Operational status and performance of the SLD CRID

Nuclear

Instruments

and Methods

in Physics Research A 371 (1996) 8-l

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NUCLEAR INSTRUMENTS & METHODS IN PHVSICS RESEARCH

Operational status and performance of the SLD CRID K. Abe”, P. Antilogusb, D. Astonb’*, K. Baird’, C. Baltayd, A. Beane, R. Ben-Davidd, T. Bienzb, F. Birdb, D-0. Caldwell”, M. Cavalli-Sforzaf, J. Colle?, P. Coylef, D. Coynef, S. Dasub, M. Dimah, A. d’oliveira’, J. Duboscq’, W. Dunwoodieb, G. Hallewellb, K. Hasegawaa, Y. Hasegawaa, J. Hubere, Y. Iwasaki”, P. Jacques’, R.A. Johnson’, M. Kalelkar’, H. Kawaharab, Y. Kwonb, D.W.G.S. Leithb, X. Liuf, A. Lu”, S. Manlyd, J. Martinez’, L. Mathys”, S. McHugh”, B. Meadows’, G. Mtillerb, D. Mullerb, T. Nagamineb, S. Narita”, M. Nussbaum’, T.J. Pavelb, R. Piano”, B. Ratcliffb, P Rensingb, A.K.S. Santha’, D. Schultzb, S. Send, J.T. Shankg, S. Shapirob, C. Simopoulosb, J. Snyder’, E. Solodovb, P. Stamer’, I. Stockdale’, F. Suekane”, N. Togeb, J. Turkd, J. Va’vrab, J.S. Whitakerg, D.A. Williamsf, S.H. Williamsb, S. Willocqd, R.J. Wilsonh, G. Word’, S. Yellin”, H. Yuta” *Department of Physics, Tohoku University, Aramaki, Sendai 980, Japan ‘Stanford Linear Accelerator Center, Stanford, CA 94309, USA ‘Serin Physics Laboratory, Rutgers University, P.O. Box 849, Piscataway, NJ 08855. USA ‘Department of Physics. Yale University, New Haven, CT 06511, USA ‘Department of Physics, University of Caltforniu, Santa Barbara, CA 93106, USA ‘Santa Cruz Inst. for Particle Physics, University of California, Santa Cruz, CA 95064. USA gDepartment of Physics, Boston University. Boston, MA 02215, USA ‘Department of Physics, Colorado State University. Fort Collins. CO 80523, USA ‘Department of Physics, University of Cincinnati, Cincinnati, OH 45221. USA

Abstract The operation and performance of the SLD CRID achieved during the recently completed 1994-95 run of the SLC will be discussed. Stable operation of liquid (C,F,,) and gas (85% C,F,, and 15% N,) radiators with good UV transparency has been achieved. Our expectations for the future SLD physics program will also be briefly discussed.

1. Introduction The design of the SLD CRID [l] is similar to the DELPHI RICH [2] in many ways, with a few notable exceptions: - the barrel gas radiator is a mixture of 85% C,F,, and 15% N,; - the endcaps have only a gas radiator; - we use charge division to obtain the TPC depth coordinate; - there is no tracking detector following the barrel. SLD took its first data in 1991 with an unpolarised electron beam, and the barrel CRID was commissioned at

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author. Fax +I

415 926 3587, e-mail

[email protected]. 0168~9002/96/$15.00 0 1996 Elsevier Science B.V. All rights reserved SSDI 0168-9002(95)01133-l

that time. In 1992 the first collisions with a longitudinally polarised (-22%) electron beam were obtained and -lOk hadronic Z” decays were observed. The endcap CRID was commissioned during the 1993 run when a further -5Ok Z” were recorded with a much improved polarisation of -63%. An improved cathode for the 1994-95 run, which ended in March, resulted in -1OOk Z0 with polarisation -77%. Operation of the CRID has been quite smooth throughout this period, and we report below on the experience gained and performance achieved during the last run. Results from the barrel will be emphasised, since the status of the endcaps has recently been presented elsewhere [3]. During the down time in 1995, the vertex detector will be upgraded with modem CCDs which will provide more hits per track and larger solid angle coverage. We expect to obtain a total of 500k more Z” with polarisation -80%

K. Abe et al. I Nucl. Instr. and Meth. in Phys. Res. A 371 (1996) 8-11

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during data taking in 1996- 1998. The combination of the CRID, the new vertex detector and high polarisation is very powerful and enables us to fully exploit flavour tagging in tests of the Standard Model and QCD.

2. Operational

status

Operation of barrel and endcaps is now routine, with experts required only to make daily checks and respond to unusual conditions detected by the monitoring systems. The fragility of the 7 pm carbon fibres in our photon detectors is a cause for concern, but broken wires have not significantly impacted the efficiency of operation. These issues were discussed elsewhere at this workshop [4]. The fluid systems have performed reliably and good UV transparency is regularly maintained. We observe excellent TPC electron drift lifetimes with TMAE, and have no evidence for loss of photoelectrons at long drift times. We now understand that the nickel getter [5] introduced to remove sulphur from the CzHh caused instabilities in the drift velocity by introduction of unpredictable amounts of CO?. Since obtaining a reliable source of C,H, with low sulphur content and removing this getter in September 1993. the behaviour of the TPC drift gas has been stable and most of the observed variation in drift velocity can be attributed to changing atmosphere pressure. As an example, Fig. I shows the correlation between TPC drift velocity. as measured by the fibre hducials, and atmospheric pressure for all data taken in the Fall of 1994. In reconstruction monitor of recently fact, an “online” acquired hadronic Z” events generally gives reasonable liquid and gas Cherenkov angle resolutions, using only the

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0.6 6c (rW

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Fig. 2. The liquid IY,distribution observed in hadronic Z” events before and after cuts to remove crosstalk from saturated dE/dx hits and other bad quality pulses. In all cases. the TMAE depth is required to be less than S cm.

detector alignment derived from 1993 data and correcting the TPC drift velocity for pressure variations.

3. Performance In the liquid radiators we use all good candidate single photoelectrons (pe’s) combinatorially to calculate Cherenkov angles (e) with respect to all relevant tracks. We demand only that the refracted ray is no more than 70” from the radiator normal. This cut artificially increases the losses from total internal reflection in a calculable way, while removing a large amount of irrelevant combinatorial background from the 6’, plots in hadronic Z” events. Fig. 2 shows the distributions before and after cuts. Total losses in signal due to the cuts are - 15% and much of the

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1993 p$.- data Monte Carlo

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Pressure (inches H20) Fig. I. TPC drift velocity versus atmospheric pressure in North barrel TPCs during October-December 1994. The line is a fit with slope constrained to the average observed in all 1994-95 data.

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Fig. 3. Observed liquid 6’cresolution as a function of dip angle of the track derived from 1993 k+t.r- events and compared with the Monte Carlo prediction.

1. REPORTS

FROM EXPERIMENTS

K. Abe et al. I Nucl. Instr. and Meth. in Phys. Res. A 371 (1996) 8-11

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remaining background is real liquid or quartz photons associated with other tracks. Our Monte Carlo adds real background from random beam crossings, which satisfactorily accounts for the rest. Fig. 3 shows the resolution of 8c in the liquid we measure from 1993 Z” + p,+l.r- data. We see 14-17 mrad (dependent on dip angle), somewhat larger than predicted by the Monte Carlo. By adding -10 mrad extra smearing in the Monte Carlo, we obtain good agreement. Results derived from hadronic Z” data are consistent with the pf p- observations. The result of fitting rings to the gas data, shown in Fig. 4, demonstrates that, of the -5; mrad observed in the raw distribution, -4 mrad seems to be the intrinsic resolution. We add 4 mrad extra smearing in the Monte Carlo to account for the rest. At present, a 6 cm TMAE depth cut is used to reduce background from liquid and quartz photons in the gas data but, even without that requirement, the raw data has a good signal/noise ratio. The average number of pe’s seen (these results are based on 1993 data, though we have good evidence that the 1994-95 data gives similar answers) are itemised below. We have corrected liquid values for total internal reflection losses and scaled them to 1 cm path length: - 15.4-tO.7 in liquid for p,+p- events averaged over all angles; - 10.5-+0.4 as above, but in hadronic Z” events; - -8: in the gas for p,‘p- events; - -8 as above, but in hadronic Z” events.

From these figures we can derive the values of N, that we actually achieve as 3952 cm-’ (from p+p- events) in the liquid radiator and -65 cm-’ in the barrel gas. We verified the correction for total internal reflection by looking at tracks close 90”. On average we see 8% less photons, which is reasonable given the gap in coverage at the mid-plane which has not been accounted for. The overall loss of -30% in hadronic Z” events compared with p+p- events is larger than the obvious losses due to our cuts (Fig. 2) and only a small part of the discrepancy is due to the TMAE depth cut. However, the Monte Carlo reproduces the effect quite well and it can be attributed to losses from overlapping pulses in the dense hadronic jet environment. In contrast, the equivalent loss in the gas is small because the mirrors focus gas rings away from the track. We have attempted to predict in some detail the numbers of photons we should observe using, as far as possible, our own measurements of transparency, reflectivity etc. [5]. The updated results obtained are as follows (the liquid value has been corrected downward, to allow for the gaps between the TPCs where photons are lost); -44 cm ’ in the liquid; - 62 cm- ’ in the barrel gas; - 70 cm-’ in the endcap gas [3]. This is consistent with our observations and lends some support to our estimate of the detector efficiency [4].

4. Conclusions

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(a)

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0.8 Acknowledgements

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(b)

Operation of the CRID is now routine, with a minimum of “surprises”. We have made considerable progress since the last workshop [5] and, although we have not yet attained the ultimate resolution of the device, the observed performance is good, and well enough understood that we are able to use the full power of particle identification [6].

This work was supported by Department of Energy contract DE-AC03-76SF00515 and other DOE and NSF grants. The speaker gratefully acknowledges the financial support of the RICH95 organizers.

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8~ (mrad) Fig. 4. The raw gas 0, distribution seen in hadronic Z” data, (a) for tracks in the range IO-11 CieV/c showing a shoulder due to kaons and, (b) for all tracks above 6 GeV/c after correcting for fitted ring centres.

References [ll SLD Design Report, sions.

SLAC-273,

UC-43D

(1984).

[Zl DELPHI Proposal, LEPC 83-3 and LEPC 84-16.

and revi-

K. Abe et al.

I Nucl. Instr.

and Meth.

S. Willocq representing the SLD CRID group, The Endcap CRID at SLD, SLAC-PUB-95-6693, presented at IEEE Nuclear Science Symp., Nov. 1994. [4] J. Va’vra. these Proceedings (1995 Int. Workshop on Ring Imaging Cherenkov Detectors, Uppsala, Sweden) Nucl. Instr. and Meth. A 371 (1996) 33. [3]

in Phys. Res. A 371 (1996)

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[5] J. Va’vra representing the SLD CRID group. Nucl. Instr. and Meth. A 343 ( 1994) 74. [6] D. Aston representing

the SLD CRID group, Ref. 141, p. 195.

I. REPORTS

FROM EXPERIMENTS