hadron mass shows patterns that are similar to those observed in p-Pb and Pb-Pb collisions at LHC energies. The obtained $p_{\rm{T}}$ distributions and yields are compared to expectations from QCD-based pp event generators as well as to predictions from thermal and hydrodynamic models. These comparisons indicate that traces of a collective, equilibrated system are already present in high-multiplicity pp collisions.

Elliptic flow of identified hadrons in small collisional systems measured with ALICE

Recent observations of long-range multi-particle azimuthal correlations in p–Pb and high multiplicity pp collisions provided new insights into collision dynamics and opened a possibility to study collective effects in these small systems. New measurements of pT-differential elliptic flow (v2) coefficient for inclusive charged hadrons as well as a variety of identified particle species in p–Pb collisions at sNN=5.02TeV are presented. The data was collected by ALICE during the LHC Run 2 data taking. Besides high precision measurements of v2(pT) for π±, K± and p(p‾), the first results for KS0, Λ(Λ‾) and ϕ are shown. In order to eliminate non-flow contamination, a pseudorapidity separation between correlated particles is applied as well as subtraction of remaining non-flow estimate based on a measurement of minimum-bias pp collisions at s=13TeV. Moreover, the reported characteristic mass ordering and approximate number of constituent quarks (NCQ) and transverse kinetic energy (KET) scaling of v2 allow to test various theoretical models, constrain the initial conditions, and probe the origin of collective behavior in small collision systems.

Nuclear modification factors of strange and multi-strange particles in pPb collisions with the CMS experiment

Recent observation of collective effects in high-multiplicity pp and pA collisions raise the question of whether QGP can also be formed in the smaller systems. Systematic studies of the strange particle abundance and nuclear modification factors can shed light on this issue. The CMS experiment has excellent strange-particle reconstruction capabilities over a broad kinematic range in pp and pPb collisions. The spectra of and hadrons have been measured in various rapidity regions as a function of pT in pp and pPb collisions at 5.02 TeV. Based on the measurements of these spectra, nuclear modification factors of , and in mid-rapidity are measured. Since pPb is an asymmetric system, the nuclear modification factor of , and in Pb-going direction are compared to those in p-going direction. These final results for nuclear modification factors measured out to high-pT can be helpful in discussing the implications of collective behavior and energy loss. In addition, the measurement of the forward-backward rapidity yield asymmetries of and Λ as a function of pT provide sensitivity to initial state effects, such as shadowing in the nuclear parton distributions.

Hydrodynamic collectivity in proton–proton collisions at 13 TeV

In this paper, we investigate the hydrodynamic collectivity in proton–proton (p–p) collisions at 13 TeV, using iEBE-VISHNU hybrid model with HIJING initial conditions. With properly tuned parameters, our model simulations can remarkably describe all the measured 2-particle correlations, including integrated and differential elliptic flow coefficients for all charged and identified hadrons (KS0, Λ). However, our model calculations show positive 4-particle cumulant c2{4} in high multiplicity pp collisions, and can not reproduce the negative c2{4} measured in experiment. Further investigations on the HIJING initial conditions show that the fluctuations of the second order anisotropy coefficient ε2 increases with the increase of its mean value, which leads to a similar trend of the flow fluctuations. For a simultaneous description of the 2- and 4- particle cumulants within the hydrodynamic framework, it is required to have significant improvements on initial condition for pp collisions, which is still lacking of knowledge at the moment.

Resonance suppression from color reconnection

We present studies that show how multi-parton interaction and color reconnection affect the hadro-chemistry in proton-proton (pp) collisions with special focus on the production of resonances using the PYTHIA8 event generator. We find that color reconnection suppresses the relative production of meson resonances such as $\rho_{0}$ and K*, providing an alternative explanation for the K*/K decrease observed in proton-proton collisions as a function of multiplicity by the ALICE collaboration. Detailed studies of the underlying mechanism causing meson resonance suppression indicate that color reconnection leads to shorter, less energetic strings whose fragmentation is less likely to produce more massive hadrons for a given quark content, therefore reducing ratios such as K*/K and $\rho_0/\pi$ in high-multiplicity pp collisions. In addition, we have also studied the effects of allowing string junctions to form and found that these may also contribute to resonance suppression.

Rope Hadronization and Strange Particle Production

Rope Hadronization is a model extending the Lund string hadronization model to describe environments with many overlapping strings, such as high multiplicity pp collisions or AA collisions. Including effects of Rope Hadronization drastically improves description of strange/non-strange hadron ratios as function of event multiplicity in all systems from e+e− to AA. Implementation of Rope Hadronization in the MC event generators Dipsy and Pythia8 is discussed, as well as future prospects for jet studies and studies of small systems.

Particle Production and Collectivity in High Multiplicity pp and pPb at the LHC

Recent unexpected evidence for collectivity in high multiplicity pp and pPb collisions at LHC energies has challenged the notion that such small systems do not exhibit any of the properties that have been used to study the quark gluon plasma in heavy ion collisions. An overview of recent results concerning particle production and collectivity in such collisions using the CMS detector at the LHC is presented.

Strange hadron production in pp, pPb, and PbPb collisions at LHC energies

Identified particle spectra provide an important tool for understanding the particle production mechanism and the dynamical evolution of the medium created in relativistic heavy ion collisions. Studies involving strange and multi-strange hadrons, such as K0S, Λ, and Ξ−, carry additional information since there is no net strangeness content in the initial colliding system. Strangeness enhancement in AA collisions with respect to pp and pA collisions has long been considered as one of the signatures for quark-gluon plasma (QGP) formation. Recent observations of collective effects in high-multiplicity pp and pA collisions raise the question of whether QGP can also be formed in the smaller systems. Systematic studies of strange particle abundance, particle ratios, and nuclear modification factors can shed light on this issue. The CMS experiment has excellent strange-particle reconstruction capabilities over a broad kinematic range, and dedicated high-multiplicity triggers in pp and pPb collisions. The spectra of K0S, Λ, and Ξ− hadrons have been measured in various multiplicity and rapidity regions as a function of pT in pp, pPb, and PbPb collisions for several collision energies. The spectral shapes and particle ratios are compared in the different collision systems for events that have the same multiplicity and interpreted in the context of hydrodynamics models.

Multiplicity dependence of jet-like two-particle correlations in pp collisions at [formula omitted] and 13 TeV with ALICE

Two-particle correlations in relative azimuthal angle (Δϕ) and pseudorapidity (Δη) have been used to study heavy-ion collision dynamics, including medium-induced jet modification. Further investigations also showed the importance of Multiple Parton Interactions (MPI) in high-multiplicity pp collisions, which are often described by pQCD-inspired models. In these proceedings the latest ALICE measurements of two-particle correlations as a function of multiplicity in pp collisions are presented using the data from Run I and Run II at the LHC.

Evidence for collective phenomena in pp collisions

Measurements of two- and multi-particle angular correlations in pp collisions at s=5, 7, and 13 TeV are presented. The data, corresponding to integrated luminosities of 1.0 pb−1 (5 TeV), 6.2 pb−1 (7 TeV), and 0.7 pb−1 (13 TeV), were collected using the CMS detector at the LHC. The second-order (v2) and third-order (v3) azimuthal anisotropy harmonics of unidentified charged particles, as well as v2 of KS0 and Λ/Λ‾ particles, are extracted from long-range two-particle correlations as functions of particle multiplicity and transverse momentum. For high-multiplicity pp events, a mass ordering is observed for the v2 values of charged hadrons (mostly pions), KS0, and Λ/Λ‾ at pT≲2 GeV/c. The v2 signals are also extracted from four- and six-particle correlations for 13 TeV pp collisions, with comparable magnitude to those from two-particle correlations. These observations strongly support the interpretation of a collective origin for the observed long-range correlations in high-multiplicity pp collisions.

Collectivity of (non-)strange hadrons in high-multiplicity pp with CMS

Observation of an enhanced long-range, near-side, two-particle correlation (known as the “Ridge”) in high-multiplicity pp and pPb collisions opened up new opportunities of exploring QCD dynamics in small collision systems. The CMS detector at the LHC has excellent capabilities of reconstructing weakly decaying strange hadrons such as , Λ and Ξ. Studies of strange hadron production and correlations in small colliding systems provide additional insights into the physical origin of the observed collective phenomena. New results for pT spectra and long-range two-particle correlations for charged particles and identified strange hadrons in high-multiplicity pp and pPb collisions are presented. The data at various collision energies for pp and pPb collisions are compared to those obtained for PbPb at similar multiplicities. Multi-particle cumulants for pp and pPb events are studied in order to more fully explore the collective nature of the long-range correlations.

Azimuthal anisotropies and initial-state fluctuations from SPS to LHC energies

The v3 coefficient, obtained using the PbAu data from the CERES detector at the top SPS energy of √sNN = 17.3 GeV, is presented. The v2 is measured over a pT range up to 100 GeV/c in PbPb collisions collected with the CMS detector. The v2{2} of charged and strange particles emitted in pp collisions shows a mass ordering effect. The v2{4} and v2{6} are comparable to the v2{2}, and thus supports the collective nature of the long-range correlations in high-multiplicity pp collisions at 13 TeV. Principle Component Analysis (PCA) of two-particle harmonics (VnΔ) is studied in PbPb and high-multiplicity pPb collisions at the LHC. The factorization breaking of the VnΔ can be attributed to the effect of initial-state fluctuations. Using a PCA, the VnΔ are characterized through the leading and sub-leading modes. The leading modes are essentially equivalent to the vn{2}. The sub-leading modes represent the largest sources of factorization breaking.

Overview of ALICE results on hadronic resonance production

The measurement of hadronic resonance production in heavy-ion collisions is a valuable tool to study the properties of the hadronic phase. In addition, these measurements contribute to the study of particle production mechanisms, such as recombination and statistical hadronization, and can give information on the parton energy loss in the hot QCD medium. Measurement of a wide set of resonances with different lifetimes is useful to better characterize the hadronic phase and the time span between chemical and thermal freezeout. Proton-proton (pp) collisions have been used extensively as a reference for the study of larger colliding systems, but recent measurements performed in high-multiplicity pp and proton-lead (p–Pb) collisions at the LHC have shown features that are reminiscent of those observed in lead-lead (Pb–Pb) collisions. Resonance measurements in small systems serve as a reference for heavy-ion collisions and contribute to searches for collective effects. An overview of recent results on hadronic resonance production measured in ALICE will be presented. Transverse momentum ( p T ) spectra, ratios of yield to that of long-lived hadrons of the K*(892) 0 and ϕ (1020) mesons in pp, p–Pb, and Pb–Pb collisions at LHC energies will be discussed. The most recent results include the measurement of resonance production in pp collisions at 7 TeV as a function of the charged-particle multiplicity, that will be compared to the results for other light hadrons as pions, kaons, protons, K S 0 , Λ, Ξ − , Ω − . In addition, the p T spectra and yields of the ρ (770) 0 meson in pp and Pb–Pb collisions at √ s NN = 2.76 TeV and of Σ(1385) ± and Ξ(1530) 0 baryons in p–Pb collisions at √ s NN = 5.02 TeV will be presented.

Identified particle production in pp collisions at = 7 and 13 TeV measured with ALICE

Proton-proton (pp) collisions have been used extensively as a reference for the study of interactions of larger colliding systems at the LHC. Recent measurements performed in high-multiplicity pp and proton-lead (p-Pb) collisions have shown features that are reminiscent of those observed in lead-lead (Pb-Pb) collisions. In this context, the study of identified particle spectra and yields as a function of multiplicity is a key tool for the understanding of similarities and differences between small and large systems. We report on the production of pions, kaons, protons, , Λ, Ξ, Ω and K*0 as a function of multiplicity in pp collisions at = 7 TeV measured with the ALICE experiment. The work presented here represents the most comprehensive set of results on identified particle production in pp collisions at the LHC. Spectral shapes, studied both for individual particles and via particle ratios as a function of pT, exhibit an evolution with charged particle multiplicity that is similar to the one observed in larger systems. In addition, results on the production of light flavour hadrons in pp collisions at = 13 TeV, the highest centre-of-mass energy ever reached in the laboratory, are also presented and compared with previous, lower energy results.

Femtoscopic Signature of Strong Radial Flow in High-multiplicityppCollisions

We report on the results of the pion HBT radii for high-multiplicity pp collisions calculated in relativistic hydrodynamics. By comparison with the ALICE data, we make an estimate of he magnitude of the radial flow and initial size of the system.

Azimuthal anisotropy harmonics from long-range correlations in high multiplicity pp collisions at CMS

Measurements of two-particle angular correlations in pp collisions at s=7 TeV are presented as a function of charged-particle multiplicities. The data, corresponding to an integrated luminosity of about 6.2 pb−1, were collected during the 2010 LHC pp run using the CMS detector. In high-multiplicity events, a long-range (|Δη|>2), near-side (Δϕ≈0) structure is found in the two-particle Δη−Δϕ correlation functions. The second-order (v2) and third-order (v3) azimuthal anisotropy harmonics of charged particles, KS0 and Λ/Λ‾ particles are extracted from long-range two-particle correlations as a function of particle multiplicity and transverse momentum, after correcting for the contribution of back-to-back jet correlations. A v2 and v3 value of about 4% and 1%, averaging over 0.3<pT<3 GeV/c, are observed in the high-multiplicity region, and are found to be smaller than values obtained in pPb and PbPb collisions at similar multiplicities.

Evidence for collectivity in pp collisions at the LHC

Measurements of two- and multi-particle angular correlations in pp collisions at sqrt(s) = 5, 7, and 13 TeV are presented as a function of charged-particle multiplicity. The data, corresponding to integrated luminosities of 1.0 inverse picobarn (5 TeV), 6.2 inverse picobarns (7 TeV), and 0.7 inverse picobarns (13 TeV), were collected using the CMS detector at the LHC. The second-order (v[2]) and third-order (v[3]) azimuthal anisotropy harmonics of unidentified charged particles, as well as v[2] of K0 short and Lambda/anti-Lambda particles, are extracted from long-range two-particle correlations as functions of particle multiplicity and transverse momentum. For high-multiplicity pp events, a mass ordering is observed for the v[2] values of charged hadrons (mostly pions), K0 short, and Lambda/anti-Lambda, with lighter particle species exhibiting a stronger azimuthal anisotropy signal below pt of about 2 GeV/c. For 13 TeV data, the v[2] signals are also extracted from four- and six-particle correlations for the first time in pp collisions, with comparable magnitude to those from two-particle correlations. These observations are similar to those seen in pPb and PbPb collisions, and support the interpretation of a collective origin for the observed long-range correlations in high-multiplicity pp collisions.

ϒ(nS) polarizations versus particle multiplicity in pp collisions at s=7 TeV

The polarizations of the Y(1S), Y(2S), and Y(3S) mesons are measured as a function of the charged particle multiplicity in proton-proton collisions at sqrt(s) = 7 TeV. The measurements are performed with a dimuon data sample collected in 2011 by the CMS experiment, corresponding to an integrated luminosity of 4.9 inverse femtobarns. The results are extracted from the dimuon decay angular distributions, in two ranges of Y(nS) transverse momentum (10-15 and 15-35 GeV), and in the rapidity interval abs(y) < 1.2. The results do not show significant changes from low- to high-multiplicity pp collisions, although large uncertainties preclude definite statements in the Y(2S) and Y(3S) cases.

Possibility of formation of a disoriented chiral condensate inppcollisions at energies available at the CERN Large Hadron Collider via the reaction-diffusion equation

There are indications of formation of a thermalized medium in high multiplicity pp collisions at LHC energy. It is possible that such a medium may reach high enough energy density/temperature so that a transient stage of quark-gluon plasma, where chiral symmetry is restored, may be achieved. Due to rapid 3-dimensional expansion, the system will quickly cool undergoing spontaneous chiral symmetry breaking transition. We study the dynamics of chiral field, after the symmetry breaking transition, for such an event using reaction-diffusion equation approach which we have recently applied for studying QCD transitions in relativistic heavy-ion collisions. We show that the interior of such a rapidly expanding system is likely to lead to the formation of a single large domain of disoriented chiral condensate (DCC) which has been a subject of intensive search in earlier experiments. We argue that large multiplicity pp collisions naturally give rise to required boundary conditions for the existence of slowly propagating front solutions of reaction-diffusion equation with resulting dynamics of chiral field leading to the formation of a large DCC domain.

Flow of strange and charged particles in pPb and PbPb collisions at LHC energies

Observation of a long-range near-side two-particle correlation (known as the“Ridge”) in high-multiplicity pPb and pp collisions opened up new opportunities of exploring novel QCD dynamics in small collision systems. Latest CMS results in pPb and PbPb collisions will be shown: (1) The multi-particle correlation in pPb collisions will be presented for the high multiplicity events, indicating the collective behavior in small collision systems. (2) Identified pT spectra of π+/π-, K+/K-, and p/p¯ in pPb collisions show a strong multiplicity dependence, which indicates radial flow at high multiplicity events. (3) The second-order anisotropy harmonics (v2) of strange particle K0s and Λ/Λ¯ are extracted from long-range correlations as a function of particle multiplicity and pT. The mass ordering effect of vn at low pT as predicted by hydrodynamics also points to the strong collective nature of expanding medium in small collision systems. Finally, the possible constituent quark number of scaling of v2 between mesons and baryons may indicate the deconfinement in small systems.