Long-Lived Particles
BND Summer School 2019
Prof. Steven Lowette Vrije Universiteit Brussel – IIHE [email protected] http://cern.ch/lowette/ @StevenLowette About these lectures
Ingredients
● the standard model
● several scenarios beyond the standard model
● particle detection techniques and modern detectors
● bias towards LHC, a current big player in long-lived particle searches
Content
● 1st lecture: phenomenology of long-lived particles and towards an experimental approach
● 2nd lecture: experimental handles, current status and new proposals
Caveats
● first time I teach on this subject :)
● very diverse subjects come together here → ask questions if any!
stop me if you get lost
Steven Lowette – Vrije Universiteit Brussel BND School 2019 – Long-Lived Particles Page 2 Long-Lived Particles
Content
● some exotics
● pheno wrap up
● building an experimental story
detector handles
unusual backgrounds ● experimental status
● future proposals
Steven Lowette – Vrije Universiteit Brussel BND School 2019 – Long-Lived Particles Page 3 Some exotics: monopoles see: PDG
● magentic monopoles are predicted since long (Dirac '31)
to explain charge quantization
to make Maxwell equations symmetric ● they have also been searched since long
nothing found so far
most recently LHC used to extend searches (MoEDAL, ATLAS data, CMS beampipe) ● very peculiar phenomenology
magnetic charge is e / (2 αEM) = 68.5 e
thus extremely ionizing
easily stopped by material, typically in electromagnetic calorimeter but not much of a shower → very narrow energy deposit strongly bent by a magnetic field
moreover, at ATLAS/CMS it bends in an unxpected direction! along z axis → all our tracking algorithms fail
Steven Lowette – Vrije Universiteit Brussel BND School 2019 – Long-Lived Particles Page 4 Some exotics: SIMPs JHEP 11 (2015) 108
● SIMP = strongly interacting massive particle
Strongly
→ high interaction cross section [but not SU(3)C]
Interacting → new light mediator; repulsive
Massive → constitutes dark matter (component)
Particle → fermionic asymmetric dark matter ● SIMP phenomenology
deposits momentum in the calorimeter
but no hadronization, more like a neutron
results in a pair of neutral jets ● also some hidden valley scenarios lead to neutral jets
potentially with missing energy
Steven Lowette – Vrije Universiteit Brussel BND School 2019 – Long-Lived Particles Page 5 Some exotics: true muonium
arxiv:1904.08458 ● true muonium (μ+μ-) remains elusive
very clean test of QED ; potential BSM physics 3 ● expected properties of a S1 (ortho-TM) state
m ~ 210MeV
cτ ~ .5mm with large boost
● phenomenology
predominantly produced in η → (μ+μ-)γ decays (BR~10-9)
decays to e+e- though material interactions would dissociate muons soft signature → low-momentum electrons
large background
Steven Lowette – Vrije Universiteit Brussel BND School 2019 – Long-Lived Particles Page 6 Some exotics: sexaquarks arXiv:1708.08951v2 [hep-ph] ● proposal for a bound state S(uuddss)
double weak decay if
absolutely stable if ● S is special
spin-0, flavour singlet, CP even, Q=0, B=2, S=-2
very compact object, almost De Broglie wavelength ● phenomenology
relatively abundant production in collisions
expected interaction rate with material very small
it will look like a soft nuclear interaction
can look inclusively for events with |ΔB|=2, |ΔS|=2 eg. in upsilon decays with lambda's or in low-momentum pp collisions can attempt to reconstruct the S 0 interaction on neutron gives KS and Λ pointing to material very soft signature, no trigger
Steven Lowette – Vrije Universiteit Brussel BND School 2019 – Long-Lived Particles Page 7 Some exotics: quirks
● a quirk is like a heavy stable charged particle
● produced in pairs, they are bound by dark gluons
like QCD, but quirks don't hadronize
leads to macroscopic oscillations of quirk pair ● phenomenology
tracks bent in non-B directions
rapid oscillation will look like a straight highly ionizing track
very slow oscillation will look like a usual HSCP
detector-size oscillation will lead to tracking algo failure need dedicated algorithms
see eg: Phys.Rev.D 96 (2017) 115015
Steven Lowette – Vrije Universiteit Brussel BND School 2019 – Long-Lived Particles Page 8 Pheno wrap up
● particles can be long-lived due to variety of effects
narrow phase space, small coupling, and mass barrier as main lifetime drivers
weak force gives us many observable long-lived particles already
● long-lived particles arise in multitude of contexts
many proposals for problems of the SM model have versions that involve longlived particles
also generic portal models for dark sectors often involve long-lived particles when decays back to SM happen
regularly very exotic proposals appear
often similar signatures arise in different contexts ← models are not easy guidelines
● take-away message
long-lived particle phenomenology is very rich
lifetimes are very sensitive to model parameters ● also: long-lived particles were not invented because we observed no BSM physics yet at the LHC
they have always been there, but have become more visible now that mainstream searches have only been placing limit
Steven Lowette – Vrije Universiteit Brussel BND School 2019 – Long-Lived Particles Page 9 Building an experimental story
● so the question then is: how to approach this richness experimentally
● signatures are very diverse...
Steven Lowette – Vrije Universiteit Brussel BND School 2019 – Long-Lived Particles Page 10 Building an experimental story
● and interplay between very different subdetectors
● No one-size-fits-all approach – decay products, lifetime, mass, boost: all dramatically affect the detector signature
...and sometimes all subdetectors must be combined for optimal results
Steven Lowette – Vrije Universiteit Brussel BND School 2019 – Long-Lived Particles Page 11 Experimental strategy
● lacking a clear top-down driving paradigm, we must work bottom-up
● we need to take an as experimental position as possible
pheno models identify a final state as interesting
then a search for this final state is built around the experimental signature rather than the model details use the detector's capabilities as the basic driver of the analysis keep the pheno model as an inspiration
● using the fact that SM long-lived particles give typically very different features than the signal sought, long-lived particle searches often manage to suppress the background to a negligible level
being statistically limited is great for discovery
good for analysis robustness
● the challenges are numerous though...
Steven Lowette – Vrije Universiteit Brussel BND School 2019 – Long-Lived Particles Page 12 Experimental challenges
● non-standard simulation
sometimes special care in event generation
GEANT challenges for R hadrons, SIMPs, quirks,... ● non-standard reconstruction
timing info
secondary vertices
displaced jets and leptons
dE/dx
veto on material interactions
... ● non-standard triggers
analysis specific, but common challenges
opportunities in scouting and parking ● non-standard backgrounds
cosmics, beam halo, spikes, noise,...
missing hits from dynamic inefficiency, broken modules,...
rare hadron decays and/or resolution tails
Steven Lowette – Vrije Universiteit Brussel BND School 2019 – Long-Lived Particles Page 13 Know your detector
● to keep it focused, I will not discuss signal simulation or trigger pecularities
very technical and very analysis specific
● I'll focus on a few low-level detector aspects
to demonstrate the potential power of rarely used techniques
to show one of the fun things of long-lived searches: detector knowledge is power ● then I'll demonstrate some of the unusual backgrounds
detector related
accelerator related
algorithmic
● this is heavily CMS (and own experience) biased, but it applies broadly
each analysis will have its own challenges
Steven Lowette – Vrije Universiteit Brussel BND School 2019 – Long-Lived Particles Page 14 Using ionization energy
● ionization energy loss is decribed by the Bethe-Bloch relation
Steven Lowette – Vrije Universiteit Brussel BND School 2019 – Long-Lived Particles Page 15 Using ionization energy
● our tracking detectors measure energy deposits from MIPs with a very good signal-over-noise
usually we just use the information that a hit occurred at a certain position to seed the tracking algo
Steven Lowette – Vrije Universiteit Brussel BND School 2019 – Long-Lived Particles Page 16 Using ionization energy
● but we have more: we measure the ionization energy
within a certain dynamic range
current CMS tracker has an analogue readout
dE/dx can be estimated also taking into account track inclination
at low momentum we can estimate the mass
Steven Lowette – Vrije Universiteit Brussel BND School 2019 – Long-Lived Particles Page 17 Using ionization energy
● effect of mass – effect of charge
● dEdx can be used to
select massive charged particles ionizing the detector
select both high-charge or low-charge particles
Steven Lowette – Vrije Universiteit Brussel BND School 2019 – Long-Lived Particles Page 18 Using timing
● excellent timing of ECAL signals
● 200ps (300ps) timing resolution in barrel (endcap) for typical Z energies
applications in delayed photons and jets
JINST 5 (2010) T03011 CMS-DP-2014/011
Steven Lowette – Vrije Universiteit Brussel BND School 2019 – Long-Lived Particles Page 19 Using timing
● also muon systems typically have good timing capabilities
muon algorithms often use that in their ID variables ● long-lived particles may need reconstructions that take into account out-of-time signals
Steven Lowette – Vrije Universiteit Brussel BND School 2019 – Long-Lived Particles Page 20 Using material veto
● background from photon conversions and nuclear interactions in tracker layers can profit strongly from a material veto
veto secondary vertices that coincide with material location ● must be obtained with a dedicated data measurement
using nuclear interactions
using conversions to muons: lower statistics but cleaner ● mostly useful in inner regions
Steven Lowette – Vrije Universiteit Brussel BND School 2019 – Long-Lived Particles Page 21 Background from beam halo
● beam halo are muons travelling along with the LHC beam
but parallel, at potentially large radius
created on the LHC collimators ● since they are in sync with the beam, their timing is off in reconstruction
● time distribution for energy deposits in the CMS ECAL
Steven Lowette – Vrije Universiteit Brussel BND School 2019 – Long-Lived Particles Page 22 Background from cosmic muons
● cosmic muons come in continuously, so their timing is random
● when severely out of time, some hits may be lost
could lead to missing tracker track, while muon or calo deposit is there ● typically angular variables are used to suppress this background
using the fact that you see two muons/tracks that belong to the same track really
● this must be the only background for which larger luminosity helps
only integrated data-taking time counts
Steven Lowette – Vrije Universiteit Brussel BND School 2019 – Long-Lived Particles Page 23 Background from sattelite bunches
● LHC bunching each 25ns is not perfect
RF has 2.5ns buckets ● by selecting out-of-time ECAL hits, you may pick up real physics from nearby RF buckets
suppression is analysis dependent Phys. Lett. B 797 (2019) 134876
in this case uses tracking
Steven Lowette – Vrije Universiteit Brussel BND School 2019 – Long-Lived Particles Page 24 Background from wrong vertex
● in standard CMS jets, we remove all charged hadrons not associated to the primary vertex
this helps significantly in reducing pileup effects ● suppose you are looking for neutral jets as a signal
if you choose a wrong primary vertex, then tracks from real QCD jets will be removed, making them neutral
such a wrong choice has only a very small probability
but the QCD cross section is huge ● must use non-standard reconstruction to suppress this
Steven Lowette – Vrije Universiteit Brussel BND School 2019 – Long-Lived Particles Page 25 Background from geometry
● in a search for fractionally charged particles, one selects tracks with many low dE/dx hits
● edges of silicon sensors don't collect all charge → low dE/dx
● tracker geometry such that presence of edge hit implies increased probability of additional edge hit(s)
this leads to increased background
suppress by rejecting tracks with hits on edges
Steven Lowette – Vrije Universiteit Brussel BND School 2019 – Long-Lived Particles Page 26 Background from radiation
● dE/dx very sensitive PXL L1 PXL L2 PXL L3 to radiation damage
● fake low dE/dx
● high particle fluxes also induce dynamic inefficiencies
some expected
some problematic
mitigation not always possible ● tracks may look detached!
Steven Lowette – Vrije Universiteit Brussel BND School 2019 – Long-Lived Particles Page 27 Background from trigger misfiring
● when timing of a L1 trigger object (“primitive”) is not sufficiently accurate, it may be associated to a wrong bunch crossing
● the trigger system may then read out the wrong bunch crossing
● tracking detectors read out in a very narrow timing window
while muon integrates in broad window ● so the event read out in the adjacent bunch crossing will look have muons, but no tracks in the tracker
a fake long-lived event!
Steven Lowette – Vrije Universiteit Brussel BND School 2019 – Long-Lived Particles Page 28 Backgrounds...
● I showed you several examples, but for sure this is not an exhaustive list
● none of these are regular physics backgrounds
● none of these can be modeled reliably in simulation
must be estimated from data ● all of these force you to take a step back from usual objects and usual tools
often they touch upon low-level detector or algorithm details
● hard, but fun!
Steven Lowette – Vrije Universiteit Brussel BND School 2019 – Long-Lived Particles Page 29 Experimental status
● the experimental and phenomenological variety leads to very diverse analyses
● only showing CMS and ATLAS
also LHCb, NA62, Belle(2)/Babar, HPS, APEX,...
Steven Lowette – Vrije Universiteit Brussel BND School 2019 – Long-Lived Particles Page 30 Experimental status
Steven Lowette – Vrije Universiteit Brussel BND School 2019 – Long-Lived Particles Page 31 Experimental status
● most experiments are conceived for prompt signals
● but where is the border between prompt and long-lived?
we can probe displacement up to experimental resolution, up to below 100um
but we also have excellent performance on b-jet identification, where B hadrons can fly millimeters before decaying ● indeed, prompt searches extend sensitivity also in long-lived territory
Steven Lowette – Vrije Universiteit Brussel BND School 2019 – Long-Lived Particles Page 32 Build your own detector
● how to build a dedicated long-lived particle detector?
● competing requirements
be as close as possible to the interaction point
cover as much solid angle as possible
instrument as deep as possible
shield as much as possible from regular particles
make it as cheap as possible
● sometimes better to focus on single purpose...
Steven Lowette – Vrije Universiteit Brussel BND School 2019 – Long-Lived Particles Page 33 Example proposal: milliQan
● proposal to search for millicharged particles
like predicted in massless dark photon models
● -3 goal: uncovered phase space at mass below mZ/2 and charge down to 10
● aim of the game: suppress backgrounds
arXiv:1607.04669 [physics.ins-det]
Steven Lowette – Vrije Universiteit Brussel BND School 2019 – Long-Lived Particles Page 34 Example proposal: milliQan
● proposal to search for millicharged particles
like predicted in massless dark photon models
● -3 goal: uncovered phase space at mass below mZ/2 and charge down to 10
● aim of the game: suppress backgrounds
● need much more sensitive detection technique
with charge goal down to 10-3, ionization loss suppressed by 10-6
counting of single photons in “large” scintillator volume
● need to go to a low-background area
out of the CMS cavern, to suppress radiation backgrounds
still shielded from cosmic muons by ~100m overburden
multiple coincidence to suppress PM dark rate coincidence
● stay relatively close to the interaction point
r2 dependence of flux
Steven Lowette – Vrije Universiteit Brussel BND School 2019 – Long-Lived Particles Page 35 Example proposal: milliQan
cost: 2 MEUR
Steven Lowette – Vrije Universiteit Brussel BND School 2019 – Long-Lived Particles Page 36 Other proposals
FASER
● search for light dark photons
● almost parallel to the beam, placed in unused side tunnel
shielding from tunnel walls ● up to 5m x 10cm instrumented with leftover spare detector parts
● under construction!
Steven Lowette – Vrije Universiteit Brussel BND School 2019 – Long-Lived Particles Page 37 Other proposals
CODEX-b
● dark photons, light scalars, dark glueballs,…
● 6 tracking layers on all faces of a large decay “box”
10x10x10m3
Steven Lowette – Vrije Universiteit Brussel BND School 2019 – Long-Lived Particles Page 38 Other proposals
MATHUSLA
● huge decay volume surrounded by tracking layers
quite expensive... ● envisaged on the surface above CMS
● search for long-lived particles flying upwards
first proposal to have sensitivity up to BBN scale (0.1s)
Steven Lowette – Vrije Universiteit Brussel BND School 2019 – Long-Lived Particles Page 39 Other proposals
AL3X
● ALICE physics program will not run until end of HL-LHC
● reuse L3 magnet and ALICE TPC
move IP 11.25m (1.5BX), put big shield in front ● run 5?
Steven Lowette – Vrije Universiteit Brussel BND School 2019 – Long-Lived Particles Page 40 Summary
Long-lived particles are fun!
● interesting, rich phenomenology
low couplings, mass barriers, phase space, and other factors yield long lifetimes
very diverse signatures in detectors ● low-level detector effects
usage of mostly unused detection techniques
dealing with unusual backgrounds ● above ~1GeV LHC is currently dominant
high-risk high-gain
new proposals of dedicated experiments
Steven Lowette – Vrije Universiteit Brussel BND School 2019 – Long-Lived Particles Page 41