Bs physics at LHCb Roger Forty CERN 1

Bs physics at LHCb Roger Forty, CERN 1. The LHCb experiment 2. Bs–Bs oscillation 3. CP violation 4. Rare decays WHEPP 8, IIT Bombay, January 2004

LHCb is a dedicated B physics experiment at the LHC

LHC environment • LHC: pp collisions at s = 14 Te. V • Bunches cross at 40 MHz frequency separated by 25 ns • sinelastic = 80 mb at high L >> 1 pp collision/crossing • Choose to run at ~ 2 × 1032 cm-2 s-1 dominated by single interactions • Makes it simpler to identify B decays from their vertex structure (and reduces radiation dose) • Beams are defocussed locally maintain optimal luminosity even when ATLAS and CMS run at 1034 Inelastic pp collisions/crossing LHCb

B production • B hadrons are mostly produced in the forward direction (along the beam) b–b correlation • Choose a forward spectrometer 10– 300 mrad • Both b and b in the acceptance: important for tagging the production state of the B hadron (PYTHIA)

Typical B event • Need to measure proper time of B decay: t = m. L / pc hence decay length L (typically ~ 1 cm in LHCb) and momentum p from decay products (which have ~ 1– 100 Ge. V) • Also need to tag production state of B: whether it was B or B Use charge of lepton or kaon from decay of the other b hadron

LHCb detector ~ 300 mrad p p 10 mrad Forward spectrometer (running in pp collider mode) Inner acceptance 10 mrad from conical beryllium beam pipe

LHCb detector Vertex locator around the interaction region Silicon strip detector with ~ 30 mm impact-parameter resolution

LHCb detector Tracking system and dipole magnet to measure angles and momenta Dp/p ~ 0. 4 %, mass resolution ~ 14 Me. V (for Bs Ds. K) Magnetic field regularly reversed to reduce experimental systematics

LHCb detector Two RICH detectors for charged hadron identification Provide > 3 s –K separation for 3 < p < 80 Ge. V

LHCb detector e h Calorimeter system to identify electrons, hadrons and neutrals Important for the first level of the trigger

LHCb detector m Muon system to identify muons, also used in first level of trigger Efficiency ~ 95% for pion misidentification rate < 1%

Design of experiment in the underground cavern Current status of magnet construction: Subdetectors also under construction Experiment will be completed ready for first LHC beam in 2007 Typical event (full GEANT simulation)

Vertex locator Constructed from silicon discs perpendicular to the beam axis, with r–f geometry Made in two halves so can be retracted during beam injection Interaction region ~1 m Vacuum vessel

RICH detectors RICH 1 detector Vertex locator RICH 2 uses a CF 4 gas radiator for ID of high momentum tracks

RICH photon detector: HPD 80 mm Test beam: e– separation 1000 pixels Typical event in the RICH 1 photon detectors Performance of particle ID

Trigger • sbb ~ 500 mb, < 1% of inelastic cross-section • Use multi-level trigger to select interesting events: high p. T electrons, muons or hadrons vertex structure and p. T of tracks full reconstruction ~ 200 Hz to tape 30– 60% efficiency

Comparison to other experiments • Enormous production rate at LHCb: ~ 1012 bb pairs per year much higher statistics than the current B factories But more background from non-b events challenging trigger and high energy more primary tracks, tagging more difficult • Expect ~ 200, 000 reconstructed B 0 J/y KS events/year cf current B-factory samples of ~ 2000 events precision on sin 2 b ~ 0. 02 in one year for LHCb (similar to expected world average precision in 2007) • Expect ~ 26, 000 reconstructed B 0 events/year cf ~ 1000 from B-factory by 2007 • But in addition, all b-hadron species are produced: B 0, B+, Bs, Bc , Lb … • Concentrate here on physics of the Bs meson (not produced at B factories) Only competition before LHC is from CDF+D 0 (lower statistics, poorer ID) • ATLAS and CMS will only have lepton trigger, poor hadron identification Direct competition will come from BTe. V, expected at the Tevatron ≥ 2009

B–B oscillation • Neutral meson with flavour eigenstates B 0 (bq), B 0 (bq) has CP eigenstates: B 1 = (B 0 + B 0) / 2 CP(B 1) = +B 1 B 2 = (B 0 B 0) / 2 CP(B 2) = B 2 • Neglecting CP violation, mass eigenstates = CP eigenstates Time development: i d B 0 = M – i G B 0 dt B 0 2 B 0 initially pure B 0 state decays as B 0 (or B 0) at time t with probability: P(t) = e Gt e DGt + e DGt ± cos Dm t 2 2 Dm = mass difference of B 1, B 2 DG = width difference of B 1, B 2 • Hence oscillatory behaviour with frequency Dm:

• In Standard Model, oscillation proceeds via box diagram: • Oscillation frequency Dm very precisely known for the B 0, Dmd = 0. 502 0. 006 ps-1 (= 3. 3 × 10 4 e. V !) but extraction of CKM element Vtd limited by knowledge of hadronic terms in the expression • If the oscillation frequency Dms could be measured for the Bs too then some uncertainty cancels in the ratio: ~ 30 fast oscillations SU(3) breaking term • DG < 1% expected for the B 0, but could be as high as ~ 10% for the Bs G

• Despite heroic effort at LEP + SLD, Dms has not yet been measured (although there is an interesting feature in the amplitude plot at ~ 18 ps-1) • Current world combined limits: Dms > 14. 4 ps-1 at 95% CL DGs < 0. 29 Gs • If CDF+D 0 don’t get there first… best channel for LHCb: Bs Ds + Expect 80, 000 reconstructed signal/year with signal/background ~ 3 • Fully reconstructed decay excellent momentum resolution Decay length resolution ~ 200 mm Proper time resolution ~ 40 fs Ds +

Tagging of production state: efficiency = 55% mistag rate = 30% Reconstructed proper-time shows clear oscillations: (for two values of Dms, with acceptance, resolution, mistag) Error on the amplitude vs Dms can make a 5 s measurement in one year for Dms up to 68 ps-1 (far beyond Standard Model expectation) Once a Bs–Bs oscillation signal is seen, the frequency is precisely determined: s (Dms ) ~ 0. 01 ps-1

CP violation • At the level of precision that will be probed by LHCb, there are two unitarity relations of the CKM matrix that are of interest: Differ at the percent level phase of Vts • Possible situation of the measurements when LHCb starts to take data: measurement of the angle will be crucial

CP asymmetry: Bs J/y f • Bs counterpart of the golden mode B 0 J/y KS • CP asymmetry arises from interference of Bs J/y f and Bs J/y f measures the phase of Bs mixing • In Standard Model expected asymmetry sin 2 c = very small ~ 0. 04 sensitive probe for new physics • Reconstruct J/y m m or e e , f K K 120, 000 signal events/year • Final state is admixture of CP-even and odd contributions angular analysis of decay products required • Define transversity angle qtr : Likelihood is sum of CP-odd and even terms L(t) = R L (t) (1+cos 2 qtr)/2 + (1 R ) L (t) (1 cos 2 qtr) • Fit for sin 2 c, R and DGs/Gs s(sin 2 c) ~ 0. 06, s(DGs/Gs) ~ 0. 02 in one year

CP asymmetry: Bs Ds K+ • Arises from interference between two tree diagrams via Bs mixing: Bs Ds+K and Bs Ds K+ B ~ 20 × 10 5 B ~ 3 × 10 5 • CP asymmetry measures 2 c ( from phase of Vub) c will be determined using Bs J/y f decays extract • Very little theoretical uncertainty Insensitive to new physics, which is expected to appear in loops • Reconstruct using Ds K K 5400 signal events/year

• Bs Ds is background for Ds K Branching ratio ~ 12 higher • Suppress it by cutting on difference in log-likelihood between K and hypotheses in RICH: Remaining contamination only ~ 10% Ds should not have CP asymmetry use it as a control channel eg to measure any possible production asymmetry of Bs and Bs

• Allow for possible strong phase difference D between the two diagrams • Fit two time-dependent asymmetries: Phase of Ds+K asymmetry is D ( 2 c) Phase of Ds K asymmetry is D ( 2 c) can extract both D and ( 2 c) Asymmetries for 5 years of simulated data s( ) ~ 14 in one year

CP asymmetry: B(s) + hh • B 0 originally proposed for measurement of angle a = b But clean extraction of a is compromised by influence of penguin diagrams • Measure time-dependent CP asymmetries for B 0 and Bs K K ACP(t) = Adir cos(Dm t) + Amix sin(Dm t) • Extract four asymmetries: Adir(B 0 ) = f 1(d, q, ) Amix(B 0 ) = f 2(d, q, , b) Adir(Bs K K ) = f 3(d’, q’, ) Amix(Bs K K ) = f 4(d’, q’, , c) deiq = ratio of penguin and tree amplitudes in B 0 d’eiq’ = ratio of penguin and tree amplitudes in Bs K K

• Assume U-spin flavour symmetry (under interchange of d and s quarks) d = d’ and q = q’ [R. Fleischer, PLB 459 (1999) 306] • Four measurements, three unknowns (taking b and c from other channels) can solve for • Plot d vs blue bands from Bs K K (95% CL) red bands from B (95% CL) ellipses are 68% and 95% CL regions ( input = 65 ) • 37, 000 reconstructed Bs K K events s( ) ~ 5 in one year • Theoretical uncertainty from U-spin assumption (can be tested) Sensitive to new physics in the penguin loops “fake” solution

Rare decays: Bs mm • Flavour-changing neutral current strongly suppressed in Standard Model B (Bs m m ) ~ 4 × 10 9 • New physics contributions could increase this significantly excellent place to look for them • eg in SUSY: [G. Kane et al, hep-ph/0310042] • Expect ~ 16 signal events/year for the Standard Model branching ratio ~ 40 background events/year (mostly from b m , b m ) ~ 4 s significance after three years • Here ATLAS and CMS are competitive, due to their higher luminosity LHCb will also study many other rare decays: B 0 K* , K*m m. . .

b s penguin decays • One of the most interesting results from B factories is for B 0 f KS • Standard Model asymmetry = sin 2 b (within ~10% theoretical uncertainty) = 0. 736 ± 0. 049 from B 0 J/y KS • Measured values: +0. 45 ± 0. 43 ± 0. 07 (Ba. Bar) 0. 96 ± 0. 50 ± 0. 10 (Belle) consistent with the Standard Model inconsistent with the Standard Model • Expect to reconstruct ~ 1000 B 0 f KS signal events/year in LHCb • However, if new physics does show up in B 0 f KS it is important to also examine other b s penguin decays: Bs ff, KK, f , … LHCb will also reconstruct large samples of all of these modes

Conclusions • LHCb is dedicated to the study of B physics, with a devoted trigger, excellent vertex and momentum resolution, and particle identification • Construction of the experiment is progressing well It will be ready for first LHC collisions in 2007 • LHCb will give unprecedented statistics for B decays, including access to the Bs meson, unavailable to the B factories • Bs–Bs oscillations will be measured precisely > 5 s for Dms up to 68 ps-1 in one year -1 s (Dms ) ~ 0. 01 ps • Many measurements of rare decays and CP asymmetries will be performed s (sin 2 b) ~ 0. 02 in one year s (sin 2 c) ~ 0. 06 s ( ) ≤ 10 • CP angles determined via channels with different sensitivity to new physics detailed test of the CKM description of the quark sector

Outlook • A possible scenario before the LHCb measurement of :

Outlook • A possible scenario after the LHCb measurement of :

Possible topics Questions that might be of interest to discuss in the workshop: • How will different New Physics scenarios affect the quantities that LHCb will measure? eg MSSM, non-minimal SUSY, or other models… effect on Dmd, Dms, DGs, sin 2 b, sin 2 c, in different channels… • What other channels would be of interest for LHCb to study? • If a large value is measured for B (Bs m m ), how can we be sure that it is a sign of SUSY? • Is the precise measurement of Dms and DGs useful? How can theoretical uncertainties in calculating them be reduced? • How might we improve the production state tagging? in addition to the standard “opposite-side” lepton and kaon tags: same-side tagging, vertex charge, jet charge… • What can be learnt from high statistics of Bc mesons and b-baryons?