Search for A’ from p0  A’ g T. Spadaro M. Mirra G. Rovelli

Goal of the present analysis Detect dark photons, feebly coupled to SM g via: L = e Amn Fmn Produce dark photons from the decay chain: K+  p+p0, p0  A’ g Assume invisible decay of the A’ (or extremely long-lived A’) Signature: one photon, missing mass peaking around the A’ mass Data sample: 4 runs of period A (6610, 6614, 6625, 6670) Reconstruction revision r1421 MC sample: private production, 11 mass bins from 30 to 130 MeV Reconstruction revision r1421 + physics-list bug fix 06/04/17 Exotic physics meeting - CERN

Status of A’ invisible searches Recent results: NA64, Babar; in future, expect results from Belle/Belle-II 06/04/17 Exotic physics meeting - CERN

Selection algorithm: “p0 tag” pnn L0 trigger used for signal search control trigger used to normalize signal counts to the p0 produced One-track-selection One good quality downstream track connected to CHOD candidate, 15 < p < 35 GeV Association to 1 and only 1 upstream track from GTK: |TGTK – TEvent(RICH)| < 400 ps Upstream and downstream tracks approach each other to better than 5 mm Point of closest approach: 115 < Zvertex < 165 m, to reduce early-decay effect Kaon momentum cut: 72 < pK < 78 GeV Pion positive identification: RICH: single ring association !MUV3: no hit connected in time and space wrt expected impact LKr, MUV1, MUV2 combined info: R. Aliberti’s tool, P(p) > 0.8 06/04/17 Exotic physics meeting - CERN

Selection algorithm: “p0 tag” Cut on the squared missing mass 0.013 < (PK - Pπ)2 < 0.023 GeV2 (PK - Pπ)2 (GeV2) Total number of events selected with control trigger: 518444 -> considering the downscale factor this leads to ~207 × 106 p0 06/04/17 Exotic physics meeting - CERN

Selection algorithm: “one g” One-photon selection: No signal from the LAV, SAC and IRC systems must be present with a time consistent with the TEvent(RICH) (standard tools) Exactly one standard cluster with E > 1.5 GeV and -5 ns < TLKr – TEvent(RICH) < 10 ns Missing momentum pmiss = pK – pp - pg must point to LKr Cut on additional veto: No activity in the LKr corresponding to auxiliary clusters of total energy above 1 GeV around the expected missing momentum extrapolated point; No RICH hits in time in addition to pion track hits Isolation: distance photon-expected vs photon found > 20 cm Remain with 166518 events from PNN trigger corresponding to a single photon inefficiency ~ 8 × 10-4 06/04/17 Exotic physics meeting - CERN

Data control trigger: 2 γ on LKr, simulate 1g loss Expected signal Search for signal in missing mass windows, from pmiss = pK – pπ - pγ Mmiss2 = Emiss2 – pmiss2 Data control trigger: 2 γ on LKr, simulate 1g loss MC: - mA’ = 30 MeV - mA’ = 60 MeV - mA’ = 90 MeV Mmiss2 (GeV2) 06/04/17 Exotic physics meeting - CERN

Exotic physics meeting - CERN Master formula Determine number of signal events: Correct for one-photon selection (esel) and trigger (etrg) efficiencies Count number of signal events (nsig) in a mass-dependent Mmiss2 window Account for mass-window acceptance (emass) Normalize to number of p0 counts from control trigger, downscale corrected Assumptions under above formula: Acceptance for single-track selection exactly cancels (that must be true) Assume control trigger efficiency for one-track events = 1 Assume nominal downscale when correcting (control trigger “re-start” to be checked) To evaluate e, account for phase space factor ~(p*)3 = (1 – mA’2/mp02)3 06/04/17 Exotic physics meeting - CERN

Efficiency corrections: one-g selection Efficiency to detect one g cluster with missing momentum towards LKr Drop at 130 MeV, understood from kinematics consideration General behavior reproduced using toy-MC simulation Multiple energy scales because of: g in LKr acceptance + isolation wrt p cluster + missing momentum pointing to LKr Include 12% inefficiency from IRC, SAC, LKr random veto losses Efficiency from MC MA’ (MeV) 06/04/17 Exotic physics meeting - CERN

Efficiency corrections: LKr veto Require no additional LKr cluster from auxiliary reconstruction: |dt| < 8 ns, distance to the expected g impact point < 40 cm, Ematch > 1 GeV Reject 53% of remaining background (events with |M2miss| < 0.001 GeV2), dominated by loss of low-energy clusters 2% random veto evaluated from control trigger + pp0 selection with p0  gg Fraction of matched events Fake veto probability Standard reconstruction Present default Auxiliary reconstruction Expected (GeV) Eaux threshold (GeV) 06/04/17 Exotic physics meeting - CERN

Efficiency corrections: vetoes Require no additional hit from RICH in time with the event Effect: reject 5.5% of the residual background (events with |M2miss| < 0.001 GeV2) Random loss at 1% checked with data From MC, inefficiency 0.5% 1.5% for MA’ = 30 MeV  130 MeV In time RICH data hits to within 800 ps distance to p ring center normalized to p ring radius 06/04/17 Exotic physics meeting - CERN

Efficiency corrections: Isolation cut To avoid LKr-energy sharing issues, require > 20 cm distance between g and expected impact from pmiss to LKr: Extremely effective for background in the negative tail (~ x10 wrt peak) Pay a price in terms of efficiency, particularly at high A’ masses (as expected) Fraction of rejected events Efficiency from MC Mmiss2 (GeV2) MA’ (MeV) 06/04/17 Exotic physics meeting - CERN

Efficiency corrections: trigger Trigger efficiency for signal obtained by reweighting MC kinematics: 1. Use data spectrum for pion energy release in LKr in momentum bins 2. Use probability from data for a photon to fire a newCHOD sector in energy bins 3. Combine g and p to simulate QX or !QX condition, apply L0 efficiency from data Fraction of events with pnn trigger bit Particles in !QX 0-g sample: two g’s on LAV 1-g sample: 1-g on LAV, !LAV12, 1g on LKr 2-g sample: !LAV, 2g on LKr, exclude condition with g’s in QX For a given total energy, efficiency depends on number of particles in the LKr Particles in QX Total energy in the LKr (MeV) 06/04/17 Exotic physics meeting - CERN

Efficiency corrections: trigger Linear dependence on the A’ mass: Higher A’ masses, lower photon energy  higher efficiency for LKr L0 Positive bias from LKr-L0 malfunctioning important at low A’ masses Efficiency from MC (reweighted w data input) MA’ (MeV) 06/04/17 Exotic physics meeting - CERN

Exotic physics meeting - CERN Expected background Evaluate expected background from negative Mmiss2 side band Assume purely symmetric resolution effects Tested using data from control trigger with p0  gg: looks OK  Use data for bkg estimate: same trigger, detector, and beam conditions  Statistics of background subtraction same order of data Data Symmetrized data Data – symmetrized data Mmiss2 (GeV2) Mmiss2 (GeV2) 06/04/17 Exotic physics meeting - CERN

Signal determination: the concept To extract signal use “CLS method”: Count data and background expected in 1-s windows around each A’ mass hypothesis Statistics: Poisson(data | signal + background) / Poisson( data | background) Vary the signal strength, evalute 90% CL coverage using toy-MC integration To evaluate compatibility of limit wrt expected fluctuations: “Expected” limit: assume no signal, vary data observed in toy-MC, evaluate limit spread Example, MA’ = 90 MeV: Count in 0.007 < Mmiss2 < 0.0095 GeV2 12 observed in data 8 background expected CLS = probability that signal is > S Limit found Median expected limit 1-s limit band 2-s limit band 90% CL coverage Signal S 06/04/17 Exotic physics meeting - CERN

Signal determination: results Limits observed are statistically compatible with fluctuations from the background-only hypothesis e Limit if Nobs = Nbkg Limit found 1-s limit band 2-s limit band MA’ (MeV) 06/04/17 Exotic physics meeting - CERN

Upper limit determination NA62 limits in an interesting region (4 runs only used) NA62 06/04/17 Exotic physics meeting - CERN

Exotic physics meeting - CERN To-do list Write extended documentation including all analysis details Checking master formula assumptions can be synergic with pnn using newly produced MC from dedicated data-based studies Check other background sources using newly produced MC do not expect relevant contribution Go for publication approval 06/04/17 Exotic physics meeting - CERN