Jets at LHCb. Gavin Salam. LHCb, CERN, 21 January CERN, Princeton & LPTHE/CNRS (Paris)

Transcription

1 Jets at LHCb Gavin Salam CERN, Princeton & LPTHE/CNRS (Paris) LHCb, CERN, 21 January 2011

2 LHCb (G. Salam) CERN, / 16 Any process that involves final-state partons gives jets Examples of physics with jets where b-tagging and/or forward detection may be crucial: forward jets: unusual mixture of high-x PDFs on low-x PDFs t t FB asymmetry: only at high rapidities is there a substantial q q initial-state asymmetry SUSY Higgs: pp b bh 4b

3 Jet finding LHCb (G. Salam) CERN, / 16

4 LHCb (G. Salam) CERN, / 16 Jets made systematic: jet definitions p π π φ K LO partons NLO partons parton shower hadron level Jet Def n Jet Def n Jet Def n Jet Def n jet 1 jet 2 jet 1 jet 2 jet 1 jet 2 jet 1 jet 2 LHC events may be discussed in terms of quarks, quarks+gluon, or hadrons A jet definition provides common representation of different levels of event complexity.

5 LHCb (G. Salam) CERN, / 16 Sequential recombination algorithms k t algorithm Catani, Dokshizter, Olsson, Seymour, Turnock, Webber Ellis, Soper 93 Find smallest of all d ij = min(kti 2,k2 tj ) R2 ij /R2 and d ib = ki 2 Recombine i,j (if ib: i jet) Repeat Bottom-up jets: Sequential recombination NB: hadron collider variables (attempt to invert QCD branching) Rij 2 = (φ i φ j ) 2 +(y i y j ) 2 rapidity y i = 1 2 ln E i+p zi E i p zi R ij is boost invariant angle R sets minimal interjet angle

6 LHCb (G. Salam) CERN, / 16 Sequential recombination algorithms k t algorithm Catani, Dokshizter, Olsson, Seymour, Turnock, Webber Ellis, Soper 93 Find smallest of all d ij = min(k 2 ti,k2 tj ) R2 ij /R2 and d ib = k 2 i Recombine i,j (if ib: i jet) Repeat NB: hadron collider variables R 2 ij = (φ i φ j ) 2 +(y i y j ) 2 rapidity y i = 1 2 ln E i+p zi E i p zi R ij is boost invariant angle R sets minimal interjet angle

7 LHCb (G. Salam) CERN, / 16 Sequential recombination algorithms k t algorithm Catani, Dokshizter, Olsson, Seymour, Turnock, Webber Ellis, Soper 93 Find smallest of all d ij = min(k 2 ti,k2 tj ) R2 ij /R2 and d ib = k 2 i Recombine i,j (if ib: i jet) Repeat NB: hadron collider variables R 2 ij = (φ i φ j ) 2 +(y i y j ) 2 rapidity y i = 1 2 ln E i+p zi E i p zi R ij is boost invariant angle R sets minimal interjet angle

8 LHCb (G. Salam) CERN, / 16 Sequential recombination algorithms k t algorithm Catani, Dokshizter, Olsson, Seymour, Turnock, Webber Ellis, Soper 93 Find smallest of all d ij = min(k 2 ti,k2 tj ) R2 ij /R2 and d ib = k 2 i Recombine i,j (if ib: i jet) Repeat NB: hadron collider variables R 2 ij = (φ i φ j ) 2 +(y i y j ) 2 rapidity y i = 1 2 ln E i+p zi E i p zi R ij is boost invariant angle R sets minimal interjet angle

9 LHCb (G. Salam) CERN, / 16 Sequential recombination algorithms k t algorithm Catani, Dokshizter, Olsson, Seymour, Turnock, Webber Ellis, Soper 93 Find smallest of all d ij = min(k 2 ti,k2 tj ) R2 ij /R2 and d ib = k 2 i Recombine i,j (if ib: i jet) Repeat NB: hadron collider variables R 2 ij = (φ i φ j ) 2 +(y i y j ) 2 rapidity y i = 1 2 ln E i+p zi E i p zi R ij is boost invariant angle R sets minimal interjet angle

10 LHCb (G. Salam) CERN, / 16 Sequential recombination algorithms k t algorithm Catani, Dokshizter, Olsson, Seymour, Turnock, Webber Ellis, Soper 93 Find smallest of all d ij = min(k 2 ti,k2 tj ) R2 ij /R2 and d ib = k 2 i Recombine i,j (if ib: i jet) Repeat NB: hadron collider variables R 2 ij = (φ i φ j ) 2 +(y i y j ) 2 rapidity y i = 1 2 ln E i+p zi E i p zi R ij is boost invariant angle R sets minimal interjet angle

11 LHCb (G. Salam) CERN, / 16 Sequential recombination algorithms k t algorithm Catani, Dokshizter, Olsson, Seymour, Turnock, Webber Ellis, Soper 93 Find smallest of all d ij = min(k 2 ti,k2 tj ) R2 ij /R2 and d ib = k 2 i Recombine i,j (if ib: i jet) Repeat NB: hadron collider variables R 2 ij = (φ i φ j ) 2 +(y i y j ) 2 rapidity y i = 1 2 ln E i+p zi E i p zi R ij is boost invariant angle R sets minimal interjet angle

12 LHCb (G. Salam) CERN, / 16 Sequential recombination algorithms k t algorithm Catani, Dokshizter, Olsson, Seymour, Turnock, Webber Ellis, Soper 93 Find smallest of all d ij = min(k 2 ti,k2 tj ) R2 ij /R2 and d ib = k 2 i Recombine i,j (if ib: i jet) Repeat R ij > R NB: hadron collider variables R 2 ij = (φ i φ j ) 2 +(y i y j ) 2 rapidity y i = 1 2 ln E i+p zi E i p zi R ij is boost invariant angle R sets minimal interjet angle

13 LHCb (G. Salam) CERN, / 16 Sequential recombination algorithms k t algorithm Catani, Dokshizter, Olsson, Seymour, Turnock, Webber Ellis, Soper 93 Find smallest of all d ij = min(k 2 ti,k2 tj ) R2 ij /R2 and d ib = k 2 i Recombine i,j (if ib: i jet) Repeat R ij > R NB: hadron collider variables R 2 ij = (φ i φ j ) 2 +(y i y j ) 2 rapidity y i = 1 2 ln E i+p zi E i p zi R ij is boost invariant angle R sets minimal interjet angle NB: d ij distance QCD branching probability α s dk 2 tj dr2 ij d ij

14 LHCb (G. Salam) CERN, / 16 A full set of IRC-safe jet algorithms Generalise inclusive-type sequential recombination with d ij = min(k 2p ti,k 2p tj ) R 2 ij/r 2 d ib = k 2p ti Alg. name Comment time p = 1 k t Hierarchical in rel. k t CDOSTW 91-93; ES 93 NlnN exp. p = 0 Cambridge/Aachen Hierarchical in angle Dok, Leder, Moretti, Webber 97 Scan multiple R at once NlnN Wengler, Wobisch 98 QCD angular ordering p = 1 anti-k t Cacciari, GPS, Soyez 08 Hierarchy meaningless, jets reverse-k t Delsart like CMS cone (IC-PR) N 3/2 SC-SM SISCone Replaces JetClu, ATLAS GPS Soyez 07 + Tevatron run II 00 MidPoint (xc-sm) cones N 2 lnn exp. All these algorithms [& much more] coded in (efficient) C++ at (Cacciari, GPS & Soyez 05-11)

15 LHCb (G. Salam) CERN, / 16 A 3-year old general-purpose jet algorithm anti-k t : repeatedly recombine pair of objects with smallest d ij = R 2 ij max(k 2 ti,k2 tj ) Hard stuff clusters with nearest neighbour Cacciari, GPS & Soyez 08 [included in FastJet]

16 LHCb (G. Salam) CERN, / 16 A 3-year old general-purpose jet algorithm anti-k t : repeatedly recombine pair of objects with smallest d ij = R 2 ij max(k 2 ti,k2 tj ) Hard stuff clusters with nearest neighbour Cacciari, GPS & Soyez 08 [included in FastJet]

17 LHCb (G. Salam) CERN, / 16 A 3-year old general-purpose jet algorithm anti-k t : repeatedly recombine pair of objects with smallest d ij = R 2 ij max(k 2 ti,k2 tj ) Hard stuff clusters with nearest neighbour Cacciari, GPS & Soyez 08 [included in FastJet]

18 LHCb (G. Salam) CERN, / 16 A 3-year old general-purpose jet algorithm anti-k t : repeatedly recombine pair of objects with smallest d ij = R 2 ij max(k 2 ti,k2 tj ) Hard stuff clusters with nearest neighbour Cacciari, GPS & Soyez 08 [included in FastJet]

19 LHCb (G. Salam) CERN, / 16 A 3-year old general-purpose jet algorithm anti-k t : repeatedly recombine pair of objects with smallest d ij = R 2 ij max(k 2 ti,k2 tj ) Hard stuff clusters with nearest neighbour Cacciari, GPS & Soyez 08 [included in FastJet]

20 LHCb (G. Salam) CERN, / 16 A 3-year old general-purpose jet algorithm anti-k t : repeatedly recombine pair of objects with smallest d ij = R 2 ij max(k 2 ti,k2 tj ) Hard stuff clusters with nearest neighbour Cacciari, GPS & Soyez 08 [included in FastJet]

21 LHCb (G. Salam) CERN, / 16 A 3-year old general-purpose jet algorithm anti-k t : repeatedly recombine pair of objects with smallest d ij = R 2 ij max(k 2 ti,k2 tj ) Hard stuff clusters with nearest neighbour Cacciari, GPS & Soyez 08 [included in FastJet]

22 LHCb (G. Salam) CERN, / 16 A 3-year old general-purpose jet algorithm anti-k t : repeatedly recombine pair of objects with smallest d ij = R 2 ij max(k 2 ti,k2 tj ) Hard stuff clusters with nearest neighbour Cacciari, GPS & Soyez 08 [included in FastJet]

23 LHCb (G. Salam) CERN, / 16 A 3-year old general-purpose jet algorithm anti-k t : repeatedly recombine pair of objects with smallest d ij = R 2 ij max(k 2 ti,k2 tj ) Hard stuff clusters with nearest neighbour Cacciari, GPS & Soyez 08 [included in FastJet]

24 LHCb (G. Salam) CERN, / 16 A 3-year old general-purpose jet algorithm anti-k t : repeatedly recombine pair of objects with smallest d ij = R 2 ij max(k 2 ti,k2 tj ) Hard stuff clusters with nearest neighbour Cacciari, GPS & Soyez 08 [included in FastJet]

25 LHCb (G. Salam) CERN, / 16 A 3-year old general-purpose jet algorithm anti-k t : repeatedly recombine pair of objects with smallest d ij = R 2 ij max(k 2 ti,k2 tj ) Hard stuff clusters with nearest neighbour Cacciari, GPS & Soyez 08 [included in FastJet]

26 LHCb (G. Salam) CERN, / 16 A 3-year old general-purpose jet algorithm anti-k t : repeatedly recombine pair of objects with smallest d ij = R 2 ij max(k 2 ti,k2 tj ) Hard stuff clusters with nearest neighbour Cacciari, GPS & Soyez 08 [included in FastJet] anti-k t gives cone-like jets without using cones And is infrared & collinear safe

27 LHCb (G. Salam) CERN, / 16 Which jet algorithm to use? For generic jet-finding, use anti-k t algorithm. not just because I m biased; but also because it works well; and it has become the standard Which R value(s) to use? ATLAS: 0.4 and 0.6 CMS: 0.5 and 0.7 Best choice depends on the physics; having two values provides powerful check on systematics

28 Forward Jets and PDFs LHCb (G. Salam) CERN, / 16

29 LHCb (G. Salam) CERN, / 16 [Forward jets] Jets going forwards LHCb probes region with interesting mix of high-x and low-x PDFs: x 1,2 2p t s e ±y cosh y p t = 200 GeV p t = 100 GeV p t = 50 GeV p t = 50 GeV p t = 100 GeV x 1 x p t = 200 GeV 8 TeV, y= TeV, y= y jets y jets

30 LHCb (G. Salam) CERN, / 16 [Forward jets] Large cross sections dσ/dp t [nb/gev] Inclusve jet spectrum 1.9<y< <y< <y<4.9 pp 7 TeV, Herwig (no UE) anti-k t R=1.0 Use of R = 1 is arbitrary p t [GeV] Going forwards, you get large x without paying the price of tiny cross sections caused by O( TeV) momentum transfers

31 LHCb (G. Salam) CERN, / 16 b jets E.g. identify the direction of each b Label the calorimeter-tower in that direction as a b-tower A b-jet is one that contained b-tower [one of several ways of assigning flavour to jets]

32 [b-jets] b-production sub-processes b b 10 flavour creation (α s 2 ) 10 flavour creation (α s 2 + αs 3 ) 10 flavour excitation (α s 3 ) 10 gluon splitting (α s 3 ) σ bb (LO) / σ ref 1 σ bb (NLO) / σ ref 1 MCFM, Tevatron flav algo α=1 σ bl (NLO) / σ ref 1 σ ll (NLO) / σ ref 1 y 3 kt < < µ/p t,avg < P t,max [GeV] P t,max [GeV] P t,max [GeV] P t,max [GeV] LHCb (G. Salam) CERN, / 16

33 [b-jets] b-jet fractions for LHCb dσ/dp t / dσ b-jet /dp t pp 7 TeV, Herwig (no UE) anti-k t R=1.0 b (and bb) jet fracion 1.9<y< <y< <y<4.9 solid: b + bb jets dashed: bb jets only p t [GeV] Can you experimentally separate b-jets from b b jets? LHCb (G. Salam) CERN, / 16

34 LHCb (G. Salam) CERN, / 16 [b-jets] b-light dijet systems Outgoing b jet follows direction of incoming b-quark. b quark light parton light parton b quark If b is the more forward jet, it s more likely to come from large-x proton rather than small-x proton Depending on accuracy of measurements & calculations, this could provide an interesting constraint on PDFs. Also on gluons, since b-pdf comes from g b b

35 LHCb (G. Salam) CERN, / 16 Conclusions LHCb s ability to do forward jets shared with ATLAS/CMS Its ability to do b-tagged forward jets is unique It was beyond the scope of this talk to do full study of what physics is possible with jets at LHCb. PDFs certainly of interest. Double v. single-b jets probing origin of heavy-flavour in jets. Maybe also top (t t asymmetry), Higgs, etc. I ll be happy to discuss further and try to point you in the direction of people who know more about specific topics.