The analysis of the CKM parameters will take a leap forward when the hadronic
B factories receive their first data. I describe the challenges faced by
B-physics at hadronic colliders and the expected reach in specific channels for
the LHCb, BTeV, ATLAS and CMS experiments.Comment: Invited talk at the Workshop on the CKM Unitarity Triangle, IPPP
  Durham, April 2003 (eConf C0304052). 6 pages LaTeX, 6 eps file

Egede, Ulrik

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CKM Reach at Hadronic Colliders

The analysis of the CKM parameters will take a leap forward when the hadronic B factories receive their first data. I describe the challenges faced by B-physics at hadronic colliders and the expected reach in specific channels for the LHCb, BTeV, ATLAS and CMS experiments.The analysis of the CKM parameters will take a leap forward when the hadronic B factories receive their first data. I describe the challenges faced by B-physics at hadronic colliders and the expected reach in specific channels for the LHCb, BTeV, ATLAS and CMS experiments

Egede,U.

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arXiv:hep-ex/0307022v1  11 Jul 2003Workshop on the CKM Unitarity Triangle, IPPP Durham, April 2003CKM03CKM Reach at Hadronic CollidersU Egede∗Imperial College London, London SW7 2AZ, United Kingdom.The analysis of the CKM parameters will take a leap forward when the hadronicB factories receive their first data. I describe thechallenges faced byB-physics at hadronic colliders and the expected reach in specific channels for the LHCb, BTeV, ATLAS and CMSexperiments.1 IntroductionWith the hadronicB factories currently under construc-tion or in the design phaseB physics will enter a new era.The LHCb, BTeV, ATLAS and CMS experiments all havea time scale forB physics from 2007 onwards. The aimof the experiments is to gain a comprehensive understand-ing of the CKM matrix for discovering physics beyond theStandard Model. The much larger statistics and the accessto Bs decays will allow to many cross checks ofCP viola-tion that are not possible at the currentB factories.By 2007 the currente+e− B factories will have collectedsamples of the order of 109 B-meson decays. This, com-bined with the data from the Tevatron will give a precisionon the value of the CKM angleβ of σ(sin 2β) = O(10−2)which is close to the systematic uncertainty from penguinpollution in the channelB → J/ψK0S . At the same timethe anticipated measurement ofBs mixing will improve thevalue of |Vtd|/|Vts| from the partial cancellation of theBdandBs form factors. The anticipated improvement in themeasurement of the apex of the unitarity triangle betweentoday [1] and 2007 is shown in figure 1.In table 1 the experimental conditions for the different ex-periments are summarised and compared to a conceptualdesign for a future+e− B factory. Several points are worthfurther comments:• At a hadron collider the ratio between thebb crosssection and the total inelastic cross section is verysmall. The ratio improves as the centre of massenergy increases thus giving the LHC experimentsan initial advantage compared to BTeV running at alower CM energy. It should also be noted that thereare significant uncertainties in thebb cross section atthe LHC energy.• The large production ofBs, Λb andBc will open upentirely new areas ofB physics where the presentdata samples are very limited.∗On behalf of the LHCb collaboration.VubVcbsin(2   )βm∆ d0 1 ρη2002VubVcbm∆ d m∆ s/0 1 ρηβ2007 ?Figure 1. The anticipated improvement in three key measure-ments of the unitarity triangle between 2002 and 2007.• LHCb will tune its luminosity to a level wheremost recorded events will have a single interaction.BTeV’s strategy is instead to cope with multipleevents in a single bunch crossing by spreading themout across a larger vertex region where the individualprimary vertices can be clearly identified.• The ATLAS and CMS experiments do not haveB physics as their primary goal and will as suchhave a much lower trigger bandwidth dedicated toB physics.2 Detector layoutFor most studies ofCP violation in B-meson decays weneed to identify the flavour of theB-meson at productiontime. The dominant contribution to this flavour tagging2 Workshop on the CKM Unitarity Triangle, IPPP Durham, April 2003LHC BTeV SuperBABARBeam type p-p p-p e+e−Status Construction Pending finance Concept√s 14 TeV 2 TeV 10.58 GeVσbb 500 µb 100 µb 1.1 nbσcc 3.5 mb 1 mb 1.3 nbσinclusive 80 mb 60 mbB+/Bd/Bs/Λb mixture 40/40/12/8 40/40/12/8 50/50/0/0Bunch separation 25 ns 132/ 96 nsSize of collision region 5.3 cm 30 cmLHCb ATLAS/CMS BTeV SuperBABARPseudorapidity coverage 2.1–5.3 -2.5–2.5 2.1–5.3L [ cm−2s−1 ] 2 × 1032 1033(1034) 2× 1032 1036< n > per bunch crossing 0.5 2 (20) 1.6/4.8nbb per 107 s 1012 5× 1012(13) 2× 1011 1010Table 1. A comparison of the beam parameters and detector coverage for the hadronicB factories and a comparison with a conceptualfuturee+e− collider.is through identification of particles from the decay of theother B-hadron created in the event. Hence the detectorneeds to be designed such that a significant part of the pro-duced pairs ofB-hadrons both end up within the detectoracceptance. The most cost effective solution to this is tomake a detector that sits as much in the forward region astechnology allows. As bothB-hadrons tend to be boostedin the same direction there is no synergetic effect from cov-ering both forward regions. This leads to the design of theLHCb and BTeV detectors as single-armed forward spec-trometers.The overall design of the LHCb detector is shown in fig-ure 2. The most essential parts of the detector are: thetrigger system which reduces the rate of events going tomass storage to an acceptable level; the vertex detectorwhich provides the trigger with secondary vertex identifi-cation and the physics with the ability to resolveBs oscilla-tions; and the particle identification system which providesthe essential pion-kaon separation required forCP violationstudies.2.1 TriggerThe single most demanding task for the hadronicB physicsexperiments will be the trigger. The combination of a crosssection minimum bias which is orders of magnitude largerthan theb cross section, with the rareB decays which areof interest requires a sophisticated trigger that can suppressrates by many orders of magnitude. With a rate of around1012 B-hadrons produced in a year the trigger also haveto be selective. This is a completely different situation tocurrente+e− colliders where allB decays are recorded.There are three main elements that allow identification ofevents with aB-hadron:• Large transverse energy or momenta with respect tothe beam axis. This is simply an indicator of a highmass particle decaying.• Vertices which are displaced from the primary ver-tex. This takes advantage of the long lifetime ofB-hadrons compared to other hadrons produced (K0SandΛ live for much longer and do not interfere withthe trigger).• High energy leptons either from semi-leptonicB-decays or in pairs fromB-hadrons with aJ/ψ in thedecay chain. This will also be the trigger for rareB→ mupm decays.The trigger strategy for LHCb and BTeV are in many waysquite different.In LHCb the aim of the first trigger level is to identifyevents with particles of high transverse energy or momen-tum with respect to the beam axis. The highpT andET arises from the decay of high mass objects and thusfavours B-hadrons to the background events with lowermass hadrons. In addition these are the type ofB mesonsevents that the final selection of events for physics analy-sis favour. At the next trigger level secondary vertices areidentified by placing cuts on the significance of the impactparameter for tracks with respect to the identified primaryvertex. The last trigger identify more specific classes ofB-decays using the results of the online reconstruction.On the other hand the aim of the BTeV trigger is to identifyall B-hadron events and write them to tape. This is done byidentifying secondary vertices by placing cuts on the sig-nificance of the impact parameter for tracks with respectto the identified primary vertex at the full 7.6 MHz rate ofthe beam crossings and then at the next trigger level per-form full secondary vertex reconstruction. The procedurewill intentionally also identify many charm events whichWorkshop on the CKM Unitarity Triangle, IPPP Durham, April 2003 3Figure 2. The design of the LHCb detector. The collision point for theprotons is within the vertex detector to the left in this drawing.are predicted to take up 50% of the rate of events going totape. We show a summary and comparison of the LHCband BTeV triggers in table 2.The trigger for ATLAS and CMS is much tighter as onlya 10 Hz output rate is allocated forB physics in the firstthree years of low-luminosity running for the two experi-ments. Both experiments rely on a single muon of highpTto trigger at the first level. The identification of muon pairsfrom eitherB→ J/ψX decays or rare two-body decays to apair of muons form the majority of the trigger. A summaryof the trigger is given in table 3.More details can be found in the talks of M. Ferro-Luzzi [2], L. Moroni [3] and A. Starodumov [4] at thisworkshop.trigger type LHCb BTeVHigh pT , highET 10 MHzImpact parameter 1 MHz 7.6 MHzDecay topology 80 kHzPhysics algorithms 40 kHzTo mass storage 200 Hz 4 kHzTable 2. A simplified comparison of the trigger levels at LHCband BTeV. The 10 MHz ingoing rate for LHCb corresponds to therate of bunch crossings with a visible interaction. In addition towhat is given above both experiments have a dedicated first leveltrigger for events with one or two highpT muons.Trigger type ATLAS CMSMuon trigger 40 MHz 40 MHzJ/ψ → ℓ+ℓ−, D+s → φπ+ 20 kHzPhysics algorithms 1 kHz 4 kHzTo mass storage 10 Hz 10 HzTable 3. A simplified comparison of theB physics trigger levelsat ATLAS and CMS.2.2 Particle identificationTo make hadronic final states useful forCP violation stud-ies it is required that we can distinguish pions and kaonsvery well. A good example is for theBs → D∓s K± de-cay to be used for the extraction of the angleγ. The de-cay Bs → D−s π+ is expected to have a branching frac-tion 15 times larger than the kaon decay thus drowning theBs → D∓s K± signal without any particle identification. Infigure 3 we illustrate the particle identification capability ofLHCb to isolate theBs → D∓s K± signal. For the two-bodyB meson decays the kaon-pion separation is also essentialfor the extraction of the angleγ from the individual mea-surements ofBd → π+π− andBs → K+K− decays.Both LHCb and BTeV use Ring ImagingCherenkov (RICH) detectors for pion and kaon identi-fication. For a given radiator the eff ctive momentumrange is limited from below by the onset of Cherenkovradiation for the pions and from above when the kaonCherenkov angle saturates causing the rings created bykaons and pions to have the same radius. This means that4 Workshop on the CKM Unitarity Triangle, IPPP Durham, April 2003Figure 3. Without particle identification from the Cherenkov de-tectors in a simulation from LHCb (top) theBs → D∓s K± sig-nal is drowned by theBs → D−s π+ decay. With particle identi-fication (bottom), the signal is dominant compared to the back-ground.more than one radiator is required. ATLAS and CMS havea very limited ability for kaon-pion separation.Kaon identification is one of the dominant sources forflavour tagging. This can either be through identifying thecharge of a kaon from the decay of the otherB createdin the event or for the tagging ofBs decays from the de-cay B∗∗s → BsK+. In addition to kaon-pion separation it isimportant to reduce the contamination of the kaon taggingsample with protons. The LHCb experiment use an aerogelradiator with refractive index of 1.03 in addition to the twogas radiators to provide kaon and proton separation downto around 2 GeV/c.3 Physics reachThe aim of giving numbers for the physics reach beforethe start-up of experiments is to assure that the detectordesign is able to give the promised results in a selection ofchannels that are thought to be representative of the physicsthat will be of interest in 2007 and beyond. In the samespirit we will here only give a few examples of the physicsthat can be addressed at the futureB factories. No attempthas been made to be comprehensive. The numbers in thissection are from [5–8] and later conference updates.In general the strategy for the experiments will be to makeseveral measurements that in independent ways test theStandard Model.As an example theβ measurement fromBd → J/ψK0S andthe measurement of|Vtd|/|Vcb| throughBd mixing are bothsensitive to new physics contributions in theBd loop di-agram. On the other hand a measurement ofγ from theBs → D∓s K± decays and of|Vub|/|Vcb| from the branchingfractions ofBd → huℓν andBd → hcX decays only probesprocesses at the tree level and are as such not expected tobe sensitive to new physics.This means we can form astandard triangle from the angleγ and|Vub|/|Vcb| measurements and anew physics trianglefrom β and |Vtd|/|Vcb|. If these two triangles do not sharethe same apex we have a sign of new physics. The principleis illustrated in figure 4.3.1 Bs mixing and the CP angleδγWithin the Standard Model the weak phaseφs in Bs mixingis given by the small value−2δγ ≡ −2λ2η. This means thatnew physics could easily show up as a larger value ofCPviolation in a decay likeBs → J/ψφ which is equivalent tothe Bd → J/ψK0S decay for the measurement of the phase2β in Bd mixing. The good calorimetry of BTeVwill alsoallow a measurement ofφs in the decayBs → J/ψη(′). Theprecision in the angleφs will depend on how fast the os-cillation frequency is forBs mixing but will in general bearound 0.02.Workshop on the CKM Unitarity Triangle, IPPP Durham, April 2003 5VubVcbVtdVts0 1 ρηβγFigure 4. An illustration of how a mismatch is possible betweenmeasurements of the unitarity triangle which are sensitiveto newphysics and measurements which are not.3.2 Extraction of the angleγThe extraction of theCP angleγ is one of the main pur-poses for the hadronicB factories. The decayBs → D∓s K±is sensitive to the angleφs + γ, where theφs part comesfrom Bs mixing and theγ part from the phase ofVub in thetree level decay. If new physics contributes toφs it willbe the same contribution as for the direct measurement ofφs and as such will not interfere with a clean measurementof γ from the tree level decay. The decayBd → D∗±π∓is the equivalent decay forBd but suffers from the prob-lem that one of the interfering decays is doubly-Cabibbo-suppressed with respect to the other; the increased statis-tics in this channel due to the large branching fraction willmore or less cancel the deterioration in sensitivity from thedoubly-Cabibbo-suppressed amplitude leading to a similaroverall sensitivity toγ.The angleγ can also be extracted with high precision froma comparison ofBd → π+pim andBs → K+K− [9]. Thismethod is sensitive to new phases introduced in the pen-guin decays and as such might not measure the StandardModel value ofγ. In table 4 we summarise the statisticalsamples available with one year of data for aγ measure-ment.Decay channel LHCb BTeV Possible new phases?Bs → D∓s K± 8 k 7.5 k NoBd → D∗±π∓ 650 k NoBd → π+π− 27 k 15 k YesBs → K+K− 38 k 19 kTable 4. The expected statistics in one year at design luiminosityfor different channels for the extraction of the angleγ. All havethe potential for an accuracy of 10◦ or better inγ.3.3 Rare decaysThe predicted branching fraction from the Standard Modelfor the Bd → µ+µ− decay is around 10−10 and for theBs → µ+µ− decay around 4× 10−9. As the decays haveto be mediated through a loop there is the possibility thatparticles from new physics will participate and dramati-cally increase the decay rate. Table 5 summarises the ex-pected number of reconstructed events given the StandardModel decay rate. As it can be seen a measurement of theBd → µ+µ− decay will be marginal even at high luminos-ity for CMS and ATLAS as there may well be significantbackground. The background levels are almost impossibleto evaluate from simulations due to the very large rejectionfactors required.Decay channel ATLAS CMS BTeV LHCbBd → µ+µ− 14 4.1 3Bs → µ+µ− 27 21 18 30B → K∗0µ+µ− 2 k 12 k 8 k 13 kTable 5. The expected reconstructed rate for Standard Model pro-duction of rare decays. Statistics are after 3 years at the nominalluminosity for LHCb and BTeV, and 3 years of low luminosity(L = 1033 cm−2s−1) for ATLAS and CMS (exceptBd → µ+µ−which is one year at high luminosity (L = 1034 cm−2s−1) for AT-LAS and CMS).4 SystematicsIn the experiments we need to control everything that cangive a flavour asymmetry and fakeCP violation. There areseveral penitential sources for a flavour asymmetry:• Since LHC is a proton-proton machine the angulardistributions and relative ratios of the different typesof B and B hadrons will be different at the percentlevel which is larger than some of the eff cts we wantto measure.• The tracking efficiency for positive and negative par-ticles will be different due to the magnetic dipolefield (positive and negative particles go through dif-ferent parts of the detector).• Particle identification will be different forK+ andK−due the the difference in nuclear interaction length.• The flavour tagging will be different due to asymme-tries in both the efficiency and mistag rates.All these effects should be measured and corrected fromanalysing the data. Separate control channels should befound for each of the different types of hadrons and careshould be taken that there is no expected directCP violationin the control channels.6 Workshop on the CKM Unitarity Triangle, IPPP Durham, April 20035 ConclusionsTheB factories at hadronic colliders will in the future pro-vide statistics of the order of 1012 bb pairs per year. Asophisticated trigger is required to reduce the backgroundfrom the much larger production of minimum bias eventsand to select the specificB decays of interest.The LHCb and BTeV detectors are optimised to cover awide range of (semi)-leptonic and hadronic decays withhigh efficiency. The ATLAS and CMS experiments will becompetitive in channels with a muon pair in the final statewhich could be of great interest in the detection of rareBdecays.The experiments should together be able to make compre-hensive measurements of theCP violating effects in thequark sector. Hopefully we will from this see that the sin-gleCP violating phase of the Standard Model is no longersufficient to explain all the data and that new phases fromNew Physics are required.6 AcknowledgementsI would like to thank the organisers of the workshop forproviding a good atmosphere for discussions. Also I wouldlike to thank P. Eerola (ATLAS), F. Palla, A. Starodu-mov (CMS) and L. Moroni (BTeV) for providing me withinformation and corrections on the other experiments. Nat-urally any remaining errors are my responsibility.References1. V. Lubicz, Unitarity Triangle Fits in the StandardModel and Sensitivity to New Physics, inWorkshop onthe CKM Unitarity Triangle, IPPP Durham, 2003.2. M. Ferro-Luzzi, The LHCb trigger strategy andperformance, inWorkshop on the CKM UnitarityTriangle, IPPP Durham, 2003.3. L. Moroni, BTeV: Strategies and sensitivities, inWorkshop on the CKM Unitarity Triangle, IPPPDurham, 2003.4. A. Starodumov,B Physics triggers at CMS, inWorkshop on the CKM Unitarity Triangle, IPPPDurham, 2003.5. P. Ballet al., (2000), hep-ph/0003238.6. S. Amatoet al., CERN-LHCC98-004(1998), LHCbcollaboration.7. G. Y. Drobychevet al., BTeV-doc316-v3(2002),BTeV collaboration.8. G. L. Bayatianet al., CERN-LHCC2002-028(2002),CMS collaboration.9. R. Fleischer, Phys. Lett.B459, 306 (1999),hep-ph/9903456.

CKM reach at hadronic colliders

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CKM Reach at Hadronic Colliders

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