Cosmology, Particle Physics and Phenomenology - CP3

Observability of new phenomenological models in High Energy experiments is delicate to evaluate, due to the complexity of the related detectors, DAQ chain and software. Delphes is a new framework for fast simulation of a general purpose experiment. The simulation includes a tracking system, a magnetic field, calorimetry and a muon system, and possible very forward detectors arranged along the beamline. The framework is interfaced to standard file format from event generators and outputs observable analysis data objects. The simulation takes into account the detector resolutions, usual reconstruction algorithms for complex objects (FastJet) and a simplified trigger emulation. Detection of very forward scattered particles relies on the transport in beamlines with the Hector software. Finally, the FROG 2D/3D event display is used for visualisation of the collision final states.

External collaborators: Severine Ovyn, Xavier Rouby from pfease company.

The top quark is the heaviest elementary particle discovered so far, and many properties related to this quark are still to be understood.
Its observation and mass measurement at Tevatron highlighted the uncommon nature of this quark. The fact that its electroweak decay is faster than the hadronization time scale implies that the top quark exists only as a free quark, so that the effects from new physics should show up very clearly by comparing measurements with the precise Standard Model preditions.
Its "re-discovery" at LHC will be a major milestone for the experiments, since the complexity of the final state demands a fairly good knowledge of the experimental apparatus and a certain degree of control of the backgrounds. Its expected large coupling to Higgs bosons will also be relevaant for the searches for higgs sectors beyond the Standard Model.

External collaborators: CMS collaboration.

At LHC, the Z boson can be produced in association with one or two b-quarks, which is here refereed as b(b)Z production. This process has been seen for the first time at LHC, and measurement of it is an important test of QCD calculations.

For the first time, we observed the Z+b final state and measured of the Z+b/Z+j cross-section ratio in 35.9/pb of pp collisions at 7 TeV, using particle flow jets and simple secondary vertex b-tagging algorithm in the definition of the signal.Emphasis is put on kinematic properties of the jets. With more luminosity, we are working on the measurement of the cross-section for the b(b)Z process, with the identification of one or two b-jets.

External collaborators: Anne-Marie Magnan (IC London), Alexandre Nikitenko (IC London), Natalie Heracleous (Aachen-I), Adrian Perieanu (Aachen-I).

High-energy photon-photon and photon-proton interactions at the LHC offer interesting possibilities for the study of the electroweak sector up to TeV scale and the search for processes beyond the Standard Model. After in-depth exploratory studies, first investigations of anomalous photon interactions in CMS are being performed.

External collaborators: CMS forward physics analysis group, R. Schicker (Heidelberg) and A. Szczurek (Krakow).

Precise determination of the absolute luminosity is crucial for many measurements in CMS. The measurement of the exclusive two-photon production of muons pairs by CMS provides a powerful method to calibrate the integrated luminosity.

External collaborators: CMS forward physics analysis group, CMS luminosity group.

ALthough the study of rare kaon radiative decays is not the main objective of NA62, both the detector and the beam are fully adapted to their study.

Among all radiative kaon decays, two channels are going to be studied in depth in order to see if NA62 could provide extra information:
and . These channels can provide information about direct CP violation and QCD-QED interplay.

External collaborators: A. Ceccucci (CERN).

A search for a yet-unobserved baryon number violation is performed using CMS data, following what suggested by the UCL-CP3 phenomenology group, who first proposed such a possibility in the top-quark system. Baryon number violation can manifest itself both in the production and in the decay process. In the latter case a top quark, produced in association with an anti-top, would decay with a certain branching ratio into a lepton and a W-boson. The analysis searches for such decays in a final state where both the other top quark and the W-boson decay hadronically, which is the most probable decay scenario. The analysis is close to being finalized and its results have been scheduled by CMS to be made public for the Moriond 2012 conference.

The CMS detector at the LHC is used to search for yet unobserved heavy (mass >100 GeV/c$^2$), quasi-stable (lifetime > 1 ns), electrically charged particles, called generically HSCPs.
HSCPs can be distinguished from Standard Model particles by exploiting their unique signature: very high momentum and low velocity, due to their mass and the available LHC collision energy. Two experimental techniques are used to measure the velocity of such particles. They make use of the Silicon Tracker and of the Barrel Muon Drift Tube detectors.

UCL members lead the analysis since 2010, when the first HSCP paper was one of the first published LHC search papers. Updated results, using part of the 2011 dataset, were produced for the Summer 2011 conferences.
The analysis, which is extremely model-independent and inclusive, doesn't find evidence of HSCP. It currently excludes the existence of stable gluinos, predicted by split supersymmetry, with a mass lower than about 800 GeV. This limit is the most stringent to date.

External collaborators: CMS collaboration.

Search for Higgs boson(s) within the Standard Model and beyond and also withing a minimal extension of the scalar sector (2HDM).

The final state under study is a Z decaying into a lepton pair associated with two b-jets. This topology is sensitive to a light SM Higgs via the associate ZH production, as well as a middle mass range SM Higgs boson via the inclusive Higgs production followed by its decay into ZZ with one Z decaying into a lepton pair and the other into bbar.

It is also very sensitive to the production of a non standard heavy Higgs boson decaying into Z plus A (pseudo scalar Higgs boson).

External collaborators: Nick van Remortel (Belgium, UA), Barbara Clerbaux (Belgium, ULB), and CMS collaboration.

Muons are particles that can be identified and measured with high precision by the CMS detector at the LHC. CMS can therefore be used to study the invariant mass spectrum of di-muon pairs and search for high-mass unstable particles (resonances) in a yet unexplored high mass range. High-mass resonances decaying into muon pairs are predicted in a number of models beyond the Standard Model of the fundamental interactions. Notable examples are heavy neutral gauge bosons predicted by grand unification theories, as well as gravitons arising in the Randall-Sundrum model of extra dimensions.

The first search for high mass resonances was published in JHEP by CMS using the data acquired in 2010. Updated results were produced using part of the 2011 dataset in Summer 2011. By combining di-electron and di-muon data, CMS has excluded the existence of resonances predicted by a number of theoretical models with masses below about 2 TeV. These limits are the most stringent to date.
S. Basegmez, G. Bruno and D. Pagano of the UCL CP3 group have contributed significantly to this analysis by being one of the three teams of the CMS Collaboration that has regularly analyzed new data to produce the updated invariant mass spectrum, by setting up the technique for identifying isolated muons and by computing the significance, including the look-elsewhere effect, of an excess observed at 120 GeV in both the di-electron and the di-muon channels. In addition, a new technique for measuring high energy muons, which is the fundamental ingredient of the entire analysis, has been developed in the past year by the team. This technique, which has been proved to be the most robust against the catastrophic energy losses that can be experienced by muons, is expected to be adopted in the search starting from the 2012 run.

The electroweak production cross section of single top quarks is an important measurement for LHC, being a potential window on "new physics" effects.
After having produced the very first measurement at 7 TeV (in t channel) with 2010 data, now we aim at: 1) competing with Tevatron on |Vtb| extraction; 2) observe the tW process for the first time; 3) study several differential distributions in order to test the existing models.

We currently have the convenership of the group.

External collaborators: CMS collaboration.

We are involved in the activities of the btag POG (performance object group) of CMS, in release and data validation and purity measurement. We are also interested in btagging in special cases like for colinear b-jets.

External collaborators: Strasbourg CMS group, CMS collaboration.

A precision test of lepton flavour universality can be performed by measuring the ratio RK of kaon leptonic decay rates and . Any deviation of the expected Standard Model prediction will be a hint of New Physics. This measurement has been performed at one percent level with the NA62 data taken in 2007 and 2008 in complete agreement with the Standard Model expectation. A prospective analysis for the improvement of this measurement with the full NA62 apparatus is underway.

External collaborators: Augusto Ceccucci (CERN), Cristina Lazzeroni (Birmingham).

The matrix element reweighting method attempts to compute the full likelihood of an observed event given a theoretical model. The method therefore measures the degree of compatibility of the event with the given model using as much information as available. MadWeight is a tool that fully automatize the computation of the event likelihood for any model implemented in MadGraph, by performing phase-space integration and providing a framework for taking into account the experimental resolution on the observed final state objects.
This project aims at validating the matrix element reweighting technique implemented in MadWeight on a number of benchmark searches. In some cases, the final goal is the efficient identification of background events. The final states that are being considered are: Zbb, single top, ttbar resonances and dimuon resonances.