DPNC - ATLAS

Physique des particules

(Français/English)

Le groupe du DPNC de l'Université de Genève est impliqué dans plusieurs analyses. Une liste de tous les résultats de la collaboration ATLAS est disponible à : https://twiki.cern.ch/twiki/bin/view/AtlasPublic.


Drell-Yan differential cross-section

Distribution of mee in data and MC.
Distribution of mee in data and MC. The dotted vertical lines indicate the range of the differential cross-section measurement.

Using the full 2011 dataset the group (P. Bell, S. Gadomski, M. Goulette, K. Nikolics, G. Pasztor, X. Wu) have performed a measurement of the Drell-Yan differential cross-section in the di-electron channel, as a function of the di-elecron invariant mass mee in the high-mass range, above the Z peak (66 > mee > 1500 GeV) [1]. The figure in this section shows the distribution of the selected events as a function of mee compared to the combined signal plus background expectation. Two data-driven methods were developed to evaluate the W + jets and QCD multi-jet backgrounds, which also found application in the search for high mass resonances decaying to two electrons, an analysis that used the same data set and event selection [2]. In addition to the background estimation, the Geneva group contributed to the overall coordination of the analysis, the preparation of the corresponding paper and many other aspects of the analysis work, including the unfolding of the detector effects and the development of a new method of measuring the electron identification efficiency at high pT.

[1] ATLAS Collaboration, Measurement of the high-mass Drell-Yan differential cross-section in pp collisions at √s =7 TeV with the ATLAS detector, ATLAS-CONF-2012-159, 2012

[2] ATLAS Collaboration, Search for high-mass resonances decaying to di-lepton final states in pp collisions at a centre-of-mass energy of 7 TeV with the ATLAS detector, arXiv:1209.2535, submitted to Journal of High Energy Physics, Sept 2012.

Di-jet resonance searches

The reconstructed dijet mass spectrum.
The reconstructed dijet mass distribution with statistical uncertainties (filled points with error bars) fitted with a smooth functional form (solid line). The bin-by-bin significance of the data-fit difference is shown in the lower panel, using positive values for excesses and negative values for deficits.

The group (C. Doglioni, F. Guescini, A. Picazio) has searched for new new phenomena in abundant jet final states [1,2]. In some exotic physics models which include quark excitations, a narrow resonance is predicted in the dijet mass at the highest masses. If observed, the resonance would suggest quark substructure. In the analysis of the full 2012 dataset taken up to October 2012, no such resonance has been found. This can be seen from the Figure in this section, which corresponds to an integrated luminosity of 13 fb-1 of ATLAS data. The fit shown on this Figure is from the background estimation. Limits on the mass of excited quarks have been set, excluding masses up to 3.84 TeV at the 95% C.L..

[1] ATLAS Collaboration, Search for New Phenomena in the Dijet Mass Distribution updated using 13 fb-1 of pp Collisions at √s=8 TeV collected by the ATLAS Detector, ATLAS-CONF-2012-148, 2012.

[2] ATLAS Collaboration, Search for New Phenomena in the Dijet Mass Distribution using 5.8 fb-1 of pp Collisions at √s=8 TeV collected by the ATLAS Detector, ATLAS-CONF-2012-088, 2012.

Search for excited leptons

Distribution of m_{e,gamma} in data and MC.
Mass of electron-photon pairs in data compared to MC background predictions. Three signal benchmark distributions are also shown. The highest mass bin includes the overflow.

If Standard Model leptons were not point-like, but instead composed of more fundamental constituents, excited lepton states could be created at the LHC. The group (B. Martin) has searched for excited electrons and excited muons, using 5 fb-1 of 7 TeV ATLAS data [1] and 13 fb-1 of 8 TeV data [2]. In the model considered, excited leptons can decay into a lepton and an electroweak gauge boson, or a lepton and a pair of fermions. We have searched for singly produced excited leptons pp→ll* (higher production rate than pair production), in the decay channel l*→lγ (low background level in the llγ final state). The figure shows the invariant mass of electron-photon pairs in 8 TeV data, where the signal would appear as a resonance. However, when the excited lepton mass approaches the new physics energy scale (compositeness scale Λ), the l* decay width becomes large and searching for a bump in the m distribution is not efficient anymore. This is why the statistical analysis is based on the mllγ spectrum, where the signal appears as an excess at high mass, independently of Λ. No excess over the Standard Model expectation is observed in data, and we set limits on the compositeness scale as a function of ml*. In the case where ml*=Λ, both excited electron and excited muon masses below 2.2 TeV are excluded at 95% C.L.

[1] ATLAS Collaboration, Search for excited leptons in proton-proton collisions at √s = 7 TeV with the ATLAS detector, arXiv:1201.3293, published in Phys. Rev. D 85, 072003 (2012).

[2] ATLAS Collaboration, Search for excited electrons and muons with 13 fb-1 of proton-proton collisions at √s = 8 TeV with the ATLAS detector, ATLAS-CONF-2012-146, 2012.

Search for WIMPs in monojet-events

Distribution of mee in data and MC.
Obtained limits on the WIMP-nucleon cross-section for different interaction operators (D1 (scalar), D5 (vector), D11 (scalar,gluon)) for varying WIMP masses. The limits are compared to results from direct detection experiments.

WIMPS (Weakly Interacting Massive Particles) are at present the best Dark Matter candidates and, if their mass is in reach of LHC energies, could be produced in proton collisions observed by ATLAS. Due to their weak interaction with Standard Model (SM) particles, WIMPs would leave the detector unseen, but if one of the incoming particles undergoes initial-state radiation such processes originate a signature of large missing transverse energy, one jet and no further activity in the event. A search for new in phenomena in such Monojet events has been performed by the group (X.Wu, J.Gramling), in data collected in proton-proton collisions at 7 TeV (2011)[1] and 8 TeV (2012)[2]. No significant access above the Standard Model expectation is observed and these results were used to set limits on the cross section of new physics processes. Following a largely model-independent approach of an effective theory describing the interaction between SM and Dark Matter particles, the limits obtained at the LHC can be compared to results from direct and indirect Dark Matter detection experiments. It shows that the copmplementary LHC limits are especially competitive for low WIMP masses.

[1] arXiv:1210.4491 [hep-ex]: "Search for dark matter candidates and large extra dimensions in events with a jet and missing transverse momentum with the ATLAS detector" (4.7 fb-1), ATLAS Collaboration

[2] ATLAS-COM-CONF-2012-190: "Search for New Phenomena in Monojet plus Missing Transverse Momentum Final States using 10 fb-1 of pp Collisions at √s = 8 TeV with the ATLAS detector at the LHC", ATLAS Collaboration

Recherche de monopôles magnétiques

Fraction de signaux à haute énergie dans le TRT versus dispersion d'énergie dans le calorimètre.
Fraction de signaux à haute énergie dans le TRT versus dispersion d'énergie dans le calorimètre. Les cercles représentent 1000 événements simulés de production de monopôles de masse 800 GeV. Les croix représentent les données d'ATLAS.

En 1931, Dirac se rendit compte que l'existence d'un seul monopôle magnétique dans l'Univers suffirait à expliquer la quantisation de la charge électrique. Il s'ensuit que les monopôles devraient porter une charge magnétique correspondant à une multitude de charges électriques élémentaires en termes de pertes d'énergie par ionisation dans la matière. Donc, un monopôle qui traverserait un détecteur apparaîtrait comme une particule extrêmement ionisante. Des recherches de production directe de monopôles ont été effectuées à chaque fois qu'un accélérateur de type nouveau ou puissance plus grande a été construit. Utilisant une partie des données prises en 2011, ATLAS a effectué la première recherche de monopôles au LHC [1]. Cette recherche exploite la signature caractéristique d'une grande fraction de signaux à haute énergie dans le transition radiation tracker (TRT) ainsi qu'une forme étroite de la déposition en énergie dans le calorimètre électromagnétique (EM) (voir figure). Cette recherche a pourtant une limitation importante: les déclencheurs (triggers) utilisés sont faits pour les électrons et les photons et exigent que la particule donne un signal dans la seconde couche du calorimètre EM, tandis que les particules hautement ionisantes comme les monopôles tendraient à perdre leur énergie et s'arrêter avant d'avoir pénétré profondément dans le calorimètre. En 2012, le groupe de Genève a développé et validé un trigger dédié aux recherches de monopôles. Ce trigger est basé sur les signaux dans le TRT plutôt que dans le calorimètre, ce qui permet de réduire l'énergie calorimétrique requise et supprime la condition sur la seconde couche du calorimètre. On s'attend donc à ce que la recherche effectuée à Genève avec les données de 2012 prises avec ce nouveau trigger surpasse largement les résultats précédents. En plus de la détection directe avec ATLAS, d'autres techniques peuvent être utilisées pour rechercher les monopôles magnétiques. Notre groupe a examiné de telles options [2] et créé de nouvelles possibilités expérimentales au LHC [3] et au-delà du LHC [4].

[1] ATLAS Collaboration, Search for magnetic monopoles in √s = 7 TeV pp collisions with the ATLAS detector, arXiv:1207.6411, Phys. Rev. Lett. 109, 261803 (2012).

[2] A. De Roeck, A. Katre, P. Mermod, D. Milstead, and T. Sloan, Sensitivity of LHC experiments to exotic highly ionising particles, arXiv:1112.2999, Eur. Phys. J. C 72, 1985 (2012).

[3] A. De Roeck, H.P. Hächler, A.M. Hirt, M. Dam Joergensen, A. Katre, P. Mermod, D. Milstead, and T. Sloan, Development of a magnetometer-based search strategy for stopped monopoles at the Large Hadron Collider, arXiv:1206.6793, Eur. Phys. J. C 72, 2212 (2012).

[4] K. Bendtz, H.-P. Hächler, A.M. Hirt, P. Mermod, P. Michael, D. Milstead, C. Tegner, T. Sloan, and S.B. Thorarinsson, Search for magnetic monopoles in polar volcanic rocks, arXiv:1301.6530, Phys. Rev. Lett. 110, 121803 (2013).


Modifié le : 15/04/2013