- Email: [email protected]
Studies for a dedicated B detector at the Fermilab collider
IUCLEAR PHYSIC.(
PROCEEDINGS SUPPLEMENTS ELSEVIER
Nuclear Physics B (Proc. Suppl.) 50 (1996) 240-247
Studies for a Dedicated B Detector at the Fermilab Collider Patricia McBride ~ ~Princeton University, Princeton NJ 08544 and Fermilab, Batavia IL 60510 The observation of CP violation in the B system is one of the great experimental challenges of the next decade. Several B factories are already planned, however, there will be many interesting measurements awaiting a second generation of B exeriments. Studies are being carried out to design a dedicated collider B experiment for the Tevatron at Fermilab. A dedicated B detector at a hadron colfider will have a physics reach beyond that of experiments scheduled to begin operation before the end of the decade.
1. I N T R O D U C T I O N Interest in B physics has grown over the past decade and the observation of CP violation in the B system is within the reach of several proposed experiments at B-factories around the world. The B factories at K E K and at SLAC will begin operating by the end of the decade just as CDF and DO begin high luminosity runs at the Tevatron at Fermilab. C L E O will have be operating at high luminosity and Hera-B is planned to begin d a t a taking in 1998. With this multitude of experiments in operation, results on CP violation and rare B-decays will be forthcoming within the next five to ten years. At Fermilab there is great interest in exploiting the large number of Bs that are produced at the Tevatron collider to explore this physics. The challenge is to design a detector that will be efficient for interesting B events and triggerable keeping in mind that most of the channels have small branching ratios of the order of 10 -5. One expects to see the first results on CP violation in the B system in the channel B ° --+ J / ~ K ° where the triggering is straightforward and the experimental signature is reasonably clean. CDF has already seen a 138 -t- 18 events in the/~+#-rr+rr - channel with a signal to background ratio of 1.1 in the first 60 pb -1 in data from Run I. [1] A dedicated B detector operating at the hadron collider at Fermilab after the year 2000 must be able to explore B physics beyond the reach of the first generation experiments. A c o m -
pilation of physics channels that will be of interest in a second generation experiment is given in Table 1. [2] [3] [4] The branching ratios for m a n y of these processes are very low and m a n y of these measurements will require high statistics which are beyond the reach of the e+e - B-factories. One hopes to take advantage of the fact that a large number of B's are produced in interactions at high energy hadron colliders. At the Fermilab collider energy of 2 TeV, the bb cross section is of order 50-100 #barns. The Tevatron operating at 1032 will yield 5 - 10 × 1011 bb pairs per year which translates to 4 - 8 x 105 B] -+ ~r+rr - decays per year. However, events containing a bb pair are only a small fraction of the total cross section at the collider with Crbg / O ' i n e l a s t i c ~ 10 -3. Efficiencies on the order of a few percent for decays of interest will be required to exploit the B physics potential at a collider exeriment. In 1994 an Expression of Interest was submitted to the Fermilab outlining the options for a dedicated B detector. [5] Currently planning studies are underway to formulate a design for a dedicated B detector for the Tevatron collider. Construction of such a detector could begin as early as 1998. 2. T H E T E V A T R O N
PROGRAM
Prior to the recent collider run at Fermilab, Run IB, the Linac energy was increased from 200 MeV to 400 MeV and the Tevatron collider has been able to operate at luminosities in the range
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P McBride~Nuclear Physics B (Proc. Suppl.) 50 (1996) 2 4 0 - 2 4 7
Table 1 Selected physics channels of interest at a hadron collider B physics experiment . Decay Mode Branching Ratio Signature B~ -4 J / ¢ K ~ 5.5 x 10 -4 g+g-~'+~rB ° -4 D+D 6 × 10 -4 K-~r+~r+K+Tr ~r B ° -4 rr%r1 x 10 -5 rr+rr B + -4 D°K + B + -4 D°K + o + B + -+ D c p + K B° B° B° B° B° B° B°
--4 D~-K + -4 D+K - 4 J/~bK* --+ D~-n + --~ DT~'+Tr+Tr-4 K ' p + # -4 p + # -
2 × 10 -4 2 x 10 -¢ 2 x 10 -4 2 1 8 5 10 2.9 1.8
× 10 - 4
× × × × ×
10 -4 10 -5 10 -3 10 -3 10 -6
× 10 - 9
of 1.5 - 2.5 x 1031 c m - 2 s e c - 1 throughout the run. Before the next run, Run II, the Main Ring will be replaced by a new accelerator known as the Fermilab Main Injector. [6] The Main Injector, a large aperture, rapid cycling proton synchrotron, will enable support of luminosities in excess of 8 x 1031 c m - 2 s e c - 1 in the Tevatron collider. Plans have been formulated over the past year to include a new 8.9 GeV antiproton accumulator ring, the Recycler, inside the Main Injector enclosure. The luminosity in the collider with the addition of the Recycler to the Main Injector project is expected to be 2 × 1032 c m - 2 s e c - 1 . It will then be possible to reach 1 x 1033 c m - 2 s e c - 1 in the collider following further modest improvements to the antiproton source. The Main Injector will begin operation near the end of the decade. The next run of the Tevatron collider RUN II which follows the commissioning of the Main Injector is scheduled to begin in 1999 or 2000. Run II luminosities are expected to reach the level of 2.0 × 103u c m - ~ s e c - 1 . The collider has two large collision halls which are currently occupied by the CDF and DO detectors. The CDF and DO collaborations are planning major upgrades to their detectors for Run II. [7] However, neither of these detectors has been designed to
241
Measurement sin 2/? sin 2/~ sin 2a
K+~r-K + K-rr+K + K+K-K +
sin 7
K+K-~r-K + K+K-zc+K g + g - K - ~ "+ K+K-Tr-Tr + K+K-Tr-Tr+Tr+Tr K%r-#+p #+#-
sin 7 B ° Mixing
FCNC
handle luminosities of 1 × 1033 c m - 2 s e c - 1 ; these high luminosities would require further detector upgrades beyond the ones currently planned for Run II. A dedicated B-detector could begin operation at the completion of Run II and run concurrently with the high luminosity program. There are studies being carried out at Fermilab to evaluate the feasibility and physics reach of a high luminosity Tevatron operating at 1 × 1033 c m - 2 s e c - 1 under a project known as TeV33. The number of interactions per crossing is very high - projected to be 9.13 interactions/crossing - in the proposed environment of TeV33 which will be problematic for vertex triggers. A dedicated B-detector could operate at a lower luminosity to decrease the number of interactions per crossing and ease demands on the trigger. At the present time, the laboratory plans to support one high-pt detector during TeV33 operation leaving the other interaction hall available for a dedicated B-detector.
2.1. Strategy for Studies At the Tevatron, the B production cross section is larger in the central region or at smaller pseudorapidity r / = - I n ( t a n ~ ) < 1.5. However, the B's produced in the forward direction
242
McBride~Nuclear Physics B (Proc. Suppl.) 50 (1996) 240-247
P
Table 2 Plans for upgrades to the Tevatron Collider at Fermilab. Run I B Run II (1993-1995) (1999-) Linac Main Upgrade Injector, Pbar Improvements Protons/bunch 2.32E+11 3.30E+11 Pbars/bunch 5.50E+10 3.60E+10 ram-mr
Run II (1999-) Antiproton ring
TeV 33 Targeting,
2.70E+11 5.50E+10
Cooling upgrades 2.40E+11 1.00E+ll
1000 36 0.18 2.03E+32 40.83 396 5.31
1000 108 0.26 1.04E+33 210.62 132 9.13
n l m - m r m
Energy Bunches Bunch length (rms) Typical Luminosity Best Luminosity Integrated Luminosity Bunch Spacing Interactions/crossing ( at 45 m b )
900 6 0.6 1.58E+31 2.50E÷31 3.18 3500 2.48
hadrons
1000 36 0.43 8.29E+31 16.72 396 2.17
a t abe T e v a t r o n
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Figure 1. ¢]7 vs 7/for B ° events at the Fermilab Collider.
GeV m c m - ;sec -1 cm-~sec -~ pb-1/week nsec
or larger pseudorapidity 1.5 > q > 4.5 are boosted making the B decay distances larger• In addition, the daughter tracks of forward going B's have higher momenta than the daughters of centrally produced B's and, therefore, the effects of multiple scattering are lessened in the forward region• A plot of/7 3' vs. 77 for B-hadrons produced in Tevatron collisions is shown in Figure 1. Solenoidal detectors such as CDF and the upgraded DO detector have good m o m e n t u m resolution out to about letal < 1.5 but the vertex resolution is degraded due to multiple scattering of the low momentum tracks in the tracking chambers and silicon• Applying a Pt cut helps the vertex resolution, but unfortunately significantly reduces the acceptance for B decays. While the geometrical acceptance is greater for 3 units of rapidity in the central region, one expects the better vertex resolution in the forward to compensate in the final analysis. We have performed a series of studies which attempts to quantify this effect. The strategy of our studies has been to examine a few selected physics channels and explore acceptance, background and resolution issues for
243
P McBride~Nuclear Physics B (Proc. Suppl.) 50 (1996) 240-247
several detector options. We plan to investigate the trigger in a second phase of the studies. Our goal is to compare various proposed detector geometries and understand their B physics potential at the analysis level. If a detector has very limited capabilities with a perfect trigger, there is not much need to investigate further. We use this strategy to make a first comparison of the detectors and have plans to review the trigger in upcoming studies. We have outlined several detector geometries for our studies: Central detectors, examples are CDF and DO, forward collider detectors with a forward dipole, an example is LHC-B, central dipole detectors, an example being the proposed BCD detector, and combinations of these detectors. The combinations could be a central solenoid with a dipole at large ]~/I or a cental dipole with smaller dipoles at large Ir/I. We made model detectors representing these various types of geometries, attempting to choose realistic detector resolutions and performance. We used a flexible parameterized detector simulation package developed at Fermilab to study the detector performance and Pythia [8] as the event generator for the studies presented below.
2.2. Looking f o r B ° Mixing One of the topics of interest for a Dedicated B experiment at a hadron collider is B8 mixing. We performed a preliminary study of B~ mixing using the channel B ° --~ J/¢f(-~- and compared the time resolution for the forward and central geometry. The simplest B, decay modes containing a ~ in the final state are the fJ~/and ~¢. Neither of these modes can be used to measure B, mixing since they are not flavor specific. It has been suggested that the Cabibbo suppressed decay B, -+ ~/--~, --+ K=F~r+ presents a possible mode to investigate mixing phenomena [9]. The sign of the kaon charge distinguishes between the decaying B, and the B,. This mode would proceed via the diagram shown in Fig. 2. The recent observation by the CLEO collaboration of B - -+ ~Tr- decays at the level expected from Cabibbo suppression provides evidence for the existence of such diagrams.
[10] They measure
NB- --+~-) B ( B - --~ ~ K - )
= (5.2 + 2.6)% ~ A2.
(1)
We can thereby make a prediction of the branching ratio B(B8 ~ ¢ I U ) = B(B~ -~ ~K*) × ~2 =
(2)
1.7 x l0 -3 x 0.05 = 8.5 x 10 -5.
u_ "~
~
- . ,g ~
c} v S or
d}(~
or K *
Figure 2. Weak decay diagrams for B, --+ ¢ ¢ and ~K*. The K* final state occurs when the virtual W - materializes as a ~d pair.
In order to quickly evaluate the potential of this decay channel one can estimate the expected rate of B ° --+ J/~bK-;- from the number of events expected for B ° --+ J/~bK ° a mode which has been carefully studied by all proposed B detectors. One expects to see about 1/15 the number of reconstructed and flavor tagged B ° --+ J / ¢ K *
as B ° -~ J/eKe,. [11] There are several advantages to using the ~bK decay mode for the study of B ° mixing. A V -+ g+g- trigger can be used to select events, and the B decay vertex contains four charged tracks coming from a single decay vertex. This is important both for background reduction and for good decay time resolution. We have made estimates of the time resolution for a possible "forward" and "central" detector at the Fermilab collider. Our model detectors consist of silicon strip detectors, tracking chambers and have a dipole field for the forward detector and a solenoidal field for the central detector. The simulation program takes into account
P McBride~Nuclear Physics B (Proc. Suppl.) 50 (1996) 240-247
244
track smearing due to multiple scattering and detector resolution, however, pattern recognition is not attempted. In Figure 3 shows a comparison of the time resolution for a forward and central detector operating at the Tevatron. The forward detector covers an q range of 1.5 < ~ < 4.5 and the tracking in the central detector covers a range ]r]l < 1.5. The large difference in the time resolution is a result of the difference in the m o m e n t u m spectra in the two 7? ranges. The daughter tracks have much lower m o m e n t u m in the central region and the subsequently multiple scattering is much more troublesome in this region.
3°°In
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Figure 3. Comparison of the time resolution for a forward collider experiment and a central detector operating at Tevatron energies for B ° --+ J / ~ K * events.
The measurement of this channel sets some requirements for a B detector. A detector with good vertex resolution will be able to take advantage of the clean J / ¢ signal and the four track B decay vertex to significantly reduce the background from generic B decays. Excellent
mass resolution will be needed to eliminate backgrounds from B~ -+ J/~bK*. Excellent particle identification will be required to identify the K and 7r in the K* decay and to remove background from other channels such as B , -+ J / ¢ ¢ . To estim a t e the reach in xs for a particular experiment requires studies not only of the vertex resolution as was done here, but of backgrounds and fitting procedures. Further studies with more detailed simulations are planned. 2.3. S i m u l a t i o n s f o r D e t e c t o r S t u d i e s In the course of our B physics studies we developed a new simulation and analysis tool. As stated above we were interested in exploring the capabilities of a dedicated B physics detector at the Fermilab collider and to see if such a detector would be competitive with other proposed experiments. We found a need for a fast generic detector simulation program to make a quantitative comparison of the physics capabilities of a variety of detector options. The program needed to be flexible, handle solenoids and dipoles and a variety of tracking detectors, but most importantly provide a fast track to physics results so that we would be able to simulate and analyze a large number of signal and background events in a relatively short time. We developed a new fast Monte Carlo package MCFast [12] for carrying out the detector studies. The package is a parameterized Monte Carlo and analysis package which gives tracking and vertex resolutions for detectors with dipole and solenoidal magnets. The goal of MCFast is to provide a general framework for the comparison of differing detector geometries. With this framework, we can model a variety of detector geometries for the Fermilab collider. We would also like to be able to make comparisons to experiments planned for the LHC. The detector simulation package is interfaced using S T D H E P [13] conventions to the standard H E P generators, ISAJET, P Y T H I A and H E R W I G which model the production of B hadrons. We use QQ, which was developed by the CLEO collaboration, for modeling the decays of B hadrons. The primary goal of MCFast is speed and flexibility which is achieved through parameteriza-
P. McBride~Nuclear Physics B (Pro+. Suppl.) 50 (1996) 240-247 tion. [15] The emphasis of the program is fast tracking which is based on the Billoir technique. [14] In evaluating the error on each particle trace in the detector, the effects of multiple scattering, detector resolution and efficiency are taken into account. The tracks are then smeared according to the calculated covariance m a t r i x and the information is immediately available for vertex studies and mass calculations. A model for hit generation has been included in the MCFast package and is under development. This will be used for trigger studies and for studies of pattern recognition and track overlap issues which will become i m p o r t a n t issues at the luminosities of future hadron colliders. Other features that are included are multiple interactions, decays in flight and 3' conversions. To verify the simulation we have compared the computed vertex resolution and mass resolution from MCFast to calculations and measurements from current experiments. Our results agree well with d a t a from C D F Run IB and the Fermilab fixed target experiment E687. We are able to simulate and analyze an interaction in a CDFlike detector operating at Tevatron energies in approximately 0.5 sec using a high speed computer with a 150 MHz R4400 processor.
of double-sided silicon planes, tracking detectors and a muon system. As mentioned above we ignored particle ID and trigger effects in these preliminary comparison studies. Our goal was to compare detectors in the as systematic a manner as possible. B ° ~ ~r+~r- is a difficult channel to isolate as the branching ratio is very small ( ~ 10 -5) and the analysis relies primarily on clean vertex separation and good mass resolution. We tried to design cuts that would take advantage of the capabilities of the vertex detectors to isolate detached vertices. One can also achieve improvements in signal to background in this channel by requiring that the two pions are both high Pt tracks but this comes with a corresponding undesirable drop in efficiency. A comparison of the decay length resolution L / ~rL is shown in Figure 4 for the central and forward detector geometries.
Lla
i05 1
2.4. C o m p a r i s o n s o f D e t e c t o r G e o m e t r i e s We have used MCFast to make a simulated comparison of B~ -+ ~r+lr - yields for various detector geometries operating at Tevatron energies. Previous studies made for a dedicated B-detector proposed for the SSC indicated that the principal background in the channel will come from bb events. [16] We generated a sample of 40,000 B~ -+ rr+~r- signal event and 300,000 generic bhadron events. We compared the results of simple analyses for isolating the signal B ° ~ rr+rr in three detector geometries: forward detector (1.5 < r~ < 4.5 in the fashion of LHC-B, a central detector 17/I < 1.5 similar to CDF RUN II, and a central dipole configuration with a large dipole magnet centered on the interaction region. We designed a analysis that could be applied to all detector configurations with only minor modifications. In order to maximize acceptance, we used a minimal pt cut on the tracks of 250 MeV. The detectors each contained multiple layers
245
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I
,
1
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zo: i
10 z
,,,
l
0
',.¢-,',','- :; , -,:',
,li]ll.n,,,,,,. I00
200
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Figure 4. Comparison of the decay length resolution L/(~L for a forward collider experiment and a central detector operating at Tevatron energies for B ° ~ ~-+ 7r- events.
246
R McBride~Nuclear Physics B (Proc. Suppl.) 50 (1996) 240-247
In the central detector, the distribution falls rapidly and is very sensitive to the placement of a cut. However, in the forward direction, the distribution is much less steep, and there is a much smaller loss of efficiency when one applies a safe cut to select clean secondary vertices. For our analysis we use a cut of T7 L of 8 for both the central and forward detectors. The smearing of prim a r y vertex by its resolution has not yet been included in this study. Other vertex cuts are added to require that the two ~r's miss the primary vertex and that the reconstructed B ° points back to the primary vertex. The results of these studies are shown in Table 3. The central dipole geometry which covers the same range in rI gives similar results to the forward geometry, but with about twice the acceptance. These results translate to an error on sin 2a of 0.17 for the forward and 0.35 for the central detector in one year assuming 100% trigger efficiency and muon tagging only. Note that the muon tagging is less than 2% efficient and improvements are expected with the addition of other tagging methods such as kaon tagging. For the measurement of sin 2/3 using B ° -+ J/~bK °, the forward and central geometries yield similar results. With a muon tag alone and a 100% efficient trigger, one expects to measure sin 2fl to 0.09 in either detector in 1 year of running at 1032cm-2sec -1. Our simulations were done for generic models of a solenoidal detector and a forward detector, therefore, these results should be considered as estimates since the physics capabilities are still functions of the exact configuration and resolutions of the tracking chambers. However, trends are becoming apparent. For measurements that depend on vertex cuts, detectors which cover the forward geometry retain efficiency as strict vertex cuts are applied. They also have excellent decay vertex resolution due to reduced multiple scattering. A good dipole magnet will give them excellent mass resolution in this region. The most promising detector geometry for a dedicated Bdetector at the Tevatron is the central dipole which covers the range 1.5 < 1771< 4.5. However, to complete the evaluation of any one detector geometry, one also must to consider tagging and trigger efficiencies in the final analysis. This work
is in progress. 3. C O N C L U S I O N S An effort is underway to design a second generation B-detector for the Tevatron collider. Studies have begun to investigate the potential of such a detector and to compare its capabilities to existing or proposed experiments for studying B physics. Simulation tools have been developed to facilitate studies of the physics reach of such a detector and to compare different geometry options. A central dipole geometry appears to be optimal for the Tevatron combining good acceptance for B decays with excellent secondary vertex resolution. Further studies are needed particularly in the area of trigger and particle ID before the design and comparisons can be completed.
4. A C K N O W L E D G M E N T S The author would like to thank the members and guests of the Fermilab Computing Division who have worked on this project. They include Paul Avery, Amber Boehnlein, Joel Butler, Paul Lebrun, Lynn Garren, Michael Procarlo, Kevin Sterner, Sheldon Stone, Torre Wenaus and Julia Yarba. The work presented here was supported by the Fermi National Accelerator Laboratory, which is operated by Universities Research Association, Inc., under contract DEAC02-76CH03000 with the U.S. Department of Energy. The author would also like to acknowledge support from the National Science Foundation.
REFERENCES 1. 2. 3. 4.
E. Meschi, in these proceedings. B D e c a y s , 2nd Edition ed. S. Stone, World Scientific, Singapore (1994). K.T. McDonald, Princeton/HEP/92-09. Proceedings of the Workshop on B
Physics at Hadron Accelerators, ed. P. McBride and C.S. Mishra, Fermilab-CONF93/267 (1993). 5. R. Edelstein, et al., Fermilab 1994-EOI-002. 6. For information on the Fermilab accelerator complex see http://www-fermi3.fnal.gov/
t? McBride~Nuclear Physics B (Proc. SuppL) 50 (1996) 240-247
247
Table 3 Acceptance for 7r+~r- in the mass region 5.0-5.6 GeV. The results for the central dipole are estimated from the forward. Studies of the Central Dipole geometry are underway. Note that the branching ratio for B ° --+ rr+rr- is 10 -5. Estimate for a Central Forward Central Dipole Detector Detector Detector 171 < 1.5 1.5 < r / < 4.5 1.5 < It/[ < 4.5 Signal events: B] --+ ~-+~rNo vertex cuts 35.5% 18.6% Silicon hits and L > 8 10.7% 13.4% All vertex cuts 6.2% 12.1% >20 % Background: from generic bb events No vertex cuts 2.2% 1.2% Silicon hits and yL > 8 0.18% 0.22% All vertex cuts 2 × 10 - 6 10 - 6 ~ 2 × 10 - 6
7. F. Dejongh, in these proceedings. 8. T. Sj6strand, Computer Physics Commun. 39 (1986) 347; T. SjSstrand, Computer Physics Commun. 43 (1987) 367. 9. S. Stone, "B Physics at the SSC," in Proc. of t h e 1991 S y m p o s i u m on t h e SSC," Corpus Christi, TX, SSCL-SR-11213 (1991) p225. 10. J. Alexander et al., (CLEO) Phys. Lett. B 341, 435 (1995); erratum ibid 347, 469 (1995). 11. P. McBride and S. Stone, in Proceedings of BEAUTY '95 - 3rd International Workshop on B-Physics at Hadron Machines, July 1995. 12. A. Boehnlein, in the Proceedings of CHEP95, Sept. 1995. 13. L. Garren, Fermilab CD Note PM0091 (or see
http ://fnpspa. fnal. gov/st dhep. html) 14. P. Billoir, NIM 225 (1984) 352. 15. For more information on MCFast
see
http.'//fnpspa.fnal.gov/simulation.html 16. O.L. Long, et al., preprint (UPR/216E, Princeton/HEP/92-07).
PROCEEDINGS SUPPLEMENTS ELSEVIER
Nuclear Physics B (Proc. Suppl.) 50 (1996) 240-247
Studies for a Dedicated B Detector at the Fermilab Collider Patricia McBride ~ ~Princeton University, Princeton NJ 08544 and Fermilab, Batavia IL 60510 The observation of CP violation in the B system is one of the great experimental challenges of the next decade. Several B factories are already planned, however, there will be many interesting measurements awaiting a second generation of B exeriments. Studies are being carried out to design a dedicated collider B experiment for the Tevatron at Fermilab. A dedicated B detector at a hadron colfider will have a physics reach beyond that of experiments scheduled to begin operation before the end of the decade.
1. I N T R O D U C T I O N Interest in B physics has grown over the past decade and the observation of CP violation in the B system is within the reach of several proposed experiments at B-factories around the world. The B factories at K E K and at SLAC will begin operating by the end of the decade just as CDF and DO begin high luminosity runs at the Tevatron at Fermilab. C L E O will have be operating at high luminosity and Hera-B is planned to begin d a t a taking in 1998. With this multitude of experiments in operation, results on CP violation and rare B-decays will be forthcoming within the next five to ten years. At Fermilab there is great interest in exploiting the large number of Bs that are produced at the Tevatron collider to explore this physics. The challenge is to design a detector that will be efficient for interesting B events and triggerable keeping in mind that most of the channels have small branching ratios of the order of 10 -5. One expects to see the first results on CP violation in the B system in the channel B ° --+ J / ~ K ° where the triggering is straightforward and the experimental signature is reasonably clean. CDF has already seen a 138 -t- 18 events in the/~+#-rr+rr - channel with a signal to background ratio of 1.1 in the first 60 pb -1 in data from Run I. [1] A dedicated B detector operating at the hadron collider at Fermilab after the year 2000 must be able to explore B physics beyond the reach of the first generation experiments. A c o m -
pilation of physics channels that will be of interest in a second generation experiment is given in Table 1. [2] [3] [4] The branching ratios for m a n y of these processes are very low and m a n y of these measurements will require high statistics which are beyond the reach of the e+e - B-factories. One hopes to take advantage of the fact that a large number of B's are produced in interactions at high energy hadron colliders. At the Fermilab collider energy of 2 TeV, the bb cross section is of order 50-100 #barns. The Tevatron operating at 1032 will yield 5 - 10 × 1011 bb pairs per year which translates to 4 - 8 x 105 B] -+ ~r+rr - decays per year. However, events containing a bb pair are only a small fraction of the total cross section at the collider with Crbg / O ' i n e l a s t i c ~ 10 -3. Efficiencies on the order of a few percent for decays of interest will be required to exploit the B physics potential at a collider exeriment. In 1994 an Expression of Interest was submitted to the Fermilab outlining the options for a dedicated B detector. [5] Currently planning studies are underway to formulate a design for a dedicated B detector for the Tevatron collider. Construction of such a detector could begin as early as 1998. 2. T H E T E V A T R O N
PROGRAM
Prior to the recent collider run at Fermilab, Run IB, the Linac energy was increased from 200 MeV to 400 MeV and the Tevatron collider has been able to operate at luminosities in the range
0920-5632/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved. PII: S0920-5632(96)00398-2
P McBride~Nuclear Physics B (Proc. Suppl.) 50 (1996) 2 4 0 - 2 4 7
Table 1 Selected physics channels of interest at a hadron collider B physics experiment . Decay Mode Branching Ratio Signature B~ -4 J / ¢ K ~ 5.5 x 10 -4 g+g-~'+~rB ° -4 D+D 6 × 10 -4 K-~r+~r+K+Tr ~r B ° -4 rr%r1 x 10 -5 rr+rr B + -4 D°K + B + -4 D°K + o + B + -+ D c p + K B° B° B° B° B° B° B°
--4 D~-K + -4 D+K - 4 J/~bK* --+ D~-n + --~ DT~'+Tr+Tr-4 K ' p + # -4 p + # -
2 × 10 -4 2 x 10 -¢ 2 x 10 -4 2 1 8 5 10 2.9 1.8
× 10 - 4
× × × × ×
10 -4 10 -5 10 -3 10 -3 10 -6
× 10 - 9
of 1.5 - 2.5 x 1031 c m - 2 s e c - 1 throughout the run. Before the next run, Run II, the Main Ring will be replaced by a new accelerator known as the Fermilab Main Injector. [6] The Main Injector, a large aperture, rapid cycling proton synchrotron, will enable support of luminosities in excess of 8 x 1031 c m - 2 s e c - 1 in the Tevatron collider. Plans have been formulated over the past year to include a new 8.9 GeV antiproton accumulator ring, the Recycler, inside the Main Injector enclosure. The luminosity in the collider with the addition of the Recycler to the Main Injector project is expected to be 2 × 1032 c m - 2 s e c - 1 . It will then be possible to reach 1 x 1033 c m - 2 s e c - 1 in the collider following further modest improvements to the antiproton source. The Main Injector will begin operation near the end of the decade. The next run of the Tevatron collider RUN II which follows the commissioning of the Main Injector is scheduled to begin in 1999 or 2000. Run II luminosities are expected to reach the level of 2.0 × 103u c m - ~ s e c - 1 . The collider has two large collision halls which are currently occupied by the CDF and DO detectors. The CDF and DO collaborations are planning major upgrades to their detectors for Run II. [7] However, neither of these detectors has been designed to
241
Measurement sin 2/? sin 2/~ sin 2a
K+~r-K + K-rr+K + K+K-K +
sin 7
K+K-~r-K + K+K-zc+K g + g - K - ~ "+ K+K-Tr-Tr + K+K-Tr-Tr+Tr+Tr K%r-#+p #+#-
sin 7 B ° Mixing
FCNC
handle luminosities of 1 × 1033 c m - 2 s e c - 1 ; these high luminosities would require further detector upgrades beyond the ones currently planned for Run II. A dedicated B-detector could begin operation at the completion of Run II and run concurrently with the high luminosity program. There are studies being carried out at Fermilab to evaluate the feasibility and physics reach of a high luminosity Tevatron operating at 1 × 1033 c m - 2 s e c - 1 under a project known as TeV33. The number of interactions per crossing is very high - projected to be 9.13 interactions/crossing - in the proposed environment of TeV33 which will be problematic for vertex triggers. A dedicated B-detector could operate at a lower luminosity to decrease the number of interactions per crossing and ease demands on the trigger. At the present time, the laboratory plans to support one high-pt detector during TeV33 operation leaving the other interaction hall available for a dedicated B-detector.
2.1. Strategy for Studies At the Tevatron, the B production cross section is larger in the central region or at smaller pseudorapidity r / = - I n ( t a n ~ ) < 1.5. However, the B's produced in the forward direction
242
McBride~Nuclear Physics B (Proc. Suppl.) 50 (1996) 240-247
P
Table 2 Plans for upgrades to the Tevatron Collider at Fermilab. Run I B Run II (1993-1995) (1999-) Linac Main Upgrade Injector, Pbar Improvements Protons/bunch 2.32E+11 3.30E+11 Pbars/bunch 5.50E+10 3.60E+10 ram-mr
Run II (1999-) Antiproton ring
TeV 33 Targeting,
2.70E+11 5.50E+10
Cooling upgrades 2.40E+11 1.00E+ll
1000 36 0.18 2.03E+32 40.83 396 5.31
1000 108 0.26 1.04E+33 210.62 132 9.13
n l m - m r m
Energy Bunches Bunch length (rms) Typical Luminosity Best Luminosity Integrated Luminosity Bunch Spacing Interactions/crossing ( at 45 m b )
900 6 0.6 1.58E+31 2.50E÷31 3.18 3500 2.48
hadrons
1000 36 0.43 8.29E+31 16.72 396 2.17
a t abe T e v a t r o n
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GeV m c m - ;sec -1 cm-~sec -~ pb-1/week nsec
or larger pseudorapidity 1.5 > q > 4.5 are boosted making the B decay distances larger• In addition, the daughter tracks of forward going B's have higher momenta than the daughters of centrally produced B's and, therefore, the effects of multiple scattering are lessened in the forward region• A plot of/7 3' vs. 77 for B-hadrons produced in Tevatron collisions is shown in Figure 1. Solenoidal detectors such as CDF and the upgraded DO detector have good m o m e n t u m resolution out to about letal < 1.5 but the vertex resolution is degraded due to multiple scattering of the low momentum tracks in the tracking chambers and silicon• Applying a Pt cut helps the vertex resolution, but unfortunately significantly reduces the acceptance for B decays. While the geometrical acceptance is greater for 3 units of rapidity in the central region, one expects the better vertex resolution in the forward to compensate in the final analysis. We have performed a series of studies which attempts to quantify this effect. The strategy of our studies has been to examine a few selected physics channels and explore acceptance, background and resolution issues for
243
P McBride~Nuclear Physics B (Proc. Suppl.) 50 (1996) 240-247
several detector options. We plan to investigate the trigger in a second phase of the studies. Our goal is to compare various proposed detector geometries and understand their B physics potential at the analysis level. If a detector has very limited capabilities with a perfect trigger, there is not much need to investigate further. We use this strategy to make a first comparison of the detectors and have plans to review the trigger in upcoming studies. We have outlined several detector geometries for our studies: Central detectors, examples are CDF and DO, forward collider detectors with a forward dipole, an example is LHC-B, central dipole detectors, an example being the proposed BCD detector, and combinations of these detectors. The combinations could be a central solenoid with a dipole at large ]~/I or a cental dipole with smaller dipoles at large Ir/I. We made model detectors representing these various types of geometries, attempting to choose realistic detector resolutions and performance. We used a flexible parameterized detector simulation package developed at Fermilab to study the detector performance and Pythia [8] as the event generator for the studies presented below.
2.2. Looking f o r B ° Mixing One of the topics of interest for a Dedicated B experiment at a hadron collider is B8 mixing. We performed a preliminary study of B~ mixing using the channel B ° --~ J/¢f(-~- and compared the time resolution for the forward and central geometry. The simplest B, decay modes containing a ~ in the final state are the fJ~/and ~¢. Neither of these modes can be used to measure B, mixing since they are not flavor specific. It has been suggested that the Cabibbo suppressed decay B, -+ ~/--~, --+ K=F~r+ presents a possible mode to investigate mixing phenomena [9]. The sign of the kaon charge distinguishes between the decaying B, and the B,. This mode would proceed via the diagram shown in Fig. 2. The recent observation by the CLEO collaboration of B - -+ ~Tr- decays at the level expected from Cabibbo suppression provides evidence for the existence of such diagrams.
[10] They measure
NB- --+~-) B ( B - --~ ~ K - )
= (5.2 + 2.6)% ~ A2.
(1)
We can thereby make a prediction of the branching ratio B(B8 ~ ¢ I U ) = B(B~ -~ ~K*) × ~2 =
(2)
1.7 x l0 -3 x 0.05 = 8.5 x 10 -5.
u_ "~
~
- . ,g ~
c} v S or
d}(~
or K *
Figure 2. Weak decay diagrams for B, --+ ¢ ¢ and ~K*. The K* final state occurs when the virtual W - materializes as a ~d pair.
In order to quickly evaluate the potential of this decay channel one can estimate the expected rate of B ° --+ J/~bK-;- from the number of events expected for B ° --+ J/~bK ° a mode which has been carefully studied by all proposed B detectors. One expects to see about 1/15 the number of reconstructed and flavor tagged B ° --+ J / ¢ K *
as B ° -~ J/eKe,. [11] There are several advantages to using the ~bK decay mode for the study of B ° mixing. A V -+ g+g- trigger can be used to select events, and the B decay vertex contains four charged tracks coming from a single decay vertex. This is important both for background reduction and for good decay time resolution. We have made estimates of the time resolution for a possible "forward" and "central" detector at the Fermilab collider. Our model detectors consist of silicon strip detectors, tracking chambers and have a dipole field for the forward detector and a solenoidal field for the central detector. The simulation program takes into account
P McBride~Nuclear Physics B (Proc. Suppl.) 50 (1996) 240-247
244
track smearing due to multiple scattering and detector resolution, however, pattern recognition is not attempted. In Figure 3 shows a comparison of the time resolution for a forward and central detector operating at the Tevatron. The forward detector covers an q range of 1.5 < ~ < 4.5 and the tracking in the central detector covers a range ]r]l < 1.5. The large difference in the time resolution is a result of the difference in the m o m e n t u m spectra in the two 7? ranges. The daughter tracks have much lower m o m e n t u m in the central region and the subsequently multiple scattering is much more troublesome in this region.
3°°In
I"-1 F o r w a r d
250
::'i~-:'.:'iC e n t r c l
200
150
100
......~';:.-:.....
50
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~.~.-..4~:' 0.1
~.,.,, , ,~:.~,.~.,~.,-., ..~..................... ,................. 0.2 0.3 0,4 0.5
Time resolution (ps)
Figure 3. Comparison of the time resolution for a forward collider experiment and a central detector operating at Tevatron energies for B ° --+ J / ~ K * events.
The measurement of this channel sets some requirements for a B detector. A detector with good vertex resolution will be able to take advantage of the clean J / ¢ signal and the four track B decay vertex to significantly reduce the background from generic B decays. Excellent
mass resolution will be needed to eliminate backgrounds from B~ -+ J/~bK*. Excellent particle identification will be required to identify the K and 7r in the K* decay and to remove background from other channels such as B , -+ J / ¢ ¢ . To estim a t e the reach in xs for a particular experiment requires studies not only of the vertex resolution as was done here, but of backgrounds and fitting procedures. Further studies with more detailed simulations are planned. 2.3. S i m u l a t i o n s f o r D e t e c t o r S t u d i e s In the course of our B physics studies we developed a new simulation and analysis tool. As stated above we were interested in exploring the capabilities of a dedicated B physics detector at the Fermilab collider and to see if such a detector would be competitive with other proposed experiments. We found a need for a fast generic detector simulation program to make a quantitative comparison of the physics capabilities of a variety of detector options. The program needed to be flexible, handle solenoids and dipoles and a variety of tracking detectors, but most importantly provide a fast track to physics results so that we would be able to simulate and analyze a large number of signal and background events in a relatively short time. We developed a new fast Monte Carlo package MCFast [12] for carrying out the detector studies. The package is a parameterized Monte Carlo and analysis package which gives tracking and vertex resolutions for detectors with dipole and solenoidal magnets. The goal of MCFast is to provide a general framework for the comparison of differing detector geometries. With this framework, we can model a variety of detector geometries for the Fermilab collider. We would also like to be able to make comparisons to experiments planned for the LHC. The detector simulation package is interfaced using S T D H E P [13] conventions to the standard H E P generators, ISAJET, P Y T H I A and H E R W I G which model the production of B hadrons. We use QQ, which was developed by the CLEO collaboration, for modeling the decays of B hadrons. The primary goal of MCFast is speed and flexibility which is achieved through parameteriza-
P. McBride~Nuclear Physics B (Pro+. Suppl.) 50 (1996) 240-247 tion. [15] The emphasis of the program is fast tracking which is based on the Billoir technique. [14] In evaluating the error on each particle trace in the detector, the effects of multiple scattering, detector resolution and efficiency are taken into account. The tracks are then smeared according to the calculated covariance m a t r i x and the information is immediately available for vertex studies and mass calculations. A model for hit generation has been included in the MCFast package and is under development. This will be used for trigger studies and for studies of pattern recognition and track overlap issues which will become i m p o r t a n t issues at the luminosities of future hadron colliders. Other features that are included are multiple interactions, decays in flight and 3' conversions. To verify the simulation we have compared the computed vertex resolution and mass resolution from MCFast to calculations and measurements from current experiments. Our results agree well with d a t a from C D F Run IB and the Fermilab fixed target experiment E687. We are able to simulate and analyze an interaction in a CDFlike detector operating at Tevatron energies in approximately 0.5 sec using a high speed computer with a 150 MHz R4400 processor.
of double-sided silicon planes, tracking detectors and a muon system. As mentioned above we ignored particle ID and trigger effects in these preliminary comparison studies. Our goal was to compare detectors in the as systematic a manner as possible. B ° ~ ~r+~r- is a difficult channel to isolate as the branching ratio is very small ( ~ 10 -5) and the analysis relies primarily on clean vertex separation and good mass resolution. We tried to design cuts that would take advantage of the capabilities of the vertex detectors to isolate detached vertices. One can also achieve improvements in signal to background in this channel by requiring that the two pions are both high Pt tracks but this comes with a corresponding undesirable drop in efficiency. A comparison of the decay length resolution L / ~rL is shown in Figure 4 for the central and forward detector geometries.
Lla
i05 1
2.4. C o m p a r i s o n s o f D e t e c t o r G e o m e t r i e s We have used MCFast to make a simulated comparison of B~ -+ ~r+lr - yields for various detector geometries operating at Tevatron energies. Previous studies made for a dedicated B-detector proposed for the SSC indicated that the principal background in the channel will come from bb events. [16] We generated a sample of 40,000 B~ -+ rr+~r- signal event and 300,000 generic bhadron events. We compared the results of simple analyses for isolating the signal B ° ~ rr+rr in three detector geometries: forward detector (1.5 < r~ < 4.5 in the fashion of LHC-B, a central detector 17/I < 1.5 similar to CDF RUN II, and a central dipole configuration with a large dipole magnet centered on the interaction region. We designed a analysis that could be applied to all detector configurations with only minor modifications. In order to maximize acceptance, we used a minimal pt cut on the tracks of 250 MeV. The detectors each contained multiple layers
245
'
I
,
1
,------, Forward Cer:h'al
103
zo: i
10 z
,,,
l
0
',.¢-,',','- :; , -,:',
,li]ll.n,,,,,,. I00
200
,i!! 300
,~00
Figure 4. Comparison of the decay length resolution L/(~L for a forward collider experiment and a central detector operating at Tevatron energies for B ° ~ ~-+ 7r- events.
246
R McBride~Nuclear Physics B (Proc. Suppl.) 50 (1996) 240-247
In the central detector, the distribution falls rapidly and is very sensitive to the placement of a cut. However, in the forward direction, the distribution is much less steep, and there is a much smaller loss of efficiency when one applies a safe cut to select clean secondary vertices. For our analysis we use a cut of T7 L of 8 for both the central and forward detectors. The smearing of prim a r y vertex by its resolution has not yet been included in this study. Other vertex cuts are added to require that the two ~r's miss the primary vertex and that the reconstructed B ° points back to the primary vertex. The results of these studies are shown in Table 3. The central dipole geometry which covers the same range in rI gives similar results to the forward geometry, but with about twice the acceptance. These results translate to an error on sin 2a of 0.17 for the forward and 0.35 for the central detector in one year assuming 100% trigger efficiency and muon tagging only. Note that the muon tagging is less than 2% efficient and improvements are expected with the addition of other tagging methods such as kaon tagging. For the measurement of sin 2/3 using B ° -+ J/~bK °, the forward and central geometries yield similar results. With a muon tag alone and a 100% efficient trigger, one expects to measure sin 2fl to 0.09 in either detector in 1 year of running at 1032cm-2sec -1. Our simulations were done for generic models of a solenoidal detector and a forward detector, therefore, these results should be considered as estimates since the physics capabilities are still functions of the exact configuration and resolutions of the tracking chambers. However, trends are becoming apparent. For measurements that depend on vertex cuts, detectors which cover the forward geometry retain efficiency as strict vertex cuts are applied. They also have excellent decay vertex resolution due to reduced multiple scattering. A good dipole magnet will give them excellent mass resolution in this region. The most promising detector geometry for a dedicated Bdetector at the Tevatron is the central dipole which covers the range 1.5 < 1771< 4.5. However, to complete the evaluation of any one detector geometry, one also must to consider tagging and trigger efficiencies in the final analysis. This work
is in progress. 3. C O N C L U S I O N S An effort is underway to design a second generation B-detector for the Tevatron collider. Studies have begun to investigate the potential of such a detector and to compare its capabilities to existing or proposed experiments for studying B physics. Simulation tools have been developed to facilitate studies of the physics reach of such a detector and to compare different geometry options. A central dipole geometry appears to be optimal for the Tevatron combining good acceptance for B decays with excellent secondary vertex resolution. Further studies are needed particularly in the area of trigger and particle ID before the design and comparisons can be completed.
4. A C K N O W L E D G M E N T S The author would like to thank the members and guests of the Fermilab Computing Division who have worked on this project. They include Paul Avery, Amber Boehnlein, Joel Butler, Paul Lebrun, Lynn Garren, Michael Procarlo, Kevin Sterner, Sheldon Stone, Torre Wenaus and Julia Yarba. The work presented here was supported by the Fermi National Accelerator Laboratory, which is operated by Universities Research Association, Inc., under contract DEAC02-76CH03000 with the U.S. Department of Energy. The author would also like to acknowledge support from the National Science Foundation.
REFERENCES 1. 2. 3. 4.
E. Meschi, in these proceedings. B D e c a y s , 2nd Edition ed. S. Stone, World Scientific, Singapore (1994). K.T. McDonald, Princeton/HEP/92-09. Proceedings of the Workshop on B
Physics at Hadron Accelerators, ed. P. McBride and C.S. Mishra, Fermilab-CONF93/267 (1993). 5. R. Edelstein, et al., Fermilab 1994-EOI-002. 6. For information on the Fermilab accelerator complex see http://www-fermi3.fnal.gov/
t? McBride~Nuclear Physics B (Proc. SuppL) 50 (1996) 240-247
247
Table 3 Acceptance for 7r+~r- in the mass region 5.0-5.6 GeV. The results for the central dipole are estimated from the forward. Studies of the Central Dipole geometry are underway. Note that the branching ratio for B ° --+ rr+rr- is 10 -5. Estimate for a Central Forward Central Dipole Detector Detector Detector 171 < 1.5 1.5 < r / < 4.5 1.5 < It/[ < 4.5 Signal events: B] --+ ~-+~rNo vertex cuts 35.5% 18.6% Silicon hits and L > 8 10.7% 13.4% All vertex cuts 6.2% 12.1% >20 % Background: from generic bb events No vertex cuts 2.2% 1.2% Silicon hits and yL > 8 0.18% 0.22% All vertex cuts 2 × 10 - 6 10 - 6 ~ 2 × 10 - 6
7. F. Dejongh, in these proceedings. 8. T. Sj6strand, Computer Physics Commun. 39 (1986) 347; T. SjSstrand, Computer Physics Commun. 43 (1987) 367. 9. S. Stone, "B Physics at the SSC," in Proc. of t h e 1991 S y m p o s i u m on t h e SSC," Corpus Christi, TX, SSCL-SR-11213 (1991) p225. 10. J. Alexander et al., (CLEO) Phys. Lett. B 341, 435 (1995); erratum ibid 347, 469 (1995). 11. P. McBride and S. Stone, in Proceedings of BEAUTY '95 - 3rd International Workshop on B-Physics at Hadron Machines, July 1995. 12. A. Boehnlein, in the Proceedings of CHEP95, Sept. 1995. 13. L. Garren, Fermilab CD Note PM0091 (or see
http ://fnpspa. fnal. gov/st dhep. html) 14. P. Billoir, NIM 225 (1984) 352. 15. For more information on MCFast
see
http.'//fnpspa.fnal.gov/simulation.html 16. O.L. Long, et al., preprint (UPR/216E, Princeton/HEP/92-07).