The Yale CDF group has been a member of the Collider Detector at Fermilab (CDF) collaboration since 1991. The CDF detector is a general purpose, cylindrical geometry device used to study collisions of protons and anti-protons at a center-of-mass energy of 1.8 TeV. It is located at Fermi National Accelerator Lab (FNAL or Fermilab) in Batavia, IL.
One of the strengths of the CDF detector is that it is a general-purpose device, allowing a broad range of physics to be studied. In addition, the collider at Fermilab is the highest energy accelerator in the world, making CDF the premier facility to search for new physics at the energy frontier. It is also a producer of billions of b-quarks each year, which allows CDF to be very competitive in studying the b-sector, a subject which is currently the focus of much of the particle physics community.
Run II physics
CDF has been significantly upgraded for the Run II data taking period, which began in 2001.
The data set collected during Run II is expected to be 20 times larger than that of Run I. While this is already a substantial increase, even larger factors will be realized for certain types of events. This is due to improvements we are making in the trigger system which will allow us to identify interesting events more quickly and effectively. For example, we will be able to identify events with evidence of long-lived (lifetimes ~1 picosecond) particles within 20 microseconds. This will allow us to collect a large number of events with B-meson decays to all-hadronic modes, which we rejected in Run I because we weren’t able to identify them quickly enough. These new capabilities will result in increases of the number of certain types of events by factors of 100 to 1,000. The increased size of the data set also allows us to extend the range of searches for new physics.
A search for Lepton + Photon + MET + b (ttbar + photon events) using 929 pb-1 of data has been done. The experimental signature for this search is a high energy lepton, a high energy photon, at least one high energy b-tagged jet, and large missing transverse momentum. This signature is possible in models based on gauge-mediated Supersymmetry and other extensions of the Standard Model. The ttbar+photon is a control sample for ttbar+Higgs production at the LHC and is a direct probe of the charge of the top quark. In the signature of Lepton+Photon+MET+b events we observe 15 events versus a standard model expectation of 14.3 +- 1.6 events. We find 10 Lepton+Photon+MET+b with H_T > 200 GeV events versus an expectation of 7.2 +- 1.0 events and we find 7 ttbar+Photon events (H_T > 200 GeV and N_jets > 2) versus an expectation of 3.6+- 0.8 events
Measuring the B_s B_sbar oscillation frequency and the B0-B0bar oscillation frequency, and hence V_ts/V_td (proportional to one of the sides of the unitarity triangle), is one of our main goals for Run II. The frequency of the B_s oscillation was already known to be fairly large (> 9/ps), so to directly observe B_s mixing requires excellent resolution on the proper time of the decay, which in turn requires excellent spatial resolution on the decay product’s tracks, and excellent momentum resolution. The decay modes most suitable for this measurement are fully reconstructed B_s decays, such as B_s -> D_s pi, with D_s -> phi pi. In the past, it was impossible to separate these events from backround in time to retain them in the final data sample. However, the B and D mesons in these events both have lifetimes of about 1 picosecond, hence the new trigger electronics will allow us to select such events and ensure they are accepted and written to tape for later analysis. This should result in approximately 10,000 of these fully reconstructed B_s decays to be recorded, allowing a measurement of the oscillation frequency up to over 30/ps, which covers the range of expected values.
A different approach to gaining the same insight is to look for the width difference in the B_s system. To do this, a time-dependent angular analysis of B_s -> J/psi phi and B_d -> J/psi K0* events was done. Using 260 pb-1 of data a measurement of DeltaGamma_s = 0.47 ps-1.
Another example of an investigation only possible with these large increases in the data set is the search for CP violation in B0 -> pi+ pi- decays. Both B0 and B0bar (particle and antiparticle) can decay into the final state pi+ pi-, which is an eigenstate of CP. Observing an asymmetry in the decay rates of B0 and B0bar into this mode would be evidence for CP non-conservation, and provides an independent measurement of one of the angles in the unitarity triangle.
The increased luminosity from the Tevatron accelerator, coupled with a greatly improved detector with new capablities will open up many exciting research avenues which weren’t possible in the past. We look forward to a rich and exciting set of physics from Run II.
Run I physics
Run I, which took place from 1992-1995, was an extremely successful period for CDF. The analysis of the data collected, which still is not complete, led to the discovery of the top quark, a precise measurement of the W-boson mass (which is related to the mass of the as-yet undiscovered Higgs boson), searches for substructure in quarks, and a few tantalizing events which can’t be explained within the Standard Model, as well as observations of time-dependent mixing in the B system and precision B lifetime measurements (which help constrain the Cabbibo-Kobayashi-Maskawa quark mixing matrix). In addition we published dozens of papers on topics ranging from measuring the proton’s quark and gluon structure functions, to tests of QCD cross-section predictions, to limits on the masses of exotic particles such as leptoquarks.
Our group has been focussing on studying the b-quark sector, and in particular measuring quantities which are related to the CKM mixing matrix which contains the information about the weak coupling of the up-type quarks (u,c,t) to the down-type quarks (d,s,b). There are several mysteries hidden within this matrix. If there are only 3 generations of quarks, conservation of probability requires the CKM matrix, V, to be unitary. One of the unitarity conditions is:
V_ud V_ub* + V_cd V_cb* + V_td V_tb* = 0,
an identity known as the unitarity triangle, as it can be drawn as a triangle in the complex plane as shown here.
If a deviation from unitarity could be proven, it would be clear evidence for new particles and/or forces. The broad-based nature of this search for new phenomena makes it attractive, as it is not tied to any one particular model of new physics. In addition, measuring the values (including the complex phases) of the CKM elements is critical for understanding the origins of CP violation (CP = Charge Conjugation x Parity). CP violation manifests itself as an asymmetry in the properties of matter and anti-matter, and is one of the three requirements enumerated by Sakharov necessary to end up with a universe which is made up of only matter from an initial condition with equal amounts of matter and antimatter.
The CKM element V_td can be measured by measuring the mixing frequency of B0 and B0bar mesons (which contain b and d quarks and antiquarks). B0 mesons are one of a very few types of particles which can actually transform into their antiparticles, a phenomenon known as “mixing”. The rate of this mixing (which our group has measured directly at CDF with our extremely high-precision tracking devices) is related to the magnitude of V_td.
Our group is pioneering a method to extract the element V_ts, which is related to V_cb, using correlations in the decay products of B_s mesons. With the next run’s data, this may also be measured directly by measuring the B_s B_sbar mixing frequency, analogously to our B_d mixing measurement.
The phases of V_ud and V_td can be measured if we observe an asymmetry in the decay rates of B0 and B0bar mesons to a CP eigenstate, such as pi+ pi- or J/psi Kshort. We do not have enough data currently to observe such asymmetries, however we fully expect to be able to do so with the upcoming data.
One such search possibility is to look for evidence of large extra spatial dimensions. In this scenario every known particle can exist in an excited (Kaluza-Klein) state where it carries momentum in at least one extra dimension. Experimentally this will look like a heavy particle which will decay into known standard model particles. A search for multiple leptons, jets and missing energy transverse to the beamline