Try Bulk read using existing ROOT IO low level interface

bulkread.C

void write()
{

int n_muon;
float muon_pt[100];

TFile f("f.root", "RECREATE");
TTree t("t","t");
t.Branch("n_muon", &n_muon, "n_muon/I");
t.Branch("muon_pt", muon_pt, "muon_pt[n_muon]/F");

auto count = 42;
for (auto i : ROOT::TSeqI(2)) {
    n_muon = 3 + i;
    for (auto j : ROOT::TSeqI(n_muon)) {
        muon_pt[j] = count++;
    }
    t.Fill();
}
t.Write();
f.Close();

}

void bulkread()
{

write();
TFile f("f.root");
auto &t = *(TTree*) f.Get("t");

auto &tbranch = *t.GetBranch("muon_pt");
tbranch.GetEntry(0);

auto tbasket = tbranch.GetBasket(0);

// With a buffer
auto tbuffer = tbasket->GetBufferRef();
float farr[100];
tbuffer->ReadFastArray(farr,3);
cout << farr[1] << endl;

// With a leaf

auto tleaf = t.GetLeaf("muon_pt");
auto p = tleaf->GetValuePointer();
cout << ((float*)p)[1] << endl;

}

Before reading from the buffer, we need to moving the TBuffer's cursor to the beginning of the first event (past some meta-data). See the call to SetOffset in TBranch::GetEntry (and the content/semantic of TBranch::fEntryOffset)

... humm but tbranch.GetEntry(0); is called so it should already have been set to the 'right' offset .... so I am not sure what is going on ....

TBranch::GetEntry should induce a call to TLeafF::ReadBasket which should find the TBuffer in the same state as line 40 of the above code (for things to work). The first debugging step is likely to be to understand the different (in particular for TBuffer::GetOffset)