9 research outputs found
Constraining R-axion models through dijet searches at the LHC
The search at hadron colliders for new massive resonances of a few 100 GeVs that couple effectively to colored states is an extremely challenging issue, due principally to the presence of large QCD multijet backgrounds at this energy, rendering the searches at the LHC particularly difficult. Recently, it was realized that these large backgrounds could be overcome by demanding one high-pT jet from initial-state radiation (ISR) and by means of novel jet-reconstruction techniques through which the resulting hadronized products of the massive resonances are reconstructed as a fat-jet, a unique large-radius jet. The ATLAS and CMS Collaborations have recently reported searches for the experimental signature of a single fat-jet in association with an ISR jet. Models of dynamical supersymmetry breaking with an spontaneously broken R-symmetry give rise to the appearance of a pseudo-Nambu?Goldstone boson called the R-axion, which naturally tends to be light. In the parameter space regions where the anomalous R-axion coupling to gluons is boosted, these models can be tested against these new LHC dijet searches. Taking into account the CMS search, we apply the qμ statistical method to the signal events against the background-only expectation and obtain the 95% C.L. exclusion limits on the most relevant model parameters for a particular messenger sector, namely, the R-axion mass ma, the decay constant fa, and the number of color messengers N, being these limits suitable to be applied to more general models with axion-like particles.Fil: Arganda Carreras, Ernesto. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Física La Plata. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Instituto de Física La Plata; Argentina. Universidad Autónoma de Madrid; EspañaFil: Medina, Anibal Damian. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Física La Plata. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Instituto de Física La Plata; ArgentinaFil: Mileo, Nicolás Ismael. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Física La Plata. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Instituto de Física La Plata; ArgentinaFil: Morales, Roberto Anibal. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Física La Plata. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Instituto de Física La Plata; ArgentinaFil: Szynkman, Alejandro Andrés. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Física La Plata. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Instituto de Física La Plata; Argentin
Interpretation of LHC excesses in ditop and ditau channels as a 400-GeV pseudoscalar resonance
Since the discovery in 2012 of the Higgs boson at the LHC, as the last missing piece of the Standard Model of particle physics, any hint of new physics has been intensively searched for, with no confirmation to date. There are however slight deviations from the SM that are worth investigating. The CMS collaboration has reported, in a search for heavy resonances decaying in tt¯ with a 13-TeV center-of-mass energy and a luminosity of 35.9 fb−1, deviations from the SM predictions at the 3.5σ level locally (1.9σ after the look-elsewhere effect). In addition, in the ditau final state search performed by the ATLAS collaboration at s = 13 TeV and L = 139 fb−1, deviations from the SM at the 2σ level have been also observed. Interestingly, both slight excesses are compatible with a new pseudoscalar boson with a mass around 400 GeV that couples at least to fermions of the third generation and gluons. Starting from a purely phenomenological perspective, we inspect the possibility that a 400-GeV pseudoscalar can account for these deviations and at the same time satisfy the constraints on the rest of the channels that it gives contributions to and that are analyzed by the ATLAS and CMS experiments. After obtaining the range of effective couplings compatible with all experimental measurements, we study the gauge invariant UV completions that can give rise to this type of pseudoscalar resonance, which can be accommodated in an SO(6)/SO(5) model with consistency at the 1σ level and in a SO(5) × U(1)P × U(1)X/SO(4) × U(1)X at the 2σ level, while exceedingly large quartic couplings would be necessary to account for it in a general two Higgs doublet model.Fil: Arganda Carreras, Ernesto. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Física La Plata. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Instituto de Física La Plata; ArgentinaFil: Da Rold, Leandro. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte; Argentina. Comisión Nacional de Energía Atómica. Gerencia del Área de Energía Nuclear. Instituto Balseiro; ArgentinaFil: Díaz, Daniel A.. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Física La Plata. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Instituto de Física La Plata; ArgentinaFil: Medina, Anibal Damian. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Física La Plata. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Instituto de Física La Plata; Argentin
Probing strangeness hadronization with event-by-event production of multistrange hadrons
This Letter presents the first measurement of event-by-event fluctuations of the net number (difference between the particle and antiparticle multiplicities) of multistrange hadrons and and its correlation with the net-kaon number using the data collected by the ALICE Collaboration in pp, p-Pb, and Pb-Pb collisions at a center-of-mass energy per nucleon pair . The statistical hadronization model with a correlation over three units of rapidity between hadrons having the same and opposite strangeness content successfully describes the results. On the other hand, string-fragmentation models that mainly correlate strange hadrons with opposite strange quark content over a small rapidity range fail to describe the data.This Letter presents the first measurement of event-by-event fluctuations of the net number (difference between the particle and antiparticle multiplicities) of multistrange hadrons and and its correlation with the net-kaon number using the data collected by the ALICE Collaboration in pp, pPb, and PbPb collisions at a center-of-mass energy per nucleon pair . The statistical hadronization model with a correlation over three units of rapidity between hadrons having the same and opposite strangeness content successfully describes the results. On the other hand, string-fragmentation models that mainly correlate strange hadrons with opposite strange quark content over a small rapidity range fail to describe the data
Charm fragmentation fractions and cross section in pPb collisions at TeV
The total charm-quark production cross section per unit of rapidity , and the fragmentation fractions of charm quarks to different charm-hadron species , are measured for the first time in pPb collisions at TeV at midrapidity ( in the centre-of-mass frame) using data collected by ALICE at the CERN LHC. The results are obtained based on all the available measurements of prompt production of ground-state charm-hadron species: , , , and mesons, and and baryons. The resulting cross section is mb, which is consistent with a binary scaling of pQCD calculations from pp collisions. The measured fragmentation fractions are compatible with those measured in pp collisions at and TeV, showing an increase in the relative production rates of charm baryons with respect to charm mesons in pp and p–Pb collisions compared with and collisions. The -integrated nuclear modification factor of charm quarks, , is found to be consistent with unity and with theoretical predictions including nuclear modifications of the parton distribution functions.The total charm-quark production cross section per unit of rapidity , and the fragmentation fractions of charm quarks to different charm-hadron species , are measured for the first time in pPb collisions at TeV at midrapidity ( in the centre-of-mass frame) using data collected by ALICE at the CERN LHC. The results are obtained based on all the available measurements of prompt production of ground-state charm-hadron species: , , , and mesons, and and baryons. The resulting cross section is mb, which is consistent with a binary scaling of pQCD calculations from pp collisions. The measured fragmentation fractions are compatible with those measured in pp collisions at and TeV, showing an increase in the relative production rates of charm baryons with respect to charm mesons in pp and pPb collisions compared with and collisions. The -integrated nuclear modification factor of charm quarks, , is found to be consistent with unity and with theoretical predictions including nuclear modifications of the parton distribution functions
Investigating strangeness enhancement in jet and medium via (1020) production in p-Pb collisions at = 5.02 TeV
This work aims to differentiate strangeness produced from hard processes (jet-like) and softer processes (underlying event) by measuring the angular correlation between a high-momentum trigger hadron (h) acting as a jet-proxy and a produced strange hadron ( meson). Measuring h correlations at midrapidity in pPb collisions at = 5.02 TeV as a function of event multiplicity provides insight into the microscopic origin of strangeness enhancement in small collision systems. The jet-like and the underlying-event-like strangeness production are investigated as a function of event multiplicity. They are also compared between a lower and higher momentum region. The evolution of the per-trigger yields within the near-side (aligned with the trigger hadron) and away-side (in the opposite direction of the trigger hadron) jet is studied separately, allowing for the characterization of two distinct jet-like production regimes. Furthermore, the h correlations within the underlying event give access to a production regime dominated by soft production processes, which can be compared directly to the in-jet production. Comparisons between h and dihadron correlations show that the observed strangeness enhancement is largely driven by the underlying event, where the ratio is significantly larger than within the jet regions. As multiplicity increases, the fraction of the total yield coming from jets decreases compared to the underlying event production, leading to high-multiplicity events being dominated by the increased strangeness production from the underlying event.This work aims to differentiate strangeness produced from hard processes (jet-like) and softer processes (underlying event) by measuring the angular correlation between a high-momentum trigger hadron (h) acting as a jet-proxy and a produced strange hadron ( meson). Measuring h correlations at midrapidity in pPb collisions at = 5.02 TeV as a function of event multiplicity provides insight into the microscopic origin of strangeness enhancement in small collision systems. The jet-like and the underlying-event-like strangeness production are investigated as a function of event multiplicity. They are also compared between a lower and higher momentum region. The evolution of the per-trigger yields within the near-side (aligned with the trigger hadron) and away-side (in the opposite direction of the trigger hadron) jet is studied separately, allowing for the characterization of two distinct jet-like production regimes. Furthermore, the h correlations within the underlying event give access to a production regime dominated by soft production processes, which can be compared directly to the in-jet production. Comparisons between h and dihadron correlations show that the observed strangeness enhancement is largely driven by the underlying event, where the ratio is significantly larger than within the jet regions. As multiplicity increases, the fraction of the total yield coming from jets decreases compared to the underlying event production, leading to high-multiplicity events being dominated by the increased strangeness production from the underlying event
Investigating baryon production in pPb collisions in jets and underlying event using angular correlations
This paper presents the first measurements of hadron(h) azimuthal angular correlations in pPb collisions at = 5.02 TeV using the ALICE detector at the LHC. These correlations are used to separate the production of associated baryons into three different kinematic regions, namely those produced in the direction of the trigger particle (near-side), those produced in the opposite direction (away-side), and those whose production is uncorrelated with the jet-axis (underlying event). The per-trigger associated yields in these regions are extracted, along with the near- and away-side azimuthal peak widths, and the results are studied as a function of associated particle and event multiplicity. Comparisons with the DPMJET event generator and previous measurements of the meson are also made. The final results indicate that strangeness production in the highest multiplicity pPb collisions is enhanced relative to low multiplicity collisions in the jet-like regions, as well as the underlying event. The production of relative to charged hadrons is also enhanced in the underlying event when compared to the jet-like regions. Additionally, the results hint that strange quark production in the away-side of the jet is modified by soft interactions with the underlying event.First measurements of hadron(h) azimuthal angular correlations in pPb collisions at = 5.02 TeV using the ALICE detector at the LHC are presented. These correlations are used to separate the production of associated baryons into three different kinematic regions, namely those produced in the direction of the trigger particle (near-side), those produced in the opposite direction (away-side), and those whose production is uncorrelated with the jet-axis (underlying event). The per-trigger associated yields in these regions are extracted, along with the near- and away-side azimuthal peak widths, and the results are studied as a function of associated particle and event multiplicity. Comparisons with the DPMJET event generator and previous measurements of the meson are also made. The final results indicate that strangeness production in the highest multiplicity pPb collisions is enhanced relative to low multiplicity collisions in the jet-like regions, as well as the underlying event. The production of relative to charged hadrons is also enhanced in the underlying event when compared to the jet-like regions. Additionally, the results hint that strange quark production in the away-side of the jet is modified by soft interactions with the underlying event
Measurement of production in PbPb collisions at = 5.02 TeV
The first measurement of and differential production with respect to transverse momentum and centrality in PbPb collisions at ~TeV is presented. The has been reconstructed via its two-charged-body decay channel, i.e., . A Blast-Wave model fit of the -differential spectra of all nuclear species measured by the ALICE collaboration suggests that the kinetic freeze-out surface is consistent with that of other nuclei. The ratio between the integrated yields of and is compared to predictions from the statistical hadronisation model and the coalescence model, with the latter being favoured by the presented measurements.The first measurement of and differential production with respect to transverse momentum and centrality in PbPb collisions at ~TeV is presented. The has been reconstructed via its two-charged-body decay channel, i.e., . A Blast-Wave model fit of the -differential spectra of all nuclear species measured by the ALICE collaboration suggests that the kinetic freeze-out surface is consistent with that of other nuclei. The ratio between the integrated yields of and is compared to predictions from the statistical hadronisation model and the coalescence model, with the latter being favoured by the presented measurements
Measurement of the production cross section of prompt baryons in pPb collisions at TeV
The transverse momentum () differential production cross section of the promptly-produced charm-strange baryon (and its charge conjugate ) is measured at midrapidity via its hadronic decay into in pPb collisions at a centre-of-mass energy per nucleonnucleon collision TeV with the ALICE detector at the LHC. The nuclear modification factor (), calculated from the cross sections in pp and pPb collisions, is presented and compared with the of baryons. The ratios between the -differential production cross section of baryons and those of mesons and baryons are also reported and compared with results at forward and backward rapidity from the LHCb Collaboration. The measurements of the production cross section of prompt baryons are compared with a model based on perturbative QCD calculations of charm-quark production cross sections, which includes only cold nuclear matter effects in pPb collisions, and underestimates the measurement by a factor of about 50. This discrepancy is reduced when the data is compared with a model in which hadronisation is implemented via quark coalescence. The -integrated cross section of prompt -baryon production at midrapidity extrapolated down to = 0 is also reported. These measurements offer insights and constraints for theoretical calculations of the hadronisation process. Additionally, they provide inputs for the calculation of the charm production cross section in pPb collisions at midrapidity.The transverse momentum () differential production cross section of the promptly-produced charm-strange baryon (and its charge conjugate ) is measured at midrapidity via its hadronic decay into in pPb collisions at a centre-of-mass energy per nucleonnucleon collision TeV with the ALICE detector at the LHC. The nuclear modification factor (), calculated from the cross sections in pp and pPb collisions, is presented and compared with the of baryons. The ratios between the -differential production cross section of baryons and those of mesons and baryons are also reported and compared with results at forward and backward rapidity from the LHCb Collaboration. The measurements of the production cross section of prompt baryons are compared with a model based on perturbative QCD calculations of charm-quark production cross sections, which includes only cold nuclear matter effects in pPb collisions, and underestimates the measurement by a factor of about 50. This discrepancy is reduced when the data is compared with a model in which hadronisation is implemented via quark coalescence. The -integrated cross section of prompt -baryon production at midrapidity extrapolated down to = 0 is also reported. These measurements offer insights and constraints for theoretical calculations of the hadronisation process. Additionally, they provide inputs for the calculation of the charm production cross section in pPb collisions at midrapidity
Measurement of the production and elliptic flow of (anti)nuclei in Xe-Xe collisions at = 5.44 TeV
Measurements of (anti)deuteron and (anti)He production in the rapidity range 0.5 as a function of the transverse momentum and event multiplicity in XeXe collisions at a center-of-mass energy per nucleonnucleon pair of = 5.44 TeV are presented. The coalescence parameters and are measured as a function of the transverse momentum per nucleon. The ratios between (anti)deuteron and (anti)He yields and those of (anti)protons and pions are reported as a function of the mean charged-particle multiplicity density, and compared with two implementations of the statistical hadronization model (SHM) and with coalescence predictions. The elliptic flow of (anti)deuterons is measured for the first time in XeXe collisions and shows features similar to those already observed in PbPb collisions, i.e., the mass ordering at low transverse momentum and the mesonbaryon grouping at intermediate transverse momentum. The production of nuclei is particularly sensitive to the chemical freeze-out temperature of the system created in the collision, which is extracted from a grand-canonical-ensemble-based thermal fit, performed for the first time including light nuclei along with light-flavor hadrons in XeXe collisions. The extracted chemical freeze-out temperature = (154.2 1.1) MeV in XeXe collisions is similar to that observed in PbPb collisions and close to the crossover temperature predicted by lattice QCD calculations.Measurements of (anti)deuteron and (anti)He production in the rapidity range 0.5 as a function of the transverse momentum and event multiplicity in XeXe collisions at a center-of-mass energy per nucleonnucleon pair of = 5.44 TeV are presented. The coalescence parameters and are measured as a function of the transverse momentum per nucleon. The ratios between (anti)deuteron and (anti)He yields and those of (anti)protons and pions are reported as a function of the mean charged-particle multiplicity density, and compared with two implementations of the statistical hadronization model (SHM) and with coalescence predictions. The elliptic flow of (anti)deuterons is measured for the first time in XeXe collisions and shows features similar to those already observed in PbPb collisions, i.e., the mass ordering at low transverse momentum and the mesonbaryon grouping at intermediate transverse momentum. The production of nuclei is particularly sensitive to the chemical freeze-out temperature of the system created in the collision, which is extracted from a grand-canonical-ensemble-based thermal fit, performed for the first time including light nuclei along with light-flavor hadrons in XeXe collisions. The extracted chemical freeze-out temperature = (154.2 1.1) MeV in XeXe collisions is similar to that observed in PbPb collisions and close to the crossover temperature predicted by lattice QCD calculations