Study of exclusive radiative B decays with LHCb Galina Pakhlova, (ITEP, Moscow) for LHCb collaboration Advanced Study Institute “Physics at LHC”, LHC Praha-2003, B Physics Day, 11/07/2003

2 Physics motivation  Constraint on CKM matrix elements |V td /V ts | from the ratios of decay rates          s      s    Search for New Physics  Direct CP violation A CP < 1% in SM (tiny u quark contribution in loop) up to ~ 10% in SM extensions            s      Mixing-induced CP violation ~0 in SM (different  polarizations in   and    decays ) up to ~ 50% in SM extensions   s         with         e.g.       s      (Dalitz plot  analysis of   s     to extract flavour blind   s   )  Exotics, super rare decays   (s)   l + l - ,   (s)  , etc  Test of QCD      n  b    etc bs(d)  W t(c,u)

3 Experimental overview  The first exclusive radiative  penguin      was observed by CLEO in 1993                  Belle                      Upper limit BABAR    barion   Upper limit CLEO Statistics at B-factories is still not sufficient for stringent test of SM in CP violation in radiative decays and for observation of b  d       (4.3 ± 0.4)×10 -5 PDG 2003      (3.8 ± 0.5)×10 -5 PDG 2003  CP    ± 0.07 PDG 2003       (1.3 ± 0.5)×10 -5 PDG 2003

4    decay vertex primaryvertex  decay distance Impact Parameter (IP)  N b hadrons    year  all types of b hadrons   bb /  inel ~    high charged and neutral multiplicity  few primary vertices  event at LHCb

5 Challenge  Electromagnetic penguins are indeed rare B decays  inclusive Br  b  s    exclusive Br are ~ smaller  b  d  are further suppressed by |V td /V ts | 2  f  b   S  f(  b     Background sources  Huge combinatorial background from generic b  b events  Minimum bias events   X    decays     separation using different shower shape  for X=V polarization can be exploited

6 Search for CP violation  Direct CP violation  no tag and lifetime analysis are required  Systematic errors are of great importance: fake (non-CP) asymmetries  different  and  production rates in pp interactions  detector asymmetries Hopefully can be calibrated with other final states e.g.    J/    Mixing-induced CP violation  need to tag  flavour  proper time resolution is critical especially for   S  due to fast oscillations ( x s >19) to be resolved

7 Can we measure radiative decays at LHCb           Direct CP  Energetic   huge soft  and   background can be effectively removed dedicated trigger selects high E T  photons at Level 0  two charged tracks     vertex reconstruction    S      s decay & mixing-induced CP  small    narrow  cut; small  yield in  decays:  background conditions are more favourable  extremely small opening angle        problem with   S vertex (proper time) resolution               b  d   small    narrow  cut   problem with combinatorial background from    in progress (not presented here)

8      reconstruction Select        combinations  Tracks are consistent with      hypothesis  Reject     tracks from all primary vertices (   IP >16)      produce secondary vertex (        PDG    MeV/c 2       PDG    MeV/c 2  E T  GeV (close to L0 trigger requirement)   E* T  GeV  Select primary vertex with minimum IP of   candidate  Require   momentum and flight direction to be consistent  (    mrad)    helicity angle: in   rest frame |cos (p B, p K )|<0.7

9 Background suppression (1) No   IP cut optimized   IP  > 16 SBSBSBSB  comb  Require large IP of      to all reconstructed primary vertex (   IP  > 16)  suppress primary tracks  especially effective background suppression in events with multiple interactions

10 Background suppression (2) Transverse energy E* T  in    rest frame  vs E T  in the lab frame E T  GeV E T  GeV powerful against low energy  and    E* T  GeV  E* T  GeV rejects also soft   candidates E T , GeV E* T  GeV      E* T  GeV bb inclusive E T , GeV

11 Background suppression (3)    reconstructed   momentum reconstructed   decay vertex reconstructed primary vertex Angle  between    momentum Angle  between    momentum and   flight direction and   flight direction  should be 0 for real    candidates (smeared by  vertex resolution)  randomly distributed for combinatorial background One of the most powerful cut!  signal ~ 60% ;  bb < 1%

12   suppression Significant suppression of fast   is expected after application of special algorithm for    separation based on shower shape (to be implemented soon) (to be implemented soon) Contribution from        is small and can be further suppressed exploiting    polarization :    helicity states 0       0                     helicity: in    rest frame cos (p B, p K ) in    rest frame cos (p B, p K )

13      : signal and background summary      : signal and background summary ~ 4 min LHCb 0 events in ( ) GeV/c 2 mass window after trigger and off-line selection with all available MC statistics ~ 10.3 M events ~ 10.3 M events ~ 54 hours LHCb    MeV/c 2 In blue:  contribution from       after trigger and off-line cuts  of      Br          PDG 2003

14      Annual yield and CP sensitivity Efficiency, [%] N year   CP  Reconstruction & acceptance selectiontriggertotal K < 0.01 BACKGROUND / SIGNAL < 90 % CL Assuming: Assuming: Br      = ( 4.3  0.4 ) × Br      = ( 4.3  0.4 ) × f  b     = 0.39 f  b     = 0.39 N      N      N      N       CP   N      N     

15   S  reconstruction Two different tasks Two different tasks  Branching ratio measurement similar to      reconstruction:  Tracks are consistent with      hypothesis  Reject     tracks from all primary vertices (   IP > 4)      produce secondary Vertex (           PDG  MeV/c 2  E T  GeV (close to L0 trigger requirements)   E* T  GeV  Select primary vertex with minimum IP of   S candidate    S momentum  and  flight direction to be consistent      mrad ( worse   S vertex resolution)    helicity : in  rest frame |cos(p B,p K )| < 0.7  Search for mixing-induced CP: selection to be re-optimized selection to be re-optimized

16   s  background suppression      narrow  <  MeV/c 2 Background is mainly due to real  Fortunately small  yield from  decays.         After L0xL1 trigger & off-line cuts bb inclusive: 0 event in mass window  GeV/c 2 from ~ 10.3 M events ( ~ 4 min LHCb)   s     s   assuming  Br        Br   s     

17   s  after trigger and off-line cuts   s  after trigger and off-line cuts Efficiency [%] N year reconstr. & accept. selectiontriggertotal K BACKGROUND / SIGNAL < % CL limited by MC statistics, expected to be better Assuming: Assuming: Br   s   = ( 4.3  0.4 ) × Br   s   = ( 4.3  0.4 ) × f  b    S  = 0.10 f  b    S  = 0.10  MeV/c 2 ~ 87 hours LHCb

18 Search for  mixing induced CP in   S  simple vertex fit    fs     fs     fs  “direction” vertex fit    fs     fs     fs    S  proper time resolution Main problem: Proper time resolution is critical:  dominated by poor  vertex resolution  kaons are almost collinear  no extra  information on   s vertex from  Improvement can be achieved:  Constrain the   s flight direction to   s momentum: (“direction” vertex fit) Improve vertex resolution Improve vertex resolution by factor > 2.5 by factor > 2.5  Select kinematical region with the better vertex (proper time) resolution: (slow  larger  opening angle)

19   s  lifetime resolution cuts re-optimization for CP violation study Achieved lifetime resolution is close to those for charged modes. Estimation of CP violation sensitivities is in progress Require: cos   , E T  GeV, tighter cut No cut E* T    S  proper time resolution, ps cos     S  proper time resolution, ps Decay angle   (between  momentum  in rest frame of   s and   s flight direction) is convenient variable to select the kinematical region that provides the better  proper time resolution  fs  fs

20 Conclusion            record statistical sensitivity in direct CP violation  (A CP )< 0.01 for one year LHCb. Stringent test of SM extensions!  systematic errors to be studied carefully   S       precise measurement of the branching fraction with ~2% accuracy for one year LHCb  For indirect CP violation   S proper time resolution is critical  advanced fit vertex procedure and dedicated cut optimization allow to hope on reasonable sensitivity Others channels of interest               |V td /V ts |       direct CP violation        s    mixing-induced CP violation           why not  under study