CMS Bulletin

PHYSICS

Since the last CMS Week, all physics groups have been extremely active on analyses based on the full 2010 dataset, with most aiming for a preliminary measurement in time for the winter conferences. Nearly 50 analyses were approved in a “marathon” of approval meetings during the first two weeks of March, and the total number of approved analyses reached 90. The diversity of topics is very broad, including precision QCD, Top, and electroweak measurements, the first observation of single Top production at the LHC, the first limits on Higgs production at the LHC including the di-tau final state, and comprehensive searches for new physics in a wide range of topologies (so far all with null results unfortunately). Most of the results are based on the full 2010 pp data sample, which corresponds to 36 pb^-1 at √s = 7 TeV. This report can only give a few of the highlights of a very rich physics program, which is listed below by physics group.

Most of these analyses profit from the particle-flow reconstruction performance. The process of integration of the particle-flow reconstruction and derived physics objects within the POGs (and relevant DPGs) is underway. The goal is to converge, for each object, towards a combination of the best of existing standard POG reconstruction algorithms and current particle flow reconstruction. This will help obtain the best objects for physics analyses together with fully consistent event description. The 2011 data reconstruction will profit from this integration.

(For related information on the Trigger, refer to the relevant section elsewhere in this Bulletin.)

Muons

The Muon POG has studied the performance of the muon HLT on 2010 data, including isolation and pile-up, and has prepared the trigger paths for the 5E32 menu. Contributions from the PAGs are greatly appreciated. In addition, comprehensive studies of the performance of muon reconstruction and identification in 2010 have been done. Work on the draft of the paper (MUO-10-004) summarising these results is nearing completion, with an approval targeted for April. This work also led to several improvements to the reconstruction algorithms – examples were shown during Physics week in February.

The tools and people put in place for the analysis of 2010 data will be used to re-assess the performance with the first 2011 data. High-p_T muons move to the top of the priority list. The resolution and momentum scale have been re-evaluated on recently-produced tracker-pointing and superpointing skims of 2010 Cosmics data, and measurements of reconstruction and trigger efficiencies on cosmic muons are in progress. The first attempts to complement the sample of cosmic muons collected in dedicated runs by cosmic muons collected during collision runs with a dedicated RPC trigger (TT25) look promising.

Tau

The Tau POG finished commissioning of the tau identification and reconstruction algorithms using 2010 data. A tau reconstruction efficiency of 50% was measured for a level of fake-rates from jets of 1%. This allowed CMS to perform in 2010 Higgs and SUSY analyses using tau decay channels that already compete with similar studies done by Tevatron experiments. Based on the commissioning results, the algorithms were further improved for 2011 analyses.

A new trigger strategy that uses particle flow objects and cross triggers, as well as tighter isolation requirements, should allow CMS to keep the trigger efficiency in 2011 high for the most important physics analyses with taus while keeping the rate sustainable for the CMS trigger system.

JetMET

The JME POG has prepared several PASs and papers based on the 2010 data. This includes jet substructure and algorithms (JME-10-013), providing the PAGs with the ability to tag jets from boosted heavy objects like Top and W, and jet resolution (JME-10-014), measured in-situ and with systematic uncertainties assigned for use by the PAGs. In addition, two critical JINST papers on the performance of MET and Jets are now in the CMS approval process. Other notable developments include:
– complete JEC with first pile-up corrections for jets for 2010 data and the start of 2011 run,
– a MET significance algorithm and algorithms to reduce the effect of ECAL holes on MET,
– jet corrections, ID, and pile-up rate reduction for triggers in a new JetMET trigger group,
– rapid feedback to PVT on DQM and validation issues coordinated tightly with DPG.

B-tagging

The BTV POG completed the characterisation of b-tagging performance with the 2010 data. We measured the performance with impact parameter, secondary vertex finding, and jet probability algorithms using di-jet events with an associated muon and the ttbar events in the datasets collected by CMS. The b-tagging efficiency is measured to be 60% for a mistag rate of around 6%. We determined the data/MC scale factors for the efficiencies and mistag rates. Many analyses and results in the Top, SUSY, and BPH PAGs were based on identification of b-jets in the events. The work of the BTV group is an integral part of the many successful measurements CMS was able to produce with the early data. The focus of the group has now shifted to studying the 2011 data and optimising the b-tagging algorithms to continue to maintain the excellent performance in events with increased pile-up.

Forward

Jets in the region at large rapidities (3 < |η| < 5) have been measured for the first time (FWD-10-003). In this region, the parton densities are probed at small momentum fractions. The measured inclusive cross-sections agree rather well with the theoretical predictions. The energy flow in the forward region is a very sensitive probe for the structure of the underlying event and multiparton interactions. With the first measurements it became clear that models describing the underlying event in the central region do not necessarily describe well the forwards region. Diffractive dissociation at highest energies is the important contribution to pile-up. The description of diffractive dissociation is tested in detail with a measurement of the energy distribution in the forward calorimeters and with charged particle spectra in the central detector. Non-trivial correlations have been observed.

BPH

The B-Physics group has performed several measurements of bottom (or beauty) quark production. Inclusive cross-section measurements are based on the identification of semi-leptonic b decays into muons (BPH-10-007) and b-jet tagging with secondary vertices (BPH-10-009). In addition, differential production cross-sections of several B mesons (B+, B0 and Bs – BPH-10-004, 005, 013) have also been measured in exclusive decay channels. The dynamics of B hadron production was further investigated by measuring the BBbar angular correlations (BPH-10-010). A novel technique based on secondary vertex identification was adopted, accessing for the first time at LHC the region of collinear B-hadron emission. Measurements of quarkonium production are also an important part of the group’s physics program. Differential cross-sections of Y(1S, 2S, 3S) mesons have been released (BPH-10-003), while the measurement of J/ψ spin alignment is expected soon. Searches for rare decays and exotic states have been initiated, leading to the observation of the exotic meson X(3872) in the decay to J/ψ and two pions.

QCD

Many QCD results have been approved recently. In the low p_T sector, the charged particle transverse momentum spectra were measured for pp collisions at √s = 0.9 and 7 TeV (QCD-10-008). The consistency of the 0.9 and 7 TeV spectra is demonstrated with an empirical x_T scaling that collapses the differential cross-sections from a wide range of collision energies onto a common curve. A measurement of the underlying activity in scattering processes was performed at √s = 7 TeV (QCD-10-010). The production of charged particles with pseudorapidity |η| < 2 and p_T > 0.5 GeV/c is studied in the azimuthal region transverse to that of the leading set of charged particles forming a track-jet. A significant growth of the average multiplicity and scalar p_T sum of those particles is observed with increasing p_T of the leading track-jet, followed by saturation above a few GeV/c.

In the high-p_T jet sector, a measurement of the inclusive jet cross section (QCD-10-011) was performed using the full 2010 dataset, extending the kinematic regime of previously published results and found to be in agreement with the perturbative QCD predictions at NLO. It also serves as a jet commissioning measurement, indicating that the superb jet reconstruction and jet understanding in CMS. Similarly, a measurement of the di-jet cross-section as a function of the di-jet invariant mass (QCD-10-025) also is found to be in agreement with the perturbative QCD predictions at NLO. The experimental systematic uncertainties on these cross-section measurements are roughly comparable to the theory uncertainties, and a future reduction of the jet energy scale uncertainty will constrain the PDF models. A measurement of the ratio of the inclusive 3-jet to 2-jet cross-sections as a function of the total jet transverse momentum in the range 0.2 TeV < H_T < 2.5 TeV was performed. Comparisons between the data and the predictions of different QCD-based Monte Carlo models for multi-jet production extend the validity of the models at LHC energies.

While several measurements with prompt photons are in progress, the measurement of the prompt isolated photon cross section performed last year was recently accepted by Physical Review Letters.

Top

The Top quark analyses based on 2010 data cover measurements of the Top-pair production cross-section utilising both the di-lepton (TOP-10-005) and the lepton+jets decay channels – without b-tagging (TOP-10-002) and with b-tagging (TOP-10-003) – as well as the first determination ever of the Top mass at a collider other than the Tevatron (TOP-10-006). Different approaches and cross-checks on the cross-section measurements were made, including one approach that simultaneously fits of many backgrounds and systematics across multiple bins of jets and tags.

Top-pair production also has been used as a probe for testing the presence of physics beyond the Standard Model in two ways: by looking for resonances in the production mechanism (TOP-10-007), as well as by looking for anomalies in the Top charge asymmetry (TOP-10-010), possible hints of signals of new physics coupling to the Top quark sector. All of the results, in terms of total or differential distributions, show impressive agreement with the expectations of the Standard Model. The limits on the presence of new physics can soon be competitive with, if not superior to, the Tevatron results, with the foreseen statistics of 2011.

Impressively, single Top electroweak production has been confirmed by CMS with the 2010 data alone. The first total single Top production cross-section in the t-channel has been measured (TOP-10-008), allowing a direct determination of |Vtb|. The quality of the detector, the simulation and the analyses will make it possible to study single Top production with great detail in 2011.

Electroweak

Many precise and novel measurements were made in the electroweak sector. Notable is the first measurement of the spin polarisation of the W boson at a proton collider, where the production of W bosons of both charges recoiling against hard jets have been measured to be predominantly left-handed, as expected from the dominant gluon-quark W+jet production process at the LHC (EWK-10-014).

The precision cross-section measurements benefit from the reduced uncertainty on the integrated luminosity, which is now reduced to 4%. The cross-section measurements of the inclusive production of W and Z bosons, previously published using 3 pb^-1 of data, have been updated (EWK-10-005). The experimental uncertainties, dominated by systematic uncertainties linked to lepton identification and selection, are now at the level of 1%, much smaller than the theory uncertainties. The ratio of W-to-Z production cross-sections, for which the luminosity uncertainty cancels, constitutes a stringent test of the theory prediction with the most recent parton densities.

The inclusive productions of W and Z bosons have also been studied in the tau-lepton channel. A significant W signal is obtained in the hadronic tau decay channels, over a background from multi-jets that is controlled from the data (EWK-11-002). The study of the Z decay into tau pairs in four different final states allows a precise estimation of the tau-lepton identification efficiency in the hadronic decay modes (EWK-10-013).

At the LHC, the production of W⁺ bosons is on average 40% larger than that of W^– bosons, an asymmetry that depends on the boson rapidity. The resulting lepton charge asymmetry has been measured as a function of lepton pseudorapidity (EWK-10-006). The precision is such that different sets of parton densities can already be distinguished.

The differential production cross-sections of the Z boson as a function of boson rapidity and transverse momentum have been measured in the electron and muon channels (EWK-10-010). The electron identification capabilities of the HF are used to extend the boson rapidity range up to |y|=3, beyond the Z rapidity plateau. The agreement with predictions is good for all boson rapidities and at large transverse momenta up to 600 GeV. In the low transverse momentum region, below 30 GeV, the data confirm the quality of the most recent tunes of the Pythia soft QCD parameters at the LHC.

Measurements of the production cross-section of Drell-Yan muon pairs have been measured in bins of the di-muon invariant mass (EWK-10-007). The agreement with predictions is good over the full mass range. The uncorrected forward-backward asymmetry of Drell-Yan pairs also was obtained in bins of mass, and good agreement is found with the simulation in both electron and muon channels (EWK-10-011). The full kinematics of Drell-Yan muon pairs has been exploited in a promising angular analysis that demonstrates the feasibility of a precision measurement of the weak mixing angle at the LHC.

The associated production of jets with W and Z bosons has been studied up to four inclusive jets. Jets are selected with a transverse energy threshold of 30 GeV (EWK-10-012). The data clearly favour theory predictions based on the full calculation of matrix elements at NLO. A first measurement of the fraction of b-jets in the Z+jet sample has been performed, with a 25% precision, based on samples of several tens of Z events with one b-tagged jet (EWK-10-015).  Several events with two b-tagged jets have been identified.

Finally the first di-boson signals at the LHC have been studied. The rate of WW production, which constitutes an irreducible background for the Higgs boson searches in that mode, was measured based on a sample of approximately thirteen di-lepton plus MET events, and found to be consistent with expectations (EWK-10-009). The production cross-sections of W+γ and Z+γ, for photon transverse momenta larger than 10 GeV, were also measured with precisions of the order of 10% (EWK-10-008). A first indication of the radiation amplitude zero in the W+γ mode comes from the charge-signed rapidity difference distribution. The WW and Wγ and Zγ measurements were used to set first limits on anomalous triple gauge couplings at the LHC.

Figure 8: Compilation of CMS electroweak and Top quark measurements compared to theory.

SUSY

A comprehensive strategy for SUSY searches has been put in place and has been exercised on the 2010 dataset. For searches in the all-hadronic final state, three complementary techniques using kinematics and detailed detector and background understanding have been pursued. These include the “alpha-T” approach (SUS-10-003), recently accepted for publication by Physics Letters B and also recently extended to include b-tagging (SUS-10-011), a new approach using the dimensionless razor (R) variable (SUS-10-009), and the traditional jets and MET approach (SUS-10-005). The reach from these searches is far beyond the Tevatron, and for the first time the limits are expressed in terms of simplified models to help model builders use the results.

The leptonic searches for SUSY include toplogies with one lepton (SUS-10-006), same-sign di-leptons (SUS-10-004), opposite-sign di-leptons (SUS-10-007), and multi-leptons (SUS-10-008). Included are hadronically decaying tau leptons in many channels. SUSY searches with photons include di-photons (SUS-10-002) and photon+lepton searches (SUS-11-002), with sensitivity surpassing previous experiments.

A wide range of searches has been performed, and limits are set on SUSY in a variety of models. Emphasis has been on data-driven methods and multiple techniques for robust analyses. CMS is ready to make a discovery in the 2011/12 data!

Exotica

The Exotica group has been extremely productive in mining the 2010 data for a variety of new physics models, delivering about 20 analyses on the complete 2010 dataset. Of these, the searches for stopped gluinos (EXO-10-003), and first and second generation leptoquarks in the leptons+jet final state (EXO-10-005,7) have been already been published while the searches for W′ (EXO-10-014,15), Z′ (EXO-10-012,13), b′ (EXO-10-018) and microscopic black holes (EXO-10-017) have been submitted for publication.

Additionally, Exotica has produced preliminary results from a number of other analyses. These include the search for first and second generation leptoquarks in the lepton+MET+jets final state (EXO-10-006,8), excited leptons (EXO-10-016), RS gravitons (EXO-10-019), LED in γγ (EXO-10-026), μμ (EXO-10-020), and in monojets (EXO-11-003), low mass ttbar resonances (EXO-10-023), a boosted Z as a search for excited quarks (EXO-10-025), lepton-jets (EXO-11-013) as a search for low-mass dark matter, and an RPV search in multi-jet resonances (EXO-11-001). Many of these results will be submitted for publication shortly as well.

With these searches, Exotica has set the most stringent limits over much of BSM model-space. The group has now begun preparing for a discovery in whichever of these analyses Nature will (hopefully) provide a signal in the 2011 data.

Higgs

The search for the Higgs launched in earnest with the 2010 data, with the completion of four analyses. This includes the Higgs search in the WW decay channel (HIG-10-003), the first paper on the Higgs boson published at the LHC. The search for MSSM H→ττ (HIG-10-002) exploits the excellent tau identification capability of CMS, significantly extending the reach beyond that of the Tevatron (see below). The searches for doubly-charged Higgs (HIG-11-001) and singly-charged Higgs in t→Hb (HIG-11-002) have already sensitivity similar to previous measurements performed at the Tevatron. Preparations are now underway to conduct an exhaustive search across many Higgs decay channels using the >1 fb ^-1 of delivered luminosity expected in 2011-’12.

Figure 9: Excluded region in the tanβ vs mA plane for the MSSM H→ττ search.