Particle Physics


The DPNC group is involved in several physics analyses. An updated list of all the results of the ATLAS collaboration is available at https://twiki.cern.ch/twiki/bin/view/AtlasPublic.

Some of the most recent results to which the group has directly contributed or led are:

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.

Search for compressed Supersymmetry

Exclusion contours for a simplified supersymmetric model
Exclusion contours for a simplified supersymmetric model as a function of gluino and neutralino mass. The analysis is sensitive to the compressed mass region near the diagonal line.

Using the full 2011 dataset the group (M. Backes) has been involved in the search for compressed supersymmetric scenarios [1]. In compressed supersymmetry the mass differences between the predicted supersymmetric particles are small and the transverse momentum spectra of the observable decay products are soft. The search is therefore based on the presence of one isolated low transverse momentum lepton in addition to jets and missing transverse energy in the final state. The figure shows the results of the search for a simplified supersymmetric model with gluino pair-production and decays via an intermediate chargino to the neutralino, which is the lightest supersymmetric particle in this case. While no excess of events over the Standard Model expectation has been found, it can be seen that the analysis is sensitive in the compressed mass region of the model parameter space near the diagonal line where the gluino and neutralino masses lie close together.

[1] ATLAS Collaboration, Further search for supersymmetry at √s = 7 TeV in final states with jets, missing transverse momentum and isolated leptons with the ATLAS detector, Phys.Rev. D86 (2012) 092002, arXiv:1208.4688 [hep-ex].

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

Search for magnetic monopoles

High-threshold TRT hit fraction versus electromagnetic energy dispersion.
High-threshold TRT hit fraction versus electromagnetic energy dispersion. The circles represent 1000 simulated single monopoles with mass 800 GeV. The crosses represent ATLAS data.

In 1931, Dirac realised that the existence of one magnetic monopole in the Universe would be sufficient to explain the quantisation of electric charge. It follows that monopoles should carry a magnetic charge equivalent to many electron charges in terms of ionisation energy loss. Thus, monopoles would appear as extremely highly-ionising particles in a detector. Searches for direct monopole production have been performed each time an accelerator of a new type or surpassing power was operated. ATLAS performed the first monopole search at the LHC, using part of the data taken in 2011 [1]. The search exploits the characteristic signature of a high fraction of high-threshold hits in the transition radiation tracker (TRT) and a narrow energy deposition in the electromagnetic (EM) calorimeter (see figure). However, this search had one severe limitation: the triggers used were designed for electrons or photons and required the particle to reach the second layer of the EM calorimeter, while highly-ionising particles such as monopoles would tend to range out before they penetrate deep into the calorimeter. In 2012, the Geneva group developed and validated a dedicated high-level trigger for monopoles. This trigger is based on TRT hits rather than calorimeter energy, which reduces the energy threshold and removes the condition on the second EM calorimeter layer. The search being performed in Geneva with the data taken in 2012 with this trigger is therefore expected to vastly surpass the previous results in sensitivity.

In addition to direct detection in ATLAS, there are several other techniques which can be used to search for monopoles. Our group investigated such options [2], creating new experimental opportunities at the LHC [3] and beyond the 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).

Last modified: 06/04/2013