Nuclear Instrumentsand Methods in PhysicsResearch A 368 (1995) 115-123
NUCLRAR
INSTRWLNTS
kRRSEARCN ‘“ti: Section A
ELSEVIER
B-physics performance of the CMS detector Danek Kotlinski a~1,Chantal Racca b*2 a Paul Schemer Insrifufe, LWD22.
CH-5232 Villigen-PSI, Switiwkmd b CRN-Srrasbourg,INtP3-CNRS ef Universiri Louis PasteurBP 28, F-67037Slmsbourg-Ceder 2. Fmnce
For the CMS Collaboration
Abstract We describe the CMS detector, with special attention to its microvertex pixel detector. Tfien, we discuss the potential of the CMS detector for the study of CP violation; the sensitivity to the unitarity triangle angle p obtained from the Bj -+ J/+Kf channel is 0.048 from a time-integrated analysis and 0.046 from a time-dependent analysis; the corresponding values for the angle LYobtained from the Bz + rr+r- channel are 0.050 and 0.070, respectively. We also discuss the possibilities to observe Bf oscillations and some rare decays.
1. Introduction
2.1. General overview and performances
It is well known that, at hadron colliders, some exclusive B-channels can be extracted with a good signal-to-noise ratio, and that decay length measurements can be performed using silicon pixel detectors. In the first phase of operation of LHC, the luminosity delivered will be of the order of 103*cm-2s-1, leading to a copious production of bb pairs ( 5 x 10” per year), opening the way for significant measurements of CP violation effects in the B system and other studies in B-physics such as Bf oscillations and rare decays.
In the following subsections, we briefly discuss the parts of the detector directly involved in our B-physics studies. The microvertex pixel detector is described in more details in Section 2.2.
2. The CMS detector The CMS detector shown in Fig. 1 has been designed to study high-m physics at the LHC. Its goal is to be able to do measurements with the highest luminosities to use the full discovery potential of the LHC machine. At low luminosity, the CMS detector can take advantage of the large number of produced b-events which are selected and identified by the muon trigger system, precisely reconstructed by the inner tracker and efficiently tagged with good impact parameter resolution and secondary vertex reconstmction capability provided by the innermost layers of the tracker, including the microvertex pixel detector.
’ E-mail
[email protected] ’ E-mail racca@frcpnl I .in2p3.fr
0168-9002/95/$09.50 @ 1995 Elsevier Science B.V. All rights reserved SSDlOl68-9002(95)00988-4
2.1.1. Magnet The choice of the magnet was the starting point of the CMS detector design. A long superconducting solenoid of length 13 m and inner diameter 5.9 m, with a uniform field of 4 T, has been chosen. The large bending power of such a magnet leads to the following benefits: - a single magnet can provide the necessary bending power for precise inner and muon tracking; - the favourable aspect ratio of the solenoid allows efficient muon detection and measurement up to rapidities of 2.4, without the need of forward toroids; - a flexible muon trigger with sharp thresholds. 2.1.2. Tracker The primary goal of the central tracking system is to reconstruct isolated high pr tracks with an efficiency better than 95% and high m tracks within jets with an efficiency of better than 90% over the rapidity range 1~1< 2.6. The momentum resolution for charged particles in the tracker has been estimated by performing a full GEANT simulation. The momentum resolution is shown in Fig. 2a as a function of m for central, intermediate and forward tracks. For tracks with pr from a few to a few tens of GeV (B-physics case), the resolution is 1% or better for the different values of 7. Fig. 2b shows the momentum resolution as a function of 7. For tracks not associated with the primary
III. GENERAL PURPOSE EXPERIMENTS
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Fig. I. A 3-D view of the CMS detector.
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Fig. 2. Momentum resolution of the nacker (a) as a function of pr and (b) BS a f~ilCti0~ of rapidity.
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Fig. 3. (a) Kf invariant mass, with combinatorialbackground coming from the same event; (b) Kf reconsmxtion efficiency.
vertex, which are important for B-physics, the resolution benefits from the pixel detector (TP design). The impact parameter resolution is presented in the subsection devoted to the microvertex pixel detector. The tracking system of CMS does not provide particle identification (r K separation). For KY an algorithm that attempts to find all tracks with transverse momentum above 350 MeV has been developed. To estimate the efficiency for Kf reconstruction, B events with Kt decays were fully simulated. Fig. 3a shows that the Kt mass peak is clearly visible and has a resolution of 8.6 MeV. The average signal-to-background ratio from the combinatorial background is about 2 : 1. Requiring at least 6 points per track, the average Kz reconstruction efficiency is about 35% (Fig. 3b). This result corresponds to events in which there are about 200 tracks per crossing including 5 5 superimposed minimum-bias events and is comparable to the values reported by UAl and CDF. 2.1.3. Muon trigger The CMS muon trigger is required to identify muons, measure their transverse momentum pr, and determine the bunch crossing from which they originate. In order to have a large safety margin in the total Level-l trigger rate, the maximum rate allowed for the inclusive single-muon trigger must be of the order of 3 kHz. Fig. 4 shows single-muon rates and corresponding trigger cuts at various luminosities. For B-physics studies, two basic triggers are considered: the inclusive single-muon trigger that requires at least one muon with a pf > 9-10 GeV in the rapidity range I$[ < 2.4; the inclusive two-muon trigger that requires at least two
muons, each with a p$ > 2.5-4.5 GeV in the rapidity range [$‘I < 2.4. The first trigger is well suited for the study of the Bj + nT+nT- channel and Bf oscillations, the second one for the study of the Bj + J/+Kf channel and the Bf + P+Psearch. 2.2. The GUS microvertex pixel detector 2.2.1. Design The two barrel layers of the CMS pixel detector will be
located at mean radii of 7.7 cm and 11.7 cm with a length of 65 cm (covering rapidity < 1.75). In order to achieve the optimal vertex position resolution in both the (r, 4) and the z coordinates the following design was adopted: - Square pixel shape 125 x 125 pm. - Enhancing the spatial resolution by analog signal interpolation. This is helped by the large drift (Lorentz angle
3 kHz
p;“’ (GeV) Fig. 4. Ch4S muon trigger rates for different values of luminosity.
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Features:
- squarepixels - 150
pm thick
- readout of electrons (n-side) - no Lorentz tilt - staggering of adjacent z-rows TScharge sharing in Q andz *~~-q15,um Fig. S. A schematic to the magnetic
view of the barrel pixel
arrays and charge spread due
field.
which results in charge spread(by 105 +m) over more than one pixel (see Fig. 5). Hence the detectors are deliberately not tilted. - Staggering the adjacent z rows by half the pixel size. Since charge is shared between adjacent z-rows, the effective pixel size in z becomes 62.5 pm. The basic detector unit (Fig. 6) has a sensitive pixel area of 1.6 cm (r, #J) x 6.4 cm (Z ). The detector thickness will be between 150 and 200 pm and will depend on detector yield and resolution requirements. Two rows of eight readout chips, each accommodating 64 x 64 pixels will be bumpbonded to the detector unit. The detector equipped with chips is fixed onto a hybrid with bond pads and input and output buses. Rows of ten detector/hybrid modules will be fixed on pairs of aluminum cooling pipes (Fig. 7). The inner (outer) layer will have 32 (48) faces. The cooling pipes of both layers are held by carbon fibre end rings located at 1 z I= 325 mm. Sparsified readout signals are brought out via kapton cables from the hybrids to the end rings where optical modulators will be placed. Cooling pipes and detector modules interconnected as indicated in Fig. 7 will provide a self-supporting structure. The water-cooling pipes have an inner cross section of 10 mm*. With an anticipated power budget of 40 FW per pixel, one pipe must remove 26 W. The temperature increase of the coolant will be 1.6’C for a fluid speed of 40 cm/s. The temperature difference across the detector plate is 3’C which is acceptable. The average radiation length per layer is 1.5% at r]=O. Due to the high radiation levels the innermost layers, representing about 35% of the total pixel area, may have to be replaced about six years after LHC start-up, when fluences of 10” /cm* are reached. 34’
at 4
T for electrons)
ing in the r-q5 direction
Fig. 6. The barrel pixel
detector
unit
Cool SUP Fig. 7. Mechanical
assembly
of pixel
detectors into barrel
layers.
2.2.2. Performance The hit resolution as a function of the z-coordinate for the barrel detector layer was calculated with a Monte-Carlo simulation. The simulation included charge generation (through ionization) along a track, charge drift and diffusion in the magnetic field of 4 T in three dimensions, pixel signal digitisation and finally the position reconstruction. In the r-4 plane, the resolution is determined by the analog interpolation between two pixels and varies slowly from 13 pm at z = 0 cm to 9 ,um at z = 32 cm. In the z direction for z = 0 the resolution is given by the ratio of l-pixel hits to 2pixelhits,29:71 (7:93)andisequalto24(18),umfora 150 ( 200) pm-thick detector (a thicker detector gives more charge spread). For large z, due to the charge sharing in z, the resolution in the z direction improves to 15 pm. For the purpose of track and vertex fitting studies, an alignment error of 15 pm is added in quadrature to the hit resolution and therefore an effective resolution of 21 pm is used. The impact parameter resolution in the r-4 plane and in the z-coordinate is shown in Fig. 8. For high-pT tracks the resolution is 20 pm in the r-4 plane and 90 pm in the z direction. The difference in the z and r-4 resolutions reflects the fact that out of the 12 central tracker layers only the first 2 (pixel layers) have a good z resolution. For tracks with pr below 10 GeVlc the resolution decreases due to the multiple scattering. The secondary vertex and the flight distance resolution was simulated for several decay channels. Its value depends on the number of tracks fitted and their average pr. For example, for B, + pp decay, the flight distance resolution in space is 120 ,um. The accurate determination of the secondary vertex can be used to suppress various backgrounds to this rare decay. 2.2.3. Readout To estimate the expected data rates in the pixel detector, simulations using the GEANT-based CMS simulation pro-
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pT GeV Fig. 8. The impact parameter resolutionin the r-9 and z planes.
gram were done. Minimum bias events and jet events were generated with the Pythia event generator. Some results of this simulation are shown in Table 1 (all numbers correspond to the first pixel layer and the nominal LHC luminosity of 10” cm-*s-I. A “column” refers to a 64-pixel column (4 direction) in a 64 x 64 pixel readout chip. A “double column” is an OR of two neighboring columns. A “detector unit” consists of 16 readout chips. The “hit-rate” is defined as the rate of events with one or more hits in a detector area (column/chip/detector-unit) within one bunchcrossing. The multiplicity gives the average (and burst) numberof pixels which tire for an averagecolumn/chip/unit hit. The burst multiplicity corresponds to events where a jet of particles hits the considered detector area. The multiplicity is larger than 1 for two reasons; one is that a track typically results in more than 1 pixel hit (this depends on the track inclination angle and is on the average 6); the second reason is that there can be more than 1 track hitting a detector area in one bunch crossing. The data rates are very high, e.g. the accumulated data rate for the first barrel layer is above 10” pixels/s (assuming that 1 pixel produces 2 bytes of information, this corresponds to a rate of 200 Gbytes/s). Such high data rates make the design of the readout architecture an ambitious task. In a readout scheme where pixels are read out only after the 1st level trigger the probability of overwriting a single pixel with additional hits within the 1st level trigTable I Summary of the data rates for the first barrel layer of the pixel detector Area
Hit rate
Multiplicity average/burst
occupancy
Single pixel Pixel column Double column Readout chip Detector unit
7 kHz 250 kHz 325 kHz 4.7 MHz 35 MHz
l.O/2.0/5.0 3.0/9.0 7.0/50 15.0/80
0.02% 0.6% 0.8% 12% 85%
ger latency (3.2 ps in CMS) is as high as 3%. Therefore a pixel-dram architecture is proposed instead. Each pair of columns is equipped with an eight-deep g-bit buffer for time stamp storage [ 1] pointed to by a 3-bit address pointer. This allows time stamps to be recorded for a&most eight crossings, which is sufficient for storing all hits of the column pair occurring during the first level trigger latency (3.2 ps). When pixels in a column pair are hit, the column OR initiates the storage of the g-bit local time stamp into the addressed buffer element. Then the pointer address is incremented to the next buffer element, awaiting the next OR. A read bit is passed up and down the double-column and all pixels with valid hits send the analog information to a data buffer element having the same address as the corresponding time stamp. The pixel drain process runs at 40 MHz and takes about 8-12 bunch-crossings to complete, depending on the number of pixels hit. Moreover, it is almost deadtime free. While pixels in a double-column are read out, one next bunch-crossing can be recorded in the same column. The information will not be lost unless the same pixels are hit. When the first level trigger asks for data, the time stamp of the triggered event is compared with those recorded in the time-stamp buffer. If the correct time-stamp is present, the corresponding analog hit information is transferred from the data buffer to the CMS data acquisition system. Data for which there was no valid first level trigger are erased by clearing a status bit associated with each time stamp buffer element. A very schematic preliminary view of the readout architecture is shown in Fig. 9. Simulations were performed in order to check that the proposed readout scheme does not result in a large data corruption. Listed below are the five main sources of errors, together with an estimate of the information loss at the full LHC luminosity: - pixels overwritten due to overlapping clusters in the same bunch-crossing (error rate
III. GENERAL PURPOSE EXPERlMENTS
Il. Kotlinski,
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Instr. and Meth. in Phys. Res. A 368 (1995)
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Bz + J/#Kt. We present the time-integrated and timedependent analyses. In this study, we use PYTHIA with gluon-fusion and gluon-splitting contributions, and we assume a~ = 0.5 mb. Momentum, angular and impact-parameter resolutions are parametrized as described previously. The tracker efficiency is assumed to be 95% for all charged particles within 77< 2.4 and with pr > 2 GeV. The expected numbers of events are calculated assuming an integrated luminosity of lo4 pb-‘. 3.1. Time-integrated analyses
timestamppointer In this section we estimate the sensitivity to sin2a and sin 2/3. The time-integrated asymmetry A is given by A=N+-N-
Nfi-N-
= DtDs/&n
xd sin24 1 +x*
2
and the corresponding sensitivity by
where q5represents either cyor j3. N+ and N- are the number of events with positivelyand negatively-charged tagging muons, Di’S are the dilution factors and xd/( 1 + xi) is the time-integration factor. 3.1.1. The Bz -+ J/#Kf channel The decay Bi --) J/$Kf, with J/t& + p+p-
Fig. 9. A preliminary simplified view of the pixel detector readout architecture.
- whole columns lost due to a busy readout (error rate =O.l%); - pixel columns lost due to insufficient space in the timestamp buffer (error rate
3. CP violation in CMS Among the many channels that test CP violation, we discuss here the two gold-plated ones: the Bi --i ?r’r- and
and Kf + + -, is the most appropriate channel to measure the angle i. k tag the Bz, the associated b-hadron is required to decay into muon + X. The trigger is defined as two low-pr muons and is parametrized with rapidity-dependent thresholds: p{ > 5.4 for 0 < $ < 1S, pf > 3.6 for 1.5 < 9’ < 2.0 and pf > 2.6 for 2.0 < 77” < 2.4. The third muon is required to reach at least the first muon chamber layer. The following selection cuts are applied: - two charged hadrons are required, with pr > 0.7 GeV and 1111< 2.4 (Kf decay); - transverse decay length of Kf between 2 and 40 cm; - mass cut with a f2a window is applied to J/e and Kt. Details of the calculation of the number of signal events can be found in Ref. [ 21. Fig. 10a shows the Mrrlrn invariant mass, calculated after the selection criteria. The number of signal events expected with Pr^” > 4 GeV is 8000. The dominant background for this channel comes from three sources: - combinatorial background which is small; - direct @I$’ production; highly suppressed by requiring a third muon ( softer pr spectrum) ; - inclusive @/(cl’production in B decays; CMS has a very good mass resolution and a mass cut can reduce this background.
D. Kotlinski, C. Racca/Nucl. lnstr. and Meth. in Phys. Res. A 368 (1995) 115-123
(b)
0 fromcascade
decays
--.otror~K,~-deeays ‘.
t
0
0
2
4
6
6
10
12
14
PT-cut Fig. IO. (a) Reconstructed mass Mp+p_a+?r-
Mp+p--R-R+ GeV
with J/g and Kt mass constraints; (b) fraction of wrong muon tags as a function of p,“.
The estimated number of background events is 800. The measured asymmetry A is affected by dilution effects. The most important of these are the mistagging of a muon due to: - mixing: oscillations of Be’s before decaying to muons; - cascadedecaysofb(b+c--+p); - muons from decays of K, r; - muons from additional b6 or cZ:in the events; - punch-through in the detector. Fig. lob shows the fraction of wrong-sign muons as a function of pf, for the different sources of mistagging previously defined. The dominant contributions come from mixing and cascade decays. Including background contamination, the dilution factor is D = 0.47 for P,m > 4 GeV. The sensitivity to sin2P is 0.048 f 0.014 (for sin2P z 0.6).
AR < 1 around the Bi direction, normalized to the pr of the B candidate; - impact parameter significance (71~> 3 for each pion; - Aa < 100 mrad, where a is the angle in the transveme plane between the direction defined by the primary and decay vertex and the reconstructed ?T?Tmomentum (see Fig. 11). Details of the calculation of the number of background and signal events can be found in Ref. [ 21. Without particle identification, two-body decays of b-hadrons fake mass peaks b
-jet
?> bg
3.1.2. The Bs --) r’rchannel The decay Bj + rr+?r- is a promising channel to measure the angle LYof the unitarity triangle. Even without particle identification, CMS can be expected to perform well, due to its excellent mass resolution. The trigger is provided by the semileptonic decay of the associated b-hadron which is used to tag the flavour. The following selection criteria are applied, in addition to the trigger cut defined by a muon with p(f > 9 GeV and Ir]( < 2.4: two opposite-sign hadrons required, with pt > 5 GeV, distance between the two pions, AR < 1 (AR = ( Av* + A4*)!“); isolation cut: I < 0.3, where the isolation parameter I is defined as the sum of the pr of the hadrons within a cone
Beam axis
/
: I.P. ‘:.-,*’ *.. ,’ I *’’ : ,’,’
x1 ~
: : : :
7c2
Fig. I I Illustration of the selection criteria for Bi + n+r-.
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)
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: from B; 5 *wc,Kn,KK and Ab k p n
’
’
’
’
’
’
’
’
1600
“O” 1400 >
,400
1200
r”
0-J
E
$
W
800 600 -
400 200 0
4.8
5.0
5.2
5.4
M x-li+
5.6
4.8
5.8
5.0
GeV
5.2
5.4
5.6
5.8
MK+71- GeV
Fig. 12. (a) Two-body backgroundsto Bz + n-r; (b) Bj -+ air signal with Z-body backgroundand combinatorialbackground.
when the charged hadrons are assigned to the pion mass (Fig. 12a). These background contaminations are all included together with the combinatorial background. Fig. 12b shows Bi + rrn- signal with all backgrounds (2-body and combinatorial). With a mass window [ - 1.%a, +2Ua], the number of signal events is 5700, the number of background events is 6100. With the set of cuts previously defined, and including dilution effects due to mixing, mistagging and background, the sensitivity to the unitarity triangle angle a is estimated to be: 6( sin 2a) = O.OSO~($~ (for sin 2n M 0.4).
3.2. Time-dependent analysis The asymmetry can be measured as a function of the proper time t/r:
A( t/r)
=
N--N+ N-+N+
=
hp
sin 24 sin (xdr/r)
The selection criteria are the same as those applied in the previous subsections. The secondary vertex resolution in the transverse plane is assumed to be 0.2 mm. The expected time-dependent precision on sin2a and sin2/3 are: 6( sin 2~) = 0.070Z~$rr~,for sin2cr = 0.4 and S(sin2/3) = 0.046~‘&$ for sin 2/? = 0.6.
4. Bz 4
@ oscillations
The observation of Bf --) Rf is a major goal in particle physics; the current limits on the value of xS are 5.6 < ns < 33.2. This observation can be a very difficult task if xS is large. To observe oscillations, both the flavour of the Bf at production and the decay time must be known. Oscillations are studied in the decay mode, Bf(@) + Dzr*, with Dr --+ @r* and 4 + K+K-. The trigger is achieved by the muon coming from the semileptonic decay of the associated b-hadron by requiring pq > 10 GeV. After a set of selection criteria described in Ref. [ 21, the expected number of signal events is x 2400 and the corresponding number of background events is 1800. To observe oscillations, the sample of signal events is divided into like-sign (D$/.L*) and unlike-sign pairs (D,~,u~). To obtain the value of xS, the distribution in proper-time of the unlike-sign pairs is subtracted from that of the like-sign pairs, and the maximum amplitude of the Fourier transform corresponds to the most probable harmonic of the oscillation. Fig. 13 shows this analysis for n, = 30. In this study, the secondary vertex resolution usv = 150 pm is obtained with the microvertex pixel detector. The dilution factor D = 0.3 does not include cascade decays and punch&roughs. With the CMS tracker design, including the microvertex pixel detector, it is likely that oscillations can be observed up to xS = 30.
D. Kotlinski.
C. Racca/Nucl.
Instr. and Meth. in Phys. Res. A 368 (1995)
Proper time t/y (ps) 70~Founer;rans;oTm 60-
!.a
‘5 = P
40-
E
30-
z
20-
time t/y4ps)
Fig. 13. Bi-B’s’ oscillations,xs = 30, uSv = 150 pm. 5.
123
- JL+P- pair isolation against accompanying charged and neutral particles; - dimuon mass cut. The background rejection factor due to the isolation depends strongly on the transverse momentum cut on the charged particles considered inside the cone ( 100 for 1 GeV and 10 for 2 GeV). Geometrical cuts, similar to the ones defined in the study of the Bi --+ ?r+rr- channel are also applied. If the branching ratio of Bz + p’p- is 2.8 x 10e9, 3 years at the luminosity of lO33cm-‘s-’ is needed to attain a 3a significance; the upper limit on the branching ratio that can be set at 90% CL is 1.4 x 10p9. These values are obtained with a pi cut of 1 GeV in the tracker isolation criterion defined previously. If this cut is 2 GeV, the upper limit that can be tested is 2.5 times worse.
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6. Conclusions We have looked on the possibility to study B-physics and CP violation with the CMS detector at LHC. Our conclusions show that a large spectrum of measurements can be achieved with a good precision, due to the large statistics of the signal events. The signal-to-background ratios are expected to be large, because of the good performances of the CMS inner tracker. The possibility of tagging b-events with electrons is also under study.
Search for rare decays: Bf -+ p+pAcknowledgements
The By + ,K+,u- decay is a Flavour Changing Neutral Current (FCNC) process forbidden at tree-level. Its branching ratio is expected to be N 2 x 10w9. The rate of this process can be used to put constraints on CKM matrix elements or on the structure of FCNC processes, or to indicate new physics. To observe this decay channel, a 2-muon trigger with pr > 4.3 GeV is considered. To reduce the major background coming from muons from two semi-leptonic decays of BB, a set of kinematical cuts is applied: - two-muon transverse momentum pF* > 12 GeV; - distance in the rapidity and azimuthal angle spacebetween two muons 0.4 < A&P < 1.2;
We greatly appreciate the help of K. Gabathuler and R. Horisbetger in preparing the pixel detector presentation, and the help of D. Denegri in preparing the physics performance presentation.
References 111M.
Wright, J. Millaud and D. Nygren, LBL-32912 (October 1992). 94-38 LHCUPI (15 December 1994) and references therein; See also the CMS presentation in the Beauty ‘94 Workshop Proceedings,Nucl. Instr. and Meth. A 351(l) ( 1994).
I21 CMS Technical Proposal CERNlLHCC
111.GENERAL PURPOSE EXPERIMENTS