PHYSICS

The Physics Groups are actively engaged on analyses of the first data from the LHC at 7 TeV, targeting many results for the ICHEP conference taking place in Paris this summer.

The first large batch of physics approvals is scheduled for this CMS Week, to be followed by four more weeks of approvals and analysis updates leading to the start of the conference in July.

Several high priority analysis areas were organized into task forces to ensure sufficient coverage from the relevant detector, object, and analysis groups in the preparation of these analyses. Already some results on charged particle correlations and multiplicities in 7 TeV minimum bias collisions have been approved. Only one small detail remains before ICHEP: further integrated luminosity delivered by the LHC! Beyond the Standard Model measurements that can be done with these data, the focus changes to the search for new physics at the TeV scale and for the Higgs boson in the period after ICHEP.

Particle Flow

The PFT group is focusing on the commissioning of the lepton identification and reconstruction after having confirmed the robustness of the reconstruction and identification of the charged hadrons, photons, and neutral hadrons at 7 TeV.

While waiting for a larger amount of leptons from the decay of W, Z and J/ψ, new Monte-Carlo studies have been conducted. They show that the reconstruction and identification of electrons and muons in the particle flow algorithm, as well as the particle-based lepton isolation, are now understood.

In EWK analyses, the particle flow muons perform in the same way as standard muons, and the containment of the particle flow electrons has been brought to the level of EGM reconstructed electrons. The particle-based electron and muon isolation now outperforms the traditional detector-based isolation approaches, and random cone studies show that the particle-based isolation efficiency can be extracted from a MinBias data sample. These new developments are available in CMSSW_3_7_0, and will be documented in PFT-10-002 and PFT-10-003.

Muon

Studies of the muon reconstruction performance expanded in scope and intensity with the advent of collisions at 7 TeV this year, as the number of muons recorded exceeded the 2009 count already on the first day of data taking and increased by several orders of magnitude over the following weeks.

Almost all of the measured distributions of muons are reproduced very well by the Monte Carlo simulation. Good progress is being made on understanding a few variables for which data and MC do not yet match perfectly. Even though about 3/4 of the muons recorded until now are expected to arise from pion and kaon decays, the very loose trigger selections used during this period made it possible to study efficiencies of muon HLT triggers by analyzing good-quality muons reconstructed offline. These efficiencies generally agree with MC predictions. First estimates of various reconstruction, identification, and trigger efficiencies obtained by applying the “tag-and-probe” method to muons from J/ψ decays became available, as well as fake rates evaluated on high-purity samples of pions (from Kshorts), kaons (from phi's), and protons (from Lambdas). Overall, the prospects for ICHEP look good.

Finally, as a reminder of the intense studies of cosmic muons conducted in 2008 and 2009, a paper describing the measurement of the charge ratio of atmospheric muons (arXiv:1005.5332) has gone through all rounds of CMS refereeing and was submitted to Phys. Lett. B.

Electron/Photon

The EGM group is carrying on with 7 TeV data the earlier activities started in 2010 with the data at 900 and 2360 GeV that were aimed at commissioning the electron and photon reconstruction and selection algorithms.

Four PASs are foreseen for the ICHEP conference that are the result of a very good collaboration between EGM and ECAL DPG. Two of them are managed by the ECAL DPG and will cover the aspects of the ECAL performance and calibration with the first 7 TeV data. The other two will instead cover the electron and photon commissioning and the measurement of the first performance on data. Given that the expected integrated luminosity that will be possible to analyze before ICHEP is expected to be of the order of 100nb-1, we consider it very unlikely to be able to cover analyses using electrons from Z decays for the measurement of selection efficiency or energy resolution.

The main objective of current electron and photon analyses is to commission all variables used in the selection and to obtain the first measurements of efficiency, purity and fake rates for given reference selections supported by the EGM group. The primary source of prompt electrons signal will be electrons from W that we expect to select with high purity in events containing a reconstructed electron or ECAL cluster, missing transverse energy, and very little additional hadronic activity. However, we have also observed a J/ψ into electrons peak that will serve to commission the reconstruction of electrons at low PT. A dedicated double low PT electron trigger is used to collect these events.

All analyses are rapidly evolving as the data arrive and we are moving now from analyses of all events, based on minimum bias triggers, to analyses using high PT photon and electron triggers. It is now becoming important to understand the turn-on curve of these triggers as well as their efficiency.

The EGM group has strong collaboration with the PFT and Physics Analysis Groups on electron commissioning and on physics analyses. Additionally, the EGM group is working with these other groups in order to define and put in place a common group skimming of electron and photon samples, expected to satisfy the needs of most of the physics groups and that could later evolve into centrally produced skims.

Tracking

With the LHC collisions at 7 TeV this year, the Tracking POG started with re-commissioning track reconstruction as for the 900 and 2360 GeV collision data, and then quickly moved toward making quantitative measurements of tracking performance.

Many tracking results from last December’s data are being published (TRK-10-001) and now with ICHEP on the horizon four separate PASs are planned to document the tracking efficiency, the tracker material, the momentum scale, and primary vertexing performance.

As CMS has accumulated more integrated luminosity, CMS has been able to reconstruct more known resonances to validate the overall performance of the tracking, e.g. K0S, Λ0, and φ peaks. With the added data, decays such as Ω- → Λ 0K-, and charm decays including D+→K-π+π+ and D*→ D0π, with D0→ K π have been reconstructed. The two plots included here show the D0→ K π peak and the D*-D0 mass difference.

These are much more than just PR plots now. From the ratio of the number of reconstructed D0→ K π and D0→ K3 π decays, the tracking efficiency can be extracted. With the data already recorded, samples of reconstructed resonances are used to study the momentum scale, and large samples of reconstructed conversions and nuclear interactions used to study the tracker material. The ability to reconstruct multiple interactions is also under study, which will be crucial as the LHC delivers higher luminosity. The results of these studies should be ready for ICHEP, but this is the first step in the program of measuring tracking performance with data.

Jet/MET

The JetMET group produced new approved results from the 7 TeV collision runs confirming the good understanding of jets and MET. Three different CMS reconstruction methods, calorimeter only, track-corrected, and particle flow, were investigated using about 0.3 nb-1 of minimum bias events recorded in 2010.

Fig.1: D0 → Kπ peak and the D*-D0 mass difference.

A sample of dijet events was selected and used to monitor jet quantities, extending the studies to larger transverse momenta compared to the previous 900-2360 GeV results. The first figure below shows the nice data-MC comparison of the dijet invariant mass. There was the same level of agreement for the other kinematic and jet quality variables. MET was cleaned of the effects of instrumental anomalies and beam induced backgrounds, thus significantly reducing tails. Inclusive and dijet events were used to monitor MET variables, which agreed well with simulation. As an example, the second figure shows the MET spread as a function of the scalar sum of the transverse energy for data and MC.

Jets and MET can be then used for the wide CMS physics program expected for ICHEP. The JetMET group is now working on determining more detailed information on data, as the jet response and MET corrections.


Fig.2: data-MC comparison of the dijet invariant mass.
Fig.3: MET spread versus scalar sum of the transverse energy for data and MC.
QCD

With the addition of 7 TeV data, the QCD group quickly published a paper (arXiv:1005.3299) based on the charged hadron distribution paper that was the first CMS publication. The preliminary analysis was done within a few hours of collecting data, with detailed checks and systematic studies done in due course.

The QCD group has also submitted a paper (arXiv:1005.3294) for publication on Bose-Einstein correlations, which measures spin-statistics correlations between same charge pions in the CMS minimum bias dataset at 0.9 and 2.36 TeV, concluding that the correlated particle emission region increases as a function of particle multiplicity.

Another particle correlation study is approved and heading towards publication, this one measuring two particle correlations in η in 0.9, 2.36 and 7 TeV data and modeling the result with a simple parameterisation that assumes particles are decay products of “clusters” which are emitted independently in the initial interaction. The effective cluster size and decay width can then be extracted from the data using this parameterisation, which shows a dependence of the cluster size on the beam energy, while the cluster decay width is roughly constant. Additional analyses on multiplicity distributions, identified particle spectra, and charged hadron spectra at high PT are well advanced and are expected to be completed for ICHEP.

One important job of the QCD group is to work on tuning the CMS Monte Carlo’s to better model the underlying event and minimum bias data. This is extremely important especially as we head into the era of pileup, where every hard scattered event will have many minimum bias events to keep it company. This effort is an LHC wide one, with a recent joint workshop held between ATLAS/CMS/ALICE and LHCb:
http://indico.cern.ch/conferenceOtherViews.py?view=standard&confId=87647

New tunes for CMS are coming and have already been shown to model better the underlying event (the part of the event that is not associated to the hard scatter). The data analysis that inspired the new tunes has been completed at 900 GeV and is well advanced for the 7 TeV data.

The QCD group, and the associated Jet Task Force spanning also the DPGs and POGs, has many jet analyses heading for ICHEP. One important analysis is the inclusive jet cross section using all four jet types: calorimeter only jets, particle flow jets, JPT (jet plus tracks) jets and track jets. This analysis is well advanced and is a nice verification of the different jet flavors, compared with NLO predictions. In addition, several dijet analyses are working towards ICHEP, two of them shared with the Exotica group and looking for new physics in dijet mass resonances or cross section ratios. Other dijet analyses include decorrelations, which is a good observable for Monte Carlo testing/tuning and for angular distributions which are sensitive to contact interactions. Jet substructure and overall event shape variables are also being studied and compared to Monte Carlo.

The QCD group is also studying photon production. Prompt photons, coming directly from the hard interaction, are a good probe of perturbative QCD. In addition, they are background to Higgs decay into two photons and other searches using photons. The inclusive photon and inclusive photon plus jet(s) spectra are two analyses targeted for ICHEP. In total the QCD group has 13 analyses specifically targeted for ICHEP and 5 that are already public (3 are published or submitted to journals).

B-Physics

The task force on measuring the inclusive b cross section is making very good progress. The analysis based on muon relative PT has first estimates of central values from 2nb-1 of data, which agree well with Pythia. The relative PT analysis having also b-tagged jets (i.e. with secondary vertices) has processed 10nb-1 and has first estimates of b-fractions, but not yet central values. The inclusive b-tagged jet cross section has first estimates of central values from 1nb-1. Measurements of final state b-jet correlations in a final state of J/ψ→μ μ plus an additional muon need at least 500nb-1 for two Delta phi bins. Alternative methods without leptons (inclusive vertex tagger) look promising and can work also with small integrated luminosity.

A second task force in the B-physics area is focused on measuring quarkonia production, J/ψ and upsilon, in their decays to muons. There are two principal motivations for the study of the J/ψ and upsilon at CMS: (1) The elucidation of the physical process (hadroproduction) that produces the J/ψ and upsilon in proton-proton collisions which is not presently understood, and (2) The J/ψ and the upsilon constitute standard candles to calibrate the detector response to low PT muons at CMS. The J/ψ cross section is expected to be about 100 nb. A plot of the invariant mass of opposite sign muon pairs for ~15 nb-1 of data is shown below. The fit (black line) is to a Crystal Ball function. The clearly visible low side tail is due to Final State Radiation. The blue dashed line is the opposite sign combinatoric background. The upsilon cross section is expected to be about ten times smaller than for the J/ψ, and so is harder to find. The radial excitations of the upsilon, known as the Y(2S) and Y(3S), are close in mass with the latter two overlapping when convolved with the CMS mass resolution. A collision having a candidate upsilon decay to muons is shown in the display below.

Fig.4: Invariant mass of opposite sign muon pairs (~15 nb-1 of data).
Fig. 5: Example of a candidate upsilon decay to muons.

Electroweak

The electroweak Physics Analysis Group focuses on the study of the production and decay of electroweak vector bosons with the CMS data.

The results aimed at the ICHEP Conference include the first measurements of W and Z production cross sections in proton-proton collisions at a 7 TeV center-of-mass energy. These measurement rely on the understanding of the selection efficiency of high transverse momentum isolated electrons and muons.

Although it is possible to determine efficiencies from the data, the ICHEP results, due to lack of statistics, will have to be based on Monte-Carlo simulation efficiency estimates. The Vector Boson Task Force has been created to carry out these cross-section measurements.

A high-purity sample of W bosons can be obtained by requiring large missing transverse energy in addition to an isolated lepton. Preliminary lepton charge asymmetry distributions will be presented at ICHEP; eventually lepton charge asymmetry in W events will provide strong constraints on the parton densities for valence quarks and sea antiquarks in the proton.

Vector bosons are also produced in association with hadronic jets, and first jet multiplicity distributions will be presented. The sample of boosted W bosons recoiling against jets can be used to measure the polarization asymmetry. Finally, depending on the available statistics by ICHEP, the first events with reconstructed Z bosons in their decay into a tau lepton pair will be presented.

Top

The Top group has recently launched two task forces en route to the first top-quark results for the summer conferences: one for the dilepton ttbar channel and another for ttbar decaying to e + jets or mu + jets. The first rough cross-section measurements in these channels can be performed starting with a minimum of 2 pb-1 of good-quality data, although probably slightly more is needed for the lepton + jets channels.

The baseline goal of these task forces is to perform a publishable cross-section measurement of ttbar production at 7 TeV using selections that are robust and simple and do not use b-tagging. These results shall be accompanied by figures showing distributions in support of the signal hypothesis of the selected sample, such as the jet and b-tagging multiplicities and the (coarsely) reconstructed top mass. The lepton + jets task force is planning, in parallel, an analysis that is also making use of b-tagging in the selection itself, in order to increase the purity.

Currently, the two task forces are synchronizing their lepton and jet reconstruction and identification within the group as well as with the EWK task forces wherever possible and reasonable, and are looking into the control samples (small jet multiplicities) to study data-driven methods of background estimation.

SUSY

The SUSY group is developing a broad set of searches based on simple, topological signatures. In early 2010, the sensitivities of many of these searches were evaluated and found that with as little as 50–100 pb-1 significant reach could be obtained beyond current Tevatron searches. In some channels, there is potential to uncover new physics with 10 pb-1 or even less. See CMS Note 2010/008 for further details on these projections and similar ones in Exotica and Higgs searches.

All of the SUSY searches are being commissioned as rapidly as possible. The initial steps in the plan involve working closely with the Physics Object Groups, especially JetMET, to understand the objects that will be used as the basis for SUSY searches.

Members of the group are contributing strongly to JetMET results that will be sent to the ICHEP conference. In addition, a SUSY PAS is being prepared for ICHEP, focusing on the QCD backgrounds. Due to the large QCD cross section and substantial theoretical uncertainties, it is critical that procedures are established quickly for suppressing and measuring this background using data-driven methods. These studies are expected to launch CMS towards a very intensive and exciting period in the fall. New people are very welcome in the SUSY group, especially those with direct detector experience.

Exotica

The current focus of the Exotica group is to produce first “pilot” results for ICHEP and position ourselves for a full-blown attack on new physics later this year, with the statistics of the order of 100 pb-1. Six analyses were identified that could extend sensitivity beyond the Tevatron limits with an integrated luminosity of 1-10 pb-1.

The first two of these analyses are searches for new physics (e.g. excited quarks, diquarks, and other strongly produced particles) in inclusive dijet events. One analysis searches for resonances which are exhibited as a “bump” in the dijet mass spectrum, while the other compares the ratio of central to forward dijet event, exploiting the fact that new physics is produced more isotropically. The next two analyses on the path for ICHEP involve the search for new long-lived heavy stable charged particles (e.g., gluinos bound into R-hadrons or stop squarks). The first analysis searches for these new particles by looking for the anomalously large ionization energy (dE/dx) that a slow moving particle would deposit in the tracker. The other analysis looks for the slowest of these HSCPs, which may stop due to nuclear interactions before exiting the CMS detector and eventually decay in the CMS calorimeter. To identify these out-of-time decays a trigger is used that is kept live only between the LHC bunch crossings (so that there are no collision backgrounds). The final two ICHEP analyses are searches for first and second generation leptoquarks (hypothesized particles, which carry both lepton and baryon number) in the eejj and µµjj final states.

Post-ICHEP, the plan is to publish pilot analyses with 10-50 pb-1 of data and, in parallel, pursue another dozen or so analyses expected to converge into a publication later this year or in early 2011. These include various searches for a fourth generation of matter, excited leptons, extra spatial dimensions, and additional gauge bosons.

Finally, the Exotica group is running a "Hotline" that identifies a handful of the most interesting events every day and reports them to a team of dedicated “scanners” allowing for a prompt feedback on very rare detector problems “hidden” in standard DQM output, as well as any hints of unexpected signatures that possibly come from new physics. Despite its short existence the Hotline has already identified a number of subtle detector and reconstruction problems reported to the corresponding DPG/POGs. Some of these resulted in new cleaning cuts and modifications to reconstruction algorithms.


by D. Acosta, G. Dissertori, G. Rolandi and the Physics Groups convenors