Comments on the First Year (2010) of the LHC
description
Transcript of Comments on the First Year (2010) of the LHC
![Page 1: Comments on the First Year (2010) of the LHC](https://reader035.fdocuments.us/reader035/viewer/2022062812/568163e6550346895dd54aec/html5/thumbnails/1.jpg)
Comments on the First Year (2010) of the LHC
Matthew StrasslerRutgers University
Work in progress(special thank you to M. Cacciari, G. Salam)
![Page 2: Comments on the First Year (2010) of the LHC](https://reader035.fdocuments.us/reader035/viewer/2022062812/568163e6550346895dd54aec/html5/thumbnails/2.jpg)
Sooooooo Good
![Page 3: Comments on the First Year (2010) of the LHC](https://reader035.fdocuments.us/reader035/viewer/2022062812/568163e6550346895dd54aec/html5/thumbnails/3.jpg)
The Fast StartSo far, LHC going well
• Original expectation (2007-2008)– Collect a lot of data (10 – 30 inv pb)– Tune up, calibrate the detectors (few months – year )– Analyze the collected data and look for signals
But this is not what it happening!
• Detectors doing extremely well– Far better than we expected 2 years ago
![Page 4: Comments on the First Year (2010) of the LHC](https://reader035.fdocuments.us/reader035/viewer/2022062812/568163e6550346895dd54aec/html5/thumbnails/4.jpg)
Implications of a Fast Start
• The detectors are already partially calibrated
• The data quality looks fantastic
• We could, if there were any signals to see, look for them already!
Can there be any? How much data do we need?
![Page 5: Comments on the First Year (2010) of the LHC](https://reader035.fdocuments.us/reader035/viewer/2022062812/568163e6550346895dd54aec/html5/thumbnails/5.jpg)
How much data do we need?• Naïve argument: Minimum 100 inverse pb to match Tevatron
– Recall• Tevatron experiments : 6 – 7 inverse fb of data each at 1.96 TeV
– though not all analyses use it all• LHC experiments for this run (through 2011) 1 inverse fb at 7 TeV
– Right now: few inverse nb !!
• Logic– LHC vs Tevatron cross sections
• gg, qg processes increase of about 30• q qbar processes increase is less
– Need 5000/30 ~ 150 inv pb or more to match statistics of Tevatron
![Page 6: Comments on the First Year (2010) of the LHC](https://reader035.fdocuments.us/reader035/viewer/2022062812/568163e6550346895dd54aec/html5/thumbnails/6.jpg)
Loopholes in the Logic• Factor 30 in ratio of LHC/TeV rates is larger for heavy colored objects
– Di-jet resonance well above TeV– Heavy gluino/squark pair production decay to stau or photon pairs
• Effect that grows very rapidly with energy– Black hole production
• There are things the Tevatron experiments haven’t done• There are things the Tevatron experiments can’t do as well
– Pairs of dijet resonances– Boosted objects of various types– Long-lived particles decaying in flight outside inner tracker– Objects with sufficiently unusual tracks
![Page 7: Comments on the First Year (2010) of the LHC](https://reader035.fdocuments.us/reader035/viewer/2022062812/568163e6550346895dd54aec/html5/thumbnails/7.jpg)
But that’s not enough• It may be that a signal is large at LHC7 (30 pb’s)
• But it may still be too small to discover soon– Large backgrounds force low-efficiency search– Can’t trigger efficiently– Requires calibration that needs lots of data– Requires background estimate that needs lots of data
• Example: heavy gluinos decaying to quarks and neutralinos– jets + missing transverse momentum (MET)
• What kinds of searches are feasible with small data samples? – 100 inv pb? 10 inv pb? 1 inv pb? Less?!
![Page 8: Comments on the First Year (2010) of the LHC](https://reader035.fdocuments.us/reader035/viewer/2022062812/568163e6550346895dd54aec/html5/thumbnails/8.jpg)
Today: Tentative Answer
Claim: 2010 is the Year of the Exotic1 inverse pb is more than enough!
Unfinished Work: I will give you one unconfirmed example to prove the pointProvocation: I challenge the audience to look for others!
• Everything I will show today is preliminary– Errors in numbers still possible– Conceptual errors regarding existing searches?– Factual errors regarding planned searches– Feedback welcome and requested!
![Page 9: Comments on the First Year (2010) of the LHC](https://reader035.fdocuments.us/reader035/viewer/2022062812/568163e6550346895dd54aec/html5/thumbnails/9.jpg)
Today: Class of large signals• Will show
– Large exotic signals not excluded yet– They may be observable in Tevatron data now– They may be observable in 1 – 10 inverse pb of LHC7 data
• Existing search strategies are good
• But a tracking-based strategy is – Complementary both for discovery and diagnosis– Possibly more sensitive in some cases– Not known to me to be on the list of early search strategies
![Page 10: Comments on the First Year (2010) of the LHC](https://reader035.fdocuments.us/reader035/viewer/2022062812/568163e6550346895dd54aec/html5/thumbnails/10.jpg)
Today: Pairs of Tracks
• Tracking: requires only … tracker (already pretty well calibrated)
• Trigger: jet (typically from initial state radiation)
• Search for pairs of objects that make isolated tracks, but– Not in typical jets– Not muons, electrons, or taus
• Tevatron has not looked in this channel yet
• Tracking search is complementary to other exotica searches– Heavy Stable Charged Particle (CHAMP/CSMP/HSCP) searches– Highly-Displaced Vertex searches– Jets-plus-MET searches
![Page 11: Comments on the First Year (2010) of the LHC](https://reader035.fdocuments.us/reader035/viewer/2022062812/568163e6550346895dd54aec/html5/thumbnails/11.jpg)
Essential Strategy• Search for events with trigger jet plus two tracks
– Tracks of moderate pT (50 – 100 GeV)– isolated from other tracks (not from energy)– Expect 100 – 1000 events / pb from QCD [detector bkgds?]
• Look for anomalous behavior– Anomalous dE/dx [atypical ionization in tracker]– Time delay [time of flight detectors show velocity < c]– Weird calorimeter deposition [not a normal EM or hadronic shower]
• Use events with one high pT track to measure backgrounds.
• AT ALL STEPS, SEPARATE MODEL INDEPENDENT FROM MODEL DEPENDENT
![Page 12: Comments on the First Year (2010) of the LHC](https://reader035.fdocuments.us/reader035/viewer/2022062812/568163e6550346895dd54aec/html5/thumbnails/12.jpg)
Benchmark Model: Unstable Gluinos
• Example to serve as existence proof– Other signals will differ greatly in their details– But I believe the tracking-based strategy can be widely applied
• Unstable gluinos with lifetimes in the 10 nanosecond range– Split SUSY [all squarks heavy] (dijet or gluon plus MET)– Gluino LSP decays via R parity (3 quarks)– Gluino LSP plus hidden sector (hidden LSP plus ???)
• e.g. hidden valley sector– Final state may be complicated; limited MET
• Many possible final states in gluino decay– Tracking strategy is (initially) largely independent of this ambiguity!
![Page 13: Comments on the First Year (2010) of the LHC](https://reader035.fdocuments.us/reader035/viewer/2022062812/568163e6550346895dd54aec/html5/thumbnails/13.jpg)
What are Tevatron Constraints?
• Three classes of constraints I know about
– Gluinos make R-hadrons (CDF search)– Gluino decays make vertices (D0 search)– Gluino decays may give jets plus MET (many searches)
![Page 14: Comments on the First Year (2010) of the LHC](https://reader035.fdocuments.us/reader035/viewer/2022062812/568163e6550346895dd54aec/html5/thumbnails/14.jpg)
Stable gluinos: R-hadron searchCDF experiment looks for stable tau sleptons and top squarks
Slepton : a heavy charge = 1 stable particle
• Look for slow “muon” (travels through detector with little energy loss)
– Muon candidate: pT > 40 GeV, |eta| < 0.7– Slow: time of flight 0.4 < v < 0.9– ISOLATION NOT REQUIRED
• R-hadrons are not well isolated from soft tracks, energy– SECOND CANDIDATE NOT REQUIRED
• R-hadrons may be neutral, often only 1 track per event
– 10 fb limit on stable tau sleptons
![Page 15: Comments on the First Year (2010) of the LHC](https://reader035.fdocuments.us/reader035/viewer/2022062812/568163e6550346895dd54aec/html5/thumbnails/15.jpg)
Stable gluinos: R-hadron search
Squark : a heavy charge = 2/3 colored stable particle• Forms charge 1 or 0 hadron (“R-hadron”) with up or down antiquark• Traveling in matter, interacts often, loses little energy, can flip charge!
• Look for slow “muon” (travels through detector with little energy loss)
– Muon candidate: pT > 40 GeV, |eta| < 0.7– Slow: time of flight 0.4 < v < 0.9– ISOLATION NOT REQUIRED
• R-hadrons are not well isolated from soft tracks, energy– SECOND CANDIDATE NOT REQUIRED
• R-hadrons may be neutral, often only 1 track per event
– 10 fb limit on stable tau sleptons– 48 fb limit on stable top squarks
![Page 16: Comments on the First Year (2010) of the LHC](https://reader035.fdocuments.us/reader035/viewer/2022062812/568163e6550346895dd54aec/html5/thumbnails/16.jpg)
Stable gluinos: R-hadron searchGluino : a heavy charge = 0 colored stable particle• Forms charge 1, -1 or 0 hadron (“R-hadron”) with quark and antiquark• Also may form baryons easily• Traveling in matter, interacts more often, loses little energy, can flip charge!
• Look for slow “muon” (travels through detector with little energy loss)– Muon candidate: pT > 40 GeV, |eta| < 0.7– Slow: time of flight 0.4 < v < 0.9– ISOLATION NOT REQUIRED
• R-hadrons are not well isolated from soft tracks, energy– SECOND CANDIDATE NOT REQUIRED
• R-hadrons may be neutral, often only 1 track per event
– 10 fb limit on stable tau sleptons– 48 fb limit on stable top squarks– MY GUESS: 200 fb limit on stable gluinos (very large errors) > 360 GeV
![Page 17: Comments on the First Year (2010) of the LHC](https://reader035.fdocuments.us/reader035/viewer/2022062812/568163e6550346895dd54aec/html5/thumbnails/17.jpg)
Convert Stau to Squark• Staus are simple
– Every stau in the acceptance window is observed• Caveat: some losses in TOF measurement
• Squarks– Squark-antiquark like B-meson system
• Only about 50% probability R-hadron is charged• Meson can flip from charged to neutral in matter• Meson can become baryon in matter (ignore)
– CDF charge flipping• Assume squark-antisquark has ½ interaction length of pion• 80% (63%) prob. of interaction in calorimeter (muon sys.)
– About 50% probability of charge flipping eac h time
![Page 18: Comments on the First Year (2010) of the LHC](https://reader035.fdocuments.us/reader035/viewer/2022062812/568163e6550346895dd54aec/html5/thumbnails/18.jpg)
Converting Squark to Gluino• Gluino has at least four states
– Isosinglet plus isotriplet from gluino-quark-antiquark– Possibly additional light isosinglet from gluino-gluon
• Charge-flipping is more common– Larger interaction cross-section (like 1 pion, not ½ pion)
• 96% (86%) prob. in calorimeter (muon system)– More likely to flip
• Especially for negative charge– Rough estimate 1/4 as likely to make a good muon track as squark
• Good muon track hits in inner and outer muon chambers• Large errors on this estimate!!
![Page 19: Comments on the First Year (2010) of the LHC](https://reader035.fdocuments.us/reader035/viewer/2022062812/568163e6550346895dd54aec/html5/thumbnails/19.jpg)
Result• Stable gluinos (or other color-adjoint particles) are limited to 200 fb
– For gluinos this means mass > 360 GeV– Large errors (difficult to quantify!)
• I have assumed gluinos that make charged R-hadrons P = 50% of time– May be much less! Dynamical question, not yet answered
• If P close to zero, use “monojet type search” (Hewett et al. 2004)– R-hadrons leave little energy– Neutral R-hadrons are virtually invisible (no track, no jet)– Monojet search gluino mass > 170 GeV in 2004 !
![Page 20: Comments on the First Year (2010) of the LHC](https://reader035.fdocuments.us/reader035/viewer/2022062812/568163e6550346895dd54aec/html5/thumbnails/20.jpg)
Gluinos decaying in flight• If sufficiently short lifetime
– Gluinos will rarely reach the muon system (no stable R-hadron signal)– Gluino decays will add jets in the calorimeter (no monojet signal)
• What about jets + unbalanced transverse momentum (MET)?– Gluino decays may have real MET– Gluino decays at funny angles or in calorimeter make fake MET
• But (thanks Yuri Gershtein, Eva Halkiadakis) jets +MET searches require high jet quality– Makes sense for SUSY searches– But discards this signal
• Does any remain? Not much (of order 1/1000 ?)
![Page 21: Comments on the First Year (2010) of the LHC](https://reader035.fdocuments.us/reader035/viewer/2022062812/568163e6550346895dd54aec/html5/thumbnails/21.jpg)
Limits on Gluinos• Straightforward to get lifetime limit from CDF R-hadron search• Not straightforward to get limit from
– Monojet and other jet-plus-MET searches at CDF/D0– Di-vertex search at D0
• Depends on final state!
Madgraph mu = m_gluinoK factor 2
< 3 events expectedin R-hadron search
![Page 22: Comments on the First Year (2010) of the LHC](https://reader035.fdocuments.us/reader035/viewer/2022062812/568163e6550346895dd54aec/html5/thumbnails/22.jpg)
Limits on GluinosDi-vertex search (thanks Andy Haas, Yuri Gershtein)• Jet > 10 GeV containing muon > 4 GeV• Two vertices (outside material) betwn 1.6 and 20 cm• 4 or more tracks reconstructed• Large invt mass or non-pointing angle for tracks
Madgraph mu = m_gluinoK factor 2
Before triggering andevent selection!!!
< 3 events expectedin R-hadron search
Gluino final state
Quarks? Muon in 1/25Gluons? Muon in 1/300
Muon often too softMuon may not point back
![Page 23: Comments on the First Year (2010) of the LHC](https://reader035.fdocuments.us/reader035/viewer/2022062812/568163e6550346895dd54aec/html5/thumbnails/23.jpg)
How to Observe? Tracks• Gluino R-hadron charged fraction P of the time
– Assume P = 0.5 (if << 0.5, need other methods)– Gluino heavy, slow radiates little R-hadron isolated from tracks– R-hadron deposits some small amount of energy in calorimeter
• Select events with two isolated tracks (NOT necessarily opposite sign)
• Isolation prescription (loose, not unique)– Two tracks not within R=0.7 of each other– No tracks within cone of R=0.7 with pT > 1/3 pT of main track– Not isolated from energy or multiple soft tracks
• Trigger?• Backgrounds? (QCD, detector)
![Page 24: Comments on the First Year (2010) of the LHC](https://reader035.fdocuments.us/reader035/viewer/2022062812/568163e6550346895dd54aec/html5/thumbnails/24.jpg)
Trigger• We do not entirely understand R-hadron spectrum, behavior in matter• I have not specified how R-hadrons decay; can vary widely• Goal is not to look just for gluinos but for many other signals with tracks
Want a robust model-independent trigger!
• Inclusive jet trigger? (Gluinos + ISR jet) Usually has high threshold. – BUT AT LHC, NOT YET.
• 1030 cm-2 s-1 – unprescaled inclusive jet threshold = 50 GeV (?)• 1031 cm-2 s-1 – unprescaled inclusive jet threshold = 100 GeV (?)
– AND AT TEVATRON, SO WHAT?• Large data set low trigger efficiency may be ok for discovery• Large cross-section maybe can use prescaled triggers
![Page 25: Comments on the First Year (2010) of the LHC](https://reader035.fdocuments.us/reader035/viewer/2022062812/568163e6550346895dd54aec/html5/thumbnails/25.jpg)
![Page 26: Comments on the First Year (2010) of the LHC](https://reader035.fdocuments.us/reader035/viewer/2022062812/568163e6550346895dd54aec/html5/thumbnails/26.jpg)
? ?
Jet Trigger
SlowSlow
![Page 27: Comments on the First Year (2010) of the LHC](https://reader035.fdocuments.us/reader035/viewer/2022062812/568163e6550346895dd54aec/html5/thumbnails/27.jpg)
Jet Trigger
Track Stub Track
![Page 28: Comments on the First Year (2010) of the LHC](https://reader035.fdocuments.us/reader035/viewer/2022062812/568163e6550346895dd54aec/html5/thumbnails/28.jpg)
Jet Trigger
Track Stub Track
![Page 29: Comments on the First Year (2010) of the LHC](https://reader035.fdocuments.us/reader035/viewer/2022062812/568163e6550346895dd54aec/html5/thumbnails/29.jpg)
Jet Trigger
Track Stub Track
Slow
High pT
High pT
![Page 30: Comments on the First Year (2010) of the LHC](https://reader035.fdocuments.us/reader035/viewer/2022062812/568163e6550346895dd54aec/html5/thumbnails/30.jpg)
Two Subsamples• Jet-plus-di-track sample
– Two isolated tracks above pT cut– One triggerable jet above trigger threshold
• Fired (or could have fired) the inclusive jet trigger• Does NOT contain either track
– ALMOST MODEL INDEPENDENT• Relatively easy to estimate trigger efficiency
• Pure di-track sample– Any other events with two isolated tracks above pT cut– VERY MODEL DEPENDENT
• Trigger efficiency could be 0% or 100%
![Page 31: Comments on the First Year (2010) of the LHC](https://reader035.fdocuments.us/reader035/viewer/2022062812/568163e6550346895dd54aec/html5/thumbnails/31.jpg)
Jet Trigger
Late
If this jet has pT>50, jet-plus-di-trackIf this jet has pT<50, pure di-track
![Page 32: Comments on the First Year (2010) of the LHC](https://reader035.fdocuments.us/reader035/viewer/2022062812/568163e6550346895dd54aec/html5/thumbnails/32.jpg)
Strategy• Use jet-plus-di-track for primary search
– Allow for one track stub• If hint is found, look in pure-di-track
– See nothing, learn nothing (since trigger efficiency unknown)– See something, adds significantly to jet-plus-di-track sample
• In both cases, use anomalous features of the tracks– Primary: Anomalous dE/dx from low velocity
• Measure in pixels (even track stub!) and TRT (ATLAS only)• NOT VERY MODEL DEPENDENT
– Secondary: Late time of arrival of track or its decay products– Secondary: Vertex at end of track stub– Secondary: Anomalous calorimetry
• MODEL DEPENDENT, BUT POWERFUL IN CEMENTING A CASE
![Page 33: Comments on the First Year (2010) of the LHC](https://reader035.fdocuments.us/reader035/viewer/2022062812/568163e6550346895dd54aec/html5/thumbnails/33.jpg)
LHC7 cross-sections (estimates!)
Ratio of event with a jet estimated from production of t-tbar-j/t-tbarLARGE UNCERTAINTIES!
Madgraph mu = m_gluinoK factor 2
Tevatron
![Page 34: Comments on the First Year (2010) of the LHC](https://reader035.fdocuments.us/reader035/viewer/2022062812/568163e6550346895dd54aec/html5/thumbnails/34.jpg)
LHC7 Cross sections for Jet-plus-di-tracksdE/dx BOTH tracks
dE/dx ONE track
2 tracks 1 track, 1 track stub
Requiring0.3 < v < 0.9
ApproximatedCMS/ATLAS tracker geometry
![Page 35: Comments on the First Year (2010) of the LHC](https://reader035.fdocuments.us/reader035/viewer/2022062812/568163e6550346895dd54aec/html5/thumbnails/35.jpg)
QCD Backgrounds• Estimate using Pythia/Herwig inclusive dijets
– Calculate for two isolated tracks (isolation as before) plus 3rd trigger jet
Conservative Upper Bound• K factor 2 (?)• Data exceeds Pythia/Herwig by ~ 2 (?) for tracks with track pT / jet pT > 0.6
Fake Hard Isolated Tracks – measure in events with one such trackFake Anomalous dE/dx – measure in events with one isolated track If backgrounds too large, can
tighten isolation requirements increase pT cuts
Thanks to G. Salam
![Page 36: Comments on the First Year (2010) of the LHC](https://reader035.fdocuments.us/reader035/viewer/2022062812/568163e6550346895dd54aec/html5/thumbnails/36.jpg)
Compare with other strategiesIs it useful to do tracking based search?
• Competitive with other methods– Stable R-hadron searches– Decaying particle searches
• Complementary to other methods– Can help diagnose a discovery
• Some signals will give many tracks, others not– Opportunity to find things (beyond gluinos) that make tracks but
would have slipped by the Tevatron searches
![Page 37: Comments on the First Year (2010) of the LHC](https://reader035.fdocuments.us/reader035/viewer/2022062812/568163e6550346895dd54aec/html5/thumbnails/37.jpg)
How far do the gluinos travel?• Charged R-hadrons with the maximum allowed lifetime from Tevatron
– Cross section (pb) to travel further than (cylindrical) radial distance– Pt>50, |eta|< 1.7, 0.6 < v < 0.95– No triggering, charge flipping or other losses included here!
If lifetime shorter than maximum, benefits tracking search relative to stable R-hadron search
Recall: jet-plus-ditrack > 1 pbPure ditracks may possibly triple it
![Page 38: Comments on the First Year (2010) of the LHC](https://reader035.fdocuments.us/reader035/viewer/2022062812/568163e6550346895dd54aec/html5/thumbnails/38.jpg)
Highly-displaced vertex searches
Jet Trigger
![Page 39: Comments on the First Year (2010) of the LHC](https://reader035.fdocuments.us/reader035/viewer/2022062812/568163e6550346895dd54aec/html5/thumbnails/39.jpg)
Highly-displaced vertex searches
Jet Trigger
![Page 40: Comments on the First Year (2010) of the LHC](https://reader035.fdocuments.us/reader035/viewer/2022062812/568163e6550346895dd54aec/html5/thumbnails/40.jpg)
Conclusions• Unstable gluino pairs: existence proof that 1 inverse pb is certainly enough
– Probably even larger signals that Tevatron has not excluded• Anomalous track-pair search seems worthwhile
– Tracks are relatively model-independent• Existence of tracks depends on production• Interactions of track in tracker are limited• Good for decays, weird calorimetry, slightly non-helical track, …
– Use jet trigger to keep trigger efficiency model independent• Efficient at low luminosity• May want to adjust trigger thresholds/prescales if early hint seen
– Complementary to other searches
• NO REASON FOR THEORISTS TO WAIT FOR 100 INVERSE PB!