Presentation on theme: "Comments on the First Year (2010) of the LHC Matthew Strassler Rutgers University Work in progress (special thank you to M. Cacciari, G. Salam)"— Presentation transcript:
Comments on the First Year (2010) of the LHC Matthew Strassler Rutgers University Work in progress (special thank you to M. Cacciari, G. Salam)
The Fast Start So far, LHC going well Original expectation ( ) – 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
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?
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
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
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?!
Today: Tentative Answer Claim: 2010 is the Year of the Exotic 1 inverse pb is more than enough! Unfinished Work: I will give you one unconfirmed example to prove the point Provocation: 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!
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
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
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
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!
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)
Stable gluinos: R-hadron search CDF 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
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
Stable gluinos: R-hadron search Gluino : 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
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
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!!
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 ) – R-hadrons leave little energy – Neutral R-hadrons are virtually invisible (no track, no jet) – Monojet search gluino mass > 170 GeV in 2004 !
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 ?)
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_gluino K factor 2 < 3 events expected in R-hadron search
Limits on Gluinos Di-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_gluino K factor 2 Before triggering and event selection!!! < 3 events expected in R-hadron search Gluino final state Quarks? Muon in 1/25 Gluons? Muon in 1/300 Muon often too soft Muon may not point back
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)
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 cm -2 s -1 – unprescaled inclusive jet threshold = 50 GeV (?) 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
? ? Jet Trigger Slow
Jet Trigger Track Stub Track
Jet Trigger Track Stub Track
Jet Trigger Track Stub Track Slow High pT
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%
Jet Trigger Late If this jet has pT>50, jet-plus-di-track If this jet has pT<50, pure di-track
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
LHC7 cross-sections (estimates!) Ratio of event with a jet estimated from production of t-tbar-j/t-tbar LARGE UNCERTAINTIES! Madgraph mu = m_gluino K factor 2 Tevatron
LHC7 Cross sections for Jet-plus-di-tracks dE/dx BOTH tracks dE/dx ONE track 2 tracks1 track, 1 track stub Requiring 0.3 < v < 0.9 Approximated CMS/ATLAS tracker geometry
QCD Backgrounds Estimate using Pythia/Herwig inclusive dijets – Calculate for two isolated tracks (isolation as before) plus 3 rd 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 track Fake 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
Compare with other strategies Is 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
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 pb Pure ditracks may possibly triple it
Highly-displaced vertex searches Jet Trigger
Highly-displaced vertex searches Jet Trigger
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!