Browsing by Subject "PHYSICS"

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  • Hoyos, Carlos; Jokela, Niko; Rodriguez Fernandez, David; Vuorinen, Aleksi (2016)
    It has been conjectured that the speed of sound in holographic models with UV fixed points has an upper bound set by the value of the quantity in conformal field theory. If true, this would set stringent constraints for the presence of strongly coupled quark matter in the cores of physical neutron stars, as the existence of two-solar-mass stars appears to demand a very stiff equation of state. In this article, we present a family of counterexamples to the speed of sound conjecture, consisting of strongly coupled theories at finite density. The theories we consider include N = 4 super Yang-Mills at finite R-charge density and nonzero gaugino masses, while the holographic duals are Einstein-Maxwell theories with a minimally coupled scalar in a charged black hole geometry. We show that for a small breaking of conformal invariance, the speed of sound approaches the conformal value from above at large chemical potentials.
  • Kokkonen, Tommi; Mäntylä, Terhi (2018)
    One well-known learning obstacle is that students rarely use the concepts in the way that scientists use them. Rather, students mix up closely related concepts and are inclined towards matter-based conceptualisations. Furthermore, some researchers have argued that certain difficulties are rooted in the student's limited repertoire of causal schemes. These two aspects are conveniently represented in the recent proposal of the systemic view of concept learning. We applied this framework in our analyses of university students' explanations of DC circuits and their use of concepts such as voltage, current and resistance. Our data consist of transcribed group interviews, which we analysed with content analysis. The results of our analysis are represented with directed graphs. Our results show that students had a rather refined ontological knowledge of the concepts. However, students relied on rather simple explanation models, but few students were able to modify their explanations during the interview. Based on the analysis, we identified three processes of change: model switch, model refinement and model elaboration. This emphasises the importance of relevant relational knowledge at a later stage of learning. This demonstrates how concept individuation and learning of relational structures occurs (and in which order) and sets forth interesting research questions for future research.
  • The ALICE collaboration; Acharya, S.; Adamova, D.; Kim, D. J.; Krizek, F.; Novitzky, Norbert; Onnerstad, A.; Parkkila, J. E.; Rytkönen, Heidi Maria; Räsänen, Sami; Saarimäki, Oskari Antti Matti; Slupecki, M.; Snellman, Tomas; Trzaska, W. H. (2021)
    The coherent photoproduction of J/psi and psi' mesons was measured in ultra-peripheral Pb-Pb collisions at a center-of-mass energy root s(NN) = 5.02 TeV with the ALICE detector. Charmonia are detected in the central rapidity region for events where the hadronic interactions are strongly suppressed. The J/psi is reconstructed using the dilepton (l(+)l(-)) and proton-antiproton decay channels, while for the psi' the dilepton and the l(+)l(-) pi(+)pi(-) decay channels are studied. The analysis is based on an event sample corresponding to an integrated luminosity of about 233 mu b(-1). The results are compared with theoretical models for coherent J/psi and psi' photoproduction. The coherent cross section is found to be in a good agreement with models incorporating moderate nuclear gluon shadowing of about 0.64 at a Bjorken-x of around 6 x 10(-4), such as the EPS09 parametrization, however none of the models is able to fully describe the rapidity dependence of the coherent J/psi cross section including ALICE measurements at forward rapidity. The ratio of psi' to J/psi coherent photoproduction cross sections was also measured and found to be consistent with the one for photoproduction off protons.
  • The ALICE collaboration; Acharya, S.; Brucken, E. J.; Chang, B.; Hilden, T. E.; Kim, D. J.; Mieskolainen, M. M.; Orava, R.; Parkkila, J. E.; Rak, J.; Räsänen, S. S.; Rytkonen, H.; Slupecki, M.; Snellman, T. W.; Trzaska, W. H.; Viinikainen, J.; Litichevskyi, V.; Vargyas, M. (2019)
    The ALICE collaboration performed the first rapidity-differential measurement of coherent J/psi photoproduction in ultra-peripheral Pb-Pbcollisions at a center-of-mass energy root s(NN) = 5.02TeV. The J/psi is detected via its dimuon decay in the forward rapidity region (-4.0 <y <-2.5) 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 750 mu b(-1). The cross section for coherent J/psi production is presented in six rapidity bins. The results are compared with theoretical models for coherent J/psi photoproduction. These comparisons indicate that gluon shadowing effects play a role in the photoproduction process. The ratio of psi' to J/psi coherent photoproduction cross sections was measured and found to be consistent with that measured for photoproduction off protons. (C) 2019 The Author. Published by Elsevier B.V.
  • Adam, J.; Brucken, E. J.; Chang, B.; Hilden, T. E.; Kim, D. J.; Kral, J.; Mieskolainen, M. M.; Rak, J.; Räsänen, S. S.; Snellman, T. W.; Trzaska, W. H.; The ALICE collaboration (2015)
    We have performed the first measurement of the coherent psi(2S) photo-production cross section in ultraperipheral Pb-Pb collisions at the LHC. This charmonium excited state is reconstructed via the psi(2S) -> l(+)l(-) and ->(2S) -> J/psi pi(+)pi(-) decays, where the J/psi decays into two leptons. The analysis is based on an event sample corresponding to an integrated luminosity of about 22 mu b(-1). The cross section for coherent psi(2S) production in the rapidity interval -0.9 <y <0.9is d sigma(coh)(psi(2S))/dy = 0.83 +/- 0.19 (stat+syst) mb. The psi(2S) to J/psi coherent cross section ratio is 0.34(-0.07)(+0.08)(stat+syst). The obtained results are compared to predictions from theoretical models. (C) 2015 CERN for the benefit of the ALICE Collaboration. Published by Elsevier B.V.
  • Cordero-Cid, A.; Hernandez-Sanchez, J.; Keus, V.; Moretti, S.; Rojas-Ciofalo, D.; Sokolowska, D. (2020)
    We study an extension of the Standard Model (SM) in which two copies of the SM-Higgs doublet which do not acquire a vacuum expectation value, and hence are inert, are added to the scalar sector. The lightest particle from the inert sector, which is protected from decaying to SM particles through the conservation of a Z(2) symmetry, is a viable dark matter candidate. We allow for CP violation in this extended dark sector and evaluate the ZZZ vertex and its CP-violating form factor in several benchmark scenarios. We provide collider signatures of this dark CP violation in the form of potentially observable asymmetries and cross sections for the f (f) over bar -> Z* -> ZZ process at both leptonic and hadronic machines.
  • Dumitru, Adrian; Mäntysaari, Heikki; Paatelainen, Risto (2021)
    Color charge correlators provide fundamental information about the proton structure. In this Letter, we evaluate numerically two-point color charge correlations in a proton on the light cone including the next-to-leading order corrections due to emission or exchange of a perturbative gluon. The non-perturbative valence quark structure of the proton is modelled in a way consistent with high-x proton structure data. Our results show that the correlator exhibits startlingly non-trivial behavior at large momentum transfer or central impact parameters, and that the color charge correlation depends not only on the impact parameter but also on the relative transverse momentum of the two gluon probes and their relative angle. Furthermore, from the two-point color charge correlator, we compute the dipole scattering amplitude. Its azimuthal dependence differs significantly from a impact parameter dependent McLerran-Venugopalan model based on geometry. Our results also provide initial conditions for Balitsky-Kovchegov evolution of the dipole scattering amplitude. These initial conditions depend not only on the impact parameter and dipole size vectors, but also on their relative angle and on the light-cone momentum fraction x in the target. (C) 2021 The Author(s). Published by Elsevier B.V.
  • Borovsky, Joseph E.; Osmane, Adnane (2019)
    Using the solar-wind-driven magnetosphere-ionosphere-thermosphere system, a methodology is developed to reduce a state-vector description of a time-dependent driven system to a composite scalar picture of the activity in the system. The technique uses canonical correlation analysis to reduce the time-dependent system and driver state vectors to time-dependent system and driver scalars, with the scalars describing the response in the system that is most-closely related to the driver. This reduced description has advantages: low noise, high prediction efficiency, linearity in the described system response to the driver, and compactness. The methodology identifies independent modes of reaction of a system to its driver. The analysis of the magnetospheric system is demonstrated. Using autocorrelation analysis, Jensen- Shannon complexity analysis, and permutation-entropy analysis the properties of the derived aggregate scalars are assessed and a new mode of reaction of the magnetosphere to the solar wind is found. This state-vector-reduction technique may be useful for other multivariable systems driven by multiple inputs.
  • Ishii, Takaaki; Järvinen, Matti; Nijs, Govert (2019)
    We establish a holographic bottom-up model which covers both the baryonic and quark matter phases in cold and dense QCD. This is obtained by including the baryons using simple approximation schemes in the V-QCD model, which also includes the backreaction of the quark matter to the dynamics of pure Yang-Mills. We examine two approaches for homogeneous baryon matter: baryons as a thin layer of noninteracting matter in the holographic bulk, and baryons with a homogeneous bulk gauge field. We find that the second approach exhibits phenomenologically reasonable features. At zero temperature, the vacuum, baryon, and quark matter phases are separated by strongly first order transitions as the chemical potential varies. The equation of state in the baryonic phase is found to be stiff, i.e., the speed of sound clearly exceeds the value c2 = 1/3 of conformal plasmas at high baryon densities.
  • Keus, Venus (2020)
    Despite great agreement with experiment, the Standard Model (SM) of particle physics lacks a viable dark matter (DM) candidate and sufficient amount of CP violation to account for the observed baryon excess in the universe. Nonminimal Higgs frameworks are economic extensions of the SM which could remedy these shortcomings. Within the framework of a three Higgs doublet model, we introduce an extended dark sector which accommodates both DM and CP violation. Such dark sources of CP violation do not contribute to the electric dipole moments and are therefore unconstrained. We present a novel mechanism in which the CP-violating dark particles only interact with the SM through the gauge bosons, primarily the Z boson. Such Z-portal dark CP violation is realized in the regions of the parameter space where Higgs-mediated (co)annihilation processes are subdominant and have negligible contributions to the DM relic density. We show that such Z-portal CP-violating DM can still thermalize and satisfy all experimental and observational bounds and discuss the implications of such phenomena for electroweak baryogenesis.
  • Huitu, Katri; Rao, Kumar; Rindani, Saurabh D.; Sharma, Pankaj (2016)
    We consider the possibility of new physics giving rise to effective interactions of the form e(+) e(-) Hf (f) over bar, where f represents a charged lepton l or a (light) quark q, and H the recently discovered Higgs boson. Such vertices would give contributions beyond the standard model to the Higgs production processes e+ e-. H l(+) l(-) and e(+) e(-) -> Hq (q) over bar qat a future e(+) e(-) collider. We write the most general form for these vertices allowed by Lorentz symmetry. Assuming that such interactions contribute in addition to the standard model production processes, where the final-state fermion pair comes from the decay of the Z boson, we obtain the differential cross section for the processes e(+) e(-) -> H l(+) l(-) and e(+) e(-) -> Hq (q) over bar to linear order in the effective interactions. We propose several observables with differing CP and T properties which, if measured, can be used to constrain the couplings occurring in interaction vertices. We derive possible limits on these couplings that may be obtained at a collider with centre-of-mass energy of 500 GeV and an integrated luminosity of 500 fb(-1). We also carry out the analysis assuming that both the electron and positron beams can be longitudinally polarized, and find that the sensitivity to the couplings can be improved by a factor of 2-4 by a specific choice of the signs of the polarizations of both the electron and positron beams for the same integrated luminosity. (C) 2016 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY license.
  • Kazakov, Ye. O.; Alcator C-Mod Team; JET Contributors; Ahlgren, Tommy Juha; Aho-Mantila, L.; Airila, M.; Asunta, O.; Björkas, Carolina Maria Isabel; Groth, M.; Hakola, A.; Järvinen, A.; Karhunen, J.; Koivuranta, S.; Lahtinen, Aki Alvar; Lasa Esquisabel, Ane; Nordlund, Kai Henrik; Safi, Elnaz (2017)
    We describe a new technique for the efficient generation of high-energy ions with electromagnetic ion cyclotron waves in multi-ion plasmas. The discussed three-ion scenarios are especially suited for strong wave absorption by a very low number of resonant ions. To observe this effect, the plasma composition has to be properly adjusted, as prescribed by theory. We demonstrate the potential of the method on the world-largest plasma magnetic confinement device, JET (Joint European Torus, Culham, UK), and the high-magnetic-field tokamak Alcator C-Mod (Cambridge, USA). The obtained results demonstrate efficient acceleration of He-3 ions to high energies in dedicated hydrogendeuterium mixtures. Simultaneously, effective plasma heating is observed, as a result of the slowing-down of the fast He-3 ions. The developed technique is not only limited to laboratory plasmas, but can also be applied to explain observations of energetic ions in space-plasma environments, in particular, He-3-rich solar flares.
  • The CMS collaboration; Sirunyan, A. M.; Eerola, P.; Kirschenmann, H.; Pekkanen, J.; Voutilainen, M.; Havukainen, J.; Heikkilä, J. K.; Järvinen, T.; Karimäki, V.; Kinnunen, R.; Lampén, T.; Lassila-Perini, K.; Laurila, S.; Lehti, S.; Lindén, T.; Luukka, P.; Mäenpää, T.; Siikonen, H.; Tuominen, E.; Tuominiemi, J.; Tuuva, T. (2018)
    A measurement of the electroweak (EW) production of two jets in association with a Z boson in proton-proton collisions at root s = 13 TeV is presented, based on data recorded in 2016 by the CMS experiment at the LHC corresponding to an integrated luminosity of 35.9 fb(-1). The measurement is performed in the lljj final state with l including electrons and muons, and the jets j corresponding to the quarks produced in the hard interaction. The measured cross section in a kinematic region defined by invariant masses m(ll) > 50 GeV, m(jj) > 120 GeV, and transverse momenta P-Tj > 25 GeV is sigma(EW) (lljj) = 534 +/- 20 (stat) fb (syst) fb, in agreement with leading-order standard model predictions. The final state is also used to perform a search for anomalous trilinear gauge couplings. No evidence is found and limits on anomalous trilinear gauge couplings associated with dimension-six operators are given in the framework of an effective field theory. The corresponding 95% confidence level intervals are -2.6 <cwww/Lambda(2) <2.6 TeV-2 and -8.4 <cw/Lambda(2) <10.1 TeV-2. The additional jet activity of events in a signal-enriched region is also studied, and the measurements are in agreement with predictions.
  • The CMS collaboration; Sirunyan, A. M.; Eerola, P.; Forthomme, L.; Kirschenmann, H.; Österberg, K.; Voutilainen, M.; Garcia, F.; Havukainen, J.; Heikkilä, J. K.; Järvinen, T.; Karimäki, V.; Kinnunen, R.; Lampen, T.; Lassila-Perini, K.; Laurila, S.; Lehti, S.; Linden, T.; Luukka, P.; Mäenpää, T.; Siikonen, H.; Tuominen, E.; Tuominiemi, J.; Tuuva, T.; Pekkanen, J.; Tumasyan, A. (2020)
    A search forWWproduction from double-parton scattering processes using same-charge electron-muon and dimuon events is reported, based on proton-proton collision data collected at a center-of-mass energy of 13 TeV. The analyzed data set corresponds to an integrated luminosity of 77.4 fb-1, collected using the CMS detector at the LHC in 2016 and 2017. Multivariate classifiers are used to discriminate between the signal and the dominant background processes. A maximum likelihood fit is performed to extract the signal cross section. This leads to the first evidence for WW production via double-parton scattering, with a significance of 3.9 standard deviations. The measured inclusive cross section is 1.41 +/- 0.28 (stat) +/- 0.28 (syst) pb.
  • The ALICE collaboration; Acharya, S.; Brucken, E. J.; Hilden, T. E.; Kim, D. J.; Krizek, F.; Litichevskyi, V.; Novitzky, Norbert; Orava, R.; Parkkila, J. E.; Rak, J.; Rytkonen, H.; Räsänen, Sami; Saarimäki, Oskari Antti Matti; Slupecki, M.; Snellman, T. W.; Trzaska, W. H.; Zhou, Z. (2020)
    Measurements of K*(892)(0) and phi(1020) resonance production in Pb-Pb and pp collisions at root s(NN) = 5.02 TeV with the ALICE detector at the Large Hadron Collider are reported. The resonances are measured at midrapidity (vertical bar y vertical bar <0.5) via their hadronic decay channels and the transverse momentum (p(T)) distributions are obtained for various collision centrality classes up to p(T) = 20 GeV/c. The p(T)-integrated yield ratio K*(892)(0)/K in Pb-Pb collisions shows significant suppression relative to pp collisions and decreases towards more central collisions. In contrast, the phi(1020)/K ratio does not show any suppression. Furthermore, the measured K*(892)(0)/K ratio in central Pb-Pb collisions is significantly suppressed with respect to the expectations based on a thermal model calculation, while the phi(1020)/K ratio agrees with the model prediction. These measurements are an experimental demonstration of rescattering of K*(892)(0) decay products in the hadronic phase of the collisions. The K*(892)(0)/K yield ratios in Pb-Pb and pp collisions are used to estimate the time duration between chemical and kinetic freeze-out, which is found to be similar to 4-7 fm/c for central collisions. The p(T)-differential ratios of K*(892)(0)/K, phi(1020)/K, K*(892)(0)/pi, phi(1020)/pi, p/K*(892)(0) and p/phi(1020) are also presented for Pb-Pb and pp collisions at root s(NN) = 5.02 TeV. These ratios show that the rescattering effect is predominantly a low-p(T) phenomenon. (C) 2020 The Author(s). Published by Elsevier B.V.
  • The ALICE collaboration; Acharya, S.; Adamova, D.; Kim, D. J.; Krizek, F.; Onnerstad, A.; Parkkila, J. E.; Rytkönen, Heidi Maria; Räsänen, Sami; Saarimäki, Oskari Antti Matti; Slupecki, M.; Trzaska, W. H. (2021)
    The first measurement of the cross section for coherent J/Psi photoproduction as a function of vertical bar t vertical bar, the square of the momentum transferred between the incoming and outgoing target nucleus, is presented. The data were measured with the ALICE detector in ultra-peripheral Pb-Pbcollisions at a centre-of-mass energy per nucleon pair root s(NN) = 5.02 TeV with the J/Psi produced in the central rapidity region vertical bar y vertical bar < 0.8, which corresponds to the small Bjorken-xrange (0.3 - 1.4) x 10(-3). The measured vertical bar t vertical bar-dependence is not described by computations based only on the Pb nuclear form factor, while the photonuclear cross section is better reproduced by models including shadowing according to the leading-twist approximation, or gluon-saturation effects from the impact-parameter dependent Balitsky-Kovchegov equation. These new results are therefore a valid tool to constrain the relevant model parameters and to investigate the transverse gluonic structure at very low Bjorken- x. (C) 2021 The Author(s). Published by Elsevier B.V.
  • The CMS collaboration; TOTEM Collaboration; Tumasyan, A.; Adam, W.; Eerola, P.; Kirschenmann, H.; Voutilainen, M.; Brücken, Erik; Havukainen, J.; Heikkilä, Jaana; Karimäki, Veikko; Kim, Minsuk; Kinnunen, R.; Kortelainen, Matti; Lampén, T.; Lassila-Perini, K.; Laurila, S.; Lehti, S.; Lindén, T.; Luukka, P.; Pekkanen, Juska; Siikonen, H.; Tuominen, E.; Tuominiemi, J.; Viinikainen, Jussi; Tuuva, T.; Berretti, Mirko; Forthomme, Laurent; Garcia, F.; Naaranoja, Tiina; Oljemark, Fredrik Robert; Österberg, K.; Saarikko, Heimo; Welti, Jan Sebastian (2022)
    A search for exclusive two-photon production via photon exchange in proton-proton collisions, pp -> p gamma gamma p with intact protons, is presented. The data correspond to an integrated luminosity of 9.4 fb(-1) collected in 2016 using the CMS and TOTEM detectors at a center-of-mass energy of 13 TeVat the LHC. Events are selected with a diphoton invariant mass above 350 GeVand with both protons intact in the final state, to reduce backgrounds from strong interactions. The events of interest are those where the invariant mass and rapidity calculated from the momentum losses of the forward-moving protons match the mass and rapidity of the central, two-photon system. No events are found that satisfy this condition. Interpreting this result in an effective dimension-8 extension of the standard model, the first limits are set on the two anomalous four-photon coupling parameters. If the other parameter is constrained to its standard model value, the limits at 95% confidence level are vertical bar zeta(1)vertical bar < 2.9 x 10(-1)3 GeV-4 and vertical bar zeta(2)vertical bar < 6.0 x 10(-13) GeV-4.
  • Annala, Eemeli; Ecker, Christian; Hoyos, Carlos; Jokela, Niko; Rodriguez Fernandez, David; Vuorinen, Aleksi (2018)
    We investigate a simple holographic model for cold and dense deconfined QCD matter consisting of three quark flavors. Varying the single free parameter of the model and utilizing a Chiral Effective Theory equation of state (EoS) for nuclear matter, we find four different compact star solutions: traditional neutron stars, strange quark stars, as well as two non-standard solutions we refer to as hybrid stars of the second and third kind (HS2 and HS3). The HS2s are composed of a nuclear matter core and a crust made of stable strange quark matter, while the HS3s have both a quark mantle and a nuclear crust on top of a nuclear matter core. For all types of stars constructed, we determine not only their mass-radius relations, but also tidal deformabilities, Love numbers, as well as moments of inertia and the mass distribution. We find that there exists a range of parameter values in our model, for which the novel hybrid stars have properties in very good agreement with all existing bounds on the stationary properties of compact stars. In particular, the tidal deformabilities of these solutions are smaller than those of ordinary neutron stars of the same mass, implying that they provide an excellent fit to the recent gravitational wave data GW170817 of LIGO and Virgo. The assumptions underlying the viability of the different star types, in particular those corresponding to absolutely stable quark matter, are finally discussed at some length.
  • Hoyos, Carlos; Jokela, Niko; Rodriguez Fernandez, David; Vuorinen, Aleksi (2016)
    We use a top-down holographic model for strongly interacting quark matter to study the properties of neutron stars. When the corresponding equation of state (EOS) is matched with state-of-the-art results for dense nuclear matter, we consistently observe a first-order phase transition at densities between 2 and 7 times the nuclear saturation density. Solving the Tolman-Oppenheimer-Volkov equations with the resulting hybrid EOSs, we find maximal stellar masses in excess of two solar masses, albeit somewhat smaller than those obtained with simple extrapolations of the nuclear matter EOSs. Our calculation predicts that no quark matter exists inside neutron stars.
  • Guzey, V.; Klasen, M. (2019)
    We compute the cross section of inclusive dijet photoproduction in ultraperipheral Pb-Pb collisions at the CERN Large Hadron Collider using next-to-leading order perturbative QCD. We demonstrate that our theoretical calculations provide a good description of various kinematic distributions measured by the ATLAS Collaboration. We find that the calculated dijet photoproduction cross section is sensitive to nuclear modifications of parton distribution functions (PDFs) at the level of 10% to 20%. Hence, this process can be used to reduce uncertainties in the determination of these nuclear PDFs, whose current magnitude is comparable to the size of the calculated nuclear modifications of the dijet photoproduction cross section.