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The upgraded D0 detector will address a variety of B physics, ranging from QCD tests involving production dynamics through the mass spectrum of the B_c system, rare decay modes, B_s mixing and CP violation. Apart from the last, the Tevatron is the only (or the pre-eminent) facility where these fundamental issues may be addressed. The D0 detector's full eta coverage is particularly well matched to measurements of B-mesons --- beauty is produced uniformly in pseudorapidity. Mesons produced centrally are typically soft, with gamma<3, putting a premium on soft lepton triggers and minimal multiple scattering. The low boost for centrally produced B's implies decay fragments which are distributed in eta. In addition much of the most interesting physics, such as mixing and CP violation, requires that both partners in the Bbar B pair be at least partially reconstructed. Physics which requires either full reconstruction of the final state or tagging of the partner B will be quite sensitive to the coverage of the tracking system. The strengths of the upgraded DO detector for B-physics, then, are its full calorimetric and muon coverage, its dedicated forward tracking systems, and its ability to trigger on low-pT leptons over the whole pseudorapidity range |eta|<3.
Although B mesons are produced copiously (about 2000 Bbar B pairs per second at L=10**32 cm-2 s-1) studies of mixing and CP violation are still likely to be limited by statistics. Final states must be fully reconstructed and the initial state must be tagged. CP violating decay modes are typically at the 10**-5 level. We will need fully reconstructed decay chains with typical branching ratios of ~ 10**-4 to obtain sufficient resolution for B_s mixing measurements. The experimental challenge is to trigger on, collect, and reconstruct these events.
D0 has had an active B physics program in Run I with an emphasis on the detector's strengths in muon detection and calorimetery. D0 has concentrated on studies of hadroproduction of b's over an extensive rapidity range. Current studies include measurements of inclusive muon and central and forward charmonium production, the heavy flavor content of jets, and Bbar B correlations. B production represents a small fraction (1/1000) of the total cross section. In Run I we have gained a great deal of understanding of how to successfully trigger and reconstruct b's. The job is especially difficult in the intermediate to large eta range. Our plans represent the results of studying the triggering and reconstruction problems over the full pseudorapidity range.
The upgrade represents a very significant improvement in D0's B physics capabilities. Magnetic tracking allows us to reconstruct the masses of hadronic final states and tag the sign of electrons and hadrons. Our J/psi mass resolution will improve by a factor of 10. The Level 1 track processor will improve the overall level of signal/noise by allowing sharp pT thresholds for lepton triggers. Combined with the A-layer muon trigger counters, the processor will substantially decrease the muon trigger pT threshold, crucial for efficient b triggering. Signal/noise will also be improved by matching muon system and central tracks at the trigger level. The forward muon pixel counters will allow muon triggering at intermediate eta. The silicon vertex detector will allow us to tag decays based on lifetimes, to study mixing, and reduce backgrounds for the whole range of heavy flavor physics. It is a crucial addition for mixing and CP violation studies. In addition the silicon disks provide forward tracking in the detector. D0 will be the first experiment to seriously explore heavy quark physics in the forward region.
However, our plans do not represent a complete B-physics program. To contain costs and meet the laboratory schedule D0 has chosen not to upgrade the Level 1 bandwidth beyond 10kHz. We have also chosen to postpone any Level 2 vertex processor. This will limit our B physics reach in Run II and present some hard choices in our trigger lists.
As examples of Run II B-physics, we consider two benchmark processes, B_s mixing and CP violation. The results shown are from recent studies using a fast simulation package. In previous studies we have demonstrated the very high reconstruction efficiency of the upgraded D0 tracking system.
A measurement of B_s mixing, along with the measured value for B_d mixing, will constrain the CKM matrix parameters rho and eta (x_s/x_d ~ 1/((1-rho)**2+eta**2) ). The large mass of the top quark suggests a value of x_s well in excess of ten; x_s can be measured by taking the difference of mixed and non-mixed events as a function of proper time: N(B_s -> B_sbar) = 1/2 N0 exp(-t/tau)(1- cos(x_s t/tau), where N0 is the number of B_s at t=0 and tau is the B_s lifetime.
We have obtained a qualitative picture of the effects of reconstruction on the lifetime resolution:
We expect that with 1 fb-1 of data, the upgraded detector should be able to measure over 2000 B_s decays with a lifetime resolution of better than 5%. This should allow us to measure values of x_s < 25.
The study of CP violation in the B system is a standard benchmark process for b physics. The error in the CKM parameter beta is delta sin(2beta) = (1+b) / D(1-2w)sqrt{N(1+b)}, where the dilution factor, D, is defined as D = x_d(1+x_d^2) , N is the number of tagged events, w is the fraction of wrong sign tags and b is the background to signal ratio. Values of D and w have been extracted from a number of studies.
We have considered three scenarios:
In accessing the resolution in sin(2beta) the increased number of events reconstructed in the forward tracker must be balanced against the degraded mass resolution in the forward direction. Background levels are obtained by scaling from CDF's B results. Expected D0 muon acceptances and efficiencies were used [17]. together with mass resolutions and reconstruction efficiencies for the upgraded tracking system given by the fast simulation package. The table below shows the results for our combined simulations of the upgraded detector.
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| | |
B -> psi Ks | Central | Central +| Full eta
| only | full tag |
----------------------|----------|----------|---------
Events in final state | 430,000 | 430,000 | 430,000
Trigger | multi-l | multi-l | multi-l
Efficiency*Acceptance | 0.0006 | 0.0011 | 0.0034
Tagged Events | 236 | 492 | 1,443
Dilution factor | 0.28 | 0.28 | 0.22
Error on sin(2beta) | 0.24 | 0.16 | 0.12
______________________|__________|__________|_________
It is clear that the CP violation measurement benefits substantially from lepton tagging and triggering ability over |eta|<3. While the advantage from covering this whole range for hadronic final state reconstruction is somewhat more modest due to the decreased resolution at high |eta|, it still yields a substantial improvement in the precision with which sin(2beta) can be measured.
Some of the other topics which we expect to focus on in our Run II B-Physics program are: