The large-angle photon veto detector for the P326 experiment at CERN R. Fantechi INFN – Sezione di Pisa on behalf of the P326/NA62 Collaboration Villa Olmo, October 8 th, 2007

2 R. Fantechi - 8/10/2007 Outline –The P326 experiment –Requirements for the photon veto –R&D on photon veto technology –Results from the test beam –Conclusions

3 R. Fantechi - 8/10/2007 The P326 experiment –Measurement of the BR(K +  + ) SM prediction is (8.0 ± 1.1)× Very clean theoretically: only short distance contributions, hadronic matrix elements could be derived from experimental data (K +  0 e +  Independent constraint  on the CKM triangle Sensitive and clean probe for new physics up to a scale of 100 TeV –Proposal submitted at CERN in 2005 R&D started immediately Hope to be approved in 2008 Construction Run in 2011 and after Located in the same hall of NA48 CERN-SPSC SPSC-P-326 CERN, Dubna, Ferrara, Florence, Frascati, Mainz, Merced, Moscow, Naples, Perugia, Protvino, Pisa, Rome, Saclay, San Luis Potosi, Sofia, Triumf, Turin

4 R. Fantechi - 8/10/2007 The technique Collect about 80 events in two years with a S/B ratio of 10 Use missing mass and particle identification to cut the background Put stringent requirements on the performance of the detectors Focus here on the  +  0 rejection

5 R. Fantechi - 8/10/2007 Photon veto requirements –High beam momentum is helpful Accepting  + with momentum less than 35 GeV gives more than 40 GeV photon energy to be detected –Need for at least rejection on  0 To be combined with missing mass cut to have total rejection factor Low energy (large angle) photons correlated with high energy (small angle ones) Wide coverage needed: 50 mr hermetic, some large angle blind zones Have different devices for different angles –Small angle calorimeters to cover the forward region (high photon energy) –The NA48 LKr calorimeter for up to 15 mrad –12 rings of large angle photon vetoes »Inefficiency for low energy photons should be good This talk

6 R. Fantechi - 8/10/2007 Large angle photon veto –It is required to have inefficiency better than above 200 Mev to be able to achieve the expected rejection factor The main limit comes anyway from the blind zones –R&D and measurements on three technologies: Lead/scintillator sandwich with WLS readout (i.e. CKM proposal) Lead/scintillating fibers as in the KLOE calorimeter Lead glass detectors used in the OPAL experiment –The R&D program Construction of a lead/scintillating fiber prototype Loan of the CKM lead/scintillator proto from Fermilab Procurement of few OPAL lead glass modules Extensive tests at the Frascati Beam Test Facility (BTF)

7 R. Fantechi - 8/10/2007 The fiber prototype KLOE-type lead/scintillating fiber calorimeter 1-mm diameter scintillating fibers 0.5-mm thick lead foils, grooved to house fibers Full-scale prototype of smallest rings 1/3 of final radial thickness Inner/outer radius: 60 cm cm Inner/outer length: cm cm Readout granularity: cells, 4.2 x 4.2 cm cm (4 cells)8.2 cm (2 cells) ~8 X 0 ~9 X 0 3 cells All fibers (same as KLOE)Fibers + 1-mm Pb wires Readout from two sides to have  measurement Hamamatsu R6427 photomultipliers

8 R. Fantechi - 8/10/2007

9 The CKM prototype Two sectors, 80 layers, 1 mm lead/5 mm scintillator WLS readout, EMI 9954B photomultipliers On loan from Fermilab Tested by the CKM Collaboration at the Jefferson Lab –Measured inefficiency for electrons: 3·10 -6 at 1 GeV

10 R. Fantechi - 8/10/2007 The OPAL lead glasses

11 R. Fantechi - 8/10/2007 BTF setup Prototypes tested at the Beam Test Facility in Frascati The BTF can provide single electrons and positrons at 50Hz rate with an energy 100 MeV< E e <750 MeV It may also provide tagged photons Several beam periods allocated to us during the first half of 2007 Hard work to understand the background and to define and build an efficient trigger system Readout Qdc 0.25 pC/ch Gate 250 ns Tdc 35 ps/ch

12 R. Fantechi - 8/10/2007 BTF Setup – The tagger Beam 90 cm Calorimeter 2 profilometers: 16 strips (each 3 fibers x 4 layers) 1 mm diameter fu and fd: 2 “fingers” 60x85x10 mm fbu: 200x130x10 mm slab with a circular hole 14 mm diameter fbd: 330x100x10 mm slab with a circular hole 14 mm diameter All four mounted on the same support and well aligned TAGG = 2 fingers ON and 2 holes OFF Tagg loose = 2 fingers ON and downstream hole OFF fu fd fbu fbd

13 R. Fantechi - 8/10/2007 The effect of the tagger

14 R. Fantechi - 8/10/2007 Collected statistics in June-July 2007

Results Still preliminary: analysis is being refined

16 R. Fantechi - 8/10/2007 Fiber prototype linearity Calibration procedure fixes position of 1e peak run by run Linearity studied by fit to multiplicity distribution: 4 Gaussians for 1-4e peaks + 2 extra peaks for background Reconstructed energy (MeV) Side A Side B ▲Side A+B Nominal energy (MeV) Energy deviation (%) Nominal energy (MeV)

17 R. Fantechi - 8/10/2007 Fiber prototype energy resolution Resolution from fits to 1e peaks for fully tagged events vs nominal energy Simple Gaussian fit extending for 1  gives reasonable  2 Reconstructed energy (MeV) Nominal energy (MeV) 200 MeV350 MeV500 MeV Energy resolution (%) Fits for side A+B

18 R. Fantechi - 8/10/2007 Fiber prototype efficiency, 200 MeV Assuming no mistag (1  ) = 11.5  4.8  3.4  10  5 No cut 1e in fingers 1e in finger& Hole veto N(events<50MeV) = 8

19 R. Fantechi - 8/10/2007 Fiber prototype efficiency, 350 MeV Assuming no mistag (1  ) = 2.3  1.3  0.8  10  5 No cut 1e in fingers 1e in finger& Hole veto N(events<50MeV) = 5

20 R. Fantechi - 8/10/2007 Fiber prototype efficiency, 500 MeV Assuming no mistag (1  ) = 7.2  5.6  3.1  10  6 No cut 1e in fingers 1e in finger& Hole veto N(events<50MeV) = 3

21 Data on CKM calorimeter: events 200 MeV 2 events below 50 MeV No cut 1e in fingers 1e in finger& Hole veto Uncalibrated Energy (MeV) Assuming no mistag (1  ) = 3.1  3.1  1.5  10  5 CKM prototype efficiency, 200 MeV

22 Data on CKM calorimeter: events 350 MeV 3 events below 50 MeV No cut 1e in fingers 1e in finger& Hole veto Uncalibrated Energy (MeV) CKM prototype efficiency, 350 MeV Assuming no mistag (1  ) = 1.4  1.0  0.6  10  5

23 Data on CKM calorimeter: events 500 MeV* 1 event below 50 MeV No cut 1e in fingers 1e in finger& Hole veto Uncalibrated Energy (MeV) CKM prototype efficiency, 500 MeV Assuming no mistag (1  ) = 5.2  0.8  3.2  10  6 cfr: CKM measurement 3·10 -6 at 1 GeV IEEE TNS 51 (2004) 2201

24 R. Fantechi - 8/10/2007 Lead glass efficiency – 200 & 500 MeV 200 MeV electrons 500 MeV electrons N(events<50MeV) = 3N(events<50MeV) = 1 Assuming no mistag (1  ) = 12.0  9.2  5.2  10  5 Assuming no mistag (1  ) = 1.1  1.8  0.7  10  5

25 R. Fantechi - 8/10/2007 Inefficiency results - Summary 1 2

26 R. Fantechi - 8/10/2007 Conclusions –The P326 experiment needs a background rejection of To reject  0 from K 2   an average inefficiency for the  0 of better than is needed Photon vetoes should be as much hermetic as possible –The Collaboration has studied and compared three technologies Lead/fiber, lead/scintillator with WLS readout and lead glass One prototype of the lead/fiber type has been built One existing lead/scintillator from Fermilab has been loaned Lead glass modules from Opal has been taken –Extensive tests with electrons has been performed at the Beam Test Facility in Frascati –The preliminary results indicate that the differences between the three technologies are small –The measured inefficiencies are well below the requirements for the P326 Photon Veto system