Search of the X17 Boson @ n_ToF Carlo Gustavino, INFN Roma

Outline: Introduction X17 search @ ATOMKI X17 search @ n_ToF Possible Setup Conclusion Introduction

A significant anomaly has been recently observed in the emission of electron-positron pairs in the 7Li(p,e+e-)8Be and 3H(p,e+e-)4He reactions.

Krasznahorkay, A.J.; et al.: "Observation of Anomalous Internal Pair Creation in 8Be: A Possible Indication of a Light, Neutral Boson". Physical Review Letters. 116 (42501): 042501 (2016). Krasznahorkay, A.J.; et al.: "New evidence supporting the existence of the hypothetic X17 particle". arXiv:1910.10459v1 [nucl-ex] (23 October 2019).

Reaction MX17(MeV) DMX17_STAT DMX17_SYST Statistical evidence 7Li(p,e+e-)8Be 16,70 0,35 0,50 >5 sigma 3H(p,e+e-)4He 16,84 0,16 0,20 >7,2 sigma v This anomaly has been interpreted as the signature of a Jp=1+ particle (hereafter X17) not foreseen in the standard model of particle physics. v X17 boson could be a mediator of a fifth force, characterized by a strong coupling suppression of protons compared to neutrons. v This evidence/scenario is presently not confirmed or excluded by other experiments or groups. 3H(p,e+e-)4He reaction @ ATOMKI

0- G + 21.1 Ep=1.7 MeV (0 )=0.5 MeV + G - E =0.9 MeV 0 (0 )=0.84 MeV p 20.2 3H + p Q=19.8 MeV Ep=0.9 MeV E0+=0.53 MeV E0-=1.8 MeV 0 0+ 3H(p,e+e-)4He setup @ ATOMKI

Plastic scintillator Carbon fiber tube v 3H adsorbed on Ti layer v 6 plastic scintillator 82x86x80 mm3 v 6 double-sided silicon strip detector (3 mm wide strips, 0.5 mm thick) Target e- v 1 mm thick carbon fiber tube Silicon strips + v Ep=0.9 MeV e Similar setup for the p(7Li,e+e-)8Be reaction X17 @ nToF

Basic idea: Exploiting the conjugate reaction:

3H(p,e+e-)4He 3He(n,e+e-)4He

0- 21.1 En=0.7 MeV G(0-)=0.5 MeV 3 + He + n 0 + 20.2 G(0 )=0.84 MeV 3H + p

Q=20.57 MeV

En=0- 2 MeV E0+=negative 0 0+ E0-=0.7 MeV Possibile setup

E.M. calorimeter (e.g. Scintillator tiles with SiPM)

TPC with MPGD e- (GEM/µMegas/RWELL) n-beam He3 target

e+

TPC: tracks and vertex recognition e.m. calorimeter: T0 for the TPC, energy measurement TPC+calorimeter: background rejection Target

DEFAULT target: commercial 3He tubes Diameter: 1 inch (25.4 mm) Pressure: 30 bar Thickness: 500 µm Material: 344 stainless steel Lenght: 10-100 cm X0= 0.029 Cost for a 20 cm long tube: 10 kEuro Tracker TPC with MPGD readout (GEM/µM/RWELL)

15-20 cm 15-20 cm

ED= 0.2÷0.5 kV/cm

ED 10 cm 15 cm ED

5 cm He3 target Gas drift volumes Ar/CO2/CF4 Ar/CO2 Central annular MPGD anular end plate cathode

Thikcness (um) X0 (cm) % X0 Cu 3 1,43 0,021 kapton 50 28,6 0,017 glue 30 33,5 0,009 MILLIFOAM/honeycomb 3000 1312,5 0,023 glue 50 33,5 0,015

Cylindrical Supports kapton 30 28,6 0,010 0,096 Layering of inner/outer cylinder including field cage (in Cu) Field cage (inner/outer) cylinders ~0.1 radiation lenghts Tracker TPC MPGD readout (GEM/µM/RWELL)

15-20 cm 15-20 cm

ED 10 cm 15 cm ED

He3 target

MPGD anular end plate

Each quadrant could be divided in n.128 r-strips (0,78 mm pitch – max length ~ 15 cm) and n.640 (128-192-320) φ-strips with a pitch of 12 – 8 – 5 mrad (roughly 0,58 – 0,62 – 0,53 mm). Globally each quadrant will be readout with 768 (analog) electronic channels 768x4=3072 channel. MPGD END PLATES

The most common MPGD as readout options:

• triple-GEM • MicroMegas • µ-RWELL

never used as TPC readout. ALICE TPC KEK TPC foreseen as Cylindrical- RWELL for CREMLIN-plus

Micromegas looks the best solution (known technology and limited ion feedback) E.M. Calorimeter

In progress: Scionix ej200 tiles with SiPM readout

e.m calorimeter+ SiPM readout

TPC+µMegas end plate s ~ 1,64 mm Target Signal/background

v The e+e pairs due to X17 have a unique signature., i.e. they produce Preliminar! e+e-pairs with Etot~9+9 MeV at large angle (~120°). Target lenght (cm) v The irreducible background is due to IPC. As the hungarian group did, 20 the signal is the net area of the IPC peak at large angle. diameter (cm) 2,54 v The 18 MeV shower of the 3He(n,g)4He reaction can be easily identified (single shower of ~18 MeV, no TPC tracks) Pressure (bar) 30 v Cosmic ray background (muon) can be measured with off spill measurements. rate p (s-1) 1,3E+06 v Gamma flash are in advance (T>1300 ns if En<1 MeV) and well rate gamma (s-1) 13 identified by the calorimeter and the TPC. v Protons due to (n,p) reaction have at very low energy (Q=764 keV) rate IPC (day-1) 66,3 and cannot cross the 500 µm steel tube for Ep<15 MeV. v Rate of X17 events is probably low àlong measuring time and R&D to increase the number of events. Conclusion (1)

A spectacular anomaly has been recently observed in the emission of electron-positron pairs in the 7Li(p,e+e-)8Be and 3H(p,e+e-)4He reaction. This anomaly has been interpreted as the signature of a particle (X17 boson) not foreseen in the standard model of particle physics. The X17 boson could be a mediator of a fifth force, characterized by a strong coupling suppression of protons compared to neutrons. This scenario may be related to Dark Matter and to the 3.6 ss anomaly of muon magnetic moment. A new measurement to confirm (or reject) the claimed is mandatory. New informations on X17 coupling are necessary to define a theoretical framework. The proposed experiment at n_ToF is a study of X17 by an indipendent collaboration, using an improved setup and exploiting a new nuclear process. The goal is to measure the parameter of X17 boson (mass, parity, life time..) and to shed light on the protophobic nature of fifth force (neutron induced reaction). Standard 3He(n,e+e-)4He and 3He(n,g)4He reactions are also of great interest in nuclear physics, providing an important experimental footing for "ab initio" calculations. Conclusion (2)

The proposed setup (large acceptance TPC and calorimeter) allows the identification of e+e- pairs at large relative angle and a background rejection better than the ATOMKI group. Long data taking and specific R&D (e.g. target density) is needed to increase statistics. N.B. The calorimeter and the TPC are very well suited also for other experiment at n_ToF. Concerning X17, a positive result of the 3He(n,e+e-)4He study could stimulate a successive study of the 7Be(n,e+e-)8Be, with the same setup.

TENTATIVE ROADMAP • February 2021: TDR of TPC and Calorimeter • May 2021: LoI to INTC requiring a beam test • October 2021: protoypes and beam test • December 2021: Submission of a proposal to INTC • May-june 2022: 3He(n,e+e-)4He measurements at n_ToF

Grazie per l'attenzione! SPARES REAZIONE 7Li(p,e+e-)8Be + 18.2 1 Ep=1030 keV 17.6 1+ Ep=441 keV 7Be + p

3.0 2+

0 0+

NIM, A808, 21 (2016) FEE & DAQ (tentative estimate)

• 768 chs/quadrant (n.128 r-strips + n.640 φ-strips) à n.6 APV25 • 3072 chs/end-plate à n.24 APV25 • 6144 chs/TPC à n.48 APV25 (n.3 x (8 master + 8 slave)) ……….. 7152 CHF • n.3 ADC …………………………………………………………………………………….. 3708 CHF • n.3 FEC …………………………………………………………………………………….. 4785 CHF • n.1 SRS-crate ……………………………………………………………………………. 809 CHF • Connectors, cables etc ……………………………………………………………... 3000 CHF

TOT 19.454 CHF L'elettronica ha una profondità di 5x192= 5 µs à drift max=20 cm

Costo Totale elettronica 40.000 CHF TPC (tentative estimate) Dear Carlo

I've made an estimate (+/- 30%) for the production of:

-Item 1 -->1 x Anode Micromegas detector : -30 cm active area with -Resistive protection -Radial and angle strips STIMA COSTI (+/- 30%, chiavi in mano) -3072 Channels -24 panasonic connectors or equivalent Costo totale (mezza camera) 21.000 CHF

-Item 2-->1 x TPC field cage : -20cm high Costo Totale (camera intera) 36.000 CHF -one Inner tube 10cm diameter -one Outer tube 30cm diameter -Glued frames at both ends for gas tight assembly with o-rings -2 resistors chain per tube -No shielding -Single layer strips on 0.2mm Flex glass/epoxy tube

-item3-->1 x Cathode circuit : -30cm active area

Item1: Design cost between 3000 to 6000 CHF (depending on symmetries we can find to simplify the layout) Tooling costs: 2500 CHF Detector production : 6000 CHF Item2: Design cost: 1000 CHF Tooling costs: 1500 CHF Field cage production : 4500 CHF Item 3: Design cost : 300CHF Tooling costs: 300 CHF Cathode production : 800 CHF

These 3 parts can be assembled with s simple screw driver. CALORIMETRO ELETTROMAGNETICO

-Bacchette EJ200 6x6x500 mm: 65 Euro l'una (minimo 26 bacchette) SCIONIX.PREVENTIVI ONLINE SiPM ADVANSID (Bias~30 Volts, 3x3mm) 21 Euro l'una (se ne compro 100). Esistono anche 4x4 A parità di prestazioni, il costo dello scintillatore scala come il quadrato del raggio interno del calorimetro à è molto importante il raggio esterno della TPC. PRIVILEGIARE UN RIVELATORE COMPATTO! Aumenta il background off beam? MPGD END PLATES Micromegas better than GeM (no ion feedback) or R-wells (too "young" technology)

KEK TPC