Experimental References

SUSY

Experimental Overview of SUSY and Results

Claus Horn's talk at the ATLAS juniors meeting

Highlights

• Motivations for SUSY: Unification of forces, solution of the hierarchy problem, candidates for dark matter, necessary for quantum-gravity
• SUSY is the only possible extension of the Poincare group
• Gauge Mediated SUSY breaking
  • LSP is always gravitino and can be very light
  • Distinct event signature: photon/tau + missing energy
  • Very predictive mass spectrum, easy to distinguish from SUGRA
• Slides 36-38 for a nice description of SUSY interactions
• SUSY introduces no new interactions. Gauginos and sfermions could to the same particles as the SM partners
• Think of chi0 as the supersymmetric Z0 and chi+/- as supersymmetric Ws
• MET depends on mass difference not mass of LSP

Event Generators

Les Houches Guidebook to MC Generators

Paper compiled during Les Houches working group meeting
Spreadsheet summarizing the MC Generators -- My version
Spreadsheet summarizing the MC Generators -- Stephanie's much better version
Link to the generators' homepages

Abstract Recently the collider physics community has seen significant advances in the formalisms and implementations of event generators. This review is a primer of the methods commonly used for the simulation of high energy physics events at particle colliders. We provide brief descriptions, references, and links to the specific computer codes which implement the methods. The aim is to provide an overview of the available tools, allowing the reader to ascertain which tool is best for a particular application, but also making clear the limitations of each tool.

Highlights

• A physicist analysing hadron collider data often obtains the most accurate theoretical predictions by combining components of many different simulation programs—minimum bias from one generator, the signal process from another, and yet more programs for background generation.
• The observable value must be insensitive to soft emissions or collinear splittings. These conditions are known as infrared safety.
• Higher order effects are added by “evolving” the event using the parton shower, which allows partons to split into pairs of other partons (this splitting is usually denoted as branching in this context). The resultant partons are then grouped together or hadronized into colour-singlet hadrons and resonances are decayed. Finally, the underlying structure of the event is generated: beam remnants, interactions from other partons in the hadrons, and collisions between other hadrons in the colliding beams (called pile-up).
• See figure 5 for a nice diagram for showering and hadronization

B-physics

B-tagging at CDF and D0: Lessons for LHC

Paper by T. Wright

Abstract The identification of jets resulting from the fragmentation and hadronization of b quarks is an important part of high- pT collider physics. The methods used by the CDF and DØ collaborations to perform this identification are described, including the calibration of the efficiencies and fake rates. Some thoughts on the application of these methods in the LHC environment are also presented.

Highlights

• Because the fragmentation is hard and the b hadrons retain about 70% of the original b quark momentum, the leptons will generally have high pT relative to the jet pT , which makes them easier to identify and separate from lepton sources in generic jets such as decays in flight of pions or kaons. The large mass of b hadrons also helps, as leptons from B decays will have ∼ 1 GeV/c of pT relative to the jet axis, while leptons and fakes in generic jets tend to be more closely aligned with the jet.
• Impact parameter resolutions are typically 40-60 µ m depending on the track pT , including a 30 µ m contribution from the beam width.
• the presence of leptons is a good signature of the presence of b hadrons in a jet. The key is to define an identification algorithm that maintains good performance even in the busy environment around the center of the jet. That means that quantities which are typically used for high- pT lepton selection, such as calorimeter energy deposition consistent with electrons or muons, cannot generally be used because of the presence of other particles nearby
• The impact parameters are signed such that tracks which cross the jet axis behind the primary vertex relative to the jet direction are negative. b-jets show a clear excess of tracks with significant positive displacement.

A Search for Anomalous Heavy-Flavor Quark Production in Association with W Bosons

Paper by the D0 collaboration

Abstract We present a search for anomalous production of heavy-flavor quark jets in association with a W boson at the Fermilab Tevatron ppbar Collider. This search is conducted through an examination of the exclusive jet spectrum of W +jets final states in which the heavy-flavor quark content has been enhanced by requiring at least one tagged jet in an event. Jets are tagged by the combined use of two algorithms, one based on semileptonic decays of b/c hadrons, and the other on their lifetimes. We compare data in e + jets (164 pb−1) and µ + jets (145 pb−1) channels, collected with the DØ detector at √s = 1.96 TeV, to expectations from the standard model, and set upper limits on anomalous production of such events.

Highlights

• At the Tevatron, the primary SM contributions to a W boson associated with HF quarks in the final state are expected to be from t¯t, W b¯b/c¯c (where the b¯b or c¯c pairs arise from gluon splitting), and W c final states, with additional contributions arising from single top quark or W Z (with Z → b¯b/c¯c) production.
• To eliminate poorly reconstructed events, the primary vertex (PV) of the event must contain at least three tracks, and its z -position (along the beam) has to be closer than 60 cm from the center of the detector. Finally, to reject multijet background, we require a reconstructed transverse mass consistent with that of the W boson, 40 < MWT < 120 GeV/c2 . In calculating MWT , we assume that the MET corresponds to the transverse energy of the neutrino.
• The soft-lepton tagging (SLT) algorithm is based on low-pT muons arising from semileptonic decays of HF quarks (via virtual W bosons) that are produced near a jet in (η, φ) space.
• Typical SLT efficiencies for b-quark jets are approximately 11%, and 0.4% for light-quark jets.

Top Physics

Measurements of Top Quark Properties

Paper by Marc-Andre Pleier. See also Francesco Spano's summary slides

Abstract This review summarises the top quark physics program pursued at Fermilab's Tevatron proton-antiproton collider, operating at a centre of mass energy of 1.96 TeV, and its two collider detectors CDF and D0. More than a decade after the discovery of the top quark at the Tevatron, it remains the only place to produce top quarks and study them directly until the Large Hadron Collider at CERN starts operation. The Tevatron's increased luminosity and centre of mass energy offer the possibility to scrutinise the properties of the heaviest fundamental particle known to date by performing new measurements that were not feasible before, like the first evidence for electroweak production of top quarks and the resulting direct constraints on the involved couplings. In addition, the precision of prior measurements has been improved to an unprecedented level, illustrated for example by the measurement of the top quark mass with a relative precision of 0.7%, marking the most precisely measured quark mass to date and allowing the prediction of the mass of the Higgs boson that still remains to be discovered. The various measurements of top quark properties provide stringent tests on the predictions performed in the framework of the Standard Model of elementary particle physics.

Highlights

• The helicity of the W boson in top quark decays can be used to test the V−A Lorentz structure of the W tb interaction (see Section 3.3.3). According to the expectation from the Standard Model, W bosons from top quark decays should be longitudinally polarised with a fraction f0≈ 70% and left-handed with a fraction f−≈ 30%, while the right-handed fraction f+ is strongly suppressed and below the per mill level [191]. For the decay of antitop quarks, the CP conjugate statement is implied, resulting in either longitudinally or right-handedly polarised W − bosons from tbar decays.
• Ways of measuring W boson helicity fractions
  • helicity angle (cosθ∗): A measurement of cos θ provides the most direct measurement of the W boson helicity but also requires the reconstruction of the top quark and W boson momenta which is challenging and involves the use of MET , exhibiting a rather poor resolution. In other words must use everything in the event
  • charged lepton pT spectrum (pℓT ): The helicity of the W boson is correlated with the charged lepton momentum distribution
  • invariant mass of b quark and charged lepton (M2ℓb ): The helicity angle distribution cos θ∗ can be approximated using the invariant mass of the system composed of the b quark and the charged lepton M2lb: Advantage is that this doesn't involve MET
  • Matrix Element method
• Measurement of B(t to Wb)/B(top to Wq): As described in Section 3.3.1, in the Standard Model framework the top quark decays basically exclusively into a W boson and a b quark due to the dominant corresponding CKM matrix element Vtb
  • Deviations of R from unity could for example be caused by the existence of a fourth heavy quark generation, non Standard Model top quark decays or non Standard Model background processes.
• Neutral Current Top Decays: A search for the top quark FCNC decay t→ Z q at the Tevatron is considered especially interesting due the large top quark mass and very distinct experimental signature
• From mass only, you can not distinguish a H+ from a Wprime. Have to look at branching ratios of X to tautau vs X to ee, since Higgs couples to the larger masses. Or look at helicity
• Since the lifetime of the top quark is so short (see Sections 3.3.2 and 7.2), it does not hadronise unlike the other quarks and hence properties like its mass can be determined directly without the complication of studying a quark embedded in a hadron.
• The top quark mass measurements described here are usually interpreted as representing the pole mass. The pole mass is the S-matrix prediction of the mass
• B quark pole mass is roughly 4 GeV, where the b quark measured mass is 5 GeV.
• Top quark does not have time to hadronize (before decaying) therefore the pole mass is very similar to the measured mass

Measurement of ttbar Production Cross Section in at Tevatron using Lepton+Jets Events with Secondary Vertex b-tagging

Paper by the CDF collaboration

Abstract We present a measurement of the t¯t production cross section using events with one charged lepton and jets from p¯p collisions at a center-of-mass energy of 1.96 TeV. In these events, heavy flavor quarks from top quark decay are identified with a secondary vertex tagging algorithm. From 162 pb −1 of data collected by the Collider Detector at Fermilab, a total of 48 candidate events are selected, where 13.5 ± 1.8 events are expected from background contributions. We measure a t¯t production cross section of 5.6 +1.2 −1.1 (stat.) +0.9 −0.6 (syst.) pb.

Highlights

• To find an event-by-event primary vertex, we first identify which of the vertices described in Section II is nearest the identified high-momentum electron or muon. For other datasets without high-momentum leptons, we use the vertex which has the highest total scalar sum of transverse momentum of associated tracks.
• A pruning stage removes tracks which contribute χ2 > 10 to the fit (or the track with the largest χ2 contribution if the total fit reduced chi-squared per degree of freedom χ2 /ndf > 5).
• Secondary vertex tagging operates on a per-jet basis, where only tracks within the jet cone are considered for each jet in the event.

Charm Physics

General Overview: Charm Mixing and CP Violation

Paper on Charm Meson Decays

Abstract We review some recent developments in charm meson physics. In particular, we discuss the theoretical predictions and experimental measurements of charmed meson decays to leptonic, semileptonic, and hadronic final states and the implications of such measurements to searches for new physics. We discuss D0 − anti-D0 mixing and CP violation in charm, as well as future experimental prospects and theoretical challenges in this area.

Highlights

• The offdiagonal mass-matrix terms induce mass eigenstates D1 and D2, which are superpositions of the flavor eigenstates D0 and anti-D0
• The charm quark system is rather unique from the theoretical point of view, as its mass places it somewhere on the border between heavy and light quark systems.
• Because all NP particles are much heavier than the SM ones, the most natural place for NP to affect mixing amplitudes is in the |C| = 2 piece, which corresponds to a local interaction at the charm quark mass scale.
• Because the 2 × 2 Cabibbo quark-mixing matrix is real, no CPV is possible in the dominant tree-level diagrams that describe the decay amplitudes. CP-violating amplitudes can be introduced in the SM by including penguin or box operators induced by virtual b quarks. However, their contributions are strongly suppressed by the small combination of CKM matrix elements Vcb Vub
  • CPV in the C = 1 decay amplitudes. This type of CPV occurs when the absolute value of the decay amplitude for D to decay to a final state f (Af ) is different from that of the corresponding CP-conjugated amplitude (termed direct CPV).
  • CPV in D0 − anti-D0 mixing matrix. Introduction of C = 2 transitions, via either SM or NP one-loop or tree-level NP amplitudes, leads to nondiagonal entries in the D0 − anti-D0 mass matrix
  • CPV in the interference of decays with and without mixing. This type of CPV is possible for a subset of final states to which both D0 and anti-D0 can decay.
• CP-violating asymmetry is expected to be at most a f ∼ 10−3 in the SM
• At the present time, there is no experimental evidence for CPV in weak decays in the charm sector
• The charm quark is the only up-type quark that can have flavor oscillations

Measurement of D0-anti-D0 Mixing using the Ratio of Lifetimes for the Decays D0 --> K-pi+, K-K+, and pi-pi+

Paper from BaBar

Abstract We present a measurement of D0-D0 mixing parameters using the ratios of lifetimes extracted from a sample of D0 mesons produced through the process D+ ! D0+, that decay to K−+, K−K+, or −+. The Cabibbo-suppressed modes K−K+ and −+ are compared to the Cabibbo-favored mode K−+ to obtain a measurement of yCP , which in the limit of CP conservation corresponds to the mixing parameter y. The analysis is based on a data sample of 384 fb−1 collected by the BABAR detector at the PEP-II asymmetricenergy e+e− collider. We obtain yCP = [1.24 ± 0.39(stat) ± 0.13(syst)]%, which is evidence of D0-D0 mixing at the 3 level, and
Y = [−0.26 ± 0.36(stat) ± 0.08(syst)]%, where
Y constrains possible CP violation. Combining this result with a previous BABAR measurement of yCP obtained from a separate sample of D0 ! K−K+ events, we obtain yCP = [1.03 ± 0.33(stat) ± 0.19(syst)]%.

Highlights

• An observation of CP violation in D0-D0 mixing with the present experimental sensitivity would provide evidence for physics beyond the SM
• A value of Rm neq 1 would indicate CP violation in mixing. A non-zero value of ϕf would indicate CP violation in the interference between mixing and decay

Statistics

Why isn't every physicist a Bayesian

Bob Cousins classic paper

-- MonicaDunford - 01 May 2009