The B-Physics Potential of Lhcb, Btev, ATLAS and CMS

Heavy Flavours 8, Southampton, UK, 1999

PROCEEDINGS

The B-Physics Potential of LHCb, BTeV, ATLAS and CMS

Neville Harnew Nuclear and Particle Physics Laboratory, University of Oxford, Oxford OX1 3RH, UK. Email: [email protected]

Abstract: The physics performance of the second generation B-physics experiments, LHCb, BTeV, ATLAS and CMS, is reviewed. The diﬀerences in detector philosophies, the CP-reach of the experiments, and the sensitivities to rare B decays are compared.

1. Introduction ∗∗∗ Im VudVub + VcdVcb + VtdVtb = 0

Prior to 2005, the first generation experiments η(1−λ2/2) to study CP-violation in the B system, BaBar, ∗ ub V α V | λ Belle, HERA-B, CDF and D0, will explore the /2) td 2 cb |V V cb parameters of the unitarity triangle. These pa- | | λ rameters, indicated in figure 1, will be deter- (1−λ γ mined with varying amounts of precision : β 0 Re ρ(1−λ2/2) 1 • The quantity sin(2β) will be well measured 0 in the “gold plated” Bd → J/ψKs channel, ∗∗ ∗ Im VtbVub + VtsVus + VtdVud = 0 perhaps to ∼0.03–0.05; η (1−λ 2 • The side opposite the angle γ will be known, λ /2) 0 ∗ | |V 0 ub V B B cb cb td assuming s s mixing is measured by CDF V V | | and D0; λ ηλ2 γ′ • The side opposite β will be measured from δγ Vts b → u Re decays, but with significant error |Vcb| 0 ρ due to theoretical uncertainty in hadronic (1−λ2/2+ρλ2) interactions; Figure 1: Two unitarity triangles shown in the com- • The quantity sin2(β + γ), i.e. sin(2α), will plex plane, with an approximation valid up to and 5 be measured, but with poor statistical pre- including O(λ ). Vij are the CKM matrix elements, cision and significant theoretical uncertainty; and λ is the sine of the Cabibbo angle.

• There will be no good or direct measurement of the angle γ.TheBssector will be value of β and less precise data, namely the mea- largely unexplored by the ﬁrst generation surement of α and the sides of the triangle. The experiments. second generation experiments – LHCb, ATLAS and CMS at the LHC, and BTeV at the Teva- The ﬁrst generation experiments will either tron – will make precision measurements of the be consistent with the Standard Model interpre- same parameters. The high statistics recorded tation, or will measure inconsistency between the by these experiments will allow the same param- Heavy Flavours 8, Southampton, UK, 1999 Neville Harnew

10 4 0 →π−π+ π Bd in 4 with π−π+ measured 10 3

10 2

10 3 θ b [rad] 2 3 1 2 1 θ [rad] 021 3 45 0 b B0 decay length [cm]

Figure 2: a) The correlation of polar angles of b and b hadrons at LHC energies, and b) the corresponding B0 decay length distribution. eters to be measured in different channels (which LHC energies. The second advantage of forward may be theoretically clean but not necessarily the geometry is that a vertex detector can be placed easiest experimentally), giving additional cross close to the interaction region. At the LHC, the checks. New parameters will also be measured mean momentum for accepted B mesons is ∼80 with precision in a number of complementary GeV/c, leading to a mean B0 decay length of channels, e.g. the angle γ. Experimental at- ∼7 mm, shown in figure 2b. Excellent proper tributes of the second generation experiments in- time resolutions of approximately 40–50 fs can clude efficient π/K identification, excellent decay be achieved. Finally, the open geometry allows time resolution, and photon detection. for easy installation and maintenance of the detector components. One disadvantage of the forward geometry 2. The second generation B physics is that minimum bias events are also strongly experiments forward peaked. This gives high track densities which result in high particle occupancies. A comparison of the LHC and the Tevatron second generation CP violation experiments is given 2.1 The LHCb Experiment in table 1. The Tevatron (Run II) will collide protons with antiprotons at 2.0 TeV, at which The LHCb detector [2] is a forward single-arm energy the bb and inelastic cross sections are ex- spectrometer, shown in figure 3. LHCb has a pected to be ∼100 µb and 50 mb, respectively [1]. precision silicon strip vertex detector for mea- The LHC will collide protons with protons at surement of B decay vertices, giving a 120 µm 14.0 TeV. The increased energy gives a bb cross resolution along the z axis. Here radial and az- section ∼5 times higher than the Tevatron, and imuthal cordinates with respect to the beam axis with an inelastic cross section which is only a fac- are measured with r and φ strips, respectively. A tor of 1.6 higher, 80 mb. Both LHCb and BTeV dipole magnet provides a field integral (B.dL) of experiments have forward geometries whereas AT- 4 Tm. There is an efficient four level trigger sys- LAS and CMS are general-purpose detectors, cov- tem (designated Level-0 to Level-3), with a ver- ering the central rapidity region. tex trigger included at Level-1. Two RICH de- There are a number of advantages associated tectors, shown in figure 4, provide particle identi- with forward detector geometry. The first ad- fication with at least a 3σπ/Kmomentum sep- vantage is the increased B acceptance; the bb aration between 1

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Tevatron LHC Energy / collision mode 2TeVpp 14 TeV pp bb cross section ∼100 µb ∼500 µb Inelastic cross section 50 mb 80 mb Ratio b b / inelastic 0.2% 0.6% Bunch spacing 132 ns 25 ns BTeV LHCb ATLAS / CMS Detector conﬁguration Two-arm forward Single-arm forward Central detector Running luminosity 2×1032cm−2s−1 2×1032cm−2s−1 ≤1×1033cm−2s−1 b b events per 107 sec 2×1011× accept. 1×1012× accept. ≤5×1012× accept. ∼2.0 0.5 (∼30% single int.) ∼2.3

Table 1: Comparison of the LHC and the Tevatron experiments.

Level-1 vertex trigger then selects detached vertices in software. This has excellent efficiency for B decays of interest : 50%, 48% and 56% 0 0 0 + − 0 for B → J/ψKs (µµ), B → π π and Bs → − + Ds K , respectively. Level-2 and Level-3 are software triggers which refine the pT and vertex requirements, and select B decay channels of interest. The combined efficiency of all trigger levels is typically 30% for reconstructable events within the spectrometer. These trigger efficiencies are “diluted” by tagging efficiencies which Figure 3: A schematic of the LHCb detector. are typically 40%, and by wrong tag fractions which are typically 30%. The importance of the LHCb RICH detec- thresholds of 4.4 and 15.6 GeV/c, respectively). tors in providing particle identification is demon- Hadron and electromagnetic calorimetry is em- strated in figure 5. This shows the effect of re- + − ployed, primarily to trigger on hadrons, electrons ducing background in the channel Bd → π π and photons. (which has a visible branching ratio < 0.7×10−5). The high B event rate means that running Without utilizing RICH information, the π+π− LHCb at low luminosity is sufficient when op- mass distribution suffers serious background from ∓ −5 erating with a highly efficient trigger, discussed the channels Bd → K π (BR. 1.5×10 ), Bs → + − −5 ∓ below. By de-tuning the beam focus, LHCb will K K (BR. 1.5×10 ), and Bs → K π (BR. 1 32 −2 −1 run at a luminosity of 2.0×10 cm s . This 0.7×10−5). These channels can themselves demon- gives an average of only ∼0.5 interactions per strate CP asymmetries and, if not rejected, can + − bunch crossing. Having a single primary ver- dominate the Bd → π π asymmetry. It can tex has distinct advantages in reducing confusion be seen that the effect of the π/K identification to the trigger, reducing particle occupancy, and provided by the RICH detectors offers almost hence improved pattern recognition in the detec- complete background rejection, with an 85% ef- tor components. Low luminosity also ensures less ficiency. The excellent mass resolution in this radiation damage. channel, 17 MeV, also greatly aids background The Level-0 trigger of LHCb comprises a muon, suppression. electron and hadron trigger, which pre-selects high 2.2 The BTeV Experiment pT (>1–2 GeV/c) tracks and clusters. A “pile- up” veto trigger ensures that >80% of events will The BTeV detector [3] is a forward-backward contain only one single beam interaction. The double arm spectrometer, shown in figure 6. The

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RICH2 RICH-1 Photodetectors ) Tracking Gas (C4 F10 chamber 300 mrad Mirror (R = 190 cm) Interaction point Aerogel 120 mrad (n = 1.03)

Window (Mylar) Beam pipe 330 mrad Mirrors

Photodetectors

Gas (CF4) 0 1 (m) 2

610812

Figure 4: Schematics of the LHCb RICH-1 and RICH-2 detectors.

decays, keeping biases to a minimum. The loca- tion of the pixel detector in the B field also fa- cilitates background rejection at the trigger level from low momentum tracks and secondary interactions. The disadvantage of the pixel detector is that it has excessive material thickness, a total of ∼1 radiation lengths, although typi- cal tracks traverse only a fraction of this. BTeV will run at the full luminosity of the Tevatron, 0 + − 32 −2 −1 Figure 5: Bs mass peak from π π combinations 2.0×10 cm s , giving an average of ∼2 in- in LHCb, showing the background from channels de- teractions per bunch crossing scribed in the text, without and with RICH informa- BTeV has a three level trigger system. Level- tion. 1 is the vertex trigger which is pipelined at 132 ns. The vertex trigger has excellent efficiency for B decays of interest : 50%, 55% and 70% for B0 → double arm gives BTeV a relative rate advantage J/ψK0(µµ), B0 → π+π− and B0 → D−K+,re- of ∼2 with respect to LHCb. A precision pixel s s s spectively. The Level-2 and Level-3 triggers are vertex detector is sited inside a dipole magnet of software triggers, which select B decays of inter- field integral 5.2 Tm. BTeV utilizes a RICH de- est. Tagging efficiencies and wrong tag fractions tector which provides particle identification be- are similar to LHCb. tween 3 and 70 GeV/c (the momentum spectrum is softer than at the LHC). A lead tungstate EM calorimeter allows reconstruction of photons and 2.3 The ATLAS/CMS Experiments 0 π ’s with high resolution. ATLAS and CMS [4, 5] are the two LHC general- The key design feature of BTeV is the uti- purpose detectors. Both detectors have tracking lization of a pixel vertex detector, shown in fig- coverage in the central rapidity region, |η| <2.5. ure 7. This detector provides excellent spatial The detectors have specialist B triggers which 33 resolution of 5–10 µm, giving a B proper time can operate up to luminosities of typically 1×10 −2 −1 resolution of ∼50 ps. The pixels have good sig- cm s . The detectors have no dedicated hadronic nal to noise, they ensure low particle occupancy, particle identification capabilities (with the ex- and the detectors are intrinsically radiation hard. ception of a limited dE/dx measurement in AT- The special feature is an ambitious vertex trigger LAS). at the first level, which selects generic B meson ATLAS and CMS incorporate three levels of

4 Heavy Flavours 8, Southampton, UK, 1999 Neville Harnew

Figure 6: A schematic of the BTeV detector.

The BTeV Baseline Pixel Detector Triplet position along beam + 4mm

400µ + 50µ + 4mm v

V 5cm 5cm V v Pixel Orientation in Triplet

-+ 6mm X -+6mm Beams Beam hole

Figure 7: A schematic of the BTeV pixel detector showing a) the layout, and b) a simulated B0 → π+π− event. trigger. High pT (5-6 GeV/c) muon and calorime- where ∆m is the mass difference of the two weak ter triggers are incorporated at Level-1. Level-2 eigenstates of the Bd − Bd system, is largely in- and 3 are software triggers which, for low and sensitive to the contribution from penguin am- modest luminosity running, allow selection of B’s plitudes. Here the sensitivities of the general- with high statistics. Neither experiment incorpo- purpose detectors compete well with those of the rates a dedicated vertex trigger at an early level. forward detectors, due to the formers’ efficient high pT muon triggers [4, 5]. The invariant 0 mass distribution of J/ψKs combinations, to- 3. B physics capabilities gether with the estimated background, is shown for ATLAS in figure 8. Table 2 demonstrates that The CP physics reach of the second generation sin(2β) can be measured to an accuracy of 0.02 experiments in specific benchmark channels for a or better in this channel by each of the second year of operation, together with the triangle pa- generation experiments in a year of operation. rameters which they measure, is summarised in table 2. A selection of important measurements 3.2 Measurement of the angle γ is described below. 0 − + 3.2.1 Bs → Ds K The channel B0 → D−K+ and its charge conju- 3.1 Measurement of the angle β via Bd → s s 0 gate states provide a measurement of the angle J/ψKs (γ − 2δγ) [7]. Four time-dependent decay rates B → J/ψK0 0 − + 0 + − The channel d s provides a “gold- are measured : Bs → Ds K , Bs → Ds K , β 0 − + 0 + − plated” measurement of sin(2 ). The decay asym- Bs → Ds K and Bs → Ds K , which yield metry : values for (γ − 2δγ) and the strong phase difference, ∆. The importance of particle identifi- B →J/ψK0−B →J/ψK0 A t d s d s ( )= 0 0 cation and good mass resolution is demonstrated Bd→J/ψKs +Bd→J/ψKs = −sin(2β)sin(∆mt), for LHCb in figure 9. In the absence of particle

5 Heavy Flavours 8, Southampton, UK, 1999 Neville Harnew

Measurement Channel LHCb BTeV ATLAS CMS

0 0 sin(2β) Bd → J/ψKs 0.011 – 0.017 0.021 0.021 0.025 0 + − sin(2α) Bd → π π 0.05 0.06 0.10 0.17 0 ◦ sin(2α), cos(2α) Bd → ρπ σ(α) ∼ 5 – – 0 ? ◦ 2β + γ Bd → D π, 3π 9 – – 0 − + ◦ ◦ γ − 2δγ Bs → Ds K 6 − 13 11 – – 0 0 ?0 ◦ γ Bd → D K 10 – – γ B− → D0K− 13◦ – – 0 ◦ ◦ δγ Bs → J/ψφ 0.6 0.9 0 − + xs Bs → Ds π < 75 < 60 < 46 < 48 0 + − Rare decay Bs → µ µ 4.4σ S.M. 4.3σ S.M. 10σ S.M. 0 ?0 Rare decay Bd → K γ 26k events 24k events – –

Table 2: Performance summary of the second generation experiments in a selection of benchmark channels for one year of operation. The quoted numbers are the errors on the parameter in question, unless specified 0 + − 0 + − otherwise. The α measurement in the Bd → π π channel assumes no penguin contribution. The Bs → µ µ channel assumes a Standard Model branching ratio of 3.5×10−9. A dash for an entry means that no significant measurement can be made. If an entry is left blank, this means that a measurement of significance is expected, but still needs to be fully studied.

Bd->J/yKs Events

2000

B0 D K 1000 Figure 9: s mass peak from s combinations in LHCb, showing the background from Dsπ,withand without RICH information.

0 in ﬁgure 10. Table 2 demonstrates that BTeV 5 5.1 5.2 5.3 5.4 5.5 and LHCb can achieve sensitivities of 6–13◦ in Mass (GeV) (γ − 2δγ) for one year of running.

0 0 ?0 3.2.2 Bd → D K Figure 8: Invariant mass distribution of J/ψ combi- 0 0 ?0 nations with the background estimate superimposed The channel Bd → D K and its charge conju- (shaded histogram). gate states provide a measurement of the angle γ. Six time-integrated decay rates are measured : 0 0 ?0 0 0 ?0 0 0 ?0 Bd → D K , Bd → D K , Bd → DCP=+1K , 0 0 ?0 0 ?0 0 ?0 identification, the signal is dominated by back- B → D K , B → D0K and B → D0 K . 0 − + d d d CP=+1 ground from Bs → Ds π decays (the Bs mix- 0 0 DCP=+1 signifies the decay of the D into a CP ing channel). eigenstate, i.e. K+K− or π+π−,otherwiseD0 → Sensitivities per year to (γ − 2δγ) depend K+π− decays are considered. Since visible branch- on (γ − 2δγ),andon∆.Anexampleofafitto ing ratios are very small (10−8 –10−7), the mea- 1000 Monte Carlo BTeV “experiments” is shown surements are only possible with a high-rate for-

6 Heavy Flavours 8, Southampton, UK, 1999 Neville Harnew

Without RICH 300

200

100

With RICH

0 4.5 5 5.5 6 + − + − m(K π K π ) [GeV/c2]

Figure 10: Example of fitting γ in the chan- 0 + − + − 0 − + Figure 11: Bd mass peak from K π K π com- nel Bs → Ds K for 1000 BTeV Monte Carlo “ex- binations in LHCb, with and without RICH informa- periments”. The values γ=45◦, sin(γ + ∆)=0.771, tion. sin(γ − ∆)=0.629 and ρ=0.5 are assumed, yielding a +11 fitted value of γ of 46−9 degrees. 0 ? ? 3.2.4 Bd → D π and D 3π The channels B0 → D?π∓ and D?3π provide ward detector utilizing particle identification (i.e. d measurements of the angle (2β+γ) via four time- LHCb and BTeV). dependent decay rates [2, 9]. Small CP asym- γ LHCb have evaluated the sensitivity of the metries, ∼1%, result from the interference of the B0 measurement in these channels. The d recon- tree diagram with the doubly-Cabibbo suppressed 0 0 ?0 structed in the Bd → D K channel is shown in decay into the same final state. LHCb have stud- figure 11, demonstrating the importance of parti- ied the sensitivity of these channels in which large cle identification. Using this method, LHCb can statistics are vital, and so are reliant on an effi- ◦ measure the angle γ to a precision of ∼10 after cient hadron trigger. The method involves an one year of running. inclusive D? reconstruction technique, yielding 0 ? ∓ 270 k events per year in the channel Bd → D π , with a signal to background of ∼7. Including − 0 − B → D K ? 3.2.3 the D a1 (a1 → 3π) channel gives an additional − 0 − 320 k events/year. The channel B → D K provides a measure- The error on (2β + γ) is shown as a function ment of the angle γ via nine time-integrated de- of (2β+γ) in figure 12, assuming no strong phase cay rates [3, 8]. CP-violation arises from the in- γ B− → D0K− difference. The angle can be measured to a terference between the decays and ◦ − 0 − 0 0 precision of 5 after five years of running. B → D K ,wheretheD and D decay into the same final state (similarly for the charge con- 3.3 Measurement of the angle α via B0 → jugate decays). It is necessary to measure the d − − 0 ρπ branching ratios BR(B → fiK )fortheD 0 0 and D decaying into at least two different final The analysis of Bd decays to three pions leads states fi in order to determine γ up to discrete to an alternative determination of the angle α. 0 + − ambiguities. Using this method, BTeV can mea- This complements the Bd → π π measure- sure the angle γ to a precision of ∼13◦ after one ment, which has large penguin uncertainties. The 0 year of running. The decay modes D0, D → interesting decays have ρ mesons in the interme- + − + − 0 + − 0 − + 0 K π and K K have been studied. diate state : Bd → ρ π , Bd → ρ π and Bd →

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0 + − pared with the Bd → π π channel, which for LHCb yields ∼ 7000 events per year, assuming a branching ratio of 7×10−6. Figure 13a shows the expected π+π−π0 invariant mass distribution after one year of LHCb 0 2 data taking. The measured Bd width is 50 MeV/c and the expected signal to background is ∼0.75. Figure 13b shows the expected signal distribution in this channel for BTeV. In principle, a nine parameter ﬁt can be performed to the time-dependent Dalitz plot, yielding α and the tree and penguin amplitudes. Pre- liminary studies from LHCb indicate that an accuracy in α of ∼5% could be reached after one year of data taking.

Figure 12: The error on (2β + γ) as a function 0 − + 3.4 Measurement of xs via Bs → Ds π of (2β + γ) after a) one and b) five years of LHCb running. Bs − Bs oscillations have been studied in the 0 − + 0 + − channels Bs → Ds π and Bs → Ds π ,and all second generation experiments can make mea- ρ0π0. A time-dependent analysis of the Dalitz surements of xs beyond Standard Model expec- plot allows the extraction of α,aswellasthetree tations. A simulation of Bs − Bs oscillations in and penguin terms separately. The interference BTeV, together with a fit to xs, is shown in fig- between the ρ+, ρ− and ρ0 amplitudes, occur- ure 14. A similar simulation for LHCb is shown ing at the intersection of the Dalitz plot bands, in figure 15. Table 2 demonstrates that xs will be allows the determination of cos 2α and sin 2α. measured to 45 or better by each of the second This removes ambiguities in the determination generation experiments in a year of operation. of α. The LHCb and BTeV collaborations have 0 + − 3.5 The Bs → µ µ rare decay 0 + − 0 performed simulation studies of the Bd → π π π B0 → µ+µ− final state. BTeV has lead tungstate electromag- The s channel is an example of a Stan- netic calorimetry which provides excellent energy dard Model rare decay process, with an expected √ ∼ × −9 resolution (∼2%/ E ⊕ 0.6%) and fine granular- branching ratio of 3.5 10 [10]. Here the high p ity for photon detection and π0 reconstruction. T di-muon triggers running at high luminosity (1×1034 cm−2s−1) give the general-purpose de- LHCb utilizes coarser granularity√ Shashlik electromagnetic calorimetry (∼10%/ E ⊕ 1.5%), but tectors a distinct advantage over the forward de- has a large 12 m lever arm for separation of the tectors to observe this channel. CMS have excel- gammas. BTeV expect ∼3000 fully tagged ρ+π− lent muon detection capabilities [5]. They expect events per year [3], and although a complete GEANT 26 signal events with 6.4 events background for −1 7 × 34 −2 −1 simulation and background rejection study still 100 fb of running (i.e. 10 sat1 10 cm s ) needs to be performed, the high resolution calorime- [11]. The expected CMS di-muon effective mass B0 try should give the experiment a clear advantage. distribution for the s decay is shown in fig- LHCb have performed a detailed study [2], and ure 16, yielding a resolution of 31 MeV. expect annual event yields of triggered, fully re- + − constructed and tagged events to be : ρ π ∼ 4. Summary 1000 (BR. = 44×10−6), ρ−π+ ∼ 200 (BR. = 10×10−6), and ρ0π0 ∼ 50 (BR. = 1×10−6), The second generation CP-violation experiments, where the branching ratios used are those theo- LHCb, BTeV, ATLAS and CMS, will measure retically predicted. These rates are to be com- CP violation parameters with huge statistics, ∼1012 bb

8 Heavy Flavours 8, Southampton, UK, 1999 Neville Harnew

Events

M(π+π-π0) [GeV/c2]

Figure 13: Simulated π+π−π0 invariant mass distributions from a) LHCb, showing the B mass peak and expected signal to noise, and b) BTeV, showing the signal only.

800 ps

5 tagged tagged as having oscillated tagged as having oscillated 600 (background only) es per 0.0 i x = 15 400 s Entr

200

0 200 150 xs = 46 es per 0.02 ps

i 100

Entr 50 0 012345 Proper time (ps)

Figure 15: Monte Carlo proper lifetime plots from 0 − + 0 0 LHCb of Bs → Ds π tagged as Bs and Bs,a)for Figure 14: Monte Carlo proper lifetime distribu- xs=15, and b) for xs=46. 0 − + 0 tions from BTeV : a) for Bs → Ds π tagged as Bs , 0 − + 0 b) for Bs → Ds π tagged as Bs, and c) the nega- tive log-likelihood of the fitted value of xs as a func- Model, and beyond. tion of xs. (Here the simulated xs value is 40, with a50fstimesmearingoftheBs decay time). pairs per year. The LHCb and BTeV experi- Acknowledgments ments will utilize vertex detectors which will give proper time resolutions of σt ∼40–50 fs, and pro- I am very grateful to the following people who vide efficient B secondary vertex triggers. K/π provided material for this review : Nick Ellis and particle identification will be essential for the mea- Roger Jones (ATLAS), Daniel Denegri and An- surement of B final states involving hadrons. drei Starodoumov (CMS), Sheldon Stone (BTeV), The second generation experiments will mea- and Tatsuya Nakada and Guy Wilkinson (LHCb). sure the angles and the sides of the unitarity tri- I also extend my thanks to the Heavy Flavour 8 angles with unprecedented precision. This will be organizers, and personally to Paul Dauncey and a unique opportunity to understand the origin of Jonathan Flynn, for making the conference in CP violation in the framework of the Standard Southampton so enjoyable.

9 Heavy Flavours 8, Southampton, UK, 1999 Neville Harnew

[8] M. Gronau and D. Wyler, Phys. Lett. B 265 (1991) 172. D. Atwood, I.Dunietz and A. Soni, Phys. Rev. Lett. 78 (1997) 3257. [9] I. Dunietz, Beauty’97, Nucl. Instr. Meth. A408 (1998) 14. [10] A. Ali, Beauty’96, Nucl. Instr. Meth. A384 (1996) 8. [11] D. Denegri private communication, and A. Starodoumov, Beauty’99, Bled, Slovenia (to be published in Nucl. Instr. Meth.).

Figure 16: The di-muon eﬀective mass distribution in CMS. Two muons are triggered with pT >4.3 GeV over the rapidity region |η| < 2.4.

References

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