- Email: [email protected]
Status of BTeV
ELSEVIER
Nuclear Physics B (Proc. Suppl.) 93 (2001) 303-306
SUPPLEMENTS
Status of BTeV Yuichi Kubotaa a School of Physics and Astronomy, University of Minnesota
116 Church St. S.E., Minneapolis, MN 55455, USA BTeV has been approved for construction by Fermi Lab to study a wide spectrum of heavy-flavour physics. This includes systematic studies of CP violating phenomena in b and c-hadron decays, studies of their rare decays and BOB', BtBi and D”bo mixing. These studies are crucial to fully understand the origin of CP violation. The BTeV experiment takes advantage of the large number of b hadrons produced at the Tevatron and state-of-the-art technologies to select and record those events in an environment with large background. This paper presents the status of the detector R&D and expected physics potential.
1. INTRODUCTION Violation of the CP symmetry was observed in the K&meson decay in 1964 [l]. Since then, all the efforts to find CP violation in non-Kg system produced null results. However, if the Standard Model holds, we should expect soon such observation in the B system by one of the current heavy-flavour experiments. CP violation is also a required ingredient to explain the abundance of matter over anti-matter in our universe. Quark flavour mixing among three generations of quark pairs, described by a unitary transformation matrix (CKM matrix) between the mass and weakinteraction eigenstates, can explain CP violation in the K” decay. But it predicts an asymmetry too small to produce the amount of matter in our universe. This suggests that more thorough studies of CP violating phenomena are needed to advance our understanding of nature. One approach is to test if the CKM matrix is unitary when all of its elements are measured including their complex phase angles. Unitarity of a matrix is equivalent to its rows or columns being orthogonal. The “8 and “b” columns are orthogonal if
This relation can be expressed geometrically on the complex plane where each of the three terms in the left-hand side of this equation corresponds 0920-5632/01/S - see front matter 0 2001 Elsewer Science B V PI1 SO920-5632(00)01123-3
to a line in the complex plane. The equation is satisfied if the three lines form a triangle. Each element of the CKM matrix corresponds to the coupling strength between the respective quarks. The term I&b, for example, represents the coupling strength between the u and b quarks. Thus the magnitude of I&, is obtained from the decay rate of the b quark to the u quark. CP violation in a decay results when two contributing decay diagrams contain combinations of CKM elements of different complex phases. Then their interference contributes in the B and B decays with opposite signs, causing CP violation. Conversely, the relative complex phases of the CKM elements can be determined from an observed CP violation in the decay. BTeV is designed to measure, among others, CP violation relevant to the orthogonality between the “d” and “b” columns. This paper describes some of the highlights of physics BTeV addresses, the major detector features and R&D. Please refer to the proposal [2] for details which are not covered in this paper.
2. DETECTOR The Tevatron will deliver 2 x 1011 b-hadron pairs every year (lo7 seconds) at a nominal luminosity of 2 x 1O32cm-2s-1. This b production rate is much higher than at the e+e- B factories (lo7 to log/year). A high-performance data acAll rights reserved
304
Y Kubota/Nuclear
Physics B (Proc
quisition system (DAQ) will be required to record the high rate b events of 100 kB in data size. These b events are buried among events containing only lighter quarks, which are produced 500 times more frequently. One of the most powerful ways to reduce this background is to require successful reconstruction of the b flight distance. BTeV must push this technology, which has been successful in charmed-particle experiments, to deal with the worse signal-to-noise ratio and higher particle multiplicity that BTeV must face. This is accomplished using the pixel silicon detectors with 50 x 400pm2 pixels. The pixel detectors are placed extremely close to the production vertices to obtain the best vertex precision. It is important to reject these background events as early as possible to keep the requirements on the DAQ system to a reasonable level. The pixel detector, with its fast readout, is the base of the trigger system to accomplish this rejection by enabling fast reconstruction of secondary vertices. The high granularity of the pixel detector leads to low occupancy and easier track and vertex reconstruction. A RICH detector using C.+Flo and Aerogel as Cerenkov radiator distinguishes r’s and K’s, which is crucial to tag the flavour of the B’s since CP violation appears as flavour asymmetry. It is also very important to distinguish different B decays such as B + mr and B + Kn. Finally, BTeV employs lead-tungstate scintillation crystals to detect gamma rays with excellent energy and position resolutions. It is necessary for BTeV to include a high-performance calorimeter to find y’s and no’s since about l/3 of the b hadron daughters are AO’S. Many of the B decay modes we are planning to use to study CP violation require detection of neutrals. For example, to measure the CP violating angle o using B -+ pn, one needs to find not only B + p’r-, which leads to all charged final state particle, but also B + p*wr. The same is true for analyses of B + mr, B + KK and B -+ DK decays. 3. R&D
Status
and
Schedule
I will describe the R&D effort of BTeV’s pixel detector in some details since it is one of the
Suppl)
93 (2001) 303-306
most critical components of BTeV. Other detector components including the calorimeter, RICH, DAQ system and trigger system hav: made significant progress in their R&D [2]. The pixel sensors identify second:Lry vertices arising from bhadron decays to separ ite b events from the rest, both at the trigger stage and in offline analyses. It must maintain high l osition resolution (less than 10 microns) with fist readout capability in a high radiation environment over the life of the experiment (10 years). The first two requirements can easily be met by a pixel detector. However, we must make surf that both the sensors and the electronics will be radiation resistant. Recent studies on the radiation llardness of pixel sensors suggest that the type of device identified as n+np+ is much less sensitile to ‘&type inversion” caused by radiation than a nore traditional p+nn+ sensors. However, this type of sensor requires a more complex fabrication. method to isolate n+ pixels from each other. Two pixel isolation methods are being studied: p stop and p spray methods. BTeV has studied two ATLAS sensors produced by CiS (Boeing North Americ:.) in a test beam recently. One used p stop methId and the other, p spray. They were bump bonded to our prototype front-end electronics, FPIXO, which has been fabricated primarily to stud;1 front-end electronics design. Fig. 1 shows the pulse height distriblltion when 227 GeV pions passed the pspray and v-stop sensors in terms of the number of electrons. Both are described well by the Landau distribution. On the average, the pspray sensor produced 23100 electrons whereas the p-stop sensor produced 30100 electrons. The p-spray sensor produced substantially smaller numbers cf electrons when the pions passed near one of t le column boundaries, which is observed as a shoulder at the lower side of the Landau peak in the right plot of Fig. 1. Although the pstop sensor suffered much smaller inefficiency, the number of ele:trons collected when the pions passed near the corners of pixels was smaller. These observations of ATLAS sensors agree with what ATLAS found [3]. The inefficiency for the p-spray sensor turned out to
Y Kubota/Nuclear Physrcs B (Proc Suppl.) 93 (2001) 303-306
FpixO-pstop Imuroved Landau fit
305
FpixO-pspraylmoroved Landan fit
Figure 1. Pulse height distributions for a) pstop and b) pspray ATLAS prototype pixel sensors. Entries below 15,000 e’s in the left plot and the shoulder at less than 10,000 e’s in the right plot indicate inefficiencies in the charge collection. be due to the particular sensor design rather than the intrinsic property of the pspray method, and a newer ATLAS p-spray sensor has an inefficiency similar to the pstop sensor. The spatial resolution of these sensors as a function of the track incident angle is shown in Fig. 2. One set of the results was obtained when charge sharing information is ignored, and the other set was found when 8-bit charge digitization was used. They agree well with the predictions from our elaborate device simulations. Controlled amounts of oxygen also make pixel sensors more radiation tolerant [4]. BTeV made a joint submission in spring 1999 to two vendors, SINTEF (Norway) and CSEM (Switzerland). The first wafers from SINTEF have been received and their measured I-V curves met our specification, particularly in terms of small leakage current and high reverse breakdown voltage (500 V or higher). These sensors will be tested for their radiation sensitivity after they are bump bonded to FPIXB electronics described below. Front-end electronics R&D started in 1997 with FPIXO using radiation-soft process, and progressed to FPIXl, which included a high-speed
Figure 2. Position resolution as a function of beam incidence angle. The triangles are the resolution when no pulse-height information is used, and the squares are when 8-bit charge information is used. The two curves are the corresponding simulation predictions.
Y Kubota/Nuclear
306
Physm
readout architecture and 2-bit flash ADC’s for each channel using 4 comparators. FPIXO contains 11 columnsx64 rows of amplifiers, which match the ATLAS prototype pixel sensors described above. FPIXl contains 18 columns x 160 rows of amplifiers, though only 90 rows were usable due to a minor design error. The noise of FPIXO was measured to be 80 to 100 e’s depending on the sensors (p spray vs. p stop). The noise of FPIXl chips is comparable to FPIXO. When they were used with an ATLAS sensor, we were able to operate them with a threshold of 1,500 e’s . Radiation-tolerant FPIX2 is being designed for implementation at two vendors (IBM and TSMC/MOSIS) using 0.25pm CMOS process, which is considered radiation hard if properly produced. Last spring, preFPIX2 containing only sample transistors and preFPIX2T containing two columns of 160 rows of amplifiers, both produced at TSMC, were irradiated using a Co60 source. The noise increased after a preFPIX2T chip received 33 MRad (more than 10 years worth of radiation) by only less than 5%, and threshold appeared to have gone down from 1100 e’s to 1000 e’s. These results on radiation tolerance are comparable to the results the RD49 experiment obtained from IBM process.
B (Proc
Suppl.)
93 (2001) 303-306
Table 1 Summary of projected physics BTeV. Parameter Decay modes CP B + n+nBs + DsKB + J/$Ks** B- + D°K-*** B- + DOK-CM* BB” B” B”
+ + + -+
KsnK--T+ p+a,o”ro
asymmetry
capabilities
of
Error
Y sin 2p
0.024 7” 0.025
y
10”
Y
< 5”
CY
N 10”
J/h** Bs + JhW**
x
0.033
Bs mixing Bs + Dsn-
X5
75*
Bs +
Rare Decays Bo + K*O/.A+/AB- + K-,LA+/.Lb + @/A-
polarization rate rate; Wilson Coefficients
? ? ?
* maximum value BTeV can measure, ** J/1(,+ **** Do + K+K-. *W@ +, K++-,
PA
Acknowledgments 4. Expected clusions
Physics
Potentials
and
Con-
We calculated the sensitivities of BTeV for various physics using GEANT-based Monte Carlo simulation including Kalman-filter track fitting and realistic shower reconstruction program for the calorimeter simulation. A track finding algorithm was also used to simulate trigger processor performance, but in the offline analysis, tracks were not reconstructed. The results, listed in Table 1, show great promise of BTeV. FNAL believes that “BTeV has the potential to be a central part of an excellent Fermilab physics program in the era of the LHC. With excitement about the science and enthusiasm for the elegant and challenging detector,” the lab gave BTeV an approval. Now it is our job to make the experiment a reality.
I am grateful to my colleagues in the BTeV collaboration who contributed to the current status of the experiment in a variety of ways. REFERENCES J.H. Christenson, J. Cronin, V. Fitch and R. Turlay, Phys. Rev. Lett. 13 (1964) 138. A. Kulyavtsev, et al. “Proposal for an Experiment to Measure Mixing, CP Violation and Rare Decays in Charm and Beauty Particle Decays at the Fermilab Collider - BTeV,” http://www-btev.fnal.gov/publac-documents/ btev_proposal/ tndex.html (2000). F. Ragusa, Nucl. Instru. and Methods A447 (2000) 184. The ROSE collaboration, CERN Report LHCC-2000-09, Dec. 1999.
Nuclear Physics B (Proc. Suppl.) 93 (2001) 303-306
SUPPLEMENTS
Status of BTeV Yuichi Kubotaa a School of Physics and Astronomy, University of Minnesota
116 Church St. S.E., Minneapolis, MN 55455, USA BTeV has been approved for construction by Fermi Lab to study a wide spectrum of heavy-flavour physics. This includes systematic studies of CP violating phenomena in b and c-hadron decays, studies of their rare decays and BOB', BtBi and D”bo mixing. These studies are crucial to fully understand the origin of CP violation. The BTeV experiment takes advantage of the large number of b hadrons produced at the Tevatron and state-of-the-art technologies to select and record those events in an environment with large background. This paper presents the status of the detector R&D and expected physics potential.
1. INTRODUCTION Violation of the CP symmetry was observed in the K&meson decay in 1964 [l]. Since then, all the efforts to find CP violation in non-Kg system produced null results. However, if the Standard Model holds, we should expect soon such observation in the B system by one of the current heavy-flavour experiments. CP violation is also a required ingredient to explain the abundance of matter over anti-matter in our universe. Quark flavour mixing among three generations of quark pairs, described by a unitary transformation matrix (CKM matrix) between the mass and weakinteraction eigenstates, can explain CP violation in the K” decay. But it predicts an asymmetry too small to produce the amount of matter in our universe. This suggests that more thorough studies of CP violating phenomena are needed to advance our understanding of nature. One approach is to test if the CKM matrix is unitary when all of its elements are measured including their complex phase angles. Unitarity of a matrix is equivalent to its rows or columns being orthogonal. The “8 and “b” columns are orthogonal if
This relation can be expressed geometrically on the complex plane where each of the three terms in the left-hand side of this equation corresponds 0920-5632/01/S - see front matter 0 2001 Elsewer Science B V PI1 SO920-5632(00)01123-3
to a line in the complex plane. The equation is satisfied if the three lines form a triangle. Each element of the CKM matrix corresponds to the coupling strength between the respective quarks. The term I&b, for example, represents the coupling strength between the u and b quarks. Thus the magnitude of I&, is obtained from the decay rate of the b quark to the u quark. CP violation in a decay results when two contributing decay diagrams contain combinations of CKM elements of different complex phases. Then their interference contributes in the B and B decays with opposite signs, causing CP violation. Conversely, the relative complex phases of the CKM elements can be determined from an observed CP violation in the decay. BTeV is designed to measure, among others, CP violation relevant to the orthogonality between the “d” and “b” columns. This paper describes some of the highlights of physics BTeV addresses, the major detector features and R&D. Please refer to the proposal [2] for details which are not covered in this paper.
2. DETECTOR The Tevatron will deliver 2 x 1011 b-hadron pairs every year (lo7 seconds) at a nominal luminosity of 2 x 1O32cm-2s-1. This b production rate is much higher than at the e+e- B factories (lo7 to log/year). A high-performance data acAll rights reserved
304
Y Kubota/Nuclear
Physics B (Proc
quisition system (DAQ) will be required to record the high rate b events of 100 kB in data size. These b events are buried among events containing only lighter quarks, which are produced 500 times more frequently. One of the most powerful ways to reduce this background is to require successful reconstruction of the b flight distance. BTeV must push this technology, which has been successful in charmed-particle experiments, to deal with the worse signal-to-noise ratio and higher particle multiplicity that BTeV must face. This is accomplished using the pixel silicon detectors with 50 x 400pm2 pixels. The pixel detectors are placed extremely close to the production vertices to obtain the best vertex precision. It is important to reject these background events as early as possible to keep the requirements on the DAQ system to a reasonable level. The pixel detector, with its fast readout, is the base of the trigger system to accomplish this rejection by enabling fast reconstruction of secondary vertices. The high granularity of the pixel detector leads to low occupancy and easier track and vertex reconstruction. A RICH detector using C.+Flo and Aerogel as Cerenkov radiator distinguishes r’s and K’s, which is crucial to tag the flavour of the B’s since CP violation appears as flavour asymmetry. It is also very important to distinguish different B decays such as B + mr and B + Kn. Finally, BTeV employs lead-tungstate scintillation crystals to detect gamma rays with excellent energy and position resolutions. It is necessary for BTeV to include a high-performance calorimeter to find y’s and no’s since about l/3 of the b hadron daughters are AO’S. Many of the B decay modes we are planning to use to study CP violation require detection of neutrals. For example, to measure the CP violating angle o using B -+ pn, one needs to find not only B + p’r-, which leads to all charged final state particle, but also B + p*wr. The same is true for analyses of B + mr, B + KK and B -+ DK decays. 3. R&D
Status
and
Schedule
I will describe the R&D effort of BTeV’s pixel detector in some details since it is one of the
Suppl)
93 (2001) 303-306
most critical components of BTeV. Other detector components including the calorimeter, RICH, DAQ system and trigger system hav: made significant progress in their R&D [2]. The pixel sensors identify second:Lry vertices arising from bhadron decays to separ ite b events from the rest, both at the trigger stage and in offline analyses. It must maintain high l osition resolution (less than 10 microns) with fist readout capability in a high radiation environment over the life of the experiment (10 years). The first two requirements can easily be met by a pixel detector. However, we must make surf that both the sensors and the electronics will be radiation resistant. Recent studies on the radiation llardness of pixel sensors suggest that the type of device identified as n+np+ is much less sensitile to ‘&type inversion” caused by radiation than a nore traditional p+nn+ sensors. However, this type of sensor requires a more complex fabrication. method to isolate n+ pixels from each other. Two pixel isolation methods are being studied: p stop and p spray methods. BTeV has studied two ATLAS sensors produced by CiS (Boeing North Americ:.) in a test beam recently. One used p stop methId and the other, p spray. They were bump bonded to our prototype front-end electronics, FPIXO, which has been fabricated primarily to stud;1 front-end electronics design. Fig. 1 shows the pulse height distriblltion when 227 GeV pions passed the pspray and v-stop sensors in terms of the number of electrons. Both are described well by the Landau distribution. On the average, the pspray sensor produced 23100 electrons whereas the p-stop sensor produced 30100 electrons. The p-spray sensor produced substantially smaller numbers cf electrons when the pions passed near one of t le column boundaries, which is observed as a shoulder at the lower side of the Landau peak in the right plot of Fig. 1. Although the pstop sensor suffered much smaller inefficiency, the number of ele:trons collected when the pions passed near the corners of pixels was smaller. These observations of ATLAS sensors agree with what ATLAS found [3]. The inefficiency for the p-spray sensor turned out to
Y Kubota/Nuclear Physrcs B (Proc Suppl.) 93 (2001) 303-306
FpixO-pstop Imuroved Landau fit
305
FpixO-pspraylmoroved Landan fit
Figure 1. Pulse height distributions for a) pstop and b) pspray ATLAS prototype pixel sensors. Entries below 15,000 e’s in the left plot and the shoulder at less than 10,000 e’s in the right plot indicate inefficiencies in the charge collection. be due to the particular sensor design rather than the intrinsic property of the pspray method, and a newer ATLAS p-spray sensor has an inefficiency similar to the pstop sensor. The spatial resolution of these sensors as a function of the track incident angle is shown in Fig. 2. One set of the results was obtained when charge sharing information is ignored, and the other set was found when 8-bit charge digitization was used. They agree well with the predictions from our elaborate device simulations. Controlled amounts of oxygen also make pixel sensors more radiation tolerant [4]. BTeV made a joint submission in spring 1999 to two vendors, SINTEF (Norway) and CSEM (Switzerland). The first wafers from SINTEF have been received and their measured I-V curves met our specification, particularly in terms of small leakage current and high reverse breakdown voltage (500 V or higher). These sensors will be tested for their radiation sensitivity after they are bump bonded to FPIXB electronics described below. Front-end electronics R&D started in 1997 with FPIXO using radiation-soft process, and progressed to FPIXl, which included a high-speed
Figure 2. Position resolution as a function of beam incidence angle. The triangles are the resolution when no pulse-height information is used, and the squares are when 8-bit charge information is used. The two curves are the corresponding simulation predictions.
Y Kubota/Nuclear
306
Physm
readout architecture and 2-bit flash ADC’s for each channel using 4 comparators. FPIXO contains 11 columnsx64 rows of amplifiers, which match the ATLAS prototype pixel sensors described above. FPIXl contains 18 columns x 160 rows of amplifiers, though only 90 rows were usable due to a minor design error. The noise of FPIXO was measured to be 80 to 100 e’s depending on the sensors (p spray vs. p stop). The noise of FPIXl chips is comparable to FPIXO. When they were used with an ATLAS sensor, we were able to operate them with a threshold of 1,500 e’s . Radiation-tolerant FPIX2 is being designed for implementation at two vendors (IBM and TSMC/MOSIS) using 0.25pm CMOS process, which is considered radiation hard if properly produced. Last spring, preFPIX2 containing only sample transistors and preFPIX2T containing two columns of 160 rows of amplifiers, both produced at TSMC, were irradiated using a Co60 source. The noise increased after a preFPIX2T chip received 33 MRad (more than 10 years worth of radiation) by only less than 5%, and threshold appeared to have gone down from 1100 e’s to 1000 e’s. These results on radiation tolerance are comparable to the results the RD49 experiment obtained from IBM process.
B (Proc
Suppl.)
93 (2001) 303-306
Table 1 Summary of projected physics BTeV. Parameter Decay modes CP B + n+nBs + DsKB + J/$Ks** B- + D°K-*** B- + DOK-CM* BB” B” B”
+ + + -+
KsnK--T+ p+a,o”ro
asymmetry
capabilities
of
Error
Y sin 2p
0.024 7” 0.025
y
10”
Y
< 5”
CY
N 10”
J/h** Bs + JhW**
x
0.033
Bs mixing Bs + Dsn-
X5
75*
Bs +
Rare Decays Bo + K*O/.A+/AB- + K-,LA+/.Lb + @/A-
polarization rate rate; Wilson Coefficients
? ? ?
* maximum value BTeV can measure, ** J/1(,+ **** Do + K+K-. *W@ +, K++-,
PA
Acknowledgments 4. Expected clusions
Physics
Potentials
and
Con-
We calculated the sensitivities of BTeV for various physics using GEANT-based Monte Carlo simulation including Kalman-filter track fitting and realistic shower reconstruction program for the calorimeter simulation. A track finding algorithm was also used to simulate trigger processor performance, but in the offline analysis, tracks were not reconstructed. The results, listed in Table 1, show great promise of BTeV. FNAL believes that “BTeV has the potential to be a central part of an excellent Fermilab physics program in the era of the LHC. With excitement about the science and enthusiasm for the elegant and challenging detector,” the lab gave BTeV an approval. Now it is our job to make the experiment a reality.
I am grateful to my colleagues in the BTeV collaboration who contributed to the current status of the experiment in a variety of ways. REFERENCES J.H. Christenson, J. Cronin, V. Fitch and R. Turlay, Phys. Rev. Lett. 13 (1964) 138. A. Kulyavtsev, et al. “Proposal for an Experiment to Measure Mixing, CP Violation and Rare Decays in Charm and Beauty Particle Decays at the Fermilab Collider - BTeV,” http://www-btev.fnal.gov/publac-documents/ btev_proposal/ tndex.html (2000). F. Ragusa, Nucl. Instru. and Methods A447 (2000) 184. The ROSE collaboration, CERN Report LHCC-2000-09, Dec. 1999.