1. The PADME experiment Seminar – Mar. 14th, 2017 Paolo Valente
The PADME Collaboration is:
INFN Roma, INFN Frascati, INFN Lecce
MTA Atomki Debrecen, University of Sofia, Cornell University
© Lucasfilm Ltd.
Seminar – Mar. 14th, 2017
Paolo Valente
– INFN Roma

2. Introduction and outline
A (short) physics preamble
The requirements for the PADME experiment and beam
The status of the experiment construction
The program of physics runs
Some idea for the future 1
In less than 1 hour
Mar. 14th, 2017
Paolo Valente

3. The dark matter problem
Original drawing by Stacy McGaugh (1995)
Astrophysics
Particle physics
Particle physics is not the only possible solution:
Modify gravity
Several hypothesized solutions
Roots are the empirical observations 2
Mar. 14th, 2017
Paolo Valente

4. Why a hidden sector?
Without modifying the SM structure: U(1)Y+SU(2)L+SU(3)C
Dark matter can’t be strong interacting (scattering cross section too high)
Cannot be electrically charged, otherwise it would not be dark!
It can be weakly interacting and massive
The WIMP has all the characteristics needed to solve the dark matter problem
Strong constraints from the LHC and direct searches at masses up to 1TeV
One notable exception: the DAMA-LIBRA experiment
What about introducing a new force?
A mediator particle, very weekly interacting with SM particles, connecting the dark matter sector
It can be light… where direct detection gets into trouble
mc=100 GeV,gc=0.6, Wc = 0.1
Standard Model
SU(3)×SU(2)×U(1)
portal
Dark Sector ?
Vector (dark photon) 3
Scalar (dark Higgs)
Fermion (sterile or heavy neutrino)
Axion or axion-like particles
Mar. 14th, 2017
Paolo Valente

5. Dark photon
The simplest hidden sector model just introduces one extra U(1) gauge symmetry and a corresponding gauge boson: the “dark photon" or U boson or heavy photon (g’ or A’)
An extra U(1) symmetry implied in many Standard Model extensions, some classes of string theory, etc.
Two types of interactions with SM particles should be considered
1. As in QED, generates interactions of the type:
Not all the SM particles need to be charged under this new symmetry
In the most general case qf is different in between leptons and quarks and can even be 0 for quarks [P. Fayet, Phys. Lett. B 675, 267 (2009).]
2. Couples to SM hypercharge through kinetic mixing operator, acquiring a (small) SM charge:
½ eFYmnF ’mn ; F ’mn=∂mA’n
Am  Am + eam ; a’ = e2a
Dark Sector
Z’?
Dark fermions c ? y ? A’ ?
h’?
Standard Model g A’ ee e 4
Mar. 14th, 2017
Paolo Valente

6. A dark matter messenger
radiopure NaI (Tl)
Nuclear recoil by the exchange of a dark photon
Independent of c mass value
Dark Matter scattering on nuclei A’ 5
Dark Matter annihilation…
… naturally lepto-philic
Mar. 14th, 2017
Paolo Valente

7. 6 Muon g-2 SM discrepancy A strong, but not the only motivation
g-2 in the Standard Model A’
3s Discrepancy between theory and experiment (3.6s, if taking into account only e+ e-  hadrons) Additional diagram with dark photon exchange can fix the discrepancy (with sub GeV A’ masses)
Contribution to g-2 from dark photon 6
A strong, but not the only motivation
Mar. 14th, 2017
Paolo Valente

8. Status of dark photon searches
“Visible” final states ( A’  l+l- )
A’-strahlung:
e- dumps
thin target: bump hunt, displaced vertices
e+e-  A’g
p0,h  A’g _
“Invisible” final states (A’  cc )
A’-strahlung:
Missing energy
e+e-  A’g , A’  cc
Mono-photon events in e+e- colliders
Fixed-target annihilations 7
Mar. 14th, 2017
Paolo Valente

9. 8 A fifth force? 8Be 18.2 1+ 17.6 1+ 3.0 2+ 0+ Mar. 14th, 2017
T=0 Ep= 1030 keV
Excitation with the
7Li(p,γ)8Be reaction
17.6 1+
T=1 Ep= 441 keV
De-excitation via
Internal pair creation
3.0 2+ 0+ 8
8Be
The highest possible radiative transitions
Mar. 14th, 2017
Paolo Valente

10. 9 A 16.6 MeV boson? Mar. 14th, 2017 Paolo Valente
Electron spectrometer
MWPC + scintillators
On resonance
Off resonance
Ep=1.04 MeV
Ep=1.10 MeV
Ep=0.8 MeV 9
Mar. 14th, 2017
Paolo Valente

11. Proto-phobic vector boson
6.8s excess, interpred as a new (vector is favored) boson
Not compatible with present limits: too high coupling unless…
A. Krasznahorkay et al., “Observation of Anomalous Internal Pair Creation in 8Be: A Possible Indication of a Light, Neutral Boson”,
Phys. Rev. Lett. 116, (2016)
J. Feng et al., “Protophobic Fifth Force Interpretation of the Observed Anomaly in 8Be Nuclear Transitions”,
Phys. Rev. Lett. 117, (2016)
8Be 10
Mar. 14th, 2017
Paolo Valente

12. More on dark sectors
Dark Sectors, SLAC, Apr. 2016
A comprehensive review can be found in the final report of “Dark Sectors ”:
J. Alexander et al., arXiv: [hep-ph] 11
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Paolo Valente

13. PADME proposal
B. Wojtsekhowski proposed to use e+ e- annihilations for searching the dark photon in the missing mass spectrum
The idea was to use the circulating e+ beam in a storage ring, striking an internal target and detecting the recoil photon
I. Rachek is leading the effort for realizing this at VEPP-3
Alternatively, use a positron beam onto an external target
M. Raggi, V. Kozhuharov and P. Valente:
Exploit the Frascati beam-test facility (BTF) extracted positrons
Use a thin, solid target
Preliminary design of the experiment and sensitivity estimate in 2014, experiment approval Sep. 2015, funding
Messina workshop, Oct. 2016 12
Adv. High Energy Phys. 2014:959802
Mar. 14th, 2017
Paolo Valente

14. 13 The PADME concept The missing mass technique has two key elements:
The positron beam: we should know
The positron annihilation point
The positron momentum vector
The detector: we should measure
The photon momentum vector
(0 , pg)
(me , 0)
(me , pe+)
A third basic element (relating beam and detector):
Which target material & thickness vs. beam intensity 13
Mar. 14th, 2017
Paolo Valente

15. The PADME concept
Which target material & thickness vs. beam intensity
Material choice
Low Z: annihilation/Bremsstrahlung ratio gets worse with 1/Z
H2 (or He) gas calls for a long target due to low density (impacting on the annihilation position resolution)
Pure Li, B thin targets not practical
Our choice is Carbon, in the form of polycrystalline detector
Thickness
Given the A’ production cross section, the compromise is luminosity vs. pile-up in the photon detector
Cross section also depends on the beam energy as well as A’ mass 14
Mar. 14th, 2017
Paolo Valente

16. Positron beam intensity vs. quality
Beam requirements referring 100 μm Carbon target, 550 MeV e+
Highest possible repetition rate
49 Hz, with dedicated LINAC
From missing mass resolution:
Momentum spread <1%
Divergence <1 mrad
Spot <1 mm (σ)
From pile-up probability vs. luminosity:
5000 e+/40 ns (limited by LINAC gun pulsing system, see further)
A longer pulse duration allows increasing the number of positrons/pulse, and also requires dedicated LINAC (see further)
>1013 eot
Positron extracted in the BTF beam-line, can be produced either in the LINAC (positron converter) or striking primary electrons onto the BTF target
Good beam quality
(already extracting e+ beam in air) 15
Mar. 14th, 2017
Paolo Valente

17. 16 DAΦNE and BTF LINAC BTF 220 MeV e- 550 MeV e+ 750 MeV e-
Damping
Ring
BTF
transfer line
Positron converter
220 MeV e-
550 MeV e+
transfer line
LINAC
750 MeV e-
BTF target
transfer line
10 m 16
To the DAΦNE
main rings
Mar. 14th, 2017
Paolo Valente

18. BTF upgrade & PADME: layout17

19. LINAC pulse width extension
Injection for DAΦNE operation requires short pulses (<10 ns)
Power supply generating pulses to the gun in the 1.5–40 ns range
New pulser for LINAC gun
Extends to the range ns
Installed and commissioned, operational since September 2016
But the electron current from the thermo-ionic gun has to be bunched and accelerated in the LINAC sections
The limiting factor in the accelerated beam pulse length comes from the falling off of the accelerating voltage due to the RF power compression (SLED device) used for reaching higher gradients 18
Mar. 14th, 2017
Paolo Valente

20. 19 LINAC RF Mar. 14th, 2017 Paolo Valente
Radio-frequency power is flat at the klystron output (over 4.5 μs)
SLED compresses the RF into a sharp (and approximately ×4 higher) peak
Optimize buncher, focussing, RF power & phases of the four stations, and timing, to compensate head-tail effects with longer pulses 19
Mar. 14th, 2017
Paolo Valente

21. LINAC pulse width extension
BCM
BCM
BCM
BCM
250 ns
Still optimization work to be done
Expect to run at 160 ns (at least) in 2018 20
Mar. 14th, 2017
Paolo Valente

22. 21 The diamond target Mar. 14th, 2017 Paolo Valente
Given the number of positrons, mm of Carbon
Given the beam spot size and shape, at least cm2 area
20×20 mm2, 50 μm detector tested on beam
16 strips per side, 1 mm pitch is the baseline design
Resolution adequate for monitoring beam spot
Digitized strips signals 21
Center of gravity
Average position
Mar. 14th, 2017
Paolo Valente

23. 22 The diamond target Readout board Vacuum vessel test Mar. 14th, 2017
Both graphite and metallic strips targets ready
Both 50 and 100 μm samples available
Readout electronics ready
Mechanics and motorized system ready (vacuum tests on-going) 22
Mar. 14th, 2017
Paolo Valente

24. 23 Magnet MBP-S series, on loan from CERN
Adjustable gap by adding/removing iron insets
Many thanks to TE-MSC-MNC, R. Lopez, D. Tommasini
Shipped to Frascati in Dec. 2015
Magnetic field mapping completed
Pole
Coils
Pole 23
Mar. 14th, 2017
Paolo Valente

25. Angular coverage vs. missing mass resolutiond1 d2
The target position determines the angular acceptance, given the magnet gap (fixed to the maximum, h=230 mm)
Target not too inside the magnet poles, due to the fringe field in the coils region
Calorimeter lateral dimensions depend on the distance from the target:
distance  diameter  cost
BTF experimental hall size limits the maximum distance to 4-5 m
On the other side, given the granularity, the calorimeter distance cannot be too small in order not to spoil the missing mass resolution 24
Calorimeter material choice is crucial
Mar. 14th, 2017
Paolo Valente

26. Fighting the background
Backgrounds: one photon + something else (not a dark photon...) going undetected:
recoil γ
recoil γ
recoil γ e+ e+ e+ e+ e- γ e- γ γ
Bremsstrahlung ≈Z2
γγ process, ≈Z
3γ process, ≈Z
Veto detectors (see further), for detecting the irradiating positron
Hole in the center of the calorimeter (very high rate at small angles)
Signature: >1 cluster in the calorimeter
Try to keep the fraction of lost photons as low as possible (below few per-mils)
Angular coverage of calorimeter at least of Δθ=100 mrad (taking into account edges for building at least a 3×3 cluster) 25
Mar. 14th, 2017
Paolo Valente

27. 26 Calorimeter g e+ The two basic requirements of the calorimeter:
1. Measure Eγ and θγ
Good energy resolution: 1-2%/√E[GeV])
High Photo-statistics
Containment
Good angular resolution: ≈1 mrad
2. Fight pile-up
Sub-ns timing resolution g
Moliére radius  granularity
Moliére radius  material
Material  radiation length  volume  cost
Material  light yield  energy and time resolution
(+ photo-sensor) e+
Target
Light yield + compactness practically limit the choice to:
LYSO (RM=2.07 cm)
BGO (RM=2.23 cm) 26
Mar. 14th, 2017
Paolo Valente

28. 27 Calorimeter Not less than 1.5M, but… Our compromise:
Assume 20×20 mm2 cells
At least 200 mm length
Our compromise:
Target to magnet iron = 200 mm
Maximum angle = 83 mrad
Target-calorimeter distance = 3 m
Calorimeter diameter
3000* cells = 560 mm
Inner hole = 5×5 cells
Total of 616 crystals (LYSO or BGO) = 50k cm3
LYSO
BGO
Not less than 1.5M, but…
Thanks to L3 collaboration, prof. S. Ting and INFN management we had access to L3 BGO crystals (one end-cap) 27
INFN Rome team during the construction of the L3 calorimeter
Mar. 14th, 2017
Paolo Valente

29. 28 Calorimeter status Mar. 14th, 2017 HZC XP1911 19 mm PMT
5×5 prototype: energy resolution 2.3% at 1 GeV
650×, tender assigned
HV supply: tender assigned
Crystals preparation completed
Extracted from L3 barrel
High-temperature annealing
Traveling to Italy on March 23th
Machining, gluing and painting: tender assigned
Mechanical structure and assembly procedure being finalized 28
Waveform digitizers: tender assigned
1 – 5 GS/s
896 channels
Calorimeter + Veto detectors + Target
Mar. 14th, 2017

30. 29 Veto detectors Time resolution better than 500 ps
Momentum resolution of few % based on impact position
Efficiency better than 99.5% for MIPs
Low energy part inside the magnet gap
High energy part close to not interacting beam 29
Mar. 14th, 2017
Paolo Valente

31. 30 Veto detectors 10×10x180 mm3 scintillator
All scintillator bars delivered
Design of the mechanics ready
Prototype of the assembly ready
Prototype electronics prototype ready
Test-beam in April
Read-out by same digitizing system as calorimeter 30
Mar. 14th, 2017
Paolo Valente

32. 31 Vacuum vessel Mar. 14th, 2017 Paolo Valente Low energy e+ veto
High energy e+ veto
Low energy
e+ veto
Not interacting e+ window
Secondary vacuum (not UHV)
Design dictated by:
Magnet constraints
Calorimeter distance
Veto detectors positioning
Target region also acting as interface to the beam-line
Target region 31
Mar. 14th, 2017
Paolo Valente

33. Background: Bremsstrahlung
The dominant background: tag with e+ veto
The scintillating bars e+ veto does not work for very soft Bremsstrahlung
Too close to the beam spot
Too high rate per single bunch
A high-resolution detector in the beam spot can improve the rejection of residual background with M2miss ≈ Ebeam
Spot enlarged by beam energy spread and divergence:
3×10 cm2 active area (<104 e+/cm2)
Silicon pixels with Timepix readout under evaluation 32
Mar. 14th, 2017
Paolo Valente

34. High energy positron/beam detector
Monte Carlo simulation:
2xN array of TimePix (Silicon pixel sensors+readout) in vacuum
Directly placed in the beam (5000 particles in 40ns)
Single bunch in TimePix array simulation
Average 1 e+/bunch/fired pixel
Expect very precise measurement of Ne+ 33
Mar. 14th, 2017
Paolo Valente

35. Background: 2 and 3 photons
The probability of losing 1 or 2 photons (given the first cluster) critically depends on the cluster definition and calorimeter angular acceptance
Recoil γ definition:
10 MeV < E < 400 MeV
30 mrad < θ < 65 mrad
ΥΥ events
1+2
2+3
1+2+3
Lost
Region 1: θ < 20 mrad
Region 2: 20 < θ < 75 mrad
Where is the cluster without and with recoil photon selection
Region 3: 75 < θ < 100 mrad
Lost: θ >100 mrad
3Υ events
1+2
2+3
1+2+3
Lost
Need for a veto detector covering the calorimeter hole:
Fast (pile-up translates in over-veto)
Moderate energy resolution 34
Mar. 14th, 2017
Paolo Valente

36. 35 Small angle detector Mar. 14th, 2017 Paolo Valente First tests:
Hamamatsu
R9880U-110
First tests:
SF-57 lead-glass bar, 20×20×200 mm3, n=1.8467
Wrapped in Teflon no optical coupling
Hamamatsu R9880U-110, operated at 950V (gain~1.5x106)
Readout with CAEN V1742 digitizer set to 5 GS/s
Interesting alternative:
PbF2: extended transparency at shorter wave- lenghts (higher Cherenkov yield)
150 ns long pulses
700 ps signal width
2.5 ns double peak separation
0.1 p.e./MeV 35
Final choice of PMT according to crystal size:
49 cells 2×2 cm2 or 25 cells 3×3 cm2
Hamamatsu
R UV
Mar. 14th, 2017
Paolo Valente

37. Beam pulse time structure
BCM4
SF-57 + R9880-U110 36
Mar. 14th, 2017
Paolo Valente

38. Background summary
e+N→e+Ng
e+e-→ gg(g)
Pile up
Pile up contribution is important but can be efficiently rejected by cuts on maximum cluster energy and M2Miss
Veto inefficiency at high missing mass when pe+≃ pe+beam
Additional positron veto detector can help rejecting residual background
Rejection of 2 and 3 photons backgrounds depend on the cluster definition and on the topology cuts
The main driver of the angular coverage choice 37
Mar. 14th, 2017
Paolo Valente

39. 38 Sensitivity Candidate Signal selection (simple and robust cuts)
Just one cluster in the calorimeter
30 mrad < θCl < 65 mrad
Emin(MA’) < ECl < Emax(MA’)
No track in the positron veto within ±2 ns
No cluster in small angle detector with E>50 MeV within ±2ns
Missing mass in the region: MA’ ± σMmiss(MA’)
Monte Carlo simulation extrapolated to 1x1013 positrons on target
Using N(A’g)=s(NBkg)
With very simple cuts, far from no-background sensitivity (improvements are possible)
Cross-section enhancement due to A’ mass 38
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Paolo Valente

40. Hand-shaking with DAFNE collisions
Running program
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
Procurement
Construction
Installation
2017
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
Commissioning
Physics Run 1
DAFNE collisions
Run 1
2018
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
Hand-shaking with DAFNE collisions
2019 39
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Paolo Valente
Galactic Standard Calendar

41. Limits on ALPs coupling to photons
Axion-like particles
Limits on ALPs coupling to photons
Primakoff
PADME can search for long-living ALPs by looking for 1 g + M2miss final states
In the visible final state agg
all production mechanisms can be exploited, extending the mass range in the region of ≈100MeV
The observables at PADME will be: egg or ggg
g-2
arXiv: v2
Bremsstrahlung
arXiv:
ALP decay to photons
ALP contribution to (g-2) +
Annihilation 40
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Paolo Valente

42. Background to ALPs searches
Invariant gg mass for all events collected by calorimeter (2×1010 e+ events) with two in-time clusters
Even without any selection cut PADME will be background free for masses > 50MeV
Main background e+e-→gg, e+e-→gg(g) has a kinematic limit at Mgg =24 MeV
Background at higher masses is due to overlapping photons from different Bremsstrahlung interactions.
Can be suppressed by using the veto detector 41
Mar. 14th, 2017
Paolo Valente

43. 42 Some disordered idea With practically the same configuration:
Scan different energies
If an effect is found, positron beam can be swept across the production threshold
Search for the proto-phobic boson at threshold
Used different targets
Optimize production with respect to target thickness and material
E.g.: jet-target with H2
Search for visible decays
Use electron and positron veto detectors inside the magnetic field as spectrometer
Studies of expected performance (resolution, background rejection, etc.) needed
Try to use the same dataset as invisible search
Running with e- beam:
Check systematics:
Reverse the magnetic field, same configuration with opposite charge, to cross-check backgrounds
Search for ALPS (egg and ggg final states)
Run in dump mode
Thick target, high Z
Look for visible decays 42
Mar. 14th, 2017
Paolo Valente

44. 43 Some disordered idea With some modification to the LINAC:
Run at threshold for proto-phobic boson production: 17 MeV ≅ √(2me×280 MeV)
Frascati LINAC possibly used without SLED compression: in principle up to 4 ms long positron pulses: up to 20× increase in statistics
Needs some modification to PADME readout and DAQ
Building a new beam-line:
Use DAFNE positron ring as a beam stretcher
Get up to 0.4 ms long pulses at 510 MeV, up to 2000× increase in statistics
Needs an extraction septum + magnetic line
Probably needs a completely new readout and DAQ system 43
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Paolo Valente

45. Outlook
A compelling physics case, but with a definite window of opportunity
Proposal in 2014, INFN approval in 2015
Since then, quickly progressing
Official start of physics run: April 2018
Almost all components available or ordered
Construction to be completed by the Fall 2017
Installation by the end of the year
Aim at covering the (g-2)m band in 6 months data taking
Estimating sensitivity for other physics cases:
Fifth force and proto-phobic X boson
ALPs
More running in 2019 and beyond desirable for:
Improving sensitivity
Extending the physics program
More ideas for longer term future 44
Mar. 14th, 2017
Paolo Valente

46. Collaboratori cercasi
Si promette molto lavoro …
… ma il divertimento è assicurato!
Ulteriori informazioni:
Mar. 14th, 2017
Paolo Valente