- Email: [email protected]
Tau physics at BaBar
PROCEEDINGS SUPPLEMENTS ELSEVIER
Nuclear Physics B (Proc. Suppl.) 76 (1999) 477-485
Tau Physics at BaBar Abraham Seiden a ~Santa Cruz Institute for Particle Physics, University of California, Santa Cruz, CA 95060, USA The next few years of nmning of the new B factories should provide data samples of r decays with more than an order of magnitude higher statistics than presently available. In addition the new detectors, such as the BaBar detector at the SLAC B factory, will have detection capabilities which will enhance their ability for r decay studies. We look at these issues as well as some of the physics opportunities and challenges provided by the expected new large data samples.
1. I N T R O D U C T I O N This past decade has been one of precision tau physics with all measurements to date in impressive agreement with the Standard Model. These results are based primarily on approximately 106 events from the sum of the four LEP experiments and approximately 3 x 108 events for a large number of publications from the CLEO group running at Cornell. In the next few years results from three new B factory detectors, BaBar at SLAC, Belle at KEK, and an upgraded CLEO at Cornell will significantly increase the number of available 7- events. At design luminosity BaBar is expecting to collect 3 × 10¢r events per year. In a few years one can expect total data samples ,~ l0 s for each of the three B factories. These large data samples will allow significant progress in r decay studies. 2. B a B a r D E T E C T O R
Although designed to optimize the measurement of CP violation in the B system, the BaBar detector [1], shown in Figure 1, should provide an excellent tool for studying r decays. Note, unlike the situation for the present generation of experiments, the colliding beams for BaBar are of unequal energy so that the center of mass is moving in the laboratory and the detector is offset relative to the collision point. The relationship between laboratory and center of mass production angles is shown in Figure 2. The detector offset provides a higher geometric efficiency, which is also more symmetrical between forward and backward directions in the center of mass. We describe the
components and expected performance of the detector below. Moving outward in radius the charged particle tracking consists of a 5 layer silicon vertex tracker followed by a small cell drift chamber containing predominantly He gas to minimize multiple scattering. The error in the impact parameter in the r-¢ and r - z planes, respectively, are shown in Figure 3. At normal incidence each error is expected to be approximately 50 # m / P (GeV). The expected fractional error in the transverse momentum measurement is shown in Figure 4. For r decays the excellent resolution will be very useful in background rejection, based on invariant mass reconstruction, when searching for Standard Model forbidden all charged decays. Figure 4b shows the center of mass angular dependence of the resolution which, because of the unequal beam energies, remains excellent over a somewhat larger solid angle backwards than forwards. Following the tracking components is the particle identification system (called the DIRC) composed of long quartz bars (arranged as a cylinder just outside the drift chamber). The Cherenkov photons produced by charged particles passing through the quartz are internally reflected and then detected outside the detector volume by a large array of phototubes. The reconstructed Cherenkov cone angle is then used to discriminate between particle types. The expected performance of the system is shown in Figure 5 as a number of standard deviations between the charged K and ~r hypotheses. The separation should be very complete for momenta below 3 GeV for the DIRC angular coverage which is
0920-5632/99/$ - sve front matter © 1999 Elsevier Science B.V. All rights reserved. PII S0920-5632(99)00510- l
478
,4. Seiden/Nuclear Physics B (Proc. Suppl.) 76 (1999) 477-485
6
4
5
I
3
2
Figure 1. Layout of the BaBar detector at date of BaBar Technical Design Report.
479
A. Selden~Nuclear Physics B (Proc. Suppl.) 76 (1999) 477-485
Detector ProTractor. 7's BACKWARD POLAR ANGLES
20F~
80°
90°
80°
FORWARD POLAR ANGLES
,.
20 °
0°
0 9
G e V on 3.109 G e V
Y(4S)
[37=0.56
e~
Figure 2. Protractor showing the relationship between center-of-mass and laboratory polar angles for photons at ~7 = 0.56.
,4. SeidenlNuclear Physics B (Proc. Suppl.) 76 (1999) 477-485
480
E
(a)
10 3 ',
-\
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lO 3 .~
(b)
!\
o
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--
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200 ~r
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,=
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,
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:*
,*
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==~,1
II
....
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-0.5
0.5
I.~,
0
0.5
cos ecru
0cm
cos
Figure 3. Resolutions for i m p a c t p a r a m e t e r s at the vertex: (a) c~y as a function of Pt at cos 0~ab = 0; (b) Crz as a function of Pt at cos Otab = 0; (c) ~xy as a function of cos 0cm for Pcm ---- 1 G e V / c ; and (d) crz as a function of cos 0c,n for Pcm -- 1 G e V / c . For (a) and (b), the dashed curve represents 50 # m / p t .
1.0
" ' 1
. . . .
i
. . . .
i
. . . .
i
. . . .
i
1.0
. . . .
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0.6
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. . . .
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. . . .
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. . . .
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. . . .
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.
.
.
.
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.
.
.
.
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.
.
.
.
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.
.
.
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COS(O~m )
Figure 4. Pt resolution at (a) 0~ab = 90 deg as a function of Pt, and (b) Pc,n = 1.0 G e V / c as a function of COS ~ c r n •
A. Seiden/Nuclear Physics B (Proc. SuppL) 76 (1999) 477-485
481
100 '
#a
'
I
. . . .
I
. . . .
I
. . . .
50
1.5
G o V / c
.
.
.
•
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,
,
,
,
,I
. . . .
-0.5
Figure 5. Predicted
K/r
I
. . . .
0
I
0.5
,
,
,
COS0
separation performance of the DIRC.
expected to be, in the center of mass, 87% in cos(0) and 93% in azimuth. The presence of very good particle identification should allow BaBar to significantly improve the knowledge about rates and mass spectra for r decays with charged kaons in the final state. Just outside the DIRC bars is a fine grained barrel crystal calorimeter whose expected resolution is shown in Figure 6. The forward solid angle is closed by an endcap of comparable performance to the barrel. The calorimeter, besides its excellent resolution, provides a large solid angle redundant trigger to that provided by the charged tracking, which will be very important to measuring the absolute efficiency for detecting r decays. The set of detectors described above sit inside a superconducting coil which provides a 1.5 Tesla solenoidal field. The iron flux return for this field is highly segmented and filled with resistive plate counters allowing very good muon detection and identification. The nearly hermetic segmented and instrumented flux return (called IFR) is shown in Figure 7. This device should allow good muon identification down to at least
600 MeV, significantly lower in momentum than in previous detectors at low energy e+e - colliders. 3. P H Y S I C S O P P O R T U N I T I E S A N D CHALLENGES We look below at some of the physics opportunities and challenges for r physics at BaBar.
3.1. Physics with kaons Major improvements in all channels containing charged kaons can be expected from BaBar due to the combination of very large statistics and excellent particle identification. Since the Cabibbo suppression for strange states is about a factor of 20 and the increase in statistics expected from each B factory is also about a factor of 20, we can expect future data samples for strange states to be comparable to present samples for non-strange states. The strange state with the most precise prediction is the Kvr final state. Comparing with a few well predicted channels, the present status for the fractional errors is [2]
482
A. Seiden/Nuclear Physics B (Proc. Suppl.) 76 (1999) 477-485
. . . . . .
I
.
.
.
.
.
.
.
.
I
.
.
.
.
2.5
,*--,
2.0
w
1.5
1.O 0.1
0.02
0.2
1
2
Photon Energy (GeV)
Figure 6. Energy resolution at cos 0c,~ = 0 as a function of photon energy. The resolution is defined as FWHM/2.36. The error bar on the first point indicates a typical uncertainty in determining the resolution. The solid line shows the target energy resolution.
Final State
Fractional Error on Branching Ratio
ev~, ~rv Kv
0.4% 1.2% 7.0%
The K g error is presently still rather large. With future B factory results we can expect a 1% test that the K v and ~rv rates agree with expectations from the ratio t~ v,d as determined in K and ~r decay. In addition, the electroweak corrections to the ratio are about 2% [3] so that a check that they contribute as expected can be made. The B factory data will allow a study of the full mass spectrum involving strange states. For massless strange quarks we would expect a branching ratio into strange states = 3.18+.02%, based on a leptonic branching ratio of 35.18 4.12% and a value for ]Vus[2 = 0.0490=t=.0003. The value that is seen is 2.78 + .15% based on adding branching ratio: K - + X = 1.66 + 0.10% branching ratio: K ° + X = 1.66 + 0.09%
and subtracting branching ratio: K - K ° + X = 0.31 + 0.04% branching ratio: K - K % r - + X -- 0.23 + 0.04%. The inclusive strange branching ratio seen is therefore [1 - (0.126 + 0.047)] x massless quark expectation. The B factory data should allow a significant improvement on the measurement of the correction factor 0.126 + 0.047, which presently has a 30% error. This factor can be written in terms of the strange quark mass at the r mass scale [4]:
8 ( m---2-'~ 211 + QCD corrections]. \mr / A precision test will require that the theoretical QCD corrections be accurately predicted, so that they create a fractional uncertainty << 30%. Note, the value of a~ at mr is known to about 6%, so this is not the dominant limitation.
A. Selden~Nuclear Physics B (Proc, Suppl.) 76 (1999) 477-485
Figure 7. The IFR detector.
483
484
A. Selden~Nuclear Physics B (Proc. Suppl.) 76 (1999) 477-485
3.2. C h a l l e n g e s o f h i g h p r e c i s i o n Two very fundamental r measurements are the electronic branching ratio, Be, and the lifetime, rr. Using v r = B e ~ r e and the accurate Standard Model prediction of Fe, we can relate these two via
Be=
rr 1632.4 + 1.3 (fsec)"
The error on the denominator, about one part in 103 , is dominantly from the uncertainty in our knowledge of the r mass. The present values for Be and rr are r~
=
2 9 0 ± 1.2 (fsec)
Be
=
17.81-4- 0.07%,
each uncertain by a factor of about 4 more than the factor relating Be to rr. The statistical errors expected from B factory measurements should allow a factor of 4 improvement in each, however, it is not at all clear that systematic effects will actually allow any improvement. To look at the systematic limitations we can use the CLEO measurements and their stated systematics. As an example of the limitations, the two most recent and most accurate CLEO measurements of Be are (with the second coming several years after the first):
Be
=
17.49±0.14:1: 0.22% [5]
Be
--
17.76+0.06±0.17% [6].
The second measurement has a much reduced statistical error, however, the reduction in the dominant systematic error is not nearly as large. To make significant improvement will require both theoretical and experimental work as both the calculation for the cross sections detected require a better understanding of higher order corrections and the uncertainties in the absolute detection efficiencies need to be improved. Turning to the lifetime, the CLEO measurement is 289.0 ± 2.8 ± 4.0 (fsec) [7]. This measurement has been made without the benefit of a silicon vertex detector and the systematic error is very predominantly from tracking and vertexing. We can therefore expect a significant improvement from the B factories based on their high
precision vertex trackers. It will be very challenging, however, to improve significantly on the present world average. 3.3. R a r e o r f o r b i d d e n d e c a y s The search for non-Standard Model phenomena at higher sensitivity will be a continuing goal of the B-factories. Some of the areas of interest are limits on lepton number violating decays and a search for CP violation in the r sector - either in the ~- structure or in v decays. Table 1 Detection efficiencies, event statistics, expected backgrounds, and upper limits for branching fractions at 90% confidence level. Decay Channel r - --+ e-e+e v - --+ p - e + e -r- -.+ p + e e r - .-} e - p + p -
r - --+ e+p p r - --+ p - g + p r - --~ e-Tr+ 7rr - -+ e - r - K + r - --). e-Tr+ K r - --+ e - K + K -
v - --+ e+~r-~r rr-
~ e+Tr-K --~ e+ K - K -
r - --~ p-lr+rr r - --+ p - w - K + ~'- ---+ p - T r + K r - -+ p - K + K -
v - --} p+Tr lr rr7"v-
--~ p + r r - K + --.+ p+ K - K -~ e-p ° -+ e - K *°
rrvrrr-
--+ e - K *° -+ e - ¢ ° -+ p - ¢ ° -+ p - K *° --+ p - K *° -+ p - ¢ °
Detection Events Expected Upper efficiency obbg limits ~ served events 10-e 17.0 16.8 19.5 16.5 19.9 15.0 13.2 13.0 13.1 11.2 15.3 14.0 13.0 8.2 6.7 6.5 4.5 8.6 7.0 4.8 14.4 9.5 9.0 7.2 10.6 6.5 6.5 4.1
1 0 0 0 0 0 0 1 3 2 0 0 1 2 1 1 2 0 1 0 0 1 2 1 2 1 1
0
0.21 0.18 0.12 0.32 0.12 0.11 0.43 0.29 0.42 0.29 0.22 0.18 0.11 0.57 0.48 0.49 0.50 0.36 0.33 0.35 0.45 0.32 0.32 0.15 0.43 0.46 0.37 0.11
2.9 1.7 1.5 1.8 1.5 1.9 2.2 3.8 6.4 6.0 1.9 2.1 3.8 8.2 7.4 7.5 15 3.5 7.0 6.0 2.0 5.1 7.4 6.9 6.3 7.5 7.5 7.0
Table 1 has the present limits on various charged modes which violate separate lepton
A. Seiden/Nuclear Physics B (Proc. Suppl.) 76 (1999) 477-485 number conservation [8]. Upper limits are typically a few times 10 -6. We can expect to lower these to 10 -7 if backgrounds can be controlled. From the table, decays with kaons already have a small number of background events and events with muons a fraction of a background event. The excellent particle identification of BaBar for kaons and muons will be essential for maximally extending the limits for modes with these particles. CP violation in the r structure can be searched for by looking for an electric dipole moment which contributes to the r pair production amplitude [9]. Such a term comes from an effective Hamiltonian of the form d 7 *a~
- ,,
It can be measured by looking at the expectation value of a CP-odd variable. For p-e final states one can look at the symmetric traceless spatial tensor Tij = (if- - ff+)i(iff- x ff+)j + (i ~ j). For a year at design £ BaBar can set a limit [10] dr ~ 10 -16 e - c m at 95% C.L. For the other leptons de
=
0.18 4- 0.16 x 10 -~6 e - c m
d,
=
3.7 4- 3.4 x 10 -19 e - c m .
In models where these arise from multiple doublets of scalar interactions, electric dipole moments are expected to scale as ,,~ m~ [11]. The r value is then about as sensitive as that of the electron and more sensitive than the muon. To look for possible CP violation in r decay a maximum rate is gotten by looking at the interference between the dominant Standard Model amplitude and a complex amplitude whose imaginary part changes sign when going from r + to r - . An interesting mode is K~" where the K*(892) can interference with K~(1430) which can have scalar exchange contributions [12]. Excellent kaon identification will be important for isolating a pure signal for this channel. Quantitative limits on CP violating amplitudes await a detailed simulation for these channels.
485
4. C O N C L U S I O N Continued progress in r physics can be expected from the data which will be taken at the soon to be operational B factories. Improvements in K identification, muon identificationi and vertexing, when compared to earlier experiments in the same energy range, combined with very large data samples should lead to significant improvements in measurements which rely on these detector components. REFERENCES 1. BaBar Technical Design Report, SLAC-R-95457. 2. All branching ratios used in text come from: "Review of Particle Physics", Eur. Phys. J. C1-4, 1 (1998). 3. W.J. Marciano and A. Sirlin, Phys. Rev. Lett. 61, 1815 (1988). 4. A. Pich, Nucl. Phys. (Proc. Suppl.) 3 9 B C 326 (1995). E. Braaten, S. Narison and A. Pich, Nucl. Phys. B 3 7 3 , 581 (1992). 5. D.S. Akerib et hi., Phys. Rev. Left. 69, 3610 (1992). 6. A. Anastassov et al., Phys. Rev. D55, 2559 (1997), Erratum; ibid. D58, 119904 (1998). 7. R. Balest et hi., Phys. Lett. B 3 8 8 , 402 (1996). 8. D.W. Bliss et al., Phys. Rev. D57, 5903 (1998). 9. W. Bernreuther, U. Lhw, J.P. Ma and O. Nachtmann, Z. Phys. C43, 117 (1989); W. Bernreuther and O. Nachtmann Phys. Rev. Lett. 63, 2787 (1989). 10. S. Yang, BaBar Note ~399 (1998). 11. H.Y. Cheng, Phys. Rev. D28, 150 (1983). 12. J.H. Kfihn and E. Mirkes, Phys. Lett. B398, 407 (1997).
Nuclear Physics B (Proc. Suppl.) 76 (1999) 477-485
Tau Physics at BaBar Abraham Seiden a ~Santa Cruz Institute for Particle Physics, University of California, Santa Cruz, CA 95060, USA The next few years of nmning of the new B factories should provide data samples of r decays with more than an order of magnitude higher statistics than presently available. In addition the new detectors, such as the BaBar detector at the SLAC B factory, will have detection capabilities which will enhance their ability for r decay studies. We look at these issues as well as some of the physics opportunities and challenges provided by the expected new large data samples.
1. I N T R O D U C T I O N This past decade has been one of precision tau physics with all measurements to date in impressive agreement with the Standard Model. These results are based primarily on approximately 106 events from the sum of the four LEP experiments and approximately 3 x 108 events for a large number of publications from the CLEO group running at Cornell. In the next few years results from three new B factory detectors, BaBar at SLAC, Belle at KEK, and an upgraded CLEO at Cornell will significantly increase the number of available 7- events. At design luminosity BaBar is expecting to collect 3 × 10¢r events per year. In a few years one can expect total data samples ,~ l0 s for each of the three B factories. These large data samples will allow significant progress in r decay studies. 2. B a B a r D E T E C T O R
Although designed to optimize the measurement of CP violation in the B system, the BaBar detector [1], shown in Figure 1, should provide an excellent tool for studying r decays. Note, unlike the situation for the present generation of experiments, the colliding beams for BaBar are of unequal energy so that the center of mass is moving in the laboratory and the detector is offset relative to the collision point. The relationship between laboratory and center of mass production angles is shown in Figure 2. The detector offset provides a higher geometric efficiency, which is also more symmetrical between forward and backward directions in the center of mass. We describe the
components and expected performance of the detector below. Moving outward in radius the charged particle tracking consists of a 5 layer silicon vertex tracker followed by a small cell drift chamber containing predominantly He gas to minimize multiple scattering. The error in the impact parameter in the r-¢ and r - z planes, respectively, are shown in Figure 3. At normal incidence each error is expected to be approximately 50 # m / P (GeV). The expected fractional error in the transverse momentum measurement is shown in Figure 4. For r decays the excellent resolution will be very useful in background rejection, based on invariant mass reconstruction, when searching for Standard Model forbidden all charged decays. Figure 4b shows the center of mass angular dependence of the resolution which, because of the unequal beam energies, remains excellent over a somewhat larger solid angle backwards than forwards. Following the tracking components is the particle identification system (called the DIRC) composed of long quartz bars (arranged as a cylinder just outside the drift chamber). The Cherenkov photons produced by charged particles passing through the quartz are internally reflected and then detected outside the detector volume by a large array of phototubes. The reconstructed Cherenkov cone angle is then used to discriminate between particle types. The expected performance of the system is shown in Figure 5 as a number of standard deviations between the charged K and ~r hypotheses. The separation should be very complete for momenta below 3 GeV for the DIRC angular coverage which is
0920-5632/99/$ - sve front matter © 1999 Elsevier Science B.V. All rights reserved. PII S0920-5632(99)00510- l
478
,4. Seiden/Nuclear Physics B (Proc. Suppl.) 76 (1999) 477-485
6
4
5
I
3
2
Figure 1. Layout of the BaBar detector at date of BaBar Technical Design Report.
479
A. Selden~Nuclear Physics B (Proc. Suppl.) 76 (1999) 477-485
Detector ProTractor. 7's BACKWARD POLAR ANGLES
20F~
80°
90°
80°
FORWARD POLAR ANGLES
,.
20 °
0°
0 9
G e V on 3.109 G e V
Y(4S)
[37=0.56
e~
Figure 2. Protractor showing the relationship between center-of-mass and laboratory polar angles for photons at ~7 = 0.56.
,4. SeidenlNuclear Physics B (Proc. Suppl.) 76 (1999) 477-485
480
E
(a)
10 3 ',
-\
v
t~
lO 3 .~
(b)
!\
o
lO 2
10 2
1
2
3 PT (GeV/c)
.~. 600
0
1
2 3 PT (GeV/c)
6oo i
500
(c)
--
(d)
I0 400
.
300
300
200 ~r
200 :---'.
loo
,=
~ 1
0
.
-1
100 0
,
....
I,,,,I~,,,
-0.5
0
..
50O
:*
,*
j
==~,1
II
....
I ....
-0.5
0.5
I.~,
0
0.5
cos ecru
0cm
cos
Figure 3. Resolutions for i m p a c t p a r a m e t e r s at the vertex: (a) c~y as a function of Pt at cos 0~ab = 0; (b) Crz as a function of Pt at cos Otab = 0; (c) ~xy as a function of cos 0cm for Pcm ---- 1 G e V / c ; and (d) crz as a function of cos 0c,n for Pcm -- 1 G e V / c . For (a) and (b), the dashed curve represents 50 # m / p t .
1.0
" ' 1
. . . .
i
. . . .
i
. . . .
i
. . . .
i
1.0
. . . .
(a)
0.8
v
v
b
(b)
0.8
0.6
0.6
0.4
0.4
o.2
b
0 . 0
. . I
0
0.5
. . . .
I
1
. . . .
I
1.5
. . . .
I
2
Pt ( G e V / c )
. . . .
I
2.5
0.2 0.0
. . . .
3
J
t_.
.
.
.
.
I
-0.5
.
.
.
.
i
0
.
.
.
.
I
.
.
.
.
0.5
COS(O~m )
Figure 4. Pt resolution at (a) 0~ab = 90 deg as a function of Pt, and (b) Pc,n = 1.0 G e V / c as a function of COS ~ c r n •
A. Seiden/Nuclear Physics B (Proc. SuppL) 76 (1999) 477-485
481
100 '
#a
'
I
. . . .
I
. . . .
I
. . . .
50
1.5
G o V / c
.
.
.
•
10
1
,
,
,
,
,I
. . . .
-0.5
Figure 5. Predicted
K/r
I
. . . .
0
I
0.5
,
,
,
COS0
separation performance of the DIRC.
expected to be, in the center of mass, 87% in cos(0) and 93% in azimuth. The presence of very good particle identification should allow BaBar to significantly improve the knowledge about rates and mass spectra for r decays with charged kaons in the final state. Just outside the DIRC bars is a fine grained barrel crystal calorimeter whose expected resolution is shown in Figure 6. The forward solid angle is closed by an endcap of comparable performance to the barrel. The calorimeter, besides its excellent resolution, provides a large solid angle redundant trigger to that provided by the charged tracking, which will be very important to measuring the absolute efficiency for detecting r decays. The set of detectors described above sit inside a superconducting coil which provides a 1.5 Tesla solenoidal field. The iron flux return for this field is highly segmented and filled with resistive plate counters allowing very good muon detection and identification. The nearly hermetic segmented and instrumented flux return (called IFR) is shown in Figure 7. This device should allow good muon identification down to at least
600 MeV, significantly lower in momentum than in previous detectors at low energy e+e - colliders. 3. P H Y S I C S O P P O R T U N I T I E S A N D CHALLENGES We look below at some of the physics opportunities and challenges for r physics at BaBar.
3.1. Physics with kaons Major improvements in all channels containing charged kaons can be expected from BaBar due to the combination of very large statistics and excellent particle identification. Since the Cabibbo suppression for strange states is about a factor of 20 and the increase in statistics expected from each B factory is also about a factor of 20, we can expect future data samples for strange states to be comparable to present samples for non-strange states. The strange state with the most precise prediction is the Kvr final state. Comparing with a few well predicted channels, the present status for the fractional errors is [2]
482
A. Seiden/Nuclear Physics B (Proc. Suppl.) 76 (1999) 477-485
. . . . . .
I
.
.
.
.
.
.
.
.
I
.
.
.
.
2.5
,*--,
2.0
w
1.5
1.O 0.1
0.02
0.2
1
2
Photon Energy (GeV)
Figure 6. Energy resolution at cos 0c,~ = 0 as a function of photon energy. The resolution is defined as FWHM/2.36. The error bar on the first point indicates a typical uncertainty in determining the resolution. The solid line shows the target energy resolution.
Final State
Fractional Error on Branching Ratio
ev~, ~rv Kv
0.4% 1.2% 7.0%
The K g error is presently still rather large. With future B factory results we can expect a 1% test that the K v and ~rv rates agree with expectations from the ratio t~ v,d as determined in K and ~r decay. In addition, the electroweak corrections to the ratio are about 2% [3] so that a check that they contribute as expected can be made. The B factory data will allow a study of the full mass spectrum involving strange states. For massless strange quarks we would expect a branching ratio into strange states = 3.18+.02%, based on a leptonic branching ratio of 35.18 4.12% and a value for ]Vus[2 = 0.0490=t=.0003. The value that is seen is 2.78 + .15% based on adding branching ratio: K - + X = 1.66 + 0.10% branching ratio: K ° + X = 1.66 + 0.09%
and subtracting branching ratio: K - K ° + X = 0.31 + 0.04% branching ratio: K - K % r - + X -- 0.23 + 0.04%. The inclusive strange branching ratio seen is therefore [1 - (0.126 + 0.047)] x massless quark expectation. The B factory data should allow a significant improvement on the measurement of the correction factor 0.126 + 0.047, which presently has a 30% error. This factor can be written in terms of the strange quark mass at the r mass scale [4]:
8 ( m---2-'~ 211 + QCD corrections]. \mr / A precision test will require that the theoretical QCD corrections be accurately predicted, so that they create a fractional uncertainty << 30%. Note, the value of a~ at mr is known to about 6%, so this is not the dominant limitation.
A. Selden~Nuclear Physics B (Proc, Suppl.) 76 (1999) 477-485
Figure 7. The IFR detector.
483
484
A. Selden~Nuclear Physics B (Proc. Suppl.) 76 (1999) 477-485
3.2. C h a l l e n g e s o f h i g h p r e c i s i o n Two very fundamental r measurements are the electronic branching ratio, Be, and the lifetime, rr. Using v r = B e ~ r e and the accurate Standard Model prediction of Fe, we can relate these two via
Be=
rr 1632.4 + 1.3 (fsec)"
The error on the denominator, about one part in 103 , is dominantly from the uncertainty in our knowledge of the r mass. The present values for Be and rr are r~
=
2 9 0 ± 1.2 (fsec)
Be
=
17.81-4- 0.07%,
each uncertain by a factor of about 4 more than the factor relating Be to rr. The statistical errors expected from B factory measurements should allow a factor of 4 improvement in each, however, it is not at all clear that systematic effects will actually allow any improvement. To look at the systematic limitations we can use the CLEO measurements and their stated systematics. As an example of the limitations, the two most recent and most accurate CLEO measurements of Be are (with the second coming several years after the first):
Be
=
17.49±0.14:1: 0.22% [5]
Be
--
17.76+0.06±0.17% [6].
The second measurement has a much reduced statistical error, however, the reduction in the dominant systematic error is not nearly as large. To make significant improvement will require both theoretical and experimental work as both the calculation for the cross sections detected require a better understanding of higher order corrections and the uncertainties in the absolute detection efficiencies need to be improved. Turning to the lifetime, the CLEO measurement is 289.0 ± 2.8 ± 4.0 (fsec) [7]. This measurement has been made without the benefit of a silicon vertex detector and the systematic error is very predominantly from tracking and vertexing. We can therefore expect a significant improvement from the B factories based on their high
precision vertex trackers. It will be very challenging, however, to improve significantly on the present world average. 3.3. R a r e o r f o r b i d d e n d e c a y s The search for non-Standard Model phenomena at higher sensitivity will be a continuing goal of the B-factories. Some of the areas of interest are limits on lepton number violating decays and a search for CP violation in the r sector - either in the ~- structure or in v decays. Table 1 Detection efficiencies, event statistics, expected backgrounds, and upper limits for branching fractions at 90% confidence level. Decay Channel r - --+ e-e+e v - --+ p - e + e -r- -.+ p + e e r - .-} e - p + p -
r - --+ e+p p r - --+ p - g + p r - --~ e-Tr+ 7rr - -+ e - r - K + r - --). e-Tr+ K r - --+ e - K + K -
v - --+ e+~r-~r rr-
~ e+Tr-K --~ e+ K - K -
r - --~ p-lr+rr r - --+ p - w - K + ~'- ---+ p - T r + K r - -+ p - K + K -
v - --} p+Tr lr rr7"v-
--~ p + r r - K + --.+ p+ K - K -~ e-p ° -+ e - K *°
rrvrrr-
--+ e - K *° -+ e - ¢ ° -+ p - ¢ ° -+ p - K *° --+ p - K *° -+ p - ¢ °
Detection Events Expected Upper efficiency obbg limits ~ served events 10-e 17.0 16.8 19.5 16.5 19.9 15.0 13.2 13.0 13.1 11.2 15.3 14.0 13.0 8.2 6.7 6.5 4.5 8.6 7.0 4.8 14.4 9.5 9.0 7.2 10.6 6.5 6.5 4.1
1 0 0 0 0 0 0 1 3 2 0 0 1 2 1 1 2 0 1 0 0 1 2 1 2 1 1
0
0.21 0.18 0.12 0.32 0.12 0.11 0.43 0.29 0.42 0.29 0.22 0.18 0.11 0.57 0.48 0.49 0.50 0.36 0.33 0.35 0.45 0.32 0.32 0.15 0.43 0.46 0.37 0.11
2.9 1.7 1.5 1.8 1.5 1.9 2.2 3.8 6.4 6.0 1.9 2.1 3.8 8.2 7.4 7.5 15 3.5 7.0 6.0 2.0 5.1 7.4 6.9 6.3 7.5 7.5 7.0
Table 1 has the present limits on various charged modes which violate separate lepton
A. Seiden/Nuclear Physics B (Proc. Suppl.) 76 (1999) 477-485 number conservation [8]. Upper limits are typically a few times 10 -6. We can expect to lower these to 10 -7 if backgrounds can be controlled. From the table, decays with kaons already have a small number of background events and events with muons a fraction of a background event. The excellent particle identification of BaBar for kaons and muons will be essential for maximally extending the limits for modes with these particles. CP violation in the r structure can be searched for by looking for an electric dipole moment which contributes to the r pair production amplitude [9]. Such a term comes from an effective Hamiltonian of the form d 7 *a~
- ,,
It can be measured by looking at the expectation value of a CP-odd variable. For p-e final states one can look at the symmetric traceless spatial tensor Tij = (if- - ff+)i(iff- x ff+)j + (i ~ j). For a year at design £ BaBar can set a limit [10] dr ~ 10 -16 e - c m at 95% C.L. For the other leptons de
=
0.18 4- 0.16 x 10 -~6 e - c m
d,
=
3.7 4- 3.4 x 10 -19 e - c m .
In models where these arise from multiple doublets of scalar interactions, electric dipole moments are expected to scale as ,,~ m~ [11]. The r value is then about as sensitive as that of the electron and more sensitive than the muon. To look for possible CP violation in r decay a maximum rate is gotten by looking at the interference between the dominant Standard Model amplitude and a complex amplitude whose imaginary part changes sign when going from r + to r - . An interesting mode is K~" where the K*(892) can interference with K~(1430) which can have scalar exchange contributions [12]. Excellent kaon identification will be important for isolating a pure signal for this channel. Quantitative limits on CP violating amplitudes await a detailed simulation for these channels.
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4. C O N C L U S I O N Continued progress in r physics can be expected from the data which will be taken at the soon to be operational B factories. Improvements in K identification, muon identificationi and vertexing, when compared to earlier experiments in the same energy range, combined with very large data samples should lead to significant improvements in measurements which rely on these detector components. REFERENCES 1. BaBar Technical Design Report, SLAC-R-95457. 2. All branching ratios used in text come from: "Review of Particle Physics", Eur. Phys. J. C1-4, 1 (1998). 3. W.J. Marciano and A. Sirlin, Phys. Rev. Lett. 61, 1815 (1988). 4. A. Pich, Nucl. Phys. (Proc. Suppl.) 3 9 B C 326 (1995). E. Braaten, S. Narison and A. Pich, Nucl. Phys. B 3 7 3 , 581 (1992). 5. D.S. Akerib et hi., Phys. Rev. Left. 69, 3610 (1992). 6. A. Anastassov et al., Phys. Rev. D55, 2559 (1997), Erratum; ibid. D58, 119904 (1998). 7. R. Balest et hi., Phys. Lett. B 3 8 8 , 402 (1996). 8. D.W. Bliss et al., Phys. Rev. D57, 5903 (1998). 9. W. Bernreuther, U. Lhw, J.P. Ma and O. Nachtmann, Z. Phys. C43, 117 (1989); W. Bernreuther and O. Nachtmann Phys. Rev. Lett. 63, 2787 (1989). 10. S. Yang, BaBar Note ~399 (1998). 11. H.Y. Cheng, Phys. Rev. D28, 150 (1983). 12. J.H. Kfihn and E. Mirkes, Phys. Lett. B398, 407 (1997).