Search

El año 2012 ha sido un año histórico para los neutrinos. Los neutrinos han dejado de ser exclusivamente objeto de estudio de los científicos y han saltado a la prensa por diferentes razones, como su supuesta pretensión de viajar más rápido que la luz o por haber sido medida su última transformación. De todo ello hablaremos en este artículo y veremos que los neutrinos nos han dado grandes sorpresas con importantes consecuencias para la física. Esas partículas minúsculas, pero extremadamente abundantes en el Universo, han conseguido romper mediante evidencias experimentales muchas de las ideas preconcebidas sobre ellas y han abierto paso a nuevos caminos más allá del Modelo Estándar de la Física de Partículas.

A measurement of the W mass and width has been performed by the DELPHI collaboration using the data collected during 1998. The data sample has an integrated luminosity of 155 pb^{-1} and an average centre-of-mass energy of 188.6 GeV. Results are obtained by applying the method of direct reconstruction of the mass of the W from its decay products in both the W+W- -> lvqq and W+W- -> qqqq channels. The W mass result for the 1998 data set is M_W = 80.387 +/- 0.087 (stat) +/- 0.034 (syst) +/- 0.017 (LEP) +/- 0.035 (FSI) GeV/c2, where FSI represents the uncertainty due to final state interaction effects in the qqqq channel, and LEP represents that arising from the knowledge of the beam energy of the accelerator. Combining this result with those previously published by the DELPHI collaboration gives the result M_W = 80.359 +/- 0.074 (stat) +/- 0.032 (syst) +/- 0.017 (LEP) +/- 0.033 (FSI) GeV/c2. The combined value for the W width is \Gamma_W = 2.266 +/- 0.176 (stat) +/- 0.056 (syst) +/- 0.052 (FSI) GeV/c2.

Searches for spontaneous R-parity violating signals at sqrt(s) = 183 GeV and sqrt(s) = 189 GeV have been performed using the 1997 and 1998 DELPHI data, under the assumption of R-parity breaking in the third lepton family. The expected topology for the decay of a pair of charginos into two acoplanar taus plus missing energy was investigated and no evidence for a signal was found. The results were used to derive a limit on the chargino mass and to constrain the allowed domains of the MSSM parameter space.

A new precise measurement of |V_{cb}| and of the branching ratio BR(\bar{B^0} -> D^{*+} \ell^- \bar{\nu_\ell}) has been performed using a sample of about 5000 semileptonic decays \bar{B^0} -> D^{*+} \ell^- \bar{\nu_\ell}, selected by the DELPHI detector at LEP I by tagging the soft pion from D^{*+} -> D^0 \pi^+. The results are: V_{cb}=(39.0 +/- 1.5 (stat.) ^{+2.5}_{-2.6} (syst. exp.) +/- 1.3 (syst. th.)) x 10^{-3} BR(\bar{B^0} -> D^{*+} \ell^- \bar{\nu_\ell})=(4.70 +/- 0.13 (stat.) ^{+0.36}_{-0.31} (syst. exp.))% The analytic dependences of the differential cross-section and of the Isgur Wise form factor as functions of the variable w = v_{B^0}.v_{D^*} have also been obtained by unfolding the experimental resolution.

Data taken by the DELPHI experiment at centre-of-mass energies of 183 GeV and 189 GeV with a total integrated luminosity of 212 pb^{-1} have been used to search for the supersymmetric partners of the electrons, muons, and taus in the context of the Minimal Supersymmetric Standard Model (MSSM). The decay topologies searched for were the direct decay ({\tilde \ell} -> \ell {\tilde \chi_1^0}), producing acoplanar lepton pairs plus missing energy, and the cascade decay ({\tilde \ell} -> \ell {\tilde \chi_2^0} -> \ell \gamma {\tilde \chi_1^0}), producing acoplanar lepton and photon pairs plus missing energy. The observed number of events is in agreement with Standard Model predictions. The 95% CL excluded mass limits for selectrons, smuons and staus are m_{\tilde {e}} \leq 87 GeV/c^2, m_{\tilde {\mu}} \leq 80 GeV/c^2 and m_{\tilde {\tau}} \leq 75 GeV/c^2, respectively, for values of \mu=-200 GeV/c^2 and tan(beta)=1.5.

Searches for pair production of gauginos and squarks in e+e- collisions at a centre-of-mass energy of 189 GeV have been performed on data corresponding to an integrated luminosity of 158 pb^{-1} collected by the DELPHI detector at LEP. The data were analyzed under the assumption of non-conservation of R-parity through a single dominant Ubar Dbar Dbar coupling between squarks and quarks. Typical final states contain between 4 and 10 jets with or without additional leptons. No excess of data above Standard Model expectations was observed. The results were used to constrain domains of the MSSM parameter space and derive limits on the masses of supersymmetric particles. The following mass limits at 95% CL were obtained from these searches: neutralino mass: m_{\tilde{\chi^0_{1}}} \geq 32 GeV chargino mass: m_{\tilde{\chi^+_{1}}} \geq 94 GeV stop and sbottom mass (indirect decay) with \Delta M > 5 GeV: m_{\tilde{t_1}} \geq 74 GeV, for \Phi_{mix}=0 rad m_{\tilde{t_1}} \geq 59 GeV, for \Phi_{mix}=0.98 rad m_{\tilde{b_1}} \geq 72 GeV, for \Phi_{mix}=0 rad. The angle \phi_{mix} is the mixing angle between left and right handed quarks.

Searches for charginos, neutralinos and sleptons at LEP2 centre-of-mass energies from 130 GeV to 189 GeV have been used to set lower limits on the mass of the Lightest Supersymmetric Particle and other supersymmetric particles within the MSSM framework. R-parity conservation has been assumed. The lightest neutralino was found to be heavier than 32.3 GeV/c^2 independent of the m_0 value. The lightest chargino, the second-to-lightest neutralino, the next-to-heaviest neutralino, the heaviest neutralino, the sneutrino and the right-handed selectron were found to be heavier than 62.4 GeV/c^2, 62.4 GeV/c^2, 99.9 GeV/c^2, 116.0 GeV/c^2, 61.0 GeV/c^2, and 87.0 GeV/c^2, respectively. These limits do not depend on m_0 or M_2 and are valid for 1 \leq tan(beta) \leq 40, in the mu region where the lightest neutralino is the LSP. If the sneutrino is heavier than the chargino the lightest neutralino has to be heavier than 32.4 GeV/c^2. The effects of mixings in the third family of sfermions on these limits are discussed. The confidence level of all limits given is 95%.

A test of the QED process e+e- -> gamma gamma (gamma) is reported. The data analysed were collected with the DELPHI detector in 1998 and 1999 at the highest energies achieved at LEP, reaching 202 GeV in the centre-of-mass. The total integrated luminosity amounts to 375.7 pb^{-1}. The differential and total cross-sections for the process e+e- -> gamma gamma were measured, and found to be in agreement with the QED prediction. 95% Confidence Level (C.L.) lower limits on the QED cut-off parameters of Lambda+ > 330 GeV and Lambda- > 320 GeV were derived. A 95% C.L. lower bound on the mass of an excited electron of 311 GeV/c^2 (for lambda_gamma = 1) was obtained. s-channel virtual graviton exchange was searched for, resulting in 95% C.L. lower limits on the string mass scale, M_S: M_S > 713 GeV/c^2 (lambda = 1) and M_S > 691 GeV/c^2 (lambda = -1).

The rapidity-rank structure of p pbar pairs is used to analyze the mechanism of baryon production in hadronic Z0 decay. The relative occurrence of the rapidity-ordered configuration p M pbar, where M is a meson, and that of p pbar adjacent pairs is compared. The data are found to be consistent with predictions from a mechanism producing adjacent-rank p pbar pairs, without requiring `string-ordered' p M pbar configurations. An upper limit of 15% at 90% confidence is determined for the p M pbar contribution.

Inclusive distributions of charged particles in hadronic W decays are experimentally investigated using the statistics collected by the DELPHI experiment at LEP during 1997, 1998 and 1999, at centre-of-mass energies from 183 to around 200 GeV. The possible effects of interconnection between the hadronic decays of two Ws are not observed. Measurements of the average multiplicity for charged and identified particles in q qbar and WW events at centre-of-mass energies from 130 to 200 GeV and in W decays are presented. The results on the average multiplicity of identified particles and on the position xi^* of the maximum of the xi_p = -log(2p/sqrt(s)) distribution are compared with predictions of JETSET and MLLA calculations.

The DELPHI detector at LEP has collected 54 pb^{-1} of data at a centre-of-mass energy around 183 GeV during 1997, 158 pb^{-1} around 189 GeV during 1998, and 187 pb^{-1} between 192 and 200 GeV during 1999. These data were used to measure the average charged particle multiplicity in e+e- -> b bbar events, _{bb}, and the difference delta_{bl} between _{bb} and the multiplicity, _{ll}, in generic light quark (u,d,s) events: delta_{bl}(183 GeV) = 4.55 +/- 1.31 (stat) +/- 0.73 (syst) delta_{bl}(189 GeV) = 4.43 +/- 0.85 (stat) +/- 0.61 (syst) delta_{bl}(200 GeV) = 3.39 +/- 0.89 (stat) +/- 1.01 (syst). This result is consistent with QCD predictions, while it is inconsistent with calculations assuming that the multiplicity accompanying the decay of a heavy quark is independent of the mass of the quark itself.

The fragmentation functions of quarks and gluons are measured in various three-jet topologies in Z decays from the full data set collected with the Delphi detector at the Z resonance between 1992 and 1995. The results at different values of transverse momentum-like scales are compared. A parameterization of the quark and gluon fragmentation functions at a fixed reference scale is given. The quark and gluon fragmentation functions show the predicted pattern of scaling violations. The scaling violation for quark jets as a function of a transverse momentum-like scale is in a good agreement with that observed in lower energy e+e− annihilation experiments. For gluon jets it appears to be significantly stronger. The scale dependences of the gluon and quark fragmentation functions agree with the prediction of the DGLAP evolution equations from which the colour factor ratio CA/CF is measured to be: CACF=2.26±0.09stat.±0.06sys.±0.12clus.,scale.

A search for stable and long-lived heavy charged particles was performed using the data taken by the DELPHI experiment at an energy of 189 GeV. The Cherenkov light detected in the Ring Imaging Cherenkov Detector and the ionisation loss measured in the Time Projection Chamber were used to identify heavy particles passing through the detector. No evidence for the production of such particles has been found, therefore exclusion limits at 95% confidence level were derived on the masses of left and right handed smuons and staus. The results were combined with previous DELPHI searches in this channel. Including previous DELPHI results, masses of left (right) handed stable smuons and staus can be excluded between 2 GeV/c^2 and 88 (87.5) GeV/c^2 at 95% CL.

Searches for neutral Higgs bosons in the Standard Model and the MSSM have been performed using data collected by the DELPHI experiment at a centre-of-mass energy of 188.7 GeV, corresponding to an integrated luminosity of 158 pb^{-1}. These analyses are used, in combination with our results from lower energies, to set new 95% confidence level lower mass bounds on the Standard Model Higgs boson (94.6 GeV/c^2) and on the lightest neutral scalar (82.6 GeV/c^2) and neutral pseudoscalar (84.1 GeV/c^2) Higgs bosons in a representative scan of the MSSM parameters. The results are also interpreted in the framework of a general two-Higgs doublet model.

The longitudinal polarization of the Λb baryon is measured at the LEP e+e− collider by DELPHI. It is determined from the charged lepton and neutrino energy spectra in 249±19 Λb semileptonic decays reconstructed in ≈3.5 million hadronic Z0 decays using Λ0-lepton correlations. The measured polarization is: PΛb=−0.49−0.30+0.32(stat.)±0.17(syst.)

Oscillations of B0s mesons have been studied in samples selected from about 3.5 million hadronic Z decays detected by DELPHI between 1992 and 1995. One analysis uses events in the exclusive decay channels: B0s -> Ds- pi+ or Ds- a1+ and B0s -> anti-D0 K- pi+ or anti-D0 K- a1+, where the D decays are completely reconstructed. In addition, B0s anti-B0s oscillations have been studied in events with an exclusively reconstructed Ds accompanied in the same hemisphere by a high momentum hadron of opposite charge. Combining the two analyses, a limit on the mass difference between the physical B0s states has been obtained: Delta(m_B0s) > 4.0 ps^{-1} at the 95% C.L. with a sensitivity of Delta(m_B0s) = 3.2 ps^{-1}. Using the latter sample of events, the B0s lifetime has been measured and an upper limit on the decay width difference between the two physical B0s states has been obtained: tau(B0s) = 1.53^{+0.16}_{-0.15}(stat.) +/- {0.07}(syst.) ps \Delta\Gamma(B0s)/\Gamma(B0s) < 0.69 at the 95% C.L. The combination of these results with those obtained using Ds+- lepton-+ sample gives: Delta(m_B0s) > 4.9 ps^{-1} at the 95% C.L. with a sensitivity of Delta(m_B0s) = 8.7 ps^{-1}. tau(B0s) = 1.46 +/- 0.11 ps and \Delta\Gamma(B0s)/\Gamma(B0s) < 0.45 at the 95% C.L.

The branching ratio for the leptonic decay of charged B mesons (B- → tau (-)[([nu])over bar](tau)) has been measured using selected leptonic tau (-) → l(-)nu (tau)[([nu])over bar](l) and hadronic tau (-) → nu X-tau decays in Z → b (b) over bar decays recorded by DELPHI at LEP1 in 1992-1995. The result, BR(B- → tau (-)[([nu])over bar](tau)) lt 1.1 x 10(-3) at the 90% confidence level, is consistent with standard model expectations and puts a constraint on the ratio tan [beta]/MH+/- lt 0.46 (GeV/c(2))(-1) in the framework of models with two Higgs doublets (type II Higgs doublet model). From the missing energy distribution in Z → b (b) over bar decays without identified leptons, the b → tau[([nu])over bar]X-tau branching ratio has been measured in the hadronic channel tau - → nu X-tau'. The result, BR(b → tau[([nu])over bar]X-tau) = (2.19 +/- 0.24 (stat) +/- 0.39 (syst))%, is consistent with the Standard Model prediction and with previous experimental measurements.

Lifetime and oscillations of B0s mesons have been studied in events with a large transverse momentum lepton and a Ds of opposite electric charge in the same hemisphere, selected from about 3.6 million hadronic Z0 decays accumulated by DELPHI between 1992 and 1995. The B0s lifetime and the fractional width difference between the two physical B0s states have been found to be: \tau_(B0s) = (1.42^{+0.14}_{-0.13}(stat.) +/- 0.03(syst.)) ps \Delta\Gamma(B0s)/\Gamma(B0s) < 0.46 at the 95% C.L. In the latter result it has been assumed that \tau_(B0s)=\tau_(B0d). Using the same sample, a limit on the mass difference between the physical B0s states has been set: Delta(m_B0s) > 7.4 ps^{-1} at the 95% C.L. with a corresponding sensitivity equal to 8.1 ps^{-1}.

Production of Sigma- and Lambda(1520) in hadronic Z decays has been measured using the DELPHI detector at LEP. The Sigma- is directly reconstructed as a charged track in the DELPHI microvertex detector and is identified by its Sigma -> n pi decay leading to a kink between the Sigma- and pi-track. The reconstruction of the Lambda(1520) resonance relies strongly on the particle identification capabilities of the barrel Ring Imaging Cherenkov detector and on the ionisation loss measurement of the TPC. Inclusive production spectra are measured for both particles. The production rates are measured to be = 0.081 +/- 0.002 +/- 0.010, = 0.029 +/- 0.005 +/- 0.005. The production rate of the Lambda(1520) suggests that a large fraction of the stable baryons descend from orbitally excited baryonic states. It is shown that the baryon production rates in Z decays follow a universal phenomenological law related to isospin, strangeness and mass of the particles.

Searches for supersymmetric partners of top and bottom quarks are presented using data taken by the DELPHI experiment at LEP in 1997 and 1998. No deviations from standard model expectations are observed in these data sets, which are taken at centre-of-mass energies of 183 GeV and 189 GeV and correspond to integrated luminosities of 54 pb^{-1} and 158 pb^{-1}. These results are used in combination with those obtained by DELPHI at lower centre-of-mass energies to exclude regions in the squark-neutralino mass plane at 95% confidence level.

An update of the searches for charginos and gravitinos is presented, based on a data sample corresponding to the 158 pb^{-1} recorded by the DELPHI detector in 1998, at a centre-of-mass energy of 189 GeV. No evidence for a signal was found. The lower mass limits are 4-5 GeV/c^2 higher than those obtained at a centre-of-mass energy of 183 GeV. The (\mu,M_2) MSSM domain excluded by combining the chargino searches with neutralino searches at the Z resonance implies a limit on the mass of the lightest neutralino which, for a heavy sneutrino, is constrained to be above 31.0 GeV/c^2 for tan(beta) \geq 1.

Searches for pair production of supersymmetric particles under the assumption that R-parity is not conserved are presented, based on data recorded by the DELPHI detector in 1998 from e+e- collisions at a centre-of-mass energy of 189 GeV. Only one R-parity violating LL\bar{E} term (i.e. one lambda coupling), which couples scalar leptons to leptons, is considered to be dominant at a time. Moreover, it is assumed that the strength of the R-parity violating couplings is such that the lifetimes can be neglected. The search for pair production of neutralinos, charginos and sleptons has been performed for both direct R-parity violating decays and indirect cascade decays. The results are in agreement with Standard Model expectations, and are used to update the constraints on the MSSM parameter values and the mass limits previously derived at sqrt(s) = 183 GeV. The present 95% C.L. limits on supersymmetric particle masses are: m_{\tilde{\chi}^0} > 30 GeV/c^2 and m_{\tilde{\chi}^\pm} > 94 GeV/c^2; m_{\tilde{\nu}} > 76.5 GeV/c^2 (direct and indirect decays); m_{\tilde{\ell}_R} > 83 GeV/c^2 (indirect decay only).

A study of b semileptonic decays into D, D pi+- and D* pi+- final states is presented. The D0, D+ and D*+ mesons are exclusively reconstructed in Z decay data recorded from 1992 to 1995 in the DELPHI experiment at LEP. The overall branching fractions are measured to be: BR(b -> D^0 \ell^- \bar{\nu}_{\ell} X) = (7.04 +/- 0.34 (stat) +/- 0.36 (syst.exp) +/- 0.17 (BR_D))% BR(b -> D^+ \ell^- \bar{\nu}_{\ell} X) = (2.72 +/- 0.19 (stat) +/- 0.16 (syst.exp) +/- 0.18 (BR_D))% BR(b -> D^{*+} \ell^- \bar{\nu}_{\ell} X) = (2.75 +/- 0.17 (stat) +/- 0.13 (syst.exp) +/- 0.09 (BR_D))% where the D0 and D+ results include also contributions from D*0 and D*+ decays. A fit to the distribution of the pi+- impact parameter to the primary interaction vertex provides a measurement of the b semileptonic branching fractions into the D0 pi+- X, D+ pi+- X and D*+ pi+- X final states. Assuming that single pion decay modes of B mesons dominate, the partial rates for \bar{B} -> D \pi \ell^- \bar{\nu_{\ell}} and \bar{B} -> D^{*} \pi \ell^- \bar{\nu_{\ell}} have been obtained, corresponding to a total branching fraction: BR(\bar{B} -> D \pi \ell^- \bar{\nu_{\ell}}) + BR(\bar{B} -> D^{*} \pi \ell^- \bar{\nu_{\ell}}) = (3.40 +/- 0.52 (stat) +/- 0.32 (syst))% . This result agrees well with the observed difference between the total B semileptonic branching fraction and the sum of the \bar{B} -> D \ell^- \bar{\nu_{\ell}} and D^* \ell^- \bar{\nu_{\ell}} branching fractions.

A measurement of the forward--backward asymmetry of e+e−→cc¯ and e+e−→bb¯ on the Z resonance is performed using about 3.5 million hadronic Z decays collected by the DELPHI detector at LEP in the years 1992 to 1995. The heavy quark is tagged by the exclusive reconstruction of several D meson decay modes. The forward--backward asymmetries for c and b quarks at the Z resonance are determined to be: \renewcommand{\arraystretch}{1.6} \begin{array}{rcr@{}l} \Afbc(\sqrt{s} = 91.235 {\rm GeV}) &=& &#;0659 \pm 0.0094 (stat) \pm 0.0035 (syst) \Afbb (\sqrt{s} = 91.235 {\rm GeV}) &=& &#;0762 \pm 0.0194 (stat) \pm 0.0085 (syst) \Afbc(\sqrt{s} = 89.434 {\rm GeV}) &=&-&#;0496 \pm 0.0368 (stat) \pm 0.0053 (syst) \Afbb(\sqrt{s} = 89.434 {\rm GeV}) &=& &#;0567 \pm 0.0756 (stat) \pm 0.0117 (syst) \Afbc(\sqrt{s} = 92.990 {\rm GeV}) &=& &#;1180 \pm 0.0318 (stat) \pm 0.0062 (syst) \Afbb(\sqrt{s} = 92.990 {\rm GeV}) &=& &#;0882 \pm 0.0633 (stat) \pm 0.0122 (syst) \end{array}

The events with two photons and missing (transverse) energy collected by the DELPHI detector at centre-of-mass energies between 130~GeV and 183~GeV have been studied to search for processes of the type e+e−→YY with the subsequent decay Y→Xγ, where X is an undetectable neutral particle. Reactions of this kind are expected in supersymmetric models, where the Y particle can be either the lightest neutralino, decaying to a photon and a gravitino, or the next-to-lightest neutralino, decaying to a photon and the lightest neutralino. To study the case of long-lived Y particles, a search for single-photon events with the reconstructed photon axis pointing far from the beam interaction region has also been performed. No evidence for a deviation from Standard Model expectations has been observed in the data and upper limits have been derived on the signal cross-section as a function of the the X and Y masses and of the Y mean decay path.

Neutral Higgs bosons were searched for in the data collected by DELPHI at a centre-of-mass energy of 183 GeV, corresponding to an integrated luminosity of 54 pb TeX . The production of the lightest neutral Higgs boson with either an on-shell Z boson or a neutral pseudo-scalar Higgs boson was analysed. Lower limits at the 95% confidence level were obtained on the Higgs boson masses. The limits are 85.7 GeV/ TeX for the Standard Model Higgs boson and 74.4 GeV/ TeX for the scalar and 75.3 GeV/ TeX for the pseudo-scalar Higgs bosons as predicted by the minimal super-symmetric extension of the Standard Model with commonly used assumptions on the model parameters, plus the assumption that the mass of the pseudo-scalar boson is greater than 20 GeV/ TeX . These results significantly improve the limits reached with previous data.

A search for charginos with masses close to the mass of the lightest neutralino is reported, based on the data collected with the DELPHI detector at LEP from 1995 to 1997 at centre-of-mass energies between 130 and 183 GeV. The signature of a photon at high transverse momentum radiated from the initial state reduces the two-photon background to acceptable rates, thus making the mass differences between a few hundred MeV/c^2 and 3 GeV/c^2 detectable. In very nearly degenerate scenarios, the lifetime of the chargino can be large enough to produce either visible secondary vertices or decays outside the detector; therefore, quasi-stable heavy charged particles and displaced decay vertices were also searched for. No excess of events with respect to the Standard Model expectations was observed, and limits in the plane of chargino-neutralino mass difference versus chargino mass are given.

Decays of W± bosons, produced at LEP2, have been used to measure the |Vcs| element of the Cabibbo-Kobayashi-Maskawa matrix. Values for |Vcs| were extracted both from the measured hadronic branching ratio of W± decays and by tagging the flavour of hadronic jets produced in W± decays. Applying the two methods to the data collected during 1996 at energies of 161 and 172 GeV, DELPHI obtains |Vcs|=0.91+0.15−0.14(stat)±0.05(syst).

This document summarizes the conclusions of the Neutrino Town Meeting held at CERN in October 2018 to review the neutrino field at large with the aim of defining a strategy for accelerator-based neutrino physics in Europe. The importance of the field across its many complementary components is stressed. Recommendations are presented regarding the accelerator based neutrino physics, pertinent to the European Strategy for Particle Physics. We address in particular i) the role of CERN and its neutrino platform, ii) the importance of ancillary neutrino cross-section experiments, and iii) the capability of fixed target experiments as well as present and future high energy colliders to search for the possible manifestations of neutrino mass generation mechanisms., Comment: 10+6 pages; Summary Document of the European Neutrino Town Meeting, Oct 22-24 at CERN; editors: Alain Blondel, Albert De Roeck, Joachim Kopp; v2: references added

The experimental confirmation of the neutrino oscillations, and therefore the non-zero neutrino mass, opened a new era of physics beyond the Standard Model of particle physics. Neutrinos do have mass, and their flavour change when propagating. Thanks to the huge effort of the neutrino physics community, many neutrino properties have been measured in the last decades, including most of the neutrino oscillation parameters. However, other properties remain unknown, such as the absolute scale of the neutrino masses, or the nature itself of their masses. The Deep Underground Neutrino Experiment (DUNE) will bring neutrino physics to a new era, measuring the oscillation parameters with unprecedented precision that will allow determining the CP violation phase in the leptonic sector and the neutrino mass ordering. It also aims at detecting neutrinos from a core-collapsing supernova within the galaxy if it occurs during the running of the experiment and performing beyond standard model searches. DUNE is a long-baseline neutrino oscillation experiment designed by an international collaboration of around 1,300 scientists. It will consist of the most powerful neutrino beam installed at Fermilab (US), a Near Detector placed downstream the beam, and a Far Detector located 1.5 km underground at the Sandford Underground Research Facility (SURF), around 1,300 km away from the beam. The Far Detector of DUNE will consist of four liquid-argon time projection chambers (LAr-TPCs) of 17 kton each. DUNE will bring the LAr-TPC technology to an unprecedented large scale. A LAr-TPC is a kind of particle detector that allows the particle-track reconstruction with millimetre precision. In a LAr-TPC, the interacting particles ionize the liquid argon along their tracks. This charge is drifted by a uniform electric field and readout. Additionally, scintillation light is produced, providing a fast signal used as a trigger and also to reconstruct the particle interaction time. The excellent imaging capabilities of the LAr-TPC technology, and the advantages of using liquid argon as a detector medium for neutrino physics, will allow DUNE to fulfil its physics goals. A full prototyping effort is being carried out by the DUNE collaboration to validate the LAr-TPC technology at the kiloton scale. In particular, two prototypes have been assembled and operated at the CERN neutrino platform: ProtoDUNE Single-Phase and ProtoDUNE Dual-Phase, of 300 ton of active mass each, and which took data from 2018 to 2020. While ProtoDUNE Single-Phase is based on a more traditional approach of a LAr-TPC based on wire readout planes, the innovative Dual-Phase approach includes a gas layer at the top, with an extraction layer and Large Electron Multipliers (LEMs) to amplify the signal. The work presented in this thesis has been carried out in the framework of this prototyping work, specifically it has been focused on the operation and performance measurement of the photon detection system of ProtoDUNE Dual-Phase. The obtained results are of interest for any liquid argon experiment. The first chapters of this thesis introduce the status of neutrino physics, the operating principles of the LAr-TPC technology and the DUNE experiment. Then, ProtoDUNE Dual-Phase and its different systems are explained with a particular focus on the photon detection system. The photon detection system is a crucial part of any LAr-TPC, since it provides the interaction time, which is needed to reconstruct the event in three dimensions and contributes to the particle calorimetric recontruction. The photon detection system can also provide a trigger for non-beam events. The ProtoDUNE Dual-Phase photon detection system consists of 36 photomultiplier tubes (PMTs) placed below the TPC active volume. Due to the monolithic design of the detector, the photon detection system must detect scintillation light from more than six meters away, being the longest light detection distance in any liquid argon TPC up to date. A good characterization of the photon detection system is crucial in order to demonstrate its capabilities. The ProtoDUNE Dual-Phase photon detection system performance is studied in detail in chapter 5. Additionally, a good understanding of the light production, propagation and detection processes is key in order to extrapolate the performance of the detector at larger scales. In this sense, the development of a Monte Carlo simulation allows identifying the main parameters affecting the detector performance. A dedicated Monte Carlo simulation of the scintillation light production, propagation and detection has been developed, including the light production from cosmic muons and low energy radiological backgrounds, and compared with data. The understanding of the radiological backgrounds is critical to correctly evaluate the physics performance of DUNE at low energies. The simulation is explained in chapter 6 and the comparison with ProtoDUNE Dual-Phase data can be found in chapter 7. Since scintillation light in liquid argon is produced at 127 nm, at which most photosensors are not sensitive, fluorescent materials are introduced in order to shift the light wavelength towards the visible range, where PMTs have maximal detection efficiency. ProtoDUNE Dual-Phase has carried out an innovative photon detection program, by testing several wavelength shifting techniques at the multi-ton scale which are the object of analysis in this thesis. ProtoDUNE Dual-Phase has been the first detector deploying the novel polyethylene naphthalate (PEN) together with the more traditional tetraphenyl butadiene (TPB) as a wavelength-shifter. Although TPB has proven its good performance in many liquid argon experiments, its deployment requires sophisticated coating setups that are difficult to scale to large surfaces. TPB coatings are also very delicate, making its handling difficult. In this sense, PEN appears as a promising alternative, since it is a thermoplastic similar to PET, very stable and easy to handle. However its wavelength-shifting efficiency is not well known, and it has been never studied in real operating conditions in large scale LAr-TPC. The PEN wavelength efficiency is measured and its performance is evaluated and compared with TPB in chapter 8. This study is critical to understand if PEN represents an effective alternative to TPB for DUNE. Xenon doping is another alternative light detection technique for large scale LAr-TPCs. The presence of xenon at the level of a few ppm (parts-per-million) in the liquid argon acts as a wavelength shifter, shifting part of the scintillation light to longer wavelengths and reducing the light attenuation with the propagation distance. The simplicity of just adding a small quantity of xenon makes it an attractive alternative for large-scale LAr-TPC that is being considered for one of the DUNE Far Detector modules. However, the light production mechanisms in xenon-doped liquid argon have not been characterized yet, and their effects in the detected light in a several-meter-drift LAr-TPC have never been measured. In this sense, a deep study is needed in order to validate the xenon-doping technique for DUNE. ProtoDUNE Dual-Phase took data with xenon-doped liquid argon contaminated with a small amount of nitrogen. The performance of the photon detection system using xenon-doped liquid argon and its comparison with pure liquid argon using ProtoDUNE Dual-Phase data is explained in chapter 9. This analysis represents a unique opportunity to study the performance of a LAr-TPC using xenon-doped liquid argon, as the ProtoDUNE Dual-Phase is the largest monolithic LAr-TPC ever operated. The observation of the proton decay is a process beyond the Standard Model that it has never been observed. Proton decay is one of the requirements of many Gran Unification Theories (GUT), and it would also help to explain the matter-antimatter asymmetry in the universe. Placed deeply underground to mitigate backgrounds and thanks to the good imaging capabilities that allow to identify the interacting particles, DUNE has a promising potential to perform proton decay searches. However, the imaging capabilities rely strongly on having a good and efficient light detection system providing the interaction time of the event. In chapter 10, the performance of a light detection system based on TPB-coated PMTs, as in ProtoDUNE Dual-Phase, is studied. The goal is to validate its capability to provide the event time of proton decay events in the presence of background events in a 10 kton Far Detector of DUNE. This is a key study since the capability to perform proton decay searches is one of the primary physics goals of DUNE.

Neutrinos are the most abundant massive particles in our Universe but its existence was only predicted 90 years ago. In fact they were not experimentally observed until 60 years ago due to their extraordinarily low cross-section and even nowadays many questions concerning these subatomic particles remain open, as their mass magnitude and ordering, their Dirac or Majorana nature or the possibility of CP-violation.The Standard Model of Particle Physics establishes that neutrinos are massless particles. Nevertheless in 2015, Takaaki Kajita and Arthur McDonald were awarded with the Nobel prize in Physics for the discovery of the phenomenon of neutrino flavour oscillations, only possible if neutrinos are massive, demonstrating that a neutrino of one particular flavour has non-zero probability to be detected with a different flavour. These oscillations are governed by six independent parameters: 3 mixing angles 12, 23 and 13, two mass squared differences and a CP phase responsible for the CP-violation in the leptonic sector, being this last parameter still unknown as well as the neutrino mass ordering...

Double Chooz is a reactor neutrino oscillation experiment whose purpose is the measurement of the still unknown mixing angle θ_13. It is based on the past CHOOZ experiment which has the best limit on this parameter. Double Chooz aims to reduce the systematic error of this measurement up to 0.6% using two identical detectors to eliminate the uncertainty on the antineutrino flux coming from reactors. The main goal of this work is to perform a calibration of the far detector to check its performance and compute the gain of the electronic channels. This is necessary to estimate the energy deposited by each event from the registered data. The first data taken with the far detector using a LEDs system have been analyzed for this study. The pulse baselines have been used to check the performance of the detector and the single photoelectron spectrum to compute the gain of all the channels. The results of this study have proven that there is no noisy channels that can be a problem for the Double Chooz data analysis. In addition, the gain of each channel has been computed obtaining an average detector gain of 50.73 digital units of charge per photoelectron. [RESUMEN] Double Chooz es un experimento de oscilaciones de neutrinos en reactores nucleares que pretende medir el ángulo de mezcla θ_13, todavía desconocido. Es el sucesor del experimento CHOOZ, que hasta ahora tiene el mejor límite en este parámetro. Double Chooz pretende reducir el error sistemático de esta medida al 0.6 %, utilizando dos detectores idénticos para así eliminar la incertidumbre en el flujo de antineutrinos emitidos por los reactores. El objetivo principal de este trabajo es realizar una calibración del detector lejano. Con esta calibración se pretende comprobar el correcto funcionamiento del detector y calcular la ganancia de los canales electrónicos. Esto es necesario para poder estimar la energía depositada por cada suceso a partir de los datos registrados. Para este estudio se han utilizado los primeros datos tomados con el detector lejano utilizando un sistema de LEDs. Se ha estudiado el baseline de los pulsos para comprobar el correcto funcionamiento del detector y los espectros de carga de un fotoelectrón para calcular la ganancia. Los resultados de este estudio han probado que en Double Chooz no hay canales ruidosos que puedan suponer un problema para el análisis. Además, se ha calculado la ganancia de cada canal obteniendo una ganancia media del detector de 50.73 unidades digitales de carga por cada fotoelectrón.