LHC-B: A dedicated B-physics detector for the LHC

IUCLEAR PHYSICS

PROCEEDINGS SUPPLEMENTS Nuclear Physics B (Proc. Suppl.) 50 (1996) 333-342

ELSEVIER

LHC-B: A Dedicated B-physics Detector for the LHC David Websdale,

Imperial College, London, UK.

For the LHC-B Collaboration

A forward collider detector, dedicated to the study of CP violation and other rare phenomena in B-physics, is proposed for the CERN LHC. Two features of the experiment are emphasized: the Cerenkov detectors, which permit the identification of a wide range of B-decay channels and the precision microvertex detector, which measures the proper-time of B-decays with a few-percent accuracy. The experiment will provide precise measurements of the angles of the unitarity triangle and a sensitivity to B,-mixing, well beyond the largest values of x, conceivable in the Standard Model.

1. I N T R O D U C T I O N The Standard Model describes quark mixing by the unitary Cabibbo-Kobayashi-Maskawa (CKM) matrix [1]. The nine complex m a t r i x elements are determined by four independent parameters, three of which can be measured in processes involving the coupling of quarks from the third generation. Precision measurements of mixing and of CP-violation in B-decay are needed to overdetermine these parameters. A failure of the overall consistency of the CKM description of quark mixing would provide a sensitive probe of possible new physics. The LHC will provide an intense source of B-hadrons. The expected b b cross-section is --~ 500 #b, leading to a production rate --~ 1012 b b per year of running at a modest luminosity of /~ = 1.5 × 1032cm-2s -1. This yield is far higher than provided by e+e - machines although the experimental conditions are less favourable, with a m i n i m u m bias background two orders of magnitude over the bb production rate. The LHC-B detector has been optimized to exploit this potential and to provide accurate measurements of CP violating asymmetries in Bdecay, with a precision of order 10 -2. An understanding of the systematic errors requires a study of m a n y different decay channels and redundancy in the method of tagging the B-flavour. The need to identify the events and to understand the effect of backgrounds emphasizes the need for particle identification and of the decay-time precision. An open-geometry forward spectrometer, similar to a fixed-target experiment is proposed. The

Lorentz boost of accepted B-mesons corresponds to mean decay paths of 7 r a m , allowing accurate measurements with a microvertex detector, and the m o m e n t u m distributions of decay products m a t c h the particle identification capabilities of Ring-Imaging Cerenkov (RICH) detectors. These features permit precise measurements of the CKM parameters involved in B-decay and mixing phenomena. Using m a n y different decay channels and trigger modes we expect to minimize systematic uncertainties in the experimental measurements and to probe the consistency of the Standard Model with the highest sensitivity. 2. D E T E C T O R S

The choice of a forward geometry is justified by optimizing the acceptance for both the B-decay under study and its partner, which is used for the flavour tag. Fig. 1 shows the polar angle distri-

1000 8O0 6O0 400 200 0

g Figure 1. Production angle (rads) of B vs. B.

0920-5632/96/$15.00 e 1996 Elsevier Science B.V. All rights reserved. PII: S0920-5632(96)00410-0

D. Websdale/Nuclear Physics B (Proc. Suppl.) 50 (1996) 333-342

334

.-" " ~

"

..

"

.......

.-- "

::i !~.~.'.':~N ~.%%%"i:!: .-1i ~.)1~!! :i~i~.,'..~ ~ ."-.1 :~:~:::.'..: ."..%'..:~..'.:: ~

- ~ t 1 ' I

I

• ..................................... ~ I

2

I

I

4

I

I

6

.-" ~ I

I

8

I

I

I

10

×

..........

~:i~:~!~"':":~"'.""::~ :: :i~:: :!/..-.e~i~,~!i~!i!i!~!..'..

~........ I

"2.-

I

I

12

I

I

14

I

I

16

I

18 21/2/~ 995

Figure 2. Side view of LHC-B detectors (scales in metres) butions of B and B, expected at LHC, calculated using the PYTHIA 5.7 [2] Monte Carlo generator. The gluon fusion process leads to forward or backward production with a very favourable correlation between B and B. A detector aperture extending from 0mi n ,.o 10 mrad to 0ms× ,.o 400 mrad accepts B-mesons having a momentum distribution extending from 20 G e V / c to a few hundred GeV/c. The mean B momentum accepted is 80 G e V / c and its mean flight path is ,-, 7 mm. Fig.2 shows the proposed layout of the LHC-B detector. It comprises a single-dipole spectrometer, equipped with a vertex detector, tracking chambers, RICH detectors for particle identification, electron and hadron calorimeters and finally a muon filter. The LHC beam pipe passes through the centre of the apparatus. Present plans foresee its installation at an existing LEP experimental area. A beam crossing point, displaced along the LHC arc by ~ 10m from its nominal position, is proposed to avoid major excavations and facilitate access to detector components.

ditions and the expected particle densities. The particle density per interaction depends on the radial distance r from the beam. A uniform rapidity density yields dn dA

a(z) r~ '

where z is the distance from the interaction point, projected along the beam direction• The parameter a(z) is shown in Fig.3. It was calculated using

•

'8

i

'

i

o

pY-I"FI I A ÷ Q E A N T

A

P~HiA

r

INNER

"13

.

i

,

0

I

200

~

I"

J

~0

J

600

Ill .

z

,

,

1

REGION

~E

0

2.1. P a r t i c l e d e n s i t i e s It has to be emphasized that the detector design proposed here could evolve, following further simulation studies and R&D in detector technologies. The current design is based on what is considered feasible now. Choices of technology are motivated by the proposed LHC-B running con-

,

I

800

1000

(cm)

Figure 3. a(z) parameter (particles/interaction) a full G E A N T [3] simulation which includes particles produced by secondary interactions in the beam pipe and detector material. The particle density is ,-~ 2.5/r 2 particles/cm 2 per interaction;

335

D. Websdale/Nuclear Physics B (Proc. Suppl.) 50 (1996) 333-342

j

cm 6 ,5

__

4

--

3 2

__

1

--

0

--

I -30

-20

1 -10

~'~------ = F0

10

20

-

-I30

-

-

] 40

-

I

I

I

50

60

70

I 80

90

cr~

Figure 4. Side-view of the silicon microvertex detector; layout of the upper silicon planes. The r.m.s length of the bunch-crossing region is 5 cm. very similar to that expected for one bunch crossing in the HERA-B [4] experiment. The experience of HERA-B will provide valuable input to the final design of the LHC-B detectors. The luminosity assumed for all calculations in this paper is L~ = 1.5 × 1032 cm -2 s -1. This can be tuned to a constant value over the lifetime of the beam fill, and it is sufficiently modest to allow full exploitation during the initial years of running the LHC. With this luminosity the mean interaction rate is 10 MHz (or 0.25 per bunch crossing). Combining particle densities with the interaction rate yields the intensities which detectors need to survive. It also determines the degree of segmentation which is required to maintain low channel occupancies. Most detectors are segmented so as to achieve < 5% occupancy; none experience higher than 10%. Simulations performed by the HERA-B [4] collaboration have demonstrated that robust pattern recognition performance is readily achievable at these levels.

cathode strip chambers [6] are two options considered for this high particle density region. The outer tracking detectors will be constructed using Honeycomb strips [7]. The r.m.s spatial precision afforded by this system is ,-~ 5 0 p m per station. Combined with the magnetic bending power this results in a mass precision O"M : 8 - - 15 MeV/c 2 for reconstructed B-mesons.

2.2. M a g n e t The proposed dipole magnet has an acceptance of + 4 0 0 m r a d for reconstructing particle momenta and +300mrad for tracks which fully traverse it. The b e n d i n ~ w e r is f B dl = 3.6 Tin.

We describe here a conservative design, using technology already available and tested in the R k D programmes for LHC/SSC detectors. It is based on silicon microstrip detectors and configured so as to allow their survival over at least one year of running before requiring replacement. Alternative possibilities using pixels, or other materials, such as GaAs, Diamond and liquid-scintillator-filled capillaries are under consideration for improved tolerance to radiation damage. The proposed layout of the vertex detector is shown in Fig. 4. Detector planes are 150 # m thick silicon, with 25 # m pitch strips and single-sided

2.3. Tracking s y s t e m Tracking chambers are situated upstream, within and downstream the magnet, grouped into 13 stations as shown in Fig.2. Independent inner and outer systems are foreseen, using technologies appropriate to the particle intensities. The inner chambers cover 40 x 40 cm 2 around the beam pipe. Microstrip Gas chambers [5] and Micro-

2.4. M i c r o v e r t e x d e t e c t o r The microvertex detector has two principal functions To provide an accurate measurement of the flight-time of B-decays. This is achieved by precise tracking of charged particles close to the beam crossing such that primary tracks and the products of secondary decays can be assigned to their vertices of origin. To provide information to the Level-2 trigger which will enrich the B-decay content of the data (see 3.2).

336

D. Websdale/Nuclear Physics B (Proc. Suppl.) 50 (1996) 333-342

AC coupled read-out. The planes are configured as shown in Fig. 5. The hole, which allows the

~ 5cm------~ 1

y Figure 5. Silicon quadrants (5x5 cm 2 each) are placed around the circulating beams which are normal to the page at the centre of the sketch.

beam to pass, ensures that no part of the silicon is closer than i cm to the beam. At this distance the annual radiation fluence is ~ 3 x 1014cm -2. The planes are installed inside the LHC vacuum pipe, protected from RF pickup by a thin aluminium foil screen. The mounting uses a Roman-pot system which permits withdrawal of the silicon during beam manipulation. A proof of principle of such an arrangement has already been demonstrated by the P238 [8] R&D project, in which silicon detectors were successfully operated inside the SppS collider. Two dimensional read-out is provided by placing x - and y-oriented strips back to back. A total of 150 silicon wafers, each provided with 1000 read-out channels, is required. The detector planes are arranged as shown in Fig. 4 covering the interaction region in both forward and backward hemispheres. The forward angular acceptance covers polar angles from 12 to 400mrads. The r.m.s precision on space points is better than 10 #m, however the ultimate performance of ~he vertex detector is limited by Coulomb scattering in the material of the first silicon plane (and its RF shield) traversed by the particles. The r.m.s precision in impact parameter, for a track

of transverse momentum PT (GeV/c) is 25 O'Ip"-10(~PT -pm, corresponding to a precision in primary vertex reconstruction, crz ~ 70pm. (Secondary vertex precision varies according to decay mode, but is typically ~ 130/m~.) 2.5. P a r t i c l e i d e n t i f i c a t i o n

B-mesons in the LHC-B detector acceptance have momenta extending from 20 G e V / c to several hundred GeV/c. Their decay products have momenta in the range 1 - 150 GeV/c, and particle identification over this range requires a combination of aerogel and two gas RICH detectors. The characteristics of the configuration shown in Fig. 2 are summarized in Table 1 and in the kinematic plot in Fig. 6 which shows the (p, 0) limit for ~r/K separation at 3~r significance. To achieve this performance Cerenkov photon

b 140

~'120

O

Gas

2 - CF4

[]

Gas

1 -

•

AerogeI-A

C5F12

100

80

o

60

40

20

0

,,,I

.... 50

I .... 1 O0

I .... 150

I ....

2OO

I ....

25G

I .... 300

I .... 350

I .... 4OO

I .... 450

500

~p mrad

Figure 6. Upper momentum limit for rr/K identification. impacts need to be measured with a precision of a few mm. Hybrid photomultipliers, which use standard photocathodes and accelerate the photoelectrons to bombard directly a segmented silicon detector, can in principle fulfil this demand. The aerogel and gas counters, upstream of the magnet, cover polar angles up to 400 mrad, while

D. Websdale/Nuclear Physics B (Proc. Suppl.) 50 (1996) 333-342 Table 1 Characteristics of the RICH counters: * required for efficient ID of Bs--+ D~-Tr+rr+Tr** required for efficient ID of Bd --+ 7r+zr- . ripe =photoelectrons per ring. ")'th ----Ethreshold/~T~ = (1 -- l / n 2 ) -1/2. Counter Aperture Momentum ~'J'pe (mrad) (GeV/c) Aerogel 100-400 14 " - 1 2 10 C5F1~ 100-400 8 60 18-38 CF4 10-120 16-140'* 18 34

.

7th 3 17 33

the downstream gas RICH identifies the high energy particles up to 120mrad. The role of the latter counter is crucial to the identification of two-body B-decays. 2.6. C a l o r i m e t e r s a n d m u o n f i l t e r The downstream arm of the LHC-B detector is completed by electromagnetic and hadron calorimeters and a muon filter. Each has a polar angle coverage up to 300 mraA. Their principal role is to provide the elements of the Level-1 high-pw triggers and to identify the leptons for Bflavour tagging. Energy resolution has not been a primary consideration and performance simulation has been based on a Pb-scintillator "Shashlik" [9] electromagnetic calorimeter and an Fescintillator "Tile matrix" [10] hadron calorimeter. The muon filter comprises an iron wall instrumented with cathode strip chambers. These technologies permit projective geometry with variable segmentation over the polar angle acceptance. The detailed design parameters will be optimized for trigger performance/cost.

3.1. L e v e l - 1 The nominal LHC-B luminosity of £ = 1.5 × 1032cm-2s -1 yields an interaction rate of 10 MHz. About 25% of all bunch crossings have a single interaction, 5% have multiple interactions. A simple pile-up rejection, cutting on total energy, Ecal < 2.2 TeV in the calorimeters, can remove 80% of multiple interactions while retaining 80% of single interactions. This is combined with the high-pT trigger channels (electron, mnon and hadron), which are used in-OR. The trigger cuts have been chosen to retain, in each channel, 1% of minimum bias (including charm) interactions. These result in PT thresholds of 1.25 and 1.5 GeV/c, respectively, for muons and electrons. For the hadron trigger, two particles with PT > 3 and PT > 2 G e V / c are required. Front-end readout of the detectors uses a 3.2/is pipe-line and the Level-1 decision is designed to be available after 2 #s. The resultant Level-1 rejection is ×50, transmitting 200 kHz to the Level-2 system. The dependence of this rate on the LHC-B luminosity is shown in Fig. 7. Without the pile-up veto the

200

0

3. T R I G G E R S

AND

EVENT

RATES

Triggering on B-hadrons is a challenge for all experiments at the LHC. The dynamics of B production exhibit no particular signature which distinguish them from minimum bias events. The LHC-B triggers focus on the B-decay properties and are designed for flexibility and efficiency. They exploit the high mass of the parent B by triggering on high-pT decay products; leptons and hadrons. They also enrich the content of events with B-decays using a vertex topology trigger.

337

I

b

I

I

2

4

6

8

Luminosity

10 ( 1 0 3:~)

Figure 7. Level-1 trigger rate vs. luminosity, including pile-up veto. The dashed curve shows the component for bunches with single-interactions. rate simply increases linearly with £. 3.2. L e v e l - 2 The Level-2 system comprises two elements. The first will use data from the tracking chambers to confirm and refine the cuts of the Level-1 highPT candidates. The second uses data from the mi-

338

D. Websdale/Nuclear Physics B (Proc. Suppl.) 50 (1996) 333-342

crovertex detector to select events based on vertex topology. One possible algorithm, which has been simulated, rejects events consistent with a single vertex. The efficiency for B's vs the Level-1 retention is shown in Fig. 8. It demonstrates, for

. . . .

i

. . . .

i

. . . .

r

Wolfenstein's [11] parameterization CKM matrix in powers of A = 0.22:

{v~d v~ V~b) VCKM =

|Vcd \ Ed

V~ V.

1 - ,k2/2

. . . .

V~ V,b )~ A2/2 -A), 2

-A

1 -

AA3(1 - p - irl)

0.6

of the

A A a ( p - ir/) A,k2 1

)

allows the unitarity condition :~ ,=,,

o.4

V~dVi~b+ cdVcb+ V~dV?b= 0 *

0.2

Bu~ar

Bg-->D,~ ~

~

o

, 0.25

0.5

Trigger

Efficiency

,

,

0.75

(B)

Figure 8. Efficiency of vertex topology trigger for minimum-bias vs. two sample physics channels. example, a x 20 rejection of Level-1 triggers, while retaining 30% of B events. 3.3.

Level-3

The input rate to the Level-3 processor farm is expected to be -.0 10kHz. A processor power of 106 MIPs can provide full event reconstruction and selection of events useful for physics analysis. These will be written to a recording medium at a rate of a few hundred per second. 3.4. E v e n t y i e l d s The numbers of events written to tape in one year of running (taken to be 107s) at the nominal LHC-B luminosity are listed in Table 2 for a selection of B-decay channels. These yields assume the expected b b cross-section of 500 pb. The branching fractions, the geometric and the trigger efficiencies are multiplicative across each row of the table. 4.

PHYSICS

PERFORMANCE

In this section we review the potential of LHC-B for measurements involving CP violation and mixing in B-decay.

V,

*

to be represented as a triangle in the p - ~1 complex plane, with vertices at (p, q), (1,0) and (0, 0). The angles at these vertices are conventionally labeled o,/3, 31. A further constraint is provided by a measurement of the ratio Xd/Xs, the Bd and Bs mixing parameters, which determines the side of the triangle opposite the angle 7. Results presented below were obtained from a simulation, using P Y T H I A 5.7 and J E T S E T 7.3 [2] for event generation and G E A N T for detector simulation. Event reconstruction uses realistic criteria for track quality, however pattern recognition algorithms are used for the microvertex detector and for the Level-2 trigger simulation. All numbers refer to one year of running and correspond to the event samples in the final column of Table. 2. These event samples are reduced following the reconstruction and the cuts used to optimize signal/background. The cuts are channel dependent and applied to invariant submass distributions, to vertex separation and to particle identification. Typical reconstruction efficiencies erec are in the range 0.2-0.4, with background/signal, RB/S, in the range 0.1-1. The main sources of background remaining at this level are due to generic b b production, not to minimum-bias events. Combinatoric backgrounds provide the main contribution to high multiplicity B-decays, whereas in low multiplicity decays (e.g. Bd -4 rrTr) it is contamination from similar channels (e.g. Bd -+ rrK) which contributes. Useful event samples are further reduced by the need to tag the B-flavour. For decay channels

D. Websdale/Nuclear Physics B (Proc. Suppl.) 50 (1996) 333-342

339

Table 2 Events accepted and triggered which are written to tape each year. The yields are calculated for a constant luminosity of £ = 1.5 x 10 32 cm -2 s -1 Efficiency Events Event Sample Visible Decay Level-1 Level-2 On Tape B.R. Acceptance 17.8% 20.6% 33.1% 110k Bd -+ 71"+Tr2.0 10- 5 14.0% 73.5% 34.5% 340k Bd ~ J / 0 K ° (p#) 2.1 10 .5 12.4% 44.6% 34.5% 183k Bd -+ J / 0 K ° (ee) 2.1 10 -5 13.7% 75.3% 42.8% 1270k Bd --+ J / 0 K * (pp) 6.3 l0 -5 12.1% 45.5% 42.8% 679k nd --+ J / O K * (ee) 6.3 10 -5 Bd --+ DOK* (KrrK:r) 8.0 10 -7 12.8% 15.4% 43.3% 3k 13.0% 14.7% 46.8% 171k Bs --+ D~- 7r+ 1.4 10 -4 9.6% 12.0% 50.8% 277k B~ --+ D~- rr+ rr+Tr3.5 10 -4 10 -5 12.6% 14.6% 47.5% 13k Bs --+ D~- K + 1.1 10 -6 12.6% 14.6% 47.5% 6k Bs -+ D + K 5.3 l0 -5 14.2% 71.8% 42.0% 246k B~ --+ J / ¢ ¢ (P#) 4.2 12.5% 44.0% 42.0% 133k Bs ~ J / ¢ ¢ (ee) 4.2 10 -5 10 -9 19.2% 84.3% 33.5% 30 B~ --+ # + # 4.0 10 -6 14.3% 70.0% 42.2% 17k Bd -+ kt+/t - K* 2.9 14.4% 18.5% 41.9% 76k Bu --+ f)°K+ (K~r) 1.5 10 -5 Bu --+DOK+(K~rlrTr) 3.1 10 -5 11.9% 14.2% 49.0% 117k

which are not self-tagging, the partner B-decay is flavour-tagged using the sign of leptons or kaons among the decay products. Typical efficiencies etag are in the range 0.4-0.5, of which a fraction ~v ~ 0.25 - 0.3 are tagged wrongly. The wrongtag fraction results in a dilution of CP violating asymmetries such that the observed a s y m m e t r y is a fraction 1 - 2w of its true value. Further complications are introduced if this fraction differs for B and B. This is studied experimentally using the so-called "control channels". 4.1. C o n t r o l c h a n n e l s a n d s y s t e m a t i c s The extraction of CP-violating decay asymmetries needs to be corrected for a possible difference in the production rates of B and B. A similar effect is introduced by differences in tagging efficiencies and wrong-tag fractions for B and B. Although it is simpler to use charged B-mesons to study these effects we have used the decay B ° --~ J / O K * , which is self-tagging at the moment of decay, no CP violation is expected and the experimental conditions are similar to those used for the CP violating decay channels. An annual yield of 227k reconstructed and tagged J/OK* -+ I+l-K+~r ~ decays allows a measure-

ment of ru, the B / B production ratio, with a precision a t = 0 . 0 0 8 and of w, the wrong-tag fraction, with precion c%=0.0018 (for ~ = 0.3). The values of Stag and ~0 depend on the specific decay channel. They have therefore been estimated separately for muon, electron and kaon tags. 4.2. sin 2a f r o m Bd --+ 7rTr The channel Bd ~ ~rrr is a demanding decay mode to extract cleanly. It has a low branching fraction (2 x 10 .5 is assumed) and, with no sub-mass constraints, the backgrounds are daunting. Particle identification, precise vertexing and the excellent LHC-B mass resolution allow the overall background/signal to be reduced to a level RB/S = 0.6. A potentially worrying background arises from Bd and Bs decays to K~r, which contribute similar numbers of events, with unknown CP asymmetries. Fig. 9 shows separately the signal Bd -+ 7r~r and background B d --~ 7rK which survive the R I C H cuts. The background is reduced to a negligible level following a tight cut on the Bd mass, which is measured with a precision ~M = 13 MeV/c 2.

340

D. Websdale/Nuclear Physics B (Proc. Suppl.) 50 (1996) 333-342

6ooI

year LHC-B will collect 520k events on tape, with 55k reconstructed and tagged. The contribution from penguin graphs is expected to be negligible in this channel and the angle /3 is given by sin 2/3 = b/(a -2~v) where w = 0.3. The precision obtained is cr(sin 2t3) ~ 0.023, which includes a 0.017 statistics contribution.

~oo

400

2100

200

4.4.

0 5

I n v o r i o n t Moes

(OeV/c

=)

Figure 9. Mass peaks for the channels Bd -+ ~rrr and Bd + 7rK reconstructed assuming the rrrr hypothesis. The 7rK contribution has been suppressed by the particle identification. A cut in the invariant mass and high m o m e n t u m particles, beyond the R I C H limit, gives additional rejection. The time-dependent CP-violating a s y m m e t r y in the decay rate can be written g b (t) -- rB X~(t) = a cos A m t + b sin A m t . Nb(t) + rBN~(t) In this expression the parameter rB is the tagged Bd/Bd production ratio, which can be measured using d a t a from the control channels. The coeflicients of the oscillation terms, a and b, are extracted from a fit to the data. If only tree graphs contribute to the decay, then a = 0 and the angle a is given by s i n 2 a = b / ( 1 - 2ha). The analysis is complicated by b-+d "penguin" graphs, which are expected to contribute at the level of 10-20%. Nevertheless the parameters a and b still determine the angle a with a precision c~(sin2a) ~ 0.04. The error has a 0.03 statistics contribution which includes also the statistical fluctuation in the background. 4.3. sin2/3 f r o m Bd --+J/~bK ° CP violation in the channel Bd --+J/~bK° will probably have been observed before LHC-B running and the distinctive dimuon signature of the J/~b decay ensures that both ATLAS [12] and CMS [13] can measure sin2/3 very well. In one

Measurement

o f t h e C P a n g l e 3'

Two independent methods have been used to extract the angle 3'- The first is similar to that described above for a , / 3 and uses mixing in the decay Bs --+ DsK (the mixing p a r a m e t e r xs is measured as described below). The time-dependent decay rates from the processes Bs,B~ -+ D;-K + and B~, Bs --+ D + K - are given, for D [ K + by Nb(~)(t )

,-~ e -r~t (cosh _4~ t + bsinh - ~

t

4- a cos Ams t + b sin Ares t) and for D+K - by the same expression, but with different p a r a m e t e r s a ~, b'. Fits to these p a r a m eters are possible due to the excellent Bs proper lifetime precision afforded by the microvertex detector, ~t = 0.04ps. They yield measurements of the CP angle 3` and of 5, the difference in the strong phases. With 3000 reconstructed and tagged events from one year's run, RB/S = 0.9 and w = 0.25, we extract values of 3` with an error ~r.y = 4 ° for 3` = 10 ° and c% = 14 o for 3` = 90 °. The errors depend on the value assumed for 5 (10 ° in the example quoted above). The second method involves a measurement of relative branching fractions for Bd --+ D°K *°, DOK.0, lJCpIX--0 TT*0 and the CP conjugate processes. The method is statistics limited. Small branching fractions result in an annual yield of only 1700 tagged events in all six channels. The parameter sin 23' can be extracted, following the procedure developed in [14], yielding a precision on 3' of ~r~ ~ 10 ° 4.5.

B~ - Bs o s c i l l a t i o n s

A m e a s u r e m e n t of the B~ oscillation p a r a m e t e r as = A m s / F s has been simulated using the decay channel Bs -+ D~-rr +. The time-dependent decay rates, N+ for Bs, Bs --+ D~-rr + are given by N+,~e-r,t(coshA--~2-ff~t-t-(l-2w)cosAmst)

.

D. Websdale/Nuclear Physics B (Proc. Suppl.) 50 (1996) 333-342

Fits to the distributions shown in Fig. 10 yield

1500

ReconstructedDecayTimes(+)

1000[

341

probe the consistency of the Standard Model of quark mixing. LHC-B achieves this programme using a modest luminosity, L: = 1.5 x 10a2cm-2s -1, two orders of magnitude below the LHC design figure. Although the clean experimental conditions of e+e - colliders cannot be matched, the complications due to event pile-up are minimal. LHC design luminosity may not be achieved during early running. LHC-B can, nevertheless, exploit fully these initial periods of operation. ACKNOWLEDGMENTS

500~ 00

1

2 t/':

3

4

Figure 10. Proper time distribution for D-re + with b-tag (top) and for D-Tr+ with b-tag (bottom). Events were generated with xs = 25 and w = 0.25. measurements of Am~, F~ and AF~. Using a tagged event sample of 35k events, R B / s "" 0.16 and w = 0.28 allows Xs to be measured up to values of 55, far beyond Standard Model expectations. 5. S U M M A R Y

AND

CONCLUSIONS

The LHC provides the opportunity to study CP violation and other physics in B-decays with the highest statistics. The LHC-B experiment proposes a forward spectrometer optimized to exploit this opportunity. It provides flexible, efficient and robust triggers, excellent decay-time precision and particle identification using RICH detectors. Many B-decay modes are accessible, allowing redundency in measurements of the CKM parameters through CP-violation asymmetries, determined by the angles (~, /3 and 7 of the unitarity triangle and through Bs mixing. The high statistics are complemented by a thorough understanding of the systematics. Measurements are obtained at the percent-level of precision which

The work reported here has been selected from the studies undertaken by members of the LHC-B collaboration, full details of which are in the Letter of Intent [15] submitted to the CERN LHC Committee. The author thanks the organising committee for planning an enjoyable and successful conference. REFERENCES

1. N. Cabibbo, Phys. Rev. Lett. 10(1963)531, N. Kobayashi and T. Maskawa, Prog. Theor. Phys. 49(1973)652. 2. H.U. Bengtsson and T. SjSstrand, Computer Physics Comm. 43(1987)367 and 46(1987)43. 3. R. Brunet al, GEANT3, CERN Program Library:W5015(1990). 4. Proposal and Technical Design Report: DESY-PRC 94/02 and 95/01. 5. RD-28 Status report, CERN/DRDC/94-45. 6. G. Charpak et al, NIM A346(1994)505, J. Seguinot et al, NIM A350(1994)430. 7. H.v.d. Graafet al, NIM A307(1991)220, F. Bakker et al, NIM A330(1993)44 8. J. Ellett et al, NIM A317(1992)28. 9. J. Bm:lier et al, NIM A348(1994)74. 10. ATLAS Technical Proposal, CERN/LHCC/94-43, 1994. 11. L. Wolfenstein, Phys. Rev. Lett. 51(1984)1945. 12. L1-M. Mir, These Proceedings. 13. Y. Lemoigne, These Proceedings. 14. M. Gronau and D. Wyler, Phys. Lett. B265(1991)172, I. Dunietz, Phys. Lett. B270(1991)75. 15. LoI for LHC-B, CERN/LHCC 95-5 (1995).